Fabrication of Large Photovoltaic Arrays in Space from Lunar Materials DOUGLAS R. SPARKS Summary The manufacturing of large silicon photovoltaic arrays, suitable for use in solar power satellites, from lunar materials is discussed in detail. The system constraints include: the use of lunar materials only; processes must be compatible with zero-gravity, therefore excluding open liquids; all processes should be based on existing technologies and if possible should be continuous for high throughput, as well as automated and easily maintainable to minimize the need for human presence. Purification of materials is covered as is the growth of single and polycrystalline silicon. Processing steps such as doping, gettering, lithography, deposition of metallic and insulating films as well as wirebonding techniques are covered. The possibility of manufacturing highly efficient pointcontact silicon photovoltaics (>27% efficiency) with the aforementioned processing constraints is covered. Introduction Due to simplicity of design and safety considerations, solar power systems are the primary power sources mentioned in recent studies of space habitation and industrialization [1-6]. As has been pointed out in a recent study by Space Research Associates for the Space Studies Institute [6], high initial costs have been the major obstacle to the development of Solar Power Satellite (SPS) systems. It was also pointed out in that study that by utilizing lunar materials instead of transporting materials from earth, the cost of such a project could be significantly reduced. Of the many solar power conversion systems surveyed, the silicon photovoltaic system utilized the highest percentage of lunar material (virtually 99%). Recent advances with point contact silicon solar cells [7], as well as over 20 years of silicon processing experience, make this system perhaps the most likely candidate for use in future large-scale space power generating systems. In this paper the fabrication of silicon photovoltaics in space will be covered. Approximately 2000 tons of silicon, excluding scrap, are used each year to produce semiconductor devices [8], Of this amount only 8% is used for discrete devices, which include solar cells. Based on current estimates [6, 9, 10], from 6000 to 15 000 tons of silicon will be required to fabricate a 10 GW SPS. The use of SPS as a significant terrestrial power source will require roughly three to eight times the amount of device grade silicon as is currently being produced. As has been mentioned in a previous paper [11], the implementation of the SPS concept will require a dramatic shift in Douglas R. Sparks, GM Hughes Electronic Corp., Kokomo, IN 46901, USA.
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