U.S. patent number RE29,812 [Application Number 05/787,533] was granted by the patent office on 1978-10-24 for photovoltaic cell.
This patent grant is currently assigned to Photon Power, Inc.. Invention is credited to John F. Jordan, Curtis M. Lampkin.
United States Patent |
RE29,812 |
Jordan , et al. |
October 24, 1978 |
Photovoltaic cell
Abstract
A large area photovoltaic cell comprising a layer of
multicrystalline cadmium sulfide, about 1 to 2 microns thick,
formed by simultaneously spraying two suitably selected compounds
on a uniformly heated plate of Nesa glass, thereafter forming a
coating of Cu.sub.2 S by spraying two suitable compounds over the
cadmium sulfide layer while the latter is heated, to form a
photovoltaic heterojunction, applying thereover a layer of CuSO4,
and applying electrodes of Cu and Zn, respectively, to separated
areas of the layer of CuSO.sub.4, and heating the cell to form a
cuprous oxide rectifying junction under the copper electrode by
reaction of the Cu electrode with the CuSO.sub.4, while diffusing
the zinc through the body of the cell. The diffusion of the zinc
provides a negative electrode coplanar with the positive copper
electrode, eliminating any need for introducing mechanically
complex provision for making a connection to the Nesa glass, while
the use of a rectifying positive electrode enables use of a layer
of CdS only 1 to 2 microns thick, rather than the usual 20 microns,
despite the fact that such thin layers tend to have pinholes, which
in the prior art render the cells inoperative but in the present
teaching do not.
Inventors: |
Jordan; John F. (El Paso,
TX), Lampkin; Curtis M. (El Paso, TX) |
Assignee: |
Photon Power, Inc. (El Paso,
TX)
|
Family
ID: |
23171755 |
Appl.
No.: |
05/787,533 |
Filed: |
April 14, 1977 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
303365 |
Nov 3, 1972 |
03902920 |
Sep 2, 1975 |
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Current U.S.
Class: |
136/258; 136/256;
136/260; 257/184; 257/51; 427/110; 427/74; 438/86; 438/94 |
Current CPC
Class: |
H01L
21/00 (20130101); H01L 31/03365 (20130101); Y02E
10/50 (20130101); Y10S 148/072 (20130101); Y10S
148/122 (20130101); Y10S 148/064 (20130101) |
Current International
Class: |
H01L
21/00 (20060101); H01L 31/0264 (20060101); H01L
31/0336 (20060101); H01L 031/06 () |
Field of
Search: |
;136/89CD,89CC,89TF
;357/16,30,59 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Weisstuch; Aaron
Attorney, Agent or Firm: Bard & Groves
Claims
What is claimed is:
1. A solar cell, comprising a transparent conductive base, a layer
of CdS microcrystals about 1 micron to 2 microns in thickness
coated on said base, a layer of Cu.sub.2 S coated over said layer
of CdS microcrystals and forming a photovoltaic heterojunction
therewith, a copper electrode superposed over a portion of said
layer of Cu.sub.2 S, a zinc electrode superposed over .[.a.].
.Iadd.another .Iaddend.portion of said layer of Cu.sub.2 S, and a
quantity of zinc diffused under said zinc electrode to provide a
conductive path from said zinc electrode to said conductive
base.
2. The combination according to claim 1, wherein .Iadd.there
.Iaddend.is provided a layer of copper sulphate between said
electrodes and said layer of Cu.sub.2 S.
3. The combination according to claim 1, wherein .Iadd.there
.Iaddend.is provided a rectifying Cu-Cu.sub.2 O junction under said
copper electrode, said junction being conductive in the direction
out of said copper electrode.
4. The combination according to claim 1, wherein said electrodes
are coplanar and interdigitated.
5. In a precursor photovoltaic cell, a conductive base, superposed
layers of microcrystals on said conductive base, said superposed
layers of microcrystals constituting a photovoltaic heterojunction,
and a rectifying junction output electrode overlying said
superposed layers of microcrystals, said rectifying junction being
non-conductive in a sense such as to prevent flow of reverse
current from said electrode into said heterojunction, said
superposed layers of microcrystals including a layer of cadmium
sulphide microcrystals and a layer of cuprous sulphide, said layer
of cadmium sulphide microcrystals .Iadd.contacting said conductive
base and .Iaddend.being about 1 to 2 microns thick, said rectifying
junction is a copper - cuprous oxide layer, and a layer of
.[.cuprous.]. .Iadd.curpic .Iaddend.sulphate being interposed
between said electrode and said heterojunction.
6. A precursor photovoltaic cell, comprising a transparent
conductive substrate, a first layer of photovoltaically active
microcrystals on said substrate, a layer of microcrystals coated
over said first layer and forming with said first layer a
photovoltaic heterojunction, an electrode coating said
heterojunction, said electrode being a rectifying coating
non-conductive in the sense from said electrode to said conductive
substrate, said photovoltaic heterojunction being constituted
essentially of cadmium sulphide microcrystals as one layer and of
cuprous sulphide as the other layer, wherein said cadmium sulphide
layer .Iadd.contacts said substrate and .Iaddend.is about 1 or 2
microns thick, said rectifying coating being a cuprous oxide -
copper rectifier, and a layer of .[.cuprous.]. .Iadd.cupric
.Iaddend.sulphate interposed between said rectifying coating and
said heterojunction.
7. In a precursor photovoltaic cell, a microcrystalline
photovoltaic heterojunction, a layer of .[.cuprous.]. .Iadd.cupric
.Iaddend.sulphate superposed over said heterojunction, a layer of
copper superposed over said layer of .[.cuprous.]. .Iadd.cupric
.Iaddend.sulphate, and copper oxide formed at the junction of said
layers of .[.cuprous.]. .Iadd.cupric .Iaddend.sulphate and copper
by interaction between said .[.cuprous.].
.Iadd.curpic.Iaddend.sulphate and said copper. .Iadd. 8. A solar
cell, comprising
a transparent conductive base,
a layer of CdS microcrystals coated on said base,
a layer of Cu.sub.2 S coated over said layer of CdS microcrystals
and forming a photovoltaic heterojunction therewith,
a metallic positive electrode superposed over a portion of said
layer of Cu.sub.2 S,
a zinc negative electrode superposed over another portion of said
layer of Cu.sub.2 S, and
a quantity of zinc diffused under said zinc electrode to provide a
conductive path from said zinc electrode to said conductive base.
.Iadd. 9. The solar cell described in claim 8, wherein said layer
of CdS microcrystals is less than 20 microns in thickness. .Iadd.
10. The solar cell described in claim 9, wherein said positive
electrode and said negative electrode are co-planar. .Iadd. 11. The
solar cell described in claim 10, further including a rectifying
junction layer interposed between said layer of Cu.sub.2 S and said
positive electrode. .Iadd. 12. The solar cell described in claim
11, wherein said rectifying junction layer includes Cu.sub.2 O
interposed between said CU.sub.2 S layer and said positive
electrode.
Description
BACKGROUND
In the art of making micro-crystal cadmium sulfide volatic cells it
has been the practice to fabricate the cadmium sulfide layer of
considerable thickness, say 20 microns. This has been deemed
necessary to assure that pin holes, or other types of defects do
not occur in the layer which if present render the cell
inoperative. It has heretofore been considered unfeasible to
utilize extremely thin layers of cadmium sulfide because a large
proportion of the cells prove defective in practice. It is one
purpose of the present invention to provide a large area
photovoltaic cell capable of being incorporated in a system
employing areas of photovoltaic generators covering areas of the
order of square miles, to enable large scale production of electric
power. In such systems the total quantity of cadmium required
becomes a problem since cadmium is in short supply in the United
States and is expensive Reduction of the feasible thickness of
cadmium compound required to fabricate a given area of cell is
therefore crucial economically and a reduction of thickness of CdS
layer by an order of magnitude or more renders ecconomically
feasible a large scale power generator of the photovoltaic type
which otherwise is not economically feasible. Utilization of
minimum cadmium per unit area of cell is rendered feasible by
utilization of a rectifying positive electrode in the cell.
It is, accordingly, a primary object of the present invention to
provide a photovoltaic cell which utilizes minimum weight of
cadmium per unit area and which can therefore be economically
utilized as a power source in a large scale electrical power
generation system. This same objective is subserved by providing a
call which has only coplanar electrodes, and also in terms of time
required to fabricate a given area of cell, a twenty micron layer
requiring twenty times as much spray time as does a one micron
layer in forming the requisite cadmium sulphide microcrystalline
layer on a substrate.
In the U.S. Pat No. 3,148,084, to Hill et al. issued Sept. 8, 1964,
a method is taught for forming a layer of cadmium sulphide
microcrystals on a glass substrate. Essentially, the method
involves spraying the glass substrate while the layer is hot, with
a cadmium salt-thiourea complex, i.e., cadmium chloride plus a
thiourea, in suitable proportions. The teaching of the patent is
that the glass may be heated by means of a hot plate and that the
spraying may take place in the atmosphere. We have found that
precisely uniform temperature of the glass plate is essential and
that a hot plate is not able to heat a glass plate uniformly
because the hot plate and the glass plate do not make perfect
contact throughout and that even slight non-uniformities of
temperature of the glass substrate produce anomalous areas of the
layer of CdS which can render an entire photovoltaic cell
inoperative. The layer of CdS must grow in the form of many tiny
crystals the anes of which are predominantly parallel. Application
of the sprayed materials at a uniform and sufficiently slow rate is
important as is uniformity of temperature to assure uniformity of
crystal growth rate and of orientation over the entire glass plate.
We have found that application of very intense ultraviolet light
over the entire CdS microcrystalline layer, as it grows, to enhance
uniformity and orientation of crystal growth improves the end
product, as evidenced by the fact that the percentage of plates
which prove imperfect is reduced. The layer of CdS may be only
about 1.0 to 2.0 microns thick, in the process of the present
invention, which is contrary to prior art practice.
In order to provide uniformity of temperature over the entire glass
plate, according to one feature of the invention, the plate is
floated during coating in melted tin, at over 700.degree. F.
In accordance with the teaching of U.S. patent to A. E. Carlson,
U.S. Pat. No. 2,820,841, issued Jan. 21, 1958, it is necessary to
superimpose Cu.sub.2 S to form a heterojunction on a layer of CdS
micro-crystals formed on Nesa glass. This is accomplished according
to the present invention by spraying on the layer of CdS while the
latter is at about 200.degree. F. to 300.degree. F., a small
quantity of copper acetate and of N,N-dimethyl thiourea, which, in
impinging against the hot CdS, forms a layer of Cu.sub.2 S about
1000. A thick thereover. Since the layer of Cu.sub.2 S is formed by
spraying cold materials which form Cu.sub.2 S only on contact with
the Cds layer, a flat layer is formed which so combines with the
exposed parts of the CdS crystals as to form the required
photovoltaic junction.
At this point, according to the teaching of Carlson, supra, it
would appear only necessary to apply an electrode to the Cu.sub.2 S
layer and a lead to the Nesa glass, to complete the fabrication of
a photovoltaic cell. A cell so fabricated is not satisfactory. Nesa
glass is conductive only because it has a coating of tin oxide.
But, tin oxide has high resistance taken along the surface of the
glass, so that a great deal of the energy generated by the cell is
lost in the tin oxide layer, and this is the more true the larger
is the cell. The problem can be ameliorated by breaking up larger
cells into smaller cells, as in FIG. 4 of Carlson et al., but only
at the cost of added complexity of fabrication.
According to the present invention, we deposit over the layer of
Cu.sub.2 S a layer of CuSO.sub.4, by spraying, and over the latter
deposit two separated electrodes of copper and zinc, respectively.
On heating the cell to about 500.degree. F. for about 12 minutes,
the CuSO.sub.4 gives up oxygen to the copper electrode, forming a
Cu.sub.2 O rectifying junction which is conductive for current flow
out of the copper electrode, but the zinc diffuses down through the
layers which it overlies, sometimes down to the layer of tin oxide
and sometimes only to but not through the CdS layer. In any event,
it has been found that if the tin oxide layer be considered to be
at ground potential, the copper electrode may be at 420. mv., while
the zinc electrode may be, in some samples at 0. mv., and in others
at minus 20. mv. The copper and zinc electrodes may be
interdigitated and the interdigitations located sufficiently close
together that the return paths for current internally of the cell
along the tin oxide layer can be short, yet the electrode and lead
system can remain simple, and easy to fabricate, requiring no
etching through the CdS - Cu.sub.2 S sandwich.
The rectifying Cu-Cu.sub. 2 O junction serves to prevent flow of
reverse currents through holes which sometimes times develop in the
CdS layer. Such holes may occur due to defects of the fabricating
process and when they occur the cell is defective because a short
circuit path to the SnOx is then available and it is the presence
of the junction which renders feasible the reduction of cadmium
usage by an order of magnitude, in comparison with prior art
cells.
SUMMARY
A photovoltaic cell, including a layer of tin oxide on a glass
base, a layer of uniformly oriented cadmium sulfide microcrystals
on the film of tin oxide a layer of Cu.sub.2 S the cadmium sulfide
so applied as to form a heterojunction, a layer of CuSO.sub.4 on
the Cu.sub.2 S layer and mutually isolated electrodes of Cu and Zn
on the CuSO.sub.4 heated to form a Cu-Cu.sub.2 O rectifying
junction, while the Zn diffuses down through the layers underlying
and thereby renders them conductive. The CdS may be or may not be
impregnated with zinc, providing either a half cell underlying the
zinc which is of zero voltage with respect to the tin oxide, or of
-20. mv. The Cu-CuSO.sub.4 junction provides oxygen for the
rectifying junction, which reduces or largely prevents a shorting
of the Cu.sub.2 S-CdS layer when the Cu.sub.2 S layer or the CdS
layer is defective due to the presence of holes in the layers. The
CdS and the Cu.sub.2 S layers are deposited by successively
spraying respectively a cadmium salt-thiourea solution and a copper
salt N,N-dimethyl thiourea solution while the glass is floating in
molten metal baths of suitable temperatures allowing CdS
microcrystals and a heterojunction with Cu.sub.2 S to develop only
on contact of each complex with a suitable heated surface.
DESCRIPTION OF DRAWINGS
FIG. 1 is a view in plan of a photovoltaic cell according to the
invention, showing interdigitated coplanar electrodes;
FIG. 2 is a view in section taken on line 2--2 of FIG. 1;
FIG. 3 is a view in section showing the mode of processing a glass
plate to form a CdS micro-crystalline layer according to the
invention; and
FIG. 4 is a view corresponding generally with FIG. 2, but in which
it is assumed that a hole or defect exists in the CdS layer of FIG.
2.
DETAILED DESCRIPTION
In FIG. 1, 10 is a plate of Nesa glass, i.e., non-conductive glass
having on one of its surfaces a thin layer 11 of tin oxide, which
is conductive. Overlying the layer 11 of tin oxide is a layer 12 of
polycrystalline CdS formed by a novel process according to the
present invention. Overlying the layer 12 of CdS is a further layer
13 of Cu.sub.2 S, also formed by a novel process according to the
present invention. Overlying the CdS layer is a thin layer 14 of
CuSO.sub.4, on which are deposited positive and negative electrodes
15, 16, of Cu and Zn respectively.
The CdS and Cu.sub.2 S layers, at their interface, form a voltage
generating heterojunction, the Cu.sub.2 S being positive and CdS
being negative, when the CdS is illuminated by light of the proper
wavelength. Specifically, the cell is responsive to sunlight.
The voltage generated at the heterojunction between the
micro-crystalline CdS and the Cu.sub.2 S is communicated via the
CuSO.sub.4 layer to the Cu electrode 15. A reaction occurs between
the CuSO.sub.4 and the Cu electrode when the latter is heated to
500.degree. F. for about 12 minutes. forming a rectifying junction
R of Cu - Cu.sub.2 O, which is conductive of current out of the
cell, so that there is no interference with operation of the cell.
The function of the rectifying junction R will be described
hereinafter.
In the prior art it was usual to utilize a tin oxide layer on glass
as the ground electrode of a CdS - Cu.sub.2 S solar cell. But
resistance is high parallel to the surface of the glass through the
thin layer of tin oxide, and therefore the efficiency of the cell
is low. To reduce losses in the cell, the cell is, according to the
prior art, slotted to provide access to the tin oxide at multiple
areas thereof, see Carlson. According to the present invention, a
zinc electrode 16 is deposited over the CuSO.sub.4, but separated
from the Cu electrode, supra. When heated, the zinc diffuses down
into the underlying layers, down to the tin oxide in some cases,
and down to the CdS in other cases. The cell is heated to about
500.degree. F. for about 12 minutes, and it is at this time that
the Cu - Cu.sub.2 O junction is also formed. The Zn diffuses to the
tin oxide and to diffuse the zinc a highly conducting path is
provided from the tin oxide to the zinc electrode 15, which now
becomes the ground of negative electrode of the cell. It is found,
in many cells, that the Zn electrode is about 20. mv. below the
voltage level of the tin oxide layer 11. This seems to imply that
the CdS is active and in conjunction with the Zn forms a negative
cell. By interdigitating the Cu and Zn electrodes, 15 and 16, as in
FIG. 1, a cell of considerably higher efficiency than that taught
by Carlson et al. is provided, and yet the fabrication is much less
costly since the electrodes are co-planar and no etching or
machining is required. Efficiency is high because paths through the
tin oxide are kept short, a concept broadly suggested in FIG. 4 of
Carlson et al. But, according to the present invention, discrete
paths to the SnOx are formed solely by doping, and the doping is
provided by the negative electrode material utilized, whereas in
Carlson machining or etching is required. The normal voltage of the
Cu electrode 15, with respect to the tin oxide layer 11, is about
420. mv. Use of Zn does not degrade this voltage, in any case, and
in most cases adds 20. mv. to the available output.
The Hill et al patent, supra, teaches that the glass plate which
forms a substrate in the present ystem must be hot, about
700.degree. F., while being sprayed, and that the spraying must be
sufficiently slow to permit uniform growth rates for the CdS
micro-crystals of the layer. It has been found that any
non-uniformities of temperature of the glass plate, producing
temperature gradients along the surface of the plate, result in
imperfect crystal growth, and therefore a defective cell. To avoid
this contingency, the glass plate 10 is sprayed, according to FIG.
3, while the plate 10 is floating in a bath of molten metal,
specifically tin. The glass plate 10 is not wet by the tin, so that
when the glass plate 10 is removed from the molten tin bath, after
it is sprayed, the underside of the plate is clean, or easily
cleaned. The spray is provided via an .[.osciallating .].
.Iadd.oscillating .Iaddend.nozzle 21, which repeatedly re-traces a
planar path designed uniformly to cover the plate 10 with spray.
The spray is a true water solution of cadmium chloride and
thiourea. As the fine droplets of the spray contact the hot surface
of the glass plate 10, the water is heated to vaporization and the
dissolved material is deposited on the plate, forming CdS, plus
volatile materials, and the CdS, if it has nucleating areas
available, grows as small crystals. The nucleating areas are
provided by the tin oxide, and if the spray is sufficiently uniform
and sufficiently slow, and if the temperature of the glass surface
is adequately high and uniform, crystal growth is uniform and all
the crystals have nearly the same spatial inclinations, so that a
uniform layer of nearly indentical micro-crystals exists. It has
been found that irradiating the crystals, as they grow, with high
intensity U.V. light, from sources 22, assists in the crystal
growing process and produces a higher yield of near perfect layers
than is otherwise the case.
It may happen that a layer of CdS micro-crystals is formed which
contains one or more holes, as 25 in FIG. 4. In such case the
Cu.sub.2 S layer, which overlies the CdS. fills the hole, and the
voltage generated at the junction between the CdS and the Cu.sub.2
S, when illuminated by radiation of appropriate wavelength may be
shorted or find a low resistance path back to the tin oxide layer.
More important, the Cu electrode 15, in its entirety, may be
shorted to ground, i.e., to the SnOx layer, through this path, and
therefore an entire cell is usually defective if one pin hole
develops anywhere in the CdS layer.
According to the invention, however, a rectifying junction is
formed at the underside of the positive copper electrode. This
junction does not substantially inhibit flow of current out of the
cell via the copper electrode, but it does inhibit flow of current
back from the copper electrode to the layer of SnOx, so that
presence of a hole in the CdS layer has no effect. The
interposition of the layer of the CuSO.sub.4 between the Cu.sub.2 S
and the Cu electrode, and subsequent heat treatment, raises the
yield of operative cells in a production run.
The method of forming a CdS layer and a Cu.sub.2 S layer is
summarized as follows. A plate of Nesa glass is floated in a tin
bath heated to 800.degree. F., to provide 700.degree. F. at the
upper .[.surfact.]. .Iadd.surface .Iaddend.of the glass plate.
CuCl.sub.2 21/2H.sub.2 O of 0.01 molar solution is employed, and an
excess of thiourea, in de-ionized water, for the reaction desired.
The desired thickness of the CdS polycrystal layer is about 1 or 2
microns.
The Cu.sub.2 S 18 is developed by floating the glass plate
previously coated with polycrystalline CdS, in a bath of molten
metal at about 200.degree.F. to 300.degree. F, and spraying with a
water solution of 0.0018 molar copper acetate and 0.001 molar of
N,N-dimethyl thiourea, to a thickness of about 1000. A. The
.[.CuSo.sub.4 .]. .Iadd.CuSO.sub.4 .Iaddend. is sprayed over the
Cu.sub.2 S layer to a thickness of about 250. A. to 1000 A and the
Cu and Zn are deposited as interdigitated electrodes. The entire
cell is then heated to 500.degree. F. for about 12 minutes causing
Cu --Cu.sub.2 O layer to form at the copper, and causing the Zn to
diffuse.
The copper and zinc electrodes may be radiation heated via separate
masks to provide optimum heating in each case for the chemical
and/or physical effects desired.
While one specific embodiment has been provided involving a CdS -
Cu.sub.2 S heterojunction, the features of the invention relating
to (1) uniformity of heating of the substrate; (2) irradiation by
ultraviolet light while the microcrystals are being formed; (3)
provision of a rectifying positive terminal formed by interaction
with an oxygen-bearing layer underlying the positive terminal; (4)
provision of a diffused co-planar negative terminal, are all
utilizable with any form of micrycrystalline heterojunction, and
are not limited to CdS - CU.sub.2 S, or to either of these.
* * * * *