Compound semiconductor solar cell and production method thereof

Details Compound semiconductor solar cell and production method thereof Compound semiconductor solar cell and production method thereof

2000-03-27

2001-10-23

Diamond, Alan (Department: 1753)

Batteries: thermoelectric and photoelectric

Photoelectric

Cells

C136S252000, C136S262000, C136S265000, C257S461000, C257S464000, C438S093000, C438S094000, C438S095000, C438S085000, C438S086000

Reexamination Certificate

active

06307148

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a compound semiconductor solar cell and a production method thereof. More particularly, it relates to a compound semiconductor solar cell having a p-n junction and a production method thereof.
2. Description of the Related Art
FIGS.
6
(
a
) and
6
(
b
) show a compound semiconductor solar cell having a p-n junction light absorbing layer according to the prior art. FIG.
6
(
a
) is a front view of the solar cell of compound semiconductor and FIG.
6
(
b
) is a longitudinal sectional view. This compound semiconductor solar cell (hereinafter called merely the “solar cell” in some cases) includes a molybdenum layer
102
on a glass substrate
100
. A p-type semiconductor layer
104
and an n-type semiconductor layer
106
are formed serially on this molybdenum layer
102
. A transparent electrode
108
is formed on the n-type semiconductor layer
106
. A comb-shaped electrode
110
is formed on the transparent electrode
108
. In this comb-shaped electrode
110
, an electrode is formed in a branched shape (a comb shaped) as shown in FIG.
6
(
a
).
The solar cell shown in FIGS.
6
(
a
) and
6
(
b
) can be produced by the method that is shown in FIGS.
7
(
a
) to
7
(
d
). First, an electrode film comprising the molybdenum layer
102
is formed on one of the surfaces of the glass substrate
100
by evaporation or sputtering. Next, an indium layer
103
is evaporated at room temperature. A copper layer
105
is further evaporated on the indium layer
103
at room temperature (process step shown in FIG.
7
(
a
)).
The metallic film comprising the indium layer
103
and the copper layer
105
are heat-treated in a hydrogen sulfide atmosphere, for sulfurization treatment, to convert the metallic film to a p-type semiconductor layer
104
of CuInS
2
. A KCN treatment for etching the surface of this p-type semiconductor layer
104
with a KCN solution containing 5 to 10% by weight of KCN is conducted. In this way, impurities formed in the p-type semiconductor layer
104
, such as Cu
x
S
y
(sulfide), are removed and the characteristics of the p-type semiconductor layer
104
are optimized and stabilized (step shown in FIG. (
b
)).
An n-type semiconductor layer
106
is further formed on the p-type semiconductor layer
104
by a chemical bath deposition method (step shown in FIG.
7
(
c
)). A transparent electrode
108
made of Al or In
2
O
3
is formed on the n-type semiconductor layer
106
by sputtering (step shown in FIG.
7
(
d
).
A comb-shaped electrode
110
made of aluminum is formed on the transparent electrode
108
. Furthermore electrode terminals (not shown) are formed on the molybdenum layer
102
in the solar cell shown in FIGS.
6
(
a
) and
6
(
b
).
In the solar cell shown in FIGS.
6
(
a
) and
6
(
b
), crystallinity inside the p-type semiconductor layer
104
can be improved by increasing as much as possible the Cu/In atomic ratio of copper (Cu) to indium (In) forming the p-type semiconductor layer
104
before the KCN treatment (the term “Cu/In atomic ratio” will hereinafter mean the Cu/In atomic ratio of copper (Cu) to indium (In) forming the p-type semiconductor layer
104
before the KCN treatment) and/or by maximizing the thickness of the p-type semiconductor layer
104
. As a result, power generation efficiency of the solar cell can be improved.
In existing solar cells of this kind, however, the Cu/In atomic ratio of the p-type semiconductor layer
104
before the KCN treatment is at most about 1.6, from the aspect of the production yield of the solar cells finally obtained, because, if the Cu/In atomic ratio is increased beyond 1.6, the p-type semiconductor layer
104
becomes more likely to peel from the electrode film
102
during the KCN treatment.
The upper limit of the P-type semiconductor layer
104
is also about 2 &mgr;m. When the thickness of the metallic film comprising the indium layer
103
and the copper layer
105
is increased so as to form a p-type semiconductor layer
104
which is more than 2 &mgr;m thick, peeling of the p-type semiconductor layer
104
occurs during the KCN treatment.
For these reasons, in the production methods according to the prior art it has been extremely difficult to improve power generation efficiency of the solar cell by improving crystallinity inside the p-type semiconductor layer
104
.
In a compound semiconductor solar cell including a p-type semiconductor layer formed mainly of copper (Cu) and indium (In), with gallium (Ga) whenever necessary, and having a p-n junction, it is therefore a main object of the present invention to provide a compound semiconductor solar cell that has a higher Cu/In atomic ratio of copper (Cu) to indium (In) before a KCN treatment than that of the prior art cells, and that has improved crystallinity inside the p-type semiconductor layer, and to provide a production method for such a solar cell.
SUMMARY OF THE INVENTION
The inventors of the present invention have conducted studies to search for the method of solving these problems, and have acquired the following information. If an indium layer is formed under heating at a step of laminating the indium layer and a copper layer on an electrode film formed on one of the surfaces of a substrate to form a metallic film, peeling of a p-type semiconductor layer can be suppressed to maximum during KCN treatment even when the thickness of the metallic film comprising the indium layer and the copper layer is increased so that a Cu/In atomic ratio of the p-type semiconductor layer becomes at least 1.8 and the thickness of the p-type semiconductor layer obtained finally becomes at least 2 &mgr;m. The present invention is completed on the basis of this observation.
In a method of producing a compound semiconductor solar cell by the steps of laminating an indium layer and a copper layer on an electrode film formed on one of the surfaces of a substrate to form a metallic film, subjecting the metallic film to sulfurization treatment or selenization treatment to form a p-type semiconductor layer made of CuInS
2
or CuInSe
2
, then subjecting the p-type semiconductor layer to KCN treatment, for removing impurities such as copper sulfide, copper selenide, etc., with a KCN solution, and forming an n-type semiconductor layer on the p-type semiconductor layer; the present invention is characterized in that the indium layer is formed under heating, or the formed indium layer is heat-treated while its surface is exposed.
In a method of producing a compound semiconductor solar cell by the steps of laminating a gallium layer or a gallium-copper alloy layer, an indium layer and a copper layer on an electrode film formed on one of the surfaces of a substrate to form a metallic film; subjecting the metallic film to sulfurization treatment or selenization treatment to form a p-type semiconductor layer made of Cu(In, Ga)S
2
or Cu(In, Ga)Se
2
then subjecting the p-type semiconductor layer to KCN treatment, for removing impurities such as copper sulfide, copper selenide, etc., with a KCN solution, and forming an n-type semiconductor layer on the p-type semiconductor layer; the present invention is characterized in that the indium layer is formed under heating, or the formed indium layer is heat-treated while its surface is exposed.
Here, the reason why Cu(In, Ga)S
2
or Cu(In, Ga)Se
2
is used is because the energy band gap of these compounds can be optimized and device performance can be improved when they contain Ga.
The gallium layer is formed on the electrode film of Mo, or the like, by sputtering or evaporation. A gallium-copper alloy layer can be used in place of the gallium layer. Gallium also plays the role of improving adhesion between the p-type semiconductor layer and the electrode film.
In the present invention, the copper layer may be heated during its formation process in the same way as the indium layer.
In the present invention having the construction described above, the heating temperature when the indium layer is formed by heating or the heating temperature when the indi

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