An optical comparison of silicone and EVA encapsulants under various spectra

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.