Introduction to Organic ElectronicsIntroduction to Organic Electronics

Marcus Halik

Organic Materials & Deviceswww.omd.uni-erlangen.de

University Erlangen-NürnbergMartensstrasse 07

91058 Erlangen (GERMANY)

"Normally most people don't think of plastic as an electrical

conductor or semiconductor, but certain organic materials have amazing electronic properties"

Bengt

Nordén

-

Chairman of the Nobel Committee for Chemistry 2000

Alan J. Heeger, Alan G. MacDiarmid and Hideki ShirakawaNobel Prize in Chemistry 2000

Conductivity in Plastics

Ene

rgy

conduction band

valence band

Conductor –band overlap

metals (Cu, Al,…)

Semiconductor –small band gap

(Si: 1.12 eV)

tunable by doping

Insulator –large band gap

(diamond: 5.2 eV)

most organic materials

ΔE

Conductor, Semiconductor, Insulator

= e-

ΔE

valenceband

conduction band

ground state(charge compensated

by nuclear charge)

excitation charge transport

each electron leaves a hole in the valence bandelectrons and holes are movable by applying an external field

+ -

Generation and Transport of Charge

Structures in Organic MaterialsPolyethylene PE

(coiled 3D structure)Polyacetylene

(flat chain structure)

90°

C-C single bondtetrahedral geometry

C-C double bondplanar geometry

Extended Systems

binding energy (σ

-

bond) =

347 kJ/molbinding energy (π

–

bond) = 260 kJ/mol

Increasing the number of double bonds

Ethylene Butadiene

E

ΔE

LUMO

HOMO

~ 2 eV

Pentacene

n = 1 n = 2 n = 3 … n

HOMO

LUMO

valence band

π

π∗

Ener

gy Orbitals “BAND”

ΔE

Organic Semiconductors

Hydrocarbons with delocalized electrons –π-conjugation

**

n

conduction band

Organic Electronics

OFET (TFT)OLED solar cell OPV memory

molecular memory, diodes, transistors etc.

thin film devices

molecular devices

Organics vs. Silicon Technology

• independent from single crystalline substrates• low temperature processing• low complexity

• low performance• new technology - new processing required

Intel© Xenon PhiTM chip – 201262 core CPU – 22nm pitch(> 5.000.000.000 transistors)

first point contact Ge-transistor (Bell Labs - 1947)

Trends in Organic Electronics

molecular electronicsthin film electronics

large feature size – flexible substratesspecialties - limited volume

large volume – low performance

supplementary products

few nanometer feature size(e.g. Green et al. Nature 445 414 (2007)

“next generation” products

Samsung 2013

Examples

OFET (TFT)

OLED

solar cell OPV

flexible memory

electrochromic cellspolymer battery

Historic Milestones

1906: Photoconductivity in anthracene single-crystals discovered

1960: TCNQ with conductivity of 100 S/cm synthesized

1963: Electroluminescence in anthracene discovered

1973: Synthesis of the first true organic metal (TTF-TCNQ) reported

1977: Doping of polyacetylene demonstrated (2000 Nobel Prize)

1982: Organic thin film light emitting diode (OLED) demonstrated

1986: Organic field effect transistor demonstrated

1986: First efficient organic solar cell demonstrated

1995: Organic integrated circuits demonstrated

2004: Seiko-Epson demonstrates 40-inch OLED panel

2004 Graphene (Nobel Prize 2010)

2009 Samsung Galaxy with AMOLED

2009 Konarka OPV bagEpson

Cathode Anode

Tang et al., APL 51, 913 (1987)

hole transport layer

electrontransport

layer

Organic Light Emitting Diodes (1987)

Organic Solar Cell (1986)

Tang et al., APL 48, 183 (1986)

hole transportlayer electron transport

layer

Tsumura et al., APL 49, 1210 (1986)

Organic semiconductor

Heavily doped silicon wafer

Thermally grown SiO2 gate dielectric

Au Source Au Drain

Organic Field Effect Transistors (1986)

Transport on Different Length Scales

pixel devices(OLED, solar cell)

5 -100 nm

μm -

cm 1-

500 μm

< 5 nm

interface devices(transistor)

charge transport on nm-scalelow charier mobility required

charge transport on μm-scalelarge charier mobility required

Polymers vs. Small Molecules

Pentacene (twice sublimed)

Poly-3-hexylthiophene (regio-regular)

• charge transport along chain and by inter-chain hopping (lattice)

• deposition from solution (pot. printing)

• charge transport through ππ- stacking overlap

• deposition from vapor phase or solution

Polymers Small Molecules

0.8 nm

Poly-3-hexylthiopheneMn ~ 30.000 g/mollength (stretched) ~ 70 nmlength (coiled) ~ 20nm

mismatches in (3,5 or 3,4 position)average conjugation length is~ 10 - 15 repeat units (6 nm!!!)

Molecular Chain and Morphology

How can we transport charge over long distances (μm-range)????

~ 70 nm

~ 20 nm

μ

= 0.1 cm2/Vs μ

= 0.0002 cm2/Vs

Orientation - P3HT

Sirringhaus

et al. Nature 401,

685 (1999)

Lattice orientation dominates the polymer performancePerformance is limited by semi-crystalline behavior

Orientation depends on:• surface (treatment)• temperature• deposition method, …

• micron-size grains with terracing• efficient ππ-interaction (HOMO overlap)

Gundlach

et al., IEEE Electr. Dev. Lett. 18, 87 (1997) Knipp

et al., J. Appl. Phys. 93, 347 (2003)Troisi

et al. JCPB 109, 1849 (2005)

side top

E

How to Assemble Molecules ?

π-systems tend to self-organize (ππ-stacking)poly-crystalline films

“Order” as General Rule for Transport

H. Faber,, Adv. Mater. 21 (2009) 3099-3104

monolayer of Zink Oxide nanoparticles (diameter 5 nm)

x

y

Molecular Order and Charge Transport

amorphous polymer semi-crystalline polymer

thin film (small molecules)

organic single crystal

week interactionsome “contact points”un-isotropic transport

poor mobility

10-4 cm2/Vs

partial order ( , )coexistence crystallineand amorphous fraction

statistically un-isotropic transport

low mobility

~ 1

cm2/Vs

strong orderbreak in orientation(grain boundaries)

un-isotropic transportgood mobility

10

cm2/Vs

very strong orderno grain boundaries

strong isotropictransport

very good mobility

> 10

cm2/Vs

OLED - Display

(press release 27.06.2013: Samsungs first OLED TV- 1.40 m diagonal curved screen- ~ $ 9.000,-

Galaxy S4 full HD

OLED - Display

RGB – mixing (green down sized)smaller feature size 50 μm (resolution limit human eye)Full HD speedpower consumption

OLED - lighting

Mesta et al. Nature Materials 12, 652–658 (2013)

OLED - lighting

complex stack precise control of layer thicknesstunable colorlarge area, flexible substrateslong term stabilitypower consumption

OPV Approaches

cathode

anode

e--donor e--acceptor

dark currentlight current

h+ e-

I

V

VOC

•

••

ISC

MPP

FF

~ VOC

OPV – Bulk Hetero Junction

P = U x I PerformanceFF = PMPP /(VOC x ISC ) Fill Factor

OPV in a Contest

OPV – Future / Challenges

significant contribution to global energy demand (15 TW / year = 15 x 1012 W)

Heliatek

Modul-Efficiency !!!!! (VOC , ISC , FF)Cost/Watt – Price!!!!Stability, CO2 - footprint transparency

0 1 20

2x10-10

4x10-10

6x10-10

8x10-10

VGS = 0V to 1.5V (0.125V steps)

Drain-source voltage (V)D

rain

curr

ent(

A)

VGS

VDS

ID

Source Drain

Gate dielectric

Substrate

Organic semiconductor

Gate electrode

Channel

OTFT - electronic switch modulated by gate voltagedigital “0” and “1” state

ICs (RFID), backplanes (diplays, sensors)

electrical output characteristics

Organic Thin Film Transistor

“0”

“1”

Important Electrical Parameter

a) Carrier mobility μ

= 0.5 cm2/Vsb) Threshold Voltage VTH = -3 Vc) Subthreshold slope 0.2 V/decaded) On/off current ratio 108a

b•

c d

transfer characteristics

“0”

“1” W/L (100μm/10μm) = 10

W/L (1000μm/10μm)= 100

ID

=1

2μeff

Ci

W

L(VG

-VT

)2

Flexible backplanes for OLED, E-Ink

homogeneous large currentsYield = 100 %

Application Fields

Sensor backplanes with integrated memory

homogeneous large currentsLong retention

Sekitani Science 2009

ICs – Dice (Simple Game)

Full logic &graphics driver in

organic electronics on plastics

Application board

Input - “start” game

Gate Count3 D-FlipFlops = 27 NANDs1 OR = 3 NANDs1 NAND = 1 NANDTotal = 31 NANDs

= 155 TFTs

Device

Circuit layout

Output - LED

Operation -

Game

Current - Mobility Speed – Mobility, Dimensions

Large mobility > 10 cm2/VsDisplay backplane

Speed larger 200 kHz (display)13.56 MHz (ICs e.g. RFID)

OTFT Challenges

Performance: a) film thickness control and homogeneityb) spatial resolution

extremely high:multilayer,interfaces,contact doping,carrier balance (< 3 nm)uncritical to high:large area devicesPixel x0 μm2 440ppiLighting cm2/m2

OLED TFTs OPV NVM

extremely high:interfaces,driving voltage,order(~ 5 nm)

high:channel length ~ speedS/G overlap few μm

uncritical:blendself-organizedinterfaces,absorption

uncritical:large area devicessupply/draincm2

high:write-erase-read voltage,signal ratio

uncritical –extremely high:integration density

Boundary Conditions

Suitable performance depends critically on control of dimensions and interfaces on nanometer scale

Technical Approaches in Thin Film Processing

throughput/expensesfilm homogeneity/ spatial resolution

approach

spin coating/spray coating excellent/ poor batch/

large area

printing (μ-CP, ink jet)

resolution ~ speed-1

medium area

PVD, CVD

critical/ good

excellent/ poor good / complex

T. Sekitani

et al. PNAS. 105, (2008) 4976-4980

Polyethylene terephthalate (PET)• Tg ~ 80°C, Tm ~ 250°C

Paper• multi component substrate• ∅

fiber ~ 10 – 20 mm

Polyethylene naphthalate (PEN)• Tg ~ 120°C, Tm ~ 265°C

Glass• smooth surface, rigid

Electronics everywhere

Sounds good – but (thermal, mechanical, solvent, optical stability;barrier properties, adhesion, costs)

Also: stainless steel, textiles

20 μm