WO2010110870A1 - Photovoltaic cells with plated steel substrate - Google Patents

Photovoltaic cells with plated steel substrate Download PDF

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Publication number
WO2010110870A1
WO2010110870A1 PCT/US2010/000854 US2010000854W WO2010110870A1 WO 2010110870 A1 WO2010110870 A1 WO 2010110870A1 US 2010000854 W US2010000854 W US 2010000854W WO 2010110870 A1 WO2010110870 A1 WO 2010110870A1
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layer
protective
solar cell
thin
coating
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PCT/US2010/000854
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French (fr)
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Anke E. Abken
Aarohi Vijh
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Xunlight Corporation
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Priority to CN2010800241027A priority Critical patent/CN102449774A/en
Publication of WO2010110870A1 publication Critical patent/WO2010110870A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/036—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes
    • H01L31/0392—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes including thin films deposited on metallic or insulating substrates ; characterised by specific substrate materials or substrate features or by the presence of intermediate layers, e.g. barrier layers, on the substrate
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/036—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes
    • H01L31/0392—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes including thin films deposited on metallic or insulating substrates ; characterised by specific substrate materials or substrate features or by the presence of intermediate layers, e.g. barrier layers, on the substrate
    • H01L31/03923—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes including thin films deposited on metallic or insulating substrates ; characterised by specific substrate materials or substrate features or by the presence of intermediate layers, e.g. barrier layers, on the substrate including AIBIIICVI compound materials, e.g. CIS, CIGS
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/036—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes
    • H01L31/0392—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes including thin films deposited on metallic or insulating substrates ; characterised by specific substrate materials or substrate features or by the presence of intermediate layers, e.g. barrier layers, on the substrate
    • H01L31/03925—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes including thin films deposited on metallic or insulating substrates ; characterised by specific substrate materials or substrate features or by the presence of intermediate layers, e.g. barrier layers, on the substrate including AIIBVI compound materials, e.g. CdTe, CdS
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/054—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means
    • H01L31/056—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means the light-reflecting means being of the back surface reflector [BSR] type
    • 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
    • Y02E10/52—PV systems with concentrators
    • 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
    • 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
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the invention relates generally to thin film photovoltaic devices, and in particular to amorphous, micro- and nanocrystalline silicon solar cell devices, which include back reflectors and metallic substrates. More specifically, the invention relates to back reflector layer stacks, which include a textured top and a highly reflective bottom layer, and to protective coatings covering mild steel substrates. Most specifically the invention relates to wet chemical processing for fabricating the back reflector stack and to the use of mild steel substrates, covered with protective coatings deposited by wet chemical processing, for the fabrication of thin film solar cell devices.
  • Fabrication of flexible thin-film (tf) solar cells may be carried out on metallic substrate material.
  • the main properties required for a metallic substrate web are 1) low contamination of the process, 2) stability at solar cell fabrication temperatures, 3) corrosion resistance, 4) flatness at processing temperature, typically about 250-300 0 C and, 5) controlled surface roughness. Additionally, ferromagnetic properties are also beneficial for supporting the substrate material in the process.
  • Stainless steel alloy substrates, for example 400-series alloys, are frequently used because they satisfy all the requirements, but they are expensive.
  • tf solar cell devices typically employ preferentially back reflectors for re-directing unabsorbed incident light back through the semiconductor layer enhancing the current collection efficiency of the device.
  • the back reflector is a stack of layers.
  • the stack usually includes a textured transparent conductive oxide top layer promoting light scattering and a highly reflective metallic bottom layer.
  • the textured top and the highly reflective bottom layer are made from materials, which are non- reactive with adjacent layer during manufacturing or operation of the device, and they are mechanically compatible with device structure and processing.
  • back reflector layers have high electrical conductivity in order to keep their contribution to the series resistance of the device low.
  • zinc oxide (ZnO) and indium oxide (ITO) are well suited as texturing layers, while silver (Ag), silver alloys and aluminum (Al) are suitable as highly reflective bottom layers in back reflectors used in said tf solar cell devices.
  • the invention provides wet chemical methods for fabricating tf solar cell devices on mild steel substrates having corrosion protection coatings and for depositing back reflectors used in tf solar cell devices.
  • the invention relates to the use of mild steel, hardened mild steel, or galvanized steel as substrates for tf solar cell devices.
  • the tf solar cell device comprises a portion of mild steel having a first surface, a protective Ni coating covering the first surface of the portion of mild steel, and at least one layer of solar cell material deposited over the protective Ni coating.
  • the invention relates to a tf solar cell device comprising one or more junctions of a tf solar cell material, a back-reflector layer stack, and a mild steel substrate covered by protective coatings. In this embodiment, at least one layer of the back-reflector stack and one of the protective coatings is deposited by a wet chemical process.
  • the invention relates to methods for wet chemical processing of highly reflective metallic and textured transparent conductive back reflector layers used in fabricating tf solar cell devices.
  • the method comprises providing a steel substrate, providing a protective coating on said steel substrate, depositing a back- reflector layer stack on said protective coating, and depositing at least one tf solar cell material layer.
  • at least one of the steps in providing the protective coating and depositing a back-reflector layer stack is performed by a wet chemical process.
  • Fig. 1 is the cross sectional view, in which the mild steel substrate is covered on both sides with a corrosion protection coating, and a back reflector layer stack including a highly reflective metallic and a textured transparent conductive oxide layer;
  • Fig. 2 is the cross sectional view, in which the mild steel substrate is covered on both sides with a corrosion protection coating comprising of a metal and a metal alloy or metal polymer composite layer, and a back reflector layer stack including a highly reflective metal and a textured transparent conductive oxide layer;
  • Fig. 3 is the cross sectional view, in which the mild steel substrate is covered on both sides with a protection coating comprising of a metal, a metal/metal alloy or metal/metal polymer composite layer stack, and a back reflector layer stack including a highly reflective metallic, a transparent oxide layer seed layer and a textured transparent conductive oxide layer;
  • a protection coating comprising of a metal, a metal/metal alloy or metal/metal polymer composite layer stack, and a back reflector layer stack including a highly reflective metallic, a transparent oxide layer seed layer and a textured transparent conductive oxide layer;
  • Fig. 4 is a cross sectional view of a Si-based thin film solar cell device comprising a mild steel substrate covered on both sides with corrosion protective coating, a back reflector layer stack including a highly reflective metallic, a textured transparent conductive oxide layer and a tf-Si (n-i-p) device covered by a transparent conductive oxide;
  • Fig. 5 is an Secondary Electron Microscope (SEM) images of plated ZnO formed in accordance with an embodiment of the present invention
  • Fig. 6 is an enhanced SEM image of Fig. 5;
  • Fig. 7 is a graph showing the results of optical measurements comparing the diffuse reflection of a sputtered ZnO/sputtered Ag back reflector stack with the diffuse reflection of electroplated ZnO/sputtered
  • the present invention at least includes solar cell material layers and solar cell devices having at least one single junction (SJ) of cadmium telluride (CdTe), amorphous silicon germanium (a-SiGe), amorphous silicon (a-Si), crystalline silicon (c-Si), microcrystalline silicon (mc-Si), nanocrystalline silicon (nc-Si), CIS, CIGS, or CIGSe.
  • SJ single junction
  • CdTe cadmium telluride
  • a-SiGe amorphous silicon germanium
  • a-Si amorphous silicon
  • c-Si crystalline silicon
  • mc-Si microcrystalline silicon
  • nc-Si nanocrystalline silicon
  • CIS CIGS
  • CIGS CIGSe
  • FIG. 1 shows a cross sectional view of a back reflector stack (10) comprising a highly reflective metallic layer (12) and a textured back reflector layer (14) that are positioned above a substrate (15) for a solar cell device.
  • the substrate (15) comprises mild steel (16) and a corrosion protection coating (18).
  • the mild steel (16) is covered on a first surface (20) and a second surface (22) with the corrosion protection coating (18).
  • the corrosion protection coating (18) is preferentially but not limited to Nickel (Ni) (protective Ni coating).
  • the highly reflective metallic layer (12) is preferentially Silver (Ag), Silver alloy, Aluminum (Al), Aluminum alloy, or combinations thereof.
  • the textured back reflector layer (14) is a textured transparent conductive oxide layer (14) such as, but is not limited to, ZnO or ITO
  • Fig. 2 depicts an embodiment of the present invention where the mild steel (16), the highly reflective metallic layer (12) and the textured back reflector layer (14) are similar to Fig. 1.
  • the mild steel (16) is covered on both surfaces (20, 22) with the protective Ni coating (18), preferentially Nickel (Ni), and a corrosion protection alloy or polymer composite coating (protective alloy coating) (26).
  • the protective alloy coating (26) is a Nickel alloy.
  • the protective alloy coating (26) may be but is not limited to Nickel-Cobalt (Ni-Co), Nickel-Phosphorous (Ni-P), Nickel-Boron (Ni-B), Nickel-Cobalt-Boron (Ni-Co-B), Nickel-Zinc (Ni-Zn), Nickel-Zinc-Cobalt (Ni- Zn-Co) or combinations thereof.
  • the protective alloy coating (26) is a polymer composite.
  • the protective alloy coating (26) is preferentially a Nickel polymer composite, such as but not limited to Ni-poly-aniline (Ni-PAN), Ni-poly-acryl-amide (Ni- PAAm), Ni-poly-pyrrole (Ni-PP) or combinations thereof.
  • Ni-PAN Ni-poly-aniline
  • Ni- PAAm Ni-poly-acryl-amide
  • Ni-PP Ni-poly-pyrrole
  • Fig. 3 depicts an embodiment where the mild steel (16), the protective Ni coating (18), the protective alloy coating (26), the highly reflective metallic layer (12) and the textured back reflector layer (14) are similar to Fig. 2.
  • a transparent conductive oxide seed layer (24) is also present and is preferentially ZnO or ITO.
  • the seed layer (24) is deposited over the reflective metallic layer (12).
  • the seed layer (24) enhances the wet chemical deposition of the textured back reflector layer (14), which is preferentially a textured transparent conductive oxide such as ZnO or ITO and prevents diffusion and migration of metal from the reflective metallic layer (12) into the solar cell material of a solar cell device.
  • Fig. 4 refers to a tf Si solar cell device (28) as an example embodiment for using the mild steel substrate (15) including the protective Ni coating (18) and the protective alloy coating (26), a highly reflective metallic layer (12) and a textured back reflector layer (14), preferably a transparent conductive oxide layer (14).
  • the tf Si solar cell device (n-i-p) includes a semiconductor material comprising a n-layer (30), an i-layer (32) and a p-layer (34) covered by a transparent conductive oxide (36).
  • the device can be a single, double or triple junction tf Si solar cell.
  • the transparent conductive oxide (36) is preferentially but not limited to ITO.
  • the tf solar cell device (28) is flexible.
  • wet chemical processing is used for the fabrication of the corrosion protective Ni coating (18), the corrosion protection alloy coating (26), the corrosion protection polymer composite coating (26), the highly reflective metallic layer (12), and the textured back reflector layer (14).
  • wet chemical processing includes electroplating, electroless reductive plating, electroless displacement (exchange) plating, chemical bath deposition, sol-gel deposition or combinations thereof.
  • the wet chemical processing uses aqueous, organic salt solutions or ionic liquids as processing media (electrolytes).
  • any of the layers formed by the deposition methods of the present invention may be deposited while the substrate (15) is moving.
  • at least one of the layers is deposited while the substrate (15) is moving.
  • it is preferred that the substrate is moving in a roll- to-roll (RTR) process.
  • the corrosion protection coating (18) is a protective Ni coating which is applied to both sides of the mild steel (16).
  • the protective Ni coating (18) is electrodeposited from a Ni plating bath containing Nickel Chloride (NiCI 2 6H 2 O), Nickel Sulfate (NiSO 4 ) or combination thereof, bath additives such as Boric Acid (H 3 BO 3 ) and brighteners.
  • the protective Ni coating (18) is deposited by an electroless deposition process because an electroless process provides improved corrosion protection for mild steel substrates (15) due to higher film density when compared to protective Ni coatings formed by electroplating.
  • the protective Ni coating (18) is deposited by electroless from a plating bath containing a Ni-salt (e.g.
  • the protective Ni coating (18) is formed on both sides of the mild steel (16) by immersing the mild steel (16) into the Ni plating bath. Surface roughness and surface defects of the mild steel substrate
  • the protective Ni coating (18) is smoothened by electropolishing. Increasing the thickness of the protective Ni coating (18) or depositing the protective alloy coating (26) onto the protective Ni coating (18) increases the smoothness of the substrate (15) and mitigates the formation of current shunting passes.
  • the protective Ni coating (18) is covered by the protective alloy coating (26).
  • the protective alloy coating (26) may be Ni-Co, Ni-Co-B, Ni-Zn, Ni-B, Ni-P or combinations thereof.
  • Ni-Co is preferentially formed by adding Cobalt Sulfate (CoSO 4 ) and Ni-Co-B by adding CoSO 4 and DMAB to a Ni plating bath.
  • Ni-Zn may be electrodeposited from a plating bath containing NiSO 4 and Zinc Sulfate (ZnSO 4 ).
  • the corrosion protection alloy coating (26) is Ni-B
  • the Ni-B layer is formed as a by-product of the Ni deposition from an electroless Ni plating bath using NaBH 4 Or DMAB as reducing agents.
  • the corrosion protection alloy coating (26) is Ni-P
  • Ni-P is formed using NaH 2 PO 2 H 2 O as a reducing agent.
  • the protective alloy coating (26) is a Ni- polymer composite.
  • the Ni-polymer composite is Ni-poly-aniline (Ni-PAN), Ni-poly-acryl-amide (Ni-PAA), Ni-poly-pyrrole (Ni- PP) or combinations thereof.
  • the Ni-polymer composite protective alloy layer (26) is formed by adding a polymer to a Ni plating bath containing a Nickel Salt (e.g. NiSO 4 ), a reducing agent (e.g. NaH 2 PO 2 H 2 O), brighteners, complexing agents and surfactants.
  • a Nickel Salt e.g. NiSO 4
  • a reducing agent e.g. NaH 2 PO 2 H 2 O
  • the Ni-polymer composite protective alloy layer (26) is electroplated from a Ni plating bath containing a Ni-salt (e.g. NiCI 2 6H 2 O, NiSO 4 or combination thereof), a monomer (e.g. aniline (C 6 H 7 N), acryl amide (C 2 H 3 CONH 2 ), pyrrole (C 4 H 5 N) or combinations thereof) and bath additives such as brighteners, complexing agents and surfactants.
  • the protective alloy coating (26) is preferentially deposited onto the protective Ni coating (18) and not directly onto the mild steel (16) in order to enhance adhesion. It should also be understood that the protective alloy coating (26) of the present invention is not limited to the Ni alloys and Ni polymer composites mentioned, as these are given as examples.
  • Back reflector stack e.g. NiCI 2 6H 2 O, NiSO 4 or combination thereof
  • a monomer e.g. aniline (C 6 H 7 N), acryl amide (C 2 H 3 CONH 2 ), pyrrol
  • the highly reflective metallic layer (12) redirects the incident light for one or more passes through the semiconductor material. And, in the case of the textured back reflector layer (14) incident light is scattered in addition to being redirected through the solar cell material layers (30, 32, 34). As a result, the short circuit current (Jsc) increases and thus the conversion efficiency of the solar cell device improves.
  • Materials used as the back reflector (stack) (10) are compatible with the manufacturing process of the solar cell device and these materials cannot react with or diffuse into adjacent layer or furthermore penetrate the semiconductor material during device fabrication or operation, which will cause device performance degradation.
  • the back reflector stack (10) is highly conductive and does not add series resistance to the solar cell device. Highly reflective metallic layer:
  • Ag, Ag-alloy, Al or combinations thereof are used as the highly reflective metallic layer (12). These layers may be applied as single layer coatings or multi layer stacks comprising a combination of said metals and alloys.
  • the highly reflective layer (12) comprises Ag
  • Ag is electroplated from a bath containing Ag-salt (e.g. Silver Nitrate (AgNOa)), complexing agents (e.g. Sodium Cyanide (NaCN), Ethylene Diamine Tetra Acetic Acid Disodium Salt Sodium Thiosulfate (Na 2 S 2 O 3 ) /Sodium Bisulfite (Na 2 S 2 O 5 )/ Sodium Sulfate (Na 2 (SO 4 )) and bath additives such as brighteners.
  • Ag-salt e.g. Silver Nitrate (AgNOa)
  • complexing agents e.g. Sodium Cyanide (NaCN), Ethylene Diamine Tetra Acetic Acid Disodium Salt Sodium Thiosulfate (Na 2 S 2 O 3 ) /Sodium Bisulfite (Na 2 S 2 O 5 )/ Sodium Sulfate (Na 2 (SO 4 )
  • bath additives such as brighteners.
  • the highly reflective layer (12) is formed by a spontaneous electroless displacement (exchange) plating process.
  • the plating process includes dissolving Ni from the protective Ni layer (18) or the protective alloy coating (26).
  • a portion of the protective Ni layer (18) or the protective alloy coating (26) is a sacrificial layer.
  • the Ag layer is formed by immersing the coated substrate (15) into an Ag plating bath.
  • the plating bath contains an Ag-salt (e.g. AgNO 3 ), complexing agents (e.g. Ammonium Hydroxide (NH 4 OH), Ci 0 H 14 N 2 Na 2 O 8 ⁇ H 2 O or combinations thereof) and brighteners (e.g. Sodium Thio Sulfate (Na 2 S 2 Oa)).
  • the Ag layer may be formed when the electroless reductive Ag plating bath contains an Ag-salt (e.g. AgNO3), a reducing agent (e.g. NaH 2 PO 2 H 2 O, N 2 H 4 H 2 O, Formaldehyd (H 2 CO), Tartaric Acid (C 4 H 6 O 6 ), DMAB) and bath additive such as brighteners and bath stabilizers.
  • a reducing agent e.g. NaH 2 PO 2 H 2 O, N 2 H 4 H 2 O, Formaldehyd (H 2 CO), Tartaric Acid (C 4 H 6 O 6 ), DMAB
  • bath additive such as brighteners and bath stabilizers.
  • Tin (II) salts e.g. Tin (II) Chloride (SnCI 2 ), Tin (II) Fluoride (SnF 2 )
  • SnF 2 Tin (II) Fluoride
  • Copper (Cu), Palladium (Pd) or combinations thereof is achieved by adding a Cu-salt (e.g. Copper Sulfate (CuSO 4 )), a Pd-salt (e.g. Palladium Chloride (PdCI 2 ), Palladium Sulfate (PdSO 4 )) or a combination thereof to the Ag-plating bath.
  • a Cu-salt e.g. Copper Sulfate (CuSO 4 )
  • Pd-salt e.g. Palladium Chloride (PdCI 2 ), Palladium Sulfate (PdSO 4 )
  • the Ag-alloy e.g. Ag-Pd-Cu
  • the Ag-alloy is formed by sequential layer deposition of Ag, Pd, Cu, i.e. the components, in the desired stoichiometric ratio by wet chemical plating followed by an annealing ("alloying") step.
  • a Al highly reflective metallic layer is formed by electroplating Al from Aluminum Chloride (AICb) and using an ionic liquids (e.g 1-Ethyl-3-Methylimidazolium Chloride ([EMIm]CI)) as the electrolyte.
  • AICb Aluminum Chloride
  • EMIm 1-Ethyl-3-Methylimidazolium Chloride
  • Textured transparent conductive oxide layer
  • the textured back reflector layer (14) is deposited over the highly reflective metallic layer (12).
  • the textured layer (14) increases irregular reflection of the incident light into the solar cell material (28).
  • the textured layer (14) acts as a diffusion barrier preventing migration of the metal from the highly reflective metallic layer into the semiconductor during fabrication or operation of the device.
  • the textured back reflector layer (14) is a textured transparent conductive oxide such as but not limited to ZnO or ITO.
  • the textured transparent conductive oxide is electroplated from an aqueous solution containing a Zn-salt (e.g.
  • Zinc Nitrate Zinc(NO 3 ) 2
  • Zinc Chloride Zinc chloride
  • Zinc Acetate Zinc Acetate
  • In-salt e.g. Indium Nitrate (ln(NO 3 ) 3
  • complexing agents e.g. Phthalic Acid (C 8 H 6 O 4 ) or combinations thereof
  • bath additives e.g Gelatin, preferentially with average molecular mass of 20,000 - 100,000, Dextrin).
  • the textured back reflector layer (14) is a textured transparent conductive oxide
  • the combination of CioHi 4 N 2 Na 2 O 8 -2H 2 O as the complexing agent and Gelatin as the bath additive is used for increasing the lifetime of the textured transparent conductive oxide plating bath, for improving the repeatability of the crystal size and grain growth of the plating and for suppressing "abnormal" (dendrites, platelets) growth.
  • the combination of bath composition, bath temperature, current density and plating time are used as control parameter for obtaining the desired grain size, crystal shape, layer density and layer thickness of the textured transparent conductive oxide layer (14).
  • the textured transparent conductive oxide layer (14) is electroplated onto a seed layer (24) comprising a transparent conductive oxide.
  • the seed layer (24) is deposited by sputtering or vacuum evaporation onto the highly reflective metallic layer (12).
  • the seed layer (24) is ZnO
  • the seed layer (24) is deposited by electroplating from a ZnO plating bath containing Zn(NO 3 ) 2 and bath additives.
  • the ZnO seed layer is deposited by chemical bath deposition using a plating bath containing a Zn-salt (e.g.
  • the seed layer (24) is plated by sol-gel deposition.
  • ZnO nano- and micro-particulates are formed from a precursor solution (sol) containing a Zn-salt (e.g. Zn(NOs) 2 , ZnCb, Zn(C 2 H 3 O 2 ) 2 or combinations thereof), organic solvents (e.g.
  • the ZnO precursor solution is applied by but not limited to spin, dip, brush, roll or spray coating forming a dense transparent oxide seed layer after removing residual solvants from the ZnO deposit (gel) by annealing.
  • the seed layer (24) comprises a transparent conductive oxide
  • the seed layer prevents migration and diffusion of metallic traces from the metallic back reflector layer (12) in to the solar cell material (28) and promotes the nucleation of the electroplated transparent conductive oxide layer (14) leading to enhanced film uniformity and adhesion.
  • the conductivity of the transparent oxide seed layer (24) may be increased by introducing dopants (e.g. Al, In, Gallium (Ga)) into the ZnO layer when adding an Al-salt (e.g. Aluminum Nitrate (AI(NO 3 ) 3 ), an In-salt (e.g.
  • the seed layer (24) and/or the textured layer (14) is formed by plating, it is preferred that a post-deposition heat treatment in air at temperatures above 130 0 C be performed.
  • the post-deposition heat treatment removes residual moisture from the textured back reflector layer (14), especially in embodiments where it is a transparent conductive oxide, and improves crystallinity of that layer.
  • a tf Si solar cell device is fabricated by providing mild steel (16) covered with the corrosion protection layers (18, 26) and the back reflector stack (10) deposited onto the protective coatings (18, 26).
  • one or more of the corrosion protection coatings (18, 26) and/or one or more of the back reflector layers (12, 14) is fabricated by using wet chemical processing, in particular electroplating, electroless plating using reducing agents or exchange reactions, chemical bath deposition or sol-gel deposition.
  • the tf Si solar cells comprise a-Si, mc-Si or nc-Si forming single, double or triple junctions.
  • Example 1 the substrate comprised a portion of grade 430 stainless steel.
  • Example 1 the substrate comprised a portion of grade 430 stainless steel.
  • Mild steel covered on both sides with a dynamically electroplated and cold rolled protective Ni layer were etched with 14.5 m NH 4 OH prior to immersing the substrate into an electroless exchange plating bath for Ag- plating.
  • the NH 4 OH etching time is 1 - 10 min, preferentially 3 min.
  • the Ag plating bath contains 3x10 2 - 5x10 '2 mol/l AgNO 3 , about 2 mol/l NH 4 OH and 0.3 - 1.5 mol/l Na 2 S 2 O 3 .
  • the Ag-plating time is 20 - 180 sec, preferentially 120 sec.
  • the formation of an Ag-layer by electroless Ni/Ag exchange was confirmed by X-ray diffraction (XRD) measurements.
  • Example 3 The procedure as described in Example 1 was repeated using mild steel, which had an electroplated protective Ni (sacrificial) layer on top of the dynamically electroplated and cold rolled Ni protection coating.
  • the sacrificial Ni layer was electroplated from a commercial Ni-strike bath at a current density of 10.6 mA/cm 2 for 1 min followed by electroplating from a commercial Ni-plating bath (Alfa-Aesar) at a current density of 16 mA/cm 2 for 15 min.
  • Alfa-Aesar commercial Ni-plating bath
  • the formation of a bright, metallic Ag layer on top of the Ni surface due to Ni/Ag exchange was confirmed by XRD measurements.
  • the introduction of the additional Ni sacrificial layer accelerates the Ni/Ag exchange reaction leading to improved process robustness compared to the procedure described in Example 1.
  • Example 3 Example 3:
  • Example 2 For experimentation the procedure for obtaining highly reflective Ag- layer as described in Example 1 was repeated using substrates comprising a portion of stainless steel covered with an electroplated protective Ni layer which also served as the Ni sacrificial layer.
  • the stainless steel substrates were degreased with a commercial degreaser for 1 min followed by etching them in 18 - 20% Hydrochloric Acid (HCI) for 10 sec before electroplating the protective Ni layer.
  • HCI Hydrochloric Acid
  • the protective Ni layer was plated from a commercial Ni-strike bath at a current density of 10.6 mA/cm 2 for 1 min followed by electroplating from a commercial Ni-plating bath (Alfa-Aesar) at a current density of 16 mA/cm 2 for 15 min.
  • Ni covered stainless steel substrates were etched in 14.5 m NH 4 OH for 10 - 20 min, preferentially 12 - 18 min, before immersing them into the Ag-plating bath for Ag layer formation following the procedure as described in Example 1.
  • XRD measurements confirm the Ni/Ag exchange reaction leading to the formation of bright, metallic Ag-layer.
  • Example 4 Mild steel was covered on both sides with a dynamically electroplated and cold rolled protective Ni layer which was then coated with a Ni layer by electroplating as described in Example 2. The samples were etched in 14.5 m NH 4 OH for 5 - 25 min, preferentially 16 - 18 min, before electroplating Ag- layer from an Ag-plating bath onto the Ni surface.
  • the Ag-plating bath contains 3x10 '2 - 5x10 2 mol/l, preferentially 4x10 "2 AgNO 3 , about 0.5 mol/l NH 4 OH, 0.6 - 1.2 mol/l NH 4 NO 3 and 0.05 - 0.2 mol/l of a complexing agent, preferably Ci O Hi 4 N 2 Na 2 O 8 H 2 O.
  • the current density used for Ag-plating on Ni is 0.2 - 5.0 mA/cm 2 , preferentially 0.5 - 1.0 mA/cm 2 , for plating times of 1 - 15 min.
  • the formation of bright, metallic Ag-coatings was confirmed by XRD-measurements.
  • Example 5 Stainless steel was covered with electroplated Ni as described in Example 3. The Ni covered substrates were etched in 14.5 m NH 4 OH for 10 - 20 min, preferentially 12 - 18 min, before electroplating Ag-layer onto the Ni-surface using the Ag-plating bath and Ag electroplating procedure as described in Example 3. The formation of bright, metallic Ag-coatings was confirmed by XRD-measurements.
  • Example 6 Stainless steel was covered with electroplated Ni as described in Example 3. The Ni covered substrates were etched in 14.5 m NH 4 OH for 10 - 20 min, preferentially 12 - 18 min, before electroplating Ag-layer onto the Ni-surface using the Ag-plating bath and Ag electroplating procedure as described in Example 3. The formation of bright, metallic Ag-coatings was confirmed by XRD-measurements.
  • Example 6 Example 6:
  • the ZnO plating bath contains 1x10 "3 - 0.4 mol/l Zn(NO 3 ) 2 , a carboxylic acid, preferentially 1x10 "7 - 1x10 "2 mol/l Phthalic Acid (C 6 H 4 -I ⁇ -(CO 2 H) 2 ) or Malic Acid (CH 2 CH(OH)(CO 2 H) 2 ) and a carbohydrate, preferentially Dextrin or Sucrose (Ci 2 H 22 Oi i).
  • the ZnO plating bath was held at temperatures of 60-90 0 C during electroplating at current densities of 0.5 - 30 mA/cm 2 .
  • Stainless steel was dynamically coated with sputtered Ag was used for depositing electroplated textured ZnO by performing the ZnO plating procedure as described in Example 6.
  • the formation of large grained, textured ZnO was confirmed by Scanning Electron Microscopy (SEM) and
  • Example 8 Stainless steel covered with dynamically sputtered Ag and dynamically sputtered ZnO seed layer were used as substrates for electroplating textured ZnO following the ZnO electroplating procedure as described in Example 6.
  • the plated ZnO substrates were annealed in air at temperatures above 130 0 C, preferentially at temperatures 250-350 0 C for 0.5 - 15h.
  • the formation of large grained, hexagonal textured ZnO was confirmed by XRD-measurements, Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Optical measurements indicate increased diffuse reflection for plated textured ZnO/sputtered ZnO/Ag back reflector layer stacks compared to sputtered ZnO/Ag back reflector stacks.
  • Example 9 Stainless steel coated with dynamically sputtered Ag were used as substrates for plating small grained ZnO from a chemical plating bath containing 0.05 - 3.0 mol/l ZnNO 3 .
  • the pH of the bath was adjusted with 14.5 m NH 4 OH to 9 - 11.
  • the bath was held for plating at temperatures of 65-85°C for plating times of about 30 min.
  • the thickness of the ZnO layer was increased by plating a second ZnO layer onto the first ZnO layer using the same plating procedure.
  • Stainless steel was dynamically coated with sputtered Ag and dynamically sputtered ZnO seed layer were used as substrates for electroplating textured ZnO.
  • the ZnO plating bath contains 1x10 '3 - 0.4 mol/l Zn(NO 3 ) 2 , 1x10 '5 - 1x10 "1 mol/l and a carbohydrate, preferentially Dextrin.
  • the ZnO plating bath was held at temperatures of 60-90 0 C during electroplating at current densities of 0.5 - 30 mA/cm 2 .
  • the plated ZnO layer were annealed in air at temperatures above 130 0 C, preferentially at temperatures 250-350 0 C for 0.5 - 15h.
  • Figs. 5 and 6 show the SEM images of a ZnO film deposited by the method of Example 10. In this Example the electrodeposition occurred at a current density of 4.4 mA/cm 2 and for a plating time of 10 min.
  • Example 11 Stainless steel coated with dynamically sputtered Ag and dynamically sputtered ZnO seed layer was electroplated with textured ZnO.
  • the ZnO plating bath contains 1x10 "3 - 0.4 mol/l Zn(NO 3 ):., 1x10 "7 - 1x10 ⁇ 2 mol/l C 6 H 4 - 1 ,2-(CO 2 H) 2 or 1x10 "5 - 1x10 '1 mol/l and Gelatin, preferentially with an average molecular mass between 20,000 - 100,000 g/mol.
  • the ZnO plating bath was held at temperatures of 40 - 90 0 C during electroplating at current densities of 0.5 - 30 mA/cm 2 .
  • Fig. 7 provides a summary of optical measurements taken for the diffuse reflection (%) of sputtered ZnO/Ag/stainless steel (36) compared to electroplated ZnO/sputtered ZnO/Ag/stainless steel at plating times of 1 minute (38), 2 minutes (40), 3 minutes (42), 5 minutes (44) and 8 minutes (46).
  • diffuse reflection increases for plated textured ZnO/sputtered ZnO/Ag back reflector layer stacks compared to sputtered ZnO/Ag back reflector stacks.
  • the substrate surface coverage with plated ZnO and the layer thickness of the textured ZnO layer is controlled by the plating time.
  • Fig. 7 shows that diffuse reflection increases with increasing plating time until saturation values for the diffused reflection are obtained.
  • Example 12 Stainless steel was dynamically covered with sputtered Al and a dynamically sputtered ZnO seed layer electroplated with textured ZnO following the ZnO electroplating procedure as described in Example 11.
  • the ZnO layer thickness was controlled by plating time. Optical measurements show an increase in diffuse reflection with increasing ZnO layer thickness controlled by the plating time.
  • Example 14 Stainless steel was dynamically coated with a sputtered Ag layer and then electroplated with textured ZnO following the ZnO electroplating procedure as described in Example 11. The growth of dense layer of small grained hexagonal ZnO was confirmed by SEM.
  • Example 14
  • Stainless steel was coated with dynamically sputtered Ag and electroplated ZnO.
  • the ZnO was electroplated from a bath containing 0.1 mol/l ZnNO ⁇ .
  • the bath temperature was held at 60 - 80 0 C using current densities of 0.5 - 1.5 mA/cm 2 , preferentially 1.0 mA/cm 2 , for a plating time of 15 - 25 min, preferentially 20 min.
  • N-doped, intrinsic and p-doped layers of tf Si were deposited on the device by plasma-enhanced chemical vapor deposition.
  • a transparent contact layer comprising ITO was deposited on the p-Si layer by sputtering.
  • the sample was then cut into individual tf Si solar cells 1000 square centimeters in size. Each cell was subjected to light-assisted shunt-passivation to reduce the effect of shunting defects. Each cell was provided with a current collecting grid on top of the ITO, and a positive bus bar in electrical communication with said grid. The cells were measured under AM1.5 illumination and were found to have an open circuit voltage of 2.2 Volts and a short-circuit current of 7 Amperes.
  • tf Si photovoltaic (PV) modules comprise two or more interconnected tf Si solar cells. Therefore, the positive bus of one cell was soldered to the back of the substrate of another cell to form a photovoltaic module using SAC solder. The performance of the tf Si module was measured under AM1.5 illumination and the module was found to have an open circuit voltage of 4.4 Volts and a short-circuit current of 7 Amperes.
  • the use of mild steel will reduce the manufacturing costs for tf solar cell devices significantly. Also, the superior hardness, flatness and stiffness of mild steel allows easy handling of the finished tf Si photovoltaic module and of the substrate during manufacturing as the formation of kinks and dimples can be avoided.
  • the higher heat conductivity of mild steel improves temperature uniformity during high temperature semiconductor layer deposition. Improved temperature uniformity improves the flatness of the substrate. The uniform temperature and the flat surface of the substrate will improve the solar cell material layer thickness uniformity and provides uniform material properties of the deposited layers yielding an improved efficiency of the solar cell device.
  • a protective coating (18, 26) over the mild steel (16) to prevent corrosion is advantageous.
  • the protective coatings (18, 26) provide a smooth and defect free surface preventing the formation of current shunting defects caused by the surface roughness of the steel substrate. These surface defects are generally responsible for nucleation issues and abnormal growth of layer deposited by wet chemical deposition onto the substrate surface. Since, in the present invention the corrosion protection coatings (18, 26) serve as the nucleation surface for the deposition of the back reflector stack (10), device performance is improved. Also, using the protective Ni coating (18) and the protective alloy coating (26) on both surfaces (20, 22) of the mild steel (16) allows the application of electrical contacts to the solar cell device by soldering without the need for welding.
  • the electrical conductivity of the substrate (15) of the present invention also be provides advantages during sputtering and PECVD as grounding of the substrate (15) is required to prevent arcing. Additionally, the substrate (15) serves as back electrode for tf-Si solar cell devices and tf- Si PV modules, and the higher conductivity reduces electrical losses.
  • Another advantage of the present invention is employing wet chemical processing for the fabrication of thin film solar cell devices. For instance, fabricating at least one of the corrosion protection layers (18, 26), the back reflector stack (10), or both, will significantly decrease manufacturing costs and allow high speed processing at low temperatures while not requiring the use of vacuum equipment. Additionally, employing wet chemical methods allows the diffuse reflection of portions of the electromagnetic spectrum to be increased and controlled at higher diffuse reflection values.

Abstract

The present invention provides an improved thin-film solar cell device and method for its fabrication. Wet chemical processing is used for the application of coatings protecting the substrate and for the formation of a back reflector having a highly reflective metallic bottom and textured top layers. Wet chemical processing includes electroplating, electroless reductive plating, electroless displacement plating, chemical bath deposition and sol-gel deposition.

Description

PHOTOVOLTAIC CELLS WITH PLATED STEEL SUBSTRATE
This PCT patent application claims the benefit of provisional patent application serial no. 61/210,986, filed March 25, 2009 and provisional patent application serial no. 61/274,243, filed August 14, 2009.
FIELD OF THE INVENTION
The invention relates generally to thin film photovoltaic devices, and in particular to amorphous, micro- and nanocrystalline silicon solar cell devices, which include back reflectors and metallic substrates. More specifically, the invention relates to back reflector layer stacks, which include a textured top and a highly reflective bottom layer, and to protective coatings covering mild steel substrates. Most specifically the invention relates to wet chemical processing for fabricating the back reflector stack and to the use of mild steel substrates, covered with protective coatings deposited by wet chemical processing, for the fabrication of thin film solar cell devices.
BACKGROUND OF THE INVENTION Fabrication of flexible thin-film (tf) solar cells may be carried out on metallic substrate material. The main properties required for a metallic substrate web are 1) low contamination of the process, 2) stability at solar cell fabrication temperatures, 3) corrosion resistance, 4) flatness at processing temperature, typically about 250-3000C and, 5) controlled surface roughness. Additionally, ferromagnetic properties are also beneficial for supporting the substrate material in the process. Stainless steel alloy substrates, for example 400-series alloys, are frequently used because they satisfy all the requirements, but they are expensive.
Additionally, tf solar cell devices typically employ preferentially back reflectors for re-directing unabsorbed incident light back through the semiconductor layer enhancing the current collection efficiency of the device. Typically the back reflector is a stack of layers. The stack usually includes a textured transparent conductive oxide top layer promoting light scattering and a highly reflective metallic bottom layer. The textured top and the highly reflective bottom layer are made from materials, which are non- reactive with adjacent layer during manufacturing or operation of the device, and they are mechanically compatible with device structure and processing. Preferably, back reflector layers have high electrical conductivity in order to keep their contribution to the series resistance of the device low. It has been found that zinc oxide (ZnO) and indium oxide (ITO) are well suited as texturing layers, while silver (Ag), silver alloys and aluminum (Al) are suitable as highly reflective bottom layers in back reflectors used in said tf solar cell devices.
However, these materials have low vapor pressures. Therefore, back reflector layers are typically fabricated by vacuum evaporation or sputtering. These processes have low deposition rates and require the use of costly vacuum equipment. Thus, the manufacturing cost for tf solar cell devices is high.
Therefore, a need exists for a low-cost metallic substrate that meets all of the above-described criteria. Additionally, a need exists for a tf solar cell back reflector stack which can be formed at high deposition rates on the metallic substrate without the need for vacuum equipment.
SUMMARY OF THE INVENTION
The invention provides wet chemical methods for fabricating tf solar cell devices on mild steel substrates having corrosion protection coatings and for depositing back reflectors used in tf solar cell devices.
In an embodiment, the invention relates to the use of mild steel, hardened mild steel, or galvanized steel as substrates for tf solar cell devices. The tf solar cell device comprises a portion of mild steel having a first surface, a protective Ni coating covering the first surface of the portion of mild steel, and at least one layer of solar cell material deposited over the protective Ni coating. In another embodiment, the invention relates to a tf solar cell device comprising one or more junctions of a tf solar cell material, a back-reflector layer stack, and a mild steel substrate covered by protective coatings. In this embodiment, at least one layer of the back-reflector stack and one of the protective coatings is deposited by a wet chemical process.
In yet another embodiment, the invention relates to methods for wet chemical processing of highly reflective metallic and textured transparent conductive back reflector layers used in fabricating tf solar cell devices. In an embodiment, the method comprises providing a steel substrate, providing a protective coating on said steel substrate, depositing a back- reflector layer stack on said protective coating, and depositing at least one tf solar cell material layer. In this embodiment, at least one of the steps in providing the protective coating and depositing a back-reflector layer stack is performed by a wet chemical process. Other advantages and aspects of the present invention will become apparent to those skilled in the art upon review of the following detailed description of the invention and the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is the cross sectional view, in which the mild steel substrate is covered on both sides with a corrosion protection coating, and a back reflector layer stack including a highly reflective metallic and a textured transparent conductive oxide layer;
Fig. 2 is the cross sectional view, in which the mild steel substrate is covered on both sides with a corrosion protection coating comprising of a metal and a metal alloy or metal polymer composite layer, and a back reflector layer stack including a highly reflective metal and a textured transparent conductive oxide layer;
Fig. 3 is the cross sectional view, in which the mild steel substrate is covered on both sides with a protection coating comprising of a metal, a metal/metal alloy or metal/metal polymer composite layer stack, and a back reflector layer stack including a highly reflective metallic, a transparent oxide layer seed layer and a textured transparent conductive oxide layer;
Fig. 4 is a cross sectional view of a Si-based thin film solar cell device comprising a mild steel substrate covered on both sides with corrosion protective coating, a back reflector layer stack including a highly reflective metallic, a textured transparent conductive oxide layer and a tf-Si (n-i-p) device covered by a transparent conductive oxide;
Fig. 5 is an Secondary Electron Microscope (SEM) images of plated ZnO formed in accordance with an embodiment of the present invention; Fig. 6 is an enhanced SEM image of Fig. 5; and
Fig. 7 is a graph showing the results of optical measurements comparing the diffuse reflection of a sputtered ZnO/sputtered Ag back reflector stack with the diffuse reflection of electroplated ZnO/sputtered
ZnO/sputtered Ag back reflector stack with varying thickness of the plated ZnO layer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
It is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly stated to the contrary. It should also be appreciated that the specific embodiments and processes illustrated in and described in the following specification are simply exemplary embodiments of the inventive concepts defined in the appended claims. For example, although the present invention will be described in connection with tf silicon (Si) solar cell material layers and solar cell devices the present invention is not so limited. As such, the present invention at least includes solar cell material layers and solar cell devices having at least one single junction (SJ) of cadmium telluride (CdTe), amorphous silicon germanium (a-SiGe), amorphous silicon (a-Si), crystalline silicon (c-Si), microcrystalline silicon (mc-Si), nanocrystalline silicon (nc-Si), CIS, CIGS, or CIGSe. Further, as used in describing the present invention, mild steel refers to a portion of mild steel, hardened mild steel, or galvanized steel. Referring now to the drawings, Fig. 1 shows a cross sectional view of a back reflector stack (10) comprising a highly reflective metallic layer (12) and a textured back reflector layer (14) that are positioned above a substrate (15) for a solar cell device. The substrate (15) comprises mild steel (16) and a corrosion protection coating (18). The mild steel (16) is covered on a first surface (20) and a second surface (22) with the corrosion protection coating (18). The corrosion protection coating (18) is preferentially but not limited to Nickel (Ni) (protective Ni coating). The highly reflective metallic layer (12) is preferentially Silver (Ag), Silver alloy, Aluminum (Al), Aluminum alloy, or combinations thereof. In an embodiment, the textured back reflector layer (14) is a textured transparent conductive oxide layer (14) such as, but is not limited to, ZnO or ITO
Fig. 2 depicts an embodiment of the present invention where the mild steel (16), the highly reflective metallic layer (12) and the textured back reflector layer (14) are similar to Fig. 1. However, in this embodiment, the mild steel (16) is covered on both surfaces (20, 22) with the protective Ni coating (18), preferentially Nickel (Ni), and a corrosion protection alloy or polymer composite coating (protective alloy coating) (26). In an embodiment, the protective alloy coating (26) is a Nickel alloy. In this embodiment, the protective alloy coating (26) may be but is not limited to Nickel-Cobalt (Ni-Co), Nickel-Phosphorous (Ni-P), Nickel-Boron (Ni-B), Nickel-Cobalt-Boron (Ni-Co-B), Nickel-Zinc (Ni-Zn), Nickel-Zinc-Cobalt (Ni- Zn-Co) or combinations thereof. In another embodiment, the protective alloy coating (26) is a polymer composite. In this embodiment, the protective alloy coating (26) is preferentially a Nickel polymer composite, such as but not limited to Ni-poly-aniline (Ni-PAN), Ni-poly-acryl-amide (Ni- PAAm), Ni-poly-pyrrole (Ni-PP) or combinations thereof.
Fig. 3 depicts an embodiment where the mild steel (16), the protective Ni coating (18), the protective alloy coating (26), the highly reflective metallic layer (12) and the textured back reflector layer (14) are similar to Fig. 2. In this embodiment, a transparent conductive oxide seed layer (24) is also present and is preferentially ZnO or ITO. The seed layer (24) is deposited over the reflective metallic layer (12). The seed layer (24) enhances the wet chemical deposition of the textured back reflector layer (14), which is preferentially a textured transparent conductive oxide such as ZnO or ITO and prevents diffusion and migration of metal from the reflective metallic layer (12) into the solar cell material of a solar cell device.
Fig. 4 refers to a tf Si solar cell device (28) as an example embodiment for using the mild steel substrate (15) including the protective Ni coating (18) and the protective alloy coating (26), a highly reflective metallic layer (12) and a textured back reflector layer (14), preferably a transparent conductive oxide layer (14). The tf Si solar cell device (n-i-p) includes a semiconductor material comprising a n-layer (30), an i-layer (32) and a p-layer (34) covered by a transparent conductive oxide (36). In certain embodiments the device can be a single, double or triple junction tf Si solar cell. The transparent conductive oxide (36) is preferentially but not limited to ITO. In an embodiment, the tf solar cell device (28) is flexible.
Wet chemical processing is used for the fabrication of the corrosion protective Ni coating (18), the corrosion protection alloy coating (26), the corrosion protection polymer composite coating (26), the highly reflective metallic layer (12), and the textured back reflector layer (14). Also, although not an exhaustive list, in the present invention wet chemical processing includes electroplating, electroless reductive plating, electroless displacement (exchange) plating, chemical bath deposition, sol-gel deposition or combinations thereof. In the present invention, the wet chemical processing uses aqueous, organic salt solutions or ionic liquids as processing media (electrolytes).
Additionally, in an embodiment, any of the layers formed by the deposition methods of the present invention may be deposited while the substrate (15) is moving. In another embodiment of the present invention, at least one of the layers is deposited while the substrate (15) is moving. For both embodiments, it is preferred that the substrate is moving in a roll- to-roll (RTR) process.
Protective Ni coating and protective alloy coating: The use of mild steel substrates (15) benefits from the application of a corrosion protection coating (18). As stated above, in an embodiment the corrosion protection coating (18) is a protective Ni coating which is applied to both sides of the mild steel (16). In this embodiment, the protective Ni coating (18) is electrodeposited from a Ni plating bath containing Nickel Chloride (NiCI26H2O), Nickel Sulfate (NiSO4) or combination thereof, bath additives such as Boric Acid (H3BO3) and brighteners.
It is preferable to deposit the protective Ni coating (18) by an electroless deposition process because an electroless process provides improved corrosion protection for mild steel substrates (15) due to higher film density when compared to protective Ni coatings formed by electroplating. Thus, in an embodiment, the protective Ni coating (18) is deposited by electroless from a plating bath containing a Ni-salt (e.g. NiCI2, NiSO4, Nickel Acetate (Ni(CH3CO2)2) or combinations thereof), a reducing agent (e.g Sodium Hypophosphite (NaH2PO2 H2O), Sodium Borohydride (NaBH4), Dimethyl Amino Borane ((CH3)2NHBH3, DMAB), Hydrazine (N2H4 H2O)), complexing agents, brighteners and bath stabilizers. The protective Ni coating (18) is formed on both sides of the mild steel (16) by immersing the mild steel (16) into the Ni plating bath. Surface roughness and surface defects of the mild steel substrate
(15) can lead to current shunting in the finished tf-Si solar cell device (28). In certain embodiments wet chemical processing is used for the deposition of the highly reflective metallic (12) and transparent conductive oxide layers (14, 24). Defect density and surface roughness of the mild steel substrate (15) impact nucleation and growth of said back reflector stack (10). Rough surfaces with a high defect density lead to abnormal growth of wet chemical deposited corrosion protection alloy or polymer composite layer (26). Therefore, in an embodiment the protective Ni coating (18) is smoothened by electropolishing. Increasing the thickness of the protective Ni coating (18) or depositing the protective alloy coating (26) onto the protective Ni coating (18) increases the smoothness of the substrate (15) and mitigates the formation of current shunting passes. As stated above, in certain embodiments the protective Ni coating (18) is covered by the protective alloy coating (26). The protective alloy coating (26) may be Ni-Co, Ni-Co-B, Ni-Zn, Ni-B, Ni-P or combinations thereof. Ni-Co is preferentially formed by adding Cobalt Sulfate (CoSO4) and Ni-Co-B by adding CoSO4 and DMAB to a Ni plating bath. In another embodiment where Ni-Zn is the protective alloy coating (26), Ni-Zn may be electrodeposited from a plating bath containing NiSO4 and Zinc Sulfate (ZnSO4). When the corrosion protection alloy coating (26) is Ni-B, the Ni-B layer is formed as a by-product of the Ni deposition from an electroless Ni plating bath using NaBH4 Or DMAB as reducing agents. When the corrosion protection alloy coating (26) is Ni-P, Ni-P is formed using NaH2PO2 H2O as a reducing agent.
In another embodiment, the protective alloy coating (26) is a Ni- polymer composite. In certain embodiments, the Ni-polymer composite is Ni-poly-aniline (Ni-PAN), Ni-poly-acryl-amide (Ni-PAA), Ni-poly-pyrrole (Ni- PP) or combinations thereof. In an embodiment, the Ni-polymer composite protective alloy layer (26) is formed by adding a polymer to a Ni plating bath containing a Nickel Salt (e.g. NiSO4), a reducing agent (e.g. NaH2PO2 H2O), brighteners, complexing agents and surfactants. In another embodiment, the Ni-polymer composite protective alloy layer (26) is electroplated from a Ni plating bath containing a Ni-salt (e.g. NiCI26H2O, NiSO4 or combination thereof), a monomer (e.g. aniline (C6H7N), acryl amide (C2H3CONH2), pyrrole (C4H5N) or combinations thereof) and bath additives such as brighteners, complexing agents and surfactants. The protective alloy coating (26) is preferentially deposited onto the protective Ni coating (18) and not directly onto the mild steel (16) in order to enhance adhesion. It should also be understood that the protective alloy coating (26) of the present invention is not limited to the Ni alloys and Ni polymer composites mentioned, as these are given as examples. Back reflector stack:
The highly reflective metallic layer (12) redirects the incident light for one or more passes through the semiconductor material. And, in the case of the textured back reflector layer (14) incident light is scattered in addition to being redirected through the solar cell material layers (30, 32, 34). As a result, the short circuit current (Jsc) increases and thus the conversion efficiency of the solar cell device improves. Materials used as the back reflector (stack) (10) are compatible with the manufacturing process of the solar cell device and these materials cannot react with or diffuse into adjacent layer or furthermore penetrate the semiconductor material during device fabrication or operation, which will cause device performance degradation. Preferentially the back reflector stack (10) is highly conductive and does not add series resistance to the solar cell device. Highly reflective metallic layer:
In an embodiment Ag, Ag-alloy, Al or combinations thereof are used as the highly reflective metallic layer (12). These layers may be applied as single layer coatings or multi layer stacks comprising a combination of said metals and alloys.
In an embodiment where the highly reflective layer (12) comprises Ag, Ag is electroplated from a bath containing Ag-salt (e.g. Silver Nitrate (AgNOa)), complexing agents (e.g. Sodium Cyanide (NaCN), Ethylene Diamine Tetra Acetic Acid Disodium Salt
Figure imgf000010_0001
Sodium Thiosulfate (Na2S2O3) /Sodium Bisulfite (Na2S2O5)/ Sodium Sulfate (Na2(SO4)) and bath additives such as brighteners.
In another embodiment, the highly reflective layer (12) is formed by a spontaneous electroless displacement (exchange) plating process. The plating process includes dissolving Ni from the protective Ni layer (18) or the protective alloy coating (26). In this embodiment, some Ni from the protective Ni layer (18) or the protective alloy coating (26) is partially removed by the Ag to form a Ag layer by the following mechanism: 2Ag+ + Ni = Ni2+ + 2Ag. In this embodiment, a portion of the protective Ni layer (18) or the protective alloy coating (26) is a sacrificial layer. The Ag layer is formed by immersing the coated substrate (15) into an Ag plating bath. The plating bath contains an Ag-salt (e.g. AgNO3), complexing agents (e.g. Ammonium Hydroxide (NH4OH), Ci0H14N2Na2O8^H2O or combinations thereof) and brighteners (e.g. Sodium Thio Sulfate (Na2S2Oa)).
In a further embodiment, the Ag layer may be formed when the electroless reductive Ag plating bath contains an Ag-salt (e.g. AgNO3), a reducing agent (e.g. NaH2PO2 H2O, N2H4 H2O, Formaldehyd (H2CO), Tartaric Acid (C4H6O6), DMAB) and bath additive such as brighteners and bath stabilizers. In an embodiment, Tin (II) salts (e.g. Tin (II) Chloride (SnCI2), Tin (II) Fluoride (SnF2)) is added to the Ag plating bath to enhance nucleation of the Ag layer. The formation of Ag alloys with e.g. Copper (Cu), Palladium (Pd) or combinations thereof is achieved by adding a Cu-salt (e.g. Copper Sulfate (CuSO4)), a Pd-salt (e.g. Palladium Chloride (PdCI2), Palladium Sulfate (PdSO4)) or a combination thereof to the Ag-plating bath. In a particular embodiment, the Ag-alloy (e.g. Ag-Pd-Cu) is formed by sequential layer deposition of Ag, Pd, Cu, i.e. the components, in the desired stoichiometric ratio by wet chemical plating followed by an annealing ("alloying") step. Wet chemical plating of the components is achieved by electroplating, electroless reductive plating, electroless exchange (displacement) plating or combinations thereof. The wet chemical formation of an highly reflective metallic layer (12) comprising Al is limited to the use of Al-salt melts or uses organic electrolytes. In an embodiment, a Al highly reflective metallic layer is formed by electroplating Al from Aluminum Chloride (AICb) and using an ionic liquids (e.g 1-Ethyl-3-Methylimidazolium Chloride ([EMIm]CI)) as the electrolyte.
Textured transparent conductive oxide layer:
In an embodiment, the textured back reflector layer (14) is deposited over the highly reflective metallic layer (12). The textured layer (14) increases irregular reflection of the incident light into the solar cell material (28). Preferentially the textured layer (14) acts as a diffusion barrier preventing migration of the metal from the highly reflective metallic layer into the semiconductor during fabrication or operation of the device. In an embodiment the textured back reflector layer (14) is a textured transparent conductive oxide such as but not limited to ZnO or ITO. The textured transparent conductive oxide is electroplated from an aqueous solution containing a Zn-salt (e.g. Zinc Nitrate (Zn(NO3)2), Zinc Chloride (ZnCI2), Zinc Acetate (Zn(C2H3O2^) or combinations thereof) or an In-salt (e.g. Indium Nitrate (ln(NO3)3), complexing agents (e.g.
Figure imgf000012_0001
Phthalic Acid (C8H6O4) or combinations thereof) and bath additives (e.g Gelatin, preferentially with average molecular mass of 20,000 - 100,000, Dextrin). In another embodiment, where the textured back reflector layer (14) is a textured transparent conductive oxide, the combination of CioHi4N2Na2O8-2H2O as the complexing agent and Gelatin as the bath additive is used for increasing the lifetime of the textured transparent conductive oxide plating bath, for improving the repeatability of the crystal size and grain growth of the plating and for suppressing "abnormal" (dendrites, platelets) growth. In another embodiment the combination of bath composition, bath temperature, current density and plating time are used as control parameter for obtaining the desired grain size, crystal shape, layer density and layer thickness of the textured transparent conductive oxide layer (14).
In certain embodiments where the textured back reflector layer (14) is a textured transparent conductive oxide, the textured transparent conductive oxide layer (14) is electroplated onto a seed layer (24) comprising a transparent conductive oxide. In an embodiment, the seed layer (24) is deposited by sputtering or vacuum evaporation onto the highly reflective metallic layer (12). In a particular embodiment where the seed layer (24) is ZnO, the seed layer (24) is deposited by electroplating from a ZnO plating bath containing Zn(NO3)2 and bath additives. In this embodiment, the ZnO seed layer is deposited by chemical bath deposition using a plating bath containing a Zn-salt (e.g. Zn(NO3)2, ZnCI2, Zn(C2H3O2J2 or combinations thereof), complexing agents (e.g. NH4OH, Ethylene Diamine (C2H4(NH2J2) or combinations thereof) and bath additives. In another embodiment, the seed layer (24) is plated by sol-gel deposition. ZnO nano- and micro-particulates are formed from a precursor solution (sol) containing a Zn-salt (e.g. Zn(NOs)2, ZnCb, Zn(C2H3O2)2 or combinations thereof), organic solvents (e.g. 1-Propanol (C3H7OH), Ethylene Glycol (C2H4 (OH)2)), sol stabilizers (e.g. Ethanol Amine (HOC2H4NH2)), bath neutralizers (e.g. Sodium Hydroxide (NaOH)) and bath additives. In a further embodiment, the ZnO precursor solution is applied by but not limited to spin, dip, brush, roll or spray coating forming a dense transparent oxide seed layer after removing residual solvants from the ZnO deposit (gel) by annealing. In the embodiments where the seed layer (24) comprises a transparent conductive oxide, the seed layer prevents migration and diffusion of metallic traces from the metallic back reflector layer (12) in to the solar cell material (28) and promotes the nucleation of the electroplated transparent conductive oxide layer (14) leading to enhanced film uniformity and adhesion. In an embodiment the conductivity of the transparent oxide seed layer (24) may be increased by introducing dopants (e.g. Al, In, Gallium (Ga)) into the ZnO layer when adding an Al-salt (e.g. Aluminum Nitrate (AI(NO3)3), an In-salt (e.g. ln(NO3)3 or Indium Sulfate (In2(SO4J3)) or a Ga-salt (e.g. Gallium Nitrate (Ga(NO3)3) or Gallium Sulfate (Ga2(SO4)3) to the chemical plating bath or to the transparent oxide precursor solution (sol). Additionally, when the seed layer (24) and/or the textured layer (14) is formed by plating, it is preferred that a post-deposition heat treatment in air at temperatures above 1300C be performed. The post-deposition heat treatment removes residual moisture from the textured back reflector layer (14), especially in embodiments where it is a transparent conductive oxide, and improves crystallinity of that layer.
In an embodiment, a tf Si solar cell device is fabricated by providing mild steel (16) covered with the corrosion protection layers (18, 26) and the back reflector stack (10) deposited onto the protective coatings (18, 26). In this embodiment, one or more of the corrosion protection coatings (18, 26) and/or one or more of the back reflector layers (12, 14) is fabricated by using wet chemical processing, in particular electroplating, electroless plating using reducing agents or exchange reactions, chemical bath deposition or sol-gel deposition. The tf Si solar cells comprise a-Si, mc-Si or nc-Si forming single, double or triple junctions.
Examples
For Examples 3 and 5-14, the substrate comprised a portion of grade 430 stainless steel. Example 1 :
Mild steel covered on both sides with a dynamically electroplated and cold rolled protective Ni layer were etched with 14.5 m NH4OH prior to immersing the substrate into an electroless exchange plating bath for Ag- plating. The NH4OH etching time is 1 - 10 min, preferentially 3 min. The Ag plating bath contains 3x102 - 5x10'2 mol/l AgNO3, about 2 mol/l NH4OH and 0.3 - 1.5 mol/l Na2S2O3. The Ag-plating time is 20 - 180 sec, preferentially 120 sec. The formation of an Ag-layer by electroless Ni/Ag exchange was confirmed by X-ray diffraction (XRD) measurements.
Example 2:
The procedure as described in Example 1 was repeated using mild steel, which had an electroplated protective Ni (sacrificial) layer on top of the dynamically electroplated and cold rolled Ni protection coating. The sacrificial Ni layer was electroplated from a commercial Ni-strike bath at a current density of 10.6 mA/cm2 for 1 min followed by electroplating from a commercial Ni-plating bath (Alfa-Aesar) at a current density of 16 mA/cm2 for 15 min. The formation of a bright, metallic Ag layer on top of the Ni surface due to Ni/Ag exchange was confirmed by XRD measurements. The introduction of the additional Ni sacrificial layer accelerates the Ni/Ag exchange reaction leading to improved process robustness compared to the procedure described in Example 1. Example 3:
For experimentation the procedure for obtaining highly reflective Ag- layer as described in Example 1 was repeated using substrates comprising a portion of stainless steel covered with an electroplated protective Ni layer which also served as the Ni sacrificial layer. The stainless steel substrates were degreased with a commercial degreaser for 1 min followed by etching them in 18 - 20% Hydrochloric Acid (HCI) for 10 sec before electroplating the protective Ni layer. The protective Ni layer was plated from a commercial Ni-strike bath at a current density of 10.6 mA/cm2 for 1 min followed by electroplating from a commercial Ni-plating bath (Alfa-Aesar) at a current density of 16 mA/cm2 for 15 min. The Ni covered stainless steel substrates were etched in 14.5 m NH4OH for 10 - 20 min, preferentially 12 - 18 min, before immersing them into the Ag-plating bath for Ag layer formation following the procedure as described in Example 1. XRD measurements confirm the Ni/Ag exchange reaction leading to the formation of bright, metallic Ag-layer.
Example 4: Mild steel was covered on both sides with a dynamically electroplated and cold rolled protective Ni layer which was then coated with a Ni layer by electroplating as described in Example 2. The samples were etched in 14.5 m NH4OH for 5 - 25 min, preferentially 16 - 18 min, before electroplating Ag- layer from an Ag-plating bath onto the Ni surface. The Ag-plating bath contains 3x10'2 - 5x102 mol/l, preferentially 4x10"2 AgNO3, about 0.5 mol/l NH4OH, 0.6 - 1.2 mol/l NH4NO3 and 0.05 - 0.2 mol/l of a complexing agent, preferably CiOHi4N2Na2O8 H2O. The current density used for Ag-plating on Ni is 0.2 - 5.0 mA/cm2, preferentially 0.5 - 1.0 mA/cm2, for plating times of 1 - 15 min. The formation of bright, metallic Ag-coatings was confirmed by XRD-measurements.
Example 5: Stainless steel was covered with electroplated Ni as described in Example 3. The Ni covered substrates were etched in 14.5 m NH4OH for 10 - 20 min, preferentially 12 - 18 min, before electroplating Ag-layer onto the Ni-surface using the Ag-plating bath and Ag electroplating procedure as described in Example 3. The formation of bright, metallic Ag-coatings was confirmed by XRD-measurements. Example 6:
Stainless steel covered with an electroplated Ni layer and electroless plated Ag layer fabricated as described in Example 3 were coated with ZnO by electroplating. The ZnO plating bath contains 1x10"3 - 0.4 mol/l Zn(NO3)2, a carboxylic acid, preferentially 1x10"7 - 1x10"2 mol/l Phthalic Acid (C6H4-I ^-(CO2H)2) or Malic Acid (CH2CH(OH)(CO2H)2) and a carbohydrate, preferentially Dextrin or Sucrose (Ci2H22Oi i). The ZnO plating bath was held at temperatures of 60-900C during electroplating at current densities of 0.5 - 30 mA/cm2.
Example 7:
Stainless steel was dynamically coated with sputtered Ag was used for depositing electroplated textured ZnO by performing the ZnO plating procedure as described in Example 6. The formation of large grained, textured ZnO was confirmed by Scanning Electron Microscopy (SEM) and
Atomic Force Microscopy (AFM).
Example 8: Stainless steel covered with dynamically sputtered Ag and dynamically sputtered ZnO seed layer were used as substrates for electroplating textured ZnO following the ZnO electroplating procedure as described in Example 6. The plated ZnO substrates were annealed in air at temperatures above 1300C, preferentially at temperatures 250-3500C for 0.5 - 15h. The formation of large grained, hexagonal textured ZnO was confirmed by XRD-measurements, Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Optical measurements indicate increased diffuse reflection for plated textured ZnO/sputtered ZnO/Ag back reflector layer stacks compared to sputtered ZnO/Ag back reflector stacks.
Example 9: Stainless steel coated with dynamically sputtered Ag were used as substrates for plating small grained ZnO from a chemical plating bath containing 0.05 - 3.0 mol/l ZnNO3. The pH of the bath was adjusted with 14.5 m NH4OH to 9 - 11. The bath was held for plating at temperatures of 65-85°C for plating times of about 30 min. The thickness of the ZnO layer was increased by plating a second ZnO layer onto the first ZnO layer using the same plating procedure.
Example 10:
Stainless steel was dynamically coated with sputtered Ag and dynamically sputtered ZnO seed layer were used as substrates for electroplating textured ZnO. The ZnO plating bath contains 1x10'3 - 0.4 mol/l Zn(NO3)2, 1x10'5 - 1x10"1 mol/l
Figure imgf000017_0001
and a carbohydrate, preferentially Dextrin. The ZnO plating bath was held at temperatures of 60-900C during electroplating at current densities of 0.5 - 30 mA/cm2. The plated ZnO layer were annealed in air at temperatures above 1300C, preferentially at temperatures 250-3500C for 0.5 - 15h. The growth of homogenous and dense layer of crystalline hexagonal ZnO with medium to small grain size was confirmed by SEM. The plated ZnO layer showed little or no "abnormal" (needles, platelets) growth of ZnO. Figs. 5 and 6 show the SEM images of a ZnO film deposited by the method of Example 10. In this Example the electrodeposition occurred at a current density of 4.4 mA/cm2 and for a plating time of 10 min.
Example 11 : Stainless steel coated with dynamically sputtered Ag and dynamically sputtered ZnO seed layer was electroplated with textured ZnO. The ZnO plating bath contains 1x10"3 - 0.4 mol/l Zn(NO3):., 1x10"7 - 1x10~2 mol/l C6H4- 1 ,2-(CO2H)2 or 1x10"5 - 1x10'1 mol/l
Figure imgf000018_0001
and Gelatin, preferentially with an average molecular mass between 20,000 - 100,000 g/mol. The ZnO plating bath was held at temperatures of 40 - 900C during electroplating at current densities of 0.5 - 30 mA/cm2. The thickness of said ZnO layer was controlled by the plating time. The growth of homogenous plated layer comprising crystalline hexagonal ZnO with medium and small grain size was confirmed by XRD and SEM. Fig. 7 provides a summary of optical measurements taken for the diffuse reflection (%) of sputtered ZnO/Ag/stainless steel (36) compared to electroplated ZnO/sputtered ZnO/Ag/stainless steel at plating times of 1 minute (38), 2 minutes (40), 3 minutes (42), 5 minutes (44) and 8 minutes (46). As indicated in Fig. 7, diffuse reflection increases for plated textured ZnO/sputtered ZnO/Ag back reflector layer stacks compared to sputtered ZnO/Ag back reflector stacks. The substrate surface coverage with plated ZnO and the layer thickness of the textured ZnO layer is controlled by the plating time. Additionally, Fig. 7 shows that diffuse reflection increases with increasing plating time until saturation values for the diffused reflection are obtained.
Example 12: Stainless steel was dynamically covered with sputtered Al and a dynamically sputtered ZnO seed layer electroplated with textured ZnO following the ZnO electroplating procedure as described in Example 11. The ZnO layer thickness was controlled by plating time. Optical measurements show an increase in diffuse reflection with increasing ZnO layer thickness controlled by the plating time.
Example 13:
Stainless steel was dynamically coated with a sputtered Ag layer and then electroplated with textured ZnO following the ZnO electroplating procedure as described in Example 11. The growth of dense layer of small grained hexagonal ZnO was confirmed by SEM. Example 14:
Stainless steel was coated with dynamically sputtered Ag and electroplated ZnO. The ZnO was electroplated from a bath containing 0.1 mol/l ZnNOβ. The bath temperature was held at 60 - 800C using current densities of 0.5 - 1.5 mA/cm2, preferentially 1.0 mA/cm2, for a plating time of 15 - 25 min, preferentially 20 min. N-doped, intrinsic and p-doped layers of tf Si were deposited on the device by plasma-enhanced chemical vapor deposition. A transparent contact layer comprising ITO was deposited on the p-Si layer by sputtering. The sample was then cut into individual tf Si solar cells 1000 square centimeters in size. Each cell was subjected to light-assisted shunt-passivation to reduce the effect of shunting defects. Each cell was provided with a current collecting grid on top of the ITO, and a positive bus bar in electrical communication with said grid. The cells were measured under AM1.5 illumination and were found to have an open circuit voltage of 2.2 Volts and a short-circuit current of 7 Amperes.
As understood by those skilled in the art, tf Si photovoltaic (PV) modules comprise two or more interconnected tf Si solar cells. Therefore, the positive bus of one cell was soldered to the back of the substrate of another cell to form a photovoltaic module using SAC solder. The performance of the tf Si module was measured under AM1.5 illumination and the module was found to have an open circuit voltage of 4.4 Volts and a short-circuit current of 7 Amperes.
As described above, in certain embodiments of the present invention the use of mild steel will reduce the manufacturing costs for tf solar cell devices significantly. Also, the superior hardness, flatness and stiffness of mild steel allows easy handling of the finished tf Si photovoltaic module and of the substrate during manufacturing as the formation of kinks and dimples can be avoided. The higher heat conductivity of mild steel improves temperature uniformity during high temperature semiconductor layer deposition. Improved temperature uniformity improves the flatness of the substrate. The uniform temperature and the flat surface of the substrate will improve the solar cell material layer thickness uniformity and provides uniform material properties of the deposited layers yielding an improved efficiency of the solar cell device.
Moreover, forming a protective coating (18, 26) over the mild steel (16) to prevent corrosion is advantageous. For instance, the protective coatings (18, 26) provide a smooth and defect free surface preventing the formation of current shunting defects caused by the surface roughness of the steel substrate. These surface defects are generally responsible for nucleation issues and abnormal growth of layer deposited by wet chemical deposition onto the substrate surface. Since, in the present invention the corrosion protection coatings (18, 26) serve as the nucleation surface for the deposition of the back reflector stack (10), device performance is improved. Also, using the protective Ni coating (18) and the protective alloy coating (26) on both surfaces (20, 22) of the mild steel (16) allows the application of electrical contacts to the solar cell device by soldering without the need for welding.
In addition to the advantageous properties of mild steel (16) and protective coatings (18, 26), summarized above, their combination provides a substrate (15) having superior ferromagnetic properties. In a RTR plasma enhanced chemical vapor deposition (PECVD) process, this contributes strongly to producing materials with improved performances at high temperature where magnets are used to maintain flatness of the substrate (15) for several reasons. First, the flatter substrate (15) provides higher production yields as non-flat surfaces often mechanically touch certain hardware in the deposition chambers causing damages on the deposited layers resulting in poorer yields. Second, the uniform temperature and the flat substrate surface will result in improved semiconductor layer thickness uniformity and will provide uniform material properties of the deposited layers.
The electrical conductivity of the substrate (15) of the present invention also be provides advantages during sputtering and PECVD as grounding of the substrate (15) is required to prevent arcing. Additionally, the substrate (15) serves as back electrode for tf-Si solar cell devices and tf- Si PV modules, and the higher conductivity reduces electrical losses.
Another advantage of the present invention is employing wet chemical processing for the fabrication of thin film solar cell devices. For instance, fabricating at least one of the corrosion protection layers (18, 26), the back reflector stack (10), or both, will significantly decrease manufacturing costs and allow high speed processing at low temperatures while not requiring the use of vacuum equipment. Additionally, employing wet chemical methods allows the diffuse reflection of portions of the electromagnetic spectrum to be increased and controlled at higher diffuse reflection values.
The above detailed description of the present invention is given for explanatory purposes. It will be apparent to those skilled in the art that numerous changes and modifications can be made without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be construed in an illustrative and not a limitative sense, the scope of the invention being defined solely by the appended claims.

Claims

CLAIMSWe claim:
1. A thin-film solar cell device, comprising: a substrate comprising a portion of mild steel having a first surface and a protective Ni coating applied to the first surface of the mild steel; and at least one layer of a thin-film solar cell material deposited over the protective Ni coating.
2. The thin-film solar cell device of Claim 1 , wherein the portion of mild steel has a second surface and wherein the protective Ni coating covers the second surface.
3. The thin-film solar cell device of Claim 1 , wherein the substrate further comprises a protective alloy coating formed over the protective Ni coating.
4. The thin-film solar cell device of Claim 1 , wherein the substrate further comprises a protective alloy coating formed over the protective Ni coating, wherein either the protective Ni coating or the protective alloy coating are partially removed.
5. The thin-film solar cell device of Claim 3, further comprising one or more junctions of the thin-film semiconductor material and a back- reflector layer stack, wherein at least one layer of the back-reflector stack or one of the protective coatings is deposited by a wet chemical process.
6. The thin-film solar cell device of Claim 3, wherein either the protective Ni coating or the protective alloy coating is a sacrificial layer.
7. The thin-film solar cell device of Claim 1 , wherein the solar cell device is flexible and the thin film solar cell material is Silicon.
8. A thin-film photovoltaic module comprising at least two thin film devices described in Claim 5, wherein the thin-film Silicon solar cell devices are fabricated in a roll-to-roll process.
9. The thin-film solar cell device of Claim 1 , further comprising a back reflector stack deposited over the Ni protective layer, wherein the diffuse reflection of the portion of mild steel, the protective Ni coating, and the back reflector stack is above 60% at approximately 830 nm of the electromagnetic spectrum.
10. The thin-film solar cell device of Claim 5, wherein the back- reflector stack comprises: a highly reflective metallic layer, one or more optically transparent and electrically conductive textured layers, at least one of the layers having being deposited by a wet chemical process.
11. The thin-film solar cell device of Claim 9, wherein the diffuse reflection is below 85% at approximately 830 nm of the electromagnetic spectrum.
12. The thin-film solar cell device of Claim 10, wherein the highly reflective metallic layer comprises Ag, Ag-alloy, Al or combinations thereof.
13. The thin-film solar cell device of Claim 10, wherein the device has one, two or three junctions comprising amorphous Silicon (a-Si), microcrystalline (mc-Si) or nanocrystalline Silicon (nc-Si).
14. The thin-film solar cell device of Claim 11 wherein one of the electrically conductive textured layer is ZnO or ITO, or their alloys.
15. The thin-film solar cell device of Claim 3 wherein the protective alloy coating comprises Ni, Ni-B, Ni-P, Ni-Zn, Ni-Co, Ni-Zn-Co, Ni-Co-B, Ni- PAN, Ni-PAAm, Ni-PP or combinations thereof.
16. The thin-film solar cell device of Claim 11 wherein the protective coatings are applied onto both sides of the substrate.
17. The thin-film solar cell device in Claim 11 , wherein the protective Ni coating is smooth.
18. The thin-film solar cell device of Claim 5, wherein the wet chemical process includes electroplating, electroless reductive plating, electroless displacement plating, chemical bath deposition, sol-gel deposition or combinations thereof and wherein the wet chemical process includes using aqueous, organic metal salt solutions and ionic liquids as processing media.
19. The thin-film solar cell device of Claim 1 , further comprising a back reflector stack deposited over the Ni protective layer, wherein the back reflector stack comprises a conductive oxide of Zn and wherein the diffuse reflectivity for the portion of mild steel, the protective Ni coating, and the back reflector stack is above approximately 65% and below approximately 85% between 600 nm and 900 nm of the electromagnetic spectrum.
20. The photovoltaic module of Claim 7, wherein the interconnection comprises soldering to the mild steel substrate coated with a protective coating.
21. A method of fabricating a thin-film solar cell device, comprising the steps of: providing a portion of steel; providing a protective coating on the steel; depositing a back-reflector layer stack on the protective coating; depositing at least one thin-film solar cell material layer; wherein at least one of the steps in providing a protective coating and depositing a back-reflector layer stack is performed by a wet chemical process.
22. The method of Claim 21 wherein the depositing the back- reflector layer stack, comprises the steps of: depositing a highly reflective metallic layer; depositing one or more optically transparent and electrically conductive textured layers, at least one of the layers being deposited by a wet chemical process.
23. The method of Claim 21 , wherein one, two or three junctions comprising amorphous Silicon (a-Si), microcrystalline (mc-Si) or nanocrystalline Silicon (nc-Si) are deposited.
24. The method of Claim 21 , wherein the electrically conductive textured layer deposited is the textured reflective back layer, wherein the textured reflective back layer is deposited by electroplating, and wherein the electroplating bath contains a metal salt, an acid, and a carbohydrate.
25. The method of Claim 21 , wherein the highly reflective metallic layer deposited comprises Ag, Ag-alloy, Al or combinations thereof.
26. The method of Claim 21 , wherein the electrically conductive textured layer deposited is the textured reflective back layer, wherein the textured reflective back layer is deposited by electroplating, and wherein the electroplating bath contains a salt of Zn, a complexing agent, and a carbohydrate.
27. The method of Claim 21 , wherein one of the electrically conductive textured layers deposited is ZnO or ITO, or their alloys.
28. The method of Claim 21 , wherein the step of providing the steel substrate comprises the additional step of hardening.
29. The method of Claim 21 , wherein the steel protective coating comprises Ni, Ni-B, Ni-P, Ni-Zn, Ni-Co, Ni-Zn-Co, Ni-Co-B, Ni-PAN, Ni- PAAm, Ni-PP or combinations thereof.
30. The method of Claim 21 , wherein the steel protective coating is applied onto both sides of the substrate.
31. The method in Claim 21 , wherein the steel protective coating is smoothened by electropolishing prior to depositing the back reflector stack.
32. The method of Claim 21 , wherein the wet chemical process includes electroplating, electroless reductive plating, electroless displacement plating, chemical bath deposition, sol-gel deposition or combinations thereof.
33. The method of Claim 21 , wherein the wet chemical process includes using aqueous, organic metal salt solutions and ionic liquids as processing media.
34. The method of Claim 21 , wherein the portion of steel is moving in a roll-to-roll process during at least one of the deposition steps.
35. The method of Claim 21 , wherein the steel substrate is mild steel.
36. The method of Claim 21 , wherein the steel substrate is stainless steel.
37. The method of Claim 21 , wherein the protective coating contains Ni and is provided by an electroless deposition process.
38. The method of Claim 21 , wherein the electrically conductive textured layer deposited is the textured reflective back layer, wherein the textured reflective back layer is deposited by electroplating, and wherein the electroplating bath contains a salt of Zn, a complexing agent, and a Gelatin having a molecular mass between 20,000 g/mol and 100,000 g/mol.
39. The method of Claim 21 , further comprising controlling the diffuse reflection of the portion of steel, the protective coating, and the back reflector stack between 70% and 80% at approximately 830 nm of the electromagnetic spectrum.
40. The method of Claim 22, further comprising controlling the bath composition, bath temperature, current density and plating time to forming a seed layer over the metallic highly reflective layer.
41. The method of Claim 38, wherein the electroplating bath is held between a temperature of 400C and 900C and a current density of 30 mA/cm2 is applied during deposition of the textured back reflector layer.
42. The method of Claim 40, further comprising immersing the steel in a Ni plating bath, wherein the bath contains an Ni salt and a reducing agent.
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