Synthesis and photovoltaic properties of two-dimensional benzodithiophene-thiophene copolymers with pendent rational naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole side chains

Article abstract of DOI:10.1039/c5ta07205k

A series of new two-dimensional copolymers, PBDTT-TABT, PBDTT-TANT and PBDTT-TSNT, with conjugated benzodiathiazole (BT) or naphthobisthiadiazole (NT) side chains were successfully synthesized and characterized for high performance polymer solar cells (PSCs). The NT unit showed a stronger electron-withdrawing ability and a larger conjugation than the BT unit, which induced a stronger ICT process between the benzodithiophene (BDT)-thiophene (T) backbone and the conjugated side chain for NT containing copolymers. As a result, PBDTT-TANT and PBDTT-TSNT showed lower-lying bandgaps and more red-shifted absorption than PBDTT-TABT. The alkylthio modification effect of conjugated side chains was also investigated in this work. This effect showed a positive role in lowering the HOMO energy level, and a negative role in elevating carrier mobility and molecular stacking properties in our two-dimensional polymeric systems. Bulk heterojunction (BHJ) PSCs were fabricated using these copolymers as the donor materials to evaluate their photovoltaic properties. The PBDTT-TABT, PBDTT-TANT and PBDTT-TSNT devices exhibited PCEs of 4.60%, 5.65% and 4.01%, respectively. In spite of the V_oc values, the highest J_sc, FF and PCE were achieved for the PBDTT-TANT device, which was attributed to its red-shifted absorption, improved carrier mobility and well-defined phase separation. It is interesting that the J_scs, FFs and PCEs of all these devices were elevated significantly when made using the solvent vapor annealing (SVA) method. The THF-SVA process would provide a driving force to facilitate the formation of a much more well-defined surface morphology, resulting in the enhanced J_sc and FF values. The PBDTT-TANT device showed the highest PCE of 8.04%, which is the top efficiency for this type of two-dimensional copolymer with donor (D)-donor (D) polymer backbones and donor (D)-acceptor (A) conjugated side chains. Our design strategy would give an instructive guide to developing high performance two-dimensional polymer donors to be used in organic photovoltaic applications.

Full text of DOI:10.1039/c5ta07205k

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Synthesis and Photovoltaic Properties of Two-dimensional
Benzodithiophene-thiophene Copolymers Pendant with Rational
Naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole Side Chains
Xiaopeng Xu, Kui Feng, Kai Li, Qiang Peng*
Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of
Chemistry, and State Key Laboratory of Polymer Materials Engineering, Sichuan University,
Chengdu 610065, P. R. China
*To whom correspondence should be addressed: Tel: +86ꢀ28ꢀ86510868; fax: +86ꢀ28ꢀ86510868;
eꢀmail: qiangpengjohnny@yahoo.com

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Abstract
A series of new twoꢀdimensional copolymers, PBDTTꢀTABT, PBDTTꢀTANT and
PBDTTꢀTSNT, with conjugated benzodiathiazole (BT) or naphthobisthiadiazole (NT)
side chains were successfully synthesized and characterized towards high
performance polymer solar cells (PSCs). The NT unit showed a stronger
electronꢀwithdrawing ability and a larger conjugation than those of BT unit, which
induced a stronger ICT process between the benzodithiophene (BDT)ꢀthiophene (T)
backbone and the conjugated side chain for NT containing copolymers. As a result,
PBDTTꢀTANT and PBDTTꢀTSNT showed the lowerꢀlying bandgaps and the more
redꢀshifted absorption than those of PBDTTꢀTABT. The alkylthio modification effect
of conjugated side chains was also investigated in this work. This effect just showed a
positive role in lowering the HOMO energy level, whereas a negative role in elevating
carrier mobility and molecular stacking property in our twoꢀdimensional polymeric
system. Bulk heterojunction (BHJ) PSCs were fabricated using these copolymers as
the donor materials to evaluate their photovoltaic properties. PBDTTꢀTABT,
PBDTTꢀTANT and PBDTTꢀTSBT devices exhibited PCEs of 4.60%, 5.65% and
4
.01%, respectively. Despite of Voc, the highest Jsc, FF and PCE were achieved for
PBDTTꢀTANT device, which was attributed to its redꢀshifted absorption, improved
carrier mobility and wellꢀdefined phase separation. It is interesting that the J s, FFs
sc
and PCEs of all these devices were elevated significantly when using a solvent vapor
annealing (SVA) method. The THFꢀSVA process would provide a driving force to
facilitate to form a much more wellꢀdefined surface morphology, resulting in the

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enhanced Jsc and FF values. PBDTTꢀTANT device showed the highest PCE of 8.04%,
which is the top efficiency for this type of twoꢀdimensional copolymers with donor
(D)ꢀdonor (D) polymer backbones and donor (D)ꢀacceptor (A) conjugated side chains.
Our design strategy would give an instructive meaning in developing high
performance twoꢀdimensional polymer donors used in organic photovoltaic
applications.
Keywords: Polymer solar cells, Twoꢀdimensional copolymers, Conjugated side
chains, Naphthobisthiadiazole, Solvent vapor annealing

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1
. Introduction
Polymer solar cells (PSCs) have attracted considerable attention in the photovoltaic
field because of their potential advantages of low cost, light weight, flexible
1
fabrication and lowꢀtemperature solution processing technique. To improve the
power conversion efficiency (PCE), significant efforts have been devoted to
2
1a,3
increasing the open circuit voltage (V ), short circuit current (J ), and fill factor
oc
sc
4
5
(
FF). After the bulk heterojunction (BHJ) architecture was introduced into PSCs, the
PCE had promptly increased in the past few years from 1% to 10% by designing
6
polymer structures and optimizing device fabrication technologies. Commonly, the
active layer of BHJ PSCs contains a phaseꢀseparated blend of a donor (D) material
1d,5
and an acceptor (D) material.
In most cases, fullerene derivatives were chosen as
the acceptor material, such as [6,6]ꢀphenylꢀC61ꢀbutyric acid methyl ester (PC61BM) or
[
6,6]ꢀphenylꢀC71ꢀbutyric acid methyl ester (PC71BM), which showed relatively weak
7
lightꢀharvest ability in visible light range. Because most of solar flux will be
harvested by the donor materials, research efforts on developing new conjugated
polymer donors are still the main streams in this field.
Most of the successful polymer donors in PSCs featured a linear and rigid main
1
c,1d
chain system with flexible side chains.
At the same time, twoꢀdimensional
conjugated polymers with conjugated side chains had always been received the
8
extensive concern of the academia. The conjugated side chains would extend the
conjugation of the polymer donors, thereby improving the absorption, charge
9
transport, and their device performance. In addition, it could also facilely adjust the

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10
energy levels of the pristine copolymers via different conjugated bridges, and
8
b
enhance the miscibility with fullerene acceptors in BHJ device fabrication. So far,
the twoꢀdimensional polymer donors usually possess some kinds of typical polymeric
9
a
8a,8b,8d,9
8c,8e
backbones, such as donor (D) , donorꢀdonor (DꢀD),
donorꢀacceptor (DꢀA)
main chain structures. Li et al. first designed and synthesized a series of
twoꢀdimensional polymer donors using polythiophenes (PT) as the backbone and
9
a
8a,9b
variable conjugated side chains of bi(phenylenevinylene) , bi(thienylenevinylene)
,
9c
and
phenothiazinevinylene ,
exhibiting
broadened
absorption
spectra,
twoꢀdimensional chargeꢀtransport properties, and enhanced device performance. Hou
et al. first attached two thiopheneꢀconjugated side chains on the skeleton of
benzo[1,2ꢀb:4,5ꢀb’]dithiophene (BDT), and synthesized a twoꢀdimensional polymer
8c
donor with a DꢀA polymeric backbone. This strategy has already been employed to
6d,11
constructing highꢀperformance polymer donor materials in PSCs.
Tan et al.
reported a twoꢀdimensional BDTꢀT copolymer with a DꢀD polymeric backbone,
PTG1, containing 4,7ꢀdithienꢀ5ꢀylꢀ2,1,3ꢀbenzodiathiazole (DTBT) as the conjugated
8
d
side chains. PTG1 showed a high hole mobility and a moderate PCE of 4.32%. In
order to reduce the bandgap and enhance the absorbance of this type of polymer
donors based on BDTꢀT backbones, isoindigo (ID) unit was introduced instead of
benzodiathiazole (BT) unit to develop a new copolymer of PBDTꢀTID, leading to an
1
2
enhanced PCE of 6.51% in an inverted device. Recently, Li et al. used fluorine
substituted BT instead of BT to decrease HOMO energy levels and increase the
interchain interaction of PTG1, and synthesized a twoꢀdimensional polymer of

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P(BDTꢀTBTF). P(BDTꢀTBTF) exhibited an elevated PCE of 6.21% compared to
1
3
PTG1.
In this work, we designed and synthesized a series of new twoꢀdimensional
BDTꢀT copolymers, PBDTTꢀTABT, PBDTTꢀTANT and PBDTTꢀTSNT, with
conjugated BT or naphtho[1,2ꢀc:5,6ꢀc]bis[1,2,5]thiadiazole (NT) side chains (Fig. 1).
As is known for acceptor moiety, NT unit is made up of two 1,2,5ꢀthiadiazole rings
fused in the skeleton of central naphthalene, which possesses a larger planarity and a
14
stronger electronꢀwithdrawing ability than those of BT unit. The introduction of NT
instead of BT would be expected to reduce the bandgap and extend the absorption of
the resulting BDTꢀT copolymers. On the other hand, alkylthio modification have been
proved to be an effective approach to further lower the HOMO energy levels and
6
a,15
elevate the device performance of BDTꢀbased copolymers in PSCs.
Thus, the
alkylthio modified thiophene side chains were also introduced on the skeleton of BDT
for comparison in this work. As expected, PBDTTꢀTANT and PBDTTꢀTSNT with NT
side chains showed an enhanced absorption than PBDTTꢀTABT with BT side chains.
However, PBDTTꢀTABT exhibited a deeper HOMO energy level than those of the
other two polymers. Overall, PBDTTꢀTANT with alkyl modified thiophene side
chains showed the best PCE of 8.04% from its conventional PSCs after the solvent
vapor annealing (SVA) treatment. The design strategy and results in this work would
give an instructive meaning in developing high performance twoꢀdimensional
polymer donors in organic photovoltaic applications.
2
. Experimental section

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2
.1 Materials
All reagents were purchased from Aladdin, Alfa Aesar, and Aldrich Co., and used as
16
received without further purification. 2ꢀ(2ꢀHexyldecyl)thiophene (3a),
16
5
ꢀ(2ꢀhexyldecyl)ꢀ2ꢀ(tributylstannyl)thiophene
(4a),
1,4ꢀdibromoꢀ2,1,3ꢀ
1
7
14
benzothiadiazole (5a), 4,9ꢀdibromonaphtho[1,2ꢀc:5,6ꢀc’]bis[1,2,5]thiadiazole (5b),
8
a
(
2,5ꢀdibromothiophenꢀ3ꢀylmethyl)phosphonic
,8ꢀdi(2,3ꢀdidecylthiophenꢀ5ꢀyl)ꢀbenzo[1,2ꢀb:4,5ꢀb’]dithiophene
synthesized according to previously reported procedures.
-[(2-hexyldecyl)thio]thiophene (3b). Under an argon protection, a solution of
acid
diethyl
ester,
1
8
4
(10),
were
2
nꢀbutyllithium (20.8 mL, 50 mmol, 2.4 M in hexane) was added dropwise into
o
thiophene (4.2 g, 50 mmol) in 100 mL of dry THF at 0 C. After the mixture was
o
stirred at 0 C for 1.5 h, sulfur powder (1.6 g, 50 mmol) was added in one portion, and
o
the resulting suspension was then stirred at 0 C for another 2 h. Subsequently, 2ꢀ
hexyldecylbromide (15.2 g, 50 mmol) was added dropwise into above solution. The
reaction mixture was stirred overnight at room temperature. After the reaction was
finished, iceꢀwater containing NH Cl was added to the reactant. The mixture was then
4
extracted with diethyl ether for three times. The combined organic layers were washed
4
with water for another three times, and dried over anhydrous MgSO . After the
removal of solvent, purification was carried out by column chromatography (silica gel;
eluent: petroleum ether) to afford compound 3b (13.90 g, yield: 82%) as colorless oil.
1
H NMR (400 MHz, CDCl , δ/ppm): 7.28ꢀ7.26 (m, 1H, ArH), 7.07ꢀ7.06 (m, 1H, ArH),
3
7
2
.96ꢀ7.94 (m, 1H, ArH), 2.80ꢀ2.78 (d, 2H, J = 15.8 Hz, CH ), 1.59ꢀ1.53 (m, 1H, CH),

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1
3
1
.40ꢀ1.25 (m, 24H, CH
δ/ppm): 136.02, 132.63, 128.54, 127.35, 44.24, 39.63, 39.54, 37.63, 32.82, 32.55,
1.96, 31.84, 29.8,7 29.69, 29.51, 29.30, 26.68, 26.56, 22.74, 14.23. Anal Calcd for
C H S (%): C, 70.52; H, 10.65. Found (%): C, 70.33; H, 10.50.
2 3 3
), 0.87ꢀ0.84 (m, 6H, CH ). C NMR (100 MHz, CDCl ,
3
20
36 2
2-[(2-hexyldecyl)thio]-5-tributhylthiophene (4b). Under an argon atmosphere,
compound 3b (1.7 g, 5.0 mmol) was dissolved in anhydrous THF (20 mL) in a
threeꢀneck flask. Then, a solution of nꢀBuLi (3 mL, 7.5 mmol, 2.5 M in hexane) was
added dropwise with stirring. After the addition was finished, the reaction mixture
was stirred for 0.5 hours. Sn(nꢀBu) Cl (2.44 g, 7.5 mmol) was subsequently added in
3
one portion to the reactant. The reaction mixture was stirred overnight at room
temperature. After that, the reaction mixture was poured into water and extracted
twice with dichloromethane. The combined organic layers were dried over anhydrous
4
MgSO , and the filtrate was concentrated to afford the yellow product. The crude
product was purified by column chromatography (silica gel; eluent: petroleum ether)
1
to afford compound 4b (2.61 g, yield: 83%) as yellowish oil. H NMR (400 MHz,
CDCl
15.8 Hz, CH
CH ), 0.90ꢀ0.87 (m, 15H, CH
35.60, 132.94, 37.69, 32.90, 31.95, 31.88, 29.94, 29.62, 29.59, 29.37, 28.96, 27.27,
3
, δ/ppm): 7.15ꢀ7.14 (m, 1H, ArH), 7.01ꢀ7.00 (m, 1H, ArH), 2.82ꢀ2.80 (d, 2H, J
), 1.60ꢀ1.59 (m, 1H, CH), 1.56ꢀ1.54 (m, 4H, CH ), 1.36ꢀ1.07 (m, 38H,
, δ/ppm): 141.09, 140.91,
=
2
2
1
3
2
3 3
). C NMR (100 MHz, CDCl
1
2
6
6.50, 26.47, 22.73, 22.71, 14.15, 13.67, 10.84. Anal Calcd for C32
1.04; H, 9.92. Found (%): C, 61.22; H, 9.98.
62 2 n
H S S (%): C,
Compound 6a. Compound 4a (2.03 g, 3.4 mmol) and compound 5a (1.0 g, 3.4

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mmol) were dissolved in 80 mL toluene, and the mixture was deoxygenated with
argon gas for 30 min. The catalytic amount of Pd(PPh was then added into the
3 4
)
reactant, and the mixture was stirred at 120 °C for 12 h. The mixture was poured into
water and extracted with dichloromethane for three times. The combined organic
4
layers were then dried over anhydrous MgSO . After concentration, the crude product
was purified by column chromatography (silica gel; eluent: petroleum ether:
1
dichloromethane = 4:1) to afford compound 6a (0.35 g, yield: 20%) as a red solid. H
NMR (400 MHz, CDCl
H, J = 19.4 Hz, ArH), 7.64ꢀ7.62 (d, 1H, J = 19.3 Hz, ArH), 6.84ꢀ6.80 (d, 1H, J = 9.1
Hz, ArH), 2.81ꢀ2.80 (d, 2H, J = 16.7 Hz, th CH ), 1.75ꢀ1.59 (m, 1H, CH), 1.35ꢀ1.26
). C NMR (100 MHz, CDCl , δ/ppm): 153.81,
51.72, 147.11, 136.04, 132.35, 128.26, 127.47, 126.58, 125.05, 112.40, 40.11, 34.62,
3.24, 33.26, 30.07, 29.78, 29.64, 29.32, 26.67, 22.75, 14.29. Anal. Calcd for
3
, δ/ppm): 7.94ꢀ7.93 (d, 1H, J = 9.1 Hz, ArH), 7.81ꢀ7.80 (d,
1
2
1
3
(
m, 24H, CH
2
), 0.87ꢀ0.84 (m, 6H, CH
3
3
1
3
C H ON Br (%): C, 59.87; H, 7.15; N, 5.37. Found (%): C, 60.58; H, 7.36; N, 5.54.
26
37
2
Compound 6b. Compound 6b was synthesized as a red solid according to the
1
similar route as that for compound 6a with a yield of 35%. H NMR (400 MHz,
CDCl
3
, δ/ppm): 9.02 (s, 1H, ArH), 8.85 (s, 1H, ArH), 8.15ꢀ8.14 (d, 1H, J = 9.1 Hz,
), 1.75
br, 1H, CH), 1.35ꢀ1.25 (m, 24H, CH ), 0.89ꢀ0.86 (m, 6H, CH ). C NMR (100 MHz,
ArH), 6.90ꢀ6.89 (d, 1H, J = 9.2 Hz, ArH), 2.86ꢀ2.84 (d, 2H, J = 16.9 Hz, CH
2
13
(
2
3
3
CDCl , δ/ppm): 153.11, 152.43, 152.14, 151.91, 148.95, 136.27, 129.58, 129.18,
1
27.67, 126.65, 125.31, 124.12, 120.93, 113.14, 40.15, 34.95, 33.32, 33.25, 32.06,
0.07, 29.68, 29.69, 29.44, 26.66, 22.71, 14.23, 14.15. Anal. Calcd for
3

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30 4 3
C H37ON S Br (%): C, 57.22; H, 5.92; N, 8.90. Found (%): C, 57.83; H, 5.45; N,
8
.75.
Compound 6c. Compound 6c was synthesized as a red solid according to the
1
similar route as that for compound 6a with a yield of 35%. H NMR (400 MHz,
CDCl
3
, δ/ppm): 9.06 (s, 1H, ArH), 8.98 (s, 1H, ArH), 8.15ꢀ8.14 (d, 1H, J = 9.7 Hz,
),
). C NMR
, δ/ppm): 166.74, 165.97, 165.68, 165.33, 154.36, 153.63, 145.67,
ArH), 7.18ꢀ7.17 (d, 1H, J = 9.7 Hz, ArH), 2.99ꢀ2.97 (d, 2H, J = 15.7 Hz, CH
2
13
1
.72ꢀ1.67 (m, 1H, CH), 1.40ꢀ1.25 (m, 24H, CH
2 3
), 0.86ꢀ0.82 (m, 6H, CH
(100 MHz, CDCl
3
1
4
43.08, 142.83, 142.78, 140.39, 138.73, 138.04, 134.82, 134.63, 127.46, 57.48, 51.78,
6.89, 45.70, 43.88, 43.43, 43.31, 43.21, 40.33, 39.52, 38.56, 37.63, 36.54, 28.05.
Anal. Calcd for C30
H, 5.79; N, 9.00.
2 4
H37ON BrS (%): C, 54.45; H, 5.64; N, 8.47. Found (%): C, 54.19;
Compound 7a. Compound 4a (0.1 g, 0.192 mmol) and 2ꢀtributhylthiophene
(0.0717 g, 0.192 mmol) were dissolved in 10 mL toluene and the mixture was
3 4
deoxygenated with argon gas for 30 min. The catalytic amount of Pd(PPh ) was
added into the solution, and the mixture was then stirred at 120 °C for 12 h. After that,
the mixture was poured into water and extracted with dichloromethane. The combined
4
organic layers were dried over anhydrous MgSO . After the removal of solvent, the
crude product was purified column chromatography (silica gel; eluent: petroleum
ether: dichloromethane = 4:1) to afford compound 7a (0.092 g, yield: 91%) as a red
1
solid. H NMR (400 MHz, CDCl
3
, δ/ppm): 8.08ꢀ8.08 (m, 1H, ArH), 7.96ꢀ7.95 (d, 1H,
J = 9.1 Hz, ArH), 7.64ꢀ7.62 (d, 1H, J = 19.3 Hz, ArH), 7.82ꢀ7.80 (d, 1H, J = 19.0 Hz,

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ArH), 7.76ꢀ7.74 (d, 1H, J = 19.0 Hz, ArH), 7.43ꢀ7.42 (d, 1H, J = 12.7 Hz, ArH),
7
.20ꢀ7.18 (d, 1H, J = 12.7 Hz, ArH), 6.85ꢀ6.84 (m, 1H, J = 9.2 Hz, ArH), 2.82ꢀ2.80 (d,
H, J = 16.8 Hz, CH ), 1.71 (br, 1H, CH), 1.32ꢀ1.27 (m, 24H, CH ), 0.87ꢀ0.84 (m, 6H,
2
2
2
13
CH ). C NMR (100 MHz, CDCl , δ/ppm): 152.61, 152.53, 146.69, 139.53, 137.05,
3
3
1
27.95, 127.67, 127.23, 126.57, 126.49, 126.30, 125.82, 125.24, 124.82, 40.11, 34.73,
40 2 3
3.34, 33.26, 32.02, 30.07, 29.74, 26.77, 22.88, 14.22. Anal. Calcd for C30H N S
3
(%): C, 68.65; H, 7.68; N, 5.34. Found (%): C, 68.60; H, 7.73; N, 5.42.
Compound 7b. Compound 7b was synthesized as a red solid according to the
1
similar route as that for compound 7a with a yield of 66%. H NMR (400 MHz,
CDCl
.10ꢀ8.09 (d, 1H, J = 9.2 Hz, ArH), 7.51ꢀ7.50 (m, 1H, ArH), 6.89ꢀ6.88 (d, 1H, J = 9.2
Hz, ArH), 2.85ꢀ2.84 (d, 2H, J = 16.8 Hz, CH ), 1.75 (br, 1H, CH), 1.36ꢀ1.25 (m, 24H,
). C NMR (100 MHz, CDCl , δ/ppm): 153.13, 152.05,
3
, δ/ppm): 8.90 (s, 1H, ArH), 8.81 (s, 1H, ArH), 8.22ꢀ8.21 (m, 1H, ArH),
8
2
13
CH
2
), 0.89ꢀ0.86 (m, 6H, CH
3
3
1
4
2
47.56, 139.07, 136.58, 128.69, 128.28, 128.07, 127.34, 126.55, 122.15, 121.06,
0.27, 34.88, 33.34, 32.02, 30.14, 33.27, 32.05, 30.08, 29.72, 29.64, 29.46, 26.66,
4 4
2.77, 14.28, 14.12. Anal. Calcd for C34H40ON S (%): C, 64.52; H, 6.37; N, 8.85.
Found (%): C, 64.13; H, 6.79; N, 8.75.
Compound 7c. Compound 7c was synthesized as a red solid according to the
1
similar route as that for compound 7a with a yield of 64%. H NMR (400 MHz,
CDCl
ArH), 7.88ꢀ7.87 (d, 1H, J = 9.4 Hz, ArH), 7.43ꢀ7.42 (d, 1H, J = 12.3 Hz, ArH),
.15ꢀ7.13 (m, 1H, ArH), 7.05ꢀ7.04 (d, 1H, J = 9.4 Hz, ArH), 2.98ꢀ2.96 (d, 2H, J =
3
, δ/ppm): 8.84 (s, 1H, ArH), 8.40 (s, 1H, ArH), 8.04ꢀ8.03 (d, 1H, J = 7.6 Hz,
7

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1
5.2 Hz, CH
2
), 1.75 (br, 1H, CH), 1.47ꢀ1.25 (m, 24H, CH
2 3
), 0.89ꢀ0.86 (m, 6H, CH ).
1
3
C NMR (100 MHz, CDCl , δ/ppm): 152.83, 151.76, 140.37, 139.68, 138.89, 132.08,
3
1
4
2
28.57, 128.36, 128.05, 127.43, 125.93, 125.54, 124.15, 124.06, 121.72, 121.15,
3.77, 37.88, 36.53, 34.57, 33.59, 32.94, 32.65, 32.05, 31.94, 30.02, 29.76, 29.45,
4 5
6.67, 26.58, 24.43, 22.72, 14.24, 14.1. Anal. Calcd for C34H40ON S (%): C, 61.41;
H, 6.06; N, 8.42. Found (%): C, 61.78; H, 6.16; N, 8.79.
Compound 8a. To the mixture of compound 7a (0.11 g, 0.21 mmol) in 30 mL dry
o
1
0
,2ꢀdichloroethane. When the solution was cooled to 0 C, anhydrous DMF (0.016 g,
.21 mmol) and POCl (0.032 g, 0.33 mmol) were added dropwise slowly in turn
3
under nitrogen atmosphere. The mixture was stirred at 85 °C for 24 h. Then, the
mixture was cooled to room temperature, and 30 mL of saturated NH Cl solution was
added. The mixture was extracted with CH Cl for several times, and the combined
organic layers were dried over anhydrous MgSO . After the solvent was removed, the
4
2
2
4
crude product was purified by column chromatography (silica gel; eluent: hexane:
dichloromethane = 1:1) to afford compound 8a (0.063 g, yield: 54%) as a red solid.
1
3
H NMR (400 MHz, CDCl , δ/ppm): 9.97 (s, 1H, CHO), 8.19ꢀ8.18 (d, 1H, J = 9.1 Hz,
ArH), 8.04ꢀ8.03 (d, 1H, J = 9.1 Hz, ArH), 7.98ꢀ7.96 (d, 1H, J = 19.2 Hz, ArH),
7
.82ꢀ7.81 (m, 2H, ArH), 6.88ꢀ6.78 (d, 1H, J = 9.2 Hz, ArH), 2.84ꢀ2.82 (d, 2H, J =
1
6.7 Hz, CH ), 1.71 (br, 1H, CH), 1.32ꢀ1.27 (m, 24H, CH ), 0.87ꢀ0.84 (m, 6H, CH ).
2
2
3
1
3
3
C NMR (100 MHz, CDCl , δ/ppm): 183.0, 152.84, 152.37, 148.88, 147.99, 143.14,
1
3
36.85, 136.56, 128.63, 128.46, 127.67, 127.41, 126.64, 124.36, 123.57, 40.17, 34.87,
3.39, 33.22, 31.94, 29.95, 29.71, 29.64, 29.44, 26.62, 22.75, 14.16. Anal. Calcd for

Page 13 of 52
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31 40 2 3
C H N OS (%): C, 67.35; H, 7.29; N, 5.07. Found (%): C, 67.57; H, 7.25; N, 5.02.
Compound 8b. Compound 8b was synthesized as a red solid according to the
1
similar route as that for compound 8a with a yield of 84%. H NMR (400 MHz,
CDCl , δ/ppm): 9.99 (s, 1H, ArH), 8.88 (s, 1H, ArH), 8.70 (s, 1H, ArH), 8.20ꢀ8.19 (m,
3
1
H, ArH), 8.08ꢀ8.07 (d, 1H, J=9.1 Hz, ArH), 7.82ꢀ7.81 (m, 1H, ArH), 6.87ꢀ6.86 (m,
H, ArH), 2.85ꢀ2.84 (d, 2H, J=16.8 Hz, CH ), 1.75 (br, 1H, CH), 1.36ꢀ1.30 (m, 24H,
). C NMR (100 MHz, CDCl , δ/ppm): 182.91, 152.91,
1
2
1
3
CH
2
), 0.89ꢀ0.86 (t, 6H, CH
3
3
1
1
3
52.83, 151.84, 151.55, 148.32, 147.87, 143.85, 136.48, 136.24, 129.27, 128.24,
27.77, 126.74, 125.73, 124.05, 123.82, 123.28, 120.64, 40.28, 34.89, 33.33, 31.91,
4 4
0.06, 29.74, 29.66, 29.42, 26.67, 22.78, 14.24, 14.13. Anal. Calcd for C35H42ON S
(%): C, 63.41; H, 6.39; N, 8.45. Found (%): C, 63.79; H, 6.22; N, 8.33.
Compound 8c. Compound 8c was synthesized as a red solid according to the
1
similar route as that for compound 8a with a yield of 30%. H NMR (400 MHz,
CDCl , δ/ppm): 10.00 (s, 1H, CHO), 9.08 (s, 1H, ArH), 8.89 (s, 1H, ArH), 8.29ꢀ8.28
3
(t, 1H, ArH), 8.17ꢀ8.16 (t, 1H, ArH),7.87ꢀ7.86 (d, 1H, J = 10.0 Hz, ArH), 6.91ꢀ6.90 (d,
1
1
H, J = 9.0 Hz, ArH), 2.87ꢀ2.85 (d, 2H, J = 16.8 Hz, CH
2
), 1.75ꢀ1.74 (m, 1H, CH),
13
.36ꢀ1.25 (m, 24H, CH
2
), 0.86ꢀ0.82 (m, 6H, CH
3
). C NMR (100 MHz, CDCl ,
3
δ/ppm): 182.92, 152.74, 152.66, 151.76, 151.48, 148.29, 147.73, 143.82, 136.46,
1
1
1
6
36.37, 129.28, 128.51, 127.93, 127.76, 126.57, 125.74, 123.88, 123.56, 123.08,
20.43, 40.12, 34.51, 34.85, 31.96, 31.07, 29.97, 29.16, 29.05, 27.64, 22.04, 14.22,
4.1. Anal. Calcd for C35
1.09; H, 5.69; N, 9.03.
4 5
H40ON BrS (%): C, 60.66; H, 5.82; N, 8.8. Found (%): C,

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Monomer 9a. (2,5ꢀDibromothiophenꢀ3ꢀylmethyl)phosphonic acid diethyl ester
(0.078 g, 0.2 mmol) and compound 8a (0.0553 g, 0.1 mmol) were dissolved in 10 mL
o
THF, and the solution was stirred at 0 C for 30 min under an argon atmosphere. After
that, potassium tertbutoxide (0.017 g, 0.15 mmol) dissolved in 5 mL THF were added
dropwise slowly to above solution. The reaction mixture was stirred for another 30
min at room temperature, and then refluxed at 50 °C for 24 h. After cooling to room
temperature, the reaction mixture was extracted with dichloromethane and washed
with dilute aqueous HCl solution. The combined organic layers were dried over
anhydrous MgSO , and then filtered. The filtrate was concentrated by rotary
4
evaporation to give a red solid. The crude product was purified by column
chromatography (silica gel; eluent: hexane: dichloromethane = 4:1) to afford 9a
1
(
0.046 g, yield: 59%) as a red solid. H NMR (400 MHz, CDCl
3
, δ/ppm): 9.97 (s, 1H,
ArH), 8.0ꢀ7.96 (m, 2H, ArH), 7.82ꢀ7.74 (m, 2H, ArH), 716ꢀ7.10 (m, 2H, ArH),
7
1
.03ꢀ6.99 (m, 1H, ArH), 6.89ꢀ6.83 (m, 2H, CH), 2.82ꢀ2.80 (d, 2H, J=16.7 Hz, CH2),
13
.71 (br, 1H, CH), 1.32ꢀ1.27 (m, 24H, CH
, δ/ppm): 152.53, 146.84, 142.86, 148.87, 139.23, 138.86, 136.97,
28.23, 127.96, 127.72, 126.46, 125.77, 124.88, 124.29, 119.89, 112.01, 110.12,
0.13, 34.74, 33.36, 33.27, 31.98, 30.04, 29.75, 29.62, 29.35, 26.62, 22.73, 14.12.
2 3
), 0.87ꢀ0.84 (m, 6H, CH ). C NMR (100
MHz, CDCl
3
1
4
Anal. Calcd for C H N S Br (%): C, 54.68; H, 5.35; N, 3.54. Found (%): C, 54.97;
3
6
42
2
4
2
H, 5.51; N, 3.22.
Monomer 9b. Compound 9b was synthesized as a red solid according to the
1
similar route as that for compound 9a with a yield of 54%. H NMR (400 MHz,

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3
CDCl , δ/ppm): 8.84ꢀ8.33 (d, 1H, J = 9.1 Hz, ArH), 8.12ꢀ8.10 (d, 1H, J = 19.4 Hz,
ArH), 7.13ꢀ7.12 (d, 1H, ArH), 7.07ꢀ7.06 (d, 1H, ArH), 7.97ꢀ6.93 (m, 1H, J = 39.9 Hz,
ArH), 6.88ꢀ6.87 (d, 1H, J = 9.1 Hz, ArH), 6.84ꢀ6.80 (d, 1H, J = 39.9 Hz, ArH),
2
.85ꢀ2.83 (d, 2H, J = 16.7 Hz, CH ), 1.74 8ꢀ1.71 (m, 1H, CH), 1.35ꢀ1.26 (m, 24H,
2
CH
2 3 42 4 2 5
), 0.87ꢀ0.84 (m, 6H, CH ). Anal. Calcd for C40H N Br S (%): C, 53.44; H, 4.71;
N, 6.23. Found (%): C, 53.51; H, 4.36; N, 6.83.
Monomer 9c. Compound 9c was synthesized as a red solid according to the
1
similar route as that for compound 9a with a yield of 54%. H NMR (400 MHz,
CDCl , δ/ppm): 9.02ꢀ8.99 (m, 1H, ArH), 8.93ꢀ8.90 (d, 1H, J = 28.9 Hz, ArH),
3
8
.38ꢀ8.29 (m, 1H, ArH), 8.20ꢀ8.12 (m, 1H, ArH), 7.19ꢀ7.12 (m, 2H, ArH), 7.02ꢀ6.85
m, 2H, ArH), 6.84ꢀ6.80 (d, 1H, J = 39.9 Hz, ArH), 2.99ꢀ2.91 (m, 2H, CH ), 1.72
). Anal. Calcd
(%): C, 51.60; H, 4.55; N, 6.02. Found (%): C, 51.65; H, 5.14, N,
(
2
8
ꢀ1.68 (m, 1H, CH), 1.41ꢀ1.25 (m, 24H, ꢀCH
Br
2 3
ꢀ), 0.86ꢀ0.82 (m, 6H, CH
for C40
H
42
N
4
2 6
S
6
.26.
Monomer 11. 4,8ꢀDi(2,3ꢀdidecylthiophenꢀ5ꢀyl)ꢀbenzo[1,2ꢀb:4,5ꢀb’]dithiophene
10) (1.91 g, 2.37 mmol) was dissolved in 50 mL of anhydrous THF under the
protection of argon. The solution was then cooled to 0 °C, and a solution of nꢀBuLi
2.37 mL, 5.92 mmol, 2.5 M in hexane) was added dropwise slowly with stirring.
(
(
After the addition was finished, the reaction mixture was warmed to ambient
temperature and stirred for 2 hours. The reaction mixture was subsequently cooled to
0
3
°C and Sn(nꢀBu) Cl (2.31 g, 7.11 mmol) was added in one portion. The reaction
mixture was stirred at 0 °C for 30 minutes, and then warmed to room temperature for

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another 8 h. After that, the reaction mixture was poured into water and extracted twice
with dichloromethane. The combined organic layers were dried over MgSO , and the
4
solvent was removed by rotary evaporation. The crude product was purified by
column chromatography (silica gel; eluent: hexane: dichloromethane = 10:1) to afford
1
compound 11 (1.51 g, yield 46%) as colorless liquid. H NMR (400 MHz, CDCl
3
,
δ/ppm): 7.75 (s, 1H, ArH), 7.25 (s, 1H, ArH), 2.84ꢀ2.80 (t, 4H, ArH), 2.62ꢀ2.58 (t, 4H,
ArH),1.75ꢀ1.71 (m, 4H, CH
2
), 1.68ꢀ1.64 (m, 4H, CH
2
), 1.61ꢀ1.56 (m, 12H, CH
2
),
1
3
1
.37ꢀ1.30 (m, 52H, CH ), 1.20ꢀ1.16 (t, 12H, CH
2
2
3
), 0.89ꢀ0.86 (m, 30H, CH ). C
NMR (100 MHz, CDCl , δ/ppm): 143.03, 141.24, 139.71, 137.93, 137.17, 136.25,
3
1
2
31.89, 129.83, 122.26, 32.11, 32.04, 31.94, 31.07, 29.73, 29.59, 29.43, 29.20, 29.11,
9.04, 28.52, 28.18, 27.43, 22.82, 14.25, 13.85, 10.95. Anal. Calcd for C74 Sn
4
H
126
S
4
(%): C, 64.34; H, 9.19. Found (%): C, 64.75; H, 9.10.
PBDTT-TABT. Monomer 9a (0.1862 g, 0.235mmol) and monomer 11 (0.3250 g,
0
.235 mmol) were dissolved in 15 mL degassed toluene, and Pd (dba) (5.67mg, 2%
2
3
3
mmol) and P(oꢀtol) (7.62 mg, 8% mmol) were added later under an argon
atmosphere. The mixture was stirred at 110 °C for 24 h in a dark environment. After
the reaction was finished, the mixture was cooled to room temperature and dropped
into 500 mL methanol slowly to form a dark precipitate. The crude copolymer was
collected by filtration, and then subjected to Soxhlet extraction successively with
methanol, acetone, and hexane in turn to remove the oligomers and impurities. The
remaining polymer was finally dissolved in chloroform and precipitated again from
n
methanol to afford PBDTT-TABT (0.242g, yield 72%) as a dark solid. M = 27.0

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1
KDa, M
w
= 53.6 KDa, PDI=1.98. H NMR (400 MHz, CDCl
3
, δ/ppm): 8.71ꢀ6.87 (br,
, CH ). Anal. Calcd
(%): C, 72.11; H, 8.02; N, 1.96. Found (%): C, 71.48; H, 7.99; N,
1
3H, ArH, CH), 2.88ꢀ2.47 (br, 10H, CH
2
), 2.03ꢀ0.86 (m, 91H, CH
3
2
for (C86
H
114
N
2
S
8
)
n
1
.97.
PBDTT-TANT. Polymer PBDTT-TANT was synthesized as a dark solid
n
according to the similar route as that for PBDTT-TABT with a yield of 69%. M =
1
1
5.9 KDa, M
w
= 26.3 KDa, PDI = 1.6. H NMR (400 MHz, CDCl
3
, δ/ppm): 8.71ꢀ6.87
(br, 13H, ArH, CH), 2.88ꢀ2.47 (br, 10H, CH
2
), 2.03ꢀ0.86 (m, 91H, CH , CH ). Anal.
2
3
Calcd for (C H N S ) (%): C, 70.17; H, 7.46; N, 3.64. Found (%): C, 70.14; H,
9
0
114
4 9 n
7
.12; N, 3.60.
PBDTT-TSNT. Polymer PBDTT-TSNT was synthesized as a dark solid
n
according to the similar route as that for PBDTT-TABT with a yield of 62%. M =
1
1
3.5 KDa, M
w
= 24.5 KDa, PDI = 1.8. H NMR (400 MHz, CDCl
3
, δ/ppm): 7.80ꢀ6.99
(
br, 13H, ArH, CH), 3.01ꢀ2.35 (br, 10H, CH ), 2.03ꢀ0.83 (m, 91H, CH , CH ). Anal.
2 2 3
114 4 10 n
Calcd for (C90H N S ) (%): C, 68.74; H, 7.31; N, 3.56. Found (%): C, 68.38; H,
7
.74; N, 3.99.
2
.2 Characterization
1
13
H and C NMR spectra were recorded on a Bruker Avanceꢀ400 spectrometer with
dchloroform as a solvent and tetramethylsilane as an internal standard. The elemental
analysis was performed on a Thermo Electron SPA Flash EA 1112 series analyzer.
Molecular weights of the copolymers were determined by using a Waters 1515 GPC
instrument with chloroform as the eluent and polystyrene as a standard.

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Thermogravimetric analysis was conducted on a TA Instrument Model SDT Q600
ꢀ1
2
simultaneous TGA/DSC analyzer at a heating rate of 10 °C min and under a N flow
ꢀ1
rate of 90 mL min . UVꢀvis spectra were obtained on a carry 300 spectrophotometer.
Cyclic voltammetry measurements were made on a CHI660 potentiostat/galvanostat
ꢀ1
electrochemical workstation at a scan rate of 50 mV s , with a platinum wire counter
electrode and an Ag/AgCl reference electrode in an anhydrous and nitrogenꢀsaturated
ꢀ1
0
.1 mol L acetonitrile solution of tetrabutylammonium perchlorate. The CHCl
3
solutions of the polymers were dropꢀcoated on the platinum plate working electrodes.
XRD patterns of the polymers were recorded on a Philips Xꢀray diffractometer
α
operated in reflection geometry at 30 mA, 40 kV with Cu K radiation. AFM images
were obtained by using a Bruker Inova atomic microscope at tapping mode. SCLC is
2
3
described by J=9ε
active layer, ꢁ is the hole or electron mobility, ε
the transport medium, ε is the permittivity of free space (8.85×10 F m ), V is the
0
ε
r
ꢁV /8L , where J is the current density, L is the film thickness of
r
is the relative dielectric constant of
ꢀ12
ꢀ1
0
internal voltage in the device and V = Vappl ꢀ Vbi ꢀ V
voltage to the device, Vbi is the builtꢀin voltage due to the relative work function
difference of the two electrodes and V is the voltage drop due to contact resistance
and series resistance across the electrodes.
a
, where Vappl is the applied
a
2
.3 Device fabrication
The device structure was ITO/PEDOT:PSS/copolymer:PC71BM/Ca/Al. A glass
ꢀ1
substrate with a preꢀpatterned ITO (sheet resistance = 15 ꢂ sq ) was first
ultrasonicated in detergent, deionized water, acetone, and isopropanol in turn, and

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then modified by UVꢀozone treatment for 20 min. After filtration through a 0.45 ꢁm
filter, PEDOT:PSS (Bay P VP AI 4083, Bayer AG) was spinꢀcoated at 4000 rpm for
6
0 s to form a thickness of 35 nm thin layer on the cleaned ITO substrate, and baked
on a hot plate at 140 °C for about 15 min. A blend film of copolymer:PC BM (1:4
71
w/w) was prepared by spinꢀcoating its mixed solvent of chlorobenzene
–
1
(
CB)/1,8ꢀdiiodoctane (DIO) (97:3, v:v) solution (concentration: 28 mg mL ) at
1
500 r. p. m. for 60 s. A Ca layer (15 nm) and an Al layer (140 nm) were finally
ꢀ6
deposited in sequence under a vacuum of 2 × 10 Torr to form the negative electrode.
Holeꢀonly devices were fabricated similar to the PSCs with a structure of
3 3
ITO/PEDOT:PSS/polymer:PC71BM (1:4)/MoO /Ag. 10 nm MoO was evaporated
onto the surface of the photoactive layer with or without surface modification before
Al evaporation. All devices were fabricated in a nitrogenꢀfilled glove box (< 0.1 ppm
O
2
and H
2
O). The thickness of films was measured using a Dektak 6 M surface
2
profilometer. The device area was 0.04 cm . The IꢀV characterization of the devices
was carried out on a computerꢀcontrolled Keithley 2400 Source Measurement system.
The EQE values were tested with a Newport Model 77890 (Newport Co. Ltd.) during
illumination with monochromatic light from a xenon lamp. A solar simulator was used
as the light source, and the light intensity was monitored by using a standard Si solar
cell. All fabrication and characterization processes, except for the EQE measurements,
were conducted in a glove box.
3
. Results and Discussion
3
.1 Materials Synthesis and Characterization

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The synthetic routes of monomers and the twoꢀdimensional copolymers are shown in
Scheme 1. The compounds 6a-6c was synthesized by the Stille coupling reaction of
the thienyltin derivatives (4a or 4b) and BT or NT based dibromides (5a or 5b). The
other bromo side of 6aꢀ6c was terminated with 2ꢀ(tributylstannyl)thiophene by Stille
coupling reaction again to afford compounds 7aꢀ7c. Using a Vilsmeier reaction,
Compounds 7aꢀ7c was synthesized through formylation of the 2ꢀposition of thiophene
unit
in
compounds
7aꢀ7c.
After
that,
7aꢀ7c
reacted
with
(2,5ꢀdibromothiophenꢀ3ꢀylꢀmethyl) phosphonic acid diethyl ester to give monomers
9
aꢀ9c, using a WittigꢀHorner reaction. Finally, the twoꢀdimensional copolymers,
PBDTTꢀTꢀDTBT, PBDTTꢀTꢀDTNT and PBDTTꢀTꢀDTSNT, were prepared by Stille
coupling polymerization in good yields using Pd (dba) /P(oꢀtol) as the catalyst
system. The molecular structures of the intermediates and copolymers were confirmed
2
3
3
1
13
by H NMR spectroscopy, C NMR spectroscopy and elemental analysis. All the
copolymers show good solubility in common organic solvents, such as chloroform,
toluene, chlorobenzene, dichlorobenzene. The molecular weights (M
polydispersity indices (PDIs) of the copolymers were determined by gel permeation
chromatography (GPC) analysis with polystyrene standard calibration.
w n
and M ) and
a
PBDTTꢀTANT and PBDTTꢀTSNT have comparable numberꢀaverage molecular
weights (M ) with 15.9 KDa and 13.5 KDa, respectively. However, PBDTTꢀTABT
n
n
exhibits relatively higher M of 27.0 KDa, which might be attributed to the decreased
steric hindrance of dibromothiophene monomer (9a) with BT containing side chain.
The thermal stabilities of the copolymers were evaluated by thermogravimetric (TGA)

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d
analysis. As shown in Fig. 2a, the decomposition temperatures (T , 5% weight loss) of
o
PBDTTꢀTABT, PBDTTꢀTANT and PBDTTꢀTSNT were measured to be 416 C, 416
o
o
C and 360 C, respectively. The results indicated that using NT instead of BT could
not significantly increase the thermal properties of BDTꢀT based copolymers while
the alkylthio flexile side chains would reduce the thermal stability of the resulting
copolymers compared to the alkyl side chains. In any event, the thermal stability of
the three copolymers is good enough for their device fabrications and evaluations.
The differential scanning calorimetry (DSC) curves of PBDTTꢀTABT, PBDTTꢀTANT
and PBDTTꢀTSNT are also shown in Fig. 2b. No glass transition was observed for all
these copolymers from their DSC heating and cooling traces in the range of room
o
temperature to 200 C. The molecular weights and the thermal analysis data are listed
in Table 1.
2
.2 Optical Properties
The UVꢀvis absorption spectra of the twoꢀdimensional copolymers in chloroform
solution and thin film are shown in Fig. 3. The corresponding absorption data of the
copolymers are summarized in Table 2. As shown in Fig. 3a, these copolymers show
two distinct absorption characteristics in solution. The absorption band in the high
energy region is owned to πꢀπ* transition of polymer backbones, and the other in the
lower energy region comes from the intramolecular charge transfer (ICT) between the
donor units of main chains and the acceptor units of side chains. The lowꢀenergy
peaks of PBDTTꢀTABT, PBDTTꢀTANT and PBDTTꢀTSNT in CHCl were located at
3
about 490 nm, 492 nm and 495 nm, respectively. PBDTTꢀTANT and PBDTTꢀTSNT

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with NT side chains show redꢀshifted absorption in contrast to that of PBDTTꢀTABT
with BT side chains. The reason is attributed to the boosted ICT properties,
originating from the stronger electronꢀwithdrawing ability of NT unit. Compared with
PBDTTꢀTANT, the absorption is redꢀshifted for PBDTTꢀTSNT in solution. This is
probably attributed to that the introduced alkylthio moiety shows an electronꢀdonating
ability, which can increase the ICT effect between the main chain and the side chain to
a certain extent. As shown in Fig. 2, similar behavior was observed from the
absorption spectra of these copolymers as thin solid films. Relative to their
counterparts in the solution state, the absorption shapes became broader in the
wavelength range of 300ꢀ800 nm, and the peaks showed slight redꢀshifts for all the
three copolymers. The reason could be explained by the enhanced aggregation and
strengthened interchain interactions between the conjugated backbones in the solid
24
state, which could facilitate charge transportation in PSCs. The optical bandgaps of
PBDTTꢀTABT, PBDTTꢀTANT and PBDTTꢀTSNT were determined from the
absorption onsets of the corresponding copolymer films to be 1.98 eV, 1.87 eV and
1
.85 eV, respectively. PBDTTꢀTANT and PBDTTꢀTSNT showed the smaller
bandgaps than that of PBDTTꢀTANT, which was ascribed to the stronger
electronꢀwithdrawing ability of NT unit, generating an enhanced ICT effect between
the main chain and the side chain. The smallest bandgap of PBDTTꢀTANT would be
expected to harvest more sunlight to increase the Jsc value in its PSCs.
3
.3 Electrochemical Properties
The energy levels are important parameters to select appropriate acceptors in BHJ

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1d
PSCs. Thus, cyclic voltammetry (CV) was employed to investigate the redox
behaviors of the copolymers and determine their energy levels. The CV experiments
were performed in a threeꢀelectrode cell, using 0.10 M tetrabutylammonium
perchlorate (nꢀBu NClO ) as the supporting electrolyte in anhydrous acetonitrile. A
4
4
platinum plate coated with a thin film of the studied copolymer, a platinum wire and
Ag/AgCl (0.1 M) were used as the work, counter and reference electrodes,
respectively. The energy level of the Ag/AgCl reference electrode was calibrated
+
19
against the Fc/Fc system to be 4.34 eV by using the previous methods. As shown in
Fig. 3, the onset oxidation potentials of PBDTTꢀTABT, PBDTTꢀTANT and
PBDTTꢀTSNT were located at 1.11 V, 1.03 V and 1.07 V, leading to the corresponding
HOMO levels of ꢀ5.45 eV, ꢀ5.37 eV and ꢀ5.41 eV, respectively. The lowꢀlying HOMO
levels of these copolymers implied their enhanced chemical stability in ambient
conditions. Furthermore, the deepened HOMO energy levels could be helpful to
obtain the relatively high V s of their PSCs because V is proportional to the energy
oc
oc
2
0
level difference between HOMO of the donor and LUMO of the acceptor.
Compared to PBDTTꢀTANT, PBDTTꢀTSNT showed a lower HOMO energy level,
6
a,14
which could be attributed to the effect of alkylthio modification.
On the other hand,
the lowest unoccupied molecular orbital (LUMO) energy levels of PBDTTꢀTABT,
PBDTTꢀTANT and PBDTTꢀTSNT were calculated to be ꢀ3.31 eV, ꢀ3.39 eV and ꢀ3.44
eV, respectively. As shown in Fig. 4b, the LUMO gaps of 0.56ꢀ0.69 eV between the
copolymers and PC71BM would afford an enough driving force to facilitate an
efficient exciton dissociation at the DꢀA interface, thereby ensuring energetically

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21
favorable electron transfer to overcome the binding energy of the intrachain exciton.
The corresponding electrochemical data of these copolymers are summarized in Table
2
.
3
.4 Theoretical Calculations
To evaluate the side chain effect on the molecular architectures and electronic
properties of the resulting copolymers, Density Functional Theory (DFT) simulations
were performed on the model monomers with three repeated donorꢀacceptor units to
verify stationary points as stable states for the optimized conformations and
22
singleꢀpoint energies, at the B3LYP/6ꢀ31G(d) level. To simplify the calculations,
long alkyl chains were replaced by the methyl group because they did not
22
significantly affect the equilibrium geometries and electronic properties. The
HOMO and LUMO wave functions of the model compounds are shown in Fig. 5.
These copolymers show similar distributions for both the HOMO and LUMO. As
shown in Fig. 5, the localization of HOMO was distributed in the electronꢀrich
skeleton of the polymer backbone while the LUMO was mainly located on the
electronꢀdeficient BT or NT containing side chains. Compared to HOMOs, HOMOꢀ1
orbitals moved from the middle part to the side parts even distributing on the side
chains to some extent, which implied that internal charge transfer was possible in
these twoꢀdimensional conjugation systems. A similar transfer trend is observed from
the LUMO to LUMO+1. The results indicated that the significant chargeꢀtransfer
character occurred between the BDTꢀT main chains and conjugated side chains, which
was consistent with the observed strong lowꢀenergy absorption band in Fig. 2. The

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HOMO and LUMO energy levels of PBDTTꢀTABT, PBDTTꢀTANT and
PBDTTꢀTSNT trimers were calculated to be ꢀ4.86/ꢀ2.66 eV, ꢀ4.80/ꢀ2.91 eV and
ꢀ
4.83/ꢀ2.98 eV, respectively. The bandgaps were then determined to be 2.20, 1.89 and
.85 eV. Obviously, the trend of simulation results were agreed well with those
obtained from CV and UVꢀvis measurements.
1
3
.5 X-ray Diffraction Analysis
To investigate an insight of the molecular stacking of the twoꢀdimensional
copolymers, Xꢀray diffraction (XRD) patterns were measured for PBDTTꢀTABT,
PBDTTꢀTANT and PBDTTꢀTSNT. As shown in Fig. 6, XRD patterns of these
copolymers exhibited two types of diffraction peaks. In the small angle region, clear
o
o
o
(
100) peaks were located at 2θ of 4.00 , 3.73 and 3.82 for PBDTTꢀTABT,
PBDTTꢀTANT and PBDTTꢀTSNT，corresponding to the lamellar packing with a
dꢀspacing (dl00) of 22.06 Å, 23.66Å and 23.10 Å. The second peaks (010) in the wide
angle region reflected the π–π stacking distances between the polymer backbones,
o
o
o
which were located at 2θ of 21.95 , 22.22 and 21.79 for PBDTTꢀTABT,
PBDTTꢀTANT and PBDTTꢀTSNT, corresponding to a πꢀπ stacking distance (d ) of
010
4
.04 Å, 4.00 Å and 4.07 Å, respectively. Because the bulk size of NT side chain is
larger than that of BT side chain, PBDTTꢀTANT possesses a longer lamellar packing
distance than PBDTTꢀTABT. After introducing alkythio group instead of alkyl group,
the flexibility of alkyl chain would be increased, confirming a little decrease of
lamellar packing distance obtained from PBDTTꢀTSNT film. On the other hand,
PBDTTꢀTANT shows the shorter πꢀπ stacking distance than PBDTTꢀTABT, which can

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be attributed to the enhanced interaction of conjugated polymer chains by choosing
the NT unit with a stronger electronꢀwithdrawing ability. Surprisingly, after
introducing alkylthio unit instead of alkyl unit, the πꢀπ stacking distance of
PBDTTꢀTSNT becomes larger even than PBDTTꢀTABT with BT side chains. The
result indicated that the effect of alkylthio modification via SꢀS interaction on the
ordered molecular stacking would not work in this type of twoꢀdimensional system,
however, the positive influence of alkylthio modification is commonly observed in
6
a,14
linear conjugated polymer donor materials.
Through the comprehensive
consideration of above XRD results, PBDTTꢀTANT shows the best stacking
properties, which can be expected to gain the highest carrier mobility in PSCs.
3
.6 Photovoltaic Properties
To study the photovoltaic properties of the resulting copolymers, PSCs with a device
structure ITO/PEDOT:PSS/copolymer:PC71BM/Ca/Al were fabricated and evaluated.
The active layers were spinꢀcoated from chlorobenzene (CB) solution of the donor
and acceptor materials. The optimized condition of device fabrication was obtained by
varying polymer:PC BM weight ratios, active layer thicknesses, and solvent
7
1
additives. The best photovoltaic properties were obtained with the blend ratio of 1:4
with 3% (v/v) 1,8ꢀdiiodooctane (DIO) as the solvent additive for all the three
copolymers based PSCs. Fig. 7 shows the current densityꢀvoltage (JꢀV) and external
quantum efficiency (EQE) curves of optimized devices. The related photovoltaic
parameters are summarized in Table 3.
As shown in Fig. 7a, the PBDTTꢀTABT:PC71BM device showed a PCE of 4.60%,

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ꢀ2
with a Voc of 1.00 V, a Jsc of 9.64 mA cm and an FF of 47.5%, which was superior to
12,13
that of PSC device based on PTG1 with a similar molecular structure.
Under the
same conditions, PBDTTꢀTANT device exhibited a higher PCE of 5.65% with a Voc
ꢀ2
of 0.98 V, a J of 10.00 mA cm and an FF of 58.0%. But a decreased PCE of 4.01%
sc
ꢀ2
with a Voc of 0.99 V, a Jsc of 8.93 mA cm and an FF of 45.3% was obtained for
PBDTTꢀTSNT device. Since the Voc is related to the HOMO of donor and LUMO of
2
acceptor energy offset, the obtained high Voc values of these devices should result
from the lowꢀlying HOMO levels of the polymer donors as determined in CV
measurements. The highest V value of 1.00 V from PBDTTꢀTABT device could be
oc
due to its deepest HOMO level as compared to those of PBDTTꢀTANT and
PBDTTꢀTSNT. Except for the decreased Voc, the simultaneously improvements of Jsc
and FF guaranteed achieving the highest PCE of 5.65% for PBDTTꢀTANT device.
The increased Jsc could be confirmed by EQE measurements. As shown in Fig. 7b, the
EQE response of these devices covered a broad range from 300 to 750 nm, which
corresponded to the absorptions of both the copolymer donor and the fullerene
acceptor. The maximum EQE values in the range of 450ꢀ650 nm reached about 63%,
6
8% and 59% for PBDTTꢀTABT, PBDTTꢀTANT and PBDTTꢀTSNT devices,
respectively. The EQE values are in good agreement with the Jsc values from JꢀV
measurements.
Solvent vapor annealing (SVA) using some polar solvents before the deposition of
cathode is an effective strategy to optimize the performance of PSCs via the formation
2
3
of more favorable phase separation. Thus, we employed tetrahydrofuran (THF)ꢀSVA

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to further improve the PCEs of the PSCs based on these twoꢀdimensional copolymers.
SVA treatment was performed in a glass Petri dish at room temperature in a glove box.
A 30 mm Petri dish was first injected about 2 mL THF and then closed for one minute
to ensure the THF vapor saturate the whole chamber. Then the active film was
transferred to the Petri dish and kept for 30 s before it was removed out. From Fig. 7a
and Table 3, in comparison with the performance of the PSCs without SVA treatments,
the PCEs of the three copolymers based PSCs increased greatly after a SVA process.
The PCEs increased from 4.60% to 6.74% for PBDTTꢀTABT device, from 5.65% to
8
.04% for PBDTTꢀTANT device, and from 4.01% to 6.00% for PBDTTꢀTSNT device,
respectively. All the PSCs showed the improvement of PCE values more than 40%,
which indicated that the THFꢀSVA treatment was an effective method to optimize the
photovoltaic property of this polymer donor system. To our knowledge, the PCE of
8
.04% is the highest value of this type of twoꢀdimensional copolymers with DꢀD
polymer backbones and DꢀA conjugated side chains. It is clearly that the V s of these
oc
devices decreased a little after the SVA treatments, which is also observed in the
2
3
previous reported work. Despite of this, the Jsc and FF showed a significant
improvement, giving rise to a large increase of PCEs for all the devices by SVA
treatments. From the Table 3, the Jsc values were improved from 9.64, 10.00 and 8.93
ꢀ2
ꢀ2
mA cm to 11.12, 13.06 and 10.25 mA cm for PBDTTꢀTABT, PBDTTꢀTANT and
PBDTTꢀTSNT devices, respectively. The EQE spectra were also measured to verify
the elevation of Jscs, which are plotted in Fig. 7b. The EQE values of all the devices
with SVA treatments exhibited higher responses than those without SVA treatments.

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The largest increase of PBDTTꢀTANT device up to 78% supported its highest Jsc of
ꢀ2
1
3.06 mA cm . The enhanced device performance after SVA treatments, especially in
sc and FF, is also confirmed by measuring the series resistance (R ) and shunt
resistance (R ). In fact, a decrease in R and/or an increase in R are essential to
J
s
sh
s
sh
25
achieving high Jsc and FF. For all the devices using SVA treatments, the R
s
and Rsh
showed the same variation trends. Take PBDTTꢀTANT device as an example, the R
s
2
2
2
decreased from 16.0 ꢂ cm to 10.8 ꢂ cm , while the Rsh increased from 549.6 ꢂ cm
2
to 629.2 ꢂ cm . The lowest R
s
and the highest Rsh of PBDTTꢀTANT device indicated
that the best ohmic contact was achieved at the interfaces of active layer/cathode,
which is beneficial for charge transport and extraction.
3
.7 Hole Mobility
To further understand the origin of enhanced Jsc with SVA treatments, the holeꢀonly
devices with a configuration of ITO/PEDOT:PSS/copolymer:PC71BM (1:4)/MoO /Ag
were fabricated in order to determine the hole mobilities of three copolymers based
3
26
blend films by spaceꢀchargeꢀlimited current (SCLC) method. The JꢀV curves of the
holeꢀonly devices are shown in Fig. 8, and the corresponding data are summarized in
Table 4. Without the SVA treatments, the hole mobilities were measured to be 2.40 ×
ꢀ4
2
ꢀ1 ꢀ1
ꢀ4
2
ꢀ1 ꢀ1
ꢀ4
2
ꢀ1 ꢀ1
1
0 cm V s , 2.93×10 cm V s and 1.33×10 cm V s for PBDTTꢀTABT,
PBDTTꢀTANT and PBDTTꢀTSNT blend films, respectively. PBDTTꢀTANT blend
film exhibited the highest hole mobility, which could be attributed to its best
crystallinity and stacking property among these copolymers, as discussed in the XRD
section. This also supports the highest Jsc value obtained for the PBDTTꢀTANT device.