GaAs/Ge tandem-cell space concentrator development

GaAs/Ge monolithic tandem two-junction concentrators are being developed by optimizing separate one-junction GaAs and Ge cells that simulate the GaAs top cell and Ge bottom cell of the tandem. Separation allows easier analysis of the tandem's top and bottom cells than if these two junctions were in series. The best GaAs top cell has an independently measured AM1.5D efficiency of 28.7% at 200 suns and 25°C (24.5% AM0 at 170 suns), a record for a monolithic cell without a prismatic cover. The Ge bottom cells have a GaAs optical filter (but no GaAs junction) to replicate the spectrum that the Ge cell sees when incorporated into a tandem. The best Ge-under-GaAs bottom cell efficiency is 4.6% AM0 at 103 suns. Evidence that the 900-1800-nm response seen from the Ge bottom cell is due to a p-n junction in the Ge and not a GaAs/Ge heterojunction is presented