Optimization of base doping for radiation hard InP solar cells
Model calculations were performed (1) to determine the optimum base doping for InP solar cells to achieve high efficiency under terrestrial conditions and (2) to quantify the difference, if any, in the optimum base doping for terrestrial and space applications. Modeling was done for a midgap trap and a shallow trap at Ev + 0.2 eV to determine the effect of trap location on the optimum base doping before radiation. After irradiation, modeling was done using the H4 trap at Ev + 0.37 eV, since the H4 defect controls the lifetime in the radiated InP cells. The highest efficiency for InP solar cells under terrestrial conditions is obtained for a base doping of 2 × 1017 cm-3 prior to irradiation and is independent of the defect energy level. Optimum base doping needs to be increased beyond 2 × 1017 cm-3 for irradiated InP cells because presence of more dopant reduces the rate of formation of radiation induced lifetime-limiting defects. The shift in the base doping depends on the amount of irradiation dose and is due to the decrease in radiation induced H4 defect concentration with increased doping.