U.S. patent application number 13/105977 was filed with the patent office on 2011-11-24 for chalcogenide-based materials and methods of making such materials under vacuum using post-chalcogenization techniques.
Invention is credited to Marc G. Langlois, Beth M. Nichols, Robert T. Nilsson, Rentian Xiong.
Application Number | 20110284134 13/105977 |
Document ID | / |
Family ID | 44534899 |
Filed Date | 2011-11-24 |
United States Patent
Application |
20110284134 |
Kind Code |
A1 |
Nichols; Beth M. ; et
al. |
November 24, 2011 |
CHALCOGENIDE-BASED MATERIALS AND METHODS OF MAKING SUCH MATERIALS
UNDER VACUUM USING POST-CHALCOGENIZATION TECHNIQUES
Abstract
The present invention provides strategies for making high
quality CIGS photoabsorbing compositions from sputtered precursor
film(s). The precursors are converted into CIGS photoabsorbing
material via a chalcogenizing treatment (also referred to as
"post-chalcogenization," including, e.g., "post-selenization" when
Se is used and/or "post-sulfurization" when S is used) using
techniques that allow the post-chalcogenizing treatment to occur
under atypically low pressure conditions. Consequently, the
strategies of the invention are readily incorporated into batch
processes or continuous processes such as roll-to-roll process
occurring under vacuum. The present invention is useful at lab,
pilot plant, and industrial scales.
Inventors: |
Nichols; Beth M.; (Midland,
MI) ; Nilsson; Robert T.; (Midland, MI) ;
Langlois; Marc G.; (Ann Arbor, MI) ; Xiong;
Rentian; (Midland, MI) |
Family ID: |
44534899 |
Appl. No.: |
13/105977 |
Filed: |
May 12, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61346515 |
May 20, 2010 |
|
|
|
Current U.S.
Class: |
148/241 ;
148/240; 148/282 |
Current CPC
Class: |
C23C 14/24 20130101;
C23C 14/5866 20130101; C23C 14/0623 20130101; C23C 14/5806
20130101; C23C 14/34 20130101 |
Class at
Publication: |
148/241 ;
148/240; 148/282 |
International
Class: |
C23C 8/62 20060101
C23C008/62 |
Claims
1. A method of making a chalcogen-containing photoabsorbing
composition, comprising the steps of: a) forming a workpiece
comprising a precursor of the chalcogen-containing photoabsorber;
b) forming a cap on the precursor, said cap comprising at least one
chalcogen; c) heating the capped workpiece to cause
chalcogenization at a pressure of below about 300 millitorr.
2. The method of claim 1, wherein the process occurs in a
roll-to-roll manufacturing process.
3. The method of claim 1, wherein the heating occurs in the
presence of an inert gas.
4. The method of claim 1, wherein the cap incorporates at least one
chalcogen in elemental form.
5. The method of claim 4, wherein the cap comprises elemental
Se.
6. The method of claim 1, wherein at least the heating step occurs
in the presence of at least one gas comprising a chalcogen.
7. The method of claim 6, wherein at least the annealing step
occurs in the presence of a gas comprising H.sub.2S or
H.sub.2Se.
8. The method of claim 1, wherein the chalcogenization treatment
converts at least a portion of the precursor to a chalcogenide
photoabsorbing material.
9. The method of claim 1, wherein the heating temperature is in the
range from about 300.degree. C. to about 600.degree. C.
10. The method of claim 1, wherein at least the heating step occurs
at a pressure of less than about 100 millitorr.
11. The method of claim 1, wherein at least the heating step occurs
at a pressure less than about 50 millitorr.
12. The method of claim 1, wherein the precursor film comprises Cu,
In, and Ga.
13. The method of claim 12, wherein the step of forming the
workpiece comprises using a sputtering target comprising Cu, In and
Ga to form the precursor.
14. The method of claim 12, wherein the step of forming the
workpiece comprises using at least one confocal target to form the
precursor.
15. The method of claim 1, wherein the precursor comprises a
sub-stoichiometric amount of at least one chalcogen.
16. The method of claim 1, wherein the heating occurs at a rate in
the range from about 5.degree. C./minute to about 400.degree.
C./minute.
17. The method of claim 16, wherein the heating occurs at a rate
having a substantially linear profile.
18. The method of claim 1, wherein the workpiece is maintained at
one or more temperatures below about 100.degree. C. prior to the
heating step.
19. The method of claim 1, wherein the step of forming the
workpiece comprises (a) cleaning a metallic support under
conditions effective to remove organic impurities and (b) forming
the workpiece on a substrate incorporating the cleaned metallic
support.
Description
PRIORITY
[0001] The present nonprovisional patent application claims
priority under 35 U.S.C. .sctn.119(e) from United States
Provisional patent application having Ser. No. 61/346,515, filed on
May 20, 2010, by Nichols et al. and titled CHALCOGENIDE-BASED
MATERIALS AND METHODS OF MAKING SUCH MATERIALS UNDER VACUUM USING
POST-CHALCOGENIZATION TECHNIQUES, wherein the entirety of said
provisional patent application is incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention relates to methods for making
chalcogenide-based photoabsorbing materials as well as to
photovoltaic devices that incorporate these materials. More
specifically, the present invention relates to methods for making
chalcogenide-based photoabsorbing materials, desirably in the form
of thin films as well as to photovoltaic devices that incorporate
these materials, in which a precursor film is prepared and then
converted to the desired photoabsorbing composition via a
chalcogenization treatment.
BACKGROUND OF THE INVENTION
[0003] Both n-type chalcogenide materials and/or p-type
chalcogenide materials have photovoltaic functionality (also
referred to herein photoabsorbing functionality). These materials
absorb incident light and generate an electric output when
incorporated into a photovoltaic device. Consequently, these
chalcogenide-based photoabsorbing materials have been used as the
photovoltaic absorber region in functioning photovoltaic devices.
Illustrative p-type chalcogenide materials often include selenides
(S), sulfides (also referred to as S; in some embodiments, SS
indicates that sulfur is being used in combination with selenium),
and/or tellurides (T) of at least one or more of copper (C), indium
(I), gallium (G), and/or aluminum (A). Specific chalcogenide
compositions may be referred to by acronyms such as CIS, CISS,
CIGS, CIGST, CIGSAT, and/or CIGSS compositions, or the like, to
represent the constituents of the composition.
[0004] Photoabsorbers based upon chalcogenide compositions offer
several advantages. As one advantage, these compositions tend to
have a very high cross-section for absorbing incident light. This
means that a very high percentage of incident light can be captured
by chalcogenide-based absorber layers that are very thin. For
example, in many devices, chalcogenide-based absorber layers have a
thickness in the range of from about 1 .mu.m to about 2 .mu.m.
These thin layers allow devices incorporating these layers to be
flexible. This is in contrast to crystalline silicon-based
absorbers. Crystalline silicon-based absorbers have a lower
cross-section for light capture and generally must be much thicker
to capture the same amount of incident light. Crystalline
silicon-based absorbers tend to be rigid, not flexible.
[0005] Making photoabsorbing chalcogenide compositions with
industrially scalable processes is quite challenging. Industry has
invested and continues to invest considerable resources in
developing this technology. According to one proposed manufacturing
technique, evaporative techniques are used to deposit the film
constituents at a high substrate temperature such that the film
reacts and crystallizes fully during growth. According to an
alternative hybrid method the films are formed by sputtering from
one or more metal targets in the presence of selenium and/or sulfur
containing gas or vapor from an evaporated source. Unfortunately,
these conventional evaporation approaches are not easily scalable
for industrial applications. Also, using only a gas as a chalcogen
source during sputtering typically requires that enough gas be used
to supply the desired chalcogen content in the precursor film plus
an overpressure of chalcogen. Using so much chalcogen-containing
gas tends to cause equipment degradation and chalcogen (e.g. Se)
buildup, target poisoning, instabilities in process control
(resulting in composition and rate hysteresis), the loss of In from
the deposited film due to volatile indium selenide compounds,
lowered overall deposition rates, and the damage of targets due to
electrical arcing.
[0006] An alternative manufacturing approach involves initially
forming a precursor film of the desired metal constituents. This
film may include one or more layers. Chalcogen(s) are incorporated
into the precursor at a later processing stage under conditions
effective to incorporate chalcogen into the film and convert it to
the desired tetragonal, chalcopyrite phase. Due to the
incorporation of chalcogen into the film after precursor formation,
this approach may be referred to as a "post-chalcogenization"
approach. This approach appears to be easier to integrate into
industrial scale processes. Yet, serious challenges remain.
[0007] As one challenge, many known approaches tend to practice
post-chalcogenization within relatively high pressure regimes, such
as on the order of a few ton to atmospheric pressure. Even in these
higher pressure regimes, retention of In and Se during
post-chalcogenization is a problem widely recognized in the
industry. It has been challenging to carry out the
post-chalcogenizing treatment without undue loss of one or both of
these materials.
[0008] It is believed that one loss mechanism occurs when In and/or
Se react to form volatile species that are lost to evaporation. The
formation of volatile species is consistent with the observation
that retention problems tend to become more severe at lower
pressures. Frustratingly, the post-chalcogenization techniques
practiced with reasonable success at higher pressures often are not
directly translatable to use at lower pressures. For instance,
undue In loss may still result when practicing a process strategy
at a lower pressure even though that same technique provides
acceptable In retention at a pressure in the range from about 10
torr to 760 torr.
SUMMARY OF THE INVENTION
[0009] The present invention provides strategies for making high
quality, chalcogenide-based, photoabsorbing compositions from
sputtered precursor film(s). The precursors are converted, into the
chalcogenide photoabsorbing material, via a chalcogenizing
treatment (also referred to as "post-chalcogenization," including,
e.g., "post-selenization" when Se is used and/or
"post-sulfurization" when S is used) using techniques that allow
the post-chalcogenizing treatment to occur under atypically and
surprisingly low pressure conditions without significant indium
loss. Consequently, the strategies of the invention are readily
incorporated into batch processes or continuous processes such as
roll-to-roll process occurring under vacuum. The present invention
is useful at lab, pilot plant, and industrial scales.
[0010] In one aspect, the present invention provides a method of
making a chalcogen-containing photoabsorbing composition,
comprising the steps of: [0011] a) forming a workpiece comprising a
precursor of the chalcogen-containing photoabsorber; [0012] b)
forming a cap on the precursor, said cap comprising at least one
chalcogen; [0013] c) heating the capped workpiece to cause
chalcogenization at a pressure of below about 300 millitorr
(mT).
[0014] In a preferred aspect of the invention, heating is carried
out in the presence of at least one gas or vapor that comprises a
chalcogen. Preferably, the gas or vapor comprises H.sub.2S or
H.sub.2Se. In still another preferred aspect of the invention,
heating is carried out in an inert gas. In yet another preferred
aspect of the invention, a solid cap incorporates a chalcogen in
elemental form. In yet another preferred aspect of the invention, a
process according to the invention occurs in a continuous and/or
semicontinuous roll-to-roll manufacturing process. Such preferred
aspects may be practiced singly, or two or more of these may be
practiced in combination.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The above mentioned and other advantages of the present
invention, and the manner of attaining them, will become more
apparent and the invention itself will be better understood by
reference to the following description of the embodiments of the
invention taken in conjunction with the accompanying drawings,
wherein:
[0016] FIG. 1 is a schematic diagram of an illustrative
photovoltaic device that incorporates principles of the present
invention;
[0017] FIG. 2 is a schematic diagram showing an exemplary structure
for a substrate that may be used in the device of FIG. 1; and
[0018] FIG. 3 is a schematic diagram illustrating how the
principles of the present invention may be used to fabricate a
chalcogen-containing photoabsorbing layer useful in the device of
FIG. 1.
DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS
[0019] The embodiments of the present invention described below are
not intended to be exhaustive or to limit the invention to the
precise forms disclosed in the following detailed description.
Rather the embodiments are chosen and described so that others
skilled in the art may appreciate and understand the principles and
practices of the present invention. All patents, pending patent
applications, published patent applications, and technical articles
cited herein are incorporated herein by reference in their
respective'entireties for all purposes.
[0020] FIG. 1 schematically shows an illustrative embodiment of a
photovoltaic device 10 that is prepared according to principles of
the present invention. Device 10 desirably is flexible to allow
device 10 to be mounted to surfaces incorporating some curvature.
In preferred embodiments, device 10 is sufficiently flexible to be
wrapped around a mandrel having a diameter of 50 cm, preferably
about 40 cm, more preferably about 25 cm without cracking at a
temperature of 25.degree. C. Device 10 includes a light incident
face 12 that receives light rays 16 and a backside face 14.
[0021] Device 10 includes a chalcogenide-containing photovoltaic
absorber region 20 formed on an underlying substrate 18. Substrate
18 generally refers to the body on which the CIGS precursor film is
formed and often incorporates multiple layers. One illustrative
embodiment of a multilayer structure for substrate 18 is shown in
FIG. 2. In the illustrative embodiment of FIG. 2, substrate 18
generally includes support 22, barrier region 23, and backside
electrical contact region 24. Support 22 may be rigid or flexible,
but desirably is flexible in those embodiments in which the
photovoltaic device may be used in combination with non-flat
surfaces.
[0022] Support 22 may be formed from a wide range of materials.
These include glass, quartz, other ceramic materials, polymers,
metals, metal alloys, intermetallic compositions, woven or
non-woven fabrics, combinations of these, and the like. Stainless
steel is preferred. The support 22 desirably is cleaned prior to
use to remove contaminants, such as organic contaminants. A wide
variety of cleaning techniques may be used. As one example, plasma
cleaning, such as by using RF plasma, would be suitable to remove
organic contaminants from metal-containing supports. Other examples
of useful cleaning techniques include ion etching, wet chemical
bathing, and the like.
[0023] The barrier region 23 helps to isolate the photovoltaic
absorber region 20 from the support 22 to prevent contamination.
For instance, barrier region 23 can help to block the migration of
Fe and other constituents from a stainless steel support 22 into
the absorber region 20. The barrier region 23 also can protect the
support 22 against Se migration if Se is used in the formation of
the photovoltaic absorber region 20. Desirably, the barrier region
23 also helps to promote adhesion between the support 22 and
overlying layers of device 10. Barrier region 23 can be formed from
one or more of a wide range of materials. Exemplary materials
include Nb, Cr, and Al.sub.2O.sub.3, combinations of these, and the
like. A film comprising Nb desirably has a thickness of at least
about 30 nm, preferably at least about 50 nm, and more preferably
at least about 100 nm. The thickness of such a film desirably is
less than about 1000 nm, preferably less than about 400 nm and more
preferably less than about 300 nm. In one embodiment, a Nb barrier
having a thickness of about 150 nm would be suitable.
[0024] The backside electrical contact region 24 provides a
convenient way to electrically couple the device 10 to external
circuitry (not shown). The backside electrical contact region 24
also helps to isolate the photovoltaic absorber region 20 from the
support 22 to minimize cross-contamination. As is the case with any
of the regions of device 10, region 24 may be formed from a single
layer or multiple layer using 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 may be used in an illustrative
embodiment. Region 24 optionally may incorporate one or more agents
such as Na, Li, H, combinations of these, and the like. Na, for
instance, is believed to favorably impact the growth of crystalline
grains in the formation of the photovoltaic absorber region 20. It
also is believed that the Na acts as a Se flux and/or mobilizing
agent to facilitate the formation of high quality
chalcogen-containing photoabsorber materials. Na might also
contribute favorably to the electronic performance of the
chalcogen-containing photoabsorbing materials.
[0025] Materials such as Na, Li, and/or the like may be
incorporated into region 24 in a variety of ways. According to one
approach, a separate layer incorporating NaF, Na-doped metals, or
the like may be incorporated into region 24. This layer may exist
on the top of region 24, be buried at any other location in region
24, or mixed evenly within the layer. According to another
approach, a Na-containing target may be co-sputtered or otherwise
concurrently supplied with one or more electrically conductive
materials to incorporate Na into at least a portion of the
resultant sputtered region 24. For instance, a target containing 97
atomic percent Mo and 3 atomic percent Na can be sputtered to form
a suitable film having a thickness of at least about 30 nm,
preferably at least about 50 nm. Such a film desirably has a
thickness of less than about 2000 nm, preferably less than about
500 nm. In one embodiment, such a target is sputtered to provide a
film that is about 350 nm thick.
[0026] Referring again mainly to FIG. 1, chalcogenide-containing
photovoltaic absorber region 20 absorbs light energy, embodied in
the light rays 16 and then photovoltaically converts this light
energy into electric energy. Region 20 can be a single integral
layer as illustrated or can be formed from one or more layers.
[0027] The chalcogenide absorber region 20 preferably incorporates
at least one IB-IIIB-chalcogenide, such as IB-IIIB-selenides,
IB-IIIB-sulfides, and/or IB-IIIB-selenides-sulfides that include at
least one of copper, indium, and/or gallium. In many embodiments,
these materials are present in polycrystalline form. Some
embodiments include sulfides or selenides of copper and indium.
Additional embodiments include selenides or sulfides of copper,
indium, and 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, and copper indium gallium sulfide
selenides. In some embodiments, such chalcogenide materials
optionally may include aluminum, tellurium, and the like. Aluminum
could be used, for instance, instead of or in addition to gallium.
Such chalcogenide materials also may be doped with other materials,
such as Na, Li, H or the like, to enhance performance. In many
embodiments, CIGS materials have p-type characteristics.
[0028] Oxygen (O) is technically a chalcogen according to its
placement in the periodic table of the elements. However, oxygen is
deemed not to be a chalcogen for purposes of the present invention
inasmuch as oxygen does not contribute to photoabsorbing
functionality to the extent of the other chalcogens such as S
and/or Se. Even though oxygen does not promote photoabsorbing
functionality to the same degree and/or in the same manner as Se or
S, oxygen may still be incorporated into chalcogenide
photoabsorbing materials purposefully for other reasons such as to
contribute to electronic properties, as an artifact of manufacture,
and/or the like. Indeed, many chalcogen materials incorporate at
least some oxygen, e.g., from O atomic percent to about 5 atomic
percent based on the total composition of the region 20.
[0029] One preferred class of chalcogenide photoabsorbing 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)
wherein, if "a" is defined as 1, then: "(b+c+d)/a"=1 to 2.5,
preferably 1.05 to 1.65 "b" is 0 to 2, preferably 0.8 to 1.3 "c" is
0 to 0.5, preferably 0.05 to 0.35 d is 0 to 0.5, preferably 0.05 to
0.35, preferably d=0 "(w+x+y)" is 1 to 3, preferably 2 to 2.8
(note, (w+x+y)<2 for substoichiometric precursor films) "w" is 0
or more, preferably at least 1 and more preferably at least 2 to 3
"x" is 0 to 3, preferably 0 to 0.5 "y" is 0 to 3, preferably 0 to
0.5 "z" is 0 to 0.5, preferably 0.005 to 0.02
[0030] The copper indium selenides/sulfides and copper indium
gallium selenides/sulfides are preferred. Illustrative examples of
such photoelectronically active chalcogenide materials may, be
represented by, the formula
CuIn.sub.(1-x)Ga.sub.xSe.sub.(2-y)S.sub.y (B)
where x is 0 to 1 and y is 0 to 2. As measured and processed, such
films usually include additional In, Ga, Se, and/or S.
[0031] Advantageously, the chalcogen-containing, photoabsorbing
materials exhibit excellent cross-sections for light absorption
that allow region 20 to be very thin and flexible. In illustrative
embodiments, a typical absorber region 20 may have a thickness of
at least about 1 .mu.m, preferably at least about 1.5 .mu.m. Region
20 desirably has a thickness of less than about 5 .mu.m, preferably
less than about 3 .mu.m.
[0032] The principles of the present invention are used to form
high quality, chalcogenide-based photovoltaic materials for use in
region 20. These principles allow these materials to be formed at
low pressure in a manner that is compatible with continuous roll to
roll manufacturing strategies that occur under vacuum. The
methodologies of the invention include at least three main stages
that can be practiced sequentially or at least partially
co-currently with one or more of the other stages. These stages
include forming a precursor of the desired chalcogenide
photoabsorbing composition, forming a chalcogen-containing cap on
the precursor, and then converting the capped precursor into the
desired photoabsorbing material.
[0033] A preferred methodology incorporating at least these three
stages is schematically illustrated in FIG. 3. Referring now to
FIG. 3, an initial stage 100 involves preparing a precursor film
112. In many embodiments, this is accomplished by using one or more
targets to sputter the constituents onto substrate 18 to form
precursor film 112. Sputtering may occur in one step or in a
sequence of two or more steps. If multiple sputtering steps are
used, these may occur sequentially, concurrently, or in overlapping
fashion. The resultant precursor film structure may include one or
more layers. If the precursor film 112 has multiple layers, the
interfaces between layers may be distinct or can be graded
transitions. The constituents of precursor film 112 may, be
incorporated into the same and/or different layers.
[0034] For purposes of illustration, a single target 110 is used in
stage 100 to form a precursor film 112 with a single layer
structure. Generally, the precursor film includes the desired metal
constituents and optionally at least a portion of the desired
chalcogen content. Consequently, target 110 would have a
composition that is effective to deposit at least the metal
constituents of the desired region 20 in the desired proportions.
For instance, if the resultant region 20 (FIG. 1) is to include Cu,
In, and Ga as metal constituents, target 110 of this illustrative
embodiment generally would include copper, indium, and gallium in
proportions effective to form precursor film 112 including these
elements. One such suitable target 110 may include about 35 to
about 50 atomic percent Cu, about 50 to about 65 atomic percent In,
and about 0 to about 30 atomic percent Ga. Other targets having
other proportions of the constituents may also be used.
[0035] A specific embodiment of an exemplary target of this type
includes 45 atomic percent Cu, 42 atomic percent In, and 13 atomic
percent Ga. In one exemplary mode of practice suitable for
incorporation in a roll to roll manufacturing strategy that occurs
in its substantial entirety under vacuum, such a target can be used
to form a precursor film 112 including Cu, In, and Ga having a
thickness of at least about 0.75 .mu.m, preferably at least about
0.8 .mu.m. Such a film desirably has a thickness of less than about
2 .mu.m, preferably less than about 1 .mu.m. Such a film 112 in
many embodiments may have a target mass of about 65 mg to about 75
mg of CIG material per 4 inch.times.4 inch area of the substrate
18. The target containing 45:42:13 atomic percent C:In:Ga could be
pre-sputtered for a suitable time period, such as about 3 minutes,
to pre-condition the target.
[0036] The precursor film 112 may be formed at any suitable
pressure. For incorporation into a roll-to-roll process, a suitable
pressure may be at least about 1 millibar (mBar), preferably at
least about 3 mBar. Such a pressure desirably is less than about 10
mBar, preferably less than about 5 mBar. A pressure of about 4 mBar
would be suitable for instance. Sputtering desirably occurs in the
presence of a suitable sputtering gas supplied at a suitable flow
rate to establish the desired pressure. An exemplary sputtering gas
is argon. It is understood that the flow rate(s) used are a
function of a number of factors including pumping speed and the
configuration of the chamber. Sputtering occurs for a suitable time
period, such as for about 28 minutes at a suitable sputtering
power, such as about 125W to form a precursor film 112. Additional
constituents such as Na, Li, Te, and/or the like may also be
incorporated into precursor film 112 via sputtering or other
techniques as well. If sputtered, these can be incorporated into
the same target 110 or supplied from additional target(s) (not
shown). Optionally, such additional constituents or even a portion
of the metal constituents can be supplied via other sources, such
as by, sputtering in the presence of one or more suitable gases or
vapors.
[0037] According to alternative embodiments, precursor film 112
could be formed from multiple targets such as a pair of confocal
targets. Using this strategy, for instance, a CuGa target and an In
target could be used confocally to form a C:I:G precursor film.
Alternatively, the precursor film may be formed from multiple
targets by passing the substrate through a plurality of sputtering
zones.
[0038] Confocal target or other multiple-target strategies such as
this are also useful in some embodiments to pre-incorporate at
least a portion of the chalcogen content of the desired region 20
(FIG. 1) into at least a portion of the precursor film 112.
[0039] For example, according to this strategy, a precursor film
incorporating overall a substoichiometric amount of selenium (i.e.,
the film includes less than one chalcogen atom for each metal atom
included in the composition) is formed from confocal first and
second targets, wherein the first target includes the composition
Cu.sub.xSe.sub.y, wherein x is approximately 2 and y is
approximately 1, and the second target incorporates Cu, In, and Ga
according to the formula CuIn.sub.pGa.sub.(1-p), wherein p
desirably is in the range from about 0.5 to about 1. Details of
incorporating chalcogen into precursor film 112 are further
described in Assignee's co-pending U.S. Provisional Patent
Application Ser. No. 61/314,840, filed Mar. 17, 2010, titled
"CHALCOGENIDE-BASED MATERIALS AND IMPROVED METHODS OF MAKING SUCH
MATERIALS" by Gerbi et al the entirety of which is incorporated
herein by reference for all purposes. Multilayer embodiments of
precursor film 112 also are further described in U.S. Provisional
Patent Application Ser. No. 61/314,840, filed Mar. 17, 2010,
bearing Attorney Docket No. DOW0027/P1, in the names of Jennifer E.
Gerbi, Marc G. Langlois, Robert T. Nilsson.
[0040] Still referring to FIG. 3, a chalcogen-containing film also
referred to herein as a cap 114 is formed in stage 102 over the
precursor film 112. Desirably, the cap 114 is in the form of a
solid cap including one or more chalcogens. In preferred modes of
practice, the cap 114 incorporates Se, Te, and/or S. Se and/or S
are preferred. Se is most preferred. If both Se and S are used, the
atomic ratio of Se to S in illustrative embodiments may be in the
range from about 1000:1 to about 1000:100, preferably about 1000:10
to about 1000:50. The chalcogens can be present in cap 114 as
compounds and/or in elemental form. The elemental form is
preferred.
[0041] The amount of chalcogen(s) incorporated into the cap 114
relative to the stoichiometric amount needed to complete region 20
can vary over a wide range. However, if the amount of chalcogen(s)
in the cap is too low, lesser amounts of In and/or Se may be
incorporated into the resultant region 20 than might be desired.
Also, the reproducibility of the stage 102 may be less than
desired. Generally, it is desirable that the cap 114 includes at
least the stoichiometric amount (1.times.) of chalcogen(s) needed
to convert substantially the entirety of the precursor film 112 to
the desired tetragonal photoabsorbing phase, taking into account
chalcogen content that might already be present in the precursor
film 112. More preferably, it is desirable that at least 2.times.,
and more preferably at least about 5.times. the stoichiometric
amount of chalcogen(s) are incorporated into the cap 114.
[0042] From a theoretical perspective, there generally is no upper
limit on the amount of chalcogen(s) used in the cap 114. Indeed,
using greater stoichiometric excess is more desirable and provides
multiple benefits. First, using a greater excess tends to produce
more uniform chalcogenide photoabsorbing materials over time as the
process is carried out more consistently. Additionally, the
reproducibility of tetragonal formation is less sensitive to
temperature fluctuations in the course of subsequent
chalcogenization. However, as a practical matter, using excessively
greater amounts of chalcogen(s) in cap 114 than is required for
stoichiometry can be wasteful without providing sufficient
incremental benefit. Consequently, it is preferred that the cap
incorporates no more than about 100.times., preferably no more than
about 60.times., preferably no more than about 30.times., of the
stoichiometric amount of chalcogen(s). In illustrative embodiments,
using a cap with 1.0.times., 1.5.times., 2.0.times., 2.5.times.,
10.times., 25.times., or 50.times. of the stoichiometric amount of
chalcogen(s) would be suitable. The stoichiometric amount of
chalcogen can be calculated based upon the relative amounts of
metal constituents in the precursor.
[0043] Often, a cap 114 with a suitable stoichiometric excess of
chalcogen(s) has a thickness in the range from about 15 .mu.m to
about 20 .mu.m. The target mass desirably involves depositing an
amount of chalcogen(s) effective to provide the stoichiometric
quantity (1.times.) relative to the precursor or the desired excess
of chalcogen(s) (e.g., about 2.times. to about 50.times. in
illustrative embodiments) relative to the precursor.
[0044] The cap 114 may be formed by evaporation from a resistively
heated stainless steel boat or ceramic crucible. The pre-weighed
substrate is suspended facedown over the crucible. Based on
calibration, the amount of selenium needed to achieve the desired
cap is loaded into the crucible. The chamber is pumped to a base
pressure of less than about 10.sup.-4 mBar, and then the crucible
is heated to greater than about 300.degree. C. (based on a
temperature reading from a thermocouple immersed in the molten
chalcogen). The full selenium load is evaporated.
[0045] In some modes of practice, it may be desirable to form a cap
114 from multiple layers in view of factors including to enhance
uniformity, to accommodate equipment limitations, to prevent
radiative heating of the substrate over extended deposition times,
and/or the like. For example, in cases of larger caps, the crucible
may be loaded two or more times to form the cap in two or more
steps. To promote uniformity, it can be desirable to rotate the
workpiece between growths. For instance, if forming cap 114 from
two elemental layers, rotating the workpiece 180.degree. between
growths may be desirable.
[0046] In a further stage 104, the capped precursor film 108 is
subjected to chalcogenization conditions effective to convert the
precursor film 112 and cap 114 to the desired final form of region
20. This stage also may be referred to as a "post-chalcogenization"
treatment to connote that at least a portion of the treatment
occurs after precursor formation. For example, if Se is being
incorporated into the precursor, the stage 104 can be referred to
as a post-selenization treatment. Likewise, if S is being
incorporated in the precursor, the stage 104 can be referred to as
a post-sulfurization. Generally, this stage 104 occurs under
thermal conditions effective to also anneal the capped film 108,
thereby converting the precursor film 112 to the desired
tetragonal, chalcopyrite phase to the extent the capped precursor
does not already have that crystal structure at this stage.
Consequently, stage 104 also may be referred to as an annealing
treatment to recognize the conversion of the precursor film 112
into the desired chalcopyrite, tetragonal phase.
[0047] Chalcogenization generally occurs by positioning the capped
film 108 in a suitable processing chamber (not shown) in which the
capped film 108 is exposed to thermal conditions effective to
convert the precursor into the photoabsorbing region 20. The
chamber can be the same or different from the chamber(s) used to
form other layers of the capped film 108. In a continuous roll-to
roll process occurring under vacuum, the capped film 108 generally
is transferred from a capping station to one or more dedicated,
downstream stations at which chalcogenization occurs. The sample
temperature desirably is maintained at a suitably low temperature
during the transfer to avoid undue loss of In and/or Se. As
exemplary guidelines, the sample temperature may be about
150.degree. C. or less, more preferably about 100.degree. C. or
less during the transfer. In an exemplary mode of practice, a
transfer temperature of about 80.degree. C. would be suitable.
[0048] The temperature of the capped film 108 is then increased to
an annealing temperature effective to cause the desired conversion.
The rate at which the temperature is increased to the annealing
temperature can impact the incorporation of In and/or Se into the
resultant product. If the temperature ramp rate is too low, undue
loss of In and/or Se may occur. On the other hand, if the ramp rate
is too fast, undue loss of In, Se, and/or Ga could occur. Other
problems that may be encountered include pinholing of the final
film. It could also be possible that damaging microbursts could
result from volatilization of constituents of the capped precursor
film 108. Balancing these concerns, it is desirable that the
temperature ramp rate is at least about 1.degree. C./min,
preferably at least about 20.degree. C./Min, more preferably at
least about 30.degree. C./min, and most preferably at least about
60.degree. C./min. Additionally, the temperature ramp rate is
desirably less than about 500.degree. C./min, preferably less than
about 400.degree. C./min, and most preferably less than about
200.degree. C./min. In one illustrative embodiment, a ramp rate of
about 30.degree. C./min to about 60.degree. C./min would be
suitable. In another illustrative embodiment, a ramp rate of
100.degree. C./min to 150.degree. C./min would be suitable.
[0049] A wide range of annealing temperatures can be used. If the
annealing temperature is too low, poor incorporation Se could be
observed. Additionally, the conversion of the precursor film to the
desired tetragonal phase may be less complete than is desired due
to unreacted starting materials or formation of stable byproducts
including binary chalcogens. On the other hand, constituents of the
precursor film 112 and/or the substrate 18 could be unduly lost or
degraded if the annealing temperature is too high. Balancing these
concerns, the annealing temperature desirably is at least about
450.degree. C., preferably at least about 500.degree. C., and less
than about 600.degree. C., more preferably less than about
550.degree. C.
[0050] Significantly, preferred modes of practice allow annealing
to occur at atypically low temperatures. Thus, in another example,
an annealing temperature of about 350.degree. C. would be suitable
such as when annealing with a Se cap in the presence of a chalcogen
containing gas such as H.sub.2Se.
[0051] Annealing generally occurs for a time sufficient to
chalcogenize the film and to convert the film to the tetragonal
phase to the desired degree. In many embodiments, annealing occurs
for a time period of at least about 1 minute, preferably at least
about 5 minutes, but less than about 36 hours, preferably less than
about 3 hours.
[0052] Desirably, a suitable control strategy is implemented in
order to maintain the annealing temperature at a steady level
during the course of annealing. Feedback control using PID
strategies would be suitable.
[0053] Advantageously, stage 104 can occur in a low-pressure regime
that is compatible with continuous roll-to-roll processes occurring
under vacuum. It is quite significant to find process conditions
that work under such low pressure inasmuch as process strategies
that work at higher pressures do not necessarily translate to
vacuum processes. Generally, the risk of In loss increases if the
pressure at stage 104 is too low. On the other hand, if the
pressure is too high, it could be harder to practically and
economically integrate stage 104 into a continuous roll-to-roll
process occurring under vacuum. Balancing these concerns, stage 104
desirably occurs at a pressure no greater than about 300 millitorr
(4e.sup.-1 mBar), preferably no greater than about 100 millitorr
(1e.sup.-1 mBar), more preferably no greater than about 50
millitorr (6e.sup.2 mBar), and most preferably no greater than
about 15 millitorr (2e.sup.-2 mBar). Preferably the pressure is at
least about 0.1 millitorr (1e.sup.-1 mBar), more preferably at
least about 1 millitorr mBar), and most preferably at least about 3
millitorr (1e.sup.-3 mBar). In an exemplary embodiment, a pressure
of about 5 millitorr (7e.sup.-3 mBar) would be suitable. In another
exemplary embodiment, a pressure of about 10 millitorr (1e.sup.-2
mBar) would be suitable.
[0054] One or more gases can be introduced into the reaction vessel
in order to help establish the desired pressure during the course
of chalcogenization and annealing. A wide variety of strategies can
be used to accomplish this. According to one strategy, annealing
and chalcogenization occur in the presence of a flow of an inert
gas such as Ar, or the like, introduced into the chamber. For
instance, annealing at 505.degree. C. in the presence of 136 sccm
Ar would be suitable. In a typical processing chamber, this flow
rate of Ar corresponds to a pressure of about 10.sup.-3 millibarr.
According to another mode of practice, annealing and
chalcogenization occur in the presence of one or more gases or
vapors incorporating one or more chalcogens introduced into the
chamber. These gases may serve as additional sources of chalcogen
to be incorporated into region 20. Exemplary gases of this type
include H.sub.2S, H.sub.2Se, mono or dialkylated S, mono or
dialkylated Se, combinations of these and the like. Combinations of
these strategies also may be used. For instance, annealing can
occur in the presence of a combination of one or more inert gases
and one or more chalcogen-containing gases.
[0055] Gases containing Se require more careful handling than gases
including S. Yet, it is still desirable to incorporate selenium
into region 20. Accordingly, a preferred mode of practice involves
capping the precursor film 112 with a Se cap 114 while annealing in
the presence of a sulfur containing gas such as H.sub.2S. This
approach is believed to yield tetraganol CIGSS (Cu:In:Ga:Se:S) and
offers significant advantages. First, the Se cap provides
sufficient Se to selenize the film while the H.sub.2S supplies
sulfur to sulfurize the film, increasing bandgap. Second, the
sulfur-containing gas is much easier to handle than a Se-containing
gas. Third, conversion of the capped precursor to CIGSS using this
strategy has been observed to occur at lower temperatures, e.g., as
low as 350.degree. C.
[0056] One factor impacting chalcogenization in some embodiments
may be whether the reactor walls are chilled or cooled. As
non-limiting examples, either hot-walled or cold-walled chambers
can be used to accomplish selenization. Cold-walled reactors are
desirably used when the stoichiometric excess of chalcogen(s) in a
cap is greater than about 5.times., preferably greater than about
7.times., more preferably greater than about 10.times.. Cooling of
at least a portion of the reactor walls can be accomplished in any
desired fashion. Water-cooling is one suitable technique. In a
typical mode of practice, the cooled walls may be maintained at a
temperature in the range of about 10 to about 25.degree. C.,
preferably 15 to about 22.degree. C. As the capped sample is
heated, at least a portion of the selenium in the cap is expected
to evaporate. Evaporated selenium generally would not be expected
to be incorporated into the final film in a chamber with chilled
walls, but rather would be generally expected to be plated out on
the cooled walls of the chamber. A benefit of using cold-wall
reactors is that the excess selenium is condensed onto the
chilled-walls thereby preventing it from entering the pumping
system or other system components and causing corrosion or damage
to them.
[0057] Hot-walled reactors are desirably used when the
stoichiometric excess (if any) of chalcogen(s) in the cap is below
about 5.times., preferably below about 4.times., more preferably
below about 3.times.. In a hot wall reactor, at least some of the
reactor walls are at temperatures above the sublimation/evaporation
points of the selenium. This is expected to create a dynamic
selenium annealing atmosphere. Selenium that evaporates from the
surface of the capped precursor generally would not be expected to
plate against the heated walls of the chamber but instead may
contribute to a gaseous selenium environment which then contributes
to the evaporation vs. reaction dynamics of the selenium at the
CIGS surface during formation.
[0058] Annealing and chalcogenization convert the capped precursor
film 108 into the desired photoabsorber region 20. After this stage
104 is completed, the workpiece 108 can be cooled down, or allowed
to cool down, and then subjected to further processing to complete
device 10. Generally, this involves cooling the sample down to
about 200.degree. C. It has been discovered that if the cooling
rate is too rapid relative to Se in the surrounding atmosphere,
elemental Se could condense at the surface of region 20, which
could unduly impair device function. On the other hand, if the
substrate cools too quickly relative to Se in the atmosphere, Se
can evolve from the surface of the substrate which may unduly
impair device function. Balancing these concerns, it is preferred
to allow the workpiece to cool passively before transferring it
from the chalcogenization chamber.
[0059] The cap 114 and the temperature ramp to the annealing
temperature singly or in combination can help allow
chalcogenization and annealing to occur at low pressure in a manner
that is compatible with roll-to-roll manufacturing strategies
occurring under vacuum. In the absence of cap 114 and/or by
practicing ramp rates that are too slow or too fast, subsequent
chalcogenization is much more difficult to carry out effectively.
Indium incorporation is poor, and the resultant region 20 may be
severely deficient with respect to In if chalcogenization and
annealing occur at low pressure, e.g., at a pressure below about
300 millitorr, preferably below about 100 millitorr, more
preferably below about 50 millitorr. This is believed to be
attributable at least in part to the formation of volatile
Indium-containing species, such as InSe and/or InS, that easily
volatilize and are lost at low pressure. Additionally, Se
incorporation also is poor, so that the resultant region 20
independently may be deficient with respect to Se.
[0060] In contrast, use of cap 114, optionally with an appropriate
temperature ramp rate, are features that singly and in combination
enable chalcogenization and annealing of CIGS precursor films to
occur at low pressures feasible for continuous vacuum roll to roll
processes. By using a cap and/or appropriate temperature ramp rate
under low pressure chalcogenization conditions, In loss is
significantly reduced, and incorporation of both In and Se into the
resultant region 20 is greatly improved.
[0061] For example, in the absence of a cap 114, it was found that
carrying out chalcogenization of a C:I:G film precursor at
350.degree. C. in the presence of H.sub.2Se or H.sub.2S gas at 5
milliTorr (7e.sup.-3 mBar) can result in In losses of almost 50%
per analysis of Cu/In ratios using ICP (inductively, coupled plasma
optical emission spectroscopy) techniques. Carrying out
chalcogenization at 500.degree. C. on a precursor film with no
chalcogen cap has been observed to result in a complete loss of In
per ICP analysis.
[0062] In contrast to this data, capping a C:I:G precursor film
(i.e., a precursor film containing Cu, In, and Ga) with elemental
Se prior to annealing in H.sub.2Se or H.sub.2S gas substantially
reduced the loss of Indium. For example, a precursor film with an
initial In/Cu ratio of about 1.3 capped with 2 times the
stoichiometric amount of elemental Se required to form a CIGS
photoabsorbing material was annealed at 350.degree. C. in 5
milliTorr H.sub.2Se. The resultant CIGS material had an In/Cu ratio
of greater than 1 per ICP analysis. The same process carried out at
10 milliTorr produced a CIGS material with an In/Cu ratio greater
than 0.9. When the same processes were carried out at 5 and 10
millitorr, respectively, using a 25.times. cap of elemental
selenium, the In/Cu ratio was greater than 1 for the resultant
films. Similar data for In retention was obtained when H.sub.2S was
used to carry out chalcogenization of precursor films containing
Cu, In, and Ga capped with 50.times. elemental selenium. Annealing
occurred in 5 millitorr H.sub.2S at 150.degree. C., 350.degree. C.,
and 500.degree. C. In all samples, the In/Cu ratio of the resultant
CIGS material per ICP analysis was greater than 1.2.
[0063] Data obtained from annealing the precursor films containing
Cu, In, and Ga, and capped with 50.times. selenium also showed that
the annealing temperature profile impacts the amount of chalcogen
that remains on or integrates into the film 112. For example, when
annealed at 150.degree. C., the resultant region 20 had a Se/Cu
ratio of only about 0.5. This is indicative of poor Se
incorporation for a Se cap that included 50.times. times the
stoichiometric amount of Se needed to convert the precursor into a
photoabsorbing CIGS material. An annealing temperature of
150.degree. C. is generally too low to successfully convert the
precursor film to the desired tetragonal CIGS material in any
event. In contrast, when annealing under otherwise identical
conditions at 350.degree. C., Se incorporation improves. The
resultant region 20 showed a Se/Cu ratio of 1.6, and XRD analysis
indicated the presence of the desired tetragonal CIGS phase. Se
incorporation improves further when annealing under otherwise
identical conditions at 500.degree. C. The resultant region 20
showed a Se/Cu ratio of 2.5. The region 20 was in the form of the
pure, tetragonal CIGS phase.
[0064] After forming photoabsorbing region 20 on substrate 18,
additional layers and features can be formed to complete the device
as shown in FIG. 1. For example, buffer region 28 generally
comprises an n-type semiconductor material with a suitable band gap
to help form a p-n junction between the absorber region 20 and the
buffer region 28. An optional window region 26 also may be present.
Optional window region 26 can help to protect against shunts.
Window region 26 also may protect buffer region 28 during
subsequent deposition of the transparent conductive layer 30. Each
of these regions is shown as a single integral layer, but can be a
single integral layer as illustrated or can be formed from one or
more layers.
[0065] One or more electrical conductors are incorporated into the
device 10 for the collection of current generated by the absorber
region 20. A wide range of electrical conductors may be used.
Generally, electrical conductors are included on both the backside
and light incident side of the absorber region 20 in order to
complete the desired electric circuit. On the backside, for
example, backside electrical contact region 24 provides a backside
electrical contact in representative embodiments. On the light
incident side of absorber region 20 in representative embodiments,
device 10 incorporates a transparent conductive layer 30 and
collection grid 36. Optionally an electrically conductive ribbon
(not shown) may also be used to electrically couple collection grid
36 to external electrical connections.
[0066] A protective barrier system 40 desirably is provided. The
protective barrier system 40 is positioned over the electronic grid
36 and helps to isolate and protect the device 10 from the
environment, including protection against water degradation. The
barrier system 40 optionally also may incorporate elastomeric
features that help to reduce the risk of damage to device 10 due to
delamination stresses, such as might be caused by thermal cycling
and or localized stress, such as might be caused by impact from
hail and or localized point load from the weight of an installer or
dropped tools during installation.
[0067] Additional details and fabrication strategies for making
layers and features 26, 28, 30, 36, and 40 are described in U.S.
Provisional Patent Application Ser. No. 61/258,416, filed Nov. 5,
2009, by Bryden et al., entitled MANUFACTURE OF N-TYPE CHALCOGENIDE
COMPOSTIONS AND THEIR USES IN PHOTOVOLTAIC DEVICES; U.S.
Provisional Patent Application Ser. No. 61/294,878, filed Jan. 14,
2010, by Elowe et al., entitled MOISTURE RESISTANT PHOTOVOLTAIC
DEVICES WITH EXPOSED CONDUCTIVE GRID; U.S. Provisional Patent
Application Ser. No. 61/292,646, filed Jan. 6, 2010, by Papa et
al., entitled MOISTURE RESISTANT PHOTOVOLTAIC DEVICES WITH
ELASTOMERIC, POLYSILOXANE PROTECTION LAYER; U.S. Provisional Patent
Application Ser. No. 61/302,667, filed Feb. 9, 2010, by Feist et
al., entitled PHOTOVOLTAIC DEVICE WITH TRANSPARENT, CONDUCTIVE
BARRIER LAYER; and U.S. Provisional Patent Application Ser. No.
61/302,687, filed Feb. 9, 2010, by DeGroot et al., entitled
MOISTURE RESISTANT PHOTOVOLTAIC DEVICES WITH IMPROVED ADHESION OF
BARRIER FILM, each of which is independently incorporated herein by
reference for all purposes in its respective entirety.
[0068] The present invention will now be described with reference
to the following illustrative examples.
EXAMPLES
Example 1
Fabrication of CIGS Solar Cell in a Roll-to-Roll Tool
[0069] Substrate preparation: [0070] A roll of 430 series stainless
steel is loaded into a continuous roll-to-roll vacuum sputtering
system. This active side of the web is first cleaned with ion
etching and then is coated by magnetron sputtering with a Nb
barrier layer, a dual layer of Mo for a back contact and a NaF
layer to act as a sodium source. The back side of the web is coated
with Cr as an adhesion layer and Mo to act as a sacrificial layer
during the selenization process. [0071] Target arrangement [0072]
The web is then translated into a Cu:In:Ga precursor sputtering
chamber. Seven pulsed DC magnetrons are arranged in four zones.
Each zone has individual gas control. The web is transported
sequentially through the four zones. Each of the first, third, and
fourth zones has 2 magnetrons. The second zone has a single
magnetron. In this example, all magnetrons are adjusted to sputter
parallel to the web. The web moves from magnetron number 1 through
7 where magnetrons 1 (Zone 1), 2 (Zone 1), 3 (Zone 2), 5 (Zone 3),
and 7 (Zone 4) have targets of composition 45:32:13 Cu:In:Ga and
magnetrons 4 (Zone 3) and 6 (Zone 4) have compositions of 35:46:20
Cu:In:Ga. In stationary tests where the web is not moving, the
paired targets show a .about.15% overlap in the sputter deposition
while the four zones are completely, separate. [0073] Deposition of
CIGS Precursor [0074] The precursor film containing Cu, In, and Ga
(CIG film) is sputtered between 6e-3 to 8e-3 mBar (4.5 to 6 mTorr)
in Argon with a maximum web temperature of .about.80.degree. C. The
flow rates of the Argon and throttling of the pumping system are
used to maintain the pressure range in each individual sputter
zone. Depending upon the configuration of each zone, it may be
desirable to adjust flow rates, pump settings, or the like, in
order to maintain desired pressure(s) in the zones. In this example
the flow rates (throttling) Ar of the individual zones are 380 sccm
(90.degree. throttle or full open) in zone 1, 600 sccm (25.degree.
open throttle) in zone 2, 650 sccm (25.degree. throttle) in zone 3
and 400 sccm (90.degree. throttle or open) in zone 4. The targets
are sputtered at 600 watts with no pulsing (straight DC). The final
film stoichiometric ratio for the precursors ranges from In/Cu
0.95-1.01 and Ga/Cu 0.32-0.34, as measured by ICP on representative
calibrations runs. [0075] Cap formation on precursor [0076] The web
carrying the CIG precursor film is translated into a selenium
evaporation unit for application of a selenium cap. The selenium
evaporation unit includes a stainless steel boat with a effusion
slit at the top with the slit being .about.1-2 cm from the
CIG-coated web face. The web temperature is not actively
controlled. The slit has enough width such that the unit can be
considered open-source configuration (i.e. no pressure of selenium
vapor is building up inside the unit). The selenium melt is heated
to 300-305.degree. C. In this experiment, ICP data shows an average
Se/Cu ratio for the cap is 14 (i.e. 7.times. stoichiometric excess
of selenium) with deviations in Se/Cu from 11 to 17 (6.5.times. to
8.5.times. stoichiometric excess of selenium) The resultant product
is a web carrying a CIG-Se stack. [0077] Annealing [0078] After
application of the selenium cap, the resultant web carrying the
CIG-Se stack is translated into a hot-wall oven. Within this oven,
the pressure is estimated to be 1-10e-3 mBar (0.75 to 7.5 mTorr)
based upon a pressure gauge outside the furnace zone but in the
same chamber. Adjacent chamber pressures are maintained at 1e-3
mBar to 8 e-3 mBar (0.75 to 6 mTorr). The heating zones are set to
achieve a maximum ramping rate of .about.60.degree. C./min to an
average web temperature of 580-585.degree. C. This temperature
range is estimated based on previous calibration of the system and
not actively measured due to the dynamic configuration intrinsic to
the continuous roll-to-roll process. The web is maintained at an
average temperature of 580-585.degree. C. for .about.20 minutes
before being cooled to .about.425.degree. C. at 60.degree. C./min
and then further to <300.degree. C. at a reduced rate of
20.degree. C./min. [0079] Data [0080] The completed CIGS films are
then transferred into additional process chambers (all within the
same vacuum system) for deposition of buffer and window layers
before being wound onto a final core. After removal of the core
from the continuous roll-to-roll system, the CIGS films are
analyzed by ICP. Final In/Cu ratios ranged from 0.86 to 0.97 while
Ga/Cu range from 0.27 to 0.33 giving final III/Cu ratios from 1.13
to 1.30. Se/Cu ratios ranged from 2.48 to 2.91--as sample
preparation for the ICP digestion is not specific to the active
side vs. the back side, it is likely that some of the
stoichiometric excess of selenium is not in the CIGS film but
rather in the form of elemental or reacted coatings on the back
side of the substrate. Upon application of a Ni--Ag grid and
mechanical scribing to form a 0.47 cm2 test cell, photoactivity of
the material was demonstrated with efficiencies typically ranging
from 5-8%.
Example 2
Fabrication of Tetragonal CIGS in the Presence of H.sub.2S Gas
(H.sub.2S) on a Cluster Tool at 6.6e-3 mBar and 350 C
[0080] [0081] Substrate preparation: [0082] A stainless steel
coupon is first cleaned by sonication in solvent baths and dried
with dry N.sub.2 gas and loaded into a multi-chamber vacuum
deposition system. The face of the substrate is further cleaned
using 300W RF plasma etching in Argon. The substrate is then
transferred under vacuum to a second chamber where a niobium
adhesion layer and molybdenum back contact layer are deposited by
sputtering. The sample is removed from vacuum. [0083] Targets
[0084] The substrate is later introduced into a precursor
sputtering chamber comprising a single, commercially available
Cu:In:Ga alloy target tilted off-axis with respect to the substrate
to give a more uniform coating thickness. The target is 45:42:13
Cu:In:Ga. The sample is rotated during deposition. [0085]
Deposition conditions The CIG precursor film is sputtered at 75
Watts, in .about.4e-3 mBar in Argon at ambient temperature. The
final film stoichiometric ratio for the precursors ranges from
In/Cu 0.90-1.3 and Ga/Cu 0.27-0.30, as measured by ICP on
representative calibrations runs. [0086] Cap formation on precursor
[0087] The substrate bearing the CIG film is removed from vacuum
and loaded into a selenium evaporation chamber for application of a
selenium cap. The selenium evaporation unit includes an open-top
ceramic cup nested in a resistive heating element. The substrate is
mounted face-down over the cup on a metal frame, and 22 g of shot
is loaded to give a 25.times. cap. The system is evacuated to
<5e-6 Torr (7e.sup.-6 mBar). The selenium crucible is heated to
>300.degree. C. for sufficient time to allow the full aliquot of
shot to be evaporated. The system is allowed to cool and then
vented for loading of a second 22 g aliquot of selenium shot. The
sample is rotated to improve film thickness uniformity before
evaporation of the second full aliquot. The process yields a
50.times. cap. [0088] Anneal conditions. [0089] After application
of the selenium cap, the substrate bearing the CIG-Se stack is
removed from vacuum and transferred to a selenization chamber. In
this chamber, the substrate is suspended face down and heated from
the back-side with a radiative graphite element. The system has
water-cooled walls (<25.degree. C. recirculating loop). The base
pressure is 3e-7 mBar prior to the run. The pressure in the system
is increased to 6.6e-3 mBar using 100% H.sub.2S gas prior to
heating. The substrate ramped up at 60.degree. C./min to about
400.degree. C. and then at about 3-5.degree. C./min to a final
temperature >500.degree. C. (estimated to be about 515.degree.
C.) by setting the power supply for the graphite heater at a
previously determined power duty cycle value. The temperature of
the substrate is maintained at >500.degree. C. for 30 minutes at
which time the power supply is turned off. The resulting film is
allowed to cooled to <80.degree. C. before transferring to a
vacuum load-lock chamber for removal. [0090] Data. [0091] The
substrate bearing the completed CIGS film is removed from the
system and analyzed by ICP for elemental composition. The In/Cu
ratio of the film is 1.3 ((In+Ga)/Cu 1.6) while the Se/Cu is 1.7.
Sulfur is not detected in the film by ICP. Analysis of the X-ray
diffraction pattern of the film shows tetragonal CIGS, InSe, Cu,
Mo, Nb, and Fe.
Example 3
Formation of CIGS in H.sub.2Se at 10 mT, 25.times. cap, 350 C
[0091] [0092] Substrate preparation [0093] The substrate is
prepared as in Example 2. [0094] Target arrangement. [0095] Targets
are used according to Example 2. [0096] Deposition conditions
[0097] Deposition of a CIG precursor on the substrate is carried
out as in Example 2. [0098] Description of cap formation on
precursor [0099] A Se cap is formed on the CIG precursor as in
Example 2. [0100] Anneal conditions. [0101] After application of
the selenium cap, the substrate bearing the CIG-Se stack is removed
from vacuum and transferred to a selenization chamber. In this
chamber, the substrate is suspended face down and heated from the
back-side with a radiative graphite element. The system has
water-cooled walls (<25.degree. C. recirculating loop). The base
pressure is 3e-7 mBar prior to the run. The pressure in the system
is increased to 1.4e-2 mBar using 100% H.sub.2Se gas prior to
heating. The substrate ramped up to >350.degree. C. at
20.degree. C./min by setting the power supply for the graphite
heater at a previously determined power duty cycle value. The
temperature of the substrate is maintained at >300.degree. C.
for 20 minutes at which time the power supply is turned off. The
resulting film is allowed to cool to <80.degree. C. before
transferring to a vacuum load-lock chamber for removal. [0102]
Data. [0103] The substrate bearing the completed CIGS film is
removed from the system and analyzed by ICP for elemental
composition. The In/Cu ratio of the film is 1.1 ((In +Ga)/Cu 1.4)
while the Se/Cu is 1.31. Analysis of the X-ray diffraction pattern
of the film shows tetragonal CIGS, InSe, Cu, Mo, Nb, and Fe.
Example 4a
Annealing of CIG Film in Argon Demonstrating Baseline for Annealing
CIG in Vacuum
[0103] [0104] Substrate Preparation. [0105] As in example 2 [0106]
Nature of target/targets--arrangement [0107] As in example 2 [0108]
Deposition conditions [0109] As in example 2 [0110] Description of
cap formation on precursor [0111] No cap is used. [0112] Anneal
conditions. [0113] The substrate bearing the sputtered CIG
precursor is transferred to a chalcogenzation chamber under vacuum.
In this chamber, the substrate is suspended face down and heated
from the back-side with a radiative graphite element. The system
has water-cooled walls (<25.degree. C. recirculating loop). The
base pressure is 6e-7 mBar (4.5e.sup.4 mTorr) prior to the run. The
pressure in the system is increased to 6.6e-3 mBar (5 mT) using
100% Ar gas prior to heating. The substrate ramped up to
>350.degree. C. (at 30.degree. C./min (highest temperature
estimated 400.degree. C.) by setting the power supply for the
graphite heater at a previously determined power duty cycle value.
The temperature of the substrate is maintained at >350.degree.
C. for 30 minutes at which time the power supply is turned off. The
resulting film is cooled to <80.degree. C. before transferring
to a vacuum load-lock chamber for removal. [0114] Data. [0115] The
substrate bearing the completed CIGS film is removed from the
system and analyzed by ICP for elemental composition. The In/Cu
ratio of the film is 1.05 while the Se/Cu was 0.04.
Example 4b
Annealing of CIG Film in H.sub.2Se Demonstrating Loss of Indium
from Surface when Annealing in Toxic Gas
[0115] [0116] Substrate preparation [0117] As in example 2. [0118]
Nature of target/targets--arrangement [0119] As in example 2.
[0120] Deposition conditions [0121] As in example 2. [0122]
Description of cap formation on precursor [0123] No cap is used.
[0124] Anneal conditions--pressure, temperature ramp [0125] The
substrate bearing the sputtered CIG precursor material is
transferred to a chalcogenzation chamber under vacuum. In this
chamber, the substrate is suspended face down and heated from the
back-side with a radiative graphite element. The system has
water-cooled walls (<25.degree. C. recirculating loop). The base
pressure is 6e-7 mBar (4.5e.sup.-4 mT) prior to the run. The
pressure in the system is increased to 6.6e-3 mBar (5 mT) using
100% H.sub.2Se gas prior to heating. The substrate ramped up to
>350.degree. C. at 30.degree. C./min (highest temperature
estimated to be about 400.degree. C.) by setting the power supply
for the graphite heater at a previously determined power duty cycle
value. The temperature of the substrate is maintained at
>350.degree. C. for 30 minutes at which time the power supply is
turned off. The resulting film is allowed to cool to <80.degree.
C. before transferring to a vacuum load-lock chamber for removal.
[0126] Data. [0127] The substrate bearing the completed CIGS film
is removed from the system and analyzed by ICP for elemental
composition. The In/Cu ratio of the film is 0:61 while the Se/Cu is
0.12.
Example 4c
Annealing of CIG Film w/2.times. cap in H.sub.2Se Demonstrating
Improved Indium Retention When Annealing a Film with a 2.times. Cap
but No CIGS Formation Yet
[0127] [0128] Substrate preparation. [0129] As in example 2. [0130]
Nature of target/targets--arrangement [0131] As in example 2.
[0132] Deposition conditions [0133] As in example 2. [0134]
Description of cap formation on precursor [0135] The substrate
bearing the CIG precursor film is removed from vacuum and loaded
into a selenium evaporation chamber for application of the selenium
cap. The selenium evaporation unit includes an open-top ceramic cup
nested in a resistive heating element. The substrate is mounted
face-down over the cup on a metal frame and 1.2 g of shot is loaded
to give a 1.times. cap. The system is evacuated to 2e-6 Torr
(3e.sup.-6 mBar). The selenium crucible is heated to 360.degree. C.
for sufficient time to allow the full aliquot of shot to be
evaporated. The system is allowed to cool and then vented for
loading of a second 1.2 g aliquot of selenium shot and rotation of
the substrate to improve uniformity of the selenium film thickness.
The evaporation process is repeated (base pressure 2e-6 Torr
(3e.sup.-6mBar), maximum temp 331.degree. C.) to yield a 2.times.
cap. [0136] Anneal conditions--pressure, temperature ramp [0137]
The substrate bearing the CIG/Se stack is transferred to a
chalcogenzation chamber under vacuum. In this chamber, the
substrate is suspended face down and heated from the back-side with
a radiative graphite element. The system has water-cooled walls
(<25.degree. C. recirculating loop). The base pressure is 6e-7
mBar (4.5e.sup.-4 mTorr) prior to the run. The pressure in the
system is increased to 6.6e-3 mBar (5 mT) using 100% H.sub.2Se gas
prior to heating. The substrate ramped up to >350.degree. C. (at
30.degree. C./min (highest temperature estimated to be about
400.degree. C.) by setting the power supply for the graphite heater
at a previously determined power duty, cycle value. The temperature
of the substrate is maintained at >350.degree. C. for 30 minutes
at which time the power supply is turned off. The resulting film is
allowed to cool to <80.degree. C. before transferring to a
vacuum load-lock chamber for removal. [0138] Data. [0139] The
substrate bearing the completed CIGS film is removed from the
system and analyzed by ICP for elemental composition. The In/Cu
ratio of the film is 1.02 while the Se/Cu is 037. X-ray diffraction
analysis of this film included peaks for a cubic CIG phase, InSe,
In.sub.4Se.sub.3, In, and Cu.
Example 5
[0140] A sample is provided including a CIG precursor film formed
on a substrate. The precursor film is capped with 25.times. Se.
Thermal annealing of the sample takes place in a vacuum quartz tube
oven (VQTO). The oven includes an 8'' diameter.times.54'' long
quartz tube inserted in a 38'' long tube furnace. The furnace has
three heat zones (left, right, and center) that are individually
controlled by Watlow controllers. The ends of the tubes have
stainless steel caps that are sealed to the tube ends via o-rings.
A vacuum rough pump in conjunction with a vacuum turbo pump provide
pressures measured down to 0.1 millitorr (1.3e-4 mBar). A mass flow
controller (MFC) connected to an argon gas cylinder provides
pressure control between 0.1 and 50 millitorr (1.3e-4 to 6.7 e-2
mBar). A liquid nitrogen trap located between the turbo pump and
the tube traps selenium vapor. Pressure is measured with a Baratron
gauge at a point along a 2-inch diameter line between the quartz
tube and the selenium trap.
[0141] The configuration includes a stainless steel carrier rack
that holds multiple samples, although only a single sample is used
in each run of this example. The rack can be inserted from the cold
end of the tube to the heated zone via a hook coupled magnetically
to the outside of the tube. Conversely, the sample can be removed
to cool rapidly with the same device. Two flexible thermocouples
are placed directly in contact with the backside of the sample for
accurate temperature monitoring. Static thermocouples are also
located at various spots in the tube oven to give the temperature
profile of the oven. All thermocouples are type "K". Controller and
static oven temperatures and pressure are recorded on a computer
using Agilent data collecting software. Temperatures connected
directly to the samples are recorded using an Omega 5309 system.
Data from these two sources are then combined into an Excel
spreadsheet.
[0142] In this example, the sample bearing the CIG-Se stack is
placed on a stainless steel carrier, which is then placed, in the
center of the oven. With the mass flow controller set to zero flow,
the endplate is attached and the tube is evacuated via the rough
and turbo pump to about 0.1 millitorr. The furnace temperature
setpoints are entered into the controllers and the recorders are
simultaneously started. The controller power to the heaters is
turned on and the tube is heated.
[0143] The pressure is maintained at about 0.1 mtorr. The substrate
is heated at a rate of about 20 C/min. to 520 C for 35 min. then
the carrier with the samples is removed to the cool end of the tube
outside the furnace and the power turned off. The sample is cooled
overnight to room temperature.
[0144] The efficiencies of four resultant CIGS cells on the
substrate are measured, and these give efficiencies of 7.12, 6.54,
5.72, and 6.85%.
Example 6
[0145] An additional sample is processed identically as in example
5, except that the capped sample is heated at a rate of 5 C/min to
the annealing temperature.
[0146] The efficiencies of four resultant CIGS cells on the
substrate are measured, and these give efficiencies of 6.84, 6.54,
6.62, and 6.89%.
[0147] Other embodiments of this invention will be apparent to
those skilled in the art upon consideration of this specification
or from practice of the invention disclosed herein. Various
omissions, modifications, and changes to the principles and
embodiments described herein may, be made by one skilled in the art
without departing from the true scope and spirit of the invention
which is indicated by the following claims.
* * * * *