WO2018209147A1 - Empilements de films métalliques multicouche pour réseaux en bardeaux de cellules solaires en silicium - Google Patents

Empilements de films métalliques multicouche pour réseaux en bardeaux de cellules solaires en silicium Download PDF

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Publication number
WO2018209147A1
WO2018209147A1 PCT/US2018/032159 US2018032159W WO2018209147A1 WO 2018209147 A1 WO2018209147 A1 WO 2018209147A1 US 2018032159 W US2018032159 W US 2018032159W WO 2018209147 A1 WO2018209147 A1 WO 2018209147A1
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WIPO (PCT)
Prior art keywords
layer
solar cell
intercalation
silicon
silicon solar
Prior art date
Application number
PCT/US2018/032159
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English (en)
Inventor
Brian E. Hardin
Daniel J. Hellebusch
Craig H. Peters
Dhea Suseno
Fillary SUSANTO
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PLANT PV, Inc.
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Filing date
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Application filed by PLANT PV, Inc. filed Critical PLANT PV, Inc.
Priority to KR1020197036327A priority Critical patent/KR20200026189A/ko
Priority to EP18799289.6A priority patent/EP3635792A4/fr
Priority to JP2019562278A priority patent/JP2020520114A/ja
Priority to CN201880045803.5A priority patent/CN111316449A/zh
Publication of WO2018209147A1 publication Critical patent/WO2018209147A1/fr

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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
    • H01L31/02—Details
    • H01L31/0224—Electrodes
    • H01L31/022408—Electrodes for devices characterised by at least one potential jump barrier or surface barrier
    • H01L31/022425—Electrodes for devices characterised by at least one potential jump barrier or surface barrier for solar cells
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
    • H01B1/14—Conductive material dispersed in non-conductive inorganic material
    • H01B1/16—Conductive material dispersed in non-conductive inorganic material the conductive material comprising metals or alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
    • H01B1/20—Conductive material dispersed in non-conductive organic material
    • H01B1/22—Conductive material dispersed in non-conductive organic material the conductive material comprising metals or alloys
    • 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
    • H01L31/02—Details
    • H01L31/02002—Arrangements for conducting electric current to or from the device in operations
    • 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
    • H01L31/0256—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 the material
    • H01L31/0264—Inorganic materials
    • H01L31/032—Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312
    • 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
    • H01L31/042—PV modules or arrays of single PV cells
    • H01L31/048—Encapsulation of modules
    • H01L31/049—Protective back sheets
    • 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
    • H01L31/042—PV modules or arrays of single PV cells
    • H01L31/05—Electrical interconnection means between PV cells inside the PV module, e.g. series connection of PV cells
    • H01L31/0504—Electrical interconnection means between PV cells inside the PV module, e.g. series connection of PV cells specially adapted for series or parallel connection of solar cells in a module
    • 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

Definitions

  • This invention relates to intercalation pastes that contain precious metal particles, intercalating particles and organic vehicle.
  • Intercalation pastes may be used to improve the power conversion efficiency of solar cells.
  • Silver based intercalation pastes are printed on aluminum layers that have moderate peel strength after firing and subsequent soldering to a tabbing ribbon.
  • Such pastes may be especially well-suited for use in silicon based solar cells that use aluminum back-surface fields (BSF).
  • BSF back-surface fields
  • the remaining 5-10% of the rear silicon surface is covered by the silver rear tabbing layer, which does not produce a field and does not make ohmic contact to the silicon wafer.
  • the rear tabbing layer is primarily used to solder tabbing ribbons to electrically connect solar cells.
  • Figure 1 is a schematic cross-section drawing of multilayer stack prior to firing, according to an embodiment of the invention.
  • Figure 2 is a schematic cross-section drawing of a fired multilayer stack, according to an embodiment of the invention.
  • Figure 3 is a schematic cross-section drawing of a fired multilayer stack in which the intercalation layer has phase separated.
  • Figure 4 is a schematic cross-section drawing of a fired multilayer stack in which the intercalation layer has phase separated into two sublayers.
  • Figure 5 is a schematic cross-section drawing of a portion of the fired multilayer stack shown in Figure 2, according to an embodiment of the invention.
  • Figure 6 is a scanning electron microscope (SEM) cross-section image of a co-fired multilayer stack, according to an embodiment of the invention.
  • Figure 7 is a scanning electron micrograph (SEM) cross-section image of a co-fired multilayer stack that has a silver-bismuth frit layer.
  • Figure 8 is a scanning electron microscope (SEM) cross-section image (in SE2 mode) of an aluminum particle layer on a silicon substrate.
  • Figure 9 is a scanning electron microscope (SEM) cross-section image (in InLens mode) of an aluminum particle layer on a silicon substrate shown in Figure 8.
  • Figure 10 is a scanning electron microscope (SEM) cross-section image (in InLens mode) of a portion of a silicon solar cell that contains a co-fired multilayer stack.
  • Figure 11 is a scanning electron microscope (SEM) cross-section image (in SE2 mode) of the portion of a silicon solar cell that contains a co-fired multilayer stack shown in Figure 10.
  • SEM scanning electron microscope
  • Figure 12 shows energy dispersive x-ray (EDX) spectra from an aluminum particle layer and from a modified aluminum particle layer, according to an embodiment of the invention.
  • EDX energy dispersive x-ray
  • Figure 13 is an EDX spectrum of the surface of a rear tabbing layer that contains a silver-bismuth intercalation layer, according to an embodiment of the invention.
  • Figure 14 shows x-ray diffraction patterns from co-fired multilayer film stacks on the rear tabbing layer of a silicon solar cell.
  • Figure 15 is a schematic cross-section drawing of multilayer film stack that includes a dielectric layer prior to firing, according to an embodiment of the invention.
  • Figure 16 is a schematic cross-section drawing of a fired multilayer film stack that includes a dielectric layer, according to an embodiment of the invention.
  • Figure 17 is a plan-view optical micrograph of co-fired multilayer film stack in which buckling has occurred.
  • Figure 18 is a screen design (not drawn to scale) that can be used during deposition of a wet metal particle layer, according to an embodiment of the invention.
  • Figure 19 is a schematic cross-section drawing of a dried metal particle layer with variable thickness deposited using the screen in Figure 18, according to an embodiment of the invention.
  • Figure 20 is a schematic cross-section drawing of a modified metal particle layer with variable thickness deposited using the screen of Figure 18 and then co-fired, according to an embodiment of the invention.
  • Figure 21 is a plan-view optical micrograph of a co-fired multilayer stack as shown in Figure 20.
  • Figure 22 is a cross-section SEM image of a portion of a fired multilayer stack with variable thickness.
  • Figure 23 is a cross-section SEM image of a portion of an aluminum particle film on a silicon substrate with a planar thickness.
  • Figure 24 is a surface topology scan of a fired multilayered stack with variable thickness.
  • Figure 25 is a surface topology scan of a fired aluminum particle layer.
  • Figure 26 is a schematic drawing that shows the front (or illuminated) side of a silicon solar cell.
  • Figure 27 is a schematic drawing that shows the rear side of a silicon solar cell.
  • Figure 28 is a schematic cross-section drawing of a solar cell module that includes a fired multilayer stack, according to an embodiment of the invention.
  • Figure 29 is a scanning electron micrograph (SEM) cross-section of the back side of a solar cell that includes a fired multilayer stack and a soldered tabbing ribbon, according to an embodiment of the invention.
  • Figure 30 is a transmission line measurement plot of a conventional silver rear tabbing layer on silicon.
  • Figure 31 is a transmission line measurement plot of a silver-bismuth intercalation layer on an aluminum particle layer that can be used as a rear tabbing layer on silicon.
  • Figure 32 is a schematic cross-sectional illustration view of solar cells arranged in a shingled architecture, according to an embodiment of the invention.
  • Figure 32A is a schematic cross-sectional illustration view of solar cells arranged in a shingled architecture, according to an embodiment of the invention.
  • Figure 32B is an enlargement of a portion of Figure 32A, showing the overlay between shingled solar cells in more detail.
  • a fired multilayer stack has a substrate, a metal particle layer on at least a portion of the substrate surface, a modified metal particle layer on at least a portion of the substrate surface, and a modified intercalation layer directly on at least a portion of the modified metal particle layer.
  • the modified intercalation layer has a solderable surface that faces away from the substrate.
  • the modified metal particle layer includes the same metal particles that are in the metal particle layer and at least one material from the modified intercalation layer.
  • the modified intercalation layer contain a precious metal and a material selected from the group consisting of antimony, arsenic, barium, bismuth, boron, cadmium, calcium, cerium, cesium, chromium, cobalt, gallium, germanium, hafnium, indium, iron, lanthanum, lead, lithium, magnesium, manganese, molybdenum, niobium, phosphorous, potassium, rhenium, selenium, silicon, sodium, strontium, sulfur, tellurium, tin, vanadium, zinc, zirconium, combinations thereof, and alloys thereof, oxides thereof, composites thereof, and other combinations thereof.
  • a precious metal and a material selected from the group consisting of antimony, arsenic, barium, bismuth, boron, cadmium, calcium, cerium, cesium, chromium, cobalt, gallium, germanium, hafnium, indium, iron, lanthanum, lead, lithium,
  • the modified intercalation layer contain a precious metal and a material selected from the group consisting of bismuth, boron, indium, lead, silicon, tellurium, tin, vanadium, zinc, combinations thereof, and alloys thereof, oxides thereof, composites thereof, and other combinations thereof.
  • the modified intercalation layer has two phases: a precious metal phase and an intercalation phase. More than 50% of the solderable surface of the modified intercalation layer may contain the precious metal phase.
  • the modified metal particle layer may include the metal particles discussed above and at least one material from the intercalation phase.
  • the intercalation phase comprises a material selected from the group consisting of antimony, arsenic, barium, bismuth, boron, cadmium, calcium, cerium, cesium, chromium, cobalt, gallium, germanium, hafnium, indium, iron, lanthanum, lead, lithium, magnesium, manganese, molybdenum, niobium, phosphorous, potassium, rhenium, selenium, silicon, sodium, strontium, sulfur, tellurium, tin, vanadium, zinc, zirconium, combinations thereof, and alloys thereof, oxides thereof, composites thereof, and other combinations thereof.
  • the precious metal phase includes at least one material selected from the group consisting of gold, silver, platinum, palladium, rhodium, and alloys, composites, and other combinations thereof.
  • the modified intercalation layer has two sublayers: an intercalation sublayer directly on at least a portion of the modified metal particle layer and a precious metal sublayer directly on at least a portion of the intercalation sublayer.
  • the solderable surface of the modified intercalation layer contains the precious metal sublayer.
  • the modified metal particle layer may include the metal particles discussed above and at least one material from the intercalation sublayer. Possible materials for the intercalation sublayer are the same as were described above for the intercalation phase.
  • Possible materials for the precious metal sublayer are the same as were described above for the precious metal phase.
  • a fired multilayer stack has a modified aluminum particle layer as its modified metal particle layer. It has a modified intercalation layer that has two sublayers: a bismuth-rich sublayer directly on the modified aluminum particle layer; and a silver-rich sublayer directly on the bismuth-rich sublayer.
  • the solderable surface of the modified intercalation layer contains the silver-rich sublayer.
  • the modified aluminum particle contains aluminum particles and may also contain at least one material selected from the group consisting of aluminum oxides, bismuth, and bismuth oxides.
  • the dielectric layer directly on at least a portion of the substrate surface.
  • the dielectric layer includes at least one material selected from the group consisting of silicon, aluminum, germanium, hafnium, gallium, and oxides, nitrides, composites, and combinations thereof.
  • the substrate surface there is a solid (e.g., eutectic) compound layer directly on the substrate surface.
  • the solid compound layer includes one or more metals selected from the group consisting of aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium and one or more materials selected from the group consisting of silicon, oxygen, carbon, germanium, gallium, arsenic, nitrogen, indium and phosphorous.
  • a portion of the substrate adjacent to the substrate surface may be doped with at least one material selected from the group consisting of aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, steel and combinations thereof.
  • a portion of the fired multilayer stack has variable thickness.
  • the fired multilayer stack may have an average peak to valley height greater than 12 ⁇ .
  • At least 70 wt% of the solderable surface of the modified intercalation layer may include a material selected from the group consisting of silver, gold, platinum, palladium, rhodium, and alloys, composites, and other combinations thereof.
  • the substrate may include at least one material selected from the group consisting of silicon, silicon dioxide, silicon carbide, aluminum oxide, sapphire, germanium, gallium arsenide, gallium nitride, and indium phosphide.
  • the substrate may include a material selected from the group consisting of aluminum, copper, iron, nickel, titanium, steel, zinc, and alloys, composites, and other combinations thereof.
  • the metal particle layer may include a material selected from the group consisting of aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, steel and alloys, composites, and other combinations thereof.
  • the precious metal may include a material selected from the group consisting of silver, gold, platinum, palladium, rhodium, and alloys, composites, and other combinations thereof.
  • the metal particle layer may have a thickness between 0.5 ⁇ and 100 ⁇ and/or a porosity between 1 and 50%.
  • the modified intercalation layer may have a thickness between 0.5 ⁇ and ⁇ ⁇ .
  • the fired multilayer stack may have a contact resistance between 0 and 5 mOhm, as determined by transmission line measurement.
  • tabbing ribbon directly on at least a portion of the solderable surface of the modified intercalation layer.
  • peel strength between the tabbing ribbon and the modified intercalation layer is greater than 1 N/mm.
  • a fired multilayer stack has a substrate, a metal particle layer on at least a portion of the substrate, a modified metal particle layer on at least a portion of the substrate, and a modified intercalation layer directly on at least a portion of the modified metal particle layer.
  • the modified intercalation layer has two sublayers: an intercalation sublayer directly on at least a portion of the modified metal particle layer and a precious metal sublayer directly on at least a portion of the intercalation sublayer.
  • the modified metal particle layer includes metal particles and at least one material from the intercalation sublayer. Possible materials for the intercalation sublayer have been described above.
  • a fired multilayer stack has a silicon substrate, an aluminum particle layer on at least a portion of the substrate, a modified aluminum particle layer on at least a portion of the substrate, and a modified intercalation layer directly on the modified aluminum particle layer.
  • the modified intercalation layer has two sublayers: a bismuth-rich sublayer directly on the modified aluminum particle layer and a silver-rich sublayer directly on the bismuth-rich sublayer.
  • the modified aluminum particle layer includes at least one material selected from the group consisting of aluminum, aluminum oxides, bismuth, and bismuth oxides.
  • a solar cell has a silicon substrate, at least one front dielectric layer directly on at least a portion of the front surface of the silicon substrate, a plurality of fine grid lines on a portion of the front surface of the silicon substrate, at least one front busbar layer in electrical contact with at least one of the plurality of fine grid lines, an aluminum particle layer on at least a portion of the rear surface of the silicon substrate, and a rear tabbing layer on a portion of the rear surface of the silicon substrate.
  • the rear tabbing layer includes a modified aluminum particle layer on a portion of the rear surface of the silicon substrate and a modified intercalation layer directly on at least a portion of the modified aluminum particle layer.
  • the modified intercalation layer has a solderable surface that faces away from the silicon substrate.
  • the modified aluminum particle layer includes aluminum particles and at least one material from the modified intercalation layer. Possible materials for the modified intercalation layer have been described above.
  • the aluminum particle layer may have a thickness between 1 ⁇ and 50 ⁇ and/or a porosity between 3 and 20%.
  • the rear tabbing layer may have a thickness between 1 ⁇ and 50 ⁇ .
  • the silicon substrate may be a monocrystalline silicon wafer with either a p-type base or an n-type base.
  • the silicon substrate may be a multi-crystalline silicon wafer with either a p-type base or an n-type base.
  • the modified intercalation layer includes two phases: a precious metal phase and an intercalation phase. More than 50% of the solderable surface may be made up of the precious metal phase.
  • the modified aluminum particle layer includes aluminum particles and at least one material from the intercalation phase. Possible materials for the intercalation phase have been described above. Possible materials for the precious metal phase have been described above.
  • the modified intercalation layer includes two sublayers: an intercalation sublayer directly on at least a portion of the modified metal particle layer, and a precious metal sublayer directly on at least a portion of the intercalation sublayer.
  • the solderable surface contains the precious metal sublayer.
  • the modified aluminum particle layer includes aluminum particles and at least one material from the intercalation sublayer. Possible materials for the intercalation sublayer have been described above. Possible materials for the precious metal sublayer have been described above.
  • the modified intercalation layer comprises two sublayers: a bismuth-rich sublayer directly on the modified aluminum particle layer, and a silver-rich sublayer directly on the bismuth-rich sublayer.
  • the modified aluminum particle layer further includes at least one material selected from the group consisting of aluminum oxides, bismuth, and bismuth oxides.
  • modified aluminum particle layer further includes bismuth and/or bismuth oxide and the weight ratio of bismuth to bismuth and aluminum (Bi:(Bi + Al)) is at least 20% higher in the modified aluminum particle layer than in the aluminum particle layer.
  • the bismuth-rich sublayer may have a thickness between 0.01 ⁇ and 5 ⁇ or between 0.25 ⁇ and 5 ⁇ .
  • the rear dielectric layer directly on at least a portion of the rear surface of the silicon substrate.
  • the rear dielectric layer includes one or more of silicon, aluminum, germanium, hafnium, gallium, and oxides, nitrides, composites, and combinations thereof.
  • the rear dielectric layer may contain silicon nitride.
  • a portion of the silicon substrate adjacent to the rear surface of the silicon substrate further comprises a rear surface field and the rear surface field is doped p-type to between 10 17 and 10 20 atoms per cm 3 .
  • a portion of the rear tabbing layer has variable thickness and may have an average peak to valley height greater than 12 ⁇ .
  • solderable surface may be silver rich.
  • the solderable surface may contain at least 75 wt% silver.
  • a tabbing ribbon soldered to a silver-rich solderable surface may have a peel strength greater than 1 N/mm.
  • a portion of the modified aluminum particle layer may have variable thickness.
  • a portion of the modified aluminum particle layer may have an average peak to valley height greater than 12 ⁇ .
  • Contact resistance between the rear tabbing layer and the aluminum particle layer may be between 0 and 5 mOhm as determined by transmission line
  • a solar cell has a silicon substrate, at least one front dielectric layer directly on at least a portion of the front surface of the silicon substrate, a plurality of fine grid lines on a portion of the front surface of the silicon substrate, at least one front busbar layer in electrical contact with at least one of the plurality of fine grid lines, an aluminum particle layer on at least a portion of the rear surface of the silicon substrate, and a rear tabbing layer on a portion of the rear surface of the silicon substrate.
  • the rear tabbing layer has a solderable surface.
  • the rear tabbing layer includes a modified aluminum particle layer on at least a portion of the rear surface of the silicon substrate, a bismuth-rich sublayer directly on at least a portion of the modified aluminum particle layer, and a silver-rich sublayer directly on at least a portion of the bismuth-rich sublayer.
  • the modified aluminum particle layer contains aluminum particles and at least one material selected from the group consisting of aluminum oxides, bismuth, and bismuth oxides.
  • a solar cell module has a front sheet, a front encapsulant layer on the rear surface of the front sheet, and a first silicon solar cell and a second silicon solar cell on the front encapsulant layer.
  • Each silicon solar cell can be any of the silicon solar cells described herein.
  • the solar cell module also has a first cell interconnect that includes a first tabbing ribbon in electrical contact with both the front busbar layer of the first silicon solar cell, and the rear tabbing layer of the second silicon solar cell, a rear sheet, a rear encapsulant layer on the rear surface of the rear sheet.
  • a first portion of the rear encapsulant layer is in contact with the first silicon solar cell and the second solar cell, and a second portion of the rear encapsulant layer is in contact with the front encapsulant layer.
  • the first cell interconnect may also include a junction box in contact with the rear sheet.
  • the junction box may contain at least one bypass diode.
  • There may also be at least one busbar ribbon connecting to the first tabbing ribbon.
  • a paste in one embodiment, contains between 10 wt% and 70 wt% precious metal particles, at least 10 wt% intercalating particles and organic vehicle.
  • the intercalating particles include one or more selected from the group consisting of low temperature base metal particles, crystalline metal oxide particles, and glass frit particles.
  • the weight ratio of the intercalating particles to the precious metal particles may be at least 1 :5.
  • the precious metal particles may include at least one material selected from the group consisting of gold, silver, platinum, palladium, rhodium, and alloys, composites, and other combinations thereof.
  • the precious metal particles may have a D50 between 100 nm and 50 ⁇ and a specific surface area between 0.4 and 7.0 m 2 /g.
  • a portion of the precious metal particles may have shapes such as a spherical shapes, flake shapes, and/or elongated shapes.
  • the precious metal particles may have a unimodal size distribution or a multimodal size distribution.
  • the precious metal particles are silver and have a D50 between 300 nm and 2.5 ⁇ and a specific surface area between 1.0 and 3.0 m 2 /g.
  • the intercalating particles may have a D50 between 100 nm and 50 ⁇ and a specific surface area between 0.1 and 6.0 m 2 /g.
  • a portion of the intercalating particles may have shapes such as spherical shapes, flake shapes, and/or elongated shapes.
  • the intercalating particles may have a unimodal size distribution or a multimodal size distribution.
  • the low temperature base metal particles may include a material selected from the group consisting of bismuth, tin, tellurium, antimony, lead, and alloys, composites, and other combinations thereof.
  • the low temperature base metal particles contain bismuth and have a D50 between 1.5 and 4.0 ⁇ and a specific surface area between 1.0 and 2.0 m 2 /g.
  • the low temperature base metal particles have a bismuth core particle surrounded by a single shell that includes a material selected from the group consisting of silver, nickel, nickel-boron, tin, tellurium, antimony, lead, molybdenum, titanium, and alloys, composites, and other combinations thereof.
  • at least some of the low temperature base metal particles have a bismuth core particle surrounded by a single shell that comprises a material selected from the group consisting of silicon oxides, magnesium oxides, boron oxides, and any combination thereof.
  • the crystalline metal oxide particles may contain oxygen and a metal selected from the group consisting of bismuth, tin, tellurium, antimony, lead, vanadium, chromium, molybdenum, boron, manganese, cobalt, and alloys, composites and other combinations thereof.
  • the glass frit particles may contain a material selected from a group consisting of antimony, arsenic, barium, bismuth, boron, cadmium, calcium, cerium, cesium, chromium, cobalt, fluorine, gallium, germanium, hamium, indium, iodine, iron, lanthanum, lead, lithium, magnesium, manganese, molybdenum, niobium, potassium, rhenium, selenium, silicon, sodium, strontium, tellurium, tin, vanadium, zinc, zirconium, alloys thereof, oxides thereof, composites thereof, and other combinations thereof.
  • a material selected from a group consisting of antimony, arsenic, barium, bismuth, boron, cadmium, calcium, cerium, cesium, chromium, cobalt, fluorine, gallium, germanium, hamium, indium, iodine, iron, lanthanum, lead, lithium,
  • the paste may have a solids loading between 30 wt% and 80 wt%.
  • the intercalating particles may make up at least 15 wt% of the paste.
  • the paste includes 45 wt% Ag particles, 30 wt% bismuth particles, and 25 wt% organic vehicle.
  • the paste includes 30 wt% Ag particles, 20 wt% bismuth particles, and 50 wt% organic vehicle.
  • the paste may have a viscosity between 10,000 and 200,000 cP at 25°C at a sheer rate of 4 sec -1 .
  • a co-firing method of forming a fired multilayer stack involves the steps of a) applying a wet metal particle layer on at least a portion of a surface of a substrate, b) drying the wet metal particle layer to form a dried metal particle layer, c) applying a wet intercalation layer directly on at least a portion of the dried metal particle layer to form a multilayer stack, d) drying the multilayer stack, and e) co-firing the multilayer stack to form the fired multilayer stack.
  • a sequential method of forming a fired multilayer stack involves the steps of a) applying a wet metal particle layer on at least a portion of a surface of a substrate, b) drying the wet metal particle layer to form a dried metal particle layer, c) firing the dried metal particle layer to form a metal particle layer, d) applying a wet intercalation layer directly on at least a portion of the metal particle layer to form a multilayer stack, e) drying the multilayer stack, and f) firing the multilayer stack to form the fired multilayer stack.
  • the wet intercalation layer has between 10 wt% and 70 wt% precious metal particles, at least 10 wt% intercalating particles, and organic vehicle.
  • the intercalating particles may include one or more selected from the group consisting of low temperature base metal particles, crystalline metal oxide particles, and glass frit particles.
  • the wet metal particle layer may include metal particles consisting of a material selected from the group comprising aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, steel and alloys, composites, and other combinations thereof.
  • the additional step involves depositing at least one dielectric layer onto at least a portion of the surface of the substrate.
  • step a) involves applying the wet metal particle layer directly on at least a portion the dielectric layer.
  • each applying step may involve a method selected from the group consisting of screen printing, gravure printing, spray deposition, slot coating, 3D printing and inkjet printing.
  • step a) involves screen printing through a patterned screen to produce a wet metal particle layer that has variable thickness.
  • steps b) and d) may involve drying at a temperature below 500°C for between 1 second and 90 minutes or at a temperature between 150°C and 300°C for between 1 second and 60 minutes.
  • Step e) may involve rapidly heating to a temperature greater than 600°C for between 0.5 second and 60 minutes in air or rapidly heating to a temperature greater than 700°C for between 0.5 and 3 seconds in air.
  • an additional step f) involves soldering a tabbing ribbon onto a portion of the fired multilayer stack.
  • a method of fabricating a solar cell involves the steps of: a) providing a silicon wafer, b) applying a wet aluminum particle layer on at least a portion of the back surface of the silicon wafer, c) drying the wet aluminum particle layer to form an aluminum particle layer, d) applying a wet intercalation layer directly on at least a portion of the aluminum particle layer to form a multilayer stack, e) drying the multilayer stack, f) applying a plurality of fine grid lines and at least one front busbar layer onto the front surface of the silicon wafer, g) drying the plurality of fine grid lines and the at least one front busbar layer to form a structure, and h) co-firing the structure to form a silicon solar cell.
  • step a) there is an additional step between step a) and step b).
  • the additional step involves depositing at least one dielectric layer onto at least a portion of the back surface of the silicon wafer.
  • step b) involves applying the wet aluminum particle layer directly on at least a portion the dielectric layer.
  • Each applying step may involve a method selected from the group consisting of screen printing, gravure printing, spray deposition, slot coating, 3D printing and inkjet printing.
  • step b) involves screen printing through a patterned screen to produce a wet aluminum particle layer that has variable thickness.
  • steps e) and g) may involve drying at a temperature below 500°C for between 1 second and 90 minutes or at a
  • Step h) may involve rapidly heating to a temperature greater than 600°C for between 0.5 second and 60 minutes in air or rapidly heating to a temperature greater than 700°C for between 0.5 and 3 seconds in air.
  • any of the solar cells, as described herein has an electrically conductive adhesive layer on at least a portion of the solderable surface of the modified intercalation layer.
  • the first solar cell has a rear tabbing layer, and the first solar cell is connected to a busbar of the second solar cell through the conductive adhesive.
  • a shingled solar cell array has a plurality of silicon solar cells arranged in line with end portions of adjacent silicon solar cells overlapping and conductively bonded to each other so that the silicon solar cells are electrically connected to one another in series.
  • Each silicon solar cell in the plurality of solar cells may be any of the solar cells described herein.
  • a portion of a first silicon solar cell overlaps a portion of a second silicon solar cell so that the front busbar layer of the second silicon solar cell is below the rear tabbing layer of the first solar cell.
  • An electronically conductive adhesive is conductively bonded to both the front busbar layer of the second silicon solar cell, and the rear tabbing layer of the first silicon solar cell to electrically connect the first silicon solar cell and the second silicon solar cell.
  • a solar cell module has a front sheet having a front surface and a rear surface, a front encapsulant layer on the rear surface of the front sheet, a plurality of shingled silicon solar cell arrays, as described herein, on the front encapsulant layer and the shingled silicon solar cell arrays are electrically connected to one another.
  • a rear sheet having a front surface and a rear surface, the rear surface of the rear sheet exposed to an outside environment, and a rear encapsulant layer on the front surface of the rear sheet, wherein a first portion of the rear encapsulant layer is on the plurality of shingled silicon solar cell arrays, and a second portion of the rear encapsulant layer is on the front encapsulant layer.
  • Some or all flexible, conductive interconnects may further comprise a junction box in contact with the rear sheet.
  • intercalation pastes comprising precious metal particles and intercalating particles, which can be printed on metal particle layers to alter the properties of the metal particle layers after they have been fired into a fired multilayer stack.
  • intercalation pastes are used to provide a solderable surface on a metal particle layer that would not have been solderable by itself.
  • Intercalation pastes may also be used to improve adhesion within a fired multilayer stack or to change the interaction of a metal particle layer with an underlying substrate.
  • Intercalation pastes are widely applicable to many applications including transistors, light emitting diodes, and integrated circuits; however, the majority of examples disclosed below will focus on photovoltaic cells.
  • SEM Scanning electron microscopy
  • EDX energy dispersive x-ray spectroscopy
  • Cross-sectional SEM images of the fired multilayer stack were prepared by ion milling. A thin epoxy layer was applied to the top of the fired multilayer stack and dried for at least 30 minutes. The sample was then transferred to a JEOL IB-03010CP ion mill operating at 5 kV and 120 uA for 8 hours to remove 80 microns from the sample edge. Milled samples were stored in a nitrogen glove box prior to SEM/EDX.
  • drying describes a thermal treatment at or below a temperature of 500°C, or below 400°C, or below 300°C for a time period between 1 second and 90 minutes or any range subsumed therein.
  • Pastes are typically applied to a substrate via screen printing or another deposition method to create a "wet" layer.
  • the wet layer may be dried to reduce or remove volatile organic species such as solvents, creating a "dried” layer.
  • firing describes heating at a temperature greater than 500°C, greater than 600°C, or greater than 700°C for a time period between 1 second and 60 minutes, or any range subsumed therein.
  • fired layer describes a dried layer that has been fired.
  • multilayer stack is used herein to describe a substrate that has two or more layers of different materials on it.
  • a "fired multilayer stack” is a multilayer stack whose layers have been dried and fired. There are several ways to fire such a multilayer stack.
  • co-firing is used to describe the treatment for multilayer stacks that are fired only once. For example, during silicon solar cell fabrication, a layer of aluminum particle paste is first applied to a substrate and dried. Then, a rear tabbing paste layer is applied on a portion of the dried aluminum particle layer then dried, resulting in a dried aluminum particle layer and a dried rear tabbing layer. During co-firing, both dried layers are fired simultaneously in one step.
  • sequential firing is used to describe the treatment for multilayer stacks that are fired multiple times.
  • a metal particle paste is applied on a substrate, dried and then fired.
  • An intercalation paste is then applied on a portion of the dried and fired metal particle paste (referred to as the metal particle layer). Then, the entire multilayer stack is dried and fired for a second time. It should be noted that embodiments of the invention that describe co-fired multilayer stacks or structures are also appropriate for multilayer stacks or structures that have been fired sequentially.
  • intercalation is used herein to describe penetration into a porous material.
  • intercalation describes the penetration of material from intercalating particles in an intercalation layer into adjacent porous dry metal particle layer(s) during the firing process, which results in an intercalating particle material coating (partial or full) on at least a portion of the metal particles.
  • modified metal particle layer is used herein to describe such a fired metal particle layer into which material from intercalating particles has penetrated.
  • the preposition "on” is used herein to mean that the layers may or may not be in direct physical contact with one another.
  • a layer is on a substrate is to say that the layer is positioned either directly adjacent to or indirectly above or adj acent to the substrate.
  • a particular layer is indirectly above or adjacent to a substrate is to say that there may or may not be one or more additional layers between the particular layer and the substrate.
  • the preposition "directly on” is used herein to mean that the layers are in direct physical contact with one another. For example, to say that a layer is directly on a substrate is to say that the layer is positioned directly adjacent to the substrate.
  • a metal particle layer consists mainly of particles of metal A
  • it can be referred to as a "metal A particle layer.”
  • a metal particle layer consists mainly of aluminum particles
  • it can be referred to as an aluminum particle layer.
  • a modified metal particle layer consists mainly of particles of metal A
  • it can be referred to as a modified aluminum particle layer.
  • solderable surface refers to a surface that can be soldered to a soldering ribbon.
  • solderable surface refers to a surface that can be soldered to a soldering ribbon.
  • materials that create a solderable surface include, but are not limited to, tin, cadmium, gold, silver, palladium, rhodium, copper, zinc, lead, nickel, alloys thereof, combinations thereof, composites thereof, and mixtures thereof.
  • a surface is solderable when at least 70 wt% of the surface consists of a material such as silver, gold, platinum, palladium, rhodium, and alloys, composites, and other combinations thereof.
  • Particles described herein can exhibit a variety of shapes, sizes, specific surface areas, and oxygen contents.
  • Particles may be spheroidal, acicular, angular, dendritic, fibrous, flaky, granular, irregular, and nodular as defined by ISO 3252.
  • spherical is used herein to refer to generally spherical shapes and may include spheroidal, granular, nodular, and sometimes irregular shapes.
  • the term “flake” refers to flaky, and sometimes angular, fibrous, and irregular shapes.
  • the term “elongated” refers to acicular, and sometimes angular, dendritic, fibrous, and irregular shapes as defined by ISO 3252: 1999.
  • Particle shape, morphology, size, and size distribution often depend on synthesis technique.
  • a group of particles may include a combination of particles of different shapes and sizes.
  • Particles that are spherical or elongated are typically described by their D50, specific surface area and particle size distribution.
  • the D50 value is defined as the value at which half of the particle population has a diameter below and half the particle population has a diameter above the value.
  • Measuring a particle diameter distribution is typically performed with a laser diffraction particle size analyzer such as the Horiba LA-950.
  • a laser diffraction particle size analyzer such as the Horiba LA-950.
  • spherical particles are dispersed in a solvent in which they are well separated and the scattering of transmitted light is directly correlated to the size distribution from smallest to largest dimensions.
  • a common approach to express laser diffraction results is to report the D50 values based on volume distributions.
  • the statistical distribution of particle sizes can also be measured using a laser diffraction particle size analyzer. It is common for precious metal particles to have either a unimodal or a multimodal particle size distribution. In a unimodal distribution, particle size is monodispersed and the D50 is in the center of a single distribution. Multimodal particle size distributions have more than one mode (or peak) in the particle size distribution. Multimodal particle size distributions can increase the tap density of a powder, which typically results in a higher green film density.
  • particles may have a flake or elongated shape as defined above.
  • the flake may have a diameter between 1 ⁇ and 100 ⁇ or between 1 ⁇ and 15 ⁇ and a thickness between 100 nm and 500 nm.
  • An elongated shape may have a diameter between 200 nm and 1000 nm and a length greater than 1 ⁇ .
  • there is no limitation on the shape of the particles any particle shape can be used, as long as its largest dimension is no larger than 50 ⁇ , 5 ⁇ , or 1 ⁇ .
  • the specific surface area of particles can be measured using the BET (Brunauer-Teller-Teller)
  • Emmett-Teller Emmett-Teller method according to DIN ISO 9277, 2003-05.
  • the specific surface area of the particles disclosed herein, and in particular of the silver and bismuth particles is determined by the following test method: BET measurement performed using TriStar 3000 (from Micromeritics Instrument Corporation) which operates based on the physical adsorption analytical technique. Sample preparation includes degassing to remove adsorbed molecules. Nitrogen is the analysis gas, and helium is used to determine the void volume of the sample tube. Micromeritics provides silica alumina for use as a reference material, along with preparation procedures and testing conditions. The measurement begins by adding a known mass of the reference material to a sample tube and mounting the sample tube on the BET apparatus manifold.
  • the thermally-stable dosing manifold, sample tubes, and dedicated tube for measuring saturation pressure (P 0 ) are evacuated.
  • the manifold is filled with helium (a non-absorbing gas) and the sample port is opened to determine the warm free space of the sample at room temperature.
  • the sample tube with the reference material are submerged in liquid nitrogen and cooled to around 77 K, and free space analysis is once again conducted.
  • Saturation pressure of the adsorptive is measured using P 0 tube, followed by nitrogen dosing into the manifold above atmospheric pressure. Pressure and temperature of nitrogen are recorded, and then the sample port is open to let nitrogen adsorb onto the sample. After some time, the port is closed to allow adsorption to reach equilibrium.
  • the amount adsorbed is the quantity of nitrogen removed from the manifold minus any residual nitrogen in the sample tube. Measured points along the adsorption isotherm are used to calculate the specific area in m 2 /g for the reference material; this procedure is repeated with any sample of interest such as the particles described herein.
  • Particles described herein have salient thermal properties: melting point and/or softening point, both of which depend on the crystallinity of the material.
  • the melting point of particles can be determined by differential scanning calorimetry with a DSC 2500 differential scanning calorimeter made by TA Instruments and using the method described in ASTM E794-06 (2012).
  • the melting point of crystalline materials can also be determined using a heating stage and x-ray diffraction. As a crystalline material is heated above its melting point, diffraction peaks begin to disappear.
  • the softening point is the temperature at which an amorphous, or glassy particle begins to soften.
  • the softening point of a glass particle may be determined using a dilatometer.
  • the softening point may also be obtained by the fiber elongation method described in ASTM C338-57.
  • a substrate, metal particle paste, and intercalation paste form a fired multilayer stack.
  • the substrate may be a solid, planar, or rigid material.
  • a substrate includes at least one material selected from the group consisting of silicon, silicon dioxide, silicon carbide, aluminum oxide, sapphire, germanium, gallium arsenide, gallium nitride and indium phosphide. Such substrates are commonly used for deposition of layers that make up transistors, light emitting diodes, integrated circuits and photovoltaic cells.
  • the substrate may also be electronically conductive and/or flexible.
  • the substrate includes at least one material selected from the group consisting of aluminum, copper, iron, nickel, titanium, steel, zinc, and alloys, composites, and other combinations thereof.
  • metal particle pastes include metal particles and organic vehicle.
  • metal particle pastes also include an inorganic binder such as a glass frit.
  • a common, commercially available metal particle paste is used.
  • Metal pastes containing aluminum commonly used on silicon solar cells are sold by Ruxing Technology (e.g., RX8252H1), Monocrystal (e.g., EFX-39), and GigaSolar Materials (e.g., Ml).
  • Metal particles may include at least one of aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, or alloys, composites, or other combinations thereof.
  • metal particles have a D50 between 100 nm and 100 ⁇ , between 500 nm and 50 ⁇ , between 500 nm and 20 ⁇ , or any range subsumed therein.
  • the metal particles may have a spherical, elongated or flake shape and may have a unimodal or multimodal size distribution.
  • Glass frit may be included in metal particle pastes in small quantities (i.e., less than 5 wt%).
  • a metal particle paste comprises 70 wt% to 80 wt% aluminum particles, less than 2 wt% glass frit, and organic vehicle.
  • an intercalation paste includes precious metal particles, intercalating particles and an organic vehicle.
  • solids loading can be used in connection with pastes to describe the amount or proportion of precious metal and intercalating particle solids in a paste.
  • the pastes described herein also include an organic vehicle, although that may not always be stated explicitly.
  • precious metal particles comprise at least one material selected from the group consisting of gold, silver, platinum, palladium, and rhodium, and alloys, composites, or other combinations thereof.
  • precious metal particles comprise between 10 wt% and 70 wt% of the paste.
  • precious metal particles have a D50 between about 100 nm and 50 ⁇ , between 300 nm and 10 ⁇ , between 300 nm and 5 ⁇ , or any range subsumed therein.
  • precious metal particles have a specific surface area ranging from about 0.4 to 7.0 m 2 /g or from about 1 to 5 m 2 /g or any range subsumed therein.
  • the precious metals may have up to 2 wt% oxygen content; the oxygen may be mixed uniformly throughout the particle or the oxygen may be found as an oxide shell that has a thickness up to 500 nm.
  • the precious metal particles may have a spherical, elongated or flake shape and have a unimodal or multimodal size distribution.
  • Silver particles are commonly used in metallization pastes in the solar industry.
  • at least some precious metal particles are silver with a D50 between 300 nm and 2.5 ⁇ and a specific surface area between 1 and 3 m 2 / g.
  • intercalating particles is used to describe particles that can deform when heated, and, when placed adjacent to a porous layer of other metal particles, can at least partially intercalate into the porous metal particle layer and phase separate from the other metal particles under the influence of heat.
  • intercalating particles have a D50 between 50 nm and 50 ⁇ , between 50 nm and 10 ⁇ , between 300 nm and 5 ⁇ , or any range subsumed therein. In one embodiment, intercalating particles have a D50 between 300 nm and 3 ⁇ .
  • intercalating particles have a specific surface area ranging from about 0.1 to 6 m 2 /g, about 0.5 to 3 m 2 / g or about 0.5 to 4 m 2 /g or any range subsumed therein.
  • intercalating particles are flakes and have a specific surface area of about 1.0 to 3.0 m 2 /g.
  • the intercalating particles may have a spherical, elongated or flake shape and may have a unimodal or multimodal size distribution.
  • intercalating particles There are three groups of particles that can be used as intercalating particles: low temperature base metal particles (LTBMs), crystalline metal oxide particles, and glass frit particles.
  • intercalating particles consist solely of low temperature base metal particles, or of crystalline metal oxide particles, or of glass frit particles.
  • intercalating particles are mixtures of particles from two or more of these groups. It is desirable that the elements of the intercalating particles have low solubility with and do not alloy with elements in adjacent metal particle layers.
  • an intercalating particle is a low temperature base metal particle.
  • low temperature base metal particles LTBMs
  • LTBMs low temperature base metal particles
  • LTBMs also contain up to 2 wt% oxygen; the oxygen may be mixed uniformly throughout the particle, or the oxygen may be found in an oxide shell that has a thickness up to 500 nm and coats or partially coats the particle.
  • the melting point of LTMBs is even lower, such as below 350°C or below 300°C.
  • LTBMs are made exclusively or essentially of bismuth, tin, tellurium, antimony, lead, or alloys, composites, or other combinations thereof.
  • intercalating particles contain only bismuth and have a D50 between 1.5 and 4 ⁇ and a specific surface area between 1 and 2 m 2 /g.
  • a LTBM intercalating particle is a bismuth core particle surrounded by a metal or metal oxide shell.
  • a LTBM intercalating particle is a bismuth core particle surrounded by a single shell made of silver, nickel, nickel alloy such as nickel boron, tin, tellurium, antimony, lead, molybdenum, titanium, composites, and/or other combinations thereof.
  • a LTBM intercalating particle is a bismuth core particle surrounded by a single shell that is an oxide of silicon, an oxide of magnesium, an oxide of boron, or any combination thereof. Any of these shells may have a thickness that ranges from 0.5 nm to 1 ⁇ , or 0.5 nm to 200 nm, or any range subsumed therein.
  • an intercalating particle is a crystalline metal oxide particle.
  • a metal oxide is any compound that has at least one oxygen atom (anion with an oxidation state of -2) and at least one metal atom. Many metal oxides contain multiple metal atoms that may all be the same or that may include a variety of metals. A wide range of metal to oxygen atom ratios is possible, as would be understood by a person with ordinary skill in the art. When metal oxides form ordered, periodic structures, they are crystalline. Such crystalline metal oxides can diffract x-ray radiation in a pattern of peaks of varying intensities characteristic of their crystal structure.
  • a crystalline metal oxide particle consists exclusively or essentially of an oxide of at least one of the following metals: bismuth, tin, tellurium, antimony, lead, vanadium, chromium, molybdenum, boron, manganese, cobalt, and alloys, composites or other combinations thereof.
  • crystalline metal oxide particles are heated, it is useful if they begin to melt (i.e., reach their melting point (TM)) at a lower temperature than the temperature at which significant interdiffusion can occur within the structure between or among metal particles of different compositions.
  • TM melting point
  • the melting point of crystalline materials in a mixed layer can be determined using a heating stage and x-ray diffraction; as a sample is heated above its melting point, diffraction peaks diminish and then disappear.
  • the intercalating particles are crystalline bismuth oxide with a D50 between 50 nm and 2 ⁇ and a specific surface area between 1 and 5 m 2 /g.
  • a crystalline metal oxide particle also contains a small amount (i.e., less than 10 wt%) of one or more additional elements that may adjust the melting point of the particle.
  • additional elements include, but are not limited to, silicon, germanium, lithium, sodium, potassium, cesium, magnesium, calcium, strontium, barium, zirconium, hafnium, vanadium, niobium, chromium, molybdenum, manganese, rhenium, iron, cobalt, zinc, cadmium, gallium, indium, carbon, nitrogen, phosphorous, arsenic, antimony, sulfur, selenium, fluorine, chlorine, bromine, iodine, lanthanum, and cerium.
  • an intercalating particle is a glass frit particle.
  • a glass frit particle consists exclusively or essentially of the combination of oxygen and at least one of the following elements: silicon, boron, germanium, lithium, sodium, potassium, cesium, magnesium, calcium, strontium, barium, zirconium, hafnium, vanadium, niobium, chromium, molybdenum, manganese, rhenium, iron, cobalt, zinc, cadmium, gallium, indium, carbon, tin, lead, nitrogen, phosphorous, arsenic, antimony, bismuth, sulfur, selenium, tellurium, fluorine, chlorine, bromine, iodine, lanthanum, cerium, oxygen, and alloys, composites, and other combinations thereof.
  • the glass frit has a softening point below 900°C or below 800°C in order to effectively deform during firing.
  • the intercalating particles are bismuth silicate glass frit particles with a D50 between 50 nm and 2 ⁇ and a specific surface area between 1 and 5 m 2 /g.
  • organic vehicle describes mixtures or solutions of organic chemicals or compounds that assist in dissolving, dispersing and/or suspending solid components in a paste.
  • organic vehicle mixtures may be used.
  • Such organic vehicles may or may not contain thixotropes, stabilizers, emulsifiers, thickeners, plasticizers, surfactants and/or other common additives.
  • Binders may be polymeric or monomeric organic compounds, or "resins," or a mixture of the two.
  • Polymeric binders may have a variety of molecular weights and a variety of polydispersity indices.
  • Polymeric binders may include a combination of two different monomeric units that are known as copolymers, in which monomeric units may either alternate singly or in large blocks (block copolymers).
  • Poly-sugars are commonly used polymeric binders and include, but are not limited to, alkyl cellulose and alkyl derivatives such as methyl cellulose, ethyl cellulose, propyl cellulose, butyl cellulose, ethylhydroxyethyl cellulose, derivatives and mixtures thereof.
  • polymeric binders include, but are not limited to, polyesters, polyethylenes, polypropylenes, polycarbonates, polyurethanes, polyacrylates (including polymethacrylate and polymethymethacrylate), polyvinyls (including polyvinylchloride, polyvinylpyrrolidone, polyvinylbutyral, polyvinyl acetate), polyamides, polyglycols (including polyethylene glycol), phenolic resins, poly-terpenes, derivatives and combinations thereof.
  • An organic vehicle binder may include between 1 and 30 wt% binder.
  • Solvents are organic species that are usually removed from pastes during processing by thermal means such as vaporization.
  • solvents that can be used in the pastes described herein include, but are not limited to, polar, non-polar, protic, aprotic, aromatic, non-aromatic, chlorinated, and non-chlorinated solvents.
  • solvents that can be used in the pastes described herein include, but are not limited to, alcohols, di-alcohols (including glycols), multi-alcohols (including glycerol), mono- and poly-ethers, mono- and poly-esters, alcohol ethers, alcohol esters, mono-and di-substituted adipate esters, mono- and poly-acetates, ether acetates, glycol acetates, glycol ethers (including ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, and triethylene glycol monobutyl ether), glycol ether acetates (including ethylene glycol monobutyl ether acetate), linear or branched saturated and unsaturated alkyl chains (including butane, pentane, hexane, octane, and decane), terpenes (including alpha-, beta-, gamma-, and 4-terpineol), 2,2,
  • an organic vehicle contains between 70 and 100 wt% solvent.
  • the proportion and composition of the binder, solvent, and any additives may be adjusted to achieve a desired dispersion or suspension of paste particles, a desired carbon content, and/or desired rheological properties, as would be understood by a person with ordinary skill in the art.
  • paste rheology can be modified by adding thixatropic agents such as Thixatrol Max®.
  • the carbon content of the organic vehicle may be increased or decreased by modifying the binder and thixatropic agent and taking into account peak firing temperature, firing profile, and airflow that will occur during thermal annealing. Minor additives may also be included.
  • a metallization paste has a viscosity between 10,000 and 200,000 cP at 25°C and at a sheer rate of 4 sec "1 as measured using a temperature-controlled Brookfield RVDV-II+ Pro viscometer.
  • intercalation pastes are shown in Table I, according to some embodiments of the invention.
  • the intercalation paste has a solids loading between 30 wt% and 80 wt%, precious metal particles make up between 10 wt% and 70 wt% of the intercalation paste, intercalating particles make up at least 10 wt%, 15 wt %, 20 wt%, 25 wt%, 30 wt%, or 40 wt% of the intercalation paste, and the ratio of intercalating particles to precious metal particles is at least 1 :5 by weight.
  • the precious metal particle content is 50 wt%
  • the intercalating particles make up at least 10 wt% of the intercalation paste.
  • the ratio of intercalating particles to precious metal particles in the intercalation paste is at least 1 :5, or 2:5, or 3:5, or 1 : 1, or 5:2 by weight.
  • an intercalation paste contains between 20 and 50 wt% precious metal particles (i.e., intercalation paste range II in Table I) and between 10 and 35 wt% intercalating particles, which may include LTBM, crystalline metal oxides, glass frits, or a combination thereof.
  • the intercalating particles are metallic bismuth particles.
  • Intercalation paste A (Table I) may contain 50 wt% silver particles, 12.5 wt% bismuth particles, and 37.5 wt% organic vehicle, resulting in a 1 :4 ratio (by weight) of intercalating particles to precious metal particles.
  • Intercalation paste C (Table I) may contain 45 wt% silver particles, 30 wt% bismuth particles, and 25 wt% organic vehicle, resulting in a 1 : 1.5 ratio (by weight) of intercalating particles to precious metal particles.
  • the intercalation paste comprises silver and bismuth particles, the notation "Ag:Bi" is used.
  • the intercalating particles are glass frit particles.
  • Intercalation paste B (Table I) may contain 45 wt% silver particles, 30 wt% bismuth-based glass frit particles, and 25 wt% organic vehicle, resulting in a 1 : 1.5 ratio (by weight) of intercalating particles to precious metal particles.
  • intercalating particles are a mixture of LTBMs, crystalline metal oxide particles, and glass frit particles.
  • Intercalation paste D (Table I) may contain 30 wt% silver particles, 15 wt% metallic bismuth particles, 5 wt% high-lead-content glass frit particles and 50 wt% organic vehicle. The formulation of the intercalation pastes can be adjusted to achieve a desired bulk resistance, contact resistance, layer thickness, and/or peel strength for a particular metal layer.
  • a method of forming an intercalation paste includes the steps of providing precious metal particles, providing intercalating particles, and mixing the precious metal particles and intercalating particles together in an organic vehicle.
  • the intercalating particles are added to the organic vehicle and mixed in a planetary mixer (e.g., Thinky AR-100) and then the precious metal particles (and additional organic vehicle, if desired) are added and mixed in the planetary mixer.
  • the intercalation paste may or may not then be milled, for example, by using a three-roll mill (e.g., Exakt 50 I).
  • the intercalation paste contains between 10 and 70 wt% precious metal particles and more than 10 wt% intercalating particles.
  • a fired multilayer stack includes a substrate on which there is at least one metal particle layer and at least one intercalation layer.
  • a fired multilayer stack is formed using a co-firing process that involves the following steps: applying a metal particle layer onto a substrate surface, drying the metal particle layer, applying an intercalation layer directly on a portion of the dried metal particle layer, drying the intercalation layer, and then co-firing the multilayer stack.
  • a fired multilayer stack is formed using a sequential firing process that involves the following steps: applying a metal particle layer onto a substrate surface, drying the metal particle layer, firing the metal particle layer, applying an intercalation layer directly on a portion of the fired metal particle layer, drying the intercalation layer and then firing the multilayer stack.
  • a portion of the intercalation layer penetrates into the metal particle layer, thereby transforming the metal particle layer into a modified metal particle layer.
  • each applying step involves a method selected independently from the group consisting of screen printing, gravure printing, spray deposition, slot coating, 3D printing and inkjet printing.
  • a metal particle layer is applied by screen printing a metal particle paste on a portion of a substrate, and an intercalation layer is applied by screen-printing an intercalation paste directly on a portion of a metal particle layer after it has been dried.
  • a portion of a substrate surface is covered by at least one dielectric layer and a metal particle layer is applied onto a portion of the dielectric layer.
  • FIG. 1 is a schematic cross-section drawing that shows a multilayer stack 100 before it is co-fired, according to an embodiment of the invention.
  • a dried metal particle layer 120 is directly on a portion of a substrate 1 10.
  • An intercalation layer 130 made up of intercalating particles and precious metal particles, as described above, is directly on a portion of the dried metal particle layer 120.
  • the intercalation layer 130 has an average thickness between 0.25 ⁇ and 50 ⁇ , between 1 ⁇ and 25 ⁇ , between ⁇ ⁇ and 10 ⁇ , or any range subsumed therein.
  • the intercalation layer 130 includes precious metal particles, intercalating particles, and optional organic binder (which may remain in the intercalation layer 130 after drying). Prior to co-firing, the precious metal particles and intercalating particles may be homogeneously distributed within the intercalation layer 130. In one arrangement, the precious metal particles and the intercalating particles are not deformed after drying (and prior to firing), retaining their original size and shape.
  • the dried metal particle layer 120 is porous and includes at least one of aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, and alloys, composites, and other combinations thereof.
  • the dried metal particle layer 120 prior to co-firing, contains metal particles, and may or may not contain organic binder, and may or may not contain non-metallic particles such as glass frit. The metal particles are typically not deformed after drying (and prior to firing), retaining their original size and shape.
  • the intercalating particles from the intercalation layer 130 intercalate into a portion of the dried metal particle 120 layer adjacent to (shown as below in Figure 1) the intercalation layer 130.
  • the portion of the dried metal particle layer 120 adjacent to the intercalation layer 130 and into which the intercalating particle material penetrates is called the "modified metal particle layer” for the purposes of this disclosure.
  • the remaining portion of the dried metal particle layer 120 that is not adjacent to the intercalation layer and into which no, or only trace amounts of intercalating particle material penetrates is referred to as the "metal particle layer" for the purposes of this disclosure.
  • particles in the dried metal particle layer 120 may sinter or melt, so that the metal particle layer has a different morphology and less porosity than the dried metal particle layer 120.
  • the changes that occur during firing and the fired multilayer stack structure are discussed in more detail below.
  • Figure 2 is a schematic cross-section drawing that shows a fired multilayer stack
  • the fired multilayer stack 200 includes a modified (due to firing) metal particle layer 222 adjacent to at least a portion of a substrate 210 and a modified (due to firing) intercalation layer 230 adjacent to the modified metal particle layer 222.
  • a modified (due to firing) metal particle layer 222 adjacent to at least a portion of a substrate 210 and a modified (due to firing) intercalation layer 230 adjacent to the modified metal particle layer 222.
  • the precious metal particles and intercalating particles in the intercalation layer form phases that phase separate from each other.
  • the precious metal particles may sinter or melt, changing the morphology and reducing the porosity of the modified intercalation layer 230.
  • the modified metal particle layer 222 includes metal particles into which material from the intercalating particles in the intercalation layer (shown as 130 in Figure 1 before firing) has penetrated, altering the material properties of a portion of the dried metal particle layer (shown as 120 in Figure 1 before firing) to form the modified metal particle layer 222.
  • the material from the intercalating particles may connect loosely packed metal particles in the modified metal particle layer 222, or it may coat metal particles that are already in contact with one another in the modified metal particle layer 222.
  • metal particle regions 220 into which little or only trace amounts of intercalation particle material have penetrated.
  • a metal particle layer 220 which is not in direct contact with the modified intercalation layer 230, does not contain an increased concentration of elements from the intercalation particles.
  • the metal particle layer 220 and the modified metal particle layer 222 form a compound(s) with the substrate 210 or dope the substrate 210 during co-firing (not shown).
  • Figure 2 indicates sharp boundaries between the metal particle layer regions 220 and the modified metal particle layer 222, it should be understood that the boundary is not generally sharp. In some arrangements, the boundary is determined by the extent of lateral diffusion of the modified intercalation layer 230 material into the metal particle layer 220 during co-firing.
  • the materials in the modified intercalation layer 230 in Figure 2 are phase separated into a phase that contains material from the intercalating particles and a phase that contains the precious metal.
  • Figure 3 is a schematic cross-section drawing that shows a fired multilayer stack 390 (equivalent to structure 200 of Figure 2) and in which the intercalation layer 330 has phase separated.
  • the fired multilayer stack 390 (within region 350 only) includes a modified (due to firing) metal particle layer 322 in region 350 between a portion of a substrate 300 and a modified (due to firing) intercalation layer 330.
  • a metal particle layer 320 that contains metal particles 392 is on the substrate 300 adjacent to the multilayer stack region 350.
  • the modified intercalation layer 330 contains two phases: a precious metal phase 335 and an intercalation phase 333 and has a solderable surface 335S. Most (at least more than 50%) of the solderable surface is made up of the precious metal phase 335. In some arrangements, the precious metal phase 335 and intercalation phase 333 do not phase separate completely during firing, so that there is also some of the intercalation phase 333 at the solderable surface 335S.
  • the modified metal particle layer 322 contains metal particles 392 and a portion of material from the intercalation phase 333. There is an interface 3221 between the modified intercalation region 330 and the adjacent metal particles 392 in the modified metal particle layer 322.
  • the interface 3221 may not be smooth and is dependent on the size and shape of the metal particles 392, as well as the firing conditions.
  • the modified metal particle layer 322 and the metal particle layer 320 may also contain a small amount of glass frit (not shown), which makes up less than 3 wt% of the layer.
  • Figure 2 are phase separated into to form a layered structure.
  • Figure 4 is a schematic cross- section drawing that shows a fired multilayer stack 400 (equivalent to the structure 200 of Figure 2) including an intercalation layer with two sublayers.
  • the fired multilayer stack 400 (within region 450 only) includes a modified (due to firing) metal particle layer 422 in region 450 between a portion of a substrate 410 and a modified (due to firing) intercalation layer 430.
  • a metal particle layer 420 that contains metal particles 402 is on the substrate 410 adjacent to the multilayer stack region 450.
  • the modified intercalation layer 430 contains two sublayers: an intercalation sublayer 433 directly on the modified metal particle layer 422 and a precious metal sublayer 435 directly on the modified intercalation layer 433.
  • the precious metal sublayer 435 has a solderable surface 435S.
  • the modified metal particle layer 422 contains metal particles 402 and some material 403 from the intercalation sublayer 433. There is an interface 4221 between the modified intercalation layer 430 (or the modified intercalation layer 433) and the top-most metal particles 402 in the modified metal particle layer 422.
  • the modified metal particle layer 422 and the metal particle layer 420 may also contain a small amount of glass frit (not shown), which makes up less than 3 wt% of the layer.
  • a metal particle layer (such as 220 in Figure 2) has an average thickness between 0.5 ⁇ and 100 ⁇ , between 1 ⁇ and 50 ⁇ , between 2 ⁇ and 40 ⁇ , between 20 ⁇ and 30 ⁇ , or any range subsumed therein.
  • Such a metal particle layer on a substrate is typically smooth with a minimum and maximum layer thickness within 20% of the average metal particle layer thickness over a 1 x 1 mm area.
  • the layer thickness and variation over the described area can be accurately measured using an Olympus LEXT OLS4000 3D Laser Measuring Microscope and/or a profilometer such as a Veeco Dektak 150.
  • a metal particle layer (such as 220 in Figure 2) is made of sintered aluminum particles and has an average thickness of 25 ⁇ .
  • the porosity of the metal particle layer can be measured using a mercury porosimeter such as a CE instrument Pascal 140 (low pressure) or Pascal 440 (high pressure) in a range between 0.01 kPa and 2 Mpa.
  • Fired metal particle layers may have porosities between 1% and 50%, between 2% and 30%, between 3% and 20%, or any range subsumed therein.
  • Fired metal particle layers made of aluminum particles and used in solar applications may have a porosity between 10% and 18%.
  • the thicknesses of an intercalation sublayer and of the precious metal sublayer were measured in an actual multilayer stack using cross-sectional SEM/EDX.
  • the sublayers were distinguished in SEM due to the differences in contrast between the intercalation and precious metal phases.
  • EDX mapping was used to identify the location of the interface shown as 4321 in Figure 4.
  • the precious metal sublayer has a thickness between 0.5 ⁇ and ⁇ , between 0.5 ⁇ and 5 ⁇ , between ⁇ and 4 ⁇ m, or any range subsumed therein.
  • the intercalation sublayer has a thickness between 0.01 ⁇ and 5 ⁇ , between 0.25 ⁇ and 5 ⁇ , between 0.5 ⁇ and 2 ⁇ , or any range subsumed therein.
  • a modified intercalation layer contains two phases: a precious metal phase and an intercalation phase
  • the intercalation phase is not solderable, so it is useful if the solderable surface 230S contains mostly the precious metal phase to ensure solderability.
  • the solderable surface contains more than 50%, more than 60%, or more than 70% precious metal phase.
  • the solderable surface of the modified intercalation layer contains mostly precious metal(s). Plan view EDX was used to determine the concentration of elements on the surface of the modified intercalation layer.
  • At least 70 wt%, at least 80 wt%, at least 90 wt%, at least 95 wt% or at least 98 wt% of the outside surface 230S of the modified intercalation layer 230 contains one or more of gold, silver, platinum, palladium, rhodium, and alloys, composites, and other combinations thereof.
  • the firing conditions, intercalating particle and precious metal particle types and sizes all affect the degree of phase separation in modified intercalation layer morphology.
  • the modified metal particle layer (shown as 222 in Figure 2) contains a much higher concentration of the intercalating particle material than does the metal particle layer (shown as 220 in Figure 2).
  • a comparison of EDX spectra taken from cross-sections of the modified metal particle layer and from the metal particle layer in an actual multilayer stack can be used to determine the concentration of material from the modified intercalation layer that has intercalated into the modified metal particle layer.
  • the weight ratio (intercalation metal to total metal) is referred to as the IM:M ratio.
  • a baseline EDX analysis was performed in a region of the metal particle layer that was at least 500 ⁇ away from the modified metal particle layer to ensure reproducible measurements.
  • a second EDX spectrum was taken from the modified metal particle layer, and the spectra were compared.
  • the IM:M ratio only the peaks of the metallic elements were considered (i.e., peaks from carbon, sulfur, and oxygen were ignored).
  • precious metals and any metallic elements from the substrate were excluded in order to prevent unreliable results.
  • the metal particle layer when the dry metal particle layer (shown as 120 in Figure 1) contained aluminum particles and the intercalation layer 130 contained bismuth and silver particles, the metal particle layer (i.e., after firing) contained approximately 1 wt% bismuth and more than 98 wt% aluminum with a Bi: (Al+Bi) (IM:M) ratio of 1 :99.
  • IM:M Bi: (Al+Bi)
  • Other intercalation metal components make up less than 0.25 wt% of the modified metal particle layer and are not considered in calculation the IM:M ratio.
  • the IM:M ratio is 1 : 10 6 , 1 : 1000, 1 : 100, 1 :50, 1 :25, or 1 : 10.
  • FIG. 5 is a schematic cross-section drawing that shows a portion of such a substrate 510, a modified metal particle layer 522 and a modified intercalation layer 530, according to an embodiment of the invention.
  • Line 502 indicates the deepest incursion of the substrate 510 into the modified metal particle layer 522.
  • Line 504 indicates the deepest incursion of the modified intercalation layer 530 into the modified metal particle layer 522.
  • sampling region 522A The region of the modified metal particle layer 522 between the line 502 and the line 504 can be referred to as sampling region 522A.
  • IM:M ratio the region of the modified metal particle layer 522 between the line 502 and the line 504
  • the IM:M ratio in a modified metal particle layer is
  • an intercalation layer containing bismuth particles is on an aluminum particle layer, and the modified metal particle layer (as analyzed in a sampling region such as that shown as 522A in Figure 5) contains 4 wt% bismuth and 96 wt% aluminum for a Bi: (Al+Bi) (or IM:M) ratio of 1 :25.
  • the Bi: (Al+Bi) ratio is 400% higher in the modified metal particle layer than in the metal particle layer.
  • an intercalation layer contains crystalline metal oxides and/or glass frits that contain more than one metal
  • the intercalation metal components are quantified by EDX and summed to determine to IM:M ratio. For example, if a glass frit contains both bismuth and lead, then the ratio is described as (Bi+Pb):(Bi+Pb+Al).
  • a fired multilayer stack also includes a solid compound layer formed from interactions between metal particles in a dried metal particle layer and a substrate during firing.
  • the solid compound layer can include, but is not limited to, alloys, eutectics, composites, mixtures, or combinations thereof.
  • a modified metal particle layer and a substrate form solid compound region(s) at their interface.
  • the solid compound region(s) may contain one or more alloys.
  • the solid compound region(s) may be continuous (a layer) or semi-continuous.
  • alloy(s) or other compounds that form may include one or more of aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, silicon, oxygen, carbon, germanium, gallium, arsenic, indium and phosphorous.
  • aluminum and silicon can form a eutectic above 660°C which, upon cooling, results in a solid aluminum- silicon (Al-Si) eutectic layer at the silicon interface.
  • the solid compound layer is a solid Al-Si eutectic layer formed on a portion of the silicon substrate. The formation and morphology of the solid Al-Si eutectic layer is well known in silicon solar cells.
  • a substrate is doped with at least one of aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, and alloys, composites, and other combinations thereof.
  • aluminum is a p-type dopant in silicon and, during firing, aluminum from an aluminum particle layer adjacent to the substrate, provides more aluminum dopant to form a highly p-type doped region in the silicon substrate, which is known as the back-surface field.
  • intercalating particles may undergo multiple phase changes as they melt and intercalate into a modified metal particle layer in a fired multilayer stack.
  • the intercalating particles may also form crystalline compounds as they intercalate into the modified metal particle layer.
  • Such crystalline compounds can improve cohesion between metal particles in the modified metal particle layer, prevent interdiffusion of certain elements, and/or reduce electrical contact resistance between metal layers in the fired multilayer stack.
  • the modified intercalation layer and the modified metal particle layer contain crystallites made up of bismuth and at least one other of oxygen, silicon and silver and alloys, composites, and other combinations thereof.
  • a precious metal phase includes at least one material selected from the group consisting of gold, silver, platinum, palladium, rhodium, and alloys thereof, composites thereof, and other combinations thereof.
  • the precious metal phase consists essentially of one or more of these materials. When one of these materials makes up the majority of the precious metal phase, the precious metal phase is described as being rich in that material. For example, if a precious metal phase, precious metal layer, or precious metal sublayer contains mostly silver, it can be referred to a silver-rich region, silver-rich layer, or silver-rich sublayer, respectively.
  • An intercalation phase contains elements from intercalating particles and may also include elements from the outside environment (e.g., oxygen) and small quantities of elements from precious metal particles in an adjacent metal particle layer, and a nearby substrate, which have been incorporated during firing.
  • elements from the outside environment e.g., oxygen
  • a wide array of elements may be in the intercalation phase depending on whether low-temperature base metals, crystalline metal oxides and/or glass frits are used as intercalating particles.
  • the intercalation phase when the intercalating particles are low temperature base metals exclusively), contains at least one material selected from the group consisting of bismuth, boron, tin, tellurium, antimony, lead, oxygen, and alloys, composites, and other combinations thereof.
  • the intercalation phase contains at least one material selected from the group consisting of oxides of bismuth, tin, tellurium, antimony, lead, vanadium chromium, molybdenum, boron, manganese, cobalt, and alloys, composites, and other combinations thereof.
  • the intercalation phase contains oxygen and at least one of the following elements: silicon, boron, germanium, lithium, sodium, potassium, cesium, magnesium, calcium, strontium, barium, zirconium, hafnium, vanadium, niobium, chromium, molybdenum, manganese, rhenium, iron, cobalt, zinc, cadmium, gallium, indium, carbon, tin, lead, nitrogen, phosphorous, arsenic, antimony, bismuth, sulfur, selenium, tellurium, fluorine, chlorine, bromine, iodine, lanthanum, cerium, and alloys, composites, and other combinations thereof.
  • the intercalation region is described as being rich in that material.
  • an intercalation region, intercalation layer, or intercalation sublayer contains mostly bismuth, it can be referred to a bismuth-rich region, bismuth-rich layer, or bismuth-rich sublayer, respectively.
  • Metal particle layers that contain mostly aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, and titanium are not solderable with mildly activated (RMA) fluxes and tin-based solders after firing. Yet, in solar cells and other devices, it is highly desirable to solder ribbons to make electrical connections with metal particle layers such as aluminum particle layers. As is disclosed herein, inventive intercalation pastes that contain precious metals such as silver and gold can be used on metal particle layers and fired in air to produce surfaces that are highly solderable.
  • Intercalation layers can be used to modify the material properties of metal particle layers in order to 1) to block diffusion of precious metals and provide a solderable surface, 2) mechanically strengthen the metal particle layer and 3) assist in etching layers beneath the metal particle layers.
  • multilayer stacks were formed using intercalation pastes that contain precious metal particles made of silver and intercalating particles made of bismuth metal or bismuth-based glass frits, and adjacent metal particle layers that contain aluminum particles.
  • Fired multilayer stacks were formed by screen printing aluminum pastes (commonly used for solar cell applications) on a bare silicon wafer, drying the sample at 250°C for 30 seconds, screen printing the intercalation paste on a portion of the dried aluminum particle layer, drying the sample at 250°C for 30 seconds and co-firing the sample using a spike fire profile with a peak temperature between 700°C and 820°C and a ramp up and cool down rate greater than 10°C/sec. All drying and firing steps were performed using a Despatch CDF 7210 furnace that is commonly used for silicon solar manufacturing.
  • SEM/EDS analysis was used to determine the elemental composition of various regions in polished, cross-sectioned fired multilayer stacks and to study the intercalation process.
  • SEM/EDX was performed with the equipment previously described using two different modes of operation.
  • SEM micrographs were imaged with a Zeiss Gemini Ultra-55 analytical field emission SEM using two modes referred to as the SE2 and Inlens.
  • the SE2 mode was operated at 5-10 kV and a working distance of 5-7 mm with the SE2 secondary electron detector and scanning cycle time of 10 seconds.
  • the brightness and contrast were varied between 0 and 50% and between 0 and 60%, respectively in order to maximize contrast between the intercalation region and the Al particles.
  • the Inlens mode was operated at 1-3 kV and a working distance of 3-7 mm with the InLens secondary electron detector and scanning cycle time of 10 seconds.
  • the brightness was set to 0% and the contrast set to around 40%.
  • intercalation pates that include 10-15 wt% intercalating particles block interdiffusion between precious metal (i.e., silver) and metal particles (i.e., aluminum).
  • Intercalation paste A (shown in Table I) includes 12.5 wt% bismuth particles and 50 wt% Ag, resulting in a ratio of intercalating particles to precious metal particles of 1 :4 by weight.
  • a fired multilayer stack was made as described above. SEM of the fired multilayer stack was performed in SE2 mode with the equipment described above at an accelerating voltage of 5 kV, a working distance of 7 mm, and 4000 times magnification.
  • Figure 6 is a scanning electron microscope cross-section image of the co-fired multilayer stack.
  • the modified intercalation layer 630 includes a bismuth-rich (intercalation phase) sublayer 632 that comprises bismuth oxide and a silver-rich (precious metal) sublayer 634.
  • the modified metal particle layer 622 contains aluminum particles 621 and intercalation phase material 623 that has spread out from the bismuth rich sublayer 632.
  • the intercalation region 632 is directly on the aluminum particles 621, at least near interface region 631.
  • the intercalation sublayer 632 seems to prevent interdiffusion of silver from the modified intercalation layer 630 and aluminum from the modified metal particle layer 622 during the co- firing process.
  • Figure 6 is an example of the layered structure described in Figure 4 above.
  • the precious metal sublayer 634 provides a surface (away from the modified metal particle layer 622) that is highly solderable.
  • the intercalation phase material 623 does not penetrate far into the modified metal particle layer 622.
  • the modified metal particle layer 622 contains mostly aluminum particles that are weakly sintered together and have poor mechanical strength after co-firing.
  • the peel strength of such a co-fired multilayer stack is less than 0.4 N/mm (Newton per millimeter) with the predominant failure mechanism between Al particles. Current solar industry standards require peel strengths greater than 1 N/mm to be considered commercially viable.
  • Intercalation paste B (shown in Table I) uses a glass frit as the intercalating particles to achieve a solderable surface.
  • Intercalation paste B contains 30 wt% bismuth-based glass frit (intercalating) particles and 45 wt% Ag, resulting in a ratio of intercalating particles to precious metal particles be 1 : 1.5 by weight.
  • the glass frit contains mainly bismuth and has a glass transition temperature of 387°C and a softening point of 419°C. SEM of the fired multilayer stack was performed in SE2 mode with the equipment described above at an accelerating voltage of 5 kV, a working distance of 7 mm, and 4000 times magnification.
  • Figure 7 is a scanning electron microscope cross-section image of such a co-fired multilayer stack, according to an embodiment of the invention.
  • a modified metal particle layer 722 contains aluminum particles 730.
  • the bismuth-based glass frit does not completely phase separate from the Ag particles, resulting in a modified intercalation layer 750 that has two phases: a precious metal phase 721 and a bismuth-based intercalation phase 740 similar to those shown in Figure 3 above.
  • a surface 750S on the modified intercalation layer 750 contains more than 50% precious metal phase 721.
  • the surface 750S is solderable with fluxes commonly used in the solar cell industry (e.g., Kester 952S, Kester 951 and Alpha NR205).
  • the overall peel strength of the fired multilayer stack is less than 0.5 N/mm, which may be due to the relatively low penetration of the bismuth intercalation phase 740 into the modified aluminum particle layer 722.
  • the morphology of the modified intercalation layer can be modified by changing the intercalating particles composition and loading in the intercalation layer.
  • Intercalation paste C (shown in Table I) contains 30 wt% bismuth particles and 45 wt% silver particles (i.e., Ag:Bi intercalation paste), resulting in a ratio of intercalating particles to precious metal particles of 1 : 1.5 by weight.
  • the increased intercalating particle content in the paste yields a higher concentration of intercalation material in the modified metal particle layer and results in a mechanically stronger fired multilayer stack.
  • Intercalation paste C was used as a drop-in replacement for a commercial silver rear tabbing paste during the fabrication of a BSF, multicrystalline, p-type solar cell. Intercalation paste C may also be referred to as a silver-on- aluminum (Ag-on-Al), rear tabbing, floating rear tabbing, or tabbing intercalation paste.
  • a suite of characterization tools was used on the resulting fired multilayer stack in order to assess the IM:M (intercalation metal: metal) ratio, precious metal surface coverage, and determine if crystallites formed in the intercalation region.
  • FIG. 8 is a scanning electron microscope (SEM) cross-section image in SE2 mode of such a fired aluminum particle layer 822 on a silicon substrate 810 taken from a region of a silicon solar cell that does not contain an intercalation layer.
  • the fired aluminum particle layer 822 is about 20 ⁇ thick and contains aluminum particles 821 and a small amount of inorganic binder (i. e., glass frit) 840.
  • An InLens mode scanning electron micrograph of the same aluminum particle layer is shown Figure 9.
  • the aluminum particle layer 922, aluminum particles 921 and the silicon substrate 910 are clearly visible, in addition to a back-surface field region 970 and a solidified aluminum- silicon (Al-Si) eutectic layer 980.
  • Figure 10 is an InLens SEM cross-section image of the same silicon solar cell used in the image shown in Figure 8 but taken from a region that contains a co-fired multilayer stack made using intercalation paste C (shown in Table I).
  • the co-fired multilayer stack 1000 contains a modified intercalation layer 1030, a modified aluminum particle layer 1022, a solidified Al-Si eutectic layer 1080, an aluminum-doped back-surface field (BSF) region 1070, and a silicon substrate 1010.
  • the BSF in the silicon substrate is doped p-type to between 10 17 and 10 20 atoms per cm 3 .
  • FIG 10 is shown Figure 11. While the InLens mode clearly shows the BSF region, SE2 mode is the preferred mode to image bismuth (intercalation phase) in the modified aluminum particle layer.
  • the co-fired multilayer stack 1100 contains a modified intercalation layer 1130, a modified aluminum particle layer 1122 and a silicon substrate 1110. A silver sublayer 1134 and a bismuth intercalation sublayer 1132 in the modified intercalation layer 1130 can also be seen.
  • the BSF region and the solidified Al-Si eutectic layer cannot be seen clearly in this image.
  • the modified aluminum particle layer 1122 contains a large quantity of bismuth intercalation material 1103, which intercalated around aluminum particles 1102 during co- firing.
  • the contrast between the bismuth and silver may not be strong enough to clearly identify the sublayers and degree of bismuth intercalation into the aluminum particle layer.
  • elemental maps of the cross-sections can be made using SEM/EDX in order to fully determine silver and bismuth placement in the co-fired multilayer stack.
  • the amount of intercalating metal (i.e., bismuth) in a modified aluminum particle layer due to intercalation can be determined by comparing EDX spectra taken from a modified aluminum particle layer region and an aluminum particle layer region in the same cross-section sample. It is most useful if the regions are more than 1 ⁇ apart from one another. A way of making this comparison has been described above as an IM:M or Bi:(Bi+Al) ratio. Such analysis can be useful in determining whether an intercalation paste was used in the fabrication of a solar cell.
  • Metallization layers in solar cells typically consist of a narrow subset of metals, which include aluminum, silver, bismuth, lead, and zinc. In commercial solar cells the aluminum particle layer contains aluminum almost exclusively.
  • intercalating particles in intercalation paste C contain bismuth exclusively, and metal particles in the metal particle layer are mostly aluminum. Comparing the ratio of bismuth to bismuth plus aluminum (Bi:(Bi+Al)) in the aluminum particle layer (i.e., which had no interaction with the intercalation paste) and the modified aluminum particle layer is a useful metric in determining whether an intercalation paste was incorporated into a solar cell.
  • the EDX spectra for these two layers was measured for approximately three minutes using the equipment described above at an accelerating voltage of 20kV, and a working distance of 7 mm.
  • the EDX spectrum for the fired aluminum particle layer 822 in Figure 8 was collected from region 898.
  • the EDX spectrum for the modified aluminum particle layer 1122 in Figure 11 was collected from region 1199. Elemental quantification was performed on these spectra using Bruker Quantax Esprit 2.0 software for automatic elemental identification, background subtraction, and peak fitting. The EDX spectra are shown in Figure 12. Aluminum and bismuth metal peak areas were quantified and wt% calculated from the EDX spectra in Figure 12 for the two layers and are summarized below in Table II. No significant quantities of any other metals could be identified in the EDX spectra.
  • the aluminum particle layer EDX spectra shown in Figure 12A yields a Bi:(Bi+Al) wt% ratio of 1 :244, and the modified aluminum particle layer spectrum shown in Figure 12B yields a Bi:(Bi+Al) wt% ratio of approximately 1 :4 as shown in Table II.
  • the Bi:(Bi+Al) wt% ratio in the modified aluminum particle layer 1122 is around 62 times higher than in the fired aluminum particle layer 822 not in contact with the Ag:Bi intercalation layer.
  • the ratio Bi:(Bi+Al) in a fired multilayer stack made is at least 20%, or at least 50%, or at least 2x, or at least 5x, or at least lOx or at least 50x higher in the modified aluminum particle layer than in the fired aluminum particle layer.
  • Plan view EDX can be used to determine the concentration of elements on the surface of a rear tabbing layer in a silicon solar cell.
  • EDX probes regions the surface to depths of about 4 ⁇ or less, making this a useful technique for identifying the degree of interdiffusion in a co-fired multilayer stack: a higher precious metal concentration means there has been less interdiffusion, and a lower precious metal concentration means there has been more interdiffusion.
  • Figure 13 is a plan view EDX spectrum taken from a surface of a rear tabbing layer that contains a Ag:Bi intercalation layer, according to an embodiment of the invention.
  • the EDX spectrum was collected using a SEM operated at an accelerating voltage of 10 kV, 7 mm working distance, and 500 times magnification.
  • the dominant peaks between 3.5 and 4 keV and the smaller peak at 0.3 keV are all identified as silver.
  • the remaining minor peaks in the spectrum are identified as the following: carbon at 0.3 keV (convoluted with a minor silver peak); oxygen at 0.52 keV; aluminum at 1.48 keV; and bismuth at 2.4 keV.
  • Element quantification was performed automatically using Bruker Quantax Esprit 2.0 software to subtract the background, identify the elemental peak, and then fit the peak intensities of the x-ray energies. The normalized weight percentages of each element are shown in Table III below. The total silver coverage on the surface of the rear tabbing layer is 96.3 weight percent (wt%).
  • Intercalation layers that contain silver and bismuth can form several unique crystal phases when fired on dried aluminum-based metal particle layers.
  • XRD can be used to distinguish between a fired multilayer stack that uses bismuth particles in the intercalation layer and a fired multilayer stack that uses a conventional silver-based tabbing paste with less than 10 wt% glass frit as the inorganic binder.
  • XRD was performed using a Bruker ZXS D8 Discover GADDS x-ray diffractometer equipped with a VANTEC-500 area detector and a cobalt x-ray source operated at 35kV and 40mA.
  • XRD patterns of fired multilayer stacks on the rear tabbing layer of a silicon solar cell are shown in Figure 14.
  • Diffractograms were measured using cobalt Ka wavelength in two 25° frames that were combined for a total window of 25-80° in 2 ⁇ . Each frame was measured for 30 minutes under x-ray irradiation. No background subtraction was performed on the two diffraction patterns in Figure 14. Patterns were normalized to unity against the largest peak, and a 0.01 background was added to the data to plot in Log(Intensity).
  • Ag:Bi intercalation layer, or rear tabbing layer in a solar cell has a different pattern compared to one formed without bismuth.
  • XRD pattem A is from a co-fired multilayer stack on the rear tabbing layer of a silicon solar cell.
  • the co-fired multilayer stack included a modified intercalation layer formed using an intercalation paste that contained approximately 45 wt% silver, 30 wt% Bi, and 25 wt% organic vehicle (as for Paste C in Table I above). Peaks 1410 are identified as silver and peaks 1420 are crystallites of bismuth oxide (B12O3).
  • XRD pattern B is from a co-fired multilayer stack on the rear tabbing layer of a silicon solar cell formed using a commercially available rear tabbing paste that contained less than 10 wt% glass frit as the intercalation layer on an aluminum particle layer.
  • the co-fired multilayer stack is dark in color, indicating significant silver-aluminum interdiffusion.
  • Peaks 1450 are identified as a silicon-aluminum eutectic phase.
  • Peaks 1460 are identified as a silver-aluminum alloy phase (i.e., Ag 2 Al).
  • Silver peaks 1410 are observed in pattern A along with a bismuth oxide compound and not in pattern B where silver is observed only as part of a silver-aluminum alloy 1450.
  • a rear tabbing layer in a silicon solar cell contains crystallites of bismuth and at least one other element such as silicon, silver, and oxides thereof, alloys thereof, composites thereof, or other combinations thereof.
  • a rear tabbing layer contains bismuth oxide crystallites.
  • an intercalation region undergoes multiple phase transformations during firing.
  • a dielectric layer is deposited onto a substrate surface before metal layers are deposited in order to passivate the substrate surface and to improve electronic properties.
  • the dielectric layer may also prevent species interdiffusion between the substrate and adjacent metal particle layer(s).
  • it may be highly desirable to etch through the dielectric layer in order to form compounds between the substrate and the metal particle layer to improve electrical conduction between the substrate and the metal particle layer.
  • Glass frits that contain bismuth and lead are known to penetrate through a variety of dielectric layers (e.g., silicon nitride) during co-firing of silicon solar cells.
  • intercalation paste D contains approximately 30 wt% silver, 20 wt% intercalating particles (15 wt% metallic bismuth particles, 5 wt% high- lead-content glass frit) and 50 wt% organic vehicle.
  • Such an intercalation paste can be especially useful if etching through a dielectric layer is desired.
  • Figure 15 shows a schematic cross-section drawing of a multilayer stack 1500 that includes a substrate 1510 coated with at least one dielectric layer 1513 prior to firing, according to an embodiment of the invention.
  • a dried metal particle layer 1520 is on a portion of the dielectric layer 1513.
  • An intercalation layer 1530 made up of intercalating particles and precious metal particles, as described above, is on a portion of the dried metal particle layer 1520. Prior to firing, the precious metal particles and intercalating particles may be homogeneously distributed within the intercalation layer 1530.
  • the dielectric layer contains at least one of silicon, aluminum, germanium, hafnium, gallium, oxides thereof, nitrides thereof, composites thereof, and combinations thereof.
  • the dielectric layer 1513 is a 75-nm thick silicon nitride layer. In another embodiment, there is a second dielectric layer (not shown) between the dielectric layer 1513 and the substrate 1510. In one arrangement, the second dielectric layer is a 10-nm thick alumina layer directly on the substrate 1510 and the dielectric layer 1513 is a 75-nm thick silicon nitride layer directly on the alumina layer.
  • Dried metal particle layer 1520 is formed by depositing metal particle paste on the dielectric layer 1513 and subsequently drying. In one arrangement, the dried metal particle layer 1520 is 20 ⁇ thick and contains aluminum particles.
  • FIG. 16 is a schematic cross-section drawing that shows a fired multilayer stack 1600 (the structure 1500 of Figure 15 after it has been fired), according to an embodiment of the invention.
  • a portion of substrate 1610 is coated with at least one dielectric layer 1614.
  • the intercalating particles that include glass frit(s) as described in reference to Figure 15
  • a modified intercalation layer 1630 melt and begin to flow, intercalating into a modified metal particle layer 1622.
  • material from glass frit particles in the modified intercalation layer 1630 penetrates into and through the metal particles in the modified metal particle layer 1622 and etches into the dielectric layer 1613 (1513 before firing), allowing some metal from the modified metal particle layer 1622 to interact chemically and electrically with the substrate 1610, forming one or more new compounds 1614.
  • Other intercalating particles e.g., bismuth particles
  • at least a portion of the precious metal particles and intercalating particles in the modified intercalation layer 1630 form phases that phase separate from each other.
  • the intercalating particles are bismuth particles and glass frit particles
  • the metal particles are aluminum.
  • An intercalation layer can cause stress in an underlying modified metal particle layer during firing, which can cause buckling or wrinkling and therefore poor layer strength and electrical communication between layers.
  • an intercalation layer may have a different coefficient of thermal expansion than an adjacent modified metal particle layer, causing the layers to expand or contract differently during firing.
  • Another source of stress in an adjacent modified metal particle layer may be the intercalation of melted intercalating particle material between the metal particles.
  • Such stresses can cause a modified metal particle layer and/or a modified intercalation layer to buckle or wrinkle.
  • Buckling or wrinkling can be described as large, periodic or non-periodic deviations in the thickness of a layer. Often this results in delamination between layers.
  • the initial thickness of the stack that contains the intercalation layer and the dried metal particle layer is approximately the same everywhere.
  • the thickness of the fired multilayer stack that contains the modified intercalation layer and the modified metal particle layer may be by as much as three times greater than the initial thickness in some regions.
  • Figure 17 is a plan-view optical micrograph of co-fired multilayer stack in which buckling has occurred.
  • a modified intercalation layer 1730 is visible.
  • the modified intercalation layer 1730 has buckled; some peak regions 1712 are indicated in Figure 17.
  • An adjacent metal particle layer 1720 has not buckled and remains smooth or approximately planar. Even though the intercalation layer 1730 has buckled, the mechanical integrity of the co-fired multilayer stack remains strong with peel strengths of more than 1 N/mm.
  • the buckling can make it challenging to make good, robust contact between the modified intercalation layer 1730 and tabbing ribbons (not shown) when they are soldered together.
  • the buckled surface of the modified intercalation layer 1730 may result in incomplete solder wetting over the extent of intercalation layer 1730, which can lower peel strength and solder j oint reliability. It may be useful to reduce or eliminate buckling in the co-fired multilayer stack to ensure successful soldering to tabbing ribbons.
  • Variable thicknesses can be incorporated into fired multilayer stacks to significantly reduce buckling and/or wrinkling of layers. When one or more layers has variable thicknesses, a non-planar interface between such layers can result.
  • An indication of variable thickness is a non-planar interface between layers in a fired multilayer film stack.
  • Variable thickness can be created by patterning a portion of a first layer and subsequently printing a second layer directly on the patterned portion of the first layer to create a non-planar interface between the two layers.
  • a layer has variable thickness as the result of having been printed using a patterned screen. After firing, the thicknesses of individual layers may be reduced, but firing does not cause layers with variable thickness to become layers with uniform thickness.
  • Variable thickness in a layer can be measured and quantified using cross- sectional SEM and surface topology techniques both before and after firing.
  • a layer can be described as having variable thickness when it has thickness variations that are at least 20% greater than or at least 20% less than the average thickness of the layer as measured within a 1 x 1 mm area.
  • Figure 18 is a screen that can be used during deposition of metal particle paste to achieve variable thickness in a dried metal particle layer according to an embodiment of the invention.
  • the screen 1800 has an open mesh 1810, and some patterned regions 1820.
  • the patterned regions 1820 contain closed areas 1821 and open areas 1822.
  • paste flows through the openings 1822 and the open mesh 1810 and is blocked by the closed areas 1821, which causes the deposited wet metal particle layer to have variable thickness.
  • the wet metal particle layer is subsequently dried to form a variable-thickness dried metal particle layer, and an intercalation paste is deposited directly on the variable-thickness dried metal particle layer.
  • Screens may be designed to ensure a continuous dried metal particle layer on a substrate surface with variable layer thicknesses overall or only in particular regions.
  • a metal particle paste is printed using a 230-mesh screen with an emulsion thickness of 5 ⁇ .
  • the pattern 1820 has a series of 100 ⁇ by 3 mm open areas 1822 adjacent to 100 ⁇ by 3 mm closed areas 1821. There is no restriction on partem type, periodicity (or lack thereof), or size. Many patterns can result in variable thickness and the patterns may be adjusted for various printing conditions and paste formulations.
  • Figure 19 is a schematic cross-section drawing of a dried metal particle layer with variable thickness deposited onto a substrate 1910 using the screen 1800 of Figure 18, according to an embodiment of the invention.
  • a dried metal particle layer 1920 outside region 1925 was formed by depositing metal particle paste through open mesh areas 1810 of the screen 1800, and then drying the metal particle paste.
  • a variable-thickness dried metal particle layer 1922 in region 1925 was deposited through a masked region 1820 of the screen 1800 and has variable thickness.
  • An intercalation paste was subsequently printed directly on the variable- thickness dried metal particle layer 1922 in region 1925 and dried to form an intercalation layer 1930.
  • Figure 20 is a schematic cross-section drawing of the structure of Figure 19 after it has been co-fired, according to an embodiment of the invention.
  • co- firing causes material from the intercalation layer 1930 (Figure 19) to intercalate into the underlying variable-thickness dried metal particle layer 1922 ( Figure 19), transforming the variable thickness metal particle layer 1922 into a variable-thickness modified metal particle layer 2022 and the intercalation layer 1930 into a modified intercalation layer 2030.
  • the modified metal particle layer 2022 has patterned thickness variations including, but not limited to, periodic bumps, ridges, edges and other feature shapes.
  • the thickness of the modified intercalation layer 1930 is often uniform, and the non-planar interface between the modified intercalation and the modified metal particle layer (due to its variable thickness) can be inferred by measuring variations in the total layer thickness of the multilayer stack.
  • Figure 21 is a plan-view optical micrograph of a co-fired multilayer stack in which metal particle paste was printed with variable thickness (in some regions) using a screen such as that shown in Figure 18.
  • An intercalation layer was printed directly on the variable thickness region of the metal particle layer, and the multilayer stack was co-fired to form a modified intercalation layer 2121 on the top surface, bordered on either side by an approximately planar metal particle layer 2120.
  • the metal particle layer 2120 has a planar top surface.
  • the surface of the modified intercalation layer 2121 is non-planar with a pattern that reflects the variations in thickness in the underlying modified metal particle layer.
  • modified intercalation layer 2121 does not show signs of buckling or wrinkling, as were seen clearly in the modified intercalation layer 1730 in Figure 17.
  • a portion of the co-fired multilayer stack has variable thickness.
  • variable thickness is to compare peak thicknesses and valley thicknesses to average layer thickness. In any layer, there can be some unintentional thickness variation, but such variations are typically less than 20% of the average layer thickness.
  • a layer can be considered to be planar (have uniform thickness) if its thickness varies by less than 20% of the average layer thickness.
  • FIG. 22 is a cross-section SEM image of a portion of a fired multilayer stack 2210 that has variable thickness, according to an embodiment of the invention. Cross-section samples were prepared and imaged using the methods described above.
  • the fired multilayer stack 2210 includes a modified intercalation layer 221 1, a modified aluminum particle layer 2212, and a silicon substrate 2213. Two interfaces on either side of the modified aluminum particle layer 2212 are identified in the image: interface 2218 between the silicon substrate 2213 and the modified aluminum particle layer 2212, and interface 2217 between the modified aluminum particle layer 2212 and the modified intercalation layer 2211. Interface 2216 is a solderable surface.
  • Figure 23 shows a silicon substrate 2322 that has a planar aluminum particle film 2321 that does not have variable thickness.
  • the thickness between the two interfaces 2217 and 2218 in Figure 22 was measured at regular intervals (e.g., 10 microns) across the sample. Thicknesses were also measured at local maximums and local minimums. Software such as ImageJ 1.50a can be used to obtain average thickness, and minimum and maximum thicknesses.
  • the peaks and valleys seen in a single cross-section sample may not be representative of an entire fired multilayer stack. Therefore, it is useful to make such measurements over several cross-section samples in order to ensure that very many peaks and valleys are measured. These are methods that would be known to a person with ordinary skill in the art.
  • the modified aluminum particle layer 2212 has an average thickness of 11.3 ⁇ , a peak thickness of 18.4 ⁇ , and a valley thickness of 5.2 ⁇ .
  • the peak thickness is 64% greater than the average thickness and the valley is 54% less than the average thickness.
  • a layer with variable thickness has a peak thickness that is at least 20%, at least 30%, at least 40%, or at least 50% greater than the average layer thickness.
  • a layer with variable thickness has a valley thickness that is at least 20%, at least 30%, at least 40%, or at least 50% less than the average layer thickness.
  • solderable surface 2216 of the modified intercalation layer 2211 is approximately parallel to the interface 2217.
  • all measurements described above for the modified aluminum particle layer 2212 can be made for the combined thicknesses of the modified aluminum particle layer 2212 and the modified intercalation layer 2211 between the solderable surface 2216 and the interface 2217. Comparisons of the thickness measurements for the two combined layers are good approximations for comparisons of the thickness measurements for the modified aluminum particle layer 2212 alone. For the combined layers in Figure 22, the peak thickness is 44% greater than the average overall thickness of 13.2 ⁇ , and the valley thickness is 43% less than the average overall thickness. This alternative method may systematically under-measure the thickness variation in a fired multilayer stack.
  • variable thickness For some applications, it may be desirable for only a portion of a fired multilayer stack to have variable thickness.
  • an aluminum particle layer on the back side of a silicon solar cell is typically planar. It may be useful to introduce variable thickness in rear tabbing layer portions (which include modified intercalation layers) on the back side of such a cell. Comparing thickness variation in a portion of the rear tabbing layer with thickness variation in a portion of the surrounding aluminum particle layer could be used to determine if layers with variable thicknesses were used on the back side of a solar cell.
  • Another useful metric to determine variable thickness in a fired multilayer film stack is average peak-to-valley height, which is the difference between the average of local maximums and the average of local minimums.
  • average peak-to-valley height is the difference between the average of local maximums and the average of local minimums.
  • surface topographic metrology methods such as profilometry, coherent scanning interferometry, and focus variation microscopy are more useful.
  • An example of a profilometer is a Bruker or Veeco Dektak 150 or equivalent.
  • Coherent scanning interferometry can be performed using an Olympus LEXT OLS4000 3D Laser Measuring Microscope. An accompanying software with these methods can automatically calculate average peak-to-valley difference.
  • profilometry is used to determine, within the same sample, the average peak-to-valley heights both for a fired multilayered stack that has variable thickness and for an aluminum particle layer that has uniform thickness.
  • a Veeco Dektak 150 was used to measure the surfaces in a 1 x 1 mm area with a 12.5 mm radius probe to produce 3D topological surface maps.
  • Figure 24 is a 3D surface topology map of the fired multilayered stack with variable thickness
  • Figure 25 and is a 3D surface topology map of the (adjacent) aluminum particle layer with uniform thickness.
  • the lightest regions in the figures indicate local maximums and the darkest regions indicate local minimums.
  • Figure 24 shows thickness variations (from -20.2 ⁇ ⁇ 15.9 ⁇ ), which would be expected for fired multilayer stack that included a variable thickness modified metal particle layer.
  • Figure 25 shows thickness variations (from -4.9 ⁇ to 5.5 ⁇ ), which would be expected for an aluminum particle layer that has uniform thickness.
  • Average peak-to-valley heights were calculated using the program Veeco Vision v4.20 which automatically identifies and averages local maximums and minimums and then subtract the difference.
  • the average peak-to-valley height for the fired multilayer stack in Figure 24 is 35.54 ⁇ and for the aluminum layer in Figure 25 is 9.51 ⁇ .
  • a layer has variable thickness when the average peak-to-valley height is greater than 10 ⁇ , greater than 12 ⁇ , or greater than 15 ⁇ , and a layer has uniform thickness when the average peak-to-valley height is less than 10 ⁇ , less than 12 ⁇ , or less than 15 ⁇ .
  • the modified intercalation layer of a co-fired variable-thickness multilayer stack such as the one shown in Figure 20
  • a tabbing ribbon when soldered to a tabbing ribbon, it has a peel strength that is twice the peel strength of a fired multilayer stack that does not have variable thickness.
  • the modified intercalation layer on the surface of such a variable-thickness fired multilayer stack is soldered to a tin-based tabbing ribbon, and they have a peel strength greater than 1.5 N/mm, or greater than 2 N/mm, or greater than 3 N/mm.
  • the thickness variations may be optimized to provide a continuous metal particle layer and back-surface field on the substrate for a silicon solar cell.
  • thickness variations may be optimized so that the contact resistance for such co-fired variable-thickness multilayer stacks is equivalent to or lower than the contact resistance for approximately planar co-fired multilayer stacks.
  • thickness variations in dry and modified metal particle layers include regions that are less than 20 ⁇ , 10 ⁇ , 5 ⁇ , or 2 ⁇ thick.
  • variable thickness layer(s) described above can be used as component(s) in any of the fired multilayer stacks described herein.
  • Variable thickness layer(s) such as variable thickness dried and modified metal particle layers, can be used on any silicon solar cells to reduce buckling of the rear tabbing layer.
  • intercalation paste that contains 45 wt% precious metal particles, 30 wt% intercalating particles, and 25 wt% organic vehicle (Paste C in Table I above) can be used as a drop-in replacement to form a rear tabbing layer in a silicon solar cell.
  • Fabrication of p-n junction silicon solar cells is well known in the art. Goodrich et al. provide a full process flow to fabricate a back-surface field silicon solar cell, which is referred to as "standard c-Si solar cell.” See Goodrich et al. "A wafer-based monocrystalline silicon photovoltaics road map: Utilizing known technology improvement opportunities for further reductions in manufacturing costs", Solar Energy Materials and Solar Cells (2013) pp.
  • a method for fabricating a solar cell electrode includes the steps of providing a silicon wafer with a portion of the front surface covered in at least one dielectric layer, applying an aluminum particle layer onto the back surface of the silicon wafer, drying the aluminum particle layer, applying an intercalation paste (rear tabbing) layer onto a portion of the aluminum particle layer, drying the intercalation paste layer, applying a plurality of fine grid lines and at least one front busbar layer onto the dielectric layer on the front surface of the silicon wafer, drying and co-firing the silicon wafer.
  • Methods such as screen printing, gravure printing, spray deposition, slot coating, 3D printing and/or inkjet printing can be used to apply the various layers.
  • Ekra or Baccini screen printers can be used to deposit the aluminum particle layer, intercalation paste layer and front side grid lines and busbar layers.
  • the solar cell has at least one dielectric layer covering at least a portion of the rear surface of the silicon wafer.
  • PERC passivated emitter rear cell
  • two dielectric layers i.e., alumina and silicon nitride
  • alumina and silicon nitride are applied to the rear side of the silicon solar cell prior to the application of the aluminum particle layer. Drying various layers may be done in a belt furnace at temperatures between 150°C and 300°C for 30 seconds to 15 minutes.
  • a Despatch CDF 7210 belt furnace is used to dry and co-fire the silicon solar cells that contain fired multilayer stacks as described herein.
  • co-firing is done using a rapid heating technique and heating to a temperature greater than 760°C for between 0.5 and 3 seconds in air, which are common temperature profiles for aluminum back-surface field silicon solar cells.
  • the temperature profile of a wafer is often calibrated using a DataPaq® system with a thermocouple attached to the bare wafer.
  • Figure 26 is a schematic drawing that shows the front (or illuminated) side of a silicon solar cell 2600.
  • the silicon solar cell 2600 has a silicon wafer 2610 with at least one dielectric layer (not shown), on top of which there are fine grid lines 2620 and front side busbars lines 2630.
  • the dielectric layer on the front side of the silicon wafer contains at least one material selected from the group consisting of silicon, nitrogen, aluminum, oxygen, germanium, hafnium, gallium, composites, and combinations thereof.
  • the dielectric layer on the front side of the silicon wafer is silicon nitride and is less than 200 nm thick.
  • FIG. 26 is a schematic drawing that shows the rear side of a silicon solar cell
  • the rear side is coated with an aluminum particle layer 2730 and has rear side tabbing layers 2740 distributed over a silicon wafer 2710.
  • the dielectric layer on the rear side comprises at least one material selected from the group consisting of silicon, nitrogen, aluminum, oxygen, germanium, hafnium, gallium, composites, and combinations thereof on the front surface of the silicon wafer.
  • the dielectric layer on the front surface of the silicon wafer is silicon nitride and less than 200 nm thick. In one embodiment there is no dielectric layer on the rear side of the silicon wafer.
  • the aluminum particle layer (after co-firing) 2730 has an average thickness between 20 and 30 ⁇ . In various embodiments, the aluminum particle layer 2730 has a porosity between 3 and 20%, between 10 and 18%, or any range subsumed therein.
  • BSF back surface field
  • solar cell architectures the rear tabbing layer is applied directly to the silicon wafer. However, to improve power conversion efficiency of the solar cell, it can be useful to print a rear tabbing layer on the aluminum particle layer.
  • an intercalation layer is applied directly onto a portion of the dried aluminum particle layer to form a rear tabbing layer 2740.
  • Figure 27 shows one possible pattem for a rear tabbing layer 2740.
  • the intercalation layer and underlying aluminum particle layer are eventually co-fired to form a fired multilayer stack as described herein.
  • the modified intercalation layer (or rear tabbing layer) 2740 has an average thickness between 1 ⁇ and 20 ⁇ , or between 2 ⁇ and 10 ⁇ , or between 2.5 ⁇ and 8 ⁇ .
  • a variable thickness metal (aluminum) particle layer can be used on the back side of silicon solar cells to reduce buckling of the rear tabbing layer and to improve adhesion and electrical contact.
  • a portion of the rear tabbing layer has variable thickness.
  • a portion of the modified aluminum particle layer has variable thickness.
  • the rear tabbing layer on the surface of such a variable-thickness modified aluminum particle layer is soldered to a tin-based tabbing ribbon, resulting in a peel strength greater than 0.7 N/mm, greater than 1.5 N/mm, greater than 2 N/mm, or greater than 3 N/mm.
  • the thickness variations can be optimized to provide a continuous metal particle layer and back-surface field on the substrate for a silicon solar cell.
  • a portion of the combined layer (modified aluminum particle layer and rear tabbing layer) in that region has a thickness that is at least 20%, 30%, or 40% greater than the average combined layer thickness measured over a 1 x 1 mm area.
  • a portion of the combined layer (modified aluminum particle layer and rear tabbing layer) in that region has a thickness that is at least 20%, 30%, or 40% less than the average combined layer thickness as measured over a 1 x 1 mm area.
  • solar cells that include any of the fired multilayer stacks discussed herein can be incorporated into solar modules.
  • solar module designs in which such solar cells be used, as would be known to a person with ordinary skill in the art.
  • the number of solar cells in a module is not intended to be limited. Typically, 60 or 72 solar cells are incorporated into commercially available modules, but it is possible to incorporate more or less depending on the application (i.e., consumer electronics, residential, commercial, utility, etc.).
  • Modules typically contain bypass diodes (not shown), junction boxes (not shown) and a supporting frame (not shown) that do not directly contact the solar cell. Bypass diodes and the junction box may also be considered part of cell interconnects.
  • FIG. 28 is a schematic cross-section drawing that shows a portion of a solar cell module, according to an embodiment of the invention.
  • the solar cell module contains at least one silicon solar cell 2840.
  • a front side 2840F of the silicon solar cell 2840 is attached to a first tabbing ribbon 2832 (which runs into and out of the page), over which there is a front encapsulation layer 2820 and a front sheet 2810.
  • a backside 2840B of the silicon solar cell 2840 is attached to a second tabbing ribbon 2834, over which there is a rear encapsulation layer 2850 and a back sheet 2860.
  • the tabbing ribbons 2832, 2834 contact adjacent solar cells electrically through soldered connections to a front side (i.e., front busbar on the front side) of one cell and to a back side (i.e., rear tabbing layer on the back side) of an adjacent solar cell.
  • a front side i.e., front busbar on the front side
  • a back side i.e., rear tabbing layer on the back side
  • Large numbers of solar cells in a solar module can be electrically coupled together using tabbing ribbons as cell interconnects.
  • Typical cell interconnects include metal tabbing ribbons that are soldered onto solar cells and metal bus ribbons that connect tabbing ribbons.
  • a tabbing ribbon is a metal ribbon that has a solder coating.
  • Such a solder-coated tabbing ribbon may have a thickness range of 20 to 1000 ⁇ , 100 to 500 ⁇ , 50 to 300 ⁇ or any range subsumed therein.
  • the width of the solder-coated tabbing ribbon may be between 0.1 and 10 mm, between 0.2 and 1.5 mm, or any range subsumed therein.
  • the length of the tabbing ribbon is determined by application, design, and substrate dimensions.
  • the solder coating may have a thickness between 0.5 and ⁇ , between 10 and 50 ⁇ , or any range subsumed therein.
  • the solder coating may contain tin, lead, silver, bismuth, copper, zinc, antimony, manganese, indium, or alloys, composites, or other combinations thereof.
  • the metal tabbing ribbon may have a thickness between 1 ⁇ and 1000 ⁇ , between 50 and 500 ⁇ , between 75 and 200 ⁇ , or any range subsumed therein.
  • the metal tabbing ribbon may contain copper, aluminum, silver, gold, carbon, tungsten, zinc, iron, tin or alloys, composites, or other combinations thereof.
  • the width of the metal tabbing ribbon may be between 0.1 and 10 mm, between 0.2 and 1.5 mm, or any range subsumed therein.
  • a tabbing ribbon is a copper ribbon that is 200 ⁇ -thick and 1 mm wide and is coated on all sides with a 20 ⁇ thick timlead (60:40 wt%) solder coating.
  • the front sheet 2810 in Figure 28 provides some mechanical support to the module and has good optical transmission properties over the portion of the solar spectrum the solar cell 2840 is designed to absorb.
  • Solar modules are positioned so that the front sheet 2810 faces a source of illumination, such as sunlight 2860.
  • the front sheet 2810 is typically made of low-iron content soda-lime glass.
  • the front encapsulant layer 2820 and the back encapsulant layer 2850 protect the solar cell 2840 from electrical, chemical and physical stressors during operation. Encapsulants are typically in the form of polymeric sheets.
  • Examples of materials that can be used as encapsulants include, but are not limited to, ethylene vinyl acetate (EVA), poly-ethylene-co-methacrylic acid (ionomer), polyvinyl butyral (PVB), thermoplastic urethane (TPU), poly-a-olefin, poly-dimethylsiloxan (PDMS), other polysiloxanes (i.e., silicone), and combinations thereof.
  • EVA ethylene vinyl acetate
  • ionomer poly-ethylene-co-methacrylic acid
  • PVB polyvinyl butyral
  • TPU thermoplastic urethane
  • PDMS poly-dimethylsiloxan
  • silicone other polysiloxanes
  • the back sheet 2860 provides protection to the solar cell 2840 from the rear side and may or may not be optically transparent. Solar modules are positioned so that the back sheet 2860 faces away from a source of illumination, such as sunlight 2860.
  • a back sheet 2860 may be a multilayer structure made of three polymeric films. DuPontTM Tedlar® polyvinyl fluoride (PVF) films are typically used in the back sheet. Fluoropolymers and polyethylene terephthalates (PET) can also be used in the back sheet. A glass sheet may also be used as the back sheet, which can aid in providing structural support to the solar module.
  • a supporting frame (not shown) may also be used to improve structure support; supporting frames are typically made of aluminum.
  • a method for forming a solar cell module is provided. Solder tabs are applied to individual solar cells (that contain any of the fired multilayer stacks described herein) either manually or by using an automated tabbing or stringing machine. Then the individual cells are electrically connected in series by directly soldering them to a tabbing ribbon. The resulting structure is referred to as a "cell string". Often multiple cell strings are arranged onto a front encapsulation layer that has been applied to a front sheet. These multiple cell strings are connected to one another using bus ribbons to create an electrical circuit. The bus ribbons are wider than the tabbing ribbons used in the cell strings.
  • the rear encapsulation material is applied to the back of the connected cell strings and the back sheet is placed on the rear encapsulation material.
  • the assembly is then sealed using a vacuum lamination process and heated (typically below 200°C) to polymerize the encapsulating material.
  • a frame is typically attached around the front sheet to provide structural support.
  • a junction box is connected to the cell interconnects and is attached to the solar module. Bypass diodes may be in the junction box or may be attached inside the module during cell interconnection processing.
  • a method of forming a solar module includes: a) providing at least one solar cell that has a front surface and a back surface; wherein, the back surface comprises a fired multilayer stack, b) soldering a portion of a tabbing ribbon onto a portion of a rear tabbing layer and front busbar layer creating a cell string, c) optionally, soldering tabbing ribbons to bus ribbons to complete the electrical circuit, d) arranging cell strings onto a front encapsulation layer that has been applied to a front sheet, e) applying a rear encapsulation layer to the cell strings and attaching a back sheet to the rear encapsulation layer to form a module assembly, f) laminating the module assembly, g) electrically connecting and physically attaching a junction box.
  • the sample is from a solar cell (that includes a novel fired multilayer stack) which had been incorporated into a solar module and was then removed as described above.
  • the image shows a metal tabbing ribbon 2932 and its solder coating 2931, which are soldered to a fired multilayer stack 2902.
  • the constituent layers of the fired multilayer stack 2902 are clearly visible.
  • the solder coating 2931 is a modified intercalation layer 2945, a modified metal particle layer 2944, and a silicon substrate 2941.
  • the layers identified in the figure can be more easily identified using EDX.
  • Intercalation pastes can be used to create a variety of fired multilayer stacks that can be used as metallization layers on the front and back side of many different solar cell architectures.
  • Intercalation pastes and fired multilayer stacks, as disclosed herein, can be used in solar cell architectures that include, but are not limited to, BSF silicon solar cells, passivated emitter and rear contact (PERC) solar cells, and bifacial and interdigitated back contact solar cells.
  • PERC solar cell architecture improves upon BSF solar cell architecture by reducing back contact surface recombination through the use of a dielectric barrier between the silicon substrate and the back contact.
  • a portion of the back side (i.e., non- illuminated) of a silicon wafer is passivated with at least one dielectric layer to reduce electrical current carrier recombination.
  • the novel fired multilayer stacks disclosed herein can be used in PERC solar cells.
  • a dielectric layer on the back side of a silicon wafer contains at least one of silicon, nitrogen, aluminum, oxygen, germanium, hafnium, gallium, composites, or combinations thereof.
  • a dielectric layer on the back side of a silicon wafer includes a 10 nm thick alumina layer on the silicon surface and a 75 nm thick silicon nitride layer on the alumina layer.
  • Commonly-used aluminum pastes e.g., Monocrystal EFX-39, EFX-85
  • Aluminum pastes designed for PERC cells do not penetrate through the dielectric layer(s).
  • small some regions of the dielectric layer are removed via laser ablation prior to deposition of the aluminum particle layer.
  • PERL passivated emitter with rear locally diffused
  • PERT passivated emitter, rear totally diffused
  • PERC cell architecture that further improve device performance. Both of these variations rely on doping the rear part of the silicon substrate to further suppress recombination at the rear contact, which serves a role analogous to the back-surface field in BSF cells.
  • the backside of the silicon substrate is doped around the opening in the dielectric that makes contact with the rear aluminum layer. Doping is usually achieved by diffusing a dopant through the dielectric openings using a boron compound or aluminum from the aluminum particles that make up the rear contact, similar to the BSF fabrication process.
  • a PERT cell is similar PERL, but the entire silicon in contact with the rear dielectric layer is doped in addition to the silicon adjacent to the dielectric openings that contact the rear contact.
  • an intercalation paste which contains intercalating particles that do not etch through dielectric layer(s) is used as the rear tabbing layer on PERC, PERL, or PERT cells.
  • the "non-etching" intercalation paste is used both to provide a solderable silver surface and to mechanically strengthen the underlying (modified) aluminum particle layer.
  • the resulting fired multilayer stack contains a silicon wafer, which is covered with at least one dielectric layer, a modified aluminum particle layer and a modified intercalation layer; for PERL or PERT, the silicon is doped only at the dielectric openings or also across the dielectric interface, respectively.
  • non-etching intercalation pastes can further reduce etching of the dielectric layer(s) and reduce surface recombination.
  • busbar pastes that are conventionally used for the rear tabbing layer in PERC cells are printed directly onto the dielectric layer and partially etch through the dielectric layer, which increases the surface recombination, during co-firing.
  • intercalation pastes can be modified to etch through the dielectric layers and assist in the diffused doping of silicon regions at the dielectric openings, according to an embodiment of the invention.
  • the "etching" intercalation paste e.g., intercalation paste D in Table I
  • the resulting fired multilayer stack contains the silicon wafer, a modified aluminum particle layer and a modified intercalation layer.
  • the fired multilayer stack may further include an Al-doped region near the silicon surface (similar to the back-surface field in BSF cells) and a solid silicon-aluminum eutectic layer at the interface between the silicon wafer and the modified aluminum particle layer.
  • intercalation pastes to etch through the dielectric layer(s) has several advantages. First, it is an inexpensive alternative to a laser ablation step, which has proven to be costly and unreliable in the past. Second, laser ablation can often remove tens to hundreds of microns of silicon substrate material and can result in large void formation between the silicon substrate and the aluminum particle layer when the wafer is co-fired. The etching intercalation paste does not cause changes in the wafer surface prior to co-firing, which results in better bond formation, reduced void formation, and better reproducibility than when laser ablation is used.
  • Intercalation pastes can be used to provide a solderable surface for cell architectures that depend on aluminum particle layers to make ohmic contact to p-type silicon, according to an embodiment of the invention.
  • these architectures include interdigitated back contact solar cells, n-type BSF cell architectures and bifacial solar cells.
  • intercalation paste C (from Table I) is applied to an Al layer on interdigitated back contact solar architecture, such as a Zebra cell.
  • intercalation pastes can replace conventional rear tabbing Ag pastes that directly contact silicon, thereby reducing the Voc of the solar cell.
  • the intercalation paste may be used on the front side (i.e., illuminated side). Intercalation pastes can also be used in conjunction with Al pastes to reduce the cost of bifacial solar cells.
  • bifacial solar cell architectures use Ag pastes that contain a small quantity of aluminum (i.e., less than 5 wt% Al) to make ohmic contact to p-type silicon layer.
  • the current bifacial architecture uses almost twice the amount of silver as the BSF architecture, which can be cost prohibitive. It can be useful to use pure aluminum pastes in bifacial architectures, but Al is not solderable.
  • Intercalation paste containing silver e.g., Paste C in Table I
  • Material properties of interest in fired multilayer stacks for use in solar cells and other electronic devices include solderability, peel strength, and contact resistance.
  • Solderability is the ability to form a strong physical bond between two metal surfaces by the flow of a molten metal solder between them at temperatures below 400°C. Soldering on the modified intercalation layer of a fired multilayer stack may be performed after heating in air to over 650°C. Soldering involves the use of flux, which is any chemical agent that cleans or etches one or both of the surfaces prior to reflow of the molten solder. Solder fluxes typically used for solar cells, designated as either RMA (e.g., Kester® 186) or R (e.g., Kester® 952), are deposited on the tabbing ribbon and dried at 70°C.
  • RMA e.g., Kester® 186
  • R e.g., Kester® 952
  • Peel strength is a metric of solder joint strength and an indicator of reliability for integrated circuit, light emitting diode and solar cell applications.
  • a solder coated metal ribbon which is between 0.8 and 20 mm wide and 100-300 um thick can be dipped into flux and dried. It is put on the modified intercalation layer and can be soldered using a solder iron at a temperature between 200° and 400°C.
  • the peel strength is the force required to peel a soldered ribbon, at a 180° angle from the soldering direction, divided by the width of the soldered ribbon, for a given peel rate.
  • the solder joints formed during the soldering process have a mean peel strength that is greater than 1 N/mm (e.g., a 2 mm tabbing ribbon would require a peel force of greater than 2N to dislodge the tabbing ribbon) at 1 mm/sec.
  • Solar cells are electrically connected by tabbing ribbons, which are soldered to the front busbars of one cell and the rear tabbing layers of an adjacent cell. It is common for the peel strength to be between 1.5 and 4 N/mm on contacts to the tabbing ribbons in commercially available solar cells.
  • a primary failure mode can be near the Al-Si interface, which can be determined using plan view SEM/EDX.
  • the peel strength is greater than lN/mm when the silver rich sublayer layer (of the modified intercalation layer) is soldered with a tin based tabbing ribbon.
  • Meier et al. describe how to use a four-point probe electrical measurement to determine the resistivity of each metallization layer on a completed solar cell. See Meier et al. "Determining components of series resistance from measurements on a finished cell", IEEE (2006) pp. 2615, which is included by reference herein.
  • the bulk resistance of a metallization layer is directly related to the bulk resistance of the material from which it is made.
  • the bulk resistance of pure Ag is 1.5xl0 "8 ⁇ -m; pure Ag metallization layers used on industrial solar cells have a bulk resistance that is 1.5 times to 5 times higher than the bulk resistance of pure Ag.
  • the bulk resistance is important for fine grid lines, which must transport current over a relatively long (i.e., more than 1 cm) length.
  • the resistance of the front busbars and rear tabbing layers are less important when the cells are tabbed in a module.
  • TLM transmission line measurement
  • TLM was used specifically to measure the contact resistance 1) between a metal particle layer and a modified metal particle layer and 2) between a modified metal particle layer and a modified intercalation layer.
  • the contact resistance of a fired multilayer stack is the sum of contact resistances 1) and 2) above.
  • the contact resistance of a fired multilayer stack is half of the y-intercept value of a linear fit of the resistance vs. distance measurements.
  • the electrical resistance between busbars was measured using a Keithley 2410 Sourcemeter in a four-point probe setup that sourced current between -0.5 A and +0.5 A and measured the voltage.
  • the contact resistance of a fired multilayer stack is between 0 and 5 mOhm, 0.25 and 3 mOhm, 0.3 and 1 mOhm, or any range subsumed therein.
  • the sheet resistance of a metal particle layer is determined by the slope of the line times the length of the electrodes.
  • the contact resistance and sheet resistance are used to numerically determine the transfer length and subsequently the contact resistivity.
  • the change in series resistance is determined by dividing the contact resistivity by the fractional area coverage of the modified intercalation layer.
  • the change in series resistance is less than 0.200 ⁇ -cm 2 , less than 0.100 ⁇ -cm 2 , less than 0.050 ⁇ -cm 2 , less than 0.010 ⁇ -cm 2 , or less than 0.001 ⁇ -cm 2 .
  • the contact resistance between a rear tabbing layer and an aluminum particle layer can affect the series resistance and power conversion efficiency of a solar cell. Such contact resistances can be measured by transmission line measurements.
  • a transmission line plot of a conventional silver rear tabbing layer on silicon with 300 ⁇ overlap with an aluminum particle layer is shown in Figure 30.
  • a transmission line plot of a modified intercalation layer, used as a rear tabbing layer, on an aluminum particle layer is shown in Figure 31.
  • the y-intercept value in Figure 31 is 1.11 mOhm as compared with a y-intercept value of 0.88 in Figure 30.
  • the contact resistance between a rear tabbing (intercalation) layer and an aluminum particle layer is 0.56 mOhm.
  • the contact resistance for a conventional rear tabbing architecture is 0.44 mOhm.
  • the contact resistance between a rear tabbing (intercalation) layer and an aluminum particle layer is between 0 and 5 mOhm, between 0.25 and 3 mOhm, or between 0.3 and 1 mOhm, or any range subsumed therein.
  • the sheet resistance of an aluminum layer is determined by the slope of the line times the length of the electrode and is approximately 9 mOhm/square in Figures 30 and 31.
  • TLM is a preferred method to accurately extract the contact resistance of a fired multilayer stack (i.e., rear tabbing layer and aluminum particle layer)
  • a four-point probe method is used by first measuring the resistance between two rear tabbing layers (RAg-to- A G ) and subsequently by moving the probes on to the Al particle layer (within 1 mm of the rear tabbing layer) to get RAHO-AI.
  • the contact resistance is determined by subtracting RA g -to-A G from RAHO-AI and dividing by two. This is not as accurate as TLM measurements but it can be approximate to within 0.50 mOhm when averaging over measurements from multiple solar cells.
  • the change in series resistance can be directly measured by fabricating a control BSF (back surface field) silicon solar cell with full Al coverage and no rear tabbing layer and fabricating a BSF silicon solar cell with full Al coverage and the Ag:Bi intercalation layer.
  • the series resistance of the cells can be derived through the current-voltage curves under various light intensities and the difference in series resistances can be ascribed to the increased contact resistance between the rear tabbing and the fired aluminum particle layer.
  • the change in series resistance in the solar cell is less than 0.200 ⁇ -cm 2 , less than 0. 100 ⁇ -cm 2 , less than 0.050 ⁇ -cm 2 , less than 0.010 ⁇ -cm 2 , or less than 0.001 ⁇ -cm 2 .
  • One benefit of using an intercalation layer on a silicon solar cell is the improvement in open-circuit voltage (Voc) caused by continuous back surface field formation in the silicon wafer.
  • the Voc gain can be directly measured by comparing a conventional BSF solar cell to the BSF solar cell containing the Ag:Bi intercalation paste, as described herein, when both devices have the same rear busbar surface area.
  • the conventional BSF silicon solar cell is fabricated with the silver based rear tabbing paste printed directly onto a silicon wafer and surrounded by the aluminum particle layer.
  • the intercalation layer e.g., fabricated using intercalation paste C
  • the Voc of both solar cells are measured through the current-voltage testing under one sun light intensity.
  • the Voc may be increased by at least 0.5 mV, by at least lmV, by at least 2 mV, or by at least 4 mV when using an intercalation layer versus a conventional, rear tabbing layer on silicon architecture.
  • the short-circuit current density (Jsc) and the fill factor may also be improved when using the intercalation layer architecture instead of the conventional rear tabbing design. Silver does not make ohmic contact to p-type silicon.
  • a silver tabbing layer directly on p-type silicon reduces current collection which can estimated by performing electroluminescence or photoluminescence measurements on complete or incomplete solar cells.
  • the increase in Jsc can also be measured by testing cells with the intercalation architecture versus the rear tabbing layer directly on silicon.
  • Another benefit is an increase in the fill factor, which can be positively changed due to an increase in the Voc, a reduction in contact resistance, and/or changes in the recombination dynamics on the rear side of the solar cell.
  • FIG. 32 is a schematic cross-sectional illustration view of solar cells 3210 arranged in a shingled architecture with the ends of adjacent solar cells 3210 overlapping and electrically connected to form a shingled array of solar cells 3200.
  • Each solar cell 3210 may be any of the solar cells described herein.
  • FIG. 32B is an enlargement of the region shown within the dotted lines in Figure 32A.
  • An electrically-conducting bonding material 3220 is used to bond adjacent solar cells to one another so that the front surface metallization feature of an underlying solar cell 3212 is conductively bonded to a rear surface metallization feature of an overlying solar cell 3214.
  • Suitable electrically conducting bonding materials 3220 may include, for example, electrically conducting adhesives, electrically conducting adhesive films and tapes, and conventional solders.
  • the electrically conducting bonding material 3220 provides a bond between the adjacent solar cells 3210, which can accommodate stress arising from mismatch between the coefficient of thermal expansion of the electrically conducting bonding material 3220 and that of the solar cells 3210.
  • the electrically conductive bonding material 3220 is applied only at particular locations along the overlapping regions of the solar cells 3210 instead of in a continuous line extending substantially the length of the edges of the solar cells 3210.
  • bonding materials are electrically conducting adhesives, electrically conducting adhesive films and tapes.
  • the electrically conductive bonding material is cured by applying heat and pressure to the array of shingled solar cells, thereby bonding adjacent overlapping rectangular silicon solar cells to each other and ensuring good electrical contact between them.
  • the electronic conductive adhesive is CP-300 made by Hitachi Chemical.
  • the solar cells in an array of shingled solar cells all have the same square or rectangular shape.
  • the solar cells in a shingled solar cell array are all rectangular with an aspect ratio between about 1 :2 to 1 :20.
  • a solar module in another aspect, includes a plurality of shingled arrays of solar cells arranged in two or more parallel rows spanning a width of a module to form a front surface of the module.
  • Each shingled array of solar cells contains a plurality of silicon solar cells as described above.
  • a flexible electrical interconnect can be used to connect adjacent shingled array of solar cells.
  • the flexible electrical interconnect may connect one end of a first shingled array of solar cells in a first row and adjacent to an edge of the module to one end of a second shingled array of solar cells in a second row and adjacent to the same edge of the module.
  • the flexible electrical interconnect may bond to the front surface of the first shingled array of solar cells at number of discrete locations with an electrically conductive adhesive bonding material.
  • the flexible electrical interconnect may run parallel to the edge of the module. At least a portion of the flexible electrical interconnect may fold around the end of the first shingled array of solar cells and be hidden from view from the front of the module.
  • Flexible interconnects may be conductively bonded to the shingled array of solar cells with, for example, the same electrically conductive bonding material as described above for use in bonding overlapped solar cells.
  • the electrically conductive bonding material may be located only at discrete positions along the edges of the shingled array of solar cells rather than in a continuous line extending substantially the length of the edge of the shingled array of solar cells, to reduce or accommodate stress parallel to the edges of the shingled array of solar cells arising from mismatch between the coefficient of thermal expansion of the electrically conductive bonding material or the interconnects and that of the shingled array of solar cells.
  • flexible interconnects are formed from or contain thin copper sheets. Flexible interconnects may be patterned with, for example, slits, slots, or holes to increase their flexibility to reduce or accommodate stress perpendicular and parallel to the edges of the shingled array of solar cells.
  • a shingled solar cell array includes a plurality of silicon solar cells arranged in line with end portions of adjacent silicon solar cells overlapping and conductively bonded to each other to electrically connect the silicon solar cells in series.
  • Each silicon solar cell may contain a silicon substrate having a front surface and a rear surface, at least one front dielectric layer directly on at least a portion of the front surface of the silicon substrate, a plurality of fine grid lines on a portion of the front surface of the silicon substrate, at least one front busbar layer in electrical contact with at least one of the plurality of fine grid lines, an aluminum particle layer on at least a portion of the rear surface of the silicon substrate, and a rear tabbing layer on a portion of the rear surface of the silicon substrate.
  • the rear tabbing layer may contain a modified aluminum particle layer on a portion of the rear surface of the silicon substrate and a modified intercalation layer directly on at least a portion of the modified aluminum particle layer.
  • the modified intercalation layer may have a solderable surface.
  • the modified intercalation layer may contain a precious metal and any of antimony, arsenic, barium, bismuth, boron, cadmium, calcium, cerium, cesium, chromium, cobalt, gallium, germanium, hafnium, indium, iron, lanthanum, lead, lithium, magnesium, manganese, molybdenum, niobium, phosphorous, potassium, rhenium, selenium, silicon, sodium, strontium, sulfur, tellurium, tin, vanadium, zinc, zirconium, combinations thereof, or alloys thereof, oxides thereof, composites thereof, or other combinations thereof.
  • the modified aluminum particle layer may contain aluminum particles and at least one material from the modified intercalation layer.
  • a portion of a first silicon solar cell may overlap a portion of a second silicon solar cell so that the front busbar layer of the second silicon solar cell is below the rear tabbing layer of the first solar cell.
  • An electronically conductive adhesive may be used to conductively bond to both the front busbar layer of the second silicon solar cell, and the rear tabbing layer of the first silicon solar cell to electrically connect the first silicon solar cell and the second silicon solar cell.
  • the front dielectric layer may include any of silicon, aluminum, germanium, hafnium, gallium, or oxides thereof, nitrides thereof, composites thereof, or combinations thereof.
  • the rear dielectric layer may be any of silicon, aluminum, germanium, hafnium, gallium, oxides thereof, nitrides thereof, composites thereof, or combinations thereof.
  • a portion of the rear tabbing layer may have variable thickness.
  • a solar cell module in another embodiment, includes a front sheet having a front surface and a rear surface, a front encapsulant layer on the rear surface of the front sheet and a plurality of shingled silicon solar cell arrays as described herein on the front encapsulant layer, wherein the shingled silicon solar cell arrays are electrically connected to one another.
  • a rear sheet having a front surface and a rear surface, the rear surface of the rear sheet exposed to an outside environment, and a rear encapsulant layer on the front surface of the rear sheet, wherein a first portion of the rear encapsulant layer is on the plurality of shingled silicon solar cell arrays, and a second portion of the rear encapsulant layer is on the front encapsulant layer.
  • the flexible, conductive interconnects may further include a junction box in contact with the rear sheet.
  • There may also be a bus ribbon connected to a portion of the shingled solar cell array and electrically connected to a bypass diode and a junction box.

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Abstract

L'invention concerne des réseaux en bardeaux de cellules solaires. Les cellules solaires utilisées pour former les réseaux en bardeaux sont fabriquées à l'aide de nouvelles pâtes d'intercalation nouvelles. Les pâtes contiennent des particules de métal précieux, des particules d'intercalation, et un véhicule organique et peuvent être utilisées pour améliorer les propriétés matérielles de couches de particules métalliques. Des formulations spécifiques ont été mises au point pour être sérigraphiées directement sur une couche de particules métalliques séchée et cuites pour former un empilement multicouche cuit. Selon certains modes de réalisation, l'empilement multicouche cuit peut graver à travers une couche diélectrique pour améliorer l'adhérence à un substrat. De telles pâtes peuvent être utilisées de façon très avantageuse en augmentant le rendement des cellules solaires en silicium, spécialement des cellules photovoltaïques à champ de surface arrière (BSF) en silicium monocristallin et polycristallin, et à contact arrière et émetteur passivé (PERC).
PCT/US2018/032159 2017-05-10 2018-05-10 Empilements de films métalliques multicouche pour réseaux en bardeaux de cellules solaires en silicium WO2018209147A1 (fr)

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EP18799289.6A EP3635792A4 (fr) 2017-05-10 2018-05-10 Empilements de films métalliques multicouche pour réseaux en bardeaux de cellules solaires en silicium
JP2019562278A JP2020520114A (ja) 2017-05-10 2018-05-10 屋根材シリコン太陽電池アレイのための多層金属膜積層スタック
CN201880045803.5A CN111316449A (zh) 2017-05-10 2018-05-10 用于搭叠的硅太阳能电池阵列的多层金属膜堆叠

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