EP2577750A2 - Traitement laser pour fabrication de cellules solaires en silicium cristallin mince à efficacité élevée - Google Patents

Traitement laser pour fabrication de cellules solaires en silicium cristallin mince à efficacité élevée

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Publication number
EP2577750A2
EP2577750A2 EP11787543.5A EP11787543A EP2577750A2 EP 2577750 A2 EP2577750 A2 EP 2577750A2 EP 11787543 A EP11787543 A EP 11787543A EP 2577750 A2 EP2577750 A2 EP 2577750A2
Authority
EP
European Patent Office
Prior art keywords
silicon
laser
approximately
crystalline silicon
oxide
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP11787543.5A
Other languages
German (de)
English (en)
Other versions
EP2577750A4 (fr
Inventor
Virendra V. Rana
Jian Jun Liang
Pranav Anbalagan
Mehrdad M. Moslehi
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Beamreach Solexel Assets Inc
Original Assignee
Solexel Inc
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Filing date
Publication date
Application filed by Solexel Inc filed Critical Solexel Inc
Publication of EP2577750A2 publication Critical patent/EP2577750A2/fr
Publication of EP2577750A4 publication Critical patent/EP2577750A4/fr
Withdrawn legal-status Critical Current

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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
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/042—PV modules or arrays of single PV cells
    • H01L31/0445—PV modules or arrays of single PV cells including thin film solar cells, e.g. single thin film a-Si, CIS or CdTe solar cells
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06—Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/062—Shaping the laser beam, e.g. by masks or multi-focusing by direct control of the laser beam
    • B23K26/0622—Shaping the laser beam, e.g. by masks or multi-focusing by direct control of the laser beam by shaping pulses
    • B23K26/0624—Shaping the laser beam, e.g. by masks or multi-focusing by direct control of the laser beam by shaping pulses using ultrashort pulses, i.e. pulses of 1ns or less
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/08—Devices involving relative movement between laser beam and workpiece
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/36—Removing material
    • B23K26/38—Removing material by boring or cutting
    • B23K26/382—Removing material by boring or cutting by boring
    • B23K26/389—Removing material by boring or cutting by boring of fluid openings, e.g. nozzles, jets
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/36—Removing material
    • B23K26/40—Removing material taking account of the properties of the material involved
    • 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
    • H01L31/18—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
    • H01L31/182—Special manufacturing methods for polycrystalline Si, e.g. Si ribbon, poly Si ingots, thin films of polycrystalline Si
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2101/00—Articles made by soldering, welding or cutting
    • B23K2101/36—Electric or electronic devices
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
    • B23K2103/16—Composite materials, e.g. fibre reinforced
    • B23K2103/166—Multilayered materials
    • B23K2103/172—Multilayered materials wherein at least one of the layers is non-metallic
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
    • B23K2103/30—Organic material
    • B23K2103/40—Paper
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
    • B23K2103/50—Inorganic material, e.g. metals, not provided for in B23K2103/02 – B23K2103/26
    • 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
    • Y02E10/546—Polycrystalline 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
    • 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

  • This disclosure relates in general to the field of solar photovoltaics, and more particularly to laser processing techniques for the production of crystalline semiconductor, including crystalline silicon, and other types of photovoltaic solar cells.
  • Laser processing offers several advantages in terms of efficiency enhancement and manufacturing cost reduction for high-performance, high-efficiency solar cell processing.
  • advanced crystalline silicon solar cells may benefit from having the dimensions of the critical features such as electrical contacts be much smaller than the current industrial practice.
  • the contact area of the front metallization to the emitter as well as the contact area of the back metal to the base needs to be low (or the contact area ratios should be fairly small, preferably much below 10%).
  • the dimensions of the various features are typically small for high efficiency.
  • the width of these regions (in particular the width of the base contact) tends to be small. Also, the dimensions of the metal contacts to these regions tend to be proportionally small.
  • the metallization connecting to the emitter and base regions then needs to be patterned to a correspondingly finer scale.
  • lithography and laser processing are the technologies that have the relatively fine resolution capability to provide the small dimensions and the control required. Of these techniques, only laser processing offers the low cost advantage required in solar cell making.
  • laser processing is a non-contact, dry, direct write method and does not require any material consumables, making it a simpler and lower cost process for solar cell fabrication.
  • laser processing is an excellent choice for environmentally benign manufacturing since it is an all-dry process which does not use any consumables such as chemicals.
  • laser processing presents the possibility of very high productivity with a very low cost of ownership.
  • Laser processing schemes are disclosed that meet the requirements of base to emitter isolation (including but not limited to shallow trench isolation) for all back-contact homo-junction emitter solar cells (such as high-efficiency back-contact crystalline silicon solar cells), opening for base doping, and base and emitter contact opening (with controlled small contact area ratios, for instance substantially below 10% contact area ratio, for reduced contact recombination losses and increased cell efficiency), selective doping (such as for base and/or emitter contact doping), and metal ablation (formation of patterned metallization layers such as creating the patterned metallization seed layer on a thin-film monocrystalline silicon solar cell prior to subsequent attachment of a backplane to the cell and its release from a reusable host template) for both front-contact and all back-contact/back-junction homo- junction emitter solar cells.
  • base to emitter isolation including but not limited to shallow trench isolation
  • back-contact homo-junction emitter solar cells such as high-efficiency back-contact crystalline silicon solar cells
  • opening for base doping and base and emitter contact opening (with
  • laser processing schemes are disclosed that are suitable for selective amorphous silicon ablation and oxide (such as a transparent conductive oxide) ablation, and metal ablation for metal patterning for hetero-junction solar cells (such as back- contact solar cells comprising hetero-junction amorphous silicon emitter on monocrystalline silicon base).
  • These laser processing techniques may be applied to semiconductor substrates, including crystalline silicon substrates, and further including crystalline silicon substrates which are manufactured either through wire saw wafering methods or using epitaxial deposition processes, that are either planar or textured/three-dimensional, where the three- dimensional substrates may be obtained using epitaxial silicon lift-off techniques using porous silicon seed/release layers or other types of sacrificial release layers.
  • These techniques are highly suited to thin crystalline semiconductor, including thin crystalline silicon films obtained using epitaxial silicon deposition on a template comprising a porous silicon release layer or other techniques known in the industry and can have any crystalline silicon absorber thickness in the range of from less than one micron to more than 100 microns (with the thin- film monocrystalline silicon solar cells preferably having a silicon thickness of less than 50 microns).
  • An all back-contact homo-junction solar cell is formed in the crystalline silicon substrate, wherein laser processing is used to perform one or a combination of the following: micromachine or pattern the emitter and base regions including base to emitter isolation as well as openings for base, provide selective doping of emitter and base, make openings to base and emitter for metal contacts, and provide metal patterning.
  • a front contacted homo- junction (emitter) solar cell may be made using laser processing for selective doping of emitter and making openings for metal contacts for both frontside and backside metallization.
  • a hetero-junction all back-contact back-contact solar cell may be made using laser processing for defining the base region and conductive oxide isolation.
  • FIG. 1 shows a scanning electron microscope (SEM) image of a shallow trench made in silicon for application in an all back contact solar cell, in accordance with the present disclosure
  • FIG. 2 shows a profile of a shallow trench in silicon for application in all back contact solar cells
  • FIGS. 3A-3D show the procedure for selecting the laser fluence to obtain damage-free silicon dioxide (or oxide) ablation.
  • FIG. 3A shows the dependence of the size of the ablation spot on the laser fluence;
  • FIG. 3B shows irregular delamination of oxide;
  • FIG. 3C shows a damage-free spot; and
  • FIG. 3D shows highly damaged silicon in the spot opening;
  • FIG. 4 shows substantially parallel rows of contacts opened in oxide using pulsed laser ablation in accordance with the present disclosure
  • FIG. 5 shows a screenshot with oxide ablation spots for metal contacts
  • FIG. 6A and 6B show the laser-ablated area formed by making ablation spots that are overlapped in both the x and y-direction;
  • FIG. 6A shows a 18 micron wide strip opened in 1000A BSG (boron-doped oxide)/500A USG (undoped oxide) for base isolation region;
  • FIG. 6B shows a 90 micron wide stripe opened in 1000A USG (undoped oxide) for base region;
  • FIG. 7A shows the threshold for oxide damage, below which metal can be removed without metal penetration of the oxide layer
  • FIG. 7B shows that after 20 scans the metal runners are fully isolated
  • FIG. 7C shows an optical micrograph of the trench formed in this metal stack
  • FIGS. 8A and 8B show a top view and a cross-sectional view of a pyramidal TFSC
  • FIGS. 9A and 9B show a top view and a cross-sectional view of a prism TFSC
  • FIGS. 10A and 10B show a process flow for creation and release of a planar epitaxial thin film silicon solar cell substrate (TFSS);
  • TFSS planar epitaxial thin film silicon solar cell substrate
  • FIGS. 11A and 11B show a process flow for planar epitaxial thin film silicon solar cell substrate in case the TFSS is too thin to be free standing or self-supporting;
  • FIGS. 12A and 12B show a process flow for micromold template (or reusable template) creation for making a 3-D TFSS ;
  • FIGS. 12C and 12D show a process flow for 3-D TFSS creation using the reusable micromold template
  • FIGS. 13 shows a process flow for making a planar front contacted solar cell where the TFSS is thick enough to be free standing and self-supporting (e.g. thicker than approximately 50 microns for smaller 100 mm x 100 mm substrates and thicker than approximately 80 microns for 156 mm x 156 mm substrates), in accordance with the present disclosure;
  • FIGS. 14 shows a process flow for making a planar front contact solar cell where the TFSS is too thin to be self supporting, in accordance with the present disclosure
  • FIG. 15 shows a process flow for making a 3-D front contact solar cell in accordance with the present disclosure
  • FIGS. 16A-16D show a process flow for making an interdigitated back contact back-junction solar cell where the TFSS is thick enough to be self supporting, in accordance with the present disclosure
  • FIG. 17 shows a process flow for making an interdigitated back-contact back- junction solar cell using thick TFSS where the in-situ emitter is not deposited. Instead, a BSG (boron-doped oxide) layer is deposited on the epitaxial silicon film and patterned to open the base isolation region, in accordance with the present disclosure;
  • BSG boron-doped oxide
  • FIG. 18 shows a process flow for making an interdigitated back-contact back- junction solar cell where the TFSS is not thick enough to be self supporting, where in-situ emitter and laser ablation of silicon is used to form the base isolation opening, in accordance with the present disclosure
  • FIGS. 19A-19H show a process flow for making an interdigitated back-contact back-junction solar cell where the TFSS is not thick enough to be self supporting, and where instead of in-situ emitter BSG (boron-doped oxide) deposition and selective laser etchback is used to form the base isolation opening, in accordance with the present disclosure;
  • BSG boron-doped oxide
  • FIG. 20 shows a process flow for making an interdigitated back-contact back- junction solar cell using a 3-D TFSS, in accordance with the present disclosure.
  • FIG. 21 shows a process flow for making an interdigitated back-contact back- junction hetero-junction solar cell, in accordance with the present disclosure.
  • the disclosed methods may be useful in the area of semiconductor device ablation, particularly crystalline silicon ablation.
  • removal of silicon with a laser involves silicon melting and evaporation that leaves undesirable residual damage in the silicon substrate.
  • This damage causes minority carrier lifetime degradation and increased surface recombination velocity (SRV) that reduces the solar cell efficiency.
  • SSV surface recombination velocity
  • wet cleaning of the silicon substrate is typically used to remove this damage layer.
  • the damage remaining in the silicon substrate upon ablating a certain thickness of it using a laser is related to the amount of laser energy absorbed in the substrate that is not used by the ablated material. If it can be managed that most of the laser energy is used in removing the material then the fraction of the incident energy that seeps into the silicon substrate is minimized, thus minimizing the laser-induced substrate damage and SRV degradation.
  • the penetration of laser energy into silicon depends on the laser pulse length (also called pulse width) and wavelength.
  • the infrared (IR) laser beam, wavelength 1.06 microns has a long penetration depth in silicon, up to about 1000 microns, while green laser beam, with a wavelength of 532 nm, penetrates only to a depth of approximately 3 to 4 microns.
  • UV laser beam with a wavelength of 355 nm, is even shorter, only about 10 nm. It is clear that using ultra-short pulses of UV or EUV wavelength limits the penetration of the laser energy into silicon. Additionally, shorter laser pulse length results in shorter diffusion of heat into silicon. While a nanoseconds pulse can lead to heat diffusion in silicon to approximately 3 to 4 microns range, the picoseconds pulse reduces it to about 80 to 100 nm, while a femtoseconds pulse is so short that typically there is no heat diffusion into silicon during the laser ablation process. Hence going to shorter pulses with a shorter wavelength lead to diminishing damage to the laser-ablated substrate. For higher production throughput, green or IR wavelengths can be used depending on the extent of laser damage acceptable.
  • FIG. 1 shows a 2.25 micron deep and nearly 100 micron wide trench made in a silicon substrate using a picoseconds UV laser beam of Gaussian profile (M 2 ⁇ 1.3), nearly 110 microns in diameter with 4 microjoule pulse energy, the laser spots overlapped nearly 15 times. . This depth of ablation was obtained using twenty overlapped scans of the laser with each scan removing about 112 nm of silicon.
  • FIG. 1 shows a 2.25 micron deep and nearly 100 micron wide trench made in a silicon substrate using a picoseconds UV laser beam of Gaussian profile (M 2 ⁇ 1.3), nearly 110 microns in diameter with 4 microjoule pulse energy, the laser spots overlapped nearly 15 times. . This depth of ablation was obtained using twenty overlapped scans of the laser with each scan removing about 112 nm of silicon.
  • FIG. 1 shows a 2.25 micron deep and nearly 100 micron wide trench made in a silicon substrate using a picoseconds UV laser beam of Gaussian profile (M 2 ⁇ 1.3),
  • FIG. 2 shows the smooth profile of a 4 micron deep and 110 micron wide trench in silicon obtained using the same picoseconds laser beam with the UV wavelength.
  • the smoothness of the profile should be noted.
  • Such an ablation of silicon is used in all back-contact back-junction solar cells to form regions that isolate base regions from emitter regions.
  • Use of a femtoseconds laser may provide further reduction of laser damage during silicon ablation.
  • the embodiments of this disclosure are also applicable to the ablation of amorphous silicon.
  • a similar scheme may be used to ablate a desired thickness of amorphous silicon using a pulsed laser beam with femtoseconds pulse length and in some embodiments a UV or green wavelength. Since ablation of amorphous silicon requires much lower energy than crystalline silicon, such a scheme may effectively be used to selectively ablate amorphous silicon films from the crystalline silicon surface for application to hetero-junction solar cells.
  • This disclosure is also applicable to oxide ablation selective to the underlying substrate, crystalline or amorphous silicon.
  • the oxide film is transparent to laser beams of wavelength down to UV. If a nanoseconds pulse length laser is used to remove the overlying oxide, the removal of oxide takes place by heating and melting of silicon underneath.
  • FIGS. 3A-3D disclose the procedure for obtaining damage-free ablation of oxide.
  • FIG. 3A shows the variation of laser spot opening in a 1000A PSG (phosphorus-doped oxide)/500A USG (undoped oxide) stack on a 35 micron thick epitaxial silicon film on a template, using a picoseconds UV laser beam.
  • the oxide layers were deposited using APCVD (atmospheric -pressure CVD) technique.
  • APCVD atmospheric -pressure CVD
  • the spot size depends on the laser fluence (J/cm 2 ).
  • the laser fluence is the laser pulse energy divided by the area of the laser beam. In this case, the laser beam was about 100 microns in diameter with a Gaussian profile (M 2 ⁇ 1.3).
  • FIG. 4 shows rows of cell contact openings that are selectively opened in the oxide for application in all back-contact (and back-junction) solar cells.
  • FIG. 5 is a close-up of these contacts.
  • the laser ablation spots can be overlapped in both x and y direction to open up an area of any desired length and width on the wafer as shown in FIGS. 6A and 6B.
  • FIG. 6A shows a 180 micron wide opening made by selectively removing the BSG (boron-doped oxide) for base isolation region using picoseconds UV laser beam with ablation spots overlapping in both x and y-direction.
  • FIG. 6B shows a 90 micron wide area opened up in USG (undoped oxide) for forming the base region.
  • the selective ablation of oxide from a silicon surface as disclosed here can be used in solar cell making in several ways.
  • this process is used to open tracks in an oxide film to expose the underlying emitter.
  • the emitter so exposed may be removed using wet etching.
  • This region is then used for base to emitter isolation and with base formed inside it.
  • this process is used to open regions that are then used for making metal contacts.
  • the oxide passivation can be used on the backside of the cells.
  • the scheme described here is then used to open contacts for the metal that is subsequently deposited on these contacts. In this manner, the metal has localized contact that is conducive to high cell efficiency.
  • contacts for both base and emitter may be opened using this scheme.
  • a doped oxide may need to be removed without causing any doping of the silicon underneath (i.e., without any appreciable heating of the doped oxide and silicon structure). Since, as described above, the oxide is removed by separation at the oxide/silicon substrate interface when using a picoseconds laser beam, the removal of oxide happens with limited pickup of the dopant from the oxide film being ablated.
  • SiN x silicon nitride
  • the selective ablation of silicon nitride (SiN x ) is used for front contacted solar cells. Using laser ablation, the contact area to the emitter surface can be reduced thereby minimizing the area where the SiN passivation is removed. This leads to higher Voc- Picosecond lasers with either UV or green wavelength are suitable for this application, although nanoseconds UV lasers can also be used.
  • FIGS. 7A-7C shows the ablation results when patterning a PVD-deposited bi- layer stack of 2400A of NiV on 1200A of Al on oxide. It is desired that the metal be removed completely between the runners without breaking through the oxide layer underneath (to prevent shunts in the cell).
  • FIG. 7A shows the threshold for pulse energy, below which this metal stack can be removed without penetration of oxide. This threshold, besides depending on the metal stack characteristics described above, depends on the laser parameters such as spot overlap obtained using a certain pulse repetition rate of the laser as well as the scan speed. With increasing pulse overlap the threshold pulse energy would decrease, because of the energy accumulation in the metal.
  • FIG. 7A shows the ablation results when patterning a PVD-deposited bi- layer stack of 2400A of NiV on 1200A of Al on oxide. It is desired that the metal be removed completely between the runners without breaking through the oxide layer underneath (to prevent shunts in the cell).
  • FIG. 7A shows the threshold for pulse energy, below which this metal stack can be removed without penetration of oxide.
  • FIG. 7B shows that using a pulse energy below the threshold for oxide damage, more than twenty scans provided complete isolation of metal runners as determined by the lOOM-ohm resistance between parallel lines.
  • FIG. 7C shows a clean 75 micron trench formed in the 2400A NiV/1200 Al metal stack.
  • the third scheme of metal ablation is applicable to highly reflective films, for example Al/Ag stack (with Al in contact with the cell and Ag on top of Al), such that most of the incident energy of the picoseconds laser is reflected and ablation is drastically reduced.
  • the surface of the reflective metal (Ag) is first dented using a long pulse length nanoseconds laser, pulse length from 10 to 800 nanoseconds, followed by picoseconds cleanup of the aluminum underneath.
  • This disclosure is also applicable to the selective doping of a substrate.
  • the pulse energy should be high enough to melt the silicon but not high enough to ablate it or the dopant layer above it. As the silicon melts, the dopant is dissolved into it.
  • the laser processing techniques described above are applicable to planar and 3-D thin-film crystalline silicon substrates.
  • the laser processes described here are suitable for any thickness of the silicon substrate. These include the current standard wafer thickness of >150 microns used for crystalline silicon solar cells. However, they become even more advantageous for thin, fragile wafers or substrates as the process in carried out without any contact with the substrate.
  • wafers thinner than 150 micron obtained from monocrystalline CZ ingots or multi-crystalline bricks using advanced wire sawing techniques or by other techniques such as hydrogen implantation followed by annealing to separate the desired thickness of wafer, or thin-film monocrystalline substrates (such as in the thickness range of from a few microns up to 80 microns) obtained using epitaxial deposition of silicon on a sacrificial separation / release layer such as porous silicon and its subsequent lift off.
  • FIGS. 8A through 9B show the 3-D thin film silicon substrates obtained using the technique described in that publication.
  • FIG. 8A shows the top view while FIG. 8B shows the cross-section of the TFSS so obtained.
  • the tips may be flat or may end in a sharp point.
  • FIGS. 9A and 9B show the TFSS with prism structure obtained using a reusable pre-structured 3D template described in the reference above.
  • the laser processes and the process flows described here are applicable to any thickness of the silicon substrate (from less than one micron to over 100 microns), we disclose here their application to solar cells made using thin silicon substrates in the thickness range of from less than 1 micron to about 80 microns, including but not limited to those that are obtained using epitaxial silicon on porous silicon surface of a reusable template as described in the '713 Application.
  • a desired thickness e.g. from about less than 10 microns up to about 120 microns
  • FIGS. 12A-12D show the process flow for obtaining three-dimensional pyramidal silicon substrates. Three-dimensional prism-shaped substrates can be obtained with similar processes, but using a lithography or screen printed pattern that provides for that structure.
  • the thin planar substrate obtained using the process flow of FIGS. 10A and 10B may be processed according to the process flow of FIG. 13 to obtain high efficiency front contacted solar cells. It should be noted for self-supporting TFSSs it is advantageous to process the template side of the TFSS first before proceeding to the other side. Since the template side of the TFSS is textured during the removal of the quasi-monocrystalline silicon remaining on the TFSS after its separation from the template it is preferably the frontside or sunnyside of the solar cell.
  • the laser processes of selective ablation of silicon oxide and silicon nitride (SiN) are used to advantage in making this front contacted solar cell.
  • FIG. 14 shows the application of various laser processes for making high efficiency front contacted solar cells using planar TFSSs where the TFSS is too thin to be free standing or self supporting during cell processing. It should be noted that in this case the non- template side surface is processed first with the TFSS on the template. Once this processing is complete the TFSS is first attached to a reinforcement plate or sheet (also called a backplane) on the exposed processed side and then separated from the template.
  • a reinforcement plate or sheet also called a backplane
  • FIG. 15 shows the application of various laser processes for making high efficiency front contacted solar cells using 3-D front TFSS.
  • pyramid tips on the template side not be sharp but end in flat ledges.
  • FIGS. 16A-16D show the laser processes used on the planar epitaxial substrate to make a back-contact/back-junction solar cell where the TFSS is self supporting (i.e., no backplane attachment to the cell).
  • the epitaxial emitter is deposited in-situ during silicon epitaxy following the deposition of the epitaxial silicon base.
  • the ablation of silicon is then used to remove the emitter from the base isolation regions.
  • four fiducials are etched into oxide to align subsequent ablation to this pattern.
  • a thermal oxide is grown to passivate the silicon surface that will become the back surface of the back-contact back-junction solar cell.
  • the epitaxial silicon film is then disconnected or released from the template (by mechanical release from the porous silicon interface).
  • the residual porous silicon layer is wet etched and the surface is textured (both can be done using an alkaline etch process). This will become the textured front surface or the sunnyside of the solar cell.
  • the thermal oxide is ablated using a picoseconds UV laser to form base openings inside the base isolation region.
  • the base opening is aligned inside the base isolation region (trench) formed by silicon ablation earlier using the fiducials that were etched in silicon earlier as mentioned above.
  • a phosphorous containing oxide layer (PSG) is blanket deposited on the surface.
  • Scanning with a nanosecond green or IR laser aligned to base opening using the fiducials in silicon causes the base to be doped. Also, the region that will have the contact openings to emitter is also doped in a similar manner using the aligned scans of nanosecond green or IR laser. Next, contact opening are made to these doped base and emitter areas using a picoseconds UV laser. Again, the alignment of these contact openings is made using fiducials in silicon.
  • a metal stack layer comprising aluminum as its first layer in contact with the cell (e.g., a stack of 1250A All 100-250A NiV/ 2250 Sn) is deposited using a suitable method such as a PVD (physical vapor deposition) technique.
  • FIG. 17 shows the laser processes used on the planar epitaxial substrate to make a back- contact solar cell where epitaxial silicon base is not deposited with an emitter layer. Instead, a boron containing oxide (BSG) layer is deposited and patterned to open the base isolation region.
  • BSG boron containing oxide
  • FIG. 18 shows a process flow using laser processes on the epitaxial substrate to make a planar back-contact/back-junction solar cell where the TFSS is not self-supporting (hence, a backplane is used). This flow uses the silicon ablation of in-situ doped emitter to form the base isolation region.
  • FIG. 19A-19H show a process flow using laser processes on the epitaxial substrate to make a planar back contact solar cell where the TFSS is not self-supporting.
  • the BSG deposition and selective laser ablation followed by thermal oxidation is used to form the emitter as well as the base isolation region.
  • FIG. 20 shows a process flow for making back contacted 3-D solar cells, it is advantageous to have the template side of pyramids end in relatively sharp points. Since the 3-D TFSS can be self-supporting to relatively low thickness (e.g., silicon as thin as about 25 microns), the process flow is similar to that shown in FIG. 16. It should be clear that we again have a choice of using the in-situ emitter followed by laser ablation of silicon, or BSG deposition and selective laser ablation followed by thermal oxidation (or thermal anneal, or thermal oxidizing anneal).
  • a hetero-junction emitter may be formed by a doped amorphous silicon layer in contact with an oppositely doped crystalline silicon base.
  • TCO transparent conducting oxide
  • Femtoseconds pulsewidth lasers with either UV or green wavelength are suitable for this application. A process flow is described in FIG. 21. Several variations of this process flow are possible.
  • Various embodiments and methods of this disclosure include at least the following aspects: the process to obtain damage- free silicon ablation of crystalline and amorphous silicon; the process to obtain oxide ablation for both doped and undoped oxides with no damage to silicon; the process to obtain fully isolated metal patterns on a dielectric surface for solar cell metallization; the process to selectively dope the emitter and base contact regions; the use of pulsed laser processing on very thin wafers, including planar and 3-D silicon substrate; the use of pulsed laser processing on substrates obtained using epitaxial deposition on a reusable template made using template pre-structuring techniques; the use of various pulsed laser processes in making front contacted homo-junction solar cells; the use of various pulsed laser processes in making all-back contacted homo-junction solar cells; and the use of various pulsed laser processes in making hetero-junction solar cells.
  • the front contact solar cells are described with p-type base and back- contact back-junction solar cells are described with n-type base
  • the laser processes described here are equally suited to the substrate with opposite doping, i.e., n-type for front contact solar cell with P + emitter, and p-type base for back-contact back-junction solar cells with p- type base and n + emitter.

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Abstract

L'invention concerne des techniques de traitement laser pour produire des types variés de cellules solaires à hétérojonction et à homojonction. Les procédés consistent à ouvrir une base et un contact d'émetteur, à réaliser un dopage sélectif et à effectuer une ablation de métal. Les techniques de traitement laser sont également appropriées pour effectuer une ablation sélective de silicium amorphe et un dopage sélectif pour des cellules solaires à hétérojonction. Les techniques de traitement laser peuvent être appliquées sur des substrats à semi-conducteurs, notamment des substrats en silicium cristallin, et d'autres substrats en silicium cristallin qui sont produits au moyen de procédés de découpage en tranches par une scie à fil ou de procédés de dépôt épitaxial, qui sont plats ou texturés/tridimensionnels. Ces techniques sont hautement appropriées pour un semi-conducteur à silicium cristallin comprenant les films en silicium cristallin mince.
EP11787543.5A 2010-05-27 2011-05-27 Traitement laser pour fabrication de cellules solaires en silicium cristallin mince à efficacité élevée Withdrawn EP2577750A4 (fr)

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PCT/US2011/038444 WO2011150397A2 (fr) 2010-05-27 2011-05-27 Traitement laser pour fabrication de cellules solaires en silicium cristallin mince à efficacité élevée

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US9508886B2 (en) 2007-10-06 2016-11-29 Solexel, Inc. Method for making a crystalline silicon solar cell substrate utilizing flat top laser beam
US8399331B2 (en) 2007-10-06 2013-03-19 Solexel Laser processing for high-efficiency thin crystalline silicon solar cell fabrication
US8637340B2 (en) 2004-11-30 2014-01-28 Solexel, Inc. Patterning of silicon oxide layers using pulsed laser ablation
US9455362B2 (en) 2007-10-06 2016-09-27 Solexel, Inc. Laser irradiation aluminum doping for monocrystalline silicon substrates
WO2013101846A1 (fr) * 2011-12-26 2013-07-04 Solexel, Inc. Systèmes et procédés permettant d'améliorer le piégeage de la lumière dans les piles photovoltaïques
GB2499192A (en) * 2012-02-02 2013-08-14 Rec Cells Pte Ltd Method for producing a solar cell with a selective emitter
JP2015516145A (ja) * 2012-04-02 2015-06-08 ソレクセル、インコーポレイテッド 高効率太陽電池構造体及びその製造方法
KR20150119262A (ko) * 2013-02-12 2015-10-23 솔렉셀, 인크. 벌크 웨이퍼를 사용한 모노리식 섬 후측 접촉 후측 접합 태양 전지
US10553738B2 (en) * 2013-08-21 2020-02-04 Sunpower Corporation Interconnection of solar cells in a solar cell module
KR20170025098A (ko) 2015-08-27 2017-03-08 삼성전자주식회사 펄스 레이저를 이용한 그래핀 홀 패터닝 방법 및 그래핀 투명전극 제조방법

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EP2577750A4 (fr) 2014-04-09

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