JPS6249754B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a solar cell with high conversion efficiency from light to electricity.

A solar cell separates pairs of electrons and holes generated in a semiconductor by light irradiation using a pn junction and extracts them to the outside as an electromotive force. Conventionally, silicon has been widely used as the material for this type of element. Although Si has the advantage of being abundant in resources, it has a forbidden band width of 1.1 eV and a wavelength of 0.5 μm.
In order to efficiently collect the solar spectrum, which has a maximum at ~2.5 eV (energy), the disadvantage is that the forbidden band width is small. In order to efficiently collect sunlight, it is desirable to use a semiconductor material with a forbidden band width of 1.4-1.6 eV. Such materials include GaAs and InP. GaAs has
Since the generated electron-hole pairs recombine on the surface and do not contribute to current, AlGaAs is usually grown epitaxially on the surface. Regarding InP, solar cells using heterojunctions with CdS have been researched. However, in the case of a heterojunction, light with an energy higher than the forbidden band width of the surface semiconductor cannot be used. For example, in CdS,
The forbidden band width is 2.5eV near the maximum of the solar spectrum.
However, it had the disadvantage of low light collection efficiency, especially on the short wavelength side.

An object of the present invention is to provide a high-efficiency solar cell that can effectively utilize sunlight by appropriately configuring an InP homojunction in order to eliminate these drawbacks.

In the present invention, a high-efficiency solar cell is realized by using InP crystal, which has a forbidden band width of about 1.4 eV and is a suitable material for solar cells, and fully takes into account its electrical and optical properties.

That is, the present invention provides a solar cell in which a first conductivity type InP layer with a low impurity concentration and a second conductivity type InP layer with a high impurity concentration are deposited on an InP single crystal substrate by epitaxial growth to form a homojunction. The thickness of the second conductivity type InP layer is 0.5 μm or less, and the first conductivity type InP layer is 2×
The film is characterized by having an impurity concentration of 10 ¹⁵ cm ^-3 or less and a film thickness of 1 μm or more, so that the depth at which the carrier can be collected is approximately equal to the depth at which most of the light is absorbed.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 1 shows an example of the configuration of the solar cell of the present invention,
Here, 1 is a p-type InP single crystal substrate, and on this substrate 1, a p-type InP layer 2 with a low impurity concentration and an n ⁺ type InP layer 3 with a high impurity concentration are deposited in this order by epitaxial growth. n ⁺ p consisting of InP
to form a homozygote. Here, by setting the thickness of the n ⁺ layer 3 to 0.5 μm or less and the thickness of the p layer 2 to 1 μm or more with an impurity concentration of 2 × 10 ¹⁵ cm ^-3 or less, the depth at which carriers can be collected can be increased. The depth at which most of the light is absorbed is made approximately equal. Furthermore, n ⁺ layer 3
A collector electrode 4 is disposed thereon, and the entire n ⁺ layer 3 and collector electrode 4 are covered with an antireflection film 5 .

Here, for convenience, the structure of the p layer 2 and the n ⁺ layer 3 that are epitaxially grown in sequence on the p-type substrate 1 will be explained ^. A layer may also be formed, with similar results to those described below.

FIG. 2 shows an energy band diagram of the solar cell of the present invention. In the solar cell of the present invention, the following conditions are set in order to increase the sunlight collection efficiency.

Since InP is a direct transition type semiconductor, its absorption coefficient is large to near the energy gap, so a film thickness of 2 to 3 μm is sufficient to absorb light sufficiently.

InP crystal has a surface recombination rate of 10 ³
It is small at ~10 ⁴ cm/sec, and the diffusion length of the minority carrier is as small as about 0.5 to 1 μm.

First, the n ⁺ layer 3 on the surface will be described. n ⁺ layer 3
10 ^{to 18} to reduce the series resistance of the solar cell
Requires a carrier concentration of ~10 ¹⁹ cm ^-3 . Also,
Since surface recombination is small in InP, there is no need for a hetero window layer like AlGaAs used in GaAs solar cells, and even light at short wavelengths can be input.

In InP, the absorption coefficient is large on the short wavelength side, so light is absorbed near the surface and carriers are generated. The loss of carriers due to surface recombination is small, but because the diffusion length of minority carriers is small, they may recombine before reaching the pn junction and no longer contribute to the current. In order to reduce such ineffective components, we considered reducing the thickness of the n ⁺ layer 3. FIG. 3 shows the wavelength dependence of the collection efficiency when the thickness of the n ⁺ layer 3 is changed. As can be seen from FIG. 3, when the thickness of the n ⁺ layer 3 is reduced to 0.2 μm, the collection efficiency increases particularly on the short wavelength side. However, considering the increase in series resistance and the difficulty of manufacturing a thin layer, the practical thickness of the n ⁺ layer 3 is about 0.5 μm or less.

Next, the p layer 2 will be described. Since the n ⁺ layer 3 has a high carrier concentration, most of the carrier depletion layer is formed in the p layer 2. When a pn homojunction is used, interfacial recombination, which is a problem in the case of a heterojunction, does not occur, so carriers generated in the depletion layer effectively contribute to the current.

Here, the depletion layer width becomes larger as the impurity concentration is lowered, and becomes approximately 0.5 μm at 10 ¹⁶ cm ⁻³ and approximately 1.3 μm at 10 ¹⁵ cm ⁻³ . Since the thickness of InP required for light absorption is at most 2 to 3 μm, by making the depletion layer width in the p layer 2 close to 1 μm, the n ⁺ layer 3
The film thickness that can collect carriers and the film thickness that can absorb light are almost the same, including the thickness of the p-layer 2 and the minority carrier diffusion length in the region where there is no electric field in the p layer 2.

As an example of the present invention, FIG. 4 shows the wavelength dependence of the collection efficiency of a pn homojunction solar cell on a p-type substrate 1 manufactured by a known liquid phase epitaxial method. Here, the impurity concentration of p layer 2 is set to 2.0×
10 ¹⁵ cm ^-3 and the impurity concentration of the n ⁺ layer 3 was 1.0×10 ¹⁹ cm ^-3 . As is clear from FIG. 4, according to the present invention, flat characteristics can be obtained from the short wavelength side to near the band gap. In this example, n ⁺
To compensate for the increased series resistance due to the thin layer 3, photolithographic techniques are used to
The collection electrodes 4 on the surface of the n ⁺ layer 3 were thin and densely arranged. Further, an antireflection film 5 using SiO was provided on the surface of the solar cell to prevent reflection. In the present invention, the impurity concentration, thickness, etc. of the p layer and n ⁺ layer (or n layer and p ⁺ layer) forming the p-n junction are changed to InP.
By determining this in accordance with the material constant of , absorbed sunlight can be converted into photocurrent extremely efficiently.

As explained above, according to the present invention, high efficiency can be achieved in a pn homojunction solar cell in which a high-quality film can be easily produced using InP.
Sunlight can be used effectively. In particular, the high-efficiency solar cell of the present invention is effective for use in outer space, where installation locations are extremely limited.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an example of the solar cell of the present invention;
Figure 2 is an energy band diagram of the solar cell of the present invention.
Fig. 3 is a characteristic curve diagram showing the wavelength dependence of collection efficiency when changing the thickness of the n ⁺ layer, and Fig. 4 is a characteristic curve showing the wavelength dependence of collection efficiency in an embodiment of the solar cell of the present invention. It is a diagram. 1... p-type InP substrate, 2... p-type InP layer, 3...
... n ⁺ type InP layer, 4 ... collection electrode, 5 ... antireflection film.

Claims (1)

[Claims] 1 An InP layer of the first conductivity type with a low impurity concentration and an InP layer of the second conductivity type with a high impurity concentration are formed on an InP single crystal substrate.
In a solar cell in which a homojunction is formed by depositing an InP layer by epitaxial growth, the thickness of the second conductivity type InP layer is 0.5 μm or less, and the first conductivity type InP layer is 2×10 A solar cell characterized by having an impurity concentration of ¹⁵ cm ^-3 or less and a film thickness of 1 μm or more, so that the depth at which carriers can be collected is approximately equal to the depth at which most of the light is absorbed.