Improved radiation resistance of epitaxial lift-off inverted metamorphic solar cells

The inverted metamorphic (IMM) solar cell has a high specific power compared to traditional germanium-based multi-junction solar cells, which may prove beneficial for space applications where costs are weight-driven. In addition, the mechanical flexibility of the IMM cell may be beneficial for new technologies, such as high-power, flexible, deployable arrays currently under development. However, IMM solar cells have not yet demonstrated radiation resistance equal to that of traditional Ge-based multi-junction cells, largely due to degradation in the InGaAs bottom subcell. A structure and process have been developed to incorporate a back surface reflector on the epitaxial lift-off (ELO) IMM solar cell, permitting the InGaAs subcell to be thinned whilst maintaining high optical absorption. The thinner subcell can better tolerate degraded base diffusion lengths following irradiation. In this manner, a significant improvement in the end of life efficiency of ELO IMM solar cells is demonstrated following irradiation with 1 MeV electrons at a fluence of 1×10¹⁵ cm^-2.