FIELD OF THE INVENTION
The present invention relates to synthesis of highly crystalline CIS or CIGS
precursor ink and related thin film absorber layers. The developed inks eliminates
the need of Na2Se, organic binders/dispersant and high temperature selenization
using highly toxic H2Se gas for deposition of thin film absorber layer.
BACKGROUND OF THE INVENTION
The solar energy plays an important role and the photovoltaic (PV) commonly used
is crystalline silicon (c-Si), due to its higher conversion efficiency of around 23%.
However, for Si, a thick (on the order of 100 µm) layer is required to efficiently
absorb the photons in the solar spectrum. To reduce recombination within the
thick layer, high-purity wafers of Si are, therefore, needed for traditional Si based
PV technology. In addition, Si technology relies on high-temperature vacuum
based process during fabrication. Expensive equipment and complicated
procedures
make it difficult to reduce the final module cost. Due to the current high cost of
solar panels, PV is not widely deployed and solar energy does not constitute a
significant percentage of everyday energy consumption.
Thin film solar cells based on chalcopyrite semiconductors are also known as
one of the alternative materials to Si solar cells. The features of chalcopyrite
semiconductors include high absorption coefficient, low light degradation, and
high radiation resistance. Copper-indium-gallium-di-selenide, Cu(In1-x Gax)Se
(where x varies from 0 to 1), often referred as CIGS is a well known p-type
chalcopyrite semiconductor. Photo absorbing CIGS has direct band gap and
photon absorption coefficient of more than 105 cm-1 in visible range. Due to
this, CIGS film having a thickness less than 1 µm can absorb more than 99%
photons of the sunlight illuminated thereon and hence cost can be reduced. In
addition to this, by variation of Ga/(In+Ga) ratio, band gap of CIGS films can
be engineered from 1.06 eV (for CuInSe2) to 1.68 eV (for CuGaSe2) to achieve
higher efficiency. CIGS based solar cells have been demonstrated to have
power conversion efficiencies of more than 19.5% [1].
Various vacuum processing techniques, which include co-evaporation,
sputtering and pulsed laser deposition (PLD) are used in prior art for
fabrication of high quality CIS or CIGS thin films. However, conventional
vacuum methods have drawbacks such as complexity in process, high
production costs and difficulty in scaling up, which are to be solved before the
commercialization of the CIGS solar cells. Furthermore, the high temperature
annealing and selenization steps used in each of these solar cell fabrication
processes are neither cost effective nor easily scalable to high volume
production. The high temperature annealing steps in these processes also
make them incompatible to make CIGS or CIS solar cells on inexpensive
flexible polymers and metallic substrates.
The power conversion efficiency of the CIS or CIGS absorber layer are 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. These techniques rely on line-of-sight
trajectories of the constituent elements, which result in non-uniform distribution of
the elements and hence variation of stoichiometric composition in absorbing layer.
To achieve a better control of the stoichiometric composition throughout the film,
the ratio of materials should be fixed in precursor solution before the thin film
deposition.
Non-vacuum deposition techniques with a capability to prepare large area uniform
thin films with control stoichiometric composition has presently gained more
attention [2-4]. Among the non-vacuum processing techniques, nanoparticle
based coating process is considered to be simple and cost-effective method for
preparing CIS or CIGS thin film without using high temperature co-evaporation or
sputtering. Nanoparticle based processes generally involves two step fabrication
method, first nanoparticles are synthesized by a chemical process, in which the
stoichiometric ratio and crystal phases can be controlled. Then the nanoparticles
are dispersed in solvents to create paints or inks.
The paints or inks could be used to form CIS or CIGS thin films using variety of
solution based coating techniques, including: dip-coating, spin coating, doctor
blade coating, roller coating, meyerbar coating, spraying, brushing, screen-
printing, contact-printing, ink-jet printing or other similar printing technology.
These techniques would be competitive in terms of processing capital costs,
efficiency of resource material usage, and processing speed. In addition, the
nanoparticles paints or inks can be applied to continuous roll-to-roll deposition
processes for large area fabrication of PV devices.
Kapur et. al. (U.S. Pat. No. 6,127,202, issued October 2000) disclosed a non-
vacuum process in which a water-based precursor ink is formulated using mixed
oxides of Cu, In and Ga, which is then coated on rigid or flexible substrates. The
resulting oxide mixture was then subjected to reduction in H2 /N2 mixture and
selenization in an H2Se/N2 atmosphere at a temperature of around 500°C. These
fabricated CIGS solar cells have efficiency in the range of 8 to 11%.
However, there are several drawbacks of metal oxides being used as precursor
materials for CIGS solar cells. First, the use of oxide-based particles in CIGS
absorber layer fabrication requires a high-temperature hydrogen reduction step to
reduce the oxides, which is potentially explosive. Second, the high temperature
selenization process for incorporating Se into CIGS layer involves highly toxic H2Se
gas, which is a serious concern in the manufacturing process. Furthermore, the
high temperature reduction and selenization steps make it impractical to make
CIGS solar cells on inexpensive polymer or metallic substrates. Finally, it is very
difficult to effectively incorporate gallium into copper indium precursor film using
metal oxide synthesis approach due to formation of gallium oxide at high
temperature annealing, which is highly stable material and difficult to reduce.
Eberspacher and Pauls (U.S. Pat. No. 6,268,014, issued July 2001) also describe
the synthesis of mixed metal oxide, sub-micron sized particles by pyrolizing
droplets of a solution, then ultrasonically spraying the resulting particles onto a
substrate. However, spray pyrolysis of micron thick layers of precursor particles
also has significant drawbacks. Ultrasonic spraying of thin layers of sub-micron
sized particles onto a substrate is an inherently non-uniform process, which leads
to formation of pockets and voids within the deposited film. This results in a solar
cell with poor and unstable optical and electronic properties.
Schultz et al. (U.S. Pat. No. 6,126,740) developed iodides based approach for
synthesis of chalcopyrite nanoparticles. CIGS nanoparticles have been prepared by
using a mixture of Cul, Ink and Gal3 in pyridine with Na2Se in methanol at low
temperature. The mixture solvent of pyridine/methanol was sprayed directly onto
a molybdenum coated soda lime glass to achieve a thin film with fixed ratios of
CIGS. However, the CIGS nanoparticles were largely amorphous, which is not
desirable for high performance photovoltaic devices. In addition to this, the
formation of sodium iodide by products and use of highly toxic Na2Se makes this
process incompatible to fabricate CIGS based efficient solar cells.
Panthani et. al. [5] prepared the CIS absorber layer by using "nanocrystal ink
method" in which a colloidal nanocrystal ink was obtained by reaction of CuCI,
InCI3 and Se in oleylamine. This approach alleviates the need for a high
temperature annealing under Se atmosphere using highly toxic H2Se gas. In
addition to this, composition control of the fabricated film is easy, as the
composition of precursor nanocrystal ink can be transferred directly to the
substrates without change in the atomic ratio of elements. Films of CuInSe2
nanocrystal ink used as absorber layer in conventional layered
Mo/CuInSe2/CdS/ZnO/ITO PV devices exhibits reproducible photovoltaic responses
with power conversion efficiencies up to -0.2%.
Guo et. al. [6] also synthesized CIS nanocrystal inks in oleylamine by "hot
injection" method. In this modified approach Se is injected into a solution of CuCI
and InCI3 in oleylamine at 285°C. This "hot injection" of Se induces abrupt super
saturation of the reaction mixture, which plays a major role in the resulting crystal
structure. The devices based on these nanocrystal inks yielded conversion
efficiency 3.2% under AMI.5 illumination. However, in oleylamine based approach
the efficiency of devices is reduced by capping ligands around the nanocrystals,
which impedes charge transport within the absorbing layer.
Liu et. al. [7] used a hydrazine solution based approach to prepare CIS absorbing
layer and corresponding solar cell devices. CIS layer was prepared by dissolution of
Cu2Se, In2Se3, and Se in hydrazine solution. In this approach, no post deposition
annealing in toxic H2Se environment is needed to obtain CIS. Instead, only a
simple heat-treatment in an inert atmosphere is required to produce good
crystallinity CIS thin film. Complete CIS devices with glass/Mo/ CuInSe2/CdS/i-
ZnO/ITO structure were fabricated by spin coating of hydrazine based solution.
The spin coated CIS film exhibit the power conversion efficiencies of as high as
12.2%, demonstrating that this new approach has great potential as a low cost
alternative for high efficiency CIS solar cell fabrication. However, in the hydrazine
solution method, the toxicity and corrosiveness of the hydrazine may cause an
environmental problem as well as a decrease in durability of fabrication
equipment, which makes this process incompatible to large area production of CIS
modules.
Thus, the solvothermal method recently emerged as a powerful tool for the
synthesis of nanostructures/microstructures, using organic amine as a solvent.
Luo et. al. [8], Jiang et. al. [9], Li et. al. [10] and Chun et. al [11] have reported
that the CIS and CIGS can be synthesized by solvothermal route. Although, its
longer reaction time (15-36 h) and difficulty in controlling the morphology of the
particles limits the synthesis of high quality CIS or CIGS nanoparticles.
Further, a simple and non-toxic mechanochemical synthesis also deserves attention,
because the CIS or CIGS powders and thin films can be directly formed via ball-
milling the elemental Cu, In, Se and Ga powders/granules. Vidhya et. al. [12]
prepared CIGS nanoparticles precursor through ball-milling the elemental powders.
The CIGS thin film was fabricated by mixing the nanoparticles precursor with an
organic binder such as ethyl-cellulose. However, HRTEM revealed that during the
process of ball milling numerous dislocations and distortions were induced, which
can critically influence the optical and electronic properties of the absorber layer.
A conclusion from the discussion of prior art, may be that the known non-
vacuum processes have still limitations, like toxic solvents, longer reaction time,
high temperature synthesis, separation, purification, annealing in highly toxic
H2Se gas environment, low yield, poor crystallinity, and presence of foreign
elements impurities caused by additives (organic binders) and suspension
stabilizers. The poor crystallinity and presence of foreign impurities may
deteriorate the electronic and optical properties of the absorber layer.
Nevertheless, synthesis of nanoparticles based on microwave heating is found to
be an efficient technique because of its various advantages, such as shorter
reaction time, low level of impurities, high crystallinity, excellent reproducibility
and high yield of product. Bensebaa et. al. [13] used a microwave synthesis
approach to prepare CuInS2 and CuInSe2 nanoparticles. Following the recent
work, Juhaiman et. al. [14] also used microwave route to synthesize CIGS
nanoparticles with variable Ga content. However use of highly toxic Na2S
(sulphur source) and Na2Se (selenium source) chemicals makes this process
incompatible to fabricate CIGS or CIS absorber layer. Recently, Wu et. al. [15]
synthesize CIS nanoparticles by proper choice of solvents and chemicals, where
use of Na2Se is avoided, but in this report no emphasis is given on CIS thin film
deposition using the prepared nanoparticles.
Thus, there is a need in the art, where highly crystalline CIS or CIGS thin film
can be deposited directly from the precursor ink with controlled stoichiometry
using less or non- toxic chemicals at very low temperature. Need of high
temperature selenization in highly toxic H2eSe gas environment and use of
organic binders/dispersant can be avoided.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention is to propose a process for
fabrication of chalcopyrite nano-inks and thin film absorber layers at low
temperature.
Another object of the invention is to propose a process for fabrication of
chalcopyrite nano-inks and thin film absorber layers at low temperature, which is
enabled to synthesise a highly crystalline nanocrystal inks, and eliminates the use
of toxic Na2Se (selenium source).
A still another object of the invention is to propose a process for fabrication of
chalcopyrite nano-inks and thin film absorber layers at low temperature, in which
the composition of the precursor nanocrystal ink developed can be transferred
directly to the thin film without change in the stoichiometry of constituent
elements.
Yet another object of the invention is to propose a process for fabrication of
chalcopyrite nano-inks and thin film absorber layers at low temperature, in which
the precursor nanocrystal inks, eliminates the prior art need of organic
binders/dispersant, and high temperature selenization using highly toxic H2Se gas
for deposition of thin film absorber layer.
SUMMARY OF THE INVENTION
Another to the present invention, there is provided a process for synthesisation of
highly crystalline CIS or CIGS precursor inks based on microwave heating . The
process avoids the use of highly toxic Na2Se as a selenium source. In the
developed precursor ink, the composition of precursor nanocrystal ink is
transferred directly to the thin film over the substrates without undergoing any
change in the stoichiometry of elements by simple drop casting method, which
interalia eliminates the separation and purification process steps. The developed
ink also eliminates the need of organic binders/dispersant and high temperature
selenization using highly toxic H2Se gas for deposition of thin film absorber layer.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1. Flow chart illustrating the microwave-assisted process for fabrication of
chalcopyrite nano-inks and related thin film absorber layers according to
the invention.
Figure 2. X-ray diffraction (XRD) pattern of CIS thin film, fabricated by drop-
casting of CIS nano-ink synthesized for 30 min microwave irradiation
time.
Figure 3. XRD pattern of CIS thin film, fabricated by drop-casting of CIS nano-ink
synthesized for 40 min microwave irradiation time.
Figure 4. UV-VIS-NIR absorbance spectra of CIS thin film, fabricated by drop-
casting of CIS nano-ink synthesized for 40 min microwave irradiation
time.
Figure 5. Size distribution pattern of CIS nanoparticles dispersed in liquid water.
Figure 6. Scanning electron microscopic (SEM) image of CIS thin film, fabricated
by drop-casting of CIS nano-ink synthesized for 40 min microwave
irradiation time.
Figure 7. XRD pattern of CIS thin film of the present invention annealed at
250°C
Figure 8 (a). XRD pattern of CIGS thin film, fabricated by drop-casting of CIGS
nano-ink synthesized for 20 min microwave irradiation time. Small
angle comparisons of CIS and CIGS thin film XRD are included in
Fig. 8 (b).
DETAILED DESCRIPTION OF THE INVENTION
The present invention teaches a process of rapid synthesis of highly crystalline CIS
or CIGS precursor ink and associated thin film absorber layers. Coating of the
precursor ink allows a precise control of stoichiometry, phase/orientation, of the
fabricated film. Embodiments of the present invention also provide an absorber
layer with several desirable properties, including but not limited to relatively high
density, high uniformity and low porosity. A right selection of the precursors in the
present embodiments to synthesize the inks avoids the need of Na2Se as a Se
source, organic binders/dispersant and high temperature selenization under highly
toxic H2Se gas. A first embodiment of the invention herein below is described
under
reference to Fig.l, in the farm of a flow chart of process adopted and precursors
used for CIS and CIGS synthesis.
Figure. 1 shows the flow chart of the microwave-assisted process for fabrication of
chalcopyrite nano-inks and related thin film absorber layers. Referring to FIG. 1,
the flow chart starts from step 100. At step 100 the Cu, In, and Se precursors are
mixed with ethylenediamine (ED) and mixture is stirred. After stirring, the
precursors are kept under microwave irradiation for different synthesis times to
get the CIS or CIGS inks, at step 101. According to step 102, CIS or CIGS thin film
is obtained by drop casting of the ink developed in step 101. The high density,
high uniformity and low porosity films are achieved by drying the film of step 102
at different temperatures at step 103.
For CuInSe2 ink synthesis, the required amount of CuCI2 (0.205 g), InCI3 (0.346 g),
and Se (0.248 g) are added to 40 ml of ED and mixed together. The mixture is
stirred and sonicated for 30 min, respectively to form a homogeneous solution.
Microwave synthesis is performed in a Microsynth System operating at microwave
power of 600W and frequency of 2.45 GHz. The solution containing CIS precursors
is sealed and heated at 180°C for 30 and 40 min, respectively.
For CuIno.7Gao.3Se2 ink synthesis, a required amount of copper(II) acetylacetonate
[Cu(acac)2], indium(III) acetylacetonate [In(acac)3], gal[ium(III) acetylacetonate
[Ga(acac)3], and Se were added to 40 ml of ED and mixed together. The solution
containing CIGS precursors was sealed and heated at 180°C for 20 min using
microwave power of 800 W.
According to the invention, group VIA elements such as Se (or S) may be
incorporated into the absorber layer before the annealing stage, using ED, which
served as both the reducing agent as well as complexing agent. A nucleophilic
attack by the amines of ED activates elemental Se to form reactive Se2" ions.
Meanwhile, ED, also reduces the Cu2+ to Cu+, because of the electron transfer
reaction. The reactive Se2- ions species react with both InCb and Cu2+
simultaneously in ED to form In2Se3 and CuSe, respectively. Finally Cu2Se and
In2Se3 could react, and CuInSe2 nanocrystal ink was produced.
According to the present invention, ED avoid the need of toxic Na2Se and H2Se
as a Se source, since the formation of reactive Se2- ions species initiate the
reaction (for CIS or CIGS synthesis) and easily incorporated into the absorber
layer without any additional step. The use of ED also avoid the use of other Se
precursor sources such as dimethyl selenide (C2H6Se), dimethyl diselenide
(CH3)2Se2, and diethyl diselenide C4H10Se2 etc.
It is within the scope of this invention that, the metals source may be a mixture
of various forms of metals and not limited. The metal compounds may be metal
halides such as copper chloride, indium chloride, gallium chloride, copper
bromide, indium bromide, gallium bromide, copper iodide, indium iodide, gallium
iodide or metal acetylacetonates such as copper acetylacetonate, indium
acetylacetonate, gallium acetylacetonate or metal sulfates such as copper sulfate,
indium sulfate, gallium sulphate or metal sulfides such as copper sulfide, indium
sulfide, gallium sulphide or metal nitrates such as copper nitrate, indium nitrate,
gallium nitrate and many more.
The CIS thin film is fabricated using nano-ink synthesize in step 101 of Figure. 1.
CIS film of thickness ~ 1.5 µm is deposited onto 25 x 25 mm glass substrates by
dropping CIS precursor ink. The procedure is repeated drop-by-drop until the
sufficient thickness of CIS film is deposited. After each deposition the substrate is
heated to a temperature around 100°C to remove the solvent and to allow
formation of dry and smooth film with well-controlled stoichiometry among the
metal compounds. CIGS thin film from the developed ink is also fabricated in the
similar fashion.
It is within the scope of this invention that, the CIS or CIGS thin films can be
deposited by variety of techniques: dip-coating, spin coating, doctor blade coating,
roller coating, meyerbar coating, spraying, brushing, screen-printing, contact-
printing, ink-jet printing or other similar printing technology.
Figure. 2 shows the XRD pattern of CIS thin film, fabricated by drop casting of CIS
nano-ink synthesized for 30 min microwave irradiation time. For the irradiation
time of 30 min, many diffraction peaks appeared in the XRD pattern, which
attributed to CuSe phases along with the characteristic peaks of CuInSe2 phases.
Figure. 3 shows the XRD pattern of CIS thin film, fabricated by drop casting of
CIS nano-ink synthesized for 40 min microwave irradiation time. As irradiation
time increases from 30 to 40 min, the secondary CuSe phase disappear and only
characteristic peaks of CuInSe2 are observed, indicating the formation of pure
chalcopyrite phase. XRD peaks around 29 = 26.6°, 44.6°, and 52.8°,
corresponding to (112), (220/204) and (116/312) planes of chalcopyrite
structure. In addition to these commonly observed orientations, the weak
orientations such as (008/004) and (316/332) are also observed in XRD pattern.
X-ray diffraction pattern of the embodiment of Fig.3 indicates that, pure
chalcopyrite phase with good crystallinity are obtained using developed ink and
the composition of precursor ink is transferred directly to the thin film without
change in the stoichiometry of elements. Incorporation of Se in absorber layer
indicates that, the developed ink eliminates the need of high temperature
selenization under highly toxic H2Se gas after deposition of thin film absorber
layer.
Thin film of Fig.l, allows fabrication of absorber layer over polymers, such as
polyimides (PI), polyamides (PA), polyetheretherketone (PEEK), polyetherimide
(PE1), polyethylene naphtalate (PEN), polyester (PET) or metal foils, such as
titanium (Ti), aluminium (Al), stainless steel (SS), molybdenum (Mo), or many
more. The use of metal and flexible substrates can greatly reduce the materials
cost of PV devices.
In order to obtain the optical property, the optical absorption spectra of fabricated
film are recorded in the range 800 - 1500 nm. Figure. 4 shows an UV-VIS-NIR
absorbance spectra of CIS thin film, fabricated by drop-casting of CIS nano-ink
synthesized for 40 min microwave irradiation time. An increase in the onset of the
absorption from 1160 nm, corresponding to a band gap of 1.06 eV, is observed,
which is in agreement with the reported values.
Surface morphology (or grain size) of the resulting CIS or CIGS absorber layer are
highly dependent on particle size present in the precursor ink. To estimate the
particle size, the separation of nanoparticles is achieved by filtration with deionized
water and ethanol as a washing agent. This process is repeated several times with
deionized water and ethanol to ensure reagent free particles. Finally, CIS
nanoparticles are dried in an oven at 80°C for 4 h. For particle size analysis 10 mg
of CIS were dispersed in 80 ml de-ionized (DI) water and sonicated for 5 min.
Figure. 5 shows a nanoparticle size distribution pattern of CIS nanoparticles
dispersed in liquid water. The particle sizes in the solvent are typically distributed
over a relatively narrow range, i.e. majority of particles lie within the range of 100-
200 nm having average particle size of 150 nm. However, in solution much smaller
particles may be present, which are not observed due to their agglomeration in
water and inhomogeneous dispersion.
Figure. 6 shows a SEM image of CIS thin film used in the embodiment [043] of the
present invention Fig. 3. The morphological investigation of CIS thin film indicate
that dense and compact film having micron size grains can be achieved using the
developed ink without any selenization at higher temperature under H2Se or Se
atmosphere. Surprisingly, the film dried at around 100°C consists of the circular
grains of size around 1-3 µm, which may lead to good electrical and optical
properties of the absorber layer. The formation of bigger size grains in the
absorber layers is described below.
According to Fig.5, the particle sizes of the prepared ink are typically distributed
over a relatively narrow range having average particle size 150 nm. The presence
of smaller size particles can significantly lower, both the melting point and the
sintering temperature required, which are the key parameters for fabrication of
dense and compact film having bigger size grains.
Reduction in the melting point is inversely proportional to the particle radius, i.e.
smaller is the particle size, lower is the melting point. It is known that the melting
point is substantially reduced, with the reduction of particle size below 500 nm
regime. The prepared nanoparticles inside the ink have narrow size distribution. It
is possible for all the particles to melt at about the same and lower sintering
temperature. Due to higher surface area, nanoparticles fuse together without any
binder to form the uniform layer. In addition to this, smaller size
nanoparticles fill the voids among the bigger size particles, which result in
formation of dense film having micron size grains even at temperature of around
100° C.
Figure. 7 shows the XRD pattern of CIS thin film of the present invention (Fig. 3),
annealed at 250°C for 30 min. XRD of CIS thin film indicates that, there is no
variation in the stoichiometry of the film after annealing at 250°C i.e. no Se loss is
observed in the annealed film, which indicates that no high temperature
selenization using toxic H2Se gas is required to fabricate the absorber layers.
In the present invention, CuIn0.7Ga035Se2 ink is also synthesized using the precursors
described in hereinbefore. Absorber layer of CIGS is prepared using the process
shown in Fig.l. XRD pattern [Fig.8 (a)] indicates that, the composition of
precursor ink was transferred directly to the thin film without change in the
stoichiometry of elements. Proper choice of precursors to synthesize the inks also
avoids the need of Na2Se, organic binders/dispersant and high temperature
selenization under highly toxic H2Se gas, similar to CIS fabrication.
Figure. 8 (b) shows a comparisons of CIS and CIGS thin film XRD in the scan
range of 24 to 30°. Peak position of the (112) orientation is found to shift to
higher values of 2 theta i.e. from 26.6° for CuInSe2 to 27.0° for CuIn0.7Ga035Se2. The
noticeable shift in the peak position is attributied to incorporation of Ga in the
lattice.
Although the present invention has been described with reference to the
preferred embodiments, thereof, it is intended that the specification and
examples be considered as exemplary only.
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WE CLAIM
1. A rapid synthesis process of CIS or CIGS precursor inks and thin film
absorber layers to produce an improved composition of precursor inks, the
precursor inks and absorber layers, comprising at least three of: copper,
indium, gallium, selenium, or a combination thereof, the process comprising
the steps of:
- mixing copper, indium, and Se precursors with ethylenediamine (ED);
- stirring and soncating the mixture for about 30 minutes to form a
homogeneous mixture;
- conducting a microwave synthesis at about 600 watt of microwave
power and frequency of 2.45 GHZ for different synthesis time-periods
to obtain CIS or CIGS ink;
- drop casting the CIS or CIGS ink to obtain a thin film of the precursor
ink; and
- Sealing the thin film solution in a container and heating at
temperature about 180°C for 30 to 40 minutes ,wherein the precursors
are mixed respectively at a ratio of 1:1.7:1.2
2. The method as claimed in claim 1, wherein the copper precursor comprises
CuCk, or Cu(acac)2.
3. The method as claimed in claim 1, wherein the indium precursor comprises
InCl3, or In(acac)3.
4. The method as claimed in claim 1, wherein the gallium precursor when used,
comprises Ga(acac)3.
5. The method as claimed in claim 1, wherein the selenium precursor comprises
of elemental Se.
6. The method as claimed in claim 1, wherein the precursors selected comprise
one of metal halides, metal acetylacetonates, metal sulfides, and metal
nitrates.
7. The method as claimed in claim 1, wherein a stoichiometric of materials can
be fixed in the precursor solution.
8. The method as claimed in any of the preceding claims, wherein composition
of precursor inks, is directly transferable to the thin film without change in
the stoichiometry of the elements.
9. The method as claimed in claim 1, wherein the drying temperature of the thin
film ranges from ambient up to about 100, 125, 150, 175, 200, 225, and
250°C.
10. The method as claimed in claim 1, wherein the thin film can be annealed up
to 250°C without any change in the stoichiometry of the elements.
11. The method as claimed in claim 1, wherein the thin film comprises micron
size grains even at drying temperature of around 100°C.
12. The method as claimed in claim 11, wherein the thin film is applicable over
polymers, or metal foils, or any flexible substrates.
13. The method as claimed in claim 1, wherein the ink comprises the
nanoparticles of ZnInxGa1-xSe2, AgInxGa1-xSe2, CuInSe2-ySy, CuInxGa1-xSe2-ySy,
ZnInxGa1-xSe2-ySy, AgInxGa1-xSe2-ySy, or a combination thereof, where x is
from 0 to 1 and y is from 0 to 2.
14. The method as claimed in claim 1, wherein incorporation of Se in the precursor
during the liquid ink synthesis is allowable.

ABSTRACT

A rapid synthesis process of CIS or CIGS precursor inks and thin film absorber
layers to produce an improved composition of precursor inks, the precursor inks
and absorber layers, comprising at least three of: copper, indium, gallium,
selenium, or a combination thereof, the process comprising the mixing copper,
indium, and Se precursors with ethylenediamine (ED); stirring and soncating the
mixture for about 30 minutes to form a homogeneous mixture; conducting a
microwave synthesis at about 600 watt of microwave power and frequency of
2.45 GHZ for different synthesis time-periods to obtain CIS or CIGS ink; drop
casting the CIS or CIGS ink to obtain a thin film of the precursor ink; and sealing
the thin film solution in a container and heating at temperature about 180°C for
30 to 40 minutes, wherein the precursors are mixed respectively at a ratio of
1:1.7:1.2.