Note: Descriptions are shown in the official language in which they were submitted.
11371~7
An array of photovoltaic cells in a back wall-
type configuration supported by a rigid transparent
vitreous substrate, such as glass, for admitting inci-
dent radiation to the cells. The substrate permits
high temperature film formation and permits a variety
of techniques to be used for subsequently removing
portions of the films during formation of the array.
The substrate also provides the structural support for
the array during handling and installation in yet a
larger scale power generating facility.
An array of photovoltaic cells wherein a plural-
ity of cells is interconnected into a desired electrical
configuration by one or more layers of elec~rically
conducting materials which overlie substantially the
entire heterojunction of each cell to minimize the in-
ternal resistance of the cell and to protect the
heterojunction from degrading environmental conditions.
The conducting layer interconnects an exposed elec-
trode region of one cell with the heterojunction
forming material of the adjacent cell while remaining
insulated from any intervening cell structure. In
one embodiment, the uppermost conducting layer is lead,
which seals the underlying materials from atmospheric
constituents, such as oxygen and water vapor, while re-
maining relatively inert to such constituents.
--2--
113';'~L~7
FIELD OF THE INVEN~ION
This invention relates generally to large area
photovoltaic cells which can be produced and inter-
connected for large scale terrestrial use and, more
particularly, to a photovoltaic panel which is formed
using mass production techniques, such as spray appli-
cation of layers, and thereafter formed into an array
of series connected solar cells and wherein the
individual cells are formed by film removal apparatus.
BACXGROUND OF THE INVENTION
The search for alternate energy sources in the
United States and throughout the world is progressing
at an ever increasing rate as the available supplies
of energy are being consumed. There are many alternate
sources of energy which might be tapped but for tech-
nological and/or cost considerations. Solar energy is
one source which is being extensively examined due to
its abundance and to an apparent absence of environ-
mentally deleterious side effects.
The technology and theory for producing basic
photovoltaic cells which generate electrical energy
in response to solar input is generally well known.
The main technical problems which are currently under
investigation deal with reducing this basic technology
to a practice which is applicable to the production of
such photovoltai,c cells at a cost which is competitive
with that required to construct and operate present
i:l3'~7
cay power generating facilities utilizing such energy
sources as oil, coal, or nuclear fission. To accom-
plish this goal, it is apparent that electrical
generating s'ations utilizing photovoltaic cells must
be fabricated using mass production techniques wherein
large areas, measured in terms of square miles, can
be literally covered with such mass-produced photo-
voltaic cells. In accordance with the present inven-
tion, large area photovoltaic panels will be formed
using production-type techniques and will thereafter
be formed into an array of series connected photo-
voltaic cells in a process suited to mass production
and in a size to generate commercial quantities of
electrical energy.
THE PRIOR ~RT
The production of first-generation photovoltaic
cells required that a single crystal of silicon or
cadmium sulfide be grown and then sliced into thin
wafers to form the semiconductor layers. By this
technique, discrete solar cells were constructed by
building up a layered cell from a plurality of dis-
crete elements bonded together to form the completed
cell. This production operation, in itself, was ex-
pensive and produced only small area photovoltaic
cells because of the requirement to form the semi-
conductor materials responsive to incident solar
radiation from single crystal materials.
1137197
To obviate the cost and size problems inherent
in the use of single crystal materials, polycrystaline
materials have been developed which are suitable for
use in forming photovoltaic cells which are conside-
rably larger than the cells which can be obtained
from single crystal materials. Typically, suitable
semiconductor materials are composed of compounds
from elements in Groups II and VI of the periodic
table. Cadmium sulfide has been found to be a par-
ticularly suitable compound which may be formed from
numerous chemical compounds containing cadmium and
sulfur and applied to a substrate in a variety of
processes to interact and form a layer of cadmium sul-
fide which exhibits semiconductor properties.
A completed photovoltaic cell which is well
known in the art includes a layer of polycrystaline
cadmium sulfide (CdS~ disposed on a suitable substrate,
and a second material which forms a heterojunction, or
"barrier layer", in cooperation with the CdS. The
material typically used to form a heterojunction with
CdS is cuprous sulfide, CuxS, where x is less than 2
for non-stoichiometric cuprous sulfide formed over
the CdS. The technology to mass produce photovoltaic
cells which incorporate the CdS - CuxS heterojunction
is rapidly developing and is not, per se, a subject of
the instant invention.
To provide for large scale terrestrial appli-
cation, the individual photovoltaic cells must be
113'7~7
formed into an interconnected array covering large
areas. Typically, a single CdS-Cu S hetero~unction
will produce an open circuit voltage of 0.40 - 0.54
volts. If a higher output voltage is desired in order
to transmit or use directly the output power from the
photovoltaic cell array, the invidiual cells may be
connected in a series arrangement to produce output
voltages of 12-24 volts, i.e., output voltages which
are equivalent to vo~tages produced by present day
storage batteries.
Early attempts to provide photovoltaic arrays
generaly consisted of taking individual photovoltaic
cells, adhering those cells to a common substrate, and
then interconnecting the photovoltaic cells with wire
conductors to complete the array. U.S. Patent No.
3,411,050 (Middleton, 11/1968) is illustrative of such prior
art. These photovoltaic arrays were custom fabricated and were
expensive to produce. The requirement to provide
physical connections for large numbers of conductor
wires further reduced the availability of surface area
for active photovoltaic power generation and thereby
reduced the overall efficiency of the photovoltaic array.
The availability of polycrystaline CdS as a component
in a photovoltain cells has greatly increased the capability
of forming a series connected array of such cells. U.S.
Patents No. 3,483,038 (Hui, 12~1969), No. 3,571,915 (Shirland
3/1971) and No. 3,713,893 (Shirland 1/1973)
are typical of recent prior art
, .,
-6-
~13'7197
attempts to provide a solar cell array. In these
prior art arrays, the polycrystaline cadmium sul-
fide layer is formed by masking and vacuum-evapora-
ting cadmium sulfide onto the surface of a suitable
substrate, which is generally a flexible plastic or
metallic foil, and then vacuum evaporating or de-
positing a slurry to produce a cuprous sulfide layer
over the cadmium sulfide and form the heterojunction.
It may be appreciated that this method is time consum-
ing and is not well adapted to mass production of
large scale panel arrays where the cells are series
connected. It should also be noted that the plastic
substrate materials require that a low-temperature
process, such as vacuum deposition, be used to form
the required layers, since the plastic cannot be suh-
jected to high temperatures.
Further, the photovoltaic arrays taught by the
above references generally utilize front wall-type
solar cells, wherein solar radiation is directly inci-
dent on the heterojunction and the substrate is
generally opaque to light. In a front wall-type solar
cell, the electrode applied to the heterojunction (the
CuxS layer~ is formed in grid-like pattern in order to
admit llght through to the heterojunction. The use of
the grid-like electrode subjects the CuxS layer to
possihle degradation during application of the grid or
du-ring subsequent exposure of the CuxS to the environ-
ment. In some fabrication techniques, the grid is
1137~37
affixed to the cell by an adhesive, whereby oxidation
of tile Cu S tends to occur when the adhesive is cured.
Also, exposure of the Cu S to the oxygen and water
vapor in the air can degrade the material during normal
cell operation.
The addition to the inefficiencies inherent in
a front wall-type solar cell from using a grid, i.e.,
covering a portion of the active heretojunction area
and a possible degradation of the heterojunction, a
front-wall type solar cell has an inherent optical
mismatch. The indices of refraction~,of cuprous sul-
fide and cadmium sulfide are 3-3.5 and about 2.5,
respectively. Accordingly, light incident on the
heterojunction at angles greater than the critical
angle for the Cu S - CdS interface, 35 to 55 depend-
ing on the particular indices of refraction, will be
reflected rather than transmitted. Further, the
abrupt large increase in the index of refraction in
passing from air to cuprous sulfide results in an
intensity of reflected light which is greater than
the intensity of the same radiation reflected from a
glass surface having a typical indes of refraction
around 1.50.
An evolving technique for producing photovoltaic cells
with polycrystaline CdS is to spray suitable solutions onto
a substrate where the solution reacts to form a film of
the desired material. U.S. Patents No. 3,880,663 (4/1975)
and No. 3,902,920 (9jl975) to Jordan et al, dis-
--8--
113~ 7
close suitable teehniques for forming large area back-
wall type photovoltaic cells by the spray method. A
glass substrate is moved through a series of spray
booths to form layered films of tin oxide, cadmium
sulfide, and perhaps cuprous sulfide. It is a feature
of these spray processes that each film is formed at
a temperature lower than that at which the preceding
film is formed. Accordingly, it would be desirable to
form the large photovoltaic panel into some number of
smaller cells, to be connected in series for increased
voltage outputs, only after all of the layers have been
formed. Such a technique would minimize the thermal
cycling of the glass and the energy required to produce
the photovoltaic panel.
The disadvantages of the prior art are over-
come by the present invention, however, and improved
methods are provided for obtaining an array of photo-
voltaie eells eonneeted in series. Further, an
improved array of series connected photovoltaic cells
on a common substrate is provided.
SUMMARY OF THE INVENTION
A method is provided for forming a large area
photovoltaic cell into a plurality of discrete photo-
voltaic cells on a common substrate. A large area
photovoltaic cell is first produced by forming layered
films over substantially an entire surface of a
common substrate. Portions of the films are there-
113'71~7
1 after selectively removed to form a plurality of smallerphotovoltaic cells. Finally, a conducting material is
applied to interconnect the photovoltaic cells into an
array.
According to the present invention, a method of
forming an electrically connected array of photovoltaic
cells mounted on a common vitreous substrate initially
having substantially the entirety of a selected surface
of the substrate covered with a first film of a
transparent and electrically-conductive material
comprises the steps of: applying at least one layer of
semiconductor material as a second film overlying the
first film; selectively removing portions of the first
film and portions of the second film to form a plurality
of individual photovoltaic cells on the vitreous
substrate and each with an exposed area of the first
film; depositing a first insulating material along an
edge portion of each of the first films adjacent the
exposed area of the first film, thereafter applying an
overlying layer of another different electrically-
conductive material onto the cells and electrically
contacting the upper surface of the second film of one
cell and the first film of an adjacent cell; and
separating the overlying different conductive material
into individual conductors in such manner as to
interconnect cells into an electrically selected array.
An interconnected array of photovoltaic ceils
forming a photovoltaic panel, comprising: a rigid
transparent vitreous substrate member; a plurality of
spaced photovoltaic cells occupying different selected
areas of one surfacee of the substrate member and each
cell having facing edges with adjacent ones of the cells;
~ 10
1~3~7~97
I each of the cells having a transparent electricall~
conductive film adjacent the substrate, a semi-conductor
film overlying the transparent film, a heterojunction
formed on the semi-conductor film, and a continuous solid
conductive layer having a first portion overlying the
heterojunction; the transparent film having an exposed
portion along the length of one of its facing edges; the
continuous solid conductive layer further having a second
portion deposited in physical and series electrical
contact conterminous along the length of the exposed
portion of the transparent film of an adjacent
photovoltaic cell.
An improved array of photovoltaic cells is produced
on a transparent vitreous substrate, such as glass or the
like. A back-wall photovoltaic cell array is thus
provided which can be formed for example by using a spray
process to produce a large area photovoltaic cell and
then removing the films to obtain a plurality of cells.
The vitreous substrate permits film formation at high
temperatures and is thereafter resistant to mechanical or
chemical film removal techniques.
A further improved array of photovoltaic cells is
provided wherein layered film form the composite
photovoltaic heterjunction structure and attached
electrodes. Substantially the entire surface area of a
substrate is covered with each film and only those
; portions of each layer are removed which must be removed
to form a plurality of photovoltaic cells on the
substrate and to form the series electrical
interconnections between the cells. The conducting
material contacting the heterojunction, seals and
protects the underlying materials while interconnecting
the photovoltaic cells into a suitable array.
11
11371~7
DESCRIPTION OF THE DRAWINGS
_ _
Preferred embodiments of the invention are shown in
the drawings wherein:
Figure 1 and lA are a cross section of a photo-
voltaic panel on which basic photovoltaic layers
have been applied.
Figures 2 and 2A are cross-sectional views of
a photocoltaic panel form which film material has been
removed to form a plurality of photovoltaic cells.
Figures 3 and 3A are cross-sectional views of
a photovoltaic panel prepared to receive an overlying
conductive coating.
Figures 4 and 4A are cross-sectional views of
a photovoltaic panel over which electrically conductive
layers have been applied.
Figures 5 and 5A are cross-sectional views of
a photovoltaic panel of series connected photovoltaic
cells sealed from the environment.
Figures 6, 6A and 6B illustrate formation of
the series connection by a slic~ng technique.
Figures 7, 7A and 7B illustrate formation of
the series connection by a "tear" strip.
Figure 8 is an isometric view of a completed
photovoltaic panel formed according to the present
invention (depth of the photovoltaic layers is
exaggerated).
Figures 9 and 9A are cross-sectional views
11371~7
showing the electrode configurations at the photo-
voltaic panel ends.
DETAILED DESCRIPTION
Referring now to the drawings and first to
Figures 1-5, there may be seen cross-sectional views,
illustrating a preferred method for forming an
interconnected solar cell array where the negative
electrode layer is formed over the entire panel and
formed into electrode areas electrically isolated
from adjacent negative electrode areas as the over-
lying heterojunction-forming films are selectively
removed. Figures lA-5A illustrate an alternate method
where the negative electrode is separated into a
plurality of negative electrode areas prior to form-
ing the overlying films.
Referring now to Figures 1-5 and first to
Figure 1, there may be seen a cross section of a sub-
strate panel 10 coated with layered films of SnOx 12,
CdS 14 and CuxS 22. These layers cooperate to form a
large area photovoltaic cell and are initially formed
over the entire substrate panel 10. At this stage, the
entire panel is, in fact, a larse photovoltaic cell
and would produce electrical power at low voltage and
high current if electrodes were now attached to the
panel.
- After the entire panel has been coated with the
semiconductor materials, the photovoltaic panel is then
/3
--~4--
11~7197
formed into a plurality of photovoltaic cells, as shown
in Figure 2. The CuxS film 22 and CdS film 14 are re-
moved from above a portion of the SnOx film 12 to expGse
a selected pattern of the SnOx film surface 16. In one
embodiment of the present invention, a strip of SnOx
approximately one millimeter wide is exposed. The width
of the exposed strip is selected to accommodate the
various insulating films and other materials formed
over the SnO~, and needed to form the electrical inter-
connections. Films 22 and 14 may be conveniently re-
moved by a tool suitable for cutting the films from the
surface, such as a tool bit or rotating cutting tool.
Referring again to Figure 2, the SnOx film 12
must be removed along one edge of the area from which
the overlying semiconductor films 22 and 14 have been
removed. The SnOx film 12 is a hard, tightly adherent
film and cannot be as readily removed by mechanical
processes as the CdS l~ and CuxS 22 films. Accordingly,
a process may be chosen which essentially vaporizes a
small portion of the film so that each photovoltaic
unit is electrically isolated at this stage from adja-
cent photovoltaic units. A preferred technique for
vaporizing the SnOx film to form gap 13 is by means
of a low voltage probe, typically at about 20 volts
d.c., which creates an electrical arc along the SnOx
to vaporize the SnOx to be removed. The SnOx film
might also be removed by means of a focused laser
beam concentrated so as to vaporize the small area of
/Y
' ~-5-
B
1137197
SnOx to be removed.. Further, it is possible to re-
move a selected portion of SnOx to form sap 13 by
conventional masking and chemical etching methods
which are conventionally employed in fabricating
semiconductor devices, such as illustrated by U.S.
Patent No. 4,009,061 to Simon.
Once a plurality of photovoltaic cells has
been formed and electrically isolated, one from
the other, the units must then be connected to form
the series array of photovoltaic cells. As shown
in Figure 3, the photovoltaic units must be prepared
to receive the overlying layers of conduc-ting materials
which are to be applied. The exposed edges of semi-
conducting layers 14 and 22 are first coated with suit-
able electrically insulating materials. It has been
found that insulating film-forming materials used in
conventional masking operations for chemical etching
may be used. A first insulating film 24 is formed
along the edge of the layers which is immediately ad-
jacent the exposed strip 16 of SnOx. A second insu-
lating film 26 is formed over the exposed edges of the
semiconducting layers of the adjacent photovoltaic unit
and to completely fill gap 13. Insulating films 24
and 26 may be formed from the same material or from
different materials where needed, as hereinbelow dis-
cussed.
Insulating films 24 and 26 may be formed from
a variety of materials to which the semiconductor
/~'
,~r--.,
1~3'~1~7
layers 14 and 22 do not react in such a manner as
to result in any degradation of the semiconducting
properties of the materials. Materials which have
been successfully used include a photo-resist
marketed under the trademark KMER by Kodak, poly-
vinyl chloride films, acrylic paint, and cellulose
film formers. Where insulating film 24 is to be
removed, the film 24 may be formed from asphalt
based printing inks or solvent based strippable
film forming materials, which are well known in
the printing industry and the etching industry.
The method of applying these insulating materials
is conventionally through a needle-like pen having
a fairly large aperture such that the insulating
material may be applied as a high solid content
slurry with just enough solvent to enable the
slurry to flow through the pen.
Referring again to Figure 3 there may be
seen an "adhesive strip" 28 formed on the surface
of the SnOx strip 16. The adhesive 28 may be ap-
plied for the purpose of insuring better electrical
contact and an adhering bond between the overlying
conducting layers, which are to be applied, and
the underlying SnOx layer 12. The need for adhesive
strip 28 is determined by the actual overlying con-
ductor material which is applied. In one embodiment,
a-rotating brass wheel is used to deposit a small
amount of brass directly on the exposed SnOx 16 by
/~
_g~;z _
1~37197
frictional contact between the rotating wheel and
exposed strip 16. Brass is particularly compatible
with an overlying copper layer. Other materials
which are suitable for forming adhesive strip 28
include zinc, indium, cadmium, tin, and bronze, and
alloys thereof.
Referring now to Figure 4 there may be seen a
photovoltaic panel with the overlying conductor layers
formed over the surface of the underlying substrate
and photovoltaic cells. It is preferred to cover the
entire substrate area with conductive ma-terials and
this may conveniently be accomplished by vacuum-evapo-
rating one or more conductive materials over the sur-
face. As shown in Figure 4, a first conductor layer
30 is vacuum-evaporated over the entire area of the substrate
and layer 30 may conveniently be copper which forms a
satisfactory bond with the CuxS layer 22 and the ad-
hesive strip 28. Finally, a layer of lead 32 may be
applied over the layer of copper 30 to further pro-
vide a conductive path for the electrical current
and to protect the copper 30 from oxidation and other
damage during subsequent fabrication of the cells into
photovoltaic structures suitable for installation in
a large scale array. It should be noted, however,
that copper and lead tend to form an alloy at the
junction of the two metals when the cell is heated
subsequent to forming both layers. Thus, a very thin
barrier film a few angstroms thick may be required at
_~_
~13~7
the junction to prevent direct contact between the lead
and copper. A suitable physical barrier may be formed
from oxidized copper, iron or inconel.
In one aspect of tile present invention the layer
of lead serves to protect the CuxS layer from degrada-
tion and prolong the life of the photovoltaic hetero-
junction. Normally, cuprous sulfide is quite susceptible
to degradation in the presence of oxygen and water, such
as would occur if the layer were exposed to the atmos-
phere for front wall-type operation. Transparent con-
ductors have not been available to cover the cuprous
sulfide layer and protect the layer. Thus, grid-like
electrode configurations have been required with a
further covering needed to seal the cell. The back
wall-type photovoltaic cell which is the subject of
the present invention does not require illumination of
the cuprous sulfide layer so a solid electrode may be
used which may also seal and protect the cuprous sulfide
layer.
It has been found that multi-layer conductors of
copper and lead provide many advantages. The copper
adheres well to the cuprous sulfide and also helps to
maintain the stoichiometry of the cuprous sulfide. E~ow-
ever, copper alone is somewhat permeable to oxygen and
water vapor. A second layer formed of lead over the
copper then seals the copper. Lead is also a conductor
and thus serves to improve the overall conductivity of
the overlying conducting material while protecting the
/~
_~_
11371~7
Cuxs .
Referring now to Figure 5 there may be seen a
cross-sectional view of a completed panel of photo-
voltaic cells which are connected in series. A
portion of overlying electrical conducting layers 30
and 32 form an electrical contact with a portion of
the exposed SnOx strip 16, which electrical contact
may be improved by means of adhesive strip 28. Con-
ducting layers 30 and 32 then extend over the CuxS
layer 22 of the adjacent photovoltaic cell and are
insulated from contact with any other portion of the
adjacent photovoltaic cell by insulation 26. Since
the SnOx layer is the negative electrode of one
photovoltaic unit and the CuxS layer forms the posi-
tive portion of the adjacent unit, the photovoltaic
units are thereby connected electrically in a series
arrangement. If desired, the layered surface of the
photovoltaic panel may then be covered with a suitable
sealant 34 for protection against exposure to detri-
mental environmental conditions.
It will be appreciated from the above discussion
that the entire operation for forming the series con-
nected photovoltaic units is one which is well adapted
to a mass production process. The steps of forming
the individual photovoltaic units, applying the insula-
tlng strips and the adhesive strip may all be done by
a suitable machine making a single pass across the sur-
face of the coated substrate. If desired, a plurality
/~
_~ _
B
~Li3~97
of devices may be used so that the entire panel is
prepared at one time and the panel need be accurately
positioned only once. The subsequent step of form-
ing the metallic conducting layers 30 and 32 by vacuum
evaporation can be readily accomplished on a produc-
tion basis, although it is more expensive than the
spray methods for forming the other films. As here-
inbelow discussed, a variety of techniques are available
for selectively removing portions of the overlying con-
ductor films 3Q and 32 so as to form the completed array.
Referring again to Figure 5, insulating strip
24 has been removed along with the portion of conductor
layers 30 and 32 overlying insulating strip 24. In one
conventional technique this is accomplished by using an
insulating film 24 (shown in Figure 4) which is removable
by means of ultrasonic vibrations whereupon the over-
lying conduetor layers 30 and 32 are deprived of their
structural backing and are also removed by the ultra-
sonic vibrations. Insulating film 26 is chosen to main-
tain integrity at the ultrasonic frequency at which film
24 is removed. Thus, selected portions of the conductive
films 30 and 32 are removed to obtain the desired electri-
cal interconnection.
Referring now to Figures lA-5A, there may be seen
a eross-section of a substrate panel 10 where the SnOx 12
areas are already formed and eleetrieally isolated from
one another. This eondition might oeeur if a de-
feetive panel is being reproeessed or if it is de-
sired to begin the CdS eoating with the SnOx already
removed. Removal of the SnO to form the isolated
eleetrode areas may be aeeomplished as hereinabove dis-
B ~
~L3~7
cussed for the step illustrated by Figure 2. Be-
cause of the progressive nature of the temperatures
used in forming a photovoltaic panel by the spray
technique, it is desirable to remove the SnOx with-
out having to cool the entire panel to roomtemperature and subsequently reheat. In such a case,
a preferred method would use the low voltage probe
method to affect film removal prior to forming the
CdS layer 14.
Once the entire substrate has been coated
with the heterojunction-forming films, CdS layer 14
and CuxS layer 22, selected portions of these films
are removed as per the discussion related to Figure
2, above. Further, as shown in Figure 2A, the re-
moved portion of CuxS film 22 and CdS film 14 is
superposed above the area from the SnOx film 12 has
been removed so that a small portion of CdS 20 re-
mains in the isolation gap which is located substan-
tially along an edge of the area from which the
overlying films have been removed.
Referring again to Figure lA and 2A, there
may been seen gap 13 filled with a portion of the
CdS 20. This occurs where the SnOx is removed prior
to forming the semiconductor films, in order to avoid
any possibility of damage to the overlying semiconductor
materials from the heat generated in film vaporization.
The CdS material 20 which fills gap 13 obtains a
different crystaline structure from the CdS microcrystals
1~3~97
which are formed directly on the SnOx layer. It is
believed that the CdS material 20 in gap 13 will
have a much higher specific resistivity than found
in CdS film 14 and will act to insulate between ad-
~acent SnOx film 12 regions~ Accordingly, it is
expected that CdS material 20 may be merely left in
gap 13 when the overlying semiconductor regions 22
and 14 are removed.
Figures 3A, 4A and 5A illustrate the steps of
forming insulating films 24 and 26, laying down insu-
lating strip 28, forming conductor layers 30 and 32,
and thereafter removing portions of the conductor
layers to produce the desired electrical interconnec-
tion. The steps are performed in a manner identical
to the steps described for Figures 3, 4 and 5 and the
resulting photovoltaic array is available for the
production of electrical energy.
As hereinabove discussed, only the preferred
method was presented for removing selected portions of
the overlying conductor films in order to separate the
photovoltaic cells and, simultaneously, form the
integral series electrical connections which provide
the interconnected array. An alternative technique to
the use of ultrasonics for the removal of one insulating
film and the overlying conductors is shown in Figures
6, 6A and 6B. As shown in Figure 6, the photovoltaic
panel has been formed and selected portions of the SnOx
layer 12 and overlying films 14 and 22 removed to produce
--~3--
113'~197
a plurality of photovoltaic cells on substrate 10.
Insulating films 24 and 26 are applied as discussed
hereinabo~re for Figure 3 except that the applicator
pens apply a larger volume of insulating film 24
whereby insulating strip 24 is formed to an eleva-
tion substantially greater than insulating strip 26.
The difference in elevation between insulating strip
24 and 26 should be such that the top portion of in-
sulating strip 24 will be higher than the top portion
of insulating strip 26 after conductors 32 and 30
have been applied, as shown in Figure 6A. It is then
possible to cut through the top portion of insulating
strip 24 and remove the overlying conductors 32 and 30
without removing the conducting films 32 and 30
from other portions of the photovoltaic panel. Thus,
an insulating region 42 is formed, as shown in Figure
6B, where the top portion of insulating strip 24 has
been removed to again provide the series interconnec-
tion between adjacent photovoltaic cells. One
advantage to this technique is that the desired inter-
connection is accomplished by merely passing the com-
pleted panel beneath a suitable cutting edge.
Referring now to Figure 7, 7A and 7B, there may
be seen yet another technique for removing conducting
layers 30 and 32 to form the series connections. Again,
a pluraiity of photovoltaic cells comprising SnOx layer
12, CdS layer 14 and CuxS 22 have been formed on sub-
strate 10 according to the methods hereinabove discussed
for Figures 1 and 2. As shown in Figure 7, insulating
~3
B ~
1137197
strips 24 and 26 have been formed. In addition, a
tear strip 44 is placed on top of insulating strip
24. Tear strip 24 may be a metallic wire or any
suitable material having sufficient tensile strength
to cut through the thin conductor layers as herein-
below discussed. As shown in Figure 7A, the con-
ductor layers 30 and 32 have again been formed over
the entire surface of substrate panel 10 and, in
particular, over tear strip 44. Tear strip 44 is
formed to extend beyond the edges of substrate panel
10 such that tear strip 44 may be pulled upward and
along insulating strip 24 to break through the over-
lying conductor layers 30 and 32 to isolate the
photovoltaic units and form the series cGnnection, as
shown in Figure 7B. Figure 7B illustrates an isola-
tion region 46 where insulation strip 24 has been
removed, but insulating material 24 may also be left
in place, if desired.
In a preferred embodiment, substrate panel 52
is a transparent vitreous material such as glass, and
the photovoltaic cells 54 are arranged on the glass in
a back-wall configuration, i.e., with the CdS nearest
the glass. The arrangement is particularly suitable
for producing the initial large area photovoltaic cell
by spray techniques. Each of the films on the glass
substrate is formed successively and at progressively
lower temperatures. Thus, the glass substrate needs
to be heated to a high temperature only once and
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thereafter only reduced to lower temperatures. Pro-
duction time is not consumed in having to repeatedly
heat and cool the glass at prescribed rates to pre~
vent excessive strains from developing. Further,
glass is heat-resistant and can withstand the
relatively high temperatures to produce the tin
oxide and cadmium sulfide films.
A glass substrate is also particularly suited
for forming the large area photovoltaic cells into
smaller cells. The rigid support provided for the
overlying films allows a cutting tool to be used for
film removal. The heat resistance of the glass also
permits the tin oxide to be removed by vaporization.
Also, glass can withstand the chemical treatment neces-
sary to remove the tin oxide by etching, if needed.
In forming the completed photovoltaic panel,
several testing steps may be desirable. In particular,
it is highly desirable to check the resistance between
adjacent photovoltaic cells once the SnOx has been re-
moved to insure the removal has been satisfactory to
electrically isolate the photovoltaic units. It is
a particular Eeature of the back wall array that
each photovoltaic cell can be individually checked
upon completing the array to particularly identify
any defe_tive cell which may be present. Further,
the panel voltage must be checked after the overlying
conducting layers have been separated to insure that
the series connection has indeed been obtained. It
should be noted that side strips (not shown) of the
substrate panel 52 which are perpendicular to the
photovoltaic cells are usually cut off after the panel
2S
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~1371~7
has been formed in order to remove those portions
which may be still electrically connected due to in-
complete removal of overlying conducting layers.
It is now apparent that the photovoltaic panel,
hereinabove described, is one well suited to provid-
ing a low cost photovoltaic cell suitable for large
scale production of electrical power. Each photovol-
taic panel covers a large area and is capable of
handling such amounts of current whereby large quanti-
ties of power may be obtained at relatively low DC
voltages of 18 -24 volts. The internal resistance of
the photovoltaic units is minimized by forming the
SnOx layer in accordance with U.S. Patent No. 3,880,633
wherein a process for forming a very low resistance SnOx
film is disclosed. The tin oxide layer produced accord-
ing to the subject patent has a sheet resistivity of
about 5 to 10 ohms per square. This sheet resistivity
allows a cell width of up to about two centimeters with-
out producing unacceptable internal power losses with-
in each cell.
Other advantages of the solar cell array accord-
ing to the present invention include forming the large
area photovoltaic cells in mass production, where spray-
ing techniques are used to produce the plurality of
layers forming the photovoltaic cells over the support-
ing substrate. Further, the active area of the entire
photovoltaic panel is maximized since only small strips
of the overlying films are removed, generally forming
no more than about ten percent of the entire panel area,
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113'~1~7
and the overlying conductors are formed as substan-
tially continuous layers whereby a low resistance
is obtained. Finally, the glass substrate inherently
seals the radiation incident surface without restric-
ting light admittance and the generally conterminous
conductors seal -the heterojunction surfaces to pro-
duce a panel which is substantially protected from
atmospheric effects. It is expected that some final
packaging may provide a final sealant for the ex-
posed edges of the photovoltaic cells and a backing
for physical protection, bu-t no special packaging and
sealing is otherwise required.
Referring now to Figures 9 and 9A, there are
more particularly depicted the terminal regions of the
completed photovoltaic panel 50 comprising the positive
terminal 60 shown in Figure 9 and the negative termi-
nal 62 shown in Figure 9A. Referring first to the
positive terminal 60 shown in Figure 9, a conductor is
placed adjacent the conductor layer 32 and over the
CuxS layer 22. In a rudimentary embodiment, conductor
61 is a solder bead, such as a tin-lead alloy, deposited
over the conductor layers 32 and 30. The volume of
solder deposited to form conductor strip 61 should be
as to maintain the current densities within the con-
ductor strip at acceptably low levels to minimi~e resis-
tance heating and energy losses. The material chosen
to~contact the conductor layer is selected to provide
a work function compatible with the conductor layer for
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1~37~g7
minimum contact losses. External connections may then
be made to terminal strip 61 by soldering, clamping
or other means of making suitable electrical contact.
Referring now to Figure 9A, terminal strip 63
is formed in contact with an exposed portion 36 of the
SnOx layer to provide a negative electrode. Terminal
strlp 63 may again be provided by an suitable connector,
such as indium solder, as hereinabove discussed. Termi-
nal strip 63 should be arranged out of contact with
the semiconductor films 14 and 22 to prevent shorting
out the films. This isolation may be obtained by simply
making exposed SnOx surface 36 wide enough to accommodate
terminal strip 63, or alternatively, by providing an
insulating strip along the exposed surfaces of the over-
lying semiconductor and conductor layers, as hereinabove
discussed for the steps for forming the series connection.
While the final means for supporting and inter-
connecting photovoltaic panel 52 into an overall network
for generating commercial quantities of electrical energy
is not the subject to which the present invention is di-
rected, it should be noted that many suitable materials
for forming terminal strips 61 and 63 exist and that such
terminal strips need not be soldered in place but may be
formed by physically urging suitable terminal strips 61
and 63 against the appropriate regions of the completed
photovoltaic panel 52. The only requirement is that the
positive terminal 60 be formed in contact with a CuxS
layer and that the negative terminal 62 be formed in
11371g7
contact with an SnOx layer and insulated from contact
with film layers overlying the SnO~.
It is therefore apparent that the present in-
vention is one well adapted to attain all of the ob-
jects and advantages hereinabove set forth together
with other advantages which will become obvious and
inherent from a description of the process and pro-
ducts themselves. It will be understood that certain
combinations and subcombinations are of utility and
may be obtained without reference to other features
and subcombinations. This is contemplated by and is
within the scope of the present invention.
As many possible embodiments may be made of
this invention without departing from the spirit or
scope thereof, it is to be understood that all mat-
ters herein set forth in the accompanying drawings
are to be interpreted as illustrative and not in any
limiting sense.
WHAT IS CLAIM~D IS:
