Space qualification of UV and IR reflecting coverslides for GaAs solar cells

As part of the space qualification effort for blue-red reflecting coverslides designed for use with GaAs solar cells, the first long-term (3000 hours) UV testing of unirradiated and 1 MeV electron-irradiated GaAs solar cells, with 4 types of multilayer-coated coverslides to reduce operating temperature, has produced some unexpected results. Important conclusions from this study, which includes two parallel tests, are as follows: (1) All of the GaAs solar cells with multilayer-coated coverslides display UV degradation. The laboratory data, extrapolated to 10 years in orbit, point to a significant loss mechanism from a combination of absorption and a reduction in optical match in such coatings from this portion of the space environment; (2) The effects of contamination in a vacuum system, on the measured degradation in solar-cell short-circuit current during a UV test, depend upon the type of coverslide coatings present on the coverslide surfaces. This has implications for both coated coverslides and optical solar reflectors (OSR's) in space; and (3) Because of the observed trends in this test and uncertainties in the extrapolation of data for multilayer coated coverslides, the use of any multilayer-coated coverslides for extended missions (greater than 1 year) cannot be recommended without prior flight testing.