Synthesis of benzoselenadiazole-based small molecule and phenylenevinylene copolymer and their application for efficient bulk heterojunction solar cells

Article abstract of DOI:10.1016/j.orgel.2009.11.010

A new low-band gap small molecule (Se-SM), which contains dithyenyl-benzoselenadiazole as central unit and terminal cyanovinylene 4-nitrophenyls, was synthesized. In addition, a new phenylenevinylene copolymer (P) containing dithyenyl-dinitrobenzothiadiazole moieties was synthesized by Heck coupling. Se-SM showed broad absorption band with long-wavelength absorption maximum (λ_a,max) at ～640 nm and optical band gap (E_g^opt) of 1.67 eV. Copolymer P had λ_a,max around 420 nm and E_g^opt of 2.31 eV. The dark current-voltage characteristics and incident photon to current efficiency spectra of the devices based on copolymer P and Se-SM indicates that both materials behave as p-type organic semiconductors. The power conversion efficiency (PCE) of the photovoltaic devices based on these materials is low. However, the PCE of the devices fabricated with the blends of copolymer P or Se-SM with PCBM, was improved significantly. The increase is attributed to the formation of bulk heterojunction with increased interfacial area. The effect of the incorporation of Se-SM molecule on the photovoltaic properties of copolymer P:PCBM blend has been also investigated. The overall PCE of the copolymer P:PCBM:Se-SM device is approximately 2.24%, which is further enhanced up to 3.16%, when the thermally annealed blend is used. Therefore, we conclude that the Se-SM molecule increases the light harvesting property of the blend and also provides efficient path for holes and electron in copolymer and PCBM phases, respectively.

Full text of DOI:10.1016/j.orgel.2009.11.010

Organic Electronics 11 (2010) 311–321
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Synthesis of benzoselenadiazole-based small molecule and
phenylenevinylene copolymer and their application for efficient bulk
heterojunction solar cells
b
b,c,
John A. Mikroyannidis a, , P. Suresh , G.D. Sharma
*
*
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.) 342005, India
c Jaipur Engineering College, Kukas, Jaipur (Raj.), India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new low-band gap small molecule (Se-SM), which contains dithyenyl-benzoselenadiaz-
ole as central unit and terminal cyanovinylene 4-nitrophenyls, was synthesized. In addi-
Received 5 August 2009
Received in revised form 22 September
2009
Accepted 4 November 2009
Available online 10 November 2009
tion,
a new phenylenevinylene copolymer (P) containing dithyenyl-dinitrobenzo-
thiadiazole moieties was synthesized by Heck coupling. Se-SM showed broad absorption
band with long-wavelength absorption maximum (ka,max) at ꢀ640 nm and optical band
gap (Egopt) of 1.67 eV. Copolymer P had ka,max around 420 nm and Eopt of 2.31 eV. The dark
g
current–voltage characteristics and incident photon to current efficiency spectra of the
devices based on copolymer P and Se-SM indicates that both materials behave as p-type
organic semiconductors. The power conversion efficiency (PCE) of the photovoltaic devices
based on these materials is low. However, the PCE of the devices fabricated with the blends
of copolymer P or Se-SM with PCBM, was improved significantly. The increase is attributed
to the formation of bulk heterojunction with increased interfacial area. The effect of the
incorporation of Se-SM molecule on the photovoltaic properties of copolymer P:PCBM
blend has been also investigated. The overall PCE of the copolymer P:PCBM:Se-SM device
is approximately 2.24%, which is further enhanced up to 3.16%, when the thermally
annealed blend is used. Therefore, we conclude that the Se-SM molecule increases the light
harvesting property of the blend and also provides efficient path for holes and electron in
copolymer and PCBM phases, respectively.
Keywords:
Low-band gap small molecule
Phenylenevinylene copolymer
Benzothiadiazole
Benzoselenadiazole
Organic photovoltaic solar cell
Bulk heterojunction
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
Power conversion efficiencies (PCEs), more than 5% have
been reported for BHJ cells, incorporating a blend of conju-
Conjugated polymer/fullerene bulk heterojunction
(BHJ) solar cells have received increased attention because
of their promising energy conversion efficiencies, solution
processability, and their low cost of fabrication [1–14].
gated polymer poly(3-hexylthiophene) (P3HT) as donor
and fullerene derivatives as electron acceptor [5–14]. How-
ever, to achieve PCE values higher than 10%, nanoscale
morphology of blend, transport properties in the device,
band gap, band alignment, and device architecture must
be optimized [4,15,16].
Since P3HT (with a band gap of about 1.9 eV) only ab-
sorbs up to 22% of the incident photons of solar spectrum
[17], low-band-gap materials were expected to harvest
more incident photons to further improve the efficiency
of organic solar cells. However, BHJ devices made from
* Corresponding authors. Address: Physics Department, Molecular
Electronic and Optoelectronic Device Laboratory, JNV University, Jodhpur
(Raj.) 342005, India. Tel.: +91 0291 2720857; fax: +91 0291 2720856
(G.D. Sharma), tel.: +30 2610 997115; fax: +30 2610 997118
(J.A. Mikroyannidis).
E-mail addresses: mikroyan@chemistry.upatras.gr (J.A. Mikroyanni-
dis), sharmagd_in@yahoo.com (G.D. Sharma).
1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2009.11.010

312
J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) 311–321
these materials with PCBM are usually show low power
conversion efficiency (<1%), with only a few of them
demonstrated efficiencies in the range of 3–6%
[18–27,9,28–35]. Most of them suffer from either (a) un-
matched energy levels with respect to PCBM and thereby
a low open circuit voltage (Voc) or (b) low hole mobility
and un-optimized morphology, leading to a low short cir-
cuit current (Jsc) and a small fill factor (FF).
Recently, several low-band gap conjugated polymers
(Eg < 1.8 eV) have been employed as electron donor
materials in BHJ solar cells [36,32]. Among these, benzo-
thiadiazole-based copolymers were the most successful,
showing a PCE in the range of ꢀ1.0–5.5% [22,24,25,32].
The reasons for such high performance might be ascribed
to the material properties, such as high hole mobility,
structural stability in the charged state, as well as a broad
absorption spectrum (Eg ꢀ1.38 eV) [22]. Moreover, a possi-
ble route to overcome the low photovoltaic performance
observed in a polymer solar cell may be the incorporation
of a low-band gap small molecule [37,38]. Small molecules
generally have high charge carrier mobility, monodisper-
sity and relatively simple synthesis [39].
Color tuning to the deep-red and near-infrared (NIR) re-
gion in conjugated polymers can be also achieved by incor-
porating low-band gap Se-containing comonomer into the
polymer backbone. Cao et al. first reported a polyfluorene
copolymer with Se-containing heterocycle, benzoselen-
adiazole (BSeD), a selenium analogue of benzothiadiazole
in different compositions [40]. He found out that incorpo-
rating Se-containing heterocycle (BSeD) into the polyfluo-
rene main chain resulted in a significant red shift in
comparison with its sulfur analogue. He also reported the
synthesis and properties of the polyfluorene copolymer
with another Se-containing heterocycle, 2,1,3-naphthose-
lenadiazole as a narrow band gap component [41]. Finally,
Cao et al. reported the synthesis of two conjugated Se-con-
taining heterocyclic monomers and their copolymerization
with 9,9-dioctylfluorene. The copolymers derived, which
had optical band gap of ꢀ1.80 eV, were used for BHJ solar
cells with PCBM and gave energy conversion efficiency
up to 1% [42]. These results indicate that synthesizing Se-
containing heterocycles with a narrower band gap moves
the absorption band edge to a further deep-red or even
NIR region without using the metallic complex ions.
Very recently, we have shown that the addition of a
low-band gap benzothiadiazole-based small molecule in
a conjugated phenylenevinylene copolymer:PCBM bulk
heterojunction solar cell improves the photocurrent and
power conversion efficiency [43]. In extension of this re-
search line, we have synthesized a new low-band gap
benzoselenadiazole-based small molecule (Se-SM) and
incorporated it with a novel phenylenevinylene copolymer
(P) for BHJ solar cells. The phenylenevinylene copolymer is
a dithyenyl-dinitobenzothiadiazole derivative which was
successfully synthesized by Heck coupling. The structures
of Se-SM and P were designed so that the energy levels
of the Se-SM lie between those of P and PCBM, in order
to avoid a charge trapping in the small molecule. Both sam-
ples contain nitro groups in the terminal or the central
units to broadening their absorption band [43,44]. More-
over, Se-SM contains two cyano-substituted vinylene
bonds. The very strong electron withdrawing effect of the
cyano group significantly affects the charge separation
and charge transfer processes as well as the band gap en-
ergy of the molecule [45]. Copolymer P carries solubilizing
hexyloxy side chains. The current–voltage characteristics
and the incident photon to current efficiency (IPCE) spectra
of the devices based on either copolymer P or Se-SM reveal
that both materials behave as p-type organic semiconduc-
tors. The PCE of the devices based on blends with PCBM is
about 0.52% and 1.30 for copolymer P and Se-SM, respec-
tively. We have shown that the addition of the Se-SM small
molecule in the copolymer P:PCBM bulk heterojunction so-
lar cell increases the efficiency of the device, which is fur-
ther improved up 3.16%, when thermally annealed
P:PCBM:Se-SM blend is used as active layer in the device.
2. Experimental
2.1. Characterization methods
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 spectro-
grade THF. TGA was performed on a DuPont 990 thermal
analyzer system. Ground samples of about 10 mg each were
examined by TGA and the weight loss comparisons were
made between comparable specimens. Dynamic TGA mea-
surements were made at a heating rate of 20 °C/min in
atmospheres of N2 at a flow rate of 60 cm3/min. Thermome-
chanical 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 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 pow-
der of sample for 3 min under 8 kp/cm2 at ambient temper-
ature. The Tg 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. Elemental anal-
yses were carried out with a Carlo Erba model EA1108
analyzer.
2.2. Reagents and solvents
1,4-Divinyl-2,5-bis(hexyloxy)-benzene was prepared
by Stille coupling reaction of 1,4-dibromo-2,5-bis(hexyl-
oxy)-benzene with tributylvinyltin [46]. N,N-Dimethyl-
formamide (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.
2.3. Synthesis of compounds and copolymer
2.3.1. Compound 4
A flask was charged with a solution of 3 (0.38 g,
1.09 mmol) in 1,2-dichloroethane (30 mL). DMF (0.26 g,

J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) 311–321
313
3.61 mmol) and POCl3 (0.55 g, 3.61 mmol) are added drop-
2.4. Electrochemical characterization
wise and the mixture was refluxed for 17 h under N2. After
cooling to room temperature and addition of dichloro-
methane (20 mL) and a saturated aqueous solution of so-
dium acetate (30 mL), the mixture was stirred for 2 h at
room temperature. The organic phase was then washed
with water and dried over MgSO4. Solvent removal and
column chromatography (silica gel/dichloromethane) gave
4 (0.27 g, 61%).
The position of the HOMO and LUMO energy levels
were estimated by cyclic voltammetry measurements. This
was performed on an Autolab Potentiostat PGSTAT-10,
equipped with a three electrode system, in an acetonitrile
solution (0.1 M) of 4-tert-butylpyridine at a potential scan
of 0.05 V/s at room temperature. Three electrode systems
were composed of a glassy carbon electrode coated with
a thin film of copolymer P, Se-SM and PCBM, Pt and Ag
electrode. The electrochemical potential was calibrated
against ferrocene/ferrocenium (Fc/Fc+).
FT-IR (KBr, cmꢁ1): 1630, 1588, 1412, 1338, 1266, 1160,
1074, 1016, 908, 780, 670, and 614.
1H NMR (CDCl3) ppm: 9.90 (s, 2H, CHO); 7.80 (m, 2H,
benzoselenadiazole); 7.23 (m, 4H, thiophene).
Anal. Calcd. for C16H8N2O2S2Se: C, 47.65; H, 2.00, N,
6.95. Found: C, 46.20; H, 1.74, N, 6.37.
2.5. Device fabrication and characterization
For the photovoltaic device fabrication, indium tin oxi-
des (ITO) coated glass substrates were cleaned with deter-
gent, de-ionized water and acetone and then dried at room
temperature. The PEDOT:PSS was spin coated upon the ITO
coated substrates and then dried at a temperature of 60 °C.
Then the active layers copolymer P, Se-SM, P:PCBM (1:1),
Se-SM:PCBM (1:1) and P:Se-SM:PCBM (1:0.5:1) were spin
coated on PEDOT:PSS coated ITO substrates from a THF at
1500 rpm. The thickness of the photoactive layers is about
100 nm. Finally, the Al electrode was thermally evaporated
on the top of the photoactive layer under vacuum
(ꢀ10ꢁ5 Torr) to make the sandwiched structure ITO/PED-
OT:PSS/organic photoactive layer/Al. The effective area of
the devices is about 25 mm2. For the thermal annealing
of photoactive layer, photoactive layer deposited on the
PEDOT:PSS coated ITO substrate was placed on a hot plate
at 100 °C for 10 min and then cooled up to room tempera-
ture, before the final Al electrode was deposited. Current–
voltage characteristics of the devices were recorded with a
Keithley electrometer with a built-in power supply in the
dark and under a white light illumination intensity of
100 mW/cm2. The 100 W halogen lamp calibrated for 1.5
AM sunlight spectrum was used as light source. The inten-
sity of the lamp was calibrated and adjusted using a energy
meter equipped with silicon detector. The incident photon
to current efficiency (IPCE) of the devices was measured by
illuminating the device with a halogen lamp using a mono-
chromator and measuring the resulting photocurrent with
a Keithley electrometer under short circuit conditions. All
the fabrications and characterization were performed in
an ambient environment.
2.3.2. Compound Se-SM
A flask was charged with a solution of 4 (0.75 g,
1.09 mmol) and 4-nitrobenzyl cyanide (0.35 g, 2.18
mmol) in anhydrous ethanol (20 mL). Sodium hydroxide
(0.20 g, 5.00 mmol) which was dissolved in anhydrous
ethanol (20 mL) was added portionwise to the stirred
solution. The mixture was stirred for 1 h at room tem-
perature under N2 and then was concentrated under
reduced pressure. Compound Se-SM precipitated as a
dark-green solid by concentration and subsequent cool-
ing into a refrigerator. It was filtered off, washed thor-
oughly with water and dried. The crude product was
purified by dissolution in THF and precipitation in meth-
anol (0.43 g, 57%).
FT-IR (KBr, cmꢁ1): 2154, 1654, 1586, 1522, 1440, 1344,
1288, 1184, 1106, 1016, 854, 702.
1H NMR (CDCl3) ppm: 8.15 (m, 4H, phenylene ortho to
nitro); 7.81 (m, 2H, benzosenadiazole); 7.70 (s, 2H, ole-
finic); 7.46 (m, 4H, phenylene meta to nitro); 7.22 (m,
4H, thiophene).
Anal. Calcd. for C32H16N6O4S2Se: C, 55.57; H, 2.33, N,
12.15. Found: C, 54.73; H, 2.12, N, 11.85.
2.3.3. Copolymer P
A flask was charged with a mixture of 9 (0.4887, 0.891
mmol) 1,4-divinyl-2,5-bis(hexyloxy)benzene (0.2944 g,
0.891 mmol), Pd(OAc)2 (0.0083 g, 0.037 mmol), P(o-tolyl)3
(0.0624 g, 0.205 mmol), DMF (5 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 dissolution in THF and
precipitation in methanol (0.46 g, 72%).
3. Results and discussion
3.1. Synthesis and characterization of small molecule Se-SM
FT-IR (KBr, cmꢁ1): 2956, 2926, 2868, 1592, 1538, 1464,
1378, 1270, 1210, 1156, 1064, 962, 874, and 702.
1H NMR (CDCl3) ppm: 7.33 (m, 4H, olefinic); 7.08 (m,
4H, thiophene); 7.04 (m, 2H, phenylene); 3.94 (m, 4H,
OCH2(CH2)4CH3); 1.80 (m, 4H, OCH2CH2(CH2)3CH3); 1.33
(m, 12H, O(CH2)2(CH2)3CH3); 0.92 (t, J = 7.7 Hz, 6H,
O(CH2)5CH3).
The Se-containing small molecule (Se-SM) was synthe-
sized by a five-step reaction sequence which is outlined
in Scheme 1. Specifically, compounds 1 [47], 2 [48] and 3
[48] were synthesized according to reported methods.
Dialdehyde 4 was prepared by formylation of 3 using
DMF and POCl3. Finally, the condensation of 4 with 4-nitro-
benzyl cyanide in ethanol in the presence of NaOH afforded
Se-SM. It precipitated from the reaction mixture as a dark-
green solid and was purified by dissolution in THF and
Anal. Calcd. for C34H36N4O6S3: C, 58.94; H, 5.24, N, 8.08.
Found: C, 57.49; H, 5.10, N, 7.85.

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J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) 311–321
Scheme 1. Synthesis of small molecule Se-SM.
precipitation into methanol. Se-SM was soluble in THF,
dichloromethane, chloroform and other common organic
solvents. Thin films of Se-SM could be obtained from its
solution by spin-coating even though it was a small
molecule.
The FT-IR and 1H NMR spectra confirmed the chemical
structure of Se-SM (see Section 2). In particular, it showed
characteristic absorption bands at 2154 (cyano group);
1586, 1440 (aromatic); 1522, 1344 (nitro group); 1654
(C@C olefinic stretching vibration); 1288, 854, 702 (thio-
phene) and 1016 cmꢁ1 (cyano-substituted trans olefinic
bond). The 1H NMR spectrum of Se-SM displayed upfield
signals at 8.15 (phenylene ortho to nitro) and 7.81 (benz-
oselenadiazole). Finally, the olefinic, the aromatic meta to
nitro and the thiophene protons resonated at 7.70, 7.46
and 7.22 ppm, respectively.
Se-SM exhibited high thermal stability, which was com-
parable with that of heat-resistant materials, due to the ab-
sence of aliphatic moieties. In particular, it had
decomposition temperature (Td) of 517 °C, char yield (Yc)
at 800 °C of 76% in N2, by TGA, and glass transition temper-
ature (Tg) of 68 °C, by TMA (Table 1).
Table 1
Thermal, optical and electrochemical properties of Se-SM and P.
Sample
Se-SM
P
a
Td (°C)
517
76
68
650
640
1.67
413
57
56
422
426
2.31
b
Yc (%)
c
Tg (°C)
d
ka,max in solution (nm)
ka,max in thin film (nm)
Eogpt (eV)
d
e
Eonset (red) (V)
HOMO (eV)
Eonset (ox) (V)
LUMO (eV)
ꢁ1.15
ꢁ5.25
0.55
ꢁ1.95
ꢁ5.1
0.40
ꢁ3.55
ꢁ2.75
Eegl (eV)
f
1.70
2.35
3.2. Synthesis and characterization of copolymer P
The HOMO and LUMO energy levels were calculated according to the
equation: ꢁELUMO = Eonset (red) + 4.7 eV and ꢁEHOMO = Eonset (ox) + 4.7 eV,
where Eonset (ox) and Eonset (red) are the onset potentials for the oxidation
and reduction processes of sample thin films vs. Ag/Ag+ electrode.
Copolymer P was synthesized by a six-step reaction se-
quence which is presented in Scheme 2. Particularly, com-
pounds 5 [49], 6 [49], 7 [50], 8 [50] and 9 [51] were
synthesized according to the literature. The Heck coupling
of dibromide 9 with an equivalent amount of 1,4-divinyl-
2,5-bis(hexyloxy)benzene gave P. This reaction took place
in DMF utilizing palladium catalyst and triethylamine as
acid scavenger. The hexyloxy side chains enhanced the
solubility of P making it very soluble in common organic
solvents. The number average molecular weight (Mn) was
12,300, by GPC, with polydispersity of 2.6.
a
Decomposition temperature corresponding to 5% weight loss in N2
determined by TGA.
b
Char yield at 800 °C in N2 determined by TGA.
Glass transition temperature determined by TMA.
ka,max: the absorption maxima from the UV–vis spectra in THF solu-
c
d
tion or in thin film.
Eogpt: optical band gap determined from the absorption onset in thin
e
film.
Eegl: electrochemical band gap determined from cyclic voltammetry.
f
Eegl = ELUMO–EHOMO.
Scheme 2. Synthesis of copolymer P.

J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) 311–321
315
Fig. 1. 1H NMR spectrum of copolymer P in CDCl3 solution. The solvent peak is denoted by an asterisk.
The FT-IR spectrum of P showed characteristic absorp-
tions at 2956, 2926 (C–H stretching of aliphatic segments);
1592, 1492, 1464 (aromatic); 1538, 1378 (NO2); 1210,
1156 (ether bond); 1270, 874, 702 (thiophene) and
962 cmꢁ1 (trans olefinic bond). The 1H NMR spectrum
(Fig. 1) displayed multiplets at 7.33, 7.08 and 7.04 ppm as-
signed to the olefinic, thiophene and phenylene protons,
respectively. Finally, the aliphatic protons labelled ‘‘d”,
‘‘c”, ‘‘b” and ‘‘a” resonated at 3.94, 1.80, 1.33 and
0.92 ppm, respectively. The small peaks between 5 and
6 ppm are assigned to the terminal CH2 = CH– groups.
Fig. 2 presents the TGA and TMA traces of P. No weight
loss was observed up to ꢀ300 °C. The Td and Yc were 413 °C
and 57%, respectively (Table 1). Finally, the Tg, which was
determined by TMA, was 56 °C. P exhibited lower thermal
stability and Tg than Se-SM owing to the presence of the
hexyloxy side chains in the former.
Fig. 2. TGA thermogram of copolymer P in N2. The inset shows the TMA
trace of this copolymer. Conditions: N2 flow, 60 cm3/min; heating rate,
20 °C/min.
Fig. 3. Normalized UV–vis absorption spectra in solution and thin film of
small molecule Se-SM (top) and copolymer P (bottom).

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J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) 311–321
Fig. 4. Current–voltage characteristics in dark and under illumination for (a) ITO/PEDOT:PSS/copolymer P/Al and (b) ITO/PEDOT:PSS/Se-SM/Al devices.
3.3. Photophysical properties
Since the HOMO level of both the copolymer P (ꢁ5.1 eV)
and Se-SM (ꢁ5.25 eV) are very close to the Fermi level of
PEDOT:PSS (ꢁ5.0 eV), forms a potential barrier of about
0.10 and 0.25 eV for hole injection from PEDOT:PSS into
the HOMO of copolymer P and Se-SM, respectively, in their
devices. This results in the formation of nearly Ohmic con-
tact at ITO/PEDOT:PSS-organic layer. However, the Fermi
level of Al (ꢁ4.3 eV) is very far from the LUMO of copoly-
mer P (ꢁ2.8 eV) and Se-SM (ꢁ3.60 eV) thus forming a po-
tential barrier of 1.5 and 0.9 eV for electron injection
from Al to LUMO of the copolymer P and Se-SM, respec-
tively, resulting a Schottky barrier contact. Therefore, the
copolymer P behaves as p-type organic semiconductor.
We have estimated the hole mobility of copolymer P
and Se-SM using the space charge limited current (SCLC)
method [52]. The J–V characteristics of ITO/PEDOT:PSS/
copolymer P/Au and ITO/PEDOT:PSS/Se-SM/Au devices in
dark on log–log scale are shown in Fig. 5. It can be seen
from this figure that in low voltage region, the log J is lin-
early dependent on the log V with a slope of unity. In the
relatively high voltage region, the log J is also linearly
Fig. 3 presents the UV–vis absorption spectra of Se-SM
and P in both dilute (10ꢁ5 M) THF solution and thin film.
They were normalized with respect the long-wavelength
absorption peak. Table 1 summarizes all photophysical
characteristics of these samples.
Se-SM showed a broad absorption bands (Fig. 3a) which
were extended up to ꢀ750 nm with long-wavelength
absorption maximum (ka,max) at 650 for solution and
640 nm for thin film. This ka,max was attributed to an
intramolecular charge transfer between the central 4,7-
di-2-thienyl-2,1,3-benzoselenadiazole and the terminal
cyanovinylene 4-nitrophenyls. Similar absorption curves
were obtained from other related compounds which have
been synthesized in our laboratory [43,44]. The thin film
absorption onset of Se-SM was located at 741 nm which
corresponds to an optical band gap (Eogpt) of 1.67 eV. The
relatively low Eogpt value of Se-SM suggests that this small
molecule is attractive for photovoltaic applications.
Copolymer
P displayed similar absorption curves
(Fig. 3b) in solution and thin film which were extended
about up to 500 nm with ka,max at ꢀ420 nm. The thin film
absorption onset was at 536 nm corresponding to Eogpt of
2.31 eV.
3.4. Electrical and photovoltaic properties of copolymer P and
Se-SM
The current–voltage (J–V) characteristics of the ITO/
PEDOT:PSS/copolymer P/Al and ITO/PEDOT:PSS/Se-SM/Al
devices in dark and under illumination intensity of
100 mW/cm2 are presented in Fig. 4. In these curves, the
forward bias corresponds to the positive and negative volt-
age applied to ITO and Al electrode, respectively. It can be
seen from this figure that the J–V characteristics show rec-
tification effect. This can be explained by the high work
function of ITO coated PEDOT:PSS and p-type semi-con-
ductivity of both copolymer P and Se-SM. This indicates
that the ITO/PEDOT:PSS/ and Al form Ohmic and Schottky
barrier with both copolymer P and Se-SM, respectively.
Fig. 5. Current–voltage curves in for devices based on copolymer P and
Se-SM sandwiched between PEDOT:PSS coated ITO and Au electrodes in
log–log scale.

J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) 311–321
317
dependent on the log V with a slope of two. The J–V curves
in this region correspond to the trap free SCLC behavior.
We estimated the hole mobility of the corresponding
copolymer P and Se-SM, according to equation
mer P. The higher value of the hole mobility for the Se-
SM also contributes to the higher photocurrent, resulting
better charge transport in the device.
In order to get more information about the formation of
Schottky barrier, we measured the incident photon to cur-
rent efficiency (IPCE) of both devices. The values of IPCE
have been estimated using the following expression
2
9
8
ðV ꢁ VbiÞ
J ¼ ereolh
ð1Þ
d3
IPCEð%Þ ¼ 1240Jsc=kPin
ð2Þ
where V is the applied voltage, J is the current density, er
and eo are the relative dielectric constant and permittivity
where Jsc (mA/cm2) is the short circuit photocurrent, Pin
(W/m2) and k (nm) is the illumination intensity and wave-
length of the monochromatic light, respectively. The IPCE
spectra of the ITO/PEDOT:PSS/copolymer P/Al and ITO/PED-
OT:PSS/Se-SM/Al devices, illuminating through the Al and
ITO side are shown in Fig. 6a and b. It can be seen from
these figures that the shape of IPCE spectra of the devices,
illuminating through the Al side closely follow the absorp-
tion spectra of the organic materials used in the device as
photoactive layer. In organic photovoltaic devices, the pho-
tocurrent is due to the exciton generation upon the photo-
excitation and their subsequent dissociation into free
carrier at the Schottky barrier or p–n junction. Since the
exciton diffusion length in most of organic semiconductors
is in the range of 10–15 nm, the excitons that reaches the
interface, only contributes to the photocurrent. When the
device is illuminated through the Al electrode, photons cor-
responding to the absorption peaks of copolymer P and
Se-SM are absorbed near the Al/copolymer P and Al/Se-
SM in their respective devices due to the high absorption
of photoactive material, generate the excitons in this region
and subsequently dissociate into free carries due to the
built-in field at the interface, resulting IPCE. However, pho-
tons corresponding to the low absorption coefficient of the
organic materials, penetrate more into the bulk of the
material. As a result, they are absorbed far from the Scho-
ttky barrier and generate excitons there. Therefore, only
excitons that are generated within the distance 10–15 nm
from the Schottky barrier (Al–organic layer interface), are
able to reach the Schottky interface, contributes to the pho-
tocurrent. In this case only few excitons reach to the Scho-
ttky barrier and results low photocurrent in comparison to
earlier case. On the other case, when the devices are illumi-
of free space (8.85 ꢂ 10ꢁ12 F/m), respectively,
lh is the hole
mobility and d is the thickness of the organic layer and Vbi
is the built-in potential which is determined by the differ-
ence between the work function of the electrodes. Using
e
r = 3.5, d = 100 nm and Vbi = 0.8 eV and fitting the curve
in high voltage region with Eq. (1), the values of hole
mobility are 2.3 ꢂ 10ꢁ5 cm2/Vs and 3.4 ꢂ 10ꢁ4 for copoly-
mer P and Se-SM, respectively.
The photovoltaic parameters, i.e., short circuit current
(Jsc), open circuit voltage (Voc), fill factor (FF) and power
conversion efficiency (g) for the devices are listed in
Table 2. It can be seen from this table that the open circuit
voltage for both devices is almost the same and the Jsc for
the ITO/PEDOT:PSS/Se-SM/Al device is higher than that
for ITO/PEDOT:PSS/copolymer P/Al device. The Voc for a
Schottky barrier device is approximately equal to the work
function difference between the electrodes used. In these
devices we have used same electrodes and therefore the
Voc is quite similar. The higher value of the Jsc for the device
based on Se-SM is attributed to the broad band absorption
spectra and the low-band gap as compared to the copoly-
Table 2
Photovoltaic parameters of the devices using different photoactive layers
sandwiched between ITO/PEDOT:PSS and Al electrodes.
Photoactive
layer
Short circuit Open
Fill
Power
current (Jsc
)
circuit
factor conversion
(FF)
(mA/cm2)
voltage (Voc
)
efficiency
) (%)
(
g
P
0.07
0.25
1.30
0.64
0.74
0.93
0.98
0.31
0.38
0.43
0.49
0.014
0.07
0.52
1.30
Se-SM
P:PCBM
Se-SM:PCBM 2.7
Fig. 6. IPCE spectra of (a) ITO/PEDOT:PSS/Se-SM/Al and (b) ITO/PEDOT:PSS/copolymer P/Al devices, illuminating through ITO and Al electrodes.

318
J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) 311–321
nated through the ITO side, only a few of the photons are
able to reach the Schottky barrier, and therefore, cause lim-
ited generation of excitons leading to low photocurrent at
the wavelength where the absorption coefficient is low.
The excitons generated near to the Ohmic contact, i.e.,
ITO/PEDOT:PSS disappear due to recombination or trapping
during their transportation through the organic layer to-
wards the Schottky barrier. At the wavelengths where the
absorption of the active layer is minimum, the light can
penetrate deeper, allowing more excitons to be generated
near the Schottky barrier, leading to higher photocurrent.
interface between the donor and acceptor. It can be seen
from Table 2 that both short circuit current and open cir-
cuit voltage for the device using Se-SM:PCBM are higher
than that for fabricated with copolymer P:PCBM. The high-
er short circuit current is attributed to the low-band gap
and broad band absorption extending up to 800 nm of
Se-SM and higher hole mobility in comparison to that for
copolymer P, resulting to greater exciton generation in
the Se-SM phase and better hole transport in the device.
In BHJ devices, if both electrodes form the Ohmic con-
tact with the photoactive layer, the open circuit is mainly
governed by the difference between the LUMO level of
acceptor and HOMO level of the donor used in the device
[54,55]. The work function of PEDOT:PSS almost matches
the HOMO level of copolymer P or Se-SM, resulting in
nearly ohmic contact for holes in the their respective BHJ
devices, under the forward bias condition. On the other
hand, the Al makes nearly Ohmic contact electron injection
in the LUMO of PCBM. For this case the open circuit voltage
is given by [54]
3.5. Photovoltaic properties of copolymer P or Se-SM blended
with PCBM
The energy levels for the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbi-
tal (LUMO) of copolymer P and Se-SM were obtained by
cyclic voltammetry with the method described by Bredas
et al. [53] and are summarized in Table 1. The crucial factor
for the effective photoinduced charge transfer in bulk het-
erojunction is the difference between the LUMO level of
the donor and acceptor components used in the bulk het-
erojunction photoactive layer. This difference should be
larger than that of the exciton binding energy, i.e., 0.3 eV.
The difference in the LUMO levels of PCBM (acceptor)
and copolymer P or Se-SM is higher. Therefore, the combi-
nation of PCBM and copolymer P or Se-SM is suitable for
the photoactive layer to be used in a BHJ PV device.
Fig. 7 shows the J–V characteristics of the devices fabri-
cated with copolymer P:PCBM and Se-SM:PCBM blends
under the illumination intensity 100 mW/cm2. The photo-
voltaic parameters of both devices are given in Table 2. The
values of the power conversion efficiency of both devices
are higher than those obtained for the devices fabricated
with pristine copolymer P and Se-SM. This indicates that
a BHJ is formed in the blend that increases the interfacial
qðVoc
ꢁ
DVbÞ ¼ ðHOMOÞdonor ꢁ ðLUMOÞacceptor
ð3Þ
where
D
Vb is the sum of the voltage losses at each contact
due to the band bending. The observed experimental val-
ues of Voc for both devices are in agreement with Eq. (3).
Since the position of the HOMO level for Se-SM
(ꢁ5.25 eV) is more negative than that for copolymer P
(ꢁ5.1 eV), results higher value of Voc for device employing
Se-SM:PCBM blend as compared to the device with copoly-
mer P:PCBM blend.
3.6. Effect of incorporation of Se-SM in the copolymer P:PCBM
blend
The photovoltaic performance of the photovoltaic de-
vices using copolymer:PCBM and Se-SM:PCBM blends is
low. A possible route to overcome this low performance
Fig. 7. Current–voltage characteristics of devices fabricated with copolymer:PCBM and Se-SM:PCBM blends under illumination.

J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) 311–321
319
P:PCBM blend, we have measured the IPCE spectra of the
devices which are shown in Fig. 9. In the case of photovol-
taic device based on blend without Se-SM, the IPCE peak is
only in the region of 380–500 nm corresponding to the
absorption spectrum of copolymer P. The increase in the
IPCE of device with copolymer P:PCBM (Fig. 9b) upon ther-
mal annealing is attributed to the increase in crystalliza-
tion of the copolymer. On the other side, the device with
copolymer P:PCBM:Se-SM exhibits two IPCE peaks appear-
ing in the wavelength region of copolymer P absorption
and also in the longer wavelength region of Se-SM
absorption.
When the Se-SM is incorporated in copolymer P:PCBM
blend, Se-SM may be located between (i) copolymer P
and PCBM phases, (ii) PCBM phases and (iii) copolymer
phases. The energy level diagram of the copolymer P,
Se-SM, and PCBM is shown in Fig. 10. As shown in this
figure, the excitons generated in Se-SM, when Se-SM is lo-
cated between copolymer P and PCBM, inject electrons into
PCBM and holes into copolymer P because the HOMO level
of Se-SM is more negative than the HOMO level of copoly-
mer P. However, when Se-SM is located at PCBM domain,
Se-SM can only inject electrons into the PCBM domain
but cannot inject holes into surrounding PCBM domain be-
cause of more negative HOMO level of PCBM. Similarly,
when Se-SM is located between the copolymer P domains
only holes are injected into the surrounding copolymer P.
Therefore, only the Se-SM molecule, which is located be-
tween the copolymer P and PCBM, can contribute to the
photocurrent.
Fig. 8. Current–voltage characteristics of ITO/PEDOT:PSS/copolymer
P:PCBM:Se-SM/Al devices.
might be to incorporate a low-band gap small molecule do-
nor in the blend of polymer P:PCBM [56,57] blend with en-
ergy levels intermediate to those polymer and PCBM, in
order to avoid the charge trapping in the small molecule.
Since the energy levels of the Se-SM are located between
the energy levels of copolymer P and PCBM, we have incor-
porated Se-SM in the copolymer P:PCBM blend to increase
the power conversion efficiency.
Fig. 8 shows the J–V characteristics of the devices based
on as cast and annealed copolymer P:PCBM:Se-SM blends
under illumination intensity of 100 mW/cm2. The photo-
voltaic parameters of the devices are summarized in
Table 3. The incorporation of Se-SM in the blend improves
both the short circuit current (Jsc) and the open circuit volt-
age (Voc). However, the Voc decreases when the annealed
photoactive blend is used. This may be due to the increase
in the crystalline nature of the copolymer that results in a
decrease of Voc because of the rise of HOMO level of the
copolymer P. The rise in the HOMO level occurs as a result
of increased inter-chain orbital delocalization [58]. The in-
crease in Jsc with the addition of Se-SM is a result of in-
creased absorption at the longer wavelength region,
which is absent in the case of the copolymer P:PCBM blend.
The absorption at the wavelength region leads increased
exciton generation in the blend, and their subsequent dis-
sociation into free charge carriers at the donor–acceptor
(D–A) interfaces formed in the blend. Since Se-SM also acts
as a donor, its incorporation also increases number of D–A
interfaces in the blend that also increases the photoin-
duced charge separation.
Fig. 9. IPCE spectra of the photovoltaic devices based on different blends
(a) copolymer P:PCBM (as cast), (b) copolymer P:PCBM (annealed), (c)
copolymer P:PCBM:Se-SM (as cast) and (d) copolymer P:PCBM:Se-SM
(annealed).
In order to get more information about the origin of the
increase in Jsc upon the addition of Se-SM in the copolymer
Table 3
Effect of Se-SM incorporation in the copolymer P:PCBM blend on photovoltaic parameters.
Photoactive layer
Short circuit current
Open circuit voltage
(Voc) (V)
Fill factor (FF)
Power conversion
efficiency (g) (%)
(Jsc) (mA/cm2)
P:PCBM:Se-SM (as cast)
P:PCBM:Se-SM (annealed)
4.1
6.1
1.05
0.92
0.52
0.54
2.24
3.16

320
J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) 311–321
Fig. 10. Energy level diagram of Se-SM and surrounding materials: (a) Se-SM molecule is located between copolymer P and PCBM and (b) Se-SM molecule
located in between two PCBM domains.
As can be seen from the IPCE spectra of the devices, the
IPCE value at 640 nm increases from 48% to 70%, when the
thermally annealed copolymer P:PCBM:Se-SM blend is
used. However, it was observed that the optical absorption
spectra do not change upon the thermal annealing. The in-
crease in the IPCE may be attributed to the enhanced
charge collection efficiency, which is improved by the for-
mation of more crystalline nature interpenetrating net-
work, upon thermal annealing. In addition to that, the
number of the Se-SM molecules in between copolymer P
and PCBM may increase upon thermal annealing because
of the increased crystalline nature of the conjugated poly-
mer and aggregation of the nano-crystalline PCBM domain.
The Se-SM molecules located in the copolymer P or PCBM
domain may be expelled from these domains into the
interface between copolymer P and PCBM, upon thermal
annealing. The combined effect of additional absorption
at longer wavelength region by Se-SM molecules and Se-
SM molecules located in between conjugated copolymer
P and PCBM results the improvement in the Jsc, thereby in-
creases the power conversion efficiency.
copolymer P or Se-SM and PCBM in their respective de-
vices. The effect of the incorporation of the Se-SM on the
photovoltaic properties of vinylene copolymer P:PCBM so-
lar cell was investigated. The broader absorption of Se-SM
molecules at the longer wavelength region effectively har-
vests the photon in this region. Moreover, the Se-SM mol-
ecules located between the copolymer
P and PCBM
contribute to the enhancement in the power conversion
efficiency due to the effective dissociation of the generated
excitons in the copolymer P phase at shorter wavelength
region. A further improvement in the power conversion
efficiency (3.16%) of the device based on thermally an-
nealed copolymer P:PCBM:Se-SM is attributed to the in-
crease in the number of Se-SM molecules in between
copolymer P and PCBM domain due to increased crystalli-
zation of the copolymer P.
Acknowledgement
P. Suresh is grateful of council for Scientific and Indus-
trial research (CSIR), Govt. of India for awarding Senior
Research fellowship. We are thankful to Dr. M.S. Roy,
Scientist and other members in his group, Defence Labora-
tory, Jodhpur for providing the cyclic voltammetry (CV)
measurement facility and fruitful discussions.
4. Conclusions
A new low-band gap Se-containing small molecule (Se-
SM) and a phenylenevinylene copolymer (P) based on dit-
hyenyl-dinitrobenzothiadiazole were synthesised and used
for BHJ solar cells. The current–voltage characteristics in
dark and IPCE spectra of devices using these materials
show that both copolymer P and Se-SM behave as p-type
organic semiconductor forming Ohmic and Schottky bar-
rier with ITO coated PEDOT:PSS and Al electrodes, respec-
tively. The power conversion efficiency of the devices
based on Se-SM and copolymer P is low. However, both
materials can be used as donors for bulk heterojunction
with PCBM due the suitable position of their LUMO level.
However, when P or Se-SM blended with PCBM, the power
conversion efficiency was significantly improved resulted
from the formation of bulk heterojunction between
References
[1] J.J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend,
S.C. Moratti, A.B. Holmes, Nature 376 (1995) 498.
[2] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995)
1789.
[3] B.A. Gregg, J. Phys. Chem. B 107 (2003) 4688.
[4] K.M. Coakley, M.D. McGehee, Chem. Mater. 16 (2004) 4533.
[5] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang,
Nat. Mater. 4 (2005) 864.
[6] W. Ma, C. Yang, Y. Gong, K. Lee, A.J. Heeger, Adv. Funct. Mater. 15
(2005) 1617.
[7] M. Reyes-Reyes, K. Kim, D.L. Carroll, Appl. Phys. Lett. 87 (2005)
083506.

J.A. Mikroyannidis et al. / Organic Electronics 11 (2010) 311–321
321
[8] Y. Kim, S. Cook, S.M. Tuladhar, S.A. Choulis, J. Nelson, J.R. Durrant,
D.D.C. Bradley, M. Giles, I. Mcculloch, C.-S. Ha, M. Ree, Nat. Mater. 5
(2006) 197.
[9] W.-Y. Wong, X.-Z. Wang, Z. He, A.B. Djurisic, C.-T. Yip, K.-Y. Cheung,
H. Wang, C.S.K. Mak, W.-K. Chan, Nat. Mater. 6 (2007) 521.
[10] S. Gunes, H. Neugebauer, N.S. Sariciftci, Chem. Rev. 107 (2007) 1324.
[11] P.W.M. Blom, V.D. Mihailetchi, L.J.A. Koster, D.E. Markov, Adv. Mater.
19 (2007) 1551.
[34] Y. Liang, Y. Wu, D. Feng, S.-T. Tsai, H.-J. Son, G. Li, L. Yu, J. Am. Chem.
Soc. 131 (2009) 56.
[35] J. Mei, K. Ogawa, Y.-G. Kim, N.C. Heston, D.J. Arenas, Z. Nasrollahi,
T.D. McCarley, D.B. Tanner, J.R. Reynolds, K.S. Schanze, ACS Appl.
Mater. Interfaces 1 (2009) 150.
[36] Y. Li, Y. Zou, Adv. Mater. 20 (2008) 2952.
[37] W.J. Belcher, K.I. Wagner, P.C. Dastoor, Sol. Energy Mater. Sol. Cells
91 (2007) 447.
[12] X. Yang, J. Loos, Macromolecules 40 (2007) 1353.
[13] A.C. Mayer, S.R. Scully, B.E. Hardin, M.W. Rowell, M.D. McGehee,
Mater. Today 10 (2007) 28.
[14] B.C. Thompson, M.J. Fréchet, Angew. Chem., Int. Ed. 47 (2008) 58.
[15] L.J.A. Koster, V.D. Mihailetchi, P.W.M. Blom, Appl. Phys. Lett. 88
(2006) 093511.
[16] M.C. Scharber, D. Muehlbacher, M. Koppe, P. Denk, C. Waldauf, A.J.
Heeger, C.J. Brabec, Adv. Mater. 18 (2006) 789.
[17] E. Bundgaard, F.C. Krebs, Sol. Energy Mater. Sol. Cells 91 (2007) 954.
[18] F. Zhang, K.G. Jespersen, C. Björström, M. Svensson, M.R. Andersson,
V. Sundström, K. Magnusson, E. Moons, A. Yartsev, O. Inganäs, Adv.
Funct. Mater. 16 (2006) 667.
[19] L.M. Andersson, F. Zhang, O. Inganas, Appl. Phys. Lett. 91 (2007)
071108.
[20] L.H. Slooff, S.C. Veenstra, J.M. Kroon, D.J.D. Moet, J. Sweelssen, M.M.
Koetse, Appl. Phys. Lett. 90 (2007) 143506.
[21] A. Gadisa, W. Mammo, L.M. Andersson, S. Admassie, F. Zhang, M.R.
Andersson, O. Inganas, Adv. Funct. Mater. 17 (2007) 3836.
[22] D. Mühlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller, R.
Gaudiana, C. Brabec, Adv. Mater. 18 (2006) 2884.
[23] Z. Zhu, D. Waller, R. Gaudiana, M. Morana, D. Muhlbacher, M.
Scharber, C. Brabec, Macromolecules 40 (2007) 1981.
[24] J. Peet, J.Y. Kim, N.E. Coates, W.L. Ma, D. Moses, A.J. Heeger, G.C.
Bazan, Nat. Mater. 6 (2007) 497.
[25] N. Blouin, A. Michaud, M. Leclerc, Adv. Mater. 19 (2007) 2295.
[26] N. Blouin, A. Michaud, D. Gendron, S. Wakim, E. Blair, R. Neagu-Plesu,
M. Belletete, G. Durocher, Y. Tao, M. Leclerc, J. Am. Chem. Soc. 130
(2008) 732.
[27] S.H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J.S. Moon, D. Moses, M.
Leclerc, K. Lee, A.J. Heeger, Nat. Photon. 3 (2009) 297.
[28] L. Liu, C.-L. Ho, W.-Y. Wong, K.-Y. Cheung, M.-K. Fung, W.-T. Lam, A.B.
Djurisic, W.-K. Chan, Adv. Funct. Mater. 18 (2008) 2824.
[29] P.-T. Wu, T. Bull, F.S. Kim, C.K. Luscombe, S.A. Jenekhe,
Macromolecules 42 (2009) 671.
[30] M.M. Wienk, M. Turbiez, J. Gilot, R.A.J. Janssen, Adv. Mater. 20 (2008)
2556.
[31] C.-P. Chen, S.-H. Chan, T.-C. Chao, C. Ting, B.-T. Ko, J. Am. Chem. Soc.
130 (2008) 12828.
[32] E. Wang, L. Wang, L. Lan, C. Luo, W. Zhuang, J. Peng, Y. Cao, Appl.
Phys. Lett. 92 (2008) 033307.
[33] J. Hou, H.-Y. Chen, S. Zhang, G. Li, Y. Yang, J. Am. Chem. Soc. 130
(2008) 16144.
[38] A. Ltaief, R.B. Chaabane, A. Bouazizi, J. Davenas, Mater. Sci. Eng. C 26
(2006) 344.
[39] M.T. Lloyd, J.E. Anthony, G.G. Malliaras, Mater. Today 10 (2007) 34.
[40] R. Yang, R. Tian, W. Yang, Q. Hou, Y. Cao, Macromolecules 36 (2003)
7453.
[41] J. Yang, C. Jiang, Y. Zhang, R. Yang, W. Yang, Q. Hou, Y. Cao,
Macromolecules 37 (2004) 1211.
[42] R. Yang, R. Tian, J. Yan, Y. Zhang, J. Yang, Q. Hou, W. Yang, C. Zhang, Y.
Cao, Macromolecules 38 (2005) 244.
[43] P. Suresh, P. Balraju, G.D. Sharma, J.A. Mikroyannidis, M.M.
Stylianakis, ACS Appl. Mater. Interfaces 1 (2009) 1370.
[44] J.A. Mikroyannidis, M.M. Stylianakis, P. Balraju, P. Suresh, G.D.
Sharma, ACS Appl. Mater. Interfaces 1 (2009) 1711.
[45] B.C. Thompson, Y.-G. Kim, T.D. McCarley, J.R. Reynolds, J. Am. Chem.
Soc. 128 (2006) 12714.
[46] Q. Peng, M. Li, X. Tang, S. Lu, J. Peng, Y. Cao, J. Polym. Sci. Part A:
Polym. Chem. 45 (2007) 1632.
[47] K. Klaus, J. Chem. Soc., Perkin Trans. 1: Org. Bio-Org. Chem. (7)
(1972–1999) 1939–1943 (1988).
[48] X. Li, W. Zeng, Y. Zhang, Q. Hou, W. Yang, Y. Cao, Eur. Polym. J. 41
(2005) 2923.
[49] F.S. Mancilha, B.A. DaSilveira Neto, A.S. Lopes, P.F. Moreira Jr., F.H.
Quina, R.S. Gonçalves, J. Dupont, Eur. J. Org. Chem. 21 (2006) 4924.
[50] E. Perzon, X. Wang, S. Admassie, O. Inganas, M.R. Andersson, Polymer
47 (2006) 4261.
[51] E. Perzon, F. Zhang, M. Andersson, W. Mammo, O. Inganas, M.R.
Andersson, Adv. Mater. 19 (2007) 3308.
[52] P.W.M. Blom, M.J.M. de Jong, M.G. van Munster, Phys. Rev. B 55
(1997) R656.
[53] J.L. Bredas, R. Silbey, D.S. Boudreux, R.R. Chance, J. Am. Chem. Soc.
105 (1983) 6555.
[54] V.D. Mihailetchi, P.W.M. Blow, J.C. Hummelen, M.T. Rispend, J. Appl.
Phys. 94 (2003) 6849.
[55] K. Kawano, N. Ito, T. Nishimori, J. Sakai, Appl. Phys. Lett. 88 (2006)
07354.
[56] A. Ltaief, R.B. Chaabane, A. Bouazizi, J. Davenas, Mater. Sci. Eng. C 129
(2006) 344.
[57] A.B. Tamayo, B. Walker, T.Q. Nguyen, J. Phys. Chem. C 112 (2008)
11545.
[58] K. Vandewal, A. Gadisa, W.D. Oosterbaan, J.V. Mance, Adv. Funct.
Mater. 18 (2008) 2064.