Synthesis and characterization of two carbazole-based alternating copolymers with 4-nitrophenylcyanovinylene pendant groups and their use as electron donors for bulk heterojunction solar cells

Article abstract of DOI:10.1039/c3ra42299b

Two new carbazole-based copolymers PT and PP carrying thienyl or phenyl, respectively, alternating segments and 4-nitrophenylcyanovinylene pendant groups attached to the carbazole nitrogen were synthesized and characterized. Their optical and electrochemical properties were investigated. Both copolymers showed broad absorption bands in the longer wavelength region, which is complementary to those of PC₆₁BM and PC₇₁BM acceptors, and showed optical band gaps about 1.64 eV and 1.69 eV for PT and PP, respectively. The deep lying HOMO energy level benefited a high open circuit voltage (V _oc), leading to an overall power conversion efficiency (PCE) of 2.19%, 2.98% 2.02% and 2.54% for bulk heterojunction (BHJ) solar cells based on PT:PC₆₁BM, PT:PC₇₁BM, PP:PC₆₁BM and PP:PC ₇₁BM blends, respectively, processed from tetrahydrofuran (THF) solvent. The BHJ solar cells based on PT:PC₇₁BM and PP:PC ₇₁BM blends processed from THF and 1-chloronaphthalene (CN) mixed solvents showed overall PCE of about 5.39% and 4.20%, respectively. The increase in the overall PCE for the BHJ devices based on solvent additives was attributed to the balanced charge transport in the devices. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra42299b

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Synthesis and characterization of two carbazole-based
alternating copolymers with
Cite this: RSC Advances, 2013, 3,
1
8821
4-nitrophenylcyanovinylene pendant groups and their
use as electron donors for bulk heterojunction solar
cells
a
b
b
cd
G. D. Sharma,* Manjeet Singh, Rajnish Kurchania,* E. N. Koukaras and J.
e
A. Mikroyannidis
Two new carbazole-based copolymers PT and PP carrying thienyl or phenyl, respectively, alternating
segments and 4-nitrophenylcyanovinylene pendant groups attached to the carbazole nitrogen were
synthesized and characterized. Their optical and electrochemical properties were investigated. Both
copolymers showed broad absorption bands in the longer wavelength region, which is complementary to
those of PC61BM and PC71BM acceptors, and showed optical band gaps about 1.64 eV and 1.69 eV for PT
and PP, respectively. The deep lying HOMO energy level benefited a high open circuit voltage (Voc), leading
to an overall power conversion efficiency (PCE) of 2.19%, 2.98% 2.02% and 2.54% for bulk heterojunction
(
BHJ) solar cells based on PT:PC61BM, PT:PC71BM, PP:PC61BM and PP:PC71BM blends, respectively,
Received 9th May 2013,
processed from tetrahydrofuran (THF) solvent. The BHJ solar cells based on PT:PC71BM and PP:PC71BM
Accepted 5th August 2013
blends processed from THF and 1-chloronaphthalene (CN) mixed solvents showed overall PCE of about
DOI: 10.1039/c3ra42299b
5
.39% and 4.20%, respectively. The increase in the overall PCE for the BHJ devices based on solvent
www.rsc.org/advances
additives was attributed to the balanced charge transport in the devices.
cells. In particular, the Voc of the BHJ solar cells is determined
1
. Introduction
by the energy gap between the highest occupied molecular
orbital (HOMO) level of the electron donor (polymer) and the
lowest unoccupied molecular orbital (LUMO) level of the
electron acceptor (fullerene). Many efforts have been focused
on the improvement of the PCE of these devices by designing
Organic solar cells (OSCs) hold promise for a cost effective,
lightweight solar energy converter, due to the simple solution
processing of the active layer. Most OSCs are based on the
bulk heterojunction (BHJ) concept with an active layer of
conjugated polymer donor and soluble fullerene derivative
acceptor blend, an anode of transparent indium doped tin
oxide (ITO) coated glass and a cathode of low work function.
The power conversion efficiency (PCE) of OSCs is determined
by the short circuit current (J ), open circuit voltage (V ) and
1
3
4
5
new materials, device structures and processing techniques.
In recent years the research devoted to this field, both in
academia and industry (i.e. Solarmer Energy Inc. Kornaka Inc.,
Mitsubishi Chemical and Heliatek), resulted in devices with
record PCE being reported for both PSCs and small molecule
2
sc
oc
6
fill factor (FF) of the device. Moreover, all of these parameters
are dependent on the electrical properties of the conjugated
polymers and the nanoscale morphology within the BHJ solar
organic cells. As a result of these efforts, so far, PSCs based on
conjugated polymers as electron donor material blended with
[6,6]-phenyl-C -butyric acid methyl ester (PC BM) as an
7
1
71
electron acceptor material have yielded more than 8% PCE
using a BHJ device structure.
The most studied polymer–fullerene BHJ active layer is that
of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 or C71
a
R & D Center for Science and Engineering, JEC group of Colleges, Jaipur Engineering
7
College Campus, Kukas, Jaipur-303101 Rajasthan, India.
E-mail: sharmagd_in@yahoo.com; gdsharma273@gmail.com
b
Department of Physics, Maulana Azad National Institute of Technology (MANIT),
Bhopal, MP, India. E-mail: rkurchania@gmail.com
butyric acid methyl ester (PC61BM or PC71BM), which favor the
c
Institute of Chemical Engineering Sciences, Foundation for Research & Technology,
charge transfer from the LUMO of the photoexcited polymer to
the LUMO of the acceptor, but it has proven difficult to
improve the PCE of polymer solar cells based on P3HT beyond
5%. This is primarily due to its intrinsic limited absorption in
the solar spectrum (band gap of ~1.90 eV) and relatively small
Hellas, Stadiou Str. Platani, Patras, 26504, Greece
d
Molecular Engineering Laboratory, Department of Physics, University of Patras,
Patras, 26500, GR, Greece
e
Chemical Technology Laboratory, Department of Chemistry, University of Patras,
Patras, GR-26500, Greece
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 18821–18834 | 18821

View Article Online
Paper
RSC Advances
energy difference between its HOMO and the LUMO of
fullerene derivatives. These factors limit the overall PCE in
nitrogen behaves as an acceptor. For the acceptor materials in
BHJ organic solar cells, PC60BM offers the advantages of good
solubility in organic solvents, high electron mobility, longer
exciton diffusion length and high electron affinity. However,
weak absorption in the visible region is the main drawback of
PC61BM, which limits its contribution to light harvesting in
the organic solar cells. In contrast, the other form of fullerene
derivative PC71BM has a greater absorption in the visible
terms of both Voc and J . Therefore, to achieve a high PCE in
sc
the polymer BHJ solar cells, it is imperative to design new
electron donating polymer materials that exhibit (i) strong and
broad absorption to harvest more sunlight, (ii) high charge
mobilities, and low lying HOMO energy levels, while keeping
8
the band gap between 1.2 and 1.9 eV. One approach for
1
4
obtaining low band gap copolymers is based on alternating
donor (D)–acceptor (A) units along the polymer backbone,
forming a partial charge separation between each moiety. This
approach increases the HOMO level and at same time
decreases the LUMO level, giving a low band gap material
and red shifting the absorption spectrum of the copolymer due
region than PC61BM.
Herein, we have used PT and PP copolymers as electron
donors along with PC60BM and PC71BM as electron acceptors
for the fabrication of BHJ solar cells. The PCE of the BHJ
polymer solar cells based on PT:PC61BM, PT:PC71BM,
PP:PC61BM and PP:PC71BM blends processed from THF
solvent was 2.19%, 2.98%, 2.02% and 2.54%, respectively.
Moreover, the BHJ devices based on PT:PC71BM and
PP:PC71BM processed from CN/THF mixed solvents yielded
PCE about 5.39% and 4.20%, respectively. This increase in the
PCE has been attributed to the increase in hole mobility
leading to the balance charge transport between hole and
electrons in the BHJ device.
9
to the intramolecular charge transfer. Many D–A type
copolymers have been used for OSCs and have achieved PCE
9
above 7%.
Recently, we have synthesized a series of novel low band
gap copolymers which contain thiophene and pyrrole rings
and used them as electron donors along with a modified
PCBM as an electron acceptor and reported PCE of 4.14% with
a solvent additive and subsequent thermal annealing of the
1
0
BHJ active layer. Carbazole based polymers are among the
most promising conjugated polymers used for optoelectronic
2. Experimental
1
1
devices due to their high hole mobility, good solution
processability, deeper HOMO energy level and air stable
features, which are derived from the nature of the carbazole
unit, such as the rigid biphenyl unit, and the delocalization of
the electron lone pair of the nitrogen atom over the aromatic
structure. D–A copolymers based on a carbazole donor unit
and various acceptors have been used as electron donor
materials for polymer solar cells and have achieved initial PCE
of 3.6% and this has subsequently been improved up to 6% by
2.1 Reagents and solvents
15
2
,5-Bis(tributylstannyl)thiophene (3a)
and 1,4-bis(tributyl-
1
6
stannyl)benzene (3b) were synthesized according to reported
methods. 4-Nitrobenzyl cyanide was synthesized from the
nitration of benzyl cyanide with concentrated nitric and
1
7
sulfuric acid. THF was dried by distillation over CaH2.
2.2 Synthesis of compounds and polymers (PT and PP)
1
2
device structure optimization.
The combination of the
3,6-Dibromocarbazole (compound 1). This compound was
unique features of the 2,7-carbazole and the advantage of
the main chain donor–side chain acceptor architecture offers
promising donor materials for high efficiency polymer solar
synthesized
by
bromination
of
carbazole
using
N-bromosuccinimide (NBS) in N,N-dimethylformamide
1
8
(
DMF) according to a reported method.
-Formyl-3,6-dibromocarbazole (compound 2). A flask was
charged with a solution of 1 (0.37 g, 1.14 mmol) in formic acid
5 mL). The mixture was stirred and refluxed for 15 h. The
1
2c
9
cells and have recently reached a PCE of 6.9%.
It was
expected that the combination of the excellent optoelectronic
properties of the carbazole electron unit along with the merits
of pendant-acceptor molecular design would lead to promising
donor materials for BHJ organic solar cells. Recently, we have
reported BHJ solar cells based on an A–D–A small molecule
(
precipitate was filtered off and washed thoroughly with water.
The crude product was recrystallized from acetonitrile (0.33 g,
8
3%).
1
H-NMR (CDCl ) ppm: 9.65 (s, 1H, formyl) ; 8.08 (m, 2H,
3
(donor) based on a 3,6-dithienyl carbazole donor unit at the
carbazole at 4,5 positions); 7.42 (m, 4H, carbazole at 1,2,7,8
positions).
Anal. Calcd for C13H Br NO: C, 44.23; H, 2.00; N, 3.97.
7 2
Found: C, 43.84; H, 2.07; N, 4.10%.
Polymer 4a. A flask was charged with a solution of 2 (0.1707
g, 0.4837 mmol) and 2,5-bis(tributylstannyl)thiophene (3a)
center and a nitrophenyl acrylonitrile acceptor unit at the
terminal ends and achieved PCE values of 3.76% and 4.96%
using PC BM and PC BM as acceptors, respectively, when
6
0
70
1
3
CN/THF was used a processing solvent. Continuing our
research work, in the present investigation we have synthe-
sized two new carbazole-based copolymers PT and PP having
thienyl or phenyl, respectively, alternating segments and
(0.3203 g, 0.4837 mmol) in dried toluene (15 mL). Nitrogen
was bubbled through this flask for 0.5 h.
4-nitrophenylcyanovinylene pendant groups attached to the
Tetrakis(triphenylphosphine)palladium(0) (0.0282 g, 0.0242
mmol) was added to the mixture and the reaction was
maintained at reflux for 48 h under nitrogen. The mixture
was subsequently filtered to remove the catalyst. It was poured
into a mixture of methanol (100 mL) and 2 N HCl hydrochloric
carbazole nitrogen. These two alternating copolymers have a
D–A architecture, since the carbazole and adjacent thiophene
or phenyl segments behave as donors, while the 4-nitrophe-
nylcyanovinylene pendant group attached to the carbazole
1
8822 | RSC Adv., 2013, 3, 18821–18834
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
acid (10 mL) and stirred for 1 h. The precipitate was filtered.
The polymer was purified by silica gel column chromatography
eluting with toluene and precipitated in methanol (0.11 g,
2H, phenylene meta to nitro); 7.42 (m, 4H, carbazole at 1,2,7,8
positions).
Anal. Calcd for (C27
15 3 2 n
H N O ) : C, 78.44; H, 3.66; N, 10.16.
8
0%).
Found: C, 78.13; H, 3.56; N, 9.97%.
1
3
H-NMR (CDCl ) ppm: 9.65 (s, 1H, formyl); 8.08 (m, 2H,
2.3 Characterization methods
carbazole at 4,5 positions); 7.42 (m, 4H, carbazole at 1,2,7,8
positions); 7.11 (m, 2H, thiophene).
1
H NMR (400 MHz) spectra were obtained using a Bruker
spectrometer. Chemical shifts (d values) are given in parts per
million with tetramethylsilane as an internal standard. The
electrochemical properties, i.e. onset oxidation and reduction
potentials of copolymers, were examined by cyclic voltammetry
Anal. Calcd for (C H NOS) : C, 74.16; H, 3.29; N, 5.09.
1
7
9
n
Found: C, 73.96; H, 3.13; N, 5.12.
Polymer 4b. A flask was charged with a solution of 2 (0.2690
g, 0.7620 mmol) and 1,4-bis(tributylstannyl)benzene (3b)
(
CH instruments). We used an electrochemical cell with a
conventional three electrode assembly and tetrabutylammo-
nium hexafluorophosphate (Bu NPF ) (0.1 M) acting as the
(0.5000 g, 0.7620 mmol) in dried toluene (20 mL). Nitrogen
was bubbled through this flask for 0.5 h.
Tetrakis(triphenylphosphine)palladium(0) (0.0176 g, 0.0151
mmol) was added to the mixture and the reaction was
maintained at reflux for 48 h under nitrogen. The mixture
was subsequently filtered to remove the catalyst. It was poured
into a mixture of methanol (100 mL) and 2 N HCl hydrochloric
acid (10 mL) and stirred for 1 h. The precipitate was filtered.
The polymer was purified by silica gel column chromatography
eluting with toluene and precipitated in methanol (0.17 g,
4
6
electrolyte in anhydrous acetonitrile solution. The voltam-
metric measurements were performed at a scan rate of 100 mV
2
s
1
at room temperature. Copolymer was deposited on the
glassy carbon electrode which was used as the working
electrode, while a Pt wire and an Ag/AgCl electrode were used
as the counter and reference electrodes, respectively. The
optical absorption spectra of the pristine copolymer (in
solution and thin film) and blended films were measured
using a Perkin Elmer UV-Vis spectrophotometer.
The crystallinity of the thin films was studied using the X-ray
diffraction (XRD) technique (PANalytical make, USA) having
Cu-Ka, as radiation source of wavelength 1.5405 A with the
film coated on quartz substrate.
85%).
1
H-NMR (CDCl
carbazole at 4,5 positions); 7.67 (m, 4H, phenylene); 7.42 (m,
H, carbazole at 1,2,7,8 positions).
Anal. Calcd for (C19 11NO) : C, 84.74; H, 4.12; N, 5.20.
3
) ppm: 9.65 (s, 1H, formyl); 8.08 (m, 2H,
4
H
n
Found: C, 84.25; H, 4.03; N, 5.16%.
Polymer PT. A flask was charged with a solution of polymer
2.4 Theoretical calculations
4
a (0.1235 g, 0.4485 mmol of the repeated unit) and
We have studied the PT and PP monomer structures
theoretically within the framework of density functional theory
4-nitrobenzyl cyanide (0.0727 g, 0.4485 mmol) in THF (20
mL). tert-BuOK (0.0300 g, 0.2674 mmol) dissolved in THF (5
mL) was added portionwise to the stirred solution. The
mixture was stirred for 1 h at room temperature under
nitrogen and then was concentrated under reduced pressure.
Water was added to the concentrate and the dark green
precipitate was filtered and washed with water. The crude
product was purified by dissolving in THF and precipitating
(DFT) and time-dependent density functional theory (TD-DFT).
We have taken into account the structural flexibility of the
structures by examining stereoisomers (rotamers) of structures
PT and PP, which however are not many, as initial geometries
for our calculations.
The first round of calculations was performed employing the
19
gradient corrected functional PBE of Perdew, Burke and
into methanol (0.14 g, 75%).
Ernzerhof. For each structure we have used several stereo-
isomers as initial geometries for the geometry optimizations.
1
3
H-NMR (CDCl ) ppm: 8.22 (m, 2H, phenylene ortho to
nitro); 8.08 (m, 2H, carbazole at 4,5 positions); 7.72 (s, 1H,
cyanovinylene); 7.47 (m, 2H, phenylene meta to nitro); 7.42 (m,
20
The TZVP basis set which is of triple-f quality was used
21
throughout. The resolution of the identity method was used
4H, carbazole at 1,2,7,8 positions); 7.11 (m, 2H, thiophene).
for the treatment of the two-electron integrals. The resulting
structures were further optimized using both the hybrid
Anal. Calcd for (C H N O S) : C, 71.58; H, 3.12; N, 10.02.
2
5
13
3
2
n
Found: C, 71.12; H, 3.10; N, 10.08%.
22
exchange–correlation functional PBE0 (without adjustable
Polymer PP. A flask was charged with a solution of polymer
parameters) of Perdew, Burke and Ernzerhof, as well as that of
2
3
4b (0.1576 g, 0.5852 mmol of the repeated unit) and
Becke, Lee, Yang and Parr, B3LYP, using the same basis set
and also taking into account solvent effects. Tight convergence
4-nitrobenzyl cyanide (0.0949 g, 0.5852 mmol) in THF (20
2
7
mL). tert-BuOK (0.0300 g, 0.2674 mmol) dissolved in THF (5
mL) was added portionwise to the stirred solution. The
mixture was stirred for 1 h at room temperature under
nitrogen and then was concentrated under reduced pressure.
Water was added to the concentrate and the dark green
precipitate was filtered and washed with water. The crude
product was purified by dissolving in THF and precipitating
criteria were placed for the SCF energy (up to 10
h
E ) and the
2
8
one-electron density (rms of the density matrix up to 10 ) as
well as for the norm of the Cartesian gradient (residual forces
both average and maximum smaller than 1.5 6 10 a.u.) and
2
5
residual displacements (both average and maximum smaller
2
5
than 6 6 10 a.u.).
The TD-DFT excited state calculations have been performed
using the same functionals on the corresponding ground state
structures. In each case we have calculated the optical gap with
and without the presence of solvents. For solvents we have
used tetrahydrofuran (THF), chloroform (CF), and acetonitrile
into methanol (0.17 g, 70%).
1
H-NMR (CDCl ) ppm: 8.22 (m, 2H, phenylene ortho to
3
nitro); 8.08 (m, 2H, carbazole at 4,5 positions); 7.72 (s, 1H,
cyanovinylene); 7.67 (m, 4H, phenylene at backbone); 7.47 (m,
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 18821–18834 | 18823

View Article Online
Paper
RSC Advances
(
AN) using the self-consistent reaction field solvent parameters
Hole or electron only devices were fabricated using the
configurations: ITO/PEDOT:PSS/PT or PP or PT:PC71BM (85
nm)/Au for holes and Al/PT or PP:PC71BM (85 nm)/Al for
electrons. Mobilities are extracted by fitting the current–
voltage curves using the Mott–Gurney relationship (space
2
4
as defined in the Gaussian 09 electronic structure package.
The first round of geometry optimization was performed using
the Turbomole package. All of the follow up calculations
were performed using the Gaussian package.
2
5
2
4
2
6
charge limited current).
2
.5 Device fabrication and characterization
The BHJ photovoltaic devices were fabricated with an indium
tin oxide (ITO) coated glass substrate as the positive electrode
and aluminium as the negative electrode. The patterned ITO
glass substrate was pre-cleaned in an ultrasonic bath of
acetone, isopropanol and then dried at room temperature. A
thin layer (60 nm) of poly(3,4-ethylenedioxythiophene) : poly
3
. Results and discussion
3.1 Synthesis of polymers
Scheme 1 outlines the synthesis of alternating copolymers PT
and PP. The copolymers were obtained in 76% and 70% yield
and had number-average molecular weights (M
(styrene sulfonate) (PEDOT:PSS, Bayon, Germany) was spin
n
) of 13 800 and
coated onto the ITO glass and baked at 80 uC for 20 min. A
tetrahydrofuran (THF) solution of blend copolymers (PT or
11 600 with a polydispersity of 2.3 and 2.1 for PT and PP,
1
respectively. These copolymers were characterized by H NMR
spectroscopy and the data have already been described in the
experimental section. Both of the copolymers were soluble in
THF, chloroform and acetonitrile solvents but not in 1,2-
dichlorobeneze.
PP):PC BM or PC BM (1 : 1 w/w) was subsequently spin
6
1
71
coated on the PEDOT:PSS layer to form a photoactive layer.
The thickness of the photoactive layers were about 80–85 nm.
The deposition of all organic layers was carried out in ambient
atmosphere. Aluminium (Al) was then evaporated onto the
2
5
3.2 Theoretical calculations
surface of the photoactive layer under vacuum of 10 Torr.
2
The active area of the device was about 5 mm . The current–
Having performed the geometry optimizations we set forth to
calculated the optical gaps of the monomers, both in the gas
phase as well as in the presence of solvents. All of the results
are given in Tables 1 and 2. Since the PBE functional belongs
to the generalized gradient approximation (GGA) family of
functionals we would expect that the corresponding calculated
highest occupied molecular orbital (HOMO)–lowest unoccu-
pied molecular orbital (LUMO) gap (HL gap) and the optical
gap would be systematically underestimated compared to
experimental values. However, from Table 1 we can see that
the PBE functional values for the optical gap of the PT
monomer are in exceptional agreement with the experimental
values. This situation has also been encountered by Jacquemin
voltage (J–V) characteristics of the devices were measured with
a computer controlled Keithley 238 source meter unit. A xenon
lamp coupled with AM1.5 solar spectrum filters was used as
the light source and the optical power at the surface of device
2
2
was 100 mW cm . All the measurements were also carried out
in ambient conditions. The incident photon to current
conversion efficiency of the device was measured by illuminat-
ing the device using a xenon lamp and a monochromator and
the resulting current was recorded with a Keithley electrometer
under short circuit conditions.
The thin film of the blend was cast from mixed solvents and
is described as follows: the blend of the solution of PT:PC71BM
and PP:PC BM (1 : 1 w/w) was first prepared using THF
2
7
et al. specifically for certain long-wavelength (560–640 nm)
anthraquinone derivative compounds. Both hybrid functionals
overestimate the optical gap by about 1.1 and 0.8 eV for PBE0
and B3LYP respectively. The situation holds for the PP
monomer as well, with a slightly higher overestimation of
7
1
solvent and stirred for 4 h. A 0.5 mL of 1-chloronaphthalene
CN) was added to the blend solution prepared from the THF
(
solvent and stirred for another 2 h.
Scheme 1 Systemic routes for the synthesis of PT and PP copolymers.
1
8824 | RSC Adv., 2013, 3, 18821–18834
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
Table 1 Calculated properties of the PT structure. Specifically HOMO and LUMO energies (eV), HOMO–LUMO gap (eV), optical gap (eV) with corresponding oscillator
strength, and dipole moment (D). Solvent effects are taken into account for tetrahydrofuran (THF), chloroform (CF) and acetonitrile (AN)
HOMO (eV)
LUMO (eV)
HL gap (eV)
Optical gap (eV)
f, osc. strength
|dipole moment| (D)
gas phase
25.39
PBE
23.69
22.87
23.01
1.84
0.17
0.44
0.33
7.34
6.85
6.97
1.70
3.32
2.92
PBE0
B3LYP
26.20
25.93
2.86
2.60
solvent THF
25.23
PBE
23.70
22.90
23.14
1.66
2.72
2.44
0.23
0.55
0.41
9.41
8.47
8.73
1.53
3.20
2.77
PBE0
B3LYP
26.10
25.91
solvent CF
25.23
PBE
23.69
22.90
23.13
1.67
2.72
2.45
0.23
0.55
0.42
9.14
8.25
8.50
1.55
3.21
2.78
PBE0
B3LYP
26.11
25.92
solvent AN
25.22
PBE
23.71
22.91
23.15
1.64
2.71
2.43
0.22
0.54
0.40
9.86
8.82
9.11
1.51
3.19
2.75
PBE0
B3LYP
26.10
25.90
the optical gaps, specifically 1.15 and 0.9 eV for PBE0 and
B3LYP respectively.
and phenyl units respectively. The p-NNB unit contributes the
remaining 21.1%, which is higher than the corresponding
value for the PT monomer. Also, the first level encountered
with a dominant contribution of 53.1% by the p-NNB is
located at 27.6 eV.
A moderate increase in the dipole moment of the monomers
is noted when solvent effects are taken into account with
respect to gas phase values. The largest increase is computed
using the PBE functional, ranging from 24.5% to 34.3%. For
the hybrid functionals the increase ranges are 20.4%–28.7%
and 21.9%–30.7% for PBE0 and B3LYP respectively. The
dependence of the dipole moment on the dielectric constant
of the solvent is monotonic but weak.
Fig. 1 shows the HOMOs and LUMOs of the two monomers.
The HOMO of both monomers extends over the carbazole unit
as well as over the thiophene or the phenyl groups for the PT
and PP monomers respectively. A small contribution to the
HOMO by the p-nitroacetonitrilebenzene (p-NNB) unit can also
be distinguished in both cases. The LUMO is highly localized
over the p-NNB units in both cases. The carbazole and
thiophene units contribute 50.2% and 36.1% to the HOMO,
respectively. The remaining 13.7% is the contribution of the
p-NNB unit which is low but not negligible. The LUMO is
significantly more localized on the p-NNB unit from which we
find a 94.6% contribution. A 5% contribution is from the
carbazole unit, but even this small value is localized on the
region near the p-NNB unit. Similar, although varying,
contributions from the subunits are found for the near
HOMO orbitals. The first level encountered with a dominant
contribution of 54% by the p-NNB is located at 27.8 eV. The
situation is analogous for the PT monomer for which we find a
We have calculated the UV/Visual absorption spectrum at
the DFT/B3LYP level of theory using THF as a solvent for both
of the PT and PP monomers, which we show in Fig. 2. The
spectra have been produced by convoluting Gaussian func-
tions with HWHM = 0.15 eV centered at the excitation wave
numbers. The spectrum for the PT monomer exhibits three
nicely distributed bands of high intensities which are centered
at 280 nm (4.43 eV), 375 nm (3.30 eV), and 507 nm (2.45 eV).
The spectrum of the PP monomer also exhibits three bands,
however with peaks of varying intensities and with two of the
bands adjacent. More specifically, two clear bands are
centered at 270 nm (4.59 eV) and 480 nm (2.58 eV). The last
band has a finer structure with peaks at 310 nm (4.0 eV) and
360 nm (3.44 eV).
Photophysical and electrochemical properties. The normal-
ized optical absorption spectra of the PT and PP copolymers
both in dilute THF solution and thin films cast from THF
solvent are shown in Fig. 3a and 3b. All the photophysical
characteristics are summarized in Table 3. The absorption
curves were broad and extended from the visible to near
infrared region of the solar spectrum. Both copolymers
displayed an absorption band in the shorter wavelength region
(460–490 nm), corresponding to the p–p* transition and in the
longer wavelength region (650–680 nm) corresponding to an
62% and 16.9% contribution to the HOMO by the carbazole
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 18821–18834 | 18825

View Article Online
Paper
RSC Advances
Table 2 Calculated properties of the PP structure. Specifically HOMO and LUMO energies (eV), HOMO–LUMO gap (eV), optical gap (eV) with corresponding oscillator
strength, and dipole moment (D). Solvent effects are taken into account for tetrahydrofuran (THF), chloroform (CF) and acetonitrile (AN)
HOMO (eV)
LUMO (eV)
HL gap (eV)
Optical gap (eV)
f, osc. strength
|dipole moment| (D)
gas phase
25.58
PBE
23.69
22.86
22.99
2.09
0.23
0.54
0.43
6.95
6.46
6.63
1.89
3.51
3.11
PBE0
B3LYP
26.36
26.10
3.00
2.77
solvent THF
25.41
PBE
23.70
22.90
23.13
1.89
2.83
2.58
0.30
0.63
0.50
8.74
7.84
8.14
1.71
3.36
2.93
PBE0
B3LYP
26.26
26.06
solvent CF
25.42
PBE
23.69
22.89
23.12
1.90
2.84
2.59
0.30
0.64
0.51
8.47
7.66
7.96
1.73
3.38
2.95
PBE0
B3LYP
26.27
26.07
solvent AN
25.40
PBE
23.72
22.91
23.15
1.87
2.82
2.56
0.29
0.62
0.49
9.11
8.12
8.45
1.69
3.34
2.90
PBE0
B3LYP
26.25
26.05
intramolecular charge transfer (ICT) from the donor (carbazole
and thiophene or phenyl) to the acceptor (4-nitrophenylcya-
novinylene) unit present in the copolymers. Moreover,
carbazole derived copolymers exhibited a much enhanced
and slightly red shifted intramolecular charge transfer
absorption at longer wavelength region, as well as a lower
oxidation potential being caused by the more efficient electron
donating capability of the nitrogen atom on the 3,6-carbazole
units. We found that the absorption peak corresponding to the
longer wavelength region for PT copolymer was bathochromi-
cally shifted. This can be attributed to the enhanced
intramolecular charge transfer due to the presence of
thiophene in the electron donating segment in PT. The
absorption peak for thin films of both copolymers was red
shifted in the longer wavelength region, relative to that for
solution. Both copolymers showed broad absorption spectra
with an extended absorption edge in the thin film. The
broader and better absorption in the solid state originates
from the better planarity of the polymer and stronger
electronic interaction between individual polymer chains in
the thin film. The optical band gap of these copolymers was
calculated from the absorption onset edge of the thin film
interaction between adjacent p-conjugated chromophores in
PT. The presence of the thiophene ring in the electron
donating segment increases its electron donating ability and
lowers the band gap, resulting in a red shift in the absorption
maximum band. Moreover, it has been well established from
our earlier reports that the presence of the cyanovinylene
4-nitrophenyl terminal moieties also affects the absorption
spectra of the materials by red shifting and broadening. Both
the copolymers exhibit a broad band absorption in the longer
wavelength region and a low band gap as compared other
carbazole based copolymers.
2
8
Cyclic voltammetry was employed to calculate the reduction
and oxidation potential of copolymers and the corresponding
LUMO and HOMO energy levels of copolymers. The onset
oxd
onset
red
onset
oxidation potential (E
) and reduction potential (E
)
observed from the cyclic voltammograms are summarized in
Table 3.The HOMO and LUMO energy levels were estimated
2
9
from the following equations:
oxd
onset
EHOMO = 2(E
+ 4.7) eV
+ 4.7) eV
red
onset
ELUMO = 2(E
opt
using Eg = 1240/lonset with lonset being the absorption onset
wavelength of the materials and were 1.64 eV and 1.69 eV for
PT and PP, respectively, which are smaller than that for P3HT
with 1.95–2.0 eV. The absorption band maxima of PT is more
red shifted as compared to PP, which indicates that there is an
excited state delocalization in the PT copolymer relative to PP
due to the presence of the thiophene ring and more exciton
The calculated values of HOMO and LUMO energy levels of
PT are 25.18 eV and 23.24 eV, respectively (Table 1). PP has
HOMO and LUMO energy levels at 25.30 eV and 23.38 eV,
respectively (Table 1). These values are in good agreement with
those theoretically estimated. The electrochemical band gap
estimated from the cyclic voltammetry measurement is 1.94 eV
1
8826 | RSC Adv., 2013, 3, 18821–18834
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
Fig. 1 (a) The frontier molecular orbitals of PT and PP monomers calculated from DFT calculations.
and 1.92 eV for PT and PP, respectively. The electrochemical
band gap is slightly higher than the optical band gap. This is a
common phenomenon in most of the organic materials due to
polymer solar cells. Therefore, we have fabricated the device
having a ITO/PEDOT:PSS/PT or PP/Al configuration and
measured their current–voltage (J–V) characteristics in dark
(Fig. 4). The J–V characteristics of both devices in the dark
showed a rectification effect, when a positive potential is
applied to the ITO/PEDOT:PSS electrode with respect to the Al
electrode. Since the HOMO level of both copolymers is very
close to the work function of PEDOT:PSS (25.1 eV), this
electrode behaves as an ohmic contact for the hole injection
from PEDOT:PSS into the HOMO level of the copolymers. On
the other hand, the LUMO level of the copolymers is very far
from the work function of the Al electrode (24. 2 eV) and
forms a Schottky barrier for electron injection from Al into the
LUMO level of copolymers. Therefore, the rectification is due
to the formation of a Schottky barrier at the interface between
Al and the copolymers in the respective devices.
3
0
their high binding energy. PT exhibits a smaller band gap
compared to PP which reflects the stronger donor property of
the electron donating segment due to the presence of the
thiophene ring, thus increasing the HOMO level more than the
LUMO level, leading to a decrease in the band gap as has been
observed by Patil et al. in the case of copolymer based on
3
1
diketopyrrolopyrrole. Moreover, the HOMO energy levels of
PT and PP are lower than that of P3HT (24.95 eV), indicating
that these copolymers could have better air stability.
Incorporation of the weak electron donating donor carbazole
unit into the push pull conjugated backbone resulted in
relatively low lying HOMO energy levels. The incorporation of a
carbazole unit into the polymer helps to generate high V in
oc
3
2
the BHJ polymer solar cells. Moreover, the offsets between
the LUMO energy levels of both PT and PP and fullerene
derivatives, i.e. PC61BM and PC71BM, were larger than 0.3 eV,
which would guarantee energetically favourable electron
transfer in the BHJ active layers under illumination.
Photovoltaic properties of BHJ solar cells. We have used
PC BM and PC BM as electron acceptors along with the PT
6
1
71
and PP as electron donors. Fig. 5 shows the optical absorption
spectra of PT:PC BM and PT:PC BM blend cast from THF
6
1
71
solvent. Similar absorption spectra were observed for
PP:PC71BM and PP:PC71BM. The absorption spectrum of the
blend shows two bands, i.e. one in the shorter wavelength
Electrical properties of PT and PP. The focus of this paper is
to use these two copolymers as electron donors for BHJ
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 18821–18834 | 18827

View Article Online
Paper
RSC Advances
Fig. 2 Theoretical UV/Vis absorption spectrum of the (a) PT and (b) PP
monomers. Solvent effects have been taken into account for tetrahydrofuran
(
THF), chloroform (CF) and acetonitrile (AN).
region corresponding to the fullerene derivative and another
in the longer wavelength region corresponding to the
intramolecular charge transfer (ICT) band of the copolymer.
A broader absorption covering from the visible to the near
infrared part of the solar spectrum is observed for both
copolymer:PC71BM blends due to the absorption of PC71BM in
the wavelength region below 550 nm. It can be seen from the
absorption spectrum of the blend with PC71BM that the
absorption band is broader than that of the blend with
PC BM, indicating a higher light harvesting efficiency and an
Fig. 3 Optical absorption spectra of (a) PT and (b) PP in THF solution and thin
film cast from THF solution.
has been attributed to the higher energy separation between
the LUMO level of the fullerene derivatives and PT, as
compared to that for PP. Since the driving force for the
photoinduced charge separation at the D/A interfaces is
directly correlated with this difference, the photoinduced
charge separation is more efficient for the device based on PT
as compared to PP as electron donor. The value of Voc for
photovoltaic devices with PP is higher than that for PT, which
is in agreement with the values of the HOMO levels of PT and
PP (as discussed earlier), since the Voc in the BHJ organic solar
cell is related to the energy difference between the LUMO level
of the acceptor and the HOMO level of the donor. For both
copolymers, the BHJ solar cells exhibited high Voc approaching
6
1
exciton generation in the copolymer:PC71BM blend. The use of
PC BM as an acceptor in the BHJ active layer can complement
7
1
the valley between the p–p* transition absorption band and
the ICT absorption band of the copolymers, thereby contribut-
ing to the photocurrent in the BHJ solar cells. Therefore, we
expect a higher Jsc for the BHJ solar cell based on an active
layer with PC BM.
7
1
The J–V characteristics of the BHJ solar cells based on the
BHJ active layers processed from THF solvent are shown in
Fig. 6a and 6b and the corresponding photovoltaic parameters,
i.e. short circuit current (J ), open circuit voltage (V ), fill
sc
oc
factor (FF) and power conversion efficiency (PCE) are
summarized in Table 4. The PCE for the organic solar cells
fabricated with PT:PC BM, PT:PC BM, PP:PC BM and
0
.80 V, which is comparable to BHJ organic solar cells based
3
0
on other pendant carbazole derived copolymers. The IPCE of
the solar cell based on a BHJ active layer with PC BM showed
6
1
71
61
71
PP:PC71BM are 2.19%, 2.98% 2.02% and 2.54%, respectively.
It can be seen from these figures and Table 4 that the
photovoltaic devices based on copolymer PT as electron donor
either using PC61BM or PC71BM as electron acceptor in the
a broader shape as compared to that for PC BM (as shown in
Fig. 7) which is also consistent with the observed values of Jsc.
6
1
The PCE for the photovoltaic devices is higher for PC BM as
71
electron acceptor as compared to its PC BM counterpart
6
0
BHJ active layer showed higher values of J , FF and PCE. This
irrespective of whether PT or PP is used as the electron donor.
sc
1
8828 | RSC Adv., 2013, 3, 18821–18834
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
Table 3 Optical and electrochemical properties of PT and PP copolymers
a
a
opt
onset
ox
NHE
onset
red
NHE
elec
l
a max in
l
a max in
Eg
E
(V) vs.
E
(eV)
HOMO
E
(V) vs.
E
(eV)
LUMO
Eg
b
c
Copolymer
solution (nm)
film (nm)
(eV)
(eV)
PT
PP
472, 646
474, 637
472, 652
474, 674
1.64
1.69
0.48
0.60
25.18
25.30
21.46
21.32
23.24
23.38
1.94
1.92
a
b
opt
g
Absorption maxima from the UV-visible absorption spectra in solution and thin film. E (eV) optical band gap estimated from the onset
c
elec
g
absorption edge in thin film.
E
(eV) electrochemical band gap estimated from the cyclic voltammetry.
2
3
The performance of the organic solar cell is mainly deter-
mined by the photon harvesting efficiency of the BHJ active
layer (light absorption and exciton generation), separation and
transport of charge carriers. Our results show that the PCE of
the devices based on PC70BM as the electron acceptor is better
than that of PC BM due to the increase in J and FF. The
of the device remains low due to the low hole mobility of PT.
The value of Voc is 0.47 V smaller than that estimated from the
energy gap (~1.23 eV) between the LUMO of PCBM and HOMO
of PT. These results indicated that the recombination
reactions between photoinduced electrons and holes are
dominant in the devices based on the blend processed from
6
1
sc
3
3
increase in the Jsc for the devices based on the PC71BM
acceptor compared to that for PC61BM can be attributed to the
higher absorption of PC BM in the visible region as compared
the THF solvent.
The PCE of the BHJ solar cells based on these two
copolymers i.e. PT and PP as electron donors and PC BM or
7
1
61
to that for PC61BM, which causes an enhancement in the light
harvesting properties. Moreover, PC71BM has higher solubility
in THF than PC BM which leads to high molecular intermix-
PC BM is still low. The two phase morphologies of the BHJ
7
1
active layer play an important role in the performance of the
BHJ photovoltaic devices. Many methods have been explored
towards the aim of achieving the nanoscale bi-continuous
6
1
ing between the donor and acceptor, resulting in an increase
in the interfacial area for exciton dissociation into free
electrons and holes. These effects enhance the Jsc and FF,
and result in higher PCE for the devices based on PC71BM as
electron acceptor. As can be seen from Table 4, the Voc is
almost the same for the devices based on PC61BM or PC71BM,
which may be attributed to the same LUMO level for both
acceptors.
3
4
35
morphology including choice of casting solvents, thermal
3
6
and solvent annealing. These strategies have been found
effective for controlling the morphology and performance of
BHJ devices based on P3HT:fullerene derivatives but are less
effective for low band gap D–A copolymers. The most versatile
and widely employed technique however is the use of a solvent
additive during film deposition, which increases the solar cell
efficiency by modifying the donor and acceptor phases. In the
case of some D–A copolymers, it has been found that
processing of the polymer:fullerene blend film from a binary
solvent mixture containing a small amount of a higher boiling
point additive, which is good solvent of fullerene but a bad
The best performance of the BHJ device was obtained from
the PT:PC71BM blend processed from THF solvent with Voc
=
2
2
0.76 V, Jsc = 8.16 mA cm , FF = 0.48 and PCE = 2.98%. The FF
Fig. 4 Current–voltage (J–V) characteristics of ITO/PEDOT:PSS/PT or PP/Al
Fig. 5 Optical absorption spectra of PT:PC61BM (THF), PT:PC71BM (THF) and
PT:PC71BM (CN/THF) thin films.
devices in the dark at room temperature.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 18821–18834 | 18829

View Article Online
Paper
RSC Advances
Fig. 7 Incident photon to current conversion efficiency (IPCE) spectra of BHJ
solar cells device using PT:PC61BM (THF cast), PT:PC71BM (THF cast) and
PT:PC71BM (CN/THF cast) blends.
the higher boiling point additive. Among various processing
additives, the diiodoalkanes were used frequently. The
incorporation of diiodoalkanes alters the phase separated
nanoscale morphology in the BHJ active layers based on the
polymer:fullerene device due to selective solubility, i.e. it is a
poor solvent for the polymer but a good solvent for fullerene
3
7,38
derivatives.
On the other hand, 1-chloronaphthalene (CN)
has good solubility for both polymer and fullerene and allowed
3
9
each component to mix well in the blend.
The J–V characteristics of BHJ polymer photovoltaic devices
based on a CN/THF processed blend are also shown in Fig. 6a
and 6b and the corresponding photovoltaic parameters are
also compiled in Table 4. It can be seen from these figures and
Table 4 that all the photovoltaic parameters i.e. Jsc, Voc and FF
have been improved for the devices processed from the CN/
THF cast blend, resulting in an overall PCE of 5.39% and
Fig. 6 Current–voltage (J–V) characteristics of the BHJ devices (a) PP and (b) PT
copolymers; PC61BM and PC71BM are used as electron donors, processed from
THF and CN/THF solvents.
solvent for the polymer, can improve the photovoltaic
performance of polymer solar cells. Initial investigations
suggested that the role of the processing additive in control-
ling the two phase nanoscale morphology was to increase the
domain size of the polymer and/or the fullerene. The domain
size of the polymer was controlled by enhancing the aggrega-
tion during the spin coating and drying while delaying the
aggregation of fullerene by virtue of its selective solubility in
4.20% for the PT:PC71BM and PP:PC71BM based devices,
respectively. The higher IPCE values of the device with the CN/
THF cast blend film (as shown in Fig. 7) also confirm the
observed higher value of Jsc. The increase in the PCE has been
attributed to the increase in the crystallinity of the BHJ active
layer and balanced charge transport in the devices, upon the
addition of CN additive, as discussed in the latter part of the
manuscript.
The optical absorption spectra of the BHJ active layer thin
films cast from THF and CN/THF are shown in Fig. 5. As
shown in this figure, in the PT:PC BM blend cast from CN/
Table 4 Photovoltaic parameters of the BHJ organic solar cells employing
7
1
different blends
THF, the absorption peak in the longer wavelength region
which corresponds to PT shows a red shift and also the
absorption coefficient increases. In addition, the PT:PC71BM
blend cast from CN/THF solution shows a vibronic shoulder
around 690 nm. These observations indicate the increase in
the crystallinity due to the enhanced interchain p–p* stacking
22
Blend
Jsc (mA cm
)
Voc (V)
FF
PCE (%)
a
PT:PC61BMa
6.38
8.16
5.88
7.05
11.25
9.67
0.78
0.76
0.82
0.80
0.84
0.87
0.44
0.48
0.42
0.45
0.57
0.50
2.19
2.98
2.02
2.54
5.39
4.20
PT:PC71BM
a
PP:PC61BMa
PP:PC71BM
4
0
b
PT:PC71BMb
as has been observed in other polymers. Similar results were
also observed for PP:PC71BM film. The improved crystallinity
of the copolymer, facilitated through self organised chain
PP:PC71BM
a
b
Cast from THF solvent. Cast from THF with CN additive.
1
8830 | RSC Adv., 2013, 3, 18821–18834
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
the overall PCE of the BHJ solar cells. The PCE of the BHJ solar
cells is governed by the balance between the transport and
recombination of charge carriers. The imbalance in the
mobilities of holes and electrons contribute to the large
recombination, which results in the lower Jsc and PCE. We
have estimated the electron and hole mobilities in the BHJ
layers processed from THF and CN/THF solvent, from the
space charge limited current (SCLC) current–voltage charac-
teristics measured in the dark for electron only and hole only
2
6
devices, respectively, using the Mott–Gurney square law and
the current is given by
2
3
r o
J = (9/8)e e m(V /d )
where V is the applied voltage corrected for built in voltage arising
Fig. 8 XRD pattern of PT (THF cast), PT:PC71BM (THF cast) and PT:PC71BM (CN/
from the difference between the work function of electrodes, e is
o
THF) thin films.
r
the permittivity of free space, e is the relative dielectric constant of
the active layer and taken as 3.5, m is the charge carrier mobility
and d is the thickness of the active layer. The hole and electron
mobilities estimated using the current–voltage characteristics of
hole only and electron only devices and the above equation are
listed in Table 5. The hole mobility for CN additive films increased
by a factor of 43 and 45, for PP:PC71BM and PT:PC71BM blends,
respectively. However, the electron mobility has been decreased by
a factor of about three. We assume that the increase in the self
ordering in the donor phase probably causes a decrease in the
ordering of the acceptor phase, which leads to the reduction in
electron mobility. The simultaneous increase in hole mobility in
the donor phase and slight decrease in the electron mobility in the
acceptor phase leads to a more balanced hole and electron
transport which is beneficial to the enhancement of Jsc and FF.
The ratio between electron mobility and hole mobility for
PT:PC71BM and PP:PC71BM blends is 122 and 146, respectively.
In the case of BHJ polymer solar cells processed with THF only, the
space charges remain in the device due to an imbalance between
the electron and hole mobilities (high ratio between electron and
hole mobilities), the photoinduced charge carriers are not fully
extracted and do not produce enough current. In this case, the
possibility of recombination between the holes and electrons at
donor–acceptor interfaces may increase, resulting in low Jsc and
FF. However, when the blends are processed from the CN/THF
solvent, this ratio is reduced to 1.54 and 1.24 for PP:PC BM and
stacking, enhances the light harvesting capabilities thus
improving the Jsc
.
Evidence for the change in the crystallinity of the BHJ active
layer, upon the CN additive, was obtained by recording the
X-ray diffraction (XRD) pattern of the thin film of PT and
PT:PC71BM processed from THF and CN/THF (Fig. 8). The
pristine PT cast from THF showed a peak at 2h = 7.17u.
However, when the PT is blended with PC BM, the diffraction
7
0
peak which corresponds to PT becomes weak suggesting an
effective mixing of PC BM with the copolymer. When the
7
1
PT:PC71BM film is processed from CN/THF, the diffraction
peak centred at 2h = 7.17u reappears and is slightly shifted to a
higher value of 2h. This indicates the blend film cast from the
THF solvent was not sufficiently crystallized. But on the
addition of a small amount of CN in the host solvent, the
blend recrystallized due to the high boiling point of CN, which
evaporates slowly. The decrease in the full width at half
maximum of the XRD pattern upon the addition of CN also
indicated an improvement in the crystallinity of PP. This is in
agreement with the increased optical absorbance of the blend
in the absorption band that corresponds to the copolymer. The
XRD peak of the blend film processed with CN additive shifted
from 2h = 7.17u (corresponding to an interlayer spacing of
14.45 A) to 2h = 7.23u (corresponding to an interlayer spacing
of 14.36 A). Similar XRD results were observed for the
PP:PC71BM blend. The reduction of interlayer spacing in the
blend film with CN additive may be beneficial for improving
the charge transport.
The charge transport i.e. the transportation of electrons and
holes towards cathode and anode plays an important role in
71
PT:PC71BM, respectively. The reduced value of this ratio reduced
the space charge and recombination of the photo-induced
electrons and holes at the interface in the BHJ solar cells and
increases the Jsc and PCE.
Table 5 Charge carrier mobilities for the BHJ active layers processed from different solvents
2
21 21
2
21 21
Blend
Hole mobility (m
h
) (cm V
s
)
Electron mobility (m
e
) (cm V
s
)
e h
m /m
a
26
24
PP:PC71BMb
PP:PC71BM
1.6 6 10
2.34 6 10
146
15.44
122
1.24
25
24
6.85 6 10
1.05 6 10
a
25
24
PT:PC71BMb
1.93 6 10
2.36 6 10
25
24
PT:PC71BM
8.68 6 10
1.08 6 10
a
b
cast from THF solvent. cast from THF with CN additive.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 18821–18834 | 18831

View Article Online
Paper
RSC Advances
Fig. 9 AFM images (3 mm 6 3mm scale) of PT:PC71BM blends cast from (a) THF and (b) CN/THF solvents.
CN additives can improve the film quality of BHJ films and
thus enhance the photovoltaic performance. The AFM images
of the PT:PC71BM cast from THF and CN/THF are shown in
Fig. 9. The PT:PC BM film cast from THF exhibits unevenly
recombination. The increase in the value of Voc for CN/THF
cast film is attributed to the lower value of J due to the
o
improved charge transport. Therefore, we assume that the CN
additive plays an important role not only in improving the
nanomorphology of the BHJ active layer but also in improving
the interface between the polymer and PCBM phases. This well
defined boundary between polymer and fullerene domains
7
1
aggregated polymer and PCBM domains with root mean
square (rms) surface roughness about 3.45 nm. This adverse
morphology reduces the effective D/A interfacial area and
limits the exciton dissociation probability resulting in exciton
4
4,45
may reduce the bimolecular charge recombination,
also improving the Voc
thus
4
1,42
loss and poor charge mobility.
addition of CN the blend film results in a smoother surface
rms surface roughness of 2.34 nm). Similar AFM images have
been observed for PP:PC BM blends. This may be because of
In contrast, with the
.
(
7
1
4. Conclusions
good solubility of the fullerene derivatives in the high boiling
point additives which improves the miscibility of copolymer
and fullerene derivatives. This increases the interfacial area of
D/A and leads to a more efficient exciton dissociation which in
turn leads to enhancement in Jsc and FF.
The dark current in the reverse bias is a measure leakage of
current related to the recombination process. We have
measured the J–V characteristics in the dark and found that
the dark current for the device based on the CN/THF cast BHJ
layer is lower than for that cast from THF. This also indicates
that addition of solvent additives provides more effective
charge transport in the device.
Two novel carbazole-based copolymers PT and PP were
synthesized and their optical and electrochemical properties
were investigated. Their optical absorption spectra were
extended up to 750 nm and the optical band gaps were 1.64
eV and 1.69 eV for PT and PP respectively. The HOMO (25.18
and 25.30 eV for PT and PP) and LUMO (23.24 and 23.38 eV
for PT and PP) energy levels of these copolymers are well
aligned for the efficient photoinduced charge transfer in BHJ
organic solar cells. The values of HOMO and LUMO levels of
these materials are similar to other D–A copolymers using
32
carbazole based copolymers. The BHJ organic solar cells
It has been reported that in BHJ polymer solar cells, the Voc
also depends on the charge transfer interaction between
polymer and fullerene, in addition to the HOMO–LUMO
based on PT:PC61BM, PT:PC71BM, PP:PC61BM and PP:PC71BM
blends showed overall PCE of 2.19%, 2.98% 2.02% and 2.54%,
respectively. Further improvement in the PCE (5.39% and
4
3
difference. The Voc for a BHJ device is given by
4.20%, for PT:PC71BM and PP:PC71BM respectively) has been
achieved when the BHJ active layer is processed from a solvent
additive i.e. CN/THF mixed solvents. The increase in the PCE
Voc = (nkT/q)LN(J /J ) + DEDA/2q
sc
o
has been attributed to the improvement in both Jsc and Voc,
Here n is the diode ideality factor, k is Boltzmann’s constant,
o
is the reverse saturation current, T is the temperature in
due to the increased crystallinity of the donor phase and
J
improvement in the hole mobility. The increase in hole
mobility and slight decrease in the electron mobility, upon the
incorporation of CN, leads to a more balanced charge
transport in the BHJ solar cells, resulting in higher overall
PCE. These values of PCE for these devices based on PT as
electron donor and PC71BM as electron acceptor are compar-
kelvin and DEDA is the interfacial energy gap between donor
and acceptor. The above equation suggests a linear depen-
dence of Voc on DEDA. Since DEDA is the same for both THF and
CN/THF cast BHJ active layers, therefore J has played a role in
o
controlling Voc. The value of Jo depends on the material
3
9
properties that determine the carrier generation, transport and
1
8832 | RSC Adv., 2013, 3, 18821–18834
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
1
2
able to those of other copolymers based on carbazole units.
7 (a) H. Y. Chen, J. H. Hou, S. Q. Zhang, Y. Y. Liang, G.
W. Yang, Y. Yang, L. P. Yu, Y. Wu and G. Li, Nat. Photonics,
Moreover, the ease of synthesis of these copolymers compared
to earlier reported copolymers based on carbazole makes them
suitable for low cost polymer BHJ solar cells.
2009, 3, 649; (b) Y. Y. Liang, Z. Xu, J. Xia, S. T. Tsai, Y. Wu,
G. Li, C. Ray and L. P. Yu, Adv. Mater., 2010, 22, E135; (c) Z.
C. He, C. M. Zhong, X. Huang, W. Y. Wong, H. B. Wu, L.
W. Chen, S. J. Su and Y. Cao, Adv. Mater., 2011, 23, 4636.
(a) S. H. Park, A. Roy, S. Beaupr ´e , S. Cho, N. Coates, J.
S. Moon, D. Moses, M. Leclerc, K. Lee and A. J. Heeger, Nat.
Photonics, 2009, 3, 297; (b) J. Hou, H. Y. Chen, S. Zhang, R.
I. Chen, Y. Yang and Y. Wu, J. Am. Chem. Soc., 2009, 131,
8
Acknowledgements
We are thankful to Prof. Y.K. Vijay for providing the
experimental facilities for the fabrication of the devices and
their characterization. We are thankful to the department of
Physics, Bansthali University, Jaipur for providing the XRD
facilities. We are also thankful to Dr S. S. Sharma, Govt.
Women’s Engineering College, Ajmer helping in device
fabrication and characterization.
15586; (c) Y. Liang, D. Feng, Y. Wu, S. T. Tsai, G. Li, C. Ray
and L. Yu, J. Am. Chem. Soc., 2009, 112, 7792; (d) R.
C. Coffin, J. Peet, J. Rogers and G. C. Bazan, Nat. Chem.,
2009, 1, 657; (e) Y. Zou, A. Najari, S. Beaupre, B. R. Aich,
Y. Tao and M. Leclerc, J. Am. Chem. Soc., 2010, 132, 5330.
(a) M. T. Dang, L. Hirsch and G. Wantz, Adv. Mater., 2011,
9
23, 3597; (b) Y. J. Cheng, S. H. Yang and C. S. Hsu, Chem.
Rev., 2009, 109, 5868; (c) J. Chen and Y. Cao, Acc. Chem.
Res., 2009, 42, 1709; (d) R. F. Service, Science, 2011, 332,
References
293; (e) D. Gendron and M. Leclerc, Energy Environ. Sci.,
011, 4, 1225; (f) J. M. Szarko, J. Guo, B. S. Rolczynski and L.
1
(a) G. Li, R. Rhu and Y. Yang, Nat. Photonics, 2012, 6, 153;
2
(
(
b) F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2009, 93, 394;
c) B. C. Thompson, P. P. Khlyabich, B. Burkhart, A.
X. Chen, J. Mater. Chem., 2011, 21, 7849; (g) L. Dou, J. You,
J. Yang, C. C. Chen, Y. He, S. Murase, T. Moriarty, K. Emery,
G. Li and Y. Yang, Nat. Photonics, 2012, 6, 180; (h) D.
J. Liporhi, B. C. K. Tee, M. Vosgueritchain and Z. N. Bao,
Adv. Mater., 2011, 23, 1771.
0 (a) J. A. Mikroyannidis, A. N. Kabanakis, P. Balraju and G.
D. Sharma, Macromolecules, 2010, 43, 5554; (b) J.
A. Mikroyannidis, A. N. Kabanakis, P. Suresh and G.
D. Sharma, J. Phys. Chem. C, 2011, 115, 7056.
1 (a) P. L. T. Boudreault, S. Beaupre and M. Leclerc, Polym.
Chem., 2010, 1, 127; (b) N. Blouin and M. Leclerc, Acc.
Chem. Res., 2008, 41, 1110; (c) N. Blouin, A. Michaud and
M. Leclerc, Adv. Mater., 2007, 19, 2295; (d) E. Zhou,
S. Yamakawa, K. Tajima, C. Yang and K. Hashimoto, Chem.
Mater., 2009, 21, 4055; (e) E. Zhou, J. Cong, K. Tajima and
K. Hashimoto, Chem. Mater., 2010, 22, 4890.
2 (a) R. Qin, W. Li, C. Li, C. Du, C. Veit, H. F. Schleiermacher,
M. Andersson, Z. Bo, Z. Liu, O. Inganas, U. Wuerfel and
F. Zhang, J. Am. Chem. Soc., 2009, 131, 14612; (b) N. Blouin,
A. Michaud, D. Gendron, S. Wakim, E. Blair, R. Neagu-
Plesu, M. Belletete, G. Durocher, Y. Tao and M. Leclerc, J.
Am. Chem. Soc., 2007, 130, 732; (c) Z. G. Zhang, H. Fan,
J. Min, S. Zhang, J. Zhnag, M. Zhang, X. Guo, X. Zhan and
Y. Li, Polym. Chem., DOI: 10.1039/c1py00119a.
E. Aviles, A. Rudenko, G. V. Shultz, C. F. Ng and L.
B. Mangubat, Green, 2011, 1, 29; (d) C. W. Schlenker and M.
E. Thompson, Chem. Commun., 2011, 47, 3702; (e) P. L.
T. Boudreaut, A. Najari and M. Leclerc, Chem. Mater., 2011,
1
1
2
3, 456; (f) P. K. Nayak, G. Garcia-Belmote, A. Kahn,
J. Bisquert and D. Cahen, Energy Environ. Sci., 2012, 5,
022.
6
2
3
G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger,
Science, 1995, 270, 1789.
(a) Y. J. Cheng, S. H. Yang and C. S. Hsu, Chem. Rev., 2009,
1
09, 5868; (b) Z. Zhu, D. Waller R. Gaudiana, M. Morana,
D. Muhlbacher, M. Scharbar and C. Brabec,
Macromolecules, 2007, 40, 1981; (c) J. Hou, H. S. Chen,
S. Zhang, G. Li and Y. Yang, J. Am. Chem. Soc., 2008, 130,
1
6144; (d) J. C. Bijleveld, V. S. Gevarts, D. D. Nuzzo,
1
M. Turbiez, M. S. G. J. Mathijssen, D. M. de Leeuw, M.
M. Wienk and R. A. Janssen, Adv. Mater., 2010, 22, E242; (e)
C. Piliego, T. W. Holcombe, J. D. Douglas, C. H. Woo, P.
M. Beaujuge and J. M. J. Frechet, J. Am. Chem. Soc., 2010,
1
32, 7595.
4
(a) G. Li, C. W. Chu, V. Shrotriya, J. Huang and Y. Yang,
Appl. Phys. Lett., 2006, 88, 253503; (b) M. S. White, D.
C. Olson, S. E. Shaheen, N. Kopidakis and D. S. Ginley,
Appl. Phys. Lett., 2006, 89, 143517; (c) H. H. Liao, L.
M. Chen, Z. Xu, G. Li and Y. Yang, Appl. Phys. Lett., 2008,
1
3 Manjeet Singh, Rajnish Kurchania, J. A. Mkiroyannidis, S.
S. Sharma and G. D. Sharma, J. Mater. Chem. A, 2013, 1,
2297.
9
2, 173303; (d) V. Shrotriya, E. H. E. Wu, G. Li, Y. Yao and
1
1
1
4 M. M. Wienk, M. Turbiez, J. Gilot and R. A. J. Janssen, Adv.
Mater., 2008, 20, 2556.
5 Y. Liu, N. Xiang, X. Feng, P. Shen, W. Zhou, C. Weng,
B. Zhao and S. Tan, Chem. Commun., 2009(18), 2499.
6 N. N. Sergeeva, A. Scala, A. Muntaz, G. O. Riordan,
J. O’Brien, G. Grassi and M. O. Senge, J. Org. Chem.,
Y. Yang, Appl. Phys. Lett., 2006, 88, 064104.
5
6
(a) F. Padinger, R. S. Rittberger and N. S. Sariciftci, Adv.
Funct. Mater., 2003, 13, 85; (b) G. Li, V. Shrotriya, J. Hunag,
Y. Yao, T. Moriarty, K. Emery and Y. Yang, Nat. Mater.,
2
005, 4, 864; (c) J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma,
D. Moses, A. J. Heeger and G. C. Bazan, Nat. Mater., 2007, 6,
97.
2
009, 74, 7140.
7 G. R. Robertson, Org. Synth, 1941, Coll. Vol. I, 396, (1922, 1,
7).
4
1
(a) M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E.
D. Dunlop, Prog. Photovoltaics, 2011, 19, 565; (b) M.
A. Green, K. Emery, Y. Hishikawa, W. Warta and E.
D. Dunlop, Prog. Photovoltaics, 2012, 20, 12.
5
1
1
8 J. C. Li, Bull. Korean Chem. Soc., 2009, 30, 951.
9 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett.,
1996, 77, 3865.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 18821–18834 | 18833

View Article Online
Paper
RSC Advances
2
0 A. Sch ¨a fer, C. Huber and R. Ahlrichs, J. Chem. Phys., 1994,
00, 5829.
Macromolecules, 2010, 43, 9376; (c) S. I. Hsu, C. M. Chen
and K. H. Wei, J. Polym. Sci., Part A: Polym. Chem., 2010, 48,
5126; (d) H. Xie, K. Zhang, C. Duan, S. Liu, F. Huang and
Y. Cao, Polymer, 2012, 53, 5675.
1
¨
¨
2
1 K. Eichkorn, O. Treutler, H. Ohm, M. Haser and
R. Ahlrichs, Chem. Phys. Lett., 1995, 240, 283.
2
2
2 C. Adamo and V. Barone, J. Chem. Phys., 1999, 110, 6158.
3 (a) A. D. Becke, J. Chem. Phys., 1993, 98, 5648; (b) C. Lee,
W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785.
34 M. H. Chen, J. Hou, Z. Hong, G. Yang, S. Sista and L.
M. Chen, Adv. Mater., 2009, 21, 4238.
3
5 C. Deibel, T. Strobel and V. Dyakonov, Adv. Mater., 2010,
2, 4097.
2
4 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M.
A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J.
L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery Jr, J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin,
V. N. Staroverov, R. Kobayashi, J. Normand,
K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J.
E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J.
J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J.
B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox,
Gaussian 03, revision C.01, Gaussian, Inc., Wallingford CT,
2
36 (a) S. E. Shaheen, C. J. Brabec, N. S. Sariciftci, F. Padinger,
T. Fromherz and J. C. Hummelen, Appl. Phys. Lett., 2001,
7
8, 841; (b) B. Walker, A. Tamayo, D. T. Duong, X. D. Dang,
C. Kim, J. Granstrom and T. Q. Nguyen, Adv. Energy Mater.,
011, 1, 221.
2
3
3
7 W. Ma, C. Yang, X. Gong, K. Lee and A. J. Heeger, Adv.
Funct. Mater., 2005, 15, 1617.
8 (a) G. Li, Y. Yao, H. Yang, V. Shrotriya, G. Yang and Y. Yang,
Adv. Funct. Mater., 2007, 17, 1636; (b) S. Miller, G. Fanchini,
Y. Y. Lin, C. Li, C. W. Chen, W. F. Su and M. Chhowalla, J.
Mater. Chem., 2008, 18, 306.
3
9 (a) J. Peet and J. Y. Kim, Nat. Mater., 2007, 6, 497; (b) J.
K. Lee, W. L. Ma, C. J. Bazan, J. Yuen, J. S. Moon, J. Y. Kim,
K. Lee, G. C. Bazan and A. J. Heeger, J. Am. Chem. Soc.,
2008, 130, 3619.
4
0 (a) C. V. Hoven, X. D. Dang, R. C. Coffin, J. Peet, T.
Q. Nguyen and G. C. Bazan, Adv. Mater., 2010, 22, E63; (b)
J. Jo, D. Gendnon, A. Najari, J. S. Moon, S. Cho, M. Leclerc
and A. J. Heeger, Appl. Phys. Lett., 2010, 97, 203303; (c)
X. Guo, C. Cui, M. Zhang, L. Huo, Y. Hunag, J. Hou and
Y. Li, Energy Environ. Sci., DOI: 10.1039/c2ee21481d; (d) Y.
P. Sun, C. H. Cui, H. Q. Wang and Y. F. Li, Adv. Energy
Mater., 2011, 1, 1058.
2004.
2
2
5 TURBOMOLE (version 5.6), Universitat Karlsruhe, 2000.
6 V. D. Mihailetchi, P. W. M. Blom, J. C. Hummelen and M.
T. Rispens, J. Appl. Phys., 2003, 94, 6849.
2
2
2
3
3
3
7 D. Jacquemin, E. A. Perp `e te, G. E. Scuseria, I. Ciofini and
C. Adamo, J. Chem. Theory Comput., 2008, 4, 123.
8 J. A. Mikroyannidis, M. M. Stylianakis, P. Suresh, P. Balraju
and G. D. Sharma, Org. Electron., 2009, 2009, 10.
9 Q. J. Sun, H. Q. Wang, C. H. Yang and Y. F. Li, J. Mater.
Chem., 2003, 13, 800.
4
4
4
1 H. C. Chen, S. W. Chou, W. H. Tseng, I. W. P. Chen, C.
C. Liu, C. Liu, C. L. Liu, C. H Chen, C. I. Wu and P. T. Chou,
Adv. Funct. Mater., 2012, 22, 3984.
2 J. H. Park, J. I. Park, D. H. Kim, J. H. Kim, J. S. Kim, J.
H. Lee, M. Sim, S. Y. Lee and K. Cho, J. Mater. Chem., 2010,
0 Y. Zhu, R. D. Champion and S. A. Jeneke, Macromolecules,
2006, 39, 871.
20, 5860.
1 C. Kanimozhi, P. Balraju, G. D. Sharma and S. Patil, J. Phys.
Chem. B, 2010, 114, 3095.
2 (a) A. Najari, P. Berrouard, C. Ottone, M. Boivin, Y. Zhou,
D. Gendron, W. O. Caron, P. Legros, C. N. Allen, S. Sadki
and M. Leclerc, Macromolecules, 2012, 45, 1833; (b) Z. Lin,
J. Bjorgaard, A. Gul Yavuz, A. Iyer and M. E. Kose, RSC Adv.,
3 (a) G. G. Malliaras, J. R. Salem, P. J. Brock and J. C. Scott, J.
Appl. Phys., 1998, 84, 1583; (b) M. D. Perez, C. Borek, S.
R. Forrest and M. E. Thompson, J. Am. Chem. Soc., 2009,
131, 9281.
44 (a) K. Vandewell, K. Tvingstedt, A. Gadisal, O. Inganas and
J. V. Manca, Nat. Mater., 2009, 8, 904; (b) S. Yamamoto,
A. Orimo, H. Ohkita, H. Benten and S. Ito, Adv. Energy
Mater., 2012, 2, 229.
2012, 2, 642; (c) S. Wen, W. Cheng, P. Li, S. Yao, B. Xu,
H. Li, Y. Gao, Z. Wang and W. Tian, J. Polym. Sci., Part A:
Polym. Chem., 2012, 50, 3758.
3
3 (a) C. H. Duan, K. S. Chen, F. Huang, H. L. Yip, S. J. Liu,
J. Zhang, A. K. Y. Jen and Y. Cao, Chem. Mater., 2010, 22,
45 (a) S. R. Cowan, A. Roy and A. J. Heeger, Phys. Rev. B:
Condens. Matter Mater. Phys., 2010, 82, 245207; (b) J.
S. Moon, M. Leclerc and A. J. Heeger, Adv. Energy Mater.,
2012, 2, 1397.
6444; (b) Z. G. Zhang, Y. L. Liu, Y. Yang, K. Y. Hou, B. Peng,
G. J. Zhao, M. Zhang, X. Guo, E. T. Kang and Y. Li,
1
8834 | RSC Adv., 2013, 3, 18821–18834
This journal is ß The Royal Society of Chemistry 2013