Synthesis of conjugated polymers with broad absorption bands and photovoltaic properties as bulk heterojuction solar cells

Article abstract of DOI:10.1016/j.polymer.2011.03.040

Two new broad absorbing alternating copolymers, poly[1-(2,6- diisopropylphenyl)-2,5-bis(2-thienyl)pyrrole-alt-4,7-bis(3-octyl-2-thienyl) benzothiadiazole] (PTPTTBT-P1) and poly[1-(p-octylphenyl)-2,5-bis(2-thienyl) pyrrole-alt-4,7-bis(3-octyl-2-thienyl)ben

Full text of DOI:10.1016/j.polymer.2011.03.040

Polymer 52 (2011) 2384e2390
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis of conjugated polymers with broad absorption bands and photovoltaic
properties as bulk heterojuction solar cells
a
b
b
,
a,
Vellaiappillai Tamilavan , Myungkwan Song , Sung-Ho Jin **, Myung Ho Hyun
*
a
Department of Chemistry, Chemistry Institute for Functional Materials, Pusan National University, Busan 690-735, Republic of Korea
Department of Chemistry Education and Interdisciplinary Program of Advanced Information and Display Materials, Pusan National University, Busan 609-735, 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 28 December 2010
Received in revised form
Two new broad absorbing alternating copolymers, poly[1-(2,6-diisopropylphenyl)-2,5-bis(2-thienyl)
pyrrole-alt-4,7-bis(3-octyl-2-thienyl)benzothiadiazole] (PTPTTBT-P1) and poly[1-(p-octylphenyl)-2,5-
bis(2-thienyl)pyrrole-alt-4,7-bis(3-octyl-2-thienyl)benzothiadiazole] (PTPTTBT-P2), were prepared via
Suzuki polycondensation with high yields. The two polymers were found to show characteristic
absorption in the visible region of the solar spectrum. Interestingly the absorption of PTPTTBT-P1 was
found to cover the visible region from 350 to 650 nm with the broad and flat absorption maximum from
16 March 2011
Accepted 19 March 2011
Available online 26 March 2011
4
40 to 510 nm in film and the absorption of PTPTTBT-P2 was found to cover the visible region from 350
Keywords:
to 950 nm with the relatively distinct absorption maxima at 425 and 522 nm and very weak absorption
maximum at 832 nm in film. The electrochemical band gaps of the polymers were calculated to be
1.88 eV and 1.87 eV, respectively, while the optical band gaps of the polymers were calculated to be
2
4
,5-bis(2-thienyl)-N-arylpyrrole
,7-bis(3-octyl-2-thienyl)benzothiadiazole
Bulk heterojunction solar cells
1.94 eV and 1.87 eV, respectively. The photovoltaic properties of polymers were investigated with bulk
heterojunction (BHJ) solar cells fabricated in ITO/PEDOT:PSS/polymer:PC70BM(1:5 wt%)/TiOx/Al config-
urations. The maximum power conversion efficiency (PCE) of the solar cell composed of PTPTTBT-
2
P1:PC70BM as an active layer was 1.57% with current density (Jsc) of 8.17 mA/cm , open circuit voltage
(Voc) of 0.52 V and fill factor (FF) of 36%.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
broader absorption with high molar absorption coefficient is
important.
Conjugated polymer-based bulk heterojunction (BHJ) solar cells
We have been interested in the preparation of pyrrole-con-
have been extensively studied because they have several advanta-
geous properties such as fast charge dissociation and charge
separation, solution processability, flexibility and cost-effective
large-area manufacturing [1e7]. In the beginning of BHJ solar cell
development, the blends of poly (3-hexylthiophene) (P3HT) and
taining
While pyrrole-containing
p
-conjugated polymers for BHJ solar cell applications.
p-conjugated polymers have been rarely
studied for BHJ solar cell applications, a pyrrole-containing olig-
omer (PTPTB) incorporating 2,5-bis(2-thienyl)-N-dodecylpyrrole
(N-dodecyl TPT) and 2,1,3-bezothiadiazole was applied to solar cell
fabrications [27,28]. The BHJ solar cells fabricated from the blends
of PTPTB oligomer and PCBM as an active layer were found to show
the PCEs of up to 1.0% [28]. In order to enhance electron dona-
ting strength and light harvesting properties of N-dodecyl TPT,
we incorporated aryl groups such as 2,6-diisopropylphenyl or
4-octylpheneyl group on the pyrrole nitrogen instead of dodecyl
group. Indeed, the molar absorptivities of N-aryl TPTs such as
1-(2,6-diisopropylphenyl)-2,5-di(5-bromo-2-thienyl)pyrrole and
[6,6]-phenyl C61 butyric acid methyl ester (PC61BM) were efficiently
employed as photoactive layers and the power conversion effi-
ciencies (PCEs) were achieved up to 5% [8e10]. The insufficient
solar flux harvesting by P3HT might be the crucial factor limiting
the PCEs. To overcome the insufficient solar flux harvesting by
P3HT, various low band gap copolymers were prepared [11e26]
and the PCE was improved up to 7.4% [23]. To improve the PCEs
further, development of new semiconducting polymers having
1-(4-octylphenyl)-2,5-di(5-bromo-2-thienyl)pyrrole were greater
than that of N-dodecyl TPT [29]. In addition, the absorption maxima
of N-aryl TPTs were red-shifted compared to that of N-dodecyl TPT
*
Corresponding author. Tel.: þ82 51 510 2245; fax: þ82 51 516 7421.
[
29]. N-Aryl TPTs have been copolymerized with tetraoctylinde-
nofluorene, 2,5-dioctyloxy phenylene or 3-octylthiophene and the
resulting -conjugated TPT-based copolymers have been applied to
** Corresponding author.
E-mail addresses: shjin@pusan.ac.kr (S.-H. Jin), mhhyun@pusan.ac.kr (M.H.
Hyun).
p
0
032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.03.040

V. Tamilavan et al. / Polymer 52 (2011) 2384e2390
2385
the fabrication of BHJ solar cells [29,30]. The BHJ solar cells fabri-
cated from the blend of the N-aryl TPT-based copolymer and [6,6]-
phenyl-C70-butyric acid methyl ester (PC70BM) as a photoactive
procedure and handled in a moisture-free atmosphere. Flash
column chromatography was performed using silica gel (Merck
1
13
Kieselgel 60, 70e230 mesh). The H and C NMR spectra were
measured using a Varian Mercury Plus 300 spectrometer with
chloroform as an internal standard. The optical studies such as
absorption and photoluminescence were performed by using
JASCO V-570 spectrophotometer and Hitachi F-4500 fluorescence
spectrophotometer. The thermogravimetric analysis (TGA) of the
polymers was performed with a Mettler Toledo Labsys TG/DSC-
1600 analyzer under a nitrogen atmosphere at a heating rate of
layer showed the PCE of 1.35% with high current density
2
(J
sc ¼ 7.4 mA/cm ) [30].
As an effort toutilize N-aryl TPTunits further in BHJ solarcells, we
were interested in synthesizing N-aryl TPT-based broad absorption
polymers. Usually the polymers developed by copolymerizing
DeAeD (donoreacceptoredonor) segment, especially thiophene-
benzothiadiazole-thiophene (TBT) segment, with electron rich unit
such as carbazole, fluorene, [3,2-b]thienothiophene, alkylthiophene
or phenothiazine have been known to show broad absorption in the
ꢀ
10 C/min. Agilent 1100 series liquid chromatography system con-
nected with PLgel 5
mm MIXED-C column was employed in the
region of 300 e700 nm, due to the combined electronic transitions
molecular weight and polydispersity index (PDI) determination,
and the gel permeation chromatography (GPC) system perfor-
mance was verified by polystyrene standard calibration. The elec-
trochemical studies of the polymers were performed on a CH
Instruments Electrochemical Analyzer. Atomic force microscopy
(AFM) images of blend films were obtained on a Veeco-Multimode
AFM operating in the tapping mode.
*
such as
pe
p
electronic transition and internal charge transfer (ICT)
between the donor and acceptor moiety [31e38]. Consequently, we
can expect that the copolymers prepared by copolymerizing N-aryl
TPT unit with TBT unit would show broad absorption. Based on this
expectation, in this study, we copolymerized the N-aryl TPT mono-
mers with an alkylated TBT monomer and obtained two copolymers
such as PTPTTBT-P1and PTPTTBT-P2 (Scheme 1). The two polymers
were found to show characteristic absorption in the visible region of
the solar spectrum. In this paper, we wish to report the synthesis of
the two newalternating copolymers, PTPTTBT-P1 and PTPTTBT-P2,
and their BHJ solar cell applications.
2.2. Device fabrication and characterization of BHJ solar cells
The BHJ solar cell devices were fabricated by the known
procedure reported previously in our laboratory [29,30] and the
method is shortly summarized as following. The active layer (80 nm
thickness) composed of electron donor (polymer) and acceptor
2
. Experimental section
(PC70BM) in a ratio of 1:5 wt% was sandwitched between the
2
.1. Materials and instruments
hole injecting and hole blocking layers such as poly(3,4-ethyl-
enedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and TiOx.
The substrate was covered with ITO anode and aluminum cathode
and the short circuit was made between the electrodes. The
All reagents were purchased from Aldrich or TCI chemicals and
used without further purification. Solvents were purified by normal
Scheme 1. Synthetic route for the synthesis of polymers PTPTTBT-P1 and PTPTTBT-P2.

2386
V. Tamilavan et al. / Polymer 52 (2011) 2384e2390
ꢀ
photovoltaic device was then subjected to annealing at 100 C for
3
0 min. The top metal electrode area comprising the active area of
2
the solar cells was 4 mm . The device performance was measured
using an AM 1.5G solar simulator (Oriel 300 W) at 100 mW/cm
2
light illumination. The light intensity was adjusted using Oriel
power meter (model No. 70260 which was calibrated using labo-
ratory standards that are traceable to the National Institute of
Standards and Technologies, USA) before the measurements were
carried out. The JeV curves were recorded using a standard source
measurement unit (Keithley 236). Thickness of the thin films was
measured using a KLA Tencor Alpha-step IQ surface profilometer
with an accuracy of ꢁ1 nm.
2.3. Synthesis of polymers
Polymers PTPTTBT-P1 and PTPTTBT-P2 were prepared as
shown in Scheme 1. The detailed synthetic procedures are as
follows.
Fig. 1. TGA curves of copolymers PTPTTBT-P1 and PTPTTBT-P2.
2
.3.1. 1-(p-Octylphenyl)-2,5-bis(5-(4,4,5,5-tetramethyl-1,3,2-
to the solution and then the whole solution was refluxed for 48 h
with vigorous stirring under argon. To the refluxing solution was
added phenyl boronic acid (50 mg) and the whole mixture was
refluxed for 6 h and then was added bromobenzene (0.1 mL) and
the resulting reaction mixture was refluxed again for 12 h. The
solvent was concentrated and cooled to room temperature. The
cooled mixture was then poured drop by drop into methanol:water
mixture (200 mL:100 mL) with vigorous stirring. The precipitate
was recovered by filtration and washed well with 2 N HCl, and then
the solid was extracted with methanol for 24 h and, then with
acetone for 24 h in a Soxhlet apparatus to afford PTPTTBT-P1 or
dioxaborolan-2-yl)-2-thienyl)pyrrole (2)
Monomer 2 was prepared in 77% yield by the same procedure as
reported for the synthesis of monomer 1 [30] by treating 1-(p-
octylphenyl)-2,5-di(2-thienyl-5-bromo)pyrrole, which was prepared
by the known procedure [29], with n-BuLi and then with 2-iso-
ꢀ
propoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. mp 169e170 C;
1
H NMR (300 MHz, CDCl
3
):
d (ppm) 7.29 (d, 2H), 7.21 (d, 4H), 6.59 (s,
2
0
H), 6.37 (d, 2H), 2.69 (t, 2H), 1.58e1.68 (m, 2H), 1.18e1.38 (m, 34H),
.89 (t, 3H); 1 C NMR (75 MHz, CDCl
3
3
): d (ppm) 144.4, 142.2, 137.7,
136.1, 130.8, 129.7, 129.6, 125.3, 111.0, 84.2, 35.9, 32.1, 31.6, 29.6, 29.5,
2
C
C
2.3, 25.1, 25.0, 22.9, 14.4; HRMS (EI, m/z) [M] Calcd for
PTPTTBT-P2 as a solid material. PTPTTBT-P1: dark brown color
38
H
H
51
B
B
2
NO
NO
4
S
S
2
671.3446, found 671.3451; Anal. Calcd for
: C, 67.96; H, 7.65; N, 2.09; S, 9.55. Found: C, 67.11; H,
1
solid. Yield (0.22 g, 83%). H NMR (300 MHz, CDCl3):
d 7.54e7.68
38
51
2
4
2
(m, 3H), 7.28 (d, 2H), 7.03 (s, 2H), 6.87 (d, 2H), 6.66 (s, 2H), 6.13 (d,
7.72; N, 2.19; S, 9.74.
2
H), 2.42e2.68 (m, 6H), 1.43e1.72 (m, 4H), 1.22 (d, 20H), 0.78e0.98
: C, 71.09; H, 6.74; N, 4.61; S,
7.57. Found: C, 70.04; H, 6.65; N, 4.05; S,17.94. PTPTTBT-P2: brown
61 3 5
(m, 18H); Anal. Calcd for C54H N S
2
.3.2. 4,7-Bis(5-bromo-3-Octyl-2-thienyl)-2,1,3-benzothiadiazole (3)
1
4
,7-Bis(3-octyl-2-thienyl)-2,1,3-benzothiadiazole (2.62 g, 5.0
1
color solid. Yield (0.22 g, 79%). H NMR (300 MHz, CDCl3):
d
7.61 (s,
H), 7.28 (d, 4H), 7.04 (s, 2H), 6.92 (d, 2H), 6.60 (d, 2H), 6.36 (d, 2H),
.51e2.62 (m, 6H), 1.51e55 (m, 6H), 1.22 (d, 30H), 0.85 (t, 9H); Anal.
: C, 71.52; H, 6.97; N, 4.47; S, 17.05. Found: C,
1.04; H, 6.68; N, 4.98; S, 17.68.
mmol), which was synthesized according to the known literature
procedure [39], was dissolved in dimethylformamide (60 mL)
under argon in the dark. To the solution was added N-bromo-
succinimide (NBS) (1.78 g, 10.0 mmol) portion wise. The resulting
2
2
65 3 5
Calcd for C56H N S
7
2
solution was stirred at room temperature under N overnight. The
solvent was concentrated by using rotary evaporator and then the
crude martial was dissolved in diethyl ether and the organic
solution was washed with water and then brine. The organic
solution was dried with sodium sulfate, filtered and the solvent
was evaporated under reduced pressure and further dried under
high vacuum. The compound was purified by flash column chro-
Table 1
Polymerization results and thermal, optical and electrical properties of PTPTTBT-P1
and PTPTTBT-P2.
PTPTTBT-P1
PTPTTBT-P2
matography using 10% ethyl acetate in hexane. Monomer 3 was
1
4 a
M
PDI
w
ꢂ 10
1.54
1.61
369
2.13
1.75
392
obtained as a yellow paste (3.0 g, 88%). H NMR (300 MHz, CDCl
3
):
a
d
7.60 (s, 2H), 7.06 (s, 2H), 2.60 (t, 4H), 1.52e1.66 (m, 4H),
TGA (T
d
)b
max (nm)
13
1
1
2
6
5
.08e1.32 (m, 20H), 0.85 (t, 6H). C NMR (75 MHz, CDCl
42.6, 133.7, 132.1, 129.8, 126.8, 113.3, 32.1, 30.6, 29.8, 29.6, 29.6,
9.5, 22.8, 14.2; HRMS (EI, m/z) [M] Calcd for C30
80.0564, found 680.0569; Anal. Calcd for C30 38Br
2.78; H, 5.61; N, 4.10; S, 14.09. Found: C, 52.21; H, 6.12; N, 4.56; S,
3.88.
3
):
d
154.1,
c
Abs
l
Solution
Filmd
Broad and flat (440e490)
Broad and flat (440e510)
ꢃ5.39
ꢃ3.51
1.88
415, 487, 805
425, 522, 832
ꢃ5.31
HOMOe (eV)
2 2 3
H38Br N S
e
LUMO (eV)
ꢃ3.44
H
2 2 3
N S : C,
f
E
E
g
g
(eV)
(eV)
1.87
g
1.94
1.87 (or 1.28)
1
a
w
Weight average molecular weight (M ) and polydispersity (PDI) of the polymers
were determined by GPC using polystyrene standards.
2
.3.3. Polymerization
Monomers 1 (0.19 g, 0.30 mmol) and 3 (0.20 g, 0.30 mmol) were
b
Onset decomposition temperature (5% weight loss) measured by TGA under N
Measured in chloroform solution.
Measured in thin film on quartz.
HOMO and LUMO energy levels were estimated from the cyclic voltammetry
2
.
c
d
e
taken for the preparation of polymer PTPTTBT-P1 and monomers 2
0.20 g, 0.3 mmol) and 3 (0.20 g, 0.3 mmol) were employed for the
preparation of polymer PTPTTBT-P2. A respective monomer
mixture in toluene (60 mL) was purged well with N for 45 min. Pd
PPh (0.02 g, 5 mol%) and aqueous 2 M K CO (7 mL) were added
(
analysis.
f
Electrochemical band gaps calculated from the HOMOeLUMO energy levels.
g
2
Optical energy band gaps estimated from the onset wavelength of the optical
absorption in thin film.
(
3
)
4
2
3

V. Tamilavan et al. / Polymer 52 (2011) 2384e2390
2387
Fig. 3. Cyclic voltammograms of polymers PTPTTBT-P1 (top) and PTPTTBT-P2
bottom) films cast on platinum working electrode in 0.1 M TBATFB/acetonitrile at
a scan rate of 100 mV/s.
(
2
2
,5-di(2-thienyl-5-bromo)pyrrole with n-BuLi and then with
-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. Electron defi-
cient TBT monomer 3 was synthesized by treating 4,7-bis-(3-octyl-
-thienyl)-2,1,3-bezothiadiazole, which was prepared via the
2
reported procedure [39], with NBS. The usual Suzuki coupling
between monomers 1 and 3 or monomers 2 and 3 and then end-
capping with phenyl boronic acid and then with bromobenzene
were employed for the preparation of alternating copolymers
PTPTTBT-P1 and PTPTTBT-P2. The two copolymers thus prepared
were purified by Soxhlet extraction and characterized by using
NMR, elemental analysis and GPC. The solubility of the copolymers
was found to be quite good in common organic solvents such as
chloroform, chlorobenzene, dichlorobenzene, tetrahydrofuran and
Fig. 2. Concentration dependent UVevisible absorption spectra of polymers PTPTTBT-
P1 (a) and PTPTTBT-P2 (b) in chloroform and normalized absorption spectra of
polymers PTPTTBT-P1 and PTPTTBT-P2 in chloroform and in thin film on quartz (c).
w
toluene. The weight average molecular weight (M ) and PDI of
4
copolymer PTPTTBT-P1 were 1.54 ꢂ 10 and 1.61, respectively, and
4
3
. Results and discussions
those of copolymer PTPTTBT-P2 were 2.13 ꢂ 10 and 1.75, respec-
tively. From the thermogravimetric analysis (TGA), the 5% weight
ꢀ
3.1. Synthesis and characterization of polymers
loss temperatures of the two copolymers were found to be 369
C
ꢀ
and 392 C, respectively, indicating the two polymers are thermally
quite stable. Fig. 1 represents the TGA curves of the copolymers. The
polymerization results and thermal properties of polymers
PTPTTBT-P1 and PTPTTBT-P2 are summarized in Table 1.
Electron rich N-aryl TPT monomer 1 was prepared via the
procedure reported previously [30]. Electron rich N-aryl TPT
monomer 2 was prepared by simply treating 1-(p-octylphenyl)-

2388
V. Tamilavan et al. / Polymer 52 (2011) 2384e2390
Fig. 4. The energy level diagram of polymers PTPTTBT-P1, PTPTTBT-P2 and PC70BM with all other materials used in BHJ solar cell.
3.2. Optical properties
solution state or at 832 nm in film state might be originated from
the donoreacceptor internal charge transfer (ICT) between the
thiophene-based units, as the electron donor, and the benzothia-
diazole moiety, as the electron acceptor [31,33,38]. According to
simple calculation with Gaussian 03W computer molecular
modeling software, the dihedral angle between the thiophene
group and the N-arylpyrrole group of the N-(2,6-diisopropyl-
phenyl) TPT unit was found to be greater than that of the
N-(p-octylphenyl) TPT unit and consequently, the coplanarity of
polymer PTPTTBT-P1 is expected to be less than that of polymer
PTPTTBT-P2. In this instance, polymer PTPTTBT-P1 seems not to
have the peak around 800 nm originated from ICT. The optical band
gap calculated from the onset absorption wavelength (638 nm) of
PTPTTBT-P1 in film state was 1.94 eV. The optical band gap
calculated from the onset wavelength (968 nm) of the very weak
absorption at 832 nm of PTPTTBT-P2 in film state was 1.28 eV.
However, the optical band gap calculated from the onset wave-
length (664 nm) of the absorption at 522 nm of PTPTTBT-P2 in film
state was 1.87 eV. The optical properties of the polymers were
summarized in Table 1.
The concentration dependent UVevisible absorption spectra of
the polymers in chloroform solution and the normalized UVevi-
sible absorption spectra of the polymers measured in chloroform
and as thin films (on quartz) at room temperature are shown in
Fig. 2 (a)e(c). The concentration dependent UVevisible spectra of
the polymers shown in Fig. 2(a) and (b) clearly show that the
absorption increase continuously as the concentration of
the polymer solution in chloroform is increased. However, the
absorption intensity of polymer PTPTTBT-P1 is about 1.5 times
greater than that of polymer PTPTTBT-P2. In addition, the
absorption spectrum of polymer PTPTTBT-P1 is quite interesting in
that it shows the quite broad absorption from 350 nm to 650 nm
with broad and flat absorption maximum from 440 nm to 490 nm
in chloroform and the quite broad absorption from 350 nm to
7
5
00 nm with broad and flat absorption maximum from 440 to
10 nm in thin film. The broad and flat absorption maximum of
copolymer PTPTTBT-P1 might be resulted from the overlap of the
*
two
pep electronic transitions of the very similar absorption
strength originated from the N-aryl TPT segment and the TBT
segment. The broad and flat absorption maximum of polymer
PTPTTBT-P1 in film is expected to induce the effective solar flux
harvesting in the region from 440 nm to 510 nm. In contrast,
polymer PTPTTBT-P2 shows the three distinct absorption maxima,
two of them in high energy part and one in low energy part of the
solar spectrum. The absorption maxima of polymer PTPTTBT-P2 in
film state were found to be at 425 nm, 522 nm and 832 nm whereas
those in solution state were found to be at 415 nm, 487 nm and
3.3. Electrochemical properties
The electrochemical properties of PTPTTBT-P1 and PTPTTBT-P2
were investigated by cyclic voltammetry (CV) analysis. The CV
experiments were conducted on a drop-cast polymer film on
a platinum working electrode in acetonitrile (ACN) containing 0.1 M
tetrabutylammonium tetrafluoroborate (TBATFB) as the supporting
electrolyte at room temperature and ambient atmosphere at a scan
rate of 100 mV/s. Ag/AgCl and platinum wire were used as the
reference and counter electrodes, respectively. The performance of
the CV instrument was calibrated using the ferrocene/ferrocenium
(FOC) redox couple as an external standard before and after the
analysis. The CV curves for PTPTTBT-P1 and PTPTTBT-P2 are
shown in Fig. 3. The HOMOeLUMO levels of the polymers were
8
05 nm, respectively. The short wavelength absorption maximum
*
is expected to be attributed to the
pep electronic transition of the
N-aryl TPT segment and the middle wavelength absorption
*
maximum is expected to be attributed to the
pep electronic
transition of the TBT segment. In polymer PTPTTBT-P2, the overlap
*
of the two
pep electronic transitions originated from the N-aryl
TPT segment and the TBT segment is not so effective because the
calculated according to the following equation [20]: EHOMO
¼
*
absorption strengths of the two
p
ep
electronic transitions origi-
½ꢃðE
ꢃ E
Þ ꢃ 4:8ꢄeV
and
ꢃ 4:8ÞꢄeV,
ox;onset vs: Ag=AgCl
onset; ferrocene vs: Ag=AgCl
nated from the N-aryl TPT segment and the TBT segment are quite
different. Consequently, two absorptions at the short and middle
wavelength are observed. The very weak absorption at 805 nm in
E
¼ ½ꢃðE
ꢃ E
LUMO
red; onset vs: Ag=AgCl
onset; ferrocene vs: Ag=AgCl
where 4.8 eV is the energy level of ferrocene below the vacuum
level and Eonset (ferrocene vs. Ag/AgCl) is 0.51 eV. The onset

V. Tamilavan et al. / Polymer 52 (2011) 2384e2390
2389
Table 2
Solar cell performance of PTPTTBT-P1 and PTPTTBT-P2 as electron donor with
PC70BM as an electron acceptor in ITO/PEDOT:PSS/PTPTTBT-P1 (or) PTPTTBT-
P2:PC70BM (1:5 wt%)/TiO
x
/Al device.
Polymer
V
oc (V)a
J
sc (mA/cm2)b
FF (%)c
PCE (%)d
PTPTTBT-P1
PTPTTBT-P2
0.52
0.51
8.17
4.15
36
34
1.57
0.71
a
Open-circuit voltage.
Short-circuit current density.
Fill factor.
b
c
d
Power conversion efficiency.
of the polymers and PC70BM are 0.79 eV and 0.86 eV, respectively.
Consequently, the electron transfer from the polymers (electron
donor) to PC70BM (electron acceptor), which is the essential
process for the charge separation of the excitons, can be allowed. In
addition, the HOMO levels of the polymers are located quite below
the LUMO level of PC70BM and their energy differences are 1.09 eV
and 1.01 eV, respectively. The energy difference between the HOMO
level of the electron donor and the LUMO level of the electron
acceptor has been known to be related to the open circuit voltage
Fig. 5. JeV characteristics of BHJ solar cells prepared from ITO/PEDOT:PSS/PTPTTBT-P1
2
(
or) PTPTTBT-P2:PC70BM (1:5 wt%)/TiO
x
/Al under AM 1.5 irradiation (100 mW/cm ).
(
V
oc) [36]. In this instance, copolymers PTPTTBT-P1 and PTPTTBT-
P2, which have the quite low HOMO levels, can induce reasonable
Voc when they are used as electron donor materials in BHJ solar
oxidation potentials (Eox,onset
) of polymers PTPTTBT-P1 and
PTPTTBT-P2 were determined to be 1.10 eV and 1.02 eV, respec-
tively, and the highest occupied molecular orbital (HOMO) energy
levels of PTPTTBT-P1 and PTPTTBT-P2 were calculated to
be ꢃ5.39 eV and ꢃ5.31 eV, respectively. The lowest unoccupied
molecular orbital (LUMO) energy levels of PTPTTBT-P1 and
PTPTTBT-P2 were calculated to be ꢃ3.51 eV and ꢃ3.44 eV,
respectively, from the onset reduction potentials (Ered,onset ꢃ0.78 V
and ꢃ0.85 V, respectively). The electrochemical band gaps calcu-
lated from the HOMOeLUMO levels of polymers PTPTTBT-P1 and
PTPTTBT-P2 were 1.88 eV and 1.87 eV, respectively. The resulting
electrochemical band gap (1.88 eV) of polymer PTPTTBT-P1 is
slightly lower than the optical band gap (1.94 eV) calculated from
the onset of absorption. The electrochemical band gap (1.87 eV) of
polymer PTPTTBT-P2 is identical to the optical band gap (1.87 eV)
calculated from the onset wavelength (664 nm) of the absorption at
cells.
3.4. BHJ solar cell properties
The photovoltaic properties of PTPTTBT-P1 and PTPTTBT-P2
were investigated by using the solution processable BHJ solar cells.
To prepare the active layer, PC70BM was used as an electron
acceptor instead of PC60BM because PC70BM has similar electronic
properties, but has the enhanced solar flux absorption at the high-
energy part of the solar spectrum compared to PC60BM [40]. The
BHJ solar cell was fabricated with the configuration of ITO/
PEDOT:PSS/PTPTTBT-P1 (or) PTPTTBT-P2:PC70BM/TiO /Al, where
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS) was used to facilitate hole extraction and titanium
x
5
22 nm. From the CV analysis, it is concluded that we can ignore the
oxide (TiO ) was used for hole blocking as well as electron
x
weak absorption at 832 nm for the optical band gap calculation for
polymer PTPTTBT-P2. The electrochemical properties of the poly-
mers are included in Table 1.
The HOMO and LUMO energy levels of the polymers are
compared to those of PC70BM and other materials used in BHJ solar
cell fabrication in Fig. 4. The LUMO levels of the polymers are higher
than that of PC70BM and the differences between the LUMO levels
extraction purposes. In our previous studies, high PCEs were
observed when the ratio of polymer:PC70BM was 1:5 wt% in the
active layer [29,30]. Consequently, in this study, the ratio of
PTPTTBT-P1:PC70BM or PTPTTBT-P2:PC70BM was also set at
1:5 wt%.
Fig. 5 shows the typical current densityevoltage (JeV) curves of
the solar cell devices made from the blend of PTPTTBT-P1 or
Fig. 6. AFM images obtained by tapping-mode on the surface for PTPTTBT-P1:PC70BM (1:4 wt%) (a) and PTPTTBT-P2:PC70BM (1:5 wt%) (b) spin coated thin film.

2390
V. Tamilavan et al. / Polymer 52 (2011) 2384e2390
PTPTTBT-P2 as the electron donor and PC70BM as the electron
acceptor measured under the AM 1.5 G irradiation (100 mW/cm )
Acknowledgments
2
from a solar simulator and the photovoltaic properties are
summarized in Table 2. The device made from PTPTTBT-P1:PC70BM
as the active layer shows the PCE of 1.57% with a short-circuit
This research was supported by the New & Renewable Energy
program of the Korea Institute of Energy Technology Evaluation
and Planning (KETEP) grant (No. 20103020010050) funded by the
Ministry of Knowledge Economy, Republic of Korea.
2
current density (Jsc) of 8.17 mA/cm , a Voc of 0.52 V, and a fill factor
(
FF) of 36% while that made from PTPTTBT-P2:PC70BM as the active
2
layer shows the PCE of 0.71% with a Jsc of 4.15 mA/cm , a Voc of
.51 V, and an FF of 34%. The two devices show quite similar Voc and
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