JP2012059953A - Manufacturing method of silicon substrate for photoelectric conversion element and manufacturing method of photoelectric conversion element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a silicon substrate for a photoelectric conversion element capable of manufacturing, in good yield, a photoelectric conversion element having a good incidence efficiency of sunlight to a photoelectric conversion layer and a high photoelectric conversion efficiency.SOLUTION: A manufacturing method of a silicon substrate for a photoelectric conversion element comprises: a step of forming a liquid composition including a first particle which has resistance for dry etching and whose average particle diameter is 0.05 μm to 0.3 μm, a second particle whose resistance is lower than the first particle and whose average particle diameter is 0.05 μm to 0.3 μm, and a binder whose resistance is lower than the first particle into a film in a coating method on one principal surface 10s of a one conductivity-type crystal silicon substrate 10 and arranging a mask material; a step of forming an irregular structure 10t which suppresses sunlight reflection by dry etching on the principal surface 10s in which the mask material was arranged; and a cleaning step of sequentially performing dry etching processing for the irregular structure 10t with a hydrogen gas and processing for immersing the structure in dilute hydrofluoric acid.

Description

  The present invention relates to a method for producing a silicon substrate for a crystalline silicon-based photoelectric conversion element and a method for producing a photoelectric conversion element obtained by using the method.

  Crystalline Si solar cells using a single crystal Si or polycrystalline Si substrate as a photoelectric conversion layer have been put into practical use. In crystalline Si solar cells, the surface of the Si substrate on the sunlight receiving side is uneven, which is called a textured structure, in order to efficiently reflect sunlight into the photoelectric conversion layer by suppressing the reflection of sunlight on the substrate surface. A structure is formed.

  In general, the wet etching method using a sodium hydroxide aqueous solution is used for monocrystalline Si and the wet etching method using a mixed solution of hydrofluoric acid and nitric acid is used for polycrystalline Si for forming a texture structure.

  However, the reflectance of sunlight with a texture structure formed by these methods is about 10% for a single crystal and about 25% for a polycrystal, making it difficult to obtain a structure that reduces the reflectance satisfactorily. FIG. 8A shows a surface SEM image of a single crystal Si texture structure used in a commercially available photoelectric conversion cell, and FIG. 8B shows a surface SEM image of a polycrystalline Si texture structure used in a commercially available photoelectric conversion cell. It is. FIG. 9 shows the result of measurement by the inventor of the wavelength dependence of the reflectance of the photoelectric conversion cell of FIG. As shown in the figure, it is difficult to obtain a texture structure with good in-plane uniformity and reproducibility due to the dependency of wet etching on the crystal orientation, especially in the case of polycrystal.

  Regardless of single crystal or polycrystal, the use of dry etching such as reactive ion etching with strong directivity is being studied as a method for forming a texture structure with good in-plane uniformity and reproducibility (patent document) 1 to Patent Document 6).

JP 60-27195 A JP-A-5-75152 JP-A-9-102625 JP 2003-197940 A JP 2010-74004 A Japanese Patent No. 4340031

  However, in the formation of unevenness by dry etching, it is necessary to clean the uneven structure surface after processing to remove etching residues. If the projections of the concavo-convex structure have a large aspect ratio and the pitch between the ridges is submicron to micron order, it is difficult to remove the etching residue well, and it cannot be removed on the surface of the concavo-convex structure. Etching residue tends to remain. The silicon substrate for photoelectric conversion elements forms a pn junction by diffusing a dopant from the surface of the concavo-convex structure after the formation of the concavo-convex structure. Therefore, the etching residue remaining on the surface of the concavo-convex structure hinders the diffusion of the dopant in the dopant diffusion step and adversely affects the formation of the pn junction, and thus it is difficult to manufacture a highly efficient photoelectric conversion element with a high yield.

  The present invention has been made in view of the above circumstances, and a crystalline Si-based photoelectric conversion element having a concavo-convex structure in which a substance that inhibits a pn junction is satisfactorily removed from the surface and incident light can be well confined. The object is to manufacture a silicon substrate for use.

  Another object of the present invention is to produce a photoelectric conversion element having high photoelectric conversion efficiency with a high yield by using the silicon substrate produced by the method for producing a silicon substrate for photoelectric conversion elements.

The method for producing a silicon substrate for photoelectric conversion elements of the present invention is a textured structure surface comprising a concavo-convex structure having a large number of convex portions that suppress reflection of sunlight on one main surface of one conductivity type crystalline silicon substrate. A method for producing a silicon substrate for a photoelectric conversion element comprising:
First particles having resistance to dry etching for forming the concavo-convex structure on one main surface of the one conductivity type crystalline silicon substrate and having an average particle diameter of 0.05 μm or more and 0.3 μm or less; A liquid composition comprising second particles having a lower resistance than the first particles and an average particle size of 0.05 μm or more and 0.3 μm or less, and a binder having a lower resistance than the first particles. Applying an object to one main surface of the silicon substrate and arranging the mask material;
Forming the concavo-convex structure by dry etching on the main surface on which the mask material is disposed;
The manufacturing method of the silicon substrate for photoelectric conversion elements characterized by having the washing | cleaning process of implementing sequentially the dry etching process by hydrogen gas, and the process immersed in dilute hydrofluoric acid in the said uneven structure.

  In the dry etching process using hydrogen gas, the dry etching process is preferably performed under the conditions of a hydrogen gas flow rate of 50 to 500 sccm, a gas pressure of 1 to 20 Pa, and a high frequency output of 50 to 300 W.

  Moreover, it is preferable that an average particle diameter of the first particles and / or the second particles is 0.1 to 0.2 μm.

It is preferable to use inorganic particles such as particles mainly composed of SiO ₂ as the first particles, and resin particles such as particles mainly composed of an acrylic resin as the second particles, and also use polyvinyl as the binder. It is preferable to use a water-soluble polymer such as alcohol or a water-dispersible polymer as a main component.

  The method for producing a silicon substrate for photoelectric conversion elements of the present invention can be preferably applied when the one-conductivity-type crystalline silicon substrate is made of polycrystalline silicon (may contain inevitable impurities).

  The dry etching is preferably reactive ion etching.

A method for producing a photoelectric conversion element of the present invention is prepared by preparing a silicon substrate produced by the method for producing a silicon substrate for a photoelectric conversion element of the present invention,
A pn junction is formed on the textured structure surface of the silicon substrate by diffusing impurities of a conductivity type different from that of the substrate,
Forming a surface electrode on the textured structure surface;
A back electrode layer is formed on a main surface opposite to the one main surface of the silicon substrate. In such a configuration, a translucent conductive layer can be directly formed on the texture structure surface as the surface electrode.

  In the method for producing a silicon substrate for a crystalline Si photoelectric conversion element of the present invention, first, at least two kinds of fine particles having different dry etching resistance (average particle size of 0.1 mm) are formed on one main surface of one conductivity type crystalline silicon substrate. After forming a concavo-convex structure on the surface of the crystalline silicon substrate by dry etching using a mask material having a thickness of 05 μm to 0.3 μm), the concavo-convex structure is dry-etched with hydrogen gas and immersed in dilute hydrofluoric acid. Are sequentially performed to clean the surface of the concavo-convex structure. In such a configuration, as the texture structure surface of the crystalline Si-based photoelectric conversion element, it is possible to effectively suppress the reflection of incident light (sunlight) and form a fine concavo-convex structure having a good light confinement effect. Etching residues generated by the formation of the concavo-convex structure can be favorably removed from the surface of the concavo-convex structure. It is known that an etching residue inhibits formation of a pn junction in the manufacture of a photoelectric conversion element.

  Therefore, according to the present invention, a silicon substrate for a crystalline Si photoelectric conversion element having a concavo-convex structure in which a substance that inhibits a pn junction is satisfactorily removed on the surface and incident light can be confined well is manufactured. be able to.

  Further, in the production of a crystalline Si-based photoelectric conversion element, by using the crystalline Si substrate manufactured by the method for manufacturing a silicon substrate for photoelectric conversion elements of the present invention, a texture structure surface having a good light confinement effect is good. A pn junction can be formed. Therefore, according to the present invention, a photoelectric conversion element having high incident light utilization efficiency and high photoelectric conversion efficiency can be manufactured with high yield.

(A)-(h) is the figure which showed typically the flow of the manufacturing method of the photoelectric conversion element of one Embodiment concerning this invention. (A) is thickness direction sectional drawing which shows the structure of the silicon substrate manufactured by the manufacturing method of the silicon substrate for photoelectric conversion elements of this invention, (b) is the figure which expanded the uneven | corrugated structure part of (a), (c). Is a cross-sectional view in the thickness direction showing the configuration of a photoelectric conversion element manufactured by the method for manufacturing a silicon substrate for photoelectric conversion elements of the present invention (A) is a surface SEM photograph of the texture structure after performing ultrasonic cleaning in pure water after forming irregularities by dry etching, and (b) is a texture when the ultrasonic cleaning time of (a) is extended by 3 minutes. Surface SEM photograph of the structure, (c) is a surface SEM photograph of the texture structure when the ultrasonic cleaning time is further extended for 5 minutes. In Example 1, the surface SEM image when the texture structure surface is cleaned by dry etching treatment with hydrogen gas (A) shows the wavelength dependence of the reflectance of the cut surface of the polycrystalline silicon substrate obtained by wire saw cutting from the polycrystalline silicon ingot, and (b) shows the wavelength dependence of the reflectance of the texture structure of Example 1. Illustration The figure which shows the wavelength dependence of the reflectance of the texture structure surface at the time of using a single-crystal Si substrate (when it is unprocessed, a cut surface). (A) is a cut surface of a single crystal silicon substrate obtained by wire saw cutting from a single crystal silicon ingot, (b) is a textured structure surface when alkali etching cleaning is performed on the surface of (a), and (c) is ( (b) a structure provided with a SiN antireflection film on the surface, (d) is a textured structure surface in which a concavo-convex structure is formed by dry etching using a mask material in the present invention, and immersion treatment in diluted hydrofluoric acid is performed, e) is the texture structure surface of Example 2. (A) And (b) is a figure which shows the relationship between the reflectance of a texture structure with respect to the light of wavelength 1000nm, and surface roughness. The surface SEM image of the texture structure of a commercially available Si type photoelectric conversion cell. (A) is a single crystal Si system, (b) is a polycrystal Si system. The figure which shows the wavelength dependence of the reflectance of the texture structure surface of the commercially available Si type photoelectric conversion cell corresponding to FIG.

"Method for manufacturing photoelectric conversion element"
Referring to FIGS. 1A to 1H, a method for manufacturing a silicon substrate for crystalline Si photoelectric conversion elements (hereinafter referred to as a silicon substrate for photoelectric conversion elements) according to an embodiment of the present invention, and crystalline Si A method for producing a photoelectric conversion element (hereinafter referred to as a photoelectric conversion element) will be described. Fig.1 (a)-(h) is a schematic sectional drawing which shows the flow of the manufacturing method of the silicon substrate 1 for photoelectric conversion elements, and the photoelectric conversion element 2 using the same.

  2A is a schematic cross-sectional view of the silicon substrate 1 for photoelectric conversion elements obtained by the method for manufacturing a silicon substrate for photoelectric conversion elements of the present invention, and FIG. 2B is a diagram showing a part of the concavo-convex structure 10t in FIG. An enlarged schematic view, (c), is a schematic cross-sectional view in the thickness direction showing the configuration of the photoelectric conversion element 2 obtained by the method for manufacturing a photoelectric conversion element of the present invention. In order to facilitate visual recognition, the scales of the respective parts are shown as being appropriately changed.

  In the method for manufacturing a photoelectric conversion element of the present invention, first, the silicon substrate 1 for photoelectric conversion element shown in FIG. 2A is manufactured, and by using this silicon substrate 1 for photoelectric conversion element, FIG. The photoelectric conversion element 2 shown in FIG.

  First, a p-type crystalline silicon substrate (wafer) 10 having a smooth surface on one principal surface (surface 10s) is prepared (FIG. 1A).

  The silicon crystal substrate 10 may be crystalline Si and may be single crystal or polycrystal. In the section of “Background Art”, the reflectance of sunlight in the texture structure of a crystalline Si photoelectric conversion element by conventional wet etching is as high as about 10% for a single crystal and about 25% for a polycrystal, and is also dry etching. He stated that the reflectance of sunlight could not be reduced sufficiently even in the texture structure formed by the above. In addition, it was described that it is difficult to obtain a texture structure with good in-plane uniformity and reproducibility in the case of polycrystal due to the crystal orientation dependence of wet etching. In the silicon substrate for a photoelectric conversion element and the method for manufacturing the photoelectric conversion element of the present invention, a texture structure surface having a very low reflectivity with single-digit reflectivity is similarly formed in both cases of single crystal and polycrystal. Therefore, a greater effect can be obtained particularly when the present invention is applied to polycrystalline Si having problems in low reflectivity and in-plane uniformity reproducibility.

  In the case where the silicon substrate 10 is single crystal silicon, an ingot formed by a pulling method or the like, and in the case of polycrystalline silicon, an ingot obtained by melting and solidifying the raw material in a crucible with a wire saw or the like has a desired thickness (for example, about 300 μm). In general, the method obtained by slicing into two. In addition, in the case of polycrystalline silicon, it can be obtained by pulling up from the melt into a plate shape.

  The smoothness of the surface 10s, which is the formation surface of the concavo-convex structure 10t, is preferably good because it affects the in-plane uniformity of the depth of the concavo-convex when the concavo-convex structure 10t, which is a subsequent process, is formed.

  In a crystalline silicon-based photoelectric conversion element, pn junction formation described later is generally performed by a technique in which a dopant of the other conductivity type is diffused from above the substrate of one conductivity type. Therefore, the thickness of the conductive layer on the lower layer side is on the order of microns, whereas the thickness of the conductive layer on the upper layer side is a very thin layer on the order of several hundred nm.

  On the other hand, as described above, a technique for cutting out an ingot using a wire saw is generally used for a silicon substrate. When a wire saw is used for cutting, a micron order damage is generally left on the cut surface. .

  Therefore, when the concavo-convex structure 10t, which is the subsequent process, is formed by dry etching while leaving these damages, the damaged portion is more fragile than the undamaged silicon substrate portion, and therefore, the damage is more damaged during the dry etching than the concavo-convex formation. The removal process of the part becomes dominant, and a concavo-convex structure is formed after damage removal. Therefore, it is difficult to form the concavo-convex structure 10t with a good pattern when forming the concavo-convex structure 10t, and the in-plane uniformity of the concavo-convex shape and depth depends on the in-plane distribution of damage existing on the surface. The variation of the is large.

  Therefore, the silicon substrate 1 for photoelectric conversion elements and the crystalline silicon substrate 10 used for manufacturing the photoelectric conversion elements 2 are not used as they are, regardless of whether they are single crystals or polycrystals. The damage must be removed.

  On the other hand, if the surface smoothness is good, and there is no damage or unevenness that affects the unevenness forming process by dry etching, such as wire saw damage, it is used without performing the above damage removal treatment. be able to.

  In the method for manufacturing the silicon substrate 1 for photoelectric conversion elements, the concavo-convex structure 10t has a plurality of first resistances against dry etching that forms the concavo-convex structure 10t on one main surface 10s (surface 10s) of the crystalline Si substrate 10. A mask material 50 including particles 51 and a plurality of second particles 52 having a dry etching resistance lower than that of the first particles 51 is provided, and irregularities are formed by dry etching using the plurality of first particles as a mask. It is formed by washing the residue.

  The method for forming the mask material 50 on the surface 10s is not particularly limited, and a sheet-shaped mask material 50 prepared in advance may be used, or the first particles 51 having resistance to the dry etching and the first A liquid composition containing a second particle 52 having a dry etching resistance lower than that of the particle 51 and a binder having a lower dry etching resistance than that of the first particle 51 is prepared, and the liquid composition is applied to the surface 10s. It may be formed by coating.

  In the present embodiment, a mode in which the mask material 50 is formed by coating film formation, which is the latter, will be described.

(Preparation of coating solution (liquid composition))
First, prior to the formation of the concavo-convex structure 10t, a first particle 51 having resistance to the dry etching, which is a raw material liquid for the mask material 50, and a second particle 52 having lower dry etching resistance than the first particle 51, A liquid composition containing a plurality of particle groups each including a binder and a binder (binder) having lower dry etching resistance than the first particles 51 is prepared. The particle group is not limited to the above two types of particles, and may include other types of particles.

  Here, the first particles have resistance to etching and are difficult to be etched by the etching process, whereas the second particles are easily etched by the dry etching process. Specifically, regarding the etching rate, for example, when the etching rate ER is “second particle etching rate / first particle etching rate”, it is preferable that ER> 5, and further ER> 10.

  The first particles are not particularly limited as long as they have etching resistance, and examples thereof include inorganic particles, organic dye / pigment particles containing inorganic elements, latex particles and capsule particles containing inorganic elements, and the like. Among these, as the first particles, inorganic particles are preferable from the viewpoints of etching resistance, availability, and handleability.

  Examples of the inorganic particles include non-metallic materials such as titanium oxide, silica (SiO2), calcium carbonate, and strontium carbonate, metals, and semiconductor materials. For example, the metal includes Cu, Au, Ag, Sn, Pt, Pd, Ni, Co, Rh, Ir, Al, Fe, Ru, Os, Mn, Mo, W, Nb, Ta, Bi, Sb, and Pb. Examples thereof include a single metal selected from the group or an alloy material composed of one or more of the metals selected from the group. Examples of the semiconductor include Si, Ge, AlSb, InP, GaAs, GaP, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, PbS, PbSe, PbTe, SeTe, and CuCl. Moreover, the microcapsule etc. which included these inorganic particles and used silica as the wall membrane material are mentioned.

  Examples of organic dye / pigment particles containing inorganic elements include metal element-containing azo dye particles and metal element-containing phthalocyanine dye particles.

Examples of latex particles or capsule particles containing inorganic elements include particles in which acrylic latex is coated with colloidal silica, particles in which acrylic latex is coated in silicate, particles in which polystyrene latex particles are coated with silica, and the like. Is not particularly limited as long as it has a difference in etching resistance with the first particles described above, but resin particles can be preferably used because of low etching resistance and easy handling. The resin particles are preferably thermoplastic resin particles, and examples thereof include acrylic resin particles, polyethylene resin particles, polypropylene resin particles, polyamide particles, polyimide particles, polyethylene terephthalate resin particles, polystyrene particles, and silicone resins.

  The average particle size of the first particles and the second particles may be the same, but the average particle size of the second particles is preferably larger than the first particles. Thereby, it becomes easy to ensure the particle | grain space | interval of the 1st particle | grains produced by the etching of 2nd particle | grains.

  The average particle diameter of the first particles and the second particles is 0.05 μm to 1 μm from the viewpoint of thinning the particle layer to be formed and obtaining the concavo-convex structure 10 t that satisfactorily reduces the reflectance of sunlight. preferable. In order to achieve a lower reflectance, the thickness is preferably 0.5 μm or less, and more preferably 0.3 μm or less. Furthermore, considering the ease of cleaning the etching residue in the subsequent process, the average particle size of the first particles and the second particles is preferably 0.1 μm to 0.5 μm, more preferably 0.1 μm to 0.5 μm. More preferably, the thickness is 0.1 μm to 0.2 μm.

  Here, the average particle diameter of a particle means the particle diameter obtained by a dynamic light scattering method, and the measuring method is as follows. In the dynamic light scattering method, it is possible to measure the particle size and particle size distribution in the submicron range or less, and the particles to be measured or dispersions thereof are dispersed by a known method such as ultrasonic irradiation in a medium. Then, after diluting it appropriately, a measurement sample is obtained. In the cumulative frequency curve of the particle size obtained by the dynamic light scattering method, the particle size having a cumulative frequency of 50% is defined as the average particle size, and the ratio of the 10% cumulative particle size to the 90% particle size is similarly determined. An example of a measuring apparatus that employs such a principle that can be used as an index of distribution is LB-500 manufactured by Horiba, Ltd.

  Moreover, it is preferable that the particle size distribution of a 1st particle and a 2nd particle is 2-50, More preferably, it is 2-10. By setting the particle size distribution in the above range, the maximum average particle size does not become too large, a flat particle layer is easily obtained, and uniform surface treatment is easily realized.

  The blending amount of the first particles and the second particles is preferably about the same or more of the second particles.

  In order to obtain a concavo-convex structure 10t capable of confining sunlight well and realizing the texture structure surface having the low reflectance described above, the coverage of the first particles on the film-formed surface of the liquid composition is 30% or more and 70%. The following is preferable. Therefore, the blending amount, blending ratio, and average particle size of the first particles and the second particles in the liquid composition are designed so that such a coverage can be achieved.

  The coverage is the ratio of the first particles covering the substrate to be processed when the particle layer is formed on the substrate to be processed, that is, the area of the first particle projected onto the substrate when viewed from the etching direction. Indicates the percentage. This coverage is measured as follows. After bonding to the substrate to be processed, the surface can be observed using a scanning electron microscope or an optical microscope, and can be calculated from the projected area.

  Although it does not restrict | limit especially as the binder 53, For example, a water-soluble polymeric material and an organic-solvent soluble polymeric material are mentioned. In particular, water-soluble ones are preferable from the viewpoint of environmental load and facility simplification.

  In addition, the binder 53 is composed of a polymerizable monomer capable of forming the above polymer material and other components constituting the particle layer to form a liquid composition for surface treatment, and is polymerized by light or heat after coating. A film may be formed by reaction, that is, a particle layer may be formed.

  As an example of the polymerizable monomer, (meth) acrylic monomer, (meth) acrylic acid C1-C12 alkyl ester, and these and a known compound as a new acrylic modifier can be used in combination. it can. Examples of the acrylic modifier include carboxy-containing monomers and acid anhydride-containing monomers. These polymerizable monomer monomers can be polymerized by a known polymerization method, and can be appropriately selected from known materials such as an initiator, a chain transfer agent, an oligomer material, and a surfactant necessary for the polymerization. Examples of the polymerizable monomer include known epoxy monomers and isocyanate monomers.

  More specifically, for example, those having a glass transition temperature of −100 to 50 ° C., a number average molecular weight of 1,000 to 200,000, preferably 5,000 to 100,000, and a degree of polymerization of about 50 to 1000 Are preferable. Examples of such include vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic ester, vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic ester, styrene, butadiene, ethylene, vinyl butyral, vinyl acetal, Examples include polymers or copolymers containing vinyl ether as a structural unit, polyurethane resins, various rubber resins, and modified polyvinyl alcohol having a weight average molecular weight of 100,000 or less.

  More preferable binders include water-soluble polyvinyl alcohol or derivatives thereof, cellulose derivatives (polyvinylpyrrolidone, carboxymethylcellulose, hydroxyethylcellulose, etc.), natural polysaccharides or derivatives thereof (starch, xanthan gum, alginsan, etc.). Gelatin, water-dispersible urethane, acrylic polymer latex and the like are also included.

  The amount of the binder is appropriately set according to the dispersibility of the particle group. For example, the amount is preferably 5% by weight to 50% by weight, more preferably 10% by weight to 30% by weight with respect to the particle group. is there.

  In addition to the above, the liquid composition contains a dispersant that can stably disperse the particle group, and, for example, a surfactant and a solvent that adjust the viscosity and surface tension of the coating liquid during production. Also good.

In particular, as a dispersant, phenylphosphonic acid, specifically “PPA” of Nissan Chemical Co., Ltd., α-naphthyl phosphoric acid, phenylphosphoric acid, diphenylphosphoric acid, p-ethylbenzenephosphonic acid, phenylphosphinic acid, aminoquinones, various Silane coupling agents, titanium coupling agents, fluorine-containing alkyl sulfates and alkali metal salts thereof can be used. In addition, nonionic surfactants such as alkylene oxide, glycerin, glycidol, alkylphenol ethylene oxide adducts, cyclic amines, ester amides, quaternary ammonium salts, hydantoin derivatives, heterocycles, phosphonium or sulfoniums, etc. Cationic surfactants, anionic surfactants containing acidic groups such as carboxylic acid, sulfonic acid, phosphoric acid, sulfate ester group, phosphate ester group, amino acids, aminosulfonic acids, sulfuric acid or phosphate esters of amino alcohol, alkyl Bedin type amphoteric surfactants and the like can also be used. As the dispersant, polyoxyethylene alkylphenyl ether, polyoxyalkylene block copolymer, polyoxyethylene alkylphenyl ether having a polymerizable unsaturated bond such as an allyl group, or the like may be selected. These dispersants (surfactants) are described in detail in “Surfactant Handbook” (published by Sangyo Tosho Co., Ltd.). These dispersants and the like are not necessarily 100% pure, and may contain impurities such as isomers, unreacted products, side reaction products, decomposition products, and oxides in addition to the main components. These impurities are preferably 30% or less, more preferably 10% or less. In the present invention, it is also preferable to use a combination of a monoester and a diester as described in the pamphlet of WO 98/35345 as a fatty acid ester.

  The solvent constituting the liquid composition is selected from solvents that dissolve the binder according to the binder type. Specifically, for example, when applying a water-soluble binder as a binder, it is preferable to apply an aqueous solvent as a solvent from the viewpoint of environmental load and simplification of equipment.

  Examples of the aqueous solvent include water and lower alcohols (methanol, ethanol, butanol, isopropyl alcohol, etc.). As the solvent, water is most preferable.

  When the particle group including the plurality of first particles 51 and the plurality of second particles 52, the binder 53, and other additives are included, they are added to the solvent, and the binder 53 is dissolved. A liquid composition is obtained by stirring and mixing so that the particle group is dispersed in a substantially uniform manner. In order to obtain a liquid composition containing a group of particles with good dispersibility, as a method of stirring and mixing, a method of mixing and dispersing with a stirring blade such as a dissolver that gives high-speed shearing, a dispersing device such as an ultrasonic disperser It is preferable to use a method of preparing by mixing and dispersing with the method described above.

(Liquid composition coating)
As shown in FIG. 1B, the liquid composition prepared as described above is applied and formed on the surface of the crystalline Si substrate 10 where the concavo-convex structure 10t is formed. The film forming method is not particularly limited, and examples of the coating method include spray method, spin coating method, dip method, roll coating method, plate coating method, doctor blade method, and screen printing method. The method and the spray method are preferable.

  The coating thickness is preferably 100 nm to 1000 nm, more preferably 300 nm to 600 nm. In the case of the spin coating method, it is preferable to make the sample stage the same as the crystalline Si substrate so that the mask material does not go around the back surface of the wafer. Moreover, when it wraps around the back surface, it is preferable to wash.

  Moreover, as the binder 53, in addition to the above, a polymerizable monomer capable of forming the polymer material may be used. In such a configuration, a liquid composition is mixed in the binder 53 with other components constituting the particle layer excluding the solvent described later, and this liquid composition is applied to the film formation surface 10s and then polymerized by light or heat. Thus, the mask material 50 can be formed.

  As an example of such a polymerizable monomer, a (meth) acrylic monomer, (meth) acrylic acid C1-C12 alkyl ester, or a compound known as an acrylic modifier having a neutrality with these may be used in combination. it can. Examples of the acrylic modifier include carboxy-containing monomers and acid anhydride-containing monomers. These polymerizable monomer monomers can be polymerized by a known polymerization method, and can be appropriately selected from known materials such as an initiator, a chain transfer agent, an oligomer material, and a surfactant necessary for the polymerization. Examples of the polymerizable monomer include known epoxy monomers and isocyanate monomers.

(Dry etching process)
Next, as shown in FIG. 1C, a dry etching process is performed on the mask material 50 to form the concavo-convex structure 10t. The dry etching method is not particularly limited, but reactive ion etching (RIE) in which etching is performed by ionizing / radicalizing the reactive gas with plasma is preferable because the etching gas is highly straight and fine patterning is possible. In particular, ICP which is inductive coupling type reactive ion etching is preferable.

As the etching gas, chlorine-based gas, fluorine-based gas, and bromine-based gas are preferable, and sulfur hexafluoride (SF ₆ ) gas is more preferable. In addition, it is preferable to use a mixed gas in which oxygen gas is mixed in these gases because etching characteristics become better. In particular, when a mixed gas of sulfur hexafluoride gas and oxygen gas is used, a finer gas is used. Since an uneven structure can be obtained, it is more preferable.

  When the etching process is performed, as shown in FIG. 1D, the binder 53 constituting the mask material 50 excluding the region covered with the first particles 51 is etched and the second particles 52 is also etched.

  Thereby, the etched region becomes a concave portion, and the non-etched region becomes a convex portion, and the concavo-convex structure 10 t is formed on the surface of the crystalline Si substrate 10. Note that the mask material 50 is etched in the region to be etched.

  Conditions for performing the dry etching process are appropriately set according to the thickness and type of the mask material 50 (type of binder 53, type of particle group, etc.). For example, in the dry etching process using the mask material 50 used in Example 1 to be described later, the relationship between the processing time and the reflectance was examined. As a result, the reflectance was 6% in the processing time of 6 minutes, and the processing time was 9 minutes. Then, a reflectance of 3% was achieved. Further, the reflectance was about 3% even at a processing time of 10 minutes. From these results, it was confirmed that the dry etching processing time determined to be able to achieve a reflectance of about 5% was about 8 minutes. Therefore, in the present embodiment, the dry etching treatment time is preferably 8 minutes or more, more preferably 8 to 10 minutes, and further preferably 8 to 9 minutes (dry etching apparatus and other devices). See Example 1 for conditions). Although it is possible, if it is too high, there is a possibility of causing plasma damage described in the next step. With the power described in the examples described later, it is possible to form irregularities without giving large plasma damage in a relatively short process of 8 to 10 minutes.

(Cleaning process)
On the surface 10 s of the concavo-convex structure 10 t after dry etching, there are etching residues including the first particles 51 and the second particles 52, the binder 53, and constituent elements of the etching gas, and further plasma damage. FIG. 3 (a) shows a surface SEM photograph after the formation of irregularities by dry etching (about 9 minutes), followed by ultrasonic cleaning (pure water, 5 minutes). In FIG. 3A, the silica residue is clearly confirmed in the upper right part. Although it is difficult to confirm with an SEM photograph, it is known that when a Si substrate is subjected to dry etching, extremely fine irregularities called a stain layer are formed on the processed surface, and the presence of this layer affects the surface physical properties. Therefore, this stain layer is called plasma damage.

  Due to the presence of the etching residue and plasma damage, the thermal diffusion of the dopant gas (for example, phosphoric acid gas) is performed when the n-type dopant (for example, phosphorus) is diffused in the subsequent pn junction formation. Due to the presence, the diffusion is inhibited, so that a good pn junction cannot be formed. Therefore, in this embodiment, the etching residue and plasma damage are removed before the diffusion of the n-type dopant.

  The etching residue is mainly the second particles (particles having high etching resistance), and other reactants (such as sulfur and fluorine compounds) with the etching gas. These residues are mainly on the convex portions 101 of the concavo-convex structure 10t, but may also remain in the concave portions 102.

  As these cleaning methods, various methods such as ultrasonic cleaning and solvent spin cleaning can be adopted so far, but it takes a long time to completely remove surface deposits. In addition, the present inventor has confirmed that the uneven shape formed is broken and the reflectivity is increased when performed under strong cleaning conditions. FIG. 3B is a surface SEM photograph when the ultrasonic cleaning time in pure water is 8 minutes, and FIG. 3C is a case where the ultrasonic cleaning time is further extended by 5 minutes (ultrasonic cleaning time 13 minutes). These are surface SEM photographs, which show the collapse of the irregular shape.

  Therefore, the present inventor has earnestly studied to achieve both a cleaning effect and damage reduction. As a result, the present inventors have found a cleaning method that can satisfactorily remove the etching residue and plasma damage and achieve high photoelectric conversion efficiency. The cleaning method will be described below.

  First, as shown in FIG. 1D, dry etching with hydrogen gas is performed from above the concavo-convex structure 10t by dry etching (arrow ↓ in the figure). Here, the dry etching with hydrogen gas (hereinafter referred to as “hydrogen etching”) is different from the hydrogen reduction treatment for removing the natural oxide film on the surface of the silicon substrate by hydrogen reduction. And a process for removing plasma damage. However, since hydrogen is applied to the silicon substrate surface, removal of the oxide film on the surface may be performed during the same process.

  The method of dry etching is not particularly limited, and it is preferable to select a method that can be carried out in the same apparatus as that for forming the concavo-convex that is the previous step because of ease of process.

  The conditions for the hydrogen etching are not particularly limited, but it is preferable to design within a range of a hydrogen gas flow rate of 50 to 500 sccm, a gas pressure of 1 to 20 Pa, and a high frequency output of 50 to 300 W. Under such etching conditions, by setting the etching time to about 1 to 5 minutes, etching residues and plasma damage can be satisfactorily removed, and etching with high running cost and high reliability is possible. Preferable etching conditions include, for example, a hydrogen gas flow rate of 100 sccm, a gas pressure of 3 Pa, a high frequency output of 100 W, and an etching time of 5 minutes.

  In hydrogen etching, it is considered that the etching resistance is in the order of etching residue (for example, silica) <plasma damage portion << crystal Si. Therefore, even if the etching residue is sufficiently removed, the projection 101 of the concavo-convex structure 10t is not likely to be etched and adversely affect the reflectance.

  The stain layer which is plasma damage can also be satisfactorily removed by the dry etching. However, in order to remove the stain layer more efficiently, it is preferable to perform wet etching with an alkali solution such as an aqueous sodium hydroxide solution after hydrogen dry etching. A preferable concentration of the alkaline solution is 0.1 to 10 wt%. In such a concentration range, the stain layer can be removed satisfactorily by setting the etching time to 10 seconds to 2 minutes.

The surface of the silicon crystal plate that has been subjected to unevenness formation and surface cleaning is very easily oxidized, and generally an oxide film of SiO ₂ is formed. Such an oxide film can be removed by hydrogen dry etching, but in order to further increase the removal rate, it is preferable to immerse in an acid solution such as dilute hydrofluoric acid following hydrogen dry etching.

  In addition, when the alkaline or acidic aqueous solution has adhered to the texture structure surface (uneven structure 10t) after washing | cleaning, it is preferable to neutralize. Furthermore, it is preferable to sufficiently dry the texture structure surface 10s when forming the pn junction in the next step.

  As shown in FIG. 2A, the cleaned silicon substrate 1 for a photoelectric conversion element has a concavo-convex structure in which one main surface 10s (surface 10s) includes a large number of needle-like convex portions 101 at a fine pitch. It is a textured surface with 10t. In such a concavo-convex structure 10t, sunlight is well confined in the photoelectric conversion layer 10, and the reflectance of the light on the texture structure surface is significantly reduced as compared with the conventional texture structure, and is incident on the texture structure surface. Light can be incident on the photoelectric conversion layer 10 with high efficiency.

  As shown in Examples described later, the present inventor performs hydrogen etching for about 3 to 15 minutes after the formation of irregularities by dry etching using the mask material 50, and performs immersion treatment in dilute hydrofluoric acid. As a result, the solar reflectance of the concavo-convex structure 10t has been successfully reduced to 5% or less, and to 1% at the lowest reflectance (see Examples, FIG. 5 and FIG. 6). It has been confirmed that such an effect can be obtained similarly when the Si substrate 10 is single crystal or polycrystalline.

  The concavo-convex structure 10t of the low-reflectivity silicon substrate 1 for photoelectric conversion elements is unique to the structure obtained by the method for producing a silicon substrate for photoelectric conversion elements found by the present inventors.

  In the above method for manufacturing a silicon substrate for photoelectric conversion elements, the variation tends to be relatively large. However, by adjusting the design of the mask material 50 and the dry etching conditions, the shape and average height of the convex portions 101 of the concavo-convex structure 10t , Pitch etc. can be designed.

  The average height of the convex portion 101 is preferably higher because a large light confinement effect can be obtained, but a longer formation time is required to increase the height of the convex portion 101. The processing time (tact time) of the formation process of the convex portion 101 is preferably short from the viewpoint of running cost. Therefore, it is preferable that the average height is low as long as a desired reflectance can be realized. As described above, the concavo-convex structure 10t of the present embodiment can be configured to include a large number of needle-like convex portions 101 at a fine pitch, so if the average height of the convex portions 101 is about 1 μm, A large light confinement effect is obtained, and the concavo-convex structure 10t having a sufficiently low reflectance can be obtained. Here, the “average height of the convex portions” means the arithmetic average height of the convex portions. In addition, the measurement of the height of a convex part shall be measured by AFM. As described above, in the method for manufacturing a silicon substrate for photoelectric conversion elements of the present invention, since the variation in the height of the convex portion 101 tends to be relatively large, the convex portion higher than the arithmetic average height is included in a range of 40% or less. There is a possibility.

  On the other hand, it can be confirmed from the SEM image of FIG. 4 that the individual heights h of the large number of convex portions 101 have variations similar to the pitch. FIG. 4 is a surface SEM image of the textured surface uneven structure 10t obtained in the examples described later, and shows the magnifications changed between (a) and (b). From the SEM image shown in FIG. 4, it is confirmed that the pitch of the convex portions 101 of the concavo-convex structure 10t is 100 to 500 nm or less, but it is also recognized that there is variation in the plane. In the concavo-convex structure 10t, the maximum height roughness Rz is preferably in the range of 0.9 μm to 3.0 μm.

  In addition, the ratio of the cross-sectional area st of the front-end | tip part of the convex part 101 with respect to the cross-sectional area sb of the bottom part of the convex part 101 in the surface substantially parallel to the back surface 10r of the photoelectric converting layer 10 with the acicular convex part 101 is 20. The convex part which is% or less is meant. Here, as shown in FIG. 2B, the bottom portion of the convex portion 101 means a portion located at the same height as the lowermost portion of the concave portion 102 closer to the tip portion among the adjacent concave portions 102. Is. The cross-sectional area st at the tip means the remaining part that is protected by a mask material in dry etching described later. In the present specification, the “height” means a distance from the bottom portion to the tip portion in a direction substantially perpendicular to the main surface opposite to the texture structure surface of the substrate.

  As described above, the silicon substrate 1 for photoelectric conversion elements can be manufactured.

  In the method for manufacturing the silicon substrate 1 for crystalline Si photoelectric conversion elements of the present invention, first, at least two kinds of fine particles (first particles having different dry etching resistances) are formed on one main surface 10 s of the one-conductivity type crystalline silicon substrate 10. After forming the concavo-convex structure 10t on the crystalline silicon substrate surface 10s by dry etching using the mask material 50 having the particles 51, the second particles 52, and the average particle size 0.05 μm to 0.3 μm, the concavo-convex structure 10t Then, a dry etching process using hydrogen gas and a process of immersing in dilute hydrofluoric acid are sequentially performed to clean the surface of the concavo-convex structure 10t. In such a configuration, the fine concavo-convex structure 10t having a good light confinement effect by effectively suppressing the reflection of incident light (sunlight) can be formed as the texture structure surface of the crystalline Si photoelectric conversion element. Etching residue generated by the formation of the concavo-convex structure 10t can be favorably removed from the surface of the concavo-convex structure.

  The thus obtained silicon substrate 1 for photoelectric conversion elements has a concavo-convex structure capable of confining incident light satisfactorily, and a substance that obstructs a pn junction is well removed on the surface. Therefore, pn defects are less likely to occur in the pn formation in the next step.

(Pn formation)
As shown in FIG. 2C, the photoelectric conversion element 2 has a light-transmitting property obtained by directly forming a film on the surface 10s of the photoelectric conversion layer 1 ′ obtained using the photoelectric conversion element silicon substrate 1 described above. Formed on the conductive layer 30, the back electrode layer 20 formed on the back surface 10r of the photoelectric conversion layer 10 (the main surface 10r opposite to the one main surface 10s), and the translucent conductive layer (surface electrode) 30 The structure includes a take-out electrode 40.

  The photoelectric conversion layer 1 ′ has a two-layer structure of a first conductivity type (p-type) Si layer 11 and a second conductivity type (n-type) Si layer 12, and a pn junction in the photoelectric conversion layer 1 ′. Is formed. As shown in FIG. 1E, such a two-layer structure is formed by diffusing a dopant from the textured structure surface (uneven structure 10t) of the cleaned silicon substrate 1 for photoelectric conversion elements. It does not restrict | limit especially as a p-type dopant and an n-type dopant, What is necessary is just a dopant ion generally used as a dopant of crystalline Si. As the p-type dopant, boron, which is a group III element, and phosphorus, which is a group V element, are preferably used as the n-type dopant.

In the present embodiment, the case where the first conductivity type Si layer 11 is a p-type layer and the second conductivity type Si layer 12 is an n-type layer will be described as an example. Good. In this configuration, as shown in FIG. 1E, an n-type dopant (phosphorus or the like) is diffused from the textured surface (uneven structure 10t) of the washed p-type silicon wafer to form a pn junction. When the n-type dopant is phosphorus, phosphorus is thermally diffused by, for example, a gas diffusion method using phosphoryl chloride (POCl ₃ ) as a diffusion source to form a pn junction (diffusion temperature is 800 ° C.).

  Next, pn separation is performed by cutting off the excess p layer and n layer that wrap around the side surface and the back surface. The separation method is not particularly limited, and wet etching or plasma etching with dilute hydrofluoric acid may be employed. Further, when phosphorus is diffused, phosphate glass is generated on the texture structure surface 10s, and therefore, it is preferably removed by immersion in dilute hydrofluoric acid.

  As already described, in the silicon substrate 1 for photoelectric conversion elements of the present embodiment, the etching residue and plasma damage are satisfactorily removed from the texture structure surface 10s (uneven structure 10t). This can be confirmed from the examples shown in FIGS. 4 (a) and 4 (b). FIGS. 4A and 4B are surface SEM images of the texture structure 10s obtained by an electron microscope. The magnification is set to 5000 times and 10,000 times. As shown in the figure, neither etching residue nor plasma damage is observed in any of the photographs.

  Therefore, in the silicon substrate 1 for photoelectric conversion elements, a good and substantially uniform pn junction can be formed on the textured structure surface 10s because there are very few etching residues and plasma damages that inhibit pn formation. Partial defects of the pn junction and in-plane non-uniformity (pn defect) greatly affect the photoelectric conversion efficiency of the photoelectric conversion element. Since a photoelectric conversion element is manufactured using the silicon substrate 1 for photoelectric conversion elements, a good and substantially uniform pn junction can be formed as described above, so that the photoelectric conversion is performed with high efficiency by the texture structure surface 10s. The sunlight incident on the layer 1 ′ can be photoelectrically converted with high efficiency.

(Electrode formation)
Next, as shown in FIG. 1 (f), the back electrode 20 is formed on the back surface 10 r, and then the surface is placed approximately on the textured structure surface 10 s of the photoelectric conversion layer 1 ′ where the pn junction is formed. Thus, the transparent conductive layer 30 to be coated is directly formed (FIG. 1G).

The back electrode layer 20 is not particularly limited, and any metal electrode can be used, but it is preferable to use highly conductive aluminum, silver, or the like.
The translucent conductive layer 30 (front surface electrode) is a layer that captures light and functions as an electrode through which the charge generated in the photoelectric conversion layer 10 flows, paired with the back electrode layer 20. The film is directly formed on the textured structure surface of the uneven structure 10t. Although it does not restrict | limit especially as the translucent conductive layer 30, ITO (indium tin oxide), metal dope zinc oxide (n-ZnO, such as ZnO: Al), etc. are preferable. The film thickness in particular of the translucent conductive layer 30 is not restrict | limited, 50 nm-2 micrometers are preferable.

  A method for forming the back electrode 20 and the translucent conductive layer 30 is not particularly limited, but may be a vapor phase method such as a sputtering method, a CVD method, an MOCVD method, an MBE method, or a liquid phase method. . The back electrode 20 may be formed by applying a silver paste or an Al paste by a screen printing method or the like and then baking.

  As described above, in the silicon substrate 1 for photoelectric conversion elements, the texture structure surface 10s can effectively suppress the reflection of incident light (sunlight) and can confine sunlight in the photoelectric conversion layer with high efficiency. Therefore, the translucent conductive layer 30 formed directly on the texture structure surface 10s as described above can be used as the surface electrode. In such a configuration, since the surface electrode is translucent, the power generation area can be increased as compared with the conventional comb electrode. Further, since the electrode can be formed immediately above the pn junction, the path to the electrode is shortened, so that the series resistance can be reduced. Therefore, highly efficient power generation efficiency can be achieved by these synergistic effects.

  Furthermore, since local electrode formation is not required unlike conventional comb electrodes, high temperature firing is not necessary. Therefore, according to the present invention, a photoelectric conversion element having high incident light utilization efficiency and high photoelectric conversion efficiency can be manufactured with high yield.

  In addition, in the photoelectric conversion element 2 of this embodiment, although it is preferable not to form an antireflection film, it is good also as a structure provided with the antireflection film.

  Finally, the extraction electrode 40 is formed on the surface of the translucent conductive layer 30 to obtain the photoelectric conversion element 2 (FIG. 1 (h)).

  Although it does not restrict | limit especially as the extraction electrode 40, The electrode in which the coating film-forming of silver, aluminum, etc. is possible is preferable. The film thickness of the extraction electrode 40 is not particularly limited and is preferably 0.1 to 3 μm. In the case where Al or silver paste is used as the extraction electrode 40, it can be formed by applying and baking after screen printing.

In the present embodiment, the mode in which the back electrode 20 is formed first has been described. However, the back electrode 20 may be formed after the light-transmitting conductive layer 30 is formed or after the extraction electrode 40 is formed. May be.
As described above, the photoelectric conversion element 2 can be manufactured.

  As described above, by using the crystalline Si substrate 1 manufactured by the method for manufacturing a silicon substrate for photoelectric conversion elements of the present invention, a good pn junction is formed on the textured structure surface 10s having a good light confinement effect. Can do. Therefore, according to the present invention, the photoelectric conversion element 2 having high incident light utilization efficiency and high photoelectric conversion efficiency can be manufactured with high yield.

  The photoelectric conversion element 2 can be preferably used for a solar cell or the like. If necessary, a cover glass, a protective film, or the like can be attached to the photoelectric conversion element 1 to form a solar cell.

(Design changes)
The present invention is not limited to the above-described embodiment, and the design can be changed as appropriate without departing from the spirit of the present invention.

Examples and comparative examples according to the present invention will be described.
Example 1
First, a coating liquid for the mask material was prepared. First, a PVA liquid (aqueous solvent) containing 3% by weight of polyvinyl alcohol (PVA) as a binder was prepared. Next, 6 parts by weight of silica (SiO ₂ ) particles (SP-03F manufactured by Fuso Chemical Co., Ltd., average particle size: 0.3 μm) are used as the first particles with respect to 100 parts by weight of the PVA liquid, and acrylic resin particles ( 4 parts by weight of MP1000 manufactured by Soken Chemical Co., Ltd. (average particle size 0.4 μm) and 0.5 parts by weight of (2-ethylhexylsulfosuccinate Na) as a dispersant were added and dispersed in a dissolver to obtain a mask material coating solution .

  A P-type polycrystalline silicon wafer (156 mm square) from which wire saw damage has been removed is coated with a mask material coating solution with a spin coater (1H-360s made by Mikasa) and dried to obtain a mask with a thickness of 0.5 μm. A material was deposited.

  Next, a concavo-convex structure was formed with an RIE apparatus (reactive ion etching apparatus: EXAM manufactured by Shinko Seiki Co., Ltd.).

Using a mixed gas of SF ₆ and O ₂ (SF ₆ : O ₂ = 1: 0.5), dry etching was performed for 8 minutes at a gas pressure of 20 Pa and a high frequency output of 150 W.

Next, in the RIE apparatus, the etching gas is replaced with H ₂ gas, dry etching is performed for 5 minutes at a hydrogen flow rate of 100 sccm, a gas pressure of 3 Pa, and a high-frequency output of 100 W, and then immersed in a 1% sodium hydroxide solution and further diluted with hydrofluoric acid. After dipping in a 10% solution, it was rinsed with pure water to obtain an uneven shape with a height of about 1 μm on the surface.

Next, phosphoryl chloride (POCl ₃ ) is gas diffused at a diffusion temperature of 800 ° C. to form an n-type layer on the surface, and pn separation is performed by plasma etching the wafer side and back surface with a mixed gas of CF ₄ and O _2. Went.

  Next, after dipping in a 10% solution of dilute hydrofluoric acid to remove the phosphate glass on the surface, an ITO transparent conductive film is formed on the surface with a plasma CVD device, and silver paste is elongated on the ITO surface for screen printing. Further, an Al paste was formed on the back surface by screen printing and then baked at a temperature of 860 ° C. to produce a photoelectric conversion element.

(Example 2)
A photoelectric conversion element was obtained in the same manner as in Example 2 except that the silicon wafer was changed to a P-type single crystal wafer.

(Comparative Example 1)
After carrying out to the phosphate glass removal step, the same procedure as in Example 1 was conducted, except that a SiN antireflection film was formed to a thickness of about 50 nm by plasma CVD and no translucent conductive layer was formed. Thus, a photoelectric conversion element was obtained.

(Comparative Example 2)
The same procedure as in Example 2 was performed except that one surface of a p-type polycrystalline silicon wafer was immersed in a mixed solution of hydrogen fluoride and nitric acid (mixing ratio 50:50 (volume ratio)) to form a texture structure on the surface. Thus, a photoelectric conversion element was produced.

(Comparative Example 3)
A photoelectric conversion element was produced in the same manner as in Example 1 except that ultrasonic cleaning was performed in pure water for 5 minutes instead of dry etching with hydrogen and immersion in a sodium hydroxide aqueous solution.

(Evaluation)
The surface reflectance and power generation efficiency of the photoelectric conversion elements obtained in Examples 1 and 2 and Comparative Examples 1 to 3 were measured. The results are shown in Table 1. In Table 1, when the power generation efficiency is ◎, it means that the power generation efficiency is 17% or more, ◯ is 16% to 14%, and Δ is 13% to 11%.

  As shown in Table 1, in the photoelectric conversion element of the present invention, it was confirmed that the surface reflectance was 1% and the power generation efficiency was good regardless of whether it was polycrystalline or single crystal.

  Moreover, in the comparative example 1, it is set as the structure which put the antireflection film in the photoelectric conversion element of this invention. In Comparative Example 1, the reflectance was the same as in Examples 1 and 2, but it was also confirmed that power generation efficiency was reduced due to the insertion of SiN as an insulating film and the locality of the surface electrode.

  Comparative Example 2 is a configuration of a conventional polycrystalline silicon solar cell, and Comparative Example 3 is an example in which the etching residue cleaning step in Example 1 is a conventional ultrasonic cleaning in pure water.

  The effectiveness of the present invention could be confirmed by the results of Examples 1 and 2 and Comparative Examples 1 to 3.

  In FIG. 4, the surface SEM image of the uneven structure 10t of the texture structure surface obtained in Example 1 is shown. As shown in FIG. 4, etching residue and plasma damage were not observed on the texture structure surface 10s (uneven structure 10t) of Example 1, and it was confirmed that the texture structure surface 10s was removed well.

  FIG. 5 shows the reflectance spectra of the polycrystalline silicon wafer surface (a) before application of the mask material and the textured structure surface (b) before n-type dopant diffusion in Example 1. As shown in FIG. 5, in the case of using a polycrystalline Si substrate, the reflectance before the texture structure formation was 20% or more in the Si absorption wavelength band of 400 nm to 1200 nm, whereas the texture structure It was confirmed that the reflectance after formation was 1 to 2%.

  FIG. 6 shows the reflectance spectrum (e) of the single crystal silicon wafer surface (a) before application of the mask material and the texture structure surface before diffusion of the n-type dopant in Example 2. FIG. 6 also shows the reflectance spectrum of the texture structure surface obtained by the conventional texture structure forming method by alkali treatment ((b) the texture structure surface of alkali treatment, (c) is (b). The surface where the SiN antireflection film is provided on the texture structure surface))). FIG. 6D shows the reflectance spectrum of the concavo-convex structure surface before the treatment in dilute hydrofluoric acid in Example 2.

  As shown in FIG. 6, when a single crystal Si substrate is used, the reflectance before forming the texture structure is 20% or more in the Si absorption wavelength band of 400 nm to 1200 nm, and the conventional alkali treatment is performed. It was confirmed that the reflectance on the texture structure surface was about 10%. Further, in the configuration in which the SiN film is provided on the texture structure surface by the conventional alkali treatment, the reflectance is 6% or less in the wavelength region shorter than 400 nm to 600 nm, but shorter on the longer wavelength side than 600 nm. It was higher than the wavelength side, and it was confirmed that the wavelength dependency of the reflectance was high. On the other hand, on the texture structure surface using the mask material, it was confirmed that the reflectance of 1 to 2% was achieved regardless of the cleaning process.

  FIG. 7 is a graph showing the results of investigation by the inventor of the relationship between the surface roughness (1.5 μm square) of the Si substrate surface and the integrating sphere reflectance for light having a wavelength of 1 μm. FIG. 7A shows the relationship between Ra (arithmetic mean surface roughness) and Rq (root mean square roughness) and reflectance, and FIG. 7B shows Rz (maximum height roughness) and Rzjis (10-point average). The relationship between the roughness, Rp (maximum peak), and Rv (maximum valley) and the reflectance is shown.

  7A and 7B, the surface roughness of the texture structure surfaces of Examples 1 and 2 and Comparative Examples 1 and 2 can be estimated. In Examples 1 and 2, since the reflectance is 1 to 3%, it is confirmed that the arithmetic average surface roughness Ra is 122 nm or more. As described in the above embodiment, the pitch (fineness) of the concavo-convex structure can be adjusted by the design of the first particles and the second particles included in the mask material. It is also confirmed that the maximum height roughness Rz is 1151 nm (1.151 μm) or more.

  As shown in Examples 1 and 2, according to the present invention, in a crystalline Si solar cell, a texture structure having a low reflectance of 6% or less was successfully produced. The structure provided with the surface electrode which consists of a conductive layer (transparent electrode layer) was implement | achieved. By realizing such an electrode layer, solar light is confined in the photoelectric conversion layer with high efficiency, and furthermore, it has succeeded in generating power in a greatly widened power generation region, and the resistance when taking out the generated charge is minimized. Made it possible to Therefore, according to the present invention, it has been possible to realize for the first time a crystalline Si-based solar cell having excellent photoelectric conversion performance with less utilization efficiency of sunlight, photoelectric conversion efficiency, and loss in the apparatus.

  The photoelectric conversion element of this invention is preferably applicable to uses, such as a photoelectric conversion element used for a solar cell, an infrared sensor, etc.

DESCRIPTION OF SYMBOLS 1 Silicon substrate for photoelectric conversion elements 1 'Photoelectric conversion layer 2 Photoelectric conversion element 10 Crystalline Si substrate 10s Surface (one main surface, texture structure surface)
10r Back surface (main surface opposite to one main surface)
10t Concavity and convexity structure (concave and convex structure surface, texture structure surface)
101 Convex part 102 Concave part 11 First conductivity type (p-type) Si layer 12 Second conductivity type (n-type) Si layer 20 Back electrode layer 30 Translucent conductive layer (surface electrode)
40 Extraction electrode 50 Mask material 51 First particle 52 Second particle 53 Binder

Claims (12)

This is a method for manufacturing a silicon substrate for a photoelectric conversion element, which has a textured structure surface having a concavo-convex structure having a large number of convex portions that suppress reflection of sunlight on one main surface of one conductivity type crystalline silicon substrate. And
First particles having resistance to dry etching for forming the concavo-convex structure on one main surface of the one conductivity type crystalline silicon substrate and having an average particle diameter of 0.05 μm or more and 0.3 μm or less; A liquid composition comprising second particles having a lower resistance than the first particles and an average particle size of 0.05 μm or more and 0.3 μm or less, and a binder having a lower resistance than the first particles. Applying an object to one main surface of the silicon substrate and arranging the mask material;
Forming the concavo-convex structure by dry etching on the main surface on which the mask material is disposed;
The manufacturing method of the silicon substrate for photoelectric conversion elements characterized by having the washing | cleaning process of implementing sequentially the dry etching process by hydrogen gas, and the process immersed in dilute hydrofluoric acid in the said uneven structure.   2. The photoelectric conversion according to claim 1, wherein in the dry etching process using the hydrogen gas, the dry etching process is performed under conditions of a hydrogen gas flow rate of 50 to 500 sccm, a gas pressure of 1 to 20 Pa, and a high frequency output of 50 to 300 W. Manufacturing method of silicon substrate for device.   3. The method for producing a silicon substrate for a photoelectric conversion element according to claim 1, wherein an average particle diameter of the first particles and / or the second particles is 0.1 to 0.2 μm.   The method for producing a silicon substrate for a photoelectric conversion element according to claim 1, wherein the first particles are inorganic particles, and the second particles are resin particles. The method for producing a silicon substrate for a photoelectric conversion element according to claim 4, wherein the first particles are particles containing SiO ₂ as a main component.   The method for producing a silicon substrate for a photoelectric conversion element according to claim 4 or 5, wherein the second particles are particles mainly composed of an acrylic resin.   The method for producing a silicon substrate for a photoelectric conversion element according to claim 1, wherein the binder is mainly composed of a water-soluble polymer or a water-dispersible polymer.   The method for producing a silicon substrate for a photoelectric conversion element according to claim 7, wherein the binder is polyvinyl alcohol.   The method for producing a silicon substrate for a photoelectric conversion element according to any one of claims 1 to 8, wherein the one-conductivity-type crystalline silicon substrate is made of polycrystalline silicon (may contain unavoidable impurities).   The method for producing a silicon substrate for a photoelectric conversion element according to claim 1, wherein the dry etching is reactive ion etching. A silicon substrate manufactured by the method for manufacturing a silicon substrate for photoelectric conversion elements according to claim 1 is prepared,
A pn junction is formed on the textured structure surface of the silicon substrate by diffusing impurities of a conductivity type different from that of the substrate,
Forming a surface electrode on the textured structure surface;
A method for producing a photoelectric conversion element, comprising forming a back electrode layer on a main surface opposite to the one main surface of the silicon substrate.   The method for producing a photoelectric conversion element according to claim 11, wherein a translucent conductive layer is formed directly on the textured structure surface as the surface electrode.