Synthesis and photovoltaic properties of low-bandgap 4,7-dithien-2-yl-2,1, 3-benzothiadiazole-based poly(heteroarylenevinylene)s

Article abstract of DOI:10.1002/pola.24704

Three novel low-bandgap copolymers containing alkylated 4,7-dithien-2-yl-2,1,3-benzothiadiazole (HBT) and different electron-rich functional groups (dialkylfluorene (PFV-HBT), dialkyloxyphenylene (PPV-HBT) and dialkylthiophene (PTV-HBT)) were prepared by Horner polycondensation reactions and characterized by ¹H NMR, gel permeation chromatography, and elemental analysis. The alkyl side chain brings these polymeric materials good solubility in common organic solvents, which is critical for the manufacture of solar cells in a cost-effective manner. The copolymers exhibit low optical bandgap from 1.48 to 1.83 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the copolymers were measured by cyclic voltammetry. Theoretical calculations revealed that the variation laws of HOMO and the LUMO energy levels are well consistent with cyclic voltammetry measurement. The bulk heterojunction photovoltaic devices with the structure of ITO/PEDOT-PSS/polymer:PCBM/LiF/Al were fabricated by using the three copolymers as the donor and (6,6)-phenyl-C₆₁-butyric acid methyl ester (PCBM) as the acceptor in the active layer. The device based on PTV-HBT:PCBM (1:4 w/w) achieved a power conversion efficiency of 1.05% under the illumination of AM 1.5, 100 mW/cm².

Full text of DOI:10.1002/pola.24704

Synthesis and Photovoltaic Properties of Low-Bandgap 4,7-Dithien-2-yl-
2,1,3-Benzothiadiazole-Based Poly(heteroarylenevinylene)s
SHANPENG WEN, JIANING PEI, PENGFEI LI, YINHUA ZHOU, WEIDONG CHENG, QINGFENG DONG, ZAIFANG LI, WENJING TIAN
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, People’s Republic of China
Received 16 February 2011; accepted 1 April 2011
DOI: 10.1002/pola.24704
Published online 27 April 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Three novel low-bandgap copolymers containing
alkylated 4,7-dithien-2-yl-2,1,3-benzothiadiazole (HBT) and differ-
ent electron-rich functional groups (dialkylfluorene (PFV-HBT),
dialkyloxyphenylene (PPV-HBT) and dialkylthiophene (PTV-
HBT)) were prepared by Horner polycondensation reactions and
characterized by 1H NMR, gel permeation chromatography, and
elemental analysis. The alkyl side chain brings these polymeric
materials good solubility in common organic solvents, which is
critical for the manufacture of solar cells in a cost-effective man-
ner. The copolymers exhibit low optical bandgap from 1.48 to
1.83 eV. The highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) energy levels of
the copolymers were measured by cyclic voltammetry. Theoreti-
cal calculations revealed that the variation laws of HOMO and
the LUMO energy levels are well consistent with cyclic voltam-
metry measurement. The bulk heterojunction photovoltaic devi-
ces with the structure of ITO/PEDOT-PSS/polymer:PCBM/LiF/Al
were fabricated by using the three copolymers as the donor and
(6,6)-phenyl-C61-butyric acid methyl ester (PCBM) as the
acceptor in the active layer. The device based on PTV-HBT:PCBM
(1:4 w/w) achieved a power conversion efficiency of 1.05% under
2
C
V
the illumination of AM 1.5, 100 mW/cm . 2011 Wiley Periodi-
cals, Inc. J Polym Sci Part A: Polym Chem 49: 2715–2724, 2011
KEYWORDS: bulk heterojunction; conducting polymers; copoly-
merization; electrochemistry; polymer solar cells
INTRODUCTION Polymer solar cells (PSCs) have great poten-
tial as the future energy source due to the advantages of low
cost, easy processing, light weight, being mechanically flexi-
ble, and suitable for large-area fabrication.1–6 The most
widely used configuration of PSCs is the bulk heterojunction
(BHJ) solar cells in which the active layer consists of a blend
of an electron donating conjugated polymer and an electron
acceptor such as (6,6)-phenyl C61-butyric acid methyl ester
(PCBM).7–10 Recently, such polymer photovoltaic cells with
power conversion efficiency (PCE) as high as 5% have been
realized by using a blend of regioregular poly(3-hexylthio-
phene) (P3HT) and PCBM as the active layer.11,12 However,
further improvement of P3HT-based PVC devices is difficult
due to P3HT’s intrinsic absorption limit. A good overlap of
the absorption of the active layer with the solar spectrum
[visible and near-infrared (IR)] is required to improve the
overall solar cell energy conversion efficiency.13 Low band
gap materials that absorb visible to IR light are favorable in
the selection of the active layer materials. In the past few
years, chemists have synthesized various low-bandgap poly-
mers consisting of alternating electron donating (D) and
electron accepting (A) units being able to harvest more pho-
tons at long wavelengths.14
the manipulation of the optical properties and electronic
structures (the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) energy lev-
els, and bandgaps). Thus, through the design and selection of
D and A building blocks, the strength of ICT can be tuned,
allowing D-A copolymers to exhibit low band gap and broad
absorption band that extends to the red and even IR region.
By using this strategy, many novel low band gap polymers
have been synthesized and used in PSCs with PCEs over 7%
combined with intensive device engineering efforts.15
Among the large variety of acceptor segments, 4,7-dithien-2-
yl-2,1,3-benzothiadiazole (DTBT), linking two thienyl units in
the 4,7-position of 2,1,3-benzothiadiazole, has been devel-
oped and copolymerized with many types of donor seg-
ments, such as fluorene,16 silafluorene,17 dialkoxypheny-
lene,18 carbezole,19 dithienosilole,20 cyclopenta[2,1-b:3,4-
b]dithiophene,21 and benzo[1,2-b:4,5-b0]dithiophene.22 The
two thiophene units were added on both ends of the 2,1,3-
benzothiadiazole to obtain relatively planar copolymers with
good charge transport properties.23 Interestingly, this type of
polymers has recently reached a PCE of 6.1% in BHJ photo-
voltaic devices.24
Link pattern (e.g., carbon-carbon single, double, or triple
bond) has a significant influence on the chemical and physi-
cal properties of polymers.25,26 Generally, low-bandgap
Intramolecular charge transfer (ICT) interaction between the
donor and acceptor units within D-A copolymers facilitates
Correspondence to: W. Tian (E-mail: wjtian@jlu.edu.cn)
C
V
Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 2715–2724 (2011) 2011 Wiley Periodicals, Inc.
SYNTHESIS AND PHOTOVOLTAIC PROPERTIES OF LOW-BANDGAP HBT, WEN ET AL.
2715

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
DTBT-based copolymers have been synthesized using the
linkage of single bonds via Suzuki or Stille cross-coupling
reaction, while the alternative copolymer linked by double
bonds via Horner reaction that bear DTBT units is still unex-
plored. The advantage of incorporating double bond, vinyl-
ene, linkages between the donor and acceptor groups inside
the conjugated copolymers is that the vinylene linkage can
increase the planarity of the copolymer backbone by elimi-
nating torsional interactions between donor and acceptor
rings, thus extend the conjugation length and decrease the
bandgap of the copolymers.27
using a Shimadzu UV-3600 spectrophotometer. Electrochemi-
cal measurements of these derivatives were performed with
a
Bioanalytical Systems BAS 10 B/W electrochemical
workstation.
Photovoltaic Device Fabrication and Characterization
The solar cells were fabricated with a device structure ITO/
PEDOT:PSS/Polymer:PCBM Blend/LiF/Al. The ITO glass sub-
strates were precleaned by detergent, acetone, and boiling in
H2O2. Highly conducting poly(3,4-ethylenedioxythiophene):-
poly(sty-enesulfonate) (PEDOT:PSS, Baytron P, Al4083) was
spin-casted (3000 rpm) at a thickness of ꢁ 40 nm from
aqueous solution (after passing through a 0.45 lm filter).
The substrate was annealed at 120 ꢀC for 15 min on hot
plate. The active layer contained a blend of copolymers as
electron donor and PCBM as electron acceptor, which was
prepared by weight ratio (1:1, 1:2, 1:3, and 1:4 w/w) in
chlorobenzene (5 mg/mL) for copolymers. The active layers
were obtained by spin coating the blend solutions at 1000
rpm for 50 s and the thickness of films was ꢁ 90 nm, as
measured with the Ambios Technology XP-2. Subsequently,
LiF (0.6 nm) and Al (100 nm) electrodes were deposited via
thermal evaporation in vacuum (5 ꢂ 10ꢃ4 Pa). The active
area was about 5 mm2. Current-voltage (J-V) characteristics
were recorded using Keithley 2400 Source Meter in the dark
and under 100 mW/cm2 simulated AM 1.5 G irradiation (Sci-
encetech SS-0.5K Solar Simulator). All the measurements
were performed under ambient atmosphere at room
temperature.
Herein, we synthesized three new p-conjugated copolymers
consisting of alkylated 4,7-dithien-2-yl-2,1,3-benzothiadiazole
(HBT) acceptor unit coupled to different electron-donating
moieties: poly(fluorenevinylene-alt-4,7-dithien-2-yl-2,1,3-Ben-
zothiadiazole) (PFV-HBT), poly(phenylenevinylene-alt-4,7-
dithien-2-yl-2,1,3-benzothiadiazole) (PPV-HBT), and poly(-
thiophenevinylene-alt-4,7-dithien-2-yl-2,1,3-benzothiadiazole)
(PTV-HBT). We reasoned that the incorporation of functional
electron-donating moieties into the HBT-based copolymers
with different electron-donating ability will bring various
degrees of ICT to the conjugated system to provide a means
of tuning the HOMO/LUMO energy levels and the bandgaps.
Indeed, we found that the energy level and absorption spec-
tra of the copolymers can be fine-tuned by changing the moi-
eties with different electron-donating ability. The photovol-
taic cells based on these three copolymers, PFV-HBT, PPV-
HBT, and PTV-HBT with a cell structure of ITO/PEDOT/
copolymer:PCBM/LiF/Al exhibit PCE of 0.33, 0.47, and
1.05% respectively under one sun of AM 1.5 solar simulator
illumination (100 mW/cm2).
Synthesis of the Monomer
3-Hexylthiophene (1)
Mg (1.20 g, 50 mmol) was placed in 10 mL of dry ether then
ꢀ
cooled to 0 C. Hexylbromide (9.2 mL, 65 mmol) was added
EXPERIMENTAL
dropwise over 2 h, and the reaction mixture was stirred for
another hour. This solution was then transferred slowly via a
cannula to a mixture of 3-bromothiophene (2.36 mL, 21
mmol) and 50 mg of Ni(dppp)Cl2 in 50 mL of dry ether
while cooling on ice. The reaction mixture was stirred for 16
h at room temperature and subsequently poured out in 500
mL of ice/water containing 10 mL of concentrated HCl. The
product was extracted with ether and the combined organic
layers were washed with plenty of water and brine, succes-
sively. The organic extracts were dried over anhydrous
MgSO4, evaporated, and purified with column chromatogra-
phy on silica gel with petroleum ether as the eluant to give
1 colorless oil 2.18 g (yeild 72%).
Materials
All starting materials were purchased from either Acros or
Aldrich Chemical Co. and used without further purification,
unless otherwise noted. In synthetic preparations, diethyl
ether and THF were dried by distillation from sodium/benzo-
phenone under nitrogen. Similarly, DMF and dichloromethane
were distilled from CaH2 under nitrogen. 4,7-dibromo-2,1,3-
benzothiadiazole,28 2,7-Bis(methylenediethyl phosphate)-9,9-
di-n-octylfluoene (M-2), 2-Methoxy-5-(2-ethylhexyloxy)-1,4-
xylylenebis(diethylphosphonate) (M-3) and 3,4-Dihexyl-2,5-
bis(methylenediethyl phosphate)thiophene (M-4)29 were
prepared according to known literature procedures.
Measurements and Characterization
1H NMR (500 MHz, CDCl3, TMS): d (ppm) 7.23 (dd, 1H),
6.95–6.90 (m, 2H), 2.62 (t, J ¼ 7.5 Hz, 2H), 1.65–1.58 (m,
2H), 1.35–1.24 (m, 6H), 0.88 (t, J ¼ 7.0 Hz, 3H). Anal. Calcd
for C10H16S: C, 71.37; H, 9.58. Found: C, 71.21; H, 9.73.
1H NMR spectra were recorded on Bruker AVANCE 500-MHz
spectrometer with chloroform-d as solvent and tetramethyl-
silane (TMS) as internal standard. The elemental analysis
was carried out with a Thermoquest CHNS-Ovelemental ana-
lyzer. The gel permeation chromatographic (GPC) analysis
was carried out with a Waters 410 instrument with tetrahy-
drofuran as the eluent (flowrate: 1 mL/min, at 35 ꢀC) and
polystyrene as the standard. The thermogravimetric analysis
(TGA) was performed on a Perkine Elmer Pyris 1 analyzer
under nitrogen atmosphere (100 mL/min) at a heating rate
of 10 ꢀC/min. UV-visible absorption spectra were measured
4-(Hexyl-2-thienyl)stannane (2)
To a solution of 3-hexylthiophene (1.8 g, 12.50 mmol) in an-
hydrous THF (40 mL) at ꢃ78 ꢀC, n-BuLi (5.5 mL, 13.75
mmol) was added dropwise, and the mixture was stirred at
this temperature under a nitrogen atmosphere for 2 h. Then
tributylchlorostannane (4.0 mL, ꢀ15.00 mmol) was added, and
the mixture was stirred at ꢃ30 C for 12 h. The mixture was
2716
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
TABLE 1 Polymerization Results and Thermal Properties of
poured into saturated aqueous NaHCO3, and the organic
phase was separated and washed with saturated aqueous
brine and then dried over an anhydrous sodium sulfate. The
solvent was removed at a reduced pressure, and the residue
was purified with column chromatography on neutral alu-
mina with petroleum ether as the eluant to give 2 colorless
oil 4.3 g (yeild 76%).
Copolymers
Polymer
Yield (%)
Mn (103)
Mw (103)
PDI
TGA (Td)
PFV-HBT
PPV-HBT
PTV-HBT
75
72
81
3.32
4.10
7.09
4.29
6.37
9.98
1.29
1.64
1.41
279
283
298
1H NMR (500 MHz, CDCl3, TMS): d (ppm) 7.11 (s, 1H), 6.89
(s, 1H), 2.57 (t, J ¼ 8.0, 2H), 1.58–1.43 (m, 8H), 1.30–1.15
(m, 12H), 1.10–0.92 (m, 6H), 0.81 (t, J ¼ 7.0 Hz, 12H). Anal.
Calcd for C22H42SSn: C, 57.78; H, 9.26. Found: C, 57.56; H,
9.42.
added. The reaction was stirred for 48 h at room tempera-
ture under nitrogen. The polymer was precipitated into 300
mL of methanol and filtered through a Soxhlet thimble,
which was then subjected to Soxhlet extraction with metha-
nol, hexane, acetone, and chloroform. The fraction from chlo-
roform was concentrated under reduced pressure and pre-
cipitated into methanol (200 mL), collected by filtration. The
final product was dried under vacuum overnight to afford
PFV-HBT as a black solid (0.27 g, 75%).
4,7-Di(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (4)
To a solution of 4,7-dibromo-2,1,3-benzothiadiazole (3) (1.91
g, 6.50 mmol) and compound 2 (6.84 g, 14.95 mmol) in THF
(60 ml), PdCl2(PPh3)2 (137 mg, 3 mol %) was added. The
mixture was refluxed under a nitrogen atmosphere for 12 h.
After removal of the solvent under reduced pressure, the
residue was purified by column chromatography on silica gel
(petroleum ether/CH2Cl2, 15:1). Recrystallization from etha-
nol gave the title compound 4 (2.68 g, 82.5%) as red solid.
1H NMR (500 MHz, CDCl3, TMS): d (ppm) 7.92 (br, 2H, -ben-
zothiadiazole), 7.80 (br, 2H, -Th), 7.64–7.28 (m, 6H, ), 7.09–
6.94 (m, 4H, -vinylic), 2.78–2.65 (m, 4H, -CH2-), 2.00–1.87
(m, 4H), 1.42–0.80 (m, 42H), 0.76–0.72 (m, 6H), 0.57 (br,
4H). Anal. Calcd for (C59H74N2S3)n: C, 78.09; H, 8.22. Found:
C, 77.76; H, 8.38.
1H NMR (500 MHz, CDCl3, TMS): d (ppm) 7.98 (s, 2H), 7.83
(s, 2H), 7.04 (s, 2H), 2.69 (t, J ¼ 7.5 Hz, 4H), 1.74–1.67 (m,
4H), 1.42–1.25 (m, 12H), 0.89 (t, J ¼ 6.5 Hz, 6H). Anal. Calcd
for C26H32N2S3: C, 66.62; H, 6.88. Found: C, 66.73; H, 6.74.
Similarly, the monomer M-3, M-4, and dicarboxyaldehyde (M-
1) were polymerized using potassium tert-butoxide to obtain
the polymers poly(phenylenevinylene-alt-4,7-dithien-2-yl-
2,1,3-benzothiadiazole) (PPV-HBT), poly(thiophenevinylene-
alt-4,7-dithien-2-yl-2,1,3-benzothiadiazole) (PTV-HBT). The
yield and molecular weights of the polymers are summarized
in Table 1.
5,50-(2,1,3-benzothiadiazole-4,7-diyl-)bis
(3-hexylthiophene-2-carbaldehyde) Monomer (M-1)
DMF (45 mL, excess) and phosphorous oxychloride (POCl3)
(10 mL, excess) were mixed together and stirred for 30 min
at 0 ꢀC. To the yellow DMF–POCI3 complex, a solution of 4
(2.00 g, 4.00 mmol) in DMF (10 mL) was added dropwise at
PPV-HBT, 72%, 1H NMR (500 MHz, CDCl3, TMS): d (ppm)
8.03–7.95 (m, 2H, benzothiadiazole), 7.83 (br, 2H, -Th), 7.52–
7.28 (m, 2H, -Ph), 7.11–6.95 (m, 4H, -vinylic), 4.06–3.86 (m,
5H, -OCH2- and -OCH3), 2.77 (br, 4H, -CH2-), 1.88–0.81 (m,
37H). Anal. Calcd for (C46H56N2O2S3)n: C,71.76; H, 7.49.
Found: C, 70.95; H, 7.89.
0
ꢀC. The reaction mixture was heated at 95 ꢀC for 24 h
under nitrogen. Then, the reaction mixture was poured into
dilute HCl solution (150 mL), and the organic layers were
extracted with chloroform. The organic layer was washed
with an aqueous solution of NaHCO3 and subsequently with
distilled water. After drying over MgSO4 and removal of or-
ganic solvent under reduced pressure, the crude product
was purified by column chromatography (petroleum ether/
ethyl acetate, 10:1) to provide 1.74 g (78%) of the title prod-
uct as a red solid.
PTV-HBT, 81%, 1H NMR (500 MHz, CDCl3, TMS): d (ppm)
7.99 (br, 2H, -benzothiadiazole), 7.81 (br, 2H, -Th), 7.15–6.80
(m, 4H, -vinylic), 2.82–2.45 (m, 8H, -CH2-), 1.74–1.28 (m,
32H, -CH2-), 0.91 (br, 12H, -CH3). Anal. Calcd for
(C46H60N2S4)n: C, 71.82; H, 7.86. Found: C, 71.34; H, 8.23.
1H NMR (500 MHz, CDCl3, TMS): d (ppm) 10.12 (s, 2H), 8.10
(s, 2H), 8.01 (s, 2H), 3.05 (t, J ¼ 8.0, 4H), 1.81–1.74 (m, 4H),
1.47–1.21 (m, 12H), 0.89 (t, J ¼6.5 Hz, 6H). Anal. Calcd for
RESULTS AND DISCUSSION
C28H32N2O2S3: C, 64.09; H, 6.15. Found: C, 63.92; H, 6.21.
Synthesis and Characterization
Synthetic routes of the monomers and copolymers are
shown in Schemes 1 and 2. Monomer (M-1) was synthesized
in a multistep synthesis. Starting from commercially available
3-bromothiophene, in two steps, 2-tributyltin-4-hexylthio-
phene (compound 2) was synthesized from the reaction of
the lithium salt of the 3-hexylthiophene with tributyltin chlo-
ride. This compound was used as starting material for the
Stille cross-coupling reaction with 4,7-dibromo-2,1,3-benzo-
thiadiazole (3) to provide compound 4. Monomer (M-1) was
obtained in good yield by using an excess of two equivalents
SYNTHESIS OF THE POLYMERS
General Procedure for Horner-Wadsworth-Emmons
Polycondensation
Poly(fluorenevinylene-alt-4,7-dithien-2-yl-2,1,3-
benzothiadiazole) (PFV-HBT)
The dicarboxyaldehyde (M-1) (0.223 g, 0.40 mmol) and
monomer (M-2) (0.28 g, 0.40 mmol) were dissolved in 20
mL anhydrous THF under nitrogen. To this solution, potas-
sium tertbutoxide (2 mL of a 1 M solution in THF) was
SYNTHESIS AND PHOTOVOLTAIC PROPERTIES OF LOW-BANDGAP HBT, WEN ET AL.
2717

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Syntheses of mono-
mer M-1. Reagents and condi-
tions: (a) (i) hexylbromide,
diethyl ether, Mg; (ii) THF,
Ni(dppp)CI2; (b) n-Buli, THF, trib-
utylchlorostannane, ꢃ78 ꢀC to
ꢃ30 ꢀC; (c) THF, PdCl2(PPh3)2; (d)
DMF, POCl3, heated at 95 ꢀC for
24 h.
of DMF/phosphoryl chloride reagent in the Vilsmeier reac-
tion. In 1H NMR spectra of M-1, as shown in Figure 1, thio-
phene proton peaked at about 7.04 ppm, disappeared after
the Vilsmeier reaction, and a new peak appeared at about
10.12 ppm, which confirmed the success of the Vilsmeier
reaction. The purity of the monomer (M-1) and the interme-
diate compounds were confirmed by 1H NMR spectroscopy
and elemental analysis. The phosphonate esters (M-2–M-4)
were synthesized by Michaelis-Arbuzov reaction.30 Mono-
mers (M-2–M-4) were copolymerized with Monomer (M-1)
by using the t-BuOK Horner coupling method. The yields of
the resulting copolymers were over 70%. All the copolymers
exhibited good solubility in common organic solvents such
weights (Mw) of PFV-HBT, PPV-HBT, and PTV-HBT were 4.3
ꢂ 103, 6.4 ꢂ 103, and 10 ꢂ 103 with PDIs (polydispersity
index, Mw/Mn) of 1.29, 1.64, and 1.41, respectively. Table 1
summarizes the polymerization results including molecular
weights, PDI, and thermal stability of the copolymers.
Thermal Properties
Figure 2 shows the TGA curves of the synthesized copoly-
mers at the heating rate of 10 ꢀC/min under a nitrogen
atmosphere. As shown in Figure 2, the decomposition tem-
peratures (Td) of the copolymers are in the range of 279–
298 ꢀC, which suggests relatively good thermal stability.
Obviously, the thermal stability of the copolymers is
adequate for the fabrication processes of PSCs and other
optoelectronic devices. The Td of the copolymers is summar-
ized in Table 1.
1
as chloroform, tetrahydrofuran, and chlorobenzene. In the H
NMR spectrum of PTV-HBT (Figure 1), the characteristic
peaks at 8.02–7.88 ppm, 7.85–7.55 ppm, and 7.15–6.80 ppm
can be assigned to the resonance of protons on the benzo-
thiadiazole ring, the thiophene ring, and the vinylene group,
respectively. The peaks due to protons in -CH2- linked to the
thiophene ring are at 2.82–2.45 ppm; the peaks at 1.74–0.91
ppm correspond to the protons of the long alkyl chain.
Optical Properties
The optical absorption spectra of PFV-HBT, PPV-HBT, and
PTV-HBT in dilute (10ꢃ5 M) chloroform solution, and thin
films are shown in Figure 3, and the main optical properties
of the copolymers are listed in Table 2. PFV-HBT with the
weak electron-donating dialkyfluorene moiety showed two
absorption bands at 424 nm and 553 nm in dilute solution
[Fig. 3(a)], which can be assigned to p-p* transition of the
The molecular weights and polydispersities of the resulting
copolymers were determined by GPC analysis with a polysty-
rene standard calibration. The weight-average molecular
SCHEME 2 Syntheses of copolymers.
2718
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
FIGURE 1 500 MHz-1H NMR
spectra of monomer M-1 and co-
polymer PTV-HBT.
conjugated polymer backbone and ICT interaction between
the fluorene donor and HBT-based acceptor.31 Similarly, the
absorption spectra of other copolymers (PPV-HBT and PTV-
HBT) in dilute solutions also showed two bands near 451,
463 nm and 563, 653 nm due to the p-p* transition and the
ICT interaction, respectively. The solution absorption spec-
trum of PTV-HBT, with an absorption maximum (kambasx) at
653 nm, is red-shifted compared with those of PFV-HBT
tively, presumably indicating the formation of a p-stacked
structure in the solid state.32,33 From the low energetic edge
of the absorption spectrum of the individual copolymer, the
abs
band gap of PFV-HBT was estimated to be 1.83 eV (k
¼
max
676 nm), while smaller band gaps of 1.64 eV and 1.48 eV
abs
max
were calculated for PPV-HBT and PTV-HBT (k
¼ 757 nm
abs
and k
¼ 835 nm), respectively. In comparison with simi-
max
lar copolymer PT3BT (without double bonds in the main
chain, Eogpt ¼1.59 eV), the absorption spectra of PTV-HBT
was red-shifted (42 nm in thin film, Eogpt ¼1.48 eV) due to
higher planarity along the polymer backbone.32(b)
abs
abs
(k
¼ 553 nm), and PPV-HBT (k
¼ 563 nm), which can
max
max
be explained by much stronger ICT effect in PTV-HBT than
that in PFV-HBT and PTV-HBT. Among the three copolymers,
there is an alternative ‘‘D*-(D-A-D)’’ structure, where D* is
the donor with varied electron-donating ability and D-A-D is
the HBT-based acceptor unit. The stronger electron-donating
ability D* possesses, the higher electronic delocalization
degree and the stronger ICT the copolymer has. Since the
order of the electron-donating abilities of the three D* is dia-
lkylthiophene > dialkyloxyphenylene > dialkylfluorene, the
strongest electron-donating ability of thiophene compared
with dialkylfluorene and dialkyloxyphenylene improves the
effective conjugation length along polymer backbone, result-
ing in an increase in the ICT strength and thus electronic
delocalization.
Figure 3(b) shows the optical absorption spectra of thin
films of the copolymers. The thin film absorption spectra are
generally similar in shape to those in dilute solution. The
maximum absorption peak of PFV-HBT at visible region
shows a 16 nm red shift between in solutions and thin films.
In the case of PPV-HBT and PTV-HBT, the absorption spectra
in thin films also exhibit 12 nm and 13 nm red shift, respec-
FIGURE 2 TGA curves of copolymers at the heating rate of
10 ꢀC/min under nitrogen atmosphere.
SYNTHESIS AND PHOTOVOLTAIC PROPERTIES OF LOW-BANDGAP HBT, WEN ET AL.
2719

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
PFV-HBT, 0.34 V for PPV-HBT and 0.23 V for PTV-HBT,
respectively. In contrast, the cathodic sweep showed onset
reduction potentials of ꢃ1.42 V (versus Ag/Agþ) for PFV-
HBT, ꢃ1.42 V for PPV-HBT, and ꢃ1.41 V for PTV-HBT.
onset
From the onset oxidation potentials (E
) and the onset
ox
onset
reduction potentials (E
) of the copolymers, HOMO and
red
LUMO energy levels as well as the energy gaps were calcu-
lated according to the following equations,29,34
onset
HOMOðeVÞ ¼ ꢃeðE
LUMOðeVÞ ¼ ꢃeðE
þ 4:73Þ
ox
onset
red
þ 4:73Þ
onset
ox
onset
red
EgecðeVÞ ¼ eðE
ꢃ E
Þ
onset
ox
onset
red
where E
and E
are the measured onset potentials rel-
ative to Ag/Agþ.
The results of the electrochemical measurements and calcu-
lated energy levels of the copolymers are listed in Table 2.
The estimated HOMO and LUMO energy levels of PFV-HBT
are ꢃ5.27 and ꢃ3.31 eV, respectively. The LUMO energy lev-
els of PPV-HBT and PTV-HBT are at ꢃ3.31 eV and ꢃ3.32 eV,
respectively, which are very similar to that of PFV-HBT. It
indicates that the substitution of D* with varied electron-
donating ability moiety has almost no effect on the reduction
potential of the copolymers and the LUMO energy level is
mainly determined by the HBT-based acceptor unit. And the
relatively low LUMO energy levels of the copolymers result
from the stronger reduction of HBT acceptor unit. On the
other hand, the HOMO energy levels of copolymers behave
quite differently. The HOMO energy levels of the copolymers
PFV-HBT, PPV-HBT, and PTV-HBT are in the range of ꢃ5.27
to ꢃ4.96 eV, which is clearly affected by the varied electron-
donating ability of the three D* due to the modulation of ICT
inside the copolymers. Generally, the stronger electron-
donating ability donor resulted in a higher HOMO energy
level. The electrochemical band gap (Eegc) was determined to
be 1.64 eV for PTV-HBT, 1.75 eV for PPV-HBT, and 1.96 eV
for PFV-HBT, which is 0.1–0.2 eV larger than the optically
determined ones (Eogpt ¼1.48–1.83 eV). This difference can
be explained by the exciton binding energy of conjugated
polymers which is thought to be in the range of 0.1–0.5 eV.35
FIGURE 3 UV-vis absorption spectra of PFV-HBT, PPV-HBT, and
PTV-HBT in chloroform solution and (b) in thin films. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Electrochemical Properties
Cyclic voltammetry of the copolymers in films was per-
formed in acetonitrile with 0.1 M tetrabutylammonium hexa-
fluorophosphate (TBAPF6) as supporting electrolyte at scan
rates of 50 mV/s. Platinum wire electrodes were used as
both counter and working electrodes, and silver/silver ion
(Ag in 0.1 M AgNO3 solution, from Bioanalytical Systems)
was used as a reference electrode. Ferrocene/ferrocenium
(Fc/Fcþ) was used as the internal standard.
Figure 5 depicts the electron-state-density distribution of the
HOMO and LUMO of geometry optimized structures (DFT
B3LYP/6-31G*) of analogous monomers PFV-HBT, PPV-HBT,
and PTV-HBT using the Gaussium 03 program. To simplify
the calculation, only one repeating unit of each polymer was
subject to the calculation, with alkyl chains replaced by C2H5
groups. The electron density distributions of LUMO levels for
The cyclic voltammagram of the copolymers are shown in
Figure 4. On the anodic sweep, all copolymers showed an ox-
idation with onset potentials of 0.54 V (versus Ag/Agþ) for
TABLE 2 Optical and Electrochemical Properties of Copolymers
abs;sol
max
abs;film
max
a (104 cmꢃ1
)
Eogpt (eV)
E
(V)
HOMO (eV)
E
(V)
LUMO (eV)
Eegc (eV)
onset
ox
onset
red
Polymers
k
(nm)
k
(nm)
PFV-HBT
PPV-HBT
PTV-HBT
424/553
451/566
463/653
431/569
459/578
475/676
7.2
6.8
8.7
1.83
1.63
1.48
0.54
0.34
0.23
ꢃ5.27
ꢃ5.07
ꢃ4.96
ꢃ1.42
ꢃ1.42
ꢃ1.41
ꢃ3.31
ꢃ3.31
ꢃ3.32
1.96
1.75
1.64
2720
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
FIGURE 5 DFT-calculated LUMO and HOMO of the geometry
optimized structures of analogous monomers of PFV-HBT,
PPV-HBT, and PTV-HBT.
in the active layer have obviously effect on the photovoltaic
performance, so BHJ PSCs with varied weight ratios (copoly-
mer:PCBM from 1:1 to 1:4) in the active layers were investi-
gated (the results are list in Table 3). The optimized per-
formances were achieved with weight ratio of copolymers:
PCBM at 1:4 (w/w). It is known that solvents used for the
preparation of the active layer have a strong impact on the
performance of the cell. Here, we chose chlorobenzene as
solvent to obtain the copolymer films with good quality.36
The current-voltage characteristics of the photovoltaic cell
based on PFV-HBT:PCBM, PPV-HBT:PCBM, and PTV-
HBT:PCBM with weight ratio (1:4 w/w) are shown in Figure
6, and the photovoltac parameters of the photovoltaic cells
are summarized in Table 3.
FIGURE 4 Cyclic voltammograms of copolymer thin films on Pt
wires in 0.1 M TBAPF6 in acetonitrile. The scan rates used were
50 mV/s.
these three polymers are nearly identical and primarily local-
ized on the HBT-based acceptor unit. Thus, the change of do-
nor units has almost no effect on the LUMO levels. However,
the electron density of HOMO is distributed over the entire
conjugated molecule (both the acceptor and donor unit),
which indicates the donor unit significantly affects the
HOMO level of the resulting polymer. The results from the
calculation follow the same trend as we observed from the
experimental values.
The cell based on PFV-HBT:PCBM (1:4 w/w) showed an
open-circuit voltage (Voc) of 0.88 V, a short-circuit current
(Jsc) 0.95 mA/cm2, and a fill factor (FF) of 0.39, giving a PCE
of 0.33%. The corresponding device parameters of the PSC
based on PPV-HBT:PCBM (1:4 w/w) are 0.62 V, 2.12 mA/
cm2, 0.36, and 0.47%, respectively. In comparison with the
Photovoltaic Properties
To investigate the photovoltaic properties of the copolymers,
the bulk-heterojunction photovoltaic cells with a structure of
ITO/PEDOT-PSS/copolymers: PCBM/LiF/Al were fabricated,
where the copolymers were used as donors and PCBM as
acceptor. The weight ratios between copolymers and PCBM
TABLE 3 Photovoltaic Properties of the Copolymer Photovoltaic Cells
Donor
Blend
Voc
Jsc
(mA/cm2)
Hole Mobility
Hole Mobility
(Polymer)
Ratio (D:A)
(V)
FF
PCE (%)
(cm2 Vꢃ1 Sꢃ1 a
)
(cm2 Vꢃ1 Sꢃ1 b
)
PFV-HBT
PPV-HBT
PTV-HBT
a
1:1
1:2
1:3
1:4
1:1
1:2
1:3
1:4
1:1
1:2
1:3
1:4
0.86
0.88
0.88
0.88
0.60
0.62
0.64
0.62
0.48
0.50
0.52
0.52
0.67
0.71
0.80
0.95
0.87
1.27
2.02
2.12
1.02
1.71
2.96
3.98
0.30
0.37
0.39
0.39
0.28
0.30
0.35
0.36
0.43
0.48
0.49
0.51
b
0.17
0.23
0.27
0.33
0.15
0.23
0.45
0.47
0.21
0.41
0.76
1.05
3.2 ꢂ 10ꢃ7
1.6 ꢂ 10ꢃ6
7.5 ꢂ 10ꢃ8
9.1 ꢂ 10ꢃ7
8.1 ꢂ 10ꢃ7
7.4 ꢂ 10ꢃ6
Measured with polymer-only devices.
Measured with polymer:PCBM (1:4) devices.
SYNTHESIS AND PHOTOVOLTAIC PROPERTIES OF LOW-BANDGAP HBT, WEN ET AL.
2721

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
device based on PPV-HBT:PCBM, the Voc of the device based
on the PFV-HBT:PCBM increases by ca. 0.29 V because the
Voc is mainly determined by the energy difference between
the HOMO of the donor and the LUMO of the acceptor, and
the relatively low-lying HOMO (ꢃ5.27 eV) of the PFV-HBT
enhances the open circuit voltage.37,38 The PCE of the device
based on PFV-HBT is slightly lower in comparison with that
based on PPV-HBT, which is primarily due to a decrease in
current density. The higher Jsc of the device based on PPV-
HBT:PCBM comparing with the device based on PFV-
HBT:PCBM could be explained by the broader absorption
and lower bandgap of PPV-HBT. Although The Voc (0.52 V) of
the PTV-HBT:PCBM (1:4) is smaller than the other two co-
polymer devices, presumably as a result of the relatively
high HOMO level of PTV-HBT, a PCE of 1.05% was still
achieved because the increased Jsc value of 3.98 mA/cm2
and FF of 0.51. The device based on PTV-HBT exhibits the
highest Jsc among the three devices due to the absorbance of
PTV-HBT matches the solar radiation better than that of
PPV-HBT and PFV-HBT. Furthermore, the amount of the
absorbed light depends not only on the edge of absorption
wavelength but also on the intensity of the absorption. Com-
paring the absorption of PFV-HBT, PPV-HBT, and PTV-HBT in
Figure 3(b), PTV-HBT have higher absorption coefficient than
PFV-HBT and PTV-HBT. That’s one of the reasons why the
device based on PTV-HBT has the higher Jsc than the devices
based on PFV-HBT and PPV-HBT. Although PTV-HBT gave the
relatively low device performance, the PCE value of 1.05% is
still much higher than that (0.13%) of the similar polymer
previously reported by Cao and coworkers.39
FIGURE 7 J-V curve in the dark of an ITO/PEDOT/Copoly-
mers:PCBM/Au device in log axis for log axis for estimating
the hole mobility of copolymers. The open triangle symbols
are the experimental data. The solid line from 0.05 to 0.60 V
means log J is fitted linearly dependent on log V with a slope
of 1. For the solid line from 0.60 to 1 V means log J is fitted lin-
early dependent on log V with a slope of 2 (SCLC area). [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
HBT:PCBM, PPV-HBT:PCBM, and PTV-HBT:PCBM. The hole
mobility values of copolymers PFV-HBT, PPV-HBT, and PTV-
HBT were (see Table 3) estimated according to the equation:
2
9
ðV ꢃ VbiÞ
In BHJ solar cells, the hole mobility in the polymer is very
important to their photovoltaic performance. We used space
charge limited current (SCLC) model to determine the hole
mobility in pure copolymers or in blends with PCBM. The
devices with a structure of ITO/PEDOT:PSS/ copolymer or
copolymer:PCBM (1:4)/Au were fabricated. Figure 7 shows
the J-V characteristics of the hole-only devices based on PFV-
J ¼ ere0lh
8
L3
where V is the applied voltage, J is the current density, er
and e0 are the relative dielectric constant and the permittiv-
ity of the free space (8.85 ꢂ 10ꢃ12 F/m), respectively, lh is
hole mobility and L is the thickness of the organic layer. The
hole mobilities of pure copolymers PFV-HBT, PPV-HBT, and
PTV-HBT were estimated to be about 3.2 ꢂ 10ꢃ7 cm2 Vꢃ1
s
ꢃ1, 7.5 ꢂ 10ꢃ8 cm2 Vꢃ1
s
ꢃ1, and 8.1 ꢂ 10ꢃ7 cm2 Vꢃ1 sꢃ1
,
respectively, while that of the blend have increased relative
to pure copolymers by ꢁ1 order of magnitude. Obviously,
The electron mobility (2 ꢂ 10ꢃ3 cm2 Vꢃ1 sꢃ1 40
)
of PCBM
was much higher than the hole mobility of polymer, resulting
in an imbalance in the hole and electron transport in the
blended film. Because of the poor hole mobility and the
imbalance of the hole and electron transport in the blend
films of PFV-HBT:PCBM and PPV-HBT:PCBM, the devices
were limited to have a low FF values 0.39 and 0.36, respec-
tively, which could be another reason for the lower PCE val-
ues of the PFV-HBT:PCBM and PPV-HBT:PCBM.41
CONCLUSIONS
We have synthesized three new low band gap alkylated 4,7-
dithien-2-yl-2,1,3-benzothiadiazole (HBT)-based copolymers
with vinyl linkages by a Horner cross coupling polymeriza-
tion. These polymers show strong absorptions in the range
of 300–900 nm and have ideal optical bandgaps (1.48–1.83
FIGURE 6 J-V curves of the copolymer photovoltaic cells based
on PFV-HBT, PPV-HBT, and PTV-HBT under the illumination of
AM 1.5, 100 mW/cm2.
2722
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
Am Chem Soc 2010, 132, 5330–5331; (c) Piliego, C.; Holcombe,
T. W.; Douglas, J. D.; Woo, C. H.; Beaujuge, P. M.; Fre´chet. J.
M. J. J Am Chem Soc 2010, 132, 7595–7597; (d) Padhy, H.;
Huang, J. H.; Sahu, D.; Patra, D.; Kekuda, D.; Chu, C. W.; Lin, H.
C. J Polym Sci Part A: Polym Chem 2010, 48, 4823–4834; (e)
Jung, I. H.; Kim, H.; Park, M. J.; Kim, B.; Park, J. H.; Jeong, E.;
Woo, H. Y.; Yoo, S.; Shim, H. K. J Polym Sci Part A: Polym
Chem 2010, 48, 1423–1432; (f) Chen, G. Y.; Chiang, C. M.;
Kekuda, D.; Lan, S. C.; Chu, C. W.; Wei, K. H. J Polym Sci Part
A: Polym Chem 2010, 48, 1669–1675.
eV). The HOMO levels of PFV-HBT, PPV-HBT, and PTV-HBT
are ꢃ5.27, ꢃ5.07, and ꢃ4.96 eV, respectively, which means
that the order of the donor strength is dialkylthiophene >
dialkyloxyphenylene
> dialkylfluorene. The photovoltaic
devices based on the polymers show the PCE in the range of
0.33–1.05%, which indicate that it is an effective way to
improve the PCE of photovoltaic devices by adjusting the
electron-donating abilities for the type of D-A copolymers.
This work was supported by the State Key Development Pro-
gram for Basic Research of China (Grant No. 2009CB623605),
the National Natural Science Foundation of China (Grant No.
20874035), the 111 Project (Grant No. B06009), and the Pro-
ject of Jilin Province (20080305).
15 Chen, H.-Y.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G.
W.; Yang, Y.; Yu, L. P.; Wu, Y.; Li, G. Nat Photonics 2009, 3,
649–653.
16 (a) Svensson, M.; Zhang, F. L.; Veenstra, S. C.; Verhees, W.
J. H.; Hummelen, J. C.; Kroon, J. M.; Inganas, O.; Andersson,
¨
M. R. Adv Mater 2003, 15, 988–991; (b) Chen, M.-H.; Hou, J. H.;
Hong, Z. R.; Yang, G. W.; Sista, S.; Chen, L.-M.; Yang, Y. Adv
Mater 2009, 21, 4238–4242.
REFERENCES AND NOTES
1 Yu, G.; Gao, J.; Hummelen, J.; Wudl, F.; Heeger, A. J. Sci-
ence 1995, 270, 1789–1791.
17 (a) Boudreault, P. T.; Michaud, A.; Leclerc, M. Macromol
Rapid Commun 2007, 28, 2176–2179; (b) Wang, E. G.; Wang, L.;
Lan, L. F.; Luo, C.; Zhuang, W. L.; Peng, J. B.; Cao, Y. Appl
Phys Lett 2008, 92, 033307.
2 Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv Mater 2009,
21, 1323–1338.
3 Helgesen, M.; Sondergaard, R.; Krebs, F. C. J Mater Chem
2010, 20, 36–60.
18 Liu, C. L.; Tsai, J. H.; Lee, W. Y.; Chen, W. C.; Jenekhe, S. A.
Macromolecules 2008, 41, 6952–6959.
4 Krebs, F. C.; Tromholt, T.; Jorgensen, M. Nanoscale 2010, 2,
873–886.
19 (a) Blouin, N.; Michaud, A.; Leclerc, M. Adv Mater 2007, 19,
2295–2300; (b) Blounim, N.; Michaud, A.; Gendron, D.; Wakim,
S.; Blair, E.; Neagu-Plesu, R.; Durocher, G.; Tao, Y.; Leclerc, M.
J Am Chem Soc 2008, 130, 732–742; (c) Chu, T.-Y; Alem, S.;
Verly, P. G.; Wakim, S.; Lu, J. P.; Tao, Y.; Beaupre´, S.; Leclerc,
M.; Be´langer, F.; De´silets, D.; Rodman, S.; Waller, D.; Gaudiana,
R. Appl Phys Lett 2009, 95, 063304.
5 Thompson, B. C.; Frechet, J. M. J. Angew Chem Int Ed Engl
2008, 47, 58–77.
6 Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem Rev 2007,
107, 1324–1338.
7 Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D.
E. Adv Mater 2007, 19, 1551–1566.
20 (a) Liao, L.; Dai, L. M.; Smith, A.; Durstock, M.; Lu, J. P.;
Ding, J. F.; Tao, Y. Macromolecules 2007, 40, 9406–9412; (b)
Huo, L. J.; Chen, H.-Y.; Hou, J. H.; Chen, T. L.; Yang, Y. Chem
Commun 2009, 37, 5570–5572.
8 Zhang, F. L.; Mammo, W.; Andersson, L. M.; Admassie, S.;
Andersson, M. R.; Inganas, O. Adv Mater 2006, 18, 2169–2173.
¨
9 Zhu, Z. G.; Waller, D.; Gaudiana, R.; Morana, M.; Mulhl-
bacher, D.; Scharber, M.; Brabec, C. Macromolecules 2007, 40,
1981–1986.
21 Moule´, A. J.; Tsami, A.; Bunnagel, T. W.; Forster, M.; Kro-
¨
nenberg, N. M.; Scharber, M.; Koppe, M.; Morana, M.; Brabec,
C. J.; Meerholz, K.; Scherf, U. Chem Mater 2008, 20, 4045–4050.
10 (a) Yip, H.-L.; Hau, S. K.; Baek, N. S.; Ma, H.; Gen, A. K.-Y.
Adv Mater 2008, 20, 2376–2382; (b) Lee, J. Y.; Choi, M. H.;
Song, H. J.; Moon, D. K. J Polym Sci Part A: Polym Chem
2010, 48, 4875–4883; (c) Chen, G. Y.; Lan, S. C.; Lin, P. Y.; Chu,
C. W.; Wei, K. H. J Polym Sci Part A: Polym Chem 2010, 48,
4456–4464; (d) 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.
22 (a) Huo, L. J.; Hou, J. H.; Zhang, S. Q.; Chen, H.-Y.; Yang, Y.
Angew Chem Int Ed Engl 2010, 49, 1500–1503; (b) Price, S. C.;
Stuart, A. C.; You, W. Macromolecules 2010, 43, 4609–4612.
23 Sonar, P.; Singh, S. P.; Sudhakar, S.; Dodabalapur, A.; Sell-
inger, A. Chem Mater 2008, 20, 3184–3190.
24 Park, S. H.; Roy, A.; Beaupre´, S.; Cho, S.; Coates, N.; Moon,
J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat Photon
2009, 3, 297–302.
11 Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Adv Funct
Mater 2005, 15, 1617–1622.
25 Liu, B.; Najari, A.; Pan, C.; Leclerc, M.; Xiao, D.; Zou, Y. P.
12 Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery,
Macromol Rapid Commun 2010, 31, 391–398.
K.; Yang, Y. Nat Mater 2005, 4, 864–868.
26 Zhang, S. M.; Fan, H. J.; Liu, Y.; Zhao, G. J.; Li, Q. K.; Li, Y.
F.; Zhan, X. W. J Polym Sci Part A: Polym Chem 2009, 47,
2843–2852.
13 (a) Liang, Y. Y.; Wu, Y.; Feng, D. Q.; Tsai, S.-T.; Son, H.-J.;
Li, G.; Yu, L. P. J Am Chem Soc 2009, 131, 56–57; (b) Liang, Y.
Y.; Feng, D. Q.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. P. J
Am Chem Soc 2009, 131, 7792–7799.
27 Ko, S.; Mondal, R.; Risko, C.; Lee, J. K.; Hong, S.; McGehee, M.
D.; Bredas, J.-L.; Bao, Z. N. Macromolecules 2010, 43, 6685–6698.
14 (a) Hou, J. H.; Chen, H.-Y.; Zhang, S. Q.; Li, G.; Yang, Y. J
Am Chem Soc 2008, 130, 16144–16145; (b) Zou, Y. P.; Najari,
28 Pilgram, K.; Zupan, M.; Skile, R. J Heterocycl Chem 1970, 7,
A.; Berrouard, P.; Beaupre, S.; Aıch, B. R.; Tao, Y.; Leclerc, M. J
¨
629–633.
SYNTHESIS AND PHOTOVOLTAIC PROPERTIES OF LOW-BANDGAP HBT, WEN ET AL.
2723

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
29 Wen, S. P.; Pei, J. N.; Zhou, Y. H.; Li, P. F.; Xue, L. L.; Li, Y.
35 Zhu, Y.; Champion, R. D.; Jenekhe, S. A. Macromolecules
W.; Xu, B.; Tian, W. J. Macromolecules 2009, 42, 4977–4984.
2006, 39, 8712–8719.
30 Egbe, D. A. M.; Tillmann, H.; Birckner, E.; Klemm, E. Macro-
36 Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S. Appl Phys Lett
mol Chem Phys 2001, 202, 2712–2726.
2001, 78, 841.
31 Zhou, E. J.; Cong, J. Z.; Yamakawa, S.; Wei, Q. S.; Naka-
mura, M.; Tajima, K.; Yang, C. H.; Hashimoto, K. Macromole-
cules 2010, 43, 2873–2879.
37 Scharher, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Wal-
¨
dauf, C.; Heeger, A. J.; Brabec, C. J. Adv Mater 2006, 18,
789–794.
32 (a) Yamamoto, T.; Komarudin, D.; Arai, M.; Lee, B. L.; Suga-
numa, H.; Asakawa, N.; Inoue, Y.; Kubota, K.; Sasaki, S.;
Fukuda, T.; Matsuda, H. J Am Chem Soc 1998, 120, 2047–2058;
(b) Yue, W.; Zhao, Y.; Tian, H. K.; Song, D.; Xie, Z. Y.; Yan, D.
H.; Geng, Y. H.; Wang, F. S. Macromolecucules 2009, 42,
6510–6518.
38 Hou, J. H.; Park, M-H.; Zhang, S. Q.; Yao, Y.; Chen, L. M.; Li,
J-H.; Yang, Y. Macromolecules 2008, 41, 6012–6018.
39 Xia, Y. J.; Deng, X. Y.; Wang, L.; Li, X. Z.; Zhu, X. H.; Cao, Y.
Macromol Rapid Commun 2006, 27, 1260–1264.
40 Mihailetchi, V. D.; van Duren, J. K. J.; Blom, P. W. M.; Hum-
melen, J. C.; Janssen, R. A. J.; Kroon, J. M.; Rispens, M. T.;
Verhees, W. J. H.; Wienk, M. M. Adv Funct Mater 2003, 13,
43–46.
33 Peng, Q.; Park, K.; Lin, T.; Durstock, M.; Dai, L. M. J Phys
Chem B 2008, 112, 2801–2808.
34 Wen, S. P.; Pei, J. N.; Zhou, Y. H.; Xue, L. L.; Xu, B.; Li, Y.
W.; Tian, W. J. J Polym Sci Part A: Polym Chem 2009, 47,
1003–1032.
41 Hou, J. H.; Chen, T. L.; Zhang, S.; Chen, H.-Y.; Yang, Y. J
Phys Chem C 2009, 113, 1601–1605.
2724
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA