U.S. patent application number 14/948813 was filed with the patent office on 2016-05-26 for method of manufacture of chalcogenide-based photovoltaic cells.
The applicant listed for this patent is Dow Global Technologies LLC. Invention is credited to Todd R. Bryden, Jeffrey L. Fenton, JR., Michael E. Mills, Gary E. Mitchell, David J. Parrillo, Kirk R. Thompson.
Application Number | 20160149069 14/948813 |
Document ID | / |
Family ID | 44626134 |
Filed Date | 2016-05-26 |
United States Patent
Application |
20160149069 |
Kind Code |
A1 |
Bryden; Todd R. ; et
al. |
May 26, 2016 |
METHOD OF MANUFACTURE OF CHALCOGENIDE-BASED PHOTOVOLTAIC CELLS
Abstract
The invention is a method of forming a cadmium sulfide based
buffer on a copper chalcogenide based absorber in making a
photovoltaic cell. The buffer is sputtered at relatively high
pressures. The resulting cell has good efficiency and according to
one embodiment is characterized by a narrow interface between the
absorber and buffer layers. The buffer is further characterized
according to a second embodiment by a relatively high oxygen
content.
Inventors: |
Bryden; Todd R.;
(Dusseldorf, DE) ; Fenton, JR.; Jeffrey L.; (Coon
Rapids, MN) ; Mitchell; Gary E.; (Prudenville,
MI) ; Thompson; Kirk R.; (Pleasanton, CA) ;
Mills; Michael E.; (Midland, MI) ; Parrillo; David
J.; (Midland, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Dow Global Technologies LLC |
Midland |
MI |
US |
|
|
Family ID: |
44626134 |
Appl. No.: |
14/948813 |
Filed: |
November 23, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13196316 |
Aug 2, 2011 |
8821139 |
|
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14948813 |
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61329728 |
Apr 30, 2010 |
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Current U.S.
Class: |
438/94 |
Current CPC
Class: |
C23C 14/0629 20130101;
H01L 21/0237 20130101; H01L 31/022466 20130101; H01L 31/0749
20130101; C23C 14/3414 20130101; H01L 31/18 20130101; H01L 21/02425
20130101; Y02P 70/50 20151101; H01L 31/1884 20130101; Y02E 10/541
20130101; H01L 31/0322 20130101; H01L 21/02485 20130101; H01L
21/02631 20130101; Y02P 70/521 20151101; H01L 31/03928 20130101;
H01L 21/02557 20130101 |
International
Class: |
H01L 31/0749 20060101
H01L031/0749; H01L 31/0224 20060101 H01L031/0224; H01L 31/18
20060101 H01L031/18; H01L 31/0392 20060101 H01L031/0392 |
Claims
1. A method comprising forming a chalcogenide based absorber layer
on a substrate forming a buffer layer comprising cadmium and sulfur
on the absorber by sputtering at a working pressure of from 0.08 to
0.12 mbar.
2. The method of claim 1 wherein the atmosphere is inert.
3. The method of claim 1 wherein sputtering is from a target of
cadmium and sulfur or compounds thereof.
4. The method of claim 1 wherein an interface having a thickness of
less than 10 nm is formed between the absorber layer and the buffer
layer wherein the interface is defined on one side by the point at
which the atomic fraction of cadmium exceeds 0.05 in energy
dispersive x-ray spectroscopy scans of cross section of the cell
and defined on a second side by the point at which the atomic
fraction of indium and selenium is less than 0.05 in energy
dispersive x-ray spectroscopy scans of cross section of the
cell.
5. The method of claim 4 wherein the interface is further defined
on the second side by the atomic fraction of copper being less than
0.10.
6. The method of claim 1 wherein the substrate is flexible and
bears a backside electrical contact.
7. The method of claim 1 further comprising forming a transparent
conductive layer over the buffer layer.
8. The method of claim 7 further comprising forming a window layer
between the transparent conductive layer and the buffer layer.
9. The method of claim 7 comprising forming an electrical
connection grid on the transparent conductive layer.
10. The method of claim 7 comprising providing a barrier layer over
the transparent conductive layer.
11. The method of claim 9 comprising providing a barrier layer over
the transparent conductive layer and the grid.
12. (canceled)
13. (canceled)
14. (canceled)
15. (canceled)
16. (canceled)
17. (canceled)
18. (canceled)
19. (canceled)
20. (canceled)
Description
FIELD OF THE INVENTION
[0001] This invention relates to a method of manufacture of
chalcogenide-based photovoltaic cells and particularly to a method
of forming a buffer layer in such cells and the cells made by this
method.
BACKGROUND OF THE INVENTION
[0002] Photovoltaic cells can be made using p-type chalcogenide
based materials as absorber layers which convert incident light or
radiation to electrical energy. These p-type chalcogenides are
typically selenides, sulfides or sulfide selenides of at least one,
and more typically at least two or three of the following metals:
Cu, In, Ga, Al (referred to herein as CIS, CISS, CIAS, CIASS, CIGS,
CIGSS, or CIAGSS depending upon the combination of elements used).
Using a CdS based buffer layer near or adjacent to the p-type
chalcogenide is also known.
[0003] It is known that CdS layers can be formed on various
substrates by chemical bath deposition, physical vapor deposition
or sputtering. See e.g. Abou-Ras et al (Thin Solid Films 480-481
(2005) 118-123) and 5,500,055. Abou-Ras specifically looked at the
effect of deposition method comparing CBD deposition to PVD
deposition and observed that CBD deposition of the CdS created more
efficient cells compared with cells made with PVD. Abou-Ras
proposes that the lack or decrease of interdiffusion at the
CIGS-CdS interface in the case of PVD deposited cells is a reason
for their decreased efficiencies.
SUMMARY OF THE INVENTION
[0004] Applicants have surprisingly found that sputtering CdS onto
the p-type chalcogenide at relatively high pressures in a
substantially inert atmosphere leads to less interdiffusion with
the underlying chalcogenide absorber than does sputtering at the
traditional low pressure sputtering conditions but also leads to
higher cell efficiencies than does the traditional low pressure
sputtering. This is particularly unexpected in view of Abou-Ras's
teaching.
[0005] Thus, according to one embodiment, the invention is a method
comprising [0006] forming a chalcogenide-based absorber layer on a
substrate, [0007] forming a buffer layer comprising cadmium and
sulfur on the absorber by sputtering in an inert atmosphere at a
working pressure of from 0.08 to 0.12 mbar (0.06 to 0.09 torr or
8-12 Pa).
[0008] This invention is also a photovoltaic cell made by the
preceding method. The cells made by this method are characterized
by interdiffusion between the CdS and absorber layer. The
interdiffusion interface region is defined at the absorber region
by the point at which the atomic fraction of cadmium exceeds 0.05
in energy dispersive x-ray spectroscopy (EDS) scans of cross
section of the cell and defined at the buffer region by the point
at which the atomic fraction of indium and selenium is less than
0.05 and preferably the atomic fraction of copper is less than
0.10. The atomic fraction is based on total atomic amounts of
copper, indium, gallium, selenium, cadmium, sulfur, and oxygen. The
grain size of the cadmium sulfide grains preferably is less than 50
nm, more preferably less than 30 nm, and most preferably less than
20 nm. Thus, according to one embodiment the invention is a
photovoltaic cell comprising a backside electrode (also referred to
herein as a backside electrical contact or backside electrical
collector), a chalcogenide-based absorber in contact with the
backside electrode, a cadmium sulfide based buffer layer on the
absorber, a transparent conductive layer located at the opposite
side of the buffer layer from the absorber layer, an electrical
collector on the transparent conductive layer, wherein the cell has
an interface between the absorber and the buffer of less than 10 nm
thickness and the buffer preferably has an average grain size of
less than 50 nm.
[0009] Surprisingly, although the cadmium sulfide preferably is
sputtered in an inert environment, the buffer layer of the cells
made by this invention has a significant amount of oxygen. Thus,
according to another embodiment this invention is a photovoltaic
cell comprising a backside electrode, a chalcogenide based absorber
in contact with the backside electrode, a cadmium sulfide based
buffer layer on the absorber, a transparent conductive layer
located at the opposite side of the buffer layer from the absorber
layer, an electrical collector on the transparent conductive layer,
wherein the atomic fraction of oxygen in the cadmium sulfide based
buffer layer is at least 0.20.
[0010] Also, surprisingly the thickness of the buffer layer may be
very low while still yielding effective cells while providing the
additional benefit of low cadmium leachate from the cell. Thus,
according to yet another embodiment the invention is a photovoltaic
cell comprising a backside electrode, a chalcogenide-based absorber
in contact with the backside electrode, a cadmium sulfide based
buffer layer on the absorber, a transparent conductive layer
located at the opposite side of the buffer layer from the absorber
layer, an electrical collector on the transparent conductive layer,
wherein the thickness of the buffer layer no greater than 30 nm,
preferably no greater than 20 nm, and most preferably no greater
than 15 nm. The amount of Cadmium that leaches from the article
under the USEPA Toxicity Characteristic Leaching Procedure Test
1311 (1992) is no greater than 1 mg/l (i.e. 1 mg of cadmium per
liter of leachant solution as specified in the protocol) preferably
no greater than 0.8 mg/l, and most preferably no greater than 0.7
mg/l.
DETAILED DESCRIPTION OF THE INVENTION
[0011] The method of this invention includes forming a chalcogenide
based absorber layer (absorbs electromagnetic radiation at relevant
wavelengths and converts to electrical energy) on a substrate and
then forming a cadmium sulfide buffer layer on that absorber. When
the method is used to form a photovoltaic device, additional layers
as are known in the photovoltaic arts also will typically be added.
For example, the substrate will typically include or bear backside
electrical contacts. A transparent conductive layer will be found
above the buffer and a electrical collector system (e.g. a grid)
will typically be located above the transparent conductive layer.
An optional window layer may be used and protective layers may be
applied over the transparent conductive layer and/or the electrical
collector. With the exception of the cadmium sulfide based buffer
layer, these additional layers may be formed by any method known in
the art.
[0012] According to one embodiment of a photovoltaic article that
may be made by processes of the invention comprises a substrate
incorporating a support, a backside electrical contact, and a
chalcogenide absorber. The article further includes a buffer region
incorporating an n-type chalcogenide composition of the present
invention, an optional front side electrical contact window region,
a transparent conductive region, a collection grid, and an optional
barrier region to help protect and isolate the article from the
ambient. Each of these components can be a single layer, but any of
these independently can be formed from multiple sublayers as
desired. Additional layers conventionally used in photovoltaic
cells as presently known or hereafter developed may also be
provided. As used occasionally herein, the top of the cell is
deemed to be that side which receives the incident light. The
method of forming the cadmium sulfide based layer on the absorber
can also be used in tandem cell structures where two cells are
built on top of each other each with an absorber that absorbs
radiation at different wavelengths.
[0013] The support may be a rigid or flexible substrate. Support
may be formed from a wide range of materials. These include glass,
quartz, other ceramic materials, polymers, metals, metal alloys,
intermetallic compositions, paper, woven or non-woven fabrics,
combinations of these, and the like. Stainless steel is preferred.
Flexible substrates are preferred to enable maximum utilization of
the flexibility of the thin film absorber and other layers.
[0014] The backside electrical contact provides a convenient way to
electrically couple article 10 to external circuitry. Contact may
be formed from a wide range of electrically conductive materials,
including one or more of Cu, Mo, Ag, Al, Cr, Ni, Ti, Ta, Nb, W
combinations of these, and the like. Conductive compositions
incorporating Mo are preferred. The backside electrical contact may
also help to isolate the absorber from the support to minimize
migration of support constituents into the absorber. For instance,
backside electrical contact can help to block the migration of Fe
and Ni constituents of a stainless steel support into the absorber.
The backside electrical contact also can protect the support such
as by protecting against Se if Se is used in the formation of
absorber.
[0015] The chalcogenide absorber preferably incorporates at least
one p-type Group 16 chalcogenide, such as Group 16 selenides,
sulfides, and selenides-sulfides that include at least one of
copper, indium, and/or gallium. In many embodiments, these
materials are present in polycrystalline form. Advantageously,
these materials exhibit excellent cross-sections for light
absorption that allow absorber to be very thin and flexible. In
illustrative embodiments, a typical absorber region may have a
thickness in the range from about 300 nm to about 3000 nm,
preferably about 1000 nm to about 2000 nm.
[0016] Representative examples of such p-type chalcogenides
absorbers are selenides, sulfides, tellurides, and/or combinations
of these that include at least one of copper, indium, aluminum,
and/or gallium. More typically at least two or even at least three
of Cu, In, Ga, and Al are present. Sulfides and/or selenides are
preferred. Some embodiments include sulfides or selenides of copper
and indium. Additional embodiments include selenides or sulfides of
copper, indium, and gallium. Aluminum may be used as an additional
or alternative metal, typically replacing some or all of the
gallium. Specific examples include but are not limited to copper
indium selenides, copper indium gallium selenides, copper gallium
selenides, copper indium sulfides, copper indium gallium sulfides,
copper gallium sulfides, copper indium sulfide selenides, copper
gallium sulfide selenides, copper indium aluminum sulfide, copper
indium aluminum selenide, copper indium aluminum sulfide selenide,
copper indium aluminum gallium sulfide, copper indium aluminum
gallium selenide, copper indium aluminum gallium sulfide selenide,
and copper indium gallium sulfide selenides. The abosber materials
also may be doped with other materials, such as Na, Li, or the
like, to enhance performance In addition, many chalcogen materials
could incorporate at least some oxygen as an impurity in small
amounts without significant deleterious effects upon electronic
properties.
[0017] One preferred class of CIGS materials may be represented by
the formula
Cu.sub.aIn.sub.bGa.sub.cAl.sub.dSe.sub.wS.sub.xTe.sub.yNa.sub.z
(A)
[0018] Wherein, if "a" is defined as 1, then: [0019]
"(b+c+d)/a"=1.0 to 2.5, preferably 1.0 to 1.65 [0020] "b" is 0 to
2, preferably 0.8 to 1.3 [0021] "c" is 0 to 0.5, preferably 0.05 to
0.35 [0022] "d" is 0 to 0.5, preferably 0.05 to 0.35, preferably
d=0 [0023] "(w+x+y)" is 2 to 3, preferably 2 to 2.8 [0024] "w" is 0
or more, preferably at least 1 and more preferably at least 2 to 3
[0025] "x" is 0 to 3, preferably 0 to 0.5 [0026] "y" is 0 to 3,
preferably 0 to 0.5 [0027] "z" is 0 to 0.5, preferably 0.005 to
0.02
[0028] The absorber may be formed by any suitable method using a
variety of one or more techniques such as evaporation, sputtering,
electrodeposition, spraying, and sintering. One preferred method is
co-evaporation of the constituent elements from one or more
suitable targets, where the individual constituent elements are
thermally evaporated on a hot surface coincidentally at the same
time, sequentially, or a combination of these to form absorber.
After deposition, the deposited materials may be subjected to one
or more further treatments to finalize the absorber properties.
[0029] Optional layers may be used on substrate in accordance with
conventional practices now known or hereafter developed to help
enhance adhesion between backside electrical contact and the
support and/or between backside electrical contact and the absorber
region. Additionally, one or more barrier layers (not shown) also
may be provided over the backside of support to help isolate device
from the ambient and/or to electrically isolate device.
[0030] The buffer region is cadmium sulfide based material
deposited on the absorber by sputtering at pressures of at least
0.08 millibar (0.06 torr, 8 Pa), more preferably at least 0.09
millibar (0.067 torr, 9 Pa), and most preferably about 0.1 millibar
(0.075 torr, 10 Pa) and no greater than 0.12 millibar (0.09 torr,
12 Pa), more preferably no greater than 0.11 millibar (0.083 torr,
11 Pa). Preferably the atmosphere is inert or a sulfur containing
gas, but is most preferably inert.
[0031] During such deposition approach, the substrate is typically
fixed to or otherwise supported upon a holder within the chamber
such as by gripping components, or the like. However, the substrate
may be oriented and affixed by a wide variety of means as desired.
The substrate may be provided in the chamber in a manner such that
the substrate is stationary and/or non-stationary during the
treatment. In some embodiments, for instance, the substrate can be
supported on a rotatable chuck so that the substrate rotates during
the deposition.
[0032] One or more targets are operably provided in the deposition
system. The targets are compositionally suitable to form the
desired cadmium sulfide composition. For instance, to form n-type
cadmium sulfide, a suitable target has a composition that includes
cadmium and sulfur-containing compounds, and is preferably 99% pure
cadmium and sulfur. Alternatively, a cadmium target can be used in
the presence of a sulfur containing gas. The resulting film is
preferably at least 10 nanometers (nm), more preferably at least 15
nm and is preferably up to about 200 nm, more preferably up to 100
nm, yet more preferably up to 30 nm, still more preferably up to 20
nm and most preferably not more than 15 nm. Since the buffer
functions effectively at these yet more, still more and most
preferred very thin layers, the amount of cadmium in the cell is
relatively low compared to prior art cells. These cells have the
added benefit of a low cadmium leachate amount.
[0033] While a small amount of interdiffusion in cells made by the
method of this invention may occur it is significantly less than is
found in when the atmosphere for sputtering is lower pressure. The
interdiffusion interface region is defined at the absorber region
by the point at which the atomic fraction of cadmium exceeds 0.05
in energy dispersive x-ray spectroscopy scans of cross section of
the cell and defined at the buffer region by the point at which the
atomic fraction of indium and selenium is less than 0.05 and
preferably the atomic fraction of copper is less than 0.10. The
atomic fraction is based on total atomic amounts of copper, indium,
gallium, selenium, cadmium, sulfur, and oxygen. The grain size of
the cadmium sulfide grains preferably is less than 50 nm, more
preferably less than 30 nm, and most preferably less than 20 nm.
According to this embodiment, the interface region is less than 10
nm, preferably less than 8 nm in thickness.
[0034] Atomic fraction can be determined from transmission electron
microscope (TEM) line scans using energy dispersive x-ray
spectroscopy (EDS). Samples for TEM analysis can be prepared by
focused ion beam (FIB) milling using a FEI Strata Dual Beam FIB
mill equipped with a Omniprobe lift-out tool. TEM analysis can be
performed for example on a FBI Tecnai TF-20XT FEGTEM equipped with
a Fischione high angle annular dark field (HAADF) scanning TEM
(STEM) detector and EDAX EDS detector. The operating voltage of the
TEM can be level suitable for the equipment for example voltages of
about 200 keV are useful with the equipment mentioned above.
[0035] Spatially-resolved EDS line scans can be acquired in STEM
mode from HAADF images. From the 100 nm long line scan, 50 spectra
can be acquired using 2 nm spot-to-spot resolution and a STEM probe
size of .about.1 nm. The full scale spectra (0-20 keV) can be
converted into elemental distribution profiles because peak
intensity (number of integrated peak counts after background
removal) is directly proportional to the concentration (in weight
percent) of a certain element. Weight percent is then converted to
atomic percentage based on mole weight of the element. Note that as
a skilled worker understands peak intensity is corrected for the
detector response and the other sample dependent factors. These
adjustments are typically made based on manufacturer's parameters
for their EDS equipment or other suitable reference standards.
Grain size can be determined by standard analysis of TEM bright and
dark field images.
[0036] The cadmium sulfide layer may contain a small amount of
impurities. The cadmium sulfide layer preferably consists
essentially of cadmium, sulfur, oxygen and copper. Preferably the
atomic fractions of cadmium and sulfur are at least 0.3, the atomic
fraction of oxygen is at least 0.2 and the atomic fraction of
copper is less than 0.15, more preferably less than 0.10.
[0037] The atmosphere for sputtering is preferably an inert gas
such as argon, helium or neon. The substrate may be placed at a
predetermined distance from and orientation relative to the
target(s). In some modes of practice, this distance can be varied
during the course of the deposition, if desired. Typically, the
distance is in the range from about 50 millimeters (mm) to about
100 mm. Preferably, the distance is from about 60 mm to about 80
mm. Prior to starting deposition, the chamber typically is
evacuated to a suitable base pressure. In many embodiments, the
base pressure is in the range from about 1.times.10.sup.-8 Torr to
about 1.times.10.sup.-6 Torr.
[0038] Conveniently, many modes of practice may be carried out at a
temperature in the range of from about 20.degree. C. to about
30.degree. C. Conveniently, many modes of practice may be carried
out under ambient temperature conditions. Of course, cooler or
warmer temperatures may be used to help control deposition rate,
deposition quality, or the like. The deposition may be carried out
long enough to provide a layer of n-type material have a desired
thickness, uniformity, and/or the like.
[0039] Optional window region which may be a single layer or formed
from multiple sublayers, can help to protect against shunts. Window
region also may protect buffer region during subsequent deposition
of the TC region. The window region may be formed from a wide range
of materials and often is formed from a resistive, transparent
oxide such as an oxide of Zn, In, Cd, Sn, combinations of these and
the like. An exemplary window material is intrinsic ZnO. A typical
window region may have a thickness in the range from about 1 nm to
about 200 nm, preferably about 10 nm to about 150 nm, more
preferably about 80 to about 120 nm.
[0040] The TCO region, which may be a single layer or formed from
multiple sublayers, is electrically coupled to the buffer region to
provide a top conductive electrode for the article. In many
suitable embodiments, the TCO region has a thickness in the range
from about 10 nm to about 1500 nm, preferably about 150 nm to about
200 nm. As shown, the TCO region is in direct contact with the
window region, but one or more intervening layers optionally may be
interposed for a variety of reasons such as to promote adhesion,
enhance electrical performance, or the like.
[0041] A wide variety of transparent conducting oxides; very thin
conductive, transparent metal films; or combinations of these may
be incorporated used in forming the transparent conductive region.
Transparent conductive oxides are preferred. Examples of such TCOs
include fluorine-doped tin oxide, tin oxide, indium oxide, indium
tin oxide (ITO), aluminum doped zinc oxide (AZO), zinc oxide,
combinations of these, and the like. In one illustrative
embodiment, TCO region has a dual layer construction in which a
first sublayer proximal to the buffer incorporates zinc oxide and a
second sublayer incorporates ITO and/or AZO. TCO layers are
conveniently formed via sputtering or other suitable deposition
technique.
[0042] The optional electrical grid collection structure may be
deposited over the TCO region to reduce the sheet resistance of
this layer. The grid structure preferably incorporates one or more
of Ag, Al, Cu, Cr, Ni, Ti, Ta, TiN, TaN, and combinations thereof.
Preferably the grid is made of Ag. An optional film of Ni (not
shown) may be used to enhance adhesion of the grid structure to the
TCO region. This structure can be formed in a wide variety of ways,
including being made of a wire mesh or similar wire structure,
being formed by screen-printing, ink-jet printing, electroplating,
photolithography, and metallizing thru a suitable mask using any
suitable deposition technique.
[0043] A chalcogenide based photovoltaic cell may be rendered less
susceptible to moisture related degradation via direct, low
temperature application of suitable barrier protection to the top
of the photovoltaic article. The barrier protection may be a single
layer or multiple sublayers. The barrier may or may not cover the
electrical grid structure.
EXAMPLE
Example 1
[0044] A photovoltaic cell is prepared as follows. A stainless
steel substrate is provided. A niobium and molybdenum backside
electrical contact is formed on substrate by sputtering. A copper
indium gallium selenide absorber is formed by a 1-stage evaporation
process where copper, indium, gallium and selenium are evaporated
simultaneously from effusion sources onto the stainless steel
substrate, which is held at .about.550.degree. C., for .about.80
minutes. This process results in an absorber stoichiometry of
.about.Cu(In.sub.0.8Ga.sub.0.2)Se.sub.2
[0045] A cadmium sulfide layer is radio frequency (re sputtered
from a CdS target (99.9+% purity) at 160 watts in the presence of
argon and varying pressures as shown in Table 1. The temperature of
the substrate is maintained at 35.degree. C. and the target to
substrate distance is .about.90 mm. The approximate thickness of
this layer is as noted in Table 1.
[0046] On the cadmium sulfide layer, an i-ZnO and Al-doped ZnO was
deposited via rf-sputtering. A front side electrical collection
grid was deposited on Al-doped ZnO.
[0047] Cells made substantially according to the above procedure
are tested for efficiency by measuring current-voltage
characteristics under illumination. As is shown, efficiency peaked
at about 0.1 millibar (10 Pa).
TABLE-US-00001 TABLE 1 CdS Sputtering thickness, Number of Mean
pressure, mbar nm samples efficiency, % 0.002 15 12 6.8 0.002 120 3
6.9 0.01 15 3 6.7 0.02 15 3 7.7 0.05 15 3 8.2 0.07 15 3 8.1 0.10 15
32 9.5 0.14 15 4 7.5
Example 2
[0048] Cross sections of photovoltaic cells which were prepared
substantially according to the method of Example 1 are prepared by
focused ion beam as described in the Detailed Description above.
See above
[0049] An atomic fraction from EDS of a cell having copper indium
gallium selenide based absorber on which cadmium sulfide is
sputtered from a 99.9% purity cadmium sulfide target in an argon
atmosphere as described above shows the atomic fraction of a cell
made by the present method by sputtering at 0.1 millibar. An atomic
fraction from EDS of a comparative cell made by sputtering at 0.002
millibar was also evaluated. These show that interdiffusion is
limited to about a 10 nm range in the samples formed at higher
pressures as in the process of the present invention but a wider
interdiffusion for the comparative cell.
Example 3
[0050] Example 1 is repeated using an absorber made by selenization
(using elemental selenium) of a sputtered layer containing the
elements copper, indium and gallium and varying the pressure during
CdS sputtering as shown in Table 2. The absorber formation process
proceeds as follows: copper, indium and gallium are deposited on a
stainless steel substrate, onto which niobium and molybdenum had
been previously deposited, via sputtering from either elemental
targets or a target made of an alloy of copper, indium and gallium.
Elemental selenium is evaporated onto the coated substrates with
the substrate temperature maintained at 100.degree. C. This coated
substrate is then heated to cause selenization of the copper,
indium, gallium precursor.
TABLE-US-00002 TABLE 2 CdS Sputtering thickness, Number of Mean
pressure, mbar nm samples efficiency, % 0.09 15 1 5.9 0.10 15 29
6.5 0.12 15 1 4.9
* * * * *