Breakeven Criteria for the GaInNAs Junction in GaInP/GaAs/GaInNAs/Ge Four-Junction Solar Cells

The four-junction GaInP/GaAs/GaInNAs/Ge solar cell structure holds the promise of efficiencies exceeding those of the GaInP/GaAs/Ge three-junction cell, which at present is the benchmark for high-efficiency multijunction cell performance. The performance of GaInNAs junctions demonstrated to date has been insufficient for the realization of these projected efficiency gains, owing to poor minority-carrier properties in the GaInNAs. However, incremental improvements in the GaInNAs junctions have brought this breakeven point within sight. In this paper, we use a semi-empirical approach to estimate the efficiency of the GaInP/GaAs/GaInNAs/Ge four-junction solar cell structure as a function of the performance parameters of the GaInNAs third junction. The results provide guidance on the extent to which the current and voltage of present-day GaInNAs junctions will need to be improved in order for the resulting four-junction cell to realize its potential for efficiencies higher than that of GaInP/GaAs/Ge benchmark.