Synthesis, optical and electrochemical properties new donor-acceptor (D-A) copolymers based on benzo [1,2-b:3,4-b′:6,5-b″] trithiophene donor and different acceptor units: Application as donor for photovoltaic devices

Article abstract of DOI:10.1016/j.orgel.2014.11.011

Two new conjugated D-A polymers P3 (PBTT-d-BTT) and P4 (PBTT-d-TPD) based on same benzo[1,2-b:3,4-b′:6,5-b″] trithiophene (BTT) donor and different acceptors monomers 5,8-dibromo-2-dodecanoylbenzo[1,2-b:3,4-b′:6,5-b″] trithiophene (d-BTT), and 1,3-dibromo-5-(2-ethylhexyl)thieno[3,4]pyrrol-4,6-dione (d-TPD) respectively, were synthesized by Stille cross-coupling reaction and characterized by gel permeation chromatography (GPC), ¹H NMR, UV-Vis absorption, thermal analysis and electrochemical cyclic voltammetry (CV) tests. Photovoltaic properties of the polymers were studied by using the polymers as donor and PC₇₁BM as acceptor with a weight ratio of polymer:PC₇₁BM 1:1, 1:2 and 1:2.5. The optimized photovoltaic device was fabricated with an active layer of a blend P3:PC₇₁BM and P4:PC₇₁BM with a blend ratio of 1:2 showed PCE 3.16% and 2.42%, respectively under illumination of AM 1.5 at 100 mW/cm² with solar simulator. The PCE of the device based on P3:PC₇₁BM processed with DIO/o-DCB has been further improved up to 4.64% with J_sc of 10.52 mA/cm² and FF of 0.58 attributed to the increase in crystalline nature of active layer and more balanced charge transport in the device, induced by DIO additive.

Full text of DOI:10.1016/j.orgel.2014.11.011

Organic Electronics 17 (2015) 167–177
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Synthesis, optical and electrochemical properties new
donor–acceptor (D–A) copolymers based on benzo
[1,2-b:3,4-b0:6,5-b00] trithiophene donor and different acceptor
units: Application as donor for photovoltaic devices
M.L. Keshtov a, , Y. Geng c, S.A. Kuklin a, A.R. Khokhlov a,b, E.N. Koukaras d,e, G.D. Sharma f,
⇑
⇑
a A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova Str., 28, Moscow 119991, Russia
b Faculty of Physics, M.V. Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia
c State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
d Molecular Engineering Laboratory, Department of Physics, University of Patras, Patras, 26504 GR, Greece
e Institute of Chemical Engineering Sciences, Foundation for Research & Technology, Hellas, Stadiou Str. Platani, Patras 26504, Greece
f R&D Center for Engineering and Science, JEC Group of Colleges, Jaipur Engineering College, Jaipur 303101, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 October 2014
Received in revised form 10 November 2014
Accepted 12 November 2014
Available online 12 December 2014
Two new conjugated D–A polymers P3 (PBTT-d-BTT) and P4 (PBTT-d-TPD) based on same
benzo[1,2-b:3,4-b0:6,5-b00] trithiophene (BTT) donor and different acceptors monomers
5,8-dibromo-2-dodecanoylbenzo[1,2-b:3,4-b0:6,5-b00] trithiophene (d-BTT), and 1,3-dibromo-
5-(2-ethylhexyl)thieno[3,4]pyrrol-4,6-dione (d-TPD) respectively, were synthesized by
Stille cross-coupling reaction and characterized by gel permeation chromatography
(GPC), 1H NMR, UV–Vis absorption, thermal analysis and electrochemical cyclic voltamme-
try (CV) tests. Photovoltaic properties of the polymers were studied by using the polymers
as donor and PC71BM as acceptor with a weight ratio of polymer:PC71BM 1:1, 1:2 and 1:2.5.
The optimized photovoltaic device was fabricated with an active layer of a blend P3:PC71BM
and P4:PC71BM with a blend ratio of 1:2 showed PCE 3.16% and 2.42%, respectively under
illumination of AM 1.5 at 100 mW/cm2 with solar simulator. The PCE of the device based
on P3:PC71BM processed with DIO/o-DCB has been further improved up to 4.64% with Jsc
of 10.52 mA/cm2 and FF of 0.58 attributed to the increase in crystalline nature of active layer
and more balanced charge transport in the device, induced by DIO additive.
Keywords:
D–A copolymers
DFT calculations
Optical and electrochemical properties
Bulk heterojunction solar cells
Power conversion efficiency
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
time [2]. In the past decade, significant progress in polymer
solar cells (PSCs) has been achieved in the development of
Organic solar cells based on bulk heterojunction active
layer concept have gain lot of attention as renewable
energy technology due to their advantages such as flexibil-
ity, low cost solution processed roll to-roll large area fabri-
cation techniques [1] and extremely short energy payback
low bandgap polymers, optimization of fabrication pro-
cessing, modification of morphology of active layer, power
conversion efficiencies of theses devices have reached
about 9–10% solar cells using BHJ concept [3] and more
than 10% for tandem organic solar cells [4]. Among them,
the development of high performance donor–acceptor
(D–A) type low bandgap conjugated copolymers have been
designed to achieve efficient PSCs, and many promising
D–A type conjugated copolymers have been developed in
⇑
Corresponding authors.
E-mail addresses: keshtov@ineos.ac.ru (M.L. Keshtov), sharmagd_in@
yahoo.com, gdsharma273@gmail.com (G.D. Sharma).
http://dx.doi.org/10.1016/j.orgel.2014.11.011
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

168
M.L. Keshtov et al. / Organic Electronics 17 (2015) 167–177
last decades [5]. Combination of already existing structures
and development of new donor and acceptor fragments to
form new low band gap donor–acceptor copolymers for
improving the understanding of the physical processes
and the efficiency of solar cells are very important. Benzo-
trithiophenes (BTT) have emerged as attractive donor units
for the designing of D–A copolymers and show promising
performance in both organic field effect transistors and
PSCs [6]. BTT donors possess high coplanarity and
and 5,8-dibromo-2-dodecanoylbenzo[1,2-b:3,4-b0:6,5-b00]
trithiophene (5) respectively with high yields. The last
have been reduced by hydrazine hydrate in
a base
conditions. The resulting compound, 5,8-dibromo-2-
dodecylbenzo[1,2-b:3,4-b0:6,5-b00] trithiophene (6) has
been transformed into desired compound 7 by halogen
exchange reaction at a-positions of thiophenes and by sub-
sequent alkylation with the trimethylstannyl electrophile.
The composition and structure of the intermediate
compounds 1–4, 6 and the desired compounds 5 and 7
have been confirmed by elemental analysis, 1H and 13C
NMR spectroscopy. In particular, typical signals of C@O
group have been observed on the 13C NMR spectrum of
compounds 2, 3, 4 and 5 in the area 193–195 ppm but
the same signal have disappeared on the 13C NMR
spectrum of compounds 6 b monomers 7. There are three
singlets in the areas 7.84–7.41 ppm, 7.53–7.03 ppm,
7.87–7.31 ppm related to the protons of thiophene moie-
ties and signals in the area 3.03–0.90 ppm related to the
aliphatic protons in the 1H NMR spectrum of compounds
5–7 respectively. Also there is multiplet related to the
eighteen protons of trimethylstannyl group in the area
0.50 ppm in the 1H NMR spectrum of desired product 7.
The ratio of the integrated intensity of the aromatic and
aliphatic protons for all intermediate compounds and
desired product corresponds to the presumed structures.
A P3 and P4 copolymers on the basis of the synthesized
extended
p-conjugation which promote intermolecular
p-stacking and charge transport in BTT-containing copoly-
mers. The large BTT-structure is planar and leads to the
efficient intramolecular packing in the solid state.
Polymers based on BTT are highly ordered, have partially
crystalline structure and exhibit good mobility of the
charge carriers. However, very few studies have been
devoted to the benzotrithiophene structures [7]. Large
BTT-structure is planar and leads to the efficient intramo-
lecular packing in the solid state. Copolymers based on
BTT are highly ordered, have partially crystalline structure
and exhibit good mobility of the charge carriers.
In this study new benzotrithiophene-containing donor
monomer 7 and different acceptors promising ‘‘building
blocks’’ were developed, and two new-containing low band
gap donor–acceptor conjugated polymers with same
benzotrithiophene donor and different donor 5,8-
dibromo-2-dodecanoyl[2,3-e:30,20-g][1]benzotrithiophene
(P3) (PBTT-d-BTT) and 1,3-dibromo-5-(2-ethylhexyl)thie-
no[3,4]pyrrol-4,6-dione (P4) (PBTT-d-TPD) were synthe-
sized and investigated their optical, electrochemical and
theoretical simulation of frontier energy levels and absorp-
tion. These copolymers were used as donor along with
PC71BM as acceptor for BHJ polymer solar cells and showed
moderate power conversion of 3.16% and 2.42%, for
P3:PC71BM and P4:PC71BM with weight ratio of 1:2, respec-
tively. To improve further the PCE, we have used DIO as a
solvent additive for P3:PC71BM processing and found that
the PCE was enhanced up to 4.64% with Jsc of 10.52 mA/
cm2 and FF of 0.58 attributed to the increase in crystalline
nature of active layer and more balanced charge transport
in the device, induced by DIO additive.
heteroaromatic thiophene-containing monomer
7 has
been developed in Stille cross-coupling reaction conditions
according to the Scheme 1.
5,8-bis(trimethylstannyl)-2-dodecylbenzo[1,2-b:3,4-b0:
6,5-b00] trithiophene (7) is the electron donor moiety in the
structure of both P3 and P4. Monomers 5,8-dibromo-2-
dodecanoylbenzo[1,2-b:3,4-b0:6,5-b00] trithiophene, 1,3-
dibromo-5-(2-ethylhexyl)thieno[3,4] pyrrol-4,6-dione are
the electron acceptor fragments. Polycondensation was
carried out in argon atmosphere in a mixture of toluene
and DMF at 115 °C for 48 h, using tetrakis(triphenylphos-
phine) palladium as a catalyst. The resulting polymers P3
and P4 were purified from metal complex catalyst resi-
dues, organotin and low molecular weight impurities by
double reprecipitation from solution in methanol and
sequential extraction with methanol, hexane, acetone
and chloroform in a Soxhlet apparatus. The yield of poly-
mers P3 and P4 was in the range 69% and 71%, respec-
tively. The copolymers are soluble in common organic
solvents such as DMF, N-methylpyrrolidone, chloroform,
THF, o-dichlorobenzene.
2. Results and discussion
2.1. Synthesis and characterization of the polymer
New monomers 5,8-dibromo-2-dodecanoylbenzo[1,2-
b:3,4-b0:6,5-b00] trithiophene (5) and 5,8-bis(trimethylstan-
nyl)-2-dodecylbenzo[1,2-b:3,4-b0:6,5-b00] trithiophene (7)
have been developed by sequential reactions according
Scheme 1 involving bromination of 3-bromothiophene fol-
lowed by acylation of obtained 2,3-dibromothiophene (1)
to form intermediate compound 2,3-dibromo-5-dodeca-
noylthiophene (2) with 89% yield. 5-Dodecanoyl-2,3-
bis(thiophen-2-yl)thiophene derivative (3) has been
synthesized by interaction of intermediate compound 2
with 2 mol of thiophene-2-boronic acid using Suzuki reac-
tion conditions with palladium catalyst leading to the 63%
yield, subsequent bromination with NBS and cyclization
with FeCl3 have led to the intermediate compound
5-dodecanoyl-2,3-bis(5-bromotiophen-2-yl)thiophene (4)
The composition and structure of copolymers P3 and P4
were confirmed by 1H NMR spectroscopy. In particular,
multiplet signals that correspond to aromatic protons of
fused thiophene fragments are presented in the 1H NMR
spectrum of polymer PI in the area dH = 8.5–6.5 ppm. Broad
resonance signals related to protons of CH2 groups which
are bounded with N have been obtained in the range
dH = 3.5–2.9 ppm. Signals observed in the range dH = 2.7–
0.56 ppm correspond to the rest protons of the alkyl
substituents of the polymer unit. 1H NMR spectrum of
polymers are quite complex, however the ratio of the inte-
grated intensity of the aromatic and aliphatic protons for
both copolymers P3 and P4, corresponds to the presumed

M.L. Keshtov et al. / Organic Electronics 17 (2015) 167–177
169
Scheme 1.
transition temperature of P3 and P4 are 303 °C and
286 °C, respectively. The good thermal stability of the
resulting polymers retards the deformation of the polymer
morphology and the degradation of the active layer at
elevated temperature which are desirable for polymer in
PSCs.
Table 1
Molecular weights and the thermal properties P3 and P4 copolymers.
Copolymer
Mn (kDa)
Mw (kDa)
Mw/Mn
Tg (°C)
T5% (°C)
P3
P4
3.3
2.4
9.7
3.6
2.9
1.5
303
286
323
330
structures of the macromolecules. The weight-average
molecular weight (Mw) and polydispersity index (Mw/Mn)
were measured by gel permeation chromatography (GPC)
using 1,2,4-trichlorobenzene as eluent at 150 °C and poly-
styrene as the internal standard. The weight-average
molecular weight (Mw) of both copolymers are in the range
of 9700–12000 with corresponding polydispersity index in
the range of 1.5–2.9. The related data are summarized in
Table 1.
3. Optical properties
The normalized UV–visible absorption spectra of dilute
solutions of P3 and P4 in o-dichlorobenzene (o-DCB) and
as well as in thin films, deposited on quartz glass, are
shown in Fig. 1a and b. Table 2 summaries the optical data
including absorption peak wavelengths (kmax), absorption
edge wavelengths (konset) and optical bandgap ðEoptÞ. Both
g
the copolymers P3 and P4 showed two absorption bands
in dilute o-DCB solution, which is the common feature of
D–A copolymers. The absorption band in shorter wave-
length region (406 and 418 nm for P3 and P4, respectively)
2.2. Thermal properties
Thermal properties of copolymers were characterized
by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) under a nitrogen atmosphere.
All Copolymer P3 and P4 exhibited good thermal
stability with 5% weight-loss temperature (T5%) in the
range 323 °C and 330 °C, respectively (Table 1). The glass
originated from the localized p–
p⁄ transition of the
conjugated backbone, whereas the absorption band at a
longer wavelength region (508 nm and 498 nm for P3
and P4, respectively) attributed to the intramolecular
charge transfer (ICT) interaction between the donor and
BTT and acceptor (d-BTT and d-TPD units for P3 and P4)

170
M.L. Keshtov et al. / Organic Electronics 17 (2015) 167–177
Fig. 2. Cyclic voltammogram of the P3 and P4 copolymers as solid film.
investigate the redox behavior on the polymer and obtain
its HOMO and LUMO energy levels. HOMO and LUMO
energy levels are key parameters for the application of
the conjugated polymers in PSCs. Therefore we measured
the HOMO and LUMO energy levels of these copolymers
by cyclic voltammetry and shown in Fig. 2. Both the
copolymers showed stable one reversible oxidation and
one reversible reduction peaks, which are a good sign for
high structural stability in the charged state. From the
onset
ox
onset oxidation potentials ðE
Þ and the onset reduction
onset
red
Fig. 1. Absorption spectra of polymers in a o-DCB solution and thin films
for (a) P3 and (b) P4 copolymers.
potentials ðE
Þ of the copolymers HOMO and LUMO
energy level as well as the energy gaps were calculated
according to the following equations:
[8]. The absorption band in longer wavelength region for
P3 is redshifted as compared to P4 may due to the stronger
ICT interaction between BTT and d-BTT. The absorption
spectra of these copolymers in thin films were similar to
the corresponding absorption spectra in solutions, with a
clear red-shift of absorption maxima and broadening of
band, which is generally observed in conjugated systems
owing to aggregation of polymer main chains and inter-
chain interactions in solid state [9]. It can be seen from
the absorption spectra of P3 that P3 showed a vibronic
shoulder at longer wavelength region, which indicated that
there were some ordered structure in the polymer film and
is benefit for the increase in mobility and photovoltaic
performance of polymer [10]. The optical bandgap was
calculated to be 1.68 eV (absorption edge ꢀ738 nm) for
P3 and 1.86 eV for P4 (absorption edge ꢀ666 nm.
onset
ox
HOMO ðeVÞ ¼ ꢁeðE
þ 4:7Þ
þ 4:7Þ
onset
red
LUMO ðeVÞ ¼ ꢁeðE
HOMO energy levels P3 and P4 were ꢁ5.17 and ꢁ5.30 eV
respectively, implying that they varied with respect to
the modulated ICT strengths resulting from the presence
of electron acceptors with various electron-accepting abil-
ities. The LUMO levels were about ꢁ3.43 eV and ꢁ3.26 eV,
for P3 and P4, respectively. The LUMO gaps of 0.57–0.74 eV
and HOMO gaps of 1.17–1.30 eV between copolymers and
PC71BM may provide a sufficient driving force for efficient
exciton dissociation/charge transfer at D–A interface, in
their corresponding devices [11]. In addition the electro-
chemical band gap (Egelec) of P3 and P4 estimated from
the difference between the onset potentials for oxidation
and reduction were 1.72 and 1.95 eV, for P3 and P4,
respectively. These values are slightly larger than the
corresponding optical band gap ðEogptÞ. The difference
between the electrochemical and optical band gaps could
4. Electrochemical properties
The electrochemical behaviors of the copolymers were
investigated by cyclic voltammetry (CV). We used CV to
Table 2
Optical and electrochemical properties of the P3 and P4 copolymers.
Eegc (eV)
ec
ðEogptÞ (eV)
sol
max
film
max
ech
Copolymer
HOMO (eV)
LUMO (eV)
ðE Þ (eV)
k
(nm)
k
(nm)
ðE Þ (eV)
ox
red
P3
P4
406,508
418,498
463,526
432,516
1.68
1.86
0.47
0.60
ꢁ5.17
ꢁ5.30
ꢁ1.15
ꢁ1.58
ꢁ3.43
ꢁ3.26
1.84
2.04

M.L. Keshtov et al. / Organic Electronics 17 (2015) 167–177
171
be explained by the exciton binding energy of the copoly-
mers and/or interfacial barriers for charge injections [12].
corresponding experimental values. The calculated wave-
lengths (using B3LYP) for the first and the highest peaks
are in good agreement with the corresponding experimen-
tal ones, overestimated at most by about 10 nm in the large
wavelength region. The transitions that are shown in
Table 3 are those with a contribution over 4% to the first
excitation. The first excitation for both structures exhibits
a clear single-configuration HOMO to LUMO character,
except in the case of SB19 for which the PBE functional also
attributes a significant contribution from a transition from
the next to HOMO state.
4.1. Theoretical calculations
We have additionally theoretically examined the P3
and P4 monomers within the framework of density func-
tional theory (DFT) and time-dependent density functional
theory (TD-DFT). At the first stage of calculations the struc-
tures were optimized employing the gradient corrected
functional PBE [13] of Perdew, Burke and Ernzerhof. The
geometry optimizations were performed initially using
the SVP basis set [14] followed by a subsequent optimiza-
tion using the triple-f quality TZVP basis set [15]. At this
stage the resolution of the identity method [16] was used
for the treatment of the two-electron integrals to increase
the computational efficiency (without loss in accuracy).
The resulting structures were further optimized using the
hybrid exchange–correlation functional B3LYP, [17] and
the same basis set. Tight convergence criteria were placed
for the SCF energy (up to 10–7 Eh) and the one-electron
density (rms of the density matrix up to 10–8) as well as
for the norm of the Cartesian gradient (residual forces both
average and maximum smaller than 1.5 ꢂ 10–5 a.u.) and
residual displacements (both average and maximum
smaller than 6 ꢂ 10–5 a.u.). Solvent effects were included
for o-dichlorobenzene using the integral equation formal-
ism variant of the Polarizable Continuum Model (IEFPCM),
as implemented in the Gaussian package [18].
In Fig.
3 we have plotted the isosurfaces (iso-
value = 0.02, B3LYP) of the HOMO and LUMO for P3 and
P4. For both monomers we additionally show MOs down
to HOMOꢁ1 for which a clear localization appears over
the benzotrithiophene moiety (in both cases). For P3 the
LUMO is highly localized over the benzotrithiophene moi-
ety and the nearby double bonded oxygen. For SB19 the
LUMO is localized over the pyrrolothiophene with a signif-
icant delocalization toward the benzotrithiophene. For
both structures the HOMOs extend on the backbone of
The excited state calculations have been performed in
the framework of time-dependent DFT to calculate the
optical gaps and the UV/Vis spectra of the monomer
structures using the B3LYP functional on the correspond-
ing ground state structures. All of the calculations were
performed using the Gaussian package [18].
For each of the two monomer structures we have calcu-
lated the corresponding optical gaps, taken here to be the
energetically lowest allowed vertical electronic excitation,
employing the PBE and B3LYP functionals. In Table 3 we
give the HOMO and LUMO energy levels, the HOMO–LUMO
gap, the optical gap along with the main contributions to
the first (allowed) excitation. We also provide the
wavelengths of the first excitation and the excitation with
the highest oscillator strengths for comparison with the
Fig. 3. Frontier and near frontier orbitals of the (left) P3 and (right) P4
monomers.
Table 3
Calculated properties of the P3 and P4 monomers. Specifically HOMO and LUMO energies (eV), HOMO–LUMO gap (eV), HL, Optical gap (eV, nm), OG, with
corresponding oscillator strengths, and dipole moment (D),
l
.
HOMO (eV)
LUMO (eV)
HL (eV)
OG (eV)
k1st/max (nm)
f
Main contributions
l (D)
P3
PBE
ꢁ4.76
ꢁ2.78
1.98
2.07
598/465
0.019
H ? L 94.5%
5.30
B3LYP
ꢁ5.51
ꢁ2.31
3.20
2.83
438/354/262/240
442/357/265/242a
0.084
H ? L 96.0%
H ? L 97.4%a
5.53
ꢁ5.44a
ꢁ2.26a
3.18a
2.80a
0.283a
6.34a
P4
PBE
ꢁ5.03
ꢁ2.88
2.15
2.55
485/344
0.261
H ? L 60.9%;
Hꢁ1 ? L 30.0%
3.21
B3LYP
ꢁ5.71
ꢁ2.51
3.20
2.89
429/294/264/243
440/297/263/244a
0.684
H ? L 98.5%
H ? L 99.3%a
3.45
ꢁ5.67a
ꢁ2.46a
3.21a
2.82a
0.864a
4.23a
a
Values when solvent effects are taken into account for o-dichlorobenzene.

172
M.L. Keshtov et al. / Organic Electronics 17 (2015) 167–177
the structures over regions that do not include the ali-
phatic moieties. The first excitation for both structures is
dominated by the HOMO ? LUMO contribution. For the
case of P3 this exhibits a charge transfer character which
is somewhat quenched by delocalization of the HOMO.
To quantify the contributions of the moieties to the frontier
orbitals we have calculated the total and partial density of
states (PDOS). The PDOS for the P3 and P4 structures are
shown in Fig. 4. We denote the benzotrithiophene and
the pyrrolothiophene moieties as BtTP (BtTPO includes
the nearby double bonded oxygen) and PTP, respectively.
We further partition the remaining parts of the structures
to the aliphatic parts collectively. The insets of the figures
are zoomed in the near frontier orbital region. For the SB18
monomer the HOMO has high, but not the same, contribu-
tions from the BtTP and BtTPO moieties with a percentage
of 62.1% and 36.5% respectively. This higher localization
over BtTP amplifies the charge transfer character of the
first excitation. For P4 the HOMO is has
a 80.8%
contribution from the BtTP and only 17.6% from PTP. In
both case the HOMO–1 has high contributions from the
BtTP moiety, 88.3% and 98.5% for P3 and P4, respectively.
For P3 the LUMO has a high, 94.3%, contribution from the
Fig. 5. Theoretical UV/Vis absorption spectrum (using the B3LYP func-
tional) of the (a) P3 and (b) P4 monomers. (HWHM = 0.25 eV).
BtTPO moiety. For P4 the LUMO has almost equal
contributions from BtTP and PTP, at 55.3% and 43.8%,
respectively. Significant contributions from the aliphatic
moieties are noticed only energetically much further down
starting around –8.3 eV (and below).
In Fig. 5 we show the UV/Visual absorption spectrum of
the P3 and P4 monomers calculated at the TD-DFT/B3LYP
level of theory. The spectra have been produced by convo-
luting Gaussian functions with HWHM = 0.25 eV centered
at the excitation wavenumbers. The calculated UV/Visual
spectra of the two monomers are very similar and in effect
reproduce the experimental spectra. In the region between
350–600 nm, that is also available experimentally, there is
one prominent bands. The high intensity band peaks at
442 nm and 440 nm, for P3 and P4 respectively. In this
region P3 exhibits a secondary strong peak (more promi-
nent in the presence of o-DCB) which can be distinguished
by the corresponding hump. At shorter wavelengths a high
intensity band is found centered around at 250 nm in both
cases.
5. Photovoltaic properties
We have investigated the photovoltaic properties of P3
and P4 copolymers by fabricating BHJ polymer solar cells
incorporating a copolymer as donor material and fullerene
derivative [6,6]-phenyl-C71-butyric acid methyl ester
(PC71BM) as the acceptor material with a device structure
of ITO/PEDOT:PSS/copolymer:PC71BM/Al. Different poly-
mer:PC71BM weight ratio, such as 1:1, 1:2 and 1:2.5 has
Fig. 4. Total and partial density of states of the (a) P3 and (b) P4
monomers. The insets are zoomed in the near frontier orbital regions.

M.L. Keshtov et al. / Organic Electronics 17 (2015) 167–177
173
LUMO energy level of PC71BM. The overall PCE of the
devices based on P3:PC71BM and P4:PC71BM active layer
is 3.16% and 2.42%, respectively. The superior performance
of P3 based device is mainly due to the higher value of Jsc
and FF as compared to that for P4. It can be seen from the
IPCE spectra of devices (Fig. 6b) that the IPCE values for the
device based on P3:PC71BM are higher than that for
P4:PC71BM, consistent with the higher value of Jsc for
P3:PC71BM based device.
Generally, in polymer solar cells, the values of Jsc and FF
are affected by hole mobility of the polymer donor materi-
als. It was reported in literature that higher hole mobility is
beneficial for better hole transport within the active layer
without large photocurrent loss caused by recombination
with opposite charges [19]. We have used space charge
limited current (SCLC) in dark method to estimate the hole
mobility in the blend thin films using hole only device
structure ITO/PEDOT:PSS/P3 or P4:PC71BM/Au. The hole
mobility was determined by fitting the dark current to
the model of a single carrier SCLC which is described by
following expression:
9
8
V2
d3
J ¼ eoerlh
The J–V curves in dark, for the hole only devices for
P3:PC71BM and P4:PC71BM are shown in Fig. 7. Fitting
the curves with above expression, the hole mobilities of
P3 and P4 are calculated to be 5.56 ꢂ 10ꢁ6 and
3.72 ꢂ 10ꢁ6 cm2/Vs, respectively.
The crystallinity and molecular ordering properties of
the copolymer films were characterized by XRD analysis
of the spin cast films on glass substrates. Fig. 8 shows the
XRD patterns of P3 and P4 films cast from o-DCB solution.
Both the polymers showed crystalline properties with two
diffraction peaks in the range 6–7° and 20–24°, which is
related to the lamellar spacing (100) and the stacking dis-
tance (010), respectively, between the coplanar conju-
gated polymers. The lamellar spacing and stacking
distance were determined from the XRD pattern are to be
24.34 and 22.16 A° and 3.67 and 3.75 A° for P3 and P4,
Fig. 6. (a) J–V curves under illumination of AM 1.5, 100 mW/cm2 (b) IPCE
spectra of the BHJ solar cells based on P3:PC71BM (o-DCB cast),
P4:PC71BM (o-DCB cast) and P3:PC71BM (DIO/o-DCB) active layers.
Table 4
Photovoltaic properties polymers P3 and P4.
Blend
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
P3:PC71BMa
P4:PC71BMa
P3:PC71BMb
P4:PC71BMb
0.80
0.92
0.76
0.88
8.58
6.26
10.52
8.14
0.46
0.42
0.58
0.50
3.16
2.42
4.64
3.58
a
DCB cast.
DIO (0.4 v%) DCB cast.
b
been investigated to optimize the photovoltaic properties
and we found the optimized ratio is 1:2. Moreover, we
have also optimized the thickness of the active layer and
found that the optimized thickness is around 90 nm. Active
layer was a blend of copolymer with PC71BM spin–coated
from o-DCB solution. Fig. 6a shows the J–V curves of the
PSCs under illumination of AM 1.5 solar irradiance
(100 mWcmꢁ2) and the corresponding photovoltaic data
are summarized in Table 4. These devices exhibited quite
respectable Voc about 0.80 V and 0.92 V for copolymer P3
and P4, respectively. The higher Voc for the device based
on P4 is attributed to the deeper HOMO level of P4 as
compared to P3 and Voc is related to the energy difference
between the HOMO energy level of the copolymer and the
Fig. 7. J–V characteristics in dark for hole only devices for P3:PC71BM
(o-DCB cast), P4:PC71BM (o-DCB cast) and P3:PC71BM (DIO/o-DCB) BHJ
active layers.

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M.L. Keshtov et al. / Organic Electronics 17 (2015) 167–177
Fig. 8. XRD pattern of pristine P3 and P4 thin films cast from o-DCB.
Fig. 9. XRD pattern of pristine P3 PC71BM thin films cast from o-DCB and
DIO/o-DCB.
respectively. As can be seen from XRD pattern that the
intensity of the peak related to (100) in more and smaller
value of spacing for P3 is than that for P4 indicates better
stacking properties, and thus exhibits higher hole mobility.
The higher crystalline nature of P3 and hole mobility may
be attributed to the increased value of Jsc and FF for the
device based on P3.
to without additive which agrees with the higher value of
Jsc. The Jsc values estimated from the integration of IPCE
spectra for the devices P3:PC71BM processed with and with
solvent additives are 10.58 mA/cm2 and 8.54 mA/cm2,
respectively and are consistent with the value calculated
from J–V characteristics under illumination.
The PCE of the polymer solar cells using above two
copolymers as donor along with the PC71BM is rather low
as compared to the reported in literature. In spite of low
bandgap and broad absorption spectrum of conjugated
copolymers, PCE of an organic solar cell also strongly
depends on the nanoscale morphology of electron donor
and electron acceptor in the active layer [20]. This nano-
scale network ensures short path lengths for exciton diffu-
sion, large interfacial area between D and A for effective
exciton dissociation into free charge carriers, and sufficient
network connectivity for effective charge transport and
collections. To achieve the ideal nanomorphology of active
layer, large number of processing techniques, such as ther-
mal annealing of active layer [20], heating with solvent
vapor [21], using solvent mixtures [22] and solvents with
high boiling point [23] has been explored. Among these
methods, adding small amounts of a processing additive
in addition to the host solvent has been found effective
for controlling the morphology of active layer.
We have also fabricated the device with solvent additive
1,8-diiodooctane DIO (0.4 v%) using P3:PC71BM and
P4:PC71BM active layers. DIO was chosen as the additive
because of its high boiling point around 170 °C and good
solubility for PC71BM which is expected to afford the
optimal morphology for achieving the high PCE [23,24].
The J–V characteristics of the device for P3:PC71BM active
are shown in Fig. 6a. The corresponding photovoltaic
parameters for devices using P3:PC71BM and P4:PC71BM
are tabulated in Table 4. The device processed with DIO
additives, Jsc is increased from 8.58 mA/cm2 to 10.52 mA/
cm2 and the FF increases from 0.46 to 0.58, leading to
enhancement of PCE from 3.16% to 4.64%. On the other
hand, Voc is slightly reduced from 0.80 V to 0.76 V, suggest-
ing that ordering of active layer, lowering of the charge sep-
arated and charge transfer state energies induced by DIO
addition, which is generally favorable for charge transport
[25]. Similar enhancement has been also observed for the
device based on P4:PC71BM. Fig. 6b shows the IPCE curves
of device based on P3:PC71BM blend processed with DIO
additives and exhibited higher values of IPCE as compared
We have measured the hole mobility of P3:PC71BM
blend films processed with DIO/o-DCB by space charge lim-
ited current (SCLC) method in dark, using hole only devices
were composed of ITO/PEDOT:PSS/blend/Au. The hole
mobility was extracted by fitting the J–V curves to the mod-
ified Mott–Gurney equation (Fig. 7). The hole mobilities of
the P3:PC71BM processed with DIO/o-DCB were found to
be 4.78 ꢂ 10ꢁ5 cm2/Vs, respectively. We have also
measured the electron mobilities for the P3:PC71BM blends
processed with and without DIO using electron only devices
(ITO/Al/ P3:PC71BM/Al) and found that the electron mobil-
ity is about 2.45 ꢂ 10ꢁ4 cm2/Vs and 2.54 ꢂ 10ꢁ5 cm2/Vs
without and with DIO, respectively.
The effect of DIO additive on the crystallinity P3:PC71BM
thin film, we have recorded the XRD pattern cast from
o-DCB and DIO/o-DCB and are shown Fig. 9. The blend film
showed similar XRD patterns as observed in pristine P3 but
the peak corresponds to (100) get broaden indicating the
crystallinity of the P3 is disturbed by the PC71BM. However,
with the addition of DIO the intensity of this peak again
increased indicating the crystalline nature of blend has
been improved.
6. Conclusion
Two new conjugated D–A copolymers P3 and P4
based on same benzo[1,2-b:3,4-b0:6,5-b00] trithiophene
(BTT) donor and different acceptors i.e. 5,8-dibromo-
2-dodecanoyl[2,3-e:3’,2’-g][1]benzotrithiophene (DDBTT)
and 1,3-dibromo-5-(2-ethylhexyl)thieno[3,4]pyrrol-4,6-
dione (DBTPD) as acceptor, respectively were synthesized
by Stille cross-coupling reaction and characterized by gel
permeation chromatography (GPC), 1H NMR, UV–Vis
absorption, thermal analysis and electrochemical cyclic
voltammetry (CV) tests. The weight-average molecular
weight (Mw) of all copolymers is in the range of 9700–
12,000 with corresponding polydispersity index in the
range of 1.5–2.9. Both the copolymer exhibited good
thermal stability with 5% weight-loss temperature (T5%
)

M.L. Keshtov et al. / Organic Electronics 17 (2015) 167–177
175
in the range 323–352 °C respectively. The glass transition
temperature of P3 and P4 are 184 °C and 196 °C,
respectively. HOMO energy levels P3 and P4 were ꢁ5.17
and ꢁ5.30 eV respectively, implying that they varied with
respect to the modulated ICT strengths resulting from the
presence of electron acceptors with various electron-
accepting abilities. The LUMO levels were ꢁ3.43,
0.125 cm2. The current–voltage curves were measured
under 100 mW/cm2 standard AM 1.5G spectrum using An
Oriel 150 W solar simulator, the light intensity was deter-
mined by silicon diode with KG-5 visible color filter. IPCE
measurements were performed under short-circuit condi-
tions with a lock-in amplifier (SR830, Stanford Research
System) at a chopping frequency of 280 Hz during illumi-
nation with a monochromatic light from a Xenon lamp.
ꢁ3.26 eV, for P3 and P4 were estimated from the onset
onset
reduction potentials ðE
Þ of polymers, and were greater
red
than that of PC71BM (ca 4.0 eV) thus we would expect
efficient charge transfer/dissociation to occur in their
corresponding devices. Photovoltaic properties of the poly-
mers were studied by using the polymers as donor and
PC71BM as acceptor with a weight ratio of polymer:
PC71BM 1:1, 1:2 and 1:2.5 and found the optimized weight
ratio is 1:2 and showed PCE of 3.16% and 2.42% for
P3:PC71BM and P4:PC71BM, respectively. We have used
DIO as solvent additive for P3:PC71BM active layer and
the PCE of the device have been increased up to 4.64% with
Jsc of 10.52 mA/cm2, Voc of 0.76 V and FF of 0.58. The
increase in Jsc and FF has been attributed to the increase
in crystalline nature of active layer and more balanced
charge transport in the device, induced by DIO additive.
7.4. Synthesis of monomers and polymers
2,3-Dibromothiophene (1): 3-bromothiophene (16.3 g;
100 mmol) was added to a suspension of NBS (17.8 g;
100 mmol) in hexane (50 ml) followed by HClO4 (70% solu-
tion in H2O; 0.7 ml; 5 mol%) addition. The reaction mixture
was stirred at room temperature for 24 h and than K2CO3
(200 mg) was added. Reaction mixture was filtrated, solids
were washed with hexane. Organic phases were concen-
trated, and residue was distilled in vacuo. Yield is 89%.
Found, %: C 19.60, H 0.88, S 13.53, Br 65.88; Calcd., %: for
C4H2Br2S, C 19.86, H 0.83, S 13.25, Br 66.05. 1H NMR
(400.13 MHz; CDCl3; d, ppm.): 7.24 (c, 1H); 6.90 (c, 1H).
2,3-Dibromo-5-dodecanoylthiophene
(2):
AlCl3
(7.142 g; 53.5 mmol) was added by small portions during
20–30 min to a cooled solution of 2,3-dibromothiophene
(10 g; 41.33 mmol) and dodecanoyl chloride (10 g;
45.56 mmol) in CH2Cl2 (100 ml) at 0 °C under argon.
Reaction mixture was stirred at 0 °C during 2 h and than
a cooled hydrochloric acid (2 M, 160 ml) was slowly added.
Phases were separated, water layer was extracted with
CH2Cl2 (2 ꢂ 100 ml), organic solution was dried with
MgSO4, filtrated and concentrated in vacuo. Residue was
purified on SiO2 column with hexane as eluent. After sol-
vent evaporation the title compound was obtained with
90% yield as orange oil, which crystallized upon standing.
Found, %: C 45.20, H 5.48, S 7.23, Br 37.19; calcd. % for
7. Experimental
7.1. Instrument
UV–vis absorption was tested on a Shimadzu UV-3600
UV–vis-NIR spectrometer. The bandgap was calculated
according to the onset absorption of UV–vis spectra
(Eg = 1240/konset eV). High temperature GPC analysis was
conducted on a PL-GPC 220 system using polystyrene as
standard and 1,2,4-trichlorobenzene as eluent at 150 °C.
CV of films was performed on a CHI660a electrochemical
analyzer with a three-electrode cell at a scan rate of
100 mV sꢁ1 (glassy carbon electrode, Pt wire and saturated
calomel electrode). Bu4NPF6 (0.1 mol Lꢁ1) was used as elec-
trolyte and anhydrous acetonitrile (drying with CaH2) was
used as the solvent.
C
16H24Br2OS, C 45.30, H 5.70, S 7.56, Br 37.67. 1H NMR
(400 MHz; CDCl3; d, ppm.): 7.46 (c, 1H); 2.79 (br.c, 2H);
1.70 (br.c, 2H); 1.24 (m, 16H); 0.87 (br.c, 3H).
5-Dodecanoyl-2,3-bis(thiophene-2-yl)thiophene (3):
Thiophenyl-2-boronic acid (1.811 g; 14.143 mmol), 2,3-
dibromo-5-dodecanoylthiophene (2) (2.5 g; 5.893 mmol),
DMF (125 ml) and aqueous solution of Na2CO3 (20 ml,
1 M, 20 mmol) were loaded into flask (250 ml), and the
mixture were degassed with argon during 40 min with
stirring. Than Pd(PPh3)4 (300 vu; 0.259 vvokm) was added
and the mixture was heated at 125 °C for 20 h under argon.
After cooling, the reaction mixture was treated with H2O
(150 ml) and extracted with CHCl3 (3 ꢂ 100 ml). Organic
phases were dried with MgSO4 and evaporated to dryness.
After purification of the residue by chromatography on
SiO2 (hexane-toluene = 2:1 as eluent) and recrystallization
from hexane, the title compound was obtained. The yield is
1.63 g (63,4%). 1H NMR (600.13 MHz; CDCl3; d, ppm): 7.70
(c, 1H); 7.37 (dd, 1H); 7.35 (dd, 1H); 7.22 (dd, 1H); 7.09 (m,
1H); 7.07 (m, 1H); 7.03 (dd, 1H); 2.90 (t, 2H); 1.78 (m, 2H);
1.28 (m, 16H); 0.9 (t, 3H). 13C NMR (150.93 MHz; CDCl3; d,
ppm): 193.33; 141.39; 140.45; 136.09; 134.34; 134.29;
132.13; 128.37; 127.88; 127.54; 127.45; 127.36; 126.37;
39.09; 31.93; 29.63; 29.50; 29.44; 29.36; 24.84; 22.71;
7.2. Materials
All reagent, unless otherwise specified, were obtained
from Aldrich and Acros and used as received. All the
solvents were freshly distilled prior to use.
7.3. Fabrication and characterization of polymer solar cell
The device with the structure of ITO/PEDOT:PSS/Blend
Film/Al. The ITO substrate was cleaned with de-ionized
water, acetone, and isopropyl alcohol successively, then
dried in oven at 120 °C overnight. After treating with UV/
ozone at room temperature for 30 min, 38 nm PEDOT:PSS
was spin-coated on the ITO substrate and dried at 120 °C
for 30 min. After that, the ITO substrate was transferred
into glove-box for spin-coating the active lay, the
concentrated of the blend solution was 8 mg/ml in
o-di-chlorobenzene (DCB). Finally, the device was
completed by evaporating LiF/Al with an area of

176
M.L. Keshtov et al. / Organic Electronics 17 (2015) 167–177
14.15. Found, %: C 67.01; H 7.15; calcd., %: for C24H30S3O, C
66.93, H 7.02.
4-b0:6,5-b00] trithiophene (6) (0.46 g; 0.8035 mmol) in THF
(30 ml) at ꢁ78 °C, and the reaction mixture was stirred at
ꢁ78 °C for 30 min. Then chlorotrimethyltin solution (1 M
in hexane; 4,1 ml; 4,1 mmol) was added dropwise and
the reaction mixture was warmed to room temperature,
treated with water (75 ml), and extracted with hexane
(3 ꢂ 50 ml). Combined organic extracts were dried with
MgSO4, filtrated and evaporated to dryness. The residue
was recrystallized from hot isopropanol to give 505 mg
(85% yield) of the title compound. 1H NMR (400.13 MHz;
CDCl3; d, ppm): 7.87 (c, 1H); 7.86 (c, 1H); 7.31 (c, 1H);
2.99 (t, 2H); 1.62 (m, 2H); 1.50–1.20 (m, 18H); 0.90
(t,3H); 0.50 (c, 18H). Found,%: C 48.40, H 6.18, S 12.53;
for C30H46S3Sn2, calcd,%: C 48.68, H 6.26, S 12.99.
5-Dodecanoyl-2,3-bis(5-bromothiophen-2-yl)thio-
phene (4): To a cooled solution of 5-dodecanoyl-2,3-
bis(thiophene-2-yl)thiophene (3) (5.890 g; 13.676 mmol)
in DMF (100 ml) a solution of NBS (5.400 g; 30.000 mmol)
in DMF (40 ml) was added dropwise at 0 °C, and the reac-
tion mixture was stirred at room temperature overnight.
After completion of the reaction, the reaction mixture
was poured onto ice (200 g), extracted with CH2Cl2
(3 ꢂ 100 ml), dried with MgSO4 and evaporated at 50 °C.
The residue was extracted with hot hexane, filtrated and
evaporated. The yield of title compound is 7.1 g (88%). 1H
NMR (600.13 MHz; CDCl3; d, ppm): 7.61 (c, 1H); 7.04 (d,
1H); 7.01 (dd, 2H); 6.85 (d, 1H); 7.09 (m, 1H); 2.97
(t, 2H); 1.76 (m, 2H); 1.32 (m, 16H); 0.90 (t, 3H). Found,
%: C 48.70, H 4.68, S 16.03, Br 26.87; calcd, %: for
Synthesis
of
P3:
5,8-bis(trimethylstannyl)-2-
dodecylbenzo[1,2-b:3,4-b0:6,5-b00] trithiophene (7), (0.6119 g;
0.8265 mmol), 5,8-dibromo-2-dodecanoyl[2,3-e:3’,2’-g][1]
benzotrithiophene (0.8265 mmol), Pd(Ph3P)4 (0.065 g;
0.056 mmol), dry toluene (15 ml) and dry DMF (2 ml) were
placed in a 25 ml flask equipped with reflux condenser and
magnetic stirrer. After stirring at 115 °C for 48 h under
argon, 2-bromothiophene (0.02 g; 0.123 mmol) and 2-(trib-
utylstannyl) thiophene (0.02 g; 0.054 mmol) was added to
the mixture and reaction was continued for 5 h. The reaction
mixture was cooled to room temperature, poured into
methanol and filtered. The polymer was dissolved in CHCl3,
precipitated from methanol, extracted in Soxlet apparatus
with methanol, hexane and chloroform and dried in vacuo.
The yield is 71%. 1H NMR (CDCl3, 400 MHz, d, ppm): 8.2–
5.9 (m, 6H), 4.4–3.6 (m, 4H), 2.48–0.17 (m, 44H).
C
24H28Br2OS3, C 48.98, H 4.80, S 16.35, Br 27.16.
5,8-dibromo-2-dodecanoylbenzo[1,2-b:3,4-b0:6,5-b00]
trithiophene (5): A solution of FeCl3 (5.190 g; 32 mmol) in
nitromethane (70 ml) was added dropwise to a solution of
5-dodecanoyl-2,3-bis(5-bromothiophen-2-yl)thiophene (4)
(8.1 g; 13.764 mmol) in dichloromethane (300 ml) at room
temperature, and the resulting mixture was stirred over-
night. After the completion of the reaction, ethanol
(300 ml) was added and the mixture was concentrated to
the volume of 300 ml and cooled to ꢁ15 °C. The solids were
filtrated and dried. After purification on SiO2 column (tolu-
ene-hexane – 1:1 as eluent) and recrystallization from this
mixture, the yield of the title compound is 6,6 g (81,8%). 1H
NMR (600.13 MHz; CDCl3; d, ppm): 7.84 (c, 1H); 7,42 (c,
1H); 7.41 (c, 1H), 3.03 (t, 2H); 1.83 (m, 2H); 1.32 (m,
16H); 0.90 (t, 3H). 13C NMR (150 MHz; CDCl3; d, ppm):
193.89; 143.18; 134.01; 133.12; 132.94; 131.70; 131.44;
129.78; 129.05; 128.24; 125.64; 125.31; 125.11; 115.78;
113.93; 39.35; 31.95; 29.68; 29.67; 29.55, 29.51; 29.39–
29.38 (br); 24.61; 22.73; 14.17. Found, %: C 49.00, H 4.38,
S 16.13, Br 26.83; calcd, %: for C24H26Br2OS3, C 49.15, H
4.47, S 16.40, Br 27.25.
Synthesis of P4: Polymer P4 was prepared similarly as
P3, from 5,8-bis(trimethylstannyl) -2-dodecylbenzo[1,2-b:
3,4-b0:6,5-b00] trithiophene (7) as donor monomer and 1,3-
dibromo-5-(2-ethylhexyl)thieno[3,4]pyrrol-4,6-dione as
acceptor. The yield is 69%. 1H NMR (CDCl3, 400 MHz, d,
ppm): 8.0–6.0 (m, 3H), 4.5–3.5 (m, 4H), 2.53–0.25
(m, 38H).
Acknowledgements
5,8-dibromo-2-dodecylbenzo[1,2-b:3,4-b0:6,5-b00]tri-
thiophene
(6):
A
mixture
of
5,8-dibromo-2-
This work is supported by the Russian foundation for
basic research (Grant Nos. 13-03-92709-IND_a, 14-03-
92003 and 13-03-91166) and DST, Government of India
(DST-RFBR joint research project). We are thankful to
Department of Physics, LNMIT, Jaipur and Material
Research Laboratory, MNIT, Jaipur for providing the facili-
ties of device fabrication and characterization.
dodecanoylbenzo[1,2-b:3,4-b0:6,5-b00] trithiophene (5) (3.5
u; 5.973 mmol), hydrazine-hydrate (15 ml; 0.3 mol) and
KOH (9 g; 153.9 mmol) in ethylene glycol (100 ml) was
stirred at 190–200 °C for 16 h. Cooled reaction mixture
poured into water, extracted with CH2Cl2 and evaporated
in vacuo. The orange solid residue was extracted with hot
hexane and filtrated through SiO2 column. The colorless fil-
trate was evaporated to give 462 mg (13.5%) of the desired
product. 1H NMR (600.13 MHz; CDCl3; d, ppm): 7.53
(c, 1H); 7.52 (c, 1H); 7.03 (c, 1H), 2.95 (t, 2H); 1.80 (m, 2H);
1.23 (m, 18H); 0.90 (t, 3H). 13C NMR (150 MHz; CDCl3; d,
ppm): 147.15; 132.65; 132.12; 131.01; 130.70; 130.43;
128.56; 125.25; 125.16; 118.49; 113.05; 112.52; 31.94;
31.27; 30.77; 29.68; 29.65 (br); 29.54, 29.37; 29.15;
22.71; 14.15. Found, %: C 50.20, H 4.88, S 16.53, Br 27.48;
calcd, %: for C24H28Br2S3, C 50.35, H 4.93, S 16.80, Br 27.91.
5,8-bis(trimethylstannyl)-2-dodecylbenzo[1,2-b:3,
4-b0:6,5-b00]trithiophene (7): n-BuLi solution (1.6 M in
hexanes; 1.2 ml; 2.046 mmol) was added dropwise to a
cooled solution of 5,8-dibromo-2-dodecylbenzo[1,2-b:3,
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