Photovoltaic devices using semiconducting polymers containing head-to-tail-structured bithiophene, pyrene, and benzothiadiazole derivatives

Article abstract of DOI:10.1002/pola.26130

An alternating copolymer composed of heal-to-tail-structured 3,4'-dihexyl-2,2'-bithiophene (DHBT) and pyrene units [poly(DHBT-alt-PYR)] was synthesized using a Stille coupling reaction for use in photovoltaic devices as a p-type donor. For the reduction of the bandgap energy of poly(DHBT-alt-PYR), 4,7-bis(3'-hexyl-2,2'-bithiophen-5-yl)benzo[c][1,2,5]thiadiazole (BHBTBT) units were introduced into the polymer. Poly(DHBT-co-PYR-co-BHBTBT)s were synthesized using the same polymerization reaction. The synthesized polymers were soluble in common organic solvents and formed smooth thin films after spin casting. The optical bandgap energies of the polymers were obtained from the onset absorption wavelengths. The measured optical bandgap energy of poly(DHBT-alt-PYR) was 2.47 eV. As the BHBTBT content in the ter-polymers increased, the optical bandgap energies of the resulting polymers decreased. The bandgap energies of poly(50DHBT-co-40PYR-co-10BHBTBT) and poly(50DHBT-co-20PYR-co-30BHBTBT) were 1.84 and 1.73 eV, respectively. Photovoltaic devices were fabricated with a typical sandwich structure of ITO/PEDOT:PSS/active layer/LiF/Al using the polymers as electron donors and [6,6]-phenyl C₇₁-butyric acid methyl ester as the electron acceptor. The device using poly(50DHBT-co-20PYR-co- 30BHBTBT) showed the best performance among the fabricated devices, with an open-circuit voltage, short-circuit current, fill factor, and maximum power conversion efficiency of 0.68 V, 5.54 mA/cm², 0.35, and 1.31%, respectively.

Full text of DOI:10.1002/pola.26130

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
Photovoltaic Devices Using Semiconducting Polymers Containing Head-
to-Tail-Structured Bithiophene, Pyrene, and Benzothiadiazole Derivatives
Ji-Hoon Kim,1 Sunyoung Lee,2 In-Nam Kang,3 Moo-Jin Park,4 Do-Hoon Hwang1
1Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University,
Busan 609-735, Republic of Korea
2Department of Applied Chemistry, Kumoh National Institute of Technology, Kumi 730-701, Korea
3Department of Chemistry, The Catholic University of Korea, Bucheon 420-743, Korea
4Polymer Science and Engineering Program, KAIST, Daejeon 305-701, Korea
Correspondence to: D.-H. Hwang (E-mail: dohoonhwang@pusan.ac.kr)
Received 5 February 2012; accepted 14 April 2012; published online
DOI: 10.1002/pola.26130
ABSTRACT: An alternating copolymer composed of heal-to-tail-
structured 3,40-dihexyl-2,20-bithiophene (DHBT) and pyrene
units [poly(DHBT-alt-PYR)] was synthesized using a Stille cou-
pling reaction for use in photovoltaic devices as a p-type do-
nor. For the reduction of the bandgap energy of poly(DHBT-alt-
PYR), 4,7-bis(30-hexyl-2,20-bithiophen-5-yl)benzo[c][1,2,5]thiadia-
zole (BHBTBT) units were introduced into the polymer. Pol-
y(DHBT-co-PYR-co-BHBTBT)s were synthesized using the same
polymerization reaction. The synthesized polymers were solu-
ble in common organic solvents and formed smooth thin films
after spin casting. The optical bandgap energies of the poly-
mers were obtained from the onset absorption wavelengths.
The measured optical bandgap energy of poly(DHBT-alt-PYR)
was 2.47 eV. As the BHBTBT content in the ter-polymers
increased, the optical bandgap energies of the resulting
polymers decreased. The bandgap energies of poly(50DHBT-
co-40PYR-co-10BHBTBT)
and
poly(50DHBT-co-20PYR-co-
30BHBTBT) were 1.84 and 1.73 eV, respectively. Photovoltaic
devices were fabricated with a typical sandwich structure of
ITO/PEDOT:PSS/active layer/LiF/Al using the polymers as elec-
tron donors and [6,6]-phenyl C71-butyric acid methyl ester as
the electron acceptor. The device using poly(50DHBT-co-
20PYR-co-30BHBTBT) showed the best performance among the
fabricated devices, with an open-circuit voltage, short-circuit
current, fill factor, and maximum power conversion efficiency
of 0.68 V, 5.54 mA/cm2, 0.35, and 1.31%, respectively.
2012
C
V
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 000:
000–000, 2012
KEYWORDS: bulk heterojunction; conjugated polymers; copoly-
merization; electrochemistry; organic solar cell; organic thin-
film transistor; polyaromatics; synthesis
INTRODUCTION Organic semiconductors have attracted
extensive scientific interest for applications in organic elec-
tronics and optoelectronics, including for organic thin-film
transistors (OTFTs) and organic photovoltaic cells (OPVs).1–
approach for improving the performance is to use low-
bandgap donor–acceptor polymeric materials; higher efficien-
cies up to 7.7% have been obtained with such polymers.20
The short-circuit current density (Jsc), open-circuit voltage
(Voc), and fill factor (FF) are the key parameters for OPV
devices, because the PCE of the device is proportional to the
values of these three parameters. Broadening the response
wavelength range of an OPV device by using conjugated side
chains21,22 or narrow-bandgap conjugated polymers23,24 is
an effective way to realize high Jsc values. Conjugated poly-
mers with low highest occupied molecular orbital (HOMO)
levels are helpful in realizing high Voc and PCE values, as the
Voc value of an OPV is directly proportional to the offset
between the HOMO level of the electron donor and the
LUMO level of the electron acceptor.25 Poly[2,7-silafluorene-
13
In particular, polymer semiconductors have been studied
because of advantages such as their chemical tenability, com-
patibility with plastic substrates, structural flexibility, me-
chanical stability, and cost-effective manufacture by using
various printing technologies such as roll-to-roll process.14,15
Increase of the power conversion efficiency (PCE) and stabil-
ity of OPVs are currently important key issues for their prac-
tical application.16
Although the conventional blends of poly(3-hexylthiophene)
and [6,6]-phenyl-C61-butyric acid methyl ester have reached
PCEs of up to 5%,17–19 further increase in the PCE is rather
difficult because of their limited photocurrent generation
and intrinsic absorption properties. Therefore, an alternative
alt-4,7-di(20-thienyl)-2,1,3-benzothiadiazole],26
poly[2,7-(9-
(20-ethylhexyl)-9-hexyl-fluorene)-alt-5,5-(40,70, -di-2-thienyl-
20,10,30-benzothiadiazole)],27 and poly[[9-(1-octylnonyl)-9H-
C
V
2012 Wiley Periodicals, Inc.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000
1

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
ride, pyrene, 2-(tributylstannyl)thiophene, bromine, trime-
thyltinchloride (1.0 M in tetrahydrofuran, THF), anhydrous
chlorobenzene, tert-butyllithium (t-BuLi, 1.7 M in hexane),
2,1,3-benzothiadiazole,
2-isopropoxy-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane, and tetrakis(triphenylphosphine)palla-
dium were purchased from Aldrich and were used without
further purification. [6,6]-Phenyl C71-butyric acid methyl
ester (PC71BM) was purchased from Nano-C.
Instrumentation
1H and 13C NMR spectra were recorded on a Varian Mercury
Plus 300 MHz spectrometer, and the chemical shifts were
recorded in units of ppm with chloroform as the internal
standard. The absorption spectra were measured using a
JASCO JP/V-570 model. The molecular weights of the poly-
mers were determined by gel permeation chromatography
(GPC) analysis relative to a polystyrene standard using a
Waters high-pressure GPC assembly (model M590). Thermal
analyses were carried out on a Mettler Toledo TGA/SDTA
851e analyzer under a N2 atmosphere with heating rates of
SCHEME 1 Synthetic routes and chemical structure of M1.
carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-
4,7-diyl-2,5-thiophenediyl]28 are three excellent examples of
this concept. Consequently, by using conjugated polymers
with lower HOMO levels and narrow bandgaps, high PCEs
have been realized in different families of conjugated
polymers.29,30
Recently, many pyrene-containing molecules and polymers
have been synthesized for diverse organic electronic devices
such as organic light-emitting diodes and OTFTs, because of
their unique electrical properties that are due to the delocal-
ized p-conjugated system.31–33 Pyrene also has a flat struc-
ture and a relatively deep HOMO energy level, so we
expected that the introduction of pyrene into conjugated
polymers could lower the HOMO energy level, and that OPV
devices using pyrene-containing polymers would show high
open-circuit voltages. In this study, an alternating copolymer
composed of head-to-tail-structured 3,40-dihexyl-2,20-bithio-
phene (DHBT) and pyrene units [poly(DHBT-alt-PYR)]
was synthesized using a Stille coupling reaction. To reduce
the bandgap energy of poly(DHBT-alt-PYR), we also intro-
ꢀ
10 C/min. Cyclic voltammetry (CV) was performed on a CH
Instruments Electrochemical Analyzer. The CV measurements
were carried out in acetonitrile solutions containing 0.1 M
tetrabutylammonium tetrafluoroborate (TBABF4) as the sup-
porting electrolyte, using Ag/AgNO3 as the reference elec-
trode, a platinum wire as the counter electrode, and a plati-
num working electrode.
Fabrication of OTFT Devices
OTFTs were fabricated using a bottom-contact geometry de-
vice (channel length L ¼ 12 mm and width W ¼ 120 mm).
The source and drain contacts were gold (100 nm), and the
dielectric was silicon oxide (SiO2) with a thickness of 300
nm. The SiO2 surface was cleaned, dried, and pretreated
with a solution of 10.0 mM octyltrichlorosilane (OTS-8) in
toluene at room temperature for 2 h under a N2 atmosphere
to produce nonpolar and smooth surfaces onto which the
polymers could be spin coated. The polymers were dissolved
in chloroform to a concentration of 0.5 wt %. Films of the
organic semiconductors were spin coated at 1000 rpm for
50 s to a thickness of 60 nm. All the device fabrication pro-
cedures and measurements were carried out in air at room
temperature.
duced
4,7-bis(30-hexyl-2,20-bithiophen-5-yl)benzo[c][1,2,5]-
thiadiazole (BHBTBT) units into the polymer. Poly(DHBT-co-
PYR-co-BHBTBT)s were synthesized using the same polymer-
ization reaction. The synthetic route and structures of the
polymers are shown in Schemes 1–3.
EXPERIMENTAL
Materials
3-Bromothiophene, 1-bromohexane, N-bromosuccinimide
(NBS), 1,3-bis(diphenylphosphino)propane nickel(II) chlo-
SCHEME 3 Synthetic routes and chemical structures of the
SCHEME 2 Synthetic routes and chemical structure of M2.
polymers.
2
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
Fabrication of Photovoltaic Devices
mmol) as the phase-transfer catalyst and 2 M aqueous sodium
carbonate solution (8 mL) were also added to the reaction mix-
ture, and the reaction solution was stirred at 80 ꢀC for 24 h.
Then, the reaction was quenched with methanol, the solvent
was removed by rotary evaporation, and the residue was puri-
fied by reprecipitation several times in methanol, affording the
product as a pale liquid (65%, yield).
Composite solutions (1:3 weight ratio) of the polymers and
PC71BM were prepared using 1,2-dichlorobenzene as the sol-
vent. The concentration was controlled adequately in the
range of 1.0–2.0 wt %. The polymer photovoltaic devices
were fabricated with a typical sandwich structure of indium
tin oxide (ITO)/(3,4-ethylenedioxythiophene):poly(styrene-
sulfonate) (PEDOT:PSS)/active layer/LiF/Al. The ITO-coated
glass substrates were cleaned using a routine cleaning proce-
dure, including sonication in detergent followed by distilled
water, acetone, and 2-propanol. A 45-nm-thick layer of
PEDOT:PSS (Baytron P) was spin coated on a cleaned ITO
substrate after exposure of the ITO surface to ozone for 10
min. The PEDOT:PSS layer was baked on a hot plate at 140
ꢀC for 10 min. The active layer was spin coated from the
predissolved composite solution after filtering through 0.45-
lm PP syringe filters. The device structure was completed
by deposition of 0.6 nm of LiF and a 120 nm of Al cathode
as the top electrode onto the polymer active layer under a
vacuum of 3 ꢁ 10ꢂ6 Torr in a thermal evaporator. The cur-
rent density–voltage (J–V) characteristics of all the polymer
photovoltaic cells were measured under the illumination of
AM 1.5 G solar light (100 mW/cm2) by an Oriel 1000 W so-
lar simulator. Electrical data were recorded using a Keithley
236 source-measure unit, and all characterizations were car-
ried out in an ambient environment. The illumination inten-
sity used was calibrated by a standard Si photodiode detec-
tor from PV Measurements, which was calibrated at NREL.
The incident photon-to-current conversion efficiency (IPCE)
was measured as a function of wavelength from 360 to 800
nm using an IPCE measurement system (PV Measurements)
equipped with a halogen lamp as the light source; calibration
was performed using a silicon reference photodiode. The
thickness of the thin film was measured using a KLA Tencor
Alpha-step IQ surface profilometer with an accuracy of 61
nm. The measurement was carried out after masking all but
the active cell area of the fabricated device. All the character-
ization steps were carried out in an ambient laboratory
atmosphere. The active area of the solar cells was 0.09 cm2.
1H NMR (300 MHz, CDCl3, d, ppm) 7.62 (d, 1H), 7.60 (d, 1H),
7.09 (s, 1H), 6.98 (d, 1H), 2.78 (m, 2H), 2.42 (m, 2H), 1.53
(m, 4H), 1.21 (m, 12H), 0.88 (t, 6H).
Synthesis of 2,20-Bis(trimethylstannyl)-3,30-
bis(hexyl)-5,50-bithiophene (M1)
A 1.7 M solution of t-BuLi (10.99 mL, 18.68 mol) in n-hexane
was added to a solution of 3,30-dihexyl-2,50-bithiophene (1)
(2.5 g, 7.47 mmol) with N,N,N0,N0-tetramethylethylenedi-
ꢀ
amine (2.17 g, 18.68 mmol) in THF (60 mL) at ꢂ78 C. The
mixture was maintained at this temperature for 2 h, and
then trimethyltin chloride (1.0 M) in THF (15.69 mL, 15.69
mmol) was added at ꢂ78 ꢀC. The mixture was heated to
room temperature and stirred for 12 h. The reaction was
quenched with methanol, and the reaction solution was
hydrolyzed and then extracted with diethylether and brine.
The organic layer was separated, dried over anhydrous
MgSO4, and concentrated using a rotary evaporator. The
yield of product obtained as a liquid was 58%.
1H NMR (300 MHz, CDCl3, d, ppm) 7.43 (s, 1H), 6.95 (s, 1H),
2.73 (t, 2H), 2.55 (t, 2H), 1.60 (m, 4H), 1.26 (m, 12H), 0.85
(t, 6H), 0.35 (t, 18H). 13C NMR (CDCl3, ppm) 137.8, 136.7,
136.4, 135.9, 131.4, 130.7, 126.1, 125.9, 35.1, 32.9, 30.1,
23.1, 22.5, 15.8, ꢂ8.17. Anal. calcd for C26H46S2Sn2: C, 47.3;
H, 7.02; S, 9.71. Found: C, 46.9; H, 6.98; S, 9.69.
4,7-Di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (4)
1H NMR (300 MHz, CDCl3, d, ppm): 7.84 (s, 2H), 7.43 (d,
2H), 7.21 (t, 2H), 6.61 (d, 2H).
4,7-Di-20-(50-bromo)-thienyl-2,1,3-benzothiadiazole (5)
1H NMR (300 MHz, CDCl3, d, ppm) 7.80 (dd, 4H), 7.13 (d,
2H).
Synthesis of Monomers and Polymers
Synthesis of 4,7-Bis(50-bromo-30-hexyl-2,20-bithiophen-5-
yl)benzo[c][1,2,5]thiadiazole (6 and M2)
2-(4-Hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lane (2),34 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole
(4),35 and 4,7-di-20-(50-bromo)-thienyl-2,1,3-benzothiadiazole
(5)36 were synthesized according to the methods described
in previous reports.
A solution of 2-bromo-3-hexylthiophene (5.3 g, 21.44 mmol)
in dry THF (30 mL) was added dropwise to magnesium
(1.56 g, 64.32 mmol) in dry THF (10 mL) and the mixture
ꢀ
was refluxed at 60 C under nitrogen gas for 2 h. Then, this
mixture was added slowly to a mixture of 4,7-bis(5-bromo-
thiophen-2-yl)benzo[c][1,2,5]thiadiazole (5) (2.45 g, 5.36
mmol) and Ni(dppp)Cl2 (0.23 g, 0.42 mmol)ꢀin dry THF (50
2-(4-Hexylthiophen-2-yl)-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (2)
1H NMR (300 MHz, CDCl3, d, ppm) 7.53 (s, 1H), 7.02 (s, 1H),
2.75 (t, 2H), 1.62 (m, 2H), 1.31 (s, 12H), 0.85 (t, 3H).
ꢀ
mL) at 0 C. The mixture was refluxed at 60 C for 12 h. The
organic layer was extracted three times with diethyl ether.
The combined organic layers were washed with brine and
dried over anhydrous MgSO4. The solvent was removed
using a rotary evaporator, and the obtained product was
purified by recrystallization from methanol to afford
BHBTBT (6). The obtained BHBTBT (2.97 g, 4.69 mmol) (6)
was dissolved in chloroform (60 mL) and acetic acid (5 mL)
in a 250-mL one-necked flask with a magnetic stirrer. NBS
Synthesis of 3,40-Dihexyl-2,20-bithiophene (3)
Tetrakis(triphenylphosphine)palladium (0.097 g, 0.084 mmol)
was inserted into a two-necked 100-mL round-bottomed flask
in a dry box. A mixture of 2-(4-hexylthiophen-2-yl)-4,4,5,5-tet-
ramethyl-1,3,2-dioxaborolane (1.0 g, 3.39 mmol), 2-bromo-3-
hexylthiophene (1.26 g, 5.08 mmol), toluene (10 mL), and THF
(20 mL) was added to the reactor. Aliquat 336 (0.113 g, 0.28
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000
3

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
(1.84 g, 10.32 mmol) was added to the solution, which was
then stirred for 2 h. The solvent was removed using the ro-
tary evaporator, and the product was purified by flash silica
gel column chromatography using a solvent (chloroform
only) and obtained as a red powder by recrystallization from
methanol. The product yield was 56%.
1H NMR (300 MHz, CDCl3, d, ppm): 8.09 (d, 2H), 7.86 (s,
2H), 7.16 (d, 2H), 6.93 (s, 2H), 2.78 (t, 4H), 1.65 (m, 4H),
1.39 (m, 4H), 1.31 (m, 8H), 0.90 (t, 6H). 13C NMR (CDCl3, d,
ppm) 150.1, 145.8, 142.1, 140.8, 138.1, 137.9, 134.2, 132.1,
130.1, 129.5, 120.1, 33.5, 32.1, 31.0, 29.5, 28.4, 13.9. Anal.
calcd for C34H34Br2N2S5: C, 51.64; H, 4.33; Br, 20.21; N, 3.54;
S, 20.27. Found: C, 51.60; H, 4.28; N, 3.21; S, 20.11.
Synthesis of 1,6-Dibromopyrene (M3)
Br2 (4.94 g, 34.7 mmol) was added to a stirred solution of
pyrene (5 g, 13.88 mmol) in carbon tetrachlꢀoride (200 mL)
in darkness. The mixture was stirred at 110 C for 12 h and
then 2 M HCl was added. The product was extracted with
chloroform. The organic phase was washed successively with
10% sodium bisulfate, 2 M HCl, and water and dried over
MgSO4. The solvent was removed to afford the product as
dark red crystals, which were recrystallized from hot hexane
to give the product (2.18 g, 63% yield).
1H NMR (300 MHz, CDCl3, d, ppm) d 8.01 (d, 2H), 7.94 (t,
2H), 7.88 (t, 2H), 7.76 (d, 2H). 13C NMR (CDCl3, d, ppm)
134.2, 131.4, 130.4, 129.1, 128.5, 127.1, 126.8, 125.4. Anal.
calcd for C16H8Br2: C, 53.37; H, 2.24; Br, 44.39. Found: C,
53.21; H, 2.20.
General Polymerization Procedure
Three copolymers were synthesized by Stille coupling poly-
merization.37 The (3,40-dihexyl-2,20-bithiophene-5,50-diyl)bis
(trimethylstannane), 4,7-bis(50-bromo-30-hexyl-2,20-bithiophen-
5-yl)benzo[c][1,2,5]thiadiazole, and 1,6-dibromopyrene mono-
mers were synthesized according to the methods reported pre-
viously. The reaction mixture of tris(dibenzylideneacetone)di-
palladium and tri-o-tolylphosphine in anhydrous chlorobenzene
(15 mL) was stirred at 90 ꢀC for 2 days. Then, an excess
amount of 2-bromothiophene and tripropyl(thiophen-2-yl)stan-
nane (the end capper) dissolved in anhydrous chlorobenzene
(1 mL) was added, and stirring was continued for 12 h. The
reaction mixture was cooled down to room temperature and
added slowly to a vigorously stirred mixture of methanol (220
mL) and 1 M HCl (10 mL). The precipitated polymer was
collected by filtration and purified by dissolution in chloroform
and reprecipitation three times in methanol. The solid was
washed for 2 days in a Soxhlet extractor using an acetone/
methanol mixture to remove any oligomers and residual cata-
lyst. The final product was obtained after drying under vacuum
at 60 ꢀC. The resulting polymers were soluble in common
organic solvents.
FIGURE 1 1H NMR spectra of (a) M1 and (b) M2 units in CDCl3
solution.
2,20-bithiophen-5-yl)benzo[c][1,2,5]thiadiazole (M2) mono-
mers are shown in Figure 1(a, b). All the proton peaks are
consistent with the corresponding monomer structures. All
the polymers were synthesized by polycondensation of
bis(arylbromide)s and bis(aryltrimethyltin)s through
a
Pd(0)-catalyzed Stille coupling reaction. The synthesized
polymers showed good solubility in common organic sol-
vents such as chloroform, toluene, and chlorobenzene. The
polymer solutions were spin coated onto glass or ITO sub-
strates and formed transparent and homogeneous thin films.
The number-average molecular weights (Mn) of the synthe-
sized polymers were determined by GPC using a polystyrene
standard. The Mn values were 5300 g/mol (polydispersity
index, PDI ¼ 1.30) for poly(DHBT-alt-PYR), 5800 g/mol (PDI
RESULTS AND DISCUSSION
Synthesis and Characterization of Monomers
and Polymers
¼
2.50) for poly(50DHBT-co-40PYR-co-10BHBTBT), and
6200 g/mol (PDI ¼ 2.00) for poly(50DHBT-co-20PYR-co-
1H NMR spectra of the 2,20-bis(trimethylstannyl)-3,30-bis
(hexyl)-5,50-bithiophene (M1) and 4,7-bis(50-bromo-30-hexyl-
30BHBTBT). The thermal stabilities of the polymers were
4
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
between the electron-withdrawing BHBTBT and electron-
donating DHBT units.
Figure 4 shows the UV–visible absorption spectra of the poly-
mer thin films. The shapes of the absorption spectra are simi-
lar to those of the corresponding polymer solutions, but both
the peak positions and onset wavelengths moved to longer
wavelength regions because of the enhanced interchain p-
interactions. The optical bandgaps (Egopt) of the polymers
were determined from the onset wavelength of the UV–visible
spectra of the polymer films. The measured optical bandgap
of poly(DHBT-alt-PYR) was 2.47 eV, which is very high for
photovoltaic applications because the polymer would absorb
only a limited portion of the sunlight. The BHBTBT-containing
ter-polymers, however, showed much lower bandgap energies
than poly(DHBT-alt-PYR). The optical bandgaps of pol-
y(50DHBT-co-40PYR-co-10BHBTBT) and poly(50DHBT-co-
20PYR-co-30BHBTBT) were 1.84 and 1.73 eV, respectively.
The UV–visible absorption properties and optical bandgap
energies of the polymers are summarized in Table 2.
FIGURE 2 TGA thermograms of the polymers with a ramping
rate of 10 ꢀC/min.
evaluated by thermogravimetric analysis (TGA) under a N2
atmosphere. The actual compositions of the resulting ter-
polymers were determined by elemental analysis using the
nitrogen content as the standard. The actual compositions of
the copolymers were not the same as the feed ratios, but we
found that the differences were not significant. These poly-
mers exhibited goodꢀstabilities, showing less than 5% weight
loss up to 371–413 C, as shown in Figure 2. The molecular
weights, thermal properties, and actual compositions of the
polymers are summarized in Table 1.
Electrochemical Properties
The electrochemical properties of the polymer films were
characterized by CV. The positions of the highest occupied
molecular orbitals (HOMOs) of the polymer films were deter-
mined by obtaining the oxidation onsets.38 A platinum elec-
trode coated with polymer was used as the working elec-
trode, a platinum wire was used as the counter electrode,
and an Ag/AgNO3 (0.01 M) electrode was used as the refer-
ence electrode. The electrochemical properties of the copoly-
mers were investigated in an electrolyte consisting of a solu-
tion of 0.1 M TBABF4 in acetonitrile at room temperature
under nitrogen at a scan rate of 50 mV/s. The measurements
were calibrated using ferrocene as the standard. The lowest
unoccupied molecular orbital (LUMO) energy levels of the
polymers were determined by combining the HOMO energy
levels obtained from CV and the optical bandgap energies
obtained from the edges of absorption.39 The onset oxidation
potentials (Eoxonset) of the three polymers were observed in
the range of 0.52–0.83 V. The energy level of the Ag/AgNO3
reference electrode was 4.78 eV below the vacuum level.38
Optical Properties
The UV–visible absorption spectra of the polymer solutions
in chloroform are shown in Figure 3. Poly(DHBT-alt-PYR)
showed a strong absorption at 412 nm, corresponding to the
p–p* transition. Interestingly, the absorption bands of the
BHBTBT-containing ter-polymers moved to longer wave-
length regions than that of poly(DHBT-alt-PYR). Poly
(50DHBT-co-40PYR-co-10BHBTBT) and poly(50DHBT-co-
20PYR-co-30BHBTBT) exhibited two strong absorption peaks
at 403 and 545 nm, respectively. The two peak positions are
the same in the two ter-polymers, but the relative peak in-
tensity at 545 nm increased with increasing BHBTBT content
in the ter-polymers. The second absorption band in the ter-
polymers might be caused by intramolecular charge transfer
Therefore, the HOMO energy levels were calculated using the
onset
equation HOMO (eV) ¼ ꢂ(Eox
þ 4.78) (eV), where Eox is
the onset oxidation potential versus Ag/Agþ. The measured
TABLE 1 Molecular Weights and Thermal Properties of Polymers
a
a
b
Polymer
Yield (%)
Mn
Mw
Td
Polymer
(g/mol)
(g/mol)
PDIa
(ꢀC)
Ratioc (DTBT mol %)
Actual Ratiod
Feed Ratio
Poly(DHBT-alt-PYR)
54
59
57
5,300
5,800
6,200
6,800
1.3
2.5
2.0
371
400
413
–
–
Poly(50DHBT-co-40PYR-co-10BHBTBT)
Poly(50DHBT-co-20PYR-co-30BHBTBT)
a
14,500
12,400
10
30
8.2
32.1
c
Mn, Mw, and PDI of the polymers were determined by GPC using poly-
Calculated by elemental analysis through calculation of the amount of
styrene standards in CHCl3.
nitrogen contained in the copolymers.
b
Temperature at 5% weight loss at a heating rate of 10 ꢀC/min under
nitrogen.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000
5

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 4 UV–visible absorption spectra of the polymer thin
FIGURE 3 UV–visible absorption spectra of the polymers in
films.
chloroform solution.
following equation: Ids ¼ (W/2L)lCi (Vgs ꢂ Vth)2, where Ids is
the drain-source current in the saturated region, W and L are
the channel width and length, respectively, l is the field-effect
mobility, Ci is the capacitance per unit area of the insulating
layer, and Vgs and Vth are the gate and threshold voltages,
respectively.40 The measured field-effect mobilities for pol-
y(DHBT-alt-PYR), poly(50DHBT-co-40PYR-co-10BHBTBT), and
poly(50DHBT-co-20PYR-co-30BHBTBT) were determined to
be 1.0 ꢁ 10ꢂ4, 4.0 ꢁ 10ꢂ6, and 1.3 ꢁ 10ꢂ6 cm2/V s, respec-
tively. The alternating copolymer, poly(DHBT-alt-PYR),
showed the highest mobility among these polymers, suggest-
ing that the regularity of the polymer structure greatly
affected the mobility. The mobilities of the ter-polymers
decreased with decreasing pyrene or increasing BHBTBT con-
tents in the polymers. The characteristics of the TFTs using
the polymers as the active layers are summarized in Table 3.
HOMO energy levels of poly(DHBT-alt-PYR), poly(50DHBT-
co-40PYR-co-10BHBTBT), and poly(50DHBT-co-20PYR-co-
30BHBTBT) were ꢂ5.61, ꢂ5.38, and ꢂ5.30 eV, respectively.
Poly(DHBT-alt-PYR) showed a relatively low HOMO energy
level with the introduction of pyrene units and would have
good oxidative stability. The HOMO energy level was
increased by reducing the pyrene contents or increasing the
BHBTBT content in the polymers. The LUMO energy levels of
the polymers were estimated from these HOMO energy lev-
els and optical bandgap energies. The LUMO energy levels,
however, moved to lower positions with increasing BHBTBT
content in the polymers. The energy diagrams of the poly-
mers, PC71BM, and electrodes are illustrated in Figure 5, and
the electrochemical properties and energy-level parameters
of the polymers are summarized in Table 2.
TFT Characteristics of the Polymer Thin Films
Photovoltaic Devices Properties
For the measurement of the field-effect mobility of the
charge carriers, OTFTs were fabricated on silicon wafers
using a bottom-contact geometry (channel length L ¼12 lm
width W ¼ 120 lm) under nitrogen. The TFT devices of the
polymers exhibited typical p-channel transistor characteris-
tics. Figure 6 shows the transfer curves of the devices fabri-
cated using the polymers as the active layer. The field-effect
mobility was calculated in the saturation regime using the
Photovoltaic devices were fabricated using the synthesized
polymers as p-type electron donors and PC71BM as an n-
type acceptor. Photovoltaic devices with ITO/PEDOT:PSS/
polymer þ PC71BM/LiF/Al configurations were fabricated
and characterized. In our preliminary study, we found that
the optimum ratio of polymer to PC71BM for these devices
was 1:3 by weight, so we prepared active layers with this ra-
tio for the photovoltaic devices.
TABLE 2 Optical and Electrochemical Properties of Polymers
kmax, abs
(nm)
Solutiona
kmax
kedge
(nm)
Filmb
Optical
HOMO
(eV)
LUMO
(eV)
(eV)c
opt
Polymers
(nm)
Filmb
Eg
Poly(DHBT-alt-PYR)
412
424
502
671
714
2.47
1.84
1.73
5.61
5.38
5.30
3.14
3.54
3.57
Poly(50DHBT-co-10PYR-co-10BHBTBT)
Poly(50DHBT-co-20PYR-co-30BHBTBT)
403, 545
403, 545
417, 570
417, 582
c
a
1 ꢁ 10ꢂ5 M in anhydrous chloroform.
Calculated from the absorption band edge of the copolymer films, Eg
¼ 1240/kedge
b
Polymer film on a quartz plate by spin casting from a solution in chlo-
roform at 1500 rpm for 30 s.
.
6
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 5 Energy band diagram of the polymers, PC71BM, ITO,
and Al electrodes.
Figure 7 shows the J–V curves of the OPVs fabricated using
the polymers under AM 1.5 illumination (100 mW/cm2). To
achieve the optimum thickness of the active layer, we fabri-
cated photovoltaic devices with different active layer thick-
nesses from 60 to 100 nm. The fabricated devices showed
the highest PCEs when the active layer thickness was 100
nm, as shown in Table 4. All the fabricated devices using the
polymers showed relatively high Voc values. The device fabri-
cated using poly(DHBT-alt-PYR) showed the highest open-
circuit potential (ꢃ0.72 V) of all those fabricated with 100-
nm-thick active layers. This is coincident with the measured
deep HOMO energy levels of the pyrene-containing polymers.
The open-circuit potential of a photovoltaic device is depend-
ent on the energy gap between the HOMO of the donor poly-
mer and the LUMO of the acceptor fullerene. Therefore, do-
nor polymers that have deep HOMO energy levels are
desirable for the fabrication of photovoltaic devices with
high open-circuit voltages.
Incidentally, the short-circuit currents of the devices are
increased upon increasing the BHBTBT contents in the poly-
mers. As shown in the UV–visible absorption spectra in Fig-
ure 3, as the number of BHBTBT units in the copolymer was
increased in the polymers, the absorption in regions of lon-
ger wavelength increased. This increasing light absorption
increased the short-circuit currents of the photovoltaic devi-
ces. The device using poly(50DHBT-co-20PYR-co-30BHBTBT)
showed the highest Jsc among the fabricated devices. This
result, however, contradicts the mobility data of the poly-
mers in the previous section, because poly(50DHBT-co-
FIGURE 6 Transfer characteristics of OTFTs fabricated using the
polymers as the active layers at a constant source-drain voltage
of ꢂ80 V. (a) Poly(DHBT-alt-PYR), (b) poly(50DHBT-co-40PYR-co-
10BHBTBT), and (c) poly(50DHBT-co-20PYR-co-30BHBTBT).
20PYR-co-30BHBTBT) showed the lowest mobility of the
three polymers. Both the mobility and light absorption may
affect the short-circuit currents of PSCs, but in our results,
TABLE 3 Summary of the Characteristics of OTFTs Fabricated Using the Polymers as the
Active Layers
Poly(50DHBT-co-40PYR-
co-10BHBTBT)
Poly(50DHBT-co-20PYR-
co-30BHBTBT)
Polymer
Poly(DHBT-alt-PYR)
Mobility (cm2/V s)
1.0 ꢁ 10ꢂ4
4.0 ꢁ 10ꢂ6
1.3 ꢁ 10ꢂ6
Ion/Ioff
104
102
103
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000
7

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
of 0.35 under AM 1.5 G irradiation (100 mW/cm2). The cor-
responding Voc, Jsc, FF, and PCE values of the devices made
under different device fabrication conditions are summarized
in Table 4. The accuracy of the photovoltaic measurements
can be confirmed by the external quantum efficiency (EQE)
of the devices.
Figure 8 shows the EQE curves of the PSCs fabricated under
the same optimized conditions as those used for the J–V
measurements. Obviously, the EQE values of poly(50DHBT-
co-20PYR-co-30BHBTBT) are higher than those of pol-
y(DHBT-alt-PYR) and poly(50DHBT-co-40PYR-co-10BHBTBT),
in agreement with the higher Jsc values of the devices
derived from poly(50DHBT-co-20PYR-co-30BHBTBT). To
evaluate the accuracy of the photovoltaic results, we calcu-
lated the Jsc values by integrating the EQE data with the AM
1.5 G reference spectrum. The Jsc values obtained by integra-
tion and the J–V measurements are rather close. The onset
wavelength of the photon-to-current conversion was 750 nm
for all the devices.
FIGURE 7 J–V curves for the polymer:PC71BM (1:3 wt %) solar
cell devices.
the light absorption (not the polymer mobility) would be the
major factor affecting the short-circuit currents of the
devices.
The surface morphologies of the polymers and PC71BM blend
films were observed by tapping-mode atomic force micros-
copy (AFM). The blend films showed almost equivalently
measured smooth surfaces [according to their root-mean-
square (RMS) roughness] with small degrees of phase sepa-
ration. As shown in Figure 9, the RMS roughness of the
active layers was 2.00 nm for poly(DHBT-alt-PYR), 0.98 nm
for poly(50DHBT-co-40PYR-co-10BHBTBT), and 1.81 nm for
poly(50DHBT-co-20PYR-co-30BHBTBT). Compared with the
poly(DHBT-alt-PYR):PC71BM blend film, the poly(50DHBT-co-
40PYR-co-10BHBTBT):PC71BM blend film showed a flatter
surface with an RMS of 0.98 nm [Fig. 9(b)], and no obvious
phase separation was observed. As poly(50DHBT-co-40PYR-
co-10BHBTBT) and PC71BM molecules have good miscibility,
an increased interfacial area was expected, and the Jsc value
of the poly(50DHBT-co-40PYR-co-10BHBTBT):PC71BM-based
device increases to 4.86 mA/cm2, which was nearly 0.15
mA/cm2 higher than that of the poly(DHBT-alt-PYR):PC71BM-
based device. In the case of the poly(50DHBT-co-20PYR-co-
The short-circuit currents of the devices with 100-nm active
layers fabricated using poly(DHBT-alt-PYR), poly(50DHBT-co-
40PYR-co-10BHBTBT),
and
poly(50DHBT-co-20PYR-co-
30BHBTBT) were 4.72, 4.86, and 5.54 mA/cm2, respectively.
Interestingly, the photovoltaic device using poly(DHBT-alt-
PYR) showed a PCE of 1.13%, even though it had a very
wide bandgap energy (ꢃ2.47 eV). This may be because of
the relatively high hole mobility of poly(DHBT-alt-PYR) and
the high Voc due to the deep HOMO energy level of the poly-
mer. To our knowledge, this would be one of the highest
PCEs among the OPVs fabricated using semiconducting poly-
mers with similar bandgap energies to poly(DHBT-alt-PYR).
The highest PCE among our fabricated devices was obtained
using the poly(50DHBT-co-20PYR-co-30BHBTBT)/PC71BM
active layer with a thickness of 100 nm. The measured PCE
was 1.31%, with Jsc of 5.54 mA/cm2, Voc of 0.68 V, and a FF
TABLE 4 Summary of the Properties of Photovoltaic Devices Fabricated Using Composites of
the Polymers and PC71BM (1:3 Ratio)
PCEa
(%)
a
a
Thickness
(nm)
Voc
Jsc
Polymer
(V)
(mA/cm2)
FFa
Poly(DHBT-alt-PYR)
60
0.76
0.74
0.72
0.67
0.70
0.67
0.67
0.69
0.68
2.35
4.33
4.72
2.47
4.62
4.86
2.65
5.44
5.54
0.29
0.32
0.33
0.28
0.30
0.32
0.28
0.33
0.35
0.52
1.03
1.13
0.47
0.98
1.05
0.50
1.23
1.31
80
100
60
Poly(50DHBT-co-40PYR-co-10BHBTBT)
80
100
60
Poly(50DHBT-co-20PYR-co-30BHBTBT)
80
100
a
Photovoltaic properties of copolymer/PC71BM-based devices spin coated from a 1,2-dichlorobenzene so-
lution of polymer.
8
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
30BHBTBT) blend film [Fig. 9(c)], uniform phase separation
was observed. However, the film from poly(50DHBT-co-
20PYR-co-30BHBTBT) and PC71BM showed clear domains
with a higher RMS value of 1.81 nm, which is helpful for
charge separation, and thus improved the FF of the device.
Thus, the large domain size for poly(50DHBT-co-20PYR-co-
30BHBTBT) suggests that even better device performance
can be expected through further device engineering, for
example, by thermal annealing, solvent annealing, and the
use of additives to promote proper phase separation.
CONCLUSIONS
A series of p-conjugated polymers composed of head-to-tail-
structured bithiophene, pyrene, and bis(bithiophene) conju-
gated benzothiadiazole derivatives were synthesized by the
polycondensation of bis(arylbromide)s and bis(aryltrimethyl-
tin)s through a Pd(0)-catalyzed Stille coupling reaction. Pol-
y(DHBT-alt-PYR) showed a large bandgap energy (2.47 eV)
even though it has a relatively high hole mobility. By intro-
ducing BHBTBT units into the polymer, we could reduce the
FIGURE 8 IPCE spectra of the polymers:PC71BM (1:3 wt %) so-
lar cell devices.
FIGURE 9 Tapping-mode AFM height images of thin films cast from (a) poly (DHBT-alt-PYR):PC71BM (1:3), (b) poly(50DHBT-co-
10PYR-co-10BHBTBT):PC71BM (1:3), and (c) poly(50DHBT-co-20PYR-co-30BHBTBT):PC71BM (1:3).
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000
9

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
15 Krebs, F. C.; Fyenbo, J.; Tanenbaum, D. M.; Gevorgyan, S.
A.; Andriessen, R.; Remoortere, B.; Galagan, Y.; Jørgensen, M.
Energy Environ. Sci. 2011, 4, 4116–4123.
bandgap energies of the resulting poly(DHBT-co-PYR-co-
BHBTBT)s. The bandgap energies of poly(50DHBT-co-40PYR-
co-10BHBTBT) and poly(50DHBT-co-20PYR-co-30BHBTBT)
were 1.84 and 1.73 eV, respectively. All the synthesized poly-
mers showed low HOMO energy levels and high open-circuit
voltages in the photovoltaic devices fabricated using them.
The device fabricated using poly(50DHBT-co-20PYR-co-
30BHBTBT) exhibited the highest PCE of 1.31% under AM
1.5 G illumination conditions (100 mW/cm2).
16 Jørgensen, M.; Norrman, K.; Gevorgyan, S. A.; Tromholt, T.;
Andreasen, B.; Krebs, F. C. Adv. Mater. 2012, 24, 580–612.
17 Dimitrakopoulos, C. D.; Purushothaman, S.; Kymissis, J.;
Callegari, A.; Shaw, J. M. Science 1999, 283, 822–824.
18 Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 281,
1741–1744.
19 Dennler, G.; Scharber, M.; Brabec, C. J. Adv. Mater. 2009,
21, 1323–1338.
ACKNOWLEDGMENTS
20 Kim, J. Y.; Kim, S. H.; Lee, H.-H.; Lee, K.; Ma, W.; Gong, X.;
Heeger, A. J. Adv. Mater. 2006, 18, 572–576.
This research was financially supported by the Ministry
of Knowledge Economy (MKE) under the New & Renew-
21 Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. Adv.
Funct. Mater. 2005, 15, 1617–1622.
able Energy Program through
a
KETEP grant (No.
20103020010050 and No. 2011T100200034) and by a
National Science Foundation (NRF) grant funded by the Korean
Government (MEST) (No. 2011-0011122) through GCRC-SOP
(Grant No. 2011-0030668).
22 Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang,
Y.; Yu, L.; Wu, L.; Li, G. Nat. Photon. 2009, 3, 649–653.
23 Hou, J. H.; Huo, L. J.; He, C.; Yang, C. H.; Li, Y. F. Macromo-
lecules 2006, 39, 594–603.
24 Zhang, Z.-G.; Liu, Y.-L.; Yang, Y.; Hou, K.; Peng, B.; Zhao, G.
J.; Zhang, M. J.; Guo, X.; Kang, E.-T.; Li, Y. F. Macromolecules
2010, 43, 9376–9383.
REFERENCES AND NOTES
25 Huo, L. J.; Hou, J. H.; Chen, H.-Y.; Zhang, S. Q.; Jiang, Y.;
Chen, T. L.; Yang, Y. Macromolecules 2009, 42, 6564–6571.
1 McCullough, R. D.; Lowe, R. D. J. Chem. Soc. Chem. Com-
mun. 1992, 1, 70–72.
26 Huo, L. J.; Tan, Z. A.; Wang, X.; Zhou, Y.; Han, M. F.; Li, Y.
F. J. Polym. Sci. Part A: Polym. Chem. 2008, 46, 4038–4049.
2 Chen, T.-A.; Rieke, R. D. J. Am. Chem. Soc. 1992, 114,
10087–10088.
27 Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Wal-
¨
3 Chen, T.-A.; Wu, X.; Rieke, R. D. J. Am. Chem. Soc. 1995,
117, 233–244.
dauf, C.; Heeger, A. J.; Brabec, C. J. Adv. Mater. 2006, 18,
789–794.
4 Cho, S.; Lee, K.; Yuen, J.; Wang, G.; Moses, D.; Heeger, A. J.;
Surin, M.; Lazzaroni, R. J. J. Appl. Phys. 2006, 100,
114503–1–114503-6.
28 Wang, E. G.; Wang, L.; Lan, L. F.; Luo, C.; Zhuang, W. L.; Peng,
J. B.; Cao, Y. Appl. Phys. Lett. 2008, 92, 033307–1–033307-3.
29 Svensson, M.; Zhang, F. L.; Veenstra, S. C.; Verhees, W. J.
H.; Hummelen, J. C.; Kroon, J. M.; Inganas, O.; Andersson, M.
R. Adv. Mater. 2003, 15, 988–991.
5 Ong, B. S.; Wu, L. Y.; Liu, P.; Gardner, S. J. Am. Chem. Soc.
2004, 126, 3378–3379.
6 McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; Macro-
nald, 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.
30 Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19,
2295–2300.
31 Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.;
Yu, L. Adv. Mater. 2010, 22, E135–E138.
7 Shaheen, S. E.; Vangeneugden, D.; Kiebooms, R.; Vander-
zande, D.; Fromherz, T.; Padinger, F.; Brabec, P. C. J.; Sariciftci,
N. S. Synth. Met. 2001, 121, 1583–1584.
32 Price, S. C.; Stuart, A. C.; Yang, L.; Zhou, H.; You, W. J. Am.
Chem. Soc. 2011, 133, 4625–4631.
33 Ashizawa, M.; Yamada, K.; Fukaya, A.; Kato, R.; Hara, K.;
Takeya, J. Chem. Mater. 2008, 15, 4883–4890.
8 Vangeneugden, D. L.; Vanderzande, D. J. M.; Salbeck, J.; Van
Hal, P. A.; Janssen, R. A. J.; Hummelen, J. C.; Brabec, C. J.;
Shaheen, S. E.; Sariciftci, N. S. J. Phys. Chem. B 2001, 105,
11106–11113.
34 Coleman, A.; Pryce, M. T. Inorg. Chem. 2008, 47, 10980– 10990.
35 Zhang, H.; Wang, Y.; Shao, K.; Liu, Y.; Chen, S.; Qiu, W.;
Sun, X.; Qi, T.; Ma, Y.; Yu, G.; Su, Z.; Zhu, D. Chem. Commun.
2006, 46, 755–757.
9 Hou, J.; Park, M. H.; Zhang, S.; Yao, Y.; Chen, L. M.; Li, J. H.;
Yang, Y. Macromolecules 2008, 41, 6012–6018.
36 Sahu, D.; Padhy, H.; Patra, D.; Kekuda, D.; Chu, C.-W.;
Chiang, I.-H.; Lin, H.-C. Polymer 2010, 51, 6182–6192.
10 Bundgaard, E.; Krebs, F. C. Macromolecules 2006, 39,
2823–2831.
37 Kim, J.; Park, S. H.; Cho, S.; Jin, Y.; Kim, J.; Kim, I.; Lee, J.
S.; Kim, J. H.; Woo, H. Y.; Lee, K.; Suh, H. Polymer 2010, 51,
390–396.
11 Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells
2007, 91, 1019–1025.
12 Li, J.-C.; Beak, M.-J.; Kwon, J.-T.; Lee, E.-W.; Lee, S.-H.; Lee,
Y.-S. Macromol. Res. 2010, 18, 1237–1240.
38 Chen, G. Y.; Cheng, Y. H.; Chou, Y. J.; Su, M. S.; Chen, C.
M.; Wei, K. H. Chem. Commun. 2011, 47, 5064–5066.
13 Jo, P. S.; Park, Y. J.; Kang, S. J.; Kim, T. H.; Park, C. Macro-
mol. Res. 2010, 18, 777–786.
39 Li, Y. F.; Cao, Y.; Gao, J.; Wang, D. L.; Yu, G.; Heeger, A. J.
Synth. Met. 1999, 99, 243–248.
14 Krebs, F. C.; Gevorgyan, S. A.; Alstrup, J. J. Mater. Chem.
2009, 19, 5442–5451.
40 Shaw, J. M.; Seidler, P. F. IBM J. Res. Dev. 2001, 45, 3–9.
10
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000