Patent application title: SELENIUM ELECTROPLATING CHEMISTRIES AND METHODS
Serdar Aksu (Santa Clara, CA, US)
Yongbong Han (Fremont, CA, US)
Bulent M. Basol (Manhattan Beach, CA, US)
IPC8 Class: AC25D1500FI
Class name: Electrolysis: processes, compositions used therein, and methods of preparing the compositions electrolytic coating (process, composition and method of preparing composition) coating contains embedded solid material (e.g., particles, etc.)
Publication date: 2009-11-19
Patent application number: 20090283411
An electroplating solution to electroplate a selenium containing film on a
conductive surface is provided. The electroplating solution includes a
solvent, a selenium source material that dissolves in the solvent; an
anti-coagulation agent that inhibits Se particle growth and promotes Se
particle dispersal. The pH value of the electroplating solution is in the
range of 2-10.
1. A solution to electroplate a selenium containing film on a conductive
surface, comprising:a solvent,a selenium source material that dissolves
in the solvent, andan anti-coagulation agent that inhibits growth of Se
particles and promotes the dispersal of such Se particles when
electroplating a selenium containing film onto a conductive surface;
wherein the pH value of the solution is in the range of 2-10.
2. The solution of claim 1, wherein the solution includes substantially no reducing agents.
3. The solution of claim 2 wherein the pH of the solution is in the range of 4-8.
4. The solution of claim 2, wherein the solvent comprises water.
5. The solution of claim 4, wherein the selenium source material comprises at least one of selenious acid, selenium oxide, and a metal selenide group material, wherein the metal selenide group material comprises a selenide selected from the group consisting of potassium selenide, sodium selenide, calcium selenide, copper selenide, indium selenide, and gallium selenide
6. The solution of claim 1, wherein the anti-coagulation agent comprises a sulfonate based material.
7. The solution of claim 5, wherein the anti-coagulation agent comprises a sulfonate based material.
8. The solution of claim 7, wherein the sulfonate based material comprises one of lignosulfonate, sodium lignosulfonate, potassium lignosulfonate, calcium lignosulfonate, ammonium lignosulfonate and lignin sulfonic acids.
9. The solution of claim 6, wherein the sulfonate based material has a concentration in the range of 2 parts per million to 500 parts per million.
10. The solution of claim 9 wherein the sulfonate based material has a concentration in the range of 5 parts per million to 125 parts per million.
11. The solution of claim 1, wherein the selenium source comprises Se dissolved in one of sulfuric acid, nitric acid and hydrochloric acid.
12. The solution of claim 1 further comprising at least one of indium, gallium and copper ions.
13. A method of electroplating a film comprising selenium on a surface of a conductive layer, comprising the steps:(a) providing an electroplating solution comprising a solvent, a selenium source to supply selenium ions to the solvent, and an additive that inhibits growth of Se particles and promotes the dispersal of such Se particles during the step of electroplating the selenium containing film on the surface of the conductive, wherein the electroplating solution has a pH value in the range of 2 and 10;(b) contacting the electroplating solution with the surface of the conductive layer and the anode;(c) establishing a potential difference between the anode and the conductive layer; and(d) electroplating the Se containing film on the surface of the conductive layer.
14. The method of claim 13 wherein the Se containing film is substantially free of aggregated Se particles.
15. The method of claim 13 wherein the conductive layer is one of copper, indium and gallium.
16. The method of claim 13 wherein the pH value of the electroplating solution is in the range of 4-8.
17. The method of claim 13, wherein the electroplating solution of step (a) comprises substantially no reducing agents.
18. The method of claim 13, wherein the selenium source comprises at least one of selenious acid, selenium oxide, and a metal selenide group material, wherein the metal selenide group material is selected from the group consisting of potassium selenide, sodium selenide, calcium selenide copper selenide, indium selenide, and gallium selenide.
19. The method of claim 13, wherein the additive comprises a sulfonate based material.
20. The method of claim 19, wherein the sulfonate based material comprises at least one of lignosulfonate, sodium lignosulfonate, potassium lignosulfonate, calcium lignosulfonate, ammonium lignosulfonate, a lignin sulfonic acid, and an alkali metal salt of a lignin sulfonic acid.
21. The method of claim 19, wherein the sulfonate based material has a concentration in the range of 2 parts per million to 500 parts per million.
22. The method of claim 21, wherein the sulfonate based material has a concentration in the range of 5 parts per million to 125 parts per million.
23. The method of claim 13, wherein the electroplating solution further comprises at least one of indium ions, gallium ions and copper ions.
24. The method of claim 13, wherein the selenium source comprises Se dissolved in one of sulfuric acid, nitric acid and hydrochloric acid.
1. Field of the Invention
The present invention generally relates to electroplating solutions and methods and, more particularly, to techniques to form Group IBIIIAVIA compound absorber layers for solar cells.
2. Description of the Related Art
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.
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)2 or CuIn1-xGax (SySe1-y)k, where 0≦x≦1, 0≦y≦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)2, a more accurate formula for the compound is Cu(In,Ga)(S,Se)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)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.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)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)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)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.
The first technique that yielded high-quality Cu(In,Ga)Se2 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)2 type compound thin films for solar cell applications is a two-stage process where metallic components of the Cu(In,Ga)(S,Se)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 CuInSe2 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)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)2 absorber.
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 CuInSe2 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.
Two-stage processing approach may also employ stacked layers comprising Group VIA materials. For example, a Cu(In,Ga)Se2 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)2 or CIGS(S) absorber layer.
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 CuInSe2 compound absorber.
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 In2Se3, J. Electroanalytical Chemistry, vol. 412, p. 95, 1996). Kemell et al. ("Electrochemical quartz crystal microbalance study of the electrodepositon mechanism of Cu2-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 CuCl2, InCl3, GaCl3 and SeO2 (U.S. Pat. No. 6,872,295). The electrochemical co-deposition of CIS films is also performed using a solution containing Cu2+, In3+, Se4+ and citrate salts, as reported in the literature (Oliveira et al., "A voltammetric study of the electrodeposition of CuInSe2 in a citrate electrolyte", Thin Solid Films, vol. 405, p. 129, 2002).
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), α-monoclinic, β-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 H2SO4 at a temperature of 100° 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.
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° 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. As the plating continues, both the number and the size of red selenium particles increase in the electroplating solution. Some particles get trapped on the cathode surface and form defects in the deposited Se film in the form of particle inclusions. FIG. 2 shows a Se layer 25 electroplated on a conductive surface 24 forming a stack 23 using prior art electroplating approaches. The conductive surface 24 may the surface of a precursor film 28, which may be formed on a base 29. As can be seen, the plated Se layer 25 comprises Se particles 26 embedded in a relatively uniform Se matrix 27. Selenium particles 26 disturb the uniformity of the electroplated Se layer 25, cause surface roughness and affect kinetics of reaction between the Se layer 25 and underlying precursor film 28 when the stack 23 is heated to form a compound absorber layer. For example, if the precursor film 28 shown in FIG. 2 is a Cu--In layer and if the stack 23 is heated up to over 400° C. to react the Cu, In and Se species to form a CIS compound film on the surface of the base 29, the reaction kinetics at a first portion "A" of the stack 23 would be very different from the reaction kinetics at a second portion "B" because the Se/(Cu+In) molar ratio in the first portion "A" is much larger than the same molar ratio in the second portion "B" due to the presence of a large Se particle within the portion "A". Such compositional differences between portions of the stack create morphological, electrical and compositional differences between corresponding portions of the compound CIS layer obtained after the reaction step, reduce its uniformity and thus reduce the efficiencies of solar cells that may be fabricated on such non-uniform layers. It should be noted that the thickness of electroplated Se layers may be in the range of 50-5000 nm whereas the size of the Se particles may range between 500-10000 nm, depending upon the length of the electrodeposition period.
As described before, Se particles typically form near the cathode surface. Some get trapped into the growing Se film on the cathode, others are swept away and precipitate on other wetted surfaces of the electrodeposition system, coating such surfaces and becoming a source of particles and defectivity throughout the electrodeposition process. This is very undesirable, especially for continuous manufacturing processes such as roll-to-roll electroplating, since deposition in such systems continues uninterrupted for several hours and even for several days. Stopping the electrodeposition process for cleaning the wetted parts to eliminate Se particles is time consuming, expensive and impractical. Furthermore, formation of Se particles, which may stay close to the surface of the cathode and grow in size depletes the dissolved Se concentration in the plating bath fast. As a result the concentration of Se in the solution decreases with time, leading to bath stability problems. The formation of colloidal red amorphous 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.
From the foregoing, there is a need in the solar cell manufacturing industry, especially in thin film photovoltaics, for better Se or Se-containing film electroplating methods that minimizes Se particle formation, minimizes growth of large Se particles and minimizes coating of the wetted parts of the electroplating system by Se and/or selenide particles.
SUMMARY OF THE INVENTION
Present invention provides in certain embodiments a solution to electroplate a selenium containing film on a conductive surface, wherein the solution comprises a solvent, a selenium source material that dissolves in the solvent, and an anti-coagulation agent that inhibits growth of Se particles and promotes the dispersal of such Se particles when electroplating a selenium containing film onto a conductive surface; and wherein the pH value of the solution is in the range of 2-10. In certain embodiments the electroplating solution is substantially free of reducing agents.
The invention provides in certain embodiments a method of electroplating a film comprising selenium on a surface of a conductive layer where the method comprises the steps of (a) providing an electroplating solution comprising a solvent, a selenium source to supply selenium ions to the solvent, and an additive that inhibits growth of Se particles and promotes the dispersal of such Se particles during the step of electroplating the selenium containing film on the surface of the conductive, wherein the electroplating solution has a pH value in the range of 2 and 10; (b) contacting the electroplating solution with the surface of the conductive layer and an anode; (c) establishing a potential difference between the anode and the conductive layer; and (d) electroplating the Se containing film on the surface of the conductive layer. In certain embodiments, the electroplating solution of step (a) is substantially free of reducing agents. In certain embodiments the resulting Se containing film is substantially free of aggregated Se particles.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a prior art solar cell structure;
FIG. 2 is a schematic view of a stack formed by electroplating a Se layer by a prior art method on a precursor film formed on a base; and
FIG. 3 is a schematic view of a stack formed by electroplating a Se layer by an embodiment of the process of the present invention on a precursor layer formed on a base.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a selenium electroplating solution that minimizes Se particle growth during Se electrodeposition. Small Se aggregates or colloids generated in the diffusion layer adjacent a cathode surface during plating are efficiently captured and dispersed in the plating solution by an additive added to the plating solution. As a result they substantially do not stay and grow near the cathode and they substantially do not get included into the growing Se layer causing non-uniformities and defects. Invention also works for electrolytes or solutions used for the electroplating of M-Se, where M may be a metal such as a material comprising at least one of In, Cu and Ga. The principles of the present invention may also be applied to electrodeposition of other group VIB elements such as tellurium and sulfur and co-deposition of such elements with Se as well as metals such as Cu, Ga and In.
Selenium plating solutions may be prepared by dissolving desired amounts of Se sources such as selenious acid or selenium oxide in water and adjusting the solution pH by adding acids or bases. Depending on the pH value it is possible to electrodeposit Se from H2SeO3 in acidic solutions, HSeO2- in neutral solutions and SeO2- in alkaline solutions through the reactions (i)-(ix) as shown below. Because the valance of Se in such species is +4, they are referred to as Se(IV+) species. During the electroplating process, Se(IV+) species are reduced to the Se(0) or Se on a cathode surface by the applied cathodic current according to the following electrochemical reduction reactions:
(i) H2SeO3+4H++4e=Se+3H2O (in acidic solutions),
(ii) HSeO2-+5H++4e=Se+3H2O (in neutral plating solutions), and
(iii) SeO32-+6H++4e=Se+3H2O (in alkaline plating solutions).
The reactions (i) through (iii) involve 4 electron exchanges during the reduction reaction to deposit elemental Se from the solution on the conductive surface. However, in addition to the reactions listed above, the following reduction reactions may also be possible:
(iv) H2SeO3+6H++6e=H2Se+3H2O (in acidic plating solutions),
(v) HSeO3-+6H++6e=HSe-+3H2O (in neutral plating solutions), and
(vi) SeO32-+6H++6e=Se2-+3H2O (in alkaline plating solutions).
The reactions (iv) through (vi) above involve 6 electron exchange and produces Se(II-) species such as H2Se, HSe-, Se2-. Such Se(II-) species may further react with the dissolved Se(IV+) species (H2SeO3 in acidic solutions, HSeO2- in neutral solutions and SeO2- in alkaline solutions) according to the following reactions:
(vii) H2SeO3+2H2Se=3Se+3H2O (in acidic plating solutions),
(viii) HSeO3-+2HSe-+3H+=3Se+3H2O (in neutral plating solutions), and
(ix) SeO32-+2Se2-+3H2O=3Se+6OH- (in alkaline plating solutions).
Because the equilibrium potentials of the reactions (i) through (iii) and the reactions (iv) through (vi) are very close to each other, these reactions may occur simultaneously even at low current densities. The reactions (vii through ix) may result in forming colloids containing elemental Se particles near the cathode surface.
As these reactions continue, more and more Se particles may form or Se may aggregate into clusters and form colloidal Se chains. Se colloids and other particles produced this way may grow in both size and number with time. The Se particles may precipitate onto the conductive surface during the electrodeposition process and get included in the growing Se film. As mentioned above such particle inclusion into electroplated Se films is undesirable, and precursor stacks utilizing Se films with Se particles and rough surface morphology have a Se content that varies locally. When such precursor layers are converted into solar cell absorbers, they yield absorbers with non-uniformities. Also, as explained before, excessive Se particle growth causes the concentration of Se in the solution to decrease fast and Se particles need to be cleaned off the plating system. In addition to unwanted increased waste of Se solid, this may also lead to difficulties in maintaining a stable electroplating bath for industrial and continuous electroplating processes.
In one embodiment of the present invention, the above described colloidal Se growth may be minimized and even eliminated by using a Se electroplating solution comprising specially formulated additives, such as an anti-coagulation additive, of the present invention that inhibits such growth of Se particles. If any particles start to grow near the cathode, they are dispersed away when they are only 5-100 nm in size. Such dispersed particles do not precipitate on and coat the surfaces of the wetted parts of the electroplating system. Particles are easily directed to a filtration unit by a pump which may continually re-cycle the solution between the filtration unit and the electroplating unit. This way, the plating solution stays substantially clear and particle-free and the wetted parts of the electroplating system stay clean. Since most of the particles cannot stay near the cathode and grow in size, Se concentration in the solution stays high, utilization of Se is improved and waste of Se is minimized. Furthermore, the electroplating solution of the present invention yields electroplated Se layers that are free of large colloidal Se particles or inclusions. This, in turn, improves the morphology, uniformity and surface roughness of the deposited Se films. It should be noted that the method of the present invention minimizes particle inclusion into the growing film. Any particles that may not be captured efficiently by the additive of the present invention may grow to a size that is much smaller than the size they would grow to without the additive.
In another embodiment, the present invention further provides a method to electroplate high quality Se layers on top of conductive surfaces with high deposition efficiency and repeatability. The present invention may be used to manufacture Group IBIIIAVIA compound solar cell absorbers including Group IB (such as Cu), Group IIIA (such as Ga and In), and Group VIA (such as Se, Te and S) elements. To manufacture a solar cell absorber layer, initially, an absorber precursor layer must be formed over a base which may include a substrate and a contact layer that is formed on a surface of the substrate. FIG. 3 shows an absorber precursor structure 100 having a conductive layer 102 and Se layer 104 electroplated on the conductive layer 102 using the present invention. The conductive layer 102 is formed on the base 106 which may comprise a substrate 108 and a contact layer 110. The contact layer 110 may comprise materials such as Mo, W, Ta, Ru, Os, Ir, etc., and the substrate 108 may be a metallic foil, glass or polymeric sheet or web. The conductive layer 102 may include at least two of Cu, In and gallium Ga. The conductive layer 102 may be in the form of a stack containing layers including Cu, In and Ga. During the selenium electroplating process, the conductive layer 102 is cathodically polarized with respect to an anodically polarized electrode, and the selenium is electrodeposited from the plating solution of the present invention onto the conductive layer 102 (cathode surface).
Selenium layer 104 can be directly plated onto Cu, In or Ga containing surfaces to form precursor stacks having various configurations. Such configurations include, but are not limited to Cu/In/Se, Cu/In/Ga/Se, Cu--Ga/In/Se etc., and they may be used to manufacture CIS or CIGS type solar cell absorber films. One method of growing CIS or CIGS absorber films using electroplating is the "two-stage" process. In the two-stage process, controlled amounts of Cu, In, Ga and Se are electrodeposited in the form of Cu, In, Ga and Se containing thin film precursor stacks such as Cu/In/Ga/Se, Cu/Ga/In/Se, In/Cu/Ga/Se, Ga/Cu/In/Se, In/Ga/Cu/Ga/Se, In/Ga/Cu/In/Se, Ga/In/Cu/Ga/Se, Ga/In/Cu/In/Se, Cu/Ga/Cu/In/Se, Cu/In/Cu/Ga/Se or the like. These stacks may then be annealed, or reacted, optionally with more Se, sulfur (S), tellurium (Te) or sodium (Na), to form a uniform thin film of the CIGS(S) alloy or compound on the contact layer. By controlling the thickness and morphology of the Cu, In, and Ga as well as Se layers within the precursor stacks, the process yield in terms of compositional control may be improved compared to the prior-art methods.
The selenium electroplating solution of the present invention may be prepared by; i) dissolving a desired amount of at least one Se source, such as selenious acid and selenium oxide, in a solvent such as water, ii) adding to the solution at least one anti-coagulation additive, and, iii) adjusting the pH of the solution by adding an acid or base. The at least one anti-coagulation additive included in the formulation of the present invention captures selenium aggregates while they are very small in size, and thereby prevents them from coagulating and growing Selenium aggregates are believed to be captured by the functional groups on the hydrocarbon chain of the anti-coagulation additives which instantly coat the surfaces of the small particles. Once the smaller Se aggregates are spatially isolated from each other by the anti-coagulation agent or additive, formation of large Se chains and larger colloidal Se particles are avoided.
The pH of the selenium electroplating solution of the present invention may be adjusted to the range of 2-10. The preferred pH range is at neutral regime of 5-6. In this preferred pH range, corrosion of the cathode surface is avoided due to the neutral pH value. It should be noted that the selenium electroplating solution of the present invention preferably does not include any reducing agents. When describing the electroplating solution of the invention as containing substantially no reducing agents, it is intended that no or substantially no reducing agents are added to the electroplating solution and, further, that any Se2- formed in the process of use of the electroplating solution is understood not to be a reducing agent for this purpose. Reducing agents such as SO2 may reduce the ionic selenium species in the solution and cause Se particle formation even before the electroplating process is initiated by applying a voltage between the cathode and the anode. In fact, the main goal of the present invention is reduction or elimination of such particle formation.
A preferred anti-coagulation additive of the present invention is a sulfonate based material. The sulfonate based materials that may be used as additives include but are not limited to lignin sulfonic acids and their alkali salts, ammonium lignosulfonate, organic poly-sulfonic acids such as poly-2-acrylamide-2-methyl-1-propanesulfonic acid, poly-anetholesulfonic acid, poly-sodium4-styrenesulfonic acid, poly-vinylsulfonic acid, poly-sodium4-styrenesulfonic acid, and poly-vinylsulfonic acid, poly-4-styrenesulfonic acid-co-maleic acid and the alkali salts of these acids.
In one embodiment, the electroplating solution of the present invention may comprise alkali salts of lignosulfonates as the anti-coagulation additives. Lignosulfonates or sulfonated lignin (CAS number 8062-15-5) are water-soluble anionic surfactants. Salts of lignosulfonate are obtained as by-products of the paper pulp industry. They contain functional groups of hydroxide (--OH), carboxylate (--COOH) in addition to sulfonate (--SO3H). Because lignosulfonates are very stable both in acidic and alkaline solutions, they may be used in plating applications with pH values ranging from acidic to alkaline regimes. Lignosulfonate prevents the clumping and settling of un-dissolved solid particles in liquid suspensions. Once attached to the Se particle surface as an anti-coagulation agent, lignosulfonate may keep the particle from being attracted to other Se particles and hence disperses the particles.
In addition to sulfonate based anti-coagulation additives, additives with carboxylate groups such as polycarboxylic acids may also be used in the practice of the present invention. Such carboxylate based additives include but are not limited to polyacrylic acid, polyepopoxysuccinic acid, copolymer of maleic and acrylic acid, and their alkali salts. Other surfactants such as sodium dodecyl sulfate, ammonium lauryl sulfate, other alkyl sulfate salts may also be included in the solution chemistry to improve wettability of the surfaces. In the following paragraphs, exemplary processes of Se electrodeposition using various embodiments of the present invention will be described.
In this example, the effect of a sulfonate based anti-coagulation additive on Se electrodeposition is demonstrated by comparing the results of a first Se electroplating process conducted without this additive (prior art method) to a second Se electroplating process using the anti-coagulation additive (an embodiment of the present invention). In both processes and in the following examples, selenium was electroplated on a conductive stack containing Cu, In and Ga, which was formed on a base comprising a stainless steel foil substrate. The top surface of the conductive stack which was coated with Se comprised more than about 80 atomic percent In.
In the first electroplating process, Se electroplating was carried out using an electroplating solution comprising H2SeO3. The pH value was adjusted to the range of 5-6. This prior art electroplating solution did not contain any anti-coagulation additive. During electroplating, the conductive stack on the base was placed in a selenium electroplating chamber and was cathodically polarized with respect to an anode. Selenium electroplating was carried out on the In-rich surface of the stack at a current density of about 10 mA/cm2. After about 27 seconds, a reddish color amorphous Se film with an average thickness of about 120 nm was formed. Examination of the surface morphology of this deposit using a scanning electron microscope revealed that large porous Se particles with open pore structure were embedded into the electroplated film. These large selenium particles with open pore structure were distributed throughout the plated surface with a density of about 50 particles per square millimeter. The size of the particles was found to be ranged from 5 to 10 micrometers.
In the second plating process, a lignosulfonate was added as an anti-coagulation agent to the above plating solution in the amount of about 7.5 ppm. Selenium electroplating was carried out on similar cathode surface under similar plating conditions. After about 36 seconds, a reddish color amorphous Se film with an average thickness of about 120 nm was formed on the indium-rich surface of the cathode. SEM examination of the Se coating showed that both the number and the size of the particles on the film surface were reduced. The Se particles observed were smaller than about 2 micrometers in size, and the areal density of such particles were found to be around 3 particles per square millimeter. These experiments demonstrated that the morphology of electroplated selenium layers can be improved by the anti-coagulation additives such as a lignosulfonate salt in the plating electrolyte. It should be noted that the conductive layer surface in this example was In-rich. However, other conductive surfaces such as Cu-rich and/or Ga-rich surfaces may also be employed. Selenium layers in this example were electroplated at 25° C. Since lignosulfonates are known to be stable up to 150° C., Se plating solutions with these additives can be heated up to higher temperatures.
Selenium particle content of the plating electrolytes was evaluated by carrying out plating process for extended periods of time. When Se electrodepositon was performed in an electroplating solution comprising H2SeO3 without any anti-coagulation additives, visible selenium particles were generated in the electroplating solution only after a couple minutes of plating and the particles started to coat the walls of the electroplating vessel. However, when the anti-coagulation additives were added to the electroplating solution, no selenium particles could be observed by naked eye in the plating solution even after one hour of plating process, i.e. solution was clear and no Se particle coating of the electroplating vessel walls was observed.
It is understood that while the above tests have been conducted for substantially pure Se film electrodeposition, the anti-coagulation additives of the present invention may be used for plating Se containing films such as indium selenide, gallium selenide, copper selenide, copper indium selenide, copper gallium selenide and copper indium gallium selenide. The principles of the present invention is also applicable to electroplating processes for other group VIB elements such as tellurium and sulfur and co-deposition of such elements with Se as well as metals such as Cu, Ga and In.
Although the present invention is described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art.
Patent applications by Bulent M. Basol, Manhattan Beach, CA US
Patent applications by Serdar Aksu, Santa Clara, CA US
Patent applications by Yongbong Han, Fremont, CA US
Patent applications in class Coating contains embedded solid material (e.g., particles, etc.)
Patent applications in all subclasses Coating contains embedded solid material (e.g., particles, etc.)