Role of Lithium in Damage and Recovery of Irradiated Silicon Solar Cells

Lithium plays a role in the degradation and recovery of diffusion length in lithium-doped silicon p/n solar cells irradiated with 1 MeV electrons. Salient experimental results are that (1) the diffusion length degrades when these cells are irradiated at a fast rate; (2) the time constant for recovery of diffusion length decreases with lithium concentration, and increases with fluence, as ϕ2/3; and (3) recovery is a temperature-sensitive process with an activation energy of (0.61 ± 0.10) eV. These results suggest the following model for the damage and recovery process. Degradation of diffusion length occurs when radiation-induced vacancies pair with lithium and other available impurities to form immobile, negatively-charged recombination sites. These sites are formed at a rate governed by the irradiation rate and the high vacancy mobility. Unpaired lithium ions diffuse to, and pair with, these recombination sites, reducing their cross sections to very low values. Thus, recovery of diffusion length occurs. Recovery follows diffusion-limited kinetics in which the reacting species are of unequal concentrations. The capture radius of the recombination site for a lithium ion is proposed to decrease with fluence as ϕ-2/3 to achieve better accord between theory and experiment.