Recovery Rate and Capacitance Measurements on Irradiated Lithium-Containing Solar Cells

Abstract
Room temperature recovery kinetics of lithium-containing p/n silicon diodes and solar-cells were experimentally investigated after irradiation by 1 MeV electrons to fluences of 1013 and 1014 e/cm2. The current physical model for the recovery process involves the diffusion of a lithium donor ion to a radiation-induced vacancy-impurity defect with negative charge and high hole-capture cross section, and the complexing of the lithium donor with this defect to form a new defect with zero charge and low hole-capture cross section. The recovery rate thus depends on the lithium ion density, the lithium-ion diffusion constant, and the defect density. Direct measurements of the density profile of lithium ions in the base region near the junction have been performed on 12 diodes. Using a capacitive technique with some novel features, effective diffusion constant for lithium ions near the junction was measured on the same diodes. Further useful information has been obtained by relating these properties to the rapid, room temperature recovery of minority-carrier diffusion length which is observed in these devices after irradiation. The results show that: · Steep gradients of lithium ion density (~1019 cm-4) exist in the base region extending, in many cells, to a distance greater than 10 micrometers from the junction; · In a group of cells made from Lopex silicon, the effective lithium diffusion constant at the junction edge ranged from 0.05 to 0.28 times the free lithium ion diffusion constant and, in a group of cells made from float-zone refined silicon, from 0.1 to 1.