Low band-gap phenylenevinylene and fluorenevinylene small molecules containing triphenylamine segments: Synthesis and application in bulk heterojunction solar cells

Article abstract of DOI:10.1016/j.orgel.2011.02.008

Starting from triphenylamine, two low band gap small molecules, PH and FL, based on phenylenevinylene and fluorenevinylene, respectively were synthesized. They were soluble in common organic solvents such as tetrahydrofuran, chloroform and dichloromethane. Their long-wavelength absorption maximum was at 605-643 nm with optical band gap of 1.64-1.66 eV. These small molecules showed a band gap lower than that of P3HT and also have deeper HOMO level, which is beneficial for improvement of the open circuit voltage. The photovoltaic properties have been investigated using the bulk heterojunction (BHJ) active layer of PH or FL with PCBM. The device based on FL:PCBM displayed higher power conversion efficiency (PCE) (1.42%) than the device based on PH:PCBM (1.02%), when the blend films were cast from chloroform solvent. Moreover, various casting solvents were used for the BHJ solar cells based on FL:PCBM blend and their effect on the photovoltaic performance was investigated. The results indicate that high boiling point solvents lead to an enhanced self-organization of FL in the active layer, which causes an increased charge transport. Finally, we have used a modified PCBM, i.e. F as electron acceptor along with FL as electron donor, to increase the light harvesting in the wavelength region below 500 nm and the PCE is about 4.38% when the BHJ (FL:F blend) device was spin casted from mixed 1-chloronaphthalene/o-dichlorobenzene solvents. The improved PCE has been attributed to the increased light absorption and higher hole mobility in the active layer, which resulted in more balanced charge transport.

Full text of DOI:10.1016/j.orgel.2011.02.008

Organic Electronics 12 (2011) 774–784
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Low band-gap phenylenevinylene and fluorenevinylene small
molecules containing triphenylamine segments: Synthesis and
application in bulk heterojunction solar cells
J.A. Mikroyannidis a, , A.N. Kabanakis a, S.S. Sharma d, G.D. Sharma b,c,
⇑
⇑
a Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-26500 Patras, Greece
b Physics Department, Molecular Electronic and Optoelectronic Device Laboratory, JNV University, Jodhpur (Raj.) 342 005, India
c R&D Centre for Engineering and Science, Jaipur Engineering College, Kukas, Jaipur (Raj.), India
d Department of Physics, Government Engineering College for Women, Ajmer (Raj.), India
a r t i c l e i n f o
a b s t r a c t
Article history:
Starting from triphenylamine, two low band gap small molecules, PH and FL, based on
phenylenevinylene and fluorenevinylene, respectively were synthesized. They were solu-
ble in common organic solvents such as tetrahydrofuran, chloroform and dichloromethane.
Their long-wavelength absorption maximum was at 605–643 nm with optical band gap of
1.64–1.66 eV. These small molecules showed a band gap lower than that of P3HT and also
have deeper HOMO level, which is beneficial for improvement of the open circuit voltage.
The photovoltaic properties have been investigated using the bulk heterojunction (BHJ)
active layer of PH or FL with PCBM. The device based on FL:PCBM displayed higher power
conversion efficiency (PCE) (1.42%) than the device based on PH:PCBM (1.02%), when the
blend films were cast from chloroform solvent. Moreover, various casting solvents were
used for the BHJ solar cells based on FL:PCBM blend and their effect on the photovoltaic
performance was investigated. The results indicate that high boiling point solvents lead
to an enhanced self-organization of FL in the active layer, which causes an increased charge
transport. Finally, we have used a modified PCBM, i.e. F as electron acceptor along with FL
as electron donor, to increase the light harvesting in the wavelength region below 500 nm
and the PCE is about 4.38% when the BHJ (FL:F blend) device was spin casted from mixed
1-chloronaphthalene/o-dichlorobenzene solvents. The improved PCE has been attributed
to the increased light absorption and higher hole mobility in the active layer, which
resulted in more balanced charge transport.
Received 19 November 2010
Received in revised form 4 January 2011
Accepted 13 February 2011
Available online 26 February 2011
Keywords:
Bulk heterojunction solar cells
Low band gap
Small molecule
Solvent effect
Triphenylamine
Phenylenevinylene
Fluorenevinylene
Ó 2011 Elsevier B.V. All rights reserved.
Abbreviations: A, acceptor; AFM, atomic force microscopy; BHJ, bulk heterojunction; CB, chlorobenzene; CF, chloroform; CN, 1-chloronaphthalene; CV,
ox
red
onset
cyclic voltammetry; D, donor; DCB, o-dichlorobenzene; DMF, N,N-Dimethylformamide; Eogpt, optical band gap; E
, onset oxidation; E
, onset
onset
reduction; FF, fill factor; HOMO, highest occupied molecular orbital; ICT, intramolecular charge transfer; IPCE, incident photon to current efficiency; ITO,
indium tin oxide; J–V, current–voltage; Jsc, short circuit current; ka,max, long-wavelength absorption maximum; LUMO, lowest unoccupied molecular
orbital; OPV, organic photovoltaic; OSCs, organic solar cells; P3HT, poly(3-hexylthiophene); PCBM, [6,6]-phenyl-C-61-butyric acid methyl ester; PCE, power
conversion efficiency; PV, photovoltaic; rms, root mean square; SCLC, space charge limited current; SM, small-molecule; THF, tetrahydrofuran; TPA,
triphenylamine; Voc, open circuit voltage; XRD, X-ray diffraction.
⇑
Corresponding authors. Tel.: +30 2610 997115; fax: +30 2610 997118 (J.A. Mikroyannidis); Address: Physics Department, Molecular Electronic and
Optoelectronic Device Laboratory, JNV University, Jodhpur (Raj.) 342 005, India. Tel.: +91 0291 2720857; fax: +91 0291 2720856 (G.D. Sharma).
E-mail addresses: mikroyan@chemistry.upatras.gr (J.A. Mikroyannidis), sharmagd_in@yahoo.com (G.D. Sharma).
1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2011.02.008

J.A. Mikroyannidis et al. / Organic Electronics 12 (2011) 774–784
775
promising materials for OSCs. D–A SMs can be considered
as efficient donor materials for OSCs because of the fact
that the absorption spectrum of theses molecules can be
extended towards longer wavelengths by intra-molecular
charge transfer (ICT) transition between donor and accep-
tor inside molecules [21] and the energy level can be easily
controlled by introducing various electron donating or
acceptor groups into the molecules.
1. Introduction
Organic semiconducting materials open the way of low-
cost, flexible, and large-scale solar cells. Recently, power
conversion efficiencies (PCEs) above 5% and to 6.1% have
been reported for bulk-heterojunction (BHJ) solar cells
[1–3]. These solar cells comprising interpenetrating net-
works of an organic donor (D) and a fullerene derivative
acceptor (A) such as [6,6]-phenyl-C61-butyric acid methyl
ester (PCBM) constitute a promising technology because
they are easy to fabricate by solution processing and are
predicted theoretically to yield PCEs of up to 10–15% if a
suitable low band gap donor material is discovered [4].
Although considerable research effort has been expended
to develop such low band-gap donor materials [5], the
highest reported efficiencies have been dominated by con-
jugated polymers such as poly(3-hexylthiophene) (P3HT)
[6–9]. This system has been explored thoroughly for the
past decade. The high efficiency of P3HT–fullerene devices
can be explained by the ability of the blend to phase sepa-
rate and crystallize into desirable BHJ morphologies after
processing, allowing for efficient charge separation and
transport [10,11]. The current effort in this area was fo-
cused on developing the low band gap polymers to harvest
the near infrared light [12]. However, polymers have prob-
lems of poor reproducibility, difficulty in purification, and
poor controllability in the molecular weight. Thus efforts
have been laid on developing solution processable low
band gap organic SM donors, because they offer advanta-
ges of easy synthesis and purification and mono-dispersion
in the molecular weight, which greatly improve the repro-
ducibility in the performance of the devices.
Most of the researchers related to BHJ solar cells have
focused on polymeric donor materials, since they generally
have better film-forming properties than non-polymeric
materials [13]. Small-molecule (SM) donor materials can
also form useable BHJ solar cells by solution processing,
although it is more challenging to obtain high-quality
films. The highest reported efficiencies for such devices
have remained lower relative to solution processed solar
cells using polymeric donor materials. SM materials, how-
ever, offer advantages over polymeric materials in terms of
ease of synthesis and purification, which greatly improve
fabrication reproducibility, as well as possessing a greater
tendency to self assemble into ordered domains, which
leads to high charge carrier mobilities [14,15]. SMs do
not suffer from batch to batch variations, broad molecu-
lar-weight distributions, end-group contamination, or dif-
ficult purification methods, which can be significant
problems for polymeric materials. These considerations
make SMs a promising class of donor material for BHJ solar
cell applications [16–24]. The PCE of solution processable
SMs has steadily improved up to in the range 2.0–4.4%
[25], due to considerable efforts towards the development
of new SMs having low band gap and semiconducting
property. Particularly, significant progress has been made
in the synthesis and processing of donor–acceptor (D–A)
SMs. Theoretical and experimental progresses in organic
solar cells (OSCs) have demonstrated that such D–A SMs
and polymers have great potential to be one class of
On the other hand, a number of polyfluorenes and their
derivatives have been studied as a photoactive structure in
polymer light-emitting devices and solar cells on account
of their large band gap, high photoluminescence, electrolu-
minescence efficiencies, good thermal/chemical stability,
and good solubility in common organic solvents. Alternat-
ing polyfluorene copolymers with various band gaps have
been synthesized, and solar cells based on some of these
copolymers have demonstrated PCEs of ꢀ1% [26,27]. A lit-
erature survey revealed that certain fluorene containing
copolymers have been synthesized and used for BHJ solar
cells very recently [28–32]. Finally, among the various
types of reported polymer semiconductors, poly(p-phenyl-
enevinylenes) (PPVs) [33–35] and their derivatives are
widely used in solar cell applications as p-type donor
materials. Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phen-
lenevinylene] (MEH-PPV) is a PPV derivative which has
been mainly used as electron donor for BHJ solar cells [36].
A series of low band gap SMs that contain cyanovinyl-
ene 4-nitrophenyl segments have been recently synthe-
sized in our laboratory and used as donors with PCBM as
acceptor for BHJ solar cells [37–41]. Additionally, we have
very recently synthesized a modified PCBM derivative F
(Scheme 1) carrying cyanovinylene 4-nitrophenyl segment
[42]. Specifically, PCBM has been hydrolyzed to carboxylic
acid and then converted to the corresponding carbonyl
chloride. The latter was condensed with 4-nitro-40-hydro-
xy-a-cyanostilbene to afford F. It showed higher solubility
than PCBM in common organic solvents due to the increase
of the organic moiety. Both solutions and thin films of F
displayed stronger absorption than PCBM in the range of
250–900 nm. BHJ solar cells based on P3HT as electron do-
nor with F as electron acceptor exhibited a PCE of 4.23%,
while the device based on P3HT:PCBM exhibited PCE of
2.93% under the same conditions [42].
Herein, we report the synthesis of two low band gap
SMs, PH and FL, based on phenylenevinylene and fluorene-
vinylene, respectively. They contain certain common
structural characteristics, namely triphenylamine (TPA)
and cyanovinylene 4-nitrophenyl segments. They are only
Scheme 1. Chemical structures of the electron acceptors (PCBM and F)
used in this investigation.

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J.A. Mikroyannidis et al. / Organic Electronics 12 (2011) 774–784
differentiated by their central unit which is 2,5-dihexyl-
oxy(phenylenevinylene) for PH and 9,9-dihexyl(fluorene-
vinylene) for FL. The aliphatic chains which are attached
to the central unit enhanced the solubility of these SMs.
They were successfully synthesized by a four-step reaction
sequence. The last step of their synthesis included the
condensation of a key-tetraaldehyde with 4-nitrobenzyl
cyanide. Both SMs possess the D–A architecture. Specifi-
cally, they carry phenylenevinylene/fluorenevinylene and
TPA as D units and cyanovinylene 4-nitrophenyl as A units
which allow an intramolecular charge transfer (ICT), thus
reducing the optical band gap (Eogpt) of the SMs. Moreover,
the presence of the TPA moieties is expected to increase
the hole mobility of both SMs. We have used these SMs
as electron donor along with PCBM or modified PCBM,
i.e. F as electron acceptor for the fabrication of BHJ PV de-
vices. The PCEs for the devices based on PH:PCBM and
FL:PCBM cast from chloroform solvent are 1.02% and
1.46%, respectively. In addition, we investigated the effect
of solvent used for the spin casting of the FL:PCBM blend
on the PV response of the device and found that the solvent
has a strong influence on the morphology of the blend
films and on the PCE of the device. Finally, we have
replaced the electron acceptor PCBM by the modified
PCBM, i.e. F, which has strong absorption at lower wave-
length region (below 500 nm) and fabricated BHJ devices
using FL as electron donor. We have achieved PCE about
3.62% and 4.38% for FL:F BHJ active layer cast from
o-dichlorobenzene (DCB) and 1-chloronaphthalene/
o-dichlorobenzene (CN/DCB) solvent, respectively. The
BHJ cast from the mixed solvents (CN/DCB) leads to an
enhanced self organization and crystallinity of FL in active
layer, which causes an increased charge transport, incident
absorption and higher carrier mobility in the active layer
thus contributing to the enhancement in the device
performance.
0.389 mmol), Pd(OAc)2 (0.0017 g, 0.007 mmol), P(o-tolyl)3
(0.0047 g, 0.015 mmol), DMF (10 mL), and triethylamine
(3 mL). The flask was degassed and purged with N2. The
mixture was heated at 90 °C for 24 h under N2. Then, it
was filtered and the filtrate was poured into methanol.
The precipitate was filtered and washed with methanol.
The crude product was purified by dissolving in THF and
precipitating into methanol (0.3072 g, yield 85%).
FT-IR (KBr, cmꢁ1): 2952, 2926 (C–H stretching of hexyl-
oxy chains); 1676 (formyl); 3030, 1590, 1506 (aromatic);
1320 (C–N stretching of triphenylamine); 1286, 1272,
1214 (ether bond); 967 (trans vinylene bond).
1H NMR (CDCl3) ppm: 9.80 (s, 4H, formyl); 7.67 (m, 8H,
TPAortho to formyl);7.30–7.2(m, 16H, other TPA; 4H, vinyl-
ene); 6.75 (s, 2H, phenylene); 3.96 (m, 4H, OCH2(CH2)4CH3);
1.81 (m, 4H, OCH2CH2(CH2)3CH3); 1.37 (m, 12H, O(CH2)2
(CH2)3CH3); 0.91 (t, 6H, O(CH2)5CH3).
Anal. Calcd. for C62H60N2O6: C, 80.14; H, 6.51; N, 3.01.
Found: C, 79.92; H, 6.43; N, 3.17.
2.2.2. Compound 3b
Compound 3b was similarly prepared in 80% yield
(0.22 g) from the reaction of 2 (0.2106 g, 0.554 mmol), with
9,9-dihexyl-2,7-divinylfluorene (0.1071 g, 0.277 mmol) in
DMF (10 mL) in the presence of triethylamine (3 mL),
Pd(OAc)2 (0.0012 g, 0.005 mmol) and P(o-tolyl)3 (0.0034 g,
0.011 mmol).
FT-IR (KBr, cmꢁ1): 2950, 2925 (C–H stretching of hexyl
chains); 1675 (formyl); 3029, 1592, 1504 (aromatic); 1315
(C–N stretching of TPA); 965 (trans vinylene bond).
1H NMR (CDCl3) ppm: 9.80 (s, 4H, formyl); 7.72 (m, 2H,
H4 of fluorene); 7.67–7.60 (m, 4H, H1 and H3 of fluorene;
8H, TPA ortho to formyl); 7.30–7.02 (m, 16H, other TPA;
4H, vinylene); 2.06 (m, 4H, CH2(CH2)4CH3); 1.11 (m, 16H,
CH2(CH2)4CH3); 0.78 (t, 6H, (CH2)5CH3).
Anal. Calcd. for C69H64N2O4: C, 84.11; H, 6.55; N, 2.84.
Found: C, 83.85; H, 6.64; N, 2.68.
2. Experimental
2.3. Small molecule PH
2.1. Reagents and solvents
A flask was charged with a solution of 3a (0.3614 g,
0.389 mmol) and 4-nitrobenzyl cyanide (0.2523 g, 1.556
mmol) in DMF (15 mL). Sodium hydroxide (0.20 g, 5.00
mmol) dissolved in ethanol (5 mL) was added portionwise
to the stirred solution. The mixture was stirred for 1 h at
room temperature under N2 and then was concentrated un-
der reduced pressure. Water was added to the concentrate
and the product precipitated as a dark green solid. It was fil-
tered, washed thoroughly with water and dried to afford PH.
It was purified by column chromatography, eluting with
a mixture of dichloromethane and hexane (1:1) (0.46 g,
80%).
4-Nitrobenzyl cyanide was synthesized from the
nitration of benzyl cyanide with concentrated nitric and
sulfuric acid [43]. It was recrystallized from ethanol.
1,4-Divinyl-2,5-bis(hexyloxy)-benzene was prepared by
Stille coupling reaction [44] of 1,4-dibromo-2,5-bis(hex-
yloxy)-benzene with tributylvinyltin [45]. 9,9-Dihexyl-
2,7-divinylfluorene was prepared by Stille coupling [44]
of 2,7-dibromo-9,9-dihexylfluorene with tributylvinyltin
[46]. N,N-dimethylformamide (DMF) and tetrahydrofuran
(THF) were dried by distillation over CaH2. Triethylamine
was purified by distillation over KOH. All other reagents
and solvents were commercially purchased and were
used as supplied.
FT-IR (KBr, cmꢁ1): 2950, 2924 (C–H stretching of
hexyloxy chains); 2158 (cyano); 1524, 1344 (nitro); 1316
(C–N stretching of TPA); 1284, 1248 (ether bond); 1584,
1504 (aromatic); 965 (trans vinylene bond); 1013
(cyanovinylene).
2.2. Preparation of small molecules
1H NMR (CDCl3) ppm: 8.19 (m, 8H, phenylene ortho to
nitro); 7.83 (s, 4H, cyanovinylene); 7.52 (m, 8H, phenylene
meta to nitro); 7.30–7.13 (m, 24H, TPA); 7.10 (m, 4H, vinyl-
ene); 6.75 (s, 2H, phenylene ortho to oxygen); 3.96 (m, 4H,
2.2.1. Compound 3a
A flask was charged with a mixture of 2 (0.2957 g, 0.778
mmol), 1,4-divinyl-2,5-bis(hexyloxy)-benzene (0.1285 g,

J.A. Mikroyannidis et al. / Organic Electronics 12 (2011) 774–784
777
OCH2(CH2)4CH3); 1.81 (m, 4H, OCH2CH2(CH2)3CH3); 1.37
(m, 12H, O(CH2)2(CH2)3CH3); 0.91 (t, 6H, O(CH2)5CH3).
Anal. Calcd. for C94H76N10O6: C, 74.98; H, 5.09; N, 9.30.
Found: C, 74.37; H, 4.82; N, 9.13.
chlorobenzene, o-dichlorobenzene, and 2% additive 1-chlo-
ronaphthalene (CN) in o-dichlorobenzene), onto the PED-
OT:PSS layer, and then dried in ambient conditions. The
concentration of PH or FL:PCBM blend solution used in this
study for spin coatingwas10 mg/mL inthe solventused. The
thickness of the active layers is approximately 90–95 nm.
The structure of the device is ITO/PEDOT:PSS/PH or
FL:PCBM/Al. The effective active area of the devices is about
20 mm2. 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 mW/cm2 in ambient conditions. A xenon light source
was used to give simulated irradiance of 100 mW/cm2
(equivalent to and AM1.5 irradiation) at the surface of the
device.
2.4. Small molecule FL
FL was similarly prepared in 83% yield (0.36 g) from the
reaction of 3b (0.2729 g, 0.277 mmol) with 4-nitrobenzyl
cyanide (0.1797 g, 1.108 mmol) in DMF (15 mL) in the
presence of sodium hydroxide (0.20 g, 5.00 mmol).
FT-IR (KBr, cmꢁ1): 2950, 2924 (C–H stretching of hexyl
chains); 2160 (cyano); 1522, 1346 (nitro); 1314 (C–N
stretching of TPA); 1584, 1506 (aromatic); 964 (trans vinyl-
ene bond); 1004 (cyanovinylene).
1H NMR (CDCl3) ppm: 8.19 (m, 8H, phenylene ortho to
nitro); 7.83 (s, 4H, cyanovinylene); 7.72 (m, 2H, H4 of flu-
orene); 7.60 (m, 4H, H1 and H3 of fluorene); 7.52 (m, 8H,
phenylene meta to nitro); 7.30–7.13 (m, 24H, TPA); 7.10
(m, 4H, vinylene); 2.06 (m, 4H, CH2(CH2)4CH3); 1.11 (m,
16H, CH2(CH2)4CH3); 0.78 (t, 6H, (CH2)5CH3).
We have also fabricated BHJ devices based on FL:F
using the same method. To measure the hole and electron
mobilities, the devices having structure ITO/PEDOT:PSS/
FL:F/Au and Al/FL:F/Al were used, respectively and the
J–V characteristics in dark have been recorded.
Anal. Calcd. for C101H80N10O8: C, 77.67; H, 5.16; N, 8.97.
Found: C, 77.38; H, 5.21; N, 9.20.
3. Results and discussion
3.1. Synthesis and characterization
2.5. Characterization methods
Scheme 2 outlines the four-step reaction for the synthe-
sis of SMs. Compounds 1 [47] and 2 [48] have been synthe-
sized according to reported methods. Compound 2 reacted
with 1,4-divinyl-2,5-bis(hexyloxy)-benzene and 9,9-dihex-
yl-2,7-divinylfluorene in a mol ratio 2:1 to afford 3a and
3b, respectively. This reaction took place in DMF using
Et3N as acid scavenger and Pd(OAc)2 as catalyst. Finally,
the condensation of 3a and 3b with 4-nitrobenzyl cyanide
in the presence of sodium hydroxide gave PH and FL,
respectively. These SMs were soluble in common organic
solvents such as tetrahydrofuran, chloroform and dichloro-
methane owing to the aliphatic chains.
IR spectra were recorded on a Perkin–Elmer 16PC FT-IR
spectrometer with KBr pellets. 1H NMR (400 MHz) spectra
were obtained using a Brucker spectrometer. Chemical
shifts (d values) are given in parts per million with tetra-
methylsilane as an internal standard. UV–Vis spectra were
recorded on a Beckman DU-640 spectrometer. Elemental
analyses were carried out with
EA1108 analyzer.
a Carlo Erba model
The electrochemical properties of both SMs, i.e. PH and
FL were examined using cyclic voltammetry (CV) (EDCA
electrochemistry system). A glassy carbon electrode was
used as working electrode. The SMs were coated on glassy
carbon electrode from tetrahydrofuran (THF) solution and
immersed in 0.1 mol/L Bu4NPF6 acetonitrile solution used
as supporting electrolyte. Cyclic voltammograms were re-
corded, using Ag/Ag+ as reference electrode at a scan rate
of 100 mV/s.
The crystallinity of the films was studied using the
X-ray diffraction (XRD) technique (panalytical make USA)
having CuKa, as radiation source of wavelength k =
1.5405 Å with the films coated on the quartz substrates.
Atomic force microscopy (AFM) images were recorded
using digital instrument nanoscope in trapping mode.
The SMs were characterized by FT-IR and 1H NMR
spectroscopy. The IR spectra showed common absorption
bands approximately at 2952, 2926 (C–H stretching of
aliphatic chains); 2128 (cyano); 1524, 1344 (nitro);
1316 (C–N stretching of TPA); 1584, 1504 (aromatic);
965 (vinylene), and 1013 cmꢁ1 (cyanovinylene). On the
other hand, the 1H NMR spectra of SMs displayed
common signals at 8.19 (phenylene ortho to nitro);
7.83 (cyanovinylene); 7.52 (phenylene meta to nitro);
7.30–7.13 (TPA) and 7.10 ppm (vinylene). Finally, the ali-
phatic chains resonated at the range of 3.96–0.91 ppm
for PH and 2.06–0.78 ppm for FL.
2.6. Device fabrication and characterization
3.2. Photophysical and electrochemical properties
All the devices were fabricated on indium tin oxide (ITO)
coatedglasssubstrates. TheITOcoatedglasssubstrateswere
cleaned with acetone in an ultrasonic bath. A thin layer
of poly(3,4-ethylene-dioxythiophene): poly(styrene sulfo-
nate) (PEDOT:PSS) (70 nm) was spin coated on it from
PEDOT:PSS aqueous solution at 2000 rpm and dried subse-
quently at 80 °C for 20 min in air, and then transferred into
a glove box. The active layer of the blend of PH or FL with
PCBM was spin coated from different solvents (chloroform,
Fig. 1 depicts the UV–Vis absorption spectra of SMs in
both dilute (10ꢁ5 M) THF solution and thin film. All photo-
physical and electrochemical characteristics of SMs are
summarized in Table 1. The solutions showed long-wave-
length absorption maximum (ka,max) or shoulder around
605 and 643 nm, while the thin films showed ka,max at
626–632 nm. The spectra were broad and extended
approximately up to 750 nm in solutions and 800 nm in

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J.A. Mikroyannidis et al. / Organic Electronics 12 (2011) 774–784
Scheme 2. Synthesis of SMs PH and FL.
thin films which is desirable for PV applications. This fea-
ture was attributed to an ICT between the electron-
donating central unit (phenylenevinylene/fluorenevinyl-
ene and TPA) and the electron-accepting terminal units
(cyanovinylene 4-nitrophenyl). The thin film absorption
onset was located at 758 and 746 nm corresponding to
an Eogpt of 1.64 and 1.66 eV for PH and FL, respectively.
The optical properties of SMs are similar thus supporting
that their central unit did not influence significantly their
photophysical characteristics.
FL, respectively. This implies that the HOMO levels are
mainly affected by the D unit in SM, while the LUMO levels
(ꢁ3.55 eV) are primarily controlled by the acceptor moie-
ties. Since the LUMO level of PCBM is ꢁ3.95 eV, the energy
difference between the LUMOs of these SMs and PCBM is
about 0.3–0.4 eV, which should be sufficient for the exciton
splitting and charge dissociation [52]. In addition, the dee-
per HOMO level of these SMs is desirable for higher open
circuit voltage (Voc) of the OPVs with organic semiconduc-
tors as D materials because the Voc is usually proportional
to the difference between the LUMO level of the acceptor
and the HOMO level of the D. The electrochemical band
gaps have been estimated from the energy difference
between the HOMO and LUMO levels and they are 1.80
and 1.75 eV for PH and FL, respectively.
The highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) energy levels
of the organic polymers and SMs are important parameters
in the design of organic photovoltaic (OPV) devices, and
ox
they can be estimated from the onset oxidation (E
)
onset
red
and reduction (E
) potentials [49]. Fig. 2 shows the cyc-
onset
lic voltammograms of PH and FL. Similar cyclic voltammo-
3.3. Photovoltaic properties
grams have been reported for other small molecules in the
ox
onset
red
onset
literature [50]. The E
and E
potentials vs. Ag/Ag+
Fig. 3(a) shows the J–V characteristics of the OPVs based
on SMs (PH or FL):PCBM cast from the chloroform solvent.
The PV parameters of the OPVs, including Voc, short circuit
current (Jsc), fill factor (FF) and power conversion efficiency
(PCE) are summarized in Table 2. The higher value of Voc
for the device based on PH:PCBM is attributed to the lower
HOMO level of PH as compared to FL. The OPV based on
the FL:PCBM blend showed higher Jsc and FF than the
PH:PCBM based device, leading to PCE of 1.46% vs. 1.02%,
respectively. However, the current density is still much
(Table 1) were used for calculation of the HOMO and LUMO
energy levels according to the following expressions [51].
ox
HOMO ¼ ꢁqðE
LUMO ¼ ꢁqðE
þ 4:7Þ eV
þ 4:7Þ eV
onset
red
onset
The values of the HOMO and LUMO levels of the PH and
FL are given in Table 1. The LUMO level of both SMs is the
same but the HOMO level is ꢁ5.35 and ꢁ5.30 eV for PH and

J.A. Mikroyannidis et al. / Organic Electronics 12 (2011) 774–784
779
Fig. 2. Cyclic voltammograms of (a) PH and (b) FL.
Fig. 1. UV–Vis absorption spectra of SMs in THF solution (top) and thin
film (bottom).
attributed to the higher value of hole mobility for the
device based on the FL film.
lower than that for the state-of-the-art polymer/fullerene
systems [13]. In order to understand the origins of the
low current, we carried out the incident photon to current
efficiency (IPCE) measurements. The IPCE spectra of the PV
devices, as shown in Fig. 3(b), are consistent with the
absorption spectra of the blend films of the active layers.
The absorptions at shorter wavelength region, below
420 nm, are mainly contributed to PCBM, while those at
longer wavelength region are contributed to SMs. The
higher Jsc of the FL:PCBM based device compared to the
PH:PCBM device is reflected on the higher IPCE values in
the wavelength region corresponding to absorption of
SMs. The higher value of the Jsc and PCE may also be
The relatively lower PCE of SM BHJ OSCs becomes the
biggest hindrance to commercial application. In the case
of OSCs based on polymers, some approaches have been
used to improve the device efficiency including thermal
annealing or utilizing additive to control the formation of
interpenetrating D/A networks at nanometer scale for effi-
cient exciton dissociation and charge transport within the
BHJ devices [53]. The solvents used for the preparation of
the active layer have shown a strong impact on the mor-
phology of the blend films, which influence the perfor-
mance of the devices. However, these methods are
Table 1
Optical and electrochemical properties of SMs.
a
a
ox
Eogpt
b
red
c
Eegl
SM ka,max in solution
ka,max in thin film
(nm)
Thin film absorption onset
(nm)
HOMO
(eV)
LUMO
(eV)
E
E
onset
onset
(nm)
(V)
(V)
(eV)
(eV)
PH (604) 643
632
626
758
746
1.64
1.66
0.65
0.60
ꢁ1.15 ꢁ5.35
ꢁ1.15 ꢁ5.30
ꢁ3.55
ꢁ3.55
1.80
1.75
FL
605 (648)
Numbers in parentheses indicate a shoulder in the UV–Vis spectrum.
a
ka,max: The absorption maxima from the UV–Vis spectra in THF solution or in thin film.
Eogpt: Optical band gap determined from the absorption onset in thin film.
Eegl: Electrochemical band gap determined from cyclic voltammetry.
b
c

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J.A. Mikroyannidis et al. / Organic Electronics 12 (2011) 774–784
Fig. 4. Absorption spectra of FL:PCBM films (1:1 w:w) prepared by spin
casting from different solvents.
wavelength is at 635 nm showing a 10 nm red-shift, with
one shoulder peak around 670 nm. Compared to the for-
mer two films absorption spectra cast from CF and CB,
the absorption spectrum of o-dichlorobenzene (DCB) cast
film becomes more distinguishable and much stronger.
The vibronic absorption is much more pronounced indicat-
ing strong interchain–interlayer interaction in FL, as well
as a good ordering of blends films. Both significant red shift
and pronounced shoulder appear in a more ordered film as
high crystallinity order involves an enhanced conjugation
length and hence a shift of the absorption spectra to lower
energies.
Fig. 5 shows the XRD results from active layer FL:PCBM
prepared by spin coating from various solvents. The XRD
intensities values were obtained from peaks at 2h = 8.26°
are 6.3, 10.5, 14.8, and 19.4 for the films cast from CF, CB,
DCB, and CN/DCB solvents, respectively. These values cor-
respond to the interchain spacing in FL associated with
the degree of crystallinity. When high boiling point sol-
vents (DCB and CB) instead of low boiling point (CF) were
used for the active layer, the increased intensities were ob-
served in the peak at 2h = 8.26°. This indicates the active
layers prepared from DCB and CB have higher ordered
structure and more improved crystallinity than CF. In the
high boiling point solvents, FL in the active layer may have
longer time to solidify due to the slow evaporation rate of
the solvents and the slow film growth rate may assist to
form the high degree of self organized structure. The
crystalline nature of the blend has been further improved
as can be seen for the XRD pattern.
Fig. 3. (a) Current–voltage (J–V) characteristics and (b) IPCE spectra of the
OSCs based on PH:PCBM and FL:PCBM cast from CF solvent under
illumination of 100 mW/cm2.
Table 2
Photovoltaic parameters of the devices based on PH:PCBM and FL:PCBM
cast from chloroform (CF).
Blend
Short circuit
Open
Fill
Power
current (Jsc
)
circuit
voltage
(Voc) (V)
factor conversion
(mA/cm2)
(FF)
efficiency
(PCE) (%)
PH:PCBM 3.7
FL:PCBM 4.3
0.78
0.74
0.41
0.46
1.02
1.46
generally not applied for SM BHJ solar cells. We have stud-
ied the effect of different solvents on the performance of
OPVs using the FL:PCBM blend.
Fig. 4 shows the UV–Vis absorption spectra of FL:PCBM
thin films for different casting solvents. By changing
the casting solvent certain differentiations appeared in
the absorption spectra. For chloroform (CF) cast film the
absorption spectra show two absorption bands around
360 and 626 nm. The absorption band having peak around
360 nm stems from PCBM [54], while the absorption band
with peak around 626 nm represents the contribution from
FL. For chlorobenzene (CB) cast film the peak absorption
Fig. 5. X-ray diffraction spectra for FL:PCBM (1:1 w/w) blend thin films
prepared from different solvents.

J.A. Mikroyannidis et al. / Organic Electronics 12 (2011) 774–784
781
3.4. Photovoltaic properties with F as electron acceptor
For the acceptor materials in OSCs, PCBM offers the
advantage of good solubility in organic solvents, high elec-
tron mobility and high electron affinity. However, weak
absorption in the visible region and a low LUMO energy le-
vel are the weak points of PCBM. Weak absorption in the
visible region limits its contribution to light harvesting in
the PV conversion. We attempted to increase the breadth
of the IPCE spectrum and the overall PCE, by fabricating
OSCs with a more light absorbing modified PCBM, i.e. F
[42]. Fig. 7(a) shows the J–V curves of the devices based
on FL:F (1:1 w/w) spin coated from DCB and CN/DCB
solvents. The Voc, Jsc, FF, and PCE of the devices are listed
in Table 4. The devices based on F as electron acceptor
show higher value of Voc as compared to that for PCBM.
This is attributed to the difference between the LUMO
energy levels in PCBM (ꢁ3.95 eV) and F (ꢁ3.75 eV) [42],
because the value of Voc in BHJ photovoltaic device is
related to the difference between the HOMO level of the
donor and the LUMO level of the acceptor. The device
fabricated with FL:F blend shows higher PCE (3.62%) as
compared to FL:PCBM (2.42%) based device, cast from
DCB. The overall PCE of the device based on FL:F has been
further improved up to 4.38% when the blend is cast from
Fig. 6. Current–voltage (J–V) characteristics of the OSCs for FL:PCBM cast
from different solvents under illumination intensity of 100 mW/cm2.
Table 3
Summary of the photovoltaic parameters of the ITO/PEDOT:PSS/FL:PCBM/
Al devices prepared from different solvents.
Solvent type
Jsc (mA/cm2)
Voc (V)
FF
PCE (%)
CF
CB
DCB
CN/DCB
4.3
5.1
6.0
7.15
0.74
0.74
0.76
0.76
0.46
0.48
0.53
0.57
1.46
1.81
2.42
3.0
The J–V characteristics under illumination intensity AM
1.5 (100 mW/cm2) for the OSCs fabricated from various
solvents employing FL:PCBM blend are shown in Fig. 6.
The PV parameters of the devices are summarized in Table
3. It is seen from this table that the Voc does not change
under various solvent conditions but the Jsc and FF show
a differentiation. Compared to the device made from CF
solvent, the device based on CB solvent leads to an increase
of PCE from 1.42% to 1.81%. Moreover, a PCE of 2.42% is
observed from the device fabricated from DCB solvent,
and the device fabricated from mixed CN/DCB solvents
possesses PCE of 3.0%. The improvement of PCE for the
devices is mainly due to the enhancement in both Jsc and
FF values. This indicates an improved charge transport
due to the expansion of the crystalline domains. The reason
for this improved crystallinity in the coated films can be
the different solvent evaporation speed during spin coat-
ing, since CF has a low boiling point of 60 °C, while the
boiling points of CB and DCB are 130 and 180 °C, respec-
tively. The film cast from DCB solvent is frozen very slowly
owing to the high boiling point of DCB. This slow freezing
of FL may cause a longer time for the FL self organization,
leading to a higher degree of ordering in the material. The
broadening of the absorption band and red shift in the
absorption peak leads to an increase of the light harvesting
by the active layer. The higher PCE for the device fabricated
from mixed solvents (CN/DCB) is attributed to the
increased ordering of the blend, which provides a more
increased D–A interfacial area. The combined effects of
the above leads to an increase in the overall PCE of the
device.
Fig. 7. (a) Current–voltage (J–V) characteristics and (b) of the OSCs for
FL:F based on FL:PCBM cast from DCB and CN/DCB solvents.

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J.A. Mikroyannidis et al. / Organic Electronics 12 (2011) 774–784
Table 4
space charge limited current (SCLC) method [56] was
adopted to determine the electron and hole mobilities in
blend films. Fig. 8(a) shows the dark J–V characteristics of
the hole only devices having structure of ITO/PEDOT:PSS/
FL:F/Au with the DCB and CN/DCB solvents casting layer
films. In order to suppress the electron injection into the
blend, a high work function metal gold (Au) was used as
cathode. The applied voltage was corrected for the built-
in voltage (Vbi). Compared to the device based on DCB
solvent cast film, the device with CN/OCB solvent cast film
shows an enhanced current density at the same driving
voltage in Fig. 8(a). This indicates that the hole mobility
is enhanced due to the self organization of FL. The J–V char-
acteristics for the electron only devices with structure of
Al/FL:F/Al are shown in Fig. 8(b). In order to suppress the
hole injection into the blend, a low work function metal
(Al) was used as anode. The electron and hole mobilities
have been measured by fitting the dark J–V characteristics
for electron and hole only devices, respectively to SCLC
model at low voltages, in which the current density is
given by
Summary of the photovoltaic parameters of the ITO/PEDOT:PSS/FL:F/Al
devices prepared from different solvents.
Solvent
Jsc (mA/cm2)
Voc (V)
FF
PCE (%)
DCB
CN/DCB
7.4
8.3
0.90
0.88
0.56
0.60
3.62
4.38
mixed CN/DCB solvents. Fig. 7(b) shows the IPCE spectra of
the devices fabricated from blends of FL:F cast from OCB
and CN/DCB solvents. The IPCE spectra are similar with
the absorption spectra of the blend. With the addition of
CN in the solvent, the IPCE maximum at 630–680 nm in-
creased from 55% to 71%, which is consistent with Jsc and
PCE. The Jsc value calculated from the integration of the
IPCE spectrum is 7.1 and 8.6 mA/cm2 for the devices based
on DCB and CN/DCB cast blends, respectively, which clo-
sely matches the Jsc values obtained from the J–V measure-
ments under white light illumination.
The increase of Jsc is mainly related to more exciton and
charge generation and more effective charge transport. It
was found that the film cast from CN/DCB exhibits stronger
absorption than that cast from DCB. The stronger absorp-
tion partially contributed to higher Jsc and IPCE of the de-
vice cast from CN/DCB as compared to the device cast
from CB. A similar phenomenon has been observed by
Yang et al. for P3HT:PCBM system [55].
V2=8d3Þ exp½0:891
ðV=dÞ ꢂ
c
0:5
J ¼ ð9eoer
l
Another important factor being responsible for in-
creased Jsc is the charge transport in active layers. The
Fig. 8. Current–voltage characteristics for (a) hole only and (b) electron
only devices based on FL:F blend films with DCB and CN/DCB solvents.
Fig. 9. AFM images of FL:F blends cast from (a) DCB and (b) CN/DCB
solvents (scale 3 ꢃ 3 m).
l

J.A. Mikroyannidis et al. / Organic Electronics 12 (2011) 774–784
783
where eoer represent the permittivity of the material,
the mobility, is the field activation factor, and d is the
thickness of the active layer and voltage V = Vappl ꢁ Vbi.
l is
Acknowledgment
c
We are thankful to Prof. Y.K. Vijay of Thin film and
Membrane Science Laboratory, University of Rajasthan
Jaipur (Raj.) for allowing us to undertake the device fabri-
cation and characterization in his laboratory.
The hole (lh) and electron (le) mobilities of the film cast
from DCB are 5.6 ꢃ 10ꢁ6 and 3.4 ꢃ 10ꢁ4 cm2/Vs, respec-
tively. The lh and le for the film cast from the CN/DCB
are about 1.8 ꢃ 10ꢁ5 and 2.2 ꢃ 10ꢁ4 cm2/Vs, respectively.
The smaller value of le/lh (60.7 and 12.2 for CB and CN/
DCB solvent cast film, respectively) indicates a more
balanced charge transport. The more balanced charge
transport contributes to the higher Jsc and higher PCE.
The difference in PV performance for the devices cast
from DCB and CN/DCB solvents is most likely due to the
change in the morphology. The effect of the solvents on
the active layer morphology was also examined by the
AFM technique. Fig. 9 compares the AFM images of the
blend films of FL:F cast from the DCB and CN/DCB solvents.
For the FL:F blend films cast from DCB solvent, its root
mean square (rms) roughness is about 0.45 nm. However,
when the film is cast from the CN/DCB solvent, the rms
roughness is about 0.78 nm. Higher roughness of the film
will give higher device efficiency. The rough surface is
probably a signature of self organization of organic semi-
conductor, which in turn enhances ordered structure for-
mation in thin films and increases the carrier mobility
[57]. The increased roughness of the active layer enhances
the D–A interfacial area leading to efficient dissociation of
the excitons into free charge carriers.
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