Damp and dry heat degradation of thermal oxide passivation of p ⁺ silicon

Thermal SiO₂ passivates both moderately and heavily doped silicon surfaces irrespective of the dopant type, which is advantageous in high-efficiency solar cell designs. Commercial photovoltaic cells are submitted to accelerated ageing tests, such as damp-heat exposure, to ensure they maintain their performance for at least 20 yr. We find damp-heat exposure causes a severe and rapid degradation of thermal SiO₂ passivation on p ⁺ silicon surfaces. The reaction is so severe that the diffused-region recombination in the degraded state is limited by the diffusion of minority carriers to the Si-SiO₂ interface not the density of interface defects D_it. Certainly, this effect renders the thermal-oxide passivation useless if employed on a solar cell. To study the cause of the degradation, we also test the effects of storage in dry heat and room ambient conditions. Examination of the rate of degradation in the tested storage conditions in comparison with modelled diffusion of moisture in SiO ₂, we find a significant correlation between the time dependent J_0e and moisture supplied to the interface, leading us to the conclusion that moisture ingression and subsequent reaction at the SiO ₂-Si interface are the cause of both damp-heat and room- ambient degradation.