WO2012148523A1 - Method of foring emitters for a back-contact solar cell - Google Patents

Method of foring emitters for a back-contact solar cell Download PDF

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Publication number
WO2012148523A1
WO2012148523A1 PCT/US2012/025264 US2012025264W WO2012148523A1 WO 2012148523 A1 WO2012148523 A1 WO 2012148523A1 US 2012025264 W US2012025264 W US 2012025264W WO 2012148523 A1 WO2012148523 A1 WO 2012148523A1
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WIPO (PCT)
Prior art keywords
solid
dopant source
state dopant
regions
substrate
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PCT/US2012/025264
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French (fr)
Inventor
Bo Li
Peter J. COUSING
David D. Smith
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Sunpower Corporation
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Publication date
Application filed by Sunpower Corporation filed Critical Sunpower Corporation
Priority to CN201280020157.XA priority Critical patent/CN103493216B/en
Priority to KR1020137030732A priority patent/KR20140041494A/en
Priority to EP12776847.1A priority patent/EP2702614B1/en
Priority to JP2014508344A priority patent/JP6026508B2/en
Publication of WO2012148523A1 publication Critical patent/WO2012148523A1/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
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/186—Particular post-treatment for the devices, e.g. annealing, impurity gettering, short-circuit elimination, recrystallisation
    • H01L31/1864—Annealing
    • 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
    • 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
    • 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
    • H01L31/18—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

  • Embodiments of the present invention are in the field of renewable energy and, in particular, methods of forming emitters for back-contact solar cells.
  • Photovoltaic cells are well known devices for direct conversion of solar radiation into electrical energy.
  • solar cells are fabricated on a semiconductor wafer or substrate using semiconductor processing techniques to form a p-n junction near a surface of the substrate.
  • Solar radiation impinging on the surface of, and entering into, the substrate creates electron and hole pairs in the bulk of the substrate.
  • the electron and hole pairs migrate to p- doped and n-doped regions in the substrate, thereby generating a voltage differential between the doped regions.
  • the doped regions are connected to conductive regions on the solar cell to direct an electrical current from the cell to an external circuit coupled thereto.
  • Figure 1 illustrates a flowchart representing operations in a method of forming emitters for a back-contact solar cell, in accordance with an embodiment of the present invention.
  • Figure 2A illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, corresponding to operation 102 of the flowchart of Figure 1 and to operation 302 of the flowchart of Figure 3, in accordance with an
  • Figure 2B illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, corresponding to operation 104 of the flowchart of Figure 1 and to operation 304 of the flowchart of Figure 3, in accordance with an
  • Figure 2C illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, corresponding to operation 106 of the flowchart of Figure 1, in accordance with an embodiment of the present invention.
  • Figure 2D illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, also corresponding to operation 106 of the flowchart of Figure 1, in accordance with an embodiment of the present invention.
  • Figure 2E illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, corresponding to operation 108 of the flowchart of Figure 1, in accordance with an embodiment of the present invention.
  • Figure 2F illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, corresponding to operation 110 of the flowchart of Figure 1, in accordance with an embodiment of the present invention.
  • Figure 2G illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, corresponding to operations 112 and 114 of the flowchart of Figure 1, in accordance with an embodiment of the present invention.
  • Figure 2H illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, in accordance with an embodiment of the present invention.
  • Figure 21 illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, in accordance with an embodiment of the present invention.
  • Figure 3 illustrates a flowchart representing operations in another method of forming emitters for a back-contact solar cell, in accordance with an embodiment of the present invention.
  • Figure 4A illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, also corresponding to operation 306 of the flowchart of
  • Figure 4B illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, also corresponding to operation 308 of the flowchart of
  • Figure 4C illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, also corresponding to operation 310 of the flowchart of
  • Figure 4D illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, corresponding to operation 312 of the flowchart of Figure
  • Figure 4E illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, corresponding to operations 314 and 316 of the flowchart of Figure 3, in accordance with an embodiment of the present invention.
  • a method includes forming, by chemical vapor deposition, a first solid-state dopant source of a first conductivity type above a substrate.
  • the first solid-state dopant source includes a plurality of regions separated by gaps. Regions of a second solid-state dopant source of a second conductivity type are formed above the substrate by printing. The regions of the second solid-state dopant source are formed in the gaps of the plurality of regions of the first solid-state dopant source but not in contact with the plurality of regions of the first solid-state dopant source.
  • the first conductivity type is opposite the second conductivity type.
  • a method in another embodiment, includes forming, by printing, a first solid-state dopant source of a first conductivity type above a substrate.
  • the first solid- state dopant source includes a plurality of regions separated by gaps.
  • a second solid- state dopant source of a second conductivity type is formed above the first solid-state dopant source by chemical vapor deposition.
  • the second solid-state dopant source is also formed above the substrate, in the gaps of the plurality of regions of the first solid-state dopant source.
  • the first conductivity type is opposite the second conductivity type.
  • the second solid-state dopant source is then patterned to form first regions of the second solid-state dopant source in the gaps of the plurality of regions of the first solid-state dopant source but not in contact with the plurality of regions of the first solid-state dopant source. Second regions of the second solid-state dopant source remain on the plurality of regions of the first solid-state dopant source.
  • the first solid-state dopant source is sufficiently thick to block driving of dopants from the second regions of the second solid-state dopant source through the plurality of regions of the first solid-state dopant source.
  • Emitters for back-contact solar cells may be formed by patterning through blanket-deposited doped films. This approach typically involves deposition of a blanket dopant-containing film, deposition of a sacrificial etch resist, etching of the dopant-containing film, and stripping of the etch resist. These multiple process operations increase the manufacturing complexity and cost for solar cell production. Since numerous operations are used, yield may also be reduced.
  • a total number of process operations used for doping back-contact solar cells is reduced by using methods described herein.
  • one or more of the embodiments of methods described herein may simplify emitter formation.
  • such improvements are achieved through selective dopant deposition which combines dopant deposition and patterning in single operation.
  • a conventional process having four operations in a portion of a manufacturing flow is reduced to two operations for that portion of the flow.
  • a particular example includes replacing the operations: depositing first solid-state dopant source, masking, etching, and then depositing second solid-state dopant source, with the operations: ink jetting first solid-state dopant source and then depositing second solid-state dopant source.
  • a conventional process having six operations in a portion of a manufacturing flow is reduced to five operations for that portion of the flow.
  • a particular example includes replacing the operations: depositing first solid- state dopant source, masking, etching, depositing second solid-state dopant source, masking, and then etching, with the operations: depositing first solid-state dopant source, masking, etching, ink jetting second solid-state dopant source, and then curing, or with the operations: ink jetting first solid-state dopant source, depositing second solid-state dopant source, curing, masking, and then etching.
  • printable dopant sources include ink-jet printable dopant source materials including, but not limited to, spin on glass-based materials or nanoparticle-based materials.
  • contacts for a back- contact solar cell may be performed using laser ablation to form holes or openings through an anti-reflective coating (ARC) layer formed above an array of p-type and n- type doped regions on the back-side of the solar cell.
  • Conductive contacts such as metal contacts, may then be formed in the openings to provide electrical coupling with the array of p-type and n-type doped regions.
  • a second conductivity-type solid- state dopant source is printed between features of an already patterned first conductivity-type solid-state dopant source.
  • Figure 1 illustrates a flowchart 100 representing operations in a method of forming emitters for a back- contact solar cell, in accordance with an embodiment of the present invention.
  • FIGS. 2A-2I illustrate cross-sectional views of various stages in the fabrication of a back-contact solar cell, corresponding to operations of flowchart 100, in accordance with an embodiment of the present invention.
  • a method of forming emitters for a back-contact solar cell includes optionally forming a thin dielectric layer 202 on a substrate 200.
  • the thin dielectric layer 202 is composed of silicon dioxide and has a thickness approximately in the range of 5-50 Angstroms. In one embodiment, the thin dielectric layer 202 performs as a tunneling oxide layer.
  • substrate 200 is a bulk single-crystal substrate, such as an n-type doped single crystalline silicon substrate. However, in an alternative embodiment, substrate 200 includes a polycrystalline silicon layer disposed on a global solar cell substrate.
  • the method of forming emitters for the back-contact solar cell also includes optionally forming a polysilicon layer 204 on the thin dielectric layer 202. It is to be understood that use of the term polysilicon layer is intended to also cover material that can be described as amorphous- or a-silicon.
  • the method of forming emitters for the back-contact solar cell includes forming (layer 205 of Figure 2C) and patterning a first solid-state dopant source 206 of a first conductivity type on the polysilicon layer 204.
  • the patterning forms gaps 208 exposing regions of the polysilicon layer 204 between a plurality of regions of the first solid-state dopant source 206, as depicted in Figure 2D.
  • forming and patterning the first solid-state dopant source 206 includes forming and patterning a layer of boron silicate glass (BSG) or a layer of phosphorus silicate glass (PSG).
  • BSG or PSG layer is formed by chemical vapor deposition as a uniform, blanket layer and then patterned by a lithography and etch process.
  • the BSG or PSG layer is formed by a chemical vapor deposition technique such as, but not limited to, atmospheric pressure chemical vapor deposition (APCVD), plasma-enhanced chemical vapor deposition (PECVD), low- pressure chemical vapor deposition (LPCVD), or ultra-high vacuum chemical vapor deposition (UHVCVD).
  • APCVD atmospheric pressure chemical vapor deposition
  • PECVD plasma-enhanced chemical vapor deposition
  • LPCVD low- pressure chemical vapor deposition
  • UHVCVD ultra-high vacuum chemical vapor deposition
  • the BSG or PSG layer is deposited already having a pattern and, thus, the forming and patterning are performed simultaneously.
  • the patterned BSG or PSG layer is formed by a screen-printing approach.
  • a solid-state dopant source is a layer of film that includes dopant impurity atoms and can be deposited above a substrate. This is in contrast to an ion implantation approach.
  • the method of forming emitters for the back-contact solar cell also includes forming, by printing, regions of a second solid-state dopant source 210 of a second conductivity type above the substrate 200.
  • the printing is performed by a technique such as, but not limited to, ink-jet printing, screen printing, or aerosol printing.
  • the regions of the second solid-state dopant source are formed by a technique such as, but not limited to, ink-jet printing, screen printing, or aerosol printing.
  • first solid-state dopant source 205/206 and the second solid-state dopant source 210 are formed on the polysilicon layer 204. However, there may be applications where a polysilicon layer distinct from a substrate is not used. Thus, in another embodiment (not shown), the first solid-state dopant source 205/206 and the second solid-state dopant source 210 are formed directly on a substrate (such as directly on substrate 200) or on a thin oxide layer on the surface of the substrate.
  • the second solid-state dopant source 210 is composed of a material such as, but not limited to, a spin-on-glass precursor material or a nanoparticle material.
  • the spin-on-glass precursor material or the nanoparticle material includes dopants of the second conductivity type disposed therein.
  • the second conductivity type is n-type and the dopant impurity atoms are phosphorus atoms.
  • the second conductivity type is p-type and the dopant impurity atoms are boron atoms.
  • the first conductivity type is opposite the second conductivity type.
  • the first conductivity type is p- type
  • the second conductivity type is n-type
  • the first solid-state dopant source 205/206 is composed of BSC
  • the first conductivity type is n- type
  • the second conductivity type is p-type
  • the first solid-state dopant source 205/206 is composed of PSG.
  • the method of forming emitters for the back-contact solar cell optionally further includes forming trenches 212 partially into the substrate 200, between the regions of the second solid-state dopant source 210 and the plurality of regions of the first solid-state dopant source 206.
  • the trenches 212 are formed in the polysilicon layer 204, in the thin dielectric layer 202, and partially in the substrate 200, as depicted in Figure 2F.
  • the method of forming emitters for the back-contact solar cell optionally further includes, subsequent to forming the trenches 212, heating 250 the substrate 200.
  • the heating drives dopants from the first and second solid-state dopant sources 206 and 210.
  • the first and second solid-state dopant sources 206 and 210 are formed on the polysilicon layer 204 and heating the substrate 200 drives dopants from the first and second solid-state dopant sources 206 and 210, respectively, into the polysilicon layer 204.
  • the first and second solid-state dopant sources 206 and 210 are formed directly on substrate 2090 or on a thin oxide on substrate 200, and heating the substrate 200 drives dopants from the first and second solid-state dopant sources 206 and 210, respectively, into the substrate 200.
  • the substrate 200 is a bulk crystalline silicon substrate, and the first solid-state dopant source 206 and the second solid-state dopant source 210 are formed on the bulk crystalline silicon substrate.
  • the bulk crystalline silicon substrate is then heated to drive dopants from the first and second solid-state dopant sources 206 and 210 into the bulk crystalline silicon substrate.
  • the method of forming emitters for the back-contact solar cell optionally further includes, texturizing portions 214 of the substrate 200 exposed by the trenches 212.
  • the texturing provides a random texture pattern.
  • the random texturing pattern may be formed by applying an anisotropic etching process to exposed regions of substrate 200 and may thus be determined by crystal planes, such single-crystalline silicon planes, of the substrate 200.
  • the heating of operation 112 hardens the second solid-state dopant source 210. Then, during the texturizing of the portions 214 of the substrate 200 exposed by the trenches 212, the hardened second solid-state dopant source acts as a mask.
  • the hardened second solid-state dopant source acts as a mask to provide selectivity to hydroxide (OH ) based etching. That is, inherent in the doping process using the printed solid-state dopant source is an ability to provide a mask for texturizing operations. It is to be understood that other doping approaches, such as atmospheric pressure chemical vapor deposition (APCVD), implantation, or laser doping, may not provide routes for such masking that are inherent in the doping process.
  • APCVD atmospheric pressure chemical vapor deposition
  • implantation implantation
  • laser doping may not provide routes for such masking that are inherent in the doping process.
  • heating the substrate 200 also includes activating the dopants from the first and second solid-state dopant sources 206 and 210, respectively, to form a plurality of polysilicon regions 220 of the second conductivity type and a plurality of polysilicon regions 222 of the first conductivity type.
  • the activating includes changing the incorporation of at least some of the dopants from interstitial to substitutional within polysilicon layer 204.
  • the first and second solid-state dopant sources 206 and 210 are also removed, as is also depicted in Figure 2H.
  • the first and second solid-state dopant sources 206 and 210 are removed by using a wet etch technique by applying a wet solution including aqueous hydrofluoric acid or another source of HF. In another such embodiment, the first and second solid-state dopant sources 206 and 210 are removed by plasma etching.
  • the method of forming emitters for the back-contact solar cell optionally further includes forming a dielectric layer 224 above the plurality of polysilicon regions 220 of the second conductivity type, above the plurality of polysilicon regions 222 of the first conductivity type, and above the exposed portions of substrate 200.
  • the dielectric layer 224 is an anti-reflective coating (ARC) layer.
  • the method of forming emitters for the back-contact solar cell optionally further includes forming, by laser abalation, a plurality of contact openings 226 to the plurality of polysilicon regions 220 of the second conductivity type and to the plurality of polysilicon regions 222 of the first conductivity type.
  • Conductive contacts 228 may then be formed in the plurality of contact openings 226 and coupled to the plurality of polysilicon regions 220 of the second conductivity type and to the plurality of polysilicon regions 222 of the first conductivity type.
  • the conductive contacts 228 are composed of metal and are formed by a deposition, lithographic, and etch approach.
  • a second conductivity-type solid-state dopant source is formed by blanket deposition over features of a printed first conductivity-type solid-state dopant source.
  • Figure 3 illustrates a flowchart 300 representing operations in a method of forming emitters for a back- contact solar cell, in accordance with another embodiment of the present invention.
  • Figures 2A, 2B, and 4A-4E, 21, and 2J illustrate cross-sectional views of various stages in the fabrication of a back-contact solar cell, corresponding to operations of flowchart 300, in accordance with an embodiment of the present invention.
  • a method of forming emitters for a back-contact solar cell includes optionally forming a thin dielectric layer 202 on a substrate 200.
  • the thin dielectric layer 202 is composed of silicon dioxide and has a thickness approximately in the range of 5-50 Angstroms. In one embodiment, the thin dielectric layer 202 performs as a tunneling oxide layer.
  • substrate 200 is a bulk single-crystal substrate, such as an n-type doped single crystalline silicon substrate. However, in an alternative embodiment, substrate 200 includes a polycrystalline silicon layer disposed on a global solar cell substrate.
  • the method of forming emitters for the back-contact solar cell also includes optionally forming a polysilicon layer 204 on the thin dielectric layer 202. It is to be understood that use of the term polysilicon layer is intended to also cover material that can be described as amorphous- or a-silicon.
  • the method of forming emitters for the back-contact solar cell includes forming, by printing, a first solid-state dopant source 252 of a first conductivity type above substrate 200.
  • the first solid-state dopant source 252 includes a plurality of regions separated by gaps 254.
  • the printing is performed by a technique such as, but not limited to, ink-jet printing, screen printing, or aerosol printing.
  • the gaps 254 expose regions of the polysilicon layer
  • the total coverage of the first solid-state dopant source 252 above the substrate 200 is in the range of 10-15% by surface area, e.g., approximately 12%.
  • the first solid-state dopant source 252 is composed of a material such as, but not limited to, a spin-on-glass precursor material or a nanoparticle material.
  • the spin-on-glass precursor material or the nanoparticle material includes dopants of the first conductivity type disposed therein.
  • the first conductivity type is n-type and the dopant impurity atoms are phosphorus atoms.
  • the first conductivity type is p-type and the dopant impurity atoms are boron atoms.
  • the method of forming emitters for the back-contact solar cell also includes forming, by chemical vapor deposition, a second solid-state dopant source 256 of a second conductivity type above the first solid-state dopant source 252.
  • the second solid-state dopant source 256 is also formed above the substrate 200, in the gaps 254 of the plurality of regions of the first solid-state dopant source 252.
  • forming the second solid-state dopant source 256 includes forming a layer of BSG or a layer of PSG.
  • the BSG or PSG layer is formed by chemical vapor deposition as a uniform, blanket layer.
  • the second conductivity type is opposite the first conductivity type.
  • the first conductivity type is p-type
  • the second conductivity type is n-type
  • the second solid-state dopant source 256 is composed of PSG.
  • the first conductivity type is n-type
  • the second conductivity type is p-type
  • the second solid-state dopant source 256 is composed of BSG.
  • the first solid-state dopant source 252 and portions of the second solid-state dopant source 256 are formed on the polysilicon layer 204.
  • the first solid-state dopant source 252 and the portions of the second solid-state dopant source 256 are formed directly on a substrate (such as directly on substrate 200) or on a thin oxide layer on the surface of the substrate.
  • the method of forming emitters for the back-contact solar cell also includes patterning the second solid-state dopant source 256 to form first regions 258 of the second solid-state dopant source 256 in the gaps 254 of the plurality of regions of the first solid-state dopant source 252 but not in contact with the plurality of regions of the first solid-state dopant source 252. Furthermore, second regions 260 of the second solid-state dopant source 256 are also formed on the plurality of regions of the first solid-state dopant source 252, as depicted in Figure 4C.
  • patterning the second solid-state dopant source 256 includes patterning a layer of BSG or a layer of PSG.
  • the BSG or PSG layer is formed by chemical vapor deposition as a uniform, blanket layer and then patterned by a lithography and etch process. In a particular such
  • the BSG or PSG layer is formed by a chemical vapor deposition technique such as, but not limited to, atmospheric pressure chemical vapor deposition (APCVD), plasma-enhanced chemical vapor deposition (PECVD), low-pressure chemical vapor deposition (LPCVD), or ultra-high vacuum chemical vapor deposition (UHVCVD).
  • APCVD atmospheric pressure chemical vapor deposition
  • PECVD plasma-enhanced chemical vapor deposition
  • LPCVD low-pressure chemical vapor deposition
  • UHVCVD ultra-high vacuum chemical vapor deposition
  • the first regions 258 of the second solid-state dopant source 256 are formed in the gaps 254 of the plurality of regions of the first solid-state dopant source 252 but not in contact with the plurality of regions of the first solid- state dopant source 252, as depicted in Figure 4C.
  • the first regions 258 of the second solid-state dopant source 256 are spaced apart from the plurality of regions of the first solid-state do
  • the method of forming emitters for the back-contact solar cell optionally further includes forming trenches 262 partially into the substrate 200, between the first regions 258 of the second solid-state dopant source 256 and the plurality of regions of the first solid-state dopant source 252.
  • the trenches 262 are formed in the polysilicon layer 204, in the thin dielectric layer 202, and partially in the substrate 200, as depicted in Figure 4D.
  • the method of forming emitters for the back-contact solar cell optionally further includes, subsequent to forming the trenches 262, heating 250 the substrate 200.
  • the heating drives dopants from the first and second solid-state dopant sources 252 and 258.
  • the first and second solid-state dopant sources 252 and 258 are formed on the polysilicon layer 204 and heating the substrate 200 drives dopants from the first and second solid-state dopant sources 252 and 258, respectively, into the polysilicon layer 204.
  • the first and second solid-state dopant sources 252 and 258 are formed directly on substrate 200 or on a thin oxide on substrate 200, and heating the substrate 200 drives dopants from the first and second solid-state dopant sources 252 and 258, respectively, into the substrate 200.
  • the substrate 200 is a bulk crystalline silicon substrate, and the first solid-state dopant source 252 and the second solid-state dopant source 258 are formed on the bulk crystalline silicon substrate.
  • the bulk crystalline silicon substrate is then heated to drive dopants from the first and second solid-state dopant sources 252 and 258 into the bulk crystalline silicon substrate.
  • the first solid-state dopant source 252 is sufficiently thick to block driving of dopants from the second regions 260 of the second solid-state dopant source 256 through the plurality of regions of the first solid-state dopant source 252.
  • it may be desirable to drive dopants from the first regions 258 of the second solid-state dopant source 256 into an underlying polysilicon layer or substrate it may not be desirable for dopants to be driven from the second regions 260 of the second solid-state dopant source 256 into the underlying polysilicon layer or substrate.
  • the first solid-state dopant source 252 it may only be desirable to drive dopants from the first solid-state dopant source 252, underlying the second regions 260 of the second solid-state dopant source 256, into the underlying polysilicon layer or substrate.
  • other parameters that may be considered for adequate blocking of the driving of dopants from the second regions 260 of the second solid-state dopant source 256 through the plurality of regions of the first solid-state dopant source 252 include, but need not be limited to, dopant concentration in the first solid-state dopant source 252, density of the first solid-state dopant source 252, identity of dopant species, and timing for the heating operation 314.
  • the method of forming emitters for the back-contact solar cell optionally further includes, texturizing portions 264 of the substrate 200 exposed by the trenches 262.
  • the texturing provides a random texture pattern.
  • the random texturing pattern may be formed by applying an anisotropic etching process to exposed regions of substrate 200 and may thus be determined by crystal planes, such single-crystalline silicon planes, of the substrate 200.
  • the heating of operation 314 hardens the first solid-state dopant source 252. Then, during the texturizing of the portions 264 of the substrate 200 exposed by the trenches 262, the hardened first solid-state dopant source acts as a mask.
  • heating the substrate 200 also includes activating the dopants from the first and second solid-state dopant sources 252 and 258, respectively, to form a plurality of polysilicon regions 220 of the first conductivity type and a plurality of polysilicon regions 222 of the second conductivity type.
  • the activating includes changing the incorporation of at least some of the dopants from interstitial to substitutional within polysilicon layer 204.
  • the first and second solid-state dopant sources 252 and 258 are also removed, as is also depicted in Figure 2H.
  • the first and second solid-state dopant sources 252 and 258 are removed by using a wet etch technique by applying a wet solution including aqueous hydrofluoric acid or another source of HF. In another such embodiment, the first and second solid-state dopant sources 252 and 258 are removed by plasma etching.
  • the method of forming emitters for the back-contact solar cell optionally further includes forming a dielectric layer 224 above the plurality of polysilicon regions 220 of the first conductivity type, above the plurality of polysilicon regions 222 of the second conductivity type, and above the exposed portions of substrate 200.
  • a plurality of contact openings 226 may then be formed, exposing the plurality of polysilicon regions 220 of the first conductivity type and to the plurality of polysilicon regions 222 of the second conductivity type.
  • Conductive contacts 228 may then be formed in the plurality of contact openings 226 and coupled to the plurality of polysilicon regions 220 of the first conductivity type and to the plurality of polysilicon regions 222 of the second conductivity type.
  • a method includes forming, by chemical vapor deposition, a first solid-state dopant source of a first conductivity type above a substrate, the first solid-state dopant source including a plurality of regions separated by gaps.
  • the method also includes forming, by printing, regions of a second solid-state dopant source of a second conductivity type above the substrate, in the gaps of the plurality of regions of the first solid-state dopant source but not in contact with the plurality of regions of the first solid-state dopant source, wherein the first conductivity type is opposite the second conductivity type.
  • the second solid-state dopant source is composed of a spin- on-glass precursor material or a nanoparticle material.
  • the first conductivity type is p-type
  • the second conductivity type is n-type
  • the first solid- state dopant source is composed of boron silicate glass (BSG).
  • BSG boron silicate glass
  • the first conductivity type is n-type
  • the second conductivity type is p-type
  • the first solid-state dopant source is composed of phosphorus silicate glass (PSG).

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Abstract

Methods of forming emitters for back-contact solar cells are described. In one embodiment, a method includes forming a first solid-state dopant source above a substrate. The first solid-state dopant source includes a plurality of regions separated by gaps. Regions of a second solid-state dopant source are formed above the substrate by printing.

Description

Method of Forming Emitters for a Back- Contact Solar Cell CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional Application No.
61/478,804, filed April 25, 2011, the entire contents of which are hereby incorporated by reference herein.
STATEMENT OF GOVERNMENT LICENSE RIGHTS
[0002] The invention described herein was made with governmental support under contract number DE-FC36-07GO17043 awarded by the United States
Department of Energy. The United States Government may have certain rights in the invention.
TECHNICAL FIELD
[0003] Embodiments of the present invention are in the field of renewable energy and, in particular, methods of forming emitters for back-contact solar cells.
BACKGROUND
[0004] Photovoltaic cells, commonly known as solar cells, are well known devices for direct conversion of solar radiation into electrical energy. Generally, solar cells are fabricated on a semiconductor wafer or substrate using semiconductor processing techniques to form a p-n junction near a surface of the substrate. Solar radiation impinging on the surface of, and entering into, the substrate creates electron and hole pairs in the bulk of the substrate. The electron and hole pairs migrate to p- doped and n-doped regions in the substrate, thereby generating a voltage differential between the doped regions. The doped regions are connected to conductive regions on the solar cell to direct an electrical current from the cell to an external circuit coupled thereto.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] Figure 1 illustrates a flowchart representing operations in a method of forming emitters for a back-contact solar cell, in accordance with an embodiment of the present invention. [0006] Figure 2A illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, corresponding to operation 102 of the flowchart of Figure 1 and to operation 302 of the flowchart of Figure 3, in accordance with an
embodiment of the present invention.
[0007] Figure 2B illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, corresponding to operation 104 of the flowchart of Figure 1 and to operation 304 of the flowchart of Figure 3, in accordance with an
embodiment of the present invention.
[0008] Figure 2C illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, corresponding to operation 106 of the flowchart of Figure 1, in accordance with an embodiment of the present invention.
[0009] Figure 2D illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, also corresponding to operation 106 of the flowchart of Figure 1, in accordance with an embodiment of the present invention.
[0010] Figure 2E illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, corresponding to operation 108 of the flowchart of Figure 1, in accordance with an embodiment of the present invention.
[0011] Figure 2F illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, corresponding to operation 110 of the flowchart of Figure 1, in accordance with an embodiment of the present invention.
[0012] Figure 2G illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, corresponding to operations 112 and 114 of the flowchart of Figure 1, in accordance with an embodiment of the present invention.
[0013] Figure 2H illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, in accordance with an embodiment of the present invention.
[0014] Figure 21 illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, in accordance with an embodiment of the present invention.
[0015] Figure 3 illustrates a flowchart representing operations in another method of forming emitters for a back-contact solar cell, in accordance with an embodiment of the present invention. [0016] Figure 4A illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, also corresponding to operation 306 of the flowchart of
Figure 3, in accordance with an embodiment of the present invention.
[0017] Figure 4B illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, also corresponding to operation 308 of the flowchart of
Figure 3, in accordance with an embodiment of the present invention.
[0018] Figure 4C illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, also corresponding to operation 310 of the flowchart of
Figure 3, in accordance with an embodiment of the present invention.
[0019] Figure 4D illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, corresponding to operation 312 of the flowchart of Figure
3, in accordance with an embodiment of the present invention.
[0020] Figure 4E illustrates a cross-sectional view of a stage in the fabrication of a back-contact solar cell, corresponding to operations 314 and 316 of the flowchart of Figure 3, in accordance with an embodiment of the present invention.
DETAILED DESCRIPTION
[0021] Methods of forming emitters for back-contact solar cells are described herein. In the following description, numerous specific details are set forth, such as specific process flow operations, in order to provide a thorough understanding of embodiments of the present invention. It will be apparent to one skilled in the art that embodiments of the present invention may be practiced without these specific details. In other instances, well-known fabrication techniques, such as lithography and patterning techniques, are not described in detail in order to not unnecessarily obscure embodiments of the present invention. Furthermore, it is to be understood that the various embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.
[0022] Disclosed herein are methods of forming emitters for back-contact solar cells. In one embodiment, a method includes forming, by chemical vapor deposition, a first solid-state dopant source of a first conductivity type above a substrate. The first solid-state dopant source includes a plurality of regions separated by gaps. Regions of a second solid-state dopant source of a second conductivity type are formed above the substrate by printing. The regions of the second solid-state dopant source are formed in the gaps of the plurality of regions of the first solid-state dopant source but not in contact with the plurality of regions of the first solid-state dopant source. The first conductivity type is opposite the second conductivity type.
[0023] In another embodiment, a method includes forming, by printing, a first solid-state dopant source of a first conductivity type above a substrate. The first solid- state dopant source includes a plurality of regions separated by gaps. A second solid- state dopant source of a second conductivity type is formed above the first solid-state dopant source by chemical vapor deposition. The second solid-state dopant source is also formed above the substrate, in the gaps of the plurality of regions of the first solid-state dopant source. The first conductivity type is opposite the second conductivity type. The second solid-state dopant source is then patterned to form first regions of the second solid-state dopant source in the gaps of the plurality of regions of the first solid-state dopant source but not in contact with the plurality of regions of the first solid-state dopant source. Second regions of the second solid-state dopant source remain on the plurality of regions of the first solid-state dopant source. The first solid-state dopant source is sufficiently thick to block driving of dopants from the second regions of the second solid-state dopant source through the plurality of regions of the first solid-state dopant source.
[0024] Emitters for back-contact solar cells may be formed by patterning through blanket-deposited doped films. This approach typically involves deposition of a blanket dopant-containing film, deposition of a sacrificial etch resist, etching of the dopant-containing film, and stripping of the etch resist. These multiple process operations increase the manufacturing complexity and cost for solar cell production. Since numerous operations are used, yield may also be reduced.
[0025] In accordance with an embodiment of the present invention, a total number of process operations used for doping back-contact solar cells is reduced by using methods described herein. As such, one or more of the embodiments of methods described herein may simplify emitter formation. In at least some embodiments, such improvements are achieved through selective dopant deposition which combines dopant deposition and patterning in single operation.
[0026] In a specific exemplary embodiment, a conventional process having four operations in a portion of a manufacturing flow is reduced to two operations for that portion of the flow. A particular example includes replacing the operations: depositing first solid-state dopant source, masking, etching, and then depositing second solid-state dopant source, with the operations: ink jetting first solid-state dopant source and then depositing second solid-state dopant source. In another specific exemplary embodiment, a conventional process having six operations in a portion of a manufacturing flow is reduced to five operations for that portion of the flow. A particular example includes replacing the operations: depositing first solid- state dopant source, masking, etching, depositing second solid-state dopant source, masking, and then etching, with the operations: depositing first solid-state dopant source, masking, etching, ink jetting second solid-state dopant source, and then curing, or with the operations: ink jetting first solid-state dopant source, depositing second solid-state dopant source, curing, masking, and then etching. Examples of printable dopant sources include ink-jet printable dopant source materials including, but not limited to, spin on glass-based materials or nanoparticle-based materials.
[0027] Following formation of doped regions in semiconductor layers or substrates by approaches described herein, the formation of contacts for a back- contact solar cell may be performed using laser ablation to form holes or openings through an anti-reflective coating (ARC) layer formed above an array of p-type and n- type doped regions on the back-side of the solar cell. Conductive contacts, such as metal contacts, may then be formed in the openings to provide electrical coupling with the array of p-type and n-type doped regions.
[0028] In an aspect of the present invention, a second conductivity-type solid- state dopant source is printed between features of an already patterned first conductivity-type solid-state dopant source. For example, Figure 1 illustrates a flowchart 100 representing operations in a method of forming emitters for a back- contact solar cell, in accordance with an embodiment of the present invention.
Figures 2A-2I illustrate cross-sectional views of various stages in the fabrication of a back-contact solar cell, corresponding to operations of flowchart 100, in accordance with an embodiment of the present invention.
[0029] Referring to operation 102 of flowchart 100, and to corresponding
Figure 2A, a method of forming emitters for a back-contact solar cell includes optionally forming a thin dielectric layer 202 on a substrate 200.
[0030] In an embodiment, the thin dielectric layer 202 is composed of silicon dioxide and has a thickness approximately in the range of 5-50 Angstroms. In one embodiment, the thin dielectric layer 202 performs as a tunneling oxide layer. In an embodiment, substrate 200 is a bulk single-crystal substrate, such as an n-type doped single crystalline silicon substrate. However, in an alternative embodiment, substrate 200 includes a polycrystalline silicon layer disposed on a global solar cell substrate.
[0031] Referring to operation 104 of flowchart 100, and to corresponding
Figure 2B, the method of forming emitters for the back-contact solar cell also includes optionally forming a polysilicon layer 204 on the thin dielectric layer 202. It is to be understood that use of the term polysilicon layer is intended to also cover material that can be described as amorphous- or a-silicon.
[0032] Referring to operation 106 of flowchart 100, and to corresponding
Figure 2C and 2D, the method of forming emitters for the back-contact solar cell includes forming (layer 205 of Figure 2C) and patterning a first solid-state dopant source 206 of a first conductivity type on the polysilicon layer 204.
[0033] In an embodiment, the patterning forms gaps 208 exposing regions of the polysilicon layer 204 between a plurality of regions of the first solid-state dopant source 206, as depicted in Figure 2D. In one embodiment, forming and patterning the first solid-state dopant source 206 includes forming and patterning a layer of boron silicate glass (BSG) or a layer of phosphorus silicate glass (PSG). In a specific embodiment, the BSG or PSG layer is formed by chemical vapor deposition as a uniform, blanket layer and then patterned by a lithography and etch process. In a particular such embodiment, the BSG or PSG layer is formed by a chemical vapor deposition technique such as, but not limited to, atmospheric pressure chemical vapor deposition (APCVD), plasma-enhanced chemical vapor deposition (PECVD), low- pressure chemical vapor deposition (LPCVD), or ultra-high vacuum chemical vapor deposition (UHVCVD). In an alternative specific embodiment, the BSG or PSG layer is deposited already having a pattern and, thus, the forming and patterning are performed simultaneously. In one such embodiment, the patterned BSG or PSG layer is formed by a screen-printing approach. It is to be understood that a solid-state dopant source is a layer of film that includes dopant impurity atoms and can be deposited above a substrate. This is in contrast to an ion implantation approach.
[0034] Referring to operation 108 of flowchart 100, and to corresponding
Figure 2E, the method of forming emitters for the back-contact solar cell also includes forming, by printing, regions of a second solid-state dopant source 210 of a second conductivity type above the substrate 200. In an embodiment, the printing is performed by a technique such as, but not limited to, ink-jet printing, screen printing, or aerosol printing. [0035] In an embodiment, the regions of the second solid-state dopant source
210 are formed in the gaps 208 of the plurality of regions of the first solid-state dopant source 206 but not in contact with the plurality of regions of the first solid-state dopant source 206, as depicted in Figure 2E. And, in a more specific embodiment, the regions of the second solid-state dopant source 210 are spaced apart from the plurality of regions of the first solid-state dopant source 206, as is also depicted in Figure 2E. In an embodiment, the first solid-state dopant source 205/206 and the second solid- state dopant source 210 are formed on the polysilicon layer 204. However, there may be applications where a polysilicon layer distinct from a substrate is not used. Thus, in another embodiment (not shown), the first solid-state dopant source 205/206 and the second solid-state dopant source 210 are formed directly on a substrate (such as directly on substrate 200) or on a thin oxide layer on the surface of the substrate.
[0036] In an embodiment, the second solid-state dopant source 210 is composed of a material such as, but not limited to, a spin-on-glass precursor material or a nanoparticle material. The spin-on-glass precursor material or the nanoparticle material includes dopants of the second conductivity type disposed therein. For example, in one embodiment, the second conductivity type is n-type and the dopant impurity atoms are phosphorus atoms. In one embodiment, the second conductivity type is p-type and the dopant impurity atoms are boron atoms.
[0037] In an embodiment, the first conductivity type is opposite the second conductivity type. For example, in one embodiment, the first conductivity type is p- type, the second conductivity type is n-type, and the first solid-state dopant source 205/206 is composed of BSC In another embodiment, the first conductivity type is n- type, the second conductivity type is p-type, and the first solid-state dopant source 205/206 is composed of PSG.
[0038] Referring to operation 110 of flowchart 100, and to corresponding
Figure 2F, in one embodiment the method of forming emitters for the back-contact solar cell optionally further includes forming trenches 212 partially into the substrate 200, between the regions of the second solid-state dopant source 210 and the plurality of regions of the first solid-state dopant source 206. In an embodiment, the trenches 212 are formed in the polysilicon layer 204, in the thin dielectric layer 202, and partially in the substrate 200, as depicted in Figure 2F.
[0039] Referring to operation 112 of flowchart 100, and to corresponding
Figure 2G, in one embodiment the method of forming emitters for the back-contact solar cell optionally further includes, subsequent to forming the trenches 212, heating 250 the substrate 200.
[0040] In an embodiment, the heating drives dopants from the first and second solid-state dopant sources 206 and 210. For example, in one embodiment, the first and second solid-state dopant sources 206 and 210 are formed on the polysilicon layer 204 and heating the substrate 200 drives dopants from the first and second solid-state dopant sources 206 and 210, respectively, into the polysilicon layer 204. However, in another embodiment, the first and second solid-state dopant sources 206 and 210 are formed directly on substrate 2090 or on a thin oxide on substrate 200, and heating the substrate 200 drives dopants from the first and second solid-state dopant sources 206 and 210, respectively, into the substrate 200. In one specific such embodiment, the substrate 200 is a bulk crystalline silicon substrate, and the first solid-state dopant source 206 and the second solid-state dopant source 210 are formed on the bulk crystalline silicon substrate. The bulk crystalline silicon substrate is then heated to drive dopants from the first and second solid-state dopant sources 206 and 210 into the bulk crystalline silicon substrate.
[0041] Referring to operation 114 of flowchart 100, and again to
corresponding Figure 2G, in one embodiment the method of forming emitters for the back-contact solar cell optionally further includes, texturizing portions 214 of the substrate 200 exposed by the trenches 212.
[0042] In an embodiment, the texturing provides a random texture pattern.
The random texturing pattern may be formed by applying an anisotropic etching process to exposed regions of substrate 200 and may thus be determined by crystal planes, such single-crystalline silicon planes, of the substrate 200. In an embodiment, the heating of operation 112 hardens the second solid-state dopant source 210. Then, during the texturizing of the portions 214 of the substrate 200 exposed by the trenches 212, the hardened second solid-state dopant source acts as a mask.
[0043] For example, in a specific embodiment, the hardened second solid-state dopant source acts as a mask to provide selectivity to hydroxide (OH ) based etching. That is, inherent in the doping process using the printed solid-state dopant source is an ability to provide a mask for texturizing operations. It is to be understood that other doping approaches, such as atmospheric pressure chemical vapor deposition (APCVD), implantation, or laser doping, may not provide routes for such masking that are inherent in the doping process. [0044] In an embodiment, referring to Figure 2H, heating the substrate 200 also includes activating the dopants from the first and second solid-state dopant sources 206 and 210, respectively, to form a plurality of polysilicon regions 220 of the second conductivity type and a plurality of polysilicon regions 222 of the first conductivity type. In one embodiment, the activating includes changing the incorporation of at least some of the dopants from interstitial to substitutional within polysilicon layer 204. In an embodiment, the first and second solid-state dopant sources 206 and 210 are also removed, as is also depicted in Figure 2H. In one such embodiment, the first and second solid-state dopant sources 206 and 210 are removed by using a wet etch technique by applying a wet solution including aqueous hydrofluoric acid or another source of HF. In another such embodiment, the first and second solid-state dopant sources 206 and 210 are removed by plasma etching.
[0045] Referring to Figure 21, in an embodiment, the method of forming emitters for the back-contact solar cell optionally further includes forming a dielectric layer 224 above the plurality of polysilicon regions 220 of the second conductivity type, above the plurality of polysilicon regions 222 of the first conductivity type, and above the exposed portions of substrate 200. In one embodiment, the dielectric layer 224 is an anti-reflective coating (ARC) layer.
[0046] Referring again to Figure 21, in an embodiment the method of forming emitters for the back-contact solar cell optionally further includes forming, by laser abalation, a plurality of contact openings 226 to the plurality of polysilicon regions 220 of the second conductivity type and to the plurality of polysilicon regions 222 of the first conductivity type. Conductive contacts 228 may then be formed in the plurality of contact openings 226 and coupled to the plurality of polysilicon regions 220 of the second conductivity type and to the plurality of polysilicon regions 222 of the first conductivity type. In an embodiment, the conductive contacts 228 are composed of metal and are formed by a deposition, lithographic, and etch approach.
[0047] In another aspect of the present invention, a second conductivity-type solid-state dopant source is formed by blanket deposition over features of a printed first conductivity-type solid-state dopant source. For example, Figure 3 illustrates a flowchart 300 representing operations in a method of forming emitters for a back- contact solar cell, in accordance with another embodiment of the present invention. Figures 2A, 2B, and 4A-4E, 21, and 2J illustrate cross-sectional views of various stages in the fabrication of a back-contact solar cell, corresponding to operations of flowchart 300, in accordance with an embodiment of the present invention.
[0048] Referring to operation 302 of flowchart 300, and to corresponding
Figure 2A, a method of forming emitters for a back-contact solar cell includes optionally forming a thin dielectric layer 202 on a substrate 200.
[0049] In an embodiment, the thin dielectric layer 202 is composed of silicon dioxide and has a thickness approximately in the range of 5-50 Angstroms. In one embodiment, the thin dielectric layer 202 performs as a tunneling oxide layer. In an embodiment, substrate 200 is a bulk single-crystal substrate, such as an n-type doped single crystalline silicon substrate. However, in an alternative embodiment, substrate 200 includes a polycrystalline silicon layer disposed on a global solar cell substrate.
[0050] Referring to operation 304 of flowchart 100, and to corresponding
Figure 3B, the method of forming emitters for the back-contact solar cell also includes optionally forming a polysilicon layer 204 on the thin dielectric layer 202. It is to be understood that use of the term polysilicon layer is intended to also cover material that can be described as amorphous- or a-silicon.
[0051] Referring to operation 306 of flowchart 300, and to corresponding
Figure 4A, the method of forming emitters for the back-contact solar cell includes forming, by printing, a first solid-state dopant source 252 of a first conductivity type above substrate 200. The first solid-state dopant source 252 includes a plurality of regions separated by gaps 254. In an embodiment, the printing is performed by a technique such as, but not limited to, ink-jet printing, screen printing, or aerosol printing.
[0052] In an embodiment, the gaps 254 expose regions of the polysilicon layer
204 between a plurality of regions of the first solid-state dopant source 252, as depicted in Figure 4A. In an embodiment, the total coverage of the first solid-state dopant source 252 above the substrate 200 is in the range of 10-15% by surface area, e.g., approximately 12%.
[0053] In an embodiment, the first solid-state dopant source 252 is composed of a material such as, but not limited to, a spin-on-glass precursor material or a nanoparticle material. The spin-on-glass precursor material or the nanoparticle material includes dopants of the first conductivity type disposed therein. For example, in one embodiment, the first conductivity type is n-type and the dopant impurity atoms are phosphorus atoms. In one embodiment, the first conductivity type is p-type and the dopant impurity atoms are boron atoms.
[0054] Referring to operation 308 of flowchart 300, and to corresponding
Figure 4B, the method of forming emitters for the back-contact solar cell also includes forming, by chemical vapor deposition, a second solid-state dopant source 256 of a second conductivity type above the first solid-state dopant source 252. The second solid-state dopant source 256 is also formed above the substrate 200, in the gaps 254 of the plurality of regions of the first solid-state dopant source 252.
[0055] In an embodiment, forming the second solid-state dopant source 256 includes forming a layer of BSG or a layer of PSG. In one such embodiment, the BSG or PSG layer is formed by chemical vapor deposition as a uniform, blanket layer. In an embodiment, the second conductivity type is opposite the first conductivity type. For example, in one embodiment, the first conductivity type is p-type, the second conductivity type is n-type, and the second solid-state dopant source 256 is composed of PSG. In another embodiment, the first conductivity type is n-type, the second conductivity type is p-type, and the second solid-state dopant source 256 is composed of BSG.
[0056] In an embodiment, the first solid-state dopant source 252 and portions of the second solid-state dopant source 256 are formed on the polysilicon layer 204. However, there may be applications where a polysilicon layer distinct from a substrate is not used. Thus, in another embodiment (not shown), the first solid-state dopant source 252 and the portions of the second solid-state dopant source 256 are formed directly on a substrate (such as directly on substrate 200) or on a thin oxide layer on the surface of the substrate.
[0057] Referring to operation 310 of flowchart 300, and to corresponding
Figure 4C, the method of forming emitters for the back-contact solar cell also includes patterning the second solid-state dopant source 256 to form first regions 258 of the second solid-state dopant source 256 in the gaps 254 of the plurality of regions of the first solid-state dopant source 252 but not in contact with the plurality of regions of the first solid-state dopant source 252. Furthermore, second regions 260 of the second solid-state dopant source 256 are also formed on the plurality of regions of the first solid-state dopant source 252, as depicted in Figure 4C.
[0058] In an embodiment, patterning the second solid-state dopant source 256 includes patterning a layer of BSG or a layer of PSG. In a specific embodiment, the BSG or PSG layer is formed by chemical vapor deposition as a uniform, blanket layer and then patterned by a lithography and etch process. In a particular such
embodiment, the BSG or PSG layer is formed by a chemical vapor deposition technique such as, but not limited to, atmospheric pressure chemical vapor deposition (APCVD), plasma-enhanced chemical vapor deposition (PECVD), low-pressure chemical vapor deposition (LPCVD), or ultra-high vacuum chemical vapor deposition (UHVCVD). In an embodiment, the first regions 258 of the second solid-state dopant source 256 are formed in the gaps 254 of the plurality of regions of the first solid-state dopant source 252 but not in contact with the plurality of regions of the first solid- state dopant source 252, as depicted in Figure 4C. And, in a more specific embodiment, the first regions 258 of the second solid-state dopant source 256 are spaced apart from the plurality of regions of the first solid-state dopant source 252, as is also depicted in Figure 4C.
[0059] Referring to operation 312 of flowchart 300, and to corresponding
Figure 4D, in one embodiment the method of forming emitters for the back-contact solar cell optionally further includes forming trenches 262 partially into the substrate 200, between the first regions 258 of the second solid-state dopant source 256 and the plurality of regions of the first solid-state dopant source 252. In an embodiment, the trenches 262 are formed in the polysilicon layer 204, in the thin dielectric layer 202, and partially in the substrate 200, as depicted in Figure 4D.
[0060] Referring to operation 314 of flowchart 300, and to corresponding
Figure 4E, in one embodiment the method of forming emitters for the back-contact solar cell optionally further includes, subsequent to forming the trenches 262, heating 250 the substrate 200.
[0061] In an embodiment, the heating drives dopants from the first and second solid-state dopant sources 252 and 258. For example, in one embodiment, the first and second solid-state dopant sources 252 and 258 are formed on the polysilicon layer 204 and heating the substrate 200 drives dopants from the first and second solid-state dopant sources 252 and 258, respectively, into the polysilicon layer 204. However, in another embodiment, the first and second solid-state dopant sources 252 and 258 are formed directly on substrate 200 or on a thin oxide on substrate 200, and heating the substrate 200 drives dopants from the first and second solid-state dopant sources 252 and 258, respectively, into the substrate 200. In one specific such embodiment, the substrate 200 is a bulk crystalline silicon substrate, and the first solid-state dopant source 252 and the second solid-state dopant source 258 are formed on the bulk crystalline silicon substrate. The bulk crystalline silicon substrate is then heated to drive dopants from the first and second solid-state dopant sources 252 and 258 into the bulk crystalline silicon substrate.
[0062] In accordance with an embodiment of the present invention, the first solid-state dopant source 252 is sufficiently thick to block driving of dopants from the second regions 260 of the second solid-state dopant source 256 through the plurality of regions of the first solid-state dopant source 252. For example, while it may be desirable to drive dopants from the first regions 258 of the second solid-state dopant source 256 into an underlying polysilicon layer or substrate, it may not be desirable for dopants to be driven from the second regions 260 of the second solid-state dopant source 256 into the underlying polysilicon layer or substrate. Instead, in those regions, it may only be desirable to drive dopants from the first solid-state dopant source 252, underlying the second regions 260 of the second solid-state dopant source 256, into the underlying polysilicon layer or substrate. Beyond thickness, other parameters that may be considered for adequate blocking of the driving of dopants from the second regions 260 of the second solid-state dopant source 256 through the plurality of regions of the first solid-state dopant source 252 include, but need not be limited to, dopant concentration in the first solid-state dopant source 252, density of the first solid-state dopant source 252, identity of dopant species, and timing for the heating operation 314.
[0063] Referring to operation 316 of flowchart 300, and again to
corresponding Figure 4E, in one embodiment the method of forming emitters for the back-contact solar cell optionally further includes, texturizing portions 264 of the substrate 200 exposed by the trenches 262.
[0064] In an embodiment, the texturing provides a random texture pattern.
The random texturing pattern may be formed by applying an anisotropic etching process to exposed regions of substrate 200 and may thus be determined by crystal planes, such single-crystalline silicon planes, of the substrate 200. In an embodiment, the heating of operation 314 hardens the first solid-state dopant source 252. Then, during the texturizing of the portions 264 of the substrate 200 exposed by the trenches 262, the hardened first solid-state dopant source acts as a mask.
[0065] In an embodiment, referring to Figure 2H, heating the substrate 200 also includes activating the dopants from the first and second solid-state dopant sources 252 and 258, respectively, to form a plurality of polysilicon regions 220 of the first conductivity type and a plurality of polysilicon regions 222 of the second conductivity type. In one embodiment, the activating includes changing the incorporation of at least some of the dopants from interstitial to substitutional within polysilicon layer 204. In an embodiment, the first and second solid-state dopant sources 252 and 258 are also removed, as is also depicted in Figure 2H. In one such embodiment, the first and second solid-state dopant sources 252 and 258 are removed by using a wet etch technique by applying a wet solution including aqueous hydrofluoric acid or another source of HF. In another such embodiment, the first and second solid-state dopant sources 252 and 258 are removed by plasma etching.
[0066] Referring to Figure 21, in an embodiment, the method of forming emitters for the back-contact solar cell optionally further includes forming a dielectric layer 224 above the plurality of polysilicon regions 220 of the first conductivity type, above the plurality of polysilicon regions 222 of the second conductivity type, and above the exposed portions of substrate 200. A plurality of contact openings 226 may then be formed, exposing the plurality of polysilicon regions 220 of the first conductivity type and to the plurality of polysilicon regions 222 of the second conductivity type. Conductive contacts 228 may then be formed in the plurality of contact openings 226 and coupled to the plurality of polysilicon regions 220 of the first conductivity type and to the plurality of polysilicon regions 222 of the second conductivity type.
[0067] It is to be understood that the operations of flowcharts 100 and 300 and the corresponding Figures are provided in the particular sequences for illustrative purposes. Other embodiments contemplated within the spirit and scope of the invention may include different fabrication sequences. For example, the specific ordering illustrated may be altered or varied, unless specified for particular embodiments. For example, the formation of trenches in the method described in association with flowchart 300 may, in other embodiments, not be necessary.
[0068] Thus, methods of forming emitters for back-contact solar cells have been disclosed. In accordance with an embodiment of the present invention, a method includes forming, by chemical vapor deposition, a first solid-state dopant source of a first conductivity type above a substrate, the first solid-state dopant source including a plurality of regions separated by gaps. The method also includes forming, by printing, regions of a second solid-state dopant source of a second conductivity type above the substrate, in the gaps of the plurality of regions of the first solid-state dopant source but not in contact with the plurality of regions of the first solid-state dopant source, wherein the first conductivity type is opposite the second conductivity type. In one embodiment, the second solid-state dopant source is composed of a spin- on-glass precursor material or a nanoparticle material. In one embodiment, the first conductivity type is p-type, the second conductivity type is n-type, and the first solid- state dopant source is composed of boron silicate glass (BSG). In one embodiment, the first conductivity type is n-type, the second conductivity type is p-type, and the first solid-state dopant source is composed of phosphorus silicate glass (PSG).

Claims

CLAIMS What is claimed is:
1. A method of forming emitters for a back-contact solar cell, the method comprising:
forming, by chemical vapor deposition, a first solid-state dopant source of a first conductivity type above a substrate, the first solid-state dopant source comprising a plurality of regions separated by gaps; and
forming, by printing, regions of a second solid-state dopant source of a second conductivity type above the substrate, in the gaps of the plurality of regions of the first solid-state dopant source but not in contact with the plurality of regions of the first solid-state dopant source, wherein the first conductivity type is opposite the second conductivity type.
2. The method of claim 1, wherein the regions of the second solid-state dopant source are spaced apart from the plurality of regions of the first solid-state dopant source, the method further comprising:
forming trenches partially into the substrate, between the regions of the second solid-state dopant source and the plurality of regions of the first solid-state dopant source; and
heating the substrate to drive dopants from the first and second solid-state dopant sources, wherein the heating hardens the second solid-state dopant source.
3. The method of claim 2, further comprising:
subsequent to the forming the trenches and to the heating, texturizing portions of the substrate exposed by the trenches, wherein the hardened second solid-state dopant source acts as a mask during the texturizing.
4. The method of claim 1, further comprising:
prior to forming the first solid-state dopant source, forming a thin dielectric layer on the substrate; and
forming a polysilicon layer on the thin dielectric layer, wherein the first solid-state dopant source and the second solid-state dopant source are formed on the polysilicon layer.
5. The method of claim 4, further comprising:
heating the substrate to drive dopants from the first and second solid-state dopant sources into the polysilicon layer.
6. The method of claim 1, wherein the substrate is a bulk crystalline silicon substrate, and wherein the first solid-state dopant source and the second solid-state dopant source are formed on the bulk crystalline silicon substrate.
7. The method of claim 6, further comprising:
heating the bulk crystalline silicon substrate to drive dopants from the first and second solid-state dopant sources into the bulk crystalline silicon substrate.
8. The method of claim 1, wherein the second solid-state dopant source comprises a spin-on-glass precursor material or a nanoparticle material.
9. The method of claim 1, wherein the first conductivity type is p-type, the second conductivity type is n-type, and the first solid-state dopant source comprises boron silicate glass (BSG).
10. The method of claim 1, wherein the first conductivity type is n-type, the second conductivity type is p-type, and the first solid-state dopant source comprises phosphorus silicate glass (PSG).
11. A solar cell fabricated according to the method of claim 1.
12. A method of forming emitters for a back-contact solar cell, the method comprising:
forming, by printing, a first solid-state dopant source of a first conductivity type above a substrate, the first solid-state dopant source comprising a plurality of regions separated by gaps;
forming, by chemical vapor deposition, a second solid-state dopant source of a second conductivity type above the first solid-state dopant source, and above the substrate in the gaps of the plurality of regions of the first solid-state dopant source, wherein the first conductivity type is opposite the second conductivity type; and
patterning the second solid-state dopant source to form first regions of the second solid-state dopant source in the gaps of the plurality of regions of the first solid-state dopant source but not in contact with the plurality of regions of the first solid-state dopant source, and second regions of the second solid-state dopant source on the plurality of regions of the first solid-state dopant source, wherein the first solid-state dopant source is sufficiently thick to block driving of dopants from the second regions of the second solid-state dopant source through the plurality of regions of the first solid-state dopant source.
13. The method of claim 12, wherein the first regions of the second solid-state dopant source are spaced apart from the plurality of regions of the first solid-state dopant source, the method further comprising:
forming trenches partially into the substrate, between the first regions of the
second solid-state dopant source and the plurality of regions of the first solid- state dopant source; and
heating the substrate to drive dopants from the first solid-state dopant source and the first regions of the second solid-state dopant source, wherein the heating hardens the first solid-state dopant source.
14. The method of claim 13, further comprising:
subsequent to the forming the trenches and to the heating, texturizing portions of the substrate exposed by the trenches, wherein the hardened first solid-state dopant source acts as a mask during the texturizing.
15. The method of claim 12, further comprising:
prior to forming the first solid-state dopant source, forming a thin dielectric layer on the substrate; and
forming a polysilicon layer on the thin dielectric layer, wherein the first solid-state dopant source and the second solid-state dopant source above the substrate in the gaps of the plurality of regions of the first solid-state dopant source are formed on the polysilicon layer.
16. The method of claim 15, further comprising:
heating the substrate to drive dopants from the first solid-state dopant source and the first regions of the second solid-state dopant source into the polysilicon layer.
17. The method of claim 12, wherein the substrate is a bulk crystalline silicon substrate, and wherein the first solid-state dopant source and the second solid-state dopant source above the substrate in the gaps of the plurality of regions of the first solid-state dopant source are formed on the bulk crystalline silicon substrate.
18. The method of claim 17, further comprising:
heating the bulk crystalline silicon substrate to drive dopants from the first solid- state dopant source and the first regions of the second solid-state dopant source into the bulk crystalline silicon substrate.
19. The method of claim 12, wherein the first solid-state dopant source comprises a spin-on-glass precursor material or a nanoparticle material.
20. The method of claim 12, wherein the first conductivity type is n-type, the second conductivity type is p-type, and the second solid-state dopant source comprises boron silicate glass (BSG).
21. The method of claim 12, wherein the first conductivity type is p-type, the second conductivity type is n-type, and the second solid-state dopant source comprises phosphorus silicate glass (PSG).
22. A solar cell fabricated according to the method of claim 12.
PCT/US2012/025264 2011-04-25 2012-02-15 Method of foring emitters for a back-contact solar cell WO2012148523A1 (en)

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