Schott’s new silicone adhesive opens window on UV absorption

Schott exec Eric Urruti explains how the company’s new UV silicon adhesive will enable solar array systems to utilize more sunlight and generate more energy, and other uses for the material beyond PV.

by Debra Vogler, senior technical editor, Photovoltaics World

February 10, 2010 – At the recent SPIE Photonics West conference, Schott North America announced its new Deep UV-200, a one-part thermally activated silicone adhesive, targeted to a variety of applications that require high-ultraviolet (UV), visible and near infrared (IR) transmission, including photovoltaic and concentrating solar arrays, and UV-LED applications.

Eric Urruti, director of R&D at Schott North America, told Photovoltaics World that the highly UV transmissive silicone adhesive, which is stable down to 200nm, will enable solar array systems to utilize more sunlight and in turn, generate more energy, due to its “significantly better” resistance to yellowing vs. comparable products. “What you see is very little deterioration over time when it’s in the light path,” he said. This attribute is critical for PV applications at much shorter wavelengths, he added, and it opens up the window to get greater absorption.

The resistance to yellowing is a critical attribute because it is indicative of aging — once the adhesive starts to yellow, it absorbs photons before they get to the PV cell. The resistance to yellowing was achieved by using as little carbon as possible, while still retaining flexibility. “If you take away the carbon, you have pure silica, which is very resistant to UV,” explained Urruti. “As you add more carbon, you slowly start to get silicone, and the material starts taking on less of a glass attribute, and more of an organic material attribute.” The more carbon there is in the silicone, the greater the amount of yellowing with age.

Originally, this product was developed for satellites, which have to be designed with end-of-life considerations in mind. Because it is known that adhesives yellow with continued exposure to UV, satellites must be designed so that they have significantly more PV cells to compensate for the loss of light absorption over time. “Our adhesive allows satellite makers to lighten the payload significantly,” noted Urruti. After 2500hrs of intense UV exposure, the new adhesive has 80% transmission at 400nm, he said. Conversely, another commonly used adhesive has only 30% transmission at that wavelength. After satellite applications, this attribute matters most to solar concentrators and rooftop PV.

Schott is currently evaluating other applications outside of solar, and examining ways to modify the properties of the adhesive. For example, Urruti said the company knows that it can modify the refractive index of the adhesive, which might be useful for optical materials applications where the refractive index needs to be very specific, yet the material has to be resistant to high doses of UV.

Other data associated with the new adhesive is that it is low gassing, has low reactivity to gamma and electron radiation, and thermal stability to 220°C. The company also says that it bonds well to a wide variety of substrates and has a long shelf life. Application for the Deep UV-200 is performed at 80°-100°C by spraying, dipping or casting. Curing is performed at 140°-°180°C for 12-24 hours, depending on the flexibility desired (elastic or rigid).

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