|Title of Invention||
A PROCESS FOR THE PREPARATION OF AMORPHOUS SILICON BASED SOLAR CELLS
|Abstract||1. A process for the preparation of an amorphous silicon based solar cell having the structure, Glass/SnO2/p-a-SiC:H/i-a- Si:H/n-a-Si:H/Al comprising of the steps of depositing the p- aSiC:H, i-aSi:H and n-a-Si:H layers on a SnO2 coated glass substrate by radio frequency assisted plasma enhanced chemical vapour deposition (PE CVD) in a single chamber apparatus using argon dilution in the range of 90% to 95% for all the layers.|
|Full Text||FIELD OF THE INVENTION
This invention relates to a process for the preparation of amorphous silicon
based solar cells.
This invention further relates to a process for the preparation of silicon based thin
film solar cells.
BACKGROUND OF THE INVENTION
Amorphous silicon based solar cells are now available commercially. A standard
commercial process involves radio frequency electric field generated discharge in
suitable process gases, which consist of silicon source gas silane (SiH4) mixed
with other gases like phosphine (PH3), diborane (B2H6), methane (CH4) and lot of
hydrogen (H2). In fact, hydrogen as a diluent gas is used in large quantities to
improve the performance of the solar cells. Silane burns spontaneously in
presence of oxygen. Moreover, hydrogen is a highly inflammable gas, which in
the presence of silane must be handled carefully to avoid outbreaks of fire.
Therefore, elaborate arrangement for venting and neutralising the spent process
gases becomes necessary for the safety of the equipment and the operating
OBJECTS OF THE INVENTION
It is therefore an object of this invention to propose a process for the preparation
of amorphous silicon based solar cells, using less hazardous gases, which will
minimise associated risk of fire.
It is a further object of this invention to propose a process for the preparation of
amorphous silicon based solar cells, which does not involve elaborate safety
arrangements for the instruments and their operating personnel and is therefore
Yet another object of this invention is to propose a process for the preparation of
amorphous silicon based solar cells, which leads to a higher rate of deposition, to
afford the cells at a faster rate.
Yet another object of this invention is to propose a process for the preparation of
amorphous silicon based solar cells, which leads to less light induced
degradation as compared to standard amorphous silicon based solar cells with
similar structure but in which the intrinsic layer deposited from hydrogen dilution
of silane gas.
These and other objects of the invention will be apparent from the ensuing
According to this invention a process is provided for the preparation of
amorphous silicon based solar cells.
In accordance with this invention, the amorphous silicon based thin film solar
cells of 1 cm2 active area, having a structure glass/Sn02/p-aSiC:H/i-a-Si:H/n-a-
Si:H/AI of which the p-type amorphous silicon carbide (p-aSiC:H), intrinsic
amorphous silicon (i-a-Si:H) and n-type amorphous silicon (n-a-Si:H) layers have
been deposited in a single chamber plasma enhanced chemical vapour
deposition system have efficiencies of the order of 8%. The average degradation
of efficiency under white light illumination for 100 hrs. is about 15%. Argon (Ar)
gas has been chosen to replace hydrogen in the standard amorphous silicon
solar cell fabrication process. Argon being totally benign, the risk of fire hazard is
minimised. Argon gas has been used as diluent instead of hydrogen gas for all
the different layers like the p-type, n-type and intrinsic silicon. Optimum radio
frequency power density lies between 10 and 353 mW/cm2 and preferably
between 20 to 80 mW/cm2. Optimum pressure is in the range of 0.1 to 0.5 Torr.
The Argon dilution used in the range of 1:99 to 100:0 in terms of silane: Argon.
Optimum ratio of silane to argon has been found between 1:99 and 20:80
preferably between 2:98 and 10:90.
The process may be applied for large area deposition of amorphous silicon solar
panels. Structure and transport properties of the Si:H based materials applied in
the different layers of the solar cell were studied by standard techniques.
The invention will now be explained in greater detail with the help of the following
An amorphous silicon based thin film solar cell having 1 cm2 active area and the
was constructed with the p-aSiC:H, i-a-Si:H and n-a-Si:H layers deposited in an
ultra high vacuum single chamber plasma enhanced chemical vapour deposition
(PECVD) apparatus using the following materials.
Substrate: Glass (corning) coated with tin oxide (Sno2) and having a haze of
Silane Semiconductor grade
Methane: Ultrahigh purity.
Argon Ultrahigh purity.
Dopants : Diborane (1%) in hydrogen
Phosphine (1%) in hydrogen
The Aluminum (Al) back electrode was deposited by vacuum evaporation.
The layers were deposited by radio frequency electric field at a frequency of
13.56 MHz. The power density Prf measured from the difference of the forward
and reflected applied rf power was between 20 and 80 mW/cm2. Deposition
temperature was between 180°C and 250°C. Deposition pressure was between
0.1 Torr and 0.5 Torr. The SiH4 percentage in the SiH4-Ar mixture was between
2% and 20%.
The a-Si:H material thus obtained was studied for various structural and
electrical transport characteristics; e.g. microstructure by SAXS method,
minority carrier diffusion Length (Ld) by steady state photocarrier grating
technique and the electron mobility-lifetime (µ) product from
photoconductivity, and the results are depicted in the figures of the
Fig 1 : SAXS intensity as a function of scattering vector (q)
Fig 2 : Variation of integrated SAXS intensity and A with Ar dilution
Fig 3 : Evolution of ut and Ld with radio frequency power density for
silane.argon = 5:95
Fig 4 : Intensity of the emission line at 414.2 nm of the SiH* at different
dilutions and Prf.
Fig 5 : Statistical distribution of the conversion efficiency ( r\ %) of a set of
Fig 6 : Normalized PV parameters vs light soaking time.
Fig 7 : Spectral response of a solar cell according to the invention
Fig 8 : Light current voltage characteristic of a solar cell according to the
Fig 1 : Shows SAXS intensity (in electron units) as a function of scattering vector
(q) for a-Si:H films deposited with different Ar dilutions and without any dilution at
low-rf power density (35 mW/cm2). A standard a-Si:H film deposited by hydrogen
dilution of silane at National Renewable Energy Laboratory, USA has also been
included for comparison.
Fig 2 : Shows the variations of the micro-structure parameters Q (representative
of nanostructural heterogeneity) and A (representative of larger scale structural
heterogeneity) as determined from SAXS of the Si:H samples deposited from
silane-argon mixtures at constant power density of 35 mW/cm2. The
corresponding values for the standard sample have also been shown.
SAXS is a powerful technique to provide information about structural
heterogeneities in terms of electron density fluctuation on a size scale up to 100
nm. SAXS intensity l(q) is plotted as a function the scattering vector q= 2n sinGA,,
the scattering angle being varied in the range 0.13°-8.8° and X being the X-ray
wavelength. I(q) plot can be divided into three distinct components:
l(q) = lL(q) + lN(q) + ld (1)
Where lL (q) is the intensity due to large size (>30 nm) scattering objects, lN(q) is
due to small objects of size between 1 nm and 30 nm and Id is a background
intensity due to the aluminum foil and the absorption by the film itself and is
almost independent of q. IL (q) follows a Porod law:
lL(q) = Aq-3 (2)
Where A is a constant.
The effect of the small size particles is taken into account in the factor Q obtained
by integrating lN(q) considering that the particles are randomly oriented in a
matrix with an uniform electron density:
The minima in both Q and A of the samples indicate a sharp fall in the samples
microstructure near 85 to 95% Argon with optimum at 93% Ar dilution. The sharp
drop of both parameters indicates that for a given Ar dilution the layers are better
ordered and should show improved transport properties. Arrows show the
optimum conditions for the materials for the best solar cells.
Figure 3 shows the variations of the electron mobility-lifetime (nx) product and of
the hole diffusion length (Ld) vs rf power density (Prf) for a 95% Ar dilution
sample. Both parameters present a maximum for Prf around 80 mW/cm2for \ix
and 40 mW/cm2 for Ld. At low Prf the simultaneous increase of these parameters
indicates that the material improves with increasing rf power. For high Prf the
carrier transport properties of the material degrades with the formation of
microcrystallites within the layers as confirmed from Raman spectroscopy and
TEM electron diffraction patterns. The optimum value of Ld is rather high (150
nm) and decreases by only 12% after light-soaking.
The plasma during deposition under different conditions was analyzed by Optical
Emission Sprotroscopy (OES). The intensity of the emission line at 414.2 nm of
the SiH* was recorded for different dilutions and Prf. The results are shown in Fig
4. We have found that, with Ar dilution of silane, optimum condition for deposition
of solar cells occurs at rf power densities just before the maximum of the SiH*
emission intensity that is in the low rf power range for 90% and 95% argon
dilutions. Intensity of scattered Ar* ion laser beam (488 nm) from the powders
formed in the plasma just vanishes at these power densities indicating the cut off
for powder formation. This justifies the growth of best device quality material. At
very low rf power densities again the quality of the material deteriorates.
The most interesting part of the above studies is that it has been possible to
obtain a device quality Si:H alloy material without hydrogen dilution of silane. The
structural and the transport property studies clearly show that the Ar diluted
materials are as good as the hydrogen diluted ones. The complete physics
behind the intricate ways by which argon influences the reactions within the
plasma and on the growth surface is not yet very clear. Balance between the
concentrations of metastable Ar* molecules and Ar+ ions in the plasma may have
an important role to determine the film properties. We have proposed that the
presence of metastable Ar* molecules in the plasma modifies the plasma
reactions and helps to produce more beneficial SiH3 precursors. Moreover,
bombardment of these metastable Ar* molecules on the growing film surface is
soft compared to that by the Ar+ ions. The former being neutral are incident on
the surface with thermal energy and therefore do not penetrate much within the
bulk. The energy released by de-excitation of these metastable molecules on the
growth surface is taken up by the precursors like SiH3. The surface mobility of the
precursors is thus greatly enhanced, which helps them to migrate over many
sites before ultimate incorporation into the film. Weak Si-Si bonds present on the
surface are also broken up by obtaining energy from the Ar* and the atoms
rearrange themselves to give stronger Si-Si bonds. Both of these processes help
in producing a more compact material.
More than 75 number of 1 cm2 diodes have been prepared in the single chamber
deposition system without any special optimization e.g. use of a buffer layer,
microcrystalline p and n layers etc. Fig. 5 shows the statistical distribution of the
conversion efficiency (r| %) of this set of diodes. The mean value of ti is
around 6.5% and the best conversion efficiency was close to 8%. For the best
diodes the short circuit current density lSc = 14.8 mA/cm2 and the open circuit
voltage V0c= 0.81 V. We want to stress that these 75 diodes were obtained in 52
runs. In the same run three samples can be deposited at a time and we have
always obtained a good homogeneity in the properties of the samples. After light
soaking during 100 h under AM1.5G the conversion efficiency decreases by 15%
and reaches saturation (See Fig. 6). Thus, the best stabilized conversion
efficiency is slightly lower than 7%. This low degradation of the order of 15%
when compared with the 30% degradation obtained on similar structure p-i-n
diodes based on hydrogen diluted standard a-Si:H, shows that the devices
obtained with Ar dilution are definitely more stable than standard devices. It is
believed that the conversion efficiency can be increased after optimization of the
doped layers as well as the P/l interface. The use of a multichamber deposition
unit where the p-aSiC:H, i-a-Si:H and n-a-Si:H layers are deposited in different
chambers, as well as texturized conductive oxide on the glass substrate should
also improve the quality of the devices.
Fig 7: Shows the typical quantum efficiency of solar cell with I- layer deposited at
93% Ar dilution and Fig. 8 shows the typical light l-V characteristic with Mayer
deposited at 93% Argon dilution.
The last interesting parameter is the deposition rate: under optimum condition
deposition rate of 1.4 A/s, slightly higher than the deposition rate of standard a-
Si:H films, has been measured.
From a complete study of the properties of a-Si:H thin films deposited from a
mixture of silane and argon the optimum conditions under which device grade
thin films can be obtained has been determined. One square centimeter devices
were produced entirely under argon dilution and show a good conversion
efficiency and an improved stability compared to devices based on standard a-
Si:H. The main result is therefore that argon dilution of silane to produce silicon
thin films by rf-PECVD glow discharge is not detrimental to the deposition of
device grade films and could be an alternative of hydrogen dilution providing a
less hazardous process.
1. A process for the preparation of an amorphous silicon based
solar cell having the structure, Glass/SnO2/p-a-SiC:H/i-a-
Si:H/n-a-Si:H/Al comprising of the steps of depositing the p-
aSiC:H, i-aSi:H and n-a-Si:H layers on a SnO2 coated glass
substrate by radio frequency assisted plasma enhanced
chemical vapour deposition (PE CVD) in a single chamber
apparatus using argon dilution in the range of 90% to 95% for
all the layers.
2. The process as claimed in claim 1, wherein the layers are
deposited on tin oxide coated glass substrate.
3. The process as claimed in claim 1, wherein the single chamber
apparatus is an ultra high vacuum single chamber apparatus.
4. The process as claimed in claim 1, wherein said p-type layer is
doped with boron.
5. The process as claimed in claim 4, wherein diborane in a
proportion of 1% diborane in hydrogen, is used for doping.
6. The process as claimed in claim 1, wherein the n-type layer is
doped with phosphorus.
7. The process as claimed in claim 6, wherein phosphine in a
proportion of 1% in hydrogen is used for doping.
8. The process as claimed in claim 1, wherein a radio frequency of
13.56 MHz is used for the deposition.
9. The process as claimed in claim 1, wherein the substrate is
maintained at a temperature in the range of 180°C to 250°C.
10. The process as claimed in claim 1, wherein the power density is
maintained between 10 to 353 mW/cm2, preferably between 20
to 80 mW/cm2.
11. The process as claimed in claim 1, wherein the pressure in the
chamber is maintained between 0.1 Torr to 0.5 Torr, preferably
at about 0.2 Torr.
12. The process as claimed in claim 1, wherein the Silane:Argon
ratio is preferably between 2:98 and 10:90.
13. An amorphous silicon based solar cell having the structure
14. The solar cell as claimed in claim 13, wherein the p-junction is
doped with boron and the n-junction with phosphorus.
15. The solar cell as claimed in claim 13, prepared by using Argon
as the diluent gas.
16. A process for the preparation of an amorphous silicon based
solar cell having the structure, Glass/SnO2/p-a-SiC:H/i-a-
Si:H/n-a-Si:H/Al substantially as herein described.
17. An amorphous silicon based solar cell having the structure
Glass/Sn02/p-a-SiC:H/i-a-Si:H/n-a-Si:H/Al prepared by the
process steps substantially as herein described.
1. A process for the preparation of an amorphous silicon based solar cell having the structure, Glass/SnO2/p-a-SiC:H/i-a- Si:H/n-a-Si:H/Al comprising of the steps of depositing the p-
aSiC:H, i-aSi:H and n-a-Si:H layers on a SnO2 coated glass substrate by radio frequency assisted plasma enhanced chemical vapour deposition (PE CVD) in a single chamber
apparatus using argon dilution in the range of 90% to 95% for all the layers.
|Indian Patent Application Number||667/CAL/2002|
|PG Journal Number||41/2010|
|Date of Filing||28-Nov-2002|
|Name of Patentee||INDIAN ASSOCIATION FOR THE CULTIVATION FOR CULTIVATION OF SCIENCE|
|Applicant Address||2A & B RAJA S C MULLICK ROAD, JADAVPUR, KOLKATA|
|PCT International Classification Number||H01L 31/06|
|PCT International Application Number||N/A|
|PCT International Filing date|