Synthesis, characterization, and application to polymer solar cells of polythiophene derivatives with ester- or ketone-substituted phenyl side groups

Article abstract of DOI:10.1002/pola.27513

J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 875-887 Four polythiophene derivatives including regiorandom polymers P1, P2, and P3 and a regioregular polymer P4, containing phenyl side chain with electron-withdrawing groups such as an ester and a ketone

Full text of DOI:10.1002/pola.27513

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Synthesis, Characterization, and Application to Polymer Solar Cells of
Polythiophene Derivatives with Ester- or Ketone-Substituted Phenyl Side
Groups
Minh Anh Truong,1 Seijiro Fukuta,2 Tomoyuki Koganezawa,3 Yu Shoji,4 Mitsuru Ueda,2
Tomoya Higashihara2,5
1Department of Organic and Polymer Materials Chemistry, Tokyo University of Agriculture and Technology, 2-24-16 Naka-Cho,
Koganei, Tokyo 184-8588, Japan
2Department of Polymer Science and Engineering, Graduate School of Science and Engineering, Yamagata University,
4-3-16 Jonan, Yonezawa 992-8510, Japan
3Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo-Cho, Sayo-Gun, Hyogo 679-5198, Japan
4Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology,
2-12-1 O-Okayama, Meguro-Ku, Tokyo 152-8552, Japan
5Japan Science and Technology Agency (JST), PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
Correspondence to: T. Higashihara (E-mail: thigashihara@yz.yamagata-u.ac.jp)
Received 25 October 2014; accepted 26 November 2014; published online 00 Month 2015
DOI: 10.1002/pola.27513
ABSTRACT: Four polythiophene derivatives including regioran-
dom polymers P1, P2, and P3 and a regioregular polymer P4,
containing a phenyl side chain with electron-withdrawing car-
bonyl groups such as an ester and a ketone at the 3-position of
the thiophene ring, were synthesized by Stille coupling reac-
tion. Bulk-heterojunction polymer solar cells (PSCs) based on
orbit energy level of P4. The short p–p stacking distance
(0.355 nm) in the parallel direction to the substrate and “face-
on” rich orientation were observed by the grazing incidence
wide-angle X-ray scattering experiment, which might reflect
higher Jsc and FF values of the P4:[6,6]-phenyl-C71-butyric acid
C
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methyl ester (PC71BM) PSC device than others.
2015 Wiley
these polymers as p-type semiconductors and [6,6]-phenyl-C61
-
Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 00,
butyric acid methyl ester (PCBM) were fabricated, and their
photovoltaic performances were evaluated for the first time.
The PSC devices based on the regioregular polymer
000–000
KEYWORDS: conducting polymers; conjugated polymers; high-
performance polymers; high Voc value; low HOMO level; poly-
mer solar cells; polythiophene; Stille coupling
P4:PCBM 5 1:2 (w/w) exhibited a high-open-circuit voltage (Voc
)
of 0.943 V because of the low-lying highest occupied molecular
develop this kind of technology in large areas, and their
widespread commercialization is limited. Instead of silicon-
based solar cells, in the past few years, polymer solar cells
(PSCs), which have a lot of potential advantages such as
light weight, flexibility, and fast/cheap roll-to-roll produc-
tion, have attracted much attention in both academic and
industrial research studies and have been rapidly developed
as one of the most promising candidates for low-cost solar
cells.4–12 In general, the most widely used device structure
of PSCs currently involves the concept of bulk heterojunc-
tions (BHJs), where the electron donor, such as conjugated
polymers, plays the role of a p-type organic semiconductor,
and the electron acceptor, such as fullerene derivatives,
plays the role of a n-type organic semiconductor. These two
types of organic semiconductor are blended and
INTRODUCTION In the recent years, several serious environ-
mental problems from climate change to environmental pol-
lution or the inadequacy of fossil fuels have been caused
by the overuse of nonrenewable resources. Among all of
the new technologies that have been developed to solve
these problems, the solar cell has been considered as one
of the most promising approaches because of its ability to
directly convert energy of light from solar radiation into
electricity by the photovoltaic effect without exhausting
toxic gases into the atmosphere. Until now, with the high
power conversion efficiency (PCE) of more than 25%,
silicon-based solar cells are still the most efficient photovol-
taic technologies.1–3 However, silicon-based solar cells have
some disadvantages, for example, the lack of flexibility and
high cost production processes, thus it is difficult to
Additional Supporting Information may be found in the online version of this article.
C
V
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FIGURE 1 Molecular structures of the polythiophene derivatives.
sandwiched between transparent indium tin oxide (ITO)
and metal electrodes.6
attained a
relatively high Voc of 0.86 V.27 Ueda and
coworkers28 have previously reported the synthesis of
poly{3-[40-(300,700-dimethyloctoxy)phenyl]thiophene} (P3PhT)
by introducing an alkoxyphenyl group at the 3-position of
each thiophene ring and successfully lowered the HOMO
energy level from 24.93 eV for P3HT to 25.13 eV for
P3PhT. A new polythiophene derivative PDCBT was recently
designed and synthesized by Ma and coworkers.29 By attach-
ing electron-withdrawing carboxylate substituents to the
side chain, it successfully decreased the HOMO level from
24.76 eV for P3HT to 25.26 eV for PDCBT. These results
indicated that the introduction of appropriate substituents is
also an effective way to lower the HOMO energy level of p-
conjugated polymers.
Among the large number of conjugated polymer systems,
which have been synthesized and investigated as donor
materials, regioregular poly(3-hexylthiophene) (P3HT) is
known as one of the most studied conjugated polymers.
Because P3HT has a semicrystalline structure, high hole
mobility, and also a favorable mixing ratio with fullerene
acceptors, the combination of P3HT:[6,6]-phenyl-C61-butyric
acid methyl ester (PCBM) as an active layer leads to solar
cell efficiencies of up to 5%.13–16 However, the open-circuit
voltage (Voc) of P3HT-based solar cells is still limited to
about 0.6 V because of the relatively high-lying highest occu-
pied molecular orbit (HOMO) level (24.76 eV)17,18 of P3HT.
Therefore, improving the Voc of polythiophene derivative-
based solar cells by a molecular structure design should be
an interesting approach to increase their PCE along with
other physical approaches.
In this study, to extend the molecular design of polythio-
phene derivatives with lower HOMO energy levels, four poly-
thiophene derivatives including regiorandom polymers P1(1-
1, 1–2, and 1–3), P2, and P3 and a regioregular polymer P4
(Fig. 1), containing a phenyl side chain with electron-
withdrawing carbonyl groups such as an ester and ketone at
the 3-position of the thiophene ring, were synthesized by
Stille coupling reaction. The chemical structure, molecular
weight, and thermal, optical, electrochemical, and photovol-
taic properties of the polymers were fully characterized. All
polymers showed a deeper HOMO energy level than that of
P3HT, and the BHJ PSC based on the regioregular polymer
P4:PCBM 5 1:2 (w/w) exhibited the high Voc value of 0.943
V. Based on the grazing incidence wide-angle X-ray scattering
(GIWAXS) experiment, a very short p-stacking distance
(0.355 nm) was observed for the P4 film with a “face-on”
rich orientation in which the p-plane tends to lie parallel to
the substrate, possibly reflecting the relatively high Jsc and
FF values of the P4:PCBM and P4:[6,6]-phenyl-C71-butyric
acid methyl ester (PC71BM) PSC devices.
There are only a few reports about the synthesis of new pol-
ythiophene derivatives with lower HOMO energy levels than
that of P3HT. Among these, the incorporation of aromatic
heterocyclic rings into a polythiophene backbone by copoly-
merization is one of the successful strategies, leading to an
enhanced oxidative stability when compared with P3HT.19–26
For example, the polythiophene derivative P3HDTTT with
fewer electron-donating groups, which was synthesized and
reported by Yang and coworkers, showed a lower HOMO
level and thus higher Voc than P3HT in PSCs.19 By attaching
the alkoxycarbonyl substitution instead of the hexyl side
chain of poly(3-hexylthienylene vinylene) (P3HTV), a new
polymer, P3CTV, which has its HOMO energy level shifted
downward by 0.21 eV when compared with P3HTV, was syn-
thesized. The solar cell device based on this polymer
2
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EXPERIMENTAL
rinsed in water, and finalized with UV–ozone treatment. As
an anode buffer layer, MoO3 was vacuum deposited with a
thickness of 20 nm on the ITO electrode. Polymer:fullerene
BHJ layer was formed on MoO3 layer by spin coating (300–
600 rpm) of a blended solution, which contains P1–P4 as a
donor and PCBM or PC71BM as an acceptor with an opti-
mized ratio in chlorobenzene. The coating process was per-
formed in a nitrogen glove box. After drying the films, the
substrate was transferred from glove box to a vacuum cham-
ber without being exposed to air. The evaporation of POPy2
was carried out at a pressure of about 2 3 1024 Pa. After
the deposition of POPy2 layer (4.5 nm), the top electrode
(Al) was successively deposited with a metal shadow mask,
which defines 5-mm stripe pattern perpendicular to the ITO
stripe. The photocurrent of the fabricated PSC devices was
investigated with a sweeping voltage using a Keithley 2400
source measurement unit controlled by a computer under
simulated solar light using an AM1.5G light source with 100
mW cm22 intensity. Incident light intensity was calibrated to
1 Sun (100 mW cm22) with a standard Si photodiode (BS-
520; Bunkoukeiki, Japan). The current density versus voltage
(J–V) characteristics was measured for an area of 0.16 cm2.
Materials
Tetrahydrofuran (THF) was dried over sodium benzophe-
none and distilled under nitrogen before use. 2,5-Bis(trime-
thylstannyl)thiophene was prepared according to the
literature.24 All other reagents and solvents were used with-
out further purification.
General Measurements
The molecular weights and dispersities (Ð) were measured
by size exclusion chromatography (SEC) on a JASCO GUL-
LIVER 1500 equipped with a pump, an absorbance detector
(UV, k 5 254 nm), and three polystyrene gel columns based
on a conventional calibration curve using polystyrene stand-
ards. CHCl3 (40 ꢀC) was used as a carrier solvent at a flow
rate of 1.0 mL min21 1H NMR and 13C NMR spectra were
.
recorded on Bruker Avance-400 spectrometer in CDCl3 cali-
brated to tetramethylsilane as an internal standard
(dH 5 0.00). Thermal analysis was performed on a Seiko
EXSTAR 6000 TG/DTA 6300 thermal analyzer at a heating
rate of 10 ꢀC min21 for thermogravimetry (TG) and a TA
Instruments Q-100 connected to a cooling system at a heating
rate of 10 ꢀC min21 for differential scanning calorimetry
(DSC). UV–vis absorption spectra were recorded using a Hita-
chi U-4100 spectrophotometer. For the thin film spectra, poly-
mers were first dissolved in CHCl3, followed by filtering
through a 0.45-lm pore size PTFE membrane syringe filter
and then drop-casted onto quartz substrate. Cyclic voltamme-
try (CV) was performed with the use of a three-electrode cell
in which ITO was used as a working electrode, and the poly-
mer film was coated on it in 0.5 3 0.7 cm2. A platinum wire
was used as an auxiliary electrode. All cell potentials were
taken in a 0.1 mol L21 acetonitrile solution of tetrabutylam-
monium perchlorate at a scan rate of 0.1 V s21 with the use
of a homemade Ag/AgCl, KCl saturated reference electrode.
GIWAXS Measurement
The samples were prepared according to the following
method: 4.0 mg of the polymers was dissolved in 0.4 mL of
CHCl3. These solutions were drop-cast on Si wafers with fil-
tration (pore size 5 0.40 mm) followed by air-drying for 30
min. Then, the samples were completely dried under vacuum
at room temperature for 1 h. The thickness of films was
almost 1 mm. GIWAXS measurements were conducted at
beamline BL19B2 of SPring-8, Japan. The sample was irradi-
ated at a fixed incident angle on the order of 0.12ꢀ through
a Huber diffractometer with an X-ray energy of 12.398 keV
(X-ray wavelength k 5 0.10 nm), and the GIWAXS patterns
were recorded with a 2D image detector (Pilatus 300K) with
the sample-to-detector distance of 174 mm.
Fabrication and Characterization of Organic Field-Effect
Transistor Devices
We used solution-based fabrication to make field-effect tran-
sistor devices with P1–P4. The source and drain electrodes
were formed on a 300-nm-thick layer of thermally grown
Synthesis of (3,7-Dimethyloctyl)24-bromobenzoate (1b)
To 4-bromobenzoic acid (6.00 g, 29.8 mmol), thionyl chloride
(30 mL) was added, and then the reaction mixture was
stirred at 90 ꢀC for 6 h. After the reaction, thionyl chloride
was removed under reduced pressure. Then, to the residue,
CH2Cl2 (75 mL), triethylamine (4.2 mL, 30.0 mmol), and 3,7-
dimethyloctan-1-ol (11.4 mL,ꢀ 59.6 mmol) were added, and
the solution was stirred at 0 C overnight. After the reaction,
the product was extracted with CH2Cl2, and the organic layer
was washed with water. After drying the solution over anhy-
drous MgSO4 followed by filtration, the filtrate was evapo-
rated under reduced pressure. The crude product was
purified by silica gel column chromatography using hexane/
ethyl acetate (EtOAc; 9/1, v/v) as an eluent to give 1b as a
colorless oil (9.86 g, 97%).
SiO2 on
a Sb-doped n-type Si substrate [capacitance
(Ci) 5 11.5 nF cm22] by photolithography of Au (90 nm)/Cr
(10 nm) layers. The standard channel length and width were
10 and 500 mm, respectively. The channels were spin coated
with a 1.0 wt % CHCl3 solution of P1–P4. The devices were
fabricated under a dry N2 atmosphere.
Fabrication and Characterization of PSC Devices
We fabricated solar cells to evaluate photovoltaic characteris-
tics of P1–P4. The cell structure was conventional as fol-
lows: glass/ITO/MoO3 (20 nm)/polymer:fullerene (100 nm)/
phenyl-dipyrenylphosphine oxide (POPy2; 4.5 nm)/Al. The
ITO layer on glass substrate was 145 nm thick with a sheet
resistance of 8 X22. The stripe pattern with 5 mm width of
the ITO layer was etched by conventional photolithographic
technique. Prior to the formation of the buffer layer, the pat-
terned ITO glass was ultrasonically cleaned using surfactant,
1H NMR (300 MHz, CDCl3, d, ppm, 25 ꢀC): 7.88 (d, J 5 8.7
Hz, ArH, 2H), 7.55 (d, J 5 8.6 Hz, ArH, 2H), 4.34 (t, J 5 7.7
Hz, AOCH2CH2A, 2H), 1.81–1.11 (m, ACH2A, 10H), 0.95 (d,
J 5 6.4 Hz, ACH3, 3H), 0.86 (d, J 5 6.6 Hz, ACH3, 6H). 13C
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NMR (75 MHz, CDCl3, d, ppm, 25 ꢀC): 165.8, 131.6, 131.0,
129.4, 127.8, 63.8, 39.1, 37.1, 35.5, 29.9, 27.9, 24.6, 22.7,
22.6, 19.6. Anal. Calcd for C17H25BrO2: C, 59.77; H, 7.32.
Found: C, 59.90; H, 7.18.
at 0 ꢀC under argon atmosphere. The reaction mixture was
stirred at 0 ꢀC for 4 h and then at room temperature for
18 h. After the reaction, the resultant pale yellow solution
was diluted with CHCl3, and 1 M aqueous NaOH was added
to quench the residual NBS. The organic layer was separated
by extraction and dried over anhydrous MgSO4. After filtra-
tion, the solvent was evaporated under reduced pressure.
The crude product was then purified by silica gel column
chromatography using hexane/EtOAc (19/1, v/v) to give 1e
as a colorless oil (1.21 g, 90%).
Synthesis of (3,7-Dimethyloctyl)-4-(3-thienyl)benzoate
(1c)
A N,N-dimethylformamide (DMF) solution (50 mL) of 1b
(5.30 g, 15.5 mmol), 3-thiopheneboronic acid (2.30 g, 18.6
mmol), K2CO3 (6.40 g, 46.6 mmol), and Pd(PPh3)4 (0.80 g,
0.77 mmol) was degasseꢀ d by three freeze–thaw–pump cycles
and then stirred at 90 C for 18 h under argon atmosphere.
After the reaction, the product was extracted with CHCl3,
and the organic layer was washed with water. After drying
the solution over anhydrous MgSO4 followed by filtration,
the filtrate was evaporated under reduced pressure. The
crude product was purified by silica gel column chromatog-
raphy using hexane/EtOAc (9/1, v/v) to give 1c as an orange
oil (5.05 g, 94%).
1H NMR (300 MHz, CDCl3, d, ppm, 25 ꢀC): 8.09 (d, J 5 8.6
Hz, ArH, 2H), 7.57 (d, J 5 8.6 Hz, ArH, 2H), 7.04 (s, ArH, 1H),
4.37 (t, J 5 7.5 Hz, AOCH2CH2A, 2H), 1.86–1.12 (m, 11H),
0.96 (d, J 5 6.4 Hz, ACH3, 3H), 0.86 (d, J 5 6.6 Hz, ACH3,
13
ꢀ
6H). C NMR (75 MHz, CDCl3, d, ppm, 25 C): 165.2, 140.0,
137.2, 130.3, 128.9, 128.7, 127.4, 110.7, 107.6, 62.7, 38.1,
36.1, 34.5, 28.9, 26.9, 23.6, 21.6, 21.5, 18.6. Anal. Calcd for
C21H26Br2SO2: C, 50.21; H, 5.22. Found: C, 50.19; H, 5.21.
1H NMR (300 MHz, CDCl3, d, ppm, 25 ꢀC): 8.05 (d, J 5 8.4
Hz, ArH, 2H), 7.62 (d, J 5 8.4 Hz, ArH, 2H), 7.50 (t, J 5 2.1
Hz, ArH, 1H), 7.37 (m, ArH, 2H), 4.35 (t, J 5 7.5 Hz,
AOCH2CH2A, 2H), 1.83–1.14 (m, 11H), 0.95 (d, J 5 6.4 Hz,
ACH3, 3H), 0.86 (d, J 5 6.6 Hz, ACH3, 6H). 13C NMR (75
MHz, CDCl3, d, ppm, 25 ꢀC): 166.8, 141.6, 140.3, 130.6,
129.4, 127.0, 126.5, 122.2, 63.9, 39.6, 37.6, 36.0, 30.4, 28.3,
25.0, 23.1, 23.0, 20.0. Anal. Calcd for C21H28SO2: C, 73.14; H,
8.12. Found: C, 72.66; H, 8.06.
Synthesis of (3,7-Dimethyloctyl)24-(2-bromo-5-iodo-3-
thienyl)benzoate (1f)
To a solution of 1d (1.44 g, 3.41 mmol) in CHCl3 and acetic
acid (30 mL, 1:1, v/v), N-iodosuccinimide (NIS; 0.84 g, 3.75
mmol) was added, and then the reaction mixture was stirred
at room temperature overnight. After the reaction, saturated
aqueous Na2S2O3 solution was added to quench the excess
amount of NIS, and the product was extracted with EtOAc
and then washed with 1 M aqueous NaOH, water, and brine.
After drying the solution over anhydrous MgSO4 followed by
filtration, the filtrate was evaporated under reduced pres-
sure. The crude product was purified by silica gel column
chromatography using hexane/EtOAc (19/1, v/v) to give 1f
as a dark red oil (1.60 g, 86%).
Synthesis of (3,7-Dimethyloctyl)-4-(2-bromo-3-
thienyl)benzoate (1d)
To a solution of 1c (2.39 g, 6.93 mmol) in THF (25 mL), N-
bromosuccinimide (NBS; 1.29 g, 7.28 mmol) was added at 0
ꢀC under argon atmosphere. The reaction mixture was
stirred at 0 ꢀC for 4 h and then at room temperature for
18 h. After the reaction, the resultant pale yellow solution
was diluted with Et2O, and 1 M aqueous NaOH was added to
quench the residual NBS. The organic layer was separated
by extraction and dried over anhydrous MgSO4. After filtra-
tion, the solvent was evaporated under reduced pressure.
The crude product was then purified by silica gel column
chromatography using hexane/EtOAc (19/1, v/v) to give 1d
as a colorless oil (2.69 g, 92%).
1H NMR (300 MHz, CDCl3, d, ppm, 25 ꢀC): 8.08 (d, J 5 8.5
Hz, ArH, 2H), 7.56 (d, J 5 8.5 Hz, ArH, 2H), 7.21 (s, ArH, 1H),
4.37 (t, J 5 7.6 Hz, AOCH2CH2A, 2H), 1.83–1.12 (m, 12H),
0.96 (d, J 5 6.4 Hz, ACH3, 3H), 0.86 (d, J 5 6.6 Hz, ACH3,
13
ꢀ
6H). C NMR (75 MHz, CDCl3, d, ppm, 25 C): 166.2, 142.2,
138.3, 138.0, 129.9, 129.7, 128.4, 112.4, 72.5, 63.7, 39.2,
37.1, 35.5, 29.9, 27.9, 24.6, 22.7, 22.6, 19.6. Anal. Calcd for
C21H26BrISO2: C, 45.87; H, 4.73. Found: C, 46.32; H, 4.75.
1H NMR (300 MHz, CDCl3, d, ppm, 25 ꢀC): 8.09 (d, J 5 8.4
Hz, ArH, 2H), 7.62 (d, J 5 8.3 Hz, ArH, 2H), 7.32 (d, J 5 5.6
Hz, ArH, 1H), 7.04 (d, J 5 5.7 Hz, ArH, 1H), 4.37 (t, J 5 7.5
Hz, AOCH2CH2A, 2H), 1.86–1.12 (m, 11H), 0.96 (d, J 5 6.4
Hz, ACH3, 3H), 0.86 (d, J 5 6.6 Hz, ACH3, 6H). 13C NMR (75
MHz, CDCl3, d, ppm, 25 ꢀC): 166.3, 140.1, 139.3, 129.6,
129.5, 128.8, 128.5, 126.3, 109.6, 63.6, 39.1, 37.1, 35.5, 29.9,
27.9, 24.6, 22.7, 22.6, 19.6. Anal. Calcd for C21H27BrSO2: C,
59.51; H, 6.37. Found: C, 59.49; H, 6.38.
Synthesis of (3,7-Dimethyloctyl)23-bromobenzoate (2b)
To 3-bromobenzoic acid (3.00 g, 14.9 mmol), thionyl chloride
(15 mL) was added, and then the reaction mixture was
stirred at 90 ꢀC for 6 h. After the reaction, thionyl chloride
was removed under reduced pressure. Then, to the residue,
CH2Cl2 (37.5 mL), triethylamine (5.7 mL, 41.0 mmol), and
3,7-dimethyloctan-1-ol (2.10 mL, 11.0 mmol) were added,
and the solution was stirred at 0 ꢀC overnight. After the
reaction, the product was extracted with CH2Cl2, and the
organic layer was washed with water. After drying the solu-
tion over anhydrous MgSO4 followed by filtration, the filtrate
was evaporated under reduced pressure. The crude product
was purified by silica gel column chromatography using
Synthesis of (3,7-Dimethyloctyl)24-(2,5-dibromo-3-
thienyl)benzoate (1e)
To a solution of 1c (0.92 g, 2.69 mmol) in CHCl3 and acetic
acid (15 mL, 1:1, v/v), NBS (1.05 g, 5.92 mmol) was added
4
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hexane/EtOAc (9/1, v/v) as an eluent to give 2b as a color-
less oil (4.78 g, 94%).
130.8, 129.7, 128.9, 128.7, 128.5, 126.2, 109.3, 63.7, 39.2,
37.2, 35.6, 30.0, 28.0, 24.7, 22.7, 22.6, 19.6. Anal. Calcd for
C
21H27BrSO2: C, 59.51; H, 6.37. Found: C, 59.45; H, 6.33.
1
ꢀ
H NMR (300 MHz, CDCl3, d, ppm, 25 C): 8.15 (t, J 5 1.8 Hz,
ArH, 1H), 7.94 (d, J 5 7.8 Hz, ArH, 1H), 7.64 (d, J 5 8.0 Hz,
ArH, 1H), 7.30 (t, J 5 7.9 Hz, ArH, 1H), 4.35 (t, J 5 7.8 Hz,
AOCH2CH2A, 2H), 1.81–1.11 (m, 11H), 0.95 (d, J 5 6.4 Hz,
ACH3, 3H), 0.86 (d, J 5 6.6 Hz, ACH3, 6H). 13C NMR (75
MHz, CDCl3, d, ppm, 25 ꢀC): 165.2, 135.7, 132.5, 132.4,
129.9, 128.1, 122.4, 64.0, 39.1, 37.1, 35.4, 29.9, 27.9, 24.6,
22.7, 22.6, 19.6. Anal. Calcd for C17H25BrO2: C, 59.77; H,
7.32. Found: C, 59.70; H, 7.13.
Synthesis of (3,7-Dimethyloctyl)23-(2-bromo-5-iodo-3-
thienyl)benzoate (2e)
To a solution of 2d (1.20 g, 2.84 mmol) in CHCl3 and acetic
acid (20 mL, 1:1, v/v), NIS (0.67 g, 2.98 mmol) was added,
and then the reaction mixture was stirred at room tempera-
ture overnight. After the reaction, saturated aqueous
Na2S2O3 solution was added to quench the excess amount of
NIS, and the product was extracted with EtOAc. After drying
the solution over anhydrous MgSO4 followed by filtration,
the filtrate was evaporated under reduced pressure. The
crude product was purified by silica gel column chromatog-
raphy using hexane/EtOAc (19/1, v/v) to give 2e as a dark
red oil (1.47 g, 95%).
Synthesis of (3,7-Dimethyloctyl)23-(3-thienyl)benzoate
(2c)
A DMF solution (30 mL) of 2b (2.69 g, 7.88 mmol), 3-
thiopheneboronic acid (1.21 g, 9.46 mmol), K2CO3 (3.26 g,
23.6 mmol), and Pd(PPh3)4 (0.45 g, 0.39 mmol) was
degassed by three freeze–thaw–pump cycles and then
stirred at 90 ꢀC for 18 h under argon atmosphere. After
the reaction, the product was extracted with CHCl3, and the
organic layer was washed with water. After drying the solu-
tion over anhydrous MgSO4 followed by filtration, the fil-
trate was evaporated under reduced pressure. The crude
product was purified by silica gel column chromatography
using hexane/EtOAc (9/1, v/v) to give 2c as an orange oil
(2.30 g, 85%).
1
ꢀ
H NMR (300 MHz, CDCl3, d, ppm, 25 C): 8.15 (t, J 5 1.6 Hz,
ArH, 1H), 8.02 (d, J 5 7.9 Hz, ArH, 1H), 7.67 (d, J 5 7.7 Hz,
ArH, 1H), 7.50 (t, J 5 7.8 Hz, ArH, 1H), 7.22 (s, ArH, 1H),
4.37 (t, J 5 6.8 Hz, AOCH2CH2A, 2H), 1.83–1.12 (m, 12H),
0.96 (d, J 5 6.4 Hz, ACH3, 3H), 0.86 (d, J 5 6.6 Hz, ACH3,
13
ꢀ
6H). C NMR (75 MHz, CDCl3, d, ppm, 25 C): 166.2, 142.3,
138.4, 133.9, 132.8, 130.9, 129.6, 129.1, 128.5, 112.1, 72.4,
63.8, 39.2, 37.1, 35.6, 30.0, 28.0, 24.7, 22.7, 22.6, 19.6. Anal.
Calcd for C21H26BrISO2: C, 45.87; H, 4.73. Found: C, 45.85; H,
4.75.
1
ꢀ
H NMR (300 MHz, CDCl3, d, ppm, 25 C): 8.26 (t, J 5 1.6 Hz,
ArH, 1H), 7.95 (d, J 5 8.0 Hz, ArH, 1H), 7.75 (d, J 5 7.8 Hz,
ArH, 1H), 7.53 (m, ArH, 1H), 7.47–7.37 (m, ArH, 3H), 4.37 (t,
J 5 6.8 Hz, AOCH2CH2A, 2H), 1.83–1.14 (m, 11H), 0.95 (d,
J 5 6.4 Hz, ACH3, 3H), 0.86 (d, J 5 6.6 Hz, ACH3, 6H). 13C
NMR (75 MHz, CDCl3, d, ppm, 25 ꢀC): 166.2, 140.9, 135.7,
130.7, 130.3, 128.5, 127.7, 127.1, 126.6, 125.8, 120.6, 63.4,
38.8, 36.8, 35.2, 29.7, 27.6, 24.3, 22.2, 19.3. Anal. Calcd for
Synthesis of 4-Bromododecanoylbenzene (3b)
To bromobenzene (7.17 g, 45.7 mmol) and aluminum chlo-
ride (3.65 g, 27.4 mmol), dodecanoyl chloride (5.00 g, 5.43
mL, 22.8 mmol) was slowly added, and the reaction mixture
was then stirred at 50 ꢀC for 1 h. After the reaction, the
reaction mixture was poured into ice water, and the product
was extracted with CH2Cl2 and washed with HCl solution (2
N) and brine. After drying the solution over anhydrous
MgSO4 followed by filtration, the filtrate was evaporated
under reduced pressure. The crude product was purified by
recrystallization from methanol:acetone (2:5, v/v) to afford
3b as a colorless plate-like solid (7.17 g, 93%).
C
21H28SO2: C, 73.14; H, 8.12. Found: C, 72.84; H, 8.03.
Synthesis of (3,7-Dimethyloctyl)23-(2-bromo-3-
thienyl)benzoate (2d)
To a solution of 2c (1.70 g, 4.95 mmol) in THF (20 mL), NBS
(0.92 g, 5.20 mmol) was added at 0 ꢀC under argon atmos-
phere. The reaction mixture was stirred at 0 ꢀC for 4 h and
then at room temperature for 18 h. After the reaction, the
solution was diluted with Et2O, and 1 M aqueous NaOH was
added to quench the residual NBS. The organic layer was
separated by extraction and dried over anhydrous MgSO4.
After filtration, the solvent was evaporated under reduced
pressure. The crude product was then purified by silica gel
column chromatography using hexane/EtOAc (19/1, v/v) to
give 2d as a colorless oil (1.49 g, 71%).
1H NMR (300 MHz, CDCl3, d, ppm, 25 ꢀC): 7.73 (d, J 5 8.5
Hz, ArH, 2H), 7.52 (d, J 5 8.5 Hz, ArH, 2H), 2.83 (t, J 5 7.4
Hz, ACOCH2CH2A, 2H), 1.63 (q, alkyl group, 2H), 1.31–1.08
(m, alkyl group, 16H), 0.89 (t, J 5 6.7 Hz, ACH3, 3H). 13C
NMR (75 MHz, CDCl3, d, ppm, 25 ꢀC): 199.8, 136.2, 132.2,
130.0, 128.3, 39.0, 32.3, 30.0, 29.9, 29.7, 24.7, 23.1, 14.5.
Synthesis of 1-(4-(Thiophen-3-yl)phenyl)dodecanone
(3c)
A
thiopheneboronic acid (1.33 g, 10.5 mmol), K2CO3 (3.61 g,
26.2 mmol), and Pd(PPh3)4 (0.50 g, 0.43 mmol) was
degassed by three freeze–thaw–pump cycles and then stirred
at 90 ꢀC for 18 h under argon atmosphere. After the reac-
tion, the product was extracted with CHCl3, and the organic
layer was washed with water. After drying the solution over
anhydrous MgSO4 followed by filtration, the filtrate was
DMF solution of 3b (2.95 g, 8.72 mmol), 3-
1
ꢀ
H NMR (300 MHz, CDCl3, d, ppm, 25 C): 8.22 (t, J 5 1.7 Hz,
ArH, 1H), 8.02 (d, J 5 7.8 Hz, ArH, 1H), 7.74 (d, J 5 7.7 Hz,
ArH, 1H), 7.53 (t, J 5 7.8 Hz, ArH, 1H), 7.34 (d, J 5 5.6 Hz,
ArH, 1H), 7.05 (d, J 5 5.7 Hz, ArH, 1H), 4.37 (t, J 5 6.8 Hz,
AOCH2CH2A, 2H), 1.86–1.12 (m, 11H), 0.96 (d, J 5 6.4 Hz,
ACH3, 3H), 0.86 (d, J 5 6.6 Hz, ACH3, 6H). 13C NMR (75
MHz, CDCl3, d, ppm, 25 ꢀC): 166.4, 140.2, 135.2, 132.9,
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evaporated under reduced pressure. The residue was puri-
fied by silica gel column chromatography using CHCl3 as an
eluent and recrystallization from CHCl3:methanol (5:1, v/v)
to give 3c as a white solid (2.72 g, 97%).
138.5, 136.3, 132.3, 130.6, 130.0, 129.8, 129.5, 127.4, 125.1,
64.0, 38.6, 37.6, 36.9, 30.3, 28.3, 25.0, 23.1, 23.0, 20.0. Anal.
Calcd for C46H56S3O4: C, 71.83; H, 7.34. Found: C, 71.32; H,
7.35.
1H NMR (300 MHz, CDCl3, d, ppm, 25 ꢀC): 7.98 (d, J 5 8.2
Hz, ArH, 2H), 7.65 (d, J 5 8.2 Hz, ArH, 2H), 7.54 (t, J 5 2.1
Hz, ArH, 1H), 7.40 (m, ArH, 2H), 2.94 (t, J 5 7.3 Hz,
ACOCH2CH2A, 2H), 1.83–1.26 (m, 19H), 0.87 (t, J 5 6.7 Hz,
ACH3, 3H). C NMR (75 MHz, CDCl3, d, ppm, 25 C): 199.9,
141.3, 140.0, 135.8, 128.8, 126.7, 126.4, 126.2, 121.9, 38.7,
31.9, 29.6, 29.5, 29.5, 29.4, 29.3, 14.0. Anal. Calcd for
Synthesis of Bis(30,70-dimethyloctyl) 4,40-(5,500-Dibromo-
[2,20,50,200-terthiophene]-3,300-diyl)dibenzoate (4b)
To a solution of 4a (0.29 g, 0.37 mmol) in DMF (8 mL), NBS
(0.13 g, 0.75 mmol) was added at 0 ꢀC under argon atmos-
13
ꢀ
ꢀ
phere, and the mixture was stirred at 0 C for 4 h and then
at room temperature for 18 h. After the reaction, the solu-
tion was diluted with Et2O, and then 1 M aqueous NaOH
was added to quench the residual NBS. The organic layer
was then washed with brine and water. After drying the
solution over MgSO4 followed by filtration, the filtrate was
evaporated under reduced pressure. The crude product was
then purified by silica gel column chromatography using hex-
ane/EtOAc (19/1, v/v) as an eluent to give 4b as a red oil
(0.35 g, 100%).
C22H30SO: C, 77.14; H, 8.83. Found: C, 77.11; H, 8.97.
Synthesis of 1-(4-(2,5-Diiodothiophen-3-
yl)phenyl)dodecanone (3d)
To a solution of 3c (0.76 g, 2.23 mmol) in CHCl3 and acetic
acid (20 mL, 1:2, v/v), NIS (1.10 g, 4.91 mmol) was added,
and then the reaction mixture was stirred at room tempera-
ture overnight. After the reaction, saturated aqueous
Na2S2O3 solution was added to quench the excess amount of
NIS, and the product was extracted with CHCl3. After drying
the solution over anhydrous MgSO4 followed by filtration,
the filtrate was evaporated under reduced pressure. The
crude product was purified by silica gel column chromatog-
raphy using hexane/CHCl3 (2/1, v/v) to give 3d as a white
solid (1.10 g, 90%).
1H NMR (300 MHz, CDCl3, d, ppm, 25 ꢀC): 7.95 (d, J 5 8.4
Hz, ArH, 2H), 7.34 (d, J 5 8.4 Hz, ArH, 2H), 7.00 (s, ArH, 1H),
6.72 (s, ArH, 1H), 4.36 (t, J 5 6.8 Hz, AOCH2CH2A, 2H),
1.86–1.12 (m, 11H), 0.96 (d, J 5 6.4 Hz, ACH3, 3H), 0.86 (d,
J 5 6.6 Hz, ACH3, 6H). 13C NMR (75 MHz, CDCl3, d, ppm, 25
ꢀC): 166.6, 148.4, 138.7, 138.5, 136.5, 133.1, 130.2, 130.1,
129.4, 128.0, 127.6, 127.2, 69.8, 39.5, 37.5, 35.9, 30.3, 28.3,
25.0, 23.1, 20.0. Anal. Calcd for C46H54Br2S3O4: C, 59.61; H,
5.87. Found: C, 59.64; H, 5.93.
1H NMR (300 MHz, CDCl3, d, ppm, 25 ꢀC): 7.99 (d, J 5 8.58
Hz, ArH, 2H), 7.53 (d, J 5 8.5 Hz, ArH, 2H), 7.10 (s, 1H), 2.99
(t, J 5 7.4 Hz, ACOCH2CH2A, 2H), 1.83–1.26 (m, 20H), 0.87
(d, J 5 6.7 Hz, ACH3, 3H). 13C NMR (75 MHz, CDCl3, d, ppm,
25 ꢀC): 199.9, 147.7, 139.5, 138.3, 136.3, 128.8, 128.2, 38.7,
31.9, 29.7, 29.4, 29.3, 24.3, 22.6, 14.1. Anal. Calcd for
General Procedure for Polymerization (P1-1, P1–2, P1–3,
P2, P3, and P4)
Pd2(dba)3 (6 mol %) and P(o-tolyl)3 (50 mol %) were added
to a toluene solution of monomer (1.0 equiv) and 2,5-bis(tri-
methylstannyl)thiophene (1.0 equiv) in argon, which was
deoxidized by bubbling with dry argon for 1 h. The solution
was refluxed for 3 days. After the reaction, the solution was
cooled down to room temperature and poured into methanol
to precipitate the polymer. The polymer was purified by
Soxhlet extraction using methanol, acetone, and CHCl3. After
evaporating CHCl3, the polymer was further purified by flash
silica gel column chromatography using CHCl3 as an eluent,
and the solution was concentrated and dried under reduced
pressure to give a polymer as a dark red solid.
C22H28I2SO: C, 44.46; H, 4.75. Found: C, 44.40; H, 4.73.
Synthesis of Bis(30,70-dimethyloctyl) 4,40-([2,20,50,200-
Terthiophene]-3,300-diyl)dibenzoate (4a)
To a mixture of 25 mL dry toluene in a two-necked flask,
2,5-bis(trimethylstannyl)thiophene (0.33 g, 0.81 mmol), com-
pound 1d (0.72 mg, 1.70 mmol), Pd2(dba)3 (0.04 g, 0.05
mmol), and P(o-Tolyl)3 (0.02 g, 0.09 mmol) were added. The
mixture was degassedꢀ by three freeze–pump–thaw cycles
and then heated to 90 C for 18 h under argon atmosphere.
After the reaction, the solution was cooled to room tempera-
ture, and the product was extracted with EtOAc and washed
with water and brine. After drying the solution over MgSO4
followed by filtration, the filtrate was evaporated under
reduced pressure. The crude product was purified by silica
column chromatography using hexane/CHCl3 (1/1, v/v) and
then hexane/EtOAc (19/1, v/v) as eluents to give 4a (1.04 g,
80%) as a colorless liquid.
Synthesis of P1-1
The title polymer, P1-1, was synthesized according to the
general procedure performed with 0.30 g (0.55 mmol) of 1f,
0.22 g (0.55 mmol) of 2,5-bis(trimethylstannyl)thiophene,
0.03 g (0.020 mmol) of Pd (PPh3)4, and 30 mL of toluene.
After purification, the polymer P1-1 (0.16 g, 68%) was iso-
lated as a dark red solid.
1H NMR (300 MHz, CDCl3, d, ppm, 25 ꢀC): 7.93 (d, J 5 8.3
Hz, ArH, 2H), 7.32 (d, J 5 8.2 Hz, ArH, 2H), 7.21 (d, J 5 5.2
Hz, ArH, 1H), 6.97 (d, J 5 5.2 Hz, ArH, 1H), 6.69 (s, ArH, 1H),
4.35 (t, J 5 6.7 Hz, AOCH2CH2A, 2H), 1.81–1.11 (m, 11H),
0.95 (d, J 5 6.3 Hz, ACH3, 3H), 0.86 (d, J 5 6.6 Hz, ACH3,
1H NMR (300 MHz, CDCl3, d, ppm, 25 ꢀC): 7.98–6.66 (m,
ArH, 8H), 4.29 (t, J 5 5.5 Hz, AOCH2CH2A, 2H), 1.86–0.77
(m, alkyl group, 22H). 13C NMR (75 MHz, CDCl3, d, ppm,
25 ꢀC): 168.1, 128.8, 128.7, 128.1, 62.7, 38.1, 36.1, 34.5,
28.9, 26.6, 25.9, 23.6, 21.7, 21.6, 18.6.
13
ꢀ
6H). C NMR (75 MHz, CDCl3, d, ppm, 25 C): 166.8, 141.0,
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Synthesis of P1–2
end-capping reaction. After purification, the polymer P3
The title polymer, P1–2, was synthesized according to the
general procedure performed with 0.36 g (0.73 mmol) of 1e,
0.30 g (0.73 mmol) of 2,5-bis(trimethylstannyl)thiophene,
0.04 g (0.03 mmol) of Pd(PPh3)4, and 30 mL of toluene.
(0.20 g, 68%) was isolated as a dark red solid.
1H NMR (300 MHz, CDCl3, d, ppm, 25 ꢀC): 7.85–6.69 (m,
ArH, 7H), 2.89 (s, ACOCH2CH2A, 2H), 1.66 (s, alkyl group,
2H), 1.18 (s, alkyl group, 17H), 0.79 (s, ACH3, 3H). 13C NMR
(75 MHz, CDCl3, d, ppm, 25 ꢀC): 200.8, 129.7, 128.6, 39.1,
32.3, 30.1, 29.9, 29.8, 29.7, 24.7, 23.1, 14.5.
After refluxing for
3 days, 2-tributylstannyl thiophene
(0.02 mL) was added to the reaction, and after 2 h, 2-
bromothiophene (0.01 mL) was added. The mixture was
stirred overnight to complete the end-capping reaction. After
purification, the polymer P1–2 (0.22 g, 71%) was isolated as
a dark red solid.
Synthesis of P4
Pd2(dba)3 (0.0203 g, 0.020 mol%) and P(o-tolyl)3 (0.0563 g,
0.18 mmol) were added to a toluene solution of 4b (0.343 g,
0.37 mmol) and 2,5-bis(trimethylstannyl)thiophene (0.152 g,
0.37 mmol) in argon, which was deoxidized by bubbling
with dry argon for 1 h. The solution was refluxed for 3 days.
1H NMR (300 MHz, CDCl3, d, ppm, 25 ꢀC): 7.92–6.66 (m,
ArH, 7H), 4.29 (t, J 5 5.5 Hz, AOCH2CH2A, 2H), 1.86–0.77
(m, alkyl group, 22H). 13C NMR (75 MHz, CDCl3, d, ppm, 25
ꢀC): 168.1, 166.7, 140.5, 139.4, 135.9, 131.3, 130.2, 129.5,
129.2, 66.6, 64.1, 39.6, 37.5, 36.0, 30.4, 30.1, 28.3, 25.0, 23.1,
23.0, 20.0.
After refluxing for
3 days, 2-tributylstannyl thiophene
(0.02 mL) was added to the reaction, and after 2 h, 2-
bromothiophene (0.01 mL) was added. The mixture was
stirred overnight to complete the end-capping reaction. After
the end-capping reaction, the solution was cooled down to
room temperature and poured into methanol to precipitate
the polymer. The polymer was purified by Soxhlet extraction
using methanol, acetone, and CHCl3. After evaporating CHCl3,
the polymer was further purified by flash silica gel column
chromatography using CHCl3 as an eluent, and the solution
was concentrated and dried under reduced pressure to give
P4 (0.243 g, 77%) as a dark red solid.
Synthesis of P1–3
The title polymer, P1–3, was synthesized according to the
general procedure performed with 0.16 g (0.33 mmol) of 1e,
0.13 g (0.33 mmol) of 2,5-bis(trimethylstannyl)thiophene,
0.01 g (0.02 mmol) of Pd2(dba)3, 0.05 g (0.16 mmol) of P(o-
Tolyl)3, and 20 mL of toluene. After refluxing for 3 days, 2-
tributylstannyl thiophene (0.02 mL) was added to the reac-
tion, and after 2 h, 2-bromothiophene (0.01 mL) was added.
The mixture was stirred overnight to complete the end-
capping reaction. After purification, the polymer P1–3
(120 mg, 70%) was isolated as a dark red solid.
1H NMR (300 MHz, CDCl3, d, ppm, 25 ꢀC): 8.00 (d, J 5 8.1
Hz, ArH, 2H), 7.41 (d, J 5 7.9 Hz, ArH, 2H), 7.07 (s, ArH, 2H),
6.74 (s, ArH, 1H), 4.36 (t, J 5 5.5 Hz, AOCH2CH2A, 2H),
1.86–1.12 (m, 11H), 0.96 (d, J 5 6.2 Hz, ACH3, 3H), 0.86 (d,
J 5 6.6 Hz, ACH3, 6H). 13C NMR (75 MHz, CDCl3, d, ppm, 25
ꢀC): 166.7, 140.5, 139.2, 135.9, 131.9, 131.2, 130.2, 129.5,
127.3, 125.3, 124.5, 64.1, 39.6, 37.5, 36.0, 31.3, 30.4, 28.3,
25.0, 23.0, 20.0.
Synthesis of P2
The title polymer, P2, was synthesized according to the gen-
eral procedure performed with 0.30 g (0.54 mmol) of 2e,
0.21 g (0.54 mmol) of 2,5-bis(trimethylstannyl)thiophene,
0.03 g (0.03 mmol) of Pd2(dba)3, 0.08 g (0.27 mmol) of P(o-
Tolyl)3, and 25 mL of toluene. After refluxing for 3 days, 2-
tributylstannyl thiophene (0.02 mL) was added to the reac-
tion, and after 2 h, 2-bromothiophene (0.01 mL) was added.
The mixture was stirred overnight to complete the end-
capping reaction. After purification, the polymer P2 (0.21 g,
70%) was isolated as a dark red solid.
RESULTS AND DISCUSSION
Monomer Synthesis
To prepare polythiophene derivatives with low HOMO levels
while maintaining narrow energy band gaps, we selected
monomers containing a phenyl group with an electron-
withdrawing ester group (1e, 1f, 2e, and 4b) and ketone
group (3d). The thiophene unit has an electron density
lower than that of P3HT because of the electron-
withdrawing group in the side chain. We expect that the new
polymers will have (1) a narrower energy band gap than
that of P3HT because the presence of a phenyl group in the
side chain will extend the conjugation length of the polymer,
and (2) a deeper HOMO level than that of P3HT because of
the effect of electron-withdrawing groups, such as ester and
ketone groups, or in other words, the new polymers will
exhibit a better oxidative stability than that of P3HT. Further-
more, 3,7-dimethyloctyl and undecanyl groups were intro-
duced to increase the solubility of the polythiophene
derivatives. These monomers were successfully prepared in
good yield in four steps [Schemes 1(a) and 2], that is, esteri-
fication of bromobenzoic acid, Suzuki-Miyaura coupling
1H NMR (300 MHz, CDCl3, d, ppm, 25 ꢀC): 7.92–6.69 (m,
ArH, 7H), 4.27 (s, AOCH2CH2A, 2H), 1.86–0.77 (m, alkyl
group, 22H). 13C NMR (75 MHz, CDCl3, d, ppm, 25 ꢀC):
165.2, 132.6, 129.7, 129.1, 127.9, 127.4, 125.8, 125.5, 62.7,
38.1, 36.1, 34.4, 28.9, 28.6, 28.2, 26.9, 23.5, 21.7, 21.6, 18.6,
13.1.
Synthesis of P3
The title polymer, P3, was synthesized according to the gen-
eral procedure performed with 0.30 g (0.56 mmol) of 3d,
0.22 g (0.56 mmol) of 2,5-bis(trimethylstannyl)thiophene,
0.03 g (0.03 mmol) of Pd2(dba)3, 0.08 g (0.28 mmol) of P(o-
Tolyl)3, and 25 mL of toluene. After refluxing for 3 days, 2-
tributylstannyl thiophene (0.02 mL) was added to the
reaction, and after 2 h, 2-bromothiophene (0.01 mL) was
added. The mixture was stirred overnight to complete the
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SCHEME 3 Synthetic route to monomer having ketone group
at the p-position.
reaction, and bromination and iodination at the 2- and 5-
positions of the 3-aromaticthiophene group, and in three
steps (Scheme 3), that is, Friedel-Crafts reaction, Suzuki-
Miyaura coupling reaction, and iodination at both the 2- and
5-positions of the 3-aromaticthiophene group. Monomer 4b
was synthesized by the Stille coupling reaction between 1d
and 2,5-bis(trimethylstannyl)thiophene [Scheme 1(b)]. The
synthesized monomers were identified by 1H and 13C NMR
analyses and elemental analysis (EA).
Polymer Synthesis
The polymers of P1 (P1-1, P1–2, and P1–3), P2, P3, and P4
were synthesized by Stille coupling reactions between the
appropriate monomers and 2,5-bis(trimethylstannyl)thio-
phene (Scheme 4). At first, P1-1 and P1–2 were synthesized
by using Pd(PPh3)4 as a typical catalyst. Changing the cata-
lyst to Pd2(dba)3/P(o-Tolyl)3 somewhat improved the molec-
ular weight (P1–3). P1–2, P1–3, P2, P3, and P4 were end-
capped with thiophene rings. The polymers were purified by
Soxhlet extraction using methanol, acetone, and chloroform.
All of these polymers have a good solubility in the common
organic solvents, such as chloroform, toluene, and THF. The
molecular weights and dispersities (Ðs) of these polymers
were determined by SEC analysis with chloroform as the elu-
ent. The number–average molecular weight (Mn), the
weight–average molecular weight (Mw), and the Ð values of
these polymers were calculated and summarized in Table 1.
SCHEME 1 Synthetic route to (a) monomers having ester
group at the p-position and (b) monomer 4b.
Thermal Properties
The thermal properties of these polymers were analyzed by
thermogravimetric analysis (TGA) and DSC, and the results
are shown in Figure 2. The decomposition temperature Td,5%
of all the polymers are higher than 348 ꢀC, demonstrating
the sufficiently high thermal stability for applications to
PSCs. The DSC was used to investigate the possible crystalli-
zation of the prepared polymers. From the DSC measure-
ments of the regiorandom copolymers P1-1, P1–2, P2, and
P3, neither melting point nor glass transition temperature
was observed (see Supporting Information Fig. S1 for
enlarged DSC curves), suggesting that these copolymers tend
to be amorphous.
SCHEME 2 Synthetic route to monomer having ester group at
the m-position.
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SCHEME 4 Synthetic routes to polymers P1–P4.
In contrast, as shown in Figure 2 (bottom), there are exo-
thermic peaks for the melting temperatures (Tm) of P1–3 at
176 ꢀC and P4 at 224 ꢀC. The Tc values at 139 ꢀC (P1–3)
and 143 ꢀC (P4) were also clearly observed, indicating the
crystallinity of P1–3 and P4. These results were caused by
the effect of the high molecular weight in both cases and the
effect of the regioregular structure in the case of P4.
FIGURE 2 TGA curves of (a) P1-1, (b) P1–2, (c) P1–3, (d) P2, (e)
P3, and (f) P4, and (g) DSC thermograms of P1–P4 at the heat-
ing rate of 10 ꢀC min21 under a nitrogen atmosphere.
Optical and Electronic Properties
TABLE 1 Physical Properties of the Synthesized Polymers
The absorption spectra of the synthesized polymers meas-
ured in both dilute chloroform and in a thin film are shown
in Figure 3. The related optical parameters are summarized
in Table 2. The solution absorption spectra of the polymers
showed a broad absorption band in the range of 300–
700 nm with maximum absorption peaks at 471 nm (P1-1),
482 nm (P1–2), 494 nm (P1–3), 485 nm (P2), 468 nm (P3),
and 486 nm (P4). The absorption spectra of the polymers in
the thin film were red-shifted ꢁ50 nm when compared with
those in solution. This result indicates the formation of a p-
stacked intermolecular packing through interchain delocali-
zation or chain planarization. The optical band gaps (Egopt) of
the regiorandom polymers obtained from the absorption
Mn
Mw
Td,5%
(ꢀC)
Polymer
(g mol21 a
)
(g mol21 a
)
Ða
P1–1
P1–2
P1–3
P2
9,120
7,840
13,800
6,140
6,590
15,500
14,000
14,200
33,300
10,000
9,880
1.54
1.81
2.42
1.63
1.50
2.34
348
358
360
380
377
373
P3
P4
36,300
a
Mn, Mw, and Ð values of the polymers were determined by SEC in
CHCl3.
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obtain a lower HOMO energy level, the ideal polymer
requires a high molecular weight and end-capping. The poly-
mer which has the ester group at the m-position of phenyl
side chain (P2) and ketone group at the p-position of the
phenyl side chain (P3) showed the same band gap level as
the P1s. This result implied that the ester group and ketone
group showed a similar effect to the band gap of the synthe-
sized polymers; however, the polymer with an ester group at
the side chain tends to exhibit a lower HOMO level than that
of the others. The ester group at the m-position of the phe-
nyl side chain did not decrease the HOMO level because of
the small electronic effect. In contrast to the regiorandom
polymers, the regioregular polymer P4 exhibited a smaller
band gap (1.90 eV), probably due to chain planarization.
According to the previous reports, P3HT has maximum
absorption peak at 456 nm in CHCl3 solution and 560 nm in
the thin film.30 The optical band gap of P3HT is 2.02 eV,31
indicating that all synthesized polymers have slightly nar-
rower band gap than P3HT. It may be caused by the
extended p-conjugation length resulting from the introduc-
tion of aromatic side groups.
The CV curves recorded for the synthesized polymers refer-
enced to an Ag/AgCl reference electrode are shown in Figure
4. The HOMO energy levels of the synthesized polymers
were determined to be lower than 25.29 eV from the corre-
ox
sponding
E
E
according to the following equation:
onset
HOMO 5 2e[Eoonxset 2 E1/2(ferrocene) 1 4.80] (eV).32 Based on
the relationship between the HOMO energy level and Egopt
estimated from the UV–vis absorption spectrum of the film
sample (Egopt 5 EHOMO 2 ELUMO), the LUMO energy levels of
the polymers were calculated to be from 23.34 eV to 23.47
eV. All the polymers with an ester group or ketone group at
the p-position of the phenyl side chain exhibited deep HOMO
energy levels from 25.31 eV (P3) to 25.40 eV (P1–3). This
is due to the high electron-withdrawing effect. In contrast,
the polymer with the ester group at the m-position of the
phenyl side chain (P2) showed an HOMO energy level up to
25.29 eV. This is due to the weak electron-withdrawing
effect through the m-position. In the case of the regioregular
polymer P4, the lower LUMO level (23.47 eV) and slightly
higher HOMO level (25.37 eV) than those of the regiorandom
polymers were observed because of the high planarity of the
main chain. A comparison of the energy levels between the
FIGURE 3 UV–vis spectra of P1–P4 in chloroform solution (top)
and in thin films (bottom).
onset of the polymer films ranged from 1.94 to 1.97 eV. In
the case of P1, which has an ester group at the p-position of
the phenyl side chain, the polymer without an end cap with
the thiophene unit (P1-1) exhibited a smaller band gap than
that of the polymer with an end cap (P1–2 and P1–3), and
the polymer with a lower molecular weight (P1-1 and P1–2)
exhibited a smaller band gap than that of the polymer with a
higher molecular weight (P1–3). This result indicates that to
TABLE 2 Optical and Electrochemical Properties of P1–P4
(nm)a
kfmilamx (nm)
k
(nm)a
kfoilnmset (nm)
HOMO (eV)
LUMO (eV)b
Egopt (eV)c
sol
max
sol
onset
Polymer
k
P1–1
P1–2
P1–3
P2
471
482
494
485
468
486
521
530
531
538
523
530
588
598
627
625
578
625
639
633
628
637
634
652
25.34
25.35
25.40
25.29
25.31
25.37
23.40
23.39
23.43
23.34
23.44
23.47
1.94
1.96
1.97
1.95
1.96
1.90
P3
P4
a
c
Measurements performed in chloroform.
Calculated using the optical band gap energy.
Estimated from the onset of the absorption in thin films.
b
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of the P1-1, P1–2, P1–3, and P2 devices were found to be
0.16, 0.32, 0.75, and 0.21%, respectively. The Voc values of
the P1-1, P1–2, P1–3, and P2 devices were found to be
0.644, 0.728, 0.926, and 0.648 V, respectively. The high Voc
value for the P1–3 devices could be explained by the low
HOMO energy level of P1–3 (25.40 eV) when compared
with those of P1-1 (25.34 eV), P1–2 (25.35 eV), and P2
(25.29 eV). Among the organic photovoltaic devices based
on the polythiophene derivatives, the Voc value of the P1–3-
based device could be one of the highest values. Because the
synthesized polymers showed no crystallinities, the hole
mobilities of these polymers are too low (ꢁ1025 cm2 V21
s21). The FF values of the P1-1, P1–2, P1–3, and P2 devices
were relatively low, probably due to the low hole mobilities.
In contrast, the P4/PCBM system exhibited a higher PCE
than that of the regiorandom polymer P1–3/PCBM system.
The PCE of this system was found to be 1.95%. The Voc
value of the P4 device was 0.943 V, slightly higher than the
Voc obtained from the P1–3 device. The Jsc and FF values of
the regioregular polymer P4 device were much improved to
4.57 mA cm22 and 0.453, respectively. When using PC71BM,
which has a stronger absorption in the long-wavelength
region than PCBM as an electron acceptor, the P4/PC71BM
system showed a Voc value of 0.930 V, a FF value of 0.391,
and an improved Jsc value of 5.43 mA cm22 because of the
stronger absorption of PC71BM in the long-wavelength
region. As a result, the PCE value was slightly improved to
1.98%.
FIGURE 4 Cyclic voltammograms of polymer films on ITO
glass performed at a scanning rate of 100 mV s21 in CH3CN/
0.1 M Bu4NClO4.
Morphological Properties
To investigate the crystalline structure of the new p-type
semiconductors, the GIWAXS measurement was performed
on the pristine polymer films. The GIWAXS image and 1D
profiles of the P4 film, which showed the best PCE, are rep-
resentatively displayed in Figure 5. The P4 film shows weak
scattering peaks, and the higher order of scattering peaks
was not observed, suggesting that the P4 film has a low
crystallinity. Focusing on the scattering corresponding to the
lamella spacing (q 5 2–4 nm21), it has a higher intensity in
the qxy direction rather than that in the ꢁqz direction. The
result proves that the lamellar direction of the P4 film is
parallel to the qxy direction; in other words, P4 film has the
polymers and PCBM revealed that the LUMO energy levels of
these synthesized polymers are about 0.8 eV higher than that
of PCBM, indicating that excitons can successfully dissociate at
the interface between these polymers and PCBM.
Organic Photovoltaic Properties
PSC devices were fabricated using the synthesized polymers
as electron donors and PCBM as an electron acceptor. The
device structure was ITO/MoO3 (10 nm)/polymer:PCBM/
POPy2 (4.5 nm)/Al. The photovoltaic performances of the
polymer:PCBM devices are summarized in Table 3. The PCEs
TABLE 3 Polymer Solar Cell Performances of the Synthesized Polymers
Polymer
Acceptor
l (cm2 V21 s21 a
)
D:A
Voc (V)b
Jsc (mA cm22 b
)
FFb
PCE (%)
P1-1
P1–2
P1–3
P2
PCBM
PCBM
PCBM
PCBM
PCBM
PC71BM
1.50 3 1025
3.03 3 1026
2.24 3 1025
1.02 3 1024
1.02 3 1024
1.02 3 1024
1:0.9
1:1
0.644
0.728
0.926
0.648
0.943
0.930
0.92
1.53
2.52
0.48
4.57
5.43
0.271
0.283
0.321
0.306
0.453
0.391
0.16
0.32
0.75
0.21
1.95
1.98
1:1
1:2
P4
1:2
P4
1:2
a
b
Hole mobilities of the synthesized polymers were investigated by fab-
ricating thin film transistor (TFT) with a bottom-contact geometry using
Au and Cr as the source and drain electrode.
Devices were fabricated with a layer structure of ITO/MoO3 (10 nm)/
polymer:PCBM/POPy2 (4.5 nm)/Al.
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the qz direction. Consequently, the PSC using P4 showed
higher Jsc and FF values than those of the other polymers.
CONCLUSIONS
In summary, we successfully synthesized four polythiophene
derivatives including regiorandom polymers P1(1-1, 1–2,
and 1–3), P2, and P3 and the regioregular polymer P4, con-
taining a phenyl side chain with an electron-withdrawing
carbonyl group such as an ester and a ketone at the 3-
position of the thiophene ring. All the polymers showed a
good solubility in common organic solvents because of the
presence of alkyl side chains. P1-1, P1–2, P1–3, P2, and P3
exhibited UV–vis absorption peaks at 521, 530, 531, 538,
and 523 nm, respectively. A photovoltaic device using the
P1–3:PCBM (1:1) film as the active layer exhibited a Voc
value of 0.926 V, a Jsc value of 2.52 mA cm22, and a FF value
of 0.321. However, only a low PCE (0.75%) was achieved
due to no crystallinity of the synthesized polymers. We
assumed that the regiorandom structure is the main reason
for the no crystallinity of the synthesized polymers. The
regioregular polymer P4 showed a better absorption than
the regiorandom polymer P1–3 in the long-wavelength
region and a similar HOMO energy level to P1–3. A photo-
voltaic device using the P4:PC71BM (1:2) film as the active
layer exhibited a high Voc value of 0.943 V, a Jsc value of 4.57
mA cm22, a FF value of 0.453, and a PCE value up to 1.98%.
The Voc value of 0.943 V would be the highest among the
polythiophene derivative-based PSCs.
ACKNOWLEDGMENTS
This work received financial support from the Mitsubishi
Chemical Group Science and Technology Research Center, Inc.
This work was partly supported by the Japan Science and Tech-
nology Agency (JST), PRESTO program (JY 220176). GIWAXS
experiments were performed at the BL19B2 of SPring-8 with
the approval of the Japan Synchrotron Radiation Research Insti-
tute (JASRI; Proposal No. 2012B1728). The authors thank Itaru
Osaka (RIKEN) for operating the GIWAXS experiments.
FIGURE 5 GIWAXS image (a) and 1D profiles (b) of the pristine
P4 film drop-casted on the Si wafer.
face-on rich orientation. In addition, the P4 film has a small
lamellar spacing in the ꢁqz direction (2.21 nm) than that in
the qxy direction (2.87 nm), suggesting the interdigitation of
the side chains in the ꢁqz direction (edge-on orientation).
Considering these results, the interdigitation might prevent
the growth of the edge-on oriented crystalline structure so
that the face-on rich structure is induced.
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