WO2013032255A1 - Method of fabricating emitter wrap through solar cell using one-step doping process - Google Patents

Method of fabricating emitter wrap through solar cell using one-step doping process Download PDF

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Publication number
WO2013032255A1
WO2013032255A1 PCT/KR2012/006962 KR2012006962W WO2013032255A1 WO 2013032255 A1 WO2013032255 A1 WO 2013032255A1 KR 2012006962 W KR2012006962 W KR 2012006962W WO 2013032255 A1 WO2013032255 A1 WO 2013032255A1
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electrode
semiconductor substrate
forming
film
passivation film
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PCT/KR2012/006962
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French (fr)
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Jae Eock Cho
Hong Gu Lee
Se-Young Seo
Deoc Hwan Hyun
Yong Hwa Lee
Gang Il Kim
Woo Won Jung
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Hanwha Chemical Corporation.
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Publication of WO2013032255A1 publication Critical patent/WO2013032255A1/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
    • H01L31/02—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
    • H01L31/022458—Electrode arrangements specially adapted for back-contact solar cells for emitter wrap-through [EWT] type solar cells, e.g. interdigitated emitter-base back-contacts
    • 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
    • H01L31/02—Details
    • H01L31/0216—Coatings
    • 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
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1804—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic Table
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/547—Monocrystalline silicon PV cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present invention relates to a method of fabricating an emitter wrap through (EWT) solar cell, and more particularly, to a method of fabricating a selective EWT solar cell by implementing doping of a light receiving surface, doping of a via hole, and doping of a back surface into a one-step doping process.
  • EWT emitter wrap through
  • a silicon solar cell has been developed from 1950, but in 1980, a high efficiency solar cell capable of remarkably increasing voltage and current while reducing surface defects of a substrate by a silicon surface passivation technology using a silicon oxide film starting to be used in Micro Electronics has arrived in earnest in 1980.
  • a solar cell should be designed in a structure capable of maximizing absorption of light.
  • a crystalline silicon solar cell has a surface textured in an uneven form to lower reflectance. The surface of the solar cell we see appears as dark blue, which is to increase a quantity of light incident to a solar cell as maximally as possible by coating an anti-reflective film.
  • the solar cell is an electric element
  • a fish bone type of a surface electrode needs to increase electric conductivity while minimizing light shading loss and therefore, a line width, the number of electrodes, and the like, according to device characteristics should be optimized.
  • An emitter wrap through (EWT) solar cell that is one of the back contact solar cell technologies can collect carriers generated by sun light at a front surface and a back surface thereof and therefore, is a solar cell capable of implementing high efficiency using a p-type low-grade substrate as described in Korean Laid-Open Patent No. 2006-0035657 unlike an interdigitated back contact (IBC) solar cell using an n-type high-grade substrate.
  • EWT emitter wrap through
  • the EWT solar cell transfers carriers collected at a front surface thereof to a back surface thereof through doped via holes and has a back emitter connected to an electrode, there is a need to implement a selective emitter making a doping concentration of via holes and a back emitter higher than that of a front light receiving surface so as to lower serial resistance.
  • An embodiment of the present invention is directed to providing a method of fabricating an EWT solar cell by implementing doping of light receiving surface having different doping levels, doping of a via hole, and doping of a back surface into a one-step doping process.
  • a method of fabricating an EWT solar cell including: a) forming via holes penetrating through opposing first and second surfaces of a p-type semiconductor substrate using laser; b) forming an anti-reflective film on the first surface of the semiconductor substrate, and forming a passivation film on the second surface of the semiconductor substrate; c) partially removing the passivation film so as to expose a part of the second surface on which an opening part of each of the via holes is formed; and d) heat-treating the semiconductor substrate under the presence of n-type impurity to dope the n-type impurity on the semiconductor substrate.
  • the via holes may be spaced apart from each other, and in step c), the passivation film may be partially removed in a strip shape so that at least two opening parts on the second surface are located in the part of the second surface exposed in a strip shape by the partial removal of the passivation film.
  • the anti-reflection film and the passivation film may each be a multiayered thin film made of semiconductor oxide, semiconductor nitride, alumina, titania, or a combination thereof.
  • a thickness of the anti-reflective film may be 10 nm to 30 nm and a thickness of the passivation may be 30 to 100 nm.
  • the first surface of the semiconductor substrate may be formed with a front emitter layer having a sheet resistance of 50 ⁇ /square to 100 ⁇ /square
  • the via holes may be formed with via hole emitters having a sheet resistance of 10 ⁇ /square to 50 ⁇ /square
  • the part of the second surfac exposed by step c) may be formed with back emitters having 10 ⁇ /square to 50 ⁇ /square.
  • the method of fabricating an EWT solar cell may further include: after step d), e) forming n-type electrodes by coating or depositing an electrode material so as to cover the opening parts of the via holes on the second surface, and forming p-type electrodes connecting the semiconductor substrate by penetrating through the passivation films by a punch through phenomenon.
  • the forming of the p-type electrodes may include: forming a first electrode connecting the semiconductor substrate by penetrating through the passivation film by the heat treatment over the passvation film; forming a second electrode over the first electrode so as to cover the first electrode without penetrating through the passivation film; and selectively connecting only the first electrode to the semiconductor substrate by the punch through phenomenon by heat treating the semiconductor substrate on which the first electrode and the second electrode are formed.
  • FIG. 1 is a process diagram showing a process of a method of fabricating a solar cell according to an exemplary embodiment of the present invention.
  • FIG. 2 is another process diagram showing a process of a method of fabricating a solar cell according to an exemplary embodiment of the present invention.
  • FIG. 3 is a view showing a substrate for explaining an example of n-type and p-type electrodes according to the present invention.
  • FIG. 4 is a view showing of a form of n-type electrode.
  • FIG. 1 is a process diagram showing a method of fabricating of an EWT solar cell according to an exemplary embodiment of the present invention.
  • the fabrication method according to the exemplary embodiment of the present invention includes: a) forming via holes 1 penetrating through opposing first and second surfaces of a p-type semiconductor substrate 100 using laser; b) forming an anti-reflective film 200 on the first surface of the semiconductor substrate 100 on which the via holes 1 are formed, and forming a passivation film 300 on the second surface of the semiconductor substrate 100; c) partially removing the passivation film 300 so as to expose a part of the second surface on which an opening part of each of the via holes 1 is formed; d) heat-treating the semiconductor substrate 100 under the presence of n-type impurity to dope the n-type impurity on the semiconductor substrate 100.
  • the semiconductor substrate 100 includes a group IV semiconductor substrate including silicon (Si), germanium, or silicon germanium (SiGe); a group III-V semiconductor substrate including gallium arsenide (GaAs), indium phosphorus (InP), or gallium phosphorus (GaP), a group II-VI semiconductor substrate including cadmium sulfide (CdS) or zinc telluride (ZnTe), or a group IV-VI semiconductor substrate including lead sulfide (PbS).
  • group IV semiconductor substrate including silicon (Si), germanium, or silicon germanium (SiGe)
  • GaAs gallium arsenide
  • InP indium phosphorus
  • GaP gallium phosphorus
  • a group II-VI semiconductor substrate including cadmium sulfide (CdS) or zinc telluride (ZnTe)
  • PbS lead sulfide
  • the semiconductor substrate includes a single crystalline, a poly crystalline, or an amorphous substrate.
  • Step a) is forming the via holes 1 penetrating through the opposing first and second surfaces of the semiconductor substrate 100 using laser, preferably, a light receiving surface receiving sun light and a back surface that is an opposite surface of the light receiving surface.
  • a diameter of the via hole 1 may be preferably 30 ⁇ m to 100 ⁇ m.
  • the semiconductor substrate 100 may be provided with the plurality of via holes 1 that are spaced apart from each other.
  • two-dimensionally arranged opening parts 1a of via holes 1 that are spaced apart from each other are disposed in an area in which n-type finger electrodes are disposed, preferably, a plurality of opening parts 1b of via holes 1 may also be formed even in a location at which n-type bus bar electrodes connecting the plurality of n-type finger electrodes are disposed.
  • the forming of the via holes 1 due to laser irradiation involves a thermal damage to the semiconductor substrate 100 and therefore, after the process of forming the via holes 1, damage removal etching for removing the damaged area may be involved.
  • the semiconductor substrate 100 on which the via holes 1 are formed is subjected to a process of forming the anti-reflective film 200 on the light receiving surface of the semiconductor substrate 100 through which the via holes 1 penetrate and the passivation film 300 on the back surface through which the via holes 1 penetrate.
  • the anti-reflective film 200 formed on the light receiving surface of the semiconductor substrate 100 means a film that serves to prevent the light received into the solar cell from being again emitted to the outside of the solar cell and to passivate surface defects acting as a trap site of carriers on the light receiving surface of the semiconductor substrate 100.
  • the anti-reflective film 200 may be a single layer thin film and when the anti-reflective action and the passivation action are made of different materials, the anti-reflective film 200 may be a multilayered thin film on which different material layers are multilayered.
  • the anti-reflective film 200 may be the multilayered thin film on which different material layers are multilayered so as to maximize the anti-reflective action and effectively passivate the defects.
  • the anti-reflective film 200 may be a single film consisting of any one selected from semiconductor oxide, semiconductor nitride, nitrogen containing semiconductor oxide, hydrogen containing semiconductor nitride, Al 2 O 3 , MgF 2 , ZnS, MgF 2 , TiO 2 , and CeO 2 or a multilayer film on which at least two films consisting of ones selected therefrom are multilayered.
  • the anti-reflective film 200 of the single layer thin film may be a silicon nitride film, a hydrogen containing silicon nitride film, or a silicon oxynitride film and the anti-reflective film 200 of the multilayered thin film includes a multilayered thin film on which films consisting of at least two selected from silicon oxide, silicon nitride, Al 2 O 3 , MgF 2 , ZnS, MgF 2 , TiO 2 , and CeO 2 are multilayered.
  • the passivation film 300 formed on the back surface of the semiconductor substrate 100 means a film that serves to passivate the surface defects acting as the trap site of carriers on the back surface of the semiconductor substrate 100.
  • the passivation film 300 includes a multilayered thin fim made of semiconductor oxide, semiconductor nitride, nitrogen containing semiconductor oxide, hydrogen containing semiconductor nitride, alumina, titania, or a combination thereof.
  • the passivation film 300 may be a silicon nitride film, a hydrogen containing silicon nitride film, a silicon oxide film, an alumina film, or a silicon oxynitride film and the passivation film 300 of the multilayered thin film include a multilayered thin film on which at least two films consisting of ones selected from a silicon nitride film, a hydrogen containing silicon nitride film, a silicon oxide film, an alumina film, a silicon oxynitride film, and a titania film are multilayered.
  • the anti-reflective film 200 and the passivation film 300 serve to suppress the diffusion of impurity to control the concentration of impurity doped on the semiconductor substrate 100, at the time of the doping of impurity in step d).
  • the anti-reflective film 200 serves to prevent n-type impurity from being partially diffused so that the light receiving surface of the solar cell may be provided with a shallow emitter during the doping process while performing both of the anti-reflection and the passivation of the surface defects and the passivation film 300 serves to form a patterned (patterned by the partial etching of the passivation film) back emitter in which only a part of the back surface exposed by the partial etching is doped with high-concentration n-type impurity by preventing the n-type impurity from being diffused during the doping process while serving as the passivation of the surface defects.
  • a thickness of the anti-reflective layer 200 is 10 to 30 nm.
  • the thickness of the passivation film 300 may be controlled to 30 to 100 nm so that n-type impurity is not doped on the back surface of the semiconductor substrate 100 at the time of the heat-treating of step d).
  • the anti-reflective films 200 and the passivation films 300 may be formed by a method of forming a thin film that is generally used in the semiconductor passivation process, for example, at least one method selected from physical vapor deposition (PVD), chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), and thermal evaporation and may also be formed by a general printing process using ink or paste.
  • PVD physical vapor deposition
  • CVD chemical vapor deposition
  • PECVD plasma enhanced chemical vapor deposition
  • thermal evaporation thermal evaporation
  • step b) in order to form the anti-reflective films 200 and the passivation films 300 and then, the n-type emitter on the back surface of the light receiving surface, the partial etching of the passivation films 300 (step C) is performed.
  • the passivation film 300 is partially etched so as to expose a part of the back surface on which the opening parts 1a of the via holes 1 are exposed.
  • the plurality of via holes 1 spaced apart from each other are arranged constantly and in order to locate the opening parts 1a of the via holes on the back surface located longitudinally or traversely in a row on the single semiconductor surface exposed in a strip shape by the partial etching of the passivation films 300, and the plurality of passivation films 300 spaced apart from each other are partially etched in a strip shape 310 so that the semiconductor surface area adjacent to all the opening parts 1a of the via holes on the same line is exposed.
  • N etching areas 310 having a strip shape connecting the opening parts 1a of the M via holes are present.
  • each of the etching areas 310 having a strip shape is formed with the n-type finger electrodes.
  • the passivation films 300 are partially etched in a strip shape for each opening part 1a of the via holes located on a straight line.
  • the passivation films 300 penetrating through the areas 310 having a strip shape may be partially etched in another strip shape 320 so that ends of the etching areas 310 having a strip shape partially etched are connected to each other.
  • the etching areas having another strip shape 320 are provided with the n-type bus bar electrodes connecting the n-type finger electrodes.
  • the areas in which the n-type bus bar electrodes are formed may also be formed with the opening parts 1b of via holes 1.
  • the passivation films 300 are etched so that the opening parts 1a of the via holes located on a straight line are located on the semiconductor surface exposed in the single strip shape 310, such that the exposed semiconductor surface is doped with the high-concentration n-type impurity to form the back emitter.
  • the exposed semiconductor surface (back emitter) is coated with an electrode material
  • the n-type electrode is formed and as another strip shape 320 penetrating through the areas 310 having the strip shape is coated with an electrode material, common electrodes (bus bar electrodes) electrically connecting a plurality of n-type electrodes are formed.
  • a width of a partial etching area having the strip shape 310 for forming the back emitter contacting the opening parts 1a and 1b of the via holes is preferably three and fourth time higher than a diameter of the via hole.
  • the partial etching of the passivation film 300 may be performed by laser ablation, mechanical scribing, or coating of etching paste that are generally used in the semiconductor process.
  • the via hole 1 is formed and the light receiving surface is formed with the anti-reflective film 200, a back surface that is an opposite surface of the light receiving surface is formed with the passivation film 300, and the partial etching of the passivation film 300 is performed, doping the semiconductor substrate 100 with the n-type impurity is performed by applying thermal energy to the semiconductor substrate 100.
  • the n-type impurity that consists of at least one selected from gases POCl 3 , P 2 O 5 and PH 3 is supplied while being mixed with inert carrier gases and the n-type impurity is doped on the semiconductor substrate 100 by heat-treating the semiconductor substrate 100 at a temperature of 800 to 900 °C for 10 to 60 minutes. In this case, removing an impurity film such as phosphor silicate glass generated by the doping heat treatment may be performed.
  • the via hole 1 of the semiconductor surface exposed by the doping heat treatment of step d) is formed with the via hole emitter having a sheet resistance of 10 ⁇ /square to 50 ⁇ /square
  • the semiconductor surface exposed by step c) is formed with the back emitter having a sheet resistance of 10 ⁇ /square to 50 ⁇ /square
  • one surface of the semiconductor substrate 100 beneath the anti-reflective film 200 is formed with the front emitter layer having a sheet resistance of 50 ⁇ /square to 100 ⁇ /square.
  • the fabrication method according to the exemplary embodiment of the present invention forms the anti-reflective film 200 of the light receiving surface and the passivation film 300 of the back surface and forms the emitter layer (selective emitter structure) having different doping levels by a one-step doping process using the same.
  • the thickness of the anti-reflective film 200 and the passivation film 300 is controlled so that some areas of the semiconductor substrate 100 are doped at the doping concentration of different specific levels by the heat treating of step d).
  • removing and additionally depositing the anti-reflective film 200 or the passivation film 300 may be performed again so as to have the optimal composition and thickness, for antireflection of sun light and passivating the semiconductor substrate 100 after the heat treatment of step d).
  • step d after the heat treating of step d), forming (re-forming the anti-reflective film) a single film consisting of any one selected from semiconductor oxide, semiconductor nitride, nitrogen containing semiconductor oxide, hydrogen containing semiconductor nitride, Al 2 O 3 , MgF 2 , ZnS, MgF 2 , TiO 2 , and CeO 2 or a multilayer film in which at least two films consisting of ones selected therefrom are multilayered on the anti-reflective film 200 of step b) may be further performed and forming (re-forming the passivation film) a single film consisting of any one selected from semiconductor oxide, the semiconductor nitride, nitrogen containing semiconductor oxide, hydrogen containing semiconductor nitride, and titania and a multilayer film in which at least two films consisting of ones selected therefrom are multilayered over the passivation film 300 may be further performed.
  • the thickness of the film is increased by forming the film made of the same material as the already formed anti-reflective film or passivation film or the film made of material different from the already formed anti-reflective film or the passivation film is further formed.
  • the method of fabricating a solar cell according to the exemplary embodiment of the present invention further includes forming the n-type electrodes 5 (n-type finger electrodes) by e) coating or depositing the electrode material so as to cover the opening part of the back surface (opposite surface of the receiving surface) of the via hole 1 and forming p-type electrodes 7 and 8 (p-type finger electrode) connecting the semiconductor substrate 102 by penetrating through the passivation film 300 by a punch through phenomenon, after) step d).
  • the n-type electrodes 5 are formed by printing and heat treating the conductive ink, wherein the conductive ink preferably includes metal particles including silver (Ag), copper (Cu), titanium (Ti), gold (Au), aluminum (Al), tungsten (W), nickel (Ni), chromium (Cr), molybdenum (Mo), platinum (Pt), lead (Pb), palladium (Pd), and an alloy thereof, more preferably, silver.
  • the conductive ink preferably includes metal particles including silver (Ag), copper (Cu), titanium (Ti), gold (Au), aluminum (Al), tungsten (W), nickel (Ni), chromium (Cr), molybdenum (Mo), platinum (Pt), lead (Pb), palladium (Pd), and an alloy thereof, more preferably, silver.
  • the p-type electrodes 7 and 8 are formed by performing the following processes.
  • the method includes a first electrode printing step of forming the first electrode 6 over the passivation film 300 by coating a first electrode material penetrating through the passivation film 300 at the time of heat-treating the semiconductor substrate, a second electrode printing step of forming a second electrode 7 over the first electrode 6 by coating a second electrode material that does not penetrate through the anti-reflective film at the time of performing the heat treating so as to cover the first electrode 6, and selectively connecting only the first electrode 6 of the first electrode 6 and the second electrode 7 to the semiconductor substrate 100 by the punch through phenomenon by heat treating the semiconductor substrate 100 on which the first electrode 6 and the second electrode 7 are formed.
  • the first electrode 6 penetrating through the passivation film 300 means that the material of the first electrode physically contacts the semiconductor substrate by interface-reacting the material of the first electrode with the anti-reflective film and the material of the first electrode contacts the p-type semiconductor substrate 100 by the punch through phenomenon.
  • the detailed mechanism related to the punch through phenomenon refers to J. Hoomstra, et al., 31st IEEE PVSC Florida 2005.
  • the first electrode 6 is formed by printing first ink including a conductive metal material and a glass frit etching the passivation film 300, wherein the conductive metal material contained in the first ink may be one or two or more material selected from silver (Ag), copper (Cu), titanium (Ti), gold (Au), aluminum (Al), tungsten (W), nickel (Ni), chromium (Cr), molybdenum (Mo), platinum (Pt), lead (Pb), palladium (Pd), and an alloy thereof, more preferably, silver, copper, nickel, aluminum, or an alloy thereof in terms of low melting point and excellent electric conductivity.
  • the conductive metal material contained in the first ink may be one or two or more material selected from silver (Ag), copper (Cu), titanium (Ti), gold (Au), aluminum (Al), tungsten (W), nickel (Ni), chromium (Cr), molybdenum (Mo), platinum (Pt), lead (Pb), palladium (Pd), and an alloy
  • the glass frit included in the first ink to etch the anti-reflective film may use lead glass including lead oxide and lead-free glass including bismuth oxide and boron oxide.
  • An example of the lead glass-based frit may include a PbO-SiO 2 -B 2 O 3 -Al 2 O 3 glass frit, PbO-SiO 2 -B 2 O 3 -Al 2 O 3 -ZrO 2 glass frit, a PbO-SiO 2 -B 2 O 3 -Al 2 O 3 -ZnO glass frit, or a PbO-SiO 2 -B 2 O 3 -Al 2 O 3 -ZnO-TiO 2 glass frit and an example of the lead-free glass-based frit may include a Bi 2 O 3 -ZnO-SiO 2 -B 2 O 3 -Al 2 O 3 glass frit, a Bi 2 O 3 -SrO-SiO 2 -B 2 O 3
  • the lead glass or the lead-free glass may include additives consisting of one or two or more selected from Ta 2 O 5 , Sb 2 O 5 , HfO 2 , In 2 O 3 , Ga 2 O 3 , Y 2 O 3 , and Yb 2 O 3 .
  • the first electrode 6 preferably includes the lead glass or the lead-free glass of 3 to 5wt%.
  • the second electrode 7 of the first electrode 6 and the second electrode 7 does not penetrate through the passivation film 300 but only the first electrode is connected to the substrate by selectively penetrates through the passivation film 300.
  • the meaning that the second electrode 7 does not penetrate through the passivation film 300 means that the material of the second electrode 7 does not interface-react with the passivation film 300 and means that the punch through of the passivation film 300 is not generated due to the second electrode 7 material even when heat energy is applied.
  • the second electrode 7 is formed by printing second ink including a conductive metal material and a glass frit that does not react with the passivation film 300, wherein the conductive metal material contained in the second ink may be one or two or more material selected from silver (Ag), copper (Cu), titanium (Ti), gold (Au), aluminum (Al), tungsten (W), nickel (Ni), chromium (Cr), molybdenum (Mo), platinum (Pt), lead (Pb), palladium (Pd), and an alloy thereof and the glass frit included in the second ink is a glass frit that does not etch the passivation film 300 and is preferably a general silica-based or phosphate-based glass that does not include B, Bi, and Pb.
  • the conductive metal material contained in the second ink may be one or two or more material selected from silver (Ag), copper (Cu), titanium (Ti), gold (Au), aluminum (Al), tungsten (W), nickel (Ni), chromium (
  • the glass frit included in the second electrode 7 has glass transition temperature 1.2 to 2 times higher than glass transition temperature (Tg) of the glass frit included in the first electrode 6 and is preferably a silica-based or phosphate-based glass that does not include B, Bi, and Pb.
  • the silica-based glass frit uses network formation component as SiO 2 and includes one or two or more materials selected from Li 2 O, Na 2 O, K 2 O, MgO, CaO, BaO, SrO, ZnO, Al 2 O 3 , TiO 2 , ZrO 2 , Ta 2 O 5 , Sb 2 O 5 , HfO 2 , In 2 O 3 , Ga 2 O 3 , Y 2 O 2 , and Yb 2 O 3
  • the phosphate-based glass frit is vanadium-phosphate-based glass that is P 2 O 5 -V 2 O 5 or zinc-antimony-phosphate-based glass that is P 2 O 5 -ZnO-Sb 2 O 3
  • the phosphate-based glass frit preferably includes one or two or more materials selected from K 2 O, Fe 2 O 3 , Sb 2 O 3 , ZnO, TiO 2 , Al 2 O 3 , and WO 3 .
  • the second electrode 7 preferably includes the silic
  • the first electrode 6 may be a dot shape or a fine line shape.
  • the first electrode 6 uses a structure consisting of a plurality of dots spaced apart from each other on a straight line as one unit and preferably has a structure in which at least two units are spaced apart from each other at a predetermined distance, and at least two units are more preferably spaced apart from each other so as to be parallel with each.
  • the second electrode 7 has a plurality of strip shapes that are spaced apart from each other, wherein the strip has a feature connecting at least two dots with each other.
  • a dot diameter of the first electrode 6 is preferably 30 ⁇ m to 300 ⁇ m and the dot diameter has a size enough to stably connect to the semiconductor substrate 100 by the punch through and minimize the damage of the passivation film 300.
  • the first electrode 6 When the first electrode 6 is a fine line, the first electrode 6 has a plurality of strip shapes that are spaced from each other so as to be parallel with each other and the second electrode 7 may be a plurality of strip shapes each enclosing the strips configuring the first electrode 6.
  • a width W 1 of the first electrode 6 having a strip shape that is a fine line may be preferably 30 ⁇ m to 300 ⁇ m.
  • the width of the first electrode 6 has a consecutive line shape by the punch through and has a size enough to connect to the semiconductor substrate and minimize the damage of the passivation film 300.
  • the width of the second electrode 7 is preferably 30 ⁇ m to 300 ⁇ m similar to the case in which the first electrode is a dot shape.
  • a width W 2 of the second electrode 7 formed over the first electrode 6 and having the strip shape covering the first electrode 6 having the dot shape or the fine line shape is preferably 50 ⁇ m to 1,000 ⁇ m.
  • the electrode configured of the first electrode 6 and the second electrode 7 has a width capable of lowering the resistance increased by the first electrode 6 locally finely contacting the semiconductor substrate, wherein the width is formed so that the electrode may have a resistance of 3 to 6 x 10 -6 ⁇ cm.
  • the p-type electrodes 8 and 7 and the n-type electrodes 5 by single heat treatment.
  • the heat treatment for forming the p-type electrodes 8 and 7 and the n-type electrodes 5 is preferably performed at 100 to 1,000°C.
  • FIG. 4 shows an example showing a shape of the p-type electrodes 8 and 7 including the first electrode formed in a fine line type and the second electrode so as to cover the first electrode and the n-type electrodes 5 formed in the partial etching area of the passivation film 300.
  • the n-type electrodes 5 have a strip shape connecting the opening parts 1a of the via holes located on a straight line and ends of the n-type electrodes 5 having the plurality of strip shapes are connected with each other by n-type common electrodes 10 (n-type bus bar electrodes).
  • the p-type electrodes 8 and 7 are located between two n-type electrodes 5 adjacent to each other and the ends of the plurality of p-type electrodes 8 and 7 are connected with each other by p-type common electrodes 20 (p-type bus bar electrodes). Therefore, the p-type or n-type electrodes having a comb shape in which the ends thereof are connected to each other by each common electrode are formed and may be preferably formed on the back surface of the semiconductor substrate 100 in an interdigitated structure or a fish bone structure in which the p-type electrodes 8 and 7 and the n-type electrodes are interdigitated with each other.
  • the p-type common electrode 20 may preferably include a first common electrode 21 connecting the ends of the plurality of first electrodes 6 with each other and a second common electrode 22 formed over the first common electrode 21 to cover the first common electrode.
  • first common electrode 21 of the first common electrode 21 and the second common electrode 22 is selectively connected with the semiconductor substrate 100 by the punch-through phenomenon.
  • the first common electrode 21 may be formed by coating a material similar to the first electrode 6 material and the second common electrode 22 may be formed by coating a material similar to the second electrode 7.
  • the n-type common electrode 10 and the p-type common electrode 20 may be formed using the method similar to the formation of the n-type electrode 5 and the p-type electrodes 6 and 7 in the printing of the n-type electrode 5 and the p-type electrodes 6 and 7 of step e) and after the n-type electrode 5, the p-type electrodes 6 and 7, the n-type common electrode 10, and the p-type common electrode 20 are formed by the printing, the heat treatment for the selective punch-through may be preferably performed.
  • step (a) texturing a surface forming micro unevenness on the surface by etching the semiconductor substrate 100 may be further performed.
  • the etching includes dry or wet etching and the textured surface includes a surface in which the micro unevenness having a reverse pyramid shape is arranged in plural.
  • fabricating a back surface field (BSF) surface layer forming the back surface field on the back surface of the semiconductor substrate 100 by coating the back surface opposite to the light receiving surface with a doping agent including the p-type impurity and heat-treating the semiconductor substrate coated with the p-type impurity doping agent may be further performed and only the first electrode of the first electrode and the second electrode of the p-type electrode may be electrically and physically connected with the BSF area formed on the back surface of the semiconductor substrate 100 by the selective punch through.
  • the method of fabricating a solar cell according to the exemplary embodiments of the present invention can fabricate the EWT solar cell of a selective emitter structure having different doping levels by using the minimum process.
  • the exemplary embodiments of the present invention can form the front emitter of the light receiving surface, the via hole emitter, and the back emitter by a one-step doping process to shorten the process costs and time and prevent efficiency from being degraded due to the deterioration in the substrate, dope the via holes and the back emitter at high concentration to reduce serial resistance of a via hole and contact resistance of a back emitter and increase the high open circuit voltage and fill factor, and dope the front emitter of the front light receiving surface at low concentration to increase the short-wavelength absorptance and increase current by a shallow junction.

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Abstract

Provided is a method of fabricating a selective EWT solar cell, including: a) forming via holes penetrating through opposing first and second surfaces of a p-type semiconductor substrate using laser; b) forming an anti-reflective film on the first surface of the semiconductor substrate, and forming a passivation film on the second surface of the semiconductor substrate; c) partially etching the passivation film so as to expose a part of the second surface on which an opening part of each of the via holes is formed; and d) heat-treating the semiconductor substrate under the presence of n-type impurity to dope the n-type impurity on the semiconductor substrate.

Description

METHOD OF FABRICATING EMITTER WRAP THROUGH SOLAR CELL USING ONE-STEP DOPING PROCESS
The present invention relates to a method of fabricating an emitter wrap through (EWT) solar cell, and more particularly, to a method of fabricating a selective EWT solar cell by implementing doping of a light receiving surface, doping of a via hole, and doping of a back surface into a one-step doping process.
A silicon solar cell has been developed from 1950, but in 1980, a high efficiency solar cell capable of remarkably increasing voltage and current while reducing surface defects of a substrate by a silicon surface passivation technology using a silicon oxide film starting to be used in Micro Electronics has arrived in earnest in 1980.
Factors affecting the efficiency of a semiconductor based inorganic solar cell that is the most general solar cell are largely divided into three.
As a first factor for increasing the efficiency of a solar cell, a solar cell should be designed in a structure capable of maximizing absorption of light. To this end, a crystalline silicon solar cell has a surface textured in an uneven form to lower reflectance. The surface of the solar cell we see appears as dark blue, which is to increase a quantity of light incident to a solar cell as maximally as possible by coating an anti-reflective film. In addition, there is a need to maximally secure a light receiving area by minimizing an area of an electrode.
As a second factor for increasing the efficiency of the solar cell, there is a need to increase a lifespan of a carrier as maximally as possible since power cannot be generated when electrons and holes excited by light in the solar cell falls to a ground state even though the absorption of light is maximally increased. Since electrons and holes called a 'carrier' are recombined by an impurity and surface defects of a substrate and disappear, there is a need to maximally increase the lifespan of carriers by performing a gettering process of using high-purity silicon or removing impurity and a passivation process of removing the surface defects. Therefore, in order to generate electricity, electrons and holes need to move to the surface electrode prior to being recombined. Today, a silicon nitride layer that is passivation reducing the surface defects of the solar cell acts as the anti-reflective film, which is very advantageous in cost reduction.
As a third factor for increasing the efficiency of the solar cell, since the solar cell is an electric element, there is a need to consider electrode disposition, material selection, and the like, capable of minimizing various electrical resistance losses during a movement of a carrier and a contact with an external electrode. In particular, a fish bone type of a surface electrode needs to increase electric conductivity while minimizing light shading loss and therefore, a line width, the number of electrodes, and the like, according to device characteristics should be optimized.
One of the most spotlighted technologies for increasing the efficiency of the crystalline silicon solar cell that is a main product of a solar cell market is a back contact solar cell. An emitter wrap through (EWT) solar cell that is one of the back contact solar cell technologies can collect carriers generated by sun light at a front surface and a back surface thereof and therefore, is a solar cell capable of implementing high efficiency using a p-type low-grade substrate as described in Korean Laid-Open Patent No. 2006-0035657 unlike an interdigitated back contact (IBC) solar cell using an n-type high-grade substrate.
Since the EWT solar cell transfers carriers collected at a front surface thereof to a back surface thereof through doped via holes and has a back emitter connected to an electrode, there is a need to implement a selective emitter making a doping concentration of via holes and a back emitter higher than that of a front light receiving surface so as to lower serial resistance.
Similar to a general standard type solar cell, in order to make a doping level different, the doping process should be performed twice, which leads to an increase in process costs and a deteriorate in a substrate to degrade efficiency. A need exists for a development of a selective emitter process by a one-time doping process.
An embodiment of the present invention is directed to providing a method of fabricating an EWT solar cell by implementing doping of light receiving surface having different doping levels, doping of a via hole, and doping of a back surface into a one-step doping process.
In one general aspect, there is provided a method of fabricating an EWT solar cell, including: a) forming via holes penetrating through opposing first and second surfaces of a p-type semiconductor substrate using laser; b) forming an anti-reflective film on the first surface of the semiconductor substrate, and forming a passivation film on the second surface of the semiconductor substrate; c) partially removing the passivation film so as to expose a part of the second surface on which an opening part of each of the via holes is formed; and d) heat-treating the semiconductor substrate under the presence of n-type impurity to dope the n-type impurity on the semiconductor substrate.
In step a), the via holes may be spaced apart from each other, and in step c), the passivation film may be partially removed in a strip shape so that at least two opening parts on the second surface are located in the part of the second surface exposed in a strip shape by the partial removal of the passivation film.
The anti-reflection film and the passivation film may each be a multiayered thin film made of semiconductor oxide, semiconductor nitride, alumina, titania, or a combination thereof.
A thickness of the anti-reflective film may be 10 nm to 30 nm and a thickness of the passivation may be 30 to 100 nm.
By the doping of step d), the first surface of the semiconductor substrate may be formed with a front emitter layer having a sheet resistance of 50 Ω/square to 100 Ω/square, the via holes may be formed with via hole emitters having a sheet resistance of 10 Ω/square to 50 Ω/square, and the part of the second surfac exposed by step c) may be formed with back emitters having 10 Ω/square to 50 Ω/square.
The method of fabricating an EWT solar cell may further include: after step d), e) forming n-type electrodes by coating or depositing an electrode material so as to cover the opening parts of the via holes on the second surface, and forming p-type electrodes connecting the semiconductor substrate by penetrating through the passivation films by a punch through phenomenon.
The forming of the p-type electrodes may include: forming a first electrode connecting the semiconductor substrate by penetrating through the passivation film by the heat treatment over the passvation film; forming a second electrode over the first electrode so as to cover the first electrode without penetrating through the passivation film; and selectively connecting only the first electrode to the semiconductor substrate by the punch through phenomenon by heat treating the semiconductor substrate on which the first electrode and the second electrode are formed.
Other features and aspects will be apparent from the following detailed description, the drawings, and the claims.
FIG. 1 is a process diagram showing a process of a method of fabricating a solar cell according to an exemplary embodiment of the present invention.
FIG. 2 is another process diagram showing a process of a method of fabricating a solar cell according to an exemplary embodiment of the present invention.
FIG. 3 is a view showing a substrate for explaining an example of n-type and p-type electrodes according to the present invention.
FIG. 4 is a view showing of a form of n-type electrode.
A method of fabricating an EWT solar cell according to an exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings. The following introduced drawings are provided by way of example so as to fully convey an idea of the present invention to a person skilled in the art to which the present invention pertains. Therefore, the prevent invention is not limited to the drawings set forth below, and may be embodied in different forms, and the drawings set forth below may be exaggerated in order to clarify the spirit of the present invention. In addition, throughout the specification, like reference numerals denotes like components.
Here, unless indicated otherwise, the terms used in the specification including technical and scientific terms have the same meaning as those that are usually understood by those who skilled in the art to which the present invention pertains, and detailed description of the known functions and constitutions that may obscure the gist of the present invention will be omitted.
FIG. 1 is a process diagram showing a method of fabricating of an EWT solar cell according to an exemplary embodiment of the present invention. As shown in FIG. 1, the fabrication method according to the exemplary embodiment of the present invention includes: a) forming via holes 1 penetrating through opposing first and second surfaces of a p-type semiconductor substrate 100 using laser; b) forming an anti-reflective film 200 on the first surface of the semiconductor substrate 100 on which the via holes 1 are formed, and forming a passivation film 300 on the second surface of the semiconductor substrate 100; c) partially removing the passivation film 300 so as to expose a part of the second surface on which an opening part of each of the via holes 1 is formed; d) heat-treating the semiconductor substrate 100 under the presence of n-type impurity to dope the n-type impurity on the semiconductor substrate 100.
In detail, the semiconductor substrate 100 includes a group IV semiconductor substrate including silicon (Si), germanium, or silicon germanium (SiGe); a group III-V semiconductor substrate including gallium arsenide (GaAs), indium phosphorus (InP), or gallium phosphorus (GaP), a group II-VI semiconductor substrate including cadmium sulfide (CdS) or zinc telluride (ZnTe), or a group IV-VI semiconductor substrate including lead sulfide (PbS).
Crystallographically, the semiconductor substrate includes a single crystalline, a poly crystalline, or an amorphous substrate.
Step a) is forming the via holes 1 penetrating through the opposing first and second surfaces of the semiconductor substrate 100 using laser, preferably, a light receiving surface receiving sun light and a back surface that is an opposite surface of the light receiving surface. A diameter of the via hole 1 may be preferably 30 ㎛ to 100 ㎛.
In step a), the semiconductor substrate 100 may be provided with the plurality of via holes 1 that are spaced apart from each other.
In detail, as shown in FIG. 2, two-dimensionally arranged opening parts 1a of via holes 1 that are spaced apart from each other are disposed in an area in which n-type finger electrodes are disposed, preferably, a plurality of opening parts 1b of via holes 1 may also be formed even in a location at which n-type bus bar electrodes connecting the plurality of n-type finger electrodes are disposed.
The forming of the via holes 1 due to laser irradiation involves a thermal damage to the semiconductor substrate 100 and therefore, after the process of forming the via holes 1, damage removal etching for removing the damaged area may be involved.
The semiconductor substrate 100 on which the via holes 1 are formed is subjected to a process of forming the anti-reflective film 200 on the light receiving surface of the semiconductor substrate 100 through which the via holes 1 penetrate and the passivation film 300 on the back surface through which the via holes 1 penetrate.
The anti-reflective film 200 formed on the light receiving surface of the semiconductor substrate 100 means a film that serves to prevent the light received into the solar cell from being again emitted to the outside of the solar cell and to passivate surface defects acting as a trap site of carriers on the light receiving surface of the semiconductor substrate 100.
Like the case in which the anti-reflective action and the passivation action are made of a single material, the anti-reflective film 200 may be a single layer thin film and when the anti-reflective action and the passivation action are made of different materials, the anti-reflective film 200 may be a multilayered thin film on which different material layers are multilayered.
Further, even when the anti-reflective action and the passivation action are made of a single material, the anti-reflective film 200 may be the multilayered thin film on which different material layers are multilayered so as to maximize the anti-reflective action and effectively passivate the defects.
Preferably, the anti-reflective film 200 may be a single film consisting of any one selected from semiconductor oxide, semiconductor nitride, nitrogen containing semiconductor oxide, hydrogen containing semiconductor nitride, Al2O3, MgF2, ZnS, MgF2, TiO2, and CeO2 or a multilayer film on which at least two films consisting of ones selected therefrom are multilayered.
As one example of the silicon solar cell, the anti-reflective film 200 of the single layer thin film may be a silicon nitride film, a hydrogen containing silicon nitride film, or a silicon oxynitride film and the anti-reflective film 200 of the multilayered thin film includes a multilayered thin film on which films consisting of at least two selected from silicon oxide, silicon nitride, Al2O3, MgF2, ZnS, MgF2, TiO2, and CeO2 are multilayered.
The passivation film 300 formed on the back surface of the semiconductor substrate 100 means a film that serves to passivate the surface defects acting as the trap site of carriers on the back surface of the semiconductor substrate 100.
Preferably, the passivation film 300 includes a multilayered thin fim made of semiconductor oxide, semiconductor nitride, nitrogen containing semiconductor oxide, hydrogen containing semiconductor nitride, alumina, titania, or a combination thereof.
As an example of the silicon solar cell, the passivation film 300 may be a silicon nitride film, a hydrogen containing silicon nitride film, a silicon oxide film, an alumina film, or a silicon oxynitride film and the passivation film 300 of the multilayered thin film include a multilayered thin film on which at least two films consisting of ones selected from a silicon nitride film, a hydrogen containing silicon nitride film, a silicon oxide film, an alumina film, a silicon oxynitride film, and a titania film are multilayered.
In the fabrication method of the exemplary embodiment of the present invention, the anti-reflective film 200 and the passivation film 300 serve to suppress the diffusion of impurity to control the concentration of impurity doped on the semiconductor substrate 100, at the time of the doping of impurity in step d).
In more detail, the anti-reflective film 200 serves to prevent n-type impurity from being partially diffused so that the light receiving surface of the solar cell may be provided with a shallow emitter during the doping process while performing both of the anti-reflection and the passivation of the surface defects and the passivation film 300 serves to form a patterned (patterned by the partial etching of the passivation film) back emitter in which only a part of the back surface exposed by the partial etching is doped with high-concentration n-type impurity by preventing the n-type impurity from being diffused during the doping process while serving as the passivation of the surface defects.
In order to form the front emitter layer having a sheet resistance of 50 Ω/square to 100 Ω/square as a surface layer on the anti-reflective film 200 and on the light receiving surface of the semiconductor substrate 100 by the heat-treating of step d), a thickness of the anti-reflective layer 200 is 10 to 30 nm.
The thickness of the passivation film 300 may be controlled to 30 to 100 nm so that n-type impurity is not doped on the back surface of the semiconductor substrate 100 at the time of the heat-treating of step d).
The anti-reflective films 200 and the passivation films 300 may be formed by a method of forming a thin film that is generally used in the semiconductor passivation process, for example, at least one method selected from physical vapor deposition (PVD), chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), and thermal evaporation and may also be formed by a general printing process using ink or paste.
In step b), in order to form the anti-reflective films 200 and the passivation films 300 and then, the n-type emitter on the back surface of the light receiving surface, the partial etching of the passivation films 300 (step C) is performed.
The passivation film 300 is partially etched so as to expose a part of the back surface on which the opening parts 1a of the via holes 1 are exposed. In detail, as shown in FIG. 2, in order to form the n-type finger electrodes on the semiconductor substrate 100, the plurality of via holes 1 spaced apart from each other are arranged constantly and in order to locate the opening parts 1a of the via holes on the back surface located longitudinally or traversely in a row on the single semiconductor surface exposed in a strip shape by the partial etching of the passivation films 300, and the plurality of passivation films 300 spaced apart from each other are partially etched in a strip shape 310 so that the semiconductor surface area adjacent to all the opening parts 1a of the via holes on the same line is exposed.
For example, when the opening parts 1a of the via holes for forming the n-type finger electrodes are arranged in an M N (M and N each are a natural number of 2 or more) matrix form, N etching areas 310 having a strip shape connecting the opening parts 1a of the M via holes are present. In this case, each of the etching areas 310 having a strip shape is formed with the n-type finger electrodes.
For forming the n-type finer electrodes, the passivation films 300 are partially etched in a strip shape for each opening part 1a of the via holes located on a straight line. In this case, the passivation films 300 penetrating through the areas 310 having a strip shape may be partially etched in another strip shape 320 so that ends of the etching areas 310 having a strip shape partially etched are connected to each other. The etching areas having another strip shape 320 are provided with the n-type bus bar electrodes connecting the n-type finger electrodes. In this case, as described above, the areas in which the n-type bus bar electrodes are formed may also be formed with the opening parts 1b of via holes 1.
The passivation films 300 are etched so that the opening parts 1a of the via holes located on a straight line are located on the semiconductor surface exposed in the single strip shape 310, such that the exposed semiconductor surface is doped with the high-concentration n-type impurity to form the back emitter. As the exposed semiconductor surface (back emitter) is coated with an electrode material, the n-type electrode is formed and as another strip shape 320 penetrating through the areas 310 having the strip shape is coated with an electrode material, common electrodes (bus bar electrodes) electrically connecting a plurality of n-type electrodes are formed.
A width of a partial etching area having the strip shape 310 for forming the back emitter contacting the opening parts 1a and 1b of the via holes is preferably three and fourth time higher than a diameter of the via hole.
The partial etching of the passivation film 300 may be performed by laser ablation, mechanical scribing, or coating of etching paste that are generally used in the semiconductor process.
Thereafter, after the via hole 1 is formed and the light receiving surface is formed with the anti-reflective film 200, a back surface that is an opposite surface of the light receiving surface is formed with the passivation film 300, and the partial etching of the passivation film 300 is performed, doping the semiconductor substrate 100 with the n-type impurity is performed by applying thermal energy to the semiconductor substrate 100.
In detail, the n-type impurity that consists of at least one selected from gases POCl3, P2O5 and PH3 is supplied while being mixed with inert carrier gases and the n-type impurity is doped on the semiconductor substrate 100 by heat-treating the semiconductor substrate 100 at a temperature of 800 to 900 ℃ for 10 to 60 minutes. In this case, removing an impurity film such as phosphor silicate glass generated by the doping heat treatment may be performed.
The via hole 1 of the semiconductor surface exposed by the doping heat treatment of step d) is formed with the via hole emitter having a sheet resistance of 10 Ω/square to 50 Ω/square, the semiconductor surface exposed by step c) is formed with the back emitter having a sheet resistance of 10 Ω/square to 50 Ω/square, and one surface of the semiconductor substrate 100 beneath the anti-reflective film 200 is formed with the front emitter layer having a sheet resistance of 50 Ω/square to 100 Ω/square.
As described above, in fabricating the selective EWT solar cell, the fabrication method according to the exemplary embodiment of the present invention forms the anti-reflective film 200 of the light receiving surface and the passivation film 300 of the back surface and forms the emitter layer (selective emitter structure) having different doping levels by a one-step doping process using the same.
In this case, as described above, the thickness of the anti-reflective film 200 and the passivation film 300 is controlled so that some areas of the semiconductor substrate 100 are doped at the doping concentration of different specific levels by the heat treating of step d).
Therefore, removing and additionally depositing the anti-reflective film 200 or the passivation film 300 may be performed again so as to have the optimal composition and thickness, for antireflection of sun light and passivating the semiconductor substrate 100 after the heat treatment of step d).
In more detail, after the heat treating of step d), forming (re-forming the anti-reflective film) a single film consisting of any one selected from semiconductor oxide, semiconductor nitride, nitrogen containing semiconductor oxide, hydrogen containing semiconductor nitride, Al2O3, MgF2, ZnS, MgF2, TiO2, and CeO2 or a multilayer film in which at least two films consisting of ones selected therefrom are multilayered on the anti-reflective film 200 of step b) may be further performed and forming (re-forming the passivation film) a single film consisting of any one selected from semiconductor oxide, the semiconductor nitride, nitrogen containing semiconductor oxide, hydrogen containing semiconductor nitride, and titania and a multilayer film in which at least two films consisting of ones selected therefrom are multilayered over the passivation film 300 may be further performed. In this case, in the re-forming of the anti-reflective film or the re-forming of the passivation film, the thickness of the film is increased by forming the film made of the same material as the already formed anti-reflective film or passivation film or the film made of material different from the already formed anti-reflective film or the passivation film is further formed.
As shown in FIG. 3, the method of fabricating a solar cell according to the exemplary embodiment of the present invention further includes forming the n-type electrodes 5 (n-type finger electrodes) by e) coating or depositing the electrode material so as to cover the opening part of the back surface (opposite surface of the receiving surface) of the via hole 1 and forming p-type electrodes 7 and 8 (p-type finger electrode) connecting the semiconductor substrate 102 by penetrating through the passivation film 300 by a punch through phenomenon, after) step d).
The n-type electrodes 5 are formed by printing and heat treating the conductive ink, wherein the conductive ink preferably includes metal particles including silver (Ag), copper (Cu), titanium (Ti), gold (Au), aluminum (Al), tungsten (W), nickel (Ni), chromium (Cr), molybdenum (Mo), platinum (Pt), lead (Pb), palladium (Pd), and an alloy thereof, more preferably, silver.
The p-type electrodes 7 and 8 are formed by performing the following processes. The method includes a first electrode printing step of forming the first electrode 6 over the passivation film 300 by coating a first electrode material penetrating through the passivation film 300 at the time of heat-treating the semiconductor substrate, a second electrode printing step of forming a second electrode 7 over the first electrode 6 by coating a second electrode material that does not penetrate through the anti-reflective film at the time of performing the heat treating so as to cover the first electrode 6, and selectively connecting only the first electrode 6 of the first electrode 6 and the second electrode 7 to the semiconductor substrate 100 by the punch through phenomenon by heat treating the semiconductor substrate 100 on which the first electrode 6 and the second electrode 7 are formed.
The first electrode 6 penetrating through the passivation film 300 means that the material of the first electrode physically contacts the semiconductor substrate by interface-reacting the material of the first electrode with the anti-reflective film and the material of the first electrode contacts the p-type semiconductor substrate 100 by the punch through phenomenon. The detailed mechanism related to the punch through phenomenon refers to J. Hoomstra, et al., 31st IEEE PVSC Florida 2005.
The first electrode 6 is formed by printing first ink including a conductive metal material and a glass frit etching the passivation film 300, wherein the conductive metal material contained in the first ink may be one or two or more material selected from silver (Ag), copper (Cu), titanium (Ti), gold (Au), aluminum (Al), tungsten (W), nickel (Ni), chromium (Cr), molybdenum (Mo), platinum (Pt), lead (Pb), palladium (Pd), and an alloy thereof, more preferably, silver, copper, nickel, aluminum, or an alloy thereof in terms of low melting point and excellent electric conductivity. The glass frit included in the first ink to etch the anti-reflective film may use lead glass including lead oxide and lead-free glass including bismuth oxide and boron oxide. An example of the lead glass-based frit may include a PbO-SiO2-B2O3-Al2O3 glass frit, PbO-SiO2-B2O3-Al2O3-ZrO2 glass frit, a PbO-SiO2-B2O3-Al2O3-ZnO glass frit, or a PbO-SiO2-B2O3-Al2O3-ZnO-TiO2 glass frit and an example of the lead-free glass-based frit may include a Bi2O3-ZnO-SiO2-B2O3-Al2O3 glass frit, a Bi2O3-SrO-SiO2-B2O3-Al2O3 glass frit, a Bi2O3-ZnO-SiO2-B2O3-La2O3-Al2O3 glass frit, a Bi2O3-ZnO-SiO2-B2O3-TiO2 glass frit, a Bi2O3-SiO2-B2O3-SrO glass frit, or a Bi2O3-SiO2-B2O3-ZnO-SrO glass frit. In this case, the lead glass or the lead-free glass may include additives consisting of one or two or more selected from Ta2O5, Sb2O5, HfO2, In2O3, Ga2O3, Y2O3, and Yb2O3. The first electrode 6 preferably includes the lead glass or the lead-free glass of 3 to 5wt%.
As described above, the second electrode 7 of the first electrode 6 and the second electrode 7 does not penetrate through the passivation film 300 but only the first electrode is connected to the substrate by selectively penetrates through the passivation film 300. The meaning that the second electrode 7 does not penetrate through the passivation film 300 means that the material of the second electrode 7 does not interface-react with the passivation film 300 and means that the punch through of the passivation film 300 is not generated due to the second electrode 7 material even when heat energy is applied.
The second electrode 7 is formed by printing second ink including a conductive metal material and a glass frit that does not react with the passivation film 300, wherein the conductive metal material contained in the second ink may be one or two or more material selected from silver (Ag), copper (Cu), titanium (Ti), gold (Au), aluminum (Al), tungsten (W), nickel (Ni), chromium (Cr), molybdenum (Mo), platinum (Pt), lead (Pb), palladium (Pd), and an alloy thereof and the glass frit included in the second ink is a glass frit that does not etch the passivation film 300 and is preferably a general silica-based or phosphate-based glass that does not include B, Bi, and Pb. More preferably, the glass frit included in the second electrode 7 has glass transition temperature 1.2 to 2 times higher than glass transition temperature (Tg) of the glass frit included in the first electrode 6 and is preferably a silica-based or phosphate-based glass that does not include B, Bi, and Pb.
The silica-based glass frit uses network formation component as SiO2 and includes one or two or more materials selected from Li2O, Na2O, K2O, MgO, CaO, BaO, SrO, ZnO, Al2O3, TiO2, ZrO2, Ta2O5, Sb2O5, HfO2, In2O3, Ga2O3, Y2O2, and Yb2O3, the phosphate-based glass frit is vanadium-phosphate-based glass that is P2O5-V2O5 or zinc-antimony-phosphate-based glass that is P2O5-ZnO-Sb2O3, and the phosphate-based glass frit preferably includes one or two or more materials selected from K2O, Fe2O3, Sb2O3, ZnO, TiO2, Al2O3, and WO3. In this case, the second electrode 7 preferably includes the silica-based or phosphate-based glass of 3 to 5wt%.
The first electrode 6 may be a dot shape or a fine line shape. In detail, when the first electrode 6 is a dot shape, the first electrode 6 uses a structure consisting of a plurality of dots spaced apart from each other on a straight line as one unit and preferably has a structure in which at least two units are spaced apart from each other at a predetermined distance, and at least two units are more preferably spaced apart from each other so as to be parallel with each.
When the first electrode 6 is a dot shape, the second electrode 7 has a plurality of strip shapes that are spaced apart from each other, wherein the strip has a feature connecting at least two dots with each other.
A dot diameter of the first electrode 6 is preferably 30 ㎛ to 300 ㎛ and the dot diameter has a size enough to stably connect to the semiconductor substrate 100 by the punch through and minimize the damage of the passivation film 300.
When the first electrode 6 is a fine line, the first electrode 6 has a plurality of strip shapes that are spaced from each other so as to be parallel with each other and the second electrode 7 may be a plurality of strip shapes each enclosing the strips configuring the first electrode 6.
A width W1 of the first electrode 6 having a strip shape that is a fine line may be preferably 30 ㎛ to 300 ㎛. The width of the first electrode 6 has a consecutive line shape by the punch through and has a size enough to connect to the semiconductor substrate and minimize the damage of the passivation film 300. In this case, the width of the second electrode 7 is preferably 30 ㎛ to 300 ㎛ similar to the case in which the first electrode is a dot shape.
A width W2 of the second electrode 7 formed over the first electrode 6 and having the strip shape covering the first electrode 6 having the dot shape or the fine line shape is preferably 50 ㎛ to 1,000 ㎛. The electrode configured of the first electrode 6 and the second electrode 7 has a width capable of lowering the resistance increased by the first electrode 6 locally finely contacting the semiconductor substrate, wherein the width is formed so that the electrode may have a resistance of 3 to 6 x 10-6 Ωcm.
After the first electrode 6 and the second electrode 7 covering the first electrode 6 are printed over the passivation film 300, heat treatment for selectively connecting only the first electrode 6 to the semiconductor substrate 100 by the punch through is performed.
In this case, after the printing of the p-type electrode and the printing of the n-type electrode are performed, it is preferable to fabricate the p-type electrodes 8 and 7 and the n-type electrodes 5 by single heat treatment. The heat treatment for forming the p-type electrodes 8 and 7 and the n-type electrodes 5 is preferably performed at 100 to 1,000℃.
FIG. 4 shows an example showing a shape of the p-type electrodes 8 and 7 including the first electrode formed in a fine line type and the second electrode so as to cover the first electrode and the n-type electrodes 5 formed in the partial etching area of the passivation film 300. As shown in FIG. 4, the n-type electrodes 5 have a strip shape connecting the opening parts 1a of the via holes located on a straight line and ends of the n-type electrodes 5 having the plurality of strip shapes are connected with each other by n-type common electrodes 10 (n-type bus bar electrodes). The p-type electrodes 8 and 7 are located between two n-type electrodes 5 adjacent to each other and the ends of the plurality of p-type electrodes 8 and 7 are connected with each other by p-type common electrodes 20 (p-type bus bar electrodes). Therefore, the p-type or n-type electrodes having a comb shape in which the ends thereof are connected to each other by each common electrode are formed and may be preferably formed on the back surface of the semiconductor substrate 100 in an interdigitated structure or a fish bone structure in which the p-type electrodes 8 and 7 and the n-type electrodes are interdigitated with each other.
Similar to the p-type electrodes 6 and 7, the p-type common electrode 20 may preferably include a first common electrode 21 connecting the ends of the plurality of first electrodes 6 with each other and a second common electrode 22 formed over the first common electrode 21 to cover the first common electrode.
Similar to the p-type electrodes 6 and 7, only the first common electrode 21 of the first common electrode 21 and the second common electrode 22 is selectively connected with the semiconductor substrate 100 by the punch-through phenomenon. The first common electrode 21 may be formed by coating a material similar to the first electrode 6 material and the second common electrode 22 may be formed by coating a material similar to the second electrode 7.
The n-type common electrode 10 and the p-type common electrode 20 may be formed using the method similar to the formation of the n-type electrode 5 and the p-type electrodes 6 and 7 in the printing of the n-type electrode 5 and the p-type electrodes 6 and 7 of step e) and after the n-type electrode 5, the p-type electrodes 6 and 7, the n-type common electrode 10, and the p-type common electrode 20 are formed by the printing, the heat treatment for the selective punch-through may be preferably performed.
In the method of fabricating a solar cell according to the exemplary embodiment of the present invention, prior to the forming of the via holes 1 (step (a)), texturing a surface forming micro unevenness on the surface by etching the semiconductor substrate 100 may be further performed. The etching includes dry or wet etching and the textured surface includes a surface in which the micro unevenness having a reverse pyramid shape is arranged in plural.
Further, in the method of fabricating a solar cell according to the exemplary embodiment of the present invention, prior to forming the anti-reflective film 200 and the passivation film 300 (step (b)), fabricating a back surface field (BSF) surface layer forming the back surface field on the back surface of the semiconductor substrate 100 by coating the back surface opposite to the light receiving surface with a doping agent including the p-type impurity and heat-treating the semiconductor substrate coated with the p-type impurity doping agent may be further performed and only the first electrode of the first electrode and the second electrode of the p-type electrode may be electrically and physically connected with the BSF area formed on the back surface of the semiconductor substrate 100 by the selective punch through.
The method of fabricating a solar cell according to the exemplary embodiments of the present invention can fabricate the EWT solar cell of a selective emitter structure having different doping levels by using the minimum process. The exemplary embodiments of the present invention can form the front emitter of the light receiving surface, the via hole emitter, and the back emitter by a one-step doping process to shorten the process costs and time and prevent efficiency from being degraded due to the deterioration in the substrate, dope the via holes and the back emitter at high concentration to reduce serial resistance of a via hole and contact resistance of a back emitter and increase the high open circuit voltage and fill factor, and dope the front emitter of the front light receiving surface at low concentration to increase the short-wavelength absorptance and increase current by a shallow junction.
The present invention is not limited to the aforementioned exemplary embodiment and an application range is various and it is apparent that various modifications can be made to those skilled in the art without departing from the spirit of the present invention described in the appended claims.

Claims (7)

  1. A method of fabricating an EWT solar cell, comprsing:
    a) forming via holes penetrating through opposing first and second surfaces of a p-type semiconductor substrate using laser;
    b) forming an anti-reflective films on the first surface of the semiconductor substrate, and forming a passivation films on the second surface of the semiconductor substrate;
    c) partially removing the passivation film so as to expose a part of the second surface on which an opening part of each of the via holes is formed; and
    d) heat-treating the semiconductor substrate under the presence of n-type impurity to dope the n-type impurity on the semiconductor substrate.
  2. The method of claim 1, wherein: in step a), the via holes are spaced apart from each other, and
    in step c), the passivation film is partially removed in a strip shape so that at least two opening parts on the second surface are located in the part of the second surface exposed in a strip shape by the partial removal of the passivation film.
  3. The method of claim 1, wherein: each of the anti-reflection film and the passivation film of is a multilayered thin film made of semiconductor oxide, semiconductor nitride, alumina, titania, or a combination thereof.
  4. The method of claim 2, wherein: a thickness of the anti-reflective film is 10 nm to 30 nm and a thickness of the passivation is 30 to 100 nm.
  5. The method of claim 3, wherein: by the doping of step d), the first surface of the semiconductor substrate is formed with a front emitter layer having a sheet resistance of 50 Ω/square to 100 Ω/square, the via holes are formed with via hole emitters having a sheet resistance of 10 Ω/square to 50 Ω/square, and the part of the second surface exposed by step c) is formed with back emitters having a sheet resistance of 10 Ω/square to 50 Ω/square.
  6. The method of claim 2, further comprising: after step d),
    e) forming n-type electrodes by coating or depositing an electrode material so as to cover the opening parts of the via holes on the second surface, and forming p-type electrodes connecting the semiconductor substrate by penetrating through the passivation films by a punch through phenomenon.
  7. The method of claim 6, wherein the forming of the p-type electrodes includes:
    forming a first electrode connecting the semiconductor substrate over the passivation film by penetrating through the passivation film by the heat treatment;
    forming a second electrode over the first electrode so as to cover the first electrode without penetrating through the passivation film; and
    selectively connecting only the first electrode to the semiconductor substrate by the punch through phenomenon by heat treating the semiconductor substrate on which the first electrode and the second electrode are formed.
PCT/KR2012/006962 2011-08-31 2012-08-31 Method of fabricating emitter wrap through solar cell using one-step doping process WO2013032255A1 (en)

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