TITLE:
A method to deposit Copper Indium Gallium Diselenide (CIGS)/ Copper Zinc Tin
Sulphide (CZTS) for superstrate CIGS / CZTS solar cells
FIELD OF INVENTION:
This invention relates to deposition of thin films in solar cells. In particular, the
invention relates to a method for depositing thin films over a substrate having
pre-deposited film of temperature sensitive material.
BACKGROUND OF THE INVENTION:
With ever increasing demand of energy and depleting fossil fuel reserves
throughout the world, solar energy can be presented as most suitable energy
alternative. To harness solar energy directly in the form of electricity, many
photovoltaic technologies have evolved. CIGS/CZTS based thin film solar cells
(TFSCs) are one of them. The advantages of CIGS/CZTS based TFSCs include
low material consumption, higher absorption coefficient, ability to work in diffused
light and possibility for the use of flexible substrate. However, economy of thin
film deposition techniques and other manufacturing processes have been a road
block for its wide scale implementation.

Thin film deposition techniques play an important role in making films of required
optical, mechanical and electronic properties in research and production
environment. In case of CIGS/CZTS TFSCs, it is required to stack multiple layers
of same or different material to make a complete device. As generation and
transport of electrons and holes is involved during operation, quality of interface
and defects within the layer play important role in determining the efficacy of
device. The techniques used to deposit these thin films vary widely in the degree
of sophistication and cost of deposition. This in turn changes the quality and
properties of the thin film being deposited. Different deposition techniques used
to deposit thin films for solar cell applications may be categorized mainly as non-
vacuum processes and vacuum processes.
Popular non-vacuum processes for depositing thin films include dip coating, spin
coating doctor blading, spraying, screen printing and electroplating; these
processes have been used extensively in research to study the properties of the
thin films and their applications [12-20]. However, relatively lower efficiencies are
achieved using these methods due to less control over environmental factors as
compared to vacuum processes.

Vacuum processes such as sputtering, thermal evaporation, chemical vapor
deposition etc have been used to make thin films [1-11]. A resistance-heated
evaporation source is relatively simple and inexpensive, but the material capacity
is very small. Sputter deposition can be used to coat large areas and complex
surfaces in production coating environments utilizing time and power for rate
control. Typically, Cu(In,Ga)Se2 (CIGS) thin films for photovoltaic devices are
deposited by co-evaporation or, alternately, by deposition of the metals with or
followed by treatment in a selenium environment. J.A. Frantz et.al. [1] described
CIGS films that are deposited by RF magnetron sputtering from a single
quaternary target without any additional selenization. The deposition power was
varied to change the film morphology of films.
Young Min Shin et.al. [2] demonstrated a three-stage co-evaporation process to
deposit Cu deficient CIGS for CIGS solar cells. In order to control the
reproducibility of CIGS film, the film was annealed in air, S, or Se. With this
annealing procedure, the Cu content of the CIGS surface was significantly
reduced and Ga content was strongly increased. An intrinsic CIGS layer with a
lower valence-band maximum and a wider ban gap was formed at the surface.
By annealing the CIGS film, the open-circuit voltage and fill factor were
significantly improved, which indicated that the surface intrinsic layer acts as a
hole-blocking layer so that the surface recombination rate was suppressed.

Bai et.al. [3] described one-step pulsed DC magnetron sputtering process to
deposit CIGS films at room temperature. Under different sputtering powers and
pulse frequencies, and the effects of sputtering voltage and current on the
composition and microstructure of the deposited GIGS films were systematically
investigated. Studies indicated the composition of the deposited films was
seriously affected by the sputtering voltage and current. In addition, the
deposition rate (corresponding to sputtering current) is considered as the key
factor of affecting the preferred orientation of the films, i.e., the preferred
orientation changed from (112) to (220)/ (204) with increasing deposition rate.
Furthermore, the electrical properties of the CIGS films deposited at different
sputtering powers were closely associated with the composition and
microstructure of the films.
Shigeru Ikeda et.al. [4] demonstrated superstrate solar cells by stacking thin films
composed of In2(Se,S)3 and CuIn(Se,S)2 layers were grown on a fluorine-doped
tin oxide (FTO)-coated glass substrate using electrodeposition of the
corresponding selenide (In2Se3 and CuInSe2) precursors followed by annealing in
H2S flow (5% in Ar). Similarly, Shintaro Osada et.al. [5] fabricated CIGS solar
cells with a superstrate structure using a lift-off process. To widen the variety of
substrate choices for CIGS solar cells, a lift-off process was developed without
an intentional sacrificial layer between the CIGS and Mo back-contact layers.

The CIGS solar cells fabricated on Mo/soda-lime glass (SLG) were transferred to
an alternative SLG substrate. The conversion efficiency of the CIGS solar cells
with the superstrate structure was 5.1%.
In the present disclosure, a modified sputtering method is discussed, which
allows deposition of CIGS/ CZTS film over temperature sensitive buffer layer. In
the disclosed method, CIGS/ CZTS deposition is carried out at in multiple steps
of low and high temperature to minimize the effect of temperature on sensitive
buffer layer. The first step deposition is carried out at lower temperature to form a
protective layer of CIGS/ CZTS over buffer layer. The second deposition is
carries out at comparatively higher temperature to achieve adhering and
crystalline CIGS/ CZTS film.
OBJECTS OF THE INVENTION:
An object of the present invention is to propose a method for depositing thin films
over a substrate having pre-deposited film of temperature sensitive material.

Another object of the present invention is to propose a method, wherein a single
sputtering target of quaternary compound of known stoichiometry is used to
deposit crystalline and uniform thin film quaternary compound at alternate low
and high temperature in same sputtering chamber with control on other
deposition parameters.
Further object of the present invention is to propose a method, wherein a layer of
material sensitive to high temperature is pre-deposited on substrate, on which
uniform thin film layer of quaternary compound of known stoichiometry is
deposited at high temperature forming electronic interface between two layers
and both layers have at least one element which is common or is from same
group in periodic table having similar properties.
BRIEF DESCRIPTION OF THE PRESENT INVENTION:
This invention relates to a method for depositing thin films over a substrate
having pre-deposited film of temperature sensitive material comprising:
heating the substrate having a thin film of temperature sensitive material in high
vacuum chamber on a rotating heating plate to a temperature range of 25-250°C,

creating the plasma by releasing 2.5 m Torr to 8 m Torr of working gas and
applying radio frequency or direct current,
depositing the plasma and then it was cert off after 2 to 20 minutes raising the
temperatures of the said coated substrate to 300 to 550°C,
creating the plasma by releasing 2-5 Torr to 8m Torr of working gas and apply
RF or DC power,
depositing the plasma and cutting off the plasma after 30-120 minutes,
subjecting the coated substrate to the step of cooling, the vacuum was gradually
released and substrate was taken out.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1 shows the schematic diagram of the apparatus used for depositing thin
films of Copper Indium Gallium Diselenide (CIGS)/ Copper Zinc Tin Sulphide
(CZTS) by the disclosed method.

Figure 2 shows different steps involved in deposition of thin films of Copper
Indium Gallium Diselenide (CIGS)/ Copper Zinc Tin Sulphide (CZTS) by the
disclosed method.
Figure 3 shows the schematic of Glass/ITO/CdS stack used as substrate used in
example 1.
Figure 4(a) shows top view SEM image of the CIGS film deposited by disclosed
method in example 1 and 4(b) shows cross-sectional view SEM image of the
CIGS film deposited by disclosed method in example 1.
DETAILED DESCRIPTION OF THE INVENTION:
The present invention discloses a novel method to deposit thin films for
multilayered devices, especially for thin film solar cell application and specifically,
devices where first layer is sensitive to high temperature and second layer to be
deposited on first layer needs high temperature for crystallinity and good
electronic properties. Both layers have at least one element which is common or
is from same group in periodic table having similar properties. A thin film of
photovoltaic material like CIGS/ CZTS / other quaternary compounds is

deposited over a substrate having pre-deposited film of Cadmium Sulphide (CdS)
or other material which is sensitive to high temperature without affecting the
properties pre-deposited film. The disclosed method improves the crystallinity of
pre-deposited film and form good electronic interface with photovoltaic material.
The embodiments of the present invention disclose a method to deposit a
continuous, uniform and adhesive film of photovoltaic material like CIGS/ CZTS
or other quaternary compounds over a substrate having pre-deposited film of
Cadmium Sulphide (CdS) or other material which is sensitive to high temperature
without affecting the properties pre-deposited film. In superstrate configuration of
thin film solar cells, shown in figure 1, 30-100 nm buffer layer of n-type material
like CdS is deposited over transparent conducting contact layer. When p-type
CIGS/ CZTS absorber layer is deposited over CdS buffer layer by sputtering at
temperature greater than 250°C, sulphur atoms start escaping from the CdS
layer and hence deteriorating its electronic properties. To address this problem, a
new method for deposition of CIGS/ CZTS layer over temperature sensitive CdS
layer has been invented.

In the preferred embodiment, a modified sputtering method to deposit multiple
layers of CIGS/ CZTS/ other quaternary compounds over a substrate having pre-
deposited film of Cadmium Sulphide (CdS) or other material which are sensitive
to high temperature at two or more different substrate temperature is disclosed. A
single composite sputtering target of quaternary compound like CIGS, CZTS of
known stoichiometry is used for deposition. Transparent conducting oxide (TCO)
and CdS buffer layer are deposited sequentially over glass substrate by other
techniques. This glass/ TCO/ buffer layer stack is used as substrate in disclosed
invention. The substrate is heated to low temperature of 25°C to 250°C in a
vacuum chamber, so that stoichiometric ratio of buffer layer remains unchanged.
The argon, nitrogen or other carrier gas is released in the working chamber, and
working pressure is maintained at 2.5mTorr to 8mTorr. The substrate is held
exactly above the target. Distance between sputtering target and the substrate
vary with diameter of target and is kept at 25% to 125% of the diameter of the
target. A continuous rotation of 1 to 10 RPM is provided to substrate to nullify the
effect of any spatial non uniformity. To create the plasma and carry target atoms/
molecules to substrate, 75 W to 175W power from direct current (DC) source or
radio frequency (RF) source is applied at target for 60 to 900 seconds. The
power supply and hence plasma is then cut off. Temperature of substrate is then

increased to relatively higher value of 300°C to 550°C, keeping other parameters
constant. The higher temperature assists in deposition of continuous, uniform
and adhesive film with less defects compared to low temperature deposition.
Plasma is again created by applying 75 W to 175W power from direct current
(DC) source or radio frequency (RF) source to deposit target atoms/ molecules
on heated substrate. The deposition is carried out for 900 to 7200 seconds.
Further depositions can be carried out at even higher temperature if required.
Compared to established low temperature sputtering method used for deposition
of CIGS/ CZTS/ other quaternary compounds on temperature sensitive substrate
for superstrate thin film solar cell, the disclosed method produces relatively
improved crystal growth and a uniform and adhesive film with less defects. Since,
in photovoltaic devices most of the electron-hole recombinations occur at the
grain boundaries, larger crystals and lesser defects decrease the probability of
recombination of the carriers. Furthermore, the disclosed method does not alter
the stoichiometry and electronic properties of temperature sensitive buffer layer
compared to established high temperature sputtering method.

In the disclosed method, the pre-deposited temperature sensitive material and
the target material have at least one common volatile element or have volatile
elements from same group in periodic table having similar properties. Initially a
layer of target material is deposited over pre-deposited layer at low temperature.
When temperature of substrate is increased to deposit another layer of target
material, the volatile element from lattice of temperature sensitive layer move to
lattice of target material layer and vice versa. Thus, the change in lattice structure
of both the layer is minimal.
Figure 1 shows the schematic diagram of the apparatus used for the disclosed
method. Substrate (B) coated with a thin layer of temperature sensitive material
(A) is held above the target on rotating substrate holder (C). Substrate rotating
motor (G) provides the rotation to substrate holder to nullify the effect of any
spatial non uniformity. Heater (F) with PID control is used to heat the substrate to
desired temperature. Gas inlets (H) are used to flow working gas in the chamber
to create plasma. RF power source (F) provides the controlled power to target
(D) to create plasma to carry target material to substrate.

EXAMPLE:
In a preferred embodiment, 70 nm thin CdS layer was deposited over ITO coated
glass by chemical bath deposition. This Glass/ ITO/ CdS stack of 25mm x 25mm
area was used as substrate for deposition of CIGS by disclosed method. Figure 3
shows the schematic of Glass/ ITO/ CdS stack used as substrate. It was
observed that the ratio of sulphur to cadmium decreased from 1 to 0.7 when the
stack was heated to temperature above 280°C. To minimize the effect of
temperature on CdS layer, initial deposition was carried out at 250°C. The stack
was placed on rotating substrate table in vacuum chamber above the target and
parallel to it at a distance of 54 mm. A circular pressed target of CIGS of
diameter 4 inches and C:I:G:S ratio 1:0.7:0.3:2 was used for the process. A
pressure of 5 x 10-6 mbar vacuum was created in the chamber by turbo molecular
pump. The temperature of the substrate was then increased to 250°C. The
substrate table was rotated constantly at 2 rotations per minute to ensure uniform
heating and deposition. The plasma was then created by releasing 3.5 mTorr of
argon and applying 125W RF power between CIGS target and its shield. After
deposition for 600 seconds at 250°C, the plasma was cut off. The temperature of
substrate was then increased to 500°C. The thin layer of CIGS over CdS

deposited at 250°C prevented sulphur from escaping the CdS layer. At 500°C,
plasma was again created by releasing 3.5 mTorr of argon and applying 125W
RF power. Deposition was continued for 3000 seconds before cutting off plasma
again. Substrate was slowly cooled, vacuum was gradually released and
substrate was taken out. The figure 4(a) and 4(b) shows the SEM images top
view and cross sectional view respectively of the CIGS film deposited by the
disclosed method. It is clearly seen that the CIGS film deposited by the
disclosed method is continuous and uniform.

WE CLAIM:
1. A method for depositing thin films over a substrate having pre-deposited film of
temperature sensitive material comprising:
heating the substrate having a thin film of temperature sensitive material in high
vacuum chamber on a rotating heating plate to a temperature range of 25-250°C,
creating the plasma by releasing 2.5 m Torr to 8 m Torr of working gas and
applying radio frequency or direct current,
depositing the plasma and then it was cert off after 2 to 20 minutes raising the
temperatures of the said coated substrate to 300 to 550°C,
creating the plasma by releasing 2-5 Torr to 8m Torr of working gas and apply
RF or DC power,
depositing the plasma and cutting off the plasma after 30-120 minutes,
subjecting the coated substrate to the step of cooling, the vacuum was gradually
released and substrate was taken out.
2. The method as claimed in claim 1, wherein, the ratio of C:I:G:S is 1:0.7:0.3:2.

3. The method as claimed in claim 1, wherein the said thin films are selected
from copper Indium Gallium Diselenide (CIGS) and
Copper Zinc Tin Sulphide (CZTS).
4. The method as claimed in claim 1, wherein the said temperature sensitive
material preferably is CdS(Cadmium sulphide).
5. The method of claim 1 wherein working gas can be nitrogen, argon or any
other inert gas.
6. The method of claim 1 wherein power source to create plasma can be direct
current (DC) source or radio frequency (RF) source.
7. The method of claim 1 wherein DC/RF power applied to create plasma can
vary from 75W to 150W.
8. The method of claim 1 wherein substrate is continuously rotated at speed of 1
to 10 RPM during deposition.

9. The method of claim 1 wherein distance between substrate and target
depends on diameter of target and can be varied from 25% to 125% of the
diameter of the target.
10. The method of claim 1 wherein a uniform, continuous and adhesive film of
quaternary compound is obtained without any other binder.
11. The method of claim 1 wherein the number of coatings over the surface of
substrate can vary from 2 to 8.