Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 661-666
Published Online July 2012 (http://www.SciRP.org/journal/jmmce)
Polymer in Sustainable Energy
Darshan Patel, Suresh P. Deshmukh*
General Engineering Department, Institute of Chemical Technology, Mumbai, India
Received January 15, 2012; revised February 22, 2012; accepted March 9, 2012
ABSTRACT
A transparent polymer based solar cell was designed and fabricated to utilize the solar energy when exposed to sunlight.
The transparent solar cell for window module was composed of a polymeric material PPV (Polyphenylene vinylene),
ITO (Indium Tin Oxide) and electrode (Al, Mg, Ca). The polymeric sheet of this cell is by casting process, and elec-
trode is applied on it by CVD (Chemical Vapor Deposition) process. The solar energy collected by this window can be
used to power up small household electrical appliances. Recently, polymeric solar cell is made by a roll-to-roll process
without using indium-tin oxide (ITO). A commercially available kapton (Polyimide) foil with an over layer of copper
was used as the substrate. Sputtering of titanium metal on to the kapton/copper in a vacuum metalizing process gave the
monolithic substrate and back electrode for the devices. The active layer was slot-die coated on to the kapton/Cu/Ti foil
followed by slot-die coating of a layer of PET, PC or PEN.
Keywords: Organic Solar Cells; Organic Photovoltaics; ITO-Free; Current Collecting Grid; Printed Anode; High
Conductive PEDOT:PSS; Polymer Solar Cells
1. Introduction
There is increasing awareness for developing renewable
energy sources in the world as the oil reserves will fi-
nally run out in future. Many countries are planning to
increase the contribution of renewable energies in total
energy supply. Earth heat, wind and ocean energy are
examples of sustainable or renewable energy sources
under investigation [1-4]. However, the utilization of
sunlight seems to be the most popular and practical ap-
proach for renewable energy realization [5]. Solar energy
can be directly converted to electrical energy by solar
cells. How to trap and use efficiently this huge energy
reservoir from the sun becomes a major challenge for
mankind. There are different kinds of solar cells being
developed such as silicon solar cells [6-8], dye-sensitized
solar cell (DSC) [9-11], organic polymer solar cells [12],
hybrid solar cells [13,14] and CIGS solar cells [15,16].
With its high solar energy conversion efficiency and ma-
ture fabrication technologies, nowadays silicon solar cell
is the most extensively used type of cell. In addition,
conventional silicon solar cells convert between 10% and
15% of the sun’s energy into electricity.
Fossil fuel alternatives, such as solar energy, are mov-
ing to the forefront in a variety of research fields. Poly-
mer-based organic photovoltaic systems hold the promise
for a cost-effective, lightweight solar energy conversion
platform, which could benefit from simple solution proc-
essing of the active layer and the very high production
speeds that can be reached by roll-to-roll printing and
coating techniques [17-22]. The function of such exci-
tonic solar cells is based on photo induced electron trans-
fer from a donor to an acceptor. Fullerenes have be-
come the ubiquitous acceptors because of their high elec-
tron affinity and ability to transport charge effectively.
The most effective solar cells have been made from bi-
continuous polymer-fullerene composites, or so-called
bulk hetero-junctions. The best solar cells currently
achieve an efficiency of about 5%, thus significant ad-
vances in the fundamental understanding of the complex
interplay between the active layer morphology and elec-
tronic properties are required if this technology is to find
viable application.
Indium-tin oxide (ITO), which is commonly used as a
transparent electrode, is one of the main cost consuming
elements in present photovoltaic devices [17-23]. The
second argument to omit ITO from OPV devices is me-
chanical flexibility. The brittle ITO layer can be easily
cracked, leading to a decrease in conductivity and as a
result degradation of the device performance. A third
argument is the multi-step patterning of the ITO layer,
which involves a lot of chemicals. A lot of effort has
been directed on the development of highly conductive
polymeric materials such as poly(3,4-ethylenedioxythio-
phene):poly(4-styrenesulphonate) (PEDOT:PSS). Replace-
ment of ITO by highly conductive PEDOT:PSS has been
intensively studied by many researchers [24-28]. How-
*Corresponding author.

D. PATEL, S. P. DESHMUKH
662
ever, organic photovoltaic devices with only a PEDOT:
PSS electrode do not provide high efficiency for large
area devices due to the limited conductivity of the PE-
DOT:PSS, which is typically up to 500 S·m−1. Improving
the conductivity of such a polymeric electrode is possible
by combining it with a metal grid, which is either ther-
mally evaporated through micro structured shadow masks
[29,30] or patterned by a lithographic method [31,32].
In Ref. [33] deposition of an Ag grid by diffusion trans-
fer reversal has been reported. Printing of the metal grid,
however, is a prerequisite for fully printed OPV devices,
enabling low-cost manufacturing. Screen printed silver
grids [20] were demonstrated in a roll-to-roll processed
inverted OPV device, where the grid is the last printed
layer in the devices. Earlier, it has been reported that,
inkjet printed current collecting grids was a part of a
composite anode in a conventional OPV device [33],
where the grid is the first printed layer in the devices.
2. Optimization of Organic Solar Cells on
the Basis of Mechanistic Principles
Efforts to optimize the performance of organic solar cells
have found their basis in the fundamental mechanism of
operation. Scheme 1 illustrates the mechanism by which
light energy is converted into electrical energy in the
devices. The energy conversion process has four funda-
mental steps in the commonly accepted mechanism [34]:
1) Absorption of light and generation of excitons; 2) dif-
fusion of the excitons; 3) dissociation of the excitons
with generation of charge; and 4) charge transport and
charge collection. Figure 1 shows a schematic represen-
tation of a typical BHJ solar cell, illustrating the compo-
nents involved in the mechanistic steps as well as a cur-
rent-voltage curve defining the primary quantities used to
validate the performance of a solar cell. The elementary
steps involved in the pathway from photo excitation to
the generation of free charges are shown in Scheme 2.
[35,36] The processes can also occur in an analogous
fashion in the case of an excited acceptor, and the details
of these mechanistic steps have been described exten-
sively in the literature [37]. The key point is that electron
transfer is not as simple as depicted in Scheme 1. The
process must be energetically favorable to form the
geminate pair in step 3 of Scheme 2 and an energetic
driving force must exist to separate this Coulombically
bound electron-hole pair.
Figure 1 shows the schematic illustration of a poly-
mer-fullerene BHJ solar cell, with a magnified area show-
ing the bicontinuous morphology of the active layer. ITO
is indium tin oxide and PEDOT-PSS is poly(3,4-ethyl-
enedioxythiophene)-polystyrene sulfonate. The typical
current-voltage characteristics for dark and light current
in a solar cell illustrate the important parameters for such
devices: Jsc is the short-circuit current density, Voc is the
open circuit voltage, Jm and Vm are the current and volt-
age at the maximum power point, and FF is the fill factor.
The efficiency (h) is defined, both simplistically as the
ratio of power out (Pout) to power in (Pin), as well as in
terms of the relevant parameters derived from the cur-
rent-voltage relationship.
Figure 1. Polymer-fullerence BHS solar cell.
Scheme 1. Mechanism of energy conversion.

D. PATEL, S. P. DESHMUKH 663
1 hv
D
AD
A


2 ,DA DA

 

3 ,,DAD A

 



4 ,DA DA

 
 
 
Scheme 2. Elementary steps in the process of photoinduced
charge separation for a donor (D) and an acceptor (A): 1)
Photoexcitation of the donor; 2) Diffusion of the exciton and
formation of an encounter pair; 3) Electron transfer within
the encounter pair to form a geminate pair; 4) Charge sepa-
ration.
It is apparent that the active layer donor-acceptor com-
posite governs all aspects of the mechanism, with the
exception of charge collection, which is based on the
electronic interface between the active layer composite
and the respective electrode. Detailed descriptions of the
steps used for device fabrication are found elsewhere
[38]. Besides the fundamental mechanistic steps, the
open circuit voltage (Voc) is also governed by the ener-
getic relationship between the donor and the acceptor
(Scheme 1) rather than the work functions of the cathode
and anode, as would be expected from a simplistic view
of these diode devices. Specifically, the energy differ-
ence between the donor and the acceptor is found to most
closely correlate with the Voc value [39,40].
It is therefore apparent that the choice of the compo-
nents in the active layer as well as its morphology, which
governs the physical interaction between the donor and
acceptor, are the primary factors affecting the perform-
ance of the device. As such, the focus of this Review is
the optimization and understanding of the electronic and
physical interactions between polymeric donors and
fullerene acceptors in BHJ solar cells. Architectural
modification (such as the use of buffer layers) or the
choices of electrodes are also critical aspects which will
be viewed as a second level of device optimization in our
discussion.
3. Manufacturing Technique
A series of organic solar cell devices were prepared on
flexible PET substrates. The layouts of the ITO-based
and ITO-free devices are shown in Figure 2.
Low conductive PEDOT:PSS was used for the prepa-
ration of the ITO-based devices. The PEDOT:PSS was
spin coated at 2000 rpm, resulting in a dry layer thick-
ness of 40 nm after baking at 150 1C for 10 min. For the
devices free from ITO, the highly conductive PEDOT:
PSS was used. Highly conductive PEDOT:PSS was spin
coated at 800 rpm, resulting in a dry layer thickness of
100 nm. Poly(3-hexylthiophene; P3HT and [6,6]-phenyl-
C61-butyric acid methyl ester (PCBM) (99%) were dis-
solved in 1,2-dichlorobenzene with a mixing ratio of 1:1
by weight. The solution was stirred for 14 h at 70˚C. The
photoactive layers were obtained by spin coating of the
blend with 4 wt% of the active materials at 1000 rpm for
30 s, which corresponds to a thickness of 220 nm. The
thicknesses of the films were measured using a Dektak
profilometer. The experiments are performed in a clean
room environment at ambient atmosphere. The metal
cathode (1 nm LiF, 100 nm Al) was thermally evaporated
in a vacuum chamber through a shadow mask. The OPV
devices were finished by thin film encapsulation in a
glove box. A picture of the device with current collecting
grid is shown in Figure 3.
Current-voltage curves were measured using simulated
AM 1.5 global solar irradiation (100 mW/cm2), using a
xenon-lamp-based solar simulator Oriel (LS0104) 150 W.
The light source was calibrated with a standard Si photo-
diode detector. UV-vis transmission/absorption spectra
were measured using a Perkin-Elmer Lambda 12 UV/vis
spectrophotometer.
Figure 4(a) shows a picture of a typical part of a
screen printed honeycomb current collecting grid, which
is part of the composite anode. The corresponding line
profile as shown in Figure 4(b) is relatively smooth. The
effective line width was 160 µm, which is a very good
match with the theoretically expected value.
(a) (b)
Figure 2. Schematic illustration of an ITO-based device and
a device with a current collec ting grid.
Figure 3. A picture of the 2 × 2 cm2 device with current
collecting grid, free from ITO.

D. PATEL, S. P. DESHMUKH
664
(a) (b)
Figure 4. Conductive grids: (a) with honey comb and (b) line
patterns used for the 2 × 2 cm2 devices.
Honeycomb structure provides more aesthetic view of
the devices. Moreover, this structure provides homoge-
neous current distribution in case of four bus-bar devices,
which were used in this study. Line pattern of the grid,
which is more applicable for module design, has been
compared with honeycomb structure. In this study the
pitch sizes (the minimum distance between two conduct-
ing lines) and grid’s surface coverage were compared.
The pitch size was 5 and 2 mm and surface coverage was
6.4% and 8% for the honeycomb and line patterns, re-
spectively.
4. Conclusions
Replacing ITO by a composite anode consisting of com-
bination of a metal grid and HC-PEDOT results in a sig-
nificant increase in efficiency for devices of 2 × 2 cm2
area. Future work will concentrate in maximizing the cell
area without substantial efficiency losses using optimized
grid structures. This will enable a substantial increase of
the active area of OPV modules, which in turn will in-
crease the final Wp/m2.
The shadow effect of the grids contributes to the lower
current density in the ITO-free devices. The sum of all
these factors explains why the current density in ITO-
based and ITO-free devices is different. Further im-
provement of the current density in ITO-free devices is
possible by decreasing the shadow effect, by minimizing
the line width in the grids and by increasing the trans-
parency of the high conductive PEDOT and optimization
of layer thicknesses. The 100 nm thick layer of high
conductive PEDOT has a relatively high absorption in
the visible region. Replacing the ITO by a high conduc-
tive PEDOT:PSS (HC-PEDOT) leads to a decrease of the
short-circuit current [18]. The result of with ITO and
ITO-free is shown in Table 1.
Organic solar cells free from ITO on flexible sub-
strates were fabricated. The alternative anode is based on
Table 1. Characteristics of photovoltaic devices.
Anode (mA/cm2) Volts
ITO 6.59 0.489
Ag Honeycomb/HC-PEDOT6.25 0.540
Ag lines/HC-PEDOT 6.36 0.540
highly conductive PEDOT:PSS in combination with
printed current collecting grids. This type of composite
anode has a significantly lower sheet resistance in com-
parison with ITO, which makes larger area devices pos-
sible without substantial efficiency losses. The experi-
ments show that the replacement of ITO by a composite
anode yields an efficiency increase by a factor of two for
devices with an active area of 2 × 2 cm2. Moreover, the
anode is entirely fabricated with solution processing and
does not require high temperature annealing nor litho
steps. All temperature treatments are compatible with
flexible substrates and roll-to-roll processing. Free from
the rather expensive vacuum deposited ITO, the compos-
ite electrode can significantly decrease manufacturing
cost. This work will ultimately contribute towards fully
printed devices, which will provide low-cost roll-to-roll
manufacturing.
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