Integration of III-V materials and devices on silicon substrates has demonstrated remarkable progress, due in large part to the development of low- dislocation-density (~106/cm2), fully relaxed germanium (Ge) epitaxial films deposited on silicon (Si) substrates by researchers from the Massachusetts Institute of Technology (MIT). Transitioning III-V materials to Si substrates offers tremendous improvements in the size, mass, strength, and cost of high-efficiency III-V photovoltaics. Using these “virtual” Ge substrate materials, researchers from MIT, the Ohio State University (OSU), and the NASA Glenn Research Center have demonstrated the highest reported open-circuit voltage values for gallium-arsenide (GaAs) cells on Si to date. In addition, single-junction GaAs cells with efficiencies greater than 17-percent air mass zero (AM0) have been demonstrated.
This GaAs/Si technology was selected to participate in a flight experiment, Materials International Space Station 5 (MISSE5), developed by the Naval Research Laboratory and designed to test advanced photovoltaic technologies in the low-Earth-orbit (LEO) space environment. The MISSE5 payload was delivered to the International Space Station (ISS) on August 2005, and astronauts attached it to a handrail on the outer surface of the P6 truss (see the photograph). The test articles aboard MISSE5 were characterized along with their operating environment (temperature, Sun angle, etc.) each orbit, with the data telemetered to ground stations via the PCSAT2 portion of the MISSE5 spacecraft. MISSE5 was returned from the ISS to Earth in September 2006.
MISSE5 shown being attached to the International Space Station (ISS).
Long description of figure 1.
The flight data were analyzed to extract temperature coefficients for the device voltage and current as a function of temperature, angle of incidence, and Earth-Sun spacing, thereby allowing the flight data to be corrected back to ground measurement conditions. The analysis indicates that all test devices operated nominally and did not show any signs of degradation. Full postflight characterization is in progress. On-orbit temperature swings were lower than anticipated (~50 K) and were somewhat lower than those of the fully populated power panel (~80 K). The MISSE5 flight data confirm ground-based thermal cycling data,simulating 1 year in LEO (6000 thermal cycles of ±80 °C), which demonstrated that even though the GaAs/Si contained microcracks because of differences in thermal expansion coefficients, the device performance was stable.
Recently, this development activity was extended to the demonstration of both lattice-matched (i.e., the cell-structure lattice is matched to the Ge final epilayer) and lattice-mismatched (LMM) tandem III-V devices on Si substrates. The LMM devices lack the microcracks normally found in thick GaAs epitaxial structures deposited on Si. Thermal cycle testing of these devices demonstrated that this crack-“free” characteristic is stable under normal LEO and geosynchronous-Earth-orbit thermal cycle environments.It appears that residual compressive strain resulting from the LMM structures being less than fully relaxed compensates for the tensile strain introduced by differences in thermal expansion coefficients between III-V materials and Si.
Glenn contact: David M. Wilt, 216-433-6293, David.M.Wilt@nasa.govLast updated: December 14, 2007
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