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M.L. Keshtov et al. / Polymer 65 (2015) 193e201
be effectively tuned by using a combination of various types of D and
A units having different electron donating and electron with-
drawing strengths via intramolecular charge transfer. Moreover, the
different structure and solubilizing groups of D and A can change
other properties such as molecular packing and solubility [8].
The ideal DeA polymer should have a combination of weak
donor and strong acceptor units, in which weak donor helps
maintain a low HOMO level whereas strong acceptor would reduce
the band gap of the polymer [9]. Among the various DeA conju-
gated polymers building blocks, 2, 1, 3-benzothiazole (BT) as
acceptor unit, due to its easy preparation, excellent stability and
electro-optical characteristics together with its ability to adopt a
quinoid structure. A DeA polymer with BT derivative as electron
withdrawing unit in which two thiophene moieties (DTBT) were
2. Experimental part
2.1. Instruments and characterization methods
1H and 13C NMR spectra of the starting compounds and co-
polymers were recorded on the spectrometer “Bruker Avance-400”
with a working frequency of 400.13 and 100.62 MHz, respectively.
IR spectra were recorded by a FT-IR spectrometer “PerkineElmer
1720-X”, TGA and DSC analysis was performed on “Per-kin-Elmer
TGA-7” and “PerkineElmer DSC 7” devices with heating rate of 20
deg/min. The absorption spectra in the range 190e1100 nm were
recorded on the spectrophotometer “Varian Cary 50.” The source of
the exciting light was a xenon lamp L8253, which is part of the
block radiator with optical fiber output radiation “Hamamatsu LC-
4.” Cyclic voltammetry measurements were performed on a
potentiostat-galvanostat AUTOLAB Type III equipped with standard
three-electrode scheme in an acetonitrile solution of 0.1 mol/L tri-
flanked into the benzene ring of BT as a p-bridge for reducing steric
hindrance and tuning the electronic properties can also be ideal
donor material of high performance OSCs. In recent years, benzo-
trithiophenes (BTT) have emerged as attractive donor units for the
designing of DeA polymers which showed promising performance
in both OFETs and polymer solar cells [7a]. This kind of donors
4 4
butylammonium perchlorate (n Bu NCIO ) at a potential scan rate
of 50 mV/s. Films of investigated polymers were deposited on a
glass surface coated with ITO and then dried and used as working
electrode. Ag/Agþ and platinum were used as reference and
counter electrodes, respectively.
possesses high coplanarity and extended
p-conjugation, which
should promote intermolecular
BTT containing polymers.
p-stacking and charge transport in
In order to accomplish this task new approach to make the
p-
2.2. Synthesis of copolymers P1 and P2
conjugated polymers was realized based on one and the same
structure of macromolecule (strong alternation of donor and
acceptor fragments, quinoid character of p-conjugation, planarity).
In view of the above mentioned new effective synthetic pathway to
obtain new class of fused thiophene containing monomers with
The synthesis and characterization of intermediate monomers
used for the synthesis of P1 and P2 were described in
Supplementary Information.
increased quinoid character of
p-conjugation and basing on such
2.2.1. Synthesis of polymer P1
monomers by selection of structural types, their composition and
sequence of alternation the conditions for directed synthesis of low
2,8-Dibromo-4,6-bis(n-octylthiophen-2-yl)
b :5,6-c ] trithiophene (10) (0.3964 g, 0.5 mmol) compound 10, 4,7-
benzo[2,1-b:3,4-
0
00
band gap
p-conjugated polymers were realized.
bis[5-(trimethylstannyl)thiophen-2-yl]-2,1,3-benzothiadiazole
In this study, we have synthesized two benzothiadiazole- and
fused bithiophene DeA copolymers P1 and P2 with same acceptor
moiety and different donor units i. e. 2,8-Dibromo-4,6-bis(n-
((0.3102 g, 0.5 mmol), Pd(Ph
3
P)
4
(0.027 g, 0.0234 mmol) and
ꢁ
toluene (20 ml) were heated under argon at 110 C for 48 h. Then 2-
bromothiophene (0.02 g) was added and the mixture was stirred at
0
00
ꢁ
octylthiophen-2-yl) benzo[2,1-b:3,4-b :5,6-c ] trithiophene and
110 C for another 5 h. After cooling to r.t. reaction mixture was
0
0
00 00
2
,5-dibrom-8-heptodecyl-10N-bisthieno
[2 ,3 :6,7;
3 ,2 :4,5]
poured into methanol (200 ml) and filtrated. Polymer was dis-
indeono[1,2-d][1,3] thiazole for P1 and P2, respectively. The later
donor is weak as compared to former. These donor monomers
solved in CHCl
rified by extraction with methanol, hexane and chloroform in
Soxlett apparatus. Yield is 88%. Calc. for C50 ,%: C,64.47;
H,4.98; N,3.00; S,27.54. Found: C, 64.28; H,4.84; N,3.13; S, 27. 69. H
NMR (400 MHz, CDCI , ppm): 8.20e7.40 (6H, Ar), 7.09e6.80 (6H,
3
and precipitated with methanol. Then it was pu-
contains highly fused thiophene unit and can form
p
ep
stacked
46 2 8
H N S
1
structure, which is favorable for increasing charge carrier mobility
and morphology of the resulted film cast from the solution pro-
cessing, which are very important for polymer solar cells. Similar
monomers are synthesized by Mullen and used them for field effect
transistor devices [7] and reported high charge carrier mobility. In
addition, Mullen et al. prepared this monomers in 7 stages (starting
3
, d
Ar), 4.25 (4H, alk), 2.30e0.86 (30H, aliph).
2.2.2. Synthesis of polymer P2
P2 was prepared similarly as P1 using 2,5-dibromo-8-
0
0
0
00 00
from 2,2 -bithiophene) and the target monomers were obtained in
pentadecyl-10H-bisthieno[2 ,3 :6,7,3 ,2 :4,5]indeno[1,2-d][1,3]
only 18% yield (by the Rh e catalyzed cyclization). In our synthetic
scheme there are 6 stages (starting from bithiophene) and the
maximum yield on the cyclization stage is achieved of 72e73%. In
addition, our synthetic stages are simpler and easily scalable to
large loading of starting materials. We modified structure of
monomers by introducing alkylthiophene fragments for increasing
charge carrier mobility, since it's known that mobility is important
for OSC too. The device based on P1:PC71BM showed higher Jsc
while P2:PC71BM exhibited higher Voc and demonstrated overall
PCE of 4.54% and 4.36%, respectively. The higher value of Jsc
attributed to the lower band gap and broader absorption profile of
P1 as compared to P2 and higher Voc for may be attributed the
deeper HOMO energy level of P2 as compared to the P1. The PSC
processed with DIO/CF showed 5.62% and 5.24%, for P1:PC71BM and
P2:PC71BM, respectively, which is attributed the fine morphology
adjustment for better exciton dissociation and charge transport in
the active layer.
thiazole compound 17 and 4,7-bis[5-(trimethylstannyl)thiophen-
2-yl]-2,1,3-benzothiadiazole
with
75%
yield.
Calc.
for
29 10 4 5
C H N S F
8,%:,C47.93; H,1.39; N, 7.71; S,22.06; F, 20.91 Found:C,
47.98; H,1.27; N, 7.64; S,22.36; F, 20.41. H NMR (400 MHz, CDCI , d,
3
1
ppm): 7.74e7.30 (6H, Ar), 7.10e6.89 (29H,alk).
2.3. Device fabrication and characterization
The photovoltaic devices using copolymers as donor and
PC71BM as acceptor were fabricated on the indium tin oxide (ITO)
coated glass substrate as follow: The ITO coated glass substrates
were cleaned continuously in ultrasonic baths containing acetone,
detergent, de-ionized water and isopropanol. Then the cleaned ITO
glass substrates were dried by high purity nitrogen gas and then
treated by UV-ozone for 10 min. The solution of PEDOT:PSS (Clevios
PVP Al 4093) was spin coated onto the cleaned ITO glass substrates
ꢁ
at 2500 rpm for 30 s and subsequently dried at 100 C for 20 min in