Single crystal silicon solar cells are the only solar cell type that has been utilized for spacecraft solar power systems. Research and development has produced continuous improvements in unit mass, efficiency, structural quality and reliability; however, great improvements would still be required for effective SPS application. Silicon cells have the advantage of more established manufacturing base, lower potential cost per cell, and abundant resource materials. Disadvantages, relative to gallium arsenide, are higher mass per unit solar cell area, greater performance degradation from thermal (temperature) and radiation effects and slightly lower efficiency. The thermal degradation effect can be minimized by designing for low cell operating temperature; i.e., without solar concentration, or by designing so that the concentrator acts as a passive radiator for cell cooling. Performance loss by radiation degradation is projected to be recovered by laser annealing the cells in place. Gallium arsenide cells have been under investigation for a number of years but significant improvements have been made since 1972. The development of the gallium-aluminum-arsenide "window" which is epitaxially grown on the basic gallium arsenide cell has led to the improvement in cell efficiency. Since most solar radiation is absorbed within 1 ym of the GaAs cell surface, it is possible to construct a very thin cell (~5 pm) with good efficiency. Consequently, the quantity of gallium needed to make the cells is significantly reduced. The advantages of gallium arsenide cells are low mass potential, resistance to degradation by thermal and radiation effects, and good efficiency. Use of solar concentration provides self-annealing of the cells at moderate temperatures. Disadvantages are relatively high cost and less technology base than silicon. Gallium availability is also a consideration. Table 1 provides an example comparison of gallium arsenide and silicon cells for a specific SPS configuration. Note that with solar concentration (CR=2), the gallium system has a cost advantage over the silicon system, but with CR=1, the silicon system would be either slightly less or competitive with gallium. Because of this close competition, silicon and gallium arsenide are both viable candidates for SPS application. Therefore, they are presented as options in the description of a reference system.
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