Optimizing the radiation resistance of InP solar cells: Effect of dopant density and cell thickness

The effect of 1 MeV electron and 10 MeV proton irradiations on the electrical properties of high-efficiency InP solar cells grown by metalorganic chemical-vapor deposition has been measured and analyzed. The base dopant levels ranged from 3×1015 to 2×1017 cm−3, and the effect of the dopant concentration on the radiation response is presented and analyzed. The radiation-induced degradation of the solar cells was measured through both dark and 1 sun air mass zero current-voltage measurements and through spectral response measurements. The radiation-induced defects in the solar-cell junction were monitored by deep-level transient spectroscopy. Thermal and photoinjection annealing experiments were performed on the irradiated cells. The highest postirradiation efficiencies measured after a fluence of 1016, 1 MeV electrons cm−2 were 11.6% which is higher than has been reported for any other solar-cell material. Although all of the cells showed high radiation resistance, the cells with a lightly doped base (essentially an n-i-p structure after irradiation) degraded more in the open-circuit voltage and less in the short-circuit current than the more heavily doped cells. Modeling indicates that the thickness of the junction and base layers is very important in determining the end of life efficiency of the cell. Using the measured data, efficiencies up to 14.1% are predicted with a thin base and a back surface reflector, following irradiation with 1016 1 MeV electrons cm−2.