Photovoltaic performance improvement of D-A copolymers containing bithiazole acceptor unit by using bithiophene bridges

Article abstract of DOI:10.1021/ma201976t

Two bithiophene-bridged D-A copolymers, PDTSBTBTz and PBDTBTBTz, based on bithiazole acceptor unit and dithienosiole (DTS) or benzodithiophene (BDT) donor unit, were synthesized by the Pd-catalyzed Stille-coupling reaction. The two copolymers exhibit good thermal stability, strong absorption in the visible region, and relatively lower HOMO energy level at ca. -5.10 eV. The hole mobilities of PDTSBTBTz and PBDTBTBTz measured by SCLC method are 1.85 × 10^-3 and 1.77 × 10^-3 cm²/(V s), respectively. Power conversion efficiency (PCE) of the polymer solar cell (PSC) based on PDTSBTBTz: PC₇₀BM (1:1, w/w) was 3.82% with J_sc = 8.68 mA/cm², V_oc = 0.72 V, and FF = 0.611, under the illumination of AM1.5, 100 mW/cm². In contrast, the PCE of the PSC based on PBDTBTBTz:PC₇₀BM (1:1, w/w) reached 4.46% with J _sc = 9.01 mA/cm², V_oc = 0.82 V, and FF = 0.603. These results indicate that bithiophene-bridged D-A copolymers are promising photovoltaic donor materials for the application in PSCs.

Full text of DOI:10.1021/ma201976t

ARTICLE
pubs.acs.org/Macromolecules
Photovoltaic Performance Improvement of DꢀA Copolymers
Containing Bithiazole Acceptor Unit by Using Bithiophene Bridges
Maojie Zhang,† Xia Guo,†,‡ and Yongfang Li*,†
†Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
‡Graduate University of Chinese Academy of Sciences, Beijing 100049, China
S Supporting Information
b
ABSTRACT: Two bithiophene-bridged DꢀA copolymers,
PDTSBTBTz and PBDTBTBTz, based on bithiazole accep-
tor unit and dithienosiole (DTS) or benzodithiophene
(BDT) donor unit, were synthesized by the Pd-catalyzed
Stille-coupling reaction. The two copolymers exhibit good
thermal stability, strong absorption in the visible region, and
relatively lower HOMO energy level at ca. ꢀ5.10 eV. The hole
mobilities of PDTSBTBTz and PBDTBTBTz measured by
SCLC method are 1.85 ꢁ 10ꢀ3 and 1.77 ꢁ 10ꢀ3 cm2/(V s),
respectively. Power conversion efficiency (PCE) of the polymer solar cell (PSC) based on PDTSBTBTz: PC70BM (1:1, w/w)
was 3.82% with Jsc = 8.68 mA/cm2, Voc = 0.72 V, and FF = 0.611, under the illumination of AM1.5, 100 mW/cm2. In contrast,
the PCE of the PSC based on PBDTBTBTz:PC70BM (1:1, w/w) reached 4.46% with Jsc = 9.01 mA/cm2, Voc = 0.82 V, and FF =
0.603. These results indicate that bithiophene-bridged DꢀA copolymers are promising photovoltaic donor materials for the
application in PSCs.
’ INTRODUCTION
In our previous work, we have synthesized two DꢀA copoly-
mers PDTSBTz (which is P3 in ref 24a) and PBDTBTz (ref 24b)
(see Scheme 1) based on bithiazole (BTz) as acceptor unit and
dithenosilole (DTS) or benzodithiophene (BDT) as donor unit
with thiophene π-bridges between the donor and acceptor units.
The two polymers exhibited hole mobilities of 3.07 ꢁ 10ꢀ4 and
6.84 ꢁ 10ꢀ4 cm2/(V s), respectively, and the polymer solar cell
based on these two polymers showed PCE of 2.86% and 3.82%,
respectively. In order to attain more extended copolymers,25 here
we synthesized two bithiophene (BT)-bridged DꢀA copolymers,
PDTSBTBTz and PBDTBTBTz, based on bithiazole as acceptor
unit (compound 3 in Scheme 2) and DTS or BDT as donor unit
by Stille-coupling reaction. The two copolymers exhibit broad
absorption, relatively lower HOMO energy level at ca. ꢀ5.10 eV
and higher hole mobilities of ca. 1.8 ꢁ 10ꢀ3 cm2/(V s). PCE of the
PSCs based on PDTSBTBTz and PBDTBTBTz reached 3.82%
and 4.46%, respectively. These results indicate that the bithio-
phene-bridged DꢀA copolymers are promising photovoltaic
donor materials for the application in PSCs.
Polymer solar cells (PSCs) have attracted considerable atten-
tion in recent years for their advantages over inorganic solar cells,
such as easy fabrication, light weight, flexibility, low cost, and so
forth.1ꢀ4 The bulk heterojunction (BHJ) PSCs have played a
leading role in realizing high efficiency by forming a donorꢀ
accept bicontinuous interpenetrating network of a conjugated
polymer donor and soluble fullerene derivative (such as [6,6]-
phenyl-C61-butyric acid methyl ester (PC60BM) or [6,6]-phenyl-
C71-butyric acid methyl ester (PC70BM)) acceptor.5 The key
issue in the studies of PSCs is the design and synthesis of high
efficiency donor and acceptor photovoltaic materials.1ꢀ4,6 For
the conjugatedpolymer donormaterials, broadabsorption spectra,
higher hole mobility, and relatively lower HOMO energy level are
key points for realizing high efficiency. One of the most popular
approaches is to design and synthesize donorꢀacceptor (DꢀA)
low-bandgap copolymers.7ꢀ22
Among the various donor (electron-rich) moieties used in the
DꢀA copolymer photovoltaic materials, dithienosilole (DTS)
and benzodithiophene (BDT) are currently two of the most
preferred donor units because of their good electron-donating
properties, rigid coplanar structure with stronger πꢀπ inter-
molecular interactions, and the possibility of side-chain manip-
ulation for suitable solubility (or processability).1,12,13,20ꢀ23 The
PSCs based on the DTS- or BDT-containing polymers as donor
and PC70BM as acceptor showed high power conversion effi-
ciency up to 6ꢀ7%.1,21,23c
’ RESULTS AND DISCUSSION
Synthesis and Thermal Stability. The general synthetic
strategy for monomer 3 and polymers is outlined in Scheme 2.
Received: August 29, 2011
Revised:
October 6, 2011
Published: October 27, 2011
r
2011 American Chemical Society
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Scheme 1. Molecular Structures of Thiophene-Bridged DꢀA Copolymers PDTSBTz and PBDTBTz and Bithiophene-Bridged
DꢀA Copolymers PDTSBTBTz and PBDTBTBTz
The alternative copolymers of PDTSBTBTz and PBDTBTBTz
were synthesized by Stille-coupling reaction. The two copoly-
mers have good solubility in common organic solvents such as
chloroform, toluene, and chlorobenzene. Molecular weights and
polydispersity indices (PDIs) of the polymers were determined
by gel permeation chromatography (GPC) analysis with a poly-
styrene standard calibration, and the results are listed in Table 1.
The number-average molecular weights (Mn) of PDTSBTBTz
and PBDTBTBTz are 9.1 and 8.6 K, respectively. Although the
molecular weights are relatively low, they are higher than those of the
corresponding thiophene-bridged DꢀA copolymers PDTSBTz24a
and PBDTBTz.24b
Thermal stability of the polymers was investigated with
thermogravimetric analysis (TGA), as shown in Figure 1. The
TGA analysis reveals that the onset temperatures with 5% weight
loss (Td) of PDTSBTBTz and PBDTBTBTz are 425 and 343 °C,
respectively. In comparison, Td of PDTSBTz is 368 °C24a and
that of PBDTBTz is 317 °C.24b Obviously, the thermal stability
of the bithiophene-bridged DꢀA copolymers is better than that
of the corresponding thiophene-bridged copolymers.
Absorption Spectra. Figure 2 shows the ultravioletꢀvisible
(UVꢀvis) absorption spectra of the polymer dilute solutions in
chloroform and films spin-coated on quartz substrates. The
detailed absorption data, including absorption maximum wave-
length of the polymer solutions and films, the absorption edge
(onset wavelength of the absorption peak, λonset) of the polymer
films, and the optical bandgap deduced from the absorption
edges, are summarized in Table 2. PDTSBTBTz and PBDTBTBTz
solutions in CHCl3 show similar absorption spectra with the
absorption maxima at 485 and 490 nm, respectively. Compared
with the absorption spectra of the polymer solutions, the
absorption maxima of PDTSBTBTz and PBDTBTBTz films
red-shifted by 65 and 45 nm, respectively, which indicates that
there was strong πꢀπ stacking and interchain interaction in
PDTSBTBTz and PBDTBTBTz films. In addition, the absorp-
tion spectra of PBDTBTBTz and PDTSBTBTz films exhibited
an apparent shoulder peak similar to that of poly(3-
hexylthiophene) (P3HT), which means that there was good
alignment in the polymer chains. Interestingly, the shoulder
peaks of PDTSBTBTz and PBDTBTBTz films are stronger than
those of their corresponding thiophene-bridged DꢀA copoly-
mers PDTSBTz and PBDTBTz films,24 indicating that the
bithiophene bridges between the DTS or BDT donor unit and
the bithiazole acceptor unit in PDTSBTBTz and PBDTBTBTz
benefitted the πꢀπ stacking and ordered aggregation of the
polymer main chains. The absorption edges (λedge) of the two
polymer films are 655 nm for PDTSBTBTz and 630 nm for
PBDTBTBTz, from which the optical bandgaps (Egopt) of the
polymers can be calculated according to Egopt = 1240/λedge. The
opt
Eg values of PDTSBTBTz and PBDTBTBTz are 1.89 and
1.96 eV, respectively.
X-ray Diffraction. Figure 3 shows the X-ray diffraction (XRD)
patterns of the copolymers PDTSBTBTz and PBDTBTBTz.
There are a sharp peaks in the XRD pattern of PDTSBTBTz at
small angle of 2θ = 5.11° with d1 = 17.3 Å. While in the XRD
pattern of PBDTBTBTz, these are only a weak peak in the low-
angle region (d1 = 17.8 Å). The peaks correspond to the distance
between main chains of the conjugated polymers separated by
the alkyl side chains.26 In comparison with regioregular P3HT
where the lamellar d-spacing is 16.4 Å,27 the copolymers have
larger lamellar d-spacing. It is obvious that the packing of
polymer chains for PDTSBTBTz is tighter than that of
PBDTBTBTz.
Electrochemical Properties. The electrochemical cyclic vol-
tammetry was performed for determining the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molec-
ular orbital (LUMO) energy levels of the conjugated polymers.28
Figure 4 shows the cyclic voltammograms of the PDTSBTBTz
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Scheme 2. Synthetic Route for Monomer 3 and Copolymers
and PBDTBTBTz films on Pt disk electrode in 0.1 mol/L Bu4NPF6
acetonitrile solution. The onset reduction potentials (jred) of
PDTSBTBTz and PBDTBTBTz are ꢀ1.79 and ꢀ1.77 V vs Ag/
Ag+, respectively, while the onset oxidation potentials (jox) are 0.36
and 0.41 V vs Ag/Ag+, respectively. HOMO and LUMO energy
levels of the polymers were calculated according to the equations29
HOMO = ꢀe(jox + 4.71) (eV) and LUMO = ꢀe(jred + 4.71)
(eV). The results of the electrochemical measurements are listed
in Table 2. The LUMO energy levels of PDTSBTBTz and
PBDTBTBTz are ꢀ2.92 and ꢀ2.94 eV, respectively, and the
HOMO energy levels of PDTSBTBTz and PBDTBTBTz are
ꢀ5.07 and ꢀ5.12 eV, respectively. The relatively lower HOMO
energy level of the copolymers is beneficial to the better stability of
the polymers against oxidation and higher open circuit voltage (Voc)
of the PSCs based on the polymers as donor.15
Hole Mobility. Hole mobility is another important parameter
for the conjugated polymer donor photovoltaic materials. Here
we measured the hole mobility of the copolymers PDTSBTBTz
and PBDTBTBTz by space-charge limit current (SCLC) meth-
od using a device structure of ITO/PEDOT:PSS/polymer/Au.
For the hole-only devices, SCLC is described by30
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
J = ð9=8Þεε0μ0V2 expð0:89 V=E0LÞ=L3
ð1Þ
where ε is the dielectric constant of the polymer, ε0 is the
permittivity of the vacuum, μ0 is the zero-field mobility, E0 is the
characteristic field, J is the current density, L is the thickness of
the blended films, V = Vappl ꢀ Vbi, Vappl is the applied potential,
and Vbi is the built-in potential which results from the difference
in the work function of the anode and the cathode (in this device
structure, Vbi = 0.2 V). The results are plotted as ln(JL3/V2) vs
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Table 1. Molecular Weights and Thermal Properties of the
Polymers
polymers
Mw
Mn
PDIa
Td (°C)a
PDTSBTBTz
PBDTBTBTz
14.7K
12.6K
9.1K
8.6K
1.61
1.46
425
343
a The 5% weight-loss temperatures under inert atmosphere.
Figure 2. Optical spectra of copolymers in CHCl3 and in films.
comparison with the PCE of 2.86%24a for PDTSBTz and 3.82%24b
for PBDTBTz, the power conversion efficiency of the bithiophene-
bridged DꢀA copolymers is improved significantly over that of their
corresponding thiophene-bridged copolymers. The better photo-
voltaic performance of the bithiophene-bridged copolymers could
be ascribed to their higher hole mobilities mentioned above.
Figure 7 demonstrates the external quantum efficiency (EQE)
curves of the PSCs based on the two copolymers. The EQE curve
of the PSC based on PDTSBTBTz:PC70BM (1:1 w/w) covers a
broad wavelength range from 350 to 700 nm with the maximum
EQE value of 59.5% at ca. 550 nm and a platform from 450 to
570 nm. In contrast, the EQE curve of the PSC based on
PBDTBTBTz:PC70BM (1:1 w/w) exhibits a higher EQE value
of 71.1% at 500 nm. The higher EQE values of the PSC based on
PBDTBTBTz agree with the higher Jsc of the PSC device
mentioned above.
In order to understand the effect of mophology of the photo-
active layers on the photovoltaic performance of the polymer
solar cells, the mophological structures of the blend films of
polymer: PC70BM was analyzed by tapping mode atom force
microscopy (AFM) measurements (as shown in Figure S1 in
Supporting Information). It is clear that the film of 1:1 blends of
polymer and PC70BM have better interpenetrating network than
that of 1:2 blend of polymer and PC70BM which is beneficial to
the exciton dissociation and charge carriers transport. This is
consistent with the higher performance of polymer solar cells
based on the blend of polymer and PC70BM with weight ratio
of 1:1.
Figure 1. TGA plots of the copolymers with a heating rate of 10 °C/
min under an inert atmosphere.
(V/L)0.5 (as shown in Figure 5). According to eq 1 and Figure 5,
the hole mobilities obtained are 1.85 ꢁ 10ꢀ3 and 1.77 ꢁ
10ꢀ3 cm2/(V s) for PDTSBTBTz and PBDTBTBTz, respec-
tively. The hole mobilities of the corresponding thiophene-
bridged copolymers PDTSBTz and PBDTBTz are 3.07 ꢁ
10ꢀ4 and 6.84 ꢁ 10ꢀ4 cm2/(V s), respectively.24 Obviously,
the hole mobility of the bithiophene-bridged copolymers PD-
TSBTBTz and PBDTBTBTz is improved over that of their
corresponding thiophene-bridged copolymers, and the higher
hole mobility of the bithiophene-bridged DꢀA copolymers
should be beneficial for the application as photovoltaic donor
materials in PSCs.
Photovoltaic Properties. To investigate and compare the
photovoltaic properties of the two copolymers, bulk heterojunc-
tion PSC devices with a configuration of ITO/PEDOT:PSS/
copolymer:PC70BM (1:1 w/w)/Ca/Al were fabricated. Figure 6
shows the JꢀV curves of the PSCs under the illumination of
AM1.5, 100 mW/cm2. The corresponding open-circuit voltage
(Voc), short-circuit current (Jsc), fill factor (FF), and power
conversion efficiency (PCE) of the devices are summarized in
Table 3. The photovoltaic properties of the corresponding
thiophene-bridged DꢀA copolymers PDTSBTz and PBDTBTz
are also listed in Table 3 for comparison. The PSCs based on the
blend of PDTSBTBTz or PBDTBTBTz and PC70BM (1:1 w/w)
exhibited higher Voc of 0.72 and 0.82 V, respectively. When
increasing PC70BM content to the weight ratio of the polymer to
PC70BM of 1:2, the Jsc and FF values of devices slightly decreased,
which could be ascribed to the reduction of the absorption of the
active layer and an unbalanced charge transporting properties
between holes and electrons due to the high PC70BM content.31
The best device based on PDTSBTBTz showed a Voc of 0.72 V, a Jsc
of 8.68 mA/cm2, and a FF of 0.611, leading to a PCE of 3.82%. The
maximum PCE of the PSC based on PBDTBTBTz reached 4.46%
with a Voc of 0.82 V, a Jsc of 9.01 mA/cm2, and a FF of 0.603. In
’ CONCLUSION
We have sythesized two new DꢀA copolymers PDTSBTBTz
and PBDTBTBTz by the Pd-catalyzed Stille-coupling reaction.
The copolymers contain dithienosilole (DTS) or benzodithioh-
pene (BDT) donor unit, bithiazole (BTz) acceptor unit, and
bithiophene (BT) bridges between them. PDTSBTBTz and
PBDTBTBTz exhibit stronger interchain interaction, lower
HOMO energy level at ꢀ5.07 and ꢀ5.12 eV, and higher hole
mobility of 1.85 ꢁ 10ꢀ3 and 1.77 ꢁ 10ꢀ3 cm2/(V s), respec-
tively. The PCE of the PSC based on PDTSBTBTz:PC70BM
(1:1, w/w) was 3.82% with Jsc = 8.68 mA/cm2, Voc = 0.72 V, and
FF = 0.611, and the PCE of the PSC based on PBDTBTBTz:
PC70BM (1:1, w/w) reached 4.46% with Jsc = 9.01 mA/cm2,
Voc = 0.82 V, and FF = 0.603, under the illumination of AM1.5, 100
mW/cm2. These results indicate that the bithiophene-bridged DꢀA
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Table 2. Optical and Electrochemical Properties of the Polymers
UVꢀvis absorption spectra
cyclic voltammetry
n-doping
solutiona
filmb
p-doping
polymers
λmax (nm)
λ max (nm)
λonset (nm)
Egopt (eV)c
jox/HOMO (V/eV)
j
red/LUMO (V/eV)
EgEC (eV)
PDTSBTBTz
PBDTBTBTz
490
485
560
530
655
630
1.89
1.96
0.36/ꢀ5.07
0.41/ꢀ5.12
ꢀ1.79/ꢀ2.92
ꢀ1.77/ꢀ2.94
2.15
2.18
a Measured in chloroform solution. b Cast from chloroform solution. c Bandgap estimated from the onset wavelength (λedge) of the optical absorption:
Egopt = 1240/λedge
.
Figure 5. ln(JL3/V2) vs (V/L)0.5 plots of the polymers for the measure-
Figure 3. X-ray diffraction patterns of copolymer films.
ment of hole mobility by the SCLC method.
Figure 4. Cyclic voltammograms of the copolymer films on a platinum
electrode measured in 0.1 mol/L Bu4NPF6 acetonitrile solutions at a
scan rate of 100 mV/s.
Figure 6. IꢀV curves of the PSCs based on PDTSBTBTz:PC70BM or
PBDTBTBTz:PC70BM with different weight ratios under the illumina-
tion of AM 1.5, 100 mW/cm2.
copolymers are promising photovoltaic donor materials for the
application in PSCs.
Measurements and Characterization. All new compounds
were characterized by H NMR spectroscopy performed on a Bruker
1
DMX-400 spectrometer. For the 1H NMR measurements, CDCl3 was
used as the solvent. Chemical shifts in the NMR spectra were reported in
ppm relative to the singlet at 7.26 ppm for CDCl3. The molecular weight
of the polymers was measured by gel permeation chromatography
(GPC), and polystyrene was used as a standard. Thermogravimetric
analysis (TGA) was performed on a Perkin-Elmer TGA-7. Mass
spectra were obtained with a Shimadzu QP2010 spectrometer. UVꢀvis
’ EXPERIMENTAL SECTION
Materials. All chemicals and solvents were reagent grades and
purchased from Aldrich, Alfa Aesar, and TCI Chemical Co. Toluene,
tetrahydrofuran, and diethyl ether were distilled to keep anhydrous
before use. The other solvents were degassed by nitrogen for 1 h prior to
use, unless otherwise stated.
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Table 3. Photovoltaic Properties of the PSCs Based on
Copolymers:PC70BM with Different Weight Ratios, under the
Illumination of AM1.5, 100 mW/cm2
coupled with a WDG3 monochromator and a 500 W xenon lamp. The
light intensity at each wavelength was calibrated with a standard single-
crystal Si photovoltaic cell. All the measurements were performed under
ambient atmosphere at room temperature.
Synthesis. The synthetic routes of the monomer and polymers are
shown in Scheme 2. Compounds 1,32ꢀ34 4,13a and 522,23 were synthe-
sized according to the procedure in the literature. The detailed synthetic
processes are as follows.
active layer
Voc (V) Jsc (mA/cm2) FF PCE (%)
PDTSBTBTz:PC70BM = 1:1
PDTSBTBTz:PC70BM = 1:2
0.72
0.67
8.68
8.28
9.01
8.61
7.85
7.84
0.611
0.555
0.603
0.576
0.535
0.57
3.82
3.08
4.46
3.97
2.86
3.82
PBDTBTBTz:PC70BM = 1:1 0.82
5,50-Bis(30-hexyl-2,20-bithiophen-5-yl)-4,40-dinonyl-2,20-bithiazole
(2). To a suspension of magnesium (0.96 g, 40.0 mmol) in anhydrous
diethyl ether (40 mL), a solution of 2-bromo-3-hexylthiophene (7.9 g,
32.0 mmol) in anhydrous diethyl ether (150 mL) was added dropwise.
The Grignard reagent prepared was then added dropwise to an ice-
cooled suspension of compound 1 (5.92 g, 8.00 mmol) and Pd-
(dppf)Cl2(200 mg) in anhydrous diethyl ether (100 mL). The reaction
mixture was stirred for 6 h at room temperature. It was then hydrolyzed
with a saturated aqueous ammonium chloride solution and followed by
the addition of diethyl ether. The red layer was washed three times with
water (200 mL) and dried over sodium sulfate. After the removal of
solvent, recrystallization from hexane gave the red solid with a yield of
82% (6.0 g). 1H NMR (400 MHz, CDCl3), δ (ppm): 7.21 (d, 2H), 7.14
(d, 2H), 7.09 (d, 2H), 6.95 (t, 2H), 2.99 (t, 4H), 2.72 (t, 4H), 2.01ꢀ1.26
(m, 42H), 0.88 (m, 12H).
PBDTBTBTz:PC70BM = 1:2 0.80
PDTSBTz:PC70BM = 1:1a
PBDTBTz:PC70BM = 1:1b
a The data from ref 24a. b The data from ref 24b.
0.68
0.86
5,50-Bis(50-bromo-30-hexyl-2,20-bithiophen-5-yl)-4,40-dinonyl-2,20-
bithiazole (3). Compund 2 (3.66 g, 4.00 mmol) was dissolved in a
mixture of chloroform (50 mL) and acetic acid (10 mL). NBS (1.50 g,
8.42 mmol) was then added to the solution which was then stirred for 1 h
in the dark. The precipitate in the reaction mixture was filtered, washed
with methanol, and dried to produce a red solid with a yield of 90% (3.87
g). 1H NMR (400 MHz, CDCl3), δ (ppm): 7.14 (d, 2H), 7.09 (d, 2H),
6.95 (t, 2H), 2.99 (t, 4H), 2.72 (t, 4H), 2.01ꢀ1.26 (m, 42H), 0.88 (m,
12H). 13C NMR (100 MHz, CDCl3), δ (ppm): 157.80, 154.88, 140.89,
136.13, 133.31, 132.93, 131.52, 127.71, 127.49, 126.86, 111.43, 77.46,
77.36, 77,15, 76.83, 32.05, 31.77, 30.69, 29.71, 29.67, 29.60, 29.47, 29,43,
29.28, 22.83, 22.74, 14.25, 14.21. Anal. Calcd (%) for C52H70Br2N2S6:
C, 58.08; H, 6.56; N, 2.61; Br, 14.86; S, 17.89. Found: C, 58.11; H, 6.58;
N, 2.67. Br, 14.80; S, 17.82. MALDL-TOF: m/z = 1075.
Synthesis of Polymer PDTSBTBTz. Compound 3 (537.5 mg, 0.5
mmol), compound 4 (372 mg, 0.5 mmol), and dry toluene (12 mL)
were added to a 50 mL two-neck round-bottom flask. The reaction
container was purged with argon for 20 min to remove O2, and then Pd
(PPh3)4 (15 mg) was added. After another flushing with argon for 20
min, the reactant was heated to reflux for 10 h. The reactant was cooled
down to room temperature, poured into MeOH (200 mL), and then
filtered through a Soxhlet thimble, which was then subjected to Soxhlet
extraction with methanol, hexane, and chloroform. Polymer was recov-
ered from the chloroform fraction by rotary evaporation as solid. The
polymer was purified by chromatography on silica gel with chloroform as
the eluent, and the polymer solution was concentrated and was poured
into MeOH. After this, the precipitates were collected and dried under
vacuum overnight. Yield: 399 mg (60%). GPC: Mw = 14.7K; Mn = 9.1K;
Mw/Mn = 1.61. 1H NMR (400 MHz, CDCl3), δ (ppm): 7.14 (m, 8H),
3.62 (d, 2H), 1.89 (m, 1H), 1.29 (m, 36H), 0.87 (m, 12H). Anal. Calcd
(%) for C76H106N2S8Si: C, 68.52; H, 8.02; N, 2.10. Found: C, 68.41; H,
7.88; N, 2.67.
Figure 7. EQE curves of the PSCs based on PDTSBTBTz:PC70BM or
PBDTBTBTz:PC70BM (1:1 w/w).
absorption spectra were obtained on a Hitachi U-3010 spectrometer.
Cyclic voltammetry was performed on a Zahner IM6e electrochemical
workstation with a three-electrode system in a solution of 0.1 M
Bu4NPF6 in acetonitrile at a scan rate of 50 mV/s. The polymer films
were coated on a Pt plate electrode by dipping the electrode into the
corresponding solutions and then drying. A Pt wire was used as the
counter electrode, and Ag/Ag+ was used as the reference electrode.
Device Fabrication and Characterization of Polymer Solar
Cells. Polymer solar cells (PSCs) were fabricated with PEDOT:PSS
modified ITO glass as a positive electrode, Ca/Al as negative electrode,
and the blend film of the polymer/PC70BM between them as a
photosensitive layer. A thin layer of PEDOT:PSS was spin-cast on
precleaned ITO-coated glass from a PEDOT:PSS aqueous solution at
2000 rpm and dried subsequently at 150 °C for 20 min to give a film with
a thickness of 40 nm. Then 1:1 or 1:2 w/w blends of polymer and
PC70BM at 10 mg/mL (according to the polymer weight) were
dissolved in o-dichlorobenezene with heating at 70 °C for at least 3 h
until all the solids dissolved completely and then stirring for more than
5 h at room temperature. The photosensitive layer was spin-coated onto
the PEDOT:PSS layer at 1000 rpm. Then a 10 nm of calcium and a
80 nm of aluminum were deposited on the polymer layer as the cathode
at a pressure of 3 ꢁ 10ꢀ5 Pa. The thickness of the photosensitive layer is
ca. 100 nm, measured on an Ambios Tech. XP-2 profilometer. The
effective area of one cell is 4 mm2. The currentꢀvoltage (IꢀV)
measurement of the devices was conducted on a computer-controlled
Keithley 236 Source Measure Unit. A xenon lamp with AM1.5 filter was
used as the white light source, and the optical power at the sample was
100 mW/cm2. The external quantum efficiency (EQE) was measured
using a Stanford Research Systems model SR830 DSP lock-in amplifier
Synthesis of Polymer PBDTBTBTz. PBDTBTBTz are synthesized
according to the same procedure as that for the synthesis of
PDTSBTBTz with monomer 5 was used instead of monomer 4. Yield:
475 mg (70%). GPC: Mw = 12.6K; Mn = 8.6K; Mw/Mn = 1.46. 1H NMR
(400 MHz, CDCl3), δ (ppm): 7.79 (m, 4H), 7.22 (m, 4H), 3.62 (d, 2H),
1.89 (m, 1H), 1.29 (m, 36H), 0.87 (m, 12H). Anal. Calcd (%) for
C78H104N2O2S8: C, 68.52; H, 8.02; N, 2.10. Found: C, 68.41; H, 7.88;
N, 2.67.
8803
dx.doi.org/10.1021/ma201976t |Macromolecules 2011, 44, 8798–8804

Macromolecules
ARTICLE
’ ASSOCIATED CONTENT
41, 8302–8305. (b) Zhou, E. J.; Yamakawa, S. P.; Tajima, K.; Yang, C. H.;
Hashimoto, K. Chem. Mater. 2009, 21, 4055–4061.
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S
Supporting Information. Figure S1. This material is
b
available free of charge via the Internet at http://pubs.acs.org.
(16) (a) Zou, Y. P.; Gendron, D.; Badrou-Aich, R.; Najari, A.; Tao,
Y.; Leclerc, M. Macromolecules 2009, 42, 2891–2894. (b) Zou, Y. P.;
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: liyf@iccas.ac.cn.
(17) (a) Zhang, M. J.; Guo, X.; Wang, X. C.; Wang, H. Q.; Li, Y. F.
Chem. Mater. 2011, 23, 4264–4270. (b) Cui, C. H.; Fan, X.; Zhang, M. J.;
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Zhang, M. J.; Sun, Y. P.; Guo, X.; Cui, C. H.; He, Y. J.; Li, Y. F.
Macromolecules 2011, DOI: 10.1021/ma201565f. (d) Min, J.; Zhang,
Z.-G.; Zhang, S. Y.; Zhang, M. J.; Zhang, J.; Li, Y. F. Macromolecules
2011, DOI: org/10.1021/ma201673m.
’ ACKNOWLEDGMENT
This work was supported by NSFC (Nos. 20874106,
20821120293, 21021091, and 50933003), the Ministry of Science
and Technology of China, and the Chinese Academy of Sciences.
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