High-voltage, low-current GaInP/GaInP/GaAs/GaInNAs/Ge solar cells

Four-junction GaInP/GaAs/GaInNAs/Ge solar cells are a widely-pursued route toward AM0 efficiencies of 35% and above, and terrestrial efficiencies of up to 40%. Extensive research into the new material system of GaInNAs has so far yielded subcells with AM0 current densities far below the ∼17 mA/cm² needed to current match the other subcells in the stack. A new multijunction structure, a 5-junction GaInP/GaInP/GaAs/GaInNAs/Ge cell, divides the solar spectrum more finely in order to relax this current matching requirement, by using an optically thin, high-bandgap GaInP top subcell, with an additional thick, low-bandgap GaInP subcell beneath it, in combination with a GaInNAs subcell. In this way, the 5-junction cell design allows the practical use of GaInNAs subcells to increase the efficiency of multijunction cells. Light I-V and external quantum efficiency measurements of the component subcells of such 5-junction cells are discussed. Experimental results are presented for the first time on GaInP/GaInP/GaAs/GaInNAs/Ge cells with the top four junctions active, with measured V_oc of 3.90 V.