FIELD OF THE INVENTION
The present invention relates to a method of fabricating a thin film amorphous
silicon solar cell. Specifically, the present invention relates to fabricating a thin
film amorphous silicon solar cell using hot wire chemical vapor deposition
(HWCVD) technique.
BACKGROUND OF THE INVENTION
With the increasing demands of energy and fast depletion of conventional energy
sources such as coal, petroleum, natural gas, the nonconventional energy
sources such as solar energy, wind energy, biomass, tidal energy, Geothermal
energy are gaining importance. Solar energy is renewable, abundant, pollution
free and eco-friendly. Among all of the non-conventional energy sources, solar
energy is the most prominent, because of zero emission of green-house gases
and air pollutes.
Solar cell is a device that converts the solar energy directly into electricity by
photovoltaic effect. In a solar cell, when light is exposed on the solar cell, the
photons are absorbed by the material and these photons transfer their energy to
bound electron to make them free. These free electron motion leads to a current

flow in the device. This direct current (DC) so generated can be used
immediately or stored in a battery.
There are many types of solar cells depending upon the constituent material
ranging from conventional wafer type crystalline solar cell to recently developed
organic solar cell. At present, silicon is the dominating material in the
photovoltaic industry. Silicon itself is used in various forms in the solar cell for
example, crystalline, poly-crystalline, microcrystalline and amorphous silicon.
Amorphous silicon solar cell is a type of a thin-film solar cell (TFSC), that is made
by depositing one or more thin layers (thin films) of photovoltaic material on a
substrate. The thickness range of such a layer is wide and varies from a few
nanometres to tens of micrometres. Thin-film solar cells are usually categorized
according to the photovoltaic material used.
Amorphous silicon solar cells have several advantages over crystalline solar cells,
e.g. Amorphous silicon is very cheap compared to crystalline silicon, and,
involves low manufacturing cost. It can be deposited in the form of a thin film
over cheap substrates from glass to flexible metal sheets. It has high absorption
coefficient compared to crystalline silicon so as to minimize material

consumption. In spite of these attributes, the problem with amorphous silicon is
that, it is inherently amorphous in nature and entails large defect density, and
dangling bonds (unsaturated bonds). For making amorphous silicon valuable for
solar cell application, hydrogen is generally incorporated in it to reduce the
dangling bond density. The hydrogenated amorphous silicon solar cell (a-Si:H)
has proved to be a viable alternative to c-Si technology.
Solar cells are conventionally deposited on glass and silicon wafers but both has
its own limitations in terms of fragility of glass, high cost & limited size of Si
wafers and most important inapplicability for roll-to roll continuous mass
production. To reduce the costs and enable roll-to-roll mass production, there
had been efforts to replace the glass substrates & Si wafers with flexible metal
substrate. Mild steel substrate offers a number of advantages when used as a
substrate. Mild steel is the cost effective substitution as it is feasible with roll to
roll technique and conventionally used in buildings rooftops & facades. These
parts of buildings have always access to sun lights. However, the change of the
substrates from glass to steel or any other metal has a profound effect on the
photovoltaic coating (solar cell) and all process parameters of depositing a solar
cell. Further, because of conductive nature of steel, one insulating dielectric
barrier coating is essential on top of steel substrate prior to solar cell deposition.
The barrier has two functions: (a) to provide electrical insulation between the

steel substrate and the monolithically Inter-connect solar cells; and (b) to reduce
the diffusion of detrimental impurities from the steel substrate into the solar
cells. To avoid short circuits and shunts between individual module cells, the
barrier must guarantee good insulation and adhesion during all deposition steps.
There had been efforts in state of art where researchers have implemented steel
as substrate for depositing the solar cell. JP2004197209 and JP2004035981
patent applications provide the details about hot wire chemical vapour deposition
(HWCVD) technique for depositing amorphous silicon semiconductor device, but
it only provides the electro-conductive surface for the device.
US4551575 teaches substrates for amorphous silicon solar cell like stain less
steel, other metal sheet by electroplating technique.US005818071A discloses
Silicon carbide layer as a metal diffusion barrier layer for integrated chips, but
does not teach deposition of silicon carbide layer having barrier as well as
insulating property by HWCVD technique.
In light of the said prior art, there is still need of an improved method of
fabricating a solar cell on mild steel. The process should ensure that deposited
barrier layer is smooth, uniform and free from pin holes. Further, the process
should make the barrier layer strongly adhesive to the steel substrate and top
layers.

OBJECTS OF THE INVENTION:
It is therefore an object of the invention to develop a method of fabricating a
thin film amorphous silicon solar cell which eliminates the disadvantages of prior
art.
Another object of the invention is to develop a method of fabricating a thin film
amorphous silicon solar cell on a mild steel substrate.
A still another object of the invention is to propose a method of fabricating a thin
film amorphous silicon solar cell using hot-wire chemical vapor deposition
(HWCVD) technique.
A still further object of the invention is to manufacture a thin film amorphous
silicon solar cell.
Summary of the invention:
Accordingly, there is provided a thin film a-Si:H solar cell, which includes:
a steel substrate, an insulating barrier layer deposited over front surface of the
steel substrate, a bottom contact layer, a multilayer n-i-p junction, and a top
transparent conducting layer deposited over the layer of the n-i-p junction.
The present invention also provides a simple and cost effective method for
fabricating the a-Si:H thin film solar cell. The method includes deposition of

insulating barrier layer; deposition of multilayer n-i-p junction and deposition of
top transparent conducting layer.
Deposition of active layers (n-i-p) is done by hot wire chemical vapor deposition
while the top transparent conducting Al-doped Zinc oxide (AZO) layer is
deposited by RF sputtering technique. Polyethyl terephthalate (PET) coating, PET
lamination, and silicon carbide (SiC) can also be used as a suitable barrier layers
on steel substrate according to the current invention.
In the present invention, Hot wire chemical vapor deposition (HWCVD) is used
for deposition of a-Si:H thin film solar cell on the steel substrates. HWCVD
technique is used for deposition of amorphous silicon solar cell because of its
high deposition rate and simple processing. Stainless steel is already known to be
a good substrate in a-Si:H based thin film silicon cells, but the diffusion of
impurities can be observed in stainless steel substrate as well. Since cost of the
mild steel is significantly lower than the stainless steel, it is cost effective to use
mild steel as a substrate in a-Si:H based solar cell.
The process of HWCVD involves dissociation of the feedstock gases into atomic
radicals at the surface of the hot filament (usually tungsten or tantalum), which
is held at a temperature around 1500°C. The reactive species are then
transported to the substrates in a low pressure ambient (typically only 20 bar for
amorphous silicon). This enables a high rate of deposition without gas phase

particle formation. It has been shown that the decomposition of Sim on the
filament surface is not due to heat but due to a chemical adsorption process in
which the filament acts as an adsorbent and a catalyst. HWCVD method has an
advantage of high deposition rate and good quality film deposition.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG.l illustrates a schematic diagram showing various thin film layers forming an
a-Si:H thin film solar cell, according to the present invention.
FIG. 2 (a) to 2 (d) shows layer by layer method of fabricating the a-Si:H solar
cell according to the present invention.
FIG.3 illustrates a schematic diagram of a-Si:H thin film solar cell deposited by
hot wire chemical vapour deposition technique in accordance with the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG.l illustrates a sectional view of the structure of a-Si:H thin film solar cell
As shown in FIG.l, the solar cell structure includes; a mild steel substrate 10, an
insulating barrier coating 11, a bottom electrode layer 12, a n-i-p junction 13 and

a top transparent conducting layer 14. The substrate is made of mild steel but
not limited to mild steel substrate only.
As illustrated in figure 1, a single junction a-Si:H thin film solar cell has a
structure of mild steel/barrier layer/Ag/AZO/N-I-P/AZO. Polyethylene
terephthalate (PET) coating, PET lamination, Silicon carbide, alumina and Silicon
Nitride thin film can be used to make a barrier layer as per the current invention.
Thickness of insulating barrier layer varies from 0.5 μm to 5 μm.
In the present invention, a smooth, stable and pin holes free silicon nitride layer
has been deposited by HWCVD technique on the steel substrate. During the
deposition of Silicon nitride layer, the metallic substrate temperature is in the range 200-
300°C temperature and filament temperature is in the range of 1700- 1900°C. The flow
rate SiH4, NH3 and N2 gases is in the ranges 1-5 sccm, 10-25 sccm and 10-30 sccm
respectively and the deposition pressure is 4 x 10-2 mbar.
Since the barrier layer isolates the back contact layer from the substrate, the
mild steel substrate disallows any current leak from the back contact layer to the
steel substrate and prevents impurities from the substrate to diffuse to active
layers of the solar cell. Table I lists the process variables in the optimization
process of thin film of Ag deposited over insulating barrier layer. Thin film of

silver (thickness ~ 300-400 nm) is deposited by DC magnetron sputtering as a
back contact layer. The deposition conditions for depositing silver film are given
in the table 1 below.

Table 1. Deposition conditions for Ag thin films
After the deposition of silver back contact layer, AZO layer is deposited. The
thickness of AZO layer is 250-400 nm. The deposition conditions for depositing
AZO layer are given in the table 2.

Table 2. Process parameters for AZO thin films
After depositing AZO layer, an n-type layer is deposited by HWCVD technique,
n-type layer has a thickness in the range of 20- 25 nm. The deposition conditions
for depositing n-type layer are given in the table 3.


Table 3 - optimum deposition parameters for n type a-Si:H layer
The intrinsic a-Si:H layer works as an absorber layer and is deposited after
deposition of n-type layer by HWCV technique. The intrinsic a-Si:H layer has a
thickness in the range of 300-400 nm. The deposition conditions for depositing i-a-
Si:H layer are given in the table 4.

Table 4. Optimum deposition parameters for intrinsic i-a-Si:H layer
P type a-Si:H layer is deposited after deposition of i-type layer by HWCV
technique. The P-type layer has a thickness in the range of 8-10 nm. The deposition
conditions for depositing p-type layer are given in the table 5.


Table 5. Optimum deposition parameters for p type a-Si:H layer
The doped p-type and n-type layers create electric field across the intrinsic i-
layer. The p-type and n-type layers are generally heavily doped. The doping level
in these layers determines the magnitude of the electric field generated. The
higher doping level gives higher electric field. The high doping level is also useful
in making low resistance ohmic contact with the Transparent conducting oxides
(TCO) and metal.
After the deposition of n-i-p junction, a transparent conducting layer of
aluminium doped zinc oxide (AZO) is deposited by RF magnetron sputtering.
The optimum deposition parameters of top AZO layers are as follows:


Table 6. optimum deposition parameters of AZO layer
In the present invention, intrinsic a-Si:H layer with the dark conductivity of 5x10"
11 Q^cm"1 and photoconductivity of 6.1xl0"5 Q^cm"1 (photoconductivity gain of
~106), highly conducting n-type a-Si:H layer with conductivity value of 3.2xl0"2
Q^cm"1 and p-type a-Si:H layer with conductivity value of 8.0xl0"3 Q^cm_1 are
used for the fabrication of a-Si:H single junction n-i-p solar cell on mild steel
substrate.
The current method of depositing a solar cell on steel substrate enables high
quality of deposited films. Further, method provides a cost friendly approach to
manufacture solar cell using mild steel as substrate.

We Claim:
l. A method for fabricating a thin film solar cell, the method comprising:
depositing an insulating barrier layer on a metallic substrate by hot wire
chemical vapour deposition (HWCVD) technique, the metallic substrate
being subjected to 200-300°C temperature and filament temperature in
the range of 1700- 1900°C, SiH4, NH3 and N2 gases' flow-rate in the
ranges 1-5 seem, 10-25 seem and 10-30 seem respectively,
depositing a thin silver metal layer on the insulating barrier layer;
depositing Aluminium doped zinc oxide (AZO) layer;
depositing a n type a-Si:H layer by HWCVD technique with a flow rate of
Sim and PH3 at 6 seem and 3 seem respectively for 80 seconds;
depositing a p type a-Si:H layer by HWCVD technique with flow rate of
S1H4 and B2H6 at 6 seem and 3 seem respectively for 45 seconds.;
depositing an intrinsic layer of a-Si:H between n type and p type a-Si:H
layer by HWCVD technique with flow rate of SiH4 at 20 seem for 10-15
minutes; and
depositing a top transparent conducting layer over n-i-p junction.
2. A method as claimed in claim 1, wherein the insulating barrier layer is
made of silicon nitride.

3. A method as claimed in claim 1, wherein the insulating barrier layer is
made of silicon carbide.
4. A method as claimed in claim 1, wherein the insulating barrier layer has a
thickness in the range of 0.5 ^im to 5 ^m.
5. A method as claimed in claim 1, wherein the n type layer is deposited in a
chamber having a base pressure of 1.0 x 10"6 mbar, and a deposition
pressure of approximately 2.6 x 10-2 mbar.
6. A method as claimed in claim 1, wherein the n-type layer is deposited by
HWCVD having filament temperature of 1650°C and substrate
temperature 150°C.
7. A method as claimed in claim 1, wherein the n-type layer has a thickness
in the range of 20- 25 nm.
8. A method as claimed in claim 1, wherein the p type layer is deposited by
HWCVD technique with filament temperature of 1700°C and substrate
temperature in the range of 150-180°C in a chamber having base pressure
3.0 x 10"6 mbar, deposition pressure of approximately 2.6 x 10~2 mbar.

9. A method as claimed in claim 1, wherein the p type layer has a thickness
in the range of 8-10 nm.
10. A method as claimed in claim 1, wherein the intrinsic a-Si:H layer is
deposited by HWCVD technique with filament temperature in the range
1450-1550°C and substrate temperature in the range 150-200°C
approximately in a chamber having a base pressure of 1.0- 5.0 x 10"6
mbar and a , deposition pressure of 1.0- 3.0 x 10"2 mbar approximately.
11. A method as claimed in claim 1, wherein the intrinsic a-Si:H layer has a
thickness in the range of 300-400 nm.
12. A method as claimed in claim 1, where the front contact layer of
Aluminium doped zinc oxide (AZO) is deposited by RF magnetron
sputtering on n-i-p junction with a deposition pressure of 5.0-9.0 X 10"4
mbar and power of 100-150 W, with Argon flow rate 5-10 seem for a
deposition time 8-10 minute.
13. A thin film solar cell manufactured by the method as claimed in the claim
1.

14. A method for fabricating a thin film solar cell, the method comprising:
depositing an insulating barrier layer on a metallic substrate by hot wire
chemical vapour deposition (HWCVD) technique, the metallic substrate
being subjected to 200-300°C and filament temperature in the range of
1700- 1900°C, and with SiH4/ NH3 and N2 gas flow rate in the ranges 1-5
seem, 10-25 seem and 10-30 seem respectively.
15. A method as claimed in claim 1, or claim 15, wherein the metallic
substrate is mild steel.

ABSTRACT

The invention relates to a method for fabricating a thin film solar cell. The method
provides depositing an insulating barrier layer and active layers (n-i-p junction) on a
metallic substrate by hot wire chemical vapour deposition (HWCVD) technique. The top
transparent conducting Aluminium doped Zinc oxide layer is deposited by RF sputtering
technique. The invention provides a cost effective method of manufacturing a solar cell
using mild steel as substrate.