Degradation of subcells and tunnel junctions during growth of GaInP/Ga(In)As/GaNAsSb/Ge 4-junction solar cells

Abstract

A GaInP/Ga(In)As/GaNAsSb/Ge 4J solar cell grown using the combined MOVPE + MBE method is presented. This structure is used as a test bench to assess the effects caused by the integration of subcells and tunnel junctions into the full 4J structure. A significant degradation of the Ge bottom subcell emitter is observed during the growth of the GaNAsSb subcell, with a drop in the carrier collection efficiency at the high energy photon range that causes a ~15% lower Jsc and a Voc drop of ~50 mV at 1-sun. The Voc of the GaNAsSb subcell is shown to drop by as much as ~140 mV at 1-sun. No degradation in performance is observed in the tunnel junctions, and no further degradation is neither observed for the Ge subcell during the growth of the GaInP/Ga(In)As subcells. The hindered efficiency potential in this lattice-matched 4J architecture due to the degradation of the Ge and GaNAsSb subcells is discussed.