Photoelectric converting device

Details Photoelectric converting device Photoelectric converting device

2001-02-09

2003-01-07

Nguyen, Nam (Department: 1753)

Batteries: thermoelectric and photoelectric

Photoelectric

Cells

C136S255000, C136S261000

Reexamination Certificate

active

06504091

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to photoelectric converting devices for converting light energy to electrical energy and, more particularly to a photoelectric converting device using a Group III-V compound semiconductor with enhanced photoelectric conversion efficiency for converting solar light energy to electrical energy especially for use in outer space.
2. Description of the Background Art
In recent years, a greater number of multi-junction solar cells mainly including semiconductors of a Group III-V compound such as GaAs are used as solar cells for outer space of a power supply source for a space craft such as a space satellite. Such solar cells can provide greater photoelectric conversion efficiency as compared with conventional silicon solar cells which have been widely used as solar cells for outer space. As such, silicon cells are suitable for use in a small satellite or a superpower satellite.
The most popular multi-junction solar cell is of the type disclosed for example in U.S. Pat. Nos. 5,223,043 and 5,405,453. The structure of such a solar cell is shown in FIG.
20
. The conventional multi-junction (two-junction) cell mainly includes a first solar cell (hereinafter referred to as “a top cell”)
104
of Ga
1-x
In
x
P formed on the solar light incidence side and a second solar cell (hereinafter referred to as “a bottom cell”) of GaAs below the top cell, which are connected by a tunnel junction
103
. GaAs or Ge single-crystal wafer is used as a substrate
101
. For a composition ratio of Ga
1-x
In
x
P of the top cell, x equals to 0.49 for the purpose of providing lattice matching with GaAs of the bottom cell. In this case, the lattice constants of the top and bottom cells are designed to be approximately equal to that of Ge of the substrate and to enable epitaxial growth on the Ge substrate relatively easily. Then, the bandgap Eg of the top cell is about 1.9 eV, and that of the bottom cell is about 1.4 eV. The conventional multi-junction solar cell has attained about 26% and about 22%, respectively at experimental and industrial product levels, as a result of characteristic testing using a light source as a solar light spectrum in outer space. Recently, a three-junction solar cell has been developed which has a pn junction also for a Ge substrate in addition to top and bottom cells.
To keep up with a dramatic progress in recent space development, the above mentioned photoelectric conversion efficiency is insufficient and higher conversion efficiency is desired. The above described conventional multi-junction solar cell has been developed from a GaAs solar cell formed on a Ge substrate, leading to the above described structure. In terms of solar energy efficiency, however, the combination of Ga
1-x
In
x
P and GaAs is not optimum for the following reasons.
The theoretical photoelectric conversion efficiency of a solar cell having two pn junctions is described for example in an article in
IEEE Transactions on Electron Devices. ED
-34, p257. The article shows a relationship between the expected value of photoelectric conversion efficiency and a range of the bandgaps of top and bottom cells based on matching of the bandgaps of top and bottom cells and incident light spectrum. In practically manufacturing a solar cell, lattice matching between top and bottom cells as well as between the bottom cell and the substrate must be achieved to provide a high-quality epitaxial layer.
FIG. 21
shows a relationship between a lattice constant and a bandgap energy for various semiconductor materials. Based on the above mentioned article,
FIG. 21
shows bandgap ranges U and L respectively for the top and bottom cells to achieve a conversion efficiency of at least 30% with respect to a solar light spectrum (AMO) in outer space. The graph shows that the combination of the materials used for the above described conventional multiunction solar cell, i.e., the combination of Ga
1-x
In
x
P and GaAs, merely provides the photoelectric conversion efficiency of no more than 30%.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a photoelectric converting device capable of providing increased photoelectric conversion efficiency by optimizing a combination of materials for top and bottom cells.
A photoelectric converting device of the present invention is provided with first and second pn junctions. The first and second pn junctions are substantially formed in semiconductors respectively represented by (Al
1-y
Ga
y
)
1-x
In
x
P and Ga
1-z
In
z
A
s
.
To achieve a convention efficiency of at least 30%, it has been said that at least the following conditions must be met.
(a) Optimization of a combination of a material for a top cell (hereinafter referred to as “a top cell material”) and a material for a bottom cell (hereinafter referred to as “a bottom cell material”).
(b) Lattice matching between the top and bottom cell materials.
(c) Lattice matching between the bottom cell and substrate materials.
(d) Matching of thermal expansion coefficients between a layer and substrate materials.
However, it is difficult to find a combination of semiconductor materials which satisfy all of these conditions and which are still inexpensive. The extensive study of each of the above conditions conducted by the present inventors have confirmed that the above conditions (a) and (b) are indispensable to provide a conversion efficiency of at least 30%.
However, the following finding was also obtained. Namely, lattice matching between the bottom cell and substrate materials are not very important, and lattice mismatching of at most about 4% can still provide a layer with good crystallinity by a crystal growth technique. (This condition, an alleviation of (c), will be hereinafter represented as (c′)).
In addition, the following finding was obtained. Matching of thermal expansion coefficients between the layer and the substrate materials is not extremely important either and, as long as the thermal expansion coefficient of the layer is at most that of the substrate, the problem of cracks to the layer caused by the difference in thermal expansion coefficient can be avoided. (This condition, an alleviation of (d), will be represented by (d′)).
The above described study confirmed that, as materials satisfying the conditions (a), (b), (c′) and (d′), semiconductors represented by (Al
1-y
Ga
y
)
1-x
In
x
P and Ga
1-z
In
z
As are respectively effective for top and bottom cells. (Al
1-y
Ga
y
)
1-x
In
x
P and Ga
1-z
In
z
As could be found mostly because of the conditions (c′) and (d′). With use of the structure of the present invention, all of the above conditions (a), (b), (c′) and (d′) can be satisfied. Consequently, a photoelectric converting device with a conversion efficiency of at least 30% can be achieved. Note that, in chemical composition representation, an element C occupies only x (≦1.0) of a site of C in a crystal grating with a chemical formula CP, while an element B occupies the remaining site of 1-x in the case of B
1-x
C
x
P including B, C, and P. For (A
1-y
B
y
)
1-x
C
x
P, B occupies only y(≦1.0) of a site of B in B
1-x
C
x
P, while A occupies the remaining 1-y For a Group III-V compound semiconductor of the present invention, InP, InAs, GaAs, GaP or the like generally has a zinc blende crystal structure. The zinc blende crystal structure is similar to a diamond structure of a semiconductor of Group IV like Ge, Si. In the photoelectric converting device of the present invention, composition ratio z, x and y of semiconductors Ga
1-z
In
z
As and (Al
1-y
Ga
y
)
1-x
In
x
P desirably fall within 0.11<z<0.29, x=−0.346z
2
+1.08z+0.484 and 131z
3
−66.0 z
2
+9.17z+0.309<y<28.0z
3
−24.4z
2
+5.82z+0.325, respectively.
Specifically, the structure optimizes bandgap energies of the top cell and bottom cell materials. The present inventors have conducted calculations of lattice constants and bandgap energies for these semiconduc

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