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
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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
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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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-
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
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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
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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
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5. References
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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
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
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17. Bijleveld, J. C.; Gevaerts, V. S.; Di Nuzzo, D.; Turbiez, M.; Mathijssen, S. G. J.; de Leeuw, D. M.; Wienk, M. M.; Janssen, R. A. J., Efficient Solar Cells Based on an Easily Accessible Diketopyrrolopyrrole Polymer. Advanced Materials 2010, 22 (35), E242-E246. 18. Chen, H. Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li, G., Polymer solar cells with enhanced open-circuit voltage and efficiency. Nature Photonics 2009, 3 (11), 649-653. 19. Cabanetos, C.; El Labban, A.; Bartelt, J. A.; Douglas, J. D.; Mateker, W. R.; Fréchet, J. M. J.; McGehee, M. D.; Beaujuge, P. M., Linear Side Chains in Benzo[1,2-b:4,5-b′] p –Thieno[3,4-c]pyrrole-4,6-dione Polymers Direct Self-Assembly and Solar Cell Performance. Journal of the American Chemical Society 2013, 135 (12), 4656-4659. 20. Manceau, M.; Bundgaard, E.; Carle, J. E.; Hagemann, O.; Helgesen, M.; Sondergaard, R.; Jorgensen, M.; Krebs, F. C., Photochemical stability of [small pi]-conjugated polymers for polymer solar cells: a rule of thumb. Journal of Materials Chemistry 2011, 21 (12), 4132-4141. 21. Mühlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.; Gaudiana, R.; Brabec, C., High Photovoltaic Performance of a Low-Bandgap Polymer. Advanced Materials 2006, 18 (21), 2884-2889. 22. Scharber, M. C.; Koppe, M.; Gao, J.; Cordella, F.; Loi, M. A.; Denk, P.; Morana, M.; Egelhaaf, H.-J.; Forberich, K.; Dennler, G.; Gaudiana, R.; Waller, D.; Zhu, Z.; Shi, X.; Brabec, C. J., Influence of the Bridging Atom on the Performance of a Low-Bandgap Bulk Heterojunction Solar Cell. Advanced Materials 2010, 22 (3), 367-370. 23. Albrecht, S.; Janietz, S.; Schindler, W.; Frisch, J.; Kurpiers, J.; Kniepert, J.; Inal, S.; Pingel, P.; Fostiropoulos, K.; Koch, N.; Neher, D., Fluorinated Copolymer PCPDTBT with Enhanced Open-Circuit Voltage and Reduced Recombination for Highly Efficient Polymer Solar Cells. Journal of the American Chemical Society 2012, 134 (36), 14932-14944. 24. Li, Z.; Tsang, S.-W.; Du, X.; Scoles, L.; Robertson, G.; Zhang, Y.; Toll, F.; Tao, Y.; Lu, J.; Ding, J., Alternating Copolymers of Cyclopenta[2,1-b;3,4-b′] p T [3 4-c]pyrrole-4,6-dione for High-Performance Polymer Solar Cells. Advanced Functional Materials 2011, 21 (17), 3331-3336. 25. Li, W.; Hendriks, K. H.; Roelofs, W. S. C.; Kim, Y.; Wienk, M. M.; Janssen, R. A. J., Efficient Small Bandgap Polymer Solar Cells with High Fill Factors for 300 nm Thick Films. Advanced Materials 2013, 25 (23), 3182-3186. 26. Liao, S.-H.; Jhuo, H.-J.; Cheng, Y.-S.; Chen, S.-A., Fullerene Derivative-Doped Zinc Oxide Nanofilm as the Cathode of Inverted Polymer Solar Cells with Low-Bandgap Polymer (PTB7-Th) for High Performance. Advanced Materials 2013, 25 (34), 4766-4771. 27. Espinet, P.; Echavarren, A. M., The Mechanisms of the Stille Reaction. Angewandte Chemie International Edition 2004, 43 (36), 4704-4734. 28. Stille, J. K., Palladium-katalysierte Kupplungsreaktionen organischer Elektrophile mit Organozinn-Verbindungen. Angewandte Chemie 1986, 98 (6), 504-519. 29. Carothers, W. H., Polymers and polyfunctionality. Transactions of the Faraday Society 1936, 32 (0), 39-49. 30. Slade Jr, P. E., INTRODUCTION. Polym Mol Weights, Pt 1 1975, 1-8. 31. Liu, J.; Zhang, R.; Sauvé, G.; Kowalewski, T.; McCullough, R. D., Highly Disordered Polymer Field Effect Transistors: N-Alkyl Dithieno[3,2-b:2′ 3′-d]pyrrole-Based Copolymers with Surprisingly High Charge Carrier Mobilities. Journal of the American Chemical Society 2008, 130 (39), 13167-13176. 32. Neto, B. A. D.; Lopes, A. S.; Wüst, M.; Costa, V. E. U.; Ebeling, G.; Dupont, J., Reductive sulfur extrusion reaction of 2,1,3-benzothiadiazole compounds: a new methodology using NaBH4/CoCl2·6H2O(cat) as the reducing system. Tetrahedron Letters 2005, 46 (40), 6843-6846. 33. Tierney, S.; Heeney, M.; McCulloch, I., Microwave-assisted synthesis of polythiophenes via the Stille coupling. Synthetic Metals 2005, 148 (2), 195-198. 34. Hunt, A. J.; Budarin, V. L.; Comerford, J. W.; Parker, H. L.; Lazarov, V. K.; Breeden, S. W.; Macquarrie, D. J.; Clark, J. H., Deposition of palladium nanoparticles in SBA-15 templated silica using supercritical carbon dioxide. Materials Letters 2014, 116 (0), 408-411.
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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
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