WO2017004624A1 - Discrete carrier selective passivated contacts for solar cells - Google Patents

Discrete carrier selective passivated contacts for solar cells Download PDF

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Publication number
WO2017004624A1
WO2017004624A1 PCT/US2016/041018 US2016041018W WO2017004624A1 WO 2017004624 A1 WO2017004624 A1 WO 2017004624A1 US 2016041018 W US2016041018 W US 2016041018W WO 2017004624 A1 WO2017004624 A1 WO 2017004624A1
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Prior art keywords
regions
silicon
tunnel dielectric
doped
backside
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PCT/US2016/041018
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French (fr)
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Mehrdad M. Moslehi
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Solexel, Inc.
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Publication of WO2017004624A1 publication Critical patent/WO2017004624A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—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
    • H01L31/06—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 characterised by potential barriers
    • H01L31/068—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 characterised by potential barriers the potential barriers being only of the PN homojunction type, e.g. bulk silicon PN homojunction solar cells or thin film polycrystalline silicon PN homojunction solar cells
    • H01L31/0682—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 characterised by potential barriers the potential barriers being only of the PN homojunction type, e.g. bulk silicon PN homojunction solar cells or thin film polycrystalline silicon PN homojunction solar cells back-junction, i.e. rearside emitter, solar cells, e.g. interdigitated base-emitter regions back-junction cells
    • 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
    • H01L31/0216—Coatings
    • H01L31/02161—Coatings for devices characterised by at least one potential jump barrier or surface barrier
    • H01L31/02167—Coatings for devices characterised by at least one potential jump barrier or surface barrier for solar cells
    • 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
    • H01L31/022441—Electrode arrangements specially adapted for back-contact solar cells
    • 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
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1804—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic Table
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/547—Monocrystalline silicon PV cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present disclosure relates in general to the fields of solar photovoltaics (PV), and more particularly to solar photovoltaic cells and fabrication methods.
  • PV solar photovoltaics
  • Known carrier selective contacts for solar cells for example metal electrode/n+ doped polysilicon/tunnel oxide/n-type silicon carrier selective contact structures, often utilize an undoped polysilicon layer deposited on a silicon surface (e.g., using an LPCVD or PECVD deposition process). The deposited undoped polysilicon layer then must be subsequently patterned doped to form both doped n+ and doped p+ patterned regions in the same continuous polysilicon layer.
  • discrete carrier selective passivated contacts for solar cells are provided which may substantially eliminate or reduces disadvantage and deficiencies associated with previously developed carrier selective contacts for solar cells.
  • a photovoltaic solar cell structure comprises a silicon substrate having a frontside and a backside, the backside having base regions and emitter regions.
  • a tunnel dielectric covers at least a portion of the base regions and at least a portion of the emitter regions.
  • Discrete silicon nanoparticle n+ doped polysilicon regions are on the tunnel dielectric covering the base regions.
  • discrete silicon nanoparticle p+ doped polysilicon regions are on the tunnel dielectric covering the emitter regions.
  • Figure 1 is a cross-sectional drawing of a back contact back junction solar cell having discrete electron- selective contact structures
  • Figure 2 is a cross-sectional drawing of a back contact back junction solar cell having discrete electron-selective contact structures and having a p-type emitter;
  • Figure 3 is a cross-sectional drawing of a back contact back junction solar cell having discrete hole-selective contact structures
  • Figure 4 is a cross-sectional drawing of a back contact back junction solar cell having combined discrete electron-selective contact structures and discrete hole-selective contact structures
  • Figure 5 is a process flow showing certain process steps for forming discrete electron- selective contacts for n-type silicon
  • Figure 6 is a process flow showing certain process steps for forming discrete hole-selective contact structures for p-type silicon
  • Figure 7 is a process flow showing certain process steps for forming discrete electron-selective contacts for n-type silicon and discrete hole- selective contact structures for p-type silicon regions;
  • Figures 8 through 16 are detailed fabrication step process flows for forming discrete carrier selective silicon nanoparticle polysilicon passivated contacts.
  • Figure 17 is a chart showing carrier-selective contact material candidates for crystalline silicon solar cells.
  • Discrete, or otherwise discontinuous, carrier selective passivated contacts solar cell solutions are fabricated using silicon nanoparticles (S P) such as either n-type-doped (e.g., phosphorus-doped) silicon (silica) nanoparticle paste/ink or p-type-doped (e.g., boron-doped) silicon (silica) nanoparticle paste/ink, or a combination thereof, printed as discrete islands on a crystalline silicon substrate covered by a suitable tunnel dielectric (e.g., approximately 0.5 nm to 2 nm of silicon oxide formed by chemical oxidation or UV-ozone oxidation).
  • S P silicon nanoparticles
  • Printing is performed by screen printing (or stencil printing) of silicon nanoparticle paste(s) or inkjet printing of silicon nanoparticle ink(s) on a tunnel dielectric layer. After thermal anneal of the substrates with printed paste(s) or ink(s), for example in an inert ambient, the silicon nanoparticle paste/ink is converted to doped (n+ with phosphorus doping, p+ with boron doping) silicon nanoparticle polysilicon.
  • S P(n+) polysilicon is annealed phosphorus-doped silicon nanoparticle paste (n+ doped) which is used as part of the electron-selective contact structure comprising Metal / S P(n+) polysilicon/ Tunnel Dielectric / n-Type silicon.
  • S P(p+) polysilicon is annealed phosphorus-doped silicon nanoparticle paste (p+ doped) which is used as part of the hole-selective contact structure comprising Metal / S P(p+) polysilicon/ Tunnel Dielectric / p-Type silicon.
  • the discrete carrier selective passivated contact solar cell solutions of the present application - either as one or a combination of electron- selective contact to n-type silicon and hole-selective contact to p-type silicon - may provide or otherwise enable the following advantages, for example: eliminating the need for heavy doping of silicon in contact regions (for instance as compared to previously known ohmic contacts that may require heavy p+, e.g., boron, doping of silicon for metal contacts to p-type silicon and heavy n+, e.g., phosphorus, doping of silicon for metal contacts to n-type silicon - the solution provided eliminates the need for these n+ doped and p+ doped regions in silicon for low-resistance contacts); a substantial reduction of contact surface recombination velocity (in cm/s) or contact Joe (in fA/cm 2 ), resulting in higher solar cell open-circuit voltage (Voc), higher solar cell efficiency, and smaller temperature coefficient of power; providing electron-select
  • FIG. 1 is a cross-sectional drawing of a back contact back junction solar cell having discrete electron- selective contact structures.
  • N-type crystalline silicon substrate 2 e.g., back contact solar cell base layer or wafer
  • ARC layer 4 e.g., A10x/H:SiN y
  • backside passivation layer 6 e.g., AlOx and/or H:SiN y and/or S1O2
  • Tunnel dielectric 8 e.g., tunnel SiOx
  • Patterned metal 12 (e.g., aluminum) is on silicon nanoparticle n+ polysilicon 10.
  • n-type silicon /tunnel dielectric 8/ silicon nanoparticle n+ polysilicon 10/metal 12 forms an electron- selective contact to n-type silicon (base) structure.
  • P-type emitter and contacts to p-type emitter are not shown in the back contact solar cell of Figure 1.
  • Figure 2 is a cross-sectional drawing of a back contact back junction solar cell (e.g., an interdigitated back contact IBC solar cell) having discrete electron- selective contact structures consistent with the solar cell of Figure 1 and having a p-type emitter 14 (e.g., boron doped silicon). Contacts to p-type emitter 14 are not shown in the back contact solar cell of Figure 2.
  • a back contact back junction solar cell e.g., an interdigitated back contact IBC solar cell
  • p-type emitter 14 e.g., boron doped silicon
  • FIG. 3 is a cross-sectional drawing of a back contact back junction solar cell having discrete hole- selective contact structures.
  • N-type crystalline silicon substrate 2 e.g., back contact solar cell base layer or wafer
  • ARC layer 4 e.g., A10x/H:SiN y
  • backside passivation layer 6 e.g., AlOx and/or H:SiN y and/or S1O2
  • Tunnel dielectric 16 e.g., tunnel SiOx
  • p-type emitter 14 e.g., boron doped silicon
  • Silicon nanoparticle p+ polysilicon 18, SNP(p+) polysilicon is on tunnel dielectric 16.
  • Patterned metal 20 (e.g., aluminum) is on silicon nanoparticle p+ polysilicon 18.
  • p-type silicon /tunnel dielectric 16/ silicon nanoparticle p+ polysilicon 18/metal 20 forms a hole- selective contact structure.
  • n-type silicon e.g., solar cell base
  • FIG. 4 is a cross-sectional drawing of a back contact back junction solar cell (e.g., an IBC solar cell) having combined discrete electron-selective contact structures and discrete hole-selective contact structures.
  • N-type crystalline silicon substrate 2 e.g., back contact solar cell base layer or wafer
  • ARC layer 4 e.g., A10x/H: SiN y
  • backside passivation layer 6 e.g., AlOx and/or H: SiN y and/or S1O2
  • Tunnel dielectric 8 e.g., tunnel SiOx
  • Silicon nanoparticle n+ polysilicon 10, SNP(n+) polysilicon, is on tunnel dielectric 8.
  • Patterned metal 12 e.g., aluminum
  • n-type silicon /tunnel dielectric 8/ silicon nanoparticle n+ polysilicon 10/metal 12 forms an electron- selective contact structure.
  • Tunnel dielectric 16 e.g., tunnel SiOx
  • p- type emitter 14 e.g., boron doped silicon.
  • SNP(p+) polysilicon is on tunnel dielectric 16.
  • Patterned metal 20 e.g., aluminum
  • silicon nanoparticle p+ polysilicon 18/metal 20 forms a hole- selective contact structure.
  • a plurality of discrete electron-selective contacts and hole-selective contacts are used in the solar cell of Figure 4.
  • Figure 5 is a process flow showing certain process steps for forming discrete electron-selective contacts for n-type silicon, for example as shown in the back contact back junction solar cell of Figure 1.
  • Figure 6 is a process flow showing certain process steps for forming discrete hole-selective contact structures for p-type silicon.
  • Figure 7 is a process flow showing certain process steps for forming discrete electron-selective contacts for n-type silicon and discrete hole- selective contact structures for p-type silicon regions, for example as shown in the back contact back junction solar cell of Figure 4.
  • the following may be advantageous, in consideration with other fabrication factors, for forming discrete carrier selective passivated contacts and discrete carrier selective passivated contact solar cells. In some instances, it may be advantageous to avoid residual damage to silicon (e.g., avoiding laser-induced damage). Additionally, in some instances it may be advantageous to limit thermal processing temperatures to less than or equal to 950°C and in some instances to as low as less than or equal to 925°C.
  • passivation layers on frontside and backside - for example APCVD boron-doped aluminum oxide (with undoped silicon oxide cap) over the solar cell substrate backside and PECVD SiNx:Hy and phosphorus front-surface field (FSF) or PECVD SiNx:Hy / aluminum oxide (with optional tunnel silicon oxide at the interface) on the solar cell substrate frontside.
  • APCVD boron-doped aluminum oxide with undoped silicon oxide cap
  • PECVD SiNx:Hy and phosphorus front-surface field (FSF) or PECVD SiNx:Hy / aluminum oxide (with optional tunnel silicon oxide at the interface) on the solar cell substrate frontside for example APCVD boron-doped aluminum oxide (with undoped silicon oxide cap) over the solar cell substrate backside and PECVD SiNx:Hy and phosphorus front-surface field (FSF) or PECVD SiNx:Hy / aluminum oxide (with optional tunnel silicon oxide at the interface) on the solar cell
  • Figures 8 through 16 are detailed fabrication step process flows for forming discrete carrier selective silicon nanoparticle polysilicon passivated contacts using efficient manufacturing process flows and standard solar fabrication equipment resulting in high efficiency interdigitated back contact back junction (IBC) solar cells.
  • the tunnel oxide may be formed, for example, as a chemical oxide or a UV ozone oxide.
  • the process flow of Figure 8 may be characterized by a lack of laser processing and the process for copper plating.
  • the process flow of Figure 9 may be characterized by a lack of laser processing and a lack of copper plating.
  • the process flow of Figure 10 may be characterized by a lack of laser processing, a lack of physical vapor deposition (PVD), and a lack of copper plating.
  • PVD physical vapor deposition
  • the process flow of Figure 11 may be characterized by one furnace step, the process for copper plating, and using pulsed laser for the opening of emitter and base contacts.
  • the process flow of Figure 12 may be characterized by one furnace step, the process for copper plating, and using resist screen printing and wet etch for the opening of emitter and base contacts.
  • the process flow of Figure 13 may be characterized by one furnace step, a lack of copper plating, and using pulsed laser for the opening of emitter and base contacts.
  • the process flow of Figure 14 may be
  • Figure 17 is a chart showing carrier-selective contact material candidates for crystalline silicon solar cells. Materials shown include dielectric and semiconductor materials.
  • n-type contacts Suitable materials for n-type contacts (electron) are shown above the silicon (Si) band gap and suitable materials for p-type contacts (hole) are shown below the silicon (Si) band gap.
  • n-type contact materials have Conduction Band Minima (CBM) close to CBM of crystalline silicon and potentially suitable p-type contact materials have Valence Band Maxima (VBM) close to CBM of crystalline silicon.
  • CBM Conduction Band Minima
  • VBM Valence Band Maxima

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Abstract

A photovoltaic solar cell structure comprises a silicon substrate having a frontside and a backside, the backside having base regions and emitter regions. A tunnel dielectric covers at least a portion of the base regions and at least a portion of the emitter regions. Discrete silicon nanoparticle n+ doped polysilicon regions are on the tunnel dielectric covering the base regions. And discrete silicon nanoparticle p+ doped polysilicon regions are on the tunnel dielectric covering the emitter regions.

Description

DISCRETE CARRIER SELECTIVE PASSIVATED CONTACTS FOR
SOLAR CELLS
CROSS-REFERENCE TO RELATED APPLICATIONS
[001] This application claims the benefit of U.S. provisional patent applications 62/188456 filed July 2, 2015 and 62/317377 filed on April 1, 2016, which are both hereby incorporated by reference in their entirety.
FIELD OF THE INVENTION
[002] The present disclosure relates in general to the fields of solar photovoltaics (PV), and more particularly to solar photovoltaic cells and fabrication methods.
BACKGROUND
[003] Known carrier selective contacts for solar cells, for example metal electrode/n+ doped polysilicon/tunnel oxide/n-type silicon carrier selective contact structures, often utilize an undoped polysilicon layer deposited on a silicon surface (e.g., using an LPCVD or PECVD deposition process). The deposited undoped polysilicon layer then must be subsequently patterned doped to form both doped n+ and doped p+ patterned regions in the same continuous polysilicon layer. Additionally, this often requires relatively heavy doping of silicon in contact regions as ohmic contacts of known carrier-selective contact solar cells may require heavy p+ (e.g., boron) doping of silicon for metal contacts to p- type silicon and heavy n+ (e.g., phosphorus) doping of silicon for metal contacts to n- type silicon - a fabrication limitation restricting fabrication solutions and requiring complex silicon substrate processing to achieve high efficiency solar cells. Background material describing known carrier-selective contacts for solar cells may be found in U.S. Pat. 7,468,485 published Dec. 23, 2008 by Richard Swanson and U.S. Pat. 7,633,006 published by Dec. 15, 2009 by Ri chard Swanson.
BRIEF SUMMARY OF THE INVENTION
[004] Therefore, a need has arisen for a carrier selective contacts for solar cells with improved efficiency and reduced fabrication complexity. In accordance with the disclosed subject matter, discrete carrier selective passivated contacts for solar cells are provided which may substantially eliminate or reduces disadvantage and deficiencies associated with previously developed carrier selective contacts for solar cells.
[005] According to one aspect of the disclosed subject matter, a photovoltaic solar cell structure is provided. A photovoltaic solar cell structure comprises a silicon substrate having a frontside and a backside, the backside having base regions and emitter regions. A tunnel dielectric covers at least a portion of the base regions and at least a portion of the emitter regions. Discrete silicon nanoparticle n+ doped polysilicon regions are on the tunnel dielectric covering the base regions. And discrete silicon nanoparticle p+ doped polysilicon regions are on the tunnel dielectric covering the emitter regions.
[006] These and other aspects of the disclosed subject matter, as well as additional novel features, will be apparent from the description provided herein. The intent of this summary is not to be a comprehensive description of the claimed subject matter, but rather to provide a short overview of some of the subject matter's functionality. Other systems, methods, features and advantages here provided will become apparent to one with skill in the art upon examination of the following FIGURES and detailed description. It is intended that all such additional systems, methods, features and advantages that are included within this description, be within the scope of any claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[007] The features, natures, and advantages of the disclosed subject matter may become more apparent from the detailed description set forth below when taken in conjunction with the drawings (dimensions, relative or otherwise not drawn to scale) in which like reference numerals indicate like features and wherein:
[008] Figure 1 is a cross-sectional drawing of a back contact back junction solar cell having discrete electron- selective contact structures;
[009] Figure 2 is a cross-sectional drawing of a back contact back junction solar cell having discrete electron-selective contact structures and having a p-type emitter;
[010] Figure 3 is a cross-sectional drawing of a back contact back junction solar cell having discrete hole-selective contact structures; [Oil] Figure 4 is a cross-sectional drawing of a back contact back junction solar cell having combined discrete electron-selective contact structures and discrete hole-selective contact structures;
[012] Figure 5 is a process flow showing certain process steps for forming discrete electron- selective contacts for n-type silicon;
[013] Figure 6 is a process flow showing certain process steps for forming discrete hole-selective contact structures for p-type silicon;
[014] Figure 7 is a process flow showing certain process steps for forming discrete electron-selective contacts for n-type silicon and discrete hole- selective contact structures for p-type silicon regions;
[015] Figures 8 through 16 are detailed fabrication step process flows for forming discrete carrier selective silicon nanoparticle polysilicon passivated contacts; and,
[016] Figure 17 is a chart showing carrier-selective contact material candidates for crystalline silicon solar cells.
DETAILED DESCRIPTION
[017] The following description is not to be taken in a limiting sense, but is made for the purpose of describing the general principles of the present disclosure. The scope of the present disclosure should be determined with reference to the claims. Exemplary embodiments of the present disclosure are illustrated in the drawings, like aspects and identifiers being used to refer to like and corresponding parts of the various drawings.
[018] And although the present disclosure is described with reference to specific embodiments, fabrication processes, and materials, one skilled in the art could apply the principles discussed herein to other solar module structures, fabrication processes, as well as alternative technical areas and/or embodiments without undue experimentation.
[019] Discrete, or otherwise discontinuous, carrier selective passivated contacts solar cell solutions are fabricated using silicon nanoparticles (S P) such as either n-type-doped (e.g., phosphorus-doped) silicon (silica) nanoparticle paste/ink or p-type-doped (e.g., boron-doped) silicon (silica) nanoparticle paste/ink, or a combination thereof, printed as discrete islands on a crystalline silicon substrate covered by a suitable tunnel dielectric (e.g., approximately 0.5 nm to 2 nm of silicon oxide formed by chemical oxidation or UV-ozone oxidation). Printing is performed by screen printing (or stencil printing) of silicon nanoparticle paste(s) or inkjet printing of silicon nanoparticle ink(s) on a tunnel dielectric layer. After thermal anneal of the substrates with printed paste(s) or ink(s), for example in an inert ambient, the silicon nanoparticle paste/ink is converted to doped (n+ with phosphorus doping, p+ with boron doping) silicon nanoparticle polysilicon.
S P(n+) polysilicon is annealed phosphorus-doped silicon nanoparticle paste (n+ doped) which is used as part of the electron-selective contact structure comprising Metal / S P(n+) polysilicon/ Tunnel Dielectric / n-Type silicon. S P(p+) polysilicon is annealed phosphorus-doped silicon nanoparticle paste (p+ doped) which is used as part of the hole-selective contact structure comprising Metal / S P(p+) polysilicon/ Tunnel Dielectric / p-Type silicon. Thus, the carrier selective passivated contacts solar cell solutions provided eliminate the need for LPCVD or PECVD polysilicon deposition and separate subsequent doping of undoped polysilicon.
[020] The discrete carrier selective passivated contact solar cell solutions of the present application - either as one or a combination of electron- selective contact to n-type silicon and hole-selective contact to p-type silicon - may provide or otherwise enable the following advantages, for example: eliminating the need for heavy doping of silicon in contact regions (for instance as compared to previously known ohmic contacts that may require heavy p+, e.g., boron, doping of silicon for metal contacts to p-type silicon and heavy n+, e.g., phosphorus, doping of silicon for metal contacts to n-type silicon - the solution provided eliminates the need for these n+ doped and p+ doped regions in silicon for low-resistance contacts); a substantial reduction of contact surface recombination velocity (in cm/s) or contact Joe (in fA/cm2), resulting in higher solar cell open-circuit voltage (Voc), higher solar cell efficiency, and smaller temperature coefficient of power; providing electron-selective contacts to n-type silicon with very low effective specific contact resistance for collection of electrons while repelling holes; providing hole- selective contacts to p-type silicon with very low effective specific contact resistance for collection of holes while repelling electrons; allows for relatively high contact area ratio (or high density of discrete contact windows) for electron-selective contacts while providing relatively low total contact surface recombination velocity (SRV) and contact Jo for high efficiency; allows for relatively high contact area ratio (or high density of discrete contact windows) for hole-selective contacts while providing relatively low total contact surface recombination velocity (SRV) and contact Jo for high efficiency; and, eliminating the need for LPCVD or PECVD polysilicon and its subsequent doping.
[021] Figure 1 is a cross-sectional drawing of a back contact back junction solar cell having discrete electron- selective contact structures. N-type crystalline silicon substrate 2 (e.g., back contact solar cell base layer or wafer) has frontside passivation and anti- reflection coating ARC layer 4 (e.g., A10x/H:SiNy) and backside passivation layer 6 (e.g., AlOx and/or H:SiNy and/or S1O2). Tunnel dielectric 8 (e.g., tunnel SiOx) is on n-type silicon. Silicon nanoparticle n+ polysilicon 10, SNP(n+) polysilicon, is on tunnel dielectric 8. Patterned metal 12 (e.g., aluminum) is on silicon nanoparticle n+ polysilicon 10. Thus, n-type silicon /tunnel dielectric 8/ silicon nanoparticle n+ polysilicon 10/metal 12 forms an electron- selective contact to n-type silicon (base) structure. P-type emitter and contacts to p-type emitter are not shown in the back contact solar cell of Figure 1.
[022] Figure 2 is a cross-sectional drawing of a back contact back junction solar cell (e.g., an interdigitated back contact IBC solar cell) having discrete electron- selective contact structures consistent with the solar cell of Figure 1 and having a p-type emitter 14 (e.g., boron doped silicon). Contacts to p-type emitter 14 are not shown in the back contact solar cell of Figure 2.
[023] Figure 3 is a cross-sectional drawing of a back contact back junction solar cell having discrete hole- selective contact structures. N-type crystalline silicon substrate 2 (e.g., back contact solar cell base layer or wafer) has frontside passivation and anti- reflection coating ARC layer 4 (e.g., A10x/H:SiNy) and backside passivation layer 6 (e.g., AlOx and/or H:SiNy and/or S1O2). Tunnel dielectric 16 (e.g., tunnel SiOx) is on p-type emitter 14 (e.g., boron doped silicon). Silicon nanoparticle p+ polysilicon 18, SNP(p+) polysilicon, is on tunnel dielectric 16. Patterned metal 20 (e.g., aluminum) is on silicon nanoparticle p+ polysilicon 18. Thus, p-type silicon /tunnel dielectric 16/ silicon nanoparticle p+ polysilicon 18/metal 20 forms a hole- selective contact structure.
Contacts to n-type silicon (e.g., solar cell base) are not shown in the back contact solar cell of Figure 3.
[024] Figure 4 is a cross-sectional drawing of a back contact back junction solar cell (e.g., an IBC solar cell) having combined discrete electron-selective contact structures and discrete hole-selective contact structures. N-type crystalline silicon substrate 2 (e.g., back contact solar cell base layer or wafer) has frontside passivation and anti-reflection coating ARC layer 4 (e.g., A10x/H: SiNy) and backside passivation layer 6 (e.g., AlOx and/or H: SiNy and/or S1O2). Tunnel dielectric 8 (e.g., tunnel SiOx) is on n-type silicon. Silicon nanoparticle n+ polysilicon 10, SNP(n+) polysilicon, is on tunnel dielectric 8. Patterned metal 12 (e.g., aluminum) is on silicon nanoparticle n+ polysilicon 10. Thus, n-type silicon /tunnel dielectric 8/ silicon nanoparticle n+ polysilicon 10/metal 12 forms an electron- selective contact structure. Tunnel dielectric 16 (e.g., tunnel SiOx) is on p- type emitter 14 (e.g., boron doped silicon). Silicon nanoparticle p+ polysilicon 18,
SNP(p+) polysilicon, is on tunnel dielectric 16. Patterned metal 20 (e.g., aluminum) is on silicon nanoparticle p+ polysilicon 18. Thus, p-type silicon /tunnel dielectric 16/ silicon nanoparticle p+ polysilicon 18/metal 20 forms a hole- selective contact structure. A plurality of discrete electron-selective contacts and hole-selective contacts are used in the solar cell of Figure 4.
[025] Figure 5 is a process flow showing certain process steps for forming discrete electron-selective contacts for n-type silicon, for example as shown in the back contact back junction solar cell of Figure 1.
[026] Figure 6 is a process flow showing certain process steps for forming discrete hole-selective contact structures for p-type silicon.
[027] Figure 7 is a process flow showing certain process steps for forming discrete electron-selective contacts for n-type silicon and discrete hole- selective contact structures for p-type silicon regions, for example as shown in the back contact back junction solar cell of Figure 4.
[028] The following may be advantageous, in consideration with other fabrication factors, for forming discrete carrier selective passivated contacts and discrete carrier selective passivated contact solar cells. In some instances, it may be advantageous to avoid residual damage to silicon (e.g., avoiding laser-induced damage). Additionally, in some instances it may be advantageous to limit thermal processing temperatures to less than or equal to 950°C and in some instances to as low as less than or equal to 925°C. Additionally, in some instances it may be advantageous to screen print boron-doped and phosphorus-doped SNP pastes over chemical oxide to form passivated n+ poly / tunnel oxide / n-type base contacts and p+ poly / tunnel oxide / p-type emitter contacts in order to simplify the passivated contact fabrication process. Additionally, in some instances it may be advantageous to use a high sheet resistance shallow emitter. Additionally, in some instances it may be advantageous to use excellent passivation layers on frontside and backside - for example APCVD boron-doped aluminum oxide (with undoped silicon oxide cap) over the solar cell substrate backside and PECVD SiNx:Hy and phosphorus front-surface field (FSF) or PECVD SiNx:Hy / aluminum oxide (with optional tunnel silicon oxide at the interface) on the solar cell substrate frontside.
[029] Figures 8 through 16 are detailed fabrication step process flows for forming discrete carrier selective silicon nanoparticle polysilicon passivated contacts using efficient manufacturing process flows and standard solar fabrication equipment resulting in high efficiency interdigitated back contact back junction (IBC) solar cells. Note the tunnel oxide may be formed, for example, as a chemical oxide or a UV ozone oxide. The process flow of Figure 8 may be characterized by a lack of laser processing and the process for copper plating. The process flow of Figure 9 may be characterized by a lack of laser processing and a lack of copper plating. The process flow of Figure 10 may be characterized by a lack of laser processing, a lack of physical vapor deposition (PVD), and a lack of copper plating. The process flow of Figure 11 may be characterized by one furnace step, the process for copper plating, and using pulsed laser for the opening of emitter and base contacts. The process flow of Figure 12 may be characterized by one furnace step, the process for copper plating, and using resist screen printing and wet etch for the opening of emitter and base contacts. The process flow of Figure 13 may be characterized by one furnace step, a lack of copper plating, and using pulsed laser for the opening of emitter and base contacts. The process flow of Figure 14 may be
characterized by one furnace step, a lack of copper plating, and using resist screen printing and wet etch for the opening of emitter and base contacts. The process flow of Figure 15 may be characterized by one furnace step, a lack of copper plating, a lack of physical vapor deposition PVD, and using pulsed laser for the opening of emitter and base contacts. The process flow of Figure 16 may be characterized by one furnace step, a lack of copper plating, a lack of physical vapor deposition PVD, and using resist screen printing and wet etch for the opening of emitter and base contacts. [030] Figure 17 is a chart showing carrier-selective contact material candidates for crystalline silicon solar cells. Materials shown include dielectric and semiconductor materials. Suitable materials for n-type contacts (electron) are shown above the silicon (Si) band gap and suitable materials for p-type contacts (hole) are shown below the silicon (Si) band gap. Potentially suitable n-type contact materials have Conduction Band Minima (CBM) close to CBM of crystalline silicon and potentially suitable p-type contact materials have Valence Band Maxima (VBM) close to CBM of crystalline silicon.
[031] The foregoing description of the exemplary embodiments is provided to enable any person skilled in the art to make or use the claimed subject matter. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the innovative faculty. Thus, the claimed subject matter is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

CLAIMS What is claimed is:
1. A photovoltaic solar cell structure, comprising:
a silicon substrate having a frontside and a backside, said backside having base regions and emitter regions;
a tunnel dielectric covering at least a portion of said base regions and at least a portion of said emitter regions;
discrete silicon nanoparticle n+ doped polysilicon regions on said tunnel dielectric covering said base regions; and
discrete silicon nanoparticle p+ doped polysilicon regions on said tunnel dielectric covering said emitter regions.
2. The photovoltaic solar cell structure of Claim 1, wherein said tunnel dielectric is a chemical oxide.
3. The photovoltaic solar cell structure of Claim 1, wherein said tunnel dielectric is a UV ozone oxide.
4. The photovoltaic solar cell structure of Claim 1, further comprising metal
electrodes contacting said discrete silicon nanoparticle n+ doped polysilicon regions and metal electrodes contacting said discrete silicon nanoparticle p+ doped polysilicon regions.
5. The photovoltaic solar cell structure of Claim 1, wherein said electrodes are
aluminum.
6. A method for making a photovoltaic solar structure, comprising:
depositing a passivation layer on said backside surface of said silicon substrate having base regions and emitter regions; exposing at least a portion of said base regions and at least a portion of said emitter regions on the backside of said silicon substrate through said passivation layer;
forming a tunnel dielectric on at least a portion of said base regions and on at least a portion of said exposed emitter regions on the backside of said silicon substrate;
depositing discontinuous regions of phosphorous doped silicon nanoparticles over said tunnel dielectric on said base regions;
depositing discontinuous regions of boron doped silicon nanoparticles over said tunnel dielectric on said emitter regions; and
transforming said discontinuous regions of phosphorous doped silicon nanoparticles into silicon nanoparticle n+ doped polysilicon and transforming said discontinuous regions of boron doped silicon nanoparticles into silicon nanoparticle p+ doped polysilicon.
7. A method for making a photovoltaic solar structure, comprising:
forming a tunnel dielectric on at least a portion of the backside of a silicon substrate;
depositing discontinuous regions of phosphorous doped silicon nanoparticles on portions of said tunnel dielectric;
depositing discontinuous regions of boron doped silicon nanoparticles on portions of said tunnel dielectric;
depositing a boron dopant on portions of the backside of said tunnel dielectric; and,
annealing to transform said discontinuous regions of phosphorous doped silicon nanoparticles into silicon nanoparticle n+ doped polysilicon, transform said discontinuous regions of boron doped silicon nanoparticles into silicon nanoparticle p+ doped polysilicon, and form a field emitter in said silicon substrate corresponding to said boron dopant.
8. The method for making a photovoltaic solar structure of Claim 7, wherein said step of depositing a boron dopant on portions of the backside of said tunnel dielectric is also deposited said boron dopant on said discontinuous regions of phosphorous doped silicon nanoparticles and said discontinuous regions of boron doped silicon nanoparticles.
9. The method for making a photovoltaic solar structure of Claim 7, wherein said step of depositing a boron dopant on portions of the backside of said tunnel dielectric is not deposited said boron dopant on said discontinuous regions of phosphorous doped silicon nanoparticles and said discontinuous regions of boron doped silicon nanoparticles.
PCT/US2016/041018 2015-07-02 2016-07-05 Discrete carrier selective passivated contacts for solar cells WO2017004624A1 (en)

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