Air Force MANTECH Contract (‘High Efficiency GaAs Cells') In the period 1982 to 1986, under an Air Force Manufacturing Technology contract, Applied Solar Energy Corporation (ASEC) demonstrated producibility by delivering 5500 cells with average efficiency of 17% (AMO). These cells met the basic space qualification tests. This program showed that production rates of 1000 2 X 2 cm cells per week could be maintained. This work also demonstrated that high throughput, metal organic chemical vapor deposition (MOCVD) reactors could be used to deposit the active GaAs (and AlGaAs) layers on GaAs substrates. The program also developed the necessary infrastructure, mainly by providing several sources of suitable GaAs substrates and CVD sources. The GaAs substrate development was interesting, leading to steady supplies of substrates tailored specifically for space cells. The substrates were grown with low dislocation density in near-rectangular shaped boats (typically 2.5 X 4.5 cm2 or 4.5 X 4.5 cm2 in size). The shape reduced wastage and led to substrates with costs per unit area less than half the costs at the time the program started. Parallel Production Efforts on GaAs Cells Before the MANTECH Program was completed (around 1984) but triggered by the results achieved to that time, an active space-cell program was begun at ASEC. This production program for 2x4 cm2 cells accelerated the build-up of the production efforts and by imposing specific user-needs, extended the range of the qualification tests. The array methods were extended to handle and bond cells in array assembly, and the GaAs cells were modified to withstand extended operation under reverse bias conditions, a possibility if parts of the array were shadowed. The scaled-up MOCVD effort also required increased attention to safety, because large quantities of toxic gases (mainly arsine, AsH3) were used. Extensive monitoring equipment and personnel protection procedures were installed. In this period, Government safety regulations were also intensified. Around 1986, Mitsubishi produced 50,000 2x2 cm2 GaAs/GaAs cells for use on a Japanese communication satellite. Increased Efficiency GaAs Cells In the early 1980s, groups at Varian, Lincoln Laboratories, Spire and Mitsubishi Corporation increased GaAs cell efficiency over 20% (up to 21.5% AMO). Most of these groups used MOCVD to deposit the GaAs layers. At this stage, GaAs cells 10-12 mils thick could be successfully processed under production conditions and interconnected into arrays. It was realized that to be competitive in future arrays, the weight of the GaAs cells must be reduced. Tests with thinner GaAs substrates were only moderately successful, and the easy cleavage of GaAs indicated that severe breakage problems could be expected if large area, thin cells were needed. Air Force Program (‘Rugged GaAs Solar Cells') In 1984, the Air Force recognized the need for lightweight GaAs cells and awarded a contract to evaluate GaAs cells grown on germanium (Ge) substrates. The atomic lattice spacing and thermal expansion coefficient of Ge are closely matched to those of
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