High Efficiency Lattice-Matched 4J Space Solar Cells on GaAs

A lattice-matched four-junction solar cell on a GaAs substrate, for space applications, is demonstrated. The solar cell incorporates MBE grown GaInP, GaAs, GaInNAsSb and GaInNAsSb junctions with band-gaps of 1.9 eV, 1.4 eV, 1.2 eV and 0.9 eV, respectively. For AMO illumination, the cell exhibited a maximum efficiency of 27%. For this performance, a high collection efficiency for the bottom cell is required. The high efficiency and current generation for the four-junction solar cell is primarily enabled by achieving a very low background doping level $(\sim 5\times 10^{14}\mathbf{cm}^{-3})$ and high charge carrier lifetimes (2–4 ns) for the GaInNAsSb bottom junction. Achieving an efficiency of 33% is deemed possible by further reduction of reflection, shadowing and transmission losses.