c• Solar Cell Manufacturing For cost considerations as well as technological requirements, the methods currently used for fabricating solar cells will not be suitable for the SPS, specifically in the area of the required solar blanket. Ref. All reports that current methods for fabricating solar blankets involve a slow, mostly handmade process. In addition to blanket fabrication, other steps in the manufacturing process require improvements. For instance, raw material processing now requires three energy intensive, high temperature cycles. Also, current crystal growth and packaging techniques present problems . Ref. A4 states that a highly automated, high rate mass production system as well as new cell processing techniques must be developed. Two processes presently being investigated for continuous ribbon growth are the web dendrite and the edge-defined film-fed growth (EFG) processes. Also some work is underway on the development of solar cells from polycrystalline silicon film, but this work is at an early, immature stage (Ref. A4). Ref. All discusses the web and EFG approaches, and also the Czochralski approach, which is characterized by large amounts of waste material. It concludes that the EFG process shows the most promise for significant cost reduction, but that finding die materials that can withstand the temperature of the process poses problems. Ref. A5 points out that significant development efforts are underway on solar cells for terrestrial use, and speculates that some of the technology developed might be applicable to SPS requirements. d. Alternative Solar Cell Approaches Several alternatives to silicon solar cells are under investigation. Gallium arsenide (GaAs) solar cells are the most advanced of these alternatives, and have demonstrated attractive characteristics. Ref. A4 reports that they have achieved efficiencies of 13.5% in the laboratory, and that in the near future, experimental cells may achieve 19% efficiency. Their thickness can be much less than that of silicon crystals because of their stronger light absorption. Also, based on limited tests, GaAs cells
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