Synthesis of π-conjugated copolymers composed of benzo[2,1,3]thiadiazole and thiophene units bearing various alkyl groups and their application to photovoltaic cells

Article abstract of DOI:10.1002/pola.24790

π-Conjugated polymers, PTOTBT, PTEHTBT, and PTt-BTBT, composed of benzothiadiazole as an electron accepting unit and terthiophene as an electron donating unit in the backbone were prepared. PTOTBT, PTEHTBT, and PTt-BTBT contained side chain groups of n-octyl, 2-ethylhexyl, and t-butyl groups, respectively. Solubility, optical and thermal properties of the polymers showed strong dependences on their side chain groups. PTEHTBT having 2-ethylhexyl groups in the side chain exhibited absorption maximum (I_max) at longer wavelength (565 nm) than PTOTBT (534 nm) and PTt-BTBT (495 nm). PTOTBT showed higher thermal stability than the others. The prepared polymers were employed to polymer solar cells (PSCs) with a configuration of ITO/PEDOT-PSS/polymer: PC₆₁BH/LiF/Al. Power conversion efficiency of the PSC-based on PTEHTBT was 1.32%.

Full text of DOI:10.1002/pola.24790

Synthesis of p-Conjugated Copolymers Composed of
Benzo[2,1,3]thiadiazole and Thiophene Units Bearing Various
Alkyl Groups and Their Application to Photovoltaic Cells
KWANG-HOI LEE,1 HO-JIN LEE,2 KOUJI KURAMOTO,3 YOHEI TANAKA,3 KAZUHIDE MORINO,1
ATSUSHI SUDO,1 TATSUO OKAUCHI,3 AKIHIKO TSUGE,3 TAKESHI ENDO1
1Molecular Engineering Institute, Kinki University, 11-6 Kayanomori, Iizuka, Fukuoka Prefecture 820-8555, Japan
2Research Fellow Laboratories Yokkaichi, JSR Corporation, 100 Kawajiri-cho, Yokkaichi, Mie Prefecture 510-8552, Japan
3Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology, Tobata-ku, Kitakyushu,
Fukuoka Prefecture 804-8550, Japan
Received 22 December 2010; accepted 21 May 2011
DOI: 10.1002/pola.24790
Published online 14 June 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: p-Conjugated polymers, PTOTBT, PTEHTBT, and PTt-
BTBT, composed of benzothiadiazole as an electron accepting unit
and terthiophene as an electron donating unit in the backbone
were prepared. PTOTBT, PTEHTBT, and PTt-BTBT contained side
chain groups of n-octyl, 2-ethylhexyl, and t-butyl groups, respec-
tively. Solubility, optical and thermal properties of the polymers
showed strong dependences on their side chain groups. PTEHTBT
having 2-ethylhexyl groups in the side chain exhibited absorption
maximum (kmax) at longer wavelength (565 nm) than PTOTBT
(534 nm) and PTt-BTBT (495 nm). PTOTBT showed higher thermal
stability than the others. The prepared polymers were employed to
polymer solar cells (PSCs) with a configuration of ITO/PEDOT-PSS/
polymer: PC61BH/LiF/Al. Power conversion efficiency of the PSC-
C
V
based on PTEHTBT was 1.32%.
2011 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 49: 3543–3549, 2011
KEYWORDS: conjugated polymers; copolymerization; functionali-
zation of polymers; photovoltaic; solar cell; UV-vis spectroscopy
INTRODUCTION Photovoltaic devices based on p-conjugated
polymers have attracted much attention due to the potential
application to low-cost, flexible, and large-area solar cells.
Currently, bulk heterojunction (BHJ) type has been utilized
for polymer solar cells (PSCs). The BHJ cell involves the
blend film of a p-conjugated polymer as an electron donor
and a fullerene derivative as an electron acceptor between
the electrodes. Very high power conversion efficiencies (5–
7% PCEs) have been achieved by BHJ solar cells.1–7 PCE is
determined by short circuit current (Jsc), open circuit current
(Voc), and fill factor (FF). For attaining enhanced Jsc, the poly-
mers should have a broad absorption in the solar spectrum
to ensure effective harvesting of the solar photons and high
charge carriers mobility.8–10 Recently, p-conjugated polymers
composed of electron donating and electron withdrawing
units in the polymer backbone have been prepared with
expecting that the absorbance can be shifted to longer wave-
length due to a formation of inter- and/or intramolecular
charge-transfer complex. In addition, morphology of the do-
nor-acceptor blend film plays very important role in an
enhancement of Jsc. Voc of OPV is proportional to the gap
between highest occupied molecular orbital (HOMO) energy
level of donor polymer and lowest unoccupied molecular
orbital (LUMO) energy level of the acceptor fullerene
material.11,12 Hence, employing polymers with lower HOMO
energy level is well-known method to give high Voc.
Benzothiadizaole (BT) unit is very frequently employed as
an electron acceptor unit in the electron donor-acceptor
polymers for PSCs, because of its planar skeleton to extend
p-conjugated length and electron accepting characteristic. A
large number of solar cells based on the polymer involving
BT units have been reported.1–3,13–22 Good solubility of con-
jugated polymer in organic solvents is necessary for the
solution processing. The solubility can be generally tailored
by the length and bulkiness of alky chains in the side chains.
The selection of appropriate alkyl chain is very important
because the electronic and optical properties of the polymer
are influenced by the side chains.
Herein, we report a new design and synthesis of p-conju-
gated polymers composed of benzothiadiazole as electron
accepting unit and terthiophene as an electron donating unit
in the backbone (Scheme 1). We examined and compared
the optical and thermal properties of the polymers by vary-
ing alkyl side groups in the side chain. The photovoltaic
characteristics of the polymers were investigated by employ-
ing them to the solar cell with a configuration of ITO/
PEDOT-PSS/polymer: PC61BH/LiF/Al.
Correspondence to: T. Endo (E-mail: tendo@mol-eng.fuk.kindai.ac.jp)
C
V
Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 3543–3549 (2011) 2011 Wiley Periodicals, Inc.
SYNTHESIS OF p-CONJUGATED COPOLYMER, LEE ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
(AM1.5G at 100 mW/cm2, highly uniform irradiation system
using 150W Xe lamp as light source, Wacom Electric) irradi-
ation. We calibrated the light intensity using a standard cell
for amorphous silicon (a-Si) solar cells. J-V characteristics
were measured using a Current-Voltage Source Meter (Keith-
ley 2410, Keithley) at room temperature in the glove box
filled with inert gas.
4,7-Bis(5-trimethylstannylthiophen-2-yl)
benzo[2,1,3]thiadiazole (3)
To a THF solution of LDA prepared from the reaction of dii-
sopropylamine (0.519 g, 0.363 moL) and n-BuLi (0.206 mL,
0.323 mmol) (1.57 M hexane solution) in THF (1 mL) at
SCHEME 1 Structures of polymers.
ꢂ
ꢁ78 C was added 1 (0.303 g, 0.101mmol) in THF (2 mL) at
EXPERIMENTAL
ꢂ
ꢂ
ꢁ78 C. After stirring the solution for 1 h at ꢁ78 C, trime-
thylstannyl chloride (90.5 mg, 0.454 mmol) was added to
the solution. After 1 h, the reaction was quenched by water
and the organic layer was extracted with ethyl acetate,
washed with brine and dried over Na2SO4. After the solvent
was removed under reduced pressure, the residue was puri-
fied by column chromatography (alumina) using hexane as
an eluent to give 63.0 mg (100%) of 3 as a reddish solid.
Materials
THF and toluene were dried over sodium/benzophenone
ketyl. Chloroform and DMF were distilled over CaH2.
Methanol and dichloromethane were used as received. 3,4-
dibromothiophene, 2-tributylstannyl thiophene, Pd(PPh3)4,
Ni(dppp)Cl2, magnesium, n-octyl bromide, 2-ethylhexyl bro-
mide, trimethylstannyl chloride, a hexane solution of n-BuLi,
N-bromosuccinimide (NBS) were used without purification.
1H NMR (CDCl3): d (ppm) 0.43 (s, -CH3, 18H), 7.27 (d, J ¼
3.6 Hz, Th-H, 2H), 7.85 (s, Ph-H, 2H), 8.17 (d, J ¼ 3.6 Hz, Th-
H, 2H,). 13C NMR (CDCl3): d (ppm) ꢁ8.2, 125.8, 125.9, 128.4,
136.1, 140.2, 145.1, 152.7.
Measurements and Characterization
1H NMR spectra were recorded on a Varian INOVA 400
NMR spectroscopy using tetramethylsilane as an internal
standard in chloroform-d (CDCl3). IR spectrum was
recorded on Thermo fishier Scientific NICOLET iS10. UV-vis
spectroscopy was measured in chloroform (1.0 ꢀ 10ꢁ5 M)
on a JASCO V570 spectrometer. Polymer films for UV-vis
measurement were prepared on a glass substrate with a
spin coater (700 rpm for 10 sec. and then 3000 rpm for
60 sec.) using 10 mg/mL chloroform solution. Number-
average (Mn) and weight-average (Mw) molecular weights
were determined by size exclusion chromatography with
TOSOH HLC-8120GPC using a calibration curve of polysty-
rene standards and THF as an eluent. Thermogravimetric
analyses (TGA) were conducted with a TA instrument
3,4-Di-(2-ethylhexyl)thiophene (5)
To a solution of 4 (0.64 g, 26.29 mmol) and Ni(dppp)Cl2
(0.71 g) in THF (100 mL) was slowly added 2-ethylhexyl
magnesium bromide solution prepared from the reaction of
2-ethylhexyl bromide (12.0 g, 62.5 mmol) and magnesium
(1.52 g, 62.5 mmol) in ethyl ether (31.3 mL) at 0 ꢂC. After
stirring overnight at room temperature, the reaction mixture
was poured into water and extracted with diethyl ether. The
combined organic layers were washed with brine and dried
over Na2SO4. After filtration volatiles were removed under
reduced pressure, and the resulting residue was purified by
distillation to afford 0.76 g (94%) of 5 as colorless oil.
ꢂ
TG-DTA6200 at a heating rate of 10 C/min under nitrogen.
1H NMR (CDCl3): d (ppm) 0.87 (m, ACH3, 12H), 1.32 (m,
ACH2A, 16H), 1.56 (m, ACHA, 1H), 2.43 (d, ACH2A, 4H),
6.85 (s, Th-H, 2H). 13C NMR (CDCl3): d (ppm) 10.8, 14.1,
23.1, 25.8, 28.9, 32.7, 33.3, 39.4, 120.7, 141.2.
HOMO levels of polymers were measured using a photo-
electron spectrometer AC-2 (Riken Keiki) by measuring of
ionic potential of polymer film in air.23,24
Device Fabrication and Characterization of PSC
The photovoltaic cell structure used in this study is ITO/
PEDOT: PSS/active layer/LiF/Al, where the active layer is
the blend film of a polymer as an electron donor and [6,6]-
phenyl C61 butyric acid hexyl ether (PC61BH) as an electron
acceptor in the weight ratio of 1:1 (w/w). PEDOT: PSS was
spin coated on the pre-cleaned ITO glass substrate and
heated at 150 ꢂC for 10 min. Subsequently, active layer was
prepared by spin casting of the blend solution of polymer
and PC61BH in chlorobenzene (10 mg/mL) on the PEDOT:
PSS layer. The thickness of active layer is about 100 nm. LiF
(1 nm) and Al anode (100 nm) were deposited on the active
layer to complete solar cell fabrication. Solar cell parameters
were estimated from current density-voltage (J-V) character-
istics under air mass 1.5 global solar simulated light
2,5-Dibromo-3,4-di-(2-ethylhexyl)thiophene (7)
To a solution of 5 (0.76 mg, 24.56 mmol) in DMF (50 mL)
was added NBS (0.87 g, 49.11 mmol). After stirred overnight
at room temperature the solution was poured into water and
extracted with ethyl acetate. The combined organic layer was
washed with water and brine, and dried over Na2SO4. After
organic solvent was removed under reduced pressure the
residue was purified by column chromatography (silica gel)
using hexane as an eluent to give 0.97 g (85%) of 7 as
colorless oil.
1H NMR (CDCl3): d (ppm) 0.87 (m, ACH3, 12H), 1.24–1.54
(m, 18H), 2.48 (d, 4H). 13C NMR (CDCl3): d (ppm) 11.1, 14.1,
23.1, 25.7, 28.9, 32.5, 33.4, 39.6, 108.5, 141.0.
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ARTICLE
3,4-Di-tert-butylthiophene (10)
PTEHTBT as a purple solid (Mw 23,000, Mw/Mn PDI 4.95).
1H NMR (CDCl3): d (ppm) 0.80-0.92 (m, ACH3,12H), 1.20-
1.68 (m, ACHA and ACH2A, 18H), 2.28-2.86 (m, ACH2A,
4H), 7.17-7.56 (m, Ar-H, 2H), 7.86-7.89 (m, Ar-H, 2H), 8.07-
8.10 (m, Ar-H, 2H)
To a solution of 9 (1.22 g, 5.25 mmol) in benzene (100 mL)
was added p-TsOH (0.20 g, 1.16 mmol). After refluxing for
1 h, the solution was washed with aqueous sodium bicarbon-
ate and dried over Na2SO4. The mixture was filtrated and
concentrated under reduced pressure, and the resulting resi-
due was purified by column chromatography (silica gel)
using hexane as an eluent to give 0.74 g (72%) of 10 as col-
orless oil.
PTt-BTBT
To a mixture of 11 (177 mg, 0.50 mmol), 3 (313 mg,
0.50 mmol), and Pd(PPh3)4 (25 mg, 0.016 mmol), which was
dried in vacuo for 10 min, toluene (8 mL) and DMF (2 mL)
were added. Argon gas was bubbled into the solution for 10
1H NMR (CDCl3): d (ppm) 1.47 (s, ACH3, 18H), 7.16 (s, Th-H,
2H). 13C NMR (CDCl3): d (ppm) 33.5, 35.3, 122.3, 150.6.
ꢂ
min, and then the solution was stirred at 120 C for 3 h. The
resulting solution was cooled to room temperature and then
poured into methanol (150 m). The resulting precipitate was
collected by filtration, dissolved in chloroform (150 mL), and
then the solution was passed through a column (silica gel,
PSQ100) using chloroform as an eluent. The obtained solu-
tion was concentrated under reduced pressure. To the result-
ing residue, methanol (300 mL) was added to give solid,
which was collected by filtration to afford 130 mg (53%) of
2,5-Dibromo-3,4-di-tert-butylthiophene (11)
To a solution of 10 (0.74 g, 3.77 mmol) in DMF (10 mL) was
added NBS (0.14 mg, 7.92 mmol). After stirring for 3h at
room temperature, the solution was poured into water and
extracted with ethyl acetate. The organic layer was washed
with water and brine, and dried over Na2SO4. The mixture
was filtrated and concentrated under reduced pressure, and
the resulting residue was purified by column chromatography
(silica gel) using hexane as an eluent to give 1.09 g (82%) of
1
PTt-BTBT as a black solid (Mw 2800, Mw/Mn 1.68). H NMR
(CDCl3): d (ppm) 1.46-1.69 (m, t-Bu,18H), 7.14-7.29 (m, Ar-
H, 2H), 7.85 (br-s, Ar-H, 2H), 8.03-8.09 (m, Ar-H, 2H)
1
11 as a white solid. H NMR (CDCl3): d (ppm) 1.59 (s, ACH3,
18H). 13C NMR (CDCl3): d (ppm) 33.6, 37.8, 107.0, 149.0.
RESULTS AND DISCUSSION
PTOTBT
Synthesis and Characterization of Polymers
The synthetic methods of monomers are shown in Scheme 2.
4,7-Bis(thiophenyl-2-yl)benzo[2,1,3]thiadiazole (1) was pre-
pared as described in the literature.25,26 4,7-Bis(5-bromo-
thiophen-2-yl)benzo[2,1,3]thiadiazole (2) was synthesized by
bromination of 1 using NBS in 66% yield.26 Dilithiated 1
prepared by using LDA was reacted with trimethylstannyl
chloride to gave 4,7-bis(5-trimethylstannylthiophen-2-yl)ben-
zo[2,1,3]thiadiazole (3), quantitatively. 3,4-Dialkylthiophenes
5 (R ¼ 2-ethylhexyl) and 6 (R ¼ n-octyl)27 were prepared
from the reaction of 3,4-dibromothophene with the corre-
sponding alkylmagnesium bromide in the presence of
Ni(dppp)Cl2 as a catalyst. Bromination of 5 by NBS gave 7 in
84% yield. We attempted to prepare 2,5-bis(trimethyl-
stannyl)-3,4-di(2-ethylhexyl)thiophene as a monomer from
the reaction of 2,5-dilithio-3,4-di(2-ethylhexyl)thiophene with
trimethylstannyl chloride. However, the reaction afforded
complex products, produced probably by the decomposition
of the target compound during a work-up process. 2,5-Dili-
thio-3,4-dioctylthophene prepared from the reaction of 6
with n-BuLi was reacted with trimethylstannyl chloride to
afford 8.28 3,4-Bis(t-butyl)thiophene 10 was prepared from
929 in the presence of p-toluenesulfonic acid in 74% yield.
Monomer 11 was prepared by bromination of 10 using NBS
in 84% yield.
To a mixture of 8 (233 mg, 0.50 mmol), 2 (313 mg, 0.50
mmol), and Pd(PPh3)4 (58 mg, 0.050 mmol), which was
dried in vacuo for 10 min, toluene (40 mL) and DMF (10
mL) were added. Argon gas bubbled into the solution for 10
min, and then the solution was stirred at 110 ꢂC for 20 h.
The resulting solution was cooled to room temperature and
then poured into methanol (150 mL). The resulting precipi-
tate was collected by filtration, dissolved in chloroform (150
mL), and then the solution was passed through a column
(silica gel, PSQ100) using chloroform as an eluent. The
obtained solution was concentrated under reduced pressure.
To the resulting residue, methanol (300 mL) was added to
give a solid, which was collected by filtration to afford 71
mg (24%) of PTOTBT as a purple solid (Mw1300, Mw/Mn
1.63). 1H NMR (CD2Cl2): d (ppm) 0.80 (br-s, ACH3, 6H),
1.20-1.59 (m, ACH2A, 24H), 2.30-2.76 (m, ACH2A, 4H),
6.82-7.89 (m, Ar-H, 6H)
PTEHTBT
To a mixture of 7 (233 mg, 0.50 mmol), 3 (313 mg, 0.50
mmol), and Pd(PPh3)4 (25 mg, 0.016 mmol), which was
dried in vacuo for 10 min, toluene (8 mL) and DMF (2 mL)
were added. Argon gas was bubbled into the solutionꢂfor 10
min, and then the solution was stirred for 2 h at 120 C. The
resulting solution was cooled to room temperature and then
poured into methanol (150 m). The resulting precipitate was
collected by filtration, dissolved in chloroform (150 mL) and
then the solution was passed through a column (silica gel,
PSQ100) using chloroform as an eluent. The obtained solu-
tion was concentrated under reduced pressure. To the result-
ing residue methanol (300 mL) was added to give a solid,
which was collected by filtration to afford 180 mg (60%) of
The target polymers were prepared by the Stille cross-cou-
pling polymerization in DMF/toluene using Pd(PPh3)4 as a
catalyst (Scheme 3). After the polymerization was terminated
the resulting solution was precipitated into methanol. The
methanol-insoluble fraction was subsequently dissolved in
chloroform and passed through
a column (silica gel,
PSQ100B) using chloroform as an eluent to remove the
metal catalyst. First, the polycondensation of 8 and 2 was
SYNTHESIS OF p-CONJUGATED COPOLYMER, LEE ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 2 Preparation of monomers.
performed. In this case, a significant amount of a chloro-
form-insoluble fraction was given, implying that the polymer
PTOTBT would have intrinsically less soluble nature. Only
the chloroform-soluble fraction was passed through a silica
gel column to obtained purified PTOTBT, of which weight
average molecular weight Mw was estimated to be 1300. IR
spectra of both the chloroform-soluble and -insoluble frac-
tions were indicated identical signal patterns, implying that
SCHEME 3 Preparation of polymers.
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ARTICLE
TABLE 1 Synthesis and Characterization of PTOTBT, PTEHTBT,
and PTt-BTBT
Molecular
Weightb
TGA (ꢂC)c
Td5 Td10
Yield (%)a
Mw
1300
Mw/Mn
PTOTBT
PTEHTBT
PTt-BTBT
a
24
60
53
1.68
4.95
1.68
396
387
255
420
421
266
23000
2800
Insoluble part in methanol.
Measured by GPC (THF).
Under N2, 10 ꢂC/min.
b
c
FIGURE 2 UV-vis spectra of PTOTBT, PTEHTBT, and PTt-BTBT
in chloroform.
the repeating units in the polymer main chain were identi-
cal. Hence, the insoluble part may be PTOTBT with large
molecular weights. Next, we designed PTOTBT analogues
bearing branched alky groups in the side chain to increase
the solubility and molecular weight. A polymer bearing 2-
ethylhexyl groups in the side chain was designed as a target
polymer PTEHTBT. The synthetic strategy for PTEHTBT is
rather different from that for PTOTBT. As described earlier,
the synthesis of a distannyl substituted 3,4-di(2-ethylhex-
yl)thiophene was not successful. Therefore, we prepared
distannyl substituted monomer 3 and dibromothiophene 7
for the preparation of PTEHTBT. The polycondensation of
3 and 7 afforded PTEHTBT in 60% yield after purification
by silica gel column. PTEHTBT exhibited increased solubil-
ity and molecular weights (Mw 23,000, Mw/Mn 4.95) com-
pared with PTOTBT. PTEHTBT showed a good solubility in
chloroform, THF, chlorobenzene, and dichlorobenzene. We
also designed PTt-BTBT bearing more bulky t-butyl groups
in the side chain. The polycondensation 3 and 11 gave PTt-
BTBT in 53% yield after silica gel column purification. The
molecular weight and yield of soluble PTt-BTBT (Mw 2800,
Mw/Mn 1.68) were higher than those of PTOTBT, implying
t-butyl group enhanced solubility more efficiently than n-
octyl group did.
ꢂ
were observed at 396, 387, and 255 C, respectively (Table 1
and Fig. 1). PT-tBTBT bearing t-butyl groups in the side
chain gave significantly poor thermal stability compared with
PTOTBT and PTEHTBT having n-octyl and 2-ethylhexyl
groups, respectively.
Optical Properties of Polymers
Optical properties of the polymers were investigated by UV-
vis spectroscopy in dilute chloroform solution and in film
state. Absorption maxima of PTOTBT, PTEHTBT, and PTt-
BTBT in chloroform were observed at 534, 565, and 495
nm, respectively (Figure 2). PTEHTBT showed absorption
maximum (kmax) at longer wavelength than PTOTBT and
PTt-BTBT, probably due to the much larger molecular
weight of PTEHTBT than the others. Interestingly, despite
the molecular weight of PTt-BTBT was about two times
larger than that of PTOTBT, kmax of PTt-BTBT appeared at
shorter wavelength than that of PTOTBT. This result can be
attributed to steric effect of polymer side chain; bulky t-butyl
group may disturb the planarity of polymer backbone to
shorten p-conjugation length. UV-vis spectra of polymer films
prepared by spin casting were measured as well. kmaxs of
PTOTBT, PTEHTBT, and PTt-BTBT films were observed at
549, 637, and 510 nm, respectively, which were red-shifted
compared to those of polymer solutions indicating the
Thermal properties of the obtained polymers were investi-
gated by TGA. Td5s of PTOTBT, PTEHTBT, and PTt-BTBT
FIGURE 3 UV-vis spectra of PTOTBT, PTEHTBT, and PTt-BTBT
FIGURE 1 TGA curves of PTOTBT, PTEHTBT, and PTt-BTBT.
films.
SYNTHESIS OF p-CONJUGATED COPOLYMER, LEE ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 Optical Properties and Energy Levels of Polymers
UV-vis (nm)
kmax CHCl3 (e)b
kmax film
HOMO (eV)c
LUMO (eV)d
Eg.opt (eV)e
a
PTOTBT
PTEHTBT
PTt-BTBT
a
534 (19,400)
565 (22,300)
495 (17,500)
549
637
510
ꢁ4.97
ꢁ5.21
ꢁ5.43
ꢁ3.32
ꢁ3.62
ꢁ3.45
1.65
1.59
1.98
Conc. ¼ 1.0 ꢀ 1 0ꢁ5 M.
Molar absorption coefficient.
Measured with AC-2.
b
c
d
e
Calculated from the equation, Eg ¼ |HOMO ꢁ LUMO|.
Calculated from the onset absorption, Eg ¼ 1240/konset
.
effective planarization and p-p interaction of the polymer
backbone in the solid state (Figure 3). Especially, PTEHTBT
showed larger shift of kmax by 72 nm in the solid state com-
pared with the others (by 15 nm). We investigated the
HOMO energy levels of the polymers with a photo-electron
spectrometer AC-2 (Riken Keiki) by measuring ionic poten-
tial of polymer film in air.23,24 The HOMO energy levels were
estimated to be ꢁ4.97, ꢁ5.21, and ꢁ5.43 eV for PTOTBT,
PTEHTBT, and PTt-BTBT, respectively. The band gaps
obtained from onset absorptions of polymer films were 1.65,
1.59, and 1.98 eV for PTOTBT, PTEHTBT, and PTt-BTBT,
respectively (Table 2).
tovoltaic parameters Voc, Jsc, FF, and PCE are summarized in
Table 3. Solar cells based on PTEHTBT and PTt-BTBT exhib-
ited higher Voc (0.82–0.85 eV) than that based PTOTBT
(0.71 eV). This result can be attributed to the low-lying
HOMO energy level of PTEHTBT (ꢁ5.21 eV) and PTt-BTBT
(ꢁ5.43 eV), compared with PTOTBT (ꢁ4.97 eV).11,12 In
terms of Jsc, the solar cell-based on PTEHTBT showed much
higher value (4.54 mA/cm2) than others (0.99–1.24 mA/
cm2). Probably, PTEHTBT with kmax at longer wavelength
may harvest more photons to enhance Jsc. PCEs of the solar
cells based on PTOTBT, PTEHTBT, and PTt-BTBT were
0.32, 1.32, and 0.21%, respectively.
Photovoltaic Properties of Polymers
The blend films of the new polymers as electron donors and
[6,6]-phenyl C61 butyric acid hexyl ester (PC61BH)30 (Fig. 4)
as an electron acceptor and applied to solar cells with a con-
figuration of ITO/PEDOT-PSS/polymer: PC61BH/LiF/Al. Poly-
mer and PC61BH were blended with a ratio of 1:1 in chloro-
benzene (10 w/v%), which was spin-coated on PEDOT-PSS
layer, pre-casted on ITO electrode. Finally, LiF and Al layers
as top electrodes were deposited on the blend film. Figure 4
shows current density-voltage (J-V) curves of the solar cells
estimated under AM1.5 at 100 mW/cm2. The associated pho-
CONCLUSIONS
Novel p-conjugated polymers composed of benzothiadiazole
and terthiophene units in the backbone were synthesized.
The polymers PTOTBT, PTEHTBT, and PTt-BTBT contain-
ing n-octyl, 2-ethylhexyl, and t-butyl groups, respectively, in
the side chain exhibited significant differences in optical
and thermal properties. The solution and film of PTEHTBT
showed UV-vis kmaxs at longer wavelength than those of
PTOTBT and PTt-BTBT, implying the optical properties of
the polymer can be controllable by varying the side chain.
PTOTB bearing linear alkyl chain showed higher thermal
stability than others. The prepared polymers were
employed into PSCs with a configuration of ITO/PEDOT-
PSS/polymer: PC61BH/LiF/Al. PCS with PTEHTBT exhibited
higher PCE (1.32%) than those based on PTOTBT (0.32%)
and PTt-BTBT (0.21%), probably due to the fact that
PTEHTBT with larger molecular weight exhibited UV-vis
kmax at longer wavelength to effectively harvest the solar
photons.
TABLE 3 Characteristic Properties of Polymer Solar Cells
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
PTOTBT
PTEHTBT
PTt-BTBT
1.24
4.54
0.99
0.71
0.82
0.85
0.36
0.35
0.25
0.32
1.32
0.21
FIGURE 4 Current density-voltage (J-V) characteristics of solar
cells based on PTOTBT, PTEHTBT, and PTt-BTBT under the
illumination of AM 1.5 G, 100 mW/cm2, and the structure of
PC61BH.
Jsc, short-circuit current density; Voc, open-circuit voltage; FF, fill factor;
PCE, power conversion efficiency.
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17 Hou, J.; Park, M.-H.; Zhang, S.; Yao, Y.; Chen, L.-M.; Li, J.-
REFERENCES AND NOTES
H.; Yang, Y. Macromolecules 2008, 41, 6012–6018.
1 Park, S. P.; Roy, A.; Beaupre´, S.; Cho, S.; Coates, N.; Moon, J.
S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat Photonics
2009, 3, 297–302.
18 Price S. C.; Stuart, A. C.; You, W. Macromolecules 2010, 43,
4609–4612.
19 Huo, L.-H.; Chen, H.-Y.; Hou, J.; Chen, T. L.; Yang, Y. Chem
2 Chen, H.-T.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.;
Commun 2009, 5570–5572.
Yu, L.; Wu, Y.; Li, G. Nat Photonics 2009, 3, 649–653.
20 Boudreault, P.-L. T.; Michaud, A.; Leclerc, M. Macromol
3 Huo, L.; Hou, J.; Zhang, S.; Chen, H.-Y.; Yang, Y. Angew
Rapid Commun 2007, 28, 2176–2179.
Chem Int Ed 2010, 1500–1503.
21 Liao, L.; Dai, L.; Smith, A.; Durstock, M.; Lu, J.; Ding, J.;
4 Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L.
Tao, Y. Macromolecules 2007, 40, 9406–9412.
J. Am Chem Soc 2009, 131, 7792–7799.
22 Blouin, N.; Michaud, A.; Leclerc, M. Adv Mater 2007, 19,
5 Kitazawa, D.; Watanabe, N; Yamamoto, S.; Tsukamoto, J
2295–2300.
Appl Phys Lett 2009, 95, 053701-1–05371-3.
23 Nakano, M.; Tsukazaki, A.; Gunji, R. Y.; Ueno, K.; Ohtomo,
A.; Fukumura, T.; Kawasaki, M. Appl Phys Lett 2007, 91,
142113-1–142113-3.
6 Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.;
Yu, L. Adv Mater 2010, 22, 1–4.
7 Liang, Y.; Wu, Y.; Feng, D.; Tsai, S.-T.; Son, H.-J.; Li, G.; Yu,
24 McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; Mac-
Donald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.;
Zhang, W.; Chabinyc, M. L.; Kline, R. J.; McGehee, M. D.;
Toney, M. F. Nat Mater 2006, 5, 328–333.
L. J Am Chem Soc 2009, 131, 56–57.
8 Hou, J.; Chen, H.-Y.; Zhang, S.; Li, G.; Yang, Y. J Am Chem
Soc 2008, 130, 16144–16145.
9 Peet, J.; Kim J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.;
25 Kim, J.-J.; Choi, H.; Lee, J.-W.; Kang, M.-S.; Song, K.; Kang,
Heeger, A. J.; Bazan, G. C. Nat Mater 2007, 6, 497–500.
S. O.; Ko, J. J Mater Chem 2008, 18, 5223–5229.
10 Li, K.-C.; Hsu, Y.-C.; Lin, J.-T.; Yang, C.-C.; Wei, K.-H.; Lin,
26 Park, M.-J.; Lee, J.; Jung, I. H.; Park, J.-H.; Hwang, D.-H.;
H.-C. J Poly Sci Part A: Polym Chem 2009, 47, 2073–2092.
Shim, H.-K. Macromolecules 2008, 41, 9643–9649.
11 Brabec, C. J.; Cravino, A.; Meissner, D.; Sariciftci, N. S.;
Fromherz, T.; Rispens, M. T.; Sanchez, L.; Hummelen, J. C. Adv
Funct Mater 2001, 11, 374–380
27 Wetering, K. V. D.; Brochon, C.; Ngov, C.; Hadziioannou, G.
Macromolecules 2006, 39, 4289–4297.
28 Lee, K.-H.; Lee, H.-J.; Kuramoto, K.; Tanaka, Y.; Morino, K.;
Sudo, A.; Okauchi, T.; Tsuge, A.; Endo, T. J Polym Sci Part A:
Polym Chem 2011, 49, 234–241.
12 Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Wal-
¨
dauf, C.; Heeger, A. J.; Brabec, C. J. Adv Mater 2006, 18, 789–794.
13 Bundgaard, E; Krebs, F. C. Sol Energy Mater Sol Cells 2007,
29 Nakayama, J.; Hasemi, R.; Yoshimura, K.; Sugihara, Y.;
91, 1019–1025.
Yamaoka, S. J Org Chem 1998, 63, 4912–4924.
14 Bundgaard, E.; Shaheen, S. E.; Krebs, F. C.; Ginley, D. S. Sol
30 Koide, K.; Komiya, R.; Han, L.; Lee, H.-J.; Arai, T. In Proceed-
ings of the 17th International Photovoltaic Science and Engi-
neering Conference (PVSEC-17), Fukuoka, Japan, December 3–
7, 2007; Okamoto, H., Ed.; Technical Digest of 17th Interna-
tional Photovoltaic Science and Engineering Conference,
Fukuoka, 2007; pp 283–284.
Energy Mater Sol Cells 2007, 91, 1631–1637.
15 Hou, J.; Chem, H.-Y.; Zhang, S.; Li, G.; Yang, Y. J Am Chem
Soc 2008, 130, 16144–16145.
16 Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Muhlbacher, D.;
¨
Scharber, M.; Brabec, C. Macromolecules 2007, 40, 1981–1986.
SYNTHESIS OF p-CONJUGATED COPOLYMER, LEE ET AL.
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