Synthesis and photovoltaic properties of low-bandgap alternating copolymers consisting of 3-hexylthiophene and [1,2,5]thiadiazolo[3,4-g]quinoxaline derivatives

Article abstract of DOI:10.1016/j.orgel.2010.01.027

Two different kinds of low-bandgap alternating copolymers consisting of 3-hexylthiophene and [1,2,5]thiadiazolo[3,4-g]quinoxaline were synthesized via the Stille coupling reaction. The copolymers show broad absorption band from visible to infrared region.

Full text of DOI:10.1016/j.orgel.2010.01.027

Organic Electronics 11 (2010) 846–853
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Synthesis and photovoltaic properties of low-bandgap alternating
copolymers consisting of 3-hexylthiophene and
[1,2,5]thiadiazolo[3,4-g]quinoxaline derivatives
Yoonkyoo Lee a, Thomas P. Russell b, Won Ho Jo a,
*
a Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea
b Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01003, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Two different kinds of low-bandgap alternating copolymers consisting of 3-hexylthiophene
and [1,2,5]thiadiazolo[3,4-g]quinoxaline were synthesized via the Stille coupling reaction.
The copolymers show broad absorption band from visible to infrared region. The bandgaps
(1.02–1.35 eV) of the copolymers are successfully tuned by changing the substituent on the
acceptor unit. The copolymers are highly soluble in common organic solvents due to three
3-hexylthiophenes in the repeating unit. By optimizing the device parameters such as the
blend ratio of copolymer to PCBM and the choice of processing solvent, the power conver-
sion efficiency reaches as high as 1.27% under the standard solar radiation condition (AM
1.5G, 100 mW/cm2).
Received 11 December 2009
Received in revised form 22 January 2010
Accepted 23 January 2010
Available online 4 February 2010
Keywords:
Bulk heterojunction organic solar cells
Low-bandgap polymer
3-Hexylthiophene
Ó 2010 Elsevier B.V. All rights reserved.
Thiadiazoloquinoxaline
1. Introduction
vests photons only up to 22% of available photons. To ab-
sorb photons at longer wavelengths, where more photon
Polymer solar cells have attracted great interest due to
their unique advantages, such as low cost, light weight,
and potential use in flexible devices. Among the concepts
for polymer photovoltaic devices, bulk heterojunction
(BHJ) solar cells have been mostly used, since they have
advantages of ease processability and high efficiency [1–
5]. The active layer of BHJ solar cells forms an interpene-
trating network of electron-donor and electron-acceptor
domains. Currently, BHJ solar cells fabricated by simple
blending of regioregular poly(3-hexylthiophene) (P3HT)
as an electron-donating polymer and [6,6]-phenyl-C61-bu-
tyric acid methyl ester (PCBM) as an electron acceptor have
reached the power conversion efficiency (PCE) as high as
5% under AM 1.5G (AM = air mass) illumination [6,7].
Although possessing several advantageous properties,
P3HT has a relatively high bandgap of 1.9 eV and thus har-
flux from the emission of the sun is found, development
of low-bandgap polymer is inevitably needed.
In recent years, donor–acceptor alternating copolymers
have attracted extensive interest because their electronic
properties could be easily manipulated [8–11]. The inter-
nal charge transfer (ICT) between donor and acceptor units
in the donor–acceptor copolymers can effectively reduce
the bandgaps of the polymers. Very recently, some new
low-bandgap polymers with high efficiency over 6% have
been reported [12,13]. The acceptor units that have been
used for donor–acceptor alternating copolymers include
2,1,3-benzothiadiazole, thieno[3,4-b]pyrazine, quinoxa-
line, benzo[1,2-c,3,4-c0]bis[1,2,5]thiadiazole, pyrazino[2,3-
g]quinoxaline, and [1,2,5]thiadiazolo[3,4-g]quinoxaline
(TQ) [14–19]. We are particularly interested in TQ as an
acceptor unit because of the strong electron-withdrawing
property of four imine groups in the TQ unit. The donor–
acceptor alternating copolymers containing TQ as the
acceptor unit have been reported to have bandgaps of
0.90–1.50 eV, depending on the electron-donating power
* Corresponding author. Tel.: +82 2 880 7192; fax: +82 2 876 6486.
E-mail address: whjpoly@snu.ac.kr (W.H. Jo).
1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2010.01.027

Y. Lee et al. / Organic Electronics 11 (2010) 846–853
847
of donor unit and the bulkiness of side group of the
acceptor unit. Although several research groups have
synthesized TQ-based low-bandgap copolymers and re-
ported the photovoltaic properties of those copolymers,
the molecular weights of the copolymers were very low
because of the lack of solubility, and therefore the power
conversion efficiencies were below 0.7% [17,20–23].
Herein we report the synthesis and characterization of a
new class of donor–acceptor alternating copolymers com-
prising 3-hexylthiophene (3HT) unit and TQ derivatives
as electron-donating and electron-withdrawing unit,
respectively. The optical and electrochemical properties
of the copolymers and the performance of the copoly-
mers/PCBM BHJ solar cells are reported.
13.73, ꢀ8.18. Anal. Calcd for C16H32S1Sn2 (%): C, 38.91; H,
6.48; S, 6.50. Found (%): C, 38.92; H, 6.47; S, 6.49.
2.2.2. 6,7-Diphenyl-4,9-di(4-hexylthien-2-yl)[1,2,5]-
thiadiazolo[3,4-g]quinoxaline (3a)
To a suspension of 5,6-diamino-4,7-di(4-hexylthien-2-
yl)-2,1,3-benzothiadiazole (0.815 g, 1.64 mmol) in 80 ml
of acetic acid, 1,2-diphenyl-1,2-ethanedione (0.862 g,
4.1 mmol) was added in one portion. When the mixture
was stirred at room temperature for 8 h, green-colored
precipitate was obtained. The precipitate was filtered,
washed thoroughly with methanol, and then dried in vac-
uum to afford green powder as product. Yield: 1.01 g (92%).
1H NMR (300 MHz, CDCl3): d (ppm) 8.86 (d, 2H), 7.83 (d,
4H), 7.44 (m, 6H), 7.30 (d, 2H), 2.80 (t, 4H), 1.78 (m, 4H),
1.48–1.35 (m, 12H), 0.93 (t, 6H). 13C NMR (75 MHz, CDCl3):
d (ppm) 153.11, 152.12, 143.23, 138.37, 135.80, 135.38,
135.04, 130.94, 130.79, 129.87, 128.27, 121.53, 32.01,
29.82, 29.37, 28.90, 22.91, 14.07. Anal. Calcd for
C40H40N4S3 (%): C, 71.41; H, 5.95; N, 8.33; S, 14.31. Found
(%): C, 71.14; H, 5.96; N, 8.30; S, 14.55.
2. Experimental
2.1. Materials
All reagents were obtained from Aldrich unless other-
wise specified and used as received. Tetrahydrofuran
(THF) was dried over sodium/benzophenone under nitro-
gen and freshly distilled before use. Hexane was dried over
calcium hydride under nitrogen and freshly distilled before
use. 5,6-Diamino-4,7-di(4-hexylthien-2-yl)-2,1,3-benzo-
thiadiazole (2), 6,7-dimethyl-4,9-di(4-hexylthien-2-yl)[1,
2,5]-thiadiazolo[3,4-g]quinoxaline (3b), and 6,7-dimethyl-
4,9-di(50-bromo-(4-hexylthien-2-yl))[1,2,5]-thiadiazolo[3,
4-g]quinoxaline (4b) were synthesized by following the lit-
erature method [24]. [6,6]-Phenyl-C61-butyric acid methyl
ester (PCBM) was obtained from Nano-C. Poly(3,4-ethyl-
enedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)
(Baytron P VP AI 4083) was purchased from H.C. Stark
2.2.3. 6,7-Diphenyl-4,9-di(50-bromo-(4-hexylthien-2-yl))-
[1,2,5]-thiadiazolo[3,4-g]quinoxaline (4a)
The compound 3a (0.890 g, 1.32 mmol) was dissolved in
40 ml of THF, to which N-bromosuccinimide (NBS)
(0.483 g, 2.71 mmol) was added in the dark. After stirring
the mixture at room temperature for 3 h, the solvent was
removed under reduced pressure. The residue was purified
by column chromatography on silica gel (1:3 chloroform/
hexane as eluent) to yield the dark-green product. Yield:
0.804 g (74%). 1H NMR (300 MHz, CDCl3): d (ppm) 8.79
(d, 2H), 7.70 (d, 4H), 7.45 (m, 6H), 2.80 (t, 4H), 1.78 (m,
4H), 1.50–1.35 (m, 12H), 0.93 (t, 6H). 13C NMR (75 MHz,
CDCl3): d (ppm) 153.02, 150.77, 141.79, 137.45, 135.22,
134.29, 134.03, 130.85, 129.44, 127.69, 119.78, 118.25,
31.87, 29.88, 29.59, 28.71, 22.66, 13.89. Anal. Calcd for
C40H38N4S3Br2 (%): C, 57.83; H, 4.58; N, 6.75; S, 11.59.
Found (%): C, 57.98; H, 4.65; N, 6.52; S, 11.79.
and passed through a 0.45
spin-coating.
lm PES syringe filter before
2.2. Synthesis
2.2.1. 2,5-Bistrimethylstannyl-3-hexylthiophene (1)
3-Hexylthiophene (5.0 g, 0.03 mol) and tetramethyl-
ethylenediamine (TMEDA) (13.5 ml, 3.0 equiv) were added
into 150 ml of anhydrous hexane. After cooled to ꢀ78 °C
with isopropanol/dry ice bath, t-BuLi (52.4 ml, 3.0 equiv)
was added dropwise to the solution over 30 min. After
the cooling bath was removed, the reaction mixture was
stirred at room temperature for 48 h under nitrogen atmo-
sphere. Then the reaction mixture was cooled back down
to ꢀ78 °C, and the reaction solution was stirred overnight
at room temperature after 92.0 ml of trimethyltinchloride
solution (3.0 equiv) was added. The product mixture was
washed with saturated solution of NaHCO3, water, and
brine, then dried over anhydrous MgSO4 and filtered. After
evaporation of the solvent under reduced pressure, the res-
idue was further purified by flash chromatography on silica
gel (99:1 hexane/triethylamine as eluent), yielding viscous
light yellow liquid as a product. Yield: 12.44 g (85%). 1H
NMR (300 MHz, CDCl3): d (ppm) 7.21 (t, 1H), 2.56 (t, 2H),
1.60 (t, 2H), 1.52–1.41 (m, 6H), 0.86 (t, 3H), 0.43–0.30
(m, 18H). 13C NMR (75 MHz, CDCl3): d (ppm) 151.88,
143.79, 137.82, 137.61, 32.77, 32.19, 31.55, 29.48, 22.08,
2.2.4. Poly(3-hexylthiophene-alt-6,7-diphenyl-4,9-bis-
(4-hexylthien-2yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline
(P(3HT-PhTDQ))
Under nitrogen atmosphere, monomer 1 (102.0 mg,
0.206 mmol) and 4a (171.3 mg, 0.206 mmol) were dis-
solved in 10 ml of anhydrous 1,2-dichlorobenzene. The
solution was flushed with N2 for 20 min, and then 7.2 mg
of Pd(PPh3)2Cl2 was added. After the reaction mixture
was stirred at 130 °C for 48 h, the polymer was precipi-
tated by addition of 70 ml of methanol. The crude product
was filtered through a Soxhlet thimble, and then subjected
to Soxhlet extraction with methanol, hexane, acetone, and
chloroform. The polymer was recovered from the chloro-
form fraction by rotary evaporation as dark green solid.
Yield: 107.7 mg (63%). 1H NMR (300 MHz, CDCl3):
d
(ppm) 9.18–8.80 (m, 2H), 7.75 (s, 4H), 7.33 (d, 6H), 7.20–
6.93 (m, 1H), 3.01–2.54 (m, 6H), 1.98–1.60 (m, 6H), 1.58–
1.21 (m, 18H), 0.98–0.65 (m, 9H). Anal. Calcd for
C50H52N4S4 (%): C, 71.75; H, 6.22; N, 6.70; S, 15.34. Found
(%): C, 71.46; H, 6.22; N, 6.67; S, 15.41.

848
Y. Lee et al. / Organic Electronics 11 (2010) 846–853
2.2.5. Poly(3-hexylthiophene-alt-6,7-dimethyl-4,9-bis-
(4-hexylthien-2yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline
(P(3HT-MeTDQ))
3. Results and discussion
3.1. Synthesis and characterization
The polymerization process was the same as that of
P(3HT-PhTDQ), except that the monomer 4b was used in-
stead of 4a. Yield: 62.5 mg (55%). 1H NMR (300 MHz,
CDCl3): d (ppm) 9.10–8.71 (m, 2H), 7.36–7.02 (m, 1H),
3.04–2.49 (m, 12H), 1.98–1.66 (m, 6H), 1.58–1.33 (m,
18H), 1.02–0.68 (m, 9H). Anal. Calcd for C40H48N4S4 (%):
C, 67.39; H, 6.74; N, 7.86; S, 18.01. Found (%): C, 67.35;
H, 6.78; N, 7.75; S, 17.96.
We synthesized alternating copolymers for preparation
of low-bandgap polymers. The synthetic routes for prepa-
ration of the monomers and polymers are shown in
Fig. 1. We used the palladium-catalyzed Stille coupling
reaction to synthesize alternating copolymers of 3HT and
TQ derivatives which are electron-donating and electron-
withdrawing unit, respectively. At first, 3HT was dilithiat-
ed with 3 equiv amount of t-BuLi and TMEDA, followed
by reaction with trimethyltin chloride to yield 2,5-
bistrimethylstannyl-3-hexylthiophene (1). In a separate
experiment, the comonomer 4a was synthesized by con-
densation reaction of diamino compound with 1,2-diphe-
nyl-1,2-ethanedione, followed by bromination with NBS
in THF. Stille coupling of 1 with 4a or 4b resulted in the
corresponding polymers, P(3HT-PhTDQ) and P(3HT-MeT-
DQ), with moderate yield. The polymers are highly soluble
in common organic solvents, such as chloroform, toluene,
and THF at room temperature. The number-average molec-
ular weight (Mn) and polydispersity index (PDI) of P(3HT-
PhTDQ) are 7200 and 1.6, respectively, and those of
P(3HT-MeTDQ) are 11,100 and 1.5, respectively.
2.3. Measurement
The chemical structures of materials used in this study
were identified by 1H NMR and 13C NMR (Avance DPX-
300). Elemental analysis was performed on EA1110 (CE
Instrument) elemental analyzer. Molecular weight and its
distribution were measured by gel permeation chromatog-
raphy (PL-GPC 50 Integrated GPC System, Polymer Labora-
tories) equipped with a refractive index detector using THF
as eluent, where the columns were calibrated against stan-
dard polystyrene samples. The optical absorption and pho-
toluminescence spectra were obtained by UV–vis–NIR
spectrophotometer (Lambda 850, Perkin-Elmer) and fluo-
rescence spectrometer (814, Photon Technology Interna-
tional), respectively. Cyclic voltammetry experiments
were carried out on potentiostat/galvanostat (Model 273
A, EG&G Princeton Applied Research) in an electrolyte
solution of 0.1 M tetrabutylammonium hexafluorophos-
phate (Bu4NPF6) in dichloromethane. Three-electrode cell
was used for all experiments. Platinum wires (Bioanalytical
System Inc.) were used as both counter and working elec-
trodes, and silver/silver ion (Ag in 0.1 M AgNO3 solution,
3.2. Optical properties
The optical absorption spectra of the two polymers in
chloroform solutions and films on glass substrate are
shown in Fig. 2a and b, respectively. Both of two polymers
have broad absorption from visible to infrared region. The
lower energy absorption peaks of P(3HT-PhTDQ) and
P(3HT-MeTDQ) in solution are 784 nm and 703 nm,
respectively. As expected, the broad and longer wave-
length absorption of the polymers is attributed to the
intermolecular charge transfer induced by the donor–
acceptor structure of the 3HT and TQ units. When the max-
imum absorption of polymers in solution is compared with
that in solid state (Fig. 2a vs. b), it reveals that the maxi-
mum absorption wavelengths (kmax) of the two polymers
are red-shifted in solid state by 76 nm and 30 nm, respec-
tively, indicating that polymer chains are associated in so-
lid state. The optical bandgaps (Eg(opt)) as determined
from the onset of the absorption spectra of P(3HT-PhTDQ)
and P(3HT-MeTDQ) are 1.02 eV and 1.35 eV, respectively.
Particularly, the bandgap of P(3HT-PhTDQ) is lower than
that of P(3HT-MeTDQ) because the electron-withdrawing
phenyl group in P(3HT-PhTDQ) makes the TQ unit more
electron-deficient while the electron-donating methyl
group in P(3HT-MeTDQ) weakens the electron-withdraw-
ing power of TQ unit. Since it has generally been accepted
that the strong electron-withdrawing power of acceptor
unit results in a decrease of bandgap for alternating
copolymer, our results are very consistent with the predic-
tion and other reports [11,25].
Bioanalytical System Inc.) was used as
electrode.
a reference
2.4. Fabrication of photovoltaic devices
Polymer solar cells were fabricated on ITO glass cleaned
by stepwise sonication in acetone and IPA, followed by O2
plasma treatment for 10 min. The substrate was spin-
coated with PEDOT:PSS at 4500 rpm for 1 min and an-
nealed at 120 °C for 30 min to yield 40 nm thick film. A
mixture of polymer and PCBM were dissolved in anhy-
drous chlorobenzene (30 mg/ml), and spin-coated on the
top of the ITO/PEDOT:PSS film at 800 rpm for 60 s. The typ-
ical thickness of the active layer was 70–80 nm. LiF
(0.5 nm) and Al (100 nm) were evaporated under vacuum
lower than 10ꢀ6 Torr on the top of active layer through a
shadow mask. The effective area of one cell was ca.
4 mm2. The current–voltage (J–V) curve of the device was
obtained on a computer-controlled Keithley 4200 source
measurement unit under AM 1.5G (100 mW/cm2) simu-
lated by an Oriel solar simulator (Oriel 91160A). The light
intensity was calibrated using a NREL-certified photodiode
prior to each measurement. The external quantum effi-
ciency (EQE) was measured using a Polaronix K3100 IPCE
measurement system (McScience) where the light inten-
sity at each wavelength was calibrated with a standard sin-
gle-crystal Si cell.
Fig. 3 compares photoluminescence (PL) spectra of
P(3HT-MeTDQ) and P(3HT-MeTDQ)/PCBM composite in
solid state. P(3HT-MeTDQ) shows strong PL with a maxi-
mum at 990 nm whereas P(3HT-PhTDQ) does not show

Y. Lee et al. / Organic Electronics 11 (2010) 846–853
849
Fig. 1. Synthetic route of alternating copolymers.
PL below 1000 nm wavelength. The PL intensity of P(3HT-
MeTDQ) is largely quenched by addition of PCBM, indicat-
ing that the charge transfer takes place effectively from the
polymer to PCBM.
ꢀ[Ered ꢀ E1/2(ferrocene) + 4.8] V, where Ered is the onset
reduction potential of polymer. The calculated HOMO
and LUMO energy levels of P(3HT-PhTDQ) are ꢀ4.82 eV
and ꢀ3.94 eV, and those of P(3HT-MeTDQ) are ꢀ4.96 eV
and ꢀ3.79 eV, respectively, from which the electrochemi-
cal bandgaps (Eg(ec)) for each polymer can be determined.
It is noted that the Eg(ec) values of the copolymers are
smaller than their corresponding values of Eg(opt). It has
generally been accepted that a LUMO–LUMO offset of
0.3–0.4 eV is necessary for efficient electron transfer from
polymer to PCBM [26]. Since the LUMO energy level of
PCBM has the value ranging between ꢀ4.0 and ꢀ4.3 eV
[27,28], the LUMO–LUMO offset between P(3HT-MeTDQ)
and PCBM is larger than 0.3 eV and therefore it is expected
that exciton can be easily dissociated at the interface be-
tween P(3HT-MeTDQ) and PCBM, whereas the LUMO–
LUMO offset between P(3HT-PhTDQ) and PCBM is small,
and thus the charge separation is difficult to occur. Fur-
thermore, the HOMO energy level of P(3HT-PhTDQ) is
3.3. Electrochemical properties
The electrochemical data of all the polymers were ob-
tained from the oxidation and reduction cyclic voltammo-
grams, as shown in Fig. 4, and are summarized in Table 1.
The onset oxidation and reduction potential of P(3HT-
PhTDQ) are 0.38 eV and ꢀ0.48 eV (vs. Ag/Ag+), respectively,
and those of P(3HT-MeTDQ) are 0.52 eV and ꢀ0.65 eV (vs.
Ag/Ag+). The HOMO energy levels of the polymers can be
calculated using the equation: HOMO = ꢀ[Eox ꢀ E1/2(ferro-
cene) + 4.8] V, where E is the onset oxidation potential
ox
of polymer and E1/2(ferrocene) is the onset oxidation po-
tential of ferrocene vs. Ag/Ag+. The LUMO energy levels
can be estimated by using the equation: LUMO =

850
Y. Lee et al. / Organic Electronics 11 (2010) 846–853
Fig. 4. Cyclic voltammograms of P(3HT-MeTDQ) and P(3HT-PhTDQ).
ported from P(3HT-MeTDQ) to PEDOT:PSS whereas holes
can be trapped at the interface between P(3HT-PhTDQ)
and PEDOT:PSS. Consequently, it can be expected that
P(3HT-MeTDQ) is better candidate for active material in
photovoltaic device.
3.4. Photovoltaic properties
The polymer solar cells were fabricated with layered
configuration of glass/ITO/PEDOT:PSS/polymer:PCBM/LiF/
Al. The current–voltage characteristics of photovoltaic de-
vices with two different polymers, P(3HT-PhTDQ) and
P(3HT-MeTDQ), are shown in Fig. 5, and the photovoltaic
parameters of open circuit voltage (Voc), short circuit cur-
rent (Jsc), fill factor (FF), and power conversion efficiency
(PCE) are summarized in Table 2. When the PCEs of
P(3HT-MeTDQ) are compared with those of P(3HT-PhTDQ),
it reveals that the PCEs of P(3HT-MeTDQ) are higher than
those of P(3HT-PhTDQ). It should be noted here that
P(3HT-MeTDQ) device reaches 0.48% of PCE at the mixing
Fig. 2. UV–vis–NIR absorption spectra of the copolymers in chloroform
(a) and in film (b).
ratio of 1:2 (polymer:PCBM) with Voc = 0.58 V, Jsc
=
1.58 mA/cm2, and FF = 0.53. The values of Voc and FF of
P(3HT-MeTDQ) are comparable to or slightly higher than
those of other TQ-based polymer solar cells [17,20–23],
while the Jsc value is lower than others. When the external
quantum efficiencies (EQE) of P(3HT-PhTDQ) and P(3HT-
MeTDQ) are compared (Fig. 6), P(3HT-MeTDQ) exhibits
7.3% efficiency at 700 nm while P(3HT-PhTDQ) shows very
low efficiency. This can be explained by the small offset of
LUMO levels of P(3HT-PhTDQ) and PCBM, which implies
that only a small fraction of the absorbed photons is disso-
ciated at the polymer/PCBM interface.
It is important to optimize the morphology of active
layer for achieving high efficiency. The AFM image of
P(3HT-MeTDQ):PCBM blend film shows smooth and
homogeneous film surface without exhibiting any charac-
teristic feature of phase separation, as can be seen in
Fig. 7a and b. This homogeneous morphology may cause
low current density because it does not provide the path-
way for charge carriers to pass through. It has recently
Fig. 3. Photoluminescence spectra of P(3HT-MeTDQ) and P(3HT-MeT-
DQ):PCBM composite films.
higher than that of PEDOT:PSS (ꢀ5.0 eV), while the HOMO
energy level of P(3HT-MeTDQ) is almost the same as that
of PEDOT:PSS, indicating that holes can be easily trans-

Y. Lee et al. / Organic Electronics 11 (2010) 846–853
851
Table 1
Optical and electrochemical properties of polymers.
Sample
Absorption
Eg(opt)a (eV)
HOMO (eV)
LUMO (eV)
Eg(ec)b (eV)
kmax(CHCl3) (nm)
kmax(film) (nm)
P(3HT-PhTDQ)
P(3HT-MeTDQ)
784
703
860
733
1.02
1.35
ꢀ4.82
ꢀ4.96
ꢀ3.94
ꢀ3.79
0.88
1.17
a
Determined from the onset of UV–vis–NIR absorption spectra.
Calculated from the cyclic voltammetry.
b
Fig. 6. External quantum efficiency spectra of polymer/PCBM solar cells.
been reported that the morphology of active layer can be
effectively modified by the use of specific additives [29–
32]. Peet and coworkers [29] have demonstrated that addi-
tion of 1,8-octanedithiol (ODT) to polymer:PCBM compos-
ite induces phase separation of active layer and thereby
improves the PCE of BHJ solar cells. Accordingly, when
we add 5 vol.% of ODT to chlorobenzene solution of
P(3HT-MeTDQ):PCBM (1:2 w/w), the morphology of active
layer shows a phase-separated structure, as shown in
Fig. 7c and d. When the surface roughness and phase mor-
phology of film with addition of ODT are compared with
those of film without ODT, as shown in Fig. 7, it is realized
that the surface roughness of film with ODT is larger than
the film without ODT and that the film morphology with
ODT exhibits more phase-separated structure as compared
to the film without addition of ODT.
Fig. 5. Current–voltage characteristics of photovoltaic devices based on
P(3HT-PhTDQ) (a) and P(3HT-MeTDQ) (b) with different mixing ratio of
copolymer to PCBM by weight.
Table 2
Photovoltaic parameters of devices testing under standard AM 1.5G conditions.
Polymer
Polymer:PCBM
Solvent
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
P(3HT-PhTDQ)
1:1
1:2
1:3
1:4
CB
CB
CB
CB
0.46
0.47
0.51
0.52
0.39
0.85
0.56
0.46
0.32
0.36
0.43
0.42
0.06
0.14
0.12
0.10
P(3HT-MeTDQ)
1:1
1:2
1:3
1:4
1:2
CB
CB
CB
CB
0.59
0.58
0.63
0.59
0.55
0.89
1.58
1.03
1.06
4.64
0.54
0.53
0.51
0.44
0.50
0.28
0.48
0.33
0.28
1.27
CB/ODT

852
Y. Lee et al. / Organic Electronics 11 (2010) 846–853
Fig. 7. AFM images of P(3HT-MeTDQ):PCBM (1:2 weight ratio) thin films cast from chlorobenzene (a, height; b, phase), and chlorobenzene with 1,8-
octanedithiol as an additive (c, height; d, phase). Scale bars are 150 nm.
To investigate the effect of phase-separated morphol-
ogy on the charge transport property, we measured hole
and electron mobilities from the space charge limited cur-
rent (SCLC) J–V curve obtained in the dark for hole-only
device (ITO/PEDOT:PSS/P(3HT-MeTDQ):PCBM/Au) and
electron-only device (Al/P(3HT-MeTDQ:PCBM)/Al) (Fig. 8).
The SCLC behavior can be analyzed using the Mott–Gurney
than that of device without ODT (0.48%). Particularly, the
addition of ODT causes a large increase in Jsc, because poly-
mer/PCBM composite is phase-separated in nanometer
scale as ODT is added, which provides effectively the path-
way for charge carriers to pass through the active layer. On
the other hand, the photovoltaic performance of P(3HT-
PhTDQ) device is not improved by addition of ODT, indicat-
ing that the low PCE of the device is attributed mainly to
the small difference between LUMO energy levels of the
polymer and PCBM.
law [33]: J = (9/8)el(V2/L3), where
constant of the medium, is the carrier mobility,
e is the static dielectric
l
V = Va ꢀ Vbi (Va, the applied bias; Vbi, the built-in potential
due to the difference in electrical contact work functions),
and L is the layer thickness. When the hole and electron
mobilities of the device with ODT, determined from the
slope of plot of J1/2 vs. V, are compared with those of the
device without ODT, it is found that both the hole and elec-
tron mobilities of the device with ODT are enhanced com-
pared to those of the device without ODT: the hole
mobility increases from 6.37 ꢁ 10ꢀ6 to 2.30 ꢁ 10ꢀ5 cm2/
Vs; the electron mobility increases from 1.10 ꢁ 10ꢀ5 to
3.60 ꢁ 10ꢀ5 cm2/Vs. This can be attributed to effective for-
mation of phase-separated structure induced by the addi-
tion of ODT, which leads to better connectivity of each
phase.
4. Conclusion
We have synthesized two types of 3-hexylthiophene-
thiadiazoloquinoxaline alternating copolymers via the
Stille coupling polymerization. Introduction of three hexyl
side groups in a repeating unit results in highly soluble
polymers with moderate molecular weight. The copoly-
mers exhibited a wide range of optical absorption up to
near-IR region. The device fabricated with P(3HT-MeTDQ)
shows the PCE of 1.27%, one of the highest values among
the TQ-based copolymers, when a small amount of ODT
is added to polymer/PCBM composite. Our results demon-
strate that P(3HT-MeTDQ) is a potential candidate for
application of near-IR absorbing polymer solar cells.
Fig. 9 compares J–V curves of devices with and without
ODT, from which Voc, Jsc, FF, and PCE are determined (Table
2). The PCE of the device with ODT (1.27%) is much higher

Y. Lee et al. / Organic Electronics 11 (2010) 846–853
853
Acknowledgement
The authors thank the Ministry of Education, Science
and Technology (MEST), Korea for financial support
through the Global Research Laboratory (GRL) program.
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