Symmetrical molecules of low band gap with a central spacer connected via ether bond with terminal 4-nitro-α-cyanostilbene units: Synthesis and application for bulk heterojunction solar cells

Article abstract of DOI:10.1016/j.orgel.2010.07.010

Two new soluble and symmetrical molecules M1 and M2 of low band gap with a central spacer which was connected at both sides via ether bond with terminal 4-nitro-α-cyanostilbene units were synthesized. The spacer of M1 and M2 consisted of dihexyloxyphenylene and n-hexylene, respectively. Their long-wavelength absorption maximum was at 590-640 nm. The thin film absorption onset for both molecules was located at 742 nm which corresponds to an optical band gap of 1.67 eV. We have fabricated bulk heterojunction (BHJ) photovoltaic devices using these molecules as donor and PCBM as acceptor. We have investigated the solvent vapor treatment effect of these blends on their morphology and photovoltaic properties. We found that the overall power conversion efficiency (PCE) for the devices based on the solvent vapor treated M1:PCBM and M2:PCBM blends is 3.05% and 1.90%, respectively, higher than those of pristine blends. This increase of PCE has been attributed to the increase in surface roughness of the blend and better balance in charge transport. The PCE has been further increased up to 3.57% and 2.42% with the thermally annealed solvent treated M1:PCBM and M2:PCBM blend, respectively. This increase of PCE may be attributed to the enhanced crystallinity of the blend and reduction of the space charge effect, improving the charge transport and collection efficiency.

Full text of DOI:10.1016/j.orgel.2010.07.010

Organic Electronics 11 (2010) 1631–1641
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Symmetrical molecules of low band gap with a central spacer connected
via ether bond with terminal 4-nitro-a-cyanostilbene units: Synthesis
and application for bulk heterojunction solar cells
a
J.A. Mikroyannidis a, , A.N. Kabanakis , S.S. Sharma d,e, Anil Kumar 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, Rajasthan 342005, India
c Jaipur Engineering College, Kukas, Jaipur, Rajasthan, India
d Thin Film and Membrane Science Laboratory, Department of Physics, University of Rajasthan, Jaipur, Rajasthan, India
e Department of Physics, Government Engineering College for Women, Ajmer, Rajasthan, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 June 2010
Received in revised form 8 July 2010
Accepted 9 July 2010
Available online 30 July 2010
Two new soluble and symmetrical molecules M1 and M2 of low band gap with a central
spacer which was connected at both sides via ether bond with terminal 4-nitro- -cyano-
a
stilbene units were synthesized. The spacer of M1 and M2 consisted of dihexyloxyphenyl-
ene and n-hexylene, respectively. Their long-wavelength absorption maximum was at
590–640 nm. The thin film absorption onset for both molecules was located at 742 nm
which corresponds to an optical band gap of 1.67 eV. We have fabricated bulk heterojunc-
tion (BHJ) photovoltaic devices using these molecules as donor and PCBM as acceptor. We
have investigated the solvent vapor treatment effect of these blends on their morphology
and photovoltaic properties. We found that the overall power conversion efficiency (PCE)
for the devices based on the solvent vapor treated M1:PCBM and M2:PCBM blends is
3.05% and 1.90%, respectively, higher than those of pristine blends. This increase of PCE
has been attributed to the increase in surface roughness of the blend and better balance
in charge transport. The PCE has been further increased up to 3.57% and 2.42% with the
thermally annealed solvent treated M1:PCBM and M2:PCBM blend, respectively. This
increase of PCE may be attributed to the enhanced crystallinity of the blend and reduction
of the space charge effect, improving the charge transport and collection efficiency.
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Small-molecule
Low band gap
Cyanovinylene
Bulk heterojunction solar cells
Solvent vapor treatment
Balance charge transport
1. Introduction
flexible devices. These materials offer the possibility to
be used by solution processing like printing, doctor blad-
There is a strong continuing interest in the development
of polymer based photovoltaic (PV) devices. Bulk hetero-
junction (BHJ) polymer PV devices have proven to be the
most efficient thus far [1]. Solar cells prepared from poly-
mer and organic materials are attractive for their potential
in low cost in manufacturing large-area, lightweight, and
ing, or spray deposition. Certain reviews for advanced
materials and processes of polymer solar cells have re-
cently been reported [2]. The key steps that occur in organ-
ic based PV devices are: light absorption to form an
exciton, separation of the exciton into holes and electrons,
and movement of the generated holes and electrons to the
electrodes. There are clearly many factors that affect each
of these steps including the optical density of the polymer
and its morphology. To achieve separation of the exciton,
the BHJ approach requires two materials of different
electron and hole affinities to be mixed. This has been
achieved using polymer blends [3–5] but the most
* Corresponding author.
** Corresponding author at: Physics Department, Molecular Electronic
and Optoelectronic Device Laboratory, JNV University, Jodhpur, Rajasthan
342005, India.
E-mail address: mikroyan@googlemail.com (J.A. Mikroyannidis).
1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2010.07.010

1632
J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) 1631–1641
common approach has been to blend the polymer with
[6,6]-phenyl-C61-butyric methyl ester (PCBM), a soluble
C60 derivative [1,6].
mers carrying 4-nitro-a-cyanostilbene units have been
synthesized in our laboratory and used for BHJ solar cells
[47] and dye-sensitized solar cells [48] with enhanced effi-
ciency. In continuation of this research line, herein we de-
scribe the synthesis and characterization of two new
symmetrical molecules M1 and M2 of low band gap with
common structural segments. Both molecules contained a
central spacer which was connected at both sides via ether
The overall power conversion efficiency (PCE) has been
steadily improving from an initial 1% via a blend of poly[2-
methoxy-5-(30,70-dimethyloctyloxy)-1,4-phenylenevinyl-
ene] (MDMO-PPV) with PCBM [7] to an impressive ꢀ5%
achieved by poly(3-hexylthiophene) (P3HT):PCBM via
extensive morphology optimization [8–10]. In order to fur-
ther improve the efficiency of BHJ polymer solar cells, the re-
search community is searching for new polymers. From the
materialsperspective, thesedonorpolymersshouldnotonly
have a low band gap, in order to increase the short circuit
current (Jsc), but also bear a low energy level of the highest
occupiedmolecularorbital(HOMO)to improvethe opencir-
cuit voltage (Voc) [11]. Typically, a low band gap polymer is
designed via a ‘‘donor–acceptor” (D–A) approach, which is
to incorporate electron-rich and electron-deficient moieties
in the polymer backbone. The low band gap is mainly caused
by the intramolecular charge transfer between donor and
acceptor units [12]. Currently, state of the art polymer based
BHJ solar cells have reached PCEs of more than 6% [13] and
6.7% [14] have been reported for single and tandem solar
cells, respectively. However, these numbers are still signifi-
cantly lower than the critical efficiencies of 10–15% [15].
In parallel efforts, organic solar cells (OSCs) based on
small-molecules with vacuum evaporating interpenetrat-
ing multilayer structures have also reached PCEs as high
as 5–6% [16]. Small-molecule 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 low (PCEs range from 0.3% to ꢀ3.0%) rela-
tive to solution-processed solar cells using polymeric do-
nor materials (ꢀ5%). Small-molecule materials, however,
offer advantages over polymeric materials in terms of ease
of synthesis and purification, which greatly improve fabri-
cation reproducibility, as well as possessing a greater ten-
dency to self assemble into ordered domains, which leads
to high charge carrier mobilities [17]. Small-molecules 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 small-molecules a promising class of donor material
for BHJ solar cell applications [18–26]. Solution-processed
small-molecules, which have been used for PV cells, have
recently been reviewed [27]. Efficient BHJ PV solar cells
bond with terminal 4-nitro-a-cyanostilbene units. The two
ether bonds are expected to increase the flexibility of the
molecules. Moreover, their aliphatic moieties render these
molecules very soluble in common organic solvents thus
allowing the facile preparation of thin films by spin cast-
ing. The molecules were successfully synthesized by a
reaction sequence the last step of which included the con-
densation of a kye-dialdehyde with 4-nitrobenzylcyanide.
We have used M1 and M2 as electron donor along with
PCBM as electron acceptor for BHJ active layer for the fab-
rication of organic PV devices. The PCE for the devices
based on pristine M1:PCBM and M2:PCBM blends, is
1.67% and 1.05%, respectively. We have investigated the ef-
fect of solvent vapor treatment of blend on the PV response
of the BHJ devices and found that the PCE has been in-
creased up to 3.05% and 1.90% for M1:PCBM and M2:PCBM
blends, respectively. This improvement of PCE has been
attributed to the increase in hole mobility and surface
roughness, leading to better charge separation and collec-
tion efficiency. The PCE has been further increased up to
3.57% and 2.42% after thermal annealing of the solvent va-
por treated M1:PCBM and M2:PCBM blends, respectively.
2. Experimental
2.1. Reagents and solvents
4-Nitrobenzylcyanide was synthesized from the nitration
of benzyl cyanide with concentrated nitric and sulfuric acid
[49]. It was recrystallized from ethanol. 4-Hydroxybenzalde-
hyde was recrystallized from distilled water. N,N-dimethyl-
formamide (DMF) and tetrahydrofuran (THF) were dried by
distillation over CaH2. All other reagents and solvents were
commercially purchased and were used as supplied.
2.2. Preparation of compounds
2.2.1. 1,4-Bis(p-formylphenoxymethyl)-2,5-bis(hexyloxy)
benzene (3)
based on symmetrical D–
p
–A–
p
–D organic dye molecules
A flask was charged with a mixture of 2 (0.1650 g,
0.355 mmol), 4-hydroxybenzaldehyde (0.1042 g, 0.853
mmol), K2CO3 (0.15 g, 1.08 mmol) and acetonitrile (25 mL).
The mixture was stirred and refluxed for 12 h under N2. It
was subsequently concentrated under reduced pressure.
Water was added to the concentrate and then it was ex-
tracted with dichloromethane. The organic layer was dried
(Na2SO4) and concentrated to afford 3 (0.1478 g, yield 76%).
FT-IR (KBr, cmꢁ1): 2930, 2870, 2854 (C–H stretching);
1684 (carbonyl of formyl); 3050, 1602, 1506, 1466 (aro-
matic); 1246, 1210, 1162 (ether bond).
[28], squaraine [29,24], phthalocyanines [30,31], oligothi-
ophene with dialkylated diketopyrrolopyrrole [32,33],
2-vinyl-4,5-dicyanoimidazole (vinazene) [34,35] and star-
shaped molecules have very recently been reported [36–
40]. The highest PCE reported to date for solar cells using
solution-processable small-molecules is 3.0% [32].
On the other hand, cyano-poly(p-phenylenevinylene)
(CN-PPV) and its derivatives with cyanovinylene moieties
are one of the most important conjugated polymers with
high electron affinity and promising materials used in
polymeric light emitting diodes and polymeric PV devices
[41–46]. Various low band gap small-molecules and poly-
1H NMR (CDCl3) ppm: 9.90 (s, 2H, formyl); 7.79 (m, 4H,
aromatic ortho to formyl); 6.96–6.85 (m, 6H, aromatic

J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) (2010) 1631–1641
1633
ortho to oxygen); 5.14 (s, 4H, OCH2Ph); 3.96 (m, 4H,
2.4. Device fabrication and characterization
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 C34H42O6: C, 74.70; H, 7.74. Found: C,
74.32; H, 7.40%.
Organic solar cell devices with the structure ITO/PED-
OT:PSS/M1 or M2:PCBM (1:1)/Al were fabricated as
follows: After spin-coating of 40 nm layer of poly (3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:
PSS) onto a pre-cleaned ITO coated glass substrates and
baking at 80 °C for 30 min, a 80 nm M1:PCBM or M2:PCBM
blend layer in THF solution was deposited on the top of the
PEDOT:PSS layer by controlled spin-coating rate. Finally, an
Al electrode (50 nm) was deposited on the top of the active
BHJ layer through the thermal evaporation method. The
effective area of the each device is 0.08 cm2. For the solvent
treatment, the spin coated blends were transferred into a
jar filled with THF. Pre-thermal annealing of the solvent
treated active layers was carried out at 120 °C for 2 min
on the hot plate before the deposition of the Al electrode.
The devices based on pure M1 or M2 were also fabricated
having structure ITO/PEDOT:PSS/M1 or M2/Al.
Current–voltage (J–V) characteristics of the PV devices
were measured using a computer controlled Keithley 238
source meter in dark as well as under illumination inten-
sity of 100 mW/cm2. A xenon light source (Oriel, USA)
was used to give a simulated irradiance of 100 mW/cm2
(equivalent to AM1.5 irradiation) at the surface of the de-
vice. The hole only device having ITO/PEDOT:PSS/M1 or
M2:PCBM/Au configuration using as cast the thermally an-
nealed active layers, were fabricated and J–V characteris-
tics in dark at room temperature, were recorded to
estimate the hole mobility.
2.2.2. Molecule M1
A flask was charged with a solution of 3 (0.1478 g, 0.270
mmol) and 4-nitrobenzylcyanide (0.0876 g, 0.540 mmol) in
ethanol (20 mL). Sodium hydroxide (0.20 g, 5.00 mmol) was
added to this solution. The reaction mixture was stirred for
1 h at room temperature under N2 and then it was concen-
trated under reduced pressure. Water was added to the con-
centrate and M1 precipitated as a dark green solid. It was
recrystallized from ethanol/water (0.15 g, yield 66%).
FT-IR (KBr, cmꢁ1): 2922, 2852 (C–H stretching); 2170
(cyano); 1510, 1346 (nitro); 3075, 1600, 1454 (aromatic);
1258, 1162, 1106 (ether bond); 1016 (trans-vinylene bond).
1H NMR (CDCl3) ppm: 8.11 (m, 4H, aromatic ortho to ni-
tro); 7.72 (s, 2H, cyanovinylene); 7.65 (m, 4H, aromatic
meta to nitro); 7.26 (m, 4H, aromatic meta to oxygen);
6.88–6.85 (m, 6H, aromatic ortho to oxygen); 5.15 (s, 4H,
OCH2Ph); 3.97 (m, 4H, OCH2(CH2)4CH3); 1.80 (m, 4H,
OCH2CH2(CH2)3CH3); 1.38 (m, 12H, O(CH2)2(CH2)3CH3);
0.90 (t, 6H, O(CH2)5CH3).
Anal. Calcd. for C50H50N4O8: C, 71.92; H, 6.04; N, 6.71.
Found: C, 71.53; H, 6.17; N, 6.90%.
2.2.3. Molecule M2
This molecule was synthesized in 70% yield from the
reaction of dialdehyde 4 with a double molar amount of
4-nitrobenzylcyanide in ethanol in the presence of NaOH
according to the procedure described for M1.
The incident photon to current efficiency (IPCE) of the
devices was measured by illuminating the device with a
xenon light source using monochromator and measuring
the resulting photocurrent with a Keithley electrometer
under short circuit conditions according to the following
expression:
FT-IR (KBr, cmꢁ1): 2936, 2860 (C–H stretching); 2160
(cyano); 1510, 1346 (nitro); 3073, 1600 (aromatic); 1250,
1174, 1106 (ether bond); 1010 (trans-vinylene bond).
1H NMR (CDCl3) ppm: 8.12 (m, 4H, aromatic ortho to ni-
tro); 7.74 (s, 2H, cyanovinylene); 7.66 (m, 4H, aromatic
meta to nitro); 7.32 (m, 4H, aromatic meta to oxygen);
6.80 (m, 4H, aromatic ortho to oxygen); 3.92 (m, 4H,
OCH2); 1.68 (m, 4H, OCH2 CH2); 1.27 (m, 4H, O(CH2)2 CH2).
Anal. Calcd. for C36H30N4O6: C, 70.35; H, 4.92; N, 9.12.
Found: C, 69.94; H, 4.80; N, 9.28%.
IPCEð%Þ ¼ 1240Jsc=kPin
where Jsc is the short circuit photocurrent and k and Pin are
the wavelength and illumination intensity of the incident
light, respectively.
2.3. Characterization methods
3. Results and discussion
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 with spect-
rograde THF. Elemental analyses were carried out with a
Carlo Erba model EA1108 analyzer.
The electrochemical cyclic voltammetry was conducted
with Pt disk, Pt wire and Ag/Ag+ electrode as working
electrode, counter electrode, and reference electrode,
respectively, in a 0.1 mol/L tetrabutylammonium hexafluo-
rophosphate (Bu4NPF6) acetonitrile solution. The M1 or M2
films for the electrochemical measurements were coated
from THF solution containing M1 or M2.
3.1. Synthesis and characterization
Scheme 1 outlines the four-step reaction sequence ap-
plied for the synthesis of the target molecule M1. In parti-
cular, 1 [50] and 2 [50] were synthesized by reported
procedures. The reaction of 2 with 4-hydroxybenzaldehyde
in a mol ratio 1:2 afforded dialdehyde 3. This reaction took
place in acetonitrile in the presence of K2CO3. Finally, the
Knoevenagel condensation [51] of 3 with a double molar
amount of 4-nitrobenzylcyanide in ethanol in the presence
of NaOH yielded M1. It is well known that this type of con-
densation forms double bonds which possess mainly trans-
conformation [52]. The facile synthesis of the other target
molecule M2 was achieved in only two steps according
to Scheme 2. Specifically, 4-hydroxybenzaldehyde reacted

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J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) 1631–1641
Scheme 1. Synthesis of molecule M1.
Scheme 2. Synthesis of molecule M2.
Fig. 1. FT-IR (top) and 1H NMR in CDCl3 solution (bottom) spectra of molecule M1.

J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) (2010) 1631–1641
1635
with 1,6-dibromohexane to afford dialdehyde 4 [53]. The
latter was subsequently condensed with 4-nitrobenzylcya-
nide to give M2. Both molecules were obtained in rela-
tively high yields (66–70%) as dark green solids which
were purified by recrystallization from ethanol/water.
The two hexyloxy side chains of M1 and the hexylene
spacer of M2 enhanced the solubility of these molecules.
Particularly, both molecules were readily soluble in com-
mon organic solvents such as THF, dichloromethane, chlo-
roform, acetone and toluene. Thin films of these molecules
could be obtained by spin casting from the solutions de-
spite their monomer nature.
Since the molecules have comparable chemical struc-
tures, their spectra showed certain common features.
Fig. 1 presents the FT-IR and 1H NMR spectra of M2 as an
example. The IR spectrum displayed characteristic absorp-
tion bands at 2936, 2860 (C–H stretching of hexylene);
2160 (cyano); 1510, 1346 (nitro); 3073, 1600 (aromatic);
1250, 1174, 1106 (ether bond) and 1010 cmꢁ1 (trans-vinyl-
ene bond). The latter appeared at longer wavelength than
usual trans-vinylene bonds (970–960 cmꢁ1) owing to the
substitution by the electron-withdrawing cyano group.
On the other hand, the 1H NMR spectrum of M2 showed
upfield signals at 8.12 and 7.74 ppm assigned to the aro-
matic ortho to nitro and the cyanovinylene respectively.
The other aromatic resonated at 7.66–6.80 ppm, while
the aliphatic at 3.92–1.27 ppm.
3.2. Photophysical and electrochemical properties
Fig. 2 depicts the UV–vis absorption spectra of mole-
cules in dilute (10ꢁ5 M) THF solution and thin film, which
were normalized with respect to the long-wavelength
absorption. Table 1 summarizes the photophysical and
electrochemical characteristics of both molecules. Their
absorption curves were broad and extended approximately
up to 750 nm. This behavior is attributable to the terminal
4-nitro-a-cyanostilbene units as it has been well estab-
Fig. 2. Normalized absorption spectra of molecules in THF solution (top)
and thin film (bottom).
lished in our previous publications [47,48]. The long-
wavelength absorption maximum (ka,max) was located at
590–640 nm. The ka,max of M2 was red-shifted relative to
that of M1 in both solution and thin film. However, both
Table 1
Optical and electrochemical properties of molecules.
molecules displayed the same thin film absorption onset
opt
at 742 nm which corresponds to an optical band gap (Eg
of 1.67 eV. This value of Eg
)
Molecule
M1
M2
opt
conforms to those of other
a
ka,max in solution (nm)
590
622
742
1.67
0.5
607
640
742
1.67
0.7
related materials [47,48].
a
ka,max in thin film (nm)
Electrochemical cyclic voltammetry has been widely
employed to investigate the redox behavior of organic
materials and to estimate their HOMO and LUMO energy
levels [54]. Fig. 3 shows the cyclic voltammograms of M1
and M2 films on Pt electrode in a 0.1 mol/L Bu4NPF6–aceto-
nitrile solution. The results of the electrochemical mea-
surements are listed in Table 1. It can be seen from Fig. 3
that there are reversible n-doping/depoping (reduction/
reoxidation) and p-doping/depoping processes in both
negative and positive potential range, respectively. The on-
set oxidation potential (Eox) for M1 and M2 is 0.5 V and
0.7 V vs. Ag/Ag+, respectively. The lower value of the oxida-
tion potential for M1 indicates higher electron donating
ability of M1 as compared to M2. In the reduction potential
region, the onset reduction potential (Ered) is ꢁ1.21 V and
Thin film absorption onset (nm)
optb
Eg
E
E
(eV)
(V)
ox
onset
red
ꢁ1.2
ꢁ1.0
(V)
onset
HOMO (eV)
LUMO (eV)
Eg (eV)
ꢁ5.2
ꢁ3.5
1.7
ꢁ5.4
ꢁ3.7
1.7
elc
a
ka,max
: the long-wavelength absorption maxima from the UV–vis
spectra in THF solution or in thin film.
b
Egopt: optical band gap determined from the absorption onset in thin
film.
c
Egel: electrochemical band gap determined from cyclic voltammetry.
ꢁ1.0 V vs. Ag/Ag+ for M1 and M2, respectively. From the
Eox and Ered of molecules M1 and M2, their HOMO and

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J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) 1631–1641
of M1 or M2 as donor with PCBM as an acceptor can be
used for efficient BHJ organic PV devices.
3.3. Electrical and photoelectrical properties of M1 and M2
The J–V characteristics of the ITO/PEDOT:PSS/M1 or M2/
Al devices in dark and under illumination are shown in
Fig. 4. In these curves, the forward bias corresponds to
the positive and negative voltage applied to PEDOT:PSS/
ITO and Al electrode, respectively. It can be seen from these
figures that the J–V characteristics show a rectification ef-
fect. The HOMO level of PEDOT:PSS (ꢁ5.2 eV) is very close
to the HOMO level of both M1 and M2. Therefore, this
electrode behaves almost as an Ohmic contact for hole
injection from the PEDOT:PSS into the HOMO of M1 and
M2 in their respective devices. However, the LUMO level
of both M1 and M2 is very far from the work function of
the Al (ꢁ4.3 eV) and forms the Schottky barrier for the
electron injection from the Al into the LUMO level of mol-
ecules. Hence, the rectification effect observed in the dark
J–V characteristics is due to the formation of Schottky bar-
rier at the Al–M1 or Al–M2 interfaces.
The charge carrier mobility of the PV materials is also an
important factor which influences the performance of or-
ganic solar cells. We have measured the hole mobilities
of these molecules using the hole only devices with ITO/
PEDOT:PSS/M1 or M2/Au structure using the SCLC model
[55]. The J–V characteristics of the devices were plotted
as ln[Jd3/(Vapp ꢁ Vbi)] vs. [(Vapp ꢁ Vbi)/d]0.5, and the hole
mobilities of M1 and M2 calculated from the intercept of
the corresponding lines are 2.3 ꢂ 10ꢁ6 and 5.1 ꢂ 10ꢁ6
cm2/Vs for M2 and M1, respectively.
Fig. 3. Cyclic voltammetry data of M1 and M2.
LUMO energy levels as well as their energy gaps were cal-
culated according to equations:
LUMO ¼ ꢁqðEred þ 4:7Þ ðeVÞ
HOMO ¼ ꢁqðEox þ 4:7Þ ðeVÞ
where the units of Eox and Ered are V vs. Ag/Ag+. The HOMO
and LUMO values of these molecules are included in
Table 1. It can be seen that the HOMO energy level of M2
(ꢁ5.4 eV) is lower than that of M1 (ꢁ5.2 eV). The LUMO le-
vel of M2 (ꢁ3.7 eV) is higher than that of M1 (ꢁ3.5 eV).
However, the electrochemical band gap (Egel) (1.7 eV) is al-
most the same for both molecules. The Egel estimated from
the electrochemical data is in good agreement with the
opt
Eg
estimated from the thin film absorption onset. The
The PV parameters, i.e. short circuit current (Jsc), open
circuit voltage (Voc), fill factor (FF) and PCE are summarized
in Table 2. It can be seen from this table that the values of Jsc
and PCE are higher for the device based on M1 as compared
to M2. This is attributed to the higher hole mobility of M1 as
compared to M2, since the Egopt and the absorption spectra
HOMO and LUMO levels of the PCBM were also estimated
from the electrochemical data and were found to be
ꢁ6.5 eV and ꢁ4.1 eV, respectively. The difference between
the LUMO level of M1 or M2 and PCBM is about 0.6 eV and
0.4 eV, respectively, which indicates that the combination
Fig. 4. Current–voltage characteristics of the devices based on pure M1 and M2 in dark (a) and under illumination (b).
Table 2
Summary of photovoltaic parameters of the ITO/PEDOT:PSS/M1 or M2/Al devices.
Active material
Short circuit current (Jsc) (mA/cm2)
Open circuit voltage (Voc) (V)
Fill factor (FF)
Power conversion efficiency (PCE) (%)
M1
M2
0.62
0.47
0.69
0.72
0.41
0.40
0.17
0.13

J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) (2010) 1631–1641
1637
of both molecules are identical. As the HOMO level of M1 is
very close to the HOMO level of PEDOT:PSS, the better hole
collection in the device based on M1 as compared to M2,
may also be the reason for the higher PCE.
3.4. Photovoltaic properties of BHJ devices
The BHJ OSCs were fabricated with a structure of ITO/
PEDOT:PSS/M1 or M2:PCBM (1:1)/Al, where M1 or M2
were used as electron donor and PCBM was used as elec-
tron acceptor. Fig. 5 shows the J–V characteristics of the de-
vices and Table 3 lists the corresponding PV parameters
(Jsc, Voc, FF and PCE) of the devices under illumination
intensity of 100 mW/cm2. It can be seen from Table 3 that
the device based on the as cast M1:PCBM blend shows bet-
ter PV response, having PCE of 1.67% as compared to 1.05%
for the device based on the as cast M2:PCBM blend. The
IPCE spectra of the devices (Fig. 6) show similar shape to
the absorption spectra of the respective blends employed
in the devices. This indicates that the visible light absorbed
by the M1:PCBM or M2:PCBM photoactive layer in respec-
Fig. 6. IPCE spectra of BHJ devices based on pristine M1:PCBM and
M2:PCBM blends.
Fig. 7. Absorption spectra of solvent vapor treated M1:PCBM and
M2:PCBM blends.
tive devices contributes to the Jsc. The maximum of IPCE for
the as cast M1:PCBM and M2:PCBM based devices are
about 55% and 42%, respectively at their absorption peak.
The IPCE values are consistent with the higher values of
Jsc and PCE of the M1:PCBM blend based organic solar cell.
The Jsc is directly related to the product of absorbed
photons and external quantum efficiency (EQE) [27]. Since
the optical absorption of both M1:PCBM and M2:PCBM is
almost the same, light absorption cannot be the factor for
increased Jsc and PCE of the device based on M1:PCBM as
compared to M2:PCBM. We assume that the difference in
EQE of the devices is responsible for the higher photocur-
rent of the M1:PCBM blend. The EQE is determined by
three processes: (i) migration/diffusion of photogenerated
excitons towards the D/A interface, (ii) excitons dissocia-
tion and charge separation at the D/A interfaces, and (iii)
collection of charge carrier at the electrodes. The process
(i) depends on the nanoscale phase separation between
the donor and acceptor components used in the BHJ active
layer. It can be seen from the AFM images (Figs. 9 and 10)
Fig. 5. Current–voltage characteristics of the devices based on pristine
M1:PCBM and M2:PCBM blend under illumination intensity of 100 mW/
cm2.
Table 3
Summary of photovoltaic parameters of BHJ devices, using layer with
different processing conditions.
Active
Short circuit
Open
Fill
Power
material
current (Jsc
)
circuit
voltage
(Voc) (V)
factor conversion
(mA/cm2)
(FF)
efficiency
(PCE) (%)
M1:PCBMa 3.76
M1:PCBMb 6.4
M1:PCBMc 7.12
M2:PCBMa 2.38
M2:PCBMb 4.15
M2:PCBMc 5.16
0.93
0.90
0.93
0.98
0.95
0.98
0.48
0.53
0.54
0.45
0.48
0.48
1.67
3.05
3.57
1.05
1.90
2.42
a
b
c
Pristine blend.
Solvent vapor treated blend.
Thermally annealed solvent treated blend.

1638
J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) 1631–1641
Generally, the performance of the organic BHJ PV de-
vices can be improved by controlling the morphology of
the BHJ active layer, because efficient photoinduced charge
generation, transport and collection of charge carriers at
each collecting electrode crucially depend on the nanome-
ter scale morphology of the films [56]. The crystallization
of the organic materials improves the light harvesting
property by extending the conjugation length and in-
creases the charge carrier mobility, which is the limiting
factor for the balance transport of the electrons and holes
dissociated from the bound excitons. The increase in the
PCE via thermal annealing is attributed to modified nano-
scale morphology, enhanced crystallization of blend and
improved transport of charges. It has also been reported
that self-organization of organic material occurs not only
under thermal annealing but also in proper ambience. Li
et al. [9] and Zhao et al. [57] reported efficient PV cells
based on P3HT/PCBM blend by controlling the growth rate
of the active layer from solution to solid state. We have
investigated the effect of thermal annealing, and combin-
ing solvent vapor treatment and thermal annealing on
the PCE of PV devices based on M1:PCBM and M2:PCBM
blends.
Fig. 7 shows the UV–vis absorption spectra of the sol-
vent vapor treated M1:PCBM and M2:PCBM blends. For
the untreated M1:PCBM blend film, the M1 absorption
peak is located at 620 nm with an absorption onset at
742 nm. Compared to the pristine M1:PCBM blend film,
the absorption peak of the solvent treated M1:PCBM blend
film shifted towards longer wavelength region and became
much stronger in this region. In addition, the absorption
spectra of the solvent treated M1:PCBM blend shows one
vibronic absorption shoulder at 680 nm, which is attrib-
uted to an enhanced conjugation length and more ordered
structure of M1. As the solvent molecules can penetrate
into the film and increase the space between the molecular
chains, the chains become mobile and self-organization
can occur to form the ordering.
Fig. 8. Current–voltage characteristics of the devices based on THF
treated M1:PCBM and M2:PCBM blend under illumination intensity of
100 mW/cm2.
that the surface roughness is higher for M1:PCBM as com-
pared to M2:PCBM, which indicates better nanoscale phase
separation in M1:PCBM blend. Therefore, we assume that
the contribution of EQE from the process (i) is higher for
M1:PCBM. For process (ii)
a sufficient large energy
difference between the LUMO of the donor and acceptor
material is required for ultra-fast photoinduced charge
transfer. This difference is higher for M1:PCBM as com-
pared to M2:PCBM blend. Photoinduced charge transfer is
comparatively faster in the former blend, resulting in high-
er EQE for the device based M1:PCBM blend. Process (iii)
depends on the percolated path for electrons and holes in
BHJ active layer, mobility of electron and hole, and on
the position of HOMO and LUMO level of the donor and
acceptor materials, respectively, relative to the work func-
tion of anode and cathode. The HOMO level of M1 is closer
to the HOMO level of PEDOT:PSS indicating higher hole
mobility for M1, which results in more efficient hole col-
lection. Therefore, processes (i–iii) are responsible for the
improved Jsc and PCE of the device based on M1:PCBM as
compared to M2:PCBM.
Fig. 8 shows the J–V characteristics of the devices under
100 mW/cm2 white light illumination with THF treatment.
The PV parameters for these PV devices are summarized in
Table 3. It can be seen that the Jsc of the PV devices with
THF vapor treatment increases to 6.4 mA/cm2 and
Fig. 9. AFM images of M1:PCBM as cast (a) and solvent treated (b) films.

J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) (2010) 1631–1641
1639
Fig. 10. AFM images of M2:PCBM as cast (a) and thermally annealed (b) films.
4.15 mA/cm2 for M1:PCBM and M2:PCBM based devices
respectively, which is a twofold increase compared to the
as produced PV device. The overall PCE of the devices based
on the solvent treated M1:PCBM and M2:PCBM has been
increased up to 3.05% and 1.90%, respectively. This
improvement is attributed to the self-organization of M1
or M2 induced by solvent vapor treatment which enhances
the optical absorption and the hole transport. The PCE of
the BHJ devices with THF treatment blends are higher than
that of pristine blends. The Jsc of the organic BHJ PV devices
is directly related to the product of light absorption by the
photoactive blend used in the device, exciton diffusion,
charge separation and charge collection efficiency. As it
can be seen from the absorption spectra of the solvent va-
por treated blends, the absorption coefficient has been in-
creased as compared to the as cast blend films and also
red-shifted. This indicates that more light is absorbed by
the solvent vapor treated blends which generated more
excitons in the photoactive blend. It is also observed that
the IPCE spectra of the devices resemble the absorption
spectra of the corresponding blend used in the device. This
indicates that the Jsc is mainly attributed to the excitons
generated because of the absorption of photons in M1 or
M2. The value of the IPCE is consistent with the values of
Jsc. The IPCE is higher for the devices based on the solvent
vapor treated blends as compared to the pristine blends,
which is consistent with the increased values of Jsc.
the blends support the increase in the phase separation
of M1:PCBM or M2:PCBM after solvent vapor treatment.
The increased phase separation leads to an increase in
the D/A interfacial area for efficient charge separation,
resulting in higher Jsc and PCE for BHJ PV devices based
on the solvent vapor treated blend films.
The Voc values of the devices based on the solvent vapor
treated blend films is lower that that of the devices based
on the as cast films. It is widely accepted that the energy
difference between the HOMO and LUMO of the acceptor
determines the Voc value of the organic BHJ PV devices.
We have already shown that the overall optical absorption
spectrum of the solvent vapor treated blend films is red-
shifted. This result can be interpreted assuming that the
red-shifted spectrum results from a lower energy state of
delocalized exciton in donor materials (M1 or M2), as re-
ported for conjugated polymers [58]. Therefore, the lower-
ing of Voc for devices based on the solvent vapor treated
blends can be explained by reduced optical band gap of
M1 or M2.
We have investigated the charge collection efficiency by
means of the space charge limited current (SCLC) measure-
ments in dark. In the case of the devices based on the as cast
blend films, the space charge limits the charge collection
efficiency, because the hole mobility is lower than the elec-
tron mobility. According to the SCLC criteria, when there is a
large difference between the hole and electron mobilities,
the half power dependence of the photocurrent on the
applied voltage takes place, and the FF can not exceed more
than 0.40. Thus, in order to obtain high charge collection
efficiency, it is necessary to balance the carrier mobility by
enhancing the hole mobility in the blend. To investigate
the space charge effects, we extract the hole and electron
mobilities from the SCLC J–V characteristics obtained in
thedarkforholeandelectrononlydevices[55a,57]. Wehave
fabricated the hole- and electron-only devices having struc-
ture ITO/PEDOT:PSS/M1 or M2:PCBM/Au and ITO/PED-
OT:PSS/M1or M2:PCBM/Al, respectively. The hole and
electron mobilities have been extracted from the J–V charac-
teristics using the SCLC model [57] and they are listed in
Table 4. It can be seen from this table that the hole mobility
has been increased about 7–8 times for the devices based on
The nanoscale morphology of the blend films has been
investigated by the atomic force (AFM) experiment. Figs. 9
and 10 show the AFM images of the as cast and solvent
vapor treated blend films. For the as cast films, an rms sur-
face roughness of 4.2 nm and 3.2 nm, for M1:PCBM and
M2:PCBM, respectively is observed. The solvent vapor trea-
ted films are much rougher that the as cast films. The sol-
vent vapor treated M1:PCBM and M2:PCBM films show
rms roughness of 7.6 nm and 5.6 nm, respectively at
5
l
m ꢂ 5
lm scan sizes. The roughness is considered to
be a signal of M1 or M2 chains self–organization and phase
separation [57]. The increase of the roughness of the blend
films after solvent vapor treatment is because of the M1 or
M2 chains, self organize into ordered structure after sol-
vent vapor treatment. The higher surface roughness of

1640
J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) 1631–1641
Table 4
Hole and electron mobility of M1:PCBM and M2:PCBM blends films evaluated using the SCLC model.
Film
Hole mobility (
l
h) (cm2/Vs)
Electron mobility (
l
e) (cm2/Vs)
le/lh
M1:PCBM (as cast)
1.2 ꢂ 10ꢁ6
9.7 ꢂ 10ꢁ6
1.5 ꢂ 10ꢁ5
0.96 ꢂ 10ꢁ6
6.8 ꢂ 10ꢁ6
9.6 ꢂ 10ꢁ6
6.4 ꢂ 10ꢁ5
6.5 ꢂ 10ꢁ5
6.9 ꢂ 10ꢁ5
6.8 ꢂ 10ꢁ5
7.0 ꢂ 10ꢁ5
7.0 ꢂ 10ꢁ5
53
M1:PCBM (solvent vapor treated)
M1:PCBM (thermally annealed solvent vapor treated)
M2:PCBM (as cast)
M2:PCBM (solvent vapor treated)
M2:PCBM (thermally annealed solvent vapor treated)
6.9
4.6
71
10.3
7.3
the solvent vapor treated blend. Moreover, the electron
mobilities with and without solvent annealing are almost
the same. The increase in hole mobility after solvent vapor
treatment is attributed to the enhance in the crystallinity
and surface roughness of the M1 or M2 as indicated from
the absorption spectra and AFM images. The mobility differ-
ence between the holes and electrons decreases remarkably
and reduces the effect of space charge for the devices based
on solvent vapor treated blends. Therefore, we assume that
the charge collection efficiency has been increased for these
devices resulting in improvement in the overall PCE of the
devices.
times as compared to the solvent treated blend, indicating
that thermal annealing can effectively activate PCBM mol-
ecules to diffuse and aggregate into clusters for better
charge transport. The increase in both electron and hole
mobilities for the device based on the thermally annealed
solvent vapor treated blends, further reduces the space
charge effect, improves the charge transport and charge
collection leading to the overall PCE of the device.
4. Conclusions
Finally, we have investigated the PV response of the de-
vices based on thermally annealed on the solvent vapor
treated blends. The J–V characteristics of these devices
are shown in Fig. 11. The PV parameters are summarized
in Table 3. Both Voc and Jsc have been increased for these
devices as compared to devices based on solvent vapor
treated blends. The PCE has been further increased up to
3.57% and 2.42% with the thermally annealed solvent trea-
ted M1:PCBM and M2:PCBM blend, respectively. This in-
crease in PCE may be attributed to a further increase in
the crystallinity of the blend upon the thermal treatment.
We have also measured the hole and electron mobility
for the devices based on the thermally annealed solvent
vapor treated blends and found that the hole mobility
has been further increased (as shown in Table 4). More-
over, the electron mobility has also been increased by 20
Two new molecules M1 and M2 of low band gap which
contained a central spacer and 4-nitro-a-cyanostilbene
terminal units were synthesized. They were soluble in
common organic solvents. Their long-wavelength absorp-
tion maximum was at 590–640 mm with thin film absorp-
tion onset at 742 nm corresponding to an optical band gap
of 1.67 eV. The electrochemical band gap estimated from
the cyclic voltammetery data is 1.70 eV, which is very close
to the optical band gap. However, the HOMO and LUMO of
M1 and M2 are at different energy levels. We have used
both M1 and M2 as electron donor for BHJ PV devices along
with PCBM as electron acceptor. The hole mobility of M1 is
higher than that of M2. The PCE for pristine M1:PCBM and
M2:PCBM is 1.67% and 1.05%, respectively. The higher va-
lue of PCE for M1:PCBM is due to the higher hole mobility
of this molecule. The devices with solvent vapor treated
shows PCE 3.05% and 1.90% for M1:PCBM and M2:PCBM,
respectively. The improved PCE has been interpreted in
terms of more balanced charge transport due to the mod-
ified nanoscale morphology and increased crystallization
of the blend. The PCE has been further increased up to
3.57% and 2.42% for the devices based on thermally an-
nealed solvent vapor treated blend M1:PCBM and
M2:PCBM, respectively. This has been attributed to the
higher balance charge transport due to both increases in
electron and hole mobilities in the blend. Our studies indi-
cate that a combination of thermal annealing and solvent
vapor treatment is an effective approach to improve the
PCE of organic BHJ PV device.
Acknowledgement
We are thankful to Prof. Y.K. Vijay, for allowing us to
undertake the device fabrication and characterization in
his laboratory. We are also grateful to the Scientists in
IUAC, New Delhi for recording the AFM images of the blend
samples.
Fig. 11. Current–voltage characteristics of the devices based on THF
treated and thermally annealed M1:PCBM and M2:PCBM blend under
illumination intensity of 100 mW/cm2.

J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) (2010) 1631–1641
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