Synthesis and photovoltaic properties of low-bandgap polymers based on N-arylcarbazole

Article abstract of DOI:10.1016/j.polymer.2011.02.029

Low-bandgap poly(2,7-carbazole) derivatives with variable N-substituent of ethyl (PEtCzBT), phenyl (PPhCzBT) and 4-diphenylaminophenyl (PTPACzBT) on the carbazoles, were synthesized through Suzuki coupling reaction. The polymers show excellent solubility

Full text of DOI:10.1016/j.polymer.2011.02.029

Polymer 52 (2011) 1748e1754
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and photovoltaic properties of low-bandgap polymers based on
N-arylcarbazole
,
a
c
Lifen Wang a, Yingying Fu b, Lei Zhu a, Guangrui Cui a, Fushun Liang a , Liping Guo , Xintong Zhang ,
*
,
a
Zhiyuan Xie b , Zhongmin Su
*
a Department of Chemistry, Northeast Normal University, Changchun 130024, China
b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
c Key Laboratory for UV-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Low-bandgap poly(2,7-carbazole) derivatives with variable N-substituent of ethyl (PEtCzBT), phenyl
(PPhCzBT) and 4-diphenylaminophenyl (PTPACzBT) on the carbazoles, were synthesized through Suzuki
coupling reaction. The polymers show excellent solubility in organic solvents (readily soluble in chlo-
roform, THF and toluene etc.), good thermal stability (5% weight loss temperature of more than 417 ꢀC),
and electrochemical properties (reversible redox process with narrow bandgap), and deep HOMO energy
levels (w5.1 eV), allowing them promising candidates in the solar cell fabrication. Bulk-heterojunction
solar cells with these polymers as electron donor and (6,6)-phenyl-C71-butyric acid methyl ester
(PC71BM) as electron acceptor exhibit high Voc (0.91e0.95 V) and good power conversion efficiency (PCE)
of 1.69% for PEtCzTB, 2.01% for PPhCzTB, and 2.42% for PTPACzTB.
Received 9 October 2010
Received in revised form
25 January 2011
Accepted 22 February 2011
Available online 1 March 2011
Keywords:
Polymer solar cells
N-arylcarbazole
Low-bandgap polymers
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
but the transport capabilities will be compromised due to the space
charge limitation [6]. Therefore, development of new LBG polymers
Polymer solar cells (PSCs) have been attracting considerable
attention in recent years due to their unique features of low cost,
light weight, and potential application in flexible large-area devices
[1e3]. The most common device structure for efficient PSCs is based
on the bulk-heterojunction (BHJ) concept which involves a thin film
blend of conjugated polymer as the electron donor and a fullerene
derivative as the electron acceptor. In order to obtain high-perfor-
mance PSCs, it is necessary to design and synthesize conjugated
polymers with desired properties, such as low-bandgap (LBG),
broad absorption range, high carrier mobility, and appropriate
molecular energy levels [4]. In the past decade, LBG conjugated
polymers have been developed and used in PSCs, and high power
conversion efficiencies of over 6% have been achieved due to their
absorption often correspond to the maximum photon flux of
sunlight spectrum [5]. However, LBG polymers often show better
absorption at a long-wavelength range, while other parts of
sunlight spectrum are being sacrificed. Meanwhile, the donor-
acceptor systems in the LBG polymers cause partial intramolecular
charge transfer (ICT) that enables manipulation of the electronic
structure (HOMO/LUMO levels) and benefits for charge-separation,
with broad and intense absorption and excellent charge transport is
still required.
Poly(2,7-carbazole) derivatives, as a type of LBG photoactive
donor materials by incorporation of electron-withdrawing group in
the main chain, have been well studied by Leclerc [7], Bo [8], and
Hashimoto [9] et al., and exhibited great promise when used with
PCBM in bulk-heterojunction photovoltaic cells. In a most recent
publication, excellent power conversion efficiency of 6.1% has been
reported by Heeger, Leclerc and co-workers [10]. In our research,
we presented a highly crystalline alternative copolymer of indolo-
carbazole and benzothiadiazole-cored oligothiophenes with a good
power conversion efficiency of 3.6% [11]. We also developed new
random copolymers of tertiary benzothiadiazole-cored oligothio-
phene, 2,20-bithiophene and 2,7-dibromocarbazole, which
provides us an opportunity to fine tune the optoelectronic prop-
erties of the molecular system and thus, a broad and strong
absorption spectrum was achieved from a single polymer chain
[12]. Almost all the work including just mentioned is based on poly
(N-alkyl-2,7-carbazole)s. Examples of poly(N-aryl-2,7-carbazole)s
in the application to PSCs were scarce [13], although they were
frequently used as light-emitting and hole-transporting materials
in organic light-emitting diodes [14]. In this study, we wish to
explore the photovoltaic performance of LBG polymers based on
N-aryl-2,7-carbazole. It was known that carbazole-based polymers
* Corresponding authors.
E-mail addresses: liangfs112@nenu.edu.cn (F. Liang), xiezy_n@ciac.jl.cn (Z. Xie).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.02.029

L. Wang et al. / Polymer 52 (2011) 1748e1754
1749
generally exhibit deep-HOMO energy level [15], so high open-
circuit voltage is expected. Thus, polymer incorporating
benzothiadiazole-centered quarterthiophene and N-phenyl-2,7-
carbazole was designed and synthesized. Furthermore, a 4-diphe-
nylaminophenyl group was introduced to the N-atom of the
carbazole moiety for further improving the hole-transporting
ability of the resulting polymer to compensate the influence of
space charge limitation in the polymer main chain [16]. Addition-
ally, a poly(N-ethyl-carbazole) was also synthesized. The molecular
structures of the three copolymers were shown in Scheme 1. The
thiophene segments in the backbone of the copolymers may help to
a Waters 410 instrument equipped with Waters HT4 and HT3
column assembly using polystyrene as standards and THF as eluent.
UVevis spectra were measured using a PerkinElmer Lambda 900
UV-vis-NIR Spectrometer. Thermal properties of the polymers were
analyzed with a Perkin-Elmer-TGA 7 instrument under nitrogen at
a heating rate of 10 ꢀC minꢁ1. Thermogravimetric analysis (TGA)
was carried out using TA instruments TGA Q500 in air. Cyclic vol-
tammetry experiments were carried out on an EG&G Princeton
Applied Research potentiostat/galvanostat (model 2273) in an
electrolyte solution of 0.1 M tetrabutylammonium perchlorate
(Bu4NClO4) in acetonitrile at 50 mV sꢁ1. A three-electrode cell was
used in all experiments, wherein platinum wires act as the working
and counter electrode and Ag/AgCl electrode as the reference. The
reference electrode was calibrated with an internal standard of
ferrocene. Polymer thin films were formed by drop-casting 1.0 mm3
of polymer solutions in chlorobenzene (1 mg/mL) on the working
electrode and were then dried in air.
tune the absorption spectrum by elongation of the
p conjugation
and the alkyl chain on the thiophene rings would ensure good
solubility.
2. Experimental section
2.1. Materials
2.3. Photovoltaic device fabrication and testing
4,7-dibromo-2,1,3-benzothiadiazole [17], 2,7-dibromocarbazole
(1) [18], 2,7-dibromo-9H-(ethyl)-9H-carbazole (2) [19],2,7-
dibromo-9-(4-nitrophenyl)-9H-carbazole (3) [20], 4-(2,7-dibromo-
carbazol-9-yl)phenylamine (4) [21], 2,7-dibromo-9-(4-phenyl)-9
H-carbazole (5) [20] and N-(4-(2,7-dibromo-9H-carbazol-9-yl)
phenyl)-N-phenylbenzenamine (6) [22] were prepared according to
the reported methods. All reagents and solvents were purchased
from JK chemicals and Alfa Chemicals. Anhydrous tetrahydrofuran
was distilled over sodium/benzophenone under Ar prior to use.
Glass slides patterned with ITO (Colorado Concept Coatings LLC)
were cleaned by sonicating sequentially in detergent, water, 1,1,1-
trichloroethane, acetone, and methanol, followed by treatment in
a low power air plasma for 15 min with a Harrick PDC-32G plasma
sterilizer. Thereafter, the ITO-coated slides were spin-coated
(500 rpm for 5 s, then 4000 rpm for 60 s) with a filtered (0.45 mm
NYL w/GMF syringe filter) aqueous solution of poly(ethylene
dioxythiophene) doped with polystyrene sulphonic acid,
PEDOT:PSS (Baytron P, H.C. Starck) and then transferred to a dry
box. The resulting thin PEDOT:PSS layer (w35 nm) was dried in an
oven under a mild N2 purge at 120 ꢀC for 1 h. A chlorobenzene
solution of polymer and PC71BM with different weight ratio of 1:1
and 1:3 for each cell was stirred at 50 ꢀC for 16 h and then spun-cast
on the PEDOT:PSS-coated slides for 90 s. LiF/Al cathode was
deposited at a vacuum level of 4 ꢂ 10ꢁ4 Pa. The thicknesses of the
LiF and Al layers are 1 and 100 nm, respectively. The effective area
of the unit cell is 12 mm2. The thickness of organic layer is deter-
mined by DEKTAK 6M Stylus profiler. Preliminary testing of each of
the ten pixels was performed to select the most promising pixel
prior to full solar simulated analysis. The best performing pixel then
underwent Voc, Isc, dark and illuminated IeV studies using an Oriel
300 W solar simulator with appropriate filters to provide AM 1.5G
(100 mW/cm2). The fill-factor (FF) was determined from the illu-
minated IeV and is the maximum power delivered divided by the
product of Voc and Isc. The EQE was measured at a chopping
frequency of 280 Hz with a lock-in amplifier (Stanford, SR830)
during illumination with the mono-chromatic light from a Xenon
lamp. Currentevoltage (IeV) characteristics were recorded using
a computer-controlled Keithley 236 source meter in the dark and
under white light (CHF-XM 500W Xenon lamp) illumination.
2.2. Measurement and characterization
NMR spectra were recorded on a Varian 500 or Bruker 400
spectrometer using tetramethylsilane as internal references.
Number- and weight-average molecular weights of the polymers
were determined by gel permeation chromatography (GPC) on
2.3.1. Monomer and polymer syntheses
The dibromonated monomer M1 was synthesized from 2,1,3-
benzothiadiazole via a divergent method according to our recent
publication
[23].
2,7-Di(1,3,2-dioxaborinan-2-yl)-9-ethyl-9H-
carbazole (M2) was prepared according to the reported method
[24].
2,7-di(1,3,2-dioxaborinan-2-yl)-9-phenyl-9H-carbazole
(M3)
[24]. To a solution of 5 (4.02 g, 10 mmol) in THF (50 mL) at ꢁ78 ꢀC
was added dropwise 2.5 M n-BuLi in hexane (8.4 mL, 21 mmol) over
15 min. The resulting mixture was stirred for 1 h while maintaining
the temperature at ꢁ78 ꢀC, after which trimethylborate (4.36 g,
42 mmol) was added and the mixture was stirred at ꢁ78 ꢀC for an
additional hour. The reaction mixture was then allowed to warm to
room temperature and stirred for 15 h. The mixture was then
Scheme 1. N-arylcarbazole-based low-bandgap deep HOMO polymers reported in this
work.

1750
L. Wang et al. / Polymer 52 (2011) 1748e1754
cooled to 0 ꢀC and 2 mol/L HCl (40 mL) added. After the mixture was
stirred for 30 min, the organic layer was separated using ether,
washed with brine three times, and finally dried over MgSO4. After
filtration and evaporation of the solvents, a light yellow viscous
liquid was obtained. Dissolving this liquid in THF followed by
precipitation to hexane gave a white solid. The solid was filtered,
washed with hexane three times, and dried briefly (30 min) under
vacuum at room temperature. The resulting compound, 9-phenyl-
9H-carbazole-2,7-diyldiboronic acid, was mixed with toluene
(100 mL) and 1,3-propylene glycol (2.28 g, 30 mmol). The mixture
was heated to reflux for 3 h; meanwhile, the water produced was
removed using a DeaneStark trap. The mixture was cooled and the
solvent was removed. The residue was purified by column chro-
matography (ethyl acetate/petroleum ether, 1/30, v/v) on silica gel.
Recrystallization from hexane gave the product as colorless crys-
methanol/water (10:1), filtered through 0.45 mm nylon filter and
washed on Soxhlet apparatus with acetone and hexanes. The
resultant polymer was collected, dried, and further purified by
passing through a short silica gel column to remove the catalyst
residues. The resulting copolymer solution was collected, concen-
trated, and precipitated into methanol. The black solid obtained
was dried overnight under vacuum at 50 ꢀC.
PEtCzTB: 309 mg, 69% yield. GPC: Mn: 9600, Mw/Mn ¼ 1.30
(relative to polystyrene standards). 1H NMR (500 MHz, CDCl3):
d
(ppm) ¼ 8.14 (br, 2H); 8.03 (br, 2H); 7.85 (br, 2H); 7.52 (br, 2H);
7.39 (br, 2H); 7.20 (br,2H); 4.44 (br, 2H); 2.94 (br, 4H); 2.85 (br, 4H);
1.81 (br, 4H); 1.73 (br, 4H); 1.53 (br, 2H); 1.48 (br, 3H); 1.26e1.39 (br,
38H); 0.86 (br, 12H). 13C NMR (100 MHz, CDCl3):
d
(ppm) ¼ 152.52,
140.57, 140.13, 139.13, 136.49, 134.20, 132.87, 131.99, 130.71, 128.51,
125.31, 125.02, 122.07, 120.75, 120.43, 109.03, 31.90, 31.14, 31.65,
30.51, 30.45, 29.68, 29.48, 29.36, 29.33, 29.05, 22.69, 14.11, 13.89,
1.01; Anal. Calcd. for (C69H89N3S5)n (%):C, 73.94; H, 8.00; N, 3.75;
Found (%): C, 74.16; H, 8.06; N, 3.79.
tals. Yield: 3.08 g (75%). 1H NMR (CDCl3, 500 MHz):
d
(ppm) ¼ 8.13
(d, 2H, J ¼ 8.0 Hz), 7.81(s, 2H), 7.69 (d, 2H, J ¼ 8.0 Hz), 7.60 (m, 4H),
7.46(d, 1H, J ¼ 7.0 Hz), 4.17 (t, 8H, J ¼ 5.5 Hz), 2.06 (m, 4H). Anal.
Calcd. (%): C, 70.12; H, 5.64; N, 3.41. Found (%): C, 70.33; H, 5.66; N,
3.45. MS (m/z): calcd. for C24H23B2NO4, 411.07; found, 412.31.
N-(4-(2,7-di(1,3,2-dioxaborinan-2-yl)-9H-carbazol-9-yl)phenyl)-
N-phenylbenzenamine (M4) [25]. To a solution of 6 (5.68 g,10 mmol)
in THF (50 mL) at ꢁ78 ꢀC was added dropwise 2.5 M n-BuLi in
hexane (8.4 mL, 21 mmol) over 15 min. The resulting mixture was
stirred for 1 h while maintaining the temperature at ꢁ78 ꢀC, after
which trimethylborate (4.36 g, 42 mmol) was added and the
mixture was stirred at ꢁ78 ꢀC for an additional hour. The reaction
mixture was then allowed to warm to room temperature and stir-
red for 15 h. The mixture was then cooled to 0 ꢀC and 2 mol/L HCl
(40 mL) added. After the mixture was stirred for 30 min, the organic
layer was separated using ether, washed with brine three times,
and finally dried over MgSO4. After filtration and evaporation of the
solvents, a light yellow viscous liquid was obtained. Dissolving this
liquid in THF followed by precipitation to hexane gave a white solid.
The solid was filtered, washed with hexane three times, and dried
briefly (30 min) under vacuum at room temperature. The resulting
compound, 9-(4-(diphenylamino)phenyl)-9H-carbazole-2,7-diyl-
diboronic acid, was mixed with toluene (100 mL) and 1,3-propylene
glycol (2.28 g, 30 mmol). The mixture was heated to reflux for 3 h;
meanwhile, the water produced was removed using a DeaneStark
trap. The mixture was cooled and the solvent was removed. The
residue was purified by column chromatography (ethyl acetate/
petroleum ether, 1/30, v/v) on silica gel. Recrystallization from
hexane gave the product as colorless crystals. Yield: 4.34 g (75%). 1H
PPhCzTB: 280 mg, 60% yield. GPC: Mn: 10600, Mw/Mn ¼ 1.40
(relative to polystyrene standards). 1H NMR (500 MHz, CDCl3):
d
(ppm) ¼ 8.18 (br, 2H); 7.98 (br, 2H); 7.83 (br, 2H); 7.63 (br, 4H);
7.51 (br, 2H); 7.45 (br, 3H); 7.14 (br, 2H); 2.86 (br, 4H); 2.70 (br, 4H);
1.75 (br, 4H); 1.65 (br, 4H); 1.46 (br, 4H); 1.36e1.25 (br, 36H); 0.87
(br, 12H). 13C NMR (100 MHz, CDCl3):
d
(ppm) ¼ 152.43, 140.54,
140.06, 139.08, 136.45, 134.23, 132.83, 131.97, 130.66, 128.45, 125.19,
124.92, 122.04, 120.73, 120.41, 108.96, 31.90, 31.14, 30.62, 30.50,
30.43, 29.70, 29.48, 29.34, 29.07, 22.68, 14.11, 13.87; Anal. Calcd. for
(C73H89N3S5)n (%): C, 75.01; H, 7.67; N, 3.60; Found (%): C, 75.24; H,
7.69; N, 3.66.
PTPACzTB: 379 mg, 71% yield. GPC: Mn: 16800, Mw/Mn ¼ 1.81
(relative to polystyrene standards). 1H NMR (500 MHz, CDCl3):
d
(ppm) ¼ 8.17 (br, 2H); 8.01 (br, 2H); 7.86 (br, 2H); 7.54 (br, 2H);
7.45 (br, 2H); 7.33e7.21 (br, 10H); 7.16 (br, 2H); 7.08 (br, 2H); 2.91
(br, 4H); 2.73 (br, 4H); 1.79 (br, 4H); 1.67 (br, 4H); 1.46 (br, 4H);
1.32e1.21 (br, 36H); 0.89 (br, 12H). 13C NMR (100 MHz, CDCl3):
d
(ppm) ¼ 152.57, 147.43, 141.78, 140.20, 139.23, 139.03, 136.55,
134.19, 132.82, 132.20, 130.72, 129.47, 128.51, 127.77, 125.39, 125.11,
124.94, 123.70, 123.53, 122.30, 121.68, 120.31, 110.42, 31.87, 31.08,
30.64, 29.66, 29.43, 29.32, 28.99, 22.66, 14.09; Anal. Calcd. for
(C86H99N3S5)n (%):C, 77.37; H, 7.47; N, 3.15; Found (%): C, 77.60; H,
7.51; N, 3.16.
3. Results and discussion
NMR (CDCl3, 500 MHz):
d
(ppm) ¼ 8.12 (d, 2H, J ¼ 7.5 Hz), 7.84 (s,
3.1. Synthesis and characterization
2H), 7.69 (d, 2H, J ¼ 8.0 Hz), 7.39 (d, 2H, J ¼ 8.5 Hz), 7.32 (m, 4H),
7.25 (d, 2H, J ¼ 7.0 Hz), 7.08 (d, 1H, J ¼ 7.0 Hz), 4.19 (t, 8H, J ¼ 5.0 Hz),
2.05 (m, 4H). Anal. Calcd. (%): C, 74.77; H, 5.58; N, 4.84. Found (%): C,
74.98; H, 5.59; N, 4.86. MS (m/z): calcd. for C36H32B2N2O4, 578.25;
found, 579.11.
The synthetic route towards monomers M1e3 was given in
Scheme 2. Compounds 1e6 were prepared according to reported
methods [17e22]. It was noteworthy that one of the key inter-
mediates, 2, 7-dibromo-9-(4-nitrophenyl)-9H-carbazole (3) was
readily prepared with a modified literature procedure [20]. As
a result, the reaction of 1 with 1.1 equiv of 4-fluoronitrobenzene
in DMF at 60 ꢀC afforded compound 3 in 92% yield. While the
reaction reported by Jian et al. needs to perform in DMF at reflux
using excess 4-fluoronitrobenzene (4.0 equiv.) and compound 3
was obtained in 86% yield [20]. Three new alternating copolymers
PEtCzTB, PPhCzTB and PTPACzTB based on 2,7-linked carbazole,
2,1,3-benzothiadiazole and 3-octylthiophene were synthesized
via the Suzuki coupling reaction. The general synthetic routes of
the polymers are shown in Scheme 3. Their chemical structures
were verified by 1H NMR, 13C NMR and elemental analysis
(for details, see supporting information). All the copolymers
were obtained as a dark-brown powder and are readily soluble
in common organic solvents such as chloroform, THF and
toluene etc.
2.3.2. General procedure for the preparation of the polymers [7b,25]
To a 50 mL one-necked flask was added tricaprylylmethy-
lammonium chloride (Aliquat 336) (20 wt% based on monomers),
the appropriate diboronate (0.4 mmol), the appropriate dibromide
(0.4 mmol), and toluene. Once all the monomers were dissolved,
2 M Na2CO3 (1.32 mmol) aqueous solution 0.66 mL was added. The
flask equipped with a condenser was then evacuated and filled
with argon three times to remove traces of air. Pd(PPh4)3
(0.002 mmol) was then added under an argon atmosphere. The
flask was again evacuated and filled with argon three times. The
mixture was then heated to reflux and maintained for 72 h under
argon. The reaction mixture was then cooled to room temperature
and the organic layer was separated, concentrated, and precipitated
into methanol. The polymer was purified by precipitation in

L. Wang et al. / Polymer 52 (2011) 1748e1754
1751
Scheme 2. Reagents and conditions: (i) C2H5Br, K2CO3, DMF, rt. (ii) (a) n-BuLi, THF, ꢁ78 ꢀC; (b) B(OMe)3, ꢁ78 ꢀC; (c) 1 M HCl, 0 ꢀC; (d) 1,3-propylene glycol, toluene, reflux; (iii) 4-F-
C6H4NO2, K2CO3, DMF, 60oC; (iv) Sn, HCl, EtOH, reflux; (v) (a) HCl, NaNO2, CH3CN, 0 ꢀC; (b) H3PO2, 80 ꢀC; (vi) PhI, CuCl, 1,10-phenanthroline, KOH, toluene, reflux.
3.2. Thermal properties
PEtCzTB, PPhCzTB and PTPACzTB in chlorobenzene solution. It
was observed that almost identical absorption profiles were
observed for the three polymers. The absorption band at shorter
wavelength centered at 381 nm corresponds to higher energy
The thermal properties of the copolymers were determined by
DSC and TGA under a nitrogen atmosphere. As shown in Fig. 1 and
Table 1, polymers PEtCzTB, PPhCzTB and PTPACzTB exhibited 5%
weight-loss temperatures (Td) of 447, 417 and 439 ꢀC, respectively.
No glass transition was observed for polymers PEtCzTB and
PPhCzTB in the DSC curves, while polymer PTPACzTB gave
a distinct glass transition temperature (Tg) at 67 ꢀC, most probably
due to the introduction of the triphenylamine moiety onto the
polymer side chain (see supporting information). No melting or
crystallization peak was observed in further heating and cooling
cycles. The good thermal stability of the polymers is desirable for
their application in PSCs.
transitions such as
pꢁp
* transition and the absorption peak
located at 536 nm was attributed to the intramolecular charge-
transfer (ICT) transition. There is an additional absorption peak at
w 315 nm for polymer PTPACzTB, which obviously arises from the
absorption of the pendent triphenylamine segment. The absorption
spectra of the polymer films on quartz plate were shown in Fig. 3
and the optical data including the absorption peak wavelength in
both solutions and films, absorption coefficients and the optical
PEtCzTB
100
PPhCzTB
3.3. Optical properties
PTPACzTB
90
The optical properties of the polymers were investigated by
Ultravioletevisible (UVevis) absorption spectroscopy in dilute
chlorobenzene (10ꢁ6 M) solutions and as spin-coated films on
quartz substrates. Fig. 2 shows the absorption spectra of the
80
70
60
50
40
0
200
400
600
800
o
C
Temperature
Scheme 3. Synthetic routes toward copolymers PEtCzTB, PPhCzTB and PTPACzTB.
Fig. 1. TGA thermograms of the copolymers, recorded at a heating rate of 10 ꢀC minꢁ1
under a N2 atmosphere.
Reagents and conditions: Pd(PPh3)4, toluene, Ar, 110 ꢀC.

1752
L. Wang et al. / Polymer 52 (2011) 1748e1754
bandgap ðEgoptÞ are summarized in Table 2. In thin film state, the
Table 1
Polymerization data and thermal properties of the copolymers.
polymers showed red-shifted and broad absorption bands. The
optical band gaps ðEgoptÞ of the three polymers were calculated from
the absorption edges in the films and found to be in the range of
1.78e1.85 eV. Clearly, the photophysical properties and energy
levels of the resultant polymers can be slightly tuned by the N-
substitution of carbazole.
Polymers Yield (%) Mn (kg mol-1
)
Mw (kg mol-1
)
PDIa Tg (ꢀC) Td (ꢀC)b
a
a
PEtCzTB
PPhCzTB
PTPACzTB 71
69
60
9.6
10.6
16.8
12.5
14.8
30.3
1.30 n.d.c
1.40 n.d.c
1.81 67
447
417
439
a
Determined by gel permeation chromatography (GPC) in THF against poly-
styrene standards.
b
Onset decomposition temperature (5% weight loss) measured by TGA.
Glass transition temperature was not detectable.
3.4. Electrochemical properties
c
To investigate the electrochemical properties of the synthesized
polymers and estimate their HOMO and LUMO energy levels, cyclic
voltammetry (CV) measurement was carried out. The HOMO and
the LUMO energy levels of the conjugated polymers were calcu-
PEtCzTB
381 nm
PPhCzTB
on
PTPACzTB
lated using the empirical equation EHOMO ¼ ꢁðE þ 4:40ÞeV and
ox
on
ELUMO ¼ ꢁðEon þ 4:40ÞeV, respectively, where E and Eon stand
536 nm
ox
for the onset rpedotentials for oxidation and reduction relatirveed to the
Ag quasi-reference electrode, respectively [26]. The cyclic voltam-
mograms of the polymers are shown in Fig. 4. Upon cathodic scan,
two couples of reversible reduction peaks were observed, corre-
sponding to the redox process of benzothiadiazole and thiophene
units, respectively. In the anodic scan, there is one couple of
oxidation peak for polymers PEtCzTB and PPhCzTB, which is
attributed to the redox process of carbazole unit. For PTPACzTB, an
additional redox peak at 0.6 V from triphenylamine moiety was
observed. The HOMO and the LUMO are listed in Table 2. One can
see that the polymers gave narrower energy gaps due to the
incorporation of electron-withdrawing benzothiadiazole units into
the polymer backbone. Meanwhile, the HOMO energy levels
PEtCzTB and PPhCzTB remain low at w5.10 eV, which are impor-
tant for achieving the large open-circuit voltage (Voc) and high air
stability. The HOMO energy level of PTPACzTB was higher (4.95 eV)
than the other two polymers, due to the introduction of the tri-
phenylamine group.
1.0
0.5
0.0
300
400
500
600
700
800
Wavelength (nm)
Fig. 2. UVevis absorption spectra of the polymers in cholorobenzene solution
(10ꢁ6 M).
0.5
PEtCzTB
PPhCzTB
3.5. Photovoltaic properties
PTPACzTB
0.4
The potential of these polymers as hole-transporting light-
absorbing components in photovoltaic cells were explored. Bulk-
heterojunction photovoltaic cells with a device structure of ITO/
PEDOT:PSS/polymers:PC71BM (1:1 or 1:3, w/w)/LiF(1 nm)/Al
(100 nm) were fabricated. The active layer was spin-coated from
chlorobenzene solution using a slow solvent evaporation process. It
was found that weight ratio between polymers:PC71BM of 1:3 gave
improved photovoltaic performances, but lower than the poly(N-
alkyl-2,7-carbazole) developed by Leclerc et al. (3.6%) [7c]. Fig. 5
shows the IeV curves of the devices under AM 1.5 simulated
solar illumination of 100 mW/cm2, and Voc, Jsc, FF, and PCE data
were collected and listed in Table 3. The corresponding power
conversion efficiency of the solar cells was 1.69% for PEtCzTB, 2.01%
for PPhCzTB, and 2.42% for PTPACzTB. It was observed that the PCE
was increased by the replacement of the ethyl group by dipheny-
laminophenyl group, which may attribute to the enhanced
0.3
0.2
0.1
0.0
300
400
500
600
700
800
Wavelength (nm)
Fig. 3. UVevis absorption spectra of the polymer films on a quartz plate.
Table 2
Absorption and electrochemical properties of the copolymers.
Polymer
labs (nm)
3
(L mol-1 cm-1
)
Egopt (eV)a
EHOMO (eV)
ELUMO (eV)
Egec (eV)b
Solution
Film
PEtCzTB
PPhCzTB
PTPACzTB
381 536
381 536
315 381 536
401 565
401 565
315 401 565
3.0 ꢂ 105
2.6 ꢂ 105
4.7 ꢂ 105
1.85
1.82
1.78
ꢁ5.12
ꢁ5.18
ꢁ4.95c
ꢁ3.16
ꢁ3.19
ꢁ3.16
1.96
1.99
1.79
a
Estimated from the absorption edge in film state.
Energy gap ¼ HOMO-LUMO.
b
c
EHOMO of PTPACzTB was calculated based on triphenylamine moiety.

L. Wang et al. / Polymer 52 (2011) 1748e1754
1753
50
40
30
20
10
0
PTPACzTB
PPhCzTB
PEtCzTB:Pc BM=1:3
71
PEtCzTB
PPhCzTB:PC BM=1:3
71
PTPACzTB:PC BM=1:3
71
400
500
600
700
-2.0 -1.5 -1.0 -0.5 0.0
0.5
1.0
1.5
Wavelength (nm)
Potential (V vs Ag/AgCl)
Fig. 6. EQE curves of PSCs incorporating polymer/PC71BM blends, each prepared at
a blend of 1:3 (w/w).
Fig. 4. Cyclic voltammograms of polymer thin films on Pt electrode in 0.1 M Bu4NClO4
acetonitrile solution at 50 mV s-1
.
HOMO energy levels. The high Voc of the three polymers are among
the best data (Voc > 0.9 V) of the LBG polymers reported in the
literature [27].
The external quantum efficiency (EQE) curves of the PSC devices
are plotted in Fig. 6. It is apparent that the device exhibits a broad
response range, covering from 350 to 700 nm, but the EQE of the
device is within 45% for almost the whole absorption range. If the
EQE of the device can be improved by increasing the thickness of
the active layer without hampering charge-separation and trans-
port properties, the device performance may be improved.
4
2
PEtCzTB:PC BM
71
PPhCzTB:PC BM
71
PTPACzTB:PC BM
71
0
-2
-4
-6
-8
4. Conclusion
In this paper, we report the synthesis and characterization of
two new N-aryl-based low-bandgap poly(2,7-carbazole)s, PPhCzTB
and PTPACzTB. Photovoltaic properties of these polymers were
investigated by fabricating bulk heterojuntion photovoltaic devices
using these polymers as electron donor and PC71BM as the acceptor.
High open-circuit voltages of 0.91e0.95 V and moderate power
conversion efficiency of 1.69% for PEtCzTB, 2.01% for PPhCzTB, and
2.42% for PTPACzTB were achieved. Of the three polymers,
PTPACzTB gave best photovoltaic performance under identical
conditions, most probably, due to the introduction of triphenyl-
amine moiety increase the hole-transporting ability of the polymer,
although the intermolecular stacking may be inhibited to some
degree. These results indicate low-bandgap poly(N-aryl-2,7-
carbazole)s are promising polymer materials for application in
polymer solar cells.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Voltage (V)
Fig. 5. Current density-voltage characteristics of polymer/PC71BM based devices under
AM 1.5 simulated solar illumination of 100 mW/cm2.
hole-transporting ability of PTPACzTB [16], although the intermo-
pep stacking in PTPACzTB is somewhat inhibited by the
introduction of triphenylamine moiety.
The open-circuit voltage (Voc) depends on the energetic differ-
ence between donor highest occupied molecular orbital (HOMO)
and acceptor lowest unoccupied molecular orbital (LUMO). The PSC
devices based on the three carbazole-based polymers gave high
open-circuit voltages of 0.91 V for PEtCzTB and 0.92 V for PPhCzTB
and 0.95 V for PTPACzTB, which are consistent with their deep
lecular
Acknowledgment
Financial support of this research by National Basic Research
Program of China (973 Programd2009CB623605), the National
Natural Science Foundation of China (20972027), the Department
of Science and Technology of Jilin Province (20080421 and
20100535), and Training Fund of NENU’S Scientific Innovation
Project (NENU-STC08013), is greatly acknowledged.
Table 3
Photovoltaic properties of polymer solar cells incorporating polymer:PC71BM blends
at 1:3 weight ratios.a,b
Polymer
Thickness (nm)
Voc (V)
Jsc (mA cm-2
)
FF
PCE (%)
PEtCzTB
PPhCzTB
PTPACzTB
75
60
60
0.91
0.92
0.95
5.64
6.58
7.08
0.33
0.33
0.36
1.69
2.01
2.42
Appendix. Supplementary data
a
Under simulated AM 1.5 solar illumination at an irradiation intensity of
100 mW/cm2.
Supplementary data related to this article can be found online at
doi:10.1016/j.polymer.2011.02.029.
b
For as-fabricated devices.

1754
L. Wang et al. / Polymer 52 (2011) 1748e1754
[11] Lu J, Liang F, Drolet N, Ding J, Tao Y, Movileanua R. Chem Commun
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