U.S. patent application number 13/347540 was filed with the patent office on 2012-11-15 for electroplating method for depositing continuous thin layers of indium or gallium rich materials.
This patent application is currently assigned to SoloPower, Inc.. Invention is credited to Serdar Aksu, Bulent M. Basol, Jiaxiong Wang.
Application Number | 20120288986 13/347540 |
Document ID | / |
Family ID | 41430346 |
Filed Date | 2012-11-15 |
United States Patent
Application |
20120288986 |
Kind Code |
A1 |
Aksu; Serdar ; et
al. |
November 15, 2012 |
ELECTROPLATING METHOD FOR DEPOSITING CONTINUOUS THIN LAYERS OF
INDIUM OR GALLIUM RICH MATERIALS
Abstract
An electrochemical deposition method to form uniform and
continuous Group IIIA material rich thin films with repeatability
is provided. Such thin films are used in fabrication of
semiconductor and electronic devices such as thin film solar cells.
In one embodiment, the Group IIIA material rich thin film is
deposited on an interlayer that includes 20-90 molar percent of at
least one of In and Ga and at least 10 molar percent of an additive
material including one of Cu, Se, Te, Ag and S. The thickness of
the interlayer is adapted to be less than or equal to about 20% of
the thickness of the Group IIIA material rich thin film.
Inventors: |
Aksu; Serdar; (Santa Clara,
CA) ; Wang; Jiaxiong; (Castro Valley, CA) ;
Basol; Bulent M.; (Manhattan Beach, CA) |
Assignee: |
SoloPower, Inc.
San Jose
CA
|
Family ID: |
41430346 |
Appl. No.: |
13/347540 |
Filed: |
January 10, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12143609 |
Jun 20, 2008 |
8092667 |
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13347540 |
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Current U.S.
Class: |
438/95 ;
257/E31.008 |
Current CPC
Class: |
C25D 3/56 20130101; C25D
5/10 20130101; Y02P 70/521 20151101; Y02P 70/50 20151101; Y02E
10/541 20130101; H01L 31/0322 20130101; C25D 3/54 20130101; H01L
31/03923 20130101; C25D 7/126 20130101 |
Class at
Publication: |
438/95 ;
257/E31.008 |
International
Class: |
H01L 31/18 20060101
H01L031/18 |
Claims
1. A method of electrodepositing a Group IIIA material rich thin
film over a surface of a base for manufacturing solar cell
precursors, the method comprising: electrodepositing an interlayer
over the surface of the base, wherein the interlayer comprises a
predetermined molar percent of at least one of In and Ga and at
least 10 molar percent of an additive material including one of Cu,
Se, Te, Ag and S, wherein the predetermined molar percent is in the
range of 20-90 percent; and electrodepositing the Group IIIA
material rich thin film on the interlayer to a predetermined
thickness, wherein the Group IIIA material rich thin film is one of
a substantially pure In film, a substantially pure Ga film and a
substantially pure In--Ga alloy, wherein the predetermined
thickness of the Group IIIA material rich thin film is less than
700 nm, and wherein the thickness of the interlayer is less than or
equal to 20% of the predetermined thickness.
2-16. (canceled)
Description
BACKGROUND
[0001] 1. Field of the Invention
[0002] This invention relates to electroplating methods and
solutions and, more particularly, to methods and electroplating
solution chemistries for electrodeposition of Group IIIA-rich
metallic thin films on a conductive surface for solar cell
applications.
[0003] 2. Description of the Related Art
[0004] Solar cells are photovoltaic devices that convert sunlight
directly into electrical power. The most common solar cell material
is silicon, which is in the form of single or polycrystalline
wafers. However, the cost of electricity generated using
silicon-based solar cells is higher than the cost of electricity
generated by the more traditional methods. Therefore, since early
1970's there has been an effort to reduce cost of solar cells for
terrestrial use. One way of reducing the cost of solar cells is to
develop low-cost thin film growth techniques that can deposit
solar-cell-quality absorber materials on large area substrates and
to fabricate these devices using high-throughput, low-cost
methods.
[0005] Group IBIIIAVIA compound semiconductors comprising some of
the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group
VIA (O, S, Se, Te, Po) materials or elements of the periodic table
are excellent absorber materials for thin film solar cell
structures. Especially, compounds of Cu, In, Ga, Se and S which are
generally referred to as CIGS(S), or Cu(In,Ga)(S,Se).sub.2 or
CuIN.sub.1-xGa.sub.x(S.sub.ySe.sub.1-y).sub.k, where
0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1 and k is approximately 2,
have already been employed in solar cell structures that yielded
conversion efficiencies approaching 20%. Absorbers containing Group
IIIA element Al and/or Group VIA element Te also showed promise.
Therefore, in summary, compounds containing: i) Cu from Group IB,
ii) at least one of In, Ga, and Al from Group IIIA, and iii) at
least one of S, Se, and Te from Group VIA, are of great interest
for solar cell applications.
[0006] The structure of a conventional Group IB IIIAVIA compound
photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te).sub.2 thin film
solar cell is shown in FIG. 1. The device 10 is fabricated on a
substrate 11, such as a sheet of glass, a sheet of metal, an
insulating foil or web, or a conductive foil or web. The absorber
film 12, which includes a material in the family of
Cu(In,Ga,Al)(S,Se,Te).sub.2, is grown over a conductive layer 13,
which is previously deposited on the substrate 11 and which acts as
the electrical contact to the device. Various conductive layers
comprising Mo, Ta, W, Ti, and stainless steel etc. have been used
in the solar cell structure of FIG. 1. If the substrate itself is a
properly selected conductive material, it is possible not to use a
conductive layer 13, since the substrate 11 may then be used as the
ohmic contact to the device. After the absorber film 12 is grown, a
transparent layer 14 such as a CdS, ZnO or CdS/ZnO stack is formed
on the absorber film. Radiation 15 enters the device through the
transparent layer 14. Metallic grids (not shown) may also be
deposited over the transparent layer 14 to reduce the effective
series resistance of the device. A variety of materials, deposited
by a variety of methods, can be used to provide the various layers
of the device shown in FIG. 1. It should be noted that although the
chemical formula for a CIGS(S) layer is often written as
Cu(In,Ga)(S,Se).sub.2, a more accurate formula for the compound is
Cu(In,Ga)(S,Se).sub.k, where k is typically close to 2 but may not
be exactly 2. For simplicity we will continue to use the value of k
as 2. It should be further noted that the notation "Cu(X,Y)" in the
chemical formula means all chemical compositions of X and Y from
(X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga)
means all compositions from CuIn to CuGa. Similarly,
Cu(In,Ga)(S,Se).sub.2 means the whole family of compounds with
Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar
ratio varying from 0 to 1.
[0007] One technique employed for growing Cu(In,Ga)(S,Se).sub.2
type compound thin films for solar cell applications is a two-stage
process where at least two ingredients or elements or components of
the Cu(In,Ga)(S,Se).sub.2 material are first deposited onto a
substrate, and then reacted with S and/or Se in a high temperature
annealing process. For example, for CuInSe.sub.2 or CIS film
growth, thin layers of Cu and In are first deposited on a substrate
and then this stacked precursor layer is reacted with Se at
elevated temperature to form CIS. If the reaction atmosphere also
contains sulfur, then a CuIn(S,Se).sub.2 or CIS(S) layer can be
grown. Addition of Ga in the precursor layer, i.e. use of a
Cu/In/Ga stacked film precursor, allows the growth of a
Cu(In,Ga)(S,Se).sub.2 or CIGS(S) absorber.
[0008] Sputtering and evaporation techniques have been used in
prior art approaches to deposit the layers containing the Group IB
and Group IIIA components of the precursor stacks. In the case of
CuInSe.sub.2 growth, for example, Cu and In layers were
sequentially sputter-deposited on a substrate and then the stacked
film was heated in the presence of gas containing Se at elevated
temperature for times typically longer than about 30 minutes, as
described in U.S. Pat. No. 4,798,660. More recently U.S. Pat. No.
6,048,442 disclosed a method comprising sputter-depositing a
stacked precursor film comprising a Cu--Ga alloy layer and an In
layer to form a Cu--Ga/In stack on a metallic back electrode layer
and then reacting this precursor stack film with one of Se and S to
form the absorber layer. Such techniques may yield good quality
absorber layers and efficient solar cells, however, they suffer
from the high cost of capital equipment, and relatively slow rate
of production.
[0009] One prior art method described in U.S. Pat. No. 4,581,108
utilizes a low cost electrodeposition approach for metallic
precursor preparation. In this method a Cu layer is first
electrodeposited on a substrate. This is then followed by
electrodeposition of an In layer and heating of the deposited Cu/In
stack in a reactive atmosphere containing Se. Various other
researchers have reported In electroplating approaches for the
purpose of obtaining In-containing precursor layers later to be
converted into CIS absorber films through reaction with Se (see for
example, Lokhande and Hodes, Solar Cells, 21 (1987) 215; Fritz and
Chatziagorastou, Thin Solid Films, 247 (1994) 129; Kim et al,
Proceedings of the 1.sup.st World Conf. on Photovoltaic Energy
Conversion, 1994, p. 202; Calixto and Sebastian, J. Materials
Science, 33 (1998) 339; Abedin et al., Electrochemica Acta, 52
(2007) 2746, and, Valderrama et al., Electrochemica Acta, 53 (2008)
3714).
[0010] A number of In electroplating baths used for depositing In
layers on various conductive substrates have been disclosed in
several references. For example, In plating baths containing
sulfamate (U.S. Pat. No. 2,458,839), cyanide (U.S. Pat. No.
2,497,988), alkali hydroxides (U.S. Pat. No. 2,287,948), tartaric
acid (U.S. Pat. No. 2,423,624), and fluoborate (U.S. Pat. No.
3,812,020, U.S. Pat. No. 2,409,983) have been developed. Some
details on such chemistries may be found in the review paper of
Walsh and Gabe (Surface Technology, 8 (1979) 87). Although it is
possible to deposit In layers using various electroplating
chemistries employing standard plating practices, unless these
layers have sub-micron thickness and smooth morphology, they cannot
be effectively used in thin film Group IBIIIAVIA compound solar
cell fabrication.
[0011] As described above, one recent application of electroplated
In films involves the formation of Cu(In,Ga)(Se,S).sub.2 or CIGS(S)
films, which are the most advanced compound absorbers for
polycrystalline thin film solar cells. An exemplary plating process
includes first electroplating a thin In layer on a Cu layer, and
then reacting this Cu/In precursor stack with Se to form a
CuInSe.sub.2, or a CIS absorber. Furthermore, to form a CIGS or
CIGS(S) type of compound absorber, Ga can also be included in the
precursor stack by plating it on the In layer or by including it in
the In layer. Zank et al. (Thin Solid Films, 286 (1996) 259), for
example, electrodeposited an In--Ga alloy layer on a Cu film
forming a Cu/In--Ga precursor stack and then obtained a CIGS
absorber layer by reacting the precursor stack with Se vapor. The
CIGS absorber was then used to fabricate a thin film solar cell
having a structure similar to the one shown in FIG. 1.
[0012] In a thin film solar cell employing a Group IBIIIAVIA
compound absorber such as CIS or CIGS, the solar cell efficiency is
a strong function of the molar ratio of the IB element(s) to IIIA
element(s), i.e. the IB/IIIA molar ratio. If there are more than
one Group IIIA materials in the composition, the relative amounts
or molar ratios of these IIIA elements also affect the solar cell
efficiency and other properties. For a Cu(In,Ga)(S,Se).sub.2
absorber layer, for example, the efficiency of the device is a
function of the molar ratio of Cu/(In+Ga). Furthermore, some of the
important parameters of the cell, such as its open circuit voltage,
short circuit current and fill factor vary with the molar ratio of
the IIIA elements, i.e. the Ga/(Ga+In) molar ratio. In general, for
good device performance Cu/(In+Ga) molar ratio is kept at or below
1.0. For ratios higher than 1.0, a low resistance copper selenide
phase, which may introduce electrical shorts within the solar cells
may form. Increasing the Ga/(Ga+In) molar ratio, on the other hand,
widens the optical bandgap of the absorber layer, resulting in
increased open circuit voltage and decreased short circuit current.
A CIGS material with a Ga/(Ga+In) molar ratio higher than about 0.3
is electronically poor. It is for this reason that the
sunlight-to-electricity conversion efficiency of a CIGS type solar
cell first increases as the Ga/(Ga+In) molar ratio in the absorber
is increased from 0 to 0.3, and then the efficiency starts to
decrease as the molar ratio is further increased towards 1.
[0013] In light of the above discussion, it should be appreciated
that if the electrodeposition process is used to introduce In into
the composition of a CIGS(S) precursor material, it is essential
that the electroplated In films have smooth morphology and uniform
thickness, in micro-scale. If micro-structure of an In film or a
In--Ga film electroplated on a Cu and optionally Ga containing
precursor layer is rough and includes protrusions and valleys or
discontinuities, the localized micro-scale Ga/(In+Ga) ratio at the
protrusions would be lower than the Ga/(In+Ga) ratio at the
valleys. Even the Cu/(In+Ga) molar ratio would be different at
these two locations. As will be described next, this kind of
micro-scale non-uniformity would yield a CIGS(S) absorber with
non-uniform electrical and optical properties after reaction of the
precursor stack with Se and/or S. The same argument also holds for
the other thin film layers (such as Cu and Ga) within the precursor
stack. However, electroplating a smooth Cu layer is relatively easy
and the problem usually lies with Ga and In electrodeposition due
to the tendency of these low melting, high surface tension elements
forming droplets rather than continuous layers when deposited in
thin film form.
[0014] Thin film CIGS(S) solar cell absorbers typically have a
thickness range of 1000-3000 nm. The amount of In that needs to be
included in such a thin absorber is equivalent to an In layer
thickness which is in the range of about 200-700 nm. For example,
for the formation of about 2000 nm thick CIGS absorber with a final
Cu/(In+Ga) ratio of 0.85-0.9 and a Ga/(Ga+In) ratio of about 0.3,
one needs to deposit about 250-300 nm thick Cu film, about 150 nm
thick Ga layer and about 450-500 nm thick In film to form a
precursor which may then be reacted with Se. Since cost lowering in
CIGS solar cell fabrication as well as the need to reduce stress in
the CIGS layer grown by the two-stage processes dictate the use of
an absorber thickness which is in the range of 1000-1500 nm, the
thickness of the In film in the above example gets reduced to about
200-300 nm level. The Ga layer thickness goes down even lower to
the 75-100 nm range. Therefore, in a two stage CIGS(S) absorber
formation approach employing an electroplated In layer, the
electroplated In film thickness will have to be much less than 1000
nm, preferably less than 700 nm, most preferably less than 500 nm.
This requirement presents many challenges for prior art In
electroplating methods and chemistries. Although these issues will
be discussed with respect to In electrodeposition, it should be
understood that they are also applicable to Ga and In--Ga alloy
electrodeposition.
[0015] Low melting Group IIIA materials such as In and Ga have high
surface tension and they grow in the form of islands or droplets
when deposited on a substrate surface in thin film form. This
behavior has been observed in prior work carried out on
electroplated In films (see for example, Chen et al., Solar Cells,
30 (1991) 451; Kim et al, Proceedings of the 1.sup.st World Conf.
on Photovoltaic Energy Conversion, 1994, p. 202; Calixto and
Sebastian, J. Materials Science, 33 (1998) 339; Abedin et al.,
Electrochemica Acta, 52 (2007) 2746, and, Valderrama et al.,
Electrochemica Acta, 53 (2008) 3714), and in work carried out on
In--Ga alloy films (see for example Zank et al., Thin Solid Films,
286 (1996) 259). As stated before, lack of planarity in sub-micron
thick In and/or Ga-rich layers presents problems for application of
such non-uniform layers to thin film solar cell manufacturing.
[0016] FIGS. 2A-2B schematically show a prior art structure in
perspective and side views, respectively. The structure includes a
typical prior art In layer 37, with sub-micron thickness which may
be electrodeposited on a surface 36 of an under-layer 33. The
under-layer 33 is formed over a base 30 having a substrate 31 and a
contact layer 32. The under-layer 33 may, for example, include Cu
and Ga and be formed on the contact layer 32. As can be seen from
FIGS. 2A and 2B, the sub-micron thick In layer 37 is discontinuous
and it includes islands 34 of In, separated by valleys 35 through
which the surface 36 of the under-layer 33 is exposed. The width of
the islands may be in the range of 500-5000 nm. If the structure of
FIGS. 2A and 2B is reacted with a Group VIA material such as Se, a
CIGS solar cell absorber 40 may be formed on the base 30 as shown
in FIG. 3. The CIGS solar cell absorber 40 has compositional
non-uniformities caused by the morphological non-uniformity of the
sub-micron thick In layer 37. Accordingly, the CIGS solar cell
absorber 40 has a first region 41 and a second region 42. The first
region 41 corresponds to the islands 34 of In of the structure of
FIG. 2A, and is an In-rich, Ga-poor region. The second region 42
corresponds to the valleys 35 of the structure of FIG. 2A, and is
an In-poor, Ga-rich region. Furthermore, the Cu(In+Ga) molar ratio
in the first region 41 is lower than the Cu(In+Ga) molar ratio in
the second region 42. It should be appreciated that when a solar
cell is fabricated on the CIGS solar cell absorber 40, the
efficiency of the solar cell would be determined by both the first
region 41 and the second region 42. The solar cell would act like
two separate solar cells, one made on the first region 41 and the
other made on the second region 42 and then interconnected in
parallel. Since the Ga/(Ga+In) as well as the Cu/(In+Ga) molar
ratios in the two regions are widely different the quality of the
separate solar cells on these regions would also be different. The
quality of the overall solar cell would then suffer from the poor
I-V characteristics of the separate solar cells formed on either
one of the first and second regions.
[0017] It should be noted that such non-uniformity problems may not
be important in applications where the electroplated In layer is
not used for the fabrication of an active electronic device such as
a solar cell. It should also be noted that the In films when
electrodeposited to thicknesses larger than about 1000 nm they may
start forming continuous layers. In such cases the islands 34 in
FIG. 2A grow horizontally as well as vertically and eventually
merge, eliminating the valleys 35. However, such thick
electroplated In layers are not useful for thin film solar cell
fabrication since they yield CIGS absorbers that are too thick
(thicker than about 3000 nm). Thick absorber layers cause excessive
stress and delamination from the base. They also add to the cost of
processing, which is not in line with the cost-lowering targets of
thin film photovoltaics. Highly efficient CIGS solar cells can be
fabricated on 1000 nm thick CIGS absorbers. Using a 3000 nm thick
CIGS absorber in a solar cell structure increases materials usage
three time and wastes effectively 67% of the materials used in
forming the CIGS absorber structure.
[0018] As can be seen from the foregoing discussion it is necessary
to develop new Group IIIA material electroplating approaches that
can yield continuous layers at thicknesses less than about 700 nm,
preferably less than about 500 nm. Such thin layers can be used in
electronic and semiconductor applications such as in processing
thin film CIGS type solar cells.
SUMMARY OF THE INVENTION
[0019] The present invention relates to electroplating methods and
solutions and, more particularly, to methods and electroplating
solution chemistries for electrodeposition of Group IIIA-material
rich thin films on a conductive surface for solar cell
applications.
[0020] In one aspect, the Group IIIA material rich thin film is
deposited on an interlayer that includes 20-90 molar percent of at
least one of In and Ga and at least 10 molar percent of an additive
material including one of Cu, Se, Te, Ag and S. The thickness of
the interlayer is adapted to be less than or equal to about 20% of
the thickness of the Group IIIA material rich thin film.
[0021] In one preferred aspect there is provided a method of
electrodepositing a Group IIIA material rich thin film over a
surface of a base for manufacturing solar cell precursors, the
method comprising: electrodepositing an interlayer over the surface
of the base, wherein the interlayer comprises a predetermined molar
percent of at least one of In and Ga and at least 10 molar percent
of an additive material including one of Cu, Se, Te, Ag and S,
wherein the predetermined molar percent is in the range of 20-90
percent; and electrodepositing the Group IIIA material rich thin
film on the interlayer to a predetermined thickness, wherein the
Group IIIA material rich thin film is one of a substantially pure
In film, a substantially pure Ga film and a substantially pure
In--Ga alloy, wherein the predetermined thickness of the Group IIIA
material rich thin film is less than 700 nm, and wherein the
thickness of the interlayer is less than or equal to 20% of the
predetermined thickness.
[0022] In another preferred aspect there is provided a precursor
layer structure for forming a Group IBIIIAVIA solar cell absorber
that comprises a conductive base layer; an interlayer formed over
the first layer, wherein the interlayer comprises a predetermined
molar percent of at least one of In and Ga and at least 10 molar
percent of an additive material including one of Cu, Se, Te, Ag and
S, wherein the predetermined molar percent is in the range of 20-90
percent; and a Group IIIA material rich thin film formed on the
interlayer layer to a predetermined thickness, wherein the Group
IIIA material rich thin film is one of a substantially pure In
film, a substantially pure Ga film and a substantially pure In--Ga
alloy, wherein the predetermined thickness of the Group IIIA
material rich thin film is less than 700 nm, and wherein the
thickness of the interlayer is less than or equal to 20% of the
predetermined thickness.
[0023] Other embodiments and aspects of the invention are described
herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a schematic view of a prior art solar cell
structure.
[0025] FIG. 2A is a perspective top view of a prior art precursor
structure formed by electroplating a sub-micron thick In layer on a
sub-layer.
[0026] FIG. 2B is a cross-sectional view of the structure of FIG.
2A taken along the line AA.
[0027] FIG. 3 is a CIGS layer formed after reaction of the
structure of FIG. 2B with Se.
[0028] FIGS. 4A-4C schematically shows electrodeposition of a
uniform In-rich layer over a continuous interlayer thus forming a
uniform stack.
[0029] FIG. 5 shows a Group IBIIIAVIA compound layer formed on a
base using the stack of FIG. 4.
DETAILED DESCRIPTION OF THE INVENTION
[0030] The present invention provides a method for forming a Group
IIIA material thin film on a conductive layer which is coated by an
interlayer to facilitate a uniform Group IIIA material thin film
growth with thickness less than about 700 nm. The Group IIIA
material film, the interlayer and the conductive layer may be a
part of a precursor stack that will eventually be reacted and
transformed into a Group IBIIIAVIA solar cell absorber. The Group
IIIA material thin film of the present invention may comprise any
one of a substantially pure In material, a substantially pure Ga
material, or an In--Ga binary alloy. The Group IIIA material thin
film is a continuous film having a thickness less than about 700
nm. In one embodiment, the Group IIIA material thin film may be
formed by an electrodeposition process on the surface of the
interlayer. Accordingly, the interlayer of the present invention is
formed on a conductive surface which may be the top surface of a
base or a precursor stack. The group IIIA material thin film may
then be formed by electrodeposition on the exposed surface of the
interlayer. The interlayer comprises 20-90 molar percent,
preferably 40-80 molar percent of at least one of In and Ga. The
balance of the interlayer composition comprises an additive
material. The additive material of the interlayer includes at least
one of Cu, Se, Te, Ag and S, preferably at least one of Cu and Te.
Other materials or impurities may also be present in the additive
material as long as their molar content does not exceed about 10
molar percent of the total additive material composition. The
process used to form the Group IIIA material thin film on the
interlayer is electrodeposition; however, in the following
description the words electroplating, plating and deposition may be
used to refer to the electrodeposition process of the In and/or Ga
layer.
[0031] An electrodeposition process of the present invention which
forms a Group IIIA material layer, or thin film, for the
manufacture of a Group IBIIIAVIA solar cell precursor structure
will be described using FIGS. 4A-4C. FIG. 5 shows the structure
with the Group IBIIIAVIA solar cell absorber, which is formed from
the precursor stack of FIG. 4C.
[0032] FIG. 4A exemplifies a first structure 100 including a first
layer 102 formed on a base 104 to initiate the precursor stack
forming process of the present invention. The first layer 102 may
preferably be formed using an electrodeposition process; however,
other deposition processes such as evaporation, sputtering and the
like may also be used to form the first layer 102. The base 104 may
be a conductive base including a substrate 106 and a contact layer
108, which will eventually form an electrical contact to the
CIGS(S) absorber after the reaction step. The substrate 106 may be
a continuous conductive material such as a metal or alloy foil,
preferably a stainless steel foil. The contact layer 108 may
comprise conductive materials such as Mo, W, metal nitrides, Ru,
Os, and Ir, which make ohmic contact to CIGS(S) type absorber
films. The first layer 102 is a conductive layer comprising Cu. The
first layer 102 may be a pure Cu layer or it may comprise In and/or
Ga. The first layer 102 may be homogeneous or it may be in the form
of a stack. Exemplary stacks forming the first layer 102 include,
but are not limited to, Cu/Ga, Cu/Ga/Cu, Cu--Ga/Cu, and the like,
stacks.
[0033] FIG. 4B shows a second structure 200 formed as the process
of the present invention proceeds. In the second structure 200, a
second layer 112 or an interlayer is formed on the top surface 110
of the first layer 102, using preferably an electrodeposition
process. The interlayer 112 is a conditioned conductive layer so
that it establishes a conditioned surface for the following Group
IIIA thin film deposition. In the context of this application, the
word conditioned refers to establishing a material composition that
not only helps forming a thin and continuous Group IIIA layer on
the interlayer but also includes constituents that do not affect
negatively the overall composition of the resulting precursor stack
and do not deteriorate the quality of the CIGS(S) absorber to be
formed. The interlayer 112 is a continuous layer with a
substantially uniform thickness which is less than 100 nm,
preferably less than 50 nm. Surface 114 of the interlayer 112
functions as an active deposition site to allow a Group IIIA
material to continuously and uniformly deposit onto the surface 114
in the subsequent step, thereby eliminating the discontinuity
problems of the prior art described above.
[0034] The interlayer 112 comprises 20-90 molar percent, preferably
40-80 molar percent of at least one of In and Ga. Presence of In
and/or Ga in the interlayer composition is important for the
interlayer to provide effective nucleation to the In and/or Ga rich
layer that will be electroplated on top of it. However, the In
and/or Ga content of the interlayer cannot be more than 90% because
the interlayer needs to be continuous to be able to provide the
effective nucleation sites. If the interlayer becomes near pure In
and/or Ga layer then it would be in the form of islands or droplets
as discussed before.
[0035] Besides In and/or Ga, the balance of the interlayer
composition is an additive material. The additive material in the
interlayer includes at least one of Cu, Se, Te, Ag and S. The most
preferred additives are Cu and Te. These additives assist in making
the interlayer a continuous film, and at the same time the In
and/or Ga in the interlayer provide high density of nucleation
sites for the In and/or Ga layer that would be electroplated on the
interlayer. Since the invention specifically targets Group
IBIIIAVIA absorber layer (compound layer) fabrication, the additive
materials are the materials that will not damage the electronic
quality of the CIGS(S) absorber. Other materials or impurities may
also be present in the additive material without exceeding about 10
molar percent of the total additive material composition. Examples
of such impurities include Sb and As. The composition of the
interlayer is largely determined by the chemical composition of the
Group IIIA material layer (layer 116 in FIG. 4C) that will be
electrodeposited onto the interlayer 112 and any other layer that
may be present in the resulting precursor stack.
[0036] In one embodiment, the interlayer 112 may be
electrodeposited out of plating electrolytes comprising at least
one of In and Ga as well as at least one additive such as Cu and
Te. By co-depositing these additives and including them into the
interlayer 112, a continuous interlayer may be obtained even at a
thickness as low as 10 nm. Although the thickness of the interlayer
112 depends on the thickness of the Group IIIA material layer that
will be electrodeposited onto the interlayer, a preferable
thickness of it may be for example less than about 50 nm so that
the amount of In, Ga and other materials that it may contain do not
become a determining factor in the overall composition, i.e. the
Cu/(In+Ga) molar ratio or Ga/(Ga+In) molar ratio, of the resulting
structure after the Group IIIA material deposition. In one
embodiment, the thickness of the interlayer is less than or equal
to about 20%, preferably less than about 10% of the thickness of
the Group IIIA material-rich layer that is deposited over the
interlayer, so that the effect of the interlayer on determining the
overall composition of the resulting precursor stack is limited.
This is important for manufacturability and repeatability of the
process.
[0037] FIG. 4C shows a third structure 300 formed after
electrodepositing a third layer 116 which is a substantially pure
Group IIIA material layer onto the interlayer 112. As opposed to
the discontinuity problems of the prior art In and/or Ga films, the
third layer 116 is a continuous thin film. By employing an
electrodeposition process that uses the interlayer 112 of the
present invention as a cathode, very thin Group IIIA material
layers having uniform thickness may be formed on the interlayer
112. The thickness of the third layer 116 may be less than about
700 nm, preferably less than about 500 nm, whereas the thickness of
the interlayer is less than about 20% of these values, i.e. less
than about 140 nm, preferably less than about 100 nm. Most
preferably the thickness of the interlayer is less than about 10%
of the thickness of the third layer 166, i.e. less than about 70
nm. In one embodiment, the Group IIIA material deposited on the
interlayer may be a substantially pure In--Ga binary alloy
electrodeposited from an electrolyte comprising In and Ga ions.
During the electrodeposition process, in an electrodeposition
chamber containing the electrodeposition electrolyte, the
interlayer 112 is cathodically polarized with respect to an anode
so that the third layer comprising In and Ga deposits onto the
surface 114 of the interlayer in a uniform manner. The chemical
composition of the third layer 116 may preferably comprise at least
90 molar percent In and/or Ga, preferably at least 95 molar percent
In and/or Ga.
[0038] Referring back to FIG. 4C, as will be appreciated, in the
third structure 300, the stack of the first layer 102, the second
layer 112 or interlayer and the third layer 116 forms a precursor
stack containing Group IB and Group IIIA elements on the base
104.
[0039] As shown in FIG. 5, in the following process step, the
precursor stack 118 is reacted with at least one Group VIA material
such as Se, Te or S to form an absorber layer 120 on the base 104.
As mentioned above the precursor stack 118 comprises Cu, In, and
Ga, and therefore reacting them with a Group VIA material forms the
absorber 120 which is a compositionally uniform Group IBIIIAVIA
compound layer.
[0040] Although the present invention is described with respect to
certain preferred embodiments, modifications thereto will be
apparent to those skilled in the art.
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