FIELD OF THE INVENTION
The present invention relates to an environment friendly synthesis process of
kesterite CZTS nanopowders in aqueous media by microwave irradiation. The
present invention relates to a process to synthesize kesterite nanopowder
compound with a formula [Cu2-aZnbSnc(S/Se)4+d] wherein 0<=a<=1, 0<=b<=l1
0<=c<=1 and -1<=d<1. The present invention also relates to a process to
produce absorber ink with the nanopowder obtained through the inventive
synthesis.
BACKGROUND OF THE INVENTION
Growing energy demands of the world has forced mankind to search for
alternative sources of energy to conventional fossil and nuclear sources. Solar
energy is one of those kinds which have the potential to act as a clean and
reliable energy source for the future. Accordingly, a vast amount of money and
effort has gone into the photovoltaic (PV) research in the last few decades.
Currently, the PV market is dominated by c-Silicon (crystalline silicon) based PV
cells with appreciable conversion efficiency. Silicon being an indirect band gap
material requires a thick layer of material (around 100 to 300 microns) for
complete absorption of the incident photons. The high thickness increases the
chances of recombination of the generated carriers. Hence, highly crystalline
wafers of Silicon, which involve complex and costly processes to manufacture,
are required to reduce the recombination of the carriers generated.
The above problems have largely been mitigated by the advent of thin film solar
cells. At present, only Cadmium Telluride (CdTe), Copper Indium Gallium

sulfide/Selenide (CIGS) and Copper Indium Sulfide (CIS) thin film solar cell
technologies have reached the commercial level of manufacturing. But the cost
of Indium (In) and Gallium (Ga) is a major hindrance to the prospects of CIGS
and CIS technologies. This is due to the restricted supplies of In and Ga on the
earth's crust and the demand for Indium as transparent conducting electrodes in
touch screens and liquid crystal displays.
Copper Zinc Tin Sulfide/Selenide (CZTS), an emerging solar absorber, has the
potential to be used as an absorber layer in thin film solar cells. CZTS has a
kesterite structure which is similar to structure of CIGS, but contains earth
abundant, non-toxic elements and has a near optimal direct band gap energy of
1.0-1.6 eV and a large absorption coefficient of ~1 x 104 cm-1. Vacuum processes
like sputtering and co evaporation have been used to make CZTS. But,
conventional vacuum methods have drawbacks such as complexity in process
high production costs and difficulty in scaling up, which are to be considered
before the commercialization of the solar cells. Also, the power conversion
efficiency of the CZTS absorber layer is highly dependent on the stoichiometric
ratio (atomic ratios) of the elements in the layer. The absorber layer must be
within a narrow stoichiometric ratio to get highly efficient cell. Achieving precise
stoichiometric composition over relatively large substrate areas is, however,
difficult using vacuum based sputtering or evaporation deposition processes.
Non vacuum processes score over vacuum processes in terms of cost and ease
of production. Hence, solution route to prepare CZTS absorber layer in thin film
solar cells has widely been the target of research. However, a lack of knowledge
of low cost and environments friendly method to prepare CZTS in large quantities
has been a major hurdle to be able to commercialize the technology.
K. Tanaka et.al. [1]., described sol-gel sulfurizing method in which precursors

were prepared from Copper (II) acetate mono-hydrate, zinc (II) acetate
dihydrate and tin (II) chloride dehydrate dissolved in a mixed solution of 2-
methoxyethanol solvent and monoethanolamine (MEA) stabilizer. The solutions
were 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
CZTS. However, the hydrogen sulfide atmosphere used is highly toxic and hence
requires careful handling. Also, the monoethanolamine brings with it the well
known problems with amines.
S C Riha et al. [2], describes hot injection method for synthesis of CZT(S/Se)
using copper (II) acetylacetonate, zinc acetate, and tin(IV) acetate combined in
appropriate proportion under inert conditions with oleylamine (OLA). Selenium
and Sulfur were separately mixed in OLA. The two solutions were injected into
triphenyl phosphine Oxide (TPO) to form C2T(S/Se). But the TPO used in the
process is highly toxic and its subsequent removal from the solution is difficult.
Hao Wei et al.[3] describe another hot injection synthesis method in which
copper acetate dehydrate, zinc chloride dehydrate, tin (II) chloride dehydrate
were taken in OLA and heated to 100°C. Selenium was taken separately in OLA
and heated to 150°C for 1 hour and injected to the metal precursor-OLA solution
to form CZTSe. The procedure further involved ultrasonication in ethanol
followed by centrifugation at 8000rpm.
The Hot injection synthesis methods used in the above two processes require
Oleylamine which acts as a capping agent to the formed CZT(S/Se). This
impedes the transport of charge when CZT(S/Se) is deposited as an absorber
layer in solar cell. Also, a pretreatment of sulfur and selenium requirement in
Oleylamine is cumbersome.

J.J. Scragg et al [5] describe a work in which p-type absorber Cu2ZnSnS4 (CZTS)
was prepared by electroplating metallic precursors sequentially onto a
molybdenum-coated glass substrate followed by annealing in a sulfur
atmosphere. Metal layers were deposited in the order Cu, Sn, Zn using cell with a
platinum counter electrode and Ag | AgCl reference electrode. Samples
consisting of a three layer stack of Cu, Sn and Zn were annealed in a sulphur
atmosphere using a quartz tube furnace at 40°C min-1 to a final temperature of
550°C, which was maintained for two hours to allow the metals to react fully
with sulfur. 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 the
stoichiometry of CZTS poses a serious challenge and there is also a chance of
formation of incomplete phases in this process.
T K. Todorov et al [6] describes a method in which Hydrazine (N2H4) is used to
make CZT(S/Se). Here, an innovative approach has been demonstrated. In this
process, individual CuS-S, SnSe-Se solutions in hydrazine were separately
prepared. Zinc powder was directly added to the tin solution. The Sulphides
and/or Selenides of metals which are dissolved/dispersed in hydrazine are spin
coated and annealed to form large grain CZTS. A record conversion efficiency of
9.6% has been reported. But this approach has its own disadvantages. The same
team later improved the efficiency to 10.1% [7]. Hydrazine is highly toxic and
flammable. Its transport is fraught with danger and hence requires expensive
equipment and processes to handle it in large scale. Also, Hydrazine being
extremely reactive, it easily erodes away the equipment used in the processes of
manufacturing.
Apart from the above processes, various other solvo thermal routes have also
been reported. A detailed review of all the processes used to make CZTS clearly

elucidate the use of hazardous chemicals. The nature of these chemicals
mandates the use of schenk line reactions which are cumbersome and require
careful handling of the chemicals. Among others, the most common solvents
used in the synthesis of CZTS are organic amines. Amines are toxic, biologically
active and not environment friendly. Hence, the use of amines in large quantities
is also not recommendable.
Last decade has seen a rise in the popularity of microwave assisted chemical
syntheses. Microwaves are particularly helpful in the reactions which require
harsh conditions to complete as they provide sufficient instantaneous energy for
the intermediates to cross the activation energy barrier. The advantages of
microwave assisted synthesis include lower bulk temperature, lower reaction
time, easy controllability, high reproducibility and easy scalability. B. Flynn et al
[8] described a method in which CZTS nanoparticles were synthesized in
ethylene glycol using microwave irradiation. But glycols are thick and pose a
problem in extracting the synthesized nanopowder from them. Also, ethylene
glycol is poisonous.
Nevertheless, synthesis of CZTS in aqueous medium definitely scores over all the
above described processes. The present invention addresses all the above issues
by presenting a scalable, reliable and reproducible method in which Dl water is
used for the synthesis of CZTS. In this innovative approach, stoichiometry of
various constituents of CZTS can be changed, comparatively lower time and
temperature is used for synthesis and highly crystalline CZTS nanopowder is
obtained. The nanopowder thus obtained is used to prepare absorber ink which
can be printed onto substrates to make CZTS absorber layer for thin film solar
cell application.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a synthesis process of
Kesterite CZTS nanopowder in aqueous media by microwave irradiation.
Another object of the invention is to propose a process to synthesize Kesterite
nanopowder compound with a formula [Cu2-aZnbSnc(S/Se)4+d] wherein
0<=a<=1, 0<=b<=1, 0<=c<=1 and -1<=d<1.
A still another object of the invention is to propose a synthesis process of
Kesterite CZTS nanopowder in aqueous media by microwave irradiation, in which
post H2S/ H2Se / Na2S/ Na2Se/S/Se environment annealing is eliminated.
A further object of the invention is to propose to synthesize Kesterite
nanopowder compound with a formula [Cu2-aZnbSnc(S/Se)4+d] wherein
0<=a<=1, 0<=b<=1, 0<=c<=1 and -1<=d<1 in which the stoichiometry of
various elements can easily be controlled and in which high temperature
annealing / heating is eliminated.
A still further object of the invention is to propose a method to prepare CZTS
inks to deposit as absorber layers by non-vacuum methods.
Yet another object of the invention is to propose a synthesis process of Kesterite
CZTS nanopowder in aqueous media by microwave irradiation, which is an
environment-friendly and low cost process.
SUMMARY OF THE INVENTION
Accordingly, the invention provides a method to prepare Kesterite Copper (Cu),

Zinc (Zn), Tin (Sn) sulfide/selenide [CZT(S/Se)] nano powders consisting of
varying degrees of earth abundant and non-toxic Cu, Zn, Sn and at least one of
Sulfur (S) and Selenium (Se) in aqueous media through microwave irradiation.
The present invention further provides a method to synthesize a kesterite
nanopowder a compound of the formula:
Cu2-aZnbSnc(S/Se)4+d wherein 0<=a<=1, 0<=b<=1, 0<=c<=1 and -1<=d<=1
in aqueous media by microwave irradiation. The invention also teaches a process
to produce absorber ink from the nano powder obtained through the inventive
synthesis process. The absorber ink can be printed onto substrates by non-
vacuum methods for solar cell application. The process completely avoids the use
of hydrazine, ethylene diamine, oleylamine or any other known amine in the
synthesis of kesterite powders. It also mitigates the use of schenk line reactions
or any other vacuum conditions required to carry out all the said processes.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows an XRD spectrum of CZTS nano powder prepared in aqueous
medium using microwave irradiation, all the peaks pertaining to
CZTS standard of JC PDS 26-0575 being shown in the XRD
spectrum.
Figure 2 - shows a Raman spectrum of the CZTS nano powder, the major
peak at 331cm-1 clearly pertains to the CZTS.
Figure 3 - shows absorption of the CZTS nano powder recorded to measure
the band gap of CZTS which is found to be around 1.25 eV.

Figure 4 - shows a particle size distribution of the CZTS ink measured using
Dynamic Light Scattering (DLS) technique, the average particle size
of the CZTS particles in the ink being around 168nm.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention discloses a method to prepare kesterite nano powders
consisting of varying degrees of earth abundant and non-toxic elements Cu, Zn,
Sn and at least one of Sulfur and Selenium (CZTS) and more particularly, a
method to prepare CZTS nanopowder in aqueous media through microwave
irradiation.
The method includes the steps of contacting a source of Copper, a source of Zinc
a source of Tin, and a source of sulphur and/or selenium in the presence of
Ammonium Hydroxide in water; stirring and heating the resultant solution in a
microwave below a pressure (in-situ reaction generated), at a temperature,
power and length of time sufficient to make the kesterite powders.
The source of Copper is any water soluble salt of Copper. The source of Zinc is
any water soluble salt of Zinc. The source of Tin is any water soluble salt of Tin.
The source of sulfur and/or selenium is selected from thiourea and/or selenourea
or the mixture thereof.
The amount of Thiourea/Selenourea or the mixture thereof can vary from 0.1 to
20 times the stoichiometric amount required to make kesterite nanopowder
including a compound of the formula:
Cu2-aZnbSnc(S/Se)4+d wherein 0<=a<=1, 0<=b<=1, 0<=c<=1 and -1<=d<=1

The metal precursors can be added according to the stoichiometry required in
the final CZTS phase.
The aqueous media includes a solution of water and ammonium hydroxide in an
amount from about 1 wt% to 99 wt%. The water and Ammonium Hydroxide
solution does not contain any other solvents including but not limited to
hydrazine, alcohols, ethers, glycols, aldehydes, ketones, alkanes, amines, cyclic
compounds or halogenated compounds. Thus, the combination of water and
ammonium hydroxide with varying amounts of Copper, Zinc, Tin, and
Thiourea/SelenoUrea resulting in required stoichiometry of kesterite
nanopowders is sufficient to make crystalline kesterite nano powders with the
help of microwave irradiation.
Microwave irradiation is used to heat the aqueous solution containing the
precursors of Copper, Zinc, Tin, Thiourea and/or Selenourea. A microwave
absorbing material such as Weflon/silicon-carbide magnet may be used as an
additional heating element during the microwave irradiation. The stirring speed
of the magnet during the reaction can vary from about 10 rpm to about 1000
rpm. The macro temperature of the reaction mixture during the reaction can vary
from about 190 °C to 240 °C. Also, the time for which the precursor solution is
exposed to microwave can vary from 40 min to 90 min. The power delivered by
the microwave can vary between 500W to 1000W depending upon the
precursors, quantity of the precursor solution and length of time of the reaction.
Once the reaction is completed, sufficient time is given for cooling of the
reaction mixture. Next, the powder is separated from the supernatant liquid by
centrifugation. The powder thus obtained is heated below 100 -200°C in
atmospheric oven for 1 to 2 hours. The powder is then used to make nano ink
which can be used for preparing CZTS layers by non-vacuum methods including

but not limited to spin coating and ink jet printing.
The particle size of the as obtained nano powders can be optimized by changing
the reaction parameters of the microwave irradiated chemical reaction. The
process is scalable owing to the uniform heating of microwaves unlike in the
conventional heating process. Hence, this method can be scaled up to few
kilograms.
As the chemicals involved are comparatively less hazardous, use of shcenk line
reactions and vacuum conditions is mitigated. Since the CZTS is formed in situ,
use of hazardous H2S/ H2Se/Na2S/Na2Se environments for making kesterite nano
powders is completely avoided.
For preparing nano ink, the oven dried nano powder is mixed with a dispersant
and a carrier liquid. The mixture is stirred for 1-5 hours and further ball milled for
another 1-10 hours. The nano powder may optionally be treated with a
surfactant before mixing it with the dispersant and the carrier liquid. This
depends on the carrier liquid used. The nano ink thus formed is used for spin
coating and ink jet printing.
The as obtained powder can be deposited as thin film using other non-vacuum
methods including but not limited to dip coating, doctor blading, spraying, screen
printing and electrophoretic deposition.
Example:
In a preferred embodiment, 0.298g of Copper (II) Chloride, 0.151g of Zinc
Chloride, 0.209 g of Tin (II) Chloride and sufficient amount of thiourea are taken

in a Teflon vessel with l-100ml of ammonium hydroxide and 30 ml of DI water.
The mixture is continuously stirred for 20 min at 500 rpm and then a Weflon
magnet is added. Next, a microwave (2.45GHz) power of 800 W is continuously
given to the mixture for 90 min. After that, the powder is collected by
centrifugation and then, analysed.
Fig I shows the X-ray Diffraction spectrum of the as obtained powder. All the
peaks which are seen clearly match with the standard CZTS JC PDS 26-0575- the
major peaks being at diffraction angle 26 = 28.5 47.33 ° and 56.17 °
corresponding to (112), (220) and (312) planes of kesterite structure. Other
minor peaks in the XRD spectrum also correspond to CZTS .These indicate a
crystalline CZTS formation.
As there is a chance for the formation of Zinc Sulfide whose XRD spectrum
matches nearly with CZTS, Raman Spectroscopy has been used for further
confirmation of the CZTS phase. Fig 2 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 optical properties of the obtained powder are studied using UV-Vis NIR
spectroscopy. The band gap of the obtained material is found out to be 1.25eV
from the absorption spectrum. Fig 3 shows the absorption spectrum of the
powder obtained in the process detailed above.
To make CZTS ink, 0.1 gram of CZTS nanopowder as obtained above is taken
with a mixture 10ml of 1,2-propanediol and 10 ml of isopropanol along with a
few drops of poly ethylene glycol (PEG). The mixture is stirred continuously for 2
hours and then ultra sonicated for another 1 hour.

Particle size analysis of the ink is carried out using the Dynamic Light Scattering
(DLS) technique. For this the ink is further diluted with isopropanol. This is to
ensure a good result in the DLS technique used to measure the particle size. Fig
4 shows the particle size distribution of the CZTS particles in the as ink prepared
as above. The average particle size obtained is around 168nm.
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.

(A) Literature References
1. KunMko Tatiaka, MasatosM Oonuki, Noriko Moritake, Hisao Uchiki,
"Cu2ZnSnS4 thin film solar cells prepared by non-vacuum processing", Solar
Energy Materials & Solar cells 93(2009) 583-587
2. Shannon C. Riha, B. A. Parkinson, Amy L. Prieto, " Compositionally tunable
Cu2ZnSn(SI-xSe)4 nanocrystals: probing the effect of Se inclusion in mixed
chalcogenides films", CHEM. SOC. 2009,131,12054-12055
3. Hao Wei, Wei Guo , Yijing Sun , Zhi Yang , Yafei Zhang , "Hot-injection
synthesis and characterization of quaternary Cu2ZnSnSe4 nanocrystals",
Materials Letters 64 (2010) 1424-1426
4. C.P. Chan, H. Lam, C. Surya, "Preparation of Cu2ZnSnS4 films by electro
deposition using ionic liquids", Solar Energy Materials & Solar Cells 94 (2010)
207-211
5. Jonathan J. Scragg, Philip J. Dale, Laurence M. Peter, Guillaume Zoppi, and
Ian Forbes, "New routes to sustainable photovoltaic: evaluation of
Cu2ZnSnS4 as an alternative absorber material", Physica Status Solidi (b)
(2008) 245, No. 9, 1772-1778.
6. Teodor K. Todorov, Kathleen B. Reuter, and David B. Mitzi, "High-Efficiency
Solar Cell with Earth-Abundant Liquid-Processed Absorber", Advanced Energy
Materials (2010), 22, E156-E159.
7. D. Aaron R. Barkhouse, Oki Gunawan, Tayfim Gokmen, Teodor K. Todorov
and David B. Mitzi, "Device characteristics of a 10.1% hydrazine-processed
Cu2ZnSn(Se,S)4 solar cell", Prog. Photovolt: Res. Appl. 2012; 20:6-11

8. Brendan Flynn, Wei Wang, Chih-hung Chang, Gregory S. Herman,
"Microwave assisted synthesis of Cu2ZnSnS4 colloidal nanoparticle inks",
Physica Status Solidi (a), (2012), DOI: 10.1002/pssa.201127734.


WE CLAIM
1. A process of synthesizing kesterite nanopowder incfuding a compound of the
formula:
Cu2-aZnbSnc(S/Se)4+d wherein 0<=a<=1, 0<=b<=1, 0<=c<=1 and -
1<=d<=1, comprising the steps of:-
contacting a source of Copper, a source of Zinc, a source of Tin, and a source
of sulphur and/or selenium in the presence of Ammonium Hydroxide in water;
stirring and heating the resultant solution in a microwave below a pressure
(in-situ reaction generated), at a temperature, power and length of time
sufficient to make the kesterite powders.
2. The method as claimed in claim 1, wherein the solvent used during the
synthesis is Dl water.
3. The method as claimed in claim 1, wherein Ammonium Hydroxide is added to
the solvent in an amount varying from about lwt% to 99 wt%.
4. The method as claimed in claim 1, wherein the Copper source consists of any
water soluble salts of Copper.
5. The method as claimed in claim 1, wherein the Zinc source consists of any
water soluble salts of Zinc.
6. The method as claimed in claim 1, wherein the Tin source consists of any
water soluble salts of Tin.
7. The method as claimed in claim 1, wherein the source of sulphur and/or
selenium is selected as Thiourea and/or Selenourea.

8. The method as claimed in claim 1, wherein microwave irradiation is used to
heat the solution of precursors in the solvent.
9. The method as claimed in claim 1, wherein reaction time can vary from 40 to
90 min depending upon the powder of microwave, nature and quantity of the
precursor solution and its pH.
10. The method as claimed in claim 1, wherein the microwave radiation of
2.45GHz is used to deliver power ranging from 400W to 1000W depending
upon the nature of precursors.
11.The method as claimed in claim 1, wherein the reaction time is not more than
90 min.
12. The method as claimed in claim 1, wherein the temperature of the precursor
solution is maintained constant during the reaction and it can vary between
190°C to 240°C.
13. The method as claimed in claim 1, wherein as obtained powder can be
deposited as thin film by non-vacuum methods including spin coating, dip
coating, doctor balding, spraying, screen printing, ink jet printing or
electrophoretic deposition.
14. The method as claimed in claim 1, wherein size of kesterite powders range
from 50 nm to 3 µm.
15. The method as claimed in claim 1, wherein kesterite powders in amounts
varying from 1mg to 1kg is produced.

16. The method as claimed in claim 1, wherein kesterite nanopowders with
optical band gap ranging from 1 Ev to 1.5 eV is formed.
17. The method as claimed in claim 1, wherein the use of hazardous H2S/ H2Se/
Na2S/ Na2Se environments for making kesterite nano powders is avoided.
18. The method as claimed in claim 1, wherein the stoichiometry of materials can
be fixed in precursor solution.
19.The method as claimed in claim 1, wherein the obtained nano powder is ball
milled with surfactant(s) for 1-5 hours followed by re-dispersion of the
powder in a carrier liquid in the presence of a dispersant using ball milling of
1-10 hours of time, the carrier liquid being any of organic diols, alcohols or a
combination thereof.
20. Non-vacuum deposition method including but not limited to spin coating, ink
jet printing, dip coating, doctor blading, spraying, screen printing and
electrophoretic deposition to deposit the nano powder produced in a method
as claimed in claim 1, in the form of 0.5 to 2 µm absorber layer.

ABSTRACT

The present invention relates to a process of synthesizing kesterite nanopowder
including a compound of the formula: Cu2-aZnbSnc(S/Se)4+d wherein 0<=a<=1,
0<=b<=1, 0<=c< = 1 and -1<=d<=1, comprising the steps of:-contacting a
source of Copper, a source of Zinc a source of Tin, and a source of sulphur
and/or selenium in the presence of Ammonium Hydroxide in water; stirring and
heating the resultant solution in a microwave below a pressure (in-situ reaction
generated), at a temperature, power and length of time sufficient to make the
kesterite powders.