two dimensional photovoltaic copolymers based on new benzothiadiazole acceptors with...
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Cite this: Polym. Chem., 2012, 3, 2933
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Two dimensional photovoltaic copolymers based on new benzothiadiazoleacceptors with diphenylamine-vinylene side chains
Qiang Peng,*ac Yingying Fu,b Xiangju Liu,c Jun Xuc and Zhiyuan Xie*b
Received 16th July 2012, Accepted 30th July 2012
DOI: 10.1039/c2py20529g
A novel acceptor of 2,1,3-benzothiadiazole (BT) with diphenylamine-vinylene side chains was firstly
designed and synthesized for building efficient low bandgap (LBG) photovoltaic copolymers in this
paper. Based on benzo[1,2-b;3,4-b]dithiophene and the designed new BT acceptor, two dimensional
(2D) like conjugated copolymers have been prepared by Stille coupling polymerization. The resulting
copolymers were characterized by NMR, elemental analysis, gel permeation chromatography,
differential scanning calorimetry and thermogravimetric analysis. UV-vis absorption and cyclic
voltammetry measurements indicate that these copolymers have good optical and electrochemical
properties. A copolymer with thiophene modified benzo[1,2-b;3,4-b]dithiophene exhibits better light
harvesting and a smaller bandgap of the two copolymers. The electronic structures of the model
compounds were also studied by DFT calculations at the B3LYP/6-31G* level. The polymer solar cells
(PSCs) were fabricated and measured with a typical structure of ITO/PEDOT:PSS/copolymer:PCBM/
Ca/Al. The primary results showed that the device based on the copolymer with thiophene modified
benzo[1,2-b;3,4-b]dithiophene had a higher efficiency of 1.88%, attributed to the smaller bandgap and
improved active film morphology. All the prepared copolymers are promising candidates for efficient
PSCs with high Voc.
Introduction
Over the past few years, there has been increasing interest in
polymer solar cells (PSCs) due to the growing demand for clean
and renewable energy. Compared to the commercial inorganic
solar cells, PSCs are expected to be cheaper because they can be
fabricated at a much lower cost with the aid of large area solution
casting or roll-to-roll manufacturing of flexible modules.1 The
power conversion efficiency (PCE) of the PSCs achieved a
significant improvement due to the introduction of the so-called
bulk heterojunction (BHJ) concept, where an active blend con-
sisted of an interpenetrating network of electron-donating
polymer donors and electron-withdrawing fullerene derivatives.2
In earlier times, the regioregular poly(3-hexylthiophene) (P3HT)
was the most extensively investigated polymer donor with high
charge carrier mobility and strong absorption in the visible
region.3 However, the PCEs of these PSCs based on P3HT as
donors and PCBM as acceptors are limited at 4–5%, because
P3HT has a relatively large bandgap and a high highest occupied
aCollege of Chemistry, Sichuan University, Chengdu 610064, P. R. China.E-mail: [email protected]; Fax: +86-028-86510868; Tel: +86-028-86510868bState Key Laboratory of Polymer Physics and Chemistry, ChangchunInstitute of Applied Chemistry, Chinese Academy of Sciences,Changchun 130022, P. R. China. E-mail: [email protected] of Environmental and Chemical Engineering, Nanchang HangkongUniversity, Nanchang 330063, P. R. China
This journal is ª The Royal Society of Chemistry 2012
molecular orbital (HOMO) energy level.4 To address these issues,
low bandgap (LBG) polymers were developed to capture more
solar photons and achieve higher PCEs.5 The push–pull effect of
the donor and the acceptor in this system will form an intra-
molecular charge transfer (ICT) and further result in the lower
energy absorption band.6 Actually, PCEs over 7% were
successfully obtained by using low bandgap copolymers in BHJ
solar cells as electron-donating materials.7
Most of the above-mentioned LBG copolymers focused on the
main chain system. In the past few years, two dimensional (2D)
like conjugated polymers with conjugated side chains have been
designed and synthesized to improve the absorption, charge
transport, as well as their device performance.8 The ‘‘side chain
conjugation’’ concept can also facilely adjust the energy levels of
the pristine copolymers via conjugated bridges,9 and enhance the
miscibility with fullerene acceptors in BHJ device fabrication.10
In order to broaden the absorption spectrum, Li and his
co-workers firstly reported some polythiophenes modified with
conjugated side chains of bi(phenylenevinylene),8a bi(thienyle-
nevinylene)8b,c and phenothiazinevinylene.8d Subsequently,
triphenylamine,9a carbazole,9b phenothiazine9c and phenan-
threnylimidazole9d,e were designed and integrated on the thio-
phene units as conjugated side chains. Huang et al. recently
attached the strong electron-withdrawing groups onto the end of
the side chains using the electron-donating conjugated bridges,
resulting in a novel two dimensional D–A copolymer system.10
By this strategy, the copolymers based on fluorene,11
Polym. Chem., 2012, 3, 2933–2940 | 2933
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silafluorene,12 carbazole,13 phenothiazine,13b dithienopyrrole13b
and cyclopentadithiophene14 with different pendant acceptor
groups of malononitrile and 1,3-diethyl-2-thiobarbituric acid
were designed and prepared for PSCs. As mentioned above, most
of the conjugated side chains were combined with the donor units
in the reported D–A copolymers, it still remains a challenge to
explore novel 2D like acceptors and understand the relationship
of their structures and properties.
In this paper, we designed and synthesized two 2D like low
bandgap copolymers, PBTBD1 and PBTBD2 (Scheme 1), with
novel 2,1,3-benzothiadiazole (BT) acceptors modified by conju-
gated diphenylamine-vinylene side chains for efficient polymer
solar cells. BT was chosen here as the parent acceptor skeleton
because its derivatives were the most promising blocks for
building high-performance polymeric donor materials.5d,15 At
present, almost all the work is focused on naked BT or alkyl and
alkoxyl modified BT framework, however, some of the impor-
tant properties of BT units modified by conjugated side chains
have not been explored fully. Thus, it is reasonable to expect that
the prepared 2D like copolymers would have good photovoltaic
properties.
Experimental part
Characterization
NMR spectra were recorded on a Bruker Avance-400 spec-
trometer with d-chloroform as the solvent and tetramethylsilane
as the internal standard. Differential scanning calorimetry (DSC)
and thermogravimetric analysis (TGA) measurements were
conducted on a TA Instrument Model SDT Q600 simultaneous
TGA/DSC analyzer at a heating rate of 10 �Cmin�1 and under a
N2 flow rate of 90 mL min�1. Cyclic voltammetry (CV)
measurements were made on an CHI660 potentiostat/galvano-
stat electrochemical workstation at a scan rate of 50 mV s�1, with
a platinum wire counter electrode and an Ag/AgCl reference
electrode in an anhydrous nitrogen-saturated acetonitrile solu-
tion (0.1 mol L�1) of tetrabutylammonium perchlorate (Bu4N-
ClO4). The copolymers were coated on the platinum plate
working electrodes from dilute chloroform solutions. UV-vis
spectra were obtained on a Carry 300 spectrophotometer. Poly-
mer solar cells were fabricated with ITO glass as an anode, Ca/Al
Scheme 1 Structures of the LBG cop
2934 | Polym. Chem., 2012, 3, 2933–2940
as a cathode and the film blend of the copolymer and [6,6]-
phenyl-C71-butyric acid methyl ester (PC71BM) as the photo-
sensitive layer. The photosensitive layer was prepared by
spin-coating a solution blend of the copolymer and PC71BM in
chlorobenzene on the ITO/PEDOT:PSS electrode. The current
density–voltage (J–V) characterization of the devices was carried
out on a computer-controlled Keithley 236 Source Measurement
system. A solar simulator was used as the light source, and the
light intensity was monitored by a standard Si solar cell. The
thickness of the films was measured using a Dektak 6 M surface
profilometer.
Materials
All reagents were purchased from Aladdin Co., Alfa Aesar Co.
and Aldrich Chemical Co., and used without further purification.
4,7-Dibromo-5-bromomethyl-2,1,3-benzothiadiazole (3), (4,7-
dibromo-2,1,3-benzothiadiazole-5-yl methyl) phosphonic acid
diethyl ester (4), 2,6-bis(tributyltin)-4,8-didodecyloxybenzo[1,2-
b;3,4-b]dithiophene, 2,6-bis(tributyltin)-4,8-bis(4-dodecyl thienyl)
benzo[1,2-b;3,4-b]dithiophene were synthesized according to the
previous literature.16,5d
Synthesis of the monomers
N-Dodecyldiphenylamine (1). Diphenylamine (6.00 g, 35.50
mmol), sodium hydroxide (14.20 g, 35.50 mmol) and dimethyl
sulfoxide (80 mL) were placed in a three-necked flask. After the
mixture was stirred for 30 min, dodecyl bromide (9.37 g, 39.05
mmol) was added dropwise to the reaction mixture slowly and
then the mixture was stirred for another 24 h at room tempera-
ture. The reaction mixture was poured into water and extracted
with methylene dichloride. Afterwards, the aqueous layer was
extracted with methylene dichloride three times. The combined
extracts were dried over anhydrous MgSO4 and evaporated. The
crude liquid was purified by column chromatography using
methylene dichloride and hexane (1 : 5) as the eluent to give 1 as
a light yellow oil (11.90 g, 99.4%). 1H NMR (CDCl3, 400 Hz,
d/ppm): 7.25 (m, 4H), 6.97 (d, 4H), 6.92 (t, 2H), 3.67 (t, 2H), 1.65
(t, 2H), 1.29–1.25 (m, 18H), 0.88 (t, 3H). 13C NMR (CDCl3,
100 MHz, d/ppm): 148.1, 129.2, 121.0, 120.9, 52.4, 31.9, 29.7,
olymers PBTBD1 and PBTBD2.
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29.6, 29.5, 29.4, 27.5, 27.1, 22.7, 14.1. HRMS calcd for C24H35N
337.2770; found 337.2763.
4-Formyl-N-dodecyldiphenylamine (2). The solution of anhy-
drous N,N-dimethylformamide (32.30 mL, 387.60 mmol) was
cooled at 0 �C in an ice bath. Phosphorus oxychloride (10.30 mL,
105.94 mmol) was added dropwise to the above solution over
30 min. After that, compound 1 (11.90 g, 35.27 mmol) in 30 mL
of 1,2-dichloroethane was added to the mixture and heated to
90 �C for 48 h. This solution was cooled to room temperature,
poured into ice water and neutralized to pH ¼ 7 by the dropwise
addition of 2 M aqueous sodium hydroxide solution. The
mixture was then extracted with methylene dichloride several
times. The combined organic layer was dried over anhydrous
MgSO4 and evaporated. The compound 2 was purified using
methylene dichloride and hexane (1 : 3) as the eluent by column
chromatography to give a yellow liquid (7.05 g, 54.7%). 1HNMR
(CDCl3, 400 Hz, d/ppm): 9.73 (s, 1H), 7.65 (d, 2H), 7.44 (t, 2H),
7.29 (d, 1H), 7.21 (d, 2H), 6.69 (d, 2H), 3.71 (t, 2H), 1.67 (t, 2H),
1.30–1.25 (m, 18H), 0.88 (t, 3H). 13C NMR (CDCl3, 100 MHz,
d/ppm): 190.2, 153.4, 145.8, 131.8, 130.1, 127.6, 126.4, 113.3,
52.1, 31.9, 29.6, 29.4, 29.3, 27.2, 26.9, 22.7, 14.1. HRMS calcd for
C25H35NO 365.2719; found 365.2713.
4,7-Dibromo-5-(N-dodecyldiphenylamine-vinylene)-2,1,3-
benzothiadiazole (5). Sodium methoxide (0.41 g, 7.44 mmol) was
added slowly to the mixture of compound 4 (3.00 g, 6.76 mmol)
in 20 mL N,N-dimethylformamide at 0 �C. After stirring for
5 minutes, compound 2 (2.47 g, 6.76 mmol) was added dropwise.
The reactant mixture was stirred for another 4 hours and poured
into saturated brine. The solution was extracted with methylene
dichloride and the combined organic layers were dried over
anhydrous MgSO4. The solvents were evaporated off to give the
crude compound. After purification by column chromatography
using methylene dichloride and hexane (1 : 1.5) as the eluent,
compound 5 was obtained as a greenish yellow solid (0.8 g,
18.0%). 1H NMR (CDCl3, 400 Hz, d/ppm): 8.17 (s, 1H), 7.42 (t,
2H), 7.37 (t, 3H), 7.18–7.13 (m, 4H), 6.82 (d, 2H), 3.71 (t, 2H),
1.68 (d, 2H), 1.31–1.25 (m, 18H), 0.88 (t, 3H). 13C NMR (CDCl3,
100 MHz, d/ppm): 154.1, 151.8, 149.3, 146.9, 139.0, 135.0, 130.5,
129.7, 128.5, 126.3, 125.3, 124.3, 121.1, 116.5, 113.2, 112.2, 52.5,
31.9, 29.6, 29.4, 29.3, 27.4, 27.1, 22.7, 14.1. HRMS calcd for
C32H37Br2N3S 653.1075; found 653.1070.
4,7-Di(3-dodecyl-2-thienyl)-5-(N-dodecyldiphenylamine-vinyl-
ene)-2,1,3-benzothiadiazole (6). A solution of compound 5
(2.09 g, 3.20 mmol) and 3-dodecyl-5-(tributylstannyl)thiophene
(4.72 g, 8.70 mmol) in freshly distilled THF (32 mL) was
degassed. The mixture was heated to reflux under an argon
atmosphere and then dichlorobis(triphenylphosphine)palladiu-
m(II) [PdCl2(PPh3)2] (45 mg, 0.06 mmol) was added. After stirring
for 12 h, the THF was removed by vacuum distillation. The
residues were purified by column chromatography using methy-
lene dichloride and hexane (1 : 4) as the eluent to give pure
compound 6 as a brick red solid (2.87 g, 90%). 1H NMR (CDCl3,
400 Hz, d/ppm): 8.06 (s, 1H), 7.92(s, 1H), 7.52(s, 1H), 7.40 (t, 2H),
7.34–7.28 (m, 5H), 7.15–7.06 (m, 4H), 6.76–6.64 (m, 2H), 3.55 (t,
2H), 2.62 (t, 4H), 1.83 (t, 2H), 1.63–1.21 (m, 58H), 0.82–0.78 (m,
9H). 13C NMR (CDCl3, 100 MHz, d/ppm): 155.3, 151.9, 149.8,
This journal is ª The Royal Society of Chemistry 2012
147.2, 143.6, 142.4, 136.1, 135.3, 131.2, 130.8, 129.5, 128.6, 127.1,
125.8, 124.7, 123.8, 123.6, 122.1, 121.5, 120.7, 116.9, 113.4, 112.6,
52.2, 31.9, 29.7, 29.6, 29.5, 29.4, 28.5, 28.4, 28.0, 27.4, 27.2, 22.3,
14.1. HRMS calcd for C64H91N3S3 997.6375; found 997.6372.
4,7-Bis(5-bromo-4-dodecyl-2-thienyl)-5-(N-dodecyldiphenyl-
amine-vinylene)-2,1,3-benzothiadiazole (7). To a solution of
compound 6 (1.56 g, 1.56 mmol) in chloroform (25 mL),
N-bromosuccimide (NBS) (0.56 g, 3.12 mmol) was added. The
mixture was then stirred for 4 h at room temperature in darkness.
The solvent was removed by vacuum distillation. The residues
were purified by column chromatography using methylene
dichloride and hexane (1 : 6) as the eluent to give pure compound
7 as a red solid (1.30 g, 72%). 1H NMR (CDCl3, 400 Hz, d/ppm):
8.11 (s, 1H), 8.03–7.96 (m, 3H), 7.75 (s, 1H), 7.53–7.45 (m, 3H),
7.33–7.28 (m, 4H), 7.03–6.86 (m, 2H), 3.69 (t, 2H), 2.38 (t, 4H),
1.89 (t, 2H), 1.72–1.34 (m, 58H), 0.89–0.74 (m, 9H). 13C NMR
(CDCl3, 100 MHz, d/ppm): 152.5, 152.2, 149.4, 147.0, 142.2,
140.3, 137.6, 137.1, 132.9, 131.4, 130.5, 128.9, 128.6, 128.1, 125.3,
124.4, 123.7, 122.8, 122.2, 121.5, 116.6, 113.1, 112.0, 53.4, 32.1,
31.3, 30.4, 29.8, 29.7, 29.6, 29.5, 29.2, 28.5, 27.4, 22.6, 14.1.
HRMS calcd for C64H89Br2N3S3 1153.4585; found 1153.4589.
Synthesis of the polymers
Polymer PBTBD1. 2,6-Bis(tributyltin)-4,8-didodecyloxybenzo
[1,2-b;3,4-b]dithiophene (0.342 g, 0.3 mmol) and monomer 7
(0.346 g, 0.3 mmol) and were dissolved in 15 mL toluene. The
solution was flushed with argon for 10 min, and then Pd2dba3(5.5 mg, 2 mol%) and P(o-tolyl)3 (7.3 mg, 8%) were added into
the flask. The flask was purged three times with successive
vacuum and argon filling cycles. The polymerization reaction
was heated to 110 �C and the mixture was stirred for 72 h under
an argon atmosphere. 2-Tributylstannyl thiophene (23.7 mL) was
added to the reaction, and then after two hours 2-bromothio-
phene (7.5 mL) was added. The mixture was stirred overnight to
complete the end-capping reaction. The mixture was cooled to
room temperature and poured slowly into 350 mLmethanol. The
precipitate was filtered and washed with methanol and hexane in
a Soxhlet extraction apparatus to remove the oligomers and
catalyst residues. Finally, the polymer was extracted with chlo-
roform. The solution was condensed by evaporation and
precipitated into methanol. The polymer was collected as a dark
purplish solid (326 mg, 70%). 1H NMR (CDCl3, 400 Hz, d/ppm):
8.20 (br, 1H) 8.04 (br, 1H), 7.50–7.41 (br, 5H), 7.25 (br, 3H),
7.08–7.02 (br, 4H), 6.81 (br, 2H), 4.26 (br, 4H), 3.66 (br, 4H),
2.93–2.83 (br, 4H), 1.85–1.61 (br, 10H), 1.17 (br, 90H), 0.79 (br,
15H). 13C NMR (CDCl3, 100 MHz, d/ppm): 154.1, 150.5, 147.6,
146.2, 144.9, 142.9, 142.1, 140.9, 139.8, 139.4, 137.0, 136.0, 135.8,
134.8, 133.1, 131.6, 131.1, 130.6, 129.7, 129.0, 128.5, 127.1, 126.8,
124.1, 123.9, 123.3, 122.5, 121.3, 120.9, 120.1, 117.5, 117.2, 116.5,
73.0, 51.4, 30.9, 29.8, 29.6, 28.7, 28.4, 26.5, 26.1, 25.2, 21.7, 13.1.
Anal. calcd for (C98H141N3O2S5)n: C, 75.77; H, 9.15; N, 2.70.
Found: C, 75.06; H, 9.07; N, 2.82%.
Polymer PBTBD2. Copolymer PBTBD2 was obtained as a
dark purplish solid with a yield of 87% from the reaction of
monomer 7 with 2,6-bis(tributyltin)-4,8-bis(4-dodecyl thienyl)
benzo[1,2-b;3,4-b]dithiophene, similar to the synthesis procedure
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described for copolymer PBTBD1. 1H NMR (CDCl3, 400 Hz,
d/ppm): 7.97–7.94 (br, 2H), 7.67–7.53 (br, 3H), 7.34–7.24 (br,
7H), 7.06–6.93 (br, 4H), 6.85–6.80 (br, 4H), 2.83 (br, 8H), 1.72–
1.61 (br, 10H), 1.17 (br, 90H), 0.93 (br, 15H). 13C NMR (CDCl3,
100 MHz, d/ppm): 154.3, 150.8, 147.5, 146.3, 146.2, 144.9, 142.3,
140.9, 139.5, 138.4, 138.0, 137.3, 136.4, 135.9, 134.9, 133.4, 132.9,
131.8, 130.9, 130.1, 129.4, 128.5, 127.1, 126.9, 126.3, 124.5, 123.9,
123.3, 123.1, 122.8, 122.3, 121.7, 121.6, 120.8, 119.0, 116.7, 51.4,
30.9, 30.7, 29.9, 29.7, 29.4, 28.7, 28.4, 26.6, 26.1, 21.7, 13.1. Anal.
calcd for (C106H145N3S7)n: C, 75.52; H, 8.67; N, 2.49. Found: C,
74.88; H, 8.55; N, 2.56%.
Results and discussion
Synthesis and characterization
The synthetic route of the monomers and copolymers is outlined
in Scheme 2. Using diphenylamine as a starting material, 4-
formyl-N-dodecyldiphenylamine (2) was synthesized via the
Vilsmeier reaction in a good yield. Bromination of 4,7-dibromo-
5-methyl-2,1,3-benzothiadiazole with N-bromosuccimide (NBS)
can easily afford 4,7-dibromo-5-bromomethyl-2,1,3-benzothia-
diazole (3), which was then reacted with triethyl phosphite to give
phosphonic acid diethyl ester (4). At last, monomer 7 was
synthesized via the Wittig–Horner reaction by the coupling of 4
and aromatic aldehyde 2. The copolymers were synthesized from
organic tin monomers and bromide 7 by the Stille coupling
reaction, using Pd2(dba)3/P(o-tolyl)3 as catalysts. Crude copoly-
mers were purified by extracting with methanol, hexane and
chloroform in this order. The resulting copolymers were
obtained by reprecipitation of the concentrated chloroform
solution in methanol several times. The structures of the copol-
ymers were confirmed with NMR spectroscopy and elemental
analysis.
These copolymers exhibit good solubility in organic solvents,
such as chloroform, toluene, xylene and chlorobenzene. The
weight-average molecular weight (Mn) and polydispersity index
(PDI) were measured by gel permeation chromatography (GPC)
using tetrahydrofuran as the eluent and polystyrene as the
internal standard. Both PBTBD1 and PBTBD2 have high Mn
values of 69.0 and 35.0 kg mol�1 with PDI values of 2.04 and
2.40, respectively. The glass transition temperatures (Tg) of the
two polymers were not observed in differential scanning calo-
rimetry (DSC) scans. Decomposition onset temperatures (Td)
(about 5% weight loss) were determined by thermogravimetric
analysis (TGA) to be 345 and 420 �C for PBTBD1 and PBTBD2,
respectively. Obviously, the combinations of such good thermal
properties are adequately suitable for avoiding the deformation
and degradation of the polymeric active layer in PSCs.
Optical properties
The UV-vis absorption properties of the obtained copolymers
were measured both in CHCl3 solutions (Fig. 1a) and thin films
spin-coated on quartz slides (Fig. 1b). As shown in Fig. 1a, the
copolymers in chloroform solutions showed broad absorptions
ranging from 300 to 800 nm with three peaks at 334, 393, 508 nm
and 335, 408, 512 nm for PBTBD1 and PBTBD2, respectively.
The two peaks ranging from 300 and 450 nm were attributed to
the absorption of the diphenylamine-vinylene side chains and the
2936 | Polym. Chem., 2012, 3, 2933–2940
p–p* electronic transitions of the conjugated polymer back-
bones. Another absorption band from 450 to 800 nm was due to
the intramolecular charge transfer transitions (ICT) between the
donor segments and the acceptor moieties.17 Compared to
PBTBD1, PBTBD2 showed slight red shifts, which were due to
the increased conjugation length by the thiophene groups
attached on the benzo[1,2-b;3,4-b]dithiophene unit. From
Fig. 1a, the relative absorbance in the region of 300–450 nm also
decreased with the reduction of conjugation. The same behavior
was observed for the absorption spectra of the copolymer films as
shown in Fig. 1b. As compared to their counterparts in the
solution state, the broad peaks in the wavelength range of 450–
900 nm showed different red-shifts of 100 and 101 nm for
PBTBD1 and PBTBD2, respectively. The reason could be
explained by the aggregation and strong interchain interactions
between the conjugated main chains of the conjugated polymers
in the solid state, which could facilitate charge transportation for
photovoltaic applications.18 The energy bandgaps calculated
from the absorption band edges of the optical absorption spectra
were about 1.67 and 1.65 eV for PBTBD1 and PBTBD2,
respectively. The results indicated that PBTBD2 had a smaller
bandgap due to the stronger ICT effect than PBTBD1, which
was useful to enhance the short circuit current (Jsc) in PSCs.19
Compared with the non-substituted benzothiadiazole based
copolymers,15d–f the obtained bandgaps were smaller, which
indicates enhanced absorption across the entire visible wave-
length region was achieved by combination of diphenylamine-
vinylene side chains on the BT skeleton, implying this strategy
can be employed to develop efficient photovoltaic materials with
improved properties.
Electrochemical properties
The energy levels of the polymeric materials, including the
highest occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO) level, were measured by cyclic
voltammetry (CV) experiments. The CV measurements were
performed using a three-electrode cell in an anhydrous CH3CN
solution of 0.1 M tetrabutylammonium perchlorate (n-Bu4N-
ClO4). A Pt plate coated with a thin film of the studied copol-
ymer, a Pt wire and an Ag/AgCl (0.1 M) were used as the work
electrode, counter electrode and reference electrode, respectively.
The energy level of the Ag/AgCl reference electrode was cali-
brated against the Fc/Fc+ system to be 4.32 eV according to the
previous methods.20 The electrochemical cell was purged with
pure argon prior to each measurement. The scans toward the
anodic and cathodic directions were performed separately at a
scan rate of 50 mV s�1 at room temperature. The reversible
oxidation (p-doping) and reduction (n-doping) processes were
observed, and the obtained curves of the copolymers were shown
in Fig. 2. Both copolymers show predominant oxidation peaks
due to the electron-donating benzodithiophene, thiophene and
diphenylamine-vinylene segments and a slight reduction peak
related to the electron-withdrawing benzothiadiazole unit. From
the onset potentials of the p-doping process, the HOMO energy
levels were determined to be �5.15 and �5.13 eV for PBTBD1
and PBTBD2, respectively. On the other hand, the LUMO
energy levels were estimated to be �3.39 and �3.46 eV for
PBTBD1 and PBTBD2 from the onset potentials of the
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Scheme 2 Synthetic route of monomers and conjugated copolymers.
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cathodical sweep, respectively. Thus, the bandgaps (Eg) of
PBTBD1 and PBTBD2 were calculated to be about 1.76 and
1.67 eV. The values and the changing trends are consistent with
those obtained by the optical method above.
Theoretical calculations
The optimal geometries and electronic structures of the conju-
gated copolymers were simulated by density functional theory
(DFT) calculations at the B3LYP/6-31G* level on model
compounds using the Gaussian 09 program suite.21 The alkyl
chains were replaced by methyl groups to simplify the calcula-
tions, which did not significantly affect the equilibrium
This journal is ª The Royal Society of Chemistry 2012
geometries and the electronic properties. As shown in Fig. 3, the
results indicated that the electron density of LUMO was mainly
localized on the benzothiadiazole segment for BD-BT1 and BD-
BT2. On the other hand, the electronic wavefunction of the
HOMO was distributed almost entirely over the conjugated
molecule, which was beneficial for obtaining a higher hole
mobility.22 The HOMO and LUMO energy levels of BD-BT1
and BD-BT2 were calculated to be �4.94 eV, �2.55 eV
and �4.89 eV, �2.53 eV, respectively. The bandgaps were
determined to be 2.39 and 2.36 eV. Obviously, the trends in
HOMOs, LUMOs and bandgaps were similar to those obtained
by the CV and UV-vis measurements. The calculated values were
somewhat different from the experimental data because of the
Polym. Chem., 2012, 3, 2933–2940 | 2937
Fig. 1 Optical absorption spectra of PBTBD1 and PBTBD2 in chlo-
roform solutions (a) and thin films on glass (b).
Fig. 2 Cyclic voltammograms of the copolymers PBTBD1 and
PBTBD2.
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part extended p-conjugation system or because just several
repeating sections were chosen from the designed copolymers.
However, the theoretical analysis is still important to provide
direct insight into the interplay between their structural modifi-
cations and the resulting electronic and optical changes.
Photovoltaic properties
Polymer solar cells (PSCs) were made and investigated in a
series of ITO/PEDOT:PSS/copolymer:PC71BM/Ca/Al devices
based on the blends of copolymer:PC71BM according to our
previous work.5d,16,18,21 Here, ITO (indium-tin oxide) glass
spin-coated with PEDOT:PSS (poly(ethylenedioxythiophene)-
poly(styrenesulfonic acid)) was used as an anode, Ca/Al as a
cathode. PC71BM was used instead of PC61BM due to its
significantly broader and stronger absorption in the visible
region. The devices were optimized by changing some conditions,
2938 | Polym. Chem., 2012, 3, 2933–2940
such as the ratio of the copolymer and PC71BM from 1 : 1 to
1 : 4, and the thickness of active layer. The best fabrication
condition was obtained by spin-coating from a 10 mg mL�1
chlorobenzene solution of copolymer/PC71BM ratio of 1 : 3
(w/w). The optical thicknesses of polymeric blend films for both
devices was about 80 nm. The photovoltaic results of these
studies are depicted in Fig. 4. In addition, hole-only devices with
the structure of ITO/PEDOT:PSS/copolymer:PC71BM (1 : 3)/Au
were fabricated to evaluate the hole mobility in the blended films
by space charge limited current (SCLC) method according to our
previous work.21 The measured hole mobilities of
PBTBD1:PC71BM and PBTBD2:PC71BM were 4.3 � 10�5 cm2
V�1 s�1 and 5.7 � 10�5 cm2 V�1 s�1, respectively.
Fig. 4 shows the current density–voltage (J–V) curves and
external quantum efficiency (EQE) spectra for PSCs under
AM1.5G illumination at 100 mW cm�2. As shown in Fig. 4a, the
PBTBD1 and PBTBD2 devices show open-circuit voltages (Voc)
of 0.95 and 0.90 V, short-circuit current densities (Jsc) of 5.11 and
5.23 mA cm�2, fill factors (FF) of 0.34 and 0.40, resulting in
PCEs of 1.64% and 1.88%, respectively. The results indicated
both devices had relatively high Voc of more than 0.9 V. To the
best of our knowledge, 0.95 V is among the highest Voc values for
PSCs based on BT-based polymers. It is known that the Voc is
dependent on the difference between the HOMO energy level of
the donor and the LUMO energy level of the acceptor.23 The Voc
of the PBTBD1 device is as expected to be higher than that of the
PBTBD2 device due to the deeper HOMO energy level of
PBTBD1. However, the lower Jsc of the PBTBD1 device results
in the lower PCE, which may be attributed to the larger bandgap
of 1.76 eV and relatively lower hole mobility compared to those
of PBTBD2. To evaluate the accuracy of measurement, the
external quantum efficiencies (EQEs) of PSCs based on the
polymeric blend with PC71BM (1 : 3) under monochromatic
illumination were measured and shown in Fig. 4b. The shapes of
the EQE curves of the devices are similar to their absorption
spectra, indicating that the excitons are mainly generated in
copolymer phases. As shown in Fig. 4b, the spectral responses of
the two devices exhibited efficient photoresponses at wavelengths
between 300 and 800 nm, with the maximum EQEs of 44% and
43% at around 500 nm. Since the EQE values are almost the
same, the broader profile of the PBTBD2 results in a higher Jsc in
the PBTBD2 device than that in the PBTBD1 devices.
Furthermore, an active film morphology with an ideal domain
size of 10–20 nm in interpenetrating bicontinuous networks is
important for achieving high-performance devices.4b,24 Fig. 5
shows the AFM height and phase topographic images of the
films of copolymer:PC71BM (1 : 3, w/w) blends by tapping-
mode. The root mean squares roughness (Ra) of the film blends
based on PBTBD1 and PBTBD2 are 3.56 and 2.72, respectively.
The observed morphologies imply the good miscibility between
the copolymers and PC71BM. From the phase images, the
polymer and fullerene domains were homogeneously distributed
throughout the blend film with a nanometer-scale inter-
penetrating network. At the same the 1 : 3 blend ratio, the
PBTBD1:PC71BM blend film showed a little larger phase sepa-
ration, which would decrease the D–A interfacial areas and
deteriorate the formation of bicontinuous interpenetrating
networks. However, the PBTBD2:PC71BM film with a smoother
surface would induce a better contact with the cathode and a
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Fig. 3 Optimized geometries and molecular orbital surfaces of the HOMO and LUMOof the model compounds, obtained at the DFT/B3LYP/6-31G*.
Fig. 4 (a) Current density–voltage curves of photovoltaic cells based on
copolymers together with PC71BM, and (b) EQE curves of photovoltaic
cells based on copolymers together with PC71BM.
Fig. 5 AFM topographic images of the film blends (copolymer:
PC71BM¼ 1 : 3, w/w). (a) PBTBD1, (b) PBTBD2. Image size: 1� 1 mm2.
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reduced internal series resistance in PSCs. The improved film
surface with nanoscaled texture benefits the internal light scat-
tering and enhanced the light absorption,25 resulting in higher Jscand FF in the PBTBD2 device relative to the PBTBD1 device.
Compared to the efficiencies of other PSCs based on BDT and
BT containing copolymers,15d–f the PCE values in this work are
still relatively low. Considering the relatively low FF, there is big
This journal is ª The Royal Society of Chemistry 2012
room for future improvement in the performance of PSCs based
on the resulting copolymer–PCBM system. Further detailed
studies of device optimization of thermal annealing, solvent/
vapor annealing, the addition of solvent additives, as well as
charge transfer and exciton separation and recombination are in
progress.
Conclusions
In summary, a novel 2,1,3-benzothiadiazole (BT) acceptor with
functional conjugated diphenylamine-vinylene side chains was
firstly designed and synthesized for building efficient low
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bandgap photovoltaic copolymers. Two dimensional (2D) like
donor–acceptor copolymers based on benzo[1,2-b;3,4-b]dithio-
phene and the new BT acceptor, PBTBD1 and PBTBD2, have
been prepared by Stille coupling polymerization. These copoly-
mers possess good solubility and thermal properties. UV-vis
measurements indicated both copolymer thin films exhibited
strong and broad absorption in the region from 300 nm to
900 nm, relatively low HOMO levels, which were beneficial for
the stability and the increase of Voc of the fabricated PSCs.
PBTBD2 had a better light harvest and smaller bandgap
compared to PBTBD1. The electronic structures of model
molecules were also studied byDFT calculations at the B3LYP/6-
31G* level. The PSCs based on these copolymers were fabricated
and measured with a typical structure of ITO/PEDOT:PSS/
copolymer:PC71BM/Ca/Al. The results showed that PBTBD2
devices had a higher efficiency of 1.88% due to the improved Jscand FF. The present results indicated that these copolymers with
functional conjugated diphenylamine-vinylene side chains would
be promising candidates for efficient PSCs with high Voc.
Acknowledgements
The work was financially supported by the National Natural
Science Foundation of China (no: 20802033), Program for New
Century Excellent Talents in University (no: NCET-10-0170),
Young Scientist of Jing Gang Zhi Xing Project and Natural
Science Foundation of Jiangxi Province (no: 2008DQ00700,
2010GZH0110), Scientific Research Foundation for Excellent
Youth Scholars (no: 2012SCU04B01) and Recruit Talents of
Sichuan University (no: YJ2011025).
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