JP2662309B2 - Compound semiconductor solar cells - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a solar cell in which a light-receiving layer is made of a compound semiconductor.

(Prior art) Compound semiconductor solar cells are more
It has features such as high energy conversion efficiency, design of absorption spectrum, and high light-condensing operation because of high-temperature operation. However, in order to use it as an energy source, further improvement in efficiency is desired, and therefore, research on this has been conventionally performed.

An example of research aimed at increasing the efficiency of compound semiconductor solar cells is described in the literature (Technical Digest of the International PVSEC3, for example).
-3, (1987), p. 771).

This research aimed to obtain a solar cell capable of photoelectrically converting light of more wavelengths in the solar spectrum by vertically stacking pn junctions having different light receiving wavelengths by a hetero growth technique. .

Specifically, as shown in the sectional view of FIG.
A first light receiving layer 17 composed of a p-GaAs layer 13 and an n ⁺ -GaAs layer 15, an n ⁺ -AlGaAs layer 19, and an n ⁺⁺ -GaA
Tunnel junction 27 composed of s layer 21, p ^++- GaAs layer 23 and p ⁺ -AlGaAs layer 25, p-AlGaAs layer 29 and n ⁺ -AlGaAs layer
This is a so-called tandem solar cell in which the second light receiving layer 35 composed of 31, 33 is vertically stacked. In FIG. 3, 37
Is a p-side electrode, and 39 is an n-side electrode.

(Problems to be Solved by the Invention) However, in the conventional compound semiconductor solar cell, since the light receiving layer has a configuration in which the semiconductor layers are simply laminated, the absorption coefficient of sunlight of the light receiving layer (hereinafter, simply referred to as absorption coefficient). There is naturally a limit to increasing the size. Therefore, in order to allow the light receiving layer to absorb the required amount of light, there is a problem that the film thickness of the light receiving layer needs to be increased to some extent (about several μm).

To obtain more efficient compound semiconductor solar cells,
By increasing the absorption coefficient of the light-receiving layer, the thickness of the light-receiving layer is reduced, thereby shortening the distance from the electron-hole pair generation region to the electrode to reduce the recombination probability of electrons and holes during traveling. Considering that it is necessary to reduce the number of electrons and holes to efficiently reach the corresponding electrodes, it is desired to solve the above problem.

The present invention has been made in view of such a point,
Accordingly, an object of the present invention is to realize a compound semiconductor solar cell with higher efficiency than the conventional one by proposing a structure that can reduce the thickness of the light receiving layer.

(Means for Solving the Problems) In order to achieve this object, according to the present invention, in a compound semiconductor solar cell having one or more light-receiving layers composed of a compound semiconductor on a substrate, A superlattice provided in a first direction along the substrate surface, wherein the conductivity type is inverted in a second direction perpendicular to the first direction and along a direction perpendicular to the substrate surface. It is characterized by comprising a lattice.

In the present invention, the superlattice may be a type II superlattice (a superlattice in which the bottom of the conduction band and the top (upper end) of the valence band are spatially separated).

The substrate in the present invention is not limited to the compound semiconductor substrate itself, but may be a substrate other than the compound semiconductor substrate, such as a silicon (Si) substrate or a germanium (Ge) substrate, and further elements formed on these substrates. This includes the case of substrates that are rare.

(Operation) According to the configuration of the present invention, a pn junction is formed in a portion where the conductivity type of the superlattice is inverted, so that the superlattice can be used as a light receiving layer. Further, since it is known that a superlattice has a large absorption coefficient (in this case, not limited to sunlight) as compared with a structure in which compound semiconductor layers are simply laminated, the light-receiving layer according to the present invention constituted by the superlattice is Even if the film thickness is smaller than that of the conventional light receiving layer, the necessary light amount can be absorbed. Since the light-receiving layer can be made thinner, the distance from the electron-hole pair generation region to the electrode is shorter than before, and the traveling distance of electrons and holes is reduced accordingly, so that the probability of electrons and holes reaching the electrode is reduced. Increase.

When the light-receiving layer is formed of a type II superlattice, the light-receiving layer can be made thinner in the same manner as described above, and in addition, electrons of the electron-hole pairs generated in the light-receiving layer And the holes reach the valence band of the other compound semiconductor layer.
Therefore, since the holes and electrons are spatially separated, the probability of recombination thereof is further reduced, and thus the probability of electrons and holes reaching the electrode is further increased.

(Example) Hereinafter, an example of a compound semiconductor solar cell of the present invention will be described with reference to the drawings. The drawings used in the description schematically show the dimensions, shapes, and arrangements of the components so that the present invention can be understood. In the following embodiments, the first direction along the substrate surface is a direction parallel to the main surface of the substrate, and the second direction is a direction orthogonal to the first direction and a direction perpendicular to the substrate surface. Is described as an example in which the direction is perpendicular to the main surface of the substrate.

First Embodiment FIG. 1 is a sectional view schematically showing a structure of a compound semiconductor solar cell according to a first embodiment.

The compound semiconductor solar cell 40 of the first embodiment includes a substrate 41
A light-receiving layer 43 comprising a superlattice of type I formed in a direction parallel to the main surface and having a conductivity type inverted in a direction perpendicular to the main surface, on the main surface; Light receiving layer
A contact layer 45 is provided on 43, one electrode 47 is provided on the contact layer 45, and another electrode 49 is provided on the back surface of the substrate 41.
It is made up of

 The substrate 41 can be composed of, for example, a p-type GaAs substrate.

When the substrate 41 is a p-type GaAs substrate, the superlattice 43 forming the light receiving layer 43 is configured by alternately arranging, for example, GaAs layers 43a and AlGaAs layers 43b in a direction parallel to the main surface of the substrate 41. The superlattice can be a superlattice in which the layers 43a and 43b are p-type portions 43ap and 43bp on the substrate 41 side, and are n-type portions 43an and 43bn in a direction perpendicular to the main surface of the substrate.

In this case, the contact 45 layer can be formed of, for example, an n ⁺ GaAs layer, and the one electrode 47 and the other electrode 49 can be formed of a conventionally known material.

Here, formation of a superlattice in a direction parallel to the main surface of the substrate can be achieved, for example, by a technique disclosed in the literature (Applied Physics Letters (Appl. Phys. Lett., 45 , (1984) p.620)). That is, using a GaAs substrate that is offset from the (100) plane in the [011] direction, the distance from a crystal step formed on the substrate surface to the next step is 1/2.
The GaAs layer and the AlGaAs layer can be alternately grown.

Further, at this time, since the growth direction of the GaAs layer and the AlGaAs layer is a direction perpendicular to the main surface of the substrate, by changing the impurity from p-type to n-type during the growth of these layers, the conductivity type inversion and That is, the p-type portions 43ap and 43bp and the n-type portions 43an and 43bn can be easily formed.

In the compound semiconductor solar cell 40 of the first embodiment, when light is incident on the light receiving layer 43, electrons and holes are generated in the light receiving layer as in the conventional solar cell. The electrons and holes are generated by an electric field formed by the pn junction, and the electrons are directed to one electrode (n-side electrode) 45 and the holes are directed to the other electrode (p-side electrode) 47. Electric power is generated and functions as a solar cell.

However, in the present invention, the light receiving layer 43 is formed of a super lattice. In general, a superlattice has a larger light absorption coefficient than that of a normal stack of semiconductor layers. Therefore, in the present invention, the film thickness required for the light-receiving layer is less than half that of the case where the superlattice is not used, so that the distance to the electrode for electrons and holes generated in the light-receiving layer is conventionally smaller. It will be shorter. For this reason, it can be expected that the probability of recombination of electrons and holes during traveling becomes smaller than before,
The realization of a light-receiving layer having high efficiency and a small thickness can be expected.

Second Embodiment Next, a compound semiconductor solar cell according to a second embodiment will be described. FIGS. 2 (A) and 2 (B) are views for explanation thereof. In particular, FIG. 2 (A) is a sectional view schematically showing a structure of a compound semiconductor solar cell 50 of a second embodiment. FIG. 8B is an energy band diagram for explaining the function of the light receiving layer 53 according to the second embodiment.

The compound semiconductor solar cell 50 of the second embodiment includes a substrate 51
A light receiving layer 53 comprising a type II superlattice formed in a direction parallel to the main surface and having a conductivity type inverted in a direction perpendicular to the main surface, on the main surface; Light receiving layer
A contact layer 55 is provided on 53, one electrode 57 is provided on the contact layer 55, and the other electrode 59 is provided on the back surface of the substrate 51.
It is made up of

 The substrate 51 can be composed of, for example, a p-type InP substrate.

When the substrate 51 is a p-type InP substrate, the superlattice 53 constituting the light receiving layer 53 has, for example, an InP layer 53a and In _x Al _1-x As in a direction parallel to the main surface of the substrate 51, where x ≦ 0.6) This is a type II superlattice constituted by alternately arranging 53b and 53b, and each layer 53a, 53
A type II superlattice in which b is p-type portions 53ap and 53bp on the substrate 51 side and n-type portions 53an and 53bn from the middle in the direction perpendicular to the main surface of the substrate can be provided.

In this case, the contact layer 45 can be composed of, for example, an n ⁺ InP layer, and the one electrode 47 and the other electrode 49 can be composed of a conventionally known material.

Forming a type II superlattice in a direction parallel to the main surface of the substrate can be performed in the same manner as in the first embodiment.

In the compound semiconductor solar cell 50 of the second embodiment, as in the first embodiment, the film thickness required for the light-receiving layer is less than half that in the case where the superlattice is not used, so that the light-emitting layer is generated in the light-receiving layer. The distance between the electron and the hole to the electrode is shorter than before. For this reason, the probability that electrons and holes recombine during traveling can be expected to be smaller than before,
This can be expected to realize a light receiving layer having a thin film thickness and high efficiency characteristics.

However, in the compound semiconductor solar cell of the second embodiment, the light-receiving layer is constituted by a type II superlattice. For this reason, the electrons and holes generated in the light receiving layer 53 (type II superlattice) are, as shown in FIG. 2B, electrons 61 in the conduction band 63 of the InP layer and holes 65 in the InAlAs layer. In the valence band 67.
That is, electrons and holes are spatially separated. Then, the electrons go to one electrode (n-side electrode) 45 and the holes go to the other electrode (p-side electrode) 47. Therefore, it can be expected that the probability that electrons and holes recombine during traveling is lower than in the case of the first embodiment.

In the above, each embodiment of the compound semiconductor solar cell of the present invention has been described. However, the present invention is not limited to the above-described embodiment, and various modifications as described below can be added.

For example, in the first embodiment, the substrate is formed of a GaAs substrate, and the superlattice is formed of GaAs / AlGaAs. However, the substrate forming material and the superlattice forming material may be other suitable materials.
In particular, the substrate is not limited to a compound semiconductor substrate, and may be a germanium substrate or a silicon substrate on which a compound semiconductor layer is grown, or a silicon substrate or a germanium substrate itself. Also in the second embodiment, the constituent materials of the substrate and the superlattice may be other suitable materials.

In the first and second embodiments, the superlattice does not need to be a lattice matching system as in the embodiment, but may be a strained superlattice. Further, the conductivity types of the substrate and the superlattice may be opposite to those of the embodiment.

In the first and second embodiments, the first direction is a direction parallel to the main surface of the substrate. However, the first direction is not limited to this and may be a direction forming an acute angle with the main surface of the substrate.

In the first and second embodiments, the light receiving layer is one layer. However, two or more light receiving layers are sequentially stacked on the substrate or via an intermediate layer (for example, a tunnel junction described in the related art). You may. In such a configuration, it is preferable to select the material of the superlattice forming each light receiving layer so that each light receiving layer has an absorption peak at a different wavelength. According to this, the sunlight spectrum can be photoelectrically converted in a wider range.

(Effects of the Invention) As is clear from the above description, the compound semiconductor solar cell of the present invention is a superlattice provided in a first direction along a substrate surface, and is provided in a direction orthogonal to the first direction. And a light-receiving layer comprising a superlattice whose conductivity type is inverted in a second direction along a direction perpendicular to the substrate surface. Since the absorption coefficient of this light-receiving layer is larger than that of the conventional light-receiving layer, the thickness of the light-receiving layer can be reduced accordingly. Therefore, since the distance to the electrode for electrons and holes generated in the light receiving layer is shorter than before, it can be expected that the probability of recombination of electrons and holes during traveling is smaller than before. The realization of compound semiconductor solar cells having high efficiency characteristics can be expected.

Further, the light receiving layer is a type II superlattice provided in a first direction along the substrate surface, in a second direction along a direction perpendicular to the first direction and along a direction perpendicular to the substrate surface. In the case of a type II superlattice of which conductivity type is inverted, electrons and holes generated by photoelectric conversion can be spatially separated. For this reason, the probability of recombination of electrons and holes during traveling can be further reduced, and a more efficient compound semiconductor solar cell can be expected.

Further, when considering the formation of the compound semiconductor solar cell of the present invention using the technique of growing a compound semiconductor single crystal on a silicon substrate, this technique is considered to be a silicon-compound when the growth thickness of the compound semiconductor layer increases. There is a problem that cracks occur in the growth layer due to a difference in thermal expansion coefficient between semiconductors. However, since the light-receiving layer of the present invention may have a small thickness, the effect of this problem is small.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the first embodiment, FIGS. 2A and 2B are diagrams for explaining the second embodiment, and FIG. 3 is a diagram for explaining the prior art. is there. 40 Compound semiconductor solar cell of the first embodiment 41 Substrate (for example, p-type GaAs substrate) 43 Light receiving layer 43a of the first embodiment using a type I superlattice, for example, GaAs layer 43b For example, AlGaAs layer 43ap, 43bp... P-type portion 43an, 43bn... N-type portion 45... Contact layer (for example, n ⁺ GaAs layer) 47... One electrode, 49. Example compound semiconductor solar cell 51: substrate (for example, p-type InP substrate) 53: light receiving layer 53a of the second embodiment using a type II superlattice, for example, InP layer 53b, for example, In _x Al _{1- x} As layer 53ap, 53bp... p-type portion 53an, 53bn... n-type portion 55... contact layer (for example, n ⁺ InP layer) 57... …… InP conduction band 65… Hole 67 …… InAlAs valence band.

Claims (2)

(57) [Claims] 1. A compound semiconductor solar cell comprising one or more light-receiving layers formed of a compound semiconductor on a substrate, wherein the light-receiving layer is a superlattice provided in a first direction along a substrate surface. A compound semiconductor solar cell, comprising a superlattice whose conductivity type is inverted in a second direction perpendicular to the first direction and perpendicular to the substrate surface. 2. The compound semiconductor solar cell according to claim 1, wherein said superlattice is a type II superlattice.