Comparison of thiophene- and selenophene-bridged donor-acceptor low band-gap copolymers used in bulk-heterojunction organic photovoltaics

Article abstract of DOI:10.1039/c2jm33735e

We report a detailed comparison of absorption spectroscopy, electrochemistry, DFT calculations, field-effect charge mobility, as well as organic photovoltaic characteristics between thiophene- and selenophene-bridged donor-acceptor low-band-gap copolymers. In these copolymers, a significant reduction of the band-gap energy was observed for selenophene-bridged copolymers by UV-visible absorption spectroscopy and cyclic voltammetry. Field-effect charge mobility studies reveal that the enhanced hole mobility of the selenophene-bridged copolymers hinges on the solubilising alkyl side chain of the copolymers. Both cyclic voltammetry experiments and theoretical calculations showed that the decreased band-gap energy is mainly due to the lowering of the LUMO energy level, and the raising of the HOMO energy level is just a secondary cause. These results are reflected in a significant increase of the short circuit current density (J_SC) but a slight decrease of the open circuit voltage (V_OC) of their bulk-heterojunction organic photovoltaics (BHJ OPVs), of which the electron donor materials are a selenophene-bridged donor-acceptor copolymer: poly{9-dodecyl-9H-carbazole-alt-5, 6-bis(dodecyloxy)-4,7-di(selenophen-2-yl) benzo[c][1,2,5]-thiadiazole} (pCzSe) or poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b;4,5-b′]dithiophene-alt-5,6- bis(dodecyloxy)-4,7-di(selenophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pBDTSe), or a thiophene-bridged donor-acceptor copolymer: poly{9-dodecyl-9H-carbazole-alt- 5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pCzS) or poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b;4,5-b′]dithiophene-alt-5, 6-bis(dodecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pBDTS); the electron acceptor material is [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Judging from our device data, the potential Se-Se interactions of the selenophene-bridged donor-acceptor copolymers, which is presumably beneficial for the fill factor (FF) of BHJ OPVs, is rather susceptible to the device fabrication conditions.

Full text of DOI:10.1039/c2jm33735e

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 21549
www.rsc.org/materials
PAPER
Comparison of thiophene- and selenophene-bridged donor–acceptor low band-
gap copolymers used in bulk-heterojunction organic photovoltaics†
a
a
c
Hung-Yang Chen,abc Shih-Chieh Yeh, Chin-Ti Chen and Chao-Tsen Chen
*
*
Received 11th June 2012, Accepted 28th August 2012
DOI: 10.1039/c2jm33735e
We report a detailed comparison of absorption spectroscopy, electrochemistry, DFT calculations, field-
effect charge mobility, as well as organic photovoltaic characteristics between thiophene- and
selenophene-bridged donor–acceptor low-band-gap copolymers. In these copolymers, a significant
reduction of the band-gap energy was observed for selenophene-bridged copolymers by UV-visible
absorption spectroscopy and cyclic voltammetry. Field-effect charge mobility studies reveal that the
enhanced hole mobility of the selenophene-bridged copolymers hinges on the solubilising alkyl side
chain of the copolymers. Both cyclic voltammetry experiments and theoretical calculations showed that
the decreased band-gap energy is mainly due to the lowering of the LUMO energy level, and the raising
of the HOMO energy level is just a secondary cause. These results are reflected in a significant increase
of the short circuit current density (JSC) but a slight decrease of the open circuit voltage (VOC) of their
bulk-heterojunction organic photovoltaics (BHJ OPVs), of which the electron donor materials are a
selenophene-bridged donor–acceptor copolymer: poly{9-dodecyl-9H-carbazole-alt-5,6-
bis(dodecyloxy)-4,7-di(selenophen-2-yl) benzo[c][1,2,5]-thiadiazole} (pCzSe) or poly{4,8-bis(2-
ethylhexyloxy)benzo[1,2-b;4,5-b0]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-di(selenophen-2-yl)benzo[c]
[1,2,5]-thiadiazole} (pBDTSe), or a thiophene-bridged donor–acceptor copolymer: poly{9-dodecyl-9H-
carbazole-alt-5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pCzS) or poly
{4,8-bis(2-ethylhexyloxy)benzo[1,2-b;4,5-b0]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl)
benzo[c][1,2,5]-thiadiazole} (pBDTS); the electron acceptor material is [6,6]-phenyl-C61-butyric acid
methyl ester (PCBM). Judging from our device data, the potential Se–Se interactions of the
selenophene-bridged donor–acceptor copolymers, which is presumably beneficial for the fill factor (FF)
of BHJ OPVs, is rather susceptible to the device fabrication conditions.
focused on improving polymer donor materials toward low
1. Introduction
band-gap energy (Eg), the energy difference between the HOMO
and LUMO levels. Low band-gap polymers have an advantage
of more light harvesting of influx photons in matching the solar
spectrum and hence a larger short-circuit current density (JSC) of
OPVs. On the other hand, the low lying HOMO energy level of
the polymer donor material is important to keep the high open-
circuit voltage (VOC) of OPVs. Many experimental or theoretical
studies have demonstrated that there is a linear relation between
the VOC and the energy difference between the LUMO level of
the electron accepting material and the HOMO level of the
electron donating material.8 It is not uncommon that a con-
flicting situation may take place in optimizing the JSC and VOC of
OPVs, if the small Eg of the electron donating polymer material is
managed by elevating its HOMO level. Here, we want to report
that replacing the thiophene p-conjugated bridge of the donor–
acceptor copolymers with selenophene is an effective approach to
increase JSC without the comprise of much VOC of BHJ OPVs.
Thiophene-containing polymers are among the best investi-
gated materials as electron donors in BHJ OPVs due to their
In the last two decades, polymer-based organic photovoltaics
(OPVs) have attracted intensive studies in both academy and
industry due to their potential value in the global solar energy
market.1 The so-called bulk-heterojunction (BHJ) has been
proven to be the most successful device structure for OPVs so
far.2 Using p-conjugated polymers as the electron donors and
soluble fullerene (such as PCBM) as the electron acceptor is most
common in BHJ OPVs.2–10 Many recent research activities have
aInstitute of Chemistry, Academia Sinica, Taipei, Taiwan 11529, Republic
of China. E-mail: cchen@chem.sinica.edu.tw; Fax: +886 2 27831237; Tel:
+886 2 27898542
bNano Science and Technology Program, TIGP, Academia Sinica, Taipei,
Taiwan 11529, Republic of China
cDepartment of Chemistry, National Taiwan University, Taipei, Taiwan
11617, Republic of China
† Electronic supplementary information (ESI) available: The drain
current–voltage plots and transistor transfer characteristics of
copolymers, details of computational study, 1H NMR spectra of
polymers. See DOI: 10.1039/c2jm33735e
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 21549–21559 | 21549

View Article Online
p-conjugated character and incredible synthetic versatility. The
current state-of-the-art thiophene-containing polymers utilized
in BHJ OPVs as electron donors have achieved power conversion
efficiencies (PCE) of over 7%.5–10 Selenophene is the chalcoge-
nophene homologue with chemical and physical properties
similar to those of thiophene. Several possible advantages of
selenophene over thiophene would be expected for application in
organic electronics.11–13 For examples, increased conductivity of
organic conductors has been found by the replacement of the
sulfur atom by the selenium atom, such as tetrathiafulvalene
(TTF) and tetraselenafulvalene (TSF) derivatives.14,15 This is due
to the intermolecular Se–Se interactions, which would facilitate
intermolecular charge transfer. Similarly, it has been shown that
selenophene-based polymers have better conductivity and charge
mobility than thiophene-based polymers due to the larger
p-overlap of the larger p-orbitals of selenium atoms.16
Furthermore, both theoretical studies and experimental evidence
indicate that selenophene-based polymers have a lower Eg.17,18
This is due to the more pronounced quinoidal character found
for selenophene than thiophene, which facilitates the p-conju-
gated connection to other structural moieties, and hence
lengthens the p-conjugation along the polymer chain.19,20 Since
the ionization potential of oligoselenophenes is around 4.6 eV,
which is only 0.05 eV lower than the calculated ionization
potential for oligothiophenes, it is not enough to account for the
0.18 eV difference of Eg.11 In other words, the heteroatom of the
polychalcogenophene alters the energy level of the material more
on the LUMO than the HOMO.
2. Results and discussion
2.1 Copolymer design and synthesis
In order to obtain high molecular weight copolymers with good
solubility, all monomers, Cz, BDT, BTS, and BTSe (Scheme 1),
were anchored with long and/or branched solubilizing alkyl
chains in their designs. One kind of alkyl chain and two kinds of
alkoxy substituent were utilized in the electron-donating
and electron-accepting units, and they are N-dodecylcarbazole,
4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b0]dithiophene, and 5,6-
bis(dodecyloxy)benzo[c][1,2,5]thiadiazole.
With incorporation of a thiophene or selenophene spacer in
these copolymers, steric hindrance between the side chains of the
donor and acceptor monomers, which is more pronounced
between benzene-based monomers such as carbazoles (Cz) and
benzo[c][1,2,5]thiadiazoles (BT), can be minimized. This mini-
mized steric hindrance, enabling a more planar and extended
p-conjugation, and thus a smaller band-gap energy.
The synthetic procedure for the corresponding monomers used
in the polymerization reaction is shown in Scheme 2. For the Cz
electron donating monomer, the boron ester group was func-
tionalized at the 2- and 7-positions of carbazole in order to
extend the electron p-conjugation of the polymer and it
(compound 1) was synthesized from 4,40-dibromo-1,10-biphenyl
according to the reported procedures.43 For another electron
donating monomer, BDT, it was synthesized from compound 2
and the trimethyltin attachment was readily achieved according
to reported procedures.44,45 Two solubilising dodecyloxyl chains
attached to the electron-accepting BT unit were in place from the
very beginning of the BT synthesis and the detailed synthetic
procedures can be found in the literature reported by Janssen
et al.46 In our synthetic design, the p-conjugating bridge, thio-
phene or selenophene, was first attached to the electron-accept-
ing BT unit instead of the electron-donating unit. They were
synthesized from 2-stannylated thiophene or 2-stannylated sele-
nophene by a Stille cross-coupling reaction with compound 3 to
give 4S and 4Se, which were then subjected to bromination to
afford BTS and BTSe, respectively.
Various thiophene- or selenophene-containing materials,
including small molecules,21,22 oligomers,23 and polymers,24–30
have been synthesized for organic field-effect transistors
(OFETs) and similar or enhanced charge mobility has been
observed for most selenophene-containing materials. However,
materials containing selenophene designed for light harvesting
are still rare.31–42 Just a couple of reports regarding thiophene-
and selenophene-bridged donor–acceptor copolymers used for
BHJ OPVs have been published very recently.41,42 In this work, a
series of thiophene- and selenophene-bridged donor–acceptor
low band-gap copolymers have been designed and synthesized.
Two different electron donor moieties, carbazole and benzo[1,2-
b:4,5-b0]dithiophene, were chosen to combine with a common
acceptor moiety, benzo[c][1,2,5]benzothiadiazole, resulting in
two pairs of copolymer couples pCzS–pCzSe and pBDTS–
pBDTSe (see Fig. 1). A detailed comparison of absorption
spectroscopy, electrochemistry, DFT calculations, field-effect
charge mobility, as well as the OPV characteristics between these
thiophene- and selenophene-bridged donor–acceptor copoly-
mers is presented herein.
Copolymers were prepared by conventional Suzuki or Stille
cross-coupling with high isolated yields of 87–93% (Scheme 1).
For the copolymers pCzS and pCzSe, the polymerization reac-
tion was readily achieved by Suzuki cross-coupling. For the
copolymers pBDTS and pBDTSe, the Stille cross-coupling
reaction was chosen to facilitate the copolymerization between
BTS or BTSe and BDT. All four copolymers were purified by
Soxhlet extraction with a washing solvent sequence of methanol,
hexane, and finally chloroform. The copolymers pCzS, pCzSe,
Fig. 1 Chemical structures of thiophene- and selenophene-bridged
donor–acceptor copolymers.
Scheme 1 Synthesis of copolymers.
21550 | J. Mater. Chem., 2012, 22, 21549–21559
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Scheme 2 Synthesis of donor moieties, Cz and BDT, and chalcogeno-
phene-attached acceptors, BTS and BTSe.
Fig.
2
Thermal decomposition diagrams for S- and Se-based
copolymers.
pBDTS, and pBDTSe are readily dissolved in common organic
solvents like chloroform, tetrahydrofuran, chlorobenzene, and
1,2-dichlorobenzne at room temperature.
pCzSe or pBDTSe compared with pCzS or pBDTS, respectively.
Such red-shifted absorption happens to both absorption bands at
short and long wavelengths, 390–440 and 530–645 nm, respec-
tively. The detailed spectroscopic data are summarized in Table 2.
Vision-wise, these four copolymers are highly colorful and
they are quite distinctive from each other (Fig. 3). Owing to the
aggregation between copolymer chains in the solid state, both
pCzS–pCzSe and pBDTS–pBDTSe couples exhibit a red-shifted
absorption in their thin film states compared with their dilute
chloroform solutions.
2.2 Physical and thermal properties
Table 1 summarizes the characterization results of all copolymers
prepared herein. All copolymers exhibit high polymerization
yields (87–93%) and high molecular weights (Mw ¼ 94.1–112.3
kg molꢀ1), even after a series of Soxhlet solvent purifications.
This can be attributed to their good solubility by adding long and
branched alkyl chains both on the donor and acceptor moiety of
the copolymers. It has been reported in the literature that solu-
bilizing alkyl chains have a very significant influence on the
polymerization yields and molecular weights of donor–acceptor
copolymers.47,48 Heteroatom substitution on these donor–
acceptor copolymers only slightly affects the polymerization
yields and molecular weights, which is due to the same polymer
backbone being shared by the S- and Se-containing copolymers.
All copolymers have thermal decomposition temperatures (Td)
over 300 ꢁC (Fig. 2), indicating the good thermal stability of these
copolymers. In addition, it can be found that similar Td values
were observed for copolymers with the same polymer backbone
(pCzS vs. pCzSe or pBDTS vs. pBDTSe). We ascribe this to the
thermal decomposition starting with the soft part (alkyl chains)
of the copolymers, which is the same for the same type of
copolymer.
Their Eg values were calculated from the onset absorption edge
of the thin film samples. pCzSe (Eg ¼ 1.86 eV) exhibits a reduced
band-gap energy of 0.1 eV when compared with that of pCzS
(Eg ¼ 1.96 eV). The same reduced band-gap energy was observed
for pBDTS (Eg ¼ 1.81 eV) and pBDTSe (Eg ¼ 1.71 eV) as well.
This result demonstrates a similar effect of narrowing band-gap
energy in the two types of donor–acceptor copolymer when the
thiophene p-conjugation bridge between the donor and acceptor
moieties is replaced by selenophene. The reduction of the band-
gap energy can be mainly attributed to the more electron rich and
more quinoidal character of selenophene than thiophene. Poly-
mers with a smaller band-gap energy are advantageous for
sunlight harvesting of OPVs.
2.4 Energy level determination
The energy levels (HOMO and LUMO) of the electron donating
material are crucial parameters governing the overall perfor-
mance of BHJ OPVs. The HOMO of the electron donating
material is closely related to the VOC of BHJ OPVs.8 On the other
hand, the LUMO–LUMO energy offset (at least 0.3 eV for P3HT
exciton)49 between the donor and acceptor materials is
2.3 Spectroscopic properties
Both UV-visible absorption and photoluminescence (PL) spectra
were significantly affected by the heteroatom substitution of these
copolymers (Fig. 3 and 4). Either in solution or as thin film, an
obvious red-shifted UV-visible absorption was observed for
Table 1 Polymerization results and thermal properties of copolymers
Yielda (%)
Mn (kg molꢀ1
)
Mw (kg molꢀ1
)
PDI (Mw/Mn)
b
b
c
Td (ꢁC)
Copolymer
pCzS
93
88
90
87
66.1
33.9
62.7
63.7
102.9
94.1
106.7
112.3
1.56
2.78
1.70
1.76
342
337
312
311
pCzSe
pBDTS
pBDTSe
a
b
c
Yield after purification by Soxhlet extraction. Determined by GPC in tetrahydrofuran (THF) at 40 ꢁC usiꢀng1 polystyrene as standard. The
decomposition temperature corresponding to a 5% weight loss determined by TGA at a heating rate of 10 ꢁC min
.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 21549–21559 | 21551

View Article Online
responsible for the dissociation of excitons, which enables the
passing of the electron from donor to acceptor and sustaining the
JSC of BHJ OPVs. Cyclic voltammetry (CV) was used to analyze
the impact of heteroatom substitution on the energy levels of the
donor–acceptor copolymers. Fig. 5 shows electrochemical cyclic
voltammograms of the copolymer couples pCzS–pCzSe and
pBDTS–pBDTSe. For all copolymers, both oxidation and
reduction potentials were measured separately and were esti-
mated from their onset potentials in the cyclic voltammograms.
The HOMO and LUMO energy levels were calculated from their
onset potentials using the ferrocene–ferrocenium redox couple
(0.4 V vs. Ag–Ag+ in the study herein) as the reference to vacuum
level (4.8 eV).50
As shown in Fig. 5 and Table 3, the selenium-containing
copolymers (pCzSe and pBDTSe) are oxidized slightly easier than
their sulfur analogues (pCzS and pBDTS) by 0.04 and 0.03 V,
respectively. On the other hand, a greater decrease of reduction
potential (0.06 and 0.07 V) was observed for pCzS–pCzSe and
pBDTS–pBDTSe, respectively. These results show that the
substitution of sulfur with selenium in these donor–acceptor
copolymers has a more significant impact on the LUMO than the
HOMO energy level. Therefore, lowering the LUMO energy level
of these copolymers is the major cause of the reduction of band-
gap energy with the change of heteroatom (S to Se). In addition,
the difference in band-gap energy acquired from electrochemical
measurements for sulfur- and selenium-containing copolymers is
0.10 eV for both copolymer couples, which is consistent with that
obtained from the optical band-gap. This also demonstrates the
same magnitude of band-gap energy reduction happens to both
copolymers when the bridged thiophene is replaced by
Fig. 3 UV-visible absorption spectra of copolymers pCzS–pCzSe (top)
and pBDTS–pBDTSe (bottom) in chloroform solution and as thin films.
The photos shown on the right-hand side display the visual colors of the
copolymers in dilute chloroform solution and as thin films.
Fig. 4 Normalized photoluminescence spectra of all copolymers in
chloroform.
Table 2 UV-visible absorption and photoluminescence data of all
copolymers
UV-visible absorption
PL
Film
CHCl3
CHCl3
lmax
(nm)
lmax
(nm)
lonset
(nm)
lmax (nm)
Eogpta (eV)
pCzS
530
552
579
599
578
596
594
644
627
658
684
726
1.96
1.86
1.81
1.71
624
644
664
686
pCzSe
pBDTS
pBDTSe
Eogpt is the band-gap energy calculated from the intersection of the
tangent on the lowest energetic edge of the copolymer thin film
a
Fig. 5 Cyclic voltammograms of copolymer couples pCzS–pCzSe (top)
and pBDTS–pBDTSe (bottom). The ferrocene–ferrocenium redox couple
is also shown as a standard (ꢀ4.8 eV vs. vacuum level) in each plot.
absorption spectrum with the baseline by Eogpt ¼ 1240/lonset
.
21552 | J. Mater. Chem., 2012, 22, 21549–21559
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Table 3 Electrochemical data and related energy levels of copolymers
Cyclic voltammetry
DFT calculations
ox
onset
CV
HOMO
CV
DE
HOMO
(eV)
a
red
onset
CV
DE
LUMO
(eV)
CV
DE
LUMO
(eV)
b
cal
HOMO
d
cal
DE
LUMO
(eV)
cal
DE
HOMO
(eV)
e
cal
LUMO
f
E
E
E
ECg Vc
(eV)
DE
(eV)
Ecgal DE
(eV) (eV)
Ecgal.
(eV)
(V)
(eV)
(V)
pCzS
0.99
ꢀ5.39
ꢀ5.35
ꢀ5.32
ꢀ5.29
ꢀ1.38 ꢀ3.02
ꢀ1.32 ꢀ3.08
ꢀ1.14 ꢀ3.26
ꢀ1.07 ꢀ3.33
2.37
2.27
2.06
1.96
ꢀ4.79
ꢀ4.77
ꢀ4.79
ꢀ4.76
ꢀ2.45
0.06
2.34
2.26
2.17
2.05
0.04
0.03
0.06
0.07
0.10
0.10
0.02
0.03
pCzSe 0.95
pBDTS 0.92
pBDTSe 0.89
ꢀ2.51
ꢀ2.62
0.09
ꢀ2.71
a
CV
DE
LUMO
CV
is the HOMO energy level difference of pCzS–pCzSe and pBDTS–pBDTSe. b DE
is the LUMO energy level difference of pCzS–pCzSe
LUMO
and pBDTS–pBDTSe. c EgCV is the energy band gap calculated from the energy level difference between the HOMO and LUMO obtained from cyclic
voltammetry. d DEgCV is the difference of band-gaps between pCzS–pCzSe and pBDTS–pBDTSe copolymer couples. e DE
is the HOMO energy
cal
HOMO
level difference of pCzS–pCzSe and pBDTS–pBDTSe. f DE
is the LUMO energy level difference of pCzS–pCzSe and pBDTS–pBDTSe.
cal
LUMO
selenophene. However, a disparity was observed for the band-gap
energy measured from spectroscopic and electrochemical
methods. This is due to the energy barrier at the interface between
the polymer thin film and the electrode surface.51 Also, the
difference can be ascribed to the exciton binding energy that
differentiates the transport band-gap (electrochemical method)
and optical band-gap (spectroscopic method).52
simplicity as well. In simulating the copolymer chain structure,
the centered donor (D) or acceptor (A) are connected to each
other, or peripheral A and D, respectively, through a p-conju-
gation bridge, either thiophene or selenophene. In addition, we
limit the long alkyl chains of the copolymer to ethyl groups for
computational simplicity. The replacement of long alkyl chains
with shorter ones does not affect the optimized structure of the
p-conjugated molecules but speeds up the computational process
significantly. The experimental details of the computational
method are described in the ESI.†
Both spectroscopic and electrochemical methods showed a
meaningful reduction of Eg for the selenophene-bridged
copolymers. Thiophene and selenophene mainly play a role as a
‘‘p-conjugation bridge’’ in extending p-conjugation and elimi-
nating steric hindrance in these donor–acceptor copolymers. The
extension of p-conjugation and elimination of steric hindrance
decrease Eg, and the magnitude of the decreased Eg is closely
related to the quinoidal character of the bridging moiety along
the donor–acceptor copolymer chain.19 According to the esti-
mated aromatic stabilization energy (ASE),20 which is 18.6 and
16.7 kcal molꢀ1 for thiophene and selenophene, respectively, it is
consistent with the fact that there is a more quinoidal character in
selenophene- than in thiophene-containing copolymers.11
Heeney et al. have reported that poly(3-hexylselenophene)
(P3HS) shows a reduction in Eg when compared with poly-
(3-hexylthiophene) (P3HT), principally due to the lowering of
the polymer LUMO, with the HOMO being almost unaffected.53
Based on a computational study of P3HT, it has been reported
that the HOMO of thiophene-based polymers has no orbital
coefficient on the sulphur heteroatom, whilst the LUMO has a
significant contribution from the sulphur heteroatom.54
Although P3HT and P3HS are not donor–acceptor type
copolymers like those studied herein, computational studies have
also been found to be adequate for probing the electron density
distribution of the HOMO–LUMO in donor–acceptor p-
conjugated copolymers.55 Therefore, it the following section, the
influence of heteroatom substitution on the HOMO–LUMO of
these copolymers is investigated by a computational study in
order to get an in-depth understanding of the relationship
between Eg and the HOMO–LUMO of pCzS–pCzSe and
pBDTS–pBDTSe copolymers.
Table 3 summarizes the calculation results of the dimer-like
model of copolymers and the comparisons with experimental
data. Although the predicted energy levels of these copolymers
are not quite the same as those obtained from experimental CV
data, certain correlations between the experimental results and
theoretical calculations are found to be consistent with each
other. For instances, DFT accurately predicts deeper lying
HOMO levels and a larger Eg for the sulphur-containing
copolymers (pCzS and pBDTS) compared to the selenium-con-
taining ones (pCzSe and pBDTSe). Moreover, DFT predicts that
the replacement of sulphur by selenium in these donor–acceptor
copolymers has a more significant impact on the LUMO than the
HOMO energy level. Specifically, the heteroatom Se substitution
results in an increase of the HOMO energy levels of 0.02 and 0.03
eV for the pCzS–pCzSe and pBDTS–pBDTSe couples, respec-
tively, which are much smaller than the 0.06 and 0.09 eV lowering
of the LUMO energy levels, respectively. This computational
result provides further confirmation of the influence of the
heteroatom substitution on the HOMO–LUMO energy levels
found by experimental methods.
The distinct electronic influence of heteroatom substitution on
the HOMO and LUMOof these copolymers can beobserved from
their electron density distribution in the p-conjugated copoly-
mers. As shown in Fig. 6, it can be clearly seen that the HOMO
electron density mostly spans over the central p–A–p–D–p
moiety, except the heteroatom sulphur (yellow color) of thiophene
or selenium (orange color) of selenophene on the p-conjugation
bridge. There is an electron density nodal plane across the sulphur
of thiophene or selenium of selenophene. Such an electron density
nodal plane accounts for the small HOMO energy level difference
between the thiophene- and selenophene-bridged copolymers. On
the other hand, the electron density of the LUMO is mainly
concentrated on p–A–p on the left-hand side, and p–A–p on the
right-hand side of the D–p–A–p–D–p–A–p model structure
2.5 Computational studies
A theoretical model of D–p–A–p–D–p–A–p, a dimer-like (D–
p–A–p)2 segment, is set up for the computational study due to its
resemblance to the copolymer structure and its computational
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 21549–21559 | 21553

View Article Online
Fig. 6 Electron density distribution of copolymer D–p–A–p–D–p–A–p model: HOMOs (left images) and LUMOs (right images).
shown in Fig. 6. Different from those of the HOMO, most of the
sulphur atoms in thiophene or selenium atoms in selenophene are
covered by electron density in the LUMO of all four model
structures. Our computational results indicate that the hetero-
atom of the p-conjugation bridge thiophene or selenophene plays
a more significant role in the LUMO than in the HOMO of these
donor–acceptor p-conjugated copolymers. These data also
suggest that the thiophene–selenophene spacer behaves like a part
of the acceptor unit, in addition to just a simple p-conjugation
extending unit. Our computational results provide insight into the
origin of reducing Eg for selenophene-bridged donor–acceptor
copolymers. Basically, our computational results are similar to
what was found for P3HT and P3HS previously,53,54 although
P3HT and P3HS are not donor–acceptor copolymers like the four
copolymers studied herein.
magnitude higher than that of pCzS (mavg ¼ 8.0 ꢂ 10ꢀ4 cm2
Vꢀ1
s
ꢀ1). However, pBDTSe exhibits a hole mobility of 3.0 ꢂ
10ꢀ4 cm2 Vꢀ1 ꢀ1, which is comparable with or somewhat
s
smaller than that of pBDTS (mavg ¼ 6.8 ꢂ 10ꢀ4 cm2 Vꢀ1
s
ꢀ1).
The enhanced mobility in pCzSe-based OFETs may be
attributed to the increased inter-polymer chain contact by Se–Se
interactions. Similar Se–Se interactions are somehow inhibited
due to the more sterically demanding 2-ethylhexyl side chains
that prevent the pBDTSe copolymer chain from close contact.
Our inference above is consistent with the fact that branched
alkyl side chains (such as 2-ethylhexyl) are better than longer but
linear alkyl side chains (such as dodecyl) in enhancing polymer
solubility.56
2.7 Photovoltaic performance
The comparison of thiophene and selenophene-bridged low band-
gap donor–acceptor copolymers applied for BHJ OPVs is pre-
sented in this section. These copolymers are designed to function
as electron donors and PCBM is employed as the electron
acceptor. Fine-tuning the thickness of the active layer is necessary
in the performance optimization of BHJ OPVs. Although it is
advantageous for light absorption by increasing the thickness of
the active layer, the optimum active layer thickness is limited by
2.6 Field-effect charge mobility properties
To understand the influence of the heteroatom substitution on
the charge transporting properties of the materials, the charge
mobility characteristics of these sulphur- and selenium-con-
taining copolymers were examined in their organic field-effect
transistors (OFETs). The detailed fabrication process and
testing of the bottom-gate, top-contact copolymer OFETs can
be found in the Experimental section. The current–voltage
output and transfer characteristics of the four copolymer
OFETs can be found in the ESI.† The copolymer OFET testing
results indicate that all four copolymers are hole transporting
with no sign of electron transport, although the electrical
characterization of the OFETs was conducted in an ambient air
environment. From the data summarized in Table 4, the
Table 4 Field-effect performances of the four copolymers
Copolymer mavg (cm2 Vꢀ1 sꢀ1
)
mmax (cm2 Vꢀ1 sꢀ1
)
Ion/Ioff
VT (V)
pCzS
8.0 ꢂ 10ꢀ4
9.6 ꢂ 10ꢀ3
6.8 ꢂ 10ꢀ4
3.0 ꢂ 10ꢀ4
9.0 ꢂ 10ꢀ4
1.1 ꢂ 10ꢀ2
8.6 ꢂ 10ꢀ4
3.1 ꢂ 10ꢀ4
1.1 ꢂ 104 11.4
4.0 ꢂ 104 ꢀ34.3
5.3 ꢂ 103 ꢀ21.3
5.7 ꢂ 102 ꢀ35.2
pCzSe
pBDTS
pBDTSe
OFETs based on pCzSe show an average hole mobility (mavg
of 9.6 ꢂ 10ꢀ3 cm2 Vꢀ1
)
s
ꢀ1, which is about one order of
21554 | J. Mater. Chem., 2012, 22, 21549–21559
This journal is ª The Royal Society of Chemistry 2012

View Article Online
the low charge carrier mobility and short diffusion length or
lifetime of photo-generated excitons in organic materials.19,57 In
order to achieve a fair comparison between the thiophene- and
selenophene-bridged donor–acceptor copolymers in BHJ OPVs,
devices were fabricated with varied concentrations and spin-
coating rates to achieve the optimal thickness.
Table 5 summarizes the photovoltaic results of S- and Se-based
BHJ OPVs fabricated under different conditions, and some of
the results are displayed in Fig. 7. It can be clearly seen that the
OPVs based on Se-containing copolymers (pCzSe and pBDTSe)
exhibit significantly higher JSC than those of the S-containing
copolymers (pCzS and pBDTS). For examples, a maximum JSC
of 9.54 mA cmꢀ2 can be achieved for pCzSe (device Se5), which is
significantly larger than the maximum JSC that can be obtained
for pCzS (6.66 mA cmꢀ2, device S2). A similar trend in JSC can
also be found between the pBDTS (maximum JSC ¼ 7.38 mA
m
ꢀ2, device S10) and pBDTSe (maximum JSC ¼ 8.62 mA mꢀ2
,
device Se10) OPVs. Furthermore, the magnitude of the enhanced
JSC of the pCzSe–pCzS couple is higher than that of the
pBDTSe–pBDTS couple. This can be explained by the much
increased hole mobility of the pCzSe–pCzS couple than the
pBDTSe–pBDTS couple. It is known that the hole mobility of
the donor polymer is responsible for the transportation of posi-
tive charges (holes) dissociated from excitons. Hence, the
enhanced JSC values of the pCzSe OPVs are not only due to
increased sunlight absorption, but also due to the increased
charge transport ability of the pCzSe copolymer.
Fig. 7 J–V curves (top) and IPCE spectra (bottom) of pCzS (device S2),
pCzSe (device Se2), pBDTS (device S7) and pBDTSe (device Se10) OPVs.
The parentheses denote the device number in Table 5.
The higher JSC for the selenophene-containing OPVs can be
attributed to their longer absorption wavelength, i.e., more influx
photons from the solar simulator are absorbed in the seleno-
phene-containing OPVs compared with the thiophene-contain-
ing OPVs. Such higher JSC values also get verified from their
incident photon-to-current efficiency (IPCE) spectra. As shown
in the bottom part of Fig. 7, the Se-based copolymers (pCzSe and
pBDTSe) exhibit broader photocurrent wavelengths and/or more
enhanced photocurrents relative to the S-based copolymers
(pCzS and pBDTS).
Table 5 Photovoltaic performances (VOC: open circuit voltage, JSC: short circuit current density, FF: fill factor, PCE: power conversion efficiency, RSH
shunt resistance, RS: series resistance) of four copolymers with different device fabrication conditions
:
Spin rate
(rpm)
Devicea
mg mLꢀ1
VOC (V)
JSC (mA cmꢀ2
)
FF (%)
PCE (%)
RSH (U cm2)
RS (U cm2)
pCzS
S1
S2
S3
S4
S5
S6
Se1
Se2
Se3
Se4
Se5
Se6
S7
5.0
10.0
10.0
10.0
15.0
15.0
5.0
10.0
10.0
10.0
15.0
15.0
8.0
8.0
8.0
10.0
10.0
8.0
8.0
8.0
10.0
10.0
1000
600
1000
1600
600
1000
1000
600
1000
1600
600
1000
700
1000
2000
600
800
700
1000
2000
600
0.75
0.83
0.83
0.84
0.82
0.83
0.79
0.76
0.78
0.79
0.77
0.80
0.73
0.72
0.70
0.72
0.71
0.65
0.66
0.63
0.65
0.65
4.59
6.66
5.23
4.62
6.44
6.58
5.62
8.84
6.22
6.55
9.54
7.44
7.37
6.97
3.93
7.38
6.96
7.47
6.39
3.47
8.62
8.46
66.8
59.3
69.4
72.2
54.9
56.8
72.3
52.5
65.4
66.9
41.0
36.1
60.3
53.6
62.2
56.8
58.8
52.6
50.7
56.2
54.5
51.3
2.30
3.28
3.01
2.80
2.90
3.10
3.21
3.52
3.17
3.46
3.01
2.15
3.24
2.69
1.71
3.02
2.91
2.55
2.14
1.23
3.05
2.82
1650
1600
2580
3320
650
860
3210
460
1690
1750
300
320
1450
740
1200
980
1490
550
680
1360
760
800
8.58
13.46
7.70
7.19
9.89
9.37
6.00
7.91
7.00
pCzSe
6.23
14.36
16.96
9.17
19.38
10.09
9.32
pBDTS
S8
S9
S10
S11
Se7
Se8
Se9
Se10
Se11
6.33
pBDTSe
13.35
24.93
7.35
8.19
8.03
800
a
For pCzS and pCzSe, the device structure is ITO/PEDOT:PSS/pCzS or pCzSe: PCBM (1 : 4)/Ca (20 nm)/Al (100 nm); and for pBDTS and pBDTSe,
the device structure is ITO/PEDOT:PSS/pBDTS or pBDTSe: PCBM (1 : 2)/Ca (20 nm)/Al (100 nm).
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 21549–21559 | 21555

View Article Online
Regarding diffusion and dissociation of copolymer-based
excitons, wavelength dependent IPCE reveals insightful infor-
mation. The photocurrent contribution from the second
absorption band around 420–430 nm of pCzSe is very
pronounced, surpassing the photocurrent contribution from the
first absorption band at longer wavelengths of 500–600 nm. A
similar enhanced IPCE can be found for the second absorption
band around 410–420 nm of pBDTSe. Such wavelength depen-
dent IPCE enhancement is not proportional to the absorption
intensity at the long and short wavelength of pCzSe or pBDTSe
shown in Fig. 3. Since such IPCE enhancement at the second
absorption band does not happen to pCzS or pBDTS, we may
make the following two inferences. Firstly, the excitons gener-
ated in the selenophene–p-conjugation bridged copolymers,
particularly with the second absorption wavelength 410–430 nm,
may be more readily separated into electrons and holes than the
thiophene–p-conjugation bridged ones. Secondly, the short
wavelength-generated excitons of pCzSe or pBDTSe may be
more efficient for exciton diffusion than the longer wavelength-
generated excitons of the respective copolymer. Our two infer-
ences are directly derived from the fundamental principle of
IPCE, also known as the external quantum efficiency (EQE) of
OPVs, documented in the literature.58
copolymers in spectroscopic, thermal, electrochemical, field-
effect charge transporting as well as photovoltaic properties.
Significant red-shifted UV-visible absorption and photo-
luminescence were observed for the selenophene-bridged
copolymers. The reduction of the band-gap energy by the
replacement of thiophene with selenophene is more due to the
lowering of the LUMO energy level than the raising of
the HOMO energy level of the copolymers. The photovoltaic
results obtained for the selenophene-bridged copolymer BHJ
OPVs show a higher JSC than that of their thiophene-bridged
copolymer counterparts, mostly due to the enhanced photocur-
rent, responsive to the exciton dissociation or diffusion at short
wavelengths and sunlight absorption at long wavelengths. The
contribution of the selenophene with higher hole mobility to the
larger JSC of the BHJ OPVs is structure (solubilizing side chain of
the copolymers) dependent. Although the selenophene-bridged
donor–acceptor copolymer OPVs exhibit a higher JSC, which is
beneficial for the PCE of OPVs, such an advantage is partially
offset by a smaller VOC and FF. The inferior VOC and FF are
ascribed to the higher HOMO energy level and the less appro-
priate nano-morphology of the PCBM blended thin film (and
hence smaller RSH), respectively.
Concerning the HOMO energy level of the copolymers and the
VOC of the OPVs, the correlation among the four copolymers
studied herein is rather clear. For most of the device data listed in
Table 5, the VOC of the thiophene-bridged copolymer OPVs is
slightly larger than those of the selenophene-bridged copolymer
OPVs. This is in general consistent with the thiophene-bridged
copolymers having slightly deeper HOMO energy levels, as
determined experimentally. With the results obtained herein, it is
fair to say that there is an advantage of selenophene rather than
thiophene in donor–acceptor copolymer BHJ OPVs, i.e., what it
gains from increased JSC is more than what it loses from
4. Experimental
4.1 General characterization methods
1H and 13C NMR spectra were recorded on a Bruker AV-400
MHz or AV-500 MHz Fourier transform spectrometer at room
temperature. Elemental analyses (on a Perkin-Elmer 2400 CHN
elemental analyzer), electron ionization (EI), or fast atom
bombardment (FAB) mass spectroscopy (MS) were performed
by the Elemental Analyses and Mass Spectroscopic Laboratory,
respectively, in-house service of the Institute of Chemistry,
Academic Sinica. The number- and weight-average molecular
weights of the polymers were determined by gel permeation
chromatography (GPC) on a Waters GPC-1515 with a 2414
refractive index detector, using THF as the eluent and poly-
styrene as the standard at 40 ꢁC. UV-vis absorption spectra were
recorded on a Hewlett-Packard 8453 diode array spectropho-
tometer. Photoluminescence (PL) spectra were recorded on a
Hitachi fluorescence spectrophotometer F-4500. Thermal
decomposition temperatures (Td’s) of the copolymers were
measured by thermogravimetric analysis (TGA) using Perkin-
Elmer TGA-7 analyzer systems. Cyclic voltammetry (CV) was
used to study the electrochemical properties of the copolymers.
The copolymer thin film coated on a platinum electrode was
studied in an anhydrous and nitrogen-saturated solution of
0.10 M tetrabutylammonium hexafluorophosphate (Bu4NPF6)
acetonitrile solution at a scan rate of 50 mV sꢀ1. Ag–Ag+ (0.1 M
of AgNO3 in acetonitrile) was used as reference electrode. For
calibration, the redox potential of ferrocene–ferrocenium (Fc–
Fc+) was measured under the same conditions. Under these
conditions, the onset oxidation potential of ferrocene
(4ox, Ferrocene) was 0.40 V versus Ag–Ag+. It is assumed that the
redox potential of Fc–Fc+ has an absolute energy level of ꢀ4.8
eV to vacuum.59 The energy levels of the highest occupied
molecular orbital (EHOMO) and the lowest unoccupied molecular
decreased VOC
.
For the fill factor (FF) aspect of OPVs, it seems to be more
sensitive to the device fabrication conditions than the effect of
heteroatom substitution of the copolymers studied herein. Most
selenophene-bridge-containing OPVs exhibit slightly lower FF
values than the thiophene-bridge-containing OPVs when the
devices are fabricated under similar conditions, for example,
device S2-6 vs. Se2-6 or device S7-11 vs. Se7-11 in Table 5. This
can be mainly attributed to the poor RSH of the devices (see data
in Table 5). Poor RSH usually indicates more current leakage
pathways due to charge recombination or trapping. In terms of
the interdigitated charge-transporting nano-morphology, the
selenophene p-conjugation bridged copolymers do not form as
good a thin film with PCBM as the thiophene p-conjugation
bridged copolymers do. However, the selenophene-containing
pCzSe devices (Se1–Se6) exhibit the widest range of FF variation
(36.1–72.3%) among the four copolymer-based OPVs. This could
be a good indication that the potential Se–Se interactions of the
selenophene-containing copolymers depend on the device fabri-
cation conditions.
3. Conclusions
In conclusion, this work has demonstrated a detailed comparison
between thiophene- and selenophene-bridged donor–acceptor
21556 | J. Mater. Chem., 2012, 22, 21549–21559
This journal is ª The Royal Society of Chemistry 2012

View Article Online
orbital (ELUMO) were then calculated according to the following
silica gel flash column chromatography using hex-
ane : dichloromethane (10 : 1) as eluent to obtain a deep orange
1
solid (1.56 g, 64%). H NMR (400 MHz, CDCl3): d (ppm) 8.82
equations:
EHOMO ¼ ꢀ[(4ox ꢀ 4ox, Ferrocene) + 4.8)] (eV)
ELUMO ¼ ꢀ[(4red ꢀ 4ox, Ferrocene) + 4.8)] (eV)
(d, 2H, J ¼ 4.0 Hz), 8.19 (d, 2H, J ¼ 5.2 Hz), 7.45–7.48 (m, 2H),
4.12 (t, 4H, J ¼ 7.2 Hz), 1.90–1.97 (m, 4H), 1.20–1.50 (m, 36H),
0.87 (t, 6H, J ¼ 6.0 Hz). 13C NMR (400 MHz, CDCl3): d (ppm)
151.5, 150.6, 138.3, 133.2, 132.8, 129.6, 119.2, 74.5, 31.9, 30.6,
29.7, 29.6, 29.5, 29.4, 25.9, 22.7, 14.1. HRMS (m/z): [M+] calcd
for C38H56O2N2SSe2 764.2393; found 764.2382. Anal. found
(Calcd): C, 59.93 (59.83); H, 7.67 (7.40); N 3.65 (3.67)%.
where 4ox and 4red are the onset oxidation and reduction
potentials vs. Ag–Ag+ of the copolymer, respectively.
4.2 Synthesis
5,6-Bis(dodecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadia-
zole (4S). Compound 4S was synthesized according to the same
Materials. All chemicals were purchased from Aldrich, Alfa
Aesar, Acros, and TCI Chemical Co., and they were used
without further purification. Solvents such as dichloromethane
(CH2Cl2), chlorobenzene, terrahydrofuran (THF), N,N-dime-
thylformamide (DMF) and toluene were distilled after drying
with appropriate drying agents. The dried solvents were stored
1
procedure as 4Se. Yield: 70%. H NMR (400 MHz, CDCl3): d
(ppm) 8.44 (d, 2H, J ¼ 3.6 Hz), 7.48 (d, 2H, J ¼ 5.6 Hz), 7.19–
7.22 (m, 2H), 4.09 (t, 4H, J ¼ 6.8 Hz), 1.84–1.93 (m, 4H), 1.20–
1.42 (m, 36H), 0.90 (t, 6H, J ¼ 7.2 Hz).
ꢀ
over 4 A molecular sieves before use. 2,7-Dibromo-9-dodecyl-
4,7-Bis(5-bromoselenophen-2-yl)-5,6-bis(dodecyloxy)benzo[c]
[1,2,5]thiadiazole (BTSe). Compound 4Se (0.61 g, 0.80 mmol)
was dissolved in 20 mL chloroform : acetic acid (1 : 1) in a two-
neck round-bottom flask. Then NBS (0.31 g, 1.76 mmol) was
added at room temperature and reacted for 16 hours. The reac-
tion mixture was then washed with deionized water, NaOH
aqueous solution and deionized water to a pH near 7. A red-
orange solid was obtained by removing solvent, followed by
purification with silica gel flash column chromatography using
hexane : dichloromethane (10 : 1) as eluent (0.70 g, 95%). 1H
NMR (400 MHz, CDCl3): d (ppm) 8.64 (d, 2H, J ¼ 4.0 Hz), 7.37
(d, 2H, J ¼ 4.4 Hz), 4.13 (t, 4H, J ¼ 7.2 Hz), 1.91–1.98 (m, 4H),
1.20–1.49 (m, 36H), 0.86 (t, 6H, J ¼ 6.0 Hz). 13C NMR (400
MHz, CDCl3): d (ppm) 151.1, 150.1, 140.1, 133.1, 132.8, 120.1,
118.7, 74.8, 31.9, 30.5, 29.7, 29.6, 29.5, 29.4, 25.9, 22.7, 14.1.
HRMS (m/z): [M+] calcd for C38H54O2N2Br2SSe2 920.0603;
found 920.0599. Anal. found (Calcd): C, 49.82 (49.57); H, 6.03
(5.91); N 3.04 (3.04)%.
9H-carbazole (1),60 4,8-di(2-ethylhexyloxy)benzol[1,2-b:4,5-b0]
dithiophene (2),61 2,6-bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)
benzo[1,2-b:4,5-b0]dithiophene (BDT)45 and 4,7-dibromo-5,6-
bis(dodecyloxy)benzo[c][1,2,5]thiadiazole (3)46 were synthesized
following procedures reported in the literature. [6,6]-Phenyl-C61-
butyric acid methyl ester (PCBM) and poly(3,4-ethyl-
enedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) were
purchased from UR and H. C. Starck, respectively. They were
used as received.
9-Dodecyl-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
9H-carbazole (Cz). To a solution of compound 1 (0.99 g, 2.00
mmol) in dried tetrahydrofuran (30 mL) in a flame-dried 150 mL
ꢁ
flask at ꢀ78 C was added dropwise 1.70 mL (4.20 mmol) of n-
butyllithium (2.5 M in hexane). The mixture was stirred at ꢀ78 ꢁC
for 1 hour and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxabor-
olane (0.90 mL, 0.819 g, 4.4 mmol) was added rapidly to the
solution. After one additional hour at ꢀ78 ꢁC, the resulting
mixture was warmed to room temperature and stirred for 16
hours. The mixture was poured into water, extracted with diethyl
ether four times and dried over magnesium sulfate. After removal
of the magnesium sulfate, the solvent was evaporated under
reduced pressure, and the residue was purified by passing through
a silica gel flash column using hexane : ethylacetate (10 : 1) as the
eluent. The product was then recrystallized in ethanol to obtain a
white solid (0.85 g, 72%). 1H NMR (400 MHz, CDCl3): d (ppm)
8.09 (d, 2H, J ¼ 7.6 Hz), 7.86 (s, 2H), 7.66 (d, 2H, J ¼ 7.6 Hz),
4.36 (t, 2H, J ¼ 7.2 Hz), 1.83–1.90 (m, 2H), 1.38 (s, 24H), 1.18–
1.30 (m, 18H), 0.85 (t, 3H, J ¼ 7.2 Hz). 13C NMR (400 MHz,
CDCl3): d (ppm) 140.4, 125.1, 124.8, 120.0, 115.2, 83.8, 42.9, 31.9,
29.6, 29.5, 29.4, 29.3, 29.2, 27.1, 24.9, 22.7, 14.1.
4,7-Bis(5-bromothiophen-2-yl)-5,6-bis(dodecyloxy)benzo[c][1,2,5]
thiadiazole (BTS). Compound BTS was synthesized according to
the same procedure as that of BTSe. Yield: 93%. 1H NMR (400
MHz, CDCl3): d (ppm) 8.34 (d, 2H, J ¼ 4.0 Hz), 7.15 (d, 2H, J ¼
4.0 Hz), 4.10 (t, 4H, J ¼ 7.2 Hz), 1.88–1.95 (m, 4H), 1.19–1.48 (m,
36H), 0.86 (t, 6H, J ¼ 6.4 Hz). 13C NMR (400 MHz, CDCl3): d
(ppm) 151.5, 150.4, 135.7, 131.0, 129.7, 117.0, 115.5, 74.6, 31.9,
30.3, 29.7, 29.6, 29.5, 29.4, 25.9, 22.7, 14.1. HRMS (m/z): [M+]
calcd for C38H54O2N2Br2S3 824.1714; found 824.1716. Anal.
found (Calcd): C, 55.20 (55.13); H, 6.58 (6.60); N 3.39 (3.37)%.
General synthetic procedures for copolymers: poly{9-dodecyl-
9H-carbazole-alt-5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl)benzo
[c][1,2,5]-thiadiazole} (pCzS) and poly{9-dodecyl-9H-carbazole-
alt-5,6-bis(dodecyloxy)-4,7-di(selenophen-2-yl)benzo[c][1,2,5]-
thiadiazole} (pCzSe). The copolymers pCzS and pCzSe were
synthesized via Suzuki cross-coupling polymerization. A repre-
sentative procedure is as follows. To a 25 mL two-neck round
bottom flask, Cz (73.4 mg, 0.125 mmol), BTS (103.4 mg, 0.125
mmol) or BTSe (115.1 mg, 0.125 mmol), dry toluene (10.0 mL),
and 1.0 mL of 20% aqueous tetraethylammonium hydroxide,
5,6-Bis(dodecyloxy)-4,7-di(selenophen-2-yl)benzo[c][1,2,5]thia-
diazole (4Se). In a 150 mL flame-dried two-neck round-bottom
flask with a condenser, compound 3 (2.12 g, 3.20 mmol), tribu-
tyl(selenophen-2-yl)stannane (3.50 g, 8.33 mmol), PdCl2(PPh3)2
(0.09 g, 0.13 mmol) and dried tetrahydrofuran (50 mL) were
added. The reaction mixture was heated to reflux for 3 days. The
reaction mixture was then cooled to room temperature and the
solvent was evaporated. The crude product was purified with
Et4NOH(aq)
,
were added. The reaction mixture was
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 21549–21559 | 21557

View Article Online
deoxygenated through three freeze–pump–thaw cycles. Then,
Pd2dba3 (4.6 mg, 0.005 mmol) and p(o-tol)3 (6.1 mg, 0.02 mmol)
were added under nitrogen and the reaction mixture was vigor-
ously stirred at 90–95 ꢁC for 72 hours. After cooling to room
temperature, the mixture was poured into methanol–water
(10 : 1, 200 mL). The precipitate was filtered through a Soxhlet
thimble, which was then subjected to Soxhlet extraction with
methanol, hexane, and chloroform. The chloroform fraction was
concentrated, re-precipitated with methanol, filtered, and then
finally dried in a vacuum oven. pCzS (116 mg, 90%). Anal. found
(Calcd): C, 74.53 (74.28); H, 8.66 (8.75); N 4.11 (4.19)%. GPC (40
ꢁC using THF as eluent): Mw ¼ 102.9 kg molꢀ1, Mn ¼ 66.1 kg
molꢀ1, PDI ¼ 1.56. pCzSe (120 mg, 87%). Anal. found (Calcd):
C, 67.79 (67.92); H, 7.68 (8.00); N 3.83 (3.83)%. GPC (40 ꢁC
tested to optimize the best active layer thickness. Finally, 20 nm
of Ca and 80 nm of Al were deposited on top of the active layer
in a vacuum of about 8 ꢂ 10ꢀ6 Torr to complete the photo-
voltaic device fabrication. All devices were encapsulated inside a
nitrogen-filled glove box.
The current density–voltage (J–V) characteristics of the BHJ
OPVs were measured in the dark and under AM 1.5G solar
illumination from a class A solar simulator (Oriel 300 W),
controlled by a programmable source meter (Keithley 2400). The
light intensity was calibrated using a Si photodiode (PVM 172;
area ¼ 3.981 cm2) from the National Renewable Energy Labo-
ratory. The power conversion efficiency (PCE) of the device was
calculated based on JSC, VOC, and FF, obtained from device
measurements that were conducted under 1-sun AM 1.5G solar
illumination. The series resistance (RS) and shunt resistance
(RSH) were estimated by taking the reciprocal from the slope of a
tangent to the J–V curve at the open- and short-circuit condi-
tions, respectively.
using THF as eluent): Mw ¼ 94.1 kg molꢀ1, Mn ¼ 33.9 kg molꢀ1
,
PDI ¼ 2.78. 1H NMR spectra of pCzS and pCzSe were acquired
(Fig. S3 and S4 in the ESI†).
General synthetic procedures for copolymers: poly{4,8-bis(2-
ethylhexyloxy)benzo[1,2-b;4,5-b0]dithiophene-alt-5,6-bis(dodecy-
loxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pBDTS)
and poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b;4,5-b0]dithiophene-
alt-5,6-bis(dodecyloxy)-4,7-di(selenophen-2-yl)benzo[c][1,2,5]-thia-
diazole} (pBDTSe). The copolymers pBDTS and pBDTSe were
synthesized via Stille cross-coupling polymerization. A repre-
sentative procedure is as follows. To a 25 mL two-neck round
bottom flask, BDT (96.5 mg, 0.125 mmol), BTS (103.4 mg,
0.125 mmol) or BTSe (115.1 mg, 0.125 mmol), and dry chlo-
robenzene (12.0 mL) were added. The reaction mixture was
deoxygenated through three freeze–pump–thaw cycles. Then,
Pd2dba3 (4.6 mg, 0.005 mmol) and p(o-tol)3 (6.1 mg, 0.02 mmol)
were added under nitrogen and the reaction mixture was reacted
For the incident photon-to-electron conversion efficiency
(IPCE) measurements, an AM 1.5G solar simulator was used to
generate the bias light. A monochromator (Newport Model
74100), which was calibrated with a National Institute of Stan-
dards and Technology calibrated photodiode and chopped at 250
Hz, was used to select the wavelengths between 400 and 750 nm
for illuminating the OPV. The photocurrent from the OPV was
measured through a lock-in amplifier (Signal Recovery 7265),
which was in turn referenced to the chopper frequency. All
electrical measurements were carried out in air.
Organic field-effect transistors (OFETs) were fabricated with a
bottom-gate, top-contact structure. The channel width (W) and
length (L) of the transistors are 2 mm and 50 mm, respectively.
The device was built on a heavily doped Si wafer with a layer of
thermally grown SiO2 (ꢃ300 nm) with a capacitance of 12 nF
cmꢀ2. The Si wafer functioned as the gate electrode while the
octyltrichlorosilane (OTS)-modified SiO2 layer acted as the gate
dielectric. The SiO2-covered Si wafer was cleaned and treated
with OTS prior to use. The OTS treatment was performed by
immersing the substrate in OTS solution (10 mL OTS in 40 mL
dried toluene) for 5 minutes, followed by rinsing with toluene,
and then air drying. The solution of S- or Se-containing copol-
ymer in 1,2-dichlorobenzene (10 mg mLꢀ1) was filtered through a
0.45 mm syringe filter and spin-coated on the substrate at 1500
rpm for 60 s at room temperature in a glove box. Subsequently,
the gold (Au) source/drain electrode pairs (70 nm) were deposited
on top of the polymer thin film by high vacuum thermal evap-
oration through a shadow mask to define the channel length (L,
50 mm) and width (W, 2 mm).
ꢁ
for 72 hours at 120 C. After cooling to room temperature, the
mixture was poured into methanol (200 mL). The precipitate
was filtered through a Soxhlet thimble, which was then sub-
jected to Soxhlet extraction with methanol, hexane, and chlo-
roform. The chloroform fraction was concentrated, re-
precipitated with methanol, filtered, and then finally dried in a
vacuum oven overnight. pBDTS (129 mg, 93%). Anal. found
(Calcd): C, 68.99 (69.02); H, 8.27 (8.33); N 2.44 (2.52)%. GPC
(40 C using THF as eluent): Mw ¼ 106.7 kg molꢀ1, Mn ¼ 62.7
ꢁ
kg molꢀ1, PDI ¼ 1.70.pBDTSe (133 mg, 88%). Anal. found
(Calcd): C, 68.99 (69.02); H, 8.27 (8.33); N 2.44 (2.52)%. GPC
(40 C using THF as eluent): Mw ¼ 112.3 kg molꢀ1, Mn ¼ 63.7
ꢁ
kg molꢀ1, PDI ¼ 1.76. 1H NMR spectra of pBDTS and
pBDTSe were acquired (Fig. S5 and S6 in the ESI†).
The electrical measurements were carried out in air using a
semiconductor parameter analyzer (Keithley 2636). The satu-
ration mobility (msat) was extracted from the slope of the square
root of the drain current plot vs. VG from the following
equation:
4.3 Device fabrication and testing
Photovoltaic devices were fabricated on patterned ITO glass
substrate. The active layer of each solar cell device is 0.04 cm2.
ITO glass substrates were sonicated sequentially in detergent,
deionized water, acetone, and isopropanol. The ITO substrates
were then baked on a hot plate at 150 ꢁC for 10 minutes. A
PEDOT:PSS thin film was spin-coated (2000 rpm, 60 s) and
W
2
ID;sat
¼
C1mLsatðVG ꢀ VTÞ
2L
ꢁ
where ID,sat is the drain-to-source saturated current; W/L is the
channel width to length ratio; Ci is the capacitance of the insu-
lator per unit area (Ci ¼ 12 nF cmꢀ2), and VG and VT are the gate
voltage and threshold voltage, respectively.
then baked at 150 C for 10 minutes. Next, an active layer was
spin coated on top of the PEDOT:PSS layer from a 1,2-
dichlorobenzene (1,2-DCB) solution of the copolymer:PCBM
blends. Different concentrations and spin-coating speeds were
21558 | J. Mater. Chem., 2012, 22, 21549–21559
This journal is ª The Royal Society of Chemistry 2012

View Article Online
32 S. K. Lee, N. S. Cho, S. Cho, S.-J. Moon, J. K. Lee and G. C. Bazan,
J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 6873.
33 D. S. Chung, H. Kong, W. M. Yun, H. Cha, H.-K. Shim, Y.-H. Kim
and C. E. Park, Org. Electron., 2010, 11, 899.
34 F. Qiao, X. Hou, Y. Lu, H. Chen, A. Liu, X. Hu and N. S. Choon,
Sol. Energy Mater. Sol. Cells, 2010, 94, 442.
35 K. A. Mazzio, M. Yuan, K. Okamoto and C. K. Luscombe, ACS
Appl. Mater. Interfaces, 2011, 3, 271.
36 J.-Y. Lee, S.-H. Kim, I.-S. Song and D.-K. Moon, J. Mater. Chem.,
2011, 21, 16480.
References
1 R. F. Service, Science, 2011, 332, 293.
2 G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science,
1995, 270, 1789.
3 C. Soci, I. W. Hwang, D. Moses, Z. Zhu, D. Waller, R. Gaudiana,
C. J. Brabec and A. J. Heeger, Adv. Funct. Mater., 2007, 17, 632.
4 R. C. Coffin, J. Peet, J. Rogers and G. C. Bazan, Nat. Chem., 2009, 1, 657.
5 H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu,
Y. Wu and G. Li, Nat. Photonics, 2009, 3, 649.
€
37 S. Haid, A. Mishra, C. Uhrich, M. Pfeiffer and P. Bauerle, Chem.
Mater., 2011, 23, 4435.
6 Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray and L. Yu,
Adv. Mater., 2010, 22, E135.
38 R. Xia, M. Al-Hashimi, W. C. Tsoi, M. Heeney, D. D. C. Bradley
and J. Nelson, Sol. Energy Mater. Sol. Cells, 2012, 96,
112.
7 T.-Y. Chu, J. Lu, S. Beaupre, Y. Zhang, J.-R. M. Pouliot, S. Wakim,
J. Zhou, M. Leclerc, Z. Li, J. Ding and Y. Tao, J. Am. Chem. Soc.,
2011, 133, 4250.
39 M. Nakano, H. Mori, S. Shinamura and K. Takimiya, Chem. Mater.,
2012, 24, 190.
8 S. C. Price, A. C. Stuart, L. Yang, H. Zhou and W. You, J. Am. Chem.
Soc., 2011, 133, 4625.
9 M.-S. Su, C.-Y. Kuo, M.-C. Yuan, U.-S. Jeng, C.-J. Su and
K.-H. Wei, Adv. Mater., 2011, 23, 3315.
40 H. A. Saadeh, L. Lu, F. He, J. E. Bullock, W. Wang, B. Carsten and
L. Yu, ACS Macro Lett., 2012, 1, 361.
41 W.-H. Lee, S. K. Son, K. Kim, S. K. Lee, W. S. Shin, S.-J. Moon and
I.-N. Kang, Macromolecules, 2012, 45, 1303.
10 H. Zhou, L. Yang, A. C. Stuart, S. C. Price, S. Liu and W. You,
Angew. Chem., Int. Ed., 2011, 50, 2995.
42 M. Shahid, R. S. Ashraf, Z. Huang, A. J. Kronemeijer, T. McCarthy-
Ward, I. McCulloch, J. R. Durrant, H. Sirringhaus and M. Heeney, J.
Mater. Chem., 2012, 22, 12817.
43 D. Patra, D. Sahu, H. Padhy, D. Kekuda, C.-W. Chu and H.-C. Lin,
J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 5479.
44 J. Hou, M.-H. Park, S. Zhang, Y. Yao, L.-M. Chen, J.-H. Li and
Y. Yang, Macromolecules, 2008, 41, 6012.
11 A. Patra and M. Bendikov, J. Mater. Chem., 2010, 20, 422.
12 S. Das and S. S. Zade, Chem. Commun., 2010, 46, 1168.
13 A. Patra, Y. H. Wijsboom, G. Leitus and M. Bendikov, Chem.
Mater., 2011, 23, 896.
14 M. R. Bryce, J. Mater. Chem., 1995, 5, 1481.
15 H. Kobayashi, H. Cui and A. Kobayashi, Chem. Rev., 2004, 104,
5265.
45 Y. Liang, D. Feng, Y. Wu, S.-T. Tsai, G. Li, C. Ray and L. Yu, J. Am.
Chem. Soc., 2009, 131, 7792.
46 M. Helgesen, S. A. Gevorgyan, F. C. Krebs and R. A. J. Janssen,
Chem. Mater., 2009, 21, 4669.
47 S. C. Price, A. C. Stuart and W. You, Macromolecules, 2010, 43, 4609.
48 H. Zhou, L. Yang, S. Xiao, S. Liu and W. You, Macromolecules,
2010, 43, 811.
49 C. Deibel, D. Mack, J. Gorenflot, D. Rauh and V. Dyakonov, Phys.
Rev. B: Condens. Matter Mater. Phys., 2010, 81, 085202.
50 Y. Li, Y. Cao, J. Gao, D. Wang, G. Yu and A. J. Heeger, Synth. Met.,
1999, 99, 243.
51 Z.-K. Chen, W. Huang, L.-H. Wang, E.-T. Kang, B. J. Chen,
C. S. Lee and S. T. Lee, Macromolecules, 2000, 33, 9015.
52 S. W. Cho, L. F. J. Piper, A. DeMasi, A. R. H. Preston, K. E. Smith,
K. V. Chauhan, P. Sullivan, R. A. Hatton and T. S. Jones, J. Phys.
Chem. C, 2010, 114, 1928.
53 M. Heeney, W. Zhang, D. J. Crouch, M. L. Chabinyc, S. Gordeyev,
R. Hamilton, S. J. Higgins, I. McCulloch, P. J. Skabara, D. Sparrowe
and S. Tierney, Chem. Commun., 2007, 5061.
54 K. Oyaizu, T. Iwasaki, Y. Tsukahara and E. Tsuchida,
Macromolecules, 2004, 37, 1257.
55 N. Blouin, A. Michaud, D. Gendron, S. Wakim, E. Blair, R. Neagu-
Plesu, M. Belletete, G. Durocher, Y. Tao and M. Leclerc, J. Am.
Chem. Soc., 2008, 130, 732.
56 T. Y. Chu, J. Lu, S. Beaupre, Y. Zhang, J. R. Pouliot, S. Wakim,
J. Zhou, M. Leclerc, Z. Li, J. Ding and Y. Tao, J. Am. Chem. Soc.,
2011, 133, 4250.
57 D. E. Markov, E. Amsterdam, P. W. M. Blom, A. B. Sieval and
J. C. Hummelen, J. Phys. Chem. A, 2005, 109, 5266.
58 P. Peumans, A. Yakimov and S. R. Forrest, J. Appl. Phys., 2003, 93,
3693.
59 J. Pommerehne, H. Vestweber, W. Guss, R. F. Mahrt, H. Bassler,
M. Porsch and J. Daub, Adv. Mater., 1995, 7, 551.
60 Y. Chen, W. Huang, C. Li and Z. Bo, Macromolecules, 2010, 43,
10216.
61 G. Zhang, Y. Fu, Q. Zhang and Z. Xie, Chem. Commun., 2010, 46,
4997.
16 D. J. Crouch, P. J. Skabara, J. E. Lohr, J. J. W. McDouall,
M. Heeney, I. McCulloch, D. Sparrowe, M. Shkunov, S. J. Coles,
P. N. Horton and M. B. Hursthouse, Chem. Mater., 2005, 17, 6567.
17 S. S. Zade and M. Bendikov, Org. Lett., 2006, 8, 5243.
18 H. Pang, P. J. Skabara, S. Gordeyev, J. J. W. McDouall, S. J. Coles
and M. B. Hursthouse, Chem. Mater., 2007, 19, 301.
19 R. Kroon, M. Lenes, J. C. Hummelen, P. W. M. Blom and B. de Boer,
Polym. Rev., 2008, 48, 531.
20 J. M1ochowski and M. Giurg, in Aromaticity in Heterocyclic
Compounds, Springer, 2009, p. 289.
21 Y. Kunugi, K. Takimiya, K. Yamane, K. Yamashita, Y. Aso and
T. Otsubo, Chem. Mater., 2003, 15, 6.
22 Y. Kunugi, K. Takimiya, Y. Toyoshima, K. Yamashita, Y. Aso and
T. Otsubo, J. Mater. Chem., 2004, 14, 1367.
23 N. S. Cho, S. K. Lee, J. H. Seo, M. Elbing, J. D. Azoulay, J. Park,
S. Cho, A. J. Heeger and G. C. Bazan, J. Mater. Chem., 2008, 18,
4909.
24 D. J. Crouch, P. J. Skabara, J. E. Lohr, J. J. W. McDouall,
M. Heeney, I. McCulloch, D. Sparrowe, M. Shkunov, S. J. Coles,
P. N. Horton and M. B. Hursthouse, Chem. Mater., 2005, 17, 6567.
25 Y. M. Kim, E. Lim, I.-N. Kang, B.-J. Jung, J. Lee, B. W. Koo,
L.-M. Do and H.-K. Shim, Macromolecules, 2006, 39, 4081.
26 D. J. Crouch, P. J. Skabara, M. Heeney, I. McCulloch, D. Sparrowe,
S. J. Coles and M. B. Hursthouse, Macromol. Rapid Commun., 2008,
29, 1839.
27 H. Kong, D. S. Chung, I.-N. Kang, J.-H. Park, M.-J. Park, I. H. Jung,
C. E. Park and H.-K. Shim, J. Mater. Chem., 2009, 19, 3490.
28 M. Al-Hashimi, J. G. Labram, S. Watkins, M. Motevalli,
T. D. Anthopoulos and M. Heeney, Org. Lett., 2010, 12, 5478.
29 Z. Chen, H. Lemke, S. Albert-Seifried, M. Caironi, M. M. Nielsen,
M. Heeney, W. Zhang, I. McCulloch and H. Sirringhaus, Adv.
Mater., 2010, 22, 2371.
30 S. P. Mishra, A. E. Javier, R. Zhang, J. Liu, J. A. Belot, I. Osaka and
R. D. McCullough, J. Mater. Chem., 2011, 21, 1551.
31 A. M. Ballantyne, L. Chen, J. Nelson, D. D. C. Bradley, Y. Astuti,
A. Maurano, C. G. Shuttle, J. R. Durrant, M. Heeney, W. Duffy
and I. McCulloch, Adv. Mater., 2007, 19, 4544.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 21549–21559 | 21559