because cadmium sulfide tends to fractionate into its elemental components when heated. The resulting film suffers from a stoichiometric imbalance of these components and, therefore, reduces efficiency. Closely controlled co-evaporation of cadmium and sulfur, to form the cadmium sulfide compound on a substrate, is possibly an answer to the fractionation problem. Since cadmium sulfide cells have already been manufactured on metallized plastic, and since the same material will be used in the SPS arrays, it seems logical to expect that the cells could be manufactured as part of the array blankets in one continuous process. This could be done with a high degree of automation and, therefore, a minimum of manpower. The deposited compound film is presently treated with a copper bearing solution to obtain efficient photovoltaic conversion. This results in an internal self-destruction mechanism and partly accounts for the efficiency degradation with time and humidity. The use of electronic ion implantation, instead of wet chemical activators, could eliminate this problem and is also naturally suited to the high vacuum environment of space. Renewed interest in cadmium sulfide has been generated in this country and in Europe as a low-cost, large-area power source for consumer applications. At least one major U.S. petroleum company is sponsoring high-level development programs and research is in progress at various universities to understand the basic operation of the cell. No new breakthroughs in efficiency have been announced as of yet, but the researchers are optimistic about success. Gallium arsenide is the most serious contender to displace silicon as the prime photovoltaic material in the future. Its advantages of higher efficiency, lower temperature sensitivity, and radiation hardness have been mentioned previously and are essential to the success of the SPS. Experimental solar cells of aluminum-gallium- arsenide have demonstrated efficiencies greater than the best production silicon cell. While gallium arsenide has a number of advantages over silicon, it also has a number of significant disadvantages. It is a binary compound instead of a single element, and is therefore more difficult to process in large defect-free crystals. Like cadmium sulfide, gallium arsenide has been known for a long time, but the major development work on it has only been done since 1970. Since that time, the demand for it has grown substantially as digital readouts for electronic hand calculators and electronic wristwatches. These applications remain the single largest use for the material, and the growth is expected to continue unabated. Gallium arsenide solar cells are not yet in production, but recent announcements by two companies are encouraging and at least a limited production line is expected within the next two years.
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