Cadmium sulfide film
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of forming polycrystalline films, and more particularly to formation of polycrystalline films comprised essentially of cadmium sulfide. 
     2. Description of the Prior Art 
     Crystalline films of cadmium sulfide are useful in the formation of photovoltaic cells, among other uses. Prior art methods of forming polycrystalline films of cadmium sulfide typically utilize cadmium chloride as the cadmium salt constituent of the crystal forming solution. For example, in Jordan, et al., U.S. Pat. No. 3,880,633, a spray method for forming a cadmium sulfide photovoltaic cell is disclosed, utilizing a solution comprising cadmium chloride and thiourea. 
     Although it can be beneficial to utilize cadmium salts other than cadmium chloride in the formation of cadmium sulfide films, such as cadmium iodide, cadmium bromide, cadmium acetate, cadmium nitrate, etc., the prior art fails to teach a method whereby a uniform, continuous cadmium sulfide film can be formed utilizing such cadmium salts. 
     The present invention overcomes this difficulty. The method of the present invention teaches inclusion of a chlorine containing compound in the solution utilized to form the cadmium sulfide film to catalyze the film forming reaction. This method allows use of cadmium salts other than cadmium chloride. 
     SUMMARY OF THE INVENTION 
     A method of forming a film comprising essentially cadmium sulfide from a solution containing a cadmium salt and a sulfur containing compound in accordance with the present invention comprises inclusion in the film forming solution of a chlorine containing compound to catalyze the reaction between the cadmium salt and the sulfur containing compound. 
     Additional compounds may be included in the solution, which may further facilitate the formation and affect the characteristics of the cadmium sulfide film. 
     Thus, it is the primary object of the present invention to provide a method for formation of cadmium sulfide films from solutions containing cadmium salts heretofore not capable of forming uniform, continuous cadmium sulfide films. 
     Another object of the present invention is to provide a method for forming cadmium sulfide films having some characteristics different from cadmium sulfide films heretofore known. 
     Yet a further object of the present invention is to provide novel solutions for forming cadmium sulfide films. 
     These and other objects, advantages, and features of the subject invention will hereinafter appear, and for the purposes of illustration, but not of limitation, exemplary embodiments of the present invention are described in the following detailed description. 
     DETAILED DESCRIPTION 
     We have found that by including a chlorine containing compound in a solution or composition utilized to form a polycrystalline film comprised essentially of cadmium sulfide, film growth is facilitated and film quality improved. Moreover, such a chlorine compound is believed to catalyze the reaction between the cadmium salt and sulfur containing compound so as to allow use of cadmium salts heretofore thought incapable of forming a uniform, continuous cadmium sulfide film. Examples of these other cadmium salts are cadmium iodide, cadmium bromide, cadmium acetate, and cadmium nitrate. 
     Although we believe that most chlorine containing compounds may be utilized to catalyze the reaction, especially those which disassociate freely in solution, we have found that hydrochloric acid and ammonium chloride provide the best results. By way of example, and not of limitation, the following exemplary solutions have been found to successfully form uniform, continuous cadmium sulfide films when utilized in a spray process, such as described in U.S. Pat. No. 3,880,633, the disclosure of which is incorporated herein by reference. 
    
    
     EXAMPLE 1 
     A solution comprising: 
     22.08 gm -- Cadmium Acetate [Cd(Ch 3  COO)2.2H 2  O] 
     6.58 gm -- Thiourea 
     8.24 gm -- Ammonium Chloride [NH 4  Cl] 
     6.0 l -- Water 
     will form a uniform, continuous cadmium sulfide film. However, a solution having the same constituents in the same proportions, but without ammonium chloride, will not form such a film. 
     EXAMPLE 1-A 
     16.30 gm -- Cadmium Acetate [Cd(CH 3  COO) 2 .2H 2  O] 
     6.58 gm -- Thiourea 
     8.24 gm -- Ammonium chloride [NH 4  Cl] 
     6.58 gm -- Aluminum chloride [AlCl 3 .6H 2  O] 
     6.0 l -- Water 
     This solution is essentially the same as the Example 1 solution, with the addition of aluminum chloride. The aluminum chloride is added to alter the physical properties of the cadmium sulfide film, in the manner and for the reasons disclosed in U.S. application Ser. No. 631,815. 
     EXAMPLE 1-B 
     29.45 gm -- Cadmium Acetate [Cd(CH 3  COO) 2 .2H 2  O] 
     6.94 gm -- Thiourea 
     14.0 ml -- HCl (concentrated) 
     6.0 l -- Water 
     This solution, which is also essentially identical to the Example 1 solution, utilizes hydrochloric acid as the catalyzing chlorine containing compound, instead of ammonium chloride. 
     EXAMPLE 2 
     30.36 gm -- Cadmium Iodide [CdI 2  ] 
     6.58 gm -- Thiourea 
     14.0 ml -- HCl (concentrated) 
     6.0 l -- Water 
     EXAMPLE 2-A 
     30.36 gm -- Cadmium Iodide [CdI 2  ] 
     6.58 gm -- Thiourea 
     8.24 gm -- Ammonium Chloride [NH 4  Cl] 
     6.0 l -- Water 
     EXAMPLE 3 
     21.2 gm -- Cadmium Sulfate [3CdSO 4 .8H 2  O] 
     6.58 gm -- Thiourea 
     14.0 ml -- HCl (concentrated) 
     6.0 l -- Water 
     EXAMPLE 3-A 
     21.2 gm -- Cadmium Sulfate [3CdSO 4 .8H 2  O] 
     6.58 gm -- Thiourea 
     8.24 gm -- Ammonium Chloride [NH 4  Cl] 
     6.0 l -- Water 
     EXAMPLE 4 
     25.57 gm -- Cadmium Nitrate [Cd(NO 3 ) 2 .4H 2  O] 
     6.58 gm -- Thiourea 
     14.0 ml --HCl (concentrated) 
     6.0 l -- Water 
     EXAMPLE 5 
     37.24 gm -- Cadmium Bromide [CdBr 2 .4H 2  O] 
     10.96 gm -- Thiourea 
     14.0 ml -- HCl (concentrated) 
     6.0 l -- Water 
     In addition to the cadmium compounds included in the above exemplary formulae, we have found that continuous sulfide films can be fabricated according to the present invention with other cadmium compounds, including cadmium fluoride, cadmium cyanide and cadmium sulfite. Moreover, cadmium compounds which also contain chlorine, such as cadmium ammonium chloride, can be utilized in the method of the present invention to supply both cadmium and chlorine. 
     Although each of the exemplary solutions utilize thiourea as the sulfur containing compound, other sulfur containing compounds may be utilized. We have found thiourea, however, to be the least expensive and best suited of such compounds for purposes of solutions such as these. Similarly, hydrochloric acid and ammonium chloride are merely exemplary chlorine containing compounds, and other chlorine containing compounds may be substituted. 
     Furthermore, although each of the above-identified exemplary solutions was developed for use in a spray process for forming a cadmium sulfide film, other well known processes for forming such films, such as dipping, vacuum deposition, or electroplating, may also be utilized. 
     It should be understood that various changes, modifications, and variations to the solutions utilized in the method of the present invention may be effected without departing from the spirit and scope of the present invention as defined in the appended claims.