It is shown that Potential Induced Degradation (PID) can be prevented by using chemically strengthened glass as module cover glass in this paper. While several factors are considered to be the causes of PID, migration of Na ions from module cover glass has been pointed out as a principal factor. It has been known that photovoltaic modules with proper materials of encapsulant, antireflection coating layer on Si wafer, and/or appropriate system conditions do not show PID. Without such proper measures, photovoltaic modules show PID under tough conditions, high temperature, high humidity, and/or high bias voltage. Chemically strengthened glass is known to have lower Na concentration in its surface compared to conventional thermally strengthened glass. Small modules containing chemically strengthened glass were prepared and operated in an accelerated PID test. Negative voltage bias of 1000 V was applied continuously to the cells in the modules by connecting shorted leads during the accelerated PID test. The small modules with chemically strengthened glass show the same maximum power under 1000 W/m 2 illumination after those accelerated PID tests with high temperature and high bias voltage as before, even though no other measures were made to the modules for preventing PID. In other words, only replacing cover glass with chemically strengthened glass prevents Na-related PID. Chemically strengthened glass as photovoltaic module cover glass is now commercially available, as "Leoflex TM " from Asahi Glass. Composition of the Leoflex TM glass is specially designed for good chemical-strengthening characteristics.

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