A novel thiophene derivative-based conjugated polymer for polymer solar cells with high open-circuit voltage

Article abstract of DOI:10.1002/cjoc.201200471

A novel D-A alternative conjugated polymer PBDTDMCT containing benzo[1,2-b:4,5-b']dithiophene (BDT) and dimethyl thiophene-3,4-dicarboxylate (DMCT), was designed and synthesized by Stille cross-coupling reaction. The copolymer exhibited excellent solubility and good thermal stability. The optical band gap determined from the onset of absorption of the polymer film was 2.10 eV. By incorporation of the ester groups into the polymer side chain, the HOMO level of polymer PBDTDMT was tuned to be deep-lying (-5.65 eV). Open-circuit voltage of polymer solar cells constructed based on PBDTDMT and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) can be tuned to achieve values as high as ca. 1.0 V.

Full text of DOI:10.1002/cjoc.201200471

FULL PAPER
DOI: 10.1002/cjoc.201200471
A Novel Thiophene Derivative-based Conjugated Polymer for
Polymer Solar Cells with High Open-circuit Voltage†
Chen, Liea,b(谌烈)
Shen, Xingxinga(沈星星)
Chen, Yiwang*,a,b(陈义旺)
a Department of Chemistry/Institute of Polymers, Nanchang University, Nanchang, Jiangxi 330031, China
b Jiangxi Key Laboratory for New Energy Chemistry, Nanchang University, Nanchang, Jiangxi 330031, China
A novel D-A alternative conjugated polymer PBDTDMCT containing benzo[1,2-b:4,5-b′]dithiophene (BDT)
and dimethyl thiophene-3,4-dicarboxylate (DMCT), was designed and synthesized by Stille cross-coupling reaction.
The copolymer exhibited excellent solubility and good thermal stability. The optical band gap determined from the
onset of absorption of the polymer film was 2.10 eV. By incorporation of the ester groups into the polymer side
chain, the HOMO level of polymer PBDTDMT was tuned to be deep-lying (－5.65 eV). Open-circuit voltage of
polymer solar cells constructed based on PBDTDMT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) can
be tuned to achieve values as high as ca. 1.0 V.
Keywords conjugated polymers, HOMO energy level, polymer solar cells, open-circuit voltage
Introduction
which is beneficial for charge transportation and ab-
sorption enhancement. Up to now, several polymers
based on BDT moiety have shown enhanced PCE
higher than 7%.[14-16]
Polymer solar cells (PSCs) have attracted great at-
tention because of their advantages such as low cost,
light weight, flexibility and large-area devices by
roll-to-roll process and so on.[1-3] So far, the bulk hetero-
junction (BHJ) structure is the most efficient architec-
ture based on the blending of polymer electron donors
and fullerene derivatives such as [6,6]-phenyl-C61-bu-
tyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-
butyric acid methyl ester (PC71BM) as electron accep-
tors.[4-6] Recently, the power conversion efficiency (PCE)
of PSCs based on poly(3-hexylthiophene) (P3HT) and
fullerene derivative has reached 5%—6%.[7,8] However,
P3HT has a relatively large band gap and high HOMO
(the highest occupied molecular orbital) energy level,
which results in relative low short-current density (Jsc)
and open-circuit voltage (Voc). An ideal polymer donor
material should have many advantages such as good
solubility, broad absorption spectra, low band gap, bal-
anced HOMO and LUMO (the lowest unoccupied mo-
lecular orbital) energy level, high hole mobility and so
on. In order to further improve performance of PSCs,
various high performance low band gap conjugated
polymers have been synthesized by alternating electron-
rich (donor) and electron-deficient (acceptor) (D-A)
copolymerization.[9-13] Benzodithiophene (BDT) build
block has attracted great interest in constructing D-A
conjugated copolymer as donor unit for high perform-
ance PSC, due to its symmetric and planar structure,
The Voc value is one of the most important parameter
for PSCs, which is determined by the energy gap be-
tween the HOMO energy level of the donor and the
LUMO energy level of the acceptor. In order to obtain
high Voc, some conjugated polymers with deeper
HOMO energy level have been studied.[17-19] The
HOMO energy level can be effectively decreased by
introducing electron-drawing groups into the polymer
main and side chains. Several groups have used fluorine
to decrease the HOMO energy level, and they obtain
higher Voc and better performance PSCs.[20-22] Recently,
high performance PSCs with high Voc have been devel-
oped by the introduction of ester group. Li et al. synthe-
sized three polythiophene derivatives PT-C1, PT-C2
and PT-C3 (Scheme 1). Their HOMO level is lower
than P3HT, and the Voc of PSCs based on the three
polymers and PC71BM blends reaches 0.8 V.[23] Cho et
al.[24] introduced methyl thiophene-3-carboxylate unit
into the polymer main chain, and synthesized polymer
PBDT-3MT (Scheme 1) possessing relatively low
HOMO level (－5.42 eV), due to the strong electron
affinity of methyl carboxylate group, which leads to a
high Voc (0.86 V) of the photovoltaic device.
Herein, in order to fine-tune the HOMO/LUMO en-
ergy level to obtain high Voc for PSC application, we
designed and synthesized a novel D-A conjugated
*
E-mail: ywchen@ncu.edu.cn; Tel.: 0086-0791-83969562; Fax: 0086-0791-83969561
Received May 15, 2012; accepted June 12, 2012; published online XXXX, 2012.
Dedicated to the 80th Anniversary of Chinese Chemical Society.
†
Chin. J. Chem. 2012, XX, 1—6
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1

Chen, Shen & Chen
FULL PAPER
polymer (PBDTDMCT) based on benzodithiophene
(BDT) as the push moiety and dimethyl-thiophene-3,4-
carboxylate (DMCT) as the pull unit. Incorporation of
carboxylate group and BDT unit into the conjugated
backbone induced the D-A polymer with a deep HOMO
energy level of －5.65 eV and a high Voc of ca. 1.0 V in
PBDTDMCT-based PSC.
absorption spectra were measured using a PerkinElmer
Lambda 750S spectrophotometer. Electrochemical
measurement of the polymer film was performed with a
Shanghai Zhongchen electrochemical workstation.
Preparation of polymer film of PBDTDMCT and the
blending films with PC71BM
Polymer PBDTDMCT was dissolved in chloroform
(15 mg/mL), and then the solution was spin-coated onto
quartz plate at the speed of 1000 r/min. The blending
film of PBDTDMCT and PC71BM (at different weight
ratio) was prepared from their dichlorobenzene solution
with the same method as PBDTDMCT film.
Experimental
Instruments
1H NMR and 13C NMR spectra were recorded on a
Bruker ARX 400-MHz spectrometer. The molecular
weight of the polymer was measured by gel permeation
chromatography (GPC), and polystyrene was used as a
standard. The thermogravimetric analysis (TGA) was
performed on a TA Q600 SDT analyzer under nitrogen
atmosphere at a heating rate of 10 ℃/min. UV-vis
Photovoltaic device fabrication and characterization
The polymer solar cells were fabricated with a de-
vice structure of ITO/PEDOT:PSS/Polymer:PC71BM
Blend/LiF/Al. The ITO glass substrates were
Scheme 1 Synthetic routes for monomer and polymer and the structures of published four alkyl thiophene-3-carboxylate-based poly-
mers, PC-T1, PC-T2, PC-T3 and PBDT-3MT
O
O
HOOC
Br
COOH
Br
O
O
a
Br
Br
S
S
2
1
O
O
O
O
b
S
S
S
c
Sn
Sn
S
S
S
O
O
3
4
5
O
O
S
d
S
*
+
5
2
*
n
S
O
O
O
O
PBDTDMCT
O
S
S
*
*
O
O
n
O
S
O
O
O
O
O
O
S
S
S
S
CH3
*
*
*
*
*
*
S
S
S
S
n
n
n
PBDTD-3MT
PT-C1
PT-C2
PT-C3
Reagents and conditions: (a) SOCl2, 85 °C; CH3OH, pyridine, 80 °C, 48 h; (b) Zn, NaOH, EtOH, reflux; (c) n-BuLi, －78 °C, r.t.; and
(CH3)3SnCl, -78 °C, r.t.; (d) Pd(PPh3)4, toluene, argon.
2
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Chin. J. Chem. 2012, XX, 1—6

A Novel Thiophene Derivative-based Conjugated Polymer for Polymer Solar Cells
pre-cleaned by detergent, deionized water, acetone and
isopropanol. After spin-coating a layer of poly(3,4-
ethylenedioxythiophene):poly(styrene-sulfonate) (PE-
DOT:PSS) onto the pre-cleaned ITO glass substrate, the
polymer/PC71BM blend solution was spin-coated. The
concentration of the polymer:PC71BM blend solution
used in this study for spin-coating was 15 mg/mL, and
dichlorobenzene was used as the solvent. Subsequently,
LiF and Al electrodes were deposited via thermal
evaporation in vacuum. The testing of the devices was
carried under the illumination of AM 1.5 G 100
mW/cm2.
7.37 (d, J＝8.4 Hz, 2H), 4.18 (d, J＝4.4 Hz, 4H), 1.84
—1.78 (m, 2H), 1.59—1.26 (m, 16H), 1.10—0.92 (m,
12H).
Synthesis of 2,6-bis(trimethyltin)-4,8-dioctyloxy-
benzo[1,2-b:3,4-b]dithiophene (5)
Compound 4 (4.2 g, 9.34 mmol) was dissolved in
anhydrous THF (100 mL) under inert atmosphere, then
n-butyl lithium (9.0 mL, 24.3 mmol) was added drop-
wise and stirred for 1 h at －78 ℃, and then stirred for
1 h at room temperature. The mixture was cooled to
－78 ℃ again, and trimethyltin chloride (28 mL, 28
mmol) was added in one portion. After stirring for 0.5 h,
the reactant was stirred for 2 h at room temperature. The
resulting mixture was poured into cold water and ex-
tracted with diethyl ether. The organic layer was washed
with water for several times, and dried over anhydrous
MgSO4. After removing solvent, the residue was re-
crystallized by isopropanol, and white solid (5.0 g, 6.5
mmol) was obtained (yield 69%). 1H NMR (CDCl3, 400
MHz) δ: 7.51 (s, 2H), 4.18 (d, J＝4.4 Hz, 4H), 1.84—
1.78 (m, 2H), 1.61—1.21 (m, 16H), 1.04—0.90 (m,
12H), 0.43 (s, 18H).
Materials
2,5-Dibromo-thiophene-3,4-dicarboxylic acid (1),[25]
8-dihydrobenzo[1,2-b:4,5-b′]dithiophene-4,8-dione
(3)[26] were prepared according to the published meth-
ods. Toluene was dried over Na/benzophenone and
freshly distilled prior to use. Other chemicals used in
this work were commercial products and used as re-
ceived.
Synthesis of dimethyl-2,5-dibromo-thiophene-3,4-
dicarboxylate (DMCT) (2)
Synthesis of poly{4,8-diethylhexyloxybenzo[1,2-
b;3,4-b]dithiophene-alt-4,6-dimethyl-thiophene-3,4-
dicarboxylate} (PBDTDMCT)
2,5-Dibromo-thiophene-3,4-dicarboxylic acid (0.5 g,
1.52 mmol) and 20 mL of thionyl chloride were put into
a 100 mL flask and heated to reflux over night. After the
reactant was cooled to ambient temperature, solvent was
removed. To the residue, 20 mL of methanol and sev-
eral drops of pyridine were added and then the reactant
was stirred at 80 ℃ for 48 h. After cooled to room
temperature, the reactant was poured into 100 mL of
cold hydrochloric acid and extracted by ethyl ether three
times. The organic layer was washed with saturated
NaHCO3 solution and water and dried by anhydrous
MgSO4. After removing of sovent, the crude product
was puried by column chromatography with petroleum
ether/ethyl acetate (V∶V＝1∶2) to provide 70 mg of
white solid (yield 13%). 1H NMR (CDCl3, 400 MHz) δ:
3.89 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ: 161.94,
133.58, 115.96, 52.76.
Compound 2 (68 mg, 0.19 mmol), compound 5 (147
mg, 0.19 mmol) and Pd(PPh3)4 (15 mg, 0.013 mmol)
were added to a 100 mL flask. The reaction container
was purged with nitrogen for 30 min and then 18 mL
toluene was added. The reaction was stirred and re-
fluxed at 110 ℃ for 16 h under nitrogen. The reactant
was cooled to room temperature and poured into MeOH
(200 mL) and then filtered. Subsequently filter residue
was subjected to a Soxhlet extraction with methanol,
hexane and chloroform successively. The fraction from
chloroform was concentrated under reduced pressure
and precipitated into methanol, collected by filtration.
The final product was dried under vaccun overnight to
afford PBDTDMCT as a dark red solid (90 mg, yield
74%). 1H NMR (CDCl3, 400 MHz) δ: 7.78 (s, 2H), 4.23
(d, J＝4.4 Hz, 4H), 3.91 (s, 6H), 1.50—1.27 (m, 18H),
1.05 (t, J＝6.4 Hz, 12H).
Synthesis of 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-
b′]dithiophene (4)
Compound 3 (3.5 g, 15.9 mmol), zinc power (2.3 g,
35.3 mmol), ethanol (15 mL), and NaOH solution (60
mL, 20%) were put into a 250 mL flask, the mixture
was stirred and heated to reflux for 3 h. And then
2-ethylhexyl bromide (9.2 g, 47.7 mmol) and a catalytic
amount of tetrabutylammonium bromide were added,
and, the mixture was refluxed for 6 h. The resulting
mixture was cooled to room temperature, poured into
water, and extracted with diethyl ether. The organic
layer was dried over anhydrous MgSO4. After removing
of solvent, the crude product was purified by silica gel
chromatography using petroleum ether as eluent, and
Results and Discussion
Synthesis and thermal stability
The general synthetic routes for the monomer di-
methyl-2,5-dibromo-thiophene-3,4-carboxylate (DMCT)
and polymer (PBDTDMCT) are shown in Scheme 1.
The synthesis of DMCT monomer started from
2,5-dibromo-thiophene-3,4-dicarboxylic acid. PBDTD-
MCT was synthesized by the Stille coupling reaction of
DMCT with 2,6-bis(trimethyltin)-4,8-bis(2-ethylhexyl-
oxy)benzo[1,2-b:4,5-b′]dithiophene (BDT). Molecular
weight determined by GPC for PBDTDMCT is 1.17×
104 g/mol, with the corresponding polydispersity index
1
light yellow oil was obtained (4.2 g, yield 60%). H
NMR (CDCl3, 400 MHz) δ: 7.48 (d, J＝8.0 Hz, 2H),
Chin. J. Chem. 2012, XX, 1—6
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3

Chen, Shen & Chen
FULL PAPER
of 1.93. Figure 1 shows the 1H NMR spectra of
PBDTDMCT. The resonance peaks at δ 7.78 and 4.23
are corresponding to the protons of the thiophene ring
and ethylhexyloxy of the BDT unit, respectively. The
peak at δ 3.91 is caused by the proton of the methyl on
DMCT moiety, and other resonance peaks can be all
assigned well, indicating the successful synthesis of the
polymer, PBDTDMCT. Furthermore, the resulted
polymer is readily dissolved in common organic sol-
vents such as THF, chloroform, and 1,2-dichloroben-
zene.
Photophysical properties
The UV-vis absorption spectra of polymer PBDTD-
MCT in chloroform solution and in film are shown in
Figure 3, and the corresponding optical properties are
summarized in Table 1. In solution, the absorption
spectrum exhibits two major absorption peaks at 386
and 458 nm, respectively. The peak at the short wave-
length is attributed to the π-π* transition of the polymer
main chain, while the peak at the long wavelength
comes from the intra-molecular charge transfer (ICT)
transition. The absorption maxima of the polymer in
film is red-shifted by 37 nm compared to that in solution,
owing to the aggregation of the conjugated polymer
main chain in solid state. Unlike traditional conjugated
polymers, e.g. P3HT, in which thermal annealing al-
ways casts positive effect on the molecular packing, the
polymer annealed at 150 ℃ exhibits the similar ab-
sorption spectrum to the as-cast film. The optical band-
gap of PBDTDMCT is determined by the absorption
edge in the UV-vis spectrum of the polymer film and
found to be 2.10 eV.
Figure 1 1H NMR spectrum of the polymer PBDTDMCT.
The thermal stability of the polymer PBDTDMCT
was investigated with thermogravimetric analysis
(TGA), the result is shown in Figure 2. The TGA result
shows that the onset temperature with 5% weight loss of
PBDTDMCT is 304 ℃, indicating that the thermal sta-
bility of PBDTDMCT is good enough for polymer solar
cells application.
Figure 3 Absorption spectra of the polymer in solution and in
film.
The PL spectra of the polymer film and the blend
film are shown in Figure 4. The polymer shows good
PL emission, indicating that the photoinduced charge
tends to recombine before transporting to the electrodes.
However, the luminescene of the blend film is greatly
quenched, suggesting that the charge transport from
polymer to PC71BM is happened. As the content of
PC71BM increased, the luminescene is quenched much
more greatly.
Figure 2 TGA thermograms of the polymer recorded under
nitrogen at a heating rate of 10 ℃/min.
Table 1 Optical and electrochemical properties of the polymer
UV-vis absorption spectra
Cyclic voltammetry
Polymer
λ
max/nm
495
λonset/nm
Egopt c/eV
Eoonxset /V
Eorendset /V
－1.01
HOMO/eV
LUMO/eV
Egec d/eV
PBDTDMCTa
590
2.10
1.25
－5.65
－3.39
2.26
c
ox
onset
a Cast from chloroform solution. b Calculated from the absorption band edge of the films, Eg＝1240/λonset. Calculated from Eg＝e( E
－ Eorendset ).
4
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Chin. J. Chem. 2012, XX, 1—6

A Novel Thiophene Derivative-based Conjugated Polymer for Polymer Solar Cells
Figure 4 Photoluminescent spectra for the polymer film and the
blend films of polymer:PC71BM at different weight ratio (excita-
tion wavelength: 400 nm).
Figure 5 Cyclic voltammograms of the polymer film.
experimental section. In order to obtain the best PSCs
performance, o-DCB was used as the solvent to form
better nanoscale morphology, and the weight ratio of
PBDTDMCT:PC71BM was optimized from 1∶1, 1∶2,
1∶3 to 1∶4. The current density-voltage characteris-
tics of the devices are shown in Figure 6, the corre-
sponding photovoltaic data are summarized in Table 2.
Electrochemical properties
The electrochemical cyclic voltammetry (CV) was
performed to determine the HOMO and LUMO energy
levels of the synthesized polymer. A platinum (Pt) elec-
trode with a polymer film by dip-coating was used as
the working electrode, a Pt wire was used as the counter
electrode, and Hg/Hg2Cl2 was used as the reference
electrode. The CV measurement of PBDTDMCT was
carried out in acetonitrile with 0.1 mol•L－ tetrabutyl-
1
ammonium tetrafluoroborate (TBAPF6). The CV curve
of PBDTDMCT is shown in Figure 5, and its electro-
chemical data are summarized in Table 1. The onset
onset
reduction potential ( E
) is －1.01 eV, while the on-
red
onset
set oxidation potential ( E
) is 1.25 eV. The HOMO
ox
and LUMO energy levels as well as the band gap of the
polymer PBDTDMCT were calculated according to the
following equations:[27]
onset
HOMO (eV)＝－e( E
LUMO (eV)＝－e( E
＋4.4)
ox
Figure 6 I-V curves of the photovoltaic devices based on
PBDTDMCT and PC71BM with different weight ratios.
onset
red
＋4.4)
Table 2 Photovoltaic properties of the polymer solar cells based
on PBDTDMCT under the illumination of AM 1.5 G, 100
mW/cm2
Egec (eV)＝e( E
－ E
)
onset
ox
onset
red
m(Polymer)∶m(PC71BM) Voc/V Jsc/(mA•cm－2) FF PCE/%
The HOMO and LUMO energy level of
PBDTDMCT are －5.65 and －3.39 eV, respectively.
PBDTDMCT exhibits deeper HOMO energy than that
of PBDT-3MT,[23] which will lead to higher Voc of the
PSCs based on PBDTDMCT and enhance the device
stability.
1∶1
1∶2
1∶3
1∶4
0.98
0.94
0.93
0.90
0.79
0.56
1.49
0.87
21.3 0.20
26.3 0.10
26.2 0.40
26.6 0.20
Photovoltaic properties
The maximum Voc value of PSCs devices fabricated
with PBDTDMCT and PC71BM blend is as high as ca.
1.0 V, due to the deep HOMO energy level of
PBDTDMCT. Among these devices, the best perform-
ance was obtained based on PBDTDMCT:PC71BM
blending weight ratio of 1∶3, with a Voc of 0.93 V, a Jsc
The photovoltaic properties of PBDTDMCT were
investigated by fabricating the PSCs with PBDTDMCT
as donor and PC71BM as acceptor with the structure of
ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al, and the
detailed device-fabrication process is described in the
Chin. J. Chem. 2012, XX, 1—6
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
5

Chen, Shen & Chen
FULL PAPER
of 1.49 mA/cm2, a fill factor (FF) of 26.2%, and a PCE
of 0.4% under the illumination of AM 1.5 G, 100
mW/cm2. At the same time, thermal annealing shows
little improvement on the device performance.
19.
[7] He, Y. J.; Chen, H. Y.; Hou, J. H.; Li, Y. F. J. Am. Chem. Soc. 2010,
132, 1377.
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Long, H.; Li, Y. F. Adv. Funct. Mater. 2012, 22, 585.
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Conclusions
In conclusion, a new D-A alternating conjugated
polymer PBDTDMCT was synthesized by Stille cou-
pling reaction of benzo[1,2-b:4,5-b′]dithiophene with
dimethyl-2,5-dibromo-thiophene-3,4-dicarboxylate.
PBDTDMCT showed good solubility and thermal sta-
bility, and exhibited very deep HOMO energy level at
－5.65 eV, due to the introduction of the dimethyl di-
carboxylate group into the thiophene ring. The PCE of
the photovoltaic device based on PBDTDMCT:PC71BM
(m∶m＝1∶3) reached 0.4%, with Voc＝0.93 V, Jsc＝
1.49 mA/cm2 and FF＝26.2%. Although the perform-
ance is too far to fulfill the requirement of PSC applica-
tion, this work has provided an effective method to in-
crease the Voc for high performance PSCs.
[16] Dou, L. T.; You, J. B.; Yang, J.; Chen, C. C.; He, Y. J.; Murase, S.;
Moriarty, T.; Emery, K.; Li, G.; Yang, Y. Nat. Photonics 2012, 6,
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[17] Lee, J. Y.; Kim, S. H.; Song, I. S.; Moon, D. K. J. Mater. Chem.
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Acknowledgements
[18] Nielsen, C. B.; Schroeder, B. C.; Hadipour, A.; Rand, B. P.; Watkins,
S. E.; McCulloch, I. J. Mater. Chem. 2011, 21, 17642.
[19] Wang, E. G.; Ma, Z. F.; Zhang, Z.; Henriksson, P.; Inganäs, O.;
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This work was supported by the National Natural
Science Foundation of China (Nos. 51073076 and
51003045).
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