Thermal cycle testing of GaAs on Si and metamorphic tandem on Si solar cells

GaAs on Si (GaAs/Si) solar cells grown on Si substrates coated with a step graded buffer of Si_xGe_1-x alloys graded to 100% Ge have demonstrated AM0 efficiencies in excess of 17%. Recently, 4 cm² devices were developed in preparation for on-orbit testing aboard Materials International Space Station Experiment number 5 (MISSES). In preparation for flight, thermal cycling life testing and thermal shock testing have been conducted to examine the stability of these thermal coefficient of expansion mismatched structures. Six thousand (6000) thermal cycles, equivalent to one year in LEO, from -80°C to +80°C have been completed with no discernable degradation in the electrical performance. The use of metamorphic III-V materials, of larger lattice parameter, has demonstrated the ability to dramatically reduce the micro crack density, presumably through strain balancing. Recently, the first demonstration of metamorphic tandem devices (1.6 eV InGaP / 1.1 eV InGaAs) on Si substrates was accomplished. The devices appeared micro crack free as determined by Nomarski optical microscopic examination and electroluminescence image analysis. Thermal shock testing of GaAs/Si devices showed micro crack formation and device electrical degradation. Thermal cycle testing of metamorphic structures (InGaP/InGaAs/Si) demonstrated a general lack of micro crack formation and improved thermal stability compared to GaAs/Si control devices.