TITLE:
Method to deposit continuous uniform adhesive films using pre-synthesized powder
material.
FIELD OF INVENTION:
The present invention relates to a method for depositing thin films.
Especially for thin film solar cell application and specifically, a method in which pre
synthesized particles are suspended in a solution containing precursors which form the
required thin film upon impinging them in atomized state onto a heated substrate i.e. the
suspended particles and the precursor solution together make a uniform adhesive film
with better optical and electronic behavior.
BACKGROUND OF INVENTION:
Thin film deposition techniques play an important role in making films of required
optical, mechanical and electronic properties in research and production environment.
The advantages of thin films are low material consumption and possibility for the use of
flexible substrate. 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. Broadly, the techniques may be divided into
vacuum and non-vacuum processes.
Vacuum processes such as sputtering, thermal evaporation, chemical vapor deposition etc
have been used to make thin films [1,9-14]. 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. But nevertheless, the cost of
vacuum deposition techniques has led the research into various scalable and easy to
follow non-vacuum deposition routes.
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
K. Tanaka et.al. [2] described sol-gel sulfurizing method in which precursors were
prepared with mixed solution of 2-methoxyethanol (2-metho) solvent and
monoethanolamine (MEA) stabilizer. The solution was spin coated at 3000 rpm for 30s
onto Mo/SLG substrates and were dried at 300°C for 5min in air using a hot plate. The

films were then annealed in N2 + H2S (5%) gas atmosphere at 500°C for 1 hour resulting
in the formation of Cu2ZnSnS4 (CZTS) thin films. Similarly, Jiang et al [3] describes
another sol-gel process in which the CZTS thin films were prepared by spin coating the
sol-gel precursor followed by annealing in a nitrogen atmosphere to make CZTS films
with an efficiency of 0.63% under AM 1.5 illumination. However, the speed of the
substrate (rpm) in spin coating affects the degree of radial (centrifugal) force applied to
the liquid resin as well as the velocity and characteristic turbulence of the air immediately
above it. Also, it requires that the resin fluid to be deposited have the right kind of
viscosity and good drying rate to form a uniform film. Spin coating defects like air
bubbles in the resin liquid, comets, streaks and flares in the deposition, swirl pattern of
the deposition etc are common. This is especially true when depositing suspended
particles or when in-organic solvents are used for spin coating.
Chen et al [4] demonstrated a dip coating process in which CuInS2 (CIS) nano crystals
are grown on TiO2 film for thin film solar cell application. Thioacetamide was added into
a 12 mL ethanol solution containing indium chloride and copper sulfate under magnetic
stirring until a clear solution was formed. Subsequently, the substrate was vertically
immersed into the solution jar which was autoclaved in a fan-forced oven at 160°C for
12 h. In dip coating, the film thickness is set by the competition among viscous force,

. capillary (surface tension) force and gravity due to which the solvents and the substrates
that can be used are limited. Also, a separate annealing step is required for the adhesion
of the film. The control of thickness in this process is also difficult as it depends on
various parameters like velocity of dipping, immersion time, substrate surface
morphology, the solution properties etc.
J.J. Scragg et al [5] describe a work in which p-type absorbe CZTS was prepared by
electroplating metallic precursors onto a molybdenum-coated glass substrate followed by
annealing in a sulfur atmosphere. Metal layers were deposited in the order of Cu, Sn, Zn
using cell with a platinum counter electrode and Ag | AgCl reference electrode and were
further heated in a sulfur atmosphere. Though, CZTS was formed, electro deposition
along with various other bath techniques mandate heating of the substrate in H2S, Na2S
and/or any other dangerous sulfur and/or selenium containing vapors. Also, controlling
She stoichiometry of CZTS poses a serious challenge and there is also a chance of
formation of incomplete phases in this process.
. Lee et al [6] describes a method in which low-cost, non-vacuum doctor-blade coating is
used for making CIS films. A further annealing step at around 400°C was made which
helped in the crystallization of CIS from Cu2-xSe and In2Se3 nuclei on the surface of the
film. The control of thickness is again a problem in the doctor blade coating. The
annealing step plays a crucial role in the formation of pin holes and cracks may be
developed in the film when the film is drying.

Other methods like Spray pyrolysis were also used to make thin films especially of metal
oxides [7]. The basic principle involved in spray pyrolysis technique is pyrolytic
decomposition of salts of the desired compound to be deposited. The temperature of the
deposition should be such that the by-products of the reaction should be volatile at the
temperature at which the substrate is put. The simple spray pyrolysis method is
sometimes modeled as analogous to Chemical Vapor Deposition (CVD) method as the
precursor solution droplets vaporize at the vicinity of substrate surface. Temperature,
spray rate and distance of nozzle tip from the substrate are some important parameters to
be optimized for good films. Though so much work has been done in spray pyrolysis area
especially for making films for thin film PV applications, the inherent disadvantages of
the process like difficult in the control of grain growth, more grain boundaries, presence
of other than targeted phases etc have been stopping it from being scaled up for mass
production.
In the present disclosure, a modified spray method is discussed which will improve the
grain size and reduce the grain boundaries of the film to be deposited by adding seed
crystals that enhance the required phase formation. Due to this, larger grains are formed
at relatively lower temperature and with lower deposition time thus avoiding the wastage
of chemical precursor. Further work in this direction may render the spray pyrolysis
method industrially viable for making thin films.

OBJECTS OF THE INVENTION:
An object of this invention is to propose a method for depositing thin films especially for
thin film solar cell application.
Another object of this invention is to propose a method for depositing adhesive thin film
in continuous and uniform manner.
Still another object of this invention is to propose a method in which the film is formed
by spraying the chemical precursor solution onto the hot substrate resulting in better grain
growth.
Further object of this invention is to propose a method when the seed crystal is nearly of
spherical shape and size varying between 50 to 500 nm.
BRIEF DESCRIPTION OF THE INVENTION:
According to this invention there is provided a method for depositing thin films
comprising:
suspending pre-synthesized seed crystals to be deposited in a precursor solution,
spraying the said solution on a hot substrate to form a uniform, continuous and adhesive
film

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1: shows the schematic diagram of the apparatus used for depositing thin films of
semiconductor material by the disclosed method.
Figure 2(a): shows the X-ray Diffraction spectrum of the CZTS seed crystals.
Figure 2(b) shows the X-ray Diffraction Spectrum of the CZTS film deposited by
disclosed method.
Figure 3: shows the Raman spectrum of the CZTS seed crystals.
Figure 4: shows the particle size distribution of the CZTS seed crystals.
Figure 5(a): shows SEM image of the CZTS film deposited by disclosed method.
Figure 5(b): shows SEM image of the CZTS film deposited by conventional spray
pyrolysis method.

DETAILED DESCRIPTION OF THE INVENTION:
The embodiments of the present invention disclose a method to deposit a continuous,
uniform and adhesive film of semiconductor material over a smooth metal, glass or
multilayer stacked substrate. The pre-synthesized seed crystal of CIS or CZTS of nearly
spherical shape and size varying between 1 to 500 nm are suspended in a solvent to form
the spray suspension. In the preferred embodiment, the chemical precursor solution for
synthesis of CIS or CZTS is used as the solvent to make the spray suspension. Chemical
precursor solution was prepared by dissolving water soluble salts of Copper (II), Indium
(III) and thiourea or Copper (II), Zinc (II), Tin (IV) and thiourea in deionized water in
varying molar ratio. A thin layer varying from 50 nm to 2 um is deposited by impinging
the spray suspension on the substrate preheated to a temperature which can vary from
200°C to 600°C. In the preferred embodiment, the spray suspension is kept in agitated
state by using a continuous low amplitude ultrasonication, probe sonication or
mechanical stirring. Rate of flow of spray suspension which can vary from 0.5 ml/minute
to 20 ml/minute is controlled by suspension pump. In the disclosed method, spray
suspension is atomized by pressurizing nitrogen, argon or compressed air in spray nozzle.
The position coordinates of the spray nozzle with respect to stationary substrate can be
controlled to cover entire substrate uniformly. The speed of the spray nozzle in x-
direction and y-direction can vary from 10mm/minute to 2000 mm/minute. The spray
nozzle is moved to cover the entire surface uniformly repeatedly; the number of
repetition can vary from 0 to 50.

Compared to other established non-vacuum methods used for deposition of thin films, the
disclosed method produces a uniform and adhesive film with relatively lesser pinholes.
The disclosed method also produces relatively improved crystal growth and hence better
optical and electronic properties. Since, in photovoltaic devices most of the electron-hole
recombinations occur at the grain boundaries, larger crystals and lesser defects decreases
the probability of recombination of the carriers.
Figure 1 shows the schematic diagram of the apparatus used for the disclosed method.
Reservoir (A) is filled with the suspension of finely powdered pre-synthesized material in
its chemical precursor solution. Hence, the reservoir contains both pre-synthesized
powder and the un-reacted chemical precursors of the material to be deposited. The
solution in the reservoir is maintained in a suspended state by an agitator (F) which
includes but not limited to continuous low amplitude ultrasonicator, probe sonicator and
mechanical stirrer. Stabilized suspension is pushed into the Atomizer (B) by a Controlled
Suspension Pump (C). The position co-ordinates of atomizer over the substrate are
controlled by a Position Controller (D). Suspension in the atomizer is atomized by a
method which includes but not limited to air pressure atomization and impinged onto the
pre-heated substrate (E) maintained at constant temperature from a distance varying from
2 inches to 3 feet.

When the suspension of pre-synthesized seed crystals of CIS/CZTS in its chemical
precursor impinges on the preheated substrate, CIS/CZTS seed crystals spread uniformly
on the substrate and act as nucleation site for reaction of un-reacted precursor solution.
The gaps between individual pre-synthesized CIS/CZTS seeds distributed on the surface
of substrate are filled by chemical precursor solution. Due to the increased temperature
on the surface and presence of CIS/CZTS seeds, the un-reacted precursor solution reacts
on the surface of substrate to form a continuous uniform adhesive film. The pre-
synthesized material particles act as nucleation site for the crystal growth. The growing
crystals are closely spaced and fuse to form a continuous, defect-free adhesive film. The
morphology and the thickness of the film can be controlled by varying the substrate
temperature and flow rate of suspension solution.
EXAMPLE:
In a preferred embodiment, CZTS nano crystals are synthesized using microwave
irradiation using a process described elsewhere [8]. Figure 2(a) and 2(b) shows the X-ray
Diffraction spectrum of the CZTS seed crystals and CZTS film deposited by disclosed
method respectively. All the peaks which are seen clearly match with the standard
kesterite CZTS JC PCS 26-0575. Raman Spectroscopy has been used for further
confirmation of the CZTS phase.

Figure 3 shows the Raman spectrum of the powder obtained in the above process. The
major peak at 331 cm"1 clearly pertains to the CZTS. The particle size distribution of the
synthesized CZTS powder (Figure 4) has been recorded using Dynamic Light Scattering
Technique (DLS).
To make the precursor solution, Cupric Chloride, Zinc Acetate and Stannic Chloride are
taken with Thiourea in an 80ml solution in the ratio of 2:1:1:12 with the base molarity
being ImM. Further, 0.2 g of pre synthesized CZTS nano powder which acts as a seed
crystal is added to the solution. The solution is kept in agitated state in reservoir by
sonication. The suspended solution is pumped at rate of 6 ml/minute into the atomizer.
The solution is then atomized using pressurized nitrogen gas and sprayed over a glass
substrate put at 350°C from a distance of 18 inches. The X-Y position of spray nozzle is
varied to cover the surface of glass uniformly to coat 5 layers maintaining constant
temperature. Further, to compare the disclosed method with conventional spray pyrolysis
method, another deposition using the same precursor solution but without the pre
synthesized powder is deposited at 350°C. All the other parameters for the deposition like
spray rate, distance of the nozzle tip from the substrate and number of layers are
maintained constant.

The Figure 5(a) and 5(b) shows the SEM images of the CZTS films deposited by the
disclosed method and conventional spray pyrolysis method respectively. It is clearly seen
that the CZTS film deposited by the disclosed method is continuous and uniform as
compared to conventional spray pyrolysis method.
The present invention can be modified and its variations can be devised by those who are
skilled in the art without departing from the spirit and scope of the invention. The present
invention, accordingly, includes all such variations and modifications to the claims given
below:

WE CLAIM:
1. A method for depositing thin films comprising:
suspending pre-synthesized seed crystals to be deposited in a precursor solution,
spraying the said solution on a hot substrate to form a uniform, continuous film.
2. The method as claimed in claim 1, wherein the said pre-synthesized seed crystals are
selected from CIS and CZTS nano crystals.
3. The method as claimed in claim 1, wherein no additional chemical or adhesive is
added to the precursor solution for making the film adhesive to the substrate.
4. The method as claimed in claim 1, wherein the said precursor solution is selected from
water soluble salts of copper (II), Indium (III) and thiourea and copper (II), Zinc (II) Tin
(IV) and thiourea dissolved in deionized water in varying molar ratio.
5. The method as claimed in claim 3, wherein the precursor solution is preferably Cupric
Chloride, Zinc Acetate and Stannic Chloride and thiourea in a ratio of 2:1:1:12 with the
base molarity being 1mM.
6. The method as claimed in claim 1, wherein preferably CZTS nano powder which acts
as a seed crystal is added to the said precursor solution under sonication.

7. The method as claimed in claim 1, wherein the substrate is preheated to a temperature
which can vary from 200°C to 600°C.
3. The method as claimed in claim 1, wherein the rate of flow of spray suspension varies
from 0.5 ml/ minute to 20 ml/minute controlled by suspension pump.
9. The method as claimed in claim 1, wherein the speed of spray nozzle in direction x & y
can vary from 10mm/minute to 2000 mm/minute.
10. The method as claimed in claim 1, wherein the said spray nozzle is moved to cover
the entire surface uniformly and repeatedly and the number of repetition can vary from 0
to 50.

ABSTRACT

A method for depositing thin films comprising:
suspending pre-synthesized seed crystals to be deposited in a precursor solution,
spraying the said solution on a hot substrate to form a uniform, continuous and adhesive
film.