intensity in W/m2, Afsa is array-specific mass in kg/m2 and D is a derating factor which accounts for the cell packing factor, space occupied by the wiring harness, diode losses, etc. A value of 0.7 was used for the latter quantity. Thicknesses used in our calculations were 10 mils for the cover glass, 2.2 mils for Si, and 3 mils for both InP and GaAs. For cell efficiencies at 25°C we used 15% for Si, 18% for InP and 19% for GaAs. These values are our best estimates of efficiencies eventually achievable in production for both GaAs and InP. Efficiencies at 60°C were obtained using the temperature coefficients —4.6X 10-2, 3.2 X 10-2 and 6.6 X 10-2%/°C for InP, GaAs and Si, respectively. The effects of storage were included by using a battery-specific energy of 100 Wh/kg and a nominal half-hour eclipse time. Such specific energies are deemed achievable using advanced sodium-sulfur batteries. In making our calculations for a geosynchronous and polar orbit, the 1 MeV performance data for both InP and GaAs were obtained from Fig. 4, while the data for silicon was obtained from reference 28. In each case, it is noted that the BOL specific power of the system containing GaAs is greater than that of InP, but that the specific power of InP soon becomes greater than that of GaAs. In addition, the specific powers of both these latter cells is significantly greater than that of Si over the entire duration of the times shown in the figures. Conclusion The results of the preceding sections illustrates that, with further development, InP solar cells have the potential to outperform both Si and GaAs in specific space radiation environments. To attain this goal, increased efficiencies are needed for InP. In addition, larger area-lower cost cells must be produced in quantity. Considering the data of Figs 1 and 3, the prospects for attaining higher efficiencies appear promising. The major cost in cell processing lies in the InP wafer used to process the cell. For example, 2 in. diameter silicon wafers can be obtained for under US$10. On the other hand, the present cost of InP wafers ranges between US$200 and US$350, depending on customer specifications. These costs should decrease when a considerable number of wafers are required for a single order. This would be the case when the cells move from research into production. Another approach lies in processing cells from extremely thin layers of InP epitaxially grown on silicon wafers. However, lattice and expansion coefficient mismatches between these two semiconductors present severe problems. Despite this, modest efforts are under way, both in the USA and Japan [30, 31], Initial experiments have produced, as expected, low efficiency cells [30, 31]. At present, it appears that transition layers are required between the InP and Si. In addition, specialized annealing techniques are expected to yield greatly increased efficiencies in these heteroepitaxial cells. At present, however, it is premature to judge the prospects for success in fabricating reasonably efficient cells from InP epitaxially deposited on silicon. However, since the long term result would be a reasonably high- efficiency, cheaper and mechanically sturdier cell, research along these lines appears to be well worth the effort. The Japanese MUSES A satellite was successfully launched in late January 1990. With reference to Fig. 3; a silicon solar cell with AMO efficiency of 20.2% (measured at NASA Lewis) was recently produced at the University of New South Wales, Kensington, Australia [32].
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