JPH0539640Y2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a solar cell element, and particularly to a structure of a solar cell element that is effective for use in a space environment such as a power source for an artificial satellite.

<Summary of the invention> The present invention operates at low temperatures by forming uneven surfaces consisting of micropentahedrons on the front and back surfaces of the semiconductor substrate constituting the solar cell element by anisotropic etching. A highly efficient solar cell element that can also extract light as output can be obtained.

<Prior art> In order to suppress the surface reflection loss of solar cell elements using silicon single crystal substrates, as shown in Kiyoshi Takahashi et al., "Solar Power Generation" (February 20, 1980), Morikita Publishing, p. 153, Anisotropic etching is performed on the light-receiving surface of a silicon single-crystal substrate to form unevenness consisting of countless pyramid-shaped micropentahedrons, and optical multiple reflection and refraction between the incident light and the pyramid surface on the surface improves the light absorption rate. There is a conventional technique to do this.

FIG. 3 shows the structure of a conventional solar cell element of this type.

1 is a silicon substrate, 2 is the front surface of the substrate, 3 is the back surface of the substrate, 4 is the PN junction surface, 5 is the front electrode, 6 is the back electrode,
7 is an antireflection film, 8 is an adhesive, and 9 is a cover glass.

<Problem to be solved by the invention> Conventionally, the back surface of a solar cell element is a smooth surface, as shown in the cross-sectional view of FIG.
The state of reflection and refraction of incident light 10 on the surface and inside of silicon substrate 1 is found from Fresnel's formula, as shown in Figure 4. Here, the refractive index of silicon substrate 1 is 3.8, and that of adhesive 8 is 1.43. In reality, anti-reflection film 7 exists on silicon substrate 1, but the angles of internal incident light 11, 12 inside the silicon substrate remain unchanged. Considering the reflection of primary incident light 11 and secondary incident light 12 on the back surface 3 of silicon substrate, total reflection occurs not only when back electrode 6 exists, but also when back surface 3 of substrate is in contact with the air. Light with a wavelength of approximately 1.2 μm or more, which has less energy than the forbidden band width of silicon,
When light is absorbed inside the silicon, it becomes heat, but as described above, the conventional example has a problem in that the light absorption rate becomes large, which tends to lead to a decrease in power generation efficiency due to an increase in element temperature.

<Means for solving the problems> The present invention was created in view of the above problems.
A PN formed by diffusing impurities of the opposite conductivity type to one main surface of a semiconductor substrate of one conductivity type.
In a solar cell element having a bond, and a front electrode and a back electrode formed on the front and back surfaces of the semiconductor substrate, the front and back surfaces of the semiconductor substrate are unevenly formed by countless minute pentahedrons by anisotropic etching. The semiconductor substrate is characterized in that the front electrode and the back electrode are formed in a comb shape or a lattice shape on the semiconductor substrate so that light can pass therethrough.

<Function> With the above-mentioned configuration, among the light incident on the silicon substrate due to multiple reflection and refraction at the countless minute pentahedrons on the surface, the light that reaches the back surface of the substrate without contributing to photoelectric conversion does not undergo total reflection because the angle of incidence with respect to the back surface of the substrate is small, and is transmitted to the outside of the substrate without being blocked by the back electrode, so that the generation of heat inside the substrate is suppressed and the element temperature is lower than in the past. In addition, since the light from the back side of the element is also converted into photocurrent, if it is used as a power source for an artificial satellite in a low earth orbit, for example, it can effectively utilize reflected light from the earth's surface in addition to the direct incident light from the sun, and the power generation efficiency is extremely high.

Therefore, a highly efficient solar cell element that operates at a lower temperature than conventional solar cells and can extract light from the back side as output can be obtained.

<Example> Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 shows the structure of an embodiment of a solar cell element according to the present invention. In this figure, the same components as those in FIG. 3 are designated by the same reference numerals.

The solar cell element of this example has a silicon substrate 1
Similar unevenness made of micropentahedrons is formed on the front surface 2 and back surface 3 of the , and both the front electrode 5 and the back electrode 6 are
It has a shape such as a comb shape or a lattice shape that does not reduce the light receiving area.

The solar cell element configured as described above is manufactured as follows.

The silicon substrate 1 used is one sliced along the <100> plane. Also, the silicon substrate 1 is P
Although either type or N type substrate may be used, a P type substrate is preferable in consideration of radiation resistance. The irregularities on the front surface 2 and back surface 3 of the substrate are created by anisotropic etching, for example,
Formed by extremely slowed alkaline etching. The size of the pyramid is on the order of several μm, which varies depending on the etching conditions. The PN junction surface 4 is formed at a depth of about 0.1 to 0.3 μm from the substrate surface 2. In addition to this, there is a P-
A P ⁺ junction or an N-N ⁺ junction may be present. The front electrode 5 and the back electrode 6 are formed into a comb shape, a lattice shape, or a similar shape in order to maximize the light-receiving area of the silicon substrate 1 and collect charges efficiently. However, it is not necessary that the front electrode 5 and the back electrode 6 have the same shape. Further, an antireflection film 7 is formed on the front and back surfaces of the silicon substrate 1. The antireflection film 7 is provided to reduce the loss of light incident on the element due to reflection, and to increase the transmittance of light transmitted from the inside of the element to the outside. The anti-reflection film may be a single layer film of a substance with a refractive index intermediate between that of the silicon substrate and the adhesive, or a multilayer film with mutually different refractive indexes. The cover glass 9 is for preventing damage to the element due to radiation.

Next, the operation and effect of the solar cell element configured as described above will be explained.

Figure 2 shows how the incident light is reflected and refracted.

The manner of reflection and refraction of the incident light 10 from the substrate surface 2 into the substrate is the same as the conventional one shown in FIG. 4. When the primary incident light 11 and the secondary incident light 12 reach the substrate back surface 3, the substrate back surface 3 and the substrate surface 2
Since similar unevenness is formed on the substrate, the angle of incidence of the incident light beams 11 and 12 on the back surface 3 of the substrate is small, so the total reflection as shown in FIG. 4 does not occur, but only partial transmission,
Part of it is a reflection. Furthermore, if the back electrode 6 is formed on the entire back surface 3 of the substrate, the above effect cannot be expected, so it is essential that the back electrode 6 has a shape similar to the front electrode 5, such as a comb shape or a lattice shape, that does not reduce the light receiving area. be.

Furthermore, the above structure provides the following effect that is difficult to obtain in the conventional example. That is, the back electrode 6 is the front electrode 5.
Since the element has a similar shape, light that enters the back surface 3 of the substrate from the back side of the element enters into the inside of the substrate in exactly the same way as light 10 that enters from the front side. Therefore, the solar cell element according to the present invention produces a photovoltaic effect not only by incident light from the front surface of the element but also by incident light from the back surface.

In addition, as a result of fabricating a prototype device with a BSF structure using a P-type silicon substrate, a photocurrent equivalent to about 70% of that for light incident from the front side was obtained even for light incident from the back side. It was also confirmed that the solar absorption rate was lower than before.

<Effects of the invention> As described above, according to the invention, among the light incident on the inside of the substrate due to multiple reflection and refraction at the countless minute pentahedrons on the surface, it reaches the back surface of the substrate without contributing to photoelectric conversion. Since the emitted light is transmitted to the outside of the substrate, heat generation inside the substrate is suppressed, and the temperature of the solar cell element can be lowered than before. In addition, since light from the back side of the element is also converted into photocurrent, especially when used as a power source for artificial satellites in low orbit, in addition to direct incident light from the sun, reflected light from the earth's surface can also be effectively used. The power generation efficiency is extremely high.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view of one embodiment of the solar cell element according to the present invention, Fig. 2 is a diagram showing how incident light is reflected and refracted in the same embodiment, and Fig. 3 is a diagram of a conventional solar cell element. The cross-sectional view, FIG. 4, is a diagram showing how incident light is reflected and refracted in a conventional example. 1...Silicon substrate, 2...Substrate surface, 3...
Back side of substrate, 4...PN junction surface, 5... Front electrode, 6
... Back electrode, 7 ... Antireflection film, 8 ... Adhesive,
9...Cover glass.

Claims (1)

[Claim for Utility Model Registration] Formed by diffusing impurities of the opposite conductivity type to one main surface of a semiconductor substrate of one conductivity type.
In a solar cell element having a PN junction and a front electrode and a back electrode formed on the front and back surfaces of the semiconductor substrate, the front and back surfaces of the semiconductor substrate are made of countless minute pentahedrons formed by anisotropic etching. What is claimed is: 1. A solar cell element characterized in that the semiconductor substrate is formed with unevenness, and the front electrode and the back electrode are formed in a comb shape or a lattice shape so that light can pass through the semiconductor substrate.