Side-chain engineering of benzodithiophene-thiophene copolymers with conjugated side chains containing the electron-withdrawing ethylrhodanine group

Article abstract of DOI:10.1039/c5ta02360b

Four benzodithiophene (BDT)-thiophene (T) copolymers with conjugated side chains containing electron-withdrawing ethylrhodanine acceptor units, PHDBDT-T-R, PEHBDT-T-R, PHDBDT-T-TR, and PEHBDT-T-TR, were designed and synthesized for investigating the effect of side chains on the physicochemical properties and photovoltaic performance of the conjugated polymers. All the four copolymers possess an identical conjugated backbone of alternative benzodithiophene-thiophene, but different side chains on BDT and thiophene units, respectively. Polymer solar cells (PSCs) with these polymers as donors and PC₇₀BM as acceptors exhibit an initial power conversion efficiency (PCE) of 0.61% for PHDBDT-T-R, 2.32% for PEHBDT-T-R, 1.46% for PHDBDT-T-TR, and 2.36% for PEHBDT-T-TR. After the treatment with 3 vol% DIO additive and with methanol, the highest PCE was increased up to 1.01%, 4.04%, 3.47%, and 4.25% for PHDBDT-T-R, PEHBDT-T-R, PHDBDT-T-TR, and PEHBDT-T-TR, respectively, with significantly increased J_sc and FF. The effects of methanol treatment on the photovoltaic performance of the PSCs can be ascribed to the increased carrier transport, improved exciton dissociation and optimized phase separation of the active layer. This work indicates that side-chain engineering plays a key role in molecular structures and optoelectronic properties.

Full text of DOI:10.1039/c5ta02360b

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DOI: 10.1039/C5TA02360B
Side-Chain Engineering of Benzodithiophene-Thiophene Copolymers
with Conjugated Side Chain Containing Electron-Withdrawing
Ethylrhodanine Group
Lixia Chen,a,b Ping Shen,*a Zhi-Guo Zhang,b Yongfang Li*b
a
College of Chemistry and Key Laboratory of Environmentally Friendly Chemistry
and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105,
China
b
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of
Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190,
China
*Correspondence to: E-mail: shenping802002@163.com (P. Shen); liyf@iccas.ac.cn
(Y. Li)
ABSTRACT
Four benzodithiophene (BDT)-thiophene (T) copolymers with conjugated side chain
containing electron-withdrawing ethylrhodanine acceptor unit, PHDBDT-T-R,
PEHBDT-T-R, PHDBDT-T-TR, and PEHBDT-T-TR, were designed and
synthesized for investigating the effect of side chains on the physicochemical
properties and photovoltaic performance of the conjugated polymers. All the four
copolymers possess an identical conjugated backbone of alternative
benzodithiophene-thiophene, but different side chains on BDT and thiophene units,
respectively. Polymer solar cells (PSCs) with these polymers as donor and PC70BM as
acceptor exhibit an initial power conversion efficiency (PCE) of 0.61% for
PHDBDT-T-R, 2.32% for PEHBDT-T-R, 1.46% for PHDBDT-T-TR, and 2.36%
for PEHBDT-T-TR. After the treatment with 3 vol % DIO additive and with
methanol, the highest PCE was increased up to 1.01%, 4.04%, 3.47%, and 4.25% for

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PHDBDT-T-R, PEHBDT-T-R, PHDBDT-T-TR, and PEHBDT-T-TR, respectively,
with significantly increased Jsc and FF. The effects of methanol treatment on the
photovoltaic performance of the PSCs can be ascribed to the increased carrier
transport, improved exciton dissociation and optimized phase separation of the active
layer. This work indicates that side-chain engineering plays a key role on molecular
structures and optoelectronic properties.
KEYWORDS: Polymer solar cells, Side-chain engineering, Ethylrhodanine,
Photovoltaic properties
Introduction
Solar energy as a green energy resource has attracted great attention. Polymer solar
cell (PSC) has been developed rapidly in recent years owing to its advantages of low
cost, light weight, easy fabrication and capability to be fabricated into large area
flexible devices.1-5 The most successful PSCs to date are the devices based on the
bulk heterojunction (BHJ) blend active layer of conjugated polymer electron donor
and fullerene derivative electron acceptor (such as [6,6]-phenyl-C61-butyric acid
methyl ester (PC60BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM)).6
The priority in the studies of PSCs is how to increase the power conversion efficiency
(PCE) of the devices. The PCE is proportional to the values of short-circuit current
density (Jsc), open-circuit voltage (Voc), and fill factor (FF) of the PSC. The Jsc
depends on the efficiencies of the light absorption of the active layer, exciton
diffusion to and dissociation at the D/A interface, charge transportation in the active
layer, and charge collection on the electrodes. Voc is mainly proportional to the
difference between the lowest unoccupied molecular orbital (LUMO) energy level of
the acceptor (e.g. PC70BM) and the highest occupied molecular orbital (HOMO)
energy level of the polymer donor. FF is related to the charge carrier mobilities and
device fabrication conditions. While in order to increase the PCE of the PSCs, it is
very important to design and synthesize new conjugated polymer donors with
narrower bandgap (broader absorption) for higher Jsc, relatively lower-lying HOMO

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energy level for higher Voc, and higher hole mobility for higher Jsc and larger FF.
Among high performance conjugated donor-acceptor (D-A) polymer photovoltaic
materials, benzo[1,2-b:4,5-b']dithiophene (BDT) has been widely investigated and
used as a donor unit due to its good electron-donating properties, rigid coplanar
structure with stronger π-π intermolecular interactions, and the possibility of side
chain manipulation for suitable solubility and processability.7-9 3-Ethylrhodanine is a
dye unit, usually used as an electron-withdrawing group, which could greatly improve
the light absorption ability, and thus, a high Jsc value could be achieved in the
corresponding photovoltaic devices.10
Equally important is that some studies have been directed towards understanding
of the effects of side chains on the photovoltaic performance of PSCs based on D-A
copolymers. Recent work suggests that for the same conjugated polymer backbone the
type (alkyl, alkoxy, aryl), shape, topology (linear, branched) and distribution (uniform,
alternating, random) of side chains have a large impact on the morphology
(crystallinity, π-stacking orientations), charge transport and photovoltaic properties of
the polymers.11-17 Thus the side chain effect should also be fully explored in the
design of new copolymers for highly efficient solar cells. In previous work, our group
have reported some side chain engineering of the BDT-bithiazole-based D-A
copolymers, a PCE of 3.71% was achieved in combination with PC70BM acceptor.11 It
is worth mentioning that high PCEs of 8-10% were achieved just by the side chains
engineering of a BDT-T-S and thieno[3,4-b]thiophene copolymers.8,17,18
On the basis of the above consideration, we herein designed and synthesized four
main chain-side chain D-A polymers (PHDBDT-T-R, PEHBDT-T-R,
PHDBDT-T-TR, and PEHBDT-T-TR, Fig. 1) based on BDT and thiophene with
alkoxy side chain onto the BDT unit and end-capping ethylrhodanine group as an
electro-withdrawing conjugated side chain on thiophene unit. For the four polymers,
we chose 2-hexlydecyloxy (HD) and 2-ethylhexyloxy (EH) as side chains on BDT
unit to systematically investigate the effects of side chain shape and size (EH vs. HD)
on solubility, light absorption, HOMO/LUMO energy levels, and charge transport
properties of the resulting polymers as well as on morphology and photovoltaic

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properties of the PSCs. On the other hand, the inserted thienylvinyl between
thiophene and end-capping 3-ethylrhodanine group gives a chance to study the
influence of the extended conjugated side chain on optoelectronic properties of the
copolymers.
Fig. 1 Chemical structures of the four copolymers.
Results and discussion
Synthesis and characterization
The chemical structures and synthesis routes of monomers and the four polymers are
outlined in Fig. 1 and Scheme 1, respectively. PHDBDT-T-R, PEHBDT-T-R,
PHDBDT-T-TR, and PEHBDT-T-TR have an identical BDT-thiophene copolymer
backbone but different side chains attached on BDT and thiophene units.
PEHBDT-T-R and PEHBDT-T-TR have ethylhexyloxyl side chains, PHDBDT-T-R
and PHDBDT-T-TR have more branched hexyloctyloxyl side chains attached on
BDT unit to further improve solubility of the resulting polymers (Fig. 1).
3-Ethylrhodanine was chosen as an end-capping side chain to build these main
chain-side chain type D-A copolymers due to its intrinsic electron-deficient character.
Compared with PEHBDT-T-R and PHDBDT-T-R, PEHBDT-T-TR and
PHDBDT-T-TR possess extended conjugated side chains with the aim to red-shift

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absorption spectra and lower bandgaps of the polymers. The choice of these different
side chains is convenient for fine-tuning of the optoelectronic properties of the
resulting polymers.
As outlined in Scheme 1, two monomers M3 and M4 based on BDT unit were
prepared according to the procedure in the literature.19 Compound 1 was synthesized
by the bromination reaction of thiophene-3-carbaldehyde. Then thiophene-based
monomer M1 containing 3-ethylrhodanine moiety was synthesized by the
Knoevenagel reaction between Compound 1 and 3-ethylrhodanine. Monomer M2
with an extended thienylvinyl moiety was synthesized via the stepwise synthetic
protocol. First, compound 2 was synthesized according to published procedures.20
Next this compound was converted into compound 3 via Vilsmeier reaction.
Monomer M2 was subsequently obtained by the Knoevenagel reaction like the M1.
Compounds 1-3 and M1, M2 were satisfactorily characterized by 1H NMR and EI or
MALDI-TOF spectroscopies. Four new D-A copolymers were synthesized
successfully via Stille-type copolymerization of a distannane monomer (M3 or M4)
with 3-ethylrhodanine-based monomers M1 and M2. Table 1 summarized the
polymerization results and thermal properties of the copolymers.
The polymers can be readily dissolved in common organic solvents, such as
chloroform, tetrahydrofuran (THF), chlorobenzene, and 1, 2-dichlorobenzene (DCB)
at room temperature or with a slightly heating. The molecular weight and
polydispersity (PDI) values of the four polymers was measured by the GPC method
(against polystyrene standards) in chloroform at room temperature (Table 1). Among
the four copolymers, PHDBDT-T-R gives the lowest weight-average molecular
weight (Mw) of 17.7 kDa with a PDI of 1.86 (Table 1), whereas PEHBDT-T-R shows
the highest Mw of 50.8 kDa with a moderate PDI of 2.68 (Table 1). PHDBDT-T-TR
and PEHBDT-T-TR have the moderate Mw of 39.5 kDa and 25.2 kDa, respectively.

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Scheme 1 Synthetic routes of the monomers and the four copolymers.
The thermal stability of conjugated polymers is very important for their
application in optoelectronic device. The thermal properties of the polymers were
investigated with thermogravimetric analysis (TGA) measurement. The four polymers
have good thermal stability with onset decomposition temperatures corresponding to
5% weight loss (Td) at above 290 °C, as shown in Table 1 and Fig. 2. PHDBDT-T-R
and PEHBDT-T-R show relatively high Td of 327 °C and 329 °C , respectively (Fig.
2), which are higher than those of PHDBDT-T-TR (316 °C ) and PEHBDT-T-TR
(299 °C ). The results suggest that the absence of a thienylvinyl moiety between the
thiophene and 3-ethylrhodanine as conjugated side chains benefits to improve the
thermal stability of the resulting polymers. Obviously, the thermal stability of the
polymers is adequate for the applications in PSCs and other optoelectronic devices.

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Fig. 2 TGA plots of PHDBDT-T-R, PEHBDT-T-R, PHDBDT-T-TR and
PEHBDT-T-TR with a heating rate of 10 °C /min under an inert atmosphere.
Table 1 Molecular weights and thermal properties of the synthesized copolymers
polymer
Yield (%) Mn (kDa)a Mw (kDa)a PDI Td (oC)b
9.5
18.9
13.8
11.6
41.6
78.3
51.1
72.6
17.7
50.8
25.2
39.5
1.86
2.68
1.82
3.41
327
329
316
299
PHDBDT-T-R
PEHBDT-T-R
PHDBDT-T-TR
PEHBDT-T-TR
a
b
Determined by GPC in CHCl3 based on polystyrene standards. Decomposition
temperature, determined by TGA in nitrogen, based on 5% weight loss.
Optical properties
Optical properties of these polymers have been investigated by UV-vis absorption
spectroscopy in dilute chloroform solution and as thin solid films (Fig. 3). The
detailed absorption data including the absorption maximum wavelength (λmax) in both
solution and films, the absorption edge (onset wavelength of the absorption peak, λedge
of the polymer films, and optical bandgaps (Egopt) are summarized in Table 2. As
shown in Fig. 3a, in solution PEHBDT-T-R exhibits two distinct absorption bands.
The first absorption band with a λmax at 435 nm, is assigned to the delocalized
excitonic π-π* transition. The second absorption band located at longer wavelength
)

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range with a λmax at 538 nm, can be attributed to an intramolecular charge transfer
(ICT) states between main-chain donor and side-chain acceptor.21 As for
PHDBDT-T-R, it shows a similar absorption shape to PEHBDT-T-R except for a
shoulder peak at the longer wavelength range. In comparison with PHDBDT-T-R and
PEHBDT-T-R, PHDBDT-T-TR and PEHBDT-T-TR with extended side chains have
just one broad absorption band ranged from 300 to 700 nm with λmax of 471 and 475
nm, respectively. This should be ascribed to that the introduction of a thienylvinyl
moiety between thiophene and 3-ethylrhodanine weakens the ICT absorption between
the main-chain donor and side-chain acceptor. The weakened ICT absorption lead to a
blue-shifted ICT band, which overlaps with π-π* transition absorption band and
finally exhibits one broad absorption band. Compared to the absorption spectra of the
polymer solutions, the absorption spectra of the films are broadened and the λmax
values are red-shifted (Fig. 3b). This phenomenon results from the enhanced
interchain interaction in the solid films, which may be related to the increased extent
of π-π* stacking of the backbones and increased polarizability of the film, or both.22
The absorption edge (λedge) of PEHBDT-T-R and PEHBDT-T-TR film are ~20 nm
red-shifted than those of the PHDBDT-T-R and PHDBDT-T-TR film, respectively.
The optical bandgaps (Egopt) of PHDBDT-T-R, PEHBDT-T-R, PHDBDT-T-TR, and
PEHBDT-T-TR are 1.92, 1.86, 1.90, and 1.84 eV, respectively, calculated from λedge
of their solid state films (Table 2). Obviously, the polymers with EH as a side chain
possess lower bandgaps than those of HD as a side chain. It may be due to that the
more branched HD side chain on BDT unit damaged the coplanarity of the main chain.
On the other hand, the result showed that the extended conjugated side chains
attached onto thiophene unit can red-shift the absorption edges and lower bandgaps,
which is favorable to improve the Jsc of PSCs based on these polymers (e.g.
PHDBDT-T-R vs PHDBDT-T-TR and PEHBDT-T-R vs PEHBDT-T-TR ) as donor.

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Fig. 3 Normalized absorption spectra of PHDBDT-T-R, PEHBDT-T-R,
PHDBDT-T-TR and PEHBDT-T-TR in (a) chloroform solution and (b) thin film on
quartz.
Electrochemical properties
Cyclic voltammetry (CV) was employed to measure the electronic energy levels of
conjugated polymers. The HOMO and LUMO energy levels of conjugated polymers
are crucial for the application as photovoltaic materials in PSCs, which can be
estimated from the onset oxidation and reduction potentials (Eox/red) in the
voltammograms. The CV of the four polymer films were measured on a Pt electrode

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in a 0.1 mol/L Bu4NPF6-acetonitrile solution (Fig. 4). The results of the
electrochemical properties are also summarized in Table 2. The CV curve was
recorded by using an Ag/AgCl electrode as a reference, which was calibrated against
the redox potential of ferrocene/ferrocenium couple (Fc/Fc+ measured as 0.39 V vs
Ag/AgCl), whose energy level was assumed to be 4.80 eV below the vacuum level.
Thus, HOMO/LUMO = −e(Eox/red + 4.41) (eV), where Eox/red is the
oxidation/reduction onset potential of the sample vs Ag/AgCl.
Fig. 4 Cyclic voltammograms of the polymer films on platinum electrode in

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-1
-1
DOI: 10.1039/C5TA02360B
acetonitrile solution containing 0.1 mol L Bu4NPF6 at a scan rate of 0.1 V s , (a) the
oxidation segments, (b) the reduction segments.
Table 2 Optical and electrochemical properties of the four copolymers
c
opt d
a
b
λedge Eg
(nm) (eV)
Eox/HOMOe Ered/LUMOe
max
max
Polymer
(nm)
(nm)
(V/eV)
(V/eV)
407,511
435, 538
471
424,530
440, 552 665
478
488
645
1.92
1.86
1.90
1.84
1.15/-5.56
1.07/-5.48
1.10/-5.51
1.04/-5.45
-1.02/-3.39
-1.08/-3.33
-0.95/-3.46
-1.03/-3.38
PHDBDT-T-R
PEHBDT-T-R
PHDBDT-T-TR
PEHBDT-T-TR
652
673
475
aMeasured in dilute chloroform solution. bMeasured on quartz plate by polymers cast
c
d
from chloroform solution. Absorption edge of the thin films. Estimated from the
opt
e
onset wavelength of the absorption spectra: Eg = 1240/λedge. Calculated according
to the equation: HOMO/LUMO = −e (Eox/red + 4.41) (eV).
Fig. 5 Energy level diagrams for the four polymers, ideal polymer and PC70BM. The
dashed lines indicate the thresholds for air stability (-5.20 eV) and effective charge
transfer to PC70BM (-3.91 eV).
On the other hand, the onset reduction potential (Ered) values are observed as -1.02,
-1.08, -0.95, and -1.03 V vs Ag/AgCl for PHDBDT-T-R, PEHBDT-T-R,
PHDBDT-T-TR, and PEHBDT-T-TR, respectively (Fig. 4b). Accordingly, the
calculated LUMO energy levels of the polymers are -3.39, -3.33, -3.46, and -3.38 eV,
respectively. Evidently, the different side chains both on BDT and thiophene units
have some effects on the LOMO energy levels of the resulting polymers. Moreover,
the LUMO energy levels of the polymers are all located within a suitable range and

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are much higher than that of PC70BM (ca. -3.91 eV),25 thus, efficient exciton
dissociation could be expected to occur in their corresponding PSCs. A complete
picture of the energy levels and band structure of the four polymers and PC70BM is
presented in Fig. 5. As can be seen from the Fig. 5, both the HOMO and LUMO
energy levels of the copolymers are close to the ideal range.
Photovoltaic properties
To investigate the effects of different side chains on the photovoltaic properties of the
BDT–thiophene-based copolymers, bulk heterojunction PSC devices with a
configuration of ITO/PEDOT:PSS/polymer:PCBM/Ca/Al was fabricated. The active
layers were spin-coated from DCB solution of polymer:PC70BM blend. The thickness
of the active layer was controlled by changing the spin speed during the spin-coating
process and measured on an Ambios Tech. XP-2 profilometer. The corresponding
open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), and power
conversion efficiency (PCE) of the devices are summarized in Table 3. Fig. 6 shows
the typical current density–voltage curves (J–V) of the PSCs based on the blends of
polymer:PC70BM under the illumination of AM1.5G, 100 mW/cm2 and Fig. 7 shows
the external quantum efficiency (EQE) curves of the PSCs.
First, the blend ratio of the polymer donor and PC70BM acceptor was optimized.
For example, when the mixing ratio of PHDBDT-T-R and PC70BM increased from 1:
1 to 1:2 to 1:3, the PCE of the device continuously increased from 0.17% to 0.25% to
0.61% (Table 3). It can be observed that an increase in the PC70BM content resulted in
a slight increase in the Voc and also elevated the Jsc and FF. It is obvious that the
optimal blend ratio for this polymer was determined to be around 1:3. The blend ratio
of the other three polymers was also optimized. It is found that the different polymers
have their own optimal blend ratio and the determined optimal blend ratios are 1:2,
1:2 and 1:1.5, for PEHBDT-T-R, PHDBDT-T-TR and PEHBDT-T-TR, respectively.
On each optimal blend ratio, the best PCE of the four polymers are 0.61%, 2.32%,
1.46% and 2.36%, respectively.
Next, we investigated the effect of the high boiling point additive 1,

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8-diiodooctane (DIO) on the photovoltaic properties of the copolymers (Table 3).
Except for PHDBDT-T-R, the other three polymers achieve improved PCEs. For
example, the Voc, Jsc and FF values of PEHBDT-T-R were improved significantly,
when a small amount of DIO (3%, by volume) was added, resulting in remarkably
improvement of PCE (Table 3). While when 3 vol % DIO was added to
PEHBDT-T-TR based cell, Jsc value was improved, Voc and FF values remain
unchanged. In these optimized conditions, the maximal PCEs of 3.31% (Voc = 0.87 V,
Jsc = 7.79 mA/cm2, FF = 48.89%), 1.64% (Voc = 0.93 V, Jsc = 3.89 mA/cm2, FF =
45.33%) and 2.80% (Voc = 0.69 V, Jsc = 8.79 mA/cm2, FF = 46.20% ) are achieved for
the PEHBDT-T-R, PHDBDT-T-TR and PEHBDT-T-TR based devices (Table 3).
Inspired by recent work26-31 on post solvent treatment, we tried to employ a polar
solvent methanol to further optimize the photovoltaic performance of PSCs. From
Table 3 and Fig. 6, one can see that the photovoltaic performance of all the
copolymers had been enhanced after the solvent treatment. For instance, the PCE of
PEHBDT-T-TR is further improved up to 4.25% with an increased Voc from 0.69 V to
0.87 V, a Jsc from 8.79 mA/cm2 to 9.36 mA/cm2, and a FF from 46.20% to 52.30%.
The improved efficiency may originate from enhanced charge transport, or improved
efficiency of carrier collection, which result from the creation of an optimal
morphology under the conditions of DIO as the processing additive and treated with
methanol. This is supported by results of the hole mobility measurements,
morphology studies, photoluminescence (PL) quenching (vide infra). The highest
PCE of 4.04% for PEHBDT-T-R with a small alkyl chain (EH) on BDT is
significantly higher than that of the analogy polymer PHDBDT-T-R (1.01%) with a
relatively bulky alkyl chain (HD) on BDT, as the same, the highest PCE of
PEHBDT-T-TR (4.25%) is higher than PHDBDT-T-TR (3.47%). The results
indicate that the alkyl chain on BDT has a great influence on the PCE of the
copolymers. In addition, the PCEs of PHDBDT-T-TR (3.47%) and PEHBDT-T-TR
(4.25%) are higher than the analogy polymers PHDBDT-T-R (1.01%) and
PEHBDT-T-R (4.04%) mainly owing to the higher Jsc of the former, which shows
that the introduction of thienylvinyl unit to extend conjugated side chains is favorable

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to improve the Jsc and PCE.
One can observe that the best cells based on the four polymers show relatively
high Voc values (above 0.85 V), which can be readily understood from their
lower-lying HOMO energy levels (Table 2). The highest Jsc values of PSCs based on
PEHBDT-T-R (8.61 mA/cm2) and PEHBDT-T-TR (9.36 mA/cm2) are higher than
those of Jsc values based on PHDBDT-T-R (3.82 mA/cm2) and PHDBDT-T-TR (7.56
mA/cm2), which are consistent with the relatively broader absorption spectra and
lower bandgaps (Fig. 2 and Table 2). Fig. 7 displays the external quantum efficiency
(EQE) curves of the photovoltaic devices under monochromatic light illumination.
PSCs based on these polymers exhibit relatively broad EQE responses ranging from
300 to 700 nm with maximum EQE values of 20% at 488 nm, 43% at 460 nm, 46% at
465 nm and 63% at 468 nm for PHDBDT-T-R, PEHBDT-T-R, PHDBDT-T-TR and
PEHBDT-T-TR, respectively. Compared to the absorption spectra of polymer films
(Fig. 2(b)), the broadened EQE responses in the visible region can be attributed to
both the intrinsic absorption of the polymers and the response of PC70BM. Although
the EQE value is not very high, the response covers a reasonable broad spectrum
(300-700 nm) and a relatively high EQE value over 50% around 470 nm, which
elucidates the obviously higher Jsc of PEHBDT-T-TR-based cells relative to the other
three polymers. Overall, the EQE results are consistent with the Jsc values of the
corresponding PSCs.
Table 3 Photovoltaic properties of the PSCs based on the four copolymers: PC70BM
with different fabricated conditions, under the illumination of AM1.5G, 100 mW/cm2.
Voc
Jsc
FF PCEmax PCEave.
Active layer
DIO MeOH
(V) (mA/cm2) (%)
(%)a
(%)b
PHDBDT-T-R:PC70BM
=1:1
PHDBDT-T-R:PC70BM
=1:2
PHDBDT-T-R:PC70BM
=1:3
PHDBDT-T-R:PC70BM
=1:3
-
-
-
-
-
-
0.63
0.68
0.74
0.65
1.00
1.08
2.14
0.65
26.72
33.32
38.66
29.90
0.17
-
0.25
0.61
0.13
-
-
-
-
3%

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PHDBDT-T-R:PC70BM
=1:3
PHDBDT-T-R:PC70BM
=1:3
PEHBDT-T-R:PC70BM
=1:1
PEHBDT-T-R:PC70BM
=1:2
PEHBDT-T-R:PC70BM
=1:2
PEHBDT-T-R:PC70BM
=1:2
PEHBDT-T-R:PC70BM
=1:2
PEHBDT-T-R:PC70BM
=1:3
PHDBDT-T-TR:PC70BM
=1:1
PHDBDT-T-TR:PC70BM
=1:2
PHDBDT-T-TR:PC70BM
=1:2
PHDBDT-T-TR:PC70BM
=1:2
PHDBDT-T-TR:PC70BM
=1:2
PHDBDT-T-TR:PC70BM
=1:3
PEHBDT-T-TR:PC70BM
=1:1
PEHBDT-T-TR:PC70BM
=1:1.5
PEHBDT-T-TR:PC70BM
=1:1.5
PEHBDT-T-TR:PC70BM
=1:1.5
DOI: 10.1039/C5TA02360B
3%
+
+
-
0.97
0.82
0.69
0.73
0.87
0.99
0.86
0.69
0.81
0.81
0.93
0.95
0.81
0.78
0.69
0.69
0.69
0.87
0.66
1.77
3.92
6.38
7.60
7.79
8.61
8.14
8.33
3.75
4.39
3.89
7.56
6.07
3.50
6.31
7.41
8.79
9.36
8.03
37.40
32.20
35.81
41.80
48.89
47.50
41.27
38.13
32.74
41.09
45.33
48.31
42.04
30.59
46.77
46.20
46.20
52.30
40.01
0.64
1.01
1.58
2.32
3.31
4.04
2.89
2.19
1.00
1.46
1.64
3.47
2.07
0.84
2.04
2.36
2.80
4.25
2.12
-
-
0.97
-
-
-
3%
3%
-
-
-
-
-
+
+
-
3.89
-
-
-
-
-
-
-
-
-
3%
3%
-
-
-
+
+
-
3.34
-
-
-
-
-
-
-
-
-
3%
3%
-
-
-
4.01
-
+
-
PEHBDT-T-TR:PC70BM
=1:2
b
aPCEmax is the highest PCE of the devices. PCEave. is the average value of four
devices.

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Fig. 6 Typical J-V curves of photovoltaic devices based on PHDBDT-T-R,
PEHBDT-T-R, PHDBDT-T-TR and PEHBDT-T-TR with the best fabricated
conditions.
Fig. 7 EQE curves of photovoltaic devices based on PHDBDT-T-R, PEHBDT-T-R,
PHDBDT-T-TR, and PEHBDT-T-TR with the best fabricated conditions.
Hole Mobility
Hole mobility is an important parameter for the photovoltaic donor materials.
Here, hole mobility of the four polymers/PC70BM blend film with the optimal D/A

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ratio were measured by using the space-charge-limited current (SCLC) method with
the hole-only device structure of ITO/PEDOT:PSS/polymer:PC70BM/Au and
estimated through the Mott–Gurney equation.33 For the hole-only devices, SCLC is
described by J = 9ε0εrμV2/8L3exp[0.891γ(V/L)0.5], where ε is the dielectric constant of
the polymers, ε0 is the permittivity of the vacuum, μ0 is the zero-field mobility, E0 is
the characteristic field, J is the current density, and L is the thickness of the film. Fig.
8 shows the ln(JL3/V2) vs (V/L)0.5 plots of the polymers measured by the SCLC
method. According to the above equation and the curves shown in Fig. 8, the
calculated hole mobilities are 1.51 × 10−5 cm2 V−1 s−1, 5.02 × 10−5 cm2 V−1 s−1, 3.04 ×
10−5 cm2 V−1 s−1, and 2.75 × 10−4 cm2 V−1 s−1 for PHDBDT-T-R, PEHBDT-T-R,
PHDBDT-T-TR, and PEHBDT-T-TR blend films, respectively. The lower hole
mobility of the PHDBDT-T-R–based blend film could be related to its lower
molecular weight (see Table 1). This should be responsible for its poor photovoltaic
performance. On the other hand, PEHBDT-T-TR–based blend film showed the
highest mobility of all, which could contribute in part to the higher Jsc and FF values
of the device.
Fig. 8 Plots of ln(JL3 V-2) vs (V0.5L-0.5) for the measurement of hole mobility in
polymer/PC70BM devices by the SCLC method.

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Morphological studies
The morphology of the photoactive layer plays an important role in the photovoltaic
performance of PSCs along with other photo-physical properties and in several
situations the photovoltaic performance of the device strongly depends on its
morphological features. To explore the effects of the methanol treatment on the
morphology and device performance of polymer/PC70BM film, we observed the
surface morphologies of the blend films using atomic force microscopy (AFM). As
shown in Fig 9a, when 3 vol % DIO was added, the active layer of PEHBDT-T-TR
displayed a root-mean-square (RMS) of 3.89 nm. After treating the active layer with
methanol, regular columnar domains formed in the phase image, and the surface of
the blend film based on PEHBDT-T-TR became relatively rough with an increased
RMS of 8.07 nm (see Fig. 9b). As some of the polar solvent molecules penetrated the
active layer, PC70BM particles redistributed and aggregated to reduce the contact
areas with the polar solvents.34 As a result, solvent-dependent PC70BM migration
within the polymer chains enabled the formation of nanoscale phase separation of the
blend films (Fig. 9b). This relatively course surface and more ordered structure are
beneficial to the charge transportation, thus leading to an increase in Jsc and FF as
well as the device efficiency. Obviously, the appropriate domain sizes and phase
separation can produce the best photovoltaic performance. Furthermore, the similar
results can be found from the other three polymers. Figs 9c, d, and e give the AFM
height images of the other three polymers after the methanol treatment. It can be seen
that the active layers of PHDBDT-T-R, PEHBDT-T-R, and PHDBDT-T-TR
displayed RMS of 3.17, 5.31, and 4.60 nm, respectively. Obviously, the active layer of
PEHBDT-T-R and PHDBDT-T-TR show relatively rougher surfaces compared with
PHDBDT-T-R. In view of the superior photovoltaic performance of PEHBDT-T-R
and PHDBDT-T-TR-based PSCs, the result further suggests that relatively larger
phase separation is more favorable for improving the photovoltaic performance.

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Fig. 9 AFM height images of a PEHBDT-T-TR: PC70BM(1:1.5, 3 vol % DIO), b
PEHBDT-T-TR:PC70BM(1:1.5, 3 vol % DIO, treated with methanol), c
PHDBDT-T-R:PC70BM (1:3, treated with methanol), d PEHBDT-T-R:PC70BM(1:2,
3 vol % DIO, treated with methanol ) and e PHDBDT-T-TR:PC70BM(1:2, 3 vol %
DIO, treated with methanol). The imaging size is 3 μm × 3 μm for each panel.
Photoluminescence
In order to gain more insight into the methanol treatment on enhanced efficiency, the
PL quenching studies in the presence of PC70BM acceptor were carried out.27,28,30 We
choose PHDBDT-T-TR as an example because its PCE changed significantly after
treating with methanol (see Table 3). Fig. 10 gives the PL spectra of the pristine
PHDBDT-T-TR, PHDBDT-T-TR:PC70BM (3 vol % DIO) with and without
methanol treatment. The pristine PHDBDT-T-TR displays the emission in the range
of 600−800 nm with a peak at 671 nm. Upon blending PHDBDT-T-TR with PC70BM,
the PL is significantly quenched as shown in Fig. 10. The PL is further quenched after
treating the PHDBDT-T-TR:PC70BM blend film with methanol. This effective PL
quenching suggests that the exciton separation is highly efficient in polymer:PC70BM
blend films. The more effective quenching in the PHDBDT-T-TR:PC70BM blend

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with methanol treatment than that without treatment implies that the feasibility of
exciton dissociation process in polymer:PC70BM blend film is increased further after
the methanol treatment. Therefore, the PL quenching studies reveal that the
polymer:PC70BM blend film with methanol treatment has more favorable features for
the higher Jsc.
Fig. 10 Photoluminescence spectra of pristine PHDBDT-T-TR, PHDBDT-T-TR:
PC70BM (3 vol % DIO) without and with methanol treatment.
Conclusions
Four main chain-side chain D-A copolymers, PHDBDT-T-R, PEHBDT-T-R,
PHDBDT-T-TR, and PEHBDT-T-TR containing the same backbone of alternative
BDT-thiophene but different side chains bearing on BDT and thiophene units were
designed, synthesized and utilized for BHJ PSCs. We found that the type and shape of
the side chains have great influences on the solubility, light absorption,
HOMO/LUMO energy levels, and charge transport properties of the resulting
polymers as well as morphology and photovoltaic properties of the PSCs. Optical and
electrochemical properties showed that the introduction of thienylvinyl unit between
the thiophene and 3-ethylrhodanine as conjugated side chains can slightly reduce

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bandgaps and elevate HOMO levels of the polymers. The BHJ PSCs based on four
polymers were prepared and the device fabrication conditions, including the different
blend ratio of the polymer donor and acceptor, the additive of 1, 8-diiodooctane (DIO)
and methanol treatment, have been optimized in detail. The effects of methanol
treatment on the enhancement in photovoltaic performance especially for the Jsc and
FF of the PSCs are demonstrated to originate from the increased exciton separation
efficiency, charge carrier mobilities and the formation of fiber-like interpenetrating
network morphologies. After systematically optimizing, the best PCEs were improved
up to 1.01%, 4.04%, 3.47%, and 4.25% for PHDBDT-T-R, PEHBDT-T-R,
PHDBDT-T-TR, and PEHBDT-T-TR, respectively, mainly due to significantly
increased Jsc and FF. This work could provide some insight into side chain
engineering to efficiently construct photovoltaic polymers.
Experiment section
Measurements and characterization
Solvents were dried and distilled from appropriate drying agents prior to use.
1
Commercially available reagents were used without further purification. H NMR
spectra and 13C NMR spectra were measured on a Bruker DMX−400 spectrometer
with CDCl3 as the solvent and (CH3)3SiH as the internal reference. Mass spectra (MS)
were recorded on a Shimadzu spectrometer and MALDI-TOF-MS spectra were
determined on a Bruker BIFLEX III mass spectrometer. Molecular weights of the
polymers were measured by gel permeation chromatography (GPC) with polystyrenes
as standard and chloroform (CHCl3) as an eluent. UV−vis absorption spectra were
obtained on a Hitachi U-3010 UV−vis spectrophotometer. Thermogravimetric
analysis (TGA) was conducted on a Perkin−Elmer TGA−7 thermogravimetric
analyzer at a heating rate of 10 °C/min and under a nitrogen flow rate of 100 mL/min.
The electrochemical cyclic voltammetry (CV) was performed on a Zahner IM6e
Electrochemical Workstation, with a glassy carbon disc coated with polymer film as
the working electrode, Pt wire as the counter electrode and Ag/AgCl electrode as the
reference electrode, respectively, in a 0.1 mol/L Bu4NPF6 acetonitrile solution at a

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scan rate of 100 mV/s. The morphology of blend films was measured using an atomic
force microscope (AFM, Digital Instrument Multimode Nanoscope IIIA) using the
tapping mode.
PSCs Device fabrication and characterization
PSCs were fabricated with ITO glass as a positive electrode, Ca/Al as a negative
electrode and the blend film of polymer/PC70BM between them as a photosensitive
layer. The patterned ITO glass was precleaned by ultrasonication with detergent water,
deionized water, acetone and isopropanol each for 20 min, then treated in an
ultraviolet-ozone chamber (Jelight Co.) for 20 min. A thin layer of PEDOT:PSS
(poly(3, 4-ethylenedioxythiophene):poly(styrenesulfonate)) was spin-cast on
precleaned ITO-coated glass from a PEDOT:PSS aqueous solution (Clevious P VP AI
4083 H. C. Stark, Germany) at 3000 rpm for 40 s and dried subsequently at 150 °C
for 15 min in air, and the thickness of the PEDOT:PSS layer is about 40 nm. After that,
transfer the device to the glovebox. The photosensitive layers, a blend solution of the
polymer and PC70BM with the poly concentration of 13.3 mg/mL in 1,
2-dichlorobenzene (DCB) was spin-coated on ITO/PEDOT:PSS electrode. Finally,
the Ca/Al cathode was deposited in vacuum on the active layer under the pressure of
ca. 9 × 10-5 Pa. The active area of the investigated device was 5 mm2. The current
density−voltage (J−V) characteristics were conducted on a computer-controlled
Keithley 236 Source−Measure Unit in a dry box under an inert atmosphere. A xenon
lamp (150 W) coupled with AM 1.5 solar spectrum filter was used as the white light
source, and the optical power at the sample was 100 mW/cm2. The external quantum
efficiency (EQE) was measured by a Stanford Research Systems model SR830 DSP
lock-in amplifier coupled with a WDG3 monochromator and a 500W xenon lamp.
The light intensity at each wavelength was calibrated with a standard single-crystal Si
photovoltaic cell. The hole mobility was calculated by fitting the dark J−V curves for
the hole-only devices to SCLC model at low voltages, in which the current density is
given by J = 9ε0εrμV2/8L3exp[0.891γ(V/L)0.5], where ε0εr represents the permittivity
of the material, μ is the mobility, γ is the field activation factor, and L the thickness of

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the active layer. The applied bias voltage is corrected for the built-in potential so that
V = Vapplied−Vbi.
Materials
N-Bromosuccinimide (NBS), thiophene-3-carbaldehyde, thiophene-2-carbaldehyde,
3-methylthiophene, 3-ethylrhodanine, t-BuOK, and Pd(PPh3)4 were purchased from
J&K Chemical or Alfa Aesar. Toluene, DMF, THF, POCl3 and ClCH2CH2Cl were
carefully dried and distilled prior to use. All other reagents and solvents used in this
work were commercially purchased and used without further purification.
2,5-Bibromo-3-(bromomethyl)thiophene,
2,5-dibromo-3-methylthiophene,
(2,5-dibromothiophen-3-ylmethyl)phosphonic acid diethyl ester28 and M3, M4 were
synthesized by the reported procedures.19 The detailed synthetic procedures of other
compounds are as follows.
Synthesis of the monomers and polymers
2, 5-Dibromothiophene-3-carbaldehyde (1). Thiophene-3-carbaldehyde (2.24 g,
20mmol) was placed in a 100mL round-bottom flask and added a bit of DMF to
dissolve. Then NBS (7.48 g, 42 mmol) was dissolved in an appropriate amount of
anhydrous DMF. The mixture was added dropwise to the flask in the dark. The
resulting solution was stirred at room temperature all day. Then, the mixture was
extracted with dichloromethane, the organic layer was washed with water and dried
over anhydrous MgSO4, and the solvent was removed under vacuum. After that, the
crude product was purified through the silica gel column with a petroleum
ether/dichloromethane mixture (4:1 by volume) as eluent. A creamy yellow solid was
obtained. Yield: 3.61 g (67%).1H NMR (400 MHz, CDCl3, δ/ppm): 9.82 (s, 1H), 7.36
13
(s, 1H). C NMR (100 MHz, CDCl3, δ/ppm): 183.0, 139.4, 128.7, 124.0, 113.3. MS
(EI): m/z: calcd for C5H2Br2OS: 269.9; found: 269.5.
(Z)-5-((2,5-Dibromothiophen-3-yl)methylene)-3-ethyl-2-thioxothiazolidin-4-one
(M1). The Monomer 1 was synthesized by the Knoevenagel reaction. Compound 1
(2.70 g, 10.0 mmol), 3-ethylrhodanine (1.62 g, 10.0 mmol) and 30 mL CHCl3 was
added in a 50 mL dual-mouth flask under argon protection. Subsequently, 0.5 mL

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piperidine was injected into the reaction mixture, and then the mixture was heated
reflux overnight. Then cooling to ambient temperature, the mixture was extracted by
chloroform, and the combined organic phase was washed with water several times,
dried over MgSO4. After that, the crude product was purified through the silica gel
column with a petroleum ether/dichloromethane mixture (2:1 by volume) as eluent to
1
give Monomer 1 as a yellow solid. Yield: 2.9 g (70%). H NMR (400 MHz, CDCl3,
13
δ/ppm): 7.59 (s, 1H), 7.13 (s, 1H), 4.21-4.15 (m, 2H), 1.286 (t, 3H, J = 7.2 Hz ). C
NMR (100 MHz, CDCl3, δ/ppm): 206.9, 191.5, 167.3, 135.4, 128.1, 124.6, 123.1,
119.9, 113.7, 39.9, 30.9, 12.2. MS (EI): m/z: calcd for C10H7Br2NOS3: 413.2; found:
413.2
(E)-2,5-Dibromo-3-(2-(thiophen-2-yl)vinyl)thiophene
(2).
(2,5-Dibromothiophen-3-ylmethyl)phosphonic acid diethyl ester (6.28 g, 14.9 mmol)
and thiophene-2-carbaldehyde (1.67 g, 14.9 mmol) were dissolved in THF (20 mL)
and the solution was stirred at room temperature for 30 min under argon atmosphere.
Then t-BuOK (1.94 g, 17.14 mmol) was dissolved in THF (20 mL) and added
dropwise to the solution. The reaction mixture was stirred at room temperature for 5 h,
and then heated to 50 °C reflux for 12 h. After cooling to room temperature, the
reaction mixture was poured into dilute aqueous HCl (20 mL) solution and extracted
with dichloromethane. The organic layer was washed with water and dried over
anhydrous MgSO4, and then the solvent was removed by rotary evaporation. The
crude product was purified by column chromatography on silica gel using petroleum
ether as eluent to afford compound 2 as a yellow solid. Yield: 3.68 g (70%). 1H NMR
(400 MHz, CDCl3, δ/ppm): 7.22 (s, 1H), 7.08 (s, 1H), 7.05 (s, 1H), 7.00 (d, 2H, J =
12.0 Hz), 6.81 (s, 1H). 13C NMR (100 MHz, CDCl3, δ/ppm): 141.8, 138.7, 127.7,
127.1, 126.7, 125.1, 124.2, 119.4, 111.8, 109.7. MS (EI): m/z: calcd for C10H6Br2S2:
350.1; found: 350.2
(E)-5-(2-(2, 5-Dibromothiophen-3-yl)vinyl)thiophene-2-carbaldehyde (3). The
compound 3 was synthesized by the Vilsmeier reaction. To the mixture of compound
2 (3.4 g, 9.7 mmol) in 1, 2-dichloroethane (25 mL) and anhydrous DMF (0.71 mL, 9.7
mmol) was added dropwise under argon atmosphere, then POCl3 (0.85 mL, 9.7 mmol)

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was added slowly. The mixture was stirred at 85 °C for 12 h. Then cooled the mixture
to room temperature, added 30 mL saturated sodium acetate solution to the mixture,
stirred 70 min at the room temperature. After that, 30 mL of water was added to
quench the reaction. The mixture was extracted with CH2Cl2, the combined the
organic phase was washed with water 3 times and dried over anhydrous MgSO4, and
then the solvent was removed under vacuum. The crude product was purified on silica
gel chromatography using a petroleum ether/dichloromethane mixture (4:1 by volume)
1
as eluent. A yellow powder was obtained. Yield: 2.39 g (65%). H NMR (400 MHz,
CDCl3, δ/ppm): 9.87 (s, 1H), 7.66 (d, 1H, J = 3.6 Hz), 7.18 (t, 2H, J = 5.6 Hz), 7.02 (d,
2H, J = 2.0 Hz). 13C NMR (100 MHz, CDCl3, δ/ppm): 182.3, 156.0, 142.0, 137.7,
136.7, 126.9, 126.8, 123.4, 122.9, 112.3, 112.2. MS (EI): m/z: calcd for C11H6Br2OS2:
378.1; found: 378.1
(Z)-5-((5-((E)-2-(2,5-Dibromothiophen-3-yl)vinyl)thiophen-2-yl)methylene)-3-
ethyl-2-thioxothiazolidin-4-one (M2). By following the similar method used for M1,
M2 was synthesized with compound 3 (1.5 g, 3.96 mmol) and 3-ethylrhodanine
(0.638 g, 3.96 mmol). The final product was obtained as a red solid. Yield: 1.48 g
1
(72%). H NMR(400 MHz, CDCl3, δ/ppm): 7.82 (s, 1H), 7.31 (d, 1H, J = 4.0 Hz),
7.17 (s, 1H), 7.13 (d, 1H, J = 4.0 Hz), 7.03 (s, 1H), 7.01-6.92 (m, 1H), 4.19 (d, 2H, J
= 7.2 Hz), 0.89 (t, 3H, J = 2.4 Hz). 13C NMR (100 MHz, CDCl3, δ/ppm): 192.0, 167.3,
149.8, 149.8, 138.2, 137.3, 135.0, 128.3, 127.1, 124.9, 123.2, 122.4, 112.5, 112.0,
40.0, 12.3. MALDI-TOF: MS (EI): m/z: calcd for C16H11Br2NOS4: 521.3; found:
521.2.
PHDBDT-T-R. The polymerization was performed by a Stille coupling reaction. In
a 50 mL flask, M1 (123 mg, 0.30 mmol) and M3 (299 mg, 0.30 mmol) were
dissolved in 10 mL toluene, and the solution was flushed with argon for 15 min, then
50 mg Pd(PPh3)4 was added into the solution. The mixture was again flushed with
argon for 20 min. The reaction mixture was heated to reflux for 24 h. The reaction
mixture was cooled to room temperature and added dropwise to 200 mL methanol.
The precipitate was collected and further purified by Soxhlet extraction with methanol,
hexane, in sequence. The hexane fraction was concentrated and added dropwise into

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methanol. Finally, the precipitates were collected and dried under vacuum overnight
to get PHDBDT-T-R as a black red solid (114 mg, yield 41.6%). 1H NMR (400 MHz,
CDCl3, δ/ppm): 8.22 (br, 1H), 8.06 (br, 1H), 7.88 (br, 1H), 7.67 (br, 1H), 4.58-3.84
(m, 6H), 2.20-1.96 (m, 2H), 1.33-0.89 (m, 63H).
PEHBDT-T-R. PEHBDT-T-R was synthesized by following the same procedure
for PHDBDT-T-R. M1 (123 mg, 0.30 mmol) and M4 (232 mg, 0.30 mmol) were
used as starting materials. Finally, PEHBDT-T-R was obtained as a dark-red solid
1
(163 mg, yield 78.3%). H NMR (400 MHz, CDCl3, δ/ppm): 8.22 (br, 1H), 8.06 (br,
1H), 7.88 (br, 1H), 7.67 (br, 1H), 4.58-3.84 (m, 6H), 2.20-1.96 (m, 2H), 1.37-0.99 (m,
31H).
PHDBDT-T-TR. PHDBDT-T-TR was synthesized by following the same
procedure for PHDBDT-T-R. M2 (156 mg, 0.30 mmol) and M3 (299 mg, 0.30 mmol)
were used as starting materials. But the precipitate was collected and further purified
by Soxhlet extraction with methanol, hexane, chloroform in sequence. The
chloroform fraction was concentrated and added dropwise into methanol. Finally,
1
PHDBDT-T-TR was obtained as a dark-red solid (157 mg, yield 51.1%). H NMR
(400 MHz, CDCl3, δ/ppm): 7.84 (br, 1H), 7.55 (br, 1H), 7.52 (br, 1H), 7.49 (br, 1H),
7.38 (br, 1H), 7.21 (br, 1H), 7.17 (br, 1H), 6.99 (br, 1H), 4.25-3.80 (m, 6H), 1.95-1.83
(m, 2H), 1.26-0.82 (m, 63H).
PEHBDT-T-TR. PEHBDT-T-TR was synthesized by following the same
procedure for PEHBDT-T-R. M2 (130 mg, 0.25 mmol) and M4 (193 mg, 0.25 mmol)
were used as starting materials. Finally, PEHBDT-T-TR was obtained as a dark-red
solid (146 mg, yield 72.6%). 1H NMR (400 MHz, CDCl3, δ/ppm): 7.84 (br, 1H), 7.55
(br, 1H), 7.52(br, 1H), 7.48 (br, 1H), 7.37 (br, 1H), 7.20 (br, 1H), 7.17 (br, 1H), 7.00
(br, 1H), 4.25-3.80 (m, 6H), 1.94-1.82 (m, 2H), 1.26-0.89 (m, 31H).
Acknowledgements
This work was supported by the Natural Science Foundation of Hunan Province,
China (Grant No. 2015JJ3019) and the Key Research Foundation of Education
Bureau of Hunan Province, China (Grant No. 14A147).

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A table of contents
Four benzodithiophene-thiophene copolymers with conjugated side chain
containing electron-withdrawing ethylrhodanine acceptor unit were designed and
synthesized. The PSCs based on the four polymers showed the highest PCE of 4.25%.