Application of two-dimensional conjugated benzo[1,2-b:4,5-b']dithiophene in quinoxaline-based photovoltaic polymers

Article abstract of DOI:10.1021/ma300060z

Two new donor-acceptor (D-A) alternative copolymers, PBDTDTQx-T and PBDTDTQx-O, were designed and synthesized to investigate the influence of two-dimensional conjugated structure on photovoltaic properties of conjugated polymers. In these two polymers, PB

Full text of DOI:10.1021/ma300060z

Article
pubs.acs.org/Macromolecules
Application of Two-Dimensional Conjugated Benzo[1,2-b:4,5-
b′]dithiophene in Quinoxaline-Based Photovoltaic Polymers
Ruomeng Duan,†,‡ Long Ye,‡,§ Xia Guo,‡,§ Ye Huang,‡,§ Peng Wang,*,† Shaoqing Zhang,‡
Jianping Zhang,† Lijun Huo,‡ and Jianhui Hou*,‡
†Department of Chemistry, Renmin University of China, Beijing, 100872, China
‡State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190
China
§Graduate University of Chinese Academy of Sciences, Beijing 100049, China
S
* Supporting Information
ABSTRACT: Two new donor−acceptor (D−A) alternative
copolymers, PBDTDTQx-T and PBDTDTQx-O, were de-
signed and synthesized to investigate the influence of two-
dimensional conjugated structure on photovoltaic properties of
conjugated polymers. In these two polymers, PBDTDTQx-O
was used as control material, which is an alternative copolymer
based on a quinoxaline derivative (DTQx) and alkoxy-
substituted benzo[1,2-b:4,5-b′]dithiophene (BDT-O) unit;
PBDTDTQx-T has an identical conjugated backbone as
PBDTDTQx-O, but a simple two-dimensional conjugated
BDT unit (BDT-T) was used to replace the alkoxy-BDT. The
polymers were characterized by TGA, UV−vis absorption,
electrochemical cyclic voltammetry, hole mobility of space-
charge-limited current (SCLC) model, and photovoltaic measurements. It was found that PBDTDTQx-T exhibits similar
molecular energy levels and higher hole mobility than PBDTDTQx-O. The power conversion efficiency (PCE) of the polymer
solar cells (PSCs) based on PBDTDTQx-T: [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) = 1/2 (w/w) reached ∼5%,
which is 60% higher than that of PBDTDTQx-O-based PSC. On the basis of these results, it can be concluded that the
application of two-dimensional conjugated structure would be a feasible approach to improve photovoltaic properties of
conjugated polymers.
way to reduce band gap of conjugated polymer is to introduce
conjugating components with strong quinoid property.23 For
instance, conjugated polymers based on thieno[3,4-b]pyrazine
(TPZ)24 and thieno[3,4-b]thiophene (TT)25−27 were designed
and these polymers exhibit narrow band gaps and promising
photovoltaic properties. Besides band gap control, the
modulation of molecular energy levels (HOMO and LUMO)
of conjugated polymers is also of great importance for
photovoltaic material design. Electron donating or withdrawing
functional groups have been used in molecular design for this
purpose. For example, the alkoxy-substituted conjugated
polymers show much higher HOMO levels than their
analogues with alkyl-substituents due to the stronger electron
donating ability of the alkoxy groups than the alkyls;28 the
carbonyl-substituted polymer exhibits lower LUMO level than
the analogue with carboxylate group, because the carbonyl
group has stronger electron withdrawing ability than the
INTRODUCTION
■
Bulk-heterojunction (BHJ) polymer solar cells (PSCs) have
attracted much attention due to its potential applications in
making large area, flexible solar panels through roll-to-roll
process.1−6 In a PSC device with BHJ structure, the active layer
is consisting of two kinds of key materials: a p-type conjugated
polymer donor and a soluble fullerene derivative acceptor. In
order to get efficient PSC devices, active layer materials with
ideal properties are requisite, and therefore, tremendous efforts
have been devoted to tuning the properties of active layer
materials through molecular structure design.7−11
As electron donor materials in PSC devices, narrow band gap
conjugated polymers are much helpful to obtain good harvest
of the sunlight, and different strategies of molecular design have
been developed. Building a donor−acceptor (D−A) alternating
structure has been proved to be an effective way to reduce band
gap of conjugated polymer materials.12−19 For example,
PCPDTBT20,21and PSBTBT22 are two representatives with
D−A structure, and these two polymers exhibit narrow band
gaps (Eg < 1.6 eV) and promising photovoltaic properties
(power conversion efficiency, PCE > 5%). Another effective
Received: January 10, 2012
Revised: March 6, 2012
Published: March 20, 2012
© 2012 American Chemical Society
3032
dx.doi.org/10.1021/ma300060z | Macromolecules 2012, 45, 3032−3038

Macromolecules
Article
Scheme 1. Molecular Structures of PBDTDTQx-O and PBDTDTQx-T
Scheme 2. Synthetic Route of PBDTDTQx-O and PBDTDTQx-T
carboxylate;25 the fluorine-substituted polymers also give
excellent examples upon this strategy.27,29
photovoltaic properties of the D−A polymers without quinoid
characteristic. Quinoxaline (Qx) is a typical electron-deficient
conjugated unit due to the strong electronegativity of the two
nitrogen atoms. Many conjugated polymers with D−A
backbone have been designed by using Qx or its derivatives
as electron-withdrawing building blocks in their backbones.
These Qx-based polymers generally exhibit interesting photo-
voltaic behaviors, and ∼6% PCE has been reported in the
polymers based on thiophene and quinoxaline.35−37 Further-
more, since Qx consists of two fused six-member ring, when it
was copolymerized with other conjugated components, the
target polymer should have much less quinoid property than
PBDTTT-polymers. Considering the above reasons, in order to
investigate the influence of 2-D-conjugated BDT on photo-
voltaic properties of D−A conjugated polymers without
quinoid property, two polymers based on Qx were designed.
In Scheme 1, molecular structures of two D−A polymers based
on BDT and the derivative of Qx, PBDTDTQx-O and
PBDTDTQx-T, are shown, and these two polymers have
identical conjugated backbones (main chain) but different side
groups: alkoxy groups and alkylthienyl groups are used on 4
and 8 positions of their BDT units, respectively. Since these
two polymers have similar molecular structures, the comparison
between them will provide useful information on the influence
of 2-D-conjugated structure on photovoltaic properties of D−A
polymers.
Two-dimensional (2-D) conjugated polymers exhibit very
interesting properties and promising photovoltaic performance.
Representatively, intensive studies about 2-D-conjugated
polythiophenes have been done.30−33 2-D-conjugated poly-
thiophenes exhibit two main absorption peaks: one originates
from the conjugated main chain, and the other is from the
conjugated side chain. Consequently, broad and strong
absorption band can be achieved in 2-D-conjugated poly-
thiophenes.30−32 On the other hand, since the 2-D-conjugated
polythiophenes have bigger conjugated plane than their one-
dimensional conjugated counterparts, better interchain π−π
overlapping can be formed and hence higher charge mobility
can be achieved.33 Recently, we designed and synthesized a new
benzo[1,2-b:4,5-b′]dithiophene (BDT) derivative with 2-D-
conjugated structure by introducing alkylthienyl groups to 4
and 8 positions of BDT unit. Initially, a polymer backbone
(PBDTTT) was selected to investigate the photovoltaic
properties of the 2-D-conjugated BDT-polymer. We found
that after introducing 2-D-conjugated structure, broader
absorption band, lower HOMO level and higher hole mobility
can be achieved, and as a result, better photovoltaic results were
recorded. For instance, PCE of the PSC devices based on the 2-
D polymers, PBDTTT-E-T and PBDTTT-C-T, are 50% and
20% higher than their analogues with one-dimensional
conjugated backbone, respectively.34
RESULTS AND DISCUSSION
As known, the backbone of PBDTTT-based polymers has
very strong quinoid property, so this kind of polymers exhibit
much narrow band gaps.34 Although it is quite clear that the 2-
D-conjugated structure is very helpful to improving the
photovoltaic properties of PBDTTT-based polymers, it is still
unknown that how does the 2-D-conjugated structure influence
■
Synthesis. The general synthetic method of these two
polymers is shown in Scheme 2. (2,3-Bis(3-(2-ethylhexyloxy)-
phenyl)-5,8-di(thiophen-2-yl)quinoxaline (compound 2) was
synthesized by Stille condensation reaction between 5,8-
Dibromo-2,3-bis(3-(2-ethylhexyloxy)phenyl)-quinoxaline
3033
dx.doi.org/10.1021/ma300060z | Macromolecules 2012, 45, 3032−3038

Macromolecules
Article
(compound 1) and 2-(tributylstannyl)thiophene with a yield of
60%, and then it was bromided by N-bromosuccininide (NBS)
to produce compound 3. The two target polymers were
prepared through Stille polycondensation reaction with a yield
of 40−60%. These two polymers exhibit excellent solubility in
commonly used solvents, including tetrahydrofuran, chloro-
form, chlorobenzene, toluene, etc. Molecular weight of the
polymers and polydispersity (PDI) was estimated by gel
permeation chromatography (GPC) method using monodis-
persed polystyrene as standard and chloroform as eluent (Table
1). The number-average molecular weight (Mn) of
PBDTDTQx-O and PBDTDTQx-T are 84K and 49.5K,
respectively.
Table 1. Molecular Weight and Thermal Stability Data of the
Polymers
a
a
a
polymer
Mw
Mn
PDI
Td (°C)
Figure 2. Absorption spectra of PBDTDTQx-T and PBDTDTQx-O
in chloroform solution and of solid films.
PBDTDTQx-O
PBDTDTQx-T
186K
80.5K
84K
49.5K
2.22
1.63
320
430
a
(λedge) of PBDTDTQx-O and PBDTDTQx-T are located at
714 and 740 nm, corresponding to band gaps (Eg) of 1.74 and
1.67 eV, respectively; the half peak positions at long wavelength
direction of PBDTDTQx-O and PBDTDTQx-T are 658 and
680 nm, respectively. PBDTDTQx-T exhibits a red-shifted
absorption spectrum than PBDTDTQx-O indicating that the π-
electrons of PBDTDTQx-T can be delocalized more effectively
than that of PBDTDTQx-O, which is ascribed to the
introduction of the 2-D conjugated structure. Besides of
absorption range, absorption coefficient of solid film is also
another important parameter to an active layer material in
PSCs. As listed in Table 2, PBDTDTQx-O and PBDTDTQx-T
exhibit much similar absorption coefficients as solid film, which
are 0.60 × 10−2/nm and 0.61 × 10−2/nm, respectively. Since
the 2-D conjugated polymer exhibits broader absorption band
and similar absorption coefficient compared to the control
polymer, when PBDTDTQx-T is used as active layer material
in PSC, the device will harvest more photons than the
PBDTDTQx-O-based device.
Mw, Mn, and PDI of the polymers were estimated by GPC using
polystyrene as standards in chloroform.
Thermal Properties. Thermal stability of the two polymers
was investigated by thermogravimetric analysis (TGA). As
shown in Figure 1, under nitrogen atmosphere, PBDTDTQx-O
Electrochemical Properties. Electrochemical cyclic vol-
tammetry (CV) is performed to evaluate molecular energy
levels of conjugated polymers. Figure 3 shows the CV plots of
PBDTDTQx-O and PBDTDTQx-T films on glassy carbon
electrode. The onset points of p-doping process of
PBDTDTQx-T and PBDTDTQx-O are both at 0.42 V,
corresponding to a HOMO level of −5.12 eV. Since it is very
hard to get a sharp n-doping signal for these two polymers, CV
curves of p-doping process were only provided here, and the
LUMO levels of the polymers were calculated by “LUMO =
HOMO + Egopt”. As known, VOC of bulk heterojunction PSCs is
directly proportional to the offset between HOMO level of the
donor and LUMO level of the acceptor, and it can be
concluded that the 2-D conjugated structure has no negative
effect on VOC of PSCs.
Photovoltaic Properties. The PSC devices with a
structure of ITO/PEDOT−PSS (Clevios 4083)/poly-
mer:PC71BM/Ca/Al were fabricated to investigate the photo-
voltaic properties of these two polymers. o-Dichlorobenzene (o-
DCB) was used to prepare the solution for spin-coating of
active layer, and the concentration of the solution used in this
work was 10 mg/mL (polymer/o-DCB). At first, different
donor/acceptor (D/A) ratios, 1:1, 1:2, and 1:3, were scanned.
We found that the optimal D/A ratio for both PBDTDTQx-O
Figure 1. TGA plots of PBDTDTQx-T and PBDTDTQx-O with a
heating rate of 10 °C/min under inert atmosphere.
shows an onset decomposition point at about 320 °C; however,
PBDTDTQx-T shows an onset decomposition point at about
430 °C. The big difference of their decomposition temperature
indicates that the 2-D-structure is much helpful to improving
thermal stability of the BDT-based polymers. The difference
scanning calorimetry (DSC) measurements for the two
polymers have not shown any endothermic or exothermic
phases in the temperature range of 25−300 °C. The DSC plots
are shown in the Supporting Information (Figure S2).
Absorption Spectra. The absorption spectra of
PBDTDTQx-O and PBDTDTQx-T in chloroform solution
and of solid films are shown in Figure 2. As listed in Table 2,
the absorption peaks of PBDTDTQx-O in solution and of solid
film are at ∼582 and ∼602 nm, respectively; the absorption
peaks of PBDTDTQx-T in solution and of solid film are both
at ∼605 nm. The absorption range of PBDTDTQx-T is red-
shifted compared to PBDTDTQx-O. The absorption edges
3034
dx.doi.org/10.1021/ma300060z | Macromolecules 2012, 45, 3032−3038

Macromolecules
Article
Table 2. Optical and Electrochemical Properties of PBDTDTQx-T and PBDTDTQx-O
λmax [nm]
λedge [nm]
absorption coefficient
film
a
polymer
solution
film
film
Egopt [eV]
HOMO [eV]
LUMO [eV]
PBDTQx-T
PBDTQx-O
LUMO = HOMO + Egopt
443, 605
421, 582
448, 605
428, 602
740
714
0.61 × 10−2/nm
0.60 × 10−2/nm
1.67
1.74
−5.12
−5.12
−3.45
−3.38
a
.
Figure 3. Cyclic voltammograms of PBDTDTQx-T and PBDTDTQx-
O film on the glassy carbon electrode in 0.1 mol/L Bu4NPF6 in
acetonitrile solution at a scan rate of 50 mV/s.
Figure 5. I−V curves of the PSCs based on PBDTDTQx-T and
PBDTDTQx-O under the illumination of AM 1.5G, 100 mW/cm2.
diiodooctane (DIO) as additive during the spin-coating
process.40 In this work, both of these two methods were
employed to optimize the device performance. We found that
no further improvement can be observed by using DIO as
additive. However, after annealing at 120 °C, JSC and fill factor
(FF) of the devices can be improved simultaneously without
affecting VOC. In the optimized device, the thickness of the
PEDOT−PSS layer is about 35 nm, the thicknesses of active
layer are 80 and 126 nm for PBDTDTQX-T and PBDTDTQX-
O-based devices, respectively. The thicknesses of Ca and Al
layers are 20 and 80 nm, respectively. For PBDTDTQx-T-
based device, a PCE of 4.95% was recorded with a VOC of 0.76
V, a JSC of 10.13 mA/cm2 and a FF of 64.3%. Since
PBDTDTQx-O has much similar structure as PBDTDTQx-T,
the optimal device fabrication conditions of these two polymers
are almost the same. However, both the VOC and the JSC of
PBDTDTQx-O-based devices are lowered than PBDTDTQx-
T-based devices, and therefore, the best PCE of the device of
PBDTDTQx-O is only 3.06%. The nanoscale morphology of
polymer/PC71BM film was tested by using atomic force
microscopy (AFM). The phase graphs are shown in Figure 4.
The surface roughness measured from the topograph image was
0.61 and 0.67 for PBDTDTQX-T and PBDTDTQX-O
respectively. Although these two kinds of polymer:PCBM
films show similar surface roughness, in comparison with the
blend of PBDTDTQx-O and PCBM, the blend of
PBDTDTQx-T exhibits much smaller domain size.
Figure 4. AFM images: (a, b) PBDTDTQX-O topography image and
phase constrast image, respectively; (c, d) PBDTDTQX-T topography
image and phase constrast image, respectively. All images are 2 μm × 2
μm.
and PBDTDTQx-T-based devices is 1/2, and Figure 5 shows
the current−voltage curves (I−V curves) of the PSCs under
illumination (AM1.5G 100 mW/cm2). The detailed photo-
voltaic data of the optimization process of PBDTDTQx-T-
based devices are listed in the Supporting Information as Table
S1. It can be seen that the VOC of PBDTDTQx-T-based devices
dropped from 0.79 to 0.73 V with increasing PC71BM content
in the blends, and the device with a D/A ratio of 1/2 showed
the highest short circuit current density (JSC) in these three
conditions. As reported, device fabrication process of PSC can
be further improved by thermal annealing38,39 or by using 1,8-
External Quantum Efficiency. As shown in Figure 6, the
external quantum efficiency (EQE) curves of the devices
indicate that the PSC devices have good response to the
sunlight at the range from 350 to 700 nm, with a peak position
at around 500 nm (Table 3). The EQE peak values of the PSC
devices of PBDTDTQx-T and PBDTDTQx-O are 58% and
44%, respectively. PBDTDTQx-T-based device shows better
EQE to the sunlight in the whole range compared to the
PBDTDTQx-O-based device. This result indicates that the
replacement of alkoxy group by alkylthienyl group is much
3035
dx.doi.org/10.1021/ma300060z | Macromolecules 2012, 45, 3032−3038

Macromolecules
Article
O indicates that the replacement of alkoxy group by alkylthienyl
group is an effective way to improve hole transport property of
the active layer, and hence, better quantum efficiency can be
obtained in the device based on the corresponding polymer
with alkylthienyl side chain.
CONCLUSION
■
Two new polymers based on Qx and BDT unit, PBDTDTQx-
O and PBDTDTQx-T, were designed, synthesized and
characterized. These two polymers exhibit broad absorption
bands and appropriate molecular energy levels as electron
donor materials in PSCs. Therefore, for the PSC device based
on PBDTDTQx-T, a PCE of 5.0% were recorded. More
important, two kinds of side groups, alkoxy group and
alkylthienyl group, were used as substituents on the BDT
units of these two polymers, and the comparison between them
provides useful information for molecular design of photo-
voltaic polymers. The results in this work give solid evidence
that the 2-D conjugated structure is beneficial to improve
photovoltaic properties of the narrow band gap D/A-polymer
system without quinoid properties. Specifically, after introduc-
ing the 2-D conjugated structure by replacing alkoxy groups
with alkylthienyl groups, higher hole mobility and higher EQE
of the PSC device was achieved, and therefore better JSC can be
realized; interestingly, although these two polymers exhibit
similar HOMO levels, the PSC device based on 2-D conjugated
polymer shows higher VOC than the alkoxy-substituted control
polymer. As a result, PCE of the device based on PBDTDTQx-
T is ∼60% higher than that of the device based on
PBDTDTQx-O. Considering all the issues mentioned above,
it can be concluded that the backbone of PBDTDTQx-T
should be a promising molecular structure for photovoltaic
polymer and the application of 2-D conjugated structure can be
seen as a feasible method in molecular design of photovoltaic
polymers.
Figure 6. EQE curves of the PSCs based on PBDTDTQx-T or
PBDTDTQx-O: PC71BM (1:2 w/w).
Table 3. Photovoltaic Properties of PSCs Based on
PBDTDTQx-T and PBDTDTQx-O, under the Illumination
of AM1.5G, 100 mW/cm2
anneal
(120 °C, D/A VOC
JSC
FF
PCE
(%)
polymer
10 min)
ratio
(V) (mA/cm2) (%)
PBDTDTQx-T
PBDTDTQx-T
PBDDTTQx-T
PBDTDTQx-T
PBDTDTQx-O
no
no
no
yes
yes
1:1
1:2
1:3
1:2
1:2
0.79
0.76
0.73
0.76
0.71
7.91
8.97
53.3
50.5
48.4
64.3
61.5
3.33
3.44
2.53
5.0
7.17
10.13
7.00
3.06
helpful to improve photovoltaic properties of the polymer.
According to the EQE curves and the solar irradiation
spectrum, the integral current density value of the PSC devices
of PBDTDTQx-T and PBDTDTQx-O are calculated, which
are 9.82 mA/cm2 and 7.0 mA/cm2, respectively. For both
PBDTDTQx-T and PBDTDTQx-O-based devices, the differ-
ence between the measured Jsc and the integral current density
values is within 4%, indicating the accuracy of our photovoltaic
measurement is reliable.
Hole Mobility. Furthermore, hole mobilities of the D/A
blend films based on these two polymers prepared by the
optimal fabrication process were measured by fabricating hole-
only device, the device structure as: ITO/PEDOT−PSS/
polymer:PC71BM/Au, and the results are shown in Figure 7.
The hole mobilities of PBDTDTQx-T and PBDTDTQx-O are
1.04 × 10−4 cm2 V−1 s−1 and 4 × 10−5 cm2 V−1 s−1, respectively.
The higher hole mobility of PBDTDTQx-T than PBDTDTQx-
EXPERIMENTAL SECTION
■
Materials. 2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)thiophen-
2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDT-T), and 2,6-bis-
(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene
(BDT-O); and the 5,8-dibromo-2,3-bis(3-(2-ethylhexyloxy)phenyl)-
quinoxaline, were purchased from Solarmer Materials Inc. and used
without any further purification. The Pd(PPh3)4 was purchased from
Frontier Scientific Inc., and the ultra dry solvents used in device
fabrication process are purchased from Aldrich. The other chemicals
are commercial available products and used without any further
purification.
Measurements. 1H NMR spectra were measured on a Bruker arx-
400 spectrometer. Absorption spectra were taken on a Hitachi U-3010
UV−vis spectrophotometer. The molecular weights of polymers were
measured by the GPC method, and polystyrene was used as a standard
by using chloroform as eluent. TGA measurements were performed on
TA Instruments Inc. TGA-2050. Thermograms were obtained on
mettle differential scanning calorimeter (DSC). The electrochemical
cyclic voltammetry experiments were conducted on Zahner IM6e
Eletrochemical workstation, with glassy carbon disk, Pt wire, and a Ag/
Ag+ electrode as the working electrode, counter electrode, and
reference electrode, respectively, in a 0.1 mol/L tetrabutylammonium
hexafluorophosphate (Bu4NPF6) in acetonitrile solution.
Synthesis. 2,3-Bis(3-(2-ethylhexyloxy)phenyl)-5,8-di(thiophen-
2-yl)quinoxaline (2). 2-(Tributylstannyl)thiophene(2.24 g, 6
mmol), compound 1 (1.40 g, 2 mmol), and 14 mL of toluene
were put into a flask. The solution was purge with argon for 10
min, then 100 mg Pd(PPh3)4 was added. The solution was
purge again by argon for 30 min the resulting mixture was
Figure 7. ln(JL3/V2) versus (V/L)0.5 plots of the polymers for the
measurement of hole mobility by the SCLC method.
3036
dx.doi.org/10.1021/ma300060z | Macromolecules 2012, 45, 3032−3038

Macromolecules
Article
heated with stirring at 110 °C for 12 h under argon
atmosphere. Then the reactant was cooled to room temper-
ature and then poured into water and extracted by diethyl ether
for several times. After removal of the solvent, the crude
product was purified by silica gel chromatography with hexane/
dichloromethane (v/v, 5/1) mixture as eluent. The pure
compound (2) was obtained as red solid (yield 60%). 1H NMR
(δ/ppm, CDCl3, 400 MHz): 8.15 (s, 2H), 7.90 (d, 2H), 7.49
(d, 2H), 7.41 (s, 2H), 7.28 (t, 4H), 7.20 (t, 2H), 6.95 (d, 2H),
3.8 (d, 4H), 1.71 (m, 2H), 1.34 (m, 16H), 0.94 (t, 12H).
2,3-Bis(3-(2-ethylhexyloxy)phenyl)-5,8-bis(5-bromothiophen-2-
yl)quinoxaline (3). To a solution of compound 2 (1.16 g, 1.65 mmol)
in 10 mL N,N-dimethylformamide (DMF), a solution of N-
bromosuccinimide (NBS) (0.60 g, 3.37 mmol) in 10 mL of DMF
was added at 0 °C. The reactant was stirred for 30 min and then
poured into water and extracted with diethyl ether (50 mL × 3). The
organic phase was dried over anhydrous MgSO4. After removal the
solvent under reduced pressure, the residues were purified by silica gel
column chromatography using hexanes as eluent. The pure product
compound 3 was obtained as deep red solid in a yield of 94% (1.33 g).
1H NMR (δ/ppm, CDCl3, 400 MHz): 8.25 (s, 2H) 7.73 (d, 2H), 7.46
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to acknowledge the financial support
from National High Technology Research and Development
Program 863, Chinese Academy of Sciences, NSFC (Nos.
2011AA050523, 20874106, 20821120293, 51173189, and
20933010).
REFERENCES
■
(1) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science
1995, 270, 1789−1791.
(2) Hoth, C. N.; Choulis, S. A.; Schilinsky, P.; Brabec, C. J. Adv.
Mater. 2007, 19, 3973−3974.
(3) Aernouts, T.; Aleksandrov, T.; Girotto, C.; Genoe, J.; Poortmans,
J. Appl. Phys. Lett. 2008, 92 (3), 033306.
(4) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv. Funct. Mater.
2001, 11, 15−26.
(s, 2H), 7.31 (t, 2H), 7.20 (t, 4H), 7.05 (d, 2H), 3.92 (d, 4H), 1.75
(m, 2H), 1.36 (m, 16H), 0.96 (t, 12H).
(5) Thompson, B. C.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2008,
47, 58−77.
Synthesis of the Polymers Using Stille Coupling Reaction. Both
PBDTDTQx-T and PBDTDTQx-O were prepared by coupling
compound 3 with the corresponding bis(trimethylstannyl)-substituted
BDT monomer by the same procedure. BDT-T (0.452 g, 0.5 mmol)
or BDT-O (0.386 g, 0.5 mmol) and compound 3 (0.430 g, 0.5 mmol)
were put into a 50 mL two-neck flask, and 10 mL of degassed toluene
was added under the protection of argon. The solution was purge with
argon for 10 min, and then 10 mg of Pd(PPh3)4 was added. After being
purged with argon for 20 min, the reaction mixture was heated with
stirring at 105 °C for 16 h. Then the reactant was cooled to room
temperature, and the polymer was precipitated by adding methanol
and then filtered through a Soxhlet thimble, which was then subjected
to Soxhlet extraction with methanol, hexane, and chloroform
successively. The polymer was recovered from the chloroform fraction
by rotary evaporation as solid. The solid was dried under vacuum for 1
day. The yields of the two polymers were both about 40%.
(6) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.;
Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photo 2009, 3, 297−
U5.
(7) Cheng, Y. J.; Yang, S. H.; Hsu, C. S. Chem. Rev. 2009, 109, 5868−
5923.
(8) Giacalone, F.; Martin, N. Chem. Rev. 2006, 106, 5136−5190.
(9) Roncali, J. Chem. Rev. 1992, 92, 711−738.
(10) Li, C.; Liu, M. Y.; Pschirer, N. G.; Baumgarten, M.; Mullen, K.
Chem. Rev. 2010, 110, 6817−6855.
(11) He, Y. J.; Li, Y. F. Phys. Chem. Chem. Phys. 2011, 13, 1970−
1983.
(12) Amb, C. M.; Chen, S.; Graham, K. R.; Subbiah, J.; Small, C. E.;
So, F.; Reynolds, J. R. J. Am. Chem. Soc. 2011, 133, 10062−10065.
(13) Price, S. C.; Stuart, A. C.; Yang, L. Q.; Zhou, H. X.; You, W. J.
Am. Chem. Soc. 2011, 133, 4625−4631.
Device Fabrication and Measurements. Polymer solar cell devices
were fabricated under conditions as follows: After spin-coating a 35
nm layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS) onto a precleaned indium−tin oxide (ITO) coated
glass substrates, the polymer/PC71BM blend solution was spin-coated.
The concentration of the polymer/PC71BM blend solution for spin-
coating was 10 mg/mL (polymer/o-dichlorobenzene). The thickness
of the active layer was controlled by changing the spin speed during
the spin-coating process and measured on profilometer (Ambios Tech.
XP-2). The devices were completed by evaporating Ca/Al metal
electrodes with an area of 4 mm2 as defined by masks. The I−V curves
are measured under the illumination of 100 mW· cm−2 AM 1.5G using
a XES-70S1 (SAN-EI Electric Co., Ltd.) solar simulator (AAA grade,
70 mm ×70 mm photobeam size). The 2 × 2 cm monocrystalline
silicon reference cell (SRC-1000-TC-QZ) was purchased from VLSI
Standards Inc.
(14) He, Z. C.; Zhong, C. M.; Huang, X.; Wong, W. Y.; Wu, H. B.;
Chen, L. W.; Su, S. J.; Cao, Y. Adv. Mater. 2011, 23, 4636−4637.
(15) Liang, Y. Y.; Xu, Z.; Xia, J. B.; Tsai, S. T.; Wu, Y.; Li, G.; Ray, C.;
Yu, L. P. Adv. Mater. 2010, 22, E135−136.
(16) Su, M. S.; Kuo, C. Y.; Yuan, M. C.; Jeng, U. S.; Su, C. J.; Wei, K.
H. Adv. Mater. 2011, 23, 3315−3316.
(17) Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.;
Neagu-Plesu, R.; Belletete, M.; Durocher, G.; Tao, Y.; Leclerc, M. J.
Am. Chem. Soc. 2008, 130, 732−742.
(18) Beaujuge, P. M.; Pisula, W.; Tsao, H. N.; Ellinger, S.; Mullen, K.;
Reynolds, J. R. J. Am. Chem. Soc. 2009, 131, 7514−7515.
(19) Huang, F.; Chen, K. S.; Yip, H. L.; Hau, S. K.; Acton, O.; Zhang,
Y.; Luo, J. D.; Jen, A. K. Y. J. Am. Chem. Soc. 2009, 131, 13886−13887.
(20) 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.
(21) Muhlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z. G.; Waller,
D.; Gaudiana, R.; Brabec, C. Adv. Mater. 2006, 18, 2884−2889.
(22) Yang, Y.; Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Li, G. J. Am.
Chem. Soc. 2008, 130, 16144−16145.
(23) Huo, L. J.; Hou, J. H. Polym. Chem. 2011, 2, 2453−2461.
(24) Zhou, E. J.; Cong, J. Z.; Yamakawa, S.; Wei, Q. S.; Nakamura,
M.; Tajima, K.; Yang, C. H.; Hashimoto, K. Macromolecules 2010, 43,
2873−2879.
(25) Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Chen, R. I.; Yang, Y.; Wu,
Y.; Li, G. J. Am. Chem. Soc. 2009, 131, 15586−15587.
(26) 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.
(27) Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Liang, Y. Y.; Yang, G. W.;
Yang, Y.; Yu, L. P.; Wu, Y.; Li, G. Nat. Photo 2009, 3, 649−653.
(28) Shi, C. J.; Yao, Y.; Yang, Y.; Pei, Q. B. J. Am. Chem. Soc. 2006,
128, 8980−8986.
ASSOCIATED CONTENT
■
S
* Supporting Information
Optimizing of PBDTDTQx-base devices, differential scanning
calorimetrythermograms, density functional theory calculations,
and XRD patterns. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: (J.H.) hjhzlz@iccas.ac.cn; (W.P.) wpeng@chem.ruc.
edu.cn.
3037
dx.doi.org/10.1021/ma300060z | Macromolecules 2012, 45, 3032−3038

Macromolecules
Article
(29) Zhou, H. X.; Yang, L. Q.; Stuart, A. C.; Price, S. C.; Liu, S. B.;
You, W. Angew. Chem., Int. Ed. 2011, 50, 2995−2998.
(30) Hou, J. H.; Tan, Z. A.; Yan, Y.; He, Y. J.; Yang, C. H.; Li, Y. F. J.
Am. Chem. Soc. 2006, 128, 4911−4916.
(31) Hou, J. H.; Huo, L. J.; He, C.; Yang, C. H.; Li, Y. F.
Macromolecules 2006, 39, 594−603.
(32) Hou, J. H.; Tan, Z.; He, Y. J.; Yang, C. H.; Li, Y. F.
Macromolecules 2006, 39, 4657−4662.
(33) Li, Y. F.; Zou, Y. P. Adv. Mater. 2008, 20, 2952−2958.
(34) Huo, L. J.; Zhang, S. Q.; Guo, X.; Xu, F.; Li, Y. F.; Hou, J. H.
Angew. Chem., Int. Ed. 2011, 50, 9697−9702.
(35) Wang, E. G.; Hou, L. T.; Wang, Z. Q.; Hellstrom, S.; Zhang, F.
L.; Inganas, O.; Andersson, M. R. Adv. Mater. 2010, 22, 5240−5244.
(36) Gadisa, A.; Mammo, W.; Andersson, L. M.; Admassie, S.; Zhang,
F.; Andersson, M. R.; Inganas, O. Adv. Funct. Mater. 2007, 17, 3836−
3842.
(37) Zhou, E.; Cong, J. Z.; Tajima, K.; Hashimoto, K. Chem. Mater.
2010, 22, 4890−4895.
(38) Zhang, M. J.; Guo, X.; Li, Y. F. Adv. Energy Mater. 2011, 1, 557−
560.
(39) Verploegen, E.; Mondal, R.; Bettinger, C. J.; Sok, S.; Toney, M.
F.; Bao, Z. A. Adv. Funct. Mater. 2010, 20, 3519−3529.
(40) Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim, J.
Y.; Lee, K.; Bazan, G. C.; Heeger, A. J. J. Am. Chem. Soc. 2008, 130,
3619−3623.
3038
dx.doi.org/10.1021/ma300060z | Macromolecules 2012, 45, 3032−3038