appear to have a high degree of radiation resistance. In addition, they are capable of operating at higher temperatures than silicon cells so that a larger degree of concentration might be used (Ref. A4). Refs. All and A13 also discuss GaAs solar cells and essentially concur with the above. Ref. A13 points out that there is a development risk associated with GaAs cells, and Ref. A4 states that their availability for large scale application is a principal issue. Other alternatives to silicon solar cells include Cadmium Sulfide (CdS) solar cells and multilayer cells. These are discussed in the most detail in Ref. A4, and the following is excerpted from that document. "The CdS solar cell has been under development for more than 10 years in both the U.S. and Europe. Present cell efficiencies are in the 6 to 8 percent range. The major advantage of this thin-film structure is that it lends itself very well to highly automated mass production assembly techniques. Early in its development, the CdS cell had a major problem in degradation and NASA dropped development of the cell for space applications several years ago. Much of the work in the recent past has been conducted in England and France and has been directed at the performance stability problem. The French claim that the stability problem has been solved but reproducibility of cell characteristics remains a problem. The prospects for increasing cell efficiency are uncertain at this time. Recently work, on the CdS system for terrestrial applications has been resumed in the U.S. under NSF sponsorship. "The multilayer cell is a device which conceptually uses a silicon solar cell as the base cell with one or two other solar cells on top of it to absorb photons in the short-wave region where silicon is not sensitive. These cover cells must be transparent to the longer wavelength photons which would pass through to the silicon cell. This concept is attractive in that projected efficiencies are in the region of 30 percent. The multilayer cell concept has long been recognized; however, virtually no fabrication and test work has been performed with such cells. The concept awaits an ingenious solution to complex fabrication problems before reaching fruition."
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