Anti-reflection (AR) plastics coatings have many practical applications, including reducing glasses glare, computer monitors and displays on your smart phone when outdoors. Now Penn State scientists have developed an AR coating that improves on existing coatings to the extent that it can create transparent plastics, such as plexiglass, virtually invisible.
"This discovery came as we tried to produce solar panels with higher efficiency," said Chris Giebink, professor of electrical engineering at Penn State. "Our approach included the concentration of light on small, highly efficient solar cells using plastic lenses and we needed to minimize their loss of reflection."
They needed an anti-reflective coating that worked well across the solar spectrum and at multiple angles as the sun was passing through the sky. They also needed a coating that could resist the weather long time out.
"We'd like to find a solution that is not available, but it was not what met our performance requirements," he said. "So we started looking for a solution."
It was a high order. Although it is relatively easy to create a coating that eliminates reflection at a certain wavelength or in a certain direction, there is none that would suit all of their criteria. For example, AR coatings are aimed at a narrow visible portion of the spectrum. But the solar spectrum is about five times as wide as the visible spectrum, so this coating will not work well for the solar concentrating system.
Reflections occur when the light moves from one medium, such as air, into a second medium, in this case plastic. If there is a difference in the refractive index that specifies how quickly light moves in a particular material, there is a large air with a refractive index of 1 and a plastic of 1.5 – then there will be plenty of reflection. The lowest index for natural coating materials such as magnesium fluoride or teflon is about 1.3. The refractive index may be quite different – from 1.3 to 1.5 mixing different materials, but the gap between 1.3 and 1 will remain.
In a post that was recently published online before printing in the magazine Nano Letters, Giebink and co-authors describe a new process to bridge the gap between Teflon and the air. They used the sacrificial molecule to form pores of nanometers in the vaporized Teflon to form a sorted Teflon-air index that blinks light into a smooth transition from 1 to 1.5 and eliminates virtually all reflections.
"Teflon, which is a polymer, is interesting when you heat it in a crucible, large polymer chains split into smaller fragments that are small enough to blast and boost the flow of vapors, they can repolymerize and create teflon," said Giebink.
When the sacrificial molecules are added to the flow, the teflon will be reformed around the molecules. Dissolving the sacrificial molecules leaves a nanoporous film that can be classified by adding more pores.
"We have worked with a number of companies looking for improved anti-reflective coatings for plastics, and some applications have been surprising," he said. "They range from eliminating glare from plastic dome that protects security cameras to eliminate scattered reflections within virtual / expanded headphones."
One unexpected application is in high-floor unmanned aircraft or unmanned aircraft. These are planes with giant wings that are covered by solar cells. Used primarily for exploration, these planes rely on sunlight to stay close to eternal flight, and so much light they receive is at the point of view where the highest reflections are. One of the companies that manufactures these solar cells examines the AR coating to see if it can improve the amount of light harvested by the UAV.
Because technology is compatible with current manufacturing practices, Giebink believes that coating technology is scalable and widely used. At this point, his test samples stood in central Pennsylvania for two years, with little change in properties. In addition, the coating is also anti-odor.
"The coating keeps a good deal of plastic, but not glass," he said. "It will not be useful for a typical roof solar panel with a glass shield, but if photovoltaics are focused on returning, plastic Fresnel lenses are critical, and we could have an impact."
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Baomin Wang et al., Anti-Reflection Coatings for Invisible Plastic Optics, Nano Letters (2019). DOI: 10.1021 / acs.nanolett.8b03886