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.