Requirements for a GaAsBi 1 eV sub-cell in a GaAs-based multi-junction solar cell

Multi-junction solar cells achieve high efficiency by stacking sub-cells of different bandgaps (typically GaInP/GaAs/Ge) resulting in efficiencies in excess of 40%. The efficiency can be improved by introducing a 1 eV absorber into the stack, either replacing Ge in a triple-junction configuration or on top of Ge in a quad-junction configuration. GaAs0.94Bi0.06 yields a direct-gap at 1 eV with only 0.7% strain on GaAs and the feasibility of the material has been demonstrated from GaAsBi photodetector devices. The relatively high absorption coefficient of GaAsBi suggests sufficient current can be generated to match the sub-cell photocurrent from the other sub-cells of a standard multi-junction solar cell. However, minority carrier transport and background doping levels place constraints on both p/n and p-i-n diode configurations. In the possible case of short minority carrier diffusion lengths we recommend the use of a p-i-n diode, and predict the material parameters that are necessary to achieve high efficiencies in a GaInP/GaAs/GaAsBi/Ge quad-junction cell.