Synthesis and characterization of quinoxaline-based thiophene copolymers as photoactive layers in organic photovoltaic cells

Article abstract of DOI:10.5012/bkcs.2011.32.2.417

A series of new quinoxaline-based thiophene copolymers (PQx2T, PQx4T, and PQx6T) was synthesized via Yamamoto and Stille coupling reactions. The M _ws of PQx2T, PQx4T, and PQx6T were found to be 20,000, 12,000, and 29,000, with polydispersity in

Full text of DOI:10.5012/bkcs.2011.32.2.417

Synthesis and Characterization of Quinoxaline-Based Thiophene Copolymers Bull. Korean Chem. Soc. 2011, Vol. 32, No. 2 417
DOI 10.5012/bkcs.2011.32.2.417
Synthesis and Characterization of Quinoxaline-Based Thiophene Copolymers
as Photoactive Layers in Organic Photovoltaic Cells
Yoon-Suk Choi, Woo-hyung Lee, Jae-Ryoung Kim,† Sang-Kyu Lee,† Won-Suk Shin, † Sang-Jin Moon,†
Jong-Wook Park, and In-Nam Kang*
Department of Chemistry, The Catholic University of Korea, Bucheon, Gyeonggi-do 420-743, Korea
*E-mail: inamkang@catholic.ac.kr
†Energy Materials Research Division, Korea Research Institute of Chemical Technology, P.O. BOX 107,
Yuseong, Daejeon 305-600, Korea
Received October 5, 2010, Accepted November 23, 2010
Aseries of new quinoxaline-based thiophene copolymers (PQx2T, PQx4T, and PQx6T) was synthesized via Yamamoto
and Stille coupling reactions. The Mws of PQx2T, PQx4T, and PQx6T were found to be 20,000, 12,000, and 29,000,
with polydispersity indices of 2.0, 1.2, and 1.1, respectively. The UV-visible absorption spectra of the polymers show-
ed two distinct absorption peaks in the ranges 350 - 460 nm and 560 - 600 nm, which arose from the π-π* transition
of oligothiophene units and intramolecular charge transfer (ICT) between a quinoxaline acceptor and thiophene donor.
The HOMO levels of the polymer ranged from ‒5.37 to ‒5.17 eV and the LUMO levels ranged from ‒3.67 to ‒3.45 eV.
The electrochemical bandgaps of PQx2T, PQx4T, and PQx6T were 1.70, 1.71, and 1.72 eV, respectively, thus yield-
ing low bandgap behavior. PQx2T, PQx4T, and PQx6T had open circuit voltages of 0.58, 0.42, and 0.47 V, and short
circuit current densities of 2.9, 5.29 and 9.05 mA/cm2, respectively, when PC71BM was used as an acceptor. For the
solar cells with PQx2T-PQx6T:PC71BM (1:3) blends, an increase in performance was observed in going from PQx2T
to PQx6T. The power conversion efficiencies of PQx2T, PQx4T, and PQx6T devices were found to be 0.69%, 0.73%,
and 1.80% under AM 1.5 G (100 mW/cm2) illumination.
Key Words: Organic solar cell, Quinoxaline, Oligothiophene, Thin film transistor, Conjugated polymers
Introduction
for OPVs.18-21 The quinoxaline moiety, which contains electron-
withdrawing nitrogen atoms, is highly electron deficient and
thus serves as an efficient electron acceptor; therefore, it easily
forms ICT complexes with other donor components.22
Here, we report the synthesis and characterization of a new
class of quinoxaline-based thiophene copolymers, specifically
poly[2,3-bis-((4-octyloxy)-phenyl)-5,8-dithien-2-yl-quinoxa-
line] (PQx2T), poly[2,3-bis-((4-octyloxy)-phenyl)-5,8-dithien-
2-yl-quinoxaline-alt-2,2'-bithiophene] (PQx4T), and poly[2,3-
bis-((4-octyloxy)-phenyl)-5,8-bis(5'-bromo-4'-hexyl-2,2'-
bithiophene-5-yl-quinoxaline)-alt-2,2'-bithiophene] (PQx6T),
that function as photoactive semiconductors in OPVs. To in-
crease the solubility of the polymers, an easily accessible octyl-
oxyphenyl ring was incorporated in the quinoxaline units. The
ICT behavior was studied by changing the number of electron-
rich thiophene rings with the electron-accepting quinoxaline
unit in the polymer backbones. Bulk heterojunction-type de-
vices with PC71BM (3'H-cyclopropa[8,25][5,6]fullerene-C70-
D5h(6)-3'-butanoic acid, 3'-phenyl-, methyl ester) acceptors were
fabricated to investigate the photovoltaic properties of the poly-
mers. Furthermore, the influence of structural variation on field-
effect mobility in OTFTs was investigated to understand charge
transport in the solar cell devices. Devices using PQx6T and
PC71BM blends as the active layer exhibited 1.80% power
conversion efficiency.
Organic electronic devices based on solution-processable
organic semiconductors have been increasingly investigated
for use in low-cost memory devices, large-area display devices,
and flexible electronic devices such as organic thin film transi-
stors (OTFTs)1-3 and organic photovoltaic cells (OPVs).4-7 In
particular, organic photovoltaic cells have attracted much scien-
tific and industrial interest because of their significant advan-
tages, including their simplicity, low production cost, and appli-
cations in renewable solar energy. The recent development of
novel polymers and processing techniques has resulted in dra-
matic improvements in the performance of OPVs, and an effi-
ciency of more than 7% has been reported.8 In the common
device structure of OPVs, a blend of an electron-donor (p-type
conjugated polymer) and electron-acceptor ([6,6]-phenyl-C61-
butyric acid methyl ester, PCBM) is used as a photoactive layer
between two electrodes.9-13 This well known structure, com-
monly referred to as a bulk heterojunction (BHJ) device, has
been the most widely used configuration for polymer-based
solar cells. Conjugated copolymers with alternating donor-
acceptor units have been extensively used to obtain low-band-
gap donor polymers, because the intramolecular charge transfer
(ICT) mechanism within the donor-acceptor structure allows
extension of the conjugation length to achieve high-perfor-
mance organic photovoltaic cells (OPVs).14-17 Low bandgap
polymers are expected to be promising materials for the de-
velopment of OPVs because of the improved absorption of the
solar photon flux. Within the class of low-bandgap polymers,
quinoxaline-based polymers are widely used as active materials
Experimental Section
Synthesis of Monomers. 4,7-Dibromo-2,1,3-benzothiadi-
azole (1),23 3,6-dibromo-1,2-phenylenediamine (2),24 1,4-dime-

418 Bull. Korean Chem. Soc. 2011, Vol. 32, No. 2
Yoon-Suk Choi et al.
thylpiperazine-2,3-dione (3),25 1-bromo-4-(octyloxy) benzene
(4),26 4,4'-bis(2-octyloxy)benzil (5),25 5,8-Dibromo-2,3-bis-(4-
octyloxyphenyl)quinoxaline (6),27 2,3-Bis(4-octyloxyphenyl)-
5,8-dithien-2-ly quinoxaline (7),27 2,3-bis(4-octyloxyphenyl)-
5,8-bis(5'-bromo-dithien-2-ly)quinoxaline (8),27 and 5,5'-bis
(trimethylstannyl)-2,2'-bithiophene28 were prepared with pre-
viously described methods.
acetone. The polymer was dissolved in small amount of toluene
and then precipitated in methanol. The resulting polymer was
further purified by Soxhlet extraction using methanol and then
dried in vacuum. The polymer yield was 50% (0.5 g) after puri-
fication. Anal. Calcd. for (C44H50N2O2S2)n: C, 75.17; H, 7.17;
N, 3.98; S, 9.12. Found: C, 72.44; H, 6.87; N, 3.74; S, 8.88.
Poly[2,3-bis(4-octyloxyphenyl)-5,8-dithien-2-ly-quinoxa-
line-alt-2,2'-bithiophene], PQx4T: A 100 mL Schlenk flask
containing anhydrous chlorobenzene (10 mL), compound 8
(0.35 g, 0.406 mmol), 5,5'-bis(trimethylstannyl)-2,2'-bithio-
phene (0.2 g, 0.406 mmol), tris(dibenzylideneacetone)dipalla-
dium(0) (0.036 g, 0.0406 mmol), and tricyclohexyl phosphine
(0.023 g, 0.0812 mmol) was kept under nitrogen atmosphere
at 130 oC for 72 h. When the reaction had finished, the reaction
mixture was precipitated from the 10 mL of HCl and 150 mL
of methanol. The polymer was dissolved in small amount of
toluene and precipitated in methanol. The resulting polymer
was further purified by Soxhlet extraction using methanol and
then dried in vacuum to give dark-green solid (0.26 g, 80 %).
Anal. Calcd. for (C52H54N2O2S4)n: C, 72.02; H, 6.28; N, 3.23;
S, 14.79. Found: C, 69.98; H, 6.42; N, 2.80; S, 13.64.
Poly[2,3-bis(4-octyloxyphenyl)-5,8-bis(5'-bromo-4'-hexyl-
2,2'-bithiophene-5-ly-quinoxaline)-alt-2,2'-bithiophene],
PQx6T: PQx6T was synthesized using the same Stille coupl-
ing procedure as for PQx4T with compound 9 (0.36 g, 0.302
mmol), 5,5'-bis(trimethylstannyl)-2,2'-bithiophene (0.15 g,
0.302 mmol), tris(dibenzylideneacetone)dipalladium(0) (0.027
g, 0.0302 mmol), tri-cyclohexylphosphine (0.017 g, 0.0604
mmol), and chlorobenzene (9.0 mL). Yield (0.30 g, 83%, dark-
ish green color). Anal. Calcd. for (C72H82N2O2S6)n: C, 72.07;
H, 6.89; N, 2.33; O, 2.67; S, 16.03. Found: C, 71.49; H, 7.00;
N, 2.16; S, 15.84.
2,3-Bis(4-octyloxyphenyl)-5,8-bis(4'-hexyl-2,2'-bithio-
phene-5-ly)quinoxaline (9): To a stirred mixture of compound
8 (2.0 g, 2.32 mmol), 4,4,5,5-tetramethyl-2-(4-octyl-thiophene-
2-ly)-[1,3,2]-dioxaborolane (1.5 g, 5.11 mmol), tetrakis(tri-
phenylphosphine) palladium(0) (0.13 g, 0.12 mmol), and Aliquat
336 (2 drops) in 30 mL of toluene and 25 mL of 2 M aqueous
sodium carbonate solution was added. The solution was reflux-
ed with vigorous stirring for 20 h under a nitrogen atmosphere.
The mixture was poured into water (100 mL) and extracted with
CHCl3. The extract was then successively washed with water
and brine. After drying over anhydrous MgSO4, the solvent
was evaporated and the residue was purified by column chro-
matography on silica gel with n-hexane: toluene (2:1) as eluent
to give compound 9 as a red solid (1.6 g, 64 %). 1H NMR (CDCl3,
300 MHz, ppm) 8,04 (s, 2H), 7.77 (d, 4H) 7.75 (d, 2H) 7.20 (d,
2H) 6.93 (d, 4H) 6.84 (s, 2H) 4.00 (t, 4H) 2.62 (t, 4H) 1.81 (m,
4H) 1.65 (m, 4H) 1.48-1.30 (m, 32H) 0.89 (t, 12H). 13C NMR
(CDCl3, 80 MHz, ppm) 160.03, 150.99, 144.13, 140.63, 137.67,
137.21, 136.61, 132.04, 131.01, 130.31, 126.60, 125.58, 124.66,
122.80, 118.96, 114.08, 48.06, 31.86, 31.75, 30.61, 30.40, 29.45,
29.34, 29.29, 29.10, 26.13, 22.70, 14.14 Anal. Calcd. for C64H78-
N2O2S4: C, 74.23; H, 7.59; N, 2.71; S, 12.39. Found: C, 74.34;
H, 7.58; N, 2.71; S, 12.38.
2,3-Bis(4-octyloxyphenyl)-5,8-bis(5'-bromo-4'-hexyl-2,2'-
bithiophene-5-ly)quinoxaline (10): To a solution of compound
9 (0.81 g, 0.78 mmol) in a CHCl3 (10 mL) was cooled to 0 oC
with an ice-water bath. And then NBS (0.27 g, 1.57 mmol) was
added. After the reaction was finished, the product was extract-
ed with CHCl3. The organic layer was dried over anhydrous
MgSO4 and the solvent removed by rotary evaporation. The
crude product was purified by crystallization with MeOH to
yield compound 10 as a red solid (0.9 g, 97%). 1H NMR (CDCl3,
300 MHz, ppm) 7.95 (s, 2H), 7.71 (d, 4H), 7.66 (d, 2H), 7.10 (d,
2H), 6.92 (s, 2H), 6.89 (d, 4H), 4.00 (t, 4H), 2.56 (t, 4H), 1.80
Instrumentation. 1H and 13C NMR spectra were recorded on
a Bruker AVANCE 400 spectrometer, with tetramethyl silane
as an internal reference. The optical absorption spectra were
measured on a Shimadzu UV-3100 UV-VIS-NIR spectrometer.
The number- and weight-averaged molecular weights of poly-
mers were determined by gel permeation chromatography
(GPC) on a Viscotek T60A instrument, using tetrahydrofuran
(THF) as eluent and polystyrene as a standard. Thermal gravi-
metric analysis (TGA) was obtained using a TA Q100 instru-
ment operated under a nitrogen atmosphere. Cyclic voltammetry
was performed on an AUTOLAB/PG-STAT12 model system
with a three-electrode cell in an acetonitrile solution of Bu4NBF4
(0.10 M) at a scan rate of 50 mV/s. Polymer film coatings on
Pt wire electrodes were formed by dipping electrodes into a
solution of the appropriate polymer. Atomic force microscopy
(AFM) measurements were obtained in tapping-mode using a
Multimode IIIa Digital Instrument AFM. Electrical charac-
teristics of the TFTs were measured under ambient conditions
using both Keithley 2400 and 236 source/measure units. All
measurements were obtained with channel lengths (L) of 100 µm
and channel widths (W) of 1000 µm. Field-effect mobilities
were extracted in the saturation regime from the slope of the
source-drain current.
(m, 4H), 1.62 (m, 4H), 1.50-1.30 (m, 32H), 0,89 (t, 12H) 13
C
NMR (CDCl3, 80 MHz, ppm) 160.08, 151.00, 142.93, 139.60,
137.47, 136.45, 132.01, 130.84, 126.42, 125.41, 123.87, 122.83,
114.03, 68.08, 31.86, 31.66, 29.65, 29.45, 29.34, 29.29, 29.01,
26.13, 22.69, 22.67, 14.13. Anal. Calcd. for C64H76Br2N2O2S4:
C, 64.41; H, 6.42; N, 2.35; S, 10.75. Found: C, 64.40; H, 6.51;
N, 2.32; S, 10.73.
Poly[2,3-bis(4-octyloxyphenyl)-5,8-dithien-2-ly-quinoxa-
line], PQx2T:The 100 mL Schlenk flask containing anhydrous
DMF (3.7 mL), bis(1,5-cyclooctadienyl) nickel(0) (0.96 g, 3.49
mmol), 2,2'-dipyridyl (0.54 g, 3.49 mmol) and 1,5-cyclooc-
tadiene (0.42 mL, 3.49 mmol) was kept under nitrogen atmo-
sphere at 78 oC for 15 min. Compound 8 (1 g, 1.16 mmol) dis-
solved in anhydrous toluene (12 mL) was added to the reaction
mixture. The polymerization was maintained at 78 oC for 36 h.
When the reaction had finished, the reaction mixture was pre-
cipitated from an equi-volume mixture of HCl, methanol and
Fabrication of the Organic Thin Film Transistor Devices.
Thin-film transistors were fabricated on silicon wafers using a
top contact geometry (channel length L = 100 µm, width W =

Synthesis and Characterization of Quinoxaline-Based Thiophene Copolymers Bull. Korean Chem. Soc. 2011, Vol. 32, No. 2 419
S
S
NaBH4
EtOH
H2N
Br
NH2
Br
Br2
N
N
1000 µm) under ambient conditions without taking special pre-
cautions to exclude air, moisture, or light. A heavily n-doped
silicon wafer with a 300 nm thermal silicon dioxide (SiO2)
layer was used as the substrate/gate electrode, with the top SiO2
layer serving as the gate dielectric. The SiO2 surface of the wafer
substrate was first cleaned with piranha solution (H2O2/H2SO4)
at 130 oC for 20 min, and the cleaned wafer was immersed in
a solution of 1.0 mM octyltrichlorosilane (OTS-8) in toluene
at room temperature for 2 h under nitrogen. The semiconductor
layer was spin-coated at 2000 rpm from a 0.5 wt % chloroben-
zene solution, to a thickness of 60 nm. Subsequently, a series
of gold source/drain electrode pairs were deposited by vacuum
evaporation through a shadow mask. Silicon oxide at the back-
side of the silicon wafer of the TFT device was removed with
HF to provide a conductive gate contact.
N
N
HBr
Br
Br
2
1
O
O
N
O
N
H
N
OEt
+
EtO
N
Et2O
H
O
3
OR1
R1O
OH
Br
OR1
2
1) Mg/ THF
KOH
DMSO
+
Br
AcOH/EtOH
2)
3
Br
O
O
5
4
R1O
OR1
R1O
OR1
R1O
OR1
Bu
SnBu
Bu
S
NBS
Fabricationof the Organic Photovoltaic Device. Composite
solutions with the quinoxaline based copolymers and PC71BM
were prepared using 1,2-dichlorobenzene (DCB). The polymer
photovoltaic devices were fabricated with a typical sandwich
structure of ITO/PEDOT:PSS/active layer/LiF/Al. The ITO-
coated glass substrates were cleaned by a routine cleaning pro-
cedure, including sonication in detergent followed by sonica-
tion in distilled water, acetone, and 2-propanol. A 45 nm thick
layer of PEDOT:PSS (Baytron P) was spin-coated onto a clean-
ed ITO substrate after exposing the ITO surface to ozone for
10 min. The PEDOT:PSS layer was baked on a hot plate at
120 oC for 60 min. The active layer was spin-coated from the
pre-dissolved composite solution after filtering through 0.45 µm
PP syringe filters. The device structure was completed by de-
positing 0.6 nm LiF and then 130 nm Al cathode as the top elec-
trode onto the polymer active layer under a 3 × 10‒6 torr. in a
thermal evaporator.
N
N
Pd(PPh3)4/Toluene
N
N
CHCl3
N
N
S
S
S
S
Br
Br
Br
Br
6
7
8
6
R1O
OR1
R1O
OR1
O
S
B
O
NBS
R2
Pd(PPh3)4
2M Na2CO3
CHCl3
N
N
N
N
S
S
S
S
S
S
S
S
Br
R2
Br
R2
9
R2
R2
10
(R1=octyl)
(R2=hexyl)
Scheme 1. Synthetic routes for quinoxaline containing monomers
R1O
OR1
Ni(COD)2 / 2,2-bipyridine
Toluene / DMF
8
Results and Discussion
N
N
S
S
n
Synthesis andThermal Properties of the Polymers. The syn-
thetic routes to the monomers are outlined in Scheme 1. The
synthetic routes to the polymers PQx2T, PQx4T, and PQx6T
are described in Scheme 2. PQx2T was synthesized by a Ni(0)-
mediated coupling reaction of compound 8. PQx4T and PQx6T
were prepared by a Stille coupling reaction of 5,5'-bis(trime-
thylstannyl)-2,2'-bithiophene and either compound 8 or 10. The
polymer yields were 50 - 83% after purification. Elemental an-
alyses of the polymers were in reasonable agreement with the
theoretical values. The resulting PQx6T showed good solubility
in common organic solvents such as chloroform, chloroben-
zene, and toluene because of the long alkyl side chain, but
PQx2T and PQx4T exhibited relatively poor solubility compar-
ed with PQx6T because of the incorporation of the rigid thiop-
hene units into the polymer backbones. The molecular weights
and polydispersity indices of the polymers were determined
by gel permeation chromatography using THF as the eluent
and polystyrene as the standard. The Mws of PQx2T, PQx4T,
and PQx6T were found to be 20,000, 12,000, and 29,000, with
polydispersity indices of 2.0, 1.2, and 1.1, respectively. The
excellent thermal stabilities of PQx2T, PQx4T, and PQx6T were
evident from their TGAprofiles, with decomposition tempera-
tures above 350 oC (Figure 1). The weight losses for PQx2T,
PQx2T
R1O
OR1
Pd2(dba)3
Sn
S
8
+
N
N
chlorobenzene
S
Sn
S
S
S
S
n
PQx4T
R1O
OR1
Pd2(dba)3
Sn
S
10
+
R2
N
N
chlorobenzene
S
Sn
S
S
S
S
S
S
n
R2
PQx6T
(R1=octyl)
(R2=hexyl)
Scheme 2. Synthetic routes for PQx2T, PQx4T, and PQx6T
PQx4T, and PQx6T were 6.9%, 2.7%, and 0.9% at 350 oC, re-
spectively. These results for the synthesized polymers are sum-
marized in Table 1.
Optical andElectrochemical Properties of the Polymers. The

420 Bull. Korean Chem. Soc. 2011, Vol. 32, No. 2
Yoon-Suk Choi et al.
Table 1. Physical properties of the PQx2T, PQx4T, and PQx6T
PQx2T
PQx2T
100
PQx4T
Td (oC)a
PQx4T
Polymer Mn (g/mol) Mw (g/mol)
Mw/Mn
PQx6T
PQx6T
90
80
PQx2T
PQx4T
PQx6T
9,900
10,000
26,000
20,000
12,000
29,000
2.0
1.2
1.1
332
399
447
aTemperature for 5% weight loss.
70
60
50
Table 2. UV-visible maximum absorption wavelength (λmax), bandgap
energy (Eg), and ionization potential (EHOMO) of the PQx2T, PQx4T, and
PQx6T
0
100
200
300
400
500
600
700
λmax (nm)
Solutiona Film
ε
Eg
EHOMO ELUMO
(eV) (eV)
Temperature (oC)
Polymer
(L/mol·cm)b (eV)c
Figure 1. TGA thermograms of the PQx2T, PQx4T, and PQx6T at a
heating rate 10 oC min‒1 under an inert atmosphere.
PQx2T
PQx4T
PQx6T
561
601
603
583
602
608
17 000
22 000
53 000
1.70
1.71
1.72
‒5.37 ‒3.67
‒5.21 ‒3.50
‒5.17 ‒3.45
(a)
aMeasurements preformed in chlorobenzene. bAbsorption coefficient
PQx2T(solution)
PQx2T (solution)
(a)
1.4
1.2
PQx4T(solution)
PQx4T (solution)
of the polymers at λmax in chlorobenzene. cElectrochemical band gap.
PQx6T(solution)
PQx6T (solution)
Eg is defined as the difference between EHOMO and ELUMO
.
1.0
0.8
ICT π-π* transition between the quinoxaline acceptor and the
oligothiophene donor.32 The short wavelength absorption peaks
of PQx2T, PQx4T, and PQx6T in chlorobenzene solution were
observed at 353, 428, and 465 nm, respectively, while those of
thin films were observed at 370, 430, and 465 nm, respectively.
It was noted that the short wavelength absorption peaks of
PQx4T and PQx6T were hugely red shifted (by about 60 - 90 nm)
compared with PQx2T in thin solid films. This red shift can be
attributed to an elongation of the effective conjugation length
upon increasing the number of thiophene rings in the polymer
backbone, which would enhance intra/intermolecular orbital
overlap, resulting in a red shift in absorption. The long wave-
length absorption peaks of PQx2T, PQx4T, and PQx6T in chlo-
robenzene solution were observed at 561, 601, and 603 nm,
respectively, while those of the thin films were observed at 583,
602, and 608 nm, respectively. To gain insight into the perfor-
mance of solar cell devices based on these polymers, we deter-
mined the energy levels of their highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) using cyclic voltammetry (CV) (Figure 3, Table 2).
The redox behavior of the polymer thin films was estimated
from CV measurements of dip-coated thin films on indium-tin
oxide (ITO)-coated glass, which formed the working electrode.
A platinum rod was used as the counter electrode, and Ag/Ag+
as the reference electrode in acetonitrile with a 0.1 M n-Bu4NPF6
supporting electrolyte solution at a scan rate of 50 mV s‒1. The
HOMO levels of PQx2T, PQx4T, and PQx6T were estimated
to be ‒5.37, ‒5.21, and ‒5.17 eV, respectively, and those of the
LUMO levels were estimated to be ‒3.67, ‒3.50, and ‒3.45 eV,
respectively. The electrochemical bandgaps, which are defined
as the difference between the HOMO and LUMO levels, of
PQx2T, PQx4T, and PQx6T were 1.70, 1.71, and 1.72 eV, res-
pectively, resulting in a low bandgap. These results indicate
that the HOMO and LUMO levels of the resulting polymers
are shifted toward the vacuum level upon increasing the number
of thiophene rings in the polymer backbones, which is due to
0.6
0.4
0.2
0.0
300
400
500
600
700
800
Wavelength (nm)
1.4
1.2
(b)
PQx2T(film)
(b)
PQx2T (film)
PQx4T(film)
PQx4T (film)
PQx6T(film)
PQx6T (film)
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
Wavelength (nm)
Figure 2. UV-visible spectra of the PQx2T, PQx4T, and PQx6T (a) in
chlorobenzene solution and (b) in thin films.
UV-visible absorption spectra of the polymers in chlorobenzene
solution and as thin solid films are presented in Table 2 and
Figure 2. All polymers showed two distinct absorption peaks
in the ranges 350 - 460 nm and 560 - 600 nm, which is a common
feature of intramolecular donor-acceptor copolymers.29-31 The
short wavelength (350 - 460 nm) absorption peak originates
from a π-π* transition of the oligothiophene units, while the
longer wavelength (560 - 600 nm) absorption peak is due to an

Synthesis and Characterization of Quinoxaline-Based Thiophene Copolymers Bull. Korean Chem. Soc. 2011, Vol. 32, No. 2 421
Table 3. Photovoltaic properties of the polymer:PC71BM devices
Polymer:PCBM
ratio (wt.)
Film
JSC
VOC PCE
(V) (%)
PQx6T
PQx6T
FF
Thickness (nm) (mA/cm2)
PQx2T (1:3)
PQx4T (1:3)
PQx6T (1:3)
69
67
65
2.90
5.29
9.05
0.41 0.58 0.69
0.33 0.42 0.73
0.42 0.47 1.80
PQx4T
PQx4T
a sandwich structure of ITO/PEDOT:PSS (50 nm)/polymer:
PC71BM/LiF (0.6 nm)/Al (130 nm). After deposition of the
PEDOT:PSS layer on the top of the anode (ITO), the active
materials were deposited via spin coating from a 1,2-dichloro-
benzene solution of polymer and PC71BM. The resulting films
were dried under vacuum at room temperature for 4 h, and then
annealed at 90 oC for 10 min to remove a solvent. In the bulk
heterojunction solar cells, the polymers were used as an electron
donor and PC71BM was used as an electron acceptor. The J-V
characteristics of the polymer solar cells prepared from the poly-
mer (PQx2T, PQx4T, and PQx6T):PC71BM blend materials are
shown in Figure 4. The open circuit voltage (VOC), short circuit
current density (JSC), fill factor (FF), and power conversion
efficiency (PCE) of these devices are summarized in Table 3.
For these devices based on the PQx2T-PQx6T:PC71BM (1:3)
blend materials, an increase in performance was observed in
going from PQx2T to PQx6T. The PCEs of the PQx2T, PQx4T,
and PQx6T devices were found to be 0.69%, 0.73%, and 1.80%,
respectively. As shown in Table 3, the PQx2T, PQx4T, and
PQx6T devices exhibited JSC values of 2.9, 5.29, and 9.05 mA/
cm2, when the polymer vs. PCBM blend ratio was 1:3. The JSC
of these polymers was found to increase as the number of thio-
phene rings in the polymer backbone increased. Because the
mobility of the donor polymer is an important parameter that
can affect the JSC values, the field-effect hole mobility was mea-
sured using an OTFT device with a top contact configuration
built on an n-doped silicon wafer. The saturation mobilities of
PQx2T, PQx4T, and PQx6T were found to be 9.0 × 10‒8, 5.0 ×
10‒7, and 2.4 × 10‒5 cm2 V‒1 s‒1, respectively (Figure 5). The JSC
should generally increase with the increasing hole mobility of
the electron-donor polymers, which enhances charge transport
in the bulk heterojunctions. Therefore, the above results indicate
that increasing the number of thiophene rings in the polymer
backbone enhances the hole mobility in the blend films, and
thus improves the PCE. In addition, the VOCs, which are related
to the difference between the HOMO level of the donor polymer
and the LUMO level of the acceptor (PCBM), of the PQx2T-
PQx6T:PC71BM (1:3) devices were found to be 0.58, 0.42, and
0.47 V, respectively. With the same electron acceptor (PC71BM
in this study), electron donors with lower HOMO give higher
PQx2T
PQx2T
-2.5
-2.0 -1.5 -1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Bias (V)
Figure 3. Cyclic voltammograms of the three polymer films on ITO
glass at a scanning rate of 50 mV s‒1 in acetonitrile containing 0.1 M
n-Bu4NBF4.
2
0
-2
-4
-6
-8
PQx2T(1:3)
PQx2T (1:3)
PQx4T(1:3)
PQx4T (1:3)
-10
-12
PQx6T(1:3)
PQx6T (1:3)
-0.2
0.0
0.2
0.4
0.6
0.8
Voltage (V)
Figure 4. J-V characteristics of photovoltaic cells prepared from poly-
mers:PC71BM (1:3) blend active materials under an illumination of
AM 1.5G, 100 mW/cm2.
10-8
PQx2T
PQx2T
PQx4T
PQx4T
PPQQxx66TT
10-9
10-10
10-11
10-12
-100
-80
-60
-40
-20
0
20
40
VOC values, however, the trend of VOC for PQx4T does not seem
VG (V)
to fit to the electrochemical potentials. This result can be under-
stood in terms of the film morphology of the blended film. Low
solubility has an impact on the film mechanical properties, the
interface resistance, and the morphology, resulting in a low
open-circuit voltage and a low fill factor.29 To investigate the
surface morphology of the blend films, we examined AFM
images of PQx2T-PQx6T:PC71BM (1:3) films spin-coated from
their corresponding 1,2-dichlorobenzene solutions, identical
Figure 5. (a) Transfer characteristics of the polymer TFT devices with
an OTS-modified surface and annealed at 100 oC, where VDS = ‒60 V.
(b) Output characteristics of PQx6T based TFT devices.
the high HOMO level of the oligothiophene groups.
Organic Photovoltaic (OPV) Characteristics of the Polymer
ThinFilm. Bulk heterojunction solar cells were fabricated with

422 Bull. Korean Chem. Soc. 2011, Vol. 32, No. 2
(a) (b)
Yoon-Suk Choi et al.
350 to 600 nm, with EQE values above 30% and a maximum
EQE of almost 40% at around 480 nm. In particular, the PQx6T:
PC71BM device yielded an EQE plot that was similar to the
absorption spectrum of PQx6T in the longer wavelength region,
indicating that excitons are mainly generated in the polymer
phase. The photovoltaic study of quinoxaline-based thiophene
polymers shows it to be a promising donor material for BHJ
solar cells.
Conclusions
(c)
In summary, we have synthesized new solution-processable
quinoxaline-based low bandgap polymers using the Yamamoto
and Stille coupling reactions. PQx6T showed good solubility
in common organic solvents such as chloroform and chloro-
benzene because of the long alkyl side chain, but PQx2T and
PQx4T showed relatively poor solubility because of the rigid
thiophene units in the polymer backbones. PQx2T, PQx4T, and
PQx6T exhibited long wavelength UV-visible absorption peaks
at 583, 602, and 608 nm, respectively. A photovoltaic device
using a PQx6T:PC71BM (1:3) film as the active layer exhibited
an open circuit voltage of 0.47 V, a short circuit current density
of 9.05 mA/cm2, a fill factor of 0.42, and a power conversion
efficiency of 1.80% under AM 1.5 G (100 mW/cm2) illumina-
tion.
Figure 6. AFM tapping mode phase images of thin films cast from
(a) PQx2T:PC71BM (1:3), (b) PQx4T:PC71BM (1:3), and (c) PQx6T:
PC71BM (1:3). Scan size is 500 nm × 500 nm.
60
PQPxQ2xT2T((11::33))
PQx4T(1:3)
PQx4T (1:3)
50
40
30
20
PQx6T(1:3)
PQx6T (1:3)
Acknowledgments. This work was supported by a grant from
the cooperative R&D Program (B551179-08-03-00) funded by
the Korea Research Council Industrial Science and Technology,
Republic of Korea, and also supported by the Catholic Univer-
sity of Korea, Research Fund, 2010. This study was supported
by a grant (M2009010025) from the Fundamental R&D Pro-
gram for Core Technology of Materials funded by the Ministry
of Knowledge Economy (MKE), Republic of Korea.
10
0
300
400
500
600
700
800
Wavelength (nm)
References
Figure 7. External quantum efficiencies of copolymer/PC71BM solar
cells.
1. Babel, A.; Jenekhe, S. A. Macromolecules 2003, 36, 7759.
2. Yoon, H.-S.; Lee, W.; Lee, J.-H.; Lim, D.-G.; Hwang, D-H.; Kang,
I.-N. Bull. Korean Chem. Soc. 2009, 30, 2371.
to the procedure used to fabricate the active layers of the solar
cell devices. We observed coarse phase separation in the images
of these polymer blends. Coarse phase separation was evident
in the images of the PQx6T:PC71BM (1:3) films, whereas the
PQx2T-PQx4T:PC71BM (1:3) films showed a smoother and
more homogeneous surface. The root-mean-square (RMS)
roughness of the PQx2T, PQx4T, and PQx6T blends was 0.56,
1.32, and 1.28 nm, respectively. These results suggest that the
differences in PCE for PQx2T-PQx6T:PC71BM (1:3) devices
may be the result of a decrease in the interfacial area and ability
for hole transportation of the donor polymers. The external
quantum efficiency (EQE), which is used to evaluate the photo-
response of the fabricated solar cells as a function of wavelength,
is shown in Figure 7. All of the devices show high photocon-
version efficiency over the wavelength range 400 - 700 nm,
which covers most of the visible wavelength range. The PQx6T:
PC71BM device shows relatively high photoconversion effici-
ency with significantly increased absorption in the range from
3. Lee, W. H.; Kong, H.; Oh, S. Y.; Shim, H. K.; Kang, I. N. J. Polym.
Sci. Part A: Polym. Chem. 2009, 47, 111.
4. Qin, Y.; Kim, J. Y.; Frisbie, C. D.; Hillmyer, M. A. Macromolecules
2008, 41, 5563.
5. Thompson, B. C.; Kim, B. J.; Kavulak, D. F.; Sivula, K.; Mauldin,
C.; Fréchet J. M. J. Macromolecules 2007, 40, 7425.
6. Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.;
Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha,
C.-S.; Ree, M. Nat. Mater. 2006, 5, 197.
7. Son, S. K.; Choi, Y. S.; Lee, W. H.; Hong, T.; Kim, J. R.; Shin, W.
S.; Moon, S. J.; Hwang, D. H.; Kang, I. N. J. Polym. Sci. Part A:
Polym. Chem. 2010, 48, 635.
8. Chen, H. Y.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G. W.;
Yang, Y.; Yu, L. P.; Wu, Y.; Li, G. Nat. Photon 2009, 3, 649.
9. Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science
1995, 270, 1789.
10. Chen, J. W.; Cao, Y. Acc. Chem. Res. 2009, 42, 1709.
11. Li, Y. F.; Zou, Y. P. Adv. Mater. 2008, 20, 2952.
12. Thompson, B. C.; Fréchet, J. M. Angew. Chem. Int. Ed. 2008, 47, 58.
13. Chen, Y. J.; Yang, S. H.; Hsu, C. S. Chem. Rev. 2009, 109, 5868.
14. Wu, W. C. ; Lee, W. Y.; Pai, C. L.; Chen, W. C.; Tuan, C. S.; Lin,

Synthesis and Characterization of Quinoxaline-Based Thiophene Copolymers Bull. Korean Chem. Soc. 2011, Vol. 32, No. 2 423
J. L. J. Polym. Sci. Polym. Phys. 2007, 45, 67.
15. Tsai, J. H.; Chueh, C. C.; Lai, M. H.; Wang, C. F.; Chen, W. C.;
Ko, B. T.; Ting, C. Macromolecules 2009, 42(6), 1897.
16. Chang, Y. T.; Hsu, S. L.; Chen, G. Y.; Su, M. H.; Singh, T. A.; Diau,
W. G.; Wei, K. H. Adv. Funct. Mater. 2008, 18, 2356.
17. Inganas, O.; Zhang, F.; Tvingstedt, K.; Andersson, L. M. Hells-
trom, S.; Andersson, M. R. Adv. Mater. 2010, 22, E100.
18. Lai, M. H.; Chueh, C. C.; Chen, W. C.; Wu, J. L.; Chen, F. C. J.
Polym. Sci. Part A: Polym. Chem. 2009, 47, 973.
19. Gadisa, A.; Mammo, W.; Andersson, L. M.; Admassie, S.; Zhang,
F.; Andersson, M. R.; Inganas, O. Adv. Funct. Mater. 2007, 17,
3836.
20. Peng, Q.; Zheng, Q. J. Polym. Sci. Part A: Polym. Chem. 2009, 47,
3399.
21. Lee, J. Y.; Shin, W. S.; Haw, J. R.; Moon, D. K. J. Mater. Chem.
2009, 19, 4938.
22. Yamamoto, T.; Zhou, Z. H.; Kanbara, T.; Shimura, M.; Kizu, K.;
Maruyama, T.; Nakamura, Y.; Fukuda, T.; Lee, B. L.; Ooba, N.;
Tomaru, S.; Kurihara, T.; Kaino, T.; Kubota, K.; Sasaki, S. J. Am.
Chem. Soc. 1996, 118, 10389.
23. Yang, R.; Tian, R.; Yan, J.; Zhang, Y.; Yang, J.; Hou, O.; Yang,
W.; Zhang, C.; Cao, Y. Macromolecules 2005, 38, 244.
24. Tsubata, Y.; Suzuki, T.; Miyashi, T. J. Org. Chem. 1992, 57, 6749.
25. Ulrich, M. W. T.; Zhou, M. J. Org. Chem. 1994, 59, 4988.
26. Gray, G. W.; Hird, M.; Lacey, D.; Toyne, K. J. J. Chem. Soc. Perkin
Trans. 1989, 2, 2041.
27. Tsami, A.; Yang, X.-H.; Farrell, T.; Neher, D.; Holder, E. J. Polym.
Sci. Part A: Polym. Chem. 2008, 46, 7794.
28. Zhang, G.; Fu, Y.; Zhang, Q.; Xie, Z. Polymer 2010, 51, 2313.
29. Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.; Neagu-
Plesu, R.; Belletete, M.; Durocher, G.; Tao, Y.; Leclerc, M. J. Am.
Chem. Soc. 2008, 130, 732.
30. Yang, J.; Jiang, C. Y.; Zhang, Y.; Yang, R. Q.; Yang, W.; Hou, Q.;
Cao, Y. Macromolecules 2004, 37, 1211.
31. Yang, R. Q.; Tian, R. Y.; Yan, J. G.; Zhang, Y.; Yang, J.; Hou, Q.;
Yang, W.; Zhang, C.; Cao, Y. Macromolecules 2005, 38, 244.
32. Lee, J-Y.; Shin, W-S.; Haw, J-R.; Moon, D.-K. J. Mater. Chem.
2009, 19, 4938.