JPH05235385A - Silicon solar cell - Google Patents

Abstract

PURPOSE:To enhance efficiency while utilizing the merit of low cost by providing the front of a semiconductor substrate with a photo-receiving plane having a structure of reducing reflection and forming an oxide film in the rear side and by setting the thickness of a cell in a specified range. CONSTITUTION:The thickness of a solar cell is 60-15mum. An N<+> diffusion layer 3 is formed in the front of a P-type silicon substrate 4, and the front shows a texture structure 7. The front is provided with N-type electrodes 2 in suitable parts, and the rear has a laminate of an oxide film 8 and a P-side electrode 6 and is spread with a P<+> diffusion layer 5 by diffusion from a window of the oxide film 8. The front structure is similar, but the P<+> diffusion layer 5 is spread over the whole rear: this is overlaid with the oxide film 8 and the P-side electrode 6, and they can be connected by perforating the oxide film 8 in suitable points. The rear can be a structure that a P<+> layer 5 serving as a BSFR layer is spread not by overlaying, but by P<+> diffusion after the oxide film 8 is locally windowed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in the structure of a silicon solar battery cell, and is particularly useful when used in a space solar battery cell which is a power source for an artificial satellite.

[0002]

2. Description of the Related Art Silicon solar cells are the most widely installed as solar cells for space, and because they are economically advantageous compared to cells made of other materials, they are used in satellite programs around the world. ..

FIG. 6 is a perspective view of an example of a BSFR solar cell which is a typical example of a conventionally manufactured space solar cell. The N ⁺ diffusion layer 3 is formed on the surface of the P-type silicon substrate 4.
Then, the antireflection film 1 is formed, and the comb-shaped N-side electrode 2 is further formed on the surface thereof. BSFR on the back
A P ⁺ diffusion layer 5 and a P-side electrode 6 to be a layer are formed.

Table 1 shows the output characteristics.

[0005]

[Table 1]

The thicker the cell thickness, the larger the short-circuit current I _SC and the maximum output P _max, but the release voltage V _OC is constant regardless of the cell thickness, and the conversion efficiency η is about 14 to 15%. It was being done.

In recent years, due to the trend toward larger satellites and higher power consumption, higher output of silicon solar cells is being demanded.
It is necessary to improve conversion efficiency.

As a part of increasing the efficiency of silicon solar cells, for example, a cell having the structure shown in FIG. 1 was prototyped by the group of the present inventors. The same parts as those in FIG. 6 are designated by the same reference numerals. The surface has a texture structure 7.

The texture structure 7 is an anisotropic etching surface of Si using an alkali such as sodium hydroxide as an etching solution, and is formed by densely forming fine pyramid shapes showing (111) planes. ..

This structure has the effect of reducing the reflection loss of the light incident from the light receiving surface and increasing the amount of light absorbed in the cell.

An SiO ₂ oxide film 8 is formed on the entire back surface, and a P ⁺ diffusion layer 5 is formed by diffusing from a locally opened portion. By forming this oxide film 8, the amount of absorption of incident light is increased by reflection on the back surface,
There is a trapping effect that reduces the back surface recombination loss of minority carriers that occur at the same time.

Further, a thin oxide film is formed on the cell surface,
It was also investigated to reduce the surface recombination loss of minority carriers on the cell surface.

Regarding a pyramid-like texture structure, for example, Conference Record of 19th IEEE Photovol
Taic Specialists Conf. 1987 p912-p917, disclosed by Martin Green and Patrick Campbell. Further, a structure in which an oxide film is formed on the front surface and the back surface of a cell having a texture structure is, for example, 19
90th International PVSEC-5 in Kyoto
Disclosed by Martin Green in its technical digest.

FIG. 7 shows an example of the results of the investigation of the relationship between the cell thickness Wd and I _SC , V _OC , and η in a two-dimensional simulation (NEDO commissioned work report in 1987 / high-efficiency crystal). Research on practical application of high efficiency solar cells and high efficiency technology research p66).

Here, the calculation is performed under the ideal conditions of multiple reflection of light on the front and back surfaces of the cell and a recombination velocity of 0 on the front and back surfaces.

From this, it can be seen that V _OC becomes higher as the cell thickness Wd is thinner, and I _SC is almost constant, so that the conversion efficiency η is also improved as the cell thickness Wd is thinner.

The improvement of V _OC is because the reduction of Wd reduces the SRH recombination loss in the bulk region and the saturation current density I ₀ .

[0018]

According to an experiment by the inventors' group, in practice, the cell thickness Wd and I _SC
It was found that the result was different from the theoretical calculation result by the above simulation.

FIGS. 8A and 8B are experimental results of output characteristics when the cell thickness Wd according to the structure shown in FIG. 1 is changed.

As shown in FIG._OCIs W
It is improved as d becomes smaller, but I _SCDeclines, the figure
Output P as shown in (b)_maxWd is around 100 μm
It turned out to be the largest.

Further, as a result of further studying the type of substrate, manufacturing conditions, etc., the cell thickness W giving the maximum value of P _max is obtained.
d is 60 to 1 unlike the known simulation result.
It was empirically revealed to be 50 μm.

It is considered that the reason for this difference is that the conditions of complete light absorption due to multiple reflection of light on the front surface and the back surface of the cell and no minority carrier recombination are actually not completely satisfied.

The inventors have further studied the optical trapping structure for perfecting the above light absorption and the reduction of minority carrier recombination, but the complete optical trapping structure causes a rise in the temperature of the cell and the conversion efficiency. It has been found that there is a problem in that the conversion efficiency may be lowered by lowering the film thickness and forming an oxide film for reducing the surface recombination, which may reduce the lifetime of the substrate.

Therefore, the conditions for complete light absorption due to multiple reflection of light on the front and back surfaces of the cell and the condition without recombination of minority carriers, which are the premise of the above theoretical calculation, are economical advantages of the silicon solar cell. It is a condition that can not be easily achieved in consideration of making use of the above, and in actual solar cell manufacturing, it is most advantageous to select the cell thickness in an appropriate range to obtain a highly efficient solar cell. We came to the conclusion that the conditions are such that

[0025]

In the solar cell of the present invention, a light receiving surface having a structure for reducing reflection is provided on the surface of a semiconductor substrate, an oxide film is formed on the back surface, and the cell thickness is 60 to 150 μm. And

[0026]

[Function] The thickness of the solar cell element is 60 to 150 μm.
By so doing, it is possible to obtain a solar cell that is economically highly efficient.

[0027]

1 to 5 are perspective views of an embodiment of a solar cell element according to the present invention, in which the cell thickness is 60 to 150 μm, respectively.
Is. In both cases, the antireflection film on the surface is omitted.

In FIG. 1, an N ⁺ diffusion layer 3 is formed on the surface of a P-type silicon substrate 4, and a texture structure 7 is formed on the surface.
Has become. The N-side electrode 2 is provided on an appropriate portion of the surface. An oxide film 8 and a P-side electrode 6 having a thickness of about 0.2 μm are laminated on the back surface, and a P ⁺ diffusion layer 5 is formed by diffusion from the windowed portion of the oxide film 8.

In FIG. 2, the surface structure is the same as that of FIG. 1, but in the structure of FIG. 2, a P ⁺ diffusion layer 5 is formed on the entire back surface and is oxidized to a thickness of about 0.2 μm. The film 8 and the P-side electrode 6 are laminated, and the P ⁺ diffusion layer 5 and the P-side electrode 6 are connected by making holes in the oxide film 8 at appropriate places.

FIG. 3 is an example in which a lattice-shaped pattern 9 is provided on the surface of the structure of FIG. 2, an inverted pyramid structure is formed by alkali etching, and the back surface has the same structure as that of FIG.

In FIG. 4, a groove-shaped pattern 10 is provided on the front surface, a plurality of grooves are formed by alkali etching, and the back surface has the same structure as in FIG.

FIG. 5 shows the surface texture of FIG.
It is an inverted pyramid shape. The back surface has the same structure as in FIG.

As for the back surface, as shown in FIGS. 1 and 5, the P ⁺ layer 5 to be the BSFR layer is not formed on the entire surface, but the oxide film 8 is locally opened and then P ⁺ diffusion is performed. It may be a partial BSF structure formed by

Further, a thin oxide film (SiO ₂ ) having a thickness of about 50 to 200 Å may be formed on the surface.

Although the description of the present invention has been mainly made up of the solar cells for space, it goes without saying that the present invention can be applied to solar cells for terrestrial use.

[0036]

EFFECTS OF THE INVENTION According to the present invention, since it is not necessary to drastically change the current manufacturing process of solar cells, the efficiency of solar cells can be improved while taking advantage of the low price of silicon solar cells. Can be planned.

[Brief description of drawings]

FIG. 1 is a perspective view of an embodiment of the present invention.

FIG. 2 is a perspective view of an embodiment of the present invention.

FIG. 3 is a perspective view of an embodiment of the present invention.

FIG. 4 is a perspective view of an embodiment of the present invention.

FIG. 5 is a perspective view of an embodiment of the present invention.

FIG. 6 is a perspective view of a conventional example.

FIG. 7 is a graph of calculation results by simulation of cell thickness and output characteristics.

FIG. 8A is a graph showing a relationship between Wd and V _OC and I _SC, and FIG. 8B is a graph showing a relationship between Wd and P _max .

[Explanation of symbols]

2 N-side electrode 3 N ⁺ diffusion layer 4 P-type silicon substrate 5 P ⁺ diffusion layer 6 P-side electrode 7 texture structure 8 oxide film

Claims (1)

[Claims] 1. A light receiving surface having a structure for reducing reflection is provided on a front surface of a semiconductor substrate, an oxide film is formed on a back surface of the semiconductor substrate, and a cell thickness is set to 60 μm or more and 150 μm or less. Silicon solar cell element that does.