InP-Based Tunnel Junctions for Microconcentrator Photovoltaics

To further improve the performance of mechanically stacked microconcentrator photovoltaic devices, we have studied high-transparency tunnel junctions for inclusion in triple junction solar cells that are fully lattice-matched to InP. These tunnel junctions are evaluated using both standalone tunnel diodes as well as full multijunction solar cells. Of particular focus herein is the p-type tunnel junction layer, which has proven challenging to integrate in multijunction solar cells with high electrical activity, a wide enough bandgap for transparency, and an abrupt doping profile. Studies include the effect of polarity, tunnel diode dopant/composition, application of a nitrogen anneal, tunnel diode growth temperature, and cladding material. Resulting InP-based triple junction devices achieved up to 370 suns-equivalent tunneling capability, which satisfies the requirements for microconcentrator photovoltaic applications in the space environment.