Final results from the MISSE5 GaAs on Si solar cell experiment

GaAs on Si (GaAs/Si) solar cells with AM0 efficiencies in excess of 17% have been demonstrated using Si substrates coated with a step-graded buffer of SixGe1-x alloys graded to 100% Ge. A year of LEO testing of this technology aboard Materials International Space Station Experiment number 5 (MISSE5) was recently competed. Electrical performance data, sun angle and thermal conditions measured on-orbit, were telemetered to ground stations daily. Ground based measurements following flight were performed on both 1cm² and 4 cm² GaAs/GaAs and GaAs/Si devices. The smaller area GaAs/Si cells showed low degradation rates for Isc, while all other cell parameters were comparable to control cells. However, the larger area GaAs/Si devices, while demonstrating similarly low Voc and FF degradation, demonstrated a larger than expected decrease in Isc. Comparison of pre and post flight QE data suggests the decrease in Isc for the large area cell may result from reduced cell active area rather than a degradation in material properties. Ground based thermal cycle testing did not replicate these results, thus differences in mounting techniques and materials may have contributed to the degradation observed on orbit for the large area device in this initial on-orbit test. Crack free GaAs/Si based devices have been demonstrated and offer a mitigation strategy for microcrack degradation.