Antireflection coating makes plastic invisible

This discovery came about as scientists were trying to make higher-efficiency solar panels.

Plastic dome coated with a new antireflection coating (right), and uncoated dome (left). IMAGE: GIEBINK LAB/PENN STATE
Plastic dome coated with a new antireflection coating (right), and uncoated dome (left). IMAGE: GIEBINK LAB/PENN STATE

Scientists at the Penn State have developed a new type of Antireflection coating that could potentially improve existing coatings to the extent that it can make transparent plastics, such as Plexiglas, virtually invisible. The technology is compatible with current manufacturing techniques, thus it has scalable and wide applications.

Scientists made this discovery while making more efficient solar panels. They came up with an approach that involves concentrating light onto small, efficient solar panels using the plastic lens.

They needed an antireflection coating that worked well over the entire solar spectrum and at multiple angles as the sun crossed the sky. They also needed a coating that could stand up to weather over long periods of time outdoors.

Although it is comparatively easy to make a coating that will eliminate reflection at a particular wavelength or in a particular direction, one that could fit all their criteria did not exist. For instance, eyeglass AR coatings are targeted to the narrow visible portion of the spectrum. But the solar spectrum is about five times as broad as the visible spectrum, so such a coating would not perform well for a concentrating solar cell system.

Reflections happen when the light goes from one medium, for example, air, into a second medium, for this situation plastic. On the off chance that the distinction in their refractive index, which indicates how quick light goes in a specific material, is extensive — air has a refractive file of 1 and plastic 1.5 — at that point, there will be a ton of reflection.

The lowest index record for a characteristic covering material, for example, magnesium fluoride or Teflon is about 1.3. The refractive index can be evaluated — slowly varied— somewhere in the range of 1.3 and 1.5 by mixing distinctive materials, however the gap somewhere in the range of 1.3 and 1 remains.

In a new study, scientists describe a new process to bridge the gap between Teflon and air. They used a sacrificial molecule to create nanoscale pores in evaporated Teflon, thereby creating a graded index Teflon-air film that fools light into seeing a smooth transition from 1 to 1.5, eliminating essentially all reflections.

When the sacrificial molecules are added to the flux, the Teflon will reform around the molecules. Dissolving the sacrificial molecules out leaves a nanoporous film that can be graded by adding more pores.

Chris Giebink, associate professor of electrical engineering, Penn State said, “The interesting thing about Teflon, which is a polymer, is when you heat it up in a crucible, the large polymer chains cleave into smaller fragments that are small enough to volatile and send up a vapor flux. When these land on a substrate they can repolymerize and form Teflon.”

“We’ve been interacting with a number of companies that are looking for improved antireflection coatings for plastic, and some of the applications have been surprising. They range from eliminating glare from the plastic domes that protect security cameras to eliminating stray reflections inside virtual/augmented -reality headsets.”

One unexpected application is in high altitude UAVs, or unmanned aerial vehicles. These are planes with giant wingspans that are coated with solar cells. Used primarily for reconnaissance, these planes rely on sunlight to stay in near perpetual flight and so a lot of the light they receive is at a glancing angle where reflections are highest. One of the companies that make these solar cells is exploring the AR coating to see if it can improve the amount of light harvested by a UAV.

Giebink said, “The coating adheres well to different types of plastics, but not glass. So, it’s not going to be useful for your typical rooftop solar panel with a protective glass cover. But if concentrating photovoltaics make a comeback, a critical part of those is the plastic Fresnel lenses, and we could make a difference there.”

Baomin Wang, graduate students Christian Ruud said, “Additional authors on the paper, Graded-Index Fluoropolymer Antireflection Coatings for Invisible Plastic Optics.”