Monolithic AlGaAs/Ge cascade solar cells

The authors describe a monolithic cascade solar cell that contains an Al_0.08Ga_0.92As high-bandgap top cell and a Ge low-bandgap bottom cell. The emitter of the Al_xGa_(1-x)As junction is compositionally graded from x=0.08 to x=0.35. A Ge/AlGaAs heterotunneling interconnect joins the two photovoltaic junctions. This cascade structure, intended for use in space, is grown on thin Ge substrates to increase the power-to-weight ratio of the device. The structure is expected to be resistant to radiation damage, and preliminary modeling indicates that beginning-of-life efficiencies of above 26% (AM0) should be possible. The program goal is an end-of-life efficiency that exceeds 24% (AM0). The authors have experimentally demonstrated Al_0.08Ga_0.92As junctions grown on GaAs that have attained an efficiency of 17% and junctions grown on Ge substrates that have reached more than 14% efficiencies.