Induced junction chalcopyrite solar cell

A new solar cell of a I-III-VI.sub.2 semiconductor material that has an inversion layer is provided. The cell comprises a substrate having an electrically conductive, first electrode, a p-conductive, polycrystalline semiconductor layer of chalcopyrite material, a barrier layer composed of an electrically non-conductive material, a second electrode, and an antireflection layer. The anti-reflection layer has stationary, positive charges that induce a negatively charged inversion layer in the boundary surface region of the semiconductor layer relative to the barrier layer. The negatively charged inversion layer serves as an emitter for a space charge zone. In an embodiment the invention comprises a semiconductor layer of copper-indium-diselenide or copper-gallium-diselenide, a barrier layer of silicon dioxide, an anti-reflection layer of silicon nitride, and cesium chloride as the stationary charges.

The present invention relates generally to solar cells. 
In order to be competitive as an alternative to conventional energy 
generators, solar cells and photovoltaic solar modules for generating 
electrical power must be cost-beneficial. Further, they must have 
efficiencies of at least 15 percent. 
Although solar cells constructed from crystalline silicon have high 
efficiencies, there is nonetheless an effort to switch to more 
cost-beneficial thin-film solar cells of amorphous or polycrystalline 
semiconductors. Such cells save material and eliminate costs. Whereas 
solar cells of amorphous semiconductor materials still do not provide 
sufficient efficiency and long-term stability, good results are currently 
achieved with thin-film solar cells of polycrystalline semiconductors. 
Recently, solar cells of polycrystalline chalcopyrite material have been 
investigated. Probably the best-known representative of this class is 
copper-indium-diselenide (CIS). CIS is a direct semiconductor having a 
band gap of 1.0 eV. Based on the position of the band gap, a theoretical 
maximum efficiency of approximately 25 percent can be expected. 
Efficiencies of nearly 15 percent have been achieved with small-area test 
cells that have been manufactured. 
An article by K. Mitchell et al. in The Conference Record of the 20th IEEE 
Photovoltaic Specialists Conference (1988), pages 1384 through 1389, 
discloses a CIS solar cell. The cell is composed of a glass substrate 
coated with molybdenum as a back side electrode, a polycrystalline, 
p-conductive CIS semiconductor layer as an absorber, a thin, n-conductive 
cadmium sulfide layer as an electron emitter, a zinc oxide layer as a 
transparent electrode, and an aluminum grid for receiving generated 
current. 
One of the greatest problems in manufacturing CIS solar cells is matching 
the different crystal lattices or, respectively, the transition between 
the absorber and the emitter layer. The lattices should not differ by more 
than a maximum of 1 percent in order to still achieve acceptable 
efficiency; greater differences result in an increased number of defects 
in the band gap. This in turn results in recombination centers for the 
charge carriers and reduces the electrical power of such a solar cell. The 
stoichiometry of the CIS material, moreover, must be varied in the region 
of the pn-junction in order to achieve a "p.sub.+ -doping." 
In addition to the problems encountered manufacturing a uniform layer 
composed of three constituents, other concerns are encountered. The use of 
toxic cadmium for the emitter layer provides a particular disadvantage for 
environmental reasons. 
SUMMARY OF THE INVENTION 
The present invention provides a chalcopyrite solar cell having high 
efficiency that overcomes the problems with respect to the lattice 
matching of the semiconductor materials used and does not use cadmium 
sulfide. 
To this end, the present invention provides solar cells having a layer 
structure comprising the following, in succession on top of one another: a 
substrate having an electrically conductive, first electrode; a 
p-conductive, polycrystalline semiconductor layer of a I-III-VI.sub.2 
chalcopyrite material; a barrier (i.e. tunneling) layer of an electrically 
non-conductive material; an antireflection layer; and a second electrode 
for receiving generated current. The antireflection layer has stationary 
positive charges at least at the boundary surface to the barrier layer. 
The stationary, positive charges induce a negatively charged inversion 
layer in the boundary surface region of the semiconductor layer relative 
to the barrier layer. The negatively charged inversion layer serves as an 
emitter for a space charge zone. 
In an embodiment, the semiconductor layer is a copper-indium-diselenide 
layer. In a further embodiment, indium can be replaced by gallium and 
selenium can be replaced by sulfur, independently of one another. 
In an embodiment, the barrier layer is composed of amorphous silicon oxide 
and has a thickness of less than 5 nm. 
In an embodiment, the antireflection layer is a silicon nitride layer. 
In an embodiment, the antireflection layer is a silicon nitride plasma 
deposited from silane (SiH.sub.4) and ammonia. 
In an embodiment, the second electrode is a close-meshed metallic grid of, 
preferably, aluminum. 
In an embodiment, the stationary, positive charges are formed by a 
plurality of layers of molecules of an ionic higher alkali metal halide. 
In a further embodiment, the higher alkali metal halide is cesium 
chloride. 
The present invention provides, in an embodiment, a solar cell having the 
following layer structure: a molybdenum-coated glass substrate; a 1 
through 2 .mu.m thick polycrystalline layer of I-III-VI.sub.2 chalcopyrite 
material; a barrier layer composed of amorphous silicon dioxide deposited 
in a maximum thickness of 2 nm; a close-meshed aluminum grid on the 
barrier layer; and a 50 through 200 nm thick, amorphous silicon nitride 
applied thereover that contains a plurality of layers of molecules of 
cesium chloride at least at the boundary surface to the barrier layer. 
Additional features and advantages of the present invention are described 
in, and will be apparent from, the detailed description of the presently 
preferred embodiments and from the drawing.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
The present invention provides a solar cell having high efficiency, that 
overcomes the problems with respect to matching the lattices, and does not 
require the use of cadmium. To this end, the cell comprises a layer 
structure of: a substrate having an electrically conductive, first 
electrode; a p-conductive, polycrystalline semiconductor layer composed of 
a I-III-VI.sub.2 chalcopyrite material; a barrier layer composed of an 
electrically non-conductive material; an anti-reflection layer; and a 
second electrode for receiving the generated current. The anti-reflection 
layer has stationary, positive charges at least at the boundary surface to 
the barrier layer that induce a negatively charged inversion layer in the 
boundary surface region of the semiconductor layer to the barrier layer. 
The inversion layer serves as an emitter for a space charge zone. 
The solar cell of the present invention utilizes principles of an MIS 
inversion layer solar cell known in solar cells composed of crystalline 
silicon that may be derived, for example, from an article by R. Hezel in 
Status Report 1990, Photovoltaik 2-4 May 1990, pages 15-1 through 15-15. 
Whereas traditional, crystalline solar cells work like semiconductor 
diodes having a metallurgical pn-junction, the MIS inversion layer solar 
cell has an induced pn-junction, analogous to a MOS (metal oxide silicon) 
transistor. 
The MIS inversion layer solar cell comprises a body of crystalline p-doped 
silicon as an absorber, a thin insulating layer of silicon oxide, and a 
cover layer of silicon nitride. The silicon nitride layer is enriched with 
positive charge carriers, for example heavy alkali metal ions, at the 
boundary surface to the silicon oxide layer. The positive charge carriers 
induce a negative charge in the boundary area region of the silicon layer 
at the other side of the silicon diode layer. 
The enrichment of electrons in the actual p-conductive semiconductor 
material is called an inversion layer. This inversion layer, similar to a 
normal, n-conductive layer, can serve as an emitter for a space charge 
zone in the p-semiconductor that is depleted of charge carriers. An 
electrical field is created that corresponds to the charge condition of 
the corresponding semiconductor layers. The charge carriers are separated 
therein and thus generate an electrical current at the adjoining 
electrical contacts. 
The present invention provides a CIS solar cell that does not require a 
pn-junction. This thereby avoids the problems associated with a lattice 
matching between the semiconductor layer and the emitter layer. An 
arbitrary barrier layer of an electrically insulating material can be 
directly generated on the chalcopyrite material semiconductor layer. The 
layer can be manufactured so that it is uniform and has few holes. For 
example, the invention provides a simple manner for producing the barrier 
layer from an insulating, amorphous oxide. No lattice matching is required 
as a result of this amorphous structure. 
The anti-reflection layer is situated above the barrier layer. The 
anti-reflection layer functions, first, as a window layer for the incident 
solar radiation and also provides a matrix for the stationary, positive 
charges. In addition to comprising a high optical band spacing in order to 
ensure transparency, the anti-reflection layer has corresponding matrix 
properties and therefore must reliably bond with metal salts, preferably 
used for the stationary, positive charges in its structure. 
In order to be able to better collect minority charge carriers (electrons) 
that are moving in the region of the positive CIS region, the second 
electrode can be directly located on the barrier layer. To this end, the 
second electrode is preferably structured so as to be of an optimally fine 
mesh. For example, the second electrode has a mesh spacing of 
approximately 200 to about 300 .mu.m and a diameter of approximately 20 to 
about 30 .mu.m. 
The semiconductor layer can be, for example, composed of 
copper-indium-diselenide. Pursuant to the present invention, indium can be 
replaced by gallium and selenium can be replaced by sulfur, independently 
of one another. 
It has been found, that compared to other CIS material, the use of gallium 
instead of indium has the advantage that the raw materials required 
therefor are more readily available. Replacing selenium with sulfur can 
yield further advantages with respect to the environmental concerns of the 
solar cell or, respectively, the method used for the manufacture thereof. 
This can be achieved without a great loss of power. 
The thickness of the semiconductor layer is selected such that the incident 
light is completely absorbed. To this end, it has been found that a layer 
thickness of approximately 1 to about 2 .mu.m is adequate. 
Preferably, the barrier layer has a thickness of less than 5 nm and is 
fabricated as thinly as possible. Amorphous silicon oxide SiO.sub.x 
(wherein x is less than or equal to 2) has been found to be well-suited 
for this purpose. SiO.sub.x a can be uniformly deposited in a layer 
thickness of a maximum of 2 nm with available processes such as a 
low-pressure glow discharge in an atmosphere containing silane (SiH.sub.4) 
and oxygen. It should be noted that other electrically insulating 
materials, however, are also fundamentally suitable insofar as they can be 
produced so as to be uniform, amorphous and thin. 
As previously stated, the second electrode is applied directly on the 
barrier layer. For example, the second electrode can be applied by 
vapor-deposition of a thin and close-meshed metallic grid (of, for 
example, aluminum) through an appropriate mask. 
The stationary charges are then applied thereover. Particularly, the 
charges are applied in the region of the surface of the barrier layer that 
is exposed in the open spaces of the electrode grid. The coating process 
can proceed by spraying, or with a spin-on technique. In this regard, an 
alkali metal halide, preferably cesium chloride, is particularly used. 
The anti-reflection layer is produced directly on this layer, which only 
has a thickness of a few layers of atoms. The anti-reflection layer is 
likewise amorphous and is preferably generated in the same glow discharge 
reactor. For example, this layer can be generated by decomposition of 
silane and ammonia, whereby a silicon nitride layer (Si.sub.3 N.sub.4) 
that is approximately 100 nm thick is generated. During the coating, the 
cesium ions diffuse into the silicon nitride layer to a depth of a few 
Angstroms where they are fixed. 
The positive, stationary space charges of the silicon nitride/cesium layer 
induce negative charges in the upper region of the semiconductor layer 
across the thin barrier layer (tunnel oxide). The electron enrichment 
denotes an inversion of the original p-conduction into an n-conduction in 
this region. This inversion layer is approximately 30 nm thick. The 
inversion layer can function as the n-emitter for generating a space 
charge zone in the semiconductor layer. 
The substrate can be any arbitrary material that, given inadequate 
electrical conductivity, is coated with the first electrode. Preferably, 
for cost purposes, a molybdenum-coated glass plate is used for this 
purpose. With respect to the adhesion of the semiconductor layer on the 
first electrode or, respectively, of the electrode on the glass, no 
problems are created with this construction; this is not necessarily true 
for other combinations of substrates and electrodes. 
Referring now to the Figure, a schematic cross section through a solar cell 
of the present invention is illustrated. The actual size of components is 
not shown true-to-scale in order to provide a clearer illustration. By way 
of example and not limitation, an example of a solar cell of the present 
invention will be set forth. 
A 1 mm thick glass plate serves as a substrate 1 on which a 2 .mu.m thick 
molybdenum layer is applied as a first electrode. A polycrystalline, 
p-conductive CIS layer is applied thereon as a semiconductor layer 3, 
through known methods. For example, the layer can be applied by the 
simultaneous evaporation of copper, indium, and selenium from separate 
crucibles. Other known methods for producing the CIS semiconductor layer 3 
can include electroplating, pyrolytic deposition, vapor-deposition or 
sputtering of copper and indium layers in succession and subsequent 
selenization in a H.sub.2 Se atmosphere, or vapor-deposition of copper 
layers, indium layers and selenium layers in succession and subsequent 
tempering. 
An approximately 2 nm thick amorphous SiO.sub.x layer 4 is deposited 
(wherein x.ltoreq.2) over the semiconductor layer 3 through a 
low-temperature glow discharge process in which silane (SiH.sub.4) and 
oxygen are decomposed. A thin and close-meshed grid 5 is applied on the 
barrier layer 4 as a second electrode by vapor-deposition of aluminum 
through an appropriate mask. The diameter and spacings of the individual 
meshes of the grid 5 are thereby selected so as to be optimally small for 
example 20 .mu.m wide with a spacing of 200 .mu.m. 
Stationary, positive charges (cesium ions) are applied in a thin layer 
including a plurality of layers of molecules on surface 4, 5 by spraying 
cesium chloride. An approximately 100 nm thick silicon nitride layer 
(Si.sub.3 N.sub.4) that functions as the anti-reflection layer 6 is 
produced thereover in a further glow discharge process by decomposition of 
silane and ammonia. During the coating process, the cesium ions diffuse 
into the silicon nitride layer a few Angstroms deep, where they are then 
fixed. In the Figure, these positive charges are illustrated by plus signs 
and are provided with the reference numeral 7. 
At the other side of the barrier layer 4, the positive charges 7 now 
produce corresponding, negative charges 8 in the uppermost layer region 11 
of the semiconductor layer 3. The negative charges 8 in the originally 
p-conductive semiconductor layer 3 form the inversion layer 11 that serves 
as an emitter for building up a space charge zone 10. This zone arises 
under the inversion layer 11 and extends, for example, up to the broken 
line indicated with reference numeral 9. 
For operating the solar cell of the present invention, the anti-reflection 
layer 6 is irradiated. The material (silicon nitride) thereof, similar to 
the silicon dioxide of the barrier layer 4, is transparent to sunlight due 
to its high band gap. An incident light quantum 12 is therefore not 
absorbed until it reaches the semiconductor layer 3, where it generates a 
charge carrier pair 13. The charge carriers 13 are separated in the field 
of the space charge zone 10, that corresponds to the field adjacent to the 
barrier layer 4, and are ultimately carried off to the electrodes 2 and 5, 
respectively. The electrons can thereby tunnel through the thin barrier 
layer 11. 
A further grid (not illustrated in the figure) is located on the surface of 
the anti-reflection layer 6, and functions to conduct the current 
initially collected in the grid 5 off. 
Although the electrical wiring of the solar cell of the invention to form a 
module is also not illustrated, this can be constructed in accordance with 
known methods for thin-film solar cells. 
The manufacturing method of the present invention has the advantage that 
all the steps can be implemented at a low-temperature in the range of 
.ltoreq.500.degree. C. 
Although the present invention provides a CIS solar cell, it can also be 
analogously used for other chalcopyrite solar cells of CuIn (Ga) Se.sub.2 
(S.sub.2). 
It should be understood that various changes and modifications to the 
presently preferred embodiments described herein will be apparent to those 
skilled in the art. Such changes and modifications can be made without 
departing from the spirit and scope of the present invention and without 
diminishing its attendant advantages. It is therefore intended that such 
changes and modifications be covered by the appended claims.