Silole-containing polymers for high-efficiency polymer solar cells

Article abstract of DOI:10.1002/pola.24870

Silole-containing conjugated polymers (P1 and P2) carrying methyl and octyl substituents, respectively, on the silicon atom were synthesized by Suzuki polycondensation. They show strong absorption in the region of 300-700 nm with a band gap of about 1.9 eV. The two silole-containing conjugated polymers were used to fabricate polymer solar cells by blending with PC₆₁BM and PC₇₁BM as the active layer. The best performance of photovoltaic devices based on P1/PC₇₁BM active layer exhibited power conversion efficiency (PCE) of 2.72%, whereas that of the photovoltaic cells fabricated with P2/PC₇₁BM exhibited PCE of 5.08%. 1,8-Diiodooctane was used as an additive to adjust the morphology of the active layer during the device optimization. PCE of devices based on P2/PC₇₁BM was further improved to 6.05% when a TiO_x layer was used as a hole-blocking layer.

Full text of DOI:10.1002/pola.24870

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ARTICLE
Silole-Containing Polymers for High-Efficiency Polymer Solar Cells
Jinsheng Song,1,2 Chun Du,1 Cuihong Li,2 Zhishan Bo1,2
1Institute of Polymer Chemistry and Physics, College of Chemistry, Beijing Normal University, Beijing 100875, China
2Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Correspondence to: Z. Bo (E-mail: zsbo@iccas.ac.cn)
Received 21 March 2011; accepted 30 June 2011; published online 28 July 2011
DOI: 10.1002/pola.24870
ABSTRACT: Silole-containing conjugated polymers (P1 and P2)
carrying methyl and octyl substituents, respectively, on the sili-
con atom were synthesized by Suzuki polycondensation. They
show strong absorption in the region of 300–700 nm with a
band gap of about 1.9 eV. The two silole-containing conjugated
polymers were used to fabricate polymer solar cells by blend-
ing with PC61BM and PC71BM as the active layer. The best per-
formance of photovoltaic devices based on P1/PC71BM active
layer exhibited power conversion efficiency (PCE) of 2.72%,
whereas that of the photovoltaic cells fabricated with P2/
PC71BM exhibited PCE of 5.08%. 1,8-Diiodooctane was used as
an additive to adjust the morphology of the active layer during
the device optimization. PCE of devices based on P2/PC71BM
was further improved to 6.05% when a TiOx layer was used as
C
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a hole-blocking layer.
2011 Wiley Periodicals, Inc. J Polym
Sci Part A: Polym Chem 49: 4267–4274, 2011
KEYWORDS: benzo[c][1,2,5]thiadiazole; conjugated polymers;
donor materials; fullerenes; hole-blocking layer; polymer solar
cells; silole; spin coating
INTRODUCTION Polymer solar cells (PSCs) have attracted
considerable attention due to their advantages such as low
cost, light weight, and easy processing. Recently, power con-
version efficiency (PCE) higher than 7%1,2 has been reported
for PSCs and lots of research results have shown that both
the material design and device fabrication process played
important roles in achieving high photovoltaic performances.
Except poly(3-hexylthiophene)s, high PCE polymer donor
materials usually have a common feature of main chain
donor acceptor alternating structure.3–12 And properties of
polymer materials such as energy level, absorption spectrum,
and solubility can be easily tuned by changing structures of
donor and acceptor repeating units.
recently, Scharber’s and Yang’s groups have also indicated
replacing the carbon by a silicon can obtain a smaller distor-
tion for the conjugated polymer backbone, which will lead to
a better packing of polymer chains.12,9
Benzothiadiazole is a promising building block for donor
materials used in PSCs, which exhibit higher PCE. In practice,
because of their limited solubility, many benzothiadiazole-
based polymers are of lower molecular weight. We have
found that the attaching of two flexible alkoxyl chains on
benzothiadiazole unit can significantly increase their copoly-
mers’ solubility without significant influence on their energy
levels. In our previous article, we have reported that PSCs
based on benzothiadiazole-containing copolymers, poly(2-(5-
(5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazol-
7-yl)thiophen-2-yl)-9-octyl-9H-carbazole), showed a PCE of
5.4%.8 The introduction of two flexible alkoxyl chains onto
the benzothiadiazole can significantly increase the solubility
of polymers without drastically decreasing the planarity of
polymer main chains.8
Silole-containing aromatic units have recently been investi-
gated as novel building blocks for conjugated polymer mate-
rials, because the r*-orbital of the silicon–carbon bond can
effectively interacts with the p*-orbital of the butadiene frag-
ment, leading to a lowest unoccupied molecular orbital
(LUMO) energy level. Silole-containing conjugated polymers
have been widely used in the field of organic light-emitting
diodes13 and field-effect transistors (FETs),14 since the
report that 2,5-di(2-pyridyl)silole derivatives exhibit promis-
ing electron-transporting properties by Tamao et al.15
Recently, Cao’s and Yang’s group have applied silole-contain-
ing polymers in the fabrication of polymer photovoltaic cells
and PCEs higher than 5% have been reached.16,4 Very
In this article, two new kinds of copolymers poly(4-(5-(9,9-di-
methyl-9H-dibenzosilole-2-yl)thiophen-2-yl)-5,6-bis(octyloxy)-
7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole) (P1) and poly(4-
(5-(9,9-dioctyl-9H-dibenzosilole-2-yl)thiophen-2-yl)-5,6-bis(oc-
tyloxy)-7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole) (P2) were
designed, synthesized, and used in solar cell device fabrica-
tion. The average performance of five solar cells based on P2
Additional Supporting Information may be found in the online version of this article.
C
V
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SCHEME 1 Synthesis of P1 and P2.
blended with PC71BM illustrated a short current density (Jsc)
of 10.67 mA cmꢀ2, an open-circuit voltage (Voc) of 0.84 V, a
fill factor (FF) of 0.59, and a PCE of 5.08% under air mass
(AM) 1.5 (100 mW cmꢀ2). When TiOx was used as an elec-
tron-blocking layer, the photovoltaic performance was greatly
improved and the PCE increased to 6.05% (Voc ¼ 0.91 V, Jsc
¼ 11.5 mA cmꢀ2, and FF ¼ 0.58), making P2 a very promis-
ing polymer for solar cell application.
calorimetry (DSC) at a heating and cooling speed of 20 ꢂC
minꢀ1 to be 134.6 and 136.8 C, respectively.
ꢂ
Electrochemical and Optical Properties
The electrochemical properties of P1 and P2 were investi-
gated by cyclic voltammetry (CV) with a standard three-elec-
trode electrochemical cell in a 0.1-M tetrabutylammonium
hexafluorophosphate solution in acetonitrile at room temper-
ature under a nitrogen atmosphere with a scanning rate of
100 mV sꢀ1. Ag/AgNO3 was used as the reference electrode
and a standard ferrocene/ferrocenium redox system was
used as the internal standard. The CV curves of P1 and P2
are shown in Figure 1. The onset oxidation potentials (Eox)
are 0.62 and 0.73 V for P1 and P2, respectively, and the
onset reduction potentials (Ered) are ꢀ1.23 and ꢀ1.20 V for
P1 and P2, respectively. The highest occupied molecular or-
RESULTS AND DISCUSSION
The synthesis of P1 and P2 is shown in Scheme 1. Starting
from 4,40-dibromo-2,20-diiodobiphenyl (1), the treatment of
Compound 1 with 4 equiv of n-BuLi in tetrahydrofuran
(THF) was followed by quenching with dichlorodimethylsi-
lane affording 2,7-dibromo-9,9-dimethyldibenzosilole (2) in a
yield of 40%. Miyaura cross-coupling of 2 and bis(pinacola-
to)diboron with Pd(dppf)2Cl2 as the catalyst precursor
afforded 9,9-dimethyl-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane-2-yl)dibenzosilole (M2) in a yield of 75%. 4,7-
Bis(5-bromothiophen-2-yl)-5,6-bis(octyloxy)benzo[c][1,2,5]-
thiadiazole8 (M1) and 9,9-dioctyl-2,7-bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolane-2-yl)dibenzosilole13,17 (M3) were pre-
pared according to literature procedures. Suzuki–Miyaura–
Schlu¨ter polycondensation was carried out in a biphasic mix-
ture of THF/toluene (1:3)/aqueous NaHCO3 with Pd(PPh3)4
as the catalyst precursor. Polymers P1 and P2 were obtained
as dark red solid in yields of 84 and 90%, respectively.
bital (HOMO) and LUMO energy levels and band gaps (Eg,ec
)
of polymers were calculated according to the following equa-
tions:
EHOMO ¼ ꢀeðEox þ 4:71Þ ðeVÞ
(1)
(2)
(3)
ELUMO ¼ ꢀeðEred þ 4:71Þ ðeVÞ
onset
red
Eg;ec ¼ Eooxnset ꢀ E
ðeVÞ
The energy levels calculated from the CV results indicated
that these new polymers are good candidates as donor mate-
rials. The LUMO energy levels of P1 and P2 are ꢀ3.48 and
ꢀ3.51 eV, which are higher than the LUMO level of PC61BM
(ꢀ4.1 eV), guaranteeing the photo-induced electron transfer
from the donor to acceptor, that is, from polymers to
PC61BM. The band gap (Eg) of P1 and P2 calculated from
the CV results is 1.85 and 1.93 eV, respectively. The optical
band gaps (Eg) of P1 and P2 were determined from the
onset of absorption to be 1.95 and 1.98 eV, respectively.
Molecular Weights and Thermal Properties
Both polymers displayed good solubility in chloroform, chlor-
obenzene (CB), 1,2-dichlorobenzene (DCB), THF and so forth
at room temperature. Molecular weights and polydispersity
indexes (PDIs) of the polymers were determined by gel per-
meation chromatography (GPC) against polystyrene stand-
ards with THF as an eluent. Number-average molecular
weights (Mn) of 79.8 and 102 kg molꢀ1 were obtained for
P1 and P2 with PDI of 1.46 and 1.66, respectively. Thermog-
ravimetric analysis (TGA) studies revealed that both the two
As shown in Figure 2, P1 and P2 displayed quite similar
absorption spectra both in solutions and films. In solutions,
the two polymers exhibited a broad absorption ranging from
300 to 650 nm with two peaks located at about 390 and
520 nm, respectively. Compared with the solution absorption
ꢂ
polymers are stable up to ꢁ330 C. Glass transition tempera-
tures of P1 and P2 were determined by differential scanning
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For P1, the optimized ratio of polymer to PC61BM was 1:4 (w/
w), the concentration of polymer is 7.5 mg mLꢀ1, the concen-
tration of DIO in DCB is 2.5% (in volume), and the spin coating
speed is 1500 rpm. The device showed a Voc of 0.71 V, a Jsc of
4.95 mA cmꢀ2, an FF of 0.50, and a PCE of 1.78%. For P2, ini-
tial test results showed that devices fabricated with a ratio of
polymer to PC61BM of 1:3 in CB and spin-coated at a speed of
1000 rpm gave a Voc of 0.87 V, a Jsc of 7.91 mA cmꢀ2, an FF of
0.59, and a PCE of 4.05%. The PCE of PSCs based on P2/
PC61BM-blend films was further increased to 4.81% by using
a solvent mixture of CB containing 2.5% (in volume) of DIO.
When PC71BM was used as the acceptor, PCEs were increased
from 1.78 to 2.72% for P1 and from 4.81 to 5.08% for P2.
These data are also summarized in Table 1. The increase of
the PCE is probably attributed to the increased Jsc, owing to
the light absorption of PC71BM in the visible region.19 The
external quantum efficiency (EQE) curve of a solar cell with
P2 and PC71BM as the active layer is shown in Figure 3, which
ranged from 300 to 720 nm and showed a broad coverage of
the solar spectrum. The photocurrent of photovoltaic cells was
recorded under monochromatic illumination. The complemen-
tary absorbance of P2 and PC71BM makes the EQE higher than
50% from 350 to 600 nm and with an onset at 720 nm. It was
reported that the introduction of a TiOx thin layer as an optical
spacer between the active layer and the top metal electrode
can redistribute the light intensity within the active layer,
block the hole transportation back to the top electrode, shield
against physical damage and chemical degradation of the
active layer, and significantly increase the PCE of the photovol-
taic devices.5,20,21 After the introduction of a TiOx layer
between the active layer and top metal electrode, the PCE of
P2 was further improved to 6.05% with a Jsc of 11.5 mA cmꢀ2
and the J–V characteristic is shown in Figure 4.
FIGURE 1 Cyclic voltammograms of polymer films in acetoni-
trile solution containing 0.1 M Bu4NPF6 and Ag/AgNO3 as the
reference electrode.
spectra, the film ones were both slightly broader and red-
shifted for about 30 nm.
Atomic force microscopy (AFM) images (5 ꢃ 5 lm2) of films
spin-coated from solutions of polymer and PC61BM contain-
ing different amount of DIO measured by AFM using the tap-
ping mode are shown in Figure 5. As shown in Figure 5(a,d),
blend films of P1:PC61BM spin-coated from DCB solutions
and blend films of P2:PC61BM spin-coated from CB solutions
Mobility Test
FET mobility measurements were carried out to detect the
hole mobility of the polymers. P1 and P2 exhibited hole
mobilities of about 4.3 ꢃ 10ꢀ4 and 3.2 ꢃ 10ꢀ3 cm2 Vꢀ1 sꢀ1
,
respectively. Mobility higher than 10ꢀ3 cm2 Vꢀ1 sꢀ1 is
desired for high-efficiency PSCs having efficient carrier trans-
port ability.18
Photovoltaic and Morphological Properties
Photovoltaic properties of P1 and P2 were first investigated
in devices with the structure of indium tin oxides (ITO)/
poly(3,4-ethylendioxythiophen):polystyrolsulfonate (PEDOT:
PSS)/active layer/LiF/Al. The active layer is a blend of poly-
mer and PC61BM. The influence of different solvent, the
ratio of polymer to PC61BM, the concentration of 1,8-diio-
dooctane (DIO) additive, and the spinning speed to the
photovoltaic performance was investigated in detail. The
optimization process was carried out in the following order:
the ratio of polymer to PC61BM, the concentration of the
additive, and the spinning speed. The results for P2 are
listed in Supporting Information Table S1 and their J–V
curves are shown in Supporting Information Figure S1.
FIGURE 2 Normalized UV–vis absorption spectra of P1 and P2
in dilute CHCl3 solutions and films on quartz substrates.
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TABLE 1 Optimized Photovoltaic Properties of PSCs Based on P1 and P2
Additive
Content
Active Layer
(vol %)
VOC (V)
TiOx
Jsc (mA cmꢀ2
)
FF
PCE (%)
P1:PC61BM ¼ 1:4
P1:PC71BM ¼ 1:4
P2:PC61BM ¼ 1:3
P2:PC71BM ¼ 1:3
P2:PC71BM ¼ 1:3
2.5
2.5
2.5
2.5
2.5
0.71
0.86
0.87
0.84
0.91
Without
Without
Without
Without
With
4.95
5.77
0.50
0.55
0.64
0.59
0.58
1.78
2.72
4.81
5.08
6.05
8.83
10.67
11.50
displayed smooth surfaces with root-mean-square (RMS)
roughness of 0.447 and 0.675 nm, respectively. Figure 5(b,e)
shows AFM images of blend films spin-coated from the solu-
tions containing 2.5% of DIO. The addition of 2.5% of DIO
could significantly increase the phase separation of blend
films and the RMS values were increased to 3.831 and 2.955
nm for P1 and P2, respectively. Although these two blend
films show similar surface topography, their phase images
are quite different as shown in Figure 5(c,f). As a comple-
ment to topographic images, phase images can provide infor-
mation on sample heterogeneity. From the phase image, it is
easy to find that the domain size formed by phase separation
of P2/PC61BM blend film is much smaller and more uniform
than that of P1/PC61BM blend film. Appropriate and uniform
domain size may favor for the efficient exciton separation
and diffusion.
2.5% of DIO) gave the highest PCE. Only blend films spin-
coated under the optimal conditions showed uniform and
spheriform domains, which disappeared by either increasing
or decreasing the concentration of DIO. From the above
results, we can easily find that the PCE is closely related to
the morphology of blend films. Only appropriate RMS number
and domain size can give the best device performance.
When TiOx spacer was used between the photo-active layer
and the top electrode, it was reported that the physical
defects of films could be prevented and the durability could
be improved.20 As shown in Figure 6(f), AFM image of blend
films covered with a TiOx layer revealed that the TiOx layer
was not a continuous and homogeneous film. It is hard for a
very thin TiOx layer to form a continuous and homogeneous
film on the blend film. Hole-blocking properties of TiOx layer
were investigated by measuring the dark J–V curves of solar
cells and the resulting dark current density versus bias volt-
age curves of P2:PC71BM blend films covered with and with-
out TiOx are summarized in Figure 7. Solar cells with the
TiOx spacer displayed better rectification property and
smaller leakage current than those without the TiOx spacer.
Under a negative bias of ꢀ2 V, holes will be injected from Al
electrode to the TiOx layer. The relative smaller current den-
sity of the device with a TiOx spacer indicated that the TiOx
film is an effective hole-blocking layer, which helps to
improve the Jsc of devices.
To deeply understand the influence of additive on the device
performance, films spin-coated from P2/PC61BM in CB con-
taining different amount of DIO were investigated in detail.
The surface topography (2 ꢃ 2 lm2) of these films obtained
by AFM using the tapping mode is shown in Figure 6(a–e).
The morphology and domain size of blend films are very sen-
sitive to the concentration of additive. With the increase of the
concentration of DIO from 0 to 5%, RMS values increased
from 0.467 to 4.953 nm, and the domain size also became
larger. Devices fabricated under optimal conditions (containing
FIGURE 3 EQE spectrum of PSCs based on P2/PC71BM from CB
FIGURE 4 Current density–voltage characteristic of PSCs based
on P2/PC71BM under AM 1.5 G illumination.
with 2.5% DIO.
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FIGURE 5 AFM topography images of P1/PC61BM (1:4, w/w) blend films spin-coated from DCB solution (a) and DCB solution with
2.5 vol % DIO (b); phase image of P1/PC61BM (1:4, w/w) blend films spin-coated from DCB with 2.5 vol % DIO (c); AFM topography
images of P2/PC61BM (1:3, w/w) blend films spin-coated from CB solution (d) and CB solution with 2.5 vol % DIO (e); phase image
of P2/PC61BM (1:3, w/w) blend films with 2.5 vol % DIO (f).
FIGURE 6 AFM topography of blend films of P2/PC61BM (1:3, w/w) spin-coated from CB solution with different DIO concentrations
(from 0 to 5 vol %; a–e). AFM topography of blend films of P2/PC61BM (using 2.5% DIO) coated with a TiOx film (f). [Color figure
can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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erage molecular weights (Mw) were measured by GPC on a
Waters GPC2410 with THF as an eluent calibrated with poly-
styrene standards. AFM images of blend films were obtained
on a Nanoscope IIIa Dimension 3100 operating in the tap-
ping mode. TGA was performed on a Perkin-Elmer Pyris 1
analyzer under nitrogen atmosphere (100 mL minꢀ1) at a
heating rate of 10 C minꢀ1. DSC measurements were per-
formed on a Mettler Toledo DSC 822e with a heating and
cooling rate of 20 C minꢀ1. Electrochemical measurements
were performed on a CHI 630A Electrochemical Analyzer.
Device Processing and Characterization
PSCs were fabricated with the device configuration of ITO/
PEDOT:PSS/active layer/LiF/Al or ITO/PEDOT:PSS/active
layer/TiOx/Al. The conductivity of ITO was 20 X sq.ꢀ1 and
PEDOT:PSS is Baytron
P VP.AI 4083. A thin layer of
PEDOT:PSS was spin-coated on the top of cleaned ITO sub-
strate at 2,400 rpm sꢀ1 and dried subsequently at 120 ꢂC
for 10 min on a hotplate. The active layer was prepared in
air by spin coating the CB or DCB solution of polymers and
PC61BM or PC71BM on the top of ITO/PEDOT:PSS. About 1
nm of LiF was thermally evaporated or 10 nm of TiOx was
spin-coated on to the active layer, following with annealing
at 80 ꢂC for 10 min in air. The top electrode was thermally
evaporated 100 nm of aluminum at a pressure of 10ꢀ4 Pa
through a shadow mask. Eight organic solar cells (OSCs) were
fabricated on one substrate and the effective area of one cell is
4 mm2. The characterization of solar cell devices was carried
out on a computer controlled Keithley 236 digital source me-
ter with an AM 1.5 solar simulator (Oriel model 96000; 100
mW cmꢀ2) as the light source.
2,7-Dibromo-9,9-dimethyldibenzosilole (Compound 2)
n-Butyllithium (6.5 cm3, 16.24 mmol, 2.5 M in hexane) was
added to a solution of 4,40-dibromo-2,20-diiodobiphenyl (2.29
g, 4.06 mmol) in dry THF (50 cm3) at ꢀ90 ꢂC under nitro-
gen atmosphere for 0.5 h. The mixture was stirred for a fur-
ther 1 h at ꢀ90 ꢂC, then dichlorodimethylsilane (1.50 cm3,
12.4 mmol) was added and the mixture was brought up to
room temperature and stirred overnight. The reaction was
quenched with distilled water and the mixture was then
extracted with diethyl ether (100 mL). Organic layer was
washed with brine (100 mL ꢃ 3), dried with anhydrous
Na2SO4, and evaporated. On purification by column chroma-
tography with hexane as the eluent, Compound 2 was
obtained as colorless crystal (0.6 g, 40%).
FIGURE 7 Photovoltaic properties of devices made from P2/
PC71BM blends with and without a TiOx layer under dark: (a) I–
V curves and (b) the absolute value of current density on log
scale.
EXPERIMENTAL
Materials and Instruments
All reagents were purchased from commercial suppliers and
used without further purification unless otherwise indicated.
THF and toluene were distilled from Na under nitrogen with
benzophenone as an indicator. Compound 1,13,14 4,7-bis(5-
bromothiophen-2-yl)-5,6-bis(octyloxy)benzo[c][1,2,5]thiadia-
zole (M1)8 and 9,9-dioctyl-2,7-bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolane-2-yl)dibenzosilole (M3)13 were prepared by
following the literature procedures.
1H NMR (400 MHz, CDCl3, d): 0.43 (s, 6H; CH3), 7.54 (dd, J
¼ 8.3, 2H; ArH), 7.63 (d, J ¼ 8.3, 2H; ArH), 7.70 (s, 2H;
ArH); 13C NMR (100 MHz, CDCl3, d): ꢀ3.25, 122.61, 122.75,
133.47, 135.78, 141.64, 145.73. Anal. Calcd for C14H12Br2Si:
C, 45.68; H, 3.29. Found: C, 45.64; H, 3.35.
Thin-layer chromatography analysis was performed using
silica gel HSG (F254) plates, and the eluted plates were
observed under a UV detector. Chromatographic purifications
were performed by flash chromatography on silica gel (200–
300 mesh). 1H and 13C NMR spectra were recorded on a
Bruker AV400 spectrometer with choroform-d1 as the sol-
vent. UV–vis absorption spectra were recorded on a UV-
1601pc spectrometer. Elemental analysis was performed on
a Vario EL elemental analysis instrument. Mn and weight-av-
9,9-Dimethyl-2,7-bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolane-2-yl)dibenzosilole (M2)
A mixture of Compound 2 (0.592 g, 1.62 mmol), 4,4,5,5-
tetramethyl-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3,
2-dioxaborolane (1.03 g, 4.06 mmol), dry potassium acetate
(1.03 g, 4.06 mmol), and dry N,N-dimethylformamide (DMF)
(20 mL) was carefully degassed before and after Pd(dppf)Cl2
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(30 mg, 36.8 lmol) was added. The mixture was stirred at 80
ꢂC for 2 days under nitrogen atmosphere. Then, the reaction
mixture was cooled to room temperature, added with water
(50 mL) and extracted with hexane (20 mL ꢃ 3). Organic
layer was washed with brine (50 mL ꢃ 3), dried with anhy-
drous Na2SO4, and evaporated to dryness. The residue was
chromatographically purified on a silica gel column eluting
with acetic ester/hexane (1:5, v/v) to afford M2 as colorless
crystal (0.56 g, 75%).
Mn and PDI measured by GPC calibrated with polystyrene
standards are 102.0 kg molꢀ1 and 1.66, respectively.
1H NMR (400 MHz, CDCl3, d): 8.57 (br, 2H), 8.00–7.82 (mbr,
6H), 7.53 (br, 2H), 4.21 (br, 4H), 2.03 (br, 4H), 1.54–0.84
(mbr, 60H); 13C NMR (100 MHz, CDCl3, d): 151.82, 150.98,
147.47, 145.91, 139.04, 133.60, 133.21, 132.14, 131.98,
132.14, 132.08, 131.99, 130.48, 128.78, 128.46, 127.78,
127.06, 122.97, 121.36, 121.23, 117.56, 74.53, 33.45, 33.41,
31.88, 30.58, 29.70, 29.67, 29.54, 29.38, 29.26, 29.22, 29.14,
29.11, 26.14, 24.01, 22.81, 22.71, 22.65, 14.22, 14.12, 14.09.
Anal. Calcd for (C58H80N2O2S3Si)n: C, 72.45; H, 8.39; N, 2.91.
Found: C, 69.28; H, 8.03; N, 2.59.
1H NMR (400 MHz, CDCl3, d): 0.42 (s, 6H; CH3), 1.37 (s, 24H;
CH3), 7.86 (d, J ¼ 7.8, 2H; ArH), 7.90 (d, J ¼ 7.8, 2H; ArH),
8.10 (s, 2H; ArH); 13C NMR (100 MHz, CDCl3, d): ꢀ3.06,
25.05, 83.95, 120.7, 137.1, 138.8, 139.5, 150.5. Anal. Calcd for
C26H36B2O4Si: C, 67.55; H, 7.85. Found: C, 67.66; H, 7.89.
CONCLUSIONS
In conclusion, low-band gap conjugated polymers (P1 and P2)
General Procedures for the Synthesis of Polymers 1 and 2
Using Suzuki–Miyaura–Schlu¨ ter Polycondensation
with
a dibenzosilole–thiophene–benzothiadiazole–thiophene
main chain were synthesized and used for PSCs. These two
polymers are of high molecular weight and good solubility in
commonly used organic solvents. PSCs were fabricated with
the blends of polymers and PC71BM as the active layer. PCEs
of the optimized photovoltaic cells are 2.72% for P1 and
5.08% for P2 under the illumination of AM 1.5 (100 mW
cmꢀ2). The performances of solar cells based on P2 were fur-
ther increased to 6.05% for PCE, 0.91 V for Voc, 11.50 mA
cmꢀ2 for Jsc, and 0.58 for FF, indicating that P2 is a promising
donor material for solar cell applications.
A mixture of 4,7-bis(5-bromothiophen-2-yl)-5,6-bis(octylox-
y)benzo[c]-[1,2,5]thiadiazole (M1), 9,9-dialkyl-2,7-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolane-2-yl)dibenzosilole (M2 or
M3), NaHCO3 (0.6 g, 71.4 mmol), H2O (3 mL), toluene (20
mL), and THF (8 mL) was carefully degassed before and af-
ter 3 mg Pd(PPh3)4 was added. The mixture was stirred and
refluxed for 1 week under nitrogen atmosphere. Phenylbor-
onic acid (10 mg, 0.08 mmol) was added; the reaction was
further refluxed for 1 day; then 1-bromobenzene (0.05 mL,
0.48 mmol) was added and the reaction was refluxed for
another 1 day. The reaction mixture was then allowed to
cool to room temperature, the whole reaction mixture was
poured into acetone (200 mL), and the resulted precipitate
was collected by filtration. The crude polymer was dissolved
in chloroform (200 mL) and filtered. The filtration was con-
centrated to about 80 mL and precipitated into acetone (300
mL). The precipitate was collected by filtration and dried
under high vacuum to afford the aimed polymer.
Financial support by the NSF of China (50603027 and
20834006), the 863 Program (2008AA05Z425), and the 973
Program (2009CB623601) is gratefully acknowledged.
REFERENCES AND NOTES
1 Chen, H.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.;
Yu, L.; Wu, Y.; Li, G. Nat Photonics 2009, 3, 649–653.
2 Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.;
Yu, L. Adv Mater 2010, 22, E135–E138.
Poly(4-(5-(9,9-dimethyl-9H-dibezosilole-2-yl)thiophen-2-
yl)-5,6-bis(octyloxy)-7-(thiophen-2-yl)benzo[c][1,2,5]-
thiadiazole) (P1)
3 Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.;
Heeger, A. J.; Bazan, G. C. Nat Mater 2007, 6, 497–500.
M1 (100 mg, 0.140 mmol) and M2 (64.4 mg, 0.139 mmol)
were used. P1 was obtained as dark solid (90 mg, 84%). Mn
and PDI measured by GPC calibrated with polystyrene stand-
ards are 79.8 kg molꢀ1 and 1.46, respectively.
4 Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Li, G.; Yang, Y. J Am
Chem Soc 2008, 130, 16144–16145.
5 Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon,
J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat Pho-
tonics 2009, 3, 297–302.
1H NMR (400 MHz, CDCl3, d): 8.54 (br, 2H), 8.00–7.75 (mbr,
6H), 7.51 (br, 2H), 4.21 (br, 4H), 2.09 (br, 4H), 1.54–1.26
(mbr, 20H), 0.89 (br, 6H), 0.56 (br, 6H); 13C NMR (100 MHz,
CDCl3, d): 151.83, 150.94, 133.40, 133.39, 131.97, 130.10,
130.06, 127.88, 121.36, 117.54, 74.55, 74.53, 31.92, 31.86,
30.54, 30.06, 29.69, 29.64, 29.52, 29.48, 29.33, 26.11, 22.90,
22.68, 14.27, 14.16, 14.11, ꢀ3.12. Anal. Calcd for
(C44H52N2O2S3Si)n: C, 69.07; H, 6.85; N, 3.66. Found: C,
67.98; H, 6.57; N, 3.42.
6 Hou, J.; Chen, H.-Y.; Zhang, S.; Chen, R. I.; Yang, Y.; Wu, Y.;
Li, G. J Am Chem Soc 2009, 131, 15586–15587.
7 Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L.
J Am Chem Soc 2009, 131, 7792–7799.
8 Qin, R. P.; Li, W. W.; Li, C. H.; Du, C.; Veit, C.; Schleier-
macher, H. F.; Andersson, M.; Bo, Z. S.; Liu, Z. P.; Inganas,
¨
O.; Wuerfel, U.; Zhang, F. L. J Am Chem Soc 2009, 131,
14612–14613.
9 Chen, H. Y.; Hou, J. H.; Hayden, A. E.; Yang, H.; Houk, K. N.;
Yang, Y. Adv Mater 2010, 22, 371–375.
Poly(4-(5-(9,9-dioctyl-9H-dibezo-silole-2-yl)thiophen-2-
yl)-5,6-bis(octyloxy)-7-(thiophen-2-yl)benzo[c][1,2,5]
thiadiazole) (P2)
10 Zou, Y. P.; Najari, A.; Berrouard, P.; Beaupre, S.; Aich, B.
R.; Tao, Y.; Leclerc, M.
5330–5331.
J Am Chem Soc 2010, 132,
M1 (100 mg, 0.140 mmol) and M3 (91.7 mg, 0.139 mmol)
were used. P2 was obtained as dark solid (120 mg, 90%).
11 Huo, L. J.; Hou, J. H.; Zhang, S. Q.; Chen, H. Y.; Yang, Y.
Angew Chem Int Ed 2010, 49, 1500–1503.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2011, 49, 4267–4274
4273

ARTICLE
WWW.POLYMERCHEMISTRY.ORG
12 Scharber, M. C.; Koppe, M.; Gao, J.; Cordella, F.; Loi, M. A.;
Denk, P.; Morana, M.; Egelhaaf, H.-J.; Forberich, K.; Dennler,
G.; Gaudiana, R.; Waller, D.; Zhu, Z.; Shi, X.; Brabec, C. J. Adv
Mater 2010, 22, 367–370.
17 Boudreault, P. L. T.; Michaud, A.; Leclerc, M. Macromol Rapid
Commun 2007, 28, 2176–2179.
18 Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.;
¨
Waldauf, C.; Heeger, A. J.; Brabec, C. J. Adv Mater 2006, 18,
789–794.
13 Chan, K. L.; McKiernan, M. J.; Towns, C. R.; Holmes, A. B.
J Am Chem Soc 2005, 127, 7662–7663.
19 Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.;
Hummelen, J. C.; Hal, P. A. V.; Janssen, R. A. J. Angew Chem
Int Ed 2003, 42, 3371–3375.
14 Usta, H.; Lu, G.; Facchetti, A.; Marks, T. J. J Am Chem Soc
2006, 128, 9034–9035.
15 Tamao, K.; Uchida, M.; Izumizawa, T.; Furukawa, K.; Yama-
20 Hayakawa, A.; Yoshikawa, O.; Fujieda, T.; Uehara, K.; Yoshi-
guchi, S. J Am Chem Soc 1996, 118, 11974–11975.
kawaa, S. Appl Phys Lett 2007, 90, 163517/1–163517/3.
16 Wang, E. G.; Wang, L.; Lan, L. F.; Luo, C.; Zhuang, W. L.;
21 Kim, J. Y.; Kim, S. H.; Lee, H. H.; Lee, K.; Ma, W. L.; Gong,
Peng, J. B.; Cao, Y. Appl Phys Lett 2008, 92, 033307/1–033307/3.
X.; Heeger, A. J. Adv Mater 2006, 18, 572–576.
4274
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2011, 49, 4267–4274