Thiadiazole quinoxaline-based copolymers with ～1.0 eV bandgap for ternary polymer solar cells

Article abstract of DOI:10.1016/j.polymer.2015.09.047

A new strong electron-deficient acceptor unit, 4,9-bis(5-bromothiophen-2-yl)-6,7-bis(5-dodecylthiophen-2-yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline (TQxT), is designed. Three TQxT based new conjugated polymers have been further developed by alternating with benzo[1,2-b:4,5-b′]dithiophene or naphtho[1,2-b:5,6-b′]difuran. Featuring ultra low bandgaps of 1.03-1.10 eV, these polymers exhibit absorption spectra beyond near-infrared (NIR) region. The polymer:PC₇₁BM (1:2) solar cells deliver a best power conversion efficiency (PCE) of 0.43%. The doping of 9 wt% PBDTT-TQxP into P3HT:PC₇₁BM (1:2) blend, however, leads to a highest PCE of 3.58% for the resultant ternary solar cells, corresponding to ～22% improvement in comparison to 2.93% PCE for P3HT:PC₇₁BM binary cells.

Full text of DOI:10.1016/j.polymer.2015.09.047

Accepted Manuscript
Thiadiazole quinoxaline-based copolymers with ∼1.0 eV bandgap for ternary polymer
solar cells
Jiangsheng Yu, Qiaoshi An, Jiefeng Hai, Xuemei Nie, Baojing Zhou, Fujun Zhang,
Weihua Tang
PII:
S0032-3861(15)30252-4
10.1016/j.polymer.2015.09.047
JPOL 18133
DOI:
Reference:
To appear in: Polymer
Received Date: 6 May 2015
Revised Date: 12 September 2015
Accepted Date: 19 September 2015
Please cite this article as: Yu J, An Q, Hai J, Nie X, Zhou B, Zhang F, Tang W, Thiadiazole quinoxaline-
based copolymers with ∼1.0 eV bandgap for ternary polymer solar cells, Polymer (2015), doi: 10.1016/
j.polymer.2015.09.047.
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ACCEPTED MANUSCRIPT
Thiadiazole quinoxaline-based copolymers with ~1.0 eV
bandgap for ternary polymer solar cells
‡a
‡b
a
a
a
Jiangsheng Yu, Qiaoshi An, Jiefeng Hai, Xuemei Nie, Baojing Zhou, Fujun
b
a
Zhang* and Weihua Tang*
a
Key Laboratory of Soft Chemistry and Functional Materials, Ministry of Education,
Nanjing University of Science and Technology, Nanjing, P. R. China. Tel: (86) 25
8
431 7311; E-mail: whtang@mail.njust.edu.cn;
b
Key Laboratory of Luminescence and Optical Information, Ministry of Education,
Beijing Jiaotong University, 100044, China. E-mail: fjzhang@bjtu.edu.cn
‡
These authors contributed equally to this work.
Keyword: Thiadiazole quinoxaline-based copolymers, Ultra low bandgaps, Ternary
polymer solar cells
ABSTRACT
A
new
,9-bis(5-bromothiophen-2-yl)-6,7-bis(5-dodecylthiophen-2-yl)-[1,2,5]thiadiazolo[3,4
g]quinoxaline (TQxT), is designed. Three TQxT based new conjugated polymers
strong
electron-deficient
acceptor
unit,
4
-
have been further developed by alternating with benzo[1,2-b:4,5-b’]dithiophene or
naphtho[1,2-b:5,6-b’]difuran. Featuring ultra low bandgaps of 1.03~1.10 eV, these
polymers exhibit absorption spectra beyond near-infrared (NIR) region. The
polymer:PC BM (1:2) solar cells deliver a best power conversion efficiency (PCE) of
7
1
0
.43%. The doping of 9 wt% PBDTT-TQxP into P3HT: PC BM (1:2) blend,
71
however, leads to a highest PCE of 3.58% for the resultant ternary solar cells,
corresponding to ~ 22% improvement in comparison to 2.93% PCE for
P3HT:PC BM binary cells.
7
1
1

ACCEPTED MANUSCRIPT
1
. Introduction
Polymer solar cells (PSCs) have attracted enormous attention due to their
predominant advantages such as light weight and the capability to fabricate flexible
large-area devices, and continue to gain importance in view of their immense
potential as promising energy source [1-6]. To date, PSCs with bulk heterojunction
(BHJ) configuration are of particular interest among various device architectures,
such as inverted and tandem devices [7-10]. Significant progress has been achieved in
PSCs, with the state-of-art power conversion efficiency (PCE) surpassing 10%
[
11-20]. However, it is still far away from their commercial application owing to the
low PCEs and short lifetime for large area devices [21].
The ideal polymers should have a strong absorption band covering the visible and
near-infrared light region, and relatively low lying HOMO energy level offsetting
with fullerenes for efficient charge separation while maximizing the open circuit
voltage (V ) and high hole mobility. To absorb photons at long wavelengths, where
oc
more photon flux from emission of the sun is found, low bandgap polymers are
necessary [22]. Some research groups have demonstrated that the theoretical
maximum of the short-circuit current density (J ) can be increased when the
sc
photocurrent response is extended. If a transparent electrode has a transmittance of
8
5% from the standard spectrum of solar irradiation (AM 1.5G) and the response is
extended to 800 nm or even 1 ꢀm, the theoretical maximum of J could be increased
sc
-2
to ~23.0 or 31.9 mA cm , respectively [23]. These calculations indicate that the
absorption spectra of polymer donors strongly affect their photovoltaic performance,
especially J . However, at least two challenges could be identified for low bandgap
sc
polymers to harvest long-wavelength photons. The reduction of polymer bandgap will
cause a shift in one of or both the lowest unoccupied molecular orbital (LUMO) and
the highest occupied molecular orbital (HOMO). This might result in the unavoidable
reduction of Voc compared with high bandgap polymers, because the energy
difference between the LUMO of the electron acceptor and the HOMO of the electron
2

ACCEPTED MANUSCRIPT
donor is found to closely correlate with the V value [22,24,25].
oc
Another challenge is to have a large enough driving force for exciton splitting.
Empirically, the overall energetic driving force for a forward electron transfer from
the electron donor to the electron acceptor is represented by the energy difference
(offset) between the LUMOs of the donor and acceptor. It appears that a minimum
energy difference of 0.3 eV is required to affect the charge separation [24,26]. This
means that the LUMO of the electron donor must be closer in energy to vacuum than
the LUMO of the electron acceptor in order to have enough driving force for exciton
dissociation at the interface of the electron donor and acceptor [25]. The LUMO
position of low bandgap polymers might be shifted away from the vacuum level,
thereby weakening the driving force. So the research and development of new
electron acceptors with lower LUMO levels than the commonly used PCBM is also
needed for achieving efficient solar cells.
For the development of low bandgap polymers, the effective strategy by alternating
electron-rich (donating, D) unit with electron-deficient (accepting, A) unit has
attracted a great concern in structure design [27-29]. Benzo[1,2-b:4,5-b’]dithiophene
(BDT) has been widely used as D unit in conjugated D-A copolymers for high
performance PSCs. BDT has a large and planar molecular structure which would help
promote backbone π–π stacking, thus benefiting charge transport. A library of
BDT-based copolymers show great potential in PSCs with enhanced mobilities and
high PCEs surpassing 8% [5,14,30-32]. Further enhancing the electron donating
ability of D unit, naphtho[1,2-b:5,6-b’]difuran (NDF) was designed by fusing two
furan units on both sides of a naphthalene ring. NDF has an extended backbone π–π
stacking, which could also benefit charge separation and transport. The remarkable
property has enabled NDF-based PSCs to achieve PCEs of ~5% [33-35]. However,
the synthesis of quinoxaline-based polymers with low bandgaps is challenging and
has
been
relatively
less
explored
[36-40].
Previous
[
1,2,5]thiadiazolo[3,4-g]quinoxaline-based polymers have proven useful as the active
materials in organic solar cells and organic field-effect transistors [39,40]. Recently,
3

ACCEPTED MANUSCRIPT
we have reported some quinoxaline-based copolymers with different bandgaps from
1
.13 eV to 1.66 eV [41-43]. Compared with these polymers with the same D unit,
thiadiazole quinoxaline-based polymers exhibit lower bandgaps than triazole
quinoxaline-based ones. The thiadiazole quinoxaline-based polymers featuring ~1.2
eV bandgap have absorptions extended to near-infrared region (beyond 1000 nm).
Further correlation studies on the structure of thiadiazole quinoxaline polymers
alternating with different D units and their physiochemical properties might provide
some insight for the design of new low bandgap polymers.
Herein,
,7-bis(5-dodecylthiophen-2-yl)-4,9-di(thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-g]quino
xaline (TQxT) was designed and synthesized. Compared with the unit of
,7-bis(3,4-bis(dodecyloxy)phenyl)-4,9-di(thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-g]qui
a
new
strong
electron-deficient
unit
of
6
6
noxaline (TQxP), the side chain is replaced by alkylthienyl group which could benefit
to enhance absorption. Three new D-A copolymers by alternating BDT (or NDF) unit
with TQxP (or TQxT) unit were designed and developed for the first time. Their
thermal, optical and electrochemical properties were systematically studied. Among
the three polymers, the best PCE of 0.43% was obtained for PBDTT-TQxP:PC BM
7
1
(1:2, w/w) cells. However, the doping of 9 wt% PBDTT-TQxP into P3HT: PC BM
71
(1:2) blend leads to 22% improvement for the binary solar cells. The ternary PSCs
-2
delivered an optimal PCE of 3.58%, with a J of 9.77 mA cm , a V of 0.60 V and a
sc
oc
FF of 61%.
2
. Experimental part
.1. Instruments and characterization methods
2
1
13
H and C NMR spectra were collected on a Bruker AVANCE 500 MHz
(
Bio-Spin Corporation, Europe) spectrometer with CDCl as the solvent. GPC
3
measurements were performed on a Waters 717-2410 with polystyrene as reference
standard and tetrahydrofuran as the eluent. TGA analyses were conducted on a TA
o
instrument TGA/SDTA851e under nitrogen at a heating rate of 20 C/min. UV-vis
4

ACCEPTED MANUSCRIPT
absorption spectra were recorded using a UV-vis instrument Evolution 220 in dilute
chloroform solutions or solid films. CV studies were conducted on an
electrochemistry workstation (CHI660D, Chenhua Shanghai) under nitrogen in a
deoxygenated acetonitrile solution of tetrabutylammonium hexafluorophosphate
(
Bu NPF , 0.1 M) at a potential scan rate of 50 mV/s. A standard three-electrode
4 6
electrochemical cell was used, where polymer film coated Pt plate, Pt slice and
saturated calomel electrode (SCE) were used as working electrode, counter electrode,
and reference electrode, respectively. The polymer films coated on platinum plate
were made from dilute polymer solutions in chloroform.
2
.2. Synthesis of monomers and polymers
All chemicals were reagent grade and purchased from Sigma-Aldrich Co., Acros or
TCI and used without further purification. THF was dried over sodium/benzophenone
and freshly distilled before use. Chloroform and dichloromethane were purified by
refluxing over CaH and freshly distilled under nitrogen for use. If not specified
2
otherwise, other solvents were used directly. Column chromatography was carried out
with 300~400 nm mesh silica.
2
.2.1. 1,2-Bis(5-dodecylthiophen-2-yl)ethane-1,2-dione (3)
Under nitrogen atmosphere, compound 1 (5g, 19.81 mmol) was dissolved in
anhydrous THF (40 mL) at 0 °C. To the solution was added with 8.3 mL of
n-butyllithium (19.81 mmol, 2.4 M). The resultant solution was stirred for 2 h.
Meanwhile, in another Schlenk tube, a LiBr (1.72 g, 19.81 mmol) solution in
anhydrous THF (40 mL) was added to a stirring suspension of CuBr (2.84 g, 19.81
mmol) in THF (40 mL) at room temperature. The first solution was slowly added to
the second one at 0 °C, and the mixture was stirred for 1 h. Oxalyl chloride (1.16 g,
9
.11 mmol) was added dropwise afterwards. The mixture was stirred at 0°C for
another 1 h and allowed to warm to room temperature overnight. The resultant
mixture was poured into a saturated solution of NH Cl. The organic phase was
4
separated, with the aqueous phase extracted with ether (2 × 100 mL). After the
removal of organic solvents, the crude mixture was chromatographed on silica gel
5

ACCEPTED MANUSCRIPT
with petroleum ether/dichloromethane (3:1) as eluent to yield 3 as an orange solid
1
(
3.05 g, 60%). H NMR (500 MHz, CDCl ): δ (ppm) 7.86 (d, J = 3.9 Hz, 2H), 6.88 (d,
3
J = 3.9 Hz, 2H), 2.88 (t, J = 7.6 Hz, 4H), 1.76-1.67 (m, 4H), 1.37-1.24 (m, 36H), 0.88
1
3
(
t, J = 6.9 Hz, 6H). C NMR (125 MHz, CDCl ): δ (ppm) 182.59, 160.06, 137.74,
3
1
36.47, 126.39, 31.89, 31.26, 30.81, 29.60, 29.47, 29.32, 29.26, 29.00, 22.67, 14.10.
2
.2.2. 6,7-Bis(5-dodecylthiophen-2-yl)-4,9-di(thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-
g]quinoxaline (9)
Compounds 8 (0.80 g, 2.42 mmol) and 3 (1.49 g, 2.66 mmol) were dissolved in 50
mL of acetic acid. The reaction mixture was purged with nitrogen and heated at 100
o
C for 12 h. The resultant mixture was concentrated via rotary evaporation and
purified by column chromatography with dichloromethane as the eluent. Further
recrystallization of the product from methanol afforded 9 as a purple black solid
1
(
1.90g, 92%). H NMR (500 MHz, CDCl ): δ (ppm) 8.88 (dd, J = 3.8, 0.9 Hz, 2H),
3
7
6
6
.71 (dd, J = 5.1, 0.9 Hz, 2H), 7.49 (d, J = 3.7 Hz, 2H), 7.32 (dd, J = 5.1, 3.9 Hz, 2H),
.75 (d, J = 3.7 Hz, 2H), 2.91 (t, J = 7.6 Hz, 4H), 1.82-1.74 (m, 4H), 1.44 (q, J = 14.7,
1
3
.8 Hz, 4H), 1.37-1.25 (m, 32H), 0.88 (t, J = 6.9 Hz, 6H). C NMR (125 MHz,
CDCl ): δ (ppm) 152.87, 151.94, 146.12, 139.17, 135.80, 134.29, 132.90, 132.03,
3
1
1
30.82, 126.74, 124.86, 120.32, 31.92, 31.49, 30.55, 29.66, 29.37, 29.22, 22.69,
4.12.
2
.2.3.
4,9-Bis(5-bromothiophen-2-yl)-6,7-bis(5-dodecylthiophen-2-yl)-
[
1,2,5]thiadiazolo[3,4-g]quinoxaline (M4)
Under nitrogen atmosphere, N-bromosuccinimide (NBS) (0.52g, 2.93 mmol) was
o
added to a solution of 9 (1.00 g, 1.17 mmol) in anhydrous THF (50 mL) at 0 C. After
being stirred overnight in darkness, the reaction mixture was poured into water and
extracted with CH Cl . The combined organic layer was washed with water and brine,
2
2
dried with MgSO , filtered, concentrated via rotary evaporation. The crude was then
4
purified with column chromatography on silica gel with petroleum
ether/dichloromethane (2:1) as eluent to obtain the title product as a purple black solid
6

ACCEPTED MANUSCRIPT
1
(
1.07 g, 90%). H NMR (500 MHz, CDCl ): δ (ppm) 8.75 (d, J = 4.2 Hz, 2H), 7.46 (d,
3
J = 3.7 Hz, 2H), 7.22 (d, J = 4.2 Hz, 2H), 6.76 (d, J = 3.7 Hz, 2H), 2.94 (t, J = 7.6 Hz,
4
6
1
2
2
H), 1.87-1.75 (m, 4H), 1.50-1.42 (m, 4H), 1.37-1.24 (m, 32H), 0.87 (t, J = 6.9 Hz,
1
3
H). C NMR (125 MHz, CDCl ): δ (ppm) 153.38, 151.39, 146.42, 138.70, 137.39,
3
33.88, 133.14, 132.31, 129.59, 124.98, 119.83, 119.38, 31.92, 31.45, 30.60, 29.69,
9.57, 29.37, 29.20, 22.68, 14.11.
.2.4. Polymer synthesis
The synthesis of PBDTT-TQxP, PBDTT-TQxT and PNDF-TQxT via Stille
coupling is shown in Scheme 1. Typically, in a schlenk tube, M1 (or M2) and M3 (or
M4) (0.2 mmol) were dissolved in dry toluene (5 mL). The catalyst Pd (dba) (5.49
2
3
mg, 0.006 mmol, 3%) and ligand P(o- tolyl) (7.30 mg, 0.024 mmol, 12%) were
3
added quickly. After flushing with nitrogen for 15 minutes, the reaction mixture was
heated to reflux for 48 h. The reaction mixture was then allowed to cool down to
room temperature before pouring into methanol (200 mL). The precipitate was
collected and Soxhlet-extracted in sequence with methanol, acetone, hexane and
chloroform. Polymer recovered from chloroform fraction was precipitated out of
methanol. The polymer was dried under vacuum. The yield and NMR results of the
polymers were listed as follows.
1
PBDTT-TQxP: a green black solid (90% yield). H NMR (500 MHz, CDCl ): δ
3
(ppm) 8.56-7.33 (br, 8H), 7.26-7.05 (br, 8H), 4.15-3.87 (br, 8H), 3.10 (br, 4H),
1
.95-1.22 (br, 120H), 0.89-0.85 (br, 18H). Molecular weight by GPC: M = 77.51
n
KDa, PDI = 3.24.
1
PBDTT-TQxT: a green black solid (70% yield). H NMR (500 MHz, CDCl ): δ
3
(ppm) 8.79-6.56 (br, 14H), 2.94 (br, 8H), 1.44-1.06 (br, 80H), 0.88 (br, 12H).
Molecular weight by GPC: M = 22.26 KDa, PDI = 2.20.
n
1
PNDF-TQxT: a green black solid (65% yield). H NMR (500 MHz, CDCl ): δ
3
(ppm) 8.83-6.82 (br, 12H), 3.06 (br, 8H), 1.28 (br, 68H), 0.88 (br, 12H). Molecular
weight by GPC: M = 14.62 KDa, PDI = 1.35.
n
2
.3. Device fabrication and characterization
7

ACCEPTED MANUSCRIPT
P3HT and PC BM were purchased from Luminescence Technology Corp and
71
Organtec Materials Inc, respectively. PSCs were fabricated with a general structure of
ITO/PEDOT:PSS/polymer:PC BM/LiF/Al. The thickness and sheet resistance of the
71
indium tin oxide (ITO) were 130 nm and 15 Ω/square, respectively. The ITO glass
substrates were sonicated sequentially in deionized water, acetone, deionized water
and ethanol. Prior to device fabrication, the cleaned substrates were dried with
nitrogen and treated with UV-ozone for 10 min for further cleaning. Firstly, the
solution of poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT/PSS,
H.C. Starck Co. Ltd.) was spin-coated at 5000 r/min for 40 s. PEDOT:PSS coated
ITO substrates were then dried at 120°C for 10 min. Then, the active blend layers
(
polymer:PC BM) were fabricated by spin-coating in a high purity nitrogen filled
71
glove box, on the top of PEDOT/PSS layer from their o-dichlorobenzene (o-DCB)
solutions. Finally, layers of LiF (1 nm) and Al (100 nm) were subsequently
evaporated using a shadow mask in vacuum to form a top anode to complete the
2
photovoltaic device fabrication. The active area of each device was about 3.8 mm ,
which is defined by the overlap of ITO anode and Al cathode. The J-V characteristics
and PCE were measured with a Keithley 4200 semiconductor characterization system
and an ABET Sun 2000 solar simulator at room temperature in a nitrogen filled
glove-box. The EQE spectra were measured by Zolix Solar Cell Scan 100. The film
morphologies were investigated with AFM on a multimode Nanoscope IIIa in tapping
mode.
3
. Results and discussion
3
.1. Synthesis and characterization
The synthetic routes for monomers M1 and M4 are outlined in Scheme 1.
,6-Bis(trimethyltin)-4,8-bis(5-dodecylthiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene
2
(M1) [44,45] and 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole-5,6-diamine (8)
[
46] were synthesized according to the reported procedure. The detailed syntheses are
described in Electronic Supplementary Information (ESI). The synthesis of
8

ACCEPTED MANUSCRIPT
(1,6-dinonylnaphtho[1,2-b:5,6-b’]difuran-2,7-diyl)-bis(trimethylstannane) (M2) [42]
and
6,7-bis(3,4-bis(dodecyloxy)phenyl)-4,9-bis(5-bromothiophen-2-
yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline (M3) [43] was adapted from our previous
report. The condensation between diamine 8 and diketone 3 afforded 9 in excellent
yield
,9-bis(5-bromothiophen-2-yl)-6,7-bis(5-dodecylthiophen-2-yl)-[1,2,5]thiadiazolo[3,4
g]quinoxaline (M4) in 90% yield. With monomers M1-M4 in hand, three new D-A
alternating copolymers were synthesized by Stille coupling reaction (Scheme 1).
(92%).
Further
bromination
of
9
achieved
4
-
R
R
S
S
i) n-BuLi,-78o
ii) 1-dodecylbromide
C
i) n-BuLi, THF
2
S
i) n-BuLi, THF
ii) SnMe3Cl
S
S
R
S
Sn
Sn
S
ii) SnCl , HCl
S
S
1
S
S
2
M1
O
O
R
R
i) n-BuLi, LiBr, CuBr
S
N
R=C H
R
12 25
S
O
O
ii)
R
R
Cl
Cl
3
S
N
S
Bu
Sn Bu
Bu
N
N
S
S
N
H
2
N
NH
2
N
N
N
Et
3
N, SOCl
2
Br2
Fuming HNO3
S
Br
Br
S
S
S
CH
2
Cl
2
HBr
Br
Br
3 2 2
Pd(PPh ) Cl , THF
con.H
2
SO
4
O
2
N
NO
2
O
2
N
NO
2
5
6
4
7
S
N
N
O
O
N
N
S
S
S
N
N
S
S
Br
Br
Fe
R
R
NBS
S
S
N
N
S
S
N
N
AcOH
AcOH
H
2
N
NH
2
S
S
S
S
8
R
R
R
R
9
M4
RO
OR
OR
R
R
S
RO
S
S
N
N
N
N
N
S
Br
Br
S
S
S
Sn
Sn
S
S
S
N
S
n
N
N
S
S
S
RO
OR
OR
M1
R
PBDTT-TQxP
R
RO
M3
R
R
R
R
S
N
N
S
S
S
Pd
2 3
(dba)
S
P(o-Tolyl)
3
S
Br
Br
N
N
S
S
Sn
Sn
Toluene
S
S
S
S
N
N
S
n
S
S
S
N
N
S
S
M1
R
R
R
PBDTT-TQxT
R
M4
S
N
N
R
R
S
S
O
Sn
Br
Br
C
9
H
19
S
S
N
N
9 19
C H
N
N
C
9
H
19
S
Sn
O
O
S
S
S
n
O
C
9
H
19
N
N
M2
R
R
S
M4
PNDF-TQxT
Scheme 1. Synthetic routes for monomers and three D-A alternating polymers.
9

ACCEPTED MANUSCRIPT
The polymers exhibit good solubility in common organic solvents such as
chloroform and o-dichlorobenzene. Molecular weight and polydispersity index (PDI)
of the polymers were determined by gel permeation chromatography (GPC) analysis.
The number-average molecular weight (M ) of polymers was estimated to be 77.51,
n
2
2.26 and 14.62 kDa for PBDTT-TQxP, PBDTT-TQxT and PNDF-TQxT,
respectively, with a corresponding PDI of 3.24, 2.20 and 1.35. PBDTT-TQxP
presented the highest M due to the existence of more solubilising alkyloxy side
n
chains.
All polymers show good thermal stability. As shown from the TGA traces in Fig. 1,
PBDTT-TQxP
,
PBDTT-TQxT and PNDF-TQxT exhibit a decompose temperature
o
(
T , 5% weight-loss temperature) at 405, 411 and 394 C, respectively. Good thermal
d
stability of polymer donors is necessary for device fabrication and applications in
optoelectronic devices. Defferential scanning calorimetry analysis of these polymers,
however, did not show any glass transition and crystallization behaviour within the
o
temperature range (0~280 C), indicating the amorphous nature of the polymers.
Fig. 1. TGA curves of these copolymers.
3
.2. Optical and electrochemical properties
In order to reveal the impact of alkylthienyl group on the optical properties of
TQxT, the UV-vis absorption spectra of M3 and M4 were recorded in chloroform
-5
solutions (~10 M). As clearly shown in Fig. 2a, TQxP solution shows one absorption
band in the range of 420-760 nm with a λmax at 532 nm. The absorption of TQxT
1
0

ACCEPTED MANUSCRIPT
solution exhibits red-shifted with two major absorption bands in the wavelength
ranges of 460-600 and 600-800 nm with a λmax at 526 and 656 nm, respectively. This
indicates that the electron-accepting TQxT unit modified with alkylthienyl group
could benefit to enhance absorption. Meanwhile, the UV-vis-NIR spectra of three
polymers in both CHCl solutions and thin films spin-coated on quartz substrates are
3
shown in Fig. 2a-b. All polymers exhibited similar absorption profile featuring two
main absorption bands. The first band at shorter wavelength (350~600 nm) is
attributed to the π-π* transition of the main chain, while the second at longer
wavelength (700~1100 nm) to the intermolecular charge transfer (ICT) interaction
between D and A units in polymers backbones.
Fig. 2. Absorption spectra of (a) two accepting monomers in solutions and
PBDTT-TQxP in both solution and film, (b) PBDTT-TQxT and PNDF-TQxT in
both solutions and films.
The absorption maxima of the polymer films are all red-shifted in comparison to
their solutions, which is a common phenomenon for conjugated polymers owing to
1
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ACCEPTED MANUSCRIPT
the aggregation of polymer main-chains in solid states. PBDTT-TQxP
,
PBDTT-TQxT and PNDF-TQxT films show a λmax at 918, 940 and 995 nm,
respectively, which is 20, 80 and 50 nm red-shifted than their solutions. Apparently,
PBDTT-TQxT or PNDF-TQxT show more extended absorption than
PBDTT-TQxP, which is in coincidence with the stronger electron-deficient nature of
TQxT unit. Interestingly, PBDTT-TQxP exhibits a broad peak from 800 to 1000 nm,
which is probably related to the higher M and the increased extent of π-π stacking of
n
the backbones. Compared with the absorption spectra of similarly-structured TQx
polymers [46], much broader absorption is observed for PBDTT-TQxT and
PNDF-TQxT in our study. The thiadiazole quinoxaline based polymers all show an
extended absorption to near-infrared region (beyond 1100 nm). However, all three
polymers show a weak absorption in 500~700 nm region of the active solar spectrum,
which may result in the low photon harvesting in PSCs. The detailed data are listed in
Table 1.
Table 1. Optoelectronic data of polymers
UV-vis absorption
Cyclic voltammetry
Solution Film
Polymer
opt
λmax
nm)
λmax
(nm)
918
940
995
b)
λonset
Eg
Eox
(V)
0.76
0.84
0.78
HOMO
(eV)
-5.16
-5.24
-5.18
LUMO
a)
b)
(
(nm) (eV)
1130 1.10
1180 1.05
1200 1.03
(eV)
PBDTT-TQxP 898
PBDTT-TQxT 860
-4.06
-4.19
-4.15
PNDF-TQxT
a)
945
opt
HOMO = - (E + 4.4) (eV); LUMO = HOMO + E (eV)
ox
g
opt
The optical bandgaps (Eg ) of the polymers were determined from the
opt
UV-vis-NIR absorption onsets of films according to equation: Eg = 1240/λonset eV.
opt
The Eg of PBDTT-TQxP
,
PBDTT-TQxT and PNDF-TQxT were thus estimated
onset
to be 1.10, 1.05 and 1.03 eV, respectively. The onset potential of oxidation (Eox
was observed to be 0.76, 0.84 and 0.78 V for PBDTT-TQxP
PNDF-TQxT, respectively (Fig. 3a). Accordingly, the HOMO energy level of
PBDTT-TQxP PBDTT-TQxT and PNDF-TQxT can be calculated to be -5.16,
)
,
PBDTT-TQxT and
,
1
2

ACCEPTED MANUSCRIPT
opt
-
5.24 and -5.18 eV, respectively. With Eg and HOMO energy level at hand, the
LUMO energy level of PBDTT-TQxP, PBDTT-TQxT and PNDF-TQxT was thus
calculated to be -4.06, -4.19 and -4.15 eV, respectively. The detailed data are also
listed in Table 1. The LUMO levels of copolymers are too close to that of PCBM to
be non-effective for charge dissociation.[37] The HOMO-LUMO energy diagrams of
PBDTT-TQxP, PBDTT-TQxT and PNDF-TQxT are compared in Fig. 3b, together
with that of PC BM.
71
Fig. 3. (a) Cyclic voltammograms of polymer films on platinum electrode, (b) HOMO
and LUMO energy levels of three polymers.
3
.3. Theoretical calculations
To simplify the theoretical calculations, long alkyl or alkyloxy side chains were
substituted by methyl or methoxy. The theoretical models were optimized initially
using HF/3-21G level. The optimal geometries and electronic structures of the
polymers were simulated with density functional theory (DFT) calculations at the
B3LYP/6-31G(d, p) level using Gaussian 09 program suite [47,48]. As shown in Fig.
4
, the electron density of LUMO mainly concentrates on the accepting segment. The
HOMO/LUMO energy levels of PBDTT-TQxP PBDTT-TQxT and PNDF-TQxT
,
were estimated at -4.49/-3.06 eV, -4.56/-3.16 eV and -4.45/-3.12 eV, respectively.
The bandgaps of the three polymers were accordingly calculated to be 1.43, 1.40 and
1
.33 eV. Obviously, the trends in bandgaps, HOMO and LUMO levels are in
agreement with those obtained by the UV–vis absorption and CV measurements. It
1
3

ACCEPTED MANUSCRIPT
showed that PNDF-TQxT had the lowest bandgap. PBDTT-TQxP presents higher
LUMO level than PBDTT-TQxT and PNDF-TQxT. As we know, good planarity is
beneficial for π-π stacking to achieve high mobility. Therefore, dihedral angles
between the repeating units in the three polymers were calculated and showed in
Table 2. Dihedral angle values of the three polymers revealed that the planarity had an
order of PNDF-TQxT > PBDTT-TQxP > PBDTT-TQxT. When the side chain of
[
1,2,5]thiadiazolo[3,4-g]quinoxaline group, alkyloxyphenyl group was replaced by
alkylthienyl group, the dihedral angles between [1,2,5]thiadiazolo[3,4-g]quinoxaline
unit and the neighbouring thienyl units were enlarged. The dihedral angles between
NDF unit and the neighbouring thienyl units were obviously decreased compared with
that between BDT unit and the neighbouring thienyl units. The calculated values are
not exactly consistent with the experimental data due to the small number of aromatic
cycles taken into account by the calculation. However, this theoretical analysis still
shows the interplay between their structural modifications and the resulting electronic
and optical changes.
Fig. 4. Caption Optimized geometries and molecular orbital surfaces of the HOMO
and LUMO of the model compounds.
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4

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Table 2. Dihedral angles of the three polymer repetition units obtained at the
DFT/B3LYP/6-31 G(d, p) level
Polymer
Structure
Dihedral angels/(degree)
O
O
O
O
S
S
S
D1
D4
N
N
N
N
N
S
S
S
D = -9.644, D = 14.213,
S
1
3
2
PBDTT-TQxP
S
S
S
S
D
= 15.987, D
4
= 3.351
N
N
D3
N
D2
S
S
S
O
O
O
O
D
D
1
= -7.541, D
= 17.668, D
2
= 17.651,
PBDTT-TQxT
PNDF-TQxT
3
4
= 12.911
D
D
1
= -0.733, D
= 13.874, D
2
= 14.115,
= -1.600
3
4
3
.4. Photovoltaic properties
The photovoltaic properties of three polymers were evaluated with configuration
ITO/PEDOT:PSS/polymer:PC BM/LiF/Al in single cells. The photoactive layers
71
were prepared by spin-coating of the blend solution of polymer:PC BM in
71
o-dichlorobenzene (o-DCB). All devices were prepared under nitrogen atmosphere
and characterized in air without encapsulation, where J-V measurements were
-2
performed under illumination of AM 1.5 G (100 mW cm ) simulated solar light.
Different polymer:PC BM weight ratios were investigated to optimize the
7
1
photovoltaic properties. The polymer/PC BM (1:2, w/w) devices showed the best
71
performance (Fig. 5).
PBDTT-TQxP/PC BM device gave a V of 0.42 V, a J of 1.92 mA cm and a
-2
7
1
oc
sc
fill factor (FF) of 53%, resulting in a highest PCE of 0.43%. The PBDTT-TQxT
,
PNDF-TQxT/PC BM devices exhibited a lower V of 0.31 V and 0.33V,
71
oc
respectively. The photovoltaic parameters of the devices are summarized in Table S1.
Both the open-circuit voltage and short-circuit current values for PBDTT-TQxP
,
1
5

ACCEPTED MANUSCRIPT
PBDTT-TQxT and PNDF-TQxT devices were very low, resulting in relatively low
PCEs. This may be explained with the absorption of polymers (Fig. 2). Although our
three polymers have a strong absorption band in longer wavelength region (700 ~1100
nm), the exciton generated by harvesting low energy photons can’t be effectively
dissociated into free charge carrier and collected by individual electrodes.[49, 50] The
ultra low bandgap caused a shift in the lowest unoccupied molecular orbital (LUMO).
The LUMOs of three polymers were closed to that of PC BM, resulting in a lack of
71
driving force for exciton splitting. This might result in the unavoidable reduction of
J and V .
sc
oc
Fig. 5. J-V characteristics of polymer:PC BM (1:2) devices
71
In order to further explore the potential of our ultra low bandgap polymers in PSCs,
the polymers were doped into P3HT:polymer:PC BM (1:2) cells to study the doping
7
1
effect on the photovoltaic performance of P3HT:polymer:PC BM ternary solar cells.
7
1
The J-V characteristic curves of the ternary PSCs based on different PBDTT-TQxP
ratios (3%, 6%, 9% and 12% by weight ratio) were measured under illumination of
-2
AM 1.5 G (100 mW cm ) simulated solar light. As showed in Fig. 6a, the PCE of
P3HT:PC BM (1:2) binary PSCs (control cell) was about 2.93%, with a J of 8.29
7
1
sc
-2
mA cm , a V of 0.62 V and a FF of 57%. After doping with PBDTT-TQxP, the
oc
ternary PSCs exhibited a similar V (~ 0.60 V) as the control cell, while the J and
oc
sc
FF of ternary cells increased and then decreased along with the increased doping ratio
1
6

ACCEPTED MANUSCRIPT
of PBDTT-TQxP. The optimized PCE of ternary PSCs was increased to 3.58% by
doping 9 wt% PBDTT-TQxP, with approximately 22% improvement compared with
-2
the control cell. The J and FF of the ternary PSCs were increased to 9.77 mA cm ,
sc
6
1%, respectively. The photovoltaic parameters of the devices are summarized in
Table 3.
Table 3. Photovoltaic properties of P3HT:polymer:PC BM ternary PSCs with
7
1
different doping ratio of ultra low bandgap polymers.
Doping ratio of polymer Voc (V) J (mA cm ) FF (%) PCE (%)
wt%
-2
sc
0
0.62
0.61
0.60
0.60
0.60
0.54
0.49
8.29
8.71
9.42
9.77
9.13
8.74
8.14
57
62
62
61
57
58
58
2.93
3.31
3.50
3.58
3.12
2.73
2.32
3
6
9
1
6
6
wt% PBDTT-TQxP
wt% PBDTT-TQxP
wt% PBDTT-TQxP
2 wt% PBDTT-TQxP
wt% PBDTT-TQxT
wt% PNDF-TQxT
In order to clarify the effect of on the performance of ternary PSCs, the EQE
spectra of ternary PSCs were measured (Fig. 6b). The EQE spectra shape of ternary
PSCs was similar with that of control cell. The ternary solar cells did not display
photocurrent response to light wavelength longer than 700 nm from the EQE spectra.
However, the EQE values of 350-650 nm were significantly fluctuated with different
doping ratios of the ultra low bandgap copolymer (PBDTT-TQxP). The variation of
EQE values with PBDTT-TQxP doping ratios is consistence with the change of Jsc
obtained from J-V characteristic curves. The PSCs with doping 9 wt% PBDTT-TQxP
show the highest EQE values almost in the whole wavelength range and the
maximum EQE value is 59% at 460 nm, while the FF of the ternary PSCs (61%)
exhibited a slight decrease compared with that of the device doped with 6 wt%
PBDTT-TQxP (62%). Therefore, the ideal PBDTT-TQxP doping ratio should play a
key role in optimizing photovoltaic performance.
1
7

ACCEPTED MANUSCRIPT
Fig. 6 (a) J-V curves and (b) EQE spectra of ternary PSCs based on P3HT:PC BM
71
with different PBDTT-TQxP doping ratios.
Ternary PSCs with P3HT:PBDTT-TQxT (or PNDF-TQxT):PC BM as the active
71
layer were also fabricated to investigate the effect of low bandgap polymer doping on
photovoltaic performance. The J-V characteristic curves and EQE spectra of ternary
PSCs doped with 6 wt% PBDTT-TQxT/ or PNDF-TQxT are showed in Fig. 7.
Unfortunately, the V decreases to 0.54 and 0.49 V, respectively. The J , V and FF
oc
sc
oc
of PBDTT-TQxT (or PNDF-TQxT) doped ternary PSCs are all lower than those of
the ternary PSCs doped with 6 wt% PBDTT-TQxP, resulting a lower PCE. The
photovoltaic parameters of the devices are also summarized in Table 3.
Fig. 7 (a) J-V curves and (b) EQE spectra of ternary PSCs based on doping 6 wt%
different ultra low polymers.
3
.5. Morphology
As a crucial factor on the performance of PSCs, the morphological features of the
film were characterized by atomic force microscopy (AFM). The AFM topography is
1
8

ACCEPTED MANUSCRIPT
shown in Fig. 8. The blend films of PBDTT-TQxP:PC BM (1:2) exhibit large
7
1
aggregation domains at micrometer scale with a root-mean-square (RMS) roughness
of 17.349 nm (Fig. 8a). The large domains exert negative effect on the exciton
separation, thus leading to a lower J and PCE. The surface images of P3HT:PC BM
sc
71
(1:2) blend film show a RMS of 13.375 nm. When doping with PBDTT-TQxP, the
resulted P3HT:PBDTT-TQxP:PC BM ternary blend films show more apparent
71
phase separation in comparison to the corresponding binary blend films. By doping
PBDTT-TQxP at 3 wt%, 6 wt%, 9 wt% and 12wt%, the ternary blend film exhibits a
RMS roughness of 16.0, 14.2, 13.4 and 11.0 nm, respectively. The P3HT:PC BM
71
blends doped with 9 wt% PBDTT-TQxP show more homogeneous films with
suitable domain size and bicontinuous interpenetrating networks for charge carrier
transport.[51-55] Serious aggregation and large domain sizes appear upon increased
doping of PBDTT-TQxP, resulting in the decrease FF. From these observations, it
can be concluded that the doping PBDTT-TQxP into P3HT:PC BM can improve the
71
phase separation to enlarge the interfacial area for efficient exciton dissociation and
the bi-continuous interpenetrating networks for efficient charge carrier transport.
Fig. 8 AFM morphology images of (a) PBDTT-TQxP:PC BM (1:2) films as cast, (b)
7
1
P3HT:PC BM (1:2) films, and (c-f) P3HT:PC BM blend films doped with
7
1
71
PBDTT-TQxP at 3 wt%, 6 wt%, 9 wt% and 12 wt%, respectively.
1
9

ACCEPTED MANUSCRIPT
4
. Conclusions
In summary,
,9-bis(5-bromothiophen-2-yl)-6,7-bis(5-dodecylthiophen-2-yl)-[1,2,5]thiadiazolo[3,4
a
strong
electron-accepting
unit,
4
-g]quinoxaline (TQxT), is firstly reported. Three new copolymers based on
thiadiazole quinoxaline accepting units have been prepared to present absorption
spectra beyond the NIR region. Featuring an ultra low bandgap of 1.03~1.10 eV, the
polymers exhibit low LUMO levels (-4.06 ~ -4.19 eV), owing to the strong
electron-accepting nature of thiadiazole quinoxaline unit. The TQxT based
copolymers may be used as organic photodetectors or applications where modulation
of NIR radiation is necessary.[39,40] Primary PSC device tests deliver a best PCE of
0
.43% for PBDTT-TQxP:PC BM (1:2) blend. However, the optimald PCE values of
71
ternary PSCs based on P3HT:PBDTT-TQxP:PC BM (1:9%:2) is 3.58%,
7
1
corresponding to 22% improvement for P3HT:PC BM binary devices. The ternary
71
solar cells did not display photocurrent response to light wavelength longer than 700
nm from the EQE spectra. The ultra low bandgap polymers mainly act as phase
modulator. The improved performance of ternary PSCs is attributed to the
contribution of PBDTT-TQxP in tuning the morphology of active layers. And the
ideal doping ratio could play a key role in optimizing photovoltaic performance. The
ternary BHJ cell approach may provide an avenue for ultra low bandgap polymers to
improve the performance of PSCs.
Acknowledgements
We gratefully acknowledged the financial support from the National Natural
Science Foundation of China (Grant No. 51573077), Program for New Century
Excellent Talents in University (NCET-12-0633), the Jiangsu Province Natural
Science Foundation for Distinguished Young Scholars (BK20130032) and the
Fundamental Research Funds for the Central Universities (30920130111006).
Appendix A. Supplementary data
Supplementary data related to this article can be found at
http://dx.doi.org/xxxxxx.
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Highlights
Thiadiazole quinoxaline-based copolymers with ~1.0 eV
bandgap for ternary polymer solar cells
Jiangsheng Yu, Qiaoshi An, Jiefeng Hai, Xuemei Nie, Baojing Zhou, Fujun Zhang and
Weihua Tang
In this paper, we demonstrate:
ꢀ
ꢀ
ꢀ
the synthesis of [1,2,5]thiadiazolo[3,4-g]quinoxaline alternating polymers with
benzo[1,2-b:4,5-b’]dithiophene or naphtho[1,2-b:5,6-b’]difuran;
the ultra low bandgaps of 1.03~1.10 eV and absorption spectra extended beyond
near-infrared (NIR) region for the polymers;
the improved power conversion efficiency for P3HT:PC BM solar cells by doping
7
1
9
wt% new polymers.

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Supporting Information
Thiadiazole quinoxaline-based copolymers with ~1.0 eV
bandgap for ternary polymer solar cells
a
b
a
a
a
Jiangsheng Yu, , Qiaoshi An, Jiefeng Hai, Xuemei Nie, Baojing Zhou, Fujun
b
a
Zhang,* and Weihua Tang*
a
Key Laboratory of Soft Chemistry and Functional Materials, Ministry of Education,
Nanjing University of Science and Technology, Nanjing, P. R. China. Tel: (86) 25
8
431 7311; E-mail: whtang@mail.njust.edu.cn;
Key Laboratory of Luminescence and Optical Information, Ministry of Education,
b
Beijing Jiaotong University, 100044, China. E-mail: fjzhang@bjtu.edu.cn
Table of Contents
1
2
3
. Synthetic routes of the monomers…………………………………………..S2
. Photovoltaic properties of polymer:PC BM (1:2) solar cells…………….…S5
7
1
. NMR spectra of intermediates and copolymers..……...……………S6
1

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. Synthesis of monomers
1
The synthetic routes for the monomers (M1, M4) were outlined in Scheme S1.
,6-Bis(trimethyltin)-4,8-bis(5-dodecylthiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene
2
(M1) and 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole-5,6-diamine (8) were
synthesized according to the similar procedure in reported literature. The detailed
synthetic procedures are as follows.
Scheme S1. Synthetic routes of the monomers.
2-Dodecylthiophene (1). Thiophene (6.00 g, 71.31 mmol) was dissolved in 100
mL of anhydrous THF under nitrogen atmosphere. The solution was cooled to -78 °C
and 29.7 mL of n-butyllithium (71.31 mmol, 2.4 M) was added dropwise. After being
o
stirred at -78 C for 1 h, 17.77 g of 1-dodecylbromide (71.31 mmol) was added in one
o
portion and the mixture was stirred at -78 C for another 1 h, then warmed to RT
overnight. The mixture was quenched by 20 mL of cool water and extracted with
diethyl ether three times. The organic layer was dried with anhydrous MgSO . After
4
the solvent was removed via rotary evaporation, the residue was purified by vacuum
1
distillation to yield the compound 1 as a colorless oil (17.10 g, 67.73 mmol, 95%). H
2

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NMR (500 MHz, CDCl ): δ (ppm) 7.11 (dd, J = 5.1, 1.2 Hz, 1H), 6.92 (dd, J = 5.1,
3
3
.4 Hz, 1H), 6.78 (dd, J = 3.4, 1.1 Hz, 1H), 2.83 (t, J = 7.7 Hz, 2H), 1.72-1.64 (m,
H), 1.36-1.27 (m, 18H), 0.89 (t, J = 7.0 Hz, 3H).
2
4,8-Bis(5-dodecylthiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene (2). Under
nitrogen atmosphere, 6.2 mL of n-butyllithium (14.98 mmol, 2.4 M) was added
dropwise to the solution of compound 1 (3.44 g, 13.62 mmol) in 50 mL of anhydrous
o
o
THF at -30 C. The mixture was then warmed to 50 C and stirred for 2 h. The system
was cooled in ice-water bath, subsequently, 4,8-dihydrobenzo[1,2-b:4,5-b’]
dithiophen-4,8-dione (1.00 g, 4.54 mmol) was added in one portion, and the mixture
o
was heated at 50 C for another 2 h. After the mixture was cooled to room temperature,
a solution of SnCl ·2H O (8.20 g, 36.32 mmol) in 20 mL of aqueous HCl (10%) was
2
2
added, and the mixture was stirred overnight. The organic layer was washed with
water and brine, dried with MgSO , filtered and concentrated via rotary evaporation.
4
Further purification was carried out by column chromatography on silica gel eluting
with petroleum ether to obtain pure compound 2 as a yellow solid (2.60 g, 3.76 mmol,
1
8
3%). H NMR (500 MHz, CDCl ): δ (ppm) 7.65 (d, J = 5.7 Hz, 2H), 7.46 (d, J = 5.7
3
Hz, 2H), 7.30 (d, J = 3.5 Hz, 2H), 6.92 (d, J = 3.5 Hz, 2H), 2.93 (t, J = 7.7 Hz, 4H),
.83-1.75 (m, 4H), 1.50-1.42 (m, 4H), 1.40-1.27 (m, 32H), 0.89 (t, J = 6.9 Hz, 6H).
,6-Bis(trimethyltin)-4,8-bis(5-dodecylthiophen-2-yl)benzo[1,2-b:4,5-b’]dithi
1
2
ophene (M1). Compound 2 (1.00 g, 1.45 mmol) and 50 mL of anhydrous THF were
added into a flask under nitrogen atmosphere. The solution was cooled to 0 °C and 1.5
mL of n-butyllithium (3.62 mmol, 2.4 M) was added dropwise. The reaction mixture
was then stirred for 2 h at room temperature and cooled to 0 °C. Subsequently, 3.6 mL
of trimethyltin chloride (3.62 mmol, 1.0 M in THF) was added in one portion and the
mixture was stirred at RT overnight. The mixture was quenched by addition of 20 mL
of water and extracted with diethyl ether three times. The combined organic phase
was dried with MgSO , filtered and concentrated via rotary evaporation. The residue
4
was recrystallized in isopropanol to obtain compound M1 as a light yellow solid (1.40
1
g, 1.38 mmol, 95%). H NMR (500 MHz, CDCl ): δ (ppm) 7.68 (s, 2H), 7.31 (d, J =
3
3
.5 Hz, 2H), 6.92 (d, J = 3.5 Hz, 2H), 2.93 (t, J = 7.7 Hz, 4H), 1.83-1.74 (m, 4H),
3

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1
.50-1.41 (m, 4H), 1.39-1.26 (m, 32H), 0.88 (t, J = 6.9 Hz, 6H), 0.39 (s, 18H).
Benzo[c][1,2,5]thiadiazole (4). To a 500 mL flask, o-phenylenediamine (10.00 g,
2.47 mmol), triethylamine (37.43 g, 369.89 mmol) and 300 mL of anhydrous CH Cl
9
2
2
were added. Thionyl chloride was added dropwise slowly and the mixture was
refluxed overnight. The solvent was removed via rotatory evaporator. Then 500 mL of
water and concentrated HCl was added to a final pH of 2. The resultant mixture was
extracted with CH Cl (3 × 200 mL). The combined organic layer was dried over
2
2
anhydrous MgSO , filtered and concentrated. The residue was recrystallized in
4
1
ethanol to obtain compound 4 (8.20g, 65%). H NMR (500 MHz, CDCl ): δ (ppm)
3
8
.02 (dd, J = 6.8, 3.2 Hz, 2H), 7.60 (dd, J = 6.8, 3.2 Hz, 2H).
,7-Dibromobenzo[c][1,2,5]thiadiazole (5). Compound 4 (3g, 22.03mmol) and
4
1
9
00 mL of HBr (48%) were added to a 500 mL flask. A solution containing Br (5 mL,
2
7.55 mmol) in 50 mL of HBr was added dropwise slowly. Then the solution was
refluxed at 100 °C for 6 h. After the solution was cooled to room temperature,
sufficient NaHSO solution (1M) was added to consume excess Br . The mixture was
3
2
filtered and washed exhaustively with water. The crude product was chromatographed
on silica gel eluting with petroleum ether/dichloromethane (2:1). Further purification
was conducted by recrystallization in ethanol to obtain compound 5 as a white
1
needled solid (6.10g, 94%). H NMR (500 MHz, CDCl ): δ (ppm) 7.73 (s, 2H).
3
4
,7-Dibromo-5,6-dinitrobenzo[c][1,2,5]thiadiazole (6). Fuming HNO (50 mL)
3
o
was added dropwise to concentrated H SO (50 mL) at 0 C. Compound 5 (3.50 g,
2
4
o
1
1.91 mmol) was added separately to the acid mixture at 0 C. The mixture was kept
at room temperature overnight. Then the mixture was poured into ice-water (500 mL)
and washed with water. The crude product was dissolved in 200 mL CH Cl . The
2
2
organic layer was washed with brine, dried with MgSO , filtered, concentrated via
4
rotary evaporation and purified by column chromatography on silica gel with
petroleum ether/dichloromethane (2:1) to give pure compound 6 as a pale yellow solid
(
1.50 g, 33%).
,6-Dinitro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole
nitrogen atmosphere, PdCl (PPh ) (91.40 mg, 0.13 mmol) was added to a mixed
5
(7).
Under
2
3 2
4

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solution of 2-tributylstannylthiophene (2.92 g, 7.81 mmol) and compound 6 (1.00 g,
o
2
.60 mmol) in 30 mL of anhydrous THF. The reaction mixture was heated at 80 C for
5 h. After being cooled to room temperature, the solvent was evaporated and the
1
crude product was purified by column chromatography on silica gel with petroleum
ether/ dichloromethane (3:2) to obtain compound 7 as an orange solid (0.80 g, 79%).
1
H NMR (500 MHz, CDCl ): δ (ppm) 7.74 (dd, J = 5.1, 1.2 Hz, 2H), 7.52 (dd, J = 3.7,
3
1
.2 Hz, 2H), 7.24 (dd, J = 5.1, 3.8 Hz, 2H).
,7-Di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole-5,6-diamine
4
(8).
Under
nitrogen atmosphere, compound 7 (1.00g, 2.56mmol) and iron dust (1.72g,
o
3
0.74mmol) were added to 80 mL of acetic acid. The mixture was heated at 75 C for
h. Then the mixture was cooled to room temperature, poured into the water and
5
extracted with ether. The organic layer was washed with water and brine, dried with
MgSO , filtered, concentrated via rotary evaporation and purified by column
4
chromatography with dichloromethane as the eluent to give compound 8 as a brown
1
solid (0.82g, 97%). H NMR (500 MHz, CDCl ): δ (ppm) 7.57 (d, J = 5.2 Hz, 2H),
3
7
.37 (d, J = 3.4 Hz, 2H), 7.27 (s, 2H), 4.40 (s, 4H).
1
. Photovoltaic properties of polymer:PC BM (1:2) solar cells
7
1
Table S1. Photovoltaic properties of binary polymers:PC BM (1:2) PSCs.
71
-2
Polymer
V [V]
oc
J [mA cm ] FF [%]
PCE [%]
sc
PBDTT-TQxP
0.42
1.92
53
0.43
PBDTT-TQxT
PNDF-TQxT
0.31
0.33
3.22
1.41
40
51
0.40
0.24
5