Novel low-bandgap oligothiophene-based donor-acceptor alternating conjugated copolymers: Synthesis, properties, and photovoltaic applications

Article abstract of DOI:10.1002/pola.24025

A series of novel soluble donor-acceptor lowbandgap-conjugated polymers consisting of different oligothiophene (OTh) coupled to electron-accepting moiety 2-pyran-4ylidenemalononitrile (PM)-based unit were synthesized by Stille or Suzuki coupling polymerization. The combination of electron-accepting PM building block with varied OTh_n (the number of thiophene unit increases from 3 to 5) results in enhanced π-π stacking in solid state and intramolecular charge transfer (ICT) transition, which lead to an extension of the absorption spectra of the copolymers. Cyclic voltammetry measurements and molecular orbital distribution calculations indicate that the highest occupied molecular orbitals (HOMO) energy levels could be fine-tuned by changing the number of thiophene units of the copolymers, and the resulting copolymers possessed relatively low HOMO energy levels promising good air stability and high-open circuit voltage (V_oc) for photovoltaic application. Bulk heterojunction photovoltaic devices were fabricated by using the copolymers as donors and (6,6)phenyl C₆₁-butyric acid methyl ester as acceptor. It was found that the highest V_oc reached 0.94 V, and the short circuit currents (J_sc) were improved from 1.78 to 2.54 mA/cm₂, though the power conversion efficiencies of the devices were measured between 0.61 and 0.99% under simulated AM 1.5 solar irradiation of 100 mW/cm ², which indicated that this series copolymers can be promising candidates for the photovoltaic applications.

Full text of DOI:10.1002/pola.24025

Novel Low-Bandgap Oligothiophene-Based Donor-Acceptor Alternating
Conjugated Copolymers: Synthesis, Properties, and Photovoltaic
Applications
YAOWEN LI, ZAIFANG LI, CHUNYU WANG, HUI LI, HONGGUANG LU, BIN XU, WENJING TIAN
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, People’s Republic of China
Received 2 February 2010; accepted 9 March 2010
DOI: 10.1002/pola.24025
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A series of novel soluble donor-acceptor low-
bandgap-conjugated polymers consisting of different oligothio-
phene (OTh) coupled to electron-accepting moiety 2-pyran-4-
ylidenemalononitrile (PM)-based unit were synthesized by
Stille or Suzuki coupling polymerization. The combination of
electron-accepting PM building block with varied OThn (the
number of thiophene unit increases from 3 to 5) results in
enhanced p–p stacking in solid state and intramolecular charge
transfer (ICT) transition, which lead to an extension of the
absorption spectra of the copolymers. Cyclic voltammetry
measurements and molecular orbital distribution calculations
indicate that the highest occupied molecular orbitals (HOMO)
energy levels could be fine-tuned by changing the number of
thiophene units of the copolymers, and the resulting copoly-
mers possessed relatively low HOMO energy levels promising
good air stability and high-open circuit voltage (Voc) for photo-
voltaic application. Bulk heterojunction photovoltaic devices
were fabricated by using the copolymers as donors and (6,6)-
phenyl C61-butyric acid methyl ester as acceptor. It was found
that the highest Voc reached 0.94 V, and the short circuit cur-
rents (Jsc) were improved from 1.78 to 2.54 mA/cm2, though
the power conversion efficiencies of the devices were meas-
ured between 0.61 and 0.99% under simulated AM 1.5 solar
irradiation of 100 mW/cm2, which indicated that this series
copolymers can be promising candidates for the photovoltaic
C
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applications. 2010 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 48: 2765–2776, 2010
KEYWORDS: conducting polymers; conjugated polymers; mor-
phology; phase separation; photophysics
INTRODUCTION Over the past decade, considerable efforts
have been directed towards the development of polymer
photovoltaic cells (PVCs). PVCs are becoming more and more
attractive because they represent a low cost and flexible
devices, and ease of processing.1–4 One of the promising
strategies for devising efficient PVCs involves the use of
interpenetrating networks bulk heterojunctions (BHJ) based
on a blend of electron-donating conjugated polymers and
soluble fullerene acceptors.5 As a classical electron-donating
material, poly(3-hexylthiophene) (P3HT), has been widely
investigated in polymer BHJ solar cells, and important pro-
gress has been made in understanding the device physics as
well as improving the device efficiency. Relatively high-per-
formance BHJ solar cells made from a mixture of P3HT and
PCBM have been reported (the maximum power conversion
efficiencies (PCE) up to 4–5%6,7), but the relatively large
bandgap of P3HT (ꢀ1.9 eV) limits the fraction of the solar
spectrum that can be harvested, and the relatively small dif-
ference between the highest occupied molecular orbital
(HOMO) of P3HT (donor) and the lowest unoccupied molec-
ular orbital (LUMO) of the fullerene (acceptor) results in a
low open circuit voltage (Voc) (ꢀ0.6 V).
Recently, several classes of low-bandgap polymers, which are
designed to make use of internal charge transfer (ICT) from
an electron-donating moiety to an electron-accepting moiety
within the polymer backbone have been developed to better
harvest the solar spectrum.8–12 The optical and electronic
properties of the copolymers could be tuned through chang-
ing the strength of ICT transition by choosing different
donor or acceptor groups inside the copolymers. So, design
and synthesis donor-acceptor (D-A)-conjugated copolymers,
which can efficiently harvest more energy of the solar spec-
trum are effective ways to obtain low-bandgap polymers for
photovoltaic applications. Furthermore, as a donor compo-
nent in photovoltaic cells, the D-A copolymers should pos-
sess two features. One is sufficient driving force for the elec-
tron transfer from the D-A copolymers to the acceptor (e.g.,
PCBM), which means that the LUMO energy level of the D-A
copolymers must be higher than that of the acceptor (at
least 0.3 eV) to have sufficient driving force for electron
transfer from the copolymer to acceptor, and the HOMO
energy level of D-A copolymers must be low enough to get
relative high Voc, which is determined by the difference
between the HOMO energy level of donor and LUMO energy
Correspondence to: W. Tian (E-mail: wjtian@jlu.edu.cn)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 2765–2776 (2010) 2010 Wiley Periodicals, Inc.
DEVELOPMENT OF POLYMER PHOTOVOLTAIC CELLS, LI ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
level of acceptor.13 The other is good miscibility with the
acceptor (e.g., PCBM) to form an active layer with interpene-
trating network. To efficiently dissociate the excitons gener-
ated in the active layer, the control of the BHJ morphology is
of crucial importance. Ideally, the phase-separation length
scale should match the exciton diffusion length of conjugated
polymer, which is ꢀ10 nm.14
low HOMO energy levels promising good air stability and
Voc for photovoltaic application. BHJ photovoltaic devices
were fabricated by using the copolymers as donors and
PCBM as acceptor. It was found that the highest Voc
reached 0.94 V, and the Jsc were improved from 1.78 to
2.54 mA/cm2, whereas the PCE of the devices were meas-
ured between 0.61 and 0.99% under simulated AM 1.5 so-
lar irradiation of 100 mW/cm2, which indicated that this
series copolymers can be promising candidates for the pho-
tovoltaic applications.
Most recently, we have synthesized a series of novel soluble
(bithiophenevinyl)-(2-pyran-4-ylidenemalononitrile) (TVM)-
based D-A conjugated polymers.15 Because of the high-oxida-
tive ability of the electron-accepting PM segment in TVM,
these copolymers have relatively deep HOMO energy level
resulting in relatively high Voc (more than 0.8 V). Besides,
the PM moiety in TVM is a strong coplanar electron-accept-
ing group, which can enhance the ICT transition and p–p
stacking in solid film leading to the increased electron affin-
ity and reduced bandgap of the conjugated system.16 How-
ever, the incorporation of noncoplanar moieties, such as fluo-
rene and phenothiazine in TVM-based copolymers can
greatly decrease the coplanarity of the copolymers and
weaken the p–p stacking in solid film, which result in lower-
ing absorption and charge transport.15 Whereas oligothio-
phene (OTh) attracted comprehensive interest and actually
has been advanced to the most frequently used p-conjugated
materials, in particular, as active components in organic
optoelectronic devices, such as organic field effect transis-
tor17,18 and solar cells,19,20 because the electronic, optical,
and electrochemical properties of OTh are intriguing,21 and
the high coplanarity of thiophene rings leads to a better p–p
interaction and excellent charge transport properties, which
are one of the most crucial assets for applications in organic
electronics. Moreover, the OThn, which are not alkylated can
increase the conjugated length, p–p stacking, and assure the
coplanarity of the molecules. The efficiency of BHJ photovol-
taic cells using different functionalized OThs as donor and
PCBM as acceptor can reach up to 3.4%.22–24 Therefore, the
combination of OTh with TVM segment may lead to a low
bandgap, and strong p–p interaction of the molecules.
EXPERIMENTAL
Materials
5,50-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,20-bithio-
phene, 2-(2,6-bis((E)-2-(5-bromo-3,4-dihexylthiophen-2-yl)vinyl)-
4H-pyran-4-ylidene)malononitrile was synthesized according to
known literature procedures.15 All reagents and chemicals were
purchased from commercial sources (Aldrich, Across, Fluka)
and used without further purification unless stated otherwise.
All solvents were distilled over appropriate drying agent(s)
before use and were purged with nitrogen.
Synthesis of 2,20-Bithiophene (1)
Mg (1.60 g, 67 mmol) was placed in 20 mL of dry ether then
cooled to 0 ꢁC. 2-bromothiophene (8.5 mL, 87.1 mmol) was
added dropwise over 0.5 h, and the reaction mixture was
stirred for another hour. This solution was then transferred
slowly via a cannula to a mixture of 2-bromothiophene
(6.5 mL, 67 mmol) and Ni(dppp)Cl2 (876 mg, 1.4 mmol) in
dry ether (50 mL) while cooling on ice. The reaction mixture
was refluxed for 24 h and subsequently poured out in
ice/water (300 mL) containing concentrated HCl (30 mL).
The product was extracted with ether and the combined
organic layers were washed with plenty of water and brine,
successively. The organic extracts were dried over anhydrous
MgSO4, evaporated and purified with column chromatogra-
phy on silica gel with petroleumether as the eluant to give
slight green solid 9.1 g (54.9 mmol, yield 82%). 1H NMR
(300 MHz, CDCl3, TMS): d(ppm) 7.218 (d, 2H, J ¼ 5.1 Hz),
7.174 (d, 2H, J ¼ 3.3 Hz), 7.008 (t, 2H, J ¼ 4.2 Hz). 13C NMR
(75 MHz, CDCl3, TMS): d(ppm) 137.360, 127.702, 124.283,
123.715. C8H6S2: C, 55.79; H, 3.64. found: C, 55.84; H, 3.58.
Herein, we synthesized three novel p-conjugated alternating
copolymers consisting of varied OThn-based donor unit
coupled to PM segment: poly{(thiophene-2,5-ylene)-alt-2-
(2,6-bis((E)-2-(3,4-dihexylthiophene-2-yl)vinyl)-4H-pyran-4-
ylidene)-malononitrile} (PTTMT), poly{(2,20-bithiophene-
5,50-ylene)-alt-2-(2,6-bis((E)-2-(5-bromo-3,4-dihexylthiophene-2-
yl)vinyl)-4H-pyran-4-ylidene)-malononitrile} (PDTTMT), poly-
{(2,20:50,200-terthiophene-5,500-ylene)-alt-2-(2,6-bis((E)-2-(3,4-
dihexylthiophene-2-yl)vinyl)-4H-pyran-4-ylidene)-malononi-
trile} (PTTTMT). The combination of electron-accepting PM
building block with varied OThn (the number of thiophene
unit increases from 3 to 5) results in enhanced p–p stack-
ing in solid state and intramolecular charge transfer (ICT)
transition, which lead to an extension of the absorption
spectra of the copolymers. Cyclic voltammetry (CV) mea-
surements and molecular orbital distribution calculations
indicate that the HOMO energy levels could be fine-tuned
by changing the number of thiophene units of the copoly-
mers, and the resulting copolymers possessed relatively
Synthesis of 2,20:50,200-Terthiophene (2)
Mg (0.56 g, 23.3 ꢁmmol) was placed in 10 mL of dry ether
then cooled to 0 C. 2-bromothiophene (2.9 mL, 30.3 mmol)
was added dropwise over 0.5 h, and the reaction mixture
was stirred for another hour. This solution was then trans-
ferred slowly via a cannula to a mixture of 2,5-dibromothio-
phene (1.3 mL, 11.6 mmol) and Ni(dppp)Cl2 (305 mg, 0.466
mmol) in dry ether (50 mL) while cooling on ice. The reac-
tion mixture was stirred for 16 h at room temperature and
subsequently poured out in ice/water (150 mL) containing
concentrated HCl (10 mL). The product was extracted with
ether and the combined organic layers were washed with
plenty of water and brine, successively. The organic extracts
were dried over anhydrous MgSO4, evaporated and purified
with column chromatography on silica gel with petroleum-
ether as the eluant to give slight yellowish solid 2.1 g
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ARTICLE
(8.3 mmol, yield 71%). 1H NMR (500 MHz, CDCl3, TMS):
d(ppm) 7.218 (d, 2H, J ¼ 5.0 Hz), 7.174 (d, 2H, J ¼ 3.5 Hz,
ATh), 7.078 (s, 2H, ACH2), 7.020 (m, 12H, ACH2). 13C NMR
(75 MHz, CDCl3, TMS): d(ppm) 137.133, 136.234, 127.845,
124.457, 124.300, 123.694. ELEM. ANAL. Calcd. for C12H8S3: C,
58.03; H, 3.25. found: C, 58.07; H, 3.20.
dihexylthiophen-2-yl)vinyl)-4H-pyran-4-ylidene)malononitrile,
ditrimethylstannyl monomer, and (PPh3)4Pd(0) (2 mol %
with respect to the monomer) were dissolved in a mixture of
toluene (15 mL). The solution was stirred under an Ar
atmosphere and refluxed with vigorous stirring for 48 h. The
resulting solution was then poured into methanol and fol-
lowed by washing with water. The precipitated solid was
extracted with methanol and acetone for 24 h in a Soxhlet
apparatus to remove the oligomers and catalyst residues,
respectively. The soluble fraction is then collected via extrac-
tion with CHCl3 for 24 h. The chloroform solution is then
concentrated to afford the copolymers. Suzuki cross-coupling
reaction was used to synthesize the copolymer of PDTTMT
shown in Scheme 2, which has been reported in ref. 15.
Synthesis of 2,5-Bis(trimethylstannyl)thiophene (M-1)
n-BuLi (7.2 mL of a 2.5 M solution in hexane, 18.4 mmol)
was added dropwise to a solution of 2,5-dibromothiophene
ꢁ
(2.0 mL, 8.4 mmol) in THF (60 mL) at ꢂ78 C with stirring.
ꢁ
After stirring for 1.5 h at ꢂ78 C, trimethyltin chloride (2 g,
21.0 mmol) in THF (10 mL) was slowly added. The reaction
mixture was stirred at ꢂ78 ꢁC for additional 0.5 h and was
then gradually warmed to room temperature and stirred
overnight. The mixture was poured out in ice/water
(150 mL), and the product was extracted with ether. The
organic layer was washed twice with water and the organic
layer was dried over MgSO4, the solvent was removed by
rotary evaporation. Recrystallization from ethanol afforded
M-1 as white crystals 2.7 g (6.6 mmol, yield 78%). 1H NMR
(300 MHz, CDCl3, TMS): d(ppm) 7.376 (s, 2H, ATh), 0.366
(s, 18H, ACH3,). 13C NMR (75 MHz, DMSO, TMS): d(ppm)
142.994, 135.781, ꢂ8.208. ELEM. ANAL. Calcd. for C10H20SSn2:
C, 29.31; H, 4.92. found: C, 29.33; H, 4.89.
Poly{(thiophene-2,5-ylene)-alt-2-(2,6-bis((E)-
2-(3,4-dihexylthiophene-2-yl)vinyl)-4H-pyran-4-ylidene)-
malononitrile}
The resulting polymer PTTMT was obtained as a dark brown
shining powder with a yield of 78%. 1H NMR (500 MHz,
CDCl3, TMS): d(ppm) 7.602 (br, 2H, -vinylic), 7.226 (br, 2H,
ATh), 6.613 (br, 2H, -PM), 6.472 (br, 2H, -vinylic), 2.741 (br,
8H, AaCH2), 1.600 (br, 8H, ACH2), 1.448 (br, 8H, ACH2),
1.358 (br, 16H, ACH2), 0.929 (br, 6H, ACH3), 0.874 (br, 6H,
ACH3). 13C NMR (125 MHz, CDCl3, TMS): d(ppm) 158.367,
155.635, 147.599, 141.020, 136.894, 134.109, 133.933,
128.563, 127.487, 117.147, 115.835, 107.217, 59.601,
32.131, 32.061, 31.981, 30.858, 30.009, 29.904, 28.476,
28.167, 27.864, 23.078, 14.505. ELEM. ANAL. Calcd. for
Synthesis of 5,500-Ditrimethylstannyl-2,20:50,200-terthiophene
(M-2)
The synthetic procedure for M-2 was similar to that for M-1
1
give a yellow crystals 3.3 g (5.8 mmol, 69.0%). H NMR (300
MHz, CDCl3, TMS): d(ppm) 7.287 (br, 2H), 7.107 (br, 2H,
APh), 7.085 (s, 2H), 0.416 (s, 18H). 13C NMR (75 MHz,
DMSO, TMS): d(ppm) 142.727, 137.524, 136.116, 135.886,
124.781, 124.144, ꢂ8.226. ELEM. ANAL. Calcd. for C18H24S3Sn2:
C, 37.66; H, 4.21. Found: C, 37.69; H, 4.19.
C48H60N2OS3: C, 73.94; H, 7.70. Found: C, 74.66; H, 7.45.
Poly{(2,20-bithiophene-5,50-ylene)-alt-2-(2,6-bis((E)-2-(5-bromo-
3,4-dihexylthioph-ene-2-yl)vinyl)-4H-pyran-4-ylidene)-malono-
nitrile} (PDTTMT) the resulting polymer PDTTMT was
obtained as a dark powder with a yield of 52%.1H NMR
(500 MHz, CDCl3, TMS): d(ppm) 7.601 (d, 2H, J ¼ 15.0 Hz, -
vinylic), 7.158 (br, 4H, -Th), 6.594 (s, 2H, -PM), 6.453 (d, 2H, J
¼ 15.5 Hz, -vinylic), 2.722 (br, 8H, AaCH2), 1.582 (br, 8H,
ACH2), 1.357 (br, 24H, ACH2), 0.928 (br, 6H, ACH3), 0.866
(br, 6H, ACH3). 13C NMR (125 MHz, CDCl3, TMS): d(ppm)
158.168, 155.648, 147.586, 145.408, 143.836, 140.886,
138.711, 134.329, 130.145, 128.415, 124.850, 117.112,
115.773, 107.134, 59.745, 32.131, 32.035, 31.955, 30.780,
29.885, 29.724, 28.834, 28.480, 28.146, 23.043, 14.484,
14.428. ELEM. ANAL. Calcd. for C52H62N2OS4: C, 72.69; H, 7.22.
Found: C, 72.86; H, 7.36.
Synthesis of 5,500-Bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-2,20-bithiophene (M-3)
n-BuLi (5.29 mL of a 2.5 M solution in hexane, 13.23 mmol)
was added dropwise into a solution of 12 (1.0 g, 6 mmol) in
THF (20 mL) at ꢂ78 ꢁC under an atmosphere of dry argon.
The mixture was stirred at room temperature for 2 h, and
then
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(3.2 mL, 15.63 mmol) was injected promptly into the flask at
ꢂ78 ꢁC. After being stirred at room temperature for one
night, the mixture was poured into water and was extracted
with ether. The extract was washed with brine and dried
over MgSO4. After evaporation of the solvent, the residue
was recrystallized from chloroform and methanol and gave a
green crystal (1.37 g, 72.0%). 1H NMR (500 MHz, CDCl3,
TMS): d(ppm) 7.525 (d, 2H, J ¼ 3.5 Hz, ATh), 7.291 (d, 2H,
J ¼ 3.5 Hz, ATh), 1.351 (s, 24H, ACH3). 13C NMR (75 MHz,
CDCl3, TMS): d(ppm) 150.438, 143.893, 133.622, 128.888,
119.346, 83.690, 55.161, 40.049, 31.742, 29.867, 28.837,
24.922, 23.560, 22.552, 14.040. ELEM. ANAL. Calcd. for
Poly{(2,20:50,200-terthiophene-5,500-ylene)-alt-2-(2,6-bis((E)-
2-(3,4-dihexylthiophen-e-2-yl)vinyl)-4H-pyran-4-ylidene)-
malononitrile}
The resulting polymer PTTTMT was obtained as a dark
green shining powder with a yield of 25%. 1H NMR (500
MHz, CDCl3, TMS): d(ppm) 7.584 (br, 2H, -vinylic), 7.150 (br,
6H, ATh), 6.634 (br, 2H, -PM), 6.451 (br, 2H, -vinylic), 2.743
(br, 8H, AaCH2), 1.561–1.383 (br, 32H, ACH2), 0.946 (br,
12H, ACH3). Anal. Calcd. for C56H64N2OS5: C, 71.41; H, 6.80.
Found: C, 71.99; H, 6.12.
C37H56B2O4: C, 75.78; H, 9.62. Found: C, 75.81; H, 9.66.
General Procedures of Polymerization
Stille cross-coupling reaction was used to synthesize the
polymers shown in Scheme 2. 2-(2,6-bis((E)-2-(5-bromo-3,4-
DEVELOPMENT OF POLYMER PHOTOVOLTAIC CELLS, LI ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthetic routes of the monomers.
Characterization
TECH 500-W solar simulator (AM 1.5 100 mW/cm2) were
measured on computer-controlled Keithley 2400 Source
Meter measurement system. All the measurements were per-
formed under ambient atmosphere at room temperature.
The elemental analysis was carried out with a Thermoquest
CHNS-Ovelemental analyzer. The gel permeation chromato-
graphic (GPC) analysis was carried out with a Waters 410
instrument with tetrahydrofuran as the eluent (flow rate: 1
mL/min, at 35 ꢁC) and polystyrene as the standard. Differ-
ential scanning calorimetry (DSC) was performed under
nitrogen flushing at a heating rate of 20 ꢁC/min with a
NETZSCH (DSC-204) instrument. Thermogravimetric analysis
(TGA) were performed on a Perkine Elmer Pyris 1 analyzer
under nitrogen atmosphere (100 mL/min) at a heating rate
of 10 ꢁC/min. 1H NMR and 13C NMR spectra were mea-
sured using a Bruker AVANCE-500 NMR spectrometer spec-
trometer and a Varian Mercury-300 NMR, respectively. UV–
visible absorption spectra were measured using a Shimadzu
UV-3100 spectrophotometer. Electrochemical measurements
of these derivatives were performed with a Bioanalytical
Systems BAS 100 B/W electrochemical workstation. Atomic
force microscopy (AFM) images of blend films were carried
out using a Nanoscope IIIa Dimension 3100. The transmis-
sion electron microscopy (TEM) measurements were con-
ducted on a HITACHI H-800 transmission electron micro-
scope operated at 200 kV.
RESULTS AND DISCUSSION
Material synthesis and structural characterization
The general synthetic routes toward the monomers are out-
lined in Scheme 1. The OThn (1, 2) were prepared by nickel-
catalyzed Grignard thiophene and bromination thiophene in
ether, affording OThn in good yield. Critical to the synthetic
strategy was bis(stannylation) or bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolane) of OThn. The monomer (M-1, M-2, M-3)
was prepared by a modified procedure with 2.2 equative n-
BuLi followed by 2.6 equative trimethyltin chloride or 2-iso-
propoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. The struc-
tures of monomer (M-1, M-2, M-3) were confirmed by 1H
NMR, 13C NMR, and ELEMENTAL ANALYSIS. The 2-(2,6-bis((E)-2-
(5-bromo-3,4-dihexylthiophen-2-yl)vinyl)-4H-pyran-4-ylide-
ne)malononitrile15 was synthesized according to the our pre-
viously reported literature, which has been confirmed that
the Knoevenagel reaction afforded the pure all-trans isomers.
The polymerization reaction was proceeded by the well-
known palladium-catalyzed Stille and Suzuki coupling reac-
tion.25 The synthetic routes of polymers are shown in
Photovoltaic Device Fabrication and Characterization
For device fabrication, the ITO glass was precleaned and
modified by a thin layer of PEDOT:PSS, which was spin-cast
from a PEDOT:PSS aqueous solution (H. C. Starck) on the
ITO substrate, and the thickness of the PEDOT:PSS layer is
about 50 nm. The active layer contained a blend of copoly-
mers as electron donor and PCBM as electron acceptor,
which was prepared by weight ratio (1:1, 1:2, 1:3, and 1:4
w/w) in chlorobenzene (4 mg/mL) for copolymers. After
spin-coating, the blend from solution at 1500 rpm, The de-
vices were completed by evaporating a 0.6 nm LiF layer pro-
tected by 100 nm of Al at a base pressure of 5 ꢃ 10ꢂ4 Pa.
The effective photovoltaic area as defined by the geometrical
overlap between the bottom ITO electrode and the top cath-
ode was 4 or 5 mm2. The thickness of the photoactive layer
was 50–60 nm, measured by the Ambios Technology XP-2.
The current-voltage (I-V) characterization of PV devices in
the dark and under white-light illumination from a SCIENCE-
1
Scheme 2. The H NMR spectra of PTTMT and its assignment
shown in Figure 1 are consistent with the proposed struc-
ture. All the copolymers, except PTTTPT, were partially solu-
ble in organic solvents (e.g., chloroform and toluene) at
room temperature and completely soluble in high boiling
point solvents (e.g., chlorobenzene) at high temperature.
PTTTMT was not soluble in most organic solvents probably
because of the absence of longer solubilizing groups on
2,20:50,200-terthiophene. Molecular weights and polydisper-
sities of the resulting copolymers were determined by GPC
analysis with the number average molecular weight (Mw) of
5900–12,600 g/mol and PDI (polydispersity index, Mw/Mn)
of 1.04–1.38. Table 1 summarized the polymerization results
including molecular weights, PDI, and thermal stability of
the polymers. The observed relative low polymerization
degrees are because of the relatively low solubility, which
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ARTICLE
SCHEME 2 Synthetic routes of the copolymers.
lead to an ineffectiveness of the Stille polymerization for pro-
Optical Properties
ducing copolymers with a high molecular weight.
The normalized UV–vis absorption spectra of PTTMT,
PDTTMT, PTTTMT, and P3HT in dilute chloroform solution
(concentration 10ꢂ5 M) are shown in Figure 3, and the main
optical properties are listed in Table 2. PTTMT with three
thiophene units showed two absorption bands at 330 nm
and 523 nm in dilute solution [Fig. 3(a)], which can be
assigned to p–p* transition of the conjugated copolymer
backbone and ICT interaction between the donor of OTh and
PM-based acceptor. Similarly, the absorption spectra of other
copolymers (PDTTMT and PTTTMT) in dilute solutions also
showed two bands near 330, 332 nm and 500, 528 nm
because of the p–p* transition and the ICT interaction,
respectively. Among these three copolymers, there is an
alternating ‘‘OThn-A’’ structure, where OThn (n ¼ 3, 4, 5) acts
as donor, and A acts as acceptor part. The solution absorp-
Thermal Properties
All the polymers exhibited good thermal stability with 5%
weight-loss temperatures (Td) higher than 296 ꢁC under N2
and high glass transition temperatures (Tg) of 112–134 ꢁC,
as revealed by TGA and the DSC, respectively, (see Fig. 2).
The high thermal stability of the resulting copolymers pre-
vents the deformation of the polymer morphology and the
degradation of active layer applied in PVCs.
tion spectrum of the three copolymers have the similar
abs
absorption peaks (k
¼ 523 nm, 500 nm, 528 nm) because
max
of the same chemical structure of the three copolymers
TABLE 1 Polymerization Results for Copolymers PTTMT,
PDTTMT, and PTTTMT
Mn Kg/mol)a
Mw (Kg/mol)a
PDI
Tg
Td
b
c
PTTMT
PDTTMT
PTTTMT
a
9.2
5.7
8.4
12.6
5.9
1.38
1.04
1.34
114
112
134
307
296
369
11.2
Calculated from GPC (eluent:THF; polystyrene standards).
b
c
Determined by DSC at a heating rate of 20 ꢁC/min under nitrogen.
Temperature at 5% weight loss by a heating rate of 10 ꢁC/min under
FIGURE 1 1H-NMR spectrum and chemical structure of PTTMT,
PDTTMT and PTTTMT in CDCl3 solution.
nitrogen.
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onset
Red
V, whereas the onset reduction potentials (E
) are almost
the same for the three copolymers (ꢂ1.28 V). The redox
potential of Fc/Fcþ, which has an absolute energy level of
ꢂ4.8 eV relative to the vacuum level for calibration is
located at 0.1V in 0.1 M TBAPF6/acetonitrile solution.29 So
the evaluation of HOMO and LUMO levels as well as the
bandgap (Eg,ec) could be done according to the following
equations:
onset
HOMOðeVÞ ¼ ꢂeðE
þ 4:7ÞðeVÞ
þ 4:7ÞðeVÞ
Ox
onset
Red
LUMOðeVÞ ¼ ꢂeðE
Eg;ec ¼ ꢂðHOMO ꢂ LUMOÞðeVÞ
onset
Ox
onset
where E
and E
are the measured potentials relative to
Red
Ag/Agþ. The electrochemical properties as well as the energy
level parameters of the copolymers are list in Table 2.
FIGURE 2 TGA thermograms of the copolymers.
The estimated HOMO and LUMO energy levels of PTTMT are
ꢂ5.44 and ꢂ3.44 eV, respectively. The LUMO energy levels of
PDTTMT and PTTTMT are ꢂ3.38 and ꢂ3.42 eV, which are
resulting in similar ICT effect. In addition, the three copoly-
mers exhibit the similar p–p* transition absorption which
may be caused by the saturation of conjugated length with
the increasing degree of polymerization,26 although the OThn
conjugated length was enhanced with the increasing number
of thiophene (n ¼ 3, 4, 5). Moreover, the relatively high
absorption coefficients (emax in the range of 27,890–71,633
[Mꢂ1 cmꢂ1]) could be calculated from Beer’s law equation
with the same dilute concentration of the copolymers in
chloroform, which assures the copolymers can absorb
enough photons.
Figure 3(b) shows the optical absorption spectra of thin
films of the copolymers and P3HT. The thin film absorption
spectra are similar in shape to those in dilute solution. From
solution to film, the spectra of PTTMT, PDTTMT, and
PTTTMT exhibit a very large red shift, which are 44–72 nm
and 52–72 nm for the maximum absorption and absorption
edge, respectively, as listed in Table 2. It implies that the
polymer chains containing planar segments of OThn and PM
can lead to the better aggregated configuration in solid state,
which could facilitate charge transport for photovoltaic appli-
cation.4 The optical bandgaps (Eg,opt) derived from the
absorption edges of the film spectra were nearly the same
(1.73, 1.72, and 1.76 eV) for the three copolymers. As
expected, the low Eg,opt of OTh-based D-A alternating high
planar conjugated copolymers, which is lower than that of
poly(3-alkylthiophene) homopolymer (ꢀ2.0 eV)27,28 is deter-
mined by the strong p-p stacking of OTh and the ICT absorp-
tion bands.
Electrochemical Properties
Figure 4 shows the CV diagrams of the polymers using
TBAPF6 as supporting electrolyte in acetonitrile solution
with platinum button working electrodes, a platinum wire
counter electrode and an Ag/AgNO3 reference electrode
under the N2 atmosphere. Ferrocene was used as the inter-
nal standard. All the polymers showed reversible or partly
FIGURE 3 Normalized absorption spectras of the synthesized
copolymers and P3HT (a) in chloroform solutions with the con-
centration of 10ꢂ5 mol/L; (b) films spin-coated from a 10 mg/mL
chloroform solution. [Color figure can be viewed in the online
issue, which is available at www.interscience.wiley.com.]
reversible redox behavior, which was attributed to the high
onset
Ox
electrical activity. The onset oxidation potentials (E
) of
the three copolymers are observed in the range of 0.45–0.74
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TABLE 2 Optical and Electrochemical Data of the Copolymers PTTMT, PDTTMT and PTTTMT
In Solutiona
In Filmb
abs
abs
onset
Ox
onset
k
[nm]
kedge
k
kedge
E
E
Electrochem.
Opticalc
max
max
Red
Polymer
(emax [Mꢂ1 cmꢂ1])
[nm]
[nm]
[nm]
(V)/HOMO (eV)
(V)/LUMO (eV)
Eg,ec (eV)
Eg,opt (eV)
PTTMT
PDTTMT
PTTTMT
a
523 (71633)
500 (27890)
528 (60497)
665
638
649
572
572
572
717
706
721
0.74/ꢂ5.44
0.64/ꢂ5.38
0.45/ꢂ5.15
c
ꢂ1.26/ꢂ3.44
ꢂ1.32/ꢂ3.38
ꢂ1.28/ꢂ3.42
2.00
1.96
1.73
1.73
1.76
1.72
1 ꢃ 10ꢂ5 M in anhydrous chloroform.
The optical bandgap (Eg,opt) was obtained from absorption edge.
b
Spin-coated from a 10mg/mL chloroform solution.
very similar to that of PTTMT. It indicates that alternating
OThn with different number of thiophene units have slight
effect on the reduction potential of the copolymers, so the
LUMO energy level is mainly determined by the PM-based
acceptor unit, and the relatively low LUMO energy levels of
the three copolymers result from the stronger reduction of
PM acceptor unit. On the other hand, the HOMO energy lev-
els of the copolymers behave quite differently. The HOMO
energy levels of the copolymers PTTMT, PDTTMT, and
PTTTMT are ꢂ5.44, ꢂ5.38, and ꢂ5.15 eV, respectively, which
are gradually increased with the enhanced conjugated length
of OThn. The higher conjugated length of OThn resulted in a
higher HOMO energy level.
HOMO and the LUMO were calculated by using the density
functional theory (DFT) as approximated by the B3LYP func-
tional and employing the 6-31G* basis set (see in Fig. 6).
Ab initio calculations on the model compound for the three
copolymers show that the small torsion angle between OThn
and A lead to the coplanar molecules, which can increase the
electrons delocalization within the molecules and enhance
the p–p stacking of the molecules in the solid state. It is in
agreement with the great red shift of copolymers between in
solutions and thin films. The HOMO state densities of the
three copolymers were distributed extent over the p-conju-
gation systems. With the increasing conjugated length of
OThn, the HOMO state densities of the copolymers are gradu-
ally located on the OThn. However, the electron density of
LUMO was stably localized on the PM segment (A). It
was also noticed that with an increase in the number of
thiophene units, the HOMO energy level increased from
ꢂ5.47 eV to ꢂ5.30 eV, ꢂ5.17 eV for PTTMT, PDTTMT,
PTTTMT, respectively, whereas the LUMO energy level
remained relatively unchanged because of its highly localized
state around the PM segment. Meanwhile, the HOMO and
LUMO energy level values of molecular orbital distribution
The HOMO energy level of the donor polymers is very im-
portant for high-performance photovoltaic cell. First, the
polymers should have good air stability with HOMO energy
level being below the air oxidation threshold (ca. ꢂ5.2 eV).30
Second, the open circuit potential (Voc) value for the photo-
voltaic cell is determined by the difference between the
HOMO energy level of donor and LUMO energy level of
acceptor. The relatively low HOMO level of the polymers can
allow a high-open circuit potential (Voc) value for the photo-
voltaic cell.31 A complete picture of the band structure of the
copolymers is presented in Figure 5. The first dashed line
indicates the threshold for air stability, and the second
dashed line represents the threshold value for an effective
charge transfer from the copolymers to PCBM (ꢂ4.3 eV).32
Both the HOMO energy levels and LUMO energy levels of the
copolymers are close to the ideal range. All the copolymers
show relatively low HOMO energy level, which is useful to
improve the air stability and Voc of photovoltaic device. In
comparison with the corresponding optical bandgaps esti-
mated from the edge of the solid-state absorption spectra
(Table 2), the values of bandgap are generally lower than
that from electrochemical determination (Table 2), which can
be explained in part by the exciton binding energy of conju-
gated copolymers.33,34
To further understand the optical and electrochemical prop-
erties of PTTMT, PDTTMT, and PTTTMT, we have carried out
the molecular orbital distribution calculations of the basic
structure units (OThn-A model) of the three copolymers
using quantum mechanical package Gaussian 03. The opti-
mum geometry and electron-state-density distribution of the
FIGURE 4 Cyclic voltammetry curves of PTTMT, PDTTMT and
PTTTMT films on platinum electrode in 0.1 mol/L TBAPF6 in
CH3CN solution, at a scan rate of 100 mV/s.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Photovoltaic Properties
To investigate the photovoltaic properties of the copolymers,
the BHJ PVCs with a structure of ITO/PEDOT-PSS/copoly-
mers: PCBM/LiF/Al were fabricated, where the copolymers
were used as donors and PCBM as the acceptor.7 The weight
ratios between copolymers and PCBM in the active layer
have obviously effect on the photovoltaic performances, so
BHJ PVCs with varied weight ratios (copolymers:PCBM from
1:1 to 1:4) in the active layers were investigated (the results
were list in Table 3), and the optimal weight ratios is 1:3
(copolymers:PCBM, w:w). It is known that solvents used for
the preparation of the active layer have a strong impact on
the performance of the cell.35 Here, we choose chloroform
for the three copolymers to obtain the films with the rela-
tively good quality. The devices were characterized in
the dark and under solar simulator AM1.5 (100 mW/cm2)
with simultaneous recording of their current-voltage charac-
teristics. The current-voltage characteristics of the photo-
voltaic cell based on PTTMT:PCBM, PDTTMT:PCBM, and
PTTTMT:PCBM with weight ratio (1:3 w/w) showed the best
performance, which are presented in Figure 7. The photovol-
taic parameters of the photovoltaic cells are summarized in
Table 3.
FIGURE 5 Band diagram for accepting PCBM and donor PTTMT,
PDTTMT and PTTTMT copolymers. Dashed lines indicate the
thresholds for air stability (ꢂ5.2 eV) and effective charge transfer
PCBM (ꢂ4.3 eV). [Color figure can be viewed in the online issue,
which is available at www.interscience.wiley.com.]
calculations are well consistent with those from electrochem-
ical test. In addition, the location of HOMO and LUMO state
density mainly on different segments indicated that the ICT
transition is accompanied by ICT from OThn moiety to PM
could be expected.
The cells based on PTTMT:PCBM (1:3 w/w) showed an open
circuit voltage (Voc) of 0.94 V, a short circuit current (Jsc) of
FIGURE 6 Molecular orbital surfaces of the HOMO and LUMO of OThn-A model obtained at B3 LYP/6-31G* level. [Color figure can
be viewed in the online issue, which is available at www.interscience.wiley.com.]
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ARTICLE
PCE (%)a
TABLE 3 Characteristic Current-Voltage Parameters from Device Testing at Standard AM 1.5G Conditions and Blend Films
Roughness of AFM Measurement
Polymer/PCBM (w/w ratio)
Voc (v)a
Jsc (mA/cm2)a
FFa
RMS (nm)b
PTTMT
PTTMT
PTTMT
PDTTMT
PTTTMT
PTTTMT
PTTTMT
a
1:2
1:3
1:4
1:3
1:2
1:3
1:4
0.84
0.94
0.86
0.90
0.72
0.78
0.8
1.84
1.78
1.57
2.39
2.12
2.54
1.21
0.3
–
0.47
0.61
0.50
0.99
0.56
0.81
0.41
0.36
0.37
0.46
0.37
0.41
0.43
4.34
–
1.64
–
4.29
–
b
Photovoltaic properties of copolymer/PCBM-based devices spin-coated
from chloroform solutions for PTTMT (1:3 w/w), PDTTMT (1:3 w/w) and
PTTTMT (1:3 w/w).
Root mean-square (RMS) roughness from AFM measurement.
1.78 mA/cm2, a fill factor (FF) of 0.36, giving a power con-
version efficiency (PCE) of 0.61%. The relative low Jsc of the
cells may be caused by the obvious aggregation in solid
state of PTTMT:PCBM blend film which leads to hindered
charge transport. However, greatly enhanced device perform-
ance was obtained for the other two cells based on
PDTTMT:PCBM (1:3 w/w) with Voc of 0.90, Jsc of 2.39 mA/
cm2, FF of 0.46, and PCE of 0.99%, and PTTTMT:PCBM (1:3
w/w) with Voc of 0.78, Jsc of 2.54 mA/cm2, FF of 0.41, and
PCE of 0.81%, respectively. The reason for the improvement
of Jsc and PCE could be explained by better p–p stacking of
PDTTMT and PTTTMT in the solid state, which may cause
the large red shift of the absorption edge kedge in film com-
paring with that in solution. Furthermore, as discussed
above, the Voc is directly governed by the difference between
the energy levels of the HOMO of the donor and the LUMO
of the acceptor. The Voc of the three copolymers were
increased with the drop of the HOMO energy level of the
copolymers. Anyhow, all the three copolymers have a satisfy-
ing Voc (around 0.80 V), which is higher than the
P3HT:PCBM-based device (0.6 V).25
PDTTMT-rich islands can be seen more clearly in Figure
8(b), which lead to an improvement of the short circuit cur-
rent for the PDTTMT:PCBM cell (2.39 mA/cm2). In the case
of PTTTMT:PCBM blend film [Fig. 8(c)], an even stronger
conjugated length (OThn, n ¼ 5) was brought into the co-
polymer, the morphology has been further modulated.
PTTTMT:PCBM blend film shows a larger PTTTMT-rich
islands and higher RMS of 4.29 nm, which indicate that self-
organized aggregates of PDTTMT via p–p stacking interac-
tions and better phase separation of blend films have formed
(whose evidence could also be found in the specific features
of the absorption spectra of the polymer thin films).
Figure 9 shows the TEM images of PTTMT:PCBM and
PTTTMT:PCBM blend films, where the dark areas are attrib-
uted to PCBM domains because the electron scattering den-
sity of PCBM is higher than that of conjugated polymer.28
From the TEM images of PTTMT:PCBM blend films [Fig.
9(a)], the copolymer with shorter conjugated length (OThn,
n ¼ 3), we can see more clearly that the substantial PCBM
grain-aggregation with the size distribution around 150 nm
and the obvious phase separation have formed. With the
Film morphology of the active layer has been found to be
one of the key elements in determining the PCE of PVC.36 To
gain better insight into what might be controlling the short
circuit current, AFM and TEM were used to examine the sur-
face topography of the blend films. Figure 8(a–c) shows the
AFM height images of PTTMT:PCBM, PDTTMT:PCBM,
PTTTMT:PCBM blend films with the same weight ratio (1:3
w/w). As is clearly evidenced by AFM, the PTTMT:PCBM
blend film shows a relatively high coarse surface with the
root mean square (RMS) of 4.34 nm [Fig. 8(a) and Table 3],
and substantial PCBM grain-aggregation are almost homoge-
neously dispersed in the PTTMT matrix, which results in a
large-scale phase separation, decreased diffusional escape
probability for mobile charge carriers and hence increased
recombination, leading to the low short circuit of
PTTMT:PCBM cell (1.78 mA/cm2). Compared with the
PTTMT:PCBM blend film, the blend film of PDTTMT consist-
ing of a longer conjugated length (OThn, n ¼ 4) and PCBM
shows a relative flat surface with the RMS of 1.64 nm and
lower degree of grain-aggregation, and the formation of
FIGURE 7 Current-voltage characteristics of copolymer photo-
voltaic cells based on PTTMT, PDTTMT and PTTTMT under illu-
mination of AM 1.5, 100 mW/cm2 white light.
DEVELOPMENT OF POLYMER PHOTOVOLTAIC CELLS, LI ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 8 Topography image obtained by tapping-mode AFM showing the morphology of the blend films spin-coated from chlo-
roform for (a) PTTMT/PCBM (w/w, 1:3) (size 5 lm ꢃ 5 lm); (b) PDTTMT/PCBM (w/w, 1:3) (size 5 lm ꢃ5 lm); (c) PTTTMT/PCBM
(w/w, 1:3) (size 5 lm ꢃ5 lm).
conjugated length increased (OThn, n ¼ 5), the TEM images
of PTTTMT:PCBM blend films [Fig. 9(b)] exhibits a finer net-
works and island-like structure allowing better charge sepa-
ration and transport, and also the Jsc of PTTTMT:PCBM (1:3
w/w) based photovoltaic device were enhanced to 2.54 mA/
cm2. However, the possibility of recombination of holes and
electrons in the network with high interfacial areas could
also be enhanced. This might be the reason why the FF values
of these devices have not reached their utmost values.29
The photovoltaic performance of PTTMT, PDTTMT, and
PTTTMT showed general increased PCEs with 0.61, 0.99,
and 0.81%, respectively. However, the relative low Jsc still
limits the efficiency of the PV cells. To increase the Jsc, the
following methods can be expected: optimization of the
device structures by introducing titanium suboxide as an
optical spacer and hole blocker,37 selecting varied and
mixture solvents for better-connected percolated networks
and nanoscale phase separation of the active layer, and
thermal annealing for better self-organization and balancing
the carrier mobility of the device.
It has been recently reported that chemical similarity
between the functional groups attached to the donor poly-
mer and the fullerene can effectively affect the formation of
films with uniform and stable nanophase morphologies.38 In
the present case, the significant morphological difference
between the three blend films suggests that the interaction
between the copolymer and PCBM can be efficiently adjust
by changing the conjugated length or the structure of the
copolymers. Therefore, it is a good method for tuning the
morphology between copolymers and PCBM by simply
changing the conjugated length or the moiety of copolymers.
Better controlling the morphology will be helpful to obtained
high photovoltaic performance.
FIGURE 9 TEM images of PTTMT:PCBM and PTTTMT:PCBM blend films with blend ratio of 1:3 w/w.
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ARTICLE
CONCLUSIONS
9 (a) Peng, Q.; Xu, J.; Zheng, W. X. J Polym Sci Part A: Polym
Chem 2009, 47, 3399–3408; (b) Yen, W. C.; Pal, B.; Yang, J. S.;
Hung, Y. C.; Lin, S. T.; Chao, C. Y.; Su, W. F. J Polym Sci Part
A: Polym Chem 2009, 47, 5044–5056.
We have designed and synthesized three novel oligothiphene
D-A conjugated copolymers consisting of PM as acceptor
coupled to OThn with different conjugated length. Optical
property investigations unequivocally indicate that these
new copolymers exhibit strong p–p stacking and the long
wavelength ICT absorption bands, which lead to an extension
and broad absorption with the optical bandgaps in the range
of 1.72–1.76 eV. Both the electrochemical properties and mo-
lecular orbital distribution calculations revealed that the
HOMO and LUMO energy levels of the resulting copolymers
can be fine-tuned by increasing the conjugated length of
comonomer OThn. Meanwhile, the morphology of the blend
films containing copolymers and PCBM can be controlled by
varying the conjugated length of OThn. The photovoltaic
devices based on the copolymers show the PCE in the range
of 0.61–0.99%. Although the power conversion efficiencies
for these unoptimized photovoltaic devices are still not suffi-
ciently high, their tunable electronic properties provide an
understanding on how the polymer structures affect the
device characteristics. Further improvements in the photo-
voltaic characteristics could potentially be achieved by ther-
mal annealing and/or the introduction of other acceptor ma-
terial such as PC71BM.
10 Zhang, S. M.; Guo, Y. M.; Fan, H. J.; Liu, Y.; Chen, H. Y.;
Yang, G. W.; Zhan, X. W.; Liu, Y. Q.; Li, Y. F.; Yang, Y. J Polym
Sci Part A: Polym Chem 2009, 47, 5498–5508.
11 Wu, P. T.; Kim, F. S.; Champion, R. D.; Jenekhe, S. A.
Macromolecules 2008, 41, 7021–7028.
12 Zhou, E. J.; Yamakawa, S. P.; Tajima, K.; Yang, C. H.; Hashi-
moto, K. Chem Mater 2009, 21, 4055–4061.
13 Leclerc, N.; Michaud, A.; Sirois, K.; Morin, J. F.; Leclerc, M.
Adv Funct Mater 2006, 16, 1694–1704.
14 Halls, J. J. M.; Pichler, K.; Friend, R. H.; Moratti, S. C.;
Holmes, A. B. Appl Phys Lett 1996, 68, 3120–3122.
15 Li, Y. W.; Xue, L. L.; Li, H.; Li, Z. F.; Xu, B.; Wen, S. P. Macro-
molecules 2009, 42, 4491–4499.
16 Zhang, S. M.; Fan, H. J.; Liu, Y.; Zhao, G. J.; Li, Q. K.; Li, Y.
F.; Zhan, X. W. J Polym Sci Part A: Polym Chem 2009, 47,
2843–2852.
17 Heeney, M.; Bailey, C.; Genevicius, K.; Shkunov, M.; Spar-
rowe, D.; Tierney, S; McCulloch, I. J Am Chem Soc 2005, 127,
1078–1079.
This work was supported by the State Key Development Pro-
gram for Basic Research of China (Grant No. 2009CB623605),
the National Natural Science Foundation of China (Grant No.
20874035), the 111 Project (Grant No. B06009), and the Pro-
ject of Jilin Province (20080305). Yaowen Li would like to thank
the financial support of the Project 20091008 from Graduated
Innovation Fund of Jilin University.
18 Lai, M. H.; Chueh, C. C.; Chen, W. C.; Wu, J. L.; Chen, F. C. J
Polym Sci Part A: Polym Chem 2009, 47, 973–985.
19 Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.;
Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCullough, I.; Ha,
C.; Ree, M. Nat Mater 2006, 5, 197–203.
20 Xia, P. F.; Lu, J. P.; Kwok, C. H.; Fukutani, H.; Wong, M. S.;
Tao, Y. J Polym Sci Part A: Polym Chem 2009, 47, 137–148.
21 Bauerle, P. In Electronic Materials: The Oligomer Approach;
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