Low bandgap EDOT-quinoxaline and EDOT-thiadiazol-quinoxaline conjugated polymers: Synthesis, redox, and photovoltaic device

Article abstract of DOI:10.1016/j.polymer.2010.03.050

Three new low bandgap conjugated copolymers with 3,4-ethylenedioxythiophene (EDOT) as donor and 2,3-bis(4-octyloxyphenyl)-quinoxaline (P1), 2,3-bis(4-octyloxyphenyl)-thiadiazol-quinoxaline (P2, P3) as acceptors were synthesized by Stille cross-coupling reaction, and their optical and electrochemical properties were studied. These polymers exhibited optical bandgap of 1.77, 1.29 and 1.13 eV, for P1, P2 and P3, respectively. Photovoltaic cells with device configuration of ITO/PEDOT: PSS/Copolymer: PCBM (1:4 w/w)/LiF/Al were fabricated. The measurements revealed an open-circuit voltage (V_oc) of 0.52 V, short-circuit current density (J_sc) of 3.24 mA/cm² and power conversion efficiency (PCE) of 0.60% for P1, and showed a V_oc of 0.33 V, J_sc of 2.11 mA/cm², PCE of 0.39% for P2.

Full text of DOI:10.1016/j.polymer.2010.03.050

Polymer 51 (2010) 2313e2319
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Low bandgap EDOT-quinoxaline and EDOT-thiadiazol-quinoxaline conjugated
polymers: Synthesis, redox, and photovoltaic device
,
b,
Guobing Zhang a, Yingying Fu b, Qing Zhang a , Zhiyuan Xie
*
*
a Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, China
b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new low bandgap conjugated copolymers with 3,4-ethylenedioxythiophene (EDOT) as donor and
2,3-bis(4-octyloxyphenyl)-quinoxaline (P1), 2,3-bis(4-octyloxyphenyl)-thiadiazol-quinoxaline (P2, P3)
as acceptors were synthesized by Stille cross-coupling reaction, and their optical and electrochemical
properties were studied. These polymers exhibited optical bandgap of 1.77, 1.29 and 1.13 eV, for P1, P2
and P3, respectively. Photovoltaic cells with device configuration of ITO/PEDOT: PSS/Copolymer: PCBM
(1:4 w/w)/LiF/Al were fabricated. The measurements revealed an open-circuit voltage (Voc) of 0.52 V,
short-circuit current density (Jsc) of 3.24 mA/cm2 and power conversion efficiency (PCE) of 0.60% for P1,
and showed a Voc of 0.33 V, Jsc of 2.11 mA/cm2, PCE of 0.39% for P2.
Received 25 December 2009
Received in revised form
15 March 2010
Accepted 27 March 2010
Available online 8 April 2010
Keywords:
Polymer solar cells
Ethylenedioxythiophene
Low bandgap polymer
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
EDOT/thiadiazol-quinoxaline have never been reported. In this
paper, we reported the first synthesis, photophysical, and elec-
Conjugated copolymers with alternating donoreacceptor
repeating units have been widely studied as low bandgap organic
semiconductor materials [1e4]. Polymers with various combina-
tions of donors and acceptors have been synthesized for opto-
electronic applications, including organic field-effect transistors,
organic photovoltaics, and organic light emitting diodes [5e10].
3,4-Ethylenedioxythiophene (EDOT) has been chosen as an effec-
tive donor unit due to its strong electron donating effects and small
steric interaction between repeating units in polymers [11e14].
Many electron acceptor units have been polymerized with EDOT to
form low bandgap copolymers [7,15e18]. Recently, quinoxaline and
thiadiazol-quinoxaline received much attention as acceptor units
for synthesis of low bandgap polymers [19,20]. The absorption
spectra of polymers with fluorene, thiophene unit as the donor and
with thiadiazol-quinoxaline unit as the acceptor were extend to the
near-infrared region [21,22]. Conjugated polymers that absorb both
in the visible and in the near-infrared regions are considered as
good candidates for polymer solar cell applications [23e25].
EDOT/benzothiadiazole copolymer and EDOT/quinoxaline
oligomer have been synthesized in the past [6,7]. However, the
donoreacceptor type copolymers of EDOT/quinoxaline and
trochemical properties of three conjugated polymers containing
EDOT as donor and quinoxaline, thiadiazol-quinoxaline as
acceptors. They are poly[(EDOT)-alt-2,3-bis(4-octyloxyphenyl)-
quinoxaline] (P1), poly[(EDOT)-alt-2,3-bis(4-octyloxyphenyl)-
thiadiazol-quinoxaline] (P2), and random poly[(EDOT)-2,3-bis
(4-octyloxyphenyl)-thiadiazol-quinoxaline] (P3) with increased
EDOT content. These donoreacceptor copolymers exhibit broad
absorption bands in the visible and near-infrared region and have
low bandgaps (1.77 eV for P1, 1.29 eV for P2, and 1.13 eV for P3).
Bulk heterojunction solar cells devices based on these polymers
as hole transport materials were studied. The power conversion
efficiencies of the polymer solar cells are 0.60% for P1, 0.39%
for P2, and 0.11% for P3 under the illumination of AM 1.5 G,
100 mW/cm2.
2. Experimental section
2.1. Materials
n-Butyllithium, tributyltin chloride, sodium borohydride and
triphenylarsine were obtained from Alfa Aesar Chemical Company.
Tris(dibenzylideneacetone)dipalladium, EDOT, and 4,40-dimethoxy-
benzl were purchased from SigmaeAldrich Chemical Company. All
materials were used as received without further purification.
Tetrahydrofuran (THF) and diethyl ether were freshly distilled over
* Corresponding authors.
E-mail addresses: qz14@sjtu.edu.cn (Q. Zhang), xiezy_n@ciac.jl.cn (Z. Xie).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.03.050

2314
G. Zhang et al. / Polymer 51 (2010) 2313e2319
Na wire under nitrogen prior to use. 2,5-Bis(tributylstannyl)-EDOT
(1) [26], 4,7-dibromobenzothiadiazol (2) [27], 3,6-dibromo-
1,2-phenylenediamine (3) [6], 4,7-dibromo-5,6-dinitro-benzothia-
diazole (4) [20,27], benzol[1,2,5]thiadiazole-5,6-diamine (5) [28],
4,40-dihydroxybenzil (6), 4,40-bis(octyloxy)benzil (7) [29], and
2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) [30] were
synthesized according to the literature procedures. All the reactions
were carried out under nitrogen atmosphere.
polymer film was coated on platinum electrode. Ferrocene was
added as an internal reference.
2.3. Device fabrication
Polymer solar cells (PSCs) with the device structures of ITO/
PEDOT: PSS/polymer: PCBM (1:4, w/w)/LiF/Al were fabricated as
follows: a ca. 40-nm-thick PEDOT: PSS (Baytron P AI 4083) was
spin-coated from an aqueous solution onto the pre-cleaned ITO
substrates, followed by drying at 120 ꢀC for 30 min in air. Then the
substrates were transferred into a nitrogen filled glove box. The
prepared solution containing a mixture of polymer: PCBM (5 mg/
mL:20 mg/mL) in chlorobenzene was spin-coated on top of the
PEDOT/PSS layer. Finally the samples were transferred into an
evaporator and 1-nm-thick LiF and 100-nm-thick Al were ther-
mally deposited under a vacuum of 10ꢁ6 Torr with area of 0.12 cm2.
The devices were encapsulated in the glove box and measured in
the air. Currentevoltage characteristics were measured using
a computer controlled Keithley 236 source meter. The photocurrent
was measured under AM 1.5 G illumination at 100 mW/cm2 from
a solar simulator (Oriel, 91160A-1000).
2.2. Characterization
Nuclear Magnetic Resonance spectra were recorded either on
a Mercury plus 400 MHz machine or on a 300 MHz machine. Gel
permeation chromatography (GPC) analyses were performed on
a Perkin Elmer Series 200 gel coupled with refractive index detector
using THF as eluent with polystyrene as standards. Thermogravi-
metric analysis (TGA) was conducteꢁd1 with a TA instrument
QS000IR at a heating rate of 20 ꢀC min under nitrogen gas flow.
Differential scanning calorimetry (DSC) analysis was performed on
a TA instrument Q2000 in a nitrogen atmosphere. All the samples
(about 10.0 mg in weight) were first heated up to 250 ꢀC and held
for 2 min to remove thermal history, followed by the cooling rate of
20 ꢀC/min to 20 ꢀC and then heating rate of 20 ꢀC/min to 250 ꢀC in
all cases. UVevis absorption spectra were recorded on a Perkin
2.4. Synthesis of polymer
Elmer model
l
20 UVevis spectrophotometer. Electrochemical
2.4.1. 2,3-Bis(4-octyloxyphenyl)-5,8-dibromoquinoxaline (8)
measurements were conducted under nitrogen in a deoxygenated
anhydrous acetonitrile solution of tetra-n-butylammonium hexa-
A solution of 3,6-dibromo-1,2-phenylenediamine (3) (1.0 g,
3.76 mmol) and 4,40-bis(octyloxy)benzil (7) (1.75 g, 3.76 mmol) in
acetic acid (100 mL) was refluxed overnight. After the mixture was
cooled to 0 ꢀC, ethanol (200 mL) was added to the solution. The
precipitate was isolated by filtration and was washed with ethanol
to afford the desired compound (1.89 g, 72.5%). 1H NMR (400 MHz,
fluorophosphate (0.1 M), using
a BAS 100B electrochemical
analyzer. A platinum electrode was used as a working electrode,
a platinum-wire was used as an auxiliary electrode, and a silver
wire anodized with AgCl was used as a reference electrode, thin
Scheme 1. Synthetic routes of monomers 1e9.

G. Zhang et al. / Polymer 51 (2010) 2313e2319
2315
Scheme 2. Synthetic routes of P1, P2, and P3.
CDCl3):
d
¼ 7.85 (s, 2H), 7.65 (d, J ¼ 8.0 Hz, 4H), 6.88 (d, J ¼ 8.8 Hz,
2,5-bis(tributylstannyl)-EDOT (1) (0.66 g, 0.92 mmol) and 2,3-bis
(4-octyloxyphenyl)-5,8-dibromoquinoxaline (8) (0.64, 0.92 mmol)
in THF (15 mL) under nitrogen. The solution was subjected to three
cycles of evacuation and nitrogen refilling. After degassing, the
reaction vessel was sealed. The reaction was heated to 80 ꢀC for
72 h. After cooling to room temperature, aqueous potassium fluo-
ride solution (20 mL, 1.0 M) was added and the mixture was stirred
for 1 h. The mixture was extracted with dichloromethane (3ꢂ). The
combined organic layer was dried over anhydrous sodium sulfate
and filtered. The filtrate was concentrated at about 10 mL in
vacuum. The solution was added slowly to methanol (250 mL) with
stir. The purple precipitate was collected by filtration. The product
was further purified by washing with methanol, hexane and then
4H), 3.99 (t, J ¼ 6.4 Hz, 4H), 1.80 (m, 4H), 1.46 (m, 4H), 1.30 (m, 16H),
0.90 (t, J ¼ 6.4 Hz, 6H). 13C NMR (100 MHz, CDCl3):
d
¼ 160.69,
153.80, 139.24, 132.70, 131.88, 130.54, 123.66, 114.59, 68.33, 32.04,
29.58, 29.46, 29.43, 26.25, 22.88, 14.33. Anal. Calcd for
C36H44Br2N2O2 (%): C, 62.07; H, 6.37; N, 4.02. Found (%): C, 62.33; H,
6.39; N, 3.54.
2.4.2. 2,3-Bis(4-octyloxyphenyl)-5,8-dibromothiadiazol-
quinoxaline (9)
A solution of benzol[1,2,5]thiadiazole-5,6-diamine (5) (1.00 g,
3.09 mmol) and 4,40-bis(octyloxy)benzil (7) (1.44 g, 3.09 mmol) in
acetic acid (100 mL) was refluxed overnight. The mixture was
cooled to room temperature. Ethanol (200 mL) was added to the
solution and the precipitate was filtrated and washed with ethanol
several times to afford a red solid (1.78 g, 76.4%). 1H NMR (400 MHz,
extracted with chloroform using
a Soxhlet extractor. After
removing solvent and drying in vacuum overnight at 45 ꢀC, the
polymer was obtained as a purple powder (0.41 g, 65.6%). 1H NMR
CDCl3):
d
¼ 7.75 (d, J ¼ 8.8 Hz, 4H), 6.91 (d, J ¼ 8.8 Hz, 4H), 4.01
(400 MHz, CDCl3):
d
¼ 8.55e8.62 (br, 2H), 7.62e7.72 (br, 4H),
(t, J ¼ 6.8 Hz, 4H), 1.80 (m, 4H), 1.44 (m, 4H), 1.30 (m, 16H), 0.90
6.52e6.64 (br, 4H), 4.45e4.60 (br, 4H), 3.72e3.82 (br, 4H),
(t, J ¼ 6.4 Hz, 6H). 13C NMR (100 MHz, CDCl3):
d
¼ 161.50, 155.69,
1.58e1.70 (br, 4H), 1.15e1.40 (br, 20 H), 0.78e0.92 (br, 6H). 13C NMR
152.35, 138.26, 132.19, 130.19, 114.62, 113.48, 68.43, 32.04, 29.58,
29.47, 29.40, 26.25, 22.89,14.34. Anal. Calcd for C36H42Br2N4O2S (%):
C, 57.30; H, 5.61; N, 7.42. Found (%): C, 57.18; H, 5.54; N, 7.14.
(75 MHz, CDCl3):
d
¼ 159.83, 150.54, 140.17, 137.73, 132.05, 131.43,
129.50, 128.78, 116.70, 114.22, 68.15, 64.90, 32.02, 29.63, 29.44,
26.33, 26.22, 22.86, 14.32. Mn ¼ 16.8 kDa, PDI ¼ 1.14.
2.4.3. Synthesis of P1 by Stille coupling reaction
2.4.4. Synthesis of P2 by Stille coupling reaction
Tris(dibenzylideneacetone)dipalladium (0.017 g, 0.018 mmol),
triphenylarsine (0.011 g, 0.036 mmol) were added to a solution of
Same polymerization method was used as P1. Compound used
were 2,3-bis(4-octyloxyphenyl)-5,8-dibromothiadiazol-quinoxaline

2316
G. Zhang et al. / Polymer 51 (2010) 2313e2319
Fig. 2. Normalized UVevis spectra of P1, P2 and P3 (a) in chloroform solution and (b)
as thin film.
Fig. 1. (a) TGA curves and (b) DSC curves of three copolymers P1, P2, and P3.
2.4.5. Synthesis of P3 by Stille coupling reaction
(9) (0.89 g, 1.18 mmol), 2,5-bis(tributylstannyl)-EDOT (1) (0.85 g,
1.18 mmol) THF (15 mL), tris(dibenzylideneacetone)dipalladium
(0.022 g, 0.024 mmol), and triphenylarsine (0.014 g, 0.048 mmol) in
this polymerization reaction. After same workup as P1, a black-green
powderwasobtained(0.56g,yield, 64.8%).1HNMR(400MHz,CDCl3):
Same polymerization method was used as P1. 2,3-Bis(4-octyloxy-
phenyl)-5,8-dibromothiadiazol-quinoxaline (9) (0.57 g, 0.75 mmol),
2,5-bis(tributylstannyl)-EDOT (1) (0.90 g, 1.25 mmol), 2,5-dibromo-
EDOT (DBEDOT) (0.15 g, 0.50 mmol), THF (20 ml), tris(dibenzylide-
neacetone)dipalladium (0.023 g, 0.025 mmol), and triphenylarsine
(0.015 g, 0.05 mmol) were used in this polymerization. After same
workup as P1, a black powder was obtained (0.75 g, yield, 60.6%). 1H
d
¼ 7.85e7.98 (br, 4H), 6.84e6.96 (br, 4H), 4.48e4.58 (br, 4H),
3.94e4.06 (br, 4H), 1.74e1.86 (br, 4H), 1.40e1.52 (br, 4H), 1.23e1.38
(br, 16H), 0.84e0.93 (br, 6H). 13C NMR (75 MHz, CDCl3):
d
¼ 160.53,
NMR (400 MHz, CDCl3):
d
¼ 7.72e7.98 (br, 4H), 6.80e6.96 (br, 4H),
152.94, 152.47, 141.24, 136.74, 132.01, 131.20, 120.56, 115.44, 114.18,
68.33, 64.89, 31.01, 29.36, 29.23, 26.05, 22.66,14.35. (Note: two peaks
in 13C NMR spectrum overlap). Mn ¼ 22.4 kDa, PDI ¼ 1.16.
4.20e4.68 (br, 8H), 3.94e4.04 (br, 4H), 1.73e1.87 (br, 4H), 1.42e1.52
(br, 4H), 1.22e1.40 (br, 16H), 1.83e0.94 (br, 6H). 13C NMR (75 MHz,
CDCl3):
d
¼ 160.77, 152.95, 152.87, 141.48, 136.98, 132.24, 132.04,
131.62, 131.44, 121.35, 115.69, 114.41, 68.35, 65.35, 64.99, 32.06,
29.95, 29.62, 29.49, 26.30, 22.91, 14.37. Mn ¼ 23.2 kDa, PDI ¼ 1.62.
Table 1
Optical and redox properties of P1, P2, and P3.
3. Results and discussion
Polymer
Solution
Film
Film
abs
max
abs
max
abs
onset
Egopt
EAb
IPc
a
red
E
l
l
l
onset
3.1. Synthesis of the polymers
(nm)
(nm)
(nm)
(eV)
(V)
(eV)
(eV)
P1
P2
P3
584
738
769
598
760
796
700
960
1100
1.77
1.29
1.13
ꢁ1.15
ꢁ0.55
ꢁ0.53
ꢁ2.99
ꢁ3.59
ꢁ3.61
ꢁ4.76
ꢁ4.88
ꢁ4.74
The synthetic routes for monomers are depicted in Scheme 1.
The 2,5-bis(tributylstannyl)-EDOT (1) was prepared from EDOT
according to our previous work [26]. The 4,7-dibromobenzothia-
diazol (2) was obtained from 2,1,3-benzothiadiazole in the presence
of HBr/Br2 mixture according to reported method [27] and
Egopt was estimated from UVevis absorption onset.
a
b
c
red
EA was obtain based on EA ¼ ꢁ(E
þ 4.14) eV.
onset
IP was obtained based on IP ¼ EA ꢁ Egopt eV.

G. Zhang et al. / Polymer 51 (2010) 2313e2319
2317
compound 2 was reduced with an excess amount of sodium
borohydride to give 3,6-dibromo-1,2-phenylenediamine (3) as
a pale yellow solid [6]. Nitration of 2 in 1/1 (vol/vol) of fuming nitric
acid and fuming sulfuric acid gave 4,7-dibromo-5,6-dinitro-ben-
zothiadiazole (4) in 25.5% yield after recrystallization in ethanol.
Reduction of 4 with iron dust in acetic acid resulted in diamine 5.
4,40-dihydroxybenzil (6) was prepared from 4,40-dimethoxybenzil
in the HBr/HAc mixture as a light tan solid. Compound 6 was
reacted with 1-bromooctane in DMF to afford diketone 7 in 72.1%
yield. The condensation of diketone 7 with diamine 3 and 5 in
acetic acid gave the monomer 8 and 9.
The polymerization of P1, P2, and P3 were depicted in Scheme 2.
Alternating copolymer P1 and P2 were prepared by Stille coupling
reaction with 1:1 monomer ratio in the presence of Pd2(dba)3 as
catalyst and triphenylarsine as ligand to give P1 (yield 65.6%) and
P2 (yield 64.8%). The crude copolymers were purified by precipi-
tating in methanol and then by washing with hexane and meth-
anol in a Soxhlet extractor. Random copolymer P3 was prepared
by using the same polymerization method as copolymer P1 and
P2 but with 2,5-bis(tributylstannyl)-EDOT (1), 2,3-bis(4-octylox-
yphenyl)-5,8-dibromothiadiazol-quinoxaline (9), and DBEDOT.
Those copolymers are soluble in common organic solvents such as
chloroform, dichloromethane, and THF at room temperature. The
number average molecular weights (Mn) of the copolymers were
16.8e23.2 kDa with a polydispersity of 1.14e1.62.
Fig. 3. Cyclic voltammograms of copolymers thin film (P1, P2 and P3) on a platinum
electrode in 0.1 M Bu4NPF6/MeCN at a scan rate of 50 mv sꢁ1 with FeCP2 as internal
standard.
3.4. Electrochemical studies
The electrochemical redox behavior of the conjugated polymers
was investigated by cyclic voltammetry (CV). The cyclic voltam-
mograms of P1, P2 and P3 were shown in Fig. 3. All the redox data
were summarized in Table 1. The potentials were internally refer-
enced to the ferrocene/ferrocenium redox couple (Fc/Fcþ), which
assumed the redox potential of ferrocene/ferrocenium has an
absolute energy level of 4.8 eV to vacuum [31e33]. The potential of
Fc/Fcþ was measured as 0.66 V. The lowest unoccupied molecular
3.2. Thermal stability
Thermal stability of copolymers was investigated by ther-
mogravimetric analysis (TGA). The TGA plots of P1, P2, and P3
were showed on Fig. 1a. The thermal decomposition tempera-
tures for P1, P2 and P3 were determined as 410 ꢀC, 392 ꢀC, and
380 ꢀC, respectively choosing 5 wt% weight loss as the onset loss
point. Three copolymers displayed adequate thermal stability for
application in polymer solar cells and other optoelectronic
devices. The DSC curves of copolymers are shown in Fig. 1b. All
three copolymers had glass transition points. The glass transition
temperatures of P1, P2, and P3 are 152 ꢀC, 143 ꢀC, and 162 ꢀC,
respectively.
orbital (LUMO) energy levels of polymers were calculated from the
red
E
and highest occupied molecular orbital (HOMO) energy levels
onset
were obtained based formula: HOMO ¼ LUMO ꢁ Egopt. Both P1 and
P2 exhibit one reversible n-doping/dedoping (reduction/reox-
idation) processes in the negative potential region, while P3 has
two reversible n-doping/dedoping processes, however, no distinct
oxidation wave was observed in the positive potential range. The
red
E
is ꢁ1.15 V for P1, ꢁ0.55 V for P2, and ꢁ0.53 V for P3, from
onset
which we can estimate the EA values (LUMO energy levels) of
ꢁ2.99, ꢁ3.59, and ꢁ3.61 eV for P1, P2, and P3, respectively. From
the Egopt and LUMO, the IP values (HOMO energy levels) of ꢁ4.76 eV
for P1, ꢁ4.88 eV for P2, and ꢁ4.74 eV for P3 were estimated. In
order to make a clear comparison, the results of HOMO and LUMO
3.3. Optical properties
The optical absorption properties of the three copolymers were
listed in Table 1. UVevis spectra were measured both in chloroform
solutions (Fig. 2a) and as thin films on glass slides (Fig. 2b). As
shown in Fig. 2a, the UVevis spectrum of P1 shows an absorption
abs
abs
peak ð
l
Þ at 584 nm. P2 shows an absorption peak ð
l
Þ at
max
max
abs
max
738 nm, and P3 shows an absorption peak ð
l
Þ at 769 nm. P2 and
P3 had 154 nm and 185 nm red-shifted respectively compared to
that of P1. Such red-shift in absorption can be explained by the fact
that the thiadiazol-quinoxaline units are stronger acceptors than
the quinoxaline unit. The UVevis spectra of P1, P2 and P3 as thin
film were showed in Fig. 2b. The thin film absorption spectra of all
three copolymers have much more broad absorption range than
solution absorption spectra. The UVevis spectra of P1, P2 and P3 as
abs
abs
thin film show a
l
at 598 nm, 760 nm and 796 nm, and the
l
max max
of P1, P2, and P3 have red-shifted 14e27 nm relative to copolymers
in solutions. The significant red-shift suggests the copolymers
aggregate very well in solid state. The optical bandgap was calcu-
lated to be 1.77 eV (absorption edge: w700 nm) for P1 and 1.29 eV
for P2 (absorption edge: w960 nm), while the absorption of P3 was
more broad and the absorption edge extended to 1100 nm, the
optical bandgap of P3 was calculated to be 1.13 eV.
Fig. 4. Energy level diagrams for P1, P2 and P3.

2318
G. Zhang et al. / Polymer 51 (2010) 2313e2319
Fig. 6. The external quantum efficiency of polymers/PC60BM.
Representative parameters of solar cells are summarized in Table 2.
The device with P1: PCBM layers (60 nm) showed an open-circuit
voltage (Voc) of 0.52 V, a short-circuit current density (Jsc) of
3.24 mA/cm2, and a fill factor (FF) of 36%, giving a power conversion
efficiency (PCE) of 0.60%. The device with P2: PCBM layers (90 nm)
demonstrated a Voc value of 0.33 V, a FF of 55%, and a Jsc value of
2.11 mA/cm2, leading to the PCE value of 0.39%, a slightly decreased
performance compare to P1. For the BHJ devices made from P3:
PCBM film (70 nm), the device exhibited almost same Voc value of
0.51 V, and a decreased Jsc value of 0.92 mA/cm2 compare to the
value of P1 and a FF of 24%, resulting in the significantly decreased
PCE of 0.11%. Compare to P1, the device with P2: PCBM demon-
strated a quite lower Voc value of 0.33 V. In our experimental
condition, the Voc of solar cells is not only determined by the energy
difference between HOMO of the donor and LUMO of the acceptor,
but also shows a strong inverse correlation with the dark current
[36]. We find that P2 has much higher dark current than P1 from
Fig. 5b. This may be the reason that the Voc value of P2 is lower than
that of P1. The external quantum efficiencies (EQE) of the solar cells
based on P2 and P3 with PCBM as acceptor were very low,
especially above 600 nm (Fig. 6), which resulted in very weak
photocurrent. The similar results have been reported by Janssen on
related polymer structures [21]. They reported that the EQE of the
cell based on thiadiazol-quinoxaline and thiophene copolymer
with PCBM acceptor was almost zero above 700 nm. The solar cell
based on P1 and PCBM has relatively high EQE compared to P2 and
P3. This is the main reason that solar cell based on P1 has higher Jsc
and PCE than those of P2 and P3.
Fig. 5. Currentevoltage characteristics of polymers/PC60BM solar cells (a) under illu-
mination (AM 1.5 G, 100 mW/cm2) and (b) dark condition.
energy levels of P1, P2 and P3 were summarized in Fig. 4. The
HOMO energy levels of the three polymers are very close. The
HOMO energy levels are relatively too high for polymers as donor
materials in bulk heterojunction solar cell devices with PCBM as
acceptor. The LUMO energy level of P1 is about 0.91 eV higher and
P2, P3 are 0.31 and 0.29 eV higher than that of PCBM (ꢁ3.90 eV)
[34]. P2 and P3 have relatively low LUMO energy level compared to
P1 because the strong electron-withdrawing units in P2 and P3
reduced the LUMO energy level of polymers [35].
3.5. Characterization of polymer solar cells
The polymer solar cells (PSCs) were fabricated with the device
structures of ITO/PEDOT: PSS/polymer: PCBM/LiF/Al. The active
layer of polymer and PCBM blend was prepared by spin-coating the
chlorobenzene solution of the polymers and PCBM (1:4, w/w) on
the ITO/PEDOT: PSS electrode. The current densityevoltage char-
acteristics with and without illumination were shown in Fig. 5.
4. Conclusions
New classes of donor/acceptor conjugated polymers comprising
EDOT units, quinoxaline units and thiadiazol-quinoxaline units have
beensuccessfullysynthesized by Stille cross-coupling polymerization
method. The copolymers of EDOT and 2,3-bis(4-octyloxyphenyl)-
quinoxaline (P1), 2,3-bis(4-octyloxyphenyl)-thiadiazol-quinoxaline
(P2, P3) have achieved relatively low bandgap (1.77e1.13 eV).
However, the HOMO energy levels of all three polymers are relatively
too high for BHJ solar cell devices with PCBM as acceptor to achieve
high open-circuit voltage and power conversion efficiency. The
performance of photovoltaic cells based on blends of copolymers and
PCBM as active layers are moderate. The power conversion efficien-
cies of 0.60% for P1, 0.39% for P2 and 011% for P3 were obtained.
Table 2
Characteristic properties of polymers solar cells.
Polymers
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
P1
P2
P3
0.52
0.33
0.51
3.24
2.11
0.92
36
55
24
0.60
0.39
0.11

G. Zhang et al. / Polymer 51 (2010) 2313e2319
2319
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Acknowledgements
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This work was supported by National Nature Science Founda-
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Shanghai municipal government (Grant no. B202 and 10ZZ15).
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The supplementary data associated with this article can be
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