TITLE:
A non-vacuum chemical process for rapid synthesis of nanostructured CIGS powders for
solar photovoltaic applications.
FIELD OF THE INVENTION:
The present invention is in the field of synthesis of nanostructured CIGS [Copper Indium
Gallium Di-Selenide, {Cu(In, Ga)Se2}] powders and its 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 CIGS powders and or its products or powders thereof with variable levels
of gallium and indium atomic ratio in the CIGS compound.
BACKGROUND OF THE INVENTION:
CIGS [Copper Indium Gallium Di-Selenide, {Cu(In,Ga)Se2}] is a p-type semiconductor
and is recognized as a promising material for fabricating lightweight and flexible high
efficiency thin-film based solar cells with long-term stability and reproducible
photovoltaic responses. CIGS material is coated in the solar cell module to absorb light
up to 800 nm of wavelength, associated with high absorption co-efficient (105 cm-1) with
higher photon energy having tunable band gap (HOMO-LUMO gap) in the range of 1.04
- 1.68 eV. CIGS 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., CIGS 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 CIGS materials by numerous
routes.

Thin Solid Films, volumes 480-481, pages 46-49 (2005), describes a solvo-thermal route
using elemental Cu, In, Se and Ga as starting chemicals in an autoclave with
ethylenediamine as a solvent, wherein spherical CIGS nanoparticles with diameter in the
range of 30-80 nm were obtained at temperatures in the range of 180-280°C and plate
like particles were obtained at 140°C. The addition of gallium to the elemental
solvothermal route for CulnSe2 particles lowered the reaction temperature for the
formation of the CIGS nanoparticles, which was studied with the Solution-Liquid -Solid
(SLS) mechanism.
Chemistry Materials volume 19, number 22 pages 5256-5261 (2007) also described the
synthesis and characterization of phthalocyanine-based soluble light-harvesting CIGS
complex through a one-pot reaction of tBu4PcM-Cl (M = Ga, In), Cul, and [(CH3)3Si]2S
using triphenyl phosphine.
Nanoscale Research Letters, volume 3, pages 21-24 (2008) describes a complex
chemistry route (Metal-Ligand Complex Route) using aqueous solution of Cul2, InCl3,
GaCl3 and Ph3P in anhydrous N, N-dimethylacetamide (DMA) with excess of
hexamethyldisilathiane as solvent thereby refluxing for 20 h at 170°C under argon gas,
followed by filtration using high vacuum wherein a soluble triphenylphosphine-
coordinated CIGS complex having a band gap of 1.24 eV with highly ordered structure
resulted.
Chemistry Materials, Volume 20, No. 22, Pages 6906-6910 (2008) reports another
complex chemistry route (Metal-Ligand Complex Route) for synthesizing CuGaSe2,
CulnSe2, and Cu(In, Ga)Se2 particles with narrow size distribution through manipulation
of reaction temperature, precursor concentrations & ligand-precursor reactivities using
commercial grade Copper, Indium, Gallium salts and Se powder using Oleylamine as
solvent.

Journal of Ceramic Processing Research, Volume 10, number 4, pages 37-442 (2009)
describes an aqueous solution route for synthesizing well crystallized CIGS particles with
crystallite size of 100 nanometer using zinc dust particles and Ga2(SO4)2, CUCl2.2H2O,
InCl3.nH2O and SeO2 as starting chemicals and ethylene glycol, dimethylsulfoxide and
dimethyformamide as various choice of solvents.
Basic Convergence Technology Research Team, Electronics and Telecommunications
Research Institute, 138 Gajeongno, Yusung-Gu, Daejeon, 305-700, Republic of Korea
reports a solution route using chloride salts of Cu, In & Ga and Selenium powders using
acetone and methanol as washing agents wherein CIGS nanoparticles with tetragonal
chalcopyrite structure resulted.
International Solar Electric Technology (ISET) 8635 Aviation Blvd., Inglewood, CA
90301 reports a co-precipitation route for synthesizing CIGS particles with specific
surface area of 15 m /g using water-based soluble precursors and co-precipitation as
hydroxides.
WO/2008/104087 describes a simple and scalable process for synthesizing small-sized
CIGS nanoparticles having a narrow particle size distribution. The process involves
reacting CuXa2, MXb3 and L2Y in water to form nanoparticles of a chalcopyrite of
formula CUw(InxGa1-x)SySe2-y, wherein Xa and Xb are the same or different and are
halogens, M is Ga or In, L is an alkali metal, Y is S or Se, w is a number from 0.8 to 1.2,
x is a number from 0 to 1, and y is a number from 0 to 2. The CIGS nanoparticles, as
described are useful for producing semiconductor films for photovoltaic devices, e.g.
solar cells, light emitting diodes and photodetectors.

US Patent number US 7829,059 B2 dated Nov 09, 2010 describes a process for
synthesizing chalcogenide nanoparticles comprising binary ternary or multinary
components by reacting a metal component with an elemental chalcogen precursor using
alkylamine solvent with boiling point above 220 DegC having particle size ranging from
5 nanometer to 1000 nanometer.
US Patent US2010/0319776 Al dated December 23, 2010 describes an CIGS-based ink
containing nanoparticles of CIGS for formation of thin film of a solar cell and its
preparation method and thereby fabricating a CIGS-based solar cell having at least one
light absorption layer formed by coating or printing the said ink containing nanoparticles
on a rear electrode and a process for manufacturing the same.
More recently, International Journal of Chemical Engineering, Volume 2011 (2011),
Article ID 545234, 8 pages reports a Large-Scale Synthesis and Characterization of
Quaternary CulnxGal-xS2 Chalcopyrite Nanoparticles via Microwave Batch Reactions
where various quaternary CulnxGal-xS2 (099.5%) of cupper chloride, gallium chloride and indium chloride
in desired molar ratio (termed as 'Solution A') depending on the indium to
gallium atomic ration in the targeted CIGS compound
• Preparing a diluted of 'Solution A' with defined concentration just by
appropriately diluting the 'Solution A' with distilled/de-ionized water
• Preparing an aqueous solution of metal (sodium) selenide by reacting selenium
powder with a suitable reducing agent (termed as 'Solution B'),

• Preparing a diluted solution of sodium selenide with defined concentration just
by appropriately diluting the 'Solution B' with distilled/de-ionized water
• Mixing the diluted 'Solution A' into the diluted 'Solution B' having appropriate
concentrations of both the solutions with continuous stirring at ambient pressure
and at a defined solution temperature
• Carrying out a precipitation reaction by mixing the above two solutions for a
period 05-10 minutes and obtaining a black coloured precipitate that resulted
immediately after mixing the two solutions
• Filtering off the resultant black coloured precipitate using distilled/de-ionized
water until the filtrate becomes free from chloride ions resulting chloride-ion-free
washed precipitate
• Drying off the washed precipitate at any temperature in the range of 50 - 90°C to
yield dried CIGS powders
• Heat treatment (annealing) of the dried CIGS powders in an inert atmosphere in
the temperature range of 100° - 300°C to improve the crystallinity of the CIGS
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 CIGS
[Copper Indium Gallium Di-Selenide, {Cu(In,Ga)Se2}] powders rapidly with variable
levels of gallium and indium atomic ratio in the CIGS 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, Indium and Gallium salts (Cu-In-Ga) with appropriate
concentrations in each case, in one hand and preparation of an aqueous solution of metal
selenides in other hand with appropriate concentration.
The mixed aqueous solution of copper, indium and gallium salts (Cu-In-Ga) are prepared
by dissolving analytical grades (minimum purity >99.5%) of copper chloride, indium
chloride and gallium chloride salts using de-ionized or distilled water in desired
concentration as per the targeted atomic ratio of 'indium-gallium' in the CIGS
compound, since any level of Indium and Gallium atomic ratio in the CIGS compound
could be prepared. The resultant aqueous solution is termed as 'Solution A'.
Aqueous solution of metal (sodium) selenide is prepared by dissolving selenium powder
in a suitable or matching reducing agent that is capable of reducing elemental selenium
into selenide ion in aqueous solution (sodium salt here) in which the redox potential for
selenium to selenide ion, i.e., E°se/se2" is 0.92 Volts. The resultant aqueous solution of
selenide ion in the form of sodium selenide 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 CIGS powder which is also crystalline.
The dried CIGS 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 CIGS compound resulting well crystalline CIGS compound
having tetragonal chalcopyrite structure.
EXAMPLE 1:
In this example, the molar ratio of indium and gallium is fixed at 0.5: 0.5 in the CIGS
compound for the synthesis. The nominal composition corresponding to the above
gallium and indium molar ratio in the CIGS compound becomes [Cu(In0.5Ga0.5)Se2].
CIGS 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, indium chloride and gallium chloride in molar ratio of 1:
0.5 : 0.5 in deionized water respectively.
The aqueous solution of sodium selenide (Solution B) is prepared by reacting selenium
powder with a matching reducing agent by considering the redox potential for selenium
to selenide ion conversion (E°se/se2" is 0.92 Volts).

The 'solution A' is mixed to the 'solution B' under constant stirring by maintaining the
solution temperature at 48 + 02 DegC. 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 DegC for a period
of 5-6 hours that resulted dried precipitates. The dried precipitate is called CIGS powders
with equimolar indium to gallium atomic ratio in the CIGS compound.
Heat treated (90°C for 1 hour) powders of CIGS materials showed primary particle size
in the range of 10-50 nm having tetragonal chalcopyrite structure in [Cu(In0.5Ga0.5)Se2]
compound.
The specific surface area of the dried CIGS powders in the BET analysis invariably
showed surface area of60±02m2/g.
EXAMPLE -2:
Molar ratio of Indium and gallium is fixed at 0.7 : 0.3 in the CIGS compound in this
example for the synthesis. The nominal composition corresponding to the above gallium
and indium molar ratio in the CIGS compound becomes [Cu(In0.7Ga0.3)Se2]. 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.
In this example, the 'solution A' is prepared by dissolving copper chloride, indium
chloride and gallium chloride in molar ratio of 1: 0.7:0.3 in deionized water.
The aqueous solution of sodium selenide (Solution B) is prepared by reacting selenium
powder with a matching reducing agent by considering the redox potential for selenium
to selenide ion conversion (E°se/se2" is 0.92 Volts).

The 'solution A' is added to the 'solution B' under constant stirring by maintaining the
temperature of the mixed solution 50 ± 02 DegC. A black colored compound out
instantaneously precipitated out from the solution. The black precipitate is filtered off and
washed repeatedly with deionized water until chloride was free from the filtrate. The
washed precipitate is then dried in an oven at a temperature of 85 ± 05 DegC for a period
of 5-6 hours that made the washed precipitates completely dry. The resulted dried
precipitate is called CIGS powders having indium to gallium atomic ratio of 7:3 in the
CIGS compound.
Heat treated (90°C for 1 hour) powders of CIGS materials showed primary particle size
in the range of 10-50 nm. The specific surface area of the dried CIGS powders in the
BET analysis invariably showed surface area of 65 ± 02 m2/g.

WE CLAIM:
1. A non-vacuum and aqueous-based process for the synthesis of nanostructured Nano-
CIGS [Copper Indium Gallium Di-Selenide, {Cu(In,Ga)Se2}] powders with variable
levels of gallium and indium atomic ratio in the CIGS compound comprising the
following steps:
• Preparing a mixed aqueous solution by dissolving analytical grades (purity
>99.5%) of copper, indium and gallium salts respectively in desired molar ratio
(Solution A) that is required for the targeted CIGS 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 metal (sodium) selenide by reacting selenium
powder with a suitable reducing agent with required molar ratio for the chemical
conversion from selenium powder to metal (sodium) selenide solution (Solution
B),
• Preparing an appropriate concentration of metal (sodium) selenide solution just by
appropriately diluting the 'Solution B' with distilled/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/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 CIGS powders
• Heat treatment (annealing) of the dried CIGS powders in an inert atmosphere in
the temperature range of 100° - 300°C to improve the crystallinity of the CIGS
powders
2. The process as claimed in claim 1, wherein the mixed aqueous solution of copper,
indium and gallium solution is prepared by dissolving analytical grades (purity
>99.5%) copper chloride, indium chloride and gallium chloride salts in de-ionized or
distilled water by maintaining molar ratio of "1.0 : 1-X : X" (where 'X' is the number
of atoms of indium in the targeted CIGS compound to be synthesized) and then
diluting the resultant mixed solution with distilled water or deionized water to obtain
diluted mixed solution with appropriate concentration/s (termed as 'solution A').
3. The process as claimed in claim 1, wherein the said metal selenide solution (sodium
selenide) is prepared by reacting selenium powder with any suitable reducing agent by
considering the redox potential for elemental selenium to selenide ion conversion
(E°se/se2- is 0.92 Volts) and thereby diluting the sodium selenide solution appropriately
with distilled or de-ionized water for preparing diluted sodium selenide solution with
appropriate concentration/s (termed as 'solution B').

4. The process as claimed in claim 1, wherein the diluted solution with appropriate
concentration of 'solution B' is mixed with the diluted solution of mixed Cu-In-Ga
solution (solution A) with appropriate concentration 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 de-ionized or distilled 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 (free from chloride
ions) 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%).
7. The process as claimed in claim 1, wherein the dried precipitate is nanostructured
CIGS [Copper Indium Gallium Di-Selenide, {Cu(In,Ga)Se2}] powder that has
tetragonal chalcopyrite structure with specific surface area in the range of60+10m2/g
comprising 10-50 nanometer of primary particles in the agglomerated particles of CIGS
compound.
8. A process as claimed in claim 6, wherein the nanostructured CIGS 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 CIGS powders and to obtain
well crystalline CIGS powders having tetragonal chalcopyrite crystal structure.

ABSTRACT

The invention describes a non-vacuum aqueous-based chemical process for synthesizing
nano-structured CIGS [Copper Indium Gallium Di-Selenide, {Cu(In,Ga)Se2}] powders
and the products thereof for solar photovoltaic applications. By following the process, a
variety of CIGS compound with variable levels of gallium and indium atomic ratio in the
CIGS compound can rapidly be prepared. The synthesized CIGS powders have
tetragonal chalcopyrite structure with specific surface area in the range of 60±10 m2/g.
Heat treatment (annealing) of the CIGS powder in any inert atmosphere, e.g., argon or
nitrogen at a temperature in the range of 100 - 300°C improves the crystallinity of CIGS
powder that results well crystalline CIGS powder. Electron microscopy analyses (TEM
& Fe-SEM) of the CIGS 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 CIGS material
following any standard procedure.