Thèse - theses.frl'injection et le transport des charges. Le greffage de polymères conjugués sur...

Université de Pau et des Pays de l’Adour

Faculté des Sciences et Techniques

Thèse

Pour obtenir le grade de :

Docteur de l’Université de Pau et Pays de l’Adour

Discipline: Chimie-Physique

Spécialité: Chimie des Polymères

Présentée par:

Hussein AWADA

Synthesis of organic-inorganic hybrids for

photovoltaic applications

Soutenue le 10 Octobre 2014 à l’IPREM

Devant le jury composé de :

Pr. Christine LUSCOMBE Université de Washington - USA Rapporteur

Pr. Eric DROKENMULLER Université Claude Bernard Lyon I Rapporteur

Dr. Frédéric CHANDEZON CEA/CNRS UMR SPrAM / Université

Joseph Fournier

Examinateur

Pr. Thierry TOUPANCE

Pr. Laurent BILLON

Université de Bordeaux 1

Université de Pau et Pays de l’Adour

Examinateur

Directeur de thèse

Dr. Christine DAGRON

Dr. Antoine BOUSQUET



Co-directeur de thèse

Examinateur

ACKNOWLEDGEMENTS

I would like to express my special appreciation and thanks to my director Professor Dr.

Laurent BILLON, co-director Dr. Christine DAGRON LARTIGAU and co-supervisor Dr.

Antoine BOUSQUET, you have been a tremendous mentor for me. I would like to thank you

for encouraging my research and for allowing me to grow as a research scientist. Your advice

on both research as well as on my career have been priceless. Without your supervision and

constant help this dissertation would not have been possible. I would like to thank the French

ministry for funding my project.

I would also like to thank my committee members, professor Christine LUSCOMBE,

professor Eric DROKENMULLER, professor Thierry TOUPANCE and Doctor Frédéric

CHANDEZON for serving as my committee members even at hardship. I also want to thank

you for letting my defense be an enjoyable moment, and for your brilliant comments and

suggestions, thanks to you.

I should not and will not forget the members of the EPCP team where I would like to express

my sincere appreciation to them due to the fact that among them I found a friendly and warm

environment.

A special thanks to my family. Words cannot express how grateful I am to my father, my

sisters and my grandparents for all of the sacrifices that you’ve made on my behalf. Your

prayer for me was what sustained me thus far.

I would also like to thank all of my friends who supported me in writing, and incented me to

strive towards my goal. Hussein MEDLIJ deserves extra thanks for explaining carefully and

quickly all what is related synthesis of polymers, your diligent work is very much

appreciated.

At the end I would like express appreciation to my beloved Waed AHMAD and my best

friend Nelly HOBEIKA, who spent sleepless nights with and were always my support in the

moments when there was no one to answer my queries.

Abbreviations

AFM, atomic force microscopy

Ar, aromatic

Au, gold

ATRP, atom transfer radical polymerization

Bipy, 2,2’-bipyridil

BHJ, bulk heterojunction

CdSe, cadmium selenium

CdTe cadmium tellurium

CNM, carbon nanomaterial

CNT, carbon nanotube

CS, charge separation

CT, charge transfer

COD, 1,5-cyclooctadiene

CP, conjugated polymer

CTP, chain transfer polycondensation

CV, cyclic voltammetry

Đ, dipersity

DA, Diels-Alder

dppe, 1,2-bis(diphenylphosphino)ethane

DPn, degree of polymerization

dppp, 1,2-bis(diphenylphosphino)propane

DSSC, dye synthesized solar cell

D/A, donor acceptor interface

EA, electron affinity

Eex, exciton binding energy

ECL, electron collecting electrode

EQE, external quantum efficiency

FF, fill factor

GO, graphene oxide

HOMO, highest occupied molecular orbital

HCL, hole collecting electrode

IPD, ionization potential

GPC, gel permeation chromatography

IR, infra-red

ITO, indium tin oxide

IQE, internal quantum efficiency

JSC, short circuit current density

LUMO, lowest unoccupied molecular orbital

MALDI-TOF, Matrix-Assisted Laser Desorption/Ionisation-time-of-flight mass spectrometry

MEH-PPV, poly[1-methoxy-4-(2-ethylhexyloxy)-p-phenylene vinylene]

Mn, average number molar mass

MW, multi-wall

NC, nanocrystals

NMR, nuclear magnetic resonance;

Ni(dppp)Cl2, 1,2-bis(diphenylphosphino)propane-dichloronickel

NP, nanoparticle

NR, nanorod

OLED, organic light-emitting diodes

OPV, organic photovoltaics

P3AT, poly(3-alkylthiophene)

P3HT, poly(3-hexylthiophene)

P3MT, poly(3-methylthiophene)

P3OT, poly(3-octylthiophene)

P4VP, poly(4-vinylpyridine)

PA, polyacetylene

PCE, power conversion efficiency

PCBM, phenyl-C61-butyric acid methyl ester

PEDOT:PSS, poly(3,4-ethylenedioxythiophene)-compl-poly(vinylbenzenesulfonic acid)

PF, polyfluorene

PFCF,poly-[4,4’-(9H-fluorene-9,9-diyl)bis(N,N-diphenylbenzenamine)(4-(9H-carbazol-9-

yl)benzaldehyde(9,9-dihexyl-9H-fluorene)

PFTPA, poly4,4’-[4-(9-phenyl-9H-fluoren-9-yl)phenylazanediyl]dibenzaldehyde-[4,4’-(9H-

fluorene-9,9-diyl)bis(N,N-diphenylbenzenamine)]-(9,9-dihexyl-9H-fluorene)

Pin, incident light power

Pout, output electrical power

PMMA, poly(methyl methacrylate)

PNIPAM, poly(N-isopropyl acrylamide)

PTM, poly(thiophene-maleimide)

PP, polyphenylen

PPE, poly(phenylene ethynylene)

PPh3, triphenylphosphine

PPV, poly(phenylene vinylene)

PSBr, poly(4-bromostyrene)

PSI, poly(4-iodostyrene)

PSCs, polymer solar cells

PPy, polypyrrole

QD, quantum dots

QE, quantum efficiency

Rs, series resistance

Rsh, shunt resistance

SAM, self-assembled monolayer

SEM, scanning electronic microscopy

SI-KCTP, surface-initiated Kumada catalyst transfer polycondensation

SiO2, silicon dioxide

SW, single wall

TEM, transmission electron microscopy

TGA, thermo-gravimetric analysis

THF, tetrahydrofuran

TiO2, titanium dioxide

TNT, trinitrotoluene

UV, ultra-violet

VOC, open circuit voltage

XPS, x-ray photoelectron induced spectroscopy

ZnO, zinc oxide

General introduction……………………….………………………………………………………………………………………………1

Table of contents chapter 1

1. Context…………………………………………………………………………………………………………………………………6

2. Polymer brushes: general features…………………………………………………………………………………….10

2.1 Grafting methodologies………………………………………………………………………………………………………..10

2.2 Anchoring groups…………………………………………………………………………………………………………………12

2.3 Structural properties of brushes……………………………………………………………………………………………13

3. Conjugated Polymer Brushes: surface chemistry…………………….……………………………………..15

3.1 “Grafting From” synthetic techniques………………………………………………………………………………….17

3.1.1 Surface initiated Kumada catalyst transfer polymerization………………………………………………….18

3.1.2 Other surface initiated polymerization method…………………………………..………………………………22

3.2 “Grafting through” synthetic techniques……………………………………………………………………………..23

3.2.1 Yamamoto surface polymerization………………………………………………………………………………………24

3.2.2 Heck surface polymerization………………………………………………………………………………………………..25

3.2.3 Sonogashira surface polymerization…………………………………………………………………………………….26

3.2.4 Other polymerization methods…………………………………………………………………………………………….28

3.2.5 Summary of the “grafting from” and “grafting through” methodologies……………………………..29

3.3 “Grafting onto” coupling techniques……………………………………………………………………………………32

3.3.1 End functionalization of conjugated polymers…………………………………………………………………….32

3.3.2 Direct substrate-polymer coupling……………………………………………………………………………………….34

3.3.3 Surface anchoring via Heck coupling…………………………………………………………………………………..36

3.3.4 Surface anchoring via cycloaddition…………………………………………………………………………………….37

3.3.5 Surface anchoring via esterification/amidification……………………………………………………………….39

3.3.6 Surface anchoring via other methods……………………………………………………………………………….....40

3.3.7 Summary of the “grafting onto” methodology……………………………………………………………………..41

3.4 Conclusion…………………………………………………………………………………………………………………………..46

4. Organic photovoltaic cells………………………………………………………………………………………………….49

4.1 General working principles of organic photovoltaic devices……………………………………………….49

4.1.1 Absorption of photons (i) and creation of excitons (ii)…………………………………………………………50

4.1.2 Diffusion of the exciton to the D/A interface (iii)…………………………………………………………………51

4.1.3 Dissociation of excitons (iv)…………………………………………………………………………………………………51

4.1.4 Charge transfer (v) and collection at electrodes (vi)…………………………………………………………….52

4.2 Photovoltaic parameters………………………………………………………………………………………………………53

4.3 Conclusion………………………………………………………………………………………………………………………….56

5. Aim and scope of the PHD…………………………….………………………………………………………………….57

6. References………………………………………………………………………………………………………………………….58


1. Introduction…………..………………………………………………………………………………………………………….70

2. Results and discussion…………………………………..………………………………………………………………….73

2.1. Synthesis and characterizations of allyl-terminated P3HT………………………………………………….73

2.2 Synthesis and characterizations of triethoxysilane-terminated P3HT………………………………….78

2.3 Specific surface area of Zinc oxide nanorods………………….………………………………………………….80

2.4. Hybrid material P3HT@ZnO nanorod characterizations…………………………………………………….82

2.5. Hybrid material properties………………………………………………………………………………………………….89

3. Perspectives.………………………………………………………………………………………………………………………92

4. Conclusion…………..…………………………………………………………………………………………………………….94

5. References………………………………………………………………………………………………………………………….95

Table of content chapter 3

1. Introduction..…………………………………………………………………………………………………………………….99

2. Low bandgap polymers….………………………………………………………………………………………………..100

3. Stille cross coupling polymerization…………………………….………………………………………………….103

4. Step growth polymerization………………………………………………………………..…………………………..104

5. Results and discussions…………………………………………………………………………………………………….106

5.1. Synthesis of monomers ……………………………………………………………………………………………………..106

5.1.1 Synthesis of 4,4‘-Bis (2-ethyl-hexyl)-5,5'-bis(trimethyltin)-dithieno[3,2-b:2',3'-d]silole

(M1)...................................................................................................................................................106

5.1.2 Synthesis of 4,7-dibromo-2,1,3-benzothiadiazole monomer (M2)……………………………………..107

5.2 Synthesis of poly[(4,4′-bis(2-ethylhexyl)dithieno-[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-

benzothiadiazole)-4,7-diyl] (PSBTBT)…………………………………………………………………………………………..108

5.3 Optical properties of PSBTBT in solution and thin films…………………………………………………..110

5.4 Polycondensation reaction from the Zinc oxide Nanorods: grafting low bandgap (PSBTBT)

…………………………………………………………………………………………………………………………………..........111

5.5 Tentative of brush formation mechanism through Stille cross coupling reaction……………….124

6. Conclusion………………………………………………………………………………………………………………………..128

7. References………………………………………………………………………………………………………………………..129


1. Introduction……………………………………………………………………………………………………………………..134

2. P3HT SAMs on ITO substrates………………………………………………………………………………………141

2.1 Preparation……………………………………………………………………………………………………………………….141

2.2 Results and discussion………………………………………………………………………………………………………142

3. Photovoltaic performance………………………………………………………………………………………………147

3.1 Fabrication……………………………………………………………………………………………………………………….147

3.2 Measurements …………………………………………………………………………………………………………………148

4. Conclusion……………………………………………………………………………………………………………………….150

5. References……………………………………………………………………………………………………………………….152

General Conlusions and Outlook……………………………………………………………………………………………...154

Experimental Part………………………………………………………………………………………………………………………157

Introduction générale

Les cellules solaires à base de polymères (CSP) ont attiré une attention considérable au cours

des dernières années en raison de leur potentiel à fournir des cellules solaires flexibles,

légères, peu coûteuses et efficaces. Les performances de ces dispositifs dépendent

principalement des constituants de la couche active, de sa morphologie et de son maintien, de

la stabilité chimique et thermique et de l’optimisation des interfaces. Récemment, les CSP ont

atteint un rendement de conversion supérieur à 10% et beaucoup de travail doit être encore

fait pour améliorer l'efficacité et la stabilité des dispositifs. Au cours de ce travail de thèse,

nous avons eu l'objectif à long terme d’améliorer les interfaces dans les dispositifs pour le

photovoltaïque organique. Dans ce domaine, les couches sont de nature chimique différentes

allant des matériaux métalliques à inorganiques et organiques. La performance et la durée de

vie des dispositifs électroniques organiques dépendent de façon critique des propriétés des

matériaux et surtout des interfaces dans le dispositif. Les problèmes d'interface entre les

électrodes et la couche active organique doivent être adressés en optimisant la stabilité,

l'injection et le transport des charges. Le greffage de polymères conjugués sur une surface

pourrait être une alternative aux techniques de dépôt physique, car elle offrirait un grand

nombre d'avantages: i) limiter le délaminage; ii) donner plus de polyvalence dans la

fabrication du dispositif en particulier lorsque plusieurs couches de même solubilité sont

utilisées; iii) permet l'orientation des chaines de polymère perpendiculairement au substrat;

iv) fournit la création d'un nouveau niveau électronique entre le polymère et la surface, et

ainsi l'amélioration des propriétés électroniques.

Deuxième exemple, dans le photovoltaïque hybride, le polymère conjugué semi-conducteur

et le matériau inorganique sont généralement préparés par mélange physique des deux

composants (donneur et accepteur) pour former l’hétérojonction en volume. Malgré la

simplicité de cette approche, il existe un risque de séparation de phases microscopique, ce qui

limite les performances de l'appareil qui en résulte. Ainsi, une nouvelle approche d'ingénierie

interfaciale fondée sur le couplage chimique du polymère aux nanoparticules est souhaitable

pour améliorer l'efficacité et la stabilité de la couche active du CSPs. Les travaux présentés

dans ce manuscrit se concentrent sur la conception de nouveaux matériaux hybrides

organique-inorganique, liés chimiquement en utilisant deux méthodes différentes de "grafting

onto» et «grafting through ».

Dans le chapitre 1, nous mettons en évidence les développements récents dans le cadre du

greffage de polymères conjugués sur différents substrats pour des dispositifs électroniques

organiques. Une vue d'ensemble des différentes méthodes de synthèse de polymères

conjugués dans la chimie macromoléculaire sur des nanoparticules et des surfaces planes est

décrite et a été publiée sous la forme d’une revue dans Progress in Polymer Science.

Dans les chapitres suivants (2 et 3), les nanoparticules de ZnO ont été fonctionnalisées

(accepteur d'électrons dans les cellules solaires hybrides) avec des polymères conjugués

(donneurs d'électrons). Dans le chapitre 2, nous démontrons une nouvelle stratégie de

greffage efficace pour ancrer le poly(3-hexylthiophène) P3HT sur des nanobatonnets de ZnO

dans une procédure « grafting onto » en une seule étape pour créer une monocouche auto-

assemblée macromoléculaire. En outre, l'influence de la masse molaire et de la densité de

greffage a été étudiée. Suite à ce travail, nous rapportons dans le chapitre 3, la première

élaboration de polymère dit à faible bande interdite (low bandgap) via l’amorçage de la

polymérisation à partir d’une surface. Une méthodologie "grafting through" a été employée

après fonctionnalisation de la surface des nanobatonnets de ZnO et a permis de préparer des

matériaux hybrides polymères low bandgap@ZnO.

Dans le chapitre 4, nous avons extropolée l’approche « grafting onto » à la modification de la

surface d'ITO par une monocouche auto-assemblée de P3HT, qui est une alternative

prometteuse au PEDOT: PSS, Le principal avantage de cette méthode est que le polymère

peut être greffé en une seule étape par dépôt spin-coating qui peut être facilement inclue dans

un procédé de fabrication des dispositifs. En outre, les performances photovoltaïques de

l’ITO greffé par du P3HT ont été comparées à celles du système classique du PEDOT: PSS

déposé sur ITO.

Enfin, un chapitre intitulé "partie expérimentale" décrit les conditions de synthèse et les

techniques analytiques utilisées au cours de ce travail.

1

General introduction

Polymer solar cells (PCSs) have attracted a considerable attention in the past few years owing

to their potential of providing flexible, lightweight, inexpensive and efficient solar cells. The

performance of such devices mainly depends on: components of the active layer as well as its

nanomorphology, chemical and thermal stabilities, and optimization of interfaces. Recently,

PSCs have achieved a power conversion efficiency exceeding 10 %1 and much work has to be

done to improve efficiency and stability of devices. During this PhD work, we had the long

term objective to improve the interface in organic photovoltaics. In this field, the devices

present superposition of layers from various chemical natures such as organic, inorganic,

metallic materials. The performance and lifetime of organic electronic devices are critically

dependent on the properties of both the materials and the device interfaces. First example,

interface issues between electrodes and the organic active layer must be addressed to optimize

stability, charge-injection, -transport and -recombination. Surface grafting with conjugated

polymers as an alternative to physical deposition techniques could be a breakthrough as it

would provide a high number of advantages: i) prevents delamination; ii) gives additional

versatility in device manufacturing especially when multiple layers with the same solubility

are used; iii) allows orientation of the polymer backbone perpendicularly to the substrate; iv)

provides the creation of a new electronic level between the polymer and the surface,

enhancing the electronic properties.

Second example, in hybrid photovoltaics, conjugated polymer and inorganic semiconductor

hybrid systems for bulk heterojunction (BHJ) are usually prepared by physically mixing the

two components (donor and acceptor). Regardless of the simplicity of this approach, there is a

risk of microscopic phase separation, thereby limiting the performance of the resulting device.

Thus a new interfacial engineering approach based on chemically linking the polymer to the

nanoparticles is desired to expect significantly improve the efficiency and stability of the

active layer of PSCs.2

These examples explain the context of the study, but the research presented in this manuscript

focuses on the chemical design and synthesis of novel organic-inorganic hybrid materials that

are chemically bonded using two different approaches “grafting onto” and “grafting through”

methodologies. Only preliminary testing of our materials has been performed in PCSs.

2

In Chapter 1, we highlight the recent developments in the grafting of conjugated polymers

onto various substrates for organic electronic devices. An overview of the various synthetic

methodologies of conjugated polymers within the chemistry of tethering macromolecular

chains onto nanoparticles and flat surfaces is described and was published in Progress in

Polymer Science.

In the following Chapters (2 and 3), ZnO nanoparticles have been functionalized (electron

acceptor in hybrid solar cells) with conjugated polymers (electron donor)

In Chapter 2, we demonstrate a novel and efficient grafting strategy to anchor poly(3-

hexylthiophene) P3HT onto ZnO nanorods in a one-step procedure via grafting-onto

technique to create a macromolecular self-assembled monolayer. In addition, the influence of

the molar mass on the grafting density was studied.

Following our previous work of grafting P3HT onto zinc oxide nanoparticles, and seeking for

conjugated polymers with better coverage of solar spectrum. We report in Chapter 3 the first

elaboration of low bandgap polymer poly[(4,4′-bis(2-ethylhexyl)dithieno-[3,2-b:2′,3′-

d]silole)-2,6-diyl-alt-(2,1,3 benzothiadiazole)-4,7-diyl] brushes via the surface initiation of an

AA/BB type step growth polymerization from zinc oxide nanoparticles. A “grafting through”

methodology was applied via surface polymerization by functionalizing ZnO nanorods with

initiating sites to prepare Core@Shell ZnO nanorods.

In chapter 4, we reported the modification of the ITO surface by P3HT self-assembled

monolayer, that is a promising alternative to PEDOT:PSS, via grafting onto technique in melt.

The major advantage of this versatile method over previously reported grafting-from

technique is that the polymer can be grafted in one simple step and easily included in a device

manufacturing procedure. Moreover, a comparison of the photovoltaic performance between

P3HT and PEDOT:PSS layer was studied by fabricating different photovoltaic devices.

Finally, a chapter called “experimental part” describes the synthesis conditions and the

analytic techniques used during this work.

1 You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C.-C.; Gao, J.;

Li, G.; Yang, Y., A polymer tandem solar cell with 10.6% power conversion efficiency. Nat Commun

2013, 4, 1446. 2 Bousquet, A.; Awada, H.; Hiorns, R. C.; Dagron-Lartigau, C.; Billon, L., Conjugated-polymer

grafting on inorganic and organic substrates: A new trend in organic electronic materials. Progress in

Polymer Science http://dx.doi.org/10.1016/j.progpolymsci.2014.03.003

Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates

Chapter 1


Abstract

This chapter highlights recent developments in the grafting of conjugated polymers onto various

substrates for organic electronic devices. The rapid development of multi-layer architectures demands

the preparation of well-defined interfaces between both compatible and incompatible materials. It is

promising therefore that interface-engineering is now known to help passivate charge trap states,

control energy level alignments, enhance charge extraction, guide active-layer morphologies, and

improve material compatibility, adhesion and device stability. In organic electronic devices,

conjugated polymers are in contact with a wide range of constituents such as metals, metal oxides,

organic materials, and inorganic particles. Covalent bonds between these materials and

macromolecules are designed to yield intimate contacts and well-defined interfaces. Following an

overview of the various synthetic methodologies of conjugated polymers, the chemistry of tethering

macromolecular chains onto nanoparticles and flat surfaces is described. The creation of functional

hybrid materials offers the potential to deliver efficient devices.


Table of contents chapter 1 1. Context ............................................................................................................................... 6

2. Polymer brushes: general features ................................................................................ 10

2.1 Grafting methodologies ............................................................................................. 10

2.2 Anchoring groups ...................................................................................................... 12

2.3 Structural properties of brushes ................................................................................ 13

3. Conjugated Polymer Brushes: surface chemistry ....................................................... 15

3.1 “Grafting From” synthetic techniques....................................................................... 15

3.1.1 Surface initiated Kumada catalyst transfer polymerization ............................... 16

3.1.2 Other surface initiated polymerization methods ................................................ 22

3.2 “Grafting through” synthetic techniques ................................................................... 23

3.2.1 Yamamoto surface polymerization .................................................................... 24

3.2.2 Heck surface polymerization ............................................................................. 25

3.2.3 Sonogashira surface polymerization .................................................................. 26

3.2.4 Other polymerization methods ........................................................................... 28

3.2.5 Summary of the “grafting from” and “grafting through” methodologies .......... 29

3.3 “Grafting onto” coupling techniques......................................................................... 32

3.3.1 End functionalization of conjugated polymers .................................................. 32

3.3.2 Direct substrate-polymer coupling ..................................................................... 34

3.3.3 Surface anchoring via Heck coupling ................................................................ 36

3.3.4 Surface anchoring via cycloaddition .................................................................. 37

3.3.5 Surface anchoring via esterification/amidification ............................................ 39

3.3.6 Surface anchoring via other methods ................................................................. 40

3.3.7 Summary of the “grafting onto” methodology .................................................. 41

3.4 Conclusion ................................................................................................................. 46

4. Organic photovoltaic cells .............................................................................................. 49

4.1 General working principles of organic photovoltaic devices .................................... 49

4.1.1 Absorption of photons (i) and creation of excitons (ii) ...................................... 50

4.1.2 Diffusion of the exciton to the D/A interface (iii). ............................................ 51

4.1.3 Dissociation of excitons (iv) .............................................................................. 51

4.1.4 Charge transfer (v) and collection at electrodes (vi) .......................................... 52

4.2 Photovoltaic parameters ............................................................................................ 53

4.3 Conclusion ................................................................................................................. 56

5. Aim and scope of the PhD. ............................................................................................. 57

6. References ........................................................................................................................ 58


6

1. Context

The field of polymers for electronic organic applications was opened in 1977 with the

discovery of the ability of conjugated polymers to be doped to cover a full range of

conductivity, from insulator to metal.1 This new class of materials exhibits conjugation that

enables both absorption within the visible light region and electrical charge transport. The

first generation of conjugated polymers was polypyrrole (PPy), polyacetylene (PA), poly(p-

phenylene) (PPP) and poly(phenylenevinylene) (PPV) (Figure 1). Some of them have since

been developed with solubilising side chains for numerous sectors such as sensor and heating

systems. In parallel, non-doped polymers in their semi-conducting form are under rapid

development and production for several applications: organic light-emitting diodes (OLEDs)

for flat panel displays and lighting; field-effect transistors for display backplanes and

disposable electronics; photodetectors; and last but not least organic photovoltaics (OPVs).

Figure 1. Representative common conjugated polymers

Organic-based devices promise low costs, and properties based on their low density,

conformability, flexibility and versatility due to the wide potential of chemical structures.

Initial work was to conceive new materials with improved control over electrical and optical

properties, along with improved processabilities; a particular target was to create soluble

conjugated polymers. Another challenge was to understand the charge carrier transport

mechanisms in molecular and macromolecular organic materials. More recently, several

research groups turned their attention toward the possibility of creating hybrid materials


7

containing both conjugated polymers (CPs) and inorganic, metal or carbon-based materials,

by covalently binding the two components. Up to now, mostly polythiophene

macromolecules have been surface initiated2 and many synthetic efforts need to be made to

find ways to covalently anchor different types of conjugated polymers (CPs) to all kind of

surfaces (metal, metal oxide, etc…).

The present manuscript deals with macromolecular engineering, with the main

objective the development of innovative synthetic paths to graft conjugated

macromolecules to different metal oxide surfaces.

The first question that arises is why consider this field? It is all about interfaces.

Organic electronic devices consist of superposed layers of different chemical natures, be they

organic, inorganic, or metallic (Figure 2). The performances and lifetimes of organic

electronic devices are critically dependent on the properties of both the active materials and,

importantly, their interfaces.

Figure 2. Schematic illustration of organic electronic devices: a) OLED; b) conventional OPV cell; and c)

inverted OPV cell [3].

Interfaces between electrodes and the organic semiconductor layers play a decisive

role in optimizing charge-injection, -transport and -recombination. For example in OLED

applications, device efficiency is dependent on the balanced injection of charge carriers. This

requires the anode and the cathode work function to be matched with the highest occupied

molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of


8

the hole transporting layer and the light emitting layer, respectively. In response to this

concern, interface engineering with small molecules and polymers has been developed and

recently reviewed in several papers.3 Similarly for OPVs, contact resistance between layers

must be minimized to reduce the device series resistance, which greatly influences the fill

factor and thus the power conversion efficiency. Moreover, physical properties such as

wetting and adhesion between layers are important for performances and lifetimes. In this

context, it is promising that interface engineering can help passivate charge traps, control

energy level alignment, guide active layer morphology, and improve material compatibilities.

Optimization of these interfacial properties can be achieved by modification of the

wettability between layers or by covalent modification of the electrode surface. Surface

grafting with polymers provides a number of advantages over physical deposition

techniques: it prevents delamination of the film; it gives additional versatility in device

manufacturing, especially when multiple layers with the same solubility are used; and it can

allow orientation of the polymer backbone perpendicular to the substrate when traditional

solution processing methods (printing, spin-coating, drop-casting, doctor-blading, etc.) do

not. This could be required to facilitate charge injection or extraction.4

Another interface that plays a decisive role is the one between electron donors and

acceptors in bulk heterojunction solar cells, a morphology obtained by co-precipitating both

components from solution 5 or during co-evaporation of small molecules.

6 The use of organic

donor and acceptor counterparts has been extensively studied. So far, fullerene and its

derivatives have given the highest power conversion efficiencies for single bulk

heterojunction, over 8 %.7 However, the poor electron mobility of n-type organic semi-

conductors compared to their p-type counterparts leads to unbalanced electron and hole

transport in active layers. The photogenerated electrons tend to accumulate at donor-acceptor

interfaces forming a space-charge region that gives rise to inefficient charge transport and

charge recombination.8 As n-type inorganic materials present a mobility typically several

orders higher than that of organic materials, their incorporation as electron acceptors and as

pathways for electronic transport in hybrid solar cells provides a route to overcome this

imbalance.9 Besides, compared with organic semiconducting materials, inorganic

semiconductors may exhibit better stability against oxygen and moisture, a key parameter for

device stability. The majority of inorganic materials are incorporated into hybrid solar cells as

nanocrystals (CdSe, CdTe, and so on) because they offer the advantages of absorbing visible

light (complementing the absorptions of the p-type polymer) and contributing to the

photocurrent. Several reviews are devoted to this subject.10

Other metallic oxide inorganic


9

semiconductors (ZnO, TiO2) exhibit a wider bandgap, again impacting the absorbed light, but

their main advantage is that their morphologies and dimensions can be tailored via synthetic

methods and in some cases (as for ZnO) vertical nanostructures are obtained.11

Active layer deposition procedures should ensure intimate mixing of acceptors and

donors within 20 nm to avoid recombination. In order to do this, great care must be taken to

tune the surface chemistry of the nanoparticles so that they do not overly aggregate and are

able to present a good interface for charge transfer.12

Nanoparticles aggregation is believed to

be one of the limiting factors of the efficiency in nanoparticle/polymer devices.13

The quality

of dispersions may be increased by modifying the surface of the nanoparticles with p-type

polymers. Moreover, grafted nanoparticles may allow morphological control during

deposition, an increase in surface areas and improved exciton dissociations. However, more

intimate mixing can reduce the connectivity of metallic oxide particles and decrease charge

carrier mobilities.14

These are the two main reasons for developing synthetic pathways that

will allow grafting of conjugated polymers. To reinforce this view, other various applications

have been reported. In chemo-sensor systems, hybrid materials exhibit improved sensitivity

and selectivity compared to conjugated polymers alone.15

Table 1 details the different

substrates grafted with CPs and their applications.

Table1. Substrates grafted with conjugated polymers and their applications.

In this chapter, we first present a résumé of the fundamentals of polymer brushes. In

the main body of this bibliographic part, we focus on three techniques: “grafting from”,

“grafting through” and “grafting onto”, applied to conjugated-polymer grafting on inorganic

and organic surfaces.


10

These grafting techniques may be used to prepare original materials with synergetic

properties to address various issues in organic and hybrid electronic devices.

2. Polymer brushes: general features

Polymer brushes, the so-called tethered polymers, first gained real attention in the

1980s when the equilibrium properties of polymer brushes yield a scaling law for the brush

conformation of coil-polymers. 16

It refers to an assembly of polymer molecules which are

attached to a surface or an interface at sufficient grafting density. The tethering is sufficiently

dense, such that the polymer chains are crowded and thus forced to stretch away from the

surface or interface to avoid overlapping. Polymer brushes have become increasingly studied

and most macromolecular and surface journals now routinely contain articles and reviews on

polymer brushes, often exploiting controlled radical polymerization techniques.17

To attain good control over the polymer mono-layer thickness and structure, so-called

“living” polymerizations, for example anionic 18

or cationic 19

polymerizations, can be used.

However, these techniques require specific experimental conditions thus making their

application difficult. Nevertheless, recent advances in controlled radical polymerizations,

have made the synthesis of well-defined and low dispersity (Đ = Mw/Mn) polymers viable. 20

Atom transfer radical polymerization (ATRP) 21

and stable free radical polymerization

(SFRP) 22

belong to this class of techniques that can be exploited. However, these techniques

cannot be used to prepare conjugated rod polymer chains; some of the specific

polymerization techniques for semi-conducting or conducting polymers will be described in

later sections. We now give a short description of the various grafting methodologies used.

2.1 Grafting methodologies

The synthesis of a dense film of polymer chains covalently bound to surfaces is an

important field of research for its ability to control and tune the surface properties. The most

straightforward methodologies for elaborating well-defined polymer layers are the “grafting

to” and “grafting from” processes (Figure 3).

The “grafting to” or “grafting onto” approach was first used by Mansky et al. and consists of

the condensation of end-functionalized polymers with the reactive groups on the substrate.23


11

This simple and direct method, resulting in the synthesis of well defined brushes, does not

usually give highly dense polymer brushes because chemisorption of the first fraction of

chains hinders the diffusion of subsequent chains to the surface by forming a macromolecular

barrier. Thus polymer chains to be grafted must diffuse through the existing polymer film to

reach the active sites on the surface. The polymer anchoring can be either performed from a

solution or a melt. However the anchoring from a melt can result in a higher grafting density

due to a screening of the excluded macromolecular barrier. 24

This limitation can be overcome by using the “grafting from” approach which can

lead to higher grafting densities. In this technique, a mono-layer of small initiator molecules

is covalently attached to a solid surface. After activation, i.e. initiation of the polymerization,

the chains grow from the surface and then the only limit to propagation is the diffusion of

small organic compounds, i.e. monomer molecules, to the growing chain-ends.25

A wide

variety of monomers can be polymerized creating a dense coating. However, the molecular

weight and the chain length distributions of polymer chains formed cannot always be

accurately controlled and measured.

A third process, called “grafting through” (Figure 3) and based on the anchoring of a

polymerizable group, is also described in the literature.26

Polymer chain initiation takes place

in the solution or in bulk and during the propagation step; the growing chains react with the

functional group attached on the surface and then further propagate with free monomers.

With this process, the length of the polymer chains grafted and the surface density are

difficult to control.

Figure 3. Schematic illustration of the attachment of polymers to surfaces.


12

2.2 Anchoring groups

A key point in the elaboration of a well-defined polymer mono-layer is determining

the nature of the anchoring group attaching the polymer to the substrate. Self-assembled

monolayers (SAMs) are ordered molecular assemblies formed by the adsorption of an active

molecule to the surface. The simplicity of this process makes SAMs inherently easy to

transfer to industrial processes; this methodology is technologically attractive for building

superlattices and for surface engineering. Order in these two-dimensional systems is

produced by a spontaneous chemical reaction at the interface, as the system approaches

equilibrium. Depending on the nature of the substrate, different chemical moieties can be

used, such as diazonium,27

sulfur,28

silicon,29

phosphorous-based anchoring functional groups

(Figure 4).30

Figure 4. Chemical structure of anchoring groups depending on the substrate nature.

In the last decade, these functional groups have been largely used for surface initiated

polymerization or “grafting onto” processes in combination with controlled radical

polymerization techniques.17a

Nevertheless, these approaches have been recently used in

organic electronics to attach conjugated polymers to inert or electro-active surfaces.

Most approaches aiming to attach polymers to a surface via the “grafting onto”

methodology use a system where the polymer carries the “anchoring” group, either as an end-

group or a side-chain. When the chain is attached by a chain-end, this approach leads to

surface-attached monolayers where the chains are oriented perpendicularly to the surface.


13

On the contrary, side-chain attachment usually leads to multiple anchorages and

accordingly, a flat conformation of chains.

2.3 Structural properties of brushes

It is generally recognized that a coil polymer brush corresponds to an array of coiled

macromolecular chains attached to a surface and in sufficient proximity so that the

unperturbed solution dimensions of the chains, i.e. as if in a good solvent, are altered. Such a

situation causes an overlap of adjacent coil chains and significantly changes the

conformational dimensions of individual polymer chains. Indeed, coiled polymer chains

extend or alter their radius of gyration to avoid unfavourable interactions. Films composed of

coil polymer chains that extend along a direction normal to the grafting surface can exhibit

properties distinctly different from chains in solution. This makes polymer brushes an

interesting field for novel properties. These new properties were achieved with the “grafting

from”, so-called surface initiated polymerization, in a process of advent and maturation,

which can permit the targeting of high grafting densities and concentrated coil polymer

brushes.

Polymer brushes can be considered as sensitive ultra-thin polymer films on a solid

substrate. Indeed, effects of end-grafting on their structure and properties have been studied

at very low and high grafting densities, respectively yielding mushroom-like structures and

concentrated brushes (Figure 5).

Figure 5. Advent of concentrated polymer brushes with surface initiated polymerization.


14

The conformation of the grafted polymer chains mostly depends on the grafting

density and the interaction of polymer chains with the surface (attraction or repulsion). At low

grafting density, tethered chains do not overlap, since the distance between the grafting sites is

larger than the size of the chains. Depending on the strength of the interaction of the polymer

segments with the surface, two cases must be distinguished. If the interaction is weak or even

repulsive, the chains form a typical random coil known as mushroom conformation. However

if the interaction is attractive, polymer chains adsorbed strongly to the underlying surface, the

polymer form a flat “Pancake”-like conformation.

For a higher grafting density value, i.e. close to 1 coil chain nm-2

, the concentrated

brush can be highly anisotropic. Theory predicts that unperturbed polymers in dry states reach

about 40% of their fully extended chain-lengths as found in swollen, solvated states. However,

in close-packing the chains are forced to take on extended, stretched states.31

Commonly used

parameter for the quantitative characterization of the transformation from “mushroom” to

brush conformation is the reduced tethered density ).

Where Rg2 is the radius of gyration of a polymer chain that measures the average size of the

chain at specific experimental conditions of solvent and temperature, is the number of

polymer chains per nm2 and .

In a good solvent, it is highly dependent on the

degree of polymerization N and expressed as Rg ~ N3/5

. However in poor solvent it shows less

dependence on N and expressed as Rg ~ N1/3

. In theta solvent, which is intermediate between

good and poor solvents, it is expressed as Rg ~ N1/2

. Thus tethered polymer chain can be

characterized by 3 major regimes: the mushroom regime at < 1, mushroom-to-brush

transition regime at 1 < < 5, and brush regime at In addition to N, the conformation

of the end tethered chains is governed by the number of polymer chains grafted per unit area of

the substrate known as grafting density. The definition of the grafting density is determined

by:

Where h is the brush thickness; , bulk density of the brush composition; Mn, number

average molar mass and Na Avogadro’s number. Here, it is important to mention that no


15

theoretical studies have been published to explain or predict the evolution of the thickness

and conformation of a mono-layer of rigid, rod-like polymer chains on solvation. Kiriy’s

group did, however, empirically demonstrated a significant doubling in swelling of a rod

P3HT monolayer.32

Reviews well-cover the area of conjugated polymer chemistry.33

Therefore, we give

an extremely brief synopsis focusing on the methods used with grafting techniques with a

detailed explanation for the synthesis of conjugated polymer brushes via such chemistries.

3. Conjugated Polymer Brushes: surface chemistry

3.1 “Grafting From” synthetic techniques

Step polymerization is a polycondensation process in which condensation reactions

between two functional groups are used to build up polymers. Typically, hetero-chain

polymers such as polyethers, polyesters, polyamides and several others are obtained via such

polymerization. In practice, this approach fails to produce CP brushes with reasonable

grafting densities since polymers form faster in solutions and hinder growth from surface.

Step polymerizations may be transformed into a chain-polymerization process by developing

suitable catalyst transfer polycondensation (CTP) methods. Chain CTP (often called chain-

growth CTP) has been successfully utilized for the controlled synthesis of conjugated

polymers including poly(thiophene),34

poly(fluorene),35

and poly(phenylene),36

as well as for

the preparation of more complex alternating copolymers.37

Mechanistically, it is generally

accepted that CTPs undergo the same oxidative addition/reductive elimination cycles that are

typical of transition metal-catalyzed cross-coupling reactions. However, their distinguishing

feature is that the oxidative addition of the catalyst occurs in an intra-chain fashion that

facilitates chain-polymerization like behavior. As a consequence, an external initiator, being a

molecule reacting exclusively during the initiation step, can be introduced into the media and

used as a starting point for the macromolecules. From this basis studies began on modifying

surfaces with conjugated polymers via the “grafting from” methodology. First the initiator is

introduced to the surface and then the polymerization is performed.


16

3.1.1 Surface initiated Kumada catalyst transfer polymerization

Kumada catalyst transfer polymerizations have been amply reviewed by Yokozawa and

Yokoyama.38

They are most prevalently used due to simplicity of use, wide applicability,2

and ease of predicting molar masses by varying repeating unit precursors and catalyst ratios.

39 The facilitation of end-group modification via Grignard

40 and other reagents

41, make this

the method of choice for grafting-techniques. An outline of the chemistry is shown in Scheme

1, as proposed by Yokozawa and colleagues.42

Importantly for grafting chemistry, the end-

groups are Br and H. The reaction revolves around the immediate generation of a dimeric

species due to condensative exclusion of 2 MgX2. This is further followed by the generation

of Ni[0] complex which is able to “walk” across the molecule and insert at the intramolecular

C-Br bond. In effect, the ligated Ni moves with the chain-end. Therefore this method is

considered as a chain growth polymerization and has been applied for the polymer

attachment to surfaces via the “grafting from” approach.

Scheme 1. Catalyst transfer condensation polymerization mechanism, as proposed by Yokozawa and

colleagues.42

In 2007 Kiriy’s group first used “grafting from” techniques with a phenylbromide surface-

anchored function to initiate Kumada polymerizations.43

They first investigated the use of

bromophenyl as initiator and 1,2-bis(diphenylphosphino)propane-dichloronickel Ni(dppp)Cl2

(Figure 6) as the catalyst in solution. However, the poor reactivity of this catalyst towards


17

unactivated arylhalides44

turned their attention towards (tetrakistriphenylphosphine)-nickel

Ni(PPh3)4. P3HT of 5000 g mol-1

with Đ = 1.2, a regioregularity of nearly 100% and a phenyl

chain-end functionalization of 98% was obtained. Poly(bromostyrene) was then deposited on

a silicon wafer, crosslinked by UV, allowed to react with the Ni complex and to polymerize

at 0 ˚C. The polymerization proceeded selectively from the immobilized initiator and not in

solution. A detailed investigation revealed that: (i) very short P3HT chains were synthesized

(the brushes being impossible to detach, it is therefore challenging to determine the real DPn);

and (ii) P3HT was grafting not only on the film-solution interface but also in the PS-Br bulk

(resulting in an interpenetrated network rather than real polymer brushes).45

Figure 6. The Nickel complexes used for surface-initiated Kumada catalyst transfer polycondensation.

These first two papers pioneered a series of studies on this promising topic and the

term “surface-initiated Kumada catalyst transfer polycondensation” (SI-KCTP) was adopted.

Locklin’s group have developed SI-KCTP to graft poly(thiophene) and poly(p-

phenylene) onto gold wafers.46

First, the substrates were functionalized with an arylbromide

moiety by reduction of a thiol group at the gold surface. Then they anchored a nickel

complex (with a 1,5 cyclooctadiene ligand, COD) Ni(COD)(PPh3)2 which was more reactive

than Ni(PPh3)4, towards the surface initiating group. Moreover, Ni(PPh3)4 did not allow

further polymerization in this case. The substrates were then immersed in a solution of ClMg-

Ar-I and ellipsometry revealed that brushes of 14 nm of PT and 42 nm of PP were obtained.


18

Some free chains in solution were observed and they were attributed to nickel species

undergoing a chain transfer from the surface to a monomer in solution.

To improve the efficiency of the nickel complex initiator, Kiriy’s group used ligand

exchange chemistry.32

A silane modified with a bromobenzene function was anchored to 460

nm and 4 nm (diameter) particles. After an unsuccessful attempt with Ni(PPh3)4, the authors

used the Et2Ni(Bipy) catalyst but only a very small amount of P3HT was grafted (was not

detected by TGA) and a large amount of free polymer was present in solution. The third

method was to first attach Et2Ni(Bipy) and then exploit a ligand exchange chemistry where

the Bipy was displaced by dppp or dppe to form Ar-Ni-(dppp)-Br and Ar-Ni-(dppe)-Br

complexes (as shown in Figure 7). In this case, after removing the catalyst and free polymer

(10 w%), grafting was clearly indicated by SEM (a shell of 19 nm) and TGA (around 13 w%

corresponding to a shell height of 20 nm). Grafted chains were detached from the particles by

dissolution of the latter in HF. GPC analysis was performed on the residues and showed a

bimodal signal with Mn = 43 000 g mol-1

and Mw = 112 000 g mol-1

. The authors claimed that

the high molar masses were related to incompletely disintegrated fragment of the particles

causing a link between chains.

Figure 7. Preparation of SiO2@P3HT hybrid particles.32

In a fine study, Sontag et al. used cyclic voltammetry to evaluate the introduction and

the efficiency of the grafted Aryl-Ni-Br complex towards Kumada coupling using a Grignard

reagent bearing a electrochemical probe (ferrocene).47

The reactive Ni(COD)2 complex was

used because of commercial availability and facile dissociation of the ligand in the presence

of -donating ligands. The results showed that introducing Ni(COD)2 with Bipy (1/1 eq) is

more efficient in anchoring the complex to the surface and grafting the probe than that found


19

with Ni(COD)2 with dppe or dppp (1/1 eq). Nevertheless, as previously mentioned, the use of

Bipy as a ligand leads to addition (step) polymerizations.32

Therefore, the complex used was

changed from Ar-Ni(Bipy)-Br to Ar-Ni(dppp)-Br in order to create an efficient chain-

polymerization initiator. The authors proposed that the ligand exchange was not very efficient

for polymerizations from gold due to disproponation of two neighbouring nickel complexes

(Scheme 2). With indium tin oxide (ITO) and SiO2 wafers, where the grafting density of the

first layer was lower than that on gold, no disproponation occurred and films of 30 to 60 nm

were produced. It is important to mention that free polymer is always observed in solution

which is a deviation from the ideal chain-growth nature.

Scheme 2. Proposed Mechanism of the surface disproportionation.47

In an attempt to increase the grafting density values, Huddleston et al. reported the

use of palladium as a viable catalyst via SI-KCTP.48

Palladium catalysts have weaker

electron-donating abilities, a more stable zero-valent oxidation states and reduced

propensities towards disproportionation.49

A (4-bromobenzyl)phosphonic acid monolayer on

ITO was subsequently reacted with Pd(PtBu3)2 (tri-tert-butylphosphine P

tBu3) and an

electrochemical probe was grafted via Kumada coupling. Cyclic voltammetry CV shows that

the surface density of the anchored probe was twice higher in the case of the Pd than the Ni

based complex. Polymerization of 2-bromo-5-chloromagnesio-3-methyl thiophene was

performed from the ITO surface in THF at 40 °C and a linear increase of current density and

UV-visible absorbance with time was found.

As a conclusion of the investigation into the initiator layer, Table 2 summarizes the

various catalysts employed. The immobilisation of the initiators was performed through

oxidative addition of Ni or Pd(0) complexes to a surface-bound aryl halogen bond, to

generate active aryl-Metal(II)-halogen species. Ni(dppp)Cl2 and Ni(PPh3)4 did not provide


20

good results as the complexes were slow to react with non-activated arylhalides. The use of

Bipy ligands leads to addition polymerizations, so its adaptation to SI-KCTP is undesirable.

Therefore ligand exchange techniques have been applied to introduce (dppp) or (dppe)

ligands (having a high efficiency towards Kumada polymerizations); i.e., an additional step

moving from the Ar-NiBipy-Br initiator is performed by using either Ni(COD)Bipy or

Et2NiBipy. It is worth noting that two systems performed well without the need for a ligand

exchange step, namely Ni(COD)2+dppe (1/1 eq) and Pd(PtBu3)2.

Table 2. Metals catalyst employed in SI-KCTP

Catalyst Surface

immobilisationa

Ligand exchange Surface Polymerization Ref

Ni(dppp)Cl2 No No No 43

Ni(PPh3)4 Yes No Yes P3HT from PS-Br or PS-I 43, 45, 50

No No No

32, 46

Ni(COD)2+PPh3

(1/4 eq) Yes No Yes PT and PP from gold wafers

46

Yes Yes with dppe Yes P3MT from ITO wafers

51

Ni(COD)2+Bipy

(1/1 eq) Yes

Yes with dppe or

dppp Yes P3MTfrom SiO2, ITO wafers

47, 52

Ni(COD)2+dppe

(1/1 eq) Yes No Yes PP from SiO2 wafers

53

NiEt2(Bipy) Yes No Yes but poor control (solution

polymerization) 32

Yes Yes with dppe

Yes PF, P3HT and P3(NH2)HT

from SiO2 particles 32,54,55

Pd(PtBu3)2 Yes No Yes P3MT from ITO

48

a Indicates whether or not the complex reacted with the surface aryl-halogen group.

Using the exact same ligand exchange methodology, Kiriy’s group broaden the scope

of SI-KCTP by varying the monomer. Poly(9,9-dioctylfluorene) was grafted from silica

particles (880 nm in diameter); TGA showed a loss of 10 % after polymer grafting

corresponding to a thickness of 25 nm.54

At the end of the polymerization, the authors observed a 30% conversion with 80% of the

polymer unbound in solution, probably coming from Ni extraction from the surface. The

brushes were then detached from the SiO2 sphere with HF and their GPC trace showed a

broad signal with Mn = 48 000 g mol-1

and Đ = 3.7 (polystyrene calibration). The grafting

density indicated that the grafted chains are in a dense brush regime, = 0.89 chain nm-2

. The

same silica particles were also modified by a water soluble conjugated polymer.55

A 3-

bromohexylthiophene was polymerized from the bromobenzene function borne by the


21

spheres. In a second step, the authors modified the brushes with potassium phthalimide and

hydrazine hydrate, to replace the pendant bromide groups by an amine, in an almost

quantitative substitution. On ionising the amine groups, the resulting particles were soluble in

water (at pH ˂ 9).

Marshall et al. developed SI-KCTP of other monomers such as 1,4-dihalogeno-2,5-

dialkoxybenzene family.53

In order to investigate the influence of the steric hindrance of the

side chains on the monomer, the authors synthesized various aryl monomers with hydrogen

(H), methyl (Me), ethyl (Et) and hexyl (Hex) side groups. They performed polymerizations

from silicon wafers and glass slides and found that a shorter side chain resulted in a thicker

layer i.e., ellipsometry and AFM indicated: H = 30 nm, Me = 17 nm, Et = 12 nm, and Hex =

4.8 nm. Moreover, they observed an absence of polymerization when brominated monomers

were used in the place of iodo-subsituted ones.

Recently ITO surfaces were functionalized by the polymerization of 5-

chloromagnesio-3-methylthiophene from a Ar-Ni(dppe)Br initiator by Luscombe group.51

A

kinetic study showed that after an induction period of 10 h the film thickness increases

linearly with time; it is noted that a typical a solution polymerisation reaches complete

monomer conversion in 2 h. This induction period was attributed to a rate determining

transmetallation step being slow due to the steric hindrance of both the surface and the bulky

(dppp) ligand. ITO was also grafted by Yang et al. in order to replace PEDOT:PSS as the

hole transporting layer in photovoltaic devices.52

It is argued that PEDOT:PSS’ acidic nature

can corrode the ITO electrode, any inhomogeneous conductivity leads to uneven charge

extraction, and insufficient electron blocking enhances charge recombination.56

The

substrates were grafted with P3MT layers via SI-KCTP from surface bound arylnickel(dppp)

bromide initiator. Solar cells were realised with four different P3MT layers of 3, 6, 9 and 20

nm in thickness.

Increasing the thickness attenuates the transmittance of P3MT films; therefore the authors

stopped their investigation at 20 nm thick films that gave 75% transmittance at 450 nm. The

results show that the performances are slightly lower than the reference cells made with

PEDOT:PSS (Figure 8), due to lower charge transport. Nevertheless, hole mobility may be

enhanced by doping the P3MT layer and the attached surfaces were found to be stable and

reusable.


22

Figure 8. Solar cell characterization in which ITO is surface modified with P3MT. 52

Finally, SI-KCTP has been used for the creation of surface microstructures.50

Patterning was performed using a poly(N-isopropyl acrylamide) (PNIPAM) microsphere

monolayer as a mask. The space between the particles was hydrophobized with a silane and

after removal of the particles poly(4-vinyl pyridine)-block-polystyrene was adsorbed on the

hydrophilic regions. The polymer was modified with iodine group located on the styrene

monomer units and the authors used Ni(PPh3)4 to catalyse the formation of P3HT side chains.

3.1.2 Other surface initiated polymerization methods

Other than the Kumada-based polymerization, there are few techniques directly adapted to

the growth of macromolecules from a surface. Kiriy’s group showed the only one example of

a palladium catalyzed Suzuki polycondensation to graft and pattern semiconducting and

fluorescent poly[9,9-bis(2-ethylhexyl)fluorene].57

Silicon wafers and glass slides were first

modified with a silane phenylbromide or PS-Br. Then the surfaces were allowed to react with

the catalyst [Pd(PtBu3)2] in toluene at 70 ˚C. XPS confirmed the presence of Pd on the

surface after rinsing. Finally the wafers were immersed in a monomer solution using THF

and aqueous sodium carbonate. After 3 h, the sample reached a thickness of 100 nm. In

contrast to their previous studies using SI-KTCP the polymerization occurred only from the


23

topmost layer of the poly(bromostyrene) film, probably because it had not swollen in the

aqueous solution.

Recently Bielawski et al. described a catalyst transfer Stille type polymerization that

enabled well-defined poly(p-phenylene ethynylene)s in a controlled, chain polymerization

manner.58

They demonstrated that in solution and using optimized conditions (catalyst,

ligand), the molar mass of the polymer increased linearly with monomer conversion while Ð

remained constant (below 1.4). They transferred this technology to the synthesis of surface-

initiated polymerizations from SiO2 nanoparticles. They first attached the [2-(4-

bromophenyl)-ethyl]-triethoxysilane, complexed the palladium catalyst and finally proceeded

to the polymerization. The polymeric material is detached from the silica surface by treating

the particles with HF; characterizations by GPC and 1H NMR indicated that the polymer had

a molar mass of 24 000 g.mol-1

.

3.2 “Grafting through” synthetic techniques

Because polycondensation often follow polyaddition mechanisms, commonly called

step polymerizations, the “grafting through” methodology has been particularly useful for the

creation of conjugated polymer brushes. As mentioned above, in the “grafting through”

method the polymerization occurs both in solution and through a surface that has been

previously functionalized with a monomer. In this review, we use the term “grafting through”

when no initiator is grafted onto the surface in a step prior to polymerization, in opposition

with “grafting from”. For example, bromophenyl moieties, often employed for the first

monolayer, are not considered initiators if no complexed catalysts are present at the start of

the polymerization; rather, the moieties are denoted surface anchored monofunctional

monomers.


24

3.2.1 Yamamoto surface polymerization

In opposition to the Kumada reaction, this method requires stoichiometric equivalents

of transition metal complexes to reductively remove halogens from aromatic monomers, most

commonly biscyclooctadiene nickel(0) [Ni(COD)2]. 59

This means that the reaction can be

prohibitively expensive, and may lead to difficulties in removing all metal pollutants.

Nevertheless, it does warrant consideration due to its wide applicability with simply achieved

monomers, the facile nature of the chemistry, and the often extremely high molar masses

obtained. The system can be used with a wide range of monomers, ranging from more classic

polyfluorenes 60

to relatively large monomers for ladder-like polymers 61

. A typical reaction

is shown in Scheme 3.

Scheme 3. Yamamoto polycondensation of dibromofluorene.

Carter’s group used Yamamoto coupling polymerizations to graft polyfluorene (PF) onto

various substrates. First Ni(0)-mediated step-growth polymerization of 2,7-dibromo-9,9-di-n-

hexylfluorene was conducted from a crosslinked polymethacrylate film containing some

bromostyrene unities.62

Due to the nature of the polymerization, both free polymer chains and

brushes were grown. The free polymer characterized in this study had a Mn = 30 000 to 70

000 g mol-1

and Đ ≈ 2. AFM and profilometry showed that the brush-length was around 6 nm

i.e. 6 repeating units. Subsequent patterning of the substrate was realized by contact molding

to give line features from 100 nm to 100 m in width and 40 nm in height. Subsequent

grafting with oligofluorene gave rise to 4 nm long hairs around the PMMA domains. They

improved the system by using a 2,7-dibromo-9-fluorenyl methacrylate instead of

bromostyrene in the initial network.63

With this change, they increased the PF thickness up to

30 nm in 30 min under microwave irradiation. Later, silicon and glass wafers were modified

by the same coupling technique except that here the anchoring sites were based on a silane

bearing a dibromofluorene function (Scheme 4).64


25

Scheme 4. Yamamoto surface polymerization.64

GPC analysis of the free polymer revealed PF of Mn = 49 000 g mol-1

with Đ = 2.08 but the

molar mass could not be linked to that of the brushes. The film thickness was 98 nm and

AFM showed an increase in the roughness from the silane to the PF surfaces, attributed to

chain dispersity. UV adsorption and fluorescence were performed on the quartz surfaces

proving the presence of PF polymer.

Finally cellulose has been modified by esterification with a bromobenzene function to

functionalize sugar units.65

Yamamoto type Ni(0) polymerization of fluorene dibromide was

performed but the final samples were polluted by Ni complexes, even after extensive washing

with Soxhlet and ultrasonication. Therefore the authors turned their attention towards Suzuki,

Heck and Sonogashira coupling.

3.2.2 Heck surface polymerization

Heck-based polymerizations typically performed with aryl halides or triflates with

aryl alkenes has generally found less use than the aforementioned methods due to the greater

difficulty in preparing the precursors. Nevertheless, the underlying chemistry, which is well

reviewed by Beletskaya and Cheprakov 66

is of interest due to the range of otherwise difficult

to obtain structures, for example see the work of Xu et al.67

. A classic example is given in

Scheme 5 to prepare PPV derivatives.


26

Scheme 5. A classic use of the Heck polymerization technique.

CdSe quantum dots are semiconductors under intense investigation as new components

for photovoltaic cells68

, light-emitting diodes,69

and bio-sensors70

. Quantum dots (QDs) are

synthesized in the presence of ligands, typically phosphine oxides, which stabilize the

particles during growth. Typically, further functionalization of these particles occurs via

ligand exchange, not trivial to perform because it can lead to surface oxidation and changes in

the QD size and size-distribution.71

Odoi et al. showed that CdSe nanocrystals can be

integrated into PPV thin films without aggregation.72

In this paper, the authors synthesized

CdSe QDs with a phosphine oxide ligand bearing a phenylbromide function (this ligand

being highly stable at 250 ˚C, the temperature required for QDs formation). The particles

were then subjected a palladium catalyzed Heck polymerization by coupling of 1,4-di-n-

octyl-2,5-divinylbezene and 1,4-dibromo-2,5-di-n-octylbenzene to yield a PPV-based

copolymer. Grafted polymers were detected by NMR and MALDI-TOF which confirmed the

formation of oligomers (3 to 6 units). TEM showed that the dispersion in PPV film was much

better when the QDs were grafted with the polymer, and indeed, characterisation by

photoluminescence indicated that there was an efficient charge transfer from the polymer to

the CdSe QDs.

3.2.3 Sonogashira surface polymerization

Much like the Heck reaction and covering Pd-catalyzed polymerizations involving

aryl alkyne and aryl halides, the Sonogashira reaction is differentiated by the use of a small

amount of copper salt which acts as a co-catalyzer. The chemistry of the reaction (scheme 6)

has been extremely well reviewed.73

It seems that the presence of oxygen does tend to

enhance secondary, homocoupling reactions, and therefore particular care is required, for

example through the employment of purified inert gases.74


27

Scheme 6. An example of Sonogashira reaction.

Schanze et al. first introduced the Sonogashira A-B type polymerization to graft silica

particles with a PPE bearing hydrophilic side chains such as oligo(oxyethylene) and sodium

sulphonate.75

In a first step, the silane bearing the monomer function (aryliodide) was

introduced together with various amounts of an unfunctionalized silane to vary the grafting

density as shown in Scheme 7. The ensuing polymerization was catalyzed with CuI and

Pd(Ph3)4. TGA clearly showed an increase in polymer content in the hybrid with an

increasing aryliodide surface density. The authors have estimated the thickness of the layer to

be 12 nm, corresponding to 10 repeating units. SEM images revealed a non-uniform surface

with evidence of aggregates as large as 50 nm. The authors mentioned that some polymer

formed in solution could be physisorbed onto the surface. This underlines the importance of

the washing procedure after grafting. The authors showed that cationic electron transfer and

energy-transfer quencher ions efficiently suppress the fluorescence of the PPE grafted

particles. This observation suggests that these hybrids could be useful for applications such as

fluorescent sensors for biological targets.

Scheme 7. Sonogashira surface polymerization.75

Feng et al. used the same procedure to graft PPE with sulphonated side chains from

silica spheres.15

TEM was used to characterize the particles and statistical measurements

showed a 24 nm increase on the sphere diameter.


28

The fluorescence quenching by electron-deficient trinitrotoluene (TNT) was studied. The

conjugated polymer-grafted silica particles showed high sensitivity towards TNT in solution,

much higher than free polymer chains. Finally on this topic, a publication from Fang et al.

presented the ability to tune the fluorescence properties of a PPE film (glass substrate) by

changing the alkyl side chains.76

In a good solvent the side chains could be solvated,

disaggregating the polymer backbones and thus increasing the fluorescence.

Cotton fibres have also been functionalized with a PPE film bearing cationic side

chains.77

The grafted fibres, characterized by SEM and fluorescence spectroscopy, have been

studied as bactericide materials for the development of antimicrobial textiles.

3.2.4 Other polymerization methods

The functionalization of silicon wafer with polyacetylene brushes was carried out by Carter’s

group using metathesis polymerizations.78

The process started with surface passivation via a

silane bearing an alkyne function. The alkyne-functionalized substrate was placed in a

toluene solution of 5-decyne with a catalytic amount of WCl6/Ph4Sn and was allowed to react

under microwave irradiation at 150 °C for 30 min. Microwaves accelerate the reaction

considerably ( from 24h to 10 min). At the end of the reaction, high molar mass polymer was

observed in the solution and the thickness increased to 41 nm. XPS revealed the presence of

tungsten due to the presence of chain-ends.

A composite nanotube-PPV was synthesized by Gilch polymerization of 1,4-

bis(chloromethyl)-2-methoxy-5-octoxy-benzene, SWCNT-COCl and tBuOK in THF.79

The

brushes were not characterized. The photoluminescence was markedly quenched upon the

doping of the SWCNT, suggesting charge transfer from the polymer to SWCNTs. Bulk

molecular heterojunction solar cells were prepared and the presence of the hybrids improved

performances. The authors attributed this behavior to intrinsic nanophase separation that

facilitated charge carrier transport, improved exciton dissociation and reduced charge

recombination.

Oxidative polymerizations have also been used to attach conjugated polymers

“through” surfaces. For this purpose a monomer, such as thiophene 80

, aniline 81

or pyrrole 82

is grafted to the surface.


29

In a second step, the polymerization occurs via the addition of an oxidant, normally FeCl3 or

ammonium persulfate. This method will not be developed in this review because of the

absence of control over the polymerization.

3.2.5 Summary of the “grafting from” and “grafting through” methodologies

Table 2 presents the classification of conjugated polymers covalently bound to substrate via

“grafting from” and “grafting through” methodologies. For the moment SI-KCTP seems to be

the most promising technology to provide well defined CP films. With this technique, P3ATs

have been grown from the surface of polymers, metal oxides and gold with some control over

molar masses and dispersities. Different anchoring groups, initiators and catalyst systems

have been developed to prepare a first layer with high coverage and to allow efficient

reactions with the monomer solution. Thick, dense and homogeneous films have been

obtained. The “grafting through” methodology has been applied to transfer solution-based

polyadditions to surface polycondensation reactions. Other kinds of polymers have been

attached to surfaces, broadening the range of the techniques available to elaborate CP brushes

such as Sonogashira, Suzuki or Heck coupling. Nevertheless a considerable amount of work

remains to be done in order to study the surface polymerization kinetics, the grafting

efficiencies and the control of the coverage layer (grafting density and location) leaving

certain area of nanoparticles (NPs) naked so the adjacent nanoparticles may interconnect one

another to form the nanoparticle network for efficient electron transport.


30

Table 3. Surface Immobilization of Conjugated Polymer by the “grafting from” and “grafting through”

methodologies.

Substrate Anchoring

group

Initiating

group Polymer

Polymerization

technique Ref

PMMA-co-PS-Br on

SiO2 wafer _

Br

poly(9,9-dihexyl

fluorene) Yamamoto

62-63

BrBr

poly(9,9-dihexyl

fluorene) Yamamoto

64

Poly(bromostyrene) on

silicon wafer

_

Br

P3AT SI-KCTP 43

P3HT SI-KCTP 45

poly(9,9-bis-2-

ethylhexylfluorene) Suzuki

57

PVP-block-poly(4-

iodostyrene) on silicon

wafer N

n

I

P3HT SI-KCTP

50

Cotton fibres (size) SiMeO

MeO

MeO

I

PPE Sonogashira

77

Cellulose

HO

O

Br

PF Yamamoto 65

PF Suzuki 65

poly(fluorene

vinylene) Heck

65

-C≡C

poly(fluorene

ethynylene

phenylene)

Sonogashira 65

SWCNT

O

Cl

poly(2-methoxy,5-

octoxy-1,4-

phenylene vinylene)

Gilch 79

Silicon wafer SiCl

Br

poly[9,9-bis(2-

ethylhexyl)fluorine] Suzuki

57


31

SiCl

Cl

Cl

S

Br

P3MT SI-KCTP 47

Silicon and quartz

wafer

SiEtO

EtO

EtO

-C≡C poly(1,2

dibutylacetylene) metathesis

78

BrBr

poly(9,9-dihexyl

fluorene) Yamamoto

64

SiCl

Cl

Cl

S

Br

PP SI-KCTP 53

Glass SiMeO

MeO

MeO

I

PPE Sonogashira

76

SiO2 NPs

(4 nm ø and 460 nm ø) SiEtO

EtO

EtO

Br

P3HT SI-KCTP

32

SiO2 NPs

(100-200 nm ø) SiEtO

EtO

EtO

Br

PPE

SI-KCTP

58

SiO2 NPs

(200 nm ø) SiMeO

MeO

MeO

I

PPE Sonogashira

15

SiO2 NPs

(300 nm ø and 5m ø) SiMeO

MeO

MeO

I

PPE Sonogashira

75

SiO2 NPs

(880 nm ø) SiEtO

EtO

EtO

Br

poly(9,9-

dioctylfluorene) SI-KCTP

54

P3HT with amino

on each unit SI-KCTP

55

ITO PO

HO

HO

Br

P3MT SI-KCTP

47-48,

52

Cl

P3MT SI-KCTP 51

CdSe QDs

(4 nm ø) PO

HO

HO

Br

PPV Heck Pc

72a,72b

Gold wafer HS- S

Br

polythiophene and

PP SI-KCTP

46

P3MT SI-KCTP 47


32

3.3 “Grafting onto” coupling techniques

The “grafting onto” methodology has also been used to graft CPs. Necessarily, a

functionalized polymer is required, and researchers have turned their attention towards how

to introduce a reactive group onto a CP. Such a group can be borne on the side chain, as

demonstrated by De Girolamo et al. who introduced diaminopyrimidine to P3HT; this

polymer developed hydrogen bonds with CdSe nanocrystals modified with thymine.83

More

commonly though, the reactive function is held at one chain-end.84

Before discussing the

“grafting onto” methodology we briefly present the methods used to end-functionalize CPs.

3.3.1 End functionalization of conjugated polymers

P3ATs have been mostly end-functionalised to enable syntheses of block copolymers,

but such techniques have also been used to introduce groups that will bind to surfaces via the

“grafting onto” methodology. Three strategies have been developed: to initiate a

polymerisation with a molecule bearing the anchoring group; to add a functional molecule at

the end of the polymerization; or to post-functionalise the H/Br terminated P3AT.

The first attempt towards in-situ functionalization was reported by Janssen, but it gave

a mixture of H/H, mono-capped, and di-capped polymer chains.85

McCullough’s group

subsequently reported an alternative pathway for the synthesis of end-functionalized P3ATs

by using a modified KCTP.40

As the GRIM method follows a chain polymerization-based

mechanism, the nickel catalyst is still bound to the P3ATs at the end of the reaction.

Therefore, a simple addition of another Grignard terminates the reaction and end-caps the

polymer. Series of polymers have been synthesized bearing functional groups at one or both

ends. This method has been demonstrated to work with a variety of Grignard reagents (i.e.,

aryl, alkyl, allyl, vinyl and so on). The reactivity of these reagents depends on their nature:

addition of allyl, ethynyl, and vinyl result in mono-functionalized polymers, all other groups

yield di-functionalized polymers, although the ethynyl group tends to give a mixture. A major

advantage of this method is that P3ATs can be obtained with higher degree of functionality in

one step. The proposed mechanism of the in-situ approach is described in Scheme 8.


33

Scheme 8. Proposed mechanism of end capping P3HT by in-situ functionalization.

The second approach is based on polymer post-modification and uses the H/Br

terminal groups of P3ATs as a reactive function to introduce desired end-groups. This

method was used to prepare H/vinyl terminated P3HT via a condensative Stille coupling of

Br-terminated P3HT and tri-n-butyltin.86

Moreover, sequential lithiation and addition of

gaseous carbon dioxide and hydrochloric acid can be used to modify both Br and hydrogen

end-groups to yield P3HT-terminated COOH.87

P3HT synthesized through McCullough,

Rieke, or GRIM routes contains a high majority of H/Br end-groups. The bromine group can

be converted to H by treating the polymer with an excess of Grignard reagents and

subsequent aqueous workup. A Vilsmeier reaction can be employed to install aldehyde

groups on both ends of the polymer chain. Furthermore the aldehyde groups can be reduced

to hydroxymethyl to obtain --dihydroxy-P3HT.88

The three post functionalization

reactions are shown in Scheme 9.

Scheme 9. Modification of P3AT chain-ends through Stille, lithiation and Vilsmeier reactions.


34

The third route to end-functionalise a P3AT is to use a functional initiator. Indeed Kiriy,43

Luscombe 89

and Koeckelberghs 90

have demonstrated that external initiation is possible

using KCTP. In 2013, Monnaie et al. used this methodology to introduce phosphonic ester,

thiol, and pyridine groups at the end of P3HT chains using the corresponding initiator.91

In

this study, gold, CdSe and iron oxide nanoparticles surfaces are then modified with these

functional macromolecules.

Based on these chemistries, researchers have the tools for introducing a function on a

CP which is able to react with a surface. Moreover, other strategies such as Heck coupling,

Huisgen cycloaddition and esterification have been used to link CPs to substrates.

3.3.2 Direct substrate-polymer coupling

In this part end- or side- functionalized CPs that have groups that can directly bind to

solid surfaces are reviewed; for the most part this concerns P3HT. Silane, thiols or

phosphonic acid (shown in Figure 4) can form self-assembled monolayers (SAM) on various

surfaces.

Spontaneous adsorption of long chain n-alkanoic acids have been studied for years but

only recently applied to conjugated polymer functionalisations. In a similar work,

Nakashima et al. reported the strong anchoring behavior (on the substrates Au, ITO, Pt and

SiO2) of three types of end-functionalized PPEs synthesized via Suzuki polycondensations

and bearing thiolacetate, isocyanide, or carboxylic acid groups.92

The results showed

selective (as opposed to the un-functionalized PPE) chemisorption of thiolacetate-PPE on

gold, isocyanide-PPE on the metals and carboxylic acid-PPE on all substrates. On metals the

chemisorption driving force is believed to be the formation of a salt between the carboxylate

anion and the cationic surface metal.25

P3HT functionalized with carboxylic acid to react with

TiO2 has also been prepared by several groups. Lohwasser et al. and Krüger et al. both

reported the synthesis of P3HT with ,-dicarboxylic acid end groups.87, 93

P3HT was

synthesized via the GRIM method, lithiated and carboxylated to yield monofunctional chain-

ends as shown in Scheme 9. Monofunctional acid carboxylic P3HT was also created via

Knoevenagel94

or Wittig reactions from the aldehyde end-functionalized P3HT.95

In all these

studies, a homogeneous layer of P3HT was observed by TEM with a polymer brush thickness

of 3 to 5 nm. Finally 2,5-dibromothiophene-3-methylcarboxylate was polymerized to obtain a


35

P3HT with methyl ester side groups, leading to carboxylic acid functions after hydrolysis.96

TiO2 nanoporous films were formed on F-doped SnO2/glass substrates by a screen-printing

method, and then coated with the polymer. DSSC devices were completed with a platinum

counter electrode and an electrolyte solution of tetrabutylammonium iodine and iodine I2.

The study showed that the structure promoted efficient excitons dissociation at the

TiO2/polymer interface, and reached PCEs as high as 3.8% playing with the hydrolysis ratio

of the polymer ester group.96

Transition metal oxides, in particular zinc oxide and titanium dioxide, are known to

interact strongly with phosph(on)ates to form relatively stable interfacial bonds.97

The

binding ability of phosphonic groups can be arranged in the order: phosphonic acid

(RPO(OH)2) ˃ phosphonic ester (RPO(OR)2) ˃ phosphine oxide (R3PO).98

Briseno et al. used

this grafting agent to anchor P3HT to ZnO microwires (several microns in length and 30-100

nm in diameter).99

The polymer was post-functionalized by reacting with butyllithium and

diethyl chlorophosphate. The nanowires were coated with phosphonic ester functionalized

P3HT by simply mixing the two components in chlorobenzene solution overnight. The

thickness of the P3HT coating ranged from 7 to 20 nm according to high resolution TEM

images. In 2012, Li et al. utilized the same procedure to functionalize ZnO nanoparticles but

with benzyl-di-n-octylphosphine oxide functionalized P3HT.100

ZnO particles quenched more

P3HT luminescence when the two components were covalently bound (P3HT-DOPO@ZnO

compared with P3HT physically mixed with ZnO). This electronic transfer associated with an

improved miscibility of the ZnO@P3HT, makes these hybrid materials suitable candidates

for photovoltaic applications.

Thiol is probably the most studied anchoring group because of its chemical affinity

for the very useful gold. It was first introduced at the end of a P3HT chain in three steps.

Initially, a hydroboration/oxidation of an allyl end-functionalized P3HT was performed to

lead to hydroxypropyl terminated P3HT. The end-group was then converted to an acetyl-

protected thiolpropyl by a Mitsunobu reaction, and finally reduced with LiAlH4. These thiol

macromolecules were reacted with CdSe quantum dots and AFM images showed an

homogeneous hybrid film.101

Post-modifications have been developed to introduce thiol

functions on the side chains of P3HT to graft on ZnO nanoparticles 102

and on the side chain

of poly(dihexylfluorene) to bind gold nanorods.103

The grafted ZnO nanorods showed better

performances in hybrid solar devices than those that were not grafted.


36

3.3.3 Surface anchoring via Heck coupling

Curiously, Heck coupling reactions have only been applied in grafting conjugated

polymers to CdSe nanocomposites (QDs and nanorods) (Figure 9). After a seminal work in

which Liu et al. were mixing nanorods with amino-terminated P3HT, several papers reported

CdSe functionalization.104

First of all, CdSe nanoparticles were synthesized in a [(4-

bromophenyl)methyl] dioctylphosphine oxide solution to anchor a bromophenyl moiety at the

surface. These bromo end-groups were used to be covalently bound to vinyl-terminated

P3HT (MALDI-TOF, 2400 g.mol-1

) via a mild palladium catalyzed Heck coupling

reaction.105

According to TGA, the authors calculated that each QD was coated by 22 chains

of P3HT. Hybrid photovoltaics cells have been elaborated with these particles, however, even

through the dense grafting maximizes the interface between donors and acceptors (permitting

fast exciton dissociations), no direct percolation between the quantum dots and the electrode

was found in spin coated films.105a

Figure 9. Different strategies using Heck surface coupling.

Nanorods (NRs) seem to present several advantages over QDs when applied to solar

cells as they possess better electrical and optical properties in terms of enhanced electron

mobility and improved absorption in the UV-visible and near IR ranges.106


37

Thus Zhang et al. utilized ligand exchange chemistry with pyridine to functionalize CdSe

NRs using either p-bromobenzyl-di-n-octylphosphine oxide (DOPO-Br) or 2-(4-bromo-2,5-

di-n-octylphenyl) ethane thiol. The vinyl-terminated P3HT (GPC, Mn= 8600 g mol-1

) used in

this case was functionalized through a Stille cross coupling reaction between the bromine end

group of P3HT and vinyl tri-n-butyltin. Heck coupling was then performed with modified

CdSe NRs to prepare the desired P3HT-CdSe NRs.86 Interestingly, the greater surface

coverage found with the thiol is attributed to a higher initial coverage of thiol ligand in

comparison with phosphine oxide ligand. This is in agreement with the greater reactivity of

thiol ligands for CdSe compared with phosphine oxides.107

Nevertheless, the ligand exchange process used in the previous study can suffer from

incomplete surface coverage.108

To overcome this limitation, the growth of NRs was

performed from NCs in the presence of phosphonic acid ligands.109

Based on these factors,

Zhao et al. reported a robust and simple route to NR@CP composites avoiding the need for

ligand exchange chemistry and increasing the number of P3HT chains per nanorod. The

anisotropic growth of CdSe particles into long rods (l = 40 nm, ø = 5 nm) was promoted by

using bromobenzylphosphonic acid ligand. The phosphonic acid functions bonded to the

surface while the bromine groups were available for further reactions with vinyl-terminated

P3HT (Mn = 4900 g mol-1

, GPC) through the efficient Heck coupling reaction.110

In each of

the previous studies, Heck coupling was performed at 50 °C for 24 h. It is important to note

that these mild coupling conditions made it possible to graft without sacrificing the stability

or photophysical properties of the two components.

3.3.4 Surface anchoring via cycloaddition

In polymer chemistry the most famous cycloaddition is the Huisgen 1,3-dipolar

cylcoaddition reaction, the so called “click chemistry”, taking place between a azide group

and an alkyne moiety to form a five membered heterocycle ( also termed as the Sharpless

‘click’ reaction.111

This promising technique was applied to graft conjugated polymers to several

inorganic surfaces such as CdSe NRs, CdTe tetrapods, reduced graphene oxide, ZnO and

SiO2 substrates. Gopalan was the first to report the synthesis of conjugated polymer brushes

of P3HT onto oxide surfaces (ZnO, SiO2) through “click chemistry”.112


38

The azide monolayers on SiO2 and ZnO surfaces were prepared from bifunctional molecules

(3-azidopropyltrimethoxysilane) containing an azide click precursor and a siloxane surface

linker (Scheme 10). In addition, ethynyl-terminated P3HT (Mn = 6000 g.mol-1

) was

synthesized using a Kumada polymerization quenched with ethynyl magnesium bromide. The

calculated surface coverage of azide monolayer is 3.2 molecule.nm-2

, but only 17% of these

azide groups reacted with the ethynyl-terminated P3HT to yield a polymer grafting density of

0.53 chain.nm-2

. Again, this is a general feature in “grafting onto” methodology that the

density decreases with the polymer molar mass. Later, CdSe NRs113

, CdTe tetrapods 114

and

graphene oxide115

were functionalized with an azide moiety borne by a silane or a phosphoric

acid anchoring group. The ethynyl-terminated P3HTs were reacted with the substrate at 55 °C

for 48 h to perform the “click chemistry” which is longer than Heck coupling. Interestingly

the more reactive catalyst system was the combination of copper iodide with N,N-

diisopropylethylamine.

Scheme 10. Surface functionalization via “click chemistry”. 112

The success of this “click chemistry” was observed with many techniques such as IR

spectroscopy presenting the disappearance of the –N3 vibration peak (2040 cm-1

) after

coupling, or dynamic light scattering showing an increase in the particles size.

The Diels-Alder (DA) reaction is an important reaction for carbon-carbon bond

formation. Nanocomposites of PPE-gold nanoparticles were created via directly grafting

maleimide functionalized gold nanoparticles onto furan side functionalized PPE by a mild

DA reaction.116

The functional monomer was copolymerized via Sonogashira

polycondensation and the DA reaction was performed at room temperature yielding to a

polymer-particles crosslinked material. Barner-Kowollik applied DA reaction to the grafting

of carbon nanotubes.117

A cyclopentadiene terminated P3HT was synthesized by post

modification in three steps from the allyl terminated polymer. Maldi-TOF MS support the

success of the synthesis with one end group fidelity of 90% and molar mass of 3000 g mol-1

.


39

The surface Diels-Alder reaction with CNT took place at 80 °C for 24 h and TGA, XPS and

TEM were the methods of choice to indicate that a 2 nm thick P3HT layer was grafted onto

the CNT. The grafting density was estimated (from XPS data, specific area of CNT and

polymer Mn) to be 0.111 mmol.g-1

which is two times higher that PMMA and PNIPAM

grafted via the same route.118

The authors explained this fact by a supramolecular affinity

between -conjugated P3HT and CNT enhancing proximity during the coupling reaction.

A comparable cycloaddition is that of the 1,3-dipolar cycloaddition of azomethine ylide

used to graft conjugated polymers onto graphene oxide (GO). This reaction is one of the most

versatile and widely applied methodologies for the functionalization of fullerene C60.119

Zhang et al. synthesized three new conjugated polyfluorene bearing aldehyde side chains

through a Suzuki coupling reaction. These polymers were successfully grafted to GO via 1,3-

dipolar cycloaddition of azomethine ylide, from N-methylglycine, to yield a highly soluble

hybrid materials.120

. The covalent grafting was confirmed by XPS and IR spectroscopy. The

hybrid material was sandwiched between ITO and Aluminum electrodes to observed

nonvolatile rewritable memory effect from the J-V curves.

3.3.5 Surface anchoring via esterification/amidification

The work with graphitic structures such as CNT or graphene gained a considerable

attention in photovoltaic applications since they possess unique mechanical and electrical

stability, good conductivity, large contact areas, and very high aspect ratios.121

On the other

hand the poor processibility of these structures and the heterogeneous matrix obtained during

their physical mixing with polymers has propelled research towards covalent surface

modifications.122

Esterification is the most developed method in achieving hybrid conjugated

polymer/carbon nanomaterials (CNM). In the following examples carboxylic acid functions

were introduced on CNM objects by acidic treatments (H2SO4/HNO3). In a second step this

surface was reacted with SOCl2 to obtain acyl-chloride functionalized CNM, a clearly very

reactive group towards esterification (Scheme 11). In parallel, conjugated polymers bearing

hydroxyl functions were synthesized and eventually grafted onto CNM via esterification. Dai

et al. introduced OH groups on both polymer ends by reduction of aldehyde moiety and

modified both CNT 88

and GO 123

. Bilayer photovoltaic devices based on solution cast CNM-

graft-P3HT/C60 showed an increase in power conversion efficiencies with respect to their


40

P3HT/C60 counterparts. The authors explain this phenomenon by an enhanced charge

transport and reduced band gap energy. Song et al. post-modified allyl terminated P3HT via

oxidation and grafted the polymer to CNT via an ester linkage. Field effect transistor

incorporating this hybrid material showed better performances than the non-grafted

counterpart.124

Finally Lee et al. synthesized a maleimide-thiophene copolymer bearing

hydroxyl groups via Suzuki coupling polycondensations. This polymer was anchored to

acylchloride functionalized CNT125

or GO126

. In these cases, the authors demonstrated both

the grafting and an improvement in solar cell performances.

Scheme 11. CNT functionalized with P3HT as reported by Jo (P3HT-OH) 124

and Dai (HO-P3HT-OH).123

P3HT bound CNT has also been realized via amidification. Specifically, carboxylic

acid surface functionalized CNTs were reacted with ethylenediamine to yield CNT-NH2.127

This material was covalently bound to carboxylic acid side P3OT chains synthesized in

several steps through oxidative polymerization of an ester monomer followed by

deprotection.128

3.3.6 Surface anchoring via other methods

A few other papers reported the surface modification of metal oxides or carbon

nanomaterials with conjugated polymer. Krebs et al. prepared a hybrid material composed of

a P3HT chromophore, a ruthenium complex taking on the role of energy transfer function and

phosphoric acid group to perform TiO2 anchoring.129

The P3HT and phenylphosphoric acid

molecules were terminated with terpyridine groups via Stille coupling, and ruthenium atoms

linked both parts by ligand-based interactions. Applications in both DSSC and hybrid


41

polymer solar device, however, did not result in high efficiencies (around 0.1 % under A.M

1.5) but did display advantageous cell performance stability.

Macromolecular grafting has also been performed via the attack of living carbanions

on nanoparticles. Geng et al. used a Gilch polymerization to prepare MEH-PPV (Mn = 27 000

g mol-1

, GPC) having a living carbanion chain-end. The grafting was achieved by adding in

the polymerization medium TiO2 nanoparticles130

, TiO2 nanorods131

, and ZnO nanorods132

after complete conversion of the monomers. The photovoltaic performance improved upon

grafting in comparison with polymer/nanoparticles blend, indicating intimate contact between

nanoparticles and polymer chains.

Preparation of P3HT-CNT has been reported by Boon et al. through imine bond

formation.133

The aldehyde-terminated P3HT (Mn = 9 600 g mol-1

) reacted with primary

amine functionalized MWCNT. In spite of the low quantity of amino groups present on the

surface of MWCNT, the TEM and AFM images showed the formation of P3HT fibrils

arranged perpendicularly to the CNT surface.

Radical attack on carbons of double bonds was finally used to graft conjugated

polymers onto carbon materials. Azide side functional polyacetylene was bound to graphene

via nitrene chemistry,134

and P3OT terminated with a chloropropionate group was anchored

to CNT via atom transfer radical addition catalyzed with copper bromide(I)/bipyridine.135

3.3.7 Summary of the “grafting onto” methodology

Table 4 gives an overview of the covalent attachment of conjugated polymer on

various substrates via the “grafting onto” methodology. Again P3HT is the category

champion being used in more than 60 % of the studies. This is explained by the possibility,

thanks to Kumada chain polymerizations, of efficient end-functionalization of the

macromolecules with a wide range of chemical groups. Being focused on either the act

grafting or the end application, most of the studies do not provide full characterization of the

polymer itself. GPC is often used to access incorrect molar masses (based on polystyrene

calibrations) and comments on real molar masses and end-functionalization efficiency are

rare. Grafting has nevertheless been performed on a wide range of substrates from carbon

nano-materials to metal oxides to metals and so on, with varying shapes, such as flat wafers,


42

nanospheres and nanorods. A real advantage of the “grafting onto” method is the possibility

to perform the grafting easily and to incorporate it in a multi-step procedure. Once the end-

functionalized polymer is obtained, in best cases it could be spin-coated and after annealing

and rinsing, be proceeded to the next step of the device fabrication. One limitation of the

“grafting onto” method is that the molar mass of the attached polymer (around 10 000 g.mol-

1) leads to layer thicknesses of around 5 nm, much lower than the thickest film accessible via

“grafting from”.


43

Table 4. Surface Immobilization of Conjugated Polymer by the “grafting onto” methodology.

Surface Anchoring

group

Polymer reactive

function: end (e)

or side (s)

Chemical

reaction Polymer

Mn

(g mol-1) Ref

graphene

oxide C

O

Cl

-OH (e) esterification PTM 3 600

c

6 000c

126

graphene - -N3 (s) radical attack PA 14500 134

GO

- C

O

H (s)

azomethine-ylide

cycloaddition

PFCF 9 200b

120b

PFTPA 17 200b

120a

C

O

Cl

-OH (e) esterification P3HT 17 500d

123

Si N3

-C≡C (e) Huisgen

cycloaddition P3HT 3 100

b

115

SWCNT

-

O

O

Cl

(e)

radical attack

CuBr

P3OT 4 350b

135

(e)

Diels-Alder


a

117

-NH2 C

O

OH (s)

amidification P3OT

6 000c

127

MWCNT

C

O

Cl

-OH (e) esterification

P3HT

17 500d

88

10 000b

124

PTM 3 600

c

6 000c

125

-NH2 C

O

H (e)

imine bond P3HT 9 600b

133

SiO2 wafer -OH C

O

OH (e)

direct coupling PPE 3 000b

92


44

Si N3

-C≡C (e) Huisgen


b

112

TiO2 wafer Ti-OH C

O

OH (e)

direct coupling P3HT 2 500

c

6 700c

94

TiO2

nanoporous

PO

EtO

EtO

N

N

N

N

N

N

(e)

supramolecular

interaction with

Ru

P3HT 2 800b 129

Ti-OH C

O

OH (s)

direct coupling P3HT 2 300 b 96

TiO2

mesoporous Ti-OH C

O

OH (e)

direct coupling P3HT 3 200a 87

TiO2 NPs

(10 nm ø) Ti-OH Carbanion (e) - MEH-PPV 27 000

b 130

TiO2 NPs

(20 nm ø) Ti-OH C

O

OH (e)

direct coupling P3HT

7 500b 93

5 500b 95

TiO2 NRs

(3 nm ø,

20 nm l)

Ti-OH Carbanion (e) - MEH-PPV 27 000b 131

ZnO NPs

(10-20 nm ø) Si N3

-C≡C (e) Huisgen


b

112

ZnO NPs

(10 nm ø) Zn-OH P O

C8H17

C8H17 (e)

direct coupling P3HT - 100

ZnO NPs Zn-OH -SH (s) direct coupling P3HT 102

ZnO NRs

(7 nm ø, 20

nm l)

Zn-OH Carbanion (e) - MEH-PPV 27 000b

132

ZnO NRs

(30 nm ø,

120 nm l)

Zn-OH -NH2 (e)

direct coupling P3HT 3 500

c

136

ZnO NRs

(30 nm ø,

100 nm l)

Zn-OH Si

OEt

OEt

OEt

(e)

direct coupling P3HT

3 000

4 000

7 000a

137

ZnO NRs

(30-100 nm

ø, x m l)

Zn-OH P O

OEt

OEt (e)

direct coupling P3HT 7 000a

99


45

ITO wafer -OH C

O

OH (e)

direct coupling PPE 3 000b

92

CdSe QDs

(4 nm ø)

- -SH (e) direct coupling P3HT 10 000a

101

PO

C8H17

C8H17

Br

-C=C (e) Heck coupling P3HT 2 400a

105b

CdSe NRs

(5 nm ø, 40

nm l)

PO

HO

HO

Br

-C=C (e) Heck coupling P3HT 4 900b

110

PO

HO

HO

N3

-C≡C (e) Huisgen


b

113

CdSe NR

(8 nm ø, 40

nm l)

PO

C8H17

C8H17

Br

HS Br

-C=C (e) Heck coupling P3HT 8 600b

86

CdTe tetrapods (5

nm arms ø,

85 nm l)

PO

HO

HO

N3

-C≡C (e) Huisgen


b

114

Au wafer -

C

O

OH

CS

O

CH3CN

(e)

direct coupling PPE

22 000b

5 000b

3 000b

92

Au NPs

(5 nm ø) HS N

O

O

O (s)

Diels-Alder

cycloaddition PPE

5 000

b

116

Au NRs (7

nm ø, 25 nm

l)

- -SH (s) direct coupling PHF e 8 400

b

103

Pt wafer - C

O

OH

CN

(e)

direct coupling PPE

5 000b

3 000b

92

Notes: a determined by MALDI-TOF;

b determined by GPC,

c determined by NMR;

d commercial product, l =

length.


46

3.4 Conclusion

The grafting of conjugated polymers to substrates is a recent but promising field of work,

developed by several research groups studying fundamental chemistry, modified interfaces,

and hybrid materials in organic electronic devices. Table 5 summarizes the polymers used to

graft substrates via the different grafting methodologies. As expected, polyalkylthiophenes

are the most often studied macromolecules, because the monomer can undergo chain

polymerizations. This allows both efficient surface initiation which are key to developing the

“grafting from” methodology, and chain-end functionalization to access the “grafting onto”

method. Each of these techniques have their specific advantages, the “grafting from” yielding

high brush surface densities and layer thicknesses, and the “grafting onto” is more versatile

and easy to carry out. Furthermore, polymers with various chemical structures, such as

polyfluorene or polyphenylene, have been grafted onto many substrates for applications in

photovoltaics, electroluminescent diodes and sensors.

A huge amount of work, however, still needs to be done to: (i) understand and control

the polymerization of conjugated monomers from the surface; (ii) improve the “grafting

through” methodology in terms of molar mass and grafting efficiency; (iii) develop versatile

“grafting onto” chemistries; (iv) study the self-assembly of silane or thiol terminated

polymers; (v) extend the range of monomers, for example to graft low-band gap polymers

and finally; (vi) increase the knowledge on the covalent bonding of hybrid materials and

interfacial properties. (vii) control the shape and morphologies of the grafted conjugated

polymer.


47

Table 5. Summary of the conjugated polymer grafted on substrates.

Polymer Chemical structure

Substrate

Grafting

from

Grafting

through

Grafting

Onto

P3AT

R = H 46

,

CH3 47

51

48, 52

C6H13 45

, 32

, 50

, 55

, 86

, 87

, 88, 93

, 105b

, 110

, 113

, 112

, 124

, 123

, 94

, 99

,101-102

, 129,

133, 136,

115,

117,

95-96,

100, 137,

114

C8H17 127

135

PS-Br 43

,45

P4VP-block-PS-I 50

silicon 47

SiO2 NPs 32

, 55

gold 46

, 47

ITO 47

, 48

, 51

, 52

SiO2 112

TiO2 87

, 94, 129

TiO2 NPs 93, 95-96

ZnO NPs 100, 102

, 112

ZnO NRs 99

,137

, 136

GO 115

, 123

CNT 88

, 117

, 124

, 127, 133

, 135

CdSe QDs 86

, 101,

105b,

110,

113

CdTe 114

PF

R = C6H3

62,63

,64

,65, 103

,

2-ethyl hexyl 57

,

C8H17 54

120

PMMA-co-PS-Br 62

, 63

silicon 64

SiO2 NPs 54

PS-Br 57

cellulose 65

gold NRs 103

GO 120

PPE

R1=R2= H 76

,

R1=R2= 2-ethylhexyloxy 58

R1=R2= hexyloxy 92

R1=OC16H33 R2= H 76

,

R1= OC3H6SO3Na R2= O-

(C2H4)3 75

R1= OC3H6SO3Na R2=H 15

R1=

R 2= H

77

SiO2 NPs 58

75

, 15

cotton 77

glass 76

silicon wafer

gold wafer

Pt wafer

ITO

92

gold NPs 116

PPV

CdSe QDs 72

CNT

ZnO NRs 132

TiO2 NPs


48

R1 = R2 = H 72b

R1 = R2 = C8H17 72a

R1 = OCH3 R2 = OC8H17 79

R1= OCH3 R2= O-(ethyl hexyl) 130

, 131

,132

79

130

TiO2 NRs 131

PP

R = H

46,

OCH3, OC2H5, OC6H13 53

quartz 53

gold 46

PA R2

R1

n R1 = R2 =C4H9

78

R1 = C5H6 R2 = (CH2)3Cl 134

silicon wafer 78

graphene 134

PTM

CNT 125

Graphite Oxide 126


49

4. Organic photovoltaic cells

Despite the numerous applications in Organic Electronics, this PhD work has chosen

to focus on the elaboration of new hybrid materials for photovoltaic application. Therefore in

this chapter are presented the basics of organic photovoltaics being mandatory for the good

comprehension of the manuscript.

The organic solar cells have a planar-layered structure, where the organic light-absorbing

layer is sandwiched between two different electrodes.

One of the electrodes must be (semi-) transparent, often

Indium–Tin-Oxide (ITO), but a thin metal layer can also

be used. The other electrode is very often aluminum

(calcium, magnesium, gold and others are also used).

The anode (ITO) is deposited on a glass substrate

(borosilicate) and is spin-coated with (PEDOT-blend-

PSS) poly (3,4-ethylenedioxythiophene)-blend-

poly(styrene sulfonate) to decrease the roughness of the

surface and increase work function. The active layer is based on a blend of donor –acceptor

(polymer-PCBM) mixture to increase the exciton diffusion efficiency.

4.1 General working principles of organic photovoltaic devices

The overall process occurring in the organic and hybrid polymer-nanoparticle

photovoltaic cell may be divided into six consecutive steps: (also presented in Figure 11)

(i) Absorption of photons.

(ii) Generation of electron-hole pairs in the photoactive material.

(iii) Diffusion of exciton in the photoactive material to the donor/acceptor interface.

(iv) Dissociation of exciton and creation of charge carriers at the boundary between

donor and acceptor materials.

(v) Transport of holes and electrons to the electrodes.

(vi) Collection of the holes and electrons by electrodes.


50

HCL: Hole colleting electrode

ECL: Electron collecting electrode

Figure 11. The main photovoltaic processes generating electrical current.

4.1.1 Absorption of photons (i) and creation of excitons (ii)

Under irradiation, the conjugated polymer in the active layer absorbs the energy of

light (photons) as it matches the difference of its energy levels (bandgap) and transforms

them into so-called photogenerated unbound charges, which are free to move in the system.

This transformation based on excitation of electrons from the Highest Occupied Molecular

Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO) and leads to the

appearance of a hole remains bounded to the electron by mutual electrostatic interaction. The

electron-hole pair is electrically neutral and called an exciton. When two charges are

localized on the same molecule or on the same monomer unit it is called Frenkel exciton

(typical of organic semiconductor), 138

while if the distance between the electron and hole

corresponds to several monomer units, it is a Wannier exciton type (typical of inorganic

semiconductor). 139

Unlike inorganic semiconductors such as silicon, conjugated polymers

have a relatively low dielectric constant, and Coulomb interaction between electrons and

holes is so strong that they form excitons (neutral) instead of free carriers. Thus, the presence

of local electrical field due to materials with different energy levels is necessary to exceed

Coulomb interaction. The creation of an exciton generally occurred on the donor (p-type

semiconductor, conjugated polymer-light absorber), but sometimes it can occurr on the

acceptor (n-type semiconductor, inorganic nanoparticles). The lifetime of an exciton is a few

nanoseconds because of dissociation; the electrons relax to the HOMO level by transferring

HCL ECL HCL ECL


51

its energy in radiative (luminescence) and non-radiative (rotation, vibration of the molecule)

form. The photon absorption yield (ɳa) is dependent on the absorption spectra band, thickness

of the photoactive layer, internal reflection, device architecture, and bandgap of the polymer.

4.1.2 Diffusion of the exciton to the D/A interface (iii)

The number of excitons that can diffuse in the organic material without recombination

with respect to the number of generated exciton due to light absorption can be calculated by

exciton diffusion yield (ɳdiff). Excitons can diffuse to DA interface with Li (distance between

photoexciton location and D/A interface) is smaller than LD (exciton diffusion length),

otherwise undesirable recombination of electrons and holes in the polymer and back electron

transfer from electron acceptor to the polymer can occur and limit the performance of the

device. The neutral exciton can diffuse randomly during their lifetime with diffusion lengths

generally limited to about 5–20 nm in conjugated polymer140

in any direction, even under a

static electric field. Nanostructuring of the donor and acceptor phases to fabricate ordered

bulk heterojunctions with controlled dimensions is an attractive approach to achieve full

exciton harvesting (Figure 12).

Figure 12. Scheme of an ordered bulk heterojunction device.

4.1.3 Dissociation of excitons (iv)

The excited electron in the conjugated polymer may be transferred into the LUMO of the

acceptor (inorganic semi-conductor) to overcome its binding energy. The difference in energy

levels at the interface D/A leads to the formation of strong electrical field to ensure the separation

Hole collecting electrode

Electron collecting electrode


52

of carrier charges (Figure 13). The driving force required for this charge separation and transfer is

that the exciton has a higher binding energy (Eex) than the difference in ionization potential IpD* of

the excited donor and the electron affinity EA of the acceptor (Eex > IpD*- EA).141

The yield of the

dissociation (ɳdiss) is dependent on the ratio of excitons that dissociate into free charges to the total

number of excitons at the donor/acceptor interface. If dissociated charges remain weakly bound at

the interface, they are referred as charge transfer (CT) state, 142

while if mobile dissociated charges

are generated, they are referred to as charge separated (CS) states.143

The dissociation yield (ɳdiss)

measures both the direct formation of CS and CT states that separate to CS states. The dissociation

efficiency can be enhanced by increasing the potential difference between the donor LUMO and

acceptor electron affinity. This can occur by applying a high total electric field across the device or

by designing morphological features that increase the distance between the electron and hole.

Figure 13. Exciton dissociation at the donor–acceptor interface (Eex ≥ IPD - EA).

4.1.4 Charge transfer (v) and collection at electrodes (vi)

The holes are transported in a conjugated polymer toward the hole collecting

electrode (ITO), while the electrons are transported through the acceptor toward the electron

collecting electrode (typically-Al). The polymers need to have a high degree of planarity for

efficient backbone stacking for a high hole mobility. Percolation within the material domains

is crucial for an efficient collection of charges carriers to the electrodes. The transport

efficiency is also influenced by the energy levels and densities of trap states in their


53

respective transport materials.144

The trap states that are typically caused by structure defects

and impurity species; there can be recombination centers leading to charge transport losses.

The collection of these charges depends on the interfaces HCL/donor semi-conductor and

ECL/acceptor semi-conductor. Thus, electrodes must be chosen so that the energy barrier to

be taken is as low as possible. In other words the Fermi level of the cathode must be close to

the (LUMO) level of the acceptor, and the Fermi level of HCL should be close to the

(HOMO) level of donor.

4.2 Photovoltaic parameters

There are some important parameters, which describe solar cell devices. These

parameters are the short circuit current density (JSC), the open circuit voltage (VOC), the fill

factor (FF), the series resistance (RS), the Shunt resistance (RSh), the Power conversion

efficiency (ɳ) and the quantum efficiency (QE).

Short-circuit current density (Jsc)

The (JSC) is the maximum current density (A.cm-2

) which flows in the device under

illumination when no voltage is applied (V= 0). The JSC is highly dependent on the number of

absorbed photons, on the morphology of the device, and on the lifetime and mobility of the

charge carriers. 145

Open-circuit voltage (Voc)

The (VOC) is the maximum voltage (V) that the device can produce under open circuit. For

bulk heterojunctions it is correlated to the difference between the HOMO of the donor

(polymer) and the LUMO of the acceptor.146

It has been found that the VOC is not very

dependent on the work functions of the electrodes.147

VOC is affected by several parameters

such as T (temperature), q (elementary charge), I (Photocurrent), I0 (reverse saturated current

density).


54

Fill Factor (FF)

The (FF) is the ratio between the maximum power output of the device (Vmax·Imax) and the

maximum theoretical power output, which can be achieved if the device is an ideal diode

(VOC·JSC). High FF can be achieved with low RS (series resistance) and high RSh (Shunt

resistance).

Series resistance (Rs)

The (RS) is another parameter that affects the (J-V) characteristics and solar performance. It

results from limited conductivity of organic layer, contact resistance between organic layer

and its corresponding electrodes, and connecting resistance between electrodes and external

circuit. The high value of (RS) can reduce the (FF) and (Jsc) but it has no impact on (VOC).

The slope of the curve (J-V) at the point VOC represents the reciprocal of the series resistance.

Shunt resistance (Rsh)

The (RSh) is related to the device structure and morphology of the film. A slight decrease in

(RSh) can lower the current flowing through the diode and thus lowering (VOC). The slope of

the curve (J-V) at the point JSC represents the reciprocal of the shunt resistance

Power conversion efficiency (PCE)

PCE is one of the most important parameters to characterize solar cell performance. It is

defined as the percentage of maximum output of electrical power available (Pout) that can be

extracted by the solar cell compared to the incident light power (Pin). Figure 14 shows the

current density-voltage (J-V) characteristic for typical organic solar cell in the dark and under

illumination. The PCE described as:


55

Figure 14. Current‐voltage (J-V) curves of an organic solar cell.

Quantum efficiency (QE)

The (QE) is an accurate measurement of the device′s sensitivity. It is often measured over a

range of different wavelengths to characterize device′s efficiency at each energy level. It is

divided into two measurements the External Quantum Efficiency (EQE) and the Internal

Quantum Efficiency (IQE).

External quantum efficiency (EQE)

The (EQE) or Incident Photon to Current Efficiency (IPCE) is another important parameter

for solar cell characterization. It is defined by the number of electrons extracted in an external

circuit divided by the number of incident photons at a certain wavelength under short circuit

conditions. This value includes the losses due to reflection at the surface and the transmission

through device.

1/Rsh

1/Rs

Under light

Dark


56

Internal Quantum Efficiency (IQE)

The (IQE) can be considered as the actually absorbed photons by the photoactive layer, EQE

can be converted into IQE.

Where Ref ( ) is the fraction of reflected light and Trans ( ) is the fraction of transmitted

light.

Air mass

The light that reaches us from the sun does not present exactly the same spectrum as that

emitted by it. This attenuation due to absorption and scattering of light in atmosphere is not

uniform and depends on the thickness of atmosphere traversed as a ratio relative to the path

vertically upward at the zenith at a given angle Z. To study these differences, a coefficient x

called air mass is introduced whose expression is:

An air mass distribution of 1.5 and within an incident power density of ~100 mW/cm-2

used

by solar industries for all standardizing testing of terrestrial solar panels corresponds to the

spectral power distribution observed when the sun’s radiation is coming from an angle to

over head of about 48°.

4.3 Conclusion

Organic solar cells can be identified as an inexpensive alternative to the inorganic

ones. Regardless of its low cost production and its easy fabrication, the power conversion

efficiency is still limited. Thus understanding the principle, mode of operation and the

photovoltaic parameters of the device is important to design new donor and acceptor units

(for better matching with solar spectrum and generation of higher number of excitons), new

device architectures ( for dissociation of higher number of excitons, less recombination and

better morphology), modifying interfaces and electrodes (for charge transport and collection)

for achieving high photovoltaic performance similar to commercial inorganic solar cells.


57

5. Aim and scope of the PhD.

Hybrid photovoltaic devices based on metal oxide nanostructures have been much reported

over the last two decades. ZnO have demonstrated to be a promising candidate due to its favorable

electronic properties and ease of fabrication. Two main strategies have been reported to design new

organic-inorganic hybrids in intimate contact for photovoltaic applications. The research in this

PhD manuscript will focus on the synthesis of two new hybrid materials by grafting classical

polymer poly(3-hexylthiophene) (P3HT) and lowband gap polymer (LBG) onto ZnO nanorods via

“grafting onto” and “grafting from” methodology, respectively. The ZnO@P3HT nanocomposites

were synthesized in one step reaction between a triethoxysilane-terminated P3HT and the

nanoparticles. While the desired ZnO@low bandgap were synthesized in three steps via Stille-cross

coupling polymerization. Moreover, we report the use of self-assembled monolayer (SAM) of

poly(3-hexylthiophene) on ITO flat surface electrode as an alternative to PEDOT:PSS. In this

work, several characterization techniques were used as: Atomic Force Microscopy (AFM), Size

Exclusion Chromatography (SEC), Thermal Gravimetric Analysis (TGA), Infrared (IR), UV-

visible spectroscopy (Uv-vis), Nuclear Magnetic Resonance (NMR), Transmission Electron

Microscopy (TEM), X-ray Photoelectron Microscopy (XPS) and device fabrication.


58

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114. Jung, J.; Pang, X.; Feng, C.; Lin, Z., Semiconducting conjugated polymer-inorganic tetrapod nanocomposites. Langmuir 2013, 29 (25), 8086-8092. 115. Meng, D.; Sun, J.; Jiang, S.; Zeng, Y.; Li, Y.; Yan, S.; Geng, J.; Huang, Y., Grafting P3HT brushes on GO sheets: Distinctive properties of the GO/P3HT composites due to different grafting approaches. Journal of Materials Chemistry 2012, 22 (40), 21583-21591. 116. Liu, X.; Zhu, M.; Chen, S.; Yuan, M.; Guo, Y.; Song, Y.; Liu, H.; Li, Y., Organic-inorganic nanohybrids via directly grafting gold nanoparticles onto conjugated copolymers through the Diels-Alder reaction. Langmuir 2008, 24 (20), 11967-11974. 117. Yameen, B.; Zydziak, N.; Weidner, S. M.; Bruns, M.; Barner-Kowollik, C., Conducting polymer/SWCNTs modular hybrid materials via Diels-Alder ligation. Macromolecules 2013, 46 (7), 2606-2615. 118. (a) Reyes-Reyes, M.; Kim, K.; Dewald, J.; López-Sandoval, R.; Avadhanula, A.; Curran, S.; Carroll, D. L., Meso-structure formation for enhanced organic photovoltaic cells. Organic Letters 2005, 7 (26), 5749-5752; (b) Mühlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.; Gaudiana, R.; Brabec, C., High photovoltaic performance of a low-bandgap polymer. Advanced Materials 2006, 18 (21), 2884-2889. 119. Hou, J.; Chen, H. Y.; Zhang, S.; Li, G.; Yang, Y., Synthesis, characterization, and photovoltaic properties of a low band gap polymer based on silole-containing polythiophenes and 2,1,3-benzothiadiazole. Journal of the American Chemical Society 2008, 130 (48), 16144-16145. 120. (a) Zhang, B.; Liu, Y. L.; Chen, Y.; Neoh, K. G.; Li, Y. X.; Zhu, C. X.; Tok, E. S.; Kang, E. T., Nonvolatile rewritable memory effects in graphene oxide functionalized by conjugated polymer containing fluorene and carbazole units. Chemistry - A European Journal 2011, 17 (37), 10304-10311; (b) Zhang, B.; Liu, G.; Chen, Y.; Zeng, L. J.; Zhu, C. X.; Neoh, K. G.; Wang, C.; Kang, E. T., Conjugated polymer-grafted reduced graphene oxide for nonvolatile rewritable memory. Chemistry - A European Journal 2011, 17 (49), 13646-13652; (c) Zhang, B.; Chen, Y.; Liu, G.; Xu, L. Q.; Chen, J.; Zhu, C. X.; Neoh, K. G.; Kang, E. T., Push-Pull archetype of reduced graphene oxide functionalized with polyfluorene for nonvolatile rewritable memory. Journal of Polymer Science, Part A: Polymer Chemistry 2012, 50 (2), 378-387. 121. Eda, G.; Chhowalla, M., Graphene-based composite thin films for electronics. Nano Letters 2009, 9 (2), 814-818. 122. Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S., The chemistry of graphene oxide. Chemical Society reviews 2010, 39 (1), 228-240. 123. Yu, D.; Yang, Y.; Durstock, M.; Baek, J. B.; Dai, L., Soluble P3HT-grafted graphene for efficient bilayer-heterojunction photovoltaic devices. ACS Nano 2010, 4 (10), 5633-5640. 124. Song, Y. J.; Lee, J. U.; Jo, W. H., Multi-walled carbon nanotubes covalently attached with poly(3-hexylthiophene) for enhancement of field-effect mobility of poly(3-hexylthiophene)/multi-walled carbon nanotube composites. Carbon 2010, 48 (2), 389-395. 125. Lee, R. H.; Lee, L. Y.; Huang, J. L.; Huang, C. C.; Hwang, J. C., Conjugated polymer-functionalized carbon nanotubes enhance the photovoltaic properties of polymer solar cells. Colloid and Polymer Science 2011, 289 (15-16), 1633-1641. 126. Lee, R. H.; Huang, J. L.; Chi, C. H., Conjugated polymer-functionalized graphite oxide sheets thin films for enhanced photovoltaic properties of polymer solar cells. Journal of Polymer Science, Part B: Polymer Physics 2013, 51 (2), 137-148. 127. Pokrop, R.; Kulszewicz-Bajer, I.; Wielgus, I.; Zagorska, M.; Albertini, D.; Lefrant, S.; Louarn, G.; Pron, A., Electrochemical and Raman spectroelectrochemical investigation of single-wall carbon nanotubes-polythiophene hybrid materials. Synthetic Metals 2009, 159 (9-10), 919-924. 128. Buga, K.; Pokrop, R.; Majkowska, A.; Zagorska, M.; Planes, J.; Genoud, F.; Pron, A., Alternate copolymers of head to head coupled dialkylbithiophenes and oligoaniline substituted thiophenes: Preparation, electrochemical and spectroelectrochemical properties. Journal of Materials Chemistry 2006, 16 (22), 2150-2164.


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129. Krebs, F. C.; Biancardo, M., Dye sensitized photovoltaic cells: Attaching conjugated polymers to zwitterionic ruthenium dyes. Solar Energy Materials and Solar Cells 2006, 90 (2), 142-165. 130. Geng, H.; Peng, R.; Han, S.; Gu, X.; Wang, M., Surface-modified titania nanoparticles with conjugated polymer for hybrid photovoltaic devices. Journal of Electronic Materials 2010, 39 (10), 2346-2351. 131. Geng, H.; Wang, M.; Han, S.; Peng, R., Enhanced performance of hybrid photovoltaic devices via surface-modifying metal oxides with conjugated polymer. Solar Energy Materials and Solar Cells 2010, 94 (3), 547-553. 132. Geng, H.; Guo, Y.; Peng, R.; Han, S.; Wang, M., A facile route for preparation of conjugated polymer functionalized inorganic semiconductors and direct application in hybrid photovoltaic devices. Solar Energy Materials and Solar Cells 2010, 94 (7), 1293-1299. 133. Boon, F.; Desbief, S.; Cutaia, L.; Douhéret, O.; Minoia, A.; Ruelle, B.; Clément, S.; Coulembier, O.; Cornil, J.; Dubois, P.; Lazzaroni, R., Synthesis and characterization of nanocomposites based on functional regioregular poly(3-hexylthiophene) and multiwall carbon nanotubes. Macromolecular Rapid Communications 2010, 31 (16), 1427-1434. 134. Xu, X.; Luo, Q.; Lv, W.; Dong, Y.; Lin, Y.; Yang, Q.; Shen, A.; Pang, D.; Hu, J.; Qin, J.; Li, Z., Functionalization of graphene sheets by polyacetylene: Convenient synthesis and enhanced emission. Macromolecular Chemistry and Physics 2011, 212 (8), 768-773. 135. Stefopoulos, A. A.; Chochos, C. L.; Prato, M.; Pistolis, G.; Papagelis, K.; Petraki, F.; Kennou, S.; Kallitsis, J. K., Novel hybrid materials consisting of regioregular poly(3-octylthiophene)s covalently attached to single-wall carbon nanotubes. Chemistry - A European Journal 2008, 14 (28), 8715-8724. 136. Chen, C. T.; Hsu, F. C.; Sung, Y. M.; Liao, H. C.; Yen, W. C.; Su, W. F.; Chen, Y. F., Effects of metal-free conjugated oligomer as a surface modifier in hybrid polymer/ZnO solar cells. Solar Energy Materials and Solar Cells 2012, 107, 69-74. 137. Awada, H.; Medlej, H.; Blanc, S.; Delville, M. H.; Hiorns, R. C.; Bousquet, A.; Dagron-Lartigau, C.; Billon, L., Versatile functional poly(3-hexylthiophene) for hybrid particles synthesis by the grafting onto technique: Core@shell ZnO nanorods. Journal of Polymer Science, Part A: Polymer Chemistry 2014, 52 (1), 30-38. 138. Wannier, G. H., The Structure of Electronic Excitation Levels in Insulating Crystals. Physical Review 1937, 52 (3), 191-197. 139. Yamashita, K.; Harima, Y.; Iwashima, H., Evaluation of exciton diffusion lengths and apparent barrier widths for metal/porphyrin Schottky barrier cells by using the optical filtering effect. The Journal of Physical Chemistry 1987, 91 (11), 3055-3059. 140. Pettersson, L. A. A.; Roman, L. S.; Inganäs, O., Modeling photocurrent action spectra of photovoltaic devices based on organic thin films. Journal of Applied Physics 1999, 86 (1), 487-496. 141. Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F., Photoinduced electron transfer from a conducting polymer to buckminsterfullerene. Science 1992, 258 (5087), 1474-1476. 142. Hwang, I. W.; Moses, D.; Heeger, A. J., Photoinduced carrier generation in P3HT/PCBM bulk heterojunction materials. Journal of Physical Chemistry C 2008, 112 (11), 4350-4354. 143. Brédas, J. L.; Norton, J. E.; Cornil, J.; Coropceanu, V., Molecular understanding of organic solar cells: The challenges. Accounts of Chemical Research 2009, 42 (11), 1691-1699. 144. Saunders, B. R.; Turner, M. L., Nanoparticle–polymer photovoltaic cells. Advances in Colloid and Interface Science 2008, 138 (1), 1-23. 145. Brabec, C. J., Organic photovoltaics: technology and market. Solar Energy Materials and Solar Cells 2004, 83 (2–3), 273-292. 146. Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J., Design rules for donors in bulk-heterojunction solar cells - Towards 10 % energy-conversion efficiency. Advanced Materials 2006, 18 (6), 789-794. 147. Frohne, H.; Shaheen, S. E.; Brabec, C. J.; Müller, D. C.; Sariciftci, N. S.; Meerholz, K., Influence of the Anodic Work Function on the Performance of Organic Solar Cells. ChemPhysChem 2002, 3 (9), 795-799

Chapter 2: Versatile Functional Poly(3-hexylthiophene) for Hybrid Particles Synthesis by the

Grafting Onto technique: Core@Shell ZnO nanorods

Chapter 2



Hussein Awada, Hussein Medlej, Sylvie Blanc, Marie-Hélène Delville†, Roger C. Hiorns

‡,

Antoine Bousquet, Christine Dagron-Lartigau*, Laurent Billon

*

IPREM CNRS-UMR 5254, Equipe de Physique et Chimie des Polymères, Université de Pau

et des Pays de l'Adour, Hélioparc, 2 avenue Président Angot, 64053 Pau Cedex 9, France.

† CNRS, Université de Bordeaux, ICMCB, 87 avenue du Dr A. Schweitzer, Pessac F-33608,

France.

‡ CNRS, IPREM CNRS-UMR 5254, Equipe de Physique et Chimie des Polymères,

Hélioparc, 2 avenue President Angot, 64053 Pau, France.

Abstract

We demonstrate an efficient strategy to anchor poly(3-

hexylthiophene) (P3HT) onto zinc oxide (ZnO) surfaces.

Synthesis of a novel triethoxysilane-terminated regioregular

P3HT is herein reported and supported by thorough

characterization. Three triethoxysilane-terminated P3HTs of

different molar masses were prepared via a hydrosilylation

reaction from allyl-terminated P3HT. MALDI-TOF and 1H NMR

were performed to characterize the polymer and show that

around 80 % of the chains are end-functionalized. These polymers were then grafted onto the

ZnO nanorods to create a macromolecular self-assembled monolayer (MSAM). This versatile

technique could be subsequently applied to different metal oxide surfaces such as silicon,

titanium or indium-tin oxide. Importantly, the influence of the molar mass on the grafting

density and the polymer shell thickness was studied via thermo gravimetric analysis and

transmission electron microscopy. The optical properties of the hybrid materials were

determined by UV-visible absorption and photoluminescence to show a quenching effect of

P3HT fluorescence by ZnO when grafted. This electronic transfer associated with an

improved miscibility of the ZnO@P3HT, makes these hybrid materials suitable candidates for

photovoltaic applications.

JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2014, 52, 30–38




1. Introduction ..................................................................................................................... 70

2. Results and discussion .................................................................................................... 73

2.1. Synthesis and characterizations of allyl-terminated P3HT. ....................................... 73

2.2 Synthesis and characterizations of triethoxysilane-terminated P3HT. ...................... 78

2.3 Specific surface area of Zinc oxide nanorods ............................................................ 80

2.4. Hybrid material P3HT@ZnO nanorod characterizations. ......................................... 82

2.5. Hybrid material properties. ........................................................................................ 89

3. Perspectives ..................................................................................................................... 92

4. Conclusion ....................................................................................................................... 94

5. References ........................................................................................................................ 95

Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the


70

1. Introduction

In the past decade, a considerable progress has been made on polymer based solar

cells, giving clues on how to control the morphology and improve the conversion efficiency.1

At present, the so-called bulk heterojunction (BHJ) is based on the intimate mixing at the

nanoscale of an electron donor, usually a conjugated polymer such as poly(3-hexylthiophene)

(P3HT), and an electron acceptor, usually a soluble modified fullerene such as phenyl-C61-

butyric acid methyl ester (PCBM). This approach represents one of the most studied type of

(OSCs) with efficiencies around 5 %.2 Such PSCs are not stable with ageing because of

PCBM aggregation in the active layer.3 To solve this problem, various inorganic nanocrystals

such as cadmium selenium CdSe4 or silicon

5 have been used as electron acceptors and studied

intensively in the field of hybrid BHJ solar cells. Metal oxide nanoparticles, and especially

titanium oxide TiO2 and ZnO,6 are of particular interest due to their ease of fabrication, non-

toxicity, good air stability and relatively low production costs. The good optical and electrical

properties of zinc oxide as wide bandgap semiconductor (~3.3 eV) with a high exciton

binding energy (~60 meV) and a good hole mobility between 5 and 30 cm2.V

-1.s

-1 makethem

suitable to replace organic acceptors.

Hybrid solar cells using ZnO nanoparticles/polymer as the BHJ were first reported by Beek et

al. in 2004. In this study the authors mixed ZnO nanocrystals with a poly(phenylene vinylene)

(PPV) and obtained an efficiency of 1.6%.7 Later the same group varied the shape, size and

concentration of the particles and showed that the best performances were obtained for

nanocrystals of 4.9 nm in diameter. The efficiency dropped from 1.6% to 0.92% when

combining the nanoparticles with P3HT8 (this polymer presents many advantageous when

compared to PPV, such as improved absorption, increased environmental stability and higher

charge mobility). This behavior was assigned to a coarse mixing of the blend and high film

roughness (Figure 1) generating current leakages.



71

Figure 1. Tapping mode AFM height images (2μm x2μm) of a) pristine P3HT (roughness = 0.8 nm), b) nc-ZnO

(15%vol):P3HT blend (roughness = 15nm).

From this conclusion, it appeared that the interface between the ZnO material and the polymer

is essential and research has turned towards its optimization. A widely used strategy to

enhance the properties of a hybrid material is to covalently attach the components; in this

case, P3HT might best be anchored to ZnO particles. Such system was chosen taking into

account the electronic composition properties, hole mobility and band gap of the P3HT with

respect to zinc oxide. (Figure 2)

Figure 2. Energy band diagram displaying HOMO and LUMO levels of P3HT donor material as well as the

valence and conduction band edge of ZnO inorganic acceptors.

This interfacial-engineering approach is desired to significantly improve the efficiency and

stability of the active layer of PSCs and could enable large-area device manufacturing using

low-cost, all-printable processes. Several studies as discussed previously in chapter 1 dealt

with different chemistries to strongly anchor a CP to surfaces (Figure 3).

Vacuum

Ener

gy re

lativ

e to

vacu

um le

vel (

eV)

P3HT

ZnO

-3.3

-5.2

-4.2

-7.6



72

Figure 3. (a)- Esterification reaction, (b)-Heck coupling and (c)-click chemistry were applied to strongly anchor

a CP (P3HT) to MWCNTs, CdSe and SiO2 surfaces, respectively.

In all these studies, two steps are required; first functionalize the surface and then make this

group polymerize or react with the end-chain modified polymer.

In this chapter, we focus on an efficient strategy to create in one step a macromolecular

self-assembled monolayer of poly(3-hexylthiophene) (P3HT) onto zinc oxide (ZnO)

surfaces (Scheme 1).

a-

b-

c-



73

Scheme 1. Synthetic procedure for ZnO@P3HT hybrids nanorods (NRs).

The versatile functional triethoxysilane moiety has been largely used by our group for many

substrates with different shapes and chemical composition in order to control the grafting of

coil polymers.9 It could be applied to different metal oxide surfaces such as titanium or

indium-tin oxide, very useful substrates for photovoltaic applications.

In conclusion, the synthesis of three triethoxysilane-terminated P3HT with different molar

mass was performed in order to study the influence of chain length on grafting density. The

polymers and the hybrid materials have been thoroughly characterized to evidence the

covalent attachment of the polymer to the metal oxide surface. Finally some preliminary

properties of the hybrid materials have been studied such as the dispersion stability in solution

and the electronic properties to anticipate their potential applications in active layer of

photovoltaic cells.

2. Results and discussion

2.1. Synthesis and characterizations of allyl-terminated P3HT.

3-Hexylthiophene was polymerized via the GRIM method 10

(Scheme 2) of 2,5-

dibromo-3-hexylthiophene (1) with isopropylmagnesium chloride to give 2-bromo-5-

chloromagnesio-3-hexylthiophene as a major product. The addition of catalyst based on

nickel 1,3-bis(diphenylphosphino)propane nickel-(II) chloride Ni(dppp)Cl2) leads to the

formation of quasi-living chain of P3HT. A second Grignard reagent (allyl magnesium

bromide) is then added after 15 min to the polymer solution to terminate the reaction.



74

Scheme 2. Synthesis of allyl-terminated rr-P3HT.

The solution was then precipitated in methanol, filtered through Soxhlet thimble and extracted

with ethanol, acetone and recovered with chloroform until the extracted solvent become

colorless. Three allyl end-functional P3HTs (P3HT-allyl) were obtained, by this method,

using the same procedure, by varying the ratio [monomer]/[catalyst] added to the mixture.

Because KCTP behaves as a chain-growth polymerization the theoretical degree of

polymerization can be calculated by the following formula:

eq.1

To elucidate the structure of the end-terminated polymer, 1H NMR was performed (Figure 3).

This spectrum is in perfect agreement with the expected structure and all peaks are attributed

to ally-terminated P3HT.11

Allyl End-group was identified by its chemical shifts and splitting

patterns. The allyl-terminated polymer shows three peaks: h ( CH2, 3.49ppm, d), i (CH, 5.98

ppm, m) and j (CH2, 5.12 ppm, t) pertaining to the end chain. The number of repeated units

can be determined by comparing the integrals corresponding to the protons bound to the -

carbon of 3-hexyl chain in the polymer main chain (d) to those corresponding to similar

protons at the ends of chain (d' and d"). For P2 the number of repeated units calculated

according to the following equation is:

eq.2

Although, the regioregularity of this polymer can be calculated based on the integrals

corresponding to protons d and d" (Id + Id") which are placed in a regular arrangement (head -

SBrBr

C6H13

SBr/H

C6H13

n

1. iPrMgCl

2. Ni(dppp)Cl2

3. RMgX

(1) P3HT-Allyl



75

to-tail) – (head -to-tail) (HT- HT), compared with the total integral corresponding to peaks d,

d' and d"(Id + Id' + Id ").The proton d' and d" would correspond to two units, that is, 4 protons.

For example, the regioregularity for polymer P2 calculated according to the following

equation:

eq.3

Figure 4. 1H NMR spectrum of allyl-terminated poly(3-hexylthiophene) with a DPn of 31 repeating units (P2).

There is also a possibility to calculate the end group functionalization, by comparing of i, j or

h to d' and d". For example, a 84% functionalization was observed for P2 according to 1H

NMR

eq.4



76

To further confirm this, MALDI-TOF investigation was carried out (see Figure 5) and

reviewed in Table 1 to determine the composition of the terminal groups of the polymer and

its real molar mass (precise and quantitative method). The molar masses determined by

MALDI-TOF are lower than those obtained by 1H NMR, in agreement with previous

studies.12

Note that the end group composition can be determined by the following equation:

(166.23)n + EG1+ EG2 eq.5

where EG1 and EG2 are the molecular weight of the terminal end groups and n is the number

of repeating units. Three populations were identified by MALDI-TOF as a representative

example, P3HT-allyl P2, consisted of a mixture of 72% of H-P2-allyl, 15% of H-P2-H and

13% Br-P2-H. The percentage of the allyl population is given in Table 1. These results are in

a good agreement with 1H NMR showing that P3HT is monofunctionalized.

Figure 5. MALDI-TOF mass spectrum of allyl-terminated P3HT P2.

999.0 2799.4 4599.8 6400.2 8200.6 10001.0

M ass (m/z )

0

2532.8

0

10

20

30

40

50

60

70

80

90

100

% In

te

ns

ity

Voyager Spec #1=>AdvBC(32,0.5,0.1)[BP = 2701.9, 2533]

2701.9

3034.02534.9

3200.0

2368.83366.1

3532.1

3699.2

3865.22203.8

2660.9 4032.3

4198.3

4364.42994.02035.7

4697.53160.05029.52742.9 3327.1

5362.71051.3 2161.73493.1 5693.7

6027.42719.92409.81997.7 3051.11081.3 3739.21388.5 6360.52515.0 3387.22115.7 2842.8 4159.21784.0 6691.84736.71059.3 5071.5 7022.71475.9 3774.4 5488.93398.0 5903.34094.7 7522.8

2777.0 2881.8 2986.6 3091.4 3196.2 3301.0

M ass (m/z )

0

100

0

10

20

30

40

50

60

70

80

90

100

% In

te

ns

ity

ISO:H(C10H14S)18C3H5

3034.5

3033.5

3036.5

3032.5

3037.5

3038.5

3039.5

3040.5

3042.5

2777.0 2881.8 2986.6 3091.4 3196.2 3301.0

M ass (m/z )

0

2496.6

0

10

20

30

40

50

60

70

80

90

100

% In

te

ns

ity

Voyager Spec #1=>AdvBC(32,0.5,0.1)[BP = 2701.9, 2533]

2867.9

2866.9

3034.02868.9

3200.02865.9

3199.02869.9 3036.0

3202.03032.0

3198.02870.9 3037.0

2827.9 3204.03031.02871.92994.02828.9

3197.03160.03039.02996.02908.0 3074.02872.9 3158.02830.9 3240.03051.12990.9 3206.03078.02881.92799.0 2925.1 3013.0 3134.02970.8 3165.92847.4 3261.02905.1 3237.12950.9 3115.83096.0 3187.9 3283.03056.8



77

For example, the peaks at 3074 Da, 3034.5 Da and 2994 Da correspond to H-(P2)18-Br, H-

(P2)18-allyl and H-(P2)18-H, respectively by applying the following equation.

(166.23)n + EG1 + EG2= 18 x (166.23)+1+80 = 3073.1

(166.23)n + EG1 + EG2= 18 x (166.23)+1+42 = 3035.1

(166.23)n + EG1 + EG2=18 x (166.23) +1+1 = 2994.14

Table 1 gives an overview of the results; more than 70% of the macromolecules were

successfully end-functionalized.

Gel Permeation Chromatography has been performed by setting the UV wavelength detection

at 450 nm. Molar masses and dispersity were extracted from GPC data and are resumed in

Table 1. Polystyrene calibrated GPCs overestimate molar masses by a factor from around 1.5

to 2. 13

The normalized GPC of all samples are reported in Figure 6.

Table 1. Macromolecular characteristics of synthesized P3HTs.

Polymer 2

Mna

g.mol-1

Mnb

g.mol-1

Mnc

g.mol-1

= %

Enda

= %

Endb

% RRa Ð

c

P1 0.087 35 3800 2700 5600 70 69 96% 1.14

P2 0.078 40 5300 3900 8000 84 72 98% 1.16

P3 0.065 47 7800 5500 11000 100 75 98% 1.1

a calculated from NMR,

b calculated from MALDI-TOF,

c calculated from SEC (polystyrene conventional

calibration), = means allyl, Si triethoxysilane and RR regioregularity.



78

Figure 6. GPC results of P1, P2 and P3 of synthesized P3HT in this study (UV detector- = 450 nm).

It should be noted that we observe a narrow peak distribution with symmetrical shape. We

observe a small shoulder for high molecular weight P3 probably due to coupling between

growing chains and Ni disproportionation when quenching the polymerization.14

.

As a conclusion, Allyl-terminated P3HTs with different molar mass, high end chain

functionalization, high regioregularities and low dispersities (Ð) (Table 1) were obtained.

2.2 Synthesis and characterizations of triethoxysilane-terminated P3HT.

A further post-functionalization via the hydrosilylation method15

was performed on allyl-

terminated P3HT under dry conditions to transform the alkene into triethoxysilane end-groups

in quantitative yields in the presence of chloroplatinic acid (H2PtCl6) (Scheme3)

Scheme3. Synthesis of rr-P3HT terminated triethoxysilane.



79

Oxidative addition of the hydrosilane (C2H5O)3SiH gives a hydrido-silyl complex which is

coordinated with the alkene end group. Then the hydrosilylation product formed after

consecutive hydrometallation and reductive elimination of the alkyl and silyl ligands.

However, due to the high sensitivity of the Si-OEt moiety to hydrolysis, the silane end-

functional polymers (P3HT-Si) were purified by several filtrations in dry ethanol under

nitrogen and stored in a glove box under inert atmosphere. Figure 7 shows a superposition of

the 1H NMR spectra of P3HT-allyl and P3HT-Si, where a complete disappearance of allylic

protons and appearance of two peaks k (CH2, 3.87 ppm, q) and l (CH3, 1.25ppm, t) was

observed.

Figure 7. 1H NMR spectrum (400MHz, CDCl3) of allyl-terminated and triethoxysilane-terminated P3HT P3.

29Si NMR performed on the polymers shows the presence of a signal at -45.4 ppm pertaining

to (EtO)3SiC group, confirming the functionalization and the absence of hydrolysed

alkoxysilane functions (Figure 8).16



80

Figure 8. 29

Si NMR spectrum (, CDCl3) of alkoxysilane-terminated poly(3-hexylthiophene) P3: -44.5

((EtO)3SiC) ppm.

2.3 Specific surface area of Zinc oxide nanorods

ZnO nanorods (length = 150 nm, width = 30 nm) were synthesized in the group of Dr. Marie-

Hélène Delville (Institue of condensed Matter Chemistry of Bordeaux-ICMCB/University of

Bordeaux). The specific surface area was calculated according to Brunauer–Emmett–

Teller (BET) theory. 17

The BET equation is expressed by:

eq.6

where P is the equilibrium pressure, P0 is the saturation pressure, V is the adsorbed gas

quantity, Vm is the monolayer adsorbed gas quantity and C is the BET constant. Vm and C

were calculated by drawing

as a function of

(Figure 9)



81

Figure 9. BET plot of zinc oxide nanorods.

The slope and the y-intercept of the straight line are 0.1779 and 0.0026, respectively.

By applying the previous equations, we can calculate Vm = 5.5438 cm3/g and C = 69.722.

Then we use this formula to calculate Specific surface area Ss.

Ss: specific surface area (m2/g), Na: Avogadro’s constant, a: cross sectional area of adsorbed

molecule (m2), m: mass of the sample (g).

The specific surface area was determined to be equal to 24 m2/g.



82

2.4. Hybrid material P3HT@ZnO nanorod characterizations

The bare ZnO particles were dispersed in THF (2 mg.mL-1

, 5 mL) by ultrasonication

for 1 h and mixed with 2 ml (an excess) of silane terminated polymer 20 mg.ml-1

. From the

ZnO nanorods specific surface area (SSA, determined by BET), we calculated that the P3HT

was introduced at an excess of 2 chains/nm2 of ZnO surface. The reaction then proceeded at

C for 12 h under inert atmosphere. The medium was cooled to RT and ZnO@P3HT was

purified by centrifugation (10000 rpm, 10 min) with removal of the supernatant containing

excess of organic component. The purification was repeated several times until the UV-visible

spectra of the THF supernatant became featureless. The precipitated particles were collected,

dried and stored under nitrogen. A change in the color of the ZnO NRs was clearly observable

from white to violet after grafting of P3HT (dry state) (Figure 10).

Figure 10. a) Picture of dry state zinc oxide b) Picture of dry state ZnO@P3HT.

FT-IR characterization was firstly used to verify the grafting of P3HT onto ZnO NRs.

Figure 11 shows the IR spectra of P3HT P1, bare ZnO particles, and hybrid ZnO@P1.

ZnO@P1 spectrum shows the characteristic frequencies of both ZnO, i.e. a broad absorption

band between 3000 and 3500 cm-1

revealing the presence of the surface hydroxyl groups, and

P3HT with a strong absorption peaks at 2960, 2923 and 2852 cm-1

, attributable to the

asymmetrical C-H stretching mode of methyl and methylene protons of the hexyl side chain

group.

a b



83

Figure 11. Infra-red spectra of P3HT (P1), bare ZnO and grafted particles ZnO@P1.

First of all, TGA of the three different P3HTs were performed to compare next to grafted

particles and the results are reported in Figure 12. Degradation under nitrogen occurs through

a single step starting at C and ending at 3 C. Finally, hen the maximum temperature

of C is reached the residual mass of the three polymers is 30% of the initial mass. This

result showed that the molar mass of P3HT has a negligible effect on the thermal degradation

temperature in agreement with previous study.18

This value has to be kept in mind for the final

calculation of the organic composition of the core@shell particles.

Thermal gravimetric analyses (T A) ere then performed under nitrogen ith a heating rate

C/min in order to examine the degradation of ZnO@P3HT NRs (Figure 13). The thermal

degradation of the organic phase will allow quantifying the amount of P3HT covalently linked

to the NRs.



84

Figure 12. Thermogravimetric analysis of silane terminated P3HT P1, P2 and P3.

Secondly, the thermal stability of crude zinc oxide nanorods showed a weight loss of 2.4%

occurring through one degradation step bet een C and 260 C related to the presence of

adsorbed water (Figure 12)

Figure 13. Thermo gravimetric analysis of bare and grafted ZnO NRs under nitrogen at a heating.

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500 600 700

Wei

ght l

oss

%

Temperature°C

P1

P2

P3

94

95

96

97

98

99

100

0 100 200 300 400 500

We

igh

t lo

ss %

Temperature C

ZnO

ZnO@P1

ZnO@P2

ZnO@p3



85

Finally, the degradation of ZnO@P3HT occurred in two degradation steps, the first one being

similar to the bare zinc oxide nanorods and second step representing the polymer degradation

(Figure 13).

The calculated weight losses for P3HT in the hybrids ZnO@P1, ZnO@P2 and ZnO@P3 were

respectively 2.7%, 3.7% and 1.9% (Table 2), calculated via the following formula:

).

Interestingly, the highest value was found when P2 was used as macromolecular grafting

agent. With the NRs specific surface area Ss is 24 m2, the polymer molar mass and the weight

fraction of P3HT in the hybrids materials (fwP3HT) can be determined by TGA.

It is possible to calculate the surface grafting density () of the polymer monolayer via the

following where Na is Avogadro constant:

Calculation for ZnO@P1

ZnO@P1 and ZnO@P2 present almost the same grafting density with respectively 0.25 and

0.24 chains/nm2, placing them in a “semi-dilute” brush regime if their behavior is comparable

to coil polymers. ZnO@P3 has a lower grafting density of 0.09 chains.nm-2

, and the chains

should have more room to fold while covering the entire surface.

This grafting density variation could be attributed to the molar mass of the macromolecular

grafting agent. P1 and P2 have a lower degree of polymerization than P3, therefore the steric

hindrance induced by a grafted P3 chain is more important than for a P1 one.



86

In the grafting onto methodology, once a few initial chains have been grafted a steric

hindrance prevents the chains in solution from reaching the surface; they must first diffuse

through the existing polymer film. This “excluded volume” barrier becomes more pronounced

as the thickness of the tethered polymer layer increases.19

UV-visible spectroscopy qualitatively dosing the P3HT content of the hybrids materials.

Normalizing the spectra to the maximum absorption wavelength of ZnO at 371 nm (Figure

14) the absorbance at = 450 nm was qualitatively compared for the three hybrid materials

prepared in chloroform solution. Relative absorbance is reviewed in Table 2. Within the three

macromolecular grafting agents, P2 was also found to be the most efficient grafting agent,

followed by P1 and finally P3, meaning that molar mass has an important role within the

grafting onto methodology.20

Figure 14. UV-Visible absorption spectra of P1, P2 and P3 grafted to ZnO NRs in chloroform solution.

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

2

360 410 460 510 560 610 660

Abs

orba

nce

Wavelength (nm)

ZnO@P1

ZnO@P2

ZnO@P3



87

In order to compare the behavior of the P3HT chains on the zinc oxide nanoparticles as done

previously by Kiriry group on organosilica particles,21

UV-Visible absorption spectra of

polymers in solution, i.e. before grafting and in thin film were recorded. (Figure 15)

Figure 15. UV-vis absorption spectra of P3HT samples in chloroform solutions (left) and as thin films (right).

In chloroform solutions, all polymers behave likely with λmax~450 nm, which is a classical

absorption of P3HT. 22 In thin films, the absorption spectra showed a red shift of the max with

a shoulder band at high wavelength indicating a polymer chain packing with a coplanar

arrangement of the adjacent thiophene rings. The observed shoulder is due to electronic

transitions between different vibrational energy levels in the conjugated polymer backbone.

The bathochromic (red shift) was enhanced with molar mass due to an increase of the

conjugation length and an easier charge transfer in the backbone.

It is interesting to note that the photophysical properties of tethered P3HT chains on

zinc oxide nanorods behaved likely to the polymer in solution (Figures 14 and 15 left). This

means that the polymer brush was solvated by the chloroform solvent molecules due to the

low grafting density.

Finally, TEM was used to determine the thickness of the grafted P3HT layer onto the ZnO

NRs surface. Figure 16 shows a clear dense and homogeneous polymer shell around ZnO NRs

leading to core@shell hybrid material. The average polymer shell thicknesses (h) were

measured from the TEM images for the three hybrids materials (Table 2). ZnO@P1,

ZnO@P2 and ZnO@P3 have a polymer shell of 3 nm, 3 nm and 4 ± 1 nm thick, respectively.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

300 350 400 450 500 550 600 650 700

No

rmal

ize

d A

bso

rban

ce (

au)

Wavelength (nm)

Thin films

P1

P2

P3

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

300 350 400 450 500 550 600

No

rmal

ize

d A

bso

rban

ce (

au)

Wavelength (nm)

In solution

p1

p2

p3



88

These very close values are not only related to the polymer molar mass but also to the grafting

density. Because the grafting density of P3 is lower than the one of P1 and P2, the P3 grafted

chains could be more folded, reducing the effect of the molar mass on the thickness.

Figure 16. TEM images for a) bare ZnO nanorods (scale bar = 20 nm), b) ZnO@P1 (scale bar = 20 nm), c)

ZnO@P2 (scale bar = 10 nm), d) ZnO@P3 (scale bar = 20 nm).

Using a phosphonic acid end-functionalized P3HT to react with Zn-OH surface moieties of

nanowires, Fréchet et al. have observed lamellar chain packing oriented parallel to the

surface, when P3HT (7000 g.mol-1

by MALDI-TOF) is grafted on the ZnO surface, and

explained this by a chain folding.23

From the estimated unit cell parameter of the P3HT and the lamellar fold length (5-10 nm),24

the authors calculated a shell thickness of to nm. If e follo Fréchet’s calculation, the

a)

b)

c)

d)



89

thickness of the P3HT brushes would be between 4 and 9 nm which is in good agreement with

the TEM measurement.

Table 2. Hybrid material characteristics.

Hybrid

Material

Mna

g.mol-1

P3HT (wt %)b Absorbance

450 nmc

b

(chains/nm2)

h d

(nm)

ZnO@P1 2700 2.7 ++ 0.25 3 ± 1

ZnO@P2 3900 3.7 +++ 0.24 3 ± 1

ZnO@P3 5500 1.9 + 0.09 4 ± 1

a calculated from MALDI-TOF

b calculated from TGA,

c calculated from UV spectroscopy at = 450 nm,

d

determined from TEM images. + is a qualitative information of the P3HT absorbance onto ZnO.

2.5. Hybrid material properties

To study the influence of the polymer shell on the particle stability, the bare and

functionalized particles were dispersed in THF by ultrasonication during 30 minutes. A first

concentration of 4 mg/mL was prepared and the sedimentation was followed visually. After 1

h, the bare ZnO solution started to be transparent as the particles aggregated at the bottom of

the container. On the contrary, grafted particles stayed dispersed even after 24 h (Figure 17).

UV-visible spectroscopy was used to quantify this phenomenon. Transmission was recorded

at = 370 nm for particles dispersion in THF (C = 0.08 mg/mL). After 800 min, the

transmission of grafted particles solution was 5% when the one for the neat particles was 20

% (Transmission started at 0%, Figure 17). This variation shows the important role of the

P3HT monolayer as a stabilizer in the good solvent medium. A similar effect is expected in a

P3HT matrix which is a good solvent of the P3HT shell.



90

Figure 17. a) UV-visible kinetic transmission at λ= 37 nm of bare ZnO (dashed line) and ZnO@P2 (plain line)

in THF (C = 0.08 mg/mL). b) Picture taken after 3 h of a well dispersed bare ZnO (left, white) and ZnO@P2

(right, orange) in THF (C = 4 mg/mL).

The optical properties of ZnO@P3HT materials were more deeply investigated on ZnO@P2

using UV-visible absorption and photoluminescence, as this one presented the best absorption

feature. Figure 18a shows the absorption spectra of bare ZnO, pure P3HT P2, grafted

ZnO@P2 and mixed ZnO/P2 in chloroform solution. The absorption band of P2 was observed

at 450 nm in agreement with literature value for P3HT.22

The grafted polymer absorbed at

around the same wavelength but the presence of ZnO particles in solution induced diffusion

artifact on the spectra (Figure 18a) that made difficult to estimate the variation of the

wavelength maximum. The bare ZnO nanorods presented a maximum at 373 nm in pure

CHCl3 and showed no discernible change after mixing with P3HT. But this characteristic band

was clearly blue shifted by 3 nm in ZnO@P2 spectrum which may be attributed to the change

in dielectric environment, revealing the intimate contact between ZnO particles and P3HT and

to energy perturbation of the quantum confined excitation.25

Photoluminescence spectra (PL) of the polymer P2, ZnO/P2 blend, and the ZnO@P2 hybrid

material, under an excitation wavelength of 450 nm are presented in Figure 18b. The ZnO/P2

mixture was prepared with the same weight ratio as for ZnO@P2.

0

5

10

15

20

25

0 100 200 300 400 500 600 700 800

% T

ran

smis

sio

n

Time (min)

a)



91

Figure 18. a) UV-visible absorption and b) photoluminescence (ex = 450 nm) spectra of chloroform solutions of

P2, bare and grafted ZnO particles, and ZnO/P2 blend (weight ratio = 96/4).

The dominant peak of P2 at 580 nm is an emission characteristic of the P3HT backbone 22

that

arises from the relaxation of excited -electron to the ground state while the shoulder around

640 nm is related to interchain states.

The addition of ZnO nanoparticles to the polymer solution, in a concentration calculated with

respect to the mass composition of ZnO@P2 (For example, ZnO/P3HT = 96.3/3.7) did not

change the photoluminescence properties of P2. It was supposed, that under these conditions,

the concentration of ZnO was too low to quench significantly the emission signal. On the

contrary, the emission spectrum of ZnO@P3HT showed a strong decrease in the PL intensity,

resulting from an efficient charge transfer from the polymer to the ZnO particles.26

The

absolute fluorescence quantum yield of P2 and ZnO@P2 have been measured (with

rhodamine B as a reference for an excitation wavelength of 500 nm) to be 0.12 and 0.03,

300 400 500 600 700

Wavelenght (nm)

P2

ZnO+P2

ZnO@P2

ZnO

Abs

orba

nce

(a.u

.)

450 500 550 600 650 700 750 800

PL

Inte

nsity

(a.u

.)

Wavelenght (nm)

P3HT

ZnO + P3HT

ZnO@P3HT

Wavelength (nm)

Wavelength (nm)



92

respectively. The intimate contact by grafting helps the quenching that occurs between ZnO

and P3HT and this property is crucial for photovoltaic devices.

3. Perspectives

The goal of our project is to test such hybrid materials in photovoltaic devices to improve the

efficiency and stability. Thus we tried to fabricate several devices using the prepared hybrid

materials. The devices based on ITO/PEDOT:PSS/P3HT-P3HT@ZnO/Ca/Al showed a short

circuit for all studied devices. The active layer was a blend of P3HT (20 mg.ml-1

) and

ZnO@P2 with a volume ratio of 1:1 and 1:2. The failure of the device was supposed to be

correlated with the size of the nanoparticles. Thus we synthesized a new batch of ZnO@P2

nanoparticles with about 20 nm diameter nanoparticles (commercial from Aldrich) and a shell

thickness ~5 nm according to TEM images presented in Figure 19.

Figure 19. TEM images for a) bare ZnO nanorods (~5 nm), b) and c) ZnO@P3HT (Mn = 8000g.mol-1

) (scale

bar = 50 nm).

Before starting any device manufacturing, PL characterization was performed to study the

charge transfer from the polymer to the nanoparticles. The results are similar to the previously

a b

c



93

synthesized particles showing an efficient electron transfer. Therefore, the particles were

suitable for PV applications (Figure 20).

Figure 20. Photoluminescence (λ = nm) spectra of chloroform solutions of P3HT, grafted ZnO particles

and a mixture composed of ZnO and P3HT.

These hybrid nanoparticles have been sent to XLIM to Dr Bouclé who performs electronic

characterization and elaboration of solar cells.

In a similar manner, we grafted P3HT onto Niobium pentoxide Nb2O5 (200 nm) synthesized

by microwave assisted hydrothermal technique to be used as polymer sensitizer in a solid

state dye sensitized solar cell. The synthetic part of nanoparticles was done by Bruna A

Bregadiolli supervised by Prof. Carlos C. F. O. Graeff at LNMD (Laboratorio de Novos

Materiais e Dispositivos) Unesp- Bauru SP – Brazil).

The desired nanoparticles Nb2O5@P3HT with a shell thickness of about 6 nm according to

TEM images (Figure 21) were prepared in our team. The electrical properties of the grafted

particles reflect that these particles are promising in solar cells. The fabrication of solid state

dye sensitized solar will be done soon by our colleagues at LNMD.



94

Figure 21. TEM images of Nb2O5 @P3HT with bare scale of 200 nm (left image) and 20 nm (right image).

4. Conclusion

This work demonstrates the efficient grafting procedure of triethoxysilane terminated

poly(3-hexylthiophene) P3HT onto zinc oxide nanorods and spherical nanoparticles but also

Niobium pentoxide particles. Three alkoxy silane-terminated regioregular P3HTs with

different molar masses were synthesized via a hydrosilylation reaction from allyl-terminated

P3HT. MALDI-TOF and 1H

NMR were performed to characterize the polymer and show that

around 80 % of the chains were end-functionalized. The raw ZnO nanorods were then grafted

with P3HT in a one-step procedure and IR spectroscopy and TGA confirmed the efficiency of

the procedure. TEM images for the hybrid materials showed a continuous and homogeneous

polymer shell of 4 ± 1 nm, not only linked to the polymer molar mass but also to the grafting

density. Finally, UV-visible absorbance and photoluminescence demonstrated the electron

transfer from irradiated P3HT to the ZnO grafted particles. This result suggests that these

hybrid core@shell materials could be suitable for the elaboration of photovoltaic active layers

by mixing ZnO@P3HT hybrids with a P3HT matrix. Also interesting, this chain-end

functionalized P3HT and this simple technique of grafting are currently applied to different

metal oxide surfaces with various shapes in order to develop more stable hybrid photovoltaic

devices.



95

5. References

1. Mühlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.; Gaudiana, R.; Brabec, C., High photovoltaic performance of a low-bandgap polymer. Advanced Materials 2006, 18 (21), 2884-2889. 2. Reyes-Reyes, M.; Kim, K.; Dewald, J.; López-Sandoval, R.; Avadhanula, A.; Curran, S.; Carroll, D. L., Meso-structure formation for enhanced organic photovoltaic cells. Organic Letters 2005, 7 (26), 5749-5752. 3. (a) Campoy-Quiles, M.; Ferenczi, T.; Agostinelli, T.; Etchegoin, P. G.; Kim, Y.; Anthopoulos, T. D.; Stavrinou, P. N.; Bradley, D. D. C.; Nelson, J., Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends. Nat Mater 2008, 7 (2), 158-164; (b) Watts, B.; Belcher, W. J.; Thomsen, L.; Ade, H.; Dastoor, P. C., A Quantitative Study of PCBM Diffusion during Annealing of P3HT:PCBM Blend Films. Macromolecules 2009, 42 (21), 8392-8397. 4. Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P., Hybrid nanorod-polymer solar cells. Science 2002, 295 (5564), 2425-2427. 5. Huang, J. S.; Hsiao, C. Y.; Syu, S. J.; Chao, J. J.; Lin, C. F., Well-aligned single-crystalline silicon nanowire hybrid solar cells on glass. Solar Energy Materials and Solar Cells 2009, 93 (5), 621-624. 6. (a) Bouclé, J.; Ackermann, J., Solid-state dye-sensitized and bulk heterojunction solar cells using TiO 2 and ZnO nanostructures: Recent progress and new concepts at the borderline. Polymer International 2012, 61 (3), 355-373; (b) Bouclé, J.; Ravirajan, P.; Nelson, J., Hybrid polymer-metal oxide thin films for photovoltaic applications. Journal of Materials Chemistry 2007, 17 (30), 3141-3153. 7. Beek, W. J. E.; Wienk, M. M.; Janssen, R. A. J., Efficient hybrid solar cells from zinc oxide nanoparticles and a conjugated polymer. Advanced Materials 2004, 16 (12), 1009-1013. 8. Beek, W. J. E.; Wienk, M. M.; Janssen, R. A. J., Hybrid solar cells from regioregular polythiophene and ZnO nanoparticles. Advanced Functional Materials 2006, 16 (8), 1112-1116. 9. (a) Ghannam, L.; Parvole, J.; Laruelle, G.; Francois, J.; Billon, L., Surface-initiated nitroxide-mediated polymerization: A tool for hybrid inorganic/organic nanocomposites 'in situ' synthesis. Polymer International 2006, 55 (10), 1199-1207; (b) Deleuze, C.; Derail, C.; Delville, M. H.; Billon, L., Hierarchically structured hybrid honeycomb films via micro to nanosized building blocks. Soft Matter 2012, 8 (33), 8559-8562. 10. Loewe, R. S.; Khersonsky, S. M.; McCullough, R. D., A simple method to prepare head-to-tail coupled, regioregular poly(3-alkylthiophenes) using grignard metathesis. Advanced Materials 1999, 11 (3), 250-253. 11. Jeffries-El, M.; Sauvé, G.; McCullough, R. D., Facile Synthesis of End-Functionalized Regioregular Poly(3-alkylthiophene)s via Modified Grignard Metathesis Reaction. Macromolecules 2005, 38 (25), 10346-10352. 12. Palaniappan, K.; Murphy, J. W.; Khanam, N.; Horvath, J.; Alshareef, H.; Quevedo-Lopez, M.; Biewer, M. C.; Park, S. Y.; Kim, M. J.; Gnade, B. E.; Stefan, M. C., Poly(3-hexylthiophene)−CdSe Quantum Dot Bulk Heterojunction Solar Cells: Influence of the Functional End-Group of the Polymer. Macromolecules 2009, 42 (12), 3845-3848. 13. (a) Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Spiering, A. J. H.; Van Dongen, J. L. J.; Vonk, E. C.; Claessens, H. A., End-group modification of regioregular poly(3-alkylthiophene)s. Chemical Communications 2000, (1), 81-82; (b) Liu, J.; Loewe, R. S.; McCullough, R. D., Employing MALDI-MS on poly(alkylthiophenes): Analysis of molecular weights, molecular weight distributions, end-group structures, and end-group modifications. Macromolecules 1999, 32 (18), 5777-5785. 14. Miyakoshi, R.; Yokoyama, A.; Yokozawa, T., Synthesis of Poly(3-hexylthiophene) with a Narrower Polydispersity. Macromolecular Rapid Communications 2004, 25 (19), 1663-1666. 15. Marciniec, B., Comprehensive Handbook on Hydrosilylation 1992, Pergamon Press Oxford.



96

16. Holzinger, D.; Kickelbick, G., Hybrid inorganic-organic core-shell metal oxide nanoparticles from metal salts. Journal of Materials Chemistry 2004, 14 (13), 2017-2023. 17. Brunauer, S.; Emmett, P. H.; Teller, E., Adsorption of Gases in Multimolecular Layers. Journal of the American Chemical Society 1938, 60 (2), 309-319. 18. Rodrigues, A.; Castro, M. C. R.; Farinha, A. S. F.; Oliveira, M.; Tomé, J. P. C.; Machado, A. V.; Raposo, M. M. M.; Hilliou, L.; Bernardo, G., Thermal stability of P3HT and P3HT:PCBM blends in the molten state. Polymer Testing 2013, 32 (7), 1192-1201. 19. Jones, R. A. L.; Lehnert, R. J.; Schönherr, H.; Vancso, J., Factors affecting the preparation of permanently end-grafted polystyrene layers. Polymer 1999, 40 (2), 525-530. 20. (a) Advincula, R. C.; Brittain, W. J.; Caster, K. C.; Rühe, J., Polymer Brushes. WILEY-VCH: 2004; (b) Ostaci, R. V.; Damiron, D.; Al Akhrass, S.; Grohens, Y.; Drockenmuller, E., Poly(ethylene glycol) brushes grafted to silicon substrates by click chemistry: Influence of PEG chain length, concentration in the grafting solution and reaction time. Polymer Chemistry 2011, 2 (2), 348-354. 21. Senkovskyy, V.; Tkachov, R.; Beryozkina, T.; Komber, H.; Oertel, U.; Horecha, M.; Bocharova, V.; Stamm, M.; Gevorgyan, S. A.; Krebs, F. C.; Kiriy, A., “Hairy” Poly(3-hexylthiophene) Particles Prepared via Surface-Initiated Kumada Catalyst-Transfer Polycondensation. Journal of the American Chemical Society 2009, 131 (45), 16445-16453. 22. (a) Xu, B.; Holdcroft, S., Molecular control of luminescence from poly(3-hexylthiophenes). Macromolecules 1993, 26 (17), 4457-4460; (b) Cruz, R. A.; Catunda, T.; Facchinatto, W. M.; Balogh, D. T.; Faria, R. M., Absolute photoluminescence quantum efficiency of P3HT/CHCl3 solution by Thermal Lens Spectrometry. Synthetic Metals 2013, 163 (1), 38-41. 23. Briseno, A. L.; Holcombe, T. W.; Boukai, A. I.; Garnett, E. C.; Shelton, S. W.; Fréchet, J. J. M.; Yang, P., Oligo- and polythiophene/ZnO hybrid nanowire solar cells. Nano Letters 2010, 10 (1), 334-340. 24. (a) Brinkmann, M.; Wittmann, J. C., Orientation of regioregular poly(3-hexylthiophene) by directional solidification: A simple method to reveal the semicrystalline structure of a conjugated polymer. Advanced Materials 2006, 18 (7), 860-863; (b) Mena-Osteritz, E.; Meyer, A.; Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W.; Bäuerle, P., Two-dimensional crystals of poly(3-alkylthiophene)s: Direct visualization of polymer folds in submolecular resolution. Angewandte Chemie - International Edition 2000, 39 (15), 2680-2684. 25. Xu, J.; Wang, J.; Mitchell, M.; Mukherjee, P.; Jeffries-El, M.; Petrich, J. W.; Lin, Z., Organic-inorganic nanocomposites via directly grafting conjugated polymers onto quantum dots. Journal of the American Chemical Society 2007, 129 (42), 12828-12833. 26. Malgas, G. F.; Motaung, D. E.; Mhlongo, G. H.; Nkosi, S. S.; Mwakikunga, B. W.; Govendor, M.; Arendse, C. J.; Muller, T. F. G., The influence of ZnO nanostructures on the structure, optical and photovoltaic properties of organic materials. Thin Solid Films 2013.

Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards

the First Low Band-Gap Polymer Brushes.

Chapter 3




1. Introduction ..................................................................................................................... 99

2. Low bandgap polymers ................................................................................................ 100

3. Stille cross coupling polymerization ............................................................................ 103

4. Step growth polymerization ......................................................................................... 104

5. Results and discussions ................................................................................................. 106

5.1. Synthesis of monomers ............................................................................................ 106

5.1.1 Synthesis of 4,4‘-Bis (2-ethyl-hexyl)-5,5'-bis(trimethyltin)-dithieno[3,2-b:2',3'-

d]silole (M1) ................................................................................................................... 106

5.1.2 Synthesis of 4,7-dibromo-2,1,3-benzothiadiazole monomer (M2) ...................... 107

5.2 Synthesis of poly[(4,4′-bis(2-ethylhexyl)dithieno-[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-

(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT). ................................................................... 108

5.3 Optical properties of PSBTBT in solution and thin films ....................................... 110

5.4 Polycondensation reaction from the zinc oxide Nanorods: grafting low bandgap

(PSBTBT) ........................................................................................................................... 111

5.5. Tentative of brush formation mechanism through Stille cross coupling reaction ... 124

6. Conclusion ..................................................................................................................... 128

7. References ...................................................................................................................... 129



99

1. Introduction

During the last few years, a tremendous progress in the photovoltaic performance of Polymer

Solar Cells PSCs was achieved via design of novel conjugated polymers (CPs). The band gap

of the polymers was continuously reduced to improve matching of the polymers absorption

with the solar spectrum in comparison with classical conjugated polymers, i.e. P3HT. The

highest power conversion efficiency of a low band gap polymer (LBG) (based on

difluorobenzothiadiazole (DFBT) unit) reported in the literature to our knowledge is 10.6 %

for tandem solar cells.1 As explained previously in chapter 1, few research groups turned their

attention toward the possibility of linking conjugated polymers to inorganic, metal or carbon-

based materials, for hybrid solar cells (HSCs) by covalently binding the two components.2

HSCs based on intimate contact between inorganic nanoparticles and conjugated polymers

have the advantages of enhancing the interfacial exciton dissociation efficiencies and being

morphologically more stable. However, the record of the power conversion efficiencies is still

limited, and an improved maximum power conversion efficiency of 4.1% was achieved.3

Believing in this interfacial-engineering approach LBG polymers could replace classical

conjugated polymers in order to achieve higher photovoltaic performance. Recently,

dithieno[3,2-b:2,3-d]silole and 2,1,3-benzothiadiazole (BT) derivatives based copolymers

have attracted attention as novel systems with high photochemical stability and low

positioned HOMO level, respectively. Among all, poly[(4,4′-bis(2-ethylhexyl)dithieno-[3,2-

b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT, P2 Figure 1), were

prepared via Stille cross-coupling polymerization depending on our interest as it shows high

hole mobility,4 good photochemical stability and good power conversion efficiency PCE of

5.1 %.5

No previous study was reported on grafting “LBG” copolymer based on donor and acceptor

units to substrates or nanoparticles. Following our previous work based on the grafting of

P3HT onto zinc oxide nanoparticles,6 we report in this chapter the first elaboration of LBG

polymer brushes via the surface initiation of an AA/BB type step growth polymerization from

zinc oxide nanoparticles.

A “grafting through” methodology was applied via surface polymerization by functionalizing

ZnO nanorods with initiating sites at the surface to prepare Core@Shell ZnO nanorods. A



100

study of the coverage of the nanorods by an organic shell was performed under different

experimental conditions (increasing the molar mass of the free polymer).

The photophysical properties of tethered polymer chains were compared to their homologues

in bulk and solution. ZnO nanorods have been chosen for their electron acceptor ability

making them good candidates for hybrid solar cell.7 The hybrid materials obtained during this

study will thus be composed of the organic donor and the inorganic acceptor covalently

bonded and of high interest for photovoltaic applications. The efficiency of the grafting

procedure has been studied by UV-Visible Absorption Spectroscopy (Uv-vis), Thermal

Gravimetric Analysis (TGA), X-ray Photoelectron Spectroscopy (XPS), and Transmission

Electron Microscopy (TEM).

2. Low bandgap polymers

This new class of materials known as “LBG” copolymers incorporates an electron

rich unit (Donor-D) and electron deficient unit (Acceptor-A) in an alternating fashion in the

polymer main chain. The D-A system exhibits partial intramolecular charge transfer (ICT)

that enables manipulation of the electronic structure (HOMO/LUMO) leading to a narrow

band gap polymer with high charge carrier mobilities.8 Common donor moieties include

thiophene,9 carbazole,

10 fluorene,

11 dibenzosilole,

12 dithieno[3,2-b:2,3-d]silole,

13 benzo[1,2-

b;3,4-b]dithiophene14

and cyclopentadithiophene groups,15

while acceptors moieties are

usually 2,1,3-benzodiathiazole (BT),16

diketopyrrolopyrrole (DPP),17

thienothiophene (TT)18

and thienopyrrolodione (TPD).19

Various copolymers were synthesized based on the combination of these donor and acceptor

units. Among the donor units, dithieno[3,2-b:2,3-d]silole (DTS)-containing polymers have

attracted attention as novel systems, with high photochemical stability,

20 in which the Si-C σ-

orbital effectively mixes with the π-orbital of the butadiene fragment to afford a low-lying

LUMO and a relatively low band gap. In addition, silicon introduction stabilizes the diene

HOMO level compared to the carbon counterparts, which should enhance the ambient

stability of silole polymers.

For example the poly[(4,4-bis(2-ethylhexyl)-cyclopenta-[2,1-b;3,4-b′]dithiophene)-2,6-diyl-

alt-2,1,3-benzothiadiazole-4,7 diyl] (PCPDTBT) (P1, Figure 1) and its derivatives are a



101

family of LBG polymers leading to conversion efficiencies around 3-4%.21

When this

polymer is modified by changing the carbon atom in the position seven of cyclopenta-[2,1-

b;3,4-b′]dithiophene by a silicon atom to give the poly[(4,4′-bis(2-ethylhexyl)dithieno-[3,2-

b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3 benzothiadiazole)-4,7-diyl] (PSBTBT) (P2), the conversion

yield was increased to 5.1% with higher crystallinity,5 improved charge transport properties

and reduced bimolecular recombination when blended with fullerene derivatives. However,

the strong stacking of Si-bridged material lead to limited solubility of the polymer in common

organic solvent at room temperature, and with high molecular weight material (Mn > 25000

g.mol-1

).22

Steve et al. reported the synthesis of a fluorinated-PCPDTBT (P3) as introducing

fluorine atoms into the heterocyclic structure resulting in higher open-circuit volatge Voc 23

taking into account the better solubility of P1 compared to P2. This fluorination causes the

Voc to exceed 0.7 V and the PCE to reach 6.16% for P3.

Figure 1. Examples of low band gap polymers based on benzodiathiazole (BT), thienopyrrolodione (TPD),

diketopyrrolopyrrole (DPP) and thienothiophene (TT) as an acceptor derivatives for P1-P2-P3, P4-P5, P6 and

P7, respectively.



102

Although, replacing 2,1,3-benzothiadiazole (BT), which possess strong electron accepting

features because of the two electron withdrawing imines (C=N) and the bridged nitrogen

atom, by thienopyrrolodione (TPD) having simple, symmetric and coplanar structure seemed

advantageous. In addition, the side chain on TPD can provide a promoted solubility. Ding et

al. synthesize P4 (CPDTTPD) that shows a broad absorption and narrow band gap. The power

conversion efficiency of this polymer reaches 6.41% with Voc = 0.75 V and very high Jsc =

14.1 mA.cm-2

.24

This efficiency was enhanced by Chu et al. that replaces

cyclopentadithiophene (CPDT) by dithienosilole unit to reach PCE = 7.3% for P5.13

This

enhancement is due to low lying LOMO level and thus results in higher Voc = 0.88 V.

Also diketopyrrolopyrrole (DPP) is a strong electron withdrawing unit that provides small

band gap and excellent charge transport properties. Li et al. copolymerized DPP with electron

donating thiophene to get polymer P6, in which a long alkyl chain was introduced to improve

the solubility and thus increase the molecular weight. The photovoltaic performance of P6

shows Jsc = 14.8 mA.cm2, FF = 0.7, and PCE = 6.9%.

25 Last but not least, the best result

reported is for polymer P7 based on thienothiophene derivatives as an acceptor unit with PCE

= 9.35 % used as an active layer for inverted PSC with PC71BM.26

The photovoltaic

characteristics of these polymers are listed in Table 1.

Table 1. Photovoltaic characteristics of some LBG polymers

Polymer Voc

(V)

Jsc

(mA.cm-2

)

FF PCE

(%)

Eg Ref

P1 0.70 11 0.47 3.2 1.4 21

P2 0.68 12.70 0.55 5.1 1.45 5

P3 0.74 14.08 0.58 6.04 1.44 23

P4 0.75 14.10 0.61 6.41 1.6 24

P5 0.88 12.20 0.68 7.3 1.73 13

P6 0.66 14.80 0.70 6.9 1.35 25

P7 0.80 15.73 0.74 9.35 1.58 26



103

3. Stille cross coupling polymerization

Stille cross coupling reaction of organic electrophiles with organostannane compounds

has grown into an extremely powerful and useful method for carbon-carbon bond formation

using a Pd0 catalyst.

27 Generally, the combination of palladium catalyst with various

phosphine ligands results in excellent yields and high efficiency. A general mechanism

proposed by Stille in 1986 28

for organostannanes (Figure 2) starts with the oxidative addition

of the low valent metal Pd0 into an organic halide (R

1-X) to form a trans Pd(II) complex (1).

In transmetalation mechanism, an organostannane (nucleophile) initially adds to the trans

metal complex where X group can coordinate to the tin via an associative substitution,

resulting in the loss of R3Sn-X and giving a palladium complex with R1 and R2 (2). Such a

complex is assumed to directly afford the cis complex that gives the final product and

regenerates the palladium catalyst after reductive elimination.

Figure 2. Mechanism of Stille cross coupling reaction.



104

This robust method is not limited to the synthesis of organic molecules and can be used to prepare

copolymers of donor and acceptor moieties if difunctional monomers R1 and R2 are used (Scheme

1).

Scheme 1. Stille cross-coupling polymerization

Achieving high molar mass polymers via Stille cross coupling reaction requires high

monomer purity and a stoichiometric equivalence of functional groups in (AA-BB approach).

4. Step growth polymerization

In step growth polymerization, the molar mass of the polymer chains build up slowly and

there is only one reaction mechanism for the formation of polymer: the difunctional

monomers first forms dimmers, then trimers, tetramers and so on to polymers (Scheme 2).

Scheme 2. Step growth polymerization of AA-BB approach

Various polymeric compositions are synthesized using a step growth polymerization process.

However, many experimental criteria must be addressed in order to achieve a linear high

molar mass polymer as:

1. high reaction conversion (> 99 %) as predicted by Carother’s equation (eq.1)

2. monomer functionality (f) equal to 2

3. functional group stoichiometry equal to 1



105

Carother’s equation 29

relates the average number degree of polymerization Xn to the

conversion p and average functionality f, for a stoechiometric ratio r = 1.

Thus, the molar mass of the polymer will be reduced if the conversion or the functionality

decreased. At 95 % conversion for difuntional monomers, Xn is then only 20. Figure 3

summarizes the impact of functional group conversion on the degree of polymerization Xn.

Figure 3. Degree of polymerization versus conversion of functional groups in step growth polymerization.30

conversion



106

5. Results and discussions

5.1. Synthesis of monomers

5.1.1 Synthesis of 4,4‘-Bis (2-ethyl-hexyl)-5,5'-bis(trimethyltin)-dithieno[3,2-b:2',3'-

d]silole (M1)

Scheme 3. Synthesis of the monomer M1.

The synthetic procedure on the monomer M1 (Scheme 3) was optimized in our group.

Starting from commercial 4,4’-Bis(2-ethyl-hexyl)-5,5’-dibromo-dithieno[3,2-b:2',3'-d]silole

(~100 % purity) a selective lithiation via lithium-bromine exchange at position 5 and 5' was

achieved by utilizing an excess of butyllithium (3.5 eq) at -80 C. Then the organolithium

intermediate reacted with excess of trimethyltin chloride (7 eq) to give the desired product.

The di-trimethyltin products are usually unstable in all types of column purification and very

sensitive to light, temperature and humidity that causes its degradation.31

Therefore, the crude

product was utilized without any further purification directly after removing the excess of

trimethyltin chloride under reduced vacuum for 24 h. The final product is pale yellow oil with

a 97% yield. 1H NMR spectra shows all the characteristic peaks of the final product (Figure

4). Integrating the signals of peak a at 7.17 ppm (difunctional stannane monomer) to peak b

(monofunctional stannane monomer) can determine the purity of this monomer, which in this

case is around 97%.

BuLi (3.5 eq)/ -80 C

(7eq)



107

Figure 4. 1H NMR spectrum (400MHz, CDCl3) of 4,4‘-Bis (2-ethyl-hexyl)-5,5'-bis(trimethyltin)-dithieno[3,2-

b:2',3'-d]silole.

5.1.2 Synthesis of 4,7-dibromo-2,1,3-benzothiadiazole monomer (M2)

Scheme 4. Synthesis of monomer M2.

The synthesis of the acceptor unit (M2) has been done following procedure reported in the

literature (Scheme 3). 32

The bromination of 2,1,3-benzothiadiazole was carried out by slow

addition of excess of bromine (Br2, 3 eq) in the presence of hydrobromic acid (HBr).

An electrophilic aromatic substitution replaced the two protons at the position 4 and 7 with

bromine atoms. Recrystallization with ethyl acetate yielded the desired product as white

crystal with a yield of 95% (purity ~100%). 1H NMR spectrum shows a singlet at 7.67 ppm

since the two protons at positions 5 and 6 have the same chemical environment (Figure 5).

a

a

b

Br2

HBr



108

Figure 5. 1H NMR spectrum (400MHz, CDCl3) of 4,7-dibromo-2,1,3-benzothiadiazole.

5.2 Synthesis of poly[(4,4′-bis(2-ethylhexyl)dithieno-[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-

(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT).

The alternating copolymer poly[(4,4′-bis(2-ethylhexyl)dithieno-[3,2-b:2′,3′-d]silole)-2,6-diyl-

alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT) was prepared via Stille cross-coupling

polymerization based on 4,7-dibromo-2,1,3-benzothiadiazole (M1) and 4,4‘-Bis (2-ethyl-

hexyl)-5,5'-bis(trimethyltin)-dithieno[3,2-b:2',3'-d]silole (M2) as presented in Scheme 5. 33

Scheme 5. Synthetic procedure of PSBTBT.

a a

a H₂O

CDCl₃



109

As previously mentioned, both monomers were obtained with a very high purity and were

copolymerized in a chlorobenzene solution using Stille cross-coupling polymerization in the

presence of (Tris(dibenzylideneacetone)dipalladium(0), Pd2(dba)3) with co-ligand tri(o-

tolyl)phosphine (o-tol)3P as catalyst system. A classical polymeri ation was done at 4 C for

24 and 48 hours. After cooling down, the solid was filtered through a Soxhlet thimble and

then subjected to Soxhlet extraction with methanol, acetone, cyclohexane, chloroform. The

cyclohexane and chloroform fractions were concentrated and precipitated into methanol, and

the precipitant was filtered and dried under high vacuum to afford PSBTBT as a dark-blue

solid. For a 24 hrs reaction, we separated a chloroform and cyclohexane fractions of PSBTBT.

Table 2. Macromolecular characteristics of synthesized PSBTBT.

Polymer

C)

Time

(h)

Mna

(g.mol-1

)

CHCl3

Đ

Mna

(g.mol-1

)

Cyclohex

Đ

Yield

(%)

PSBTBT-1 140 24 19 000 2.47 9 000 2 81

PSBTBT-2 140 48 25 300 2.88 10 000 2.3 93

a calculated from SEC (polystyrene conventional calibration).

Using these experimental conditions, we are able to achieve high molar masses polymer with

high yields, thus we plan to start grafting conjugated polymer PSBTBT onto zinc oxide

nanoparticles by applying the same experimental strategy.



110

5.3 Optical properties of PSBTBT in solution and thin films

Uv-visible absorption spectra of the chloroform fractions of PSBTBT synthesized are shown

in Figure 6 and summarized in Table 3.

Figure 6. UV-visible absorption spectra of PSBTBT in solution and thin film.

The samples were prepared in chloroform solutions. For polymer PSBTBT-1 with the lowest

molar mass we observe a maximum absorption wavelength of 667 nm. As the molar mass

increases for PSBTBT-2, we observe a red shift of 5 nm in solution. Moreover a shoulder was

observed at 730 nm for all samples attributed to the strong π-π interaction of PSBTBT

molecules.4-5

When the measurement was performed on solid films a red shift of 10 nm and

higher levels of π-π stacking were observed. The slight shifts in solid state suggest that the

backbone of PSBTBT is rather planar even in solution in agreement with previous studies.4-5

The optical band gap was ~1.52 eV. (Optical band gap estimated from the low energy band

edge in the optical spectrum, Eg = 1240/λonset.)

Table 3. UV-visible absorption characteristics of PSBTBT polymer.

Sample name λ ax n )

CHCl3

Solution Thin film Mn (g/mol)

PSBTBT-1 667 676 19 000

PSBTBT-2 672 680 25 300

0

0,2

0,4

0,6

0,8

1

350 450 550 650 750 850

No

rmal

ize

d a

bso

rban

ce

Wavenlength (nm)

PSBTBT in film

PSBTBT-1 PSBTBT-2

0

0,2

0,4

0,6

0,8

1

350 450 550 650 750 850

No

rmal

ize

d A

bso

rban

ce

Wavelength ( nm)

PSBTBT in solution

PSBTBT-1 PSBTBT-2



111

5.4 Polycondensation reaction from the zinc oxide Nanorods: grafting low bandgap

(PSBTBT)

The functionalization of ZnO nanoparticles with PSBTBT was realized in three steps (Scheme

6). First [2-(4-bromo-phenyl)-ethyl]-triethoxysilane was anchored to zinc oxide nanorods

(length = 150 nm, ø = 30 nm and specific surface area = 24 m2.g

-1). Then palladium catalyst

was linked to the surface as initiating site to start the copolymerization. In the last step, the

grafting through polymerization of PSBTBT from the particle surface was achieved..

Scheme 6. Synthetic procedure of PSBTBT@ZnO.

a) Grafting of the phenyl bromine moiety

For this purpose, a [2-(4-bromo-phenyl)-ethyl]-triethoxy-silane (Si-PhBr) was synthesized

(procedure and characterization in experimental part) via a standard hydrosilylation of 4-

bromostyrene and triethoxysilane in the presence of chloroplatinic acid (catalyst). Vacuum

distillation offered the desired product (yield = 80%, purity ~ 100%). The product was used to

modify ZnO nanorods surface in order to introduce a phenyl bromine moiety that is necessary

for the immobilization of the Pd catalyst. For this purpose Si-PhBr was dissolved in

anhydrous toluene and the mi ture was reflu ed for 24 h at 2 C. After several repeated



112

centrifugations in EtOH and THF to remove the ungrafted organosilane, the functionalization

of the ZnO nanoparticles has been followed by X-ray Photoelectron Spectroscopy (XPS).

The corresponding binding energy (B.E) and atomic percentage are reported in Table 4. The

Zn 2p3/2 component is located at 1021.6 eV which is representative of the divalent zinc in

ZnO. The O 1s peak of the surface OH species and O-2

ions in the defective sublattice is

located at 532.3 eV. The anchorage of the [2-(4-bromo-phenyl)-ethyl]-triethoxy-silane to the

ZnO nanorods is highlighted by the XPS signature of silicium and bromine covalently linked

to the carbon ring at 102.5 eV (characteristic of Si-O3) and 70.4 eV, respectively.

Furthermore, the increase in the carbon content C 1s due to the appearance of two signals at

283.4 and 291.4 eV characteristics of carbon–silicon bond and sp2 of carbon-carbon bond was

detected (Figure 7 and Table 4).

Figure 7. XPS spectra of a) bare ZnO and b) ZnO-PhBr.



113

Thermal gravimetric analysis TGA was done under air atmosphere with a heating rate of 10

C/min. Zinc oxide nanorods exhibited a weight loss of 2.5% which could be related to

adsorbed molecules on the surface of particles (Figure 8). The modified particles showed a

weight loss of 4 %. A degradation of 1.5% started at 3 C related to the degradation of

organosilane residues (Si-PhBr).

Figure 8. Thermal gravimetric analysis of ZnO and ZnO-Ph-Br under air atmosphere ( C/min).

To calculate the grafting density, we apply the following equation.

Where MPhBr is the molecular weight of the organic part of the initiator CH2-Ph-Br, Mn = 185

g.mol-1

. SSA is the specific surface area measured by BET, SSA = 24 m2/g.

fwPhBr is the mass fraction of the organic part in the hybrid materials ZnO@PhBr measured

with TGA.

b) Catalyst grafting

In a second step, the palladium catalyst (Tris(dibenzylideneacetone)dipalladium(0),

Pd2(dba)3) was anchored to the substrate by creation of a phenyl-Pd(dba)2-Br complex.

Between each step, treated particles were purified from unreacted Si-PhBr or Pd2(dba)3 and

byproducts by repeated redispersion/centrifugation cycle. The solvent used to link the catalyst

95,5

96

96,5

97

97,5

98

98,5

99

99,5

100

0 100 200 300 400 500 600

We

igh

t lo

ss %

Temperature °C

ZnO-PhBr

Zinc oxide



114

to the surface should be anhydrous and the particle color changed from white to grey.

Importantly, the particles were dried and stored in the glovebox since the palladium catalyst is

sensitive to H2O. In order to prove the incorporation of palladium, XPS analysis was

performed. The appearance of palladium signal without modification of the other core peaks

has been noted. The Pd 3d core peak is split in two components due to spin orbit coupling [Pd

3d5/2 (binding energy = 338.3 eV) and Pd 3d3/2 (binding energy = 335.4 eV)] mostly assigned

to Pd2+ 34

on the surface of nanoparticle with a small amount of Pd0 35

(Figure 9 and Table 4).

Figure 9. Pd 3d XPS spectrum of phenyl-Pd(dba)2-Br.

Table 4. Ionization energy and surface chemical composition percentage determined by XPS.

c) Grafting from polymerization

Then the prepared particles were sonicated for 1 h before adding the monomers (M1 and M2)

and co-ligand (P(o-tol)3) . The mixture was heated at 150 C under nitrogen. After 20 min, the

beginning of the reaction (oligomers formation) was clearly observed due to change in

solution color from grey to dark brown as shown in Figure 10. The Stille Cross-Coupling

polymerization carried on for 2h and polymer formation was identified with a dark blue

solution. The particles were purified by several centrifugations in chloroform solution, and

some free polymer chains were detected in solution. The molar mass of the free polymer

chains was 3 600 g.mol-1

with Ð = 1.2 obtained according to GPC.

Atom C (1s) O (1s) Zn (2p) Br (3d) Si (2p) Pd (3d)

IE (eV) 285 532.3 1021.6 70.4 102.8 335.4

ZnO 19.4 45.6 35.0 - - -

Substrate ZnO@PhBr 23.9 43.4 26.9 1.8 3.7 -

ZnO@PhPdBr 21.4 41.7 26 1.4 8.2 0.9

Binding energy (eV)



115

Figure 10. a) Brown solution at the beginning of the reaction b) dark-blue color solution indicates the synthesis

of polymer c) clear supernatant solution after several centrifugations and precipitation of the grafted particles.

Under the same experimental conditions, we repeated the same experiment twice by

increasing the reaction time to 4 h and 6 h. The increase in the molar mass was clearly seen

with a color change of the mixture to dark green and an increase of the viscosity for both

samples. This result was proved by GPC and UV-visible absorption (Figure 12 and 14). In

these two cases, cleaning the particles became more difficult as we obtained high molar

masses hardly soluble in THF or chlorobenzene (Figure 11). Thus, several extra dispersion-

centrifugation cycles (more than 30 in the case of polymer PSBTBT-6h) were done in

chlorobenzene solutions in both samples. For 6h of experiment, we obtained an insoluble

polymer even in chlorobenezne as shown in Figure 11b.

Figure 11. Polymer PSBTBT is insoluble in a) THF for 4h and 6h reactions b) chlorobenzene after 6h

polymerization.

i) Analysis of free polymer chains

The molar mass of the free polymer chains pertaining to polymerization 4 and 6 h were

obtained by GPC in THF (with a conventional calibration of polystyrene standard) and were

approximately the same Mn = 10 500 g.mol-1

, Ð = 1.25. This was a surprising result from the

analysis of the color and solubility differences between these two samples. A closer look to

a b



116

the experimental condition and the GPC trace gave further information (Figure 12). First the

two polymers, as previously mentioned were not totally soluble in THF, and after the usual

pre-injection filtration, a non negligible polymer fraction was eliminated. It is logical to

believe that the smallest chains of the samples were dissolved in THF and that filtration

removed the highest molar masses. This assumption is also visualized in Figure 13 where we

observe a difference in color between the samples used for GPC (blue solution) and high Mn

fraction (green solution) obtained from chlorobenzene centrifugations (not soluble in THF).

Furthermore, the low dispersity obtained (1.25) is not characteristic of a step growth

polymerization, usually higher than 2.5 (without Soxhlet extraction).

Figure 12. GPC chromatograms of PSBTBT-2h, 4h and 6h (UV-detector λ = 660 nm).

Thus, the molar mass cannot be estimated from GPC with THF as the eluent. However,

literature showed that this polymer start to be insoluble in chlorobenzene for Mn > 25 000

g.mol-1

. 22

Therefore, we think that PSBTBT-4h and PSBTBT-6h has a Mn slightly lower and

higher than 25000 g.mol-1

, respectively.

Figure 13. PSBTBT polymer with blue and green color for 4h and 6h reactions, respectively (chlorobenzene).

A qualitative estimation of the molar masses can be obtained from UV-visible absorption

spectra (Figure 14). It is clearly shown that the maximum wavelength (λmax) was red shifted

0

20 25 30 35

No

rmal

ize

d U

v d

ete

cto

r si

gnal

Retention voulme (ml)PSBTBT-2h PSBTBT-4h PSBTBT-6h



117

from 600 nm to 675 nm and then to 698 nm for 2 h, 4 h and 6 h reactions, respectively. The

shift is related to an increase in π-π stacking due to the increase in the molar mass of the

polymer. Thus increasing the reaction time is accompanied with an increase in the molar mass

of PSBTBT.

Figure 14. UV-visible absorption spectra of free PSBTBT obtained after 2h, 4h and 6h reactions.

ii) Analysis of the grafted particles

It is interesting to observe that the photophysical properties of the LGB polymer brushes

significantly differed from the same free polymer in solution. Figure 15 presents the UV-

visible absorbance spectra in CHCl3 of the grafted ZnO@PSBTBT-2h nanorods, a mixture of

bare ZnO nanoparticles/free PSBTBT-2h and finally the free polymer alone.

Figure 15. UV-visible spectra of the ZnO@PSBTBT-2h (plain line), ZnO + PSBTBT-2h (dash) and PSBTBT-

2h (dot) in chloroform solution.



118

The first feature to observe was the presence in the UV region at 370 nm of the ZnO

absorption peak. The free polymer exhibited a maximum absorption at a wavelength of 605

nm which was the same as that of the ZnO + PSBTBT-2h mixture and indicating that the

macromolecules were well solvated in CHCl3 and behaved independently without any

interactions at this concentration. On the contrary, upon grafting the maximum absorbance is

red-shifted to a wavelength of 680 nm with a clear shoulder at 750 nm. This bathochromic

shift reflects a significant planarization of the polymer backbone and π- π stacking resulting in

efficient delocali ation of the π-conjugated electrons. This behavior is related to the high

grafting density of the polymer brushes which forces the macromolecules to be extended and

in close contact one from each other. Kiriy has already observed the same effect for a P3HT

brushes created on silica particles via the “grafting from” methodology.36

On the contrary,

when the grafting density is lower, as in the case when the “grafting onto” methodology is

used, the anchored polymer is swollen and its maximum absorption remains the same as that

of the corresponding free polymer (see previous chapter with P3HT). 6, 37

In the case of

samples 4 h and 6 h, the shift between grafted and free polymer is much lower because the

absorption of the free polymer is already red shifted due to a low solubility in chloroform. In

Figure 16, the absorption spectra of ZnO@PSBTBT-2h, 4h and 6h are superposed. For

ZnO@PSBTBT-4h we observe a red shift of ~ 20 nm and a higher absorption of the grafted

polymer in comparison with ZnO@PSBTBT-2h. For ZnO@PSBTBT-6h we observe a

maximum absorption at 780 nm related to very strong π-π interaction and 5 times more

absorption for the PSBTBT polymer in comparison with ZnO@PSBTBT-2h and

ZnO@PSBTBT-4h.

Figure 16. UV-visible absorption of ZnO@PSBTBT after 2h, 4h and 6h reaction.

0

0,2

0,4

0,6

0,8

1

1,2

1,4

300 350 400 450 500 550 600 650 700 750 800 850 900

Nor

mal

ized

abs

orpt

ion

Wavelength

ZnO@PSBTBT-2h

ZnO@PSBTBT-4h

ZnO@PSBTBT-6h



119

This red-shift is correlated with the better orientation of the polymer backbone after grafting

in comparison with free polymer is solution. The polymer brushes are in the dense brush

regime (high grafting density) due to the surface initiated “grafting-through” methodology.

To evaluate the amount of PSBTBT grafted to the nanoparticles, thermal gravimetric

analysis of ZnO@PSBTBT was performed under air with a heating rate of C/min. First of

all, TGA of the free PSBTBT is reported in Figure 17. Degradation under oxygen occurred

through two steps starting at 4 C and ending at 5 C. inally, when the ma imum

temperature of C is reached the residual mass of the three free polymers is 18 % of the

initial mass.

Figure 17. Thermal gravimetric analysis of free PSBTBT under air at a heating rate C/min.

The thermal stability of the PSBTBT polymer seemed to decrease upon grafting. The

degradation of the organic phase occurs in single step starting at 3 C and ending at 5 C as

shown in Figure 18. The weight loss for PSBTBT polymer (Table 6) in the hybrids

ZnO@PSBTBT-2h, 4h and 6h were respectively 3.33, 4.14 and 8.22 %. This increase in the

weight loss of the grafted particles has to be related with the increase in the molar mass of the

free polymer in solution. For 2h and 4h reactions we can observe a 0.8 % difference, while for

6 h reaction the much higher weight loss is in agreement with the UV-visible result. At this

point, we assumed that the free PSBTBT-6h being insoluble in chlorobenzene is not

completely removed from the grafted particles in spite of the numerous cycle dispersion-

centrifugation carried out. Therefore, the high absorbance and mass loss could be attributed to

the presence of free polymer adsorbed on the grafted particles.

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500 600 700 800

We

igh

t lo

ss %

Temperature C

PSBTBT



120

Figure 18. Thermal gravimetric analysis of the hybrid materials ZnO@PSBTBT under air at a heating rate

C/min.

In order to characterize the samples obtained after the Stille Cross-Coupling

polymerization, we previously investigate the XPS signals of the synthesized PSBTBT. We

identify on the survey spectrum (not shown here) the oxygen, carbon, bromide, silicium,

sulphur, tin and nitrogen elements. The O1s, C 1s, Br 3d, Si 2p, S 2p, Sn 3d and N 1s core

peaks have been recorded. The XPS analysis of the pure PSBTBT allow us to obtain the

reference binding energies of these core peaks. Thus, we can confirm the anchorage of this

polymer via the Stille Cross-Coupling polymerization. The Si 2p3/2 component located at

100.7 eV is attributed to the silicium surrounding by four carbon atoms in the PDTSBT

polymers. The components at higher B.E. (101.4 eV) are related to an oxygenated

environment of the silicium corresponding to the silane moiety. The bromine atom bonded to

a carbon of the monomer is identified by a B.E. of Br 3d5/2 component at 70.7 eV. The sulphur

atoms are present in two environments in the PDTSBT polymers. The thiophenic environment

is assigned to the B.E. of 164.1 eV. Due to the influence of the two nitrogen atoms setting in

the first neighbourhood, the B.E. of the third sulphur is higher (165.4 eV). The nitrogen atoms

are well characterized by a N 1s peak at 399.7 eV. The C 1s core peaks, due to the surface

contamination carbon and to the polymer, can be decomposed into four peaks: the main peak

at 285.0 eV associated with C-C or C-H bonds, the peak at 284 eV with the Si-C bonds, the

peak at 286.2 eV with C-N/C-S bonds and the peak at 287.6 eV with S-C-C-S environments.

We also identify a small peak characteristic of the tin atom (486.9 eV).

84

86

88

90

92

94

96

98

100

0 50 100 150 200 250 300 350 400 450 500 550 600 650

We

igh

t lo

ss %

Temperature C

ZnO

ZnO@PSBTBT-2h

ZnO@PSBTBT-4h

ZnO@PSBTBT-6h



121

The Stille Cross-Coupling polymerization has been carried out for 2h, 4h and 6h. No

evolution of chemical environments of the sulfur, palladium and nitrogen atoms has been

observed after the polymerization on the ZnO nanorods. Anyway, we can note an evolution of

the Pd° component with the polymerization duration time, with a maximum for 4h of

polymerization. This could be due to the formation of Pd0 clusters that haven’t been removed

by purification. The formation of those nanoparticles can be explain by the fact that the

Pd2(dba)3 catalyst commonly used for in Stille polymerization is also a precursor for the

synthesis of Pd nanoparticles.35

As expected, we observe two doublets for the Si 2p core peaks attributed to the two

environments of the the silicium in the ZnO@LBG materials, the Si-O3 and the Si-C

environments characterized, respectively, by a B.E. of Si 2p3/2 of 101±0.1 eV and 102.5±0.1

eV (Figure 19).

Figure 19. Si 2p XPS spectrum of all samples.

From the composition, we can clearly observe different factor which indicates that the

polymerization time has a direct influence on the length of the polymer chains. Indeed, the

atomic percentages of the specific atoms C, S, and N contain in the polymer are increasing

whereas the oxygen and zinc content is decreasing with the polymerization time (Table 5).



122

Moreover, in the same manner, the atomic percentage of the Si-C component (284 eV) of the

Si 2p peak is increasing with the reaction time (I(Si102.5eV)/I(Si101eV) = 2.5, 1.2, 1.1) compared

by the SiO3 signal which is constant.

Table 5. Ionization energy and surface chemical composition percentage determined by XPS.

Atom C

(1s)

O

(1s)

Zn

(2p)

Br

(3d) Si-O Si-C

Pd

(3d)

S

(2p)

Sn

(3d)

N

(1s)

IE (eV) 285 530.5 1022.5 70.1 102.8 101.1 335.4 164 487.4 399.7

ZnO 19.4 45.6 35.3 - - - - - - -

ZnO@PhBr 20.3 43.4 26.9 1.8 3.9 - - - - -

ZnO@PhPdBr 21.4 41.7 26 1.4 8.2 - 0.9 - - -

ZnO@PSBTBT-2h 31.1 37.4 23.2 1.7 2.8 0.5 1 1.2 0.1 1.2

ZnO@PSBTBT-4h 49.1 26.4 12.9 2.0 1.2 1 1.1 3.12 - 2.1

ZnO@PSBTBT-6h 57 20.8 9.4 0.8 1.7 1.6 1.1 4.6 - 2.8

To calculate the thickness of the grafted layer and to check the coverage of the nanoparticles

we performed TEM analysis (Figure 20).

Figure 20. TEM images for a) bare ZnO nanorods, b) ZnO@PSBTBT-2h, c) ZnO@PSBTBT-4h, d)

ZnO@PSBTBT-6h (scale bar = 50 nm).

c) d)



123

A dense and homogenous polymer shell leading to core@shell hybrid materials was

visualized in all grafted samples. An average shell thickness about 6 nm was observed (Table

6). In all images, we did not see a clear effect of increasing the time of the reaction on the

shell thickness in contradiction with UV-visible, TGA and XPS results. We assume that

grafting longer polymer chains with increasing the reaction time is correlated with increasing

the dispersity of the grafted polymer chains, resulting in only a slight change in the shell

thickness. This assumption will be detailed in the next paragraph by explaining the

mechanism for tethered polymer chain formation.

Moreover in some images, we observe the presence of dark spots that become more evident

with increasing the molar mass of the free polymer chains (Figure 21a). They may be due to

the presence of palladium catalyst in agreement with XPS analysis. If this is the case, this

underlines a drawback of the applied strategy and should encourage scientists to focus on

functionalizing low band gap polymer in order to apply grafting onto technique (ability to get

rid of catalyst). Furthermore, Figure 21b shows the presence of free polymer chains for the

ZnO@PSBTBT-6h. This is in agreement with our observation in TGA and UV-visible

spectroscopy. This fact comes from the high molar mass of the free polymer synthesized

which was even insoluble in chlorobenzene and that we were unable to remove it with our

cleaning procedure.

Figure 21. TEM images for ZnO@PSBTBT-6h to show a) presence of catalyst b) presence of free polymer

chains.



124

Table 6. Hybrid material characteristics.

Hybrid material Reaction

time

Mn

(g.mol-1

)a

LBG

weight

%b

Absorbance

(600-900 nm)c

Shell

thickness

(nm)d

Đe

ZnO@PSBTBT

2h 3500 3.33 + 4 ± 1 nm +

4h 10 500 4.14 ++ 4 ± 1 nm ++

6h > 20 000 8.233 +++ 5 ± 1 nm +++ a determined on the free polymer chains by GPC in THF (calibrated with PS standard),

b calculated from TGA,

c calculated from UV-vis spectroscopy,

d determined from TEM images. e

dispersity (the explanation is given in

the 5.4 part of this chapter). + is a qualitative information.

5.5. Tentative of brush formation mechanism through Stille cross coupling reaction

In the first step of the catalytic reaction, an exchange of ligand between Pd2(dba)3 and P(o-

tol)3 generates the reactive Pd0 species (Scheme 7). P(o-tol)3 is superior to other co-ligands

because of the large cone angle (194°) which results in the release of steric strain in the

transmetallation step. urthermore, the phosphine groups form sigma bonds (σ) with the metal

by donating the lone pair on the phosphorus to the empty d orbital of the metal. The donation

of the lone pair increases the electron density of the metal. Therefore the oxidative addition is

favored as the metal becomes more nucleophilic.38

Scheme 7. Generation of the reactive Pd0 catalyst.

The following step is an oxidative addition of the organohalide (R-X) to the Pd0 to form a Pd

II

complex. The organohalides are susceptible to nucleophilic attack from the metal due to the

presence of a good leaving group.

The third step is a transmetallation step occurred (Scheme 8), it is not well understood but this

has been described as the rate-determining step.39

The organostannane with a tin atom bonded

to an allyl or aryl group can coordinate to palladium via one of these bonds. Then, a cleavage

+

Step 1: Ligand exchange



125

of the R-Sn bond occurs and R transferred to the palladium complex after elimination of

halide group.

Scheme 8. The organostanne monomer anchors the surface (Transmatellation step).

The reductive elimination is an intermolecular reaction, a cyclic transition state of a

cis/trans isomerization of the Pd (II) complex resulting in cis-R/R' Pd complex needed for

reductive elimination (Scheme 9). The Pd+2

catalyst is removed from the surface and gains

two ligands to regenerate and the catalytic cycle can begin again.

Scheme 9. Cis/trans isomerization and reductive elimination step.

Once the palladium catalyst is released from the surface, it reacts with a M2 monomer

in solution. This activated M2 monomer would then react either with a surface tin moiety bore

by the attached M1 or with a M1 present in solution providing free dimer (Scheme 10). From

this point, polymerization occurs both on surface and in solution, leading to the existence of

free and grafted chains.



126

Scheme 10. Reaction process either in solution or onto surface.

After a while, the scheme 11 presents what the media could look like. Here

macromolecules dispersity is high, either in solution or on surface. Average molar mass has

increased but slowly like a step-growth polymerization behaves. At this point steric hindrance

of the grafted chains and of the free polymer plays a role like in the “grafting onto”

methodology. Indeed, as few initial chains have been grafted, the polymer chains in solution

to be grafted must diffuse through the existing polymer film to reach the reactive sites on the

surface. This “e cluded volume barrier” becomes more pronounced as the thickness of the

tethered polymer layer increases.

Scheme 11. High dispersity for grafted polymer chains.

In case of hybrid materials ZnO@PSBTBT-2h, we obtained low molar mass polymer. In this

case, conversion is low and the dispersity of polymer chains is relatively narrow. Thus, the

excluded volume barrier is less pronounced and the tethered polymer chains are extended and

behave as a brush regime (Scheme 12).



127

Scheme 12. Grafting PSBTBT polymer through step growth polymerization for short polymerization time.

On the contrary, in the case of ZnO@PSBTBT-4h and 6h, we obtained high highmolar mass

polymer. For long polymer chains, the volume barrier becomes more pronounced. Thus,

steric hindrance at the surface began to play a role and hide some anchoring sites. Therefore

increasing the molar mass, the brush dispersity increases. As a consequence some attached

macromolecules will have to fold over the surface preventing again active sites on the surface

from further extension (Scheme 13). We can estimate an evolution of the dispersity Ð

(ÐZnO@PSBTBT-6h> ÐZnO@PSBTBT-4h > ÐZnO@PSBTBT-2h). Therefore, the steric hindrance induced by

grafted chains is more important for 6h and 4h, than that of 2h. This could explain why we

observe the same shell thickness by TEM images.



128

Scheme 13. Brush conformation for the three hybrid materials.

6. Conclusion:

In summary, PSBTBT “LBG” polymer has been covalently grafted onto zinc oxide

nanorods via Stille Cross Coupling polymerization. Three batches of the hybrid materials

were synthesized by increasing the molar mass of free polymer in bulk. According to GPC,

free polymer chains ranging between 3 500 g.mol-1

and more than 25 000 g.mol-1

were

synthesized. The grafting density is high because the UV-visible spectra of the brushes are

similar to free polymer in films. Increasing the molar mass of the grafted polymers was

confirmed by TGA, UV-visible and XPS. TEM images for the hybrid materials showed a

continuous and homogeneous polymer shell of 5 ± 1 nm, not only linked to the polymer molar

masses but also to dispersities. The drawbacks of the applied method are the presence of a

residue of palladium catalyst, difficulty to control the molar mass and hardness to remove free

polymer chains. Thus applying a grafting-onto technique by functionalizing low band gap

could be advantageous. Therefore, we start working on functionalizing PSBTBT with strong

anchoring group.



129

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17. Bijleveld, J. C.; Gevaerts, V. S.; Di Nuzzo, D.; Turbiez, M.; Mathijssen, S. G. J.; de Leeuw, D. M.; Wienk, M. M.; Janssen, R. A. J., Efficient Solar Cells Based on an Easily Accessible Diketopyrrolopyrrole Polymer. Advanced Materials 2010, 22 (35), E242-E246. 18. Chen, H. Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li, G., Polymer solar cells with enhanced open-circuit voltage and efficiency. Nature Photonics 2009, 3 (11), 649-653. 19. Cabanetos, C.; El Labban, A.; Bartelt, J. A.; Douglas, J. D.; Mateker, W. R.; Fréchet, J. M. J.; McGehee, M. D.; Beaujuge, P. M., Linear Side Chains in Benzo[1,2-b:4,5-b′] p –Thieno[3,4-c]pyrrole-4,6-dione Polymers Direct Self-Assembly and Solar Cell Performance. Journal of the American Chemical Society 2013, 135 (12), 4656-4659. 20. Manceau, M.; Bundgaard, E.; Carle, J. E.; Hagemann, O.; Helgesen, M.; Sondergaard, R.; Jorgensen, M.; Krebs, F. C., Photochemical stability of [small pi]-conjugated polymers for polymer solar cells: a rule of thumb. Journal of Materials Chemistry 2011, 21 (12), 4132-4141. 21. Mühlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.; Gaudiana, R.; Brabec, C., High Photovoltaic Performance of a Low-Bandgap Polymer. Advanced Materials 2006, 18 (21), 2884-2889. 22. Scharber, M. C.; Koppe, M.; Gao, J.; Cordella, F.; Loi, M. A.; Denk, P.; Morana, M.; Egelhaaf, H.-J.; Forberich, K.; Dennler, G.; Gaudiana, R.; Waller, D.; Zhu, Z.; Shi, X.; Brabec, C. J., Influence of the Bridging Atom on the Performance of a Low-Bandgap Bulk Heterojunction Solar Cell. Advanced Materials 2010, 22 (3), 367-370. 23. Albrecht, S.; Janietz, S.; Schindler, W.; Frisch, J.; Kurpiers, J.; Kniepert, J.; Inal, S.; Pingel, P.; Fostiropoulos, K.; Koch, N.; Neher, D., Fluorinated Copolymer PCPDTBT with Enhanced Open-Circuit Voltage and Reduced Recombination for Highly Efficient Polymer Solar Cells. Journal of the American Chemical Society 2012, 134 (36), 14932-14944. 24. Li, Z.; Tsang, S.-W.; Du, X.; Scoles, L.; Robertson, G.; Zhang, Y.; Toll, F.; Tao, Y.; Lu, J.; Ding, J., Alternating Copolymers of Cyclopenta[2,1-b;3,4-b′] p T [3 4-c]pyrrole-4,6-dione for High-Performance Polymer Solar Cells. Advanced Functional Materials 2011, 21 (17), 3331-3336. 25. Li, W.; Hendriks, K. H.; Roelofs, W. S. C.; Kim, Y.; Wienk, M. M.; Janssen, R. A. J., Efficient Small Bandgap Polymer Solar Cells with High Fill Factors for 300 nm Thick Films. Advanced Materials 2013, 25 (23), 3182-3186. 26. Liao, S.-H.; Jhuo, H.-J.; Cheng, Y.-S.; Chen, S.-A., Fullerene Derivative-Doped Zinc Oxide Nanofilm as the Cathode of Inverted Polymer Solar Cells with Low-Bandgap Polymer (PTB7-Th) for High Performance. Advanced Materials 2013, 25 (34), 4766-4771. 27. Espinet, P.; Echavarren, A. M., The Mechanisms of the Stille Reaction. Angewandte Chemie International Edition 2004, 43 (36), 4704-4734. 28. Stille, J. K., Palladium-katalysierte Kupplungsreaktionen organischer Elektrophile mit Organozinn-Verbindungen. Angewandte Chemie 1986, 98 (6), 504-519. 29. Carothers, W. H., Polymers and polyfunctionality. Transactions of the Faraday Society 1936, 32 (0), 39-49. 30. Slade Jr, P. E., INTRODUCTION. Polym Mol Weights, Pt 1 1975, 1-8. 31. Liu, J.; Zhang, R.; Sauvé, G.; Kowalewski, T.; McCullough, R. D., Highly Disordered Polymer Field Effect Transistors: N-Alkyl Dithieno[3,2-b:2′ 3′-d]pyrrole-Based Copolymers with Surprisingly High Charge Carrier Mobilities. Journal of the American Chemical Society 2008, 130 (39), 13167-13176. 32. Neto, B. A. D.; Lopes, A. S.; Wüst, M.; Costa, V. E. U.; Ebeling, G.; Dupont, J., Reductive sulfur extrusion reaction of 2,1,3-benzothiadiazole compounds: a new methodology using NaBH4/CoCl2·6H2O(cat) as the reducing system. Tetrahedron Letters 2005, 46 (40), 6843-6846. 33. Tierney, S.; Heeney, M.; McCulloch, I., Microwave-assisted synthesis of polythiophenes via the Stille coupling. Synthetic Metals 2005, 148 (2), 195-198. 34. Hunt, A. J.; Budarin, V. L.; Comerford, J. W.; Parker, H. L.; Lazarov, V. K.; Breeden, S. W.; Macquarrie, D. J.; Clark, J. H., Deposition of palladium nanoparticles in SBA-15 templated silica using supercritical carbon dioxide. Materials Letters 2014, 116 (0), 408-411.



131

35. Hu, G. Z.; Nitze, F.; Jia, X.; Sharifi, T.; Barzegar, H. R.; Gracia-Espino, E.; Wagberg, T., Reduction free room temperature synthesis of a durable and efficient Pd/ordered mesoporous carbon composite electrocatalyst for alkaline direct alcohols fuel cell. RSC Advances 2014, 4 (2), 676-682. 36. Senkovskyy, V.; Tkachov, R.; Beryozkina, T.; Komber, H.; Oertel, U.; Horecha, M.; Bocharova, V y A bs F C y A “H y” P y(3-hexylthiophene) Particles Prepared via Surface-Initiated Kumada Catalyst-Transfer Polycondensation. Journal of the American Chemical Society 2009, 131 (45), 16445-16453. 37. Li, F.; Du, Y.; Chen, Y.; Chen, L.; Zhao, J.; Wang, P., Direct application of P3HT-DOPO@ZnO nanocomposites in hybrid bulk heterojunction solar cells via grafting P3HT onto ZnO nanoparticles. Solar Energy Materials and Solar Cells 2012, 97 (0), 64-70. 38. Stille, J. K.; Lau, K. S. Y., Mechanisms of oxidative addition of organic halides to Group 8 transition-metal complexes. Accounts of Chemical Research 1977, 10 (12), 434-442. 39. Stille, J. K., The Palladium-Catalyzed Cross-Coupling Reactions of Organotin Reagents with Organic Electrophiles [New Synthetic Methods (58)]. Angewandte Chemie International Edition in English 1986, 25 (6), 508-524.

Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells

Chapter 4



1. Introduction: ................................................................................................................. 134

2. P3HT SAMs on ITO substrates .................................................................................. 141

2.1 Preparation ................................................................................................................. 141

2.2 Results and discussion ................................................................................................ 142

3. Photovoltaic performance ............................................................................................ 147

3.1 Fabrication .................................................................................................................. 147

3.2 Measurements ............................................................................................................. 148

4. Conclusion ..................................................................................................................... 151

5. References ..................................................................................................................... 152


134

1. Introduction

To enter the market of fabricating integrated photovoltaic devices, polymer solar cells (PSCs)

should reach power conversion efficiency higher than 10% on large modules and should be

more stable in time.1 In order to reach such performance researchers have worked on

designing new narrow bandgap polymer to improve photon harvesting,2 optimizing the

morphology,3,4

and designing novel device architectures 5. An important consideration is the

optimization of the interfaces found in a PSC. Indeed an extensive interfaces study will help

to avoid many losses of conversion in the device such as electron-hole recombination,6 charge

leakage due to imperfect diodes,5b

inefficient exciton dissociation7 and surface energy

mismatches that lead to interfacial dewetting.8

A PSC (Figure 1) is composed of several layers deposited on glass substrate; 1) an

indium-doped tin oxide (ITO) layer as high work function electrode (hole collecting layer),2)

a hole transporting layer (HTL) such as poly (3,4-ethylenedioxythiophene)-blend-poly(styrene

sulfonate) (PEDOT:PSS), 3) an active layer (Polymer-blend-acceptor) and finally 4) a top

metal electrode with low work function (electron collecting layer).

Figure 1. Different layers of PSCs.

Most PSCs comprise an active layer with a bulk heterojunction (BHJ) wherein an electron

donating polymer and an electron accepting fullerene derivative form nanoscaled

interpenetrating networks allowing efficient exciton dissociation and charge carrier transport.

Improving the power conversion efficiency is a challenge, it is a product of open circuit

voltage (Voc), short circuit current density (Jsc) and fill factor (FF). The Voc is limited by the

energy level difference between the HOMO of the polymer donor and the LUMO of fullerene

acceptor. However, low recombination as well as matching the Fermi levels of the hole

collecting electrode and electron collecting electrode to the HOMO of the donor and the

ITO

PEDOT:PSS

P3HT:PCBM

Active layer

Al

1)

2)

3)

4)


135

LUMO of the acceptor respectively, are required to obtain the highest Voc. Jsc is determined

by the light harvesting and the charge separation efficiency under large extraction fields, and

FF is determined by the device series resistance, the dark current and the charge

recombination/extraction rate under low internal fields. The series resistance (Rs) in a solar

cell is attributed to the bulk conductivity of each of the functional layers and the contact

resistance between them. Materials with high charge carrier mobility and ohmic contact at the

interfaces are required to obtain low Rs affecting the Jsc. Another important parameter, the

shunt resistance (Rsh), is determined by the quality of the thin films and their interfaces. Low

Rsh originates from the loss of charge carriers through leakage paths including pinholes in the

films and the recombination and trapping of the carriers during their pass through the cell

leading to a decrease in device performance. Therefore to improve all these factors,

interface engineering is essential.

Unlike inorganic solar cells where ohmic contacts can be made by surface doping, PCS

requires alternative strategies for the interface engineering. Specifically, poor ohmic contacts

between the polymer BHJ and the transparent conducting oxides are due to the mismatch of

work function, the presence of interfacial dipoles

as well as high densities of interfacial trap

states. ITO composed of In:Sn (90:10 atomic ratio) is the most widely used electrode for a

variety of optoelectronic technologies. It shows electrical conductivities rivaling metal thin

film, and good transparency in the visible region. However, due to annealing after sputter

deposition of ITO, the surface is highly polar with a variable surface roughness and work

function. A hole transporting layer is often necessary to optimize the interface properties

between anode and active layer. The most commonly used hole transporting layer (HTL) is

PEDOT:PSS which ensures ohmic contact between active layer and anode,9 enhances hole

collection,10

and increases open-circuit voltage 11

. PEDOT:PSS is a water soluble

polyelectrolyte system with excellent film formability, high electrical conductivity (ca. 103

S.cm-2

),12

high visible light transmittance, and good thermal stability. Even though

PEDOT:PSS exhibits these advantages, its acidic nature (pH~1) can corrode the ITO

electrode, leading to chemical instability at the interface.13

Furthermore, the spin-coated film

of PEDOT:PSS presents large microstructural and electrical inhomogeneities with insufficient

electron blocking capacity 13a

which reduces the short circuit density (JSC) in polymer solar

cells. Finally, Its hydrophilic nature is also responsible for water penetration and diffusion in


136

the device, leading to device degradation and a decrease of the solar cell performances.14

(Figure 2)

Figure 2. Different mechanisms of PSCs degradation due to water and oxygen penetration or impurities

diffusion. PEDOT:PSS is the main entrance for water in the device.14

These issues illustrate the need for a new interfacial material that has a greater charge

blocking characteristics and allows strong adhesion between the active layer and the anode

surface. Among the electrode interlayer materials used to replace PEDOT:PSS, transition

metal oxides are promising because of their better environmental stability, higher optical

transparency, easy synthesis routes, their ability to efficiently extract charge carriers 15

and

their compatibility with high volume roll-to-roll processing 16

. For example, MoO3 has been

proved to enhance the open circuit voltage and fill factor of solar cell devices.17

Organic HTLs have also been developed to replace PEDOT:PSS such as

chlorobenzene and silane derivatives, benzoic acid derivatives and polymers (Scheme 1).


137

Scheme 1. Different types of organic HTLs used in OPVs a) chlorobezene derivatives b) aminopropyltriethoxysilane, and

(trichloro(3,3,3,-trifluoropropylsilane) c) benzoic acid derivatives d) poly[9,9-dioctylfluorene-co-n-[4-(3-methylpropyl)]-

diphenylamine] (TFB), 5,5'-bis[(p-trichlorosilylpropylphenyl)phenylamino]-2,2'-bithiophene (PABTSi2) e) Poly(3-

methyl)thiophene

One of the emerging technologies to enhance interfacial properties is to grow

molecular self-assembled monolayer (SAMs) on ITO surface. The wettability of the substrate

is modified by replacing the hydroxyl surface groups by organic molecules. Moreover, a

variation in the electrode work function () and in the overall efficiency of the device was

observed in previous studies.

In 2006, Khodabakhsh et al. utilized three types of chlorobenzene derivatives to modify the

ITO substrate (= 4.7eV): 4-chlorobenzoylchloride (CBC), 4-chlorobenzenesulfonyl

chloride (CBS), 4-chlorophenyldichlorophosphate (CBP). The work function measured by

Kelvin Probe Technique increased with the magnitude of the molecule dipole moment ( CBS

(5.1 eV) > CBC (4.94 eV) > CBP (4.9 eV)). The results showed an increase in the short

circuit photocurrent density Jsc and fill factor FF when ITO increased while remaining the

voltage open circuit Voc constant. As a result, the PV device based on copper phthalocyanine

(CuPc):C60 heterojunction showed an optimized PCE from 0.16% to 1.27% after modifying

the ITO layer by CBS.18

a) b) c)

d)e)


138

Kim et al. correlated these variations not only to the electrode work function but also to its

surface energy. A lower anode surface energy leads to a better active layer wettability

(because of the hydrophobicity of the active layer) and improved bulk heterojunction

morphology. They measured the surface energy of ITO substrates modified with

aminopropyltriethoxysilane (SAM-NH2) and (trichloro(3,3,3,-trifluoropropylsilane) (SAM-

CF3) to have a value of 46.5 and 28.7 mJ.m-2

, respectively. As shown in Table 1, the solar cell

efficiency increases with the increase of anode work function. Optical microscopy showed

that ITO modification with SAM-NH2 resulted in severe PCBM aggregation in the active

layers due to higher surface energy in comparison with SAM-CF3.19

Table 1. Work function of the various SAM-treated ITO substrates and the electrical properties of the photovoltaic

devices.

HTL Work

Function

(eV)

Active layer Device

Architecture

Voc

(V)

Jsc

(mA.cm-²)

FF PCE

(%)

Ref

Bare ITO 4.5 CuPC/C60 BHJ 0.485 1.27 0.26 0.16 18

CBC 4.94 CuPc/C60 BHJ - < 3.55 - - 18

CBS 5.1 CuPc/C60 BHJ 0.45 5.88 0.48 1.27 18

CBP 4.9 CuPc/C60 BHJ - < 3.55 - - 18

Bare ITO 4.7 P3HT/PCBM BHJ 0.36 5.98 0.35 0.75 19

SAM-NH2 4.35 P3HT/PCBM BHJ 0.55 5.71 0.3 0.95 19

SAM-CF3 5.16 P3HT/PCBM BHJ 0.6 13.87 0.38 3.15 19

Bare ITO 4.8 ClAlPc/C60 Bilayer 0.47 5.47 0.52 1.32 20

SubPc/C60 Bilayer 0.56 4.21 0.46 1.1 20

BBA 4.88 ClAlPc/C60 Bilayer 0.78 6.43 0.55 2.72 20

SubPc/C60 Bilayer 0.7 4.48 0.55 1.7 20

CBA 5.02 ClAlPc/C60 Bilayer 0.79 6.84 0.59 3.25 20

SubPc/C60 Bilayer 0.71 4.54 0.53 1.7 20

FBA 5.05 ClAlPc/C60 Bilayer 0.8 5.94 0.57 2.74 20

SubPc/C60 Bilayer 0.95 4.44 0.5 2.2 20

Beaumont et al. utilized benzoic acid with different withdrawing group at the para-position:

Bromine (BBA), Fluorine (FBA) and Chlorine (CBA). The work function was found to be


139

FBA (= 5.05 eV) > CBA (= 5.02 eV) > BBA (= 4.88 eV) increasing simultaneously

with the increase of electronegativity of the withdrawing groups (F > Cl > Br). An increase of

the power conversion efficiency from 1.3 to 3.3% of bilayer OPV devices based on

(ITO/donor/C60/BCP (Bathocuprine)/Al) and (ITO/SAMs/donor/C60/BCP/Al), respectively

with two types of donors: chloroaluminium phthalocyanine (ClAlPc) and boron sub-

phthalocyanine (SubPc) were reported and listed in Table 1.20

These improvements were

attributed to the better compatibility of ITO electrode with the overlaying active layer and to

the improved alignment between work function of the electrode and HOMO donor which

results in better ohmic contact.

Alexander et al. studied the use of a conjugated polymer: poly[9,9-dioctylfluorene-co-n-[4-(3-

methylpropyl)]-diphenylamine] (TFB) (2eq) mixed with 5,5'-bis[(p-

trichlorosilylpropylphenyl)phenylamino]-2,2'-bithiophene (PABTSi2) (1eq) as a spin coated

crosslinked interfacial layer. This homogenous conductive film (~ 10 nm) with hole field

effect mobility of 5 x 10-4

cm2.V

-1 s

-1 is covalently crosslinked by the silane moieties forming

a thermally and chemically stable film. Moreover, it possessed high-lying HOMO level to

block electron leakage/recombination at the ITO anode. The OPVs based on the active layer

P3HT:PCBM exhibits a PCE of 3.14% compared with a PCE of 1.46% for a PEDOT:PSS

based device.

This result attracted the interest of scientists toward conjugated polymer brushes

grafted to the ITO as they provide excellent stability since they are covalently linked to the

surface. Moreover, the chemical structures of such macromolecular SAMs can be altered to

increase the compatibility within an improved energy level alignment that creates a higher

degree of uniformity at the electrode/organic interface.21

Luscombe et al. reported the grafting of poly(3-methylthiophene) P3MT on ITO using

surface initiated Kumada Catalyst-Transfer Polycondensation (SI-KTCP) from surface-bond

arylnickel (II) bromide initiator (grafting-from technique).22

They demonstrated a control of

the film thickness by varying the monomer concentration from 0.03 to 0.18 M creating a

polymer layer ranging between 30 and 265 nm, respectively. The absorbance values of the

maximum wavelength λ ~ 500 nm (for different thicknesses) were smaller than expected for

similar thickness, revealing a low grafting density of P3MT. They discovered the possibility

to change the work function by increasing the relative amount of oxidized thiophene units.

Then Li Yang et al. studied the same hole transporting layer (P3MT) with varying layer

thickness (3, 6, 9, 20 nm) as HTLs.23

The photovoltaic performance for undoped P3MT and


140

doped P3MT were tested and compared to PEDOT:PSS and bare ITO by choosing

P3HT:PCBM as an active layer at a weight ratio 1:1 (Table 2).

The insertion of P3MT layer causes an important increase in the Voc attributed to the

modification in the work function of ITO electrode. For the undoped P3MT a lower fill factor

(FF) and short circuit current (Jsc) related to the low mobility and poor charge transport in the

polymer backbone limit the power conversion efficiency of the device. This issue was

addressed by doping P3MT layer to raise the efficiency from 1.12% (for bare ITO) to 2.51%

(for ~ 9 nm doped-P3MT). As the thickness of the layer increased to 20 nm a drop in the

efficiency to 1.27% was observed meaning that thin HTL is better.

This feature indicates that polythiophene as interfacial layer is promising.

Table 2. Photovoltaic properties of devices based on Bare ITO, ITO/PEDOT:PSS and (doped, undoped)

ITO/P3MT.

From these different studies useful information can be extracted on an “ideal” HTL. It should

present:

- thin and packed layer to ensure light transmittance and enhance compatibility with

overlaying organic active layer, respectively.

interfacial

layer Thickness

(nm) Voc (V)

Jsc (mA.cm

-²)

FF PCE (%)

ITO ----

0.27 8.61 0.484 1.12

PEDOT:PSS --- 0.53 8.8 64.8 3.02

undoped

P3MT

doped P3MT

~3 0.39 7.14 0.368 1.03

~6 0.45 6.57 0.401 1.18

~9 0.49 7.54 0.294 1.07

~20 0.45 5.26 0.435 1.03

~3 0.45 6.81 0.475 1.46 ~6 0.49 7.45 0.551 2.03

~9 0.55 8.39 0.545 2.51 ~20 0.47 5.81 0.465 1.27


141

- thermal and chemical stabilities

- interfacial energy level matching between anode and the active layer

- enhance hole collection by altering the work function of electrode

- facilitate charge transport from BHJ to anode depending on the effective molecular

arrangement of the π-conjugated systems to form conductive pathways.

In this context we report the grafting of P3HT (better solubility and crystallinity

than P3MT) by using the grafting onto technique to create a macromolecular SAMs in a

facile way. The major advantage of this versatile method over previously reported grafting-

from technique is that the polymer can be grafted in one simple step and easily included in a

device manufacturing procedure. Indeed there is no need for the use of catalyst or the

preparation of the initiator layer. Moreover the polymer grafted has a controlled molar mass

and a narrow molar mass distribution resulting in the elaboration of well-defined polymer

brushes.

2. P3HT SAMs on ITO substrates:

2.1 Preparation

Indium tin oxide (ITO) - coated glass electrodes (10 Ω/sq, Kintec), were successively cleaned

in acetone, ethanol and iso-propanol for 15 min under ultrasound at 40 °C. After drying the

substrates with air flow, UV-ozone treatment (15 min) was applied to the substrates in order

to increase the hydrophilic nature of the surface and to remove residual organic

contamination. The same experimental procedure developed in Chapter 2 was applied for the

synthesis of two rr-P3HTs terminated-triethoxysilane with different molar masses (Table 3).

Table 3. Macromolecular characteristics of rr-P3HT terminated-triethoxysilane.

Polymer n Ni(dppp)Cl2

(mmol)

Mna

(g.mol-1

)

% RRb Ð

a Si%

Endb

P1-Si 0.1 30 6500 97% 1.2 80

P2-Si 0.05 60 11000 98% 1.14 100

acalculated from SEC (polystyrene conventional calibration),

b calculated from

1H NMR


142

The grafting of the polymers-Si onto the cleaned substrates was then performed from melt

(Figure 3). A layer of P3HT-Si was dip-coated on the cleaned ITO substrate and annealed at

170 °C for 3h under inert atmosphere.

The grafted substrates were subjected to ultrasonication in chloroform for 15 min 3 times to

remove the free polymer (ungrafted) and dried under nitrogen. The grafted substrates were

stored in the glove box under nitrogen to prevent any degradation of the SAMs layer. The

grafted SAMs were analyzed by UV-Visible Spectroscopy, Contact Angle Measurement, X-

ray Photoelectron Microscopy (XPS) and Atomic Force Microscopy (AFM).

Figure 3. Procedure of grafting P3HT (SAMs) onto ITO substrates.

2.2 Results and discussion

P3HT with two different molar masses were grafted onto cleaned ITO substrates to study the

effect of chain length onto the layer properties.

UV-visible Transmission was first used to verify the grafting of SAMs on ITO (Figure 4). The

optical properties of the SAMs were investigated by studying the wavelength and intensity of

transmission peaks. First, the tethered polymer chains behave likely to the polymer in film

where Polymer P1 (6500 g/mol, Ð = 1.2) has a transmission minimum peak observed at 516

nm which is red shifted in the case of P2 (11000 g/mol, Ð = 1.1) to 544 nm with a relatively

higher π-π stacking band (better packing) demonstrated by the appearance of a clear shoulder

around = 600 nm. The bathochromic effect caused by the increase in the conjugation length

reveals a better delocalization of electron that lowers the band gap. Moreover, the increase in

Grafting P3HT onto ITO


143

the shoulder means we have a higher degree of structural ordering in case of higher molar

mass polymer.

If we compare these UV spectra with the one reported by Luscombe 22

and Locklin 23

, where

the maximum absorption reported was at 450 and 500 nm respectively, (lower than those of

P3HT films), we could attribute this blue shift to a loss of regioregularity of the tethered

polymer chain as side chain length has no effect on optical properties. In addition, in both

studies there is an absence of the π-π stacking shoulder in comparison with our study (where

annealing at 170 °C was applied to graft the polymer) reveals that tethered P3HT chains attain

better crystallinity upon annealing or it has a better packing than tethered P3MTchains .

Figure 4. UV-visible transmission spectra of the two P3HT SAMs layers, and bare ITO.

Another point to mention is that the transmittance increased from P1 to P2, meaning that the

amount of grafted polymer was more important when P1 was used as SAM (Figure 5). This

fact directly induces that the density of the P1 grafted layer was higher than that of P2, which

is in agreement with the previous study on zinc oxide nanorods.24

Indeed P2 has a higher

molar mass and at equal grafting density this should result in a higher quantity of grafted

polymer. The steric hindrance induced by a grafted polymer P2 with higher molar mass is

more important than for P1.

89

91

93

95

97

99

101

300 400 500 600 700

Tran

smit

tan

ce

Wavelength (nm)

ITO

P1-g-ITO

P2-g-ITO


144

Figure 5. Schematic drawing of the proposed brush conformation for grafted P1-Si and P2-Si onto ITO.

Surface modification with P3HT SAMs materials changed the wettability properties of the

substrate surface by replacing the hydroxyl terminal group on the bare ITO with hydrophobic

carbon polymer chain. Changes in wettability can be detected by measuring the static contact

angle of water on treated substrate. The greater the contact angle is the more the surface

hydrophobicity is (Figure 6).

Figure 6. Contact angle images of cleaned ITO substrate (left) and SAM grafted substrate (right).

The bare ITO substrate has a water contact angle of 41.5°, whereas ITO grafted by P3HT

sample shows a contact angle of 88.5 . This enhancement of the water repellency character

(increase in the contact angle) should improve compatibility with a better contact between the

active layer and the ITO substrate. For further work, AFM images of the active layer

deposited on the modified and unmodified ITO could prove the compatibility between the two

layers.

ITO-Substrate P3HT-grafted-ITO substrate

Angle= 88.5°Angle = 41.5°


145

The bare and modified ITO substrates were analysed by X-ray photoelectron spectroscopy

(XPS) in order to identify the surface chemical composition (Table 4).

The atomic percentage of In /Sn for all samples doesn’t change ~5.8. The binding energies of

In3d5/2 and Sn3d5/2 are equal to 445.1 and 478.2 eV, respectively. These values are

characteristic of Indium and Tin atoms of the oxides In2O3 and Sn2O3.25

The presence of

oxygen and carbon for bare ITO is due to the presence of some impurities on the substrates.

The success of the grafting of P3HT SAMs is demonstrated by: 1) the appearance of Si2p

(binding energy =103.1 eV, characteristic of silicon element in silane function) and S2p3/2

peak (binding energy =163.8 eV, characteristic of sulfur atom in the thiophene ring), 2) the

decrease in the atomic content of In3d and Sn3d and 3) the increase in the C1s atomic content.

The ratio

=

and the higher atomic content of carbon and sulfur for P1 SAM

layer confirms that the number of tethered chains for P1 is higher (higher grafting density)

than that of P2 in agreement with the Uv-visible absorption. The atomic ratio sulfur/silicon

determined by XPS is much lower than the estimated value from the structure depending on

the number of units.

Table 4. Ionization energy and Surface chemical composition obtained from XPS.

IE (eV) ITO

% atomic P1@ITO

% atomic P2@ITO

% atomic

C (1s) 285,0 26,3

62,5

48,66

In (3d) 445,1 26,6 7,0 16,43

Sn (3d) 487,2 4,6 1,2 2,84

O (1s) 530.5 41,2 19,0 26,3

S (2p) 163,8 - 5,1 3,3

Si (2p) 103,1 - 4,0 1,54

To measure the thickness of the macromolecular SAM layer on the grafted material, Atomic

Force Microscopy analysis was performed. In fact, subsequent analyses of SAM layer (P2)

grafted onto ITO did not determine brush thickness of the layers due to the high roughness of

the ITO. To address this difficulty, the study was performed on a silicon wafer having a very

low surface roughness to easily evaluate the thickness of the grafted layer (Figure 7). Silicon

wafers were cleaned with piranha solution consisting of a mixture of varying concentration of

H2SO4 and H2O2 to remove organic residue from surfaces.


146

Figure 7. AFM topography image (upper image) and cross section (bottom image) of a bare Silicon wafer.

According to the AFM image of grafted wafer with P2 (Figure 8), a homogeneous layer of

an average 5 nm thickness was achieved in agreement with the results obtained in

chapter 2 for grafting P2 onto zinc oxide. The grafting density of tethered P3HT brushes

was calculated using the following equation:

where h = 5 nm is the brush thickness, = 1.1 g.cm-3

the density of P3HT, Mn = 5500 g.mol-1

according to MALDI-TOF MS, the corresponding grafting density σ is 0.6 chains per nm2

confirming that polymer is in the brush regime in agreement with previous study.26

The dense layer obtained is significant for the deposition of the active layer in order to

achieve a uniform and homogenous coverage.


147

Figure 8. AFM topography 3D image (upper image) and cross section (bottom image) of 5 nm brush thickness

of P3HT SAM.

Briefly, we can conclude that we succeeded to modify the ITO surface with two different

molar masses of P3HT in a one step procedure. The optical properties of tethered polymer

chains demonstrate that we have a higher grafting density for lower molar mass polymer

(proved by XPS) but lower π-π stacking interaction. A dense layer with about 5 nm thickness

was achieved according to AFM images makes these substrates suitable for photovoltaic

applications.

3. Photovoltaic performances

3.1 Fabrication

To examine the influence of SAMs interlayer between the active layer (P3HT:PCBM)

and the anode (ITO electrode), solar cells were fabricated at IMS laboratory (laboratoire de

l' Intégration du Matériau au Système) at the university of Bordeaux in collaboration

with Dr Sylvain Chambon.


148

Three types of organic solar cells were fabricated and tested according to the

following procedure (Figure 9). The previous prepared substrates with SAMs as a hole

selective layer is compared to ITO substrate without any modification and to ITO coated with

the water dispersion of poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate)

(PEDOT:PSS, Sigma-Aldrich, spin-coated at 4000 rpm during 50 s, followed by a thermal

treatment at 100 °C for 30 min to remove residual moisture, layer thickness was around 40

nm). All further device elaboration and characterization steps were carried out under inert

atmosphere (N2) in glovebox. The active layer was composed of P3HT (50 000 g.mol-1

):

PCBM mixed in a 1:1 weight ratio in chlorobenzene (C = 20 mg.ml-1

) and solubilized on a hot

plate at 50 °C overnight. The solution was then spin-coated on the hole selective layer (1000

rpm over 50 s), and the samples were left to dry for about one hour for an efficient solvent

annealing. Finally, a calcium (20 nm), aluminum (80 nm) top electrode (cathode) was

thermally evaporated under secondary vacuum (10-6

mbar) through a shadow mask. The

current density-voltage (J-V) characteristics of the cells were measured with a Keithley 2400

under illumination using an AM1.5 solar simulator set at 100 mW/cm², with an IL1400BL

calibrated radiometer.

Figure 9. Structures of the three fabricated types of organic photovoltaic devices.

3.2 Measurements

The representative current-voltage (J-V) curves of the Hero devices under illumination and in

dark are presented in Figure 10. Moreover, the key photovoltaic characteristics are

summarized in Table 5.

ITO

Glass substrate

BHJ

CaAl

Glass substrate

BHJ

CaAl

ITO

Glass substrate

CaAl

PEDOT:PSS

BHJ

ITO

PEDOT:PSS as hole selective layer ITO as hole selective layer P1or P2 as Hole Selective Layer


149

-1,0 -0,5 0,0 0,5 1,0

1E-5

1E-4

1E-3

0,01

0,1

1

10

100

1000

ITO

PEDOT:PSS

P2

P1

J (

mA

.cm

-2)

V (V)

For all the devices Jsc has the same range of value between 10-12 mA.cm-2

. However, P1-

grafted-ITO and P2-grafted-ITO compared to reference devices present a lower current

density due to higher series resistance extracted in the dark revealing low conductivity of the

grafted layers. For both grafted ITO, Voc were higher than that of bare ITO. The Voc for P1-

grafted-ITO (0.5V) was closed to that of PEDOT:PSS@ITO (0.53V) demonstrating the

existence of an efficient hole selective layer. For P2-grafted-ITO, the Voc was slightly lower

(0.45V), probably due to inhomogeneities of grafted layer creating pinholes and shunts.

Figure 10. Characteristic J-V curves of devices prepared with different HTLs based on P3HT-PCBM as an

active layer under illumination (upper figure) and under darkness (bottom figure).

0,0 0,2 0,4 0,6

-15

-10

-5

0

5

10

15

20

ITO

PEDOT:PSS

P2

P1

J (

mA

.cm

-2)

V (V)


150

The shunt resistance extracted in dark is around -0.5V. Its intensity is higher for P1-grafted-

ITO compared to PEDOT:PSS. This also proves the efficient hole selectivity of the P1-

grafted-ITO layer in PSCs.

To explain these results, the work function of the different substrates were measured by

Kelvin probe microscopy (performed by Dr Sylvain Chambon) showing a decrease of the

work function from 5.15 eV for bare ITO and to 4.65 eV P3HT-grafted-ITO. This variations

that limits the device performance, as it creates an energy mismatch between the work

function of ITO electrode and the HOMO level of P3HT. The overall photovoltaic

performance of the P1-g-ITO and P2-g-ITO did not reach yet the PCE of the PEDOT:PSS

devices (4.16%) due to lower Jsc and FF caused by the high value of Rs. Thus the low

conductivity of the grafted layer prevents its optimization to reach the high performance

observed with PEDOT:PSS. In order to improve the conductivity, doping of the grafted layer

could be applied, as shown in the literature.23

Table.5 The photovoltaic characteristics of the average and hero devices in brackets.

HTL

Jsc (mA.cm

-2)

Voc (V)

FF PCE (%)

Rs (Ω)

Rsh (Ω)

Leakage

current @ -1V

(mA.cm-2

)

ITO 11.54

(11.91) 0.36

(0.38) 0.53

(0.57) 2.17

(2.56) 19 1.1E+05 8E-1

PEDOT:

PSS 11.57

(11.74) 0.53

(0.53) 0.66

(0.67) 4.03

(4.16) 15 3.9E+05 1.8E-1

P1 10.56

(10.68) 0.45

(0.50) 0.49

(0.54) 2.36

(2.88) 46 2.1E+06 1.7E-3

P2 10.08

(10.03) 0.41

(0.45) 0.51

(0.56) 2.13

(2.52) 34 3.9E+05 11


151

4. Conclusion:

We have successfully modified the surface of ITO with P3HT brushes as an alternative to

PEDOT:PSS as a hole selective layer in organic photovoltaic device. The grafting density for

the lower molar mass P3HT (6 500 g.mol-1

) appeared to be higher than that of P2 (11 000

g.mol-1

) in agreement with the previous study on zinc oxide nanorods. According to AFM, a

thickness of 5 nm with a grafting density of 0.6 chains per nm2 was achieved. An increase of

the Voc and Rsh revealed that the layer is efficient for hole selectivity compared to bare ITO,

but less efficient than PEDOT:PSS. However low FF and Jsc due to high series resistance and

low conductivity limits the performance of the device. Finally doping the P3HT SAMs layer

could be a way to achieve better characteristics to replace PEDOT:PSS. Also the elaboration

of double brushes using of fluorinated conjugated molecules and P3HT could increase the

work function of the electrode and thus improve the power conversion efficiency.


152

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15. Wang, Z. J.; Qu, S. C.; Zeng, X. B.; Liu, J. P.; Zhang, C. S.; Tan, F. R.; Jin, L.; Wang, Z. G., Hybrid bulk heterojunction solar cells from a blend of poly(3-hexylthiophene) and TiO2 nanotubes. Applied Surface Science 2008, 255 (5, Part 1), 1916-1920. 16. Jackson, W. B.; Kim, H.-J.; Kwon, O.; Yeh, B.; Hoffman, R.; Mourey, D.; Koch, T.; Taussig, C.; Elder, R.; Jeans, A. In Roll-to-roll fabrication and metastability in metal oxide transistors, 2011; pp 795604-795604-11. 17. (a) Murase, S.; Yang, Y., Solution Processed MoO3 Interfacial Layer for Organic Photovoltaics Prepared by a Facile Synthesis Method. Advanced Materials 2012, 24 (18), 2459-2462; (b) Meyer, J.; Hamwi, S.; Kröger, M.; Kowalsky, W.; Riedl, T.; Kahn, A., Transition Metal Oxides for Organic Electronics: Energetics, Device Physics and Applications. Advanced Materials 2012, 24 (40), 5408-5427. 18. Khodabakhsh, S.; Sanderson, B. M.; Nelson, J.; Jones, T. S., Using Self-Assembling Dipole Molecules to Improve Charge Collection in Molecular Solar Cells. Advanced Functional Materials 2006, 16 (1), 95-100. 19. Kim, J. S.; Park, J. H.; Lee, J. H.; Jo, J.; Kim, D.-Y.; Cho, K., Control of the electrode work function and active layer morphology via surface modification of indium tin oxide for high efficiency organic photovoltaics. Applied Physics Letters 2007, 91 (11), -. 20. Beaumont, N.; Hancox, I.; Sullivan, P.; Hatton, R. A.; Jones, T. S., Increased efficiency in small molecule organic photovoltaic cells through electrode modification with self-assembled monolayers. Energy & Environmental Science 2011, 4 (5), 1708-1711. 21. Hains, A. W.; Ramanan, C.; Irwin, M. D.; Liu, J.; Wasielewski, M. R.; Marks, T. J., Designed Bithiophene-Based Interfacial Layer for High-Efficiency Bulk-Heterojunction Organic Photovoltaic Cells. Importance of Interfacial Energy Level Matching. ACS Applied Materials & Interfaces 2009, 2 (1), 175-185. 22. Doubina, N.; Jenkins, J. L.; Paniagua, S. A.; Mazzio, K. A.; MacDonald, G. A.; Jen, A. K. Y.; Armstrong, N. R.; Marder, S. R.; Luscombe, C. K., Surface-initiated synthesis of poly(3-methylthiophene) from indium tin oxide and its electrochemical properties. Langmuir 2012, 28 (3), 1900-1908. 23. Yang, L.; Sontag, S. K.; LaJoie, T. W.; Li, W.; Huddleston, N. E.; Locklin, J.; You, W., Surface-Initiated Poly(3-methylthiophene) as a Hole-Transport Layer for Polymer Solar Cells with High Performance. ACS Applied Materials & Interfaces 2012, 4 (10), 5069-5073. 24. Awada, H.; Medlej, H.; Blanc, S.; Delville, M.-H.; Hiorns, R. C.; Bousquet, A.; Dagron-Lartigau, C.; Billon, L., Versatile functional poly(3-hexylthiophene) for hybrid particles synthesis by the grafting onto technique: Core@shell ZnO nanorods. Journal of Polymer Science Part A: Polymer Chemistry 2014, 52 (1), 30-38. 25. Hanyš, P.; Janeček, P.; Matolín, V.; Korotcenkov, G.; Nehasil, V., XPS and TPD study of Rh/SnO2 system - Reversible process of substrate oxidation and reduction. Surface Science 2006, 600 (18), 4233-4238. 26. Paoprasert, P.; Spalenka, J. W.; Peterson, D. L.; Ruther, R. E.; Hamers, R. J.; Evans, P. G.; Gopalan, P., Grafting of poly(3-hexylthiophene) brushes on oxides using click chemistry. Journal of Materials Chemistry 2010, 20 (13), 2651-2658.

154

General Conclusions and Outlook

The main aim of this work, which was the synthesis of covalently grafted conjugated polymer

brushes on inorganic surfaces, was successfully performed. These new designed organic-

inorganic hybrids were chosen based on their electrical and optical properties that make them

suitable candidates for photovoltaic applications. Thus, two different Core@Shell ZnO

nanorods (ZnO@P3HT and ZnO@PSBTBT) were developed to highlight the effect of

grafting methodology and shell properties on the desired nanocomposites.

In the first stage, three triethoxysilane-terminated regioregular P3HTs with different molar

masses with high end group functionalization were synthesized via a hydrosilylation reaction

from allyl-terminated P3HT. Then a one-step procedure of condensation was needed to graft

the P3HT bearing strong anchoring group to the surface of zinc oxide nanorods via a “grafting

onto” methodology to yield the desired nanocomposite ZnO@P3HT. In the second stage,

PSBTBT low band gap polymer has been covalently grafted onto zinc oxide nanorods in three

steps procedure via a surface initiated step growth polymerization (grafting through) to

synthesize ZnO@PSBTBT hybrid materials.

The two applied methods seemed efficient; a homogenous shell was observed on TEM

images. The major advantage of the simple and robust direct “grafting onto” method over

“grafting through” is that well defined polymers with controlled molar masses can be used for

grafting, resulting in the synthesis of well defined brushes. Furthermore, it overcomes the

drawbacks of the “grafting through” methodology where we were unable to get rid of the

catalyst and free polymer chains for high molar masses polymer. That makes this process

easier to handle and more compatible with device fabrication. On the other hand, with UV-

visible spectroscopy we can assume that the polymer shells for PSBTBT and P3HT are in the

brush (behaves like polymer in film) and semi-dilute regimes (behaves like polymer in

solution), respectively. This highlights on the advantage of “grafting through” over “grafting

onto” method in term of grafting density. The two synthesized hybrid materials seemed to be

suitable candidates for photovoltaic applications. In that sense, these hybrid materials were

sent to XLIM to Dr Bouclé who will perform electronic characterizations and elaboration of

solar cells. However, we are convinced that conditions should be improved to decrease the

grafting density essential to avoid the complete coverage that is not beneficial for electron

transport.

155

Finally, the elaboration of self assembled monolayer brushes (P3HT) on the ITO surface was

achieved by applying grafting onto technique in melt as an alternative to PEDOT:PSS.

Preliminary testing the photovoltaic performances showed an increase of the Voc and Rsh in

comparison to bare ITO and revealed that the P3HT SAM layer is an efficient for hole

selectivity. In spite of that, the photovoltaic characteristics of SAM layer did not reach yet the

high performance of the PEDOT:PSS layer. Thus, an elaboration of double brushes using

fluorinated conjugated molecules and P3HT, or doping the P3HT layer, or test another

conjugated polymer could be useful to improve the power conversion efficiency of polymer

solar cells.

This research work shows the potential of the applied grafting methods concerning the

synthetic chemistries of monomers, polymers and hybrid nanomaterials and opens broad

prospects for the future. First, the versatile synthetic method (in stage one) and its simple

technique of grafting can be applied to different metal oxide surfaces with various shapes in

order to develop the quantity of materials interesting for organic electronic applications.

Second, the field of grafting low band gap polymers with different optical properties can be

started to improve the efficiency of solar cells.

Conclusions générales et perspectives

L'objectif principal de ce travail, qui était la synthèse de brosses de polymères conjugués

greffés de manière covalente sur des surfaces inorganiques, a été atteint avec succès. Ces

nouveaux matériaux hybrides organiques-inorganiques ont été conçus en fonction de leurs

propriétés électriques et optiques qui en font des candidats appropriés pour les applications

photovoltaïques. Ainsi, deux types de matériaux ont été réalisés à partir de polymères

différents P3HT et PSBTBT greffés sur de l’oxyde de zinc. La méthodologie a été démontrée

pour réaliser les nanocomposites souhaités.

Dans la première étape, trois P3HTs régioréguliers de différentes masses molaires,

fonctionnalisés par des triéthoxysilanes , ont été synthétisés par une réaction d'hydrosilylation

après modification de la fonction terminale allyle du P3HT. Ensuite, une étape de

condensation a permis de greffer le P3HT portant un groupe d'ancrage à la surface de

nanotubes d'oxyde de zinc par l'intermédiaire de la technique «grafting onto», pour obtenir le

nanocomposite ZnO@P3HT souhaité. Dans la seconde étape, un polymère à faible bande

interdite PSBTBT a été greffé de façon covalente sur des nanobatonnets de ZnO par une

procédure en trois étapes. Cette méthode consiste en la polymérisation amorcée à partir de la

surface du ZnO (greffage) pour faire la synthèse de matériaux hybrides à base de

ZnO@PSBTBT.

Les deux méthodes appliquées ont été efficaces ; une couche homogène de polymère a été

observée sur les images de microscopie TEM. Le principal avantage de la méthode simple et

robuste et directe de "grafting onto" sur la méthode "grafting through" est que des polymères

de masses molaires contrôlées peuvent être utilisés pour le greffage, aboutissant à la synthèse

de brosses de dimensions bien définies. En outre, elle permet de surmonter les inconvénients

de la méthode «grafting through" où il est difficile d’éliminer les traces de catalyseur et

d’obtenir des polymères de masses molaires élevées. Cela rend ce processus plus facile à

manipuler et plus compatible avec la fabrication des cellules. D'autre part, la caractérisation

par spectroscopie UV-visible nous permet de supposer que les brosses de polymère pour

PSBTBT et P3HT sont respectivement dans un régime de brosse et semi-dilué. Cela met en

évidence l'avantage de la technique "grafting through" sur celle de "grafting onto" en terme

de densité de greffage. Les deux types de matériaux hybrides synthétisés semblent être des

candidats potentiels pour les applications photovoltaïques. En ce sens, ces matériaux hybrides

ont été envoyés au Dr Bouclé (XLIM, Limoges) qui effectuera les caractérisations électriques

en cellules solaires. Cependant, nous sommes convaincus que les conditions doivent être

améliorées pour réduire la densité de greffage pour éviter le recouvrement complet des

nanoparticules d’oxyde métallique, qui n'est pas bénéfique pour le transport des électrons.

Enfin, l'élaboration de brosses de monocouches auto-assemblées (P3HT) sur la surface d'ITO

a été réalisée en appliquant la technique de greffage en tant qu'alternative au PEDOT: PSS.

Les tests préliminaires des performances photovoltaïques ont montré une augmentation de la

tension de circuit ouvert et la résistance Shunt, en comparaison à l’ITO « nu » et a ainsi

révélé que la monocouche de P3HT est un moyen efficace pour la sélectivité des trous. En

dépit de cela, les caractéristiques photovoltaïques de l’ITO modifié par la monocouche de

P3HT n'ont pas atteint celles obtenues avec la couche de PEDOT: PSS. Ainsi, l’élaboration

de brosses doubles à l'aide de molécules conjuguées fluorés et P3HT, ou le dopage de la

couche P3HT, ou l’utilisation d’un autre polymère conjugué pourrait être des stratégies pour

améliorer l'efficacité de conversion de puissance des cellules solaires polymères.

Ce travail de recherche montre le potentiel des méthodes de greffage à la synthèse de

nanomatériaux hybrides et ouvre de larges perspectives pour l'avenir. Tout d'abord, le

procédé de synthèse polyvalent (en une étape), et sa technique simple de greffage peut être

appliquée à différentes surfaces d'oxydes métalliques, de différentes formes, afin de

développer la quantité de matières intéressantes pour des applications électroniques

organiques. Deuxièmement, le domaine de greffage des polymères de faible bande interdite

avec des propriétés optiques différentes peut être utilisé pour améliorer l'efficacité des

cellules solaires.

Experimental Part

157

Experimental Part

1. Materials

All reactions were performed under pre-dried nitrogen using flame-dried glassware and

conventional Schlenk techniques. Syringes used to transfer reagents or solvents were purged

with nitrogen prior to use. Chemicals and reagents were used as received from Aldrich

(France) and ABCR (Germany) and stored in the glove box. Solvents (Baker, France) were

used as received; THF was distilled over sodium and benzophenone under nitrogen.

2. Instrumentations

1H and

29Si Nuclear Magnetic Resonance (NMR) spectra were recorded using a Bruker

400MHz instrument in CDCl3 at ambient temperature.

Gel Permeation Chromatography (GPC) was performed using a bank of 4 columns (Shodex

KF801, 802.5, 804 and 806) each 300 mm x 8 mm at 30 °C with THF eluent at a flow rate of

1.0 ml min-1

controlled by a Malvern pump (Viskotek, VE1122) and connected to Malvern

VE3580 refractive index (RI) and Malvern VE3210 UV-visible detectors. Calibration was

against polystyrene standards.

Thermal gravimetric analysis (TGA) was performed on a TGA Q50, TA Instruments at a

heating rate of 10 °C min-1

under nitrogen. UV-visible absorption spectra were recorded on a

Shimadzu UV-2450PC spectrophotometer.

MALDI-MS spectra were performed by the CESAMO (Bordeaux, France) on a Voyager mass

spectrometer (Applied Biosystems). The instrument was equipped with a pulsed N2 laser (337

nm) and a time-delayed extracted ion source. Spectra were recorded in the positive-ion mode

using the reflectron and with an accelerating voltage of 20 kV. Samples were dissolved in

THF at 10 mg/ml. The DCTB matrix T-2-[3-(4-t-butylphenyl)-2-methyl-2-propenylidene]

malononitrile solution was prepared at a concentration of 10 mg.mL-1

in THF. The solutions

were combined in a 10:1 volume ratio of matrix to sample. One to two microliters of the

obtained solution were deposited to the sample target and vacuum-dried. C. Absalon from

CESAMO (University of Bordeaux)

158

Emission Spectroscopy (Photoluminescence): Corrected steady-state emission and excitation

spectra were recorded at 1 nm resolution using a photon counting Edinburgh FLS920

fluorescence spectrometer with a xenon lamp. The concentrations in CHCl3 were adjusted to

an absorbance around 0.1 at 450 nm (excitation wavelength) in a 1 cm quartz fluorescence

cell (Hellma). Done by Sylvie Blanc

Transmission Electronic Microscopy. Analysis of the core@shell nanoparticles shape and the

thickness of the P3HT monolayer were obtained by Transmission Electron Microscopy

(TEM) with a JEOL JEM-2100 FX transmission electron microscope, using an accelerating

voltage of 200 kV at room temperature. Done by Marie-Hélène Delville from ICMCB

(University of Bordeaux).

Atomic Force Microscopy (AFM). AFM images were obtained on a microscope Veeco, di-

Innova model «fashion tapping». These analyzes were performed by Sadia Radiji from

IPREM-EPCP (University of Pau).

3. Chapter 2: Experimental Part

3.1 Synthesis of allyl-terminated P3HT

Allyl-terminated P3HTs of high regioregularities were

synthesized using literature procedures.1 The GRIM

method was applied to synthesize the desired polymer in a

flamed-dried 100 mL round flask bottom under inert

atmosphere at room temperature. Initially 2,5-dibromo-3-hexylthiophene (1) (3.06 mmol) and

freshly distilled THF 10 mL were added into the flask. After mixing for several minutes,

isopropyl magnesium chloride (3.06 mmol) was then added via a syringe and stirred for 2h at

room temperature. The reaction mixture was diluted to 50 mL with dried THF, and 1,3-

bis(diphenylphosphino)propane nickel-(II) chloride Ni(dppp)Cl2 (0.087 mmol for P1, 0.078

for P2 and 0.065 for P3) was added. The polymerization proceeded for 10 min before adding

allyl magnesium bromide (1.53 mmol) and then the reaction continued for another 30 min to

ensure high end-group functionalization before quenching with methanol. The resulting solid

polymer was washed by Soxhlet extraction using ethanol and acetone, and recovered with

chloroform. The three Allyl-terminated P3HT with number average molar masses (Mn

1 M. Jeffries-El, G. Sauvé, R. D. McCullough, Macromolecules 2005, 38, 10346-10352.

159

according to GPC) are P3HT (P1) [5600 g/mol, Ð = 1.14], P3HT (P2) [8000g/mol, Ð = 1.16],

P3HT (P3) [11000 g/mol, Ð = 1.1] were synthesized using the same procedure and varying

the amount of catalyst. Yield =50 %. 1H

NMR (400 MHz, CDCl3, δ (ppm): 6.98 (s, 1H), 6.0

(m, 1H), 5.15 (m, 2 H), 3.52 (d, 2H), 2.8 (t, 2H), 1.7 (q, 2H), 1.3-1.5 (m, 6H), 0.92 (t, 3H).

Figure 1. 1H NMR spectrum of allyl-terminated poly(3-hexylthiophene) with a DPn of 47 repeating units (P3).

3.2 Synthesis of triethoxysilane-terminated P3HT

In a flame-dried 50 mL flask, 100 mg of allyl-terminated

P3HT (2 eq) was mixed with 4 mg of H2PtCl6 (catalyst, 1

eq) and 15 mL of THF. The solution mixture was

degased for 15 min to avoid air. Under stirring 0.3 mL (0.26 g, 100 eq) of triethoxysilane was

added drop wise. The mixture was stirred for 30 min at room temperature before heating at

C for 5h. Finally the polymer was precipitated twice in dry ethanol, filtered under nitrogen

and stored in the glove box to avoid hydrolysis/condensation of the polymer end chain. Yield

> 90 %. 1H NMR (400 MHz, CDCl3), δ (ppm): 6.98 (s, 1H), 3.87 (q, 6H), 2.8 (t, 2H), 1.7 (q,

2H), 1.3-1.5 (m, 6H), 1.25 (t, 9H), 0.92 (t, 3H). 29

Si NMR (, CDCl3): -45.4 ((EtO)3SiC) ppm.

160

Figure 2. 1H NMR spectrum of alkoxysilane-terminated poly(3-hexylthiophene) with DPn = 47.

3.3 Grafting triethoxysilane-terminated P3HT onto ZnO nanorods

ZnO nanorods (NRs) were dispersed in THF (2 mg.mL-1

, 5 mL) by ultrasonication for 1 h. 2

ml solution of triethoxysilane-P3HT (20 mg.ml-1

) in THF was added to the mixture. From the

ZnO nanorods specific surface area (SSA, determined by BET), we calculated that the P3HT

was introduced at an excess of 2 chains/nm2

C for 12 h under inert

atmosphere. The medium was cooled to RT and ZnO@P3HT was purified by centrifugation

(10,000 rpm, 10 min) with removal of the supernatant containing excess of organic

component. The purification was repeated several times until the UV-visible spectra of the

THF supernatant became featureless (no P3HT absorption around 450 nm). The precipitated

particles were collected, dried and stored under nitrogen. A change in the color of the ZnO

NRs was clearly observable from white to violet after grafting of P3HT (dry state).

161


4.1 Synthesis of [2-(4-bromo-phenyl)-ethyl)]-triethoxysiliane

In a flame-dried 20 ml round flask bottom, 2.8 g (15.3 mmol, 1 eq) of 4-bromostyrene was

charged with 14 mg of chloroplatinic acid (H2PtCl6 catalyst, 0.027 mmol, 0.003 eq) and 2 ml

of absolute ethanol. Under stirring 7 g of triethoxysilane (42.6 mmol, 4 eq) was added

dropwise. The mixture was stirred at 70 °C for 5h. Finally the product was purified by

vacuum distillation. The resulting product (yield= 80%) is a mixture of Markonikov (17%)

and anti-Markovnikov adducts (83%).

Figure 3.1H NMR spectrum of [2-(4-bromo-phenyl)-ethyl)]-triethoxysilane

162

1H NMR (400 MHz, CDCl3) of the major adduct, δ (ppm): 7.4 (d, 2H), 7.1 (d, 2H), 3.9-3.83

(q, 6H), 2.74-2.7 (m, 2H), 1.22 (t, 9H), 0.99-0.96 (m, 2H). 1H NMR (400 MHz, CDCl3) of the

minor adduct, 7.39 (d, 2H), 7.08 (d, 2H), 3.77-3.75 (q, 6H), 2.66-2.63 (m, 2H), 1.4 (d, 3H),

1.16 (t, 9H).13

C NMR (CDCl3, 100MHz) of major adduct , δ (ppm): δ 143.56, 129.8, 129.3,

119.3, 58.34, 28.7, 18.4, 12.47. 13

C NMR (CDCl3, 100MHz) of minor adduct, δ 143.1, 131.17,

129.6, 118.4, 59.16, 25.76, 18.25, 15.4.

4.2 Synthesis of 4,4‘-Bis (2-ethyl-hexyl)-5,5 '-bis(trimethyltin)-dithieno[3,2-

b:2 ',3 '-d]silole

To a solution of 4 4’-Bis(2-ethyl-hexyl)- ’-dibromo-dithieno[3,2- b:2',3'-d]silole (1.2 g,

2.51 mmol) in anhydrous THF (8 ml), a 2.5 M solution of n-butyllithium in hexane (4 ml, 10

mmol) was added slowly at -78°C under nitrogen. The reaction proceeded for 2h at -78°C

before adding trimethyltinchloride (16 ml, 16 mmol) in one portion. After removing the

cooling bath, the mixture reaches ambient temperature and continues stirring for overnight.

Then the mixture was poured into 50 ml of deionized (DI) water and extracted by 60 ml of

diethyl ether. The organic layer was washed with DI water (5 × 20 ml), dried over anhydrous

Na2SO4, filtered and the solvent was removed by rotary evaporation. The crude product was

placed under high vacuum for 72 hours yielding a yellow-brown viscous oil (yield = 96%)

which was used in the next step without any further purification.

1H NMR (CDCl3, 400MHz), δ (ppm): δ 7.03(s, 2H), 1.68(m, 2H), 1.4-1.13(m, 16H), 0.90(t,

6H), 0.83(t, 6H), 0.74(m, 4H).13

C NMR (CDCl3, 100MHz), δ (ppm): δ 154.66, 143.89,

137.91, 137.42, 35.91, 35.61, 28.97, 28.89, 23.05, 17.83, 14.20, 10.85, -8.18.

3.3 Synthesis of 4,7-dibromo-2,1,3-benzothiadiazole monomer (M2)

The synthesis has been done following procedure reported in the literature.2

2 Neto, B. A. D.; Lopes, A. S.; Wüst, M.; Costa, V. E. U.; Ebeling, G.; Dupont, J., Tetrahedron Letters 2005, 46

(40), 6843-6846.

163

4.3 Surface functionalization of Zinc Nanorods by [2-(4-bromo-phenyl)-

ethyl)]-triethoxysiliane. (ZnO-PhBr)

First zinc oxide nanorods with an average specific surface area of 24 m2/g were dried at 110

°C in an oven for 24h to remove adsorbed water. Then in 10 ml flamed-dried round flask

bottom we disperse the nanoparticles in 4 ml of anhydrous toluene in ultrasonication bath for

1h. After complete dispersion of the nanoparticles we add 200 mg (0.57 mmol) of [2-(4-

bromo-phenyl)-ethyl)]-triethoxysiliane to the mixture and refluxed for 24h at 120 °C. The

modified nanoparticles were purified by several centrifugations and redispersions in toluene.

The nanoparticles stored in the glovebox after drying the solvent under reduced pressure.

4.5 Preparation of the ini tiating sites on ZnO nanorods (ZnO-C6H4–Pd2(dba)2–

Br).

In nitrogen filled glovebox, 10 ml high pressure tube equipped with a sealed septum was

charged with magnetic stirrer, 100 mg of the modified ZnO NRs, 2 ml of anhydrous THF, and

10 mg (0.011 mmol) of (Pd2(dba)3). The mixture was heated at 60 °C for 6h. The

nanoparticles were cleaned by several centrifugations in anhydrous THF solutions. The

desired product were dried under reduced pressure and stored in the glovebox to be used

directly.

4.6 Polycondensation reaction from the Zinc oxide Nanorods: grafting low

bandgap (PSBTBT)

In a 10 mL high pressure tube equipped with a sealed septum were added 100 mg of (ZnO-

C6H4–Pd2(dba)3–Br , 4,7-dibromo-2,1,3-benzothiadiazole (BT) (59.2 mg, 0.2 mmol), 4,4‘-Bis

(2-ethyl-hexyl)-5,5'-bis(trimethyltin)-dithieno[3,2-b:2',3'-d]silole (DTSSn) (150 mg, 0.2

mmol), tri(o-tolyl)phosphine (6.135 mg, 0.1eq) and dissolved in 2 ml of anhydrous

chlorobenzene solution in the glovebox. The mixture was sonicated for 1h to disperse

Br2

HBr

164

completely the nanoparticles. Then the tube was subjected to heating at 150 °C for 2h. After

cooling down the mixture the particles were cleaned by several centrifugations in chloroform

solution then dried and stored in the glovebox. The molar mass of the free polymer chain is

3600 g.mol-1

with Ð = 1.2. Under the same experimental conditions we repeat this experiment

with increasing the reaction time to 4h and 6h. The increase in the molar mass was clearly

seen by the color change of the mixture to greenish and the increase in viscosity for both

samples and was proved by GPC and UV-visible.

4.7 Synthesis of ly[(4 4’-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-

diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl], PSBTBT by classical

polymerization

In a 10 mL high pressure tube equipped with a

sealed septum were added 4,7-dibromo-2,1,3-

benzothiadiazole (BT) (118.47 mg, 0.4mmol), 4,4‘-Bis

(2-ethylhexyl)-5,5'-bis(trimethyltin)-dithieno[3,2-b:2',3'-d]silole (DTSSn) (300 mg,

0.4mmol), tris(dibenzylideneacetone) dipalladium(0) (7.38 mg, 0.02 eq), tri(o-tolyl)phosphine

(12.27 mg, 0.1eq) and dissolved in 2 ml of anhydrous chlorobenzene solution in the glovebox.

The tube was subjected to heating at 140 °C for 24h. After cooling to room temperature, the

resulting viscous liquid was dissolved in hot chlorobenzene then added slowly into a

vigorously stirred cold methanol. The solid was filtered through a Soxhlet thimble and then

subjected to Soxhlet extraction with methanol, acetone, cyclohexane, chloroform. The

cyclohexane and chloroform fractions were concentrated and precipitated into methanol, and

the precipitant was filtered and dried under high vacuum to afford PSBTBT as a dark-blue

solid (Yield= 81%).

GPC (THF, PS Standards): (Chloroform) fraction Mn = 19000 g mol-1

, Đ = 2.47

(Cyclohexane) fraction Mn = 9000 g mol-1

, Đ = 2

The reaction proceeded again under the same conditions for longer time 48 h to afford

PSBTBT as a dark-blue solid (Yield= 93%).

GPC (THF, PS Standards): (Chloroform) fraction Mn = 25300 g mol-1

, Đ = 2.88

(Cyclohexane) fraction Mn = 9700 g mol-1

, Đ = 2.3

165

Figure 6.

1H NMR spectrum of PSBTBT (Mn = 9000 g mol

-1, 400 MHz, in C2D2Cl4, 80 °C).


5.1 Preparation of P3HT SAMs on ITO substrates

Indium tin oxide (ITO) - l l (1 Ω/ q K ) w ly l

in acetone, ethanol and iso-propanol for 15 min under ultrasound at 40 °C. After drying the

substrates with air flow, UV-ozone treatment (15 min) was applied to the substrates in order

to increase the hydrophilic nature of the surface and to remove residual organic

contamination. The same experimental procedure developed in chapter 2 was applied for the

synthesis of P1-Si and P2-Si with different macromolecular characteristics. Then grafting of

the polymers-Si onto the cleaned substrates was performed from melt. A layer of P3HT-Si

was dip coated on the cleaned ITO substrate and annealed at 170 °C for 3h under inert

atmosphere. The grafted substrates were subjected to ultrasonication in chloroform for 15 min

3 times to remove the free polymer (ungrafted) and dried under nitrogen. The grafted

substrates were stored in the glove box under nitrogen to prevent any degradation of the

SAMs.

a

b

ab

166

5.2 Fabrication of photovoltaic devices

Three types of organic solar cells were fabricated and tested according to the

following procedure (Figure 9). The previous prepared substrates with SAMs as a hole

selective layer is compared to ITO substrate without any modification and to ITO coated with

the water dispersion of poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate)

(PEDOT:PSS, Sigma-Aldrich, spin-coated at 4000 rpm during 50 s, followed by a thermal

treatment at 100 °C for 30 min to remove residual moisture, layer thickness was around 40

nm). All further device elaboration and characterization steps were carried out under inert

atmosphere (N2) in glovebox. The active layer was composed of P3HT (50 000 g.mol-1

):

PCBM mixed in a 1:1 weight ratio in chlorobenzene (C = 20 mg.ml-1

) and solubilized on a hot

plate at 50 °C overnight. The solution was then spin-coated on the PEDOT:PSS layer (1000

rpm over 50 s), and the samples were left to dry for about one hour for an efficient solvent

annealing. Finally, a calcium (20 nm), aluminum (80 nm) top electrode (cathode) was

thermally evaporated under secondary vacuum (10-6

mbar) through a shadow mask. The

current density-voltage (J-V) characteristics of the cells were measured with a Keithley 2400

under illumination using an AM1.5 solar simulator set at 100 mW/cm², with an IL1400BL

calibrated radiometer.

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