U.S. patent application number 11/278664 was filed with the patent office on 2006-07-27 for method of forming semiconductor compound film for fabrication of electronic device and film produced by same.
Invention is credited to Bulent M. Basol.
Application Number | 20060165911 11/278664 |
Document ID | / |
Family ID | 23086905 |
Filed Date | 2006-07-27 |
United States Patent
Application |
20060165911 |
Kind Code |
A1 |
Basol; Bulent M. |
July 27, 2006 |
Method of Forming Semiconductor Compound Film For Fabrication of
Electronic Device And Film Produced by Same
Abstract
A process of forming a compound film includes formulating a
nano-powder material with a controlled overall composition and
including particles of one solid solution The nano-powder material
is deposited on a substrate to form a layer on the substrate, and
the layer is reacted in at least one suitable atmosphere to form
the compound film. The compound film may be used in fabrication of
a radiation detector or solar cell.
Inventors: |
Basol; Bulent M.;
(US) |
Correspondence
Address: |
PILLSBURY WINTHROP SHAW PITTMAN LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Family ID: |
23086905 |
Appl. No.: |
11/278664 |
Filed: |
April 4, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10474259 |
May 19, 2004 |
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PCT/US02/11047 |
Apr 11, 2002 |
|
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11278664 |
Apr 4, 2006 |
|
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60283630 |
Apr 16, 2001 |
|
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Current U.S.
Class: |
427/458 |
Current CPC
Class: |
C23C 10/06 20130101;
C23C 8/02 20130101; C23C 12/00 20130101; C23C 10/02 20130101; H01L
31/18 20130101; Y02E 10/541 20130101; H01L 31/0322 20130101 |
Class at
Publication: |
427/458 |
International
Class: |
B05D 1/04 20060101
B05D001/04 |
Claims
1. A precursor composition to form a compound layer, comprising: at
least one set of particles, wherein each particle within the at
least one set comprises a solid-solution selected from the group of
Cu--Ga solid solution, Cu--In solid solution. In--Ga solid solution
and Cu--In--Ga solid solution.
2. The precursor composition of claim 1 including two sets of
particles wherein each particle within the first set comprises
Cu--Ga solid solution and each particle within the second set
comprises In--Ga solid solution.
3. The precursor composition of claim 1 further including at least
one type of metallic particles selected from the group of In
particles, Ga particles and Cu particles.
4. The precursor composition of claim 2 further including at least
one type of metallic particles selected from the group of In
particles, Ga particles and Cu particles.
5. The precursor composition of claim 3 wherein each particle
within the at least one set of particles comprises Cu--Ga solid
solution and the metallic particles are In particles.
6. The precursor composition of claim 1 further comprising Group
VIA particles of at least one of Se, Te, S, Se--S, Se--Te, and
Te--S.
7. The precursor composition of claim 3 further comprising Group
VIA particles of at least one of Se, Te, S, Se--S, Se--Te, and
Te--S.
8. The precursor composition of claim 1 further comprising a p-type
dopant.
9. The precursor composition of claim 3 further comprising a p-type
dopant.
10. The precursor composition of claim 1 further comprising a
solvent within which the at least one set of particles are
dispersed.
11. The precursor composition of claim 6 further comprising a
solvent within which the at least one set of particles and Group
VIA particles are dispersed.
12. The precursor composition of claim 3 further comprising a
solvent within which the at least one set of particles and the at
least one type of metallic particles are dispersed.
13. The precursor composition of claims 10, 11 or 12 wherein the
solvent is water.
14. The precursor composition of claim 1 wherein the Cu/(In+Ga)
molar ratio is in the range of 0.7-1.0.
15. The precursor composition of claim 1 wherein the Ga/(Ga+In)
molar ratio is in the range of 0.05-0.3.
16. The precursor composition of claim 3 wherein the Ga/(Ga+In)
molar ratio is in the range of 0.05-0.3.
17. The precursor composition of claim 3 wherein the Cu/(In+Ga)
molar ratio is in the range of 0.7-1.0.
18. A method of forming a photovoltaic device, the method
comprising: providing a precursor composition; and depositing a
layer of the precursor composition on a substrate, wherein the
precursor composition comprises of at least one set of particles,
and wherein each particle within the at least one set comprises a
solid-solution selected from the group of Cu--Ga solid solution,
Cu--In solid solution, In--Ga solid solution and Cu--In--Ga solid
solution.
19. The method of claim 18 wherein depositing the layer of
precursor composition comprises using a dry and/or wet process.
20. The method of claim 18 wherein depositing the layer of
precursor composition comprises using electrostatic powder
deposition wherein prepared powder particles are coated with poorly
conducting or insulating materials that can hold charge.
21. The method of claim 20 wherein charged powders are deposited on
substrates that are biased in a way to attract the charged
particles.
22. The method of claim 21 wherein the material coating the powder
particles are organic materials characterized by thermal properties
such that upon heating, the organic materials substantially burn or
evaporate away without leaving any residues that would
deleteriously affect the compound film to be grown.
23. The method of claim 18 further comprising using a carrier
liquid with the precursor composition
24. The method of claim 18 wherein the carrier liquid is either a
water-based or an organic solvent.
25. The method of claim 18 further comprising adding an additive to
the carrier liquid and the precursor material to form a thick
paste.
26. The method of claim 18 further comprising adding an additive to
the carrier liquid and the precursor material to form a thin
dispersion.
27. The method of claim 18 further comprising adding an additive of
either an ionic and/or non-ionic dispersants.
28. The method of claim 18 further comprising adding at least one
additive selected from the following: thickening agents,
pH-adjustment agents, surfactants, and/or binders.
29. The method of claim 18 further comprising adding at least one
additive wherein the additive is a volatile material that once
evaporated out of the layer of precursor composition, leaves
substantially no residue behind that would have deleterious effect
on the compound film.
30. The method of claim 18 wherein depositing the layer of
precursor composition comprises using at least one of the following
techniques: screen printing, ink jet printing, ink deposition by
doctor-blading, and/or reverse roll coating.
Description
[0001] This application is a division of and claims the benefit of
priority to U.S. patent application Ser. No. 10/474,259 filed under
35 USC 371 on May 19, 2004 from PCT International Application
Serial No. US02/11047 filed Apr. 11, 2002, which claims the benefit
of priority to U.S. Provisional Application Ser. No. 60/283,630
filed Apr. 16, 2001, the entire disclosures of which are fully
incorporated herein by reference for all purposes.
FIELD OF THE INVENTION
[0002] The present invention relates to methods of preparing
polycrystalline thin films of semiconductors for active or passive
electronic component and device applications especially for the
fabrication of radiation detectors and solar cells.
BACKGROUND
[0003] Solar cells convert sunlight directly into electricity.
These electronic devices are commonly fabricated on silicon (Si)
wafers. However, the cost of electricity generated using
silicon-based solar cells is rather high, compared to electricity
generated by the traditional methods, such as fossil-fuel-burning
power plants. To make solar cells more economically viable,
low-cost, thin-film growth techniques that can deposit high-quality
light-absorbing semiconductor materials need to be developed. These
techniques need to employ cost-effective approaches to grow solar
cell absorbers on large-area substrates with high throughput and
high materials utilization. Therefore, low-cost and high-efficiency
thin film solar cell fabrication requires: i) a solar cell absorber
material that fundamentally has the capability to yield
high-efficiency devices, ii) a low-cost deposition technique that
can deposit this absorber material in the form of a high-quality
thin film on a low-cost substrate. Both of these ingredients are
necessary for manufacturing high-efficiency, low-cost solar cell
structures. When solar cells formed on large-area substrates are
interconnected, modules with higher voltage and power output are
obtained.
[0004] Copper-indium-sulfo-selenide, Cu(In,Ga)(S,Se).sub.2,
compounds are excellent absorber materials for thin-film solar cell
structures provided that their structural and electronic properties
are good. An important compositional parameter of
Cu(In,Ga)(S,Se).sub.2 thin films is the metals molar ratio of
Cu/(In+Ga). The typically acceptable range of this molar ratio for
high-efficiency solar cell absorbers is about 0.70-1.0, although in
some cases when the compound is doped with a dopant such as sodium
(Na), potassium (K) or lithium (Li), this ratio can go even lower.
If the Cu/(In+Ga) molar ratio exceeds 1.0, however, a
low-resistivity copper selenide, sulfide or sulfo-selenide phase
precipitates and deteriorates the performance of the device due to
electrical shorting paths it creates through the absorber.
Therefore, control of the Cu/(In+Ga) ratio is important for any
technique that is used for the preparation of Cu(In,Ga)(S,Se).sub.2
films for radiation detector or solar cell applications. The
Ga/(In+Ga) ratio is also important to control since this ratio
determines the bandgap of the absorber, which can be varied from
about 1 eV (for CuInSe.sub.2) to 2.43 eV (for CuGaS.sub.2). In
principal the Ga/(In+Ga) ratio may vary from zero in
CuIn(S,Se).sub.2 to 1.0 in CuGa(S,Se).sub.2. However, laboratory
experience to date has shown that best device efficiencies are
obtained for Ga/(In+Ga) ratios in the range of 0.1-0.3.
[0005] Although important to control, the Cu/(In+Ga) and Ga/(In+Ga)
ratios are not the only factors that influence the electronic
properties of Cu(In,Ga)(S,Se).sub.2 compound thin films. The
compositional ratios of a compound film may be within the
acceptable ranges, but solar cells fabricated on this film may
still have poor light-to-electricity conversion efficiencies.
Cu(In,Ga)(S,Se).sub.2 compound thin films used in high-efficiency
solar cell structures, besides having the right composition, also
need to have good morphology and large-grain structure. For
example, a typical high-quality Cu(In,Ga)(S,Se).sub.2 thin film is
1.0-3.0 .mu.m thick; it is dense and it has columnar grains with
widths of at least 0.5 .mu.m.
[0006] One approach that yielded high-quality Cu(In,Ga)Se.sub.2
films for solar cell applications is co-evaporation of Cu, In, Ga
and Se onto heated substrates in a vacuum chamber [see for example,
Bloss et al., "Thin Film Solar Cells", Progress in Photovoltaics,
1995, vol. 3, page 3]. Absorbers grown by this technique are
typically dense and they have large columnar grains. The Cu/(In+Ga)
ratio and the Ga/(In+Ga) ratio are closely controlled during
deposition by monitoring and controlling the individual evaporation
rates of Cu, In and Ga. Consequently, Cu(In,Ga)Se.sub.2 solar cells
fabricated on co-evaporated absorbers yielded small,
laboratory-size solar cells with close to 19% conversion
efficiency. Although there are now concentrated efforts to apply
this technique to the fabrication of large-area Cu(In,Ga)Se.sub.2
modules, the method is not readily adaptable to low-cost production
of large-area films, mainly because control of Cu/(In+Ga) and
Ga/(In+Ga) ratios by evaporation over large-area substrates is
difficult, materials utilization is low and the cost of vacuum
equipment is high.
[0007] Another technique for growing Cu(In,Ga)(S,Se).sub.2 type
compound thin films for solar cell applications is the two-stage
processes where at least two components of the compound are first
deposited onto a substrate, and then reacted with each other and/or
with a reactive atmosphere in a high-temperature annealing process.
U.S. Pat. No. 4,798,660 issued to J. Ermer et al. teaches a method
for fabricating a CuInSe.sub.2 film comprising sequentially
depositing a film of Cu on a substrate by DC magnetron sputtering
and depositing In on said film of Cu and heating the composite film
in the presence of Se to form the compound. This approach is
schematically depicted in FIG. 1, as it would be applied to the
growth of a Cu(In,Ga)Se.sub.2 thin film. In FIG. 1, a Cu sub-layer
12 is first deposited on a substrate 10 which has a metallic film
11, such as molybdenum (Mo) on its surface. Then a (In+Ga)
sub-layer 13 is deposited over the Cu sub-layer 12 to form the
composite layer 14. Later in the process, the complete structure of
FIG. 1 is heated in the presence of Se vapors to convert the
composite layer 14 into a Cu(In,Ga)Se.sub.2 compound layer.
[0008] Karg et al. in U.S. Pat. No. 5,578,503 teach an alternate
two-stage technique where a Cu film, an In or Ga film and a S or Se
film are deposited on a substrate to form a stacked layer and then
this stacked layer is heated rapidly to form the compound. The
stacked structure of this prior art approach is depicted in FIG. 2,
as it would be applied to the growth of a Cu(In,Ga)Se.sub.2 solar
cell absorber. In FIG. 2, a Cu sub-layer 22 is first deposited over
the metal film 21 which is previously coated on substrate 20. This
is followed by the depositions of a (In+Ga) sub-layer 23 and a Se
sub-layer 24. All the sub-layers form the stacked layer 25 which is
then converted into a Cu(In,Ga)Se.sub.2 compound layer when the
whole structure of FIG. 2 is rapidly heated in a rapid thermal
processing (RTP) furnace.
[0009] Yet another two-stage processing approach is taught in
European Patent application No EP0838864A2 by K. Kushiya et al. In
that method a stacked precursor comprising a Cu--Ga sub-layer and
an In sub-layer was employed. This method is schematically shown in
FIG. 3 as it would be applied to the growth of a Cu(In,Ga)Se.sub.2
absorber. According to FIG. 3, a Cu--Ga sub-layer 32 is first
deposited over the metal contact layer 31 which was previously
deposited on the surface of substrate 30. An In sub-layer 33 is
then deposited over the Cu--Ga sub-layer 32. The structure of FIG.
3 is then annealed in the presence of Se vapors to convert the
multi-layer 34 into a Cu(In,Ga)Se.sub.2 layer. Further annealing in
H.sub.2S yields a Cu(In,Ga)(S,Se).sub.2 film.
[0010] All of the prior art two-stage techniques reviewed above
yielded high-quality compound films in terms of their structural
and electronic properties. Large-scale manufacturing, however, also
requires strict control of the material composition over large-area
substrates. This means that in the two-stage processes that utilize
a stack of various sub-layers, the uniformity and thickness of each
sub-layer have to be individually controlled over large-area
substrates. This is very difficult. DC magnetron sputtering
techniques which are commonly used to deposit the sub-layers of
FIGS. 1, 2 and 3 are expensive vacuum techniques with low materials
utilization. Therefore, their cost is high.
[0011] Since compositional control, especially the control of the
Cu/(In+Ga) ratio is important for Cu(In,Ga)(S,Se).sub.2 compounds,
attempts have been made to fix this ratio in an initial material,
before the deposition process, and then transfer this fixed
composition into a thin film formed using this initial material. T.
Arita et al. in their 1988 publication [20th EEE PV Specialists
Conference, 1988, page 1650] described a screen printing technique
that involved mixing and milling pure Cu, In and Se powders in the
compositional ratio of 1:1:2 and forming a screen printable paste,
screen printing the paste on a substrate, and sintering this film
to form the compound layer. They reported that although they had
started with elemental Cu, In and Se powders, after the milling
step the paste contained the CuInSe.sub.2 phase. Solar cells
fabricated on the sintered layers had very low efficiencies because
the structural and electronic quality of these absorbers were
poor.
[0012] Thin layers of CuInSe.sub.2 deposited by a screen printing
method were also reported by A. Vervaet et al. [9th European
Communities PV Solar Energy Conference, 1989, page 480]. In that
work a CulnSe.sub.2 powder was used along with Se powder to prepare
a screen printable paste. Layers formed by screen printing were
sintered at high temperature. A difficulty in this approach was
finding a suitable fluxing agent for dense CulnSe.sub.2 film
formation. Therefore, solar cells fabricated on the resulting
layers had poor conversion efficiencies.
[0013] U.S. Pat. No. 5,985,691 issued to B. M. Basol et al
describes another particle-based method to form a Group IB-IIIA-VIA
compound film, where IB=Cu, Ag, Au, IIIA=In, Ga, Al, Tl, and VIA=S,
Se, Te. The described method includes the steps of preparing a
source material, depositing the source material on a base to form a
precursor, and heating the precursor to form a film. In that
invention the source material includes Group IB-IIIA
alloy-containing particles having at least one Group IB-IIIA alloy
phase, with Group IB-IIIA alloys constituting greater than about 50
molar percent of the Group IB elements and greater than about 50
molar percent of the Group IIIA elements in the source material.
The powder is milled to reduce its particle size and then used in
the preparation of an ink which is deposited on the substrate in
the form of a precursor layer. The precursor layer is then exposed
to an atmosphere containing Group VIA vapors at elevated
temperatures to convert the film into the compound. The precursor
films deposited using this technique were porous and they yielded
porous CuInSe.sub.2 layers with small-grain regions as reported by
G. Norsworthy et al. [Solar Energy Materials and Solar Cells, 2000,
vol. 60, page 127]. Porous solar cell absorbers yield unstable
devices because of the large internal surface area within the
device. Also small grains limit the conversion efficiency of solar
cells.
[0014] PCT application No. WO 99/17889 (Apr. 15, 1999) by C.
Eberspacher et al. describes methods for forming solar cell
materials from particulates where various approaches of making the
particulates of various chemical compositions and depositing them
on substrates are discussed.
[0015] As the above brief review of prior art demonstrates, there
have been attempts to use Cu(In,Ga)Se.sub.2 compound powders, oxide
containing particles, and Cu--(In,Ga) alloy powders with
(In,Ga)-rich compositions, to form precursor layers which were then
treated at high temperatures to form Cu(In,Ga)Se.sub.2 compound
films. These techniques were successful in demonstrating
compositional control. In other words the overall composition of
the powder was directly transferred into the precursor layer and
then into the compound layer. As discussed previously however,
composition is only one of the important parameters of high-quality
solar cell absorbers. The other important parameters are the
morphology and the grain size, which directly influence the
electronic properties of these films. Solar cell absorbers need to
be dense layers with large grain size. This requires precursor
layers that are dense and compositionally uniform both in
micro-scale and macro-scale. Repeatability and the overall yield of
the process further requires the quality of the source material or
the initial powder material to be repeatable. This means that the
chemical composition and the phase content of the individual
particles comprising the powder need to be well controlled and
repeatable.
[0016] To this end, there is a need for a low-cost method that has
the capability to form Cu(In,Ga)(S,Se).sub.2 thin films over
large-area substrates with controlled compositional uniformity,
good structural and electronic properties in a repeatable manner
with high yield.
SUMMARY OF THE INVENTION
[0017] In accordance with the invention, a method of forming a
Cu(In,Ga)(S,Se).sub.2 semiconductor compound film includes the
steps of formulating a nano-powder material with controlled phase
content and overall composition, depositing the nano-powder on a
substrate to form a micro-layer and heating and reacting the
micro-layer in suitable atmosphere(s) to form a
Cu(In,Ga)(S,Se).sub.2 semiconductor compound film. The nano-powder
material may contain nano-size particles of only solid solutions or
elemental forms of Cu, In, and Ga. Additionally particles of at
least one of Se, S and Te may be included in the nano-powder. The
nano-powder material may also contain a dopant to enhance
p-typeness of the semiconductor compound film. The
Cu(In,Ga)(S,Se).sub.2 semiconductor compound film of the present
invention may be used in electronic device structures including
solar cells.
[0018] More generally, according to the invention, a process of
forming a compound film includes formulating a nano-powder material
with a controlled overall composition and having particles of one
solid solution. The nano-powder material is deposited on a
substrate to form a layer on the substrate, and this layer is
reacted in at least one suitable atmosphere to form the compound
film. The invention also concerns a semiconductor compound film for
use in fabrication of a radiation detector or solar cell made by
such a process, as well as a powder material, for use in preparing
such a compound film on a substrate, comprising particles of a
copper-gallium solid solution which are sub-micronic in size.
According to one preferred embodiment, the compound film has a
Cu/(In+Ga) compositional range of 0.7-1.0 and a Ga/(In+Ga)
compositional range of 0.05-0.3.
[0019] The foregoing and additional features and advantages of this
invention will become further apparent from the detailed
description and accompanying drawings that follow. In the figures
and written description, numerals indicate the various features of
the invention, like numerals referring to like features.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic representation of a composite layer
employed in a prior art two-stage technique for the growth of a
Cu(In,Ga)Se.sub.2 compound film.
[0021] FIG. 2 is a schematic representation of a stacked layer
employed in another prior art two-stage technique for the growth of
a Cu(In,Ga)Se.sub.2 compound film.
[0022] FIG. 3 is a schematic representation of a multi-layer
structure employed in yet another prior art two-stage technique for
the growth of a Cu(In,Ga)Se.sub.2 compound film.
[0023] FIG. 4 is the general structure of a Cu(In,Ga)(S,Se).sub.2
compound solar cell that may be constructed using the compound
absorbers grown by the method of the present invention.
[0024] FIG. 5 is a flow chart showing the steps of a method used to
grow Cu(In,Ga)(S,Se).sub.2 compound thin films, in accordance with
the present invention.
[0025] FIG. 6 is a schematic drawing showing the composition of the
nano-powder.
[0026] FIG. 7 shows the micro-layer obtained as a result of
depositing the precursor containing the nano-powder.
[0027] FIG. 8a is a schematic drawing of copper-gallium phase
diagram.
[0028] FIG. 8b is a schematic drawing of copper-indium phase
diagram.
[0029] FIG. 8c is a schematic drawing of gallium-indium phase
diagram.
DETAILED DESCRIPTION
[0030] The general structure of a Cu(In,Ga)(S,Se).sub.2 compound
solar cell that may be constructed using the compound absorbers
grown by the method of present invention is shown in FIG. 4. The
device is fabricated on a substrate that includes a base 40 and a
conductive contact layer 41. The base 40 may be made of various
conductive or insulating, rigid or flexible materials, such as
glass or flexible foils made of Mo, Ti, stainless steel, polyimide,
mica and like. The p-type compound absorber film 42 is deposited
over the conductive contact layer 41 which is traditionally made of
Mo but may also be made of Ta, W, their alloys or nitrides. It is
important that the conductive contact layer 41 is stable and does
not appreciably react with other elements within the solar cell
structure or the reaction environments used to form the solar cell
structure. An n-type transparent window layer 43 is formed on the
p-type compound absorber film 42, through which light enters the
device. Metallic finger patterns 44 may be deposited over the
window layer 43, if necessary.
[0031] Although the present invention is described for the growth
of Cu(In,Ga)(S,Se).sub.2 as the p-type compound absorber film 42,
tellurium may also be included into the composition to grow
Cu(In,Ga)(S, Se, Te).sub.2 compounds. The compound layer may
additionally contain dopants such as potassium (K), sodium (Na),
lithium (Li), phosphorous (P), arsenic (As), antimony (Sb) and
bismuth (Bi) to enhance its electronic properties.
[0032] The transparent window layer 43 may have one or more layers
made of materials commonly used in solar cell window layer
structures. For example the transparent window layer may contain
layers of cadmium-sulfide, cadmium-zinc-sulfide, zinc-selenide,
zinc-oxide, indium-tin-oxide, indium-zinc-oxide and tin oxide,
among many others.
[0033] The preferred cell structure of FIG. 4 is commonly referred
to as a "substrate-type" structure. A "superstrate-type" structure
can also be constructed by depositing a window layer first on a
transparent base such as glass, then depositing the
Cu(In,Ga)(S,Se).sub.2 compound absorber film, and finally forming a
back ohmic contact to the device by a conductive layer. In this
"superstrate-type" structure, which is commonly known in the field,
light enters the device again from the window layer.
[0034] FIG. 5 shows the general steps of the compound film growth
process of the present invention. In this figure process steps are
identified in rectangular boxes and the results are identified in
circles. Accordingly, the first step in the film growth process of
the present invention is precursor formulation 50. The second
process step is precursor deposition 51, which involves depositing
the formulated precursor onto the surface of a substrate to form a
micro-layer 52. The micro-layer 52 is then converted into the
Cu(In,Ga)(S,Se).sub.2 compound layer 54 through the third process
step which is identified as reaction 53 in FIG. 5.
[0035] Precursor used in precursor formulation 50 step comprises a
nano-powder, i.e. a powdered material with nano-meter size
particles. Compositions of the particles constituting the
nano-powder used in precursor formulation 50 step are important for
the repeatability of the process and the quality of the compound
films grown by the method of the present invention. As
schematically shown in FIG. 6 the nano-powder 60 of the present
invention is a collection of small particles. These particles are
preferably near-spherical in shape and their diameters are less
than about 200 nm, and preferably less than about 100 nm.
Alternately, nano-powder 60 may contain particles in the form of
small platelets. The nano-powder 60 preferably contains
copper-gallium solid solution particles 61, and at least one of
indium particles 62, indium-gallium solid-solution particles 63,
copper-indium solid solution particles 64, and copper particles 65.
Alternately, the nano-powder may contain copper particles and
indium-gallium solid-solution particles. The nano-powder 60 may
additionally contain Group VIA particles 66. Group VIA particles 66
may be made of at least one of Se, S and Te or their alloys or
solid solutions.
[0036] Precursor formulation 50 step of FIG. 5 involves processes
or operations that transform the nano-powder 60 into a form that is
suitable for deposition onto a substrate. Therefore, precursor
formulation 50 step may, for example, involve mixing the
nano-powder with well known solvents, carriers, dispersants etc. to
prepare an ink or a paste that is suitable for deposition onto a
substrate. Alternately, nano-powder particles may be prepared for
deposition on a substrate through dry processes such as dry powder
spraying, electrostatic spraying or processes which are used in
copying machines and which involve rendering charge onto particles
which are then deposited onto substrates.
[0037] After precursor formulation 50 step, the precursor, and thus
the nano-powder constituents have to be deposited onto a substrate
in the form of a micro-layer through the precursor deposition step
51. As indicated before, precursor deposition step may involve dry
or wet processes. Dry processes include electrostatic powder
deposition approaches where the prepared powder particles may be
coated with poorly conducting or insulating materials that can hold
charge. Charged powders may then be deposited on substrates that
are biased in a way to attract the charged particles. The materials
coating the powder particles may be selected from amongst organic
materials so that upon heating they substantially burn or evaporate
away without leaving any residues that would deleteriously affect
the compound films to be grown. Some examples of wet processes are
screen printing, ink jet printing, ink deposition by
doctor-blading, reverse roll coating etc. In these approaches the
nano-powder may be mixed with a carrier which may typically be a
water-based or organic solvent. Well-known additives may then be
added into the formulation to form either a thick paste (for screen
printing) or a thin dispersion (for ink or slurry deposition).
Examples of these additives include a large variety of ionic and
non-ionic dispersants marketed by companies such as Rohm and Haas,
thickening agents, pH-adjustment agents, surfactants, binders etc.
It is preferred that the carrier be a volatile material that once
evaporated out of the wet micro-layer, leaves substantially no
residue behind that would have deleterious effect on the compound
film. The same is also valid for all the other chemical agents used
in the precursor formulation. Some examples of the commonly used
carriers are water, alcohols, ethylene glycol, etc.
[0038] Once the carrier and other agents in the precursor
formulation are totally or substantially evaporated away, the
micro-layer 52 is formed on the substrate 70 as shown in FIG. 7.
The micro-layer 52 is made of the nano-size particles of the
nano-powder distributed in a dense matrix. It should be noted that
the most preferred shape of nano-powder particles is spherical or
near spherical. For a uniform size distribution, spherical
particles can give about 75% volume density when closely packed
into a film. If the particle sizes are varied, i.e. if there is a
size distribution of particles, this packing density can be made
even higher by filing the gaps between larger particles with
smaller particles. Another preferable shape of the particles is
flat plates. With this shape it is possible to get films with over
90% density. The substrate 70 in FIG. 7 is a general
representation. As explained before the substrate 70 preferably
comprises a conductive layer 71 on its surface.
[0039] The nano-size particles of the present invention contain
soft, elemental and solid-solution materials such as In and In--Ga
solid solutions. Therefore, the preferred method of preparing an
ink or a paste include process steps of mixing and dispersing but
not milling or grinding. In other words the preferred size of the
particles in the present invention is less than about 200 nm. This
way no mechanical grinding or milling should be necessary for
further size reduction although such processes may also be
employed. If milling or mechanical grinding is not used in
precursor formulation step 50, mixing, blending, sonicating etc.
steps may be carried out to form a uniform mixture or dispersion
ready to be deposited on the substrate in the form of a
micro-layer. The substrate temperature during the precursor
deposition 51 step may be in the 20-400 C range. Preferably it
should be in the 20-150 C range so that the deposition process can
be carried out in air rather than in an oxygen-free atmosphere.
[0040] Once the micro-layer 52 is formed on the selected substrate
it has to be reacted to form the compound film. The reaction step
involves at least one annealing process. Annealing may involve
furnace-annealing, RTP or laser-annealing, microwave annealing,
among others. Annealing temperatures may be in 350-600 C and
preferably between 400-550 C. If the micro-layer 52 contains Group
VIA particles 66, the annealing atmosphere may be inert. In this
case the Group VIA particles react with the metallic particles to
form the compound. If there is no Group VIA particles in the
nano-powder and thus in the micro-layer, or if the amount of Group
VIA particles is not adequate then the reaction step should employ
an atmosphere with the vapors of at least one of Group VIA elements
(Se,S,Te). It should be understood that the reaction step may
consists of two or more annealing processes each with different
temperature time profiles and different atmospheres. For example,
in the case of metallic micro-films there may be three annealing
processes. In the first process which may be carried out at
temperatures of 100-300 C, the micro-layer is annealed in inert
atmosphere or vacuum to dry the film and to get any impurities out
or to fuse and alloy the film and reduce the porosity. The second
annealing process may employ a Se-containing atmosphere to grow a
Cu(In,Ga)Se.sub.2 film. There may be a third annealing process
where the Cu(In,Ga)Se.sub.2 film is heated in a S-containing
atmosphere to convert it into a Cu(In,Ga)(S,Se).sub.2 layer.
Alternately, the sequence of the second and third heating processes
may be changed to form first a S-containing compound and then the
Cu(In,Ga)(S,Se).sub.2 compound The first heating process may
altogether be skipped.
[0041] Although a Cu(In,Ga)(S,Se).sub.2 compound is used as an
example, other compounds in the same family (such as compounds
containing Al, Tl, Ag and/or Te) may also be formed using the
present invention. Similarly compounds from other groups such as
Group IIB(Cd,Zn)-IVA(Si,Ge,Sn)-VA(P,As,Sb) and Group IB-VA-VIA
compounds may also be grown using the present invention provided
that solid solutions exist in their respective binary diagrams.
[0042] Referring back to FIG. 6, the copper-gallium solid solution
particles 61 have the general chemical composition of
Cu.sub.1-xGa.sub.x, where x.ltoreq.0.22, preferably x.ltoreq.0.18.
Indium-gallium solid solution particles 63 have the general
chemical composition of In.sub.1-yGa.sub.y, where y.ltoreq.0.04,
preferably y.ltoreq.0.03. Copper-indium solid solution particles 64
have the general chemical composition of Cu.sub.1-zIn.sub.z, where
z.ltoreq.0.11, preferably z.ltoreq.0.02. These compositional limits
assigned to the particles of the present invention assure that all
the metallic particles in the formulation of the nano-powder 60 are
either pure metals (such as Cu and In) or solid solutions which can
be repeatably manufactured. This point will now be clarified
further by referring to the phase diagrams replicated in FIGS. 8a,
8b and 8c.
[0043] FIG. 8a schematically shows a copper-gallium binary phase
diagram [P. R. Subramanian and D. E. Laughlin, in Binary Alloy
Phase Diagrams, page 1410]. As can be seen from this figure the
maximum solid solubility of Ga in Cu lies at about 22 atomic % Ga
at the peritectoid temperature of 620 C, and 18 atomic % Ga near
room temperature. There is, on the other hand, very little or no
solubility of Cu in Ga. This means that the dotted region 80 in
FIG. 8a contains the single-phase copper-gallium solid solution
with properties that are close to those of pure copper. The portion
of the phase diagram of FIG. 8a which lies outside the dotted
region 80 is very complex with multiple Cu-rich and Ga-rich alloy
phases such as Cu.sub.2Ga (33% Ga), Cu.sub.9Ga.sub.4 (29% Ga),
Cu.sub.3Ga (25% Ga), Cu.sub.3Ga.sub.2 (40% Ga) and CuGa.sub.2 (67%
Ga). Consequently any Cu--Ga alloy with Ga-content of about higher
than 22 atomic percent (such as those alloys utilized in U.S. Pat.
No. 5,985,691) would most likely contain multiple Cu--Ga alloy
phases unless the selected composition exactly matches to the
composition of one of the stable phases. Even then, synthesis of
the single phase material which lies in the middle of a phase
diagram with multiple phases is very challenging and often requires
very high accuracy, long heating cycles etc. Fabrication of
particles/powders containing only simple solid solutions, on the
other hand, is simple and repeatable. Elemental powders of Cu and
In can be prepared in nano-sizes without difficulty. Particles of
copper-gallium, copper-indium, indium-gallium solid-solutions can
also be prepared using the same techniques used for the preparation
of elemental powders. This means that such solid solution powders
can be fabricated in a repeatable manner with high yield and small
particle size. Use of such powders then assure the quality and
repeatability of the compound forming process of the present
invention. One of the techniques successfully used for making Cu
nano particles, for example, involve sputtering copper out of a Cu
target into inert gas at high pressure. The sputtered Cu
nano-clusters quench in the inert gas and get collected. This
technique can easily be adapted to prepare solid solution
particles, such as Cu--Ga solid-solution particles with Ga content
less than 22 atomic percent, because sputtering targets of solid
solutions can be readily fabricated just like elemental metals.
When sputtered into inert gas, solid-solution nano-particles with
compositions similar to that of the target can be obtained. If, on
the other hand, a target of complex alloy phases such as a Cu--Ga
alloy target with Ga content over 22% is attempted to be used for
the same purpose, different portions of the target would contain
different phases with varying sputtering yields. The resulting
particles would not be uniform in composition. The method of melt
spraying which was used in U.S. Pat. No. 5,985,691, on the other
hand would give relatively large particles (typically larger than 1
micron) with different phases each of which have different melting
points and Cu/Ga metallic ratios.
[0044] The phase diagram for copper-indium is shown schematically
in FIG. 8b [P. R. Subramanian and D. E. Laughlin, "The Cu--In
system", Bulletin of Alloy Phase Diagrams, 1989, vol. 10, No. 5,
page 554]. Various phases identified in this diagram include
Cu-rich and In-rich alloy compositions such as Cu.sub.11In.sub.9
(45% In), CuIn.sub.2 (66% In), Cu.sub.16In.sub.9 (36% In),
Cu.sub.9In.sub.4(31% In), Cu.sub.7In.sub.3 (30% In), and
Cu.sub.7In.sub.4 (36% In). There is negligible solid solubility of
Cu in In, whereas the maximum solubility of In in Cu is about 11
atomic % and it is at about 574 C. Near room temperature this
solubility is reduced to about 2%, as can be seen from FIG. 8b.
Therefore, solid solution with up to 11% indium content can be
easily synthesized at 574 C and then the material can be quenched
down to room temperature to preserve this composition. If the In
content of the solution is limited to only 2%, on the other hand,
material synthesis is even easier and there is no need for special
processing or even quenching, and the resultant material is assured
to be a single-phase solid solution. Accordingly, the shaded region
85 in FIG. 8b corresponds to the single-phase copper-indium solid
solution which has properties very close to that of Cu. Powders of
the solid-solution compositions can be prepared in nano sizes in a
repeatable manner with controlled phase (thus composition)
content.
[0045] The schematic gallium-indium phase diagram of FIG. 8c [T. J.
Anderson and I. Ansara, J. Phase Equilibria, vol. 12, No. 1, 1991,
page 64] suggests that there is hardly any solubility of In in Ga.
However Ga solid solubility in In at about 70 C is about 4% and
then drops to about 3% at room temperature. This single-phase solid
solution region is shown in FIG. 8c as the shaded region 88. It
should be pointed out that there are some older reports putting the
solid solubility limit of Ga in In to much higher value of 18% [M.
Hansen, "Constitution of Binary Alloys", second edition, 1958, page
745], which would increase the shaded region 78 of FIG. 8c
accordingly.
[0046] The phase diagrams of FIGS. 8a, 8b and 8c, and the above
discussion demonstrate the complexity of the Cu--Ga, Cu--In, Ga--In
alloy systems for all compositions except where solid-solutions
exist. It is this complexity that causes uncontrolled variations in
processes utilizing Cu--In or Cu--Ga alloy films or alloy
particles. The following examples can demonstrate this.
EXAMPLE 1
[0047] Based on experimental results to date, the best
compositional range for Cu(In,Ga)(S,Se).sub.2 absorbers are
Cu/(In+Ga)=0.7-1.0 and Ga/(In+Ga)=0.05-0.3. Consider the case of
growing an absorber with Cu/(In+Ga)=1.0 and Ga/(In+Ga)=0.25, i.e. a
compound film of CuIn.sub.0.75Ga.sub.0.25(S,Se).sub.2. If the
metallic precursor film of this composition were to be prepared by
the prior art technique of U.S. Pat. No. 5,985,691, Cu--Ga, Cu--In,
or Cu--In--Ga alloy particles would be utilized where the alloy
phase(s) would contain over 50 atomic % of the required Cu and over
50 atomic % of the required Ga and In. These alloy particles would
have compositions that are outside the solid-solution regions
indicated in FIGS. 7a and 7b. In other words, the alloy particles
would contain multi-phases which during processing could change. It
should be noted that the situation worsens for Cu/(In+Ga) ratios of
smaller than 1.0. In fact this has been observed in experiments
with such alloy particles. In a recent publication [B. M. Basol,
"Low cost techniques for the preparation of Cu(In,Ga)(S,Se).sub.2
absorber layers", Thin Solid Films, 2000, vol. 361, page. 514] the
multi-phase nature of alloy particles containing more than 50
atomic % of the required In was shown. It was also discovered that
the phases present changed upon ball milling the powder for the
purpose of particle size reduction for ink formulation.
Furthermore, milling resulted in irregular particle shapes which
yielded porous films.
EXAMPLE 2
[0048] Using the present invention the compound layer with a
composition close to CuIn.sub.0.75Ga.sub.0.25(S,Se).sub.2 may be
grown in a repeatable manner and with high quality. In this case a
copper-gallium solid solution powder is used where the composition
is Cu.sub.0.78 Ga.sub.0.22. This powder is mixed with a powder of
indium-gallium solid solution of In.sub.0.97Ga.sub.0.03
composition. The amounts of each powder are selected so that one
mole of the first and one mole of the second are intimately mixed.
After forming the micro-film the overall composition of the
micro-film yields
Cu.sub.0.78Ga.sub.0.22+In.sub.0.97Ga.sub.0.03=Cu.sub.0.78In.sub.0.97Ga.su-
b.0.25. Therefore in the compound layer, Cu/(In+Ga)=0.64 and
Ga/(In+Ga)=0.20, which are desirable compositional parameters for
high quality compound layers when doped.
EXAMPLE 2a
[0049] In the above example if 1.28 mole of Cu.sub.0.78Ga.sub.0.22
solid solution powder is mixed with 0.77 mole of
In.sub.0.97Ga.sub.0.03 solid solution powder and a micro film of
these powders is obtained. After the reaction step the compound
would have the composition of 1.28 Cu.sub.0.78Ga.sub.0.22+0.77
In.sub.0.97Ga.sub.0.03=CuIn.sub.0.75Ga.sub.0.3, which means,
Cu/(In+Ga)=0.95 and Ga/(Ga+In)=0.29. These are close to perfect
compositional parameters required for undoped compound films
yielding high efficiency solar cells. This example demonstrates the
flexibility and power of the present invention in changing film
composition using just solid solutions. It should be appreciated
that amounts of the two solid solutions could be changed to obtain
various compound composition. Elemental powders of Cu, In or Ga
could also be added in the formulation to further adjust these
compositions as desired.
EXAMPLE 3
[0050] If the desired Ga content in the micro-film and thus in the
compound film is less than the solid solubility of Ga in Cu, then
there is no need to use the indium-gallium solid solution. Pure In
powder can be used in this case. For example, to grow a
CuIn.sub.1-kGa.sub.k(S,Se).sub.2 compound with k<0.22, the
nano-powder may contain Cu.sub.1-kGa.sub.k solid-solution particles
and pure In particles where one mole of the copper-gallium
solid-solution powder is mixed with (1-k) mole of pure In powder.
Using low melting point pure In or In--Ga solid solution particles
in formulations containing Cu--Ga solid solution particles is
attractive because upon heating the micro-layer, these low melting
phases form a liquid fusing region around the high-melting Cu--Ga
solid solution particles.
EXAMPLE 4
[0051] Cu--In and Cu--Ga solid solutions may both be used in the
formulation of the nano-powder. For example, a
Cu.sub.1-xGa.sub.x+Cu.sub.1-yIn.sub.y+m In mixture would allow
growth of a wide range of compositions of the compound. Since
x.ltoreq.0.22 and y.ltoreq.0.11, the value of m can be adjusted to
yield a range of Ga/(In+Ga) ratios. Of course the Ga amount can be
easily reduced in this composition. However, for Cu/(In+Ga)=1.0,
maximum amount of Ga that can be included in the film using this
formulation can be calculated as follows:
Cu.sub.0.78Ga.sub.0.22+Cu.sub.0.89In.sub.0.11+m In is the
formulation that would yield 0.78+0.89=0.22+0.11+m m=1.34
Ga/(In+Ga)=0.22/(0.22+0.11+1.34)=13%
[0052] Therefore, similar calculations may be used to formulate
nano-powders containing elemental powders and solid-solution
powders and yielding compound films with various compositions after
the reaction step.
EXAMPLE 5
[0053] Copper-gallium solid solution particles, Cu particles and In
particles may also be used to obtain micro-layers and compound
layers of various compositions. In this case
[0054] wCu.sub.1-xGa.sub.x+u Cu+n In would be the formulation of
the nano-powder and the composition of the micro-layer and the
compound would have Cu/(In+Ga) ratio=[u+w(1-x)]/(n+wx), and
Ga/(In+Ga) ratio=wx/(n+wx), where x.ltoreq.0.22
[0055] Above examples demonstrate the flexibility of the present
invention to yield high quality absorber layers with compositional
ranges that are most important for high efficiency solar cell
fabrication. The present invention uses particles of elements and
solid-solutions to mix and formulate nano-powders which are then
deposited in thin film form and converted into the desired
compound. Mixtures of particles that can be successfully used in
this invention include but are not limited to the following
mixtures of binary solid solutions and elements (binary solid
solutions are solid solutions of two elements):
[0056] [CuGa+In] (meaning CuGa solid solution particles mixed with
In particles), [CuGa+InGa] (meaning CuGa solid solution particles
mixed with InGa solid solution particles), [CuGa+InGa+In] (meaning
CuGa solid solution particles mixed with InGa solid solution
particles and In particles), [CuGa+InGa+Cu] (meaning CuGa solid
solution particles mixed with Inga solid solution particles and Cu
particles). [CuGa+InGa+Cu+In] (meaning CuGa solid solution
particles mixed with InGa solid solution particles, Cu particles
and In particles), [CuGa+Cu+In] (meaning CuGa solid solution
particles mixed with Cu particles and In particles), [CuGa+CuIn+In]
(meaning CuGa solid solution particles mixed with CuIn solid
solution particles and In particles), [CuGa+CuIn+InGa] (meaning
CuGa solid solution particles mixed with CuIn solid solution
particles and InGa solid solution particles), [CuGa+CuIn+InGa+In]
(meaning CuGa solid solution particles mixed with CuIn solid
solution particles, InGa solid solution particles and In
particles), [CuGa+CuIn+Cu+In] (meaning CuGa solid solution
particles mixed with CuIn solid solution particles, Cu particles
and In particles), [CuGa+CuIn+InGa+Cu] (meaning CuGa solid solution
particles mixed with CuIn solid solution particles, InGa solid
solution particles and Cu particles), and [CuGa+CuIn+InGa+Cu+In]
(meaning CuGa solid solution particles mixed with CuIn solid
solution particles, InGa solid solution particles, Cu particles and
In particles). By selecting relative amounts of particles in the
mixture a whole range of compositions can be obtained by this
approach including the ranges necessary for solar cell fabrication.
In certain cases a film may be fabricated with high Cu content
(outside the range acceptable for solar cell fabrication) but then
another film with composition of low Cu can be deposited over the
first film bringing the overall composition into the acceptable
range. These approaches are well known in the field. It should be
noted that the above example formulations did not include elemental
Ga powders. If this is done, the number of possible powder
formulations that can be used increases even further. However,
since Ga is a low-melting material that can oxidize easily, the
preferred method of the present invention is introduction of Ga
into the formulation in the form of a CuGa or InGa solid solution.
Also there is no data available on ternary solid solution region
for the CuGaIn alloy system. The present invention foresees the use
of particles of such ternary solid solutions. For example, it may
be possible to dissolve some In in CuGa solid solution to obtain
one or more CuGaIn solid solutions. In such case a powder of this
material can be mixed with the powders of the elements and/or the
other binary solid solutions listed above.
[0057] If the composition of the nano-powder does not contain a
group VIA powder, the micro-layer which contains only metallic
phases need to be reacted to form the compound film. This reaction
step may be carried out in the vapor of Group VIA element (Se, S,
Te) or other sources such as H.sub.2-- Group VIA-type gases such as
H.sub.2S, H.sub.2Se or their mixtures at elevated temperatures.
Once the compound layers are formed solar cells and other
electronic devices may be fabricated on these layers using methods
well known in the field.
[0058] The foregoing disclosure has been set forth merely to
illustrate the invention and is not intended to be limiting. Since
modifications of the disclosed embodiments incorporating the spirit
and substance of the invention may occur to persons skilled in the
art, the invention should be construed to include everything within
the scope of the appended claims and equivalents thereof.
* * * * *