Synthesis and photovoltaic properties of copolymers based on benzo[1,2-b:4,5-b′]dithiophene and thiophene with electron-withdrawing side chains

Article abstract of DOI:10.1002/pola.24799

Six alternating conjugated copolymers (PL1-PL6) of benzo[1,2-b:4,5- b′]dithiophene (BDT) and thiophene, containing electron-withdrawing oxadiazole (OXD), ester, or alkyl as side chains, were synthesized by Stille coupling reaction. The structures of the polymers were confirmed, and their thermal, optical, electrochemical, and photovoltaic properties were investigated. The introduction of conjugated electron-withdrawing OXD or formate ester side chain benefits to decrease the bandgaps of the polymers and improve the photovoltaic performance due to the low steric hindrance of BDT. Bulk heterojunction polymer solar cells (PSCs) were fabricated based on the blend of the as-synthesized polymers and the fullerene derivative [6,6]-phenyl-C ₆₁-butyric acid methyl ester (PC₆₁BM) in a 1:2 weight ratio. The maximum power conversion efficiency of 2.06% was obtained for PL5-based PSC under the illumination of AM 1.5, 100 mW/cm².

Full text of DOI:10.1002/pola.24799

Synthesis and Photovoltaic Properties of Copolymers Based on
Benzo[1,2-b:4,5-b0]dithiophene and Thiophene with
Electron-Withdrawing Side Chains
YUJUAN NIE,1 BIN ZHAO,1,2 PENG TANG,1 PENG JIANG,1 ZONGFANG TIAN,1 PING SHEN,1,2 SONGTING TAN1,2
1College of Chemistry and Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education,
Xiangtan University, Xiangtan 411105, People’s Republic of China
2Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province, Xiangtan University,
Xiangtan 411105, People’s Republic of China
Received 27 March 2011; accepted 23 May 2011
DOI: 10.1002/pola.24799
Published online 14 June 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Six alternating conjugated copolymers (PL1–PL6) of
benzo[1,2-b:4,5-b0]dithiophene (BDT) and thiophene, containing
electron-withdrawing oxadiazole (OXD), ester, or alkyl as side
chains, were synthesized by Stille coupling reaction. The
structures of the polymers were confirmed, and their thermal,
optical, electrochemical, and photovoltaic properties were
investigated. The introduction of conjugated electron-with-
drawing OXD or formate ester side chain benefits to decrease
the bandgaps of the polymers and improve the photovoltaic
performance due to the low steric hindrance of BDT. Bulk
heterojunction polymer solar cells (PSCs) were fabricated
based on the blend of the as-synthesized polymers and the
fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester
(PC61BM) in a 1:2 weight ratio. The maximum power conversion
efficiency of 2.06% was obtained for PL5-based PSC under the
2
C
V
illumination of AM 1.5, 100 mW/cm .
2011 Wiley Periodicals,
Inc. J Polym Sci Part A: Polym Chem 49: 3604–3614, 2011
KEYWORDS: benzo[1,2-b:4,5-b0]dithiophene; bulk heterojunction;
conjugated polymers; electrochemistry; electron-withdrawing
side chains; oxadiazole; photophysics; polymer solar cell
INTRODUCTION Polymer solar cells (PSCs) have attracted
much attention due to their unique features such as low cost,
light weight, solution processibility, fast roll-to-roll production,
and applications in large area flexible panels.1–4 So far, the
most successful PSCs are bulk heterojunction (BHJ) devices,
which use an electron-rich polymer and electron-deficient
fullerene as the photoactive layer.5 The most commonly
used PSCs consist of poly(3-hexylthiophene):[6,6]-phenyl C60-
butyric acid methyl ester (P3HT:PC61BM) and achieved power
conversion efficiency (PCE) around 5%.6 To exploit high-
performance photovoltaic materials, great efforts have been
devoted to low-bandgap polymers and broad-absorption poly-
mers.7,8 Although significant advances have been achieved in
improving the PCEs of PSCs by using new active layer materi-
als,9–16 there is still much to do before the realization of prac-
tical applications of PSCs. Limiting factors include the narrow
absorption band of conjugated polymers, the low photon–elec-
tron conversion rate, the unmatched molecular energy levels,
and the low-charge-carrier mobility of the polymer photovol-
taic materials.
energy levels. In the past 2 years, a series of new donor–
acceptor low-bandgap polymers based on BDT have been
reported.14–22 For example, the PSCs based on a BDT copoly-
mer PBDTTT have achieved a series of high PCE values in
the range of 5–7%,17–22 and a new world record of 9.2% was
presented in the later investigation.23 Usually, these low
bandgap polymers were constructed as donor–acceptor (D-A)
alternating copolymers, which were BDT as donor units and
electron-deficient groups as acceptor units. Almost all of the
electron-deficient groups were inlayed in the main chain of
the polymers to reduce the bandgaps. However, there are few
reports about incorporating electron-deficient moieties as side
chains for reducing the bandgap of the polymers.
Oxadiazole (OXD)-based polymers have been extensively
studied in polymer light-emitting diodes but rarely in
PSCs.24–27 In our previous study, the polymers based on fluo-
rene and thiophene with OXD moieties were synthesized and
applied in PSCs.28 However, the bandgaps of the polymers
were larger than 2.4 eV, which limited the exploitation of
sunlight and the application in high-performance PSCs. In
this study, we report the synthesis and photophysical, elec-
trochemical, and photovoltaic properties of the alternating
copolymers PL1–PL6 (Scheme 1) with electron-withdrawing
side chains. First, the polymers of PL1, PL2, and PL3,
Recently, benzo[1,2-b:4,5-b0]dithiophene (BDT)-based conju-
gated polymers exhibited promising photovoltaic properties in
BHJ PSCs because of their inherent small steric hindrance,
high hole mobility and tunable frontier molecular orbital
Correspondence to: B. Zhao (E-mail: xtuzb@163.com)
C
V
Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 3604–3614 (2011) 2011 Wiley Periodicals, Inc.
3604
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
SCHEME 1 Molecular structures of PL1–PL6.
containing the electron-rich BDT in the main chain and the
electron-deficient OXD moiety in the side chain, were designed
and synthesized to decrease the bandgaps of the resulted
polymers because a strong donor BDT unit was used to sub-
stitute a weak donor fluorine moiety. To evaluate the steric
hindrance effect of side chain on the effective conjugation
length and the absorption spectrum, an additional thiophene
unit with/without hexyl group was introduced to the back-
bone of the polymers PL2 and PL3. Simultaneously, butyl for-
mate was introduced to PL5 as side chain in order to survey
the effects of different electron-withdrawing groups. Finally,
nonconjugated butyl acetate and hexyl side chains were intro-
duced in PL4 and PL6, respectively, to further investigate the
effects of (non-)conjugated electron-withdrawing side chains
on the optoelectronic properties of the polymers.
hexyl bromide, 3-hexylthiophene, tetra-n-butylammonium
hexafluorophosphate, and n-butyllithium were purchased from
Pacific ChemSource or Alfa Aesar. THF and toluene were dis-
tilled from sodium-benzophenone to remove oxygen and water.
DMF was dried and distilled under reduced pressure. All other
commercially available materials were used as received unless
noted otherwise. Column chromatography was carried out on
silica gel (200–300 mesh).
Analytical Measurements
1H NMR and 13C NMR spectra were measured with Bruker
AVANCE 400 spectrometer. MALDI-TOF mass spectrometric
measurements were performed on Bruker Bifiex b MALDI-
TOF. FTIR spectra were obtained on Perkin-Elmer Spectra
One. UV–vis spectra and photoluminescence (PL) spectra of
the polymers were measured on Perkin-Elmer Lamada 25
spectrometer and Perkin-Elmer LS-50 luminescence spec-
trometer, respectively. Thermogravimetric analyses (TGA)
were performed under nitrogen at a heating rate of 20 ꢀC/
min with Netzsch TG 209 analyzer, and differential scanning
calorimetric measurements (DSC) were recorded with TA
EXPERIMENTAL
Materials and Characterization
Methyl 4-hydroxybenzoate, thiophene-3-carboxylic acid, thio-
phene-3-acetic acid, 3-hexylthiophene, 1-bromooctane, 2-ethyl-
SYNTHESIS AND PHOTOVOLTAIC PROPERTIES OF COPOLYMERS, NIE ET AL.
3605

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
ꢀ
DSCQ10 instrument at a heating rate of 20 C/min. The aver-
ture, then stirred over 6 h until the color changed to orange.
The resulting mixture was cooled to room temperature,
diluted with water, and extracted with chloroform. The
organic extraction was dried with anhydrous sodium sulfate
and evaporated in vacuum. The crude product was purified
on silica gel chromatography using petroleum ether as
eluent, 2.0 g light yellow oil to yield (50% yield).
age molecular weight and polydispersity index (PDI) of the
polymers were determined using Waters 1515 gel permea-
tion chromatography (GPC) analysis with THF as eluent and
polystyrene as standard. Cyclic voltammetry (CV) was
conducted on an electrochemistry workstation (CHI660A,
Chenhua Shanghai) with the polymer film on Pt plate as the
working electrode, Pt wire as the counter electrode, and
saturated calomel electrode (SCE) as a reference electrode in
a 0.1 M tetra-n-butylammonium hexafluorophosphate aceto-
nitrile solution at a scan rate of 100 mV/s.
1HNMR (400 MHz, CDCl3, d/ppm): 7.49–7.46 (d, 2 H), 7.38–
7.35 (d, 2 H), 4.18–4.16 (m, 4 H), 1.9–1.7 (m, 2 H), 1.7–1.38
(m, 16 H), 1.1–0.91 (m, 12 H).
2,6-Bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-
Fabrication and Characterization of PSCs
b:4,5-b0]dithiophene (M1). Compound
2
(1.238 g,
The photovoltaic cells were constructed in the traditional
sandwich structure through several steps. The indium tin
oxide (ITO)-coated glass substrates were cleaned by succes-
sive ultrasonic treatment in acetone and isopropyl alcohol,
then treated with a nitrogen-oxygen plasma for 5 min.
2.78 mmol) was dissolved in 40 mL of anhydrous THF under
an inert atmosphere, then n-butyl lithium (2.80 mL, 2.5 M in
hexane) was added dropwise to the mixture and stirred for
ꢀ
1 h at ꢂ78 C, then stirred for 0.5 h at ambient temperature.
The mixture was cooled to ꢂ78 ꢀC again, and trimethyltin
chloride (4.2 mL, 2 M in hexane) was added. After removing
the cooling bath, the reactant was stirred at ambient temper-
ature for 2 h. Subsequently, it was poured into 200 mL of
cool water and extracted by ether for three times. The
organic layer was washed by water for two times and then
dried by anhydrous MgSO4. After removing solvent under
vacuum, the residue was recrystallized by isopropanol. M1
(1.71 g, 80% yield) was obtained as needle crystal.
Poly(3,4-ethylene
dioxythiophene)-poly(styrenesulfonate)
(PEDOT:PSS, from Bayer AG) was spincoated on a cleaned
ITO/glass substrate giving a film with thickness about
40 nm, which was measured by Ambios Technology XP-2
surface profilometer, and it was dried subsequently at
150 ꢀC for 30 min. The photoactive layer of polymer and
PC61BM was prepared by spin-coating the chlorobenzene
solution of the polymer and PC61BM (1:2, w/w) with the
polymer concentration of 1ꢀ2 mg/mL on the ITO/PEDOT:PSS
electrode and dried at 150 C for 30 min, in a nitrogen-filled
glove box. The cathode of devices, consisting of 10 nm of Ca
and 100 nm of Al, was thermally deposited on the top of
photoactive layer at 5 ꢁ 10ꢂ4 Pa. The active area of the
device is 9 mm2. Current density–voltage (J–V) characteris-
tics were measured by a Keithley 2602 source meter under
100 mW/cm2 irradiation using a 500 W Xe lamp equipped
with a global AM 1.5 filter for solar spectrum simulation,
and the incident light intensity was calibrated using a
standard Si solar cell. The measurement of monochromatic
incident photon-to-current conversion efficiency (IPCE) was
performed using a Zolix DCS300PA Data acquisition system.
1H NMR (400 MHz, CDCl3, d/ppm): 7.51 (s, 2 H), 4.34–4.28
(d, 4 H), 1.94–1.86 (m, 2 H), 1.64–1.27 (m, 16 H), 1.00–0.90
(m, 12 H), 0.43 (s, 18 H). 13C NMR (100 MHz, CDCl3, d/
ppm): 143.3, 140.3, 133.8, 132.9, 128.0, 75.66, 40.72, 30.58,
29.26, 23.95, 23.14, 14.12, 11.32. MALDI-TOF MS (m/z):
calcd for C32H54O2S2Sn2 772.16; found, 772.13.
2-(2,5-Di(4-hexylthiophen-2-yl)thiophen-3-yl)-5-(4-(octylox-
y)phenyl)-1,3,4-oxadiazole (4). M2 (1.00 g, 1.95 mmol),
compound 3 (1.8 g, 3.9 mmol), and 30 mL anhydrous DMF
were mixed under an inert atmosphere. Then Pd(PPh3)4
(50 mgꢀ) was added, and the mixture was refluxed for 36 h
at 100 C. The reaction mixture was cooled to room temper-
ature, extracted with chloroform, and washed with distilled
water. After the solvent was removed, the residue was puri-
fied by silica gel column chromatography with petroleum
ether/dichloromethane (2:1 by volume). The product 4
(95% yield) was obtained as yellow solid.
Synthesis of Monomers and Polymers
Synthesis of the Monomers
Synthetic routes of the monomers and polymers were shown
in Scheme 1.
Benzo[1,2-b: 4,5-b0]dithiophene-4,8-dione (1),17,18 2-(2,5-bibro-
mothiophene-3-yl)-5-(4-(octyloxy)phenyl)-1,3,4-oxadiazole
(M2),28 3-hexyl-5-tributylstannyl-thiophene (3),29 and 2-(2,5-bis
(5-bromothiophen-2-yl)thiophen-3-yl)-5-(4-(octyloxy)phenyl)-
1,3,4-oxadiazole (M4)28 were synthesized according to litera-
ture procedures. All other compounds were synthesized
following procedures described below.
1H NMR (400 MHz, CDCl3, d/ppm): 7.94–7.91 (d, 2 H), 7.7
(s, 1 H), 7.4 (s, 1 H), 7.09 (s, 1 H), 7.04(s, 1 H), 6.98–6.95 (d,
2 H), 6.88(s, 1 H), 4.04–3.9 (t, 2 H), 2.64–2.5 (t, 4 H), 1.68–
1.59 (m, 6 H), 1.41–1.25 (m, 22 H), 0.94–0.89 (t, 9 H).
2-(2,5-Bis(5-bromo-4-hexylthiophen-2-yl)thiophen-3-yl)-5-
(4-(octyloxy)phenyl)-1,3,4-oxadiazole (M3). N-Bromosucci-
nimide (NBS; 0.57 g, 3.2 mmol) dissolved in 10 mL of
anhydrous DMF, then it was added dropwise to a mixture of
4 (1.03 g, 1.5 mmol) in anhydrous CHCl3 (15 mL) in the
dark. The mixture was stirred for 2 h, then poured into
100 mL of water, extracted with chloroform, and washed
with water. The crude product was purified on silica gel
4,8-Bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b0]dithiophene (2).
A mixture of benzo[1,2-b:4,5-b0]dithiophene-4,8-dione (2.0 g,
9.0 mmol), zinc dust (1.3 g, 20 mmol), ethanol (8.0 mL), and
aqueous sodium hydroxide solution (30 mL, 20%) was
stirred and refluxed for 2 h under an inert atmosphere, and
then 2-ethylhexyl bromide (4.84 mL) and a catalytic amount
of tetrabutylammonium bromide were added into the mix-
chromatography using
a
petroleum ether/ethyl acetate
3606
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
mixture (5:1 by volume) as eluent. M3 (98% yield) was
obtained as golden yellow solid.
131.7, 118.8, 111.2, 72.79, 65.03, 30.69, 30.59, 19.20, 18.60,
13.61, 13.35.
1H NMR (400 MHz, CDCl3, d/ppm): 7.93–7.90 (d, 2 H), 7.4
(s, 1 H), 7.59 (s, 1 H), 7.27(s, 1 H), 6.98–6.96 (d, 2 H), 6.93
(s, 1 H), 4.04–3.9 (t, 2 H), 2.89–2.5 (t, 4 H), 2.04–1.58 (m, 6
H), 1.40–1.24 (m, 22 H), 0.94–0.86 (t, 9 H). 13C NMR (100
MHz, CDCl3, d/ppm): 162.1, 143.3, 142.9, 136.7, 130.2,
128.7, 125.6, 124.3, 115.0, 111.5, 77.30, 77.18, 76.98, 76.67,
68.32, 31.77, 31.57, 29.72, 29.65, 29.57, 29.29, 29.18, 29.12,
28.96, 28.86, 28.25, 27.82, 26.71, 25.98, 22.61, 22.53, 17.50,
17.28, 14.01, 13.51. MALDI-TOF MS (m/z): calcd for
2,5-Dibromo-3-hexylthiophene (M7). NBS (6.6 g, 37.1
mmol) and 3-hexyl thiophene (3 g, 17.8mmol) were reacted
by following the similar method as for M5, M7 was obtained
(98% yield).
1H NMR (400 MHz, CDCl3, d/ppm): 6.78 (s, 1 H), 2.52–2.48
(t, 2 H), 1.54–1.49 (m, 2 H), 1.33–1.29 (m, 6 H), 0.91–0.87
(t, 3 H). 13C NMR (100 MHz, CDCl3, d/ppm): 143.0, 131.0,
110.4, 108.0, 31.65, 29.61, 29.57, 28.88, 22.65, 14.14.
C40H50Br2N2O2S3 847.14; found, 847.07.
Synthesis of the Copolymer
Butyl 2-(thiophen-3-yl)acetate (5). Thiophene-3-acetic acid
(2.5 g, 17.6 mmol), n-butyl alcohol (13 mL, excessive), ben-
zene (40 mL), and several drops of H2SO4 were carefully
added into a 100-mL flask, and then the mixture was refluxed
to remove water via a Dean-Stark trap for 24 h. The solution
was cooled and poured into a saturated solution of NaHCO3
and subsequently extracted with dichloromethane and washed
by water three times. The organic extraction was dried with
anhydrous MgSO4 and evaporated in vacuum. The crude prod-
uct was purified on silica gel chromatography using petroleum
ether/ethyl acetate mixture (5:1 by volume) as eluent. Com-
pound 5 (96% yield) was obtained as light brown oil.
Poly{2,6-(4,8-bis(2-ethylhexyloxy))benzo[1,2-b:4,5-b0]
dithiophene-alt-2,5-[(thio-phen-3-yl)-5-(4-(octyloxy)phenyl)-
1,3,4-oxadiazole]} (PL1). M1 (193 mg, 0.25 mmol) and M2
(128 mg, 0.25 mmol) were dissolved in 25 mL of toluene,
and the solution was flushed with argon for 15 min, then 30
mg of Pd(PPh3)4 was added into the solution. The mixture
was again flushed with argon for 30 min. The polymerization
was carried out at 110 ꢀC for 2 days. Finally, the reaction
mixture was cooled to room temperature and slowly added
to methanol (100 mL). The precipitate was collected by fil-
tration from methanol and further purified by Soxhlet extrac-
tion with methanol, hexane, and chloroform in sequence. The
chloroform fraction was evaporated by rotary evaporation
and drying under vacuum for 1 day. The final product was
obtained as deep red solid (174 mg, 87% yield).
1H NMR (400 MHz, CDCl3, d/ppm): 7.3–7.28 (d, 1 H), 7.15
(s, 1 H), 7.05–7.04 (d, 1 H), 4.12–4.08 (t, 2 H), 3.65 (s, 2 H),
1.65–1.5 (m, 2 H), 1.41–1.31 (m, 2 H), 0.98–0.90 (t, 3 H).
1H NMR (400 MHz, CDCl3, d/ppm): 7.40–8.50(br, 4 H), 6.75–
7.10 (br, 3 H), 3.90–4.22 (br, 6 H), 0.79–1.77 (br, 45 H). GPC
Mn ¼ 23.0 k, Mw/Mn ¼ 4.2.
Butyl 2-(2,5-dibromothiophen-3-yl)acetate (M5). NBS
(6.1 g, 33 mmol) was dissolved in 25 mL of anhydrous DMF,
then the solution was added dropwise in the dark to a mix-
ture of compound 5 (3.25 g, 16.4 mmol) in CHCl3 (25 mL)
Poly{[2,6-(4,8-bis(2-ethylhexyloxy))benzo[1,2-b:4,5-b0]
dithiophene]-alt-2,5-[(2,5-di(4-hexylthiophen-2-yl)thiophen-
3-yl)-5-(4-(octyloxy) phenyl)-1,3,4-oxadiazole]} (PL2). By
following the similar method as for PL1, PL2 was synthe-
sized with M1 (193 mg, 0.25 mmol) and compound M3
(211 mg, 0.25 mmol). The final product was obtained as
deep red solid (175 mg, 58% yield).
ꢀ
solvent. The reactant was stirred for 22 h at 60 C and then
cooled to room temperature; the production was extracted
with chloroform and washed with water. The crude product
was purified on silica gel chromatography using a petroleum
ether/dichloromethane mixture (1:1, by volume) as eluent,
and light brown oil M5 (95% yield) was obtained at last.
1H NMR (400 MHz, CDCl3, d/ppm): 7.26 (s, 1 H), 4.14–4.1 (t,
2 H), 3.56 (s, 2 H), 1.65–1.58 (m, 2 H), 1.41–1.32 (m, 2 H),
0.95–0.91 (t, 3 H). 13C NMR (100 MHz, CDCl3, d/ppm):
169.5, 134.6, 131.42, 110.9, 110.6, 65.03, 34.98, 30.59,
19.09, 13.65.
1H NMR (400 MHz, CDCl3, d/ppm): 6.9–8.0 (br, 9 H), 3.9–
4.3(br, 6 H), 2.6–3.0 (br, 6 H), 1.29–2.1 (br, 44 H), 0.67–1.19
(br, 21 H). GPC Mn ¼ 11.0 k, Mw/Mn ¼ 2.1.
Poly{[2,6-(4,8-bis(2-ethylhexyloxy))benzo[1,2-b:4,5-b0]
dithiophene]-alt-2,5-[(2,5-di(thiophen-2-yl)thiophen-3-yl)-
5-(4-(octyloxy) phenyl) -1,3,4-oxadiazole]} (PL3). PL3 was
synthesized with M1 (193 mg, 0.25 mmol) and M4 (169 mg,
0.25 mmol). The final product was obtained as deep red
solid (159 mg, 66% yield).
2,5-Dibromothiophene-3-carboxylic acid (6). By following
the similar method as for M5, compound 6 was synthesized
from NBS (6.42 g, 36.1 mmol) and thiophene-3-carboxylic
acid (2.2 g, 17.2 mmol). The product was obtained as yellow
oil and used without further purification.
1H NMR (400 MHz, CDCl3, d/ppm): 7.1–7.7 (br, 11 H), 3.5–
4.2 (br, 6 H), 1.25–2.1 (br, 30 H), 0.8–1.2 (br, 15 H). GPC Mn
¼ 5.4 k, Mw/Mn ¼ 3.4.
Butyl 2,5-dibromothiophene-3-carboxylate (M6). Compound
6 (2.5 g, 8.74 mmol) and n-butyl alcohol (6.5 mL, excessive)
were reacted by following the similar method as for com-
pound 5. M6 was obtained as light yellow oil (98% yield).
Poly{2,6-(4,8-bis(2-ethylhexyloxy))benzo[1,2-b:4,5-b0]
dithiophene-alt-2,5-[butyl thiophene-3-acetate]} (PL4). PL4
was synthesized with M1 (193 mg, 0.25 mmol) and M5
(88.5 mg, 0.25 mmol). The final product was obtained as
deep red solid (114 mg, 71% yield).
1H NMR (400 MHz, CDCl3, d/ppm): 7.36 (s, 1 H), 4.32–4.26
(t, 2 H), 1.79–1.58 (m, 2 H), 1.53–1.42 (m, 2 H), 1.0–0.84 (t,
3 H). 13C NMR (100 MHz, CDCl3, d/ppm): 160.63, 132.1,
SYNTHESIS AND PHOTOVOLTAIC PROPERTIES OF COPOLYMERS, NIE ET AL.
3607

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
1H NMR (400 MHz, CDCl3, d/ppm): 7.40–8.50 (br, 2 H),
6.75–7.10 (br, 1 H), 3.90–4.22 (br, 8 H), 0.79–1.77 (br, 37 H).
GPC Mn ¼ 12.0 k, Mw/Mn ¼ 3.1.
stability of the six copolymers are good enough for the appli-
cation of PSCs.
Photophysical Properties
Poly{2,6-(4,8-bis(2-ethylhexyloxy))benzo[1,2-b:4,5-b0]
dithiophene-alt-2,5-[butyl thiophene-3-carboxylate]}
PL5. PL5 was synthesized with M1 (193 mg, 0.25 mmol)
and M6 (85 mg, 0.25 mmol). The final product was obtained
as deep red solid (99 mg, 63% yield).
The photophysical properties of the polymers were investi-
gated by UV–vis and fluorescence spectroscopy in diluted
CHCl3 solutions and solid films on quartz plates. The correla-
tive properties data of the polymers were summarized in Ta-
ble 2. As shown in Figure 2(a), the absorption peaks (kmax
)
1H NMR (400 MHz, CDCl3, d/ppm): 7.40–8.50 (br, 2 H),
6.75–7.10 (br, 1 H), 3.90–4.22 (br, 6 H), 0.79–1.77 (br, 37 H).
GPC Mn ¼ 10.3 k, Mw/Mn ¼ 1.7.
of PL1, PL2, and PL3 in the visible region are ascribed to
the p–p* transition derived from the polymer backbone,30
and those in the UV region are attributed to the absorption
of the side chains. The kmax of PL3 solution and thin film is
476 and 520 nm, respectively, which are red-shifted about
29 and 33 nm compared with the polyfluorene analogue P3
in our previous work.28 It implies that the polymer based on
the BDT should be more appropriate for PSCs. Simultane-
ously, the maximum absorption peaks of PL1 solution and
thin film are 490 and 520 nm, respectively. It is interesting
that the kmax of PL1 is red-shifted over 100 nm than that of
the polyfluorene analog P2.28 This is because 4,9-bisalkoxy-
BDT without substituent on 1, 3, 5, and 7 positions is less
distortion and increase the effective conjugation length of
the polymer backbone. Compared with PL1 and PL3, the
kmax of PL2 is obviously blue-shifted because of the steric
hindrance between long alkyl side chain and OXD side chain,
which results in distortion of the polymer backbone and a
decrease in effective conjugation length.
Poly{2,6-(4,8-bis(2-ethylhexyloxy))benzo[1,2-b:4,5-b0]
dithiophene-alt-2,5-[3-hexyl-thiophene]} PL6. PL6 was syn-
thesized with M1 (193 mg, 0.25 mmol) and M7 (81 mg,
0.25 mmol). The polymer was obtained as deep red solid
(102 mg, 67% yield).
1H NMR (400 MHz, CDCl3, d/ppm): 7.40–8.50 (br, 2 H),
6.75–7.10 (br, 1 H), 3.90–4.22 (br, 6 H), 0.79–1.77 (br, 41 H).
GPC Mn ¼ 10.8 k, Mw/Mn ¼ 1.3.
RESULTS AND DISCUSSION
Synthesis and Characterization
Six alternating copolymers, based on BDT and thiophene
with OXD, esters or hexyl alkyl side chains, were synthesized
by the Stille coupling reaction. The general synthetic strategy
for monomers and polymers is outlined in Scheme 2. The
key monomers, M1, M2, M3, and M4, were synthesized
according to the reported methods.27 M5, M6, and M7 were
obtained by esterification and bromination. Their chemical
structures were confirmed with 1H NMR, 13C NMR, and
MALDI-TOF. All the polymers were readily dissolved in chlo-
roform, THF, and toluene at room temperature. Table 1 sum-
marized the polymerization results and thermal properties
of the polymers. The number-average molecular weight (Mn)
of PL1 is higher than that of PL2 due to the larger steric
hindrance alkly side chains of PL2. Although the steric hin-
drance of PL3 is smaller than that of PL1, the Mn of PL3 is
the lowest with a larger the PDI, which would be attributed
to the inferior solubility of M4. The other three polymers
PL4, PL5, and PL6 show similar Mn. The Mn of polymers,
PL1-PL6, is in the range from 5 to 23 kg/mol, with the PDIs
ranging from 1.3 to 4.2.
As shown in Figure 2(b), PL4, PL5, and PL6 exhibit the
absorption peaks in the visible region attributed to the p–p*
transition, but there are not obvious absorption peaks in the
UV region. The maximum absorption peaks of PL5 and PL6
are same at 471 nm in diluted CHCl3 solution, which are
obviously red-shifted than that of PL4. The absorption spec-
tra of the polymers PL4, PL5, and PL6 in thin films are
broadened and red-shifted in comparison with those in
diluted CHCl3 solutions due to effect of interchain p-stacking.
Moreover, compared with PL4 and PL6, the absorption peak
of PL5 in thin film is red-shifted to 516 nm, and the absorp-
tion band is obviously broadened due to conjugated effect of
the butyl formate on thiophene and stronger intermolecular
interaction of PL5. The absorption onset wavelengths of
PL1–PL6 are 614, 605, 623, 577, 597, and 583 nm corre-
sponding to the optical bandgaps of 2.02, 2.05, 1.99, 2.15,
2.08, and 2.13 eV, respectively. The optical bandgaps of the
polymers are summarized in Table 2.
Thermal Properties
The thermal properties of the polymers were investigated by
DSC and TGA measurements. The glass transition tempera-
tures (Tg) of PL1–PL6 are in the range of 99–117 ꢀC, and
corresponding results are summarized in Table 1. As shown
in Figure 1, the TGA curves reveal that the degradation tem-
peratures (Td) of 5% weight loss of PL1–PL6 are 355, 320,
PL spectra of the polymers in the diluted CHCl3 solutions are
shown in Figure 3. All emission data were obtained by the
excitation at the absorption peaks derived from the polymer
backbones, and the PL peaks are listed in Table 2. In CHCl3
solution, PL1 exhibits only one maximum emission peak at
595 nm. With the additional thiophene units in the main
chain, the PL spectra of PL2 and PL3 show blue-shift and
broad shoulder emission peaks compared with PL1, and the
maximum emission peaks of PL2 and PL3 are blue-shift to
567 and 569 nm, respectively. On the other hand, the PL
spectra of PL4 and PL6 show similar narrow emission peaks
ꢀ
355, 227, 350, and 351 C, respectively. The Td of PL4 is the
lowest in the six polymers because of the weak interaction
of acetic ester side chain with the conjugated main chain,
and the relatively lower Td of PL2 is owing to the high-den-
sity hexyl alkyl side chain compared with PL1 and PL3.
However, it is obvious that the heat resistance and thermal
3608
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
SCHEME 2 Synthetic routes of the monomers and the corresponding polymers.
(ꢃ535 nm) with weak shoulder emission. Compared with
Electrochemical Properties
PL4 and PL6, PL5 exhibits broader PL spectrum and the
maximum emission peak red-shifted to 563 nm, due to the
conjugated effect of butyl formate side chain. All the results
clearly indicate that the photophysical properties of these
polymers could be tuned by changing side chain groups.
However, all the polymers have no detectable PL emission in
thin films due to fluorescence quenching.
The electrochemical properties of the polymers were investi-
gated by CV to estimate the redox behavior of the polymers
and estimate the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) energy lev-
els. For comparison, optical bandgaps (Eogpt) of the polymers
were calculated from the onset wavelength of optical absorp-
tion and listed in Table 2. The onset potentials for oxidation
SYNTHESIS AND PHOTOVOLTAIC PROPERTIES OF COPOLYMERS, NIE ET AL.
3609

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Molecular Weights and Thermal Properties of the
Polymers
Polymers Yields (%) Mn (ꢁ103)a PDI Tg (ꢀC)b Td (ꢀC)c
PL1
PL2
PL3
PL4
PL5
PL6
a
87
58
66
71
63
67
23.0
11.0
4.2
2.1
3.4
3.1
1.7
1.3
102
112
100
99
355
320
355
227
350
351
5.40
12.0
10.3
10.8
99
117
Determined by GPC in THF based on polystyrene standards.
Determined by DSC at scan rate of 20 ꢀC/min under nitrogen.
b
c
Decomposition temperature, determined by TGA in nitrogen, based
on 5% weight loss.
(Eox) and reduction (Ered) of the polymers were measured
by CV, where SCE electrode was used as the reference elec-
trode, and the HOMO and LUMO energy levels and electro-
chemical bandgaps (Eg) of polymers were calculated with
the following empirical equations.31,32
FIGURE 1 TGA curves of the polymers at scan rate of 20 ꢀC/
min under nitrogen atmosphere. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
mers based on fluorene.28 Furthermore, the bandgap of PL3
is lower than PL1 due to the additional thiophene units in
the conjugated main chain. On the other side, the HOMO
energy levels of PL4, PL5, and PL6 are ꢂ5.55, ꢂ5.43, and
ꢂ5.50 eV, respectively. When compared with PL4 and PL6,
PL5 shows a higher HOMO energy level. The LUMO energy
level of PL4, PL5, and PL6 are –3.30, –3.43, and –3.31 eV,
and the corresponding Egs are 2.25, 2.00, and 2.19 eV,
respectively. It is obviously that conjugated butyl formate
side chain could effectively reduce the bandgap of the poly-
mer due to the strong abilities of electron-withdrawing. The
LUMO energy levels of all the polymers are higher than that
of PCBM, so these polymers are promising candidates for the
effective applications of PSCs.
EHOMO ¼ ꢂeðEox þ 4:4Þ ðeVÞ
ELUMO ¼ ꢂeðEred þ 4:4Þ ðeVÞ
Eg ¼ EHOMO ꢂ ELUMO ðeVÞ
All the results were summarized in Table 2. Figure 4 shows
the CV curves of the polymers on the Pt electrode. It can be
seen that all the polymers show reversible reduction proc-
esses except for PL3, but the oxidation processes of all the
polymers are irreversible. The HOMO energy levels of PL1,
PL2, and PL3 are –5.42, –5.35, and –5.44 eV, respectively.
When compared with PL1 and PL3, PL2 with hexyl side
chain on thiophene shows a higher HOMO energy level,
which indicates the planarity of PL2 main chain is disrupted
because of the steric hindrance of OXD and hexyl side chains.
Correspondingly, the LUMO levels of PL1, PL2, and PL3 are
–3.42, –3.31, and –3.45 eV, and the calculated Eg are 2.00,
2.04, and 1.99 eV, respectively. It can be seen that the Eg is
very close to the corresponding Eogpt, and the bandgaps of
these polymers are obviously lower than that of the poly-
Photovoltaic Properties
The BHJ photovoltaic cells of the polymers were prepared
and investigated by blending the polymers and PC61BM. The
used device structure was ITO/PEDOT:PSS (40 nm)/poly-
mer:PC61BM (1:2, w/w)/Ca (10 nm)/Al (100 nm). The PCE
TABLE 2 Optical and Electrochemical Properties of the Polymers
Solutiona
kmax (nm)
Filma kmax (nm)
Polymers
UV
PL
UV
Eogpt (eV)b
HOMO (eV)
LUMO (eV)
Eg (eV)
PL1
PL2
PL3
PL4
PL5
PL6
a
490
450
476
464
471
471
595
567
569
535
563
538
520
481
520
494
516
503
2.02
2.05
1.99
2.15
2.08
2.13
ꢂ5.42
ꢂ5.35
ꢂ5.44
ꢂ5.55
ꢂ5.43
ꢂ5.50
ꢂ3.42
ꢂ3.31
ꢂ3.45
ꢂ3.30
ꢂ3.43
ꢂ3.31
2.00
2.04
1.99
2.25
2.00
2.19
Measured in chloroform solution.
b
Bandgap estimated from the onset wavelength of optical absorption.
3610
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
and the fill factor (FF) were calculated according to the fol-
lowing equations:
PCE ¼ Voc ꢁ Jsc ꢁ FF=Pmax
FF ¼ Vmax ꢁ Jmax=Voc ꢁ Jsc
where Voc (V), Jsc (mA/cm2), FF, and Pmax (mW/cm2) are the
open-circuit potential, short-circuit current density, fill factor,
and incident-light power, respectively. Vmax (V) and Jmax
(mA/cm2) are the voltage and current density at the point of
the maximum power output, respectively.
The input photon to converted current efficiency (IPCE)
curves of the devices based on PL1–PL6 is shown in Figure
5. It can be seen that all the polymers showed the broad
IPCE curves from 350 to 650 nm, which are coincident
FIGURE 2 UV–vis absorption spectra of the polymers (a) PL1–
PL3 and (b) PL4–PL6 in chloroform solutions and in the films.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
FIGURE 4 Cyclic voltammograms of the polymer films on plati-
num electrode in 0.1 mol/L Bu4NPF6 acetonitrile solution.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
FIGURE 3 Photoluminescence spectra of the polymers in the
diluted CHCl3. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary. com.]
SYNTHESIS AND PHOTOVOLTAIC PROPERTIES OF COPOLYMERS, NIE ET AL.
3611

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
known, Voc of photovoltaic cells is related to the difference
of the LUMO of the electron acceptor and the HOMO of the
electron donor, therefore, the lower HOMO of the polymers
could be beneficial to a higher Voc.33 As shown in Table 2,
the HOMO levels of these polymers are lower than ꢂ5.3 eV,
so the Voc of all cells should be higher than 1.0 V in theory,
which is higher than that of the cell based on P3HT. When
compared with PL2, the HOMO levels of PL1 and PL3 are
lower, so the Vocs of PL1 (0.67 V) and PL3 (0.75 V) are
higher than that of PL2 (0.55 V). The HOMO levels of PL4
and PL6 are lower than that of PL5, but the Vocs of them
(0.57 and 0.59 V) are lower than that of PL5 (0.75 V). As
shown in Figure 7, the bicontinuous phase separations of
PL4: PC61BM and PL6: PC61BM blend films are very uncon-
spicuous, and the surface root mean squares (RMS) of them
(0.7 nm) are higher than that of PL5: PC61BM blend film
(0.5 nm). Therefore, the lower Vocs of the devices based on
PL4 and PL6 could be explained by the smaller interfacial
area and larger RMS.
FIGURE 5 IPCE curves of the polymer solar cells based on PL1–PL6.
[Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
The Jsc is determined by the amount of absorbed light and
the internal conversion efficiency.7,33,34 The Jsc values for
PL1, PL2, and PL3 are 7.40, 1.62, and 6.88 mA/cm2, respec-
tively. The lowest Jsc value of PL2 could be attributed to the
disrupted planarity of the polymer backbone by the large
steric hindrance between the OXD side chain and thiophene
alkyl side chains. Simultaneously, another important factor
for low Jsc value is attributed to the weak bicontinuous
phase separation of PL2: PC61BM blend film because more
alkyl side chains of PL2 could lead to excellent miscibility of
PC61BM in the polymer PL2.
with the absorption band of the polymers. The maximum
IPCE value of PL1 is ꢃ33% at 410 nm, and the maximum
IPCE value of PL2 is only ꢃ17% at 412 nm. Although the
IPCE values of PL3 and PL5 are lower than that of PL1
between 420 and 600 nm, the maximum IPCE values of PL3
and PL5 are 48% at 387 nm and 58% at 363 nm, respec-
tively, which are all larger than the maximum IPCE value of
PL1 (33% at 410 nm). The higher IPCE values might be
attributed to more effective intramolecular cascade energy
transfer from BDT donor to OXD or butyl formate side chain
acceptor via conjugated polymer backbone.
Moreover, though the absorption spectra of PL1 and PL3 are
similar, the Jsc value of PL3 (6.88 mA/cm2) is lower than
that of PL1 (7.4 mA/cm2), which may be ascribed to the
lower Mn of PL3.35 Simultaneously, the bicontinuous phase
separation of PL1: PC61BM and PL3: PC61BM blend films
shows obviously difference, and the surface root mean
square (RMS) of PL3: PC61BM (1.5 nm) is lower than that of
PL1: PC61BM (1.9 nm), which may increase the charge car-
rier mobility.36 Therefore, the PCE of the cell based on PL3
is higher than that of PL1. It is likely that the photovoltaic
performance of PL3 could be further improved by increasing
Figure 6 shows J–V curves of the PSCs, and the correspond-
ing data (Voc, Jsc, FF, and PCE) are listed in Table 3. As we
the Mn of PL3. When compared with P2 (Voc ¼ 0.68 V, Jsc
¼
0.15 mA/cm2, FF ¼ 0.29, and PCE ¼ 0.03%) and P3 (Voc¼
0.73 V, Jsc ¼ 6.27 mA/cm2, FF ¼ 0.32 and PCE ¼ 1.49%) in
TABLE 3 Photovoltaic Performance of the PSCs Based on the
Polymers/PC61BM Blends
Polymers/PC61BM
(1 : 2, w/w)
Jsc
(mA/cm2)
PCE
(%)
Voc (V)
FF
PL1
PL2
PL3
PL4
PL5
PL6
0.67
0.55
0.75
0.57
0.75
0.59
7.40
1.62
6.88
4.26
6.62
4.93
0.36
0.29
0.38
0.33
0.41
0.30
1.77
0.25
1.98
0.81
2.06
0.86
FIGURE 6 J–V curves of the photovoltaic cells based on PL1–
PL6 under the illumination of AM 1.5, 100 mW/cm2. [Color fig-
ure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
3612
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
FIGURE 7 AFM phase images of PL1–PL6. All the images are 2 lm ꢁ 2 lm.
our previous report,28 PL1 and PL3 show more excellent
photovoltaic performance due to the less steric hindrance of
BDT in the polymer backbone. The Jsc values for PL4, PL5,
and PL6 are 4.26, 6.62, and 4.93 mA/cm2, respectively. On
one hand, the absorption spectrum of PL5 is obviously
broader than that of PL4 and PL6 [Fig. 2(b)], which is favor-
able for light-harvesting. On the other hand, the PL5: PC61BM
blend film shows better bicontinuous phase separations than
that of PL4: PC61BM and PL6: PC61BM blend films, which
leads to a larger interfacial area for exciton charge separation
(Fig. 7). As a result, the Jsc value and PCE of the cell based on
PL5 (2.06%) is higher than that of the other two polymers.
The Jsc and Voc of the cells based on PL4 and PL6 are similar,
which indicates there is no significant different effects on the
photovoltaic performance of the polymers between the non-
conjugated butyl acetate and hexyl group side chains. It can be
concluded that electron-withdrawing group could effectively
decrease the bandgap and improve photovoltaic performance
of the polymer only when it is attached onto the polymer back-
bone as a conjugated side chain.
bandgap of the polymers and improve the photovoltaic per-
formance of the corresponding cells. The PCEs of the devices
based on PL3 and PL5 reached 1.98 and 2.06%, respectively.
The results indicate that PL3 and PL5 were more promising
for the potential application of photovoltaic cells.
This work was supported by the National Nature Science
Foundation of China (Nos. 50973092, 51003089, 21004050) and
Specialized Research Fund for the Doctoral Program of Higher
Education of China (Nos. 20094301120005, 20104301110003).
REFERENCES AND NOTES
1 Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv Funct
Mater 2001, 11, 15–26.
2 Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem Rev 2007,
¨
107, 1324–1338.
3 Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv Mater 2009,
21, 1323–1338.
4 Brabec, C. J.; Gowrisanker, S.; Halls, J. J. M.; Laird, D.; Jia,
S. J.; Williams, S. P. Adv Mater 2010, 22, 3839–3856.
CONCLUSIONS
5 Yu, G.; Gao, J.; Hummelen, J. C.; Wudi, F.; Heeger, A. J. Sci-
In summary, six alternating benzo[1,2-b: 4,5-b0]dithiophene
and thiophene derivatives containing different side chains
have been synthesized. These polymers show good solubility
and thermal stabilities. When compared with analogous poly-
fluorene, the polymers based on BDT show less steric hin-
drance even though the side chain is rigid OXD. The incorpo-
ration of electron-withdrawing OXD or butyl formate in
conjugated side chain would effectively decrease the
ence 1995, 270, 1789–1791.
6 Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery,
K.; Yang, Y. Nat Mater 2005, 4, 864.
7 Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D.
E. Adv Mater 2007, 19, 1551–1566.
8 Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T. -Q.;
Dante, M.; Heeger, A. J. Science 2007, 317, 222–225.
SYNTHESIS AND PHOTOVOLTAIC PROPERTIES OF COPOLYMERS, NIE ET AL.
3613

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
9 Zhang, J.; Cai, W. Z.; Huang, F.; Wang, E.; Zhong, C. M.; Liu,
S. J.; Wang, M.; Duan, C. H.; Yang, T. B.; Cao, Y. Macromole-
cules 2011, 44, 894–901.
22 Son, H. J.; Wang, W.; Xu, T.; Liang, Y. Y.; Wu, Y.; Li, G.; Yu,
L. P. J Am Chem Soc 2011, 133, 1885–1894.
23 Service, R. F. Science 2011, 332, 293.
10 Chu, T. Y.; Lu, J. P.; Beaupre´, S.; Zhang, Y. J.; Pouliot, J.-R.;
Wakim, S.; Zhou, J. Y.; Leclerc, M.; Li, Z.; Ding, J. F.; Tao, Y.
J Am Chem Soc 2011, 133, 4250–4253.
24 Schulz, B.; Kaminorz, Y.; Brehmer, L. Synth Met 1997, 84,
449–450.
25 Yu, W. L.; Meng, H.; Pei, J.; Lai, Y. H.; Chua, S. J.; Huang, H.
11 Woo, C. H.; Beaujuge, P. M.; Holcombe, T. W.; Lee, O. P.;
W. Chem Commum 1998, 18, 1957–1958.
Fre´chet, J. M. J. J Am Chem Soc 2010, 132, 15547–15549.
26 Yu, W. -L.; Meng, H.; Pei, J.; Huang, W.; Li, Y. F.; Heeger, A.
12 Miller, S. A.; Stuart, A. C.; Womick, J. M.; Zhou, H. X.; You,
J. Macromolecules 1998, 31, 4838–4844.
W.; Moran, A. M. J Phys Chem C 2011, 115, 2371–2380.
27 Meier, M.; Buchwald, E.; Karg, S. Posch, P.; Greczmiel, M.;
¨
13 He, F.; Wang, W.; Chen, W.; Xu, T.; Darling, S. B.; Strzalka,
Strohriegl, P.; Rieb, W. Synth Met 1996, 76, 95–99.
J.; Liu, Y.; Yu, L. P. J Am Chem Soc 2011, 133, 3284–3287.
28 Zhao, B.; Liu, D. X.; Peng, L.; Li, H.; Shen, P.; Xiang, N.; Liu,
14 Najari, A.; Beaupre´, S.; Berrouard, P.; Zou, Y. P.; Poulio, J. -
R.; Lepage, P. C.; Leclerc, C. M. Adv Funct Mater 2011, 21,
718–728.
Y. J.; Tan, S. T. Eur Polym J 2009, 45, 2079–2086.
29 Liu, Y. J.; Xiang, N.; Feng, X. M.; Shen, P.; Zhou, W. P.;
Weng, C.; Zhao, B.; Tan, S. T. Chem Commun 2009, 2499–2501.
15 Price, S. C.; Stuart, A. C.; Yang, L. Q.; Zhou, H. X.; You, W.
30 Buvat, P.; Hourquebie, P. Macromolecules 1997, 30,
J Am Chem Soc 2011, 133, 4625–4631.
2685–2692.
16 Piliego, C.; Holcombe, T. W.; Douglas, J. D.; Woo, C. H.; Beauj-
31 Li, X. Z.; Zeng, W. J.; Zhang, Y.; Hou, Q.; Yang, W.; Cao, Y.
uge, P. M.; Fre´chet, J. M. J. J Am Chem Soc 2010, 132, 7595–7597.
Eur Polym J 2005, 41, 2923–2933.
17 Liang, Y. Y.; Feng, D. Q.; Wu, Y.; Tsai, S. -T.; Li, G.; Ray, C.;
32 Agrawal, A. K.; Jenekhe, S. A. Chem Mater 1996, 8,
Yu, L. P. J Am Chem Soc 2009, 131, 7792–7799.
579–589.
18 Hou, J. H.; Park, M. H.; Zhang, S. Q.; Yao, Y.; Chen, L. -M.;
33 Mihailetchi, V. D.; Blom, P. W. M.; Hummelen, J. C.;
Li, J. -H.; Yang, Y. Macromolecules 2008, 41, 6012–6018.
Rispens, M. T. J Appl Phys 2003, 94, 6849–6854.
19 Chen, H. Y.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G.
W.; Yang, Y.; Yu, L. P.; Wu, Y.; Li, G. Nat Photonics 2009, 3,
649–653.
34 Ravirajan, P.; Haque, S. A.; Durrant, J. R.; Bradley, D. D. C.;
Nelson, J. Adv Funct mater 2005, 15, 609–614.
35 Goh, C.; Joseph Kline, R.; McGehee, M. D. Appl Phys Lett
20 Hou, J. H.; Chen, H. -Y.; Zhang, S. Q.; Chen, R. I.; Yang, Y.;
2005, 86, 122110.
Wu, Y.; Li, G. J Am Chem Soc 2009, 131, 15586–15587.
36 Mikroyannidis, J. A.; Tsagkournos, D. V.; Sharma, S. S.;
21 Szarko, J. M.; Guo, J. C.; Liang, Y. Y.; Lee, B. D.; Rolczynski,
B. S.; Strzalka, J.; Xu, T.; Loser, S.; Marks, J. C.; Yu, L. P.; Chen,
L. X. Adv Mater 2010, 22, 5468–5472.
Vijayc, Y. K.; Sharma, G. D.
4679–4688.
J Mater Chem 2011, 21,
3614
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA