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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
Université de Pau et Pays de l’Adour
Université de Pau et Pays de l’Adour
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
Table of contents chapter 2
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
Table of content 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……………………………………………………………………………………………………………………….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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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,
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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”.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
cycloaddition P3HT 3 000
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
44
Si N3
-C≡C (e) Huisgen
cycloaddition P3HT 5 900
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
cycloaddition P3HT 5 900
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
cycloaddition P3HT 5 100
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
cycloaddition P3HT 5 500
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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).
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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:
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
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.
Chapter 1: Conjugated-Polymer Grafting on Inorganic and Organic Substrates
58
6. References
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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
Chapter 2: Versatile Functional Poly(3-hexylthiophene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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
Chapter 2: Versatile Functional Poly(3-hexylthiophene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
Table of contents chapter 2
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
Grafting Onto technique: Core@Shell ZnO nanorods
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.
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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-
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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.
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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.
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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.
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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)
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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.
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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.
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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.
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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)
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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.
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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)
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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)
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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.
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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.
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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.
Chapter 2: Versatile Functional Poly(3-hexylthiopehene) for Hybrid Particles Synthesis by the
Grafting Onto technique: Core@Shell ZnO nanorods
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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.
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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.
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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)
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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.
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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.
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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)
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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.
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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).
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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)
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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.
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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.
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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).
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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.
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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.
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
129
7. References
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Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
130
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.
Chapter 3: Surface Initiated Polymerization of A-A/B-B type Conjugated Monomers: Towards
the First Low Band-Gap Polymer Brushes.
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
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
Table of content 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
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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)
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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).
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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)
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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°
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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.
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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.
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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.
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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)
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
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.
Chapter 4: Poly(3-hexylthiophene) brushes as hole transporting layer of Organic Solar Cells
152
5. References
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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. Chapter 3: Experimental Part
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-ethyl- hexyl)-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. Chapter 4: Experimental Part
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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