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.

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Under the AM1-5g spectrum, the efficiency of a c-Si module can be increased by 0.5-1.5% (relative) by using a silicone encapsulant rather than EVA. This increase is primarily due to photons of wavelengths less than -400 nm being transmitted by silicone but blocked by EVA. The highest increase in efficiency arises for cells with a good 'blue response' and for silicones of a higher refractive index. In this work we show that when calculated for the AM1-5g spectrum, the optical advantage of silicone over EVA tends to be an underestimate of what can be expected in the field. This is because incident spectra are frequently 'bluer' than the AM1-5g spectrum, particularly in summer and on cloudy days. In such cases, a larger fraction of photons have a wavelength less than 400 nm. With ray tracing, we find that the relative advantage of silicone over EVA is increased by an additional 0.3% on a sunny summer day in Phoenix AZ., and by 0.7% on a cloudy summer day in Brownsville TX. Still greater increases are expected for maritime climates and for installations with high albedo.

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