Synthesis and simultaneously enhanced photovoltaic property of poly[4,4,9,9-tetra(4-octyloxyphenyl)-2,7-indaceno[1,2-b:5,6-b′] dithiophene-alt-2,5-thieno[3,2-b]thiophene]

Article abstract of DOI:10.1016/j.polymer.2012.12.006

An alternating conjugated polymer derived from 4,4,9,9-tetra(4- octyloxyphenyl)indaceno[1,2-b:5,6-b′]dithiophene and thieno[3,2-b] thiophene was synthesized by Stille coupling reaction. The copolymer shows extensive absorption from 360 nm to 590 nm with optical band gap of 2.12 eV. The highest occupied molecular orbit (HOMO) and the lowest unoccupied molecular orbit (LUMO) energy levels of the copolymer, determined by cyclic voltammetry (CV), are about -5.29 eV and -3.01 eV, respectively. A field-effect hole mobility of 8.1 × 10^-4 cm²/(V s) and an on/off ratio of 2.0 × 10³ are obtained in the field-effect transistors based on the copolymer. The J_sc, V_oc and FF of the polymer photovoltaic cells (PVCs) based on the copolymer were enhanced simultaneously via the inserting of alcohol soluble conjugated polymer interlayer between active layer and metal cathode, and the maximal power conversion efficiency of 4.19% was achieved in the modified devices under an AM 1.5 simulator (100 mWcm^-2).

Full text of DOI:10.1016/j.polymer.2012.12.006

Polymer 54 (2013) 607e613
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and simultaneously enhanced photovoltaic property of poly
[4,4,9,9-tetra(4-octyloxyphenyl)-2,7-indaceno[1,2-b:5,6-b0]dithiophene-alt-
2,5-thieno[3,2-b]thiophene]
,
b
c
a
a
b
b
Yangjun Xia a , Yuanyuan Gao , Yong Zhang , Junfeng Tong , Jianfeng Li , Huijuan Li , Dejia Chen ,
*
Duowang Fan a
,
*
a Key Laboratory of Optoelectronic Technology and Intelligent Control of Ministry Education, Lanzhou Jiaotong University, Lanzhou 730070, PR China
b College of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, PR China
c Institute of Optoelectronic Materials and Technology, South China Normal University, Guangzhou 510631, PR China
a r t i c l e i n f o
a b s t r a c t
An alternating conjugated polymer derived from 4,4,9,9-tetra(4-octyloxyphenyl)indaceno[1,2-b:5,6-b0]
dithiophene and thieno[3,2-b]thiophene was synthesized by Stille coupling reaction. The copolymer
shows extensive absorption from 360 nm to 590 nm with optical band gap of 2.12 eV. The highest
occupied molecular orbit (HOMO) and the lowest unoccupied molecular orbit (LUMO) energy levels of
the copolymer, determined by cyclic voltammetry (CV), are about ꢀ5.29 eV and ꢀ3.01 eV, respectively. A
field-effect hole mobility of 8.1 ꢁ 10ꢀ4 cm2/(V s) and an on/off ratio of 2.0 ꢁ 103 are obtained in the field-
effect transistors based on the copolymer. The Jsc, Voc and FF of the polymer photovoltaic cells (PVCs)
based on the copolymer were enhanced simultaneously via the inserting of alcohol soluble conjugated
polymer interlayer between active layer and metal cathode, and the maximal power conversion effi-
ciency of 4.19% was achieved in the modified devices under an AM 1.5 simulator (100 mWcmꢀ2).
Ó 2012 Elsevier Ltd. All rights reserved.
Article history:
Received 8 August 2012
Received in revised form
1 November 2012
Accepted 1 December 2012
Available online 6 December 2012
Keywords:
Indaceno[1,2-b:5,6-b0]dithiophene
Photovoltaic cells
Thieno[3,2-b]thiophene
1. Introduction
chemical structure of the BDT-based CPs. The PCEs of 5e9% have
been achieved in the PVCs with BDT-based CPs as electron donor
In recent years, the conjugated polymers (CPs) derived from
heteroacene with fused thiophene have emerged as the forefront of
polymer heterojunction photovoltaic cells [1]. Many CPs based on
heteroacene with two fused thiophene have been synthesized, and
the polymer photovoltaic cells (PVCs) with power conversion effi-
ciencies (PCEs) of 3e9% have been achieved in last decade. Among
them, the narrow band gap CPs based on benzo[1,2-b:4,5-b0]
dithiophene (BDT) have been proven the most promising electron
donor materials for high performance PVCs and BDT has been
proven to be the most promising electron donor building blocks for
CPs used in high performance PVCs. For example, in 2009, Yu et al.
had reported a CP derived from benzo[1,2-b:4,5-b0]dithiophene
(BDT) and thieno[3,4-b]thiophene derivatives, and named PTB1.
The PCE of 4.8% was achieved in the devices based on the PTB1 and
PC61BM blend [2]. After that, Yu et al. [3] Yang et al. [4] Hou et al. [5]
Li et al. [6] and Huang et al. [7] have reported more promising CPs
based on BDT through the optimization of the energy level and
materials.
Whereas electron-rich heteroacenes with more fused thiophene
units not only hold promise as a suitable donor constituent in the
building of donoreacceptor type narrow band gap copolymers for
heterojunction blend PVCs, but also the presence of large coplanar
structure and extended
p-conjugation of heteroacenes with more
fused thiophene units are beneficial to enhance the rigidity of the
molecular backbone and the degree of conjugation, thus enhance
pep overlap and pep effective approaches to increase the hole
mobility of CPs [8]. Then, CPs based on heteroacenes with more
fused thiophene units have been pinned much hope on developing
more promising electron donor materials as comparison to the
notable BDT-based copolymers. Unfortunately, the synthesis
procedures of heteroacenes with more fused thiophene units
suitable to use as building blocks to CPs encountered tedious
synthesis, and more importantly difficult introduction of suitable
alkyl chains for solubility [9].
As an attractive heteroacene with more fused thiophene units,
indaceno[1,2-b:5,6-b0]dithiophene (IT) not only holds larger
coplanar fused structure, which enables enhanced
p-conjugation,
* Corresponding authors. Tel.: þ86 (0) 931 4956022; fax: þ86 (0) 931 4956058.
E-mail addresses: yjxia73@126.com (Y. Xia), fanduowang@163.com (D. Fan).
pep
intermolecular interaction and improved charge carrier
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.12.006

608
Y. Xia et al. / Polymer 54 (2013) 607e613
mobility, but also has two sites for attaching side alkyl chains to
optimize the solubility of the CPs. Therefore, IT can be used as
a promising building block to develop novel CPs for high perfor-
mance PVCs. For example, Ko et al. and Zhang et al. have reported
series of CPs derived from 4,4,9,9-tetra(4-hexylphenyl)indaceno
[1,2-b:5,6-b0]dithiophene, respectively. And the PCEs of 3e6.2%
have been achieved in the PVCs based on the copolymers [10]. On
the other hand, we have demonstrated that the open circuit
voltage, short current density and fill factor of PVCs with narrow
band gap CPs derived from indeno[1,2-b]fluorene e a similar fused
coplanar structure unit to IT, can be simultaneously enhanced via
the incorporation of polyelectrolyte interlayer between active layer
and metal cathode [11]. Wu et al. [12,13a] and Bazan et al. [13b] also
have demonstrated that the open circuit voltage (Voc), short current
density (Jsc) and fill factor (FF) of PVCs based on CPs derived from
benzo[1,2-b:4,5-b0]dithiophene and/or carbazole-based CPs, can be
simultaneously enhanced via the method through the incorpora-
tion of alcohol soluble conjugated polymer interlayer or poly-
electrolytes interlayer between active layer and metal cathode,
respectively. Unfortunately, it has been found that the validity of
the method on the increase of Jsc, Voc and FF of the PVCs, have only
taken effects on some specific donor materials system such as
fluorene, carbazole and benzo[1,2-b:4,5-b0]dithiophene etc [11e14].
In order to broaden the materials systems, the method through
which the photovoltaic parameters of PVCs such as Jsc, Voc and FF
are increased via the incorporation of polyelectrolyte or alcohol
soluble conjugated polymer interlayer between active layer and
metal cathode, and optimize the PVCs from IT-based CPs, further
development of novel IT-based CPs and implement of devices
configuration with modified cathode, is interesting and promising
for the achievement of high performance PVCs from IT-based
copolymers.
standard. UVevisible absorption spectra were measured on a UV-
2550 spectrophotometer (Shimadzu). Photoluminescence (PL)
spectra were taken by 970 CRT spectrofluorometer (Sanco,
Shanghai) through 500 nm excitation. Thermogravimetric analysis
(TGA) was measured on a TGA 2050 (TA instruments) thermal
analysis system under a heating rate of 10 ꢂC minꢀ1 and a nitrogen
flow rate of 20 mL minꢀ1
.
Cyclic voltammetry (CV) was measured on a CHI 660 electro-
chemical workstation (Shanghai Chenhua Co.) at a scan rate of
50 mV/s with a nitrogen-saturated solution of 0.1 M tetrabuty-
lammonium hexafluorophosphate (Bu4NPF6) in acetonitrile
(CH3CN) with platinum and Ag/AgCl electrodes as the working and
reference electrodes, respectively.
2.3. Fabrication and characterization of field-effect transistor
Heavily p-doped Si wafers with 100 nm thermal SiO2 grown,
were used both as the substrate and the gate electrode. The
substrates were cleaned followed by a wet-cleaning process inside
an ultrasonic bath, beginning with deionized water, followed by
acetone and isopropanol for 10 min, and dried with nitrogen. Then,
the substrates were treated with oxygen plasma for 5 min to
remove any residual organic materials and to create a high density
of silanol groups at the surface. On the other hand, to improve the
chemical and electrical properties of the gate dielectric, a self-
assembled monolayer (SAM) octyltrichlorosilane (OTS) film on
the wafers were prepared as the following procedure [19], begin-
ning with immersed the wafers in a 0.1 M solution of the OTS in
toluene at 60 ꢂC for 30 min, then rinsed with toluene and 2-
propanol, and dried in a vacuum oven. After that, the films of
PITT were deposited on pre-treated SiO2/Si substrates by spin-
coating a 10 mg mLꢀ1 solution of the polymer in chlorobenzene,
followed by annealing under nitrogen to remove the residual
solvent. The source and drain electrodes were defined by thermally
evaporating gold (70 nm) through a shadow mask on top of the
organic thin film forming top contact geometry transistors. Channel
length and width of the obtained OFET were 0.1 and 10 mm,
respectively. The OFET was characterized under ambient conditions
by a semiconductor parameter analyzer (model: 4155B).
In this paper, we present our results on the synthesis and char-
acterization of an alternating copolymer derived from 4,4,9,9-tetra(4-
octyloxyphenyl)indaceno[1,2-b:5,6-b0]dithiophene and thieno[3,2-b]
thiophene. The resulted polymer, named PITT, is soluble in common
organic solvents and exhibits extensive absorption from 330 nm to
600 nm. The chemical structure, molecular weight, thermal stability,
field-effect charge carrier property and electrochemical properties of
the copolymer have been characterized. Moreover, the application of
the copolymer as electron donor material for PVCs was investigated,
and the optimization of the devices through the cathode modification
through alcohol soluble conjugated polymer interlayer was
implemented.
2.4. Fabrication and characterization of PVCs
A patterned indium tin oxide (ITO) coated glass with a sheet
resistance of 10e15
U/square was cleaned by a surfactant scrub,
followed by a wet-cleaning process inside an ultrasonic bath,
beginning with deionized water, followed by acetone and iso-
propanol. After oxygen plasma cleaning for 5 min, a 40 nm thick
2. Experimental section
2.1. Materials
poly(3,4-ethylenedioxythiophene):poly(styrene
sulfonate)
(PEDOT:PSS) (Bayer Baytron 4083) anode buffer layer was spin-
casted onto the ITO substrate and then dried by baking in
a vacuum oven at 80 ꢂC overnight. The active layer, with a thickness
in the 70e80 nm range, was then deposited on top of the PEDOT:PSS
layer, by spin-casting from a chlorobenzene solution containing
PITT/PC61BM (1:2 w/w), PITT/PC61BM (1:4 w/w), PITT/PC71BM(1:4
w/w). The PFN solution in methanol was spin-coated on the top of
the obtained active layer to form a thin interlayer of 5 nm. The
thickness of the PEDOT:PSS and active layer were verified by
a surface profilometer (DektakXT, Boyue In. Co.). Determination of
the thickness of the PFN interlayer followed as previously described
[18], and thickness control was achieved by adjusting the concen-
tration of the solution and the spin speed (between 600 and
2500 rpm). The thickness of the PFN interlayer was determined
using a surface profilometer, in combination with extrapolation
from an absorbanceethickness curve that assumes a linear depen-
dence of absorbance at 380 nm on the film thickness. Finally,
All reagents, unless otherwise specified, were obtained from
Aldrich, Acros, and TCI Chemical Co., and used as received. All the
solvents were further purified under a nitrogen flow. 1-bromo-4-
octyloxybenzene [15], diethyl 2,5-dibromoterephthalate [16] and
2,5-bis(trimethystannyl)thieno[3,2-b]thiophene [17] were prepared
following the published procedures and characterized by 1H NMR and
GCeMS before use. Poly [(9,9-bis(30-(N,N-dimethylamino)propyl)-
2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) were prepared and
characterized by the procedure as reported reference [18].
2.2. General methods
1H NMR spectra were recorded on a Bruker DRX 400 spec-
trometer operating at 400 MHz and were referred to tetrame-
thylsilane (TMS). Analytical GPC was obtained using a Waters GPC
2410 in tetrahydrofuran (THF) via a calibration curve of polystyrene

Y. Xia et al. / Polymer 54 (2013) 607e613
609
a 100 nm aluminium layer was thermally evaporated with a shadow
2.5.4. 2,7-dibromo-4,4,9,9-tetra(4-octyloxyphenyl)indaceno[1,2-
b:5,6-b0]dithiophene (IV) [20]
mask under vacuum of 3 ꢁ 10ꢀ5 Pa. The overlapping area between
the cathode and anode defined a pixel size of device of 0.15 cm2. The
thickness of the evaporated cathode was monitored by a quartz
crystal thickness/ratio monitor (SI-TM206, Shenyang Sciens Tech-
nology Co.). Except for the deposition of the PEDOT:PSS layers, all
the fabrication processes were carried out inside a controlled
atmosphere in a nitrogen drybox (Etelux Co.) containing less than
1 ppm oxygen and moisture. The PCEs of the resulting polymer solar
cells were measured under 1 sun, AM 1.5G (Air mass 1.5 global)
condition using a solar simulator (XES-70S1, San-EI Electric Co.)
(100 mW cmꢀ2). The current densityevoltage (JeV) characteristics
were recorded with a Keithley 2410 source-measurement unit. The
spectral responses of the devices were measured with a commercial
EQE/incident photon to charge carrier efficiency (IPCE) setup (7-
SCSpecIII, Bejing 7-star Opt. In. Co.). A calibrated silicon detector
was used to determine the absolute photosensitivity.
Compound III 2.0 g was dissolved into a 100 mL acetate acid and
stirred for 20 min. Then two drops of concentrated sulfuric acid was
added, and the mixture was refluxed for 4 h. After the reaction was
cooled to room temperature, 100 mL distilled water was added. The
resulted solution was extracted with ethyl acetate for three times.
The combined organic phase washed with saline solution and dried
with anhydrous Na2SO4. Then the solvent was removed under
reduced pressure, and the product was purified with chromatog-
raphy (petroleum ether/ethyl acetate ¼ 30:1, V/V) to afford 0.87 g
white solid (yield 45%). 1H NMR (400 MHz, CDCl3),
d (ppm): 7.28 (s,
2H), 7.10 (d, J ¼ 8.8 Hz, 8H), 6.96 (s, 2H), 6.77 (d, J ¼ 8.8 Hz, 8H), 3.90
(t, J ¼ 6.8 Hz, 8H),1.74 (m, 8H),1.42 (m, 8H),1.31e1.26 (m, 32H), 0.87
(t, J ¼ 7.2 Hz, 12H). FABeMS: Cacld for C72H88Br2O4S2, 1241.4,
Found: 1241.
2.5.5. Synthesis of the polymer
2.5. Synthesis procedure
A mixture of toluene (6 mL) and N,N-dimethylformamide
(0.5 mL) was added to a 25 mL two-neck flask containing mono-
mers IV (372.30 mg, 0.3 mmol), 2,5-bis(trimethylstannyl)thieno
[3,2-b]thiophene (139.65 mg, 0.3 mmol) and Pd(PPh3)4 (4.0 mg).
The solution was refluxed with vigorous stirring for 36 h under an
argon atmosphere. At the end of polymerization, the polymer was
2.5.1. Diethyl 2,5-di(thiophen-2-yl)terephthalate (I) [20]
A solution of anhydrous zinc chloride (5.4 g, 40 mmol) sus-
pended in 30 mL anhydrous THF was added dropwise into 30 mL of
2-thienylmagnesium bromide (1 M in THF) solution at 0 ꢂC, and the
reaction was kept at 0 ꢂC for another 0.5 h under argon atmosphere.
A solution of diethyl 2,5-dibromoterephthalate (4.38 g, 11.5 mmol)
and Pd(PPh3)4 (226 mg) in 100 mL THF was added in one portion.
After the mixture was refluxed overnight, the reaction mixture were
cooled to room temperature and quenched by distilled water, then
extracted with diethyl ether for three times. The organic phase was
combined and the solvent was removed under reduced pressure.
The product was purified by chromatography (petroleum ether/
ethyl acetate ¼ 8:1, V/V) to give 4.29 g white solid with yield of 96%.
end-capped
with
2-tributylstannylthiophene
and
2-
bromothiophene to remove bromo and trimethystannyl end
groups. The mixture was then poured into methanol, and the
precipitated material was collected and extracted with ethanol,
acetone, hexane and toluene in a Soxhlet extractor. The solution of
the copolymer in toluene was condensed to about 5 mL and then
poured into methanol (500 mL). The precipitation was collected
and dried under vacuum overnight (yield: 75%). 1H NMR (400 MHz,
CDCl3),
d (ppm): 6.98e7.45 (m, 22H); 3.93 (t, 8H); 1.74 (m, 8H);
1H NMR (400 MHz, CDCl3),
d
(ppm): 7.81 (s, 2H), 7.38 (dd, J ¼ 5.2,
1.41e1.26 (m, 40 H); 0.87 (t, 12H); Anal. Found: C 75.78, H 7.37,N
2.49,1 S 14.28; Mn ¼ 16,500 g/mol with a polydispersity index (PDI)
of 1.47.
1.6 Hz, 2H), 7.10e7.06 (m, 4H), 4.22 (q, J ¼ 7.2 Hz, 4H), 1.15 (t,
J ¼ 7.2 Hz, 6H). GCeMS: Cacld for C20H18O4S2, 386.48, Found: 386.15.
2.5.2. Diethyl 2,5-bis(5-bromothiophen-2-yl)terephthalate (II)
[10b]
A solution of N-bromosuccinimide (NBS, 2.0 g, 11.2 mmol) dis-
solved in 20 mL chloroform was added dropwise into the solution
of I (2.0 g, 5.2 mmol) in the 90 mL chloroform and 35 mL acetic acid
at room temperature under dark. After the reaction was carried on
for another 3 h, 80 mL distilled water was added and the mixture
was extracted with chloroform for three times. The combined
organic was dried with anhydrous Na2SO4, the solvent was
removed under reduced pressure. Then the yellow solid was
recrystallized from acetone to led 2.59 g yellow flake (yield, 92%).
3. Results and discussion
3.1. Synthesis and characterization
The general synthetic route toward the monomers and copol-
ymer is outlined in Scheme 1. Diethyl 2,5-di(thiophen-2-yl)tere-
phthalate (I) was synthesized by 2-thienylzinc chloride and diethyl
2,5-dibromoterephthalate through the Pd(PPh3)4 catalyzed
coupling reaction, then I was brominated by NBS under dark to led
II. Then II was reacted with 4-octyloxyphenylmagnesium bromide
to led a corresponding alcohol III. Without further purification,
compound III was then treated to acid-mediated to afford the IV in
45% yield [20]. 2,5-Bis(trimethylstannyl)thieno[3,2-b]thiophene
was prepared following the modified procedures [17]. All the
compounds were characterized with 1H NMR spectra and GCeMS
or FABeMS. The alternating conjugated copolymer (PITT) was
synthesized by the palladium-catalyzed Stille coupling reaction and
1H NMR (400 MHz, CDCl3),
d
(ppm): 7.77 (s, 2H), 7.04 (d, J ¼ 4.0 Hz,
2H), 6.84 (d, J ¼ 3.6 Hz, 2H), 4.24 (q, J ¼ 7.2 Hz, 4H),1.21 (t, J ¼ 6.8 Hz,
6H). C20H16Br2O4S2, Calcd:44.13; H, 2.96; Br, 29.36; S, 11.78. Found:
C, 44.09; H, 2.94; Br, 29.4; S, 11.81.
2.5.3. 2,5-bis(5-bromothien-2-yl)-1,4-bis(1,1-di(4-octyloxyphenyl-
hydroxylmethyl) benzene (III) [20]
A solution of 4-octyloxyphenylmagnesium bromide prepared by
1-bromo-4-octyloxybenzene (18.0 g, 63.15 mmol) and magnesium
(1.82 g, 75.8 mmol) in 80 mL THF, was added slowly to a solution of
II (4.3 g, 7.9 mmol) at 0 ꢂC under argon atmosphere. After the
reaction was carried on for another 1.5 h at 0 ꢂC, then refluxed
overnight, the reaction was quenched by water and extracted with
ethyl acetate. The combined organic phase was dried with anhy-
drous Na2SO4, the solvent was removed under reduced pressure.
The resulted product was used for the next step directly without
further purification.
end-capping
reactions
were
performed
using
2-
tributylstannylthiophene and 2-bromothiophene [21]. The ob-
tained copolymer is soluble in common organic solvents, such as
toluene, THF and chloroform etc. The number-average molecular
weight of PITT determined by GPC using a polystyrene standard, is
16,500 g molꢀ1 with a polydispersity index (Mw/Mn) of 1.47. Free-
standing films can be easily obtained from the solution of PITT in
toluene. Furthermore, the copolymer shows excellent thermal
stability (5%, degradation at 380 ꢂC, Fig. 1).

610
Y. Xia et al. / Polymer 54 (2013) 607e613
Scheme 1. The synthetic route of the monomer and polymer.
3.2. Optoelectronic properties of the copolymer
THF solution. The maximal absorption and shoulder absorption
peaks of the copolymer in solid thin film are red-shifted in web
version to around 515 and 541 nm, respectively. The copolymer
shows extensive light absorption from 350 to 583 nm in THF
solution and 350e590 nm in solid thin film. The optical band gap
(Eg) of PITT estimated from the onset of absorption edge in solid
thin film is about 2.12 eV. The PL emission peaks of the copolymer
in THF solution and solid thin film are around 580 and 592 nm,
respectively (Fig. 2). The solid state photoluminescence of the
copolymer with a maximal emission at 590 nm is totally quenched
by the addition of [6,6]-phenyl-C61 butyric acid methyl ester
(PC61BM) (Fig. 2b). This efficient photoluminescence quenching is
the consequence of ultra-fast photo-induced charge transfer from
the polymer to PC61BM. It indicates that the PITT is a viable electron
donor material for PVCs [21].
Fig. 2 shows the normalized UVevis absorption and photo-
luminescence spectra of the polymer in the solution of THF and
solid thin film. There are one absorption peak at around 510 nm and
a shoulder absorption peak at around 535 nm for the copolymer in
The electrochemical behavior of the copolymer was investigated
by cyclic voltammetry (Fig. 3). We can record two oxidation process
and one reduction process in the copolymer solid thin film. The
reduction potential is at around ꢀ1.35 V. Two oxidation potentials
are observed at around 0.83 eV and 1.32 eV. HOMO and LUMO
levels of the copolymer calculated by empirical formulas
(EHOMO ¼ ꢀe(Eox þ 4.46) (eV)) and ELUMO ¼ ꢀe(Ered þ 4.46) (eV))
[22] are ꢀ5.29 eV and ꢀ3.11 eV, respectively. The electrochemical
band gap of the copolymer of 2.18 eV is similar to the optical band
gap of the copolymer (Eopt ¼ 2.12 eV).
3.3. Field-effect transistor performances
The field-effect carrier mobility of the polymer was investigated
by fabricating and evaluating thin film field-effect transistors (FETs)
based on the top contact geometry. Fig. 4a displays the plots of
Fig. 1. TGA plots of PITT in nitrogen.

Y. Xia et al. / Polymer 54 (2013) 607e613
611
Fig. 3. Electrochemical curve of the copolymer.
Fig. 2. Normalized absorption and PL spectra of the copolymer in solid thin film and
the THF solution (5 ꢁ 10ꢀ5 mol Lꢀ1) (a), normalized absorption and PL spectra of the
blend of PITT and PC61BM (b).
source-drain current (IDS) as a function of sourceedrain voltage
(VDS) at different gate voltages (VG) from 0 to ꢀ40 V. On the
application of negative VG, characteristic transistor behavior is
observed, indicating that the fabricated FET holds p-channel char-
acteristics. As the VDS increases, the current IDS approach into the
saturation regime could be described by Equation (1):
IDS ¼ ðW=2LÞC0mðVG ꢀ VT Þ2
(1)
where
m is the field-effect hole mobility, W is the channel width
(10 mm), L is the channel length (0.1 mm), C0 is the capacitance per
unit area of the gate dielectric layer (SiO2, 100 nm, C0 ¼ 34.5 nF/
cm2), and VT is the threshold voltage. The saturation region field-
effect mobility was thus calculated from the transfer characteris-
1=2
DS
tics of the OFETs involving plotting I
vs. VG (Fig. 4b). The PITT
devices show typical p-channel FET characteristics with good
drainecurrent modulation and well defined linear and saturation
regions. The hole mobility of PITT was calculated in the saturated
Fig. 4. The output at different gate voltages (VG) (a) and transfer characteristics in the
saturation regime (b) for OFETs using spin-coated PITT on SiO2/Si substrates.

612
Y. Xia et al. / Polymer 54 (2013) 607e613
Table 1
3.4. Solar cell performances
Device characteristics of photovoltaic cells based on PITT/PCBM blend.
The potential of the PITT to be employed as electron donor
material for PVCs were explored in a device configuration of ITO/
PEDOT:PSS/active layer/Al, with PC61BM and PC71BM as electron
acceptor materials, respectively. The obtained PVCs were tested
under AM 1.5G illumination at an irradiation intensity of
Device
Voc (V)
Jsc
FF (%)
he (AM1.5
(mA/cm2)
100 mW/cm2) (%)
PITT:PC61BM (1:2)
PITT:PC61BM (1:4)
PITT:PC71BM (1:4)
PITT:PC71BM (1:4)
(with PFN interlayer)
0.80
0.80
0.80
0.88
3.23
4.82
6.97
7.68
43
48
54
62
1.11
1.85
3.01
4.19
100 mW cmꢀ2
, and the resulting device performances are
summarized in Table 1. The PCEs of 1.11%e1.85% and the Voc of 0.80
and 0.80 V, Jsc of 3.23 and 4.82 mA cmꢀ2 and fill factor (FF) of 43%
and 48% are achieved in PVCs based on PITT/PC61BM blend (w:w,
1:2 and 1:4), respectively. When PC71BM was used as the electron
acceptor material, the PCEs of the devices from PITT/PC71BM
(w:w; 1:4) are improved to 3.01% with Voc of 0.80 V, Jsc of
6.97 mAcmꢀ2 and FF of 54%.
regime at VDS ¼ ꢀ30 V to be 8.1 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1 with a current
on/off ratio of 2.0 ꢁ 103. The result is consistent with the mobility of
IT-based copolymers reported by Ko et al. [10b]. The high hole
mobility of the PITT is expected to facilitate charge transport and
reduce recombination loss in the PVCs, therefore it can lead to an
improved device performance.
When a thin layer of PFN (5 nm) was inserted as interlayer
between the active layer and metal electrode, the device perfor-
mances are improved in all aspects (Table 1). The PCE of the PVCs,
with PC71BM as electron acceptor material and PFN as interlayer
between active layer and metal cathode, were improved from 3.01%
to 4.19%. The Voc of 0.88 V, Jsc of 7.68 mA cmꢀ2, FF of 62%, and the
incident photons to charge efficiency (IPCE) of the PVCs based on
the PITT/PC71BM blend with PFN interlayer, were simultaneously
enhanced as comparison of the parameters for PVCs without PFN
interlayer (Fig. 5a and 5b). The improvement of device performance
may be attributed to an enhanced built-in potential across the
device due to the existence of interface dipole between the active
layer and cathode by the PFN layer, improvement to chargee
transport properties, elimination of the buildup of space charge,
and reduced recombination loss due to increase in built-in field and
charge carrier mobility [12]. The results indicated that the perfor-
mance of PVCs with IT-based conjugated polymers as electron
donor materials could be enhanced via the incorporation of alcohol
soluble conjugated polymer interlayer between active layer and
metal cathode. It should open up new opportunities to improve the
performance of PVCs from IT-based CPs.
4. Conclusion
An alternating copolymer derived from 4,4,9,9-tetra(4-
octyloxyphenyl) indaceno[1,2-b:5,6-b0]dithiophene and thieno
[3,2-b]thiophene was synthesized, and the chemical structure,
molecular weight, thermal stability, field-effect charge carrier
property and electrochemical properties of the copolymer have
been characterized. The application of the copolymer as electron
donor materials for polymer photovoltaic cells (PVCs) was inves-
tigated. On the other hand, the optimization of the devices through
the cathode modification by alcohol soluble conjugated polymer
interlayer, was also implemented. The Voc, Jsc and FF of the PVCs
based on PITT were enhanced simultaneously in the PVCs with PFN
interlayer, and the maximal power conversion efficiency reaches
4.19% with an open circuit voltage (Voc) of 0.88 V, short circuit
current densities (Jsc) of 7.68 mAcmꢀ2 and fill factor (FF) of 62% in
the devices with PFN as cathode modification interlayer under an
AM 1.5 simulator (100 mWcmꢀ2). The results indicated that the
performance of PVCs with IT-based conjugated polymers as elec-
tron donor materials could be enhanced via the incorporation of
alcohol soluble conjugated polymer interlayer between active layer
and metal cathode. It should open up new opportunities to improve
the performance of PVCs from IT-based CPs.
Acknowledgments
The authors are deeply grateful to National Natural Science
Foundation of China (20704021, 61166002 U1174001), the Gansu
Fig. 5. a. JꢀV characteristics of photovoltaic devices based on the blend of copolymer
and PC71BM. b. IPCE curves of the devices based on copolymer and PC71BM.

Y. Xia et al. / Polymer 54 (2013) 607e613
613
(d) Hou JH, Park M-H, Zhang SQ, Yao Y, Chen L-M, Li J-H, et al. Macromolecules
2008;41(16):6012e8.
[6] (a) Shi QQ, Fan HJ, Liu Y, Hu WP, Li YF, Zhan XW. Macromolecules 2011;
44(23):9173e9;
(b) Zhang MJ, Guo X, Zhang Z-G, Li YF. Polymer 2011;52(24):5464e70.
[7] Wang M, Hu XW, Liu P, Li W, Gong X, Huang F, et al. J Am Chem Soc 2011;
133(25):9638e41.
Province Natural Foundation (1107RJZA154, 1111RJDA009),
the Natural Science Foundation of Guangdong Province
(S2011010003400), the China Postdoctoral Science Foundation, the
China Postdoctoral Science Special Foundation, the ‘‘Qinglan’’ talent
project of Lanzhou Jiaotong University for financial support.
[8] (a) Sirringhaus H, Brown PJ, Friend RH, Nielsen MM, Bechgaard K, Langeveld-
Voss BMW, et al. Nature (London) 1999;401(6754):685e8.
(b) Österbacka R, An CP, Jiang XM, Vardeny ZV. Science 2000;287(5454):
839e42;
(c) McCulloch I, Heeney M, Bailey C, Genevicius K, Macdonald I, Shkunov M,
et al. Nat Mater 2006;5(4):328e33.
[9] Takimiya K, Shinamura S, Osaka I, Miyazaki E. Adv Mater 2011;23(38):
4347e70.
References
[1] (a) Nielsen CB, Fraser JM, Schroeder BC, Du JP, White AJP, Zhang WM, et al. Org
Lett 2011;13(9):2414e7;
(b) Wang B, Tsang S-W, Zhang WF, Tao Y, Wong MS. Chem Commun 2011;
47(33):9471e3;
(c) Kim JY, Lee K, Coates NE, Moses D, Nguyen T-Q, Dante M, et al. Science
2007;317(5835):222e5;
(d) He F, Wang W, Chen W, Xu T, Darling SB, Strzalka J, et al. J Am Chem Soc
2011;133(10):3284e7;
(e) Chen C-H, Hsieh C-H, Dubosc M, Cheng Y-J, Hsu C-S. Macromolecules 2010;
43(2):697e708;
(f) Bijleveld JC, Shahid M, Gilot J, Wienk MM, Janssen RAJ. Adv Funct Mater
2009;19(20):3262e70;
(g) Hou JH, Chen H-Y, Zhang SQ, Li G, Yang Y. J Am Chem Soc 2008;130(48):
16144e5;
(h) Chu T-Y, Lu JP, Beaupré S, Zhang YG, Pouliot J-R, Wakim S, et al. J Am Chem
Soc 2011;133(12):4250e3.
[10] (a) Chen C-P, Chan S-H, Chao T-C, Ting C, Ko B-T. J Am Chem Soc 2008;
130(38):12828e33;
(b) Chan S-H, Chen C-P, Chao T-C, Ting C, Lin C-S, Ko B-T. Macromolecules
2008;41(15):5519e26;
(c) Zhang Y, Zou JY, Yip H-L, Chen K-S, Zeigler DF, Sun Y, et al. Chem Mater
2011;23(9):2289e91.
[11] Xia YJ, He ZC, Tong JF, Li B, Wang CL, Cao Y, et al. Macromol Chem Phys 2011;
212(11):1193e201.
[12] He ZC, Zhong CM, Huang X, Wong W-Y, Wu HB, Chen LW, et al. Adv Mater
2011;23(40):4636e43.
[13] (a) Huang F, Wu HB, Cao Y. Chem Soc Rev 2010;39:2500e21;
(b) Seo JH, Gutacker A, Sun YM, Wu HB, Huang F, Cao Y, et al. J Am Chem Soc
2011;133(22):8416e9.
[14] He C, Zhong CM, Wu HB, Yang RQ, Yang W, Huang F, et al. J Mater Chem 2010;
20(2617):2617e22.
[15] Goldfinger MB. Patent No. US7671083B2; 2007.
[16] Liu B, Yu W-L, Lai Y-H, Huang W. Chem Mater 2001;13(6):1984e91.
[17] (a) Seitz DE, Lee S-H, Hanson RN, Bottaro JC. Synth Commun 1983;13(2):
121e8;
(b) van Pham C, Macomber RS, Mark Jr HB, Zimmer H. J Org Chem 1984;
49(26):5250e3.
[18] Huang F, Wu HB, Wang DL, Yang W, Cao Y. Chem Mater 2004;16(4):708e16.
[19] Pan HL, Wu YL, Li YN, Liu P, Ong BS, Zhu SP, et al. Adv Funct Mater 2007;
17:3574e9.
(i) Huo LJ, Chen H-Y, Hou JH, Chen TL, Yang Y. Chem Commun 2009;37:
5570e2.
[2] Liang YY, Wu Y, Feng DQ, Tsai S-T, Son H-J, Li G, et al. J Am Chem Soc 2009;
131(1):56e7.
[3] (a) Liang YY, Feng DQ, Wu Y, Tsai S-T, Li G, Ray C, et al. J Am Chem Soc 2009;
131(22):7792e9;
(b) Liang YY, Xu Z, Xia JB, Tsai S-T, Wu Y, Li G, et al. Adv Mater 2010;22(20):
E135e8.
[4] (a) Huo LJ, Hou JH, Zhang SQ, Chen H-Y, Yang Y. Angew Chem 2010;122(8):
1542e5;
(b) Dou LT, You JB, Yang J, Chen C-C, He YJ, Murase S, et al. Nat Photon 2012;
6(3):180e5.
[5] (a) Huo LJ, Zhang SQ, Guo X, Xu F, Li YF, Hou JH. Angew Chem 2011;123(41):
9871e6;
[20] Wong K-T, Chao T-C, Chi L-C, Chu Y-Y, Balaiah A, Chiu S-F, et al. Org Lett 2006;
8(22):5033e6.
[21] Blouin N, Michaud A, Leclerc M. Adv Mater 2007;19(17):2295e300.
[22] de Leeuw DM, Simenon MMJ, Brown AR, Einerhand REF. Synth Met 1997;
87(1):53e9.
(b) Hou JH, Chen H-Y, Zhang SQ, Chen RI, Yang Y, Wu Y, et al. J Am Chem Soc
2009;131(43):15586e7;
(c) Chen H-Y, Hou JH, Zhang SQ, Liang YY, Yang GW, Yang Y, et al. Nat Photon
2009;3(11):649e53;