WO2015033864A1

WO2015033864A1 - Method for manufacturing solar cell

Info

Publication number: WO2015033864A1
Application number: PCT/JP2014/072695
Authority: WO
Inventors: 熊谷　晃
Original assignee: 株式会社ジェイ・イー・ティ
Priority date: 2013-09-06
Filing date: 2014-08-29
Publication date: 2015-03-12
Also published as: JP2015053398A; TW201523898A

Abstract

Provided is a method for manufacturing a solar cell, which is capable of providing the surface of a silicon substrate with more uniform texture. A method for producing a solar cell (1), wherein a porous layer (3) is formed on the surface of a silicon substrate (2) by etching using metal ions (5), is characterized by comprising: a step wherein the silicon substrate (2) is immersed in a first aqueous solution containing a strong acid and the metal ions (5), and the metal ions (5) are caused to adhere to the surface of the silicon substrate (2) by electroless plating; and a step wherein the silicon substrate (2), to the surface of which the metal ions (5) have been caused to adhere, is immersed in a second aqueous solution containing hydrofluoric acid and hydrogen peroxide solution, and the porous layer (3) is formed on the surface of the silicon substrate (2) by a catalytic reaction of the metal ions (5).

Description

Manufacturing method of solar cell

The present invention relates to a method for manufacturing a solar cell, and more particularly to a method for forming a porous layer on the surface of a silicon substrate.

As an alternative energy source such as coal and oil, sunlight is attracting attention as a clean and inexhaustible energy source, and the spread of solar cells that convert the light energy of the sunlight into electrical energy is further expected.

An infinite number of fine irregularities (hereinafter referred to as “textures”) having a role of effectively taking in sunlight are formed on the surface of the solar cell. In the case of single crystal silicon, the texture of the pyramid structure can be easily obtained by etching the Si (100) surface using an alkaline solution. On the other hand, in the case of polycrystalline silicon, since various crystal orientations appear on the surface of the silicon substrate, it is difficult to form a uniform texture on the entire surface of the silicon substrate like single crystal silicon.

As a method of forming a texture on the surface of a silicon substrate made of polycrystalline silicon, a porous layer is formed on the surface of the silicon substrate by immersing the silicon substrate in a mixed aqueous solution of an oxidant and hydrofluoric acid containing metal ions. A forming method (for example, Patent Document 1) is disclosed. A first step of immersing the silicon substrate in a mixed aqueous solution of an oxidant containing metal ions and hydrofluoric acid to form a porous layer on the surface of the silicon substrate; and the silicon substrate surface that has undergone the first step. And a second step of forming a texture by dipping in a mixed acid mainly composed of hydrofluoric acid and nitric acid to form a texture (for example, Patent Document 2) is disclosed.

According to Patent Document 2, the silicon substrate having a texture formed by the method of Patent Document 1 has a problem that the surface of the silicon substrate is discolored although the reflectance is low, and as a result, the characteristics of the solar cell are greatly deteriorated. . On the other hand, in the method of Patent Document 2, the surface of the silicon substrate that has undergone the same process as in Patent Document 1 is immersed in a mixed acid mainly composed of hydrofluoric acid and nitric acid to form a texture. In addition, a clean silicon surface can be obtained while maintaining the effect of reducing the reflectance, and the metal at the bottom of the hole can be removed. It is described in Document 2.

Japanese Patent No. 3925867 Japanese Patent No. 4610669

However, in both of

Patent Documents

1 and 2, by immersing a silicon substrate in a mixed aqueous solution of an oxidant containing metal ions and hydrofluoric acid, adhesion of metal ions and etching proceed simultaneously on the silicon substrate surface. Therefore, it is difficult to simultaneously manage the adhesion amount of metal ions and the etching amount. The variation in the adhesion amount of metal ions causes variation in the density of holes formed by etching. Moreover, the variation in the etching amount causes the variation in the size of the hole formed by the etching. Variations in the adhesion amount and etching amount of metal ions not only occur on the entire surface of the silicon substrate, but also occur between a plurality of silicon substrates using the same mixed aqueous solution due to deterioration of the mixed aqueous solution. Therefore, due to variations in the amount of metal ions attached and the amount of etching,

Patent Documents

1 and 2 have a problem that it is difficult to form a texture having a uniform size and density on the surface of the silicon substrate.

Therefore, an object of the present invention is to provide a method for manufacturing a solar cell capable of more uniformly forming a texture on the surface of a silicon substrate.

The method for manufacturing a solar cell according to the present invention is the method for manufacturing a solar cell in which a porous layer is formed on the surface of a silicon substrate by etching using metal ions, wherein the silicon is added to the first aqueous solution containing a strong acid and metal ions. A step of immersing the substrate and attaching the metal ions to the surface of the silicon substrate by electroless plating; and attaching the metal ions to the surface in a second aqueous solution containing hydrofluoric acid and hydrogen peroxide. And dipping the silicon substrate and forming the porous layer on the surface of the silicon substrate by a catalytic reaction of the metal ions.

According to the present invention, the dispersibility of metal ions is improved by immersing a silicon substrate in a first aqueous solution containing a strong acid and metal ions, and attaching the metal ions to the surface of the silicon substrate. Can be more uniformly attached to the surface of the silicon substrate.

It is a perspective view which shows the whole structure of the solar cell which concerns on this embodiment. It is a flowchart which shows the manufacturing method of the solar cell which concerns on this embodiment. FIG. 3A is a cross-sectional view showing a method for manufacturing a solar cell according to the present embodiment step by step, FIG. 3A is a state in which metal ions are attached, FIG. 3B is a state in which a porous layer is formed, and FIG. FIG. 3D is a diagram illustrating a state where metal ions are removed. 4A and 4B are SEM images of the surface of a silicon substrate manufactured by the manufacturing method according to the present embodiment, in which FIG. 4A shows a state in which a porous layer is formed, and FIG. 4B shows a state in which the porous layer is etched with a mixed acid mainly containing hydrofluoric acid. FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. A solar cell 1 shown in FIG. 1 includes a silicon substrate 2 that performs photoelectric conversion and a porous layer 3 in which the surface of the silicon substrate 2 is processed into a textured shape, and is incident from a light receiving surface on which the porous layer 3 is formed. Light is converted into electrical energy in the silicon substrate 2. In the porous layer 3, light incident on the surface of the silicon substrate 2 is repeatedly transmitted and reflected, and as a result, more light is guided into the silicon substrate 2 than on the flat silicon substrate surface. It is generally known that the fine texture formed in the porous layer 3 can efficiently confine incident light when the height and density are uniform rather than nonuniform. Note that the texture height refers to the difference in level of the unevenness, and the density refers to the number of concaves or convexes per unit area. The solar cell 1 according to the present embodiment is characterized in that a fine texture is uniformly formed in the porous layer 3 as compared with the conventional one, and the other configuration is the same as the conventional one.

Actually, although not shown, the solar cell 1 has a diffusion layer, an antireflection film, and a grid electrode sequentially formed on the light receiving surface side where the porous layer 3 is formed with respect to the p-type silicon substrate. An electric field layer and a back electrode are formed in this order. The antireflection film is formed on the surface of the porous layer 3 in order to suppress light reflection. The antireflection film is composed of, for example, a single layer structure of a titanium oxide (TiO ₂ ) film or a silicon nitride (SiN) film formed by a chemical vapor deposition (CVD) method or the like.

Next, a method for forming the porous layer 3 will be described with reference to FIG. In the following description, the concentration is mass%.

First, in step SP1, the silicon substrate 2 is immersed in a first aqueous solution in which a strong acid and a metal are mixed, and the metal ions 5 are adhered to the surface of the silicon substrate 2 by electroless plating (FIG. 3A). Since the first aqueous solution contains a strong acid and is acidic, the metal easily becomes the metal ion 5. The metal ions 5 do not aggregate due to the repulsive force of the potential, and the dispersibility is improved.

As the strong acid, for example, hydrofluoric acid, hydrochloric acid, sulfuric acid and the like can be used. For example, Ag can be applied as the metal. When Ag is applied as the metal, the first aqueous solution can be generated using AgNO ₃ as the metal-containing agent. The volume ratio in this case is HF (concentration 50%): AgNO ₃ (concentration 3E-4M): H ₂ O = 400 ml to 4000 ml: 10 ml to 40 ml: 10000 ml to 20000 ml. The electroless plating conditions can be, for example, an immersion time of 300 seconds and a temperature of the first aqueous solution of 26 degrees.

The silicon substrate 2 has a natural oxide film removed beforehand. The method for removing the natural oxide film on the silicon substrate 2 is not particularly limited, and for example, a sputtering method, a plasma method, or the like can be used.

As described above, in the method for manufacturing the solar cell 1 according to this embodiment, prior to the formation of the porous layer 3 by etching, the silicon substrate 2 is immersed in the first aqueous solution containing metal ions, and the silicon substrate 2 Metal ions were allowed to adhere to the surface. As a result, the metal ions can be uniformly attached to the surface of the silicon substrate 2 as compared with the conventional case where the metal ions are attached and etched simultaneously.

Moreover, in the manufacturing method of the solar cell 1 according to the present embodiment, the silicon substrate 2 is immersed in a first aqueous solution containing a strong acid and metal ions, and the metal ions 5 are attached to the surface of the silicon substrate 2. did. Thereby, the dispersibility of the metal ions 5 can be improved, and the metal ions 5 can be more uniformly attached to the surface of the silicon substrate 2.

Further, since the metal ions 5 are well dispersed in the first aqueous solution, the metal ions 5 adhere more reliably to the surface of the silicon substrate 2. Therefore, the metal ions 5 have a large bonding force with the surface of the silicon substrate 2 and can be prevented from being lost due to water washing or the like.

Further, by knowing in advance the correlation between the metal ion concentration and the amount of metal ions 5 attached to the surface of the silicon substrate 2, the amount of metal ions 5 attached can be controlled. Therefore, by managing the concentration of the metal ions 5 to be introduced into the first aqueous solution, the metal ions 5 can be uniformly attached within the surface of one silicon substrate 2.

Next, water washing is performed in step SP2, and in step SP3, the silicon substrate 2 is immersed in a second aqueous solution containing hydrofluoric acid and hydrogen peroxide water. In the silicon substrate 2, the hydrogen reduction reaction of the hydrogen peroxide solution by the catalytic action of the metal ions 5 attached to the surface proceeds. Then, electrons are extracted from the surface of the silicon substrate 2 in contact with the metal ions 5 in order to compensate for the increase in the amount of electron consumption. As a result, holes are generated in the silicon substrate 2 and cause oxidative dissolution of the silicon substrate 2.

In this way, an infinite number of holes (concaves) 6 are formed on the surface of the silicon substrate 2, and the porous layer 3 is formed by the holes 6 and portions where the holes 6 are not formed (convex) (FIG. 3B). In this case, the volume ratio of the second aqueous solution is HF (concentration 50%): H ₂ O ₂ (concentration 30%): H ₂ O = 400 ml to 4000 ml: 400 ml to 2000 ml: 10000 ml to 20000 ml. By controlling the immersion time of the silicon substrate 2, the size of the holes 6 constituting the porous layer 3 is controlled. For example, if the hole 6 is large, the difference in height from the portion where the hole 6 is not formed becomes large, and thus the height of the texture becomes large. Note that the second aqueous solution is only intended to form the porous layer 3 on the surface of the silicon substrate 2 by etching, and is not intended to attach the metal ions 5 to the surface of the silicon substrate 2. Does not contain.

The concentration of the hydrogen peroxide solution in the second aqueous solution is preferably suppressed to a concentration that does not suppress the etching of the metal ions 5 in order to form the porous layer 3 more reliably. Specifically, the concentration of the hydrogen peroxide solution is preferably 25 to 50% with respect to the concentration of hydrofluoric acid. Since the hydrogen peroxide solution has a stronger ability to take electrons from the silicon substrate than the metal ions, if the concentration of the hydrogen peroxide solution is higher than 50%, the etching rate by the hydrogen peroxide solution is higher than the etching rate by the catalytic action of the metal ions. As a result, the entire surface of the silicon substrate is oxidized and becomes a mirror surface, and the porous layer is not formed. Further, when the concentration with respect to hydrofluoric acid is outside the above range, that is, when the concentration with respect to hydrofluoric acid is higher than 50%, and when the concentration with respect to hydrofluoric acid is less than 25%, The reflectance cannot be reduced.

In this embodiment, after the metal ions 5 are attached to the surface of the silicon substrate 2, the silicon substrate 2 is immersed in a second aqueous solution containing hydrofluoric acid and hydrogen peroxide solution, and the catalytic reaction of the metal ions 5 is performed. Thus, the porous layer 3 was formed on the surface of the silicon substrate 2. In this way, by controlling the immersion time of the silicon substrate 2 and the ratio of hydrofluoric acid and hydrogen peroxide solution, the size of the holes formed by the catalytic reaction can be controlled. Therefore, in the present embodiment, a texture having a uniform height can be formed on the surface of the silicon substrate 2.

After the metal ions 5 are attached to the surface of the silicon substrate 2, the silicon substrate 2 is immersed in a second aqueous solution in which the concentrations of hydrofluoric acid and hydrogen peroxide water are controlled for a predetermined time, and porous on the surface of the silicon substrate 2. Since the layer 3 is formed, the texture can be more uniformly formed on the surface of the silicon substrate 2.

The second aqueous solution that forms the porous layer 3 deteriorates in proportion to the number of processed silicon substrates 2 and needs to be replaced. However, in this embodiment, the second aqueous solution contains metal ions 5. Since it is a separate body from the first aqueous solution, it is not necessary to discard the metal ions 5 as in the prior art, so that the management of the aqueous solution can be simplified.

The following steps may be added after step SP3. That is, in step SP4, water washing is performed, and in step SP5, the silicon substrate 2 is immersed in a third aqueous solution containing hydrofluoric acid and nitric acid and etched (FIG. 3C). Thereby, in this embodiment, while removing the metal ion 5, the silicon substrate 2 which has the porous membrane 3A which has a bigger hole can be obtained (FIG. 3D). In this case, the volume ratio of the third aqueous solution is HF (concentration 50%): HNO ₃ (concentration 69%): H ₂ O = 100 ml to 500 ml: 600 ml to 3000 ml: 10000 ml to 50000 ml. The immersion time can be 240 seconds to 360 seconds.

Next, in step SP6, the substrate is washed with water, and then the silicon substrate 2 is immersed in an alkaline chemical solution to remove the stain film (step SP7). The stain film is a black-brown film formed on the surface of the silicon substrate 2 by etching. Finally, the silicon substrate 2 provided with the porous layer 3A according to the present embodiment can be obtained by washing with water (step SP8).

(Modification)
The present invention is not limited to the above-described embodiment, and can be appropriately changed within the scope of the gist of the present invention. For example, by using an acid capable of removing the natural oxide film as the strong acid, for example, hydrofluoric acid, in step SP1, the metal ions 5 can be attached while removing the natural oxide film. In this case, it is not necessary to use a silicon substrate from which a natural oxide film has been removed in advance, and a silicon substrate having a natural oxide film can be used.

(Example)
Next, examples of the method for manufacturing the solar cell 1 according to the above embodiment will be described. In this embodiment, a p-type silicon substrate is used as the silicon substrate 2. Hydrofluoric acid as a strong acid and AgNO ₃ as a metal-containing agent were mixed to prepare a first aqueous solution. The volume ratio was adjusted to HF (concentration 50%): AgNO ₃ (concentration 3E-4M): H ₂ O = 1000 ml: 40 ml: 10000 ml. The silicon substrate 2 was immersed in a first aqueous solution, and the metal ions 5 were attached to the surface of the silicon substrate 2 by electroless plating. The plating conditions in this case were an immersion time of 300 seconds and a temperature of the second aqueous solution of 26 degrees. Further, the second aqueous solution was allowed to flow around the silicon substrate 2 by the pump circulator.

Subsequently, the second aqueous solution in which the volume ratio of hydrofluoric acid and hydrogen peroxide water was adjusted to HF (concentration 50%): H ₂ O ₂ (concentration 30%): H ₂ O = 1200 ml: 600 ml: 10000 ml The silicon substrate 2 was immersed, and the porous layer 3 was formed on the surface of the silicon substrate 2 by the catalytic reaction of the metal ions 5. FIG. 4A shows an SEM image (Scanning Electron Microscope) of the surface of the silicon substrate 2 on which the porous layer 3 is thus formed. As is clear from this figure, according to the method for manufacturing the solar cell 1 according to this example, the porous layer 3 can be more uniformly formed on the surface of the silicon substrate 2.

Next, the porous layer 3 was added to a third aqueous solution in which the volume ratio of hydrofluoric acid and nitric acid was adjusted to HF (concentration 50%): HNO ₃ (concentration 69%): H ₂ O = 400 ml: 3000 ml: 6000 ml. The formed silicon substrate 2 was immersed and etched. An SEM image of the surface of the silicon substrate 2 etched in this manner is shown in FIG. 4B. As is clear from this figure, according to the method for manufacturing the solar cell 1 according to this example, the porous layer 3A having larger pores on the surface of the silicon substrate 2 can be formed more uniformly.

DESCRIPTION OF SYMBOLS 1 Solar cell 2 Silicon substrate 3 Porous layer 5 Metal ion 6 Hole

Claims

In a method for manufacturing a solar cell in which a porous layer is formed on the surface of a silicon substrate by etching using metal ions,
Immersing the silicon substrate in a first aqueous solution containing a strong acid and metal ions, and attaching the metal ions to the silicon substrate surface by electroless plating;
The silicon substrate having the metal ions attached to the surface thereof is immersed in a second aqueous solution containing hydrofluoric acid and hydrogen peroxide, and the porous surface is formed on the surface of the silicon substrate by a catalytic reaction of the metal ions. And a step of forming a layer.
2. The solar cell according to claim 1, wherein in the step of attaching the metal ions to the surface of the silicon substrate, the adhesion amount of the metal ions is controlled based on a concentration of the metal put into the first aqueous solution. Production method.
The method for manufacturing a solar cell according to claim 1 or 2, wherein the second aqueous solution has a hydrogen peroxide concentration of 25% to 50% with respect to hydrofluoric acid.
4. The method according to claim 1, further comprising the step of immersing and etching the silicon substrate on which the porous layer is formed in a third aqueous solution containing hydrofluoric acid and nitric acid. The manufacturing method of the solar cell of description.
5. The method for manufacturing a solar cell according to claim 1, further comprising a step of immersing and etching the silicon substrate in an alkaline chemical solution.
In the step of forming the porous layer on the silicon substrate surface,
The size of the pores constituting the porous layer is controlled by controlling the immersion time of the silicon substrate and the ratio of the hydrofluoric acid and the hydrogen peroxide solution. 5. The method for producing a solar cell according to claim 1.
The electroless plating is performed while flowing the second aqueous solution containing the metal ions in the step of attaching the metal ions to the silicon substrate surface. The manufacturing method of the solar cell of description.
The method for producing a solar cell according to any one of claims 1 to 7, wherein the strong acid in the first aqueous solution is hydrofluoric acid.