Barry Ketola

An optical comparison of silicone and EVA encapsulants for conventional silicon PV modules: A ray-tracing study

Ray-trace simulation is used to quantify the optical losses of photovoltaic modules containing silicon cells. The simulations show that when the modules encapsulant is silicone rather than ethylene vinyl acetate (EVA), the modules short-circuit current density under the AM1-5g spectrum is 0.7–1.1% higher for screen-printed multi-cSi cells, 0.5–1.2% higher for screen-printed mono-cSi cells, and 1.0–1.6% higher for high-efficiency rear-contact cells, depending on the type of silicone. This increase is primarily due to the transmission of short-wavelength light (≪420 nm) and is therefore greatest when used with low UV-absorbing glass and cells of a high IQE at short wavelength. We also quantify absorption in the glass, EVA and silicone at longer wavelengths and describe the influence of an encapsulants refractive index on escape losses.

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.

Improved spectral response of silicone encapsulanted photovoltaic modules

In this work the benefit of using optically superior silicone encapsulant materials over the incumbent ethylene vinyl acetate is demonstrated. Optical characterization of the two materials demonstrates improved transmission in the UV region of the solar spectrum. Single cell mini-modules were prepared using two different manufacturers of screen printed textured monocrystalline Si PV cells. Spectral response measurements demonstrate a 0.74–1.03% relative increase in short circuit current density when silicones are used rather than ethylene vinyl acetate, clearly in the region of improved UV transparency. IV measurements demonstrate a 0.31–1.45% improvement in current for silicone.

Reliability of Silicone Encapsulants in high Pollution Environments.

Photovoltaic cells and their encapsulants are subjected to harsh environments with extremes in solar radiation, thermal/humidity cycling, mechanical stress, electrical bias and increasing levels of pollution. Volcanic action and the escalating use of fossil fuels have increased the threat of atmospheric sulfurous gaseous on silver and other metal contact materials. Twenty years ago Cuddihy et. al., concluded that elastomeric silicone encapsulants were the only technology to meet all the physical, chemical, spectral, and mechanical demands of photovoltaic modules. In the decades since the decision to excluded silicone in this application for cost reasons, the thermal and photo-degradation of Ethylene Vinyl Acetates, EVAs, continues to be well documented. It can be concluded from recent work on water vapor transmission rates that sufficient water can penetrate todays EVA cells to fuel the corrosion processes mitigating hard fought efficiency gains. Today silicones are in wide-spread use as frame or junction-box sealants in the PV industry to protect against water/moisture ingress into EVA or electrical connections, as well as to protect bypass diodes and soldering points in junction box potting applications. Future forecast are cure speeds will be ramped up, and PV module prices will be progressively driven down, is it time to reconsider this proven silicone technology? Dow Corning Corporation has expanded their studies to include the growing list of sulfur-based industrial pollutants. The results of continuous corrosion rate monitoring of silicone encapsulated silver circuits on glass substrates reconfirms the benefits of silicone encapsulants in this application