Spectral response of electron-irradiated homoepitaxial InP solar cells

The Naval Research Laboratory has presented numerous reports on the radiation hardening of the two-terminal InP/Ga_0.47In_0.53As tandem solar cells for space use. To date, most work has been done on n⁺p devices. However, in order to facilitate the heteroepitaxial growth of these tandems on substrates such as Si or Ge, the polarity of the subcells may need to be reversed. With this in mind, spectral response measurements on irradiated homoepitaxial p⁺n InP solar cells were carried out for several fluences of 1 MeV electrons. The quantum efficiency data degraded at high wavelengths but no significant degradation was observed at low wavelengths. An important material parameter in solar cell devices is the minority carrier diffusion length (L) for which L_P was measured in the present study. This parameter was measured as a function of electron irradiation using the steady-state short-circuit current method. The degradation of L_P was plotted as a function of electron fluence and the data were fit to the standard semi-empirical model which relates ΔL to the damage constant, K _L. The fit yielded values for K_L and the unirradiated minority carrier diffusion length, L₀