U.S. patent application number 12/642691 was filed with the patent office on 2010-06-10 for selenium containing electrodeposition solution and methods.
This patent application is currently assigned to SoloPower, Inc.. Invention is credited to Serdar Aksu, Cyprian E. Uzoh.
Application Number | 20100140098 12/642691 |
Document ID | / |
Family ID | 42229873 |
Filed Date | 2010-06-10 |
United States Patent
Application |
20100140098 |
Kind Code |
A1 |
Uzoh; Cyprian E. ; et
al. |
June 10, 2010 |
SELENIUM CONTAINING ELECTRODEPOSITION SOLUTION AND METHODS
Abstract
The present inventions relate to selenium containing
electrodeposition solutions used to manufacture solar cell absorber
layers. In one aspect is described an electrodeposition solution to
electrodeposit a Group IB-Group VIA thin film that includes a a
solvent; a Group IB material source; a Group VIA material source;
and at least one complexing that forms a complex ion of the Group
IB material. Also described are methods of electroplating using
electrodeposition solutions.
Inventors: |
Uzoh; Cyprian E.; (San Jose,
CA) ; Aksu; Serdar; (San Jose, CA) |
Correspondence
Address: |
PILLSBURY WINTHROP SHAW PITTMAN LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
SoloPower, Inc.
San Jose
CA
|
Family ID: |
42229873 |
Appl. No.: |
12/642691 |
Filed: |
December 18, 2009 |
Related U.S. Patent Documents
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Application
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Filing Date |
Patent Number |
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12121687 |
May 15, 2008 |
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12642691 |
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12371546 |
Feb 13, 2009 |
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12121687 |
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Current U.S.
Class: |
205/96 ; 205/198;
205/316 |
Current CPC
Class: |
C25D 3/54 20130101; C25D
5/18 20130101; C25D 5/10 20130101; Y02P 70/521 20151101; C25D 3/58
20130101; Y02E 10/541 20130101; C25D 3/56 20130101; H01L 31/0322
20130101; C25D 9/08 20130101; Y02P 70/50 20151101 |
Class at
Publication: |
205/96 ; 205/316;
205/198 |
International
Class: |
C25D 5/18 20060101
C25D005/18; C25D 3/38 20060101 C25D003/38; C25D 9/04 20060101
C25D009/04 |
Claims
1. An electrodeposition solution to electrodeposit a Group IB-Group
VIA thin film, comprising: a solvent; a Group IB material source
that dissolves in the solvent and provides a Group IB material; a
Group VIA material source that dissolves in the solvent and
provides a Group VIA material; and at least one complexing that
forms a complex ion of the Group IB material wherein such complex
ion dissolves in the solvent; an adhesion promoting agent; a
corrosion inhibitor; and wherein the pH of the electrodeposition
solution is in the range of 1-13.
2. The electrodeposition solution of claim 1, wherein the Group IB
material source comprises a copper source and the Group VIA
material comprises a selenium source.
3. The electrodeposition solution of claim 2, wherein the copper
source of copper ions comprises at least one of dissolved copper
metals and dissolved copper salts, wherein the copper salts include
copper-chloride, copper-sulfate, copper-acetate, copper-oxide,
copper-hydroxide, copper-nitrate, copper-phosphate,
copper-tetraflouroborate, copper-citrate, copper-gluconate,
copper-sulfamate, and copper-carbonate.
4. The solution of claim 1, wherein the Group Se material source
comprises at least one of dissolved elemental Se, acids of Se, and
dissolved Se compounds, wherein the Se compounds include oxides,
chlorides, sulfates, nitrates, perchlorides and phosphates of
Se.
5. The solution of claim 4 wherein the complexing agent includes at
least one of a carboxylate functional group and an amine functional
group.
6. The solution of claim 5 wherein the complexing agent comprises
one of an acid and an alkali metal salt of the acid and an ammonium
salt of the acid, and wherein the acid comprises one of tartaric
acid, citric acid, acetic acid, malonic acid, malic acid, succinic
acid, ethylenediamine (EN), ethylenediamine tetraacetic acid
(EDTA), nitrilotriacetic acid (NTA), and
hydroxyethylethylenediaminetriacetic acid (HEDTA).
7. The solution of claim 6 wherein the complexing agent comprises
an alkali metal salt of the acid is selected from the group of
sodium and potassium salts tartaric acid, citric acid, acetic acid,
malonic acid, malic acid, succinic acid, ethylenediamine (EN),
ethylenediamine tetraacetic acid (EDTA), nitrilotriacetic acid
(NTA), and hydroxyethylethylenediaminetriacetic acid (HEDTA).
8. The solution of claim 6 wherein the complexing agent comprises
an ammonium salt of the acid is selected from the group of ammonium
salts of tartaric acid, citric acid, acetic acid, malonic acid,
malic acid, succinic acid, ethylenediamine (EN), ethylenediamine
tetraacetic acid (EDTA), nitrilotriacetic acid (NTA), and
hydroxyethylethylenediaminetriacetic acid (HEDTA).
9. The electrodeposition solution of claim 4, wherein the amount of
the complexing agent is in the range of 5 mM/L to 0.55 M/L.
10. The electrodeposition solution of claim 1, wherein the adhesion
promoting agent comprises organic compounds with at least one amine
functional group and at least one aminopropyl group.
11. The electrodeposition solution of claim 11, wherein the
adhesion promoting agent comprises at least one of
1,4-Bis(3-aminopropyl) piperazine and N,N' Bis(3-aminopropyl)
ethylenediamine and any possible mixture of these.
12. The electrodeposition solution of claim 7, wherein the amount
of the adhesion promoting agent is in the range of 1.0 mM/L to 40
mM/L.
13. The electrodeposition solution of claim 1, wherein the
corrosion inhibitor comprises a multihydric alcohol.
14. The electrodeposition solution of claim 13, wherein the
corrosion inhibitor is glycerol.
15. The electrodeposition solution of claim 13, wherein the amount
of the corrosion inhibitor is in the range of 20 mM/L to 500
mM/L.
16. The electrodeposition solution of claim 1 further comprising a
grain refiner including one of chloride, pyrophosphate and
sulfite.
17. The electrodeposition solution of claim 1 further comprising a
conductivity improving agent including ammonium sulfate.
18. The electrodeposition solution of claim 1, wherein the amount
of the conductivity improving agent is in the range of 5 mM/L to
0.55 M/L.
19. A method of electrodepositing an adherent film comprising
copper selenide alloy material on a conductive layer, comprising:
providing an electrodeposition solution comprising a solvent, a
copper ion source, a selenium ion source, at least one complexing
agent and at least one adhesion promoting agent, the adhesion
promoting agent suppressing the formation of colloidal particles on
the substrate and in the plating solution, wherein the
electrodeposition solution has a pH value in the range of 1-13;
contacting the electrodeposition solution with the surface of the
conductive layer and an anode; establishing a potential difference
between the anode and the conductive layer; and electrodepositing
the copper selenide film on the surface of the conductive
layer.
20. The method of claim 19, wherein the conductive layer is one of
indium, gallium, selenium copper and their alloys.
21. The method of claim 19 wherein the copper ion source is copper
sulfate pentahydrate, and the selenium ion source is selenium
oxide.
22. The method of claim 19, wherein said adhesion promoting agent
comprises organic compounds with at least one amine functional
group and at least one aminopropyl group.
23. The method of claim 19, wherein the step of electrodepositing
includes varying electrodeposition current density during the
electrodeposition.
24. The method of claim 19, wherein the step of electrodepositing
includes pulsing the current density between a high and a low
current density for desirable time intervals to electrodeposit
laminated selenide films containing different copper
concentrations.
25. The method of claim 19 wherein said copper selenide film has a
graded copper composition within the film thickness.
26. The method of claim 19 wherein the step of electrodepositing is
performed using one of galvanostatic electrodeposition,
potentiostatic electrodeposition , and various combinations of
current and voltage mode.
27. A method of electrodepositing a precursor on a conductive
layer, comprising: electrodepositing a copper selenide film on the
conductive layer using an electrodeposition solution, the
electrodeposition solution comprising a copper ion source, a
selenium ion source, at least one complexing agent, at least one
adhesion promoting agent; depositing a thin film including a Group
IB material, at least one Group IIIA material onto the copper
selenide film, wherein the step of depositing uses a physical vapor
deposition technique.
28. The method of claim 27 wherein the thin film further includes a
sodium salt.
29. The method of claim 27 wherein the Group IB material includes
copper, and the Group IIIA material includes gallium and indium.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation in Part of U.S. patent
application Ser. No. 12/121,687 FILED May 15, 2008 entitled
"SELENIUM ELECTROPLATING CHEMISTRIES AND METHODS" (SP-049), and
this application is a Continuation in Part of U.S. patent
application Ser. No. 12/371,546 filed Feb. 13, 2009 entitled
"ELECTROPLATING METHODS AND CHEMISTRIES FOR DEPOSITION OF
COPPER-INDIUM-GALLIUM CONTAINING THIN FILMS" (SP-051), which claims
priority to U.S. Provisional Application No. 61/150,721, filed Feb.
6, 2009, entitled "ELECTROPLATING METHODS AND CHEMISTRIES FOR
DEPOSITION OF COPPER-INDIUM-GALLIUM CONTAINING THIN FILMS"
(SP-051P), and this application is a Continuation in Part of U.S.
patent application Ser. No. 12/______ filed on Dec. 18, 2009
entitled "ENHANCED PLATING CHEMISTRIES AND METHODS FOR PREPARATION
OF GROUP IBIIIAVIA THIN FILM SOLAR ABSORBERS" (SP-098) and this
application is a Continuation in Part of U.S. patent application
Ser. No. 12/______ filed on Dec. 18, 2009 entitled "ELECTROPLATING
METHODS AND CHEMISTRIES FOR DEPOSITION OF COPPER-INDIUM-GALLIUM
CONTAINING THIN FILMS", (SP-101), all of which are expressly
incorporated herein by reference.
BACKGROUND
[0002] 1. Field of the Inventions
[0003] The present inventions relate to selenium containing
electrodeposition solutions used to manufacture solar cell absorber
layers.
[0004] 2. Description of the Related Art
[0005] 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 the 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.
[0006] 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. It should be noted that although the
chemical formula for CIGS(S) 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] The structure of a conventional Group IBIIIAVIA compound
photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te).sub.2 thin film
solar cell is shown in FIG. 1. A photovoltaic cell 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. An absorber
film 12, which comprises a material in the family of
Cu(In,Ga,Al)(S,Se,Te).sub.2 is grown over a conductive layer 13 or
contact layer, which is previously deposited on the substrate 11
and which acts as the electrical contact to the device. The
substrate 11 and the conductive layer 13 form a base 20 on which
the absorber film 12 is formed. Various conductive layers
comprising Mo, Ta, W, Ti, and their nitrides 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
the 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, CdS/ZnO or
CdS/ZnO/ITO stack is formed on the absorber film 12. 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. The preferred
electrical type of the absorber film 12 is p-type, and the
preferred electrical type of the transparent layer 14 is n-type.
However, an n-type absorber and a p-type window layer can also be
utilized. The preferred device structure of FIG. 1 is called a
"substrate-type" structure. A "superstrate-type" structure can also
be constructed by depositing a transparent conductive layer on a
transparent superstrate such as glass or transparent polymeric
foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te).sub.2 absorber
film, and finally forming an ohmic contact to the device by a
conductive layer. In this superstrate structure light enters the
device from the transparent superstrate side.
[0008] The first technique that yielded high-quality
Cu(In,Ga)Se.sub.2 films for solar cell fabrication was
co-evaporation of Cu, In, Ga and Se onto a heated substrate in a
vacuum chamber. However, low materials utilization, high cost of
equipment, difficulties faced in large area deposition and
relatively low throughput are some of the challenges faced in
commercialization of the co-evaporation approach. Another technique
for growing Cu(In,Ga)(S,Se).sub.2 type compound thin films for
solar cell applications is a two-stage process where metallic
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
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. If the reaction atmosphere also contains
sulfur, then a CuIn(S,Se).sub.2 layer can be grown. Addition of Ga
in the precursor layer, i.e. use of a stack such as a Cu/In/Ga
stacked film precursor, allows the growth of a
Cu(In,Ga)(S,Se).sub.2 absorber.
[0009] 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 are sequentially
sputter-deposited on a substrate and then the stacked film is
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. U.S. Pat. No. 6,092,669 described sputtering-based
equipment for producing such absorber layers.
[0010] Two-stage processing approach may also employ stacked layers
comprising Group VIA materials. For example, a Cu(In,Ga)Se.sub.2 or
CIGS film may be obtained by depositing In--Ga-selenide and
Cu-selenide layers in a stacked manner and reacting them in
presence of Se. Similarly, stacks comprising Group VIA materials
and metallic components may also be used. In--Ga-selenide/Cu stack,
for example, may be reacted in presence of Se to form CIGS. Stacks
comprising metallic elements as well as Group VIA materials in
discrete layers may also be used. Selenium may be deposited on a
metallic precursor film comprising Cu, In and/or Ga through various
approaches to form stacks such as Cu/In/Ga/Se and Cu--Ga/In/Se. One
approach for Se layer formation is evaporation as described by J.
Palm et al. ("CIS module pilot processing applying concurrent rapid
selenization and sulfurization of large area thin film precursors",
Thin Solid Films, vol. 431-432, p. 514, 2003) in their work that
involved preparation of a Cu--Ga/In metallic precursor film by
sputtering and evaporation of Se over the In surface to form a
Cu--Ga/In/Se stack. After rapid thermal annealing and reaction with
S, these researchers reported formation of Cu(In,Ga)(Se,S).sub.2 or
CIGS(S) absorber layer.
[0011] Evaporation is a relatively high cost technique to employ in
large scale manufacturing of absorbers intended for low cost solar
cell fabrication. Potentially lower cost techniques such as
electroplating have been reported for deposition of Se films.
Electroplating can be used for depositing substantially pure Se
thin films as well as for co-depositing Se with Cu, In and Cu
metallic components. One specific method for the former case
involves depositing a metallic precursor comprising Cu and In on a
substrate and then electroplating a Se layer over the Cu and In
containing layer to form a Cu--In/Se stack. This stack may then be
heated up to form a CuInSe.sub.2 compound absorber.
[0012] Binary metal selenide film preparation by electrodepositon
has been reported in various publications. For example, Massaccessi
et al. carried out In--Se electroplating from indium sulfate and
selenious acid solutions ("Electrodeposition of indium selenide
In.sub.2Se.sub.3, J. Electroanalytical Chemistry, vol. 412, p. 95,
1996). Kemell et al. ("Electrochemical quartz crystal microbalance
study of the electrodepositon mechanism of Cu.sub.2-xSe thin film",
Electrochemical Acta, vol. 45, p. 3737, 2000) used a thiocyanate
solution to deposit copper selenide thin films. In addition to
preparation of substantially pure selenium films and binary metal
selenides, one-step electroplating process has also been used to
deposit the entire precursor film in the form of Cu--In--Se or
Cu--In--Ga--Se. For example, CIGS films have been prepared with
electrochemical co-deposition method from acidic solutions
containing CuCl.sub.2, InCl.sub.3, GaCl.sub.3 and SeO.sub.2 (U.S.
Pat. No. 6,872,295). The electrochemical co-deposition of CIS films
is also performed using a solution containing Cu.sup.2+, In.sup.3+,
Se.sup.4+ and citrate salts, as reported m the literature (Oliveira
et al., "A voltammetric study of the electrodeposition of
CuInSe.sub.2 in a citrate electrolyte", Thin Solid Films, vol. 405,
p. 129, 2002).
[0013] Electrochemical deposition techniques have been developed to
deposit pure Se films in both amorphous and metallic crystalline
forms. Selenium can assume four allotropic modifications in its
solid state; amorphous (also called vitreous), -monoclinic,
-monoclinic and a hexagonal (so-called metallic) phase. The
amorphous selenium is composed of irregular arrays of selenium
chain molecules, while the monoclinic modifications consist of Se
ring molecules. Vitreous and monoclinic modifications of Se are
insulators and they are generally red in color. The hexagonal phase
of selenium is gray in appearance and therefore called "gray"
selenium. Hexagonal modification is a semiconductor due to the
ordered arrangement of selenium chains facilitating electronic
conduction. A. Von Hippel et al. (U.S. Pat. No. 2,649,409)
disclosed that gray crystalline metallic Se may be electroplated
using an acidic electrolyte composed of saturated selenium dioxide
in 9 molar H.sub.2SO.sub.4 at a temperature of 100.degree. C. Since
plating of metallic Se requires use of high temperature solutions
and highly acidic solution formulations, they are not very suitable
for large scale production.
[0014] Typically Se deposits obtained from low temperature
solutions are of amorphous nature. A. Graham et al.
("Electrodeposition of amorphous Se", J. Electrochemical Society,
vol. 106, p. 651, 1959) have established that amorphous Se layers
with thicknesses up to 500 nm can be plated using acidic (pH
0.7-0.9) or alkaline (pH 7.5-8.0) electrolytes in the temperature
range between 20 to 40.degree. C. A common problem associated with
electrodeposition of amorphous Se films is that the current plating
processes are known to produce colloidal Se which is mostly
produced near the cathode surface. These colloidal Se particles
aggregate and get larger in size with time. The formation of Se
particles is not only observed in electrodeposition of pure Se
layers but also occurs electroplating of metal selenides such as
In--Se, Ga--Se, Cu--Se, Cu--In--Se, Cu--Ga--Se etc. The generation
of colloidal particles might also be present in plating
applications where other group VIB elements such as tellurium and
sulfur are electrodeposited either in the form of pure elemental
layers or co-deposited with Se such as sulfur-selenium layers,
tellurium-selenium layers and sulfur-tellurium-selenium layers, or
co-deposited with metals such as In, Cu and Ga, or co-deposited
with Se and metals such as In, Cu and Ga.
[0015] From the foregoing, there is a need in the solar cell
manufacturing industry, especially in thin film photovoltaics, for
better Se-containing electrodeposition solutions to deposit high
quality Se containing films.
SUMMARY OF THE INVENTION
[0016] The present inventions relate to selenium containing
electrodeposition solutions used to manufacture solar cell absorber
layers, and methods of using the same.
[0017] In one aspect is described an electrodeposition solution to
electrodeposit a Group IB-Group VIA thin film, comprising: a
solvent; a Group IB material source that dissolves in the solvent
and provides a Group IB material; a Group VIA material source that
dissolves in the solvent and provides a Group VIA material; and at
least one complexing that forms a complex ion of the Group IB
material wherein such complex ion dissolves in the solvent; an
adhesion promoting agent; a corrosion inhibitor; and wherein the pH
of the electrodeposition solution is in the range of 1-13.
[0018] In another aspect is described a method of electrodepositing
an adherent film comprising copper selenide alloy material on a
conductive layer, comprising: providing an electrodeposition
solution comprising a solvent, a copper ion source, a selenium ion
source, at least one complexing agent and at least one adhesion
promoting agent, the adhesion promoting agent suppressing the
formation of colloidal particles on the substrate and in the
plating solution, wherein the electrodeposition solution has a pH
value in the range of 1-13; contacting the electrodeposition
solution with the surface of the conductive layer and an anode;
establishing a potential difference between the anode and the
conductive layer; and electrodepositing the copper selenide film on
the surface of the conductive layer.
[0019] In a further aspect is described a method of
electrodepositing a precursor on a conductive layer, comprising:
electrodepositing a copper selenide film on the conductive layer
using an electrodeposition solution, the electrodeposition solution
comprising a copper ion source, a selenium ion source, at least one
complexing agent, at least one adhesion promoting agent; and
depositing a thin film including a Group IB material, at least one
Group IIIA material onto the copper selenide film, wherein the step
of depositing uses a physical vapor deposition technique.
BRIEF DESCRIPTION OF THE DRAWING
[0020] These and other aspects and features of the present
invention will become apparent to those of ordinary skill in the
art upon review of the following description of specific
embodiments of the invention in conjunction with the accompanying
figures, wherein:
[0021] FIG. 1 is a schematic view of a prior art solar cell
structure;
[0022] FIG. 2A is a schematic view of a precursor stack
electrodeposited on a base; and
[0023] FIG. 2B is a schematic view of a CIGS absorber layer formed
when the precursor stack shown in FIG. 2A is reacted.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] The preferred embodiment, as describe herein, provide
electrodeposition (also called electroplating or plating) solutions
(also called baths or electrolytes) and methods to
co-electrodeposit (also called electrodeposit or electroplate or
plate from now on) uniform, smooth, continuous and compositionally
repeatable Group IB-Group VIA alloy or mixture films. The
electrodeposition solution compositions and methods of the present
invention suppresses or eliminate the formation of selenium
particles in the electrodeposition solution, thereby allowing
electrodeposition of defect-free and highly adherent, low stress
Group IB-Group VIA alloy or mixture thin films at a high deposition
rate in both conformal and non-conformal fashions. Such films can
be used in the preparation of Group IBIIIAVIA compound
semiconductors.
[0025] In one embodiment, for example, the Group IB material may be
Cu and the Group VIA material may be Se to electroplate a copper
selenide, Cu--Se film. Such Cu--Se films may be used to form one or
more layers of a precursor stack of a Cu(In,Ga)(Se).sub.2 absorber
layers. The stoichiometry or composition of such films, e.g. Group
IB/Group VIA atomic ratio, may be controlled by varying the
composition of the electrodeposition solution and by modification
of the appropriate plating conditions. For example, the composition
of Se in the deposited copper-selenide film may be tailored to a
specific composition or varied in a desirable manner within the
thickness of the film. The disclosed electrodeposition Cu--Se
electrodeposition solutions may be used to deposit a copper
selenide films containing more than, for example, 95% Se at pH
values lower than 3 as well as deposit highly copper rich selenide
at pH values higher than 7.5. Through the use of embodiments
described herein it is possible to form micron or sub-micron thick
Cu--Se films on conductive surfaces for the formation of solar cell
absorbers.
[0026] The electrodeposition solution of the present invention
comprises a source of Group IB material, such as a copper source, a
source of Group VIA material such as a selenium source, at least
one complexing agent, an adhesion promoting agent, a corrosion
inhibitor such as multihydric alcohol, for example glycerol, a pH
adjuster and a solvent such as water. The pH range of the plating
solution is between 1 and 13. The pH of the plating bath can be
modified, for example, depending on the nature of the substrate,
and the copper content of the copper selenide film.
[0027] A Cu--Se electrodeposition solution, or electrodeposition
solution or solution hereinbelow, of the present invention includes
known soluble copper salts, for example, sulfates, chlorides,
nitrates, oxides, hydroxides, tetrafluoroborates, citrates, acetate
gluconate, phosphates, sulfamates, carbonates, copper amine salts
or the like as the Cu source. A copper sulfate source may
preferably be used as the Cu source in this invention. Cu source
may comprise stock solutions prepared by dissolving copper metal
into its ionic form in a suitable solvent such as water. The copper
ions concentration may be in the range of 1-500 mM/L and preferably
5-100 mM/L.
[0028] In the Cu--Se electrodeposition solution, selenium oxide may
provide the Group VIA material source although other Group VIA
compounds or elemental Group VIA materials may also be used for
this function as long as they dissolve in the electrodeposition
solution. In certain embodiments, the Group VIA material source may
be compounds of Se, S, and Te such as acids of Se, S, and Te as
well as oxides, chlorides, sulfates, nitrates, perchlorides, and
phosphates of Se, S, and Te. The Se source may preferably include
soluble selenium salts or acid or combination of both, for example,
sodium selenate and selenious acid. The selenium content in the
Cu--Se electrodeposition solution may be in the range of 50-1.5
M/L, preferably 75-700 mM/L.
[0029] The Cu--Se electrodeposition solution may include at least
one complexing agent. The complexing agents may be water soluble
complexing agents with carboxylate and/or an amine group. A blend
of two or more complexing agents can be included to provide full
complexation of Group IB ions including Cu ions. Complexing agents
can be selected from organic acids such as citric acid, tartaric
acid, maleic acid, malic acid, oxalic acid succinic acid,
ethylenediamine (EN), ethylenediaminetetra acetic acid (EDTA),
nitrilotriacetic acid (NTA), and
hydroxyethylethylenediaminetriacetic acid (HEDTA). The salts of
these acids with alkali metals and alkali earth metals can be also
used as the source for complexing agents. Sodium citrate, potassium
tartrate and calcium oxalate can be used to provide citrate,
tartrate and oxalate complexing ions, respectively. Ammonium salts
of these complexing agents can be also used to provide the
complexing ions. For example, ammonium citrate, either in the form
of ammonium citrate dibasic or in the form of ammonium citrate
tribasic form was found to be a very useful source material to
provide citrate complexing ions in the electrodeposition solution.
The complexing agent content of the electrodeposition solution may
preferably be in the range of 5 mM/L to 0.55 M/L, more preferably
between 0.1M to 0.4 M/L. When ammonium citrate is used as the
complexing agent source, the ammonium citrate content of the
electrodeposition solution may vary between 0.05 M/L and 0.4 M/L,
and preferably between 0.08 M/L and 0.35 M/L. It is also possible
to prepare this electrodeposition solution for other Group IB
materials. Silver (Ag) or gold (Au) ion sources may also be added
to Cu--Se electrodeposition solutions to deposit layers including
them. For example, in one embodiment, an electrodeposition solution
including Cu, Ag and Se may be prepared to electrodeposit
Cu--Ag--Se layers. In this case, an additional completing agent for
Ag may or may not be added to the electrodeposition solution.
[0030] The electrodeposition solution of the present invention
might further include other compounds, specifically added to
increase the conductivity of the electrodeposition solution. The
conductivity improving agents might be selected from ammonium salts
of inorganic acids, alkali metal salts of inorganic acids, and
alkali metal earth salts of inorganic acids. The conductivity
improving agents might include but not limited to ammonium sulfate,
potassium sulfate, sodium sulfate, magnesium sulfate, ammonium
nitrate, potassium nitrate, sodium nitrate, magnesium nitrate,
ammonium chloride, potassium chloride, sodium chloride, magnesium
chloride. For example, ammonium sulfate was found to be a very
useful source material to high conductivity in the bath. The
conductivity improving agent content of the electrodeposition
solution may preferably be in the range of 5 mM/L to 0.55 M/L, more
preferably between 0.1M to 0.4 M/L. When ammonium sulfate is used
as the conductivity improving agent source, the ammonium sulfate
content of the electrodeposition solution may vary between 0.05 M/L
and 0.4 M/L, and preferably between 0.08 M/L and 0.35 M/L.
[0031] The adhesion promotion agent added to the Cu--Se electro
deposition solution enhances Cu--Se film formation and its adhesion
to the surface of the underlying layer during the electrodeposition
process. Adhesion promoting agents may be selected from a group of
organic compounds with amine functional groups, which can form very
strong chealation with copper ions. Preferably, organic compounds
with amine functional groups, which also contain at least one
aminopropyl group are desirable because of their ability to form
very stable complexes with copper ions. An exemplary adhesion
promoting agent preferably includes an organic amine molecule with
at least one nitrogen atoms, and preferably with opposing multiple
nitrogen atoms or nitrogen bearing group. For example, organic
compounds with piperazine functional groups may be used as adhesion
promoting agents in this invention. The adhesion promoting agents
not only improve the adhesion of the film but also largely suppress
the formation of powdery or colloidal Se particles or aggregates
that might form on the cathode and within the electrolyte solution
during the plating process. Therefore, they also provide a large
increase in the cathodic efficiency for plated films. Examples of
suitable adhesion promotion agents of this invention included but
not limited to 1,4-Bis(3-aminopropyl) piperazine and N, N'
Bis(3-aminopropyl) ethylenediamine. Alternatively, any possible
mixture of these adhesion promoting agents may be used in the
electrodeposition solution of the present invention. The amount of
the adhesion promoting agents in the electrodeposition solution is
in the range of 0.01 mM/L to 200 mM/L, preferably in the range of
1.0 mM/L to 40 mM/L.
[0032] Another key constituent of the Cu--Se electrodeposition
solution is an organic compound, which acts as a corrosion
inhibitor when electrodeposition is conducted on a galvanically
more active surface, for example Cu--Se plating over In and Ga
metal or alloy thin films comprising In and Ga metals or alloys
comprising In and Ga. Such organic corrosion inhibitors of the
present invention might be selected from multihydric alcohols.
Specifically, glycerol was found to be particularly useful for the
Cu--Se electrodeposition solution. The addition of glycerol into
the Cu--Se electrodeposition solution, suppresses/eliminates the
dissolution or partial dissolution of some substrate thin film
material during the electrodeposition step. The glycerol enhances
substrate wetting and improves coating uniformity by dispersing any
particulates in the solution as well as suppressing their adhesion
to the substrate surface. When added at high concentrations, in the
presence of adhesion promoting compounds with piperazine functional
groups, glycerol may also refine the microstructure of the plated
Cu--Se film, by forming smaller grains in a uniform fashion with a
good surface coverage. The concentration of the multihydric
alcohols, such as glycerol in the present invention may vary
between 20 mM/L to 500 mM/L and preferably between 30 mM/L to 250
mM.
[0033] The Cu--Se electroplating solution of the present invention
might further include grain-refiners. To obtain a layer with a
microstructure with finer grains, inorganic additives with
chloride, pyrophosphate and sulfite ions might be added to the
electrodeposition solution. The chloride additives may include
known water soluble chloride compounds, for example, sodium
chloride, ammonium chloride. The pyrophosphate additives may
include for example potassium pyrophosphate. The sulfite additives
may include sodium sulfite. It is also preferable that the
concentration of the sulfite, or chloride, or pyrophosphate and
their various combinations does not exceed 1% of the Cu--Se
electrodeposition solution. Excessive amount of chlorides,
pyrophosphates and sulfites might produce defective films and may
lead to premature excessive selenium precipitation in the
electrodeposition cell.
[0034] The Cu--Se electrodeposition solutions of present invention
may be prepared in a wide range of pH value between 1 and 13. The
specific pH of the electrodeposition solution depends on factors
such as the nature of the substrate, level of conformity desired in
the film, and the copper content of the copper selenide film. A pH
range between 1 and 5, more preferably between 1.5 to 3, is
suitable for plating operations to deposit Se rich Cu--Se films
with a high Se/Cu ratio, greater than 10. In this acidic pH regime,
the pH of the plating bath may be adjusted with a pH adjuster,
which could be as simple as an inorganic acid such as sulfuric acid
or sulfonic acid or a pH buffer couple chosen for the pH of the
particular application. Increasing pH of the electrodeposition
solution allows deposition of Cu--Se films with higher Cu content.
A highly Cu rich Cu--Se film may be deposited at pH values greater
than 7.5. The pH of the solution in the alkaline regime can be
adjusted by addition of sodium hydroxide, potassium hydroxide or
ammonium hydroxide. Alkaline buffer couples could be also employed
to adjust the pH. Such alkaline pH buffer systems include but not
limited to monopotassium phosphate/dipotassium phosphate, boric
acid/sodium hydroxide, sodium bicarbonate/sodium carbonate,
monosodium tellurate/disodium tellurate, monosodium
ascorbate/disodium ascorbate, and dipotassium
phosphate/tripotassium phosphate. The pH value of the
electrodepositon solution also regulates the conformation of the
Cu--Se film
[0035] Electrodeposited Group IBVIA layers using the
electrodeposition solution can be employed in the preparation of
precursor layers in a two-stage process where such precursor layers
are reacted in a high temperature annealing process. During
reaction, sulfur and/or more selenium might present to produce the
desired compound form. FIG. 2A shows an exemplary precursor stack
100 which will be utilized below to describe various uses of the
electrodeposition solution of the present invention. In this
embodiment, the precursor stack 100 may include a multilayer
structure including a first layer 102, a second layer 104 and an
optional third layer 106. In this embodiment, at least a portion of
the precursor stack 100 is electrodeposited using the Cu--Se
electrodeposition solution of the present invention. During the
process, initially, the first layer 102 may be formed over a base
101 which may include a substrate 101A and a contact layer 101B
formed over the substrate. The second layer 104 is electrodeposited
on the first layer 102 and the third layer 106 may be
electrodeposited on the second layer. Principles of the
electrodeposition process are well known and will not be repeated
here for the sake of clarity. The contact layer 101B may be made of
a molybdenum (Mo) layer deposited over the substrate 101A or a
multiple layers of metals stacked on a Mo layer; for example,
molybdenum and ruthenium multilayer (Mo/Ru), or molybdenum,
ruthenium and copper multilayer (Mo/Ru/Cu). To form a contact layer
having multi layers, for example, Ru layer may be electrodeposited
on the Mo layer, and similarly the Cu layer may be electrodeposited
on the Ru layer to form the contact layer. The substrate 101A may
be a flexible substrate, for example a stainless steel foil, or an
aluminum foil, or a polymer. The substrate may also be a rigid and
transparent substrate such as glass.
[0036] As shown in FIG. 2B, in the following step, the precursor
stack 100 is reacted in a reactor to transform it to an absorber
layer 108 i.e. a compound layer. As will be described below, the
composition of the precursor stack determines the composition of
the resulting absorber layer or compound layer. In one embodiment,
the Cu--Se electrodeposition solution of the present invention may
be used to prepare Group IBIIIAVIA compound semiconductors such as
CuInSe.sub.2, CuGaSe.sub.2 CuAlSe.sub.2, Cu(In, Ga) Se.sub.2,
Cu(In, Al, Ga) Se.sub.2, Cu(In, Al, Ga) (Se, S).sub.2 and Cu(In,
Ga) (Se, S).sub.2, which can be used as solar absorber layers. In
this case, the first layer 102 and the second layer 104 of the
precursor stack may comprise Group IB, Group IIIA and Group VIA
materials, i.e., Cu, In, Ga and Se to form a Group IBIIIAVIA
compound layer.
[0037] In this embodiment, the first layer 104 may be configured as
a stack including a Cu-film, an In-film and a Ga-film, which will
be shown with Cu/In/Ga insignia hereinbelow. This and similar
insignia will be used throughout the application to depict various
stack configurations, where the first material (element or alloy)
symbol is the first film, the second material symbol is the second
film deposited on the first film and so on. For example, in the
Cu/In/Ga stack: the Cu-film, as being the first film of the stack,
may be electrodeposited over the contact layer or another stack;
the In-film (the second film) is electrodeposited onto the Cu-film;
and the Ga-film (the third film) is deposited onto the In-film. In
the first layer 102, the order of such films 102 may be changed,
and the first layer 102 may be formed as a Ga/Cu/In stack or
In/Cu/Ga stack. Furthermore, the first layer 102 may be formed as a
stack of four films, such as Cu/Ga/Cu/In or Cu/In/Cu/Ga. In another
embodiment, the first layer 102 may be formed as a (Cu--In--Ga)
ternary alloy film or as a stack including (Cu--In) binary alloy
film and (Cu--Ga) binary alloy film. Such alloy binary or ternary
alloy films may have any desired compositions. The first layer 102
may be formed by any possible combinations of the above given
stacks of films, binary films and ternary alloy films.
[0038] Referring back to FIG. 2A, the second layer 104 includes a
Cu--Se alloy film electrodeposited using the electrodeposition
solution of the present invention. Alternatively, the second layer
may include a mixture films including at least one of the films
included in the first layer and at least one film of Cu--Se. The
third layer 106 is an optional layer to add more Se to the
precursor stack 100. If more Se is desired in the precursor, it
could be added as the third layer 106. Preferably, the second layer
includes a Cu--Se film.
[0039] Alternatively, the Cu--Se electrodeposition solution of the
present invention may be used to form precursors for absorber
layers comprising as ternary compounds such as CuGaSe.sub.2,
CuInSe.sub.2, CuAlSe.sub.2. In this embodiment, the first layer 102
may comprise a Group IIIA material, such as one of In, Ga, or Al,
and the second layer 104 comprises (Cu--Se) film. Precursor stacks
including Ga/(Cu--Se), In/(Cu--Se) and Al/(Cu--Se) can be prepared
by electrodepositing the (Cu--Se) layer over Ga, In and Al films,
respectively, which are deposited on, for example, a contact layer
such as Mo. Such layers can be later reacted at high temperature in
a S and/or additional Se-containing environment to form
CuGaSe.sub.2, CuInSe.sub.2, CuAlSe.sub.2, respectively.
[0040] The copper selenide (Cu--Se) film of the present invention
may be deposited at very high deposition rates. The
electrodeposition current density may be greater than 30
mA/cm.sup.2 as compared to typical 2 mA/cm.sup.2 of the prior art.
Cu--Se film is very uniform and exhibits excellent adhesion and
around pH=2. The Cu--Se film composition is independent of plating
current density between 5 and 30 mA/ cm.sup.2. The copper content
of the Cu--Se film may be varied between less than 6% and more than
50% by, for example: adjusting pH alone; adjusting copper content
of the Cu--Se electrodeposition solution; adjusting selenium
content of bath; and the addition of sulfonic acid or EDTA. The
Cu--Se electrodeposition solution is capable of electrodepositing a
film including very high Se content. For example, the Cu--Se
electrodeposition solution can deposit copper selenide films with
less than 6% copper on indium substrates with minimal indium loss.
For copper rich continuous and adherent Cu--Se films, the selenous
acid may be replace with sodium selenate and Cu--Se films plated at
pH between 7 and 13, and preferably at a pH between 8 and 11.
EXAMPLE
[0041] An exemplary Cu--Se electrodeposition solution was prepared
and electrodeposited on a thin film of In with approximately 3000
Angstrom thickness, which was electroplated over a Mo/Cu containing
stack on stainless steel substrate. The Cu--Se electrodeposition
solution includes the following composition: 5 g/L copper sulfate
pentahydrate; 30 g/L sodium citrate; 60 g/L sodium sulfate; and 50
g/L selenious acid. The pH of the electrodeposition solution was
adjusted to 2 by adding sulfuric acid. During the
electrodeposition, a potential difference is established between
the metallic surface and an anode such as Pt coated Ti electrode. A
plating current density of about 30 mA/cm.sup.2 is applied for 60
seconds. The electrodeposition process resulted in a Cu--Se film
having a thickness of about 330 nm. Generation of Se particles or
aggregates was observed emanating from the cathode surface during
the deposition step. In the following experiment, the addition of
1.25 ml/liter 1,4-Bis(3-aminopropyl) piperazine as the adhesion
promoting agent to this electrodeposition solution produced an
extremely adherent film and no visible Se particle or aggregate
generation during the plating process. This provided a large
increase in the cathodic efficiency yielding a thickness of 562
nm.
[0042] The Cu--Se electrodeposition solutions may also be used to
form graded Cu--Se films, or to vary the copper composition
profile, within the film thickness, by varying electrodeposition
current density during the electrodeposition. The plating current
density may be graded from a high current density and terminated
with a much lower current density or vice versa. The much lower
current density Cu--Se film containing comparatively higher copper
percentage than the film plated at higher current density. The
current may be pulsed between a higher and a low current density
for desirable time intervals to coat a laminated selenide films
containing with different copper concentrations. For example, at a
fixed pH, a Cu--Se layer may be deposited at 25 mA/sq.cm for 60 s,
the current density is the reduced to 2.5 mA/sq.cm for 30 s. The
selenide film deposited at 25 mA/sq.cm may contain about 8% copper
and the film deposited at 2.0 mA/sq.cm may contain about 14% copper
in the layer, producing a Cu--Se alloy with a copper rich surface.
The laminate may be symmetrical or assymetrical, the initial alloy
coating may be relatively copper rich or copper poor with respect
to the subsequent Cu--Se material coated.
[0043] The Cu--Se electrodeposition solution may be adapted for
multi-cell operations, where individual cell may dispose variant
chemistries of the Cu--Se electrodeposition solution. Such
multi-cell plating approach may be used to vary the copper
composition profile within the coating thickness. The substrate may
be plated sequentially or non-sequentialy across the various
plating cells to achieved the desired alloy profile. The deposited
Cu--Se films exhibit low stress characteristics. For example, more
than 3 micron Cu--Se film may be electrodeposited on an indium film
without film cracking or any local delamination. The Cu--S alloy
films of this invention may be plated galvanostatically, the said
alloys may also be deposited potentiostatically or various
combinations of current and voltage mode.
[0044] The Cu--Se film of the present invention may be used to
replace a significant portion of the evaporated selenium in the
precursor, resulting in a very significant reduction in the
operating cost of selenium evaporators. In another embodiment of
this invention, the electrodeposited Cu--Se film may be covered or
coated with other thin film materials, for example, sodium
fluoride, indium, gallium, binary or ternary photovoltaic alloy
material using PVD deposition techniques prior to the selenization
step. As mentioned before, the Group IB material may also include
silver (Ag) or gold (Au). For example
[0045] Although the present invention is described with respect to
certain preferred embodiments, modifications thereto will be
apparent to those skilled in the art.
* * * * *