Monolithic multiple solar cells

Abstract

A monolithic multiple solar cell includes at least three partial cells, with a semiconductor mirror placed between two partial cells. The aim of the invention is to improve the radiation stability of said solar cell. For this purpose, the semiconductor mirror has a high degree of reflection in at least one part of a spectral absorption area of the partial cell which is arranged above the semiconductor mirror and a high degree of transmission within the spectral absorption range of the partial cell arranged below the semiconductor mirror.

Claims

1. A solar cell comprising: a first cell formed of a plurality of first layers, at least two of the first layers have a different doping from one another; a second cell formed of a plurality of second layers, at least two of the second layers have a different doping from one another; and a Bragg reflector formed of a plurality of third layers and arranged between the first cell and the second cell, the third layers partially reflecting incident light back to the first cell and to allow a portion of the incident light to pass to the second cell, wherein the solar cell does not include a multi-quantum well.

2. The solar cell according to claim 1, wherein the solar cell is a monolithic solar cell.

3. The solar cell according to claim 1, wherein the plurality of third layers contain at least 10 layers.

4. The solar cell according to claim 1, wherein the plurality of third layers have at least two layers that have a different thickness or a different refraction index or are made of different materials.

5. The solar cell according to claim 1, wherein the Bragg reflector is directly adjacent to the first cell.

6. The solar cell according to claim 1, wherein the Bragg reflector is directly adjacent to the second cell.

7. The solar cell according to claim 1, wherein the Bragg reflector has materials with a band gap energy that is equal with or greater than the first cell.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:

(2) FIG. 1 shows a schematic configuration of a multiple solar cell with integrated semiconductor mirror,

(3) FIG. 2 shows a reflection of an ideal semiconductor mirror, and

(4) FIG. 3 shows a simulated reflection of a semiconductor mirror.

DETAILED DESCRIPTION

(5) The schematic configuration of a multiple solar cell 10 with a semiconductor mirror integrated according to the invention can be seen in FIG. 1. The solar cell 10 comprises m partial cells 12, 14, 16, 18, which have been epitaxially applied on a substrate 20. Between the (n)th cell 16 with m>n and the (n+1)th cell 18, a semiconductor mirror 22 is integrated, which has also been deposited during the epitaxy of the solar cell structure. Suitable epitaxy processes to be considered are those that are suitable for the deposition of numerous layers of different materials. MOVPE, MBE (molecular beam epitaxy), or VPE (vapor phase epitaxy) can be mentioned as examples, without causing as a consequence a limitation of the teaching of the invention.

(6) The multiple solar cell 10 is especially a triple solar cell, wherein the upper cell is made of Ga.sub.0.5In.sub.0.5P, the central cell is made of Ga.sub.0.99In.sub.0.01As, and the lower cell is made of Ge. The semiconductor mirror 22, which comprises several layers, is integrated in particular between the lower cell made of Ge and the central cell made of Ga.sub.0.99In.sub.0.01As. The layered structure is such that at least two layers of different materials, different thicknesses, and different indices of refraction are provided.

(7) Through the selection of the materials, layer thicknesses, and indices of refraction, in the ideal case a reflection behavior as that shown in FIG. 2 is achieved. Maximum reflection is thus obtained for energies greater than the band gap energy of the nth partial cell, that is, maximum reflection is achieved in the exemplary embodiment of the triple cell with a Ga.sub.0.99In.sub.0.01As central cell having a band gap energy of >1.4 eV or 880 nm. For energies that are lower than the band gap energy of the nth cell, the reflection is minimal or the transmission is maximal. Transmission losses through absorption in the semiconductor mirror can be prevented or kept negligibly small by selecting suitable materials, for example, GaAs and AlGaAs.

(8) The uppermost layer of the semiconductor mirror 22 can be made of GaInP and at the same time be the rear side field for the Ga.sub.0.99In.sub.0.01As central cell located above. GaInP is used as material, since it has very good properties as rear side passivation. The remaining layers of the semiconductor mirror 22 in the exemplary embodiment are made of three different materials: Ga.sub.0.99In.sub.0.01As, Al.sub.0.2Ga.sub.0.8As, and Al.sub.0.8Ga.sub.0.2As. There is an essential difference with respect to the Bragg reflector, which is made of only two different materials. Furthermore, various layer thicknesses are also in the example, while in the classic Bragg reflector all the layers of one material have the same thickness.

(9) A further essential characteristic of the layer sequence of the semiconductor mirror 22 is that it reaches, on the one hand, a high reflection for energies above the band edge of the partial cell located above it, but has, on the other hand, also a low reflection or high transmission for lower energies. The reflection of the semiconductor mirror on the boundary surface to the Ga.sub.0.99In.sub.0.01As partial cell disposed above is shown in FIG. 3 to illustrate this. A high reflection is achieved for wavelengths between 800 nm and 900 nm. For wavelengths greater than 900 nm, however, the reflection is low. This is an important property of the semiconductor mirror, which ensures that the current generation in the Ge lower cell is not substantially reduced by the semiconductor mirror 22.

(10) The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.