TITLE:
A non-vacuum chemical process for rapid synthesis of nanostructured CZTS powders for
solar photovoltaic applications.
FIELD OF THE INVENTION:
The present invention relates to the synthesis of nanostructured CZTS [Di - Copper
Zinc Tin Tetra - Sulphide, {Cu2(Zn.Sn)S4}] powders and products thereof for solar
photovoltaic applications. More specifically, the present invention deals with the
disclosure of a non-vacuum aqueous-based chemical process for rapid synthesis
of nanostructured CZTS powders and or its products or powders thereof with
variable levels of zinc and tin atomic ratio in the CZTS compound.
BACKGROUND OF THE INVENTION:
CZTS [Di - Copper Zinc Tin Tetra - Sulphide, {Cu2(Zn, Sn)S4}] is a p-type semiconductor
and is recognized as a promising and inexpensive emerging solar cell material that
contain only earth - abundant elements which in turn present an opportunity to
significantly reduce the production cost of photovoltaic devices by replacing
expensive rare earth elements indium and gallium in CIGS. CZTS material is
coated in the solar cell module to absorb light associated with high absorption co-

efficient (104 cm-1) and has a near optimal band gap of 1.45 - 1.65 eV, abundant /
cheap materials, nontoxicity and adoptability to various growth techniques. CZTS
class of materials having nanostructure in the particles are also termed as "Quantum
Dots" since the colour and absorption characteristics changes with the crystallite
size of the material.
Synthesis of the core solar absorbing material i.e., CZTS powders have gained a
lot of attention in the research community throughout the world and various
research reports, articles and patents available that describe the synthesis of
CZTS materials by numerous routes.
2010 NNIN REU Research Accomplishments, pages 112 - 113, describes a
Aerosol Deposition (AD) route to synthesize CZTS powders using both metallic
precursor powders as well as chlorides of copper, tin and zinc with tetraethylene
glycol as solvent, wherein nano particles of the phase kesterite phase of CZTS
with near stoichiometric composition and grain size 200 nm was obtained at a
the solvent boiling temperature (317°C). The combination of CuCl2, Zn, Sn and S
powders appears to be the most promising precursors for producing kesterite phase
nanoparticles.

Solar Energy Materials and Solar Cells, Volume 101, June 2012, Pages 87-94,
described synthesis of CZTS nanoparticles using Cul, ZnCl2 and Snl4 dispersed in
oleylamine solvent for 30 min to 170 °C followed by mixing with sulfur in
oleylamine solution at room temperature. The reaction solution was heated to
210°C for 90 min. and the prepared nanoparticles exhibit diameters between 7
and 35 nm and kesterite crystal structure.
EP patent Publication number EP2432842 A1 dated March 28, 2012 describes a
process of synthesizing CZTS nanoparticles and nano ink for solar cell applications
using of copper chloride, zinc chloride, tin chloride and free sulfer with oleylamine
as solvent. Part of the synthesis procedure involves an argon atmosphere and
the temperature of the final reaction is 230°C.The particle size of thus prepared
CZTS nanopartcles ranged 10-50 nm.
J. Am. Chem. Soc. 2009, 131 (34), pp 12054 - 12055, reports a solution-based
direct synthesis of Cu2ZnSnS4 nano crystals by the hot injection method. A
stoichiometric amount of copper (II) acetylacetonate, zinc acetate, tin (IV) acetate
under vacuum at 150°C in oleylamine and sulfur in oleylamine precursor was used
as injecting component. An average particle size of 12.8 ± 1.8 nm was formed
with a band gap of 1.5eV.

J. Am. Chem. Soc. 2009, 131 (35), pp 12554 - 12555, explains a process to
synthesize CZTS nano crystals by high - temperature arrested precipitation in
the coordinating solvent, oleylaminec, copper(ll) acetylacetonate [Cu(acac)2],
zinc acetate [Zn(02CCH3)2], tin(ll) chloride dehydrate [SnCI2.2H2O] and
elemental sulfur at 280°C under inert atmosphere. The particles are crystalline
with an average particle diameter of 10.6 ± 2.9 nm and a slightly irregular,
faceted shape.
J. Am. Chem. Soc. 2009,131 (33),pp 11672 -11673, reports to synthesize CZTS
nanocrystals by hot injection of a solution of in elemental sulfur oleylamine in to
an oleylaminesolution containing copper(ll) acetylacetonate, zinc
acetylacetonate and tin(IV) bis(acetylacetonate) dibromide at 225°C.The
particle size of the nanocrystals were in the range of 15 - 25 nm with a band
gap of 1.5eV.
Solar Energy Materials and Solar Cells, Volume 95, issue 10, Oct. 2011, Pages
2855 - 2860, described to synthesize Cu2ZnSnS4 (CZTS) thin film by a non -
vacuum process based on the sulfurization of precursor coatings, on Mo - coated
SLG and SLG substrates, consisting of a sol - gel solution of copper(ll) acetate
monohydrate, zinc(ll) acetate monohydrate and tin(ll) chloride dehydrate in
solvent media, 2 - methoxyethanol under H2S ± N2 atmosphere with different
H2S concentration.

United States Patent US8414862 explains preparation of CZTS and its
analogs in ionic liquids for PV cell applications using copper (II) acetate
monohydrate, zinc (II) acetate dihydrate, tin (II) chloride and
tetraethylammonium sulfide with tributylmethylammonium methylsulfate;
1,2,4-trimethylpyrazolium methylsulfate; or 1,2,3-trimethylpyrazolium
methylsulfate as ionic liquids, wherein the process composition is heated to
150 - 200 under nitrogen. XRD analysis of black powder confirmed the presence
of CZTS.
In this contest, the present invention describes a non - vacuum aqueous -
based chemical process for synthesizing nanostructured CZTS [Di-Copper
Zinc Tin Tetra-Sulfide, {Cu2(Zn,Sn)S4}] powders with varied zinc and tin atomic
ratio in the CZTS compound.
OBJECTS OF THE INVENTION:
An object of this invention is to propose a non-vacuum aqueous-based chemical
process for rapid synthesis of nanostructured CZTS [Di-Copper Zinc Tin Tetra-
Sulfide, {Cu2(Zn,Sn)S4}] powders with variable levels of zinc and tin atomic ratio in
the CZTS compound.

Another object of this invention is to propose a process for preparing CZTS
powder having tetragonal kesterite crystal structure with specific surface area in
the range of 70±5 m2/g.
Still another object of this invention is to propose a process for preparing CZTS
powder with primary particles in the range of 10-50 nanometers.
Further object of this invention is to propose a process to carry out the
synthesis of nanostructured CZTS [Di-Copper Zinc Tin tetra-Sulfide,
{Cu2(Zn,Sn)S4}] powders with varied zinc and tin atomic ratio in the CZTS
compound in a reproducible manner.
BRIEF DESCRIPTION OF THE INVENTION:
According to this invention, there is provided a non-vacuum and aqueous-based
process for the synthesis of nanostructured nano-CZTS [Di-Copper Zinc Tin Tetra-
Sulfide, {Cu2(Zn,Sn)S4}] powders with variable levels of zinc and tin atomic ratio in
the CZTS compound comprising the following steps:
Preparing a mixed aqueous solution of Cu-Zn-Tin salts by dissolving analytical
grades (purity >99.5%) cupper chloride, zinc chloride and tin chloride in desired
molar ratio (termed as 'Solution A') depending on the zinc to tin atomic ratio in
the targeted CZTS compound;

Preparing a diluted of 'Solution A' with defined concentration just by
appropriately diluting the 'Solution A' with distilled or de-ionized water
Preparing an aqueous solution of metal (sodium) sulfide by dissolving
analytical grade sodium sulfide (purity >99.5%) powder in desired molar ratio
(termed as 'Solution B').
Preparing a diluted of sodium sulfide with defined concentration just by
appropriately diluting the 'Solution B' with distilled or de-ionized water
Heating and stirring both 'Solution A' and 'Solution B' separately at a defined
solution temperature for 20 - 30 minutes
Mixing the diluted 'Solution A' into the diluted 'Solution B' with continuous
stirring at ambient pressure and at a defined solution temperature to form a
black precipitate;
Carrying out a precipitation reaction by mixing the above two solutions for a
period of 05 - 10 minutes and obtaining a black coloured precipitate that
resulted immediately after mixing the two solutions

Subjecting the black precipitate to the step of washing and filtration using
distilled or de-ionized water until the filtrate becomes free from chloride ions
resulting in chloride-ion-free washed precipitate;
Drying off the washed precipitate at any temperature in the range of 50-90°C to
yield dried CZTS powders;
subjecting the dried CZTS powders to the step of heat treatment in an inert
atmosphere in the temperature range of 100°-300°C to improve the crystallinity of
the CZTS powders.
DETAILED DESCRIPTION OF THE INVENTION:
According to the present invention and in order to accomplish the above
objects, there is provided an aqueous-based chemical process for synthesizing
nanostructured CZTS [Di-Copper Zinc Tin Tetra-Sulfide, {Cu2(Zn,Sn)S4}] powders
rapidly with variable levels of zinc and tin atomic ratio in the CZTS compound
which is disclosed in this invention.
In a more particular embodiment of the present invention, the aqueous-based
chemical process is also a non-vacuum process which comprises the preparation

of a mixed aqueous solution of copper, zinc and tin salts (Cu-Zn-Sn) with
appropriate concentrations in each case, in one hand and preparation of an
aqueous solution of metal sulfides in other hand with appropriate
concentration.
The mixed aqueous solution of copper, zinc and tin salts (Cu-Zn-Sn) are
prepared by dissolving analytical grades (minimum purity >99.5%) of copper
chloride, zinc chloride and tin chloride salts using de-ionized or distilled water
in desired concentration as per the targeted atomic ratio of 'zinc-tin' in the
CZTS compound, since any level of zinc and tin atomic ratio in the CZTS
compound could be prepared. The resultant aqueous solution is termed as
'Solution A'.
Aqueous solution of metal (sodium) sulfide is prepared by dissolving analytical
grade (minimum purity >99.5%) sodium sulfide powder using de-ionized or
distilled water in desired concentration as per the targeted CZTS compound
and the solution is termed as 'Solution B'.
Further aspect of the process is to carry out a precipitation reaction between
the 'Solution A' and "Solution B' at any temperature in the range of 40-60°C by
mixing the solutions with appropriate concentrations and under constant stirring
and ambient atmosphere, in which a black colored precipitate is obtained
immediately after mixing the solutions.

The resultant precipitate is to be washed several times with de-ionized or
distilled water until the precipitate becomes free from chloride ions in the
filtrate and the washed precipitate that is free from chloride ions are to be
collected.
The washed and chloride-free precipitate is to be dried in an oven in air or in
any inert atmosphere at any temperature in the range of 60-90°C for several
hours until the precipitate becomes dry which are free from moisture. The
resultant dried precipitate is called CZTS powder.
The dried CZTS powder is heat treated (annealed) at any temperature in the
range of 100- 300°C in an inert atmosphere like, argon, nitrogen etc in order to
improve the crystallinity of the resultant CZTS compound resulting well
crystalline CZTS compound having tetragonal kesterite crystal structure.
Example 1:
In this example, the molar ratio of zinc and tin is fixed at 1.0: 1.0 in the CZTS
compound for the synthesis. The nominal composition corresponding to the
above zinc and tin molar ratio in the CZTS compound becomes [{Cu2(Zn,Sn)S4}].
CZTS compound having this composition is widely used in solar photovoltaic
application for creating solar absorption layer on a various thin-film or multi-
junction solar cells.

In this example, the 'Solution A' is prepared by dissolving analytical grades
(minimum purity >99.5%) copper chloride, zinc chloride and tin chloride in molar
ratio of 2.0:1.0:1.0 in de- ionized water respectively.
The 'Solution B' is prepared by dissolving analytical grade (minimum purity
>99.5%) sodium sulphide powder in de-ionised water such that copper and
sulfer atomic ratio to be 2.0: 4.0 in the CZTS compound.
Both the solutions were heated to 40 - 60°C separately under constant stirring.
The 'solution A' is mixed to the 'solution B' under constant stirring by
maintaining the solution temperature at 40 - 60°C. A black colored compound
instantaneously precipitated out from the solution. The black precipitate is
filtered off and washed repeatedly with de- ionized water until chloride ion was
free from the rejected filtrate. The washed precipitate is then dried in an oven at
a temperature of 80 + 05°C for a period of 5-6 hours that resulted dried
precipitates. The dried precipitate is called CZTS powders with equimolar zinc
to tin atomic ratio in the CZTS compound.
Heat treated (90°C for 1 hour) powders of CZTS materials showed primary
particle size in the range of 10-50 nm having tetragonal kesterite structure in
[{Cu2(Zn,Sn)S4}]compound.

The specific surface area of the dried CZTS powders in the BET analysis
invariably showed surface area of 74 + 02 m2/g.
Example 2:
Molar ratio of zinc and tin is fixed at 0.4: 0.6 in the CZTS compound in this
example for the synthesis. The nominal composition corresponding to the above
zinc and tin molar ratio in the CZTS compound becomes [Cu2(Zn0.4,Sn0.6)S4]. This
composition is also often used in solar photovoltaic application for tuning the band
gap in the material for creating solar absorption layer on a solar cell.
The 'solution A' is prepared by dissolving copper chloride, zinc chloride and tin
chloride in molar ratio of 2.0: 0.4:0.6 in de-ionized water.
In this example, the 'solution B' is prepared by dissolving sodium sulphide powder
in de- mineralised such that copper chloride to sodium sulphide in the atomic
ratio 2.0:4.0.
Both the solutions were heated to 40 - 60°C separately under constant stirring.
The 'solution A' is added to the 'solution B' under constant stirring by
maintaining the temperature of the mixed solution by maintaining the solution
temperature of 40 - 60°C. A black colored compound out instantaneously

precipitated out from the solution. The black precipitate is filtered off and washed
repeatedly with de-ionized water until chloride was free from the filtrate. The
washed precipitate is then dried in an oven at a temperature of 85 + 05 °C for a
period of 5-6 hours that made the washed precipitates completely dry. The
resulted dried precipitate is called CZTS powders having zinc to tin atomic ratio
of 4:6 in the CZTS compound.
Heat treated (90°C for 1 hour) powders of CZTS materials showed primary particle
size in the range of 10-50 nm. The specific surface area of the dried CZTS powders
in the BET analysis invariably showed surface area of 63 + 02 m2/g.

WE CLAIM:
1. An aqueous-based non-vacuum chemical process for the synthesis of
nanostructured CZTS [Di-Copper Zinc Tin Tetra-Sulfide, {Cu2(Zn,Sn)S4}] powders
with variable levels of zinc and tin atomic ratio in the CZTS compound
comprising the following steps:
preparing a mixed aqueous solution by dissolving analytical grades (purity
>99.5%) of copper, zinc and tin salts respectively in desired molar ratio (Solution
A) that is required for the targeted CZTS compound
preparing an appropriate concentration of 'Solution A' just by appropriate
diluting the 'Solution A' with distilled/de-ionized water
preparing an aqueous solution of sodium sulfide by dissolving sodium sulfide
powder in distilled or de- ionized water with required molar ratio for the
chemical conversion from sodium sulfide powder to sulfer (Solution B).
preparing an appropriate concentration of sodium sulfide solution just by
appropriately diluting the 'Solution B' with distilled or de- ionized water

mixing the diluted 'Solution A' into the diluted 'Solution B' with maintenance
of appropriate concentrations of both the solutions with continuous stirring at
ambient atmosphere and at a defined temperature
carrying out a precipitation reaction by mixing the above two solutions at
ambient atmosphere for a period 05-10 minutes and obtaining a black
coloured precipitate that resulted immediately after mixing the two solutions
also at a defined range of temperature
filtering off the resultant black coloured precipitate using distilled or de-
ionized water until the filtrate becomes free from chloride ions and obtaining
chloride-ion free precipitate
drying off the precipitate (free from chloride ion) in an oven at around 90°C to
yield dried CZTS powders
subjecting the dried CZTS powders to the step of heat treatment in an inert
atmosphere in the temperature range of 100° - 300°C to improve the crystallinity
of the CZTS powders

2. The process as claimed in claim 1, wherein the mixed aqueous solution of
copper, zinc and tin solution is prepared by dissolving analytical grades (purity
>99.5%) copper chloride, zinc chloride and tin chloride salts in distilled or de-
ionized water by maintaining molar ratio of 2.0 :1.0 :1.0 in the CZTS compound
to be synthesized and then diluting the resultant mixed solution with distilled
water or de-ionized water to obtain diluted mixed solution with appropriate
concentration/s (termed as 'solution A).
3. The process as claimed in claiml, wherein the said metal sulfide solution
(sodium sulfide) is prepared by dissolving sodium sulfide powder in distilled or
de-ionized water under constant stirring and heating at a temperature range of
40°C - 60°C and thereby diluting the sodium sulfide solution appropriately with
distilled or de-ionized water for preparing diluted sodium sulfide solution with
appropriate concentration/s (termed as 'solution B').
4. The process as claimed in claim 1, wherein diluted solution of mixed Cu-Zn-
Sn solution (solution A) with appropriate concentration is mixed with the
diluted solution with appropriate concentration of 'solution B' in the
temperature range of 40-60°C under constant stirring in ambient atmosphere
with the yield of a black colored precipitate

5. The process as claimed in claim 1, the resultant precipitate is filtered off and
washed with distilled or de-ionized water so that the filtrate becomes free from
chloride ions for obtaining washed precipitate which is free from chloride ions
6. The process as claimed in claim 1, wherein the washed precipitate (chloride
ion free) is dried off in an oven in the temperature range of 50 - 90°C for a
couple of hours to several hours so that the precipitate becomes free from
moisture (<0.1%) with the yield of dried precipitate.

7. The process as claimed in claim 1, wherein the dried precipitate is
nanostructured CZTS [Di-Copper Zinc Tin Tetra-Sulfide, {Cu2(Zn,Sn)S4}] powder
that has tetragonal kesterite structure with specific surface area in the range of
70 + 5 m2/g comprising 10-50 nanometer of primary particles in the
agglomerated particles of CZTS compound.
8. A process as claimed in claim 6, where in the nanostructured CZTS powder
is heat treated (annealed) in an inert atmosphere in the temperature range of
100° - 300°C in the range of 1-5 hours to improve the crystallinity of the CZTS
powders and to obtain well crystalline CZTS powders having tetragonal
kesterite crystal structure.

9. The process as claimed in claim 1, wherein the derived CZTS powder is
suitable for photovoltaic applications for forming light absorption layer either
by coating or printing the material with desired stoichiometry following any
standard procedure in solar photovoltaic and allied applications.

ABSTRACT

A non-vacuum chemical process for rapid synthesis of nanostructured CZTS
powders for solar photovoltaic applications.
The invention describes a non-vacuum aqueous-based chemical process for
synthesizing nano-structured CZTS [Di - Copper Zinc Tin Tetra - Sulphide, {Cu2(Zn,Sn)S4]
powders for solar photovoltaic applications. By following the process, a variety of
CZTS material with variable levels of zinc and tin atomic ratio in the CZTS
compound can rapidly be prepared. The synthesized CZTS powders have
tetragonal kesterite structure with specific surface area in the range of 70 + 5 m2/g.
Heat treatment (annealing) of the CZTS powder in any inert atmosphere, e.g.,
argon or nitrogen at a temperature in the range of 100 - 300°C improves the
crystallinity of CZTS powder since as-synthesized CZTS powders are poorly
crystalline. Electron microscopy analyses (TEM & Fe-SEM) of the CZTS powder show
the nanostructure in the material with primary particles in the range of 10 - 50
nanometers and is suitable for photovoltaic applications for forming light
absorption layer either by coating or printing the CZTS material following any
standard procedure.