Analysis of structure change of Si solar cells irradiated with high fluence electrons

An anomalous increase in short-circuit current of Si space solar cells, followed by an abrupt decrease and cell failure has been induced by fluences greater than ${10}^{16}$ ${\mathrm{cm}}^{\mathrm{-2}}$ of 1 MeV electrons. This can be explained by a reduction in carrier concentration of the base region, in addition to a decrease of minority-carrier diffusion length. A change in the spectral response has been observed along the change in the short-circuit current. The spectral response has been modeled to account for radiation-induced changes in the cell structure. The results show that the junction depth increases when the degradation occurs. Also, spectral response after cell failure has been explained by conduction-type conversion of the base layer. This conversion is confirmed by a cross-sectional electron-beam-induced current signal. A mechanism for these phenomena has been proposed, which consists of the generation of vacancies and the vacancy-mediated diffusion of phosphorous.