Photovoltaic Cells

It is known to form photovoltaic cells by coating on a hot sheet of Nesa glass, or glass previously coated with SnO.sub.x, a thin film of CdS, by spraying a water solution of compounds which form a layer of CdS microcrystals on the SnO.sub.x, and providing a Cu.sub.x S heterojunction as a layer on the CdS layer, and forming electrodes on the film of Cu.sub.x S. In accordance with out prior application, referred to hereinabove, the Cu.sub.x S layer was formed by spraying a Cu.sub.x S forming solution on the CdS film while the substrate was hot. In accordance with the present application, the Cu.sub.x S may instead be formed by dipping or by electroplating, or by a combination of both, at or near room temperature.

Photovoltaic cells have heretofore utilized relatively thick films of CdS, or have resorted to other expedients to obviate the difficulty that CdS films generally permit permeation by Cu.sub.x S is formed by dipping or electroplating, i.e., by ion exchange. This permeation provides short circuits between the Cu.sub.x S layer and the SnO.sub.x, the latter constituting the negative electrode of the cell, rendering the cell inoperative. In order to convert solar energy to electrical energy on a large scale, square miles of solar cells may be required. Since Cd is a rare and expensive metal, it becomes important to form photovoltaic cells with minimum quantities of CdS and hence with extremely thin CdS films. Moreover the cells must be reliably fabricated and have long life.

We have produced CdS photovoltaic cells in which the layer of CdS is about 2 to 4 microns thick, yet which show zero shorting permeation, the layers being highly impervious to Cu.sub.x S or Cu containing solution, in an ion exchange process. We have heretofore used the method of spraying a CdS forming solution on a glass coated with SnO.sub.x, intermittently and covering only a small portion of the glass at a given point in time, while maintaining the surface of the glass at uniform and constant temperature in the range between 500.degree. F and 1100.degree. F. According to the present invention, in one embodiment, multiple sprays are required to form the CdS layer. Each spray comprises a solution containing a cadmium compound and a sulphur containing compound. However, to one spray is added AlCl.sub.3 .multidot. 6H.sub.2 O, in proportions such that the Al content is 10 to 50 molar percent of the total metal ion content of the solution, and the quantity of sulphur containing compound is increased as required to combine with the Al. In a superposed spray solution, optionally no AlCl.sub.3 .multidot. 6H.sub.2 O is included, or a very much reduced quantity of AlCl.sub.3 .multidot. 6H.sub.2 O, such that the layer formed by the superposed spray contains little or no impregnated Al. Instead of applying two discrete films, one formed from a spray containing a large quantity of aluminum compound and the other containing little or none, a single film may be formed by gradually decreasing the aluminum compound content of the solution being sprayed in proceeding from the stratum of the film adjacent to the SnO.sub.x layer to the stratum of the film adjacent to the exposed surface of the CdS film. In this manner the stratum of the CdS film adjacent to the SnO.sub.x layer is impregnated with significantly greater quantities of Al than the stratum of the film adjacent to the exposed surface of the CdS film. The quantity of the sulphur containing compound is proportioned to the quantity of the aluminum compound used.

After heat treating at a temperature in the range 450.degree. C to 550.degree. C, it is found that the portion of the CdS layer in which Al is impregnated is extremely hard and highly adherent to the SnO.sub.x layer, so that it can only with difficulty be removed by application of acid or by scraping and is highly impervious to chemicals involved in forming a Cu.sub.x S layer by ion exchange, or to Cu.sub.x S, and inhibits diffusion of Cu through a CdS layer. The impregnation of Al in the CdS film in the relatively large quantities resulting from use of solutions containing Al in a quantity representing 10 to 50 molar percent of the total metal ion content of the solution does not constitute a doping procedure, such as disclosed in Middleton, et al., U.S. Pat. No. 3,411,050. Rather it comprises a new compound or material, or a new crystalline form, having properties quite distinct from those of CdS or CdS impregnated with only small amounts of Al. It has been found that if the entire film of CdS is heavily impregnated with Al the cell remains operative, but at reduced efficiency. In an alternative embodiment, hydrochloric acid is added to the solution utilized to form the CdS layer containing aluminum. Also disclosed is an alternative embodiment of the spray process wherein a spray comprised of essentially uniform droplets is utilized.

A method of fabricating a photovoltaic cell comprising spraying on a hot insulating transparent substrate coated with a transparent conducting film, a solution comprising a solvent, a cadmium salt, a sulphur containing compound and an aluminum containing soluble compound in heavy concentration. Thereafter a further solution comprising a cadmium salt and a sulphur containing compound, having little or no included aluminum may be sprayed onto the substrate. The aluminum concentration in the resulting CdS layer may be concentrated most heavily in the underside of the CdS layer, i.e., be dispersed throughout the CdS layer, but with a much heavier concentration at the bottom of the layer than occurs at the top, where no Al may occur or only little, or the Al may be dispersed equally throughout. The spray process is conducted while the sprayed surface of the substrate is maintained at a constant temperature in the range about 500.degree. F to 1100.degree. F. After the spray process is completed, the coated substrate is heated in an oxygen containing atmosphere to about 525.degree. C, then cooled to approximately room temperature. Thereafter, the exposed surface of the CdS film is converted to Cu.sub.x S, either by dipping, electroplating, or a combination of both to form a heterojunction. The exposed surface of the CdS film may also be converted to Cu.sub.x S by spraying thereon a suitable copper containing solution. Thereafter, an electrode is formed over the Cu.sub.x S layer.

The disclosure of the co-pending parent application is hereby imported by reference into this application, the process as there disclosed being applied to a glass substrate as the glass substrate travels along a tank containing a molten salt or a molten metal. The sheet of glass may be supported solely by the molten liquid or partially by the molten liquid and partially by extraneous supports, so that its bottom surface is immersed in the liquid and the liquid continuously supplies heat to the substrate. In the course of spraying the substrate with solution, the upper exposed surface of the substrate is cooled by the spray. It is then necessary to conduct the spraying operation intermittently, and over so small a portion of the substrate surface at any instant of time, that the substrate surface can acquire from the molten material enough heat to recover the temperature of the exposed surface of the substrate between the sprays applied to any area. This process provides a uniform layer thickness, and also facilitates maintenance of constant temperature, or maintains the temperature more nearly constant than is otherwise feasible.

Subject to the considerations stated in the immediately preceding paragraph, glass is assumed to be at approximately the temperature of the heated liquid, the temperature of which is such as to maintain the exposed surface of the glass at a temperature in the range 500.degree. F to 1100.degree. F, and it is assumed for the purpose of the present invention that the glass has been coated with SnO.sub.x in a thin transparent layer, as by the methods taught in our parent application. Spraying is advantageously accomplished under intense ultraviolet light, which is absorbed by CdS and enhances the internal energy of the CdS crystals.

In a first example of the invention, two solutions are prepared. The first solution may be in the proportion:

2 liters Water

60 cc: 1 Molar CdCl.sub.2 solution

74 cc: 1 Molar thiourea solution

1.95 gm: AlCl.sub.3 .multidot.6 H.sub.2 O

the second solution employed is in the proportion:

5 liters Water

150 cc: 1 Molar thiourea solution

150 cc: 1 Molar CdCl.sub.2 solution

2.5 cc: Hydrochloric acid

While thiourea is specified as a component, its function is to produce sulphur. Other compounds which are soluble in water, and which give up sulphur, can be substituted. Specifically NN DMthiourea has been employed, but thiourea is the least expensive compound which has been found satisfactory. The specific quantity of AlCl.sub.3 .multidot. 6 H.sub.2 O may be varied over a wide range, provided that the quantity of sulphur available is proportionally varied. The purpose of Al inclusion is to provide at least a stratum of CdS by combination of CdCl.sub.2 with thiourea which is heavily impregnated with an aluminum compound, the exact nature of which is unknown. The aluminum content of the solution may be varied from 10 to 50 molar percent of the total metal ion content of the solution. Even higher molar percentages of aluminum may be employed, but higher percentages have not been found to produce superior performance. The CdS film having Al impregnated therein formed with a solution having an Al content of from 10 to 50 molar percent of the total metal ion content of the solution is found to have properties quite different from CdS films containing no aluminum or relatively small amounts of aluminum. The layer is extremely hard, impervious to Cu.sub.x S or Cu, and is highly adherent to the SnO.sub.x. The reason for these physical properties is not known.

The second solution contains no aluminum, on the example cited, but in fact some aluminum may be included in the form of AlCl.sub.3 .multidot. 6 H.sub.2 O. Inclusion of HCl is optional, and for reasons unknown increases slightly the output of the cell, but so far as is known does not otherwise affect the operation of the cell.

Similar to EXAMPLE I, but with the solutions being differently comprised. The first solution may be in the proportion:

8 liters: Water

18.63 gm: CdCl.sub.2 .multidot. 21/2 H.sub.2 O

8.77 gm: Thiourea

6.96 gm: AlCl.sub.3 .multidot. 6 H.sub.2 O

2 cc: Hydrochloric acid

The second solution may be in the proportion:

4 liters: water

24.70 gm: CdCl.sub.2 .multidot. 21/2 H.sub.2 O

10.96 gm: Thiourea

A solution of water, CdCl.sub.2, thiourea and AlCl.sub.3 .multidot. 6 H.sub.2 O is sprayed on glass coated with SnO.sub.x, but as the spraying proceeds the proportion of AlCl.sub.3 .multidot. 6 H.sub.2 O to CdCl.sub.2 is gradually decreased, for example, logarithmically. For example, as much as 50 molar percent aluminum may be present in the solution forming the lowermost part of the CdS film, and zero or substantially zero molar percent aluminum in the solution forming the upper surface of the film. As the quantity of AlCl.sub.3 is varied, the quantity of thiourea is varied proportionately, to satisfy both the Cd and the Al in forming the final compounds.

In all cases, the preferred end result is that directly in contact with the SnO.sub.x is a layer of CdS heavily impregnated with aluminum and at the upper surface of the layer of CdS, which is to be converted to Cu.sub.x S to form a heterojunction, there is no aluminum or very little aluminum. Two distinct layers may be employed, or a decreasing percentage of aluminum in proceeding from bottom to top of the layer.

The solutions are sprayed on the glass intermittently and slowly in successive passes over a considerable period of time, of the order of 10-40 minutes for the first coating and 10-40 minutes for the second coating, in Examples I and IA, and in the order of 20-60 minutes in the case of the graduated layer, and intermittently and over only a small portion of the glass at any one instant so that the glass sheet always remains at the same average temperature during the spraying, despite the heat removed from the glass in the spraying process. The total thickness of the layer formed in this way is between about 2 to 4 microns, or less. The coated plate is, after spraying is completed, heated to a temperature of about 450.degree. C to 550.degree. C for approximately 15 minutes.

It should be noted that although the preferred method of forming a CdS film which is impregnated with Al is via the spray method above described, other methods of forming such a film may be employed. Thus, the CdS film, which is impregnated with Al, may be formed by vacuum deposition, dipping, electroplating, or any other suitable means by which films are deposited on a substrate, as known in the art. Although we are of the belief that the best such films are formed using the spray method, these other well-known methods of forming a film may also be utilized.

After the CdS layer has been formed, the bath is slowly cooled, and the coated product may then be removed.

To complete a photovoltaic cell, the exposed surface of the CdS layer is converted to Cu.sub.x S by dipping the previously cooled cell into an appropriate solution at room temperature, by electroplating, by a combination of dipping and electroplating, or by spraying thereon a suitable copper containing solution.

The solution employed to form a Cu.sub.x S layer by dipping may be in proportions as follows:

100 cc : Water

7 cc : HNO.sub.3 (5-1)

1.5 gm : (L+)-Tartaric Acid

1.5 gm : CuCl

1.5 gm : H.sub.4 Ce(SO.sub.4).sub.4

1.5 gm : NaCl

The tartaric acid is a typical organic acid and for it may be substituted citric or lactic acid. The presence of H.sub.4 Ce(SO.sub.4).sub.4 appears to add an increment of about 50 millivolts of voltage to the final cell, but the manner in which it functions in the process is not understood. For NaCl may be substituted another halide, as NH.sub.4 Cl. The HNO.sub.3 is optional.

Another solution which may be utilized in the dipping process is:

700 cc : Water

7 gm : Citric acid monohydrate

30 gm : NH.sub.4 Cl

3.5 gm : CuCl

As with the EXAMPLE I solution, another halide may be substituted for NH.sub.4 Cl.

A preferred solution employed in electroplating to form Cu.sub.x S is:

700 cc : Water

3.9 gm : Citric acid monohydrate

2.0 gm : NH.sub.4 Cl

7.5 gm : Cupric acetate pentahydrate

The NH.sub.4 Cl may be omitted, but seems to improve the quality of the cells by a small factor. Plating is accomplished at a current density of 0.5 ma per cm.sup.2 for 2-5 minutes, with the CdS layer negative with respect to a copper anode.

700 cc : Water

10 gm : Cupric acetate pentahydrate

15 cc : Acetic acid (20%)

700 cc : Water

7 gm : Citric acid monohydrate

1.2 gm : NH.sub.4 Cl

5 gm : Cupric acetate pentahydrate

As indicated above, a combination of dipping and electroplating may be used in forming the Cu.sub.x S layer. Alternatively, a suitable solution may be sprayed onto the CdS film to form the Cu.sub.x S layer.

After the Cu.sub.x S formation process is completed, a deposit of copper may be applied over the Cu.sub.x S to a thickness of about 7,000 A and aluminum or another suitable metal is then coated over the copper to protect the copper from oxidation and from the effects of moisture and airborne impurities.

The present method can be employed with Nesa glass as the starting material, obviating the need for coating with SnO.sub.x, but Nesa glass has a sheet resistivity per square of about 50-75 ohms per square, whereas by our methods very low resistivity coatings may be produced, i.e., of the order of 1-20 ohms per square. The use of low resistivity coatings of SnO.sub.x increases the efficiency of the cell by decreasing the amount of energy which is lost in the SnO.sub.x.

Reference has been made hereinabove to the use of very intense ultraviolet light while spraying the cadmium sulphide forming solution. The precise intensity of the irradiation has not been measured, but four 150 watt ultraviolet lamps have been placed within a few inches of a small surface being sprayed, and the ultraviolet light found to be advantageous. If there is no irradiation during spraying, the cells sometimes have relatively lower outputs than occurs in the presence of intense ultraviolet light.

It also appears to be important to the process that the spraying occur by depositing droplets which are as uniform as possible. If the spray consists of many small and many large droplets, the very small droplets are evaporated by the intense heat, approximate to the exposed surface of the substrate, and only the larger droplets reach the substrate. This causes some wastage of CdS and it implies that the rate at which the spray is applied must take into account the nonuniformity of the droplets.

In accordance with the method of our parent application, supra, the glass substrate will be moving longitudinally along a trough or tank containing molten material, and the spray will occur by transverse passes across the substrate as the glass moves, so that the total quantity of spray reaching any given small area of the substrate will be uniform. The method allows for the fact that spray out of an air gun or out of an electrostatic spray gun does not have a uniform pattern.

After a cell is completed, including electrodes, it is heated to 400.degree. F to 500.degree. F.

The present cell is to be exposed to solar radiation via its glass substrate. The presence of Al in the CdS does not materially affect transparency of the CdS-Al layer, so that the heterojunction may be exposed via the latter.

It should be understood that various changes, modifications, and variations in the structure and function of the present invention may be effected without departing from the spirit and scope of the present invention as defined in the appended claims.