Materials Sciences and Applicatio ns, 2011, 2, 1702-1707
doi:10.4236/msa.2011.212227 Published Online December 2011 (http://www.SciRP.org/journal/msa)
Effect of PEDOT:PSS Layer and ITO Ozonization
in Arylenevinylene-co-Pyrrolenevinylene (AVPV)
Based Solar Cell Devices
Ankur Solanki1,2, S. Sundar Kumar Iyer2,3, Ashish Garg1,2
1Department of Materials Science & Engineering, Indian Institute of Technology, Kanpur, India; 2Samtel Center for Display Tech-
nologies, Indian Institute of Technology, Kanpur, India; 3Departmental of Electrical Engineering, Indian Institute of Technology,
Kanpur, India.
Received September 26th, 2011; revised November 1st, 2011; accepted November 17th, 2011.
ABSTRACT
Arylenevinylene-co-pyrrolenevinylene (AVPV) is polymer oligomer system derived from arylbridged bispyrroles which
has been explored for photovoltaic devices. In this paper, we show the dependence of the photovoltaic device parame-
ters on the anode surface treatment in an organic single layer photovoltaic device based on AVPV as an electron donor .
Since the total quantum efficien cy includes the charge collection efficien cy at the electrodes, experiments were carried
out to vary the anode (ITO) characteristics, achieved by using ITO with or without ozonization and with or without
PEDOT:PSS (Polyethylene dioxythiophene:Polystyrene sulphonic acid) layer. Devices fabricated on ITO anode (with-
out ozonization and without PEDOT:PSS) exhibited the maximum current density (Jsc = 1.3 µA·cm–2) as compared to
those devices where ITO was ozonized as well as had a PEDOT:PSS layer (Jsc = 0.1 µA·cm–2) measured under 1 sun
illumination of AM 1.5 through a calibrated solar simulator.
Keywords: AVPV, Organic Poly mers, Ozonization, PEDOT:PSS, Organic Solar Cells
1. Introduction
Photovoltaic solar cells based on organic compounds are
promising candidates for solar energy conversion. They
have the potential for cost effectiveness, mechanical
flexibility and easy processing [1-3]. However, in order
to compete with the inorganic thin film solar cells, power
conversion efficiencies of more than 10% must be
achieved. While current research efforts have led to the
efficiencies above 8% [4] There is a lack of certainty on
the maximum achievable efficiencies and concerns on
the device reliability and environmental stability of the
organic solar cells (OSC) [5,6]. Moreover, for large area
solar cell devices, development of materials which are
environment friendly and manufacturing processes which
are ecologically favourable is required.
As reviewed by Gunes et al., [7] a variety of polymers
have been used to fabricate the organic solar cells and the
results are dependent on the energy band alignment of
the polymers with respect to other layers in the device.
The internal power conversion efficiency of organic solar
cell is mainly a product of five factors viz., 1) absorption
efficiency, 2) exciton diffusion efficiency, 3) exciton
dissociation efficiency, 4) charge transport efficiency and
5) charge collection efficiency [8,9]. It can be easily es-
timated that the polymers having low band gap (<2 eV)
absorb the light in the visible range (300 nm - 700 nm) of
solar spectrum. Since the first report of low band gap
conjugated polymer, polyisothianaphthene (PITN) [10],
much work has been carried out to prepare polymers with
low band gap energy [11,12]. Low band gap conjugated
polymers may have higher conductivity (carrier mobility)
and therefore offer potential applications in transistors,
transparent conductors, non-linear optical devices and
smart windows.
In addition, stability of the active layer and rapid deg-
radation of properties has been a major concern for OSC
devices especially under light illumination and simulta-
neous exposure to oxygen or water vapor and an under-
standing of these aspects is crucial for fabricating long
lasting devices. Degradation of organic solar cells device
can occur due to various mechanisms such as diffusion of
oxygen and moisture resulting in active layer and elec-
trode degradation as reviewed by Jorgensen et al. [13].

Effect of PEDOT:PSS Layer and ITO Ozonization in Arylenevinylene-co-Pyrrolenevinylene 1703
(AVPV) Based Solar Cell Devices
During last few years, researchers have designed and
synthesized a large number of π-conjugated arylenevi-
nylene-co-pyrrolenevinylene (AVPV) oligomer system
derived from arylbridged bispyrroles [14]. Bispyrroles
are fluorescent dyes and have been used as the building
blocks for fabricating the low band gap polymeric sys-
tems. In our previous work, we introduced a derivative of
AVPV and investigated its candidature for the solar cells
in single layer device structure [15]. The structure of this
molecule is shown in Figure 1. The presence of alternate
single and double bond in structure facilitate the conduc-
tion of electrons in this polymer. The device performance
of photovoltaic cell is characterized by the short-circuit
current density (Jsc), open-circuit voltage (Voc) and fill
factor (FF). For single layer device structure, maximum
ideal Voc is limited by the work function difference of
electrodes used as cathode and anode [5,16,17]. In this
work, we have a systematic study on the effect of varia-
tions in anode surface characteristics on charge collection
efficiency. The study shows that total internal efficiency
in single layer organic solar cell is also limited by the
work function of used electrodes and HOMO (highest
occupied molecular orbital)—LUMO (lowest occupied
molecular orbital) levels of polymer [18,19].
2. Experimental Details
Device Fabrication
The OSC devices were fabricated using AVPV as an
active layer. A schematic of the device structure before
encapsulation is shown in Figure 2. Transparent indium
tin oxide (ITO)-coated glass substrates were used as an-
ode for this study. Sheet resistance of the ITO layers was
16 Ω·sq-1. ITO typically has a transitivity of 80% - 90%
in the UV range of light [1]. The ITO was patterned and
then subjected to ozonization for selective experiments.
The workfunction of ITO was measured as 4.8 eV prior
to ozonization and 5.1 eV after ozonization. These values
are in good agreement with the reported data [20,21].
For a few devices, thin film of PEDOT:PSS was spin
coated on top of an ITO-coated substrates at 3000 rpm
for 90 s. This layer was dried in the nitrogen ambient at
110˚C for 1 h.
To study the effect of variation in the anode character-
istics, four different device structures were fabricated i.e.
substrates with and without ozonization and with and
without PEDOT:PSS. Active layers were spin coated at
1000 rpm from a solution containing AVPV polymer.
Concentration of AVPV solution was 10 mg·cc-1 in
chlorobenzene. The active layer was dried in a vacuum
drying chamber at 120˚C for 1 h. For optical absorption
measurements, a single layer of AVPV was deposited on
a bare glass substrate whose results are shown in Figure 3.
Figure 1. Molecular structure of Arylenevinylene-co-pyr-
rolenevinyle ne (AVPV).
Figure 2. Schematic of the organic solar cell device struc-
ture.
300 400 500 600 700
0.5
0.6
0.7
0.8
0.9
1.0
Absorption (normalized)
Wavelength (nm)
Figure 3. Normalized absorption spectrum of AVPV film in
Chlorobenzene.
Top contacts of calcium-aluminium were evaporated
on the top of the active layer in high vacuum of 10-6 mbar
to form the cathode. The final active device area for each
cell was 0.2 cm2. Finally, the devices were encapsulated
with epoxy in a nitrogen environment.

Effect of PEDOT:PSS Layer and ITO Ozonization in Arylenevinylene-co-Pyrrolenevinylene
1704
(AVPV) Based Solar Cell Devices
The encapsulated devices were electrically character-
ized with a Keithley 4200 parameter analyser. Light charac-
teristics were measured under 1 sun illumination of AM
1.5 through a calibrated solar simulator.
3. Results and Discussion
3.1. Light Absorption
To make a good quality organic solar cell, absorption
spectrum of the organic material should match the Solar
spectrum, as closely as possible. In addition, thickness of
the active layer should be sufficient to achieve higher
absorption. Generally, organic materials have higher ab-
sorption coefficients than inorganic semiconductors such
as silicon [22]. Low band gap materials always tends
absorb sunlight towards lower wavelength.
Figure 3 shows the normalized absorption spectrum
taken on thin films of single layer AVPV, spun coated on
a clean glass substrate. The figure shows that AVPV film
has two broad peaks at ~423 nm and ~540 nm respect-
tively. The broad nature of these peaks suggests that
AVPV absorb mainly in the visible region (between 400 -
700 nm of the solar spectrum). This compares fairly well
with the absorption spectra of well established polymer
materials such as P3HT [23] and justifies the use of this
polymer for the OSC devices as reported by us previ-
ously [15].
3.2. Effect of Variations in the Anode Surface
As a rule, the internal quantum efficiency ( 
IQE) of OSCs
depends on the collection efficiency of carriers at elec-
trode. Collection of the charge carriers at the electrodes
depends on the alignment of HOMO of the donor poly-
mer and work function of the anode as well as alignment
of LUMO of the polymer and work function of the cath-
ode. As the difference between metal work function and
LUMO or HOMO level reduces, it enhances the number
of collected charge carriers at electrode and hence the
internal quantum efficiency.
To the determine the effect of surface characteristics
of anode, four different device structures of single layer
AVPV devices, (A) ITO with ozonization and with PE-
DOT:PSS, (B) ITO with ionization and without PE-
DOT:PSS, (C) ITO without ionization and with PE-
DOT:PSS and (D) without ionization without PE-
DOT:PSS were fabricated. The energy band diagram of
each of these device structures are shown in Figure 4.
These device structures are fabricated to understand the
change in the device performance after changes in the an-
ode surface by means of ozonization, leading to change in
the work function of ITO, as well as use of PEDOT:PSS.
Figure 4. Energy band diagrams of single layer AVPV de-
vices: (a) ITO with ionization and with PEDOT:PSS; (b)
ITO with ionization and without PEDOT:PSS; (c) ITO
without ionization and with PEDOT:PSS, and (d) ITO
without ionization and without PEDOT:PSS.

Effect of PEDOT:PSS Layer and ITO Ozonization in Arylenevinylene-co-Pyrrolenevinylene 1705
(AVPV) Based Solar Cell Devices
As our cyclic voltammetry measurements showed, the
Highest Occupied Molecular Orbital (HOMO) level of
AVPV lies at 4.77 eV while PEDOT:PSS has a work
function of 5.2 eV. Hence, as shown in the schematic
band diagram in Figure 4(a), at the time of collection at
anode, holes face a barrier of 0.43 eV. This indicated
towards a reduction in the number of collected holes and
thus may have an effect on the solar cell efficiency. In
this context, ozonization of ITO also affects the collec-
tion because after ozonization as we found that ITO work
functions shift from 4.8 eV to 5.1 eV.
J-V characteristics for all these devices measured un-
der 1 sun illumination of AM 1.5 through a calibrated
solar simulator and the results are shown in Figure 5. We
find that the devices without ozonization and without
PEDOT:PSS exhibit the highest short circuit current
density Jsc, and devices with ozonization and with PE-
DOT:PSS exhibit the lowest Jsc among these structures.
To perform a comparative study, the efficiency deter-
mining parameters are mentioned as a function of anode
in Table 1. The table shows that the photovoltaic devices
made with ionization with PEDOT:PSS are inferior in
comparison to the devices from all other three type of
devices. As shown in Figure 4(a), for the device struc-
ture (A), after the exciton dissociation, holes face a bar-
rier of 0.43 eV for the collection at anode as the accumu-
lation should take place in the part of the device near
anode.
This barrier reduces the hole collection at anode ham-
pering the collection efficiency and hence total internal
efficiency. Likewise, as the anode work function changes,
barrier for collection of holes reduces from Figures 4(a)-
(d) and as a result the current density increases. The ex-
tracted device parameters for each of the devices (short
circuit current density, Jsc, open circuit voltage, Voc, Fill
Factor, FF, series and shunt resistances, Rs and Rsh re-
spectively) are shown in Table 1. In the following sec-
tions, we discuss these effects separately on each of the
device parameter.
3.2.1. Effect on Jsc
Jsc is mainly controlled by efficiency of absorption of
light, exciton dissociation, charge transport and charge
collection. For all the four devices, absorption, dissocia-
tion and transportation are likely to be similar because of
similar device thicknesses. The only variation is in the
anode characteristics and band positions at the anode
interface (Figure 4). The figure shows that as for the ITO
anode without PEDOT:PSS and without ozonization
(Device D), the ITO work function lies at 4.8 eV which
helps in aligning the HOMO level of AVPV with the
ITO work function facilitating an easy transport of holes
to the anode and hence increasing the Jsc. In contrast the
devices which have a PEDOT:PSS layer, the holes face a
larger barrier of 0.43 eV.
3.2.2. Effect on Voc
For a single layer OSC device, Voc is determined by the
difference in the work function of both the electrodes
[16]. This implies that the devices where ITO is ozonized
and is further coated with PEDOT:PSS layer should
show highest Voc (because of voltage drop at anode and
active layer interface). But in these devices, charges also
accumulate near the electrode and thus hampering the Voc.
In the device “D”, no accumulation is likely so Voc was
found to be maximum. Less Voc for device “B” having
ITO with ozonization without PEDOT:PSS and device
“C” without ozonization and with PEDOT:PSS. This
appears to suggest that anode roughness may also have
an effect on the Voc as PEDOT:PSS has a smoothening
effect on ITO.
Figure 5. J-V characteristics of single layer AVPV devices
with anode variations under 1 Sun illumination.
Table 1. A comparison of device parameters for all four
types of OSC devices with variations in anode characteris-
tics.
Devices Jsc (A·cm-2)
 10-7 Voc (V)FF (%) Rs (106 ·cm-2)
Rsh
(106 ·cm-2)
A 1.1 1.2 24 1.7 5.0
B 2.1 1.0519 1.8 2.8
C 4.3 1.0518 1.1 1.4
D 13 1.4 9.1 0.6 0.2

Effect of PEDOT:PSS Layer and ITO Ozonization in Arylenevinylene-co-Pyrrolenevinylene
1706
(AVPV) Based Solar Cell Devices
3.2.3. Effect on FF
Fill Factor (FF) is actually the filling factor (ratio of
maximum power to the theoretical maximum power)
determined by the fraction of the photo-generated charge
carriers that actually reach the electrodes, when the built-
in field is lowered toward the open circuit voltage. In fact,
there is a competition between charge carrier recombina-
tion and charge transport. Highest shunt resistance for
device “D” results in highest recombination and hence
least FF [24].
4. Conclusions
In conclusion, we showed the effect of surface character-
istics of anode in the single layer devices on the photo-
voltaic properties of the devices. Since the total quantum
efficiency also includes the charge collection efficiency
at the electrodes the energy level diagram of the device
suggested that the alignment of ITO anode work function
and HOMO level of donor is an important condition for
better collection of holes under illumination. We found
that the devices fabricated on ITO anode (without
ozonization and without PEDOT:PSS) exhibited the
maximum current density (Jsc = 1.3 µA·cm-2) as compared
to those devices where ITO was ozonized as well as had a
PEDOT:PSS layer (Jsc = 0.1 µA·cm-2 ). This is due to re-
duced barrier to the hole transport to the anode from the
active layer when ITO is not ozonized and PEDOT:PSS
layer is absent.
5. Acknowledgements
This work was financially supported by the Department
of Science and Technology, Government of India, New
Delhi.
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