Synthesis and photovoltaic performances of benzo[1,2-b:4,5-b']dithiophene- alt-2,3-diphenylquinoxaline copolymers pending functional groups in phenyl rings

Article abstract of DOI:10.1002/pola.26477

Two donor/acceptor (D/A)-based benzo[1,2-b:4,5-b′]dithiophene-alt-2, 3-biphenyl quinoxaline copolymers of P1 and P2 were synthesized pending different functional groups (thiophene or triphenylamine) in the 4-positions of phenyl rings. Their thermal, photo

Full text of DOI:10.1002/pola.26477

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Synthesis and Photovoltaic Performances of Benzo[1,2-b:4,5-b’]
dithiophene-alt-2,3-diphenylquinoxaline Copolymers Pending Functional
Groups in Phenyl Rings
Hua Tan, Xianping Deng, Junting Yu, Jianhua Chen, Kaixuan Nie,
Ying Huang, Yu Liu, Yafei Wang, Meixiang Zhu, Weiguo Zhu
College of Chemistry, Xiangtan University, Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education,
Xiangtan 411105, China
Correspondence to: Dr. W. Zhu (E-mail: zhuwg18@126.com)
Received 3 August 2012; accepted 5 November 2012; published online 4 December 2012
DOI: 10.1002/pola.26477
ABSTRACT: Two donor/acceptor (D/A)-based benzo[1,2-b:4,5-
b0]dithiophene-alt-2,3-biphenyl quinoxaline copolymers of P1
and P2 were synthesized pending different functional groups (thi-
ophene or triphenylamine) in the 4-positions of phenyl rings.
Their thermal, photophysical, electrochemical, and photovoltaic
properties, as well as morphology of their blending films were
investigated. The poly(4,8-bis((2-ethyl-hexyl)oxy)benzo[1,2-b:4,5-b’]
dithiophene)-alt-(2,3-bis(4’-bis(N,N-bis(4-(octyloxy) phenylamino)-
1,1’-biphen-4-yl)quinoxaline) (P2) exhibited better photovoltaic per-
formance than poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b’]
dithiophene)-alt-(2,3-bis(4-(5-octylthiophen-2-yl)phenyl)quinoxaline)
(P1) in the bulk-heterojunction polymer solar cells with a configu-
ration of ITO/PEDOT:PSS/polymers: [6,6]-phenyl-C71-butyric acid
methyl ester (PC71BM)/LiF/Al. A power conversion efficiency of
3.43%, an open-circuit voltage of 0.80 V, and a short-circuit cur-
rent of 9.20 mA cmꢀ2 were achieved in the P2-based cell under
the illumination of AM 1.5, 100 mW cmꢀ2. Importantly, this
power conversion efficiency level is 2.29 times higher than that
in the P1-based cell. Our work indicated that incorporating tri-
phenylamine pendant in the D/A-based polymers can greatly
improved the photovoltaic properties for its resulting poly-
C
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mers. 2012 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym.
Chem. 2013, 51, 1051–1057
KEYWORDS: benzo[1,2-b:4,5-b’]dithiophene; quinoxaline; D-A
copolymer; polymer solar cells; bulk heterojunction
INTRODUCTION Polymeric solar cells (PSCs) have attracted
considerable attention in recent years due to their potentials
of low-cost manufacture, easy processability over large area
via printing or roll-to-roll coating technologies, and favorable
compatibility with flexible substrates. In these developed
PSCs, the devices with bulk heterojunction (BHJ) structure
exhibited some encouraged photovoltaic performance, where
an electron-rich polymer and an electron-deficient fullerene
were blended in the photoactive layer.1–8 As the BHJ struc-
ture not only provides a large-area donor-acceptor interface
for charge separation but also can form a discontinuous
interpenetrating network for charge transport, the BHJ-based
PSCs are available to present high power conversion effi-
ciency (PCE). Up to now, the highest PCE of 9.2% was
reported by Cao and coworkers in this type of PSCs.9
order to enhance the absorption to sunlight, the ideal poly-
mers need to harvest more additional solar photons at lon-
ger wavelengths, particularly in the near-IR region, and ex-
hibit much broader spectra. Therefore, one feasible approach
was to design various D-A copolymers with electron-rich do-
nor (D) unit and electron-deficient acceptor (A) unit. Owing
to an additional intramolecular charge transfer from the D
unit to the A unit, these D-A copolymers presented wider
absorption spectral bands. By tuning the structures of D and
A units, some D-A copolymers showed improved photovoltaic
properties with PCE as high as 3–8%.12–16
Quinoxaline is a typical electron-deficient conjugated unit due
to the strong electronegativity of its two nitrogen atoms and
usually employed to construct conjugated polymers with D-A
backbone in the main chain.17–20 For example, Yang et al.21
reported a series of soluble thiophene-alt-quinoxaline D-A
polymers pending a hole-transporting carbazole moiety as a
side chain and obtained the PSCs with a PCE of 0.40%. Lee
et al. presented a series of dioctyloxybenzo[1,2-b;3,4-b’]dithio-
phene-alt-bis(2-thienyl)-2,3- diphenylbenzo[g]quinoxaline low
However, the photovoltaic levels of these reported copoly-
mers in the devices have not met commercial requirements
for application. The unideal results were considered to be
mainly related to the spectral mismatch between the copoly-
mer’s absorption and the terrestrial solar spectra.10,11 In
Additional supporting information may be found on the online version of this article.
C
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2012 Wiley Periodicals, Inc.
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TABLE 1 Polymerization Results and Thermal and Optical Properties of P1 and P2
Mn (ꢂ104)
Mw (ꢂ104)
PDIa
2.04
1.96
Td (C)
356
Absorptionc (kmax/nm) (emax/dm3 molꢀ1 cmꢀ1
300 (2.5 ꢂ 106), 385 (2.4 ꢂ 106), 586 (1.2 ꢂ 106)
299 (3.6 ꢂ 106), 402 (3.2 ꢂ 106), 612 (1.3 ꢂ 106)
)
a
a
b
˚
Polymer
P1
1.61
3.29
P2
2.73
5.34
363
a
c
Mn,Mw, and PDI of the polymers were determined by GPC using poly-
Measured in the DCM at 298 K.
styrene standards.
b
Td were measured by TGA at a temperature of 5% weight loss for the
polymers.
bandgap copolymers. By attaching methoxy or fluorine groups
into the phenyl rings, this kind of copolymers exhibited a
maximum PCE of 1.42% with an open-circuit voltage (Voc) of
0.8 V, a short-circuit current (Jsc) of 5.73 mA cmꢀ2, and a fill
factor (FF) of 30.9% under illumination of AM 1.5, 100 mW
Thermal Stability
The thermal property of P1 and P2 was investigated by ther-
mogravimetric analysis (TGA). Figure S2 shows the TGA
curves of two copolymers. These data are summarized in Ta-
ꢁ
ble 1. The degradation temperatures (Td) of 356 C and 363
cm
.
ꢀ2 22 Recently, Hou and coworkers designed a kind of diocty-
ꢁC are observed for P1 and P2 with 5% weight loss, respec-
tively. Obviously, the P2 polymer with pendent triphenyl-
amine group exhibited better thermal property than the P1
polymer with pendent thiophene group. It means that intro-
ducing the electron-donor triphenylamine group into the
side chain of quinoxaline unit has a positive effect on the
thermal stability of its resultant copolymer.
loxybenzo[1,2-b;3,4-b’]dithiophene-alt-bis(2-thienyl)-2,3-diphe-
nylquinoxaline conjugated polymers and exhibited a PSC with a
PCE of 3.06%. By further applying two-dimensional conjugated
benzo-[1,2-b:4,5-b’]dithiophene in the above conjugated poly-
mers, his group achieved an improved PSC with a PCE as high
as 5.0%.23 Meanwhile, Zou et al. developed another type of
indacenodithiophene-alt-quinoxaline copolymers and obtained
inverted PSCs with a maximum PCE of 6.38%. To my best
knowledge, this is the highest PCE value reported in PSCs using
quinoxaline-based conjugated polymer as donors up to now.24
Dispersible Property
In general, photovoltaic properties of PSCs are greatly
affected by the surface morphology of the photoactive layer.
Good morphology is available to form excellent interpene-
trating networks in the active layer and further facilitate
charge separation and transportion of photogenerated exci-
tons, finally increase the FF level of the devices. Therefore, it
is necessary to investigate dispersible property of the poly-
mer donor in the PC71BM acceptor. Based on this considera-
tion, the typical blend films of P1 and P2 in PC71BM were
made, in which the weight ratio of polymer and PC71BM was
of 1:3. Their surface morphologies were recorded by atomic
force microscopy (AFM) in a tapping mode as shown in Fig-
ure 2. The roughness of 0.363 nm and 0.639 nm for the
P1:PC71BM and P2:PC71BM blend films are observed, respec-
tively. Compared to the reported results, both P1 and P2
polymers have a good dispersibility in the PC71BM matrix.
In this article, in order to explore the effect of the pending func-
tional groups on the photovoltaic performance of the quinoxa-
line-based conjugated polymers, two D-A-based benzo[1,2-b:4,5-
b0]dithiophen-alt-2,3-biphenyl quinoxaline copolymers of P1
and P2 were designed and synthesized pending the functional
groups of thiophene or triphenylamine in the 4-positions of the
phenyl rings. Here, the P1 is poly(4,8-bis ((2-ethylhexyl)oxy)
benzo[1,2-b:4,5-b’]dithiophene)-alt-(2,3-bis(4-(5-octylthiophen-
2-yl)phenyl)quinoxaline) and the P2 is poly(4,8-bis((2-
ethylhexyl)oxy)benzo[1,2-b:4,5-b’]dithiophene)-alt-(2,3-bis(4’-bis
(N,N-bis(4-(octyloxy)phenylamino)-1,1’-biphen-4-yl)quinoxaline).
Their photophysical, electrochemical, and photovoltaic properties
were studied. The best performances were observed in the BHJ
PSC devices using P2 copolymer with pendent triphenylamine unit
as the active layer. A maximum PCE of 3.43% with a Voc of 0.80 V, a
Jsc of 9.20 mA cmꢀ2 and a FF of 47.0% was achieved under the illu-
mination of AM 1.5, 100 mW cmꢀ2. Our work indicated that photo-
voltaic properties of the D-A quinoxaline-based copolymers can be
improved by attaching the functional group of triphenylamine.
Optical Property
The UV-Vis absorption spectra of P1 and P2 in dilute DCM
and their neat films are shown in Figure 1. Their UV-Vis
absorption data are also summarized in Table 1. Similar
absorption spectra with two p-p* transitions of the D and A
units in the 260–480 nm range and an intramolecular charge
transfer transition in the 500–750 nm range are exhibited
for the P1 and P2 polymers. However, the absorption inten-
sity and peak site have a significant difference with various
units introduced in the 2,3-positions of quinoxaline group.
The maximum absorption peak located at 612 nm for P2,
which has 26 nm red shift with respect to that for P1 in
DCM. Furthermore, the absorption intensity and the extinc-
tion coefficient of P2 are bigger than those of P1 in DCM.
The enhanced absorption intensity and the red-shifted
absorption spectrum are displayed for P2 instead of P1 in
their neat films. It indicates that introducing the triphenyl-
amine unit in the side chain of the D-A-based polymers has
RESULTS AND DISCUSSION
Synthesis and Characterization of the Polymers
4,7-dibromo-2,1,3-benzothiadiazole was reduced with zinc in
acetic acid to afford 1,4-bibromo-2,3-diaminobenzene, con-
densation of 1,4-bibromo-2,3-diaminobenzene with diketone
2 or 4 to obtain the M2 and M3, respectively. The M2 and
M3 were subsequently copolymerized with 2,6-bis(trimethyl-
tin)-4,8-bis(2-ethyl-hexyloxy)-benzo[1,2-b:4,5-b’]dithiophene
to afford P1 and P2, respectively. Because of the side chains,
the P1 and P2 showed good solubility in common organic
solvents, such as chlorobenzene, o-dichlorobenzene, and
dichlomethane (DCM).
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potential and reduction potential than P1. The EHOMO and
ELUMO levels of P2 are 0.02 eV and 0.07 eV higher than the
corresponding levels of P1, respectively. This result indicates
that introducing triphenylamine moieties into the side chain
of the D-A-based copolymers can further tuning the EHOMO
and ELUMO levels for the resulting polymers.
Photovoltaic Properties
As PC71BM has similar electronic properties to PC60BM but a
considerably higher absorption coefficient in the visible region,
here it was chosen as an electron acceptor instead of PC60BM.
The photovoltaic performance of these devices is listed in Ta-
ble 3 at different processing condition. While the blending
ratios of polymer and PC71BM were tuned from 1:1 to 1:3, the
devices exhibited significantly increased PCE, Jsc, and FF. After
annealed, the resulting devices further presented increased
PCE and Jsc. The PCE from 1.71% to 3.43% and Jsc from 7.59 to
9.20 mAcmꢀ2 were observed in the P2-based devices with
increasing ratios from 1:1 to 1:3 under the annealed condition,
when the P1-based devices displayed an elevatory PCE from
0.46% to 1.50% and Jsc from 2.87 to 7.81 mAcmꢀ2 at the same
condition, respectively. Therefore, adding ratio of PC71BM and
annealing are available to increase the PCE and Jsc. As the ther-
mal annealing processes is available to improve the morphol-
ogy of the active layer greatly, it should result in excellent
interpenetrating networks in the active layer and further facili-
tate charge separation and transportion of photogenerated
excitons. Therefore, the devices exhibited higher efficiency
under annealing condition. For intuitive comparison, the J-V
characteristics of the P1- and P2-based BHJ PSCs are shown in
Figure 3 at a blending ratio of 1:3 between polymer and
PC71BM under the illumination of AM 1.5 G (100 mW/m2)
from a solar simulator. Obviously, the P2-based devices pre-
sented better photovoltaic performance than the P1-based
devices under annealing condition. Incorporating triphenyl-
amine group into the side chain of benzo[1,2-b:4,5-b’] dithio-
phene-alt-2,3-biphenyl quinoxaline copolymer has positive
FIGURE 1 Normalized UV-Vis absorption spectra of the P1 and
P2 in DCM and their neat films at room temperature.
better effect to improve the absorption properties of its
resulting polymers (Fig. 1).
Electrochemical Study
In order to further understand the influence of the pendent
groups to the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) energy
levels (EHOMO and ELUMO), the electrochemical behaviors of
P1 and P2 films were investigated by cyclic voltammetry
(CV). The resulting CV data are listed in Table 2. The onset
oxidation potential (Eox) and reduction potential (Ered) were
observed in a range of 0.68–0.75 V and ꢀ0.85 to ꢀ0.87 V
versus SCE, respectively. According to the empirical formu-
las: EHOMO ¼ ꢀ(Eox þ 4.40) eV and ELUMO ¼ ꢀ(Ered þ 4.40)
eV, the HOMO and LUMO energy levels for these polymers
were obtained Finally, the electrochemical band gaps (Eg) of
P1 and P2 were calculated based on the formula of Eg
ELUMO ꢀ EHOMO. It is found that P2 has a decreased oxidation
¼
FIGURE 2 Tapping-mode AFM images of the P1 (a) and P2 (b) films on quartz glass.
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TABLE 2 Electrochemical Properties of P1 and P2
Polymer
P1
Ered (V)
Eox (V)a
EHOMO (eV)b
ꢀ5.27
ꢀ5.25
ELUMO (eV)b
ꢀ3.72
ꢀ3.65
Eg (eV)c
1.72
Eel (eV)c
1.65
1/2
1/2
ꢀ0.87
0.85
0.68
0.75
P2
1.75
1.70
a
Potential values are reported by cyclic voltammetry.
b
c
ELUMO ¼ ꢀ(4.40 þ Ered1/2) eV,Eg ¼ ELUMO ꢀEHOMO.
Estimated according to the UV-Vis absorption spectra of the polymer
films at 298 K.
effect on improving the photovoltaic performance for its
resulting polymer in the cells. The highest PCE of 3.43%, a Jsc
of 9.20 mA cmꢀ2, and a FF of 47.0% were obtained in the P2-
based solar cells at a blending ratio of 1:3.
in the BHJ PSCs. The maximum PCE of 3.43% with a Voc of
0.80 V and a Jsc of 9.20 mA cmꢀ2 were achieved in these
devices under the illumination of AM 1.5, 100 mW cmꢀ2
.
Our work indicated that incorporating triphenylamine pend-
ant into the side chain of the D-A-based copolymers can
greatly improved the photovoltaic properties for its resulting
polymers in the BHJ PSCs.
We noted that the experimental Voc of the P1- and P2-based
devices was 0.66 and 0.80 V under this blending ratio,
respectively. In general, there is
a linear relationship
between Voc and the energy gap between the HOMO level of
the donor polymer and LUMO level of an acceptor. Based on
this rule, the theoretical Voc is estimated to be about 0.95 V
(using ꢀ4.0 eV for the LUMO energy level of PC71BM).24
Compared to the theoretical Voc, the experimental Voc is
reduced and has some deviations of 0.15–0.29 V. The lower
experimental Voc is closely related not only to the dark cur-
rent-voltage curve of the diode but also to the interface
interaction between their domains, which influences the
charge photogeneration efficiency and recombination.25,26
EXPERIMENTAL
Materials and Measurements
All manipulations were performed under dry nitrogen flow.
All reagents were obtained from Aldrich and directly used
without further purification. Scheme 1 illustrates the syn-
thetic route of the polymers.
1H and 13C NMR spectra were recorded on a Bruker DRX
400 spectrometer using tetramethylsilane as a reference in
deuterated chloroform solution at 298 K. MALDI-TOF mass
spectrometric measurements were performed on Bruker
Bifiex III MALDI-TOF. Molecular weight was determined
using a Waters GPC 2410 in tetrahydrofuran via a calibration
curve of polystyrene as standard. UV-Vis absorption spectra
were recorded on a HP-8453 UV visible system. CV was
carried out on a CHI660A electrochemical work station in a
three-electrode cell dipped in a 0.1M tetrabutylammonium
hexafluorophosphate (Bu4NPF6) solution in acetonitrile
under nitrogen protection at a scan rate of 100 mV/s and
room temperature. In this three-electrode cell, a platinum
rod, platinum wire, and calomel electrode were used as a
working electrode, counter electrode, and reference elec-
trode, respectively. The morphology of the polymer/PCBM
blend film was investigated by an AFM on a Veeco, DI Multi-
mode NS-3D apparatus in a trapping mode under normal air
condition at room temperature. Tributyl(5-octyl- thiophen-2-
On the other hand, by comparison with the device performance
of the previous quinoxaline conjugated polymers-based solar
cells,27 we observed that the P2-based cells exhibited signifi-
cantly increased PCE level, as well as a parallel Voc and Jsc val-
ues, respectively. However, the FF level here was not improved.
It is considered that the FF value could be improved by using
different solvents,28 adding various additives,29 and/or using a
buffer layer.30 Hence, to further improve it, some optimized
work for the devices need be carried out.
CONCLUSIONS
In summary, two D-A-based benzo[1,2-b:4,5-b’]dithiophene-
alt-2,3-biphenyl quinoxaline copolymers of P1 and P2 were
made with functional thiophene or triphenylamine units in
the 4-positions of phenyl rings. The P2 copolymer with
pendent triphenylamine unit exhibited the best performance
TABLE 3 Photovoltaic Properties of P1 and P2
Polymer
P1
Polymer:PC71BM
Thickness (nm)
Annealing
No
Jsc (mA/cm2)
2.63
Voc (V)
0.66
0.68
0.66
0.66
0.76
0.74
0.80
0.80
PCEmax (PCEaverag) (%)
0.40 (0.33)
FF (%)
23
1:1a
1:1b
1:3a
1:3b
1:1a
1:1b
1:3a
1:3b
80
80
80
80
80
80
80
80
Yes
2.87
0.46 (0.39)
24
No
6.19
1.19 (1.07)
29
Yes
7.81
1.50 (1.44)
29
P2
No
6.26
1.43 (1.26)
30
Yes
7.59
1.71 (1.60)
30
No
7.60
2.92 (2.67)
48
Yes
9.20
3.43 (3.17)
47
a
b
˚
Annealed at 150 C for 15 min.
Unannealed.
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unit in the dark. All device characterizations were performed
under an ambient atmosphere at room temperature.
Synthesis of Materials
Synthesis of M2
A mixture of 2 (0.60 g, 1.00 mmol), 3,6-dibromobenzene-1,2-
diamine(0.27 g, 1.00 mmol), solid potassium carbonate, and
ethanol (25 mL) was vigorously stirred at 75 ꢁC for 24 h under
a nitrogen atmosphere. After cooling, the reaction solution was
extracted with dichloromethane and water. The organic layer
was collected and dried over magnesium sulfate. The solvent
was removed by rotary evaporation and the residue was puri-
fied by column chromatography [silica gel, petroleum ether/
dichloromethane (5/1) as eluent] to give yellow solid, 1HNMR
(400 MHz, CDCl3, ppm): 7.92 (d, J ¼ 3.6 Hz, 2H), 7.74 (d, J ¼ 8.0
Hz, 4H), 7.69 (d, J ¼ 8.8Hz, 4H), 7.31 (d, J ¼ 3.2 Hz, 2H), 6.81 (d,
J ¼ 3.0 Hz, 2H), 2.85-2.82 (t, 4H), 1.73-1.70 (m, 4H), 1.44-1.29
(br, 20H), 0.90 (t, 6H). 13C NMR (CDCl3, 100MHz), d (ppm):
153.4, 146.8, 140.7, 139.3, 138.1, 136.3, 136.1, 136.0, 133.1,
133.0, 130.8 , 130.2, 129.6, 128.5, 125.3, 125.2, 125.1, 123.7,
123.6, 31.9, 31.6, 30.3, 29.7, 29.3, 29.2, 29.1, 22.7, 14.1.TOF-MS
(m/z): Calcd for C44H48Br2N2S2, 828.2; found, 829.2.
FIGURE 3 J-V characteristics of the bulk heterojunction poly-
meric solar cells with
a structure of ITO/PEDOT:PSS/poly-
mer:PC71BM (1:3, wt/wt)/LiF/Al under the illumination of AM
1.5 G from a solar simulator (1000 W mꢀ2).
yl)stannane (1), 3,6-dibromobenzene-1,2-diamine, 4-(oct-
yloxy)-N-(4-(octyloxy)phenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)benzen amine (3), 2,6-bis(trime-
thyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-
b0]dithiophene (M1) were synthesized by the published pro-
cedures.21–23
Synthesis of M3
1HNMR (400 MHz, CDCl3, ppm): 7.90 (d, J ¼ 4.0 Hz, 2H),
7.75 (d, J ¼ 8.4 Hz, 4H), 7.58 (d, J ¼ 8.8Hz, 4H), 7.46 (d, J ¼
8.0 Hz, 4H), 7.09 (d, J ¼ 8.6 Hz, 8H), 7.00 (d, J ¼ 8.8 Hz,
4H), 6.85 (d, J ¼ 8.4 Hz, 4H), 3.95-3.92 (t, 8H), 1.81-1.78 (m,
8H), 1.46-1.29 (br, 40H), 0.90 (t, 12H). 13C NMR (CDCl3,
100MHz), d (ppm): 155.7, 153.8, 148.8, 142.1, 140.6, 139.3,
136.0, 132.9, 131.6, 130.7, 127.5, 126.8, 126.2 , 123.7, 120.4,
115.4, 68.4, 31.8, 29.4, 29.3, 26.1, 22.7, 14.1. TOF-MS (m/z):
Calcd for C88H102Br2N4O4, 1438.6; found, 1438.3.
Polymer Solar Cell Fabrication and Test
The photovoltaic cells were constructed with a traditional
sandwich structure through the following steps. The iridium-
tin oxide (ITO)-coated glass substrates were cleaned by a se-
ries of ultrasonic treatments for 10 min in acetone, following
by deionized water, then 2-propanol. The substrates were
dried under a stream of nitrogen and subjected to the treat-
ment of Ar/O2 plasma for 5 min. A filtered aqueous solution
of poly(3,4-ethylenedioxy-thiophene)-poly-(styrenesulfonate)
(PEDOT:PSS) (Bayer AG) was spun-cast onto the ITO surface
at 2000 rpm for 30 s and then baked at 150 ꢁC for 30 min
to form a PEDOT:PSS thin film with a thickness of 30 nm. A
blend of polymer and PC71BM (1:3 wt/wt, 20 mg/mL for
polymer) was dissolved in chlorobenzene, filtered through a
0.45-lm poly(tetrafluoroethylene) filter and spun-cast at
3000 rpm for 30 s onto the PEDOT:PSS layer. The substrates
were dried under N2 at room temperature and then
annealed at 150 ꢁC for 15 min in a nitrogen-filled glovebox.
The devices were completed after thermal deposition of a
10-nm lithium fluoride and a 0.5-nm aluminum film as the
cathode at a pressure of 6 ꢂ 10ꢀ4 Pa. The active area was 9
mm2 for each cell. The thicknesses of the spun-cast films
were recorded by a profilometer (Alpha-Step 200, Tencor
Instruments).
General Procedures of Stille Coupling Reaction
Taking the preparation of P1 as an example: a mixture of
monomers M1 (77.2 mg, 0.1 mmol), M2 (82.8 mg, 0.1
mmol), and Pd(PPh3)4 (10 mg) in toluene (4 mL) was
degassed with nitrogen flow and stirred at 90 ꢁC for 48 h
under nitrogen atmosphere. After cooled to room tempera-
ture (RT), tributyl(thiophen-2-yl)stannane (37 mg, 0.10
mmol) was added and the mixture was refluxed for 12 h.
Then 2-bromothiophene (32 mg, 0.20 mmol) was injected
and the mixture was refluxed for another 12 h. The mixture
was cooled to RT and poured into 200 mL MeOH to form
precipitate. The precipitate was collected and dissolved in
DCM. The filtrate was poured into 200 mL MeOH again to
provide precipitate. Then, the precipitate was washed with
acetone in a Soxhlet apparatus for 48 h to give P1 as black
solid. The polymerization results are summarized in Table 1.
P1: black solid, 90.1mg, yield: 81.1%. 1H NMR (CDCl3, 400
MHz, ppm): 8.26–7.47 (br, 12H), 7.39–7.10 (br, 2H), 6.93–
6.70 (br, 2H), 4.45–3.96 (br, 4H), 3.03–2.71 (br, 4H), 1.97–
1.17(m, 42H), 1.16–0.59 (br, 18H). 13C NMR (CDCl3,
100MHz), d (ppm): 152.4, 150.8, 146.3, 144.3, 141.3, 140.2,
138.4, 136.0, 135.3, 133.0, 131.4, 131.2, 130.8 , 130.2, 129.8,
128.3, 127.5, 125.4, 125.2, 125.1, 123.4, 123.1, 40.9, 32.0,
31.7, 30.5, 29.5, 29.3, 29.2, 23.2, 22.7, 14.1. Mn¼1.61 ꢂ 104
g molꢀ1; Mw¼3.29 ꢂ 104 g molꢀ1; PDI¼2.04.
Device characterization was carried out under an AM 1.5G
irradiation with an intensity of 100 mW/cm2 (Oriel 91160,
300 W) calibrated by a NREL-certified standard silicon cell.
Current density-voltage (J-V) characteristics were measured
by a computer-controlled Keithley 2602 source measurement
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SCHEME 1 Synthesis of the P1 and P2 by the Stille cross-coupling reaction.
P2: black solid, 152.3mg, yield: 88.3%. 1H NMR (CDCl3, 400
MHz, ppm): 8.22–7.81 (br, 24H), 7.80–7.24 (d, J ¼ 7.0 Hz,
2H), 7.21–6.93 (d, J ¼ 8.0 Hz, 6H), 6.92–6.70 (d, J ¼ 8.0 Hz,
6H), 4.07–3.79 (br, 8H), 3.61–3.43 (br, 4H), 2.03 (t, 12H),
1.67–1.08 (br, 60H), 1.07–0.53 (br, 18H). 13C NMR (CDCl3,
100MHz), d (ppm): 155.5, 148.5, 144.3, 140.7, 139.1, 138.9,
137.5, 137.0, 136.8, 132.4, 132.1, 131.5, 130.9, 130.8, 127.3,
126.6, 126.0, 123.7, 120.8, 115.4, 68.3, 31.9, 30.6, 29.5, 29.3,
26.2, 22.7, 14.3, 14.1. Mn¼2.73 ꢂ 104 g molꢀ1; Mw¼5.34 ꢂ
104 g molꢀ1; PDI¼1.96.
ACKNOWLEDGMENTS
Thanks to the financial supports from the National Natural Sci-
ence Foundation of China (Grant Nos. 21172187, 91233112
and 50973093), the Innovation Group and Xiangtan Joint
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