JPH03206669A - Solar cell - Google Patents

Abstract

PURPOSE:To obtain a high conversion efficiency without damaging a mechanical strength even when an active layer is made thin by a method wherein a reflection layer and a transverse-direction epitaxial growth operation are used. CONSTITUTION:An Si oxide film 2 is formed on the surface of an Si substrate 1 and is patterned. Then, an wavy surface is obtained by an anisotropic etching operation. The film 2 is removed. An Si oxide film 2 to be used as a reflection layer is formed; it is patterned; opening parts 3 are formed. An epitaxial growth layer 4 is formed on the surface of the Si substrate 1 exposed in the opening parts 3 by a transverse-direction epitaxial growth operation; the whole surface of the film 2 is covered. P or the like is diffused from the surface; an n-type diffusion layer 5 is formed; electrodes are formed on the surface and the rear to form a solar cell.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a solar cell that can convert incident light energy into electrical energy with high conversion efficiency.

[Prior Art] FIGS. 4(a) to 4(d) are schematic cross-sectional views showing the structure of an example of a conventional high-efficiency solar cell according to its manufacturing process. In FIG. 4, 1 is a single crystal silicon substrate (hereinafter abbreviated as silicon substrate), 2 is a silicon oxide film,
5 is a diffusion layer.

The silicon substrate 1 is p-type and has a <100> plane orientation, and both sides of the silicon substrate 1 are covered with silicon oxide films 2 (see FIG. 4(a)).
). Next, the silicon oxide film 2 is patterned by photolithography or other methods, and a part of it is removed by etching, and patterning is performed on both sides of the silicon substrate 1 at regular intervals, alternating between the front and back sides. Figure 4 (
b)). Next, anisotropic etching is performed from both sides of the silicon substrate 1 using KOH or the like (FIG. 4(C)). Since the silicon substrate 1 has a <100> plane orientation, a plane with a (111) plane orientation appears by anisotropic etching. This etching reduces the silicon substrate 1 to a thickness of approximately 50 um.
Make it so that Thereafter, the remaining silicon oxide film 2 is removed, impurities such as phosphorus are diffused on the surface to form an n-type diffusion layer 5 (FIG. 4(d)), and electrodes (not shown) are formed on the front and back surfaces. to form a solar cell.

When light enters the solar cell formed in this way from the surface, the light entering the silicon substrate 1 is absorbed internally while repeating reflection between the front and back surfaces, and the surface of the silicon substrate 1 Since the light reflected by the silicon substrate 1 also enters the silicon substrate 1 again, the incident light can be efficiently absorbed. In this structure, the effective thickness of the silicon substrate 1 due to the light confinement effect is about four times or more than the actual thickness. For example, for a 50 um thick one, it will cost about 200 um.
A light absorption amount equivalent to that obtained when using a flat silicon substrate having a thickness of ILm or more can be obtained.

However, since the actual thickness of solar cells is thin, the percentage of carriers generated by light absorption that are annihilated and lost before reaching the electrodes is reduced, resulting in improved performance as a solar cell. .

However, in this structure, the silicon substrate 1 itself is processed into a thin film having a thickness of about 50 LLm, which has low mechanical strength and is easily damaged, making it difficult to handle.

Furthermore, since the crystal is processed into a wavy shape, stress is concentrated at the corner portions, making crystal defects more likely to occur in these portions, impairing the characteristics of the solar cell.

[Problems to be solved by the invention] Conventional solar cells are constructed as described above, and therefore have problems such as poor mechanical strength, difficulty in handling in the process, and inability to obtain high efficiency as predicted by theory. There was an issue.

This invention was made to solve the above problems, and aims to obtain a solar cell that can achieve high conversion efficiency even when it has a wavy structure without impairing mechanical strength.

[Means for Solving the Problems] The solar cell of the present invention includes anisotropic etching of one main surface of a single crystal or polycrystalline silicon substrate, a reflective layer provided on the surface, and a reflective layer provided on a part of the reflective layer. An epitaxial growth layer used as an active layer is formed on the reflective layer by lateral epitaxial growth from the opening.

[Function] In the present invention, by using a reflective layer and lateral epitaxial growth, even if the active layer is made thin, mechanical strength is not impaired and high conversion efficiency can be obtained.

[Example] FIGS. 1(a) to 1(g) are cross-sectional views showing the structure of an example of a solar cell according to the present invention according to its manufacturing process. In this figure, 1 is a single crystal silicon substrate (hereinafter abbreviated as silicon substrate), 2 is a silicon oxide film,
3 is an opening provided in this silicon oxide film 2, 4 is an epitaxial growth layer formed by lateral epitaxial growth, and 5 is a diffusion layer.

The silicon substrate 1 is a p-type with high impurity concentration and low resistance, and the surface orientation is <100>. A silicon oxide film 2 is formed on the surface of a silicon substrate 1 (FIG. 1(a)), and the silicon oxide film 2 is patterned by photolithography or other methods, and is etched away leaving a part of it (FIG. 1(a)). b)). Next, by performing anisotropic etching using K○H or the like, a wavy surface similar to that described for the conventional example is obtained. however,
In the case of this invention, the entire back surface of the silicon substrate 1 is covered with an oxide film, and anisotropic etching is not performed (Fig. 1(C)).
). Silicon oxide film 2 used as an etching mask
The pattern is removed once, and a new silicon oxide film 2 is provided to be used as a reflective layer (FIG. 1(d)). A part of the silicon oxide film 2 is patterned and removed by photolithography or other methods to form an opening 3 (FIG. 1(e)).

Furthermore, by a method such as a vapor phase method or a liquid phase method, a bitaxially grown layer 4 is formed by lateral epitaxial growth using the surface of the silicon substrate 1 exposed in the opening 3 as a seed.
(Fig. 1(f)). At this time, since the surface of the silicon substrate 1 exposed in the opening 3 is a (1.11) plane that appeared by anisotropic etching, in epitaxial growth using this as a seed, the aspect ratio, that is, the grown epitaxial growth layer 4 opening 3 for the layer thickness of
The ratio of the length of the silicon oxide film 2 grown laterally on the silicon oxide film 2 can be increased, and the entire surface of the silicon oxide film 2 can be easily covered with the epitaxially grown layer 4. The epitaxial growth layer 4 is p-type and grows to a thickness of about 50 iLm. Subsequently, phosphorus or the like is diffused from the surface to form an n-type diffusion layer 5 (FIG. 1(g)), and electrodes (not shown) are provided on the front and back surfaces to form a solar cell.

Epitaxial growth layer 4 that becomes an active layer as a solar cell
is a thin film with a thickness of 50 μm, but it does not need to be mechanically self-supporting as shown in the conventional example.
Since it is integrated with the silicon substrate 1, its mechanical strength as a whole is sufficiently strong, and there is no problem in handling it in the manufacturing process. Furthermore, since the geometrical structure is exactly the same as that shown in the conventional example, the features of the wave-shaped structure as a solar cell similar to those described in the conventional example can be effectively utilized. Furthermore, since the active layer is formed on the silicon substrate 1 and stress is not directly applied to it, defects due to stress will not occur and the characteristics of the solar cell will not be impaired. The silicon substrate 1 with high impurity concentration and low resistance also plays a role as an electrode on the back side of the solar cell.

Furthermore, in the case of the present invention, processing of the wavy structure is performed on the silicon substrate 1 before forming the epitaxial growth layer 4, which is the active layer, so that not only anisotropic etching but also mechanical friction can be used. . That is, the first
In the process shown in Figure (C), the surface that appears on the slope of the wavy structure is
It can be tilted from the exact <111> plane. For example, if the wavy structure of the silicon substrate 1 is processed so that its apex angle is 67'' and epitaxial growth is performed on it,
Since the epitaxial growth layer 4 grows with the (1 1 1> plane exposed on the surface, the apex angle of the epitaxial growth layer 4 is 70.5°, so the structure shown in FIG. 2 can be realized. In this case , the optical confinement efficiency is further improved, and the effective optical path length is increased by the epitaxial growth layer 4.
The thickness is 20 times or more.

In the above embodiment, a p-type epitaxial growth layer 4 is provided on a p-type silicon substrate 1, and an n-type diffusion layer 5 is provided on the surface of the p-type epitaxial growth layer 4. A similar effect can be obtained by providing an n-type epitaxial growth layer 4 and providing a p-type diffusion layer on its surface.

Furthermore, if a layer with a high impurity concentration of the same conductivity type as the epitaxial growth layer 4 is provided on the interface side of the epitaxial growth layer 4 with the reflective layer (silicon oxide film 2), the characteristics of the solar cell are further improved. Further, although the case where the silicon oxide film 2 is used as the reflective layer is shown, a silicon nitride film or a laminated film of an oxide film and a nitride film may be used, and the material thereof is not limited. The use of a silicon nitride film has the advantage that the aspect ratio can be increased when performing lateral epitaxial growth, compared to the case of an oxide film. Alternatively, a metal film may be used as the reflective layer. In this case, there is an advantage that the electrical connection between the active layer (epitaxially grown layer 4) and the silicon substrate 1 is improved. Further, although the drawings show that the openings 3 are provided one at a time near the center of the slope portion of the wavy structure, the position, size, shape, interval, etc. of the openings 3 are not specified. Furthermore, although a method using impurity diffusion has been described for forming a pn junction as a solar cell, other methods may be used, such as a heterojunction using a microcrystalline thin film.

Further, in the above embodiment, a case where a wavy structure is formed on a single crystal substrate is shown, but similar effects can be obtained even if a polycrystalline substrate is used.

FIGS. 3(a) to 3(f) are cross-sectional views showing the manufacturing process when the present invention is applied based on a polycrystalline substrate.

The polycrystalline silicon substrate 11 is an aggregate of single crystal grains separated by a large number of grain boundaries 6, as shown in FIG. 3(a). When this surface is anisotropically etched using, for example, KOH, the (1 1 1) plane of each crystal grain appears preferentially (FIG. 3(b)). For example, a silicon oxide film 2 is provided as a reflective layer on this surface (FIG. 3(c)), an opening 3 is formed in this (FIG. 3(d)), and epitaxial growth is performed. If anisotropic etching is not performed, planes with various crystal orientations appear on the surface of the polycrystalline silicon substrate 11, so lateral epitaxial growth cannot be effectively performed. Since the (111.) plane preferentially appears on the silicon surface exposed in the opening 3 serving as a seed, lateral epitaxial growth becomes active and it becomes possible to cover the entire surface of the silicon oxide film 2 (
Figure 3(e)). After that, a diffusion layer 5 is provided (Fig. 3(f)
))． Electrodes are formed to form a solar cell. Alternatively, after the step shown in FIG. 3(f), an amorphous silicon pin-structured solar cell may be formed thereon to form a stacked solar cell.

[Effect of the invention 1 As explained above, the present invention provides a method in which one main surface of a single crystal or polycrystalline silicon substrate is anisotropically etched, a reflective layer is provided on the surface, and a reflective layer is provided on a part of the reflective layer. Since the epitaxial growth layer used as the active layer is formed on the reflective layer by lateral epitaxial growth from the opening, a high-performance solar cell can be obtained even with a wavy optical confinement structure without sacrificing mechanical strength. can.

[Brief explanation of drawings]

Figures (a) to (g) are cross-sectional views showing the structure of a solar cell according to an embodiment of the present invention according to the manufacturing process;
The figure is a sectional view showing another embodiment of the present invention, FIG. 3(a)
-(f) are cross-sectional views showing the structure of a solar cell according to still another embodiment of the present invention according to the manufacturing process, and FIG.
a) to (d) are schematic cross-sectional views showing the structure of a conventional solar cell according to the manufacturing process. In the figure, 1 is a silicon substrate, 2 is a silicon oxide film, 3 is an opening, 4 is an epitaxial growth layer, 5 is a diffusion layer, and 6 is a crystal grain boundary. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Claims] One principal surface of a single-crystalline or polycrystalline silicon substrate is anisotropically etched, a reflective layer is provided on the surface, and an active layer is formed on the reflective layer by lateral epitaxial growth from an opening provided in a part of the reflective layer. A solar cell characterized by forming an epitaxially grown layer used as a layer.