Note: Descriptions are shown in the official language in which they were submitted.
`- 10771~
The invention relates to systems and methods for
generating electrical power from light radiation, and speci-
fically from solar radiation.
The increasingly aggravated inadequacy of fossil
fuels for energy generation of all types has led to many
efforts to tap alternative energy sources. A particularly
attractive alternative source is light radiation, and par-
ticularly solar rad-lation, which comprlses enormous amounts
of easily accessible energy and is largely untapped.
Among the most important ex~sting devices for converting
solar energy into electricity are devices of the type de-
veloped in the space effort. These devices comprise net-
works of s~aller area thin monocrystalline layers connected
in serles. These devices have relatively high efficlency
in terms of power generation in relation to weight. This
criterion, however, is substantially inapplicable to the
problem of power generation for normal commercial and
consumer purposes, in which the criterlon of usefulness
is related to economic factors, such as power generation
per unit cost. Under this criterion of efficiency, units
which are useful in the space effort are impractical.
These units are comprised of photoelectric material which
is substantially monocrystalline and must be grown from
crystalline solutlon, wlth a high fa~lure rate. These
constraints limit such units to small dimensions and re-
quire many such units to provide even a minimal power
source.
Devices and fabrication techniques utilizlng
polycrystalline semiconductive materials have generally
proYen inadequate due to high production costs. Among
the contributing factors to these high costs is the
requirement of use of structural materials of high heat
resistance due to the high temperature utilized with
~/5 7~
1~77~
these fabrication techniques. Moreover, such devices
generally utilize metallic internal conductors, thus fur-
ther increasing costs. Also, the failure rate in fabri-
cating such devices is relatively high because of penetra-
tion by impurities, in the course of fabrication. Further,
control of the deposition of semiconductive material in
such process presents substantial problems.
There has been a recent attempt to form photo-
voltaic power generating means with a compound semi-
conductor material having an N-type region and a P-type
region and in which the N-type and P-type regions were
doped. In this case, the first semiconductor section,
constituting an N-type section, was formed by a vapor
phase deposition of a metal, such as cadmium with the
addition of sulfur to provide a cadmium sulfide layer.
The second, or P-type, semiconductor section was formed
by dipping the material into a hot aqueous solution of
CuCl which caused formation of CuxS. However, the results
were poor since the cadmium sulfide was sometimes porous
giving rise to shorted junctions. In addition, a large
amount of unused cadmium was required in the deposition,
thereby creating a substantially expensive photo-voltaic
power generating means, oftentimes of low efficiency.
U.S. Patent No. 3,573,177 issued March, 1971 to
William McNeill describes a prior art technique by which
polycrystalline cadmium, zinc, or cadmium-zinc sulfide or
selenide is formed by electrochemical deposition on a Cd or Zn
or Cd, Zn anode and where sulfur or selenium is provided from a
solution containing $= or Se= ions and which polycrystal-
line material is usable as a semiconductor material. Theconcept of forming thin films of semiconductor materials
by electrochemical techni~ues is relatively new and due,
in part, to the teachings in the aforesaid McNeill patent.
1077161
In accordance with the McNeill patent, electrochemical
discharge o~ ions, such as those ylelded by sulfides or
selenides dissolved -in an eleckrolyte, occur with respect
to cadmium or zinc actlng as an anocle in an electrolytic
cell. This electrochemical discharge converts the zinc
or cadmium, or the alloys of these metals, to the corres-
ponding sulfldes or sul~oselenides.
The McNeill patent has advanced the art of pro~
ducing semiconductive materials by electrochemical tech-
niques and presents many advantages, including the abilityto apply ~ilms of semiconductor materials to irregularly
shaped substrates which were not thoroughly cleaned.
Nevertheless, the McNeill patent su~fers ~rom many limi-
tations in that the end product is not necessarily capa-
ble o~ functloning as a P-N semiconductor ~unction mater-
ial necessary in the operation of a photo-voltaic cell or
simllar diode. McNeill is essentially concerned with the
manufacture o~ non-~unction semiconductor ~ilms, such as
those found in the electro-luminescent panels, electro-
sonic transducers and photosensitive conductors.
The essence of the McNeill patent lies primarily
with anodic plating, with discharge of S= and Se ions.
However, it is not known that the McNeill process can be
applied to e.g. discharge of Te= ions which is more ideal
in the case of photo-voltaic cells. Yet it is such a
discharge, forming CdTe and a ZnTe that is of prime
importance for the manu~acture o~ solar cells since cad-
mlum telluride has a dlrect band-gap uniquely optimized
for sunlight at 1.5eV.
There has also been a proposed prior art tech-
nique ~or electrochemically precipitating metals at a
cathode for producing a selenium recti~ier. This tech-
nique is reported in an article entitled, "Electrochemische
--3--
1~771~1
~bscheidung von Metallseleniden", by H. ~on Gobrecht, H.D.
L-less and A. Tausend, in Ber. ~eutsche Bunsengesellschaft
6F (1973), page 930. Thls article does not describe the
production of photo-voltaic power generating cells. In
accordance with this prlor art technique, deposition of
the less noble component and the more noble component
must be very care~ully controlled due to the difference
in standard precipitation potentials. The more noble
component had to be added in carefully controlled small
dose9 in order to operate with this technique.
Therefore, there has been, and is, a well recog-
nized, but unfulfilled need for photo-voltaic power
generating means having relatively high power generating
capability per dollar of cost to produce and having a
form suitable for commercial and consumer use, and for a
method of producing such means.
It is, therefore, the primary ob~ect of the pres-
ent invention to provide a photo-voltaic power generating
means in the form of a power generating cell which is
cons~ructed of semiconductor material having an N-type
region and a P-type region.
It is another obJect of the present invention to
provide a photo-voltaic power generating means of the
type stated which operates with a relatively high degree
of efficiency and which can also be made at a relativel~
low cost, compared to convenbional and proposed photo-
voltaic power generating means.
It is a further object of the present invention
to provide a photo-voltaic power generating means which
can be produced in the form of a relatively flat sheet for
disposltion upon a surface which is located to receive
solar radiation.
It is an additional object of the present
77~ti1
invention to provide a low-temperature method of producing
photo-voltaic power generating means and which eliminates
the high temperature operation which was heretofore
employed to produce such power generating means having
semiconductor materials.
It is also an object of the present invention to
provide a method of cathodically depositing semiconductor
forming material at the cathode of an electrolytic cell.
It is yet another object of the present invention
to provide a photo-voltaic power generating means which is
created by cathodically depositing semiconductor forming
material at the cathode of an electrolytic cell to produce a
semiconductor compound which is photoreactive.
It is another salient object of the present invention
to provide a method of producing photo-voltaic power generating
means of the type stated which is highly efficient and
substantially eliminates material waste.
In accordance with one broad aspect, the invention
relates to a method of preparing a photo-voltaic power
generating cell comprising the step of depositing electro-
chemically on an electrode a coating of at least one semi-
conductor compound from an electrolytic bath including the
components of said semiconductor compound, said compound being
capable of forming a semiconductor junction, being transmissive
to light radiation and being capable of forming electron-hole
pairs upon being irradiated with photons, said components
being formed of at least one of the metal elements of Class IIB
and non-metal elements of Class VIA of the Periodic Table of
Elements.
With the above and other objects in view, our invention
resides in the novel features of form, construction, arrangement,
B -5-
.
77~61
and combination of parts presently described and pointed out
in the claims.
Having thus described the invention in general
terms, reference will now be made to the accompanying drawings
in which:
FIGURE 1 is a somewhat simplified perspective view
of a photo-voltaic power generating means in accordance with
the present invention;
FIGURE 2 is a somewhat schematic side-elevational
view of a power cell in the power generating means of Figure l;
FIGURE 3 is a side-elevational view, somewhat
similar to Figure 2, and showing a slightly modified form
2Q
~ -5a-
1~771~i1
of photo-voltaic power cell;
FIGURE ~ is a perspective view, partially broken
away and shown in sectlon, and showing a preferred power
cell construction in accordance with the present invention;
FI~URE 5 is a schematic electrical circuit view
showing an equivalent electrical network for a solar
energy operated cell in accordance with the present in-
vention;
FIGURE 6 is a schematic side-elevational view
showing one method for forming a pboto-voltaic power cell
ln accordance with the present invention;
FIGURE 7 is a schematic side-elevational view,
8 omewhat similar to Figure 6, and showing another modified
form of creating a photo-voltaic power cell in accordance
wit, the present invention;
FIGURE 8 is a schematic side-elevational vlew,
somewhat similar to Figure 6, and showing another method
for forming a photo-voltaic power cell in accordance with
the present invention;
FIGURE 9 is a somewhat schematic side-elevational
vlew, somewhat simllar to Figure 6, and showlng still
another modified form of method for creating a photo-
voltaic power cell in accordance with the present inven-
tion;
FIGURE 10 is a schematic side-elevational view,
somewhat slmilar to Figure 9, and showing yet another
modified form of method for creating a photo-voltaic
power cell utilizing a plurality of anodes in accordance
with the present invention;
FIGURE 11 is a schematic side-elevational view,
somewhat similar to Figure 10, and showing another
modified method of the present invention which also
utilizes a pair of anodes;
--6--
-` ~077~ti1
FIGURE 12 is a schema~lc side-elevational view,
somewhat simllar to Figure 11, and showing an additional
modifled form of the present invention for creating a
photo-voltaic power cell in accordance with the present
invention;
FIGURE 13 is a schematic diagrammatic view show-
ing the steps ut-llized in the method of the present inven-
tion.
Re~errlng now in more detail and by reference
characters to the drawingsJ 20 designates a photo-voltaic
power generating means, as depicted in Figure 1, in a
form suitable for commercial and consumer use and con-
figured as a sheet or panel 22. This panel 22 is si~ed
to be disposed upon a surface 24 as shown as the roof of
a dwelling. In the depicted application, the photo-
voltaic power source 20 generates power as a consequence
of having solar radiation incident thereupon. The inven-
tion may~ of course, be utilized in a wide range of other
applicationsJ including heavy stationary installations,
vehicles, and laboratory uses, with other light sources
and in other configurations.
The power generating means 20 comprises as a
ma~or integral component thereof, a photo-voltaic power
generating cell 26, (Figure 2) which is formed of semi-
conductor material. In this respect, the sheet or panel
22 may be comprised of a plurality of series-connected
cells, such as the cells 26. The cell 26 in its simplest
form includes an N-type region 28 and a P-type region 30,
which are separated by a ~unction 32, in a manner to be
hereinafter described in more detail. While the present
invention is erfective with a hetero-~unction, it is also
possible to produce the N-type region 2~ and the P-type
region 30 with homo-junction therebetween.
.
.
10771~;1
The term "photo-voltaic" as used herein refers
to a compound semiconductor which is capable of generating
electrlcal power when the compound semiconductor is sub-
Jected to the incidence of solar radiation or similar
forms of light radiation. The semiconductor in its
simplest form is often referred to as a "cell". In ~any
cases the term "cell" is also used to encompass not only
the compound semiconductorl but the substrate and ter-
minals or electrodes as well. In each case the cell will
have two region~, e.g., an N-type region and a P-type
region, e~tablishing a junction therebetween.
The N-type region 28 ls formed of an N-type
material which may comprlse any of a number of well-known
compositions which exhibit N-type semiconductor proper-
ties. The P-type region 30 is formed of a P-type material
formed of any well known composition which exhibits P-
type properties. In a preferred aspect of the invention,
the cation is preferably cadmium or zinc and the anion
is sulfur, selenium or tellurium.
Figure 3 illustrates a modified form of photo-
voltaic power generating cell 34 which comprises a sub-
strate 36 formed of a relatively inert electrically non-
conductive material which is preferably transparent to
llght radiation. The substrate 36 may be formed of a
relatlvely low-cost material, such as any of a number of
plastics, and particularly that plastic sold under the
trademark "Mylar" and the material sold under the trade
name "Kapton" of relatively low heat resistance and of
low cost. However, any of a number of other substrate
materials may also be used in accordance with the present
invention, and include any fiber-reinforced plastic sub-
strates, such as for examp~e, epoxy resin impregnated
fiberglas substrates, or the like. In essence, the
--8--
~771~1
substrate should be one which is relatively inert with
respect to electrical conductivity and may be without sub-
stantial heat resistAnce.
Bonded to one flat surface of the substrate 36
is an electrlcally conductive metal electrode 38 which
may be composed of a relatlvely inert electrically con-
ductive metal, such as stainless steel, nickel or the
like. In thls case, it can be observed that the electrode
38 comprises a thin layer o~ sheet, although the electrode
10 38 may take other forms and may have other positions in -
accordance wlth the present invention. The electrode 38
may also be secured to the substrate 36 by any of a
variety of ~nown techniques, such as metal vapor deposi- -
tion, electrolytic deposition, or the like. Otherwise,
the electrode 38 may be prefabricated as a strip and
bonded to the substrate 36 by means of conventional ad-
heslves, etc.
Secured to the exposed flat surface of the
electrode 38 is a photo-electric power cell 40 which ls
substantially similar in construction to the power cell
26. In this case, the photo-voltaic cell 40 ls comprised
of a compound semiconductor material. This cell 40 is
provided with an N-type section region 44, similar to the
N-type section 28, and a P-type section 46, similar to
the P-type section 30, with a ~unction 48 therebetween.
An electrlcally conductive cover sheet 50 is secured to
the outer eurface of the section 30.
The N-type region 44 and the P-type region 46
are ~imilarly ~ormed in the same manner as the N~type
region 28 and the P-type region 30 were formed in the
cell 26. Moreover, in this case, the N-type region 44
and the P-type region 46 may also have a homo-junction 48
therebetween, as in the case where the N-type and khe
.
77~
P-type regions are formed of substantially the same
material. Otherwlse, these two regions may be formed of
different materials and have a hetero~unction 4~ there-
between. Finally, an electrically conductive connector
52 is connected to the metal electrode 3~ and an elec-
trically conductlve connector 54 is connected to the
sheet 50. This cell 34 operates in substantially the
same manner as the cell 26 and will generate a current
flow through a load connected across the connectors or
terminals 52 and 51~ when solar radiation is incident upon
the cell 34.
The cell 26 or the cell 34 may be ¢ompletely
enveloped within and contained by a container, or similar
form of contalner means, or so-called envelope (not shown),
which is formed of a material transparent to light radia-
tion. The container means may be formed of any of a
number of known materials capable of passing solar radia-
tion and including all forms of light radiation as, for
example/ plastic material including polyethylene sheets.
Polybutyral sheets and other forms of plastics, as well
as other electrically non-conductive like transparent
materials, may also be used in the formation of the con-
tainer.
The cell 40, as well as the previously described
cells, may be formed of cadmium sulfide or cadmium sel-
enide, and preferably of cadmium telluride. In the
depicted embodiment, the cell 40 is in the form of a thin
layer, although in other applications the cell 40 and the
N-type and P-type regions 44 and 46 may be configured in
any appropriate manner. ~s described in detail below,
the thickness of the cell 40 is readily controllable
through the method of fabrication of the photo-voltaic
power generating means in accordance with the invention,
--10--
1~7716~
thereby affordlng substantial economy. In any evenk, the
cell 40, as well as the other layers of the cell 40, as
described below, may be of the order of 1-20 microns in
thlckness, although the cells will prererably range in
thickness between about 0.1 to 40 microns.
The top surface of the container means for the
cells 26 or 34 and cover sheet 50 would be transmissive
of light and comprises an element of a light path 56 as
schematically illustrated in Figure 3 of the drawings.
An additional or alternative light path (not shown) may
be provided through the lower surface of the cover in
which the substrate 36 and electrode 38 may comprlse grid
structures to permit access of light and will be very
thln to minimlze internal electrical reslstance.
In accordance wlth the above, it may be observed
that the photo-voltaic power generating means of the
present invention may be dlsposed in contact with llght
receiving surfaces, such as roofs of structures, in the
form of continuous panels which may be fltted to the size.
These features, among other previously descrlbed and
hereinafter described in more detail, constitute a sub-
stantial advantage of the invention in terms of ease of
use and economy.
As indicated previously, the cover means over
the photo-voltaic cell would be transmlssive of llght and
comprise~ an element of the path 56 glvlng access to the
cell 40, that is through the P-type semlconductor region
46. In this case, the P-type semlconductor reglon 46 is
the region which is exposed to light radiatlon, although
it should be observed that the N-type region 44 could also
be the outermost region exposed to light radiatlon through
the path 56.
The leads ~2 and 54 connect the cell 40, and
-11-
10771~1
hence the power ~ neratin~ means, to an external load
(not shown). This load may comprise, for example, the
main power source of a vehicle or of electrical systems
within a vehicle. In operation, light traversing the light
path 56 strikes the photo-cell 40 and causes a movement
of electrons from the semiconductor material of the P-
type region ~6 across the ~unction 1~8 to the N-type region
44, under well-known phenomena of photo interaction with
semiconductive materials. Consequently, a migration of
electrons to the plus termlnal 52 occurs and a current
appears in the leads and ln the load.
In the embodiment of the cell illustrated in
Figure 3, where the light path is designated 56, at least
the electrode 50 must be transparent. In this case, the
electrode 38 and the substrate 36 would not have to be
transparent. However, -ln accordance with the present
invention, the cell could have a transparent electrode 38
and a transparent substrate 36, formed of a conductive
glass or transparent plastic substrate, as described in
more detail hereinafter. In this construction, the cell
would respond to a light path designated as 56' in Figure
3 of the drawings. However, it should be recognized that
all components of the cell 34 could be transparent.
Where the cell 34 is constructed so that it
responds to the light path 56, the narrow band gap mater-
ial will be incidental to the electrodes 38 and the wide
band gap material will be incidental to the electrode 50.
When the cell 34 is constructed so that it responds to
the light path 56'~ the wide band gap material will be
incidental to the electrode 38 and the narrow band gap
material will be incidental to the electrode 50. In
essence, the wide band gap material will always face the
source of light. In the present invention, a CdS layer
1077~ti1
has a wider band gap than the CdSe layer, which in turn,
has a wider band gap than a CdTe layer.
In accordance with the invention, the electrode
50 need not be ~ormed of a metal~ but could be formed of
a conductive transparent o~ide as hereinafter described.
Again, the electrode 38 could be formed of a conductive
transparent oxide. In addition the substrate 36 could be
conducting and constitute an electrode, thereby elimin-
atlng the necessity of the electrode 38.
One of the preferred embodiments of a photo-
voltalc power cell construction in accordance with the
present invention, is more fully illustrated in Figure 4
of the drawings. In this case, the preferred embodiment
of the power cell is designated by reference numeral 57
and lncludes a substrate 58 which is electrically non-
conductive, such as a glass substrate. This substrate 58
i8 preferably relatively thick with about a thickness of
one-eighth inch. A metallic electrode 60 is disposed on
one flat surface of the substrate 58 and this electrode
60, in the form of a grid, is comprised of a plurality o~
parallel spaced apart transversely extending strips 62
and a plurality of parallel transversely spaced apart
longitudinal extending strips 64. In the preferred as-
pect of the invention, the strips 62 and the strips 64
are ~ocated in an essentially perpendicular relationship.
A conductive coating 66 is applied to the sur-
face of the substrate 58 in which the metal grid 60 is
applied and this substrate 58 is preferably an electric-
ally conductive coating comprised of stannous oxide doped
with antimony, or indium oxide doped with tin. The
- electrically conductive coating essentially completely
covers the entire inner surface of the substrate 58
except for the portions of the grid in contact therewith
1~7~
and completely covers the metal gr-ld 60 and is in electri-
cal contact therewith while the grid 60 is on the inner
surface of the substrate 58.
This last substrate 58 may be formed of any
transparent substrate material as, for example, polymethyl-
methacrylate, or the like. In addition, the substrate 58
could actually form part Or a basic cell buildlng block in
the ~orm of a glass roof or wall tile. In any event, it
is important that the substrate is surficiently trans-
parent to admit the passage of light, when the cell isoriented for passage o~ light through the substrate.
The coating 66 which also faces the source of
solar energy ~s coated upon the substrate 58, preferably
by vapor coating, in the form of a uniform thin film of
electrically conductive material which is preferably
antimony-doped tin oxide, or indium oxide doped with tin.
In accordance wlth the present invention, it has been
found that it is not necessary to use a metal as an elec-
trode and that a relatively thick transparent substrate
can serve as the electrode when made electrically conduc-
tive through application of a conductive oxide. The con-
ductive oxide is an N-type material and therefore the
conductive oxide must be in contact with the N-region or
otherwise another ~unction would be established.
The grid 60, often referred to as a "bridge I~J iS
on the surface of the substrate 58 and located between the
substrate 58 and the tin oxide coating 66 in order to lower
the ohmic resistance. In this way, the grid 60 becomes a
first electrode which has a resistivity well below one
3 ohm per square inch. A terminal 68 extending fromone
portion of the grid 60 serves as a first terminal for
making electrical connection to the cell. A flat bus bar
(not shown) may ~so extend around the periphery of the
-14-
10'~7~
terminal portions of the grid 60 to serve as one o~ the
two terminals for providing the electrical connection to
the cell.
The photo-voltaic cell 57 will also include the
cell structure of the type illustrated in Figure 2 of the
drawings~ In this case, the cell structure includes an
N-type section 70, equivalent to the N-type section 44
in Figure 3, and a P-type section 72, equivalent to the
P-type section 46 in Figure 3 of the drawings, with a
lQ ~unction 74 therebetween. The N-type section 70 may be
comprised of, e.g. cadmium selenide or cadmium telluride,
whereas the P-type section may be formed of the same
material wlth a homo-junction or a different material with
a heterojunction. A relatively thin metallic film 76
is applied to the outer surface of the P-type section 72
in the manner as illustrated in Figure 4. This outer
metallic film 76 constitutes the rear electrode assembly,
and is provided with an electrically conductlve lead wire
78.
It can be observed that the construction of the
c ell of Figure 4 enables light to pass through the sub-
strate 58, thereby eliminating the need of a third metal
substrate. Moreover, it can be observed that the grid 60
is also electrically conductive and in conductive relation-
ship to the N-type region 20 through the conductive trans-
parent oxide film 66. The outer metallic film may have a
reflective surface facing the substrate 50 SQ as to cause
reflection of the light which entered the cell and thereby
: cause greater energy conversion efficiency.
A relatively high efficiency value for a poly-
crystalline photo-voltaic cell can be achieved by a com-
bination of factors including the use of a thin layer of
cadmium telluride facilitating maximum bransport o~ photons
-15-
.
--` 1077161
to the junction region. The use of a thin film of modi-
fied metal, as for example, vapor deposited nickel, performs
as an anti~reflection coating on the surface of the glass
facing the sun rays. The dlsclosed structure is quite
effective ln that it reduced ohmic losses in the two elec-
trodes and the semiconductor material.
This form of cell structure is highly advan-
tageous over previous prior art cell structures of the
single crystal type in that the cell structures described
hereln include substantial economies which become possible
through the deposition of thin layers of one or more costly
active materials on an inexpensive glass or transparent
plastic substrate. In accordance with this latter embodi-
ment of the invention, this embodiment provides the ability
to make a large integrated area cell without the necessary
recourse to intraconnecting a multiplicity of small or
independent units in a connected arrangement. More0~er,
this cell structure includes the possibility of the employ-
ment of printed circuit type conductors to connect a plur-
ality of indlvidual cells on a tile or similar substratein series or series-parallel arrangement.
Figure 5 illustrates, in schematic form, a pre-
ferred electrical configuration of at least one or more
cells connected in accordance with the present invention.
It has been well established that absorption of photons
having wave lengths shorter than the optical band-gap
creates electronic-hole pairs in a crystal lattice of the
semiconductor material. A built-in field provided by the
P-N ~unction, e.g. the P-N ~unction 32 or the junction ~8,
or otherwise, a Schottky batrier, separates the electrons
and the holes generating a photovoltage which biases the
~unction in a forward direction. Thus, in this way, a
solar cell of the type proposed by the present invention
-16-
'' : : :'
,
107~71ti1
can be represented hy the equivalent circuit in Flgure 5
o~ the drawings.
More fully considerlng Figure 5, it can be ob-
served that each of the cells are designated by reference
numeral 80 which functions as a current generator per unit
area. These cells have a diode 82 connected in parallel
therewith in the manner illustrated in Figure 5 of the
drawings. The diodes 82 are of unit area with respect to
the current generators, such as the photo-voltaic cells 80.
In this respect, it can be observed that while the cells 80
are connected across the diodes 82 ln parallel relation-
ship, the opposed terminals of the diodes 82 are connected
to a positive line 84 and a ground line 86. Resistors 88
and 90 represent th~ sheet resistance of the electrodes and
of the ad~acent electrically neutral portions of the semi-
conductors bordering the built-in field region. Resistors
g2 and 94 are representative of the contact resistances
per unit area of the neutral regions with these electrodes
and the reslstance per unit area of these neutral regions.
Each cell has similar resistive functions and diode func-
tions in the manner as illustrated in Figure 5 of the
drawings.
For optimal conversion efficiency, the resis-
tances 88 and 90, as well as resistances 92 and 94, the
latter of which constitute parasitic resistances, should
be made as small as possible. The selection of the semi-
- conductor material for optimizing similar energy conversion
thus involves maximizing the effective full type conver-
sion into electron-pairs for solar radiation. In other
words, this efficiency is created by maximizing the current
generator, e.g., the solar cells 80 and maximizing the
forward resistance of the various diodes 82. The maxi-
mization is required with respect to the solar cells 80
-17-
1077~L6~
and the diodes 82 and t~lese requirements are interrelated
resultlng in a compromise on the band-gap of the semlcon-
ductor material which is chosen with decreasing band-gap
as more radiation is absorbed. However, the internal
resistance of the barrier decreases, leading to optimal
band-gaps for cadmlum telluride of approximately 1.5eV for
solar radiation conversion at the earth's surface on a
cloudy day.
The technique for making the photo-voltaic power
cells in accordance with the present invention is more
fully illustrated in Figures 6-12 of the drawings with a
schematic flow diagram thereof illustrated in Figure 13 of
the drawings. In essence, the present invention provldes
for the controllable electrochemical production of Junc-
tions of cadmium and zinc-type compound semiconductors used
as photo-voltaic cells. In accordance with the invention,
a semiconductor compound material is formed at the cathode
where both the more noble components and the less noble
components are discharged.
Referring now to Figure 6, 100 designates a con-
tainer, such as a beaker, formed of a relatively inert
material. Located within the container 100 is a cathode
102 which is slmilarly formed of a relatively inert mater-
ial, nickel as shown. However, any other form of metal
electrode which is inert to the reaction, such as steel
or glass or plastic provided with a conductive oxide coat-
ing, for example, may be used. Also located w~thin th~
container 100 is an anode 104 which may also be inert, or
otherwise the anode may be formed of cadmium or zinc or
selenium or tellurium, as hereinafter described. As
illustrated, the anode 104 is formed of an inert platinum
material. Both of the electrodes 102 and 104 are disposed
within an electDolyte 106, as hereinafter described, and
-18-
~077~
both the anode and the cathode are e:Lectrically connected
through a source of electrical current 108.
This particular arrangement of Figure 6 repre-
sents a simplified system which illustrates the formation
of a coating at the cathode 102. ~y way of example, it is
possible to electrochemically deposit sulfur on the nickel
aathode 102 to form a sulfur coatlng, designated as S in
Figure 6, through the use of an electrolyte such as S02 in
H20. In this way, the reaction which proceeds is represented
by:
4e~ ~ 4H+ + H2S03 = 3H20 + S-
This reaction demonstrates that sulfur is reduced during
deposition at the cathode. Similarly, the H2S03 is oxi-
dized to H2S0~ at the anode. In this case, deposition
would occur pre~erably at about 10C to about 20C, with
about three to six volts applied across the anode and
cathode, along with a current density of 0.1 amp. per
square centimeter. An optimal depositio~ of the sulfur
occurs from a 1 mg 1~1 solution of S02 in water.
In the event that it was desired to form cadmium
sulfide, as opposed to a mere sulfur layer at the cathode,
the electrolyte could be changed from S02 in water to S02
-~ 3CdS04.4H20. In the ionic dissociation of the cadmium
sulfate in water, positively charged cadmium ions are
formed. ~hese cadmium ions are attracted to and dis-
charged at the cathode on which sulfur is also being
deposited simultaneously. Thus, the cadmium and the sulfur
will combine as they are simultaneously discharged at the
cathode to form a layer of cadmium sulfide on the cathode.
In this way, it is possible to form a film of cadmium
sulfide with any desired stoichiometry, as established
through the concentrations of the solutes used in the
electrolyte.
-19-
:1077~
Figure 7 -1llustrates a system similar to Figure
6, except t~lat in t~lls case the cathode which is employed
will constitute the metal upon which a coating is desired
to be formed. It can be observed that the cathode 102 is
*ormed o~ cadmium and wlth the aforementioned reaction,
sulfur can be catho~lcally deposited as along with cad-
mium a film upon the cadmium cathode to obtain, for example,
cadmium sulflde.
Figure 8 illustrates another embodiment of the
method of making a cadmium sulfide compound film on an
inert cathode, which in this case, is shown as glass having
a conductive oxide coating thereon. Again, the anode is
also formed of an inert material, such as platinum. In
order to produce the cathodic coating of cadmium sulfide,
the sulfur is introduced into a solution of the electro-
lyte in the form of S02 in H20, and the cadmium is intro-
duced in the form of 3CdS04.4H20, dissolved in this solu-
tion as previously described. It should also be observed
that cadmium telluride and cadmium selenide, etc,, zinc
sulfide, zinc selenide and zinc telluride could be formed
in the same way. Thus, in order to form a cadmium tel-
luride coating on the cathode 102, the electrolyte would
constitute tellurous acid as the source of tellurium and
cadmium sulfate as the source of cadmium. In this way,
the positively charged cadmium ions which are thus formed
would be discharged at the cathod. In like manner, the
tellurium would be deposited at the cathode and simultan-
eously react with the cadmium to form the cadmium telluride
film.
It can be observed that it would be necessary to
plate out the cadmium and the tellurium, or the other com-
ponents used, in the desired stoichiometric amounts. How-
ever, the standard voltage required for plating out the
-20-
1o7~
cadmium and tellurium would be different. For example, a
more negatlve voltage would be needed for the less noble
component as, for example, cadmium, than for the more noble
component, as for example, tellurium or selenium. While
there ls somewhat of a compensating effect with respect to
the deposition voltages when a semiconductor component is
formedJ it is usua]ly desirable to decrease the concen-
tration of the more noble component. Thus, in the case
Or producing cadmium telluride, the amount of tellurium
1~ in solution would be decreased with respect to the amount
of cadmium.
In order to form a cadmium telluride or similar
photo-voltaic device, as illustrated in Figures 2, 3 or
4, a first layer of cadmium telluride would be plated on
the nickel anode 104, in the manner as previously described.
The film thus formed on the cathode would be produced as
an N-region or a P-region, depending upon the ratio of the
cadmium and tellurium. After forming the first cadmium
telluride layer on the cathode, as for example the glass
with oxide coating cathode in Figure 8, a second electrolyte
9 imilarly including the same compositions to produce the
source of cadmium ions and the tellurium ions would also
be used. However, the concentration ratio of the cadmium
and tellurium in the second solution would be different
from that of the first solution in order to form the other
of the P-type region or N-type region. Thus, for example,
if a first film of cadmium telluride were placed onto the
nickel cathode 102 with, e.g., 50.01% cadmium~ this film
would constitute the N-type layer 28. When the second
film of cadmium telluride from the second electro}yte is
placed on the first film, this second film could have a
lower concentratlon of cadmium as, for example, 49.99%.
In this case, the second film would function as, and
-21-
-10771~;1
constitute, the P-kype layer 30~ Thus, it can be observed
that by merely controlling the stoichiometry of the metal
component, e.g., cadmium, and the nonmetal component, e.g.
tellurlum, or otherwise the ions of any other metal and
nonmetal components used in accordance with the present
invention, it is possible to produce elther an N-type layer
or a P-type layer. In accordance with this exampleJ it
can be observed that the two films thus formed on the
n ickel cathode 102 will form a homojunction 32 therebetween.
It should be observed in accordance with the
present invention that it is possible to produce the N-
type region and the P-type region from different materials
with a hetero~unction therebetween. In this case, e.g.,
cadmium selenide would be formed as a first film on the
anode 102, which is glass with a conductive oxide coating
as shown. Thereafter, the electrolyte would be changed
to plate out, e.g. cadmium telluride. The cadmium tel-
luride would then be plated onto the first layer of cadmium
selenide. In this way, the concentrations of the cadmium
with respect to the tellurium and the selenium would be
stoichiometrically ad~usted so as to create both an N-
type region and a P-type region. Thus~ the cadmlum selenide
layer could operate as either a P-type region or an N-
type region, but more preferably an N-type region, and
the same holds true of the cadmium telluride layer which
would preferably be a P-type layer.
Figure 9 illustrates another alternative technique
for producing a cathodically formed film in accordance
with the present lnvention. In this case, the cathode is
also inert as, for example, the glass with the conductive
oxide coating as shown. The anode, ln this case, would be
formed of the metal component as, for example, a solid
cadmium or zinc sheet, or otherwise a cadmium or zinc-
-22-
..
~' - . ' ' ' -
10771~;1
plated æheet. The electrolyte would be comprised of those
materials which provided the nonmetal component of the com-
pound. Thus, in the case of sulfur, the electrolyte would
comprlse a solution of S02 in water. In this way, cadmium
sulfide would be formed at the cathode. Again, tellurous
acid could be used as the electrolyte and, in which case,
cadmium telluride would be formed on the nickel cathode.
With cadmium sulfide, it has been found that the
cadmium sulfide can be formed on the cathode with a layer
10 of a thickness of about 5 mlcrons for preferred results.
These layers are obtained from a 5~ solution of S02 at
about 45C. f
With the embodiment of Figure 9, as well as some
of the other embodiments herein, it is also possible to
utilize cadmium and similar metal anodes containing dopents.
Thus, indium as a donor dopant could be combined with the
cadmium as a cadmium-indium alloy to be used as the
anode. In this way, the electro-chemical process of the
invention has the advantage of forming a cadmium sulfide
20 film which contains indium in solid solution. By choosing
the proper concentration of the cadmium-indium alloy, the
indium concentration in the cadmium sulfide, or otherwise
cadmium telluride, etc., can be regulated.
Thus, it can be observed that thoee systems
illustrated in Figures 6, 7, 8 and 9 are all effective
in forming the desired photo-voltaic film material on the
cathode. Moreover, in each of these systems, by changing
the electrolyte it is possible to form a second film in
the same manner as previously described. Thus, if the
30 two films are formed of the same material with one
being of the N-type and the other being of the P-type,
they will form a homojunction therebetween, and with
different materials they will form a hetero~unction
-23-
therebetween.
As also previously describedl the N-type region
and the P-type region can be formed merel-y by adjustlng
the stoichiometry of the components used. However, it is
also possible to use any of several dopants in the two
regions. Thus, one of the regions could be doped with
indium, aluminum or gallium, etc., as donors, or with
p hosphorus, arsenic or antimony, etc., as acceptors.
The present inventlon is primarily effective for
use in produclng cathodically formed films with cadmium
and zinc ions and sulfur, tellurium and selenium ions. In
addition, mixed crystals of the types Cd(S,Se), Cd(S,Te),
Cd(Se,Te), Cd,Z~(Te), Cd,Hg(Te) and Cd,Mg(Te), etc., can
be produced. Thus, any combination of mixed crystals formed
of ions of cadmlum, mercury, magnesium, zinc and any form
o~ mixed crystals, as, for example, those formed of ions
of sulfur, selenium and tellurium may be produced by the
present invention. These substances may be pure or doped
with those donors or acceptDrs as previously described or
any other form of effective and acceptable donor or accep-
tor.
As indicated above, electrolytic deposition on a
conducting cathode permits ions from both the metal and
nonmetal components in the electrolyte to be simultaneously
discharged at the cathode and formed a semiconductor com-
pound material on the cathode. As also indicated, S02 may
be used as the electrolyte in order to form a sulfide
layer, as prevlously described. Cadm~um sulfate is also
used in combination wlth the S02 in order to form the
cadmium sulfide layer. When ~orming the various cadmium
salt films as semiconductor compounds, various acids,
such as ~2SeO3, H2S03 or H2TeO3 may be used, or otherwise
the alkaline salts o~ these acids may be used with an
-24-
10771ti1
inert anode. In add-ltion, solutlons in acid of S02,
SeO2 or TeO2 may be utilized with an inert anode. The
compositlon of the deposited film is controlled through
the composition of the electrolyte as described. Alter-
natively, it is possible to use as an electrolyte a solu-
tion o* S02, SeO2 or TeO2 in water with an anode of Cd
(Cd,Zn), (Cd,Hg) or (Cd,Mg), etc.
The ions formed by the metal components, e.g.,
cadmium and zinc, and the ions formed by the nonmetal
components, e.g., sulfur, selenium and tellurium, in solu-
tion cannot necessarily be characterized as single cations
and anions. Generally, the cadmium and zinc in solution
will form single cations since they are generally posi-
tively charged, e.g., Cd and Zn++. In many cases the
nonmetal components provide ions, e.g., S and Se .
Tellurium, for example, can adopt several valence states
as Te . However, TeO3= complex ions can be formed. More-
over, Te~4 ions could be formed with TeO2 in hydrofluoric
- acid. In this case, TeF4 would form which dissociates to
produce Te~4 in solution.
The electrochemical principles which might be
; applicable to explain the plating of both the metal and
nonmetal components as a semiconductor compound on the
cathode are not fully understood. Nevertheless, it has
been established that these components do plate out at
the cathode to form a semiconductor compound. With re-
spect to the ions of the metal components, these ions
would normally be attracted to and discharged at the
cathode. The reasons for the discharge of the ions of the
nonmetal components is more complex.
The nonmetal components present ions in solution
in the presence of hydrogen. Thus, for example, the non-
metal components are introduced in an acid form, in most
-25-
1077161
cases presenting~ an available source of hy~rogen. It has
been theorized that the hydrogen in proximity to the
cathode alds in the reduction of the nonmetal ions in
proximity to the cathode. Thus, for example, TeO3= -~ 6H+
provides Te which are available at and become discharged
at the cathode. Nevertheless, whlle the exact principles
may not be fully understood, it has been established that
the cathodic formation of the semiconductor compound
material does occur.
The method for producing the photo-voltaic power
cells of the present invention can also be effectively
operated with a plurality of anodes, as illustrated in the
arrangements of Figures 10 and 11. In this case, the
method would also utilize a container 110, such as a beaker,
equivalent to the container 100. Moreover, in the arrange-
ment illustrated in Figure 10, a relatively inert cathode
112 as, for example, a cathode formed of glass with a
conductive oxide coating, as shown, would also be utilized,
along with a neutral anode 114 formed of an inert material
as, for example, platinum as shown. In addition, a second
anode 116 formed of cadmium would be utilized. The two
anodes 114 and 116 are connected to the cathode 112 through
a source of electrical current 118. Potentiometers 120 and
122 are respectively connected to the anodes 114 and 116
and to the source 118, in the configuration as illustrated
in Figure 10. Also, the cathode 112, along with the
anodes 114 and 116, are slso disposed in a suitable elec-
trolyte 124, as those electrolytes heretofore described
and as hereinafter described.
The anode 116 which is formed of cadmium may
otherwise be a cadmium-plated anode. In like manner, the
anode 116 could be formed of an alloy of cadmium with a
desired dopant. Tellurium ions would be provided in
-26-
,
1~77161
solut-lon as, for example, hy a tellurous acid composltion.
By carefully controlling the current flow to the respectlve
anodes 11l~ and 116, it is possible to introduce cadmlum
into solution fr~m the anode 116. In this way, the tellur-
ium ions contained in the tellurous acid will be discharged
at the cathode 112, and in like manner the cadmium entered
into solution from the anode 116 will also be discharged
at the cathode 112. In order to form cadmium sulfide
or cadmium selenide, H2S03 would be used to form the cad-
mium sulfide and H2SeO3 would be used to form the cadmiumselenide.
Again, a first layer could be formed on the
cathode 112 or other inert cathode, and which would either
constitute an N-type or P-type region according to the
amount of cadmium introduced from the anode 116 into solu-
tlon. The amount of cadmium introduced via the cadmium
anode can be controlled by adJustment of the two potenti-
ometers 120 and 122. Thereafter, a second layer could be
formed on the first-mentioned layer in order to form
either a P-type region or an N-type region which is oppo-
site to the first deposited layer. In all cases where two
anodes are employed in the arrangement as illustrated in
Figure 10, or otherwise the arrangement illustrated in
Figure 11 as hereinafter described, the ratio of the metal
ion to the nonmetal ion or molecule in the compound which
is deposited is determined by the currents flowing through
the respective anodes to the single cathode. Moreover,
it can also be observed that it is equally easy to provide
semiconductor films with a homojunction as well as a
hetero~unction. By merely changing the electrolyte to
form the second layer, it will be possible to form the
heterojunction materlals.
Figure 11 illustrates an arrangement whereby a
-27-
,
., ~o ~7~6~ :
nonmetal anode 126 is used in place of the cadmium anode
116. Moreover, the electrolyte 124 would be replaced by
an electrolyte 128 containlng cadmium ions in solution.
As indicated, the cadmium could be introduced in the solu-
tion as, ~or example, from a solution of cadmium salts.
In accordance with this arrangement, it is possible t~
carefully control the amount of tellurium introduced into
solution through adjustment of the respective potentio-
meters 120 and 122.
The tellurium anode of Figure 11 could also be
formed as an alloy with, e.g., antimony, phosphorus or
arsenic, etc., to provide a dopant. In either case, the
use of the two anodes provides a means to continually
supply the mlnority component in order to slowly replenish
the same in solution. ~eplenishing of the minority com-
ponent, generally the more noble component, is usually
required when there is a large ratio between the concen-
trations of the ma~ority and minority components in the
electrolyte. In the case where two anodes are not employed,
and where a large ratio does exist, the minority component
could be slowly add~d on a continued basis, as by dripping
the same into the electrolyte, based on the ratio of de-
pletion of the minority component.
In any of these embodiments illustrated in Figures
10 and 11 it is possible to provide the P-type region and
N-type region by stoichiometrically adjusting the amount
of cadmium with respect to the nonmetals, such as tellur-
ium, selenium, sulfur, etc. Otherwise, it is posslble to
introduce dopants into the solutions. In the more pre-
ferred form, the dopant could actually be contained withinthe material formed in one o~ the anodes as an alloy there-
of, as, for example, cadmlum-indium alloys as an anode.
With respect to the use of the three or more
-2~-
.
1077~
electrodes, it should be understood that plating could
occur on one of the electrodes, whlch may not constitute a
cathode per se. By properly adjusting the components in
the electrolyte and by adjusting the potentlal applied to
the electrodes, it ls actually possible to perform anodic
deposition and cathodic depositlon at the same time.
Figure 12 illustrates an arrangement with three electrodes
wlth one of the electrodes 129 being formed of a sulfur-
containing mater~al and the other of the electrodes 130
being formed of a cadmium-containing material. A third
electrode 132 is also provided and is preferably of an
inert material. Again, by mere adjustment of two poten-
tiometers, e.g., the two potentiometers 120 and 122, it
is possible to care~ully regulate the amount of cadmium
and sulfur ions which are introduced into solution and
which are discharged at the electrode 132 in order to form
a cadmium sulfide film, as illustrated.
While cadmium sulfide has been described herein
as an example, any of the other metal and nonmetal com-
ponents could be used. Moreover, in this embodiment, theelectrodes cannot be defined as cathodes and anodes in the
classical sense. By way of example, the electrode 129
could have, e.g., a negative 2-volt potential, the elec-
trode 132 could have, e.g., a positive 2-volt potential,
and the electrode 130 could have, e.g., a positive 4-volt
potentlal. In this way, cadmium from the electrode 130
would be discharged and plate out on the electrode 132
through a cathodic process and sulfur from the electrode
129 would be discharged and plate out on the electrode 132
through an anodic process.
As used herein, the terms "inert" or "relatively
inert", as, ~or example, with an "inert anode" or "inert
cathode", refer to a material which is inert with respect
-29-
107~16~
to the reactants being employed. Thus, in the case of an
inert cathode, such as a nickel cathode, the cathode would
be inert and nonreactive with respect to the electrolyte
or any of the ions introduced therein in order to for~ the
semiconductor film on the cathode.
The present invention is highly effective in
obtaining relatively thin films by use of the electro-
chemical technlques described herein. In this instance,
films with a thickness ranging from about 0.1 micron to
about 40 microns and larger can be obtained as described
above. Thus, the use of the term "thin" or "relatively
thin" with respect to the film thickness will be based on
film thicknesses within the range of about 0.1 micron to
about 40 microns, or perhaps greater.
While the present invention is effective with
those materials described above, and which can be made in
accordance with the processes of the present invention,
one of the most effective materials thus found for use in
the production of the photo-voltaic cells is that formed
of cadmium telluride. It has been found that photo-voltaic
cells based on P-N homojunctions have an expected energy
conversion efficiency that is a function of the band-gap
of the material used with the optimum band-gap occurring
near approximately 1.5eV. Moreover, it has been found
that cadmium telluride provides a band-gap in this range.
In addltion, the cadmium telluride provides a reasonably
high efficiency and also a lower cost with respect to
other materials which might be employed. Cadmium telluride
is also stable in air, is nontoxic and can withstand
temperature variations of several hundred degrees above
and below ambient temperatures without decomposing. More-
over, cadmium telluride is preferred inasmuch as it is
neither deliquescent nor hygroscopic, and furthermore is
-30-
~.o77~
not subJect to d.isproportionation uncler conditions of
expected terrestrial operation.
Figure 13 illustrates the st;eps employed in the
method of producing 'he photo-voltaic power cells in
accordance with the present invention. These steps were
actually delineated in connection with the previous de-
scription. However, referring to Figure 13, lt can be
observed that a metal cathode is introduced into the
electrolyte and an anode is introduced into the electro-
lyte. The meth~ includes the formation of molecules orions of the nonmetallic component in the electrolyte and
the formation of ions of the metallic component in the
e lectrolyte. As indicated previously, these ions could
be introduced into solution in several different ways,
and the ions of both components would be discharged at
the cathode during the application of the electric field.
Both ions and molecul~s can migrate to the cathode, and
upon application of the electric field they are discharged
and form a coatlng ln the form of a compound semiconductor,
as previously described. As indicated above, the coating
would be firsk formed with a f~rst region such as an N-
type region or a P-type region. The coating would then be
provided with a second region which is the opposite of the
first region.
Finally, in the making of the photo-voltaic power
cells, conductive terminals could be applied to both the
N-region and the P-region. Otherwise, these terminals
could be applied to layers in contact with the N-region
and the P-region in connection with the embodiments illus-
trated in Figures 3 and ~ of the drawings.
One of the unique results which can be obtainedin accordance with the present invention is that either
a homo~unction or a heteroJunction can be established
1077161
between the P~t-~pe regiorl and che N-t~Tpe region. In this
way, problems of material waste and impurities are sub-
stantially reduced, and almost completely eliminated.
Furthermore, all of the heretofore required stringent
control procedures used in the formation of photo-voltaic
cells and similar semiconductor materials can be com-
pletely obv-lated.
Another one of the unique aspects of the present
invent-lon is that the reactions heretofore described may
be carried out or close to room temperature. Moreover,
and as lndicated, the processes described herein result
in very little, if any, waste material. In addition, the
processes can be carried out with very little concentra-
tions of the required ions.
It should be understood that each of the photo-
voltaic power generating means described above do not
operate on the basis of a galvanic cell and, hence, do
not require the need of an electrolyte in their operation.
Moreover, in order to obtain a greater efficiency of
electron travel across the surface of the cell, it is
possible to use a conductive coating, such as a trans-
parent electrolyte coating or any other form of trans-
parent electrolytic gel, for transference of the gener-
ated electrons to the electrical contact as, for example,
in connection with the embodiment illustrated in Figure 4.
The configuration and method of fabrication of
the photo-voltaic power generating means in accordance
with the present invention lend themselves to continuous
processes of production of substantial lengths and areas
30 of such power generating means. In many applications, ~ -
power generating means may be wound upon a roller or
other storage means and simply laid out as a sheet or
surface covering areas exposed to light, such as roofs
-32-
-- ~077~
and walls e~posed to solar radiation.
Thus, there has been illustrated and described
novel photo-voltaic power generat-lng means and methods of
use and methods of fabricaking such power generating
means with a relatively high degree of efficiency and
which fulfill all of the ob'ects and advantages sought
therefor. Many changes, modifications, variations and
other uses and applications of the power generating means
and methods described herein will become apparent to those
skilled in the art after consldering this specification
and the accompanying drawings. All such changes, modi-
flcations, variations and other uses and applications
which do not depart from the spirit and scope of the
invention are deemed to be covered by the invention which
is limited only by the following claims.
-33-
,
. ~: .
