CA1137197A

CA1137197A - Solar cell array

Info

Publication number: CA1137197A
Application number: CA000308173A
Authority: CA
Inventors: John F. Jordan; Curtis M. Lampkin
Original assignee: Photon Power Inc
Current assignee: Photon Power Inc
Priority date: 1977-09-08
Filing date: 1978-07-26
Publication date: 1982-12-07
Also published as: EG13954A; GB1575888A; NL7808630A; TR20403A; IL55165A; ZA783886B; AU3899878A; GR63166B; OA06048A; IN149318B; ES473061A1; FR2405557A1; JPS5441686A; PT68530A; DE2839038C2; IT1105538B; YU213178A; FR2405557B1; IL55165A0; DE2839038A1

Abstract

ABSTRACT OF THE DISCLOSURE

A method for producing an array of photovoltaic cells responsive to incident radiation by forming heterojunction-forming material layers over a transparent substrate panel having a transparent electrically conductive coating and thereafter removing selected portions of the materials to form a plurality of cells on a common substrate is disclosed.
The cells are then electrically interconnected by depositing electrically conducting materials over substantially the entire panel and removing only those portions of the deposited materials required to form series electrical connections.
An array of photovoltaic cells in a back wall-type configuration supported by a rigid transparent vitreous substrate, such as glass, for admitting incident radiation to the cells is also disclosed. 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.

Description

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.

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.

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 , ., ~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-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.

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 --~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

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 ~

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 _ ~_ , l\
~it ,~ ~

~1~'7~97 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 _~_ ~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, G

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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 ~7 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:

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. 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 said substrate covered with a first film of a transparent and electrically-conductive material, comprising the steps of:
applying at least one layer of a semiconductor material as a second film overlying said first film;
selectively removing portions of said first film and portions of said second film to form a plurality of individual photovoltaic cells on said vitreous substrate and each with an exposed area of said first film;
depositing a first insulating material along an edge portion of each of said first films adjacent said exposed area of said first film;
thereafter applying an overlying layer of another different electrically-conductive material onto said cells and electrically contacting the upper surface of said second film of one cell and said first film of an adjacent cell; and separating said overlying different conductive material into individual conductors in such manner as to interconnect cells into an electrically selected array.

2. The method according to claim 1, wherein portions of said first film are selectively removed to form a plurality of electrically isolated areas of said transparent conductive film on said substrate prior to application of said second film.

3. The method according to claim 2, wherein portions of said second film are selectively removed to expose the regions of said substrate where said first film has been removed and also areas of said first film adjacent said regions from which said first film has been removed.

4. The method according to claim 1, wherein portions of said second film are removed to expose portions of said first film and thereafter portions of said exposed portions of said first film are removed to form a plurality of electrically-isolated photovoltaic cells each having a remaining exposed area of said first film.

5. The method according to claim 3, wherein exposed edges of said films forming said plurality of photovoltaic cells are masked except that at least a portion of each exposed area of said first film is left still exposed, said layer of said different conductive material is applied over said plurality of photovoltaic cells and into electrical contact with said still exposed portions of said first film, and said layer of said different conductive material is separated into a plurality of conductors at the regions of separation between the cells in such manner as to connect every cell in series-connected electrical relationship with at least one adjacent cell.

6. The method according to claim 5, wherein before said layer of said different conductive material is applied, a quantity of a preselected material is applied to each still exposed portion of said first film to promote bonding between said first film and said layer of said different conductive material when said electrical contact is made therebetween.

7. An interconnected array of photovoltaic cells forming a photovoltaic panel, comprising:
a rigid transparent vitreous substrate member;
a plurality of spaced photovoltaic cells occupying different selected areas of one surface of said substrate member and each cell having facing edges with adjacent ones of said cells;
each of said cells having a transparent electrically conductive film adjacent said substrate, a semi-conductor film overlying said transparent film, a heterojunction formed on said semi-conductor film, and a continuous solid conductive layer having a first portion overlying said heterojunction;
said transparent film having an exposed portion along the length of one of its facing edges;
said continuous solid conductive layer further having a second portion deposited in physical and series electrical contact coterminous along the length of said exposed portion of said transparent film of an adjacent photovoltaic cell.

8. The array according to claim 7, wherein said photovoltaic cells each include a CdS-CuxS heterojunction.

9. The array according to claims 7, wherein said vitreous substrate is glass.

10. An array according to claim 7, wherein said plurality of interconnected photovoltaic cells further comprises a plurality of first transparent electrically conductive film areas each formed on a different selected portion of one surface of said substrate member and electrically isolated from other first transparent conductive film areas formed on adjacent portions of said surface of said substrate member; a plurality of first semiconductor films each overlying and substantially covering all but an edge portion of a different respective one of said first transparent conductive film areas;
a plurality of second semiconductor films coextensively overlying and covering a different respective one of said first semiconductor films and forming a corresponding plurality of photovoltaic heterojunctions on said panel; and a plurality of second electrically-conductive material layers each disposed on and substantially covering a different respective one of said second semiconductor films and extending beyond said respective second semiconductor film into electrical contact with said edge portion of said first transparent conductive film area respective to the next adjacent second semiconductor film for electrically connecting said heterojunctions in series.

11. The array according to claim 10, wherein said first semiconductor films are electrically insulated from said second electrically-conductive material layers.

12. The array according to claim 10, wherein said plurality of photovoltaic cells occupy successive parallel transverse strips of said panel.

13. The array according to claim 10, 11 or 12, further including a bond-enhancing material interposed at each contact junction between a first transparent conductive film area and a second electrically-conductive material layer.

14. The array according to claim 7 or 10, wherein said transparent electrically conductive film is SnOx.

15. The array according to claim 11 or 12, wherein said first semiconductor film is a compound of an element selected from Groups II and VI of the Periodic Table.

16. The array according to claim 10, 11 or 12, wherein said first semiconductor film is CdS.

17. The array according to claim 10, 11 or 12, wherein said second semiconductor film is CuxS.

18. The array according to claim 10, 11 or 12, wherein each said second electrically conductive material layer forms a seal over the respective second semiconductor film.

19. The array according to claim 10, 11 or 12, wherein each said second electrically-conductive material layer is a composite layer comprising a first component layer of copper disposed over said second semiconductor film, and a second component layer of lead disposed coextensively over said layer of copper.

20. The array according to claim 10, 11 or 12, wherein each said second electrically-conductive material layer is a positive electrode and includes at least one component layer of electrically-conductive material capable both of exposure to environmental conditions without substantial degradation and of retarding the entry of environmental oxygen and water vapour to said second semiconductor film.

21. The array according to claim 10, 11 or 12, wherein each said second electrically-conductive material layer is a positive electrode and includes at least one component layer of electrically-conductive material capable both of exposure to environmental conditions without substantial degradation and of retarding the entry of environmental oxygen and water vapour to said second semiconductor film, and wherein said second electrically conductive material layer includes lead.

22. The array according to claim 8, wherein said CdS is next to said transparent electrically conductive film.

23. The method according to claim 4, wherein exposed edges of said films forming said plurality of photovoltaic cells are masked except that at least a portion of each exposed area of said first film is left still exposed, said layer of said different conductive material is applied over said plurality of photovoltaic cells and into electrical contact with said still exposed portions of said first film, and said layer of said different conductive material is separated into a plurality of conductors at the regions of separation between the cells in such manner as to connect every cell in series-connected electrical relationship with at least one adjacent cell.

24. The method according to claim 23, wherein before said layer of said different conductive material is applied, a quantity of preselected material is applied to each still exposed portion of said first film to promote bonding between said first film and said layer of said different conductive material when said electrical contact is made therebetween.

25. The method according to claim 1, wherein said step of applying said overlying layer of another different electrically-conductive material further comprises the step of:
applying a second insulating material over edges of said first and second films opposing said exposed areas of said first film.

26. The method according to claim 1, wherein said step of selectively removing portions of said first film and portions of said second film further comprises the steps of:
selectively removing a portion of said second film to expose at least a portion of said first film; and thereafter selectively removing a portion of said exposed first film to form a plurality of electrically isolated photovoltaic cells each having a remaining exposed area of said first film.

27. The array of photovoltaic cells according to claim 7 further including an adhesion-enhancing material formed at the junction between said transparent electrically conductive film and said solid conductive layer.