Development of metamorphic triple-junction solar cells for low temperature, low intensity operation in space

Space missions to outer planets such as Mars or Jupiter place special requirements on the solar generator as intensities and temperatures can be much lower compared to the earth orbits. Additionally, the spectral conditions on a planet like Mars are depending on the specific landing point and are changing significantly over time. This leads to the question about the best solar cell technology for such low intensity, low temperature (LILT) missions. In this paper the performance of triple-junction solar cells was investigated for five typical Mars scenarios. A state-of-the-art lattice-matched triple-junction solar cell is compared to a metamorphic cell consisting of Ga0.35In0.65P, Ga0.83In0.17As and Ge. Theoretical calculations suggest that the efficiency of the metamorphic devices can be up to 21 % higher under extreme Mars operating conditions with temperatures down to −120 °C and intensities of only 22 W/m². Experimentally this was confirmed with even 25 % higher efficiencies measured for the metamorphic 3-junction solar cell under these conditions. The IV-characteristics of the metamorphic devices were found to be well behaved even at the lowest intensities, suggesting that dislocations due to the lattice-mismatched buffer structure are not leading to low shunt resistances. The metamorphic triple-junction solar cell turns out to be an extremely interesting alternative to the lattice-matched structure for LILT conditions.