Towards thermally stable poly (3-hexylthiophene) based photovoltaic devices

8/4/2019 Towards thermally stable poly (3-hexylthiophene) based photovoltaic devices

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TOWARDS THERMALLY STABLE POLY (3-HEXYLTHIOPHENE)

BASED PHOTOVOLTAIC DEVICES

NAUMAN MITHANI

CHEM 481

Supervisor: DR. STEVEN HOLDCROFT

Term 1104


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INTRO: need for alternative/renewable energy

2

eventuality

economics

smoggreenhouse extreme weather


3/24

INTRO: energy consumption forecast

3Source: Arvizu, D; National Renewable Energy Laboratory: Milken/Sandia workshop for financial & capital market leaders, 2007/10/23.


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INTRO: photovoltaic materials - organic vs.inorganic

4

lightness of weight

flexibility

large scale

low cost

printability / substrate

flexibility

molecular engineering

band gap control

lower

efficiency

poorer

stability


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INTRO: choice oforganic materialsefficiency & promise

5

organic photovoltaic materials

-conjugated polymers

P3HT & PCBM *

poly (3-hexylthiophene)(P3HT)

donor (D)/hole carrier

[6,6]-phenyl C61 butyric acid methyl ester(PCBM)

acceptor (A)/electron transporter

* Yao, Y.; Hou, J.; Xu, Z.; Li, G.; Yang, Y. Advanced Functional Materials2008, 18, 1783.


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INTRO: mechanism ofphoto (photon)voltaic(charge)

6

diffusion to interface2 exciton dissociation3exciton generation1


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INTRO: mechanism ofphoto (photon)voltaic(charge) {contd.}

7

charge transport photocurrent5free electron/hole pairs4


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INTRO: the mish-mash ORbulk heterojunction (BHJ)

8

scale of exciton diffusion is (tens of)

nanometres

exciton must reach interface in


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INTRO: prelude to project

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P3HT & PCBM are ultimately immiscible

eventual microscopicscalephase segregation (103 over limit)

interruption of bi-continuity (discrete aggregates)

P3HT & PCBM blend

Thermal annealing: somePCBMpost-functionalisedonto P3HT

serves to tethersome PCBM to P3HT

serves asaccelerated lifetime evaluation of (thermal) stability

P3HT-(graft-PCBM)& PCBM blend

Gholamkhass, B.; Peckham, T. J.; Holdcroft, S. Polymer Chemistry2010, 1, 708.

excess post-functionalisation

interruption of P3HT chains self-organisation & semi-crystallinity

decrease in electronhole mobility

performance & stabilitybi-continuity& semi-crystallinity

annealing duration/temp. must be optimised to avoidphase segregation

100 m

10 m


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EXPT.: synthesis ofpoly 2%(3-bromo-4-hexylthiophene), 1

10Gholamkhass, B.; Peckham, T. J.; Holdcroft, S. Polymer Chemistry2010, 1, 708.

where extent ofpost-functionalisation,

n = 0.02

P3HT98% regioregular (H-T)

1

NBS (2% by mole)


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EXPT.: P3HT-2%(TEMPO-styrene) macroinitiator, 2

Gholamkhass, B.; Peckham, T. J.; Holdcroft, S. Polymer Chemistry2010, 1, 708. 11

n = 0.02

1 2


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EXPT.: synthesis ofP3HT-2%(graft-(ST-stat-CMS)), 3


n = 0.02

2 3

x:y = 1:1

nitroxide-mediated living

radical polymerisation


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RESULTS & CHARACTERISATION: 1H-NMR ofP3HT-2%(graft-(ST-stat-CMS)), 3

13

CMS:ST provided by

m:k

1:12

93.1:99.0


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RESULTS & CHARACTERISATION:aggregation ofP3HT-2%(graft-(ST-stat-CMS)), 3

14

UV-Vis spectra of3 in CHCl3

Polymer aggregation

induced by MeOH

max of P3HT backbone

shifts from 452 nm to

530/570 nm -* stacking of the

P3HT chains

semi-crystalline

arrangement of chains

occurrence of vibrionic

side bands

2% graftdensity is acceptable towards P3HT self-organisation

long-range -conjugation (hole mobility) is NOT hampered


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EXPT.: synthesis ofP3HT-2%(graft-(ST-stat-N3MS)), 4


n = 0.02

3 4

x:y = 1:1


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EXPT.: blendofP3HT-2%(graft-(ST-stat-N3MS)) + PCBM (1:1)


Step 2: spin-coat on quartz substrate

slow evaporation

bulk

heterojunction

&Step 1:

freePCBM


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EXPT.: 1:1 blendofP3HT-2%(graft-(ST-stat-(NMS-graft-PCBM))) + PCBM


in excess relative

to number of

reaction sites on 4

blend of

4

5

freePCBM

graftedPCBM


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pre-annealed graftcopolymer blend post-annealed graftcopolymer blend

RESULTS & CHARACTERISATION: optical microscope photographs ofblend, pre- & post-annealed


100 m

10 m1:1

P3HT+PCBM

system

microscopic

phase

separation

discrete PCBM aggregates

semi-crystalline P3HT domain


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P3HT abs.PCBM abs.

RESULTS & CHARACTERISATION: UV-Vis spectra ofblend, pre- & post-annealed


pre-anneal: P3HT max blue-shifts

from 525 nm to 500 nm

-* stacking of the P3HT chains

is hindered

post-anneal: P3HT max appears to

be less blue-shifted

-* stacking of the P3HT chains

is lesshindered

same relative intensity of vibrionic side-bands to abs. maximum suggests semi-crystalline

arrangement stilloccurs


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CONCLUSION

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1:1 P3HT(copolymer) + PCBM blends do NOT suffer microscopic phase separation

Thermal annealing was successful: greater degree of self-organisation of P3HT

as shown by red-shifting in UV-Vis spectra relative to pre-annealed blends

temperature of 150 oC and duration of 1 hr. are acceptable

P3HT(copolymer)+PCBM blends exhibit thermal stability


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FUTUREWORK

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Photoluminescence spectroscopy: better understanding of SPR such as electron hole formation

efficiency by PL quenching

Detailed morphology study by Transmission Electron Microscopy: determination of P3HT(copolymer)

and PCBM domains, and changes after annealing

Addition ofhigh boiling point additive e.g. 1,8-octanedithiol*to solution in film-casting: selective

dissolution of PCBM

enhanced solubility of PCBM uniform film formation, no spots of insolubility

Photovoltaic performance parameters: hole mobility, current densityvs.potential, external quantum

efficiency(EQE) and overallpower conversion efficiency(PCE)

Optimisation ofblend ratio

quantitative analysis by Thermo-gravimetric analysis (TGA) & NMR

*Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim, J. Y.; Lee, K.; Bazan, G. C.; Heeger, A. J. Journal of the American Chemical

Society2008, 130, 3619.


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ACKNOWLEDGEMENTS

22

Bobak Gholamkhass

Dr. Steven Holdcroft

Dr. Timothy Peckham

Dr. Mahesh Kulkarni

Dr. Jaclyn Brusso

Kristen Soo

Graeme Suppes

Owen Thomas

Martyn Stocker

Jessica Luo

Emily Tsang

Ami Yang


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Fin


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Towards thermally stable poly (3-hexylthiophene) based photovoltaic devices

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