WO2005059985A1 - Process for producing silicon substrate with porous layer - Google Patents

Abstract

[PROBLEMS] To provide a process for producing a solar cell with high productivity, in which a porous layer can be formed at low cost without generation of hazardous gas as a by-product. [MEANS FOR SOLVING PROBLEMS] There is provided a process characterized in that a polycrystalline silicon substrate is immersed in a silver-ion-containing aqueous solution consisting of a mixture of oxidizer, such as hydrogen peroxide, and hydrofluoric acid to thereby form a porous silicon layer at the surface of the substrate, or characterized in that the substrate is occasionally further subjected to alkali treatment so as to remove an upper porous layer, thereby forming a texture structure.

Description

 Specification

 Method for manufacturing a silicon substrate with a porous layer

 Technical field

 The present invention belongs to a method for producing a silicon substrate with a porous layer, and particularly relates to a method for efficiently forming a silicon substrate for a solar cell that requires a texture structure.

 Background art

 [0002] When sunlight reaches a silicon substrate that is a cell in a solar battery, it is divided into light that enters the substrate and light that is reflected by the substrate surface. Of these, only light entering the interior contributes to the photovoltaic effect.

 Thus, conventionally, in a crystalline solar cell, the energy conversion efficiency has been improved by selectively etching the surface of the substrate with an alkali so as to form a so-called textured shape in which a large number of pyramids are connected. This is different from the case where the substrate surface is flat, even if the light reflected by the slope of the pyramid is received by the slope of the adjacent pyramid and incident by refraction there. It is.

 [0003] As a method of forming the texture shape on a silicon substrate that is not a single crystal such as a polycrystalline silicon substrate or an amorphous silicon substrate, a machining method (Patent Document 1) and a reactive ion etching method ( Patent document 2) is known. In addition, a technique of using porous silicon as a texture structure has been proposed. Known methods for producing porous silicon on a silicon substrate include an electrochemical reaction method (Patent Document 4) and a chemical etching method (Patent Documents 3 and 5). The electrochemical reaction method involves immersing a silicon substrate in a hydrofluoric acid aqueous solution and causing an electrochemical reaction using a silicon substrate as an electrode. As a result, silicon on the surface of the silicon substrate serving as an electrode elutes to form a porous layer. The chemical etching method is a method for forming a porous layer by immersing a silicon substrate in a hydrofluoric acid aqueous solution containing an oxidizing agent such as nitric acid, chromic acid, or a metal redox pair. Furthermore, in recent years, methods utilizing the oxidation action by metal ions (Non-Patent Documents 1 and 2) have also been proposed.

[0004] Patent Document 1: Japanese Patent Laid-Open No. 9148603 Patent Document 2: JP-A-9-102625

 Patent Document 3: Japanese Patent Laid-Open No. 9-167850

 Patent Document 4: Japanese Patent Laid-Open No. 7-230983

 Patent Document 5: US Patent No. 5421958

 Non-Patent Document 1: K. Peng et al., Adv. Funct. Mater. 13 (2003) 127

 Non-Patent Document 2: P. Gorostiza et al., J. Electroanal. Chem. 469 (1999) 48

 Non-Patent Literature 3: Proceedings of the 50th Joint Conference on Applied Physics, 28a-ZC-5 Non-Patent Literature 4: 3rd World Conference on Photovoltaic Energy onvension, Abstracts for the Technical Program, 4LN-D-08

 Disclosure of the invention

 Problems to be solved by the invention

[0005] However, in the machining method, it is necessary to cut one sheet at a time, and it takes too much time to form a desired number of grooves, resulting in high costs. In reactive ion etching, the gases such as SF, CF, and C1 introduced into the etching chamber are corrosive.

 6 4 2

 The equipment must be made of materials that can withstand this, and the number of processed sheets at a time is small, resulting in high costs. Of the methods using porous silicon, the electrochemical reaction method requires a current generator and is expensive. The chemical etching method and the method using the oxidization effect by metal ions are expensive because they consume a large amount of metal ions such as nitric acid, chromic acid, and metal redox.

 Therefore, an object of the present invention is to provide a method for producing a silicon substrate with a porous layer at low cost and without adversely affecting the environment.

 Means for solving the problem

In order to solve the problem, the method for manufacturing a silicon substrate with a porous layer according to the present invention includes immersing the silicon substrate in a mixed aqueous solution of an oxidizing agent and hydrofluoric acid containing metal ions. A porous silicon layer is formed on the surface of the substrate.

Examples of the metal ion include one or more ions selected from silver, copper, nickel, platinum, palladium, and gold. According to the method of the present invention, a metal is deposited on the surface of a silicon substrate in a liquid containing metal ions, and the metal is a catalyst for reducing an oxidizing agent such as hydrogen peroxide or hydrogen. The oxidant quickly receives electrons from the silicon substrate. This leaves holes in the substrate. These holes promote the dissolution of the substrate material in the acid and liquid. As a result, the surface of the substrate is a double layer composed of a porous layer consisting of a large number of small pores with a diameter of about several nanometers and a porous layer containing a large number of large pores with a diameter of about several hundreds of nm below it. It becomes. The invention's effect

 [0007] According to the method of the present invention, metal ions are only precipitated and function as a catalyst, so a small amount is sufficient. In addition, since it is possible to immerse a large number of substrates in the same solution at the same time, it is highly productive and does not require expensive equipment or labor. Therefore, the porous layer can be formed at low cost. Also, since the reaction proceeds slowly, it is easy to control the thickness of the porous layer. Furthermore, if hydrogen peroxide, oxygen or ozone is used as the oxidizing agent, water is the only by-product of these reduction reactions (H 0 + 2H + + 2e → 2H 0, 0 + 4H + +

 2 2 2 2

 4e → 2H 0, 0 + 2H + + 2e → H 0 + 0), so it does not pollute the environment.

 2 3 2 2

 Brief Description of Drawings

 FIG. 1 is a scanning electron micrograph of the surface of a silicon substrate with a porous layer in Example 1.

 FIG. 2 is a graph showing the results of measuring the reflectance of the substrate of Example 1 and a control substrate.

 FIG. 3 is a scanning electron micrograph of the surface of a silicon substrate with a porous layer in Example 2.

 FIG. 4 is a graph showing the results of measuring the reflectance of a substrate of Example 2, a control substrate, and a comparative substrate.

 BEST MODE FOR CARRYING OUT THE INVENTION

[0009] The substrate having the double layer can be used as a gas sensor, a biosensor, a low dielectric constant film, a light emitting element, or an electron emitting element. The upper porous layer can also be used for solar cells as an antireflection film, and when the upper porous layer is dissolved in an alkaline aqueous solution, it becomes a textured surface consisting of many irregularities with a diameter of about several hundred nm, Either way, it is suitable as a substrate for solar cells.

 Example 1

[0010] A p-type polycrystalline silicon substrate doped with boron, sliced to an average thickness of 350 μm Prepared what was made. The layer damaged by the blade during slicing was removed by immersing in 6% NaOH aqueous solution at 80 ° C for 10 minutes. The specific resistance was 0.5-2 Ωcm. This substrate was ultrasonically washed in acetone for 5 minutes and then washed with pure water. Next, containing silver perchlorate AgCIO of 10- ⁴ M, 10: 1 mixture water and 10% hydrofluoric acid and 30% hydrogen peroxide

 Four

 The substrate was immersed in the liquid for 10 minutes. Figure 1 shows the results of observing the surface of the obtained substrate with a scanning electron microscope.

 As seen in FIG. 1, a porous layer consisting of many small pores having a diameter of about several nm was formed on the surface of the substrate. The resulting substrate was measured for reflectance at wavelengths from 300 nm to 800 nm using an ultraviolet-visible spectrophotometer (UV-2450) and an integrating sphere for reflection spectrum measurement. As a control, the same measurement was performed for the substrate immersed in the above mixed aqueous solution. Figure 2 shows the measurement results. In the figure, the solid line is this example, and the broken line is the control. As can be seen in Figure 2, the formation of the porous layer significantly reduced the reflectivity compared to before the formation.

 Example 2

 [0012] The substrate obtained in Example 1 was further washed with pure water, and immersed in a 1% NaOH aqueous solution for 10 minutes to remove the upper porous layer. Next, after washing with pure water again, the silver remaining on the surface was removed by immersing in 30% nitric acid for 30 minutes. Figure 3 shows the results of observation of the surface of the substrate thus obtained with a scanning electron microscope.

As seen in FIG. 3, a porous layer composed of a large number of pores having a diameter of about 500 nm-1 μm was formed on the surface of the substrate. The reflectance of this substrate was measured in the same manner as in Example 1. Figure 4 shows the measurement results. For comparison, in place of immersing in the above mixed aqueous solution for 10 minutes, except for immersing in 80% 6% NaOH aqueous solution containing 0.8 mol / L of isopropyl alcohol for 10 minutes (alkali etching method) The reflectance of the substrate treated under the same conditions as 1 was also measured. In the figure, solid line force S This example, the broken line is the control described in Example 1, and the thin solid line is the comparative example. As shown in Fig. 3 and Fig. 4, the upper porous layer is removed to expose the lower porous layer, and a texture structure is formed on the surface. The reflectivity was lower than that. Industrial applicability

 According to the present invention, the porous layer can be formed on the surface of the silicon substrate by a method suitable for mass production: useful for the spread of various sensors and solar cells.

Claims

The scope of the claims  [1] Silicon with porous layer, characterized in that a porous silicon layer is formed on the surface of a substrate by immersing the silicon substrate in a mixed aqueous solution of an oxidant and hydrofluoric acid containing metal ions A method for manufacturing a substrate.  [2] The method according to claim 1, wherein the metal ion is one or more ions selected from among silver, copper, nickel, platinum, palladium and gold. [3] The method according to claim 1, wherein the oxidizing agent is one or more selected from hydrogen peroxide, oxygen, and ozone.  4. The method according to claim 1, wherein the silicon substrate is immersed in the mixed aqueous solution and then immersed in an alkaline aqueous solution.