After several months' work, successful results were obtained. The growth conditions were changed to lower temperatures, and slower growth rates, along with some changes in substrate preparation. The quality of the GaAs layers has improved steadily and it is now possible to grow high efficiency GaAs cells on the passive-Ge substrate. These passive-Ge cells do not have excess Voc, although the Voc values and other I-V parameters are typical of the values obtained for high efficiency GaAs/GaAs cells. Also, these cells have the same low temperature coefficient as GaAs/GaAs cells. Despite the presence of a low density of growth defects not observed for growth on GaAs substrates, the GaAs/Ge cell performance has equalled that of co-grown and coprocessed GaAs/GaAs cells. The best cells have exceeded 20% (Table I). These same tests also showed that similar efficiencies were obtained for all four 2x2 cm2 cells grown on the 4.5X4.5 cm2 substrates used (Table II). This indicates that with an optimized grid design (a minor modification), the same high efficiencies can be projected for 4 X 4 cm2 GaAs/Ge cells. Air Force MANTECH Program (‘Rugged, Thin GaAs Solar Cells') In 1988, the Air Force awarded a MANTECH program to develop and produce high efficiency GaAs cells on thin Ge substrates. The goals were 18-20% efficiency, areas 4x4 cm2 to 6x6 cm2, and thickness 3-4 mils. The program also called for demonstration of panel technology for these cells, using either welding or soldering. Although the program was planned when active-Ge substrate cells were considered promising, when ASEC received the contract, it was decided to use the passive-Ge substrate approach. The good results reported in the previous section are encouraging for the MANTECH Program. Production of GaAs/Ge Cells The production effort outlined above was satisfactorily completed, the total output for the deliverables being over 35 Kw, with average cell efficiences of 16.5-17%. For follow-on production runs, GaAs/Ge cells were promising, and cells with the passive-Ge structure are undergoing qualification testing, with production to follow validation that all qualification requirements are met.
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