2.1.4.2 Silicon Solar Calls The Boeing reference design (1,2) assumed that 50 micron thick silicon solar cells with an efficiency of 15.75X would be available by 1985(2,p52). This has proved to be somewhat optimistic and such a cell has been projected for 1990 in this study. The state of the art in high efficiency silicon solar cells is changing so rapidly that any attempt to predict future achievements is risky at best. In any event it seems unlikely that standard silicon cell technology will produce efficiencies in excess of 20% for this application. This would imply that the reference cell used in this study is conservative by a factor of no more than 30% (this assumes, of course, that the projected values for the reference cell are in fact achieved). In the silicon planar option, the solar conversion system constitutes a majority of the overall system mass. Any significant improvement in the efficiency of silicon cells would, therefore, be of great value. The non-lunar mass would not be greatly affected, however, as the conversion system accounts for only a small percentage of the SPS total of non-lunar material. Of potential significance is the maturing of some alternate silicon cell approaches such as multi-gap and amorphous cells. Amorphous cells, to date, have achieved only relatively low efficiencies but are much less massive than standard cells(9,p67). Multi-gap cells offer higher efficiency but at the cost of higher mass(9,pp100-103). In either case it is the mass efficiency that will finally determine their relative merit. While neither of these two technologies are competitive now, it seems likely that one or both will become so in the future. A good example of current laboratory achievements in silicon cell technology appears in a paper by S. Matsuda et al (6). The National Space Development Agency of Japan and the Sharp Corporation collaborated to produce BSFR (Back Surface Field, Back Surface Reflector) silicon cells approximately 50 microns thick that operate at 13.23% efficiency (at AMO and 28 C). In addition these cells were exposed to a 1 Mev electron fluence of 1E+15 e/cm2 after which they were annealed at 60 C for 24 hours thereby restoring them to 82% of BOL (Beginning of Life power).
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