Electron and proton irradiation-induced degradation of epitaxial InP solar cells

The degradation of epitaxial, shallow homojunction n + p InP solar cells under 1 MeV electron and 3 MeV proton irradiation is presented. The data measured under 3 MeV proton irradiation are analyzed in terms of displacement damage dose which is the product of the particle fluence and the calculated non-ionizing energy loss (NIEL)[1]. A characteristic proton degradation curve is derived from which the cell degradation under any energy proton irradiation can be calculated. The data measured under 1 MeV electron irradiation is also analyzed in terms of displacement damage dose. The electron irradiation-induced degradation is correlated with the proton degradation curve by determining electron to proton dose ratios for each of the photovoltaic (PV) parameters. A comparison of the characteristic degradation curves for InP and GaAs/Ge solar cells, which was determined previously, shows InP to be intrinsically more resistant to displacement energy deposition. The base carrier concentration was measured during the irradiations, and significant carrier removal was observed. When analyzed as a function of displacement damage dose, the reduction in carrier concentration under both the 1 MeV electron and the 3 MeV proton irradiation is shown to follow the same degradation curve. From this common degradation curve, a characteristic carrier removal rate is calculated for InP under any irradiation. The junction dark current was also measured during both irradiations, and the data were fit to a three-term diode dark current equation. From the fits, the diffusion current is determined as a function of particle fluence. Changes in the diffusion current under electron and proton irradiation are shown to correlate in terms of displacement damage dose in the same way as the cell maximum power. The junction recombination current is also determined from the dark current data, and the results show the energy level of the dominant radiation-induced recombination center to be approximately the same in both the electron and proton irradiated samples. In addition, the dark current analysis indicates that the relative changes in the hole and electron lifetimes are essentially the same under both the electron and the proton irradiations. Based on these results and the overall correlation between the electron and proton damage, a detailed description of the mechanism of the radiation response of InP is developed which describes the cell degradation under any particle irradiation.