taking into consideration the mechanism of PID in n-type Si PV modules, PID can be basically avoided by using Na-free front cover substrates (14,15) or encapsulant whose volume resistivity is high (16-19), to diminish migration of Na toward the surface of Si cell. For instance, when a chemically strengthened glass whose Na in the surface of glass is exchanged by K is used as the cover front glass instead of a conventional soda lime glass, no degradation of crystalline Si PV module by PID was observed