JP2000216416A - Solar battery structure and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily form a high-quality silicon polycrystalline thin film on a substrate and increase the conversion efficiency of a solar battery by forming a porous layer between the substrate and a silicon PN junction structure. SOLUTION: A solar battery structure comprises a silicon PN junction 4 disposed on a substrate 1 and a current collector electrode 5 disposed on the silicon PN junction 4. In this structure, a porous silicon layer 2 is disposed between the substrate 1 and the silicon PN junction 4. To be more specific, the substrate 1 is a heat-resisting substrate made of ceramic, carbon, etc., and the porous silicon layer 2 is formed on the heat-resisting substrate 1. Since the porous silicon layer 2 is porous material, a substantial refractive index decreases according to the concentration of pores and the porous silicon layer 2 could be a reflection layer. Part of light not absorbed by a second polycrystalline silicon layer 3 and the silicon PN junction 4 is reflected on the porous silicon layer 2 towards the second polycrystalline silicon layer 3, thereby making an optical path length longer and increasing the conversion efficiency.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a thin-film solar cell structure and a method for manufacturing the same.

[0002]

2. Description of the Related Art With the recent increase in energy demand and the decrease in reserves of fossil energy such as petroleum and coal, the importance of solar cells that effectively utilize solar energy, which has not been fully utilized in the past, is increasing. . At present, a solar cell mainly employs a substrate obtained by cutting a silicon ingot with a thickness of several hundred microns and having a pn junction and an electrode formed thereon.
Since a large amount of silicon is used in this method, it is expected that silicon will be insufficient when mass-producing solar cells. One method for solving this problem is, for example, the 21st IEEE Photovoltaic Specialist Conference (21st IEEE Photovoltaic).
Specialists Conference) (1
990) As described on pages 699-699, a polycrystalline silicon film having a thickness of about 100 μm is formed on a heat-resistant substrate such as carbon graphite at a high temperature of 1000 ° C. or more, and a pn junction is formed on the polycrystalline silicon film at a high temperature. In some cases, there is a method in which a layer serving as a light absorption layer is further grown to form a pn junction, and electrodes are formed on these layers to form a solar cell, and a conversion efficiency of 10 to 12% is obtained. In this method, the amount of silicon material used is smaller than that in the above-described method of cutting from an ingot.

[0003]

However, when a polycrystalline silicon film is formed on a substrate such as ceramic or carbon at a high temperature, impurities diffuse from the substrate into the polycrystalline silicon film to deteriorate the film quality. In order to prevent impurities from the substrate from diffusing on the light incident side opposite to the substrate serving as the light absorbing layer. For this reason, the first layer of polycrystalline silicon is close to 100 μm, and it has been a problem to further reduce the thickness of the silicon film.

The present invention has been made in view of the above circumstances, and a high-quality silicon polycrystalline thin film can be easily obtained on a substrate, the conversion efficiency of a solar cell can be improved, and economical and resource saving of the solar cell can be achieved. It is an object of the present invention to provide a solar cell structure that is effective for the formation of a solar cell and a method for manufacturing the same.

[0005]

In order to achieve the above object, the present invention comprises at least a silicon PN junction structure provided on a substrate and a current collecting electrode provided on the silicon PN junction structure. In the solar cell structure, a porous layer is provided between the substrate and the silicon PN junction structure.

The present invention is also characterized in that in the above solar cell structure, the porous layer is a porous silicon layer.

Further, the present invention is characterized in that in the above solar cell structure, the effective refractive index of the porous layer is smaller than the effective refractive index of the silicon PN junction structure.

The present invention also relates to a method for manufacturing a solar cell in which a polycrystalline silicon layer is formed on a heat-resistant substrate, wherein the first polycrystalline silicon layer is formed on the heat-resistant substrate and then the first polycrystalline silicon layer is formed. The method is characterized in that the crystalline silicon layer is made porous, and then a second polycrystalline silicon layer is grown on the porous silicon layer.

[0009] The heat-resistant substrate referred to herein is CVD.
Means a substrate that can withstand the heating temperature required for the growth of silicon crystals.

In the present invention, in manufacturing a solar cell, in forming a polycrystalline silicon layer on a heat-resistant substrate, a first polycrystalline silicon layer is formed on the substrate and then the first polycrystalline silicon layer is formed. The crystalline silicon layer is made porous, and then a second polycrystalline silicon layer is formed on the porous silicon layer by growth. When the porous silicon layer is heated in a hydrogen atmosphere or the like, the surface of the porous layer closes the pores and returns to the original crystalline state. Therefore, the second polycrystalline silicon layer on the substrate serving as the light absorbing layer is formed of a crystal having a diameter reflecting the crystallinity of the first polycrystalline silicon layer. In addition, if the heat-resistant substrate has a porous layer, even if the heat-resistant substrate contains a large amount of impurities and is exposed to high temperatures, diffusion of the impurities from the substrate is prevented in the porous layer, and Difficult to diffuse into the polycrystalline silicon layer. Therefore, even if the first polycrystalline silicon layer is thin, a high-purity film suitable for solar cell production can be obtained as the second polycrystalline silicon layer on the substrate serving as a light absorbing layer, and the thick first polycrystalline silicon No layers are required.

[0011] A solar cell manufactured by the present manufacturing method is a solar cell comprising at least a polycrystalline silicon layer provided on a heat-resistant substrate and a collecting electrode provided on the polycrystalline silicon layer. In the above, the structure was such that porous silicon was provided between the heat-resistant substrate and the polycrystalline silicon layer. Therefore, even if the first polycrystalline silicon layer is thin, a high-purity silicon layer suitable for a light absorption layer of a solar cell is obtained on the light incident side, and the whole can be made thin. Furthermore, since the porous silicon layer is porous, the effective refractive index decreases in accordance with the pore density, and the porous silicon layer can be a reflective layer for light entering from the silicon layer having a high refractive index. Therefore, the light that has not been sufficiently absorbed by the second polycrystalline silicon layer or the junction is reflected again in the direction of the second polycrystalline silicon layer to increase the optical path length, thereby contributing to an improvement in conversion efficiency.

[0012]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIGS. 1A to 1E are schematic sectional views showing a method for manufacturing a solar cell according to the present invention.
Is a porous silicon layer, 2 'is a first polycrystalline silicon layer,
Reference numeral 3 denotes a second polycrystalline silicon layer, 4 denotes a junction, 5 denotes a current collecting electrode, and 6 denotes a back surface electrode. Note that the antireflection film is omitted. Hereinafter, the manufacturing method will be described step by step. FIG. 1 (a)
, A first polycrystalline silicon layer 2 ′ is formed on a heat-resistant substrate 1. As the heat-resistant substrate 1, a substrate such as a ceramic substrate or a carbon substrate can be used. The first polycrystalline silicon layer 2 'can be formed by, for example, depositing a polycrystalline thin film containing silicon as a main component from a melt of an alloy of silicon and another metal, for example, aluminum, indium, tin, or the like. For example, a method of spraying silicon particles onto a substrate in an inside or an inert gas and melting the silicon particles to form a polycrystal can be used. The polycrystalline silicon film thus formed has a large grain size. In FIG. 1B, the first polycrystalline silicon layer 2 'is made porous to form a porous silicon layer 2. A typical example of the method is anodization. Anodization is a method in which silicon is electrolyzed in a hydrofluoric acid aqueous solution or an alcohol solution, and is a method of forming pores on the surface.
Also, many fine holes can be formed by performing patterning using a resist frequently used in the semiconductor industry and performing dry etching or the like. Next, FIG.
2C, a second polycrystalline silicon layer 3 is grown on the porous silicon layer 2. As a growth method, a chemical vapor deposition method (CVD) or the like is preferable. At this time, by heating to a high temperature of about 1000 ° C., the surface of the porous silicon layer 2 reproduces the crystallinity reflecting the crystal state of the first polycrystalline silicon layer 2 ′. Therefore, the second polycrystalline silicon layer 3 has crystallinity reflecting the crystal state of the first polycrystalline silicon layer 2 '. Therefore, the first polysilicon layer 2 '
Is preferably large in the surface direction of the substrate 1. Then Figure 1
In (d), a silicon PN junction 4 is formed. The silicon PN junction may be formed by diffusion of a dopant, or may be a heterojunction with amorphous silicon. Further, at the time of the growth shown in FIG. 1C, it may be manufactured by growing silicon having different polarities. Next, in FIG. 1E, a current collecting electrode 5, a back surface electrode 6, and an antireflection film (not shown) are formed thereon. Further, if integration is required, a process for integration can be performed.

FIG. 2 is a schematic sectional view showing a solar cell structure of the present invention, wherein 1 is a substrate, 2 is a porous silicon layer, 3 is a second polycrystalline silicon layer, 4 is a silicon PN junction, and 5 is a collector. The electrode 6 is a back electrode. Note that the antireflection film is omitted. That is, in a solar cell structure in which the silicon PN junction 4 is provided on the substrate 1 and the current collecting electrode 5 is provided on the silicon PN junction 4, between the substrate 1 and the silicon PN junction 4 The porous silicon layer 2 is provided. The effective refractive index of the porous silicon layer 2 is set smaller than the effective refractive index of the silicon PN junction 4. In FIG. 2, only one solar cell is formed on the substrate 1,
It is also possible to divide the solar cell structure on the substrate 1 into an integrated structure in which electrodes are connected. As the substrate 1, a heat-resistant substrate such as ceramic or carbon can be used. On these heat-resistant substrates 1 there is a porous silicon layer 2. The size of the hole in the porous layer may be such that the hole is closed when epitaxial growth is performed at a high temperature in the step of FIG. 1C and a polycrystalline silicon film can be formed on the porous silicon layer 2. The maximum diameter is determined by the processing temperature, that is, the heat resistance of the substrate 1 used. Further, since the porous silicon layer 2 is porous, the effective refractive index is reduced according to the pore density, and the porous silicon layer 2 can be a reflective layer. Therefore, the light that has not been sufficiently absorbed by the second polycrystalline silicon layer 3 or the silicon PN junction 4 is reflected again in the direction of the second polycrystalline silicon layer 3 to increase the optical path length, thereby improving the conversion efficiency. Can also contribute. Example 1 A ceramic substrate was used as a heat-resistant substrate 1, and a first polycrystalline silicon layer 2 'was deposited at 750 ° C by supplying silicon to a liquid in which aluminum and silicon were dissolved by a sputtering method. First polycrystalline silicon layer 2 '
Had a thickness of 5 μm and an average particle size of 10 μm. This was immersed in a liquid composed of hydrofluoric acid, alcohol, and water, and a current was applied to form a porous silicon layer 2. Next, the film was held at 950 ° C. in a hydrogen atmosphere by a CVD apparatus, and silane was flowed to grow the second polycrystalline silicon layer 3 to 12 μm. At this time, the silicon PN junction 4 was formed by first flowing boron and then phosphorus. The collecting electrode 5
In addition, the back electrode 6 and the antireflection film were formed to produce the structure shown in FIG. The conversion efficiency of this solar cell was 13%. Example 2 A carbon substrate was used as the heat-resistant substrate 1, and a first polycrystalline silicon layer 2 'was produced by spraying silicon powder on the substrate in an argon atmosphere at 900 ° C. The thickness of the first polysilicon layer 2 'is 10 .mu.m and the average grain size is 13 .mu.m.
Met. This was immersed in a liquid composed of hydrofluoric acid, alcohol and water, and an electric current was applied to form a porous silicon layer 2. Next, this film is formed in a hydrogen atmosphere in
After maintaining at 50 ° C., silane was flowed to grow a P-type second polycrystalline silicon layer 3 to 15 μm. Next, plasma was applied at 500 ° C. to form an N-type amorphous silicon layer and form a silicon PN junction 4. A transparent electrode, a current collecting electrode 5, and a back electrode 6 were formed thereon to produce the structure shown in FIG. The conversion efficiency of this solar cell was 15%.

[0015]

According to the solar cell of the present invention, since the thickness of the silicon layer on the substrate can be reduced, the amount of the silicon raw material used can be significantly reduced as compared with the conventional method. In addition, since the porous layer prevents the impurities of the substrate from diffusing into the silicon PN junction structure, a high quality silicon PN junction structure can be obtained. Further, the refractive index n1 of the silicon PN junction structure and the refractive index of the porous layer can be improved. The relationship with the rate n2 is n1
By setting> n2, the porous layer can be used as a reflective layer, so that the conversion efficiency of the solar cell can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view illustrating a method for manufacturing a solar cell according to an embodiment of the present invention.

FIG. 2 is a schematic sectional view showing a solar cell structure according to an embodiment of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 heat-resistant substrate 2 porous silicon layer 2 ′ first polycrystalline silicon layer 3 second polycrystalline silicon layer 4 silicon PN junction 5 current collecting electrode 6 back electrode

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takumi Yamada 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Within Japan Telegraph and Telephone Corporation (72) Inventor Masami Tachikawa 3-192-1, Nishishinjuku, Shinjuku-ku, Tokyo No. Nippon Telegraph and Telephone Corporation F-term (reference) 5F051 AA03 DA03 FA14 GA04 GA06 GA20

Claims (4)

[Claims] 1. A solar cell structure comprising at least a silicon PN junction structure provided on a substrate and a current collecting electrode provided on the silicon PN junction structure, wherein the solar cell structure comprises: A solar cell structure comprising a porous layer provided between the solar cells. 2. The solar cell structure according to claim 1, wherein the porous layer is a porous silicon layer. 3. The solar cell structure according to claim 1, wherein the effective refractive index of the porous layer is smaller than the effective refractive index of the silicon PN junction structure. 4. A method for manufacturing a solar cell in which a polycrystalline silicon layer is formed on a heat-resistant substrate, wherein the first polycrystalline silicon layer is formed on the heat-resistant substrate, and then the first polycrystalline silicon layer is formed. And then growing a second polycrystalline silicon layer on the porous silicon layer.