JPS5861681A - Solar battery - Google Patents

Abstract

PURPOSE:To eliminate current leakage resulting from P-N junction and to obtain a large photo response characteristic by providing an oxide film between the semiconductor thin film and semiconductor substrate at the light sensible surface. CONSTITUTION:An oxide film 32 is formed on the P type single crystal Si substrate 1. The N type polycrystal Si film 42 is then formed over the layer 32. Thereafter the electrodes 3, 4 are formed on both front and rear sides. When the electron and hole pair excited and generated by the light beam within the substrate 1 moves in the direction to the film 32 by the diffusion, the hole receives a force in the direction as becoming far from the film 32 due to a bending of energy band in the vicinity of film 32, while electron receives a force in the direction as becoming near to the film 32. Therefore, the electron and hole are isolated and the hole remains in the substrate 1 while the electron moves to the film 32. The film 32 is very thin and the electron having reached the vicinity of film 32 passes easily through the film 32 and reaches the film 42. The electron and hole pair generated in the substrate 1 in the above process is converged as the carrier. The electron and hole pair generated in the film 42 is also converged as the carrier by the similar process.

Description

[Detailed description of the invention] The present invention relates to solar cells.

Solar cells, as transducers that can directly convert light into electricity, have the potential to play a role in the use of solar energy, which is an energy source that can replace fossil energy such as oil. Many resources are required, including the necessary energy, and therefore the price of solar cells is high.0Also, if you try to obtain the energy spent on manufacturing solar cells by generating electricity with solar cells, , it takes a long period of 10 to 20 years,
2 - This period is comparable to the lifetime of a solar cell. In other words, when viewed as a power source, solar cells were never economical.

One way to solve this problem is to increase the conversion efficiency of solar cells.

If the conversion efficiency is high, more electricity can be generated from the same amount of solar energy, and the price of electricity will therefore be lower.

The most effective way to increase conversion efficiency is to increase the lifetime of minority carriers in the substrate and increase the output current by improving the optical spectrum response.

Here, a conventional solar cell will be explained using figures. Since the solar cell according to the present invention is particularly aimed at improving the optical spectrum response, the explanation of the conventional solar cell will also be centered on this point.

FIG. 1 is a cross-sectional perspective view of a conventionally used general solar cell. 1 is a P-type silicon substrate, 2 is an n-swelled diffusion layer,
3 is a front electrode, and 4 is a back electrode.

The light is reversely incident on the side where the n-swelled diffusion layer 2 is located, and the light enters the n-type diffusion layer 2.
and is absorbed by the p-type silicon substrate 1, and electron-hole pairs are generated. Some of the generated electron-hole pairs recombine and disappear and no current is generated, but if the electron-hole pairs reach the pN junction at the interface between the p-type silicon substrate and the n-swelled diffusion layer, Electrons and holes are separated, generating an electric current. The above is a general photocurrent generation process. The n-swelled diffusion layer 2 has a very high impurity concentration and generally has many crystal defects. Therefore, the lifetime of minority carriers in the n-swelled diffusion layer 2 is very short. For this reason, the electron-hole pairs generated in the n-type diffusion layer do not contribute much to the generation of charge current.

For these reasons, it is a good method to improve the optical spectrum response to reduce the thickness of the n-swelled diffusion layer 2 as much as possible, and this has been widely practiced in the past. However, it is extremely difficult to form a thin diffusion layer uniformly, and even if a thin diffusion layer can be formed uniformly, such a thin diffusion layer will be difficult to form when forming the surface electrode 3. easily penetrates, and the p-type silicon substrate 1 and the n-swelled diffusion layer 2 are short-circuited. Therefore, in the conventional solar cell, the n-swelled diffusion layer 2
The thickness of the grooves remained at about 2 microns to 0.5 microns, and it was impossible to make them more grooved.

From the point of view of uniformly forming a thin diffusion layer, the method shown in FIG. 2 is also used. FIG. 2 is a cross-sectional view of its structure, and 12 is an n-type epitaxial layer. The same components as those of the conventional solar cell shown in Fig. 1 are given the same numbers.The features of the solar cell shown in Fig. 2 are as follows.
An n-type epitaxial layer 12 is formed on a p-type silicon substrate 1 to form a 7) N junction.A uniform epitaxial layer can be formed with a thickness of about 0.5 microns, so The solar cell shown in Figure 2 is formed by epitaxial growth, which is expected to improve the optical spectral response. :, the PNN junction leakage current is large, which causes a problem that a process must be provided to reduce the leakage current of the PN junction, such as performing heat treatment again after forming the epitaxial layer. Furthermore, the solar cell shown in FIG. 2 does not provide any solution to the problem of PN junction short circuit during the formation of surface electrodes.

As one method for improving optical spectral response, there is also a solar cell using a Schottky junction as shown in FIG.

FIG. 3 is a sectional view of its structure, and 22 is a Schottky electrode. Further, the same components as those of the solar cell shown in FIG. 1 are given the same numbers.

In FIG. 3, since the Schottky electrode 22 must transmit light, it is formed sparingly.

However, if the thickness is too small, it becomes difficult to form a film, and the thickness of the Schottky electrode 22 cannot exceed a certain level. Therefore, reflection of light by the Schottky electrode 22 is unavoidable, resulting in a decrease in optical spectrum response.

An object of the present invention is to provide a solar cell having a novel structure that compensates for the drawbacks of such conventional solar cells.

That is, to provide a solar cell that increases the optical spectrum response, especially the response to light at the peak wavelength, has no leakage current at the PN junction, and also solves the problem of PN junction short circuit that easily occurs when forming surface electrodes. The purpose is to

FIG. 4 shows a structural sectional view of the solar cell according to the present invention.

In the figure, 32 is an oxide film, and 42 is an n-type polycrystalline silicon film (hereinafter abbreviated as n-poly film). Components that are the same as those of the conventional solar cell shown in FIG. 1 are given the same numbers. In order to facilitate explanation of the operation of the solar cell shown in FIG. 4, an outline of the energy band structure is shown in FIG. 6. 5
o is the Fermi level, 51 is an electron-hole pair, 52 is an electron, 5
3 is a hole. Further, the same parts as those of the solar cell according to the present invention shown in FIG. 4 are given the same numbers.

The solar cell shown in FIG. 4 is manufactured, for example, as follows. That is, a p-type single crystal silicon substrate having a specific resistance of about 10 cmO is heat treated in an oxidizing atmosphere to form an oxide film of about 30 angstroms. Although the components of the atmosphere and the heat treatment temperature vary depending on the conditions such as the shape of the furnace used for the heat treatment, for example, the heat treatment is performed in a dry, oxygen atmosphere at 600°C for 110 minutes.

Next, a resistivity oO01Ω/e-type multilayer crystal silicon film with a thickness of 100 angstroms is formed using a well-known CVD technique. Finally, electrodes are formed on both the front and back sides to complete the process.

The solar cell in the present invention operates as described below. That is, when electron-hole pairs 51 excited and generated by light move in the P-type silicon substrate 1 by diffusion toward the oxide film 32, the holes move away from the oxide film due to the bending of the energy band near the oxide film. The electrons receive a force in the direction of the oxide film 3
Therefore, the electron-hole pair is separated, the hole remains in the P-type silicon substrate, and the electron moves toward the oxide film 32.

Since the oxide film 32 is very thin, with a thickness of several tens of angstroms, the electrons 52 that have reached the vicinity of 1/(J+) of the oxide film 32 easily pass through the oxide film 32 due to the tunnel effect, and become n -Poly film 42 on the film.In the process described above, P
The electron-hole pairs generated in the shaped silicon substrate 1 are collected as carriers, and the electron pressure (L pairs) generated in the n-poly film 42 are also collected as carriers in the process described above.

That is, the holes 53 that have reached the vicinity of the oxide film 32 pass through the oxide film 32 due to the tunnel effect and reach the inside of the P-type silicon substrate 1.

The solar cell according to the present invention is characterized by the presence of the oxide film 32, and that the light-receiving surface is composed of two semiconductor films on the n-poly film 42.

Due to the presence of the oxide film 32, there is no need to worry about leakage current at the P and N junctions as in the solar cell shown in FIG. That is, due to the presence of the oxide film 32, there is no PN junction in the solar cell according to the present invention shown in FIG. 6, and leakage current due to the PN junction is likely to occur.

When the light-receiving surface is composed of two semiconductor films on the n-poly film 42, electron-hole pairs can be generated even in the n-poly film 42, and the Schottky solar cell shown in FIG. This eliminates the problem of light reflection. Also, n-
It is extremely easy to form a uniform thin film on the poly film 42, and it is not difficult to form it to a thickness of 0.1 micron using the current CDV technology. In other words, the solar cell of the present invention shown in FIG. 4 can have a larger optical spectrum response to short wavelength light than the conventional solar cell shown in FIG. 1.

Furthermore, when forming the surface electrode, due to the presence of the oxide film 32,
It is possible to prevent the n-po17 film 42 and the P-type silicon substrate 1 from being short-circuited.

As described above, the solar cell according to the present invention can increase the optical spectrum response particularly to short wavelength light by making it possible to form an extremely thin and uniform semiconductor film on the light receiving surface. Moreover, it has the effect of reducing leakage current caused by the PN junction. By providing an oxide film between the semiconductor substrate on the light-receiving surface and the semiconductor substrate, the PN junction short circuit that occurs during electrode formation can be reduced. It also has the effect of eliminating problems0

[Brief explanation of drawings]

Fig. 1 is a cross-sectional perspective view of a conventional general solar cell, Fig. 2 is a structural cross-sectional view of a solar cell in which a PN junction is formed by epitaxial growth, Fig. 3 is a structural cross-sectional view of a Schottky solar cell, and Fig. 4 is a structural cross-sectional view of a solar cell with a PN junction formed by epitaxial growth. The figure is a cross-sectional view of the structure of the solar cell according to the present invention, and FIG. 5 is a schematic diagram showing the energy band of the solar cell according to the present invention. 1... P-type silicon substrate, 32... Oxide film, 42... N-type polycrystalline silicon film. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 4 Figure 4 Figure 5

Claims (1)

[Claims] a semiconductor substrate having either p or n conductivity type;
an oxide film formed on the semiconductor substrate; and a conductive layer 14 formed on the oxide film and having a conductivity opposite to that of the semiconductor substrate. A solar cell characterized by comprising a semiconductor film having a