Materials Sciences and Applicatio n, 2011, 2, 1014-1021
doi:10.4236/msa.2011.28137 Published Online August 2011 (http://www.SciRP.org/journal/msa)
Synthesis and Properties of Polythiophène
Benzylidene and Their Photovoltaic Applications
Reguig Bendoukha Abdelkarim1,3, Ahmed Yahiaoui1*, Aïcha Hachemaoui1, Mohammed Belbachir2，
Abdelbasset Khelil3
1Laboratoire de chimie Organique, Macromoléculaire et des Matériaux (LCOMM), Université de Mascara, Faculté des Sciences
et de la Technologie, Mascara, Algérie; 2Laboratoire de Chimie des Polymères, Université d’Oran, Oran, Algérie; 3LPCM2E,
Universite´ d’Oran Es-Senia, Algérie.
Received March 19th, 2011; revised April 11th, 2011; accepted May 9th, 2011.
ABSTRACT
Research on organic solar cells has a craze importance because they show very interesting properties including their
flexibility and the opportunity to be made into large surfaces. However, their stability and performance should be sig-
nificantly improved compared to their current state. A nominal output of around 10% will be the goal for the coming
years. The use of organic materials for photovoltaic applications is the subject of intense research in recent years. This
work is based in part on the development of new conjugated polymers. In this paper, we present the synthesis and
characterization of poly [(thiophene-2,5-diyl)-co-(benzylidene)] PTB catalysed by Maghnite-H+, used in the active layer
of the solar cell organic heterojunction with PCBM (derivative of C60) was used as a junction of the solar cell:
Glas/ITO/BCP/C60/PTB/Au/Al. A current density of short circuit of about Jcc 0.1mA/cm² was obtained for this struc-
ture with a yield of around 0.15%.
Keywords: Polymerization, Conjugated Polymers, UV-Vis Spectroscopy, IR Spectroscopy, Yield Calculation, Solar Cell
Organic Heterojunction.
1. Introduction
In the past few decades, conjugated polymers with an
extended p-conjugation have received considerable at-
tention due to their electronic and photonic applications,
such as light-emitting diodes [1,2], photovoltaic cells
(PVCs) [3,4], and thin film transistors [5,6]. Polythio-
phenes (PTs) are the most promising conjugated poly-
mers because of their relatively high charge carrier mo-
bility and long wavelength absorption in comparison
withother conjugated polymers [7]. For example, regio-
regular poly(3-hexylthiophene) (P3HT) have exhibited
excellent properties in PVCs [8,9]. However, P3HT only
absorbs a part of the visible light and exhibits the rela-
tively low open-circuit voltage (Voc) [10]. To further
improve the related properties and explore the full poten-
tial applications of these materials, chemical modifica-
tions of PTs have been performed actively. One success-
ful strategy to achieve broader absorption of PTs is based
on the pioneering work by Li and coworkers [11,12].
They synthesized PTs with conjugated bi(thienyleneviny-
lene) as side chain and made the absorption band in the
region from 350 nm to 650 nm, resulting in a good power
conversion efficiency (PCE) of 3.18%. PTs with sub-
stituents other than alkyl groups have also been investi-
gated, among which those with electrondonating alkoxy
groups have displayed promising optical properties [13].
Actually, the PCE of polymer photovoltaic devices is
determined by three main factors: the efficiency of exci-
ton generation, the efficiency of exciton dissociation into
free charge carriers, and the efficiency of their unhin-
dered collection by the electrodes. To increase the device
efficiency, the active layer should absorb as many of the
incident photons as possible to generate a maximum of
excitons [14,15], therefore, low band gap polymers are
necessary since the absorption of the active layer should
match the solar spectrum well. The factors that influence
the bandgap of a polymer are conjugated length, solid
state ordering, and the presence of electron withdrawing
or -donating moieties. The effective conjugated length,
which is dependent upon the torsion angle between the
repeating units along the polymer backbone, can be con-
trolled by choosing sterically hindered units along the
polymer back-chain or by introducing bulky side-chains

Synthesis and Properties of Polythiophène Benzylidene and Their Photovoltaic Applications1015
to twist the units out of plane [16,17]. While, the Voc of
a PVCs based on polymer and (6,6)-phenyl C61-butyric
acid methyl ester (PCBM) blend system is determined by
the difference between the HOMO of the polymer and
lowest unoccupied molecular orbital (LUMO) of PCBM
[18,19]. Therefore, the HOMO level is also an important
parameter to be considered when designing a new elec-
tron-donating polymer.
Triphenylamine (TPA) is a preferred electrondonating
moiety with excellent hole transporting properties and
their derivatives have been widely investigated for al-
most two decades [20,21]. Owing to the noncoplanarity
of the three phenyl substituents, TPA derivatives can be
viewed as 3D systems and the amorphous character of
these materials offers possibilities to developactive mate-
rials for PVCs with isotropic optical and charge-transport
properties [22]. Recently, Roncali, J. and coworkers [23]
synthesized various series of star-shaped molecules
based on TPA small molecules with combinations of
thienylenevinylene conjugated branches and lectron-
withdrawing indanedione or dicyanovinyl groups, which
have been applied to organic PVCs as donor materials
and got PCE of 1.20%.To our knowledge, the application
of TPA-based polymers for photovoltaic devices has
been scarcely considered.
Here, we report the synthesis and photovoltaic proper-
ties of a new conjugated polymer derived from PT, poly
[(thiophene-2,5-diyl)-co-(benzylidene)] (PTB) a novel
soluble obtained in a single step polymerization through
a very simple acid-catalyzed condensation of thiophene
and benzaldehyde in the presence of an exchanged clay
montmorillonite called Mag-H as catalyst. The PTB was
used for the photoactive layer; ITO and aluminum (Al)
are used as metal electrodes.
Poly [(thiophene-2,5-diyl)-co-(benzylidene)]PTB is
soluble in dichloromethane CH2Cl2, was characterized
by infrared spectroscopy and Ultra violet spectroscopy.
2. Experimental
2.1. Reagents
Dichloromethane and methanol (98%) were purchased
from Aldrich and used as received. Thiophene: was pur-
chased from Aldrich Chemical Co. and distilled under
reduced pressure. Benzaldehyde: was purchased from
Aldrich Chemical. Raw-Maghnite (Algerian montmoril-
lonite clay) was procured from BENTAL (Algerian So-
ciety of Bentonite).
2.2. Polymer Preparatio
The condensation of benzaldehyde with thiophene in the
presence of Mag-H+ as a catalyst was carried out by
condensation in bulk under nitrogen for 6 h.
Each mixture was prepared with a weighted quantity
of Mag-H+ dried just before use for 1 h in a drying oven
at 100˚C. Benzaldehyde (6 × 10–3 mol) and thiophene (6
× 10–3 mol) were mixed and 1g of Mag-H+ was added.
The reaction was carried out at 25˚C for 6 h. The poly-
mer film was washed with water and methanol several
times, and finally dried under vacuum at room tempera-
ture for 24 h. The yield was 50%. The resulting polymers
were characterized by FTIR and UV-Visible spectros-
copy.
Anal. Calcd for (C22H14S2)n C, 77.16; H, 4.12; S,
18.72. Found: C, 78.63; H, 4.53; S, 15.93. IR (film on
NaCl, cm–1): 3027, 1598, 1492, 1443, 1294, 1231, 1176,
1155, 1105, 1073, 1028, 901, 801, 751, 696, 668 (Figure
1).
2.3. Development of the Photovoltaic Cell
Photovoltaic cells consist of a molecular active layer
sandwiched between an anode of ITO (thickness 100 nm)
and an aluminum cathode. The cell size was determined
by the size of the aluminum cathode (evaporated through
a mask of 0.25 cm²). A layer 260Å béthocuproine PCO
(C26H2ON2), sandwiched between the ITO layer and the
acceptor is primarily intended to protect this last layer of
oxygen diffusion from ITO. Toward the cathode, a layer
of 20Å of gold (Au) avoids the recombination of excitons
in organic-metal interface (Figure 3). The bathocuproine
BCP thickness and C60 respectively 260 and 1650Å
were being deposited on the ITO layer in the laboratory
by the technique of thermal evaporation (TE).
We conducted the following cells, using the active layer
in the donor-acceptor pair TMP-C60:
The layer of PTB was dissolved in a solution of di-
chloromethane and deposited onto the layer of C60 by
the method of spin coating.
So we made a solar cell junction with the following:
Glass/ITO/BCP/ C60/PTB/Au/Al
3. Results and Discussion
3.1. Spectroscopic Characterization:
The Figure 1 presents the FTIR spectrum of PTB and
shows the appearance of a strong absorption at 1640 cm–1
which is attributed to the stretching vibration of conju-
gated C=C and the stretching vibration of aromatic in
thiophene .A distinct peack near 737,37 cm–1 is due the
out of plane vibration C
 -H characteristic of the
 -linkage in thiophene rings .
The UV-vis absorption spectra were recorded with an
OPTIZEN UV-2120 spectrophotometer (Figure 2)
shows the optical absorption spectrum of polymer: PTB
in CH2Cl2 solution. The colours of the polymer solutions
were almost grey or black. The absorption spectrum in

Synthesis and Properties of Polythiophène Benzylidene and Their Photovoltaic Applications 1016
Figure 1. FTIR spectrum of PTB.
Figure 2. Optical absorption spectra of PTTM in CH2Cl2.
(a)
(b)
Figure 3. (a) Molecular structure of PTB, (b) structure of printed diodes with “P N” junction

Synthesis and Properties of Polythiophène Benzylidene and Their Photovoltaic Applications 1017
Figure 2 shows two major absorption bands. The band in
range of 280 - 300 nm is assigned to the π - π* transition
of the aromatic heterocyclic since it corresponds to the
same band as its precursor, and the band in the range of
620 - 650 nm is assigned to the π - π* band gap transition
[9].
3.2. Photovoltaic Properties
To investigate the photovoltaic properties of the poly-
mers, the bulk heterojunction solar cells with a structure
of ITO/BCP/C60/PTB/Au/Al were fabricated where the
polymer was used as donor and C60 as acceptor. The
active layers were prepared by spin coating
Current-voltage characteristics of solar cells in the
dark and under illumination of 100 mW/cm2 white light
from a xenon lamp (Jobin Yvon, FL-1039) were meas-
ured on the computer-controlled Keithley 2400 Sour-
ceMeter system measurement. All measurements were
carried out under ambient atmosphere at room tempera-
ture.
The Current–voltage characteristics in the dark and
under illumination were measured. One can see that in
our case (Figure 4) a photovoltaic effect is revealed but
with a small but significant yield was obtained. This
structure has a good recovery, a VOC of 0.26 V and a
JSC 1,6 mA/cm². In addition, there is a clear improve-
ment of the FF (36%), giving a yield of around 0.15%.
After a performing a photovoltaic cells, and in order to
well understand and control the key physical processes
that determine the performance of organic solar cells, we
must know all the physical parameters, such as series
resistance, parallel resistance, the different saturation
currents, photo-current and ideality factor.
We were led to use electrical models [24], equivalent
circuit to a diode, which allowed us to model our solar
cells in the dark and under illumination.
3.3. Electrical Equivalent Circuit of a
Photoelectric Cell
Solar cells are generally equivalent to a simple circuit
with a single diode in parallel with a resistor Rp, and a
series resistance Rs .
In literature, the most frequent expression is:
exp 1
SS
S
Tp
VRI VRI
II nu R




 




(1)
where n is the ideality coefficient.
The solution of equation (1) is:



exp
pSp S
SppS T
TT
RIR I I
VRRIRInuW
nu nu



 





(2)
where W is the Lambert function defined by


.
Wx
Wxe x
3.4. In the Case of the Diode Under
Illumination
PN junction under illumination can be diagrammed by a
current generator Iph (current proportional to incident
light) in parallel with the diode delivering a current
-2000200 400
-5,0
-2,5
0,0
2,5
5,0
7,5
10,0
12,5
15,0
17,5
20,0
22,5
25,0
Densit?de courant (mA/cm2)
Tension (V)
Figure 4. Current–voltage characteristics of polymer photovoltaic cells based on PTB (Substrate/ ITO / PCB /C60 /PTB/ Al)
blend system in the dark (----) and underillumination (----) of 100 mW/cm2 white light.

Synthesis and Properties of Polythiophène Benzylidene and Their Photovoltaic Applications 1018
Figure 5. Diagram of a real diode.
(a) (b)
Figure 6 electric circuit of a PN junction under ideal illumination (a) connected to a load resistor and its equivalent circuit
(b).
R
p
(a) (b)
Figure 7. Case of a PN junction with a series resistance (a) and shunt resistor in series (b).
exp 1
S
qV
iI kT








( darkness).
The overall current is given by the following equation:
exp 1
S
qV
ph
I
I
kT



 


I (3)
We obtain the following equivalent circuit of an idéal
solar cell (Figure 6).
When the junction is connected to a load resistor ,
the flow current through brings up a voltage drop
(open circuit, et
C
R
C
R
C
R CO VV

).
The orientation of the current
p
h
I
to produces a
voltage V inducing a bias across the junction so that the
current
C
R
I
is opposite to
p
h
I
From another point of view, we can consider that the
direction of photo-current in the load resistance induces a
voltage across the junction which generates a direct cur-
rent
I
in the opposite direction
p
h
I
, so that, generally
we dont observe the entire current
p
h
I
, but only the
quantity:
p
h
IIi

 (4)
3.5. Real IV Characteristic of a Photovoltaic Cell
When the contact resistance (resistivity electrodes and
interfaces metal-organic materials) and ohmic losses (due
to the resistivity of the organic layers) generate signifi-

Synthesis and Properties of Polythiophène Benzylidene and Their Photovoltaic Applications1019
cant resistance from the load resistor, we must associate
series resistance in the equivalent circuit.
S
If we denote Vj the voltage across the junction, the
voltage V across the cell is reduced to:
R
j
S
VV RI (5)
In the first quadrant we have:
exp 1exp1
S
Sph ph
VRI
qV
I
IIqI
kT kT



 
   
 
 
(6)
In addition, when leak currents occur through the cell,
we can take into account this new component by insert-
ing a parallel resistance (Rp). When this resistance be-
comes very large (Rp → ∞), these leak currents become
negligible . We obtain: 0
p
i
exp 1
S
Sp
p
VRI VRI
II qI
kT R
 


 


S
h
(7)
Already in the case of an ideal cell we do not find the
total current
p
h but only i-ph. In the real case
this reduction is quite pronounced, this is due to the shunt
resistor
IiI
p
R which introduces a leak current
p
i and is
given by the following equation:
ph p
I
iI i (8)
Solar cells are generally equivalent to a simple circuit
with a single diode in parallel with a resistor
p
R, and a
resistor in series.
S
R
expexp 1
SS
Sp
Tp
VRI VRI
h
I
II
nu R

 

 




(9)





exp
Sp pphS
pSph
pS
T
TT
VRRIRI I
RII I
RI
nu Wnu nu
 








 









(10)
With n: ideality factor,
,
T
kT
uWLambert Function
q

3.6. Theoretical Simulations to Determine the
Parameters.
We performed theoretical calculations of simulations to
determine the parameters of the I-V curve under illumi-
nation (Figure 8).
The analytical simulations were used to calculate the
values of various parameters. The performed values are in
good agreement with experimental results. According to
values found from the filted curve on the experimental
curve (under illumination), we can confirm that we have a
good diode. The yield of 0.15% is confirmed by these
values.
In conclusion, we have prepared some conjgugated
poly(thiophene benzylidene) by polycondensation of
benzaldehyde and thiophene catalyzed by Maghnite-H+ ,
the conjugated aromatic backbone through the insertion
-0,4-0,20,0 0,2 0,4 0,6 0,8
-1,0x10-3
0,0
1,0x10-3
2,0x10-3
3,0x10-3
4,0x10-3
5,0x10-3
6,0x10-3
7,0x10-3
8,0x10-3
J (A/cm2)
V(volt)
Theorique
Experimentale
UT = 0.02585
n = 2 .6
Rp = 20000
rs = 4 0
JS = 3*10-6
Jph = 1.16*10-4
Figure 8. Current–voltage characteristics of polymer photovoltaic cells based on PTB (Substrate/ ITO / PCB /C60 /PTB/ Al)
blend system (----)experimentaland (____) theoretical curves.

Synthesis and Properties of Polythiophène Benzylidene and Their Photovoltaic Applications 1020
Table 1. Photovoltaic Characteristics of our Polymer and those of literature [25].
Voc (V) Jsc (mA/cm2) FF PCE (%)
PTB 0.26 1.6 0.36 0.15
P3T-DDTPA 0.72 0.38 0.32 0.086
P1 0.71 0.08 0.24 0.011
P2 0.74 0.12 0.25 0.013
P3 0.74 0.10 0.25 0.019
PTB: poly (thiophene-co-benzylidene 1:1)
P3T-DDTPA: Poly ((E)-4-(dodecyloxy)-N-(4-(dodecyloxy)phenyl)-N-(thiophen-3-yl)vinyl)phenyl)aniline)
P1: poly ((E)-4-(2-(2,5-Dibromothiophen-3-yl)vinyl)-N,Nbis(4-(dodecyloxy)phenyl)aniline-co-3-hexylthiophene 10:1)
P2: poly ((E)-4-(2-(2,5-Dibromothiophen-3-yl)vinyl)-N,Nbisidodecyloxy)phenyl)aniline-co-3-hexylthiophene 2:1)
P3: poly ((E)-4-(2-(2,5-Dibromothiophen-3-yl)vinyl)-N,Nbis(4-(dodecyloxy)phenyl)aniline-coi3-hexylthiophene 1:1)
P4: poly ((E)-4-(2-(2,5-Dibromothiophen-3-yl)vinyl)-N,Nbis(4-(dodecyloxy)phenyl)aniline-co-3-hexylthiophene 1:4.8)
of benzylidene between two thiophene rings.
Such results may serve primarily to illustrate a new
strategy to increase the low band gap polymers through
the arrangement of different aromatic heterocyclic in
conjugated polymer backbones.
The manufacture of organic solar cell with heterojunc-
tion, Glass/ITO/BCP/C60/PTB/Au/Al shows that our cell
is Promising with a VOC of 0.26 V and a 1.6-JSC mA/cm².
In addition, there is a clear improvement of the FF (36%),
giving a yield of about 0.15%. PTB gave the best results
compared to the 3-hexylthiophene copolymers [25], with
the PCE of 0.15% because of the relative low HOMO
level of the polymer. We note here that this high effi-
ciency is obtained due to the high Jsc and Voc. photo-
voltaic parameters are summarized in Table 1.
These results it was confirmed by calculations of
theoretical simulations to determine the parameters of the
curve under illumination. Further improvements could be
made for the PTB from these polymers by optimizing the
polymer/C60 entering the morphology of the active layer
4. Acknowledgements
The authors wish to thank the National Agency for De-
velopment and Research of Algeria for the financial sup-
port.
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