Thin film III–V solar cells on Mo foil

Space bound photovoltaics utilize crystalline III–V material systems to achieve extremely high conversion efficiencies. The measure of these devices is not ultimate conversion efficiency but specific power density, which can be limited by the bulky non-contributing substrates they must be epitaxially grown on. By replacing these substrates with thin metal foil, the overall weight of the device can be significantly reduced. This can be achieved by deposition of amorphous Ge on a metal foil and recrystallizing it through a thermal process to create a large-grain polycrystalline surface allowing for epitaxial growth. In this paper, the changes in device characteristics in the transition from single-crystal GaAs to polycrystalline Ge, as well as adhesion layers and annealing conditions for the recrystallization of Ge on Mo foil, was studied. It was shown that cells grown on poly-Ge substrates exhibited an increased JSC compared to those grown on crystalline Ge substrates. A process window for recrystallization was observed between 700°C and 850°C. Delamination of the Ge was observed using both W/Ti and Cr as a barrier layer to Mo, but Cr exhibited promising recrystallization results in need of further study.