Synthesis of thieno[3,4-b]pyrazine-based and 2,1,3-benzothiadiazole-based donor-acceptor copolymers and their application in photovoltaic devices

Article abstract of DOI:10.1021/ma100039q

Two kinds of thieno[3,4-6]pyrazine-based monomers, 2,3-dimethyl-5,7-di(2- bromothien-5-yl)-thieno[3,4-b]pyrazine and 2,3-diphenyl-5,7-di(2-bromothien-5- yl)-thieno[3,4-p]pyrazine, were synthesized via an improved synthetic route. These two monomers and 4,7-di(2-bromothien-5-yl)-2,l,3-benzothiadiazole were copolymerized with three donor segments (fluorene, carbazole, and indolocarbazole) separately by a Suzuki cross-coupling reaction to give six types of 5,7-dithien-2-yl-thieno[3,4-p]pyrazine (DTTP)-based donor- acceptor (D-A) copolymers (TP1-6) and three types of 5,7-dithien-2-yl-2,l,3- benzothiadiazole (DTBT)-based D-A copolymers (PF-DTBT, PC-DTBT, and PIC-DTBT). The optical properties, electrochemical behavior, and energy levels of these nine copolymers were investigated. The photovoltaic performance of the copolymers was compared and discussed considering their energy levels.

Full text of DOI:10.1021/ma100039q

Macromolecules 2010, 43, 2873–2879 2873
DOI: 10.1021/ma100039q
Synthesis of Thieno[3,4-b]pyrazine-Based and 2,1,3-Benzothiadiazole-Based
Donor-Acceptor Copolymers and their Application in Photovoltaic
Devices
Erjun Zhou,† Junzi Cong,† Shimpei Yamakawa,‡ Qingshuo Wei,‡ Motoshi Nakamura,‡
Keisuke Tajima,‡ Chunhe Yang,† and Kazuhito Hashimoto*,†,‡
†HASHIMOTO Light Energy Conversion Project, ERATO, Japan Science and Technology Agency (JST), and
‡Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan
Received January 7, 2010; Revised Manuscript Received February 10, 2010
ABSTRACT: Two kinds of thieno[3,4-b]pyrazine-based monomers, 2,3-dimethyl-5,7-di(2-bromothien-5-
yl)-thieno[3,4-b]pyrazine and 2,3-diphenyl-5,7-di(2-bromothien-5-yl)-thieno[3,4-b]pyrazine, were synthe-
sized via an improved synthetic route. These two monomers and 4,7-di(2-bromothien-5-yl)-2,1,3-benzothia-
diazole were copolymerized with three donor segments (fluorene, carbazole, and indolocarbazole) separately
by a Suzuki cross-coupling reaction to give six types of 5,7-dithien-2-yl-thieno[3,4-b]pyrazine (DTTP)-based
donor-acceptor (D-A) copolymers (TP1-6) and three types of 5,7-dithien-2-yl-2,1,3-benzothiadiazole
(DTBT)-based D-A copolymers (PF-DTBT, PC-DTBT, and PIC-DTBT). The optical properties, electro-
chemical behavior, and energy levels of these nine copolymers were investigated. The photovoltaic
performance of the copolymers was compared and discussed considering their energy levels.
Introduction
[2,1-b:3,4-b]dithiophene,12 dithienosilole,13 dithieno[3,2-b:20,30-d]-
pyrrole,14 and indolo[3,2-b]carbazole.15 When these materials have
In the past decade, polymer solar cells (PSCs) have received a
great deal of attention as a potential renewable energy source
because of their advantages such as low cost, light weight, and
capability of fabricating flexible large-area devices.1 To achieve high
performance in optoelectronic devices, it is critical to tune physical
properties such as solubility, band gap, molecular energy level, and
carrier mobility by chemically modifying the polymer structure.
For photovoltaic polymers, the most widely investigated one is
regioregular poly(3-hexylthiophene) (P3HT). Power conversion
efficiency (PCE) of 4 to 5% has been achieved in PSCs using the
blend of P3HT and [6,6]-phenyl-C61-butyric acid methyl ester
(PCBM) by thermal annealing after device fabrication to con-
struct the nanomorphology of the interpenetrating network.2
However, P3HT has a relatively large band gap (∼2.0 eV) and
only harvests photons with wavelength below 650 nm, which is a
small portion of the whole solar spectrum. Therefore, the design
and preparation of novel photovoltaic polymers with broader
absorption of solar light, especially in the longer wavelength
region, are important challenges to be addressed at present.3
Many works have demonstrated that alternating electron-rich
(donor) and electron-deficient (acceptor) units in the polymer
chain is an effective strategy for tuning the properties of poly-
mers, especially to reduce the band gap.4 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.5
been applied to PSCs, PCEs in the range of 0.41 to 6.1% have been
achieved. 5,7-Dithien-2-yl-thieno[3,4-b]pyrazine (DTTP) is an iso-
mer of DTBT and has also been studied for designing photovoltaic
polymers.16 However, a detailed comparison of the two isomers in
the D-A copolymers has not been done, and the effects of different
substituted groups at the 2,3-position of thieno[3,4-b]pyrazine (TP)
segment remains unclear.
In this article, we report an improved route for the synthesis of
two types of 2,3-disubstituted DTTPs: 2,3-dimethyl-5,7-di(2-
bromothien-5-yl)-thieno[3,4-b]pyrazine and 2,3-diphenyl-5,7-
di(2-bromothien-5-yl)-thieno[3,4-b]pyrazine. We also report the
copolymerization of these two acceptor segments with three well-
known donor segments: fluorene, carbazole, and indolo[3,2-
b]carbazole. Furthermore, the preparation of copolymers com-
bining DTBT and these three donor segments was also conducted
for comparison. The optical, electrochemical, and photovoltaic
properties of these nine D-A copolymers are compared in detail.
Results and Discussion
Material Synthesis. The reported synthetic route to mono-
mers 5a16a and 5b16b (Scheme 1) is as follows: reduction of
30,40-dinitro-2,20:50200-terthiophene (2) with SnCl2 to produce
30,40-diamino-2,20:50200-terthiophene (3) and condensation of
3 with 1,2-diketones to yield 2,3-disubsititued-5,7-di(2-
thienyl)-thieno[3,4-b]pyrazine, which is brominated with N-
bromosuccinimide (NBS) to afford the target compound 5.
The difficulty with this route is in the last step, where mono-,
di-, and tribrominated products are formed, which are
difficult to separate because of their poor solubility and
similar Rf values. Furthermore, if one wants to obtain
monomer 5 containing different substituted groups such as
5a and 5b, one must repeat the bromination step and the
time-consuming separation of the brominated products.
Among the large variety of acceptor segments, 4,7-dithien-2-
yl-2,1,3-benzothiadiazole (DTBT), linking two thiophene rings in
the 4,7-position of 2,1,3-benzothiadiazole (BT), has been devel-
oped and copolymerized with many types of donor segments,
such as pyrrole,6 didecyloxyphenylene,7 fluorene,8 carbazole,9
silafluorene (or dibenzosilole),10 indeno[2,1-a]indene,11 cyclopenta-
*Corresponding author. E-mail: hashimoto@light.t.u-tokyo.ac.jp.
r 2010 American Chemical Society
Published on Web 02/22/2010
pubs.acs.org/Macromolecules

2874 Macromolecules, Vol. 43, No. 6, 2010
Zhou et al.
Scheme 1. Reported Synthetic Routes to 4,7-Di(2-bromothien-5-yl)-2,1,3-benzothiadiazole17 and 2,3-Disubstituted-5,7-di(2-bromothien-5-yl)-
thieno[3,4-b]pyrazine16a,b
Scheme 2. Improved Synthetic Routes to 2,3-Disubstituted-5,7-di(2-bromothien-5-yl)-thieno[3,4-b]pyrazine
Here an improved synthetic route (Scheme 2) was devel-
oped. First, 30,40-dinitro-2,20:50200-terthiophene was bromi-
nated with NBS in DMF to afford only the dibrominated
product 6. Reduction of 6 with iron in acetic acid produced
the diamine 7, which was found to react with 1,2-diketones
very effectively without purification to afford the target
compounds 5a and 5b. This proposed synthetic route pro-
vides more facile access to the target compounds than the
previous one. The yield from 6 to monomer 5 was acceptable
(∼70%), and the purification was much simpler. The total
yield from 2 to 5a was also improved from ∼22% by the
previous route16b to ∼60% by the present route.
absorption in the shorter wavelength range, which might be
due to an electronic transition related to the peripheral
phenyl rings. Compared with the DTBT-based polymers
(PF-DTBT, PC-DTBT, and PIC-DTBT), the 2,3-dimethyl-
DTTP-based polymers (TP1, TP3, and TP5) and the 2,3-
diphenyl-DTTP-based polymers (TP2, TP4, and TP6) had
broader absorption ranges. TP1, TP3, and TP5 had absorp-
tion maxima in the longer wavelength range at 577-590 nm
with absorption coefficients that were ∼20% less, and TP2,
TP4, and TP6 had the maxima at 635-651 nm with absorp-
tion coefficients that were ∼40% less, which were red-shifted
by about 50 and 100 nm relative to the DTBT-based poly-
mers, respectively. The shape and the position of the absorp-
tion peaks and the absorption coefficients seemed to be
determined mainly by the identity of the acceptor segment,
and the effects of the donor segment were small. Because of
the differences in the absorption spectra, the copolymers
showed quite different colors in solution: red for DTBT-
based polymers (PF-DTBT, PF-DTBT, and PF-DTBT),
blue for 2,3-dimethyl-DTTP-based polymers (TP1, TP3,
and TP5), and green for 2,3-diphenyl-DTTP-based polymers
(TP2, TP4, and TP6).
The absorption spectra of the nine polymers in thin films
are shown in Figure 2, and the optical properties are sum-
marized in Table 2. The shape of the peaks and the trend of
the absorption wavelength were generally similar to those in
solution. The absorption maxima in the films were red-
shifted by 2-23 nm compared with those in solution, sug-
gesting that intermolecular interactions were present in the
solid state but were probably not as strong as those for
regioregular P3HT.
Nine D-A copolymers were synthesized from the combi-
nation of three donor units (8, 9, and 10) with three acceptor
units (1, 5a, and 5b) by the Suzuki cross-coupling reaction
(Scheme 3). To avoid the formation of a large insoluble
fraction of polymers, we used a feed molar ratio of donor to
acceptor segments that was 100:95. The yield, number-
average molecular weight (Mn), weight-average molecular
weight (Mw), and polydispersity indices (PDIs) are summar-
ized in Table 1. Most polymerization reactions yielded over
50% after purification by Soxhlet extraction. (See the Ex-
perimental Section.) The reactions involving the carbazole
showed slightly lower yield; this result could be attributed to
the poor solubility of the resulting polymers, the alkyl chain
of which does not provide sufficient flexibility to compensate
for the rigidity of the acceptor units.
All polymers exhibited excellent thermal stability with
onset decomposition temperatures around 300-387 °C in
air measured shown by thermogravimetric analysis (TGA).
The data are summarized in Table 1. The high thermal
stability of these polymers is adequate for application in
PSCs or other optoelectronic devices.
Optical Properties. The UV-vis absorption spectra of the
nine copolymers in CHCl3 solutions are shown in Figure 1.
All polymers exhibited two major absorption bands in the
ranges of 300-500 and 500-800 nm. The peak at longer
wavelength could be attributed to the intramolecular charge-
transfer (ICT) transition, and the other at shorter wave-
length was the result of a higher energy transition. The
2,3-diphenyl-DTTP-based polymers showed higher relative
Electrochemical Properties. The electrochemical proper-
ties of the nine polymers were investigated by cyclic voltam-
metry (CV). The CV curves were recorded referenced to a
Ag/Agþ (0.01 M of AgNO3 in acetonitrile) electrode, which
was calibrated against the ferrocene-ferrocenium (Fc/Fcþ)
redox couple (4.8 eV below the vacuum level). The cyclic
voltammetric curves are shown in Figure 3. The 2,3-diphenyl-
DTTP-based polymers (TP2, TP4, and TP6) showed
reversible reduction, whereas others exhibited irrever-
sible reduction at lower potentials. This indicates that the

Article
Macromolecules, Vol. 43, No. 6, 2010 2875
Scheme 3. Synthetic Routes to Three DTBT-Based D-A Copolymers and Six DTTP-Based D-A Copolymers
2,3-diphenyl-DTTP-based polymers can accept electron ea-
sier than the others and are stable in the reduced state. Con-
sidering the differences between the 2,3-diphenyl-DTTP and
2,3-dimethyl-DTTP, the existence of phenyl rings at the 2,3-
positions is probably responsible for the higher stability of the
reduced state compared with the methyl derivatives. All poly-
mers exhibited quasi-reversible behavior in electrochemical
oxidation. The onset potential of DTTP-containing polymers
was lower than that of DTBT-contained polymers. The HOMO
PIC-DTBT), the six DTTP-based polymers (TP1-6) had similar
LUMO energy levels, but the HOMO energy levels shifted
upward by 0.13 to 0.3 eV, which reduce the band gap of the
DTTP-based polymers. This might be due to better delocaliza-
tion of electrons in the HOMO of the DTTP-based polymers
because the three thiophene rings in the DTTP unit impart the
molecule with better planarity compared with the one benzene
and two thiophene rings in DTBT. For all nine polymers,
the electrochemical band gap (EEgC) was slightly larger than
the optical band gap (Eogpt), which was calculated from the onset
of the absorption spectra in the films.
and the LUMO energy levels of the conjugated polymers were
ox
calculated from the value of the onset oxidation potential (E
)
on
and the onset reduction potential (Eroend); the HOMO and the
LUMO are listed in Table 2. Compared with the three corre-
sponding DTBT-based polymers (PF-DTBT, PC-DTBT, and
Photovoltaic Properties. Photovoltaic devices were fab-
ricated in a typical sandwich structure of glass-ITO-
PEDOT/PSS-polymer/PCBM-Al, where ITO and Al were

2876 Macromolecules, Vol. 43, No. 6, 2010
Zhou et al.
Table 1. Yield, Molecular Weight, PDI, and Decomposition
Temperature of Nine Polymers
solar light (100 mW/cm2). The corresponding open-circuit
voltage (VOC), short-circuit current (ISC), fill factor (FF), and
PCE of the devices are summarized in Table 3.
polymer
yield Mn (kg/mol)a Mw (kg/mol)a PDIa Tdb (°C)
The results indicate that the VOC values of 0.80 to 1.02 V
for the photovoltaic devices based on the three DTBT-based
polymers were high, whereas those for polymers based on
TP1-6 had lower VOC values of 0.56 to 0.70 V. This difference
in VOC could result from the upward shift of the HOMO
energy levels of DTTP-based polymers compared with
DTBT-based polymers because the VOC is related to the
energy difference between the LUMO of the acceptor
(PCBM) and the HOMO of the donor (the conjugated
polymer).19 In addition, all devices based on the six DTTP-
based polymers exhibited lower PCEs than the DTBT-based
polymers, which could be understood from two aspects:
VOC, which decreased because of an increase in the HOMO
energy level of the DTTP-based polymers, and ISC, which
decreased probably because of the poor charge separation or
transport of the DTTP-based polymers.
PF-DTBT
TP1
TP2
PC-DTBT
TP3
TP4
PIC-DTBT 79%
TP5
TP6
54%
52%
57%
42%
32%
36%
11.3
4.2
5.8
16.5
6.6
7.8
17.5
12.3
14.4
15.8
5.7
1.4
1.4
2.0
1.5
1.6
1.8
1.9
1.7
1.8
361
300
340
371
382
317
344
381
387
11.7
24.8
10.6
14.0
33.9
20.9
26.4
73%
75%
a Determined by GPC with polystyrene as standard and CHCl3 as
eluent. b Onset decomposition temperature with 1% weight loss deter-
mined by TGA in air.
To compare the contribution of polymer absorption to
ISC, we measured the external quantum efficiency (EQE) of
the devices based on three fluorene-containing polymers
(PF-DTBT, TP1, and TP2) under the illumination of mono-
chromatic light (Figure 5). The shape of the EQE curves of
the devices was similar to that of the absorption spectra,
indicating that all wavelengths absorbed by the polymers
contributed to photovoltaic conversion. The maximum peak
in the EQE curve was red-shifted from 530 (PF-DTBT) to
600 (TP1) and 640 nm (TP2), but the value of EQE decreased
from 34% for PF-DTBT to 21% for TP1 and 16% for TP2.
With increasing thickness of the active layer, the absorption
could be improved, but the EQE value and ISC decreased,
possibly because the distance for the charge transport was
too great. Further investigation on the charge transport
properties in both the pristine polymer film and blend film
are in progress.
Figure 1. UV-vis absorption spectraofcopolymersinCHCl3 solution.
For bulk-heterojunction polymer solar cells, it has been
proposed that the surface morphology of the active layer
strongly affects photovoltaic properties.16j,20 The morpho-
logy of the blend films was observed by atomic force micro-
scopy (AFM), as shown in Figure 6. All films, except TP5,
exhibited low roughness and no significant aggregation,
suggesting that those polymers were highly compatible with
PCBM molecules in the films cast from the chlorobenzene
solutions. The rough surface and coarse phase separation of
the TP5/PCBM layer explain, at least in part, the lowest PCE
of TP5, and suggest there is still high potential to improve the
photovoltaic performance by developing suitable methods
for controlling the morphology of TP5/PCBM.
The PCEs of the photovoltaic devices based on PF-DTBT/
PCBM and PC-DTBT/PCBM were 2.67 and 3.05%, respec-
tively, which were lower than the PCE values of similar
materials in the literature.8,9 Therefore, the preliminary
photovoltaic performance still has high potential to be
improved by intensive device optimization, such as chang-
ing the acceptor or cathode metal, controlling film morpho-
logy by slow growth and/or using mixed solvents. Further
attempts to optimize the photovoltaic properties are in
progress.
Figure 2. UV-vis absorption spectra of polymer films on a quartz
plate.
used as anode and cathode, respectively, in diode-mode
operation. The detailed layout and dimensions are shown
in Scheme 4. The area of overlap between Al and ITO was
approximately 3 ꢀ 5 mm2. To avoid the so-called “edge
effect” and evaluate the PCEs of the photovoltaic devices
accurately,18 we defined the effective area of the PSCs using a
metal photomask (2 ꢀ 3 mm2) during irradiation of simu-
lated solar light.
Conclusions
Six thieno[3,4-b]pyrazine-based donor-acceptor (D-A) co-
polymers (TP1-6) and three BT-based D-A copolymers (PF-
DTBT, PC-DTBT, and PIC-DTBT) were synthesized by Suzuki
cross-coupling reaction. The absorption peaks and absorption
coefficients of the nine polymers were mainly dependent on the
Figure 4 shows the I-V curves of the devices based on each
of the nine polymers under illumination of AM 1.5 simulated

Article
Macromolecules, Vol. 43, No. 6, 2010 2877
Table 2. Optical and Electrochemical Properties, And Derived Energy Level of Nine Polymers
UV-vis absorption spectrum
solution
cyclic voltammetry
film
p-doping
n-doping
absorption coefficient
(L mol-1 cm-1
ox
on
red
polymer
λmax (nm)
)
λmax (nm)
Eogpt (eV)
E
/HOMO (V)/(eV)
E
/LUMO (V)/(eV)
on
EEg C (eV)
PF-DTBT
TP1
TP2
PC-DTBT
TP3
TP4
PIC-DTBT
TP5
TP6
386/537
412/588
425/639
391/545
414/590
428/651
397/531
417/577
431/635
4.19 ꢀ 104
3.31 ꢀ 104
2.59 ꢀ 104
4.15 ꢀ 104
3.25 ꢀ 104
2.57 ꢀ 104
4.00 ꢀ 104
3.20 ꢀ 104
2.52 ꢀ 104
387/547
414/600
409/641
392/551
415/603
434/674
398/540
418/586
430/644
1.85
1.61
1.55
1.72
1.57
1.46
1.91
1.65
1.51
0.76/-5.56
0.46/-5.26
0.47/-5.27
0.68/-5.48
0.44/-5.24
0.43/-5.23
0.65/-5.45
0.50/-5.30
0.52/-5.32
-1.33/- 3.47
-1.35/-3.45
-1.27/-3.53
-1.34/-3.46
-1.35/-3.45
-1.25/-3.55
-1.25/-3.55
-1.33/-3.47
-1.21/-3.59
2.09
1.81
1.74
2.02
1.79
1.68
1.90
1.83
1.73
Scheme 4. Layout and Dimensions of Photovoltaic Device
identity of the acceptors and the effects of the donor segments
(fluorene, carbazole, and indolocarbazole) were small. The high-
lying HOMO energy levels of six DTTP-based polymers were
responsible for the lower VOC of the PSCs. The results for the
photovoltaic devices indicate that not only the position of
absorption peaks but also the molecular energy levels should be
considered carefully in the design of photovoltaic polymers.
terthiophene (0.992 g, 2.0 mmol) and benzil (0.84 g, 4 mmol).
The target compound was purified by column chromatography
(silica gel; eluent: 50:50 chloroform/hexane v/v) and isolated as a
1
black-blue solid. Yield: 0.82 g (67%). H NMR (CDCl3, 400
MHz, δ): 7.60-7.55 (m, 4H), 7.43-7.31 (m, 8H), 7.05 (m, 2H).
MALDI-TOF MS (m/z): calcd for C26H14Br2N2S3, 607.89;
found, 607.42.
Synthesis of the Polymers by Suzuki Cross-Coupling Reaction.
Monomers 8, 9, or 10 (0.4 mmol), monomers 1, 5a, or 5b (0.38
mmol), and toluene (20 mL) were added to a 50 mL double-
necked round-bottomed flask. The reaction container was
purged with N2 for 30 min to remove O2. Pd(PPh3)4 (2%, 10
mg) was added and heated to 110 °C for 10 min. Tetraethylam-
monium hydroxide (1.8 mL, 20% by weight in H2O) was
introduced, and the mixture was heated under reflux overnight.
Phenylboric acid (100 mg in 1 mL of THF) was added. After 2 h,
bromobenzene (1 mL) was introduced. The mixture was allowed
to reflux for 2 h and cooled to room temperature. Then, the
polymer was precipitated by slow pouring of the mixture into
MeOH, filtered, and Soxhlet extracted with ether and CHCl3.
The CHCl3 solution was passed through a column packed with
alumina, Celite, and silica gel. The column was eluted with
CHCl3. The combined polymer solution was concentrated to
30 mL and was poured in methanol (300 mL). The precipitate
was collected and dried under vacuum overnight. The yield, 1H
NMR data (CDCl3, 400 MHz), and molecular weight are as
Experimental Section
Synthesis. 9,9-Di(2-ethylhexyl)fluorene-2,7-diboronic acid
bis(1,3-propanediol) ester solution (monomer 8, 0.5 M in to-
luene, Aldrich) and other chemicals were purchased from Alfa,
Aldrich, or Wako and used without further purification. The
following compounds were synthesized according to the proce-
dure in the literature: 4,7-di(2-bromothien-5-yl)-2,1,3-ben-
zothiadiazole (monomer 1),17 5,500-dibromo-30,40-dinitro-2,20:
50200-terthiophene (6),21 2,7-bis-(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)-N-(1-decylundecyl)carbazole (monomer 9),22 and
3,9-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-5,11-di(1-decyl-
undecyl)indolo[3,2-b]carbazole (monomer 10).15
Synthesis of 2,3-Dimethyl-5,7-di(2-bromothien-5-yl)-thieno-
[3,4-b]pyrazine (5a). 5,500-Dibromo-30,40-dinitro-2,20:50200-terthio-
phene (1.488 g, 3.0 mmol), iron powder (2.18 g, 39 mmol), and
acetic acid (60 mL) were mixed and heated at 60 °C for 30 min.
The mixture was cooled and filtered in a 100 mL flask. Then, 2,3-
butanedione (0.77 g, 9.0 mmol) was added to the flask. The
mixture was heated at 60 °C for 5 h under a N2 atmosphere.
After cooling, water was added, and the solid was collected by
filtration and washed with CH3OH. Finally, the crude com-
pound was purified by column chromatography (silica gel;
eluent: chloroform/hexane 50/50 (v/v)) and isolated as a deep
red solid. Yield: 1.1 g (72%). 1H NMR (CDCl3, 400 MHz, δ):
7.22 (d, 2H), 7.02 (d, 2H), 2.64 (s, 6H). MALDI-TOF MS (m/z):
calcd for C16H10Br2N2S3, 483.84; found, 483.51.
1
follows. PC-DTBT: Yield: 54%. H NMR (δ): 8.19 (br, 2H),
7.95 (br, 2H), 7.74 (br, 6H), 7.49 (br, 2H), 2.14 (br, 4H), 1.0-0.5
(br, 30H). Mn = 11.3K; PDI = 1.4. TP1: Yield: 52%. 1H NMR
(δ): 7.8-7.4 (m, 10H), 2.74 (br, 6H), 2.11 (br, 4H), 1.0-0.5 (m,
30H). Mn = 4.2K; PDI =1.4. TP2: Yield: 57%. 1H NMR (δ):
7.8-7.3 (m, 20H), 2.11 (br, 4H), 1.0-0.5 (m, 30H). Mn = 5.8K;
PDI = 2.0. PC-DTBT: Yield: 42%. 1H NMR (δ): 8.1-7.4 (m,
12H), 4.70 (br, 1H), 2.40 (br, 2H), 2.02 (br, 2H), 1.4-0.7 (m,
38H). Mn = 16.5K; PDI = 1.5. TP3: Yield: 32%. 1H NMR (δ):
8.1-7.3 (m, 10H), 4.68 (br, 1H), 2.74 (br, 6H), 2.41 (br, 2H), 2.03
(br, 2H), 1.4-0.7 (m, 38H). Mn = 6.6K; PDI = 1.6. TP4: Yield:
Synthesis of 2,3-Diphenyl-5,7-di(2-bromothien-5-yl)-thieno-
[3,4-b]pyrazine (5b). A procedure similar to the synthesis of 3a
was used starting from 5,500-dibromo-30,40-dinitro-2,20:50200-
1
36%. H NMR (δ): 8.1-7.3 (m, 20H), 4.70 (br, 1H), 2.41 (br,

2878 Macromolecules, Vol. 43, No. 6, 2010
Zhou et al.
Figure 4. I-V curves of nine polymer solar cells based various poly-
mers under illumination of AM 1.5 at 100 mW/cm2.
Figure 5. EQE of PSCs based on three fluorene-based polymers.
Table 3. Device Characteristics of PSCs Based on Nine Copolymers
active layer
VOC (V)
ISC (mA/cm2)
FF
PCE (%)
PF-DTBT/PCBM
TP1/PCBM
TP2/PCBM
PC-DTBT/PCBM
TP3/PCBM
TP4/PCBM
PIC-DTBT/PCBM
TP5/PCBM
TP6/PCBM
1.02
0.60
0.64
0.80
0.56
0.62
0.94
0.68
0.70
5.12
3.97
4.07
7.66
4.41
3.94
3.77
2.55
3.94
0.51
0.57
0.34
0.50
0.41
0.37
0.41
0.49
0.46
2.67
1.37
0.89
3.05
1.02
0.90
1.47
0.84
1.27
Figure 3. Cyclic voltammograms of nine polymers films on a platinum
plate in acetonitrile solution of 0.1 mol/L [Bu4N]PF6 (Bu = butyl) at a
scan rate of 50 mV/s. To highlight differences between polymers, dashed
lines are drawn as a guide for the eyes.
HSV-100 (Hokuto Denkou) potentiostat. A Pt plate coated with
a thin polymer film was used as the working electrode. A Pt wire
and an Ag/Agþ (0.01 M of AgNO3 in acetonitrile) electrode
were used as the counter and reference electrodes (calibrated
against Fc/Fcþ), respectively. AFM measurement was carried
out using a Digital Instrumental Nanoscope 31 operated in
tapping mode.
Fabrication and Characterization of Polymer Solar Cells.
PSCs were constructed in the traditional sandwich structure
through several steps. ITO-coated glass substrates were cleaned
by ultrasonication sequentially in detergent, water, acetone, and
2-propanol. After drying the substrate, PEDOT/PSS (Baytron
P), was spin-coated (4000 rpm for 30 s) on ITO. The film was
dried at 150 °C under a N2 atmosphere for 5 min. After the
substrate was cooled, a chlorobenzene solution of the polymer
and PCBM mixture was spin-coated. After the chlorobenzene
was dried, the substrate was transferred into an evaporation
2H), 2.02 (br, 2H), 1.4-0.7 (m, 38H). Mn = 7.8K; PDI = 1.8.
PIC-DTBT: Yield: 79%. 1H NMR (δ): 8.4-7.3 (m, 14H), 4.74 (br,
2H), 2.50 (br, 4H), 2.06 (br, 4H), 1.4-1.0 (br, 64H), 0.85 (m, 12H).
Mn = 17.5K; PDI = 1.9. TP5: Yield: 73%. 1H NMR (δ): 8.4-7.2
(m, 12H), 4.75 (br, 2H), 2.75 (br, 6H), 2.50 (br, 4H), 2.06 (br, 4H),
1.4-0.8 (m, 76H). Mn = 12.3K; PDI = 1.7. TP6: Yield: 75%. 1H
NMR (δ): 8.4-7.3 (m, 14H), 4.71 (br, 2H), 2.48 (br, 4H), 2.07 (br,
4H), 1.4-0.7 (m, 76H). Mn = 14.4K; PDI = 1.8.
Characterization. 1H NMR (400 MHz) spectra were mea-
sured using a JEOL Alpha FT-NMR spectrometer equipped
with an Oxford superconducting magnet system. Absorption
spectra were measured using a Shimadzu MPC-3100 spectro-
photometer. Cyclic voltammograms (CVs) were recorded on an

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