Synthesis and characterization of a new perylene bisimide (PBI) derivative and its application as electron acceptor for bulk heterojunction polymer solar cells

Article abstract of DOI:10.1016/j.orgel.2012.07.002

A symmetrical perylene bisimide derivative (PBI) with 2-(4-nitrophenyl) acrylonitrile groups at the 1,7 bay positions of perylene and solubilizing cyclohexyl units was synthesized and characterized. The absorption spectrum of PBI was broad with the most prominent peak at 655 nm and optical band gap of 1.72 eV. The electrochemical investigation indicates that PBI has a LUMO energy level of -3.9 eV which is similar to that of PCBM or PC₇₀BM. Bulk heterojunction solar cell fabricated using a blend of poly(3-hexylthiophene) (P3HT) and PBI (1:1 w/w) as active layer cast from THF exhibited power conversion efficiency (PCE) at 1.56%. However, the device with P3HT:PBI blend deposited from mixed solvent (DIO/THF) improved the PCE to 2.78% which further increased to 3.17% on using the thermal annealed active layer. The improvement in the PCE is attributed to the enhanced crystallinity of the blend (particularly P3HT) and increase in hole mobility leading to balanced charge transport.

Full text of DOI:10.1016/j.orgel.2012.07.002

Organic Electronics 13 (2012) 3118–3129
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Synthesis and characterization of a new perylene bisimide (PBI)
derivative and its application as electron acceptor for bulk
heterojunction polymer solar cells
G.D. Sharma a, , M.S. Roy c, J.A. Mikroyannidis b, K.R. Justin Thomas d
⇑
a R&D Center for Science and Engineering, Jaipur Engineering College, Kukas, Jaipur, Rajasthan 303001, India
b Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-26500 Patras, Greece
c Defence Laboratory, Jodhpur, Rajasthan 342011, India
d Organic Material Laboratory, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A symmetrical perylene bisimide derivative (PBI) with 2-(4-nitrophenyl)acrylonitrile
groups at the 1,7 bay positions of perylene and solubilizing cyclohexyl units was synthe-
sized and characterized. The absorption spectrum of PBI was broad with the most promi-
nent peak at 655 nm and optical band gap of 1.72 eV. The electrochemical investigation
indicates that PBI has a LUMO energy level of ꢀ3.9 eV which is similar to that of PCBM
or PC70BM. Bulk heterojunction solar cell fabricated using a blend of poly(3-hexylthioph-
ene) (P3HT) and PBI (1:1 w/w) as active layer cast from THF exhibited power conversion
efficiency (PCE) at 1.56%. However, the device with P3HT:PBI blend deposited from mixed
solvent (DIO/THF) improved the PCE to 2.78% which further increased to 3.17% on using the
thermal annealed active layer. The improvement in the PCE is attributed to the enhanced
crystallinity of the blend (particularly P3HT) and increase in hole mobility leading to bal-
anced charge transport.
Received 7 December 2011
Received in revised form 28 June 2012
Accepted 2 July 2012
Available online 15 July 2012
Keywords:
Polymer solar cells
Bulk heterojunction
Perylene bisimide derivative
Power conversion efficiency
Solvent additive effect
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
the analysis of the fundamental processes of photon–elec-
tron conversion and on device technology has generated
Organic solar cells (OSCs) are attractive over the con-
ventional inorganic based solar cells due to their useful-
ness in developing large area, light weight, cost effective
panels on flexible substrates by means of simple and low
environment impact technologies [1,2]. They also offer eas-
ier processing methodologies for the application of organic
materials as active layers. Considerable advances have re-
cently been made in increasing the power conversion effi-
ciency (PCE) of bulk heterojunction (BHJ) polymer solar
cells [3], in which an electron donor polymer and an elec-
tron acceptor material are blended to make the active pho-
tovoltaic layer. Multidisciplinary research effort focused on
tremendous progress leading to an increase of PCE from
0.10% to 5.0% in less than a decade [1a–c,4]. During the last
few years progress in the synthesis of low band gap conju-
gated polymers has produced further increase of PCE to
values in the range of 7–8% [5]. For instance PCEs higher
than 8% have been recently achieved using such photoac-
tive layers [6].
To date the fullerene derivatives are the most widely
studied acceptors for organic BHJ solar cells and such
explorations have led to the improvement in solar cell per-
formance and donor design. The most commonly em-
ployed fullerene derivatives include [6,6]-phenyl C61
butyric acid methyl ester (PC60BM) [7] and [6,6]-phenyl
70-butyric acid methyl ester (PC70BM) [8]. With PC60BM
-
C
⇑
Corresponding author. Tel.: +91 01426 247345; fax: +91 01426
511240.
efficiencies have reached as high as 5%, when blended with
P3HT [9]. Despite its success PC60BM is not an ideal
E-mail address: sharmagd_in@yahoo.com (G.D. Sharma).
1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2012.07.002

G.D. Sharma et al. / Organic Electronics 13 (2012) 3118–3129
3119
material for OSC as it absorbs light poorly in the red region
2. Experimental detail
and its high electron affinity may cause energy losses when
blend with low band gap polymers [10]. These losses have
been reduced to some extend with PC70BM, which posses
relatively stronger absorption than PC60BM in the red re-
gion [11]. However, the almost negligible absorption of ful-
lerene acceptors in the solar spectral region indicates that
the light harvesting of BHJ solar cells using them is limited
by the optical absorption of the donor materials. In addi-
tion, fullerene acceptors have poor photochemical stability
in air and their spherical geometry leads to morphological
instability of blends with linear conjugated polymers.
Therefore, the development of new non-fullerene acceptor
materials is an important goal in organic BHJ photovoltaic
devices.
2.1. Reagents and solvents
N,N-Dimethylformamide (DMF) and N-methyl-2-pyr-
rolidone (NMP) were dried by distillation over CaH2. 4-
Hydroxybenzaldehyde was recrystallized from distilled
water. 4-Nitrobenzyl cyanide was synthesized by the nitra-
tion of benzyl cyanide with a mixture of concentrated ni-
tric and sulfuric acids [18]. It was recrystallized from
ethanol. All other reagents and solvents were commer-
cially purchased and were used as supplied.
2.2. Characterization methods
Several non-fullerene small molecules have been syn-
thesized and studied as acceptors in BHJ organic solar
cells [12] and the PCE is <2% [12f]. Among n-type organic
materials, perylene bisimide (PBI) derivatives have large
molar absorption coefficients, excellent electron accept-
The absorption spectra were recorded using a Perkin El-
mer UV–vis spectrophotometer. The IR spectra of the com-
pound were recorded on a Perkin Elmer 16PC FT-IR
spectrometer with KBr pellet. 1H NMR (400 MHz) spectra
were obtained using Bruker spectrometer. Thermal gravi-
metric analysis (TGA) was performed on a DuPont 990 ther-
mal analyzer system. Ground samples of about 10 mg each
was examined by TGA and the weight loss comparison was
made between comparable specimens. Thermo-mechanical
analysis (TMA) was recorded on a DuPont 943 TMA using a
loaded penetration probe at a scan rate of 20 °C/min in N2
with a flow rate of 60 cm3/min. The thermo-mechanical
analysis (TMA) experiments were conducted at least in
duplicate to ensure the accuracy of the results. The TMA
specimens were pellets of 10 mm diameter and ꢁ1 mm
thickness prepared by pressing powder of sample for
3 min under 8 kp/cm2 at ambient temperature. The glass
transition temperature is assigned by the first inflection
point in the TMA curve, and it was obtained from the onset
temperature of this transition during the second heating.
Electrochemical properties (onset oxidation and reduc-
tion potentials) of PBI were obtained by cyclic voltammet-
ric (EDCA instrument) measurements. The PBI was coated
onto a glassy carbon electrode, which was used as the
working electrode, and immersed in 0.1 M Bu4NPF6 aceto-
nitrile solution used as supporting electrolyte. Cyclic vol-
tammogram was recorded using Ag/Ag+ as a reference
electrode at a scan rate of 100 mV/s.
ing properties and a conducting direction along
p–p
stacking axis [13]. These materials are among the most
studied non-fullerene acceptors for polymer BHJ solar
cells [13b,14] and achieved PCE as high as 1.87% [15].
Mikroyannidis et al. have synthesized an alternating
phenylenevinylene copolymer bearing perylene bisimide
moieties along the backbone which behaved as n-type
organic semiconductor [16]. The photovoltaic device
based on the BHJ with poly (3-phenyl hydrazone thio-
phene) (PPHT) showed PCE of 1.67% which was further
improved to 2.32% upon thermal annealing. Moreover,
Mikroyannidis et al. have synthesized two symmetrical
perylene bisimides carrying anthracene or pyrene termi-
nal moieties which were used as sensitizers for quasi-so-
lid state dye-sensitized solar cells and the PCE shoot up
to 3.42% [17].
As an extension of our research work herein investi-
gated new symmetrical perylene bisimde with 2-(4-nitro-
phenyl)acrylonitrile segments and solubilizing cyclohexyl
rings. Incorporation of 2-(4-nitrophenyl)acrylonitrile units
on the perylene core has led to the broadening of the
absorption spectrum up to 780 nm in thin film. From
the current–voltage characteristics of the ITO/PBI/Al de-
vice it was found that PBI behaves as the n-type organic
semiconductor, which forms nearly Ohmic contact and
Schottky barrier for electron injection with the Al and
ITO electrodes, respectively. The photovoltaic properties
of the blend of PBI (electron acceptor), with a P3HT (elec-
tron donor), were systematically investigated. The PCE of
the PSC based on P3HT:PBI cast from THF solvent is
1.56%. We have also studied the current–voltage charac-
teristics of the BHJ devices prepared from P3HT:PBI cast
from mixed solvent DIO/THF and subsequent thermal
annealing. They displayed PCE of 2.78% and 3.17%, respec-
tively. The higher PCE observed for the device with the
BHJ deposited from the mixed solvent and further
improvement on subsequent thermal annealing have
been attributed to the increase in crystallinity of the
blend and the enhancement in the hole mobility. The la-
ter is expected to balance the charge transport in the pho-
toactive layer.
2.3. Synthesis of compounds
2.3.1. 1,7-dibromoperylene-3,4,9,10-tetracarboxydianhydride
(1)
This compound was synthesized by the reaction of perr-
ylene-3,4:9,10-tetracarboxylic dianhydride with bromine
in sulfuric acid in the presence of a catalytic amount of io-
dine according to a reported method [19]. The 1,7-dibromo
derivative was the predominant isomer which was ob-
tained as a residue in the reaction and used without further
purification in the next step.
2.3.2. N,N0-bis(cyclohexyl)-1,7-dibromo-perylene-3,4:9,10-
bis(dicarboximide) (2)
This compound was synthesized by the aminolysis of
compound 1 with excess of cyclohexylamine (mol ratio

3120
G.D. Sharma et al. / Organic Electronics 13 (2012) 3118–3129
1:3) in N-methyl-2-pyrrolidone (NMP) in the presence of
glacial acetic acid [19].
acterized as a minimum due to the absence of imaginary
frequency. The time-dependent DFT (TD-DFT) [23] compu-
tations at the same level (B3LYP/6–31G⁄) was further car-
ried out to determine first 10 vertical excitations to the
excited state of the molecule. The computations in THF/
DMF were performed by applying polarizable continuum
model (PCM) using the integral equation formalism variant
(IEFPCM) to describe the electrostatic solute–solvent inter-
actions by the creation of solute cavity via a set of overlap-
ping spheres [24]. The absorption spectra were simulated
by using the 10 lowest spin-allowed singlet transitions,
mixed Lorentzian-Gaussian lineshape (0.5) and an average
full-width at half maximum of 3000 cmꢀ1 for all peaks
[25].
2.3.3. Compound 3
Compound 2 (0.20 g, 0.281 mmol) was dissolved in N,N-
dimethylformamide (DMF) (15 mL) by heating. 4-
Hydroxybenzaldehyde (0.10 g, 0.819 mmol) and K2CO3
(0.12 g, 0.868 mmol) were added to this solution. The mix-
ture was stirred and refluxed under N2 for 20 h. Then, it
was concentrated under reduced pressure and water con-
taining few drops of hydrochloric acid was added to the
concentrate. The precipitate was filtered, washed thor-
oughly with hot water to dissolve excess of 4-hydroxy-
benzaldehyde and dried to afford 3 (0.194 g, 87%).
FT-IR (KBr, cmꢀ1): 2928, 2852 (C–H stretching of cyclo-
hexyl ring); 1694, 1654 (imide carbonyl and formyl); 1236
(ether bond).
2.5. Device fabrication and characterization
1H NMR (DMSO-d6) ppm: 9.81 (s, 2H, formyl); 8.18 and
7.48 (m, 6H, perylene); 7.79 (m, 4H, aromatic ortho to for-
myl); 6.88 (m, 4H, aromatic meta to formyl); 2.62, 1.97–
1.44 and 1.44–0.72 (m, 22H, cyclohexyl ring).
Anal. Calcd. for C50H38N2O8: C, 75.55; H, 4.82; N, 3.52.
Found: C, 74.87; H, 4.76; N, 3.63.
The polymer solar cells were fabricated on the indium
tin oxide (ITO) coated glass substrates. The indium tin
oxide (ITO) coated glass substrates were cleaned by ultra-
sonic treatment in de-ionized water, acetone, detergent,
and isopropyl alcohol sequentially. A thin layer (thickness
60 nm) of poly (3,4-ethylenedioxythiophene: poly (styrene
sulfonate) (PEDOT:PSS) (Bayer Baytron) was spin coated on
the ITO coated glass substrate, followed by heating for
15 min. The mixture solution of P3HT and PBI with weight
ratios of 1:0.5, 1:1,1:1.5 in THF solvent were prepared and
was then spin coated onto the PEDOT:PSS layer. We have
prepared 5 devices for each weight ratio. The thickness of
the active layers was about 90–95 nm. For the mixed sol-
vent 1.5% by volume of DIO was mixed into the blend solu-
tion in THF. For thermal annealing, the blend film was
placed on a hot plate and annealed at a temperature of
120 °C for 2 min. Finally, the aluminum (Al) top electrode
was thermally deposited on the active layer at a vacuum
of 10ꢀ5 Torr. The effective area of each device was
20 mm2. All the devices are fabricated in air atmosphere
and without encapsulation.
Current–voltage (J–V) characteristics of the devices
were measured using a computer controlled Keithley 238
source meter in dark and under illumination intensity of
100 W/cm2 under ambient conditions. A xenon light source
(100 W) coupled with optical filter (AM 1.5) was used to
give a stimulated irradiance of 100 mW/cm2 at the surface
of the device. The hole and electron devices having struc-
ture ITO/PEDOT:PSS/P3HT:PBI/Au and Al/P3HT:PBI/Al
were fabricated, respectively to measure the hole and elec-
tron mobilities in the BHJ active layer.
2.3.4. Perrylene bisimide (PBI)
A flask was charged with a solution of compound 3
(0.1874 g, 0.236 mmol) and 4-nitrobenzyl cyanide
(0.765 g, 0.471 mmol) in DMF (20 mL). Sodium hydroxide
(0.10 g, 2.5 mmol) dissolved in ethanol (10 mL) was added
portion wise to the stirred solution. The mixture was stir-
red for 1 h at room temperature under N2 and then concen-
trated under reduced pressure. Water was added to the
concentrate and PBI precipitated as a dark green solid. It
was recrystallized from DMF-water (0.192 g, 75%).
FT-IR (KBr, cmꢀ1): 2928, 2852 (C–H stretching of cyclo-
hexyl ring); 2172 (cyano); 1703 (imide carbonyl); 1525,
1345 (nitro); 1235 (ether bond).
1H NMR (DMSO-d6) ppm: 8.22 (m 4H, aromatic ortho to
nitro); 8.18 and 7.48 (m, 6H, perylene); 7.72 (s, 2H, cyano-
vinylene); 7.47 (m, 4H, aromatic meta to nitro); 7.05–6.78
(m, 8H, aromatic meta and ortho to ether oxygen); 2.62,
1.97–1.44 and 1.44–0.72 (m, 22H, cyclohexyl ring).
Anal. Calcd. for C66H46N6O10: C, 73.19; H, 4.28; N, 7.76.
Found: C, 72.68; H, 4.41; N, 7.92.
2.4. Computational methods
DFT calculations were performed using the Gaussian
09W [20] (Revision-A.02) program suite and employing
default optimization conditions for tight SCF convergence
criteria. The geometry of the dye in air was optimized
using density functional theory (DFT) [21] using hybrid
B3LYP [22] density functional and the 6–31G⁄ basis set.
Further the structure was simplified by replacing the N-
cyclohexyl group with N-methyl unit to facilitate the com-
putations. The starting structure of the dye was composed
from the crystal structural data available for perylene-
3,4:9,10-bis(dicarboximide) core from the literature. This
alternation in the structure does not influence the elec-
tronic structure of the dye significantly. The optimized
structure was subjected to vibrational analysis, and char-
3. Results and discussion
3.1. Synthesis of PBI
Scheme 1 outlines the synthesis of PBI. In particular,
perrylene bisanhydride, perylene-3,4,9,10-tetracarboxy-
dianhydride was brominated to afford the 1,7-dibromo
derivative 1 as a predominant product [19]. The latter
was subsequently condensed with cyclohexylamine to give
the corresponding bisimide 2 [19]. This compound reacted
with 4-hydroxybenzaldehyde in DMF in the presence of

G.D. Sharma et al. / Organic Electronics 13 (2012) 3118–3129
3121
Scheme 1. Synthesis of perylene bisimide (PBI).
K2CO3 to afford the bisether 3. Finally, the target PBI was
obtained from the condensation of 3 with 4-nitrobenzyl
cyanide in the presence of sodium hydroxide utilizing a
mixture of DMF and ethanol as reaction medium. PBI
was characterized by FT-IR and 1H NMR spectroscopy.
The two cyclohexyl rings enhanced the solubility of PBI
in common organic solvents.
from the intersection of absorption and emission is about
1.78 eV.
To obtain information about the origin of the electronic
transitions observed for the dye we have undertaken
TDDFT computations at gas, THF and DMF phases. The
computed positions of the vertical excitations, their oscil-
lator strengths, compositions of vertical transitions in
terms of molecular orbitals and orbital energies are listed
in Table 1a and Table 1b. The electronic distributions ob-
served for selected molecular orbitals contributing signifi-
cantly to the prominent electronic excitations in the dye
are presented in Fig. 1.
3.2. Thermal properties of PBI
The thermal characterization of PBI was accomplished
by thermogravimetric analysis at a heating rate of 5 °C/
min. The PBI exhibited good thermal stability, the decom-
position temperature, defined as a temperature corre-
sponding to 5% weight loss is found at 370 °C. The
characteristics yield of 49% at about 780 °C. The TMA trace
showed a distinguishable step from the onset of which the
glass transition temperature (Tg) was determined to be
82 °C. The small glass transition temperature of PBI sug-
gests a low degree of crystallinity in solid state. The good
thermal stability is positive aspect for using PBI as electron
acceptor in organic photovoltaic devices.
There are two prominent absorptions predicted from
the computations for the dye in gas phase as well as in sol-
vent medium. The lower energy absorption corresponds to
the HOMO to LUMO transition while the higher energy
transition occurs due to the excitations from HOMO ꢀ 1
or HOMO ꢀ 2 to LUMO + 1 or LUMO + 2. Since the frontier
molecular orbitals, HOMO and LUMO are mainly derived
from the central perylene-3,4:9,10-bis(dicarboximide)
core and the HOMO ꢀ 1, HOMO ꢀ 2, LUMO + 1 and
LUMO + 2 orbitals contributed by the 2-(4-nitro-
phenyl)acrylonitrile arms, the longer wavelength absorp-
tion can be attributed to the
p–
p⁄ transition localized
3.3. Optical and electrochemical properties of PBI
within the perylene-3,4:9,10-bis(dicarboximide) unit
while the shorter wavelength absorption assigned to the
Fig. 3 shows the normalized absorption spectra of PBI in
both dilute (10ꢀ5 M) THF solution and thin film. They were
normalized with the long wavelength absorption peak. The
absorption spectra of PBI had same pattern in solution and
thin film with two maxima (ka,max) around 425 nm and
610–630 nm. It should be emphasized that the absorption
spectra of this material is broad and covered a wide range
of UV–vis with optical onset about 725 nm. Generally, per-
ylene bisimide usually shows vibronic bands in solution
and thin films [26], but we have not observed any vibronic
peak in PBI, this may be due to the bulkier substitutions at
bay positions suppress the vibronic band. The presence of
2-(4-nitrophenyl) acrylonitrile moieties in the molecule
contributed to the broadening of its absorption as has been
well established in the literature [27]. PBI displayed thin
film absorption onset around 725 nm, corresponding to
optical band gap 1.72 eV. The optical band gap estimated
p–
p⁄ transition localized in the 2-(4-nitrophenyl)acryloni-
trile unit. The simulated absorption spectra of the dye in
the different media are shown in Fig. 2. The absorption
profile predicted closely resembles to that experimentally
observed for the PBI in DMF. A slight blue shift observed
for the theoretical values is intriguing. It is probably due
to the underestimation caused due to the error in modeling
the long range electronic coupling in larger molecules by
DFT.
The ionization potential is the energy difference be-
tween the cationic and the neutral states based on the
optimized geometry for the neutral states while the elec-
tron affinity is the energy difference between the anionic
and the neutral states based on the optimized geometry
in the neutral form. The vertical ionization potential of
PBI is ꢀ7.06 eV and the electron affinity is estimated at

3122
G.D. Sharma et al. / Organic Electronics 13 (2012) 3118–3129
Table 1a
Computed vertical transitions their oscillator strength and assignments in terms of molecular orbital contributions, frontier orbital energies and dipole
moments for the PBI.
Medium kmax
Oscillator
Strength
Assignment
Dipole moment
(Debye)
Vacuum 539.5 0.694
376.8 1.493
HOMO ? LUMO(94%)
2.41
HOMO ꢀ 1 ? LUMO + 2 (50%) HOMO ꢀ 1 ? LUMO + 1 (17%) HOMO ꢀ 4 ? LUMO (15%)
HOMO ꢀ 2 ? LUMO + 1 (7%)
THF
567.2 0.825
402.4 1.268
395.4 0.513
HOMO ? LUMO (97%)
3.23
HOMO ꢀ 1 ? LUMO + 2 (64%) HOMO ꢀ 1 ? LUMO + 1 (23%) HOMO ꢀ 2 ? LUMO + 2 (8%)
HOMO ꢀ 2 ? LUMO + 1 (58%) HOMO ꢀ 1 ? LUMO + 1 (16%) HOMO ꢀ 1 ? LUMO + 2 (16%)
HOMO ꢀ 2 ? LUMO + 2 (5%)
DMF
570.4 0.830
408.1 1.197
400.5 0.529
HOMO ? LUMO (97%)
3.46
HOMO ꢀ 1 ? LUMO + 2 (65%) HOMO ꢀ 1 ? LUMO + 1 (23%) HOMO ꢀ 2 ? LUMO + 2 (7%)
HOMO ꢀ 2 ? LUMO + 1 (58%) HOMO ꢀ 1 ? LUMO + 1 (17%) HOMO ꢀ 1 ? LUMO + 2 (15%)
HOMO ꢀ 2 ? LUMO + 2 (6%)
The electrochemical properties of the PBI were studied
by cyclic voltammetry. Fig. 4 shows the cyclic voltammo-
gram of the PBI. The onset oxidation and reduction poten-
tials were 1.05 V and 0.8 V, Vs Ag/Ag+ respectively. The
highest occupied molecular orbital (HOMO) (EHOMO) and
lowest unoccupied molecular orbital (LUMO) (ELUMO) en-
ergy levels of the material were estimated according to
the following expression [30]:
Table 1b
Computed values of HOMO, LUMO and band gap of the PDI.
Medium
HOMO (eV)
LUMO (eV)
Band gap (eV)
Vacuum
THF
DMF
ꢀ6.10
ꢀ5.85
ꢀ5.80
ꢀ3.61
ꢀ3.40
ꢀ3.37
2.49
2.45
2.43
ox
2.63 eV. These values suggest a reasonable electron affinity
for the molecule due to the presence of bisimide function-
ality. According to Marcus theory [28], the charge transfer
rate (KCT) is described as:
EHOMO ¼ ꢀðE
ELUMO ¼ ꢀðE
þ 4:70ÞeV
þ 4:70ÞeV
onset
ð1Þ
red
onset
The values of the HOMO and LUMO energy levels of PBI
are ꢀ5.80 eV and ꢀ3.6 eV, respectively. The electrochemi-
cal band gap of the PBI is 2.20 eV, which is higher than that
estimated from optical band gap (1.72 eV and 1.78 eV esti-
mated from the absorption onset and intersection of emis-
sion and absorption spectra) and smaller that that
estimated from DFT calculation 2.45 eV. The LUMO level
of PBI is comparable with that of PCBM (LUMO = 3.75–
4.0 eV). The higher value of electrochemical band gap is
generally found in the organic semiconductors due to their
high binding energy. The difference between the LUMO
levels of the donor and acceptor materials used for BHJ ac-
tive layer, play an important role in the photo-induced
charge transfer in the BHJ organic solar cells. This differ-
ence must be higher than the exciton binding energy i.e.
0.4–0.6 eV [31]. The difference between the LUMO level
of PBI and P3HT (ꢀ3.0 eV) is more than the exciton binding
energy. These results indicate that PBI can be potentially
utilized as an n type material (electron acceptor) in a BHJ
polymer solar cell.
4p
2V2
4pkBTk
k
ꢀ4k
T
B
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
KCT
¼
e
h
where k is the reorganization energy, V is the charge trans-
fer integral, T is the temperature, kB and h are the Boltz-
mann
and
Planck
constants
respectively.
The
reorganization energy k corresponds to the sum of geome-
try relaxation energies upon going from the neutral-state
geometry to the charged-state geometry and vice versa.
This parameter can be used to estimate the feasibility of
hole and electron transfer possibilities of organic materials.
Smaller the value of this quantity larger will be the charge
transfer propensity. So, the hole and electron reorganiza-
tion energies of the molecule were computed by using
the equations:
kþ ¼ ½ENðGCÞ ꢀ ENðGNÞꢂ þ ½ECðGNÞ ꢀ ECðGCÞꢂ
kꢀ ¼ ½ENðGaÞ ꢀ ENðGNÞꢂ þ ½EaðGNÞ ꢀ EaðGaÞꢂ
where EN, EC and EA are the energies of the neutral, cationic
and anionic states; GN, GC, GA are the optimized geometries
of the neutral, cationic and anionic states respectively. The
hole and electron reorganization energies are 0.30 eV and
0.28 eV, respectively. The hole reorganization energy of
PBI is higher than that reported for the typical hole trans-
port materials N,N0-diphenyl-N,N0-bis(3-methylphenyl)-
3.4. Electrical properties of the PBI
The current–voltage (J–V) characteristics of the ITO/
PEDOT:PSS/PBI/Al device display rectification effect as
shown in Fig. 5. The LUMO of the PBI is about ꢀ3.6 eV
which is close to the work function of the Al electrode
(ꢀ4.1 eV). However, the HOMO of PDI (ꢀ5.80 eV) is rela-
tively far from the HOMO level of PEDOT:PSS (5.0 eV). Con-
sequently, the rectification effect is due to the band
bending at the interface PEDOT:PSS-PBI that forms the
Schottky barrier for hole injection from PEDOT:PSS to the
(1,10-biphenyl)-4,40-diamine
and
1,4-bis-(carbazol-
yl)biphenyl [29]. The lower electron reorganization energy
realized for PDI as compared to the hole reorganization en-
ergy indicate that it may possess better electron transport-
ing capability.

G.D. Sharma et al. / Organic Electronics 13 (2012) 3118–3129
3123
Fig. 2. Simulated (fwhm = 3000 cmꢀ1) absorption spectra of the PBI from
the computed peak positions and their oscillator strengths.
Fig. 3. UV–vis optical absorption spectra of PBI in THF solution and thin
film form.
HOMO of the PBI. The forward bias corresponds to the neg-
ative and positive potential applied to the Al and ITO elec-
trode respectively and the device acts as electron only
device. Therefore, PBI behaves as n-type organic semicon-
ductor and has been used as an electron acceptor for the
BHJ photovoltaic device as discussed in the following part
of the paper.
The charge carrier mobility of materials either used as
donor or acceptor in the organic PV is also important fac-
tor, which influences the short circuit current (Jsc), fill fac-
tor (FF) and power conversion efficiency (PCE) of the
device. We have fabricated the device having configuration
Al/PDI/Al. In this device, both Al electrodes form nearly Oh-
mic contact with PDI and electrons are injected from the
bottom electrode into the LUMO of PDI and collected by
the top electrode, when bottom electrode is negative with
respect to top electrode. The J–V characteristic of the de-
vice was plotted in log–log scale and is shown in Fig. 6. It
can be seen from this figure that the in the low voltage re-
gion (up to 0.25 V), J is linearly dependent on V with a slope
of unity, which corresponds to Ohmic region. In the
Fig. 1. Electronic distributions observed for the PBI in selected frontier
molecular orbitals contributing significantly to the electronic transitions.

3124
G.D. Sharma et al. / Organic Electronics 13 (2012) 3118–3129
relatively high voltage (beyond 0.25 V), J is linearly depen-
dent on voltage with slope about two. Therefore, the J–V
characteristics in dark, in higher voltage region, correspond
to the space charge limited current (SCLC) behavior with
trap free limit. The electron mobility of PDI was estimated
fitting J–V characteristics with the following equation in
higher voltage region [32]:
coefficient as compared to the THF cast blend, which is fur-
ther enhanced when the DIO/THF cast film is thermally an-
nealed. As it has been reported earlier, the thin film
absorption of P3HT also red shifts and displays more dis-
tinct vibronic structures, when a solvent with high boiling
point is added to the solution before spin casting [33].
Upon addition of the DIO to the blend solution in THF,
the absorption band corresponds to the P3HT shifts to-
wards higher wavelength (red shift) and becomes broader.
A further red shift in the absorption band has also been ob-
served for thermally annealed blend. A high degree of
P3HT crystallinity as indicated by a red shift in the absorp-
JSCLC ¼ ð9=8Þeoer
l
ðV2=d3Þ
ð2Þ
where JSCLC is the current density in SCLC region, V is the
applied voltage, er and eo are the relative dielectric con-
stant of the organic layer and permittivity of the free space
tion band enhances the interchain
p–
p⁄ stacking and also
(8.85 ꢃ 10ꢀ12 F/m), respectively,
l
is the charge carrier
increases the absorption intensity. During the spin coating
process, the blend solution solidified into film structure, as
majority of solvent evaporate from the blend solution. The
blend film cast from the mixed solvent (DIO/THF) showed
higher crystallinity than cast from THF solvent.
To investigate the electron accepting ability or photolu-
minescence quenching (PLQ) of PBI, we measured the pho-
toluminescence quenching (PLQ) of P3HT as a function of
acceptor concentration. The PLQ can be estimated using
following expression:
mobility, and d is the thickness of the organic layer. Using
er = 3.5, d = 90 nm, the electron mobility fitting the Eq. (2)
with J–V characteristics in SCLC region is about
5.4 ꢃ 10ꢀ4 cm2/V s. This value is fairly comparable the va-
lue of electron mobility for fullerene derivatives. This indi-
cates PBI can be used as an electron acceptor along with
conjugated polymers for the fabrication of BHJ organic so-
lar cells.
3.5. Optical properties of P3HT:PDI blend
gD=A
gD
PLQ ¼ 1 ꢀ
ð3Þ
It is well known that P3HT shows an absorption peak
around 520 and an absorption onset around 620 nm [10].
As shown in Figs. 2 and 3 the PBI shows an absorption peak
around 610–630 nm and onset around 725 nm. These two
materials show complementary optical absorption spectra
with respect to each other. The normalized optical absorp-
tion spectra for the P3HT:PBI blend cast from THF, DIO/
THF and DIO/THF (thermally annealed) are shown in
Fig. 7. It can be seen from this figure that the optical
absorption spectra of P3HT:PDI film show a continuous
broad absorption band from 400 nm to 680 nm which ex-
tended up to 725 nm. In particular, the absorption peak
around 520 nm corresponds to P3HT, which is associated
where
g
D and
g
D/A are the PLQ of the donor and D/A blends,
respectively. Fig. 8 shows the PLQE plotted as a function of
PBI concentration. It shows that PBI quenches efficiently
even at low concentration. PBI at a concentration corre-
sponding to 1.5% (by wt.) quenches the PL of P3HT with
an efficiency of over 80%. The results show that photo-in-
duced charge transfer from P3HT to PBI can be considered
highly efficient.
with the interchain
p–
p⁄ transition, whereas the peak
around 630 nm corresponds mainly to PBI. This indicates
using the P3HT:PBI blend, the light harvesting can be in-
creased in the BHJ polymer solar cell. The optical absorp-
tion spectra of the blend cast from DIO/THF solvent
shows broader absorption spectra and high absorption
Fig. 4. Cyclic voltammogram of PBI thin film coated on a glassy carbon
electrode in Bu4NBF6/acetonitrile at a scan rate of 100 mV/s.
Fig. 5. J–V characteristics of ITO/PEDOT:PSS/PBI/Al device in dark and
under illumination intensity of 100 mW/cm2.

G.D. Sharma et al. / Organic Electronics 13 (2012) 3118–3129
3125
3.6. Photovoltaic properties of P3HT:PBI blend
The enhancement in the PCE is attributed to the both in-
creased Jsc and FF. The enhanced PCE observed for these
processing conditions may result from better charge car-
rier photogeneration and transport facilitated by the
change in the nanoscale morphology that forms intercon-
nected well known pathways for charge transport. The
use of DIO in processing of P3HT:PBI blend may modifies
the phase distribution in the blend films. The increased va-
lue of PCE and Jsc have been attributed to the enhanced va-
lue of the absorption coefficient of the blend, the broader
absorption band and the improved crystallinity of the film
cast from DIO/THF solvent as compared to the device based
on THF cast blend. As already discussed, the thin film
absorption of P3HT:PBI red shifts and become more broad-
er, when DIO is added to the blend solution in THF before
spin coating. This increases the intermolecular ordering
and crystalline nature of the P3HT and is a factor for im-
proved PV performance of the device. A similar phenomena
has been reported for P3HT: fullerene derivatives blends
and other BHJ polymer solar cells [33].
The IPCE spectra of the devices are shown in Fig. 10. It
can be seen from that the IPCE spectra of the devices clo-
sely resembles with optical absorption spectra of respec-
tive blends, indicating the contribution of both P3HT and
PBI in the photogeneration. The IPCE spectrum is more
broaden for devices B and C as compared to device A. The
IPCE values are higher for the wavelength corresponds to
absorption maxima of P3HT. The values of Jsc has been esti-
mated from the integrated IPCE spectra are 4.60 mA/cm2,
6.78 mA/cm2 and 7.65 mA/cm2 for device A, B and C,
respectively. These values are very closely to the experi-
mentally observed values.
In BHJ PV devices, the composition of electron donor
and electron acceptor materials in the BHJ active layer
has a large effect on the PCE of the device, because there
should be a balance between the absorbance and charge
transporting network of the active layer. When the accep-
tor content is too low, the electron transporting ability will
be limited. Moreover, when the acceptor content is too
high, the absorbance and hole transporting ability of the
active layer will be decreased. We have fabricated the de-
vices with different weight ratios (1:0.5, 1:1 and 1:1.5) of
P3HT and PBI and found that the optimized photovoltaic
results are for 1:1 weight ratio. Therefore, to keep the bal-
ance between the absorbance and charge transporting abil-
ity, we have investigated the photovoltaic properties of
P3HT:PBI (1:1 wt./wt.) in BHJ solar cells processed from
THF solution and DIO additive in THF solution (1.5% by vol-
ume). We have denoted the devices as follows:
ITO=PEDOT : PSS=P3HT : PBIðTHFcastÞ=Alðdevice AÞ
ITO=PEDOT : PSS=P3HT : PBIðDIO=THFcastÞ=AlðdeviceBÞ
ITO=PEDOT : PSS=P3HT : PBIðDIO=THF cast and
subsequently thermally annealed=Alðdevice CÞ
The current–voltage characteristics of these devices are
shown in Fig. 9. The photovoltaic parameters of devices
fabricated with and without DIO additives, including short
circuit current (Jsc), open circuit voltage (Voc), fill factor (FF)
and power conversion efficiency (PCE) are collected in Ta-
ble 2. A PCE of 1.56% was obtained without DIO additive.
However, the PCE of the device processed from the mixed
solvent, i.e. DIO/THF, is about 2.78%, which is further in-
creased up to 3.17%, when the thermally annealed blend
processed from DIO/THF solvent is used as active layer.
It can be seen from the Table 2 that the enhancement in
the overall PCE of the device is mainly, due to the increase
in both Jsc and FF. Since the Jsc of the BHJ PSCs is a result of
light absorption, exciton generation, exciton dissociation
and charge transport [34]. The charge transport across
the BHJ active layer towards the respective electrodes
mainly depends upon the electron and hole mobilities.
We have measured the hole and electron mobilities of
P3HT:PBI blends cast under different conditions, using
Fig. 6. Dark current–voltage characteristics of the Al/PBI/Al device in log–
log scale at room temperature.
Fig. 7. Optical absorption spectra of THF cast, DIO /THF cast and
thermally annealed DIO/THF cast P3HT:PBI films.

3126
G.D. Sharma et al. / Organic Electronics 13 (2012) 3118–3129
SCLC measurements with hole only /electron only devices.
Fig. 11 shows the variation of dark current of ITO/PED-
OT:PSS/P3HT:PBI/Au devices with corrected bias voltage,
which is determined by the difference between the work
function of Au and the HOMO level of P3HT. In the trap free
region (V > 0.7 V), where all the traps are filled, the SCLC
behavior can be characterized by the Mott-Gurnay square
law (Eq. (2)). The hole mobilities in the blends were esti-
mated by fitting the J–V curve with the SCLC model. The
hole mobility of P3HT:PBI blend film cast from THF solvent
is 8.4 ꢃ 10ꢀ6 cm2/V s and it increases to 7.6 ꢃ 10ꢀ5 cm2/V s
and 1.5 ꢃ 10ꢀ4 cm2/V s with DIO/THF and DIO/THF (ther-
mally annealed), respectively. The increase in hole mobility
is attributed to the enhanced
p–
p⁄ stacking and chain
ordering of P3HT as supported by the optical absorption
spectra and XRD data [10]. We have also measured the
electron mobility in the blends cast under different condi-
tions, for electron only devices having structure Al/
P3HT:PBI/Al and observed similar electron mobilities (in
the range 3.8–5.6 ꢃ 10ꢀ4 cm2/V s). With the hole mobility
enhancement and a similar electron mobility, the ratio be-
tween the electron mobility and hole mobility reduced i.e.
44 for THF cast, 6.0 for DIO/THF cast and 3.1 for DIO/THF
cast (thermally annealed). The higher value of this ratio
attribute to the unbalance charge transport, the accumula-
tion of hole near the anode takes place and the photocur-
rent is space charge limited [35,36]. A smaller value of
the ratio between electron and hole mobilities indicates a
more balanced charge transport in the device, which con-
tributes to higher Jsc and PCE. Hence the increase in the
PCE with the addition of DIO was attributed to the in-
creased hole mobility induced by DIO and additional ther-
mal treatment.
In organic BHJ solar cells, exciton dissociation is a two
step process, where an exciton is initially separated to a
bound polaron pair through an ultrafast charge transfer
from donor to acceptor, and then the bound polaron pair
dissociates into free charge carriers at heterojunction. To
actively generate the bound polaron pair, the photogener-
ated exciton need to meet acceptor molecules within the
diffusion length that is in the range of 10–20 nm. In addi-
tion, to generate photocurrent the bound polaron pairs
need to migrate to the heterojunction where they sepa-
rated into free carriers.
Fig. 9. J–V characteristics under illumination intensity of 100 mW/cm2 of
the BHJ photovoltaic devices A, B and C.
We have investigated the crystallinity of the P3HT:PBI
blend cast under different conditions via measuring the
X-ray diffraction patterns as shown in Fig. 12. We have ob-
served P3HT:PBI film cast from THF solvent showed the
peak (100) (2h = 5.62°) peak, which is related to the main
chain side chains separations of P3HT [37]. We have also
recorded the XRD pattern of PBI film cast under different
casting conditions and observed that there is no significant
change in the XRD pattern which may be due to the ampr-
phous nature of PBI. When the film was deposited from
mixed solvent (DIO/THF), the height of the diffraction pat-
tern peak observed around same angle is increased and is
further increased when the film cast from DIO/THF is ther-
mally annealed. The observed increase in the intensity of
diffraction peak also suggests an increase in the crystallin-
ity of P3HT, when the blend is cast from mixed DIO/THF
solvent and additional thermal annealing. We have calcu-
lated the d-spacing and crystal size from the information
of (100) peak using the Bragg’s and Scherrer’s equation,
respectively [38]. Among the three P3HT:PBI thin films,
investigated P3HT:PBI (THF cast) has the least crystallinity
nature than P3HT:PDI films. The value of crystal size for
the blend film is 12.6 nm, 13.8 and 14.5 nm cast from
THF, DIO/THF and DIO/THF (thermally annealed), respec-
tively. The increase in the crystal size for the film cast from
DIO/THF film as compared to THF cast film is due to the in-
crease in the crystallization of P3HT [39]. The thermal
annealing further improves the crystallization, as observed
from the XRD data (diffraction peak corresponding to (100)
further intensify). The increase in the crystal size also pro-
vides further insight onto effective charge transport and
also provides high internal effective field for charge separa-
tion, resulting in higher value of PCE.
To investigate the reason for the increase in the PCE of
the BHJ organic solar cell based on the blend cast from
the DIO/THF solvent, we quantitatively analyzed the pho-
tocurrent (Jph) against the effective applied bias voltage.
The Jph is defined as Jph = Jlight ꢀ Jdark, where Jlight and Jdark
are the current densities of the device measured under
illumination and in the dark, respectively. The effective
Fig. 8. Photoluminescence quenching of P3HT with various concentration
of PBI in the P3HT:PBI blend.

G.D. Sharma et al. / Organic Electronics 13 (2012) 3118–3129
3127
Table 2
Photovoltaic parameters of ITO/PEDOT:PSS/P3HT:PBI (1:1)/Al devices, in which P3HT:PBI blend processed in different solvents.
Device
Short circuit current (Jsc) (mA/cm2)
Open circuit voltage (Voc) (V)
Fill factor (FF)
Power conversion efficiency (PCE) (%)
A
B
C
4.48 ( 0.10)
6.6 ( 0.14)
7.4 ( 0.09)
0.76 ( 0.06)
0.78 ( 0.04)
0.78 ( 0.05)
0.46( 0.02)
0.52( 0.02)
0.55( 0.01)
1.56 ( 0.10)
2.78( 0.15)
3.17( 0.18)
Fig. 12. XRD profiles of P3HT:PBI thin films cast under different
conditions.
Fig. 10. IPCE spectra of BHJ photovoltaic A, B and C devices.
applied voltage is defined as Veff = Vo ꢀ V, where Vo is the
compensation voltage, defined as the voltage, where
Jph = 0 and V is the applied voltage. As shown in Fig. 13,
the photocurrent (Jph) in all three devices linearly increases
with the voltage at low effective field Veff 6 0.3 V) and
gradually saturates, independent of V, at high effective
field (Vo–V > 0.3 V). The Jph could be estimated by Jph = q
Fig. 13. Effective photocurrent density (Jph = Jlight ꢀ Jdark) as a function of
effective voltage (Vo–Vappl) of device A, B and C.
G(T,E)d in the saturation regime for Veff > 0.3, where q is
the electronic charge, G(T,E) is the generation rate of free
charge carrier [40,41]. In the high voltage region, the most
of the bound electron – hole pairs are separated into free
charge carriers and the Jph is saturated to Jsat = qGmax d.
We have estimated the generation rates of free charge car-
riers form the saturation current and the value is
1.2 ꢃ 1027 mꢀ3sꢀ1, 4.3 ꢃ 1027 mꢀ3 sꢀ1 and 6.5 ꢃ 1027 mꢀ3
-
Fig. 11. Current–voltage characteristics of ITO/PEDOT:PSS/P3HT:PDI/Au
devices, cast from THF solvent, DIO/THF solvent and thermally annealed
DIO/THF cast films.
sꢀ1 for A, B and C devices, respectively, which correlate
well to the PCE values of devices.

3128
G.D. Sharma et al. / Organic Electronics 13 (2012) 3118–3129
Soc. (131) (2008) 56;
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