Indolo[3,2-b]carbazole-based alternating donor-acceptor copolymers: Synthesis, properties and photovoltaic application

Article abstract of DOI:10.1039/b912258c

Two new donor-acceptor (D-A) type copolymers, poly{5,11-di(1-decylundecyl) indolo[3,2-b]carbazole-3,9-diyl-alt-4,7-dithien-2-yl-2,1,3-benzothiadiazole- 5′,5″-diyl} (PIC-DTBT) and poly{5,11-di(1-decylundecyl)indolo[3,2-b] carbazole-3,9-diyl-alt-4,7-di(2,2′-bithien-5-yl)-2,1,3-benzothiadiazole- 5′,5″-diyl} (PIC-DT2BT) were synthesized. The polymers have an alternating structure consisting of a large-size fused aromatic ring, indolo[3,2-b]carbazole (IC), as the donor segment and 4,7-dithien-2-yl-2,1,3- benzothiadiazole (DTBT) or 4,7-di(2,2′-bithien-5-yl)-2,1,3- benzothiadiazole (DT2BT) as the acceptor segment. The absorption spectra and the electrochemical properties of PIC-DTBT and PIC-DT2BT indicate that these copolymers have suitable energy levels for photovoltaic application. Polymer photovoltaic devices based on blends of the copolymers and [6,6]-phenyl C ₆₁ butyric acid methyl ester (PCBM) have a high open-circuit voltage (>0.9 V) under the illumination of AM 1.5 (100 mW cm^-2). The higher power conversion efficiency (PCE) of PIC-DT2BT (2.07%) compared to that of PIC-DTBT (1.47%) suggests that DT2BT could be a promising building block for the efficient D-A type copolymers if the low solubility is overcome by the molecular design of the donor segment. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b912258c

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Indolo[3,2-b]carbazole-based alternating donor–acceptor copolymers:
synthesis, properties and photovoltaic application
a
Erjun Zhou, Shimpei Yamakawa, Yue Zhang, Keisuke Tajima, Chunhe Yang and Kazuhito Hashimoto
b
b
b
a
ab
*
Received 22nd June 2009, Accepted 20th August 2009
First published as an Advance Article on the web 3rd September 2009
DOI: 10.1039/b912258c
Two new donor–acceptor (D–A) type copolymers, poly{5,11-di(1-decylundecyl)-
indolo[3,2-b]carbazole-3,9-diyl-alt-4,7-dithien-2-yl-2,1,3-benzothiadiazole-50,500-diyl} (PIC-DTBT)
and poly{5,11-di(1-decylundecyl)indolo[3,2-b]carbazole-3,9-diyl-alt-4,7-di(2,20-bithien-5-yl)-
2,1,3-benzothiadiazole-50,500-diyl} (PIC-DT2BT) were synthesized. The polymers have an alternating
structure consisting of a large-size fused aromatic ring, indolo[3,2-b]carbazole (IC), as the donor
segment and 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT) or 4,7-di(2,20-bithien-5-yl)-2,1,3-
benzothiadiazole (DT2BT) as the acceptor segment. The absorption spectra and the electrochemical
properties of PIC-DTBT and PIC-DT2BT indicate that these copolymers have suitable energy levels
for photovoltaic application. Polymer photovoltaic devices based on blends of the copolymers
and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) have a high open-circuit voltage (>0.9 V) under
the illumination of AM 1.5 (100 mW cmꢀ2). The higher power conversion efficiency (PCE) of
PIC-DT2BT (2.07%) compared to that of PIC-DTBT (1.47%) suggests that DT2BT could be
a promising building block for the efficient D–A type copolymers if the low solubility is overcome by
the molecular design of the donor segment.
was bridged with an alkylamino group, and the alternating
copolymer combined with DTBT as the acceptor segment
(PDTP-DTBT). The PSC based on PDTP-DTBT have
a moderate PCE of 2.18%,12 which completed the synthesis and
evaluation in PSCs of the possible combinations for this class of
materials, as summarized in Table 1.
Introduction
In the past few years, polymer solar cells (PSCs) have attracted
growing attention1 as a potential renewable energy technology
based on their simple and low-cost manufacturing processes and
capability to fabricate flexible large-area devices.2 From the early
days of these studies, poly(p-phenylenevinylene)s (PPVs)3 and
polythiophenes (PTs)4 have often been used as the electron donor
materials in bulk heterojunction type PSCs. However, the upper
limits of the PCEs obtained by using PPV or P3HT seem to have
been reached due to their relatively high-lying HOMO energy
levels, large band gaps and the limited possibilities to alter their
electronic properties through structural modifications.
To further improve the absorption and electronic properties of
the polymers used in PSCs, extension of the conjugation plane in
the polymer chains would be a promising strategy. However, the
solubility of the polymers in the organic solvents would decrease
if the strategy was adopted without careful consideration of the
molecular design. In fact, 4,7-di(2,20-bithien-5-yl)-2,1,3-benzo-
thiadiazole (DT2BT), in which two extra unsubstituted thio-
phene rings are attached to the ends of DTBT, has not been
investigated as an acceptor segment, likely due to the poor
solubility of the resulting polymers. However, the extended
conjugation of the thiophene units is expected to improve the
device performance. Therefore, it is necessary to explore the
material designs that allow for both solubility of the polymer and
extension of the conjugations of the main chain.
Therefore, the design of novel photovoltaic polymers with
low-band-gaps and broader absorptions is one of the major
challenges today.5 A promising strategy for the design of low-
band-gap polymers is to use the strategy of incorporating donor
(electron-rich) and acceptor (electron-deficient) segments alter-
natively in a conjugated polymer chain.6 For the acceptor
segments, 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT) is one
of the most effective building blocks to date for tuning the
energy levels and the absorption spectra of the resulting D–A
copolymers for use in photovoltaic devices (Table 1). For
the donor segments, (hetero)aromatic structures based on
biphenyl or bisthiophene with a bridging atom (C, S, or N)
have been extensively studied.7–11 Recently, we synthesized
dithieno[3,2-b:20,30-d]pyrrole (DTP) in which the bisthiophene
In this work, we synthesized a derivative of indolo[3,2-b]-
carbazole (IC) and used it as the donor segment in D–A copol-
ymers. IC has a fused ring structure of carbazole that could
extend the conjugation and also promote the close packing of the
molecules in the films. After their first utilization in the organic
electronics reported by Ong et al.,13 IC units have been utilized in
field effect transistors (FETs)14 and in PSCs.15 In the present
molecular design, two bulky 1-decylundecyl groups were
attached to the nitrogen atoms of the IC segments to compensate
for the low solubility induced by the extension of the conjugation
in the main chain. This donor design enabled the use of not only
DTBT but also DT2BT as the acceptor segment, resulting in the
aHashimoto Light Energy Conversion Project, ERATO, Japan Science and
Technology Agency (JST), 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656,
Japan. E-mail: hashimoto@light.t.u-tokyo.ac.jp
bDepartment of Applied Chemistry, School of Engineering, The University
of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
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successful synthesis of two new soluble D–A type copolymers,
poly{5,11-di(1-decylundecyl)indolo[3,2-b]carbazole-3,9-diyl-alt-
4,7-dithien-2-yl-2,1,3-benzothiadiazole-50,500-diyl} (PIC-DTBT)
and poly{5,11-di(1-decylundecyl)indolo[3,2-b]carbazole-3,9-diyl-
alt-4,7-di(2,20-bithien-5-yl)-2,1,3-benzothiadiazole-50,500-diyl} (PIC-
DT2BT), respectively. Photovoltaic devices were fabricated with
the polymers as the electron donor and [6,6]-phenyl C61 butyric
acid methyl ester (PCBM) as the acceptor using the blend bulk-
heterojunction approach.
Material synthesis
4,7-Di(2-bromothien-5-yl)-2,1,3-benzothiadiazole (monomer 1)
was synthesized according to reported procedures.16 A similar
route was used to obtain 4,7-di(50-bromo-2,20-bithien-5-yl)-2,1,3-
benzothiadiazole (monomer 4). 3,9-Dibromoindolo[3,2-b]carba-
zole (monomer 6) was synthesized by following a reported
method.14a The alkylation reaction was carried out in a mixed
solvent (THF and DMSO) with the tosylate derivative
(compound 5). The long alkyl chains with a Y-shaped structure
(1-decylundecyl) were used to give a high solubility to the IC
segments. Compound 8 was synthesized through a double
lithiation of compound
7 with n-BuLi and subsequent
quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane (Scheme 1). The alternating copolymers of PIC-DTBT
and PIC-DT2BT were synthesized via a Suzuki coupling reac-
tion. PIC-DTBT and PIC-DT2BT are readily soluble in common
organic solvents such as chloroform, THF, toluene and chloro-
benzene. The molecular weight and polydispersity index (PDI) of
the copolymers were determined by gel permeation chromatog-
raphy (GPC) analysis with a polystyrene standard calibration.
PIC-DTBT and PIC-DT2BT had weight-averaged molecular
weights (Mw) of 33.9 kg molꢀ1 and 13.7 kg molꢀ1, respectively,
and PDIs of 1.94 and 1.65, respectively. The lower molecular
weight of PIC-DT2BT compared to that of PIC-DTBT is
possibly the result of the poorer solubility and lower reactivity of
the monomer 4 in the Suzuki reaction compared to the monomer
1 (Scheme 2).
Optical properties
The normalized UV-vis absorption spectra of PIC-DTBT and
PIC-DT2BT in CHCl3 solution are shown in Fig. 1. The
absorption of both PIC-DTBT and PIC-DT2BT have two
absorption bands, which is in accordance with other D–A
copolymers.7–12 The peak at the longer wavelength could be
attributed to the intramolecular charge-transfer (ICT) transition,
and the other is possibly the result of higher energy transitions.
The shapes of the absorption spectra in a film are similar to those
in solutions and are shown in Fig. 2. The optical data are
summarized in Table 2. The absorption peak of the ICT transi-
tion of PIC-DT2BT is at a wavelength 10 nm longer than that of
PIC-DTBT both in solutions and films, indicating the extension
of the conjugation by the introduction of the two additional
thienyl units. The absorption peaks at the longer wavelength in
the films of both polymers have a red-shift of 10 nm compared
with those in solution, indicating the presence of intermolecular
interactions in the solid state. The interactions seem to be not as
strong as those of P3HT or other D–A type polymers, which is
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Scheme 1 Synthetic routes for acceptor and donor segments.
likely an effect of the two bulky side chains at the IC segments.
The optical band gaps (Eogpt) of the two polymers are estimated
from the absorption onset and summarized in Table 2.
same for both polymers, indicating fast relaxation to stable
excited states in which the electrons are localized in the IC units.
The PL peak of PIC-DT2BT has a red shift of 10–12 nm relative
to that of PIC-DTBT, which provides further evidence of the
extended conjugation resulting from the introduction of the
thiophene units.
The photoluminescence (PL) spectra of the two copolymers in
CHCl3 solution are also shown in Fig. 1. The PL spectra with
excitations at two different absorption maxima are almost the
Scheme 2 Synthetic route for D–A copolymers using Suzuki reactions.
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Fig. 1 Uv-vis absorption spectra and PL spectra of two copolymers in
Fig. 3 Cyclic voltammogram of the polymer films on a platinum plate in
an acetonitrile solution of 0.1 mol Lꢀ1 [Bu4N]PF6 (Bu ¼ butyl) at a scan
CHCl3 solution.
rate of 50 mV sꢀ1
.
observed, which was different from the behavior of the poly-
thiophene derivatives.17 From the values of the onset oxidation
ox
on
red
on
potential (E ) and onset reduction potential (E ) of the poly-
mers, the HOMO and LUMO energy levels as well as the elec-
trochemical band gaps (EEgC) were calculated (Table 2). EgEC
coincides with the optical band gaps estimated from the UV-vis
absorption onset for both copolymers.
Compared with other copolymers containing indolo[3,2-
b]carbazole,15 the HOMO energy levels of PIC-DTBT and PIC-
DT2BT are quite low-lying. It is now widely accepted that the
maximum VOC of photovoltaic device is related to the energy
difference between the LUMO of the acceptor and the HOMO of
the donor,18 therefore, PSCs based on PIC-DTBT and PIC-
DT2BT are expected to have high open-circuit voltages (VOC).
Fig. 2 Uv-vis absorption spectra of two copolymers in films.
Photovoltaic properties
The bulk heterojunction PSCs were fabricated with a device
structure of ITO/PEDOT:PSS/Polymer:PCBM/Al. In order to
accurately evaluate the PCEs of the photovoltaic devices, it is
essential to measure the device area correctly.19 For this purpose,
the effective areas of the PSCs, contributing to photocurrent,
were defined using a metal photomask during irradiation with
simulated solar light (Scheme 3).
Electrochemical properties
The electrochemical properties of the two copolymers films on
a Pt electrode were investigated by cyclic voltammetry (CV). The
CV curves were recorded and referenced to an Ag/Ag+ (0.01 M
AgNO3 in acetonitrile) electrode. In order to obtain accurate
redox potentials, the reference electrode was calibrated by the
ferrocene/ferrocenium (Fc/Fc+) redox couple (4.8 eV below the
vacuum level). As shown in Fig. 3, both the copolymers under-
went reversible oxidation–reduction processes. In the reduction–
re-oxidation processes, however, a reduction peak was not
It has been proposed that building an interpenetrated network
of donors and acceptors inside a film is critical for obtaining
high-performance photovoltaic devices.20 Building inter-
penetrated networks relates to many factors such as the ratio of
Table 2 Optical and electrochemical properties of PIC-DTBT and PIC-DT2BT
UV-vis absorption spectra
Cyclic voltammograms
In film
p-doping
n-doping
In CHCl3
lmax/nm
ox
on
red
on
Polymer
lmax/nm
l
onset/nm
Eogpt/eV
E
/V
HOMO/eV
E
/V
LUMO/eV
EEgC /eV
PIC-DTBT
PIC-DT2BT
397, 531
415, 541
398, 540
418, 551
650
670
1.91
1.85
0.65
0.63
ꢀ5.45
ꢀ5.43
ꢀ1.25
ꢀ1.18
ꢀ3.55
ꢀ3.62
1.90
1.81
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Scheme 3 Schematic representation of the polymer/fullerene bulk-het-
erojunction PSCs.
the polymer to PCBM in the blend and post-annealing treatment
at suitable temperatures. In this system, the ratio of PIC-DTBT
(or PIC-DT2BT) to PCBM was varied between 1 : 1 and 1 : 4
(wt/wt). The devices that were post-annealed at 110 ꢁC for 10 min
produced the best results in each case. This post-annealing
treatment has been reported to induce moderate phase separa-
tion between the polymers and PCBM, which results in the
construction of better pathways for both holes and electrons in
the case of the P3HT:PCBM system. Fig. 4 shows atomic force
microscopy (AFM) phase images of PIC-DTBT/PCBM (1 : 3,
wt/wt) composite films with post-annealing at 110 ꢁC and 130 ꢁC.
The film had low roughness (Ra ¼ 0.29 nm) and no significant
aggregation after post-annealing at 110 ꢁC. However, post-
annealing temperatures higher than 110 ꢁC promoted the
aggregation of PCBM and the formation of large PCBM crystals
that possibly degraded device performance. Similar phenomena
were also observed in the PIC-DT2BT/PCBM system.
Fig. 5 I–V curves of the PSC based on PIC-DTBT and PCBM with
different mixing ratios under the illumination of AM 1.5, 100 mW cmꢀ2
.
efficiency (PCE) for different ratios of polymers and PCBM is
summarized in Table 3.
The PCEs of photovoltaic devices based on PIC-
DT2BT:PCBM were higher than those of the PIC-DTBT:PCBM
system for all mixing ratios: this can be attributed to the higher
values of ISC and FF. These results suggest that increasing the
number of unsubstituted thienyl units in D–A copolymers could
be simple and effective method for improving the photovoltaic
performance of PSCs, especially for DTBT-based polymers.
The VOC values of the devices based on PIC-DTBT:PCBM
and PIC-DT2BT:PCBM are around 0.9 V, which is almost the
maximum value deduced from the difference between the
HOMO levels of the copolymers (PIC-DTBT: ꢀ5.45 eV; PIC-
DT2BT: ꢀ5.43 eV) and the LUMO level of PCBM (-4.3 eV).1e
The values of VOC are similar with those of some D–A deriva-
tives, such as DTBT copolymerized with fluorene (ꢂ1.0 V),7
dibenzosilole (0.9–0.97 V)8 or carbazole (0.88 V),9 but higher
than those of other D–A derivatives, such as DTBT copoly-
merized with cyclopenta[2,1-b:3,4-b0]dithiophene (0.6 V),10
dithienosilole (0.44 V)11 or dithieno[3,2-b:20,30-d]pyrrole
(0.52 V)12 The comparison between these analogs suggests that
the VOC values of photovoltaic devices are also related to the
fused structure of the donor in the copolymers with DTBT. It
seems that bridged biphenyl segments give higher VOC than the
bridged bisthiophene segments. This result will help us to design
new photovoltaic polymers with higher VOC. Interestingly, the
VOC of the cells with PIC-DT2BT (0.88–0.96 V) is higher than
that with P(InCzTh2BTD) (0.69 V) that has the same conjugated
polymer backbone with different alkyl chains15a This result
suggests not only the main chain but also the side chain of the
polymer can affect the VOC, possibly through the main chain
distortion or the change of the mixing morphology in the films.
Similar phenomena can be also found in the PSCs with poly-
thiophene derivatives that showed the variations of VOC by
modifying the side chains.5a,17,21 Considering the high PCEs of
copolymers of DTBT in combination with fluorene, dibenzosi-
lole or carbazole (Table 1), copolymers of these donors segments
in combination with DT2BT are expected to be promising if the
problem of solubility can be solved by material design.
Fig. 5 shows the I–V curve of the best devices based on
PIC-DTBT:PCBM (1 : 3, wt/wt) and PIC-DT2BT:PCBM
(1 : 3, wt/wt) under the illumination of AM 1.5 (100 mW cmꢀ2).
The thicknesses of the active layers were 60 and 70 nm, respec-
tively, measured by surface profilometry. The photovoltaic
performance as measured by the open-circuit voltage (VOC),
short-circuit current (ISC), fill factor (FF) and power conversion
Fig. 4 AFM phase images of PIC-DTBT/PCBM (1 : 3, wt/wt) composite
film with post-annealing (a, b) at 110 ꢁC and (c, d) at 130 ꢁC.
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Table 3 Device characteristics of PSCs based on PIC-DTBT:PCBM and PIC-DT2BT:PCBM
Active layer (wt/wt)
VOC/V
ISC/mA cmꢀ2
FF
PCE (%)
PIC-DTBT:PCBM (1 : 1)
PIC-DTBT:PCBM (1 : 2)
PIC-DTBT:PCBM (1 : 3)
PIC-DTBT:PCBM (1 : 4)
PIC-DT2BT:PCBM (1 : 1)
PIC-DT2BT:PCBM (1 : 2)
PIC-DT2BT:PCBM (1 : 3)
PIC-DT2BT:PCBM (1 : 4)
0.98
0.90
0.94
0.92
0.96
0.88
0.90
0.88
1.24
3.33
3.77
2.75
2.83
4.21
4.83
3.59
0.26
0.43
0.41
0.40
0.30
0.45
0.48
0.47
0.32
1.29
1.47
1.01
0.82
1.68
2.07
1.48
which is 40% higher than that of PIC-DTBT (1.47%). Both
polymers had high VOC values of 0.9 V, which can be attributed
to the large difference between the HOMO level of the polymers
and the LUMO level of PCBM. These results suggest that the
DT2BT acceptor segment could be a promising building block
for the design of efficient D–A-type photovoltaic polymers.
Experimental
Materials and characterization
All chemicals were purchased from Alfa, Aldrich or Wako and
used without further purification. 4,7-Di(2-bromothien-5-yl)-
2,1,3-benzothiadiazole (monomer 1),16 toluene-4-sulfonic acid
1-decylundecyl ester (monomer 5)22 and 3,9-dibromoindolo[3,2-b]-
carbazole (monomer 6)14a were synthesized according to similar
procedures in the literature.
1H NMR (400 MHz) spectra were measured using a JEOL
Alpha FT-NMR spectrometer equipped with an Oxford super-
conducting magnet system. Gel permeation chromatography
(GPC) was performed on a Shimadzu Prominence system
equipped with a UV detector using CHCl3 as the eluent. The
sample solutions were filtered with a PTFE filter (pore size: 0.2
mm) before injection. Absorption spectra were measured using
a SHIMADZU spectrophotometer MPC-3100. Cyclic voltam-
mograms (CVs) were recorded on an HSV-100 (Hokuto Den-
kou) potentiostat. A Pt plate coated with a thin polymer film was
used as the working electrode. A Pt wire and an Ag/Ag+ (0.01 M
of AgNO3 in acetonitrile) electrode were used as the counter and
reference electrodes (calibrated by Fc/Fc+), respectively. AFM
measurements were carried out using a Digital Instrumental
Nanoscope 31 operated in tapping mode.
Fig. 6 EQEs of PSCs based on PIC-DTBT:PCBM (1 : 3, wt/wt) and
PIC-DT2BT:PCBM (1 : 3, wt/wt).
Fig. 6 shows the external quantum efficiency (EQE) plots of
the devices under the illumination of monochromatic light. The
shapes of the EQE curves of the devices are similar to the
absorption spectra, indicating that all the absorption wave-
lengths of the polymers contributed to the photovoltaic conver-
sion. The EQE value of PIC-DT2BT was higher than that of
PIC-DTBT for all wavelengths from 350 nm to 700 nm, reflecting
the higher ISC of PIC-DT2BT. However, the EQEs of both PIC-
DT2BT and PIC-DTBT were still lower than that of the
P3HT:PCBM system. Although the reason for this is not yet
clear, it is possibly due to the low hole mobility of the polymers.
We attempted to measure the FET mobility of the pristine
polymer films but failed, probably due to the hole mobility being
too low or poor hole-injection efficiency to the low-lying HOMO
levels. Further attempts to investigate the charge-transport
properties in both the pristine polymer film and blend film are in
progress.
Fabrication and characterization of polymer solar cells
ITO-coated glass substrates were cleaned by ultrasonication in
detergent, water, acetone, and 2-propanol, sequentially. After
drying, the substrate was further cleaned with ozone for 15 min
before PEDOT:PSS (Baytron P) was spin-coated (4000 rpm for
ꢁ
30 s) onto it. The film was dried at 150 C under an N2 atmo-
Conclusion
sphere for 10 min. After cooling the substrate, a chlorobenzene
solution of polymer and PCBM mixture (1 : 1 to 1 : 4, wt/wt) was
spin-coated. Al electrodes were then evaporated under high
vacuum (approximately 2 ꢃ 10ꢀ4 Pa) in a ULVAC UPC-260F
vacuum evaporation system. The thickness of the Al electrode
was approximately 40 nm. Post-annealing was carried out at
110 ꢁC for 10 min inside a nitrogen-filled glove box. The current–
voltage characteristics of the photovoltaic cells were measured
The combinations of IC with DTBT or DT2BT resulted in two
D–A-type conjugated polymers. The polymers had good solu-
bility in organic solvents and compatibility with PCBM in blend
films. Both optical and electrochemical measurements showed
that the band gap of PIC-DT2BT was smaller than that of PIC-
DTBT. The photovoltaic device based on a PIC-DT2BT:PCBM
bulk heterojunction had a power conversion efficiency of 2.07%,
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using the Keithley 2400 I–V measurement system. The
measurements were conducted under the irradiation of AM 1.5
simulated solar light (100 mW cmꢀ2, Peccell Technologies PCE-
L11). Light intensity was adjusted by a standard silicon solar cell
with an optical filter (Bunkou Keiki BS520). The external
quantum efficiency (EQE) of the devices was measured on
a Hypermonolight System (Bunkoh-Keiki SM-250F).
1 : 1 v/v) was added dropwise through an addition funnel over 2 h
at room temperature. After stirring at room temperature over-
night, the reaction mixture was poured into water and the
aqueous layer was extracted three times with hexane.
The combined organic layers were dried over MgSO4, and the
solvents were removed by rotary evaporation. The crude
compound was purified by column chromatography (silica gel;
eluent: hexane), and compound 7 was isolated as a yellow
powder. Yield: 3.40 g (85%). 1H NMR (CDCl3, 400 MHz):
d(ppm) 8.12–7.31 (m, 8H), 4.65–4.50 (m, 2H), 2.40–2.27 (m, 4H),
1.99 (br, 4H), 1.4–0.8 (m, 76H). MALDI-TOF MS (m/z) 1001.43
(M+). Anal. calcd for C60H94Br2N2: C, 71.83; H, 9.44; N, 2.79;
found: C, 72.06; H, 9.63; N, 2.74.
Synthesis of 5-tri-n-butylstannyl-2,20-bithiophene (2)
n-Butyllithium (12.5 mL, 20 mmol, 1.6 M in hexane) was added
via a syringe to a solution of 2,20-bithiophene (3.32 g, 20 mmol) in
anhydrous THF (80 mL) at ꢀ78 ꢁC under an N2 atmosphere.
After the reaction was stirred for 2 h at ꢀ78 ꢁC, tri-n-butyl-
stannyl chloride (6.0 mL, 22 mmol) was added dropwise. After
one additional hour at ꢀ78 ꢁC, the resulting mixture was warmed
to room temperature and stirred overnight. The mixture was
poured into water and extracted with CHCl3. The combined
organic layers were dried over MgSO4, and the solvents were
removed by rotary evaporation. The product was dried under
high vacuum, and the resulting 8.40 g of greenish viscous oil were
used for subsequent preparation of 4,7-di(2,20-bithien-5-yl)-
2,1,3-benzothiadiazole without purification.
Synthesis of 3,9-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
5,11-di(1-decylundecyl)indolo[3,2-b]carbazole (8)
n-BuLi (4.1 mL, 6.56 mmol, 1.6 M in hexane) was added via
a syringe to a solution of 3,9-dibromo-5,11-di(1-decylundecy-
l)indolo[3,2-b]carbazole (3.009 g, 3 mmol) in anhydrous THF
ꢁ
(80 mL) at ꢀ78 C under an N2 atmosphere. After the reaction
was stirred for 2 h at ꢀ78 ꢁC, 2-isopropoxy-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (1.8 mL, 8.82 mmol) was added rapidly via
a syringe. After one additional hour at ꢀ78 ꢁC, the resulting
mixture was warmed to room temperature and stirred overnight.
The mixture was poured into water and extracted with CHCl3.
The combined organic layers were dried over MgSO4, and the
solvents were removed by rotary evaporation. Lastly, the crude
compound was purified by column chromatography (silica gel;
eluent: chloroform–hexane 10 : 90 v/v) and isolated as a pale-
yellow sticky solid. Yield: 2.0 g (61%).
Synthesis of 4,7-di(2,20-bithien-5-yl)- 2,1,3-benzothiadiazole (3)
5-Tri-n-butylstannyl-2,20-bithiophene (8.4 g), 4,7-dibromo-2,1,3-
benzothiadiazole (1.47g, 5 mmol) and toluene (100 mL) were
added to a 200 mL double-neck round-bottom flask. The reac-
tion container was purged with N2 for 30 min to remove O2.
Pd(PPh3)4 (5%, 290 mg) was added, and the reaction mixture was
heated to 110 ꢁC and stirred overnight. After cooling, methanol
(100 ml) was added, and the mixture was filtered and washed
with water, methanol and hexane. The crude product was
recrystallized in CHCl3 to afford compound 3 as a golden brown
solid. Yield: 1.84 g (79%). 1H NMR (CDCl3, 400 MHz): d(ppm)
8.07 (br, 2H), 7.88 (br, 2H), 7.4–7.3 (m, 4H), 7.07 (br, 2H).
MALDI-TOF MS (m/z) 464.1 (M+).
1H NMR (CDCl3, 400 MHz): d(ppm) 8.23–7.63 (m, 8H), 4.74
(br, 2H), 2.39(m, 4H), 2.01 (m, 4H), 1.4–1.0 (m, 70H), 0.82
(m, 30H). MALDI-TOF MS (m/z) 1096.06 (M+).
Synthesis of poly{5,11-di(1-decylundecyl)indolo[3,2-b]carbazole -
3,9-diyl-alt-4,7-dithien-2-yl-2,1,3-benzothiadiazole-50,500-diyl}
(PIC-DTBT)
Synthesis of 4,7-di(50-bromo-2,20-bithien-5-yl)-2,1,3-
benzothiadiazole (4)
Monomer 1 (117 mg, 0.255 mmol), monomer 8 (280mg, 0.255
mmol) and toluene (10 mL) were added to a 50 mL double-neck
round-bottom flask. The reaction container was purged with N2
for 30 min to remove O2. Pd(PPh3)4 (3%, 9 mg) was added, and
In a 1 L round-bottom flask, compound 3 (465 mg, 1.0 mmol)
was dissolved in a mixed solvent of CHCl3 (500 mL) and acetic
acid (100 mL), and the flask was covered with aluminium foil.
N-Bromosuccinimide (374 mg, 2.1 mmol) was added in portions,
and the reaction mixture was stirred overnight at room temper-
ature. Then, methanol (100 mL) was added, and the mixture was
filtered and washed with water, methanol, hexane and CH2Cl2.
The precipitate was collected as a dark blue solid. Yield: 0.48 g
(77%). MALDI-TOF MS (m/z) 621.8 (M+).
ꢁ
the reaction mixture was heated to 110 C for 10 min. Tetrae-
thylammonium hydroxide (0.9 mL, 20% by weight in H2O) was
added, and the mixture was heated under reflux overnight.
Phenylboronic acid (100 mg in 1 mL THF) was added, and after
2 h, bromobenzene (1 mL) was added. The mixture was allowed
to reflux for 2 h and cooled to room temperature. The polymer
was precipitated by slowly adding the mixture into MeOH,
filtered and Soxhlet extracted with ether and CHCl3. The CHCl3
solution was passed through a column packed with alumina,
celite and silica gel. The column was eluted with CHCl3. The
combined polymer solution was concentrated to 30 mL and
poured into methanol (300 mL). The precipitate was then
collected and dried under vacuum overnight. Yield: 0.23 g (79%).
1H NMR (CDCl3, 400 MHz): d(ppm) 8.42–7.30 (m, 14H), 4.74
(br, 2H), 2.50 (br, 4H), 2.06 (br, 4H), 1.4–1.0 (br, 64H),
Synthesis of 3,9-dibromo-5,11-di(1-decylundecyl)indolo[3,2-b]-
carbazole (7)
In a flame-dried 100 mL double-neck round-bottom flask, 3,9-
dibromoindolo[3,2-b]carbazole (1.656 g, 4.0 mmol) and 2.24 g
(40 mmol) freshly powdered potassium hydroxide was dissolved
in anhydrous dimethyl sulfoxide (DMSO, 30 mL), and a solution
of 5.60 g (12 mmol) of monomer 6 in DMSO and THF (40 mL,
7736 | J. Mater. Chem., 2009, 19, 7730–7737
This journal is ª The Royal Society of Chemistry 2009

View Article Online
J. M. Kroon, D. J. D. Moet, J. Sweelssen and M. M. Koetse, Appl.
Phys. Lett., 2007, 90, 143506.
8 (a) P. T. Boudreault, A. Michaud and M. Leclerc, Macromol. Rapid
Commun., 2007, 28, 2176; (b) E. G. Wang, L. Wang, L. F. Lan,
C. Luo, W. L. Zhuang, J. B. Peng and Y. Cao, Appl. Phys. Lett.,
2008, 92, 033307.
0.85 (m, 12H). Anal. calcd. for C74H100N4S3: C 77.84, H 8.83,
N 4.91; found: C 77.60, H 8.77, N 4.90.
Synthesis of poly{5,11-di(1-decylundecyl)indolo[3,2-b]carbazole-
3,9-diyl-alt-4,7-di(2,20-bithien-5-yl)-2,1,3-benzothiadiazole-
50,500-diyl} (PIC-DT2BT)
9 (a) N. Blouin, A. Michaud and M. Leclerc, Adv. Mater., 2007, 19,
ꢀ
2295; (b) S. H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates,
J. S. Moon, D. Moses, M. Leclerc, K. Lee and A. J. Hegger, Nat.
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A procedure similar to the synthesis of PIC-DTBT was used
starting from monomer 4 (127 mg, 0.204 mmol) and monomer 8
(224 mg, 0.204 mmol). Yield: 0.22 g (82%). 1H NMR (CDCl3, 400
MHz): d(ppm) 8.40–7.22 (m, 18H), 4.71 (br, 2H), 2.48 (br, 4H),
2.04 (br, 4H), 1.4–1.0 (br, 64H), 0.82 (m, 12H). Anal. calcd. for
C82H104N4S5: C 75.41, H 8.03, N 4.29; found: C 75.20, H 8.12,
N 4.27.
ꢀ
€
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J. Mater. Chem., 2009, 19, 7730–7737 | 7737