WO2004023567A2 - Method of manufacturing a solar cell - Google Patents

Method of manufacturing a solar cell Download PDF

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Publication number
WO2004023567A2
WO2004023567A2 PCT/JP2003/011203 JP0311203W WO2004023567A2 WO 2004023567 A2 WO2004023567 A2 WO 2004023567A2 JP 0311203 W JP0311203 W JP 0311203W WO 2004023567 A2 WO2004023567 A2 WO 2004023567A2
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WO
WIPO (PCT)
Prior art keywords
semiconductor substrate
substrate
paste
porous film
etching
Prior art date
Application number
PCT/JP2003/011203
Other languages
French (fr)
Other versions
WO2004023567A3 (en
Inventor
Takahiro Mishima
Naoki Ishikawa
Makiko Emoto
Original Assignee
Ebara Corporation
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Filing date
Publication date
Application filed by Ebara Corporation filed Critical Ebara Corporation
Priority to AU2003263592A priority Critical patent/AU2003263592A1/en
Publication of WO2004023567A2 publication Critical patent/WO2004023567A2/en
Publication of WO2004023567A3 publication Critical patent/WO2004023567A3/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic System or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
    • H01L21/302—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to change their surface-physical characteristics or shape, e.g. etching, polishing, cutting
    • H01L21/306—Chemical or electrical treatment, e.g. electrolytic etching
    • 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
    • H01L31/0236—Special surface textures
    • H01L31/02363—Special surface textures of the semiconductor body itself, e.g. textured active layers
    • 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
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy

Definitions

  • the present invention relates to a method of manufacturing a solar cell comprising a semiconductor substrate having an antireflection structure (i.e., textured structure) on a sunlight incident surface thereof, and more particularly to a method of forming such a textured structure on a substrate.
  • an antireflection structure i.e., textured structure
  • Solar cells are generally formed of a semiconductor material such as silicon, and light-receiving surfaces of solar cells are in the form of flat mirror surfaces. Accordingly, in order to reduce reflection from a light-receiving surface of a solar cell to enhance the efficiency of receiving sunlight, an antireflection film such as a silicon nitride film is formed on the light-receiving surface of the solar cell. Such an antireflection film can reduce reflection from a light-receiving surface of a solar cell to some degree. However, since a light-receiving surface of a solar cell is in the form of a flat mirror surface, as described above, reflection cannot be reduced sufficiently even though an antireflection film is formed on the light-receiving surface of the solar cell. From this point of view, a textured structure having fine irregularities may be formed on a surface of a solar cell to collect more sunlight into the solar cell to enhance the efficiency of receiving sunlight .
  • Such a textured structure has a pattern with a myriad of fine irregularities (convexes and concaves) formed on a surface of a substrate used for a solar cell .
  • a textured structure is formed by immersing a surface, having a (100) crystalline plane, of a substrate into a special etching liquid such as acid or alkali.
  • a special etching liquid such as acid or alkali.
  • a pattern of irregularities is produced in the form of a pyramid corresponding to a (111) crystalline plane.
  • Another method of forming a textured structure comprises forming a texture pattern of a resist film on a surface of a substrate by lithography, and then etching the substrate to transfer the texture pattern of the resist film to the surface of the substrate .
  • Other knownmethods include amethod of forming a textured structure on a substrate by etching after mechanically forming grooves in the substrate with a dicing saw, and a method of forming a textured structure on a substrate by laser etching. These methods are applicable to a silicon substrate or the like irrespective of its crystalline plane. However, these methods require complicated processes and hence high costs. Additionally, these methods may cause a surface of a substrate to be mechanically damaged so as to deteriorate the performance of a solar cell.
  • a porous film having a resistance to etching is formed on a semiconductor substrate for a solar cell with the use of a paste.
  • the semiconductor substrate having the porous film, which serves as a mask, is etched so as to form an antireflection structure having fine irregularities on the semiconductor substrate.
  • the paste should preferably contain particles having an alkali resistance or an acid resistance.
  • the paste may comprise a mixture of glass particles and an organic binder.
  • the glass particles may have a particle diameter of 10 ⁇ m or less .
  • the organic binder may comprise ethyl cellulose or terpineol.
  • the paste may comprise a mixture of low-density polyethylene particles and an organic binder.
  • the low-density polyethylene particles may have a particle diameter of 4 to 10 ⁇ m.
  • the porous film maybe formed on the semiconductor substrate so as to cover a surface of the semiconductor substrate at a ratio of 40 to 99%, preferably 80 to 98%.
  • the paste may be screen-printed on the semiconductor substrate.
  • the semiconductor substrate may comprise amonocrystalline substrate, a polycrystalline substrate, or a glass (amorphous) substrate .
  • a textured structure can readily be formed on a surface of a crystalline or amorphous semiconductor substrate for a solar cell, irrespective of the crystal structure of the semiconductor substrate. Therefore, it is possible to provide a solar cell which can receive more sunlight and has a high degree of efficiency.
  • the porous film has a resistance to etching and can readily be formed by screen-printing a paste containing particles having an alkali resistance or an acid resistance, and burning or heat hardening the paste.
  • a textured structure having fine irregularities can easily be formed on the surface of the substrate by immersing the substrate with the porous film in an etching liquid.
  • a textured structure can be formed irrespective of the crystalline plane of the semiconductor substrate. Therefore, the present invention is applicable to a crystalline substrate having a (111) crystalline plane.
  • FIGS. 1A through ID are schematic views showing processes of forming a textured structure on a surface of a crystalline substrate by etching; and
  • FIG. 2 is a schematic view showing an example of a porous film.
  • FIGS. 1A through ID and FIG. 2 A method of manufacturing a substrate for a solar cell according to the present invention will be described below with reference to FIGS. 1A through ID and FIG. 2.
  • FIGS. 1A through ID show a method of manufacturing a substrate for a solar cell according to the present invention.
  • FIG. 1A shows a semiconductor substrate 11 to be used for a solar cell.
  • the semiconductor substrate 11 is doped with impurities into an n-type or p-type .
  • the semiconductor substrate 11 may comprise a monocrystalline silicon substrate having a (111) crystalline plane.
  • the semiconductor substrate 11 may comprise a glass substrate, which is to be used for an amorphous silicon solar cell, or a polycrystalline silicon substrate.
  • the semiconductor substrate 11 may comprise a thin glass or polycrystalline silicon substrate mounted on a support substrate such as a ceramic or a metal plate .
  • silicon is used as a semiconductor material for a solar cell in the present embodiment, other materials such as gallium arsenide may be used instead of silicon.
  • a textured structure (antireflection structure) having fine irregularities (convexes and concaves) is formed on a sunlight incident surface 11a of the substrate 11.
  • a method of forming such a textured structure will be described below.
  • a paste for forming a porous film is applied onto the sunlight incident surface 11a of the semiconductor substrate 11 in accordance with a screen-printing method or the like. As shown in FIG. IB, the semiconductor substrate 11 with the paste is heated to form a porous film 13 having a resistance to etching (i.e., etching resistance).
  • FIG.1C shows an example in which the semiconductor substrate 11 having the porous film 13 is immersed in an acid/alkali etching liquid 17.
  • the semiconductor substrate 11 is etched so as to form a textured structure 15 on the sunlight incident surface 11a of the semiconductor substrate 11, as shown in FIG. ID.
  • a paste comprises a mixture of glass particles having a particle diameter of 10 ⁇ m or less and an organic binder such as ethyl cellulose or terpineol, which is generally used in a paste.
  • the glass particles have an alkali resistance or an acid resistance and serve as a material for a porous film.
  • a porous film having an etching resistance is formed on the semiconductor substrate 11.
  • any materials that can be expected to have an etching resistance may be used as a material for the porous film.
  • ceramics, graphite, TeflonTM, and polyethylene can be used as a material for the porous film.
  • theporosityofthe porous film canbe adjustedbychanging components and/or contents of the components in the paste . As a result, the reflectance of the textured structure can be adjusted.
  • the paste is screen-printed on the sunlight incident surface 11a of the semiconductor substrate 11 with a stainless mesh of about #400.
  • the semiconductor substrate 11 having the paste thereon is rapidly heated to about 600°C and then cooled.
  • a porous film 13 is formed on the semiconductor substrate 11 as shown in FIG. 2.
  • the porous film 13 contains glass particles 14 and pores therebetween.
  • the glass particles 14 are etched at an etching rate that is different than an etching rate at which silicon is etched.
  • the glass particles 14 are bonded to each other in a porous state. Since glass particles 14 have an etching resistance, an etching liquid affects the sunlight incident surface 11a of the substrate 11 through the pores in the porous film 13. As a result, a textured structure having fine irregularities is formed on the sunlight incident surface 11a.
  • the glass particles 14 may be etched depending upon the particular composition of the etching liquid.
  • the porous film 13 can be used as an effective film having an etching resistance.
  • a textured structure having good characteristics can be produced on the surface 11a of the substrate 11.
  • a porous film may be formed on a rear face of the substrate under the same conditions as described above to produce a substrate having textured structures formed on both surfaces.
  • the porous film 13, serving as a mask, should preferably cover the surface 11a of the substrate 11 at a ratio of 40 to 99%, more preferably 80 to 98% in view of a ratio of reducing light reflection to form a good textured structure.
  • the substrate 11 is immersed in an alkali solution 17 such as NaOH, which has been heated to about 70 °C, for several tens of seconds while the porous film 13 on the substrate 11 serves as amask for etching.
  • the sunlight incident surface 11a of the substrate 11 is etched through pores in the porous film 13.
  • a textured structure 15 is formed on the sunlight incident surface 11a of the substrate 11.
  • the substrate to be etched comprises silicon
  • the material for the etchingmask (porous film) comprises glass
  • the etching liquid comprises an alkali solution such as NaOH.
  • the etching mask (porous film) covers the sunlight incident surface of the substrate at a ratio of 80 to 98%
  • the etching liquid etches not only the sunlight incident surface of the substrate, but also the glass included in' the etchingmask. Therefore, the etching mask (porous film) can be removedby the etching liquidwhen irregularities have been formed on the surface of the silicon substrate.
  • a textured structure (antireflection structure) 15 having fine irregularities can be formed on the sunlight incident surface 11a of the substrate 11, as shown in FIG. ID.
  • a conventional material used for screen-printing does not have a sufficient resistance to the NaOH solution.
  • a paste for the porous film 13 according to the present invention contains particles having an alkali resistance or an acid resistance, and is burned or heat-hardened to bond the particles to each other. Therefore, the porous film 13 according to the present invention has sufficient resistance to etching. Further, the porous film 13 can readily be removed with the use of HF solution. In the first example, NaOH solution is used as the etching liquid.
  • etching liquid various kinds of acid or alkali solutions can be used as the etching liquid as long as the porous film has an etching resistance in view of particles having an alkali/acid resistance which is contained in the paste.
  • etching may be performed not only by an acid/alkali solution, but also by an etching gas, gas plasma, or the like.
  • a paste comprises a mixture of low-density polyethylene particles having a particle diameter of 4 to 10 ⁇ m and an organic binder of 20%.
  • the paste is screen-printed on a sunlight incident surface 11a of a semiconductor substrate 11 with a stainless mesh of about #400.
  • the semiconductor substrate 11 having the paste thereon is rapidly heated from a roomtemperature to 120 °C in 3 seconds with the use of an infrared heating furnace, held at 120 °C for 2 seconds, and then cooled from 120°C to 60°C in 5 seconds, so that a porous film 13 is formed on the semiconductor substrate 11.
  • the porous film 13 mainly contains low-density polyethylene particles .
  • the porous film 13 obtained under the above conditions covers the sunlight incident surface 11a of the substrate 11 at a ratio of about 94%.
  • the semiconductor substrate 11 having the porous film 13 serving as an etching mask is immersed in a mixture liquid having a HF-HN0 3 ratio of 1:20 for 5 seconds.
  • the semiconductor substrate 11 is etched via pores between the low-density polyethylene particles in the porous film 13.
  • a textured structure 15 is formed on the sunlight incident surface 11a of the semiconductor substrate 11, as shown in FIG. ID.
  • the semiconductor substrate 11 with the porous film 13 as an etching mask is then immersed in acetone within an ultrasonic cleaning device for 10 minutes to remove the porous film 13 on the semiconductor substrate 11.
  • the chemical liquid (acetone) is replaced with a new one, the semiconductor substrate 11 is immersed in the chemical liquid for 10 minutes to remove the porous film 13 completely.
  • the semiconductor substrate 11 is rinsed with running water for 2 minutes, and heated and dried at 110 °C for 5 minutes .
  • the processes are completed.
  • the maximum processing temperature for producing the porous film 13 is about 120°C, damage to the semiconductor substrate can considerably be reduced, and the cost of the apparatus can be reduced.
  • the paste can be formed simply by mixing particles having an etching resistance to a medium for etching a surface of a semiconductor substrate .
  • the porous film can readily be formed by applying the paste on the semiconductor substrate by a simple process, such as screen-printing, and then heat hardening or burning the paste.
  • a textured structure can readilybe formed on the semiconductor substratebyusing a general etching liquid (or gas) .
  • the method according to the present invention does not need expensive equipment and can achieve a high level of productivity.
  • the substrate 11 having the textured structure 15 formed thereon is then processed for formation of p-n junctions or the like to manufacture a solar cell.
  • the solar cell thus produced can sufficiently reduce light reflection to achieve a high trapping efficiency and a high photoelectric conversion efficiency. Therefore, a solar cell, which is easy to use and has a high photoelectric conversion efficiency, can be provided at a low cost.
  • the substrate may comprise a polycrystalline substrate, or a glass (amorphous) substrate.
  • the present invention is applicable not only to a substrate having a (111) crystalline plane as described in the above embodiment, but also to a substrate havingother crystallineplanes .
  • NaOH solution and HF/HN0 3 mixture are used as the etching liquid in the above examples, other acidic or alkali solutions, various etching gas, etching plasmamaybe used for etching the substrate .
  • a textured structure can readily be formed on a surface of a semiconductor substrate, irrespective of a surface orientation
  • a porous filmhaving an etching resistance is formed on a surface of a substrate by screen-printing, and the surface of the substrate is etched through pores in the porous film, so that a textured structure can easily be formed on the surface of the substrate with a high level of productivity at a low cost.
  • the present invention is suitable for manufacturing a solar cell comprising a semiconductor substrate having an antireflection structure (i.e., textured structure) onasunlight incident surface thereof.
  • an antireflection structure i.e., textured structure

Abstract

A porous film having a resistance to etching is formed on a semiconductor substrate for a solar cell by using a paste. The semiconductor substrate having the porous film, which serves as a mask, is etched so as to form an antireflection structure having fine irregularities on the semiconductor substrate. The paste should preferably contain particles having an alkali resistance or an acid resistance.

Description

DESCRIPTION
METHOD OF MANUFACTURING SOLAR CELL
Technical Field
The present invention relates to a method of manufacturing a solar cell comprising a semiconductor substrate having an antireflection structure (i.e., textured structure) on a sunlight incident surface thereof, and more particularly to a method of forming such a textured structure on a substrate.
Background Art
Solar cells are generally formed of a semiconductor material such as silicon, and light-receiving surfaces of solar cells are in the form of flat mirror surfaces. Accordingly, in order to reduce reflection from a light-receiving surface of a solar cell to enhance the efficiency of receiving sunlight, an antireflection film such as a silicon nitride film is formed on the light-receiving surface of the solar cell. Such an antireflection film can reduce reflection from a light-receiving surface of a solar cell to some degree. However, since a light-receiving surface of a solar cell is in the form of a flat mirror surface, as described above, reflection cannot be reduced sufficiently even though an antireflection film is formed on the light-receiving surface of the solar cell. From this point of view, a textured structure having fine irregularities may be formed on a surface of a solar cell to collect more sunlight into the solar cell to enhance the efficiency of receiving sunlight .
Such a textured structure has a pattern with a myriad of fine irregularities (convexes and concaves) formed on a surface of a substrate used for a solar cell . For example, such a textured structure is formed by immersing a surface, having a (100) crystalline plane, of a substrate into a special etching liquid such as acid or alkali. Specifically, when a surface, having a (100) crystalline plane, of a substrate is etched by the special etching liquid, a pattern of irregularities is produced in the form of a pyramid corresponding to a (111) crystalline plane. However, in a case of a crystalline substrate having a (111) crystalline plane, such a textured structure cannot readily be formed with the use of the aforementioned etching liquid. Another method of forming a textured structure comprises forming a texture pattern of a resist film on a surface of a substrate by lithography, and then etching the substrate to transfer the texture pattern of the resist film to the surface of the substrate . Other knownmethods include amethod of forming a textured structure on a substrate by etching after mechanically forming grooves in the substrate with a dicing saw, and a method of forming a textured structure on a substrate by laser etching. These methods are applicable to a silicon substrate or the like irrespective of its crystalline plane. However, these methods require complicated processes and hence high costs. Additionally, these methods may cause a surface of a substrate to be mechanically damaged so as to deteriorate the performance of a solar cell.
Disclosure of Invention
The present invention has been made in view of the above drawbacks. It is, therefore, an object of the present invention to provide a method of manufacturing a solar cell in which a textured structure can readily be formed on a surface of a semiconductor substrate so as to trap more sunlight in the solar cell, irrespective of a crystal structure According to an aspect of the present invention, a porous film having a resistance to etching is formed on a semiconductor substrate for a solar cell with the use of a paste. The semiconductor substrate having the porous film, which serves as a mask, is etched so as to form an antireflection structure having fine irregularities on the semiconductor substrate. The paste should preferably contain particles having an alkali resistance or an acid resistance. The paste may comprise a mixture of glass particles and an organic binder. The glass particles may have a particle diameter of 10 μm or less . The organic binder may comprise ethyl cellulose or terpineol.
Alternatively, the paste may comprise a mixture of low-density polyethylene particles and an organic binder. The low-density polyethylene particles may have a particle diameter of 4 to 10 μm.
The porous filmmaybe formed on the semiconductor substrate so as to cover a surface of the semiconductor substrate at a ratio of 40 to 99%, preferably 80 to 98%. The paste may be screen-printed on the semiconductor substrate.
The semiconductor substrate may comprise amonocrystalline substrate, a polycrystalline substrate, or a glass (amorphous) substrate . According to a method of manufacturing a solar cell as described above, a textured structure can readily be formed on a surface of a crystalline or amorphous semiconductor substrate for a solar cell, irrespective of the crystal structure of the semiconductor substrate. Therefore, it is possible to provide a solar cell which can receive more sunlight and has a high degree of efficiency.
Particularly, the porous film has a resistance to etching and can readily be formed by screen-printing a paste containing particles having an alkali resistance or an acid resistance, and burning or heat hardening the paste. Further, a textured structure having fine irregularities can easily be formed on the surface of the substrate by immersing the substrate with the porous film in an etching liquid. According to the present invention, a textured structure can be formed irrespective of the crystalline plane of the semiconductor substrate. Therefore, the present invention is applicable to a crystalline substrate having a (111) crystalline plane. Thus, it is possible to inexpensivelymanufacture a solar cell using a silicon onocrystalline substrate, which can achieve a high photoelectric conversion efficiency, and has a textured structure, which can achieve a high efficiency of receiving sunlight . The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.
Brief Description of Drawings FIGS. 1A through ID are schematic views showing processes of forming a textured structure on a surface of a crystalline substrate by etching; and FIG. 2 is a schematic view showing an example of a porous film.
Best Mode for Carrying Out the Invention
A method of manufacturing a substrate for a solar cell according to the present invention will be described below with reference to FIGS. 1A through ID and FIG. 2.
FIGS. 1A through ID show a method of manufacturing a substrate for a solar cell according to the present invention. FIG. 1A shows a semiconductor substrate 11 to be used for a solar cell. The semiconductor substrate 11 is doped with impurities into an n-type or p-type . For example, the semiconductor substrate 11 may comprise a monocrystalline silicon substrate having a (111) crystalline plane. The semiconductor substrate 11 may comprise a glass substrate, which is to be used for an amorphous silicon solar cell, or a polycrystalline silicon substrate. Alternatively, the semiconductor substrate 11 may comprise a thin glass or polycrystalline silicon substrate mounted on a support substrate such as a ceramic or a metal plate . Although silicon is used as a semiconductor material for a solar cell in the present embodiment, other materials such as gallium arsenide may be used instead of silicon.
A textured structure (antireflection structure) having fine irregularities (convexes and concaves) is formed on a sunlight incident surface 11a of the substrate 11. A method of forming such a textured structure will be described below.
First, a paste for forming a porous film is applied onto the sunlight incident surface 11a of the semiconductor substrate 11 in accordance with a screen-printing method or the like. As shown in FIG. IB, the semiconductor substrate 11 with the paste is heated to form a porous film 13 having a resistance to etching (i.e., etching resistance).
Then, the semiconductor substrate 11 having the porous film 13 thereon is immersed in an acid/alkali etching liquid or exposed to an acid/alkali etching gas or plasma . FIG .1C shows an example in which the semiconductor substrate 11 having the porous film 13 is immersed in an acid/alkali etching liquid 17. Thus, the semiconductor substrate 11 is etched so as to form a textured structure 15 on the sunlight incident surface 11a of the semiconductor substrate 11, as shown in FIG. ID.
A first specific example will be described below. In the first example, a paste comprises a mixture of glass particles having a particle diameter of 10 μm or less and an organic binder such as ethyl cellulose or terpineol, which is generally used in a paste. The glass particles have an alkali resistance or an acid resistance and serve as a material for a porous film. With such a paste, a porous film having an etching resistance is formed on the semiconductor substrate 11.
Although glass particles have an alkali resistance or an acid resistance are used as a material for the porous film in the first example, any materials that can be expected to have an etching resistance may be used as a material for the porous film. For example, ceramics, graphite, Teflon™, and polyethylene can be used as a material for the porous film. Further, theporosityofthe porous filmcanbe adjustedbychanging components and/or contents of the components in the paste . As a result, the reflectance of the textured structure can be adjusted.
The paste is screen-printed on the sunlight incident surface 11a of the semiconductor substrate 11 with a stainless mesh of about #400. The semiconductor substrate 11 having the paste thereon is rapidly heated to about 600°C and then cooled. As a result, a porous film 13 is formed on the semiconductor substrate 11 as shown in FIG. 2. The porous film 13 contains glass particles 14 and pores therebetween. The glass particles 14 are etched at an etching rate that is different than an etching rate at which silicon is etched. The glass particles 14 are bonded to each other in a porous state. Since glass particles 14 have an etching resistance, an etching liquid affects the sunlight incident surface 11a of the substrate 11 through the pores in the porous film 13. As a result, a textured structure having fine irregularities is formed on the sunlight incident surface 11a.
At that time, the glass particles 14 may be etched depending upon the particular composition of the etching liquid. However, when the shape of the porous film 13 is adjusted so that the porous film 13 has a proper thickness and a proper ratio of covering the surface 11a of the substrate 11, the porous film 13 can be used as an effective film having an etching resistance. As a result, a textured structure having good characteristics can be produced on the surface 11a of the substrate 11. According to an intended use, a porous film may be formed on a rear face of the substrate under the same conditions as described above to produce a substrate having textured structures formed on both surfaces.
The porous film 13, serving as a mask, should preferably cover the surface 11a of the substrate 11 at a ratio of 40 to 99%, more preferably 80 to 98% in view of a ratio of reducing light reflection to form a good textured structure. As shown in FIG. 1C, the substrate 11 is immersed in an alkali solution 17 such as NaOH, which has been heated to about 70 °C, for several tens of seconds while the porous film 13 on the substrate 11 serves as amask for etching. Thus, the sunlight incident surface 11a of the substrate 11 is etched through pores in the porous film 13. As a result, as shown in FIG. ID, a textured structure 15 is formed on the sunlight incident surface 11a of the substrate 11.
It is assumed that the substrate to be etched comprises silicon, the material for the etchingmask (porous film) comprises glass, and the etching liquid comprises an alkali solution such as NaOH. In a case where the etching mask (porous film) covers the sunlight incident surface of the substrate at a ratio of 80 to 98%, the etching liquid etches not only the sunlight incident surface of the substrate, but also the glass included in' the etchingmask. Therefore, the etching mask (porous film) can be removedby the etching liquidwhen irregularities have been formed on the surface of the silicon substrate. Thus, it is possible to dispense with a process of removing the etching mask,' thereby greatly reducing the number of processes and the number of the chemical liquids used.
As described above, a textured structure (antireflection structure) 15 having fine irregularities can be formed on the sunlight incident surface 11a of the substrate 11, as shown in FIG. ID. A conventional material used for screen-printing does not have a sufficient resistance to the NaOH solution. However, a paste for the porous film 13 according to the present invention contains particles having an alkali resistance or an acid resistance, and is burned or heat-hardened to bond the particles to each other. Therefore, the porous film 13 according to the present invention has sufficient resistance to etching. Further, the porous film 13 can readily be removed with the use of HF solution. In the first example, NaOH solution is used as the etching liquid. However, various kinds of acid or alkali solutions can be used as the etching liquid as long as the porous film has an etching resistance in view of particles having an alkali/acid resistance which is contained in the paste. Alternatively, etching may be performed not only by an acid/alkali solution, but also by an etching gas, gas plasma, or the like.
A second specific example will be described below. In the second example, a paste comprises a mixture of low-density polyethylene particles having a particle diameter of 4 to 10 μm and an organic binder of 20%. The paste is screen-printed on a sunlight incident surface 11a of a semiconductor substrate 11 with a stainless mesh of about #400. The semiconductor substrate 11 having the paste thereon is rapidly heated from a roomtemperature to 120 °C in 3 seconds with the use of an infrared heating furnace, held at 120 °C for 2 seconds, and then cooled from 120°C to 60°C in 5 seconds, so that a porous film 13 is formed on the semiconductor substrate 11. The porous film 13 mainly contains low-density polyethylene particles . The porous film 13 obtained under the above conditions covers the sunlight incident surface 11a of the substrate 11 at a ratio of about 94%.
The semiconductor substrate 11 having the porous film 13 serving as an etching mask is immersed in a mixture liquid having a HF-HN03 ratio of 1:20 for 5 seconds. Thus, the semiconductor substrate 11 is etched via pores between the low-density polyethylene particles in the porous film 13. As a result, a textured structure 15 is formed on the sunlight incident surface 11a of the semiconductor substrate 11, as shown in FIG. ID. The semiconductor substrate 11 with the porous film 13 as an etching mask is then immersed in acetone within an ultrasonic cleaning device for 10 minutes to remove the porous film 13 on the semiconductor substrate 11. After the chemical liquid (acetone) is replaced with a new one, the semiconductor substrate 11 is immersed in the chemical liquid for 10 minutes to remove the porous film 13 completely. Then, the semiconductor substrate 11 is rinsed with running water for 2 minutes, and heated and dried at 110 °C for 5 minutes . Thus, the processes are completed. When an experiment was carried out under the above conditions, it was confirmed that, with regard to reflection properties of the textured structure of the semiconductor substrate, the reflectivity could largely be reduced as compared to a mirror-finished silicon substrate, which had not been processed. Because the maximum processing temperature for producing the porous film 13 is about 120°C, damage to the semiconductor substrate can considerably be reduced, and the cost of the apparatus can be reduced.
According to the method of manufacturing a semiconductor substrate for a solar cell described above, it is easy to prepare the paste, whichever paste of the first or second example is used. Specifically, the paste can be formed simply by mixing particles having an etching resistance to a medium for etching a surface of a semiconductor substrate . Further, the porous film can readily be formed by applying the paste on the semiconductor substrate by a simple process, such as screen-printing, and then heat hardening or burning the paste. A textured structure can readilybe formed on the semiconductor substratebyusing a general etching liquid (or gas) . Thus, the method according to the present invention does not need expensive equipment and can achieve a high level of productivity.
The substrate 11 having the textured structure 15 formed thereon is then processed for formation of p-n junctions or the like to manufacture a solar cell. The solar cell thus produced can sufficiently reduce light reflection to achieve a high trapping efficiency and a high photoelectric conversion efficiency. Therefore, a solar cell, which is easy to use and has a high photoelectric conversion efficiency, can be provided at a low cost.
Although a silicon monocrystalline substrate is used as the substrate in the above embodiment, the substrate may comprise a polycrystalline substrate, or a glass (amorphous) substrate. The present invention is applicable not only to a substrate having a (111) crystalline plane as described in the above embodiment, but also to a substrate havingother crystallineplanes . Although NaOH solution and HF/HN03 mixture are used as the etching liquid in the above examples, other acidic or alkali solutions, various etching gas, etching plasmamaybe used for etching the substrate .
As described above, according to the present invention, a textured structure can readily be formed on a surface of a semiconductor substrate, irrespective of a surface orientation
(crystalline plane) of the semiconductor substrate. Specifically, a porous filmhaving an etching resistance is formed on a surface of a substrate by screen-printing, and the surface of the substrate is etched through pores in the porous film, so that a textured structure can easily be formed on the surface of the substrate with a high level of productivity at a low cost. Thus, accordingto the present invention, it is possible toprovide a solar cell having a high photoelectric conversion efficiency at low manufacturing cost.
While the present invention has been described in detail with reference to the preferred embodiment thereof, it will be apparent to those skilled in the art that many modifications and variations may be made therein without departing from the spirit and scope of the present invention.
Industrial Applicability The present invention is suitable for manufacturing a solar cell comprising a semiconductor substrate having an antireflection structure (i.e., textured structure) onasunlight incident surface thereof.

Claims

1. A method of manufacturing a solar cell, said method comprising: forming a porous film, having a resistance to etching, on a semiconductor substrate for a solar cell by employing a paste; and etching the semiconductor substrate having the porous film, which serves as a mask, to form an antireflection structure having fine irregularities on the semiconductor substrate.
2. The method according to claim 1, wherein the paste contains particles having at least one of an alkali resistance and an acid resistance.
3. The method according to claim 1, wherein the paste comprises a mixture of glass particles and an organic binder.
4. The method according to claim 3, wherein the glass particles have a particle diameter of 10 μm or less.
5. The method according to claim 3, wherein the organic binder comprises at least one of ethyl cellulose and terpineol.
6. The method according to claim 1, wherein the paste comprises a mixture of low-density polyethylene particles and an organic binder.
7. The method according to claim 6, wherein the low-density polyethylene particles have a particle diameter of 4 to 10 μm.
8. The method according to claim 1, wherein said forming operation comprises forming the porous film on the semiconductor substrate so as to cover a surface of the semiconductor substrate at a ratio of 40 to 99%.
9. The method according to claim 1, wherein said forming operation comprises forming the porous film on the semiconductor substrate so as to cover a surface of the semiconductor substrate at a ratio of 80 to 98%.
10. The method according to claim 1, wherein said forming operation comprises screen-printing the paste on the semiconductor substrate.
11. The method according to claim 10, wherein said forming operation further comprises at least one of burning and heat hardening the paste.
12. The method according to claim 1, wherein the semiconductor substrate comprises at least one of a monocrystalline substrate, a polycrystalline substrate, and a glass substrate.
13. A method of manufacturing a semiconductor substrate for a solar cell, said method comprising: forming a porous film, having a resistance to etching, on a semiconductor substrate by employing a paste; and etching the semiconductor substrate having the porous film, which serves as a mask, to form a structure having fine irregularities on the semiconductor substrate.
14. The method according to claim 13, wherein the paste contains particles having at least one of an alkali resistance and an acid resistance.
15. The method according to claim 13, wherein the paste comprises a mixture of glass particles and an organic binder.
16. The method according to claim 15, wherein the glass particles have a particle diameter of 10 μm or less.
17. The method according to claim 15, wherein the organic binder comprises at least one of ethyl cellulose and terpineol.
18. The method according to claim 13, wherein the paste comprises a mixture of low-density polyethylene particles and an organic binder.
19. The method according to claim 18, wherein the low-density polyethylene particles have a particle diameter of 4 to 10 μm.
20. The method according to claim 13, wherein said forming operation comprises forming the porous film on the semiconductor substrate so as to cover a surface of the semiconductor substrate at a ratio of 40 to 99%.
21. The method according to claim 13, wherein said forming operation comprises forming the porous film on the semiconductor substrate so as to cover a surface of the semiconductor substrate at a ratio of 80 to 98%.
22. The method according to claim 13, wherein said forming operation comprises screen-printing the paste on the semiconductor substrate.
23. The method according to claim 22, wherein said forming operation further comprises at least one of burning and heat hardening the paste.
24. The method according to claim 13, wherein the semiconductor substrate comprises at least one of a monocrystalline substrate, a polycrystalline substrate, and a glass substrate.
PCT/JP2003/011203 2002-09-06 2003-09-02 Method of manufacturing a solar cell WO2004023567A2 (en)

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WO2012039830A1 (en) * 2010-09-20 2012-03-29 Sunpower Corporation Method of fabricating a solar cell
DE102010044132A1 (en) * 2010-11-18 2012-05-24 Schott Ag Method for applying structure in surface of silicon wafer, involves applying mask base material to surface of solar cell, coating selected pattern with plunger, and contacting liquid etching medium with etching mask
CN102496660A (en) * 2011-12-30 2012-06-13 常州亿晶光电科技有限公司 Acid-base combined monocrystalline silicon solar cell texturing method
DE102011084346A1 (en) 2011-10-12 2013-04-18 Schott Solar Ag Process for treating silicon wafers, treatment liquid and silicon wafers
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EP2323173A4 (en) * 2008-09-05 2017-11-22 LG Chem, Ltd. Paste and manufacturing methods of a solar cell using the same
TWI586617B (en) * 2010-04-30 2017-06-11 康寧公司 Anti-glare surface and method of making
US8445309B2 (en) 2010-08-20 2013-05-21 First Solar, Inc. Anti-reflective photovoltaic module
US8658454B2 (en) 2010-09-20 2014-02-25 Sunpower Corporation Method of fabricating a solar cell
US9263622B2 (en) 2010-09-20 2016-02-16 Sunpower Corporation Method of fabricating a solar cell
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DE102011084346A1 (en) 2011-10-12 2013-04-18 Schott Solar Ag Process for treating silicon wafers, treatment liquid and silicon wafers
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JP2013143469A (en) * 2012-01-11 2013-07-22 Aurotek Corp Manufacturing method for nano-fine structure

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