Polythiophenes bearing electron-withdrawing groups in the side chain and their application to bulk heterojunction solar cells

Article abstract of DOI:10.1002/pola.24444

Soluble polythiophenes bearing strong electron withdrawing groups, dicyanoethenyl [-CH=C(CN)₂] (PTDCN) and cyano-methoxycarbonylethenyl [-CH=C(CO₂Me)CN] (PTCNME), in the side chains have been prepared. Optical band gaps calculated from onset absorption were 1.70 eV and 1.73 eV for PTDCN and PTCNME, respectively. Highest occupied molecular orbital energy levels measured with a surface analyzer (AC-2) were -5.53 eV and -5.29 eV for PTDCN and PTCNME, respectively, which were much lower than that of poly(3-hexylthiophene) (-4.81 eV). To investigate photovoltaic properties, bulk heterojunction polymer solar cells based on PTDCN and PTCNME were fabricated with a structure of ITO/PEDOT:PSS/active layer/LiF/Al, where the active layer was a blend film of polymer and [6,6]-phenyl C₆₁ butyric acid hexyl ester (PC₆₁BH). Solar cell parameters were estimated from current density-voltage (J-V) characteristics under the illumination of AM1.5 at 100 mW/cm². The solar cell based on the blend film of PTCNME:PC ₆₁BH (1:1) showed power conversion efficiency (PCE) of 0.72% together with the open current voltage (V_oc) of 0.61 V, the short current density (J_sc) of 3.90 mA/cm², and the fill factor of 30.3%. The PCE of a solar cell fabricated from PTDCN in a similar way was 0.56%. Copyright

Full text of DOI:10.1002/pola.24444

Polythiophenes Bearing Electron-Withdrawing Groups in the Side Chain
and Their Application to Bulk Heterojunction Solar Cells
KWANG-HOI LEE,1 HO-JIN LEE,2 KOUJI KURAMOTO,3 YOHEI TANAKA,3 KAZUHIDE MORINO,1
ATSUSHI SUDO,1 TATSUO OKAUCHI,3 AKIHITO 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 13 May 2010; accepted 14 October 2010
DOI: 10.1002/pola.24444
Published online 11 November 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Soluble polythiophenes bearing strong electron
withdrawing groups, dicyanoethenyl [ACH¼C(CN)2] (PTDCN)
blend film of polymer and [6,6]-phenyl C61 butyric acid hexyl
ester (PC61BH). Solar cell parameters were estimated from cur-
rent density–voltage (J–V) characteristics under the illumination
of AM1.5 at 100 mW/cm2. The solar cell based on the blend film
of PTCNME:PC61BH (1:1) showed power conversion efficiency
(PCE) of 0.72% together with the open current voltage (Voc) of
0.61 V, the short current density (Jsc) of 3.90 mA/cm2, and the
and
cyano-methoxycarbonylethenyl
[ACH¼C(CO2Me)CN]
(PTCNME), in the side chains have been prepared. Optical
band gaps calculated from onset absorption were 1.70 eV and
1.73 eV for PTDCN and PTCNME, respectively. Highest occu-
pied molecular orbital energy levels measured with a surface
analyzer (AC-2) were ꢀ5.53 eV and ꢀ5.29 eV for PTDCN and
PTCNME, respectively, which were much lower than that of
poly(3-hexylthiophene) (ꢀ4.81 eV). To investigate photovoltaic
properties, bulk heterojunction polymer solar cells based on
PTDCN and PTCNME were fabricated with a structure of ITO/
PEDOT:PSS/active layer/LiF/Al, where the active layer was a
fill factor of 30.3%. The PCE of a solar cell fabricated from
C
V
PTDCN in a similar way was 0.56%.
2010 Wiley Periodicals,
Inc. J Polym Sci Part A: Polym Chem 49: 234–241, 2011
KEYWORDS: conjugated polymers; copolymerization; polythio-
phene; solar cell; Stille coupling reaction; thin films
INTRODUCTION Organic photovoltaics (OPVs) based on p-
conjugated polymers are of interest because of their poten-
tial to be low cost, lightweight, and flexible. Bulk heterojunc-
tion (BHJ) devices based on a p-conjugated polymer as a do-
nor and a fullerene derivative as an acceptor have emerged
as the most efficient solar cells to date.1 BHJ solar cells using
p-conjugated polymers have afforded very high power con-
version efficiencies (PCEs; 5–7%).2 PCE depends on three
factors: short-circuit current (Jsc), open-circuit current (Voc),
and fill factor (FF). For attaining enhanced Jsc, the polymers
should have a broad absorption in the solar spectrum to
ensure effective harvesting of the solar photons and high
charge carriers mobility.3 Generally, such a broad absorption
can be achieved by designing more planar molecular geome-
tries and more rigid structures in p-conjugated polymer.
Recently, p-conjugated polymers composed of electron-donat-
ing and electron-withdrawing units in the polymer backbone
have been prepared with expecting that the absorbance can
be shifted to longer wavelength because of the formation of
inter- and/or intramolecular charge-transfer complex. In
addition, morphology of the donor–acceptor blend film plays
very important role in an enhancement of Jsc. The optimiza-
tion of the phase separation and domain size of polymer and
fullerene in the blend film has been performed by several
methods such as varying component ratio of polymer and
fullerene, annealing of blend film, and addition of cosolvents
or additives.4 Voc of OPV is proportional to the gap between
highest occupied molecular orbital (HOMO) level of donor
polymer and lowest unoccupied molecular orbital (LUMO)
level of the acceptor fullerene material.5 Currently, polymers
with lower HOMO energy level are used as electron-donating
polymer for polymer solar cells (PSCs) with high Voc. Several
strategies to lower HOMO energy level of polymers have
been reported so far. Hou et al.6 currently reported that
P3HDTTT bearing an alkyl group in the side chain of terthio-
phene repeating unit showed lower HOMO energy level than
poly(3-hexylthiophene) (P3HT). HOMO energy level of the
polymer was lowered with decreasing the number of elec-
tron-donating alkyl group in the polythiophene side chain.
The Voc of PSC based on P3HDTTT exhibited higher value
than that based on P3HT, implying that the Voc value directly
depends on the HOMO energy level of a donor polymer. The
introduction of strong electron-withdrawing groups such as
nitro, cyano, halogen, ester, and ketone into the side chain is
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, 234–241 (2011) 2010 Wiley Periodicals, Inc.
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ARTICLE
also well-known method to lower HOMO energy level of the
corresponding polymers. Heeger and coworkers7 reported
the preparation of poly(3,4-dicyanothiophene) (PDCTh) from
2,5-diiodo-3,4-dicyanothophene by in situ thermal polymer-
ization. PDCTh showed very low HOMO energy level of ꢀ6.7
eV because of electron-deficient cyano groups substituted
directly on thiophene unit. Casalbore-Miceli et al.8 reported
the polythiophene composed of 3,30-dialkoxy-2,20-bithio-
phene unit and 3-dicyanoethenylthiophene in the main chain
(PolyCN), which was prepared by chemical and electrochemi-
cal polymerization. They described an intramolecular charge
transfer between the oxygen lone pair of the alkoxy group
and the dicyanoethenyl group, which led an appearance of
absorption peak in the longer wavelength. However, in terms
of HOMO energy level, PolyCN showed slightly lowered
HOMO energy level (ꢀ5.1 eV) compared with P3HT (ꢀ4.8 to
ꢀ5.1 eV).
sion chromatography with TOSOH HLC-8120GPC using a cali-
bration curve of polystyrene standards and THF as an
eluent. TGAs were conducted with a TA instrument TG-
DTA6200 at a heating rate of 10 ꢂC/min under nitrogen.
HOMO levels of polymers were measured using a photo-elec-
tron spectrometer AC-2 (Riken Keiki) by measuring ionic
potential of polymer film in air.14
Device Fabrication and Characterization
of Polymer Solar Cell
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]-phe-
nyl C61 butyric acid hexyl ester (PC61BH) as an electron
acceptor in the weight ratio of 1:1 or 1:2 (wt/wt).
PEDOT:PSS was spin coated on the precleaned ITO glass sub-
ꢂ
strate and heated at 150 C 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) characteristics under air mass 1.5 global solar simulated
light (AM1.5G at 100 mW/cm2, highly uniform irradiation
system using 150W Xe lamp as light source, Wacom Electric)
irradiation. We calibrated the light intensity using a standard
cell for amorphous silicon (a-Si) solar cells. J–V characteris-
tics were measured using a Current–Voltage Source Meter
(Keithley 2410, Keithley) at room temperature in the glove
box filled with inert gas.
Herein, we report the synthesis and characterization of novel
polythiophenes PTDCN and PTCNME composed of a thio-
phene unit substituted with long alkyl chains that assured
the high solubility of the polymers and terthiophene units
with strong electron-withdrawing groups, 2,2-dicyanoethenyl
[ACH¼¼C(CN)2]
and
2-cyano-2-methoxycarbonylethenyl
[ACH¼¼C(CO2Me)CN] in the side chains. Their optical and
thermal properties were also investigated by UV–vis spec-
troscopy and thermogravimetric analysis (TGA). We also
describe the photovoltaic properties of the polymers by their
utilization for fabricating solar cells with the structure of
ITO/PDOT:PSS/polymer:PCBH/LiF/Al.
EXPERIMENTAL
Synthesis
Materials
2-(5,500-Dibromo-[2,20;50,200]terthiophen-30-
ylmethylene)malononitrile (5)
THF and toluene were dried over sodium/benzophenone
ketyl and distilled before use. Chloroform and DMF were dis-
tilled over CaH2. Methanol and dichloromethane were used
as received. 3-Thiophenecarboxaldehyde, 3,4-dibromothio-
phene, 2-tributylstannyl thiophene, Pd(PPh3)2Cl2, Pd(PPh3)4,
magnesium, bromooctane, trimethylstannyl chloride, a hex-
ane solution of n-BuLi, N-bromosuccinmide (NBS), acrylro-
nitrile, cyanoacetic acid, and piperidine were used without
purification. 2,5-Dibromo-3-thiophenecarboxyaldehyde (1),9
[2,20;50,200]terthiophene-30-carboxyaldehyde (3),10 5,500-dibromo-
[2,20;50,200]terthiophene-30-carboxyaldehyde (4),11 cyanoacetic
acid methyl ester,12 and 3,4-dioctylthiophene (8)13 were pre-
pared according to the literature.
To a solution of 4 (0.33 g, 0.76 mmol) in acetonitrile (5 mL)
were added malononitrile (59.4 mg, 0.90 mmol) and piperi-
dine (3 lL) at room temperature. After refluxing for 30 min,
the solution was cooled to room temperature. Methanol (10
mL) was added into the resulting solution to give a precipi-
tate, which was collected by filtration and then recrystallized
from chloroform/methanol to afford 0.33 g (76%) of 5 as a
reddish solid.
1
ꢂ
mp: 155.5–156.5 C. H NMR (400 MHz, CDCl3, d, ppm): 6.94
(d, J ¼ 4.0 Hz, th-H, 1H), 7.03 (d, J ¼ 4.0 Hz, th-H, 1H), 7.04
(d, J ¼ 4.0 Hz, 1H), 7.19 (d, J ¼ 4.0 Hz, th-H, 1H), 7.77 (s, th-
H, 1H), 7.94 (s, >C¼¼CHA, 1H). 13C NMR (100 MHz, CDCl3, d,
ppm): 82.30, 112.74, 113.82, 113.95, 117.34, 121.54, 125.98,
130.25, 130.95, 131.11, 131.73, 133.11, 135.97, 137.97,
144.84, 149.84. Anal Calcd for C16H6Br2N2S3: C 39.85, H
1.25, Br 33.14, N 5.81, S 19.95. Found: C 39.48, H 1.09, Br
32.74, N 5.64, S 19.54.
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 spectrometer
was measured in chloroform (1.0 ꢁ 10ꢀ5 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 s and then 3000 rpm for 60 s) using 10 mg/mL
chloroform solution. Number-average (Mn) and weight-aver-
age (Mw) molecular weights were determined by size exclu-
2-Cyano-3-(5,500-dibromo-[2,20;50,200]terthiophen-30-yl)
Acrylic Acid Methyl Ester (6)
To a solution of 4 (1.00 g, 2.30 mmol) and cyanoacetic acid
methyl ester (0.30 g, 3.00 mmol) in acetonitrile (30 mL) was
added piperidine (5 lL) at room temperature. After refluxing
BULK HETEROJUNCTION POLYMER SOLAR CELLS, LEE ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 1 1H NMR spectra of monomers 5 (a) and 6 (b) in CDCl3 at room temperature.
for 1 h, the solution was cooled to room temperature to
afford a precipitate, which was collected by filtration and
washed with methanol. Obtained solid was purified by
recrystallization from chloroform/methanol to afford 0.96 g
(81%) of 6 as an orange solid.
2,5-Bis(trimethylstannyl)-3,4-dioctylthiophene (9)
To a solution of 8 (3.22 g, 10.4 mmol) in hexane (10 mL)
was added n-BuLi (13.9 mL, 21.9 mmol) (1.58 M in hexane).
The solution was refluxed for 30 min and then cooled to
ꢂ
ꢀ45 C. After Me3SnCl (4.78 g, 24.0 mmol) was added to the
ꢂ
solution at ꢀ45 C, the solution was stirred at the tempera-
1
ꢂ
mp: 193.5–195.0 C. H NMR (400 MHz, CDCl3, d, ppm): 3.94
(s, ACH3, 3H), 6.94 (d, J ¼ 4.0 Hz, th-H, 1H), 7.02 (d, J ¼ 4.0
Hz, th-H, 1H), 7.03 (d, J ¼ 4.0 Hz, 1H), 7.15 (d, J ¼ 4.0 Hz,
th-H, 1H), 8.05 (s, th-H, 1H), 8.32 (s, >C¼¼CHA, 1H). 13C
NMR (100 MHz, CDCl3, d, ppm): 53.36, 102.50, 113.34,
115.43, 116.48, 122.59, 125.55, 129.63, 130.97, 131.15,
131.55, 134.00, 136.53, 137.04, 143.74, 145.61, 163.15. Anal
Calcd for C17H9Br2NO2S3: C 39.63, H 1.76, Br 31.01, N 2.72,
O 6.21, S 18.67. Found: C 39.69, H 1.62, Br 30.98, N 2.58, S
18.20.
ture for 2 h. The reaction was finished by adding water and
the mixture was extracted with ether. The extract was dried
over MgSO4 and organic solvent was removed under reduced
pressure to give a reside which was purified by column
chromatography (silica gel, hexane—5% Et3N) to afford 6.11
g (92%) of 9 as a colorless oil.
1H NMR (400 MHz, CDCl3, d, ppm): 0.35 (s, ASnMe3, 18H),
0.89 (t, J ¼ 6.6 Hz, ACH3, 6H), 1.29–1.52 (m, ACH2A, 24H),
2.51–2.60 (m, ACH2A, 4H). 13C NMR (100 MHz, CDCl3, d,
SCHEME 1 Preparation of monomers.
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ARTICLE
SCHEME 2 Preparation of PTDCN and PTCNME.
ppm): ꢀ7.89, 14.11, 22.69, 29.26, 29.48, 30.23, 31.19, 31.90,
afforded terthiophene carboxyaldehyde 3 in 81% yield.10
Dibromoterthiophene 4 was obtained by the bromination of
3 with NBS in 62% yield.11 Knoevenagel reaction of 4 with
malononitrile or cyanoacetic acid methyl ester afforded
monomers 5 and 6 in 88% and 81% yields, respectively. 1H
NMR spectra of dibromoterthiophene monomers 5 and 6 are
shown in Figure 1. Distannylthiophene monomer 9 was pre-
pared as follows: first, the reaction of 3,4-dibromothiophene
7 with octylmagnesium bromide in the presence of Ni cata-
lyst afforded dioctylthiophene 8, quantitatively.13 Lithiation
of 8 with n-BuLi followed by an addition of trimethylstannyl
chloride gave 9 as colorless oil in 92% yield (Scheme 1).
32.96, 138.68, 151.00.
PTDCN
To a mixture of 5 (0.32 g, 0.50 mmol), 9 (0.24 g, 0.50
mmol), and Pd(PPh3)4 (11.6 mg, 0.010 mmol) were added
anhydrous toluene (10 mL) and anhydrous DMF (2.5 mL).
The solution was bubbled with argon for 10 min and then
stirred for 48 h at 120 ꢂC under argon atmosphere. After
cooling to room temperature, the solution was poured into
hexane (300 mL) to afford black precipitate, which was col-
lected with filtration. The collected solid was dissolved in
THF (50 mL) and then purified by column chromatography
(silica gel, PSQ100B) using THF as an eluent. After organic
solvent was removed under reduced pressure, hexane (200
mL) was added to give precipitate, which was collected with
filtration to afford 0.19 g (59%) of PTDCN as a black solid.
Preparation of PTDCN and PTCNME
PTDCN and PTCNME were prepared by the Stille cross-cou-
pling polymerization using Pd(PPh3)4 as a catalyst. The reac-
tions of bis(trimethylstannyl)thiophene 9 with dibromoter-
thiophenes 5 and 6 afforded PTDCN and PTCNME in 61%
and 63% yields, respectively (Scheme 2). Polymers were
purified by reprecipitation from hexane to remove oligomeric
compounds and subsequent column chromatography (silica
gel, PSQ100B) using THF as an eluent to remove metal cata-
lyst. Molecular weight (Mw) and distribution (Mw/Mn) of the
resulting polymers were 2.20 ꢁ 104 and 3.3 for PTDCN and
2.30 ꢁ 104 and 3.8 for PTCNME, respectively (Table 1). The
resulting polymers showed good solubility in organic sol-
vents such as THF, chloroform, chlorobenzene, and dichloro-
benzene. The structures of PTDCN and PTCNME were
assigned by 1H NMR and IR spectroscopies (Figs. 2 and 3).
IR spectra of PTDCN and PTCNME exhibited CN stretching
bands at 2222 cmꢀ1 and 2220 cmꢀ1, respectively. In addi-
tion, the peak intensity of CN bond of PTDCN was stronger
than that of PTCNME. A strong signal assignable to C¼¼O
1H NMR (400 MHz, CDCl3, d, ppm): 0.89 (br, ACH3, 6H),
1.18–1.49 (br, ACH2A, 16H), 1.55–1.80 (br, ACH2A, 8H),
2.51–2.79 (m, ACH2A, 4H), 6.90–7.26 (m, th-H, 4H), 7.91
(br, th-H, 1H), 8.02 (br, >C¼¼CHA, 1H). IR (ATR, cmꢀ1): 3064
(aromatic CAH), 2922, 2852 (aliphatic CAH), 2222 (CBN).
UV–vis (chloroform) kmax, nm (e): 439 (22,900). Anal Calcd
for C36H40N2S4: C 68.74, H 6.41, N 4.45, S 20.39. Found: C
66.28, H 6.11, N 3.80, S 17.35.
PTCNME
PTCNME was prepared under the same conditions as that
for the preparation of PTDCN using 6 (0.32 g, 0.50 mmol)
and 9 (0.26 g, 0.50 mmol). After purification by column
chromatography, 0.27 g (81%) of PTCNME was obtained as a
black solid.
1H NMR (400 MHz, CDCl3, d, ppm): 0.88 (br, ACH3, 6H),
1.15–1.52 (br, ACH2A, 16H), 1.68 (br, ACH2A, 8H), 2.51–
2.85 (m, ACH2A, 4H), 3.95 (s, ACH3, 3H), 6.90–7.24 (m, th-
H, 4H), 8.10 (br, th-H, 1H), 8.48 (br, >C¼¼CHA, 1H). IR (ATR,
cmꢀ1): 3061 (aromatic CAH), 2922, 2851 (aliphatic CAH),
2220 (CBN), 1726 (C¼¼O). UV–vis (chloroform) kmax, nm (e):
452 (23,500). Anal Calcd for C37H43NO2S4: C 67.13, H 6.55,
N 2.12, O 4.83, S 19.37. Found: C 64.56, H 6.55, N 2.02,
S 16.99.
stretching band of PTCNME was observed at 1726 cmꢀ1
.
Thermal stability of the polymers was investigated with TGA
TABLE 1 Synthesis and Characterization of PTDCN
and PTCNME
Molecular Weightb
TGA (ꢂC)c
Td5 Td10
Polymer
Yield (%)a
Mw
Mw/Mn
2.2 ꢁ 104
2.3 ꢁ 104
3.3
3.8
405
319
435
374
RESULTS AND DISCUSSION
PTDCN
PTCNME
a
61
63
Preparation of Monomers
2,5-Dibromo-3-thiophenecarboxyaldehye (1) was prepared
by the reaction of 3-thiophenecarboxyalhedye with NBS.9
The Stille coupling of 1 with 2-tributylstannyl thiophene
Insoluble part in hexane.
Measured by GPC (THF).
Under N2, 10 ꢂC/min.
b
c
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE
2
1H NMR spectra of
PTDCN (a) and PTCNM (b) in
CDCl3.
under nitrogen atmosphere. Both polymers were stable up
to 250 ꢂC. Td5s of PTDCN and PTCNME were observed at
405 ꢂC and 319 ꢂC, respectively. PTDCN with rigid dicyano
groups showed higher thermal stability than PTCNME. Glass
transition temperature (Tg) by differential scanning calorime-
try measurement was not detectable for both polymers.
were measured by the same methods to be 1.90 eV and
ꢀ4.81 eV, respectively. Energy diagram of PTDCN, PTCNME,
and P3HT are shown in Figure 5. It is clearly shown that poly-
thiophenes bearing strong electron-withdrawing substituent
in the side chain significantly reduce HOMO energy levels.
PTDCN exhibited much lower HOMO energy level than P3HT
by 0.72 eV. Optical band gaps of PTDCN and PTCNME are
narrower than that of P3HT by about 0.2 eV, implying that
the LUMO energy levels of PTDCN and PTCNME are signifi-
cantly lowered as well by introducing strong electron-with-
drawing substituents. PTDCN showed lower HOMO energy
level than PTCNME by 0.24 eV. This result suggests dicyano
ethenyl group works more effectively to reduce HOMO
energy level than a single cyano ethenyl group. Our current
results are comparable with those of PolyCN described in
the Introduction section. The band gap of PTDCN (1.70 eV)
was larger than that of PolyCN (1.3 eV). This result can be
Optical Properties of PTDCN and PTCNME
Optical properties of PTDCN and PTCNME are summarized
in Table 2. Absorption spectra obtained in solution and film
states are shown in Figure 4. First, UV–vis spectra of a solu-
tion state were measured in chloroform to give absorption
maxima at 439 nm and 452 nm for PTDCN and PTCNME,
respectively. These results imply that methyl ester group in
the side chain of polythiophene was more effective for elon-
gation of p-conjugation length than cyano group in the solu-
tion state. PTCNME also showed slightly higher molar
absorption coefficient than PTDCN (Table 2). Absorption
spectra of both polymer films prepared by spin cast on a
glass substrate were measured as well. Absorption maxima
of polymer films appeared at 473 nm and 466 nm for
PTDCN and PTCNME, respectively. Absorption maxima of
both polymers were observed at longer wavelength than
those in a solution state, indicating the effective planariza-
tion and p–p interaction of the polymer backbone in the
solid state. Interestingly, PTDCN film revealed an absorption
maximum at longer wavelength than that of PTCNME, which
is a different result from that in a solution state. This may
be attributed to the formation of more planar polymer struc-
ture by rigid dicyanoethenyl groups in a film state. Both
polymer films also showed broad shoulders at 600 nm and
560 nm for PTDCN and PTCNME, respectively. Optical band
gaps calculated from the absorption onset of polymer films
are 1.70 eV and 1.73 eV for PTDCN and PTCNME, respec-
tively. HOMO levels of both polymers were measured to be
ꢀ5.53 eV for PTDCN and ꢀ5.29 eV for PTCNME. For a com-
parison, optical band gap and HOMO energy level of P3HT
explained with
a structural difference. PolyCN involves
alkoxy groups as a strong electron-donating group and dicya-
noethenyl groups a strong electron-withdrawing group in the
side chain, which may lead the formation of intramolecular
charge-transfer complex to give an absorption maximum in
FIGURE 3 IR spectra of PTDCN and PTCNME.
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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
PTDCN
PTCNME
a
439 (2.29 ꢁ 104)
452 (2.35 ꢁ 104)
Conc. ¼ 1.0 ꢁ 10ꢀ5 M.
473
466
ꢀ5.53
ꢀ5.29
ꢀ3.83
ꢀ3.56
1.70
1.73
b
c
Molar absorption coefficient (Mꢀ1/m).
Measured with AC-2.
d
e
Calculated from the equation, Eg ¼ |HOMO ꢀ LUMO|.
Calculated from the onset absorption, Eg ¼ 1240/konset
.
the longer wavelength. On the other hand, PTDCN contains
soluble dialkyl groups, which are weaker electron-donating
groups than alkoxy groups. Therefore, the formation of weak
intramolecular charge-transfer complex is predicted for
PTDCN. In terms of HOMO energy level, PTDCN revealed
much lower energy level than PolyCN (ꢀ5.1 eV) probably
because of no alkoxy groups as electron-donating substitu-
ents in the side chain of PTDCN.
Evaluation of Solar Cells Based on PTDCN and PTCNME
We previously investigated the PCE of solar cells based on
the blend film of P3HT as electron donor and [6,6]-phenyl
C61 butyric acid methyl ester (PC61BM) or PC61BH as an
electron acceptor.15 Our previous results showed the solar
cell based on P3HT:PC61BH blend gave higher PCE than that
based on P3HT:PC61BM blend. The higher miscibility
between P3HT and PC61BH may lead the increase in PCE.
Our new polymers PTDCN and PTCNME were blended with
PC61BH as an electron acceptor and applied to solar cells
with a configuration of ITO/PEDOT-PSS/polymer:PC61BH/
LiF/Al (Fig. 6). Polymer and PC61BH were blended with a ra-
tio of 1:1 or 1:2 (wt/wt) in chlorobenzene (10 wt/vol %)
and spin coated on PEDOT-PSS layer, which was precasted
on ITO electrode. The devices were finalized by the deposi-
tion of LiF and Al layer as top electrodes on the blend film.
Figure 7 shows current density–voltage (J–V) curves of the
solar cells estimated under AM1.5 at 100 mW/cm2. The
associated photovoltaic parameters Voc, Jsc, FF, and PCE are
summarized in Table 3. For comparison, the photovoltaic pa-
rameters of the solar cell based on P3HT are included. Solar
cells based on PTDCN showed higher Voc (0.69–0.73 V) com-
pared with those based on PTCNME (0.60–0.61 V). This
result is attributed to the lower HOMO energy level of
PTDCN than that of PTCNME (Fig. 4). Jsc of solar cell was sig-
nificantly affected by the blend ratio of polymer:PC61BH; Jscs
FIGURE 4 UV–vis spectra of PTDCN and PTCNME in solution
(a) and film states (b).
FIGURE 5 HOMO and LUMO energy levels of P3HT, PTDCN,
FIGURE 6 The structures of solar cell and PC61BH.
and PTCNME.
BULK HETEROJUNCTION POLYMER SOLAR CELLS, LEE ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 8 Absorption spectra of blend films of PTDCN:PC61BH
¼ 1:1 and PTCNME:PC61BH ¼ 1:1.
FIGURE 7 Current density–voltage (J–V) characteristics of PV
cells based on polymer PC61BH with a ratio of 1:1 and 1:2.
seems that the higher charge mobility of regioregular P3HT
(97%) and the charge balance of the P3HT:PC61BH device
led enhanced Jsc and FF. It was also found that although
band gaps of PTDCN and PTCNME are narrower than that of
P3HT, the absorption intensity of P3HT:PC61BH film was
stronger than those of PTDCN:PC61BH and PTCNME:PC61BH,
which may lead lowered Jsc and PCE of the solar cells with
PTDCN and PTCNME compared to that with P3HT.
of solar cells with a blend ratio of polymer:PC61BH ¼ 1:1
were relatively higher than those with polymer:PC61BH ¼
1:2. PCEs of solar cells based on both PTDCN and PTCNME
are 0.56–0.72% and 0.22–0.30% for polymer:PC61BH ¼ 1:1
and 1:2 blend, respectively. When the ratio of PC61BH was
increased in the blend, almost no change in Voc and FF, and
a drastic decrease of Jsc were observed. This is attributable
to the relatively weak absorbance of polymer:PC61BH ¼ 1:2
blend film in UV–vis spectrum compared with that of poly-
mer:PC61BH ¼ 1:1 blend film. Solar cells based on PTDCN
revealed higher PCEs than those of PTCNME under the same
fabrication conditions. This result can be explained with UV–
vis spectra of polymer:PC61BH blend films. Unnormalized
UV–vis spectra of polymer and PCBH blend films are shown
in Figure 8. Absorbance of PC61BH was observed at 334 nm
with a strong intensity. Both spectra showed very similar
absorption pattern except for the shoulder peak at around
600 nm. The blend film of PTCNME exhibited stronger
absorption at around 600 nm than that of PTDCN, which
might lead higher Jsc and PCE for PTCNME (Table 3). In
terms of Voc, the solar cells with new polymers PTDCN and
PTCNME exhibited higher values than that with P3HT
because of lower HOMO levels of PTDCN and PTCNME than
P3HT. However, Jsc and FF of the solar cells with PTDCN and
PTCNME gave much lower values than that with P3HT. It
CONCLUSIONS
We prepared novel polythiophenes composed of dioctylthio-
phene as a soluble unit and a terthiophene having strong
electron-withdrawing groups, dicyanoethenyl (PTDCN) and
cyano-methoxycarbonylethenyl groups (PTCNME) via Stille
cross-coupling polymerization. By using strong electron-with-
drawing substitutes in the side chain, both polymers showed
very low HOMO energy levels of ꢀ5.53 eV and ꢀ5.29 eV for
PTDCN and PTCNME, respectively. Band gaps of PTDCN and
PTCNME were 1.70 eV and 1.73 eV, respectively.ꢂ Both poly-
mers possessed high thermal stability up to 250 C. The BHJ
PSCs fabricated by using blend films of polymers as the elec-
tron donor and PC61BH as the electron acceptor exhibited
0.56% and 0.72% of PCEs for PTDCN and PTCNME, respec-
tively, under the illumination of AM1.5 at 100 mW/cm2.
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TABLE 3 Characteristic Properties of Polymer Solar Cells
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