Huge demand of silicon in photovoltaic cells caused a shortage of silicon which results in demand for new technology in this field and so another revolutionary cheap method is innovated namely thin film solar cell. In this paper, various types of thin film solar cells are reviewed. They have less efficiency and also low cost compared to 1st generation solar cell. They are based on silicon Thin film implies that less material is used which makes the solar cells cheaper.

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THIN FILM SOLAR CELLS : A REVIEW
PROF. R. D. PARMAR
Lecturer, Electrical Engineering Department, C.U.Shah Polytechnic, Surendranagar,
Gujarat, India
raghuvir_dhirubha@yahoo.com
ABSTRACT: Huge demand of silicon in photovoltaic cells caused a shortage of silicon which results in demand
for new technology in this field and so another revolutionary cheap method is innovated namely thin film solar
cell. In this paper, various types of thin film solar cells are reviewed. They have less efficiency and also low cost
compared to 1st generation solar cell. They are based on silicon Thin film implies that less material is used
which makes the solar cells cheaper.
Keywords: a-Si, CdTe, CIS, CIGS
1. INTRODUCTION:
Second Generation Solar Cells are thin film solar
cells. In this case no c-Si wafers are used but very
thin layers of silicon, which are deposited on glass or
a flexible substrate.Typical production size 1*1
m^2.Typical thickness about 1 to 4 micro meter. So
to absorb same amount of light they require almost
99% less material than crystalline solar cell. These
solar cells are manufactured using cheaper processing
technology hence the materials have more defects
resulting in lower performances.The other advantages
of thin films are; high automation and production
efficiency,ease of building integration and improved
appearance, good performance at high ambient
temperature and reduced sensitivity to overheating.
For utility production, thin film technologies will
require more land than crystalline silicon
technologies in order to reach the same capacity due
to their lower efficiency. So, land availability and
cost must be taken into consideration when thin film
technology is considered. Although the solar cells
efficiency is lower, due to the lower cost price per
area,the cost-price per Watt of the second generation
PV technology is significant lower.Thin films are
made by depositing extremely thin layers of
photosensitive materials in the micrometre (µm)
range on a low-cost backing, such as glass, stainless
steel or plastic.
The first generation of thin film solar cell produced
was a-Si. To reach higher efficiencies, thin
amorphous and microcrystalline silicon cells have
been combined with thin hybrid siliconcells. With II-
VI semiconductor compounds, other thin film
technologies have been developed, including
cadmium telluride (CdTe) and copper-indium-
gallium-diselenide (CIGS).[1]
2. TYPES OF THIN FILM CELL SOLAR
CELLS
2.1 Silicon based
2.1.1. Single junction amorphous silicon:
Because a-Si alloy absorbs light more efficiently,so
a-Si solar cell thickness can be up to 300 times less
than that of conventional cells, so material cost
reduces[7]. Introduction of the triple junction
modules provide relatively high levels of effiiciency
and stability.[2] A p-i-n junction structure is used
with the n- and p-type regions creating a field in the
i-layer due to their work-function difference (Green,
2003).Individual cells deposited onto a glass sheet
are laterally connected in series.Since the amorphous
silicon is not very conductive, a key feature of the
technology is the use of a transparent conductive tin
oxide layer between the silicon and the glass. The
strength of a-Si technology is its simplicity combined
with the use of benign and abundant silicon.[3]
2.1.2. Multiple junction amorphous silicon
If two or more cells are stacked on top of one
another,inferior quality material of thinner layers of
amorphous silicon can be used.If the bandgaps of the
lower cells are smaller than that of the upper cells,
this also gives a performance boost as well.
One of the approach to reduce the bandgap from the
quite high values is to make alloy of hydrogenated
amorphous silicon (1.7 eV) with germanium.One of
such known module is having 3 cell stack with the 2
underlying cells made from a-Si alloyed with
germanium.This gives nomina lmodule performance
in the 6-7% range which is better than the best of the
single junction a-Si approaches.The glass top cover
sheet is replaced it by Tefzel, a transparent co-
polymer of tetrafluoroethane and ethylene in the form
of a film.Mechanical support is provided by a thin
Galvalume rear metal sheet with a hot-dip
aluminium-zinc coating.
The cells themselves are deposited onto a continuous
roll of another stainless steel sheet of 2.5 km
length.Only the three stacked cells are connected
together during this operation, with strips of these cut
from the stainless steel.These strips are then
connected together as in a wafer-based module.
Compared to the usual glass-encapsulated
modules,material cost would be appreciably higher
due to Tefzel,Galvalume,stainless steel and the
additional required insulating layers.

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Recent approach is to combine an amorphous silicon
top cell with a bottom cell consisting of a
two-phase mixture of amorphous and
microcrystalline silicon.The bandgap of the lower
cell is similar to that of wafer-based cells (1.1
eV).Apart from the use of two cells, which improves
module performance to the 8-10% range, the
technology otherwise resembles that of amorphous
silicon with its associated strengths and weaknesses.
Although the mixed-phase material is more stable
than the amorphous material, the overall stability of
the device is determined by the latter, giving no
grounds for expecting improved stability
performance from the “hybrid” combination, despite
early claims to this effect [3]
Cells of different band gaps are stacked together,In a
triple junction cell.The top cell uses a-Si alloy with
an optical gap of 1.8 eV,which captures the blue
photons.Green photons are absorbed by an
amorphous silicongermanium (a-SiGe) alloy with
about 10-15% Ge and optical gap of 1.6 eV in as
intrinsic layer in the middle cell.The bottom cell
captures the red and infrared photons and uses an i
layer of a-SiGe alloy with about 40-50% Ge,
corresponding to an optical gap of 1.4 eV. Light that
is not absorbed in the cells gets reflected from the
aluminum/zinc oxide (Al/ZnO) back reflector, which
is usually textured to facilitate light trapping.[7]
The resulting thin film photovoltaic product has the
ability to capture a greater percentage of the incident
light energy which is one of the keys to higher
efficiencies and higher energy output, especially at
lower irradiation levels and under diffused light.The
cell is deposited using a vapor deposition process at
low temperatures; the
energy payback time is therefore much smaller than
that for the conventional technology. [7]
Once the solar cell material has been provided with
suitable electrodes, the cells are encapsulated in UV
stabilized, weather-defying polymers. This
laminating process incorporates a fluoropolymer on
the top side. The bottom side of the finished product
is a polyester material suitable for adhesives.[7]
2.1.3 Crystalline silicon on glass
With the help of high temperature process an
amorphous silicon layer is converted to a
polycrystalline layer, The resulting films have
properties similar to those of the polycrystalline
wafers that nowdominatethe commercial solar
module market.As it is more conductive, transparent
conducting oxide is not needed.So cost reduces a-si
stability problem is eliminated. Other advantages are
;ruggedness and a fault tolerant metallisation
approach.
2.2 Chalcogenide-Based Cells
2.2.1 Cadmium Sulphide
The first thin-film solar cell prodeuced on for large-
scale were based on cadmium sulphide.Due to
attributed to stability issues with the cells and the
appearance of amorphous silicon as an apparently
superior contender commercialisation was
unsuccessful.In case Cds(cadmium sulfide) bandgap
is of the order of 2.5 ev.
2.2.2 Cadmium Telluride
A layer of cadmium sulphide is deposited from
solution onto a glass sheet coated with a transparent
conducting layer of tin oxide. This is followed by the
deposition of the main cadmium telluride cell by as
variety of techniques including close-spaced
sublimation, vapour transport, chemical spraying, or
electroplating.[3].One of the biggest problem with
cadmium telluride is that Cadmium is hazardous and
telluride is rare available.
CdTe cells are a type of II-VI semiconductor thin
film that has a relatively simple production process,
allowing for lower production costs.It has an energy
payback time of eight months, the shortest time
among all existing PV technologies. In case of CdTe,
bandgap is of the order of 1.5 ev and absorption
coefficient is 10 times that of Si.Indeed, thin film
solar cells based on CdTe show the highest
performance among thin film solar cells in terms of
efficiency and on the module level. Today, CdTe
cells have achieved a dominant position in the thin
film and have a market-leading cost-per watt.
However, these materials are toxic and less abundant
than silicon. CdTe cells and modules have reached
16.5 % and 10.2% efficiency respectively [5]..
2.2.3 Copper-Indium Diselenide (CIS and CIGS)
CIS technology have 19% efficiency small cells, but
has proved difficult to commercialise. It involves
deposition onto a glass substrate and then
interconnected.An additional glass top-cover is then
laminated to the cell/substrate combination. For
CIGS cells, the fabrication process is more
demanding and results in higher costs and
efficiencies compared to CdTe cells. CIGS solar cells
and modules have achieved 19.5 and 13%
efficiencies , respectively.Thus they offer highest
efficiency of all thin film technologies.
In case of CIGS cell, over glass/ss/polymer, a layer of
Mo of size 0.5-1.5 micro meter, above that layer
another layer of CulnGase2 of size 1.5-2.0 micro
meter, above that layer another layer of Cds of size
0.03-0.05 micrometer, above that a layer of ITO/Zno
of size 0.5-1.5 micro meter, above that MgF2 of size
0.1 micro meter is deposited.
3. CONCLUSION :
Among these commercially available thin film
technologies, a-Si is the most important in terms of
production and installation.Multicrystalline thin film
on glass (CSG) is a promising thin film technology
which is now entering industrial production.
Microcrystalline technology, in particular the
combination of amorphous silicon and
microcrystalline silicon (a-Si/m-Si), is another
approach with encouraging results. Thin film
technologies are growing rapidly. In recent years,
thin film production units have increased from pilot
scale to 50 MW lines, with some manufacturing units

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in the gigawatt (GW) range. As a result, thin films
technologies are expected to increase their market
share significantly by 2020. It is difficult to predict
which of the thin film technologies will reach higher
market shares in the long-term.
REFERENCES :
1. International Energy Commission, Energy
Technology Perspectives: Scenarios and Strategies to
2050, 2008
2.Gregg,A.,Blieden,R.,Chang, A.and Ng H.
Performance Analysis of Large Scale,Amorphous
Silicon Photovoltaic Power Systems,31st Institute of
electrical and electronics engineers,photovoltaic
specialist conference and exhibition,january 3-7,
2005, Lake Buena Vista, Florida USA.
3. Thin film solar cells: Review of technologies and
commercial status by Marin A.Green, ARC
photovoltaic centre of excellence, Univesity of new
south wales.sydney 2052, Australia, ANZSES Solar
2005
4.An EPRI technology innovation white paper on “
Solar photovoltaics : expanding electric generation
options”,december 2007.
5.Noufi,R.and Zweibel,K.(2006) High-efficiency
CdTe and CIGS Thin film Solar Cells: Highlights
and Challenges,Conference Paper,WPEC4,Hawaii.
6. Quaschning, Volker, Technical and Economical
System Comparison of Photovoltaic and
Concentrating Solar Thermal Power Systems
Depending on Annual Global Irradiation. Solar
Energy, 2006
7.Mario pagliaro,giovanni palmisano and rosaria
ciriminna Flexible solar cells by WILEY –VCH
Verlag GmbH & Co.KGaA @2008
8.Photovoltaic solar energy : development and
current research by European commission
Luxmbourg ,2009
9.”Photovoltaic power generation” – Prepared by
Thomas penick and Bill Louk,december 4,1998
10.‘‘A Strategic Research Agenda for photovoltaic
solar energy technology by european
commission,Luxembourg,2007
11.” Photovoltaic soalr cells :An overview of state of
the art cell development and environmental issues.”
by R.W.Miles,K.M.Hynes,I.Forbes,Northumbria
photovoltaics applications centre,University of
Northumbria,school of engineering,U.K.,2005
12.Solar generation : solar electricity for over one
billion people and two million jobs by
2020,september 2006,GREENPEAKS,European
photovoltaic industry association 13 .”
Microcrystalline silicon and micromorph tandem
solar cells” by H.Keppner, J.Meier,P. Torres,D
.Fischer,A.Shah ,1997
14.Research opportunities in crystalline silicon
photovoltaics for the 21st
century by
H.A.Atwater,B.Sopori &T.Ciszek, L.C.Feldman,
J.Gee,A.Rohatgi,NREL,1999
14 ch.4 ,crystalline silicon solar cell byMartin
A.Green,photovoltaic special research centre,
Univesity of new south wales, sydney, N.S.W.,
Australia
REVIEW PAPER PUBLISHED ON : 02/08/2010