Side-Chain Engineering of Benzodithiophene-Fluorinated Quinoxaline Low-Band-Gap Co-polymers for High-Performance Polymer Solar Cells

Article abstract of DOI:10.1002/chem.201403153

A new series of donor-acceptor co-polymers based on benzodithiophene and quinoxaline with various side chains have been developed for polymer solar cells. The effect of the degree of branching and dimensionality of the side chains were systematically inve

Full text of DOI:10.1002/chem.201403153

DOI: 10.1002/chem.201403153
Full Paper
&
Donor–Acceptor Systems
Side-Chain Engineering of Benzodithiophene-Fluorinated
Quinoxaline Low-Band-Gap Co-polymers for High-Performance
Polymer Solar Cells
Xiaopeng Xu,[a] Yulei Wu,[b] Junfeng Fang,*[b] Zuojia Li,[a] Zhenguo Wang,[a] Ying Li,[a] and
Qiang Peng*[a]
Abstract: A new series of donor–acceptor co-polymers
based on benzodithiophene and quinoxaline with various
side chains have been developed for polymer solar cells. The
effect of the degree of branching and dimensionality of the
side chains were systematically investigated on the thermal
stability, optical absorption, energy levels, molecular packing,
and photovoltaic performance of the resulting co-polymers.
The results indicated that the linear and 2D conjugated side
chains improved the thermal stabilities and optical absorp-
tions. The introduction of alkylthienyl side chains could effi-
ciently lower the energy levels compared with the alkoxyl-
substituted analogues, and the branched alkoxyl side chains
could deepen the HOMO levels relative to the linear alkoxyl
chains. The branched alkoxyl groups induced better lamel-
lar-like ordering, but poorer face-to-face packing behavior.
The 2D conjugated side chains had a negative influence on
the crystalline properties of the co-polymers. The per-
formance of the devices indicated that the branched alkoxyl
side chains improved the Voc, but decreased the Jsc and fill
factor (FF). However, the 2D conjugated side chains would
increase the Voc, Jsc, and FF simultaneously. For the first time,
our work provides insight into molecular design strategies
through side-chain engineering to achieve efficient polymer
solar cells by considering both the degree of branching and
dimensionality.
derivatives.[3] A low-band-gap (LBG) co-polymer featuring elec-
tron donor/acceptor (D/A) units in the main chain may be the
most successful candidate, which can form a push–pull config-
uration and facilitate intramolecular charge transfer (ICT) be-
tween the donor and the acceptor parts.[1c,d,4] The rational se-
lection and optimization of D/A building blocks allow the band
gaps to be finely tuned, as well as the energy levels and light-
absorption abilities of the designed LBG co-polymers.[5] More-
over, the particular push–pull structure can reduce intrachain
p–p stacking distances, and thus induce, improve charge-carri-
er mobility.[6] By using this design strategy, the power conver-
sion efficiencies (PCEs) of the PSCs reached 7–9% for a single
junction and 9–11% for tandem device architectures with the
newly developed LBG polymer materials.[2f,7,8]
Introduction
Polymer solar cells (PSCs) based on bulk heterojunction (BHJ)
structures, incorporating conjugated polymers as donor mate-
rials and fullerene derivatives as acceptors, have attracted
much attention due to their promising applications in fabricat-
ing flexible and large-area solar panels by using low-cost solu-
tion-processing methods.[1] Tremendous efforts have been
made toward developing highly efficient BHJ PSCs, including
the design and preparation of new p-type polymeric donors
and n-type accepting materials, by changing various additives
to finely tune the morphology of the active layer, by optimiz-
ing device fabrication techniques, and by adopting novel
device architectures.[2] Electron-donating conjugated polymers
have received extensive attention because they make a greater
contribution to harvesting photons in BHJ PSCs than fullerene
In addition to band-gap control and molecular energy level
modulation, side-chain engineering has been receiving increas-
ing attention for the design of high-performance photovoltaic
polymers. Previously, alkyl side chains were introduced onto
conjugated backbones as solubilizing groups to afford the pre-
pared polymers with good solubility and processability.[9] Gen-
erally, the solubility of the polymers can be improved by at-
taching longer alkyl side chains; however, the molecular pack-
ing, blend morphology, fullerene miscibility, and optoelectronic
performance will also be influenced.[9] In the last few years,
branched alkyl chains have also been studied, including the
length, size, and position of the branching point relative to the
polymer backbone.[10] In addition to the alkyl side chains, the
[a] X. Xu,+ Dr. Z. Li, Z. Wang, Prof. Y. Li, Prof. Q. Peng
Key Laboratory of Green Chemistry and Technology
of Ministry of Education, College of Chemistry
Sichuan University, Chengdu 610064 (P.R. China)
Fax: (+86)28-86510868
E-mail: qiangpengjohnny@yahoo.com
[b] Y. Wu,+ Prof. J. Fang
Ningbo Institute of Materials and Technology
and Engineering, Chinese Academy of Sciences
Ningbo 315201 (P.R. China)
E-mail: fangjf@nimte.ac.cn
[+] These authors contributed equally to this work.
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use of alkyl-substituted aromatic side groups on the polymer
backbone has been demonstrated to extend the p conjugation
from the backbone to the lateral substituents, leading to 2D
conjugated systems.[11,12] The variable conjugated side chains
have been introduced onto the D[11] and A[12] skeletons to pro-
cess enhanced absorption, charge transport, and device per-
formance. For example, Li et al. designed and prepared a series
of polythiophenes with conjugated side chains of bi(phenyl-
enevinylene),[13] bi(thienylenevinylene),[11a,14] and phenothiazi-
nevinylene;[15] these exhibited broadened and strengthened
absorption spectra, improved charge mobility, and enhanced
PCEs. Hou et al. first introduced conjugated alkylthienyl side
chains onto the benzodithiophene (BDT) skeleton, leading to
a more planar configuration than that of BDT with alkoxyl side
chains;[11d] this had a positive effect on energy level modula-
tion and p–p stacking properties between polymer main
chains; hence improving solar cell performance.[11] Ge et al. re-
ported diketopyrrolopyrrole (DPP)-based co-polymers with
furan side chains on the BDT core; these exhibited smaller opti-
cal band gaps, broader absorption ranges, and deeper HOMO
energy levels than those of the corresponding alkoxyl-substi-
tuted counterparts.[16] Recently, side chains composed of one
or more thiophene units linked by single or double bonds
were studied to further extend the p conjugation and raise the
efficiency of related PSCs.[17] It is notable that the rational
choice of suitable side chains is very important for conjugated
polymers to achieve high photovoltaic performance.[18] Howev-
er, no work has been done to consider the degree of branch-
ing and dimensionality at the same time. Thus, a systematic in-
vestigation of the side-chain engineering is needed to under-
stand the structure–property relationship of D–A-type photo-
voltaic co-polymers.
DPP,[20] thiazolothiazole (TTZ),[21] thienopyrroledione (TPD),[17b][22]
and thieno[3,4-b]thiophene (TT),[2c,19b,23] and substantial PCEs
surpassing 7% had been achieved. Derivatives of Qx have been
widely used as an acceptor unit for building D–A co-polymers
because of the electron-withdrawing property of the imine ni-
trogen atoms.[24] Recently, the incorporation of an electron-
withdrawing fluorine atom onto the Qx skeleton has become
more attractive.[7b,25] The small size of the fluorine atom would
not increase steric interactions; however, its strong electron-af-
finity nature is helpful for lowering the HOMO and LUMO
energy levels to maintain almost the same optical band gap as
their corresponding non-fluorinated analogues.[2d,19d,26] Fluori-
nation would also induce molecular coplanarity and molecular
packing through intra- and/or intermolecular interac-
tions;[8c,11b,25,27] this would result in higher carrier mobilities and
better morphology. From the viewpoints of the degree of
branching and dimensionality studied herein, we would like to
report the overall effect of side chains on the thermal stability,
optical absorption, energy level, molecular packing, hole mobi-
lity, and photovoltaic performance of BDT–Qx-based co-poly-
mers. Although previous studies have been reported concern-
ing alkyl, alkoxyl, and alkylthienyl side chains, there have not
been any reports on a systematic investigation into the degree
of branching and dimensionality of the side chains simultane-
ously with identical conjugated backbones. Polymer PBDTTL-
BFQ, with 2D and linear conjugated side chains, demonstrated
the best performance of up to 5.00% PCE, which could be of
great importance for the future design and synthesis of poly-
mer donor materials in photovoltaic applications.
Based on the above considera-
tions, we present herein the syn-
thesis and characterization of
a series of BDT–quinoxaline (Qx)
conjugated
co-polymers,
PBDTAL-BFQ,
PBDTAB-BFQ,
PBDTTL-BFQ, and PBDTTB-BFQ
(Scheme 1), in which variable
side chains with different de-
grees of branching and dimen-
sionality have been attached
onto the BDT skeleton. BDT is
chosen as a weak electron-do-
nating unit because of its struc-
tural symmetry and rigid fused
aromatic system, which pro-
motes enhanced electron deloc-
alization and molecular pack-
ing.[19] According to reports in
the literature, a variety of BDT-
based co-polymers have been
designed and prepared with var-
ious electron acceptors, includ-
ing benzothiadiazole (BT),[2d,11b,d]
Scheme 1. Design strategy and molecular structures of the co-polymers described herein.
benzotriazole
(BZ),[8c,11e,19d]
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pared by
a condensation reaction of 4 with 1,2-bis[3-
Results and Discussion
(octyloxy)phenyl]ethane-1,2-dione.[24,25] A Stille cross-coupling
reaction of dibromide 5 with 2-(tributylstannyl)thiophene yield-
ed intermediate 6. Bromination of 6 with NBS in DMF afforded
the target monomer BFQ. Co-polymers PBDTAL-BFQ, PBDTAB-
BFQ, PBDTTL-BFQ, and PBDTTB-BFQ were then prepared
through Stille coupling polymerization by using [Pd2(dba)3]/
P(o-tolyl)3 as catalysts to give the co-polymers in good yields.
The crude polymers were precipitated into methanol and fil-
tered, followed by sequential Soxhlet extraction with metha-
nol, hexane, and chloroform to remove byproducts and oligo-
mers. The purified co-polymers were obtained from the chloro-
form fraction precipitated into
Synthesis and characterization
The molecular structures of the co-polymers and the synthetic
routes to the monomers and co-polymers are shown in
Schemes 1 and 2, respectively. All organic tin monomers, in-
cluding monomer BDTTB, were synthesized according to our
previously reported procedures.[11b,12] 4,7-Bibromo-5-fluoro-
2,1,3-benzothiadiazole (3) was converted into the correspond-
ing diamine compound (4) by means of a reductive reaction in
the presence of Zn/acetic acid. Then, compound 5 was pre-
methanol again, filtered, and
dried under vacuum for 24 h.
The chemical structures of the
resulting co-polymers were con-
firmed by 1H NMR spectroscopy
and elemental analysis. These
polymers exhibit excellent film-
casting properties and good sol-
ubility in some common organic
solvents, such as chloroform,
THF, chlorobenzene (CB), and di-
chlorobenzene
(DCB).
The
number-average
molecular
weights (Mn) and polydispersity
indices (PDIs) of these polymers
were measured by gel permea-
tion chromatography (GPC) by
using THF as the eluent and
polystyrene as the internal stan-
dard. The Mn values of PBDTAL-
BFQ, PBDTAB-BFQ, PBDTTL-
BFQ, and PBDTTB-BFQ were de-
termined to be 28, 8, 44, and
46 kDa, respectively, with PDIs in
the range of 2.1–4.2.
The thermal stability of conju-
gated polymers plays a very im-
portant role in PSCs. The thermal
properties were investigated by
thermogravimetric
analysis
(TGA). As shown in Figure 1a
and Table 1, the degradation
temperatures (Td) of PBDTAL-
BFQ, PBDTAB-BFQ, PBDTTL-
BFQ, and PBDTTB-BFQ with 5%
weight loss were measured to
be about 332, 326, 434, and
4238C, respectively. From the re-
sults, it is easy to see that in-
creasing the degree of branch-
ing of the side chains decreases
the thermal stability of the rele-
vant co-polymers. We also found
that 2D conjugated side chains
Scheme 2. Synthetic routes of the monomers and co-polymers. NBS=N-bromosuccinimide, dba=dibenzylidenea-
cetone.
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Optical properties
The UV/Vis absorption properties of the polymers were investi-
gated in chloroform (10ꢀ5 m) and as thin solid films prepared
by drop-casting on quartz slides from a 10 mgmLꢀ1 solution in
chloroform. The corresponding absorption data of the co-poly-
mers are summarized in Table 2. As shown in Figure 2, these
polymers exhibit similar UV/Vis absorption spectra due to the
consistency of their backbone structures. The first intense
band ranges from l=300 to 480 nm and the second ranges
from l=500 to 750 nm. The higher energy absorption band
corresponds to the p–p* transition of the conjugated back-
bone, whereas the lower one should be attributed to ICT be-
tween the D (BDT) and A (Qx) in the polymer backbone.[4] Both
in CHCl3 and in the solid state, polymers PBDTAB-BFQ and
PBDTTB-BFQ, with branched alkoxyl side chains, possess blue-
shifted absorption relative to that of the linear alkoxyl-substi-
tuted side-chain analogues. This is probably because the
branched alkoxyl substituents in the BDT skeleton disrupt the
planar conformation of the resulting co-polymers to some
extent. Going from 1D alkoxyl side chains to 2D alkylthienyl
side chains, the maximum peaks are redshifted greatly, which
signifies the extension and delocalization of the p-electron
system. Consequently, polymer PBDTTL-BFQ has a relatively
good absorption at long wavelengths with the main peaks at
l=359, 445, and 616 nm in solution and at l=365, 447, and
652 nm in the thin film. From the onset absorption edges, op-
tical band-gap energies of the thin films were determined to
be 1.75, 1.79, 1.70, and 1.74 eV for PBDTAL-BFQ, PBDTAB-BFQ,
PBDTTL-BFQ, and PBDTTB-BFQ, respectively. All of these co-
polymers had LBGs with broad absorption bands in the visible
region, which implied the significance of harvesting more solar
photons in their PSCs. The co-polymers with linear alkoxyl side
chains have lower band gaps than those of co-polymers with
branched alkoxyl side chains. Furthermore, replacing alkoxyl
side chains with alkylthienyl side chains could efficiently
reduce the band gaps of the relevant co-polymers; this implied
that polymer PBDTTL-BFQ, with 2D conjugated linear side
chains, should exhibit the best light-harvesting ability and
would be expected to obtain high Jsc values in the fabricated
PSCs.
Figure 1. TGA and differential scanning calorimetry (DSC) curves of the co-
polymers.
Table 1. Molecular weights and thermal properties of the co-polymers.
[a]
[a]
[b]
Co-polymers
Yield
[%]
Mw
Mn
PDI[a]
Td
[kDa]
[kDa]
[8C]
PBDTAL-BFQ
PBDTAB-BFQ
PBDTTL-BFQ
PBDTTB-BFQ
73
78
59
78
95
17
185
101
28
8
44
46
3.4
2.1
4.2
2.2
332
326
434
423
[a] Molecular weights and PDIs were determined by GPC in THF by using
polystyrene as standards. [b] Onset decomposition temperature mea-
sured by TGA under N2.
based on alkylthienyl-substituted BDTs were much more stable
than their alkoxyl-substituted analogues. Therefore, the ther-
mal stability was mainly affected by the 2D conjugated side
chains, and the branched alkyl side chains made a slightly
more negative difference. On
Electrochemical properties
The energy levels are crucial for the selection of appropriate
acceptors in BHJ PSCs.[28] The HOMO and LUMO energy levels
the other hand, no endo- or
exothermal signals were ob-
Table 2. Optical and electrochemical data of the co-polymers.
served from DSC measurements
in the range of 0–2308C from
the second heating run (Fig-
ure 1b), which implied that
these polymers were thermally
robust. The data for molecular
weights and thermal analysis are
listed in Table 1.
sol
film
[a]
[b]
Co-polymers
Abs. lmax [nm]
Abs. lmax [nm]
Eg [eV]
HOMO [eV]
LUMO [eV]
Eg [eV]
PBDTAL-BFQ
PBDTAB-BFQ
PBDTTL-BFQ
PBDTTB-BFQ
417, 584
408, 565
429, 627
420, 584
1.75
1.79
1.70
1.74
ꢀ5.18
ꢀ5.19
ꢀ5.21
ꢀ5.30
ꢀ2.90
ꢀ2.88
ꢀ3.08
ꢀ3.05
2.28
2.31
2.13
2.25
359, 445, 616
355, 427, 580
365, 447, 652
360, 440, 598
[a] The optical band gap was estimated from the wavelength of the optical absorption edge of the co-polymer
film. [b] The electrochemical band gap was calculated from the LUMO and HOMO energy levels.
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Figure 3. a) Cyclic voltammograms of the co-polymers at a scan rate of
50 mVsꢀ1 and b) the electronic energy level diagram of the co-polymers and
phenyl-C61-butyric acid methyl ester (PC71BM).
Figure 2. Normalized UV/Vis absorption spectra of the co-polymers in
chloroform (a) and as thin solid films (b).
of the co-polymers were evaluated by cyclic voltammetry (CV)
of their thin films. CV measurements were carried out on
a CHI660 potentiostat/galvanostat electrochemical workstation
at a scan rate of 50 mVsꢀ1, with a platinum wire counter elec-
trode and an Ag/AgCl reference electrode in a 0.1 molLꢀ1 solu-
tion of Bu4NClO4 in anhydrous, nitrogen-saturated CH3CN. A
platinum plate coated with a thin film of the studied co-poly-
mer was used as the working electrode. The energy level of
the Ag/AgCl reference electrode was calibrated against the fer-
rocene/ferrocenium (Fc/Fc+) system (4.34 eV).[29] The HOMO
and LUMO energy values (in eV) were calculated by using
Equations (1) and (2), respectively:
to be 2.28, 2.31, 2.13, and 2.25 eV for PBDTAL-BFQ, PBDTAB-
BFQ, PBDTTL-BFQ, and PBDTTB-BFQ, respectively. The differ-
ence between the optical and electrochemical band gaps
could be explained by the exciton binding energy of the co-
polymers and/or the interfacial barriers for charge injec-
tion.[24d][30] The results indicated that the branched alkoxyl side
chains were beneficial to deepen the HOMO energy levels,
compared with the linear alkoxyl chains, which gave the co-
polymers desired high Voc values for PSCs.[31] On the other
hand, the introduction of 2D alkylthienyl conjugated side
chains could further lower the HOMO and LUMO energy levels,
as well as the band gaps. This deep-lying HOMO level was at-
tributed to the conformational distortion and enhanced aro-
matic population of the conjugated backbone caused by high
steric hindrance.[11g] To make a clear comparison, the electronic
energy level diagrams of the co-polymers and PC71BM are de-
scribed in Figure 3b. The LUMO gaps of 0.92–1.12 eV and
HOMO gaps of 1.18–1.30 eV between the co-polymers and
PC71BM should provide a sufficient driving force to guarantee
efficient exciton dissociation at the D–A interface, which would
ensure energetically favorable electron transfer.[32]
EHOMO ¼ ꢀeðEonset,ox þ 4:34Þ
ELUMO ¼ ꢀeðEonset,re þ 4:34Þ
ð1Þ
ð2Þ
in which Eonset,ox and Eonset,re are the onset oxidation and reduc-
tion potentials, respectively, versus Ag/AgCl.[29]
As shown in Figure 3a, all of these co-polymers exhibited re-
versible oxidation and reduction waves, which implied their
potential to transport positive and negative charges. On the
basis of the onset potentials, the HOMO and LUMO energy
levels were estimated to be ꢀ5.18 and ꢀ2.90 eV for PBDTAL-
BFQ, ꢀ5.19 and ꢀ2.88 eV for PBDTAB-BFQ, ꢀ5.21 and
ꢀ3.08 eV for PBDTTL-BFQ, and ꢀ5.30 and ꢀ3.05 eV for
PBDTTB-BFQ. The electrochemical band gaps were calculated
Theoretical calculations
To evaluate the degree of branching and dimensionality im-
pacts of the side chains on the molecular architectures and
electronic properties of the polymers, DFT calculations were
performed to verify stationary points as stable states for the
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optimized conformations and single-point energies, at the
B3LYP/6-31G(d) level.[25] Three repeating units (trimers: BDTAL-
BFQ, BDTAB-BFQ, BDTTL-BFQ, and BDTTB-BFQ) of the co-
polymers were studied as the model compounds for simula-
tions of PBDTAL-BFQ, PBDTAB-BFQ, PBDTTL-BFQ, and
PBDTTB-BFQ. To simplify the calculations, long alkyl chains of
dodecyl, 2-ethylhexyl, and 2-octyldodecyl groups were re-
placed by amyl and isoamyl groups because they did not sig-
nificantly affect the equilibrium geometries or electronic prop-
erties.[25] The HOMO and LUMO wavefunctions of the model
compounds are shown in Figure 4. The calculated HOMOs fea-
tured extended delocalization along the entire conjugated
backbones. The electron density mainly distributed in the
middle part of the conjugated molecular skeleton, which was
affected by both the donor and acceptor units. However, the
LUMOs were mainly focused on the Qx skeletons, resulting
from the effect on the acceptor units. Compared with the
HOMO and LUMO, the HOMOꢀ1 and LUMO+1 seemed to be
distributed from the middle part to two side parts, which im-
plied that the internal charge transfers were possible in BDT–
Qx conjugated systems. Thus, LBGs of PBDTAL-BFQ, PBDTAB-
BFQ, PBDTTL-BFQ, and PBDTTB-BFQ would be obtained by
these efficient internal charge-transfer processes from D seg-
ments to A segments. The HOMO and LUMO energy levels of
PBDTAL-BFQ, PBDTAB-BFQ, PBDTTL-BFQ, and PBDTTB-BFQ
trimers were calculated to be ꢀ4.71/ꢀ2.51, ꢀ4.74/ꢀ2.53,
ꢀ4.65/ꢀ2.48, and ꢀ4.66/ꢀ2.49 eV, respectively. The band gaps
were then determined to be 2.20, 2.21, 2.17, and 2.17 eV. From
the calculation results, the effect of side chains on the band
gaps was similar to those results obtained from CV and UV/Vis
evaluations. The introduction of branched alkoxyl side chains
will deepen the HOMO energy levels and increase the band
gaps. On the contrary, changes to the dimensionality by conju-
gated side chains will decrease the band gaps, while keeping
low-lying HOMO energy levels.
Crystallinity and hole mobility
The crystallinities and molecular ordering properties of the co-
polymer films were investigated by XRD analyses of the spin-
cast films onto glass substrates. Figure 5a shows the XRD pat-
terns of the corresponding co-polymer films cast from solu-
tions in CB. As shown in Figure 5a, all films displayed some-
what crystalline properties with two diffraction peaks in the
ranges of 3–5 and 20–308, which were related to the lamellar
spacings (dl) and the stacking distances (dp) between coplanar
conjugated polymers. For the alkoxyl-substituted co-polymers,
co-polymer PBDTAB-BFQ with branched alkoxyl side chains ex-
hibits higher crystallinity than that of PBDTAL-BFQ. The dl and
dp spacings were calculated to be 25.58 and 22.23 ꢁ and 3.59
and 3.64 ꢁ for PBDTAL-BFQ and PBDTAB-BFQ, respectively.
The increased degree of branching would help to induce la-
mellar-like ordering of the resulting co-polymer, separated by
the attached alkyl side chains. Conversely, branched alkyl side
chains would prevent the face-to-face packing of the polymer
backbones due to greater steric hindrance. After the introduc-
Figure 4. Molecular structures and optimized molecular orbital surfaces of the LUMO+1, LUMO, HOMO, and HOMOꢀ1 for the model trimers, obtained by
Gaussian 09 at the B3LYP/6-31G(d) level.
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J ¼ 9e0ermhV2=8d3
ð3Þ
The voltage (V)–current density (J) curves of the fabricated
hole-only devices of the co-polymer blends are shown in Fig-
ure 5b. According to Equation (3) and the results shown in Fig-
ure 5b, the hole mobilities of PBDTAL-BFQ, PBDTAB-BFQ,
PBDTTL-BFQ, and PBDTTB-BFQ are calculated to be 2.6ꢂ10ꢀ6
,
1.2ꢂ10ꢀ5, 3.8ꢂ10ꢀ6, and 4.7ꢂ10ꢀ6 cm2 Vꢀ1 sꢀ1, respectively. The
PBDTAB-BFQ film exhibited the highest hole mobility under
the same conditions; this is attributed to the best crystallinity
and molecular packing properties, as described by the XRD re-
sults. The other three blend films show similar hole mobilities
at a level of 10ꢀ6 cm2 Vꢀ1 sꢀ1. The results also indicated that the
introduction of branched side chains was helpful for increasing
the hole mobilities of the relevant co-polymers. By considering
of the XRD results, p–p stacking may have a more important
effect on the carrier mobility than lamellar stacking in our co-
polymers based on BDT and Qx.
Photovoltaic properties
To investigate the effects of side chains on the photovoltaic
properties of these co-polymers, BHJ PSC devices with a con-
ventional configuration of ITO/PEDOT:polystyrene sulfonate
(PSS)/polymer:PC71BM/Ca/Al and an inverted configuration of
ITO/ZnO/poly({9,9-bis[3’-(N,N-dimethylamino)propyl]-2,7-fluo-
rene}-alt-2,7-(9,9-dioctylfluorene)) dibromide (PFN) /poly-
mer:PC71BM/MoO3/Al were fabricated and evaluated. PC71BM
was chosen here instead of PC61BM because of its significantly
broader and stronger absorption in the visible region.[34] The
device fabrication processes are described in detail in the Ex-
perimental Section. The fabrication conditions were optimized
by changing many parameters, including the choice of solvent,
annealing conditions, blend ratio of the polymer and PC71BM,
and processing additive effect. The active layers were finally
spin-coated from the co-polymers and solutions of PC71BM in
CB and the optimal weight ratio of co-polymer to PC71BM was
1:1 (w/w) with 3% 1,8-diiodooctane (DIO) additive. DIO was
chosen as the additive because it had a high boiling point of
1688C and good ability to solvate the PC71BM acceptor, which
was expected to afford an optimal film morphology for obtain-
ing a high PCE.[35] The addition of DIO would decrease the Voc
value of the related PSCs, which could be attributed to lower-
ing of the charge-separated and charge-transfer-state energies
during the addition process.[36] Generally, all conventional devi-
ces with 3% DIO additive exhibit better performances than
those of PSCs without any additives. On the other hand, the
thermal annealing process had no positive effect on improving
the photovoltaic performance in this work. All solar cell devices
were evaluated under simulated 100 mWcmꢀ2, AM 1.5G illumi-
nation. The current density–voltage (J–V) curves are plotted in
Figure 6a and b, and the photovoltaic data of Voc, Jsc, FF, and
PCE are summarized in Table 3 for a clear comparison.
Figure 5. a) J–V characteristics of co-polymer/PC71BM-based hole-only devi-
ces measured at ambient temperature. b) XRD patterns of the co-polymer
films on silicon wafers.
tion of alkylthienyl conjugated side chains, the crystalline prop-
erties of the 2D co-polymers decreased. The lamellar-like order-
ing and p–p stacking also seemed to decrease, however, the
effect of the degree of branching was the similar to that of the
alkoxyl-substituted analogues. As calculated from peaks (100)
and (010), the dl and dp spacings were determined to be 27.75
and 24.79 ꢁ and 3.61 and 3.79 ꢁ for PBDTTL-BFQ and
PBDTTB-BFQ, respectively. Through comprehensive consider-
ation of the XRD results, co-polymer PBDTAB-BFQ shows the
best stacking properties, and thus, can be expected to process
the highest carrier mobility.
Generally in PSCs, the Jsc value and fill factor (FF) are some-
what affected by the hole mobility of the polymer donor mate-
rials. Previous work showed that higher hole mobility would
enable better hole transport within an active layer without
large photocurrent loss caused by recombination of opposite
charges.[12,20c,33] The space-charge limited current (SCLC)
method was widely employed to investigate the vertical carrier
transport properties of the PSCs. To obtain a realistic simula-
tion for PSCs, we chose blend films of co-polymer:PC71BM (1:1)
to fabricate the hole-only devices and evaluate the hole mobi-
lity by using the SCLC method.[12,20c,33] The structure of the
hole-only device was indium tin oxide (ITO)/poly(3,4-ethylene-
dioxythiophene) (PEDOT)/polymer/MoO3/Au, whereas the mo-
bility was determined by fitting the dark current to the model
of a single-carrier SCLC, which was described by Equation (3),
according to our previous work.[12]
For conventional devices, the PSC based on PBDTAL-BFQ
exhibited a Voc of 0.64 V, a Jsc of 9.44 mAcmꢀ2, and an FF of
47.5%, resulting in a PCE of 2.86%. For polymer PBDTAB-BFQ,
the Voc value improved from 0.64 to 0.70 V, which was consis-
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Table 3. Performance of the PSCs tested under AM 1.5G simulated illumi-
nation.
Co-polymers
Voc [V]
Jsc [mAcmꢀ2
]
FF [%]
PCE [%]
PBDTAL-BFQ[a]
PBDTAL-BFQ[b]
PBDTAB-BFQ[a]
PBDTAB-BFQ[b]
PBDTTL-BFQ[a]
PBDTTL-BFQ[b]
PBDTTB-BFQ[a]
PBDTTB-BFQ[b]
0.64
0.66
0.70
0.70
0.70
0.74
0.79
0.88
9.44
7.94
7.63
47.5
67.9
33.2
45.8
57.5
70.0
47.9
63.9
2.86
3.55
1.78
2.41
4.80
5.00
2.16
2.64
7.51
11.87
10.91
5.71
4.71
[a] Data obtained by using
a conventional configuration of ITO/PE-
DOT:PSS/polymer:PC71BM/Ca/Al. [b] Data obtained by using an inverted
configuration of ITO/ZnO/PFN/polymer:PC71BM/MoO3/Al.
voltaic performances and longer ambient stability because
a more stable, high work-function metal electrode is used and
the acidic PEDOT:PSS layer is replaced by other stable anode
buffer layers.[20c,27,37] As shown in Figure 6b, the inverted devi-
ces based on PBDTAL-BFQ, PBDTAB-BFQ, PBDTTL-BFQ, and
PBDTTB-BFQ exhibit PCEs of 3.55, 2.41, 5.00, and 2.64%, re-
spectively. The efficiencies of all inverted PSCs are larger than
their corresponding conventional devices; this is mainly due to
the increased Voc and FF values. The significantly increased Voc
and FF values should be attributed to the improved interface
between the active layer and ITO electrode and decreased
work functions of ITO by ZnO layer, which might supply effi-
cient electron collection and decrease electron–hole
recombination.[7c,38]
In conventional and inverted PSCs, the PBDTTL-BFQ devices
exhibited the best photovoltaic performance, which was fur-
ther verified by external quantum efficiency (EQE) evaluations
and surface morphology studies of the active blend films. The
EQE curves of the PSCs fabricated under optimal conditions
are provided in Figure 6c. The PBDTTL-BFQ device had
a higher maximum EQE value of 51% at l=460 nm than those
devices based on PBDTAL-BFQ, PBDTAB-BFQ, and PBDTTB-
BFQ. Moreover, the PBDTTL-BFQ device exhibited a significant-
ly broader spectral response range, within l=300–750 nm,
compared with those based on the other three co-polymers;
this is in good agreement with optical absorption spectra of
the thin solid films, as discussed in the section on optical prop-
erties. Accordingly, the PBDTTL-BFQ devices should show
a larger photocurrent and a higher overall PCE than the other
devices. The surface morphology of polymer:PC71BM film was
also investigated by using AFM. The AFM height (left) and
phase (right) images are shown in Figure 7. The average sur-
face roughnesses (Ra) measured from the topographic images
were 1.79, 2.98, 2.53, and 3.84 nm for PBDTAL-BFQ, PBDTAB-
BFQ, PBDTTL-BFQ, and PBDTTB-BFQ, respectively, blend films
cast from CB with ratios of 1:1 (by adding 3% DIO). The phase
images of the active blending films show uniform D–A inter-
penetrating networks with appropriate domain sizes. As shown
in Figure 7, the domain sizes of the PBDTTB-BFQ:PC71BM
active layer are a little bigger. The lowest Jsc value of the
PBDTTB-BFQ device was probably caused by the poor phase
Figure 6. J–V curves of co-polymer:PC71BM (1:1) based PSCs with a conven-
tional (a) and an inverted structure (b) under AM 1.5G illumination,
100 mWcmꢀ2. c) EQE curves of co-polymer:PC71BM (1:1) based PSCs.
tent with the CV measurements, that is, PBDTAB-BFQ had
a lower HOMO level than that of PBDTAL-BFQ. However, the
Jsc and FF values dropped, which resulted in a lower PCE of
1.78%. Similar trends were found for PSCs based on PBDTTL-
BFQ and PBDTTB-BFQ. The results indicated that the introduc-
tion of branched alkoxyl side chains could improve the Voc, but
decrease the Jsc and FF values of the PSCs. On the contrary, the
introduction of 2D conjugated side chains would increase the
Voc, Jsc, and FF values simultaneously, which would result in
higher efficiencies than the alkoxyl-substituted analogues.
PSCs based on the polymer of BDTTL-BFQ exhibited the high-
est Voc of 0.79 V. Overall, devices composed of PBDTTB-BFQ
showed the best performance of 4.80% PCE with a Voc of
0.70 V, a Jsc of 11.87 mAcmꢀ2, and an FF of 57.5%, as a result
of the significantly enhanced Jsc and FF values. Relative to con-
ventional PSCs, inverted solar cell devices display better photo-
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required to obtain high FF and
Jsc values in the PSCs.[39] Herein,
the large difference in absorp-
tion may play a more significant
role on the photovoltaic proper-
ties than that of the carrier mo-
bility.
Conclusion
We used Stille coupling polymer-
ization to prepare a series of
new conjugated polymers fea-
turing BDT donors, with different
side chains, co-polymerized with
monofluorinated Qx moieties.
Our results showed that subtle
changes to the degree of
branching and dimensionality of
the side chains altered their mo-
lecular coplanarity and the elec-
tron state of the related co-poly-
mers, which simultaneously in-
fluenced their thermal stability,
optical band gap, energy levels,
and molecular packing of the re-
sulting co-polymers. The linear
and 2D conjugated structures of
the side chains were helpful at
increasing the thermal stabilities
and optical absorptions of the
relevant co-polymers. Moreover,
the introduction of 2D alkyl-
thienyl conjugated side chains
could lower the HOMO and
LUMO energy levels, whereas
the branched alkoxyl side chains
were beneficial at deepening the
HOMO energy levels compared
with the linear alkoxyl chains.
The increased degree of branch-
ing would also help to induce la-
mellar-like ordering of the result-
ing co-polymers; however, the
branched alkoxyl side chains
would prevent face-to-face pack-
ing of the polymer backbones.
Figure 7. AFM topographic images of the film blends (polymer:PC71BM=1:1, w/w): PBDTAL-BFQ (a,b), PBDTAB-
BFQ (c,d), PBDTTL-BFQ (e,f), and PBDTTB-BFQ (g,h). Image size: 5ꢂ5 mm2.
separation and narrow, low spectral response, as discussed
above. Although the morphology of the PBDTAB-BFQ:PC71BM
layer was also comparable to that of PBDTTL-BFQ:PC71BM, its
related PSCs could not achieve high performance, although
the blended film had the highest hole mobility, as determined
from the SCLC evaluation. The hole mobility could be influ-
enced by morphology, field, recombination, or carrier density
in the photoactive layer under the operating conditions. Our
results proved that high hole mobility alone could not guaran-
tee high device performance; substantial carrier mobility was
The 2D conjugated side chains would decrease the crystalline
properties of the co-polymers. The photovoltaic results indicat-
ed that the introduction of branched alkoxyl side chains could
improve the Voc, but decrease the Jsc and FF values of the
PSCs. On the contrary, the introduction of 2D conjugated side
chains would increase the Voc, Jsc, and FF simultaneously, which
would result in higher efficiencies than their alkoxyl-substitut-
ed analogues. Finally, PSCs based on the polymer BDTTL-BFQ
exhibited the highest Voc of 0.88 V. Overall, the PBDTTB-BFQ
devices showed the best performance of 5.00% PCE with a Voc
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of 0.74 V, a Jsc of 10.91 mAcmꢀ2, and an FF of 70.0%, which re-
sulted from the significantly enhanced Jsc and FF values. This
work demonstrates a good example for tuning the thermal sta-
bility, absorption range, energy level, charge transport, and
photovoltaic properties of the designed co-polymers by side-
chain engineering of the degree of branching and dimension-
ality, which can offer a simple and effective method to improve
the efficiency of PSCs.
Materials
All chemicals were purchased as reagent grade from Aladdin,
Adamas, Aldrich, Alfa Aesar, and Acros Chemical Co., and used
without further purification. All solvents were freshly distilled im-
mediately prior to use. 4,8-Dehydrobenzo[l,2-b:4,5-b’]dithiophene-
4,8-dione (1), 4,7-dibromo-5-fluoro[2,1,3]benzothiadiazole (3) and
the four organic tin monomers were synthesized according to pro-
cedures reported previously.[11b,12]
4,8-Bis[5-(2-octyldodecyl)-2-thenyl]benzo[1,2-b:4,5-b’]dithio-
phene (2)
Experimental Section
Measurements and characterization
2-(2-Octyldodecyl)thiophene (3.64 g, 10 mmol) in anhydrous THF
(15 mL) was added to a three-necked, round-bottomed flask
equipped with a condenser. When the solution was cooled to 08C
under an argon atmosphere, n-butyllithium (4.4 mL, 11 mmol, 2.5m
in hexane) was added dropwise. The reactant solution was then
heated to 508C and kept at the same temperature for 2 h. Then,
NMR spectra were collected on a Bruker ARX 400 NMR spectrome-
ter with CDCl3 as the solvent and tetramethylsilane (TMS) as the in-
ternal standard. Elemental analysis was performed on a Carlo Erba
116 elemental analyzer. Molecular weights of the co-polymers were
determined by using a Waters 1515 GPC instrument with chloro-
form as the eluent and polystyrene as a standard. TGA was con-
ducted on a TA Instrument Model SDT Q600 simultaneous TGA/
DSC analyzer at a heating rate of 108Cminꢀ1 under a N2 flow rate
of 90 mLminꢀ1. UV/Vis spectra were obtained on a Cary 300 spec-
trophotometer. CV measurements were made at room temperature
on a CHI660 potentiostat/galvanostat electrochemical workstation
at a scan rate of 50 mVsꢀ1, with a platinum wire counter electrode
and an Ag/AgCl reference electrode in an anhydrous nitrogen-satu-
rated solution of Bu4NClO4 in acetonitrile (0.1 molLꢀ1). The redox
couple of Fc/Fc+ was used as an external standard. The co-poly-
mers were coated on the platinum plate working electrodes from
dilute solutions in chloroform. XRD patterns of the polymers were
recorded on a Philips X-ray diffractometer operated in reflection
geometry at 30 mA, 40 kV with CuKa radiation. The AFM measure-
ments were performed on a Veeco Nanoscope V microscope in
tapping mode.
4,8-dihydrobenzo[1,2-b:4,5-b’]dithiophen-4,8-dione
(0.55 g,
2.5 mmol) in anhydrous THF (10 mL) was added dropwise. The mix-
ture was stirred at 508C overnight, and then cooled to ambient
temperature. SnCl2·H2O (4.5 g, 20 mmol) in HCl (8 mL; 10%) was
added, and the mixture was stirred for another 2 h. Then ice water
was added and extracted with dichloromethane. The combined ex-
tracts were dried over anhydrous MgSO4 and evaporated. The
crude product was purified by column chromatography (SiO2, pe-
troleum ether) to give 2 as a light-yellow oil (0.98 g, 42.8%).
1H NMR (400 MHz, CDCl3): d=7.67 (d, J=5.7 Hz, 2H; ArH), 7.47 (d,
J=5.7 Hz, 2H; ArH), 7.32 (d, J=3.5 Hz, 2H; ArH), 6.91 (d, J=3.5 Hz,
2H; ArH), 2.88 (d, J=6.6 Hz, 4H; -CH2-), 1.75 (m, 2H; -CH-), 1.43–
1.20 (m, 64H; -CH2-), 0.93–0.86 ppm (m, 12H; -CH3); 13C NMR
(100 MHz, CDCl3): d=145.7, 139.0, 137.2, 136.5, 127.7, 127.4, 125.4,
124.1, 123.4, 40.0, 34.7, 33.4, 32.0, 30.0, 29.7, 29.69, 29.66, 29.4,
26.7, 22.7, 14.2 ppm; elemental analysis calcd (%) for C58H90S4: C
76.08, H 9.91; found: C 76.05, H 9.96.
PSC device fabrication and characterization
2,6-Bis(tributyltin)-4,8-bis[5-(2-octyldodecyl)-2-thenyl]ben-
Conventional PSCs were fabricated with ITO glass (15 Wcmꢀ2) as
the anode, Ca/Al as the cathode, and the active layer of the co-
polymer and PC71BM as the photosensitive layer. After spin-coating
a 35 nm layer of PEDOT:PSS (Baytron P VP Al 4083) onto the pre-
cleaned ITO substrate, the photosensitive layer was subsequently
prepared by spin-coating the solution (and adding 3% DIO) of the
co-polymer and PC71BM (w/w) in CB on the ITO/PEDOT:PSS elec-
trode. For the inverted devices, a ZnO thin film (about 40 nm) was
deposited on the surface of ITO glass. The ZnO layer was pretreat-
ed with UV–ozone for 10 min and the conjugated polyelectrolyte
of PFN (5 nm) was spin-coated on top of the ZnO layer. The photo-
active layer was then spin-coated on top of this layer from a solu-
tion in CB (containing 3% DIO). Subsequently, about 10 nm MoO3
and 100 nm Ag were deposited in turn through shadow masks by
thermal evaporation. The device area was 0.04 cm2. The I–V charac-
terization of the devices was carried out on a computer-controlled
Keithley 236 Source Measurement system. The EQE values were
measured at a chopping frequency of 280 Hz with a lock-in amplifi-
er (Stanford, SR830 DSP) during illumination with monochromatic
light from a xenon lamp. A solar simulator was used as the light
source, and the light intensity was monitored by using a standard
Si solar cell. The thickness of the films was measured by using
a Dektak 6m surface profilometer. All fabrication and characteriza-
tion processes, except for the EQE measurements, were conducted
in a glove box.
zo[1,2-b:3,4-b’]dithiophene (BDTTB)
A solution of 2 (0.92 g, 1.0 mmol) in anhydrous THF (20 mL) was
added to a three-necked, round-bottomed flask. The mixture was
cooled to 08C under an argon atmosphere. n-Butyllithium (1.0 mL,
2.5 mmol, 2.5m in hexane) was added dropwise and the solution
was stirred for 2 h at ambient temperature. The mixture was
cooled to 08C, and tributyltin chloride (0.96 g, 3.0 mmol) was
added slowly. The mixture was stirred at 08C for 30 min, and then
slowly warmed to room temperature. After the solution was stirred
overnight, deionized water (50 mL) was added to terminate the re-
action. The mixture was then extracted with diethyl ether, and the
combined organic layers were dried over anhydrous MgSO4. After
removing the desiccant, the filtrate was concentrated. The crude
product was finally purified by column chromatography (SiO2, pe-
troleum ether, containing 1% triethylamine) to give BDTTB as
1
a yellow oil (0.61 g, 41%). H NMR (400 MHz, CDCl3): d=7.30 (t, J=
13.4 Hz, 2H; ArH), 7.35 (d, J=3.4 Hz, 2H; ArH), 6.89 (d, J=3.5 Hz,
2H; ArH), 2.89 (d, J=6.6 Hz, 4H; ArH), 1.80–1.70 (m, 2H; -CH-),
1.68–1.47 (m, 12H; -CH2-), 1.45–1.20 (m, 76H; CH2-), 1.15 (t, 12H;
-CH2-), 0.94–0.80 ppm (m, 30H; -CH3); 13C NMR (100 MHz, CDCl3):
d=145.2, 143.2, 141.5, 138.3, 137.3, 131.6, 127.5, 125.3, 122.2, 34.8,
33.4, 32.0, 30.1, 29.8, 29.8, 29.5, 29.4, 29.2, 29.1, 29.0, 27.6, 27.5,
27.3, 27.1, 26.8, 22.8, 14.2, 13.8, 10.9, 8.8 ppm; elemental analysis
calcd (%) for C82H142S4Sn2: C 65.94, H 9.58; found: C 65.96, H 9.62.
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0.45 g,2.5 mmol) was added to the solution, the mixture was
heated at 508C for 24 h. The solution was cooled to room temper-
ature and poured into a mixture of deionized water (150 mL) and
dichloromethane (150 mL). The organic layer was separated and
washed with deionized water several times. The solution was dried
over anhydrous MgSO4, and concentrated by reduced pressure dis-
tillation. The residue was finally purified by column chromatogra-
phy (SiO2, petroleum ether/dichloromethane=3:1) to give mono-
mer BFQ as an orange solid (0.65 g, 66%). 1H NMR (400 MHz,
CDCl3): d=7.78 (d, J=13.7 Hz, 1H; ArH), 7.71 (d, J=3.6 Hz, 1H;
ArH), 7.54 (s, 1H; ArH), 7.51 (s, 1H; ArH), 7.43 (d, J=4.1 Hz, 2H;
ArH), 7.23–7.19 (m, 2H; ArH), 7.11 (d, J=4.1 Hz, 1H; ArH), 7.07 (t,
3H; ArH), 6.98 (m, 2H; ArH), 4.04 (q, 4H; -O-CH2-), 1.81 (m, 4H;
-CH2-), 1.54–1.27 (m, 20H; -CH2-), 0.90 ppm (t, J=6.9 Hz, 6H; -CH3);
13C NMR (100 MHz, CDCl3): d=159.8, 159.43, 159.40, 157.3, 151.8,
150.43, 150.40, 139.03, 138.97, 137.54, 137.51, 137.2, 137.1, 133.4,
133.3, 133.1, 130.6, 130.5, 130.0, 129.8, 128.9, 128.7, 125.7, 123.1,
123.0, 118.4, 117.9, 117.8, 117.4, 117.3, 115.7, 115.6, 115.14, 115.06,
114.98, 114.8, 68.39, 68.36, 31.9, 29.54, 29.45, 29.38, 26.3 ppm; ele-
mental analysis calcd (%) for C44H47Br2FN2O2S2: C 60.14, H 5.39, N
3.19; found: C 60.12, H 5.43, N 3.24.
3,6-Dibromo-4-fluoro-1,2-phenylenediamine (4)
Under the protection of argon, compound 3 (1.0 g, 3.2 mmol), zinc
powder (2.5 g, 12 mmol), and glacial acetic acid (30 mL) were
added to a round-bottomed flask. The mixture was heated at
reflux for 3 h. After cooling to room temperature, a solution of
NaOH (10%) was added until the pH became alkaline. The mixture
was then extracted with diethyl ether (4ꢂ100 mL). The combined
extracts were washed with a solution of NaOH and then deionized
water, and dried over anhydrous MgSO4. The solvent was evaporat-
ed under reduced pressure to give 4 as a brown solid (0.72 g,
79%). The product was used in the next step directly with no fur-
ther purification.
5,8-Dibromo-6-fluoro-2,3-bis(3-octyloxyphenyl)quinoxaline
(5)
Compound 4 (1.5 g, 5.28 mmol), 1,2-bis(3-octyloxyphenyl)-ethane-
1,2-dione (2.47 g, 5.28 mmol), and glacial acetic acid (30 mL) were
added to a round-bottomed flask. Under the protection of argon,
the mixture was briefly warmed to 508C, and then stirred at room
temperature for 2 h. The precipitate was collected by filtration,
washed with ethanol, and dried to afford compound 4 as a white
1
PBDTAL-BFQ
solid (2.8 g, 74%). H NMR (400 MHz, CDCl3): d=7.94 (d, J=8.0 Hz,
1H; ArH), 7.24–7.21 (m, 4H; ArH), 7.17 (t, J=7.1 Hz, 2H; ArH), 6.94
(m, 2H; ArH), 3.86 (t, J=6.6 Hz, 4H; -O-CH2-), 1.73 (m, 4H; -CH2-),
1.42–1.29 (m, 20H; -CH2-), 0.89 ppm (t, J=6.9 Hz, 6H; -CH3);
13C NMR (100 MHz, CDCl3): d=160.6, 159.1, 158.0, 154.5, 153.2,
139.5, 139.0, 138.9, 136.3, 129.4, 124.3, 123.3, 123.0, 122.6, 122.5,
116.7, 116.5, 115.8, 68.1, 31.9, 29.3, 29.2, 26.1, 22.7, 14.2 ppm; ele-
mental analysis calcd (%) for C36H43Br2FN2O2: C 60.51, H 6.07, N
3.92; found: C 60.48, H 6.05, N 3.98.
2,6-Bis(tributyltin)-4,8-bis(dodecyloxy)benzo[1,2-b:3,4-b]dithiophene
(0.3282 g, 0.2883 mmol), monomer BFQ (0.2526 g, 0.2883 mmol),
and degassed toluene (12 mL) were added to a two-necked flask.
After being degassed with argon several times, [Pd2(dba)3] (5.5 mg,
2 mol%) and P(o-tol)3 (7.3 mg, 8%) were added and the solution
was degassed with argon several times again. The reaction solution
was subsequently heated to 1108C for 20 h. Tributylstannylthio-
phene (23.7 mL) was added to the reaction and then after 2 h, 2-
bromothiophene (7.5 mL) was added. The mixture was stirred over-
night to complete the end-capping reaction. The reactant mixture
was slowly dropped into methanol (400 mL) to allow precipitation
of the crude polymer. The precipitate was filtered, and washed
with methanol and hexane in a Soxhlet apparatus to remove oligo-
mers and catalyst residue. Finally, the co-polymer was extracted
with chloroform. The solution was condensed by evaporation and
precipitated into methanol again. The polymer PBDTAL-BFQ was
then collected as a dark-purple solid (0.267 g, 73%). 1H NMR
(400 MHz, CDCl3): d=8.5–6.5 (m, 15H; ArH), 4.6–3.3 (m, 8H; -O-
CH2-), 2.5–0.5 ppm (m, 76H); elemental analysis calcd (%) for
(C78H99O4N2S4F)n: C 73.43, H 7.82, N: 2.20; found: C 70.22, H 8.03, N
2.27.
2,3-Bis(3-octyloxyphenyl)-6-fluoro-5,8-di(thiophen-2-yl)qui-
noxaline (6)
A mixture of 5 (0.64 g, 0.90 mmol), tributyl(2-thienyl)stannane
(0.84 g, 2.25 mmol), [Pd(PPh3)2Cl2] (25 mg, 0.036 mmol), and de-
gassed toluene (10 mL) was heated at reflux overnight. The mix-
ture was cooled to room temperature and the solvent was re-
moved under reduced pressure. The residue was purified by
column chromatography (SiO2, petroleum ether/dichlorome-
thane=3:2) to give 6 as a yellow solid (0.54 g, 83.6%). 1H NMR
(400 MHz, CDCl3): d=8.01 (d, J=13.3 Hz, 2H; ArH), 7.89 (dd, J=
3.7, J=1.0 Hz, 1H; ArH), 7.58–7.54 (m, 2H; ArH), 7.37 (d, J=
11.52 Hz, 2H; ArH), 7.26–7.19 (m, 6H; ArH), 6.94 (m, 2H; ArH), 3.91
(t, J=6.6 Hz, 4H; -O-CH2-), 1.75 (m, 4H; -CH2-), 1.44–1.29 (m, 20H;
-CH2-), 0.90 ppm (t, J=6.9 Hz, 6H; -CH3-); 13C NMR (100 MHz,
CDCl3): d=161.1, 160.2, 159.09, 159.05, 158.6, 157.7, 153.6, 152.0,
151.6, 151.6, 150.6, 139.8, 139.7, 139.6, 139.5, 139.4, 139.3, 137.3,
136.92, 136.89, 134.8, 134.3, 133.7, 133.6, 132.2, 132.1, 131.81,
131.77, 130.3, 130.2, 130.1, 129.8, 129.3, 129.24, 129.20, 129.16,
129.02, 128.99, 127.4, 127.0, 126.7, 126.6, 126.4, 122.9, 122.79,
122.75, 122.6, 117.1, 116.8, 116.7, 116.6, 116.52, 116.49, 116.4, 116.1,
115.82, 115.76, 115.72, 68.3, 68.1, 31.9, 29.4, 29.3, 29.21, 29.18,
26.11, 26.09, 22.7, 14.2 ppm; elemental analysis calcd (%) for
C44H49FN2O2S2: C 73.30, H 6.85, N 3.89; found: C 73.29, H 6.88, N
3.92.
PBDTAB-BFQ
Co-polymer PBDTAB-BFQ was obtained as a dark-purple solid in
a yield of 78% from the reaction of 2,6-bis(tributyltin)-4,8-bis(2-eth-
ylhexyloxy) benzo[1,2-b:3,4-b]dithiophene with monomer BFQ, sim-
ilar to the procedure described for co-polymer PBDTAL-BFQ.
1H NMR (400 MHz, CDCl3): d=8.4–6.5 (m, 15H; ArH), 4.5–3.5 (m,
8H; -O-CH2-), 2.5–0.5 ppm (m, 64H); elemental analysis calcd (%)
for (C70H83O4N2S4F)n: C 72.25, H 7.19, N 2.41; found: C 68.49, H 7.24,
N 2.15.
PBDTTL-BFQ
Co-polymer PBDTTL-BFQ was obtained as a dark-purple solid in
a yield of 59% from the reaction of 2,6-bis(tributyltin)-4,8-bis(5-do-
decyl-2-thenyl) benzo[1,2-b:3,4-b]dithiophene with monomer BFQ,
similar to the procedure described for co-polymer PBDTAL-BFQ.
1H NMR (400 MHz, CDCl3): d=9.0–5.5 (m, 19H; ArH), 4.4–3.5 (m,
5,8-Bis(5-bromothiophen-2-yl)-6-fluoro-2,3-bis(3-octyloxy-
phenyl)quinoxaline (BFQ)
Compound 5 (0.81 g, 1.1 mmol) was dissolved in DMF (20 mL) at
room
temperature.
After
N-bromosuccinimide
(NBS,
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[8] a) J. You, C. C. Chen, Z. Hong, K. Yoshimura, K. Ohya, R. Xu, S. Ye, J. Gao,
G. Li, Y. Yang, Adv. Mater. 2013, 25, 3973–3978; b) K. Li, Z. Li, K. Feng, X.
Xu, L. Wang, Q. Peng, J. Am. Chem. Soc. 2013, 135, 13549–13557; c) W.
Li, A. Furlan, K. H. Hendriks, M. M. Wienk, R. A. J. Janssen, J. Am. Chem.
Soc. 2013, 135, 5529–5532; d) G. Dennler, M. C. Scharber, T. Ameri, P.
Denk, K. Forberich, C. Waldauf, C. J. Brabec, Adv. Mater. 2008, 20, 579–
583.
4H; -O-CH2-), 3.3–2.6 (m, 4H; -CH2-), 2.3–0.5 ppm (m, 76H); ele-
mental analysis calcd (%) for (C86H103O2N2S6F)n: C 73.36, H 7.37, N
1.99; found: C 73.85, H 7.60, N 2.24.
PBDTTB-BFQ
Co-polymer PBDTTB-BFQ was obtained as a dark-purple solid in
a yield of 78% from the reaction of monomer BDTTB with mono-
mer BFQ, similar to the procedure described for co-polymer
[9] a) P. Sonar, S. P. Singh, Y. Li, M. S. Soh, A. Dodabalapur, Adv. Mater. 2010,
22, 5409–5413; b) T. Lei, J. H. Dou, J. Pei, Adv. Mater. 2012, 24, 6457–
6461.
1
[10] a) Z. Chen, M. J. Lee, R. S. Ashraf, Y. Gu, S. Albert-Seifried, M. M. Nielson,
B. Schroeder, T. D. Anthopoulos, M. Heeney, I. McCulloch, H. Sirringhaus,
Adv. Mater. 2012, 24, 647–652; S. Albert-Seifried, M. M. Nielson, B.
Schroeder, T. D. Anthopoulos, M. Heeney, I. McCulloch, H. Sirringhaus,
Adv. Mater. 2012, 24, 647–652; b) I. Meager, R. S. Ashraf, S. Mollinger,
B. C. Schroeder, H. Bronstein, D. Beatrup, M. S. Vezie, T. Kirchartz, A.
Salleo, J. Nelson, I. McCulloch, J. Am. Chem. Soc. 2013, 135, 11537–
11540; c) A. T. Yiu, P. M. Beaujuge, O. P. Lee, C. H. Woo, M. F. Toney,
J. M. J. Frꢄchet, J. Am. Chem. Soc. 2012, 134, 2180–2185.
PBDTAL-BFQ. H NMR (400 MHz, CDCl3): d=8.8–6.5 (m, 19H; ArH),
4.2–3.5 (m, 4H; -O-CH2-), 3.3–2.6 (m, 4H; -CH2-), 2.2–0.5 ppm (m,
108H); elemental analysis calcd (%) for (C102H135O2N2S6F)n: C 75.04,
H 8.34, N 1.72; found: C 75.15, H 8.32, N 1.90.
Acknowledgements
[11] a) J. Hou, Z. Tan, Y. Yan, Y. He, C. Yang, Y. Li, J. Am. Chem. Soc. 2006, 128,
4911–4916; b) Q. Peng, X. J. Liu, D. Su, G. W. Fu, J. Xu, L. M. Dai, Adv.
Mater. 2011, 23, 4554–4558; d) L. Huo, S. Zhang, X. Guo, F. Xu, Y. Li, J.
Hou, Angew. Chem. 2011, 123, 9871–9876; Angew. Chem. Int. Ed. 2011,
50, 9697–9702; e) J. Min, Z. G. Zhang, S. Zhang, Y. Li, Chem. Mater.
2012, 24, 3247–3254; f) Y. Huang, X. Guo, F. Liu, L. Huo, Y. Chen, T. P.
Russell, C. C. Han, Y. Li, J. Hou, Adv. Mater. 2012, 24, 3383–3389; g) L.
Huo, L. Ye, Y. Wu, Z. Li, X. Guo, M. Zhang, S. Zhang, J. Hou, Macromole-
cules 2012, 45, 6923–6929.
[12] a) Q. Peng, S. L. Lim, I. H. Wong, J. Xu, Z. K. Chen, Chem. Eur. J. 2012, 18,
12140–12151; b) Q. Peng, Y. Y. Fu, X. J. Liu, J. Xu, Z. Y. Xie, Polym. Chem.
2012, 3, 2933–2940.
[13] J. H. Hou, L. J. Huo, C. He, C. H. Yang, Y. F. Li, Macromolecules 2006, 39,
594–603.
This work was supported by the NSFC (21272164, 51273208),
the 863 Project (no.: 2013AA031901), the Hundred Talent Pro-
gram of the Chinese Academy of Sciences, the Starting Re-
search Fund of Team Talent (Y10801A01) in NIMTE, the Youth
Science and Technology Foundation of Sichuan Province
(2013JQ0032), and the Fundamental Research Funds for the
Central Universities (2012SCU04B01, YJ2011025).
Keywords: conjugation
·
donor–acceptor
systems
·
electrochemistry · polymers · solar cells
[14] Y. P. Zou, W. P. Wu, G. Y. Sang, Y. Yang, Y. Q. Liu, Y. F. Li, Macromolecules
2007, 40, 7231–7237.
[15] E. J. Zhou, Z. A. Tan, Y. Yang, L. J. Hou, Y. P. Zou, C. H. Yang, Y. F. Li, Mac-
romolecules 2007, 40, 1831–1837.
[16] a) Y. Wang, F. Yang, Y. Liu, R. Peng, S. Chen, Z. Ge, Macromolecules 2013,
46, 1368–1375; b) Y. Wang, Y. Liu, S. J. Chen, R. X. Peng, Z. Y. Ge, Chem.
Mater. 2013, 25, 3196–3204.
[17] a) C. Y. Kuo, W. Y. Nie, H. Tsai, H. J. Yen, A. D. Mohite, G. Gupta, A. M. Dat-
telbaum, D. J. William, K. C. Cha, Y. Yang, L. Wang, H. L. Wang, Macromo-
lecules 2014, 47, 1008–1020; b) H. S. Chung, W. H. Lee, C. E. Song, Y.
Shin, J. Kim, S. K. Lee, W. S. Shin, S. J. Moon, I. N. Kang, Macromolecules
2014, 47, 97–105.
[18] C. Cabanetos, A. E. Labban, J. A. Bartelt, J. D. Douglas, W. R. Mateker,
J. M. J. Frꢄchet, M. D. McGehee, P. M. Beaujuge, J. Am. Chem. Soc. 2013,
135, 4656–4659.
[19] a) Y. Liang, D. Feng, Y. Wu, S. T. Tsai, G. Li, C. Ray, L. Yu, J. Am. Chem. Soc.
2009, 131, 14612–14613; b) H. Y. Chen, J. Hou, S. Zhang, Y. Liang, G.
Yang, Y. Yang, L. Yu, Y. Wu, G. Li, Nat. Photonics 2009, 3, 649–653; c) Y.
Liang, D. Feng, Y. Wu, S. T. Tsai, G. Li, C. Ray, L. Yu, J. Am. Chem. Soc.
2009, 131, 7792–7799; d) S. C. Price, A. C. Stuart, L. Yang, H. Zhou, W.
You, J. Am. Chem. Soc. 2011, 133, 4625–4631.
[20] a) L. Dou, J. Gao, E. Richard, J. You, C. C. Chen, K. C. Cha, Y. He, G. Li, Y.
Yang, J. Am. Chem. Soc. 2012, 134, 10071–10079; b) W. Li, W. S. C. Roe-
lofs, M. M. Wienk, R. A. J. Janssen, J. Am. Chem. Soc. 2012, 134, 13787–
13795; c) Q. Peng, Q. Huang, X. B. Hou, P. P. Chang, J. Xu, S. J. Deng,
Chem. Commun. 2012, 48, 11452–11454.
[21] a) L. Huo, X. Guo, S. Zhang, Y. Li, J. Hou, Macromolecules 2011, 44,
4035–4037; b) S. Subramaniyan, H. Xin, F. S. Kim, S. A. Jenekhe, Macro-
molecules 2011, 44, 6245–6248; c) M. Yang, B. Peng, B. Liu, Y. P. Zou,
K. C. Zhou, Y. H. He, C. Y. Pan, Y. F. Li, J. Phys. Chem. C 2010, 114, 17989–
17994.
[1] a) S. Gꢃnes, H. Neugebauer, N. S. Sariciftci, Chem. Rev. 2007, 107, 1324–
1338; b) Y. J. Cheng, S. H. Yang, C. S. Hsu, Chem. Rev. 2009, 109, 5868–
5923; c) Y. F. Li, Y. P. Zou, Adv. Mater. 2008, 20, 2952–2958; d) J. W. Chen,
Y. Cao, Acc. Chem. Res. 2009, 42, 1709–1718.
[2] a) G. Li, V. Shrotriya, J. S. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang,
Nat. Mater. 2005, 4, 864–868; b) S. H. Park, A. Roy, S. Beaupre, S. Cho,
N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee, A. J. Heeger, Nat.
Photonics 2009, 3, 297–302; c) Y. Liang, Z. Xu, J. Xia, S. T. Tsai, Y. Wu, G.
Li, C. Ray, L. Yu, Adv. Mater. 2010, 22, E135–E138; d) H. Zhou, L. Yang,
A. C. Stuart, S. C. Price, S. Liu, W. You, Angew. Chem. 2011, 123, 3051–
3054; Angew. Chem. Int. Ed. 2011, 50, 2995–2998; e) Z. C. He, C. M.
Zhong, X. Huang, W. Y. Wong, H. B. Wu, L. W. Chen, S. J. Su, Y. Cao, Adv.
Mater. 2011, 23, 4636–4637; f) J. You, L. Dou, K. Yoshimura, T. Kato, K.
Ohya, T. Moriarty, K. Emery, C. C. Chen, J. Gao, G. Li, Y. Yang, Nat.
Commun. 2013, 4, 1446–1455.
[3] a) W. L. Ma, C. Y. Yang, X. Gong, K. Lee, A. J. Heeger, Adv. Funct. Mater.
2005, 15, 1617–1622; b) G. Li, V. Shrotriya, J. S. Huang, Y. Yao, T. Moriar-
ty, K. Emery, Y. Yang, Nat. Mater. 2005, 4, 864–868.
[4] a) J. Roncali, Chem. Rev. 1997, 97, 173–206; b) R. C. Coffin, J. Peet, J.
Rogers, G. C. Bazan, Nat. Chem. 2009, 1, 657–661; c) H. Y. Chen, J. H.
Hou, A. E. Hayden, H. Yang, K. N. Houk, Y. Yang, Adv. Mater. 2010, 22,
371–375.
[5] S. A. Jenekhe, L. Lu, M. M. Alam, Macromolecules 2001, 34, 7315–7324.
[6] a) L. Bꢃrgi, M. Turbiez, R. Pfeiffer, F. Bienewald, H. Kirner, C. Winnewisser,
Adv. Mater. 2008, 20, 2217–2224; b) Y. Li, S. P. Singh, P. Sonar, Adv.
Mater. 2010, 22, 4862–4866; c) H. N. Tsao, D. M. Cho, I. Park, M. R.
Hansen, A. Mavrinskiy, D. Y. Yoon, R. Graf, W. Pisula, H. Spiess, K. Mꢃllen,
J. Am. Chem. Soc. 2011, 133, 2605–2612.
[7] a) T. Yang, M. Wang, C. Duan, X. Hu, L. Huang, J. Peng, F. Huang, X.
Gong, Energy Environ. Sci. 2012, 5, 8208–8214; b) H. C. Chen, Y. H.
Chen, C. C. Liu, Y. C. Chien, S. W. Chou, P. T. Chou, Chem. Mater. 2012,
24, 4766–4772; c) Z. He, C. Zhong, S. Su, M. Xu, H. Wu, Y. Cao, Nat. Pho-
tonics 2012, 6, 591–595; d) I. Osaka, T. Kakara, N. Takemura, T. Kogane-
zawa, K. Takimiya, J. Am. Chem. Soc. 2013, 135, 8834–8837; e) H. Choi,
J. P. Lee, S. J. Ko, J. W. Jung, H. Park, S. Yoo, O. Park, J. R. Jeong, S. Park,
J. Y. Kim, Nano Lett. 2013, 13, 2204–2208; f) C. H. Cui, W. Y. Wong, Y. F.
Li, Energy Environ. Sci. 2014, 7, 2276–2284.
[22] a) Y. Zhang, S. K. Hau, H. L. Yip, Y. Sun, O. Acton, A. K. Y. Jen, Chem.
Mater. 2010, 22, 2696–2698; b) A. A. Paraecattil, S. Beauprꢄ, M. Leclerc,
J. E. Moser, N. Banerji, J. Phys. Chem. Lett. 2012, 3, 2952–2958;
[23] H. J. Son, W. Wang, T. Xu, Y. Liang, Y. Wu, G. Li, L. Yu, J. Am. Chem. Soc.
2011, 133, 1885–1894.
[24] a) A. Gadisa, W. Mammo, L. M. Andersson, S. Admassie, F. Zhang, M. R.
Andersson, O. Inganꢅs, Adv. Funct. Mater. 2007, 17, 3836–3842; b) L. J.
&
&
Chem. Eur. J. 2014, 20, 1 – 14
www.chemeurj.org
12
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
Lindgren, F. L. Zhang, M. Andersson, S. Barrau, S. Hellstrom, W. Mammo,
E. Perzon, O. Inganꢅs, M. R. Andersson, Chem. Mater. 2009, 21, 3491–
3502; c) J. Zhang, W. Cai, F. Huang, E. Wang, C. Zhong, S. Liu, M. Wang,
C. Duan, T. Yang, Y. Cao, Macromolecules 2011, 44, 894–901; d) P. T. Wu,
F. S. Kim, R. D. Champion, S. A. Jenekhe, Macromolecules 2008, 41,
7021–7028; e) E. G. Wang, L. T. Hou, Z. Q. Wang, S. Hellstrom, F. L.
Zhang, O. Inganꢅs, M. R. Andersson, Adv. Mater. 2010, 22, 5240–5244;
f) A. V. Patil, W. H. Lee, E. Lee, K. Kim, I. N. Kang, S. H. Lee, Macromole-
cules 2011, 44, 1238–1241; g) E. Zhou, J. Z. Cong, K. Tajima, K. Hashimo-
to, Chem. Mater. 2010, 22, 4890–4895.
[32] C. J. Brabec, C. Winder, N. S. Sariciftci, J. C. Hummelen, A. Dhanabalan,
P. A. van Hal, R. A. J. Janssen, Adv. Funct. Mater. 2002, 12, 709–712.
[33] a) V. Shrotriya, Y. Yao, G. Li, Y. Yang, Appl. Phys. Lett. 2006, 89, 0635051–
0635053; b) Y. Y. Liang, L. P. Yu, Acc. Chem. Res. 2010, 43, 1227–1236;
c) X. Guo, N. Zhou, S. J. Lou, J. W. Hennek, O. R. Ponce, M. R. Butler, P. L.
Boudreault, J. Strzalka, P. O. Morin, M. Leclerc, N. J. T. Lopez, M. A.
Ratner, L. X. Chen, R. P. Chang, A. Facchetti, T. J. Marks, J. Am. Chem. Soc.
2012, 134, 18427–18439.
[34] M. M. Wiek, J. M. Kroon, W. J. H. Verhees, J. Knol, J. C. Hummelen, P. A.
van Hal, R. A. J. Janssen, Angew. Chem. 2003, 115, 3493–3497; Angew.
Chem. Int. Ed. 2003, 42, 3371–3375.
[35] a) J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger, G. C.
Bazan, Nat. Mater. 2007, 6, 497–500; b) J. K. Lee, W. L. Ma, C. J. Brabec,
J. Yuen, J. S. Moon, J. Y. Kim, K. Lee, G. C. Bazan, A. J. Heeger, J. Am.
Chem. Soc. 2008, 130, 3619–3623.
[25] a) A. Iyer, J. Bjorgaard, T. Anderson, M. E. Kçse, Macromolecules 2012, 45,
6380–6389; b) H. C. Chen, Y. H. Chen, C. H. Liu, Y. H. Hsu, Y. C. Chien,
W. T. Chuang, C. Y. Cheng, C. L. Liu, S. W. Chou, S. H. Tung, P. T. Chou,
Polym. Chem. 2013, 4, 3411–3418; c) Y. Z. Lu, Z. G. Xiao, Y. B. Yuan, H. M.
Wu, Z. W. An, Y. B. Hou, C. Gao, J. S. Huang, J. Mater. Chem. C 2013, 1,
630–637; d) P. Y. Yang, M. J. Yuan, D. F. Zeigler, S. E. Watkins, J. A. Leeb,
C. K. Luscombe, J. Mater. Chem. C 2014, 2, 3278–3284.
[36] D. Di Nuzzo, A. Aguirre, M. Shahid, V. S. Gevaerts, S. C. J. Meskers, R. A. J.
Janssen, Adv. Mater. 2010, 22, 4321–4324.
[26] a) Y. Zhang, S. C. Chien, K. S. Chen, H. L. Yip, Y. Sun, J. A. Davies, F. C.
Chen, A. K. Y. Jen, Chem. Commun. 2011, 47, 11026–11028; b) S. Al-
brecht, S. Janietz, W. Schindler, J. Frisch, J. Kurpiers, J. Kniepert, S. Inal, P.
Pingel, K. Fostiropoulos, N. Koch, D. Neher, J. Am. Chem. Soc. 2012, 134,
14932–14944.
[27] Y. C. Yang, R. M. Wu, X. Wang, X. P. Xu, Z. J. Li, K. Li, Q. Peng, Chem.
Commun. 2014, 50, 439–441.
[28] Y. F. Li, Acc. Chem. Res. 2012, 45, 723–733.
[29] a) Y. F. Li, Y. Cao, J. Gao, D. L. Wang, G. Yu, A. J. Heeger, Synth. Met. 1999,
99, 243–248; b) Q. Peng, Z. Y. Lu, Y. Huang, M. G. Xie, S. H. Han, J. B.
Peng, Y. Cao, Macromolecules 2004, 37, 260–266; c) Q. Peng, J. B. Peng,
E. T. Kang, K. G. Neoh, Y. Cao, Macromolecules 2005, 38, 7292–7298.
[30] N. S. Sariciftci, Primary Photoexcitations in Conjugated Polymers: Molecu-
lar Excitions vs. Semiconductor Band Model, World Scientific, Singapore,
1997;
[37] a) C. H. Hsieh, Y. J. Cheng, P. J. Li, C. H. Chen, M. Dubosc, R. M. Liang,
C. S. Hsu, J. Am. Chem. Soc. 2010, 132, 4887; b) Z. Xu, L. M. Chen, G.
Yang, C. H. Huang, J. Hou, Y. Wu, G. Li, C. S. Hsu, Y. Yang, Adv. Funct.
Mater. 2009, 19, 1227; c) S. Chen, C. E. Small, C. M. Amb, J. Subbiah, T. H.
Lai, S. W. Tsang, J. R. Manders, J. R. Reynolds, F. So, Adv. Energy Mater.
2012, 2, 1333–1337.
[38] a) X. F. Xu, Y. X. Zhu, L. J. Zhang, J. M. Sun, J. Huang, J. W. Chen, Y. Cao,
J. Mater. Chem. 2012, 22, 4329–4336; b) Y. Dong, X. W. Hu, C. H. Duan,
P. Liu, S. J. Liu, L. Y. Lan, D. C. Chen, L. Ying, S. J. Su, X. Gong, F. Huang, Y.
Cao, Adv. Mater. 2013, 25, 3683–3688.
[39] a) B. Qu, D. Tian, Z. Cong, W. Wang, Z. An, C. Gao, Z. Gao, H. Yang, L.
Zhang, L. Xiao, Z. Chen, Q. Gong, J. Phys. Chem. C 2013, 117, 3272–
3278; b) J. E. Parmer, A. C. Mayer, B. E. Hardin, S. R. Scully, M. D. McGe-
hee, M. Heeney, I. McCulloch, Appl. Phys. Lett. 2008, 92, 1133091–
1133093.
[31] a) Q. Peng, K. Park, T. Lin, M. Durstock, L. M. Dai, J. Phys. Chem. B 2008,
112, 2801–2808; b) C. Y. Li, T. CN. Blouin, A. Michaud, D. Gendron, S.
Wakim, E. Blair, R. Neagu-Plesu, M. Belletete, G. Durocher, Y. Tao, M. Le-
clerc, J. Am. Chem. Soc. 2008, 130, 732–742.
Received: April 18, 2014
Published online on && &&, 0000
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FULL PAPER
Bit on the side: By considering the
degree of branching and dimensionality
simultaneously, a systematic investiga-
tion of side-chain engineering was per-
formed to explore the effect on the
thermal stability, optical absorption,
energy levels, molecular packing, and
photovoltaic properties of the resulting
co-polymers (see figure).
&
Donor–Acceptor Systems
X. Xu, Y. Wu, J. Fang,* Z. Li, Z. Wang,
Y. Li, Q. Peng*
&& – &&
Side-Chain Engineering of
Benzodithiophene-Fluorinated
Quinoxaline Low-Band-Gap Co-
polymers for High-Performance
Polymer Solar Cells
&
&
Chem. Eur. J. 2014, 20, 1 – 14
www.chemeurj.org
14
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!