GB1575888A

GB1575888A - Solar cell array

Info

Publication number: GB1575888A
Application number: GB22915/78A
Authority: GB
Original assignee: Photon Power Inc
Current assignee: Photon Power Inc
Priority date: 1977-09-08
Filing date: 1978-05-26
Publication date: 1980-10-01
Also published as: GR63166B; BR7805816A; IT7850646A0; IL55165A; YU213178A; OA06048A; NL186209C; IT1105538B; DE2839038C2; FR2405557B1; IL55165A0; AU517645B2; AU3899878A; ZA783886B; DE2839038A1; IE781675L; ES473061A1; JPS5441686A; EG13954A; JPS6146993B2

Description

(54) SOLAR CELL ARRAY (71) WE, PHOTON POWER INC., a corporation organised and existing under the laws of the State of Delaware, of 10767 Gateway West, El Paso, Texas 79935, United States of America, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement:- This invention relates generally to large area photovoltaic cells which can be produced and interconnected for large scale terrestrial use and, more particularly, to a photovoltaic panel which is formed using mass production techniques, such as spray application of layers, and thereafter formed into an array of series-connected solar cells and wherein the individual cells are formed by film removal apparatus.

The search for alternative 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 alternative sources of energy which might be tapped but for technological and/or cost considerations. Solar energy is one source which is being extensively examined due to its abundance and to an apparent absence of environmentally 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 techical problems which are currently under investigation deal with reducing this basic technology to a practice which is applicable to the production of such photovoltaic cells at a cost which is competitive with that required to construct and operate presentday power generating facilities utilizing such energy sources as oil, coal, or nuclear fission. To accomplish this goal, it is apparent that electrical generating stations 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 massproduced photovoltaic cells. In accordance with the techniques of the present invention, large area photovoltaic panels can be formed using mass production-type techniques and each can thereafter be formed into an array of series-connected photovoltaic cells in a process suited to mass production and in a size to generate commercial quantities of electrical energy.

The production of first-generation photovoltaic cells required that a single crystal of silicon or cadmium sulphide 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 discrete elements bonded together to form the completed cell. This production operation, in itself, was expensive 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, polycrystalline materials have been developed which are suitable for use in forming photovoltaic cells which are considerably 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 sulphide has been found to be a particularly suitable compound which may be formed from numerous chemical compounds containing cadmium and sulphur and applied to a substrate in a variety of processes to interact and form a layer of cadmium sulphide which exhibits semi conductor properties.

A completed photovoltaic cell which is well known in the art includes a layer of polycrystalline cadmium sulphide (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 sul phide, Cuts, where x is less than 2 for non-stoichiometric cuprous sulphide formed over the CdS. The technology to mass produce photovoltaic cells which in corporate the CdS - CuxS heterojunction is rapidly developing and is not, per se, a subject of the instant invention.

To provide for large scale terrestrial application, the individual photovoltaic cells must be formed into an interconnected array covering large areas. Typically, a single CdS - CuxS heterojunction 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 indi vidual cells may be connected in a series arrangement to produce output voltages of 12-24 volts, i.e., output voltages which are equivalent to voltages produced by present day storage batteries.

Early attempts to provide photovoltaic arrays generally consisted of taking indi vidual photovoltaic cells, adhering those cells to a common substrate, and then in terconnecting the photovoltaic cells with wire conductors to complete the array. U.S.

Patent No. 3,411,050 is illustrative of such prior art. These photovoltaic arrays were custom fabricated and were expensive to produce. The requirement to provide physi cal connections for large numbers of con ductor wires further reduced the availability of surface area for active photovoltaic power generation and thereby reduced the overall efficiency of the photovoltaic array.

The availablity of polycrystalline CdS as a component in a photovoltaic cell has greatly increased the capability of forming a series-connected array of such cells. U.S.

Patents No. 3,483,038, No. 3,571,915 and No. 3,713,893 are typical of recent prior art attempts to provide a solar cell array.

In these prior art arrays, the polycrystalline cadmium sulphide layer is formed by mask ing and vacuum-evaporating cadmium sul phide onto the surface of a suitable sub strate, which is generally a flexible plastics or metallic foil, and then vacuum evaporat ing or depositing a slurry to produce a cup rous sulphide layer over the cadmium sul phide and form the heterojunction. It may be appreciated that this method is time consuming 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 plastics substrate materials require that a low-temperature process, such as vacuum deposition, be used to form the required layers, since the plastics cannot be subjected to high temperatures.

Further, the photovoltaic arrays taught by the above references generally utilize front wall-type solar cells, wherein solar radiation is directly incident 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 gridlike pattern in order to admit light through to the heterojunction. The use of the gridlike electrode subjects the CuxS layer to possible degradation during application of the grid or during subsequent exposure of the CuxS to the environment. In some fabrication techniques, the grid is affixed to the cell by an adhesive, whereby oxidation of the CuxS tends to occur when the adhesive is cured. Also, exposure of the CuxS to the oxygen and water vapour in the air can degrade the material during normal cell operation.

In addition to the inefficiencies inherent in a front wall-type solar cell due to using a grid, i.e., covering a portion of the active heterojunction area, and possible degradation of the heterojunction, a front wall-type solar cell has an inherent optical mismatch.

The indices of refraction of cuprous sulphide and cadmium sulphide are 3-3.5 and about 2.5 respectively. Accordingly, light incident on the heterojunction at angles greater than the critical angle for the CuxS - CdS interface, 35 to 55" depending 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 sulphide 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 index of refraction around 1.50.

An evolving technique for producing photovoltaic cells with polycrystalline 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. 3,880,633 and No. 3,902,920 to Jordan et al, disclose suitable techniques for forming large area backwall 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 sulphide, and perhaps cuprous sulphide. 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 photo voltaic 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 reduced or overcome by the present invention, and improved methods are provided for obtaining an array of photovoltaic cells cdnnected in series. Further, an improved array of series-connected photovoltaic cells on a common substrate is provided.

According to the present invention, there is provided 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 thereafter applying an overlying layer of another different electrically-Conductive material on to said cells and substantially over the entirety of said selected surface of said substrate and separating said overlying different conductive material into individual conductors in such manner as to interconnect said cells into an electrically connected array.

The invention further provides an interconnected array of photovoltaic cells forming a photovoltaic panel, comprising: a rigid transparent vitreous substrate member, and a plurality of photovoltaic cells occupying different areas of one surface of said substrate member and comprised of superposed electrically-conductive and semiconductor film layers on said substrate member which have been separated into individual areas corresponding to said plurality of cells, said cells being functionally responsive only to solar rays traversing said substrate member, and wherein said photovoltaic cells are integrally series-connected by electrically conductive interconnections formed from electrically conductive component film layers of said photovoltaic cells.

By "integrally connected" is meant that the electrical-interconnections between the cells are provided by portions of the elect Finally conductive layers applied to the substrate in forming :the cells and not by sep rat ly-applied or additional conductors.

-In a preferred technivque embodying the invention, a large area a photovoltaic cell is first produced by forming layered films over substantially an entire surface of a transparent substrate. Portions of the films are thereafter selectively removed to form a plurality of smaller photovoltaic cells.

Finally, a conducting material is applied to interconnect the photovoltaic cells into an array. This conducting material contacting the heterojunction seals and protects the underlying materials while interconnecting the cells.

The array of photovoltaic cells can be produced on a transparent vitreous substrate, such as glass. A back-wall photovoltaic cell array is thus provided which can be formed by using a spray process to produce a large area photovoltaic cell and then partially 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.

Thus, it is preferred to produce a seriesconnected photovoltaic array by first forming the various films which form the photovoltaic cell heterojunction over the entire substrate and thereafter removing selected portions of the films to form the array. By substantially covering the entire panel with each material layer, one obviates the need for elaborate masking techniques, and by providing a photovoltaic cell array in which all the photovoltaic cells are formed on a glass substrate, back-wall illumination is achieved.

It is aimed to provide a photovoltaic array in which the film area which must be removed to form the plurality of individual photovoltaic cells, and which is not therefore available as an active power generating area, is greatly reduced. The electricallyconducting film on the substrate and the CdS film can be formed by means of a spray technique covering the entire panel and portions of the sprayed-on films thereafter removed to form individual photovoltaic cells.

Portions of the layered films which form the large area photovoltaic cell can be removed by cutting through the films.

One may also produce a series-connected array of photovoltaic cells by first forming a plurality of electrically-isolated conductive films on a substrate panel and then applying at least one semiconductor film over the entire substrate panel to cover the plurality of conductive films, selected portions of the semiconductor film thereafter being cut through to form the plurality of photovoltaic cells.

By these means is achieved an advan tageous apparatus for generating electrical energy, comprising a rigid transparent vitreous- substrate member, and a -plurality of interconnected photovoltaic cells formed on and functionally responsive to solar rays traversing the substrate member.

The solar cell panels obtained can be conveniently interconnected into a power generating array to cover an extremely large area and provide for large scale production of electrical power.

Particular techniques according to the invention, given by way of example, will be understood from the following detailed description, wherein reference is made to the accompanying drawings, in which: Figures 1 and 1A are cross sections of photovoltaic panels on which the basic photovoltaic layers have been applied by alternative processes.

Figures 2 and 2A are cross-sectional views of the photovoltaic panels from each of which the film material has been removed to form a plurality of photovoltaic cells.

Figures 3 and 3A are cross-sectional views of the photovoltaic panels each prepared to receive an overlying conductive coating.

Figures 4 and 4A are cross-sectional views of the photovoltaic panels over which further electrically conductive layers have been applied.

Figures 5 and 5A are cross-sectional views of the photovoltaic panels of series-connected photovoltaic cells sealed from the environment.

Figures 6, 6A and 6B illustrate formation of the series connection by a slicing 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 showing the electrode configurations at the photovoltaic panel ends.

Referring now to the drawings and first to Figures 1-5, there may be seen crosssectional views, illustrating a preferred method for forming an interconnected solar cell array, where the negative electrode layer is formed over the entire panel, and is separated into electrode areas electrically isolated from adjacent negative electrode area after the overlying heterojunctionforming films have been applied. Figures IA-SA illflstrate an alternative method where the negative electrode is separated into a plurality of negative electrode areas prior to forming the overlying films.

Referring now to Figures 1-5 and first to Figure 1, there may be seen a cross section of a substrate panel 10 coated with layered films of SnOx 12, CdS 14 and Cu,S S 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 large 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 formed into a plurality of photovoltaic cells, as shown in Figure 2. The CuxS film 22 and CdS film 14 are removed from above a portion of the SnOx film 12 to expose a selected pattern of the SnOx film surface 16. In one embodiment, 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 to be formed over the SnOx and needed to form the electrical interconnections. Films 22 and 14 may be removed by any 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 SnO film 12 is a hard, tightly adherent film and cannot be as readily removed by mechanical processes as the CdS 14 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 adjacent photovoltaic units. A preferred technique for vaporising 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 focussed laser beam concentrated so as to vaporize the small area of SnOx to be removed. Further, it is possible to remove a selected portion of SnO to form gap 13 by masking and chemical etching methods which are conventionally employed in fabricating semiconductor devices, such as illustrated by U.S. Patent No. 4,009,601 to Simon.

Once a plurality of photovoltaic cells has been formed and electrically isolated, one from the other, the units must 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 conducting materials which are to be applied. The exposed edges of semiconducting layers 14 and 22 are first coated with suitable elect rically-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 strip 24 is formed along the edge of the layers which is immediately adjacent the exposed strip 16 of Snowy. A second insulating strip 26 is formed over the exposed edges of the semiconducting layers of the adjacent photovoltaic unit and to completely fill gap 13. Insulating strips 24 and 26 may be formed from the same material or from different materials where needed, as hereinbelow discussed.

Insulating strips 24 and 26 may be formed from a variety of materials to which the semiconductor 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, polyvinyl chloride films, acrylic paint, and cellulose film formers.

Where insulating strip 24 is to be removed, the strip 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 ]arge 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 a "bonding strip" 28 on the surface of the SnO, strip 16. The strip 28 may be applied 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 strip 28 is determined by the actual overlying conductor material which is applied. In one embodiment, a rotating brass wheel is used to deposit a small amount of brass directly on the exposed SnO 16 by 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 bonding 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 materials and this may conveniently be accomplished by vacuum-evaporating one or more conductive materials over the surface. 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 bonding strip 28. Finally, a layer of lead 32 may be applied over the layer of copper 30 to further provide 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 of a few angstroms thick may be required at the junction to prevent direct contact between the lead and copper. A suitable physical barrier may be formed from oxidized copper, iron or Inconel (Registered Trade Mark).

In one aspect, the layer of lead serves to protect the CuxS layer from degradation and prolong the life of the photovoltaic heterojunction. Normally, cuprous sulphide is quite susceptible to degradation in the presence of oxygen and water, such as would occur if the layer were exposed to the atmosphere for front wall-type operation. Transparent conductors have not been available to cover the cuprous sulphide layer and protect the layer. Thus, grid-like electrode configurations have been required with a further covering needed to seal the cell. Th e back wall-type photovoltaic cell which is the subject of the present description - does not require illumination of the cuprous sulphide layer so a solid electrode may be used which may also seal and protect the cuprous sulphide layer.

It has been found that multi-layer conductors of copper and lead provide many advantages. The copper adheres well to the cuprous sulphide and also helps to maintain the stoichiometry of the cuprous sulphide. However, copper alone is somewhat permeable to oxygen and water vapour. 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 Cuts.

Referring now to Figure 5 there may be seen a cross-sectional view of a completed panel of photovoltaic 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 may be improved by means of bonding strip 28. Conducting 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 CxS layer forms the positive 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 detrimental environmental conditions.

It will be appreciated from the above discussion that the entire operation for forming the series connected photovoltaic units is one which is well adapted to a mass production process. The steps of forming the individual photovoltaic units, applying the insulating strips and the bonding strip may all be done by a suitable machine making a single pass across the surface of the coated substrate. If desired, a plurality 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 forming the metallic conducting layers 30 and 32 by vacuum evaporation can be readily accomplished on a production basis, although it is more expensive than the spray methods for forming the other films. As hereinbelow discussed, a variety of techniques are available for selectively removing portions of the overlying conductor films 30 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 strip 24 (shown in Figure 4) which is removable by means of ultrasonic vibrations whereupon the overlying conductor layers 30 and 32 are deprived of their structural backing and are also removed by the ultrasonic vibrations. Insulating strip 26 is chosen to maintain integrity at the ultrasonic frequency at which strip 24 is removed. Thus, selected portions of the conductive films 30 and 32 are removed to obtain the desired electrical interconnection.

Referring now to Figures lA-SA, there may be seen a cross-section of a substrate panel 10 where the SnO 12 areas are already electrically isolated from one another. This condition might occur if a defective panel is being reprocessed or if it is desired to begin the CdS coating with the SnOx already removed. Removal of the SnOx to form the isolated electrode areas may be accomplished as hereinabove discussed for the step- illustrated by Figure 2. Because of - the progressive nature of the temperatures -used in t forming - a- photovoftaic panel by the spray - technique, it is desirable to remove the SnOx without having to cool the entire panel to room temperature and subsequently reheat. In such a case, a preferred method would use the low voltage probe technique to effect 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 removed portion of CuxS film 22 and CdS film 14 is superposed above th moved to produce a plurality of photovoltaic cells on substrate 10. Insulating strips 24 and 26 are applied as discussed hereinabove for Figure 3 except that the applicator pens apply a larger volume of insulating strip 24 whereby insulating strip 24 is formed to an elevation substantially greater than insulating strip 26. The difference in elevation between insulating strips 24 and 26 should be such that the top portion of insulating 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 interconnection between adjacent photovoltaic cells. One advantage to this technique is that the desired interconnection is accomplished by merely passing the completed 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 plurality of photovoltaic cells comprising SnO, layer 12, CdS layer 14 and CuxS 22 have been formed on substrate 10 according to the methods hereinabove discussed for Figures 1 and 2, As shown in Figure 7, insulating 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 hereinbelow discussed. As shown in Figure 7A, the conductor layers 30 and 32 have again been formed over the entire surface of substrate panel 10 and, in par titular, 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 overlying conductor layers 30 and 32 to isolate the photovoltaic units and form the series connection, as shown in Figure 7B. Figure 7B illustrates an isolation region 46 where insulation strip 24 has been removed, but the insulating material 24 may also be left in place, if desired In a preferred embodiment, substrate panel 52 (Figure 8) is a transparent vit reous - 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 photovoltaic cells 54 occupy successive parallel transverse strips of the panel 52. 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, theglass substrate needs to be heated to a high temperature only once and thereafter only reduced to lower temperatures. Production time is not consumed in having to repeatedly heat and cool the glass at prescribed rates to prevent excessive strains from developing. Further, glass is heatresistant and can withstand the relatively high. temperatures to produce the tin oxide and cadmium sulphide 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 necessary 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 removed to insure the removal has been satisfactory to electrically isolate the photovoltaic units. It is a particular feature of the back wall array that each photovoltaic cell can be individually checked upon completing the array to particularly identify any defective 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 has been formed in order to remove those portions which may be still electrically connected due to incomplete removal of overlying conducting layers.

It is now apparent that the photovoltaic panel, hereinabove described, is one well suited to providing a low cost photovoltaic cell suitable for large scale production of electrical power. Each photoyoltaic panel covers a large area and is capable of handling such amounts of current whereby large quantities of power may be obtained at relatively low DC voltages of 18 - 24 volts.

The internal resistance of the photovoltaic units is miriimized by forming the SnOy layer in - accordance with U.S. Patent No.

3,880,633 wherein a process for forming à very low resistance SnO film is disclosed.

The tin oxide layer produced according 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 without producing unacceptable internal power losses within each cell.

Other advantages of this solar cell array include forming the large area photovoltaic cell in mass production, where spraying techniques are used to produce the plurality of layers forming the photovoltaic cells over the supporting 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, and the overlying conductors are formed as substantially continuous layers whereby a low resistance is obtained. Finally, the glass substrate inherently seals the radiation incident surface without restricting light admittance and the conductors seal the heterojunction surfaces to produce a panel which is substantially protected from atmospheric effects. It is expected that final packaging may provide a final sealant for the exposed edges of the photovoltaic cells and a backing for physical protection, but no special packaging and sealing is otherwise required.

Referring now to Figures 9 and 9A, there are depicted the terminal regions of the completed photovoltaic panel 50 comprising the positive terminal 60 shown in Figure 9 and the negative terminal 62 shown in Figure 9A. Referring first to the positive terminal 60 shown in Figure 9, a conductor 61 is placed on the conductor layer 32. In a rudimentary embodiment, conductor 61 is a soldered 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 such as to maintain the current densities within the conductor strip at acceptably low levels to minimize resistance heating and energy losses. The material chosen to contact the conductor layer 32 is selected to provide a work function compatible with the conductor layer for 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 strip 63 may again be provided by any suitable material, such as indium solder, as hereinabove discussed. Terminal 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 overlying semiconductor and conductor layers, as hereinabove discussed for the steps for forming the series connection.

While the final means for supporting and interconnecting photovoltaic panel 52 into an overall network for generating commercial quantities of electrical energy is not the subject to which the present invention is directed, 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 clamping 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 electrical connection with a Cu,S layer and that the negative terminal 62 be formed in contact with an SnOx layer and insulated from contact with the layers overlying the SnO WHAT WE CLAIM IS: 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 thereafter applying an overlying layer of another different electrically-conductive material on to said cells and substantially over the entirety of said selected surface of said substrate and separating said overlying different conductive material into individual conductors in such manner as to interconnect said cells into an electrically connected 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.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

The tin oxide layer produced according 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 without producing unacceptable internal power losses within each cell.

Other advantages of this solar cell array include forming the large area photovoltaic cell in mass production, where spraying techniques are used to produce the plurality of layers forming the photovoltaic cells over the supporting 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, and the overlying conductors are formed as substantially continuous layers whereby a low resistance is obtained. Finally, the glass substrate inherently seals the radiation incident surface without restricting light admittance and the conductors seal the heterojunction surfaces to produce a panel which is substantially protected from atmospheric effects. It is expected that final packaging may provide a final sealant for the exposed edges of the photovoltaic cells and a backing for physical protection, but no special packaging and sealing is otherwise required.

Referring now to Figures 9 and 9A, there are depicted the terminal regions of the completed photovoltaic panel 50 comprising the positive terminal 60 shown in Figure 9 and the negative terminal 62 shown in Figure 9A. Referring first to the positive terminal 60 shown in Figure 9, a conductor 61 is placed on the conductor layer 32. In a rudimentary embodiment, conductor 61 is a soldered 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 such as to maintain the current densities within the conductor strip at acceptably low levels to minimize resistance heating and energy losses. The material chosen to contact the conductor layer 32 is selected to provide a work function compatible with the conductor layer for 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 strip 63 may again be provided by any suitable material, such as indium solder, as hereinabove discussed. Terminal 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 overlying semiconductor and conductor layers, as hereinabove discussed for the steps for forming the series connection.

While the final means for supporting and interconnecting photovoltaic panel 52 into an overall network for generating commercial quantities of electrical energy is not the subject to which the present invention is directed, 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 clamping 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 electrical connection with a Cu,S layer and that the negative terminal 62 be formed in contact with an SnOx layer and insulated from contact with the layers overlying the SnO WHAT WE CLAIM IS: 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 thereafter applying an overlying layer of another different electrically-conductive material on to said cells and substantially over the entirety of said selected surface of said substrate and separating said overlying different conductive material into individual conductors in such manner as to interconnect said cells into an electrically connected 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 or 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.
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, and a plurality of photovoltaic cells occupy- ing different areas of one surface of said substrate member and comprised of superposed electrically-conductive and semiconductor film layers on said substrate member which have been separated into individual areas corresponding to said plurality of cells, said cells being functionally responsive only to solar rays traversing said substrate member, and wherein said photovoltaic cells are integrally series-connected by electrically conductive interconnections formed from electrically conductive component film layers of said photovoltaic cells.
8. The array according to claim 7, comprising a transparent electrically conductive film of material interposed between said substrate member and said semiconductor film layers of said photovoltaic cells, said transparent conductive film being separated into individual areas corresponding to said plurality of cells.
9. The array according to claim 7 or claim 8, wherein said cells are electrically connected in series array on said substrate member.
10. The array according to any one of claims 7 to 9, wherein said photovoltaic cells each include a CdS - CuxS heterounction.
11. The array according to claims 8 and 10, wherein said CdS is next to said transparent electrically conductive film.
12. The array according to any one of claims 7 to 11, wherein said vitreous substrate is glass.
13. An array according to any one of claims 7 to 12, wherein said plurality of interconnected photovoltaic cells 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 each 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.
14. The array according to claim 13, wherein said first semiconductor films are electrically insulated from said second electrically-conductive material layers.
15. The array according to claim 13 or claim 14, wherein said plurality of photovoltaic cells occupy successive parallel transverse strips of said panel.
16. The array according to any one of claims 13 to 15, further including a bondenhancing material interposed at each contact junction between a first transparent conductive film area and a second electrically-conductive material layer.
17. The array according to any one of claims 8, 11, or 13 to 16, wherein said transparent electrically conductive film is Snow.
18. The array according to any one of claims 13 to 17, wherein said first semiconductor film is a compound of an element selected from Groups II and VI of the Periodic Table.
19. The array according to claim 18, wherein said compound is CdS.
20. The array according to any one claims 13 to 19, wherein said second semiconductor film is Cuts.
21. The array according to any one of claims 13 to 20, wherein each said second electrically-conductive material layer forms a seal over the respective second semiconductor film.
22. The array according to any one of claims 13 to 21, wherein each said second electrically-conductive material layer is a composite layer comprising: a first component layer of copper disposed over said second semi-conductor film, and a second component layer of lead disposed coextensively over said layer of copper.
23. The array according to any one of claims 13 to 21, 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.
24. The array according to claim 23, wherein said second electrically-conductive material layer includes lead.
25. A method of forming a seriesconnected array of photovoltaic cells on a common substrate, substantially as described herein with reference to Figures 1 to 5, or Figures 1A to SA, or Figures 6 to 6B, or Figures 7 to 7B, of the accompanying drawings.
26. An interconnected array of photovoltaic cells forming a photovoltaic panel, substantially as described with reference to Figures 5, 8, 9 and 9A or Figures 6B, 8, 9 and 9A or Figures 7B, 8, 9 and 9A of the accompanying drawings.