A copolymer based on benzo[1,2-b:4,5-b′]dithiophene and quinoxaline derivative for photovoltaic application

Article abstract of DOI:10.1016/j.reactfunctpolym.2012.07.006

A D-A-D copolymer (PBDTQx) with a bandgap of 1.78 eV, containing alkoxy-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) as donor and quinoxaline derivative (Qx) as acceptor, was synthesized by Stille coupling reaction. In order to study the photovoltaic property of PBDTQx, polymer solar cells (PSCs) were fabricated with PBDTQx as the electron donor blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) as the electron acceptor. The power conversion efficiency (PCE) of PSC was 1.01% for an optimized PBDTQx: PC₆₁BM ratio of 1:5, under the illumination of AM 1.5, 100 mW/cm². The results indicated that PBDTQx was a promising donor candidate in the application of polymer solar cells.

Full text of DOI:10.1016/j.reactfunctpolym.2012.07.006

Reactive & Functional Polymers 72 (2012) 897–903
Contents lists available at SciVerse ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
0
A copolymer based on benzo[1,2-b:4,5-b ]dithiophene and quinoxaline derivative
for photovoltaic application
Haimei Wu a,b, Bo Qu c, , Zhiyuan Cong , Hongli Liu , Di Tian , Bowen Gao , Zhongwei An , Chao Gao
⇑
b
b
b
d
b
b,d,
⇑
,
c
c
d
c,
⇑
a,
⇑
Lixin Xiao , Zhijian Chen , Huanhuan Liu , Qihuang Gong , Wei Wei
a
Institute of Advanced Materials, Nanjing University of Posts and Telecommunications, Nanjing 210003, People’s Republic of China
Xi’an Modern Chemistry Research Institute, Xi’an, Shaanxi 710065, People’s Republic of China
State Key Laboratory for Artificial Microstructures and Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, People’s Republic of China
State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Science (CAS), Xi’an, Shaanxi 710119,
b
c
d
People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 April 2012
Received in revised form 27 June 2012
Accepted 22 July 2012
Available online 31 July 2012
A D–A–D copolymer (PBDTQx) with a bandgap of 1.78 eV, containing alkoxy-substituted benzo[1,2-b:4,5-
b ]dithiophene (BDT) as donor and quinoxaline derivative (Qx) as acceptor, was synthesized by Stille cou-
pling reaction. In order to study the photovoltaic property of PBDTQx, polymer solar cells (PSCs) were fab-
ricated with PBDTQx as the electron donor blended with [6,6]-phenyl-C61-butyric acid methyl ester
0
(
PC61BM) as the electron acceptor. The power conversion efficiency (PCE) of PSC was 1.01% for an opti-
2
mized PBDTQx: PC61BM ratio of 1:5, under the illumination of AM 1.5, 100 mW/cm . The results indicated
that PBDTQx was a promising donor candidate in the application of polymer solar cells.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Polymer solar cell
D–A–D polymers
Heterojunction
Quinoxaline
Thiophene
1
. Introduction
Polymer bulk-heterojunction (BHJ) solar cells with an increas-
levels and absorption properties by controlling the intermolecular
charge transferring from the donor to the acceptor. Recently, many
kinds of D–A–D copolymers have been developed and excellent
photovoltaic properties with PCE as high as 3–8% have been
achieved [10]. Among these conjugated copolymers, polymers
ing demand on cheap renewable energy source have drawn broad
attention in recent years, owing to their advantages of light weight,
low cost, easy fabrication and large-area devices through roll to roll
printing [1–5]. In this type of device, a phase separated blend of
electron-accepting material and electron-donating material is used
as the active layer. [6,6]-phenyl-C-61-butyric acid methyl ester
0
based on benzo[1,2-b:4,5-b ]dithiophene (BDT) units as electron
donors and electron-deficient units like thieno[3,4-b]thiophene
(TT) [11,12], N-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD) [13],
and 4,7-dithiophene-2-yl-2,1,3-benzothia-diazole (DTBT) [14],
have attracted much attention in the PSC field and promising pho-
tovoltaic properties have been achieved for this type of polymers.
Meanwhile, as a type of acceptors, quinoxaline can effectively
tune the band gap and energy levels due to the electron-deficient
N-heterocycle. For example, APFO-15-based device had a low
HOMO of ꢀ5.36 eV and a high Voc of 1.0 V [15], quinoxaline-based
copolymer TQ1 exhibited a Voc of 0.9 V and a very low-lying HOMO
of ꢀ5.7 eV [16], and PECz–DTQx showed a high PCE of 6.07% with
PFN as the cathode interlayer [17]. Although copolymers based on
BDT or quinoxaline exhibited promising photovoltaic performance
[18], the copolymers based on BDT and quinoxaline is less reported
in photovoltaic fields to the best of our knowledge.
(
PC61BM) is the common-used acceptor material and poly(3-hexyl-
thiophene) (P3HT) [6–8] is a representative conjugated donor
material. However the shortcoming of P3HT is the quite large band
gap and high HOMO level, which may limit the absorption from the
solar spectrum and reduce the value of Voc, respectively. The power
conversion efficiency (PCE) of polymer solar cells (PSCs) based on
the thin film blended with PC61BM is limited to 4–5% [9].
To achieve high performance BHJ polymer solar cells, one
efficient way is to design alternating donor–acceptor–donor
(
D–A–D) copolymers combined electron-rich (donor) and elec-
tron-deficient (acceptor) moieties, which can modulate the energy
In this work, we synthesized a copolymer (PBDTQx) using BDT
as electron-donating moiety and quinoxaline as electron-accepting
moiety, respectively. The photovoltaic performance under AM1.5
⇑
Corresponding authors. Address: Xi’an Modern Chemistry Research Institute,
Xi’an Shaanxi 710065, People’s Republic of China (C. Gao). Fax: +86 29 88291488.
E-mail addresses: bqu@pku.edu.cn (B. Qu), chaogao74@gmail.com (C. Gao),
qhgong@pku.edu.cn (Q. Gong), weiwei@njupt.edu.cn (W. Wei).
2
conditions with a Voc of 0.80 V, Jsc of 2.67 mA/cm , a fill factor
1
381-5148/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.reactfunctpolym.2012.07.006

8
98
H. Wu et al. / Reactive & Functional Polymers 72 (2012) 897–903
(
FF) of 0.47, and a power conversion efficiency (PCE) of 1.01% was
2.1.1. 3,6-Dibromo-1,2-phenylenediamine (2)
achieved by the copolymer (PBDTQx:PC61BM, 1:5 w/w, in chloro-
benzene solution) due to its interpenetrating network morphology
of PBDTQx:PC61BM blend film.
4,7-Dibromo-2,1,3-benzothiadiazole (1) (8.82 g, 0.03 mol) was
dissolved in ethanol (190 ml), to the suspension was added por-
tionwise sodium borohydride (21 g, 0.55 mol) at 0 °C, and the mix-
ture was stirred for 20 h at room temperature. Then evaporation of
the solvent, 200 ml water was added, and the mixture was ex-
tracted with ethyl acetate. The extract was washed with brine
and dried over anhydrous magnesium sulfate. After evaporation
of the solvent, the residue was purified by column chromatography
on silica gel using hexane/ethyl acetate (25:1) as eluent to afford
2
. Materials and methods
2.1. Synthesis of the polymer
3
,6-dibromo-1,2-phenylenediamine (5.01 g) as a pale solid in 63%
All chemicals and reagents are obtained from Aldrich and Alfa
1
yield. H NMR (CDCl
3
, 500 MHz) d (ppm): 6.80(s,2H), 3.60(s,4H).
Aesar. THF is dried over Na/benzophenone ketyl and freshly dis-
tilled prior to use. The synthetic routes of monomers and copoly-
mer are shown in Scheme 1. Monomers of 3,6-dibromo-1,2-
phenylenediamine (2) and 1,2-bis(3-(octyloxy)phenyl)ethane-1,2-
dione (5) were prepared according to the published literature
1
3
C NMR (CDCl
MS: m/z = 263.
3
, 500 MHz) d (ppm): 133.85, 123.08, 109.66. GC–
2.1.2. 1,2-Bis(3-(octyloxy)phenyl)ethane-1,2-dione (5)
[
(
15] and dissolved in acetic acid to obtain 5,8-dibromo-2,3-bis(3-
A THF solution of anhydrous LiBr (3.19 g, 36.8 mmol, 15 ml) was
added to a suspension of CuBr (2.64 g, 18.4 mmol) in THF (15 ml)
under nitrogen. The mixture was stirred at room temperature until
it became homogeneous and was then cooled to 0 °C in an ice-
water bath. A freshly prepared solution of 3-octyloxybenzene mag-
nesium bromide in THF (5.23 g, 18.4 mmol, 20 ml) was added
dropwise to the mixture. After 10 min, oxalyl chloride (1.04 g,
8.22 mmol) was added and the mixture was stirred at 0 °C for
20 min, Then the reaction was quenched with saturated aqueous
octyloxy)phenyl)- quinoxaline (9) in high yield of 97%. 2,6-Bis(tri-
0
methyltin)-4,8-bis(2-ethylhexyl)benzo[1,2-b:3,4-b ]dithiophene
was adopted as a electron donor and prepared according to the lit-
erature [19]. PBDTQx was synthesized by Stille polycondensation
of monomer 8 and 9. The molecular weight and polydispersity in-
dex of the resulting polymer were recorded by GPC analysis, which
shows
1
2
a
large number-average molecular weight (M
92.86 kDa, with the corresponding polydispersity index (PDI) of
.88.
n
) of
4
NH Cl and extracted with ethyl acetate, the combined organic
Scheme 1. Synthesis routes of the copolymer PBDTQx.

H. Wu et al. / Reactive & Functional Polymers 72 (2012) 897–903
899
layer was washed with brine and dried over anhydrous magnesium
sulfate. After removal of the solvent under reduced pressure, the
residue was purified by column chromatography on silica gel using
filtration and was Soxhlet-extracted in order with methanol, hex-
ane, and then with chloroform. The chloroform solution was con-
centrated to a small volume, and the polymer was precipitated
by pouring this solution into methanol. Finally, the polymer was
collected by filtration, dried under vacuum at 50 °C overnight
hexane/ethyl acetate (100:1) as eluent to afford compound 5
1
(
2.03 g) as a white solid in yield 53%. H NMR (CDCl
3
, 500 MHz) d
(
ppm):7.51(s,2H), 7.45(m,2H), 7.38(t,2H, J = 8 Hz), 7.19(ddd,2H,
and afforded PBDTQx (247 mg) as a deep-blue solid in yield
1
J = 5 Hz),
1
4.00(t,4H,
J = 6.5 Hz),
1.79(m,4H),
1.46(m,4H),
84.3%.
H
NMR (CDCl
3
,
500 MHz)
d
(ppm): 7.68(m,2H),
13
3
.33(m,16H), 0.89(t,6H, J = 5 Hz). C NMR (CDCl , 500 MHz) d
7.40(br,4H), 7.15(br,2H), 7.01(br,4H), 3.77(m,8H). GPC (tetrahydro-
furan, polystyrene standard): M = 192.86 kDa, M = 556.36 kDa,
PDI = 2.88. Anal. Calcd for (C62 (%): C 75.87, H 8.22, N
(
6
ppm): 194.56, 158.36, 133.91, 129.88, 122.80, 122.15, 113.72,
8.39, 31.81, 29.33, 29.23, 29.13, 26.01, 22.67, 14.11. GC–MS: m/
z = 466.
n
w
80 2 4 2 n
H N O S )
2.85. Found (%):C75.28, H 8.13, N 2.90.
2
.1.3. 2,6-Bis(trimethyltin)-4,8-bis(2-ethylhexyl)benzo[1,2-b:3,4-
0
b ]dithiophene (8)
2.2. Measurement and characterization
0
In a flask, 4,8-dihydrobenzo[1,2-b:4,5-b ]dithiophen-4,8-dione
6) (6.6 g, 30 mmol) and zinc powder (4.29 g, 66 mmol) were
(
All compounds were characterized by nuclear magnetic reso-
nance spectra (NMR) recorded on a Bruker AV 500 spectrometer
in CDCl at room temperature. Molecular weights and distributions
3
added to 90 ml of water, then 18 g of NaOH was added into the
mixture. After the solution was refluxed for 1 h, 1-bromo-2-ethyl-
hexane (17.4 g, 90 mmol) and a catalytic amount of tetrabutylam-
monium bromide were added and heated to reflux for another 6 h.
Then, cold water (300 ml) was added and the mixture was ex-
tracted with ethyl acetate, the extract was dried over anhydrous
magnesium sulfate, the residue was purified by column chroma-
tography on silica gel to obtain 4,8-bis(2-ethylhexyl-
oxy)benzo[1,2-b:3,4-b]dithiophene (7) in a 65% yield. Compound
of the copolymer were determined using GPC, THF as eluent and
polystyrene as standard. The absorption spectra were taken by a
Unico UV-2102 scanning spectrophotometer. Thermogravimetric
analysis (TGA) of the polymer was recorded on a Universal V2.6D
TA instruments. The electrochemical cyclic voltammetry was con-
ducted on a CHI 660D Electrochemical Workstation with Pt disk, Pt
+
plate, and Ag/Ag electrode as working electrode, counter elec-
7
(6.1 g, 13.6 mmol) was added into THF (200 ml) under nitrogen,
trode, and reference electrode respectively in a 0.1 mol/L tetrabu-
n-butyllithium (36.3 mmol, 2.2 M) was added dropwise to the mix-
ture at ꢀ80 °C and stirred for 1 h, the cooling bath was removed,
and the reactant was stirred at ambient temperature for another
tylammonium hexafluorophosphate (Bu
4
NPF
6
)
acetonitrile
solution. Polymer thin films were formed by drop-casting chloro-
form solution (analytical reagent, 1 mg/mL) onto the working elec-
trode, and then dried in the air. Atomic force microscopy (AFM)
images were collected in air under ambient conditions using the
MultiMode scanning probe microscope (Agilent Technologies
5500).
1
h. Trimethyltin chloride (8 g, 40.4 mmol) was added in one por-
tion at ꢀ80 °C and the reactant was stirred at ambient temperature
overnight. After 200 ml cold water was added and the mixture was
extracted with hexane, the organic layer was dried over anhydrous
magnesium sulfate, after the evaporation of solvent, the residue
was recrystallized by ethyl alcohol and obtained 2,6-bis(trimethyl-
0
tin)-4,8-bis(2-ethylhexyl)benzo[1,2-b:3,4-b ]dithiophene 8 (6.15 g,
2.3. Solar cell device fabrication and characterization
7
7
1
.96 mmol) in a 58.3% yield. 1H NMR (CDCl
.52 (s,2H), 4.23 (d,4H, J = 10 Hz), 1.79(m,2H), 1.54–1.36 (m,16H),
3
, 500 MHz) d (ppm):
The organic photovoltaic cells were fabricated with ITO glass as
the anode, Al as the cathode and the blend films of PBDTQx: PC61-
BM as the photosensitive layer. The ITO glass was pre-cleaned and
modified by a thin layer of PEDOT:PSS which was spin-coated from
a PEDOT:PSS aqueous solution (Bayer), and the thickness of the
PEDOT:PSS layer was about 40 nm. The photosensitive layer was
prepared by spin-coating a blend solution of the PBDTQx and PC61-
BM in chlorobenzene (CB) with the concentration of 30 mg/ml on
top of ITO/PEDOT:PSS substrate and the thickness of the active
layer was about 80 nm. Then the blend system was put in glove
box overnight, Finally, 0.5-nm thick LiF layer and 85-nm thick Al
1
3
.06 (t,6H, J = 7.5 Hz), 0.96 (t,6H, J = 7.5 Hz), 0.40(s,18H). C NMR
500 MHz) (ppm): 143.41, 140.41, 133.91, 132.89,
28.03, 40.73, 30.60, 29.30, 23.97, 23.23, 14.24, 11.41, ꢀ6.97,
8.32, ꢀ9.95.
(
CDCl
3
,
d
1
ꢀ
2.1.4. 5,8-Dibromo-2,3-bis(3-(octyloxy)phenyl)quinoxaline (9)
A mixture of compound 2 (2.28 g, 8.58 mmol) and compound 5
(
4 g, 8.58 mmol) were added into the acetic acid (70 ml), the mix-
ture was briefly warmed to 60 °C, and then the solution was stirred
at room temperature for another 2 h. The precipitate was collected
by filtration, washed with ethanol, and dried to afford 5,8-dibro-
ꢀ6
layer were evaporated in sequence under the vaccum of 3 ꢁ 10
-
2
mo-2,3-bis(3-(octyloxy)phenyl)quinoxaline 9 (5.69 g) as a white
Torr. The effective area of the device was 0.0314 cm . The current
density–voltage (J–V) curves were obtained using a Keithley 2611
source-measure unit. The photocurrent was measured with a solar
simulator (Newport Thermal Oriel 69911 300 W, 4 in.ꢁ4 in. beam
1
solide in yield 97%.
3
H NMR (CDCl , 500 MHz) d (ppm):
7
.91(s,2H), 7.23(d,4H, J = 10 Hz), 7.18(d,2H, J = 10 Hz), 6.94(d,2H,
J = 10 Hz), 3.87(t,4H, J = 6.5 Hz), 1.75(q,4H), 1.25–1.52(m,20H),
1
3
2
0
1
1
.91(t,6H, J = 5.5 Hz). C NMR (CDCl
3
, 500 MHz) d (ppm): 159.07,
size) with AM 1.5 G illumination at 100 mW/cm . A calibrated
54.05, 139.31, 139.14,133.09, 129.33, 123.72, 122.56, 116.57,
15.77, 68.12, 31.85, 29.36, 29.29, 29.13, 26.04, 22.70, 14.13.
mono silicon diode is used as a reference.
2.1.5. Synthesis of PBDTQx
3. Results and discussion
In a 50 ml dry flask, monomer 8 (231.6 mg, 0.3 mmol) and 9
(
209 mg, 0.3 mmol) were dissolved in degassed toluene (10 ml),
the mixture was flushed with nitrogen for 30 min, tris(dibenzyli-
deneacetone)dipalladium(0) (Pd (dba) (5.5 mg) and tri(o-
tolyl)phosphine (P(o-Tol) ) (7.3 mg) were added, and flushed with
3.1. Thermal properties
2
3
)
The thermal properties of PBDTQx were determined by TGA
(Fig. 1). The copolymer exhibited a reasonable thermal stability
3
nitrogen for another 30 min. Then the mixture was vigorously stir-
red at 100 °C for 24 h under nitrogen. After cooling down, the solu-
tion was poured into methanol. The polymer was collected by
d
with 5% weight-loss temperature (T ) of 310 °C. Obviously, the
thermal property of PBDTQx is adequate for further application
in photovoltaic solar cells and other optoelectronic devices.

9
00
H. Wu et al. / Reactive & Functional Polymers 72 (2012) 897–903
4
1
00
PBDTQx
3
2
1
0
1
9
8
7
6
0
0
0
0
ox
E
= 0.54V
onset
red
onset
-
E
= -1.56V
-2
-
-
3
4
0
100
200
300
400
500
-5
-
2.5 -2.0 -1.5 -1.0 -0.5
0.0
0.5
1.0
o
Temperature ( C)
+
Potential / V vs. Ag/Ag
Fig. 1. Thermogravimetric analysis plot of PBDTQx.
Fig. 3. Electrochemical cyclic voltammetry curves of PBDTQx.
potential range. The HOMO and LUMO energy levels of the polymer
were calculated from the onset oxidation potentials (Eox) and the
onset reduction potentials (Ered) and by assuming the energy level
1
.0
+
0.8
0.6
0.4
0.2
0.0
of ferrocene/ferrocenium (Fc/Fc ) to be ꢀ4.72 eV below the vac-
+
uum level. The formal potential of Fc/Fc was measured as
+
ec
0.09 eV against Ag/Ag . The energy gap ðE Þ of the polymer was
g
calculated from the HOMO and LUMO energy levels. The calculat-
ing equations are as follows:
E
E
E
HOMO ¼ ꢀeðEox þ 4:72Þ
ð1Þ
ð2Þ
ð3Þ
PBDTQx in CHCl3
PBDTQx film
LUMO ¼ ꢀeðEred þ 4:72Þ
4
00
500
600
700
800
900
ec
g
¼ eðEox ꢀ Ered
Þ
Wavelength/nm
+
where the units of Ered and Eox are V vs Ag/Ag . The electrochemical
Fig. 2. Absorption spectra of PBDTQx solution and film.
potentials and energy levels of PBDTQx are shown in Table 1. The
Eox is 0.54 V and the Ered is ꢀ1.56 V. The HOMO and LUMO energy
3
.2. Absorption properties
levels of PBDTQx are ꢀ5.26 and ꢀ3.16 eV, respectively. The electro-
chemical energy bandgap is 2.1 eV, which is somewhat larger than
the optical bandgap estimated from UV–vis spectra (1.78 eV). The
higher electrochemical bandgap than optical bandgap is common
phenomenon for the conjugated polymers because of the energy
barriers of the charge transfer at electrodes during the electrochem-
ical measurement. It is worthy to note that the HOMO of PBDTQx is
relatively low, which indicates that the polymer should have good
air stability for its HOMO energy level being below the air oxidation
threshold (ꢀ5.2 eV). Meanwhile, this low energy level might be
benefited to the photovoltaic performance of the polymer solar cells
because HOMO energy level of donor and LUMO of acceptor have
direct relation with the Voc of device. In addition, the LUMO energy
level of PBDTQx is significantly higher than that of PC61BM (–
3.91 eV, as shown in Fig. 4), it facilitate electron transferring from
the excitons of PBDTQx to the LUMO of PC61BM at the interface of
the donor/acceptor.
The absorption properties of PBDTQx were investigated both in
CHCl solution and film as shown in Fig. 2. The copolymer showed
two main absorption peaks at 378 and 598 nm in UV–vis spectra.
The absorption peak at 378 nm could be assigned to the
transition of their conjugated backbone, while the other peak at
3
ꢂ
p–p
5
98 nm could be ascribed to the strong intermolecular charge
transfer between BDT donor and quinoxaline acceptor. The absorp-
tion edge of the polymer film was 696 nm, about 22 nm red shift
in comparison with that of the polymer solution as seen in Fig. 2,
1
which implied that
formed. The optical bandgap of copolymer can be calculated to be
.78 eV from the absorption onset of the film. It is strange that the
p stacked structure in the solid state might be
1
absorption maximum of the polymer film is less than that of its chlo-
roform solution. The derivation of the absorption maximum for film
and for solution may result from the big steric hindrance caused by
the two octyloxyl group in the donor (BDT) and in the acceptor
(
quinoxaline), which may even affect the aggregation state proper-
3.4. Photovoltaic properties
ties of the polymer.
In order to investigate the photovoltaic properties of the copoly-
3
.3. Electrochemical properties
mer, the photovoltaic cells were fabricated with a typical configu-
ration of ITO/PEDOT:PSS/polymer:PC61BM/LiF/Al, in which PBDTQx
was used as donor, PC61BM as acceptor. The active layer of the de-
vice was fabricated with chlorobenzene (CB) solution of copolymer
and PC61BM by spincoating technique. The concentration of blend
solution of the polymer and PC61BM in CB was 30 mg/ml. Weight
ratios of the donor and acceptor varied from 1:2 to 1:6. The J–V
curves of the PBDTQx-based PSC with different D/A ratios under
Fig. 3 showed the cyclic volatmmogram of PBDTQx film on Pt
electrode. It could be seen that there were p-doping/dedoping
oxidation/re-reduction) processes at positive potential range and
(
n-doping/dedoping(reduction/oxidation) processes at negative
1
For interpretation of color in Figs. 2–8, the reader is referred to the web version of
2
this article.
illumination of AM1.5G (100 mW/cm ) were shown in Fig. 5. As

H. Wu et al. / Reactive & Functional Polymers 72 (2012) 897–903
901
Table 1
0.0
Electrochemical and optical properties of the PBDTQx.
PBDTQx:PC61BM)=1:4
PBDTQx:PC61BM)=1:4,with 2.5% DIO
PBDTQx:PC61BM)=1:5
ec
g
opt
g
(eV)a
Polymer
Eox (V)
Ered (V)
EHOMO (eV)
E
LUMO (eV)
E
(eV)
E
-0.5
PBDTQx:PC61BM)=1:5,with 2.5% DIO
PBDTQx 0.54
1.56
ꢀ5.26
ꢀ3.16
2.1
1.78
-1.0
-1.5
-2.0
-2.5
-3.0
a
Optical band gaps determined from the onset of electronic absorption of the
opt
polymer films ðEg ¼ 1240=k ðnmÞÞ.
Donor
3.16eV
-
Acceptor
3.91eV
0.0
0.2
0.4
0.6
0.8
-
Voltage (V)
-
4.3 eV
LiF/Al
Fig. 6. J–V curves of the PSCs based on PBDTQx: PC61BM in CB with and without
PBDTQx
2
2.5 vol.% DIO under illumination of AM1.5G, 100 mW/cm .
-
4.80eV
ITO
PC BM
6
1
Table 3
-5.26eV
Photovoltaic results of the PSCs based on PBDTQx:PC61BM with and without DIO
under the illumination of AM1.5G (100 mW/cm ).
2
-
5.93eV
sc (mA/cm2)
D/A ratio
Solvent
V
oc (V)
J
FF
PCE (%)
1:4
CB
0.73
0.84
1.45
1.90
0.49
0.43
0.52
0.68
Fig. 4. Energy level diagrams for PBDTQx and PC61BM.
CB + 2.5%DIO
1:5
CB
0.79
0.80
1.44
2.67
0.45
0.47
0.51
1.01
CB + 2.5%DIO
0.0
ingly. The PCE of 1:4-device and 1:5-device was gauged to be 0.68%
and 1.01%, respectively. The J–V curves of the PBDTQx–based solar
cells (1:4,1:5) with and without DIO were shown in Fig. 6 and
summarized in Table 3. Both Voc and Jsc were optimized with the
addition of DIO, and Jsc of 1:4-device and 1:5-device was increased
by 31% and 84% compared with that of devices without DIO doping.
To investigate the surface morphology of the blend poly-
mer:PC61BM = 1:4 and 1:5 with and without the additive DIO,
atomic force microscopy (AFM) studies had been carried out (Figs. 7
and 8). It can be seen that evident aggregation of the blend films
and the poor miscibility between polymer and PC61BM (Fig. 7a
and c). After the addition of DIO, the resulting blend films showed
a more uniform and finer domain structure (Fig. 7b and d) and an
average domain size of 1:4-device and 1:5-device was 30–50 nm
and 20–30 nm (Fig. 8), respectively, the latter is closed to ideal do-
main size and favorable for efficient exciton dissociation and
-
0.4
-0.8
-1.2
PBDTQx:PC BM=1:2
61
PBDTQx:PC BM=1:3
61
PBDTQx:PC BM=1:4
-1.6
61
PBDTQx:PC BM=1:5
61
PBDTQx:PC BM=1:6
61
-2.0
-
0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Voltage (V)
Fig. 5. J–V curves of the PSCs based on PBDTQx: PC61BM with different donor/
acceptor ratios (1:2,1:3,1:4,1:5 and 1:6), under illumination of AM1.5G, 100 mW/
2
cm .
charge transport. As a result, the Jsc increased from 1.44 to
2
2
.67 mA/cm , and the Voc and FF are all increased, above-men-
Table 2
tioned factors made the device performance was greatly improved
and reached 1.01%. Although the PBDTQx-based photovoltaic de-
vice showed low Jsc of 2.67 mA/cm , it had a reasonable FF of
0.47 and a high Voc of 0.80 V. If the morphology of blend film can
be optimized by thermal annealing or solvent annealing, the im-
proved photovoltaic performance is prospective.
Photovoltaic results of the PSCs based on PBDTQx and PC61BM with different weight
ratio under the illumination of AM1.5G (100 mW/cm ).
2
2
D/A ratio
V
oc (V)
J
sc (mA/cm2)
FF
PCE (%)
1
1
1
1
1
:2
:3
:4
:5
:6
0.77
0.65
0.73
0.79
0.70
0.82
1.72
1.45
1.44
1.07
0.35
0.43
0.49
0.45
0.50
0.22
0.48
0.52
0.51
0.37
4
. Conclusion
A copolymer (PBDTQx) based on BDT and quinoxaline was syn-
thesized by stille coupling reaction. PBDTQx had a narrow bandgap
of 1.78 eV and a low HOMO level of ꢀ5.26 eV, which facilitated
high Voc of PBDTQx-based PSCs. With the concentration being
30 mg/ml, the highest PCE of 1.01% was obtained when the blend-
ing ratio of PBDTQx and PC61BM reached 1:5 and DIO doping ratio
was 2.5%. Although the copolymer PBDTQx-based photovoltaic de-
vice showed relatively low Jsc, it had a reasonable high Voc of
0.80 V. If the morphology of blend film can be further optimized,
the improved photovoltaic performance is prospective.
listed in Table 2, the optimized performance was achieved with the
weight ratio of polymer:PC61BM being 1:4, which was slight higher
than that of 1:5-device.
1
,8-Diiodooctane (DIO) is one of processing solvent additives,
which can improve the morphology of the active layer [20,21],
DIO doping ratio was controlled to be 2.5% (v/v) in the film
deposition, and the device performance could be improved accord-

9
02
H. Wu et al. / Reactive & Functional Polymers 72 (2012) 897–903
(a) polymer:PC61BM (1:4)
(
b) polymer:PC61BM (1:4)
With DIO
(
c) polymer:PC61BM (1:5)
(
d) polymer:PC61BM (1:5)
With DIO
Fig. 7. AFM images for polymer:PC61BM films (1:4,1:5) prepared without and with DIO (2.5 vol.%) (5
panels, and the corresponding phase images in the right panels.
lm ꢁ 5 lm). (a–d). The topography of each film is showed in the left

H. Wu et al. / Reactive & Functional Polymers 72 (2012) 897–903
903
(a) polymer:PC61BM (1:4)
With DIO
(b) polymer:PC61BM (1:5)
With DIO
Fig. 8. AFM images for polymer:PC61BM films (1:4,1:5) prepared with DIO (2.5 vol.%) (0.5
and the corresponding phase images in the right panels.
lm ꢁ 0.5
lm). (a and b). The topography of each film is showed in the left panels,
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[
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Appendix A. Supplementary material
(
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1H NMR,13C NMR spectra of 2, 5, 8, 9 and 1HNMR of PBDTQx.
This material is available free of charge via the Internet at http://
www.sciencedirect.com. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
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