Polymer solar cells fabricated with 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4, 5-b′]dithiophene and alkyl-substituted thiophene-3-carboxylate-containing conjugated polymers: Effect of alkyl side-chain in thiophene-3-carboxylate monomer on the device performance

Article abstract of DOI:10.1016/j.polymer.2012.07.007

Four alkyl-substituted thiophene-3-carboxylate containing donor-acceptor (D-A) copolymers were designed, synthesized, and characterized. Thiophene-3-carboxylate was used as a weak electron acceptor unit in the copolymers to provide a deeper highest occupied molecular orbital (HOMO) level for obtaining a higher open-circuit voltage in polymer solar cells (PSCs). The resulting bulk heterojunction PSCs, made of the copolymers and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM), exhibited different short circuit currents (J_SCs) and open-circuit voltages (V_OCs), depending on the length of alkyl side-chain in the thiophene-3-carboxylate unit. Among all fabricated photovoltaic (PV) devices, PC2:PC₇₁BM (1:1 wt. ratio) showed the highest efficiency with the highest J_SC of 10.5 mA/cm². Although PC5:PC₇₁BM (1:1) displayed the highest V_OC of 0.93 V, the device efficiency was observed to be poor, which is due to poor nanophase segregation. This comparison shows that the side-chain of thiophene carboxylate in these copolymers plays a very important role in the device efficiency.

Full text of DOI:10.1016/j.polymer.2012.07.007

Polymer 53 (2012) 3835e3841
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
0
Polymer solar cells fabricated with 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b ]
dithiophene and alkyl-substituted thiophene-3-carboxylate-containing
conjugated polymers: Effect of alkyl side-chain in thiophene-3-carboxylate
monomer on the device performance
a
a
a
b
b,
a,
*
Min Ju Cho , Jangwon Seo , Kai Luo , Kyung Hwan Kim , Dong Hoon Choi **, Paras N. Prasad
a
Department of Chemistry, University at Buffalo, State University of New York, Buffalo, NY 14260, USA
Department of Chemistry, Research Institute for Natural Sciences, Korea University, Seoul 136-701, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 May 2012
Received in revised form
Four alkyl-substituted thiophene-3-carboxylate containing donoreacceptor (DeA) copolymers were
designed, synthesized, and characterized. Thiophene-3-carboxylate was used as a weak electron acceptor
unit in the copolymers to provide a deeper highest occupied molecular orbital (HOMO) level for
obtaining a higher open-circuit voltage in polymer solar cells (PSCs). The resulting bulk heterojunction
PSCs, made of the copolymers and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), exhibited
different short circuit currents (JSCs) and open-circuit voltages (VOCs), depending on the length of alkyl
side-chain in the thiophene-3-carboxylate unit. Among all fabricated photovoltaic (PV) devices,
26 June 2012
Accepted 5 July 2012
Available online 16 July 2012
Keywords:
2
PC2:PC71BM (1:1 wt. ratio) showed the highest efficiency with the highest JSC of 10.5 mA/cm . Although
Bulk heterojunction
Copolymerization
PCBM
PC5:PC71BM (1:1) displayed the highest VOC of 0.93 V, the device efficiency was observed to be poor,
which is due to poor nanophase segregation. This comparison shows that the side-chain of thiophene
carboxylate in these copolymers plays a very important role in the device efficiency.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
capability is directly related to the value of JSC. Recently, efficient
low-bandgap copolymers with broad absorption spectra were
Polymer solar cells (PSCs) are of interest because of their
potential to be fabricated by low cost of device fabrication using
solution-processing technology such as spin-coating, drop-casting,
and ink-jet printing, etc. They also offer potential possibility for
future flexible photovoltaic (PV) devices [1e5]. To achieve a high
power conversion efficiency (PCE), highly crystalline polymer
donors and organic soluble fullerene acceptors have often been
employed for solution-processed bulk heterojunction (BHJ) PSCs,
which have been shown to have a PCE approaching 5.0e8.5%
developed by using electron donating units, such as cyclopenta[2,1-
0
0
0
b:3,4-b ]dithiophene [14,15], dithieno[3,2-b:2 ,3 -d]silole [16], and
benzo-[1,2-b:4,5-b]dithiophene [10,13], and electron withdrawing
units, such as 2,1,3-benzothiadiazole [17], diketopyrrolo[3,4-c]-
pyrrole-1,4-dione [18], thieno[3,4-b]thiophene [10,13], and thieno
[3,4-c]pyrrole-4,6-dione [19]. VOC is also one of the important
factors to obtain a high PCE value. VOC of PSC is commonly
proportional to the difference between the HOMO of the electron
donating polymeric unit and the LUMO of the electron accepting
PCBM within the active layer [20]. Therefore, it is desirable for
efficient PSCs to design an electron donor polymer with a lower
HOMO level. Some kinds of donoreacceptor (DeA) copolymers
have been successfully used for achieving high VOCs in PSCs by
choosing fluorene, silafluorene, and carbazole derivatives as elec-
tron donating groups. These derivatives consist of well-known
wide-band-gap semiconducting materials. However, most of
these polymers have suffered from a low JSC in PSCs, except for
some specific polymers [8,9]. Among these copolymers, only a few
[
(
6ꢀ10]. PCE is the product of the short-circuit current density
J
SC), the open circuit voltage (VOC), and the fill factor (FF) divided by
the incident light intensity. Therefore, for achieving improved JSC, it
is necessary that the polymeric donors should have a broad
absorption from the visible to near-infrared (NIR) wavelength range
of the solar spectrum [11e13], because the light absorbing
*
Corresponding author. Tel.: þ1 716 645 6800; fax: þ1 716 645 6945.
polymers, containing
a
thieno[3,4-c]pyrrole-4,6-dione (TPD)
*
* Corresponding author.
acceptor unit, showed relatively higher VOC and JSC compared to the
conventional P3HT:PCBM BHJ PSC [21e24]. Although the TPD-
E-mail addresses: dhchoi8803@korea.ac.kr (D.H. Choi), pnprasad@buffalo.edu
(
P.N. Prasad).
0
032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.07.007

3836
M.J. Cho et al. / Polymer 53 (2012) 3835e3841
based polymers with PCBM exhibited a high efficiency in PSC, the
synthesis of TPD is quite complicated to produce a desirable
amount of corresponding polymers. It is quite intriguing that the
alkyl-substituted thiophene-3-carboxylate (TC) groups act as
electron-withdrawing units in the main-chain of conjugated
copolymers. The TC unit with different alkyl groups can be easily
prepared through a simple synthetic procedure. In the past, several
research groups reported that PSCs with TC-based polymers and
PCBM exhibited a much better PV performance, with moderate
high PCE and VOC values than P3HT:PCBM-based PSCs. For example,
Hu et al. reported a new regiorandom PCTBDT which is an alter-
nating copolymer containing benzodithiophene (BDT) and hexyl
thiophene-3-carboxylate [25]. The PSC obtained from
PCTBDT:PCBM blends exhibited PCE value up to 2.32%, with VOC of
The film samples were fabricated by spin-coating on silicon wafer,
followed by drying at 90 C for 10 min.
ꢁ
2.3. Organic thin film transistor (OTFT) device fabrication and
characterization
For the characterization of TFT performance, a bottom gate top
contact device geometry was employed. On a heavily n-doped Si/
SiO substrate, spin-coated films of copolymers were prepared
2
from chloroform as the solvent. Surface modification was carried
out with octyltrichlorosilane (OTS) to make the dielectric surface
hydrophobic. The source and the drain electrodes were then ther-
mally evaporated (100 nm) through a shadow mask, with channel
width and length of 1500
mm, and 100 mm, respectively. All the field
0
.80 V. Zhao’s group demonstrated PL5 copolymer consisting of
effect mobilities were extracted in the saturation regime using the
BDT and butyl thiophene-3-carboxylate. The PCE and VOC values of
the PV devices based-on PL5 were measured to be 2.06% and 0.75 V,
respectively [26]. In our previous study, we also synthesized the TC-
based copolymer (PBDT-3MT) containing BDT and methyl thio-
phene-3-carboxylate for fabricating PSCs. The PV device composed
of PBDT-3MT:PCBM blends exhibited a high PCE value of 4.52%,
with VOC ¼ 0.86 V [27]. Herein, we have synthesized a series of
2
relationship
m
sat ¼ (2IDSL)/(WC(V
G
ꢀVth) ), where IDS means satu-
dielectric, V is gate
ration drain current, C is capacitance of SiO
2
G
bias, and Vth is threshold voltage. The device performance was
evaluated in air using a 4200-SCS semiconductor characterization
system.
0
2.4. Photovoltaic device fabrication and characterization
copolymer bearing 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b ]
dithiophene and the TC unit, and investigated the effect of alkyl
side chain (e.g. methyl, butyl, octyl, and 2-ethylhexyl groups) in the
TC unit on the PV device performances. Also, we systematically
studied the effect of alkyl side-chain in TC unit of these copolymers
on the optical properties, film morphology, carrier mobility in
organic thin film transistor (OTFT), as well as on the photovoltaic
performance. These results provide useful information on how to
control the VOC and PCE value of OPV devices, based-on the struc-
tural variation in copolymers containing TC unit.
All devices reported here were fabricated on indium tin oxide
(
ITO) coated glass substrates. A 30 nm layer of PEDOT:PSS was spin
ꢁ
coated on the top of ITO and baked at 120 C for 10 min. Subsequent
device processing was performed in a glove box under nitrogen
atmosphere. A polymer:PC71BM blend film was spin coated on top
of the PEDOT:PSS layer using a o-dichlorobenzene (o-DCB) solution
ꢁ
and was dried at 90 C for 10 min. Finally, a w100 nm thick lithium
fluoride (LiF)/aluminum (Al) electrode was deposited by a thermal
ꢀ
6
ꢀ7
2
evaporator (vacuum at 10 e10 Torr) to produce a 0.0425 cm
active area using a mask. The IeV characteristics were measured by
2
. Experimental section
a computerized Keithley 2400 source meter in the dark and under
2
AM 1.5G illumination at 100 mW/cm supplied by a solar simulator
2.1. Materials
(
Oriel, 1000 W). The spectral response was measured using
a tungstenehalogen light source combined with a monochromator
Spectra Pro 2300, Acton Research). The light was filtered through
All reagents were purchased from the SigmaeAldrich Co. and
(
used without further purification, unless stated otherwise. Reagent
grade solvents used in this study were freshly dried under standard
distillation methods. The 2,5-dibromothiophene-3-carboxylic acid
the monochromator for the UV, visible and IR wavelength range to
remove harmonics and was modulated using an optical chopper at
350 Hz. The resulting photocurrent in the photovoltaic cell was
(1), 2b, 3, and PC2 (PBDT-3MT) were prepared according to pub-
measured using a lock-in amplifier (SR-830, Stanford Research).
The power of the monochromatic light was measured by using
a calibrated optical power meter and a photodetector (Coherent
Fieldmax II with UV to IR sensor heads and a Thorlabs PM100). All
equipment and data acquisition were controlled by LabView.
lished procedures [27,28].
2.2. Instrumental analysis
1H NMR spectra were recorded on a Varian NMR 400 and
5
00 MHz spectrometer using deuterated chloroform purchased
from Cambridge Isotope Laboratories, Inc. The molecular weight of
the polymers was determined on a Waters 1525/2414 gel perme-
ation chromatography (GPC) with polystyrene as a standard.
Thermal gravimetric analysis (TGA) was conducted on a Mettler
2.5. Synthesis of alkyl 2,5-dibromothiophene-3-carboxylates:
general procedure
2,5-Dibromothiophene-3-carboxylic acid (1) (1.0 mmol) and
K CO (3.0 mmol) were dissolved in 20 mL of dried dime-
2 3
ꢁ
TGA50 thermal analysis system at a heating rate of 10 C/min. The
absorption spectra of the film samples and of chloroform solutions
were obtained using a Shimadzu UV-3101 PC spectrophotometer in
the wavelength range of 300e800 nm. The redox property of the
synthesized copolymers was examined by cyclic voltammetry
thylformamide (DMF), and alkyl bromide/iodide (2.0 mmol) was
added in portions. The mixture was warmed and stirred at 80 C for
10 h. After the reaction was complete, the mixture was poured into
200 mL of water bearing 5 mL of 1 M HC1. The solution was washed
ꢁ
(
0
Model: EA161 eDAQ). The electrolyte solution employed was
.10 M tetrabutylammonium hexafluorophosphate (Bu NPF ) in
2 4
with ether, and dried with Na SO . The ether was removed with
a rotary evaporator and the products (2bee) were purified by silica-
4
6
freshly dried acetonitrile. The Ag/AgCl and Pt wire (0.5 mm in
diameter) electrodes were utilized as the reference and the counter
electrodes, respectively. The scan rate was at 50 mV/s. Atomic force
microscopy (AFM, Advanced Scanning Probe Microscope, XE-100,
PSIA), operating in the tapping mode with a silicon cantilever,
was used to characterize the surface morphologies of the samples.
gel column chromatography.
2.5.1. Butyl-2,5-dibromothiophene-3-carboxylate (2c)
2c was synthesized with 2,5-dibromothiophene-3-carboxylic
acid (1) and 1-bromobutane by following the general procedure.
1
The final product, colorless oil, was obtained with a 92% yield. H

M.J. Cho et al. / Polymer 53 (2012) 3835e3841
3837
ꢁ ꢁ
2 3 3 4
Scheme 1. Synthetic procedure for monomers and polymers: i) alkyl bromide or alkyl iodide, K CO , DMF, 80 C, 10 h, ii) Pd(PPh ) , toluene/DMF (9:1, v/v), 110 C, 60 h.
NMR (400 MHz, CDCl
3
,
d, ppm): 7.38 (s, 1 H), 4.29 (t, 2 H), 1.78e1.58
7.20e6.75 (br, 4H), 4.20e3.82 (br, 4H), 1.85e1.42 (br, 18H),
(
m, 2 H), 1.52e1.41 (m, 2 H), 0.91 (t, 3 H).
1.30e0.89 (br, 12H). GPC: M
w
¼ 8.2 kDa, M ¼ 4.3 kDa, PDI ¼ 1.9.
n
ꢁ
TGA: T
d
¼ 326 C.
2
.5.2. Octyl-2,5-dibromothiophene-3-carboxylate (2d)
2
d was synthesized with 2,5-dibromothiophene-3-carboxylic
2.6.2. Synthesis of PC3 (4c)
4c was synthesized with monomer
acid (1) and 1-bromooctane by following the general procedure.
3
and butyl-2,5-
The final product, a white powder, was obtained with an 88% yield.
dibromothiophene-3-carboxylate (2c) by following the general
1
H NMR (400 MHz, CDCl
3
,
d
, ppm): 7.40 (s, 1H), 4.31 (t, 2H),
procedure. The final product, a red powder, was obtained with
1
1.79e1.56 (m, 2 H), 1.35e1.18 (m, 10 H), 0.90 (t, 3H).
a 75% yield. H NMR (500 MHz, CDCl
3
,
d
, ppm): 7.95 (br, 1H),
7.73e7.35 (br, 2H), 4.38e4.08 (br, 6H), 1.89e1.25 (br, 22H),
2
.5.3. 2-Ethylhexyl-2,5-dibromothiophene-3-carboxylate (2e)
0.90e1.21 (br, 15H). GPC: M
w
¼ 40.9 kDa, M ¼ 15.3 kDa, PDI ¼ 2.7,
n
ꢁ
2
e was synyhesized with 2,5-dibromothiophene-3-carboxylic
TGA: T
d
¼ 318 C.
acid (1) and 2-ethylhexyl-1-bromide by following the general
procedure. The final product, colorless oil, was obtained with an
2.6.3. Synthesis of PC4 (4d)
4d was synthesized with monomer
dibromothiophene-3-carboxylate (2d) by following the general
1
8
2
2% yield. H NMR (400 MHz, CDCl
H), 2.11e1.89 (m, 1H), 1.31e1.21 (m, 8H), 0.89e0.82 (m, 6H).
3
,
d
, ppm): 7.44 (s, 1H), 4.32 (t,
3
and octyl-2,5-
procedure. The final product, a red powder, was obtained with
1
2.6. Synthesis of copolymers: general procedure
a 71% yield. H NMR (500 MHz, CDCl
3
,
d, ppm): 7.93 (br, 1H),
7.71e7.57 (br, 2H), 4.32e4.21 (br, 6H), 1.91e1.42 (br, 22H),
To a 10 mL three-necked flask, bis(trimethylstannyl)-monomer
1.30e0.82 (br, 23H). GPC: M
w
¼ 81.0 kDa, M
n
¼ 28.7 kDa, PDI ¼ 2.8,
ꢁ
(
(
0.2 mmol), dibromo-monomer (0.2 mmol), and Pd(PPh
3
)
4
TGA: T
d
¼ 327 C.
0.012 g, 0.01 mmol) were dissolved in anhydrous toluene (18 mL)
and dimethylformamide (DMF) (2 mL). The reaction mixture was
ꢁ
heated to 110 C and stirred for 60 h. The crude product was
collected by precipitating in methanol (200 mL), and subjected to
Soxhlet extractions with acetone and CHCl
3
in succession. The pure
PC1
PC2
PC3
PC4
PC5
1
00
polymer was collected from the CHCl
the solvent.
3
extracted after removal of
8
6
4
2
0
0
0
0
0
2.6.1. Synthesis of PC1 (4a)
4
a was synthesized with 4,8-bis(2-ethylhexyloxy)benzo[1,2-
0
b:4,5-b ]dithiophene (3) and 2,5-dibromothiophene (2a) by
following the general procedure. The final product, a red powder,
1
was obtained with a 64% yield. H NMR (400 MHz, CDCl
3
,
d
, ppm):
Table 1
Physical properties of copolymers.
ꢁ
Polymer
Yield [%]
M
n
[kDa]
M
w
[kDa]
PDI
T
d
[ C]
0
200
400
600
800
PC1
PC2
PC3
PC4
PC5
64
77
75
71
66
4.3
26.9
15.3
28.7
142.4
8.2
92.0
40.9
81.0
209.7
1.9
3.4
2.7
2.8
1.5
326
320
318
327
317
o
Temperature ( C)
Fig. 1. Thermogravimetric analysis (TGA) plot of the copolymers with a heating rate of
ꢁ
10
C/min in an atmosphere of N
2
.

3838
M.J. Cho et al. / Polymer 53 (2012) 3835e3841
a
b 1.0
0.5
0.4
0.3
0.2
0.1
0.0
PC1
PC1
PC2
PC3
PC4
PC5
PC2
PC3
PC4
PC5
0.8
0.6
0.4
0.2
0.0
300
400
500
600
300
350
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Fig. 2. UVevis absorption spectra of copolymers in the o-DCB solution (a) and in the film state (b).
2
.6.4. Synthesis of PC5 (4e)
tetrahydrofuran (THF) as eluent and polystyrene as the standard
are listed in Table 1.
4
e was synthesized with monomer 3 and 2-ethylhexyl-2,5-
dibromothiophene-3-carboxylate (2e) by following the general
procedure. The final product, a red powder, was obtained with
3.2. Thermal properties
a 66% yield. 1H NMR (500 MHz, CDCl
,
d, ppm): 7.94 (br, 1H),
3
7
.75e7.53 (br, 2H), 4.35e4.12 (br, 6H), 1.89e1.18 (br, 27H), 1.15e0.81
The thermal stability of the polymers was characterized by
thermogravimetric analysis (TGA). As shown in Fig. 1 and Table 1,
(
br, 18H). GPC: M
w
¼ 209.7 kDa, M ¼ 142.6 kDa, PDI ¼ 1.5, TGA:
n
ꢁ
T
d
¼ 317 C.
the polymers, PC1ꢀ5 are thermally stable, with 5% weight-loss
ꢁ
temperatures of 326, 320, 318, 327, and 317 C, respectively. The
thermal stability of the copolymers satisfied the requirement for
their applications to PSCs and other electronic devices.
3
. Results and discussion
3.1. Synthesis and characterization
3.3. Optical properties
The general synthetic procedure for the monomers and the
copolymers is outlined in Scheme 1. Monomers, 2bꢀe were easily
The UVevis absorption spectra of the copolymers in dilute
chloroform solution and in thin films are shown in Fig. 2 and
Table 2. In the solution of PC1ꢀ5, the weak absorption peak at short
synthesized by using 2,5-dibromothiophene-3-carboxylic acid (1)
2 3
and alkyl bromide/iodide in the K CO base. In order to investigate
the effect of alkyl side-chain in TC unit of copolymers on PV device
performance, we selected the methyl, butyl, octyl, and 2-ethylhexyl
groups as side-chain moieties in the TC unit. Polymers, PC2ꢀ5 were
synthesized by using the Stille coupling reaction of monomer 3
with monomers 2bꢀe in a mixture of toluene/DMF co-solvent in
wavelength (
l
¼ 350ꢀ425 nm) originates from the
p p* transition
ꢀ
of the benzodithiophene units, while the strong absorption peak at
long wavelength (
¼ 450e550 nm) is due to intra-molecular
charge transfer (ICT) between the donor and the acceptor units.
In the film state, the maximum absorption peaks (
are red-shifted by about 33e42 nm compared with those in solu-
tion. It implies that PC3ꢀ5 in solution have random coil structures
l
lmax) of PC3ꢀ5
the presence of Pd(PPh
3
)
4
as a catalyst. In particular, PC1 containing
only thiophene unit was also synthesized with the same method as
a control sample, to compare the effect of the carboxylate substit-
uents of PC2ꢀ5 on the PSC performances. All copolymers showed
good solubility in common organic solvents such as chloroform,
THF, toluene, and o-dichlorobenzene (o-DCB). The molecular
weights and the polydispersity indices (PDIs) of copolymers were
measured by gel permeation chromatography (GPC) with
0.5
0.4
0.3
0.2
0.1
PC1 5 deg
PC2 5 deg
PC1 90 deg
PC2 90 deg
Table 2
Photo and electrochemical properties of copolymers.
a
HOMOb LUMOc
opt
g
[eV]
d
Polymers Absorbance [nm]
Solution Film
500, 534 505, 545 596
l
cut-off
E
[V]
ox
D
E
[
nm]
[eV]
[eV]
PC1
PC2
PC3
PC4
PC5
0.92 ꢀ5.32
1.01 ꢀ5.41
1.02 ꢀ5.42
1.03 ꢀ5.44
1.10 ꢀ5.50
ꢀ3.24
ꢀ3.34
ꢀ3.31
ꢀ3.36
ꢀ3.38
2.08
2.07
2.11
2.08
2.12
503
478
473
476
516
511
515
509
600
589
596
585
0.0
300
350
400
450
500
550
600
650
a
The oxidation potential (Eox) was determined by E oo xn set.
Wavelength (nm)
b
c
onset
HOMO ¼ ꢀeð4:4V þ E
Þ.
ox
opt:
ꢁ
LUMO ¼ E
g
þ HOMO.
Fig. 3. UVevis absorption spectra of PC1 and PC2 in the o-DCB solution at 5 C and
d
opt:
ꢁ
E
g
¼ 1240/
l
cut-off
.
90 C.

M.J. Cho et al. / Polymer 53 (2012) 3835e3841
3839
spectral shape at high temperature are similar to those of the
PC3ꢀ5 bearing relatively long and bulky alkyl side-chain groups.
Therefore, the resulting absorption behaviors indicate that PC1 and
PC2 have a very strong inter- and intra-chain interaction between
the polymer chains even in the solution state [29]. These results
mean that the methyl carboxylate group in PC2 did not hinder
geometrical interactions and its optical behavior of PC2 in solution
and in solid film states is similar to that of PC1 having a relatively
planar backbone structure.
PC5
PC4
PC3
PC2
PC1
3.4. Electrochemical properties
Electrochemical analysis was performed to determine the redox
ionization potentials of the synthesized copolymers. The highest
occupied and lowest unoccupied molecular orbital (HOMO/LUMO)
energy levels can be calculated from the electrochemical potentials,
together with the absorption spectra. The cyclic voltammograms
(CV) were recorded in a solution sample, and the potentials were
-
1.8
-1.2
-0.6
0.0
0.6
1.2
1.8
+
Potential (V vs Ag/Ag )
Fig. 4. Cyclic voltammogram of copolymer films on the platinum plate electrode in the
acetonitrile solution containing 0.1 M Bu
þ
4
NPF
6
at a scan rate of 50 mV/s.
obtained relative to an internal ferrocene reference (Fc/Fc ). Eox of
copolymers was obtained from the onset potentials of oxidation
waves versus SCE. Fig. 4 presents the cyclic voltammograms of the
copolymer films coated on the Pt plates. All polymers exhibit one
quasi-reversible oxidation and one irreversible reduction
processes; therefore, we were unable to estimate their LUMO
energies accurately. In order to determine the LUMO levels, we
Table 3
OFET device performance of copolymers.
2
ꢀ1 ꢀ1
Polymers
W/L ratio
I
on/Ioff
V
th
m
FET (cm V )
s
-
-
-
-
-
3
PC1
PC2
PC3
PC4
PC5
15
15
15
15
15
104
103
103
104
102
ꢀ4
ꢀ2
ꢀ9
ꢀ9
ꢀ23
1.00 ꢂ 10
4.18 ꢂ 10
2.68 ꢂ 10
8.45 ꢂ 10
3.35 ꢂ 10
5
5
5
5
combined the oxidation potential in CV with the optical bandgap
opt
energy ( E
g
) resulting from the absorption edge in their absorption
spectra. Comparing to the HOMO energy value of PC1 (ꢀ5.32 eV),
the PC2ꢀ5 containing carboxylate unit showed deeper HOMO
values of ꢀ5.41, ꢀ5.42, ꢀ5.44, and ꢀ5.50 eV, respectively. These
results show that the alkyl carboxylate groups in copolymer PC2ꢀ5
are favorable to make a lower lying HOMO energy value compared
to that of PC1. We also found that the HOMO value of TC-based
copolymers was slightly increased by increasing the number of
alkyl length into TC unit.
and have the limited conjugation in the polymer mainꢀchain
because of the steric hindrance of relatively longer alkyl side-chain
in polymer backbone. However, steric hindrance in the polymer
films (PC3ꢀ5) is suppressed by intermolecular interactions
between polymer backbones upon aggregation. In contrast, lmaxs of
PC1 and PC2 in the film states showed a smaller red-shift
(
Dl ¼ 11ꢀ13 nm), as compared to those in solution (Fig. 2). Inter-
3.5. OTFT properties
estingly, PC1 and PC2 showed strong thermochromic behaviors in
ꢁ
the oꢀDCB solution (Fig. 3). At temperature over 90 C, PC1 and PC2
The charge transport properties of copolymers were investi-
gated by solution processed field-effect transistors (FETs). Thin
films of the copolymers on n-octyltrichlorosilane (OTS)-treated
showed a blue-shifted absorption band at lmaxs of 482 and 488 nm,
respectively. However, by decreasing the temperature, the
absorption spectra of PC1 and PC2 were significantly red-shifted
and the spectral shapes at a low temperature are considerably
similar to those of the film state. Also, the absorption range and the
SiO /Si wafers were deposited by spin-casting a solution of copol-
2
ymers in chloroform. All the devices displayed typical p-type
channel TFT performance. Copolymer PC1 exhibited good device
a
b
12
4
0.98
2
0
0
.96
.94
1
0
8
6
4
2
0
-
-
-
-
2
4
6
8
JSC
0.92
0.90
VOC
PC1
PC2
PC3
PC4
PC5
0
.88
.86
0
-
-
10
12
-
0.84
0.2
0.0
0.2
0.4
0.6
0.8
1.0
PC2
PC3
PC4
PC5
Voltage (V)
Device
Fig. 5. Current densityevoltage (JeV) characteristics of PV devices based on copolymer:PC71BM (1:1 wt %) blends (a) and dependence of VOC and J SC of the different polymer:PCBM
solar cells (b).

3840
M.J. Cho et al. / Polymer 53 (2012) 3835e3841
Table 4
shows the current densityevoltage (JeV) characteristics of the
Photovoltaic performances of copolymers under illumination of 100 mW/cm2 white
light.
fabricated PV cells under simulated AM 1.5G solar illumination at
2
1
00 mW/cm . The results of the device performance are summa-
Polymers
Polymer:PC71
BM [wt. %]
J
SC [mA/cm2]
VOC [V]
FF
PCE [%]
rized in Table 4. Among the PV devices with PC1ꢀ5, the device
composed of PC2:PC71BM (1:1) showed the highest PCE of 4.52%.
PC1
PC2
PC3
PC4
PC5
1:1
1:1
1:1
1:1
1:1
5.16
0.75
0.86
0.91
0.96
0.96
0.57
0.50
0.33
0.43
0.32
2.20
4.52 [27]
1.80
1.40
0.92
Compared to the PC1:PC71BM (1:1) device (VOC ¼ 0.75 V and
10.50
6.08
3.42
2.97
2
J
SC ¼ 5.16 mA/cm ), the PC2-based device exhibited a better OPV
2
performance, yielding a VOC of 0.86 V and JSC of 10.5 mA/cm [27].
For a better understanding of the origin of the PV performance,
electrochemical analysis and atomic force microscopy (AFM) were
performed. The high VOC value of the PC2-based PV device corre-
sponds to the lower-lying HOMO level (ꢀ5.43 eV), compared to PC1
of ꢀ5.32 eV. As shown in Fig. 6a, the AFM surface image of the as-
cast PC1:PC BM (1:1) blend film showed larger crystalline domain
performance, with mobility up to 1.0 ꢂ 10ꢀ cm V
3
2
ꢀ1 ꢀ1
s
, and the
ꢀ
5
hole mobility for PC2ꢀ5 are evaluated to be 4.18 ꢂ 10
,
ꢀ
5
ꢀ5
ꢀ5
2
ꢀ1 ꢀ1
s , respectively
2
.68 ꢂ 10 , 8.45 ꢂ 10 , and 3.35 ꢂ 10 cm V
71
(Table 3). These results indicate that the alkyl chain of TC unit might
size compared to the PC2:PC71BM (1:1) blend film [27]. It is likely
that the PC2 polymer introducting methyl carboxylate could have
much better compatibility with PC71BM. Although the carrier
interrupt the crystallinity of polymer film, resulting in reducing the
hole mobility of PC2ꢀ5.
ꢀ
3
2
ꢀ1 ꢀ1
mobility (
m
¼ 1 ꢂ10 cm V
s ) of PC1 is much higher than the
3.6. Photovoltaic properties
other copolymers, the coarse domain of the film surface could
decrease the interfacial area for exciton dissociation and effective
percolation for charge transport in the blend film, thus lowering the
The photovoltaic properties of copolymers were studied in BHJ
PSCs using PC71BM as an acceptor in a conventional device
configuration of ITO (indium tin oxide)/poly(3,4-ethylene dioxy-
thiophene):poly(styrene sulfonate)(PEDOT:PSS)/polymer:PC71BM
JSC of the PC1:PC71BM device. Also, compared to the PC2-based
device, the devices with PC3ꢀ5 showed higher VOC values of 0.91,
0.96, and 0.96 V, respectively. These VOC results are in good
agreement with the measured HOMO values of copolymers in the
cyclic voltammogram (CV) studies. However, by increasing the alkyl
chain length in the TC unit, JSCs of PC3ꢀ5 devices largely decreased
as can be seen in Fig. 5b. It can be also supported by the difference
in the AFM surface images of polymer:PC71BM blend films. As
(
1:1 wt. ratio)/LiF)Al. In this study, the total concentration of
polymer:PC71BM(1:1 wt%) blend solution used for spin-coating
active layer was 20 mg/mL, and o-dichlorobenzene was used as
the solvent. All devices were post-annealed in a N
2
atmosphere for
ꢁ
10 min at 90 C, before the deposition of the electrode. Fig. 5a
Fig. 6. AFM topographic view (10 ꢂ 10
mm) of the spin cast active layer; (a) PC1:PC71BM(1:1), (b) PC3:PC71BM(1:1), (c) PC4:PC71BM(1:1), and (d) PC5:PC71BM(1:1).

M.J. Cho et al. / Polymer 53 (2012) 3835e3841
3841
shown in Fig. 6, these films showed different domain size and the
root mean square (RMS) roughness values of PC2ꢀ5 are 1.0 nm [27],
particularly thank for the support by the NRF funded by the Ministryof
Education, Science and Technology (NRF2012R1A2A1A01008797 and
20120005860).
6
.5 nm, 8.7 nm and 9.7 nm, respectively. The formation of larger
domains and poor nanophase segregation can decrease the JSC
value due to a decrease in the interfacial area for the exciton
dissociation between the polymer and PC71BM. Herein, it could be
understood that the photovoltaic properties of the TC-based
copolymers are much dependent on alkyl chain length and bulki-
ness of the TC monomer unit.
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[
[
[
[
This work was financially supported by the Air Force Office of
Scientific Research (Grant No. FA95500910361) and the National
ScienceFoundation(GrantNo. DMR0702372). Partialsupportwasalso
provided by the National Research Foundation of Korea Grant funded
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