Photo-physics and applications of colloidal quantum dots

Photo-physics and applications of colloidal quantum dots

A thesis submitted to The University of Manchester for the degree of Doctor of Philosophy

in the Faculty of Engineering and Physical Sciences

2010

Stuart K Stubbs

The Photon Science Institute School of Physics and Astronomy

Stuart Stubbs PhD Thesis 2

Contents List of tables........................................................................................................................... 4 List of figures ......................................................................................................................... 4 List of abbreviations............................................................................................................... 7 Abstract ................................................................................................................................ 10 Declaration ........................................................................................................................... 11 Copyright statement ............................................................................................................. 12 Publications and presentations ............................................................................................. 13 Acknowledgements.............................................................................................................. 15 Chapter 1: Introduction and Background............................................................................. 16

1. Introducing Nanocrystal Quantum Dots ...................................................................... 16 1.1. Motivation to study multiple exciton generation (MEG).......................................... 18 1.2. Introduction to hybrid quantum dot organic light-emitting diodes (QD-OLEDs).... 20 1.3. Nanocrystal literature review .................................................................................... 21 1.4. Multiple exciton generation (MEG) literature review ..............................................29 1.5. Hybrid quantum dot organic light emitting devices literature review ...................... 34

Chapter 2: Theory ................................................................................................................ 46 2.1 Quantum dot theory.............................................................................................. 46

2.1.1 Theoretical description of quantum dots...................................................... 46 2.1.2 Confinement regimes ................................................................................... 47 2.1.3 The Particle-in-a-sphere model .................................................................... 48 2.1.4 Applying the model to quantum dots........................................................... 49 2.1.5 Valence band substructure ........................................................................... 51 2.1.6 Exciton fine structure and the “dark” exciton.............................................. 52

2.2 Multiple exciton generation (MEG) theory..........................................................55 2.2.1 Multiple excitons in bulk semiconductors ................................................... 55 2.2.2 Multiple exciton generation in semiconductor nanocrystals........................ 57

2.3 Hybrid quantum dot organic light-emitting devices ............................................ 62 2.3.1 Charge injection and energy transfer in QD-OLEDs................................... 63 2.3.2 Photometry and colour ................................................................................. 66

2.3.2.1 Projected area and solid angle.................................................................. 67 2.3.2.2 Radiometric and photometric quantities .................................................. 68 2.3.2.3 Colour Science ......................................................................................... 71

Chapter 3: Spectroscopic methods ....................................................................................... 75 3 Introduction to optical spectroscopy ........................................................................ 75 3.1 Continuous wave techniques...................................................................................... 75

3.1.1 Steady state absorption spectroscopy........................................................... 75 3.1.2 Steady state fluorescence spectroscopy ....................................................... 80

3.2 Fluorescence lifetime measurements ................................................................... 82 3.2.1 Overview of time-correlated single photon counting................................... 83 3.2.2 TCSPC electronics ....................................................................................... 85 3.2.3 The Instrument Response Function (IRF).................................................... 87 3.2.4 Analysing lifetime data ................................................................................ 88 3.2.5 TCSPC systems............................................................................................ 89

3.2.5.1 Mini-tau based systems............................................................................ 89 3.2.5.2 Femtosecond laser and microchannel plate TCSPC ................................ 91

3.3 Femtosecond transient absorption........................................................................ 94 3.3.1 Optical scheme............................................................................................. 94


3.3.2 White light continuum generation................................................................97 3.3.3 Laser system................................................................................................. 99

3.3.3.1 Millennia and Tsunami .......................................................................... 100 3.3.3.2 Spitfire Pro and Empower......................................................................102 3.3.3.3 TOPAS-C ............................................................................................... 105

3.3.4 Detection and control ................................................................................. 107 Chapter 4: Spectroscopic results ........................................................................................ 109

4.1 Introduction to results ........................................................................................ 109 4.2 Quantum dots under study ................................................................................. 109 4.3 Preliminary MEG studies using TCSPC............................................................ 111

4.3.1 Photoluminescence and absorption spectroscopy...................................... 111 4.3.2 Time correlated single photon counting (TCSPC)..................................... 114 4.3.3 Discussion .................................................................................................. 119

4.4 MEG studies on InP NQDs with ultrafast transient absorption ......................... 121 4.4.1 Sample information.................................................................................... 121 4.4.2 Photoluminescence and absorption spectroscopy...................................... 122 4.4.3 Single exciton lifetimes using TCSPC....................................................... 123 4.4.4 Ultrafast transient absorption of InP NQDs............................................... 125

4.5 Multiexciton dynamics of type II structures ...................................................... 140 4.5.1 Sample Information.................................................................................... 141 4.5.2 Photoluminescence and absorption spectroscopy...................................... 141 4.5.3 TCSPC on type I and II dots ...................................................................... 142 4.5.4 Multiexcitons in type II NQDs...................................................................143 4.5.5 Discussion .................................................................................................. 144

4.6 MEG studies on “green” PbS NQDs.................................................................. 146 4.6.1 Sample Information.................................................................................... 146 4.6.2 Photoluminescence and absorption spectroscopy...................................... 146 4.6.3 MEG studies on PbS .................................................................................. 147

Chapter 5: Methods and techniques in hybrid quantum dot light emitting devices........... 153 5.1. Scope and aims of project ....................................................................................... 153 5.2. Methods and techniques for production and testing ............................................... 153

5.2.1. Spin coating...................................................................................................... 154 5.2.2. Thermal vacuum evaporation........................................................................... 156 5.2.3. Profilometry ..................................................................................................... 159 5.2.4. Atomic Force Microscopy (AFM) ................................................................... 160 5.2.5. Characterisation techniques ............................................................................. 161 5.2.6. Spin and evaporator trials................................................................................. 162

5.3. Initial device builds ................................................................................................. 163 5.4. Conclusions............................................................................................................. 181

Chapter 6: QD-OLED development................................................................................... 183 6. Introduction................................................................................................................ 183 6.1. Improvements to fabrication methods..................................................................... 184 6.2. Cadmium containing QD-OLEDs........................................................................... 186 6.3. Cadmium-free QD-OLEDs ..................................................................................... 196

Conclusions and further work ............................................................................................ 209 Word count: 66,850


List of tables

Table 2.1 Table showing fundamental quantities in radiometry and photometry

P69

Table 6.1 Table showing measured maximum luminance and luminous efficiency at different device brightness’ for the CdSe devices

P194

List of figures

Fig. 1.1 Core-shell quantum dot P17 Fig. 1.2 Theoretical limits to efficiency for single gap devices for different

numbers of excitons created per absorbed photon P20

Fig. 1.3 Absorption and second derivative of absorption for various sizes of quantum dot

P24

Fig. 1.4 Experimental data and calculated pair states are shown as a function of 1st excited state energy

P25

Fig. 1.5 Calculated band edge exciton structure against effective radius P27 Fig. 1.6 Schematic of possible mechanism for intraband relaxation mediated by

surface Cd atoms P28

Fig. 1.7 a) graphical representation of conservation of energy limited carrier multiplication and b) experimentally determined QE for PbSe and PbS dots plotted along QE derived from ideal conservation of energy theory

P30

Fig. 1.8 Transient absorption trace showing the effects of stirring on PbSe dots P33 Fig. 1.9 Energy level diagram of hybrid QD-OLED P36 Fig. 1.10 Electroluminescence spectra of red QD-OLED for different dot

thicknesses P40

Fig. 1.11 Flexible QD-OLED device shown bent at maximum brightness P41 Fig. 2.1 Diagram showing continuum of states in semiconductor and discrete

states in a nanocrystal P46

Fig. 2.2 Band diagrams for a) a direct gap semiconductor and b) a nanocrystal P50 Fig. 2.3 Valence band structure for a) zinc blende semiconductor lattice and b)

wurtzite (hexagonal) crystal lattice P52

Fig. 2.4 Energy level diagram showing the effects on the band edge exciton when a uniaxial crystal lattice and prolate shape dominates and when the exchange interaction in small nanocrystals dominates.

P54

Fig. 2.5 Energy level diagram showing band edge exciton fine structure P55 Fig. 2.6 Cooling mechanism of “hot” electrons and holes in bulk

semiconductors P56

Fig. 2.7 Energy level diagram showing the processes competing with MEG P58 Fig. 2.8 Schematic cross section showing the architecture of a typical device P63 Fig. 2.9 Energy level diagram of a tri-layer device P64 Fig. 2.10 Diagram showing the concept of projected area P68 Fig. 2.11 Plot of photopic spectral luminous function P70 Fig. 2.12 The three CIE 1931 colour matching functions P72 Fig. 2.13 CIE 1931 colour space P73 Fig. 3.1 Simple representation of a two level system in an atom or molecule P76 Fig. 3.2 Diagram showing principle behind spectrophotometer schemes P78


Fig. 3.3 Schematic of the Fluorolog-3 spectrofluorimeter P82 Fig. 3.4 Schematic of TCSPC technique P83 Fig. 3.5 Diagram showing forward and reverse mode in TCSPC P84 Fig. 3.6 Electronics used in TCSPC P85 Fig. 3.7 Graphs showing the distribution of PMT pulse amplitudes and the count

rate plotted against CFD threshold P86

Fig. 3.8 Mini-tau fluorescence lifetime spectrometer P90 Fig. 3.9 Schematic of modified TCSPC set-up P91 Fig. 3.10 Experimental set-up for the femtosecond TCSPC set-up P93 Fig. 3.11 Optical set-up of the femtosecond transient absorption experiment P95 Fig. 3.12 WLC spectra in sapphire optimised at 600 nm and 900 nm P99 Fig. 3.13 Diagram showing the layout of the femtosecond laser system P100 Fig. 3.14 System used in the Tsunami to compensate for GVD P101 Fig. 3.15 Schematic showing the set-up of the regenerative amplifier P104 Fig. 3.16 Arrangement for beams in an OPA P106 Fig. 4.1 Photoluminescence and absorption spectra for CdSe and InP NQDs P113 Fig. 4.2 Instrument response function of TCSPC set-up using MCP-PMT P115 Fig. 4.3 Full fluorescence decay of CdSe Nanodot 640 P115 Fig. 4.4 First 40 ns of fluorescence decay of CdSe Nanodot 640 P116 Fig. 4.5 First 1 ns of fluorescence decay of CdSe Nanodot 640 P116 Fig. 4.6 Time decay curve for InP NQD sample P117 Fig. 4.7 First 1 ns of time decay curve for InP NQD sample P128 Fig. 4.8 PL decay of CdSe NQDs for excitation above and below MEG

threshold P119

Fig. 4.9 Absorption spectra for common solvents that NQDs can be dissolved in P122 Fig. 4.10 PL and absorption spectra taken for the three InP samples used in the

TA MEG study P123

Fig. 4.11 Time decays for the three sizes of InP samples P125 Fig. 4.12 Absorption transients for the large core InP NQD at a range of fluences

for a photon energy equal to 1.4 times the band gap. P127

Fig. 4.13 Selection of transients for pump photon energy of 2.6Eg at low fluences showing persistence of fast time component and comparison of lowest fluence decays for excitation at 1.4 and 2.6Eg.

P128

Fig. 4.14 Average number of electron hole pairs generated per absorbed photon at a range of photon energies.

P130

Fig. 4.15 Average exciton multiplicity plotted against photon energy as a multiple of the NQDs band gap

P132

Fig. 4.16 Transients taken for the large core dot at high and low fluence for excitation at 1.4Eg and 2.6Eg where the quantum dot solution is either static (black lines) or stirred at 1000 rpm

P133

Fig. 4.17 Absorption transients taken on the medium core dot for a range of fluences at the photon energy of 2.76 eV.

P134

Fig. 4.18 Plots of R against fractional change in transmission for the medium core dot for various pump photon energies and a plot of the average number of electron hole pairs created per absorbed photon as a function of hυ/Eg

P136

Fig. 4.19 Transients for the small core dot for two photon energies taken at very similar fluence levels

P137


Fig. 4.20 Plots of R against fractional change in transmission for the small core dot for various pump photon energies and a plot of the average number of electron hole pairs created per absorbed photon as a function of hυ/Eg

P138

Fig. 4.21 Plot of quantum efficiency as a function of hυ/Eg for all three sizes of InP NQD

P139

Fig. 4.22 Diagram showing the structure of the type II dot with the energy level alignment of type I and type II systems

P140

Fig. 4.23 PL and absorption spectra for the type I and type II NQDs P142 Fig. 4.24 PL decays of the type I and type II NQDs P143 Fig. 4.25 Absorption transients taken of the type II NQDs P144 Fig. 4.26 Amplitudes of the fast (~ 30ps) and slow (~300 ps) time components

are plotted as a function of fractional change in transmission P145

Fig. 4.27 Absorption spectrum of the PbS NQDs dispersed in hexane. The first exciton maximum is at ~ 1.3 eV

P147

Fig. 4.28 Absorption transients taken at approximately the same fluence but for excitation above and below the threshold for MEG in PbS (~ 3Eg)

P148

Fig. 4.29 Plots of average number of excitons per photo excited PbS NQD, R, as a function of fractional change in transmission for various pump photon energies. The average exciton multiplicity as a function of hυ/Eg is also shown

P149

Fig. 4.30 Average exciton multiplicity of PbS NQDs as measured in Manchester compared with the bulk values plotted against absolute photon energy and band gap normalised photon energy.

P151

Fig. 5.1 Diagram showing general design of spin coater systems P155 Fig. 5.2 Picture showing the Edwards 306 thin film thermal evaporator P158 Fig. 5.3 2-D scan of the polymer hole injection layer using the stylus profiler P159 Fig. 5.4 General AFM set-up showing optical detection scheme P160 Fig. 5.5 Spins trials conducted on hole transport polymers PVK and poly-TPD P163 Fig. 5.6 Diagram showing regions of ITO remaining after etching and the

deposited aluminium strip P164

Fig. 5.7 Energy level diagram for the initial QD-OLED design P167 Fig. 5.8 Photograph of one of the initial LEDs showing brown discoloration of

the Al cathode P168

Fig. 5.9 Photographs showing first successful EL from a device P168 Fig. 5.10 Diagram of the recess chuck used to give a uniform film P169 Fig. 5.11 Normalised EL of an early device compared with ETL PL P172 Fig. 5.12 EL spectra of device containing TPD and using the phase separation

technique P173

Fig. 5.13 Current density plotted against bias for the QD-OLED described above P174 Fig. 5.14 Photographs of device using phase separation and using TPD P174 Fig. 5.15 Energy level diagram for device structure

ITO/PEDOT/TPD/QD/Alq3/LiF/Al P176

Fig. 5.16 Normalised EL spectra for the device structure ITO/PEDOT/TPD:QD/Alq3/LiF/Al

P178

Fig. 5.17 EL spectra for devices that are baked and unbaked. P180 Fig. 5.18 Current-voltage curves for the baked and unbaked devices P180 Fig. 6.1 Photographs showing the best red CdSe device P186 Fig. 6.2 EL spectra of the red CdSe devices for thinner and thicker NQD layers P188


Fig. 6.3 Photographs showing the best yellow CdSe device P189 Fig. 6.4 EL spectra of the yellow CdSe devices for thinner and thicker NQD

layers P190

Fig. 6.5 TFPL of the various transport layer with the absorption spectra of different sized NQDs to show the spectral overlap

P192

Fig. 6.6 Photograph showing the blue and green CdSe containing devices P193 Fig. 6.7 EL spectra for the red, green and blue CdSe containing devices P194 Fig. 6.8 CIE coordinates for the emission of the best CdSe QD-OLED devices P195 Fig. 6.9 Energy level diagram for the InP containing devices P197 Fig. 6.10 EL spectra for an early InP containing device P199 Fig. 6.11 Normalised EL spectra for the InP device with a thinner HIL P200 Fig. 6.12 Current-voltage and luminous efficiency curves for different HIL P201 Fig. 6.13 Tapping mode AFM of various layers in the device structure P202 Fig. 6.14 EL spectra for devices without NQDs and for different ETL thicknesses P203 Fig. 6.15 Energy level diagram for the device containing no NQDs P204 Fig. 6.16 EL spectra for the device containing TOPO capped InP NQDs P205 Fig. 6.17 EL spectra for the device containing a new batch of TOPO capped InP

NQDs with different HTL thicknesses P207

Fig. 6.18 Current-voltage and luminous efficiency curves for the new batch of TOPO capped InP NQDs

P207

List of abbreviations

ADC Analogue to digital converter

AFM Atomic force microscopy

Alq3 Tris-(8-hydroxyquinoline) aluminium

AOM Acousto-optic modulator

BBO Beta barium borate

CFD Constant fraction discriminator

CIE Commission internationale de l’Elairage

CM Carrier multiplication

CPA Chirped pulse amplification

EA Electron affinity

ehp Electron-hole pair

EIL Electron injection layer

EL Electroluminescence

EML Emissive layer

EQE External quantum efficiency

ETL Electron transport layer


FPD Flat panel display

FRET Förster resonance energy transfer

FTM Film thickness monitor

FWHM Full width at half maximum

GVD Group velocity dispersion

HBL Hole blocking layer

HDA Hexadecylamine

HOMO Highest occupied molecular orbital

HTL Hole transport layer

I.I. Impact ionisation

IP Ionisation potential

IRF Instrument response function

ITO Indium tin oxide

LBO Lithium triborate

LCD Liquid crystal display

LO Longitudinal optical

LUMO Lowest unoccupied molecular orbital

MBE Molecular beam epitaxy

MCA Multi-channel analyser

MCP Multichannel plate detectors

MEG Multiple exciton generation

ML Monolayer

MOCVD Metal organic chemical vapour deposition

NC Nanocrystal

ND Neutral density

NQD Nanocrystals quantum dot

OPA Optical parametric amplifier

PL Photoluminescence

PLE Photoluminescence excitation

PMT Photo-multiplier tube

PPV p-paraphenylene vinylene (PPV)

PV Photovoltaic


PVD Physical vapour deposition

PVK polyvinylcarbazole

QD Quantum dot

QD-OLED Quantum dot organic light emitting diode

RF Radio frequency

SPM Self phase modulation

TA Transient absorption

TAC Time to amplitude converter

TCSPC Time correlated single photon counting

TOP Trioctylphosphine

TOPO Trioctylphosphine oxide

TPD N, N’-diphenyl-N, N’-bis(3-methylphenyl)-(1,1’-biphenyl)-4,4’-diamine

TRPL Time resolved photoluminescence

TTS Transit time spread

UV Ultraviolet

WLC White light continuum

ZC Zero crossing (level)

Abstract


Abstract The work presented in this thesis was submitted to The University of Manchester for the degree of Doctor of Philosophy in June 2010 by Stuart K Stubbs and is entitled “Photo-physics and applications of colloidal quantum dots”. In this thesis the results of spectroscopic studies on various colloidal quantum dots, particularly related to the measurement and characterisation of multiple exciton generation are presented. Research conducted with Nanoco Technologies Ltd. that involved the design and development of hybrid quantum dot organic light emitting diodes for use in flat panel display technology is also presented. Cadmium selenide (CdSe), indium phosphide (InP), and lead sulphide (PbS) type I and cadmium selenide/cadmium telluride type II colloidal quantum dots were characterised using steady state photoluminescence and absorption spectroscopy. The fluorescence lifetimes of the decay of single excitons was measured in these quantum dots using time correlated single photon counting. An ultrafast transient absorption spectrometer was designed, built, and used to observe the picosecond dynamics of the decay of multiexcitons. These absorption transients were analysed in order to extract the quantum efficiency of producing multiple excitons per absorbed photon. The characteristic signature for multiple exciton generation was first found in CdSe using a time correlated single photon counting set-up. Results from the transient absorption spectrometer demonstrated efficient multiple exciton generation in InP for the first time as well as in PbS, where the efficiency was found to agree with values obtained by other research groups. The absorption transients taken for the type II CdSe/CdTe type II quantum dots demonstrated some novel decay dynamics that could not be attributed to the generation of multiple excitons. Quantum dot organic light emitting diodes were fabricated using Nanoco Technologies high quality cadmium based quantum dots and were shown to demonstrate bright, colour saturated emission originating from the quantum dot layer only. Using quantum dots of different sizes and structures red, green and blue devices were made and shown to be appropriate both in terms of brightness and chromaticity for the use as the red, green and blue pixels of a flat panel display. Because heavy metals like cadmium are restricted or banned from commercial products in many countries, Nanoco Technologies heavy metal free quantum dots, made from InP, were also incorporated in devices. Devices are demonstrated that emit from the quantum dot layer only, albeit at a lower luminance and efficiency than that found in the cadmium containing devices. This was the first demonstration of a heavy metal free, hybrid quantum dot organic light emitting diode.

Declaration


Declaration

No Portion of the work referred to in this thesis has been submitted in support

of an application for another degree or qualification of this or any other

university or other institute of learning.

Copyright statement


Copyright statement

i. The author of this thesis (including any appendices and/or schedules to this

thesis) owns certain copyright or related rights in it (the “Copyright”) and s/he

has given The University of Manchester certain rights to use such Copyright,

including for administrative purposes.

ii. Copies of this thesis, either in full or in extracts and whether in hard or electronic

copy, may be made only in accordance with the Copyright, Designs and Patents

Act 1988 (as amended) and regulations issued under it or, where appropriate, in

accordance with licensing agreements which the University has from time to

time. This page must form part of any such copies made.

iii. The ownership of certain Copyright, patents, designs, trade marks and other

intellectual property (the “Intellectual Property”) and any reproductions of

copyright works in the thesis, for example graphs and tables (“Reproductions”),

which may be described in this thesis, may not be owned by the author and may

be owned by third parties. Such Intellectual Property and Reproductions cannot

and must not be made available for use without the prior written permission of

the owner(s) of the relevant Intellectual Property and/or Reproductions.

iv. Further information on the conditions under which disclosure, publication and

commercialisation of this thesis, the Copyright and any Intellectual Property

and/or reproductions described in it may take place is available in the University

IP Policy (see

http://www.campus.manchester.ac.uk/medialibrary/policies/intellectual-

property.pdf), in any relevant Thesis restriction declarations deposited in the

University Library, The University Library’s regulations (see

http://www.manchester.ac.uk/library/aboutus/regulations) and in The

University’s policy on presentation of Theses

Publications and presentations



Journal articles

S. K. Stubbs, S. J. O. Hardman, D. M. Graham, B. F. Spencer, W. R. Flavell, P. Glarvey, O.

Masala, N. L. Pickett, and D. J. Binks, “Efficient carrier multiplication in InP

nanoparticles”, Physical Review B 81, 081303 (2010)

Akhtar, Javeed; Malik, M. Azad; O'Brien, Paul; Wijayantha, K. G. U.; Dharmadasa, R.;

Hardman, Samantha J. O.; Graham, Darren M.; Spencer, Ben F.; Stubbs, Stuart K.; Flavell,

Wendy R.; Binks, David J.; Sirotti, Fausto; Kazzi, Mario El; Silly, Mathieu, “A greener

route to photoelectrochemically active PbS nanoparticles”, Journal of Materials Chemistry,

20(12): p. 2336-2344, (2010)

Conference presentation

S. K. Stubbs, D. J. Binks, F. Aslam, C. Q. Nguyen, A. Malik, P. O’Brien, C. C. Byeon, D-

K. Ko and J. Lee “Optical Characterisation of CdSe Nanocrystal Quantum Dots Grown

from New Single Source Precursors” Conference on Lasers and Electro-Optics (Pacific

Rim), Seoul, August 2007.

Conference posters

F. Aslam, D. Graham, D. Binks, P. Dawson, S. K. Stubbs, N. Pickett, P. O'Brien, C. Byeon,

D-K. Ko, J. Lee “Electric field dependent photoluminescence studies of nanoparticle

sensitized photorefractive polymers” 5th International Conference on Semiconductor

Quantum Dots (QD2008) Gyeongju, Korea, May 11-16, 2008.

D. J. Binks, D. M. Graham, S. J. O. Hardman, S. K. Stubbs, A. Thomas and P. Dawson

"Investigation of InP nanoparticles as photo-absorbers for solar cells", ESF Research

Conferences: Nanotechnology for Sustainable Energy, Obergurgl, Austria, June 2008

Stuart Stubbs, Darren Graham, Samantha Hardman, David Binks, Philip Dawson, Wendy

Flavell, Nigel Pickett, “Investigating multiple exciton generation in quantum dots for



photovoltaics”, Future generation solar cells – research and exploitation, Daresbury science

and innovation campus, Cheshire, 4th November 2008

S. J. O. Hardman, D. M. Graham, B. F. Spencer, W. R. Flavell, S. K. Stubbs, D. Binks, F.

Sirotti, M. El Kazzi, M. Silly, J. Aktar, M. A. Malik and P. O'Brien "An Investigation into

the Electronic Structure of Nanoparticulate Lead Sulphide and the Implications for Novel

Hybrid Photovoltaic Cell Design", One Day Quantum Dot Meeting, University of Sheffield,

January 13th 2009

S. K. Stubbs, S. J. O. Hardman, D. M. Graham, W. R. Flavell, D. -K. Ko, K. Presland, M.

Afzaal, P. O’Brien and D. J. Binks, “Ultrafast charge dynamics of type II CdSe/CdTe/CdS

colloidal quantum dots”, Institute of Physics conference Quantum Dot 2010, Nottingham

University, 26-30 April 2010

S. K. Stubbs, S. J. O. Hardman, D. M. Graham, B. F. Spencer, J. Akhtar, M. A. Malik, J.

Thomas, P. O’Brien, D. Binks, and W. R. Flavell, “Efficient carrier multiplication in PbS

colloidal quantum dots synthesisd by environmentally benign methods”, Institute of

Physics conference Quantum Dot 2010, Nottingham University, 26-30 April 2010

Acknowledgements


Acknowledgements

Foremost I would like to thank my supervisor Dr. David Binks for all his help and support

over the last three years. The constant guidance and willingness to engage in impromptu

meetings to discuss and explain both theory and results with me were invaluable. Thank

you for so much of your time.

I would also like to thank the two postdocs without whom none of this would have been

possible. Darren Graham for patiently teaching me how to conduct experiments with the

laser, spending so much time “swearing in the dark” fixing it, and being available for so

many interesting discussions. To Sam Hardman for all the very long hours she put in

making sure everything ran smoother than it ever really should of, for constantly reassuring

me everything was going to be ok and for teaching me how to make things look pretty.

All of the lovely people at Nanoco also deserve my thanks, particularly Nigel Pickett and

Michael Edelman for taking a chance on me and giving me the opportunity to work in such

an exciting environment. Also thanks goes to Zugang Liu for being a pleasure to work with

and for teaching me some tricks of the trade, all the guys in the optoelectronics group and

all the brilliant chemists making the quantum dots.

Special thanks must go to my parents for being so reliably brilliant and whose support over

the years both financially and emotionally set me down the right path. To all my friends

including members of trouser and claude as well as those in the laser physics group, past

and present, for so many laughs and particularly for the drinks.

Chapter 1 Introduction


Chapter 1: Introduction and Background

1. Introducing Nanocrystal Quantum Dots

A quantum dot (QD) is simply described as a nanostructure in which the electrons,

holes or excitons (electron-hole bound pairs) experience quantum confinement in all three

spatial dimensions. In recent years they have received a great deal of attention from

researchers wanting to exploit their great technological potential and to investigate new

fundamental physics. It is the ability to tune the optical and electronic properties that gives

quantum dots great potential for use in optoelectronic devices such as in laser technology,

photovoltaics and nanoscale electronics. They represent a situation between that of bulk

and molecular materials, they maintain the crystalline structure of the bulk but the 3-

dimensional quantum confinement leads to discrete atom-like energy spectra. This leads to

them often being viewed as “artificial” atoms with their energy levels labelled using

atomic-like notation (1S, 1P etc.), although they are typically made up of 100s to 100,000s

of atoms.

The quantum confinement in QDs is manifested as a blue shift of the absorption

band edge compared with that of the bulk material [1] and some discrete nature of the

absorption spectrum, which is continuous in bulk. In fact as the QD is reduced in size the

degree of quantum confinement is increased leading to a larger energy gap. Thus it is

possible to tune the emission and absorption properties by controlling the size of the QD for

specific applications or experiments. In optical applications the large photoluminescence

(PL) quantum yields that are routinely found make QDs particularly attractive. Device

applications and high quantum yields require high quality quantum dots. It is therefore

important to fabricate dots that have a high degree of monodispersity are highly crystalline,

and with atomic precision growth.

Nanoparticles of semiconductors and metals can be synthesised using many

different techniques often dependant upon the material. Self assembled quantum dots are

grown by physical processes that involve high-energy input known as molecular beam

epitaxy (MBE) and metal organic-chemical-vapour-deposition (MOCVD). Dots produced

from these techniques are usually in the size ranges from 10 to 50 nm. Other techniques

such as precipitation in molten glasses, sputtering, ion implantation and chemical methods

have all been used to produce QDs [2, 3]. Ultimately however, for commercial and

practical use it is necessary to be able to economically fabricate QDs in large quantities.



Using colloidal chemical synthesis it is possible to fabricate on a mass scale what we call

nanocrystal quantum dots (NQDs) cheaply. NQDs produced using this method are typically

less than ~10 nm in diameter, have size dispersions as small as 5%, and can be grown with

great accuracy. This method has a low-energy input and is easily carried out at bench top

temperatures and pressures. The general structure of a NQD is an inorganic core coated

with organic ligand molecules. The functions of the organic ligands are to provide chemical

passivation of surface dangling bonds, to prevent the NQDs from growing further and

forming aggregates, and assist solubility and ease of manipulation [4]. The large degree of

control possible, particularly in relation to the surface properties to alter physical and

chemical properties make NQDs stand out from the epitaxial QDs. The growth of layered

structures is routine and can improve photoluminescence efficiency (fig. 1.1). It is also easy

to see that whilst epitaxial QDs are bound to a substrate, NQDs are colloidal and so

freestanding allowing for incorporation into all manner of organic and inorganic materials.

Fig. 1.1 Core-Shell quantum dot (taken from [5])



1.1. Motivation to study multiple exciton generation (MEG)

The use of nanoparticles as natural laboratories to investigate quantum systems and

fundamental physical processes has gone hand in hand with the recognition of their

potential as materials for optoelectronic devices. In a world where the effects of energy

consumption and generation are the biggest problem facing the human race, discovering

ways of producing sustainable and cheap electricity has taken on a more vital role than ever

before. The renewed interest in the process of producing more than one electron-hole pair

(exciton), a process known as multiple exciton generation (MEG), in semiconductor

materials, via excitation with an energetic photon, is hardly surprising given its potential

application in high efficiency photovoltaics. With global power consumption currently at

16 TW (16 x 1012 W) and set to increase to over 30 TW by 2050, the limiting factor for the

wide scale roll out of photovoltaic (PV) systems for electricity production has been cost.

The figure of merit when comparing PV methods of generating electricity relates to the cost

per peak watt of electrical power produced, where peak watt refers to the solar radiation

power that reaches the earth’s surface at mid-day with no cloud cover. This figure

combines the costs for the full life cycle and includes the initial PV module manufacturing

cost, installation costs, “balance of systems” costs (components needed for full PV system

e.g. inverters, batteries), the lifetime of the module, the module efficiency etc. Currently PV

electricity generation cannot compete with other forms of generation like coal and nuclear

due primarily to the high cost associated with manufacturing solar cells of high enough

efficiency due to a lack of high throughput processes or roll to roll capabilities. This feeds

through to make the actual cost of electricity for both large scale power plant style

installations (21-46 ¢/kWh) and smaller residential style systems (37-81 ¢/kWh) many

times higher than typical rates in the US, for example, (9-18 ¢/kWh) where electricity is

relatively cheap compared to Europe[6].

There are different approaches to tackling the issues with PV power generation,

which are separated into 1st, 2nd, and 3rd generation solar cells. 1st generation solar cells

account for over 85% of solar generated electricity and are based on crystalline silicon

usually doped to produce n-type and p-type semiconductors. They hold the record for

efficiency of a single junction solar cell of ~ 25 % [6] but are expensive due to the use of

highly pure, single-crystal silicon wafers. 2nd generation solar cells are based on thin film

technology where semiconductors such as cadmium telluride and amorphous silicon are



deposited using solution-based processes and can be produced in high volumes at a low

cost. The efficiencies of this type are usually fairly low meaning that the ability to produce

large volumes cheaply is the main advantage here. 3rd generation solar cell schemes are

centered on beating the theoretical power conversion efficiencies to give very high

efficiency cells. Thanks to Shockley and Queisser [7] it has long been known that there is a

maximum thermodynamic efficiency with which an ideal solar cell can convert incident

sunlight into electrical power. Their analysis found that for semiconductors with band gaps

of ~ 1.2 eV a maximum efficiency of ~ 31% can be achieved under ideal conditions where

radiative recombination is the only loss mechanism of charge carriers. For absorption of

photons in the solar spectrum with energy above the band gap, the excess energy will be

distributed between the electron and hole depending upon their effective masses. These

“hot” carriers will then relax to the band edge through electron-phonon scattering before

emitting phonons. As the solar radiation spectrum is to a good approximation a broad black

body spectrum, this broad wavelength range means that large amounts of energy will either

not be used or will be wasted. This loss as heat can be identified as the predominant loss

factor causing this 31% energy conversion limit. Quantum dots could act as photoabsorbers

that, through MEG (also known as carrier multiplication), can give internal quantum

efficiencies above 100 % for excitation by the photon energies in the solar spectrum.

Through similar methods to that used by Shockley and Queisser the efficiency of an ideal

solar cell utilizing MEG was calculated by Hanna and Nozik [8]. Here they found that a

power conversion efficiency of 44 % is possible in a cell based on a single junction MEG

absorber (figure 1.2). In this graph Mmax corresponds to the situation where the maximum

possible number of multiple excitons are created from the excess energy (i.e. n excitons are

created by photons with energy n times the band gap). Where only 2 excitons can be

created for photon energies of increasing multiples of the band gap (M2) the efficiency is

still very high and not much smaller than that for Mmax.

The work carried out here involves measuring the efficiency of carrier

multiplication (CM) in various colloidal nanocrystals and determining the lifetime of the

resultant multiexcitons by developing ultrafast transient absorption and ultrafast

photoluminescence experiments.



Fig. 1.2 Theoretical limits to efficiency for single gap devices for different numbers of

excitons created per absorbed photon. Mmax is the limit when only considering energy

conservation, M1 corresponds to producing only one exciton and is the Shockley-Queisser

limit, M2 is for the creation of 2 excitons when excited by photons with energy greater than

twice the band gap. L2 and L3 are for MEG thresholds of 2 and 3 times the band gap

respectively, with linear increase in quantum yield thereafter. (Taken from [8]).

1.2. Introduction to hybrid quantum dot organic light-emitting diodes (QD-

OLEDs)

As emissive materials where the wavelength can be tuned across the entire visible

range and can give spectrally narrow and efficient luminescence, QDs are very well suited

for use in electroluminescent (EL) devices. Colloidal nanocrystals, being solution-based,

also offer the possibility of using inexpensive processing techniques such as spin coating

over large areas and on a variety of substrates as in organic systems but with the benefit of

the photo-stability of the QD semiconductor material. Potential future technologies with

applications in lighting and flat panel displays have previously been dominated by organic

light emitting diodes (OLEDs) which boast high colour contrast, and lower energy use than

current liquid crystal display technology. However, the tuneable nature and saturated

emission of QDs make hybrid organic quantum dot light-emitting devices (QD-OLEDs)

stand out in terms of colour quality, an important factor in both lighting and display

technology. The work described here was carried out with Nanoco Technologies Ltd. with

the initial aim of proving this technology using Nanoco’s proprietary quantum dots, and to



build up knowledge and experience to show that QD-OLEDs can be made with the

necessary performance for display technologies. Many of the techniques used to produce

and characterise QD-OLEDs (spin coating and printing deposition of organics and NQDs,

I-V curves etc.) are complementary to thin film photovoltaics and these two technologies

work through many of the same physical processes. Also, many of the charge transport

materials used to transfer the electron and hole to the emissive layer in QD-OLEDs can also

be used in thin film photovoltaics to separate the electron and hole and transport them to the

electrodes as photocurrent. As well as the expected benefits of high power efficiencies and

colour purity the design of these LEDs opens up novel applications for flat panel displays

(FPD) and lighting. As much of the device is fabricated using solution processes this

technology is compatible with already mature techniques such as ink jet printing allowing

very large area displays to be printed. Also envisaged for this technology is the use of

flexible substrates with the thin films deposited on to give next generation flexible, curved

or even disposable displays. These characteristics lead to QD-OLED technology promising

not only to penetrate the current $40 billion a year display market but also to open up new

opportunities from novel applications.

1.3. Nanocrystal literature review

Although interest in nanoscale structures has only increased dramatically in the last

25 years it was Faraday in his experiments on nanoscale gold who noticed that the colour of

these particles was dependant upon their size [9]. Research into small semiconductor

particles and their size effects goes back to the 1960s, but it was not until 1982 that Ekimov

et al. discovered that the optical spectra in semiconductor nanocrystal-doped glasses was

size dependant [10]. This was represented by showing the shift in the first exciton peak in

the absorption spectrum as a function of nanocrystal size. Henglein independently found

this in colloidal nanocrystals. It was at this time, and in the following decades that the level

of research and publication of papers concerning QDs grew rapidly and this new field

expanded dramatically. It was clear that quantum size effects influenced the optical and

electronic properties of semiconductor quantum dots. It was in 1982 that Al. L Efros and A.

L. Efros described a simple theoretical model involving three different size regimes to

include the effect of size quantization on interband absorption in a semiconductor sphere

[11]. The size regimes described were what are now known as the strong, intermediate and



weak confinement regimes. Where the radius of the nanocrystal, a, is smaller than the Bohr

radii of the electron (ae), hole (ah) and exciton (aexc) the charge carriers are strongly

confined within the nanocrystal quantum dot (NQD) and so is called the strong

confinement regime (a < ae, ah, aexc). The intermediate confinement regime is that whereby

the NC radius is between the Bohr radii of the two charge carriers (i.e. ah < a < ae, aexc) and

so only one is strongly confined whereas the other is not. For ae, ah, < a < aexc the centre-of-

mass motion of the exciton alone is confined and so this is known as the weak confinement

regime (see section 2.1.2).

It was the work of Brus in the following year (1983) that described a theoretical

model of the electronic structure of semiconductor colloids as a function of size [12]. This

work considers one mobile charge carrier in a crystallite and asks the question “At what

point will a single carrier in thermal equilibrium with the lattice begin to sense the finite

size of the crystal?” The question of understanding what happens to a semiconductor in a

nanometre size regime has driven research into NQDs from a fundamental physics and

practical perspective. The model proposed here considers the nanoparticle as a sphere of

constant potential containing a particle of mass m, and cites the major assumptions of the

model as the use of the effective mass, and the use of an interaction potential. So it is here

the effective mass model for nanocrystallites was developed. In 1984 Brus added to the

theoretical model by considering the situation whereby the sphere contains both a positive

and negative charge as would be the case for an exciton in a NQD [13]. The model involves

looking at how bulk states change when the crystallite approaches small sizes of the order

~5 nm. The energy of the lowest exciton state is approximated and shown to be dependant

upon bulk electronic properties and strongly upon size.

Although NQD synthesis is not really the focus of the work here its importance in

understanding QDs is not to be underestimated. QD research was for a time limited in its

scope by large size and shape dispersions, defects in the NQD structure particularly at the

surface, and a poor level of crystallinity. These effects will limit the interpretation of

experimental data and the application of any model as there will be difficulties in

distinguishing effects due to quantum confinement and those due to variations in the

nanocrystal samples. Clearly for confident interpretation of experimental results and

comparison between research groups it is important to be able to synthesize dots with a

high degree of control of its physical properties. For the work detailed in this thesis the



emphasis is upon nanocrystals prepared via wet chemistry methods, which have been

shown to be relatively simple and economic to produce in high quality. In this project the

dots were fabricated by the Chemistry department at The University of Manchester and

Nanoco technologies Ltd. and were supplied dispersed in a number of organic solvents. The

general method involves pyrolysis of metal organic precursors in hot coordinating solvents.

The various stages of nucleation and subsequent growth of nanoparticles were studied by

La Mer and Dinegar [14] and the reader is referred to this paper as well as a review by

Murray et al. [15]. Combined with size-selective precipitation, which separates larger from

smaller nanocrystals, this method can give size dispersions of < 5%. In NQD research the

II-VI direct-gap semiconductor compound CdSe has taken a place as the model for

investigating NQD electrical and optical phenomena. This is partly because of the quality

with which CdSe NCs can be fabricated from a number of different precursors and also as it

was the first system to be produced that had the necessary level of monodispersity and

quality [16]. The paper by Murray, Norris and Bawendi (reference 14) boasts NCs

produced from a single reaction in macroscopic quantities with small size dispersion,

uniform shape, high levels of crystallinity, and good surface passivation.

Optical spectroscopies have been used, since their advent, to investigate the

electronic properties of NQDs. The large scope of experiments and ability to investigate the

internal energy levels of NQDs is entirely due to the fundamental characteristics of

semiconductors. A photon incident upon a direct gap semiconductor will be absorbed if its

energy is sufficient to excite an electron from the valence band into the conduction band

and an electron-hole pair (ehp) or exciton can be created in the material. In the same respect

when an electron in the conduction band recombines with a hole in the valence band a

photon will be emitted with energy equal to the difference between the two energy levels.

Size quantization effects result in atomic-like quantization of energy levels within the

nanocrystal so unlike in bulk where any photon with energy larger than the band gap will

be absorbed, in NQDs only photons with discrete energies will create an exciton. Probing

these processes was shown to be possible using absorption and luminescence spectroscopy

as was carried out by Ekimov et al. on CdSe which with a Bohr radius of ~5 nm is in the

strong confinement regime [17]. A blue shift of the luminescence is seen in the smallest

dots investigated here (2.1 nm radius) but for the largest (25 nm radius) the spectra are the

same as for bulk indicating no quantum confinement. Absorption and PL spectra show



discrete features corresponding to electronic transitions for the three smaller samples of

radii 3.8 nm, 2.6 nm, and 2.1 nm. In Ekimov et al.’s work they present calculations of the

energy levels which consider the effect of valence band degeneracy as well as spherical

confinement, the Coulomb interaction and conduction band non-parabolicity. By taking the

second derivative of the absorption spectrum for the three smallest dot sizes to reveal any

hidden features and comparing the position of these features with their theoretical

calculations they are able to assign the allowed transitions to the features. This is shown

below in figure 1.3 where the calculated transitions are shown as vertical lines whose

height indicates the theoretical transition strength.

Fig. 1.3 Absorption and second derivative of absorption for dots sizes 3.8 nm (top), 2.6 nm

(middle), 2.1 nm (bottom). Inset indicates labelling of transitions (taken from [11]).

Following their work on producing high quality NQDs Norris and Bawendi went on

to investigate and understand the size dependence of the electronic structure in NQDs for



the first time [18] using the technique of photoluminescence excitation (PLE) spectroscopy

whereby the wavelength of the excitation is scanned over a range whilst a small portion of

the spectrum is observed. This will result in absorption information from a subset of dots,

which will have smaller size dispersion than the sample as a whole. The authors apply this

extensively to a large number of dot sizes within the range 1.2 – 5.3 nm and use an

extension to the particle in a sphere model that includes a lifting of the valence band

degeneracy due to electron-hole exchange interaction, crystal shape asymmetry and the

lattice structure. From this the size dependence of the electronic energy levels is calculated

and compared with the experimental data; they then use this to assign the PLE features to

ehp states. Figure 1.4 below is taken from their paper [12] and shows the good agreement

between experiment and theory for these lowest transitions. In this graph the energy of the

first excited state is used on the x axis as a measure of NQD size to eliminate significant

error that would be introduced by using average radii found from transmission electron

microscopy. The energy relative to the first excited state is plotted on the y axis.

Fig. 1.4 Experimental data (circles) and calculated pair states (lines) are shown as a

function of 1st excited state energy, which is a size dependent label (taken from [12]).

The success seen in the production and characterisation of CdSe NQDs encouraged

researchers to look at dots made from different materials. The electronic structure of lead

salts such as PbS and PbSe for example were investigated in a similar way to CdSe [19].



This type of NC was an attractive prospect for research since with electron and hole Bohr

radii of ~10 nm study of the strong confinement regime is much easier than in Cd based

dots. The focus of Norris and Bawendi’s paper (reference 19) is upon enhanced non linear

optical properties due to the strong quantum confinement, and so potential applications for

NQDs was now adding to motivation for further research. As well as new materials to

improve characteristics different NC systems were developed. Obviously for photonic

applications a high quantum yield is important. Core NQDs typically show quantum yields

of 10% at room temperature. This is due to non-radiative recombination processes

occurring at the surface of the dots. By overcoating the NCs in a semiconductor material

with a higher band gap these surface states are passivated resulting in quantum yields as

high as 50% [20, 21]. In references 17 and 18 ZnS is used as the higher band gap material

to overcoat CdSe NCs, and the effect upon the optical properties of this shell is

characterised. This has the added advantage of protecting the NC making it more stable and

so workable for incorporation into devices such as LEDs and solar cells.

The use of CdSe nanocrystals as model quantum dot systems led to further advances

in the understanding of semiconductor nanocrystal systems. The observation of long

emission lifetimes relative to bulk exciton recombination times at temperatures of the order

10 K had long puzzled researchers. It was thought that the two lifetimes should be of

similar magnitude and so this phenomenon was attributed to interactions of the electron or

hole with the surface. This assumption was thought reasonable as NQDs have such a large

surface-to-volume ratio and were likely to have defects and other trap sites on the surface.

Carriers trapped at the surface would have a small overlap with their charge pair and so

their recombination time would be responsible for the long time constants observed. This

was not found to be the case in Nirmal and Norris’ work in observing the “dark exciton”

(see section 2.1.6) [22]. The long radiative lifetimes can be explained using the fine

structure of the band edge exciton. The exciton fine structure investigated by Norris et al.

has been shown to be size dependant and is a result of the CdSe hexagonal lattice structure,

the NCs being slightly prolate (i.e. not spherical), and the electron hole exchange

interaction that becomes important for quantum confinement [23]. PLE and fluorescence

line narrowing experiments were conducted at 10 K and revealed a fine structure in the first

absorption peak. As shown in figure 1.5 this fine structure results in 5 sublevels labelled by

|Nm| (projection of N along unique crystal axis) and upper and lower levels with the same



|Nm| are distinguished by U and L superscripts respectively. An exciton relaxing into the

lowest NQD state, |Nm| = 2 will result in long emission lifetimes as it is optically forbidden

or “dark” and can only return to the ground state via inefficient phonon transitions. The

dark exciton model is further supported by evidence that it can easily explain the

luminescence stokes shift and the reduction in lifetime in the presence of a magnetic field

as shown in [16].

Fig. 1.5 Calculated band edge exciton structure. Solid lines indicate optically active states

and dashed lines indicate optically passive states. (Taken from [17]).

The applications envisaged for NQDs in optoelectronic devices are strongly

dependant upon the dynamics of the exciton and its subsequent recombination at the band

edge. The design of any device or application will therefore require an understanding of the

relaxation kinetics, for example, a band edge quantum dot laser would require a fast

intraband relaxation to the lasing states. Nanocrystal dynamics are very different from those

in bulk where relaxation is via Fröhlich interactions with longitudinal optical (LO) phonons

resulting in sub-ps relaxation times [24]. In NQDs we find what is known as the “phonon

bottleneck” whereby the separation between energy levels is larger than LO phonon

energies. Thus energy and momentum conservation means that electrons cannot relax via

single phonon interactions. This theoretical prediction means that long intraband dynamics

for electrons were expected. The valence band states are separated by much smaller

energies than the conduction band and so it was also expected that the holes would not be

affected by the phonon bottleneck and so hole decay would be driven by the phonon

lifetime (10’s of picoseconds). A slow intraband relaxation time due to the phonon

bottleneck is contrary to what has been observed by the majority of groups [25]. Sub-ps

relaxation times are found for the 1P-1S electron relaxation [26], which seem to refute the



existence of a phonon bottleneck. To explain this non-phonon mediated fast relaxation,

Auger type e-h energy transfer was proposed by Efros et al. [27]. Transient absorption

experiments where conducted by Klimov et al. which separated the electron and hole by

using hole trapping pyridine and thiophenol [26]. In these studies the 1Se – 1Pe relaxation

time in the absence of a hole was slowed to a few ps. This was explained by the e-h

coupling being reduced and so leading to longer relaxation times. Despite this slowing of

the relaxation rate for the electron the interaction with the surface trapped hole was

expected to be much smaller than that found by Klimov et al. Guyot-Sionnest et al. [19]

however, show using electrochemical reduction that a fast relaxation time is found even in

the absence of holes. It would seem therefore, that another efficient mechanism must be

mediating intraband relaxation. Here Guyot-Sionnest suggests that the interband relaxation

is much more dependant upon surface ligands than was previously thought. Two relaxation

mechanisms involving electronic (fig. 1.6) and vibrational processes are proposed for the

interaction with the surface.

Fig. 1.6 Schematic of possible mechanism for intraband relaxation mediated by surface Cd

atoms (taken from [19])

The Auger process explanation was not sufficient to explain the seeming lack of a

phonon bottle neck in lead chalcogenides where small and almost identical effective masses

for the electron and hole mean that the valence band and conduction band are nearly

symmetric. Observations by Schaller et al. on PbSe dots of different sizes did not find

evidence of a phonon bottleneck and in fact found the rate of relaxation to increase as the

quantum confinement was increased (QD diameter decreased) [28]. A comparison of the

temperature dependence of the intraband relaxation rates in both CdSe and PbSe reveals the

mechanism to be temperature-independent in CdSe, an observation consistent with Auger



mediated processes, but a temperature dependence in PbSe points to efficient multi phonon

emission and not to surface or defect interactions. The relaxation in lead chalcogenide

nanocrystals was modelled [29] using time domain ab initio simulations and it was found

that a phonon bottleneck does not exist in CdSe and PbSe NQDs due to a high density of

states at higher energies and in larger dots. Schaller et al. also find that multiple individual

excitations combine into distinct bands so that the difference between state energies is the

same as phonon energies for all but the lowest excitation energies and smallest dots.

Despite this they conclude that as the relaxation of charge carriers to the band edge is on

the order of picoseconds as found experimentally [28], it should not effect the MEG

process which occurs on a timescale with an upper bound of 250 fs [30]. They also confirm

here that multiple excitons can be directly excited upon absorption of a single photon.

1.4. Multiple exciton generation (MEG) literature review

Over 50 years ago the process of an energetic electron using its excess energy to

create one or more extra excitons through a scattering process called impact ionisation (I.I.)

was proposed as a mechanism to surpass the Shockley/Queisser limit. In bulk

semiconductors, however, this process is very inefficient until photon energies of many

multiples the band gap are used. Therefore, it was not of great interest for research into

photovoltaics due to its impractical use in the bulk materials such as silicon and germanium.

It was in fact the questions surrounding the phonon bottleneck hypothesis in nanocrystals

that led to investigations into creating multiple excitons; it was believed that as the size of a

nanoparticle was reduced the electronic energy gaps would increase and so relaxation via

interactions with phonons would slow down. It was these studies that led Nozik to suggest

that impact ionization might be more efficient in nanocrystals where quantum confinement

effects would increase the interaction between charge carriers [31].

It was not until 2004, however, that Schaller and Klimov [32] demonstrated for the

first time that carrier multiplication (CM) could occur with high efficiency in PbSe NCs.

They directly observed that the fast decay of biexcitons persisted even at low fluence when

excited with high energy photons. Upon analysing the data they found that MEG occurred

very quickly and found the threshold for MEG to be three times the band gap (3Eg), where

2 excitons were created per absorbed photon. Further work was done on lead chalcogenide

nanocrystals by Ellingson et al. in Nozik’s group that confirmed efficient MEG in PbSe and



found quantum yields of 300 % (3 excitons per absorbed photon) at photon energies four

times the band gap [30]. They also showed in the same paper that MEG occurred in PbS

dots suggesting that carrier multiplication was indeed a phenomenon that occurred in all

QDs. Further evidence of this generality is seen in another paper by Schaller [33] where

PbSe and CdSe QDs are directly compared using transient absorption (TA). Here the

efficiency for MEG is found to be similar in CdSe and PbSe despite the very different

energy level structures and relaxation mechanisms. MEG has also been reported in QDs

made from the semiconductor materials PbTe [34], InAs [35], and Si [36] with some very

high internal quantum efficiencies reported. In PbSe, for example, 7 excitons were created

per absorbed photon with an energy that is 7.8 times the band gap [37]. This impressive

result is shown below and is noted to sit on the “staircase” of ideal QEs. This provides

evidence to support the theory that MEG is governed by simple energy conservation, where

a photon with energy 2 x Eg will produce 2 excitons and so photons with additional

multiples of the band gap will incrementally produce an extra exciton.

Fig. 1.7 a) graphical representation of conservation-of-energy-limited carrier multiplication

whereby increasing the photon energy by Eg results in a 100% increase in quantum

efficiency and b) experimentally determined QE for PbSe and PbS dots plotted along QE

derived from ideal conservation of energy (black lines) theory. (Modified from [37].)

Not all investigations into MEG have found the efficiency to be only limited by

conservation of energy. The fact that the threshold for MEG in the above case is found to

be ~ 3Eg for both PbS and PbSe seems at odds with the energy requirements for MEG. The

energy threshold for MEG has been found to vary between studies for the same type of dot

a) b)



and different dot materials have given different MEG thresholds. For example, early studies

into MEG in PbSe conducted by Schaller gave an MEG threshold of 3Eg for three different

PbSe sizes [32], whereas Ellingson reported a threshold as low as 2Eg [30], and Schaller’s

initial value was later corrected to a slightly lower value of ~ 2.85Eg [38]. Variation is seen

in other lead salt quantum dot materials, for example in PbTe [34], and values of ~ 2.5Eg

have also been measured for the MEG threshold in CdSe QDs [33] which also does not

support a threshold limited only by energy conservation. An alternative model was

therefore proposed by Schaller [33] whereby the threshold for MEG depended upon the

excess energy of a single charge carrier, as the energy in excess of the band gap will be

distributed between the electron and hole according to the inverse of their effective masses.

Despite the variation in results found experimentally for Pb chalcogenide QDs no report

has refuted the existence of MEG in this type of dot, in fact, as a response to reports

challenging the existence of MEG in other QD materials, an independent group confirmed

that “In spite of recent doubts carrier multiplication does occur in PbSe nanocrystals” in

their paper with this same title [39].

The majority of investigations into MEG to date have used the ultrafast transient

absorption (TA) technique where by the pump-induced bleach of the lowest excited state

(1S) is used as a measure of the state occupancy [37] or the pump induced absorption for

intraband transition energies is monitored [40]. In these experiments MEG can be identified

by the emergence of a fast decay component, identical to the Auger recombination decay

rate, which persists in the limit of vanishing fluence for excitation with photons above the

threshold for MEG. Photoluminescence measurements can also be conducted on QDs to

monitor this same multiexciton decay component using time correlated single photon

counting (TCSPC). The use of time-resolved photoluminescence (TRPL) experiments

allows the fast decay component to be monitored as in the TA. The first demonstration of

MEG using TCSPC [41] showed that it is a complementary technique and found that it

gave the same CM threshold for CdSe QDs of ~ 2.5Eg as previously found in TA. In

reference 41 Schaller et al. also use time resolved PL measurements to demonstrate MEG

spectrally for the first time, by measuring a red shift of the PL at times shortly after photo-

excitation which is indicative of the presence of multiple excitons. Not all groups have

found MEG to be as efficient as that reported in the studies mentioned thus far, with some

researchers finding no evidence of carrier multiplication at all. A report by Nair et al. in



2007 [42] failed to observe multiple excitons in both CdSe and CdTe using a time-resolved

photoluminescence technique that utilises a streak camera ( ~ 2 ps time resolution). Despite

exciting with photon energies > 3.1Eg, well above the threshold previously found for CdSe,

they see no additional fast component when compared with lower photon energies despite

observing the fast component at high fluence. Inconsistent results have also been reported

for other nanocrystal materials. MEG was first reported in InAs nanocrystals and

characterised using three complementary optical techniques. Interband TA, time-resolved

terahertz spectroscopy and quasi continuous wave excitation spectroscopy were all used

and a threshold for MEG of 2.14Eg was extracted. The same group however, later reported

on the absence of carrier multiplication in a study using TA [43], and also reported the

time-resolved terahertz spectroscopy results taken initially could not be reproduced on

another batch of InAs NCs [44]. Efficient MEG has, however, been observed in InAs NCs

by Schaller and Klimov [35] where the threshold was found to be defined by the energy

conservation limit i.e. 2Eg.

The variations in threshold and efficiency of MEG in the Pb chalcogenide dots as

well as the discrepancies and non-reports of carrier multiplication in CdSe, CdTe, and InAs

need to be addressed before a full understanding of MEG can be achieved. Several ideas

have been proposed as to the origin of these inconsistent results and the variations in

efficiency, threshold and even observation of MEG seem to point towards sample and batch

differences. The influence of the surface is an ever-present unknown in nanocrystals as it is

not well-characterised or understood, but is known to have a large impact on various

processes in QDs. As mentioned earlier Guyot-Sionnest et al. have found that the

passivating ligands have a role to play in carrier relaxation [25] and so it is likely that MEG,

which also depends upon the way “hot” carriers relax, will be affected. Differences in

measured MEG efficiency as a result of experimental methods were ruled out in the case of

InAs by Ben Lulu et al. [43] where they point out differences between the structure of the

dots used by Schaller (InAs/CdSe core/shell) and the QDs used in their study

(InAs/CdSe/ZnSe core/shell/shell) as a possible cause of the variation. Having previously

observed MEG in a separate batch of dots they also consider batch differences but decide

somewhat ambiguously that this is not likely to be the cause. To settle some of these

arguments side by side studies using TRPL and TA to reliably measure the MEG efficiency,

as well as investigations into sample to sample variability and a study into NC



photoionisation effects were all conducted by McGuire et al. [45]. They find that both TA

and TRPL give MEG yields that agree and so exclude differences due to experimental

methods. They make 2 samples of PbSe dots with the same band gap but from different

reducing agents and dispersed in different solvents, and find different MEG yields between

the two dots with variation of ~ 30%. They also suggested photoionisation may be long

lived and so lead to charging in a large fraction of the nanoparticles as earlier MEG studies

had been conducted on static samples. This would give exaggerated MEG yields and so

they used rigorous sample stirring to prevent build-up of charge due to exposure to multiple

laser pulses. Although some samples showed no difference between static and stirred

configurations, the PL amplitude was affected at different times in some samples leading to

different carrier multiplication yields (figure 1.8).

Fig. 1.8 TA trace showing the effects of stirring on a PbSe sample with Eg = 0.63 eV. The

authors suggested the increase in CM efficiency in the static case is in fact due to NC

ionization. (Modified from [45])

The case for efficient MEG in NCs has recently been supported firstly by work

looking into how efficient MEG is in various PbSe films [46] that had been chemically

treated prior to observing MEG and secondly in the demonstration of a NC-based

photoconductive detector that shows >100% internal photoconductive gain [47]. The first

of these two reports is of importance to the MEG debate as for MEG to be practically

utilised in a solar cell the nanocrystals will need to be deposited as the active layer, for

example as the intrinsic region in a p-i-n structure. To make use of the additional charges



created through MEG the QDs in the layer must be electronically coupled so as to separate

the charges to get useful photocurrent. This must occur after the MEG process but before

Auger recombination of the biexciton, so it is important to ensure that MEG still occurs in

these coupled films if it is to be utilised in photovoltaic devices. In reference 46 Beard et al.

observed efficient MEG in isolated QDs in solution, untreated films, and in films treated

with hydrazine to enhance the electronic coupling between dots in the film. Further to this

they found no reduction in MEG efficiency for the coupled film, an important result as a

reduction in efficiency had been expected on the basis of reduced quantum confinement

due to carrier delocalisation. The second report is of particular importance as all the reports

of MEG previous to this had been using spectroscopic techniques to indirectly observe and

measure MEG. The most conclusive demonstration of carrier multiplication in QDs would

be through the fabrication of an optoelectronic device where either the external or internal

quantum efficiency of the photocurrent was greater than 1. Here Sukhovatkin et al. report

the internal gain is seen to increase drastically for photon energies greater than 2.7 times the

band gap which represents an important step towards photovoltaics exploiting MEG.

1.5. Hybrid quantum dot organic light emitting devices literature review

Organic light emitting devices (OLEDs) have been at the centre of intensive

research for several decades because of the potential commercial gains in their application

to display technologies. By combining the strengths of OLED technology with nanocrystals,

tuning of the emission colour by changing NC size, high colour purity and photo stability is

possible whilst maintaining the solution processable advantage. Early devices were only

slightly more complex than the simplest device which is capable of emitting light i.e. a film

of nanocrystals set between two metal electrodes, one of which is transparent to allow the

light to couple out. The first demonstration of electroluminescence (EL) in polymer-

embedded nanocrystals came in 1994 where a device was made consisting of indium tin

oxide (ITO), used as the transparent anode, a hole transporting polymer, p-paraphenylene

vinylene (PPV), a layer of CdSe nanocrystals as the emitting layer and a magnesium

cathode [48]. The brightness of these devices is usually quoted in photometric units; here

Colvin, Schlamp and Alivisatos report EL of 100 cd/m2 (candelas per square metre), a

brightness just visible under normal room lights, with an external quantum efficiency

(EQE) of just 0.01%. Another significant observation made for this device was the voltage



dependence of the recombination zone. This manifested as emission being dominated by

the CdSe QDs at low voltages and EL from the PPV layer dominating at higher voltages.

The use of a conjugated polymer here is important as without it the device would suffer

from low efficiencies and high turn on voltages. In reference 48 Colvin et al. explain that

the thin QD layer would undergo dielectric breakdown at the operating voltages and so the

polymer enhances hole injection whilst imparting electrical stability. Conjugated polymers

have attracted a lot of attention from researchers independently as they show

semiconducting properties that result from delocalised π-orbitals along the polymer

backbone [49]. The massive potential of these materials in their own right has meant that

many other polymers have been fabricated with properties tailored to meet the demands of

their application.

In 1995, the year after the initial demonstration of a QD-OLED device, an

alternative design was studied that used a blend of the hole conducting polymer

polyvinylcarbazole (PVK) and a small molecule electron transporter t-Bu-PBD (an

oxadiazole derivative) [50]. The nanocrystals are incorporated into the blend to act as the

trapping and recombination sites for the injected charges. Here Dabbousi et al. find even

lower EQE (0.0005 %) but see that the I-V characteristics are unchanged upon

incorporation of the QDs. The low efficiency is explained by the offset between the highest

occupied molecular orbital (HOMO) of PVK and the valence band maxima of the CdSe

nanocrystals, meaning that hole injection into the dots will be unfavourable and so the

majority of charges will pass through the film without forming an exciton. Moving on from

simply demonstrating EL in hybrid-OLEDs researchers naturally concentrated on

understanding and enhancing the processes involved in order to build more efficient EL

devices. The first work in this direction was carried out by the Alivisatos group that first

demonstrated EL in hybrid QD-OLEDs [51]. Here they made use of advances in

nanocrystal fabrication that gave core/shell structures designed to remove non-radiative

decay pathways by overcoating the cores with wider band gap materials like ZnS or CdS

which confines the exciton to the centre of the QD. The Alivisatos group use CdSe/CdS as

they explain that whilst ZnS does indeed increase photoluminescence efficiency it also

places a large potential barrier for charge injection in the device stack. They report that CdS

overcoated nanocrystals give EQE’s of 0.22% at a brightness of 600 cd/m2. The smaller

conduction band offset between CdSe/CdS compared to CdSe/ZnS means that the electron



is not confined to the core to the same degree. The energy level diagram of the device built

in this study is shown below (figure 1.9) and is typical of the designs used in these early

works.

Fig. 1.9 Energy level diagram of hybrid QD-OLED. Electron affinity (EA) and ionization

potential of the QDs are extrapolated from bulk values using confinement theory. (Taken

from [51]).

The bilayer devices described above have misleading figures for brightness and

efficiency as a significant contribution to the EL came from the organic polymer layers.

Emission only from the QDs is the aim for display technology and any emission from other

layers will reduce the colour quality of the display. In these devices the NC films acted as

both the electron transport layer and the exciton recombination zone. In order to achieve

efficient electroluminescence from the QDs alone a multilayer device design should allow

control of the separate elements of charge injection, transport and emission. In order to

achieve this a group at Massachusetts Institute of Technology (MIT) decided to remove the

nanocrystal film from the charge transport processes by placing a QD monolayer between

the organic electron and hole transport layers [52]. The ability to deposit monolayers of

QDs by spin coating was also demonstrated. This is achieved by use of a phase separation

process whereby the dots are spin coated from a mixture of the small molecule N, N’-

diphenyl–N, N’–bis(3-methylphenyl)-(1,1’-biphenyl)-4,4’-diamine (TPD) and the dots in

chloroform. By varying the conditions of spin coating it was possible to obtain a single

monolayer of QDs on top of a 35 nm thick TPD film. In this way the charges would be

transported to the QD monolayer where they would form an exciton which would



recombine to emit a photon. As an electron transport material they used tris-(8-

hydroxyquinoline) aluminium (Alq3), a material used in OLEDs as both an emitter and

electron transport layer. As Alq3 and TPD also emit they add a further layer between the

dots and the Alq3 by thermal evaporation of 3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-

1,2,4-triazole (TAZ) that has energy levels that should act to block holes before they reach

the Alq3 and help confine the excitons to the dot layer. Here they achieved a brightness of

2000 cd/m2 at 1.6 cd/A in the device that did not contain TAZ. This represents a 25 fold

improvement over the previous best, however at higher voltages there is still significant

contribution from the organic layers albeit reduced in the TAZ device. Further studies into

devices comprising a monolayer of QDs were conducted again by the Bulovic group at

MIT where core/shell CdSe/ZnS of varying core and shell thickness are used [53]. For

excitons to recombine in the nanocrystals one of two processes must occur; either charges

are directly injected in the QD or excitons are transferred from the organic matrix to the

dots via Förster resonance energy transfer (FRET). In reference 51 Schlamp et al. find that

increasing the shell thickness of ZnS leads to an increase in the EQE, with an increase in

shell thickness of 0.5 nm leading to a three fold increase in efficiency. This therefore

supports the Förster transfer model, as a thicker shell would lead to a lower rate of carrier

injection but the increase in shell thickness is much smaller than the Förster radius and so

should have a minimal effect on the Förster transfer rate. Other groups had success with

completely solution processable tri-layer devices [54] and found that in comparison with

the bilayer structures they showed improved performance which was attributed to a more

balanced charge injection scheme.

Obviously for any full colour display the demonstration of red, green and blue

pixels in QD-OLEDs with high efficiency and saturated emission is necessary. The use of a

monolayer of quantum dots yielded the best devices seen at the time with the MIT group

improving their original monolayer device with the demonstration of a red emitting QD-

OLED with an EQE >2% and maximum brightness of 7000 cd/m2 [55] although

contribution from the hole transport layer (HTL) and electron transport layer (ETL) was

still observed. The fabrication of green emitting LEDs was presented by Steckel, also at

MIT, using a monolayer of dots between a HTL, a hole blocking layer (HBL) and the ETL

[56]. Good performance and emission almost entirely from the QD layer was presented, at

higher voltages EL contributions of ~ 2.6% were seen from the HTL. Saturated emission



was also reported for red emitting LEDs by Zhao et al. at the University of Washington

[57], also using a QD monolayer. Their approach utilised a thermally polymerized HTL to

prevent any issues arising from degradation of the emissive dot and transport layers due to

sequential spin coating of layers from solvents. This paper represented suppression of the

EL from the polymer and small molecule transport layers at high voltages for the first time.

EL from the QDs showed a full width at half maximum (FWHM) of only 30 nm, an

important step in demonstrating the colour purity advantage of this technology. Although

demonstrations of red through to green wavelengths up to this point had been numerous,

blue QD-OLEDs had been difficult to produce in part due to limitations in producing high

quality blue nanocrystals. For example, in CdSe, the best characterised and most easily

produced QD system, sizes of ~ 2 nm are needed for blue emission. Dots of this size are

hard to produce with small size dispersions, high quantum yields and overcoated with

higher band gap materials as explained by Steckel et al. [58]. Further to this, dots of this

size will have a small absorption cross-section meaning that Förster transfer will be less

efficient when they are incorporated into devices. Steckel et al. make progress in this area

by using CdS/ZnS core/shell nanocrystals that have narrow PL centred at ~ 470 nm and

quantum yields of 20 – 30 %. Although encouraging, a significant contribution to the

electroluminescence comes from the organic transport layers even at low current densities.

An impressive result for blue QD-OLEDs came from Jun and Jang in 2005 by again

seeking to find quantum dot structures that gave higher quality blue emission [59]. In this

paper, Jun and Jang produce dots in an alloy phase by slowly growing ZnS at high

temperatures which allowed the ZnS to diffuse into the CdSe core. In this same report they

observe emission primarily from the QDs even up to voltages as high as 14 V giving a high

quality blue colour to the device. The EL is at the same position as the PL and the

maximum EQE is measured to be 1.5 cd/A, which is a 10 fold improvement on previous

reports. The small contribution to the EL that does not come from the dots originates in the

Alq3 layer and so the authors suggest that as it is an efficient emitter and due to energy level

alignment, Alq3 cannot be used in blue LEDs and suggest the use of alternate ETLs.

Originally the use of a single monolayer of quantum dots as the emissive layer was

motivated by a study carried out by Leatherdale et al. [60] where photoconductivity

experiments revealed that when compared with the organic semiconductors, thick QD films

did not conduct efficiently. This was used to explain the poor performance of early bi layer



devices where the QD layer was also used as the ETL and so would lead to poor charge

balance in these devices, manifested as higher turn on voltages and reduced carrier injection

efficiencies. This approach led to some impressive examples of QD-OLEDs with high

advances in performance as detailed above, but this scheme was not without its

disadvantages as noted by Sun et al. [61]. The use of a single monolayer will clearly mean

a lower density of QDs in the devices and so fewer sites where exciton recombination will

give the desired emission wavelength, also having such a thin active region will mean

excitons are not confined within it as effectively. It is therefore suggested that using a

monolayer will put limitations on output power, maximum luminance and spectral purity.

Sun et al. from the Chinese academy of Sciences, achieved record performance in 2007

through the use of thick QD layers and show that improved performance is possible by

selecting the correct structure of dot, number of monolayers and optimising ETL and HTL

thickness for each colour device [61]. In terms of luminance and luminous efficiency they

achieved 9064 cd/m2 and 2.8 cd/A for red, 3200 cd/m2 and 1.8 cd/A for orange, 4470 cd/m2

and 1.3 cd/A for the yellow and 3700 cd/m2 and 1.1 cd/A for the green. They also report

that FWHM of the EL to be ~ 30 nm for all the devices with the emission remaining stable

even at high voltages and see no component from the organic transport layers. They found

that the optimum number of monolayers (ML) in each device depended upon the size and

shell structure of the dots used (figure 1.10) where they used CdSe/ZnS core-shell dots for

the green and yellow devices and CdSe/CdS/ZnS for the orange and red devices. The

optimum thickness for the red device was found to be 2 monolayers of dots, and ~ 2.5, ~ 4,

and ~ 7 MLs for the orange, yellow and green devices respectively.

Brightness levels for modern displays are on the order of 100s of cd/m2 and so the

brightness and colour purity of QD-OLEDs has reached levels that would produce

extremely high quality displays. Lifetimes of these devices still need drastic improvements,

however, with only a few groups reporting lifetimes on the order of 100s of hours [61],

several orders of magnitude less than would be necessary. Attempts to improve on the best

performance have been ongoing with Niu et al. in the Washington group improving on the

best efficiency for red devices through thermal annealing of the QD layer [62]. This

improved the EL performance 3 fold to 4.24 cd/A at 100 cd/m2 whilst degrading the PL

efficiency. Annealing the film at 180 ˚C was shown to remove some of the ligand and gave



more densely packed and ordered films resulting in overall better morphology from the QD

films.

Fig. 1.10 EL spectra of red QD-OLED for different thicknesses when operated at 3000

cd/m2. The inset shows the same but for green devices. (Taken from [61]).

Other researchers looked into simplifying the device design to ultimately reduce the

complexity of manufacturing processes but also to allow higher current densities at lower

voltages due to the use of a single layer [63]. In this case CdSe/ZnS QDs were dispersed in

the polymer poly(9,9-dioctylfluorene) (PFO) to complement the emission and hole and

electron transporting properties of both materials. In reference 63 Campbell and Crone

produce good quality red devices, with slightly worse performance from the green and as

admitted by the authors, interactions with the PFO in the blue devices yielded poor EL. For

nearly all the devices described thus far the ETL and hole blocking layers have required the

use of vacuum thermal evaporation which on a commercial scale would involve expensive

vacuum set-ups. Also the dot and HTL are degraded by the use of solvent processing on top

of them. This was the motivation behind Stouwdam’s and Janssen’s work [64] where they

use ZnO nanocrystals as the ETL, deposited from solution by spin coating. As ZnO NCs

can be dissolved in a number of solvents it is possible to choose solvents such that the

layers beneath are not damaged. This allows the use of spin coating or inkjet printing to be

used for all layers in the device which offers significant advantages in terms of

manufacturing. Also it has been suggested that by using inorganic materials for the charge



transport layers the LEDs should become more stable. Stouwdam and Janssen also fabricate

red, green and blue devices although with performance well below the best and put this

down to low QY for their dots and lack of device optimisation. Work has also been done on

developing printing methods to deposit the ordered and patterned layers of dots that would

be needed for full colour displays and high efficiency devices have been fabricated using

this technique [65, 66]. An impressive demonstration of the potential of this technology has

recently been published whereby high efficiency red, green and blue devices were

fabricated on the flexible substrate PET (figure 1.11) [67]. When compared directly with

devices produced on glass substrates the output is found to be lower. This is a result of the

low temperature deposition of the conductive indium tin oxide (ITO) layer necessary on the

PET leading to a higher roughness. This will have a pronounced effect on the thin transport

and emissive layers that are on the order of nanometres thus creating pin hole defects and

the like.

Fig. 1.11 Flexible QD-OLED device shown bent at maximum brightness. (Taken from

[67])

Quantum dots have also been incorporated into LEDs with different functionalities

to that described so far. The QDs in the devices has also been investigated to produce white

light QD-OLEDs. This is of interest in solid state lighting where massive savings in energy

use can be made through widespread use of more efficient lighting. QD-OLEDs are ideally

suited for use as back lights in LCD panels as they offer thinner displays with better colour

rendering properties and lower power consumption. A number of schemes have been

implemented using emission from the dots combined with other layers to obtain broad,

white EL [68, 69], or by utilising nanocrystals with deliberately broad emission [70] or by

using three different sizes of quantum dot for the red, green and blue emission [71]. These



three schemes have all shown some success and although interesting in their own right the

focus in this thesis is on monochromatic devices for flat panel display applications. They

have also been used for their unique electrical properties to improve the efficiency of

OLEDs without being the main emissive component [72].

So by combining the strengths of quantum dot technology and that of organic LEDs

hybrid light emitting devices can be made with advantages over other display technologies.

The emission derived from the quantum dots in the hybrid devices is spectrally very narrow

with typical FWHM of ~ 30 nm leading to excellent colour purity. Also the ability to

produce quantum dots over such a wide range of wavelengths allows the number of colours

that can be displayed on current displays to be exceeded considerably through mixing of

various amounts of red, green and blue. The best performance for these hybrid devices still

lags way behind that of OLEDs, although OLEDs have had many more years of research,

and hybrid devices have improved rapidly in a comparatively short length of time. Further

advantages over traditional displays are gained from the pixel either being on or off

allowing huge contrast ratios to be obtained as no backlight is necessary. This as well as the

low operating voltages will ultimately lead to more efficient, less power-hungry displays

whilst the solution processing techniques allow for low cost manufacturing. The

incorporation of inorganic nanocrystals should also allow for more stable devices and so

longer lifetimes when compared with OLEDs as degradation related to the organic

molecules spending significant time in excited states will be reduced. There are a number

of problems that need to be overcome before this technology can become commercial with

one of the largest problems being the use of heavy-metal-containing nanoparticles. The

most widely available quantum dot material is CdSe. These QDs have the highest quality

and performance and are the best characterised. Materials containing heavy metals, even in

very small amounts, are banned from commercial products in many countries and studies

into the toxicology of QDs have highlighted the dangers of heavy-metal-containing

nanocrystals [73]. Most of the hybrid QD-OLEDs to date have made use of heavy metal

containing nanocrystals and so an important step for the commercial application of this

technology is to produce LED using heavy-metal-free quantum dots. A better

understanding of the charge transfer processes occurring between the dots and organic

layers and the interaction leading to recombination is also needed. A moderate amount of

research has been directed in this area with experiments on charge transfer processes [74-



76] and elementary interactions between QDs and organic layers [77] as well as attempts to

theoretically model the QD-OLED system [78]. These have revealed the nature of some of

the mechanisms involved to aid fabrication of higher performance devices.

References

1. Fernee, M.M.J., et al. Nanotechnology, 2003. 14(9): p. 991-7. 2. Lipovskii, A., et al. Applied Physics Letters, 1997. 71(23): p. 3406-3408. 3. Brus, L.E. Applied Physics A, 1991. 53: p. 465-474. 4. Klimov, V.I., ed. Semiconductor and Metal Nanocrystals: Synthesis and Electronic

and optical properties. Optical Engineering, ed. B.J. Thompson. 2004, Marcel Dekker.

5. EvidentTechnologies. 2007 [cited 2007 4th May]; Available from: http://www.evidenttech.com/.

6. Hillhouse, H.W. and M.C. Beard. Current Opinion in Colloid & Interface Science, 2009. 14(4): p. 245-259.

7. Shockley, W. and H.J. Queisser. Journal of Applied Physics, 1961. 32(3): p. 510-519.

8. Hanna, M.C. and A.J. Nozik. Journal of Applied Physics, 2006. 100(7): p. 074510-8. 9. Faraday, M. Philosophical Transactions of the Royal Society of London, 1857. 147:

p. 145-181. 10. Ekimov, A.I. and A.A. Onushchenko. Soviet Physics. Semiconductors, 1982. 16(7):

p. 775-778. 11. Efros, A.L. and A.L. Efros. Soviet Physics. Semiconductors, 1982. 16(7): p. 772-

775. 12. Brus, L.E. Journal of Chemical Physics, 1983. 79(11): p. 5566-5571. 13. Brus, L.E. The Journal of Chemical Physics, 1984. 80(9): p. 4403-4409. 14. LaMer, V.K. and R.H. Dinegar. J. Am. Chem. Soc., 1950. 72(11): p. 4847-4854. 15. Murray, C.B., C.R. Kagan, and M.G. Bawendi. Annual Review of Materials Science,

2000. 30(1): p. 545-610. 16. Murray, C.B., D.J. Norris, and M.G. Bawendi. J. Am. Chem. Soc., 1993. 115(19): p.

8706-8715. 17. Ekimov, A.I., et al. Journal of the Optical Society of America. B, Optical physics,

1993. 10: p. 100-107. 18. Norris, D.J. and M.G. Bawendi. Physical Review B, 1996. 53(24): p. 16338. 19. Kang, I. and F.W. Wise. Journal of the Optical Society of America. B, Optical

physics, 1997. 14(7): p. 1632-46. 20. Hines, M.A. and P. Guyot-Sionnest. J. Phys. Chem., 1996. 100(2): p. 468-471. 21. Dabbousi, B.B.O., et al. The journal of Physical Chemistry. B, Materials, surfaces,

interfaces & biophysical, 1997. 101(46): p. 9463-75. 22. Nirmal, M., et al. Physical Review Letters, 1995. 75(20): p. 3728. 23. Norris, D.J., et al. Physical Review B, 1996. 53(24): p. 16347. 24. Klimov, V.I., et al. Physical Review B, 1999. 60(19): p. 13740. 25. Guyot-Sionnest, P., Brian Wehrenberg, and D. Yu. The Journal of Chemical

Physics 2005. 123(7): p. 07.



26. Klimov, V.V.I. The Journal of Physical Chemistry. B, Materials, surfaces, interfaces & biophysical, 2000. 104(26): p. 6112-23.

27. Efros, A.L., V.A. Kharchenko, and M. Rosen. Solid State Communications, 1995. 93(4): p. 301-305.

28. Schaller, R.D., et al. Physical Review Letters, 2005. 95(19): p. 196401. 29. Prezhdo, O.V. Chemical Physics Letters, 2008. 460(1-3): p. 1-9. 30. Ellingson, R., et al. Nano letters, 2005. 5(5): p. 865-871. 31. Nozik, A.J. Annual Review of Physical Chemistry, 2001. 52: p. 193-231. 32. Schaller, R.D. and V.I. Klimov. Physical Review Letters, 2004. 92(18): p. 186601-4. 33. Schaller, R.D., M.A. Petruska, and V.I. Klimov. Applied Physics Letters, 2005.

87(25): p. 253102-3. 34. Murphy, J.E., et al. Journal of the American Chemical Society, 2006. 128(10): p.

3241-3247. 35. Schaller, R.D., J.M. Pietryga, and V.I. Klimov. Nano Letters, 2007. 7(11): p. 3469-

3476. 36. Beard, M.C., et al. Nano Letters, 2007. 7(8): p. 2506-2512. 37. Schaller, R.R.D. Nano letters, 2006. 6(3): p. 424-9. 38. Rupasov, V.I. and V.I. Klimov. Physical Review B (Condensed Matter and

Materials Physics), 2007. 76(12): p. 125321-6. 39. Trinh, M.T., et al. Nano Lett., 2008. 8(6): p. 1713-1718. 40. M.C. Beard and R.J. Ellingson. Laser & Photonics Review, 2008. 2(5): p. 377-399. 41. Schaller, R.D., et al. J. Phys. Chem. B, 2006. 110(50): p. 25332-25338. 42. Nair, G. and M.G. Bawendi. Physical Review B (Condensed Matter and Materials

Physics), 2007. 76(8): p. 081304-4. 43. Ben-Lulu, M., et al. Nano Letters, 2008. 8(4): p. 1207-1211. 44. Pijpers, J.J.H., et al. The Journal of Physical Chemistry C, 2008. 112(12): p. 4783-

4784. 45. McGuire, J.A., et al. Accounts of Chemical Research, 2008. 41(12): p. 1810-1819. 46. Beard, M.C., et al. Nano Letters, 2009. 9(2): p. 836-845. 47. Sukhovatkin, V., et al. Science, 2009. 324(5934): p. 1542-1544. 48. Colvin, V.L., M.C. Schlamp, and A.P. Alivisatos. Nature, 1994. 370(6488): p. 354-

357. 49. Klimov, V.I., Semiconductor and Metal Nanocrystals: Synthesis and Electronic and

optical properties. Optical Engineering, ed. B.J. Thompson. Vol. 87. 2004: Marcel Dekker.

50. Dabbousi, B.O., et al. Applied Physics Letters, 1995. 66(11): p. 1316. 51. Schlamp, M.C., X. Peng, and A.P. Alivisatos. Journal of Applied Physics, 1997.

82(11): p. 5837-5842. 52. Coe, S., et al. Nature, 2002. 420(6917): p. 800-803. 53. Coe-Sullivan, S., et al. Organic Electronics, 2003. 4(2-3): p. 123-130. 54. Chaudhary, S.S. Applied Physics Letters, 2004. 84(15): p. 2925-7. 55. Coe-Sullivan, S., et al. Advanced Functional Materials, 2005. 15: p. 1117-1124. 56. Steckel, J.S., et al. Angewandte Chemie International Edition, 2006. 45: p. 5796 –

5799. 57. Zhao, J., et al. Nano Letters, 2006. 6(3): p. 463-467. 58. Steckel, J.S., et al. Angewandte Chemie International Edition, 2004. 43(16): p.

2154-2158. 59. Jun, S. and E. Jang. Chemical Communications, 2005(36): p. 4616-4618.



60. Leatherdale, C.A., et al. Physical Review B, 2000. 62(4): p. 2669. 61. Sun, Q., et al. Nature Photonics, 2007. 1(12): p. 717-722. 62. Y. H. Niu, et al. Advanced Materials, 2007. 19(20): p. 3371-3376. 63. Campbell, I.H. and B.K. Crone. Applied Physics Letters, 2008. 92(4): p. 043303-3. 64. Stouwdam, J.W. and R.A.J. Janssen. Journal of Materials Chemistry, 2008. 18(16):

p. 1889-1894. 65. Kim, L., et al. Nano Letters, 2008. 8(12): p. 4513-4517. 66. Anikeeva, P.O., et al. Nano Letters, 2009. 0(0). 67. Zhanao Tan, et al. Journal of Applied Physics, 2009. 105(3): p. 034312. 68. Xuan, Y., et al. Nanotechnology, 2006. 17(19): p. 4966-4969. 69. Ahn, J.H., et al. Nanotechnology, 2007. 18(33): p. 335202. 70. Tang, A.-W., et al. Journal of Luminescence, 2007. 122-123: p. 649-651. 71. Anikeeva, P.O., et al. Nano Letters, 2007. 7(8): p. 2196-2200. 72. Ryu, S.Y., et al. Nanotechnology, 2009. 20(6): p. 065204. 73. Hardman Ron. Environmental Health Perspectives, 2006. 114(2): p. 165. 74. Kucur, E., et al. The Journal of Chemical Physics, 2004. 120(3): p. 1500-1505. 75. Chin, P.T.K., R.A.M. Hikmet, and R.A.J. Janssen. Journal of Applied Physics, 2008.

104(1): p. 013108-6. 76. StoÌˆferle, T., U. Scherf, and R.F. Mahrt. Nano Letters, 2009. 9(1): p. 453-456. 77. Huang, H., et al. Nano Letters, 2007. 7(12): p. 3781-3786. 78. Kohary, K.K. Journal of Applied Physics, 2006. 100(11): p. 114315-1.

Chapter 2 Theory


Chapter 2: Theory

2.1 Quantum dot theory

2.1.1 Theoretical description of quantum dots

As has been introduced in the previous chapter, quantum dots represent a unique

situation between that of molecules and bulk materials. They are discrete particles whose

size, on the order of nanometres, is such that they are said to confine the wave functions of

the electron, hole and exciton. Quantum confinement has been observed in low dimensional

structures for many years. In the case of confinement in one dimension we get a 2-D

structure known as a quantum well, for confinement in two dimensions, a 1-D structure is

obtained called a quantum wire. In the case for quantum dots we get quantum confinement

of the charge carrier wave functions in all three spatial dimensions yielding a quasi-zero-

dimensional structure. The potential barrier represented by the physical limit of the dot will

act to confine the charge carriers if its size approaches the natural length scale of the

electron, hole or exciton, termed the Bohr radius. The quantum dot crystal structure remains

the same as that for the bulk semiconductor yet this quantum size effect changes the

continuous conduction and valence bands found in bulk into discrete atomic-like states. For

this reason semiconductor nanocrystals have been dubbed artificial atoms, but unlike atoms

the degree of quantization is a direct result of the size of the particle and therefore these

states can be tuned by varying the nanocrystal size.

Fig. 2.1 Diagram showing bulk semiconductor with continuum of states in conduction and

valence band separated by energy gap and nanocrystals with discrete states separated by

larger energy gap due to the quantum size effect. (Taken from [1])

Chapter 2 Theory


2.1.2 Confinement regimes

The size of the nanoparticle relative to the Bohr radii of the electron, hole and

exciton determines the size regime it is said to occupy. It is possible to calculate the

electron and hole Bohr radii using the relationship ae, h = εrћ/me,he2 where εr is the relative

dielectric constant, ћ is the reduced Planck constant, me,h is the electron or hole effective

mass, and e is the elementary charge. The Bohr radius of the exciton can be found from the

equation

0*0 a

m

ma rb ε= , (2.1)

where m* is the reduced effective mass of the electron hole pair, m0 is the rest mass of

the electron and a0 is the Bohr radius of the hydrogen atom.

Three size regimes can be defined and are known as the strong, intermediate and

weak confinement regimes. The size regime a nanoparticle is in will have an influence on

the energy levels of the electron and hole as well as the dynamics of the charge carriers.

Quantum dots in the weak confinement regime will only confine the centre of mass motion

of the exciton and corresponds to the situation when the radius of the nanocrystal, a, is

larger than the Bohr radii of the electron (ae) and hole (ah) but is smaller than that of the

exciton (aexc) (i.e. ae, ah < a < aexc). Features in the optical spectra of this regime are

therefore due to the quantisation of the exciton centre of mass motion as the exciton

binding energy will be greater than the energy quantisation of the electron and hole. When

the radius of the nanocrystal is larger than the Bohr radii of the hole but smaller than that of

the electron and exciton (ah < a < ae, aexc) as found in materials where the electron and hole

effective masses differ greatly, the QD is said to be in the intermediate confinement regime.

In this case the hole will be strongly confined to the centre of the dot and the energy levels

will mainly be determined by the quantisation of the electron motion. The strong

confinement regime is found when the radius of the nanocrystal is smaller than the Bohr

radii of the electron, hole and exciton. Here the charge carriers are strongly confined within

the nanocrystal and the separation between the quantum size levels of both carriers is now

of the order ћ/me,he2 which is large compared with the coulomb interaction between the

electron and hole [2]. The regime that the nanocrystal is in will depend upon the material

being used and of course the size of the nanocrystal itself. For example, in CdSe

nanocrystals the electron radius ae ~ 3 nm and the hole radius is ah ~ 1 nm whilst the Bohr

Chapter 2 Theory


radius of the exciton is ab ~ 5 nm. Therefore CdSe nanocrystals are typically found to be in

the strong or the intermediate confinement regime depending upon their size. III-V

semiconductor quantum dots such as InP with a Bohr radius of ~ 11 nm can be made small

enough such that they are in the strong confinement regime, IV-VI semiconductors

typically have even larger Bohr radii such as in PbS where ab ~ 18 nm.

2.1.3 The Particle-in-a-sphere model

In order to describe the size dependant electronic properties of nanocrystals

quantitatively it is necessary to go beyond the simple description of different confinement

regimes. A relatively simple model was formulated in work by Efros and Brus [2, 3]

commonly known as the particle-in-a-sphere model. In this model a number of

approximations are used so as to consider the electrons and holes as particles inside an

infinite spherical potential well of radius a. The approach used to describe it here is that

same as that used by Norris [4]. It considers a particle of mass m0 within a spherical

potential well of radius a, that is 0 within the well and infinite outside the well.

>∞≤

=ar

arrV

0)( (2.2)

The Schrodinger equation can be solved and separated into an angular part and a

radial part where the angular solutions are the spherical harmonics known from the model

of the hydrogen atom and the radial function can be described by Bessel functions [4]

yielding

r

rkjCr

mllnl

mln

),()(),,( ,

,,,

φθφθψ

Υ= (2.3)

where )( , rkj lnl is the lth order spherical Bessel function, mlΥ is a spherical harmonic and C

is a normalization factor. Due to the boundary conditions, the wave number, k, cannot take

any value and so is determined by

ak ln

ln,

,

α= (2.4)

where ln,α is the nth zero of the Bessel function, lj . The eigenvalues of the Hamiltonian

representing the energy of the particle are therefore [4]

Chapter 2 Theory


20

2,

2

0

2,

2

, 22 amm

kE lnln

ln

αhh== (2.5)

The electron and hole levels can be described as atomic like orbitals labelled using the

principal quantum number, n, and the angular momentum l as 1S, 2S, 1P etc. This general

model reveals the strong energy dependence on nanocrystal size observed is a result of the

1/a2 term seen here.

2.1.4 Applying the model to quantum dots

The general model described above requires a number of approximations to allow

the situation found in a real nanocrystal to be considered as a particle in a sphere problem.

As lattice spacing in semiconductors such as CdSe are on the order of 6 Ǻ, a quantum dot

on the order of several nanometres in diameter will contain thousands of atoms and so

electrons and holes are not simply particles experiencing a potential in empty space. One

simple approach to this is to assume that as the nanocrystals contain a large number of

atoms they will retain their bulk lattice structure and so the effective mass approximation

can be used. In a bulk crystal with a periodic lattice the electron wavefunctions can be

described as plane waves or Bloch functions that are modified by the influence of the

periodic potential of the crystal lattice. Bloch functions are written according to Bloch’s

theorem as [4]

)exp()()( rkirur nknk

rrrr ⋅=Ψ (2.6)

where )(runk

r is a function accounting for the periodicity of the lattice and n is an index

describing the particular band with wave vector k . Also, under the effective mass

approximation the bands described by the energy of these wavefunctions as a function of k

are assumed to have simple parabolic bands. This means that in CdSe and CdS for example,

which are direct-gap semiconductors, the band diagram will have the form shown in figure

2a (minimum band gap at k = 0). As such, the energy of the conduction and valence bands

are given by

gceff

ck E

m

kE +=

2

22h

veff

vk m

kE

2

22h−= (2.7)

Chapter 2 Theory


where Eg is the direct semiconductor band gap and the energies are relative to the valence

band maxima.

Fig. 2.2 Bands are parabolic under the effective mass approximation as shown here where

(a) is a simple 2 band model for a direct gap semiconductor and (b) shows discrete electron

and hole levels found in nanocrystals as a result of quantization of the bulk bands. (Taken

from [4]).

So the influence of the lattice is accounted for by simply treating the electron and

hole as free particles with different masses, where they are still described by plane waves

that are modulated by the periodic potential of the lattice. As such, we write the single

particle wavefunctions of either the electron or hole as a linear combination of Bloch

functions just as we would in a bulk sample

)exp()()( rkruCr nknkk

sp

rrrr ⋅∑=Ψ ) (2.8)

where the coefficients Cnk ensure that the function takes into account the spherical

boundary conditions of the nanocrystal [4]. By assuming that unk depends weakly on k we

can take the periodic potential outside the sum resulting in

)()()exp()()( 00 rfrurkiCrur spnnkk

nsp

rrrrrr =⋅∑=Ψ (2.9)

where )(rf sp

r is the single particle envelope function and 0nu can be determined from the

tight binding approximation. If the above assumptions hold then the energy levels of the

Chapter 2 Theory


electron and hole are described as in figure 2.2b by envelope functions which can be found

from the particle-in-a-sphere solutions, and by inclusion of the effective mass their energy

can be found from equation (2.5). So by combining the above single particle wavefunctions

and treating the electron and hole as separate particles in a sphere the states will take the

form of equation (2.3) and the energy of the electron-hole pair can be calculated from [3]

cceff

Ln

veff

Lngeehhehp E

mmaELnLnE eehh −

++=2

,2

,

2

2

2)(

ααh (2.10)

Here the first term takes into account the band gap of the semiconductor, the second

term is the confinement term taking into account the quantum energy of localisation and the

third term is incorporated as a result of the Coulomb attraction and is added as a first order

energy correction. Under the strong confinement approximation the electron and hole can

still be treated as independent as the Coulomb attraction varies as 1/a and so will be

dominated by the confinement term which varies as 1/a2. Thus, the coulomb correction is

equal to ae εβ /2 where ε is the dielectric constant and β is close to 1.8 for pair states where

the electron is in the 1Se and 1.7 for transitions to 1Pe and 1De. Under this formulation, as

seen in the left hand side of equation 10, states are labelled using the quantum numbers

nhLhneLe.

2.1.5 Valence band substructure

There are fundamental differences between the conduction and the valence band

which have not been taken into account in the model above. Describing the conduction

band in CdSe, for example, as simple parabolic bands is reasonably accurate as it arises

from Cd 5s orbitals which are spherically symmetric and non-degenerate. The valence band

is more complicated, arising in CdS and CdSe from 3p and 4p orbitals of sulphur and

selenium which are not spherically symmetric and are degenerate. As such the valence band

is not best described using the effective mass approximation. In order to account for this, in

CdSe for example, we assume that it has the ideal zinc blende or diamond-like band

structure where, due to strong spin-orbit coupling, the valence band degeneracy is lifted.

When considering the valence band the quantum number J, which is the total angular

momentum, is used to describe the sub-bands that result from the strong spin-orbit coupling.

Chapter 2 Theory


As J = L ± S, where L (= 1 for p orbitals) is the orbital contributions to the angular

momentum and S (= ½ for electron) is the spin contribution to the angular momentum, it

can take values of 3/2 and 1/2 as shown in figure 2.3a.

Figure 2.3 a) valence band structure for a zinc blend semiconductor lattice and b) the band

structure for a wurtzite (hexagonal) crystal lattice where the resultant crystal field causes

the A and B bands to split. (Taken from [4]).

In this figure we see three distinct bands which are called the heavy hole (hh) light

hole (lh) and split off (so) band, also called A, B and C respectively. At k = 0 the J = 3/2

band is degenerate with the different curvatures corresponding to different effective masses,

hence the names light hole and heavy hole. For semiconductors that are better described by

a wurtzite or hexagonal crystal structure two different effects contribute to split the hh and

lh sub-bands. Firstly, the structure leads to a slightly prolate NQD shape resulting in a

crystal field and secondly there is no longer inversion symmetry in the crystal which will

further split the A and B bands. As these terms can be relatively small they are often

neglected in calculating the energy levels in QDs [4].

2.1.6 Exciton fine structure and the “dark” exciton

Although the absorption spectra of quantum dots are well understood and electronic

transitions can be easily assigned to their features [5] there was for a time some contention

as to the nature of the emitting states. The problem centred around the experimental

evidence which showed that even in high quality nanocrystals the radiative recombination

Chapter 2 Theory


lifetime was very long at low temperatures (~ 1 µs at 10 K [6]) despite it occurring on the

order of nanoseconds in bulk. High quality samples are mentioned here as initially this long

lifetime was explained in terms of carriers interacting with the surface and becoming

localized in surface traps. This would lead to a reduced overlap of the electron and hole

wavefunctions and so a lower recombination rate. Thus, in higher quality samples this long

lifetime cannot be attributed to trap states and so the idea of a band edge exciton fine

structure was proposed. In CdSe for example, the lowest excited state is the 1S3/21Se state

which, under the assumptions of it having a zinc blende crystal structure and a spherical

shape is eightfold degenerate [5]. As has been mentioned, CdSe NQDs in fact have a

slightly prolate shape and is more accurately described as having a wurtzite crystal

structure. So as a result of going from spherical to uniaxial symmetry the band edge

degeneracy is lifted and is split into two four-fold degenerate states (figure 2.4, left hand

side). The exchange interaction between electrons and holes has negligible impact in bulk

semiconductors, but as it is proportional to the spatial overlap between the electron and

hole, its effect will be greatly enhanced in quantum dots [7]. This therefore splits the band

edge exciton and mixes electron and hole spin states. In bulk it acts to split the 8-fold

degenerate band edge exciton into an optically active 3-fold degenerate state with total

angular momentum 1, and a 5-fold degenerate optically passive state with total angular

momentum 2 (figure 2.4, right hand side). By including both of these effects the good

quantum number is the total angular momentum projection on the unique crystal axis Nm,

and the band edge is split into one level with 2=mN , two with 1=mN and two

with 0=mN .

As was shown in figure 1.5 in chapter 1 the splitting of the band edge exciton is size

dependent and can be used to explain some of the characteristics observed in absorption

and photoluminescence spectra. Each sub level is labelled by its angular momentum

projection on the crystal axis, with a superscript of U (upper) or L (lower) to denote

whether it comes from the N = 1 or N = 2 manifold respectively. Thus, the question of the

long luminescence lifetimes found at low temperatures can be quantitatively explained

through the exciton fine structure.

Chapter 2 Theory


Fig. 2.4 Energy level diagram showing the effects on the band edge exciton when a

uniaxial crystal lattice and prolate NQD shape dominates (left) and when the exchange

interaction in small nanocrystals dominates (right). When both effects are included the five

sublevels labelled by Nm are formed. (Taken from [4]).

An exciton in a nanocrystal will quickly relax to the lowest band edge state, which, is

the 2=mN state in figure 2.5 below. As the transition to the ground state from here would

involve two units of angular momentum it is optically forbidden, passive or “dark”. It can

recombine, however, by some process that will flip the electron spin projection or through a

LO phonon-assisted transition [8] and as this will typically be fairly inefficient the long

luminescent lifetimes can be explained. The exciton fine structure can also be used to

explain the large size-dependent Stokes shift. As transitions to this exciton ground state

( 2=mN ) is one photon forbidden, the nanocrystal will not absorb into this state. It has

been shown that the U1± and the U0 carry nearly all of the oscillator strength so these

states absorb photons readily. A transition to the 0=mN or the 1=mN state will

therefore be followed by relaxation into the optically passive 2=mN emitting state, and so

the energy difference between these states will give rise to the observed size dependent

Stokes shift.

Chapter 2 Theory


Fig. 2.5 Energy level diagram showing band edge exciton fine structure where optically

“bright” states appear as solid lines and optically “dark” states appear as dashed lines and

arrowed lines show transitions between the ground state and excited states. (Taken from

[9])

2.2 Multiple exciton generation (MEG) theory

2.2.1 Multiple excitons in bulk semiconductors

In bulk semiconductors the process of absorbing light and the subsequent dynamics

of the charge carriers are well known. Upon absorption of a photon of sufficient energy by

a semiconductor crystal a valence band electron will be excited to the conduction band

leaving a positively charged hole in the valence band. These charge carriers will effectively

be free as the binding energy for the electron hole pair will be too low to form an exciton at

room temperature. If the semiconductor absorbs a photon with energy in excess of the band

gap, this excess energy will be distributed among the charge carriers according to their

effective masses. Momentum conservation will dictate that more of this energy will be

imparted to the carrier with the smaller effective mass and will be manifested as kinetic

energy. The excess energy of the electron under this situation can be found from [10]

)1)(( **hege mmEhvE +−=∆ (2.11)

where *em and *

hm are the effective masses of the electron and hole respectively.

The dynamics for this scenario can be more clearly considered by looking at the

case for excitation by an ultrashort laser pulse of monoenergetic photons in excess of the

band gap. Immediately following excitation the carrier distribution of electrons and holes

will not be in equilibrium until a number of interactions with themselves and the crystal

lattice have occurred. They firstly have to separately undergo inelastic carrier-carrier

scattering to redistribute the energy amongst them according to a Boltzmann distribution.

Chapter 2 Theory


As the kinetic energy distribution will correspond to a temperature above that of the crystal

lattice they are often called “hot” electrons or holes. The electron and hole distributions will

then need to cool in order to reach equilibrium with the lower temperature of the crystal

lattice. This can occur through Fröhlich interactions whereby longitudinal optical phonons

are emitted and carry away the excess kinetic energy of the carriers until the temperature of

the carrier distribution and the lattice matches (figure 2.6).

Fig. 2.6 Cooling mechanism of “hot” electrons and holes in bulk semiconductors. (taken

from [11])

Recombination of the electrons and holes may then occur, either radiatively or non-

radiatively, to return the system to the equilibrium present before excitation. Alternatively

the charge carriers could undergo spatial separation to form a photocurrent as required for

solar cell operation. If the excess energy of these “hot” carriers could be turned into useful

work before their energy is lost through phonon emission then photovoltaic devices could

be made with efficiencies which surpass the Shockley/Queisser limit [12]. As a charge

carrier receives more excess energy the rate of the inelastic scattering process will increase

to the point where if it’s kinetic energy is larger than the semiconductors band gap it can

scatter with a valence band electron, exciting it across the band gap into the conduction

band [13]. This can be thought of as an inverse Auger process as one highly energetic

electron hole pair will relax to the band edge by transferring some of its energy to create

Chapter 2 Theory


another exciton; this is known as impact ionization (I.I.). In bulk semiconductors the

threshold for I.I. is larger than that required for energy conservation alone because, as with

all scattering processes in bulk, both energy and crystal momentum must be conserved.

Also due to relatively weak Coulomb interactions, free carriers, not bound electron-hole

pairs are initially produced in bulk semiconductors and so the probability of the whole

process will be low. Clearly carrier cooling through fast phonon emission will directly

compete with I.I. and the continuum of states in the conduction and valence band in bulk

means energy loss by phonon scattering is very efficient. As such, it is not until the excess

kinetic energy of the electron is multiples of the band gap that the rate of I.I. will be

increased to the extent that it competes with phonon scattering rates. In the case of bulk

silicon the efficiency of I.I. only reaches 5 % at 3.6 times the band gap (hv = 4 eV) and is

still only 25 % at 4.4 times the band gap (hv = 4.8 eV) [14]. This will therefore not

contribute to improved solar cell efficiencies for the photon ranges present in the solar

spectrum.

2.2.2 Multiple exciton generation in semiconductor nanocrystals

To understand why the MEG process is so much more efficient in quantum dots the

idea of the phonon bottleneck, as discussed in chapter one, will be revisited here.

Semiconductor nanocrystals have discrete quantized energy levels with a large energy

spacing on the order of 100 meV [13]. As such the expected scenario is that carrier cooling

via emission of phonons would be significantly reduced as the phonon energy (~ 30 meV)

is only a fraction of the energy spacing. It can therefore only proceed via inefficient multi-

phonon emission; this effect is known as the “phonon bottleneck”. Experimentally the hot

carrier relaxation, although slow compared with bulk (sub-ps time scale), was not found to

be as slow as expected for a true phonon bottleneck. An Auger cooling mechanism was

proposed to explain this whereby the electron relaxes to the band edge by imparting its

energy to the hole which then cools to the band edge through the denser valence band. In

quantum dots such as Pb chalcogenides where the electron and hole effective masses are

almost the same, the Auger mechanism will not be operative and so a slower carrier

relaxation is expected. However, in PbSe for example, although the carrier relaxation rate

was found to be slower than that for CdSe and InP, it was still of the order of picoseconds

which is too fast for the phonon bottleneck explanation [15]. Although a complete

Chapter 2 Theory


understanding of carrier cooling in quantum dots is still lacking, recent calculations on

PbSe [16] have shown that the density of states is much greater than was previously

thought. In this sense we can see that for MEG to become competitive in quantum dots it

must occur faster than the carrier relaxation (picoseconds). In fact, for MEG to give an

increase in photocurrent it must also have a higher rate than electron transfer out of the dot

or to trap states and Auger recombination (figure 2.7). Despite this seeming lack of a

phonon bottleneck the carrier cooling in quantum dots is slowed relative to bulk and as

multiple excitons are generated in a time with an upper bound of 250 fs [17] MEG can

compete favourably with carrier relaxation.

rET

Eg hv

rcc rMEG

rMEG

rMEG > rET, rcc

rET

Eg hv

rcc rMEG

rMEG

rMEG > rET, rcc

Eg hv

rcc rMEG

rMEG

rMEG > rET, rcc

Fig. 2.7 Energy level diagram showing the processes competing with MEG where rMEG, rcc,

rET are the rates of MEG, carrier cooling, and electron transfer respectively.

The reduction of the carrier cooling rate due to the discrete nature of the quantum

dot energy levels is not the sole reason for increased efficiency of MEG in quantum dots. A

result of quantum confinement in quantum dots is enhancement of the electron-hole

coulomb interaction. This is because of the forced overlap in the carriers electronic

wavefunctions and has the effect of increasing the rate of Auger processes, as these are

driven by the coulomb interaction, including the inverse Auger process of MEG. The

threshold is further reduced as the crystal momentum in nanocrystals is not a good quantum

number. As nanocrystals confine charge carriers in all three spatial dimensions, the carriers

Chapter 2 Theory


location is well defined and so according to Heisenberg’s uncertainty principle the

momentum of the carriers will be uncertain. This means that for the MEG process only

energy must be conserved as the requirement for momentum conservation will be relaxed.

This lower threshold for MEG in nanocrystals relative to bulk was thought to

require a photon with energy of twice the band gap by using a simple energy conservation

model. Experimental observations have found this threshold to vary somewhat for different

materials, with CdSe having a ~ 2.5Eg threshold and PbSe having a threshold of ~ 2.9Eg

[18]. Another model has therefore been proposed [19] where the threshold for MEG is

determined by the excess energy of a single charge carrier, which must possess enough

energy to create additional excitons. As has been described, for excitation with a photon

with energy in excess of the band gap, the excess energy will be shared between the

electron and hole according to their effective masses. Using the assumption that the

effective mass of the hole, mh, is greater than or equal to that of the electron, me, a simple

condition for the onset of MEG is found:

gh

eth E

m

mhv )2( += (2.12)

This would appear to match the thresholds found experimentally, as PbSe has very similar

hole and effective masses we would expect a threshold of about 3Eg which is comparable

with the ~ 2.9Eg measured, and as CdSe has very different electron and hole effective

masses the calculated threshold of ~ 2.3Eg is again comparable with the 2.5Eg measured.

Observing MEG in quantum dot systems is challenging due to the rapid processes

involved. Therefore the recombination dynamics that result from the creation of multiple

excitons must be used as a characteristic signature of MEG. This method exploits the

difference between the recombination dynamics of single excitons and multiexcitons and

was first proposed by Schaller and Klimov [20]. Single exciton decay is characterised by

slow radiative recombination which can be tens of nanoseconds in CdSe dots or

microseconds in Pb chalcogenides. The presence of multiple excitons in nanocrystals will

lead to strong exciton-exciton interactions due to the enhanced Coulomb interaction

induced by confinement. Thus multiexciton decay is dominated by Auger recombination in

which the electron and hole recombine non-radiatively by transferring the energy to another

charge carrier. Auger recombination depends upon carrier density. As the number of

electron hole pairs in the dot is increased so too does the rate of decay of the multiexciton

Chapter 2 Theory


state. For dots containing multiple excitons Auger recombination proceeds via a series of

discrete decay rates that correspond to the decay of the N, N-1, …until the single exciton

state [21]. The Auger recombination rate increases according to the number of excitons

present in the nanocrystal with a lifetime on the order of 10s to 100s of picoseconds. This

large difference between single and multiexciton lifetimes affords a means by which MEG

can be confirmed by the emergence of a fast time component with the characteristic

lifetime of Auger recombination when each nanocrystal is excited by only one photon with

energy above the threshold for MEG. Quantum dots excited by ultra-short laser pulses

represent an ideal situation for studying Auger recombination as they will absorb integer

numbers of photons where the proportion of NCs that have absorbed m photons will be

described by a Poisson distribution according to

)exp(!

)( 00 N

m

NmP

m

−= (2.13)

where 0N is the average number of photons absorbed per nanocrystal per pulse and is

calculated from the product of the NC absorption cross section, σa, and the pump photon

fluence, jp: pa jN σ=0 . Thus, by using pulsed laser excitation and observing the

emergence of the Auger recombination we can extract quantitative information on the

quantum yield of MEG. To observe carrier dynamics on the time scale of picoseconds

femtosecond transient absorption is often used. This is a pump-probe technique in which

the pump pulse will excite the NC and a probe pulse, which is delayed relative to the pump,

will be tuned to the 1S exciton transition and monitors the pump-induced absorption

changes. Therefore, the fractional change in transmission at the 1S absorption edge is

proportional to the total number density of excitons in the sample and its initial peak value

represents the number of excitons created by the pulse [4]. Biexciton will decay quickly

due to Auger recombination and so after several biexciton lifetimes Auger recombination

will be complete and each nanocrystal will contain approximately one exciton. As over this

time frame there will not be significant single exciton decay the fractional change in

transmission here corresponds to the number density of initially photo-excited nanocrystals

[18]. So by monitoring the initial amplitude and the amplitude at times long enough such

that the biexcitons will have decayed it is possible to extract the exciton quantum yield [22].

We can arrive at a relationship between the exciton quantum yield and the ratio of these

two values by considering the processes occurring due to the pump and probe beams. The

Chapter 2 Theory


number density of absorbed photons, Nex, will be equal to the total incident photon fluence

minus the number of transmitted photons over the absorption depth,

βσ )exp(1(0 lNJ

N QDpumpex

−−= (2.14)

where J0 is the photon fluence at the front of the cuvette, β is the absorption depth, pumpσ is

the absorption cross section at the pump wavelength, NQD is the density of QDs in solution

and l is the cuvette path length. The absorption induced by the pump beam will lead to a

fractional change in transmission, and by substituting equation (2.14) into this we find that

for no pump probe delay (t = 0) and no MEG occurring this can be written as

βσσ llNJ

T

T QDpumpprobe

t

))exp(1(0

0

−−=∆

=

, (2.15)

where probeσ is the absorption cross section at the probe wavelength. As has been

mentioned, at long times compared to the Auger lifetime any nanocrystal that originally had

more than one exciton will now have only one exciton per excited NC. It is therefore

possible to use Poisson statistics to calculate the number of excited nanocrystals to be

)]exp(1[ JNN pumpQDex σ−−= , which leads to the fractional change in transmission being

given by

lJNT

TpumpQDprobe

t

))exp(1( σσ −−=∆

∞=

. (2.16)

At this point we must consider the two conditions for when the sample has high or

low optical density. For the case of high absorbance the absorption depth, β, will be 1/αpump,

and at low absorbance it will equal the length of the cuvette l. In both of these limiting

cases the ratio of the fractional change in transmission at early and late times, R, is found to

be the same and is expressed as

))exp(1()(

0

00 J

JJR

pump

pumppump σ

σσ

−−= . (2.17)

Chapter 2 Theory


It is possible to incorporate MEG into this equation as we know that at early times Nex is

increased by the quantum yield of producing multiexcitons. Over the time range considered

here single exciton decay will be small but its effect can be easily included as a correction

of the form [ ]τδ )(exp earlylate tt −= ;

))exp(1(

..)(

0

00 J

QYJJR

pump

pumppump σ

δσσ

−−= . (2.18)

From equation (2.18) it is possible to see that in the limit of vanishing fluence the ratio, R,

will tend to δ.QY and so by extrapolating to low fluence we can get a quantum yield of

exciton production per absorbed photon. This method allows the absorption cross section to

be extracted and so avoids any problems that could arise from independent evaluation of

the absorption cross section.

2.3 Hybrid quantum dot organic light-emitting devices

Hybrid quantum dot organic light-emitting devices, or QD-OLEDs, make use of the

characteristic properties of quantum dots in order to make displays with advantages over

other display technologies. The architecture of hybrid OLED devices is similar to that

found in OLEDs but here the quantum dots are used as the emissive layer where all

recombination and light generation should take place. The organic materials act as the

charge transport matrix to deliver the electrons and holes to the quantum dot layer. By

utilizing the best characteristics of both organic and inorganic technologies they offer the

possibility of displays with superior colour purity, high brightness and long lifetimes due to

the inorganics and the low power consumption and cheap and easy manufacturing available

from the organics. A typical QD-OLED structure is shown in figure 2.8. It is made up of a

transparent anode (ITO – indium tin oxide) upon which hole-injecting (HIL) and hole-

transporting (HTL) layers are deposited, usually by spin coating. The emissive layer (EML)

is formed from a layer of quantum dots also deposited via spin coating followed by thermal

evaporation of the electron transport layer (ETL), electron injection layer (EIL) and finally

the metallic cathode.

Chapter 2 Theory


Metallic cathode

SubstrateITO - anode

HILHTL

QD - EMLETLEIL

V+

−Metallic cathodeMetallic cathode


HILHTL

QD - EMLETLEIL

V+

−


HILHTL

QD - EMLETLEIL

V+

−Metallic cathode

Fig. 2.8 Schematic cross section showing the architecture of a typical device stack.

QD-OLEDs are typically operated as forward bias diodes as shown above and the

efficiency of electroluminescence will depend on a few distinct processes. Under an applied

bias electrons and holes will be injected from the contacts into the organic layers. These

charges will then need to be transported to the vicinity of the emissive quantum dot layer

where the electrons and holes will need to undergo some form of charge or energy transfer

from the organic material to quantum dots. Once in an excited state the dots will need to

release this energy in the form of photons. The choice of organic materials for supporting

matrix is very large as a great many materials have been developed for use in OLEDs. They

need to be selected to obtain exciton recombination in the QD layer whilst suppressing any

unwanted processes that may reduce the efficiency. Organic in this context encompasses

two types of organic materials, polymers and “small molecules”, with energy level schemes

similar in some ways to semiconductors with a gap between allowed states defined as the

difference between the highest occupied molecular orbital (HOMO) and the lowest

unoccupied molecular orbital (LUMO). As these materials often show a higher

conductivity for one type of charge carrier over another, different electron transporting and

hole transporting materials are needed. Often able to support the formation of excitons as

well as undergo efficient luminescence themselves, the position of energy levels in these

materials is an important consideration in device design.

2.3.1 Charge injection and energy transfer in QD-OLEDs

To achieve spectrally narrow emission in QD-OLEDs it is necessary to ensure that

any excitons generated will recombine in the quantum dot. The two important processes

involved in fulfilling this requirement are direct charge injection of both electrons and holes

Chapter 2 Theory


or alternatively a non-radiative energy transfer mechanism to the quantum dots from

excitons formed in the organic layers. By considering the energy level alignment of the

various components in the device stack (figure 2.9) it is possible to gain an understanding

of how excitons are formed in the quantum dot. To satisfy the various functions in the

device the different layers will have different energy levels. The wide variety of OLED

materials available means that different materials can be selected to optimize the desired

processes. As we want electrons and holes to be transported through the device to the

quantum dots, the energy levels need to be matched in order to achieve this. For example it

can be seen in figure 2.9 that the potential energy step for injection of the electron and hole

from the electrodes into the ETL and HTL is relatively small and so injection is not

impeded to a large degree. The LUMO position of the ETL material, B, means that there is

in fact no potential barrier for charge injection of electrons into the quantum dot conduction

band. This means the only barrier for electron injection into the dots comes from the

organic ligands which passivate the QD surface [23]. As the ligands will only form a very

thin insulating layer on the surface of the dots ( ~ 5 Ǻ) it should be possible for charge

carriers to tunnel through this layer.

A BQD

CathodeAnode

Vacuum level = 0 eV

Ene

rgy

A BQD

CathodeAnode

Vacuum level = 0 eV

Ene

rgy

A BQD

CathodeAnode

Vacuum level = 0 eV

Ene

rgy

Fig. 2.9 Example of an energy level diagram for a tri-layer device where A is an hole

transport layer and B is an electron transport layer. Solid circles represent electrons in the

conduction band (LUMO) and open circles represent holes in the valence band (HOMO)

with the blue regions showing the band gaps of the different layers. All energy levels are

relative to the 0 eV vacuum level and the arrows represent the movement of charges.

Chapter 2 Theory


For injection of holes into the quantum dot valence band there is a large difference

between the HOMO of the HTL and the top of the valence band in the dots. This is

therefore a significant potential step for the holes to overcome and so this is likely to

impede hole injection into the dots. This scenario will result in a reduced external quantum

efficiency of the device for several reasons. For a dot to emit a photon an electron and a

hole need to be injected into the dot where they will form an exciton which will then

undergo radiative recombination. In figure 2.9 there will be an imbalance of charge carriers

in the quantum dots, which will act in this case as electron traps but will be relatively

unlikely to trap holes. This could lead to the quantum dots becoming charged due to the

accumulation of electrons, therefore, when an exciton is formed in the dot it will decay via

non-radiative Auger recombination. In Auger recombination the energy released by the

recombination of the exciton is given to the unpaired electron, exciting it to a higher energy

level. As this Auger process occurs on far shorter time scales (~ 100s of ps) than radiative

recombination (10s of ns) the result is rapid quenching of quantum dot luminescence [24].

This accumulation of electrons at QD sites will also lead to localized electric fields in the

device which will act to reduce current for a given bias meaning conduction in QD-OLEDs

is dominated by space-charge limited conduction. As Auger recombination is very efficient,

particularly in smaller dots, it is important to balance the number of electrons and holes

being injected into the quantum dots. In reality this is a very hard criterion to fulfil as

charge conduction in organic materials is usually quite low and depending upon the device

design, the dots are likely to trap one of the charge carriers more effectively than another.

The energy transfer mechanism that can contribute to the electroluminescence (EL)

involves the formation of excitons in the organic layers and subsequent non-radiative

resonant energy transfer of the exciton to the QD. This is known as Förster resonance

energy transfer (FRET) or dipole-dipole resonance energy transfer. The efficiency of this

transfer is dependent upon the spectral overlap of the emission of the organic transport

layers, which here act as the donors, and the absorption of the quantum dots which act as

the acceptors. It also has a high dependence upon distance between the donor and acceptor

where the rate of non-radiative energy transfer is inversely proportional to the 6th power of

this separation [25]. The efficiency of this transfer can be found from the equation

660

60

rR

RE

+= (2.19)

Chapter 2 Theory


where R0 is known as the Förster distance and is defined as the distance at which the rate of

energy transfer is equal to the rate of radiative decay and r is the physical separation

between donor and acceptor. Quantum dots are ideal energy acceptors due to the ability to

tune their broad absorption spectrum and their large absorption cross-sections.

Investigations into energy transfer from organic donor materials to quantum dots have

shown that efficient energy transfer in these systems is possible [24]. In Förster energy

transfer the requirement that spin is maintained would appear to limit the efficiency of QD-

OLED operation in that only singlet excitons can be used for light generation. However,

energy transfer of triplet excitons from organic materials to CdSe/ZnS core-shell quantum

dots has been observed [26] and seen to enhance the luminescence intensity of the quantum

dots. As the electron and hole exchange interaction and spin orbit coupling leads to mixing

of the electron and hole spin states in quantum dots the result is a “dark” exciton state (as

described in chapter 1) that sits slightly lower in energy than the emissive exciton state. As

these states are only slightly separated in energy, even for the smallest of dots, thermal

mixing should mean that triplet states can undergo radiative recombination and contribute

to the luminescence intensity. Dexter exchange is another mechanism for exciton transfer

that requires that only total spin is maintained. As it requires an overlap of the donor and

acceptor wavefunction the range over which this can occur is much smaller than that for

Förster exchange with its rate reducing exponentially with distance [26]. In order to avoid

any luminescence from the organic transport layers they will need to transfer all their

energy into the quantum dots which can be a problem in devices employing energy transfer.

There is still some debate in the literature as to the precise nature and efficiency of energy

transfer from organic materials to quantum dots [27] and where it has been observed, the

dominant process has not been identified. To further advance this hybrid technology a more

detailed understanding of the interactions and energy transfer processes between the

organic materials and the semiconductor quantum dots is needed.

2.3.2 Photometry and colour

In order to compare the performance of hybrid QD-OLEDs between different

research groups as well as for different materials and device structures at different

wavelengths it is important to have a practical and consistent system of units and

nomenclature. The measurement of light in the optical range of ultraviolet, visible and

Chapter 2 Theory


infrared wavelengths is known as radiometry. Photometry is also the measurement of

optical radiation but in this case it is limited to the radiant power that can be detected by the

human eye, i.e. about 380 - 770 nm. The response of the eye is clearly very important for

display technology as the eye is more sensitive to green light than blue, for example, and so

any display will need to be designed with this in mind. In practice this means that green

light will appear brighter to an observer than blue light of the same radiance. In radiometry

the radiant energy of light is represented in terms of absolute power and we have quantities

such as radiant energy in joules, radiant flux or power in watts, radiant intensity in watts per

steradian and so on. Each of these quantities have a photometric equivalent which is simply

the radiometric version weighted by the spectral response of the eye.

2.3.2.1 Projected area and solid angle

The measurement of light from a source can be influenced by the area being

measured as well as the angle or distance we measure it from. It is therefore, important to

consider this as many radiometric and photometric units are defined in terms of a solid

angle. Projected area is defined as the rectilinear projection of a surface of any shape on a

plane normal to the unit vector. It can be found by integrating over the differential form of

the cross sectional area dA.cosθ where θ is the angle between the line of sight and the

surface normal. For example, the area of a flat rectangle is the length multiplied by the

width but its projected area will depend upon the angle it is viewed from as shown in figure

2.10. Thus as the area is at an angle to the line of sight of the observer it will appear smaller.

The concept of a solid angle can be defined by first considering the idea of a plane angle. A

plane angle is formed when two straight lines intersect at a point and is defined by the

space between the two lines measured in degrees or radians. A radian is defined as the

angle between 2 radii of a circle that cuts off on the circumference an arc equal in length to

the radius. A circle therefore subtends a plane angle of 2π radians. A solid angle is really

just the 3 dimensional equivalent of the plane angle and is given the units steradian. A

steradian is defined as the solid angle which, having its vertex at the centre of a sphere, cuts

off an area on the surface of the sphere equal to that of a square with sides of length equal

to the radius of the sphere [28]. It is also defined as the ratio between the spherical area

projected onto a unit sphere and the square of the radius. Any sphere will subtend a solid

angle of 4π steradians (sr.).

Chapter 2 Theory


n

Area A

Projected area A0

θ

n

Area A

Projected area A0

θ

Fig. 2.10 Diagram showing the concept of projected area where n is the normal to the plane.

2.3.2.2 Radiometric and photometric quantities

The fundamental quantities in radiometry and their corresponding photometric

quantities require the definition of radiant energy although radiant energy is rarely used

alone in measurements. Radiant energy, Q, is the amount of energy propagating onto,

through or emerging from a specified surface of given area in a given time [29]. The

important radiometric quantities are summarised in the table below with their photometric

equivalents. In the table below dS is an element of the area of the surface and dω is an

element of the solid angle. As the light from many sources will have some spectral

dependence it is common to give the above quantities the suffix spectral (e.g. spectral

radiant energy) to denote that the value is a function of wavelength. Symbols can be given a

lambda subscript where the total quantity will be equal to integrating the above quantities

over wavelength. In the table below the base SI unit of luminous intensity, the candela, is

introduced. The candela is so called as historically candles had been used as standard light

sources when photometry was in its infancy. Later they were replaced by carbon filament

vacuum lamps in the early 20th century which were in turn superseded by a crucible

containing liquid platinum at its freezing point [29]. Today the candela represents the

intensity of a theoretical point source and is defined by the following; the candela is the

luminous intensity, in a given direction, of a source that emits monochromatic radiation of

frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per

steradian. This is the definition adopted by the 16th international commission of weights

and measures in 1979. This definition of the candela along with the weighting factor that

takes into account the spectral response of the eye are all that is needed to allow conversion

between radiometric and photometric measurements.

Chapter 2 Theory


Radiometric quantity Equation SI unit Photometric

quantity

SI unit

Radiant Energy, Q joule, J Luminous energy,

Qv

lm.s

Radiant flux, Φ dQ/dt watts, W Luminous flux, Φv Lumen, lm

Radiant flux density,

E or M

dΦ/dS W/m2 Luminous flux

density, Ev or Mv

lm/m2 = lux,

lx

Radiant intensity, I dΦ/dω W/sr Luminous intensity,

Iv

lm/sr =

candela, cd

Radiance, L d2Φ/dSdωcosθ W/m2sr Luminance, Lv lm/m2sr =

cd/m2 or nits

Table 2.1 Table showing fundamental quantities in radiometry and photometry with units.

The quantities in photometry represent measures of the impression of colour and

light detected by the human eye, that is, how the visible spectrum is perceived by the eye.

The eye is a complex and sensitive vision system but the actual light detection is done by

the retina, which is made up of two different photoreceptors called rods and cones. The

cones are responsible for vision in light conditions and the perception of colour known as

photopic vision, whilst the rods are mainly used for vision in dark conditions known as

scotopic vision. The spectral response of the eye under photopic conditions was quantified

in 1924 by the Commission Internationale de l’Eclairage (CIE). This was achieved by

finding a standard spectral response curve formed empirically from many observer’s

impressions of the visual brightness of monochromatic light sources under controlled

conditions [29]. The curve is shown in figure 2.11 below and has its maximum at 555 nm

and a very small efficiency for wavelengths below 380 and above 760 nm. For dark

conditions the peak spectral sensitivity will shift to the blue and a separate curve for

scotopic vision is used. The original curve has been improved over time to include a

slightly larger range of wavelengths and to include data for 1 nm intervals.

Chapter 2 Theory


350 400 450 500 550 600 650 700 7500.0

0.2

0.4

0.6

0.8

1.0

Spe

ctra

l lum

inou

s ef

ficie

ncy,

V

Wavelength (nm)

Photopic spectral luminous efficiency

Fig. 2.11 Photopic spectral luminous efficiency function V(λ), plotted from data taken from

[29].

In order to calculate the relevant photometric quantity, Xv, from its equivalent

radiometric quantity, Xλ, it is possible to use the spectral luminous efficiency function for

photopic vision V(λ) in the following equation:

∫= λλλ dVXX v )(683 , (2.20)

where the integral is across the measured spectrum [29]. Some of the quantities and units

introduced in table 1 warrant some explanation as to their physical meaning. Luminous flux

also known as luminous power has units of lumens (lm) which is an SI derived unit defined

from the candela as 1/683 watts of radiant power at a frequency of 540 x 1012 hertz.

Alternatively, an isotropic point source with a luminous intensity in all directions of one

candela will emit one lumen of luminous flux per unit solid angle. The lumen is analogous

to the watt differing only in that it is weighted by the spectral response of the eye.

Luminous flux density can relate to two slightly different situations known as illuminance,

Ev, which is the photometric equivalent of irradiance and is used when the light is incident

upon a surface, whereas luminous exitance, Mv, the equivalent of radiant exitance is used

when the light is emerging from the surface. Both are measured in lumens per square metre

(lm/m2) which is also known as a lux (lx). Luminance is an important photometric quantity,

equivalent to radiance. It has units of lumens per square metre per steradian (lm.m-2.sr-1)

Chapter 2 Theory


but is more commonly given units of candela per square metre (cd/m2) which are known as

nits. It is important as luminance is what we actually perceive and so is closely related to

what we often refer to as “brightness” when characterising displays or luminaries.

2.3.2.3 Colour Science

Perception of colour by an observer is the result of the physical process of detecting

the radiant flux entering the eye by the cones in the retina and the interpretation of that

information by both the eye and the brain. It is a subjective idea that will vary from person

to person but is consistent enough to allow the meaningful analysis of colour perception.

Systems for measuring and describing colour have developed and changed over many years

and numerous systems have been used. It has been found that any colour can be reproduced

through the mixing of three primary sources with particular spectral flux distributions [29].

A system which specifies a colour in terms of three standard spectral distributions, also

known as colour stimuli, is known as a trichromatic system. The three primary stimuli used

are most often chosen to be red, green and blue to approximate the three types of cones

found in the retina with the most important and widely used tristimulus system being that of

the Commission Internationale de l’Eclairage. The system adopted by the CIE in 1931 has

become the standard way of defining colour although it has been amended and improved

upon since that time. Standard curves are defined based on psychophysical measurements

which describe the sensitivity of the standard observer for the three colour stimuli and are

called the colour matching functions )(λx , )(λy and )(λz . They are related to the primary

colour stimuli X, Y, and Z and represent the amount of each primary stimuli need to

stimulate a perceived colour that is the same as the perceived colour of monochromatic

light of a particular wavelength. They are shown in figure 2.12 and act as weighting factors

on spectral distributions to find the wavelength they would correspond to. For light with

intensity that is a function of wavelength I(λ), the three responses can be found from the

integral over the entire spectrum with each of the weighting factors as below and so are

known as the tristimulus values for the spectrum I(λ):

∫

∫

∫

=

=

=

λλλ

λλλ

λλλ

dzIZ

dyIY

dxIX

)()(

)()(

)()(

(2.21)

Chapter 2 Theory


350 400 450 500 550 600 650 700 7500.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Rel

ativ

e m

agni

tude

Wavelength (nm)

x y z

Fig. 2.12 The three CIE 1931 colour matching functions plotted from CIE data for a

standard observer.

In order to display this clearly the CIE XYZ three-dimensional colour space is used

to represent all perceivable colours known as the human gamut. The tristimulus values can

be transformed to give chromaticity coordinates in this colour space as shown below.

ZYX

Zz

ZYX

Yy

ZYX

Xx

++=

++=

++=

(2.22)

It is common to represent the colour on the two dimensional CIE chromaticity diagram xyY

diagram (figure 2.13) as only x and y are needed to describe a colour where the Y denotes

the brightness or luminosity and can be found using equation 2.20. Monochromatic colours

of light sit on the locus of the xyY colour space whilst white sits towards the centre.

Wavelengths of any two colours can be mixed to create the perception of any of the colours

lying on a line connecting the two in the colour space. In the terminology of colorimetry x,

y and z only depend upon the hue, which is the closeness of a colour to one of the perceived

Chapter 2 Theory


colours (red, yellow, green, blue, purple), and saturation, which describes how much the

colour is mixed with white (i.e. pink is a less saturated version of red), and so is

independent of luminance. To define the amount of colours in the human gamut that a

display with the typical red, green and blue components can reproduce we can work out the

chromaticity coordinates for each component and draw a triangle in the colour space

between them. The amount of colours the display will be able to produce will be contained

within the triangle with colours towards the locus allowing the display to show more

colours and so a more realistic looking display. The triangle shown in figure 2.13 is the

standard colour space formed from the use of the three types of phosphor used in cathode

ray tube televisions as defined by the national television system committee (NTSC).

Fig. 2.13 CIE 1931 colour space where the triangle indicates the standard NTSC gamut and

the circles indicate the positions of QD-OLEDs made by Sun et al. (Taken from [30])

Chapter 2 Theory


References

1. McGuire, J.A., et al. Accounts of Chemical Research, 2008. 41(12): p. 1810-1819. 2. Efros, A.L. and A.L. Efros. Soviet Physics. Semiconductors, 1982. 16(7): p. 772-

775. 3. Brus, L.E. Journal of Chemical Physics, 1983. 79(11): p. 5566-5571. 4. Klimov, V.I., ed. Semiconductor and Metal Nanocrystals: Synthesis and Electronic


5. Ekimov, A.I., et al. Journal of the Optical Society of America. B, Optical physics, 1993. 10: p. 100-107.

6. Nirmal, M., C.B. Murray, and M.G. Bawendi. Physical Review B, 1994. 50(4): p. 2293.

7. Nirmal, M., et al. Physical Review Letters, 1995. 75(20): p. 3728. 8. Efros, A.L. Physical Review B, 1992. 46(12): p. 7448. 9. Norris, D.J., et al. Physical Review B, 1996. 53(24): p. 16347. 10. Nozik, A.J. Annual Review of Physical Chemistry, 2001. 52: p. 193-231. 11. Nozik, A.J. Chemical Physics Letters, 2008. 457(1-3): p. 3-11. 12. Hanna, M.C. and A.J. Nozik. Journal of Applied Physics, 2006. 100(7): p. 074510-8. 13. M.C. Beard and R.J. Ellingson. Laser & Photonics Review, 2008. 2(5): p. 377-399. 14. Wolf, M., et al. Journal of Applied Physics, 1998. 83(8): p. 4213-4221. 15. Harbold, J.M., et al. Physical Review B, 2005. 72(19): p. 195312. 16. Prezhdo, O.V. Chemical Physics Letters, 2008. 460(1-3): p. 1-9. 17. Ellingson, R., et al. Nano letters, 2005. 5(5): p. 865-871. 18. Schaller, R.D., M.A. Petruska, and V.I. Klimov. Applied Physics Letters, 2005.

87(25): p. 253102-3. 19. Pijpers, J.J.H., et al. The Journal of Physical Chemistry C, 2008. 112(12): p. 4783-

4784. 20. Schaller, R.D. and V.I. Klimov. Physical Review Letters, 2004. 92(18): p. 186601-4. 21. Klimov, V.I., et al. Science, 2000. 287(5455): p. 1011-1013. 22. Beard, M.C., et al. Nano Letters, 2007. 7(8): p. 2506-2512. 23. Anikeeva, P.O., et al. Nano Letters, 2009. 9(7): p. 2532-2536. 24. Anikeeva, P.O., et al. Physical Review B (Condensed Matter and Materials

Physics), 2008. 78(8): p. 085434-8. 25. Aaron R. Clapp, I.L.M.H.M. ChemPhysChem, 2006. 7(1): p. 47-57. 26. Anikeeva, P.O., et al. Chemical Physics Letters, 2006. 424(1-3): p. 120-125. 27. Clapp, A.R., et al. Journal of the American Chemical Society, 2005. 127(4): p.

1242-1250. 28. Thompson, A. and B.N. Taylor. NIST special publication 2008. SP811. 29. McCluney, W.R., Introduction to Radiometry and Photometry. 1994: Artech house

Inc. 30. Sun, Q., et al. Nature Photonics, 2007. 1(12): p. 717-722.

Chapter 3 Spectroscopic methods


Chapter 3: Spectroscopic methods

3 Introduction to optical spectroscopy

Spectroscopy deals with the production and investigation of spectra. Specifically

optical spectroscopy is concerned with the study of the optical processes involved in

absorption and emission of light by matter. Optical spectroscopy has the advantage of being

non-destructive and not requiring physical contact with the objects under investigation.

Originally it was concerned with the wavelength dependence of the absorption and

emission of light by matter but with advancing technology new techniques have emerged in

spectroscopy. The massive advances in laser technology for example, with their narrow

bandwidth allowed improved spectral sensitivity and ultra short pulsed lasers open up ultra

fast time resolution down to the attosecond regime. This allows the study of processes that

could not previously be resolved such as in chemical reactions or charge dynamics.

Advances in detector design have also had a significant impact on modern spectroscopy

allowing the measurement of small signals. Many different types of photo-detector are now

available and are chosen for the best sensitivity, efficiency, time resolution or spectral

range as required from the experiment. Computer power has also increased dramatically in

the modern age, trivialising instrument control and data acquisition, allowing a huge

increase in throughput for experiments. This chapter will explain the different experimental

methods used firstly to characterise quantum dots and will then move on to discuss the

building up of an ultra fast transient absorption experiment that was used to investigate

MEG.

3.1 Continuous wave techniques

3.1.1 Steady state absorption spectroscopy

The measurement of the amount of light that is absorbed by a material as a function

of wavelength has become a very common method for characterising materials. It is used in

many scientific disciplines from physics to microbiology and can be used, for example, to

distinguish different compounds from each other or to identify materials. In this case we are

mainly concerned with what is sometimes referred to as ultraviolet-visible spectroscopy

(UV-vis) which concerns the spectroscopy of photons in the visible part of the spectrum as

well as in the ultraviolet and near infra-red (NIR). It is possible to consider a two-level



system as shown in figure 3.1 below and use this to represent the simplest case for energy

levels in atoms and molecules. The energy levels in atoms, molecules and indeed quantum

dots can only take certain quantized values. By observing the frequency of light that a

material absorbs the structure of the electronic energy levels can be found. In addition, it is

the selection rules and transition oscillator strengths which determine whether a transition

occurs between two electronic energy levels with appreciable probability. The selection

rules depend upon the symmetry of the states in the quantum mechanical system and the

probability of a transition from one state to another is given by the square of the dipole

matrix element [1].

E1

E2

Ephoton = hv∆E = E2 – E1

E1

E2

Ephoton = hv∆E = E2 – E1

Fig. 3.1 Simple representation of two level system in an atom or molecule having 2 states

with energies E1 and E2 and a difference in energy of ∆E between them.

If light is incident upon an absorbing medium then the amount of that light which is

absorbed will depend upon the density of absorbing centres, the efficiency of those

absorbing centres and the amount of medium the light travels through. The Beer-Lambert

law is used to relate the experimentally measured absorbance to the characteristics of the

absorbing species. The intensity of light entering the absorbing medium, I0, will be more

than the light exiting the absorbing medium, I, as some proportion of the photons will have

been absorbed. The ratio of the intensity before and after the medium will yield the

transmittance of the sample which is the relative amount of light that passes through the

sample. As an analytical technique absorption spectroscopy is useful in a great many areas

of science and engineering and we find that the transmittance is often written using one of

the following representations [2];



)()'(

0

0

1010

)exp()exp(

lcl

I

IT

lnlI

IT

⋅⋅−− ===

⋅⋅−=−==

εα

σα

(3.1)

In the above α is known as the absorption coefficient and has the units of inverse length

(cm-1), however, it is converted to different parameters depending upon the type of medium.

In physics when describing absorption at the molecular level it is common to use the

absorption cross-section, σ, measured in cm2. σ does not depend upon concentration and

represents the probability of the absorption process. In this case n is the number density of

absorbing centres such as dye molecules or quantum dots. Alternatively in chemistry it is

common to define a molar absorption coefficient, ε, with units of molar concentration per

centimetre, M-1cm-1, known as the extinction coefficient or molar absorptivity. In this case

c represents the concentration of absorbing molecules in the medium. The use of a base e or

base 10 is simply a case of convention and a constant factor can be used to convert between

them. Similarly the difference between α and α’ in equation 3.1 is simply a constant factor

due to the use of different bases. The absorbance or optical density of a material, A, is

defined in terms of the transmittance of a sample as the negative logarithm (natural or base

10) of the transmittance

( ) )log(log 0 TIIA −=−= (3.2)

which gives us the following set of equations for absorbance [2];

lcA

lnA

⋅⋅=⋅⋅=

εσ

(3.3)

where absorbance is a dimensionless parameter. It is the wavelength dependence of these

parameters which is of interest in absorption spectroscopy.

Measuring the absorption of materials is a very common procedure. In modern

research many systems designed specifically for this purpose are commercially available

and are known as absorption spectrometers or spectrophotometers. Practically, such an

instrument passes a beam of light through a sample; the intensity of this light must be

measured before and after it has passed through the sample. In this way the transmittance or

the absorbance can be calculated and this is done for a range of wavelengths in order to

build up a spectrum. Such an instrument requires the ability to expose the sample to

monochromatic light that can be tuned over a wide range of wavelengths, a detector that



can measure the intensities of the light over the same range of wavelengths, and an optics

scheme for routing the light to the sample from the light source and then to the detector.

There are in general three different set-ups for spectrophotometers with the first being the

most simple known as a single channel scheme. In this set-up (figure 3.2a) the

monochromatic light is sent through the sample and is incident upon a single detector

which measures the intensity as the wavelength is scanned. It involves taking two separate

scans per sample, the first is done with no sample in place to take the base line

measurement and the second is done with the sample in place and the absorbance calculated

as in equation 3.2. The act of measuring the baseline and the sample spectra one after the

other will introduce inaccuracy due to any instability in the system. Ideally both of these

spectra would be measured at the same time, which naturally leads to the two channel

spectrophotometer scheme (figure 3.2b). In this type of spectrophotometer the

monochromatic light from the light source is split and half is passed through a reference

sample and half is passed through a sample channel. The transmittance can then be

calculated immediately, however, differences between the split beams will be introduced as

splitting it 50:50 at all wavelengths is practically very hard as many mirrors and optical

components have some spectral response. It is therefore necessary to take a response

spectrum prior to the measurements with no sample in place to correct for any difference

between the two channels.

Light source Monochromator

Sample

Detector


Sample

Detector 1Reference

Detector 2

M1

a)

b)


Sample

DetectorLight source Monochromator

Sample

Detector


Sample

Detector 1Reference

Detector 2

M1


Sample

Detector 1Reference

Detector 2

M1

a)

b)

Fig. 3.2 Diagram showing principle behind a) single channel and b) two channel

spectrophotometers



One of the main advantages of this method lies in it being insensitive to changes

due to variations in the light source output. Any change will not affect the result as both

sample and reference beams will change by the same amount and so their ratio will remain

unchanged. In the two channel scheme there is also a further advantage when extracting the

absorbance of a material that is dissolved in some solvent. The solvent will have its own

absorption spectrum as will the cuvette used to hold the sample. By making up a reference

cuvette containing only the solvent and a sample cuvette containing the sample dissolved in

the solvent and placing them in the reference and sample channels respectively we take the

ratio between the two (corrected for the base line spectrum) yielding the absorption

spectrum of the sample alone [2]. Although this same measurement can be done in a single

channel spectrophotometer, for a series of samples in different solvents it would be required

to measure the base line for each new solvent used, whereas in the two channel scheme it

will only need to be taken once. The third possible scheme involves the use of an array

detector such as a CCD and does not require the light source to be scanned in wavelength.

The advantage of this approach is that the “white” light is passed through the sample and

the resulting spectrum is incident upon the array using some grating or monochromator

system which will then take a snapshot of the entire spectrum in one go. Whilst arrays

allow spectra to be taken rapidly compared to the step by step wavelength scanning the

spectrum resolution is limited by the dispersion of the monochromator and the distance

between the photosensitive elements of the detector.

The two absorption spectrometers used during this work are both double beam

spectrophotometers. The first is a Perkin Elmer Lambda 1050 UV/Vis/IR spectrometer

which has a spectral range from 175 to 3300 nm. It incorporates an all reflecting optical

system with double holographic grating monochromators for UV/Vis and NIR. The very

large spectral range is achieved using two pre-aligned sources, one of which is a tungsten-

halogen lamp for the visible and near-infrared combined with a deuterium lamp used for the

ultraviolet range. In order to detect over this range it uses a three detector accessory

comprised of a photomultiplier tube (PMT) used for the ultraviolet/visible range from 175 –

860 nm (the system requires purging below 190 nm due to oxygen absorption), a Peltier

cooled wideband InGaAs detector covering the 860 – 2500 nm range (water vapour absorbs

between 1350 – 1450 nm, 1850 – 1950 nm and 2520 – 3000 nm so purging is necessary for

these ranges) and a Peltier cooled PbS detector which covers the 2500 – 3300 nm range.



The second absorption spectrometer used was a Varian Cary 1E UV/Vis double

beam spectrophotometer that also contains both tungsten-halogen and deuterium lamps for

the visible and ultraviolet ranges respectively. The design of this system is the same in

principle to the Lambda 1050 spectrophotometer in that light from these sources is directed

through an entrance slit into a monochromator. This light is split and passed through the

sample and reference and directed onto a photomultiplier tube as a detector. This

combination of light sources, monochromator and PMT detector gives this system a

wavelength range of 190 – 900 nm.

3.1.2 Steady state fluorescence spectroscopy

Emission spectroscopy is, in many ways, a complementary technique to absorption

spectroscopy and vice versa. Whilst both give important information in their own right,

together they can give a deeper understanding of the sample under study when used

together as they will each reveal features that may be hidden in the other. Emission

spectroscopy represents the energy spectra of the electronic transitions from an excited state

to the ground state, whereas absorption spectroscopy observes the transitions from the

ground state to some excited state. Photoluminescence (PL) spectroscopy can give a wealth

of information including band gap parameters, exciton binding energies, PL efficiencies,

layer thicknesses, impurity levels, and constituents of compounds. If we return to the 2

level system shown in figure 3.1 we can see that upon absorption of light (or indeed any

way of exciting the sample) an electron in the ground state is excited and undergoes a

transition to the excited state (absorption). Once in this excited state it can release the

energy it has gained through spontaneous emission of a photon and so return to the ground

state. Practically the emission of light from a substance is known as luminescence. This is

split into two different types known as fluorescence and phosphorescence. The difference

between these two types of luminescence lies in the total spin of the states involved in the

transitions. Fluorescence involves singlet states which have total spin quantum number 0,

and as the ground state is often a singlet state the transition from the excited state to the

ground state is said to be spin allowed. Physically this corresponds to the electron in the

excited state being paired with an electron in the ground state that has opposite spin. If the

excited state has a total spin of 1 then it is said to be a triplet state and can be formed

through intersystem crossing. As spin must be conserved in electronic transitions it is



forbidden for the electron to undergo a triplet to singlet state transition. Physically this

means the excited state electron has the same spin orientation as the ground state, and so

emission of a photon, known in this case as phosphorescence takes place over much longer

lifetimes (seconds) when compared to fluorescence (nanoseconds) [3].

The optical design of an emission spectrometer, also known as a fluorimeter or

spectrofluorometer, is fundamentally quite similar to a spectrophotometer as an initial

optical absorption process is used to excite the sample. Obviously it is the amount of light

absorbed by the sample which is measured in absorption spectroscopy whereas in emission

spectroscopy it is the light emitted by the sample across its spectrum which is measured.

The general set-up usually involves a lamp and a monochromator which combined make up

the excitation source. This light is directed onto the sample which is excited and emits light

in all directions. Fluorimeters are designed so as to collect as much of this emission as

possible usually using mirrors to minimise chromatic aberration. The collected light is

directed through slits, a monochromator, and onto a detector. One of the fundamental

differences when taking emission spectra compared to absorption spectra is that in general

very low light levels need to be detected and so for this reason photomultiplier tubes are

often implemented. Array detectors can be used for faster acquisition times but at the

expense of sensitivity. In this work a Horiba Jobin Yvon Fluorolog-3 model FL3-22iHR

spectrofluorometer (figure 3.3) is used to take photoluminescence spectra of various

quantum dots in solution. It comprises double grating monochromators on both the

excitation and emission arms for increased stray light rejection as well as giving a boost to

sensitivity meaning that the slits can be opened twice as wide without reducing the

resolution. As the excitation source, a 450 W xenon arc lamp is used as it has a spectrum

which closely mimics that of daylight. This lamp gives significant output through the UV,

visible and even near infra-red wavelengths. For detection it uses a Hamamatsu R928P red

sensitive photomultiplier tube that has a wavelength range of 185 – 900 nm with a peak

sensitivity of 400 nm. It also has the option of being cooled by liquid nitrogen which will

have the effect of giving fewer dark counts, useful in time-resolved photoluminescence but

not implemented here.



Fig. 3.3 Schematic of the Fluorolog-3 spectrofluorimeter.(taken from [4]).

3.2 Fluorescence lifetime measurements

Time-resolved emission measurements are important for characterising materials as

they provide information on the dynamics of charge carriers in an excited system. The

lifetimes measured can be related to how carriers relax to the emitting state as well as the

radiative and non-radiative processes that occur when the electrons and holes recombine.

One of the most widely-used techniques for determining the photoluminescence decay time

is known as Time Correlated Single Photon Counting (TCSPC). It is significantly more

accurate and gives superior sensitivity and dynamic range than any other method used to

obtain fluorescent lifetimes. This has allowed it to be used in situations where very low

signal levels are involved, even to the extreme of single molecules. Despite being in use for

over two decades, TCSPC remains a technique used in cutting edge research. The

incorporation of picosecond and femtosecond lasers alongside fast multichannel plate

detectors (MCP) and constant fraction amplifiers and discriminators (CFDs) allow the

measurement of lifetimes down to a few picoseconds.



3.2.1 Overview of time-correlated single photon counting

TCSPC is a digital counting technique which relies upon the detection of single

photons which are time correlated to a reference signal produced by an excitation light

pulse. The sample is typically excited by a rapidly pulsing light source and the time taken

from the excitation pulse to the detection of the first fluorescence photon is recorded by

adding a count to a histogram where the x-axis channels correspond to time. This histogram

will represent the probability distribution for the detection of a photon. The probability of

detecting a photon at a particular time is proportional to the fluorescence intensity (also

proportional to the population of the excited state), and so our measurement yields the

decay of the PL intensity versus time. The electronics involved act as a very fast timing

mechanism which is started by the start signal pulse from the excitation source and stopped

by an electrical pulse generated by the PMT when it detects a photon. This general scheme

is shown in figure 3.4 for one of many events needed to build up the optical waveform. The

reference signal from the detector has a frequency dependant upon the repetition rate of the

laser whereas the photon pulses from the detector are randomly distributed. As TCSPC is a

statistical method a high repetition rate (>kHz) light source is required to quickly obtain a

sufficient number of time measurements for a high statistical precision and to avoid long

accumulation times.

Fig 3.4 Schematic of TCSPC technique. (Taken from [5]).

TCSPC can be operated in two different modes known as forward and reverse mode

to make optimum use of the experimental resources (figure 3.5). Forward mode is as shown

in figure 3.4 where the excitation pulse from the light source is connected to the start input

and the signal from the random pulses from the detector is connected to the stop input. In



TCSPC in the case of high repetition rate but low level signals for each excitation pulse the

probability of detecting a photon in each signal period is less than one. As a result there are

many signal periods in which a photon is not detected and some where one photon is

detected. For periods where a photon is not detected the TCSPC electronics is started but

not stopped and so the electronics must reset every so often for the next signal period. This

causes a problem for high repetition rate lasers (e.g. Ti:Sapphire at 50 to 100 MHz) where

the vast majority of the time the electronics will be started by the high start rate of the laser

but not stopped by the much lower photon detection rate. This can be avoided by operating

the set-up in reverse start-stop mode whereby the photon detection starts the timing and is

stopped by the next reference pulse from the excitation light source. In this way the

electronic timing works at the much lower rate of the photon detection events. A shifting

delay is required so that the light pulses do not arrive at the stop input before the start

pulses from the detector. The time axis is reversed in this case so that photon events with

short retardation times are still shown on the left of the time axis on the histogram.

Figure 3.5 Diagram showing forward and reverse mode with effect of delay shown. (taken

from [5]).



3.2.2 TCSPC electronics

The accurate measurement of the time at which a photon hits the detector is crucial

to TCSPC and is achieved using an electronic system; a schematic representation of this

system is shown in figure 3.6. In order to record the time between the excitation light

source and the detection of a single photon the electronics must trigger with a high degree

of accuracy on the input pulses from the reference light source pulses and the detector

pulses. This is complicated by the fact that neither of these inputs is stable; the reference

pulse amplitude can change between pulses which may have slightly different intensities,

and due to the random amplification process used in single photon counters the detector

pulse has a significant amplitude jitter [6]. As a result triggering using the leading edge of

these pulses introduces a timing jitter since trigger times depend upon the peak height. In

order for the trigger times to be independent of the amplitude of the input pulse, constant

fraction discriminators (CFDs) are used on both the reference and detector inputs. CFDs

trigger at a constant fraction of the pulse amplitude. In practice this is achieved by splitting

the incoming signal and delaying one of the components with respect to the other. These

are then be fed into the inputs of a comparator which triggers when the difference between

these signals crosses the baseline.

Figure 3.6 Electronics used in TCSPC: constant fraction discriminator (CFD), electrical

delays (Del), time-to-amplitude converter (TAC), amplifier (Amp), analogue-to-digital

converter (ADC) and digital memory (Mem). (Taken from [5]).

In practice each CFD will have an adjustable threshold which can be used to reduce

the impact of thermal dynode pulses and the electronic background, and sets the minimum

amplitude of the input pulses that will trigger the CFD. Clearly of these three components it

is only the regular photon pulses that we wish to record and so the threshold must be set so

as to reject the unwanted pulses. There is an optimum CFD threshold that will cut off the



low amplitude electronic noise and the thermal dynode pulses but will allow the majority of

pulses caused by photon events. The reality is that these three components are not always

distinct and the optimum valley between the thermal dynode and photon pulses may be

hard to define meaning that a compromise must be reached between the instrument

response function (IRF) and the counting efficiency (figure 3.7). By observing the

dependence of the count rate on the discriminator threshold it is possible to find this

optimum CFD threshold. At very low threshold the count rate is large as both the electronic

noise and thermal dynode pulses are detected. Ideally there will be a point where the count

rate flattens or causes a plateau and this point will represent the optimum threshold.

Increasing the threshold further will reject more pulses and reduce the efficiency but may

give lower jitter by eliminating more background noise. The reference CFD has a much

smaller impact on time resolution than the detector CFD as the laser excitation source will

have much smaller amplitude jitter[6]. In fact, it is often not used at all in systems utilising

very stable and well-known excitation sources and so the threshold will have little influence

on the shape of the IRF.

Figure 3.7 Graphs showing the distribution of PMT pulse amplitudes (left) and the count

rate plotted against the CFD threshold (right). (Taken from [6]).

Nominally, the CFD triggers when the difference between the input pulse and a

delayed copy of itself crosses through zero volts. In this situation, spurious signals can

cause some jitter to be introduced by the zero crossing (ZC) discriminator. This can result

in ripples in the recorded curves or a double structure in the IRF. This jitter can be reduced,

however, by adding a small offset (a few mV) to the ZC level so that it is slightly different



from zero volts. Older TCSPC systems were subject to a large influence on the shape of the

IRF from the ZC level; advances in technology mean that in newer systems it has a smaller

impact as the slope of the base line transition is much steeper due to faster responding

detectors and faster CFDs.

The triggering pulses from the two CFDs are then sent to a time-to-amplitude

converter (TAC) which combined with the analogue-to-digital converter makes up the

timing mechanism. A start pulse sent to the TAC switches a current source on, which then

charges a capacitor. The stop pulse switches off this current and the voltage across the

capacitor represents the time between the start and stop pulses. The TAC output pulse is

amplified, stretching the time axis and this analogue signal is further processed by the

analogue-to-digital converter (ADC). The ADC samples the TAC voltage and the output is

a digital representation of the time at which the photon was detected. This means that

measuring voltages in certain ranges (e.g. 0 ≤ V1 < ∆V) add a count to time bins

corresponding to the photon arrival time. By adding incrementally to these bins the photon

distribution over time is built up. The resolution of the ADC dictates the number of discrete

time values possible and the time resolution is equal to the time range being observed over

the resolution of the ADC in channels.

3.2.3 The Instrument Response Function (IRF)

An instrument response function is a time scan measurement which characterises

the time resolution of a TCSPC set-up; it can be used as a measure of the quality of the

instrument and to correct data for the effect of the instrument. The IRF shape is governed

by the width of the exciting light pulses, the transit time spread (TTS) or response of the

detector, the jitter introduced by the TCSPC electronics and any pulse dispersion in the

optical system. Typically it is the detector and light source (electronic jitter is typically < 25

ps) which limit the temporal resolution rather than the TCSPC method itself. Any lifetime

measurements taken are a convolution of the fluorescence decay and the IRF. This has a

large impact on the early times in longer decays and samples with lifetimes approaching the

IRF width. A common approach to this is to take the IRF for any lifetime measurements

and to use a deconvolution procedure to extract the actual lifetime data. This improves the

accuracy of fits at short times in decays. After the IRF has decayed, the data is due only to

sample decay. An IRF is typically taken using a scattering sample to direct some of the



excitation light into the path of the detector. This is often achieved using a cuvette with

identical geometry to the sample filled with dilute milk. When using the minitau

fluorescence lifetimes spectrometer (see section 3.2.5), for example, the maximum pulse

width of the diode laser at 10 MHz is ~ 90 ps [7], the detector pulse width is ~200 ps [8],

and using a maximum value for the timing jitter of the electronics as 25 ps, a sum of

squares gives a value of 220 ps for the IRF. The measured IRF may be different to this

estimate which does not take into account the dispersive effects of the optics as the

horizontal path length of the laser in a 10 mm cuvette can add 45 ps of transit time spread

to the IRF alone [6].

3.2.4 Analysing lifetime data

The mathematical expression that describes an exponential growth or decay process

used to fit the lifetime data is;

)/exp()( τtBAtf i −∑+= (3.4)

where A is the background, Bi are the pre exponential factors, and τi are the characteristic

lifetimes.

Commonly, two different procedures can be used in fitting to lifetime data which

are known as ‘tail fit’ analysis and ‘deconvolution fit’ analysis. In a tail fit analysis the

statistical noise and the sample excitation process are not included in the fit. Thus this

routine only fits to a region where no further sample signal generation occurs, that is, in a

region where the exciting light pulse has disappeared. This can be used to analyse long

decay times as it only fits to lifetimes that dominate in the longer time region. In reality the

exciting light pulse is not infinitely short and so the sample has a finite rising edge due to

the exciting light pulse. This initial rise and decay may contain important information of

short lifetimes and so it is important to include the IRF as this will determine the initial part

of the sample response. A deconvolution fit makes use of the convolution, F(t), of the IRF,

r(t), and the sample decay model, f(t):

∫ −= ττ dtftrtF )()()( . (3.5)

As mentioned previously, this requires the IRF to be separately measured which

allows the fit to include the rising edge of the data, and so eliminate the effects due to noise

and the exciting light pulse. In both the tail fit and deconvolution analyses, the best fit



values of Bi and τi are found by minimising the “goodness of fit” parameter 2gχ . The

goodness of fit is often scaled by the number of free parameters so that different time scan

fits can be compared and is known as the “reduced chi squared” given by;

∑−

=k

kkk n

SFw

222 ][χ (3.6)

where k is the index for each data point to be fitted, and S represents the raw data and wk is

weighting factor that is dependent upon the type of noise introduced from the data

collection method. In photon counting Possion noise statistics determine the standard

deviation of each data point as its square root kk Sw 1= which is used as the weighting

factor in the above. For data of this type a reduced chi squared value of 1 indicates an ideal

fit [5]. Other indications of a good fit use the difference between the fitted curve and the

raw data to plot what are known as the residuals which should be randomly distributed

around zero.

3.2.5 TCSPC systems

3.2.5.1 Mini-tau based systems

One of the TCSPC systems used in this thesis was based on modified commercially-

bought system. This was an Edinburgh Instruments Ltd. fluorescence lifetime spectrometer

which was made up of a sample chamber (figure 3.8), a Hamamatsu H7422

thermoelectrically cooled photomultiplier tube (PMT) and a plug-in computer card, a

TCC900, which contains all the electronics needed for the TCSPC technique.



Figure 3.8 Mini-tau fluorescence lifetime spectrometer. (Taken from [7]).

A picosecond pulsed diode laser emitting at 405 nm was also purchased from Edinburgh

instruments and had a pulse width of ~ 90 ps with pulse repetition rates between 20 MHz

and 20 KHz available. The spectral range was limited to samples which could be excited by

the 405 nm laser and to emission that can be detected in the range of the PMT, 300 – 870

nm. A variable attenuator allows for control of the laser power and a filter wheel containing

band pass filters enables different wavelength emission to be separated out.

A limitation of this set-up was that using only band pass filters it was not possible to

monitor the fluorescence lifetime at specific wavelengths, and a limited choice of 45 nm

band pass filters meant it was not possible to effectively select all the emission required. An

Acton research SpectraPro-500i 0.5 metre focal length triple grating imaging

monochromator was therefore incorporated into the set-up. This involved creating a light

tight plate for mounting the PMT onto the output slit of the monochromator and detaching

the laser from the sample chamber. A cuvette holder and imaging optics were set-up as in

figure 3.9 for front face excitation by the laser, 30 cm and 10 cm planar convex lenses with

2” diameters were used to couple as much light into the spectrometer as possible and to



match the monochromator f-number as closely as possible. This set-up allows us to monitor

the fluorescent lifetimes of quantum dots with emission across the range of the PMT.

Monochromator

PMT

Laser

Cuvette holder

F = 30 cm

F = 10 cm

Monochromator

PMT

Laser

Cuvette holder

F = 30 cm

F = 10 cm

Figure 3.9 Schematic of modified TCSPC set-up

3.2.5.2 Femtosecond laser and microchannel plate TCSPC

An alternative TCSPC set-up which used the North West Science Fund (NWSF)

laser (see section 3.3.3) and a microchannel plate (MCP) PMT was also used; the narrow

pulse width of the laser (~ 100 fs) and the small transit time spread (TTS) of MCPs allow

faster lifetimes to be observed. In this way TCSPC can be used to observe the fast Auger

recombination of biexcitons that represents the signature of MEG. This set-up was built up

at Daresbury laboratory along with the postdoctural researchers Dr. Darren Graham and Dr.

Samantha Hardman. The MCP PMT used here is a Hamamatsu R3809U-50 which has a

TTS of ≤ 25 ps and a spectral range of 180 – 850 nm. MCP detectors have many

microscopic channels with a conductive coating. When a high voltage is applied along the

channels, the walls of the channel act as secondary emission targets. These detectors allow

the photocathode to be placed very close to the initial microchannel plate resulting in

reduced transit time spread of the photoelectrons. The NWSF laser used as the excitation

source is set on two optical tables stacked on top of each other. In this experiment, we

utilised the top deck. A Spectra-Physics Millennia Pro s-series diode pumped, CW visible

laser gives an output of 12 W of 532 nm light. This is used to pump a Spectra-Physics

Tsunami mode-locked Ti:Sapphire laser, which can output several watts of pulsed laser



light at around an 80 MHz repetition rate and can be wavelength tuned from 720 – 850 nm

for the mirror set used here (the Tsunami can be tuned over larger ranges by changing the

mirror set). As the fluorescent lifetimes being measured here are typically 10s to 100s of

nanoseconds long the 12.5 ns between pulses will not allow the sample to completely relax

between photoexcitation events. An APE PulseSelect dual is therefore incorporated as a

pulse picker to allow variation of the Tsunami pulse repetition rate. This makes use of an

acousto-optic modulator (AOM) system whereby a fused silica crystal is cemented onto a

piezoelectric transducer. A radio frequency (RF) signal sent to the piezoelectric transducer

causes acoustic waves to be generated in the fused silica which reflect off the top surface

and interfere to cause a standing wave pattern of varying density (and therefore refractive

index) in the crystal. This acts as a diffraction grating to incoming light pulses which are

diffracted out of the main beam path; by pulsing the RF signal at a particular frequency

certain pulses are ejected to leave pulses with a lower repetition rate. Dual stages can be

used to increase the overall contrast ratio but at the expense of diffraction efficiency. The

output of the laser in this case was 81.19 MHz which is used as a seed signal for the pulse

picker that can alter the repetition rate by a user-defined division ratio. The division ratio is

set to 203 to achieve a laser repetition rate of ~ 400 kHz chosen so as to maintain a pulse

period that is over 5 multiples of the full decay, as is the convention for TCSPC. The output

from the pulse picker is then sent to a Spectra-Physics GWU-FHG flexible harmonic

generator which is used to generate the second and third harmonics required for the pump

pulse. The GWU-FHG contains a lithium triborate (LBO) crystal for second harmonic

generation and a Beta Barium Borate (BBO) crystal for third harmonic generation. Second

harmonic generation of the 800 nm light from the Tsunami is achieved when the electric

field of the laser pulse causes a non-linear response in the induced electrical polarization of

the LBO crystal. The result is an output of light pulses with the same frequency as the

incoming light and twice this frequency (called the second harmonic) corresponding in this

case to 400 nm. To generate the 240 nm light required to pump above the threshold

required for MEG the Tsunami output is tuned to 720 nm which is sum frequency mixed in

the BBO crystal with its second harmonic. Lenses couple the fluorescence into the

monochromator by closely matching its f-number as before. Neutral density filters are used

to reduce the laser power and colour glass filters are used to block residual scattered laser

light.



Monochromator

MCP PMT

Pump laser

532 nm

Ti:Sapphireoscillator

Pulse picker

Flexible harmonic generator

Delay & gate generator

CFD – set to leading edge

Coax delay box

TAC

startstop

Amp CFDCount rate on mini-tau

Colour glass and ND filters

outputPC with MCA card

SampleMonochromator

MCP PMT

Pump laser

532 nm


Pulse picker




Coax delay box

TAC

startstop

Amp CFDCount rate on mini-tau



Monochromator

MCP PMT

Pump laser

532 nm


Pulse picker




Coax delay box

TAC

startstop

Amp CFDCFDCount rate on mini-tau



Sample

Figure 3.10 Experimental set-up for the femtosecond TCSPC experiment.

In this set-up modular electronics are implemented on both the start and stop signal

lines. For the start pulse the trigger is taken from the pulse picker as this determines the

repetition rate of the laser pulses. The output from the pulse picker is not a simple pulse and

so this signal is first sent to an Ortec DAGG3 delay and gate generator. This performs two

functions; first the leading edge of the input signal can trigger a user-defined delay allowing

us to move our pulses around in the time window, and secondly after this delay an output

pulse is generated with defined characteristics. Here a square wave is used as the trigger

pulse and is sent to an Ortec 584 CFD, however, as we know all our pulses will be almost

identical square waves, the CFD is set to trigger on the leading edge and is not used as a

CFD. The coaxial cable delay box is used to give us a greater range of delay than is

available from the delay and gate generator, and essentially consists of a box filled with

coaxial cables of varying lengths which can be switched in or out to add a time delay to the

signal. For example, a metre length of the commonly available 50 Ω cables will add a delay

of roughly 5 ns. The stop pulse from the MCP is first amplified by a SR445 Stanford

Research Systems preamplifier to the range of the CFD which is an Ortec model 583

constant fraction differential discriminator. This is operated as a CFD and the timing

outputs are fed to both a counter on the mini-tau card to observe the stop rate and to the

stop input of an Ortec 567 TAC. The voltage from the TAC is then sent to a computer with



an Ortec Maestro 32 multichannel analyser (MCA) emulator card. This card operates using

a set number of channels over which the time scale of the TAC (voltage range) is

distributed. The voltages passed to the MCA from the TAC put one count into a channel

corresponding to a time bin that depends upon voltage. The familiar TCSPC histogram is

built up in this way by passing a train of voltages from the TAC to the MCA.

3.3 Femtosecond transient absorption

Transient absorption (TA) is both a powerful and widely-used technique used to

monitor MEG with many advantages over TCSPC. Although TCSPC is exceptionally

sensitive to weak light sources, the temporal resolution in TA is limited only by the width

of the laser pulse, and with modern ultra-fast laser systems offering sub 100 fs pulses the

time resolution obtainable is unrivalled. By combining this with lock-in or gated box car

integrators it is possible to detect very small signals as well as tiny modulations in these

signals. Femtosecond transient absorption is a pump-probe technique whereby a sample is

excited or pumped by a femtosecond laser pulse and the transmission of a probe pulse is

monitored as a function of the time delay between the two pulses. The recording of the

differential transmission of the probe pulse is enabled through the use of a synchronous

chopper in the pump beam. This is locked into half the repetition rate of the laser. In this

way the amplitude of the probe when the pump is present and absent is measured, giving

∆T, which is related to the change in absorption. By using a delay stage to control the

relative delay between the pump and probe the change in absorption can be plotted as a

function of time.

3.3.1 Optical scheme

The general optical set-up for the TA is shown in figure 3.11. Simple TA set-ups

utilising single wavelengths (degenerate TA) split before the sample are possible but in this

scheme we have a tuneable pump pulse to allow us to pump both above and below the

threshold for MEG for a range of dots with different band gaps. In order to also probe at the

band gap for a range of nanocrystals we use the high pulse energies from the amplifier to

allow us to create a white light continuum (WLC) in a 2 mm sapphire plate. The desired

wavelength of the WLC that is to be monitored can then be selected using the

monochromator. Approximately 5 % of the 800 nm, 1 kHz repetition rate output from a



regenerative amplifier is used to generate the WLC as shown in figure 3.11, however it is

first passed through a variable attenuation reflective ND filter, used to control the average

power incident upon the sapphire crystal.

800 nm from amplifier

Pump pulse from OPA

Variable ND filter

Delay stage with retro reflector

Sapphire plate

D-shaped mirror

Lens

800 nm from amplifier

Pump pulse from OPA

Variable ND filter

Delay stage with retro reflector

Sapphire plate

D-shaped mirror

Lens

Figure 3.11 Optical set-up of the femtosecond transient absorption experiment. The blue

beam represents the pump pulse whilst the red indicates the 800 nm pulse produced by the

amplifier and the yellow indicates the white light continuum. The red box surrounds the

system for WLC generation.

The 800 nm light is then sent along a Newport ILS series 200 mm delay stage via a broad

band hollow retroreflector; this greatly simplifies delay stage alignment as it results in an

output beam parallel to the incident beam independent of the angle of incidence. In the TA

scheme used here the probe is delayed with respect to the pump (only the relative delay

between the two is important) with a time delay equal to the twice the length of the step (as

the beam passes along the delay stage twice) divided by the speed of light. In this way a

step of 0.5 mm will give a total 1 mm path length difference and so time delay steps of 3.33



ps are easily possible. This means the time resolution of the system is only limited by the

pulse width of the laser. The path lengths of both the pump and the probe are well-matched

firstly by measuring out the beams paths (the pump beam travels longer through the OPA)

and then are finely adjusted using the manual delay stage on the pump beam. After passing

through the WLC generator (the red box in figure 3.11) the probe goes through a telescope

system consisting of 500 mm and 250 mm focal length achromatic doublet lenses placed

apart at a distance equal to the sum of their focal lengths. This focuses the probe beam

waist at the sample to ~ 200 µm, which is much smaller than the pump beam and ensures

that the probe is incident upon a uniformly-excited region. This will also reduce the beam

diameter to allow both the sample and reference beams to be sent through the

monochromator onto the detectors. Just after the 500 mm achromatic lens is a UV reflective

ND filter used to split the white light into a sample and reference beam, both of which are

sent through the monochromator onto 2 Vishay Si PIN high speed photodiodes. Only one

detector would be needed in an ideal system but as we are measuring both small signals and

small modulations in those signals (as low as ~ 1 µV changes in a 1 mV signal), any other

fluctuations in the system appear as a change in absorption. The small shot to shot noise of

the laser can be negated by instead observing changes between these sample and reference

beams. A variable reflective neutral density filter wheel was placed in the reference beam

to allow easy balancing of the sample and reference beam before scanning in the presence

of the pump. The reference beam is reflected off a D-shaped mirror into the

monochromator whilst the sample beam passes over the top. Both beams are then sent

through a short pass filter which cuts off the residual 800 nm light from the amplifier which

is orders of magnitude more intense than the WLC as well as any near infra-red

wavelengths produced in the sapphire. They are subsequently directed through a 60 mm

achromatic doublet lens to focus both beams into the monochromator and to match the f-

number as closely as possible. The monochromator is a SpectraPro 2500i 0.5 metre

imaging spectrometer, used to select the wavelength to be monitored, and inverts the

sample and reference beams whilst keeping them distinct. Irises are used firstly to align the

beams but then once aligned serve the purpose of spatial filtering of both scattered light and

sample emission at the monitoring wavelength. This is achieved by setting the diaphragm

of the iris to the size of the probe beam both behind the sample and in front of the lens

collimating the probe light. UV-enhanced aluminium mirrors were used on the pump beam



path for their enhanced reflectivity in the UV range whereas silver mirrors were used in the

probe beam path.

3.3.2 White light continuum generation

The white light continuum generator is shown in figure 3.11 in a red box and

contains a 100 mm focal length lens which is used to focus the beam into the sapphire plate

to increase the power density and therefore surpass the threshold for continuum generation.

The white light is then collected by a second 100 mm lens that acts to re-collimate the light

for delivery into the sample. Despite being a phenomenon that is still not fully understood it

is used remarkably widely in scientific research [2]. White light continuum generation

relies on the use of ultra-short high-power laser pulses where the electric field becomes

comparable to the materials atomic field. When this is the case higher order nonlinear terms

in the polarization expansion become comparable to or exceed the linear term. It is the third

order term in the polarisation expansion which causes the main processes leading to

continuum generation. These have been identified as self-focusing and self-phase

modulation (SPM), although they cannot explain certain characteristics of the WLC which

is the topic of current research [9]. As the non-linear index of refraction depends upon the

intensity of the laser, for a Gaussian beam profile where the central region is more intense

than the outer edges, this region will experience a higher index of refraction than the outer

edges. Thus, the central region will be travelling more slowly due to the higher refractive

index and as such the beam will start to focus.

The threshold power for continuum generation has been shown to be coincident

with the threshold for self-focusing [10] which is thought to be due to the large increase in

intensity as the beam focuses. This threshold depends upon the linear and non linear

refractive indices of the material and the specific wavelength of the laser. Since a sapphire

plate is used here we can calculate this critical power using the equation [10]

20

0

8

77.3

nnPcrit π

λ= , (3.7)

where n0 and n2 are the linear and non-linear index of refraction respectively. The high

intensities lead to a high probability of multi-photon excitation; plasma is formed from the

free electrons, which acts to limit the self-focusing and so the beam becomes collimated

through the rest of the medium.



The wide frequency spectrum that results from the narrow laser pulse is due to an

effect known as self-phase modulation of the light pulse however, other effects combine to

extend the ranges in the red and blue tails of the continuum. The time differential of the

non-linear phase is proportional to the pulse intensity variation in time as a result of the

non-linear refractive index dependence upon intensity. As the induced frequency change is

dependent upon the non-linear refractive index variation in time it is also dependent upon

the pulse intensity variation in time. Therefore the frequency at the front of the pulse is red

shifted (stokes shifted) and is blue shifted (anti-stokes shifted) at the back of the pulse. As

mentioned SPM is influenced by a number of physical processes such as the generation of

free electrons, and is a function of the laser intensity profile; these also act to alter the

refractive index. Combined with the increased intensity caused by the self-focusing, the

spectral deviation is enhanced further.

For the purposes of TA it is important that the WLC is both very stable and

spectrally flat. The WLC was optimised by moving the input 10 cm lens on an xy stage and

varying the power using a variable ND filter placed before the delay stage. In this way a

power that was 110 % of the threshold was found to give the most stable continuum. The

WLC can be optimised for the probe wavelength required, which in this case corresponds

to the quantum dot band gap. Sapphire is well suited for producing a WLC as it has a high

laser induced damage threshold, is optically transparent from the UV to the near IR, and

produces a continuum from ~ 400 nm to 1000 nm. WLC spectra are shown in figure 3.12

optimised for probing at 600 and 900 nm, here ~ 800 µW of the 800 nm output from the

regenerative amplifier is focused into the sapphire.



880 900 920 940 960 980 10000

2

4

6

8

10

Spe

ctra

l int

ensi

ty (

arb.

uni

ts)

Wavelength (nm)

Optimised for 900 nm probe

400 500 600 700 800 9000.01

0.1

1

10

100

Spe

ctra

l int

ensi

ty (

arb

. uni

ts)

Wavelength (nm)

Optimised for 600 nm probe

Figure 3.12 WLC spectra in sapphire optimised for a probe wavelength of 600 nm and 900

nm.

3.3.3 Laser system

In pump-probe experiments it is the laser which determines many of the

characteristics of the TA set-up, and so this is therefore central to transient absorption

experiments. The laser system used here is the aforementioned NWSF laser system; this is

composed of numerous modules only some of which are used here. The full laser system is

set out over two optical tables stacked on top of one another and is shown in figure 3.13. In

this experiment the Tsunami Ti:sapphire oscillator is pumped by the Millennia pump laser,

as described before, to produce mode-locked ~100 fs pulses at 800 nm with a repetition rate

of ~ 81.2 MHz. The output from this is then sent down a periscope to the bottom deck of

the laser system. The Tsunami output is then used as the seed pulse for a Spectra Physics

SpitfirePro Ti:Sapphire regenerative amplifier which is pumped by a Spectra Physics

Empower Q-switched laser that gives an output of 532 nm with an average power of 15 W.

The Spitfire Pro can then output pulses at 1 kHz with a pulse energy of ~ 1 mJ and a pulse

width of ~ 100 fs; a high energy beam splitter is used to split this beam so that 5 % of this

beam is sent to form the probe beam of the TA experiment whilst the other 95 % is used to

pump a tuneable optical parametric amplifier known as a TOPAS-C, which combined with

frequency mixing crystals gives a spectral range from 240 nm to 20,000 nm.



Millennia pump laser

Tsunami, Ti:Sapphireoscillator

Pulse pickerFlexible harmonic generator

Empower pump laser

Spitfire Pro, Ti:Sapphire, Regenerative amplifier system

Top deck

Bottom deck

Periscopes to bottom deck

Mirror turrets

Seed in

Pump inOutput

Black anodised beam tubes

To experiment

λ/2 wave plate

Polariser cube

TOPAS-C, OPA

Frequency mixing crystals

Variable ND filter

Millennia pump laser

Tsunami, Ti:Sapphireoscillator

Pulse pickerFlexible harmonic generator

Empower pump laser

Spitfire Pro, Ti:Sapphire, Regenerative amplifier system

Top deck

Bottom deck

Periscopes to bottom deck

Mirror turrets

Seed in

Pump inOutput

Black anodised beam tubes

To experiment

λ/2 wave plate

Polariser cube

TOPAS-C, OPA

Frequency mixing crystals

Variable ND filter

Figure 3.13 Schematic showing the layout of the NWSF laser over its two decks. Mirror

turrets show direction of beams which are enclosed in black anodised tubes for safety

3.3.3.1 Millennia and Tsunami

The Millennia pump laser comprises a water-cooled laser head and a power supply

unit that contains two 40 W diode laser modules that are fibre-coupled to the laser head.

The laser head itself contains a neodymium doped yttrium vanadate (Nd:YVO4) laser

crystal that is end-pumped by the two diode laser modules. The Nd3+ ion has a very well

matched absorption band with the output of the diode laser enabling efficient coupling. The

vanadate crystal will then emit a laser beam at 1064 nm that is resonant with the optical

cavity. The 532 nm output is achieved through intra-cavity frequency doubling in an LBO

non-linear crystal yielding efficient conversion efficiencies (as the 2ω output is dependent

upon the square of the fundamental peak power). The LBO crystal is non-critically phase-

matched and housed in an oven to keep it at a constant optimum temperature for 532 nm



output. Non-critical phase matching allows for collinear fundamental and second harmonics

as well as the ability to maintain maximum conversion efficiency through temperature

control without the need for realignment [11].

The ultra-short pulses produced in mode-locked lasers will have a frequency

bandwidth related to the pulse duration via the time-bandwidth product. As the pulse

traverses the optics of the laser cavity the different frequencies in this distribution will

experience slightly different refractive indices. Thus each frequency will travel with a

different speed in the medium and the frequencies will therefore spread in time as they

propagate. This difference in transit time as a function of wavelength is known as group

velocity dispersion (GVD). GVD can be positive as in Ti:sapphire where high frequencies

travel faster than lower ones or negative for the opposite case of low frequencies travelling

with the higher velocity. This spread of laser wavelengths in time, also known as the chirp,

is also influenced by the non-linear index of refraction of the Ti:sapphire crystal. As

explained earlier the intensity dependent non-linear refractive index leads to self-phase

modulation of the laser pulse, further adding to the chirp of the pulses. A reasonably simple

technique that utilises prism pairs (figure 3.14) is used in the Tsunami to introduce negative

GVD and so compensate for the positive GVD and SPM. The first prism spreads the

wavelengths so that the long wavelengths travel through more optical material than the

shorter wavelengths. Moving the second and third prism alters the GVD by changing the

amount of material the light must travel through. Using this set-up, micrometer control of

the slit and prisms gives easy control over wavelength selection and pulse width.

Figure 3.14 Four prism sequence used to compensate for the positive GVD in the laser

system by introducing negative GVD to the pulse. The tuning slit is used for wavelength

selection. (taken from [13]).



To achieve variable attenuation on the output of high power femtosecond lasers an

optical system known as a Glan-laser prism is often used. This is a very simple scheme that

uses a half-wave plate in a rotation mount and a polariser cube. The calcite, air spaced

polariser used here has been orientated so that horizontally polarised light is transmitted

through with minimal loss and vertically polarised light is totally internally reflected and

exits through the side window of the polariser (to the spitfire). By placing a half-wave plate

in front of the polariser cube, the vertically polarised output of the Tsunami can be rotated

through rotation of the half wave-plate. In this way we can control the proportion of the

laser power that is transmitted straight through (horizontally polarised) or reflected out of

the side window (vertically polarised) through rotation of the half-wave plate.

3.3.3.2 Spitfire Pro and Empower

The Spitfire Pro regenerative amplifier is specifically used to amplify the energy

contained within the Tsunami pulses from the nanojoule level to the millijoule level. This is

a requirement for many applications involving non-linear optics and here is used to pump

the TOPAS-C OPA and produce the white light continuum. Along with the Spitfire the

actual system for amplification requires a seed laser (Tsunami) supplying the pulses for

amplification, and a pump laser (Empower) used to supply the energy for amplification. A

particular problem associated with femtosecond pulse amplification is circumvented in the

Spitfire using a technique known as chirped pulse amplification (CPA). The peak power of

amplified pulses is limited by the damage threshold of the laser crystal and optical

components, and, as before, the non-linear refractive index of Ti:sapphire would cause a

catastrophic self focusing of the beam leading to massive peak powers. CPA uses the

relationship between pulse duration and bandwidth to first stretch the ultra short, high peak

power pulse, so that its peak power is now much lower. The stretched pulse is then passed

through the regenerative amplifier until it saturates the gain of the Ti:sapphire crystal. The

stretched, amplified pulse is then sent through a compressor which reverses the initial

stretching to yield pulses with close to the initial pulse duration but much higher pulse

energies.

By using similar principles to those employed by the Tsunami prism system for

reducing GVD, the Spitfire pulse stretcher makes use of gratings to disperse the spectrum

of frequencies in the pulse and then direct them over different path lengths to spread the



pulse in time (chirping). Two gratings are often used for this purpose and arranged in the

stretcher such that longer wavelengths have a smaller optical path length than the shorter

wavelengths. The opposite is the case for the compressor where the longer wavelengths

now travel the furthest distance and so the shorter wavelengths catch up the longer and the

pulse is compressed in time. In this way positive GVD is applied to the input pulses by the

stretcher and negative GVD is applied to the amplified pulses in the compressor. In the

spitfire, a simplifying design is implemented whereby only one grating is used in the

stretcher and only one is used in the compressor. A retroreflector is used to direct the beam

onto the grating multiple times to achieve greater dispersion, and the same principle is used

in reverse for the compressor which also corrects for dispersion occurring in the amplifier

cavity. The large footprint of the Spitfire is partly due to the gratings being separated by ~ 1

m in order to provide a large dispersion.

Regenerative amplification of pulses from the stretcher involves the trapping of

pulses from the ~ 80 MHz pulse train at the rate of ~ 1 kHz. The subsequent amplification

of these pulses allows the gain of the amplifier to be concentrated in fewer pulses [12]. The

pulse selection exploits the polarisation characteristics of the seed pulses and makes use of

Pockels cells and wave plates to form fast optical switches. A Pockels cell is an electro-

optic switch made from a material that becomes birefringent when an external electric field

is applied. This means that the components of a linearly polarised beam will travel at

different speeds along the mediums so called “fast” and “slow” axis. The phase difference

introduced between these two components leads to a change in polarisation of the incoming

light. Commonly a voltage which causes a λ/4 or λ/2 retardation is used in combination

with a passive wave plate, as is the case here. The stretched seed pulse is vertically

polarised when directed into the amplifier cavity and is incident upon the Ti:sapphire rod

(figure 3.15). The rod is cut at the Brewster’s angle for horizontally polarised light and so

the pulse is reflected off towards the cavity mirror and towards the input Pockels cell.

Incoming pulses are rejected from the cavity or trapped and amplified depending upon the

state of the input Pockels cell. If the Pockels cell is off it has no effect on the pulse

travelling through it, and the pulse will make a double pass through the λ/4 wave plate

rotating the polarisation 90˚ giving horizontal polarisation. The pulse can now pass through

the Ti:sapphire rod, making one round trip by passing through the inactive output Pockels

cell and back through the Ti:sapphire. Now once it passes through the input Pockels cell



and it makes a double pass through the λ/4 plate, its polarisation is returned to vertical and

so it is reflected out of the cavity by the Brewster surface of the laser rod. The pulse passes

back through the stretcher and is blocked by the Faraday isolator.

Figure 3.15 Schematic showing beam paths for seed pulses from stretcher to amplifier to

compressor. The stretcher and compressor are in the top left and right respectively and the

regenerative amplifier is along the bottom. (Modified from [14]).

From the above case we see that for the pulse to remain in the amplifier cavity it

must be horizontally polarised, otherwise when it reaches the Brewster angle of the

Ti:sapphire it will be reflected out. In order to select and trap a pulse the input Pockels cell

must be turned on to give λ/4 retardation once the pulse to be selected has made a double

pass through the λ/4 wave plate (now having horizontal polarisation) and has passed

through the inactive Pockels cell. As the pulse now has horizontal polarisation it will pass

through the Ti:sapphire rod, through the inactive output Pockels cell and will make a return

trip through the cavity. The high energy pump pulse will precede the seed pulse and excite

the rod to population inversion. The seed pulse will experience gain through stimulated



emission each time it makes a pass through the cavity. As the input Pockels cell is now

active, the selected pulse will experience 180˚ polarisation rotation once it makes a double

pass through both the Pockels cell and wave plate. Therefore, it remains horizontally

polarised and will make multiple passes through the cavity until it has saturated the gain in

the Ti:sapphire rod. After this has happened, the output Pockels cell is switched on and so a

double pass through it flips the pulses polarisation to vertical which is then ejected to the

compressor by the horizontal polariser. All other pulses are rejected during the

amplification of the selected pulse as upon propagation through the input Pockels cell (now

on) it will experience a 45˚ rotation. Therefore, passing twice through the λ/4 wave plate

and twice through the activated Pockels cell flips its polarisation 180˚ to remain vertical

and so it is reflected off the Ti:sapphire out of the cavity.

The Empower pump laser in this case is a Q-switched high power laser with a

diode-laser-pumped neodymium-doped lithium yttrium fluoride which emits at 1053 nm.

Q-switching is achieved using an AOM inside the cavity that acts to switch the laser cavity

between high and low Q values. The gain in the laser crystal is concentrated into a series of

high powered Q-switched pulses ideal for pumping the Ti:sapphire crystal in the amplifier.

The 1053 nm radiation of the Nd:YLF laser is frequency doubled to 527 nm by using a

lithium triborate (LBO) crystal in an intracavity design which combined with the Q-

switched pulses exposes the LBO to high powers, resulting in efficient frequency doubling.

3.3.3.3 TOPAS-C

The Light Conversion Ltd. TOPAS-C is a travelling wave optical parametric

amplifier of a white light continuum and can be used, in combination with second harmonic,

sum frequency and difference frequency generator crystals, to produce any wavelength

between 230 nm and 20,000 nm. The output from the Spitfire is split such that 95 % is used

to pump the TOPAS-C and 5 % is sent to form the probe in the TA experiment. A number

of distinct stages are used in the TOPAS-C itself to produce a signal wave with vertical

polarisation that can be tuned between 1150 and 1600 nm and an idler wave that can be

tuned between 1600 and 2600 nm. A very small fraction of the ~ 1 mJ pump pulses (from

the Spitfire) is focused into a sapphire plate to create a femtosecond white light continuum

in the same way as described in section 3.3.2. The amplification process is made up of two

stages and this WLC is used as the seed pulse for the first parametric amplification stage.



As has been mentioned, light waves of different frequencies under conditions such that they

undergo a non-linear interaction in a non-linear crystal will generate a third wave to

conserve energy and momentum. This scenario can be used to amplify a weak “signal”

beam through interaction with a strong “pump” beam with the production of a resultant

beam known as the “idler”. The condition of energy conservation means that the

relationship between the pump, signal and idler frequencies must be [15];

isp ωωω += (3.8)

where ωp is the pump frequency, ωs is the signal frequency and ωi is the idler frequency.

An optical parametric amplifier is one such device that makes use of the above

concept (shown in figure 3.16) whereby the signal wave is amplified at the expense of the

pump. This is due to the production of a travelling wave at the signal frequency when the

pump and idler wave mix; under proper phase matching conditions this increases the

amplitude of the signal frequency [12]. The ability to tune the wavelength of the device lies

in the ability to change the phase-matching condition so that the pump photons can be

divided up between the signal and idler in a great many ways. As already explained, in the

first amplification stage of the TOPAS-C the WLC is used as the seed and a significant part

of the split 800 nm input pulses are used as the pump.

Signal

IdlerPump

Signal

IdlerPump

Figure 3.16 Arrangement for beams in an OPA showing the amplification of the signal and

idler beams at the expense of the pump

The pump and the WLC must be focused to the same point in the beta barium borate

(BBO) non linear crystal; here a non-collinear scheme is used to allow the easy separation

of the signal beam after the crystal. The particular part of the dispersed WLC that is

overlapped in space with the pump is easily controlled by adding a delay to the WLC in

relation to the pump. Phase-matching at different wavelengths can be achieved by tuning



the BBO crystal angle. This signal beam is then collimated and sent to the second power

amplification stage in another BBO crystal. Here approximately 90 % of the input 800 nm

pump is used to amplify the signal produced in the pre-amplifier stage. The pulses must

once again be overlapped in space and time but here a collinear arrangement is used such

that the beams can be separated as desired at a later stage. Outside of the TOPAS-C box

are turrets containing BBO non-linear crystals; by using these in combination with the

signal, idler and residual pump it is possible to generate the second harmonic of the signal

or idler (SHS, SHI), the sum frequency of the idler or signal with the pump (SFI, SFS), the

second harmonic of the second harmonic of the signal or idler (SHSHS, SHSHI), or the

second harmonic of the sum frequency of both the signal and idler (SHSFS, SHSFI).

3.3.4 Detection and control

As mentioned ealier the sample beam and reference beam in the TA experiment are

coupled through an imaging spectrometer onto a pair of Si photodiodes. The pump beam

was modulated with a mechanical chopper synchronized to the 2nd sub-harmonic of the

laser repetition rate. A digital lock-in amplifier (Stanford Research Systems SR830) was

phase-locked with the mechanical chopper and amplified any difference in signal between

the reference and sample probe beams. The system used for data acquisition and

experiment control is based upon a Labview programme that was written for this purpose.

This allowed simultaneous control over the delay line and lock-in controls and monitored

the output from the lock-in as a function of the delay line position. In this way an average

light intensity for the change in absorption when the pump is present compared to when it

is absent was measured. Typical scan parameters were the range of the delay line (transient

time range), the step size of the delay line (time increment), the dwell time at each data

point, and the number of scans to average. Due to the length of time needed for each scan at

a certain pump wavelength and fluence a compromise was met between signal-to-noise

ratio and time per scan.



References

1. Klimov, V.I., ed. Semiconductor and Metal Nanocrystals: Synthesis and Electronic and optical properties. Optical Engineering, ed. B.J. Thompson. 2004, Marcel Dekker.

2. Tkachenko, N.V., Optical spectroscopy: methods and instrumentations. 1 ed. 2006, Oxford: Elsevier. 307.

3. Lakowicz, J.R., Principles of Fluorescence Spectroscopy. 3rd ed. 2006: Springer US.

4. Horiba, Fluorolog-3 operation manual. 2002, Jobin Yvon Inc. 5. Operating instructions TCC900 computer module for TCSPC. 2002, Edinburgh

Instruments,. 6. Becker, W., Advanced Time-Correlated Single Photon Counting techniques.

Springer series in Chemical Physics, ed. A.W. Castleman, J.P. Toennies, and W. Zinth. 2005: Springer Berlin Heidelberg.

7. Operating instructions EPL-series Picosecond pulsed diode lasers. 2006: Edinburgh Instruments.

8. Operating instructions Mini-tau fluorescence lifetime spectrometer. 2004: Edinburgh Instruments.

9. Nagura, C., et al. Appl. Opt., 2002. 41(18): p. 3735-3742. 10. Brodeur, A. and S. Chin. Journal of the Optical Society of America B 1999. 16(4): p.

637-650. 11. Spectra-Physics, ed. User manual Millennia Pro s-series 2006. 12. Koechner, W., Solid State Laser Engineering. Springer series in optical sciences, ed.

W.T. Rhodes. 2006: Springer. 13. Spectra-Physics, ed. User manual Tsunami mode locked Ti:sapphire laser. D ed.

2002. 14. Spectra-Physics, ed. User manual Spitfire Pro regenerative amplifier. B ed. 2005. 15. Davis, C.C., Lasers and Electro-optics: fundamentals and engineering. 2002:

Cambridge university press.

Chapter 4 Spectroscopic results


Chapter 4: Spectroscopic results

4.1 Introduction to results

All of the quantum dots studied during the course of this PhD have been produced

using wet chemistry techniques that require a low energy input and are often referred to as

colloidal nanocrystals or nanocrystal quantum dots (NQDs). The use of passivating ligands

on their surfaces gives a degree of control over the way the dots interact with their

environment and allows them to be manipulated as large molecules [1]. Unlike quantum

dots grown using high energy input methods such as molecular beam epitaxy (MBE) and

metal organic chemical vapour deposition (MOCVD) these dots are free-standing, distinct,

and can be made soluble in many polar and non-polar solvents through manipulation of the

surface ligand. The quantum dots studied in this work were obtained both from the

Chemistry department at the University of Manchester and grown by Javeed Akhtar (PbS)

and Katayune Presland (type II NQDs) and from Nanoco Technologies Ltd., a company

situated in Manchester that produces high quality NQDs and grown by Dr. Ombretta

Masala and Dr. Paul Glarvey. Before building a dedicated experimental set-up for the

investigation of MEG in quantum dots it was felt that a good first step was to characterise

and investigate the suitability of these quantum dots for MEG purposes. Considering the

climate at the time in MEG studies, with many contentious issues surrounding the

efficiency and even existence of MEG [2, 3], it was also decided (depending upon the

biexciton lifetimes in these dots) to attempt to observe MEG using TCSPC as well as TA

spectroscopy. Transient absorption experiments were conducted on various NQDs made

from different materials and with different structures including novel and cadmium-free

systems.

4.2 Quantum dots under study

Two types of quantum dot sample were available from Nanoco; cadmium selenide

and indium phosphide based NQDs. The CdSe NQDs produced at Nanoco are part of the

NanoDotTM range of cadmium-containing dots with peak photoluminescence wavelengths

ranging from 480 nm to 640 nm. Despite the wide range and large volume of research

conducted on CdSe quantum dots, conducting experiments upon them is still useful. They

represent the most well-characterised quantum dot system as well as providing some of the

best quality quantum dots available. They can therefore be used to build experience and



confidence in experimental studies without the added complications associated with poor

quality dots in terms of defects and trap states as well as large size dispersions. In addition

to this the reports of the absence of MEG have meant that independently confirming the

presence or absence of MEG in previously studied materials is still of interest and will add

to the wider evidence available to the MEG community.

InP NQDs from Nanoco are not currently commercially available and although they

cannot match the quality of the CdSe in terms of stability, size dispersion and quantum

yield they are still routinely produced with PL quantum yields of over 50 %. They are

available over a similar range of wavelengths but have the advantage for use in applications

such as solar cells as they are deemed non-toxic, being free of heavy metals such as

cadmium and are compliant with RoHS directive 2002/95/EC on the restriction of the use

of certain hazardous substances in electrical and electronic equipment. There is a number of

novel quantum dot systems in which MEG could be studied but InP is an excellent

candidate for this study. A theoretical study conducted by Luo, Franceschetti and Zunger

aimed to screen an array of semiconductor materials by introducing a figure of merit to

indicate their potential for MEG [4]. This figure is based upon the ratio of the biexciton to

the single exciton density of states and is proportional to the ratio between the rate of

impact ionisation and the competing process of Auger recombination. The greater this ratio

is found to be, the more likely is MEG to occur within that material. Based on this the

authors found that NQDs made from the semiconductor materials PbSe, Si, GaAs, CdSe,

and InP had particularly high figures of merit with PbSe, CdSe and InP being ~ 3 orders of

magnitude higher than most other materials. Clearly CdSe and PbSe have been extensively

studied with no reports of the absence of MEG in PbSe whilst InP has to date not been

thoroughly investigated. The large ratio between the electron and hole effective masses in

InP is also advantageous as a MEG threshold of 2.1Eg is relatively low when compared

with the 2.9Eg and 2.5Eg found in PbSe and CdSe. This has been shown to be a favourable

characteristic when using MEG to produce efficient solar cells [5].

Quantum dots with different structures and made from different materials are

fabricated in the School of Chemistry at The University of Manchester and are also studied

here. PbS colloidal quantum dots are produced and are interesting as they are particularly

well-suited for use in photovoltaics because lead and sulphur are both cheap and abundant

and therefore suitable for mass production. In addition to this PbS NQDs have been shown



to exhibit efficient MEG [6] and have an absorption edge which is size-tuneable across

much of the visible/near IR region allowing optimal use of the solar spectrum. As a further

motivation the synthesis method used to produce these PbS NQDs uses olive oil as both the

solvent and the capping agent, eliminating the need for expensive and toxic chemicals. This

is important as most NQD synthesis routes involve the use of capping agents such as TOPO

which have been shown to be both cytotoxic and genotoxic[7]. Also available from the

School of Chemistry are quantum dot heterostructures or type II quantum dots. The

effective band gap of a colloidal quantum dot can be reduced by forming a heterostructure

from semiconductor materials with staggered band gaps. The photoluminescence (PL) and

absorption spectra can then be red-shifted to a spectral range that would be unobtainable

using either of the semiconductor materials alone. In addition to this the electron and hole

will be spatially separated leading to a reduced overlap between their respective

wavefunctions and so to an increase in single [8] and multiple [9] exciton lifetimes. As a

result of these characteristics it has been suggested that type II colloidal quantum dots may

be favourable for use in solar cells exploiting CM [10]. It may be possible to tune the band

gap of materials with low CM thresholds such that they absorb a greater proportion of the

solar spectrum. Also, longer multiexciton lifetimes give a solar cell a better chance of

extracting these charges for use as photocurrent before they relax via the Auger mechanism.

4.3 Preliminary MEG studies using TCSPC

4.3.1 Photoluminescence and absorption spectroscopy

A sample of the CdSe Nanodot 640 was received from Nanoco which had a graded

core-shell structure of CdSe/CdZnS/ZnS and was passivated by a 2:1 mix of tri-n-

octylphosphine oxide (TOPO): hexadecylamine (HDA) and with a peak PL wavelength of

~ 640 nm and a PL quantum yield, measured at Nanoco, of 33 %. Also received was a

sample of InP core dots with peak PL wavelength of ~ 660 nm, that were passivated by

myristic acid, and had a PL quantum yield also measured at Nanoco as 54 %. Although

MEG was expected to be more efficient in smaller dots due to the increased Coulomb

interaction and so increased rate of impact ionisation [11], larger dots with a longer

wavelength and so smaller band gap were chosen so as to allow excitation photon energies



of over twice the band gap. Both samples were dispersed in toluene and placed in far UV-

rated quartz cells.

The PL and absorption spectra are shown in figures 4.1 a) and b) for the CdSe

Nanodot 640 quantum dot and for the InP core quantum dot respectively. A photon

excitation energy of ~ 3.1 eV (400 nm) was selected using the excitation monochromator

for the PL measurements. The peak wavelength for the CdSe Nanodot 640 sample was

found to be at a wavelength of 637 nm (~ 1.95 eV) with a full width at half the maximum

(FWHM) of 36 nm. For the InP dots a Gaussian fit to the peak revealed the maximum to be

at 630 nm (~ 1.96 eV) with a FWHM of 90 nm. The absorption spectrum for the CdSe

quantum dot shows a distinct peak at 2.03 eV (~ 610 nm) and another distinct feature

forming a plateau at around 2.4 eV. In the InP case the absorption onset is a slow rise and

the first absorption feature starts to level off but does not form a peak or plateau. The centre

of this feature is found using a Gaussian fit to be ~ 2.15 eV. The absorbance then increases

with energy but with no further discernable features.



1.75 2.00 2.25 2.50 2.750.0

0.5

1.0

1.5

2.0

2.5

Wavelength (nm)

Nor

mal

ised

Inte

nsity

(a.

u.)

Photon Energy (eV)

Absorption Photoluminescence

700 650 600 550 500 450

1.5 2.0 2.5 3.00.0

0.5

1.0

1.5

2.0

2.5

Photoluminescence Absorption

Wavelength (nm)

Nor

mal

ised

Inte

nsity

(a.

u.)

Photon Energy (eV)

800 750 700 650 600 550 500 450

Excitation energy = 3.1 eV

b)

a)

Excitation energy = 3.1 eV

Figure 4.1 Photoluminescence and absorption spectra for a) CdSe/CdZnS/ZnS Nanodot 640

and b) InP core quantum dot samples.



4.3.2 Time correlated single photon counting (TCSPC)

TCSPC was conducted on the CdSe and InP samples using the ultrafast laser and

MCP-PMT as described in section 3.2.5.2. In the first instance the lifetimes of the dots was

investigated to assess the suitability of these dots for studying MEG. For this purpose

NQDs with a lifetime that can be reasonably described by a mono-exponential would be the

ideal case. In reality NQDs with lifetimes that are described by two or three exponentials

can be used as long as the lifetime of the single exciton and any other process is on the

order of nanoseconds. To put it another way, there must be no other fast processes whose

presence could confuse or prevent the identification of the biexciton lifetime (~ 10s ps) and

so MEG. Here the CdSe and InP NQDs were excited at 400 nm by frequency-doubling the

800 nm Tsunami output in the GWU-FHG. Survey scans revealed the transients decayed to

the background level after about 500 ns. In order to maintain the accuracy of the measured

lifetimes we must ensure that the sample is allowed to completely relax between excitation

events. As a rule of thumb five times the total length of the decay is often used for the pulse

period of the laser, and a division ratio of 203 is used in the pulse picker to reduce the ~ 81

MHz repetition rate to ~ 400 kHz giving a pulse period of ~ 2.5 µs. The monochromator

entrance and exit slits are set to 1 mm and the wavelength is first set to 400 nm when taking

the IRF from a scatterer and then to the peak of the PL for each dot. The MCP is Peltier-

cooled to -30˚C with water used as the heat exchange and is set to -3.3 kV, close to its

maximum, to give the smallest FWHM of the IRF. The IRF is shown in figure 4.2 and is

found to have a FWHM of 117 ps and roughly follows the shape of the detector response.

In order to observe the decay at early times in as much detail as possible the time base is set

to 50 ns and the maximum number of channels of 8192 available from the MCA is used,

each channel then represents a time bin of 6.1 ps. The full decay in a 500 ns time window is

shown in figure 4.3 for the CdSe Nanodot 640 and the decay in a 50 ns time window (with

fit) as well as the first 1.5 nanoseconds in figures 4.4 and 4.5 respectively.



0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.41

10

100

1000

10000

FWHM = 117 ps

Cou

nts

Time (ns)

MCP-PMT Instrument response

Figure 4.2 Showing the instrument response function for the TCSPC set-up utilising the

femtosecond laser and the MCP PMT. Taken using scattering sample and monochromator

set to exciting wavelength of 400 nm.

0 100 200 3001

10

100

1000

Cou

nts

Time (ns)

Nanodot 640CdSe/CdZnS/ZnSdetection at 1.953 eV (635 nm)

Figure 4.3 Full decay of nanodot 640 CdSe NQD with excitation at 400 nm and detection at

the peak of the PL (635 nm).



0 10 20 30 4010

100

1000

Cou

nts

Time (ns)

Nanodot 640Excitation = 3.1 eVDetection = 1.953 eV

Figure 4.4 First 40 ns of CdSe decay showing a flat mono-exponential decay for detection

at the peak photoluminescence wavelength.

0.0 0.5 1.0 1.5100

1000

Nanodot 640Excitation = 3.1 eVDetection = 1.953 eV

Cou

nts

Time (ns)

Figure 4.5 First nanosecond of CdSe decay to show the presence of any fast components.

There are none observed for detection at the PL peak.



The InP sample also decays to the background level in ~ 500 ns and so the same

repetition rate of ~ 400 kHz is used. In order to prevent pulse pile (the detection of more

than one pulse in the same time bin) the stop rate is kept to less than 2 % of this start rate.

The detection wavelength is set to the PL peak of the InP sample which is 1.956 eV or 633

nm. Figure 4.6 shows the time decay curve over the first 40 ns of the decay and figure 4.7

shows the time decay curve for the first nanosecond of the decay. Both show flat mono-

exponential decays and also show that the single exciton lifetime does not appreciably

decay over the early times we will be looking at when attempting to measure MEG.

0 10 20 30 4010

100

1000

Inte

nsity

Time (ns)

InP SampleExcitation = 3.1 eVDetection = 1.956 eV

Figure 4.6 Time decay curve for the InP NQD sample for excitation and detection

conditions shown.

As studies into the efficiency of MEG had already been conducted on CdSe NQDs

[12] and due to the fact that the CdSe sample was of the highest quality it was a good

candidate to test for MEG. Exciting with the third harmonic output of the GWU-flexible

harmonic generator allows excitation above the MEG threshold with a photon energy that is

2.7 times the band gap of the CdSe NQD. This is conducted at a photon fluence that for

excitation at 1.6 times the band gap had resulted in a flat mono-exponential decay even at



early times. As such this fluence is of a level whereby the probability of absorbing more

than one photon is negligible.

0.0 0.5 1.0 1.510

100

1000

Inte

nsity

Time (ns)

InP SampleExcitation = 3.1 eVDetection = 1.956 eV

Figure 4.7 First nanosecond of InP decay when excited by 400 nm radiation and detected at

peak of photoluminescence.

The laser pulse period is again set to ~ 2.5 µs and the monochromator is set to the

steady state PL maximum of 637 nm. Figure 4.8 shows PL decays taken for excitation at

1.6 and 2.7 times the band gap, that is, above and below the threshold for MEG. By

normalising the decays at a time that is longer than the Auger recombination time constant

but at times when there has not been significant single exciton decay we can quantify the

MEG efficiency. Exponential fits to the two decays (shown in red) reveal an additional

decay feature for excitation at 2.6Eg with a time constant of 150 ± 10 ps.



5.00 5.25 5.50 5.75 6.00 6.25 6.50 6.75 7.00

200

400

600

800

1000

1200

1400

CdSe 2.6 Eg

CdSe 1.6 Eg

Detection at 1.953 eV

Inte

nsity

Time (ns)

Figure 4.8 Photoluminescence decay monitored at steady state absorption maximum for

excitation with photons of energy 1.6 (green) and 2.6 (blue) times the band gap.

Exponential fits to the decays are shown in red.

4.3.3 Discussion

Figures 4.1 a) and b) show some stark differences between the PL and absorption

spectra of the CdSe sample and the InP samples obtained. Common to both however, is the

characteristic blue shift of the spectra to higher energies relative to bulk due to quantum

confinement (Bulk band gap is 1.74 eV in CdSe and 1.34 eV in InP). The narrow

photoluminescence peak (FWHM ~ 35 nm) and distinct features in the absorption spectrum

of the CdSe sample is indicative of a small size dispersion. As the band gap of individual

dots is modified depending upon the degree of quantum confinement a smaller size

distribution will give narrow and more spectrally pure emission. Higher quality samples

will reveal more of their electronic structure [1] as is the case here for the CdSe.

Comparison with theory and the application of the selection rules (∆L = 0) allows us to

identify some of the optical transitions. The first exciton peak in the absorption spectra at

2.03 eV can be attributed to the 1Sh – 1Se transition and the higher energy feature at ~ 2.5

eV can be cautiously attributed to the 1Ph – 1Pe. As was described in the theory section, it

is the lifting of the degeneracy of the split off 1S3/2 hole state which causes the observed



stokes shift. The strong optical transition between the high energy hole states and the 1Se

state is responsible for the band edge absorption where as a far weaker transition between

the low energy holes states and the 1Se state is responsible for the band edge emission. In

the InP sample it is much more difficult to distinguish any features in the absorption

spectrum due to the large inhomogeneous line broadening. The large FWHM in the PL of ~

90 nm shows the much larger size dispersion of the sample and lower quality in comparison

to the CdSe. In order to find the position of the band from this very broad absorption

spectrum a Gaussian was fitted to the broad absorption onset and its peak was used to give

the position of the 1S band edge exciton.

The PL decays, taken below the threshold for MEG, in both samples are found to be

well described by single exponential functions. The lifetimes extracted from this fit were

found to be 22.5± 0.2 ns and 27.0± 0.5 ns for the CdSe and InP samples respectively. This

indicates that after excitation to higher energy levels the charge carriers quickly relax to the

band edge, probably through Auger-type electron-hole energy transfer, and then recombine

to emit a photon on a nanosecond time scale. Figures 4.5 and 4.7 also reveal the decay to be

very flat and mono-exponential at early times. This is good for the identification of MEG

which relies on the identification of a fast time component due to the decay of biexcitons.

The additional initial feature found in figure 4.8 for excitation at 2.6 times the band

gap is tentatively attributed to multiple exciton generation due to the absorption of a single

photon. The biexciton lifetime of 150± 10 ps is similar to the biexciton lifetime measured

by others for CdSe NQDs using the same method [13]. These dots were dispersed in

toluene which absorbs some light and emits with its own characteristic lifetime. This was

measured to be ~ 8 ns however, and so is thought to have little influence over the time

range studied but the situation is far from ideal. As CdSe has been characterised extensively

by others this first glimpse of MEG was a good preliminary observation before moving

onto building up the TA experiment.



4.4 MEG studies on InP NQDs with ultrafast transient absorption

Access to high quality InP NQDs puts us in an advantageous position when trying

to observe MEG in this NQD system. Many studies of novel phenomena in quantum dots,

especially those studying charge dynamics, can be hindered or prevented entirely if high

quality samples are not available. The presence of traps, strain between layers, poor surface

passivation and excess un-reacted product or ligand will all introduce effects which may

prevent or simply mask the signature of any process under investigation. A full study with

the aim of observing and characterising MEG in InP NQDs is conducted for the first time.

4.4.1 Sample information

In this study three new samples of InP NQDs were obtained from Nanoco

technologies. These samples comprised InP cores of different diameters with a

core/shell/shell structure with zinc sulphide inner shells and zinc oxide outer shells, with

undecylenic acid (UA) used as the passivating ligand. The three sizes were chosen to give

“large”, “medium” and “small” core samples where the large core is chosen so as to allow

excitation with photon energies that are multiples of its band gap (the shortest laser

wavelength is 240 nm). The quantum yields as measured by Nanoco were given as 25 %,

42% and 65% for the large, medium and small core samples respectively. A suitable

solvent that did not absorb at the ultraviolet pump wavelengths was needed; as the dots are

only soluble in certain solvents absorption spectra were taken of these to ensure they had

the appropriate characteristics (figure 4.9). As can be seen, toluene and chloroform absorb

to some degree the UV pump wavelengths and so only hexane is suitable for the transient

absorption on these dots. The dots were therefore dispersed in hexane and placed in UV

quartz, 10 mm path length cuvettes.



200 250 300 350 4000.0

0.5

1.0

1.5

2.0

2.5

3.0

Abs

orba

nce

(O.D

.)

Wavelength (nm)

Chloroform Toluene Hexane

Figure 4.9 Absorption spectra for solvents that the InP dots are soluble in showing the

absorption onset for toluene, chloroform and hexane to be 290, 250 and 220 nm

respectively.


The photoluminescence and absorption spectra are shown in figure 4.10 for all three

samples of InP dots which had the structure of InP/ZnS/ZnO:UA and differed only in size.

For the absorption spectra a hexane blank was used in the reference arm of the

spectrophotometer and the slits were set to 2 mm. For the PL spectra a photon energy of 3.1

eV was used as the excitation wavelength for all the dots and the slits were also set to 2 nm.

These spectra reveal the PL peak of the small, medium and large dots to be ~ 2.21±0.01 eV

(561 nm), 1.96±0.01 eV (632 nm), and 1.91±0.01 eV (650 nm). As before these InP dots

also show large FWHM on the PL of 90, 82 and 92 nm for the small medium and large dot

respectively. With such broad features in the absorption spectrum finding the position of

the 1st absorption peak can be difficult. To be accurate and consistent when identifying this

peak a Gaussian fit is done to the first absorption edge and its peak position is used as the

position of the 1S absorption feature. Although this is an ad hoc method it gives an

absorption edge that agrees very well with that chosen independently by eye as well as

giving the same FWHM as found in the PL (both represent the same size dispersion).



Previous studies on InP nanoparticles with similar PL have given an empirical relationship

between the maximum of the 1S absorption and the PL maximum showing that the

absorption maximum lies 100 to 300 meV higher in energy [1]. It is also found that the

position of the fitted peak, as well as that found by eye agrees with this empirically found

relationship. This allows the positions of the absorption maximum and so the effective band

gap to be taken as 2.0, 2.1 and 2.4 eV for the large medium and small NQDs respectively.

As the PL and absorption peaks of the large and medium core dot are very close this shows

that they are actually quite similar in size but are named large and medium for convenience.

1.5 2.0 2.5 3.0 3.50.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Large core Medium core Small core Gaussian fits

PL

Inte

nsity

(ar

b. u

nits

)

Energy (eV)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Abs

orba

nce

800 700 600 500 400

Wavelength (nm)

Figure 4.10 Photoluminescence and absorption spectra taken for the three InP samples with

the Gaussian fits shown in red. For the PL, excitation was at the photon energy of 3.1 eV.

4.4.3 Single exciton lifetimes using TCSPC

The TCSPC set-up used here is as described in section 3.2.5.1 where a 405 nm

pulsed diode laser is used as the excitation source and the PL is coupled through a 0.5 metre

imaging monochromator and photons are detected by the Hamamatsu H7422 PMT. The

spectrometer was first set to 405 nm and a quartz cuvette filled with water was used to take

the IRF of the system. The FWHM of the IRF in this case was found to be ~ 400 ps, thus



using this system we are limited to observing single exciton lifetimes. The monochromator

slits are set to ~ 2 mm for the IRF and the lifetime scans. The monochromator is then set to

the PL peak of the relevant dots for observation of their lifetimes. Survey scans reveal that

these dots decay to background in about 400 ns and so a repetition rate of 500 kHz is

chosen for the laser as this will give a pulse period of 2 µs. All decays are accumulated to

10,000 counts and a GG435 filter is used to block any scattered laser light from entering the

monochromator. All external light sources such as indicator LEDs and computer monitors

are blocked or switched off for all scans which are taken in the dark to minimise

background counts. The number of channels is set to the maximum available on the mini-

tau TCC900 card of 4096. A TAC time range of 500 ns gives 122 ps/channel.

Reconvolution fits are performed on the three decays to allow for the effect of the

instrument. The fitting region is chosen so as to avoid the low intensity tail section where a

large spread of points from the mean will lead to poor quality fits. The best fit is found by

looking for an improvement in the reduced chi squared with the addition of another

exponential function, as well as by checking that each function has a significant

fluorescence intensity (and so is real) and making sure the residuals are randomly

distributed around zero. The decays are shown in figure 4.11 for the three sizes of dots. The

decay for the large core NP could be well-described by a tri-exponential function with

decay constants of 8.5±0.2 ns, 30.2±0.4 ns and 99±2 ns with relative amplitudes of 32%,

58% and 10%, respectively. The decay for the medium core NP could also be well-

described by a tri-exponential function with decay constants of 5.3±0.4 ns, 23.9±0.4 ns and

65.6±1.2 ns with relative amplitudes of 3%, 51% and 47%, respectively. For the small core

NP, a bi-exponential function with time constants (amplitudes) 65±1 ns (91%) and 20±1 ns

(9%) was sufficient to characterize the decay. The lifetimes are all in the nanosecond

regime and are best described by multiple exponentials as has been found in InP dots

previously [14].



0 50 100 150 200 2501

10

100

1000

10000

Large core Medium core Small core

Inte

nsity

(ar

b. u

nits

)

Time (ns)

Figure 4.11 Time decays for the large, medium and small core InP quantum dot systems for

excitation at 3.05 eV and detection at the PL peak.

4.4.4 Ultrafast transient absorption of InP NQDs

The characterisation and analysis of the MEG phenomenon relies on the ability to

observe the signature of MEG, that is, the emergence of the fast Auger recombination

lifetime due to the decay of biexcitons. The persistence of this feature in the limit of

vanishing fluence for pump photon energies equal to multiples of the band gap indicates

that multiple excitons are being generated from one energetic photon. As such the

unrivalled time resolution of transient absorption is ideal for resolving the picosecond time

component due to Auger recombination. The TA experiment was built up as described in

section 3.3 by Dr. Samantha Hardman and myself and was used to study the three InP dots.

In TA it is the pump-induced absorption change at the band gap which is monitored

and so for each of the NQDs the monochromator is moved to the band gap wavelength

found from the relevant absorption spectrum. The white light continuum generated in the

sapphire plate can be optimised for different wavelengths by changing the focusing of the

beam into the plate. The monitoring wavelength of the probe beam is optimised for each

dot so as to obtain maximum signal. It is important that the optical delay stage is well-

aligned as any movement of the probe beam as the delay stage is scanned would appear as a



change in signal on the detector. The delay stage is fine tuned everyday and then checked

by scanning the delay stage with no sample to ensure a flat noise level is obtained. The

sample and reference arms of the probe are then balanced in the absence of the pump beam

and the transmission is noted. The lock-in amplifier is phase locked with the second sub-

harmonic of the amplifier output using the mechanical chopper. The signal monitored

therefore, is the small change in transmission caused by the presence of the pump or ∆T.

By stepping the translation stage it is possible to build up the fractional change in

transmission as a function of delay between the pump and the probe. It is important to

ensure that a uniformly excited area is probed when taking the transients and so for this

reason the pump beam is kept much larger than the probe. The beam diameter (assuming a

Guassian beam profile) is defined here as the distance from the beam axis in which 1/e2 of

the total power is contained. The pump beam diameter is measured using an iris and power

meter which finds the beam diameter to be ~ 4 mm. The probe beam however is too small

to be measured using this approach. The probe beam is focused down using a 500 mm lens

and so by measuring the diameter before focusing is calculated to give a spot size of ~ 200

µm. Figure 4.12 shows the absorption transients taken for the large InP NQD for a range of

absorbed fluences and a pump photon energy of 2.75 eV. This pump energy corresponds to

1.4 times the measured band gap (1.4Eg) and so is below the threshold for MEG. To limit

the effect that photoionisation of the NQDs could have on our analysis of the yield of MEG,

all samples were stirred using a Variomag mini magnetic stirrer at 1000 rpm.

To be confident the pump fluence is at a level whereby the average number of

photons absorbed per nanocrystal, pa jN .0 σ= , is less than one a rough calculation is

performed. This uses the measured optical power of the pump (< 0.2 mW) and the beam

diameter as measured above. The pulse repetition rate (1 kHz) can be used to measure the

energy per pulse and the focal spot area along with the photon energy can then be used to

calculate the per pulse pump fluence. At the lowest fluence used, this calculation finds

0N ~ 0.02 for excitation at 450 nm and 0N ~ 0.01 for excitation at 240 nm. The number

of nanocrystals in the population that have absorbed more than one photon will therefore be

negligible at the lowest fluence levels.



0 100 200 300 400 5000.0

0.5

1.0

1.5

2.0

2.5

3.0

∆T/T

(x

10-2)

Time (ps)

Increasing Pump Fluence

Figure 4.12 Pump induced fractional transmittance change for the large core dot for pump

pulses with a range of fluences and photon energies of 1.4 times the band gap.

The transients shown above are the average of multiple scans (up to 20 and as low

as 5 averages were taken for the high and low fluence decays respectively) of the delay

stage over its chosen range. It can be seen that in the case of low fluence the decay is flat

and decays very little for over the entire time scan range. As the pump fluence is increased

the emergence of an initial decay becomes evident and the relative amplitude of this

component increases with the fluence. A mono-exponential fit to this initial decay feature

yields a time constant of 41±3 ps, which is similar to the biexciton lifetime measured

previously for InP [15]. As there is insufficient photon energy absorbed by the dot for the

excitation of multiple excitons the initial Auger component here can be identified as the

decay of biexcitons produced via the sequential absorption of photons. This will be

identical to the lifetime of biexcitons created through impact ionisation after absorption of a

photon with energy in excess of the MEG threshold. Thus if this component is observed for

pumping with higher energy photons at low fluence levels (for which it was absent in the

below threshold case), then multiple excitons are being created through the absorption of

one highly energetic photon. This is shown in figures 4.13 a) and b) that show transients

taken for pump photon energy of 5.2 eV which is equal to 2.6 times the band gap. The



transients in figure 4 b) were taken at fluence levels where the fast decay feature had

vanished when under 1.4Eg excitation (bottom transient in figure 4.12). This is because the

fluence levels here are low such that the probability of a single NQD absorbing more than

one photon has become negligible.

0 100 200 300 400 5000.0

0.2

0.4

0.6

0.8

1.0

1.2

∆T/T

(x

10-2)

Time (ps)

hυ = 5.2 eV ( 2.6 E

g)

Increasing pump fluence

0 100 200 300 400 500 600 700 800 900 10000.00

0.05

0.10

0.15

∆T/T

(X

10-2

)

Time (ps)

2.6 Eg

1.4 Eg

b)

a)

Figure 4.13 a) Selection of transients for pump photon energy of 2.6Eg at low fluences

showing persistence of fast time component and b) comparison of lowest fluence decays for

excitation at 1.4 and 2.6Eg.



Transients were taken over a range of fluences for pump photon energies of ~ 2.8, 4,

4.4, 4.8, and 5.2 eV. This means the large dot is excited by photons that are 1.4, 2.0, 2.2,

2.4 and 2.6 times the band gap. The calculated threshold for MEG in InP is found from

equation 2.12 to be 2.1Eg and so these photon energies lie both above and below this

threshold. As described in section 2.2.2 the absorption change at the band gap due to the

pump gives a change in transmission proportional to the total number density of excitons in

the sample [16]. At early times this will be equal to the total created by the pulse and after 5

biexciton lifetimes will be equal to the number of initially excited nanoparticles. So in order

to extract the quantum yield of MEG it is necessary to find the ratio of the initial amplitude

to the amplitude at long times called R. This is done by fitting an exponential decay to the

first 200 ps of the transients, as after this point they plateau to a near constant value. By

fixing the decay constant to the 41 ps found for the biexciton lifetime we can extract the

amplitude and the background from the exponential decay function. This ratio, R, can be

plotted as a function of fractional change in transmission. By fitting equation 2.18 the

quantum efficiency of MEG can be extracted by finding the value of R in the limit of

vanishing ∆T(0)/T. The number of photons absorbed per NQD, Jpumpσ , is proportional to

the fractional change in transmission at t=0 as

T

T

QY

kJpump

∆=σ , (4.1)

where k is a constant of proportionality and QY is included to take into account how the

quantum yield of MEG affects the number of absorbed photons. A correction factor δ is

calculated to account for the small drop in single exciton population over the 200 ps

measurement window using the PL decay constants discussed earlier. For the large core

sample it is 1.01 and so this is incorporated into the fits. Figures 4.14 a) – e) show graphs of

R as a function of ∆T(0)/T for the 5 pump photon energies in the large core dot.



e)

0.1 1 10

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

R

∆T/T (x 10-3)

2.4 x Eg

0.1 1 100.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

R

∆T/T (x 10-3)

2.2 x Eg

d)

0.1 1 100.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

R

∆T/T (x 10-3)

2.6 x Eg

c)

b)

0.1 1 100.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

R

∆T/T (x 10-3)

2.0 x Eg

0.1 1 100.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6R

∆T/T (x 10-3)

1.4 X Eg

a)

Figure 4.14 Average number of electron-hole pairs generated per absorbed photon of

energy a) 1.4 b) 2.0 c) 2.2 d) 2.4 and e) 2.6 time Eg plotted as a function of fractional

change in transmission. The fit of equation 2.18 to the data is shown in red.



The trend seen for all photon energies is that larger values of R are obtained for high

fluence and smaller values as ∆T(0)/T is reduced. For excitation at 2.0Eg and 1.4Eg we find

that as the fluence is decreased the average number of excitons per photo-excited NQD

tends to unity. This is consistent with biexcitons being generated at high fluences due to a

significant probability of absorbing multiple photons but once the fluence reaches a level

whereby this probability becomes negligible one absorbed photon produces on average

only one exciton. For excitation with photon energies > 2.2Eg, in the limit of low fluence it

is observed that R tends to a value significantly greater than one indicating that an average

of more than one exciton is generated per absorbed photon. The number of additional

excitons for excitation at 2.4 and 2.6 times the band gap is found to be 0.15 ± 0.02 and 0.18

± 0.03 respectively. The quantum efficiency of MEG is often quoted as a percentage by

multiplying the average number of electron-hole pairs produced per absorbed photon by

100 (QEMEG = 100 Nehp). Here this corresponds to 115 % at 2.4Eg and 118% at 2.6Eg.

Figure 4.15 shows a plot of the average exciton multiplicity against photon energy as a ratio

of the band gap (hυ/Eg). The average value for the data points below 2.0Eg is 0.04 ± 0.01;

by taking this into account as an offset, a linear fit to the three data points above 2.0Eg gives

an MEG threshold of hυth = (2.1 ± 0.2)Eg. This threshold is in agreement with both the

value calculated using equation 2.12 according to the energy partition model and is also

consistent with conservation of energy. The slope efficiency with which MEG increases is

found from the gradient of this linear fit. A gradient of 0.3 ± 0.1 means that for every

multiple of the band gap increase in photon energy, on average 0.3 additional excitons are

created in the NQD.



1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.61.00

1.05

1.10

1.15

1.20

1.25

Neh

p, ave

rage

exc

iton

mul

tiplic

ity

Photon energy/Band gap

InP large core

Figure 4.15 Average exciton multiplicity plotted against photon energy as a multiple of the

NQD band gap. Linear fit to photon energies > 2.0Eg is shown in red.

In order to address some of the contentious issues relating to the measurement of

MEG it was important to be sure that the fast time constant occurring in the large dot was in

fact a result of MEG. One factor that has been suggested to artificially enhance the apparent

efficiency of MEG is photoionisation [17, 18] whereby energetic charge carriers created

either through excitation or when biexcitons recombine become trapped in a long-lived

surface state. The subsequent formation of an exciton in the now charged dot will form a

trion with the other charge. This will also decay rapidly with a lifetime similar to biexcitons

and thus will act to exaggerate the apparent yield of MEG. Rigorous stirring of the samples

should prevent or reduce the effects of photoionisation as a result of exposure to multiple

laser pulses; thus a study into the effects of stirring was conducted. This was conducted on

the large core dot at both high and low fluence and for different pump photon energies

where the dots were either stirred at 1000 rpm or were not.



0 200 400 600 800 1000 12000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

∆T/T

(x

10-3)

Time (ps)

Static Stirred

Excitation = 5.2 eV

0 200 400 600 800 1000 12000

2

4

6

8

10

12

14

16

18

∆T/T

(x

10-3)

Time (ps)

Static Stirred

Excitation = 2.8 eV

b)

a)

Figure 4.16 Transients taken for the large core dot a) at high and low fluence for excitation

at 1.4Eg b) and for 2.6Eg where the quantum dot solution is either static (black lines) or

stirred at 1000 rpm (red lines).

Figure 4.16 shows the results for excitation at photon energies of 2.8 eV and 5.2 eV.

The figure indicates that in these NQDs there is no apparent change when stirred or not

stirred for excitation at 2.8 eV. For excitation at 5.2 eV a small difference between the



static and stirred transients can be seen. This difference however, is small compared to the

noise level of these traces and it is further noted that there is no significant difference in the

fits derived from these two transients. This allows us to conclude that our MEG

measurements are not appreciably influenced by charging of the dots due to photoionisation.

Further to this as there is no significant change at the higher photon energies photo-

degradation of the sample does not appear to pose a problem.

Transients were then taken for the medium core dot and so the white light

continuum was optimised to probe the band gap of this sample found from its absorption

spectrum to be ~ 2.4 eV. The monochromator was also set to 515 nm to monitor the

absorption change at the band gap of the dot. The quantum dots were again dispersed in

hexane and placed in a 10 mm path length UV-rated quartz cuvette which was stirred at

1000 rpm. This was pumped with the same range of photon energies as for the large core

dot. In this way it is possible to show that the initial fast time component is due to MEG as

this depends upon hυ/Eg whereas photoionisation would depend only upon hυ. Figure 4.17

shows absorption transients for a range of fluences at the pump photon energy of 2.8 eV.

As before it shows a flat mono-exponential for low fluence and the emergence of a fast

time component as the fluence is increased. The biexciton lifetime of this dot was measured

to be 52 ± 1 ps.

0 100 200 300 400 500 6000

1

2

3

4

5

6

7

8

∆T/T

(x

10-3)

Time (ps)

Excitation = 1.3 Eg

Increasing fluence

Figure 4.17 Absorption transients taken on the medium core dot for a range of fluences at

the photon energy of 2.8 eV which here corresponds to 1.3Eg.



Transients were taken for various fluences at four of the same photon energies as in

the large core of ~ 2.8, 4, 4.4, and 5.2 eV which here correspond to exciting at ~ 1.3, 1.9,

2.1, and 2.5 times the band gap. R was then found as before by fitting an exponential decay

function to the first 250 ps so as to preserve the same 5 times ratio between the fitting range

and the biexciton lifetime as was used for the large core dot. R is then plotted as a function

of the fractional change in transmittance as before and equation 2.18 is fitted through the

points in order to extract the quantum efficiency of MEG. The correction factor δ in this dot

is calculated to be 1.006 and so is fixed at this value for the fits. The result of these fits are

shown in figure 4.18 a) – d) where, as before, when the photon energy is ≤ 2.1Eg high

fluence yields on average multiple excitons per photo excited NQD but the average number

of excitons per NQD tends to unity when the fluence is reduced through several orders of

magnitude. In d) we see that when the photon energy is above the threshold for MEG in the

limit of low fluence, the average number of electron-hole pairs is found to be 1.20 ± 0.03.

This means the quantum efficiency of MEG for excitation at 2.5 times the band gap is

120 % meaning that an additional 0.20 ± 0.03 excitons are created per absorbed photon.

To help confirm and support the observation of MEG it was decided to investigate

an NQD whose size was too small to be excited by multiples of its band gap by the photon

energy range available here. As the band gap of the small dot is found at 2.4 eV, the

shortest wavelength possible from the TOPAS-C of 240 nm would only allow excitation at

2.1Eg. This would serve to show that MEG efficiency is dependent upon the ratio of photon

energy to the band gap and not on photon energy alone as has been suggested by some

authors [19]. If the picosecond time component was found in the small NQDs at low

fluence and when excited below the threshold for MEG then it could be attributed to

photoionisation (which would depend only upon photon energy). For the small core dot the

monochromator is set to monitor at the band edge and the white light continuum is

optimised for this wavelength. The dots are stirred as before at 1000 rpm and the same five

pump wavelengths as used on the large core dot were used here. The pump photon energies

of 2.8, 3.1, 4.0, 4.8, and 5.2 eV will in this dot excite it at 1.1, 1.3, 1.7, 2.0, and 2.1

multiples of the band gap.



1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

1.00

1.05

1.10

1.15

1.20

1.25

Neh

p, ave

rage

exc

iton

mul

tiplic

ity

Photon Energy / Band gap

0.1 1 10

1.0

1.2

1.4

1.6

R

∆T/T (x 10-3)

2.5 Eg

0.1 1 10

1.0

1.2

1.4

1.6

R

∆T/T (x 10-3)

2.1 Eg

e)

d)

0.1 1 10

1.0

1.2

1.4

1.6

R

∆T/T (x 10-3)

1.9 Eg

1 10

1.0

1.2

1.4

1.6R

∆T/T (x 10-3)

1.3 Eg

c)

a) b)

Figure 4.18 Plots of R against fractional change in transmission for the medium core dot for

pump photon energies of a) 1.3 b)1.9 c) 2.1 and d) 2.5 time the band gap with fits to

equation 2.18 shown in red. The average number of electron-hole pairs created per

absorbed photon as a function of hυ/Eg is shown in e).



A fit to the high fluence 1.1Eg transient yields a biexciton lifetime for this dot of 69

± 9 ps. Figure 4.19 shows transients taken at the pump photon energies (as in the large core

sample) of 5.2 and 4 eV. Both transients appear flat across the full length of the decay at a

fluence which yielded the fast time component in the large core sample for 5.2 eV

excitation.

0 100 200 300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

∆T/T

(x

10-3)

Time (ps)

5.2 eV (2.1 Eg)

4 eV (1.7 Eg)

Figure 4.19 Transients for the small core dot for two photon energies taken at very similar

low fluence levels.

By fitting the biexciton lifetime to the first 350 ps of these decays to preserve the

five times the lifetime fitting range as used before, R can be plotted against the fractional

change in transmission as before where the correction factor for this dot is found to be 1.01.

These plots along with the fit of equation 2.18 are shown in figure 4.20 a) – e) and the

extracted quantum efficiency of MEG is plotted as a function of hυ/Eg in f). As can be seen

in f) there is no trend for the exciton multiplicity as the photon energy is increased over the

same wavelength range as for the large core sample. In particular at the photon energy of

5.2 eV the value of R(J→0) is found to be 1.03 ± 0.02 which is significantly smaller than

the value of 1.18 ± 0.03 found for the large core NQD at the same absolute photon energy.

This further supports the model that MEG is dependent upon the multiple of the band gap

represented by the photon energy and not upon absolute photon energy.



0.1 1 10

1.0

1.2

1.4

1.6

R

∆T/T (x 10-3)

1.7 Eg

0.1 1 10

1.0

1.2

1.4

1.6R

∆T/T (x 10-3)

1.1 Eg

0.1 1 10

1.0

1.2

1.4

1.6

R

∆T/T (x 10-3)

1.3 Eg

f)e)

a)

c)

b)

1.0 1.2 1.4 1.6 1.8 2.0 2.21.00

1.05

1.10

1.15

1.20

1.25

Neh

p, A

vera

ge e

xcito

n m

ultip

licity


Small core InP

0.1 1 10

1.0

1.2

1.4

1.6

R

∆T/T (x 10-3)

2.1 Eg

0.1 1 10

1.0

1.2

1.4

1.6

R

∆T/T (x 10-3)

2.0 Eg

d)

Figure 4.20 Plots of average number of excitons per photo excited small core NQD, R, as a

function of fractional change in transmission for pump photon energies a) 1.1 b) 1.3 c) 1.7

d) 2.0 and e) 2.1 times Eg. The average exciton multiplicity as a function of hυ/Eg is also

shown in f).



Figure 4.21 shows plots of quantum efficiency of MEG for all three sizes of dot as a

function of hυ/Eg. It is clear to see that that the quantum efficiency of MEG in the three

NQDs depends upon hυ/Eg and not on photon energy alone. The threshold found for the

large core dot of hυth = (2.1 ± 0.2)Eg is within experimental error of the threshold predicted

by both the energy partition model, which predicted a threshold for MEG of 2.1Eg, and that

predicted by the conservation of energy alone, 2Eg. Therefore this result cannot distinguish

between the energy partition model and the energy conservation model as it is consistent

with both. A low threshold has been shown to be a clear advantage [5] for solar cells

utilising MEG and is much lower than the 3Eg threshold that has been found for the Pb

chalcogenide NQDs [20]. The slope efficiency found here is approximately 3 times lower

than that observed for Pb chalcogenide and CdSe NQDs. As the figure of merit for these

three materials was calculated to be similar [4] this result is unexpected and may warrant

further studies.

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6100

105

110

115

120

125

Large core Medium core Small core

Qua

ntum

effi

cien

cy o

f ME

G (

%)

Photon energy/ Band Gap

Figure 4.21 Plot of quantum efficiency as a function of hυ/Eg for all three sizes of InP NQD.

The linear fit to the large core sample data points above threshold is shown in red.



4.5 Multiexciton dynamics of type II structures

The band gap of the NQD is another parameter that is critical to the enhancement of

solar cell efficiency by MEG. The band gap required to maximally exploit the solar

spectrum has been calculated to be ~ 0.9 – 1.1 eV [21]; in comparison the bulk band gap of

InP is 1.34 eV and the value for InP NQD is typically in the range 1.5 – 2.0 eV [1].

However, the effective band gap of a NQD can be reduced by the use of type II core/shell

structure, in which the electron and hole are localised within the core and shell respectively

(or vice versa). The band gap is then the difference between the top of the core valence

band and the bottom of the shell conduction band (or vice versa); in practice, the effective

band gap can be tuned not only by choice of core and shell material, and core size but also

by shell thickness [8]. This type of structure (figure 4.22) also has the advantage in that the

spatial separation of the electron and hole that it produces increases the lifetime of both

biexcitons and single excitons thereby increasing the probability that photogenerated

carriers may be extracted from the NQD before recombination [9].

Type I

CdSe

CdS

CdSe

CdTe

Type II

CdSe

CdTe

CdS

Type I

CdSe

CdS

CdSe

CdTe

Type II

CdSe

CdTe

CdS

Figure 4.22 Diagram showing the structure of the type II dot received from the school of

Chemistry and the energy level alignment of type I and type II systems as a function of

distance from the centre of the NQD.



4.5.1 Sample Information

Investigations into type II structures were conducted on the cadmium-based type II

nanoparticles available from the School of Chemistry at the University of Manchester. The

type II sample received had a CdSe core shelled with CdTe and CdS. With this structure the

CdSe/CdTe interface acts as a type II heterojunction and the CdS shell acts as a type I

heterojunction so as to keep the charges away from any surface traps. A type I structure

was also received so as to be able to compare between type I and type II systems made in a

similar way and had a CdSe core with a CdS shell. Both NQDs were passivated by

trioctylphosphine (TOP), dispersed in hexane and placed in sealed screw top cuvettes as the

type II dots have been found to degrade rapidly when exposed to air. The band alignment of

these materials means that in the type II dots it is possible to make NQDs that absorb and

emit in the near infrared.


Figure 4.23 a) and b) shows the photoluminescence and absorption data for both the

type I and type II dots. The PL was taken as before on the Fluorolog spectrometer with

excitation at a photon energy of 3.1 eV (400 nm) and the PL peak wavelength was found to

be ~ 660 nm (~ 1.88 eV) for the type I dot with a FWHM of 34 nm and ~ 740 nm ( ~1.68

eV) for the type II dot with a FWHM of 60 nm. The absorption edge for the type I dot is

found at 630 nm which means it has a band gap of ~1.97 eV and the absorption edge of the

type II dot is found to be approximately 680 nm indicating an effective band gap for this

sample of 1.82 eV. The discrete features in the type I dot reveal the good size dispersion

and electronic transitions occurring in this dot whereas in the type II dot absorption

spectrum a smearing out of the excitonic features is observed. This smearing out of the

optical transitions is due to a reduction in the oscillator strengths of the individual

transitions and is consistent with the transition to the type II regime as has been found

previously [8].



1.50 1.75 2.00 2.25 2.50

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5


Nor

mal

ised

Inte

nsity

(ar

b. u

nits

)

Photon energy (eV)

850 800 750 700 650 600 550 500

Type II CdSe/CdTe/CdS

Wavelength (nm)

1.75 2.00 2.25 2.50 2.75

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5


Nor

mal

ised

Inte

nsity

(ar

b. u

nits

)

Photon energy (eV)

750 700 650 600 550 500 Wavelength (nm)

Type I CdSe/CdS

b)a)

Figure 4.23 Photoluminescence and absorption spectra for the a) type I CdSe/CdS and b)

type II CdSe/CdTe/CdS NQDs.

4.5.3 TCSPC on type I and II dots

The single exciton dynamics were investigated using the same TCSPC set-up as for

the InP NQDs and as is described in section 3.2.5.1. As before a pulsed diode laser at 405

nm is used as the excitation source and combined with the 0.5 m monochromator and

Hamamatsu PMT gave an IRF with FWHM of ~ 400 ps. The monochromator is set to 660

and 740 nm respectively for the type I and type II dots to monitor the fluorescence at the

peak of their photoluminescence spectra. Survey scans on the two dots revealed that the

type I sample decayed to the background count level in ~ 300 ns and the type II dots

decayed to the background level in ~ 500 ns. The pulse repetition rate for the laser was thus

set to 400 kHz so as to maintain 5 multiples of the full decay time period between each

laser pulse. Figure 4.24 a) and b) show the time decays recorded for the type I and type II

dots respectively with the fits to these shown in red. Investigation by eye of the two decays

shows that the Type II dot has a longer lifetime than the type I dot which is consistent with

the NQD causing spatial separation of the electron and hole. Both decays can be well-

described using bi-exponential fits yielding time constants of 6.1 ± 0.1 ns with 15 %

relative intensity and 31.9 ± 0.2 ns with 85 % relative intensity for the type I dot, and 4.3 ±

0.1 ns (13 %) and 41.3 ± 0.2 ns (87%) for the type II dot.



0 50 100 1501

10

100

1000

Inte

nsity

(A

rb. u

nits

)

Time (ns)

Type II time decay Fit to decay

0 50 100 1501

10

100

1000

In

tens

ity (

Arb

. uni

ts)

Time (ns)

Type I time decay Fit to decay

b)a)

Figure 4.24 showing the time decays taken for the a) type I CdSe/CdS and the b) type II

CdSe/CdTe/CdS NQDs with fits to both shown in red.

4.5.4 Multiexcitons in type II NQDs

To observe the dynamics of multiple excitons in the type II NQDs the transient

absorption experiment can be used in a similar way to that used for investigating MEG in

the InP NQDs. The white light continuum is optimised and the monochromator set to

observe at the effective band edge of the type II dots. The effective band gap of the type II

dots at 1.8 eV meant that the shortest UV wavelength available from the TOPAS-C of 240

nm will lead to excitation at 2.8Eg. This is above the threshold for MEG calculated for a

CdSe dot of ~ 2.3Eg from equation 2.12 and that measured as 2.5Eg [22], however, it is not

yet known how the type II structure may affect this. Transients are first taken for a low

energy pump photon of 1.5Eg (450 nm) to allow the biexciton lifetime to be measured

before pumping with high energy photons to produce and measure MEG. These transients

are shown in figure 4.25 and are the average of 10 scans of the delay stage taken at the

different fluence levels. Immediately obvious from the transients is the large amplitude at

short times and that the decay does not reach a plateau as in the InP NQDs. Fitting two

exponential decays to these transients yields a very good fit with two time constants of 37 ±

1 ps and 339 ± 14 ps. Taking the ratio of the short time component amplitude to the long

time component amplitude (R) as before suggests that on average 4 fluence created excitons

were present in the dot. As the absorbed fluence range used here is similar to that used in

other dots there is doubt as to whether these decay signatures correspond to the Auger

decay of multiexcitons.



0 200 400 600 800 1000 1200

0

5

10

15

20

25

30

∆T/T

(x

10-3)

Time (ps)

Increasing fluence

Type II CdSe/CdTe/CdSPump photon energy = 2.75 eV

Figure 4.25 Absorption transients taken of the type II CdSe/CdTe/CdS NQDs with the

pump at 450 nm corresponding to excitation at 1.5Eg

4.5.5 Discussion

It is hard to confidently attribute the results found here to any particular process and

only limited conclusions can be drawn from these data. It was also not possible to observe

the way the dynamics changed for higher energy pump photons as at these wavelengths the

shape of the transient changed in sequential scans. Also, there was visible degradation of

the sample at these energies in the form of markings on the cuvette where the pump laser

had been incident. For the transients taken here one possible scenario is that the fluence of

the laser is high enough for this dot to allow 4 photons to be absorbed leading to the

creation of 4 excitons. The multiexcitons would then sequentially decay through Auger

recombination which being dependent upon carrier density will have discrete step-like rates

(see section 2.2.2). The ~ 30 ps time constant would then be due to the Auger

recombination of one multiexciton and the ~ 300 ps time constant would be due to the

Auger recombination of the other. If this ~ 300 ps time constant is due to the decay of

biexcitons its lifetime is approximately double that found in type I CdSe structures

previously by others [23]. This lengthening of the Auger recombination time would be

consistent with the spatial separation of the electron and hole in the type II dot. A doubling



of the biexciton lifetime when compared to type I NQDs is similar to what has been found

when comparing other type I and II systems [24]. Other scenarios are possible however,

such as the formation of a trion in a significant population of the NQDs. A trion contains

two electrons and a hole (or two holes and an electron) which decay to a single electron

[25]. The formation of fluence-created multiexcitons is expected to scale with the pump

intensity according to their Poisson probability where !/)exp()( 00 mNNmPm −= .

Plotting the amplitudes of the fast and slow components for the decays as a function of

fractional change in transmission (proportional to0N ) is not found to follow this trend

(figure 4.26). The spread of the results makes it difficult to extract a trend but neither the

fast nor slow decay amplitude seem to follow the superlinear or sublinear growth expected

from multiple or single excitons. This would seem to indicate that the fast and slow decay

components here are not likely to be due to the Auger recombination of multiexcitons. The

fast component could actually be the decay of a trion to a single electron and the long time

constant would then be caused by the reduced absorption that results due to state filling by

this electron, also possible is that the two decay components are actually the decay of

negative and positive trions which have been shown to have different decay rates [26].

More work is required to further investigate the charge dynamics observed here; this will

certainly require more stable type II NQDs.

0 1 2 3 4 5

0.000

0.005

0.010

0.015

Fast decay Slow decay

As, A

f am

plitu

des

(arb

. uni

ts)

∆T/T (x 10-3)

Figure 4.26 Amplitudes of the fast (~ 30ps) and slow (~300 ps) time components plotted as

a function of fractional change in transmission.



4.6 MEG studies on “green” PbS NQDs

Hitherto there has been a great deal of theoretical and experimental work conducted

on lead chalcogenide NQDs with numerous demonstrations of carrier multiplication [6, 12].

Although an absence of MEG in Pb chalcogenide NQDs has never been reported there have

been contentious results claiming the MEG process in these dots to be less efficient than

previously measured [17, 19]. The interest in the Pb chalcogenides stems from them being

good candidates for use in photovoltaic devices. As well as being cheap and abundant

materials, their band gap is tuneable across much of the NIR spectrum allowing them to

make optimal use of the solar radiation spectrum.

4.6.1 Sample Information

The PbS NQDs studied here were again received from the School of Chemistry at

the University of Manchester. These NQDs were fabricated using a novel “green” process

that does not use the expensive, toxic solvents and passivating ligands (TOPO) usually used

to produce Pb chalcogenides and replaces both with cheap and environmentally benign

olive oil [27]. This method has led to PbS NQDs being produced that maintain good size

dispersions, have band gaps between 0.88 and 1.72 eV yet are more environmentally

friendly than the Pb chalcogenides normally produced. Band gaps in this range will allow

for excitation at many multiples of the band gap with the lowest pump wavelength available.

The samples were dispersed in hexane and placed in a UV quartz cuvette.


The absorption spectrum was taken for these NQDs as before using the Lambda

1050 spectrophotometer (figure 4.27). It was not possible to take the photoluminescence

spectra of these NQDs as the wavelength range of the PMT of the Jobin Yvon Fluorolog

spectrometer only extends as far as 850 nm. The band gap energy was taken from the

absorption edge which was found to be at ~ 1.28 eV. The threshold for MEG in PbS

according to the energy partition model calculated from equation 2.12 is ~ 3Eg due to the

fact that the effective mass of the electron and hole in PbS are approximately the same. It

was not possible to take the single exciton photoluminescence decay as the available set-up

also used a PMT with a spectral sensitivity only up to 850 nm. Fluorescent lifetimes taken

of similar PbS dots but of a size such that they emit below 850 nm were found to have



single exciton lifetimes on the order of microseconds as found previously for PbS NQDs

[28, 29].

0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.500.0

0.5

1.0

1.5

2.0

2.5

Abs

orba

nce

(O.D

.)

Photon energy (eV)

PbS NQD with oleic acid passivation

Figure 4.27 Absorption spectrum of the PbS NQDs dispersed in hexane. The first exciton

maximum is at ~ 1.3 eV.

4.6.3 MEG studies on PbS

Transient absorption spectroscopy was conducted on these dots as before but this

time a different range of photon energies were used so as to excite both above and below

the threshold (~ 3Eg). The sample was pumped using the output of the TOPAS-C at photon

energies of ~ 2.5, 4.0, 4.5, and 5.2 eV which corresponds to pumping at ~ 1.9, 3.1, 3.5, and

4.0 times the band gap. The white light continuum is optimised for the near IR and the

monochromator is set to monitor the band edge of the PbS NQD as found from the

absorption spectra. The samples are stirred at 1000 rpm as for the other NQDs. In these PbS

NQDs we observe that for excitation below the threshold for MEG (1.9Eg) all fluences

yield transients which are flat across the entire time range. When exciting above the

threshold for MEG the emergence of a fast time component that persists in the limit of

vanishing fluence is found, and is thus attributed to the decay of biexcitons created through

MEG. Figure 4.28 shows absorption transients taken at approximately the same fluence but

for pump photon energies that are above and below the threshold for MEG. From this we

see a fast time component with a significant amplitude that is completely absent for

excitation below the threshold for MEG.



0 100 200 300 400 500 600

0.0

0.5

1.0

1.5

2.0

∆T/T

(x

10-3)

Time (ps)

2.5 eV (1.9 x Eg)

4.5 eV (3.5 x Eg)

Figure 4.28 Absorption transients taken at approximately the same fluence but for

excitation above (red) and below (black) the threshold for MEG in PbS (~ 3Eg).

An exponential fit to this fast time component yields a biexciton lifetime of 38 ± 4

ps and the analysis of this components amplitude at different fluences for a range of

wavelengths allow us to extract R as before. The plots of R against fractional change in

transmission are shown in figure 4.29 a) – d) with the fits of equation 2.18 shown in red. In

this PbS NQD we find that for excitation with photon energies that are < 3Eg in the limit of

vanishing fluence the average number of electron hole pairs tends to one. The maximum

quantum yield of MEG measured was 138 ± 4 % and was for hυ = 4Eg corresponding to the

shortest wavelength used.



d)

e)

c)

b)

1.5 2.0 2.5 3.0 3.5 4.0

0.9

1.0

1.1

1.2

1.3

1.4

1.5

Ne

hp, A

vera

ge e

xcito

n m

ultip

licty


0.1 1 10

0.8

1.0

1.2

1.4

1.6

1.8

R

∆T/T (x 10-3)

3.5 Eg

0.1 1 101.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

R

∆T/T (x 10-3)

4 Eg

0.1 1 100.8

1.0

1.2

1.4

1.6

1.8

R

∆T/T (x 10-3)

3.1 Eg

0.1 1 10

0.6

0.8

1.0

1.2

1.4

1.6

1.8

R

∆T/T (x 10-3)

1.9 Eg

a)

Figure 4.29 Plots of average number of excitons per photo-excited PbS NQD, R, as a

function of fractional change in transmission for pump photon energies a) 1.9 b) 3.1 c) 3.5

and d) 4 times Eg. The average exciton multiplicity as a function of hυ/Eg is also shown in

e).



4.6.4 Discussion

The result presented here clearly demonstrates MEG occurring in PbS quantum dots

with a threshold of 3Eg; this is consistent with the value calculated from equation 2.12 and

so with the energy partition model. Unlike the InP NQDs a clear distinction can be made;

this threshold is not consistent with the value of 2Eg as would be expected when taking into

account only energy conservation. These measurements are also interesting in respect to the

wider debate on MEG and specifically, Pb chalcogenide NQDs, which has been stimulated

by experiments conducted on bulk PbSe and PbS by Pijpers et al. [30]. These

measurements on bulk PbSe and PbS have shown that the average exciton multiplicity is

larger in bulk for a given photon energy than that found in NQDs. At first glance this result

may appear to demonstrate that as the number of electron hole pairs generated in the bulk

material for a given photon energy is higher than that found in NQDs there is no reason to

use NQDs in photovoltaics utilising the MEG process. Plotting the MEG efficiency as a

function of photon energy is interesting from a fundamental physics perspective. As MEG

can be thought of as a competition between impact ionisation and other carrier relaxation

processes this demonstrates that the excess energy of the charge carriers is critical (figure

4.30a). This approach, however, does not take into account the band gap of the different

materials which is an important consideration for PV applications. The larger band gap in

the NQDs as compared with the bulk means that the band edge excitons are higher in

energy in the NQDs than those in bulk. Normalising the excitation photon energy by the

band gap reverses the trend seen in figure 4.30 a) and as is shown in b) the average exciton

multiplicity is higher in NQDs for lower hυ/Eg. Similar arguments to these have been

suggested previously [19, 31] and reveal that a greater proportion of the photon energy is

used in MEG in NQDs and that a smaller amount is wasted as heat than in the bulk scenario.

This ratio between the amount of energy converted into excitons and the amount wasted as

heat is the most important when considering application of MEG to solar cells. These

considerations are also supported in reference 31 where PbSe NQDs are shown to have a

higher exciton multiplicity and a lower threshold for MEG than bulk silicon when tuned to

have the same band gap (1.2 eV).



0 1 2 3 4 50.5

1.0

1.5

2.0

2.5

3.0

3.5

Neh

p, A

vera

ge e

xcito

n m

ultip

licity

photon energy (eV)

Bulk PbS Nanocrystal PbS

a)

1 2 3 4 5 6 7 8 9 10 11 120.5

1.0

1.5

2.0

2.5

3.0

3.5

Neh

p, A

vera

ge e

xcito

n m

ultip

licity


Bulk PbS Nanocrystal PbS

b)

Figure 4.30 The average exciton multiplicity as measured in the PbS NQDs in Manchester

() and the values measured for bulk PbS (Eg = 0.42 eV) in reference 30 () plotted as a

function of a) absolute photon energy and b) band gap normalised photon energy.



References

1. Klimov, V.I., ed. Semiconductor and Metal Nanocrystals: Synthesis and Electronic and optical properties. Optical Engineering, ed. B.J. Thompson. 2004, Marcel Dekker.

2. Ben-Lulu, M., et al. Nano Letters, 2008. 8(4): p. 1207-1211. 3. Nair, G. and M.G. Bawendi. Physical Review B (Condensed Matter and Materials

Physics), 2007. 76(8): p. 081304-4. 4. Luo, J.-W., A. Franceschetti, and A. Zunger. Nano Letters, 2008. 8(10): p. 3174-

3181. 5. Hanna, M.C. and A.J. Nozik. Journal of Applied Physics, 2006. 100(7): p. 074510-8. 6. Ellingson, R., et al. Nano letters, 2005. 5(5): p. 865-871. 7. Hoshino, A., et al. Nano Letters, 2004. 4(11): p. 2163-2169. 8. Jack Li, J., et al. Chemical Physics, 2005. 318(1-2): p. 82-90. 9. Oron, D., M. Kazes, and U. Banin. Physical Review B (Condensed Matter and

Materials Physics), 2007. 75(3): p. 035330-7. 10. Stubbs, S.K., et al. Physical Review B, 2010. 81(8): p. 081303. 11. Klimov, V.V.I. The Journal of Physical Chemistry. B, Materials, surfaces,

interfaces & biophysical, 2000. 104(26): p. 6112-23. 12. Schaller, R.D., M.A. Petruska, and V.I. Klimov. Applied Physics Letters, 2005.

87(25): p. 253102-3. 13. Schaller, R.D., et al. J. Phys. Chem. B, 2006. 110(50): p. 25332-25338. 14. Micic, O.I., et al. Applied Physics Letters, 1996. 68(22): p. 3150-3152. 15. Ellingson, R.J., et al. J. Phys. Chem. B, 2002. 106(32): p. 7758-7765. 16. Nozik, A.J. Chemical Physics Letters, 2008. 457(1-3): p. 3-11. 17. McGuire, J.A., et al. Accounts of Chemical Research, 2008. 41(12): p. 1810-1819. 18. Sukhovatkin, V., et al. Science, 2009. 324(5934): p. 1542-1544. 19. Nair, G., et al. Physical Review B, 2008. 78(12): p. 125325. 20. Schaller, R.R.D. Nano letters, 2006. 6(3): p. 424-9. 21. Shockley, W. and H.J. Queisser. Journal of Applied Physics, 1961. 32(3): p. 510-

519. 22. M.C. Beard and R.J. Ellingson. Laser & Photonics Review, 2008. 2(5): p. 377-399. 23. Klimov, V.I., et al. Science, 2000. 287(5455): p. 1011-1013. 24. Nanda, J., et al. The Journal of Physical Chemistry C, 2007. 111(42): p. 15382-

15390. 25. Pandey, A. and P. Guyot-Sionnest. The Journal of Chemical Physics, 2007.

127(11): p. 111104-4. 26. Califano, M. ACS Nano, 2009. 3(9): p. 2706-2714. 27. Akhtar, J., et al. Journal of Materials Chemistry, 2010. 20(12): p. 2336-2344. 28. Warner, J.J.H. Nanotechnology, 2005. 16(2): p. 175-9. 29. Rogach, A.L., et al. Small, 2007. 3(4): p. 536-557. 30. Pijpers et al., Nature Physics, 2009. 5(11): p. 811-814. 31. Delerue, C., et al. Physical Review B, 2010. 81(12): p. 125306.

Chapter 5 Methods and techniques in hybrid QD-OLEDs


Chapter 5: Methods and techniques in hybrid quantum dot light emitting devices.

5.1. Scope and aims of project

The scope and aims of this project initially took the form of a feasibility study

looking into whether Nanoco’s proprietary NQDs could be incorporated into hybrid

quantum dot organic light-emitting devices (QD-OLEDs) for use in display technology. All

device fabrication in this work was conducted at Nanoco’s research laboratory in

Manchester. In time the relative successes and failures as well as the acquisition of

knowledge, experience and skill in device manufacture allowed the project to move on

from this preliminary stage. As device performance improved, test and measurement

equipment was purchased to enable precise characterisation of devices. In time a dedicated

set-up for the production and testing of QD-OLEDs was bought which allowed devices to

be made and tested in clean and inert atmospheres using nitrogen filled glove boxes. At first

the wide range of high quality cadmium-containing NQDs available were used to build up

the knowledge and experience required to produce QD-OLEDs. As electroluminescence

had already been demonstrated using CdSe and because of the high quality and range of

dots available from Nanoco, CdSe NQDs were well suited for this stage of the project.

Ultimately however, the toxicity of heavy metal containing NQDs [1] as described in

chapter 1 means that many countries around the world ban their use in commercial products.

To be used in display technology these hybrid QD-OLEDs must make use of heavy-metal-

free NQDs. Therefore the knowledge gained in the production of CdSe QD-OLEDs is used

to make devices utilising the novel InP NQD systems available from Nanoco.

5.2. Methods and techniques for production and testing

The methods and techniques described here for the production and testing of QD-

OLEDs are appropriate for research purposes making small batches of QD-OLEDs. Spin

coating and thermal evaporation are the primary techniques used to deposit the thin films

required. These would not be appropriate for large scale manufacture of QD-OLEDs or

actual displays; spin coating on a large scale would be wasteful of material and thermal

evaporation would be costly. Printing techniques would be necessary for large scale

manufacture of displays; printing technology is currently at a very advanced stage so when



both technologies are ripe they should be easily combined. These however, are not the

focus of the current project and so are not mentioned further.

5.2.1. Spin coating

Spin coating has become an important technique used in research since the first

observations that viscous liquids placed on a rotating disk could be used to produce thin

films of uniform thickness [2]. For the QD-OLED design used here a combination of

organic and non-organic materials is used to make up the hole and electron transporting

layers. The organic materials can be dissolved in various solvents to form solutions of

various concentrations and viscosities. Spin coating is a simple, cheap and effective way of

forming thin films from solutions and so is used when depositing the organic and quantum

dot layers. The general process involves dispensing an excess of some viscous solution

onto the centre of a substrate which is then spun at high velocity such that centripetal forces

cause the liquid to spread across the surface. The majority of the solution will be flung off

the side but a thin film will be left behind whose thickness will depend upon parameters

such as spin speed, viscosity and volatility of the liquid as well as environmental factors.

Four stages in the spin coating process can be identified during which different processes

dominate. Clearly the first stage involves the dispensing of the actual fluid. This can be via

a static dispense or a dynamic dispense. Static dispense is simply the depositing of a large

excess of the material in the centre of the substrate prior to spinning. Dynamic dispense

involves dispensing the solution whilst the substrate is spun at a low spin speed which will

act to spread the fluid over the surface of the substrate. It is important that the solution fully

wets the substrate to prevent incomplete coverage or non-uniformity and so dynamic

dispense can help with fluid or substrates that do not have good wetting characteristics.

The next step is when the substrate is accelerated up to the spin speed set by the

operator. The majority of the solution will be flung off the sides during this stage and the

fluid will begin to dry. The degree of acceleration can have a large effect upon the final

film quality. Excessive forces between the now rotating substrate and the top surface of the

solution can cause a swirling pattern for example, but some degree of twisting is necessary

to coat all parts of the substrate. The third stage is when the substrate is spinning with a

constant velocity by which point the fluid has formed a thin film and is co-rotating with the

substrate. There will be a slow thinning of the film during this stage which is dominated by



the viscous forces of the fluid. This will eventually lead to the fourth stage whereby the

dynamics of the film become dominated by the solvent evaporation rate. As the fluid dries

it will become more viscous until the centripetal force is balanced by the viscous force and

the fluid can no longer move across the surface. At the edges of the substrate the film can

often be found to be thicker and non-uniform. This is a result of the fluid flowing out to the

edges where it must form droplets before being thrown off. Thus, the outer edge of the

substrate will be thicker and as square substrates are used here the airflow will be slightly

different over the corners causing non-uniformity; the edge region can not be used for

device fabrication for these reasons.

Substrate

Vacuum chuck

Fluid dispenser

Substrate

Vacuum chuck

Fluid dispenser

Figure 5.1 Diagram showing general design of spin coater systems.

The final thickness and quality of the film is dependent upon the properties of the

fluid such as viscosity, solvent volatility, surface tension and on the spin process such as

acceleration, final spin speed, and spin time. Therefore for each material it is important to

carry out a spin trial where factors such as the concentration (viscosity), solvent etc. are

kept constant and only the spin parameters are altered to find the best spin conditions. The

general trend for final film thickness, d, is that as spin speed, ω, increases the final film

thickness will decrease according to a power law md −∝ ω where m is between -1 and -0.5

as found by Sukanek [3]. A number of spin coaters were used as the project progressed; a

Chemat KW-4A spin coater was used for the initial device builds constructed in air, higher

end programmable SCS (Speciality Coating Systems) G3P-8 spin coaters were then used in

a glove box environment for subsequent device builds. The higher standard SCS spin

coaters give better spin accuracy and repeatability as well as greater spin ranges and more

flexible programming capabilities. The deposition of different layers via spin coating can



only be achieved if the consecutive layers are deposited from solvents that will not dissolve

or compromise the layer underneath. In practice this can be done by using solvents with

different polarities for the alternate layers but care must be taken as some materials can be

partially dissolved due to the relative polarity of the solvents used.

5.2.2.Thermal vacuum evaporation

The emitting layer of quantum dots are spun down from an organic solvent as

described in the previous sections to form a layer of close-packed but loosely-bound NQDs.

The efficiency of these devices has been shown to be dependent upon forming uniform and

smooth layers particularly for the emitting NQD layer [4]. The delicate nature of the NQD

layer means that further solvent deposition by spin coating would damage and deteriorate it

and reduce the device performance. For this reason the electron-transporting materials and

the metal cathode are deposited by thermal vacuum evaporation. Thermal vacuum

evaporation is a physical vapour deposition (PVD) process used to make electrically-

conductive films, mirror coatings and optical interference coatings. All PVD processes

allow controlled atomistic deposition of materials by vaporising solids and liquids in

vacuum which are then transported to the substrate where they condense [5]. Thermal

vacuum evaporation (often referred to as just vacuum evaporation) specifically entails the

use of a vacuum chamber which is pumped down to low pressures (typically 10-5 – 10-8

mbar) and a thermal vaporisation source, such as a heated tungsten boat or wire. The heated

boat or wire creates a vapour of the material which travels directly to the substrate. The

vapour pressure of the material can be controlled through the amount of heating until a

deposition rate of several ångström per second is achieved. Rotary and diffusion pumps are

often used together in vacuum evaporation set-ups to achieve the relatively high vacuums

required. The level of vacuum can impact on the quality of the film as gaseous

contaminants could collide with the material vapour causing non-uniformity of the film or

reacting with them to form unwanted products. The vacuum also acts to increase the mean

free path of the evaporated molecules such that they have a direct path to the substrate.

A number of different vacuum evaporators were used as the project progressed;

initially a reconditioned Edwards 306 thin film deposition system was used (figure 5.2).

This system uses a glass bell jar as the vacuum chamber and the combination of a rotary

and turbo molecular pump can pump the bell jar down to the 10-8 mbar level. A water



cooling system and a liquid nitrogen trap on the high vacuum pipeline are used to improve

vacuum level and reduce pump-down times considerably. A Pirani gauge is used to

measure the higher pressures in the roughing pipeline and a Penning gauge is used to

measure the pressure once it falls below ~ 10 -3 mbar. The materials are vaporised using

resistive heating by passing high currents through tungsten wire bent into baskets or spirals

or through the use of a ceramic crucible with a tungsten wire spiralled around it. Early on in

the QD-OLED development described here a wire basket was used, however molten

materials such as aluminium would flow and drip off the wire and the possibility of

contaminants on the tungsten (oxides etc.) led us to switch to the crucible and wire design.

In general the current is slowly increased with a mechanical shutter covering the substrate

as contaminants on the filaments and impurities in the source material are the first to

evaporate as these are mostly on the surface of the materials and filaments. Only once there

is significant vapour flux is the shutter opened to deposit the film. To deposit films in

defined areas shadow masks can be used. The design of these began with a simple design

that held a microscope slide with a hole down the middle to deposit a strip of aluminium.

The masks became more complex later with different masks required for the electron

transport layer (ETL) and the aluminium cathode; these masks were made from very thin

layers of aluminium and cut precisely via laser cutting to obtain the best quality masks

possible.

The deposition rate is another important parameter in depositing high quality thin

films. The deposition rate and final film thickness can be monitored using a film thickness

monitor (FTM) which commonly consists of a quartz crystal. Quartz is a piezoelectric

material and can be made to resonate by applying a suitably modulated voltage. This

resonant frequency will depend upon factors such as crystal orientation and thickness. The

change in resonant frequency is directly proportional to the mass added to the crystal and so

by monitoring this change the layer thickness can be found. This will still require

calibration and will also depend upon the density of the material being deposited. By

measuring the thickness independently (using profilometry or AFM) and comparing this

with the value measured by the FTM a “tooling” factor can be applied so that the FTM

displays the actual thickness deposited.



Figure 5.2 Picture showing the Edwards 306 thin film coater showing bell jar with

deposition set-up inside.

There are several issues related to the deposition system above for use in QD-

OLEDs. The first is that to deposit different layers the vacuum must be broken exposing the

previously deposited layers to oxygen and moisture. Ideally the entire QD-OLED would be

made in an inert nitrogen-filled atmosphere and so once the project advanced all device

fabrication was done in the glove box environment and an Edwards Auto 500 evaporator

built into a glove box was used. This gives a number of advantages, the first being a four

source turret allowing four different materials to be deposited in sequence without the need

to break the vacuum improving device quality and increasing throughput. A rotating

substrate holder also increases film quality and uniformity by preventing geometrical

shadowing whereby raised and lowered features in the underlying film or in the shadow

mask will cause thickness variation of the film. A source and substrate shutter gives

optimal control as the deposition rate can be finely tuned before exposing the substrate.

Finally a programmable deposition computer and feedback loop on the heater element gives

a constant vapour flux by dynamically changing the applied current to keep the deposition

rate constant.



5.2.3.Profilometry

The thickness and quality of the spun or evaporated films have been shown to have

a large impact on the efficiency of the final QD-OLED [6]. A stylus profiler scans a sharp

silicon tip in contact with a surface in order to detect information about its topography. The

scan speed, length and stylus force can be appropriately selected for the material being

measured and as the high precision gantry is scanned any surface variations will cause a

vertical movement of the tip. In our case the stylus profiler is mainly used to measure the

thickness of a layer in order to conduct a spin trial on a new material or to calibrate the film

thickness monitor in vacuum evaporation. In this project a Veeco, Dektak 8 surface profiler

allowed for quick measurement of step heights as well as 3D plots of surfaces. It was

mainly used to measure the thickness of the hard and relatively smooth polymers that make

up the hole injection (HIL) and hole transport layers (HTL) as well as the evaporated ETL

and metal cathode layers. A material is deposited onto smooth and polished glass slides

specifically for spin and evaporator trials and a scratch is made in the film using a fine

needle. Scanning the stylus over this feature will then reveal the total thickness of the

deposited layer. An example of the 2D plots used for thickness measurement is shown in

figure 5.3 where a scratch made in a layer of the hole injection material PEDOT:PSS has

been scanned.

0 100 200 300 400 500-5

0

5

10

15

20

25

30

35

Ver

tical

dis

tanc

e (n

m)

Horizontal distance (µm)

Figure 5.3 2D scan using the dektak 8 surface profilometer of a scratch made using a needle

in the polymer PEDOT:PSS used as the HIL. The thickness is found to be ~ 25 nm.



5.2.4.Atomic Force Microscopy (AFM)

As mentioned previously, the layer of NQDs forms a layer that is only loosely

bound to the substrate. As such the use of profilometry to measure the thickness or surface

roughness of the NQDs was not possible as its contact method would scratch through the

NQD layer. Scanning probe microscopy (SPM) techniques have been successful in

resolving the topography of surfaces with nanoscale resolution for many years now. The

first SPM technique was developed in 1981 [7] and is known as scanning tunnelling

microscopy (STM). The drawback to this technique was that it required the surface being

investigated to be electrically conductive to allow a tunnel current to be measured. AFM

uses direct contact with the sample and precise measurement of forces on the tip to build up

a picture of the surface. There are a number of modes possible in AFM and the general

principle is the same for them all; a tip on the end of a cantilever experiences forces as it is

scanned across the sample surface. The cantilever will bend in response to the attractive

and repulsive forces it experiences as the surface topography changes. To detect these

changes an optical system using a laser reflected off the cantilever (figure 5.4) and incident

upon a position-sensitive photodiode is used. Any change in the cantilever position will be

amplified in the deflection of the laser beam on the photodiode allowing very sensitive

detection.

Lasermirror

Position sensitivephoto detectorPiezo scanner

SurfaceCantilever

Lasermirror

Position sensitivephoto detectorPiezo scanner

SurfaceCantilever

Figure 5.4 General AFM set-up showing optical detection scheme.



Contact mode AFM has a limitation in that as the tip is scanned, lateral and

adhesive forces between the tip and the surface can drag material in samples that are easily

damaged or loosely held by the substrate. In tapping mode the cantilever is made to

oscillate at its resonant frequency using a piezo stack causing the laser beam to trace out a

regular pattern on the photo diode and so generate a sinusoidal signal. The cantilever is then

moved in so that it just taps the surface; the forces experienced by the tip would then cause

its oscillation amplitude to be reduced or increased. As the tip is scanned over features on

the surface the amplitude of oscillation is kept constant by altering the tip sample separation

through the use of a feedback loop. In this way accurate topographical information can be

extracted whilst maintaining sample integrity.

5.2.5.Characterisation techniques

The electroluminescence spectra of the devices at this stage and throughout the

project were measured using either an Ocean Optics USB 4000 which is a miniature fibre

optic spectrometer with a CCD array sensitive to wavelengths from 200 to 850 nm or using

the Fluorolog 4 system described in chapter 3 but with the excitation wavelength blocked

off. The EL spectra were often taken at a number of voltages or luminance values (such as

100 cd/m2) to observe how the emission changed. In some experiments the

photoluminescence of the different materials was taken when in solution as well as the thin

film photoluminescence (TFPL) as this could reveal information about where the emission

originated in the device; all PL and TFPL spectra were taken using the Fluorolog 4. Once

the QD-OLEDs were being made with repeatable and consistently high luminance a

common test procedure was adopted. This involved taking current-voltage-luminance (IVL)

measurements and using these to calculate the luminous efficiency. Current and voltage

measurements were taken using a Keithley 2400 IV source meter which can rapidly switch

between source voltage/measure current and vice versa. It can be programmed to quickly

step through a source voltage range whilst measuring current. It has a current compliance

function which is used here to protect the QD-OLEDs from destruction. In this mode the

source metre will source voltage but if the user-set current compliance value is exceeded

the source metre will automatically switch to a constant current source thus protecting the

device. The luminance of the QD-OLEDs was initially found by measuring the radiant flux

in watts using an Anritsu ML93A optical power metre which is sensitive from 0.38 to 1.8



µm and then converting this using the photopic function to luminous flux according to

equation 2.20 with units of lumens. The solid angle can also be calculated when the

detector with a defined area is at a certain distance and angle from the QD-OLED. The

angle here is kept at 90˚ and the detector is placed 2 cm away from the surface of the QD-

OLED. The luminance in cd/m2 (as well as other photometric quantities) can then be

calculated from the equation shown in table 2.1; a Matlab programme was written to

perform these calculations. In later stages, to ensure accurate and repeatable measurements,

as well as reducing the time taken for these measurements a Topcon BM-7 luminance

colorimeter was used. This was eventually incorporated with the IV source metre with both

controlled by Labview to take simultaneous IVL measurements. The colorimeter measures

the radiant flux in a user-defined field of view and will automatically convert this to give a

measure of luminance in cd/m2 as well as CIE colour coordinates.

5.2.6. Spin and evaporator trials

The EL spectrum, brightness (luminance), and efficiency can be greatly affected by

the thickness of the electron and hole transport layers as well as the thickness of the NQD

layer [8]. As the organic charge-transporting polymers and small molecule materials are

often efficient emitters by themselves the relative thickness of the different layers must be

optimised to obtain pure QD emission and high efficiency. As has been discussed the

thickness of the layers in spin coating are dependent upon many factors, the foremost of

which are spin speed and concentration. As making up lots of solutions would be time

consuming and wasteful it was decided to keep the concentrations (in mg/mL) constant for

the different materials and to vary the spin speed to vary the thickness. It was found that a

certain concentration would allow for a range of thicknesses to be reasonably achieved. At

this concentration spinning beyond a certain speed would not give a thinner film and

spinning too slowly would not yield thicker films of a high quality. In fact it was observed

that for spin speeds below ~ 1000 rpm a suitably uniform film could not always be formed.

By conducting spin trials it was possible to find a concentration that gave the desired

thickness range and to spin at a range of speeds to find the relationship between film

thickness and spin speed for this material. This was conducted on polished spin trial glass.

The surface roughness of the previous layer, however, will impact on the subsequent film

thickness. Therefore for layers deposited on top of previously-deposited layers (e.g. HTL),



the layer below was first characterised such that its layer thickness was known before

measuring the total thickness of the two layers to find the thickness of the overlying

material. The results of spin trials conducted on the hole transport polymers poly(9-

vinylcarbazole) (PVK) and poly(N,N’-bis(4-butylphenyl)-N,N’-bis(phenyl) benzidine

(poly-TPD) are shown in figures 5.5 a) and b) respectively. The two data sets in b) are for

different concentrations of poly-TPD dissolved in chlorobenzene, where it is found that a

more concentrated solution gives a thicker film for the same spin speed. The PVK layers

were also dissolved in chlorobenzene with a concentration of 15 mg/mL. Fits to the data

sets yield equations for the power law as shown where the thickness varies approximately

as ~ ω-0.5 which as explained in reference 3 agrees with a simple evaporation model

whereby the “wet” stage persists for much of the process and so thickness is dominated by

the evaporation rate. Spin trials on the NQDs found m to vary between -0.8 and -0.9 which

is indicative of the evaporation stage, where the film dries, happening very quickly and so

evaporation has little effect for the rest of the process [3].

1000 2000 3000 4000 5000 600020

25

30

35

40

45

50

55

12 mg/mL 10 mg/mL Fit y = 3065x-0.55

Fit y = 2232x-0.55

Film

thic

knes

s (n

m)

Spin speed (RPM)

poly-TPDa)

1000 1500 2000 2500 3000 3500 400040

50

60

70

80

90

Poly(9-vinylcarbazole) - PVK Power law fit y = 2662x-0.5

Film

thic

knes

s (n

m)

Spin speed (RPM)

b)

Figure 5.5 Spins trials conducted on hole transport polymers a) PVK and b) poly-TPD for

constant concentration. The fits to the data set are shown in red and blue.

5.3. Initial device builds

In order to design a first device the literature was drawn upon to decide what

materials and device stack should be implemented. The starting point with all devices is the

transparent anode through which light is coupled out. Here, as in OLEDs, glass substrates

with a conductive layer of indium tin oxide (ITO) form the anode which injects holes into



the device stack. ITO is used extensively in current flat panel displays because it can be

deposited as transparent colourless thin films that are also highly conductive. ITO glass

microscope slides were purchased from Sigma-Aldrich with a resistivity of ~ 100 ohm per

square. These were cut using a diamond scribe to pieces with dimensions 25 x 25 mm. In

order to form an active region on the device the ITO was etched by masking off regions

using tape and dropping the slides in a nitric acid and water solution for 2 minutes. Once a

strip of aluminium was deposited the overlap with the ITO formed electrodes as shown in

figure 5.6 allowing for 2 ‘pixels’ on each piece of ITO coated glass. The ITO slides are

subjected to a cleaning procedure that involves washing in soapy water followed by

sonication in acetone for 10 minutes and sonication in propan-2-ol for a further 10 minutes.

ITO anode

ITO anode

Al cathode

ITO anode

ITO anode

Al cathode

Figure 5.6 Diagram showing regions of ITO (blue) remaining after etching and the

deposited aluminium strip, where the two overlap the QD-OLED is formed.

It has become common in organic, polymer and hybrid QD light emitting devices to

modify the ITO-only anode by depositing a layer of the hole conducting polymer poly(3, 4-

ethylenedioxythiophene):poly(styrenesulphonic acid)(PEDOT:PSS). The ITO/PEDOT:PSS

then acts as the anode in the device having several advantages over ITO alone. The main

function of the PEDOT:PSS layer is to increase the work function of the anode from 4.6 eV

for the washing treatments used here to 5 eV to improve hole injection. It also acts to

reduce the high surface roughness of the ITO which can cause pin-hole defects which lead

to unstable conduction through the layers. PEDOT has a relatively high conductivity and is

transparent to visible wavelengths in thin films. Combined with PSS it is made soluble in

water and so is deposited from an aqueous dispersion. It is insoluble to organic solvents so

the following HTL can be deposited without compromising the PEDOT:PSS layer.



Introduction of the PEDOT:PSS layer has been shown to increase device lifetime

considerably and so is almost universally used in these types of LEDs [9]. Two types of

PEDOT:PSS were tried at this stage both purchased from Sigma-Aldrich, a PEDOT:PSS

formulation and conductive grade PEDOT:PSS 1.3 wt % dispersion in H2O. The

formulation, which was more viscous, was dropped on the slides and spun at 800 rpm for

10 seconds to spread it across the slide and then at 3000 rpm for 30 seconds to give a thin

uniform layer. The dispersion being less viscous was spun at 800 rpm (10s) and 2600 rpm

(30s). The slides were then placed in a drying oven at ~ 75˚C for 1 hour to evaporate any

remaining water.

A number of hole transporting polymers had been tried by different research groups

before this project was started. A paper shared the results of cyclic voltametry in studying

the ionisation potentials (IP) and electron affinities (EA) of a number of candidate polymers

for the HTL concluded that poly- (2- methoxy-5 (2’-ethylhexoxyphenylenevinylene)(MEH-

PPV), a more soluble derivative of PPV, has the most favourable energy level positions for

charge transfer into CdSe NQDs [10]. A solution of MEH-PPV (Sigma-Aldrich) was made

up using 15 mg of MEH-PPV dissolved in ~ 4 ml of toluene. The solution was briefly

sonicated and then filtered using glass wool and a pipette to remove excess MEH-PPV. The

solution was applied to the device by spin coating at 700 rpm (10 s) then 2600 rpm (30s),

the devices were again placed in a drying oven at ~ 75 ˚C for 1 hour.

The next layer in the device structure is the emitting nanocrystal layer which is also

to be spun down. The interface between MEH-PPV and a CdSe NQD normally promotes

charge separation, the opposite of the exciton formation required in LEDs. However,

charge injection into the device and the consequent charge build up at the interface causes

significant injection of electrons and holes in the NQDs and hence exciton formation. For

efficient LED operation, charge re-separation and non-radiative recombination must be

minimized. Capping the NQDs with surfactants such as trioctylphospine oxide (TOPO) has

been shown to reduce the rate of charge transfer [11]. The addition of an overlayer or shell

of wider bandgap semiconductor, usually CdS or ZnS, confines the exciton more to the

centre of the NQD and hence reduces non radiative recombination associated with surface

states. Greenham has argued that a CdS (bulk band gap = 2.6 eV) shell will produce better

results because ZnS (bulk band gap = 3.7 eV) presents too large a barrier to charge

injection into a CdSe dot [12]. Efficient QD-OLEDs have been fabricated, however, using a



CdSe/ZnS core-shell structure [13]. Also CdSe/ZnS NQDs were available from Nanoco

with very high PL quantum yields indicative of few non-radiative decays and so made good

emitting materials. The NQDs used here had PL peaks at ~ 600 nm which allowed their

emission to be easily distinguishable from the green emission of the MEH-PPV. The NQDs

used initially were dissolved in octane and spin coated onto the devices using dynamic

dispense as the solvent evaporated very quickly and if the film dries to any extent before

spinning starts it could cause imperfections in the layer. As NQD solutions are typically not

very viscous slow spin speeds were used to deposit several monolayers of dots. In this case

a short 5 second 700 rpm spreading step was used before spinning at 1500 rpm for 30

seconds to achieve a thin uniform layer. These were again placed in the drying oven at

~75˚C for 30 minutes to evaporate all the solvent.

The cathode in the QD-OLEDs needs to have a low work function in order to

promote electron injection into the dots or electron transporting layer (ETL). In these early

designs no ETL was used and device production was done in the open air. In metals a low

work function can often mean high reactivity with oxygen and water and so corrosion of

the metal cathode could be a severe limiting factor. Aluminium was chosen as the metal

cathode in this early work because it has a sufficiently low work function (4.2 eV) to allow

electron injection into the conduction band of the NQDs but is not so low that it is rapidly

oxidised on exposure to air. The Edwards 306 evaporator was pumped down to a pressure

of 10-6 mbar and the current across the tungsten basket filament was increased slowly up to

40 A to evaporate all the aluminium. The evaporator thickness monitor measured deposited

layers of 200 – 400 nm. The energy level diagram of the device stack described here is

shown in figure 5.7.



Ф = 4.2 eV

Vacuum E = 0 eV

Ф = 4.6 eV

ITO PEDOT

IP = 5 eV

MEH-PPV

Eg = 2.4 eV

EA = 2.6 eV

CdSe/ZnS NCs

Eg = 2.0 eV

EA = 4.4 eV

Al

Ф = 4.2 eV

Vacuum E = 0 eV

Ф = 4.6 eV

ITO PEDOT

IP = 5 eV

MEH-PPV

Eg = 2.4 eV

EA = 2.6 eV

CdSe/ZnS NCs

Eg = 2.0 eV

EA = 4.4 eV

Al

Figure 5.7 Energy level diagram for the initial QD-OLED design showing the work

function (Φ), ionisation potentials (IP), electron affinities (EA), and the band gaps (Eg) of

the different materials. As the positions of the energy levels are size dependent in

CdSe/ZnS only approximate values are given. ITO [8], PEDOT [8], MEH-PPV [10], NCs

[8] and Al [10] values are obtained from the literature.

Initial attempts at building these devices displayed the current voltage behaviour of

a typical p-n junction but no electroluminescence was observed. The device was found to

exhibit strong photoluminescence over the entire surface indicating that a good coverage of

NQDs had been deposited. A brown discoloration was noticed on the aluminium cathode

which became more pronounced over the course of a few days (figure 5.8). It was assumed

that some unanticipated reaction had taken place between the aluminium and one of the

organic components, possibly also involving atmospheric oxygen. Devices were then made

using NQDs that had been subjected to a cleaning procedure to remove any by-products of

the reactions used to synthesise them as well as removing a large proportion of the excess

ligand. This involved precipitating the dots in methanol, centrifuging the solution,

discarding the solvent, and then dissolving the dots in their original solvent again. A

number of these devices produced weak, green electroluminescence (figure 5.9).



Figure 5.8 Photograph of one of the initial LEDs showing brown discoloration of the Al

cathode.

Figure 5.9 Photographs showing first successful EL from a device.

The EL from the above devices was not visible under normal room light conditions

and could only be observed in a dark room. As a result the EL spectra could not be taken

but this first demonstration still represented an important step. The EL here is green with

some patches of orange indicating recombination is occurring predominantly in the MEH-

PPV layer. After this initial demonstration it was important to get to a point whereby

devices could be made that consistently demonstrated electroluminescence and could be

easily compared without the extraneous effects of poorly spun layers, bad evaporations, low

purity materials etc. A lot of work was done making devices that yielded only small

iterative gains in device performance yet built up the skill and experience required to make

devices in a way that was as consistent and scientifically rigorous as possible. During this



time the quality of spin coated layers was improved by fine-tuning the spin conditions for

each layer as well as the concentrations used. The spun-down films would often have

imperfections such as ‘comet’ features, a central circular mark the shape of the chuck, pin

holes in the film or a swirl pattern. The comet features and pin holes in the film can be

caused by dust particles on the substrate or particles in the spinning solution which then act

to resist the normal flow of the fluid across the substrate. These are therefore reduced by

filtering any spun-down layers through PTFE membrane sterile syringe filters with pore

sizes of 0.45 µm for the PEDOT:PSS which form particles in solution of ~ 70 nm [14] and

do not filter through the smaller 0.2 µm filters that are used for the other layers (HTL and

NQDs). The substrate is also subject to a ‘drop and drag’ cleaning procedure using lens

cleaning tissues and propan-2-ol to remove dust and the device interaction with the

environment is minimised where possible. The circular chuck mark, being the same size as

the vacuum chuck beneath is thought to be due to either the physical bowing of the glass

due to a strong vacuum or temperature difference between the vacuum chuck and the

solution on the substrate causing a thicker central region. This was easily solved by getting

a custom-milled recess chuck made that was specifically designed to take the square

substrates used here (figure 5.10). The swirl pattern was eliminated by carefully choosing

the acceleration and spin speed conditions for each material. In some cases this problem

was trivially solved by reducing the acceleration.

Figure 5.10 Diagram of the recess chuck used to give uniform film.

In terms of the thermal evaporation it was noticed that as the current was increased

the vacuum level dropped slightly before recovering again; this was due to de-gassing of

the filament and material. These impurities could affect the film quality and so the shutter

was kept in place whilst increasing the current slowly. This allowed the impurities to be



evaporated off and the vacuum to recover as well as allowing all the material to reach the

same temperature avoiding hot spots before exposing the substrate.

To further improve the EL output the simple design used here was modified to

incorporate the electron transport material tris-(8-hydroxyquinoline)aluminium (Alq3). This

material has been shown to have good electron transport capability [8] and also serves to

move the recombination zone away from the metal cathode. Confining the recombination to

layers away from the metal cathode improves device performance as this reduces

quenching of excitons in close proximity to the injecting metal contacts [15, 16]. Alq3 was

thermally evaporated prior to the cathode using a separate mask to deposit it over a

controlled area. Alq3 was purchased from Sigma-Aldrich and was then purified via

sublimation before being incorporated into the QD-OLEDs. This was found to improve the

efficiency of the LEDs. The relatively low solubility of MEH-PPV caused problems in

device fabrication as filtration often removed solid MEH-PPV material leading to

ambiguity in the concentration. This led to variable film thickness as well as poor quality

films due to the high solid content of the solution. N, N’-diphenyl-N, N’-bis(3-

methylphenyl)-(1, 1’-biphenyl)-4, 4’-diamine (TPD) had been used in some of the best

demonstrations of QD-OLEDs up to this time and so was chosen as an alternate hole

transport material [17]. Solutions of TPD in chloroform were found to give a clearer and

more uniform film than the MEH-PPV greatly increasing device throughput. At this point

there was no access to a profilometer and so the TPD was spun down according to the spin

conditions used by other researchers [18]. The EL observed from most of the working

devices was green suggesting it originated from the organic charge transport layers (Alq3).

This means that the injected holes had been transported by the QD layer to the Alq3 where

they met electrons, formed excitons and recombined emitting green photons. To circumvent

this problem and confine emission to the NQD layer only many groups have used hole

blocking layers (HBL) made from materials with suitable HOMO levels [13, 19]. The

problem is likely to be more of an issue for smaller dots emitting in the green or blue as the

top of the valence band will have an energy in the range of 7 eV making it energetically

favourable for holes to cross into the ETL. 2, 9-dimethyl-4,7-diphenyl-1,10-phenanthroline,

also called bathocuproine (BCP) was obtained from Sigma-Aldrich and was incorporated as

a HBL. A different shadow mask for evaporation was also designed so as to make better

use of materials and to get more individual QD-OLED pixels onto each ITO piece. Using



this 4 pixels were now formed on each substrate which gave us a better chance of

producing a working device as pin holes distributed randomly would short a number of the

LED pixels.

Many devices were made during the time these improvements were implemented

and experience was being gained. These did often demonstrate weak EL with contributions

from both organic (green EL) and the NQDs (red/orange EL) although the emission was too

weak for spectra to be taken. To observe the EL from these devices large voltages were

required often leading to more device degradation the longer the device was operated. The

emission was also rarely uniform with brighter and darker patches on pixels probably

corresponding to non-uniform layers of dots or charge transport layers. In order to achieve

the efficient and uniform emission required, a phase separation technique, first introduced

by a group at MIT, was attempted [17]. As described in chapter 1 this technique involves

spinning down a mixture of an NQD solution and a TPD solution. During spinning the

quantum dots separate from the TPD and rise to the surface to produce a monolayer of dots

evenly distributed throughout the layer. The same concentrations and spin conditions as

used by the MIT group were implemented with varying success. The above improvements

were applied and with more experience of device fabrication EL was more consistently

observed. It was however, a device with sequentially deposited layers which gave the first

measureable EL. The ITO was etched and cleaned as before and PEDOT:PSS HIL and the

TPD HTL were spun down and baked. The NQDs had a CdSe/ZnS core-shell structure with

HDA as the passivating ligand, the peak PL emission was found to be at 560 nm. After the

NQDs had been spun down the device was transferred to the vacuum chamber for

evaporation. BCP (14 nm), Alq3 (55 nm) and Al (150 nm) were deposited at vacua of ~ 10-6

mbar as before and the device was then tested in air. The EL spectrum was taken using the

Ocean Optics USB 4000 and is shown in figure 5.11 along with the PL spectra of Alq3 in a

chloroform solution. The electroluminescence matches the photoluminescence of the Alq3

indicating that the EL originates from the Alq3 layer. This was unexpected as poor

conduction in multilayers of NQDs has been observed previously [20] and the use of BCP

should improve electron injection and block holes before they enter the Alq3 layer. This

result would seem to suggest that the BCP is not acting as a hole blocking material and

through inspection of the energy levels (EA = -2.9 eV, Eg = 3.5 eV) the HOMO level is not

thought to be deep enough to effectively block electrons. The radiant power of this device



was measured and the photometric quantities were calculated; the luminance was calculated

to be ~ 4 cd/m2 and the device was found to have a luminous efficiency of 1.8 x 10-4 cd/A.

400 450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Nor

mal

ised

inte

nsity

(ar

b. u

nits

)

Wavelength (nm)

Alq3 photoluminescence

QD-OLED electroluminescenceDevice stack - ITO/PEDOT/TPD/QD/BCP/Alq

3/Al

Figure 5.11 Normalised electroluminescence (red) for the device stack indicated and

normalised photoluminescence spectra (green) for Alq3 dissolved in chloroform.

After this time the surface resistivity of the some etched ITO slides was tested and

despite the specified value of 70 – 100 Ω/square, it was measured using a digital multimeter

as 2 kΩ. Un-etched slides were tested and the resistivity was measured as ~ 90 Ω/square

and so it was surmised that the acid creeps under the tape and impacts on the ITO layer.

The high resistivity here would act to limit the current being injected into the device and so

reduce device performance. A new etching procedure was then implemented that used nail

varnish to mask off the required areas before being submerged in a fresh acid solution. The

nail varnish could then be removed using acetone and the slides could be washed according

the procedure described earlier. The resistivity was still found to increase to about 180

Ω/square. As the devices typically degraded and were destroyed upon testing they were

also encapsulated in a nitrogen-filled glove box using a glass cover slide and Araldite

epoxy resin before testing. This was hoped to increase the lifetime by excluding oxygen

and water and allowing full testing to be conducted without excessive degradation. Devices

were then fabricated on the newly etched slides after they had been washed and dried and

the PEDOT:PSS was spun down as before. In this case the phase separation technique was



again tried using a 1 mL chloroform solution containing 10 mg of TPD and 30 mg of NQDs.

The quantum dots were again a CdSe/ZnS core shell structure with HDA as the passivating

ligand and had a PL peak of ~ 620 nm and FWHM of ~ 32 nm. BCP (~ 15 nm) was

deposited as a strip down the centre of the ITO such that each pixel had a region of BCP

and no BCP; this was to test whether the BCP areas were preventing emission. As the EL

so far has suggested that the charge balance in the device favours the holes with

recombination occurring in the ETL, a thinner layer of Alq3 (20 nm) was deposited. A 150

nm layer of Al was also deposited to form the top cathode. Upon testing the device was

found to emit red EL at voltages as low as 3.5 V from the regions that did not have a BCP

layer. In the regions with BCP the Al was observed to have a white lustre indicative of

some reaction taking place, possibly due to the BCP. The EL spectrum of the device at a

bias of 6 V as well as the PL of the NQDs used in the device are shown in figure 5.12

clearly showing the red EL does indeed originate from the NQD layer. The NQD emission

is found to be red shifted by 4-5 nm in the QD-OLEDs compared to the QD emission in

chloroform. This is attributed to electronic energy transfer in the close packed film from

smaller dots (larger band gaps) to larger dots which have a smaller band gap. This has been

observed in mixed films of small and large dots and has been explained using the Förster

resonance transfer mechanism [21].

500 550 600 650 700 750

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Nor

mal

ised

inte

nsity

(ar

b. u

nits

)

Wavelength (nm)

PL spectrum CdSe/ZnS(HDA) EL spectrum

Device stack - ITO/PEDOT/TPD:QD/BCP/Alq3/Al

Figure 5.12 Graph showing the electroluminescence spectrum (red) of the device stack

shown where TPD:QD indicates these layers were produced using the phase separation

technique. The PL of the NQDs used in chloroform is also shown in black.



Figure 5.13 shows the current-voltage characteristics of this QD-OLED with an

ohmic region for low bias followed by a region with a power law dependence of current on

voltage. This is consistent with a trap-assisted space-charge-limited conduction mechanism

in CdSe thin films as found in previous reports [22]. In this mechanism, at first, charge

carriers are captured at charge trap sites and do not meet and recombine to produce EL. A

steady state between charge injection and trapping is eventually reached such that charge

carriers can now travel to the emissive layer to recombine. However, charge accumulation

in traps and at potential steps in the device produces internal electric fields which act to

drive charge in the reverse direction. At this stage the instability of the device prevented a

more in depth investigation of luminance and current voltage characteristics. Photographs

of the QD-OLED are shown in figure 5.14 for a) 5 V and b) at a maximum brightness of 68

cd/m2 when at a bias of 12 V.

1 1010-6

10-5

10-4

10-3

10-2

Cur

rent

den

sity

(A

/cm

2 )

Voltage (V)

Figure 5.13 Current density plotted against bias for the QD-OLED described above.

Fig 5.14 Photograph for the QD-OLED described for a bias of 5 V (left) and at 12 V (right).



It was decided to try to repeat the preparation of the above devices as a striation

pattern was observed and it was felt they could be improved upon. A striation pattern is a

common defect found in spin coating and had been observed in previous batches of the

QD-OLEDs. The striation pattern is due to thickness variations of the film with their

direction indicating the direction of fluid flow during the spin coating. The process that

causes this thickness variation has been ascribed to the early evaporation of solvents at the

surface increasing the surface tension in a localised region. The surface tension of this layer

may then be higher than the solution below and random fluctuations lead to areas of higher

and lower surface tension becoming amplified. Areas of higher surface tension draw fluid

inwards whilst the spaces around this are now able to evaporate solvent more easily and

become thinner [23]. To avoid this a less volatile solvent was used to make up the

TPD:NQD solutions used for the phase separation. This was limited by how soluble the

dots and TPD were and toluene was found to give clear and smooth films which lacked any

striation pattern. Upon repeating the same device structure as before no striation pattern

was found and the emission was smooth and uniform across the pixel. The device still gave

a very similar maximum luminance (~ 70 cd/m2) at a bias of 12 V and as the current was

also very similar there was no real increase in efficiency.

In attempts to increase efficiency the energy level diagram for the current device

stack was analysed(figure 5.15). The large potential step (1.4 eV) between the aluminium

cathode and the Alq3 will act as a source of charge imbalance in the devices. A lower work

function metal could be used but these are usually highly reactive and have poor corrosion

resistance. To improve electron injection into the Alq3 a number of low work function

materials have been implemented in OLEDs such as calcium [8], barium [24], or

combinations of low work function metals like magnesium with protective layers of silver

[25]. Thin layers of insulating materials deposited between the ETL and cathode have also

been shown to improve electron injection in OLEDs [26]. Lithium fluoride with a wide

band gap energy of 12 eV is a strong insulating material which - when deposited as a thin

film with an Al overlayer - has been shown to enhance electron injection into the Alq3 layer

[27]. Some authors claim photoemission experiments have shown it to cause a downward

band bending of the Alq3 [27] reducing the potential barrier between Al and Alq3 although

most research now attributes this to the release of free lithium upon evaporation of the

cathode effectively doping the organic layer [28]. A work function of 2.9 eV makes lithium



fluoride a good candidate for electron injection into Alq3, and it has also been shown to be

more efficient than the Ca or Mg cathodes often used in OLEDs [29].

Ф = 2.9 eV

Ф = 4.2 eV

Vacuum E = 0 eV

Ф = 4.6 eV

ITO PEDOT

IE = 5 eV

TPD

IE = 5.4 eV

EA = 2.1 eV

CdSe/ZnS NCs

Eg = 2.0 eV

EA = 4.4 eV

Al

Alq3

IE = 5.5 eV

EA = 2.8 eVLiF/AlФ = 2.9 eV

Ф = 4.2 eV

Vacuum E = 0 eV

Ф = 4.6 eV

ITO PEDOT

IE = 5 eV

TPD

IE = 5.4 eV

EA = 2.1 eV

CdSe/ZnS NCs

Eg = 2.0 eV

EA = 4.4 eV

Al

Alq3

IE = 5.5 eV

EA = 2.8 eVLiF/Al

Figure 5.15 Energy level diagram of device stack with work functions of LiF/Al and Al

shown for comparison. Approximate values for the NQDs energy levels are given as before

and the energy levels for TPD, Alq3 and LiF are taken from references [8, 27, 30].

ITO slides were cut, etched and cleaned and PEDOT was spun down according to

the procedures used previously. The phase separation technique was used to deposit the

TPD(10 mg/mL):NQD(30mg/mL) layer from toluene and used the same NQDs (CdSe/ZnS

PL ~ 620 nm) as in the last devices. The Alq3 (25 nm) was evaporated over the majority of

the device (using a square shadow mask) and the vacuum was then broken to change over

masks for the evaporation of the LiF and Al. A thin ~ 1 nm layer of LiF was carefully

deposited followed by ~ 140 nm of Al; all evaporations were done at a pressure of ~ 10-6

mbar. After encapsulation in a nitrogen-filled glove box the devices were brought out into

the air for characterisation.

The emission from these devices was not completely uniform with some brighter

and darker regions to each pixel, and the turn on voltages were found to be low (~ 3 V).

The EL spectra for this device was taken using the Fluorolog 4 spectrometer with the slits

set to 2 nm and the excitation source blocked and is shown in figure 5.16 taken at different

bias voltages. A turn on voltage of ~ 3 V was measured and the luminance was found to

increase with increasing voltage up to a maximum luminance of 110 cd/m2 at an external



bias of 11 V. To the eye the emission appears orange rather than red due to the contribution

of green Alq3 EL but at its maximum brightness is easily visible under normal room lights.

The luminous efficiency at the maximum brightness of 110 cd/m2 was measured as 0.05

cd/A. The spectra of this device in figure 5.16 show that a proportion of the emission

originates from the Alq3 layer and show that the contribution of Alq3 to the EL increases

with applied bias. The presence of Alq3 EL indicates that whilst the majority of excitons

recombine in the NQDs, a number do recombine on the Alq3 with an increasing probability

as bias is increased. The addition of the LiF electron injection layer and the increase in

efficiency observed would seem to be consistent with the direct charge injection model. By

incorporating LiF it would appear that better electron and hole balance has been achieved

by more efficiently transporting electrons to the NQD layer. The presence of Alq3 emission

would suggest either hole leakage through the NQD layer or that excitons are formed on the

Alq3 in a region that is further than the Förster radius from the NQDs. A close-packed

monolayer of dots should organise into a hexagonal-close-packed structure but an

incomplete monolayer would have vacancies and cracks which the Alq3 would fill when it

is deposited. This would give a direct route between the TPD and the Alq3; no AFM or

profilometry was available to clarify the situation, but this seems the most likely scenario as

the phase separation technique requires tuning to the particular materials and solvents [31].

Further to this, increasing the applied bias will act to widen the exciton generation region

meaning more excitons are formed on the Alq3 yet are too far away to efficiently transfer to

the NQDs. Unfortunately it was not possible to take the I-V curve for this device as when

the applied bias was increased past 11 V the device ceased to emit and was permanently

destroyed.



450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ised

EL

(arb

. uni

ts)

Wavelength (nm)

5 V 6 V 7 V

Device stack - ITO/PEDOT/TPD:NQD/Alq3/LiF/Al

Figure 5.16 Normalised electroluminescence spectrum for the device stack shown at

different applied voltages.

At this point the supply of NQDs used above had been exhausted; this throws up a

problem in QD-OLED development and design. Batch to batch differences between NQDs

made to have the same characteristics cannot be avoided. This means that comparing

devices made using different batches of dots will introduce unknown variables that are hard

to account for. Bearing this in mind devices were made using NQDs with a graded shell

structure designed to reduce the strain between layers with different lattice constants and so

give fewer defects and higher quantum yields. This type of dot may even reduce the barrier

to charges being injected into them as the electron and hole wavefunctions will be less

confined to the core and so spread throughout the volume of the dot. The NQDs had a

structure of CdSe/CdZnS/ZnS and were capped with a 1:1 ratio of TOPO/HDA, the PL

peak was found to be at 559 nm with a FWHM of 35 nm and a PLQY of 60%. It was also

decided at this point that baking the samples in the glass drying oven was not appropriate as

there was no control over the temperature and it was open to the water and oxygen in the air.

Thermal annealing of the NQD layer had also been shown to yield improved performance

from QD-OLEDs by improving the morphology of the QD layer as some surface ligand is

lost and the NQDs form a more close packed and ordered film [6]. The device fabrication

procedure was altered so that the TPD:QD layer was now transferred to the glove box after

spinning where the device was heated on a hot plate at 70˚C for 20 minutes. As the boiling



point of toluene is 110˚C and the glass transition temperature of TPD is tg = 63˚C it was

hoped annealing at 70˚C would release trapped solvent and give more amorphous films.

The PEDOT:PSS layer could not be thermally annealed in the glove box as it was spun

down from an aqueous solution which would ruin the oxygen and moisture free

environment of the glove box. This was therefore done in the glass drying oven as before.

After etching and cleaning the ITO slides the PEDOT:PSS was spun down and annealed as

described previously. The new graded shell dots and TPD were made up into a solution of

toluene in the same ratio as for the previous dots (TPD(10mg/mL):NQD(30mg/mL)). This

was then spun down at 3000 rpm for 30 seconds and one LED was transferred to the glove

box where it was annealed at 70˚C for 20 minutes whilst the other device was left untreated.

The devices were then transferred to the evaporation chamber with square masks allowing

deposition of Alq3 over the central region of the device. At a pressure of 3 x 10-6 mbar 25

nm of Alq3 was deposited after which the vacuum was broken and the mask changed for

deposition of the LiF (1 nm) and Al (140 nm) cathode at 3 x 10-6 mbar. It was noted that a

white lustre was formed upon deposition of the Al which could again be due to some

reaction between the different layers. Although lustre on the Al was observed in the

previous batch of devices it was to a lesser extent than here and the fact that the previous

dots were washed whereas these were not seems to be the only difference. The devices

were encapsulated in the glove box as before and brought out for optical and electrical

testing.

Electroluminescence was observed from both the baked and un-baked devices and

both had high turn on voltages of 6 V for the baked device and 14 V for the un-baked

device. The electroluminescence for both devices at an applied bias of 18 V is shown in

figure 5.17. In the baked device it can be seen that the emission originates from both the

Alq3 (~ 50%) centred at ~ 520 nm and the NQDs (~50%) at around 560 nm. The EL

spectrum for the un-baked device has more noise as the luminance was lower and also has a

lower contribution to the EL from the NQD layer. The emission from both QD-OLEDs

appeared green to the eye but was patchy and non-uniform particularly in the non-baked

device. The luminance for the non-baked device was so low that it was difficult to measure;

the maximum was found at an applied bias of 24 V and calculated to be ~ 2 cd/m2. The

baked device was many times brighter with its maximum luminance being at 20 V and

calculated to be 321 cd/m2. Despite the large voltages required the current density was still



found to be low relatively low (see figure 5.18) and so a high luminous efficiency of 2.67

cd/A was calculated. The un-baked device had very unstable current flow (inset figure

5.18) which was reflected in the unstable emission with many of the pixels developing

shorts and being permanently destroyed.

450 500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0

1.2

Nor

mal

ised

EL

(arb

. uni

ts)

Wavelength (nm)

Baked device at 18 V Un baked device at 18 V

Device stack - ITO/PEDOT/TPD:NQD/Alq3/LiF/Al

Figure 5.17 Electroluminescence spectra for the baked and un-baked devices at an external

bias of 18 V. Both have components from the Alq3 and the NQD layer.

0 5 10 15 20 25

0.00

0.01

0.02

0.03

0.04

0 5 10 15 20 250.000

0.002

0.004

0.006

0.008

0.010

0.012

Cur

rent

den

sity

(m

A/c

m2 )

Voltage (V)

Un-baked device

Cur

rent

den

sity

(m

A/c

m2 )

Voltage (V)

Baked device

Figure 5.18 I-V curve for the baked device where data points are simply connected and the

unstable I-V curve for the unbaked device is shown inset.



The current flow in the unbaked device was much lower for a given applied bias

indicating that the resistance in this device is much higher than in the baked device where

the turn on voltage is also considerably lower. As the turn on voltage was found to be high

in both these devices however, it is hypothesised that as the dots had not been washed a

large excess of ligand was thought to be acting as an insulator and so limiting current. This

is consistent with the results here as baking has been shown to result in a loss of some of

the NQD surface ligands [6] and to higher current densities and improved charge transport

and injection into the NQDs. The results here clearly show that baking of the TPD:NQD

layer in a nitrogen filled glove box is hugely beneficial and in this case resulted in a device

that was over 100 times brighter than for when this layer was not baked. Despite the high

brightness a significant component of the EL originates in the Alq3 layer and this is

probably due to an increased leakage current through the thin QD layers as well as at voids,

grain boundaries and interstitial spaces.

5.4. Conclusions

This initial proof of concept stage demonstrated that it was possible to fabricate

QD-OLEDs from the quantum dots available at Nanoco. In order to develop the devices for

display applications it was important to build consistent and stable devices in a controlled

environment. More precise and controlled deposition of layers, as well as consistent testing

procedures will allow experimental trends to be more easily identified. The results so far

where the majority of emission is from the QD layer and with luminance values > 100

cd/m2 are encouraging. Typical mobile phone displays will have a luminance of 50 – 150

cd/m2 with monitors and flat screen televisions requiring luminance levels > 500 cd/m2.

The goals for the project after this stage were to produce devices with pure quantum dot

emission with brightness levels suitable for display applications. Further to this, the QD-

OLEDs must be stable to a degree such that they can be operated at standard luminance

levels (100, 400 and 1000 cd/m2) to allow complete characterisation and for use as

demonstrations.



References

1. Hardman Ron. Environmental Health Perspectives, 2006. 114(2): p. 165. 2. Emslie, A.G., F.T. Bonner, and L.G. Peck. Journal of Applied Physics, 1958. 29(5):

p. 858-862. 3. Sukanek, P.C. Journal of The Electrochemical Society, 1991. 138(6): p. 1712-1719. 4. Zhao, J., et al. Nano Letters, 2006. 6(3): p. 463-467. 5. Mattox, D.M., Handbook of Physical Vapor Deposition (PVD) Processing. 1998,

William Andrew Publishing/Noyes. 6. Y. H. Niu, et al. Advanced Materials, 2007. 19(20): p. 3371-3376. 7. Binnig, G. and H. Röhrer. Reviews of Modern Physics, 1987. 59(3): p. 615. 8. Sun, Q., et al. Nature Photonics, 2007. 1(12): p. 717-722. 9. Berntsen, A., et al. Optical Materials, 1998. 9(1-4): p. 125-133. 10. Kucur, E., et al. The Journal of Chemical Physics, 2004. 120(3): p. 1500-1505. 11. Ginger, D.D.S. Physical review. B, Condensed matter, 1999. 59(16): p. 10622-9. 12. Klimov, V.I., ed. Semiconductor and Metal Nanocrystals: Synthesis and Electronic


13. Coe-Sullivan, S., et al. Organic Electronics, 2003. 4(2-3): p. 123-130. 14. Kok, M.M.d., et al. physica status solidi (a), 2004. 201(6): p. 1342-1359. 15. Kalinowski, J. Journal of Physics D: Applied Physics, 1999. 32(24): p. R179. 16. Larkin, I.A., et al. Physical Review B, 2004. 69(12): p. 121403. 17. Coe, S., et al. Nature, 2002. 420(6917): p. 800-803. 18. Huang, H., et al. Nano Letters, 2007. 7(12): p. 3781-3786. 19. Rizzo, A., et al. Applied Physics Letters, 2007. 90(5): p. 051106-3. 20. Leatherdale, C.A., et al. Physical Review B, 2000. 62(4): p. 2669. 21. Kagan, C.R., et al. Physical Review Letters, 1996. 76(9): p. 1517. 22. Caruge, J.-M., et al. Nano Letters, 2006. 6(12): p. 2991-2994. 23. Birnie, D.P. Journal of materials research, 2001. 16(1): p. 1145-1154. 24. Stouwdam, J.W. and R.A.J. Janssen. Journal of Materials Chemistry, 2008. 18(16):

p. 1889-1894. 25. Anikeeva, P.O., et al. Nano Letters, 2009. 9(7): p. 2532-2536. 26. Shakya, P. and et al. Journal of Physics D: Applied Physics, 2008. 41(8): p. 085108. 27. Hung, L.S., C.W. Tang, and M.G. Mason. Applied Physics Letters, 1997. 70(2): p.

152-154. 28. Heil, H., et al. Journal of Applied Physics, 2001. 89(1): p. 420-424. 29. Liu, Z., O.V. Salata, and N. Male. Synthetic Metals, 2002. 128(2): p. 211-214. 30. Anikeeva, P.O., et al. Physical Review B (Condensed Matter and Materials

Physics), 2008. 78(8): p. 085434-8. 31. Coe-Sullivan, S., et al. Advanced Functional Materials, 2005. 15: p. 1117-1124.

Chapter 6 QD-OLED development


Chapter 6: QD-OLED development

6. Introduction

The initial device builds described in the last chapter represented a period when a

lot of knowledge and experience was gained that could only be achieved by making QD-

OLEDs. Promising results were obtained from some of the builds but in any one batch of

devices there were many failed LEDs partly due to the oxygen and moisture in the

environment in which they were made. Ideally they should be made in a clean room

environment to avoid particulate contaminants or impurities making their way into the

device and creating shorts or areas of higher or lower electric field. Also, as many of the

materials used are sensitive to both oxygen and water it is important that the HTL and the

NQDs are deposited in an inert atmosphere. From a device design perspective many of the

QD-OLEDs made using the phase separation approach show a large contribution to the EL

from the charge transport layers. This is unacceptable as one major advantage to using QD-

OLEDs in display applications is the colour purity that should be possible using NQDs as

the emitter. Devices relying upon charge injection mechanisms require that no single carrier

is found in excess on the NQDs. Balancing the number of charges reaching the NQDs

would be very difficult and different potential steps between the transport layers and the

NQDs would nearly always lead to favourable injection of one charge carrier over another.

Also the large volume of work required when fine tuning the phase separation technique for

different batches of dots and new solutions means that depositing dots in multiple layers is

much more practical. This change in approach was supported in Sun’s paper [1] where the

highest performing QD-OLEDs at the time were fabricated using different numbers of

monolayers depending upon the size of the dots used.



6.1. Improvements to fabrication methods

At this stage an MBraun 150B-G dedicated dual glove box system comprising two

glove boxes connected by an antechamber to pass objects between them with a built in

Edwards Auto-500 evaporator was bought by Nanoco technologies. The glove box system

was installed in the optoelectronics development lab at Nanoco and commissioned for use.

These glove boxes have a nitrogen-filled atmosphere and by using filters and vacuum

pumps achieves a level of < 1 ppm for both oxygen and water. The G3-P speciality coating

systems spin coaters were used in the glove box to deposit the HTL and the NQD layers,

and thermal annealing was now done on a hot plate in the glove box. The devices were then

passed through the antechamber into the adjoining glove box that contained the evaporator.

The ETL, EIL and the metal cathode could then all be deposited without ever exposing the

device to air or moisture; even upon breaking the vacuum to change shadow masks between

the ETL and EIL the devices are simply exposed to nitrogen. Encapsulation was also done

in the glove box using a UV-curing epoxy resin obtained from Nagase ChemteX that was

used to hold down glass caps which had a slight recess on one side where an oxygen and

moisture absorbing getter was placed. The active area of the devices was masked off and

the resin exposed to a UV lamp for ~ 10 minutes during which time it cured. This was

hoped to increase device stability and lifetime and allow rigorous and full optical and

electrical testing. The PEDOT:PSS is an aqueous solution so was spun down in a laminar

flow glove box filled with clean air.

In order to be able to analyse the affects of improvements and changes to device

design it is important that the other components are the same in all other LEDs. As the

etching and cutting of ITO coated glass slides was felt to be a source of variation (in terms

of resistivity and quality of etching) ITO slides were purchased from Cambridge Display

Technologies (CDT). CDT use photolithography to remove the ITO with high precision

making small features and well-defined electrode areas possible. These had a surface

resistivity of ~ 20 Ω/square and allowed for up to 8 LEDs to be deposited on each 25 x 25

mm2 substrate. These had already been subjected to a wet cleaning procedure and are

packaged in air tight anti-static bags to maintain their cleanliness. Researchers looking into

the stability of polymer LEDs (PLEDS) found using XPS that chemical cleaning of the ITO

was not sufficient for good device performance [2]. Berntsen et al. found in reference 2 that

using a UV-ozone treatment significantly improved device performance and this has



become a standard procedure for treating ITO slides used in OLEDs, PLEDs, PHOLEDs

(Phosphorescent OLED) and QD-OLEDs. The UV-ozone treatment has two effects; firstly

it removes any remaining organic contaminants and secondly it modifies the work function

through increased oxygen content at the surface. Whilst this is favourable for hole injection

the presence of oxygen is found to degrade the polymers in contact with them, thus the use

of PEDOT:PSS in combination with a UV-ozone treatment should lead to devices with

improved performance that are also more stable.

Another source of device failure and instability was the tendency of the TPD to

crystallise leading to films with a poor morphology that would often lead to shorts in the

places where the TPD crystallised. This has been previously attributed to its exposure to

oxygen and water even in the low levels found in a glove box [3]. It was decided to replace

TPD with poly-TPD as this had been demonstrated to produce extremely high quality QD-

OLEDs [1]. There are a number of reasons as explained by Sun et al. (ref. 1) that poly-TPD

yields improved device performance the first of which being that its HOMO level of 5.2 eV

reduces the barrier for hole injection from PEDOT:PSS. It is also known to have very good

hole transporting properties and does not require high annealing temperatures which could

damage the underlying PEDOT:PSS layer. Further to this its intrinsic resistance to non-

polar organic solvents means that the NQDs can be spun down as a separate layer from a

suitable solvent such as toluene without damaging the poly-TPD layer.

Also purchased at this time were a Dektak 8 stylus profiler and a Deeco digital

instruments dimension 3100 scanning probe microscope that could be used for thickness

monitoring and calibration of the various layers. In conjunction with the stylus profiler and

AFM each layer could now have a detailed film thickness study done, allowing layers of a

precise thickness and surface roughness to be spun down; a combination of literature

reports and experimentation allowed us to find the best thickness for each layer. The film

thickness monitor in the evaporator was also calibrated for each layer and after several

attempts the new fabrication line was validated for use in producing all subsequent QD-

OLEDs.

As mentioned previously the large excess of organic ligand used to passivate the

surface of the NQDs acts as an insulating layer through which the charge carriers have to

tunnel. Therefore a cleaning procedure is implemented on all the dots used in the QD-

OLEDs to remove as much of the excess ligand as is possible without impacting



significantly on the PL quantum yield. The cleaning procedure used by the NQD growers at

Nanoco (Dr. Ombretta Masala and Dr. Steve Daniels) is an extension of the method tried

previously whereby the dots are precipitated out of solution by the addition of methanol and

are then collected via centrifugation. The precipitate is then re-dissolved in chloroform and

centrifuged to remove any insoluble material. This procedure is conducted under nitrogen

and is repeated up to 5 times before dissolving the NQDs in toluene for use in the EL

devices.

6.2. Cadmium containing QD-OLEDs

Once the new fabrication line had been built up and commissioned and the various

improvements above had been implemented, it was important to validate all the processes

by making reproducible, high quality LEDs. Cadmium-based NQDs were initially used on

the new fabrication line with the aim of producing devices to demonstrate bright and

saturated EL and so prove the viability of using Nanoco’s NQDs for future display

technology. It was quickly possible to produce clear and uniform thin films of the various

layers and the optimum thicknesses and deposition conditions for each layer were found by

using the literature as a guide followed by experimentation.

Figure 6.1 Photographs showing the device containing a 20 nm layer of red dots at 5 V

(left) and 9 V (right).



Two types of graded shell nanocrystals that had been cleaned with 5 iterations of

the cleaning procedure were tried in the device stack. An NQD with red PL centred at 620

nm (FWHM = 35 nm) with the structure CdSe/CdS/CdZnS/ZnS (HDA/TOPO) and a

yellow emitting NQD with peak PL = 580 nm (FWHM = 30 nm) and a structure

CdSe/CdZnS/ZnS (HDA/TOPO) were used.All devices were completed using the new

fabrication line according to the conditions described below which had been found to give

the best results. After treating the slides with UV-ozone for 10 minutes the Aldrich PEDOT

formulation was deposited as a 50 nm layer and annealed at 200˚C for 20 minutes in air

(laminar flow glove box) and then 20 minutes in the nitrogen-filled glove box. The poly-

TPD was spun down from chlorobenzene at a concentration of 10 mg/ml at 1500 rpm for

60 seconds to obtain a 40 nm layer. This layer was annealed at 110˚C for 1 hour, all of

which was done in the nitrogen-filled glove box. The NQDs were deposited after spin trials

on each batch to deposit two different layer thicknesses. The NQDs were deposited in

layers of 20 nm and 8 nm and were annealed on a hot plate again at 110˚C for 20 minutes.

All of the devices were then transferred to the evaporation chamber where a 35 nm

layer of Alq3 was deposited followed by a 1.2 nm LiF layer and 100 nm aluminium layer.

The devices were then encapsulated in the glove box and left to completely cure before

removing them for testing. The EL from these QD-OLEDs was found to be a bright red due

to the large contribution from the NQD layer as shown in the photograph above (figure 6.1).

The emission was easily visible in room lights and the maximum brightness of 1628 cd/m2

was found at 9 V which is more than bright enough for display purposes. The EL spectrum

is shown in figure 6.2 at a range of voltages for the device with the thicker (a) and the

thinner (b) layer of NQDs as taken by the USB4000 fibre optic spectrometer.



400 450 500 550 600 650 700 7500.0

0.2

0.4

0.6

0.8

1.0

1.2

Nor

mal

ised

EL

(arb

. uni

ts)

Wavelength (nm)

4V 5V - 59 cd/m2, 0.47 cd/A 7V - 773 cd/m2, 0.53 cd/A 9V - 1628 cd/m2, 0.317 cd/A

NQDs -CdSe/CdS/CdZnS/ZnS (HDA/TOPO), PL = 620 nmLayer thickness = 20 nm

a)

400 450 500 550 600 650 700 7500.0

0.2

0.4

0.6

0.8

1.0

1.2NQDs -CdSe/CdS/CdZnS/ZnS (HDA/TOPO), PL = 620 nmLayer thickness = 8 nm

Nor

mal

ised

EL

(arb

. uni

ts)

Wavelength (nm)

5V - 81.7 cd/m2, 0.22 cd/A 6V 7V - 1842 cd/m2, 0.8 cd/A 8V

b)

Figure 6.2 Electroluminescence spectra for the red emitting devices with a) a 20 nm and b)

8 nm NQD layer where the component at ~ 520 nm is from the Alq3 layer. The measured

luminance and luminous efficiency is shown in the legend.



As described above the yellow emitting device was made in exactly the same way

as the red and was part of the same batch of devices, as a result it was exactly the same but

for the structure of the NQDs used. The emission from these devices was observed to be a

bright yellow with a turn on voltage as low as 3 V (figure 6.3). The device with the thinner

NQD layer had a much larger contribution to the EL from the Alq3 layer which due to the

high brightness and reasonable efficiency is attributed to hole leakage through the NQD

layer. Upon taking the EL spectra using the fibre optic spectrometer pure, colour saturated

emission is measured with almost negligible contribution from Alq3 for the device with a

20 nm layer of dots and only a small component in the device with a 8 nm layer of dots

(figure 6.4). The maximum luminance found for the device with a 20 nm layer was 2298

cd/m2 at an applied bias of 7 V with a luminous efficiency of ~ 1 cd/A. In the device with

an 8 nm layer the maximum luminance of 2470 cd/m2 was also found at 7 V with a

luminous efficiency of ~ 4 cd/A.

Figure 6.3 Photographs showing the yellow emitting device under bright room lights (left)

and in a darkened room (right) at an applied bias of 5 V.



400 450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

1.2NQDs -CdSe/CdZnS/ZnS (HDA/TOPO), PL = 580 nmLayer thickness = 20 nm

Nor

mal

ised

EL

(arb

. uni

ts)

Wavelength (nm)

4V 5V - 343 cd/m2, 1.07 cd/A

6V - 1580 cd/m2, 1.62 cd/A 7V - 2298 cd/m2, 1.08 cd/A 8V

Voltage

a)

400 450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

1.2NQDs -CdSe/CdZnS/ZnS (HDA/TOPO), PL = 580 nmLayer thickness = 8 nm

Nor

mal

ised

EL

(arb

. uni

ts)

Wavelength (nm)

4V 5V - 186 cd/m2, 2 cd/A 6V - 1041 cd/m2, 3.8 cd/A 7V - 2470 cd/m2, 3.9 cd/A

b)

Figure 6.4 Electroluminescence spectra for the yellow emitting devices with a) a 20 nm and

b) 8 nm NQD layer where the component at ~ 520 nm is from the Alq3 layer. The measured

luminance and luminous efficiency is shown in the legend.



These devices showed the best performance yet from the QD-OLEDs and a low turn

on voltage of ~ 3 V indicates that the rigorous cleaning procedure and the use of high purity

materials had removed any insulating materials. All the devices showed high brightness,

appropriate for display technology and uniform emission over the entire pixel. The devices

were also stable enough to undergo full testing and then could be used repeatedly for

demonstration purposes. The EL spectra for the devices with thicker or thinner layers serve

to show that the EL can be tuned so that only emission from the NQD layer is obtained. For

each dot size it was necessary to optimise the NQD layer thickness so as to get only dot

emission whilst maintaining a high efficiency as was found by Sun et al. in [1]. In the red

LEDs the NQDs are larger than the yellow NQDs and so a layer of the same thickness will

correspond to fewer monolayers of dots. This can explain the increased contribution from

Alq3 found in the QD-OLEDs made with the large dots as hole leakage is likely to be larger

due to a lower density of NQDs for recombination.

Up to this point many of the QD-OLEDs made with smaller dots to emit in the

green or blue were found to have a large contribution to the emission from the Alq3 ETL.

The poor performance of BCP meant another electron transport/hole blocking layer was

needed to improve the performance of the green and blue devices. 2,2’,2’’-(1,3,5-

benzenetriyl)-tris(L-phenyl-1-H-benzimidazole) (TPBi) is one such material that has been

used to fabricate efficient red, green and blue devices [3-5]. In TPBi the position of the

LUMO (-2.7 eV) and the HOMO (-6.3 eV) are such that the barrier to electron injection is

small and similar to that of Alq3 and the deeper HOMO makes it more likely that holes will

be blocked and so confined to the emissive NQD layer. By looking at the emission and

absorption spectrum of the organic layers and the NQDs respectively the higher efficiencies

reported using TPBi can be explained (figure 6.5). As explained in chapter 2 any excitons

formed on the organic transport layers can be transferred to the dots via FRET with an

efficiency that depends both upon distance between the donor (organic layers) and the

nanocrystals (acceptor) and the spectral overlap between the donor emission and the

acceptor absorption. Figure 6.5 shows that exciton transfer is possible from TPBi to all

sizes of NQD with complete spectral overlap and that poly-TPD is well suited to transfer

excitons to all sizes of quantum dot but with reduced efficiency in green and blue emitting

dots. The spectral overlap between the NQDs absorption and the emission of the Alq3

shows that whilst exciton transfer from the Alq3 to the red and orange dots is favourable



there is only partial or no overlap for the case of green and blue emitting NQDs. In fact the

spectral overlap for the blue dots suggests that FRET from the dots to the Alq3 is likely to

occur thus reducing QD emission whilst increasing Alq3 emission. The TPBi should

therefore move the exciton formation region away from the Alq3 whilst allowing efficient

transfer of excitons to blue and green NQDs.

350 400 450 500 550 600 650 700 750 8000.0

0.5

1.0

1.5

2.0

2.5

nor

mal

ised

abs

orpt

ion

norm

alis

ed P

L

Wavelength (nm)

pTPD TFPL TPBi TFPL Alq

3 TFPL

Orange NQD Red NQD Green NQD

Figure 6.5 Thin film photoluminescence are shown for TPBi, p-TPD, and Alq3 along with

the absorption spectra of red, orange and green NQDs to demonstrate spectral overlap

between donors and acceptors.

TPBi was purchased from Luminescence Technology Corp. and was deposited

through vacuum evaporation between the NQD and the Alq3 layer; thickness calibration

was conducted using the AFM. Devices were fabricated using graded shell green NQDs

with the structure CdS/CdZnS/ZnS (HDA:TOPO), where the device stack was built up as

before with PEDOT:PSS (50 nm)/p-TPD (50 nm)/NQDs (18 nm)/TPBi (20 nm)/Alq3 (30

nm)/LiF(1.2 nm)/Al (100 nm). It was also decided to use the TPBi to make an orange

device that demonstrated pure NQD emission. This was made in exactly the same way as

the green device with the layer thickness kept constant for each layer but with the NQD

structure CdSe/CdZnS/ZnS (HDA:TOPO). A blue device was also produced using NQDs

with the structure ZnCdS/ZnS and oleic acid (OA) as the passivating ligand with PL peak at



443 nm, a FWHM of 25 nm and a PLQY of 76 %. This device was made using a 40 nm

layer of TPBi and a 10 nm layer of Alq3 as attempts to make blues using this dot previously

had given ~ 50 % emission from Alq3. The top of the valence band is deeper in blue dots in

comparison to red and so it is possible that the TPBi will not block holes as efficiently with

these small dots. The thicker TPBi layer is designed to move the exciton formation region

away from the Alq3 and so make exciton transfer from the TPBi into the dots more

favourable. The emission from both the green and orange device was observed to be bright

and colour-saturated, with the electroluminescence originating from the NQD layer only

even at higher voltage. The blue device, however, gave only a small emission component

from Alq3 and appeared to the eye as an electric blue colour (figure 6.6). The

electroluminescence spectra are shown in figure 6.7 and are similar to the solution PL

spectra of the dots with an EL peak at 535 nm and a FWHM of ~ 37 nm for the green, EL

peak at 595 nm and a FWHM of ~ 35 nm for the orange and an EL peak at 445 nm with a

FWHM of ~ 25 nm. The presence of Alq3 emission in the blue device is likely due to holes

not being confined to the dot layer and being transported to the TPBi/Alq3 interface. Here

they will recombine with an electron in the Alq3 layer due to the barrier for electron

injection into TPBi from Alq3.

Figure 6.6 Photographs showing the blue and green devices emitting in the dark at an

applied bias of 9 V and 6 V respectively.



400 450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

1.2

Nor

mal

ised

EL

(arb

. uni

ts)

Wavelength (nm)

CdS/CdZnS/ZnS (HDA:TOPO), PL = 530 nm CdSe/CdZnS/ZnS (HDA:TOPO), PL = 590 nm ZnCdS/ZnS (OA), PL = 443 nm

Figure 6.7 Electroluminescence spectra for the devices utilising TPBi as an electron

transport/hole blocking layer showing pure saturated emission originating from the

quantum dot layers only for the green and orange with only a small Alq3 component in the

blue device.

Luminous efficiency (cd/A) at luminance of

QD-OLED colour

Maximum

luminance

(cd/m2)

100 cd/m2 400 cd/m2 1000 cd/m2

Orange (ELmax = 595 nm) 1021±1 5.04±0.02 2.91±0.02 1.09±0.01

Green (ELmax = 535 nm) 805±1 4.41±0.02 3.00±0.02 -

Blue (ELmax = 445 nm) 232±1 0.51±0.01 - -

Table 6.1 Table showing measured maximum luminance and luminous efficiency at

different device brightness’.

Both the green and orange devices had low turn on voltages of around 3 V and a

high maximum luminance. The current efficiency of these devices was also measured and

is presented in table 6.1 above. The high efficiency of these LEDs is attributed to the



function of the TPBi which as well as allowing efficient exciton transfer to the dots acts to

reduce the concentration of electrons at the NQDs. The increased TPBi thickness and

interface between TPBi and Alq3 leads to a better charge balance between electrons and

holes. This will result in less quenching of the NQD emission via the Auger mechanism.

The result here shows that both resonant energy transfer and charge injection play a role in

the efficiency of these devices.

The selection of a red, green and blue colour pixel that displays colour saturated

emission will define the gamut of colours that any display can produce. The CIE 1931

chromaticity coordinates of the best LEDs produced here can be found and used to show

the high colour quality available from this technology. The positions of some of the devices

on the CIE 1931 RGB colour space are shown in figure 6.8 with the current colour space

standard for HDtv (Rec 709) indicated by the triangle. As can be seen all of the colours are

beyond that of the high-definition colour standard. The blue device could be improved upon

through the use of a slightly longer wavelength dot (~ 470 nm).

CIE 1931 Chromaticity Coordinates

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.2 0.4 0.6 0.8

x

y

Red NQD device

Yellow NQD device

Green NQD device

Blue NQD device

HDtv colourtriangle

Figure 6.8 CIE coordinates for the emission of the best QD-OLED devices with the HDtv

colour triangle shown in black.



6.3. Cadmium-free QD-OLEDs

To be used in display applications in commercial products hybrid QD-OLEDs will

need move away from the heavy metal NQDs that are currently used in all demonstrations

of EL from QD-OLEDs. Access to the high quality cadmium-free NQDs based on indium

phosphide at Nanoco Technologies puts us in a unique position to investigate their use in

QD-OLEDs. InP-based NQDs also show promise to give even better device performance as

the positions of the conduction and valence band edges appear more favourable. As InP

nanocrystals have a shallower valence band maximum than CdSe the potential barrier to

holes being injected into the NQDs should be reduced. This situation is shown in figure 6.9

where the approximate energy levels for CdSe and InP dots are shown using values found

from the literature [6]. As for the CdSe QD-OLEDs, the InP NQDs act as electron traps

whilst the holes experience no barrier at the interface between the dots and the Alq3.

However, as the top of the valence band is not as deep in InP dots compared to CdSe the

use of a hole blocking layer may be more feasible.

Devices containing InP NQDs were attempted in the early development of the QD-

OLEDs using the phase separation technique and a device structure of

ITO/PEDOT/TPD:NQD/Alq3/Al. The dots used had a core/shell/shell structure of

InP/ZnS/ZnO with myristic acid (MA) used as the passivating ligand. The peak PL was

measured to be 595 nm with a FWHM of 86 nm and a quantum yield of 55%. These

devices only displayed weak and non-uniform electroluminescence at voltages of around 10

V. Increasing the voltage either led to sparks or to only marginally brighter EL even up to

21 V but the EL was still not at a level that could be measured. The devices were made in

the open air and the techniques had not yet been perfected so it was not surprising that these

devices were of low quality. Cadmium-containing devices had been produced at this time

with the same device stack as described here with luminance as high as 100 cd/m2. As the

energy level positions of the conduction and valence band in the InP NQDs are more

favourable for charge injection from the organic layers it is concluded that there must be

some sort of extra barrier to charge injection. These dots were not washed and so a large

excess of the myristic acid ligand could be acting as an insulator to charge injection. The

core-shell-shell structure is also designed to confine the exciton, electron, and hole to the

InP core of the dot through the use of wide band gap materials but will concomitantly act as

a barrier to charge injection. It is also possible that the phase separation process had not



worked correctly as at the time there was no access to an AFM to confirm that monolayer

coverage had been achieved.

EA = 2.3 eV

IE = 5.2 eV

Ф = 2.9 eV

Vacuum E = 0 eV

Ф = 4.6 eV

ITO PEDOT

IE = 5 eV

poly-TPD InP NCs

Eg = 2 eV

EA = 4 eV

Alq3

IE = 5.5 eV

EA = 2.8 eV

LiF/Al

EA = 2.3 eV

IE = 5.2 eV

Ф = 2.9 eV

Vacuum E = 0 eV

Ф = 4.6 eV

ITO PEDOT

IE = 5 eV

poly-TPD InP NCs

Eg = 2 eV

EA = 4 eV

Alq3

IE = 5.5 eV

EA = 2.8 eV

LiF/Al

Figure 6.9 Energy level diagram showing the energy levels of the various layers used in the

initial InP device stack. The positions of the InP nanocrystals are again approximate as they

depend on the size but are taken here from [6] and the levels for poly-TPD are taken from

[1].

Further unsuccessful attempts were made to incorporate InP NQDs into QD-OLED

devices, and it was noted that all of the dots supplied had up to 3 ZnS/ZnO shells which

would act as a considerable barrier to charge injection. These devices either did not show

any emission at all or had green EL that originated from the Alq3 layer. The first cadmium

free (CF) QD-OLEDs to demonstrate measureable EL ambitiously used blue InP/ZnS/ZnO

(MA) NQDs with peak PL at 470 nm with a FWHM of 90 nm. These were made once

many of the improvements to the deposition processes had been implemented and after

treating the pre-etched slides in UV-ozone for 10 minutes the Aldrich PEDOT:PSS was

spin coated in the laminar flow glove box at 6000 rpm for 60 seconds which was found to

give a 50 nm layer. This was then annealed on the hot plate at 200˚C in clean air for 20

minutes and then in nitrogen for a further 20 minutes. The poly-TPD was then dissolved in

chlorobenzene at a concentration of 10 mg/ml and spun at 1500 rpm for 60 seconds to

achieve a layer thickness of ~ 40 nm. This layer was annealed at 110˚C for 1 hour in the



nitrogen-filled glove box before depositing the NQDs as a separate layer. Spinning the dots

at a speed of 2000 rpm for 60 seconds at a concentration of 15 mg/ml was found to give a

layer 20 nm thick. This layer was then also annealed at 110˚C for 20 minutes before

transferring the devices to the evaporation chamber. 5 nm of BCP and 30 nm of Alq3 are

sequentially evaporated before changing the shadow mask and depositing 1.2 nm of LiF

and 100 nm of Al. The devices were encapsulated using the UV curing resin in the glove

box and left to completely cure over night. Upon testing the devices were found to emit

unsaturated green EL which appeared as a light yellow/green to the eye as a result of the

wide spectral range of the emission. The voltage required in these devices to drive EL was

again found to be high and as is seen in figure 6.10 had a number of components. The main

component is at ~ 520 nm, characteristic of Alq3 emission, indicating that the BCP layer is

not acting to confine holes in the dot layer. The emission component at ~ 600 nm cannot be

attributed to exciton recombination in the NQDs nor does it match the energy gaps found in

the transport materials. It is possible for charges to meet at the interface between two

materials and form an exciplex [7]. This usually occurs when it is energetically favourable

for the electron and hole to stay in their respective materials due to an energy barrier

between the two materials. From the energy level diagram no interface seems to match the

emission energy (~2 eV) of this EL. As these are blue NQDs the energy levels will be

significantly shifted from that in figure 6.9 and, as the electron effective mass is much

smaller than the holes, the majority of this shift will be in the conduction band (CB). This

makes the transition between the CB of the dots and the HOMO of the Alq3 the most likely

source of an exciplex. This again would suggest the BCP layer was not acting as is

expected. The maximum luminance of 40 cd/m2 was found at an applied bias of 14 V.



400 500 600 700 8000

200

400

600

800

1000

1200

1400

1600

Inte

nsity

(ar

b. u

nits

)

Wavelength (nm)

12 V 14 V 16 V

Device stack - ITO/PEDOT/pTPD/QD/Alq3/LiF/Al

QD - InP/ZnS/ZnO (MA)

Figure 6.10 Electroluminescence spectrum for the device stack shown at different applied

bias. The maximum luminance was found at 14 V.

From the experience gained with the cadmium-containing OLEDs it was known

washing the dots to remove excess ligand and un-reacted products increased the efficiency

of the devices considerably. The high voltages required in the above devices are indicative

of some current-limiting process and so the cadmium free NQDs should receive the same

treatment. Prior to the cleaning treatment the InP/ZnS/ZnO (MA) dots had a PL maximum

of ~585 nm with a FWHM of 93 nm and a PL quantum yield of 74%. At this point as the

source of PEDOT:PSS from Aldrich was nearly exhausted another formulation of PEDOT

from H.C. Stark known as Clevios (previously Baytron) was tested alongside the Sigma-

Aldrich formulation. The appropriate spin and annealing conditions were supplied by the

manufacturer but spin trials still needed to be conducted to ensure the layer deposited was

of the correct thickness. The substrates were treated with the UV-ozone before depositing

the two PEDOT:PSS formulations in layers with thicknesses of 50 and 80 nanometres. A

40 nm layer of poly-TPD was spun down from chlorobenzene and annealed at 110˚C for

one hour. The InP dots were spun down from toluene at 1500 rpm for 60 seconds to achieve

a layer of 20 nm. The Alq3, LiF and aluminium were then evaporated as before to deposit

layers of 35, 1.2, and 100 nm respectively. The devices were encapsulated in the nitrogen



glove box and made ready for testing. The turn on voltage for observing emission in these

devices was much lower than for the previous cadmium-free QD-OLEDs indicating that

washing the NQDs and the removal of excess ligand improved the performance. The EL in

all cases however, showed significant emission from the Alq3 layer, although the NQD

emission was found to be dominant for the first time. The trend for both types of PEDOT

and for the different thicknesses was that as the bias voltage was increased the contribution

of Alq3 to the EL spectra also increased (figure 6.11). The Alq3 contribution was also

slightly larger in the devices with thicker PEDOT layers.

400 450 500 550 600 650 700 750 8000.00

0.25

0.50

0.75

1.00

1.25

Nor

mal

ised

EL

(arb

. uni

ts)

Wavelength (nm)

4 V 5 V 6 V

voltage

Device - ITO/PEDOT/pTPD/QD/Alq3/LiF/Al

50 nm Aldrich PEDOT:PSS

Figure 6.11 Normalised EL for thinner Sigma-Aldrich PEDOT device at different bias

voltages.

The increased PEDOT:PSS thickness is likely to slow the transport of holes to the

NQD layer and as the NQDs act as traps to electrons but not holes this increases the

concentration of electrons relative to the holes in the NQD layer. In the devices with thicker

PEDOT layers the increased electron concentration in the NQDs leads to an increased rate

of Auger-assisted quenching of the NQD EL and so a higher contribution from Alq3.

Excitons are likely to be formed in the Alq3 layer close to the NQDs and Förster resonant

energy transfer is probably due to the overlap between the absorption spectrum of the



nanocrystals and the PL spectrum of Alq3 (see figure 6.5). As the current in the devices is

increased the concentration of electrons and so the rate of Auger-assisted quenching of the

QD EL will lead to a higher contribution from the Alq3 as is observed here. At increased

voltage the exciton generation region will also become wider leading to a greater

proportion of excitons created further than the Förster transfer distance away from the

NQDs leading to increased Alq3 emission. The maximum luminance of 511 cd/m2 and 780

cd/m2 is found at 7 V in both the thinner and thicker Sigma Aldrich PEDOT devices

respectively. The Clevios PEDOT devices have a maximum luminance of 500 cd/m2 and

560 cd/m2 at 6 V in the thinner and thicker devices respectively. The maximum luminance

of these devices has at least 50 % emission from Alq3 but by calculating the luminous

efficiency (figure 6.12 b) we see that both Clevios devices appear to give a greater

efficiency than the Sigma Aldrich PEDOT formulation.

2 3 4 5 6 7 80.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Lu

min

ous

effic

ienc

y (c

d/A

)

Voltage (V)

50 nm Aldrich 80 nm Aldrich 50 nm Clevios 80 nm Clevios

0 1 2 3 4 5 6 7 8

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

Cur

rent

(m

A)

Voltage (V)

50 nm Aldrich 80 nm Aldrich 50 nm Clevios 80 nm Clevios

a) b)

Figure 6.12 Graphs showing a) the I-V curves and b) the luminous efficiency for the

devices using different PEDOT formulations where the rest of the device stack is the same.

Despite the low efficiency and large contribution from Alq3, to the best of our

knowledge these represent the first demonstrations of InP-containing and indeed, cadmium-

free hybrid QD-OLEDs. To maintain Nanoco’s competitive edge in this area and to protect

Nanoco’s IP it was not possible to publish these results in a peer-reviewed journal in

accordance with the non-disclosure agreement (NDA).

As the performance of the cadmium-free devices was significantly inferior to the

cadmium-containing devices, yet with no differences between the device structures, our

attention turned to possible differences between the NQDs. The myristic acid ligand used in



the fabrication of the InP dots is the most obvious difference and could be a source of either

insulation to charge or may result in poor deposition or wetting of the NQD layer. To

investigate this, detailed AFM images were taken of the poly-TPD film (50 nm) deposited

on the PEDOT:PSS (50 nm) as in the device stack and also once a 15 nm layer of the InP

dots had been deposited. The various films were treated in the same way as they were when

building up an LED. The image taken of the PEDOT:PSS/pTPD layers is shown in figure

6.13 below where the total thickness was found to be ~ 100 nm as was expected from the

two 50 nm layers. Despite this it was noted that the mean roughness, Ra, of this layer at ~ 2

nm was higher than had been expected. Upon deposition of the NQDs a striation pattern not

visible by eye is observed showing thinner and thicker regions of the device. The average

total thickness is measured as 110 nm and the mean roughness is found to increase to ~ 3

nm. Exposure to a UV lamp and observation of red photoluminescence reveals that the dots

had indeed been deposited.

PEDOT/pTPD PEDOT/pTPD/NQDs 132.84 nm

-17.16 nm

PEDOT/pTPD PEDOT/pTPD/NQDsPEDOT/pTPD PEDOT/pTPD/NQDs 132.84 nm

-17.16 nm

Figure 6.13 Tapping mode AFM images taken for the polymer layers without (left) and

with (right) the NQDs deposited upon them. Lighter shades indicate higher features whilst

darker indicate lower.

As described in chapter 5 this striation effect is usually a result of the early

evaporation of solvent but as the dots here were spun from toluene, which had previously

given excellent films for the cadmium dots, it may be due to a large excess of the myristic

acid ligand. Attempts to spin down myristic acid only onto the pTPD layer were found to

be very difficult due to wetting issues between the MA and pTPD. The surface tension of

the MA was found to be very high resulting in poor films with incomplete coverage. It is



therefore possible that the use of this ligand may have degraded the performance of the

LEDs either chemically, electrically, or through poor layer morphology. As a result of these

findings it was decided to try two samples of InP NQDs passivated by different ligands.

One had the structure InP/In2O3/ZnS/ZnO and was passivated using undecylenic acid and

had peak PL of 512 nm with a FWHM of 49 nm. The other had the structure InP/ZnS and

was passivated using TOPO, as in the CdSe NQDs, and had a peak PL at 587 nm with a

FWHM of 80 nm. The charge balance of the InP devices was also investigated by making

devices where TPBi alone made up the electron transport and hole blocking layer, and

devices which incorporated both Alq3 and TPBi. These devices made use of the Clevios

PEDOT:PSS formulation and had the device structure PEDOT:PSS (50 nm)/p-TPD (50

nm)/ NQD (18 nm)/ and then either TPBi (40 nm)/Alq3 (10 nm) or TPBi (50 nm); both sets

had a LiF (1.2 nm)/Al (100 nm) cathode evaporated onto them. A device incorporating no

NQDs was also fabricated with the other layers the same to act as a control and to help

assess the processes occurring in the devices. The EL from this device without NQDs is

shown in figure 6.14

400 450 500 550 600 650 700 7500.0

0.2

0.4

0.6

0.8

1.0

1.2

Nor

mal

ised

EL

(arb

. uni

ts)

Wavelength (nm)

TPBi (50):Alq3 (0)

TPBi (40):Alq3 (10)

No NQDs

NQDs - InP/In2O

3/ZnS/ZnO (UA) PL = 512 nm

Figure 6.14 Electroluminescence spectra for the device without NQDs at 5 V (blue) and for

those devices containing the undecylenic acid capped dots with and without Alq3.



EA = 2.3 eV

IE = 5.2 eV

Ф = 2.9 eV

Vacuum E = 0 eV

Ф = 4.6 eV

ITO PEDOT

IE = 5 eV

poly-TPD TPBi

IE = 6.2 eV

EA = 2.7 eV

Alq3

IE = 5.5 eV

EA = 2.8 eV

LiF/Al

EA = 2.3 eV

IE = 5.2 eV

Ф = 2.9 eV

Vacuum E = 0 eV

Ф = 4.6 eV

ITO PEDOT

IE = 5 eV

poly-TPD TPBi

IE = 6.2 eV

EA = 2.7 eV

Alq3

IE = 5.5 eV

EA = 2.8 eV

LiF/Al

Figure 6.15 Energy level diagram for the device containing no NQDs.

By using the energy level diagram for the device without NQDs shown in figure

6.15 it is possible to identify the source of this emission. The emission from the organic-

only device was observed to be a very bright blue with a maximum brightness of over 1000

cd/m2 and a low turn on voltage of ~ 3 V. In this device electrons and holes are likely to

meet at the pTPD/TPBi junction and due to the high potential barrier for hole injection

from the p-TPD into TPBi, (as compared with the potential barrier for electron injection

from TPBi into p-TPD) the majority of excitons are likely to be formed on the p-TPD

molecules. The lack of EL at 355 nm (3.5 eV, the band gap of TPBi), would seem to

support this and suggests that any excitons that are formed on the TPBi are efficiently

transferred via the Förster mechanism to p-TPD. Whilst the majority of the EL is from p-

TPD there are other components which could be the result of exciplex formation between

the two layers.

The devices containing the UA-capped NQDs in comparison demonstrate extremely

weak EL and require very high voltages before any emission is observed at all (max

luminance = 23 cd/m2 at 14 V). It is also noted that these devices are very unstable (unlike

the control device) with the EL degrading and eventually disappearing in a very short time

period. The weak EL from the device containing Alq3 would appear to originate entirely

from this layer. Perhaps in this situation a large excess of electrons entirely quenches the

NQD luminescence and a small proportion of holes traverse the dots to the TPBi/Alq3



interface where they recombine in the Alq3. The devices containing no Alq3 clearly have a

component at ~ 510 nm which must originate from the NQDs; the 430 nm component is

probably from the pTPD. The size of these NQDs will mean that there is only partial

overlap between the emission from pTPD and the absorption spectra of the dots leading to a

reduced efficiency of exciton transfer leading to the 430 nm emission seen here.

The devices containing the InP dots capped with TOPO demonstrate EL which is

almost entirely from the NQDs yet is still weak with a maximum luminance of 30 cd/m2 at

14 V. This would seem to suggest that either the structure of the undecylenic acid capped

NQDs or the undecylenic acid ligand is causing the lack of QD EL. This could be a result

of the large energetic barriers presented by the In2O3/ZnS/ZnO layers preventing or

hindering direct charge injection into the NQDs or that the acid ligand is damaging or

leading to poor morphology of the NQD layer. As seen in figure 6.16 devices with and

without Alq3 emit mostly from the NQDs but a small component at ~ 430 nm, likely from

the pTPD, is present in both, having a larger contribution in the device without Alq3.

350 400 450 500 550 600 650 700 7500.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ised

EL

(arb

. uni

ts)

Wavelength (nm)

TPBi (40):Alq3 (10)

TPBi (50):Alq3 (0)

NQDs - InP/ZnS (TOPO) PL = 590 nm

Figure 6.16 Electroluminescence spectra for the devices containing the TOPO-capped dots

with the core-shell structure with (red) and without (black) Alq3.



The results here as well as those for previous InP QD-OLEDs would seem to

indicate that the use of acid ligands result in poor devices where the EL is mostly from the

organic charge transport layers. The structures used previously also had a number of shells

which may have acted as a barrier to charge injection into NQDs. The use of a simple core-

shell structure with TOPO as the passivating ligand has however, produced devices that

emit almost entirely from the quantum dot layer and generate measurable

electroluminescence that is observable under normal room lights.

To follow on from the success of the above devices another batch of the core-shell

InP/ZnS NQDs was made using TOPO as the passivating ligand. This batch of NQDs had a

high PL quantum yield of 74% and a PL peak wavelength of 592 nm with a FWHM of 67

nm. In an attempt to balance the numbers of electrons and holes in the NQD layer the p-

TPD layer was this time reduced in thickness. Changing the electron transport layer

thickness and material was found to give only moderate gains, and so by increasing the

number of holes at the NQD layer the hope was that there would be fewer unpaired

electrons and so less quenching of the NQD emission. Devices were therefore made using

the device structure that was most successful for the cadmium dots (PEDOT (50 nm)/pTPD

(50 nm)/NQD (18 nm)/TPBi (20 nm)/Alq3 (30 nm)) and a device structure with only a 22

nm layer of pTPD. As can be seen from the EL spectra below, devices made with both

thicknesses of p-TPD gave pure NQD EL with no other components to the emission.

The luminance-current-voltage (LIV) data for these devices revealed that the thicker

layer of pTPD led to devices that were less stable, gave a lower luminous efficiency, and

reached a lower maximum brightness (figure 6.18) than for devices using a thinner layer of

pTPD. The lower turn on voltage of ~ 6 V for these devices in comparison to the ~ 10 V

found for the previous batch of InP/ZnS seems to suggest that either the cleaning procedure

was more effective in the second batch of dots or possibly that these NQDs were simply of

better quality. The maximum luminance was ~ 40 % higher in the device with a thinner

layer of poly-TPD and at a luminance of 100 cd/m2 the thinner poly-TPD device had a

luminous efficiency of 0.71 cd/A in comparison to the 0.29 cd/A measured for the thicker

poly-TPD device. The higher efficiencies found in the device with a thinner layer of pTPD

is attributed to a better balance of electrons and holes in the emitting NQD layer. To the

best of our knowledge this again represents the first demonstration of pure quantum dot



emission from a cadmium-free QD-OLED but under the terms of the non-disclosure

agreement cannot be published in a peer reviewed journal.

450 500 550 600 650 700 750 8000.0

0.2

0.4

0.6

0.8

1.0N

orm

alis

ed E

L (a

rb. u

nits

)

Wavelength (nm)

50 nm pTPD 22 nm pTPD

NQDs - InP/ZnS (TOPO), PL = 592 nm

Figure 6.17 Electroluminescence spectra for the devices containing the second batch of

TOPO-capped dots with the core-shell structure with a thin (red) and thick (black) pTPD

layer.

0 2 4 6 8 10 12 14 160.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Current density

Cur

rent

den

sity

(A

/cm

2 )

Applied bias (V)

50 nm poly-TPD layer

0

25

50

75

100

125

150

Luminance

Lum

inan

ce (

cd/m

2 )

a)

0 2 4 6 8 10 12 14 16

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Current density

Cur

rent

den

sity

(A

/cm

2 )

Applied bias (V)

22 nm poly-TPD layer

0

25

50

75

100

125

150

175

200

Luminance

Lum

inan

ce (

cd/m

2 )

b)

Figure 6.18 Current-voltage-luminance characteristics for the devices containing the new

batch of InP/ZnS (TOPO) dots with a) a 50 nm and b) a 22 nm layer of poly-TPD.



References

1. Sun, Q., et al. Nature Photonics, 2007. 1(12): p. 717-722. 2. Berntsen, A., et al. Optical Materials, 1998. 9(1-4): p. 125-133. 3. Anikeeva, P.O., et al. Nano Letters, 2009. 9(7): p. 2532-2536. 4. Zhao, J., et al. Nano Letters, 2006. 6(3): p. 463-467. 5. Y. H. Niu, et al. Advanced Materials, 2007. 19(20): p. 3371-3376. 6. M. Pientka, et al. Nanotechnology, 2004. 15: p. 163-170. 7. Kohary, K.K. Journal of materials science: Materials in electronics, 2007. 20(1): p.

10-14.

Conclusions and further work



Nanocrystal quantum dots are exciting, low dimensional structures that are

interesting for both research looking into new fundamental physics and as novel optical

materials with great technological potential. NQDs are central to the research presented in

this thesis which can be split into two distinct sections. The first section investigated the

photo-physics and optical properties of NQDs specifically related to the observation and

characterisation of the multiple exciton generation phenomena in different types of NQD.

The motivation for this was to assess the possible use of NQDs exploiting MEG in novel

high efficiency photovoltaics. The second section grew from a project conducted with

Nanoco Technologies to demonstrate the use of their proprietary quantum dots in hybrid

organic light emitting devices and also served to show colloidal quantum dots in working

optoelectronic devices.

In order to investigate the MEG process in quantum dots optical spectroscopy was

used with both time correlated single photon counting and ultrafast transient absorption

experiments designed and built up for the purpose of characterising MEG. These

complementary techniques have been used to observe MEG in nanocrystals with TCSPC

also being used to observe the single exciton dynamics. The TA set-up allowed us to probe

quantum dots with band edges through the visible spectrum to the near infra-red and

allowed the NQDs to be excited by photons with an energy as high as 5.2 eV. It has been

shown that efficient MEG occurs in InP nanoparticles when excited with a photon energy

greater than or equal to 2.1 times the band gap. The highest measured quantum efficiency

of MEG was 118 %, corresponding to an average of 1.18 excitons produced per absorbed

photon with energy equal to 2.6 times the band gap. The threshold for MEG measured here

is consistent with both the conservation of energy model and the energy partition model and

is noted to be lower than the values found for other NQD materials such as PbSe (2.85Eg)

or CdSe (2.5Eg). This was the first time MEG had been measured and characterised in InP

NQDs and resulted in the work being published as a rapid communication in the journal

Physical Review B. The controversy surrounding the efficiency and even observation of

MEG in quantum dot systems makes this measurement in a new dot material a good

contribution to the debate.



To make optimum use of the solar spectrum a low MEG threshold is required

otherwise only marginal gains in efficiency would be made in a solar cell utilising MEG.

The range of band gaps available from InP NQDs (1.5 – 2.0 eV) [1] limit their ability to

utilise the MEG process in high efficiency solar cells. A band gap of 0.9 – 1.1 eV would

make the best use of the range of photon energies emitted by the Sun [2]. The use of a type

II structure, however, can reduce the effective band gap of an NQD and additionally could

lead to longer biexciton lifetimes such that the probability of extracting the charge carriers

before recombination increases. To investigate this CdSe/CdTe/CdS type II NQDs were

used as a model system and the transient absorption set-up was used to observe the carrier

dynamics. The absorption transients for this quantum dot were well described by two

exponentials with time constants of 37 ± 1 ps and 339 ± 14 ps. A large ratio between the

amplitude of the fast and slow time components was found for excitation with photons of

energy below the threshold for MEG and at fluence levels where the absorption of more

than one photon had a low probability. A number of possible mechanisms are proposed to

explain these results but more experimentation is needed for these to be conclusive. Further

work on type II dots is proposed with the need for new batches of type II quantum dots that

are both stable under UV excitation and when stored. The quantum dots investigated here

were found to degrade and eventually precipitate when subjected to deep UV excitation and

their PL quantum yield reduced over time. The use of CdSe and CdTe as either the core or

shell is recommended as the type II structure as these materials, having been well-

characterised and investigated, can act as a model system before applying any insights to

other materials.

The MEG efficiency in PbS nanocrystals was also investigated using the transient

absorption set-up. There are numerous reports of MEG in Pb chalcogenide quantum dots

with PbS being one of the first materials in which MEG was demonstrated [3]. The

motivation here is based upon the way these quantum dots were made. The method

involves the use of greener chemical methods where olive oil acts as both the organic

capping agent and coordinating solvent with the reactions carried out at lower temperatures

than the common synthesis approach which uses long chain amines such as TOP and

TOPO. This method was developed by researchers in the School of Chemistry at the

University of Manchester and is described in detail in our jointly published paper [4]. The

threshold for MEG in the PbS quantum dots investigated here was found to be 3±0.2Eg.



This is within error of the threshold found by Ellingson and Beard of 2.8Eg. The maximum

quantum yield of MEG was measured to be 138 ± 4 % at a pump photon energy of 4Eg.

Investigations into MEG in PbS NQDs also conducted by Ellingson and Beard found

efficiencies of ~ 155 % at 4Eg [3], whereas more recent measurements found a maximum

MEG efficiency of 125 % even when excited by photons with energy 5Eg [5]. The PbS

nanocrystals investigated in this thesis were fabricated using a different technique to the

ones in the literature which may account for the difference. Experiments on PbSe and PbS

quantum dots have shown a batch-to-batch variability in the MEG yields of up to 30 % [6]

which would also more than account for the difference between the values reported here.

Differences in the surface properties of quantum dots would result in different single and

possibly multiexciton dynamics; the ever present unknown influence of the surface is an

area in need of investigation so that its effect can be quantified.

The controversy surrounding the efficiency of MEG in nanocrystal quantum dots

stem, in part, from the fact that nearly all attempts to measure and characterise MEG are

based upon indirect spectroscopic methods. The most credible argument would come from

a measurement demonstrating an internal or external quantum efficiency of over 100 %

with respect to the number of photons incident. Currently there have been only two

attempts to demonstrate enhanced photocurrent in an optoelectronic device utilising MEG.

The first conducted at the National Renewable Energy Laboratory (NREL) by Nozik’s

group reported no appreciable MEG photocurrent from a PbSe based nanocrystal solar cell

[7]. Despite a group at the University of Toronto obtaining a significant increase in internal

gain in a PbS NQD-based photodetector for photon energies above 2.7Eg, the internal

photoconductive gain is only marginally over 100 % for very deep UV excitation. This

result, however, does indicate that the NQDs in this device demonstrate MEG and that this

gives real benefits in terms of increased photocurrent. Further work on producing

nanocrystal-based optoelectronic devices which use the MEG process to increase the

internal and external quantum efficiency is necessary to move the debate on. This will

require both nanocrystal and device design that can make use of the multiple charge carriers

before Auger recombination takes place. As the rate of Auger decay is very high with

lifetimes on the order of ~ 100 ps, this will necessitate rapid extraction of both charge

carriers or excitons.



The second section of work presented in this thesis involved the fabrication of

hybrid QD-OLEDs which demonstrated the viability of Nanoco Technologies quantum dot

technology for display purposes. Devices have been made using a number of different

designs and upon optimisation and investigation a device structure was found that gave

QD-OLEDs that demonstrated bright, colour-saturated emission that originates from the

quantum dot layer only. Through experimentation both direct charge injection into the

quantum dots and Förster resonant energy transfer are found to contribute to the

electroluminescence. It is thought however, that whilst it may be hard to precisely balance

the numbers of electrons and holes reaching the NQD layer, schemes which optimise the

FRET efficiency between layers may be more easily realised. A more detailed

understanding of the processes occurring between organic materials such as polymers and

small molecules would help show how brightness and efficiency can be increased further.

Cadmium-containing devices have been fabricated with low turn on voltages and quantum-

dot-only electroluminescence at a luminance that is more than sufficient for display

purposes. Red, green and blue devices have been produced which could feasibly make up

the red, green and blue pixels of a flat panel display device. Such a device would appear

more realistic due to the larger number of colours in the human gamut that it could

reproduce. In a very short period of time the performance of these devices reached similar

levels to that of the best devices demonstrated with luminance values of 1000s cd/m2 and

luminous efficiency values of several cd/A.

As has been set out the use of heavy metals in consumer electronics is strictly

controlled in the majority of large economies around the world. As a result the numerous

demonstrations of QD-OLEDs which all contain cadmium quantum dots cannot be used

commercially. For this reason incorporating InP quantum dots into the devices is a

necessary step towards producing a consumer product. This has been demonstrated here

with the best success coming from a device that emitted electroluminescence from the

quantum dot layer only. To maintain the benefit of using quantum dots in these types of

devices the narrow and spectrally pure emission from the quantum dots is absolutely vital.

Although the efficiency and brightness was found to be much lower than that for the

cadmium containing devices, this performance is likely to be far from what is possible. The

precise reasons why the InP dots do not perform as well as the Cd-based dots will need to

be investigated. This results presented here however, are an important step in QD-OLED



development as it is the first time that heavy-metal-free QD-OLEDs have been reported.

The project up to this time was concerned mainly with proving the benefits and

viability of Nanoco’s quantum dots and so the devices were assessed in terms of luminance

and luminous efficiency. Although qualitative trends in device lifetimes have been noted,

where in general increases in device efficiency have led to more stable devices, rigorous

scientific measurement of device lifetime has not been conducted. For a real display device

a requirement that it can operate for 10s of thousands of hours without the brightness

dropping below 50 % is common. As such an experimental set-up to measure the lifetime

of these devices is necessary to more fully characterise the performance of these devices.

As well as competing with other QD-OLEDs this technology will also have to compete

with both current display technologies, such as LCD and plasma, and future display

technologies such as OLEDs, polymer LEDs (PLEDs), and phosphorescent LEDs

(PHOLEDs). These latter technologies have had over a decade more research and so it is

not surprising that QD-OLEDs still demonstrate efficiencies that are an order of magnitude

lower. QD-OLEDs have numerous advantages over other types of display; the main

advantage being the more lifelike and saturated colours possible, as has been demonstrated

here. They also offer the possibility of more stable devices owing to the inherent stability of

the semiconductor materials the quantum dots are made from when excited in comparison

to organics. The role of the ligands is one area that requires further investigation, as found

here, when found in excess they both degrade the device quality in terms of morphology

and act as a barrier to charge injection into the QDs. Even more promising is the possibility

of using functionalised ligands which play a positive role in the operation of the devices.

This will involve ligand exchange, an already active area of research on colloidal quantum

dots and close collaboration between device physicists and chemists.

In summary the work presented here has shown that a variety of NQD structures can

demonstrate efficient multiple exciton generation which could enable the production of

third generation photovoltaics. These PV devices could be fabricated cheaply using wet

chemistry processes and have high efficiencies such that they become economically viable

for electricity generation. NQDs have also been incorporated into QD-OLEDs which

provide a viable alternative to current display technologies as well as opening up new

opportunities for display technology. Examples of this include flexible displays, transparent



heads up displays (HUD), on the curved surface of a car windscreen for example or for

large signage on the sides of buildings.

References 1. Klimov, V.I., ed. Semiconductor and Metal Nanocrystals: Synthesis and Electronic


2. Hanna, M.C. and A.J. Nozik. Journal of Applied Physics, 2006. 100(7): p. 074510-8. 3. Ellingson, R., et al. Nano letters, 2005. 5(5): p. 865-871. 4. Akhtar, J., et al. Journal of Materials Chemistry, 2010. 20(12): p. 2336-2344. 5. Nair, G., et al. Physical Review B, 2008. 78(12): p. 125325. 6. McGuire, J.A., et al. Accounts of Chemical Research, 2008. 41(12): p. 1810-1819. 7. Law, M., et al. Nano Letters, 2008. 8(11): p. 3904-3910.

Photo-physics and applications of colloidal quantum dots

Documents

Transcript of Photo-physics and applications of colloidal quantum dots