012 Thesis final -...

N° d’ordre : 4227

THÈSE

Présentée à

L’UNIVERSITÉ DE BORDEAUX 1 ÉCOLE DOCTORALE DES SCIENCES CHIMIQUES

Par HARIKRISHNA EROTHU

Pour obtenir le grade de

DOCTEUR SPÉCIALITÉ : CHIMIE POLYMÉRE

SYNTHESIS AND PHOTOVOLTAIC APPLICATIONS

OF NOVEL COPOLYMERS BASED ON POLY(3-HEXYLTHIOPHENE)

Date de soutenance : 25 Février 2011

Devant la commission d’examen formée de :

M. M. L. TURNER Professeur, Université de Manchester, UK Rapporteur Mme L. LUTSEN Directrice de recherche, IMEC, Belgique Rapporteur M. P. HUDHOMME Professeur, Université de Angers, France Examinateur Mme L. VIGNAU Maitre de conférences, IPB, Bordeaux Examinatrice M. E. CLOUTET Chargé de Recherche CNRS, LCPO Directeur de thése M. H. CRAMAIL Professeur, Université de Bordeaux 1 Directeur de thése M. R. C. HIORNS Chargé de Recherche, CNRS, PAU Invité

-2011-

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Abstract

Synthesis and photovoltaic applications of novel copolymers based on poly(3-hexylthiophene)

Abstract : The performance of organic photovoltaic cells mainly depends on the active layer nano-morphology. Rod-coil block copolymers (BCPs) are well known in their ability to self-assemble into well-ordered nanoscopic morphologies. BCPs containing electron-donor and acceptor segments are of particular interest for use in photovoltaic cells because electronic light-excited states exist over distances similar to the typical size of block copolymer domains (~10 nm). Therefore, we designed novel donor-acceptor BCPs to exploit this coincidence in dimensions. This thesis is focused on BCPs based on regioregular poly(3-hexylthiophene) (rr-P3HT) due to its high hole mobility and good processibility from various solvents. Simplified and versatile syntheses of donor-acceptor rod-coil di- and tri- BCPs consisting of the donor block P3HT (rod) and polystyrene or poly(4-vinylpyridine) (coil) blocks to carry the acceptor C60 in different ways were developed. These materials were used as additives to stabilize the nano-morphology of reference P3HT: [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) based devices. Photovoltaic characterizations were then tied to copolymer structural data with the help of AFM and a range of complementary characterization techniques.

Keywords : Organic photovoltaic cells, rod-coil block copolymers, regioregular poly(3-hexylthiophene) (P3HT), GRIM polymerization, functional poly(3-hexylthiophene), controlled radical polymerization, anionic polymerization, C60, nano-structuration. -------------------------------------------------------------------------------------------------------

Synthèse et application en cellules solaires organiques de nouveaux copolymères à base de poly(3-hexylthiophène)

Résumé : Dans cette étude, des copolymères à blocs rigide-flexible comprenant des segments donneur [poly(3-hexylthiophène) régiorégulier, (rr-P3HT)] et accepteurs d’électrons (C60) ont été synthétisés. L’auto-assemblage en masse de ces copolymères à blocs avait pour objectif d’atteindre des morphologies dont la taille des domaines coïncide avec la distance idéale de transport de l’exciton (~10 nm) en vue d’utiliser ces systèmes comme matériaux de couche active dans les cellules photovoltaïques organiques de type P3HT-PCBM. La maîtrise et l'optimisation des conditions de synthèse de rr-P3HT de fonctionnalité terminale bien définie nous ont permis d'accéder à différentes architectures de copolymères linéaires di- et triblocs, constitués de P3HT comme bloc rigide et de polystyrène ou poly(4-vinylpyridine) comme bloc ‘flexible’. La fonctionnalisation du bloc flexible avec des dérivés du fullerène (C60 ou PCBM) a ensuite été réalisée et ces copolymères utilisés comme additifs pour stabiliser la morphologie de la couche active des cellules solaires organiques de type P3HT/PCBM. Les caractéristiques photovoltaïques des matériaux ainsi préparés ont été déterminées et corrélées aux analyses morphologiques de la couche active. Mots-clés : cellules photovoltaïques organiques, copolymères à blocs rigide-flexible, poly(3-hexylthiophène) régiorégulier (rr-P3HT), polymérisation GRIM, poly(3-hexylthiophène) fonctionnel, morphologie, compatibilisation.

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Acknowledgements

I would like to express my sincere thanks to Prof. Yves GNANOU and Prof. Henri CRAMAIL to permit me as a PhD student in LCPO. I am very grateful to Prof. Henri CRAMAIL and Dr. Eric CLOUTET for giving a chance to do my PhD under their very kind supervision in LCPO and also for their encouragement and valuable suggestions during my research, writing publications and thesis. It is my great pleasure to express my sincere thanks to Dr. Roger C Hiorns, my research advisor for his excellent guidance, cooperation and encouragement throughout my research work. He has nurtured me with excellent scientific input to carry out my research work and also with his valuable discussions and insights, which helped me in the successful completion of my thesis. I am very grateful for his great advices for my personal life also.

I would like to thankful to my collaborators Dr. Laurence VIGNAU, Habiba and Mafoudh from IMS, Bordeaux, for their excellent help to characterize my polymers and also for giving the best results. I express my sincere thanks to external Jury members, Prof. Michael Turner, Prof. L. Lutsen, Prof. P. HUDHOMME and Dr. Laurence VIGNAU for accepting my thesis evaluation.

Its very great pleasure to express my sincere thanks to my previous PhD supervisor (late) Prof. G. Sundararajan (IIT Madras, India) for giving me an opportunity to work in his excellent group and also for introducing me to this great field of conducting polymers. Its very pleasure as well as great honour to thank Prof. Pierre Dixneuf (PHD) for his kind support at my difficult times and I am very grateful for his immense encouragement, inspiration, and hospitality during my stay in France. My sincere thanks to Prof. A. K. Mishra (IIT Madras, India) for all his kind help as well as for his valuable research guidance and also my heartful thanks to Prof. U. V. Varadaraju for his kind personal help during my stay at IIT Madras, India. I wish to thank all my teachers and professors for their guidance and inspiration. I am indeed thankful to my other colleagues in LCPO particularly Mathieu Urien, Bertrand, Jean, Maryliine and Mumtaz for their initial support to handle the instruments etc at the beginning of my PhD work and also my office-mates; Cedric,

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Chantal, Vincent, Jennyfer and also Autumn for their kind personal help. I am extremely grateful to Emmanuel IBARBOURE for his excellent help in AFM and DSC, Nicolas GUIDOLIN for his kind help in GPC, Michele Schappacher for his good teaching in NMR, Christelle Absalon for her wonderful job for MALDI-TOF mass results. I am very thankful to Catherine ROULINAT (especially for her kind personal help during all my stay at Bordeaux), Corine GONCALVES de CARVALHO, Mimi, Bernadette GUILLABERT and Nicole GABRIEL for their excellent help in the administrative work. My sincere thanks to many great people from LCPO especially Cyril Brochon (Expert in anionic polymerization) who helped me a lot, Vijayakrishna (Expert in RAFT) and also Feng, Jerome, Flu, Stephane, Stephanie, Julie, Gabriel, Celia, Dargie, Samira, Katerina.... from LCPO who helped me during my research work. It was very good discussions with my Indian friends: Dakshina, Dynesh, Arvind, Anil when we used to go for coffee every evening. I am equally thankful to all permanent and non-permanent members of LCPO and IMS, Bordeaux for their kind help and moral support during my research work. I am really very grateful to all Indians in Bordeaux, which I passed excellent time during my PhD. I cannot forget the time we passed together with my Indian friends during trips, dinner, lunch parties, Indian festivals and other occasions. I am very thankful to all Indian friends who helped me necessary times particularly Srikumar, Veena, Ujwala, Vidya, Ramana, Aroun, Laxmireddy, Srinivas, Gowda, kamal, Arka, Amol, Mythili, Basabdatta and also all my friends at IIT Madras. Finally, my heartful thanks to my dear mother (Lakshmiswarajyam), father (Brahmaiah), brother (Nani), sister (Vani) and all my in-laws, relatives for their love and moral supports. I am extremely thankful to my best friend cum wife, Dr. Anitha who stood always with me at difficult times in my life and gave me endless love, encouragment and positive support to finish my doctorate successfully. Last but not least, I am really happy to thank my sweet daughter, Haritha for being with us during my doctorate time and also for giving more happiness to finish my thesis soon. My sincere thanks to many other people (space doesn’t allow me to mention names) who helped me in my life to reach this position.

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Dedication

I feel very pleasure to dedicate this thesis to my dear parents, grandparents,

my dear wife (Anitha), my dear sweet daughter (Haritha) for their love, moral

support and also to my dear teachers, professors for their inspiration,

encouragement.

HARIKRISHNA EROTHU

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Abbreviations

A Electron acceptor

AFM Atomic Force Microscopy

AIBN α,α'-azobisisobutyronitrile

AlCl3 Aluminum chloride

ATRA Atom Transfer Radical Addition

ATRP Atom Transfer Radical Polymerisation

AM Air Mass number

a.u. Arbitrary unit

Br2 Bromine

n-BuLi n-butyl lithium

s-BuLi sec-butyl lithium

t-BuLi tert-butyl lithium

Bu4NI Tetrabutylammonium iodide

CaH2 Calcium hydride

CB Conduction Band

CdS Cadmium(II) sulfide

CdTe Cadmium telluride

CMC Critical Micelle Concentration

CRP Controlled Radical Polymerisation

CuBr2 Copper(II) bromide

CuCl2 Copper(II) chloride

CuI Copper(I) iodide

D Electron donor

Đ Dispersity (Mw/Mn)

DBU 1,8-Diazabicyclo[5.4.0]undec-7-ene

o-DCB ortho-Dichlorobenzene

DEH-PPV Poly(diethylhexyloxy-p-phenylenevinylene)

DIEA or DIPEA N,N-Diisopropylethylamine

DMSO Dimethyl sulfoxide

DMF N,N’-dimethylformamide

DSC Differential Scanning Calorimetry

EA Electronic Affinity

EQE External Quantum Efficiency

FeCl3 Ferric chloride

FF Fill Factor

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GPC Gel Permeation Chromatography (also known as

Size Exclusion Chromatography or SEC)

GRIM Grignard Metathesis

HBr Hydrobromic acid

HCl Hydrochloric acid

HH Head-to-Head

HT Head-to-Tail

H2O2 Hydrogen peroxide

H2SO4 Sulfuric acid

HOMO Highest Occupied Molecular Orbital

I Current

I2 Iodine

IP Ionization potential

IPCE Incident Photon to Current Efficiency

Isc short-circuit current

i-PrMgCl iso-propylmagnesium chloride

IR Infra Red

ITO Indium tin oxide

LDA Lithium Diisopropylamide

LUMO Lowest Unoccupied Molecular Orbital

Mn Number average molecular weight

Mw Weight average molecular weight

MALDI-TOF Matrix Assisted Laser Desorption Ionisation - Time Of Flight

NBS N-bromosuccinimide

NMP N-methyl pyrrolidone

NMR Nuclear Magnetic Resonance

Ni(dppp)Cl2 1,3-bis(diphenylphosphino)propane Nickel(II) chloride

OPV Organic Photovoltaic

OSC Organic Solar Cell

PA Polyacetylene

PPA Poly(phenylacetylene)

PAT Poly(alkylthiophene)

PCBM [6,6]-Phenyl-C61-Butyric acid Methyl ester

PCE Photo Conversion Efficiency

PEO Poly(ethylene oxide)

PI Polyisoprene

PEDOT:PSS Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate)

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PLED Polymer Light Emitting Diode

PMA Poly(methylacrylate)

PMDETA N,N,N’,N’’,N’’-pentamethyldiethylenetriamine

PPP Poly(para-phenylene)

PPV Poly(p-phenylenevinylene)

PPy Polypyrrole

PS Polystyrene

PT Polythiophene

PTSA p-toluenesulfonic acid

PVA Poly(vinyl alcohol)

P3AT Poly(3-alkylthiophene)

P3HT Poly(3-hexylthiophene)

P2VP Poly(2-vinylpyridine)

P4VP Poly(4-vinylpyridine)

RAFT Reversible Addition-Fragmentation Chain Transfer

Polymerization

RR Regioregularity

T Temperature

Tc Crystallization temperature

Tg Glass transition temperature

Tm Melting temperature

TBAF.3H2O Tetra-n-butylammonium fluoride trihydrate

t-BuMgCl tert-butylmagnesium chloride

TEA Triethylamine

TEM Transmission Electron Microscopy

TGA Thermo Gravimetric Analysis

THF Tetrahydrofuran

TMS Tetramethylsilane

TT Tail-to-Tail

UV Ultra Violet

VB Valence Band

V Voltage

Voc Open-circuit voltage

δ Chemical shift

λ Wavelength

η Conversion efficiency

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Table of contents

Section Title Page

Abstract........................................................................................ 1 Acknowledgements..................................................................... 3 Dedication.................................................................................... 5 Abbreviations............................................................................... 7 Table of Contents........................................................................ 11 General Introduction................................................................... 17

Chapter 1 Literature Review................................................................. 25

1.1 Polymers as semiconductors................................................................ 27 1.1.1 Electrical conductivities of conjugated polymers........................... 27 1.1.2 Archetypal polymeric semiconductors........................................... 30 1.1.3 Electronic structures of conjugated polymers................................ 31 1.1.3.1 Undoped polymers......................................................... 31 1.1.3.2 Doped polymers............................................................. 33 1.1.4 Improving the processibilities of conjugated polymers.................. 34

1.2 Organic photovoltaics............................................................................ 35 1.2.1 Solar energy.................................................................................. 35 1.2.2 Definition, history and development of photovoltaic cells.............. 37 1.2.2 Operating principle of organic photovoltaic cell............................. 42 1.2.4 Efficiency characteristics of organic photovoltaic cells.................. 43 1.2.5 Organic photovoltaic active layer architectures............................. 46 1.2.6 Novel low band-gap polymers....................................................... 49

1.3 Synthesis of the archetypal conjugated polymer, poly(3-hexyl thiophene) (P3HT) ............................................................ 53

1.3.1 Regioregularity.............................................................................. 53 1.3.2 McCullough and Rieke methods.................................................... 54 1.3.3 Grignard metathesis (GRIM) polymerisations leading to P3HT..... 55 1.3.4 Chain-growth condensation polymerisations leading to P3HT...... 56 1.3.5 Chain-end capping of P3HT using GRIM...................................... 57

1.4 Organic photovoltaic cells and block copolymers.............................. 59 1.4.1 Importance of morphology of active layer...................................... 59 1.4.2 Controlling morphology of active layer in blends........................... 60

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1.4.3 Self-assembly behaviour of rod-coil block copolymers.................. 63 1.4.4 Why block copolymers in photovoltaic cells? ............................... 65

1.4.5 Synthesis and self-assembly of exampled rod-coil block copolymers.................................................................................... 67

1.4.5.1 Copolymers based on poly(p-phenylene vinylene)s....... 68 1.4.5.2 Copolymers based on polythiophenes........................... 71

1.5 References............................................................................................... 77

Chapter 2 Towards comb copolymers based on P3HT via ω-acetylene-P3HT and ω-vinyl-P3HT macromonomers................................................................ 87

2.1 Introduction............................................................................................. 89 2.2 Syntheses of monomers......................................................................... 91

2.2.1 Synthesis of 3-hexylthiophene....................................................... 91 2.2.2 Synthesis of 2,5-dibromo-3-hexylthiophene.................................. 92

2.2.3 Synthesis of 2-bromo-3-hexyl-5-iodo-thiophene............................ 93 2.3 Synthesis and characterization of regioregular P3HT......................... 95

2.3.1 Regioregular α,ω-diH-P3HTs........................................................ 95 2.3.2 Regioregular, end-functionalised ω- and α,ω-P3HTs.................... 97

2.4 Synthesis of ω- or α,ω-alkynyl-P3HT by the GRIM method................ 98 2.4.1 Synthesis of ω-ethynyl-P3HT (P2) and α,ω-pentynyl-P3HT(P3) .. 99

2.5 Synthesis of regioregular ω-vinyl-P3HTs by the GRIM method......... 103

2.6 Synthesis of mono-functionalised-P3HT by externally added Ni-catalyst initiator....................................................................................... 105

2.6.1 Synthesis of the Ni-initiator: [(Ph)Ni(PPh3)2-Br] (4) ..................... 106 2.6.2 Synthesis of mono-functionalised P3HT by “small molecule” Ni-

initiator [(Ph)Ni(PPh3)2-Br] ............................................................ 107 2.7 Syntheses and characterizations of polyacetylene-graft-P3HT.......... 112

2.7.1 Phenylacetylene polymerisation by [Rh(nbd)Cl]2 catalyst.............. 113 2.7.2 Polymerisation of ω-alkynyl-P3HTs by [Rh(nbd)Cl]2 catalyst......... 115 2.7.3 Attempted copolymerisation of ω-acetylene-P3HT with phenyl

acetylene....................................................................................... 118 2.7.4 Attempted polymerisation of ω-vinyl-P3HTs.................................. 119

2.8 Conclusions............................................................................................. 120 2.9 References............................................................................................... 121

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Chapter 3 Block copolymers based on P3HT and PS or P4VP 123

3.1 Introduction............................................................................................. 125 3.2 Synthesis of azide-terminated polystyrene.......................................... 127

3.2.1 Principle of atom transfer radical polymerisation (ATRP) ............. 127 3.2.2 Synthesis of azide initiator............................................................. 128 3.2.3 Synthesis of α-azido polystyrenes................................................. 131

3.3 Synthesis of block copolymers P3HT-block-PS and PS-block-P3HT-block-PS by “Click” chemistry.................................................... 133

3.3.1 History and principle of “click” chemistry....................................... 133 3.3.2 Synthesis of copolymers P3HT-b-PS and PS-b-P3HT-b-PS......... 134 3.3.3.1 Triblock copolymers PS-b-P3HT-b-PS........................... 134 3.3.3.2 Diblock copolymers P3HT-b-PS..................................... 138

3.4 Synthesis of donor-acceptor and acceptor-donor-acceptor block copolymers P3HT-block-PS-C60 and C60-PS-block-P3HT-block -PS-C60............................................................................................................. 143

3.4.1 Grafting of fullerene by atom transfer radical addition (ATRA) ..... 143 3.4.2 Synthesis of P3HT-b-PS-C60 and C60-PS-b-P3HT-b-PS-C60......... 143

3.5 Synthesis and characterization of block copolymers P4VP-block-P3HT-block- P4VP.................................................................................... 147

3.5.1 Synthesis of α,ω-difunctionalised-P3HT by GRIM polymerisation 148 3.5.2 Synthesis of triblock copolymer P4VP-block-P3HT-block-P4VP

by anionic polymerisation.............................................................. 154 3.5.2.1 Introduction to anionic polymerisation............................ 154 3.5.2.2 A short history of anionic polymerisation........................ 155 3.5.2.3 Synthesis of P4VP-b-P3HT-b-P4VP............................... 156

3.6 Physical characterisation di- and triblock copolymers....................... 158 3.6.1 P3HT-b-PS and PS-b-P3HT-b-PS block copolymers with and

without C60 chain-ends.................................................................. 158 3.6.2 P4VP-b-P3HT-b-P4VP block copolymers..................................... 163


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Chapter 4 Photovoltaic performances and morphological characterizations of block copolymers........................ 169

4.1 Introduction............................................................................................. 171 4.2 Photovoltaic performances of synthesized P3HTs (P1, P1a, P1b

and Plextronics P3HT) ........................................................................... 173 4.3 Photovoltaic performances of block copolymers................................ 177

4.3.1 Diblock copolymer P3HT-block-PS as compatibilizer in the mixture of P3HT-blend-PCBM....................................................... 178

4.3.2 Donor-acceptor diblock copolymer P3HT-block-PS-C60 as compatibilizer in the mixture of P3HT-blend-PCBM...................... 182

4.3.3 Acceptor-donor-acceptor triblock copolymer C60-PS-block-P3HT-block-PS-C60 as compatibilizer in the mixture of P3HT-blend-PCBM.................................................................................. 186

4.3.4 Triblock copolymer P4VP-block-P3HT-block-P4VP as a compatibilizer in the mixture of P3HT-blend-PCBM...................... 188


Chapter 5 Experimental Section ......................................................... 193

1 Materials................................................................................................... 197 1.1 Purification of Solvents.................................................................... 197 1.2 Purification of Monomers................................................................. 197 1.3 Chemicals........................................................................................ 197

2 Synthesis................................................................................................. 199 2.1 Monomers...................................................................................... 199 2.1.1 3-Hexylthiophene................................................................ 199 2.1.2 2,5-Bibromo-3-hexylthiophene............................................ 199 2.1.3 2-Bromo-3-hexylthiophene.................................................. 200 2.1.4 2-Bromo-3-hexyl-5-iodo-thiophene..................................... 200 2.2 Regioregular P3HTs (P1-P6) by the Grignard metathesis

(GRIM) ............................................................................................ 201 2.2.1 α,ω-DiH-P3HTs (P1, P1a, P1b and P1c) ............................ 201 2.2.2 Chain-end functionalised w-P3HTs or ω-P3HTs................. 202 2.2.2.1 ω-Ethynyl, ω-vinyl-P3HTs and α,ω-pentynyl-

P3HTs................................................................... 202 2.2.2.2 α,ω-Diformyl and α,ω-dihydroxy-P3HTs................ 203

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2.3 Mono-functionalised P3HTs (P7-P8) by externally added Ni-catalyst initiator............................................................................. 205

2.3.1 Ni-initiator: [(Ph)Ni(PPh3)2-Br] ............................................ 205 2.3.2 Mono-functionalised P3HTs by small molecule Ni-initiator. 205 2.4 Azide-terminated Polystyrene...................................................... 206 2.4.1 Azide initiator for ATRP....................................................... 206 2.4.1.1 3-Azido-1-propanol................................................ 206 2.4.1.2 3-Azidopropyl-2-bromoisobutyrate........................ 206 2.4.2 α-Azido-polystyrenes (PS1-PS6) ....................................... 207 2.5 Block copolymers P3HT-block-PS and PS-block-P3HT-block-

PS by Click Chemistry.................................................................. 208 2.5.1 Triblock copolymers PS-b-P3HT-b-PS................................ 208 2.5.2 Diblock copolymers P3HT-b-PS.......................................... 209 2.6 P3HT-block-PS-C60 and C60-PS-b-P3HT-b-PS-C60 by ATRA....... 210 2.7 Triblock copolymers P4VP-block-P3HT-block-P4VP by

anionic polymerisation................................................................. 211 2.8 Polyacetylene-graft-P3HT (PA-graft-P3HT) ................................ 212 2.8.1 Phenylacetylene polymerisation by [Rh(nbd)Cl]2 catalyst... 212 2.8.2 Polymerisation of ω-alkynyl-P3HTs by [Rh(nbd)Cl]2

catalyst ............................................................................... 212 2.8.3 Attempted copolymerisation of ω-acetylene-P3HT with

phenyl acetylene................................................................. 213 3 Characterization...................................................................................... 213 4 Photovoltaic device fabrication and characterization......................... 215

General conclusions........................................................................................ 217 Appendix........................................................................................................... 223 Publications and Conferences........................................................................ 227

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General Introduction

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General Introduction

The continuous use of fossil fuels (coal, oil, gas) that produces CO2, which

increases global warming, is drastically damaging our environment. At current

rates of consumption, CO2 levels are projected to reach considerably higher

levels than the “moderately stringent” limit of 550 ppm set by

Intergovernmental Panel on Climate Change (IPCC).1 The growing global

energy demand and the depletion of conventional fossil fuels means finding

an alternative to non-renewable resources. It is essential to develop research

into the materials that will enable renewable energy sources (such as nuclear,

wind, hydropower, biomass and solar) that will improve the quality of life.

From the considerations of energy sustainability and environmental

protection, solar energy is the largest carbon-neutral energy source to be

explored and can be utilized much more extensively.2 It is known that the

earth receives more energy from the sun in one hour than is required for all

human needs in a year. Therefore, if harvested economically, solar power is

clearly the most rational energy source to produce the step-change in energy

provision required to shape our world for the future environmental, economical

and technological demands. This energy source has also the advantage of

being available everywhere on the planet and enjoy a huge energy potential.

Today, solar cells based on silicon are about 99% of global production of

photovoltaic cells. But the present-day silicon technology has some

disadvantages: the silicon purification processes are extremely expensive and

the silicon availability is limited due to its extensive use in the field of

microelectronics. Consequently, the production cost of silicon solar panels is

too high to be economically viable. This is the major motivation for the

development of organic photovoltaic materials and devices (organic solar cells

or OSCs), which are envisioned to exhibit advantages such as low cost, high

device flexibility, and cheap fabrication from highly abundant materials.3,4

This new generation of photovoltaic cell is based on the discovery of

semiconducting polymers in 1977 by A. McDiarmid, H. Shirakawa and A.

Heeger, who won the Nobel Prize in chemistry, 2000. In 1986, C. W. Tang

introduced the first efficient bilayer OSC with 1% efficiency5 that was a

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monumental shift from inorganic photovoltaics, which are thick, rigid and

fragile. OSCs have very interesting advantages, related to the nature of these

materials, such as the possibility of flexible modules, their low cost, but also

their shape, which can be controlled by roll-to-roll printing on a large scale.6

The simplest and to date, most successful technique is based on

solution-processed bulk heterojunction (BHJ) OSCs composed of electron-

donating semiconducting polymers and electron-withdrawing fullerides as

active layers.7 In the past 20 years, many modifications to OSCs have been

performed and introduced new photoactive materials, deposition techniques,

device architectures and electrode materials.8,9 These changes brought to

certified power conversion efficiencies of nearly 8% as reported recently10

which is an impressive milestone. However, the efficiency of OPVs is still

significantly lower than their inorganic counterparts, such as silicon, CdTe and

copper indium gallium selenide (CIGS), which prevents practical applications

in large scale. However, the scientific community as a whole accepts that

OSCs must overcome the 10% efficiency benchmark to become commercially

attractive. If the field continues to develop at the same dynamic rate,

expectations that the 10% efficiency milestone will be reached by 2011.

There are many factors limiting the performance of the BHJ solar

cells.11 Several physical processes occur successively in OSCs; absorption of

photons, creation of excitons, exciton diffusion exciton dissociation, and finally

the transport of charge carriers to the electrodes. However, the morphology of

the films in OSCs plays a very important role and affects both the process of

dissociation of the exciton but also the charge transport to electrodes. The

majority of OSC research carried out over the past 17 years have focused on

the donor/acceptor bulk heterojunction approach using conjugated

semiconducting polymer as the donor, and a fullerene derivative as the

acceptor. The most successful system consists of a physical mixing of poly(3-

hexylthiophene) (P3HT) as donor and a fullerene derivative, [6,6]-phenyl-C61-

butyric acid methyl ester (PCBM), as acceptor. However, it is very difficult in

this case to control the morphology of the active layer, the two compounds

(donor and acceptor) behave independently of one another, leading to

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completely random structures. Several solutions are proposed to overcome

this problem, the use of double-cable polymers or the use of block copolymers

(BCPs), which is the main subject of this thesis since BCPs have the ability to

self-organize to form nanoscale structures to optimize various parameters of

the photovoltaic process and also the exciton distance exactly coincides well

with the typical size of block copolymer domains. Since the early 2000s, BCPs

for photovoltaic applications has generated a lot of research and multiple

strategies, which depend on the nature of each block that has been

developed. Though these BCPs provide access to morphologies whose

structure and size are favorable for photovoltaic process, the synthesis of

these new materials suffer, in general, from complex methods, and

performances that are still rather low.

Thesis overview The motivation of the research work described here was to develop a

simplified and versatile synthesis of BCPs, to understand the microstructure of

functional BCPs and to explore the use of these materials as active layer or

compatibilizer in OSCs. This work is mainly focused on BCPs based on P3HT

due to their high hole mobility, their chemically tunable electronic properties

and their processibility from various solvents.12-14 Chain-end functionalised

P3HTs were used as the building blocks for the syntheses of BCPs containing

other blocks.

Chapter 1 begins with an introduction to the main families of

conjugated polymers, their electronic structures and mechanism of electrical

conduction. The second part of Chapter 1 gives brief update on organic

photovoltaics; history, development of OSCs, the operating principle of an

OSC, various device architectures currently used in OSCs and focused review

on low band-gap polymers. Then a review of the archetypal conjugated

polymer, P3HT, which is the main focus in this thesis, looks at the various

synthetic methods and the mechanism leading to regioregular P3HT. Finally,

a detailed review on the interest of BCPs in the field of OSCs and the

literature on rod-coil BCPs are explained.

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Chapter 2 explores the synthesis of regioregular P3HTs, and

regioregular end-functionalised ω- and α,ω-alkynyl, and alkenyl P3HTs. It

explores the synthesis of a small molecule Ni-catalyst initiator based on

phenyl bromide for an attempt to prepare purely monofunctionalised P3HT.

Attempts were then made to synthesize PA-graft-P3HT using synthesized

macromonomers of P3HTs.

Chapter 3 describes the synthesis of donor-acceptor rod-coil block

copolymers in which rod block is P3HT and the coil block polystyrene (PS) or

poly(4-vinylpyridine) (P4VP) for their application in photovoltaics. It explains

the two different synthetic approaches to obtain donor-acceptor block

copolymers. In one case, the acceptor fullerene (C60) is covalently attached to

the insulating block polystyrene (PS), and in the other case, weak

supramolecular interactions produced by complex formation between

insulating block poly(4-vinylpyridine) (P4VP) and a C60 derivative (PCBM) is

explored. The di- and tri-block copolymers P3HT-b-PS and PS-b-P3HT-b-PS

were synthesized by 1,3-dipolar Huisgen addition, known as "click" chemistry

from P3HT functionalized alkyne and azide functionalized PS. Then fullerene

(C60) was then attached to these block copolymers by atom transfer radical

addition (ATRA) to obtain the donor-acceptor copolymers. The other tri block

copolymers of ABA coil-rod-coil, P4VP-b-P3HT-b-P4VP in which rod block is

P3HT and the coil block is P4VP were synthesized by anionic polymerisation

from quenching of living P4VP chains with P3HT di-functionalized aldehyde.

Finally, it describes the physical characterization of all the synthesized

copolymers.

Chapter 4 explores the photovoltaic characterization of some of the

synthesized materials in this thesis. First, it describes the photovoltaic

performances of synthesized P3HTs of different molecular weights, P1 (25

kg/mol), P1a (50 kg/mol), P1b (100 kg/mol) and compared the performances

with the commercially available P3HT (Plextronics, 50 kg/mol). It was

therefore necessary to characterize and optimize the P3HTs performance in

mixture with PCBM for a reference and compare its power and photovoltaic

characteristics with the addition of block copolymers as compatibilizers to the

23

reference P3HT-blend-PCBM. Finally some of the copolymers were examined

as active layer or compatibilizers in the OPV devices.

Chapter 5 shows all the detailed experimental methods for the

synthesis and characterization of monomers, regioregular P3HTs, chain-end

regioregular end-functionalised P3HTs by the GRIM method, “small molecule”

Ni-catalyst initiator, monofunctionalised P3HTs, graft copolymers PA-graft-

P3HT, di- and tri-block copolymers P3HT-b-PS, P3HT-b-PS-C60 and PS-b-

P3HT-b-PS, C60-PS-b-P3HT-b-PS-C60, P4VP-b-P3HT-b-P4VP.

References:

1. http://www.ipcc.ch/ipccreports/tar/wg3/pdf/2.pdf.

2. Lewis, N. S. Global Energy Prospective. Solar Energy Workshop; US

Department of Energy: Washington, DC, 2005.

3. Lewis, N. S. Science 2007, 315, 798.

4. Brabec, C. J. Organic Photovoltaics: Technology and Market. Sol. Energy

Mater. Sol. Cells 2004, 83, 273–292.

5. Tang, C. W. Two-layer Organic Photovoltaic Cell. Appl. Phys. Lett. 1986, 48,

183–185.

6. Brabec, C. J.; Durrant, J. R. MRS Bull. 2008, 33, 670–675.

7. Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995,

270,1789.

8. Sun, S.; Sariciftci, N. S.; Eds. Organic Photovoltaics: Mechanisms, Materials,

and Devices; Taylor & Francis: Boca Raton, FL, 2005.

9. Thompson, B. C.; Fréchet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58–77.

10. Liang, Y.; Xu, Z.; Xia, J.; Tsai S-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater.

2010, 22, E135–E138.

11. Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21, 1323.

12. Nalwa, H. S.; Ed. Handbook of Organic Conductive Molecules and Polymers;

J. Wiley & Sons: New York, 1996.

13. McCullough, R. D. Adv. Mater. 1998, 10, 93–116.

14. Osaka, I.; McCullough, R. D. Acc. Chem. Res. 2008, 41, 1202–1214.

25

Chapter 1: Literature Review

26

Contents

1.1 Polymers as semiconductors................................................................ 27 1.1.1 Electrical conductivities of conjugated polymers........................... 27 1.1.2 Archetypal polymeric semiconductors........................................... 30 1.1.3 Electronic structures of conjugated polymers................................ 31 1.1.3.1 Undoped polymers......................................................... 31 1.1.3.2 Doped polymers............................................................. 33 1.1.4 Improving the processibilities of conjugated polymers.................. 34

1.2 Organic photovoltaics............................................................................ 35 1.2.1 Solar energy.................................................................................. 35 1.2.2 Definition, history and development of photovoltaic cells.............. 37 1.2.2 Operating principle of organic photovoltaic cell............................. 42 1.2.4 Efficiency characteristics of organic photovoltaic cells.................. 43 1.2.5 Organic photovoltaic active layer architectures............................. 46 1.2.6 Novel low band-gap polymers....................................................... 49

1.3 Synthesis of the archetypal conjugated polymer, poly(3-hexyl thiophene) (P3HT) ............................................................ 53

1.3.1 Regioregularity.............................................................................. 53 1.3.2 McCullough and Rieke methods.................................................... 54 1.3.3 Grignard metathesis (GRIM) polymerisations leading to P3HT..... 55 1.3.4 Chain-growth condensation polymerisations leading to P3HT...... 56 1.3.5 Chain-end capping of P3HT using GRIM...................................... 57

1.4 Organic photovoltaic cells and block copolymers.............................. 59 1.4.1 Importance of morphology of active layer...................................... 59 1.4.2 Controlling morphology of active layer in blends........................... 60 1.4.3 Self-assembly behaviour of rod-coil block copolymers.................. 63 1.4.4 Why block copolymers in photovoltaic cells? ............................... 65

1.4.5 Synthesis and self-assembly of exampled rod-coil block copolymers.................................................................................... 67

1.4.5.1 Copolymers based on poly(p-phenylene vinylene)s....... 68 1.4.5.2 Copolymers based on polythiophenes........................... 71

1.5 References............................................................................................... 77

27

1.1 Polymers as semiconductors 1.1.1 Electrical conductivities of conjugated polymers Conductive polymers are organic polymers that possess both metallic

conductivity and processibility. Generally, current is defined as the net flow of

charge through a material in a definite direction for a given time and the

charge carriers can be free electrons or holes. A hole may be described as a

vacancy previously occupied by an electron. Hence, a hole is oppositely

charged to an electron and in the presence of applied electric field, hole

moves in the opposite direction of an electron.1 The conduction mechanism

varies depending on the material in which the three most common categories

are metals, semiconductors and insulators. Metals readily conduct current and

normally “electron sea” model is used to explain the flow of current. In metals,

the delocalization of the valence electrons due to the nature of the metallic

bond, the electrons can move easily in the “sea” under an applied electric

field.2 In semiconductors, the electrons are more strongly bound to the nuclei

of their associated atoms.

The best understood of semiconductors are probably those based on

inorganic materials. Indeed, much of the theory for organic materials arose

from that previously developed in this area. Generally charge transport in

inorganic semiconductors is demonstrated by band theory.1,3,4 The underlying

concept is that regular covalent bonding creates a crystal structure which

allows to form bands where charge is transported. The valence electrons,

which are bound to the nuclei of the semiconducting atoms, create the

valence band. A sufficient energy is given to an electron to overcome its

attraction to the nuclei, and then the electron can enter to the conduction band

where it can move freely in the semiconductor by creating a hole in the

valence band. The energy difference between the valence and conduction

bands is called as the band gap energy (Eg). Semiconductors are having

small band gap energies (Eg < 4 eV) and therefore, a considerable amount of

electrons can be transferred from the valence band (VB) to the conduction

band (CB).1-4 Inorganic insulators are also having same type of band structure

as semiconductors but Eg of an insulator is very high. Therefore, electrons are

28

not promoted to the CB and then no flow of current observed in insulators.

Most of the organic small molecules and polymers are insulating materials

due to their very large band gaps. In organic solid, individual molecules are

prepared by covalent bonds. Generally intermolecular interactions (van der

Waals interactions) are much weaker in organic solid than in inorganic solid

and prohibit the formation of band-like transport.5 But, organic semiconductors

have characteristic bonding patterns in which alternate arrangement of

carbon-carbon bonds between single and double bonds (“conjugation”) in

such a way that only three atoms covalently bound to each carbon nucleus

(sp2-hybridization).6 Thus this hybridization permits valence p-orbital electrons

to become delocalized and contribute to the current.

The conductivities of representative conjugated polymers both in the

neutral and doped states are shown in Figure 1.1.

Figure 1.1 Electronic conductivities of conjugated polymers with different degrees of doping. [Reference: Skotheim TA, Elsenbaumer RL and Reynolds JR (eds), Handbook of Conducting Polymers, 2nd edition, Marcel Dekker Inc, New York (1998)].

29

While conductivity in conjugated polymers is typically at least one order

of magnitude lower than that of metals, the ability to control conjugated

polymers from insulating to conducting and their ease of processing has led to

some unique applications.

The electrical conductivity (σ) of a conjugated polymer is equal to the

inverse of its specific resistivity (ρ), which is a measure of the ability of the

conjugated polymer to conduct an electrical charge. Electrical conductivity is

determined by measuring the resistance (R) to charge transport through a

known volume, where L is the length over which resistance is being measured

and A is the cross-sectional area through which the current passes:

σ =1/ρ = L/(R·A) Eq. 1.1 From the above discussion it is clear that conducting polymers are

alternative source of semiconducting materials that can be used to replace

relatively expensive and environmentally dangerous inorganic semiconducting

materials. They are excellent candidates for electroluminescent devices,

rechargeable batteries, sensors, electrochromic windows, photovoltaic

devices, photodiodes etc. due to their interesting opto-electronic properties.

The great discovery, high conductivities of doped polyacetylene (PA) in

1970s7,8 by MacDiarmid, Heeger and Shirakawa who received chemistry

Nobel prize in 2000, encouraged researchers for optimizing their electronic

properties.9 Especially, π-Conjugated organic compounds are very important

in the electro-optics field because of their non-linear optical (NLO) behaviour

and photoconductivity.10-13 Hence the development of these conjugated

polymers brought their use in various applications such as electroluminescent

diodes (PLED),14,15 photovoltaic cells,16-20 stable electronic memories,21

polymer field effect transistors (PFET)22-25 and so on.26-28

30

1.1.2 Archetypal polymeric semiconductors Since the discovery in 19778 that PA demonstrates conductivity about

103 S cm-1 by doping with Br2, I2 or AsF5, it triggered a real interest in the field

of conducting polymers. But the application of PA is limited because of its

poor solubility and low thermal stability. To avoid these problems, researchers

have developed many aromatic conjugated polymers such as poly(para-

phenylene) (PPP),29 polythiophene (PT),30 polypyrrole,31 and others shown in

Figure 1.2. Recently, many conjugated polymers resulting from these parental

structures have been synthesized such as poly(3,4-ethylenedioxythiophene)

(PEDOT), one of the most widely explored.32-35 This polymer shows a high

conductivity (ca 300 S cm-1), quasi-transparency in the form of film and very

high stability in the oxidized state.33-36 Another great discovery by Friend and

colleagues in 1990 was the green electroluminescence of undoped poly(p-

phenylenevinylene) (PPV)37 and it was followed in 1991 by the manufacture of

the first blue polymer light emitting diode (PLED) made up of poly(9,9'-di-n-

hexylfluorene).38

n S nn N

H

n

trans-polyacetylene poly(para-phenylene) polythiophenepolypyrrole

n n S n

poly(p-phenylene vinylene) poly(p-phenylene ethynylene) poly(thienylene vinylene)

S

OO

nn

polyfluorene poly[3,4-(ethylenedioxy)thiophene]

Nn

polycarbazoleH

SHN

HN

n nn

poly(phenylenesulfide) poly(diphenylamine) polyaniline Figure 1.2 Chemical structures of principal families of conjugated polymers.

31

1.1.3 Electronic structures of conjugated polymers 1.1.3.1 Undoped polymers As I mentioned earlier, the majority of conjugated polymers consist of a

regular alternate single and double bonds that allow the π-delocalization of

single 2pz valence electrons at each carbon atom along the polymer skeleton.

The π-delocalization, is influenced by the geometry of the system, is

maximum if π-conjugated system is planar and any deviation from planarity

results the reduction in conjugation.39 The electronic structure of these

systems depends on their different levels of molecular orbitals and

predominantly the value of their HOMO (Highest Occupied Molecular Orbital)

and LUMO (lowest unoccupied Molecular Orbital). The HOMO represents

together the highest occupied energy levels and the LUMO represents the

lowest unoccupied energy levels. This energy difference corresponds to a π-

π* transition in simple molecules and band gap in the polymers.

The HOMO and LUMO levels of a conjugated polymer depend on the

degree of conjugation, i. e. the number of monomer units (Figure 1.3). If the

repeating units become very high, it passes a series of discrete levels where

the energy levels are grouped into two bands, the valence band (VB) and the

conduction band (CB). All HOMO group together to form the VB and all LUMO

combine to form the CB, the energy difference between these two levels is

called as band gap or forbidden band (Eg) as shown in Figure 1.3. This value

Eg determines the electronic properties of the conjugated polymers and limits

the polymers as semi-conductors instead of metallic conductors. Therefore,

the electrons from VB must overcome this band gap (Eg) to move. This band

gap can also be defined as the difference between the energy to pull an

electron from the highest point of the CB (i.e. the ionization potential, or IP)

and the energy required to inject an electron into the lowest point of the VB

(i.e. the electron affinity, or EA). Eg is usually in between 0.8-4.0 eV that

coincides with the energy of visible light. So the electrons can interact with

light and therefore this property is explored in various opto-electronic

applications. Many parameters such as chain planarity, in-chain defects and

also impurities can further change the energy levels of the bands.40-47

32

Figure 1.3 Molecular orbital diagram (π-levels) as a function of the number of monomer units. (Reference: A.J. Attias, Techniques de l'Ingénieur, E1862, 2002).

According to this model, it is thus possible to categorize materials with

respect to their band gap size. Intrinsic semiconductors have a band gap

values between 0 and 3 eV. The insulators have a similar band structure as

that of the semiconductors but their bandgap is too high (>4 eV). The majority

of conjugated polymers are semiconductors and band gaps of some

conjugated polymers are given here; trans-polyacetylene (PA) (1.4 - 1.5

eV),48,49 polythiophene (PT) (2.0 - 2.1 eV),50,51 poly(p-phenylene) (PPP) (2.7

eV),52 poly(p-phenylene vinylene) (PPV) (2.5 eV),53 polypyrrole (PPy) (3.2

eV),54 poly(3,4-ethylenedioxythiophene) (PEDOT) (1.6 eV ).55,56

The Peierls effect means that the conjugation of conductive polymers

permits them to have two electronic resonance structures. If they are

equivalent in energy, then the system is called “degenerate” as in the case of

trans-PA shown in Figure 1.4. But if the two resonance structures are not

energetically equal, then the energy levels of the system are called “non-

degenerate” which is the case for PPP, PT and PPV. The majority of the

conjugated polymers exist in two resonance forms: aromatic and quinoid

forms which are not equal in energy. There is a ground state aromatic form

and a more excited quinoid state in PT shown in Figure 1.5.42

33

a.

n n b.

Figure 1.4 (a) Mesomeric structures of trans-PA and (b) Potential energy curve for PA showing two energetically equivalent structures (degenerate).

Figure 1.5 Total energy curve for PT showing two energetically inequivalent structures (non-degenerate). 1.1.3.2 Doped polymers To combine the mechanical properties of polymers with the conducting

properties of metals, one can introduce a load into semiconductor polymer by

a process known as “doping”. This process involves a charge-transfer redox

reaction in which the introduction of electron-withdrawing (p-type doping) or

electron-donating (n-type doping) impurities into the polymer and is mainly

carried out by chemical or electrochemical ways. However, semiconducting

polymers do not undergo easily reversible and controllable doping. In the case

Ene

rgy

Deformation coordinate

34

of chemical doping with a chemical oxidant (p-type doping) or reductant (n-

type doping), a neutral conjugated polymer is transformed into a poly(cation)

or poly(anion) respectively. At the same time, a counter-ion is associated to it

inorder to maintain the overall electro-neutrality of the system, as shown in the

following examples (Eqs 1.2 and 1.3).57

• p-type doping:

(π-polymer)n + 3/2 ny(I2) → [(π-polymer)+y (I3-)y]n Eq 1.2

• n-type doping:

(π-polymer)n + [Na+ (C10H8)-•]y → [(Na+)y (π-polymer)-y]n + (C10H8)0 Eq 1.3

1.1.4 Improving the processibilities of conjugated polymers The precessibility of many conjugated polymers is often suffered by their poor

solubility and mechanical properties. With the introduction of some side

groups, such as alkyl58-60 and poly(ethylene oxide) (PEO)61 chains, or some

polar groups such as quaternary sulfonates62,63 or ammoniums,64 these

problems can be rectified (Figure 1.6).

S n

C6H13

S n

S n

H3C OO m

SO3- Na+

S n

OO

O

SO3- Na+

n

C6H13 C6H13poly(3-hexylthiophene) (P3HT)poly(9,9'-di-n-hexylfluorene)

poly[3-oligo(ethylene oxide)-4-methylthiophene]

sodium poly[2-methoxybutylenesulfonate-(3,4-ethylenedioxythiophene)]

sodium poly(3-butylenesulfonate-thiophene)

Figure 1.6 Conjugated polymers with pendant groups to improve the handling properties.

The conjugated polymers combined with flexible coil-like polymers

such as polystyrene,65-67 polyisoprene,68-70 poly(methyl methacrylate),67 and

poly(ethylene oxide)66,70 brought a radical change in the properties of these

35

polymers. As well as improving mechanical properties and solubilities, the

resulting rod-coil copolymers based on the structure shown in Figure 1.7

produced an extraordinary range of supra-macromolecular structures. This

self-assembly of diblock and triblock conjugated copolymers will be detailed

later in the Section 1.4.3.

Figure 1.7 Schematic representation of rod-coil di- and triblock copolymers.

1.2 Organic photovoltaics 1.2.1 Solar energy Due to continuous industrialization and growth of the human

population, the energy consumption in 2050 is expected to be 28‐35 TW

which cannot be met with the energy sources currently in hand. Most of our

present energy is derived from fossil fuels (coal, oil, gas) but the supply is

finite and the energy derived from combustion of fossil fuels produces CO2

which is supposed to be responsible for the acceleration of global warming

(Figure 1.8).71 This limited supply of fossil fuel sources and the negative

long‐term effects of CO2 call for the development of renewable energy

resources. Providing energy from non‐CO2‐emissive sources is required to

prevent global warming that might induce irreversible climate changes.72

Figure 1.8 (a) World market energy consumption in exajoule (EJ = 1018 J); and (b) World carbon dioxide emission in billion metric ton (BMT).71

36

Extensive studies have been done in exploring various renewable

energy sources to respond the growing energy demand. The expansion is

limited for hydropower because most sites are already utilized whereas in the

case of wind power, the ideal location to install wind turbines depends

critically on geographic and climate conditions since the generated power is

relative to the cube of the wind speed. And also, most of these places are far

away from population and industry. Sunlight strikes everywhere at the surface

of the earth with 165 000 TW of power that corresponds to 1000 W/m2.73 In

other words, the earth receives approximately 430 EJ per hour from the sun

which is equivalent to all human present needs in one year. Hence,

comparing the other energy sources and global consumption, solar energy is

the most attractive and abundant (Table 1.1).

Energy source EJ/year

Solar energy 430 (per hour)

Hydropower 1.9

Wind power 0.4

Geothermal 0.04

Global consumption 480

Table 1.1 Comparison of renewable energy sources and global consumption.71, 74

Thus harvesting energy directly from sunlight and converting into

electrical energy using photovoltaic (PV) technology is increasingly

recognized as part of the solution to the growing energy challenge and a

fundamental factor of the future global renewable energy production.75

The intensity of sunlight that reaches Earth’s outer surface of the

atmosphere is referred as air mass zero (AM0) and is equal to 1353 W/m2.76

After passing through the Earth’s atmosphere, the intensity of light decreases

due to absorption and scattering of light by dust particles. Obviously, the

amount of solar radiation that reaches a terrestrial observer depends on the

person’s exact location on the Earth. The commonly used standard for

obtaining power conversion efficiencies of photovoltaics is air mass 1.5

(AM1.5) because it is representative of the sunlight available in most of United

States and Europe. Specifically, it represents the average sunlight incident on

a south-facing position at 37° N during a year.77 The solar spectrum at AM0

37

and AM1.5 are shown in Figure 1.9. It shows that most of the spectral

irradiance is at wavelengths of less than 2000 nm. There is a large amount of

sunlight present at wavelengths 750 nm < λ < 2000 nm (equivalent to 1.65 to

0.62 eV, respectively) that cannot be absorbed by many of the prototypical

semiconductors due to their relatively wide band gaps. Therefore, an active

area of current research is finding low band gap organic semiconductors (Eg <

1.5 eV) so that more incident radiation can be harvested.78 The AM1.5

spectrum can be obtained by the use of lamps and filters in the laboratory and

solar simulators are commercially available.79

Figure 1.9 Solar spectra for AM0 and AM1.5 air mass conditions.77

1.2.2 Definition, history and development of photovoltaic cells The “photovoltaic effect” is the conversion of absorbed solar photons directly

into electrical energy and was first discovered in 1839 by the French physicist

A. E. Becquerel. He found that a photocurrent emerged when platinum

electrodes, covered with silver bromide or silver chloride, was illuminated in

aqueous solution.80 The term "photovoltaic" (PV) comes from the Greek word

“phōs” meaning "light", and "voltaic", meaning electric, from the name of the

Italian physicist Volta, after whom a unit of electro-motive force, the volt, is

named. The term "photo-voltaic" has been in use in English since 1849.81

38

Smith and Adams’ work on the photoconductivity of selenium in 187382

and 187683 respectively, lead to further understanding PV effect. However, it

was not until 1883 that the first solar cell was built, by Charles Fritts, who

coated selenium with an extremely thin layer of gold to form junctions and the

device was around 1% efficient. Subsequently Russian physicist Aleksandr

Stoletov built the first solar cell based on the outer photoelectric effect

(discovered by Heinrich Hertz earlier in 1887). Albert Einstein explained the

photoelectric effect in 1905 for which he received the Nobel Prize in Physics

in 1921. Russell Ohl patented the modern junction semiconductor solar cell in

194684, which was discovered while working on the series of advances that

would lead to the transistor. The first inorganic silicon-based solar cell with an

efficiency of 6% was discovered by Pearson, Fuller and Chapin at Bell

Laboratories in 1954.85 During the 1960s and 1970s, the terrestrial installation

of PV cells opened the initial market of daily utilization.86 Over the years, the

efficiency of the crystalline silicon cell has recently attained 25 %.87

The field of photovoltaics is at the moment dominated by silicon-based

solar cells. Due to the large availability of the used material and extensive

knowledge from the microelectronics industry, crystalline silicon solar cells

currently have a 90% market share.88 The main drawback of this type of

devices is the high purity needed for proper device operation. The energy,

and thus costs, needed in the fabrication process limits its usefulness as an

alternative energy source. Second generation photovoltaics are under active

investigation in order to further reduce the cost of produced electricity. This is

so-called thin film photovoltaic technology that includes cadmium sulphide

(CdS), cadmium telluride (CdTe), chalcogenides such as copper indium

diselenide (CIS) or copper indium gallium selenide (CIGS), amorphous and

nanocrystalline silicon. Such inorganic semiconductor materials are more

absorbing than crystalline silicon and can be processed into thin film directly

onto large area substrates using techniques such as sputtering, physical

vapour deposition, and plasma‐enhanced chemical vapour deposition. The

fabrication of low cost inorganic thin film solar cells with efficiencies ranging

from 10‐19% have been demonstrated in the laboratory89 but the controlled

manufacturing still remains a challenge. The best laboratory efficiencies of

39

solar cells obtained for various materials and technologies are shown below

(Figure 1.10).

Figure 1.10 The best laboratory efficiencies of solar cells obtained for various materials and technologies. [Source: NREL] In the meantime, the research based on the photo-electronic properties

of organic molecules and devices accelerated after the photoconductivity was

discovered in anthracene by Pochettino in 190690 and Volmer in 1913.91

During 1950-60s, the development of organic materials as photoreceptors

extended the possibility of organic molecules as electronic materials.92 The

first organic heterojunction solar cell based on a copper phthalocyanine and a

perylene tetracarboxylic acid derivative was reported by Tang in 1986 (Figure

1.11).93

Figure 1.11 Chemical structures of archetypal organic molecules.

40

After the discovery of the ultrafast charge transfer between poly[2-

methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and

buckmisterfullerene (C60) by Sariciftci et al. in 1992,94 there has been a rapid

increase in efficiencies from less than 0.1% to greater than 7% in 2010 for

laboratory scale devices.95,96a Recently, National Energy Renewable

Laboratory (NREL) has announced that Konarka’s latest organic based

photovoltaic (OPV) solar cells have demonstrated a world record efficiency of

8.3%.96b It is therefore hoped that the research will deliver higher efficiencies

and stabilities appropriate to long-term industrialisation in the near future

(Table 1.2).

The pace of development of organic photovoltaic devices (OPV) makes

them the expected low-cost alternative to their more expensive inorganic

counterparts. OPVs are particularly attractive because of their ease of

processing, mechanical flexibility and potential for low cost fabrication of large

area devices. In addition, their material properties can be substantially

adapted by modifying their chemical structure, resulting in greater

customization compared to traditional inorganic solar cells.

The field of organic photovoltaics can be divided into three classes

spanning small molecules,97,98 dye‐sensitized99‐101 and polymer based solar

cells. π‐conjugated polymers in OPVs are an especially attractive alternative

to traditional silicon‐based solar cells because they are strong absorbers of

visible light, in even <100 nm thin film devices, and can be deposited onto

flexible substrates over large areas using wet‐processing techniques such as

spin‐coating, printing or roll‐to‐roll coating.102‐115

41

Year Author Research contribution Ref.

1986 C.W. Tang First organic heterojunction PV cell. 93

1991 Hiramoto et al. First dye/dye bulk heterojunction PV cell. 116

1993 Sariciftci et al. First polymer/C60 heterojunction PV cell. 117

1994 Yu et al. First bulk polymer/C60 heterojunction PV cell. 118

1995 Yu et al./Halls et al. First bulk polymer/polymer heterojunction PV cell. 119,

120

2000 Peeters et al. Oligomer-C60 dyads/triads PV cells. 121

2001 Schmidt-Mende et al. Self-organized liquid crystalline PV cell. 122

2001 Ramos et al. Double cable polymer PV cells. 123

2002 Brabec et al. First low-band gap polymer/PCBM PV cell with η = 1% 124

2003 Wienk et al. First bulk heterojunction polymer/PC70BM PV cell 125

2009 Sung et al. Bulk heterojunction polymer solar cell η = 6.1% 126

2010 Yongye Liang et al. Bulk Heterojunction Polymer Solar Cell η = 7.4% 95

2010 Solarmer OPV Record efficiency η = 8.13 % 96a

2010 Konarka OPV World record efficiency η = 8.3 % 96b

Table 1.2 Milestones of Organic PV cells.

The dye-sensitized solar cell (DSC) was introduced by O’Reagan and

Grätzel in 1991 and consists of a nanoporous titanium oxide (TiO2) layer.101

The main disadvantage of DSC’s is the use of the liquid electrolyte, which

causes stability problems.127

Small molecule solar cells are fabricated by thermal evaporation of a

donor and acceptor material in either a double layer structure93 or a bulk

heterojunction similar to polymer solar cells.128 The advantage of small

molecule cells is the large control of the deposition enabling for instance

combinations of bilayer and bulk heterojunctions. On the downside the

vacuum based deposition does not comply with the concept of a low cost and

high throughput fabrication techniques.

Polymer solar cells are based on π -conjugated polymers as electron

donors. Modification of the molecular structure allows to modify chemical and

physical properties and have resulted in a number of well performing

materials with different band gaps and energy levels such as poly(3-

hexylthiophene) (P3HT),129 poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta‐

42

[2,1‐b;3,4‐b′]‐dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)] (PCPDTBT),130

poly[9,9-didecanefluorene-alt-(bis-thienylene)benzothiadiazole] (PF10TBT)131

and poly[3,6-bis-(4’-dodecyl-[2,2’]bithiophenyl-5-yl)-2,5-bis-(2-ethyl-hexyl)-2,5-

dihydropyrrolo[3,4-]pyrrole-1,4-dione] (PBBTDPP2).132 As an acceptor either

another semiconducting polymers,133,134 inorganic materials135 or fullerenes136

can be used.

1.2.3 Operating principle of organic photovoltaic cells When the HOMO‐LUMO levels are appropriately matched between proximate

electron donor and acceptor material, absorption of light by either of the

materials can lead to photoinduced charge transfer between the materials.

For example, upon light absorption in the donor material an electron is excited

from the HOMO into the LUMO to obtain an exciton. From this excited state,

the electron may be transferred into the LUMO of the acceptor resulting in

free charge carriers. The driving force for this photoinduced charge transfer is

the difference in ionization potential (ID) of the excited donor and the electron

affinity (EA) of the acceptor, minus Coulombic correlations.18 After the

photoinduced charge transfer, the positively charged hole remains on the

donor material whereas the electron is located on the acceptor material.

Finally the free charge carriers need to be transported to the respective

electrodes to create a photovoltaic effect. At this point the donor material

serves to transport the holes whereas the electrons travel within the acceptor

material. The charge carrier transport is driven by internal electric fields

across the photoactive layer caused by the different work function electrodes

for holes and electrons.

43

The complete process starting from an absorbed photon to charges

collected at the electrodes is mainly divided into four steps (Figure 1.12).137

1.2.4 Efficiency characteristics of organic photovoltaic cells The solar cell performance and electrical characteristics are determined by

measuring the current density to voltage (J-V) characteristics, both in dark

and under illumination. In the dark, there is almost no current flowing until

external voltages larger than the open circuit is applied. Figure 1.13 shows J-

V characteristics of an organic solar cell under illumination. From the J-V

curve, four parameters can be obtained. The current density under

illumination at zero applied bias is called the short circuit current density (Jsc),

Figure 1.12 Operative process in an OPV (From G. Chidichimo and L. Filippelli International Journal of Photoenergy 2010, 123534)

An exciton after a photon absorbed by the donor material (1). This exciton diffuses

towards a donor/acceptor interface where the electron is transferred to the acceptor

material (2). Even though the hole and electron are now on different materials they are

still strongly bound by Coulombic interactions and need to be dissociated into free

carriers (3). Then finally they are transported through the two respective phases and are

collected at the electrodes (4). During each of the above-mentioned processes energy

can be lost due to various loss mechanisms: all photons are not absorbed by the active

layer, not only due to limitations of the bandgap but also due to the often limited thickness

of the active layer; excitons decay when created too far from the D-A interface; geminate

recombination of the bound electron hole pair can occur; and bimolecular recombination

of free charge carriers during transport to the electrodes.

44

when the current density under illumination is zero the cell is at the open

circuit voltage (Voc). The Voc is limited by the energy difference between the

HOMO of the donor material and the LUMO of the acceptor.

Figure 1.13 Current-voltage (I-V) characteristics and the corresponding power-voltage curve for a solar cell under illumination.138 The essential parameters determining the photovoltaic performance are shown: Jsc is the short-circuit current, Voc is the open-circuit voltage, Jmp and Vmp are the current and voltage, respectively, at which a given device’s electrical power output is the maximum, Pmax, the fill factor (FF) is a graphic measure of the “squareness” of the I-V curve, and the power conversion efficiency (PCE) is defined as the ratio of maximum power output (Pmax) to power input (Pin).

The Voc of a conjugated polymer/PCBM solar cell can be estimated by:

Voc = [−ELUMO (A) − EHOMO (D)] − 0.4V Eq 1.4

where EHOMO(D) is the oxidation potential of the polymer (donor), ELUMO(A) is

the reduction potential of PCBM and the value 0.4 V is the approximate

voltage loss at the interfaces.139,140

The maximum power the device can produce is characterized by the

maximum power point (MPP). The MPP is determined by:

Eq 1.5

where Vmax and Jmax are the voltage and current at the MPP.

The fill factor (FF) is the ratio between the MPP and the maximum theoretical

power output:

45

Eq 1.6

With the value of FF, the power conversion efficiency (η) can be written as

Eq 1.7

where Pin is the incident light power.

The power conversion efficiency has to be determined under standard

test conditions which includes the temperature of the solar cell (25 ºC), an

illumination intensity of 1000 W/m2 and a spectral distribution of the

illumination source (AM1.5).141 Since the spectrum of the used illumination

source is in general not the same as the AM1.5 solar spectrum, the mismatch

factor (M) for the measurement has to be determined using the equation142

Eq 1.8

where ER(λ) and ES(λ) are the AM1.5 solar spectrum and spectrum of the

used illumination source and SR(λ) and ST(λ) are the spectral responses of a

reference cell and the tested cell, respectively.

To determine the spectral response of the tested cell, Incident photon-to-

current efficiency (IPCE) measurements can be done. Figure 1.14 shows an

example of an IPCE characterisation, also known as External Quantum

Efficiency (EQE) measurement, which besides determining the mismatch

factor of the measurement is also very useful for determining loss

mechanisms in solar cells.

46

Figure 1.14 External Quantum Efficiency (EQE), also known as Incident Photon-to-Current Efficiency (IPCE) of a polymer : fullerene solar cell, measured at short circuit conditions.

The incident photon to current efficiency (IPCE) is the ratio of the

number of charge carriers collected at short circuit per incoming photon of a

given energy shining on the device. The IPCE can be calculated by:

Eq 1.9

Where e is the elementary charge (1.602 × 10‐19 C) and PPhotons is the number

of photons.

1.2.5 Organic photovoltaic active layer architectures There are three predominant architectures that have been used in the

fabrication of organic solar cells (OSCs). The first generation was based on a

single organic layer sandwiched between two different metal electrodes

(Figure 1.15a).143 The current was generated due to the potential difference

induced by the asymmetric work functions of the electrodes under light

irradiation. Because of the large exciton binding energy in organic

semiconductors,144 the difference in the work functions is usually not high

enough to produce sufficient photoinduced charge generation. And also, the

exciton diffusion distance is low (~5-20 nm),145-148 and only excitons

47

generated in the region close to the electrodes can convert into separate

charges that can be collected.

(a) Single layer (b) Bilayer (c) Bulk-heterojunction

Figure 1.15 Different structures employed in OSCs.

In order to improve the efficiency of OSCs, the bilayer heterojunction

(Figure 1.15b) structure, where two separate layers for the electron and hole

transporting organic materials are stacked between the electrodes was

introduced. Electrostatic forces result at the interface between the electron

and the hole transporting materials due to the difference in the electron affinity

of the electron transporting material and the ionization potential of the hole

transporting material. When this local electric field is strong enough to exceed

the exciton binding energy, the excitons are dissociated into electrons and

holes. The first bilayer heterojunction solar cell produced 1%, reported by

Tang in 198693 is also limited by the exciton diffusion length as excitons

formed at positions further away from the donor-acceptor interface than the

exciton diffusion length have a lower probability of generating free charge

carriers.

The limitation of the bilayer approach was overcome with the

development of the bulk heterojunction (BHJ),136,149 where the photoactive

layer consists of an intimately mixed blend of the donor and acceptor material

(Figure 1.15c and Figure 1.16b), which indeed led to a major increase in

generated free charge carriers upon light absorption. Ideally, a nanoscale

interpenetrating bicontinous network of donor and acceptor materials are

created within the entire photoactive layer, ensuring that every generated

exciton can reach the donor‐acceptor interface. At the same time, the

constructed BHJ should ensure a direct or percolating pathway of the charge

carriers to the respective electrodes in order to effectively transport and

48

collect the charges. In this approach, efficient charge separation can be

achieved within the exciton lifetime, and geminate recombination is greatly

reduced. For an optimal morphology, the electron D and A materials must be

interpenetrated with a domain size close to the typical exciton diffusion length

in the material.

The first report of photoinduced charge transfer from a conjugated

polymer, poly[2‐methoxy‐5‐(2‐ethylhexyloxy)phenylene vinylene] (MEH‐PPV),

to a buckminsterfullerene (C60) in1992 by Sariciftci et al.,18 has guided the

development of the field of polymer–fullerene BHJ solar cells. Now most of

the polymer solar cells are based on the BHJ concept as proposed by Yu et

al.136 The most widely studied system is that based on P3HT (shown in Figure

1.18a) as the electron donor. The acceptor molecule is generally a modified

fullerene (C60), the archetypal product being [6,6]-phenyl-C61-butyric acid

methyl ester (PCBM, Figure 1.18a),150 in order to increase its solubility but

retain its electronic behaviour. The bulk heterojunction solar cells based on

P3HT (Figure 1.18b) and the fullerenes [60]PCBM and [70]PCBM where

efficiencies reported generally are in the range of 4‐5%.151‐153

The typical device architecture of a BHJ solar cell based on P3HT and

PCBM is shown in Figure 1.16(b). First a layer of hole conducting

poly(3,4‐ethylenedioxythiophene)-blend-poly(styrenesulfonate) (PEDOT-

blend-PSS) is spin coated on a glass substrate coated with the transparent

electrode indium‐tin oxide (ITO). The PEDOT:PSS layer improves the surface

roughness of the substrate and improves and stabilizes the electrical contact

between ITO and the active layer. Subsequently, a mixture of the donor and

acceptor material is spin-coated from a suitable organic solvent. During

evaporation of the solvent a phase separation of the donor and acceptor

material take place with the formation of an interpenetrating network within the

photoactive layer. Finally a thin hole blocking layer of lithium fluoride (LiF) and

a layer of aluminium (Al) is evaporated on top as the back electrode.

49

O

OMe

S

C6H13

n

P3HT PCBM (a)

(b)

Figure 1.16 (a) Chemical structures of P3HT and PCBM; (b) Representation of a typical BHJ device based on P3HT (red) and PCBM (blue). 1.2.6 Novel low band-gap polymers To improve efficiencies further towards 10% new materials are needed

because the P3HT:PCBM system is approaching optimal device performance.

The main disadvantage of P3HT is the poor matching of its absorption

spectrum with the solar emission spectrum. The band gap of P3HT is around

1.9 eV, limiting the absorbance to wavelengths below 650 nm. Since the

photon flux reaching the surface of the earth from the sun has a maximum of

approximately 1.8 eV (700 nm) P3HT is only able to harvest up to 22.4%

(Figure 1.17) of the available solar photons.155,156 So by decreasing the band

gap of the active material, it is possible to harvest a larger amount of the solar

photons and thereby increase the power conversion efficiency.

50

Figure 1.17 Photon flux from the sun (AM1.5) as a function of the wavelength. The percentage of the total photon flux and the corresponding maximum obtainable current density is displayed on the right y‐axis.155,156 Novel promising polymer materials are shown in Figure 1.18. The

highest reported photovoltaic performances in blends with PCBM are listed in

Table 1.3. One of the most promising low band gap polymers to date is

PCPDTBT based on a benzothiadiazole acceptor unit and the planar

cyclopentadithiophene (CPDT) as the donor unit. Zhu et al. have reported

power conversion efficiencies up to 3.5% for BHJ solar cells based on

PCPDTBT and [70]PCBM with a maximum EQE of 38% around 700 nm and

over 25% in the range 400 to 800 nm.157 Further optimizing of the processing

conditions, by incorporating a few volume percent of alkanedithiol in the

solution used to process the films of PCPDTBT:[70]PCBM, improved the PCE

up to 5.5% through improving the BHJ morphology.158 Upon optimization, the

short circuit current was enhanced up to 16.2 mA/cm2, which is among the

highest reported to date. Silole derivatives of CPDT (PSBTBT, Figure 1.18)

showed a hole mobility of 3 × 10‐3 cm2/(V s), 3 times higher than that for

PCPDTBT.159 Efficiencies up to 5.1% have been reported for solar cells based

on PSBTBT:[70]PCBM blends.

51

b)

Figure 1.18 Novel donor materials used in polymer solar cells: (a) PTB7 and PC71BM95; and (b) recently discovered low band gap polymers (Table 1.3).

Polymer Acceptor η (%) Ref. PTB7 [71]PCBM 7.4 95

PCDTBT [70]PCBM 6.1 160

PTPTBT [70]PCBM 4.3 162

PTB4 [60]PCBM 6.1 161

PBBTDPP2 [70]PCBM 4.0 163

PCPDTBT [70]PCBM 5.5 158

PSBTBT [70]PCBM 4.7 159

Table 1.3 Efficiencies of some low bandgap polymers in blends with PCBM.

Recently, power conversion efficiency of 6.1% was reported for a BHJ

solar cell based on a blend of the polymer

poly[N‐9’’‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4’,7’‐di‐2‐thienyl‐2’,1’,3’‐benzot

hiadiazole) (PCDTBT, Figure 1.20) and [70]PCBM.160 The

PCDTBT:[70]PCBM solar cell demonstrated the best performance of any

single junction polymer solar cell studied to date. PCDTBT (Figure 1.18) is

52

based on a 4,7‐dithienylbenzothiadiazole unit and a soluble carbazole unit

that gives it an optical band gap around 1.88 eV. It should be pointed out that

the high performance is not reached via reduction of the band gap, but

through the deep HOMO level of the polymer, mainly fixed by the carbazole

moiety, which leads to higher values for the open circuit voltage. The latest

report of highly efficient polymer solar cells involve PTB4161 (Figure 1.18) that

is based on thieno[3,4‐b]thiophene and benzodithiophene units resulting in a

optical band gap around 1.63 eV. Fine tuning of the structure and electronic

properties has been done by introducing electron‐withdrawing fluorine to the

thieno[3,4‐b]thiophene unit, which reduce the HOMO energy level of the

polymer. A power conversion efficiency of over 6% was achieved in solar cells

based on fluorinated PTB4:[60]PCBM blends. After an extensive structural

optimization, Yongye Liang et al., further developed a new polymer from the

PTB family, PTB7, which exhibited an excellent photovoltaic effect. The

structure of PTB7 is shown in Figure 1.18 (a). The branched side chains in

ester and benzodithiophene render the polymer good solubility in organic

solvents. The weight average molecular weight (Mw) of PTB7 is 97500 g mol-1

and a dispersity (Đ = Mw/Mn) of 2.1. A PCE of about 7.4% has been achieved

from PTB7/PC71BM [Figure 1.18 (a) PC71BM1/4phenyl-C71-butyric acid

methyl ester] solar cell devices, which is the first polymer solar cell showing a

PCE over 7% to date.95

Although the performance of polymer solar cells has increased steadily

as indicated in Table 1.3, further improvements in efficiency are required for

large-scale commercialization. Aside from the power conversion efficiency,

processing and stability are two other important aspects that have to be

addressed with equal intensity for the success of polymer solar cells. With the

knowledge of low band gap materials that have been demonstrated, it is clear

that for the long-term stability of devices (required for large scale, i.e. greater

than 1 m2 installations), other routes to develop more stable morphologies will

be required. If the device stabilities can be improved then this will permit

OPVs to be more widely used in the market.

53

1.3 Synthesis of the archetypal conjugated polymer, poly(3-hexylthiophene) (P3HT) The most widely studied substituted polythiophenes are poly(3-

alkythiophene)s (P3AT)s. Poly(3-alkylthiophene) represents the most

important conjugated polymer in recent years, which was used in various

applications like light-emitting diodes, field-effect transistors and plastic solar

cells because of its excellent optical and electrical properties. Early reports for

the polymerization of unsubstituted thiophene were metal-catalyzed

condensation reactions.164,165 Another frequently used method by iron (III)

chloride was oxidative polymerization of thiophene monomers.166 Although

these methods are successful for polymerizing unsubstituted thiophene,

polythiophenes are not soluble in common organic solvents. Then the

researchers have focused by developing new synthetic techniques for

substituted polythiophenes.

This Section will consider the chemistry of P3HT, an important part of

this thesis’s work. P3HT is widely considered a “standard” for photovoltaic

devices, morphological studies and the manipulation of materials within OPV

active layers. This is because it can be prepared with predetermined

molecular weights,167 chemically modified,168 has high solubility in common

organic solvents, has a well understood electronic behaviour,169 and exhibits a

semi-crystallinity which both enhances interfacial interactions with the electron

acceptor molecule and facilitates charge transfer through crystalline

domains.170

1.3.1 Regioregularity The optoelectronic properties of P3ATs are mainly dependent on the

regiochemical couplings along the polymer chain. So it is necessary to define

the concept of regioregularity of P3ATs. As 3-alkylthiophene is an

asymmetrical molecule, there are three possible couplings between the

thiophene repeat units during polymerisation. These sequences are head-tail

(HT or 2-5’), head-head (HH or 2-2’) or tail-tail (TT or 5-5’) as in the case of

poly (3-hexylthiophene) (P3HT) shown in Figure 1.19. If the thiophene rings

54

are coupled in a HH manner Figure 1.19 (b) with twisted out of conjugation

due to steric repulsion between alkyl chains, it leads to regio-irregular P3HT

that reduces the electrical conductivity of the polymer. Otherwise, the

thiophene rings are coupled in a consecutive HT manner Figure 1.19 (a)

during polymerization leads to regio-regular P3HT that adopts coplanar

conformation resulting in a lower energy. This arrangment gives a highly

conjugated low bandgap polymer. The regioregularity can be defined as the

percentage of head-tail sequences of 3-hexylthiophene units and brought a

certain arrangement of polymer chains. Regioregularity plays a crucial role on

the electronic properties of P3ATs. In order to achieve good crystal packing

and electrical transport properties, the P3ATs must be regioregular; this

means that almost all of the linkages along the polymer chain are of the same

type (usually head-to-tail).

S

C6H13

S

C6H13

S

C6H13

S

C6H13

S

C6H13

S

C6H13

a b c

1 1 1

2 2 2

3 3 34 4 4

555

1' 1' 1'

2'2'2'

3' 3' 3'4' 4'4'

5' 5' 5'

Head-to-tail (HT) Coupling Head-to-Head (HH) Coupling Tail-to-tail (TT) Coupling

Figure 1.19 Regiochemical couplings of P3HTs: (a) head-to-tail (HT or 2-5’) (b) head-to-head (HH or 2-2’) (c) tail-to-tail (TT or 5-5’).

1.3.2 McCullough and Rieke methods In 1992, two methods the McCullough method171 and the Rieke method172

were introduced for the synthesis of regioregular P3ATs which is shown in

Scheme 1.1. In the McCullough method, 2-bromo-5-bromomagnesio-3-

alkylthiophene [Scheme 1.1(a)] monomers are cross-coupled together with

1,3-bis(diphenylphosphino)propane nickel (II) chloride [Ni(dppp)Cl2]. This was

the first report for the synthesis of regioregular P3ATs with 90% head-to-tail

regioselectivity whereas the Rieke method uses activated (Rieke) zinc to

generate the substituted thiophene monomer, which was then coupled with a

nickel catalyst [Scheme 1.1(b)]. Both methods generated regioregular P3ATs

with tunable molecular weights and low PDIs, but they required cryogenic

temperatures and highly reactive metals limit the large-scale production of

55

P3ATs. Several methods for the synthesis of poly(3-hexylthiophene) were

developed after the interest of this polymer in the field of organic

electronics.173

S

R

Br

1. LDA

2. MgBr2 S

R

BrBrMg

Ni(dppp)Cl2

S

R

n

S

R

Br

Rieke Zn

S

R

BrBrZn

Ni(dppe)Cl2

Br

a

b

S

R

n

Scheme 1.1 Synthesis of poly(3-alkylthiophene)s (P3ATs) by (a) McCullough method171 and (b) Rieke method.172

1.3.3 Grignard metathesis (GRIM) polymerisations leading to P3HT Later on, McCullough and coworkers in 1999 again retooled his method and

discovered another method for the synthesis of regioregular P3ATs by

Grignard metathesis (GRIM) (Scheme 1.2).174 This is the most commonly

employed method for synthesizing well-controlled, highly regioregular, and

economical poly(3-alkylthiophenes). In this method, reaction of 2,5-dibromo-3-

hexylthiophene with alkyl Grignard reagents gives two metallated

regioisomers (A:B) in 85:15 or 75:25 ratio via a magnesium exchange reaction

(Scheme 1.2). Then addition of Ni(dppp)Cl2 to this reaction mixture produces

P3HT with more than 95% regioregularity. [GC-MS analysis after addition of

Ni(dppp)Cl2 showed that only the A isomer is incorporated within the polymer

while B is not consumed.]

S

C6H13

Br Br

RMgX

S

C6H13

XMg Br S

C6H13

Br MgX

+

85% 15%

Ni(dppp)Cl2

S

C6H13

n

A B Scheme 1.2 Synthesis of poly(3-hexylthiophene)s (P3HTs) by Grignard metathesis (GRIM) McCullough method.174

56

1.3.4 Chain-growth condensation polymerisations leading to P3HT Yokozowa and his coworkers found that Mn values of P3HT were controlled

by feed ratio of [monomer]/[Ni catalyst] when the polymerization was carried

out at room temperature and used the exact amount of isopropylmagnesium

chloride for the formation of 2-bromo-5-chloromagnesio-3-hexylthiophene

from the corresponding bromoiodothiophene175 (Scheme 1.3) leading to P3HT

with dispersity around 1.1 even upto Mn of 28700 g mol-1 when the

polymerization was quenched with hydrochloric acid.176

S

C6H13

I Br

i-PrMgCl THF

S

C6H13

ClMg Br

1. Ni(dppp)Cl2, THF

2. 5M HClS

C6H13

n

Scheme 1.3 Synthesis of poly(3-hexylthiophene)s (P3HTs) by Yokozawa et al.175,176

McCullough and Yokozawa independently demonstrated the GRIM

polymerization of 3-alkylthiophenes follows in a living chain growth

mechanism instead of the traditionally accepted step growth

polycondensation. As a result, low dispersities (1.1-1.3) and well-defined

molecular weights can be prepared by controlling the feed ratio of monomer to

the Ni catalyst.177-179 After detailed investigation of the polymerization of M2a

(Scheme 1.4), Yokozowa et al., proposed a mechanism called as “Catalyst

transfer condensation polymerization” which is shown in Scheme 1.4.180

According to this, first Ni(dppp)Cl2 reacts with two equivalents of M2a and

forms a dimer of M2a in situ which is chain initiator with the zero-valent Ni(0)

complex. The Ni(0)-complex without diffusing into reaction mixture is inserted

into intramolecular C-Br bond by reductive elimination involving C-C bond

formation. Again another M2a reacts with this Ni, then coupling reaction and

transfer of the Ni catalyst to the next C-Br bond. In this way, growth will

continue with the Ni catalyst moves to the polymer end group.178 So this

reaction can be done both at room temperature and on a large scale, the

Grignard metathesis/Kumada-Corriu coupling has become the most broadly

used method for the synthesis of predetermined high molecular weight

P3ATs.

57

SBrClMg

C6H13

Ni(dppp)Cl2

M2a

SBr

C6H13

SBr

C6H13

NiL2

SBr

C6H13

SBr

C6H13

NiL2

SBr

C6H13

S

C6H13

NiL2 Br M2a

SBr

C6H13

S

C6H13

NiL2

SBr

C6H13

SBr

C6H13

S

C6H13

NiL2SBr

C6H13

SBr

C6H13

S

C6H13

NiL2

S

C6H13

BrM2a

SBr

C6H13

S

C6H13

NiL2

S

C6H13

Br 5M HCl

SBr

C6H13

S

C6H13

n-1H

(L2 = dppp)

Scheme 1.4 Mechanism of Catalyst Transfer Condensation Polymerization of P3HT proposed by Yokozawa et al.180

Regioregular poly(3-hexylthiophene) (P3HT) is widely studied for

electronic and photovoltaic devices because of its high hole mobility, high

solubility in common solvents and good chemical stability.181 Hence this GRIM

synthetic route was chosen for its ease of implementation and also it leads to

highly regioregular P3HT.

One of the main benefits of this chemistry is that once the

polymerization is finished, the chain-ends remain active and can be used to

perform end-capping reactions with Grignard reagents. This technique will be

widely exploited in this thesis.

1.3.5 Chain-end capping of P3HT using GRIM To synthesize copolymers based on P3HT, it is essential to functionalize.

Jeffries-El et al.177d showed that it was possible to do so by adding a second

Grignard reagent at the end of polymerization. This provides access to a wide

variety of different terminal functional groups (vinyl, ethynyl, aryl, aldehyde

and amine...). Scheme 1.5 describes the reaction mechanism of

functionalization of P3HT. The addition of a second Grignard reagent on a

58

living P3HT allows, firstly, to stop the chain growth, but also to functionalize

the P3HT.

SBrMg Br

C6H13

+ Ni ClCl

L

L

THF/R.T.

After severalcatalytic cycles

SBr

C6H13

S

C6H13

S

C6H13 n

Ni RLL

SBr

C6H13

S

C6H13

S

C6H13 n

Ni BrL

L

RMgX

SBr

C6H13

S

C6H13

n

Ni(0)L

L S

C6H13

R+

Associated pair

Monocapped Polymer

reductive elimination

SNi

C6H13

S

C6H13

nS

C6H13

R L

LBr

oxidative addition

SNi

C6H13

S

C6H13

nS

C6H13

R L

LR SR

C6H13

S

C6H13

nS

C6H13

RNi(0)L

L+

reductive elimination

n

M1a4

5

6 7Dicapped Polymer

RMgX

Scheme 1.5 Proposed Mechanism for the end-capping reaction of P3HTs.177d

The authors have shown to synthesize a mono or difunctionalization of

polymers depending on the nature of Grignard reagent and not depending on

its concentration in the medium. Jeffries-El et al177d observed that species

containing a double or triple bond led to mono-adducts P3HT whereas others

particularly aromatics give polymers with diadducts which is shown in Scheme

1.5. This is because the alkenyl and alkynyl may react with Ni(0) to form a

stable π -complex, which prevents any further reaction with terminal bromine

of P3HT. However, it should not be generalized because many functionalised

Grignard reagents remain to be tested in this scheme to access other types of

functional groups at the end of P3HT chains. In order to ultimately prepare

block copolymers from functional P3HT, two types of Grignard reagents were

used177d to obtain mono and di-functionalised P3HT in this thesis work.

59

1.4 Organic photovoltaic cells and block copolymers 1.4.1 Importance of morphology of active layer in OPVs In organic solar cells, the morphology of the active layer is critical and affects

several physical parameters of the photovoltaic process.182 First, it enables

the transformation of light-formed excitons into separate positive and negative

charges by providing interfaces between donor and acceptor domains.

Second, well-organised domains of the respective materials enhance

percolation of charges to the electrodes. Third, it allows control over the

mechanical properties of the materials that are destined for use in roll-to-roll

processing.5 Good organization of donor and acceptor materials allows them

to limit electron-hole recombination by generating a phase separation whose

characteristic size is equivalent to the diffusion length of excitons, but also

optimizes carrying loads by creating channels conduction to the electrodes.

An idealized morphology is shown in Figure 1.20. The presence of a thin layer

of acceptor material in contact with the cathode material and contact the

donor to the anode may minimize charge recombination at the electrodes.

Figure 1.20 Ideal morphology of BHJ structure for organic solar cells.16

Various parameters influence the morphology of the active layer. They

include the structure of the materials, the concentration of donor and

acceptor, the solvent used to make films, the concentration of the solutions,

the deposition temperatures and subsequent heat treatments. These

parameters can be classified into two broad classes that are the

thermodynamic parameters and kinetic parameters. The thermodynamic

parameters correspond to the nature and properties of the initial solution

60

(materials, ratio between donor and acceptor materials-solvent interaction).

The kinetic parameters for their work mainly during the film formation

(evaporation time of solvent, crystallization of materials, thermal annealing).

1.4.2 Controlling morphology of active layer in blends In the case of donor-acceptor blends, the morphology of the films is very

difficult to control because it depends on many parameters. The two

components of the mixture are very different, behave independently of each

other, and/or cooperatively for example in the formation of eutectic mixtures

leading to a wide variety of structures.183 It is however possible to improve the

morphology by optimizing the deposition conditions, such as concentrations of

donor and acceptor in the solvent, the rate of deposition, and the use of

thermal and solvent annealing.

P3HT tends to be organized in fibrillar form, but this organization is

highly dependent on the deposition techniques and solvents used.184-188

Indeed, spin-coating, which allows a very fast evaporation of the solvent,

leads to less ordered structures than the dip-coating and drop-casting, which

allows a slow evaporation of the solvent and thus lead to more orderly

arrangements (Figure 1.21).

Dip-coating Drop-casting Spin-coating

Figure 1.21 AFM phase images of thin layers obtained from P3HT (Mn=1.9 kDa) using different processing techniques in chloroform.185

Ideally, there should be an optimzation of the evaporation time so that

it is short enough to limit phase separation and long enough to facilitate the

crystallization of materials.189 The choice of solvent also determines the

morphology of the films. Firstly, the constituents of the mixture will have

61

varying affinities with respect to the organic solvents chosen which effects the

formation of films. Secondly, the solvent will have a particular boiling point that

will lead to a particular evaporation time resulting in different structures that

are more or less organized. Thus, for the system P3HT:PCBM, the films

deposited from solvents of high boiling points (tetrahydronaphthalene,

dichlorobenzene) have, in the absence of heat treatment, a better structure

and therefore better mobility of charges that those made from solvents with

low boiling temperatures (chloroform, chlorobenzene).188

Another way to improve the morphology is by thermal annealing of

devices before or after the addition of the final electrode interface. The

purpose of this annealing is both to nano-structure the films, playing on the

ability of species to reptate and crystallize, but also to assess the stability of

the active layers. In the case of the P3HT:PCBM system, numerous studies

have shown the beneficial effect of annealing on the morphology and the

concomitant increase in efficiency.184,190-196 However, the temperature and the

annealing time varies greatly from one study to another. This is partly due to

the nature of P3HT used (molecular weight, dispersity, regioregularity,...) and

the variations in equipment and processes in each laboratory. Neverthless, a

paper by Yang et al.196 revealed by transmission electron microscopy (TEM),

an improvement of the morphology after annealing with better crystallization

of the two components and the formation of a network of interconnected fibrils

P3HT (Figure 1.22).

Figure 1.22 TEM images of films P3HT: PCBM (a) general view, (b) zoom and (c) schematic representation of the morphology of the active layer before and after annealing.196

62

This trend was also confirmed by X-ray diffraction measurements, which show

a marked increase in the crystallinity of P3HT after annealing.193,195 This nano-

structuring results in a significant increase in the short circuit current and thus

the cell efficiency. Indeed in some cases, the energy conversion efficiency is

increased ten-fold after thermal treatment.193 The increase of short-circuit is

due to both the mobility of charge carriers increased when the materials are in

a crystalline state, but also to a decrease in recombination of excitons by

optimizing the size of crystalline domains (≈ 15 nm).

Annealing enhances the charge carrier mobility197,198 (see Figure 1.23) in

combination with changing the recombination behavior from a Langevin type

into a non-Langevin type recombination mechanism.199 X-ray, AFM and TEM

investigations enabled a microscopic picture of the annealing process to be

developed,196,200 which is considered to take place in three subsequent steps:

(i) annealing softens the P3HT matrix, which (ii) allows PCBM molecules to

diffuse out of disordered P3HT clusters and form larger fullerene aggregates,

before (iii) the now fullerene-free P3HT matrix recrystallizes into larger fibrillar

type crystals, which are embedded in a matrix considered to consist of PCBM

nano-crystals and amorphous P3HT.

Figure 1.23 I–V curves of P3HT/PCBM solar cells under illumination with white light of 800 Wm−2 : as produced solar cells (filled squares), annealed solar cells (open circles) and cells simultaneously treated by annealing and applying an external electric field (open triangles).197

63

Despite these improvements, the ideal morphology (shown in Section 1.4.1) is

far from being reached which severely limits the conversion efficiencies.

Several solutions are being proposed to overcome this problem by curing of

the active layer,201-203 the use of double cable polymers,204-207 or the use of

block copolymers,208 which is one of the main subjects of this thesis.

1.4.3 Self-assembly of rod-coil block copolymers Block copolymer consists of different adjacent blocks that are derived

from different monomers or from the same monomer with different

composition. These different A and B blocks in a block copolymer tend to

minimize their contact surface that cannot separate in a macroscopic scale

unlike simple mixtures and therefore forced to self-organise into domains (10-

50 nm) which are close in size to the length of each block.209,210 The Flory-

Huggins parameter (χAB), characterizes the incompatibility between the two

blocks, positive value of this term represents repulsion between the chains

and negative value represents compatibility between the blocks.211

The research on “microphase separation” has been widely described

both theoretically and experimentally over 30 years.212,213 Most of the

theoretical research, initiated by Meier in 1969,214 related linear coil-coil

diblock copolymers which are now generally well understood. In the 1990s,

Matsen and Bates215 proposed a theoretical phase diagram (Figure 1.24a)

depending on the volume fraction (f) of each component and the product, χN

(N = total degree of polymerization). Various thermodynamically stable

microstructures have been predicted: they are lamellae (L), cylinders

organized in a hexagonal arrangement (H), body-centered cubic (QIm3m),

closed-packed spheres (CPS) and bicontinuous cubic (gyroid) phase of an

Ia3d symmetry (QIa3d), as shown in Figure 1.24b.

64

(a)

(b)

Figure 1.24 (a) Phase diagram of coil-coil diblock copolymer 215; and (b) schematic representation of block copolymer morphologies.211b

In the 1980s, Semenov and Valencino216 researched theoretical

studies on the bulk behaviour of rod-coil diblock copolymers and proposed

two phases: a nematic phase and a smectic A phase and they also introduced

the smectic C phase after subsequent developments217,218 as shown in Figure

1.25. Later on, Williams and Fredrickson in 1992 reported on non-lamellar

structures of rigid segments with high coil block volume fractions (fcoil > 0.9),

which are called as "hockey pucks" (Figure 1.25e).219

Figure 1.25 Rod-coil copolymers self-assembly (a) nematic phase, (b) bilayer smectic-A phase, (c) monolayer smectic-A phase, (d) monolayer smectic-C phase and (e) "hockey pucks".219

65

In 1993, Williams and Halperin220 reported lamellar, cylindrical and

spherical micro-segregated structures and later, Matsen221 described the

influence of the rigidity on the lamellar phases. Different research groups222-224

reported that only morphologies where the presence of flexible coil blocks on

the outer side of an interface are thermodynamically stable and the suitable

introduction of compatible homopolymers, the domain sizes of rod-coil block

copolymers can be controlled.225 Stupp and coworkers226 introduced the

interesting concept of “mushroom” shaped nano-aggregates which is good in

various applications, for example in photovoltaics where the crystallisation of

the rod domain is very important for charge transfer.

1.4.4 Why block copolymers in photovoltaic cells? Block copolymers (BCPs) are currently exploited in photovoltaic cells for two

reasons. The first reason was because the excited electronic state termed

exciton227 (see Section 1.2.3) for a typical polymer can exist over a distance of

around 5-20 nm228,229 and this distance coincides extremely well with the

typical size of block copolymer domains.230 As the domain width can often be

tailored simply by varying the length of the polymer, it should be relatively

simple to design block copolymers that can have dimensions correct for

exciton formation and conversion into charges as per the ideal structures

shown in Figure 1.20 of Section 1.4.1. The increasing interest in BCPs is their

ability to obtain a tunable nano scale self-assembly.231 With the help of

modern synthetic chemistry; one can design BCPs with specific lengths and

geometries to obtain variety of ideal nanostructures which may enhance the

device efficiencies and also the introduction of rod blocks in BCPs bring a

competition between crstallization and microphase separation which can

further affects the morphology. An example of a target system, donor-

acceptor rod-coil BCP connected by a linker is shown in Figure 1.26.

The second main reason for the use of block copolymers has been as

stabilisers or compatibilizers in solar cells. Attempts to increase efficiencies,

while making active-layers more stable and less sensitive to environmental

perturbations during their preparation, have used the self-organising

66

properties of copolymers containing opposing segments of donors and

acceptors.232,233 In these systems, like-polymer blocks self-assemble into

excitonic-scale domains that can provide pathways through which charges

may pass to the electrodes as previously mentioned.

Figure 1.26 A target device based on conjugated rod-coil block copolymers where domains are sized to be around 10 nm to enhance exciton capture and charge percolation to electrodes.

Notable examples reported by Hadziioannou234 and Fréchet235

incorporating fullerene, and Thelakkat236 and Emrik237 using perylene, have

shown how meso-scale separation of different moieties have been feasible. In

general, however, these systems have not been able to show high

efficiencies, and often suffer from complex multi-step syntheses, an important

point when considering industrialisation. The synthesis of block copolymers of

P3HT and polynorbornenes carrying high concentrations of C60 reported by

Fréchet is of particular relevance (Figure 1.27).235 Even though the

copolymers did not enhance efficiencies when mixed as an optimised portion

with P3HT-blend-PCBM, it was demonstrated that through compatibilization

the physical stability of the active layer could be dramatically improved. This is

important as devices exposed to sunlight over long periods of time can easily

reach temperatures above the first glass transition temperature (Tg) of P3HT

(at ca 50 ˚C).238-240

67

Figure 1.27 Chemical structure of P3HT-based diblock copolymers incorporating fullerene by Fréchet et al.235 It is apparent that there is a necessity to find a facile route to rod-coil donor-

acceptor copolymers that can both compatibilize and enhance organisation of

the P3HT-blend-PCBM layer, which is main object of this thesis. Examples of

specific copolymers for both these types are detailed in the following sections. 1.4.5 Synthesis and self-assembly of exampled rod-coil block copolymers Conjugated rod-coil BCPs have good immiscibility between rod and coil

segments which allows the formation of domains even with quite low molar

masses unlike coil-coil BCPs.241 Because of the lower solubilities of the rod

blocks, the synthetic procedures are very complex compared to coil-coil

systems. Therefore first, rod block is prepared and then the coil block is

attached by condensation reactions, or rod macro-initiators can be used for

the polymerisation of coil blocks.

The use of BCPs can optimize several parameters of organic

photovoltaic process, based on the ability of these species to self-organizing.

In recent years, many reseach groups have addressed the block copolymers

consisting of rod-coil coupled rigid block, because of their ability to self-

organize and their better formatting patterns (thanks to the presence of a

flexible block which increases their solubility). Examples are copolymers

based on poly(p-phenylene)242-244, poly(p-phenylene ethynylene)245-247,

polyfluorene248-251, poly(p-phenylene vinylene)(PPV)252-257 and poly(3-

hexylthiophene) (P3HT)258-264, which is the main subject of this thesis.

68

1.4.5.1 Copolymers based on poly(p-phenylene vinylene)s

Chemists have prepared BCPs made of p-type block such as PPV or its

soluble derivatives and for the n-type, preparing polymers that carry electron

acceptors as pendant, grafted groups attached to coil polymers such as PS.

Hadziioannou and coworkers contributed considerably in the synthesis and

self-assembly of block copolymers based on rod-coil derivatives of PPV for

photovoltaic application. Their work was based on an alkoxy-subsituted PPV

(MEH-PPV) coupled to a fullerene substituted PS (Figure 1.28a).265 Alkoxy-

subsituted PPV was used as macro-initiator for obtaining copolymers of PS

and poly(4-chloromethylstyrene) with predetermined molecular weights by

controlled radical polymerisations. The chloromethyl groups were then

attached to C60 by atom transfer radical addition (ATRA). This copolymer as

active layer showed better photovoltaic performance and improvement in the

short circuit current (0.15 to 5.8 µA/cm2) than the mixture with the same donor

and acceptor (Figure 1.28b).

Figure 1.28 (a) Chemical structure of D-A block copolymer PPV-b-P(S-stat-C60MS); (b) Photovoltaic performance of copolymer PPV-b-P(S-stat-C60MS) (B) compared with a blend of donor homopolymer and acceptor polymer (A) under illumination.265

Macromonomer for the polymerisation of acrylates266 was achieved by

the preparation of an alkyloxylated PPV with an alkoxyamine chain-end.

Another facile route was found that the addition of the alkoxyamine to the

alkyloxylated PPV using Grignard chemistry for the synthesis dialkyloxylated

PPV macro-initiator carrying chlorostyrene groups.267 To reduce crosslinking

reactions with C60, the chloromethyl groups were first converted to azido

69

groups before addition to C60. This method again produced rod-coil BCPs

carrying pendant C60s on the coil block. They have performed the effect of C60

grafting density of the PS on the material’s electronic properties and then, an

increase in electron charge mobilities with density was found.268

The morphology of the dialkyloxylated PPV-PS based rod-coil BCPs

was further investigated and it was found that the π -stacking of the PPV

resulted a thermodynamically stable lamellar structure of the conjugated

blocks within PS matrix.269 N. Sary et al.270 have synthesized and studied the

self-organization properties of rod-coil BCPs based on PPV-b-P4VP [poly(2,5-

di(2'-ethylhexyloxy)-1,4-phenylenevinylene)-block-poly(4-vinylpyridine)] shown

in Figure 1.29 (a). These copolymers were synthesized by anionic radical

method in which the chains of PPV-aldehyde were used to deactivate “living”

P4VP chains. The living polymerization of 4-vinylpyridine can perfectly control

the size of chains and the authors were able to synthesize copolymers PPV-b-

P4VP with different sizes of P4VP (copolymers Px, with x the volume fraction

of P4VP). After heat treatment in several steps to promote self-assembly of

copolymers, the morphologies were determined by TEM [Figure 1.29 (b)]. The

resulting structures are very different and depend strongly on the proportion of

each block in the copolymer. Indeed, for P55 (55% of coil), the observed

morphology corresponds to a lamellar phase of 20 nm. This phase is

organized in grains, in the order of micrometers. However, the interfaces of

these areas, the PPV channels appear distorted [Figure 1.29 (b)]. By

increasing the proportion of P4VP, the structure is changed to hexagonal

phase formed by sticks PPV [white on Figure 1.29 (b) P80]. However, for 88%

of coil (P88), the structure is disorganized and has a nodular phase. This work

therefore shows the great variety of morphologies that can be obtained by

choosing P4VP as a coil block.

From solid-state phase diagram for PPV-b-P4VP, It was found that the

lamellar phase dominated over a proportion of PPV to P4VP blocks as

expected because of the liquid-crystalline nature of the rod-blocks. But

hexagonal and spherical microphase separated morphologies are also

possible at high volume ratios of P4VP due to dominated force towards

macrophase separation.

70

Figure 1.29 (a) Chemical structure of copolymers PPV-b-P4VP; (b) TEM images (annealed) of various morphologies obtained for different proportions of coil block.270

Another report regarding rod-coil copolymers, an alkyloxylated PPV rod

connected with the coil block, which is a combination of azido-styrene units

and butyl acrylate groups, attached with C60 as shown in Figure 1.30.271 The

solid-state morphology of this copolymer showed that the fullerene still

directed to aggregate even it was bonded to the flexible coil blocks. The

utilization of high crystalline PTs instead of PPVs recommended as one of the

best solutions to overcome this problem.

Figure 1.30 Chemical structure of the PPV based rod coil copolymer with C60s attached to the coil block via a tertiary amine bridge.271 The efficiency of BHJ based on composite of dialkyloxy-substituted PPV

(MEH-PPV) as donor and cyano-substituted PPV as an electron acceptor was

around 2 %272,273 which is very high compared to present block copolymer

based systems.

71

1.4.5.2 Copolymers based on polythiophenes Since PPV copolymers have limitations in the device efficiency; the research

on PT copolymers, especially P3HT has been widely explored in organic

electronics. McCullough and colleagues were synthesized for the first time

P3HT based ATRP macro-initiators which was then used for the synthesis of

tri BCPs, PS-b-P3HT-b-PS and PMA-b-P3HT-b-PMA with coil block PS,

poly(methyl acrylate) (PMA). They were also successful in synthesizing

polyurethane elastomer based on P3HT using Vilsmeier-Hack formylation of

P3HT chain ends274 as shown in Scheme 1.6.274

Scheme 1.6 Synthetic routes for P3HT-based polyurethane elastomers and also triblock copolymers via Vilsmeier-Hack formylation reaction.274

The GRIM method via chain-growth condensation polymerisation

(shown in Scheme 1.4 of Section 1.3.4) has been widely explored in the

synthesis of rod-coil block copolymers because it has many advantages; (i) by

varying the ratio of Ni-catalyst to monomer one can predetermine the length of

the rod block (ii) since the width of the domains is directly associated to the

length of the rod polymers, it helps to design the target domain size and also

(iii) the polymerisation can be terminated with functionalized Grignard

reagents which lead to block copolymer chemistry (Scheme 1.5). Some of the

important examples are given below.

The synthesis of rod-coil BCPs P3HT-b-PMA were successful with

narrow dispersities (Đ < 1.3) by GRIM method as shown in Scheme 1.7.275

Another recent report by Dante et al. also used GRIM method for synthesizing

72

P3HT-macroinitiator for reversible addition-fragmentation chain transfer

(RAFT) chemistry to obtain copolymer, P3HT-b-PS-C60 as shown in Figure

1.31a.276 AFM images showed a fibrillar structure and also localised

conductivity measurements revealed that the lighter, darker areas on the

surface corresponding to P3HT, PS and C60 respectively as shown in Figure

1.31b.

Scheme 1.7 Synthesis of the ATRP macro-initiator based on P3HT and the synthesis of copolymers P3HT-b-PMA.275a

(a) (b)

Figure 1.31 (a) The chemical structure of rod-coil copolymer based on P3HT carrying C60, P3HT-b-PS-C60 (b) AFM image (topographic) of the same copolymer.276

73

Sary et al.,277 recently reported rod-coil BCPs based on P3HT which is

good for organic solar cell devices. They have synthesized diblock

copolymers P3HT-b-P4VP varying molecular weights of P4VP to achieve

domains suitable for excitons capture as well as charge transfer. They

anticipated that P4VP domains could interact with PCBM due to

supramolecular interactions retaining the self-assembly of BCP when

copolymer P3HT-b-P4VP with mixed PCBM (Figure 1.32a). But they observed

very low efficiency around 0.03% and it was found that the P4VP has a

tendency to wet the PEDOT-blend-PSS substrate and thus disturbing the

vertical profile of the device. This problem was resolved using an inverse

structure (Figure 1.32b) and they obtained an efficiency of around 1.2%.

Thus, it was found that casting conditions and environment effect on the self-

assembly of block copolymers.

(a) (b)

Figure 1.32 (a) Supramolecular interactions between P3HT-b-P4VP and PCBM; and (b) Inverted photovoltaic structure.277

P3HT macro-initiator was again used for the synthesis of rod-coil BCPs

consisting of poly(perylene bisimide acrylate)278 as electron acceptors (Figure

1.33) with suitable chain lengths for domain formation. The device efficiency

increased after annealing in the order of ca 0.5% because of ordered micro-

phase separation.278c It was an Important observation that the BCPs could

work as compatibilisers in the blend of P3HT and perylene bisimide active

layer. The OSC device efficiency was increased around 50% by restricting the

excessive crystallisation of perylene.278d Similar systems have also been

74

prepared using P3HT macro-initiator by either ATRP279a or RAFT279b

methods, shown in Figure 1.33. It was observed that the quenching of P3HT

photoluminescence is better in the block copolymers as active layers in OSCs

than in the simple blends of P3HT and PCBM.279a

Figure 1.33 Double-crystalline block copolymers based on poly(3-hexylthiophene) P3HT and perylene bisimide by Emrick et al. (a), Frechet et al. (b), Segalman et al. (c) and Thelakkat et al. (d).278f Our group used chain-end chemistry via GRIM method to synthesize

alkynyl-P3HT and then by “click chemistry” approach between α,ω-dipentynyl-

P3HT and azide-terminated-PS (obtained by ATRP), we could synthesize rod-

coil P3HT-b-PS diblock and PS-b-P3HT-b-PS triblock copolymers

successfully as shown in Scheme 1.8.168g This work represents the first

example of "click chemistry" on conjugated polymers based on P3HT.

75

S

C6H13

m

N NN O Br

nNN

NOBrn

O O

S

C6H13

m

N NN O Br

n

O

P3HT-b-PS

PS-b-P3HT-b-PS

S m

C6H13

N3 O

O

Brn

CuI, DBU

THF

Scheme 1.8 Synthetic route to rod-coil di- and triblock copolymers based on P3HT and PS by “Click chemistry”.168g

Recently, we again explored the chain-end chemistry of P3HT to

prepare the multi-block rod-coil copolymers incorporating fullerene as a repeat

unit, which is shown in Figure 1.34(a).280 These copolymers, poly{(1,4-

fullerene)-alt-[1,4-dimethylene-2,5-bis(cyclohexylmethylether)phenylene]}-

block-poly(3-hexylthiophene) (PFDP-b-P3HT), were obtained using

condensation reactions between functionalised-P3HT and novel fullerene

polymer. AFM images revealed that fibrillated wires like structures are formed

[Figure 1.34(b)]. The sizes of the resulting domains are around 20 nm, which

is expected in which the P3HT chain length could be predestined and around

9 C60 repeat units are presented in the C60 coiled block. The photovoltaic

performances of these multi-block copolymers for OSCs are in progress.

(a)

NiBr (dppp)BrMg O

Oi)

ii) HCl, THFiii) methanol

O

O

BrBr+

CuBr, pyridine

toluene, 115 ˚C

K2CO3, toluene, 18-crown-6,

85 ˚C

O

O

Br

m

O

O Br

S

C6H13

O O

n

O

O

Br

m

H

p

O

O

S

C6H13

O O

n

HHS

C6H13

n

Br

76

(b)

Figure 1.34 (a) Chemical structure of multi-block rod-coil copolymers, PFDP-b-P3HT and (b) AFM phase image of PFDP-b-P3HT annealed at 220 °C (500 x 500 nm).280 Rod-coil BCPs have not reached high efficiencies sofar due to several

reasons; processing and optimisation, especially correlating the absorption

spectrum to that of the solar spectrum, the effective interfacial structures

between the block copolymer and the electrodes, and perfecting the size,

position and type of insulating, covalent link between the two blocks that will

optimise charge transfer through space between the blocks.230e This indicates

that it is essential to develop efficient routes for cost-effective organic

photovoltaic devices based on block copolymers.

Hence, it is very important to find a simplified and versatile synthesis of rod-coil donor-acceptor copolymers that can both stabilize and enhance the organisation of the P3HT-blend-PCBM layer, which may result high efficiency OSC devices , which is the main objective of this thesis .

77

1.5 References 1 Kittel, C. Introduction to Solid State Physics, 8th Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2005. 2 Callister, W. D. Materials Science and Engineering: An Introduction, 7th Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2006. 3 Pierret, R. F. Advanced Semiconductor Fundamentals, 2nd Ed.; Prentice Hall: Upper Saddle River, NJ, 2002. 4 Sze, S. M. Physics of Semiconductor Devices, 2nd Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 1981. 5 Sun, S.; Sariciftci, N. S.; Eds. Organic Photovoltaics: Mechanisms, Materials,

and Devices; Taylor & Francis: Boca Raton, FL, 2005. 6 Hagen, K.; Ed. Organic Electronics: Materials, Manufacturing, and

Applications; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2006.

7 Ito, T.; Shirakawa, H.; Ikeda, S. J. Polym. Sci. Pt A Polym. Chem. Ed. 1974, 12, 11.

8 Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J. Chem. Commun. 1977, 578. 9 Skotheim, T. A.; Reynolds, J. R.; editors. Handbook of Conducting Polymers,

3rd Edition, vols 1 and 2, New York, CRC Press, 2007. 10 Yesodha, S. K.; Pillai, C. K. S.; Tsutsumi, N. Prog. Polym. Sci. 2004, 29, 45. 11 Cho, M. J.; Choi, D. H.; Sullivan, P. A.; Akelaitis, A. J. P.; Dalton, L. R. Prog.

Polym. Sci. 2008, 33, 1013. 12 Moliton, A. Optoelectronics of molecules and polymers, New York, Springer,

2005. 13 McCulloch, I.; Yoon, H. Encyclopedia of Materials: Science and Technology, 2008, 8476. 14 Choi, M-C.; Kim, Y.; Ha, C-S. Prog. Polym. Sci. 2008, 33, 581. 15 Akcelrud, L. Prog. Polym. Sci. 2003, 28, 875. 16 Günes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324. 17 Thompson, B. C.; Fréchet, J. M. J. Angew. Chem. Int. Ed. 2008, 47, 58. 18 Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474. 19 Campoy-Quiles, M.; Ferenczi, T.; Agostinelli, T.; Etchegoin, P. G.; Kim, Y.;

Anthopoulos, T. D.; Stavrinou, P. N.; Bradley, D. D. C.; Nelson, J. Nat. Mater. 2008, 7, 158.

20 Park, S. H.; Roy, A.; Beaupré, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A, J. Nature Photonics 2009, 3, 297.

21 Ling, Q-D.; Liaw, D-J.; Zhu, C.; Chan, D. S-H.; Kang, E-T.; Neoh, K-G. Prog. Polym. Sci. 2008, 33, 917. 22 Singh, Th. B.; Sariciftci, N. S.; Jaiswal, M.; Menon, R. ; Handbook of Organic Electronics and Photonics. Nalwa, H. S. editor 2008, 3, 153. 23 Singh, Th. B.; Sariciftci, N. S. Annu. Rev. Mater. Res. 2006, 36,199. 24 Dimitrakopoulos, C. D.; Mascaro, D. J.; IBM J. Res & Dev. 2001, 45,11. 25 Fréchet, J. M. J. Prog. Polym. Sci. 2005, 30, 844. 26 McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537. 27 Adhikari, B.; Majumdar, S. Prog. Polym. Sci. 2004, 29, 699. 28 Guimard, N. K.; Gomez, N.; Schmidt, C. E. Prog. Polym. Sci. 2007, 32, 876. 29 Ivory, D. M.; Miller, G. G.; Sowa, J. M.; Shacklette, L. W.; Chance, R. R.;

Baughman, R. H. Chem. Phys. 1979, 71, 1506. 30 Tourillon, G.; Garnier, F.; Electroanal. Chem. Interfacial Electrochem.

1982, 135, 173. 31 Diaz, A. F.; Kanazawa, K. K. J. Chem. Soc. Chem. Commun. 1979, 635.

78

32 (a) Bayer, A. G. Eur. Patent No. 339 340, 1988; (b) Kirchmeyer, S.; Reuter, K. Journal of Material Chemistry 2005, 15, 2077.

33 Jonas, F.; Schrader, L. Synth. Metal. 1991, 41, 831. 34 Heywang, G.; Jonas, F. Adv. Mater. 1992, 4, 116. 35 Winter, C.; Reece, C.; Hormes, J.; Heywang, G.; Jonas, F. Chem. Phys. 1995, 194, 207. 36 Dietrich, M.; Heinze, J.; Heywang, G.; Jonas, F. J. Electroanal. Chem. Phys. 1994, 369, 87. 37 Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347, 539. 38 Ohmori, Y.; Uchida, M.; Muro, K.; Yoshino, K. Jpn. J. Appl. Phys. 1991, 30(12B), L1941. 37 Kippelen, B.; Marder, S. R.; Hendrickx, E.; Maldonado, J. L.; Guillemet, G.;

Volodin, B. L.; Steele, D. D.; Enami, Y.; Sandalphon; Yao, Y. J.; Wang, J. F.; Rockel, H.; Erskine, L.; Peyghambarian, N. Science 1998, 279, 54.

38 Ballantyne, A. M.; Chen, L.; Dane, J.; Hammant, T.; Braun, F. M.; Heeney, M.; Duffy, W.; McCulloch, I.; Bradley, D. D. C.; Nelson, J. Adv. Funct. Mater. 2008, 18, 2373.

39 Babel, A.; Zhu, Y.; Cheng, K-F.; Chen, W-C.; Jenekhe, S. A. Adv. Funct. Mater. 2007, 17, 2542. 40 Jaiswal, M.; Menon, R. Polym. Int. 2006, 55, 1371. 41 Kirova, N. Polym. Int. 2008, 57, 678. 42 Moliton, A.; Hiorns, R. C. Polym. Int. 2004, 53, 1397. 43 Roncali, J. Chem. Rev. 1992, 92, 711. 44 Kertesz, M.; Choi, C. H.; Yang, S. Chem. Rev. 2005, 105, 3448. 45 Zade, S. S.; Bendikov, M. Chem. Eur. J. 2007, 13, 3688. 46 Hiorns, R. C.; Iratçabal, P.; Bégué, D.; Khoukh, A.; de Bettignies, R.; Leroy, J.; Firon, M.; Sentein, C.; Martinez, H.; Preud’homme, H.; Dagron-Lartigau, C. J. Polym. Sci. Pt. A Polym. Chem. 2009, 47, 2304. 47 Ouhib, F.; Dkhissi, A.; Iratçabal, P.; Hiorns, R. C.; Khoukh, A.; Desbrières, J.;

Pouchain, C.; Dagron-Lartigau, C. J. Polym. Sci. Pt. A Polym. Chem. 2008, 46, 7505.

48 Burroughes, J. H.; Friend, R. H.; Silbey, R.; editor. Conjugated Polym.: Kluwer Academic Press, Dordrecht, 1991, 555.

49 Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. Nature 1993, 365, 628.

50 Chung, T. C.; Kaufman, J. H.; Heeger, A. J. Physical Review B Condensed Matter. 1984, 30, 702. 51 Sato, M.; Tanaka, S.; Kaeriyama, K. Synth. Met. 1986, 14, 279. 52 Grem, G.; Leditzky, G.; Ullrich, B.; Leising, G. Synth. Met. 1992, 51, 383. 53 Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Markes, R. N.;

Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Brédas, J. L.; Löglund, M.; Salaneck, W. R. Nature 1999, 397, 121.

54 Brédas, J. L.; Dekker, M.; editor. Handbook of Conducting Polymers. New York, 1986, 2, 859. 55 Pei, Q.; Zuccarello, G.; Ahlskog, M.; Inganäs, O. Polymer 1994, 35, 1347. 56 Akoudad, S.; Roncali, J. Synth. Met. 1998, 93, 111. 57 Heeger, A. J. Angew. Chem. Int. Ed. 2001, 40, 2591. 58 Hotta, S.; Rughooputh, S. D. D. V.; Heeger, A. J.; Wudl, F. Macromolecules

1987, 20, 212. 59 Elsenbaumer, R. L.; Jen, K. Y.; Oboodi, R. Synth. Met. 1986, 15, 169. 60 Fukuda, M.; Sawada, K.; Yoshino, K. J. Polym. Sci. Pt. A Polym. Chem. 1993, 31, 2465. 61 Levesque, I.; Leclerc, M. Chem. Mater. 1996, 8, 2843.

79

62 Patil, A. O.; Ikenoue, Y.; Wudl, F.; Heeger, A. J. J. Am. Chem. Soc. 1987, 109, 1858. 63 Stéphan, O.; Schottland, P.; Le Gall, P-Y.; Chevrot, C. J. Chem. Phys. 1998,

95, 1168. 64 Balanda, P. B.; Ramey, M. B.; Reynolds, J. R. Macromolecules 1999, 32, 3970. 65 François, B.; Zhong, X. F. Synth. Met. 1991, 41, 955. 66 Marsitzky, D.; Brand, T.; Geerts, Y.; Klapper, M.; Müllen, K. Macromol. Rapid Commun. 1998, 19, 385. 67 François, B.; Widawski, G.; Rawiso, M.; Cesar, B. Synth. Met. 1995, 69, 463. 68 Radzilowski, L. H.; Stupp, S. I. Macromolecules 1994, 27, 7747. 69 Hiorns, R. C.; Martinez, H. Synth. Met. 2003, 139, 463. 70 Hiorns, R. C.; Holder, S. J.; Schué, F.; Jones, R. G. Polym. Int. 2001, 50, 1016. 71 International Energy Outlook 2009, Energy Information Administration, 2009, http://www.eia.doe.gov/oiaf/ieo/ 72 Solomon, S.; Plattner, G. K.; Knutti, R.; Friedlingstein, P. Proc. Natl. Acad. Sci. U. S. A. 2009, 106 (6), 1704‐1709. 73 Smalley, R. E. MRS Bull. 2005, 30 (6), 412‐417. 74 Renée M. Nault, Basic energy sciences workshop on solar energy utilization,

Argonne National Laboratory, 2005. 75 Lewis, N. S. Science 2007, 315 (5813), 798‐801. 76 Thekaekara, M. P. Suppl. Proc. 20th Annu. Meet. Inst. Environ. Sci. 1974. 77 Solar Spectral Irradiance:Air Mass 1.5. http://rredc.nrel.gov/solar/spectra/am1.5/ (June 2009), National Renewable Energy Laboratory (NREL) website. 78 Roncali, J. Chem. Rev. 1997, 97, 173–205. 79 Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Adv. Funct.

Mater. 2006, 16, 2016–2023. 80 Becquerel, A.E. Compt. Rend. Acad. Sci. 1839, 9, 145. 81 Alfred Smee 1849. Elements of Electro-Biology, or The Voltaic Mechanism of

Man; of Electro-Pathology, Especially of the Nervous System.... London: Longman, Brown, Green, and Longmans.

82 Smith, W. Nature, 1873, 7, 303. 83 Adams, W.G.; Day, R.E. Proc. R. Soc. Lond. 1876, 25, 113. 84 "Light sensitive device" U.S. Patent 2,402,662 85 Chapin, M.; Fuller, C.S.; Pearson, G.L. J. Appl. Phys. 1954, 25, 676. 86 Perlin, J. From space to earth: The story of solar electricity. Havard University

Press, Cambridge, MA, 2002. 87 University of New South Wales (2008, October 24). Highest Silicon Solar Cell

Efficiency Ever Reached. Science Daily. Retrieved September 22, 2010, from http://www.sciencedaily.com/releases/2008/10/081023100536.htm

88 Slaoui, A.R.; Collins, T. MRS Bull. 2007, 32, 211. 89 Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W. Prog. Photovolt: Res. Appl. 2009, 17 (5), 320‐326. 90 Pochettino, A. Acad. Lincei Rend. 1906, 15, 355. 91 Volmer, M. Ann. Physik 1913, 40, 775. 92 Borsenberger, P.M.; Weiss, D.S. Organic photoreceptors for imaging

systems, Marcel Dekker, New York, 1993. 93 Tang, C. W. Appl. Phys. Lett. 1986, 48, 183. 94 Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science, 1992, 258,

1474. 95 Liang, Y.; Xu, Z.; Xia, J.; Tsai S-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater.

2010, 22, E135–E138. 96 (a) http://www.pv-

tech.org/news/_a/new_polymers_push_solarmers_opv_efficiency_to_record_8.

80

13/ [accessed 13th September 2010]; (b) http://www.gizmag.com/world-record-efficiency-for-organic-photovoltaic-solar-cells/17186/ [accessed 5th December 2010].

97 Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93 (7), 3693‐3723. 98 Peumans, P.; Forrest, S. R. Appl. Phys. Lett. 2001, 79 (1), 126‐128. 99 Bai, Y.; Cao, Y.; Zhang, J.; Wang, M.; Li, R.; Wang, P.; Zakeeruddin, S. M.;

Gratzel, M. Nat. Mater. 2008, 7 (8), 626‐630. 100 Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Sekiguchi, T.;

Grätzel, M. Nat. Mater. 2003, 2 (6), 402‐407. 101 O'Regan, B.; Gratzel, M. Nature 1991, 353 (6346), 737‐740. 102 Blankenburg, L.; Schultheis, K.; Schache, H.; Sensfuss, S.; Schrodner, M.

Sol. Energy. Mater. Sol. Cells 2009, 93 (4), 476‐483. 103 Dennler, G.; Lungenschmied, C.; Neugebauer, H.; Sariciftci, N. S.; Labouret,

A. J. Mater. Res. 2005, 20 (12), 3224‐3233. 104 Krebs, F. C. Sol. Energy Mater. Sol. Cells 2009, 93 (4), 394‐412. 105 Krebs, F. C.; Jørgensen, M.; Norrman, K.; Hagemann, O.; Alstrup, J.;

Nielsen, T. D.; Fyenbo, J.; Larsen, K.; Kristensen, J. Sol. Energy Mater. Sol. Cells 2009, 93 (4), 422‐441.

106 Krebs, F. C. Sol. Energy Mater. Sol. Cells 2009, 93 (4), 465‐475. 107 Krebs, F. C.; Alstrup, J.; Spanggaard, H.; Larsen, K.; Kold, E. Sol. Energy

Mater. Sol. Cells 2004, 83 (2‐3), 293‐300. 108 Krebs, F. C.; Spanggaard, H.; Kjaer, T.; Biancardo, M.; Alstrup, J. Mater. Sci.

Eng. B 2007, 138 (2), 106‐111. 109 Krebs, F. C. Sol. Energy Mater. Sol. Cells 2009, 93, 1636‐1641. 110 Krebs, F. C. Org. Electron. 2009, 10, 761‐768. 111 Krebs, F. C.; Gevorgyan, S. A.; Alstrup, J. J. Mater. Chem. 2009, 19, 5442‐5451. 112 Lungenschmied, C.; Dennler, G.; Neugebauer, H.; Sariciftci, S. N.; Glatthaar,

M.; Meyer, T.; Meyer, A. Sol. Energy Mater. Sol. Cells 2007, 91 (5), 379‐384. 113 Niggemann, M.; Zimmermann, B.; Haschke, J.; Glatthaar, M.; Gombert, A. Thin

Solid Films 2008, 516 (20), 7181‐7187. 114 Tipnis, R.; Bernkopf, J.; Jia, S.; Krieg, J.; Li, S.; Storch, M.; Laird, D. Sol. Energy Mater. Sol. Cells 2009, 93 (4), 442‐446. 115 Zimmermann, B.; Glatthaar, M.; Niggemann, M.; Riede, M. K.; Hinsch, A.;

Gombert, A. Sol. Energy Mater. Sol. Cells 2007, 91 (5), 374‐378. 116 Hiramoto, M.; Fujiwara, H.; Yokoyama, M. Appl. Phys. Lett., 1991, 58,

1062. 117 Sariciftci, N. S.; Braun, D.; Zhang, C.; Srdanov, V. I.; Heeger, A. J.; Stucky, G.;

Wudl, F. Appl. Phys. Lett. 1993, 62, 585. 118 Yu, G.; Pakbaz, K.; Heeger, A. J. Appl. Phys. Lett. 1994, 64, 3422. 119 Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995,

270, 1789. 120 Halls, J. J. M.; Walsh, C. A.; Greenham N. C. Nature, 1995, 376, 498. 121 Peeters, E.; Van Hal, P.A.; Knol, J.; Brabec, C. J.; Sariciftci, N. S.; Hummelen,

J. C.; Janssen, R. A. J. J. Phys. Chem. B 2000, 104, 10174. 122 Schmidt-Mende, L.; Fechtenkötter, A.; Müllen, K.; Moons, E.; Friend, R. H.;

MacKenzie, J. D. Science 2001, 293, 1119. 123 Ramos, A. M.; Rispens, M. T.; Hummelen, J. C.; Janssen, R. A. J. Syn.

Metals 2001, 119, 171. 124 Brabec, C. J.; Winder, C.; Sariciftci, N. S. Adv. Funct. Mater. 2002, 12,

709.

81

125 Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H. Angew. Chem. Int. Ed. 2003, 42, 3371.

126 Park, S. H.; Roy, A.; Beaupré, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nature Photonics 2009, 3, 297.

127 Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissörtel, F.; Salbeck, J.; Spreitzer, H.; Grätzel, M. Nature 1998, 395, 583. 128 Xue, J.; Rand, B. P.; Uchida, S.; Forrest, S. R. Adv. Mater. 2005, 17, 66. 129 Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. Adv. Funct. Mater. 2003, 13, 85. 130 Muhlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.; Gaudiana, R.;

Brabec, C. Adv. Mater. 2006, 18, 2884. 131 Slooff, L. H.; Veenstra, S. C.; Kroon, J. M.; Moet, D. J. D.; Sweelssen, J. S.;

Koetse, M. M. Appl. Phys. Lett. 2007, 90, 143506. 132 Wienk, M. M.; Turbiez, M.; Gilot, J.; Janssen, R. A. J. Adv. Mater. 2008 20, 2556. 133 Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.;

Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498. 134 McNeill, C. R.; Abrusci, A.; Zaumseil, J.; Wilson, R.; McKiernan, M. J.; Burroughes, J. H.; Halls, J. J. M.; Greenham, N. C.; Friend, R. H. Appl. Phys. Lett. 2007, 90, 193506. 135 Moet, D. J. D.; Koster, L. J. A.; de Boer, B.; Blom, P. W. M. Chemistry of Materials, 2007, 19, 5856. 136 Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995,

270, 1789. 137 Cheng, Y-J.; Yang, S-H.; Hsu, C-S. Chem. Rev. 2009, 109, 5868–5923. 138 Martijn Lenes, PhD thesis 2009, University of Groningen, The Netherlands. 139 Gadisa, A.; Svensson, M.; Andersson, M. R.; Inganas, O. Appl. Phys. Lett. 2004, 84 (9), 1609‐1611. 140 Mihailetchi, V. D.; Blom, P. W. M.; Hummelen, J. C.; Rispens, M. T. J. Appl. Phys. 2003, 94 (10), 6849‐6854. 141 IEC-904-3, IEC Standard 142 Kroon, J. M.; Wienk, M. M.; Verhees, W. J. H.; Hummelen, J. C. Thin Solid Films 2002, 403, 223. 143 Whrle, D.; Meissner, D. Adv. Mater. 1991, 3, 129. 144 Knupfer, M. Appl. Phys. A 2003, 77, 623. 145 Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 3693. 146 Halls, J. J. M.; Pichler, K.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Appl. Phys. Lett. 1996, 68 (22), 3120‐3122. 147 Haugeneder, A.; Neges, M.; Kallinger, C.; Spirkl, W.; Lemmer, U.; Feldmann,

J.; Scherf, U.; Harth, E.; Gugel, A.; Mullen, K. Physical Review B 1999, 59 (23), 15346‐15351.

148 Pettersson, L. A. A.; Roman, L. S.; Inganas, O. J. Appl. Phys. 1999, 86 (1), 487‐496.

149 Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376 (6540), 498‐500.

150 Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv. Funct. Mater. 2001, 11, 15. 151 Ko, C. J.; Lin, Y. K.; Chen, F. C.; Chu, C. W. Appl. Phys. Lett. 2007, 90(6). 152 Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y.

Nat. Mater. 2005, 4 (11), 864‐868. 153 Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. Adv. Funct. Mater. 2005, 15 (10), 1617‐1622. 154 (a) Kim Y, Ballantyne AM, Nelson J, Bradley DDC, Organic Electronics

10:205 (2009); (b) de Cuendias, A.; Hiorns, R. C.; Cloutet, E.; Vignau L.;

82

Cramail, H. Polym. Int. 2010, 11, 1-25. 155 Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007, 91 (11), 954‐985. 156 Kroon, R.; Lenes, M.; Hummelen, J. C.; Blom, P. W. M.; de Boer, B. Polym. Rev. 2008, 48 (3), 531‐582. 157 Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Muhlbacher, D.; Scharber, M.;

Brabec, C. Macromolecules 2007, 40 (6), 1981‐1986. 158 Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C. Nat. Mater. 2007, 6 (7), 497‐500. 159 Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Li, G.; Yang, Y. J. Am. Chem. Soc. 2008, 130 (48),16144‐16145. 160 Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.;

Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3 (5), 297‐302. 161 Liang, Y. Y.; Feng, D. Q.; Wu, Y.; Tsai, S. T.; Li, G.; Ray, C.; Yu, L. P. J. Am. Chem. Soc. 2009, 131 (22), 7792‐7799. 162 Yu, C. Y.; Chen, C. P.; Chan, S. H.; Hwang, G. W.; Ting, C. Chem. Mater.

2009, 21 (14), 3262‐3269. 163 Wienk, M. M.; Turbiez, M.; Gilot, J.; Janssen, R. A. J. Adv. Mater. 2008, 20

(13), 2556‐2560. 164 Yamamoto, T.; Sanechika, K.; Yamamoto, A. J. Polym. Sci., Polym. Lett. Ed.

1980,18, 9-12. 165 Lin, J. W. P.; Dudek, L. P. J. Polym. Sci., Poly. Chem. 1980, 18, 2869- 2873. 166 Leclerc, M.; Diaz, F. M.; Wegner, G. Makromol. Chem. 1989, 190, 3105. 167 (a) Yokoyama, A.; Miyakoshi, R.; Yokozawa, T. Macromolecules 2004, 37, 1169; (b) Sheina, E. E.; Liu, J.; Iovu, M. C.; Darin, W.; Laird, D. W.; McCullough, R. D. Macromolecules 2004, 37, 3526; (c) Yokozawa, T.; Yokoyama, A. Chem. Rev. 2009, 109 (11), 5595; (d) Hiorns, R. C.; de Bettignies, R.; Leroy, J.; Bailly, S.; Firon, M.; Sentein, C.; Khoukh, A.; Preud’homme, H.; Dagron-Lartigau, C. Adv. Funct. Mater. 2006, 16, 2263. 168 (a) Jeffries-El, M.; Sauvé, G.; McCullough, R. D. Macromolecules 2005, 38,10346; (b) Liu, J.; McCullough, R. D. Macromolecules 2002, 35, 9882; (c) Liu, J.; Loewe, R. S.; McCullough, R. D. Macromolecules 1999, 32, 5777; (d) Bronstein, H. A.; Luscombe, C. K. J. Am. Chem. Soc. 2009, 131, 12894; (e) Kaul, E.; Senkovskyy, V.; Tkachov, R.; Bocharova, V.; Komber, H.; Stamm, M.; Kiriy, A. Macromolecules 2010, 43, 77; (f) Hiorns, R. C.; Khoukh, A.; Gourdet, B.; Dagron-Lartigau, C. Polym. Int. 2006, 55, 608; (g) Urien, M.; Erothu, H.; Cloutet, E.; Hiorns, R. C.; Vignau, L.; Cramail, H. Macromolecules 2008, 41, 7033. 169 Lan, Y-K.; Yang, C. H.; Yang, H. C. Polym. Int. 2010, 59, 16. 170 (a) Brinkmann, M.; Rannou, P. Macromolecules 2009, 42, 1125; (b) Zen, A.; Saphiannikova, M.; Neher, D.; Grenzer, J.; Grigorian, S.; Pietsch, U.; Asawapirom, U.; Janietz, S.; Scherf, U.; Lieberwirth, I.; Wegner, G. Macromolecules 2006, 39, 2162; (c) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Fréchet, J. M. J.; Toney, M. F. Macromolecules 2005, 38, 3312; (d) Chirvase, D.; Parisi, J.; Hummelen, J. C.; Dyakonov, V. Nanotechnology 2004, 15, 1317; (e) Yang, X.; Loos, J.; Veenstra, S. C.; Verhees, W. J. H.; Wienk, M. M.; Kroon, J. M.; Michels, M. A. J.; Janssen, R. A. J. Nano Letters 2005, 5(4), 579. 171 McCullough, R. D.; Lowe, R. D. J. Chem. Soc., Chem. Commun. 1992, 1, 70–

72. 172 Chen, T. A.; Wu, X.; Rieke, R. D. J. Am. Chem. Soc. 1992, 114, 10087-10088. 173 McCullough, R.D. Adv. Mater. 1998, 10, 93.

83

174 Loewe, R. S.; Khersonsky, S. M.; McCullough, R. D. Adv. Mater. 1999, 11, 250–253.

175 Yokoyama, A.; Miyakoshi, R.; Yokozawa, T. Macromolecules 2004, 37, 1169–1171.

176 Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. Macromol. Rapid Commun. 2004, 25, 1663.

177 (a) Jeffries-EL, M.; Sauvé, G.; McCullough, R. D. Adv. Mater. 2004, 16, 1017– 1019; (b) Sheina, E. E.; Liu, J.; Iovu, M. C.; Laird, D. W.; McCullough, R. D. Macromolecules 2004, 37, 3526–3528; (c) Iovu, M.C.; Sheina, E.E.; Gil, R.R.; McCullough, R.D. Macromolecules 2005, 38, 8649; (d) Jeffries-EL, M.; Sauvé, G.; McCullough, R. D. Macromolecules 2005, 38, 10346–10352. 178 Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. J. Am. Chem. Soc. 2005, 127, 17542. 179 Yokozawa, T.; Yokoyama, A. Prog. Polym. Sci. 2007, 32, 147. 180 Yokozawa, T.; Yokoyama, A. Chem. Rev. 2009, 109, 5595-5619. 181 Osaka, I.; McCullough, R. D. Acc. Chem. Res., 2008, 41, 1202-1214. 182 Lindner, S. M.; Hüttner, S.; Chiche, A.; Thelakkat, M.; Krausch, G. Angewandte Chemie International Edition 2006, 45 (20), 3364. 183 Muller, C.; Toby, A. M.; Ferenczi, M. C-Quiles.; Jarvist, M. F.; Bradley, D. D. C.; Smith, Paul.; Stingelin-Stutzmann, N.; Nelson, J Adv. Mater. 2008, 20, 3510– 3515. 184 Al-Ibrahim, M.; Ambacher, O.; Sensfuss, S.; Gobsch, G. Applied Physics

Letters 2005, 86, 201120. 185 Verilhac, J. M.; LeBlevennec, G.; Djurado, D.; Rieutord, F.; Chouiki, M.;

Travers, J. P.; Pron, A. Synthetic Metals 2006, 156, 815. 186 Mihailetchi, V.D.; Xie, H.; de Boer, B.; Popescu, L.M.; Hummelen, J.C.; Blom,

P.W.M.; Koster, L.J.A. Applied Physics Letters 2006, 89, 012107. 187 Zhao, Y.; Xie, Z.; Qu, Y.; Geng, Y.; Wang, L. Applied Physics Letters 2007, 90,

043504. 188 Janssen, G.; Aguirre, A.; Goovaerts, E.; Vanlaeke, P.; Poortmans, J.; Manca, J.

The European Physical Journal Applied Physics 2007, 37, 287. 189 Berson, S.Thesis 2007, University of Grenoble. 190 Huang, J.; Li G.; Yang, Y. Applied Physics Letters 2005, 87, 112105. 191 Li, G.; Shrotriya, V.; Yao Y.; Yang, Y. J. Appl. Phys. 2005, 98, 043704. 192 Kim, Y.; Choulis, S.A.; Nelson, J.; Bradley, D.D.C.; Cook, S.; Durrant, J.R.

Applied Physics Letters 2005, 86, 063502. 193 Ma, W.; Yang, C.; Gong, X.; Li, K.; Heeger, A.J. Advanced Functional Materials

2005, 15, 1617. 194 Padinger, F.; Rittberger, R.S.; Sariciftci, N.S. Advanced Functional Materials

2003, 13, 85. 195 Zhokhavets, U.; Erb, T.; Hoppe, H.; Gobsch, G.; Sariciftci, N.S. Thin Solid

Films 2006, 496, 679. 196 Yang, X.; Loos, J.; Veenstra, S. C.; Verhees, W. J. H.; Wienk, M. M.; Kroon, J.

M.; Michels, M. A. J.; Janssen, R. A. J. Nano Lett. 2005, 5, 579. 197 Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. Adv. Funct. Mater. 2003, 13, 1. 198 Mihailetchi, V. D.; Xie, H.; de Boer, B.; Koster, L. J. A.; Blom, P. W. M. Adv.

Funct. Mater. 2005, 15, 1260. 199 Pivrikas, A.; Juska, G.; Mozer, A. J.; Scharber, M.; Arlauskas, K.; Sariciftci, N.

S.; Stubb, H.; Österbacka, R. Phys. Rev. Lett. 2005, 94, 176806. 200 Erb, T.; Zhokhavets, U.; Gobsch, G.; Raleva, S.; Stuhn, B.; Schilinsky, P.;

Waldauf, C.; Brabec, C. J. Adv. Funct. Mat. 2005,1,1193. 201 Drees, M.; Hoppe, H.; Winder, C.; Neugebauer, H.; Sariciftci, N.S.; Schwinger,

W.; Schäffler, F.; Topf, C.; Scharber, M.C.; Zhu, Z.; Gaudiana, R. J. Mater. Chem. 2005, 15, 5158.

202 Nierengarten, J.F.; Setayesh, S. New J. Chem. 2006, 30, 313.

84

203 Zhou, E.; Tan, Z.; Yang, Y.; Huo, L.; Zou, Y.; Yang, C.; Li, Y. Macromolecules 2007, 40, 1831.

204 Cravino A.; Sariciftci, N.S. J. Mater. Chem. 2002, 12, 1931. 205 Cravino, A.; Zerza, G.; Maggini, M.; Bucella, S.; Svensson, M.; Andersson,

M.R.; Neugebauer, H.; Brabec, C.J.; Sariciftci, N.S. Monatshefte für Chemie 2003, 134, 519.

206 Cravino, A. Polymer International 2007, 56, 943. 207 Tan, Z.; Hou, J.; He, Y.; Zhou, E.; Yang C.; Li, Y. Macromolecules 2007, 40,

1868. 208 Hadziioannou, G. MRS Bulletin 2002, 27, 456. 209 (a) Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Chem. 1990, 41, 525; (b)

Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, 52, 32. 210 Hadziioannou, G.; Picot, C.; Skoulios, A.; Ionescu, L. M.; Mathis, A.; Duplessix,

R.; Gallot, Y.; Lingelser, J. P. Macromolecules 1982, 15, 2560. 211 (a) Segalman, R. A. Materials Science and Engineering R 2005, 48, 191; (b)

Darling, S. B. Prog. Polym. Sci. 2007, 32, 1152. 212 Klok, H-A.; Lecommandoux, S. Adv. Mater. 2001, 13, 1217. 213 Noshay, A.; McGrath, J. E. Block Copolymers. Academic Press, New York

1977. 214 Meier, D. J. J. Polym. Sci. B 1969, 34,1821. 215 Matsen, M. W.; Bates, F. S. Macromolecules 1996, 29, 1091. 216 Semenov, A. N.; Vasilenko, S. V. Sov. Phys. JETP A 1986, 63, 70. 217 Semenov, A. N. Cryst. Liq. Cryst. 1991, 209, 191. 218 Semenov, A. N.; Subbotin, A. V. Sov. Phys. JETP A 1992, 74, 690. 219 Williams, D. R. M.; Fredrickson, G. H. Macromolecules 1992, 25, 3561. 220 Williams, D. R. M.; Halperin, A. Phys. Rev. Lett. 1993, 71, 1557. 221 Matsen, M. W. J. Chem. Phys. 1996, 104, 7758. 222 Borsali, R.; Lecommandoux, S.; Pecora, R.; Benoît, H. Macromolecules 2001,

34, 4229-4234. 223 Müller, M.; Schick, M. Macromolecules 1996, 29, 8900. 224 Semenov, A. N. Sov. Phys. JETP A 1985, 61, 733. 225 Tao, Y.; Olsen, B. D.; Ganesan, V.; Segalman, R.A. Macromolecules 2007, 40,

3320. 226 Stupp, S. I.; LeBonheur, V.; Walker, K.; Li, L. S.; Huggins, K. E.; Keser, M.;

Amstutz, A. Science 1997, 276, 384. 227 Kirova, N. Polym. Int. 2008, 57, 678. 228 Nunzi, J-M. C. R. Physique 2002, 3, 523. 229 Watkins, P. K.; Walker, A. B.; Verschoor, G. L. B. Nano Letters 2005,

5(9),1814. 230 (a) Darling, S. B. Energy Environ. Sci. 2009, 2, 1266; (b) Sun, S.; Fan, Z.;

Wang, Y.; Haliburton, J. Journal of Materials Science 2005, 40, 1429; (c) Gratt, J. A.; Cohen, R. E. J. Appl. Polym. Sci. 2004, 91, 3362; (d) Sun, S-S. Solar Energy Materials & Solar Cells 2003, 79, 257; (e) Shah, M.; Ganesan, V.; Macromolecules 2010, 43, 543.

231 Botiz, I.; Darling, S. B. Materials Today 2010,13 (5), 42-51. 232 Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44, 6725. 233 Sun, S.-S.; Zhang, C.; Ledbetter, A.; Choi, S.; Seo, K.; Bonner, J. C. E.; Drees,

M.; Sariciftci, N. S. Appl. Phys. Lett. 2007, 90, 043117. 234 Barrau, S.; Heiser, T.; Richard, F.; Brochon, C.; Ngov, C.; van de Wetering, K.;

Hadziioannou, G.; Anokhin, D. V.; Ivanov, D. A. Macromolecules 2008, 41, 2701; (b) Adamopoulos, G.; Heiser, T.; Giovanella, U.; Ould-Saad, S.; van de Wetering, K.; Brochon, C.; Zorba, T.; Paraskevopoulos, K. M.; Hadziioannou, G. Thin Solid Films, 2006, 371, 511-512; (c) Heiser, T.; Adamopoulos, G.; Brinkmann, M.; Giovanella, U.; Ould-Saad, S.; Brochon, C.; van de Wetering, K.; Hadziioannou, G. Thin Solid Films, 2006, 219, 511-512.

85

235 a) Sivula, K.; Ball, Z. T.; Watanabe, N.; Fréchet, J. M. J. Adv. Mater. 2006, 18, 206; b) Ball, Z. T.; Sivula, K.; Fréchet, J. M. J. Macromolecules 2006, 39, 70.

236 (a) Lindner, S. M.; Hüttner, S.; Chiche, A.; Thelakkat, M.; Krausch, G. Angew. Chem. Int. Ed. 2006, 45, 3364; (b) Sommer, M.; Lang, A. S.; Thelakkat, M. Angew. Chem. Int. Ed. 2008, 47, 7901; (c) Sommer, M.; Hüttner, S.; Wunder, S.; Thelakkat, M. Adv. Mater. 2008, 20, 2523.

237 Zhang, C.; Cirpan, A.; Russell, T. P.; Emrick, T. Macromolecules, 2009, 42(4), 1079.

238 Katz, E. A.; Faiman, D.; Tuladhar, S. M.; Kroon, J. M.; Wienk, M. M.; Fromherz, T.; Padinger, F.; Brabec, C. J.; Sariciftci, N. S. J. Appl. Phys. 2001, 90, 5343.

239 Liu, C.; Oshima, K.; Shimomura, M.; Miyauchi, S.; Synth. Met. 2006, 156, 1362. 240 Hiorns, R. C.; de Bettignies, R.; Leroy, J.; Bailly, S.; Firon, M.; Sentein, C.;

Khoukh, A.; Preud’homme, H.; Dagron-Lartigau, C. Adv. Funct. Mater. 2006, 16, 2263.

241 Leclère, P.; Hennebicq, E.; Calderone, A.; Brocorens, P.; Grimsdale, A. C.; Müllen, K.; Brédas, J. L.; Lazzaroni, R. Prog. Polym. Sci. 2003, 28, 55.

242 Widawski, G.; Rawiso, M.; Francois, B. Nature 1994, 369, 387. 243 Tsitsilianis, C.; Voyiatzis, G. A.; Kallitsis, J. K. Macromolecular Rapid

Communications 2000, 21, 1130. 244 Leclère, P.; Calderone, A.; Marsitzky, D.; Francke, V.; Geerts, Y.; Müllen, K.;

Brédas, J. L.; Lazzaroni, R. Advanced Materials 2000, 12, 1042. 245 Francke, V.; Räder, H.J.; Geerts, Y.; Müllen, K. Macromolecular Rapid

Communications 1998, 19, 275. 246 Tsolakis, P.K.; Kallitsis, J.K.; Godt, A. Macromolecules 2002, 35, 5758. 247 Leclère, P.; Surin, M.; Jonkheijm, P.; Henze, O.; Schenning, A.P.H.J.; Biscarini,

F.; Grimsdale, A.C.; Feast, W.J.; Meijer, E. W.; Müllen, K.; Brédas, J. L.; Lazzaroni, R. European Polymer Journal 2004, 40, 885.

248 Marsitzky, D.; Klapper, M.; Mullen, K. Macromolecules 1999, 32, 8685. 249 Kong, X.; Jenekhe, S.A. Macromolecules 2004, 37, 8180. 250 Lin, H. C.; Lee, K. W.; Tsai C. M.; Wei, K. H. Macromolecules 2006, 39, 3808. 251 Stalmach, U.; de Boer, B.; Videlot, C.; van Hutten, P. F.; Hadziioannou, G. J.

Am. Chem. Soc. 2000, 122, 5464. 252 Heiser, T.; Adamopoulos, G.; Brinkmann, M.; Giovanella, U.; Ould-Saada, S.;

Brochon, C.; Wetering, K. v. d.; Hadziioannou, G. Thin Solid Films 2006, 219, 511-512.

253 Sary, N.; Rubatat, L.; Brochon, C.; Hadziioannou, G.; Ruokolainen, J.; Mezzenga, R. Macromolecules 2007, 40, 6990.

254 Barrau, S.; Heiser, T.; Richard, F.; Brochon, C.; Ngov, C.; Wetering, K. v. d.; Hadziioannou, G.; Anokhin, D. V.; Ivanov, D. A. Macromolecules 2008, 41, 2701.

255 Van der Veen, M. H.; Boer, B. d.; Stalmach, U.; Wetering, K. v. d.; Hadziioannou, G. Macromolecules 2004, 37, 3673.

256 Boer, B. d.; Stalmach, U.; vanHutten, P.F.; Melzer, C.; Kranikov, V. V.; Hadziioannou, G. Polymer 2001, 42, 9097.

257 Iovu, M. C.; Jeffries-El, M.; Sheina, E. E.; Cooper, J. R.; McCullough, R. D. Polymer 2005, 46, 8582.

258 Sauvé, G.; McCullough, R. D. Adv. Mater. 2007, 19, 1822. 259 Iovu, M. C.; Zhang, R.; Cooper, J. R.; Smilgies, D. M.; Javier, A. E.; Sheina, E.

E.; Kowalewski, T.; McCullough, R. D. Macromol. Rapid Commun. 2007, 28, 1816.

260 Iovu, M. C.; Jeffries-El, M.; Zhang, R.; Kowalewski, T.; McCullough, R. D. J. Macromol. Sci. Part A 2006, 43, 1991.

261 Radano, C. P.; Scherman, O. A.; Stingelin-Stutzmann, N.; Muller, C.; Breiby, D. W.; Smith, P.; Janssen, R. A. J.; Meijer, E. W. J. Am. Chem. Soc. 2005, 127, 12502.

86

262 Muller, C.; Goffri, S.; Breiby, D. W.; Andreasen, J. W.; Chanzy, H. D.; Janssen, R. A. J.; Nielsen, M. M.; Radano, C. P.; Sirringhaus, H.; Smith, P.; Stingelin-Stutzmann, N. Advanced Functional Materials 2007, 17, 2674.

263 Dai, C. A.; Yen, W. C.; Lee, Y. H.; Ho, C.C.; Su, W. F. J. Am. Chem. Soc. 2007, 129, 11036.

264 Moliton, A. Optoelectronics of molecules and polymers, New York, Springer, 2005.

265 (a) de Boer, B.; Stalmach, U.; van Hutten, P. F.; Melzer, C.; Krasnikov, V. V.; Hadziioannou, G. Polymer 2001, 42, 9097; (b) Stalmach, U.; de Boer, B.; Videlot, C.; van Hutten, P. F.; Hadziioannou, G. J. Am. Chem. Soc. 2000, 122, 5464.

266 Stalmach, U.; de Boer, B.; Post, A. D.; van Hutten, P. F.; Hadziioannou, G. Angew. Chem. Int. Ed. 2001, 40(2), 428.

267 van der Veen, M. H.; de Boer, B.; Stalmach, U.; van de Wetering, K. I.; Hadziioannou, G. Macromolecules 2004, 37, 3673.

268 Adamopoulos, G.; Heiser, T.; Giovanella, U.; Ould-Saad, S.; van de Wetering, K. I.; Brochon, C.; Zorba, T.; Paraskevopoulos, K. M.; Hadziioannou, G. Thin Solid Films 2006, 371, 511-512.

269 Sary, N.; Mezzenga, R.; Brochon, C.; Hadziioannou, G.; Ruokolainen, J. Macromolecules 2007, 40, 3277.

270 Sary, N.; Rubatat, L.; Brochon, C.; Hadziioannou, G.; Ruokolainen, J.; Mezzenga, R. Macromolecules 2007, 40, 6990.

271 Barrau, S.; Heiser, T.; Richard, F.; Brochon, C.; Ngov, C.; van de Wetering, K.; Hadziioannou, G.; Anokhin, D. V.; Ivanov, D. A. Macromolecules 2008, 41, 2701.

272 (a) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498; (b) Yu, G.; Heeger, A. J. J. Appl. Phys. 1995, 78, 4510.

273 Friend, R. H. Pure. Appl. Chem. 2001, 73(3), 425. 274 Liu, J.; Sheina, E.; Kowalewski, T.; McCullough, R. D. Angew. Chem. Int. Ed.

2002, 41(2), 329. 275 (a) Iovu, M. C.; Jeffries-EL, M.; Sheina, E. E.; Cooper, J. R.; McCullough, R. D.

Polymer 2005, 46, 8582; (b) Sauvé, G.; McCullough, R. D. Adv. Mater. 2007, 19, 1822.

276 Dante, M.; Yang, C.; Walker, B.; Wudl, F.; Nguyen, T-Q. Adv. Mater. 2010, 22, 1835.

277 Sary, N.; Richard, F.; Brochon, C.; Leclerc, N.; Lévêque, P.; Audinot, J-N.; Berson, S.; Heiser, T.; Hadziioannou, G.; Mezzenga, R. Adv. Mater. 2010, 22, 763.

278 (a) Sommer, M.; Lang, A. S.; Thelakkat, M. Angew. Chem. Int. Ed. 2008, 47, 7901; (b) Sommer, M.; Hüttner, S.; Steiner, U.; Thelakkat, M. Appl. Phys. Lett. 2009, 95, 183308; (c) Zhang, Q.; Cirpan, A.; Russell, T. P.; Emrick, T. Macromolecules 2009, 42(4), 1079; (d) Rajaram, S.; Armstrong, P. B.; Kim, B. J.; Fréchet, J. M. J. Chem. Mater. 2009, 21(9), 1775; (e) Tao, Y.; McCulloch, B.; Kim, S.; Segalman, R. A. Soft Matter. 2009, 5, 4219; (f) Sommer, M.; Hüttner, S.; Thelakkat, M. J. Mater. Chem. 2010, 20, 10788-10797.

279 (a) Lee, J. U.; Cirpan, A.; Emrick, T.; Russell, T. P.; Jo, W. H. J. Mater. Chem. 2009, 19, 1483; (b) Yang, C.; Lee, J. K.; Heeger, A. J.; Wudl, F. J. Mater. Chem. 2009, 19, 5416.

280 Hiorns, R. C.; Cloutet, E.; Ibarboure, E.; Khoukh, A.; Bejbouji, H.; Vignau, L.; Cramail, H. Macromolecules 2010, 43, 6033.

87

Chapter 2: Towards comb copolymers based on P3HT

via ω-acetylene-P3HT and ω-vinyl-P3HT

macromonomers

88

Contents

2.1 Introduction............................................................................................. 89 2.2 Syntheses of monomers......................................................................... 91

2.2.1 Synthesis of 3-hexylthiophene....................................................... 91 2.2.2 Synthesis of 2,5-dibromo-3-hexylthiophene.................................. 92

2.2.3 Synthesis of 2-bromo-3-hexyl-5-iodo-thiophene............................ 93 2.3 Synthesis and characterization of regioregular P3HT......................... 95

2.3.1 Regioregular α,ω-diH-P3HTs........................................................ 95 2.3.2 Regioregular, end-functionalised ω- and α,ω-P3HTs.................... 97

2.4 Synthesis of ω- or α,ω-alkynyl-P3HT by the GRIM method................ 98 2.4.1 Synthesis of ω-ethynyl-P3HT (P2) and α,ω-pentynyl-P3HT(P3) .. 99

2.5 Synthesis of regioregular ω-vinyl-P3HTs by the GRIM method......... 103

2.6 Synthesis of mono-functionalised-P3HT by externally added Ni-catalyst initiator....................................................................................... 105

2.6.1 Synthesis of the Ni-initiator: [(Ph)Ni(PPh3)2-Br] (4) ..................... 106 2.6.2 Synthesis of mono-functionalised P3HT by “small molecule” Ni-

initiator [(Ph)Ni(PPh3)2-Br] ............................................................ 107 2.7 Syntheses and characterizations of polyacetylene-graft-P3HT.......... 112

2.7.1 Phenylacetylene polymerisation by [Rh(nbd)Cl]2 catalyst.............. 113 2.7.2 Polymerisation of ω-alkynyl-P3HTs by [Rh(nbd)Cl]2 catalyst......... 115 2.7.3 Attempted copolymerisation of ω-acetylene-P3HT with phenyl

acetylene....................................................................................... 118 2.7.4 Attempted polymerisation of ω-vinyl-P3HTs.................................. 119


89

2.1 Introduction

Much research effort has been devoted to the design and synthesis of

polymers with extended π-conjugation due to their potential applications in

non-linear optics and opto-electronic applications. Polyacetylene (PA) is

perhaps the best-known conjugated polymer and exhibits metallic conductivity

upon doping.1 But its intractability and instability have greatly limited its scope

for practical applications. The PA’s with appropriate backbone-pendant

combinations show various functional properties such as liquid crystallinity,

photoconductivity, light emission, photoresistance, helical chirality and optical

nonlinearity.2

Our research group (late Prof. G. Sundararajan’s group) at IIT madras,

India was interested in synthesizing novel linear and tribranched polymers

spanned by polyphenylacetylenes (polyPAs), with redox-active ferrocene

and/or (arene)Cr(CO)3 as end-groups through a simple metathesis route and

studying the existence of an electronic communication between the metal

centers through electrochemical studies.3,4 Polycyclic aromatic compounds

such as naphthalene and anthracene are well-known emitters and their

photophysical properties are well-established and exploited for various

applications. In this connection, we were interested to synthesize

polyphenylacetylenes (PPA) with one end anthracene and other end

naphthalene or substituted phenyl groups as donor/acceptors by metathesis

route using the classical metathesis polymerization catalyst W(CO)6 under

photolytic condition (Scheme 2.1) and also to study their photophysical

properties with respect to their intramolecular electron-transfer interactions.

The chain lengths of these conjugated linear polymers can be varied by

changing the number of equivalents of PA.

ArX1. W(CO)6

UV

2. n PA

UV

3. End capping with

Anthracene

ArX

X, Y = CN, NO2, NMe2, OMe, Me

Ar = Phenyl, Naphthyl

PA = Phenylacetylelne

CPh

Yn

Scheme 2.1 Synthesis of Donor-acceptor conjugated polymers spanned by polyPAs.

90

Given this past experience in PA chemistry in India that it seemed an

opportune moment to exploit the well-defined P3HT chemistry, discussed in

the Introduction, with that of PA. P3HT is known for its excellent optical and

electrical properties and its use in various applications such as light-emitting

diodes, field-effect transistors and plastic solar cells (again, see Section 1.3

for more information). The reason was to see if their combined use could lead

to new structures and architectures in the solid state that might be appropriate

to photovoltaic applications. We expected that the incorporation and grafting

of P3HT chains to a PA backbone might lead to conjugated polymers with

new properties that would result in microphase separated morphologies

(Figure 2.1b). We anticipated that the PA-graft-P3HT chains might have better

interactions with PCBM in the solar cells while retaining crystalline zones and

thus increase the efficiencies through improved exciton formation and charge

collection (Figure 2.1).

Figure 2.1 Schematic representation of (a) P3HT-blend-PCBM and (b) (PA-graft-P3HT)-blend-PCBM.

In this Chapter, we therefore describe the synthesis of two thiophene

monomers required for the preparation of regioregular P3HTs, the synthesis

of regioregular P3HTs, and regioregular end-functionalised ω- and α,ω-

alkynyl, and alkenyl P3HTs by the GRIM method. In an attempt to prepare

monofunctionalised P3HT, we also prepared a small molecule Ni-catalyst

initiator based on phenyl bromide. Attempts were then made to synthesize

PA-graft-P3HT using synthesized macromonomers of P3HTs.

91

2.2 Syntheses of monomers

For the synthesis of regioregular P3HT, two types of thiophene monomers

were prepared. They were 2,5-dibromo-3-hexylthiophene (M1) and 2-bromo-

3-hexyl-5-iodothiophene (M2), as shown in Scheme 2.2. In both cases, the

first step was to prepare 3-hexylthiophene (3).

C6H13BrDiethylether

MgC6H13MgBr

S

Br

Ni(dppp)Cl2Diethylether S

C6H13

1 2 3

S

C6H13

3

NBS, acetic acid

Dichloromethane

1. NBS, THF

2. PhI(OAc)2, I2Dichloromethane

S

C6H13

Br Br

S

C6H13

I Br

2,5-dibromo-3-hexylthiophene(M1)

2-bromo-3-hexyl-5-iodothiophene(M2)

Scheme 2.2 Syntheses of thiophene monomers, 2,5-dibromo-3-hexylthiophene (M1) and 2-bromo-3-hexyl-5-iodothiophene (M2). 2.2.1 3-Hexylthiophene (3) The synthesis of 3-hexylthiophene was inspired from the synthesis of 3-(2-

ethylhexyl) thiophene proposed by Somanathan et al.5 as shown in Scheme

2.2. Hexylmagnesium bromide (2) was prepared by the Grignard reaction of

Mg with hexylbromide (1). This active Grignard species (2) was added in a

flask containing bromothiophene and 1,3-bis(diphenylphosphino)propane

nickel(II) [Ni(dppp)Cl2] in diethyl ether to give 3-hexylthiophene (3). Very high

yields, around 90% were obtained. These Grignard reactions are very

sensitive and must be performed under anaerobic conditions to prevent their

92

degradation. Figure 2.2 shows the 1H NMR spectrum of 3, which matches the

structure of hexylthiophene. All peaks are attributed.

Figure 2.2 1H NMR (400 MHz, CDCl3) spectrum of 3-hexylthiophene (3).

2.2.2 2,5-Dibromo-3-hexylthiophene (M1) The synthesis of first monomer, 2,5-dibromo-3-hexylthiophene (M1) was

performed by mixing 3-hexylthiophene (3) and N-bromosuccinimide (NBS)

(2.2 equivalents) in a mixture of acetic acid and dichloromethane. This

reaction leads to the formation of three species by bromination on positions 2,

4 and 5, to yield the mono-, di- and tribrominated products. Only the

dibromothiophene (M1) is useful for the polymerization; to separate the

various reaction products, then distillation seems to be the most appropriate

method. However, M1 is a compound with a high boiling point (around 300 °

C), and it was therefore distilled at ca 170 ˚C under high vacuum to avoid

subjection to excessive heat. The 1H NMR spectrum of M1 is shown in Figure

2.3 and corresponds to what was expected. The thiophene unit no longer

bears aromatic protons, except that in position 4 corresponding to a peak at

6.80 ppm in 1H NMR spectrum, confirming the dibromination of

hexylthiophene. The relative integration of this peak with the peak

93

corresponding to the proton in position 2 located at 7.1 ppm (a witness to the

monobromothiophene impurity) indicated a purity of 99.5%. It is very

important to obtain a very pure product in order to avoid irregularities in the

chain polymerization. That is why this product was distilled 2-3 times. This

reaction leads to molar yields of ca 70%.

Figure 2.3 1H NMR (400 MHz, CDCl3) spectrum of 2,5-dibromo-3-hexylthiophene

(M1).

2.2.3 2-Bromo-3-hexyl-5-iodo-thiophene (M2) The second monomer, 2-bromo-3-hexyl-5-iodothiophene (M2) was prepared

from the 3-hexylthiophene (3) in two steps.9 The first step was to synthesize

2-bromo-3-hexylthiophene by the reaction of NBS with 3-hexylthiophene, the

second is to add an iodine atom at position 5 of the thiophene ring via a

reaction with iodine (I2) in the presence of iodobenzene diacetate. The

resulting product is distilled twice to remove iodobenzene and also passed

through silica gel column by cyclohexane as eluent to obtain the purest

possible monomer. The 1H NMR spectrum of M2 is shown in Figure 2.4. This

spectrum is very similar to that of M1. Only the position 4-thiophene proton is

slightly offset, moving from 6.8 ppm (M1) to 7.0 ppm (M2), due to the slightly

different environment in the 5 position of thiophene, which has a bromine

94

atom in the case of M1 and an iodine atom in the case of M2. The purity of M2

is around 97%.

Figure 2.4 1H NMR (400 MHz, CDCl3) spectra of 2-bromo-3-hexylthiophene and 2-bromo-3-hexyl-5-iodo-thiophene (M2).

95

2.3 Synthesis and characterization of regioregular P3HTs 2.3.1 Regioregular α,ω-diH-P3HTs

Here is the general procedure for the preparation of α,ω-diH-P3HTs (P1, P1a,

P1b, P1c), which is shown in Scheme 2.3. The reaction conditions for all the

principal P3HT polymers are shown in Table 2.1. In a flask 2,5-dibromo-3-

hexylthiophene M1 was dissolved in THF and stirred under nitrogen. Tert-

butylmagnesium chloride was added and the mixture was stirred at room

temperature for 3 h. The catalyst Ni(dppp)Cl2 was added in one-shot and the

mixture was allowed to stir for 24 h at room temperature. Termination and

removal of bromine chain ends was accomplished by slow addition of LiAlH4.

The reaction was quenched by addition of HCl and the polymer was

recovered by precipitation in ethanol and filtered into a soxhlet extraction

thimble. Following extensive Soxhlet washing with methanol, hexanes and

chloroform, α,ω-diH-P3HTs were recovered from the Soxhlet filter with

chloroform. The reaction conditions for all synthesized α,ω-diH-P3HTs are

summarized in Table 2.1. (See experimental section for complete detailed

procedures and purifications).

S

C6H13

Br Br

1. tBuMgCl2. Ni(dppp)Cl2

3. LiAlH44. HCl (aq) THF, R.T.M1

S

C6H13

H Hn

P1, P1a, P1b, P1c

Scheme 2.3 Synthesis of regioregular α,ω-diH-P3HTs.

Figure 2.5 shows the 1H NMR spectrum of P1 after purification, which

is representative NMR spectrum of all samples. This spectrum is in perfect

agreement with the expected structure and all peaks are attributed to P3HT.

The peak at 6.98 ppm corresponds to aromatic proton of thiophene 4 position)

for regioregular P3HT (HT-HT). The protons corresponding to regioirregular

sequences are 7.00 ppm (TT-HT), 7.02 ppm (HT-HH) and 7.05 ppm (TT-HH)

respectively.5 Therefore it is possible to determine the degree of

regioregularity of a P3HT by integration of these peaks. Similarly, the peak at

2.80 ppm corresponds to the proton bound to α-carbon of 3-hexyl chain in an

96

HT-HT regioregular arrangement whereas the peak at 2.60 ppm corresponds

to the same proton in a regio-irregular arrangement. The relative integration of

these two peaks can also give percentage of regioregularity (HT-HT coupling).

Table 2.1 collects the values of different regioregularity P3HT synthesized,

which are between 92% and 98%. The normalized SECs of all the samples

are overlayed in Figure 2.6.

Debrominated P3HT (α,ω-diH-P3HTs)

P3HT

Monomer M1 (g)

Grignard reagent

tBuMgCl (mL)

Ni(dppp)Cl2 (mol %)

Polymeri-ation time

(h)

LiAlH4 (1 M sol.

THF)

Mn (g/mol)

Đ

RR %

P1 2.83 8.6 0.21 24 4.3 30 000 1.6 96

P1a 3.01 9.2 0.10 24 5.0 50 000 1.7 98

P1b 6.0 18.4 0.06 24 10.0 117 000 1.7 97

P1c 3.14 9.5 1.70 2 10.0 7 000 1.1 92

Table 2.1 Reaction conditions and macromolecular characteristics determined by SEC (THF, UV-254 nm) against polystyrene standards and NMR (RR) of α,ω-diH-P3HTs. [Note: Dispersity, Đ = Mw/Mn]

Figure 2.5 Representative 1H NMR (400 MHz, CDCl3) spectrum of α,ω-diH-P3HT.

97

Figure 2.6 Normalised SECs (UV detection) of α,ω-diH-P3HTs in THF against PS standards.

It should be noted that the values of dispersity are relatively high. This

is most likely due to polymers aggregating as their molecular weight increases

during their formation, thus disrupting the chain-growth polymerization and

leading to varying chain-lengths.

2.3.2 Regioregular, end-functionalised ω- and α,ω-P3HTs

Here is the general procedure for the typical end-capping reaction (GRIM

method). In a flask, 2,5-dibromo-3-hexylthiophene (M1) was dissolved in THF

and stirred under nitrogen. tert-Butylmagnesium chloride (1 M in THF) was

added via syringe and the mixture stirred at room temperature for 2.5 h. Then

Ni(dppp)Cl2 was added in one portion and the mixture stirred for 30-60 min at

room temperature. The Grignard functionalization reagent (50-60 mol % of

monomer) was added via syringe to the reaction mixture and stirred for

additional 30-60 min at room temperature. Finally the reaction was quenched

by adding conc. HCl (5 M) and then poured into methanol to precipitate the

polymer. The polymer was filtered into an extraction thimble and then washed

by Soxhlet extraction with methanol, pentane and chloroform. The polymer

was isolated from the chloroform extraction and concentrated, dried under

reduced pressure.

98

2.4 Synthesis of ω- or α ,ω-alkynyl-P3HT by the GRIM method

Alkynyl functionalized P3HTs were prepared using alkynyl-

magnesiumbromide as end-capping Grignard reagent in the polymerization.

All the synthesized alkynyl-P3HTs from P2 to P3a are shown in Table 2.2.

Two different Grignard reagents (ethynyl-MgBr and pentynyl-MgBr) were

tested to evaluate the effect of distance between the triple bond and π -

conjugated backbone of P3HT. In addition, both groups should lead to P3HT

near (majority) mono-adducts, according to the work performed by Jeffries-El

et al.6 The Grignard reagents used were: (i) ethynylmagnesium bromide to

synthesize ω-ethynyl-P3HT; and (ii) (5-chloromagnesio-1-pentynyl)

trimethylsilane to synthesize, once deprotected, pentynyl-P3HT (see Scheme

2.4). In the latter case, the Grignard reagent, not available commercially, was

obtained by reacting (5-chloro-1-pentynyl)trimethylsilane with magnesium in

THF and we have observed dipentynyl-P3HT as the major product (Scheme

2.4).

Following purification, the P3HT was deprotected by the action of basic

tetrabutylammonium trihydrate (TBAF.3H2O). The trimethylsilane group

protects the alkyne function, most noticeable in the case of pentynyl

functionalization. In both cases (ethynyl and pentynyl), the Grignard reagent

was added at the end of polymerization and the reaction was left for 15-30

min before precipitation in methanol.

S

C6H13

Br Br


THF, R.T.

3. BrMg

3. ClMg SiMe3

4. TBAF.3H2O, THF

S

C6H13

H/Br n

S

C6H13

n

M1

P2

P3

Scheme 2.4 Synthesis of P3HT-terminated with acetylene [ω-ethynyl-P3HT (P2) and α,ω-pentynyl-P3HT (P3)] by GRIM method.

99

2.4.1 Synthesis of ω-ethynyl-P3HT (P2) and α,ω-pentynyl-P3HT (P3)

In a typical experiment, M1 was dissolved in THF and stirred under

nitrogen. Tert-butylmagnesium chloride was added, and the mixture was

stirred at room temperature for 2.5 h. The mixture was then diluted with THF,

Ni(dppp)Cl2 was added, and the mixture stirred for 30 min at room

temperature. The termination of the polymers with the respective Grignard

functionalization agent was carried out in a one-shot addition using 50-60 mol

% with respect to the monomer. As mentioned above, ω-ethynyl-P3HT (P2),

the Grignard reagent was ethynylmagnesium bromide. For α,ω-pentynyl-

P3HT (P3), the reagent was (5-chloromagnesium-1-pentynyl) trimethylsilane

which was prepared separately by the reaction of (5-chloro-1-

pentynyl)trimethylsilane and magnesium in THF (30 mL) for 1 day at room

temperature. P3HTs were purified by a series of precipitation from methanol

solution and analyzed by SEC, 1H NMR, DSC and MALDI-TOF.

Alkynyl Functionalised P3HT

P3HT

Monomer

M1 (g)

Grignard

reagent

tBuMgCl

(mL)

Ni(dppp)Cl2

(mol %)

Polymerization time

(min)

Grignard reagent

used for

functionalisation

Mn

(g/mol)

Đ

RR

(%)

P2 4.50 13.7 0.98 40 Ethynyl-MgBr 14 000 1.1 97

P2a 4.56 14.0 1.80 60 Ethynyl-MgBr 9 000 1.2 95

P2b 3.33 10.0 1.71 30 Ethynyl-MgBr 7 700 1.1 95

P2c 3.0 9.2 1.80 30 Ethynyl-MgBr 8 500 1.1 94

P2d 1.43 4.3 1.68 60 Ethynyl-MgBr 3 500 1.1 90

P2e 3.19 9.5 1.69 60 Ethynyl-MgBr 2 500 1.1 90

P3 5.68 17.4 1.69 60 ClMg(C5H6)Si(Me)3

TBAF.3H20

8 000 1.1 98

P3a 1.6 4.8 1.69 30 ClMg(C5H6)Si(Me)3

TBAF.3H20

6 200 1.1 97

Table 2.2 Reaction conditions, and macromolecular characteristics determined by SEC (THF, UV-254 nm) against polystyrene standards and NMR (RR) of alkynyl-functionalised-P3HTs.

100

Representative 1H NMR spectra of P2 and P3 for alkynylP3HTs, which

correspond to ethynyl and pentynyl functionalized P3HT are shown in Figure

2.7. These spectra characterize the chain ends of polymers, P2 and P3.

Figure 2.7 (a), which shows the 1H NMR spectrum of P2, confirms the

expected structure. Indeed, this is a typical spectrum of regioregular P3HT

has an additional peak around 3.52 ppm that corresponds to the ethylinic

proton. This is also confirmed by the 13C NMR of P2, the peak at 67.98 ppm

assigned alkynyl carbon (-C ≡ CH), which usually appears in a range between

60 and 90 ppm.16,17

The 1H NMR spectrum P3 (Figure 2.7 (b)) shows the expected peaks.

The alkynyl proton (-C ≡ CH) appeared around 3.49 ppm and the protons of

the pentynyl group carbons α, β and γ are to 2.5 ppm, 1.9 ppm and 2.3 ppm

respectively and also the peak at 68.94 ppm in 13C NMR, which can be

attributed to pentynylic carbon (-C ≡ CH). The difference between the alkyne

protons of P2 and P3 is certainly due to the delocalization of electrons by

lower insertion of an alkyl chain between the thiophene and the triple bond in

the case of P3.

The NMRs were also used, as decribed in Section 2.3.1, to determine

regioregularities of the samples. It is noticeable that while most samples

display high regioregularities, P2d and P2e are rather lower. This is probably

due to their considerably lower molecular weight. It is known (see Section 1.3)

that the mechanism for the formation of P3HT results in a regio-irregularity at

the chain-ends and, at lower molecular weights, the concentration of this

irregularity becomes relatively high.

In addition, Figure 2.8 shows the MALDI-TOF mass spectra (Matrix

Assisted Laser Desorption Ionization - Time Of Flight) of P2b and P3. In the

case of ω-ethynyl-P3HT P2b [Figure 2.8 (a)], the spectrum shows a major

population (85%), which corresponds to the mono functionalized P3HT. It was noted that the presence of difunctionalised α,ω-diethynyl-P3HTs might be of

concern when attempting the graft copolymers discussed in Section 2.7.2 as they could lead to crosslinking reactions. α,ω-Di-pentynyl-P3HT P3 [Figure

101

2.8 (b)] was found to be, by far, the major population (85%), in contrast to that

found in the literature where lower concentrations of difunctionalised P3HT

was found, due to possible triple bond complexation with the Ni.6 This would

effectively exclude the use of α,ω-dipentynyl-P3HT from the planned graft

copolymer formation reactions.

(a)

(b)

Figure 2.7 1H NMR (400 MHz, CDCl3) spectra of (a) ω-ethynyl-P3HT (P2) and (b) α,ω-pentynyl-P3HT (P3) [Note that the peaks at ca 2.6 ppm are due to chain-end alkyl α-Hs on the P3HT (see Section 2.3.1)].

102

Figure 2.8 shows two examples of calculation of molar masses that agree well

with molecular weights of the corresponding molecular peaks.

(a)

(b)

Figure 2.8 MALDI-TOF mass spectra of: (a) ω-ethynyl-P3HT (P2b); and (b) α,ω-pentynyl-P3HT (P3).

103

2.5 Synthesis of regioregular ω-vinyl-P3HTs by the GRIM method Chain-end vinyl functionalized P3HT was considered a valid target to

investigate the possibility for obtaining mono-functionalised P3HT

macromonomer for graft copolymers. Functionalization of vinyl P3HT can

achieve the copolymerizations to obtain novel copolymers. For instance, it has

been shown to be possible to synthesize a macro-initiator of ATRP-P3HT

bromoester from ω-vinyl-P3HT.7 The Grignard reagent we used in this case

was vinyl-magnesium bromide. It was added to the growing chains of P3HT

for 15 min before being precipitated in methanol (Scheme 2.5). Figure 2.9

shows the 1H NMR spectrum of P6. The vinyl protons appear at around 5.1

ppm, 5.5 ppm and 6.8 ppm. In addition, MALDI-TOF mass characterizations

(Figure 2.10) confirmed that this polymer resulted in a 100% mono-addition to

the P3HT as first reported by Jeffries-El et al.6

S

C6H13

Br Br


3. THF, R.T.

M1S

C6H13

Br/H n

P6, P6a, P6b

BrMg

Scheme 2.5 Synthesis of ω-vinyl-P3HTs (P6, P6a, P6b) by GRIM method.6 ω-Vinyl-P3HTs of different molecular weights and regioregularities by

varying the amount of Ni-catalyst used in the polymerization were synthesized

as detailed in Table 2.3. The normalised SECs of the samples are shown in

Figure 2.11. We observed a small hump for all the functionalised P3HTs at

high molecular weights probably due to coupling between growing chains and

Ni disproportionation when quenching the polymerisation.8

ω-Vinyl-P3HT

Ni(dppp)Cl2

(mol %)

Mn

(g/mol)

Đ

Regioregularity

(RR %)

P6 2.5 5 500 1.2 90

P6a 1.8 7 400 1.1 94

P6b 1.7 9 000 1.2 96

Table 2.3 Macromolecular characteristics determined by SEC (THF, UV-254 nm) against polystyrene standards and NMR (RR) of ω-vinylP3HTs.

104

Figure 2.9 1H NMR (400 MHz, CDCl3) spectrum of ω-vinyl-P3HT (P6).

Figure 2.10 MALDI-TOF mass spectrum of ω-vinyl-P3HT (P6).

105

Figure 2.11 Normalised SECs (THF, UV-254 nm) of ω-vinyl-P3HTs (P6, P6a, P6b) against PS standards. To conclude, this section has presented the synthesis of regioregular

P3HTs and showed that it was difficult to precisely control the parameters of

the synthesized polymers. For the functionalization, the results are different

from those published by other groups and showed that the mono-or di-

functionalization is not only dependent on the chemical nature of the groups

but also on the reactivity of functional groups.

In addition, alkyne terminated P3HTs have been synthesized which

made possible subsequent coupling reactions using "click" chemistry to form

rod-coil block copolymers (as detailed in Chapter 3) and also attempts to

synthesize polyacetylene-graft-polythiophene using ethynyl-P3HT and vinyl-

P3HT (see Section 2.7).

2.6 Synthesis of mono-functionalised-P3HT by externally added Ni-catalyst initiator Using the well-known GRIM method, we were able to synthesize

regioreglar P3HTs and also regioregular end-functionalized P3HTs

successfully. However, as mentioned above, the drawback of this method is

that it was not completely possible to ensure the preparation of totally mono-

ethynyl functionalised P3HTs. And this is very important for the synthesis of

106

novel polymer architectures such as brushes, starlike and block copolymers.

Recently, reports in which “external” initiators for the synthesis of

conjugated polymer brushes of regioregular P3HT via surface-initiated

catalyst-transfer polycondensations have been demonstrated.9,10 Senkovskyy

et al., in particular, demonstrated the chain-growth polymerization of P3HT by

both small-molecule and surface initiators as shown in Scheme 2.6.9 This

method, which appeared to allow the formation of completely mono-

functionalised P3HTs, inspired us to prepare regioregular mono-ethynylated

P3HTs of controlled molecular weights and use these macromonomers to

synthesize graft copolymers, which was one of our major aims. Therefore we

attacked this route.

Scheme 2.6 Catalyst-transfer polycondensation of 1a initiated by small molecules or macroinitiators according to Senkovskyy et al.9

2.6.1 Synthesis of the Ni-initiator: [(Ph)Ni(PPh3)2-Br] (4) The extremely reactive complex [Ni(PPh3)4] reacts easily with various

arylhalides to produce the desired adducts [(Ar)Ni(PPh3)2-Br].11 In a flame-

dried Schlenk flask; to a solution of [Ni(PPh3)4] in dry toluene, bromobenzene

was added at room temperature under argon atmosphere. Then the

homogeneous mixture was allowed to stir for about 30 min and allowed to

107

stand for overnight. The original deep red colour of the reaction mixture

gradually changed to brownish yellow colour with the precipitation of

[(Ph)Ni(PPh3)2-Br] (4) as yellow crystals that were, rather tediously, filtered

under argon atmosphere and washed with dry pentane (Scheme 2.7).

Br [Ni(PPh3)4]-2PPh3, RT, Ar

Ni[PPh3]2-Br+Dry toluene

4 Scheme 2.7 Synthesis of the “small molecule” Ni-initiator.

2.6.2 Synthesis of mono-functionalised P3HT by “small molecule” Ni- initiator [(Ph)Ni(PPh3)2-Br] We adopted the synthetic procedure of Senkovskyy et al.,9 for the

synthesis of mono-functionalized P3HTs, using “small molecule” Ni-initiator

[(Ph)Ni(PPh3)2-Br]. In a typical polymerization, the Ni-catalyst initiator (4)

solution in dry toluene was added to Grignard regio-isomer of M2a at 0 °C

under argon and the reaction mixture was stirred at 0 °C for about 6 h under

argon. At the end of the polymerization, the reaction was quenched with 5 M

HCl or a functional Grignard reagent to result in chain-end capping. We used

the protected Grignard reagent (5-chloromagnesio-1-pentynyl) trimethylsilane

with the aim of preparing α-Ph-ω-pentynyl-P3HT (P7, P7a). The former case,

with HCl led to α-Ph-ω-H-P3HT (P8, P8a, P8b) (Scheme 2.8). The reaction

conditions and the details of characterised mono-functionalised P3HTs from

P7 to P8b are shown in Table 2.4. The polymers’ regioregularities are quite

low while the dispersities are high when compared with those prepared by the

GRIM method. When the polymerization temperature was raised from 0 °C to

RT (Table 2.4), the regioregularity decreased and the dispersities increased,

in agreement with the observations of Senkovskyy et al., which means that

the polymerization for high molecular weight P3HTs was not well-controlled by

this method. The detailed experimental procedures are given for all polymers

in the experimental section. The polymers were characterized by 1H NMR,

SEC and MALDI-TOF.

108

S

C6H13

I Br

iPrMgCl

S

C6H13

ClMg Br S

C6H13

R

Ni[PPh3]2-Br1.

2. R-MgBr3. 5M HCl n

M2 M2a

THF, Ar, 0 oC

P7, P7a (R = Pentynyl)P8, P8a, P8b (R = H)

Scheme 2.8 Synthesis of mono-functionalised P3HT by “small molecule” Ni-initiator [(Ph)Ni(PPh3)2-Br] according to Senkovskyy et al.9

Mono-functionalised P3HT by [(Ph)Ni(PPh3)2-Br]

P3HT Monomer

M2 (g)

Grignard reagent tPrMgCl 2 M (mL)

Ni-initiator (4) (mg)

Polyme-rization

time (h)

Grignard reagent for endcapping

Target structure

Mn (g/mol) Đ

RR

(%)

P7 1.28 1.74 100 6

(0 ˚C)

ClMg(C5H6)Si(Me)3

TBAF.3H20

α-Ph-ω-ethynyl-

P3HT 6 500 1.7 92

P7a 0.58 0.8 40 6

(0 ˚C)

ClMg(C5H6)Si(Me)3

TBAF.3H20

α-Ph-ω-ethynyl-

P3HT 6 200 1.6 88

P8 0.50 0.7 35 6

(0 ˚C) 5 M HCl α-Ph-ω-H-P3HT 8 600 1.4 90

P8a 0.60 0.8 40 3 (RT) 5 M HCl α-Ph-ω-H-P3HT 5 300 1.7 88

P8b 0.50 0.7 35 3 (RT) 5 M HCl α-Ph-ω-H-P3HT 5 900 1.6 89

Table 2.4 Reaction conditions, molecular weights (Mn) and regioregularity (RR) of mono-functionalised-P3HTs prepared using the “small molecule” Ni-initiator. From 1H NMR and MALDI-TOF techniques, we have investigated the

initiation efficiency for the polymerization and also obtained the information

about both the starting groups as well as end-groups of resulting P3HT

products. Representative 1H NMR (Figure 2.12) and MALDI-TOF (Figure 2.13)

mass spectra of P7 and P8 for P3HTs, which correspond to α-Ph-ω-pentnyl-

P3HT and α-Ph-ω-H-P3HT, respectively. Figure 2.14 shows the normalised

overlay SECs of P3HTs P7 and P8. Figure 2.12(a) shows the 1H NMR

spectrum of P8, confirms the expected structure. From the integration of

signals in the aromatic region and a detailed assignment of the 1H NMR of the

product showed that the obtained P3HT had around 70% α-phenyl groups

along with Ph/H, Ph/Br and a small amount of H/H end-groups, thus agreeing

109

with Senkovskyy’s et al.8 results. However, the MALDI-TOF mass spectra

characterisations of the obtained P3HTs indicate results which are in

disagreement with those obtained by Senkovskyy et al.9 Figure 2.13 (a)

MALDI-TOF spectrum of P8 shows that the Ph-initiated P3HT (thus giving

Ph/H or Ph/Br chain-ends) is not a major portion and in fact the majority

population is α-H-ω-bromo-P3HT, probably due to chain-transfer reactions.

This contradicts the results of Senkovskyy et al.9 and the reason for this is

probably the poor resolution of their MALDI-TOF spectra which made

impossible a correct assignment of chain-ends. A considerable amount of

work was performed by our service (CESAMO, Bordeaux) in order to

distinguish between these chain-ends. It should be noted that there is a

collusion in the molar masses of phenyl and bromo chain-end groups (Ph is

2571 g mol-1, Br is 2574 g mol-1).

Figure 2.12 (b) shows the 1H NMR spectrum of P7 (α-Ph-ω-pentynyl-

P3HT) that confirms the expected structure. The alkynyl proton (-C ≡ CH)

appears at around 3.49 ppm and the protons of the pentynyl group carbons α,

β and γ are at 2.5 ppm, 1.9 ppm and 2.3 ppm, respectively. Integration of the

peaks in the aromatic region show that the majority of the portion is due to

H/H end-groups and that this functionalized P3HT has Ph-initiating groups on

only around 50% of the P3HT chains. This is confirmed in the NMR spectrum,

as the intensity of peak (f) at 6.9 ppm is high in the case of P7, which is not

the case in P8. This further confirmed from MALDI-TOF mass spectrum

results of P7 [Figure 2.13 (b)]. From this mass spectrum, we have observed

that the product α-Ph-ω-pentynyl-P3HT (P7) was a mixture, which

corresponds to end groups H/H (major population), Ph/Pentynyl, H/Pentynyl

and Ph/H (minor population). The chain-end functionalisation was not very

efficient here. We did not separate the fractions from this mixture. However,

we nevertheless attempted to use this chain-end mono functionalized P3HT to

prepare graft copolymers (see Section 2.7).

110

(a)

(b)

Figure 2.12 1H NMR (400 MHz, CDCl3) spectrum of: (a) α-Ph-ω-H-P3HT (P8); and (b) α-Ph-ω-pentynyl-P3HT (P7).

111

(a)

(b)

Figure 2.13 MALDI-TOF mass spectrum of: (a) α-Ph-ω-H-P3HT (P8); and (b) α-Ph-ω-pentynyl-P3HT (P7).

112

Figure 2.14 Normalised SECs (UV detection) of P3HTs, P7 and P8 in THF against PS standards. In this section, we have synthesized mono-functionalised P3HT by

external “small molecule” Ni-initiator. We could not reproduce the results

reported by Senkovskyy et al.9 even though considerable time and care was

expended to do so. It is proposed that the results were essentially the same

as Senkovskyy et al.’s but that the interpretations differ. The synthesis of

regioregular P3HT by this “external” initiation method was not as efficient as

GRIM method. The low regioregularities and high dispersities of P3HT were

obtained using Senkovskyy et al.’s procedure. More recently Luscombe’s

research group also investigated methods for the external initiation of P3HT

including the effect of varying substituents on the intiating aryl halide12 and

they were very successful in synthesizing P3HTs with high regioregularity and

narrow dispersity by an improved “external” initiation method.13

2.7 Syntheses and characterizations of polyacetylene-graft-P3HT

Graft copolymers derived from poly(macromonomers) and branched polymers

generally exhibit unique properties in terms of their organisation and assembly

both in solution and in bulk as compared to their linear counterparts.14,15

Generally graft copolymers can be achieved mainly by three methods:

grafting-onto, in which side chains are first synthesized and then attached to a

multifunctional linear backbone;16 grafting-from, in which grafting of monomer

113

from a linear macroinitiator;17 grafting-through (macromonomer method), in

which macromonomers are copolymerized with low molecular weight

comonomers.18 In a particular case, homopolymerisation of macromonomers

produces comb polymers or polymer brushes.19 Substituted PAs have attracted considerable interest due to their unique

properties. PA-based grafted copolymers are more stable due to the

protection of the main chain by side chains as unsubstituted PAs decompose

gradually in solution. Here we use macromonomer alkynyl-P3HTs synthesized

earlier for the synthesis of polyacetylene-graft-P3HT (PA-g-P3HT). These

graft copolymers may be interesting candidates for applications in organic

electronics.

2.7.1 Phenylacetylene polymerisation by [Rh(nbd)Cl]2 catalyst It is well known that substituted acetylenes polymerize with transition

metal catalysts.20-23 Among various catalysts used, Rh based catalysts attract

particular interest as they efficiently polymerize mono-substituted acetylenes,

especially phenylacetylene derivatives.23-31 Particularly Rh complex catalyst,

[Rh(norbornadiene)Cl]2 can stereoregularly polymerize monosubstituted

acetylenes to produce corresponding polyacetylenes with cis-transoid

structure in high yields under mild conditions. 23,26,27

In this section, the use of an Rh catalyst to polymerize

macromonomers alkynyl-P3HTs (P7 and P2b) is described with the reaction

conditions reported elsewhere.23

Before doing this, we optimized reaction conditions by polymerizing the

monomer phenylacetylene to produce poly(phenylacetylenes) (PPA) with the

Rh-based catalyst and triethylamine (TEA) as a co-catalyst in THF at room

temperature for about 24 h (Scheme 2.9). The polymerization in THF

proceeded smoothly to obtain PPAs in moderate yields after precipitation in

methanol. The obtained PPAs with different molecular weights and

dispersities are detailed in Table 2.5 and are soluble in all organic solvents.

The PPAs are characterized by 1H NMR and SEC. Figure 2.15 (a) shows the

114

representative 1H NMR spectrum of PPA2 in CDCl3 at room temperature. This

spectrum features a typical cis-transoid structure because it exhibits very

sharp lines indicating highly regular structure.26 This was clearly evidenced

that the ratio of an integrated area due to =C-H proton at 5.85 ppm and five

phenyl protons observed at 6.5-7.0 ppm estimated as 1:5. Figure 2.15 (b)

shows the normalised overlay SECs of PPA1 and PPA2. The optimized

reaction conditions were then used for the polymerizations of alkynyl-P3HTs.

[Rh(nbd)Cl]2/TEA

THF, RT, 24 hrs

Hn

PA PPA Scheme 2.9 Synthesis of poly(phenylacetylene) (PPA) by Rh-based catalyst.23

Monomer [Rh(nbd)Cl]2

(mg)

Polymer Mn(g mol-1) Đ

Phenylacetylene 25 PPA1 52 000 2.5

Phenylacetylene 20 PPA2 58 000 2.1

Table 2.5 Molecular weight characteristics (SEC, UV-254 nm) of PPAs by Rh catalyst.

(a)

115

(b)

Figure 2.15 (a) Representative 1H NMR (400 MHz, CDCl3) spectrum of PPA2 and (b) Normalised SECs (UV detection) of PPAs (PPA1 and PPA2) in THF and against PS standards. 2.7.2 Polymerisation of ω-alkynyl-P3HTs by [Rh(nbd)Cl]2 catalyst

Here two types of macromonomers are utlised: namely ω-ethynyl-

P3HT (P2b) prepared by the GRIM method and α-phenyl-ω-pentynyl-P3HT

(P7) synthesized by the “external” small molecule Ni-initiator. Each is

subjected to homo-polymerization by the Rh-based catalyst in a highly original

manner to obtain polyacetylene-graft-P3HT (PA-g-P3HT). The

polymerizations were performed at room temperature in THF under the

optimized reaction conditions used for the synthesis of PPAs and SEC was

used to monitor the reaction performance. The macromonomer α-phenyl-ω-

pentynyl-P3HT (P7) in which the ethynyl group is far from the conjugated

P3HT backbone did not undergo polymerization even after 24 h and there

was no polymerisation though the reaction was forced by increasing the

temperature. But the macromonomer ω-ethynyl-P3HT (P2b) in which the

ethynyl group is directly attached to thiophene ring was able to undergo

polymerization in the presense of the Rh-based catalyst and produced the

graft copolymer, PA-g-P3HT shown in Scheme 2.10. 1H NMR and SEC were

used to confirm the graft copolymers’ structures. The molecular weights and

dispersities of the graft copolymers are given in Table 2.6.

116

S

C6H13 n [Rh(nbd)Cl]2/TEA

THF, RT S

C6H13 n

m

P2b = Ethynyl-P3HTP7 = Pentynyl-P3HT

PA-graft-P3HT for P2bNo reaction for P7

Scheme 2.10 Synthesis of PA-g-P3HT by Rh-based catalyst.23

Experiment

SEC, Mn (g mol-1) Graft copolymer PA-g-P2b

(Macromonomer, P2b)

Đ

1 34 800 (7 700) 1.4

2 32 100 (7 700) 1.4

3 23 400 (7 700) 1.3

Table 2.6 Molecular weights (Mn) and dispersities (Đ) of PA-g-P3HTs by Rh catalyst. The SECs shown in Figure 2.16 (c) confirm that SECs of α-phenyl-ω-

pentynyl-P3HT (P7) and reaction mixture of P7 with Rh catalyst shows that

the curves are exactly overlayed indicating that P7 was not able to polymerize

in the presence of the Rh-based catalyst.

However, Figure 2.16 (a) showing the representative 1H NMR

spectrum of PA-g-P3HT and confirms the expected structure following the

polymerisation of ω-ethynyl-P3HT (P2b). The disappearance of ethynylic-

proton of ω-ethynyl-P3HT (P2b) at 3.49 ppm indicates that the

macromonomer P2b participated in the polymerization by Rh catalyst. But due

to the low concentration of =C-H proton compared to polymer, it was not able

to identify in the 1H NMR spectrum even at longer scans and high relaxation

time. The formation of graft copolymer further confirmed by the SEC. Figure

2.16 (b) represents the normalised overlay SECs of macromonomer P2b and

PA-g-P3HT in which graft copolymer peak shifted towards high molecular

weight along with small amount of unreacted macromonomer, ω-ethynyl-

P3HT (P2b).

117

(a)

(b) (c)

Figure 2.16 (a) Representative 1H NMR (400 MHz, CDCl3) spectrum of PA-g-P3HT; (b) Normalised overlay SECs of ω-ethynyl-P3HT (P2b) and PA-g-P2b demonstrating copolymerisation reaction; and (c) Normalised overlay SECs of α-phenyl-ω-pentynyl-P3HT (P7) and reaction mixture of P7 by Rh catalyst showing non-reaction. All SECs (UV detection) in THF and against PS standards.

To achieve complete homopolymerization of the macromonomer ω-

ethynyl-P3HT (P2b), reaction conditions were varied by changing parameters

such as the reaction temperature, reaction times and dilution. It should be

noted that it was found that despite the high molecular weight of the products,

the solubilities of the graft copolymers were high in common organic solvents.

118

2.7.3 Attempted copolymerisation of ω-acetylene-P3HT with phenyl acetylene

In general, polymerization of macromonomer with low molecular weight

monomer helps to achieve better yields.32 Since we were not successful to

achieve complete homopolymerization of macromonomer ω-ethynyl-P3HT,

this idea prompted us to give a try for copolymeriztion of ω-ethynyl-P3HT with

low molecular weight monomer phenylacetylene. So we have attempted the

copolymerization of the macromonomer, ω-ethynyl-P3HT (P2c) with

phenylacetylene (PA) by varying the amount of PA in the reaction mixture

from 10% to 50% to produce graft copolymers, poly(P2c-co-PA) which is

shown in Scheme 2.11 and monitored the efficiency of the reaction by SEC.

But unfortunately, we were again unsuccessful to obtain expected graft

copolymers by complete copolymerization of ω-ethynyl-P3HT (P2c) with

phenylacetylene.

Figure 2.17 (b) showing the overlayed SECs of macromonomer, ω-

ethynyl-P3HT (P2c) with reaction mixture of P2c and PA (10%) by Rh catalyst

clearly indicates that obtained copolymer is a mixture of products with

remaining unreacted macromonomer, ω-ethynyl-P3HT (P2c) that is significant

amount. To obtain complete copolymerization; we varied the amount of PA

(from 10% to 50%) in the reaction mixture for the copolymerization of P2c, but

the efficiency of copolymerization was decreased by retaining the significant

macromonomer in the reaction mixture which was shown in Figure 2.17 (a).

S

C6H13 n

PA(10% - 50%)

[Rh(nbd)Cl]2/TEA

THF, RT

Expected graft copolymerPoly(P2c-co-PA)

S

C6H13 n

x y

P2c = Ethynyl-P3HT

Scheme 2.11 Copolymerization of macromonomer, ω-ethynyl-P3HT (P2c) with phenylacetylene (PA) by [Rh(nbd)Cl2] catalyst.32

119

(a) (b)

Figure 2.17 (a) Normalised overlay SECs of ω-ethynyl-P3HT (P2c) and reaction mixture of P2c and PA (10%-50%) by Rh catalyst demonstrating copolymerisation reaction; and (b) Normalised overlay SECs of ω-ethynyl-P3HT (P2c) and reaction mixture of P2c and PA (10%) by Rh catalyst showing uncomplete reaction (for clarity). All SECs (UV detection) in THF and against PS standards.

2.7.4 Attempted polymerisation of ω-vinyl-P3HTs

Finally we have made attempts to prepare graft copolymers by other

methods namely RAFT method and olefin polymerization by Ni-metallocene

catalysts from ω-vinyl-P3HT (Scheme 2.12) and monitored the reaction

efficiency by SEC. But even after 2 days also, we have not observed

formation of graft copolymers by SEC. Figure 2.18, the SECs of

macromonomers, ω-vinyl-P3HTs (P6 and P6a) and their reaction mixtures

shows that the curves are exactly overlayed indicating that ω-vinyl-P3HTs

were not able to polymerize by RAFT and olefin polymerization methods. The

reason for this was probably the conjugation of the chain-end and the steric

encoumbrance of the vinyl moiety. Therefore other routes are envisaged.

S

C6H13

Br/H n

1. RAFT method CTA, AIBN, THF

2. Olefin polymerization Ni-catalyst, MAO, Toluene

No graft copolymerP3HT

nXP6 or P6a

Scheme 2.12 Synthesis of P3HT grafted copolymers from ω-vinyl-P3HT by RAFT and Olefin polymerization methods.

120

(a) (b)

Figure 2.18 (a) Normalised overlay SECs of ω-vinyl-P3HT (P6) and reaction mixture of P6 by RAFT polymerisation demonstrating no-reaction; and (b) Normalised overlay SECs of ω-vinyl-P3HT (P6a) and reaction mixture of P6a by olefin polymerisation demonstrating no-reaction. All SECs (UV detection) in THF and against PS standards. 2.8 Conclusions In this chapter we have synthesized regioregular P3HTs and also chain

end-functionalised P3HTs with narrow dispersities by the GRIM method. A

small molecule Ni-initiator was also synthesized and utilized to prepare

completely mono-functionalised P3HTs. But we could not reproduce

Senkovskyy et al.’s results. We obtained a mixture of products when we used

the “external” initiator whereas the GRIM method produced better results. We

were somewhat more successful for our attempts to prepare P3HT grafted

copolymers by alkynyl-P3HTs. Meanwhile we found that conjugation and

steric hindrance play a key role for the polymerization of alkynyl-P3HTs by Rh

catalyst and also in the polymerization of ω-vinyl-P3HTs by RAFT and olefin

polymerization methods.

For the efficient polymerization of P3HT-substituted acetylenes, the

acetylene group should be directly attached to the aromatic group; however, it

is probable that a spacer is necessary to separate the bulk of the aromatic

acetylene group from P3HT due to conjugation and steric hindrance.

Therefore further investigations are required.

121

2.9 References 1 Shirakawa, H. Angew. Chem. Int. Ed. 2001, 40, 2575. 2 Jacky, W. Y. L.; Tang, B. Z. Acc. Chem. Res. 2005, 38, 745. 3 (a) Dhanalakshmi, K.; Sundararajan, G. J. Organomet. Chem. 2002, 645, 27; (b) Dhanalakshmi, K.; Sundararajan, G. Polym. Bull. 1999, 42, 683. 4 Santhosh, N. S.; Sundararajan, G. Org. Lett. 2006, 8 (4), 605–608. 5 Somanathan, N.; Radhakrishnan, S.; Mukundan, T.; Schmidt, H. W.

Macromolecular Materials and Engineering 2002, 287, 236. 6 Jeffries-EL, M.; Sauve, G.; McCullough, R. D. Macromolecules 2005, 38,

10346–10352. 7 Iovu, M.C.; Jeffries-EL, M.; Sheina, E.E.; Cooper, J.R.; McCullough, R.D.

Polymer 2005, 46, 8582. 8 Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. Macromol. Rapid Commun. 2004,

25, 1663. 9 Senkovskyy, V.; Khanduyeva, N.; Komber, H.; Oertel, U.; Stamm, M.; Kuckling,

D.; Kiriy, A. J. Am. Chem. Soc. 2007, 129, 6626. 10 Sontag, S. K.; Marshall, N.; Locklin, J. Chem. Commun. 2009, 3354. 11 Hiday, M.; Kashiwagi, T.; Ikeuchi, T.; Uchida, Y. J. Organomet. Chem. 1971,

30, 279. 12 Doubina, N.; Ho, A.; Jen, A. K.-Y.; Luscombe, C. K. Macromolecules 2009, 42,

7670–7677. 13 Bronstein, H. A.; Luscombe, C. K. J. Am. Chem. Soc. 2009, 131, 12894-12895. 14 (a) Zhang, M.; Mueller, A. H. E. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 3461-3481; (b) Hadjichristidis, N.; Pitsikalis, M.; Iatrou, H.; Pispas, S. Macromol. Rapid Commun. 2003, 24, 979-1013; (c) Ito, K.; Kawaguchi, S. Adv. Polym. Sci. 1999, 142, 129-178.

15 a) Desvergne, S.; Heroguez, V.; Gnanou, Y.; Borsali, R. Macromolecules 2005, 38, 2400-2409; (b) Zhang, B.; Zhang, S.; Okrasa, L.; Pakula, T.; Stephan, T.; Schmidt, M. Polymer 2004, 45, 4009-4015; (c) Viville, P.; Leclere, P.; Deffieux, A.; Schappacher, M.; Bernard, J.; Borsali, R.; Bredas, J.-L.; Lazzaroni, R. Polymer 2004, 45, 1833-1843; (d) Liu, Y.; Abetz, V.; Mueller, A. H. E. Macromolecules 2003, 36, 7894-7898; (e) Qin, S.; Matyjaszewski, K.; Xu, H.; Sheiko, S. S. Macromolecules 2003, 36, 605-612.

16 (a) Gacal, B.; Durmaz, H.; Tasdelen, M. A.; Hizal, G.; Tunca, U.; Yagci, Y.; Demirel, A. L. Macromolecules 2006, 39, 5330-5336; (b) Li, A.; Lu, Z.; Zhou, Q.; Qiu, F.; Yang, Y. Polymer 2006, 47, 1774-1777; (c) Ryu, S. W.; Hirao, A. Macromolecules 2000, 33, 4765-4771; (d) Schappacher, M.; Deffieux, A. Macromolecules 2000, 33, 7371-7377.

17 (a) Lee, H.-i.; Jakubowski, W.; Matyjaszewski, K.; Yu, S.; Sheiko, S. S. Macromolecules 2006, 39, 4983-4989; (b) Muthukrishnan, S.; Zhang, M.; Burkhardt, M.; Drechsler, M.; Mori, H.; Mueller, A. H. E. Macromolecules 2005, 38, 7926-7934; (c) Cheng, G.; Boeker, A.; Zhang, M.; Krausch, G.; Mueller, A. H. E. Macromolecules 2001, 34, 6883-6888; (d) Borner, H. G.; Beers, K.; Matyjaszewski, K.; Sheiko, S. S.; Moller, M. Macromolecules 2001, 34, 4375-4383.

18 (a) Nguyen, S.; Marchessault, R. H. Macromolecules 2005, 38, 290-296; (b) Cai, Y.; Hartenstein, M.; Mueller, A. H. E. Macromolecules 2004, 37, 7484-7490; (c) Nagai, A.; Ochiai, B.; Endo, T. Macromolecules 2004, 37, 4417-4421; (d) Batis, C.; Karanikolopoulos, G.; Pitsikalis, M.; Hadjichristidis, N. Macromolecules 2003, 36, 9763-9774; (e) Schulze, U.; Fonagy, T.; Komber, H.; Pompe, G.; Pionteck, J.; Ivan, B. Macromolecules 2003, 36, 4719-4726; (f) Breitenkamp, K.; Simeone, J.; Jin, E.; Emrick, T. Macromolecules 2002, 35, 9249-9252.

122

19 Morandi, G.; Montembault, V.; Pascual, S.; Legoupy, S.; Fontaine, L. Macromolecules 2006, 39, 2732-2735.

20 Masuda, T.; Sanda, F. Polymerization of substituted acetylenes. In Handbook of metathesis; Grubbs, R.H., Ed.; Wiley-VCH: Weinheim, Germany, 2003; Vol. 3, Chapter 11, 375 p.

21 Sedlacek, J; Vohlidal, J. Collect. Czech. Chem. Commun. 2003, 68, 1745-1790.

22 Choi, S.-K.; Gal, Y.-S.; Jin, S.-H.; Kim, H. K. Chem. Rev. 2000, 100, 1645-1682.

23 Tabata, M.; Sone, T.; Sadahiro, Y. Macromol. Chem. Phys. 1999, 200, 265-282.

24 Furlani, A.; Napoletano, C.; Russo, M.V.; Camus, A.; Marsich, N. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 75-86.

25 Furlani, A.; Napoletano, C.; Russo, M.V.; Feast, W. J. Polym. Bull. 1986, 16, 311-317.

26 Tabata, M.; Yang, W.; Yokota, K. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1113-1120.

27 Tabata, M.; Yang, W.; Yokota, K. Polym. J. 1990, 22, 1105-1107. 28 Mastrorilli, P.; Nobile, C. F.; Gallo, V.; Suranna, G. P.; Farinola, G. J. Mol.

Catal. A: Chem. 2002, 184, 73-78. 29 Tang, B. Z.; Poon, W. H.; Leung, S. M.; Leung W. H.; Peng, H.

Macromolecules 1997, 30, 2209-2212. 30 Kishimoto, Y.; Itou, M.; Miyatake, Y.; Ikariya, T.; Noyori, R. Macromolecules

1995, 28, 6662-6666. 31 Aoki, T.; Kokai, M.; Shinohara, K.; Oikawa, E. Chem. Lett. 1993, 22, 2009. 32 Zhang, W.; Shiotsuki, M.; Masuda, T. Macromolecules 2007, 40, 1421-1428.

123

Chapter 3: Block copolymers based on

poly(3-hexylthiophene) (P3HT) and polystyrene (PS) or

poly(4-vinylpyridine) (P4VP)

124

Contents

3.1 Introduction............................................................................................. 125 3.2 Synthesis of azide-terminated polystyrene.......................................... 127

3.2.1 Principle of atom transfer radical polymerisation (ATRP) ............. 127 3.2.2 Synthesis of azide initiator............................................................. 128 3.2.3 Synthesis of α-azido polystyrenes................................................. 131

3.3 Synthesis of block copolymers P3HT-block-PS and PS-block-P3HT-block-PS by “Click” chemistry.................................................... 133

3.3.1 History and principle of “click” chemistry....................................... 133 3.3.2 Synthesis of copolymers P3HT-b-PS and PS-b-P3HT-b-PS......... 134 3.3.3.1 Triblock copolymers PS-b-P3HT-b-PS........................... 134 3.3.3.2 Diblock copolymers P3HT-b-PS..................................... 138

3.4 Synthesis of donor-acceptor and acceptor-donor-acceptor block copolymers P3HT-block-PS-C60 and C60-PS-block-P3HT-block -PS-C60............................................................................................................. 143

3.4.1 Grafting of fullerene by atom transfer radical addition (ATRA) ..... 143 3.4.2 Synthesis of P3HT-b-PS-C60 and C60-PS-b-P3HT-b-PS-C60......... 143

3.5 Synthesis and characterization of block copolymers P4VP-block-P3HT-block- P4VP.................................................................................... 147

3.5.1 Synthesis of α,ω-difunctionalised-P3HT by GRIM polymerisation 148 3.5.2 Synthesis of triblock copolymer P4VP-block-P3HT-block-P4VP

by anionic polymerisation.............................................................. 154 3.5.2.1 Introduction to anionic polymerisation............................ 154 3.5.2.2 A short history of anionic polymerisation........................ 155 3.5.2.3 Synthesis of P4VP-b-P3HT-b-P4VP............................... 156

3.6 Physical characterisation di- and triblock copolymers....................... 158 3.6.1 P3HT-b-PS and PS-b-P3HT-b-PS block copolymers with and

without C60 chain-ends.................................................................. 158 3.6.2 P4VP-b-P3HT-b-P4VP block copolymers..................................... 163


125

3.1 Introduction Rod-coil block copolymers are well-known in their ability to self-assemble

into well-ordered nanoscopic morphologies that can be tuned in size and

shape by varying the molecular weight and size of the individual blocks.

Block copolymers (BCPs) containing electron-donor and electron-acceptor

are of particular interest for their application in photovoltaic cells because

the exciton distance exactly coincides well with the typical size of block

copolymer domains (see Section 1.4.4). The solar cells efficiency does not

only depend on the charge conduction but also on the efficiency of exciton

dissociation that is mainly related to the amount of electron donor-electron

acceptor interfaces. The synthesis of block copolymers containing both

electron donor-electron acceptor moieties is therefore necessary and also

to retain their self-assemble behaviour of BCPs by incorporating fullerenes

into their insulating part of BCPs which is challenging now. So our aim is

to design donor-acceptor block copolymers to exploit the coincidence in

dimensions between the formation of domains and exciton mean pathways

in polymer photovoltaic cells and also to use these block copolymers as

compatibilizers and stabilizers in the active layer of photovoltaic cells.

Here we have used two different approaches to obtain donor-

acceptor block copolymers in which acceptor domain fullerene (C60) is

covalently attached by grafting to the insulating block polystyrene (PS)

and/or weak supramolecular interactions produced by complex formation

between insulating block poly(4-vinylpyridine) (P4VP) and fullerene

derivative (PCBM).

This section describes the synthesis of donor-acceptor rod-coil

block copolymers in which rod block is poly(3-hexylthiophene) (P3HT) and

the coil block polystyrene (PS) or poly(4-vinylpyridine) (P4VP) for their

application in photovoltaics. The di- and tri-block copolymers P3HT-b-PS

and PS-b-P3HT-b-PS were synthesized by 1,3-dipolar Huisgen addition,

known as "click" chemistry from alkyne functionalized P3HT and azide

functionalized PS which was described by our group in 2008.1 This

method, developed since the early 2000s, has many advantages and is an

126

innovation in the field of organic electronics. The fullerene (C60) was then

grafted onto these block copolymers by atom transfer radical addition

(ATRA) to obtain the donor-acceptor copolymers. The other tri block

copolymers of ABA coil-rod-coil, P4VP-b-P3HT-b-P4VP in which rod block

is poly(3-hexylthiophene) (P3HT) and the coil block is poly(4-vinylpyridine)

(P4VP) were synthesized by anionic polymerization from quenching of

living P4VP chains with P3HT di-functionalized aldehyde. All these BCPs

are schematically represented in Figure 3.1.

C60

a

b

c

d

P3HTPS

P3HT

P3HT

P3HT

PS or P4VP PS or P4VP

PS

PSPS

C60

C60

Figure 3.1. Schematic representation of rod-coil block copolymers (a) P3HT-b-PS (b) PS-b-P3HT-b-PS or P4VP-b-P3HT-b-P4VP (c) P3HT-b-PS-C60 (d) C60-PS-b-P3HT-b-PS-C60.

These copolymers may have a great interest in the field of organic

photovoltaics by their ability to organize themselves. This structuring of the

active layer can achieve favorable morphologies at different physical

processes taking place within the organic solar cell (OSC), and thus

increase the solar cell performances. To our credit, we have obtained good

improvement in the photoconversion efficiency (PCE) as these block

copolymers used as compatibilizers instead of using as donor materials in

the blends of solar cell devices.

127

3.2 Synthesis of azide terminated-polystyrene 3.2.1 Principle of atom transfer radical polymerisation (ATRP) Azide terminated polystyrenes were synthesized by atom transfer radical

polymerization (ATRP) technique described in this paragraph. The name

atom transfer radical polymerization (ATRP) comes from the atom transfer

step, which is the key elementary reaction responsible for the uniform

growth of the polymeric chains. In a conventional radical polymerization,

the stages of initiation, propagation and chain termination occur at the

same time to prevent the growth of chains. Increasing the lifetime of a

propagating radical, however, reduced the likelihood of irreversible

termination to obtain well-defined polymers. The principle of controlled

radical polymerization (CRP) is based on a temporal deactivation of

macro-radical growth, so as to form dormant species in equilibrium with

active chains. In the case of a conventional radical polymerization, the

lifetime of a growing chain may be less than a second, it can reach several

hours in CRP.2

Atom transfer radical polymerization (ATRP) based on the

mechanism of Karasch addition3 was proposed simultaneously by

Matyjaszewski4 and Sawamoto5 in 1995. This method proceeds via

transfer of a halogen (eg Br or Cl) carried by a halogenated initiator (RX) to

a transition metal (eg copper) complexed by a ligand typically polyamine.

This complex will alternately capture and release the halogen atom leading

to redox equilibrium between the metal species (eg CuI and CuII). During

this exchange, the radical R• formed, reacts with the monomer M to give

the active species •RM radical which momentarily returns its "dormant"

halogenated (RMX). Growth intervenes between each cycle of reduction /

oxidation occured by the chain ends. The ligand of the metal / ligand

complex plays an essential role because it enables the solubilization of the

complex and makes the metal more easily oxidized by its donor character

(Figure 3.2).6 The polymers synthesized under these conditions can

achieve 100% conversion and molar masses well defined (DPn = [M]0 / [I]0)

with narrow distributions (1< Đ <1.3). Extensive studies have shown that

128

the various components of the system like the monomer, initiator, metal,

ligand, solvent and additives are the parameters that influence the control

of the polymerization.

R-Mm-X CuI / Lkact

kdeact

R-Mm X-CuII / L

kp

monomer

termination

kt

Mm+n

L - ligand X - halogen atom (Br or Cl)kact << kdeact

Figure 3.2 Mechanism of Transition-Metal-Catalyzed ATRP.6

Our objective here was to synthesize different molar masses of

polystyrene functionalized with azide (PS-N3) by ATRP through the use of

a specific initiator having an azide moiety. These polymers would then be

“clicked” with P3HTs to give the BCPs shown in Figure 3.1. The first step

was to obtain a polymerization initiator bearing the azide group, and

second to perform the polymerization of styrene using this initiator under

standard ATRP conditions.

3.2.2 Synthesis of azide initiator The synthesis of the azide initiator (6) involves in two steps, as shown in

Scheme 3.1.

Br OH

NaN3, Bu4NI18 Crown-6

2-ButanoneReflux, 24h

N3 OHBr

O

Br

THF,TEA25 οC, 3h

N3 O Br

O

4 5 6

Scheme 3.1 Synthesis of ATRP initiator bearing an azido group.

The first step was to synthesize 3-azido-1-propanol (5) from 3-

bromo-propanol (4) by nucleophilic substitution. Several test reactions

were conducted by treating 4 with sodium azide (NaN3) and

tetrabutylammonium iodide (Bu4NI) in acetone, following a literature

procedure.7 However, these reactions leading to the expected product (5)

with very low yields of around 15%. Fernandez-Santana et al.8 have

129

reported by adding reagents a crown ether, the dicyclohexano-18-crown-6

in the presence of butanone gives better yields. Crown ethers are

compounds with specific properties of complexation of cations, which give

rise to the catalysis of nucleophilic substitution. They can make extremely

nucleophilic azide ion by trapping the cation Na+ in their cavity.9 This

procedure works very effectively and leads to 3-azido-1-propanol (5) with a

yield of around 83%. Figure 3.3 shows the 1H NMR spectrum of 5 that

corresponds perfectly to the expected structure.

Figure 3.3 1H NMR (400MHz, CDCl3) spectrum of 3-azido-1-propanol (5). The second step is the synthesis of ATRP initiator, 3-azidopropyl-2-

bromoisobutyrate (6) from 3-azido-1-propanol (5) (Scheme 3.1). This

reaction proceeds by esterification of an alcohol (5) with an acyl bromide,

(α-bromoisobutyryl bromide) in the presence of triethylamine (TEA),

according to a literature procedure.6 However, in this study, the authors

used the acyl bromide in excess of alcohol (1.5 equivalents). The first tests

carried out by following strictly the procedure led to a mixture of products 6

and acyl bromide, which are impossible to separate by column

chromatography due to the close nature of the two components. However,

these two compounds must be separated because the acyl bromide can

130

also initiate the polymerization of styrene, which would lead to non-

functionalized polystyrenes. Several eluents were tested, such as

dichloromethane or mixtures of toluene / ethyl acetate / hexane / ethyl

acetate at different ratios with little success, these methods leading to the

production of a few milligrams of the expected product.

However, by introducing 1.1 equivalents of acyl bromide compared

to 5, the reaction leads exclusively to the expected product 6, as shown by

the 1H NMR spectrum in Figure 3.4. This spectrum perfectly matches to

the expected structure both in chemical shift and peak integration. This

spectrum confirms the unique presence of 6, which overcomes the step of

separation by column chromatography leading to very long time and

obtaining little product. In addition, this synthesis occurs with the yield of

81% that is quite good to do several polymerization reactions of styrene by

varying its molar mass.

Figure 3.4 1H NMR (400MHz, CDCl3) spectrum of 3-azidopropyl-2-bromoisobutyrate (6).

131

3.2.3 Synthesis of α-azido-polystyrenes

Polystyrenes of different molecular weights were synthesized, in order to

study subsequently the influence of the length of coil insulating block on

the photovoltaic performance. This reaction involves the polymerization of

styrene using the initiator 6 (bearing the azide function) in the presence of

catalytic system CuBr/Bipyridine at 130 ºC (Scheme 3.2). The reaction was

stopped abruptly by lowering the temperature of the reaction at 0 ºC.

N3 O Br

O

6

Styrene

CuBr/2,2-Bipyridyl130 οC

N3 O

O

Brn

α-azido-PS

Scheme 3.2 Synthesis of Polystyrenes terminated with azide function.

Various chain length PSs were obtained by varying the

polymerization time. The polymerization times and the obtained molecular

weights (PS1 to PS6) are summarized in Table 3.1. The SEC calibration

was performed against polystyrene standards, and therefore in contrast to

P3HT, require no coefficient to obtain “real” values.

Polystyrene-N3

(PS) Polymerization

Time, min Mn (SEC, g mol-1)

Đ

PS1 12 2600 1.08

PS2 25 3800 1.17

PS3 10 1900 1.21

PS4 30 4500 1.30

PS5 11 2000 1.11

PS6 30 5200 1.29

Table 3.1 Characteristics of the synthesized α-N3-ω-bromo-polystyrenes (SEC in THF, UV-254 nm).

As expected, the polymerization time is used to vary the molecular

weight of polystyrene (Table 3.1). In this study, short reaction times were

chosen at low conversion rates and to avoid side reactions, but also to

generate low molecular weight PS (between 2000 g/mol and 5200 g/mol).

132

Indeed, given that the target rod-coil copolymers are intended for

use in electronics, the proportion of insulating PS in these macromolecules

should not be too high. It should be noted that the state of purification of

the copper bromide (CuBr) played an important role on the characteristics

of polymers. We have observed PS with increased dispersities when using

unpurified CuBr for the polymerizations (eg. PS1, PS3 and PS5 in Table

3.1).

Figure 3.5 shows the 1H NMR spectrum of PS2, characteristic of all

samples. The spectrum perfectly matches the expected structure. The

polymers were purified on alumina column and by precipitation in

methanol. This procedure allowed us to obtain relatively pure PS as shown

in the NMR spectrum of PS2. In addition, the presence of the chain-end,

as evidenced by the peak at 4.5 ppm (proton in Figure 3.5), can be

considered for subsequent fullerene grafting.

Infrared spectroscopy further confirmed the presence of the azide

function (N3), required for coupling reactions. It has a characteristic intense

signal (νN3) at 2100 cm-1. The IR spectrum of PS2 is shown in Figure 3.6,

which is also characteristic of all the synthesized PS.

Figure 3.5 1H NMR (400MHz, CDCl3) spectrum of PS2.

133

Figure 3.6 Representative IR spectrum of polystyrene PS2.

To conclude, this Section described the synthesis of azide-

terminated polystyrenes of varying molecular weights. The most difficult

step was involved in the synthesis of the ATRP initiator.

3.3 Synthesis of block copolymers P3HT-block-PS and PS-block-P3HT-block-PS by “Click” chemistry 3.3.1 History and principle of “click” chemistry Since Kolb, Finn, and Sharpless introduced “click” chemistry in

200110, there has been an extensive growth in this area of chemistry. The

term relates to the chemical reactions generating substances quickly and

simply by linking two different units.

Among these reactions, the most popular is the Huisgen cycloaddition,

which is a 1,3-dipolar addition between the azide function (N3: 1,3-dipole)

and alkyne function (triple bond: dipolarophile), leading to the formation of

a triazole ring. Copper catalysis can also exclusively obtain the 1,4-

disubstituted regioisomer as shown in Figure 3.7. This click reaction has

many advantages. In particular, it leads to pure products, requires very

simple reaction conditions, gives high yields, generates no toxic

byproducts and can be applied to many domains10, which are very

interesting points for a potential future industrialization of OSCs.

134

H

N NR'

N

R

+1 2

3

45

AlkyneDipolarphile

Azide1,3-Dipole

1

2

3NN

N

R

R'

H 45

1,4-Triazole

[Cu]

Figure 3.7 Mechanism of copper catalyzed Huisgen 1,3-dipolar cycloaddition. Since 2005, "click" chemistry has been extended to polymer science

and numerous studies have shown an interest in the synthesis of block

copolymers.7,11-13 However, very few reports in the synthesis of block

copolymers concern conjugated polymers.1a,14,15 For all the above reasons,

this click chemistry path was chosen for the synthesis of block copolymers

P3HT-b-PS and PS-b-P3HT-b-PS from a P3HT end functionalized alkyne

and an azide-terminated polystyrene respectively.

3.3.2 Synthesis of copolymers P3HT-b-PS and PS-b-P3HT-b-PS 3.3.2.1 Triblock copolymers PS-b-P3HT-b-PS We synthesized di- and triblock copolymers using "click" chemistry

between polystyrenes terminated with azides and P3HT-alkynes. For the

synthesis of triblock copolymers PS-b-P3HT-b-PS, we chose α,ω−

pentynylP3HT (P3 and P3a) reacted with different molecular weight

polystyrenes (PS1, PS2, PS3 and PS4, Table 3.2). Scheme 3.3 shows the

coupling reactions for P3 and P3a which were performed in THF at 40 °C,

using the catalytic system CuI/DBU. This work describes the first example

of "click" chemistry on conjugated polymers1a but the main difficulty was in

choosing the solvent system and catalyst/ligand. Most studies reported so

far for obtaining block copolymers by "click" chemistry used

dimethylformamide (DMF) as a solvent of choice for these cycloaddition

reactions.13 However, P3HT being not soluble in DMF, so the relatively

polar solvent THF was chosen as the reaction solvent to dissolve both the

P3HT and PS.1

135

S m

C6H13

P3 or P3a

+N3 O

O

Brn

CuI, DBU

THF, 40 oC

PS (2 eq.)

S

C6H13

m

N NN O Br

nNN

NOBrn

O O

PS1-b-P3-b-PS1

PS2-b-P3-b-PS2

PS3-b-P3a-b-PS3

PS4-b-P3a-b-PS4 Scheme 3.3 Synthesis of Triblock copolymers PS-b-P3HT-b-PS by Click chemistry.1a

α,ω-Dipentynyl-P3HT (P3 and P3a) was reacted with stoichiometric

amounts (2eq.) of PS1, PS2 and PS3, PS4 respectively according to the

synthesis shown schematically in Scheme 3.3, in order to obtain triblock

copolymers PS-b-P3HT-b-PS. The products obtained were characterized

by SEC, 1H NMR and infrared spectroscopy. Indeed, Figure 3.8 shows the

typical SEC chromatograms of homopolymers P3, PS1 and the triblock

copolymer PS1-b-P3-b-PS1. These chromatograms show an increase in

molecular weight for the final product, with a shift towards lower elution

time, confirmed by the Mn values in many of these species, collected in

Table 3.2. In addition, the chromatogram of the copolymer is monomodal

with small shoulder at high molecular weight region may be due to the

aggregation of P3HT and has a low dispersity of 1.21, demonstrating the

formation of a unique population of copolymer. In addition, similar curves

were obtained in the case of copolymerizations of P3 with the copolymers

PS2 and also P3a with the copolymers PS3 and PS4, confirming the

formation of triblock copolymers (PS2-b-P3-b-PS2, PS3-b-P3a-b-PS3 and

PS4-b-P3a-b-PS4) of varying both P3HT and PS-block lengths (Table 3.2).

The efficiency of "click" chemistry coupling reaction was also

confirmed by 1H NMR; the spectrum of copolymer PS1-b-P3-b-PS1 is

shown in Figure 3.9. This spectrum is typical of the four copolymers

obtained, and demonstrates the structure of expected triblock copolymer.

Indeed, all peaks corresponding to PS and P3HT are present in the

136

spectrum. In addition, the peak at 7.51 ppm can be attributed to the proton

of triazole ring (f), formed during the cycloaddition. The disappearance of

alkynyl proton at 3.49 ppm and peaks at 1.9 ppm, 2.3 ppm and 2.5 ppm,

corresponding to the protons of the pentynyl group, also confirmed the

formation of triblock copolymers.

α-Azido-PS α ,ω -PentynylP3HT PS-b-P3HT-b-PS PS Mn, SEC

(g/mol)

Đ P3HT Mn, SEC

(g/mol)

Đ Copolymer Mn, SEC

(g mol-1)

Đ

PS1 2 600 1.08 P3 8 000 1.1 PS1-b-P3-b-PS1 12 800 1.21

PS2 3 800 1.17 ‘’ “ ‘’ PS2-b-P3-b-PS2 13 200 1.37

PS3 1 900 1.21 P3a 6 200 1.1 PS3-b-P3a-b-PS3 9 800 1.20

PS4 4 500 1.30 ‘’ “ ‘’ PS4-b-P3a-b-PS4 10 900 1.30

Table 3.2 Molecular weight characteristics of synthesized homopolymers azido-PS, α,ω-pentynylP3HT and triblock copolymers PS-b-P3HT-b-PS (SEC in THF, PS as standards). Infrared spectroscopy further confirmed the formation of copolymers

by click chemistry, from P3 and P3a. Indeed, the signal at 2100 cm-1,

corresponding to the azide function present on the spectra of PS1 and the

mixture of P3+PS1 (Figure 3.10), disappears completely in the spectrum

for the click reaction product between P3 and PS1, indicating that the

efficiency of the reaction high. This feature is also well verified for the

products of reactions between P3, P3a and polystyrenes PS1, PS2, PS3,

and PS4, respectively.

Figure 3.8 Normalised overlay SECs (THF, UV-254 nm) of homopolymers azido-PS1, α,ω-pentynylP3HT (P3) and triblock copolymer PS1-b-P3-b-PS1.

137

Figure 3.9 Representative 1H NMR (400 MHz, CDCl3) spectrum of triblock copolymer PS1-b-P3-b-PS1.

Figure 3.10 Overlayed IR spectra of homopolymers azido-PS1; mixture of azido-PS1+α,ω-PentynylP3HT (P3) and click product of triblock copolymer PS1-b-P3-b-PS1.

138

3.3.2.2 Diblock copolymers P3HT-b-PS We are also interested to study the influence of the chain ends of P3HT, in

this instance the alkyne function to see the effect of conjugation between

thiophene unit and ethynyl functional group. In the case of ethynyl-P3HT

(P2), the proximity of the P3HT to the ethynyl group reduced its reactivity

towards the azide group, because of its involvement in the conjugation of

the P3HT chain. In the case of α,ω-PentynylP3HT (P3), the alkyne function

is not influenced by the electronic conjugation of the P3HT because of the

alkyl spacer between P3HT and alkynyl functional groups, and should be

able to react freely with the azide terminated polystyrenes.

Hence cycloaddition reactions from ω-ethynyl-P3HT (P2) did not

work, as evidenced by the SEC and infrared spectroscopy. Figure 3.11 (a)

shows the typical chromatograms SEC of P2, PS2 and the product of the

reaction between these two homopolymers under initial “click” reaction

conditions. The chromatogram of the final compound is a combination of

the two chromatograms starting homopolymers (P2 and PS1); it is strictly

identical to that of the P2, with the same shoulder due to the aggregation

of P3HT. These test reactions therefore showed that the SEC product is a

mixture of two homopolymers, and therefore that the copolymerization has

not worked successfully by initial "click" reaction conditions.

This result was also further confirmed by infrared spectroscopy. The

IR spectrum of reaction product between P2 and PS2 (Figure 3.12) has a

signal at 2100 cm-1, indicating the presence of the azide function of the

homopolymer PS2, which confirms the failure of the reaction. For these

reactions, two different catalyst systems/ligand were used namely

CuI/DBU, identical reactions which were successful by "click" chemistry

from P3, but also CuBr/PMDETA, very commonly used in such coupling

reactions.13

139

Figure 3.11 Normalised overlay SECs (THF, UV-254 nm) of (a) homopolymers azido-PS2, ω-EthynylP3HT (P2) and click reaction product of P2 and PS2; (b) homopolymers azido-PS2, ω-ethynyl-P3HT (P2) and diblock copolymer P2-b-PS2; and (c) close view for overlay of ω-ethynyl-P3HT (P2) and diblock copolymer P2-b-PS2.

Figure 3.12 Overlayed IR spectra of homopolymers azido-PS2; mixture of azido-PS2 and ω-ethynyl-P3HT (P2) and click product of diblock copolymer P2-b-PS2.

140

Furthermore, P2 and P3 with similar characteristics (regioregularity

and dispersity) are very similar, only the chain ends are responsible for the

success or failure of the reaction. In case of P3, alkyne function, separated

from the conjugated polymer by a short alkyl chain, is not constrained and

reacts easily with the azide of polystyrene. Regarding P2, the alkyne

function is linked directly to the conjugated chain in position 2 and

necessarily involved the conjugation of P3HT, and therefore the

delocalization of electrons from the alkyne on the P3HT chain reaction

seems to prevent "click" reactions to take place.

More recently, Benanti et al.,14 and Tao et al.,15 have shown that it

is possible to obtain copolymers from ethynylP3HT by varying “click”

chemistry reaction conditions i.e., reaction temperature, Cu catalyst/ligand

with sonication to enhance the solubility of ethynylP3HT and Jatsch et

al.,16 showed that it was also possible to perform "click" chemistry

reactions on ethynyl-oligothiophenes.

The inspiration from the literature mentioned above gave a way to

synthesize diblock copolymers P3HT-b-PS successfully from ethynylP3HT

by varying the click reaction conditions. We have finally optimized reaction

conditions using CuI and Hunig’s base diisopropylethylamine [(i-pr)2NEt,

DIEA or DIPEA] in THF at 50 °C and also with the help of sonication to aid

ethynyl-P3HT dissolution in THF. So we have chosen ω-ethynyl-P3HT of

different molecular weights, P2 and P2a reacted with polystyrene PS2

(Table 3.3). Finally, two different rod-coil diblock copolymers have been

synthesized from P2 and P2a, namely P2-b-PS2 and P2a-b-PS2, as in

Scheme 3.4. This study represents the modification of our literature

procedure1, which is the first example of synthesis of exclusively rod-coil

diblock copolymers, by "click" chemistry and shows the effectiveness of

this type of reaction.

141

SBr/H m

C6H13

+N3 O

O

Brn

PS2P2 or P2a

CuI, DIEA

THF, 50 oCSonication

SBr/H

C6H13

m

N NN

O

O

Brn

P2-b-PS2

P2a-b-PS2

Scheme 3.4 Synthesis of Diblock copolymers P3HT-b-PS by Click chemistry.

The diblock copolymers obtained were characterized by SEC, 1H

NMR and infrared spectroscopy. Indeed, Figure 3.11 (b) and (c) shows the

SEC chromatograms of homo polymers P2, PS2 and the diblock

copolymer P2-b-PS2. These chromatograms showed little increase of

molar mass for the final product, with a slight shift towards lower elution

time, confirmed by the values of molar masses collected in Table 3.3. In

addition, the chromatogram of the copolymer is monomodal and has a low

dispersity of 1.27, which would tend to indicate the formation of a unique

population of copolymer. In addition, similar curves were obtained in the

case of copolymerizations of P2a with the copolymers PS2, confirming the

formation of diblock copolymer (P2a-b-PS2) whose length of P3HT block

varied (Table 3.3).

The reaction efficiency of coupling "click" chemistry can also be

confirmed by 1H NMR, the spectrum of copolymer P2-b-PS2 is shown in

Figure 3.13. This typical spectrum demonstrates the expected structure of

diblock copolymer. Indeed, all peaks corresponding to PS and P3HT are

present in the spectrum. In addition, the formation of diblock copolymers

was further confirmed by the disappearance of alkynyl proton at 3.52 ppm

and appearance of the new peak at 7.51 ppm that corresponds to the

proton of triazole ring (f) formed during the cycloaddition.

α-Azido-PS ω-EthynylP3HT P3HT-b-PS PS Mn, SEC

(g/mol)

Đ P3HT Mn, SEC

(g/mol)

Đ Copolymer Mn, SEC

(g/mol)

Đ

PS2 3 800 1.17 P2 14 000 1.1 P2-b-PS2 14 900 1.27

PS2 3 800 1.17 P2a 9 000 1.2 P2a-b-PS2 10 200 1.36

Table 3.3 Characteristics of synthesized homopolymers azido-PS, ω-ethynyl-P3HT and diblock copolymers P3HT-b-PS (SEC in THF, PS as standards).

142

Figure 3.13 Representative 1H NMR (400 MHz, CDCl3) spectrum of diblock copolymer P2-b-PS2.

Infrared spectroscopy further confirmed the formation of diblock

copolymers by click chemistry, from P2 and P2a. Indeed, the signal at

2100 cm-1, corresponding to the azide function and present on the spectra

of PS2 and the mixture P2/PS2 (Figure 3.12), disappears completely in the

spectrum of click reaction product between P2 and PS2, which means the

complete disappearance of the azide function and therefore “click”

chemistry worked very good by varying the reaction conditions. This

feature is also well verified for the products of reactions between P2a and

polystyrene PS2.

All detailed experimental procedures for the synthesis of di-and tri-block

copolymers are described in the experimental section (see chapter-5).

143

3.4 Synthesis of donor-acceptor and acceptor-donor- acceptor block copolymers P3HT-block-PS-C60 and C60-PS-block-P3HT-block-PS-C60

In this part, our objective was to take the advantage of a bromine end

group in the previously described block copolymers (P3HT-b-PS-Br and

Br-PS-b-P3HT-b-PS-Br) to attach C60 moiety by ATRA for synthesizing

P3HT donor based block copolymers attached with C60 acceptor moieties.

3.4.1 Grafting of fullerene by atom transfer radical addition (ATRA) Many studies have reported the grafting of polymer chains on the fullerene,

by different synthetic routes.17 Only few reports concerned the

incorporation of C60 at the chain end of conjugated polymers. In particular,

Gu et al.18 have synthesized oligomers terminated by C60 through

cycloaddition reactions with N-methylglycine targeting photovoltaic

applications.

In the early 2000s, the research groups of Li19,20 and Mathis21,22

simultaneously proposed the grafting polymer radicals onto C60. Indeed

chains of PS-Br, converted into macro-radicals by the reaction of the

system CuBr/bipyridine, are added to the fullerene by the atom transfer

radical addition mechanism (ATRA) (similar to ATRP, but involving the

intra-C60 transfer of a radical site). So this synthetic route was chosen for

the grafting of synthesized di-and tri-block copolymers to the fullerene by

the reaction of macro-radicals (via the PS block) on the C60.

3.4.2 Synthesis of P3HT-b-PS-C60 and C60-PS-b-P3HT-b-PS-C60

The ATRA of C60 was achieved onto the diblock copolymers P2-b-PS2,

P2a-b-PS2 and triblock copolymers PS1-b-P3-b-PS1, PS2-b-P3-b-PS2

respectively, syntheses described in the prior section. The C60 attached

copolymers are synthesized according to a procedure reported by Zhou et

al.19, shown in Scheme 3.5. The block copolymers P3HT-b-PS and PS-b-

P3HT-b-PS are treated with C60 and the system CuBr/bipyridine in

chlorobenzene used as solvent. According to the reported procedure, it

144

enables access to mono-addition of C60 on copolymers. In addition, the C60

is used in all these reactions with large excess compared to the copolymer

to promote this mono addition. After mono- and di-addition of C60 to block

copolymers, there was slight increment in the molecular weights of these

block copolymers with retained good dispersity (Table 3.4) which indicated

the efficiency of the reaction. The synthesized C60-attached di-and tri-

block copolymers were characterized by 1H NMR, SEC and DSC.

S

C6H13

mN N

NO

O

n

S

C6H13

m

N NN O

nNN

NOn

O O

P2-b-PS2-C60

P2a-b-PS2-C60

C60-PS1-b-P3-b-PS1-C60


S

C6H13

m

N NN

O

O

Brn

P2-b-PS2-Br

P2a-b-PS2-Br

S

C6H13

m

N NN O Br

nNN

NOBrn

O O

Br-PS1-b-P3-b-PS1-Br


CuBr/Bipyridine

Chlorobenzene, 110 oC

CuBr/Bipyridine

Chlorobenzene, 110 oC

Scheme 3.5 Synthesis of C60-attached di-and tri-block copolymers by ATRA.

Block copolymer Mn, SEC

(g/mol)

Đ C60-attached block

copolymer

Mn, SEC

(g/mol)

Đ

P2-b-PS2-Br 14 900 1.27 P2-b-PS2-C60 15 400 1.43

P2a-b-PS2-Br 10 200 1.36 P2a-b-PS2-C60 11 000 1.38

Br-PS1-b-P3-b-PS1-Br 12 800 1.21 C60-PS1-b-P3-b-PS1-C60 13 000 1.26

Br-PS2-b-P3-b-PS2-Br 13 200 1.37 C60-PS2-b-P3-b-PS2-C60 14 000 1.37

Table 3.4 Characteristics of di- and triblock copolymers, C60-attached di and tri block copolymer (SEC in THF, UV-254 nm, PS as standards). Figure 3.14 shows the UV-visible absorption spectrum of product of

the ATRA reaction between PS2-b-P3-b-PS2 and C60. This spectrum

consists of a large contribution with maximum absorption around 480 nm,

which corresponds to the P3HT segment, and two peaks at 210 nm and

330 nm, due to fullerene, and absorptions below 220 nm and at 258 nm,

145

corresponding to the PS block. The presence of the characteristic peaks of

fullerene confirms the grafting of the polymer as residual C60 was removed

by precipitation in THF, and then filtered off by repeated passing through

an alumina column (C60 is not soluble in THF). The addition of fullerene to

the triblock copolymer PS2-b-P3-b-PS2 was also confirmed on the

absorption spectrum by the contribution at 330 nm due to reacted C60.19

The reaction was carried out in chlorobenzene with a large excess of C60

compared to the copolymers produced expected mono-C60 diblock

copolymers and di-C60 triblock copolymers. The representative SECs of

PS2-b-P3-b-PS2 and its ATRA product, show unimodal distributions with a

single population in the reaction of C60 with PS2-b-P3-b-PS2 (Figure 3.15).

The representative 1H NMR spectra of mono-C60 diblock copolymers and

di-C60 triblock copolymers are shown in Figure 3.16.

Figure 3.14 UV-visible absorption spectrum (film) of C60-PS2-b-P3-b-PS2-C60 (the product of ATRA reaction between triblock copolymer PS2-b-P3-b-PS2 and C60).

Figure 3.15 Overlayed SECs of C60-PS2-b-P3-b-PS2-C60 and triblock copolymer PS2-b-P3-b-PS2 (THF, UV-254 nm, PS as standards).

146

(a)

(b)

Figure 3.16 Representative 1H NMR (400 MHz, CDCl3) spectra of (a) P2-b-PS2-C60 and (b) C60-PS2-b-P3-b-PS2-C60.

147

Two types of C60-functionalized block copolymers have thus been

obtained as shown in Figure 3.17. These block copolymers containing a

rod-coil and coil-rod-coil both with an acceptor (C60) and an electron donor

(P3HT) were examined for organic photovoltaic devices as compatibilizers

in P3HT/PCBM blends (see Chapter 4).

C60

P3HT

P3HT

PS

PSPS

C60

C60

Figure 3.17 Shematic representation of donor-acceptor copolymer P3HT-b-PS-C60 and acceptor-donor-acceptor copolymer C60-PS-b-P3HT-b-PS-C60.

3.5 Synthesis and characterization of block copolymers

P4VP-block-P3HT-block-P4VP This section describes facile and efficient synthesis of well-defined ABA

triblock coil-rod-coil copolymers, P4VP-b-P3HT-b-P4VP in which the rod

block is P3HT and the coil block is P4VP. These copolymers are

schematically represented in Figure 3.18. These copolymers were

synthesized by anionic polymerisation from quenching of living P4VP

chains with P3HT di-functionalized aldehyde and the coupling was very

effective due to the higher electrophilicity of the carbon in aldehyde group

of P3HT. This route has been adapted from reported literature for the

synthesis of a PPV-based block copolymer.23 Since poly(4-vinylpyridine)

(P4VP) is capable of complexing with a C60 charge transfer between the

nitrogen of the pyridine and the electrophilic fullerene,24 these copolymers

are of particular interest to test their potentiality in photovoltaics application

due to supramolecular interactions between P4VP and PCBM (Figure

3.18) in the active layer of solar cell devices. We have obtained flexible

electron acceptor sequence novel copolymers P4VP-b-P3HT-b-P4VP.

148

Figure 3.18 Schematic representation of coil-rod-coil block copolymers, P4VP-b-P3HT-b-P4VP having supramolecular interaction with PCBM in a solar cell device.

3.5.1 Synthesis of α ,ω -difunctionalised-P3HT by GRIM polymerisation Here we describe the synthesis of three different α,ω-difunctionalised-

P3HTs by the GRIM method (Scheme 3.6).25 To synthesize α,ω-

difunctionalised P3HTs, monomer (M1) was dissolved in THF and stirred 5

min under nitrogen. tert-Butylmagnesium chloride was added, and the

mixture was stirred at room temperature for 2.5 h. The mixture was then

diluted with THF, Ni(dppp)Cl2 was added, and the mixture allowed to stir

for 30-120 min at room temperature depending upon the functional group

(Table 3.5). The termination of the polymers with the respective Grignard

functionalization agent was carried out in a one-shot addition using 70-90

mol % with respect to the monomer. In all the cases, the mixture was

stirred for an additional 30-60 min and then poured into methanol to

precipitate the polymer. After precipitating in methanol, P3HTs were

purified by soxhlet extraction with methanol, acetone, pentane respectively

and finally pure polymers were extracted with THF. All the functionalised

polymers (Table 3.5) were characterized by SEC and 1H NMR.

The deprotection of polymers was performed as follows. In the case

of α,ω-diphenylformyl-P3HT (P4 and P4a), the Grignard reagent was 4-

(1,3-Dioxan-2-ylphenyl) magnesium bromide and the deprotection was

done by overnight refluxing the polymer in THF with pyridinium p-

toluenesulfonate (PTS). In the case of α,ω-diethylformyl-P3HT (P4b), the

Grignard reagent was 4-(1,3-dioxan-2-ylethyl) magnesium bromide and the

deprotection was not successful with PTS under same conditions used for

149

P4. It was successful, however, with concentrated HCl and overnight

refluxing in THF. For the synthesis of α,ω-diphenylhydroxy-P3HT (P5),

the Grignard reagent was 4-(2-tetrahydro-2H-pyranoxy)-phenyl

magnesium bromide and the complete deprotection was observed by

refluxing the polymer with concentrated HCl in THF for 18 h. All the

deprotected α,ω-difunctionalised P3HTs were again purified by soxhlet

with methanol and extracted with chloroform and characterised by SEC, 1H

NMR, DSC and MALDI-TOF techniques.

S

C6H13

Br Br


THF, R.T.

S

C6H13

n

M1

O

OBrMg

OBrMg O

3.

4. PTS, THF, reflux CHOOHC

S

C6H13

n OHHO

S

C6H13

nCHOOHC

3.

4. 5M HCl, THF, reflux

O

OBrMg3.

4. 5M HCl, THF, reflux

P4, P4a

P4b

P5 Scheme 3.6 Synthesis of α,ω-difunctionalised P3HTs [α ,ω-diphenylformyl-P3HT (P4 and P4a), α ,ω-diethylformyl-P3HT (P4b) and α ,ω-diphenylhydroxy-P3HT (P5)] by GRIM method.25

α ,ω -Difunctionalised P3HT

P3HT

Monomer 2,5-dibromo-3-

hexylthiophene (M1 g)

Grignard

reagent tBuMgCl

(mL) 1M sol.

Ni(dppp)Cl2

mol %

Poly time

min

Grignard reagent used for

functionalisation

Mn (g mol-1)

Đ

RR

%

P4 3.31 10.1 1.70 40 4-(1,3-Dioxan-2-

ylphenyl)-MgBr/PTS 7 000 1.1 98

P4a 3.22 9.8 1.15 120 4-(1,3-Dioxan-2-

ylphenyl)-MgBr/PTS 14 000 1.1 98

P4b 3.22 9.9 2.20 30 4-(1,3-Dioxan-2-

ylethyl)-MgBr/HCl 5 800 1.1 98

P5 3.0 9.2 1.35 120 4-(2-tetrahydro-2H-

pyranoxy)-phenyl

MgBr/HCl

11 000 1.2 95

Table 3.5 Reaction conditions, and molecular weight characteristics (GPC, THF, UV-254 nm) and regiorgularities (NMR) of α ,ω-difunctionalised-P3HTs.

150

In all the cases, 1H NMR and MALDI-TOF techniques were used to

find out the chain end-functionalisation of all the polymers after the

deprotection step. Representative 1H NMR spectra of P4 and P4b, which

correspond to ethyl- and phenyl aldehyde-difunctionalized-P3HT

respectively are shown in Figure 3.19. Figure 3.19 (a), shows the 1H NMR

spectrum of P4 and confirms the expected structure. Indeed, this is a

typical spectrum of regioregular P3HT with three additional peaks at 7.69

ppm, 7.93 ppm and 10.05 ppm corresponding to end-groups phenyl and

aldehyde protons respectively. This was also further confirmed by MALDI-

TOF mass characterisation [Figure 3.20 (a)] that showed the major

population corresponding to aldehyde di-functionalised P3HT. 1H NMR

spectrum of polymer P4b [Figure 3.19 (b)] showed all the peaks

correponding to P3HT and also three additional peaks around 2.5 ppm, 3.1

ppm and 9.85 ppm which corresponds to ethyl-CHO chain end functional

group, again confirming the expected structure. According to MALDI-TOF

mass spectrum in the case of P4b, the polymer product was a mixture of

mono, di-protected aldehyde-P3HT and deprotected aldehyde-P3HT.

Figure 3.21 shows the 1H NMR spectrum of regioregular α,ω-

diphenylhydroxy-P3HT (P5) which is a typical spectrum of regioregular

P3HT with three additional peaks at 6.89 ppm, 7.35 ppm and 7.52 ppm

corresponding to the chain-end groups with phenyl and hydroxy protons (-

OH) respectively and confirming the difunctionalised polymer. The MALDI-

TOF mass spectrum of this polymer, P5 was not clear due to its high

molecular weight. However, the deprotection step was effective in all the

above cases as confirmed by the 1H NMR spectra, which showed the

disappearance of peaks correponding to protective groups.

151

(a)

(b)

Figure 3.19 1H NMR (400 MHz, CDCl3) spectra of: (a) α,ω-diphenylformyl-P3HT (P4); and (b) α,ω-diethylformyl-P3HT (P4b).

152

(a)

(b)

Figure 3.20 MALDI-TOF mass spectra of: ((a) α,ω-diphenylformyl-P3HT (P4); and (b) α,ω-diethylformyl-P3HT (P4b).

153

Figure 3.21 1H NMR (400MHz, CDCl3) spectrum of α,ω-diphenylhydroxy-P3HT (P5).

We thus successfully synthesized three regioregular P3HTs with

different functional groups by GRIM method. We have chosen α,ω-

diphenylformyl-P3HT (P4) instead of α,ω-diethylformyl-P3HT (P4b) to

synthesize the copolymers P4VP-block-P3HT-block-P4VP by anionic

polymerization since P3HT (P4b) was not completely functionalized

according to MALD-TOF mass analyses; and the also phenylaldehyde of

P4 is more electrophilic towards ‘living” anionic centres than the ethyl-

aldehyde of P4b. Another polymer, α,ω-diphenylhydroxy-P3HT (P5) was

used to synthesize donor-acceptor multiblock copolymers incorporating

fullerene backbone repeat units in collaboration with my colleague Dr.

Roger C Hiorns in our lab. The photovoltaic characterizations of these

multiblock copolymers as compatibilizers in the active layer of P3HT-blend-

PCBM based devices are in progress.

154

3.5.2 Synthesis of triblock copolymer P4VP-block-P3HT-block- P4VP by anionic polymerisation 3.5.2.1 Introduction to Anionic polymerisation Anionic polymerization is one of the most common controlled

polymerization techniques for coil-like polymers, and is generally termed

”living”. The definition of “living” is "polymers that retain their ability to

propagate for a long time and grow to a desired maximum size while their

degree of termination or chain transfer is still negligible".26 In order to use

living anionic polymerizations, an unsaturated bond (such as a vinyl group

or cyclic structure) must be present in the monomer. These

polymerizations proceed via organometallic sites i.e. carbanions with

metallic counterions.27 The C-M+ ion pair (where C- is the propagating

carbanion and M+ is the metal gegen ion) is a reactive centre (Scheme

3.7). The carbanion is extremely sensitive and contact with water or air or

impurities will arrest the polymerisation. Polymerisations are normally

conducted in a dry solvent under a high vacuum or a dry inert gas. The

“living” polymer is coloured, varying from a deep blood red for those of α-

methylstyrene to a light orange for those of dienes.

CR

R'CH2 M C

R

R'CH2 C

R

R'CH2 C

R

R'CH2 M

Scheme 3.7 A typical propagation of the anionic polymerisation.

Polymers generated in this manner have finely tuned molecular

weights, very narrow molecular weight distributions, and can be used to

produce block copolymers and end-functionalized polymers. These

reactions are defined such that irreversible termination and chain transfer

are not present. Flory advanced the idea of the probability of growth of

polymer chains, and stipulated that without termination, the molecular

weight of a polymer should approach that of the 'Poisson' distribution,

shown in equation 3.1.28

Mw/Mn = Đ ≅ 1+1/N Eq 3.1

155

where Mw is the weight-average molecular weight, Mn is the number-

average molecular weight, and N is the number of repeat units in the

polymer. Therefore, the molecular weight distribution approaches unity as

the molecular weight of the polymer increases. For example, a modest

molecular weight polymer with N = 1000 has Đ = 1.001.

3.5.2.2 A short history of anionic polymerisation The interest in anionic polymerization started in early 1910, when Matthew

and Strange polymerised isoprene using an alkali metal in a

heterogeneous reaction and observed a viscous product. However, the

mechanism of the reaction was not understood at that time.29 Later on,

Ziegler et al. in their series of publications suggested that the addition of

two sodium atoms to the unsaturated double bonds of diene producing two

carbon–sodium linkages and in 1934 Ziegler proposed a mechanism which

is now accepted through propagation via insertion of monomer into the

carbon–sodium linkage.30 The nature of the linkage between carbon and

metal was not understood clearly at that time, though it was believed that it

is a covalent bond.

Michael Szwarc who first demonstrated the anionic polymerization

of styrene using sodium naphthalenide in tetrahydrofuran (THF).31,32 He

suggested that the initiation occurs via electron transfer from the sodium

naphthalenide radical anion to styrene monomer. A new styryl radical

anion forms upon addition of an electron from the sodium naphthalenide

and subsequent rapid dimerization yielding dimeric-dicarbanion (Scheme

3.8), which starts the propagation of styrene. Michael Szwarc also

characterized the living behavior of the polymerization as ‘‘living

polymerization’’ and called the polymers as ‘‘living polymers’’.32 Here, the

term ‘living’ refers to the ability of the chain-ends of these polymers

retaining their reactivity for a sufficient time enabling continued propagation

without termination and transfer reactions. Although several reports of

anionic polymerization of vinyl monomers were available in the literature,

Szwarc’s first report of living anionic polymerization of styrene free from

156

termination and transfer reactions in THF marks the beginning of lively

research activities in this field.31,32

Na +CH CH2 THF +

HC CH2

Na

CH-CH2-CH2-CH NaNa

CH-CH2-CH-CH2-CH2-CH-CH2-CHn-2 n-2

NaNaCH CH2n

THF/-78 oC

Scheme 3.8 Anionic polymerization of styrene using sodium naphthalenide as initiator in THF.33 3.5.2.3 Synthesis of P4VP-b-P3HT-b-P4VP We have synthesized block copolymers P4VP-b-P3HT-b-P4VP by an

anionic convergent route by termination with di-functionalized P3HT

(Section 3.5.1). P4VP was synthesized by anionic polymerization of 4-

vinylpyridine using an initiator (1) obtained by reaction of sec-butyllithium

on α-methylstyrene (Scheme 3.9). This initiator (1) was chosen over sec-

butyllithium for two reasons - its steric hindrance associated with the

ternary character of the carbanion formed to ensure that the anionic

polymerization takes place on the vinyl groups of vinyl-pyridine without

secondary reaction on the aromatic rings and - its color can show the

presence of the anions in the reactive medium.

The anionic polymerization of 4-vinylpyridine was stopped by the

addition of well-defined aldehyde di-functionalised-P3HT (P4), synthesized

by GRIM method as a quencher in the reaction medium (Scheme 3.9).

Thus the anions of the growing P4VP reacted by nucleophilic substitution

on the terminal aldehydes to form the corresponding copolymer with the

alcohol functions on both sides. To obtain pure copolymers instead of

undesired chain deactivation during quenching, we used an excess of

P4VP living chains (10 eq.) with respect to the P3HT “quencher” (P4). The

excess of P4VP homopolymer was then removed by washing the organic

phase several times (at least 3 times) with acidic water (pH = 4) in which

the P4VP was protonated and soluble whereas the block copolymer was

157

not. The efficiency of quenching was easily determined by 1H NMR since

the aldehyde peak of homo P3HT (P4) at 10.05 ppm disappears

completely in the final purified copolymers. We synthesized three

copolymers of varying P4VP molecular weights. The molecular weights of

copolymers were not determined by SEC in THF and DMF also due to the

strong aggregations of P4VP in solvents used in SEC. The copolymers

were characterized by NMR only. With the integrating ratio of P3HT H1

(6.98 ppm) and P4VP H2 (6.42 ppm) in 1H NMR, we estimated the

molecular weights of P4VP in the copolymers. The results of these

copolymers are summarized in Table 3.6 and the representative 1H NMR

spectrum of copolymers, P4VP-b-P3HT-b-P4VP (3) was shown in Figure

3.22.

Sec-BuLi

-78 oC, THF

C- Li+ N C-

N N

m

HLi+

-78 oC, THF(+HMTP)

S PhCHOOHCPh

C6H13

n

N

m

OH

N

m

OHS

C6H13

n

-78 oC to RT overnight

P4

P4VP-b-P4-b-P4VP

1 2

Scheme 3.9 Synthesis of copolymers P4VP-b-P3HT-b-P4VP by anionic polymerization.

Polymer Mn

(g/mol)*

Mn (g/mol)*

P3HT

Mn (g/mol)*

P4VP P3HT (P4) 4 200 4 200 0

Copolymer

P4VP-b-P4-b-P4VP (1) 6 800 4 200 2 600

P4VP-b-P4-b-P4VP (2) 7 200 4 200 3 000

P4VP-b-P4-b-P4VP (3) 9 400 4 200 5 200 Table 3.6 Characteristics of copolymers P4VP-b-P3HT-b-P4VP (*molecular weights determined by 1H NMR).

158

We have synthesized three different molecular weights copolymers,

P4VP-b-P3HT-b-P4VP by varying the P4VP chain length using convergent

anionic polymerization. The photovoltaic studies of these copolymers will

be described in the chapter-4 in which we used these copolymers as

donors and also as surfactants in the P3HT/PCBM blends.

Figure 3.22 Representative 1H NMR (400 MHz, CDCl3) spectrum of copolymer, P4VP-b-P3HT-b-P4VP (3). 3.6 Physical characterisation di- and triblock copolymers This section describes the characterization of synthesized copolymers by

UV-visible absorption spectroscopy and differential scanning calorimetry

(DSC). These techniques help to determine the physical properties of

these synthesized materials and thus attempt to assess their potential

application in organic photovoltaics.

3.6.1 P3HT-b-PS and PS-b-P3HT-b-PS block copolymers with and without C60 chain-ends Figure 3.23 shows the overlayed UV-visible absorption spectra of rod-coil

di-and coil-rod-coil tri-block copolymers P2-b-PS2, PS2-b-P3-b-PS2 and

and their corresponding C60-attached block copolymers P2-b-PS2-C60, C60-

PS2-b-P3-b-PS2-C60 whose syntheses is described in previous section.

159

The absorption spectra of di-and tri-block copolymers, identical to each

other, consist of two regions of characteristic absorptions, that of P3HT,

between 330 nm and 660 nm, and that of PS below 230 nm. The

absorption of P3HT block copolymers does not change significantly by

addition of the PS block, but a slight blue shift of absorption maximum with

increasing the number of the coil blocks in the case triblock copolymer; the

spectrum shape remains the same marked with two shoulders at 550 nm

and 600 nm. The peak at 600 nm is also attributed to inter-chain

interactions, and indicates a stacking of P3HT chains. The attachment of

the PS block does not alter the self-organization of P3HT, which retains its

electronic properties.

Figure 3.23 Overlayed UV-visible absorption spectrum (films made by spin-coating using o-DCB as solvent) of (a) P2-b-PS2 and P2-b-PS2-C60 (b) PS2-b-P3-b-PS2 and C60-PS2-b-P3-b-PS2-C60.

For copolymers carrying C60 chain-ends, there are two new peaks

around 250 nm and 320 nm confirming the attachment of fullerene to block

copolymers. Moreover, the absorption of P3HT block is blue shifted with a

shift of 35 nm and maximum at 490 nm for C60-PS2-b-P3-b-PS2-C60 vs

525 nm for PS2-b-P3-b-PS2, which means a significant loss of absorption

of radiation to high wavelengths whereas in the case of P2-b-PS2, a slight

blue shift is observed which is only due to one C60. In addition, the

shoulders, structuring of P3HT, disappear completely in the case of the

C60-PS2-b-P3-b-PS2-C60 with the addition of C60 that is thus indicated to

disrupt the self-organization of P3HT chains.

160

The synthesized di- and tri-block copolymers and their C60-attached

copolymers were characterized by differential scanning calorimetry (DSC)

to determine their characteristics and thus their thermal properties.

Measurements were performed from 0 °C to 300 °C at the heating rate of

10 °C/min. The samples underwent two heating and cooling cycles. Figure

3.24 shows the representative overlayed DSC thermograms of copolymer

P2-b-PS2-C60 with their corresponding diblock copolymer P2-b-PS2 and

homopolymer P2 (cooling and second heating cycle). The melting

temperature (Tm) and crystallization temperature (Tc) measured for their

copolymers (P2-b-PS2-C60 and P2-b-PS2) are lower than for the original

polymer (P2), as shown in Figure 3.24. All values of characteristic

temperatures and enthalpies of homopolymers, di- and triblock copolymers

and their C60-attached copolymers are summarized in Table 3.7.

The block copolymers P2-b-PS2, P2a-b-PS2, PS1-b-P3-b-PS1and

PS2-b-P3-b-PS2 thermograms reveal relatively similar with both

amorphous behavior with a glass transition temperature due to the PS

block, but also showed semi crystal-melting peak and crystallization peak,

due to the block P3HT (Table 3.7). The presence of these two phases

indicates a thermodynamic incompatibility between the two blocks (P3HT

and PS) and therefore the self-assembly of the synthesized copolymers.

The crystallization and melting temperature are decreased for the diblock

P2a-b-PS2 due to decrease in molecular weight of P3HT compared to P2-

b-PS2, which agreed with literature. The length of PS present in the

copolymer PS2-b-P3-b-PS2 thus significantly alters the behavior of

semicrystalline P3HT block compared to PS1-b-P3-b-PS1 (Table 3.7).

161

Figure 3.24 Overlayed DSC curves of donor-acceptor copolymer P2-b-PS2-C60 with their corresponding diblock copolymer P2-b-PS2 and homopolymer P2.

Crystallization temperature

Melting temperature Homopolymer / Block

Copolymer / C60-attached

Block Copolymer Tc

(°C)

Enthalphy

(J/g)

Tm

(°C)

Enthalphy

(J/g)

P2 203 12.5 231 16.5

P2a 192 15.4 215 14.8

P3 188 17.0 208 16.1

P2-b-PS2 194 14.9 225 12.2

P2a-b-PS2 183 14.9 210 12.6

PS1-b-P3-b-PS1 171 7.5 200 5.5

PS2-b-P3-b-PS2 164 6.6 203 3.7

P2-b-PS2-C60 192 13.0 222 6.5

P2a-b-PS2-C60 181 11.0 208 9.2

C60-PS1-b-P3-b-PS1-C60 175 7.2 202 6.7

C60-PS2-b-P3-b-PS2-C60 163 7.9 199 3.9

Table 3.7 Crystallization, melting temperature and enthalphy values of homopolymers, di-and tri-block copolymers and their C60-attached copolymers.

162

The thermograms of C60-attached copolymers are different from

their corresponding copolymers (eg. Figure 3.25). As expected, the

addition of fullerene alters the crystallization properties of the copolymer.

The melting temperature (Tm) and crystallization temperature (Tc)

measured are lower than for the original copolymer, as shown in Table 3.7

(eg. Tm = 222 °C, Tc = 192 °C for P2-b-PS2-C60; Tm = 225 °C, Tc = 194 °C

for P2-b-PS2). In the case of C60-PS2-b-P3-b-PS2-C60 (Tm = 199 °C, Tc =

162 °C), there was significant decrease in the melting and crystallization

temperatures compared to P2-b-PS2-C60 (Tm = 222 °C, Tc = 192 °C). This

temperature decrease in the case of the P2-b-PS2-C60 indicates a lower

macromolecular order due to the attaching of C60 and thus the loss of good

crystalline properties of P3HT block. But it clearly indicates that the

number of C60 increases in the copolymers, the crystallinity of the

copolymers decreasing dramatically as it is shown in Figure 3.25 (Tc = 194

°C for P2-b-PS2, Tc = 192 °C for P2-b-PS2-C60, Tc = 163 °C for C60-PS2-b-

P3-b-PS2-C60).

Figure 3.25 Overlayed DSC curves of diblock copolymer P2-b-PS2, donor-acceptor copolymer P2-b-PS2-C60, triiblock copolymer PS2-b-P3-b-PS2 and acceptor-donor-acceptor copolymer C60-PS2-b-P3-b-PS2-C60 showing crystallization temperatures.

163

3.6.2 P4VP-b-P3HT-b-P4VP block copolymers Figure 3.26 shows the representative overlayed UV-visible

absorption spectra of coil-rod-coil tri-block copolymers P4VP-b-P3HT-b-

P4VP (3) and homopolymer P3HT (P4) at room temperature and annealed

at 180 °C, respectively. The absorption spectrum of P3HT in block

copolymers changed significantly by the addition of P4VP block, a slight

blue shift of absorption maximum (shifted 560 nm - 500 nm) in the case of

tri block copolymer was observed. Unfortunately the shoulder around 600

nm, which is related to vibronic absorption decreased in the case of the

copolymer at room temperature and also for samples annealed at 180 ºC

indicating disruption of the P3HT crystalline order. The grafting of P4VP

block to P3HT significantly disturbs the self-assembly of P3HT and hence

affects its electronic properties.

Figure 3.26 Overlayed UV-visible absorption spectrum (film) of triblock copolymer with their corresponding homopolymer (a) P4VP-b-P3HT-b-P4VP (3) and P3HT (P4) at RT (b) P4VP-b-P3HT-b-P4VP (3) and P3HT (P4) at 180 °C.

The thermal properties of synthesized tri-block copolymers, P4VP-

b-P3HT-b-P4VP were characterized by differential scanning calorimetry

(DSC). Measurements were performed from 0 °C to 300 °C at the heating

rate of 10 °C/min. The samples underwent two cycles of heating and

cooling cycle. Figure 3.27 shows the DSC thermograms of the cooling and

heating (second cycle). All values of characteristic temperatures and

enthalpies of reactions are summarized in Table 3.8.

164

The DSC thermograms (Figure 3.27) of copolymers P4VP-b-P3HT-

b-P4VP (1) and P4VP-b-P3HT-b-P4VP (3) are relatively similar, displaying

both amorphous behavior with a glass transition temperatures due to the

P4VP block, but also showing semi crystal-melting peak and crystallization

peak, due to the P3HT block (Table 3.8). The presence of these two

phases indicates a thermodynamic incompatibility between the two blocks

(P3HT and P4VP) and therefore the synthesized copolymers probably self-

organize into discrete structures. The crystallization and melting

temperature of copolymers are decreased compared to homopolymer

P3HT (P4) due to introduction of P4VP coil block and also the length of

P4VP present in the copolymers significantly alters the melting

temperature of P3HT (P4) compared to P4VP-b-P3HT-b-P4VP (Table 3.8).

The representative DSC overlay (Figure 3.27) of copolymers P4VP-

b-P4-b-P4VP (3), P4VP-b-P4-b-P4VP (1) with their corresponding homo

polymer P3HT (P4) clearly shows that the attachment of P4VP coil block

significantly changes the melting and crystallization temperatures of P3HT

present in the copolymer [eg. Tm = 194 °C, Tc = 171 °C for P4VP-b-P3HT-

b-P4VP (3); Tm = 206 °C, Tc = 186 °C for P3HT (P4)].

Crystallization temperature

Melting temperature Homopolymer /

Copolymer Tc (°C) Enthalphy (J/g) Tm (°C) Enthalphy (J/g) P3HT (P4) 186 16.9 206 13.6

P4VP-b-P4-b-P4VP (1) 169 2.7 199 1.8

P4VP-b-P4-b-P4VP (2) 183 3.7 205 3.1

P4VP-b-P4-b-P4VP (3) 171 4.0 194 2.9

Table 3.8 Crystallization, melting temperature and enthalphy values of tri-block copolymers P4VP-b-P3HT-b-P4VP (1), (2), (3) with homo polymer P3HT (P4).

The length of coil block P4VP present in the copolymers also played

a significant role in their melting and crystallization temperatures of

copolymers. As P4VP chain length increases, especially the melting

temperatures of copolymers are significantly decreased [eg. Tm = 194 °C

165

for P4VP-b-P4-b-P4VP (3), Tm = 199 °C for P4VP-b-P3HT-b-P4VP (1) and

Tm = 206 °C, Tc = 186 °C for P3HT (P4)]. There was a dramatic change in

the crystallization temperatures of copolymers with the introduction of

P4VP coil blocks to P3HT (P4) [eg. Tc = 171 °C for P4VP-b-P3HT-b-P4VP

(3); Tc = 183 °C for P4VP-b-P3HT-b-P4VP (2) and Tc = 186 °C for P3HT

(P4)] which is shown in Table 3.8.

Figure 3.27 Overlayed DSC curves of triblock copolymers, P4VP-b-P4-b-P4VP (1), P4VP-b-P4-b-P4VP (3) with their corresponding homopolymer P3HT (P4).

166

3.7 Conclusions This chapter describes the synthesis of donor-acceptor block copolymers

based on P3HT, PS and P4VP by two different approaches. In the first

approach, we synthesized the di- and tri-block copolymers, P3HT-b-PS

and PS-b-P3HT-b-PS by "click" chemistry between polystyrene terminated

azide and P3HT alkyne in the presence of copper catalysts. This study

represents the modification of the reported literature1a by our group, which

is the first example of synthesis of exclusively rod-coil block copolymers,

by "click" chemistry. However, the influence of the conjugated chain of

P3HT on the alkyne function is very important and necessary to introduce

a separation between the two entities to achieve efficient coupling reaction

or by varying the “click” chemistry conditions with the help of sonication,

one can achieve the expected copolymers.

The C60-attached copolymers (P3HT-b-PS-C60 and C60-PS-b-P3HT-

b-PS-C60) were obtained by ATRA of bromine terminated PSs. This

reaction was performed by reacting the copolymers P3HT-b-PS and PS-b-

P3HT-b-PS with C60 in the presence of CuBr/bipyridine in chlorobenzene.

In the second approach; the triblock copolymers, P4VP-b-P3HT-b-

P4VP that contained the donor P3HT blocks and acceptor domains P4VP

coil blocks were synthesized via anionic polymerization. All these

copolymers were then characterized by UV-visible absorption

spectroscopy and differential scanning calorimetry, to assess their physical

properties. These measures have enabled to determine their characteristic

temperatures (glass transition, melting, crystallization), very important

elements with respect to their potential application into organic

photovoltaics, which will be further discussed in Chapter-4.

167

3.8 References 1 (a) Urien, M.; Erothu, H.; Cloutet, E.; Hiorns, R. C.; Vignau, L.; Cramail, H.

Macromolecules 2008, 41, 7033-7040; (b) Urien, M. PhD thesis, 2008,

IMS, University of Bordeaux 1.

2 (a) Nicolay, V. T.; Matyjaszewski, K. Chem. Rev. 2007, 107, 2270-2299;

(b) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921-2990.

3 Kharash, M. S.; Jensen, E. V.; Urry, W. H. Science 1945, 102, 128.

4 Wang, J.-S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614.

5 Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules

1995, 28, 1721.

6 Xia, J.; Zhang, X.; Matyjaszewski, K. ACS Symp. Ser. 2000, 760, 207.

7 Quémener, D.; Davis, T. P.; Barner-Kowollik, C.; Stenzel, M. H. Chem.

Comm. 2006, 5051.

8 Fernandez-Santana, V.; Gonzalez-Lio, R.; Sarracent-Perez, J.; Verez-

Bencomo, V. Glycoconjugate Journal 1998, 15, 549.

9 Carey, F.A.; Sundberg, R.J. Chimie Organique Avancée Tome 2:

Réactions et Synthèses, Ed. De Boeck Université, 1997.

10 Kolb, H. C., Finn, M. G.; Sharpless, K. B. Angew. Chem. Int. Ed. 2001, 40,

2004.

11 Agut, W.; Taton, D.; Lecommandoux, S. Macromolecules 2007, 40, 5653.

12 Opsteen, J. A.; Hest, J. C. M. v. Chem. Comm. 2005, 57.

13 Binder, W. H.; Sachsenhofer, R. Macromol. Rapid Commun. 2007, 28, 15.

14 Benanti, T. L.; Kalaydjian, A.; Venkataraman, D. Macromolecules 2008,

41, 8312-8315.

15 Tao, Y.; Mcculloch, B.; Kim, S.; Segalman, R. A. Soft Matter, 2009, 5,

4219–4230.

16 Jatsch, A.; Kopyshev, A.; Mena-Osteritz, E.; Bauerle, P. Organic Letters,

2008, 10, 961-964.

17 Wang, C.; Guo, Z.-X.; Fu, S.; Wu, W.; Zhu, D. Progress in Polymer

Science 2004, 29, 1079.

18 Gu, T.; Tsamouras, D.; Melzer, C.; Krasnikov, V.; Gisselbrecht, J.-P.;

Gross, M.; Hadziioannou, G.; Nierengarten, J.-F. Chem. Phys. Chem.

2002, 3, 124.

19 Zhou, P.; Chen, G.-Q.; Li, C.-Z.; Du, F.-S.; Li, Z.-C.; Li, F.-M. Chem.

Commun. 2000, 9, 1948.

168

20 Zhou, P.; Chen, G.-Q.; Li, C.-Z.; Du, F.-S.; Li, Z.-C.; Li, F.-M.

Macromolecules 2001, 33, 1948.

21 Audoin, F.; Nunige, S.; Nuffer, R.; Mathis, C. Synthetic Metals 2001, 121,

1149.

22 Audoin, F.; Nuffer, R.; Mathis, C. Journal of Polymer Science: Part A:

Polymer Chemistry 2004, 42, 3456.

23 Sary, N.; Rubatat, L.; Brochon, C.; Hadziioannou, G.; Ruokolainen, J.;

Mezzenga, R. Macromolecules 2007, 40, 6990.

24 Laiho, A.; Ras, R. H. A.; Valkama, S.; Ruokolainen, J.; Osterbacka, R.;

Ikkala, O. Macromolecules 2006, 39, 7648-7653.

25 Jeffries-EL, M.; Sauve, G.; McCullough, R. D. Macromolecules 2005, 38,

10346–10352.

26 Szwarc, M. J. Polym. Sci. Polym. Chem. ix, 1998, 36.

27 Hsieh, H. L.; Quirk, R. P. Anionic Polymerization, Principles and Practical

Applications; Marcel Dekker, Inc.: New York, NY, 1996.

28 Flory, P. J. J. Am. Chem. Soc. 1940, 62, 1561.

29 Matthews, F. E.; Strange, E. H. British Patent, 1910, 24, 790.

30 Ziegler, K. Angew. Chem. 1936, 49, 499.

31 Szwarc, M.; Levy, M.; Milkovich, R. J. Am. Chem. Soc. 1956, 78, 2656.

32 Szwarc, M. ‘Living’ polymers. Nature (London) 1956, 178, 1168.

33 Baskaran, D.; Muller, A.H.E. Prog. Polym. Sci. 2007, 32, 173–219.

169

Chapter 4: Photovoltaic performances and

morphological characterizations of block

copolymers

170

Contents

4.1 Introduction............................................................................................. 171 4.2 Photovoltaic performances of synthesized P3HTs (P1, P1a, P1b

and Plextronics P3HT) ........................................................................... 173 4.3 Photovoltaic performances of block copolymers................................ 177

4.3.1 Diblock copolymer P3HT-block-PS as compatibilizer in the mixture of P3HT-blend-PCBM....................................................... 178

4.3.2 Donor-acceptor diblock copolymer P3HT-block-PS-C60 as compatibilizer in the mixture of P3HT-blend-PCBM...................... 182

4.3.3 Acceptor-donor-acceptor triblock copolymer C60-PS-block-P3HT-block-PS-C60 as compatibilizer in the mixture of P3HT-blend-PCBM.................................................................................. 186

4.3.4 Triblock copolymer P4VP-block-P3HT-block-P4VP as a compatibilizer in the mixture of P3HT-blend-PCBM...................... 188


171

4.1 Introduction

The efficiencies of bulk heterojunction solar cells decrease with exposure

to heat and light.1,2 This is in part due to introduction of thermal and

radiative energy into the film causing changes in the morphology of the

film. Researchers have explored the use of additives to stabilize the

polymer-fullerene microstructures and observed some improvements in the

life-time of the devices.1-3 This is very important because devices are

exposed to sunlight for long periods of time. Generally, block copolymers

(BCPs) have been used as stabilizers or compatibilizers to avoid excessive

macrophase separation of polymer blends and also to produce the nano

and microstructured materials.4,5 Keeping this knowledge in mind, we have

used some of the synthesized block copolymers as additives that can both

compatibilize and enhance the organisation of the P3HT-blend-PCBM

active layer.

The synthesized materials were examined for photovoltaic

characterization using the structure shown schematically in Figure 4.1.

Indium-doped tin oxide (ITO) coated glass substrates were sonicated

sequentially in water, acetone, ethanol and isopropanol for 10 min, and

then dried with compressed nitrogen. The substrates were then exposed to

a UV-ozone treatment for 5 min. Immediately following this procedure, an

aqueous poly(ethylene dioxythiophene)-blend-poly(styrene sulfonate)

(PEDOT-blend-PSS) solution was spin-coated onto the ITO substrates

with a speed rate of 1000 rpm for 1 min and then annealed at 110 ºC for 10

min under vacuum. The active layer solution containing P3HT-blend-

PCBM (1:1 ratio, 20 mg in 1 mL) or P3HT-blend-PCBM-blend-

(%)copolymer in o-dichlorobenzene (ODCB) was stirred overnight to form

an homogeneous solution. Then these solutions were filtered (PTFE

membrane, 20 µm pore size) and spin-coated onto the PEDOT:PSS

coated substrates at a rotation rate of 1000 rpm for 1 min. Finally, the

device was completed by a thermal evaporation of aluminium cathode

under a secondary vacuum (10-6 mbar) through a shadow mask. The

active surface of the device for all the solar cells was 10 mm2. The

172

annealing process was carried out under an inert atmosphere by keeping

the cells directly onto a temperature-controlled hot plate. Cell

performances were evaluated following free cooling to ambient

temperature. Current-tension curves were recorded using a Keithley 4200

SCS, under an illumination of 100 mW cm-2 from a KHS Solar Celltest 575

solar simulator with an AM1.5 G filter. The luminous intensity was verified

against an IL1400 radiometer.

Figure 4.1 Schematic representation of organic solar cell device.

In all the studies we presented below, the thickness of the active

layer was not varied and was adjusted to 100 nm (± 10 nm) for the sake of

comparison. The heat treatments were applied after the deposition of the

cathode. They were performed in all cases for 5-10 min at the desired

temperature. It should be noted that all the series of manipulations were

repeated at least twice to ensure reproducibility of results.

173

4.2 Photovoltaic performances of synthesized P3HTs (P1, P1a, P1b and Plextronics P3HT) First, we have examined the photovoltaic performances of synthesized

P3HTs of different molecular weights, P1 (25 kg/mol), P1a (50 kg/mol),

P1b (100 kg/mol) and compared the performances with the commercially

available P3HT (Plextronics, 50 kg/mol). It was therefore necessary to

characterize and optimize the P3HTs performance in mixture with PCBM

for a reference and compare its power and photovoltaic characteristics with

the addition of block copolymers as compatibilizers to P3HT-blend-PCBM.

Regarding the weight ratio of P3HT: PCBM, the best results were obtained

for the ratio of P3HT: PCBM equivalent to 1:1. We have maintained this

ratio in all the photovoltaic studies.

It is necessary to optimize all parameters (ratio, annealing

temperature,..) of materials used and it is not necessary to apply a pre-

established formula, because all P3HTs are different in their chemical and

physical characteristics (molecular weight, dispersity, the transition

temperature,...) and therefore give various electronic properties. It is also

known that thermal treatment applied to the components can have a great

influence on the morphology of the active layer and therefore on device

performance. Figure 4.2 shows the photovoltaic parameters (Voc, Jsc, FF

and photo conversion efficiency) of all the P3HTs (P1, P1a, P1b and

Plextronics) used in the mixture P3HT:PCBM as a function of annealing

temperature. This Figure 4.2 shows first that the annealing improves the

cell performance. Indeed, maximum photo conversion efficiency around

2.8 % is achieved for P3HT (P1a) at an annealing temperature of 180 °C,

which is good, compared to the efficiency of 0.75 % of the unannealed

device. This value is greater than those of the similar molecular weights of

P3HT (Plextronics). All the values Voc, Jsc, FF and efficiency of P3HT (P1a)

were better than all the P3HTs especially P3HT (Plextronics) of similar

molecular weight which is shown in Table 4.1. Though the Voc of P3HT

(P1b) is slightly higher than P3HT (P1a), we have observed the maximum

current, 7.7 mA/cm2 and fill factor, 0.62 for P3HT (P1a). The increase in

174

these parameters (Jsc and FF) with the annealing is well-known, and is

mainly due to better structuring of the active layer.

Figure 4.2 Photovoltaic characteristics; open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF) and photo conversion efficiency (η) of the P3HTs (P1, P1a, P1b, Plextronics) used in the mixture P3HT-blend-PCBM at different annealing temperatures.

Voc (V) Jsc (mA/cm2) FF η (%) P3HT

(Mn, kg/mol) RT 180 ºC RT 180 ºC RT 180 ºC RT 180 ºC

P1 (25) 0.39 0.49 4.02 6.86 0.50 0.60 0.80 2.02

P1a (50) 0.40 0.56 3.61 7.73 0.52 0.62 0.75 2.76

P1b (100) 0.45 0.58 2.58 7.32 0.46 0.50 0.59 2.12

Plextronics (50) 0.47 0.55 3.18 7.50 0.49 0.59 0.73 2.48

Table 4.1 Photovoltaic characteristic values of Voc, Jsc), FF and η of the P3HTs in the mixture P3HT-blend-PCBM for unannealed (RT) and annealed (180 ºC) devices.

175

Figure 4.3 shows the UV-visible absorption spectra of P3HT/PCBM

blends [where the P3HTs used are P1, P1a, P1b and P3HT (Plextronics)]

for unannealed and annealed (180 ºC) devices. The absorption bands at

270 nm and 330 nm are characteristic absorption peaks of PCBM. The

absorption peaks between 450 nm and 650 nm correspond to the

absorption of P3HT. The spectrum of the annealed device exhibits better-

defined peaks due to P3HT with contributions around 516 nm (maximum),

550 nm (shoulder) and 600 nm (shoulder). Its absorption maximum is

slightly red shifted relative to P3HT absorption spectrum of the unannealed

device. In addition, the peak at 600 nm behaves independently of the other

peaks without any wavelength shift. Brown et al.6 have shown that this

contribution at 600 nm could be attributed to interchain interactions and

therefore was is due to intrachain 0-0 vibronic transition. This peak helps to

quantify the stacking ("packing") of P3HT chains. The absorption spectra

of the mixture P3HT:PCBM (Figure 4.3) after annealing show peaks at 550

nm and 600 nm, much better resolved and intense than at room

temperature, indicating better organization of polymer chains and a better

"π-stacking". In addition, after annealing, the absorption spectrum is more

intense and wider, which means an increase in the number of charge

carriers in the active layer and thus a better Jsc (Table 4.1).

Figure 4.3 Normalised UV-Visible absorption spectra of P3HT:PCBM blends (P3HT = P1, P1a, P1b and Plextronics) for unannealed and annealed (180 ºC) devices.

176

This strong structuring of P3HT chains is further illustrated in Figure

4.4 which presents the AFM image (phase and height) of a device based

on P3HT (P1a) obtained by spin coating in o-DCB annealed at 180 ºC. This

image, which provides information on the self-organization of P3HT chains,

shows a good morphology in which P3HT chains are organized in a fibrilar

structure. The literature commonly reports two types of possible structures

for P3HT/PCBM mixtures, namely the formation of "nano-rods' for low

molecular weight P3HT and fibrillar structures for P3HT of higher

molecular weights (higher than 20 000 g/mol).6,7 The structure obtained

here is composed of small fibrils stacked at a small micron scale.

Figure 4.4 AFM images (tapping mode) of: (a) phase; and (b) height of the fibrillar structure of the film P3HT(P1a):PCBM made by spin coating in o-DCB and annealed at 180 ºC. The photovoltaic performance of the in-house prepared P3HT (P1a)

was slightly better than that of P3HT (Plextronics) even though their

molecular weights are directly comparable. Figure 4.5 shows I-V

characteristics of solar cells based on the synthesized P3HTs and the

P3HT from Plextronics. Indeed, it was shown that as the molecular weight

of P3HT increased, the better the photovoltaic results, especially with

respect to FF and Jsc values.9,10 This is partly due to the crystallization

properties and hence self-organization of P3HT, which very dependent on

the chain length. However, P1b (100 kg/mol) was probably of too high a

molecular weight and therefore unable to self-organise easily. Hence

P3HT (P1a) was used as the reference for all the following photovoltaic

177

studies where block copolymers are used as additives to the P3HT-blend-

PCBM mixture.

Figure 4.5 I-V characteristics of solar cells based on the P3HTs (P1, P1a, P1b, and Plextronics) in the mixture of P3HT-blend-PCBM used as active layer in the dark and under illuminations. 4.3 Photovoltaic performances of block copolymers This section describes some of the use of the synthesized di- and triblock

copolymers (P3HT-b-PS, P3HT-b-PS-C60, C60-PS-b-P3HT-b-PS-C60 and

P4VP-b-P3HT-b-P4VP) as stabilizers or compatibilizers in a mixture of

P3HT-blend-PCBM active layer for organic photovoltaic cells. P1a (P3HT-

50 kg/mol) was used as the matrix donor in all the photovoltaic studies of

block copolymers. The PCBM used as the acceptor in this study had a

purity of 99.5 % (Solaris). Our aim in this study was to force the active

layer to self-organize with the addition of these block copolymers at

different proportions (0-5 %) into the mixtures of P3HT-blend-PCBM. In

that case a more favorable morphology can be achieved for efficient

charges to electrodes in organic photovoltaic process and also to see the

effect of these block copolymers as additives on device efficiencies.

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4.3.1 Diblock copolymer P3HT-block-PS as compatibilizer in the mixture of P3HT-blend-PCBM Initially, the diblock copolymer, P2-b-PS2, shown in Figure 4.6, was used

as an additive at different proportions (0.5%, 1.0%, 1.5%, 2.5% and 5.0%)

in a mixture of P1a-blend-PCBM (1:1). The solar cell devices were tested

at different annealing temperatures, from room temperature up to 195 ºC.

The best performance for the mixture P1a-blend-PCBM-blend-(P2-b-PS2)

(0-5 %), both the P1a-based devices alone and also those based on P2-b-

PS2 was observed following annealing at 167 ºC.

S

C6H13

60

N NN

O

O

Br36

P2-b-PS2

P3HTPS

Figure 4.6 Chemical structure of diblock copolymer, P2-b-PS2.

Figure 4.7 shows the photovoltaic characteristics of devices based

on the mixture P1a-blend-PCBM-blend-(P2-b-PS2) (0-5 %) based on the

amount of copolymer added. Indeed, the mixture for P1a-blend-PCBM

(0%) alone has an efficiency of 2.90%, it increases with the addition of

copolymer and reaches a significant maximum efficiency of 3.7% for P1a-

blend-PCBM-blend-(P2-b-PS2) (1.0%) following annealing at 167 ºC. The

efficiency on further additions of copolymer decreased to 2.6% for the

addition of 5.0% copolymer. While the FF had a maximum value of 0.61

with 1.0% copolymer addition, the Voc remained relatively constant at

between 0.54 V and 0.56 V. Only the short-circuit current (Jsc) increased

dramatically with the addition of copolymer (P2-b-PS2) and reached a

maximum value of 11 mA/cm2 at 1.0% copolymer blend (the best in our

studies), but decreased on further addition of copolymer. Figure 4.8

represents the I-V characteristics of solar cells based on the mixture P1a-

blend-PCBM-blend-(P2-b-PS2) (0-5 %). It was shown that the addition of

179

copolymer (1.0%) to P1a-blend-PCBM (0%) significantly changes the

photovoltaic parameters involved, especially photo conversion efficiency,

(from 2.9-3.7%) and short-circuit current, (from 8.7-11.0 mA/cm2).

Figure 4.7 Photovoltaic characteristics: Voc, Jsc, FF and η of the mixture P1a-blend-PCBM-blend-(P2-b-PS2) (0-5 %) of unannealed and annealed (167 ºC) devices.

Figure 4.8 I-V characteristics of solar cells based on the the mixture P1a-blend-PCBM-blend-(P2-b-PS2) (0-5 %) annealed at 167 ºC in the dark and under illumination.

180

Figure 4.9 shows the UV-Visible absorption spectra (film) of the

mixture P1a-blend-PCBM-blend-(P2-b-PS2) (0-5 %) at room temperature

(left side) and at annealing temperature 175 ºC (right side). The absorption

bands at 270 nm and 330 nm, the characteristic absorption peaks of

PCBM, did not change on addition of copolymer, whereas the absorption

peaks between 450 nm and 650 nm, corresponding to absorptions by

P3HT slightly red shifted with addition of copolymer compared to the P1a-

blend-PCBM alone at room temperature. The spectra of annealed (175 ºC)

samples are much better defined than that unannealed and the peaks at

550 nm (maximum) and 600 nm (shoulder) corresponding to P3HT are

better resolved, indicating that the addition of copolymer helps to organize

P3HT.

Figure 4.9 Normalised UV-Visible absorption spectra (film) of the mixture P1a-blend-PCBM-blend-(P2-b-PS2) (0-5 %) unannealed (left) and annealed (175 ºC) (right). The addition of copolymer structuring and its effect on P3HT chains

is further illustrated in Figure 4.10 which presents the AFM images (phase

and height) of a device based on P1a-blend-PCBM-blend-(P2-b-PS2) (0, 1

and 5 %) obtained by spin coating in o-DCB annealed at 180 ºC. Indeed,

the addition of 1% copolymer leads to an improvement of the structure,

with fibrils perfectly stacked together and in which areas of disorder

present in the mixture at 0% disappear. This structure is extremely regular,

and whose areas are well defined, promoting the mobility of charges and

limiting the exciton recombination. This contributes to the increase in FF

and of the Jsc and explains that the maximum energy conversion efficiency

(3.7%) was obtained for the mixture of 1% copolymer. At 5%, the structure

181

is completely disorganized with randomly tangled fibrils. Such structures

lead to a substantial decrease in the Jsc, and explain the drop in

performance for samples with 1.5%, 2.0% and 5.0% copolymers.

Figure 4.10 AFM images (tapping mode) of phase (left) and height (right) of the fibrillar structure of the film P1a-blend-PCBM-blend-(P2-b-PS2) (0, 1 and 5 %) made by spin coating from o-DCB and annealed at 180 ºC.

182

4.3.2 Donor-acceptor diblock copolymer P3HT-block-PS-C60 as compatibilizer in the mixture of P3HT-blend-PCBM Here the donor-acceptor diblock copolymer P2-b-PS2-C60 which is shown

in Figure 4.11 has been used as an additive at different proportions (0.5%,

1.0%, 1.5%, 2.5% and 5.0%) in a mixture of P1a-blend-PCBM (1:1). The

best performance for the mixture P1a-blend-PCBM-blend-(P2-b-PS2-C60)

(0-5 %), both the P1a-based devices and also those based on P2-b-PS2-

C60 was observed after annealing at 180 ºC.

S

C6H13

N NN

O

O

P2-b-PS2-C60

60

36

P3HTPS

C60

Figure 4.11 Chemical structure of donor-acceptor diblock copolymer, P2-b-PS2-

C60.

Figure 4.12 shows the photovoltaic characteristics of devices based

on the mixture P1a-blend-PCBM-blend-(P2-b-PS2-C60) (0-5 %) based on

the amount of copolymer added for unannealed and annealed devices.

The efficiency for the mixture P1a-blend-PCBM (0%) alone was 3.0%,

which significantly increased to maximum efficiency of 4.0% with addition

0.5 % of copolymer at 180 °C and started decreasing the efficiency of the

device on further addition of copolymer to reach a lower efficiency of 2.9%

for 5.0% copolymer. Here surprisingly the FF reached the maximum value

of 0.65 with 0.5% copolymer addition and the Vocs are in turn relatively

constant at between 0.53 V and 0.56 V. But the Jsc increases dramatically

with the addition of copolymer, P2-b-PS2-C60 and reaches a maximum

value of 12.0 mA/cm2 for 0.5% copolymer blend to decrease upon further

addition of copolymer. Figure 4.13 represents the I-V characteristics of

solar cells based on the mixture P1a-blend-PCBM-blend-(P2-b-PS2-C60)

183

(0-5 %). It was shown that the addition of copolymer (0.5%) to P1a-blend-

PCBM (0%) significantly changes the photovoltaic parameters involved,

especially the photo conversion efficiency, (from 3.0-4.0%), the fill factor

with a maximum value of 0.65 and the maximum current Jsc, 12.0 mA/cm2

which are the best results in our PV studies.

Figure 4.12 Photovoltaic characteristics; Voc, Jsc, FF, and η of the mixture P1a-blend-PCBM-blend-(P2-b-PS2-C60) (0-5 %) of unannealed and annealed (180 ºC) samples.

Figure 4.13 I-V characteristics of solar cells based on the the mixture P1a-blend-PCBM-blend-(P2-b-PS2-C60) (0-5 %) of annealed (180 ºC) device in the dark and under illumination.

184

Figure 4.14 shows the UV-Visible absorption spectra (film) of the

mixture P1a-blend-PCBM-blend-(P2-b-PS2-C60) (0-5 %) in unannealed

and annealed (180 ˚C) devices. For the former, the intensity of the

absorption bands at 270 nm and 330 nm, corresponding to PCBM, and the

absorption peaks between 450 nm and 650 nm, corresponding to the

absorption of P3HT, increase with addition of copolymer compared to P1a-

blend-PCBM alone at room temperature. The absorption spectra of the

annealed device with (0-5 %) copolymer is much better resolved and the

intensity of peaks at 550 nm (maximum) and 600 nm (shoulder)

correponding to P3HT is increased and slightly red shifted compared to

room temperature, whereas the intensity of peaks corresponding to C60

decrease. This indicates that the addition of copolymer helps to improve

the organization of P3HT and hence lead to higher efficiences.

Figure 4.14 Normalised UV-Visible absorption spectra (film) of the mixture P1a-blend-PCBM-blend-(P2-b-PS2-C60) (0-5 %) of unannealed an annealed (180 ˚C) devices. The morphology of the devices was investigated by AFM images (0-

5 % copolymer) as shown in Figure 4.15. The addition of 0.5% copolymer

to P1a-blend-PCBM (annealed, 180 ºC) achieved a better fibrilar

morphology which resulted in the high FF and the best observed efficiency

whereas further addition of copolymer (5%) disrupted the P3HT fibrillar

structure chains (height images of Figure 4.15) leading to a reduced FF

and lower energy conversion efficiencies.

185

Figure 4.15 AFM images (tapping mode) of phase (left side) and height (right side) of the film P1a-blend-PCBM-blend-(P2-b-PS2-C60) (0, 0.5, 1 and 5 %) made by spin coating from o-DCB and annealed at 180 ºC.

186

4.3.3 Acceptor-donor-acceptor triblock copolymer C60-PS-block- P3HT- block-PS-C60 as compatibilizer in the mixture of P3HT- blend-PCBM The acceptor-donor-acceptor triblock copolymer, C60-PS2-b-P3-b-PS2-C60,

shown in Figure 4.16, was used as a compatibilizer at different proportions

(0%, 0.5%, 1.0%, 1.5%, 2.0%, 4.0%, 5.0% and 7.0%) in a mixture of P1a-

blend-PCBM (1:1).

S

C6H13

30

N NN ONN

NO

O O


3636

C60P3HT

PSPS

C60

Figure 4.16 Chemical structure of the acceptor-donor-acceptor triblock copolymer, C60-PS2-b-P3-b-PS2-C60. The photovoltaic characteristics of the devices as a function of the

amount of compatibilizer in the film P1a-blend-PCBM-blend-(C60-PS2-b-

P3-b-PS2-C60) (0-7 %) both unannealed and annealed (180 ºC) are shown

in Figure 4.17. There is a dramatic decrease in the Jsc with addition of

copolymer for annealed devices. In fact, the addition of 0.5% copolymer

reduces the Jsc value from 9.5 to 7.3 mA/cm2 (Table 4.2) and this value

continues to decrease with the further addition of copolymer. However, for

unannealed devices, the Jsc value significantly increased with 0.5%

addition of copolymer from 3.1 to 6.3 mA/cm2 but started decreasing on

further addition of copolymer to reach a low value of 1.0 mA/cm2 with 7%

weight copolymer. On the other hand, the Voc of annealed devices remains

approximately constant (Voc ≈ 0.50 V at 180 ºC) whereas at room

temperature it varies 0.28 to 0.42 V. The FF of the devices both for

unannealed devices and after annealing also gradually decreased with the

increasing amount of the copolymer added.

Because Voc and FF were approximately constant, the efficiency

values followed the trend of the Jsc and decreased with increasing the

amount of copolymer (0-7 %). But unannealed devices, with 0.5% addition

of copolymer increases the efficiency which nearly doubles from 0.67 to

187

1.27 %, although this decreased on further addition of copolymer to reach

η = 0.11% at 7% copolymer. This is an interesting result as 1.27 %

efficiency can be reached without annealing.

Figure 4.17 Photovoltaic characteristics; Voc, Jsc, FF and η of the mixture P1a-blend-PCBM-blend-(C60-PS2-b-P3-b-PS2-C60) (0-5 %) for room temperature (black line) and annealed (180 ºC) devices (red line).

Voc (V) Jsc (mA/cm2) FF η (%) Copolymer

(%) RT 180 ºC RT 180 ºC RT 180 ºC RT 180 ºC

0 0.42 0.51 3.13 9.50 0.53 0.62 0.67 3.05

0.5 0.40 0.52 6.36 7.33 0.43 0.56 1.27 2.13

1.0 0.40 0.53 4.48 7.48 0.38 0.56 0.68 2.26

1.5 0.37 0.53 4.36 6.77 0.37 0.54 0.61 1.95

2.0 0.39 0.53 4.79 7.72 0.43 0.51 0.81 2.12

4.0 0.33 0.51 1.40 7.25 0.27 0.34 0.13 1.29

5.0 0.28 0.51 2.10 6.01 0.26 0.41 0.15 1.29

7.0 0.38 0.54 1.06 7.52 0.28 0.29 0.11 1.19

Table 4.2 Photovoltaic characteristic values of Voc, Jsc, FF and η of the mixture P1a-blend-PCBM-blend-(C60-PS2-b-P3-b-PS2-C60) (0-7 %) for unannealed (RT) and annealed (180 ºC) devices.

188

The decreasing short-circuit current on increasing weights of C60-

PS2-b-P3-b-PS2-C60 suggested that the compatibilizer directly affected the

charge transport in the devices. The reason for this may be due to the low

molecular weight of the copolymer used in these blends as it has been

shown that polythiophene-C60 bulk heterojunctions composed of higher

molecular weight P3HT have better device performances10 and also it has

been indicated in our case that the donor-acceptor diblock copolymer, P2-

b-PS2-C60 of high molecular weight P3HT has improved the Jsc when at

0.5% copolymer (Section 4.3.2.).

4.3.4 Coil-rod-coil triblock copolymer P4VP-block-P3HT-block-P4VP as compatibilizer in the mixture of P3HT-blend-PCBM The coil-rod-coil triblock copolymer, P4VP-b-P4-b-P4VP (1), which is

shown in Figure 4.18, is of particular interest in which it can have

supramolecular interactions from its P4VP block with PCBM in the blended

device. It was tested both as donor (P4VP-b-P4-b-P4VP):PCBM (1:1) and

also as a compatibilizer at varying proportions (0.5% and 1.0%) in a

mixture of P1a-blend-PCBM (1:1).

Figure 4.18 Chemical structure of the coil-rod-coil triblock copolymer, P4VP-b-P4-b-P4VP. Initially, we used the copolymer as a compatibilizer and followed

photovoltaic characteristics of the devices as a function of the amount of

compatibilizer in the film P1a-blend-PCBM-blend-(P4VP-b-P4-b-P4VP) (0,

0.5% and 1.0%) both for unannealed and annealed (175 ºC) devices, as

shown in Table 4.3. There is a decrease in the Jsc on addition of copolymer

189

for annealed devices. In fact, the addition of 0.5% copolymer reduced the

Jsc value from 6.8 to 6.6 mA/cm2 (Table 4.3) and this value decreased

dramtically with the 1.0% addition of copolymer to 1.8 mA/cm2 whereas

unannealed devices, the Jsc value significantly increased on 0.5% addition

of copolymer from 3.6 to 4.9 mA/cm2 but decreased on further additions to

a very low value of 0.76 mA/cm2 at 1% weight copolymer. The Voc and the

FF of the devices both unannealed and annealed (175 ºC) also gradually

decreased with the increasing amount of the copolymer added. Finally the

efficiency decreased with increasing copolymer (0-1 %) reaching the low

value of 0.11% with 1% copolymer.

We also attempted using the copolymer as donor material in

unannealed and annealed (167 ºC and 175 ºC) devices (P4VP-b-P4-b-

P4VP):PCBM (1:1). The photovoltaic characteristic values of unannealed

devices were Voc = 0.47, Jsc = 3.19 mA/cm2, FF = 0.49 and η = 0.74 and

annealing the devices at 167 ºC showed a slight increment in the Jsc to

3.43 mA/cm2 (Table 4.3). There was dramatic decrease of these values

(Voc = 0.07, Jsc = 0.12 mA/cm2, FF = 0.24 and η = 0.002) when the device

annealed at 175 °C.

Voc (V) Jsc (mA/cm2) FF η (%) Copolymer

(%) RT 175 ºC RT 175 ºC RT 175 ºC RT 175 ºC

0 0.40 0.56 3.61 6.87 0.52 0.62 0.75 2.36

0.5 0.29 0.38 4.92 6.67 0.30 0.38 0.45 0.98

1.0 0.15 0.17 0.76 1.81 0.23 0.34 0.03 0.11

Copolymer:PCBM RT 167 ºC RT 167 ºC RT 167 ºC RT 167 ºC

1:1 0.47 0.45 3.19 3.43 0.49 0.50 0.74 0.74

at 175 ºC ‘’ 0.07 ‘’ 0.12 ‘’ 0.24 ‘’ 0.002

Table 4.3 Photovoltaic characteristic values of Voc, Jsc, FF and η of the mixture P1a-blend-PCBM-blend-(P4VP-b-P4-b-P4VP) (0, 0.5 and 1 %) at room temperature (RT) and at annealing temperature (175 ºC) and device based on (P4VP-b-P4-b-P4VP)-blend-PCBM (1:1) at room temperature, annealing temperaures 167 ºC and 175 ºC.

190

The annealing of devices (P4VP-b-P4-b-P4VP)-blend-PCBM (1:1)

at high temperatures did not improve the efficiencies. The low values found

are similar to those reported with a near-similar polymer (η =0.017-

0.026%) using a standard device.11 The low efficiency for this device may

be due to the preferential wetting of one of the copolymer blocks (P4VP) at

the PEDOT:PSS interface during the film formation. P4VP tends to

preferentially wet oxides or charged surfaces.12 The efficiency of the device

using this type of copolymer (P4VP-b-P4-b-P4VP) may be enhanced by an

inverted PV device, as already observed in the literature.11

4.4 Conclusions In this section, we have described the photovoltaic characterization of

synthesized di- and triblock copolymers P2-b-PS2, P2-b-PS2-C60, C60-PS2-

b-P3-b-PS2-C60 and P4VP-b-P3HT-b-P4VP used as compatibilizer in a

mixture of P1a-blend-PCBM active layer for organic photovoltaic cells. The

first study involves the optimization of P3HT mixed with PCBM, for which

maximum yield is obtained for P1a among all the synthesized P3HTs and

also commercial P3HT of similar molecular weight from Plextronics

annealing at 180 ºC. The block copolymers were then tested as

compatibilizers (0-5 %) in combination with P1a and PCBM (1:1) based

devices. The device based on P1a-blend-PCBM-blend-(P2-b-PS2) using

1% addition of copolymer has achieved the highest short-circuit current 11

mA/cm2 and also highest photoconversion efficiency, 3.7% at annealing

temperature of 167 ºC in our solar cell studies. At 5% addition of this

copolymer P2-b-PS2, the structure is completely disorganized and tangled

fibrils random structures lead to a substantial decrease in the Jsc, and

explains the drop in performance for the mixtures of 1.5%, 2.0% and 5.0%.

This is explained by taking into account the degree of crystallinity of this

copolymer, which is superior to others, reflecting its ability to facilitate

better mobility of charge carriers in the active layer. The donor-acceptor

diblock copolymer, P2-b-PS2-C60, has been characterized in organic solar

cell as compatibilizer by adding 0-5% amounts to P1a-blend-PCBM (1:1)

based devices. Surprisingly it was shown that the addition of copolymer

191

(0.5%) to P1a-blend-PCBM (0%) at 180 ºC annealing significantly changes

the photovoltaic parameters involved especially photoconversion

efficiency, (from 3.0-4.0%), fill factor with maximum value of 0.65, but the

maximum short-circuit current, 12.0 mA/cm2 was observed at 0.5%

addition of copolymer. In this case, we have observed an excellent fibrilar

morphology. The 5% weight addition of copolymer disrupts the fibrillar

structure of the P3HT chains (height images of Figure 4.15) which lead to

reduced fill factor and reduced energy conversion efficiencies.

In the case of triblock copolymers, the devices based on P1a-blend-

PCBM-blend-(C60-PS2-b-P3-b-PS2-C60) (0-7 %) (annealed, 180 ºC)

showed dramatic decreases in the Jsc with addition of copolymers whereas

unannealed devices showed Jsc values significantly increased with 0.5%

copolymer, but started decreasing on further addition of copolymer to

reach a low value of 1.0% at 7% copolymer. Neverthless, with 0.5%

copolymer the efficiency nearly doubled from 0.67 to 1.27%. The devices

based on P1a-blend-PCBM-blend-(P4VP-b-P4-b-P4VP), in which the

copolymer was used as a compatibilizer (0, 0.5% and 1.0%) showed

dramatic decreases in Jsc with the addition of copolymer for annealed

devices. The Voc and the FF of the devices both unannealed and annealed

also gradually decreased with the increasing copolymer. We have also

used the copolymer, P4VP-b-P4-b-P4VP as donor material in the device,

(P4VP-b-P4-b-P4VP):PCBM (1:1) at room temperatures and at annealed

temperatures 167 ºC and 175 ºC, but only low photovoltaic efficiences were

found.

We have observed that the addition of triblock copolymers as

compatibilizers disrupts the molecular structure of P3HT chains and also

the disorganization of the active layer, resulting in low efficiencies. But in

the case of diblock copolymers as compatiblizers, we have observed the

enhancement of Jscs and efficiencies with respect to P3HT-blend-PCBM

device alone. This might be due to the nano-domain constraints placed

upon such systems by tri-block copolymers.

192

4.5 References 1 Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324-

1338.

2 Kim, B. J.; Miyamoto, Y.; Ma, B.; Fréchet, J. M. J. Adv. Mater. 2009, 19,

1-9.

3 Ball, Z. T.; Sivula, K.; Fréchet, J. M. J. Macromolecules 2006, 39, 70–72.

4 Hiemenz, P. C.; Lodge, T. P. Polymer Chemistry; CRC Press: Boca Raton,

FL, 2007.

5 Sperling, L. H. Introduction to Polymer Science, 3rd Ed.; John Wiley &

Sons, Inc.: Hoboken, NJ, 2001.

6 Brown, P. J.; Thomas, D. S.; Köhler, A.; Wilson, J. S.; Kim, J.-S.; Ramsdale,

C.

M.; Sirringhaus, H.; Friend, R. H. Physical Review B 2003, 67, 064203.

7 Verilhac, J.-M.; LeBlevennec, G.; Djurado, D.; Rieutord, F.; Chouiki, M.;

Travers, J.- P.; Pron, A. Synthetic Metals 2006, 156, 815.

8 Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Frechet, J. M. J.;

Toney, M. F. Macromolecules 2005, 38, 3312.

9 Schilinsky, P.; Asawapirom, U.; Scherf, U.; Biele, M.; Brabec, C. J. Chem.

Mater. 2005, 17, 2175.

10 Ma, W.; Kim, J. Y.; Lee, K.; Heeger, A. J. Macromol. Rapid Commun. 2007,

28,

1776–1780.

11 Sary, N.; Richard, F.; Brochon, C.; Leclerc, N.; Lévêque, P.; Audinot, J. N.;

Berson, S.; Heiser, T.; Hadziioannou, G.; Mezzenga, R. Adv. Mater. 2010,

22, 763–768.

12 Malynych, S.; Luzinov, I.; Chumanov, G. J. Phys. Chem. B 2002, 106, 1280.

193

Chapter 5: Experimental Section

195

Contents

1 Materials................................................................................................... 197 1.1 Purification of Solvents.................................................................... 197 1.2 Purification of Monomers................................................................. 197 1.3 Chemicals........................................................................................ 197

2 Synthesis................................................................................................. 199 2.1 Monomers...................................................................................... 199 2.1.1 3-Hexylthiophene................................................................ 199 2.1.2 2,5-Bibromo-3-hexylthiophene............................................ 199 2.1.3 2-Bromo-3-hexylthiophene.................................................. 200 2.1.4 2-Bromo-3-hexyl-5-iodo-thiophene..................................... 200 2.2 Regioregular P3HTs (P1-P6) by the Grignard metathesis

(GRIM) ............................................................................................ 201 2.2.1 α,ω-DiH-P3HTs (P1, P1a, P1b and P1c) ............................ 201 2.2.2 Chain-end functionalised w-P3HTs or ω-P3HTs................. 202 2.2.2.1 ω-Ethynyl, ω-vinyl-P3HTs and α,ω-pentynyl-

P3HTs................................................................... 202 2.2.2.2 α,ω-Diformyl and α,ω-dihydroxy-P3HTs................ 203 2.3 Mono-functionalised P3HTs (P7-P8) by externally added Ni-

catalyst initiator............................................................................. 205 2.3.1 Ni-initiator: [(Ph)Ni(PPh3)2-Br] ............................................ 205 2.3.2 Mono-functionalised P3HTs by small molecule Ni-initiator. 205 2.4 Azide-terminated Polystyrene...................................................... 206 2.4.1 Azide initiator for ATRP....................................................... 206 2.4.1.1 3-Azido-1-propanol................................................ 206 2.4.1.2 3-Azidopropyl-2-bromoisobutyrate........................ 206 2.4.2 α-Azido-polystyrenes (PS1-PS6) ....................................... 207 2.5 Block copolymers P3HT-block-PS and PS-block-P3HT-block-

PS by Click Chemistry.................................................................. 208 2.5.1 Triblock copolymers PS-b-P3HT-b-PS................................ 208 2.5.2 Diblock copolymers P3HT-b-PS.......................................... 209 2.6 P3HT-block-PS-C60 and C60-PS-b-P3HT-b-PS-C60 by ATRA....... 210 2.7 Triblock copolymers P4VP-block-P3HT-block-P4VP by

anionic polymerisation................................................................. 211 2.8 Polyacetylene-graft-P3HT (PA-graft-P3HT) ................................ 212 2.8.1 Phenylacetylene polymerisation by [Rh(nbd)Cl]2 catalyst... 212

196

2.8.2 Polymerisation of ω-alkynyl-P3HTs by [Rh(nbd)Cl]2

catalyst ............................................................................... 212 2.8.3 Attempted copolymerisation of ω-acetylene-P3HT with

phenyl acetylene................................................................. 213 3 Characterization...................................................................................... 213 4 Photovoltaic device fabrication and characterization......................... 215

General conclusions........................................................................................ 217 Appendix........................................................................................................... 223 Publications and conferences........................................................................ 227

197

1. Materials 1.1 Purification of Solvents Solvents were distilled over their respective drying agents under reduced

pressures and stored under inert atmosphere. Tetrahydrofuran (THF, JT

Baker, 99%) was first distilled over calcium hydride (CaH2) and then cryo-

distilled over sodium benzophenone just before use. Dichloromethane

(DCM, Xilab, 99%) was cryo-distilled after refluxing 1 h over CaH2.

Diethylether (JT Baker, 99%) and toluene (JT Baker, 99%) were first

distilled over calcium hydride (CaH2) and then cryo-distilled over polystyryl-

lithium just before use. Chlorobenzene (Aldrich, 99%) was distilled after

stirring with CaH2 overnight. Methanol (Xilab, 99%) was cryo-distilled after

refluxing overnight over Mg turnings. Triethylamine (TEA, Aldrich, 99%)

was cryo-distilled following overnight stirring over KOH pellets. Hexane (JT

Baker, 95%), acetic acid (Aldrich, 99%), ethylacetate (JT Baker, 99%) and

diisopropylethylamine (DIEA, Aldrich, 99%) were used as received without

purification.

1.2. Purification of Monomers Styrene (Aldrich, 99%), α-methylstyrene (Aldrich, 99%) and 4-vinylpyridine

(Aldrich, 99%) were cryo-distilled over CaH2 just prior to polymerisation.

2,5-Dibromo-3-hexylthiophene and 2-bromo-3-hexyl-5-iodothiophene were

distilled 2-3 times to obtain 100% purity (1H NMR) for polymerisation and

stored at 4 ºC.

1.3. Chemicals Bromohexane (98%), 3-bromothiophene (97%), 1,3-

bis(diphenylphosphino)propane nickel(II) chloride [Ni(dppp)Cl2], N-

bromosuccinimide (NBS, 99%), iodine, iodobenzene diacetate (98%),

sodium bicarbonate (NaHCO3), sodium sulfate (Na2SO4), sodium

thiosulfate (Na2S2O3), 3-bromo-1-propanol (97%), sodium azide (NaN3, ≥

99.5%), tetrabutylammonium iodide (Bu4NI, 98%), dicyclohexano-18-

crown-6 (18-crown-6, 98%), 2-butanone (≥ 99.9%), α-bromoisobutyryl

198

bromide (98%), bipyridine (≥ 99.9%), copper(I) iodide (CuI, ≥ 99.9%), 1,8-

diazabicyclo[5.4.0]undec-7-ene (DBU, 98%), N,N,N',N",N"-

pentamethyldiethylenetriamine (PMDETA, 99%), C60 (99.9%), styrene (≥

99.9%), triethylamine (TEA, 99.5%), ammonium chloride (NH4Cl), and

potassium hydroxide (KOH), were used as received from Aldrich without

purification. Mg turnings (98%, Aldrich) were dried overnight in an oven at

150 ºC before use.

Tert-butylmagnesium chloride (tBuMgCl, 1 M solution in THF), iso-

propylmagnesium chloride (i-PrMgCl, 2 M solution in THF),

vinylmagnesium bromide (2 M solution in THF), ethynylmagnesium

bromide (0.5 M solution in THF), (5-chloro-1-pentynyl)trimethylsilane (97

%), 4-(2-tetrahydro-2H-pyranoxy)phenylmagnesium bromide were used as

received from Aldrich without purification. 4-(1,3-Dioxan-2-

ylphenyl)magnesium bromide was purchased from Rieke Metals Inc.

Copper(I) bromide (CuBr, Aldrich, 98%) was purified by stirring

overnight in a mixture of acetic acid and water (1:1), then filtered, rinsed

sequentially with ethanol and ether and dried in vacuum oven for about 24

h at 40 ˚C. It was kept under an inert atmosphere until use. Typically, CuBr

(2 g) was washed with a mix of acetic acid (50 mL) and water (50 mL).

199

2. Synthesis All reactions were carried out under a dry argon atmosphere, using flame-

dried glassware.

2.1 Monomers 2.1.1 3-Hexylthiophene: In a 500 mL flask equipped with condenser

magnesium (15.13 g, 0.62 mol) and dry diethyl ether (210 mL) were

introduced into the flask and cooled to 0 ºC. A solution of bromohexane

(82.25 g, 70 mL, 0.5 mol) was added slowly by dropping funnel. The

resulting mixture was stirred under nitrogen for 3 h and transferred to a

dropping funnel (250 mL) fitted to another 1 L flask equipped with

condenser containing 3-bromothiophene (50 g, 28 mL, 0.306 mol) and

Ni(dppp)Cl2 (0.75 g, 1.38 mmol) in dry diethyl ether (100 mL). After cooling

with an ice bath, the Grignard reagent was added dropwise and the

resulting adduct was allowed to warm to room temperature and stirred for

3 days under nitrogen. To terminate the reaction, slightly acidified (HCl)

water was added (100 mL) slowly. To recover the monomer, chloroform

(200 mL) was added, and the organic layer washed 3 times with water and

dried over Na2SO4. The crude product was distilled under reduced

pressure to obtain in the pure form as a colorless oil (45 g, 87%).

Characterization of 3-hexylthiophene: 1H NMR (400 MHz, CDCl3): δH

0.90 (t, 3H, (CH2)5-CH3), 1.32 (m, 6H, CH2-CH2-(CH2)3-CH3), 1.63 (q, 2H,

CH2-CH2-(CH2)3-CH3), 2.63 (t, 2H, CH2-CH2-(CH2)3-CH3), 6.92 (m, 1H, CH

Ar), 6.95 (m, 1H, CH Ar), 7.23 (m, 1H, CH Ar).

2.1.2 2,5-dibromo-3-hexylthiophene: NBS (80.65 g, 0.453 mol) was

added to a stirred solution of 3-hexylthiophene (42.15 g, 0.25 mol) in acetic

acid (320 mL) and CH2Cl2 (320 mL). The mixture was stirred at room

temperature for 24 h under nitrogen. The organic layer was washed five

times with water, five times with a saturated aqueous NaHCO3 solution,

dried over Na2SO4, filtered and concentrated. The crude product was

recovered as pale yellow oil by two successive secondary vacuum

distillations (5 x 10-4 mbar, 100 ºC) (58 g, 70%).

200

Characterization of 2,5-dibromo-3-hexylthiophene: 1H NMR (400 MHz, CDCl3): δH 0.89 (t, 3H, (CH2)5-CH3), 1.30 (m, 6H, CH2-CH2-(CH2)3-CH3),

1.55 (q, 2H, CH2-CH2-(CH2)3-CH3), 2.51 (t, 2H, CH2-CH2-(CH2)3-CH3), 6.78

(s, 1H, CH aromatic). δ C 14.10 ((CH2)5-CH3), 22.60 ((CH2)4-CH2-CH3)),

26.96 (-CH2-(CH2)4-CH3), 28.81 ((CH2)2-CH2-(CH2)2-CH3), 29.51 ((CH2)3-

CH2-CH2-CH3), 31.60 (CH2-CH2-(CH2)3-CH3), 107.95 (C2 Ar), 110.32 (C5

Ar.),130.98 (C4 Ar), 143.02 (C3 Ar).

2.1.3 2-Bromo-3-hexylthiophene: NBS (20.54 g, 115.4 mmol) was

added to the stirred solution of 3-hexylthiophene (19.43 g, 115.4 mmol) in

THF (200 mL) at 0 ºC, and the mixture was stirred at 0 ºC for 1 h. After

addition of water, the mixture was extracted with Et2O. The organic layer

was washed successively with 10% aqueous Na2S2O3, 10% aqueous

KOH, and water, and dried over anhydrous Na2SO4. Distillation (5 x 10-4

mbar, 100 ºC) and filtering through cotton gave the pure product as

colorless oil (28 g, 95%).

Characterization of 2-bromo-3-hexylthiophene: 1H NMR (400 MHz, CDCl3): δH 0.89 (t, 3H, (CH2)5-CH3), 1.31 (m, 6H, CH2-CH2-(CH2)3-CH3),

1.57 (q, 2H, CH2-CH2-(CH2)3-CH3), 2.56 (t, 2H, CH2-CH2-(CH2)3-CH3), 6.78

(d, 1H, CH5 Ar), 7.18 (d, 1H, CH4 Ar). 2.1.4 2-Bromo-3-hexyl-5-iodothiophene: Iodine (12.51 g, 49.2 mmol)

and iodobenzene diacetate (17.33 g, 53.8 mmol) were added successively

to a stirred solution of 2-bromo-3-hexylthiophene 3 (22.17 g, 89.68 mmol)

in CH2Cl2 (200 mL) at 0 ºC, and the mixture stirred at room temperature for

4 h. 10% Aqueous Na2S2O3 was added, and the mixture extracted with

Et2O. The organic layer was washed with 10% aqueous Na2S2O3 and dried

over anhydrous Na2SO4. After filtration, the solvent and iodobenzene were

removed by evaporation under reduced pressure. The residue was distilled

to remove traces of iodobenzene and then purified by silica gel column

chromatography (eluent:cyclohexane) to give the pure product as pale

yellow oil (32.26 g, 96%).

Characterization of 2-bromo-3-hexyl-5-iodothiophene: 1H NMR (400 MHz, CDCl3): δH 0.89 (t, 3H, (CH2)5-CH3), 1.30 (m, 6H, CH2-CH2-(CH2)3-

201

CH3), 1.55 (q, 2H, CH2-CH2-(CH2)3-CH3), 2.52 (t, 2H, CH2-CH2-(CH2)3-

CH3), 6.96 (s, 1H, CH4 Ar).

2.2 Regioregular P3HTs (P1-P6) by the Grignard metathesis (GRIM) 2.2.1 α ,ω -diH-P3HTs (P1, P1a, P1b and P1c) (representative method): To a three-necked round bottom flask 2,5-dibromo-3-hexylthiophene M1 (3

g, 9.2 mmol) was dissolved in THF (20 mL) and stirred under nitrogen. Tert-butylmagnesium chloride (9.3 mL, 9.2 mmol, 1 M in THF) was added

and the mixture was stirred at room temperature for 3 h. The mixture was

then diluted to 80 mL with THF, Ni(dppp)Cl2 (0.0054 g, 0.01 mmol) was

added at once and the mixture was allowed to stir for 24 h at room

temperature. Termination and removal of bromine chain ends was

accomplished by the slow addition of LiAlH4 (4.6 mL, 4.6 mmol, 1 M in

THF). After another 16 h, excess LiAlH4 was quenched by slow dropwise

addition of HCl (10 mL, 1 M) (caution: this step may evolve rapid H2), the

polymer was recovered by precipitation in ethanol (1 L) and filtered into a

Soxhlet extraction thimble. Following extensive Soxhlet washing with

methanol, hexane and chloroform, α,ω-diH-P3HT was recovered from the

Soxhlet filter with chloroform. Following precipitation in ethanol and filtered

with G4 crucible, drying under reduced pressure overnight at 70 ˚C, the

polymer was stored under an inert atmosphere and protected from light.

Debrominated P3HT (α ,ω -diH-P3HTs)

P3HT

Monomer M1

[g, (mmol)]

Grignard reagent t-BuMgCl

(1 M in THF) [mL, (mmol)]

Ni(dppp)Cl2

[g, (mmol)] Polymerization

time

(h)

LiAlH4 (1M in THF)

[mL, (mmol)]

Mn (g mol-1,

GPC)

Đ

P1 2.83

(8.6)

8.6

(8.6)

0.01

(0.02) 24 4.3 (4.3) 30 000 1.6

P1a 3.01

(9.2)

9.2

(9.2)

0.0054

(0.01) 24 5.0 (5.0) 50 000 1.7

P1b 6.0

(18.4)

18.4

(18.4)

0.006

(0.01) 24 10.0 (10.0) 117 000 1.7

P1c 3.14

(9.62)

9.5

(9.5)

0.090

(0.166) 2 10.0 (10.0) 7 000 1.1

Table 1: Reaction conditions and macromolecular characteristics determined by SEC (THF, UV 254 nm) against polystyrene standards of α,ω-diH-P3HTs (Note: Dispersity, Đ = Mw/Mn).

202

2.2.2 Chain-end functionalized ω-P3HTs or α,ω-P3HTs (P2-P6)

(typical end-capping reaction): In a three-necked round bottom flask; 2,5-

dibromo-3-hexylthiophene (M1, 3 g, 9.2 mmol) was dissolved in THF (20

mL) and stirred under nitrogen. tert-Butylmagnesium chloride (9.3 mL, 9.2

mmol, 1 M in THF) was added via syringe and the mixture stirred at room

temperature for 2.5 h. Following dilution to 80 mL with THF, Ni(dppp)Cl2

(0.068 g, 0.125 mmol) was added in one portion and the mixture stirred for

30-60 min at room temperature. The Grignard reagent (50-60 mol % of

monomer) with respect to the functionalisation was added via syringe to

the reaction mixture and stirred for an additional 30-60 min at room

temperature. Finally the reaction was quenched by adding conc. HCl (5 M)

and then poured into methanol (800 mL) to precipitate the polymer. The

polymer was filtered into an extraction thimble and then washed by Soxhlet

extraction with methanol, hexane and chloroform. The polymer was

isolated from the chloroform extraction and concentrated under reduced

pressure, precipitated into methanol, filtered with G4 crucible, dried

overnight under vacuum and finally stored under inert atmosphere

protecting from light.

2.2.2.1 ω-Ethynyl, ω-vinyl-P3HTs and α,ω -pentynyl-P3HTs: The deprotection of polymers was performed as follows. In the case of P2,

P2a, P2b, P2c, P2d and P2e; the Grignard reagent used for

functionalization was ethynyl-magnesium bromide, leading to the ω-

ethynylP3HTs, whereas for P6, P6a and P6b; vinyl-magnesium bromide is

used to obtain ω-vinylP3HTs (Table 2). For these two functional groups,

the Grignard reagent is commercially available which is not the case of

α,ω-pentynylP3HTs (P3 and P3a), in which the terminating Grignard agent,

5-chloromagnesio-1-pentynyl)trimethylsilane, was synthesized by the

following procedure. In a 50 mL two-necked flask were introduced (5-

chloro-1-pentynyl)trimethylsilane (4.97 g, 28.4 mmol), magnesium turnings

(0.972 g, 40 mmol) and THF (30 mL). The mixture was stirred for 24 h at

25 ºC and then added to the end of the P3HT polymerization . The

polymers were then deprotected to obtain α,ω-pentynyl-P3HTs after the

203

purification steps. The polymers were dissolved in THF and then cooled to

-20 ºC. TBAF.3H2O solution (0.20 M) was added slowly to the medium at

this temperature, then the mixture is stirred 4 h at room temperature. The

solution is then passed through a short silica column to remove excess

TBAF. α,ω-pentynylP3HTs were then recovered by precipitation in

methanol and dried under reduced pressure.

2.2.2.2 α ,ω -Diformyl and α ,ω -dihydroxy-P3HTs: In the case of α,ω-diphenylformyl-P3HT (P4 and P4a), the Grignard

reagent was 4-(1,3-dioxan-2-ylphenyl) magnesium bromide and the

deprotection was done by overnight refluxing the polymer (0.70 g, 0.166

mmol) in THF with pyridinium p-toluenesulfonate (PTS) (0.160 g, 0.637

mmol). In the case of α,ω-diethylformyl-P3HT (P4b), the Grignard reagent

was 4-(1,3-dioxan-2-ylethyl) magnesium bromide and the deprotection was

performed successfully by refluxing the polymer (0.05 g) overnight in THF

(20 mL) with concentrated HCl (0.6 mL). For the synthesis of α,ω-

diphenylhydroxy-P3HT (P5), the Grignard reagent was 4-(2-tetrahydro-

2H-pyranoxy)-phenyl magnesium bromide and the complete deprotection

was observed by refluxing the polymer (0.75 g) with conc. HCl (2.0 mL) in

THF (150 mL) for 18 h. All the deprotected α,ω-difunctionalised P3HTs

(Table 2) were again purified by Soxhlet washing with methanol and

extraction with chloroform.

204

Chain-end Functionalised P3HT (ω- or α ,ω-P3HTs)

P3HT

Monomer

M1 [g, (mmol)]

Grignard reagent

t-BuMgCl (1 M in THF)

[mL, (mmol)]

Ni(dppp)Cl2

[g, (mmol)]

Polym.

time (min)

Grignard reagent

used for functionalisation

Mn

(g mol-1, GPC)

Đ

P2 4.50

(13.7)

13.7

(13.7)

0.074

(0.136) 40 Ethynyl-MgBr 14 000 1.1

P2a 4.56

(14.0)

14.0

(14.0)

0.137

(0.252) 60 Ethynyl-MgBr 9 000 1.2

P2b 3.33

(10.2)

10.0

(10.0)

0.095

(0.175) 30 Ethynyl-MgBr 7 700 1.1

P2c 3.0

(9.2)

9.2

(9.2)

0.090

(0.166) 30 Ethynyl-MgBr 8 500 1.1

P2d 1.43

(4.38)

4.3

(4.3)

0.04

(0.073) 60 Ethynyl-MgBr 3 500 1.1

P2e 3.19

(9.78)

9.5

(9.5)

0.09

(0.166) 60 Ethynyl-MgBr 2 500 1.1

P3 5.68

(17.4)

17.4

(17.4)

0.160

(0.295) 60

ClMg(C5H6)Si(Me)3/

TBAF.3H20 8 000 1.1

P3a 1.6

(4.9)

4.8

(4.8)

0.045

(0.083) 30

ClMg(C5H6)Si(Me)3/

TBAF.3H20 6 200 1.1

P4 3.31

(10.15)

10.15

(10.15)

0.0976

(0.18) 40

1,3-dioxan-2-yl)

phenyl-MgBr/PTS 7 000 1.1

P4a 3.22

(9.8)

9.8

(9.8)

0.06

(0.11) 120

(1,3-dioxan-2-yl)

phenyl-MgBr/PTS 14 000 1.1

P4b 3.22

(9.8)

9.8

(9.8)

0.117

(0.215) 30

(1,3-dioxan-2-yl)

ethyl-MgBr/HCl 5 800 1.1

P5 3.0

(9.19)

9.2

(9.2)

0.068

(0.125) 120

4-(2-tetrahydro-2H-

pyranoxy)phenyl-

MgBr/HCl

11 000 1.2

P6 2.09

(6.4)

6.4

(6.4)

0.09

(0.166) 30 Vinyl-MgBr 5 500 1.2

P6a 2.78

(8.52)

8.5

(8.5)

0.078

(0.144) 45 Vinyl-MgBr 7 400 1.1

P6b 3.25

(9.96)

9.9

(9.9)

0.09

(0.166) 120 Vinyl-MgBr 9 000 1.2

Table 2: Reaction conditions, and macromolecular characteristics determined by SEC (THF, UV-254 nm) against polystyrene standards of chain-end functionalized ω-P3HTs or α,ω-P3HTs (P2-P6).

205

2.3 Mono-functionalised P3HTs (P7-P8) by externally added Ni- catalyst initiator 2.3.1 Ni-initiator [(Ph)Ni(PPh3)2-Br]: In a flame-dried Schlenk flask, to a solution of [Ni(PPh3)4] (0.2 g, 0.18

mmol) in dry toluene (3 mL), bromobenzene (0.1 mL, 0.95 mmol) was

added at room temperature under argon atmosphere. Then the

homogeneous mixture was allowed to stir for about 30 min and allowed to

stand for unperturbed overnight. The original deep red colour of the

reaction mixture gradually changed to brownish yellow colour with the

precipitation of [(Ph)Ni(PPh3)2-Br] (4), the yellow crystals of which were

filtered under argon atmosphere and washed with dry pentane (yield:

0.055 g).

2.3.2 Mono-functionalised P3HTs by small molecule Ni-initiator: In a typical polymerization; 2-Bromo-3-hexyl-5-iodothiophene (M2) (0.586

g, 1.57 mmol) was placed in a round-bottomed flask equipped with a

magnet stirrer bar, and the atmosphere was replaced with argon. Dry THF

(30 mL) was added via a syringe, and the mixture was cooled to 0 ºC.

Afterwards, isopropylmagnesium chloride (2.0 M solution in THF, 0.80 mL,

1.57 mmol) was added via a syringe, and the mixture was stirred at 0 ºC for

1 h. A solution of Ni-catalyst initiator (4) in dry toluene (40 mg in 2 mL, 3.50

mol %) was added via a syringe at 0 ºC, and then the mixture was stirred

for 6 h at 0 °C. At the end of the polymerization, the reaction was

quenched with 5 M HCl or a functional Grignard reagent to result in chain-

end capping. We used the protected Grignard reagent (5-chloromagnesio-

1-pentynyl) trimethylsilane with the aim of preparing α-Ph-ω-pentynyl-

P3HT (P7, P7a). The former case, with HCl led to α-Ph-ω-H-P3HT (P8,

P8a, P8b) (Table 3). The reaction conditions leading to the characterised

mono-functionalised P3HTs from P7 to P8b are shown in Table 3.

206

P3HT Monomer

M2

[g, (mmol)]

Grignard reagent tPrMgCl

(2 M in THF) [mL, (mmol)]

Ni-initiator [(Ph)Ni(PPh3)2Br]

[g, (mmol)]

Polymerization time

(h)

Grignard reagent for endcapping

Target structure

P7 1.28

(3.43)

1.74

(3.43)

0.1

(0.135)

6

(0 ˚C)

ClMg(C5H6)Si(Me)3/

TBAF.3H20 α-Ph-ω-

ethynyl-P3HT

P7a 0.58

(1.57)

0.8

(1.57)

0.04

(0.054)

6

(0 ˚C)

ClMg(C5H6)Si(Me)3/

TBAF.3H20 α-Ph-ω-

ethynyl-P3HT

P8 0.50

(1.34)

0.7

(1.34)

0.035

(0.047)

6

(0 ˚C)

5 M HCl

(10 drops) α-Ph-ω-H-

P3HT

P8a 0.60

(1.60)

0.8

(1.60)

0.04

(0.054) 3 (RT)

5 M HCl


P3HT

P8b 0.50

(1.34)

0.7

(1.34)

0.035

(0.047) 3 (RT)

5 M HCl


P3HT Table: 3 Reaction conditions of mono-functionalised-P3HTs (P7-P8) prepared using the small molecule Ni-initiator. 2.4 Azide-terminated Polystyrene 2.4.1 Azide initiator for ATRP: 2.4.1.1 3-Azido-1-propanol: 3-bromo-1-propanol (4) (10 g, 72 mmol),

NaN3 (7 g, 108 mmol), Bu4NI (4 g, 11 mmol) and dicyclohexano-18-crown-

6 (20 mg, 0.07 mmol) were dissolved in 2-butanone (50 mL) and the

mixture was stirred under reflux for 24 h. The mixture was then filtered, the

solid rinsed with acetone and the combined solutions were concentrated.

After distillation, the pure product (5) was obtained as a colorless oil (6 g,

83 %).

Characterization of 3-azido-1-propanol: 1H NMR (400 MHz, CDCl3): δH1.81 (q, 2H, CH2-CH2-CH2), 2.02 (s, 1H, CH2-OH), 3.43 (t, 2H, CH2-N3),

3.72 (t, 2H, CH2-OH); δ C 31.44 (CH2-CH2-CH2), 48.47 (CH2-N3), 59.84

(CH2-OH).

2.4.1.2 3-Azidopropyl-2-bromoisobutyrate: A solution of α-

bromoisobutyryl bromide (11.95 g, 6.43 mL, 52 mmol, 1.05 eq.) in THF (50

mL) was added dropwise to a solution of 5 (5 g, 49.5 mmol) and

triethylamine (6.5 g, 9 mL, 64.4 mmol) in THF (50 mL) at 0 ºC. After

complete addition, the reaction mixture was stirred for 2 h at 25 ºC. Excess

acid bromide was quenched by addition of degassed methanol (50 mL).

The formed triethylammonium bromide salt was filtered off and the solution

207

was concentrated. The crude product was dissolved in CH2Cl2, washed 3

times with a saturated ammonium chloride solution and 3 times with

distillated water. The organic layer was dried over sodium sulfate

(Na2SO4), filtered and concentrated, yielding pale yellow oil, which was

dried under vacuum (9 g, 81%).

Characterization of 3-azidopropyl 2-bromoisobutyrate: 1H NMR (400 MHz, CDCl3): δH1.92 (s, 6H, (CH3)2C), 1.96 (q, 2H, CH2-CH2-CH2), 3.44 (t,

2H, CH2-N3), 4.27 (t, 2H, CH2-O-C(=O)); δC 27.97 (CH2-N3), 30.70 ((CH3)2-

C), 48.03 (CH2-CH2-CH2), 55.66 (C-Br), 62.74 (CH2-O), 171.53 (C=O).

2.4.2 α-Azido-polystyrenes (PS1-PS6): Here is the general procedure for the synthesis of polystyrenes terminated

with azide functional group (PS1-PS6). Experiments were carried out

varying the polymerization time to achieve different molar mass of

polystyrenes, which was showed in the Table 4 below. In a typical

experiment; 3-azidopropyl-2-bromoisobutyrate (6) (0.25 g, 1 mmol), freshly

purified CuBr (0.143 g, 1 mmol), 2,2’-bipyridyl (0.468 g, 3 mmol) and

styrene (5 g, 5.5 mL, 48 mmol) were added to a Schlenk flask. The mixture

was stirred for 5 min and degassed three times by freeze-pump-thaw

cycles to remove residual oxygen. The polymerization reaction was

performed at 130 ºC. The reaction was stopped by dropping the

temperature of the Schlenk flask to 0 ºC. The solution was then dissolved

in THF and passed through a basic alumina column. After being

concentrated, the solution was precipitated in methanol and the polymer

was dried overnight under vacuum and characterized by FTIR and 1H

NMR, SEC and DSC.

208

PS

Initiator

[mg, (mmol)]

CuBr

[mg, (mmol)]

Bipyridine

[mg, (mmol)]

Styrene

[g, (mmol)]

Polymerisation

time (min) Mn

(SEC, g/mol) Đ

PS1 260

(1.04)

148

(1.03)

487

(3.12)

5

(48) 12 2600 1.08

PS2 250

(1.0)

143

(1.0)

468

(3.0)

5

(48) 25 3800 1.17

PS3 250

(1.0)

143

(1.0)

468

(3.0)

5

(48) 10 1900 1.21

PS4 250

(1.0)

143

(1.0)

468

(3.0)

5

(48) 30 4500 1.30

PS5 500

(2.0)

286

(2.0)

936

(6.0)

10

(96) 11 2000 1.11

PS6 260

(1.04)

148

(1.03)

487

(3.12)

5

(48) 30 5200 1.29

Table 4: Reaction conditions and characteristics of the synthesized α-N3-ω-bromo-polystyrenes (PS1-PS6, SEC in THF, UV-254 nm). 2.5 Block copolymers P3HT-block-PS and PS-block-P3HT-block-PS by Click Chemistry 2.5.1 Triblock copolymers PS-b-P3HT-b-PS For the synthesis of triblock copolymers PS-b-P3HT-b-PS, α,ω-

pentynylP3HT of different molecular weights (P3 and P3a) was reacted

with different molecular weight α-azido-polystyrenes (PS1, PS2, PS3 and

PS4, Table 5). In a typical experiment for the synthesis of PS1-b-P3-b-

PS1, α,ω-pentynylP3HT (P3, 250 mg, 0.052 mmol), PS1 (405 mg, 0.156

mmol) and CuI (37 mg, 0.259 mmol) were introduced to a 50 mL round-

bottom flask, evacuated for 10 min and backfilled with nitrogen (3 cycles).

A solution of degassed DBU (1,8-Diazabicyclo[5.4.0]undec-7-ene, 316 mg,

2.08 mmol) in THF (25 mL) was added and the flask was placed in a

constant temperature oil bath at 50 ºC for 5 days. The solution was passed

through a neutral alumina column in order to remove copper salt. After

concentration, the product was recovered by precipitation in methanol,

dried under reduced pressure, and then three times dissolved in a

minimum of THF and precipitated in acetone to remove unreacted PS and

low molar mass P3HT. Further drying under reduced pressure yielded pure

copolymers characterized by SEC, FTIR and 1H NMR.

209

PS-b-P3HT-b-PS P3 or P3a

[mg, (mmol)] Azido-PS

[mg, (mmol)] CuI

[mg, (mmol)]

DBU [mg, (mmol)]

PS1-b-P3-b-PS1 250 (0.052)

405 (0.156)

37 (0.259)

316 (2.08)

PS2-b-P3-b-PS2 250 (0.052)

600 (0.157)

37 (0.259)

316 (2.08)

PS3-b-P3a-b-PS3 125 (0.034)

275 (0.068)

20 (0.105)

210 (1.38)

PS4-b-P3a-b-PS4 70 (0.02)

104 (0.054)

40 (0.210)

210 (1.38)

Table 5: Reaction conditions for the synthesized triblock copolymers, PS-b-P3HT-b-PS. 2.5.2 Diblock copolymers P3HT-b-PS For the synthesis of diblock copolymers P3HT-b-PS; ω-ethynyl-P3HT of

different molecular weights (P2 and P2a) reacted with α-azido-polystyrene

(PS2, Table 6). In a typical experiment for the synthesis of P2-b-PS2;

ω-ethynyl-P3HT (P2, 219 mg, 0.022 mmol), PS2 (280 mg, 0.073 mmol),

CuI (40 mg, 0.210 mmol), DIPEA (diisopropylethylamine, 565 mg, 4.38

mmol) and THF (30 mL) were introduced to a 50 mL round-bottom flask,

evacuated for 10 min and backfilled with nitrogen (3 cycles). Then the

reaction mixture was subjected for sonication (2 h) to aid ethynyl-P3HT

dissolution in THF (clear orange solution) and the flask was placed in a

constant temperature oil bath at 50 ºC for 5 days. The solution was passed

through a neutral alumina column in order to remove copper salt. After

concentration, the product was recovered by precipitation in methanol,

dried under reduced pressure, and then three times dissolved in a

minimum of THF and precipitated in acetone to remove unreacted PS and

low molar mass P3HT. Further overnight drying under reduced pressure

yielded pure copolymers characterized by SEC, FTIR and 1H NMR.

P3HT-b-PS P2 or P2a [mg, (mmol)]

Azido-PS2 [mg, (mmol)]

CuI [mg, (mmol)]

DIPEA [mg, (mmol)]

P2-b-PS2 219 (0.022)

280 (0.073)

40 (0.210)

565 (4.38)

P2a-b-PS2 425 (0.077)

500 (0.131)

100 (0.526)

742 (5.75)

Table 6: Reaction conditions for the synthesized diblock copolymers, P3HT-b-PS.

210

2.6 P3HT-block-PS-C60 and C60-PS-b-P3HT-b-PS-C60 by ATRA The reaction conditions for all the synthesized C60-attached di-and tri-block

copolymers by ATRA were given in the Table 7. In a typical experiment for

the synthesis of P2-b-PS2-C60; C60 (28 mg, 0.039 mmol), CuBr (12 mg,

0.084 mmol), 2,2’-bipyridine (26 mg, 0.017 mmol) and P2-b-PS2-Br (91

mg, 0.0065 mmol) were introduced to a 50 mL round-bottom flask and

dissolved in 20 mL of chlorobenzene (freshly distilled over CaH2). The

mixture was stirred for 5 min and degassed three times by freeze-pump-

thaw cycles to remove residual oxygen. Then the reaction was performed

at 110 ºC for about 24 h. After 24 h, the mixture was dropped into THF

(200 mL) to precipitate unreacted fullerene that was then removed along

with copper salts by passing the solution through a neutral alumina

column. Once precipitated from THF in excess methanol, the polymer was

recovered by filtration, redissolved in THF and again passed through a

fresh column. This procedure was then repeated for three times, following

precipitation in methanol and dried under reduced pressure at 40 ºC for 3

days yielded pure C60-attached copolymers.

C60-attached block copolymer

Block copolymer [mg, (mmol)]

C60 [mg,

(mmol)]

CuBr [mg,

(mmol)]

Bipyridine [mg,

(mmol)]

P2-b-PS2-C60 P2-b-PS2-Br

[91, (0.0065)]

28

(0.039)

12

(0.084)

26

(0.017)

P2a-b-PS2-C60 P2a-b-PS2-Br

[113, (0.012)]

51

(0.071)

22

(0.015)

48

(0.031)

C60-PS1-b-P3-b-PS1-

C60


[150, (0.015)]

65

(0.09)

22

(0.015)

58

(0.037)

C60-PS2-b-P3-b-PS2-

C60


[150, (0.012)]

52

(0.072)

21

(0.014)

47

(0.030)

Table 7: Reaction conditions for the synthesized C60-attached di-and tri-block copolymers by ATRA.

211

2.7 Triblock copolymers P4VP-block-P3HT-block-P4VP by anionic polymerisation The reaction conditions for all the synthesized triblock copolymers P4VP-b-

P3HT-b-P4VP by anionic polymerisation were given in the Table 8.

Preparation of initiator (we prepared three times the desired amount of

initiator): In a 50 mL flame dried Schlenk flask, 10 mL of distilled THF was

cooled at -78 ºC under argon and then freshly distilled α-methylstyrene (0.5

mL, 3.8 mmol) was added. A few drops of sec-butyllithium (1.4 M in

cyclohexane) were added until the persistence of light red color and the

desired amount of sec-butyllithium (2.2 mL, 3.0 mmol) was rapidly injected

to form the initiator. The reaction mixture was stirred for about 15 min and

then kept at -78 ºC which is stable.

Polymerization of 4-vinylpyridine (4-VP) and synthesis of triblock copolymer, P4VP-b-P4-b-P4VP (3): In a three-neck 500 mL flame dried

flask, 200 mL of freshly distilled THF (1 mL HMTP) was cooled at -78 ºC

under argon and 4-vinylpyridine previously purified by two distillations (3.9

mL, 36.2 mmol) was then added. A few drops of initiator were added to the

stirred solution until persistent yellow coloration obtained and then the

required amount of initiator (4 mL of the initiator solution) was immediately

injected. The polymerization was left at -78 ºC for about 30 min. Aldehyde

end-functionalized P3HT (P4) quencher was dried by azeotropic

distillation: P3HT (P4) was dissolved in distilled toluene and evaporated

under reduced pressure three times before it actually dissolved in 10 mL of

distilled toluene. Finally, polymerization of 4-vinylpyridine was quenched by

rapid addition of aldehyde-end functionalized P3HT (P4, 0.1 g, 0.024

mmol) into the reactive medium. The reaction mixture is slowly allowed to

return to room temperature and left it for overnight. The solvents are then

evaporated under vacuum and the obtained polymer was redissolved in 50

mL of chloroform. According to the reported procedure, a large excess of

living anionic P4VP chains were used. To separate the copolymer P4VP-b-

P3HT-b-P4VP (3) from the P4VP homopolymer, the P4VP was protonated

by washing several times (at least 3 times) the organic phase with 100 mL

of HCl/H2O (pH=4). The organic phase was washed three times with

212

distilled water, dried over Na2SO4, filtered and the solvent was evaporated

under reduced pressure to yield pure copolymer P4VP-b-P3HT-b-P4VP

(3).

Copolymer Initiator

[mL, (mmol)] 4-VP

[mL, (mmol)] P3HT (P4)

[mg, (mmol)]

P4VP-b-P4-b-P4VP (1) 10 (4.82)

7.80 (72.38)

200 (0.048)

P4VP-b-P4-b-P4VP (2) 4 (0.95)

1.95 (18.09)

100 (0.024)

P4VP-b-P4-b-P4VP (3) 4 (0.95)

3.90 (36.19)

100 (0.024)

Table 8: Reaction conditions for the synthesized triblock copolymers, P4VP-b-P3HT-b-P4VP by anionic polymerisation.

2.8 Polyacetylene-graft-P3HT (PA-graft-P3HT) 2.8.1 Phenylacetylene polymerisation by [Rh(nbd)Cl]2 catalyst Polymerization was carried out under argon atmosphere in a Schlenk tube

equipped with a three-way stopcock. A typical polymerization procedure is

as follows: A distilled THF solution (8.0 mL) of phenylacetylene (0.465g,

4.56 mmol) was added to a triethylamine (TEA) solution (2.0 mL) of

[Rh[(nbd)Cl]2 (0.02 g, 0.043 mmol) and then polymerization was carried

out at 30 ºC for 24 h. The polymer was precipitated in methanol, filtered

and dried under vacuum to obtain pure polymer.

2.8.2 Polymerisation of ω-alkynyl-P3HTs by [Rh(nbd)Cl]2 catalyst In a typical polymerization of ω-ethynyl-P3HT (P2b); to a 50 mL flame dried

Schlenk flask contained P2b (0.2 g, 0.049 mmol) in a distilled THF solution

(25 mL) was added a triethylamine (TEA) solution (5.0 mL) of [Rh(nbd)Cl]2

(0.01 g, 0.021 mmol) and then polymerization was carried out at 30 ºC for

24 h. The polymer was precipitated in methanol, filtered and dried under

vacuum to obtain pure graft copolymer, PA-graft-P3HT.

213

2.8.3 Attempted copolymerisation of ω-acetylene-P3HT with phenyl acetylene This is a typical copolymerization of ω-ethynyl-P3HT (P2c) with

phenylacetylene (50 %). To a 50 mL flame dried Schlenk flask; P2c (0.4 g,

0.08 mmol), phenylacetylene (0.008 g, 0.08 mmol) and 30 mL distilled THF

were introduced. The reaction mixture was stirred and sonicated for 30

min. Now the catalyst solution [Rh[(nbd)Cl]2 (0.0062 g, 0.0134 mmol) in

TEA (5.0 mL) was injected rapidly to the reaction mixture and then the

polymerization was carried out at 30 ºC for 48 h. The polymer was

precipitated in methanol, filtered and dried under vacuum.

3. Characterization

♦ 1H, 13C and 2D-HMQC NMR spectra were recorded using a Bruker

AC-400 NMR spectrometer (400 MHz). All the samples are analyzed in

CDCl3 solution.

♦ Fourier Transform InfraRed measurements (FTIR) spectra were

performed on a Bruker Tensor 27 spectrometer having a beam

diameter of 0.6 mm, a resolution of 4 cm-1 and a spectral range

between 4000 cm-1 and 400 cm-1. The different samples were

analyzed qualitatively after evaporation of a drop of solution containing

1 g/mL on an ATR cell. The spectra were all corrected by the reference

spectrum.

♦ The relative molecular weights of polymers and copolymers

synthesized were determined by size exclusion chromatography (SEC)

at room temperature in THF. These tests were performed on a system

equipped with a Waters pump type 880-PU. It includes a set of three

columns Tosohaas TSK-gel (styrene-divinylbenzene) in series, a

differential refractometric detector (Varian RI-4) and a UV absorption

detector (Jasco 875, λ = 254 nm). The values of molecular weights

were evaluated against a series of well-defined polystyrenes.

214

♦ MALDI-TOF mass spectra were performed at the Center for Study and

Structural Analysis of Organic Molecules by C. Absalon (CESAMO,

University of Bordeaux 1) on a Voyager mass spectrometer (Applied

Biosystems). The unit is equipped with a pulsed nitrogen laser (λ = 337

nm). The spectra were made in positive ionization mode with

reflectron, and an acceleration voltage of 20 kV. The samples were

dissolved in THF at 5 mg/mL. Dithranol matrix was prepared by

dissolving 10 mg of product in 1 mL of dichloromethane. A cationizing

agent solution (NaI) in methanol (10 mg/mL) was also prepared. These

different solutions were combined in a 10:1:1 ratio

(matrix:sample:cationizing agent). µL of this solution was deposited on

the target and dried under vacuum.

♦ Differential scanning calorimetry (DSC) analyses were performed

using the DSC Q100 (TA Instruments). The samples were subjected to

two heating cycles (50 ºC to 250 ºC) and a cooling cycle. All

measurements were made at a constant speed of 10 ºC/min.

♦ The UV-visible absorption spectra of films were obtained on the

spectral range from 200 nm to 900 nm on two spectrometers: a Varian

Cary 3E and SAFAS UVmc2. The spectra were all corrected by the

reference spectrum. The films were prepared by deposition of

solutions of polymers and copolymers by spin-coating with a SCS

P6700 device on quartz plates. In each series of experiments, the

thickness is kept constant and verified by profilometry (KLA Tencor,

Alpha Step IQ).

♦ Atomic force microscopy (AFM) images were recorded in air with a

Nanoscope IIIa microscope operating in tapping mode (TM). The

probes were commercially available as silicon tips with a spring

constant of 42 N m-1, resonance frequency of 285 kHz, and a typical

radius of curvature in the 10-12 nm range. Both the topography and

the phase signal images were recorded at Centre de Recherche Paul

Pascal (CRPP) by E. Ibarboure (LCPO).

♦ TGA measurements were taken using PERKIN ELMER

Thermogravimetric Analyzer (TGA 7).

215

4. Photovoltaic device fabrication and characterization Organic solar cells were fabricated on indium tin oxide (ITO)

substrates on glass (Merck Display) cleaned by successive ultrasonic

baths in water, acetone, ethanol and isopropanol and then treated with UV-

ozone. The substrates were then covered by a 40 nm thick layer of

PEDOT-blend-PSS deposited by spin-coating at 4000 rpm for 1 min and

then annealed at 110 ºC under rotary pump vacuum for 1 h. The P3HT-

blend-PCBM-blend- (%copolymer) was deposited by spin coating at 1200

rpm for 90 s from anhydrous chlorobenzene solution. A 1:1 P3HT-blend-

PCBM weight ratio was used throughout all experiments and (0-5 %) of

copolymer added to P3HT-blend-PCBM. Film thicknesses were measured

using an Alpha-step IQ profilometer and all found to be ca 100 nm. The

aluminium cathode was thermally evaporated under a secondary vacuum

(10-6 mbar) through a shadow mask. The active surface area of the device

was 10 mm2. The annealing process was carried out under an inert

atmosphere by placing the cells directly onto a controlled hot plate.

The Current-voltage curves and conversion efficiencies of the solar

cells were recorded using a Keithley 4200 SCS, under an illumination of

100 mW/cm2 from a KHS Solar Celltest 575 solar simulator with an AM1.5

G filter. The luminance intensity was checked against an IL1400

radiometer.

217

General Conclusions

219

General Conclusions The objective of this research work was to develop a simplified and

versatile synthesis of novel block copolymers (BCPs) based on poly(3-

hexylthiophene) (P3HT), to understand the microstructure of the resulting

functional BCPs, and to examine the use of these materials as active

layers or compatibilizer in organic solar cells (OSCs). These materials

were chosen for their ability to self-organize and thus to form structures

that can optimize certain physical parameters in the photovoltaic process,

such as the dissociation of excitons or transportation of charges to the

electrodes.

We have prepared two types of BCPs based on P3HT: (i)

polyacetylene-graft-P3HT (PA-g-P3HT) graft copolymers using

macromonomer alkynyl-P3HTs; and (ii) donor-acceptor rod-coil di- and tri-

BCPs in which P3HT was chosen as the donor block (rod), and

polystyrene (PS) or poly(4-vinylpyridine) (P4VP) were chosen as coil

blocks to carry the acceptor C60 in different ways. The first stage of the

thesis was to develop a simple and versatile synthesis of such copolymers,

which could meet the requirements of a potential industrialization. The

second stage of the thesis was to utilize these BCPs in different

proportions as compatibilizers in P3HT-blend-PCBM devices to enhance

the photo conversion efficiency (PCE).

In the first stage, we synthesized high molecular weight

regioregular P3HTs and also highly regioregular (above 95%) chain end-

functionalised P3HTs with narrow dispersities by the GRIM method. A

“small molecule” Ni-initiator was also synthesized and utilized to prepare

completely mono-functionalised P3HTs. But we could not reproduce

Senkovskyy et al.’s results. We obtained a mixture of products when we

used the so-called “external” initiator, whereas the GRIM method produced

better results. We were somewhat more successful in our attempts to

prepare P3HT grafted copolymers by alkynyl-P3HTs. We found that

conjugation and steric hindrance play a key role for the polymerization of

alkynyl-P3HTs by Rhodium based catalyst and also in the polymerization

220

of ω-vinyl-P3HTs by RAFT and olefin polymerization methods. For the

efficient polymerization of P3HT-substituted acetylenes, the acetylene

group should be directly attached to the aromatic group; however, it is

probable that a spacer is necessary to separate the bulk of the aromatic

acetylene group from P3HT due to conjugation and steric hindrance.

Therefore further investigations are required.

The synthesis of donor-acceptor block copolymers based on P3HT,

PS and P4VP by two different approaches was successful. Azide

terminated polystyrenes of different molecular weights were successfully

synthesized by atom transfer radical polymerization (ATRP). In the first

approach, we synthesized the di- and tri-block copolymers, P3HT-b-PS

and PS-b-P3HT-b-PS by "click" chemistry of polystyrene terminated azide

and P3HT alkyne in the presence of copper catalysts. This study

represents the modification of the reported literature by our group, which is

the first example of synthesis of exclusively rod-coil block copolymers, by

"click" chemistry. However, the influence of the conjugated chain of P3HT

on the alkyne function is very important and necessary to introduce a

separation between the two entities to achieve efficient coupling reaction

or by varying the “click” chemistry conditions with the help of sonication,

one can achieve the expected copolymers. The C60-attached copolymers

(P3HT-b-PS-C60 and C60-PS-b-P3HT-b-PS-C60) were obtained by atom

transfer radical addition (ATRA) of bromine terminated PSs. This reaction

was performed by reacting the copolymers P3HT-b-PS and PS-b-P3HT-b-

PS with C60 in the presence of CuBr/bipyridine in chlorobenzene.

In the second approach; the triblock copolymers, P4VP-b-P3HT-b-

P4VP which contained the donor P3HT blocks and acceptor domains

P4VP coil blocks were successfully synthesized via anionic polymerization.

The molecular weight of these polymers was identified by 1H NMR only

since the solubility of these triblock copolymers was not high in THF and

DMF (the solvents used in our SECs). All these copolymers were then

characterized by UV-visible absorption spectroscopy and differential

scanning calorimetry, to assess their physical properties. These measures

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have enabled to determine their characteristic temperatures (glass

transition, melting, crystallization), very important elements with respect to

their potential application in OSCs. Finally the synthesized di- and tri-BCPs P2-b-PS2, P2-b-PS2-C60,

C60-PS2-b-P3-b-PS2-C60 and P4VP-b-P3HT-b-P4VP were used as

compatibilizers in a mixture of P1a-blend-PCBM active layer for OSCs.

The first study involved the optimization of P3HT mixed with PCBM, for

which a maximum efficiency was obtained for P1a, i.e. a higher PCE was

found that with the commercial sample (Plextronics) of a similar molecular

weight, and when annealing at 180 ºC. The BCPs were then tested as

compatibilizers (0-5 %) in combination with P1a and PCBM (1:1) based

devices. The device based on P1a-blend-PCBM-blend-(P2-b-PS2) using

1% addition of copolymer achieved the highest short-circuit current (11

mA/cm2) and also highest PCE (3.7%) at an annealing temperature of 167 ºC in our solar cell studies. At 5% addition of this copolymer P2-b-PS2, the

structure was completely disorganized and tangled fibrils random

structures lead to a substantial decrease in the Jsc, and explains the drop

in performance for the mixtures of 1.5%, 2.0% and 5.0%. This is explained

by taking into account the degree of crystallinity of this copolymer, which is

superior to others, reflecting its ability to facilitate better mobility of charge

carriers in the active layer.

The donor-acceptor diblock copolymer, P2-b-PS2-C60, was

characterized in OSC as compatibilizers by adding 0-5% amounts to P1a-

blend-PCBM (1:1) based devices. Surprisingly it was shown that the

addition of copolymer (0.5%) to P1a-blend-PCBM (0%) at 180 ºC annealing

significantly changes the photovoltaic parameters involved especially PCE,

(from 3.0-4.0%), fill factor with maximum value of 0.65, but the maximum

short-circuit current, 12.0 mA/cm2 was observed at 0.5% addition of

copolymer. In this case, we have observed an excellent fibrilar

morphology. The 5% weight addition of copolymer disrupts the fibrillar

structure of the P3HT chains (height images of Figure 4.15) which lead to

reduced fill factor and reduced energy conversion efficiencies.

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In the case of triblock copolymers, the devices based on P1a-blend-

PCBM-blend-(C60-PS2-b-P3-b-PS2-C60) (0-7 %) (annealed, 180 ºC)

showed decreases in the Jsc with addition of copolymers whereas

unannealed devices showed Jsc values significantly increased with 0.5%

copolymer, but started decreasing on further addition of copolymer to

reach a low value of 1.0% at 7% copolymer. Nevertheless, with 0.5%

copolymer the efficiency nearly doubled from 0.67 to 1.27%. The devices

based on P1a-blend-PCBM-blend-(P4VP-b-P4-b-P4VP), in which the

copolymer was used as a compatibilizer (0, 0.5% and 1.0%) showed

decreases in Jsc with the addition of copolymer for annealed devices. We

have also used the copolymer, P4VP-b-P4-b-P4VP as donor material in

the device, (P4VP-b-P4-b-P4VP):PCBM (1:1) at room temperatures and at

annealed temperatures 167 ºC and 175 ºC, but only low PCEs were found.

Hence we have observed that the addition of triblock copolymers as

compatibilizer disrupts the molecular structure of P3HT chains resulting in

low efficiencies. But in the case of diblock copolymers as compatiblizers,

we have observed the enhancement of Jscs and efficiencies with respect to

P3HT-blend-PCBM device alone. This might be due to the nano-domain

constraints placed upon such systems by tri-block copolymers, and would

tend to indicate that di-block copolymers, under certain circumstances,

would be better for use in OSCs than tri-block materials.

This research work of thesis has shown the potential application of

rod-coil and donor-acceptor BCPs as a compatibilizer in the field of OSCs

and opens broad prospects for the future. First, concerning the synthetic

chemistry, a simplified and versatile synthesis of rod-coil block copolymers

based on P3HT conjugated block was developed by the well-known GRIM

method. This method can be applied to obtain BCPs with various coil

blocks. Moreover, the concept of compatibilizing blends donor-acceptor

seems to be very promising, especially with diblock copolymers. To

improve the best current performance in triblock copolymers, it seems that

we should synthesize a copolymer of high molecular weight P3HT in order

to structure the active layer blend P3HT:PCBM.

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Appendix

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Table 1: Glass transition temperature of all synthesized polystyrenes (PS1-PS6).

Polystyrene-N3

(PS)

Mn (SEC,

g mol-1)

Glass transition

Temperature, Tg (ºC)

PS1 2600 87

PS2 3800 88

PS3 1900 84

PS4 4500 89

PS5 2000 84

PS6 5200 93 (a)

(b)

Figure1: Representative DSC curves of Polystyrene-N3 (PS-N3) showing glass transition temperature, Tg (ºC) (a) PS1 and (b) PS2.

227

Publications and Conferences

Publications :

1. Poly(3-hexylthiophene) Based Block Copolymers Prepared by “Click” Chemistry Urien, M.; Erothu H; Cloutet, E.; Hiorns, R. C.;

Vignau, L.; Cramail, H. Macromolecules 2008, 41, 7033-7040. 2. Poly(3-hexylthiophene) Based Donor-acceptor Block Copolymers

for Photovoltaics. Erothu H; Urien, M; Mafoudh, R; Cloutet, E.; Hiorns,

R. C.; Vignau, L.; Cramail, H. (manuscript in preparation) 3. Photovoltaic characterization of multiblock copolymers containing

polythiophenes and polyfullerenes. Hiorns, R. C.; Erothu H; Habiba,

B.; Cloutet, E.; Hiorns, R. C.; Vignau, L.; Cramail, H. (manuscript in

preparation)

4. Block Copolymers Based on Poly(3-hexylthiophene) and Poly(4-vinylpyridine) by anionic polymerisation for Photovoltaics.

Erothu H; Mafoudh, R; Brochon, C; Cloutet, E.; Hiorns, R. C.; Vignau,

L.; Cramail, H. (manuscript in preparation)

5. Photovoltaic characterization of novel Polyacetylene-grafted-Poly(3-hexylthiophene) Based Block Copolymers. Erothu H; Urien,

M, Cloutet, E.; Hiorns, R. C.; Vignau, L.; Cramail, H. (manuscript in

preparation)

Conferences (oral and poster presentations) : 1. Synthesis and Photovoltaic application of Block copolymers Based

on Poly(3-hexylthiophene) and Polystyrene (poster presentation)

Harikrishna Erothu, M. Urien, R. Mafoudh, Roger C. Hiorns, Eric

Cloutet, Laurence vigneau and Henri Cramail, MACRO 2010, 43rd

IUPAC World Polymer Congress, 11-16 July 2010, SECC, Glasgow,

UK.

2. Synthesis of Poly(3-hexylthiophene) grafted Polyacetylene and their Photovoltaic Characteristics (poster presentation)

Harikrishna Erothu, Roger C. Hiorns, Eric Cloutet and Henri Cramail

MNPC 2009, 19-23 October 2009, Arcachon, France.

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3. Synthesis of Poly(3-hexylthiophene) Based Copolymers for Organic Electronics (oral presentation)

Harikrishna Erothu, M. Urien, Roger C. Hiorns, Eric Cloutet and Henri

Cramail, Journées GFP Sud-Ouest, 5-6 February 2009, Eauze, Gers,

France.

4. Synthesis of polyacetylene-grafted-poly(3-hexylthiophene) for organic photovoltaic cells (poster presentation)

Harikrishna Erothu, Roger C. Hiorns, Eric Cloutet and Henri Cramail

GFP Polymeres & Photovoltaics,14-15 octobre 2008, ENSCPB, Pessac,

France.

5. Synthesis of polyacetylene-grafted-poly(3-hexylthiophene) for organic photovoltaic cells (poster presentation)

Harikrishna Erothu, Roger C. Hiorns, Eric Cloutet and Henri Cramail,

XXIII International Conference on Organometallic Chemistry ICOMC

2008, July 13-18, Rennes, France.

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