Synthesis and photovoltaic behaviors of benzothiadiazole- and triphenylamine-based alternating copolymers

Article abstract of DOI:10.1016/j.polymer.2011.12.026

A series of donor-acceptor (D-A) alternating copolymers (P1, P2 and P3) with thiophene-benzothiadiazole-thiophene-triphenylamine main chain have been synthesized by Suzuki polycondensation. P1, P2, and P3 possess medium optical band gaps of 1.99, 1.97 and 1.93 eV, respectively. Bulk heterojunction polymer solar cells (BHJ PSCs) with these polymers as donor and PC₇₁BM as acceptor showed power conversion efficiency (PCE) in the range of 2.1-2.8%. The highest PCE of 2.8 % was achieved for P1 with short circuit current (J _sc) of 7.8 mA/cm². This study offers a useful and important insight for designing triphenylamine derivative-based polymers used for efficient PSCs.

Full text of DOI:10.1016/j.polymer.2011.12.026

Polymer 53 (2012) 324e332
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and photovoltaic behaviors of benzothiadiazole- and triphenylamine-
based alternating copolymers
,
a
a
a
,
b
,
c,
Ming Wang a, Cuihong Li b , Aifeng Lv , Zhaohui Wang , Zhishan Bo
**
*
, Fengling Zhang
***
a State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
b College of Chemistry, Beijing Normal University, Beijing 100875, China
c Department of Physics, Chemistry and Biology, Linköping University, SE-58183, Linköping, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
A
series of donoreacceptor (DeA) alternating copolymers (P1, P2 and P3) with thiophene
Received 28 September 2011
Received in revised form
14 December 2011
Accepted 17 December 2011
Available online 21 December 2011
ebenzothiadiazoleethiopheneetriphenylamine main chain have been synthesized by Suzuki poly-
condensation. P1, P2, and P3 possess medium optical band gaps of 1.99, 1.97 and 1.93 eV, respectively.
Bulk heterojunction polymer solar cells (BHJ PSCs) with these polymers as donor and PC71BM as acceptor
showed power conversion efficiency (PCE) in the range of 2.1e2.8%. The highest PCE of 2.8 % was ach-
ieved for P1 with short circuit current (Jsc) of 7.8 mA/cm2. This study offers a useful and important insight
for designing triphenylamine derivative-based polymers used for efficient PSCs.
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
Conjugated polymers
DeA copolymers
Polymer solar cells
1. Introduction
The overlapping of polymer absorption spectrum with the solar
photon flux peak is required for harvesting more solar photons
Green and renewable solar energies are regarded as promising
energy sources to solve the world energy crisis. Polymer solar cells
(PSCs) have been attracting considerable attention over the past 15
years because of their unique advantages of light-weight, low cost,
and potential for making flexible and large area devices using roll-
to-roll process [1e6]. PSCs based on the concept of bulk hetero-
junction (BHJ) have been intensively investigated worldwide and
have become the most successful active layer structure in organic
photovoltaic devices to date [7]. BHJ PSCs are generally fabricated
with a light-absorption blending layer containing an electron-
to generate photocurrent. Polymers with a band gap of 1.5e1.8 eV
are considered suitable for high efficiency PSCs [15]. However,
decreasing the band gap of polymers will result in either lower
open circuit voltage (Voc) or lower driving force for exciton disso-
ciation as Voc is mainly governed by the energy level difference
between the lowest unoccupied molecular orbital (LUMO) of
the acceptor and the highest occupied molecular orbital (HOMO)
of the donor and short circuit current (Jsc) depends on LUMO energy
level difference between the donor and the acceptor [16,17]. For
a given acceptor, HOMO and LUMO energy levels of polymers must
be matched with those of the acceptor to balance Voc and Jsc.
Noticeably, a high Voc is more readily obtained through medium
band gap (1.7e2.0 eV) polymers with a low-lying HOMO energy
level while keep properly high LUMO energy level. Based on this
design strategy, great progress has been made in synthesis and
application of medium band gap polymers in BHJ PSCs. Power
conversion efficiencies (PCEs) of 5.2%, 6.1%, and 7.1% have been
reached for PBDTTTZ (2.0 eV),[18] PCDTBT (1.9 eV) and PBnDT-FTAZ
(2.0 eV) [19,20], respectively, indicating their great potential for the
next generation high performance PSCs. Thus, the seemingly
overlooked medium band gap polymers warrant further
exploration.
donating conjugated polymer as
a donor and an electron-
accepting fullerene derivative as an acceptor. 6,6-phenyl-C61
-
butyric acid methyl ester (PC61BM) and 6,6-phenyl-C71-butyric acid
methyl ester (PC71BM) are still the most widely used and efficient
acceptors in BHJ PSCs so far [8e11]. Thus, much research efforts
have been focused on the molecular engineering of electron-
donating conjugated polymers [12e14].
* Corresponding author. State Key Laboratory of Polymer Physics and Chemistry,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
Tel./fax: þ86 01062206891.
** Corresponding author.
*** Corresponding author.
Triphenylamine (TPA) derivative has been regarded as a prom-
ising unit for efficient photovoltaic materials due to excellent
E-mail addresses: licuihong@bnu.edu.cn (C. Li), zsbo@iccas.ac.cn (Z. Bo), fenzh@
ifm.liu.se (F. Zhang).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.12.026

M. Wang et al. / Polymer 53 (2012) 324e332
325
electron-donating and high hole-transporting properties [21e24].
It has been reported that the TPA derivative has become an efficient
building block as donor units in small molecular solar cells and dye
sensitized solar cells [25,26]. Moreover, the two-dimensional
5e6% [19,33,34], we report here the synthesis of a series of inter-
mediate band gap DeA copolymers based on TPA derivatives and BT
segments. As shown in Scheme 1, alkoxy or alkyl side chains are
attached to the TPA unit (P1 and P2) to not only ensure the poly-
mer’s solubility in common organic solvents but also get more
insight into the influence of substituents on the photovoltaic
performances. Besides, diphenylamine unit is also introduced to
polymer backbones (P3) to investigate the structureeproperty
relationships. Meanwhile, BT unit carrying two octyloxy chains
was used to improve the solubility and processability of the
resulting polymers [34e36]. The results indicate P1, P2, and P3
possess medium band gaps of 1.99, 1.97 and 1.93 eV, respectively.
The devices based on the blend of each polymer and PC71BM
showed the best photovoltaic performance with PCE of 2.1e2.8%.
Among these three polymers, P1 exhibits the highest PCE of 2.8%
with Voc of 0.68 V, Jsc of 7.8 mA/cm2, and fill factor (FF) of 0.53.
conjugated structures by combining TPA with linear p-conjugated
systems may improve the isotropic charge transportation [27],
which is extremely important for PSCs. Cao et al. have reported
a series of two-dimensional conjugated donoreacceptor (DeA)
copolymers based on TPA [28e30]. They have shown that such
two-dimensional conjugated polymers gave promising photovol-
taic performance with PCE of 2e4%. Different from their TPA-
containing DeA copolymers, in which the acceptor positioned
at the end of TPA units, there are only few reports on TPA-containing
main chain donor acceptor alternating copolymers utilized in BHJ
PSCs [31,32]. Since benzothiadiazole (BT) has been proved to be an
excellent electron-deficient unit in polymer solar cells with PCEs of
OC8H17
OC8H17
Br
N
OH
i
iv
iii
ii
N
N
N
O
O
B
Br
Br
Br
Br
B
Br
Br
Br
Br
iv
1
2
O
O
M2
C8H17
Br
N
C8H17
v
N
N
O
O
B
B
Br
Br
O
O
M3
3
C8H17
N
C8H17
N
H
N
H
N
iv
vii
vi
O
O
n
B
B
Br
+
Br
Br
Br
O
O
M4
4
5
OC8H17
N
C8H17
O
S
OC8H17
S
C8H17
viii
O
N
N
Br
S
S
N
S
Br
N
N
O
O
C8H17
O
OC8H17
B
S
B
O
O
P1
M2
M1
C8H17
C8H17
O
S
OC8H17
S
C8H17
viii
N
N
Br
S
n
S
N
N
+
+
S
Br
N
N
O
O
O
C8H17
O
OC8H17
B
S
B
O
O
P2
M3
M1
C8H17
S
O
S
OC8H17
S
N
N
C8H17
N
Br
S
C8H17
viii
n
N
S
Br
N
N
O
C8H17
O
OC8H17
B
S
B
O
O
P3
M4
M1
Scheme 1. Synthesis of the monomers and the copolymers. Reagents and reaction conditions: (i) n-BuLi, ꢀ78 ꢁC, 1 h, and then tri-iso-propyl borate, overnight to room temperature;
(ii) KOH/H2O, H2O2, 50 ꢁC for 3 h, and HCl, pH ¼ 2, 53% yield; (iii) 1-bromooctane, butanone, K2CO3, reflux for 20 h, 69% yield; (iv) PdCl2(dppf), potassium acetate, bis(pinacolato)
diboron, DMF, 80 ꢁC for 72 h; (v) n-BuLi, ꢀ78 ꢁC for 1 h, then 1-iodooctane, overnight, 26% yield; (vi) NBS, 0 ꢁC for 6 h, 99% yield. (vii) NaH, 1 h, and then 1-bromooctane, reflux for
24 h, 95% yield; (viii) Pd(PPh3)4, NaHCO3, THF/H2O, reflux.

326
M. Wang et al. / Polymer 53 (2012) 324e332
2. Experimental section
a hotplate before transferred into a glove box. The active layer was
prepared by spin-coating the solution of polymers and PC71BM on
the top of ITO/PEDOT:PSS. The top electrode was thermally evap-
orated, with a 0.6 nm LiF layer, followed by 80 nm of aluminum at
a pressure of 10ꢀ6 Torr through a shadow mask. Four cells were
fabricated on one substrate with an effective area between
0.04e0.05 cm2. The measurement of devices was conducted in air.
Currentevoltage characteristics were recorded using a Keithley
2400 Source Meter under AM 1.5 illumination with an intensity of
100 mW/cm2 from a solar simulator (Model SS-50A, photo Emis-
sion Tech., Inc.). The temperature while measuring the JeV curves
was approximately 25 ꢁC.
2.1. Materials and instruments
Unless otherwise noted, all chemicals were purchased from
commercial suppliers and used without further purification.
Solvents were dried using standard procedures. The catalyst
precursor Pd(PPh3)4 was prepared according to the literature[37]
and stored in a Schlenk tube under nitrogen. Tetrahydrofuran
(THF) and diethyl ether (Et2O) were distilled from sodium with
benzophenone as an indicator under nitrogen atmosphere.
Hexane and dichloromethane (CH2Cl2) were distilled from CaH2.
Chloroform was distilled before use. All reactions were performed
under an atmosphere of nitrogen and monitored by thin layer
chromatography (TLC) on silica gel 60 F254 (Merck, 0.2 mm).
Column chromatography was carried out on silica gel (200e300
mesh). 1H and 13C NMR spectra were recorded on a Bruker DM
300 or AV 400 spectrometer in CDCl3. UVevis absorption spectra
were obtained on a SHIMADZU UV-visible spectrometer model
UV-1601PC. Elemental analyses were performed on a Flash EA
1112 analyzer. Thermal gravimetric analysis (TGA) and differential
scanning calorimetry (DSC) measurements were performed on
TA2100 and PerkineElmer Diamond DSC instrument, respectively,
under a nitrogen atmosphere at a heating rate of 10 ꢁC/min to
record TGA and DSC curves. Matrix-assisted laser desorption
ionization time of flight (MALDI-TOF) mass spectra were recorded
on a BIFLEXIV mass spectrometer. Elemental analysis was per-
formed on a Vario EL elemental analysis instrument. Number-
average (Mn) and weight-average (Mw) molecular weights were
measured by gel permeation chromatography (GPC) on a PL-GPC
50 with THF as an eluent and calibrated with polystyrene stan-
dards. Atomic force microscopy (AFM) images of blend films were
obtained on a Nanoscope IIIa Dimension 3100 operating in the
tapping mode. The film thickness was determined by a Dektak 6M
surface profilometer. Electrochemical measurements were per-
formed on a CHI 630A Electrochemical Analyzer with a standard
three-electrode electrochemical cell in a 0.1 M tetrabutylammo-
nium tetrafluoroborate solution in CH3CN at room temperature
with a scanning rate of 0.1 V/sꢀ1. A glassy carbon working elec-
trode, a Pt wire counter electrode, and an Ag/AgNO3 (0.01 M in
CH3CN) reference electrode were used. The experiments were
calibrated with the standard ferrocene/ferrocenium (Fc) redox
system and assumption that the energy level of Fc is 4.8 eV below
vacuum.
2.4. Monomer and polymer synthesis
2.4.1. Monomer synthesis
2.4.1.1. 4-(Bis(4-bromophenyl)amino)phenol (1). n-Butyllithium (n-
BuLi) (9 mL, 2.5 M in hexane, 21 mmol) was added dropwise over
30 min under a nitrogen atmosphere to a stirred solution of tris(4-
bromophenyl)amine (10 g, 21 mmol) in anhydrous THF (300 mL)
at ꢀ78 ꢁC. The solution was stirred at ꢀ78 ꢁC for 1 h, and then tri-
iso-propyl borate (5.9 mL, 25 mmol) was added in one portion. The
reaction mixture was stirred overnight and allowed to slowly warm
to room temperature. After quenched by water (100 mL), the
organic layer was separated and the aqueous layer was extracted by
diethyl ether (50 mL ꢂ 3). The combined organic layer was dried
over magnesium sulfate (MgSO4) and the solvents were removed
by rotary evaporator. The crude product of boronic acid was ob-
tained as a residue, which was directly used in the next step
without purification. To a mixture of the boronic acid crude
product, KOH (2.5 g, 45 mmol), and water (50 mL) was added
dropwise H2O2 (30%, 7.6 g, 68 mmol) at 0 ꢁC. After the addition of
H2O2, the mixture was heated to 50 ꢁC and stirred for 3 h.
Dichloromethane (100 mL) was added and the aqueous layer was
acidified by hydrochloric acid (HCl) to pH ¼ 2. The organic layer was
separated and dried over MgSO4 and the solvents were removed by
rotary evaporator. The crude product was purified on silica gel
column eluting with petroleum ether/dichloromethane (2:1 v/v
followed by 1:2 v/v) to afford the title compound as a colorless solid
(4.6 g, 53%). The compound was unstable under ambient condition,
so only NMR characterizations were performed. 1H NMR (400 MHz,
CDCl3): d C
7.30 (m, 4H), 6.98 (m, 2H), 6.88 (m, 4H), 6.78 (m, 2H); 13
NMR (100 MHz, CDCl3):
116.8, 114.8.
d 152.9, 147.0, 140.1, 132.4, 127.8, 124.5,
2.2. Organic field-effect transistors characteristics
2.4.1.2. 4-Bromo-N-(4-bromophenyl)-N-(4-(octyloxy)phenyl)amine
(2). Compound (4.6 g, 10.9 mmol), 1-bromooctane (2.54 g,
1
The thin film organic field-effect transistors (OFET) of all three
polymers were investigated by spin-coating method. Each polymer
with the concentration of 10 mg/mL in 1,2-dichlorobenzene (1,2-
DCB) was spun on OTS-treated Si/SiO2 substrates. Au electrodes
were thermally deposited through a multiple finger configuration
13.1 mmol), and butanone (50 mL) were added into a flask and
degassed. Potassium carbonate (K2CO3) (3.03 g, 22 mmol) was
added under nitrogen atmosphere, and the reaction mixture was
refluxed for 20 h and then was filtered off. The solid was washed
with dichloromethane (200 mL ꢂ 3) and the combined filtrate was
concentrated and purified on silica gel column to afford the title
compound as a colorless oil (4 g, 69%). 1H NMR (400 MHz, CDCl3):
mask with channel length 50 mm and width 2500 mm separately.
Measurements were performed in air using a Keithley 4200-SCS
semiconductor parameter analyzer and a Micromanipulator 6150
probe station in a clean and shield box.
d
7.30 (d, 4H), 7.00 (d, 2H), 6.88 (d, 4H), 6.83 (d, 2H), 3.93 (t, 3H),1.44
(m, 2H), 1.30 (m, 10H), 0.87 (t, 3H); 13C NMR (100 MHz, CDCl3):
156.6, 147.1, 139.7, 132.4, 127.7, 124.5, 115.8, 114.7, 68.5, 32.1, 29.6,
d
2.3. Fabrication and characterization of PSCs
29.5, 29.4, 26.3, 22.9, 14.3. Anal. calcd. for C26H29Br2NO: C, 58.77; H,
5.50; N, 2.64; found: C, 58.65; H, 5.54; N, 2.39.
PSCs were fabricated with the device configuration of ITO/
PEDOT:PSS/Polymer:PC71BM (1:3)/LiF/Al. The conductivity of ITO is
2.4.1.3. 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-N-(4-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-N-(4-(octyloxy)
phenylamine) (M1). A mixture of compound 2 (2.0 g, 3.77 mmol),
bis(pinacolato)diboron (2.2 g, 8.7 mmol), potassium acetate (2.6 g,
26.5 mmol), and DMF (20 mL) was carefully degassed before and
20
U
/,. PEDOT:PSS is Baytron Al 4083 from H.C.Starck and was
m PVDF film before use. A thin layer of
filtered with a 0.45
m
PEDOT:PSS was spin-coated on top of cleaned ITO substrate at
3000 rpm/s for 60 s and dried subsequently at 130 ꢁC for 15 min on

M. Wang et al. / Polymer 53 (2012) 324e332
327
after [1,10-bis(diphenylphosphino)ferrocene]palladium(II) chloride
(PdCl2(dppf)) (31 mg, 0.038 mmol) was added under nitrogen
atmosphere. The reaction mixture was stirred at 80 ꢁC for 72 h and
then ethyl acetate (200 mL) and water (100 mL) were added. The
organic layer was separated, washed with brine (50 mL ꢂ 3), dried
over anhydrous MgSO4, and evaporated to dryness. The crude
product was purified on silica gel column eluting with petroleum
ether/ethyl acetate (3:1, v/v) increasing to 1:1 (v/v) to give the title
compound as a colorless oil (1.46 g, 62%). 1H NMR (400 MHz,
over MgSO4 and evaporated to dryness. The crude product was
purified on silica gel column eluting with petroleum ether to give
the title compound as a colorless oil (5.1 g, 95%). 1H NMR (400 MHz,
CDCl3):
d
7.34 (m, 4H), 6.84 (m, 4H), 3.60 (t, 2H), 1.60 (m, 2H), 1.26
147.0, 132.6,
(m, 10H), 0.87 (t, 3H); 13C NMR (100 MHz, CDCl3):
d
123.6, 114.2, 52.7, 32.3, 30.1, 27.9, 27.6, 27.4, 23.0, 14.5. Anal. calcd.
for C20H25Br2N: C, 54.69; H, 5.74; N, 3.19; found: C, 55.20; H, 5.95;
N, 2.99.
CDCl3):
(m, 2H), 1.33 (m, 34H), 0.90 (t, 3H); 13C NMR (100 MHz, CDCl3):
156.6, 150.6, 139.9, 136.0, 128.3, 122.0, 115.6, 83.8, 68.5, 32.1, 29.6,
29.5, 29.3, 25.1, 22.9, 14.3. Anal. calcd. For C38H53B2NO5: C, 72.97; H,
8.54; N, 2.24; found: C,72.89; H, 8.46; N, 2.12.
d
7.65 (d, 4H), 7.04 (m, 6H), 6.83 (m, 2H), 3.94 (t, 2H), 1.47
2.4.1.8. 4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-N-(4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-N-octylphenylamine
(M3). M3 was synthesized by the same procedure as M1. The title
compound was obtained as a colorless oil in a yield of 64 %. 1H NMR
d
(400 MHz, CDCl3):
2H), 1.30 (s, 24H), 1.24 (m, 10H), 0.88 (t, 3H); 13C NMR (100 MHz,
CDCl3): 150.3, 136.2, 120.2, 83.7, 52.3, 32.0, 29.6, 27.6, 27.2, 25.1,
d 7.71 (d, 4H), 7.00 (d, 4H), 3.73 (t, 2H), 1.58 (m,
2.4.1.4. 4-Bromo-N-(4-bromophenyl)-N-(4-octylphenyl)phenylamine
(3). n-BuLi in hexane (9 mL, 2.5 M in hexane, 21 mmol) was added
dropwise over 30 min to a stirred solution of tris(4-bromophenyl)
amine (10 g, 21 mmol) in anhydrous THF (300 mL) at ꢀ78 ꢁC
under a nitrogen atmosphere. The solution was stirred for 1 h
at ꢀ78 ꢁC, and then 1-iodooctane (6 g, 25 mmol) was added in one
portion. The reaction mixture was stirred overnight and allowed to
slowly warm to room temperature. After the reaction mixture was
filtered, the solid was washed with petroleum ether (200 mL). The
combined organic layers were washed with water (100 mL), dried
over Mg2SO4, and evaporated to dryness. The crude product was
purified on silica gel column eluting with petroleum ether to give
the title compound as a colorless oil (2.8 g, 26%). 1H NMR
d
22.9, 14.3. Anal. calcd. for C32H49B2NO4: C, 72.06; H, 9.26; N, 2.63;
found: C, 72.12; H, 9.28; N, 2.52.
2.4.2. General procedures for the synthesis of polymers P1, P2, and
P3 by Suzuki polycondensation
A mixture of M1 (0.25 mmol), M2 (M3 or M4) (0.25 mmol),
NaHCO3 (0.3 g, 35.7 mmol), H2O (3 mL) and THF (20 mL) was
carefully degassed before and after 0.01 mmol Pd(PPh3)4 was
added under nitrogen atmosphere. The mixture was stirred and
refluxed for 2 days under nitrogen atmosphere. Phenylboronic acid
(10 mg, 0.08 mmol) was added; the reaction was further refluxed
for 1 h; then 1-bromobenzene (0.05 mL, 0.48 mmol) was added;
and the reaction was refluxed for another 1 h. The reaction mixture
was then allowed to cool to room temperature, chloroform
(100 mL) and water (100 mL) was added. The organic layer was
separated and the aqueous layer was extracted with chloroform
(50 mL ꢂ 2). The combined organic layers were concentrated to
30 mL and poured into acetone (200 mL), and the resulted
precipitate was collected by filtration. The crude product was dis-
solved in chloroform again and precipitated into acetone. This
operation was repeated for three times. The precipitate was
collected by filtration and dried under high vacuum to afford the
aimed polymer.
(400 MHz, CDCl3):
(m, 4H), 2.57 (t, 3H), 1.61 (m, 2H), 1.30 (m, 10H), 0.90 (t, 3H). 13C
NMR (100 MHz, CDCl3): 146.8, 144.5, 138.9, 132.3, 132.2, 129.6,
d 7.32 (m, 4H), 7.08 (d, 2H), 6.98 (d, 2H), 6.92
d
125.1, 115.1, 35.5, 32.0, 31.6, 29.6, 29.5, 29.4, 22.8, 14.2. Anal. calcd.
for C26H29Br2N: C, 60.60; H, 5.67; N, 2.72; found: C, 62.03; H, 5.83;
N, 2.64.
2.4.1.5. 4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-N-(4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-N-(4-octylphenyl)phe-
nylamine (M2). M2 was synthesized using the same procedure as
M1 as a colorless oil in a yield of 51%. 1H NMR (400 MHz, CDCl3):
d
7.66 (d, 4H), 7.03 (m, 8H), 2.57 (t, 2H), 1.58 (m, 2H), 1.30 (m, 34H),
0.88 (t, 3H); 13C NMR (100 MHz, CDCl3):
d
146.7, 144.4, 138.9, 132.2,
2.4.2.1. Polymer P1. Dark red solid was obtained (143 mg, 62%). 1H
129.5, 124.9, 124.5, 114.9, 35.4, 31.9, 31.5, 29.5, 29.4, 29.3, 22.7, 14.1.
Anal. calcd. for C38H53B2NO4: C, 74.89; H, 8.77; N, 2.30; found: C,
75.06; H, 8.78; N, 2.26.
NMR (400 MHz, CDCl3):
4.14 (m, 4H), 4.06 (m, 2H), 1.91 (m, 6H), 1.48 (m, 14H), 1.21 (m, 16H),
0.81 (m, 9H); 13C NMR (100 MHz, CDCl3):
150.6, 150.1, 149.8, 137.9,
d 8.46 (m, 8H), 7.35 (m, 4H), 7.13 (m, 4H),
d
133.1, 132.5, 130.7, 129.5, 125.7, 122.6, 116.4, 73.5, 31.5, 30.8, 29.5,
28.6, 25.1, 25.0, 22.1, 21.7, 13.1. Anal. calcd. for C56H67N3O3S3: C,
72.61; H, 7.29; N, 4.54; found: C, 72.64; H, 7.38; N, 4.38.
2.4.1.6. Bis(4-bromophenyl)amine (4). A solution of N-bromo-
succinimide (NBS) (10.53 g, 59.2 mmol) in DMF (50 mL) was added
dropwise over 30 min to a stirred solution of diphenylamine (5 g,
29.6 mmol) in DMF (50 mL) at 0 ꢁC. The resulting solution
continued to be stirred at 0 ꢁC for 6 h. Water was added and the
precipitate was filtered and dried under vacuum to afford the title
compound as a colorless solid (9.6 g, 99%). 1H NMR (400 MHz,
2.4.2.2. Polymer P2. Dark red solid was obtained (166 mg, 73%). 1H
NMR (400 MHz, CDCl3):
7.16 (m, 8H), 4.18 (m, 4H), 2.61 (m, 2H), 2.00 (m, 4H), 1.64 (m, 4H),
1.33 (m, 22H), 0.85 (m, 9H); 13C NMR (100 MHz, CDCl3):
151.6,
d 8.51 (m, 2H), 7.61 (m, 4H), 7.38 (m, 2H),
d
CDCl3):
d
7.37 (d, 4H), 6.91 (d, 4H), 5.63 (s, NH); 13C NMR
141.8, 132.4, 119.6, 113.5. Anal. calcd. for
150.9, 147.2, 145.5, 144.6, 138.8, 132.9, 131.9, 128.9, 127.8, 126.6,
125.3, 123.9, 123.6, 122.3, 117.4, 35.5, 31.9, 31.8, 31.5, 30.5, 29.6, 29.5,
29.4, 29.3, 29.2, 26.1, 22.7, 14.1. Anal. calcd. For C56H67N3O2S3: C,
73.88; H, 7.42; N, 4.62; found: C, 73.21; H, 7.40; N, 4.41.
(100 MHz, CDCl3):
d
C12H9Br2N: C, 44.07; H, 2.77; N, 4.28; found: C, 44.26; H, 2.89; N,
4.02.
2.4.1.7. 4-Bromo-N-(4-bromophenyl)-N-octylphenylamine (5). A solu-
tion of compound 4 (4 g, 12.2 mmol) in THF (100 mL) was degassed
before and after sodium hydride (0.38 g, 15.9 mmol) was added
under nitrogen atmosphere. The mixture was stirred for 1 h at
room temperature. 1-Bromooctane (2.83 g, 14.7 mmol) was added
and then the reaction was refluxed for 24 h. The reaction was
quenched by water and the organic layer was washed with brine
(100 mL) and separated. The combined organic layers were dried
2.4.2.3. Polymer P3. Dark red solid was obtained (116 mg, 56%). 1H
NMR (400 MHz, CDCl3): d 8.44 (d, 2H), 7.57 (d, 4H), 7.29 (d, 2H), 7.02
(d, 4H), 4.11(m, 4H), 3.70 (m, 2H),1.90 (m, 4H), 1.67 (m, 2H),1.44 (m,
8H), 1.26 (m, 22H), 0.80 (m, 9H); 13C NMR (100 MHz, CDCl3):
d
150.6, 150.0, 146.2, 144.7, 131.7, 130.9, 126.6, 125.8, 121.0, 120.1,
116.4, 73.4, 51.4, 30.8, 29.4, 28.6, 28.4, 28.3, 26.6, 26.1, 25.1, 21.7, 21.6,
13.1. Anal. calcd. for C50H63N3O2S3: C, 71.99; H, 7.61; N, 5.04; found:
C, 70.3; H, 7.69; N, 4.89.

328
M. Wang et al. / Polymer 53 (2012) 324e332
low molecular weight oligomers. The purified polymers were ob-
tained as dark red solids in yields of 62% for P1, 73% for P2, and 52%
for P3. The polymers display good solubility in chloroform, chlo-
robenzene (CB), 1,2-dichlorobenzene (DCB), tetrahydrofuran (THF),
1,2,4-trichlorobenzene (TCB), etc. at elevated temperature.
Number- and weight-average molecular weights (Mn and Mw) and
polydispersity indexes (PDI) of polymers were measured by gel
permeation chromatography (GPC) with THF as an eluent and
calibrated with polystyrene standards. The Mn of P1, P2 and P3
were 97,000, 5800 and 5400 kg/mol, with the corresponding PDI of
1.54, 2.24 and 2.49, respectively. The thermal behaviors of the
copolymers under nitrogen atmosphere were characterized by
thermogravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC). As shown in Fig. 1, all three polymers exhibited good
thermal stability and are suitable for application in PSCs. The
molecular weights and the thermal properties of the polymers are
listed in Table 1.
Fig. 1. TGA plots of P1, P2 and P3 with a heating rate of 10 ꢁC/min in the nitrogen.
3.2. Optical and electrochemical properties
The UVevis absorption spectra of the polymers both in
chloroform solution and in the solid state at 20 ꢁC are shown in
Fig. 2. In chloroform solutions, the three polymers all exhibit two
distinct absorption bands. The band located at about 382 nm is
Table 1
Polymerization results and thermostability of polymers.
Polymer
Yield
(%)
Mn
Mw
PDI
Td
Tg
(g/mol)a
(g/mol)a
(ꢁC )b
(ꢁC )c
corresponding to the
pep transition of their conjugated back-
P1
P2
P3
62
73
56
97,000
5800
5400
150,000
13000
13500
1.54
2.24
2.49
329
276
331
109
53
142
bones. The peaks at longer wavelength are at 525, 515, 525 nm for
P1, P2 and P3, respectively, which can be attributed to strong
intermolecular charge transfer (ICT) interaction between the
donor and the acceptor units. Compared with P2, the maximum
absorption band of P1 shows a slight red-shift of 10 nm, which
might be due to the incorporation of the alkoxy chains in the end
of TPA as a stronger donor resulting in stronger ICT interaction. For
P3, the introduction of diphenylamine units into the polymer
backbone has no obvious influence on the absorption spectra in
solution in contrast to that of P1. The absorption bands of these
three polymers all red-shift and become broader when they go
from the solutions to the solid states. The maximum absorption
bands in films are 558, 550 and 562 nm for P1, P2 and P3,
respectively. The red shift that occurs from the solution to the film
state is attributed to the aggregation of the polymer chains or the
a
Mn, Mw and PDI of polymers were determined by GPC using polystyrene stan-
dards with THF as eluent.
b
Decomposition temperature determined by TGA in N2 based on 5% weight loss.
Glass transition temperature determined by DSC in N2 at a heating rate of 10 ꢁC/
c
min.
3. Results and discussion
3.1. Synthesis and characterization
The synthetic routes toward polymers P1, P2, and P3 are out-
lined in Scheme 1. Benzothiadiazole based monomer (M1) and TPA-
based diboronic ester monomers (M2, M3 and M4) were synthe-
sized according to the modified literature procedures [34]. Three
polymers were synthesized by Suzuki polycondensation between
TPA-based monomer and M1 in a biphasic mixture of THF/aqueous
NaHCO3 with Pd(PPh3)4 as the catalyst precursor. After polymeri-
zation, the obtained polymers were dissolved in a small amount of
chloroform and precipitated into acetone for three times to remove
orderly
pep stacking formed in the solid states. The absorption
red-shifts for P1, P2 and P3 are 29, 34 and 36 nm, respectively.
Larger red-shift for P3 can be attributed to the replacement of
phenyl group in TPA unit by alkyl chains to deduce the twist of the
polymer backbone and increase the aggregation effect in solid
state. The onsets of the absorption spectra in films are 624, 628
a
b
1,0
1,0
0,8
0,6
0,4
0,8
0,6
0,4
WE 1 in film
0,2
WE1 in solution
0,2
WE 2 in film
WE 3 in film
0,0
WE2 in solution
WE3 in solution
0,0
400
500
600
700
400
500
600
700
Wavelength(nm)
Wavelength(nm)
Fig. 2. UVevis absorption spectra of P1, P2, and P3 in chloroform solutions (a) and in films (b).

M. Wang et al. / Polymer 53 (2012) 324e332
329
Table 2
respectively. The related electrochemical parameters, HOMO
and LUMO energy levels are summarized in Table 2. P2 shows lower
lying HOMO energy levels than P1, which is attributed to alkyl
groups with weaker electron-donating ability than alkoxy groups
for P1. P3 exhibited higher HOMO energy level among the three
polymers, which was due to the weaker electron delocalization
ability of alkyl chains than the phenyl group in P1 and P2. Lower
lying HOMO energy levels are expected to a higher Voc, since Voc
linearly depends on the difference between the HOMO energy level
of the donor polymer and the LUMO energy levels of the acceptor.
Therefore, higher Voc for P2 will be expected in solar cells compared
to the other two polymers. Furthermore, three polymers all posses
high LUMO energy levels, which can ensure enough driving force
for charge separation.
Electrochemical and optical properties of the polymers.
polymer
In solution
In film
Eg,opt
(eV)
HOMO
(eV)
LUMO
(eV)
lmax [nm]
lmax [nm]
lonset [nm]
P1
P2
P3
383, 529
378, 516
383, 526
388, 558
388, 550
388, 562
624
628
640
1.99
1.97
1.93
e5.29
e5.31
e5.13
e3.30
e3.34
e3.20
and 640 nm for P1, P2 and P3, respectively, corresponding to the
optical band gaps of 1.99, 1.97 and 1.93 eV. The absorption
maximum in solutions and in films and Eg,opt of the polymers are
summarized in Table 2.
The positions of the HOMO and LUMO energy levels have
pronounced influence on Voc, the driving force for charge separa-
tion, and the overall photovoltaic performance. Thus, cyclic vol-
tammetry (CV) was used to determine the electrochemical
properties of polymers. From the oxidation potentials obtained
in the electrochemical measurements, the HOMO energy level can
be estimated according to the equation EHOMO ¼ ꢀe(Eox þ 4.71)
(eV), where Eox is the onset oxidation potential in volts vs Ag/Agþ.
For P1, P2 and P3, the HOMO energy levels are calculated to
be ꢀ5.29, ꢀ5.31 and ꢀ5.13 eV, respectively, according to the
onset oxidation potentials. Because the reduction potentials could
hardly be obtained, the LUMO energy levels often are difficult
to determine. Therefore, with the combination of the optical
ban gap and the HOMO energy levels, the LUMO energy levels
of the resulting polymers can be calculated according to the
equation ELUMO ¼ EHOMO þ Eg,opt. The LUMO energy levels of three
polymers are ꢀ3.30, ꢀ3.34 and ꢀ3.20 eV for P1, P2 and P3,
3.3. Transport properties
The transport properties of these three polymers were investi-
gated by fabricating organic thin film field-effect transistors
(OFETs). Typical p-channel field-effect transistor behavior was
obtained. The hole mobility (
regime from the derivative plots of the square root of sourceedrain
current (ISD versus gate voltage (VG) through equation
m) was estimated in the saturated
)
ISD ¼ ðW=2LÞCimðVG ꢀ VTÞ2 where W is the channel width, L is the
channel length, Ci is the capacitance per unit area of the gate
dielectric layer (SiO2, 500 nm, Ci ¼ 7.5 nF/cm2), and VT is the
threshold voltage. A hole mobility of 8.21 ꢂ10ꢀ3 cm2 Vꢀ1 sꢀ1 is
observed for P1 based OFETs. The hole motilities of P2 and P3 are
6.20 ꢂ 10ꢀ4 and 6.98 ꢂ 10ꢀ4 cm2 Vꢀ1 sꢀ1, respectively, which are
one order of magnitude lower than P1. The output curves are
-7
2.5x10
-6
6.0x10
P2
P1
-100 V
-7
-100 V
-6
2.0x10
5.0x10
-6
4.0x10
-80 V
-80 V
-7
1.5x10
-6
3.0x10
-7
1.0x10
-6
-60 V
2.0x10
-60 V
-8
-6
5.0x10
1.0x10
-40 V
-40 V
-20, 0, 10 V
0.0
-20, 0, 10V
0.0
0
-20
-40
-60
(V)
-80 -100
0
-20
-40
-60
-80 -100
V
(V)
V
-8
-8
-8
-8
-8
-8
-9
3.5x10
P3
-100 V
3.0x10
2.5x10
2.0x10
1.5x10
1.0x10
5.0x10
-80 V
-60 V
-40 V
-20,-10,0 V
-80 -100
0.0
0
-20
-40
-60
(V)
V
Fig. 3. The output curves at different gate voltages (VG) for OFETs using spin-coated P1, P2 and P3 on OTS-treated Si/SiO2 substrates.

330
M. Wang et al. / Polymer 53 (2012) 324e332
shown in Fig. 3. The transfer curves of the spin-coated film of P1, P2
and P3 on OTS-treated Si/SiO2 substrates are shown in Fig S1 in the
Supporting Information.
3.4. Photovoltaic properties
Bulk heterojunction photovoltaic devices were fabricated.
PC71BM was chosen as the acceptor because it has similar electronic
properties as PC61BM, but a higher absorption coefficient in the
visible region with a broad peak from 440 to 530 nm, which
compensates the absorption valley of the polymers. The solvent
used for device fabrication is important because the solubility of
the polymers has a pronounced influence on the blend film
morphology that affects charge separation and transport in devices.
To optimize the morphology of the active layers, solar cells were
fabricated from the blends of each polymer and PC71BM in 1,2-DCB,
CB, and TCB solutions, respectively. The optimized weight ratio
between polymer and PC71BM is 1:3. The photovoltaic parameters
of the solar cells and the thicknesses of the corresponding active
layers are listed in Table 3.
The devices of P1:PC71BM ¼ 1:3 fabricated from TCB or CB
solution both show PCE of 2.1% with Voc of 0.76e0.77 V, Jsc of
5.4e5.7 mA/cm2 and fill factor (FF) of 0.46e0.50. Slightly higher
PCE of 2.3 % with a Voc of 0.77 V, Jsc of 6.9 mA/cm2, and FF of 0.43 is
achieved by the device fabricated from P1:PC71BM ¼ 1:3 in DCB
solution. PCEs of solar cells fabricated from P2:PC71BM ¼ 1:3 in TCB,
CB, and DCB solution are 2.0, 2.1 and 2.1%, respectively. It is striking
that the performance of the solar cells are not extremely sensitive
to the solvents, which can simplify the preparation of devices and it
is unnecessary to select one kind of the solvent strictly. Such
insensitivity might be due to the two-dimensional structure of TPA-
containing polymers P1 and P2, since the two-dimensional struc-
ture can suppress the aggregation of polymer chains to some extent
and endow the polymers with better solubility in solutions. PCEs
of the devices fabricated from P3:PC71BM ¼ 1:3 in CB and DCB
solution are 1.7 and 1.8%, respectively. The film spin-coating from
P3:PC71BM ¼ 1:3 in TCB solution show extremely rough and large
visible particles, which might be due to the poor solubility of
P3 in TCB solution. Therefore, the device fabricated from
P3:PC71BM ¼ 1:3 in TCB solution provided unsatisfactory results.
Noticeably, the solar cell fabricated from P2:PC71BM ¼ 1:3 in DCB
solution exhibits higher Voc of 0.80 V than that of P1 with 0.77 V
and P3 with 0.71 V, which is consistent with its deeper HOMO
energy level among these three polymers. Fig. 4a shows the current
densityevoltage (JeV) curves of the three polymer devices
fabricated from DCB solution. The device fabricated from
P1:PC71BM ¼ 1:3 in DCB solution shows the highest PCE of 2.3%
Fig. 4. JeV curves (a) and EQE (b) of PSCs fabricated from P1:PC71BM ¼ 1:3,
P2:PC71BM ¼ 1:3 and P3:PC71BM ¼ 1:3 in DCB solutions.
although P1 possesses medium Voc of 0.77 V among these three
polymers. It is obvious that higher Jsc for P1 accounts for the better
photovoltaic performance. Jsc can be improved mainly from the
increased light absorption, the good hole mobility of the polymer
for charge transfer and optimal film nanoscale morphology for
charge separation and transport. Obviously, all polymer films have
similar absorption spectra and intensity with almost superimposed
absorption profiles, higher Jsc for P1 was attributed to, at least
partially, the higher hole mobility of P1 than that of P2 and P3. To
evaluate the accuracy of measurements, the external quantum
efficiency (EQE) curves of the PSC devices fabricated from the blend
of the corresponding polymer and PC71BM in DCB solution were
measured. EQE curves are shown in Fig. 4b and present shallower
valleys from 400 to 500 nm compared to the corresponding
absorption spectra (Fig. 2b), which indicates PC71BM compensates
the absorption valley of the polymers and the contribution from
PC71BM to the photocurrent is very pronounced. PSC devices based
on three polymers all exhibited excellent response in their
absorption range from 350 to 750 nm. The highest EQE value of the
devices reached 48% at about 490 nm for P1 based PSCs. All current
intensity (Jsc) values calculated from the integration of the EQEs of
the devices agree well with the Jsc values obtained from the JeV
measurements.
Table 3
Summary of the photovoltaic properties of BHJ PSCs based on P1, P2, and P3.
The active layer
Solvent
Thickness
(nm)
Voc (V)
Jsc
FF
PCE
(%)
(mA/cm2)
P1:PC71BM
CB
DCB
TCB
DCB
90
75
50
70
0.76
0.77
0.77
0.68
5.7
6.9
5.4
7.8
0.46
0.43
0.50
0.53
2.1
2.3
2.1
2.8
(1% DIO)
CB
DCB
TCB
DCB
(2% DIO)
CB
DCB
P2:PC71BM
P3:PC71BM
100
85
60
0.83
0.80
0.82
0.71
5.3
5.5
4.7
5.8
0.46
0.48
0.52
0.61
2.1
2.1
2.0
2.5
75
80
65
65
0.68
0.71
0.68
5.3
4.9
4.6
0.48
0.52
0.63
1.7
1.8
2.0
DCB
(2% DIO)

M. Wang et al. / Polymer 53 (2012) 324e332
331
a
2
0
b
P1 DIO 1%
P2 DIO 2%
P3 DIO 2%
60
50
40
30
20
10
0
P1 DIO 1%
P2 DIO 2%
P3 DIO 2%
-2
-4
-6
-8
-0.2
0.0
0.2
0.4
0.6
0.8
400
500
600
700
800
Wavelength/nm
Voltage [V]
Fig. 5. JeV curves (a) and EQE (b) of the PSCs fabricated from P1:PC71BM ¼ 1:3, P2:PC71BM ¼ 1:3 and P3:PC71BM ¼ 1:3 in DCB solution containing DIO.
Processing additives have considerable influence on the
photovoltaic performance. 1,8-Diiodooctane (DIO), which is
a commonly used processing additive, was chosen to optimize the
morphology and expected to enhance the photovoltaic perfor-
mance of the resulting PSCs [4,38e40]. The photovoltaic parame-
ters of solar cells are shown in Table 3. With the addition of DIO, the
devices fabricated from the blend of each polymer and PC71BM in
DCB solution all show slight enhanced photovoltaic performance.
For P1, the PCE is improved from 2.3 to 2.8 % with Voc of 0.68 V, Jsc of
7.8 mA/cm2 and FF of 0.53. PCE of the device from P2 based PSCs is
increased from 2.1 to 2.5%. Both the devices based on P1 and P2
exhibit improved PCEs, which is attributed to the increased Jsc and
FF with the addition of the additive DIO. For P3 based PSCs, there is
a slight decrease for Jsc although the PCE is increased a little from
2.1 to 2.3%. The JeV curves of these devices are shown in Fig. 5a. EQE
of the devices fabricated from the blend of each polymer and
PC71BM in DCB solution containing the additive DIO is also shown
in Fig. 5b. The high EQE of 56% for P1 was observed at 477 nm,
which reflects higher Jsc for P1 based PSCs.
The morphology of the blend films spin-coated from the poly-
mers and PC71BM in DCB solution without or with DIO was inves-
tigated by tapping-mode atomic force microscopy (AFM). DIO has
a higher boiling point than the host solvent, PCBM tends to remain
in solution (during drying) longer than the blend polymer, since the
Fig. 6. AFM images (5 ꢂ 5
P1, d for P2 and f for P3).
m
m2) of the active layers from the blend of the polymer and PC71BM (polymer: PC71BM ¼ 1:3) without (a for P1, c for P2 and e for P3) and with DIO (b for

332
M. Wang et al. / Polymer 53 (2012) 324e332
fullerenes are selectively dissolved in DIO, which can realize the
control of the phase separation and the resulting morphology of the
BHJ material [4,38e40]. As shown in Fig. 6, all three blend films
from DCB, (P1 (a), P2 (c) and P3 (e)) show very smooth surfaces and
no large domains can be observed, which indicate no obvious phase
separations occur. It is well known that a certain degree of phase
separation is necessary for providing a percolation pathway for
charge transport to the electrodes. The root-mean-square (rms)
roughness for the polymers and PC71BM blend films are 0.940,
0.423 and 0.785 nm for P1, P2 and P3, respectively. Therefore, it can
be concluded that the designed polymers P1, P2 and P3 possess the
optimized geometry. The incorporation of TPA group can construct
the two-dimensional architecture and would create large free space
between the polymer chains, where fullerene molecules can be
distributed well over the polymer network, thus no evident phase
separation was observed. With the addition of DIO, the images
exhibited rms has been increased to 2.38 and 2.63 nm from 0.94
and 0.423 nm for P1 (b) and P2 (d), respectively. The increasing of
the roughness indicates the phase separation takes place, which
reduces the recombination of charges, leading to higher Jsc and FF
for P1 and P2 based PSCs with the addition of DIO. For P3 (f), the
image of the blend film with DIO exhibited a rougher film with rms
of 2.92 nm in contrast to the rms of 0.785 nm without the additive.
Such large phase separation can decrease the interface for exciton
dissociation, which caused a little drop of Jsc. The increase of FF
compensates the loss of Jsc resulting in the slight improvement of
the photovoltaic performance for P3 based PSCs. An inter-
penetrating bicontinuous network between polymer and PCBM
with an ideal domain size of 10e20 nm is desirable for high-
performance PSCs. Both larger and smaller phase separations are
not favorable for efficient charge separation and transportation.
Large phase separation increases the distance to the nearest
interface where the exciton may dissociate before they recombine,
which leads to low photocurrent. While, smaller domain sizes
between polymer and PCBM increase the possibility of charge
recombination. Only appropriate phase separation can balance
charge generation and recombination and result in the optimized
photovoltaic performance. The influence of DIO on Voc is more
pronounced for P1 and P2 (Voc decrease almost 0.1 V) than P3 (only
reduced by 0.03 V), which indicates that P3 is less sensitive to
processing additive.
2009CB623603) F.Z. thanks the financial support for the collabo-
ration with ICCAS from Swedish Research Council (VR) and for
visiting Fraunhofer ISE from VINNOVA (VINNMER).
Appendix. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.polymer.2011.12.026.
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4. Conclusion
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We have successfully designed and synthesized a series of
alternating copolymers based on benzothiadiazole and triphenyl-
amine (TPA) derivatives: P1, P2 and P3. These three polymers
all possess medium optical band gaps, which are 1.99, 1.97
and 1.93 eV, resꢀp1ectively. The field-effect hole mobilities are
8.21 ꢂ10ꢀ3 cm2 V sꢀ1 for P1, 6.20 ꢂ 10ꢀ4 cm2 Vꢀ1 sꢀ1 for P2, and
6.98 ꢂ 10ꢀ4 cm2 Vꢀ1 sꢀ1 for P3. PCEs of higher than 2.0% have been
obtained for photovoltaic cells fabricated from P1, P2, and P3
blended with PC71BM. The highest PCE of 2.8% for P1 with short
circuit current (Jsc) of 7.8 mA/cm2 was achieved. The photovoltaic
performance of P1 and P2 is not sensitive to the choice of the blend
solvent due to the incorporation of two-dimensional TPA units. This
study offers a useful and important insight for designing TPA
derivative-based polymers for efficient PSCs.
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Acknowledgement
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1783.
We thank the financial support by the NSF of China (20834006
and 50821062) and 973 Program (2011CB935703 and