Si cells have dominated. As production experience with GaAs-based cell grows, further cost reduction can be projected. Also, more complex cell designs, especially with wraparound or wrapthrough contacts will become available. The same processing has also been applied to make GaAs cells with very high efficiency under high concentrations (over 24% at 100-times concentrations of AMO sunlight, and over 27% at 500 X terrestrial sunlight). Several spacecraft array designs use concentrators operating between 20 X and 100 X, and plans to use these arrays are based on use of GaAs cells. There is also parallel development of optimized cascade cells, some based on GaAs/Ge technology, and these are being evaluated for higher power output in both flat-plate and concentrator arrays for future spacecraft applications. Finally, high efficiency (18.5% AMO) has been obtained for GaAs cells grown on Si substrates, and these cells have advantages in reduced weight. Present tests are evaluating whether these efficiencies can be obtained over large areas, and also whether the cells can withstand the temperature cycling experienced in most space applications. If GaAs/Si cells prove to be effective, they will add considerable choice in GaAs- related technology for the array designers. Conclusions This paper has traced the evolution of GaAs cell designs for space use, and has shown how the cell performance and production experience have been steadily increased to meet an increasing number of array demands. This evolution has taken about ten years, and this appears to be a typical cycle for the application of advanced technology to provide spacecraft power. The engineering community responsible for maintaining and extending technology for space power is considered to be conservative. Much of this conservatism stems from a good record of success in designing and manufacturing arrays which have usually exceeded their predicted period of operation. Array designers believe that detailed attention to many reliability criteria during cell manufacture and assembly into arrays is an essential feature ensuring the good record. This period, during which possible advantages in applying GaAs technology to space power were assessed and incorporated into the design options available to array designers for advanced missions, has provided valuable experience to both cell and array producers. Further advances can be expected.
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