Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/54—Electroplating: Baths therefor from solutions of metals not provided for in groups C25D3/04 - C25D3/50
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F10/00—Individual photovoltaic cells, e.g. solar cells
  • H10F10/10—Individual photovoltaic cells, e.g. solar cells having potential barriers
  • H10F10/16—Photovoltaic cells having only PN heterojunction potential barriers
  • H10F10/167—Photovoltaic cells having only PN heterojunction potential barriers comprising Group I-III-VI materials, e.g. CdS/CuInSe2 [CIS] heterojunction photovoltaic cells
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F77/00—Constructional details of devices covered by this subclass
  • H10F77/10—Semiconductor bodies
  • H10F77/12—Active materials
  • H10F77/126—Active materials comprising only Group I-III-VI chalcopyrite materials, e.g. CuInSe2, CuGaSe2 or CuInGaSe2 [CIGS]
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/56—Electroplating: Baths therefor from solutions of alloys
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/50—Photovoltaic [PV] energy
  • Y02E10/541—CuInSe2 material PV cells

Definitions

• This invention relates to gallium (Ga) electroplating methods and chemistries to deposit uniform, defect free and smooth Ga films with high plating efficiency and repeatability. Such layers may be used in fabrication of electronic devices such as thin film solar cells.
• Gallium is an element that is used in semiconductor and electronics industries. Gallium is generally recovered as a by-product from Bayer-process liquors containing sodium aluminate (see for example, U. S. Patent 2,793,179 and U.S. Patent 2,582,377). Although electrodeposition is a common method to recover bulk Ga (see for example, U.S. Patent 3,904,497) out of basic or acidic solutions, or to purify bulk Ga, there have not been many applications for this material where thin films were deposited with controlled uniformity, morphology and thickness. Therefore, only a few electroplating bath chemistries and processes were developed and reported for the deposition of thin layers of Ga on substrates for electronic applications.
• Ga- chloride solutions with pH values varying between 0 and 5 were evaluated by S. Sundararajan and T. Bhat (J. Less Common Metals, vol.l 1, p. 360, 1966) for electroplating of Ga films.
• Prior-art Ga electroplating techniques utilizing simple electrolytes operating under acidic or basic pH values are not suitable for the above mentioned electronics applications for a variety of reasons, including that they result in poor plating efficiencies and films with rough morphology (typically surface roughness larger than about 20% of the film thickness).
• Gallium is a difficult metal to deposit without excessive hydrogen generation on the cathode because Ga plating potential is high.
• Hydrogen generation on the cathode causes the deposition efficiency to be less than 100% because some of the deposition current gets used on forming the hydrogen gas, rather than the Ga film on the substrate or cathode.
• Hydrogen generation and evolution also causes poor morphology and micro defects on the depositing films due to the tiny hydrogen bubbles sticking to the surface of the depositing film, masking the micro-area under them, and therefore impeding deposit on that micro-area. This causes micro-regions with less than optimum amount of Ga in the film stack. Poor plating efficiencies inherently reduce the repeatability of an electrodeposition process because hydrogen generation phenomenon itself is a strong function of many factors including impurities in the electrolyte, deposition current densities, small changes on the morphology or chemistry of the substrate surface, temperature, mass transfer etc. As at least one of these factors may change from run to run, hydrogen generation rate may also change, changing the deposition efficiency.
• Electrodeposition of Ga out of low pH aqueous electrolytes or solutions may suffer from low cathodic efficiencies arising from the presence of a large concentration of H + species in such electrolytes. Therefore, hydrogen gas generation may be expected to lessen at higher pH values.
• Ga forms oxides and hydroxides which may precipitate as reported in the literature. Only at extremely alkaline pH values these oxides/hydroxides dissolve as soluble Ga species. Therefore, it becomes possible to electrodeposit Ga in a bath of pH>14 containing Ga salts as was done in prior-art techniques using high concentrations of KOH and NaOH in the bath formulation.
• the present invention relates to gallium (Ga) electroplating methods and chemistries to deposit uniform, defect free and smooth Ga films with high plating efficiency and repeatability. Such layers may be used in fabrication of electronic devices such as thin film solar cells.
• the present invention provides a solution for application on a conductor.
• the solution includes a Ga salt, a complexing agent, and a solvent, wherein the solution provides by electrodeposition a sub-micron thickness Ga containing film on the conductor.
• the solution further includes one or both of a Cu salt and an In salt, and the pH of the solution is substantially 7 or higher.
• the present invention provides a method to electroplate Ga films onto conductive surfaces at high deposition efficiency and repeatability.
• Two particular conductive surfaces used in this invention are Cu and In surfaces.
• the present invention may be used to manufacture Cu/In/Ga, Cu/Ga/In, In/Ga/Cu and other metallic stacks, which in turn may be employed in processing CIGS(S) type solar cell absorbers.
• a recently developed "two-stage" processing method for growth of CIGS(S) thin films for example, controlled amounts of Cu, In and Ga are electrodeposited in the form of Cu, In and Ga containing thin film stacks such as Cu/In/Ga, Cu/Ga/In, In/Cu/Ga, Ga/In/Cu, Ga/Cu/In, Cu/Ga/Cu/In, Cu/In/Cu/Ga etc. stacks, on a base such as a substrate coated with a conductive contact layer. These stacks are then reacted with Se and/or S to form a thin film of the CIGS(S) compound on the contact layer. Details of such a processing approach may be found in the following patent applications, each of which are expressly incorporated by reference herein:
• the thickness of Ga layers in such stacks is typically sub-micron, more typically in the range of 50-200 nm.
• the electrical and optical properties of the compound semiconductors such as CIGS(S) are highly sensitive to the stoichiometry or composition of the material. Specifically, these properties strongly depend on the Cu/(In+Ga) and Ga/(Ga+In) molar ratios throughout the film. Efficiency of solar cells fabricated on such compound semiconductor layers, in turn, depends on the optical and electrical properties of the layers.
• the typical acceptable CIGS(S) film composition has a Cu/(In+Ga) molar ratio in the 0.8- 1.0 range whereas the Ga/(Ga+In) molar ratio may be in the range of 0.1-0.3.
• copper layers may be electroplated or sputter deposited on a base comprising a substrate which, on its surface may have a conductive contact film such as a Mo layer and/or a Ru-containing layer.
• the substrate may be a metallic foil, glass or polymeric sheet or web.
• the Ru containing layer on the substrate surface may be a Ru layer, a Ru-alloy layer, a Ru compound layer or a stack containing Ru such as a Mo/Ru stack or in general a M/Ru stack, where M is a conductor or semiconductor.
• Gallium electroplating on the Cu surface (or the In surface) can be carried out at various current densities, such as at 5, 10, 20, 30, 40 and 50 mA/cm 2 ,using the electrolytes of the present invention. Both DC and/or variable (such as pulsed or ramped) voltage/current waveforms may be used for electroplating the Ga layer.
• this invention provides a class of Ga plating baths containing complexing agents.
• Complexing agents complex the Ga in the bath, forming complexes which may be in general represented by Ga k+ (L m" ) n .
• the value of "k” may be 3.
• Complexing agents may serve multiple purposes. Among these are: i) reduction of free Ga ion concentration in the bath, ii) reduction of Ga salt precipitation, and, iii) maintenance of a stable pH.
• Ga salts may be dissolved in a basic solution at reasonably large concentrations of 0.1-1. OM, without precipitation, so that hydrogen generation is reduced and deposition efficiency is enhanced.
• Some of the advantages for the bath compositions of the present invention are: i) since the pH is typically higher than 7, preferably higher than 9, hydrogen generation is reduced, ii) since the pH is preferably lower than 14, excessive corrosion problems are avoided, iii) complexed Ga species form small grained smooth Ga deposits in a repeatable manner.
• the invention will now be described by presenting several examples.
• the electroplating experiments in these examples were carried out using a potentiostat/galvanostat (EG&G Model 263 A).
• the substrates for the plating tests included stainless steel and soda-lime glass, both coated with a 500 nm thick Mo layer followed by a Ru layer which had a thickness in the range of 5-100 nm.
• a 50-200 nm of Cu layer was electroplated on the Ru surface.
• Gallium was then electroplated on the Cu surface and the results were evaluated.
• the surface areas for the substrates were varied from several cm 2 to several hundreds cm 2 to understand the suitability of the method for large scale manufacturing.
• the uniformity and the plating efficiency were evaluated by dissolving various portions of the film and using Atomic Absorption Spectroscopy to measure the Ga amounts in the dissolved samples.
• Example 1 (citrate as the complexing agent):
• a set of exemplary aqueous plating baths were prepared containing 0.2 - 0.5 M GaCl 3 , and 0.5 - 0.8 M sodium citrate (Na 3 C 6 H 5 O 7 ). The pH was adjusted to a range between 10 and 13.
• Gallium was electrodeposited on the copper surface at current densities of 30-50 mA/cm 2 . Highly adherent Ga films with surface roughness of ⁇ 10 nm were obtained for a thickness of 100 nm. The plating efficiency was measured and found to be in the 85-100% range, the higher current density yielding more efficient deposition process.
• Gallium was also plated on other metal surfaces also using the citrate containing complexed baths.
• An aqueous plating bath was formulated with 0.2 M GaCl 3 and 0.4 Molar EDTA. The pH was adjusted to the range of 12-14 using NaOH. The plating tests were carried out on electroplated copper surfaces at current densities of 10-50 mA/cm". All Ga films were shiny with smooth morphology. Surface roughness was ⁇ 10 nm for 100 nm thick films. In this case the deposition efficiency was found to be higher at current densities around 20-30 mA/cm compared to lower and higher current density values. These efficiency values were in the range of 75-95%.
• Example 3 (Glycine as the complexing agent): [00026] An aqueous plating bath was formulated with 0.2 M GaCl 3 and 0.5 M Glycine. The pH was adjusted to the range of 11-13 using NaOH. The plating tests were carried out on the surfaces of electroplated copper at current densities of 10-50 mA/cm 2 . All Ga films were shiny with smooth surfaces. Surface roughness was ⁇ 10 nm for 100 nm thick layers. In this case the deposition efficiency was found to be in the range of 75- 90% at 20-30 mA/cm". Efficiency went down at lower and higher current density values.
• citrate EDTA
• glycine EDTA
• glycine glycine
• Citrates used may be organically modified such as triethyl citrate and tributyl citrate.
• complexing agents include but are not limited to tartrates (such as sodium tartrate, lithium tartrate, potassium tartrate, sodium potassium tartrate, diethyl tartrate, dimethyl tartrate, dibutyl tartrate, diisopropyl tartrate, and ammonium tartrate), oxalates (such as sodium, potassium and lithium oxalates), ammonia and ammonium salts, ethylenediamine, nitrilotriacetic acid and its salts, hydroxyethylethylenediaminetriacetic acid and its salts, aminobutyric acids and their salts, amino acids including alanine, valine, leucine, isoleucine, praline, phenylalanine, tyrosine, tryptophan, lysine, arginine, histidine, aspartate, glutamate, serine, threomine, cysteine, methionine, asparagine, and glutamine.
• tartrates such as sodium tartrate, lithium tartrate, potassium
• the preferred Ga plating bath compositions of this invention have a pH value of higher than 7, preferably higher than 9, and most preferably in the range of 9 to 14.
• the above examples employed simple aqueous chemistries with water as the solvent.
• water is the preferred solvent in the formulation of Ga plating baths of the preferred invention
• organic solvents may also be added in the formulation, partially or wholly replacing the water.
• organic solvents include but are not limited to glycerin, alcohols, ethylene glycol, ethylene carbonate, propylene carbonate, acetonitrile, formamide, dimetyl sulfoxide, sulfolane etc.
• the examples above utilized DC voltage/current during the Ga electrodeposition process. It should be noted that pulsed or other variable voltage/current sources may also be utilized to obtain the high plating efficiencies and high quality Ga deposits employing the Ga plating baths of the present invention.
• the temperature of the Ga electroplating baths may be in the range of 5- 150 C depending upon the nature of the solvent. It is preferable to keep this temperature below the boiling point of the solvent.
• the preferred bath temperature for water-based formulation is in the range of 10-60 C. The most preferred range is 15-30 C.
• the electroplating baths of the present invention may comprise additional ingredients. These include, but are not limited to, grain refiners, surfactants, dopants, other metallic or non-metallic elements etc.
• organic additives such as surfactants, suppressors, levelers, accelerators etc. may be included in the formulation to refine its grain structure and surface roughness. There are many such additives commonly used in the field.
• Organic additives include but are not limited to polyalkylene glycol type polymers, propane sulfonic acids, coumarin, saccharin, furfural, acryonitrile, magenta dye, glue, SPS, starch, dextrose, etc.
• the Ga layers produced using the bath compositions of the present invention were employed to fabricate exemplary all-electroplated metallic stacks on bases comprising stainless steel substrates coated with Mo/Ru or only Ru layers. These stacks had various deposition sequences yielding base/Cu/Ga/In, base/Cu/Ga/Cu/In, base/Cu/In/Cu/Ga and base/Cu/In/Ga structures.
• An indium sulfamate-based plating bath marketed by Indium Corporation of America was utilized for In film depositions. The stacks were reacted in a tube furnace at 500 C for 50 minutes with Ar+H?Se gas mixture, forming Cu(In,Ga)Se 2 absorbers.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electroplating And Plating Baths Therefor (AREA)
• Photovoltaic Devices (AREA)

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• 2006
  • 2006-09-27 US US11/535,927 patent/US7507321B2/en not_active Expired - Fee Related
• 2007
  • 2007-09-25 JP JP2009530550A patent/JP2010505045A/ja active Pending
  • 2007-09-25 KR KR1020097008547A patent/KR20090085583A/ko not_active Ceased
  • 2007-09-25 WO PCT/US2007/079356 patent/WO2008039736A1/en not_active Ceased
  • 2007-09-25 EP EP07814990A patent/EP2094882A4/en not_active Withdrawn
  • 2007-09-25 CN CNA2007800391728A patent/CN101528987A/zh active Pending
  • 2007-09-27 TW TW096135964A patent/TW200829725A/zh unknown
• 2009
  • 2009-03-16 US US12/404,690 patent/US20090173634A1/en not_active Abandoned

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