Synthesis and photovoltaic properties of two-dimensional low-bandgap copolymers based on new benzothiadiazole derivatives with different conjugated arylvinylene side chains

Article abstract of DOI:10.1002/chem.201200730

A new series of 2,1,3-benzothiadiazole (BT) acceptors with different conjugated aryl-vinylene side chains have been designed and used to build efficient low-bandgap (LBG) photovoltaic copolymers. Based on benzo[1,2-b:3,4-b']dithiophene and the resulting n

Full text of DOI:10.1002/chem.201200730

FULL PAPER
DOI: 10.1002/chem.201200730
Synthesis and Photovoltaic Properties of Two-Dimensional Low-Bandgap
Copolymers Based on New Benzothiadiazole Derivatives with Different
Conjugated Aryl-vinylene Side Chains
Qiang Peng,*[a, c] Siew-Lay Lim,[b] Ivy Hoi-Ka Wong,[b] Jun Xu,[c] and Zhi-Kuan Chen*[b]
Abstract: A new series of 2,1,3-benzo-
thiadiazole (BT) acceptors with differ-
ent conjugated aryl-vinylene side
chains have been designed and used to
build efficient low-bandgap (LBG)
photovoltaic copolymers. Based on
benzo[1,2-b:3,4-b’]dithiophene and the
resulting new BT derivatives, three
two-dimensional (2D)-like donor (D)–
acceptor (A) conjugated copolymers
have been synthesised by Stille cou-
pling polymerisation. These copolymers
were characterised by NMR spectro-
scopy, gel-permeation chromatography,
thermogravimetric analysis and differ-
ential scanning calorimetry. UV/Vis ab-
sorption and cyclic voltammetry meas-
urements indicated that their optical
and electrochemical properties can be
facilely modified by changing the struc-
tures of the conjugated aryl-vinylene
side chains. The copolymer with
phenyl-vinylene side chains exhibited
the best light harvesting and smallest
bandgap of the three copolymers. The
basic electronic structures of D–A
model compounds of these copolymers
were also studied by DFT calculations
at the B3LYP/6-31G* level of theory.
Polymer solar cells (PSCs) with a typi-
cal structure of indium tin oxide (ITO)/
poly(3,4-ethylenedioxythiophene) (PE-
copolymer:ACHTNGUTRENNU[G 6,6]-phenyl-C61ACHTUGNTRNEN(UNG C71)-butyric
acid-methyl ester (PCBM)/calcium
(Ca)/aluminum (Al) were fabricated
and measured under the illumination
of AM1.5G at 100 mWcmÀ2. The re-
sults showed that the device based on
the copolymer with phenyl-vinylene
side chains had the highest efficiency
of 2.17% with PC71BM as acceptor.
The results presented herein indicate
that all the prepared copolymers are
promising candidates for roll-to-roll
manufacturing of efficient PSCs. Suita-
ble electronic, optical and photovoltaic
properties of BT-based copolymers can
also be achieved by fine-tuning the
structures of the aryl-vinylene side
chains for photovoltaic application.
DOT):poly(styrenesulfonate)
(PSS)/
Keywords: conjugation · donor–ac-
ceptor systems · polymers · solar
cells · synthesis design
Introduction
acceptor by using the typical bulk-heterojunction (BHJ)
device structure.[3] However, the mismatch between the ab-
sorption of the devices and the terrestrial solar spectrum
limited their performance in PSCs. To achieve enhanced ab-
sorption, low-bandgap (LBG) conjugated polymers were de-
veloped by incorporating electron-rich donor (D) and elec-
tron-deficient acceptor (A) segments into the polymer back-
bones.[4,5] The push–pull interaction of the donor and accept-
or moieties can result in a lower-energy absorption band
due to intramolecular charge transfer (ICT).[6] Based on this
concept, PCEs of over 7% were obtained by using LBG
conjugated copolymers in bulk-heterojunction (BHJ) solar
cells as electron-donating materials.[7]
Harvesting energy directly from the sun is one of the most
efficient approaches to address the growing global energy
needs. Recently, polymer solar cells (PSCs) have received
increasing attention because of their flexibility, facile proces-
sibility, light weight and low production cost.[1] Considerable
effort has been focused on improving the power conversion
efficiency (PCE) of PSCs from around 1 to 9% for potential
commercial applications.[2] In earlier times, high PCEs of 4–
5% were achieved by blending P3HT as a donor and
[6,6]phenyl-C61-butyric acid methyl ester (PC61BM) as an
To date, most LBG copolymers have linear main-chain
structures. In the past few years, two-dimensional (2D)-like
copolymers with conjugated side chains were proposed to
enhance absorption, charge transport and device performan-
ce.[4a,8] Li and co-workers designed and prepared a series of
polythiophenes with conjugated side chains of bi(phenylene-
vinylene),[8a] bi(thienylenevinylene)[8b,c] and phenothiazinevi-
nylene,[8d] which exhibited broadened absorption spectra
and enhanced power conversion efficiencies. Other interest-
ing work with 2D-like polythiophenes in PSCs included the
use of conjugated side chains of triphenylamine,[9a] carbazo-
le,[9b] phenothiazine[9c] and phenanthrenylimidazole.[9d,e] Very
[a] Prof. Q. Peng
College of Chemistry, Sichuan University
Chengdu, 610064 (P. R. China)
[b] Dr. S.-L. Lim, Dr. I. H.-K. Wong, Prof. Z.-K. Chen
Institute of Materials Research and Engineering
3 Research Link, Singapore, 117602 (Singapore)
Fax : (+86)28-86510868
E-mail: qiangpengjohnny@yahoo.com
zk-chen@imre.a-star.edu.sg
[c] Prof. Q. Peng, M. Sc. J. Xu
School of Environmental and Chemical Engineering
Nanchang Hangkong University
Nanchang, 330063 (P. R. China)
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recently, Huang et al. reported a novel 2D D–A conjugated
copolymer system in which the strong electron-withdrawing
acceptor groups were attached to the end of the side chains
connecting the electron-donating conjugated main chain
through a bridge.[10] By this method, low bandgaps can be ef-
fectively obtained and relatively low highest-occupied mo-
lecular orbital (HOMO) energy levels can be easily main-
tained.[11] The BHJ solar cells based on these copolymers ex-
hibit high PCE values of over 4%.[10,11] Although previous
work has proved that 2D-like conjugated copolymers are
promising candidates for building efficient LBG photovolta-
ic copolymers, it still remains a challenge to develop new
systems and understand the relationship between their struc-
tures and properties.
Results and Discussion
Synthesis and characterisation: The synthetic routes to the
monomers and copolymers are outlined in Scheme 1. 4,7-Di-
bromo-5-methyl-2,1,3-benzothiadiazole (3) was prepared by
bromination of 5-methyl-2,1,3-benzothiadiazole with Br2 and
HBr (47%) according to the procedure used for the synthe-
sis of 4,7-dibromo-2,1,3-benzothiadiazole.[16c] 4,7-Dibromo-5-
bromomethyl-2,1,3-benzothiadiazole (4) was synthesised
from compound 3 by further bromination with NBS by
using benzoyl peroxide (BPO) as the radical initiator. Ben-
zothiadiazole 4 was subsequently treated with triethyl phos-
phite to give phosphonic acid diethyl ester 5. 2D-Like ben-
zothiadiazole derivatives 6a–c were synthesised by the
Wittig–Horner reaction by coupling 5 with various aromatic
aldehydes, which then underwent the further coupling reac-
tions and brominations to monomer 8a–c. The copolymers
were synthesised in good yields from organic tin compounds
In this work we have designed and synthesised a series of
2D-like D–A conjugated copolymers, PBDBT1, PBDBT2
and PBDBT3 (Figure 1), with novel 2,1,3-benzothiadiazole
and bromides 8 by Stille coupling reactions with [Pd
2ACHTUNGTRENNUNG(dba)3]/
P
ACHTUNGTRENNUNG
PBDBT3 were purified by washing with methanol and
hexane in turn and then extraction with chloroform, respec-
tively. The chloroform solution was concentrated and repre-
cipitated in methanol several times to give the resulting co-
polymer. The structures of the copolymers were confirmed
by NMR spectroscopy.
Owing to presence of the attached dodecyl and dodecy-
loxy side chains, these copolymers exhibit good solubility in
organic solvents such as chloroform, tetrahydrofuran, tol-
uene, xylene and chlorobenzene. The number-average mo-
lecular masses (Mn) and polydispersity indices (PDI) were
measured by gel-permeation chromatography (GPC) using
THF as the eluent and polystyrenes as the internal stand-
ards. The results show that PBDBT1–PBDBT3 have high
number average molecular mass values ranging from 39200
to 66100 Da with the PDI values ranging from 2.08 to 2.57
(Table 1). The glass transition temperatures (Tg) of these co-
Figure 1. Structures of the LBG copolymers PBDBT1, PBDBT2 and
PBDBT3.
(BT) acceptors modified with conjugated aryl-vinylene side
chains for efficient BHJ solar cell application. The parent
skeleton of the BT acceptor was chosen because it has been
widely employed and proven to be one of the most promis-
ing acceptor blocks for building high-performance polymer
solar cell materials.[12] In fact, LBG copolymers containing
BT and various donor segments, such as fluorene,[13] sila-
Table 1. Molecular weights and thermal properties of the copolymers.
[a]
[c]
Copolymers
Yield [%]
Mn
PDI[a,b]
Td [8C]
Tg[d] [8C]
PBDBT1
PBDBT2
PBDBT3
73
80
91
39200
66100
52400
2.08
2.57
2.21
339
333
345
119
99
106
ACTHNUTRGNEUNG
fluorene,[14] carbazole,[15] indolo[3,2-b]carbazole,[16] dithieno-
silole,[17] dithieno[3,2-b:2’,3’-d]pyrrole,[18] benzo[1,2-b:4,5-
b’]dithiophene,[19] showed high PCEs ranging from 2 to 7%.
Clearly, almost all the work was focused on naked BT or
alkyl- and alkoxy-modified BT frameworks, but not all of
the important properties of the BT unit and its derivatives
have been explored fully. Thus, it is reasonable to expect
that the prepared copolymers would have good photovoltaic
properties. The effects of different conjugated aryl-vinylene
side chains, donor–acceptor strength and backbone planarity
on the absorption spectra and electronic and optoelectronic
properties of the resulting copolymers have also been inves-
tigated. To our knowledge, this is the first report of the use
of 2D-like benzothiadiazole units as acceptors with conju-
gated side chains for the construction of efficient LBG co-
polymers for PSC application.
[a] Molecular masses and polydispersity indices were determined by GPC
in THF with polystyrene as standards. [b] The PDI is defined as Mw/Mn.
[c] Onset decomposition temperature measured by TGA under N2.
[d] Glass transition temperature measured by DSC under N2.
polymers (Figure 2a) were determined to be in the range
99–1198C by differential scanning calorimetry (DSC). The
copolymers exhibit good thermal stability with decomposi-
tion onset temperatures (Td; about 5% weight loss) of 333–
3458C, as measured by thermogravimetric analysis (TGA;
Figure 2b). Clearly, the combination of such good thermal
properties renders these copolymers suitable for application
in PSCs, light-emitting diodes and other optoelectronic devi-
ces.
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Photovoltaic Properties of Benzothiadiazole Copolymers
FULL PAPER
Scheme 1. Synthetic routes to the monomers and conjugated copolymers.
Table 2. Optical and electrochemical data of the copolymers.
Optical properties: The electronic absorption properties of
the copolymers obtained were measured both in CHCl3 sol-
ution (Figure 3a) and as thin films spin-coated on to quartz
slides (Figure 3b). The spectroscopic data are listed in
Table 2. As shown in Figure 3a, the absorption spectra of
these copolymers in CHCl3 present two absorption bands, as
is commonly observed in conjugated D–A copolymers.[5]
The broad distinct peak in the wavelength range 450–
800 nm arises from intramolecular charge-transfer transi-
tions (ICT) between the donor segments and the acceptor
moieties.[20] Another peak ranging from 350 to 450 nm can
be attributed to the p–p* electronic transitions of the conju-
gated polymer backbones. PBDBT1, PBDBT2 and
PBDBT3 have absorption maxima at 402 and 562, 393 and
539, and 410 and 579 nm, respectively. Relative to PBDBT1
and PBDBT2, the absorption maxima of PBDBT3 show
slight redshifts, which can be attributed to the higher
formed ICT from the donor to the acceptor. It is surprising
that PBDBT2 with an electron-withdrawing carbonyl group
[a]
ox
onset
red
onset
[b]
Copolymers Absorption Eg
E
[V]/
E
[V]/
Eg
THF
max
film
max
l
l
[eV]
HOMO
[eV]
LUMO [eV] [eV]
[nm] [nm]
402, 417, 1.67
562 603
393, 413, 1.71
539 598
410, 420, 1.66
579 606
PBDBT1
PBDBT2
PBDBT3
0.75/À5.15
0.87/À5.27
0.68/À5.08
À0.78/À3.62 1.54
À0.75/À3.65 1.62
À0.83/À3.57 1.51
[a] The optical band gap was estimated from the wavelength of the opti-
cal absorption edge of the copolymer film. [b] The electrochemical band
gap was calculated from the LUMO and HOMO energy levels.
in the dodecanoylthienyl-vinylene side chain has the most
blue-shifted absorbance spectrum. The results also indicate
that the conjugated side chains can easily tune the absorp-
tion behaviour of BT-based D–A copolymers, which is very
useful for the design and preparation of efficient photovolta-
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Q. Peng, Z.-K. Chen et al.
ic materials with enhanced absorption and suitable energy
levels. When the polymer solutions were heated to 808C,
the absorption peaks were slightly blueshifted (Figure 3a),
which indicates the partial disaggregation of the polymer
backbone at high temperature.
As shown in Figure 3b, similar behaviour was observed
from the absorption spectra of these copolymers as solid
thin films. Relative to their counterparts in the solution
state, the broad peaks in the wavelength range of 450–
900 nm are redshifted by 41, 59 and 27 nm for PBDBT1,
PBDBT2 and PBDBT3, respectively. This can be explained
by the formation of p-stacked structures in the solid state,[21]
which could facilitate charge transportation for photovoltaic
applications. Although PBDBT2 has a narrow absorption
band, it presents the best p-stacked structures with the lon-
gest redshifted maxima. This may be because the carbonyl
group increases the interaction between the polymer main
chains. The energy band gaps calculated from the absorption
band edges of the optical absorption spectra are around
1.67, 1.71 and 1.66 eV for PBDBT1, PBDBT2 and
PBDBT3, respectively. This indicates that PBDBT3 with
weaker electron-donating phenyl-vinylene side chains shows
a stronger ICT effect than PBDBT1 and PBDBT2, leading
to a smaller bandgap, which is useful for increasing the short
circuit current (Jsc) in PSCs.[3a,22] As expected, the optical
bandgaps and broad absorption bands across the entire visi-
ble wavelength region of the BT-based copolymers can be
fine-tuned by modifying the structures of the conjugated
side chains, which suggests that this strategy can be em-
ployed to develop efficient photovoltaic materials with im-
proved properties.
Figure 2. a) DSC and b) TGA curves of the copolymers (recorded under
a nitrogen atmosphere at a heating rate of 108C minÀ1).
Electrochemical properties: Cyclic voltammetry (CV) was
employed to determine the highest-occupied (HOMO) and
lowest-unoccupied molecular orbital (LUMO) energy levels
of the polymeric materials. These energy levels are crucial
for the selection of appropriate acceptor materials in active
blends in PSCs. The CV experiments were carried out by
using a three-electrode cell in an anhydrous CH3CN solution
of 0.1m tetrabutylammonium perchlorate (nBu4NClO4). A
platinum plate coated with a thin film of the studied copoly-
mer, a platinum wire and Ag/AgCl (0.1m) were used as the
work, counter and reference electrodes, respectively. The
energy level of the Ag/AgCl reference electrode was cali-
brated against the Fc/Fc+ system to be 4.40 eV by using
previous methods.[23] The resulting oxidation and reduction
curves of the copolymer thin films are shown in Figure 4
and the electrochemical redox data are summarised in
Table 2. The cell was purged with pure argon prior to each
scan. The scans in the anodic and cathodic directions were
performed separately at a scan rate of 50 mVsÀ1 at room
temperature. As shown in Figure 4, all the copolymers show
a predominant oxidation peak (p doping) due to the elec-
tron-donating benzodithiophene, thiophene and aryl-vinyl-
ene segments and a slight reduction peak (n doping) related
to the electron-withdrawing benzothiadiazole unit. From the
onset potentials of the p-doping process, the HOMO energy
Figure 3. Optical absorption spectra of the copolymers a) in CHCl3 solu-
tion and b) as thin film on glass.
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Photovoltaic Properties of Benzothiadiazole Copolymers
FULL PAPER
the D–A model molecules have HOMO and LUMO energy
levels in the range of À5.02 to À5.15 eV and À2.55 to
À2.63 eV, respectively. The band gaps were thus determined
to range from 2.47 to 2.52 eV. Clearly, the trends in the
HOMOs, LUMOs and band gaps are similar to those ob-
tained by CV and UV/Vis measurements of the copolymers.
Although the calculated values are somewhat different from
the experimental data because of the part-extended p-conju-
gation system or because only several repeating sections
were selected from the copolymers, the theoretical analysis
of the geometric structures and electronic structures is still
important to gain a direct insight into the interplay between
their structural modifications and the resulting electronic
and optical changes.
Figure 4. Cyclic voltammograms of the copolymers.
levels were determined to be À5.15, À5.27 and À5.08 eV for
PBDBT1, PBDBT2 and PBDBT3, respectively. These re-
sults indicate that carbonylthienyl-vinylene can lower the
HOMO level in this polymer system compared with thienyl-
vinylene and phenyl-vinylene, which is expected to increase
the open-circuit voltage (Voc) of the PSCs.[2d] On the other
hand, the LUMO energy levels were estimated to be À3.62,
À3.65 and À3.57 eV for PBDBT1, PBDBT2 and PBDBT3
from the onset potentials of the cathodic sweep, respectively.
Thus, the band gaps (Eg) of PBDBT1, PBDBT2 and
PBDBT3 were calculated to be around 1.54, 1.62 and
1.51 eV. The values and their trends are consistent with
those obtained by the optical method described above.
Photovoltaic properties: To verify the potential of the co-
polymers for photovoltaic application, bulk heterojunction
solar cells were fabricated and investigated according to our
previous work[2d,5d,20] with a conventional configuration of
glass/indium tin oxide (ITO)/poly(3,4-ethylenedioxythio-
phene) (PEDOT):poly(styrenesulfonate) (PSS)/copolymer:-
ACHUTNGRENUN[G 6,6]-phenyl-C61ACHTUNGTREN(NUGN C71)-butyric acid-methyl ester (PCBM)/cal-
cium (Ca)/aluminum (Al). The devices were optimised by
changing some conditions, such as the ratio of the copoly-
mer to PCBM and the thickness of the active layer. The
active layers were spin-coated from 1,2-dichlorobenzene
(DCB) solutions of the copolymers and PCBM, and the op-
timal weight ratio of copolymer to PC61BM was found to be
1:1 for PBDBT1, PBDBT2 and PBDBT3. The optimal
thicknesses for blend films of PBDBT1, PBDBT2 and
PBDBT3 with PCBM were about 110, 85 and 75 nm, respec-
tively. The representative characteristics of the solar cells
are summarised in Table 3. Note that the device based on
PBDBT3 with phenyl-vinylene side chain exhibits much
better photovoltaic performance than those made from
PBDBT1 and PBDBT2 with thienyl-vinylene side chains.
Figure 6 shows the current density–voltage (J–V) curves
and external quantum efficiency (EQE) spectra for the
PSCs under AM1.5G illumination at 100 mWcmÀ2. As
shown in Figure 6a, the PBDBT1 and PBDBT2 devices
Theoretical calculations: DFT/B3LYP/6-31G* has been
found to be an accurate method for calculating the optimal
geometries and electronic structures of many molecular sys-
tems. To better understand the evolution of the oxidation
and reduction potentials, the electronic structures of the
conjugated copolymers, molecular simulation on D–A
model compounds was performed through DFT calcula-
tions[24] at the B3LYP/6-31G* level of theory by using the
Gaussian 09 program suite.[25] To make computation possi-
ble, the alkyl groups were replaced by methyl groups in the
calculations because they do not significantly affect the
equilibrium geometries or electronic properties. The frontier
molecular orbitals and optimised molecular geometries are
illustrated in Figure 5. Ab initio calculations on the model
compounds indicate that they favour planar conformations,
which enables the electrons to be delocalised across the
whole conjugated molecular system for higher carrier mobi-
lity.[26] As shown in Figure 5, the electron density of the
LUMO is mainly localised on the benzothiadiazole segment
for BD-BT1 and BD-BT3. Owing to the electron-withdraw-
ing nature of the carbonyl group, the LUMO of BD-BT2 is
delocalised over the conjugated section from the benzothia-
diazole to the carbonyl group through the thienyl-vinylene
linker. At the same time, as a result of the carbonyl group,
the electron density of the HOMO of BD-BT2 is distributed
over the entire conjugated molecule including the conjugat-
ed side chain, which lowers the HOMO energy level giving
rise to the enhanced Voc in PSCs. The calculations show that
Table 3. Characteristic current density–voltage parameters derived from
devices tested under standard AM1.5G conditions.
Copolymer Copolymer/PCBM
(w/w)
VOC
[V]
JSC
FF PCE
[%]
[mAcmÀ2
]
PBDBT1
PBDBT1
PBDBT1
PBDBT1
PBDBT2
PBDBT2
PBDBT2
PBDBT2
PBDBT3
PBDBT3
PBDBT3
PBDBT3
1:1[a]
1:2[a]
1:4[a]
1:1[b]
1:1[a]
1:2[a]
1:4[a]
1:1[b]
1:1[a]
1:2[a]
1:4[a]
1:1[b]
0.69
0.66
0.69
0.71
0.84
0.83
0.75
0.84
0.66
0.62
0.61
0.66
4.82
3.97
3.98
5.51
3.49
2.60
1.67
3.98
5.37
4.66
4.41
6.61
0.38 1.27
0.36 0.94
0.41 1.11
0.35 1.38
0.35 1.03
0.39 0.84
0.40 0.50
0.38 1.26
0.54 1.92
0.42 1.19
0.44 1.18
0.50 2.17
[a] Using PC61BM as acceptor. [b] Using PC71BM as acceptor.
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Q. Peng, Z.-K. Chen et al.
Figure 5. Optimised geometries and molecular orbital surfaces of the HOMOs and LUMOs of the model compounds obtained by DFT at the B3LYP/6-
31G* level of theory.
show open-circuit voltages (Voc) of 0.69 and 0.84 V and
short-circuit current densities (Jsc) of 4.82 and 3.49 mAcmÀ2,
(The Voc and Jsc values can be read from the J–V curves, and
the PCE and FF can be calculated from Voc and Jsc values
tween the HOMO energy level of the donor and the
LUMO energy level of the acceptor.[27] However, the lowest
Jsc of the PBDBT2 device results in the lowest PCE, which
may be attributed to the larger bandgap of 1.71 eV com-
pared with those of PBDBT1 and PBDBT3. Furthermore,
the dodecanoyl unit with a carbonyl group may act as an
electron trap compared with the dodecyl and dodecyloxy
units. The PCEs were achieved without any pre- and/or
post-treatments, such as thermal annealing, solvent/vapour
annealing or the addition of solvent additives. Thus, consid-
ering the relatively low FF values of 0.35–0.54, there is big
room for future improvement in the performance of PSCs
based on the copolymer/PCBM system.
from the equations of PCE=VocJscFF/Pmax and FF=VmaxJmax
/
VocJsc. Here, Pmax is a fixed value and VmaxJmax can also be
calculated from the J–V curves.), which results in PCEs of
1.27 and 1.03% with fill factors (FF) of 0.38 and 0.35, re-
spectively. Under the same conditions, the PBDBT3 device
exhibits a higher PCE of 1.92% with a Voc of 0.66 V, a Jsc of
5.37 mAcmÀ2 and a FF of 0.54. Of the fabricated devices,
the PBDBT2 device has the highest Voc due to its lowest
HOMO energy level, as described previously. Such results
are expected as the Voc is dependent on the difference be-
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Photovoltaic Properties of Benzothiadiazole Copolymers
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Figure 6. a) Current density–voltage curves of photovoltaic cells based on
copolymers and PC61BM under AM1.5 illumination and b) EQE curves
of photovoltaic cells based on copolymers and PC61BM.
Figure 7. AFM topographic images of the blend films (copoly-
mer:PC60BM, 1:1, w/w) of a) PBDBT1, b) PBDBT2 and c) PBDBT3.
Image size: 2ꢁ2 mm2.
To evaluate the accuracy of the measurements, the EQEs
of the PSCs based on the copolymers blended with PC61BM
(1:1) under monochromatic illumination were measured and
are shown in Figure 6b. The shapes of the EQE curves of
the devices based on copolymer:PC61BM are similar to their
absorption spectra, which indicates that most of the absor-
bed photons of the copolymers contribute to the photocur-
rent generation. That is, the excitons are mainly generated
in copolymer phases. As shown in Figure 6b, the spectral re-
sponses of the three copolymer:PC61BM based devices ex-
hibit an efficient photoresponse at wavelengths between 300
and 800 nm with maximum EQEs of 42, 30 and 22% at
around 500 nm. Meanwhile, the Jsc values calculated from
the EQEs show a consistent trend with the measured values.
Clearly, the broader profile and higher EQE value of
PBDBT3 result in a higher Jsc for the PBDBT3 device com-
pared with the PBDBT1 and PBDBT2 devices.
AFM is one of the key factors in determining the per-
formance of polymer solar cells.[3b,28] An ideal domain size
of 10–20 nm of copolymer and PCBM with an interpenetrat-
ing bicontinuous network has been found to be important
for achieving high-performance devices. However, both
larger (>10–20 nm) and smaller (<10–20 nm) domain sizes
of the blend films will limit charge transfer and separation.
Figure 7 shows the atomic force microscopy (AFM) topo-
graphic images of the films of copolymer:PC61BM (1:1, w/w)
blends recorded in tapping mode. All the films exhibit typi-
cal cluster structures with many aggregated domains, and
root-mean-square (RMS) roughnesses of 1.80, 1.55 and
2.07 nm were obtained for PBDBT1, PBDBT2 and
PBDBT3, respectively. The larger roughness of
PBDBT3:PC61BM compared with the PBDBT1 and
PBDBT2 blend films shows the higher degree of ordered
structure, which indicates reduced internal series resistance
and more efficient charge separation in the PSCs. The film
surface with higher roughness and nanoscaled texture also
benefits the internal light scattering and enhances light ab-
sorption.[29] All this results in the higher Jsc value and effi-
ciency of the PBDBT3 device relative to the PBDBT1 and
PBDBT2 devices. Further detailed investigations on the
charge transfer and exciton separation and recombination
are in progress and will be reported elsewhere.
PC71BM has a similar electronic structure to PC61BM, but
a considerably stronger absorption in the visible region.[30]
Thus, the copolymer-based PSCs can be expected to exhibit
improved performance when PC71BM is used in place of
PC61BM as the acceptor because of the compensation for
the poor absorption of the copolymer:PC61BM blend in the
400–500 nm range. To understand the effect of PC71BM on
the photovoltaic properties, PSCs with different copoly-
mer:PC71BM ratios were fabricated and investigated. The
optimal ratio was found to be 1:1 for all the PC71BM-based
devices. The J–V and EQE curves of the PSCs are plotted in
Figure 8, and the photovoltaic parameters of Jsc, Voc, FF and
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Q. Peng, Z.-K. Chen et al.
PBDBT3, were synthesised by Stille coupling polymerisa-
tion. Their optical and electrochemical properties can be
facilely affected by adjusting the structures of the conjugat-
ed aryl-vinylene side chains attached to the BT units. UV/
Vis absorption and cyclic voltammetry measurements show
that PBDBT3 has the best light harvesting and smallest
bandgap of the three copolymers. The basic electronic struc-
tures of the D–A model compounds of these copolymers
were also studied by DFT calculations at the B3LYP/6-31G*
level of theory. PSCs based on these copolymers were fabri-
cated with a typical structure of ITO/PEDOT:PSS/copoly-
mer:PCBM/Ca/Al and illuminated with AM1.5G at
100 mWcmÀ2. The results showed that PBDBT3 devices
have higher efficiencies of 1.92 and 2.17% compared with
the PBDBT1 and PBDBT2 devices using PC61BM and
PC71BM as acceptors, respectively. The results presented
herein indicate that these copolymers are promising candi-
dates for roll-to-roll manufacturing of PSCs. Suitable elec-
tronic, optical and photovoltaic properties of BT-based co-
polymers can be achieved by fine-tuning the structures of
the aryl-vinylene side chains for application in PSCs.
Experimental Section
Figure 8. a) Current density–voltage curves of photovoltaic cells based on
copolymers and PC71BM under AM1.5 illumination and b) EQE curves
of photovoltaic cells based on copolymers and PC71BM.
Materials and instruments: NMR spectra were recorded on a Bruker
Avance-400 spectrometer with [D]chloroform as the solvent and tetrame-
thylsilane as the internal standard. Cyclic voltammetry was performed on
a CHI660 potentiostat/galvanostat electrochemical workstation at a scan
rate of 50 mVsÀ1 with a platinum wire counter electrode and a Ag/AgCl
reference electrode in an anhydrous nitrogen-saturated 0.1 m acetonitrile
solution of tetrabutylammonium perchlorate (nBu4NClO4). The copoly-
mers were coated on platinum plate working electrodes from dilute
chloroform solutions. UV/Vis spectra were obtained on a Carry 300 spec-
trophotometer. Thermogravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC) were performed on a TA Instrument Model SDT
Q600 simultaneous TGA/DSC analyser at a heating rate of 108CminÀ1
PCE are summarised in Table 3. As shown in Figure 8a, all
of the copolymer devices exhibit some improvement in the
PCEs, by 7.8–22%, compared with the corresponding
PC61BM-based devices. The PBDBT1 and PBDBT2 devices
show Voc values of 0.71 and 0.84 V, Jsc values of 5.51 and
3.98 mAcmÀ2, and FFs of 0.35 and 0.38, which results in
PCEs of 1.38 and 1.26%, respectively. The PBDBT3 device
still shows the highest PCE of 2.17% with a Voc of 0.66 V, a
Jsc of 6.61 mAcmÀ2 and a FF of 0.50. Figure 8b shows that
the poor absorption in the range of 400–500 nm is efficiently
compensated. Clearly, the increased absorption enhances
light harvesting when PC71BM is used instead of PC61BM,
which results in further improved device efficiency. The Voc
in these devices changes just a little, which indicates that the
Voc obtained has almost achieved its limit. The FF value re-
flects the fraction of photogenerated charge carriers, howev-
er, the FFs in this work are not very high. Further optimisa-
tion of the electrodes and device performance are underway
to gain more information.
and under a N2 flow rate of 90 mLminÀ1
.
Copolymer and [6,6]phenyl-C61-butyric acid methyl ester or [6,6]phenyl-
C71-butyric acid methyl ester (PC61BM or PC71BM) were dissolved in an-
hydrous 1,2-dichlorobenzene (DCB) in the desired blend ratio. The
blend solution was heated on a magnetic hotplate stirrer at 708C for 2 h,
followed by stirring at 408C overnight. The final concentration of the pol-
ymer in the blend ranged from 8–10 mgmLÀ1. Devices were prepared on
indium tin oxide (ITO) patterned glass substrates.
All reagents were purchased from Aladdin Reagent Co., Alfa Aesar Co.
and Aldrich Chemical Co. unless stated otherwise. Commercial chemicals
were used without further purification. PC61BM and PC71BM were pur-
chased from Nano-C, Inc. 4-Dodecyloxybenzaldehyde, 2-dodecyl-5-thio-
phenecarbaldehyde and 2,6-bis(trimethyltin)-4,8-bis(dodecyloxy)ben-
zo[1,2-b:3,4-b]dithiophene were synthesised according to the literature.[8a,
b,19a]
The ITO patterned glass substrates were first sonicated in a detergent
(Hellmanex) bath for 30 min, followed by two rounds of sonication in de-
ionised water for 10 min. This was followed by successive sonication in
acetone and isopropanol for 15 and 20 min, respectively. The substrates
were dried under flowing nitrogen and then placed in an oven at 808C
for 1 h. The substrates were subjected to UV ozone treatment for 10 min
before a PEDOT:PSS (CLEVIOS P VP Al 4083) layer of 40–45 nm
thickness was spin-coated onto the ITO surface. The PEDOT:PSS-coated
substrates were then annealed at 1208C for 10 min. Next, the active layer
was spin-coated in an inert glovebox (N2 atmosphere). Lastly, 25 nm of
calcium and 80 nm of aluminium were deposited on the active layer by
Conclusion
A new type of 2,1,3-benzothiadiazole (BT) acceptor with a
conjugated aryl-vinylene side chain has been designed and
used to build efficient low bandgap (LBG) photovoltaic co-
polymers. Three 2D-like donor (D)–acceptor (A) conjugat-
ed copolymers based on benzo[1,2-b:3,4-b’]dithiophene and
the resulting new BT derivatives, PBDBT1, PBDBT2 and
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Photovoltaic Properties of Benzothiadiazole Copolymers
FULL PAPER
thermal evaporation under vacuum at 10À6 Torr. The active area of the
device was 0.09 cm2. The thickness of the film was measured by using a
surface profiler (KLA-Tencor P10 surface profiler). Current density–volt-
age (J–V) measurements were carried out in an inert environment
(MBraun glovebox, N2 atmosphere) under 1 Sun (AM1.5G) conditions
by using a solar simulator (SAN-EI Electric XES-301S 300W Xe Lamp
JIS Class AAA) at an intensity of 100 mWcmÀ2. The external quantum
efficiency (EQE) as a function of wavelength was measured with a
home-built setup consisting of an Oriel 300-W Xe lamp in combination
with an Oriel Cornerstone 130 monochromator and a SRS 810 lock-in
amplifier (Stanford Research Systems). The number of photons incident
on the device was calculated for each wavelength by using a calibrated
silicon diode as reference.
rotary evaporation. The residues were purified by recrystallisation in tet-
rachloromethane to give pure compound 4 as a pale-yellow solid. Yield:
3.78 g (67%); m.p. 159–1608C; 1H NMR (400 MHz, CDCl3): d=7.96 (s,
1H), 4.74 ppm (s, 2H); 13C NMR (100 MHz, CDCl3): d=155.36, 152.52,
141.21, 135.55, 115.10, 113.23, 24.58 ppm; HRMS (EI+): m/z calcd for
C7H3N2SBr3: 383.7567 [M+]; found: 383.7562; elemental analysis calcd
(%) for C7H3N2SBr3: C 21.73, H 0.78, N 7.24, S 8.29; found: C 21.66, H
0.83, N 7.27, S 8.34.
(4,7-Dibromo-2,1,3-benzothiadiazol-5-ylmethyl)phosphonic acid diethyl
ester (5): A solution of compound 4 (4.06 g, 10.49 mmol) in triethyl phos-
phite (20 mL) was heated at reflux for 12 h. Excess triethyl phosphite
was removed by vacuum distillation. The residue was purified by column
chromatography (silica gel; eluent: ethyl acetate) to afford pure com-
pound
5 as a white solid. Yield: 1.95 g (41.1%); m.p. 112–1148C;
2-Dodecanoylthiophene (1): A solution of thiophene (2 g, 0.023 mol) in
freshly distilled methylene dichloride (70 mL) was added to a 250 mL
flask under an argon atmosphere. Dodecanoyl chloride (5.46 g,
0.026 mol) was then slowly added dropwise to the solution. The mixture
was stirred at room temperature for 30 min and then cooled to 08C. Sub-
sequently, anhydrous AlCl3 (5.46 g, 0.026 mol) was added in batches. The
mixture was warmed to room temperature and stirred overnight. Then
2m hydrochloric acid (100 mL) was added to end the reaction and the
mixture was extracted with CHCl2 (3ꢁ100 mL). The combined organic
layers were dried over MgSO4 and the solvents were removed by rotary
evaporation. Lastly, the crude compound was purified by column chro-
matography (silica gel; eluent: CH2Cl2/hexane, 1:1, v/v) and isolated as a
pale-yellow oil. Yield: 5.32 g (86%); 1H NMR (400 MHz, CDCl3): d=
7.70 (d, 1H), 7.61 (d, 1H), 7.12 (t, 1H), 2.88 (t, 2H), 1.78–1.72 (m, 2H),
1.35–1.26 (m, 16H), 0.89–0.86 ppm (t, 3H); 13C NMR (100 MHz, CDCl3):
d=193.46, 144.56, 133.25, 131.62, 128.02, 39.45, 31.92, 29.63, 29.50, 29.45,
29.36, 24.81, 22.70, 14.12 ppm; HRMS (EI+): m/z calcd for C16H26OS:
266.1704 [M+]; found: 266.1702; elemental analysis calcd (%) for
C16H26OS: C 72.12, H 9.84, S 12.03; found: C 72.08, H 9.86, S 12.08.
1H NMR (400 MHz, CDCl3): d=7.95 (d, 1H), 4.15–4.11 (d, 4H), 3.59–
3.54 (d, 2H), 1.32–1.29 ppm (t, 6H); 13C NMR (100 MHz, CDCl3): d=
153.4, 151.9, 135.9, 134.8, 115.3, 136.1, 62.6, 34.6, 33.2 ppm; HRMS (EI+):
m/z calcd for C11H13N2O3SPBr2: 441.8751 [M+]; found: 441.8755; elemen-
tal analysis calcd (%) for C11H13N2O3SPBr2: C 29.75, H 2.95, N 6.31, S
7.22; found: C 29.79, H 2.99, N 6.36, S 7.28.
4,7-Dibromo-5-(5-dodecyl-2-thienylvinyl)-2,1,3-benzothiadiazole
(6a):
Sodium methoxide (0.12 g, 2.1 mmol) was added slowly to a mixture of
compound 5 (0.85 g, 1.9 mmol) in N,N-dimethylformamide (7 mL) at
08C. After stirring for 5 min, 2-dodecyl-5-thiophenecarbaldehyde (0.54 g,
1.9 mmol) was added dropwise. The mixture was stirred for another 4 h
and then poured into saturated brine (100 mL). The solution was extract-
ed with CHCl2 (3ꢁ100 mL) and the combined organic layers were dried
over MgSO4. The solvents were evaporated to give the crude compound.
After purification by column chromatography (silica gel; eluent: CH2Cl2/
hexane, 1:1.5, v/v), compound 6a was obtained as a greenish yellow solid.
1
Yield: 0.69 g (63.5%); m.p. 111–1128C; H NMR (400 MHz, CDCl3): d=
8.14 (s, 1H), 7.35–7.28 (t, 2H), 7.06 (d, 1H), 6.74 (d, 1H), 2.83 (t, 2H),
1.71 (t, 2H), 1.36–1.23 (m, 18H), 0.88 ppm (t, 3H); 13C NMR (100 MHz,
CDCl3): d=154.0, 151.9, 148.6, 139.3, 138.3, 130.2, 129.1, 128.3, 125.2,
122.8, 113.4, 112.7, 31.9, 31.4, 30.6, 29.6, 29.5, 29.1, 22.7, 14.1 ppm; HRMS
(EI+): m/z calcd for C24H30N2S2Br2: 568.0217 [M+]; found: 568.0213; ele-
mental analysis calcd (%) for C24H30N2S2Br2: C 50.53, H 5.30, N 4.91, S
11.24; found: C 50.48, H 5.24, N 4.94, S 11.26.
2-Dodecanoyl-5-thiophenecarbaldehyde (2):
A mixture of 2-dodeca-
noylthiophene (1; 1.33 g, 5 mmol), N,N-dimethylformamide (1 mL) and
1,2-dichloroethane (8.4 mL) was cooled to 08C in an ice bath. Phospho-
rus oxychloride (1.2 mL) was slowly added dropwise. The reactant solu-
tion was then heated at reflux overnight. After cooling the mixture to
room temperature, a saturated aqueous solution of acetic acid (20 mL)
was added. The mixture was stirred for a further few hours and extracted
with CH2Cl2 (3ꢁ100 mL). The combined organic layers were dried over
MgSO4 and the solvents removed by rotary evaporation. The residue was
purified by column chromatography (silica gel; eluent: CH2Cl2/hexane,
1:1.5, v/v) to give pure compound 2 as a pale-yellow oil. Yield: 1.23 g
4,7-Dibromo-5-(5-dodecanoyl-2-thienylvinyl)-2,1,3-benzothiadiazole (6b):
Compound 6b was obtained as a yellow solid in a yield of 77% from the
reaction of 5 with 2-dodecanoyl-5-thiophenecarbaldehyde (2) similar to
the procedure described for 6a. M.p. 88–898C; 1H NMR (400 MHz,
CDCl3): d=7.90 (s, 1H, Ph-H), 7.50 (d, 1H), 7.29 (d, 1H), 7.22 (d, 1H),
7.10 (d, 1H), 2.79 (m, 2H), 1.66 (m, 2H), 1.49–1.27 (m, 16H), 0.86 ppm
(d, 3H); 13C NMR (100 MHz, CDCl3): d=152.9, 151.0, 139.5, 137.5,
137.0, 135.2, 131.6, 129.4, 127.9, 127.3, 126.0, 125.9, 112.7, 30.9, 30.4, 28.8,
28.7, 28.6, 28.4, 28.3, 28.2, 24.6, 21.7, 13.1 ppm; HRMS (EI+): m/z calcd
for C24H28N2OS2Br2: 582.0010 [M+]; found: 582.0013; elemental analysis
calcd (%) for C24H28N2OS2Br2: C 49.32, H 4.83, N 4.79, S 10.97; found: C
49.35, H 4.86, N 4.82, S 11.05.
1
(83.4%); H NMR (400 MHz, CDCl3): d=9.67 (s, 1H), 7.56 (d, 1H), 7.19
(d, 1H), 2.56 (t, 2H), 1.78–1.72 (m, 2H), 1.48–1.27 (m, 18H), 0.88 ppm
(t, 3H); 13C NMR (100 MHz, CDCl3): d=189.9, 141.64, 138.2, 132.4,
130.6, 127.1, 31.9, 29.7, 29.6, 29.4, 29.3, 28.6, 27.8, 22.7, 14.1 ppm; HRMS
(EI+): m/z calcd for C17H26O2S: 294.1654 [M+]; found: 294.1658; elemen-
tal analysis calcd (%) for C17H26O2S: C 69.34, H 8.90, S 10.89; found: C
69.37, H 8.94, S 10.96.
4,7-Dibromo-5-methyl-2,1,3-benzothiadiazole (3): A solution of 5-methyl-
2,1,3-benzothiadiazole (3 g, 19.97 mmol) in 47% HBr (37.5 mL) was
heated to 1258C, and then bromine (3.06 mL, 59.91 mmol) in HBr
(3 mL) was slowly added dropwise. The mixture was stirred for 6 h and
then cooled to room temperature. A saturated sodium bisulfite aqueous
solution was then added to remove the excess bromine. The precipitate
was filtered and recrystallised in ethanol to afford pure compound 3 as a
pale-yellow solid. Yield: 5.08 g (82.6%); m.p. 150–1518C; 1H NMR
(400 MHz, CDCl3): d=7.76 (s, 1H), 2.61 ppm (s, 3H); 13C NMR
(100 MHz, CDCl3): d=153.42, 151.53, 140.42, 135.14, 114.21, 112.60,
22.64 ppm; HRMS (EI+): m/z calcd for C7H4N2SBr2: 305.8462 [M+];
found: 305.8467; elemental analysis calcd (%) for C7H4N2SBr2: C 27.30,
H 1.31, N 9.10, S 10.41; found: C 27.35, H 1.38, N 9.16, S 10.46.
4,7-Dibromo-5-(4-dodecyloxyphenylvinyl)-2,1,3-benzothiadiazole
(6c):
Compound 6c was obtained as a yellow solid in a yield of 54% from the
reaction of 5 with 4-dodecyloxybenzaldehyde similar to the procedure de-
scribed for 6a. M.p. 124–1258C; 1H NMR (400 MHz, CDCl3): d=7.67 (s,
1H), 7.11 (d, 2H), 6.94 (d, 1H), 6.85 (d, 1H), 6.76 (d, 2H), 3.93 (t, 2H),
1.76 (t, 2H), 1.45–1.25 (m, 18H), 0.87 ppm (t, 3H); 13C NMR (100 MHz,
CDCl3): d=160.2, 138.8, 134.9, 134.6, 134.1, 130.6, 130.5, 128.8, 128.2,
125.0, 122.7, 115.0, 114.6, 113.4, 68.2, 31.9, 29.7, 29.6, 29.4, 29.2, 26.1, 22.7,
14.1 ppm; HRMS (EI+): m/z calcd for C26H32N2OSBr2: 578.0602 [M+];
found: 578.0605; elemental analysis calcd (%) for C26H32N2OSBr2: C
53.80, H 5.56, N 4.83, S 5.52; found: C 53.85, H 5.62, N 4.88, S 5.59.
4,7-Bis(4-dodecyl-2-thienyl)-5-(5-dodecyl-2-thienylvinyl)-2,1,3-benzothia-
diazole (7a): A solution of compound 6a (1.83 g, 3.2 mmol) and 3-dodec-
yl-5-(tributylstannyl)thiophene (4.72 g, 8.7 mmol) in freshly distilled THF
(32 mL) was degassed. The mixture was heated at reflux under an argon
4,7-Dibromo-5-bromomethyl-2,1,3-benzothiadiazole (4):
A mixture of
compound (4.49 g, 14.6 mmol), benzoyl peroxide (BPO; 90 mg,
3
0.39 mmol) and N-bromosuccinimide (NBS; 2.6 g, 14.6 mmol) in tetra-
chloromethane (86 mL) was heated at reflux for 17 h under an argon at-
mosphere. The solution was filtered and the filtrate concentrated by
atmosphere and then [PdCl2ACTHUNGTER(UNNG PPh3)2] (45 mg, 0.06 mmol) was added.
After 12 h, the THF was removed by vacuum distillation. The residue
was purified by column chromatography (silica gel; eluent: CH2Cl2/
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Q. Peng, Z.-K. Chen et al.
hexane, 1:3, v/v) to give pure compound 7a as a brick-red solid. Yield:
2.27 g (78%); m.p. 87–888C; 1H NMR (400 MHz, CDCl3): d=8.13 (s,
1H), 8.00 (s, 1H), 7.37–7.33 (d, 1H), 7.28 (s, 1H), 7.22 (s, 1H), 7.19 (s,
1H), 7.07 (s, 1H), 6.98 (d, 1H), 6.71 (d, 1H), 2.79 (t, 2H), 2.71 (t, 4H),
1.73–1.66 (m, 6H), 1.37–1.26 (m, 54H), 0.88 ppm (m, 9H); 13C NMR
(100 MHz, CDCl3): d=155.7, 151.7, 147.1, 144.4, 143.1, 140.4, 138.7,
136.1, 135.9, 132.1, 129.4, 127.3, 126.3, 125.7, 125.1, 124.9, 124.4, 123.3,
122.7, 121.6, 31.9, 31.6, 30.7, 30.6, 30.5, 29.8, 29.7, 29.6, 29.5, 29.4, 29.2,
22.7, 14 ppm; HRMS (MALDI): m/z calcd for C56H84N2S4: 913.5595
[M+H]+; found: 913.5588; elemental analysis calcd (%) for C56H84N2S4:
C 73.63, H 9.27, N 3.07, S 14.04; found: C 73.66, H 9.33, N 3.14, S 14.10.
C56H80N2OS4Br2: C 61.97, H 7.43, N 2.58, S 11.82; found: C 61.93, H 7.41,
N 2.63, S 11.88.
4,7-Bis(5-bromo-4-dodecyl-2-thienyl)-5-(4-dodecyloxyphenylvinyl)-2,1,3-
benzothiadiazole (8c): Compound 8c was obtained as a red solid in a
yield of 73% from the reaction of 7c with NBS similar to the procedure
described for 8a. M.p. 52–548C; 1H NMR (400 MHz, CDCl3): d=8.10 (s,
1H), 7.82 (s, 1H), 7.46 (d, 2H), 7.36 (d, 1H), 7.23 (d, 1H), 7.09 (s, 1H),
6.91 (d, 2H), 3.98 (t, 2H), 2.64 (t, 4H), 1.80 (t, 2H), 1.69–1.63 (m, 4H),
1.47–1.25 (m, 54H), 0.87 ppm (t, 9H); 13C NMR (100 MHz, CDCl3): d=
159.0, 152.1, 151.4, 142.9, 139.3, 138.3, 137.9, 137.2, 135.5, 132.5, 130.5,
129.2, 128.1, 127.7, 126.8, 124.2, 118.3, 118.2, 113.8, 110.8, 68.3, 35.5, 32.0,
31.9, 29.8, 29.7, 29.5, 29.4, 29.3, 29.1, 26.5, 26.2, 22.9, 22.7, 14.1 ppm;
HRMS (MALDI-TOF): m/z calcd for C58H84N2OS3Br2: 1079.4191
[M+H]+; found: 1079.4183; elemental analysis calcd (%) for
C58H84N2OS3Br2: C 64.42, H 7.83, N 2.59, S 8.90; found: C 64.46, H 7.85,
N 2.67, S 8.99.
4,7-Bis(4-dodecyl-2-thienyl)-5-(5-dodecanoyl-2-thienylvinyl)-2,1,3-benzo-
thiadiazole (7b): Compound 7b was obtained as a red oil in a yield of
74% from the reaction of 6b with 3-dodecyl-5-(tributylstannyl)thiophene
similar to the procedure described for 7a. 1H NMR (400 MHz, CDCl3):
d=7.99 (d, 1H), 7.95 (s, 1H), 7.38 (d, 1H), 7.23 (t, 1H), 7.17 (m, 2H),
7.10 (m, 1H), 7.06–7.02 (m, 2H), 2.72–2.61 (m, 6H), 1.70 (m, 6H), 1.36–
1.26 (m, 52H), 0.92 ppm (t, 9H); 13C NMR (100 MHz, CDCl3): d=155.8,
151.8, 151.7, 144.4, 143.2, 141.0, 138.8, 138.5, 136.6, 135.7, 131.8, 130.4,
129.9, 129.5, 128.5, 128.0, 127.7, 126.8, 124.4, 122.6, 121.6, 32.0, 31.2, 30.7,
30.6, 30.5, 30.4, 29.9, 29.7, 29.6, 29.5, 29.4, 29.3, 28.3, 28.2, 26.8, 22.7,
14.2 ppm; HRMS (MALDI): m/z calcd for C56H82N2OS4: 927.5388
[M+H]+; found: 927.5392; elemental analysis calcd (%) for
C56H82N2OS4: C 72.52, H 8.91, N 3.02, S 13.83; found: C 72.49, H 8.93, N
3.06, S 13.86.
PBDBT1: 2,6-Bis(trimethyltin)-4,8-bis(dodecyloxy)benzo[1,2-b:3,4-b]di-
thiophene (0.265 mg, 0.3 mmol) and compound 8a (0.321 g, 0.3 mmol)
were dissolved in toluene (15 mL). The solution was flushed with argon
for 10 min and then [Pd2dba3] (5.5 mg, 2 mol%) and PACTHUNTRGNEU(GN o-tolyl)3 (7.3 mg,
8%) were added to the flask. The flask was purged three times by succes-
sive vacuum and argon filling cycles. The polymerisation reaction mixture
was heated at 1108C with stirring for 72 h under an argon atmosphere. 2-
Tributylstannylthiophene (23.7 mL) was added to the reaction and then
after 2 h, 2-bromothiophene (7.5 mL) was added. The mixture was stirred
overnight to complete the end-capping reaction. It was then cooled to
room temperature and then slowly poured into methanol (350 mL). The
precipitate was filtered and washed with methanol and hexane in a Soxh-
let apparatus to remove oligomers and catalyst residue. Finally, the poly-
mer was extracted with chloroform. The solution was condensed by evap-
oration and precipitated into methanol. The polymer was collected as a
dark-purple solid. Yield: 0.27 g (73%); 1H NMR (400 MHz, CDCl3): d=
8.11–8.05 (br, 2H), 7.52 (br, 2H), 7.33 (br, 3H), 6.98 (br, 1H), 6.70 (br,
1H), 4.28 (br, 4H), 2.93–2.65 (br, 6H), 1.86–1.64 (br, 10H), 1.19 (br,
90H), 0.78 ppm (br, 15H); 13C NMR (100 MHz, CDCl3): d=152.8, 150.8,
148.8, 148.0, 147.1, 145.4, 143.3, 143.1, 140.8, 139.6, 137.2, 135.6, 134.7,
133.2, 132.7, 131.5, 131.3, 130.8, 129.2, 126.3, 125.7, 124.4, 123.0, 122.0,
117.6, 114.0, 110.8, 73.0, 30.9, 30.6, 30.0, 29.7, 28.7, 28.4, 28.2, 25.2, 21.7,
4,7-Bis(4-dodecyl-2-thienyl)-5-(4-dodecyloxyphenylvinyl)-2,1,3-benzothia-
diazole (7c): Compound 7c was obtained as a brick-red solid in a yield
of 76% from the reaction of 6c with 3-dodecyl-5-(tributylstannyl)thio-
1
phene similar to the procedure described for 7a. M.p. 91–938C; H NMR
(400 MHz, CDCl3): d=8.20 (s, 1H), 8.01 (s, 1H), 7.45 (t, 2H), 7.41 (s,
1H), 7.24 (d, 2H), 7.18 (s, 1H), 7.07 (s, 1H), 6.90 (d, 2H), 3.98 (t, 2H),
2.74–2.68 (m, 4H), 1.80–1.69 (m, 6H), 1.46–1.26 (m, 54H), 0.89–0.86 ppm
(m, 9H); 13C NMR (100 MHz, CDCl3): d=159.4, 155.8, 151.7, 144.4,
143.1, 138.8, 136.7, 136.0, 131.9, 131.8, 129.6, 129.4, 128.2, 126.3, 124.7,
124.5, 123.2, 122.5, 121.6, 31.9, 30.7, 30.6, 30.5, 29.7, 29.6, 29.5, 29.4, 29.3,
26.1, 22.7, 14.1 ppm; HRMS (MALDI): m/z calcd for C58H86N2OS3:
923.5980 [M+H]+; found: 923.5975; elemental analysis calcd (%) for
C58H86N2OS3: C 75.43, H 9.39, N 3.03, S 10.42; found: C 75.47, H 9.44, N
3.08, S 10.49.
˜
13.0 ppm; IR (KBr): n=2923, 2851, 2340, 1638, 1458, 1384, 1261, 1116,
800, 621 cmÀ1; elemental analysis calcd (%) for (C90H134N2O2S6)n: C
73.61, H 9.20, N 1.91, S 13.10; found: C 73.18, H 9.33, N 2.12, S 13.32.
4,7-Bis(5-bromo-4-dodecyl-2-thienyl)-5-(5-dodecyl-2-thienylvinyl)-2,1,3-
benzothiadiazole (8a): NBS (0.56 g, 3.12 mmol) was added to a solution
of compound 7a (1.43 g, 1.56 mmol) in chloroform (25 mL). The mixture
was then stirred in the dark for 4 h at room temperature. The solvent was
removed by vacuum distillation. The residue was purified by column
chromatography (silica gel; eluent: CH2Cl2/hexane, 1:8, v/v) to give pure
PBDBT2: Copolymer PBDBT2 was obtained as a dark-purple solid in a
yield of 80% from the reaction of 8b with 2,6-bis(trimethyltin)-4,8-bis(-
dodecyloxy)benzo[1,2-b:3,4-b]dithiophene similar to the procedure de-
scribed for copolymer PBDBT1. 1H NMR (400 MHz, CDCl3): d=8.01
(br, 2H), 7.50 (br, 2H), 7.41 (br, 3H), 7.07–7.00 (br, 2H), 4.28 (br, 4H),
2.94–2.65 (br, 6H), 1.87–1.76 (br, 10H), 1.18 (br, 90H), 0.80 ppm (br,
15H); 13C NMR (100 MHz, CDCl3): d=154.4, 152.8, 152.0, 148.8, 145.6,
145.2, 144.8, 144.0, 142.9, 139.7, 138.6, 137.3, 136.9, 134.9, 134.0, 133.3,
132.8, 131.1, 130.8, 130.0, 129.0, 128.4, 127.6, 127.4, 126.8, 124.1, 123.3,
122.0, 121.2, 117.8, 112.4, 73.0, 30.9, 30.0, 29.7, 28.7, 28.4, 25.2, 21.7,
13.1 ppm; IR (KBr): n˜ =2922, 2850, 2340, 1742, 1637, 1569, 1459, 1363,
compound 8a as
a red solid. Yield: 1.15 g (69%); m.p. 60–618C;
1H NMR (400 MHz, CDCl3): d=8.02 (s, 1H), 7.81 (s, 1H), 7.33 (d, 1H),
7.18 (d, 1H), 7.07 (s, 1H), 6.99 (d, 1H), 6.73 (d, 1H), 2.80 (t, 2H), 2.65
(t, 4H), 1.70–1.65 (m, 6H), 1.37–1.26 (m, 54H), 0.87 ppm (t, 9H);
13C NMR(100 MHz, CDCl3): d=152.7, 150.9, 144.7, 142.4, 141.6, 140.2,
138.8, 133.3, 132.5, 131.9, 131.4, 124.0, 123.2, 122.9, 122.8, 114.5, 111.7,
110.8, 38.0, 24.8, 32.0, 31.5, 30.4, 30.1, 29.7, 29.6, 29.5, 29.4, 29.3, 29.2,
29.7, 27.9, 22.7, 14.1 ppm; HRMS (MALDI-TOF): m/z calcd for
C56H82N2S4Br2: 1069.3806 [M+H]+; found: 1069.3811; elemental analysis
calcd (%) for C56H82N2S4Br2: C 62.78, H 7.71, N 2.61, S 11.97; found: C
62.83, H 7.75, N 2.66, S 12.05.
1261, 1101, 1049, 798, 619 cmÀ1
; elemental analysis calcd (%) for
(C90H132N2O3S6)n: C 72.92, H 8.98, N 1.89, S 12.98; found: C 72.49, H
9.16, N 2.02, S 13.14.
PBDBT3: Copolymer PBDBT3 was obtained as a dark-purple solid in a
yield of 91% from the reaction of 8c with 2,6-bis(trimethyltin)-4,8-bis(-
dodecyloxy)benzo[1,2-b:3,4-b]dithiophene similar to the procedure de-
4,7-Bis(5-bromo-4-dodecyl-2-thienyl)-5-(5-dodecanoyl-2-thienylvinyl)-
2,1,3-benzothiadiazole (8b): Compound 8b was obtained as a red solid in
a yield of 73% from the reaction of 7b with NBS similar to the proce-
dure described for 8a. M.p. 39–408C; 1H NMR (400 MHz, CDCl3): d=
7.84 (d, 1H), 7.71 (d, 1H), 7.49–7.41 (dd, 1H), 7.37–7.34 (d, 1H), 7.13
(m, 1H), 7.10–7.05 (m, 2H), 2.67–2.61 (m, 6H), 1.65–1.61 (m, 6H), 1.37–
1.25 (m, 52H), 0.87 ppm (t, 9H); 13C NMR (100 MHz, CDCl3): d=155.3,
151.5, 151.3, 143.1, 142.1, 140.9, 138.4, 138.1, 136.7, 135.6, 131.5, 131.2,
129.9, 128.6, 127.8, 127.3, 126.9, 125.6, 124.2, 123.0, 112.0, 31.9, 31.2, 30.3,
29.9, 29.8, 29.7, 29.6, 29.5, 29.4, 29.3, 29.2, 29.0, 28.3, 22.7, 14.1 ppm;
HRMS (MALDI-TOF): m/z calcd for C56H80N2OS4Br2: 1083.3598
[M+H]+; found: 1083.3592; elemental analysis calcd (%) for
1
scribed for copolymer PBDBT1. H NMR (CDCl3, 400 Hz): d=8.19–8.05
(br, 2H), 7.48–7.27 (br, 7H), 6.88 (br, 2H), 4.26–3.94 (br, 6H), 2.94–2.84
(br, 4H), 1.82–1.75 (br, 10H), 1.20 (br, 90H), 0.80 ppm (br, 15H);
13C NMR (CDCl3, 100 MHz): d=158.8, 152.7, 150.6, 144.4, 142.9, 142.5,
140.8, 139.8, 139.4, 137.1, 135.2, 134.8, 134.0, 133.3, 132.7, 132.1, 131.5,
131.1, 130.4, 129.4, 129.1, 127.4, 126.7, 125.4, 124.2, 121.7, 120.8, 117.5,
114.0, 111.0, 73.0, 67.2, 30.9, 30.0, 29.7, 28.7, 28.4, 21.7, 25.2, 13.1 ppm; IR
(KBr): n˜ =2923, 2852, 2340, 1637, 1458, 1382, 1262, 1168, 887, 852, 815,
622 cmÀ1; elemental analysis calcd (%) for (C92H136N2O3S5)n: C 74.74, H
9.27, N 1.89, S 10.84; found: C 74.32, H 9.03, N 2.06, S 10.98.
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ÝÝ
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Photovoltaic Properties of Benzothiadiazole Copolymers
FULL PAPER
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Adv. Mater. 2009, 21, 2093–2097.
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This work was financially supported by the NSFC (No: 20802033), the
Program for New Century Excellent Talents in University (No: NCET-
10-0170), Young Scientist of Jing Gang Zhi Xing Project of Jiangxi Prov-
ince (No: 2008DQ00700), NSFJX (No: 2010GZH0110), the Scientific Re-
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Received: March 5, 2012
Published online: && &&, 0000
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
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These are not the final page numbers!

Photovoltaic Properties of Benzothiadiazole Copolymers
FULL PAPER
2D Photovoltaic copolymers: New
benzothiadiazole acceptors with differ-
ent conjugated aryl-vinylene side
chains have been designed and used to
build 2D-like low-bandgap copolymers
(see figure). It was found that their
optical and electrochemical properties
can be facilely modified by changing
the structures of the conjugated aryl-
vinylene side chains. The photovoltaic
properties imply that all the prepared
copolymers are promising candidates
for roll-to-roll manufacturing of effi-
cient polymer solar cells.
Polymer Solar Cells
Q. Peng,* S.-L. Lim, I. H.-K. Wong,
J. Xu, Z.-K. Chen* ........... &&&& — &&&&
Synthesis and Photovoltaic Properties
of Two-Dimensional Low-Bandgap
Copolymers Based on New Benzothia-
diazole Derivatives with Different
Conjugated Aryl-vinylene Side Chains
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!