A new series of random conjugated copolymers containing 3,4-diphenyl-maleimide and thiophene units for organic photovoltaic cell applications

Article abstract of DOI:10.1016/j.polymer.2012.04.014

A series of random conjugated copolymers (labeled PMLTQT, PMLT2T, and PMLT3T) consisting of 3,4-diphenyl-maleimide and various thiophene derivatives has been designed and synthesized via Stille cross-coupling for application in polymer solar cells. These copolymers were readily soluble in common organic solvents, thermally stable from 405 to 437 °C upon heating, and exhibited good absorption in the UV and visible regions from 300 to 650 nm. The intensities of the PL emission spectra of these copolymers in a solid film were dramatically quenched by the addition of 50 wt% [6,6]-phenyl C₆₁ butyric acid methyl ester (PC₆₁BM). Their electrochemical properties indicated that the highest occupied molecular orbital levels of these copolymers were in the range of -5.63-5.73 eV, characteristic of better air stability and a high open-circuit voltage (V_oc) suitable for application to photovoltaic cells. Bulk heterojunction photovoltaic devices composed of an active layer of electron-donor copolymers blended with the electron acceptor PC₆₁BM or [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) at a weight ratio of 1:3 were investigated. The photovoltaic device containing PMLT3T and PC₇₁BM (1:3, w/w) as the active layer afforded the best performance among these copolymers, with a V_oc of 0.74 V, J_sc of 7.4 mA cm^-2 and a PCE of 1.20% under AM 1.5 G simulated solar light. Copyright

Full text of DOI:10.1016/j.polymer.2012.04.014

Polymer 53 (2012) 2334e2346
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
A new series of random conjugated copolymers containing 3,4-diphenyl-
maleimide and thiophene units for organic photovoltaic cell applications
,
b
c
c
Li-Hsin Chan a , Shiang-Yin Juang , Ming-Chou Chen , Yu-Jou Lin
*
a Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Nantou 54561, Taiwan, ROC
b Department of Applied Chemistry, National Chi Nan University, Nantou 54561, Taiwan, ROC
c Department of Chemistry, National Central University, Chung-Li 32504, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of random conjugated copolymers (labeled PMLTQT, PMLT2T, and PMLT3T) consisting of 3,4-
diphenyl-maleimide and various thiophene derivatives has been designed and synthesized via Stille
cross-coupling for application in polymer solar cells. These copolymers were readily soluble in common
organic solvents, thermally stable from 405 to 437 ꢀC upon heating, and exhibited good absorption in the
UV and visible regions from 300 to 650 nm. The intensities of the PL emission spectra of these copol-
ymers in a solid film were dramatically quenched by the addition of 50 wt% [6,6]-phenyl C61 butyric acid
methyl ester (PC61BM). Their electrochemical properties indicated that the highest occupied molecular
orbital levels of these copolymers were in the range of ꢁ5.63e5.73 eV, characteristic of better air
stability and a high open-circuit voltage (Voc) suitable for application to photovoltaic cells. Bulk heter-
ojunction photovoltaic devices composed of an active layer of electron-donor copolymers blended with
the electron acceptor PC61BM or [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) at a weight ratio of
1:3 were investigated. The photovoltaic device containing PMLT3T and PC71BM (1:3, w/w) as the active
layer afforded the best performance among these copolymers, with a Voc of 0.74 V, Jsc of 7.4 mA cmꢁ2 and
a PCE of 1.20% under AM 1.5 G simulated solar light.
Received 17 January 2012
Received in revised form
1 April 2012
Accepted 11 April 2012
Available online 19 April 2012
Keywords:
Organic photovoltaic cell
3,4-diphenyl-maleimide
Random copolymer
Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
1. Introduction
[12e14], the mismatch or narrow absorption of polythiophenes (PTs)
with the solar spectra might hinder further enhancement of the PCE.
During the past decade, polymer solar cells (PSCs) have been
attracting extensive attention because they are flexible, lightweight,
and potentially low-cost materials suitable for large-area
manufacturing, and they present a renewable energy alternative to
fossil energy [1e6]. In the past two years, significant progress has
been achieved on polymer/fullerene hybrid systems based on the
concept of bulk heterojunction (BHJ) [7e9]. For most BHJ cells, the
photoactive layer of a PSC is based on a blend of an electron-donating
polymer and an electron-accepting fullerene derivative [6,6],-phenyl
C61 butyric acid methyl ester (PC61BM) or [6,6]-phenyl C71 butyric
acid methyl ester (PC71BM). Regioregular poly(3-hexylthiophene)
(rr-P3HT) is one such electron-donating polymer that is most widely
investigated in PSC applications [10,11]. A high power conversion
efficiency (PCE) of around 4% has been achieved with a BHJ-structure
PSC based on rr-P3HT and PC61BM [11]. Although the rr-P3HT:
PC61BM blend system represents encouraging progress in PSCs
To further improve the absorption properties of polymers, the
intramolecular charge transfer interactions between electron-
donor and electron acceptor moieties have been extensively
applied to the development of low bandgap
with better PSC performance. Among several
p
-conjugated polymers
-conjugated D-A
p
polymer systems, aromatic heterocycles such as benzothiadiazole-
[15e19], diketopyrrolopyrrole- [20,21], quinoxaline- [22e24], and
thienopyrazine-based [25e27] derivatives have been widely used
as electron-accepting moieties in absorbing light of longer wave-
lengths. Maleimide is an electron-deficient heterocyclic ring and
similar to the larger members of phthalimide, naphthalimide, and
peryleneimide, which are known as n-type organic semiconducting
materials for organic transistors [28e31]. Furthermore, N-alkyl
derivatives of 3,4-bis(4-bromophenyl)maleimide BrML (Scheme 1)
are ideal monomers that can form polyarylenes with high molec-
ular weights through various types of polycondensation reactions.
In several studies, various N-alkyl derivatives of BrML have been
utilized in the preparation of 3,4-diphenyl-maleimide-based poly-
aryl macromolecules [32e36]. Recently, we have developed effi-
cient and highly fluorescent small molecules of arylamine-
* Corresponding author. Tel.: þ886 49 2910960 4908; fax: þ886 49 2912238.
E-mail address: lhchan@ncnu.edu.tw (L.-H. Chan).
0032-3861/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2012.04.014

L.-H. Chan et al. / Polymer 53 (2012) 2334e2346
2335
R
N
O
O
C8H17
S
S
S
S
S
S
C8H17
m
C8H17
n
C8H17
PMLTQT
H17C8
C8H17
S
Sn Cl
Br
Sn
S
Br
S
S
S
Br
n-BuLi
THF
S
Sn
Br
O
H17C8
C8H17
Pd(PPh3)4
toluene
BTT
S
R
N
Br
R
N
R
O
O
O
O
B B
N
O
O
O
C8H17
O
O
PdCl2(dppf)
KOAc
DMSO
Pd(PPh3)4
K2CO3
toluene
S
S
Br
Br
O B
B O
C8H17
H17C8
TML
O
O
BrML
BML
*
R =
NBS
HOAc
CH2Cl2
R
R
S
N
Br
N
O
O
O
O
Br
S
2T
S
S
S
S
S
S
S
C8H17
H17C8
C8H17
H17C8
S
Br
Br
Sn
Sn
S
n
m
BrTML
Pd(PPh3)4
toluene
PMLT2T
S
S
Sn
Sn
Br
Br
S
S
3T
Pd(PPh3)4
toluene
R
N
O
O
S
S
S
S
S
C8H17
H17C8
S
S
m
n
PMLT3T
Scheme 1. Synthetic routes for monomers as well as copolymers PMLTQT, PMLT2T, and PMLT3T.

2336
L.-H. Chan et al. / Polymer 53 (2012) 2334e2346
substituted 3,4-diphenyl-maleimide for use in red organic light-
emitting diodes (OLEDs) [37e40], in addition to orange-yellow to
red fluorescent polymers derived from maleimides [41e43]. We
demonstrated that N-alkyl-3,4-diphenyl-maleimide (ML) resem-
bles such electron-deficient heterocyclic arenes as benzoselena-
diazole, naphthoselenadiazole, and thiophene dioxide, and can be
mixture was poured into iced water and extracted with DCM. After
drying over magnesium sulfate, the extract was concentrated. The
solution mixture was purified by flash silica gel column (hexanes/
triethylamine, 10:1) to yield a yellow viscous oil (1.81 g, 86%). 1H
NMR (CDCl3, 300 MHz,
1.59e1.51 (m, 4H), 1.29e1.24 (m, 20H), 0.89e0.85 (m, 6H), 0.37 (s,
18H). 13C NMR (CDCl3, 75 MHz,
/ppm): 142.97, 137.43, 136.96,
d
/ppm): 7.02 (s, 2H), 2.52 (t, 4H, J ¼ 7.8 Hz),
involved in low bandgap
p-conjugation in the formation of long-
d
wavelength-emissive fluorene copolymers. More recently, we
have developed randomly configured PT derivatives that show
promising results for application in PSCs [44,45].
135.34, 32.05, 31.08, 30.89, 29.72, 29.54, 29.39, 28.90, 22.83, 14.27.
HRMS calculated for C30H54S2Sn2: 718.1711, found 718.1696.
Based on the above discussion, a series of random copolymers
containing 3,4-diphenyl-maleimide and different thiophene deriva-
tives has been designed and synthesized via Stille coupling with
random configurations for application in PSCs. To generate copoly-
mers for use in PSCs, we first utilized a 3,4-diphenyl-maleimide
derivative copolymerizing with quaterthiophene. The 3,4-diphenyl-
maleimide derivative was designed as an electron-accepting unit,
whereas an electron-sufficient thiophene was the electron-donating
unit. Adonor-acceptor(D-A)copolymercouldbereadilysynthesized.
In order to enhance the ability to harvest the light of the solar spec-
trum, a thiophene unit was further converted into a 3,4-diphenyl-
maleimide unit. The 3,4-diphenyl-maleimide-thiophene derivative
was then copolymerized with different fused thiophene derivatives
to improve the coplanarity and conjugation lengths. Several groups
have reported that introduction of fused thiophene derivatives into
the polymer backbone can reduce steric hindrance, extend conju-
gation, enhanceabsorption, andimprove charge transport properties
[46e51]. Hence,theresultingrandomcopolymersexhibitelectronD-
A architectures, which promisethemeritsoflow bandgapsandbroad
absorption bands. In this work, the influence of coplanarity and
number of thiophenes for these copolymers on their photophysical
and electrochemical properties were investigated in detail. More-
over, the morphological and photovoltaic (PV) characteristics of the
copolymer/fullerene derivative-blend films were also discussed.
2.1.2. 2,5-Dibromothieno[3,2-b]thiophene (2T)
To an ice-cooled solution of thieno[3,2-b]thiophene (0.70 g,
5.0 mmol) in DMF (20 mL), NBS (1.95 g, 11.0 mmol) was added
portionwise, and the mixture was stirred for 3 h. The resulting
mixture was extracted with CH2Cl2. The organic layer was washed
with water, dried over MgSO4, and evaporated. The residue was
recrystallized in mixed solvent (ethanol:water ¼ 5:1, v/v) to yield
a colorless crystal (0.96 g, 65%). 1H NMR (CDCl3, 300 MHz,
d/ppm):
7.17 (s, 2H). 13C NMR (CDCl3, 75 MHz,
d/ppm): 113.60, 121.79,138.24.
HRMS calculated for C6H2Br2S2: 295.7965, found 295.7972.
2.1.3. 2,6-Dibromo-dithieno[3,2-b; 20,30-d]thiophene (3T)
Dithieno[3,2-b; 20,30-d]thiophene was prepared according to
a literature preparation [59]. To a solution of dithieno[3,2-b; 20,30-d]
thiophene (0.98 g, 5.0 mmol) in DMF (20 mL), NBS (1.95 g,
11.0 mmol) was added portionwise and then the mixture was
stirred at room temperature overnight. The light yellow suspension
was treated with water and filtered off. Subsequently, silica column
chromatography with hexane yielded a light yellow solid (1.50 g,
85%). 1H NMR (300 MHz, CDCl3): 7.27 (s, 2H). 13C NMR (CDCl3,
75 MHz, d/ppm): 112.32,123.19,130.79,139.02. HRMS calculated for
C8H2Br2S3: 351.7685, found 351.7671.
2.1.4. PMLTQT
PMLTQT was synthesized via a Stille coupling route, as shown in
Scheme 1. BrML (0.39 g, 0.75 mmol), 5,5-dibromo-2,20-bithiophene
(0.24 g, 0.75 mmol), and BTT (1.07 g, 1.50 mmol) were dissolved in
15 mL of dry toluene under a nitrogen atmosphere. Then, Pd(PPh3)4
(53 mg, 3.0 mol% with respect to ditin monomer) was added to the
reaction mixture and the reactions were continued at 85e90 ꢀC for
at least 48 h. After the mixture was cooled to room temperature, it
was poured into a solution (400 mL) of methanol and deionized
water (10:1). Fibrous precipitates were obtained by filtration. The
precipitate was then dissolved in DCM, and reprecipitated first from
water and then from methanol. Further purification was performed
by Soxhlet extractions with methanol and hexane. The resulting
polymer PMLTQT was obtained as a red solid with isolated yields of
68% after drying under vacuum. Gel permeation chromatography
(GPC) (THF): Mw ¼ 10.4 kg/mol and PDI ¼ 1.5. 1H NMR (CDCl3,
2. Experimental details
2.1. Chemical materials
The starting materials 2,5-bis(trimethyl-stannyl)thiophene and
bis(pinacolato)diboron, reagents, andchemicals were purchased from
Aldrich, Alfa, and TCIChemicalCo. Theywere used as received without
any further purification. All solvents, including diethyl ether,
dichloromethane (DCM), tetrahydrofuran (THF) or dimethylforma-
mide (DMF), and toluene were freshly distilled over appropriate
drying agents prior to use and purged with nitrogen. 2-bromo-3-
octylthiophene [52], 5,5-dibromo-2,20-bithiophene [53,54], 3,30-
dioctyl-2,20-bithiophene [55], thieno[3,2-b]thiophene [46,56], and
dithieno[3,2-b:20,30-d]thiophene [57,58] were synthesized according
to methods in the literature. N-(2-Ethylhexyl)-3,4-bis(4-bromoph
enyl)maleimide (BrML), N-(2-ethylhexyl)-bis[4-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)phenyl]maleimide (BML), N-(2-ethylhexyl)-
3,4-bis{4-[(5-bromo-3-octylthien-2-yl)phenyl]}maleimide (BrTML),
and N-(2-ethylhexyl)-3,4-bis{4-[(3-octylthien-2-yl)phenyl]}maleim
ide (TML) have been known before and synthesized in our lab
[38,41e43].
300 MHz,
(br, 2H), 2.59e2.52 (m, 8H), 1.85e1.75 (br, 1H), 1.70e1.53 (m, 8H),
1.43e1.20 (m, 40H), 0.98e0.85 (m,18H). 13C NMR (CDCl3, 75 MHz,
d/ppm): 7.61e7.56(br, 8H), 7.09e7.04 (m, 8H), 3.57e3.53
d
/
ppm): 171.07, 143.93, 143.56, 142.80, 137.02, 136.72, 136.23, 135.92,
135.55, 134.86, 131.96, 131.46, 130.57, 129.16, 129.00, 128.69, 127.86,
127.68, 127.53, 125.70, 125.47, 125.34, 124.28, 42.31, 38.46, 31.92,
30.67, 29.43, 29.27, 29.08, 28.61, 23.94, 23.04, 22.71, 14.15, 10.48.
Anal. Calcd. for [(C48H61NO2S2)0.48 þ (C32H40S4)0.52]: C, 73.63; H,
7.74; N, 1.04; S, 15.06. Found: C, 72.02; H, 7.79; N, 0.86; S, 16.16.
2.1.1. 1,10-(3,30-Dioctyl[2,20-bithiophene]-5,50-diyl)bis[1,1,1-
trimethylstannane] (BTT)
n-Butyllithium (2.5 mL of a 2.5 M solution in hexane, 6.25 mmol)
was added dropwise to a solution of 5,50-dibromo-3,30-dioctyl-2,20-
bithiophene (1.62 g, 2.95 mmol) in THF (15 mL) at ꢁ78 ꢀC for 2 h,
after which trimethyltin chloride (1.76 g, 8.80 mmol) was added
into the flask. After stirring for 10 min at ꢁ78 ꢀC, the reaction
mixture was allowed to stand overnight at room temperature. The
2.1.5. PMLT2T
PMLT2T was obtained as a red solid with a yield of 78% from the
reaction of BrTML (0.62 g, 0.68 mmol), 2,5-bis(trimethyl-stannyl)
thiophene (0.56 g,1.36 mmol), 2T (0.20 g, 0.68 mmol), and Pd(PPh3)4
(47 mg, 3.0 mol% with respect to ditin monomer) according to the
procedure described for the synthesis of PMLTQT. GPC (THF):

L.-H. Chan et al. / Polymer 53 (2012) 2334e2346
2337
Mw ¼ 47.0 kg/mol and PDI ¼ 4.3. 1H NMR (CDCl3, 300 MHz,
d
/ppm):
indium tin oxide (ITO) were sequentially patterned lithographically,
cleaned with detergent, ultrasonicated in acetone and isopropyl
alcohol, dried on a hot plate at 120 ꢀC for 5 min, and treated with
oxygen plasma for 5 min. The hole-transporting material poly(3,4-
ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS,
7.70e7.58(br, 8H), 7.35e7.50 (br, 6H), 7.09e6.90 (br, 2H), 3.57e3.53
(br, 2H), 2.70e2.52 (br, 4H), 1.85e1.82 (br, 1H), 1.70e1.53 (br, 4H),
1.43e1.10 (m, 28H), 0.95e0.70 (m, 12H). 13C NMR (CDCl3, 75 MHz,
d/
ppm): 171.09, 140.55, 139.50, 135.99, 135.03, 130.21, 128.95, 127.66,
126.53,124.30,123.62,109.98, 42.29, 38.44, 31.85, 30.85, 30.59, 29.54,
29.39, 29.24, 29.06, 28.62, 27.21, 23.94, 23.00, 22.67,14.12,10.47. Anal.
Calcd. for [(C52H63NO2S3)0.53 þ (C10H4S3)0.47]: C, 71.21; H, 6.48; N,
1.36; S, 17.65. Found: C, 69.67; H, 6.27; N, 1.30; S, 16.30.
Clevios P-VP AI4083) was passed through a 0.45-mm filter prior to
being deposited on the ITO-coated glass by the spin-coating method
(5000 rpm). The sample was then dried on a hot plate at 150 ꢀC for
30 min. A mixture solution of fullerene derivatives (both PC61BM
and PC71BM were used) and the copolymer with various weight
ratios (w/w) [36 mg/mL in o-dichlorobenzene (o-DCB)] was stirred
2.1.6. PMLT3T
PMLT3T was obtained as a deep red solid with isolated yields of
72% from the reaction of BrTML (0.63 g, 0.69 mmol) 2,5-
bis(trimethyl-stannyl)thiophene (0.57 g, 1.38 mmol), 3T (0.24 g,
0.69 mmol), and Pd(PPh3)4 (48 mg, 3.0 mol% with respect to ditin
monomer) according to the procedure described for the synthesis of
PMLTQT. GPC (THF): Mw ¼ 15.0 kg/mol and PDI ¼ 1.9.1H NMR (CDCl3,
overnight and then filtered through a 0.2-mm poly(tetrafluoro-
ethylene) (PTFE) filter. The composite film-based photoactive layer
of the copolymer/fullerene derivative was formed above the PEDOT:
PSS layer by spin-coating (600 rpm, 60 s) of the mixture solution. By
using AFM, film thickness of the photoactive layer in PSCs was found
in the range of 65e82 nm. The Ca-based (30 nm)/Al (100 nm)
cathode was thermally deposited onto the photoactive thin film in
a high-vacuum chamber. The active area of the PSC was 4 mm2. After
Al electrode deposition, the PSCs were examined under ambient
conditions. Current densityevoltage (IeV) curves of the PSCs were
measured on a programmable electrometer with current and
voltage sources (Keithley 2400) under the illumination of
100 mW cmꢁ2 solar light from an AM 1.5G solar simulator (Oriel
solar simulator). The illumination intensity was calibrated by using
a standard Si photodiode detector equipped with a KG-5 filter. The
output photocurrent was adjusted to match the photocurrent of the
300 MHz, d/ppm): 7.71e7.61 (m, 8H), 7.51e7.40 (br, 6H), 7.06e6.90
(br, 2H), 3.65e3.45 (br, 2H), 2.73e2.57 (br, 4H), 1.87e1.74 (br, 1H),
1.70e1.55 (br, 4H),1.54e1.26 (m, 28H), 0.99e0.78 (m,12H). 13C NMR
(CDCl3, 75 MHz, d/ppm): 171.07,143.87,143.51,142.80,141.79,140.55,
136.70, 136.21, 135.99, 135.79, 135.55, 135.01, 131.96, 131.44, 130.55,
130.22, 129.47, 128.93, 127.85, 127.65, 126.52, 125.69, 125.46, 125.34,
124.27,116.75,109.98, 42.30, 38.45, 31.89, 30.86, 30.65, 29.56, 29.42,
29.25, 29.08, 28.63, 27.22, 23.94, 23.02, 22.69, 14.13, 10.48. Anal.
Calcd. for [(C52H63NO2S3)0.56þ(C12H4S4)0.44]: C, 70.37; H, 6.31; N,
1.33; S, 18.76. Found: C, 69.94; H, 6.21; N, 1.33; S, 17.60.
Si reference cell to obtain a power density of 100 mW cmꢁ2
.
2.2. Characterization of copolymers
3. Results and discussion
General. 1H and 13C NMR spectra were recorded on a Varian Unity
Inova 300WB NMR spectrometer at room temperature. Elemental
analyses were performed on an Elementar Vario EL III elemental
analyzer. The number- and weight-average molecular weights of
polymers were determined by GPC on a Waters GPC-1515 with
refractive index detector 2414, using THFas the eluent and polystyrene
as the standard. The glass transition temperatures (Tg) and thermal
decomposition temperatures (Td) of the copolymers were determined
by differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) with PerkineElmer DSC-6 and TGA-7 analyzer systems,
respectively. Both thermal analyses were performed with a scanning
(both heating and cooling) rate of 10 deg/min in an atmosphere of
nitrogen. UVevisible absorption and fluorescence spectra were
recorded by HewlettePackard 8453 and Hitachi F-450 spectropho-
tometers, respectively. The thin films were spin-cast from toluene
solution on the glass substrate and subjected to vacuumed drying at
room temperature for at least 16 h before experimental measure-
ments. The redox potentials of the polymers were determined bycyclic
voltammetry (CV) with an electrochemical analyzer CHI 612D (scan-
ning rate: 50 mV sꢁ1), equipped with platinum (Pt) electrodes and an
Ag/Agþ (0.10 M AgNO3 in MeCN) reference electrode in an anhydrous
N2-saturated solution of 0.1 M Bu4NClO4 in MeCN. Bu4NClO4 (98%, TCI)
was recrystallized three times from mixed solvent methanol/water
(1:1) and then dried at 100 ꢀC under reduced pressure. A Pt plate
coated with a thinpolymer filmwasusedastheworkingelectrode, and
a Pt wire and an Ag/Agþ electrode were used as the counter and
reference electrodes, respectively. The electrochemical potential was
calibrated against Ferrecene/Ferroceneþ. The morphologies of the
polymer solid films were studied with an atomic force microscope
(AFM, Seiko SII SPA400) operated in the tapping mode.
3.1. Characterization of copolymers
The synthetic routes towards the monomers and copolymers are
outlined in Scheme 1. BrML was synthesized according to the
literature. Another monomer, BML, was easily derived from BrML
by reacting it with bis(pinacolato)diboron catalyzed by PdCl2(dppf)
(dppf
¼
1,10-bis(diphenylphosphino)ferrocene). BrTML was
obtained in high yield by NBS (N-bromosuccinimide) bromination
of TML, which in turn was easily prepared by Suzuki cross-coupling
between BML and 2-bromo3-octylthiophene. PMLTQT was
synthesized by Stille coupling of BrML, BTT, and 5,5-dibromo-2,20-
bithiophene. PMLT2T and PMLT3T were synthesized with the same
method by coupling BrTML, 2,5-bis(trimethyl-stannyl)thiophene,
and 2T or 3T, respectively. The chemical structures of these conju-
gated polymers were characterized by 1H NMR spectra, as shown in
Fig. 1. In the 1H NMR spectrum of PMLTQT, there was a peak at
7.60 ppm (peak a), which is attributed to the hydrogens on the
phenyl group of BrML. Moreover, there were peaks from 7.04 to
7.09 ppm (peak b, c), which were assigned to the hydrogens of the
thiophenes. Thus, the repeat unit ratio (m/n) of PMLTQT calculated
from the integral areas was about 1.09:1. By the same concept, the
peaks at 7.60 and 7.06 ppm (peak a, b) for PMLT2T were attributed
to the phenyl and alkyl thiophene moieties of BrTML. The peak at
7.45 ppm (peak c) is assigned to the hydrogens of 2T and unsub-
stituted thiophenes. Therefore, the repeat unit ratio (m/n) calcu-
lated from the integral areas of the above-mentioned peaks for
PMLT2T was about 1.14:1. Calculating the integral areas from the
hydrogens assigned to BrTML as well as 3T and unsubstituted
thiophenes in the 1H NMR spectrum of PMLT3T with the same
method (peak a, b and peak c), the repeat unit ratio (m/n) was about
1.27:1.
2.3. Fabrication and characterization of PSCs
The molecular weights and polydispersity indices (PDIs, Mw/Mn)
were characterized by GPC with THF as the eluent and polystyrenes
as the internal standards. The results summarized in Table 1. These
All PSCs were prepared by using the following device fabrication
procedure. Glass substrates [Sanyo, Japan (8U/sq.)] coated with

2338
L.-H. Chan et al. / Polymer 53 (2012) 2334e2346
Fig. 1. 1H NMR spectra of (a) PMLTQT, (b) PMLT2T, and (c) PMLT3T.
copolymers were readily soluble in common organic solvents such
as chloroform, THF, and toluene. The number-average molecular
weights (Mn) of PMLTQT, PMLT2T, and PMLT3T were found to be 6.9,
11.0, and 8.0 kg molꢁ1 respectively, with corresponding poly-
dispersity indices of 1.50, 4.27, and 1.87.
shortening the alkyl substituent of these copolymers can effectively
enhance the values of Tg.
3.2. Optical properties of the conjugated copolymers
The operational stability of an optoelectronic device is directly
related to the thermal stability of the conjugated polymers. Thus,
a high Tg and high Td are desirable for the application of a conju-
gated polymer to PSCs. As shown in Table 1, the values of Td (at
which a 5% weight loss occurred) of PMLTQT, PMLT2T, and PMLT3T
were in the range of 405e431 ꢀC under an atmosphere of nitrogen.
It was apparent that these copolymers exhibited good thermal
stability, which was adequate for their application to PSCs.
Regarding the DSC experiments, PMLTQT exhibited a glass transi-
tion centered at 45 ꢀC, whereas PMLT2T and PMLT3T showed
thermal transitions temperatures at 85 and 90 ꢀC, respectively. The
rigidity of PMLT2T and PMLT3T results in their values of Tg
becoming higher than PMLTQT. This suggests that appropriately
Fig. 2 shows thenormalizedabsorptionspectra ofcopolymersand
blended copolymer/PC61BM in both toluene solution and thin films.
The photophysicalpropertiesof these copolymersare summarized in
Table 2. In solution, absorption peaks were observed at 435, 428, and
457 nm for PMLTQT, PMLT2T, and PMLT3T, respectively. These
copolymers displayed a broad absorption, which originated from the
p-
p* transition of its conjugated main chain and intramolecular
charge transfer (ICT) interactions between the donors (thiophene
derivatives units) and acceptors (maleimide units) along the main
chain [60,61]. Compared to PMLT2T, PMLTQTshows a slight red-shift
in the absorption peakdan effect attributed to the longer effective
conjugated length in the main chain. However, it was noted that
PMLT2T has a broader absorption range than PMLTQT. This implies
the presence of a copolanar moiety in the main chain that increases
the conjugation length as well as the ICT interactions [62]. As a result,
the fact that PMLT3T exhibited the absorption peak with the longest
wavelength could be attributed to the coplanarity of 3T monomer. If
correct, this suggests that the incorporation of a coplanar moiety into
the polymer main chain would lead to the capture of sunlight at
longer wavelengths and contribute to the performance of the resul-
tant PSC devices. In general, conjugated polymers with relatively
broad absorptionwere favorable for the harvesting of solar light. This
broad absorption characteristic was expected to improve the
Table 1
Molecular weights and thermal properties of copolymers.
a
a
b
Copolymers Mn (kg/mol) Mw (kg/mol) PDI (Mw/Mn) Tg (ꢀC) Td (ꢀC)
PMLTQT
PMLT2T
PMLT3T
6900
11,000
8000
10,400
47,000
15,000
1.50
4.30
1.87
45
90
85
431
405
417
a
Mn, Mw and PDI of the polymers were determined by GPC using polystyrene
standards in THF.
b
Temperature of 5% weight loss.

L.-H. Chan et al. / Polymer 53 (2012) 2334e2346
2339
absorption efficiency of the photoactive layer and, thus, induce
a larger photocurrent in PSCs.
absorption range in the UVevis region. The normalized UVeVis
absorption spectra of PMLTQT/PC61BM-, PMLT2T/PC61BM-, and
PMLT3T/PC61BM-blend films in various weight ratios are also
shown in Fig. 2. The absorption band of the conjugated polymer
ranged from 350 to 600 nm, whereas the absorption band of
PC61BM ranged from 300 to 375 nm. Moreover, the absorption
intensity of the conjugated polymer decreased as the PC61BM
content increased for the conjugated polymer/PC61BM-based blend
films. From a photon-absorption viewpoint, employing less PCBM
in the photoactive layer was preferred. However, the typical stoi-
chiometry of a polymer/PCBM blend is 1:4 by weight, which has
The same trend was observed in thin films of these copolymers,
also as shown in Fig. 2. The maximal absorption wavelengths of
PMLTQT, PMLT2T, and PMLT3T were observed at around 454, 446,
and 472 nm, respectively. A red-shift and an enhancement of the
full-width at half-maximum of the absorption bands were
observed for these copolymers, as compared to those in solution.
These results were attributed to the interaction of and p-p stacking
between the polymer chains. In particular, highly coplanar fused
thiophene rings introduced to the main chain resulted in a broader
a
d
PM LTQT (solution)
1.2
PM LTQ T (film )
PM LTQ T:PC61BM =1:1 (film )
1.2
PM LTQT:PC61BM =1:1 (solution)
1.0
0.8
0.6
0.4
0.2
0.0
PM LTQT:PC61BM =1:3 (solution)
PM LTQ T:PC61BM =1:3 (film )
1.0
0.8
0.6
0.4
0.2
0.0
300
400
400
400
500
600
700
800
300
300
300
400
400
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
b
e
PM LT2T (film )
PM LT2T:PC61BM =1:1 (film )
PM LT2T (solution)
PM LT2T:PC61BM =1:1 (solution)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
PM LT2T:PC61BM =1:3 (film )
PM LT2T:PC61BM =1:3 (solution)
300
500
600
700
800
500
600
700
800
Wavelength (nm)
Wavelength (nm)
c
f
PM LT3T (solution)
PM LT3T:PC61BM =1:1 (solution)
PM LT3T (film )
PM LT3T:PC61BM =1:1 (film )
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
PM LT3T:PC61BM =1:3 (solution)
PM LT3T:PC61BM =1:3 (film )
300
500
600
700
800
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 2. UVevis absorption spectra of copolymers and copolymer/PC61BM (w/w, 1:1 and 1:3) in solution (aec) and thin films (def).

2340
L.-H. Chan et al. / Polymer 53 (2012) 2334e2346
Table 2
a
PM LTQ T (solution)
Optical and electrochemical properties of copolymers.
PM LTQ T:PC61BM =1:1 (film )
Copolymers In solutiona (nm) In filmb (nm) HOMO/LUMOc (eV) Egecd (V)
abs
max
em
max
abs
max
em
PM LTQ T:PC61BM =1:3 (film )
l
l
l
l
max
PMLTQT
PMLT2T
PMLT3T
435
428
457
619
620
622
454
446
472
631
634
643
ꢁ5.63/ꢁ3.55
ꢁ5.67/ꢁ3.58
ꢁ5.73/ꢁ3.65
2.08
2.09
2.08
120
80
40
0
a
Measured in toluene solution.
Casted from o-DCB solution.
The energy levels were calculated according to: HOMO ¼ ꢁeðE ꢁE
b
c
ox
ferrocene
þ4:8Þ
on
on
red
ferrocene
on
ðeVÞ, LUMO ¼ ꢁeðE ꢁE
þ4:8ÞðeVÞ.
on
d
Estimated using empirical equations: Egec ¼ EHOMO ꢁ ELUMO
.
been found to be optimal for devices in several PSC systems [63]. A
high proportion of PCBM limits optical absorption in the composite
layer because PCBM absorption is quite inefficient in the visible
region [64]. In addition, the maximal absorption wavelengths of the
conjugated polymer/PC61BM-blend films varied with PC61BM
content. Noticeably, the maximal absorption wavelength showed
a blue shift as the blending content of PC61BM was further
enhanced. A blue shift of the maximal absorption wavelength was
observed for the polymer/PC61BM-blend (w/w ¼ 1:1) film as
compared to the polymer film. This was attributed to the excellent
compatibility between the polymer and PC61BM. The intercalating
of PC61BM into the polymer chains led to perturbing the aggrega-
tion between copolymer chains. Hence, the maximal absorption
wavelength of the polymer showed a blue shift for the polymer/
PC61BM-blend (w/w ¼ 1:1) film.
500
600
700
800
900
Wavelength (nm)
PM LT2T (solution)
PM LT2T:PC61BM =1:1 (film )
b
PM LT2T:PC61BM =1:3 (film )
30
20
10
0
The photoluminescence (PL) spectra of the copolymers PMLTQT,
PMLT2T, and PMLT3T in toluene solution and in the copolymer/
PC61BM-based blend films spin-coated from o-dichlorobenzene
with various weight ratios are shown in Fig. 3. The PL emission
maxima of PMLTQT, PMLT2T, and PMLT3T in solution were observed
at 619, 620, and 622 nm, respectively. The three copolymers
exhibited almost the same PL emission maxima. This is because the
fused thiophene rings were introduced intothe PMLT2Tand PMLT3T
polymer backbone, which can enhance the coplanarity and ICT
interactions along the main chains. Hence, the emission peaks of
PMLT2Tand PMLT3Tare slightly red-shifted in comparisonwith that
of PMLTQT. A similar trend was found in solid films, which showed
maximum emission at wavelengths of 631, 634, and 643 nm for
PMLTQT, PMLT2T, and PMLT3T, respectively (Table 2). The PL emis-
sions were almost completely quenched upon the addition of
PC61BM (w/w ¼ 1:1) for the copolymers, as shown in Fig. 3. The same
quenched PL emissions were observed for the polymer-blend film
with higher PC61BM content (w/w ¼ 1:3). The highly efficient PL
quenching phenomena suggest that the excitons generated by the
absorbed photons completely dissociate to free charge carriers
(electrons and holes). Moreover, this achieves effective charge
transfer from the copolymer to PC61BMda basic requirement for
preparing a PSC with excellent PV performance [65].
500
600
700
800
900
Wavelength (nm)
PM LT3T (solution)
PM LT3T:PC61BM =1:1 (film )
c
PM LT3T:PC61BM =1:3 (film )
60
40
20
0
3.3. Electrochemical properties of conjugated copolymers
Cyclic voltammetry (CV) was employed to investigate electro-
chemical behavior and to estimate the highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
energy levels of the conjugated polymers. The oxidation and
reduction behaviors in the CV curves of the copolymers are shown
in Fig. 4. The oxidation potentials of PMLTQT, PMLT2T, and PMLT3T
were 1.31, 1.35, and 1.38 V, respectively. From these values, the
HOMO levels of the copolymers were calculated according to the
follow equation:
500
600
700
800
900
Wavelength (nm)
Fig. 3. PL spectra of copolymers (a) PMLTQT, (b) PMLT2T, and (c) PMLT3T in solution
and copolymer/PC61BM (w/w, 1:1 and 1:3) in thin films.
where 4.80 eV is the energy level of ferrocene below the vacuum
level Hence, the corresponding HOMO levels were ꢁ5.63, ꢁ5.67,
and ꢁ5.73 eV for PMLTQT, PMLT2T, and PMLT3T, respectively. In the
same way, the reduction potentials of PMLTQT, PMLT2T, and
ꢀ
ꢁ
ox
ferrocene
on
HOMO ¼ ꢁe E ꢁ E
þ 4:8 ðeVÞ
on

L.-H. Chan et al. / Polymer 53 (2012) 2334e2346
2341
to avoid excessive computation demand, since this has only
minimal effect on the electronic properties. Fig. 5 shows the
geometries and the HOMO and LUMO surface plots for these
copolymers. For each copolymer, the HOMO state density is
distributed entirely over the thiophene units, while the electron
density of LUMO is mainly localized on the maleimide units. These
results indicated that the HOMO-LUMO transition for the three
copolymers is mainly accompanied by charge transfer from the
electro-rich segments to maleimide units due to electron deficiency
of maleimide units. The insertion of fused thiophene rings can
provide improved planarity and greater conjugation length as
demonstrated by the optimized geometry of PMLT2T and PMLT3T
in Fig. 5. The calculated HOMO levels were ꢁ4.97 eV for all copol-
ymers, while the LUMO levels were ꢁ2.62, ꢁ2.67, ꢁ2.68 eV for
PMLTQT, PMLT2T, and PMLT3T, respectively. Although discrep-
ancies exist between the calculation and experimental results, the
trend of variation in the LUMO energies are similar.
PM LTQT
PM LT2T
PM LT3T
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
+
Potential (V vs Ag/Ag )
3.4. Atomic force microscope investigation
The film morphology of the active layer has been found to be the
one of the key elements in determining the PCE of PSCs [11,72,73].
Therefore, the compatibility and morphology of the conjugated
copolymer/PCBM composite films were investigated using AFM
microscopy. The topography (aec) and phase (def) nanoscale
images of the PMLTQT/PC61BM, PMLT2T/PC61BM, and PMLT3T/
PC61BM-blend films are shown in Fig. 6. As shown in Fig. 6, the
copolymer/PC61BM (w/w ¼ 1:3) films displayed low levels of root-
mean-square (RMS) (0.39, 0.36, and 0.27 nm for PMLTQT, PMLT2T
and PMLT3T, respectively), which implies that all blend films
exhibited satisfactory miscibility. PMLTQT/PC61BM showed the
highest surface roughness among the blend films; this suggests
that the introduction of fused thiophene rings into the polymer
main chain enhanced the compatibility by providing sufficient
room for PC61BM intercalation. However, the PMLT2T/PC61BM
Fig. 4. Cyclic voltammograms of copolymer films on platinum plates in acetonitrile
solution of 0.1 mol Lꢁ1 Bu4NClO4.
PMLT3T were found to be ꢁ1.01, ꢁ0.98, and ꢁ0.91 V, respectively.
The corresponding LUMO levels were estimated with the equation:
ꢀ
ꢁ
red
ferrocene
on
LUMO ¼ ꢁe E ꢁ E
þ 4:8 ðeVÞ
on
As a result, the LUMO levels were ꢁ3.55, ꢁ3.58, and ꢁ3.65 eV for
PMLTQT, PMLT2T, and PMLT3T, respectively. The electrochemical
bandgaps ðEgec
Þ
calculated from the differences between the
oxidation and reduction potentials were found to be around 2.08 eV
for these polymers. Although the electrochemical bandgaps were
almost identical for these polymers, the HOMO levels were lowered
by incorporation of the fused thiophene moiety into the polymer
backbone. Delocalization of electrons from the fused thiophene
unit into the backbone is less favorable than that from a single
thiophene ring, because the resonance stabilization energy of the
fused ring is larger than that of the single thiophene ring. This
reduced delocalization along the backbone results in a lowering of
the polymer HOMO level [66]. In BHJ solar cells, the Voc value is
directly proportional to the difference between the HOMO level of
the polymer and the LUMO level of the PCBM derivatives [67].
Superior efficiencies were obtained with PSC devices fabricated
from PMLT2T and PMLT3T with low HOMO levels. Given that the
reduction potential of the D-A structure copolymer is mainly
determined by the electron-accepting moiety, all copolymers that
possess the same maleimide moiety with electron-accepting ability
showed similar reduction potentials. The electrochemical proper-
ties of the copolymers are summarized in Table 2. All the copoly-
mers showed good air stability with a HOMO energy level below
the air oxidation threshold (ca. ꢁ5.2 eV), and the relatively low
HOMO level of the copolymers can allow a high open-circuit
voltage (Voc) for the PV cell [68,69] Meanwhile, the difference in
the LUMO energy levels between the donor and acceptor (e.g.,
PCBM) is more than 0.3 eV, which implies a sufficient driving force
for exciton dissociation [70,71].
To provide further insight into the fundamentals of molecular
architecture, the geometries and electron-state density distribu-
tions of three copolymers were performed by using Q-Chem
employing the B3LYP functional with 6-31G* basic set. Note that
the calculations were performed on model systems of copolymers
where all alkyl chain substituents were replaced with ethyl groups
Fig. 5. Calculated HOMO and LUMO levels and surface plots for PMLTQT, PMLT2T, and
PMLT3T at B3LYP/6-31G* level.

2342
L.-H. Chan et al. / Polymer 53 (2012) 2334e2346
Fig. 6. Topographic (aec) and phase (def) images of PMLTQT/PC61BM (w/w ¼ 1:3) (a, d), PMLT2T/PC61BM (w/w ¼ 1:3) (b, e), and PMLT3T/PC61BM (w/w ¼ 1:3) (c, f) in the tapping
mode. The area was 300 nm ꢂ 300 nm.

L.-H. Chan et al. / Polymer 53 (2012) 2334e2346
2343
Fig. 7. Topographic (aec) and phase (def) images of PMLTQT/PC71BM (w/w ¼ 1:3) (a, d), PMLT2T/PC71BM (w/w ¼ 1:3) (b, e), and PMLT3T/PC71BM (w/w ¼ 1:3) (c, f) in the tapping
mode. The area was 300 nm ꢂ 300 nm.

2344
L.-H. Chan et al. / Polymer 53 (2012) 2334e2346
Table 3
blend film showed clearer grain aggregation and homogeneous
dispersion of PC61BM than the other blend films, which resulted in
large-scale phase separation between the copolymer and PC61BM,
and hence decreased the Jsc of the PMLT2T/PC61BM-based device.
This may explain why the performance of PMLTQT and PMLT3T
were superior to that of PMLT2T. Fig. 7 shows the topography (aec)
and phase (def) nanoscale images of the PMLTQT/PC71BM, PMLT2T/
PC71BM, and PMLT3T/PC71BM blend films. As shown in Fig. 7,
PMLTQT/PC71BM (w/w ¼ 1:3) films displayed relatively high levels
of RMS compared to the other films (0.89, 0.48, and 0.39 nm for
PMLTQT, PMLT2T, and PMLT3T, respectively). A large degree of
phase segregation appeared again in the PMLTQT/PC71BM blend
film; however, a clear nanoscale-segregated phase was observed in
both PMLT2T/PC71BM and PMLT3T/PC71BM blend films. The
purported ideal morphology is comprised of a nanoscale inter-
penetrating network between donor and acceptor, which enables
a large interface area for exciton dissociation and continuous
percolating paths for hole and electron transport to the corre-
sponding electrodes [74]. It is important to note that the PMLT2T/
PC71BM and PMLT3T/PC71BM blend films showed a relatively
smooth morphology in the AFM height image; therefore, the
PMLT2T- and PMLT3T-based solar cells gave a higher Jsc, suggesting
the appropriate coarse phase separation between the copolymers
and PC71BM in these cells. Clearly, the suitable morphology of the
blend films is consistent with the higher PV performance of the
PSCs. This result indicates that the introduction of fused thiophene
rings into the polymer main chain enhanced the percolation of
PC71BM and thus enabled a smooth phase to be obtained.
Photovoltaic performances of the bulk heterojunction solar cells.
Active layer(w/w ratio)
Voc (V)
Jsc (mA/cm2)
FF
h(%)
PMLTQT:PC61BM (1:3)
PMLT2T:PC61BM (1:3)
PMLT3T:PC61BM (1:3)
PMLTQT:PC71BM (1:3)
PMLT2T:PC71BM (1:3)
PMLT3T:PC71BM (1:3)
0.76
0.78
0.80
0.70
0.70
0.74
4.7
2.7
4.9
0.16
0.18
0.21
0.19
0.20
0.22
0.57
0.38
0.82
0.85
1.08
1.20
6.2 (6.28)a
7.9 (7.98)a
7.4 (7.38)a
a
Calculated from the EQE spectrum.
short-circuit current density (Jsc), fill factor (FF), and PCE are
summarized in Table 3. As shown in Fig. 8, all devices exhibited
a high Voc in the range of 0.76e0.80 V, having benefited from the
deep HOMO energy levels of the copolymers, which correlated with
their HOMO levels. In organic solar cells, the Voc is generally gov-
erned by the energy difference between the HOMO of the donor
and the LUMO of the acceptor [67]. As expected, the Voc of the
PMLT3T/PC61BM-based (w/w ¼ 1:3) PSC was higher than that of the
PMLTQT/PC61BM-based (w/w ¼ 1:3) PSC owing to the deeper
HOMO level of PMLT3T. Compared to the absorption spectra of the
blend films (see Fig. 2), they are almost identical. Therefore, the PCE
difference of these devices is ascribed to the morphology of the
blend films, which lead to different Jsc values for the corresponding
devices. The larger Voc and higher Jsc values led to a superior PCE
value; thus, the PMLT3T/PC61BM-based (w/w ¼ 1:3) PSC exhibited
the highest PCE among all PSCs. The relatively low Jsc for the
PMLT2T-based device could be explained by the large number of
carrier recombinations due to the clear grain aggregation in the
blend film. Solar cell efficiency was further optimized by using the
1:3 blending ratio of copolymers with the acceptor PC71BM. Higher
current is usually obtained by replacing PC61BM with PC71BM as an
acceptor in the active layer, as the latter has higher absorption in
the visible region [6]. When PC71BM was used instead of PC61BM as
the electron acceptor in the devices, significant improvements in Jsc
were recorded. However, slightly lower Voc values were observed in
the copolymer/PC71BM based devices. A similar drop in the Voc
could be explained by the formation of a charge transfer complex
(CTC) in a polymer/PCBM film, which reduces the effective LUMO
level of PCBM [75]. An optimized PCE of 1.20% could be achieved by
using PMLT3T with PC71BM (1:3 blend ratio). Although PMLT3T/
3.5. PV properties of PSCs
The PSCs were fabricated from copolymer/PC61BM- and copol-
ymer/PC71BM-blend films at the weight ratio of 1:3 via the spin-
coating method, where the blending ratio of copolymers with the
acceptor PC61BM or PC71BM were optimized. The PCE performance
of the PSCs was observed when the photoactive layer was spin-
coated (600 rpm) from the o-DCB solution. The photocurrent
densityevoltage (IeV) characteristics of the PSCs prepared from
copolymer/PC61BM and copolymer/PC71BM, measured under the
illumination of simulated AM 1.5 G conditions (100 mW/cm2), are
shown in Fig. 8. The PV properties of these PSCs, including the Voc
,
PMLTQT/PC71BM
2
0
PMLT2T/PC71 BM
60
PMLT3T/PC71 BM
50
-2
40
30
20
10
0
PMLTQT/PC61BM
-4
-6
PMLT2T/PC 61 BM
PMLT3T/PC 61 BM
PMLTQT/PC 71 BM
PMLT2T/PC 71 BM
PMLT3T/PC 71 BM
-8
-10
400
500
600
700
800
0.0
0.2
0.4
0.6
0.8
Wavelength (nm)
Voltage (V)
Fig. 9. EQE curves of the photovoltaic cells with (copolymer:PC71BM ¼ 1:3) as the
Fig. 8. Current density-potential characteristics of illuminated (AM 1.5 G, 100 mW/
active layer.
cm2) bulk heterojunction solar cells.

L.-H. Chan et al. / Polymer 53 (2012) 2334e2346
2345
PC71BM-based (w/w ¼ 1:3) PSC exhibited the highest PCE among
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maleimide-based copolymers for PSCs, such as broadening of the
absorption band to long wavelengths by the introduction of an
electron-donating low bandgap as well as high coplanarity como-
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Fig. 9 depicts the external quantum efficiency (EQE) curves of
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In summary, we synthesized three new random D-A conjugated
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the visible region. By incorporating fused thiophene rings into the
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can be enhanced. The electrochemical data indicated the fused
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Improved compatibility was observed for both PMLT2T/PC61BM (or
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to the PMLTQT/PC61BM (or PC71BM) blend film. PV devices based on
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copolymers, which implies that better compatibility would lead to
better film morphology thus enhance the PSC efficiency. The PV
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electron-accepting unit for constructing D-A copolymers. A higher
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Financial support from the National Science Council of Taiwan
(ROC) through NSC 97-2113-M-260-003-MY2, and NSC 99-2113-M-
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Prof. Chin-Ti Chen and Dr. Hung-Yang Chen at Institute of Chem-
istry, Academia Sinica, for assistance in the EQE measurements.

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