Synthesis and photovoltaic properties of two-dimensional conjugated polythiophene derivatives presenting conjugated triphenylamine/thiophene moieties

Article abstract of DOI:10.1016/j.polymer.2012.07.010

In this study we synthesized three two-dimensional (2-D) polythiophene derivatives (PTs), namely PTBPTPA, PTStTPA, and PTCNStTPA, featuring three different conjugated units - biphenyl (BP), stilbene (St), and cyanostilbene (CNSt), respectively - in the po

Full text of DOI:10.1016/j.polymer.2012.07.010

Polymer 53 (2012) 4091e4103
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and photovoltaic properties of two-dimensional conjugated
polythiophene derivatives presenting conjugated triphenylamine/thiophene
moieties
,
Hsing-Ju Wang a, Yuan-Peng Chen a, Yung-Chung Chen b, Chih-Ping Chen c, Rong-Ho Lee a
**
,
*
,
Li-Hsin Chan d, Ru-Jong Jeng e
a Department of Chemical Engineering, National Chung Hsing University, 250, Kuo Kuang Rd., Taichung 402, Taiwan
b Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan
c Material and Chemical Laboratories, Industrial Technology Research Institute, Chutung 310, Taiwan
d Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Puli, Taiwan
e Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study we synthesized three two-dimensional (2-D) polythiophene derivatives (PTs), namely
PTBPTPA, PTStTPA, and PTCNStTPA, featuring three different conjugated unitsdbiphenyl (BP), stilbene
(St), and cyanostilbene (CNSt), respectivelydin the polymer backbones and presenting conjugated tri-
phenylamine/thiophene (TPATh) moieties on the side chains. In addition, we also synthesized three
conjugated BP-, St-, and CNSt-based main-chain-type conjugated polymers (PTBP, PTSt, and PTCNSt,
respectively). Incorporating the St and CNSt moieties into the polymer backbones and appending TPATh
units induced high degrees of intramolecular charge transfer within the conjugated frameworks of the
polymers, thereby resulting in lower band gap energies and red-shifting of the maximal UVeVis
absorption wavelengths. Moreover, the energy levels of the highest occupied molecular orbitals of the
BP-, St-, and CNSt-based main-chain-type and 2-D PTs were lower than that of P3HT, implying that they
would be applicable for the preparation of polymer solar cells (PSC) with greater open-circuit voltages.
The photovoltaic performances of PSCs fabricated from blends of the 2-D PTs and the fullerene derivative
PC61BM were superior to those of PSCs based on the main-chain-type polymer/PC61BM blends.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 9 April 2012
Received in revised form
24 June 2012
Accepted 6 July 2012
Available online 26 July 2012
Keywords:
Polythiophene derivatives
Photovoltaic property
Polymer solar cell
1. Introduction
(PC71BM)das the electron acceptor [4e6]. The photovoltaic (PV)
performances of polymer solar cells (PSCs) have improved
Conjugated polymers consisting of an electronically delocalized
backbone are significant technological materials used in opto-
electronics [1e3]. Solar cells incorporating bulk heterojunctions
significantly in the past decade through optimization of the device
structure, interfacial layer engineering, and processing conditions
[7e10]. Although photo-energy conversion efficiencies
(h) of
(BHJs) based on low-band gap
p
-conjugated polymers have
greater than 7% have been achieved for PSCs based on blends of
polythiophene derivatives and fullerene derivatives [11e16], the
efficiencies of PSCs remain too low for commercial applications;
several key factors must be resolved if a theoretical efficiency of
10% is to be realized [17].
received much interest because of their promise for the devel-
opment of solution-processable, highly flexible, large-area, low-
cost, light-weight solar modules [4e6]. The photoactive layer in
a typical BHJ cell is based on a blend of a conjugated polymer as
the electron donor and
derivativedfor example, [6,6]-phenyl-C61-butyric acid methyl
ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester
a
high-electron-affinity fullerene
Efficient photon-to-charge conversion in PSCs would require
a conjugated polymer exhibiting a combination of suitable optical,
electrochemical, and electronic properties. Generally, a high short-
circuit current (JSC) and a large open-circuit voltage (VOC) are both
required if a PSC is to display a high photo-energy conversion
efficiency (h). A conjugated polymer with a broad absorption band
and low band gap will harvest more solar radiationda favorable
feature for a PSC exhibiting a high value of JSC [18e20]. Moreover,
* Corresponding author. Tel.: þ886 4 22854308; fax: þ886 4 22854734.
** Corresponding author. Tel.: þ886 2 33665884; fax: þ886 2 33665237.
E-mail addresses: rhl@dragon.nchu.edu.tw (R.-H. Lee), rujong@ntu.edu.tw
(R.-J. Jeng).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.07.010

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H.-J. Wang et al. / Polymer 53 (2012) 4091e4103
high charge mobility is also required of the polymer to achieve high
values of JSC in PSCs [19e21]. Introducing strong electron-donating
or -withdrawing moieties into the polymer backbone is an effective
means of decreasing the band gap energy of the conjugated poly-
mer. Unfortunately, the energy level of the highest occupied
molecular orbital (HOMO) of the polymer would increase upon
decreasing the band gap energy. In a typical PSC cell, the value of
VOC will correlate proportionally to the difference in energy
between the HOMO of the electron-donating conjugated polymer
and the lowest unoccupied molecular orbital (LUMO) of the
electron-accepting fullerene derivative [22]. Therefore, the value of
VOC of a PSC would decrease upon decreasing the band gap energy
of the polymer [23].
In recent years, several groups of chemists have proposed the
use of two-dimensional (2-D) conjugated polymers, namely
conjugated polymers presenting electron-donating and -with-
drawing pendent groups, for PSC applications [20,24e35]. 2-D
conjugated polymers sometimes exhibit two absorption bands in
the UV and visible regions, therefore harvesting a greater amount of
solar light [20,25,35]; indeed, high values of JSC have been obtained
for PSCs fabricated from such 2-D conjugated polymers. Li et al.
have reported 2-D polythiophene derivatives (2-D PTs) presenting
electron-donating triphenylamine (TPA) moieties as side chain
units [34]. The presence of the TPA moieties twists the main chain
morphologies and PV performances of PSCs based on these TPATh-
containing 2-D PT/PC61BM blends.
2. Experimental details
2.1. Chemical materials
All reagents were purchased from Aldrich, Alfa, or TCI Chemical
and used as received. Dichloromethane (DCM), tetrahydrofuran
(THF), and toluene were freshly distilled over appropriate drying
agents prior to use and then purged with N2. The syntheses of 4,40-
bis(trimethylstannyl)biphenyl (1), (E)-1,2-bis(4-bromophenyl)
ethane (2), (E)-1,2-bis[4-(trimethylstannyl)phenyl]ethene (3), (Z)-
2,3-bis(4-bromophenyl)acrylonitrile (4), and (Z)-2,3-bis[4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]acrylonitrile (5) are
presented in Scheme 1. Compound (4-bromobenzyl)phosphonic
acid diethyl ester, 2,5-dibromo-3-dodecanethiophene (6) and (E)-
4-{5-[2-(2,5-dibromothiophen-3-yl)vinyl]thien-2-yl}-N,N-diphe-
nylaniline (7) were synthesized according to previously described
procedures [35,40]. Detailed procedures for the syntheses of the
conjugated copolymers PTBP, PTBPTPA, PTSt, PTStTPA, PTCNSt, and
PTCNStTPA are displayed in Scheme 2.
2.1.1. 4,40-Bis(trimethylstannyl)biphenyl (1)
out of planar
p
-conjugation and decreases the effective conjugation
A solution of 4,40-dibromophenyl (3.12 g, 10.0 mmol) in dry THF
(60 mL) was stirred at ꢀ78 ꢁC under N2 for 10 min and then n-BuLi
(2.5 M in hexane, 10.0 mL, 25.0 mmol) was added dropwise. The
mixture was maintained at ꢀ78 ꢁC with continued stirring for
a further 3 h, at which point a solution of trimethyltin chloride
(4.23 g, 25.0 mmol) in THF (10 mL) was added dropwise. After
warming to room temperature and stirring for 8 h, MeOH was
added to quench the reaction. The solution was partitioned
between DCM and water; the organic phase was collected, dried
(MgSO4), filtered, and evaporated to dryness. The residue was re-
crystallized (MeCN) to provide a white solid (3.17 g, 66%). 1H
length of the polymer backbone. The HOMO energy levels of such
TPA-containing 2-D PTs were lower than that of poly(3-
hexylthiophene) (P3HT). Accordingly, higher values of VOC were
observed for the corresponding PSCs. Nevertheless, the enhanced
band gap energies of these TPA-containing 2-D PTs did not favor the
harvesting of solar light [34].
Our objective in this present study was to develop a series of 2-D
PTs featuring efficient conjugation lengths and deep HOMO energy
levels for the preparation of PSCs exhibiting high values of JSC and
VOC. We synthesized three 2-D PTs, namely PTBPTPA, PTStTPA, and
PTCNStTPA, featuring different conjugated unitsdbiphenyl (BP),
stilbene (St), and cyanostilbene (CNSt), respectivelydin their
polymer backbones and each presenting conjugated triphenyl-
amine/thiophene (TPATh) pendent moieties. The conjugated poly-
mer containing electron-donating moiety TPA has been widely
studied for optoelectronic applications [36e39]. Although the
HOMO energy levels of 2-D PTs incorporating electron-donating
moieties are deeper than those of their parent PTs [34], the pres-
ence of bulky moieties would decrease the effective conjugation
lengths of their polymer backbones [34]. We suspected that
NMR (400 MHz, CDCl3)
d
(ppm): 0.27 (s, 18 H), 7.53 (d, J ¼ 7.4 Hz,
4 H), 7.55 (d, J ¼ 7.4 Hz, 4 H). EIMS (m/z): calcd for C18H26Sn2,
479.82; found, 480.0. Anal. Calcd for C18H26Sn2: C, 45.06; H, 5.42.
Found: C, 45.41; H, 5.65.
2.1.2. (E)-1,2-Bis(4-bromophenyl)ethane (2)
A mixture of (4-bromobenzyl)phosphonic acid diethyl ester
(2.71 g, 11.0 mmol) and potassium tert-butoxide (3.37 g, 30.0 mmol)
in THF (20 mL) was stirred in an ice-water bath for several minutes
and then 4-bromobenzaldehyde (1.85 g,10.0 mmol) was added. The
solution was maintained at 0 ꢁC and stirred continuously for
a further 5 h. Then, the reaction was quenched through the addition
of ice water. The mixture was partitioned between DCM and water
and then the organic phase was collected, dried (MgSO4), filtered,
and evaporated to dryness. The residue was recrystallized (MeOH/
THF) to provide a white solid (2.43 g, 72%). 1H NMR (400 MHz,
twisting of the main chain out of planar
p-conjugation in the
presence of bulky moieties could be suppressed though incorpo-
ration of rigid conjugation units (BP, St, CNSt) into the backbone.
Thus, we expected PTBPTPA, PTStTPA, and PTCNStTPA to display
a high degree of intramolecular charge transfer between the
polymer backbones and the electron-donating pendent moieties.
Consequently, the 2-D PTs PTBPTPA, PTStTPA, and PTCNStTPA
should maintain deep HOMO energy levels and higher values of
VOC; effective tuning of energy levels to modulate the absorbance
profit would result in higher values of JSC. For the sake of compar-
ison, we also synthesized three main-chain-type conjugated poly-
mers, namely PTBP, PTSt, and PTCNSt, based on BP, St, and CNSt
units, respectively. We have used UVeVis absorption spectroscopy
and cyclic voltammetry (CV) to study the effects of the rigid
conjugation segment (BP, St, CNSt) on the photophysical and elec-
trochemical behavior, respectively, of the TPATh-containing 2-D
PTs. We have also used atomic force microscopy (AFM) to study
the morphologies of the TPATh-functionalized 2-D PT/PC61BM and
main-chain-type PT (PTBP, PTSt, PTCNSt)/PC61BM blends. Herein,
we also discuss the effect of the chemical structure on the
CDCl3)
d
(ppm): 7.09 (s, J ¼ 7.4 Hz, 2 H), 7.24 (d, J ¼ 7.4 Hz, 4 H), 7.45
(d, 4 H). EIMS (m/z): calcd for C14H10Br2, 338.3; found: 338.1. Anal.
Calcd for C14H10Br2: C, 44.20; H 2.96. Found: C, 44.32; H, 2.975.
2.1.3. (E)-1,2-Bis[4-(trimethylstannyl)phenyl]ethene (3)
A solution of 2 (2.77 g,10.0 mmol) in dry THF (60 mL) was stirred
at ꢀ78 ꢁC under a N2 atmosphere and then n-BuLi (2.5 M in hexane,
10 mL, 25 mmol) was added dropwise. The mixture was maintained
at ꢀ78 ꢁC with continued stirring for a further 3 h, at which point
a solution of trimethyltin chloride (4.23 g, 25.0 mmol) in THF
(10 mL) was added dropwise. After warming to room temperature
and stirring for 8 h, water was added to quench the reaction. The
mixture was partitioned between DCM and water; the organic
phase was dried (MgSO4), filtered, and evaporated to dryness. The

H.-J. Wang et al. / Polymer 53 (2012) 4091e4103
4093
Trimethyltin chloride
Me3Sn
SnMe3
Br
Br
Br
n-BuLi, THF
Compound 1
Br
t-BuOK
OEt
Br
+
Br
CHO
P
THF
O
EtO
Compound 2
SnMe
3
Trimethyltin chloride
n-BuLi, THF
Me3Sn
Compound 3
Br
K2CO3
THF
Br
Br
+
CHO
Br
CN
CN
Compound 4
O
O
Bis(pinacolate)diboron
B
O
O
B
CN
Compound 5
Pd(PPh ) Cl2, 1,4-dioxane
3 2
Scheme 1. Synthesis of the conjugation units BP, St, and CNSt.
residue was recrystallized (MeCN) to provide a white solid (2.98 g,
C27H33B2NO4, 457.18; found, 457.3. Anal. Calcd for C27H33B2NO4: C,
70.86; H, 7.22; N, 3.06. Found: C, 70.93; H, 7.25; N, 3.12.
59%). 1H NMR (400 MHz, CDCl3)
d
(ppm): 0.28 (s, 18 H), 7.09 (s, 2 H),
7.46 (d, J ¼ 8 Hz, 4 H), 7.52 (d, J ¼ 8 Hz, 4 H). MS (m/z): calcd for
C20H28Sn2, 505.86; found, 506.0. Anal. Calcd for C20H28Sn2: C, 47.49;
H, 5.58. Found: C, 47.57; H, 5.75.
2.1.6. PTBP
A solution of 1 (0.72 g, 1.5 mmol) and 6 (0.62 g, 1.5 mmol) in dry
toluene (20 mL) was purged with N2 and subjected to three freeze/
pump/thaw cycles to remove traces of O2. Pd(PPh3)4 (30 mg, 1 mol
%) was added to the mixture, which was then stirred and heated
under reflux for 48 h. After cooling to room temperature, the
mixture was poured into MeOH (100 mL) and the precipitated
material was filtered through a Soxhlet thimble. Soxhlet extractions
were performed sequentially with MeOH, hexane, acetone, and
CHCl3. The polymer was recovered through rotary evaporation of
the CHCl3 fraction. Drying under vacuum for 24 h provided an
orange-red solid (44%). Gel permeation chromatography (GPC;
2.1.4. (Z)-2,3-Bis(4-bromophenyl)acrylonitrile (4)
4-Bromobenzaldehyde (1.85 g, 10 mmol) was added to an ice-
water-cooled mixture of 4-bromophenylacetonitrile (2.35 g,
12.0 mmol) and K2CO3 (0.560 g, 5.00 mmol) in THF (30 mL). After
5 h, the reaction was quenched through the addition of ice water
and the mixture partitioned between DCM and water. The organic
phase was dried (MgSO4), filtered, and evaporated to dryness. The
residue was purified chromatographically (SiO2; EtOAc/hexanes,
1:10) to provide a white solid (2.69 g, 74%). 1H NMR (400 MHz,
CDCl3)
d
(ppm): 7.43 (s, 1 H), 7.55e7.60 (m, 6 H), 7.60e7.71 (m, 2 H).
THF): weight-average molecular weight (Mw), 13.9 kg molꢀ1
;
MS (m/z): calcd for C15H9NBr2, 362.8; found, 362.9. Anal. Calcd for
C15H9NBr2: C, 49.60; H, 2.48; N, 3.86. Found: C, 49.95; H, 2.52; N,
3.89.
polydispersity index (PDI), 2.03. 1H NMR (400 MHz, CDCl3)
d
(ppm):
0.86 (t, 3 H), 1.26 (br, 18 H), 1.57e1.62 (m, 2 H), 2.55e2.75 (t, 2 H),
7.12 (s, 1 H), 7.28e7.71 (br, 8 H). Anal. Calcd for C28H34S: C, 83.57; H,
8.45; S, 7.96. Found: C, 83.62; H, 8.38; S, 7.87.
2.1.5. (Z)-2,3-Bis[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
phenyl]acrylonitrile (5)
2.1.7. PTSt
A mixture of 4 (3.63 g, 10.0 mmol), KOBut (4.91 g, 50.0 mmol),
and bis(pinacolate)diboron (6.35 g, 25.0 mmol) in a dry 1,4-dioxane
(50 mL) was deoxygenated with N2 for 30 min. Pd(PPh3)2Cl2
(0.140 g, 0.200 mmol) was added to the mixture, which was then
stirred and heated under reflux for 24 h. After cooling to room
temperature, the mixture was partitioned between EtOAc and
water. The organic phase was dried (MgSO4), filtered, and evapo-
rated to dryness. The residue was purified chromatographically
(SiO2; EtOAc/hexanes, 1:10) to provide a white solid (3.02 g, 66%).
Using the same procedure as that described for the synthesis of
PTBP, the reaction of 3 (0.77 g, 1.5 mmol) and 6 (0.62 g, 1.5 mmol)
provided
a
yellowish solid (0.22 g, 51%). GPC (THF): Mw,
(ppm): 0.80 (t,
10.5 kg molꢀ1; PDI, 2.18. 1H NMR (400 MHz, CDCl3)
d
3 H), 1.18 (br, 18 H), 1.56e1.61 (m, 2 H), 2.30e2.62 (t, 2 H), 7.09e7.20
(br, 3 H), 7.25e7.65 (br, 8 H). Anal. Calcd for C30H36S: C, 84.10; H,
8.41; S, 7.47. Found: C, 83.92; H, 8.58; S, 7.39.
2.1.8. PTBPTPA
1H NMR (400 MHz, CDCl3)
(d, J ¼ 8 Hz, 2 H), 7.86 (d, J ¼ 7 Hz, 6 H). MS (m/z): calcd for
d
(ppm): 1.34 (s, 24 H), 7.59 (s, 1 H), 7.65
Using the same procedure as that described for the synthesis of
PTBP, the reaction of 1 (0.72 g, 1.5 mmol), 6 (0.31 g, 0.75 mmol), and

4094
H.-J. Wang et al. / Polymer 53 (2012) 4091e4103
toluene (30 mL) was deoxygenated with N2 for 1 h. The mixture
S
Br
Br
S
was heated at 90 ꢁC and stirred under a N2 atmosphere for 48 h.
After cooling to room temperature, the mixture was poured into
MeOH (100 mL) and the precipitated material then filtered into
a Soxhlet thimble. Soxhlet extractions were performed sequentially
with MeOH, hexane, acetone, and CHCl3. The polymer was recov-
ered from the CHCl3 fraction through rotary evaporation. Drying
under vacuum for 24 h provided a black-red solid (0.30 g, 43%). GPC
(THF): Mw, 12.34 kg molꢀ1; PDI, 2.27. 1H NMR (400 MHz, CDCl3)
Br
S
Br
C12H25
+
+
X
X
A
Compound 6
Compound 1, 3, or 5
N
d
(ppm): 0.87 (t, 3 H), 1.26 (br, 18 H), 1.56e1.61 (m, 2 H), 2.55e2.75
Compound 7
S
(t, 2 H), 7.15 (s, 1H), 7.40e7.80 (br, 8 H), 7.94 (s, 1 H). Anal. Calcd for
C31H35NS: C, 82.10; H, 7.73; N, 3.09; S, 7.05. Found: C, 82.32; H, 7.58;
N, 3.12; S, 6.89.
S
A
A
m
n
2.1.11. PTCNStTPA
Stille or Suzuki
Polymerization
Using the same procedure as that described for the synthesis of
PTCNSt, the reaction of 5 (0.69 g, 1.5 mmol), 6 (0.31 g, 0.75 mmol),
and 7 (0.45 g, 0.75 mmol) provided an orange-red solid (0.38 g,
51%). GPC (THF): Mw, 10.09 kg molꢀ1; PDI, 1.29. 1H NMR (400 MHz,
S
CDCl3)
d (ppm): 0.85 (br, 3H), 1.26 (br, 18 H), 1.56e1.62 (m, 2 H),
N
2.55e2.75 (t, 2H), 6.95e8.12 (br, 33 H). Anal. Calcd for
(C31H35NS)1.27(C43H28N2S2)1: C, 81.60; H, 5.95; N, 3.77; S, 8.65.
Found: C, 81.98; H, 6.23; N, 2.93; S, 8.08.
X
X
A
Copolymers
2.2. Characterization
m:n = 1:0 PTBP
Me3Sn
SnMe3
1H NMR (400 MHz) and 13C NMR (150 MHz) spectra were
recorded using a Varian Unity Inova Spectrometer. The average
molecular weights of the polymers were measured using the GPC
method. GPC was performed using a Waters chromatography
system (717 plus Autosampler), two Waters Styragel linear
columns, polystyrene as the standard, and THF as the eluent. The
glass transition temperatures (Tg) and thermal decomposition
temperatures (Td; at which a 5% weight loss occurred) of the
copolymers were determined through differential scanning calo-
rimetry (TA Instruments, DSC-2010) and thermogravimetric anal-
ysis (TA Instruments, TGA-2050), respectively. Both thermal
analyses were performed under a N2 atmosphere at scanning (both
heating and cooling) rates of 10 ꢁC minꢀ1. The temperatures at the
intercept of the curves in the thermogram (endothermic,
exothermic, or weight loss) and the leading baseline were taken as
estimates of the onset values of Tg and Td. Absorption spectra were
measured using a Hitachi U3010 UVeVis spectrometer. Fluores-
cence spectra were measured using a Varian Cary Eclipse lumi-
nescence spectrometer. Redox potentials of the polymers were
determined through CV using a CHI 611D electrochemical analyzer
(scanning rate: 50 mV sꢀ1) equipped with 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 MeOH and water (1:1) and
then dried at 100 ꢁC under reduced pressure. A Pt plate coated with
a thin polymer film was used as the working electrode; a Pt wire
and an Ag/Agþ electrode were used as the counter and reference
electrodes, respectively. The electrochemical potential was cali-
brated against ferrocene/ferrocenium. The morphologies of the
conjugated copolymer/PC61BM blend films were studied using an
atomic force microscope (AFM, Seiko SII SPA400) operated in the
tapping mode, and a transmission electron microscope (TEM, JEOL
JEM-1400).
m:n = 1.24:1 PTBPTPA
Compound 1
m:n = 1:0 PTSt
SnMe3
Me3Sn
m:n = 1.85:1 PTStTPA
Compound 3
m:n = 1:0 PTCNSt
O
O
B
O
B
m:n = 1.27:1 PTCNStTPA
O
CN
Compound 5
Scheme 2. Synthesis of the main-chain-type and 2-D PTs.
7 (0.45 g, 0.75 mmol) provided a dark-red solid (0.45 g, 61%). GPC
(THF): Mw, 13.4 kg molꢀ1; PDI, 2.22. 1H NMR (400 MHz, CDCl3)
(ppm): 0.85 (br, 3 H), 1.24 (br, 18 H), 1.57 (br, 2 H), 2.55e2.75 (m,
2 H), 7.03e7.83 (br, 31 H). Anal. Calcd for (C28H34S)1.24(C40H27NS2)1:
C, 82.74; H, 6.38; N, 1.28; S, 9.57. Found: C, 82.88; H, 6.47; N, 1.17;
S, 9.15.
d
2.1.9. PTStTPA
Using the same procedure as that described for the synthesis of
PTBP, the reaction of 3 (0.77 g, 1.5 mmol), 6 (0.31 g, 0.75 mmol), and
7 (0.45 g, 0.75 mmol) in dry toluene (20 mL) provided a red solid
(0.42 g, 53%). GPC (THF): Mw, 13.54 kg molꢀ1; PDI, 2.02. 1H NMR
(400 MHz, CDCl3)
d (ppm): 0.85 (br, 3 H), 1.26 (br, 18 H), 1.62e1.65
(m, 2 H), 2.55e2.75 (t, 2 H), 7.00e7.88 (br, 33 H). Anal. Calcd for
(C30H36S)1.85(C42H29NS2)1: C, 83.40; H, 6.81; N, 0.99; S, 8.78. Found:
C, 83.35; H, 6.77; N, 1.02; S, 8.68.
2.3. Fabrication and characterization of PSCs
2.1.10. PTCNSt
Glass substrates [Sanyo, Japan (8 U ,
ꢀ1)] coated with indium
A stirred mixture of 5 (0.69 g, 1.5 mmol), 6 (0.61, 1.5 mmol),
Pd(PPh3)4 (0.03 g, 1 mol %), 2 M K2CO3(aq) (5 mL), and anhydrous
tin oxide (ITO) were sequentially patterned lithographically,
cleaned with detergent, ultrasonicated in acetone and isopropyl

H.-J. Wang et al. / Polymer 53 (2012) 4091e4103
4095
alcohol, dried on a hot plate (120 ꢁC, 5 min), and treated with
oxygen plasma for 5 min. The hole-transporting material poly(3,4-
ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS; Cle-
dodecanethiophene segment had the opposite effect. A higher
content of St/TPATh segments would lead to precipitation at the
initial stages of polymerization, resulting in the polymer being
removed during the purification procedure. Therefore, we obtained
PTStTPA with a higher m/n ratio. Using GPC with THF as the eluent
and polystyrenes as internal standards, we determined the values
of Mw and PDI (Mw/Mn) of the conjugated polymers to be
10.09e13.90 (kg molꢀ1) and 1.19e2.27, respectively (Table 1). All of
the conjugated copolymers, expect for PTSt, were soluble in
common organic solvents, including DCM, THF, toluene, and o-DCB.
The relatively poor solubility of PTSt was due to the high planarity
of its St groups [42].
vios P-VP AI4083) was passed through a 0.45-mm filter prior to
being deposited on the ITO-coated glass using the spin-coating
method (3000 rpm). The sample was then dried at 150 ꢁC for
30 min in a glove box. A solution of PC61BM and a conjugated
copolymer [weight ratio, 1:1; 15 mg mLꢀ1 in o-dichlorobenzene (o-
DCB)] was stirred overnight and then filtered through a 0.2-mm
polytetrafluoroethylene (PTFE) filter. A film of the photoactive layer
(copolymer/PC61BM composite) was formed above the PEDOT:PSS
layer through spin-coating (1000 rpm, 30 s) of the mixture solution.
The photoactive layers were thermally treated at 80 ꢁC for 1 h or
140 ꢁC for 10 min. The film thicknesses of the photoactive layer in
PSCs were determined (AFM) to be approximately 110 nm. The LiF
(1 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 deposition of the Al electrode, the PSC
was encapsulated using UV-curing glue (Nagase, Japan). Current
densityevoltage (IeV) curves of the PSCs were recorded using
a programmable electrometer, equipped with current and voltage
sources (Keithley 2400), under illumination (100 mW cmꢀ2) with
solar light from a Peccell AM 1.5G solar simulator. The illumination
intensity was calibrated using a standard Si photodiode detector
equipped with a KG-5 filter. The output photocurrent was adjusted
to match the photocurrent of the Si reference cell to obtain a power
The operational stability of an optoelectronic device is directly
related to the thermal stability of its conjugated polymers. Thus,
high values of Tg and Td are desirable for a conjugated polymer to be
applied in PSCs. We obtained the values of Tg and Td of the conju-
gated copolymers through thermal analyses using DSC and TGA
(Table 1). The values of Td of the conjugated copolymers were in the
range 373e422 ꢁC. The char yields at 800 ꢁC of the conjugated
copolymers were in the range 22e54%. Higher residues were
observed for the 2-D PTs as compared to that of the main-chain
type PTs. Generally, the incorporation of bulky TPATh pendent
units into the polymer backbone improved the thermal decompo-
sition properties of the conjugated copolymers. As a result, the
values of Td for PTBPTPA and PTStTPA were higher than those of
PTBP and PTSt. Notably, however, the value of Td of PTCNStTPA was
lower than that of PTCNSt, presumably because of the relatively low
molecular weight of PTCNStTPA. In addition, the presence of cyano
groups resulted in the values of Td of PTCNSt and PTCNStTPA being
lower than those of the other copolymers. We determined all of the
transition temperatures from the second round of DSC heating
scans. DSC analysis revealed superior values of Tg (ca. 100e153 ꢁC)
for our copolymers, making them suitable for PSC applications
[43,44]. Only one endotherm glass transition was observed for each
conjugated copolymer, implying that the dodecyl chain-containing
and TPATh segments were distributed homogeneously. The pres-
ence of rigid TPATh pendent units with high polarity hindered the
molecular motion of the polymer backbone; therefore, the values of
Tg for PTBPTPA and PTCNStTPA were higher than those for PTBP and
PTCNSt [45]. Notably, however, PTStTPA exhibited a lower value of
Tg than that of the PTSt. For PTStTPA, the incorporation of a low
content of TPATh units did not favor a high value of Tg. Conversely,
the presence of TPATh pendent units enhanced the free volume
between the polymer backbones. Therefore, the value of Tg for
PTStTPA was lower than that of PTSt. In addition, the high polarity
of the cyano moieties enhanced the interactions between the
polymer chains, resulting in the values of Tg of PTCNSt and
PTCNStTPA being higher than those of PTBP and PTBPTPA, respec-
tively [46]. Furthermore, unlike the high planarity of St units in the
polymer backbone, the presence of cyano groups resulted in steric
hindrance and twisting of the CNSt units, resulting in poorer
packing of the polymer backbones [47]. Therefore, the value of Tg of
PTCNSt was lower than that of PTSt. On the other hand, PTCNStTPA
density of 100 mW cmꢀ2
.
3. Results and discussion
3.1. Characterization of polymers
We used 1H NMR spectroscopy to determine the chemical
structures of the conjugated polymers. 1H NMR spectra of the main-
chain type and 2-D PTs are shown in Figure S1-1eS1-6 (see sup-
porting information). Chemical shifts in the range 6.9e8.2 ppm
represented the signals of the protons in the vinylene, phenyl,
and thiophene groups. The signals of the protons in the dodecyl
chains were observed at chemical shifts ranging from 0.8 to
2.8 ppm. For the 2-D PTs PTBPTPA, PTStTPA, and PTCNStTPA, the
repeat unit ratio (m/n) was modulated by controlling the feed ratio
of the conjugation unit (BP, St, CNSt), 6, and 7. The actual values of
m/n were determined from the relative integral areas of the peaks
at 6.9e8.2 ppm (protons in the vinylene, phenyl, and thiophene
groups) and 0.8e2.8 ppm (protons in the dodecyl chains) in the 1H
NMR spectra of the copolymers. The repeat unit ratios (m/n) of the
conjugated copolymers PTBPTPA, PTStTPA, and PTCNStTPA were
approximately 1.24:1, 1.85:1, and 1.27:1, respectively (Table 1). The
repeat unit ratios (m/n) were close to 1:1 for the copolymers
PTBPTPA and PTCNCtTPA, but approximated 2:1 for the copolymer
PTStTPA. We attribute the higher m:n ratio of PTStTPA to the poor
solubility of the St/TPATh segment [41] inhibiting the propagation
of the polymer chains; the good solubility of the St/3-
Table 1
Molecular weights and thermal properties of the main-chain-type and 2-D PTs.
Polymer
Mw (kg molꢀ1
)
PDI
Repeat unit ratio (m:n)a
Td (ꢁC)
Char yields at 800 ꢁC (%)
Tg (ꢁC)
PTBP
PTSt
PTCNSt
PTBPTPA
PTStTPA
PTCNStTPA
13.90
10.50
12.34
13.41
13.54
10.09
2.03
2.18
2.27
2.22
2.02
1.29
1:0
1:0
1:0
1.24:1
1.85:1
1.27:1
403
414
406
421
422
373
49
25
22
54
42
39
100
153
126
130
125
141
a
Calculated from the 1H NMR spectra of the polymers.

4096
H.-J. Wang et al. / Polymer 53 (2012) 4091e4103
Table 2
Optical and electrochemical properties of the main-chain-type and 2-D PTs.
abs
abs
abs
Egopt
E
HOMO
(eV)
LUMO
(eV)
ox
Polymer
l
l
l
max
max
onset
1=2
(nm)a
(nm)b
(nm)c
(eV)d
(V)
PTBP
PTSt
PTCNSt
PTBPTPA
PTStTPA
PTCNStTPA
376
407
431
391
407
434
387
409
447
395
414
443
468
502
539
506
525
563
2.65
2.47
2.30
2.45
2.36
2.20
1.00
0.80
1.00
0.74
0.76
0.55
ꢀ5.61
ꢀ5.41
ꢀ5.61
ꢀ5.35
ꢀ5.37
ꢀ5.16
ꢀ2.96
ꢀ2.94
ꢀ3.31
ꢀ2.90
ꢀ3.01
ꢀ2.96
a
Maximal absorption wavelength of the polymer in solution.
Maximal absorption wavelength of the polymer as a thin film.
Onset of UVeVis absorption of the polymer as a thin film.
b
c
d
Calculated from the onset absorption
(lonset) of the polymer thin film:
Egopt ¼ 1240/lonset
.
than those of PTBP and PTSt. We also observed the influences of the
coplanarity of the St unit and the polarity of the cyano groups on
the effective conjugated lengths of the conjugated copolymers
incorporating the TPATh pendent moieties. The maximal absorp-
tion wavelength and full width at half-maximum of PTCNStTPA
were greater than those of PTBPTPA and PTStTPA. Furthermore, the
red-shifts and full widths at half-maximum of the absorption bands
of the copolymers in the solid films were greater than those in
solution, presumably because of noncovalent interactions (e.g.,
pep stacking) between the polymer chains.
We determined the band gap energies ðEgoptÞ of the conjugated
polymers in their thin film state from the onset wavelengths of
their absorption bands. As listed in Table 2, the values of Egopt for
PTBP, PTSt, PTCNSt, PTBPTPA, PTStTPA, and PTCNStTPA were 2.65,
2.47, 2.30, 2.45, 2.36, and 2.20 eV, respectively. Because its phenyl
rings rotate freely, the BP units provided low conjugation lengths,
thereby minimizing electron transfer in its polymer backbones.
Conversely, the extended planar structure of the St units ensured
effective conjugation in its polymer backbones [42]. Notably, the
introduction of cyano units as electron-withdrawing groups in the
polymer backbones enhanced the intramolecular charge transfer
capacity of the conjugated polymers [49,50]. The value of Egopt for
PTBP was higher than that of PTCNSt. In addition, the incorporation
of electron-donating pendent units induced a higher degree of
intramolecular charge transfer within the conjugated framework of
the polymers, thereby decreasing the band gap energy of the
conjugated polymers [51,52]. Consequently, the values of Egopt of
PTBPTPA, PTStTPA, and PTCNStTPA were lower than those of PTBP,
PTSt, and PTCNSt, respectively. In general, a higher-intensity of
UVeVis absorption and a lower value of Egopt are favorable for
a conjugated polymer to harvest solar light. A strong absorption
Fig. 1. Normalized UVeVis absorption spectra of the main-chain-type and 2-D PTs ((a)
in o-DCB solution and (b) as solid film).
exhibited a higher value of Tg relative to that of PTStTPA, due to the
lower TPATh content of PTStTPA. Thus, the thermal stability of each
conjugated copolymer was affected by the planarity and polarity of
its conjugated units (BP, St, CNSt) and the bulk of its pendent
groups.
3.2. Optical properties of the conjugated copolymers
Fig. 1 presents the normalized UVeVis absorption spectra of the
copolymers in o-DCB solutions and as solid films. Table 2 summa-
rizes the photophysical properties of the copolymers. The absorp-
tions of the conjugated copolymers in solution and as films ranged
intensity and a broad absorption band should improve the
absorption efficiency of the photoactive layer, thereby enhancing
the photocurrent generated in a PSC. Apart from that, the Egopt
values of PTBPTPA, PTStTPA, and PTCNStTPA incorporated with
conjugated unit (TBP, St, CNSt) are larger than that of the TPATh
moiety-containing thiophene-derivatized copolymers [35]. This is
due to the fact that the conjugated units (TBP, St, CNSt) exhibit
a larger energy gap than that of the thiophene unit.
from 300 to 600 nm. The absorptions were attributed to the pep*
transitions of the conjugated main chains. In solution, the maximal
absorption wavelengths for the conjugated polymers PTBP and PTSt
appeared near 376 and 407 nm, respectively. This red-shifting of
the absorption band for PTSt relative to PTBP implies that the St
units in PTSt had a longer conjugation length and better coplanarity
Fig.
2 presents normalized UVeVis absorption spectra of
PTStTPA/PC61BM blend films. The absorption band of the conju-
gated polymer ranged from 350 to 700 nm, while that of PC61BM
ranged from 300 to 375 nm [53]. A red-shift of the absorption bands
was observed for the PTStTPA/PC61BM blend films as compared to
that of the PTStTPA film. This implies that the dilution of the small
molecules PC61BM resulted in the decreased main chains aggre-
gation of PTStTPA. Consequently, the effective conjugation length of
PTStTPA was enhanced by the blending of PC61BM [35]. Moreover,
the absorption intensity of the conjugated polymer decreased upon
increasing the PC61BM content in the PTStTPA/PC61BM blend films.
and p-electron delocalization along the polymer backbone than did
the BP units in PTBP. The poor coplanarity of the BP units in the
polymer backbone of PTBP was due to steric repulsion between the
ortho hydrogen atoms of the two neighboring phenyl groups [48].
Consequently, the maximal absorption wavelength of PTSt was
greater than that of PTBP. Moreover, introducing the electron-
deficient cyano groups into the polymer backbone enhanced the
intramolecular charge transfer capacity of the polymer chains. As
a result, the maximal absorption wavelength of PTCNSt was greater

H.-J. Wang et al. / Polymer 53 (2012) 4091e4103
4097
PTStTP A:P CBM (1:0)
1.0
0.8
0.6
0.4
0.2
0.0
PTStTP A:P CBM (1:1)
PTStTP A:P CBM (1:2)
PTStTP A:P CBM (1:3)
PTStTP A:P CBM (1:4)
350
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 2. Normalized UVeVis absorption spectra of PTStTPA/PC61BM blend films.
For the other conjugated polymer/PC61BM blend films, the
decreases in absorption intensity upon increasing the PC61BM
content were similar to those of the PTStTPA/PC61BM blend films.
From the point of view of photon absorption, it would be preferable
to employ less PC61BM in the photoactive layer; a high proportion
of PC61BM limits optical absorption in the composite layer because
its absorption is quite inefficient in the visible region. We observed
similar absorption behavior for the blend films based on the other
conjugated polymers and PC61BM.
Fig. 3 displays photoluminescence (PL) spectra of the copoly-
mers in solution and as thin films. The maximal PL wavelengths
em
ð
l
Þ increased upon increasing the effective conjugation length of
max
em
max
the conjugated polymers. The values of
l
of the polymers con-
taining St units (PTSt, PTStTPA) were greater than those of the BP-
containing polymers (PTBP, PTBPTPA). Moreover, the presence of
em
max
cyano groups resulted in the values of
l
of PTCNSt and
PTCNStTPA being greater than those of PTSt and PTStTPA, respec-
tively. The 2-D PTs incorporating TPATh pendent moieties
em
(PTBPTPA, PTStTPA, PTCNStTPA) exhibited greater values of
l
Fig. 3. PL spectra of the main-chain-type and 2-D PTs ((a) in o-DCB solution and (b) as
solid film).
max
than did the main-chain polymers (PTBP, PTSt, PTCNSt). In addition,
we observed bathochromic shifts of the maximum emission
wavelengths and broadening of the emission peaks for the copol-
ymers as solid films, relative to those in solution, due to
pe
p
3.3. Electrochemical properties of the conjugated copolymers
stacking and interchain aggregation in the polymer films. Notably,
however, a hypsochromic shift appeared for the maximum emis-
We employed CV to investigate the electrochemical behavior of
the conjugated polymers and to estimate their HOMO and LUMO
energy levels. Fig. 4 presents CV traces of the conjugated polymer
films in MeCN, recorded at a scanning rate of 50 mV sꢀ1 Table 2
summarizes the electrochemical properties of the copolymers.
The oxidation potentials of PTBPTPA, PTStTPA, and PTCNStTPA were
em
sion wavelength of the thin film of PTCNStTPA (
l
¼ 599 nm)
max
em
relative to that in solution (in o-DCB;
l
¼ 623 nm). On the
max
contrary, the maximal emission wavelength of PTCNStTPA as thin
em
max
film state (
l
¼ 599 nm) presented a bathochromic shift relative
em
to that in toluene (
l
¼ 587 nm). An apparent solvent-induced
max
solvatochromism was observed in a highly polar solvent, o-DCB.
Similar fluorescence properties of bipolar conjugated polymers in
relation to the polarity of the solvent have been investigated in the
literature [54,55].
0.74, 0.76, and 0.55 V, respectively; those of PTBP, PTSt, and PTCNSt
ox
were 1.00, 0.80, and 1.00 V, respectively. Thus, the values of E
of
1=2
the 2-D PTs (PTBPTPA, PTStTPA, PTCNStTPA) were lower than those
of the main-chain polymers (PTBP, PTSt, PTCNSt). We attribute the
ox
Next, we investigated the PL spectra of the conjugated polymer/
PC61BM blend films. The PL emissions were almost completely
quenched upon the addition of PC61BM (w/w, 1:1) to all of the
copolymer blend films (see Supporting Information, Fig S2). These
highly efficient PL quenching phenomena suggest that the excitons
generated by the absorbed photons dissociated completely to free
charge carriers (electrons, holes). Thus, effective charge transfer
existed from the conjugated copolymer to the electron acceptor
PC61BMda basic requirement for preparing a PSC exhibiting
satisfactory PV performance [56].
relatively lower values of E
of the 2-D PTs containing TPATh
1=2
moieties to the increased effective conjugation length resulting
from the presence of those TPATh moieties [57]. Moreover, the
ox
value of E
of PTSt was lower than that of PTBP. The presence of
1=2
the St unit enhanced the effective conjugation length of the poly-
mer backbone and decreased the oxidation potential of the
conjugated polymer. Conversely, the presence of a cyano group on
the St unit decreased the oxidation capacity of the polymer back-
bone and enhanced the oxidation potential of the polymer. As
ox
a result, the value of E
of PTCNSt was higher than that of PTSt.
1=2

4098
H.-J. Wang et al. / Polymer 53 (2012) 4091e4103
Bu4NClO4/MeCN solution was 0.19 V. Hence, the corresponding
HOMO energy levels were ꢀ5.61 eV for PTBP, ꢀ5.41 eV for
PTST, ꢀ5.61 eV for PTCNST, ꢀ5.35 eV for PTBPTPA, ꢀ5.37 eV for
PTStTPA, and ꢀ5.16 eV for PTCNStTPA. Because no reversible n-
doping process was observable in the CV spectrum, we estimated
the LUMO energy levels from the HOMO energy levels and the
values of Egopt from the UVeVis absorption spectra, using the
equation
ꢀ
ꢁ
LUMO ¼ HOMO þ Egopt ðeVÞ
Accordingly, the LUMO energy levels were ꢀ2.96 eV for
PTBP, ꢀ2.94 eV for PTST, ꢀ3.31 eV for PTCNST, ꢀ2.90 eV for
PTBPTPA, ꢀ3.01 eV for PTStTPA, and ꢀ2.96 eV for PTCNStTPA.
Typically, the introduction of electron-withdrawing cyano groups
decreases the value of Egopt by lowering the LUMO energy level,
while the introduction of electron-donating TPATh groups
decreases the value of Egopt by increasing the HOMO energy level.
According to donor/acceptor orbital hybridization of a bipolar
conjugated polymer, the HOMO of the donor segment will interact
with the LUMO of the acceptor segment to yield a lower value of
Egopt [59,60]. The HOMO energy levels of the main-chain-type
polymers and TPATh-containing 2-D PTs based on BP, St, and
CNSt units were lower than that of P3HT (ca. ꢀ4.7 to ꢀ4.9 eV)
[29,30]. In general, the HOMO energy level of a p-type conjugated
polymer is an important parameter affecting the performance of
a BHJ cells. High open circuit voltages (VOC) are typically obtained
for PSCs fabricated from conjugated polymers with low HOMO
energy levels. Here, the HOMO energy levels of PTBPTPA, PTStTPA,
and PTCNStTPA were slightly higher than those of PTBP, PTSt, and
PTCNSt, respectively. The introduction of the TPATh moieties did
not twist the main chains out of planar p-conjugation and did not
curtail the effective conjugation length of the polymer backbones.
Conversely, the electron-donating ability of the TPATh moiety
resulted in good conjugation and intramolecular charge transfer
with the BP, St, and CNSt units. Therefore, the HOMO energy level
increased slightly after the incorporation of the TPATh moieties.
Nevertheless, for the BP-, St-, and CNSt-based conjugated poly-
mers, low HOMO energy levels were maintained after attachment
of the TPATh pendent moieties. This effect of the incorporated
TPATh moieties on the backbones of the conjugated polymers is
quite different from the results published previously in the liter-
ature [34]. Li et al. reported that the effective conjugation length of
PTs was curtailed after the incorporation of bulky triphenylamine-
containing side chains, due to twisting of the main chains out of
Fig. 4. Electrochemical properties of the main-chain-type and 2-D PTs.
Notably, PTCNStTPA exhibited two quasi-reversible oxidation peaks
in the positive potential region. The first oxidation potential, near
0.55 V, was presumably due to oxidation of the electron-donating
TPATh pendent unit. Moreover, the potential of the oxidation
peak of the TPATh pendent moieties was lower than those of
PTBPTPA and PTStTPA, presumably because of strong intra-
planar
p-conjugation [34]; consequently, the HOMO energy level
was lowered after grafting the bulky pendent groups onto the PT
[34]. On the other hand, lower HOMO levels were observed for the
2-D PTs (PTBPTPA, PTStTPA, and PTCNStTPA) as compared to those
of the TPATh moiety-containing thiophene-derivatized copoly-
mers [35].
molecular
a donore
electron
transfer
in
polymers
possessing
p
eacceptor structure [58]. We suspect that the second
oxidation potential, near 0.89 V, was due to the non-planarity of the
CNSt units in the PTCNStTPA main chain, thereby minimizing the
electronic communication of the polymer backbone. As a result, the
oxidation of the CNSt unit occurred at higher potential [47]. The
oxidation-onset potentials for the main-chain-type and 2-D PTs
incorporating the BP, St, and CNSt units were greater than those of
P3HT [34], suggesting that their air stabilities were greater than
3.4. Morphologies of conjugated polymer/PC61BM blend films
The performance of a PSC is strongly dependant on the
morphology of the conjugated polymer/fullerene-derivative
composite film [61]. To avoid recombination of excitons, the P/N
heterojunction phase must be controlled under nanoscale distri-
bution, due to the diffusion range of the charge carrier being
approximately 3e10 nm [62,63]. We used AFM microscopy to
investigate the compatibility and morphology of the conjugated
polymer/PC61BM composite films. Fig. 5 displays topographic and
phase images of the PTBP/PC61BM, PTSt/PC61BM, and PTCNSt/
PC61BM blend films (1:1, w/w) after they had been annealed at
80 ꢁC for 1 h Table 3 summarizes the roughnesses of these
ox
that of P3HT. From the values of E , we calculated the HOMO
1=2
energy levels of the conjugated copolymers using the equation
ꢀ
ꢁ
ox
ox
HOMO ¼ ꢀe E
ꢀ E
;
ferrocene þ 4:8 ðeVÞ
1=2
1=2
Here, 4.80 eV is the energy level of ferrocene below the vacuum
ox
level; the value of E
of ferrocene/ferrocenium in the 0.1 M
1=2

H.-J. Wang et al. / Polymer 53 (2012) 4091e4103
4099
Fig. 5. Topographic (a, c, e) and phase (b, d, f) images of PTBP/PC61BM (a, b), PTSt/PC61BM (c, d), and PTCNSt/PC61BM (e, f) blend films (1:1, w/w) that had been annealed at 80 ꢁ
C
for 1 h.
composite films. For the PTBP/PC61BM blend film, we observed
a phase-separated interpenetrating network with sizable PC61BM
domains. A certain degree of phase separation is critical for efficient
formation of free carriers, which might provide optimal PV
properties for the resulting PSC. For the PTSt/PC61BM blend film, the
poor solubility of PTSt resulted in serious separation of the PTSt and
PC61BM phases. For the PTCNSt/PC61BM blend film, we observed
good miscibility of the PTCNSt and PC61BM units. We did, however,
observe nanoparticles (<100 nm) on the surface of PTCNSt/PC61BM
blend film, presumably because of the presence of non-soluble
polymer material [64,65]. Fig. 6 displays topographic and phase
images of the PTBPTPA/PC61BM, PTStTPA/PC61BM, and PTCNStTPA/
PC61BM blend films (1:1, w/w) after they had been annealed at
80 ꢁC for 1 h. The PC61BM molecules were distributed in a homo-
geneous manner in the composite films based on the 2-D PTs
(PTBPTPA, PTStTPA, PTCNStTPA), suggesting that the presence of
the TPATh pendent moieties enhanced the miscibility of the
conjugated polymer and PC61BM. The bulky side chains also
provided sufficient free volume for the PC61BM units to intercalate
into the polymer chains, enhancing the compatibility of the poly-
mer and fullerene derivative. Nevertheless, several nanoparticles
were present on the surface of the PTStTPA/PC61BM blend film,
presumably because of the relatively poor solubility of PTStTPA. In
Table 3
PV performances of PSCs based on conjugated polymer/PC61BM blends.
PSC
Polymer/PC61BM
(1:1, w/w)
RMS
VOC
(V)
JSC
FF
h (%)
(nm)a
(mA cmꢀ2
)
PSC I
PTBP/PC61BM
PTSt/PC61BM
2.37
2.49
5.51
0.79
1.69
0.88
3.02
0.68
0.83
0.83
0.86
0.86
0.65
0.83
0.69
0.04
0.17
1.38
0.90
1.99
1.73
0.26
0.12
0.28
0.32
0.31
0.27
0.33
0.12
0.01
0.04
0.38
0.24
0.36
0.49
PSC II
PSC III
PSC IV
PSC V
PSC VI
PSC VII
PTCNSt/PC61BM
PTBPTPA/PC61BM
PTStTPA/PC61BM
PTCNStTPA/PC61BM
PTStTPA/PC61BMb
a
RMS: the root-mean-square (RMS) value of polymer/PC61BM blend film
measured by AFM.
b
The photo-active layer was annealed under 140 ꢁC for 10 min.

4100
H.-J. Wang et al. / Polymer 53 (2012) 4091e4103
Fig. 6. Topographic (a, c, e) and phase (b, d, f) images of PTBPTPA/PC61BM (a, b), PTStTPA/PC61BM (c, d), and PTCNStTPA/PC61BM (e, f) blend films (1:1, w/w) that had been annealed
at 80 ꢁC for 1 h.
addition, the surface roughnesses of the 2-D PT-based blend films
were lower than those of the blend films based on main-chain-type
polymers. Because the incorporation of the TPATh moieties
enhanced the miscibility of the conjugated polymer and PC61BM,
the 2-D PT-based blend films exhibited lower surface roughnesses.
and ƞ of the PTBP/PC61BM blend-based PSC I were much greater
than those of the PTSt/PC61BM and PTCNSt/PC61BM blend-based
PSCs (PSC II and PSC III, respectively). We attribute the poorer PV
performances of PSC II and PSC III to the phase separation and low
quality of the photoactive layer. Phase separation, which results in
current leakage in PSCs, decreased the values of JSC and FF for PSC II
and PSC III [66]. Therefore, the values of ƞ for the PSCs based on the
PTSt/PC61BM and PTCNSt/PC61BM blends were lower than that of
the PTBP/PC61BM blend-based PSC. Conversely, the values of VOC of
PSC II and PSC III were slightly larger than that of PSC I. Although
the value of VOC is determined primarily by the difference between
the LUMO energy level of the electron acceptor (in this case,
PC61BM) and the HOMO energy level of the electron donor (the
conjugated polymer), it is also affected by the morphology of the
photoactive layer [67] (Fig. 8).
3.5. PV properties of PSCs based on conjugated polymer/PC61BM
blend films
We fabricated PSCs incorporating films of the conjugated poly-
mer/PC61BM (1:1, w/w) blends prepared from o-DCB solutions
(15 mg mLꢀ1) using an optimized spin-coating procedure. The
photoactive layers based on the conjugated polymer/PC61BM
blends were thermally treated at 80 ꢁC for 1 h Fig. 7 displays the
photocurrent densityevoltage plots of the PSCs. Table 3 summa-
rizes the PV properties of these PSCs, including their open-circuit
voltages (VOC), short-circuit current densities (JSC), fill factors
(FFs), and photo-energy conversion efficiencies (ƞ). The values of JSC
Notably, the PV performances of the PSCs based on the 2-D PT
(PTBPTPA, PTStTPA, PTCNStTPA)/PC61BM blends (PSCs IVeVI) were
much better than those of the PSCs based on the main-chain-type

H.-J. Wang et al. / Polymer 53 (2012) 4091e4103
4101
PTBP and PTSt. We attribute the lower values of VOC of PSCs I and II
to the poorer thin film quality PTBP and PTSt [67]. Notably, the PV
performance of PSC V was poorer than that of PSC IV, even through
the conjugation length in PTStTPA was larger than that in PTBPTPA;
we suspect that the film quality of the PTStTPA/PC61BM blend was
poorer than that of the PTBPTPA/PC61BM blend. Moreover, the value
of JSC for PSC VI was higher than that for PSC IV, presumably
because of the enhanced effective intramolecular charge transfer
and light-harvesting capacity of the PTCNStTPA/PC61BM blend-
based PSC. In contrast, however, the PTCNStTPA/PC61BM-based
PSC VI provided a value of ƞ that was lower than that of PSC IV
because of its lower values of VOC and FF. We attribute the lower
value of VOC of PSC VI to the higher HOMO energy level of
PTCNStTPA. Incorporating electron-withdrawing cyano groups not
only enhanced the effective intramolecular charge transfer of the
polymer backbone but also decreased the hole mobility in
PTCNStTPA [23]. Accordingly, PSC VI provided a lower FF.
In addition, the thermal annealing effect on the PV performance
of the 2-D PT/PC61BM blend film based PSC was also studied. The
photo-active layer of PTStTPA/PC61BM blend film (w/w ¼ 1:1) based
PSC VII was thermally annealed above the Tg of the copolymer
(140 ꢁC for 10 min). The photocurrent densityevoltage plot of PSC
VII is shown in Fig. 7. The PV properties of PSC VII are summarized
in Table 3. The values of JSC and ƞ of the PSC VII were much greater
than those of the PSC V. The enhancement of PV performance of
PTStTPA/PC61BM blend based PSC was further investigated by the
observation of morphological change of the photo-active layer
annealed at 140 ꢁC for 10 min. AFM images of the PTStTPA/PC61BM
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
PTBP/PC BM (80 C 1 h)
PTSt/PC BM (80 C 1 h)
PTCNSt/PC BM (80 C 1 h)
PTBPTPA/PC BM (80 C 1 h)
PTStTPA/PC BM (80 C 1 h)
PTCNStTPA/PC BM (80 C 1 h)
PTStTPA/PC BM (140 C 10 min)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Voltage (V)
Fig. 7. Photocurrent densityevoltage plots of PSCs fabricated from the conjugated
polymer/PC61BM blends (1:1, w/w) that had been annealed at 80 ꢁC for 1 h or 140 ꢁC
for 10 min.
conjugated polymer (PTBP, PTSt, and PTCNSt)/PC61BM blends (PSCs
IeIII). Several factors are responsible for this behavior, including the
effective conjugation length and hole mobility of the conjugated
polymer and the morphology of the photoactive layer. The presence
of electron-donating TPATh pendent units enhanced the effective
conjugation length and hole mobility of the conjugated polymer,
thereby improving the solar light absorption capacity and hole
mobility in the photoactive layer of the corresponding PSC [35].
Moreover, the compatibility of the conjugated polymer and PC61BM
improved upon appending the TPATh moieties to the polymer
backbone. Their presence also had a positive effect on the charge
separation and transporting capacity in the photoactive layer.
Accordingly, the JSC and ƞ of the PSCs based on the TPATh-
presenting 2-D PTs were much better than those of the PSCs
based on the main-chain-type conjugated polymers. Moreover,
higher values of VOC were observed for the TPATh-containing 2-D
PTs based PSCs IV and PSC V as compared to those of the main-
chain-type PTs based PSCs I and II, despite the fact that the
HOMO levels of PTBPTPA and PTStTPA were higher than those of
blend film indicate that
a phase-separated interpenetrating
network with sizable PC61BM domains was formed after annealing.
A certain degree of phase separation might provide better charge
separation for the resulting PSC. TEM images of the PTStTPA/
PC61BM blend film after thermal annealed at 80 ꢁC for 1 h or 140 ꢁC
for 10 min are shown in Fig. 9. It was found that the PC61BM unit
was uniformly distributed in blend film after being annealed at
80 ꢁC for 1 h (Fig. 9(a)). Moreover, the morphology of phase sepa-
ration became obvious for the blend film annealed at 140 ꢁC for
10 min (Fig. 9(b)). The change in morphology resulted in a large
interfacial area for efficient charge generation [68]. Therefore, the
photo-current density and photo-conversion efficiency of the
PTStTPA/PC61BM blend film based PSC were enhanced significantly
after the photo-active layer was thermal annealed at 140 ꢁC for
10 min.
Fig. 8. Topographic (a) and phase (b) images of PTStTPA/PC61BM (1:1, w/w) that had been annealed at 140 ꢁC for 10 min.

4102
H.-J. Wang et al. / Polymer 53 (2012) 4091e4103
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Appendix A. Supplementary material
Supplementary material associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.polymer.
2012.07.010.

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