New conjugated copolymers based on benzo[1,2-b; 3,4-b′]dithiophene and derivatives of benzo[g]quinoxaline for bulk heterojunction solar cells

Article abstract of DOI:10.1002/pola.24477

A series of new low-band gap copolymers based on dioctyloxybenzo[1,2-b;3,4- b′] dithiophene and bis(2-thienyl)-2,3-diphenylbenzo[g]quinoxaline monomers have been synthesized via a Stille reaction. The effect of different functional groups attached to bis(2-thienyl)-2,3-diphenylbenzo[g]quinoxaline was investigated and compared with their optical, electrochemical, hole mobility, and photovoltaic properties. Polymer solar cell (PSC) devices of the copolymers were fabricated with a configuration of ITO/ PEDOT: PSS/copolymers: PCBM (1:4 wt ratio)/Ca/Al. The best performance of the PSC device was obtained by using PbttpmobQ as the active layer. A power conversion efficiency of 1.42% with an open-circuit voltage of 0.8 V, a short-circuit current (JSC) of 5.73 mA cm ^-2, and a fill factor of 30.9% was achieved under the illumination of AM 1.5, 100 mW cm^-2.

Full text of DOI:10.1002/pola.24477

New Conjugated Copolymers Based on Benzo[1,2-b; 3,4-b0]dithiophene
and Derivatives of Benzo[g]quinoxaline for Bulk Heterojunction
Solar Cells
PO-I LEE, STEVE LIEN-CHUNG HSU, JUNG FENG LEE, HUNG-YI CHUANG, PIYUN LIN
Department of Materials Science and Engineering, National Cheng-Kung University, Tainan, Taiwan 701-01, Republic of China
Received 14 September 2010; accepted 31 October 2010
DOI: 10.1002/pola.24477
Published online 2 December 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A series of new low-band gap copolymers based on
dioctyloxybenzo[1,2-b;3,4-b0] dithiophene and bis(2-thienyl)-2,3-
diphenylbenzo[g]quinoxaline monomers have been synthesized
via a Stille reaction. The effect of different functional groups
attached to bis(2-thienyl)-2,3-diphenylbenzo[g]quinoxaline was
investigated and compared with their optical, electrochemical, hole
mobility, and photovoltaic properties. Polymer solar cell (PSC)
devices of the copolymers were fabricated with a configuration of
ITO/ PEDOT: PSS/copolymers: PCBM (1:4 wt ratio)/Ca/Al. The best
performance of the PSC device was obtained by using PbttpmobQ
as the active layer. A power conversion efficiency of 1.42% with an
open-circuit voltage of 0.8 V, a short-circuit current (JSC) of 5.73
mA cmꢀ2, and a fill factor of 30.9% was achieved under the illumi-
nation of AM 1.5, 100 mW cmꢀ2
.
2010 Wiley Periodicals, Inc.
C
V
J Polym Sci Part A: Polym Chem 49: 662–670, 2011
KEYWORDS: benzodithiophene; benzoquinoxaline; conjugated poly-
mers; copolymerization; monomer; polymer; solar cell; synthesis
INTRODUCTION In recent years, polymer solar cells (PSCs)
have attracted great attention as a new generation of renew-
able energy sources due to flexibility, low cost, light weight,
and easy manufacturing.1–4 The bulk heterojunction solar
cells that are composed of an electron-donating conjugated
polymer blended with an electron acceptor have played a
major role in reaching high efficiencies.5–7 At present, the
most widely investigated PSCs are based on blends of
regioregular poly(3-hexylthiophene) (P3HT) with 6.6-phenyl
exhibits a large planar conjugated structure, and its copolymers
have a high hole mobility.23 Bis(2-thienyl)-2,3-bis-(4-phenyl)-
benzo[g]quinoxaline is a monomer with a D–A molecular struc-
ture, and can be used to lower the band gap of copolymers.
We will discuss the properties of the copolymers with different
electron-withdrawing and electron-donating functional groups.
EXPERIMENTAL
Materials
C61-butyric acid methyl ester (PC61BM), which have achieved
8-Dihydrobenzo[1,2-b:4,5-b0]dithiophen-4,8-dione (1) was
prepared according to the published method.24 Trimethyltin
chloride, and 2-(tributylstannyl)-thiophene (8) were obtained
from Aldrich Chemicals. 1-Bromooctane, bromine, 2,3-diamino-
naphthalene (4), 4,40-dimethoxybenzil (6a), benzil (6b), 4,40-
difluorobenzil (6c), tetrakis(triphenylphosphine) palladium
[Pd(PPh3)4], N-bromosuccinimide (NBS), and bis(triphenyl-
phosphine)palladium(II) dichloride [PdCl2(PPh3)2] were pur-
chased from Acros Organics. Tetrabutylammonium bromide
(TBAB) was obtained from TCI. n-Butyllithium was obtained
from Strem Chemicals. Toluene, dichlorobenzene, and N,N-
dimethylformamide (DMF) were obtained from TEDIA. All
reagents were used as received.
a power conversion efficiency (PCE) of 4 to 6%.8–10 How-
ever, the PCE of a P3HT/PC61BM blended system is limiting,
because its absorption wavelength is less than 650 nm. To
improve device performance, many researches have devel-
oped conjugated polymers with low-band gap, high mobility,
and broader absorption of the solar spectrum.11–18
Internal charge transfer (ICT) from an electron donor (D) to
electron acceptor (A) has been used to synthesize low-band
gap conjugated polymers.19,20 Several new D–A polymers
were developed, which exhibited good performance. Conju-
gated copolymers containing 3,6-dithiophen-2-yl-2,5-dihydro-
pyrrolo[3,4-c]pyrrole-1,4-dione were applied to PSCs, and a
high PCE of 4.45% was achieved.21 Biniek et al.22 reported
[3,2-b]thienothiophene-alt-benzothiadiazole copolymer for
photovoltaic applications with PCE up to 5.2%.
Measurements and Characterization
1H and 13C NMR spectra were recorded on a Brucker
Advance 600 spectrometer, and the deuterated CDCl3, and
DMSO were used as solvents, and the chemical shifts were
reported in ppm. The molecular weights and distributions of
the polymers were obtained by using a Waters GPC 2414 in
In this work, we developed a series of copolymers based on
dioctyloxybenzo [1,2-b;3,4-b0]dithiophene and bis(2-thienyl)-
2,3-diphenylbenzo[g]quinoxaline. Benzo[1,2-b;3,4-b0]dithiophene
Correspondence to: S. L.-C. Hsu (E-mail: lchsu@mail.ncku.edu.tw)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 662–670 (2011) 2010 Wiley Periodicals, Inc.
662
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
tetrahydrofuran (THF) via a calibration curve of polystyrene
standards. Thermal stability was analyzed using a TA instru-
ment thermogravꢀim1 etric analyzer (TGA) Q500 at a heating
removed, and the reaction was stirred at ambient tempera-
ture for 2 h. Then, it was poured into 200 mL of cool water
and extracted by ether three times. The organic layer was
washed with water two times, and then dried by anhydrous
MgSO4. After removing the solvent under vacuum, the
residue was recrystallized from ethyl alcohol three times.
The yield was 62%.
ꢁ
rate of 10 C min under nitrogen. The glass transition tem-
perature (Tg) was determined on a Perkin Elmer Instruments
Model 4000 differential scanning calorimeter (DSC) at a
heating rate of 10 ꢁC minꢀ1 under nitrogen. UV–vis absorp-
tion spectra were recorded on a Hitachi U-2001 spectropho-
tometer. The voltage–current was controlled by a Keithley
2400 power source. Cyclic voltammetry (CV) was carried out
on a potentiostat/galvanostat Autolab PGSTAT30 with a
platinum electrode at a scan rate of 50 mV sꢀ1 against an
Ag/AgCl reference electrode with a nitrogen-saturated solu-
tion of 0.1 M tetrabutylammonium hexafluorophosphate in
acetonitrile. The films of the copolymer/PC71BM were placed
on a 200 mesh copper grid, and examined with a JEOL JEM-
1200EX TEM using an acceleration voltage of 200 KV. Hole
mobility was investigated by fitting dark J–V curves to the
space-charge-limited current (SCLC) model25 with a device
structure of ITO/PEDOT:PSS/copolymers:PC61BM (1:4 wt
ratio)/Au, where the SCLC is described by
1H NMR (600 MHz, CDCl3): 7.52 (s, 2H), 4.30 (t, 4H), 1.89
(m, 4H), 1.59 (m, 4H), 1.42ꢂ1.31 (m, 16H), 0.90 (t, 6H), 0.45
(s, 18H). ELEM. ANAL. (%) calcd: C, 49.76; H, 7.05; Found: C,
50.11; H, 7.20; MS (FAB): m/z 772 (Mþ) (calcd. 772.3).
1,4-Dibromonaphthalene-2,3-diamine (5)
Compound (4) (1 g, 6.32 mmol) was dissolved in 35 mL of
glacial acetic acid. Bromine (0.8 mL, 14.5 mmol) in 30 mL of
glacial acetic acid was added slowly, and the mixture was
stirred for 2 h at room temperature. The precipitate was
filtered and washed with aqueous K2CO3 solution and water.
The yield was 85%.
1H NMR (600 MHz, DMSO): 7.79 (d, 2H), 7.26 (d, 2H), 5.65
(s, 4H).
J ¼ 9e0erlV2=8L3
5,10-Dibromo-2,3-bis-(4-methoxyphenyl)benzo[g]
quinoxaline (7a)
where e0 is the permittivity of free space, er is the dielectric
constant of the polymer, l is the hole mobility, V is the
voltage drop across the device, and L is the thickness of the
active layer.
A mixture of compound (5) (0.954 g, 3 mmol), compound
(6a) (0.81 g, 3 mmol), and ethanol (30 mL) was refluxed
under nitrogen for 12 h. The precipitate was collected by
filtration, washed with ethanol, and then vacuum dried. The
yield was 65%.
Monomer Synthesis
4,8-Dioctyloxybenzo[1,2-b;3,4-b0]dithiophene (2)23
1H NMR (600 MHz, DMSO): 8.56 (d, 2H), 7.82 (d, 2H), 7.61
(d, 4H), 7.01 (d, 4H), 3.82 (s, 6H).
The synthetic route of the monomers is shown in Scheme 1.
Compound (1) (2.2 g, 10 mmol) and zinc powder were sus-
pended in 25 mL of water, and, then, 6 g of NaOH was added
into the mixture under nitrogen. The mixture was heated to
reflux and stirred for 1 h. During the reaction, the color of the
mixture changed from yellow to red, and then to orange. Subse-
quently, 1-bromooctane (5.8 g, 30 mmol) and a catalytic amount
of TBAB were added into the mixture, and the reaction solution
was refluxed for 12 h. The reaction mixture was poured into
cold water and extracted by diethyl ether three times. The or-
ganic layer was dried over anhydrous MgSO4, and then the sol-
vent was removed. The crude product was recrystallized from
alcohol to obtain compound (2). The yield was 67%.
5,10-Dibromo-2,3-diphenylbenzo[g]quinoxaline (7b)
The synthetic route of compound (7b) was similar to com-
pound (7a). The difference was that the compound (6a) was
replaced by compound (6b). The yield was 68%.
1H NMR (600 MHz, DMSO): 8.62 (d, 2H), 7.88 (d, 2H), 7.62
(d, 4H), 7.48 (t, 2H), 7.43 (t, 4H).
5,10-Dibromo-2,3-bis-(4-fluorophenyl)benzo[g]
quinoxaline (7c)
The synthetic route of compound (7c) was similar to com-
pound (7a). The difference was that the compound (6a) was
replaced by compound (6c). The yield was 64%.
1H NMR (600 MHz, DMSO): 8.26 (d, 2H), 7.88 (d, 2H), 7.68
(d, 4H), 7.30 (d, 4H).
1H NMR (600 MHz, CDCl3): 7.48 (d, 2H), 7.36 (d, 2H), 4.30
(t, 4H), 1.89 (m, 4H), 1.55 (m, 4H), 1.41ꢂ1.29 (m, 16H), 0.90
(t, 6H).
2,6-Bis(trimethyltin)-4,8-dioctyloxybenzo[1,2-b;3,
4-b0]dithiophene (3)23
5,10-Di-2-thienyl-2,3-bis-(4-methoxyphenyl)benzo[g]
quinoxaline (9a)
Compound 2 (2.68 g, 6 mmol) and 100 mL of THF were
added into a flask under an inert atmosphere. n-Butyllithium
(13.2 mmol, 1.6 M in n-hexane) was added dropwise into
the solution at room temperature, and after being stirred for
1 h at room temperature, a great deal of white solid precipi-
tate appeared in the flask. Then, 14 mmol of trimethyltin
chloride (14 mL, 1 M in n-hexane) was added in one portion,
and the reactant turned clear rapidly. The cooling bath was
Compound (7a) (1.1 g, 2 mmol) and compound (8) (1.8 g,
4.8 mmol) were dissolved in 70 mL of DMF, and
PdCl2(PPh3)2 was added into the solution. The reaction mix-
ture was heated to 120 ꢁC for 48 h under nitrogen atmo-
sphere. After removing the solvent under reduced pressure,
the crude product was washed with methanol three times.
The yield was 56%.
DERIVATIVES OF BENZO[g]QUINOXALINE, LEE ET AL.
663

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthesis of monomers.
1H NMR (600 MHz, DMSO): 8.22 (d, 2H), 7.94 (d, 2H), 7.63
(d, 2H), 7.47 (d, 4H), 7.41 (d, 2H), 7.36 (t, 2H), 6.92 (d, 4H),
3.77 (s, 6H).
1H NMR (600 MHz, DMSO): 8.23 (t, 2H), 7.93 (d, 2H), 7.65
(t, 2H), 7.48 (d, 4H), 7.42 (d, 2H), 7.39–7.32 (m, 8H).
5,10-Di-2-thienyl-2,3-bis-(4-fluorophenyl)benzo[g]
quinoxaline (9c)
The synthetic route of compound (9c) was similar to com-
pound (9a). The difference was that the compound (7a) was
replaced by compound (7c). The yield was 58%.
5,10-Di-2-thienyl-2,3-diphenylbenzo[g]quinoxaline (9b)
The synthetic route of compound (9b) was similar to
compound (9a). The difference was that the compound (7a)
was replaced by compound (7b). The yield was 59%.
664
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
1H NMR (600 MHz, DMSO): 8.22 (d, 2H), 7.93 (d, 2H), 7.65
(t, 2H), 7.52 (d, 4H), 7.41 (d, 2H), 7.35 (d, 2H), 7.20 (d, 4H).
(600 MHz, CDCl3): 160.69, 151.95, 144.08, 139.76, 137.51,
135.34, 132.98, 131.7, 131.43, 129.86, 127.18, 126.86,
125.01, 116.05, 113.71, 74.10, 55.29, 31.86, 30.64, 29.35,
26.15, 22.67, 14.07.
5,10-Bis(5-bromo-2-thienyl)-2,3-bis-(4-methoxyphenyl)
benzo[g] quinoxaline (10a)
Compound (9a) (0.834 g, 1.5 mmol) was dissolved in 40 mL
of DMF under nitrogen atmosphere, and NBS (0.54 g, 3
mmol) was added in one portion. The mixture was stirred
overnight at room temperature. After removing the solvent
under reduced pressure, the solid was washed with metha-
nol several times. Finally, the precipitate was recrystallized
from a mixture of dichloromethane and methanol at room
temperature. The yield was 60%.
PbtbQ
Monomer feed ratio: compound (3) (0.386 g, 0.5 mmol) and
compound (10b) (0.327 g, 0.5 mmol). The yield was 72%.
1H NMR (600 MHz, CDCl3): 8.54 (s), 7.74–7.58 (broad),
7.44–7.35 (broad), 4.34–4.41 (broad), 1.99 (broad), 1.66–
1.30 (broad), 0.84–0.91 (broad). 13C NMR (600 MHz, CDCl3):
152.45, 144.10, 139.95, 138.80, 137.22, 135.33, 133.22,
132.69, 131.80, 130.34, 129.82, 128.18, 127.12, 125.01,
116.14, 74.10, 31.87, 30.65, 29.36, 26.15, 22.68, 14.09.
1H NMR (600 MHz, DMSO): 8.24 (d, 2H), 7.65 (d, 2H),
7.46 (m, 6H), 7.25 (d, 2H), 6.94 (d, 4H), 3.79 (s, 6H).
ELEM. ANAL. (%) calcd.: C, 57.15; H, 3.10; N, 3.92; Found: C,
56.49; H, 3.02; N, 4.00. MS (FAB): m/z 714 (Mþ) (calcd.
714.5).
PbtFbQ
Monomer feed ratio: compound (3) (0.386 g, 0.5 mmol) and
compound (10c) (0.345 g, 0.5 mmol). The yield was 63%.
1H NMR (600 MHz, CDCl3): 8.53 (s), 7.70–7.58 (broad),
7.50–7.40 (broad), 7.05 (broad), 4.38–4.34 (broad), 1.97
(broad), 1.64–1.29 (broad), 0.91–0.84 (broad). 13C NMR
(600 MHz, CDCl3): 164.38, 162.71, 151.10, 144.08, 140.01,
137.07, 136.81, 135.24, 134.68, 133.30, 132.66, 131.83,
130.31, 129.56, 128.99, 127.28, 126.56, 125.02, 116.19,
115.37, 74.06, 31.86, 30.62, 29.34, 26.13, 22.67, 14.07.
5,10-Bis(5-bromo-2-thienyl)-2,3-diphenylbenzo[g]
quinoxaline (10b)
The synthetic route of compound (10b) was similar to
compound (10a). The difference was that the compound
(9a) was replaced by compound (9b). The yield was 55%.
1H NMR (600 MHz, DMSO): 8.25 (d, 2H), 7.66 (d, 2H), 7.47–
7.44 (m, 6H), 7.39 (d, 2H), 7.34 (t, 4H), 7.25 (d, 2H). ELEM.
ANAL. (%) calcd: C, 58.73; H, 2.77; N, 4.28 Found: C, 57.98; H,
2.74; N, 4.31. MS (FAB): m/z 654 (Mþ) (calcd. 654.4).
PbttpmobQ
Monomer feed ratio: compound (3) (0.386 g, 0.5 mmol),
compound (10a) (0.179 g, 0.25 mmol), and compound (11)
(0.061 g, 0.25 mmol). The yield was 68%.
5,10-Bis(5-bromo-2-thienyl)-2,3-bis-(4-fluorophenyl)
benzo[g] quinoxaline (10c)
The synthetic route of compound (10c) was similar to com-
pound (10a). The difference was that the compound (9a)
was replaced by compound (9c). The yield was 62%.
1H NMR (600 MHz, DMSO): 8.25 (d, 2H), 7.69 (d, 2H), 7.51
(d, 4H), 7.45 (d, 2H), 7.22–7.27 (m, 6H). ELEM. ANAL. (%)
calcd.: C, 55.67; H, 2.34; N, 4.06 Found: C, 54.96; H, 2.31; N,
3.97. MS (FAB): m/z 690 (Mþ) (calcd. 690.4).
1H NMR (600 MHz, CDCl3): 8.50 (s), 7.70–7.41 (broad), 6.87
(broad), 4.34 (broad), 3.82 (s), 1.96 (broad), 1.65–1.29
(broad), 0.92–0.83 (broad). 13C NMR (600 MHz, CDCl3):
160.68, 151.91, 144.08, 137.04, 136.05, 135.34, 132.95,
132.69, 132.39, 131.79, 129.58, 126.86, 125.02, 116.04,
113.70, 74.06, 55.28, 31.90, 30.64, 29.37, 26.15, 22.69,
14.17.
General Procedure of Copolymer Synthesis
Fabrication and Characterization of Solar Cell Devices
Patterned indium tin oxide (ITO) slides were cleaned with
detergent, deionized water, acetone, and 2-propanol in an
ultrasonic bath and dried on a hot plate at 150 ꢁC for
30 min, and then treated with UV-ozone for 30 min before
use. A 50–60 nm layer of poly(styrene sulfonic acid)-doped
poly(ethylene dioxythiophene) [PEDOT: PSS] (Baytron P VP
AI 4083) was spin-coated on the ITO, and baked for 30 min
at 150 ꢁC. The copolymer blended with PCBM (1:4 by
weight) solution in dichlorobenzene was spin coated on top
of PEDOT: PSS, and dried on a hot plate. Finally, a thin layer
of calcium (10 nm) was deposited by thermo-evaporation
under a vacuum of 10ꢀ5 torr, and followed by a layer of Al
(100 nm). The voltage–current characteristics were analyzed
using a Keithley 2400 sourcemeter under one sun illumina-
tion of AM 1.5 (100 mW cmꢀ2) from a solar simulator
(Model Class A 91160A, Newport-Oriel Instruments). The
solar-simulator illumination intensity was measured using a
standard monocrystalline silicon photovoltaic calibrated by
The synthetic route is shown in Scheme 2. The synthesis of
copolymer PbtmobQ is taken as an example. Compound (3)
(0.386 g, 0.5 mmol) and compound (10a) (0.357 g, 0.45
mmol) were dissolved in toluene (8 mL) and DMF (2 mL),
and purged under a nitrogen atmosphere for 30 min. Sub-
sequently, Pd(PPh3)4 (25 mg) was added into the flask, and
the reaction mixture was stirred at 120 ꢁC for 24 h. After
being cooled to room temperature, the reaction mixture was
poured into methanol. The collected solid was re-dissolved
in chloroform and filtered to remove the metal catalyst.
Then, the copolymer solution was concentrated and precipi-
tated in a large amount of methanol. The solid was further
purified by a Soxhlet extractor for 24 h using acetone as a
ꢁ
solvent, and dried in a vacuum oven at 80 C overnight. The
yield was 60%.
1H NMR (600 MHz, CDCl3): 8.50 (s), 7.72–7.67 (broad), 7.58
(broad), 7.40 (m), 6.88 (broad), 4.41–4.34 (broad), 3.81 (s),
1.99 (broad), 1.66–1.26 (broad), 0.82–0.91 (broad). 13C NMR
DERIVATIVES OF BENZO[g]QUINOXALINE, LEE ET AL.
665

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 2 Synthesis of copoly-
mers.
the National Renewable Energy Laboratory. The device mea-
surement was carried out in air at room temperature.
bility of the polymers. PbtmobQ and PbttpmobQ have higher
molecular weights than PbtmobQ and PbttpmobQ, which
could be due to their better solubility in the polymerization
RESULTS AND DISCUSSION
TABLE 1 Polymerization Results and Thermal Properties
Synthesis and Characterization
of the Copolymers
The synthetic route of the copolymers via palladium cata-
lyzed Stille reaction26,27 is shown in scheme 2. The colors
of the resulting alternating copolymers are dark. The weight-
average molecular weights (Mw), number-average molecular
weights (Mn), and polydispersity index (PDI) of the copoly-
mers were determined by gel permeation chromatography
(GPC) against polystyrene standard in THF are listed in Ta-
ble 1. The methoxy group in PbtmobQ and PbttpmobQ was
introduced to increase the electron donating ability and solu-
Copolymer
Mn
Mw
PDI
Td (ꢁC)a
PbtmobQ
PbtbQ
24,600
12,600
12,000
20,100
47,500
25,900
22,800
41,100
1.93
2.05
1.90
2.04
341
337
334
349
PbtFbQ
PbttpmobQ
a
Temperature of 5% weight loss measured by TGA in nitrogen.
666
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
TABLE 2 Electrochemical Properties of Copolymers
Eg
Eg
Eonset
Eonset
HOMO LUMO (elec)a (opt)b
Copolymer (ox) (V) (re) (V) (eV)
(eV)
(eV)
(eV)
PbtmobQ
PbtbQ
0.61
0.65
0.70
ꢀ0.92
ꢀ0.86
ꢀ0.78
ꢀ0.64
ꢀ4.92
ꢀ4.96
ꢀ5.01
ꢀ5.1
ꢀ3.39 1.53
ꢀ3.45 1.51
ꢀ3.53 1.48
ꢀ3.67 1.43
1.78
1.65
1.59
1.64
PbtFbQ
PbttpmobQ 0.79
a
Eg (elec) ¼ ꢀ(HOMO ꢀ LUMO).
b
Optical band gaps are estimated from the onset of absorption,
Eg (opt) ¼ 1240/konset
.
Optical Properties
The UV–vis absorption spectra of the copolymers as spin
coated films on quartz and in dichlorobenzene are shown in
Figure 2. PbtmobQ, PbtbQ, and PbtFbQ exhibited two absorp-
tion regions, which is a common phenomenon on donor–
acceptor copolymers.28 It can be observed that the absorp-
tion peaks of PbtFbQ show red shift in comparison with
PbtmobQ and PbtbQ at long wavelength region (500ꢂ900
nm), due to the electron-withdraw effect of fluorine. The
spectrum of PbttpmobQ shows broad and strong absorption
peaks ranging from 300 to 800 nm because of copolymeriza-
tion with three monomers. The absorption peaks of the
FIGURE 1 TGA curves of copolymers.
solvents. The thermal stability of the copolymers was investi-
gated by TGA under a nitrogen atmosphere as shown in
Figure 1. The thermal decomposition temperatures (Tds, 5%
weight loss) of PbtmobQ, PbtbQ, PbtFbQ, and PbttpmobQ are
341, 337, 334, and 349 ꢁC. The result indicates that all
copolymers have good thermal stability.
FIGURE 2 UV–vis spectra of copolymer in solid state (A) and in
solution (B).
FIGURE 3 Cyclic voltammograms of copolymers.
DERIVATIVES OF BENZO[g]QUINOXALINE, LEE ET AL.
667

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
intermolecular interactions in the solid state. The optical
band gaps (Egopt) of the copolymers (Table 2), derived from
the absorption edge, range from 1.59 to 1.78 eV.
Electrochemical Properties
The redox behaviors of the copolymers were investigated by
CV, and their cyclic voltammograms are showed in Figure 3.
The highest occupied molecular orbital (HOMO) and the
LUMO energy values of the copolymers are listed in Table 2.
Comparing PbtmobQ, PbtbQ, and PbtFbQ, the introduction of
the electron-withdrawing fluorine and electron-donating
methoxy groups changed the energy levels of the copolymers
slightly. Comparing PbttpmobQ and PbtmobQ, less electron-
withdrawing benzoquinoxaline groups in the polymer back-
bone led to the lowering of the HOMO level.
Hole Mobility
FIGURE 4 Dark J–V curves of the hole-only devices with
Figure 4 shows the J–V curves of the devices with the
copolymers:PC61BM (1:4 wt ratio) blended films. The hole
mobilities of PbtmobQ, PbtbQ, PbtFbQ, and PbttpmobQ
blends were 4.18 ꢃ 10ꢀ5, 4.04 ꢃ 10ꢀ5, 3.07 ꢃ 10ꢀ5, and
copolymers:PC61BM (1:4 wt ratio)-blended films.
copolymer films [Fig. 2(A)] were similar to the spectra in so-
lution [Fig. 2(B)] with red shifts. The higher absorption at
long wavelength (450ꢂ700 nm) was attributed to the strong
8.05 ꢃ 10ꢀ5 cm2 Vꢀ1
s
ꢀ1, respectively. PbttpmobQ exhibits
the best hole mobility, because intermolecular packing may
be enhanced when planar thiophene segments replaced large
bis(2-thienyl)-2,3-bis-(4-phenyl)benzo[g]quinoxaline units in
the copolymer backbone.
Photovoltaic Properties
The bulk heterojunction PSCs with configuration of ITO/
PEDOT: PSS/copolymers: PCBM (1:4 wt ratio)/Ca/Al were
fabricated. Figure 5 shows the J–V curves of the devices. The
open-circuit voltage (VOC), short-circuit current (JSC), fill fac-
tor (FF), and PCE are summarized in Table 3. The PCEs of
PC61BM based devices are 0.73 (PbtmobQ), 0.53 (PbtbQ),
0.56 (PbtFbQ), and 0.87 (PbttpmobQ), respectively. As listed
in Table 3, the VOC and FF of devices are close, but the Isc
increases from 2.21 (PbtbQ) to 3.92 mA cmꢀ2 (PbttpmobQ).
The device with PbttpmobQ exhibits the highest value of Isc.
This is because PbttpmobQ has a broader absorption spec-
trum, which can harvest more sunlight, and higher hole
mobility. To improve PCE, PC71BM was used to replace
PC61BM. The PCEs of PbtmobQ and PbttpmobQ were achieved
at 1.04 and 1.42%. The significantly higher Isc is due to the
higher absorption of PC71BM in the visible region.29
TABLE 3 Photovoltaic Performances of the PSCs Based
on the Copolymers
Copolymer/
Isc
(mA cmꢀ2
PC61BM (1:4)
VOC (V)
)
FF (%)
PCE (%)
PbtmobQ
PbtbQ
0.85
0.8
2.9
29.7
30.1
29.8
29.7
28.9
30.9
0.73
0.53
0.56
0.87
1.04
1.42
2.21
2.34
3.92
4.50
5.73
PbtFbQ
0.8
PbttpmobQ
PbtmobQa
PbttpmobQa
0.75
0.8
FIGURE 5 Current density–voltage characteristics of the PSCs
0.8
based on copolymesr:PC61BM (A) and copolymers:PC71BM (B)
under the illumination of AM 1.5, 100 mW cmꢀ2
.
PC71BM.
a
668
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
2 Coakley, K. M.; McGehee, M. D. Chem Mater 2004, 16,
4533–4542.
3 Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Adv Funct
Mater 2005, 15, 1617–1622.
4 Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T. Q.;
Dante, M.; Heeger, A. J. Science 2007, 317, 222–225.
5 Winder, C.; Sariciftci, N. S. J Mater Chem 2004, 14, 1077–
1086.
6 Mihailetchi, V. D.; Koster, L. J. A.; Blom, P. W. M.; Melzer, C.;
Boer, B. D.; Duren, J. K. J. V.; Janssen, R. A. J. Adv Funct
Mater 2005, 15, 795–801.
7 Riedel, I.; Hauff, E. V.; Parisi, J.; Martin, N.; Giacalone, F.;
Dyakonov, V. Adv Funct Mater 2005, 15, 1979–1987.
8 Reyes, M. R.; Kim, K.; Carroll, D. L. Appl Phys Lett 2005, 87, 083506.
9 Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery,
K.; Yang, Y. Nat Mater 2005, 4, 864–868.
10 Kim, K.; Liu, J.; Namboothiry, M. A. G.; Carroll, D. L. Appl
Phys Lett 2007, 90, 163511.
11 Hou, J.; Tan, Z.; Yan, Y.; He, Y.; Yang, C.; Li, Y. J Am Chem
FIGURE 6 TEM image of PbttpmobQ:PC71BM (1:4 wt ratio) film.
Soc 2006, 128, 4911–4916.
12 Becerril, H. A.; Miyaki, N.; Tang, M. L.; Mondal, R.; Sun, Y.
S.; Mayer, A. C.; Parmer, J. E.; McGehee, M. D.; Bao, Z. J Mater
Chem 2009, 19, 591–593.
The dispersion state of the active layer is important to
photovoltaic performance. Figure 6 shows the TEM bright-
field image of the PbttpmobQ:PC71BM (1:4 wt ratio)-blended
film. A phase separation between the PC71BM (dark areas)
and polymer matrix can be observed. The agglomerations of
PC71BM are connected to each other and become pathways
in the polymer films. The continuous pathways provide both
charges to reach the respective electrodes for effective
charge collection.30,31 The PC71BM domains exhibit a broad-
size distribution (500ꢂ1500 nm), and the large scale of
phase separation reduces the donor–acceptor interface for
excitons to dissociate.32,33
13 Liang, Y.; Wu, Y.; Feng, G.; Tsai, S. T.; Son, H. J.; Li, G.; Yu,
L. J Am Chem Soc 2009, 131, 56–57.
14 Gedefaw, D.; Zhou, Y.; Hellstrom, S.; Lindgren, L.; Ander-
¨
sson, L. M.; Zhang, F.; Mammo, W.; Inganas, O.; Andersson,
¨
M. R. J Mater Chem 2009, 19, 5359–5363.
15 Ahmed, E.; Kim, F. S.; Xin, H.; Jenekhe, S. A. Macro-
molecules 2009, 42, 8615–8618.
16 Song, S.; Jin, Y.; Park, S. H.; Cho, S.; Kim, I.; Lee, K.;
Heeger, A. J.; Suh, H. J Mater Chem 2010, 20, 6517–6523.
17 Song, J.; Zhang, C.; Li, C.; Li, W.; Qin, R.; Li, B.; Liu, Z.; Bo,
CONCLUSIONS
Z. J Polym Sci Part A: Polym Chem 2010, 48, 2571–2578.
18 Song, S.; Park, S. H.; Jin, Y.; Park, J.; Shim, J. Y.; Kim, I.;
Lee, H.; Lee, K.; Suh, H. J Polym Sci Part A: Polym Chem 2010,
48, 4567–4573.
In this article, we have developed four new low-band gap
copolymers based on dioctyloxybenzo[1,2-b;3,4-b0]dithiophene
and bis(2-thienyl)-2,3-diphenylbenzo[g]quinoxaline. The band
gaps of the copolymers are 1.78, 1.65, 1.59, and 1.64 eV, res-
pectively. The energy levels of the copolymers were changed
slightly by substituting different side chains of bis(2-thienyl)-
2,3-diphenylbenzo[g]quinoxalines. The solar cell device based
on PbttpmobQ:PC71BM (1:4 wt ratio) exhibits the best PCE of
1.42% with a VOC of 0.8V, JSC of 5.73 mA cmꢀ2, and FF
19 Zhang, Q. T.; Tour, J. M. J Am Chem Soc 1998, 120, 5355– 5362.
20 Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.;
Neagu-Plesu, R.; Belletete, M.; Duricher, G.; Tao, Y.; Leclerc, M.
J Am Chem Soc 2008, 130, 732–742.
21 Huo, L.; Hou, J.; Chen, H. Y.; Zhang, S.; Jiang, Y.; Chen, T.
of 30.9% under the illumination of AM 1.5, 100 mW cmꢀ2
.
L.; Yang, Y. Macromolecules 2009, 42, 6564–6571.
The results indicate that the four copolymers are promising
candidates for PSCs.
22 Biniek, L.; Chochos, C. L.; Leclerc, N.; Hadziioannou, G.; Kallit-
sis, J. K.; Bechara, R.; Leveque, P.; Heiser, T. J Mater Chem 2009,
19, 4946–4951.
The financial support provided by the National Science Council
(Taiwan, ROC) through project NSC98-2627-E-006-001 is
greatly appreciated.
23 Hou, J.; Park, M. H.; Zhang, S.; Yao, Y.; Chen, L. M.; Li, J. H.;
Yang, Y. Macromolecules 2008, 41, 6012–6018.
24 Sloocum, D. W.; Gierer, P. L.
J Org Chem 1976, 41,
REFERENCES AND NOTES
3668–3673.
1 Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. Adv Funct
25 Chang, Y. T.; Hsu, S. L.; Su, M. H.; Wei, K. H. Adv Mater
Mater 2003, 13, 85–88.
2009, 21, 2093–2097.
DERIVATIVES OF BENZO[g]QUINOXALINE, LEE ET AL.
669

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
26 Liang, Y.; Feng, D.; Wu, Y.; Tsai, S. T.; Li, G.; Ray, C.; Yu, L.
30 Yang, X.; Duren, J. K. J. V.; Janssen, R. A. J.; Michels, M. A.
J Am Chem Soc 2009, 131, 7792–7799.
J.; Loos, J. Macromolecules 2004, 37, 2151–2158.
27 Hou, J.; Chen, T. L.; Zhang, S.; Huo, L.; Sista, S.; Yang, Y.
31 Yang, X.; Loos, J.; Veenstra, S. C.; Verhees, W. J. H.; Wienk,
M. M.; Kroon, J. M.; Michels, M. A.; Janssen, R. A. J. Nano Lett
2005, 5, 579–583.
Macromolecules 2009, 42, 9217–9219.
28 Wang, E.; Wang, M.; Wang, L.; Duan, C.; Zhang, J.; Cai,
W.; He, C.; Wu, H.; Cao, Y. Macromolecules 2009, 42,
4410–4415.
32 Sivula, K.; Ball, Z. T.; Watanabe, N.; Frechet, M. J. Adv
Mater 2006, 18, 206–210.
29 Chen, C. P.; Chan, S. H.; Chao, T. C.; Ting, C.; Ko, B. T.
33 Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem Rev 2007,
¨
J Am Chem Soc 2008, 130, 12828–12833.
107, 1324–1338.
670
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA