Determination of moisture ingress rate through photovoltaic encapsulants

Encapsulant materials provide protection and electrical isolation of the solar components in photovoltaic (PV) modules from the environment. However, some photovoltaic devices are sensitive to low levels of moisture and the rate of moisture ingress into a module can decrease its performance significantly during the lifetime of a module in the field. The moisture penetrates through the encapsulant to the module´s metal components and power degradation and metal corrosion can occur. The rate of moisture ingress is a characteristic property of an encapsulant and is a function of both the water vapor transmission rate (WVTR) and the water vapor diffusion coefficient (D). The WVTR and D of commercial, partially neutralized poly(ethylene co-methacrylic acids) [ionomers] were measured and compared with poly(ethylene vinyl acetate) [EVA] and polyolefin encapsulants at various temperatures up to 85°C. The experimental data correlated well to a Fickian diffusion model. The rate of moisture ingress can be modeled for glass/glass modules and for the encapsulant region between the glass and a single crystalline-Silicon cell.