Radiation-induced two-step degradation of Si photoconductors and space solar cells

Since two-step degradation, i.e., gradual degradation at low fluence and steep at higher fluence, was found in Si space solar cells, extensive studies have been conducted to clarify the mechanism. Recent studies indicate that the steep degradation results from a bulk process. Either decrease in carrier mobility or carrier exhaustion has been proposed for the mechanism. Independently of the device degradation, we observed a similar two-step degradation in photoconductivity of bulk Si, and have investigated the relevant phenomena. Taking advantage of the bulk simplicity, the steep degradation mechanism has been experimentally evaluated. Consequently, a possibility of mobility change is excluded. A rapid decrease in photocarrier lifetime is evident, though the carrier exhaustion simultaneously occurs. The drop of lifetime is explained in terms of Fermi-level-dependent recombination cross-section of deep centers. As the Fermi-level jump is induced by the carrier exhaustion, the cross-section may steeply increase. The origin of two-step degradation in the solar cells is also discussed, based on the bulk degradation.