It is projected that the major portion of the necessary solar cell cost reduction will be attained simply through the implementation of mass production and the associated learning curves as shown in Exhibit 53. Technology advances, however, will also be needed in at least the following areas: • New silicon material sources • New crystal growing processes • New solar cell fabrication processes. The key problem in the EFG continuous crystal growth technique is to find die materials that can withstand the temperature of the process and yet maintain the efficiency of the solar cell produced. (2) Improved Solar Cell Performance Substantial data are available on the performance of silicon solar cells. Present silicon solar cells are about 200 microns thick and have efficiencies of 15%. Increases in efficiency are considered feasible even with reduced thicknesses. Present technology uses silicon solar cells mounted on rigid substrates with cover glasses bonded to the solar cell to achieve radiation shielding. Advanced technology based on a "roll out" blanket design which exhibits weight-to-power ratios of about 30 Ib/kW (14 kg/kW), have been fabricated. With improved fabrication techniques, reductions in thickness to less than 4 mils (100 microns), and use of solar concentrators, solar cell array weights of about 3 Ib/kW (1.4 kg/kW) are projected to be achievable in 10 years. These projections are based on reasonable improvements for single-crystal silicon solar cells and successful achievement of the goals of the National Photovoltaic Conversion Program being conducted by ERDA. The maximum theoretical efficiency of a silicon solar cell is about 22 percent. Solar cells used on past space programs have efficiencies in the range of 10-12 percent. Values on the order of 18-20 percent (new cells with air mass zero and concentration ratios of one) have been postulated for the SSPS. Achieving the higher level of efficiency desir- able for the SSPS will require technology advances to:
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