The effect of damp-heat and UV aging tests on the optical properties of silicone and EVA encapsulants



The absorption coefficient α(λ) of polydimethylsiloxane (PDMS) silicone and ethylene vinyl acetate (EVA) is monitored during exposure to damp-heat (DH) and UV accelerated aging tests. In the DH test, the samples were exposed to 85% relative humidity at 85°C for 1325 h, satisfying the DH components of the IEC, UL, and IEEE's humidity-freeze and DH tests. It is shown that moisture introduced by the DH exposure scatters incident light, reducing transmission through test samples and complicating optical experiments. It was therefore necessary to dry out the samples to assess the effect of DH exposure on α(λ). By this procedure, it was found that DH exposure caused an increase of α(λ) in both PDMS and EVA, where small absorption shoulders were introduced at wavelengths of 250–500 nm; this translates to a 0.14% and 0.39% loss in module efficiency for the silicone and EVA, respectively. The weight gain due to moisture was 0.035% for PDMS and 0.28% for EVA. In the UV test, the samples were held at 60 ± 5°C and exposed to 52 kWh/m2 between the wavelengths of 280 and 385 nm, and 5.0 kWh/m2 between 280 and 320 nm, satisfying the pre-conditioning UV tests of the IEC reliability standard. The UV exposure did not affect α(λ) of either the PDMS or the EVA. Copyright © 2010 John Wiley & Sons, Ltd.