Anti-reflective (AR) coatings on plastics have a myriad of practical applications including reduced glare on eyewear, computer monitors and display on your smartphone when outdoors. Penn State researchers have now developed an antiretroviral coating that improves existing coatings to the point where transparent plastics, such as the Plexiglas, are virtually invisible.
"This finding came as we tried to produce more efficient solar panels," said Chris Giebink, associate professor of electrical engineering at Penn State. "Our approach involved concentrating light in small, high-efficiency solar cells using plastic lenses, and we needed to minimize the loss of reflexes."
They needed an antireflective coating that would work well across the solar spectrum and at various angles when the sun would cross the sky. They also needed a coating that would weather the weather for long periods of time outdoors.
"We would like to find a solution ready for use, but there was not one that met our performance requirements," he said. "So we started looking for our own solution."
This was a difficult task. Although it is comparatively easy to make a coating that eliminates reflection at a particular wavelength or in a particular direction, one that could fit into all of your criteria did not exist. For example, the RA coatings for glasses are directed to the visible portion of the spectrum. But the solar spectrum is about five times wider than the visible spectrum, so such a coating would not work well for a concentrated solar cell system.
Reflections occur when light travels from a medium, such as air, to a second medium, in this case the plastic. If the difference in refractive index, which specifies how fast light travels in a given material, is large – the air has a refractive index of 1 and plastic of 1.5 – so there will be plenty of reflection. The lowest index for a natural coating material, such as magnesium fluoride or Teflon, is about 1.3. The refractive index can be classified – slowly varying – between 1.3 and 1.5, mixing different materials, but the difference between 1.3 and 1 remains.
In an article recently published online before printing in the journal Nano Letters, Giebink and co-authors 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 graduated Teflon-index film that illuminates the smooth transition from 1 to 1.5, essentially eliminating all reflections.
"The interesting thing about Teflon, which is a polymer, is when you heat it in a crucible, the large polymer chains break down into smaller fragments that are small enough to volatilize and send a stream of steam. a substrate they can repolymerize and form Teflon, "said Giebink.
When the sacrificial molecules are added to the flow, the Teflon will reform around the molecules. The dissolution of the sacrificial molecules leaves a nanoporous film that can be classified by adding more pores.
"We are interacting with several companies looking for improved anti-reflective coatings for plastic, and some of the applications have surprised us," he said. "They range from eliminating the brightness of the plastic domes that protect the security cameras to eliminating glare lost in virtual / augmented reality headphones."
An unexpected application is in high altitude UAVs or unmanned aerial vehicles. These are aircraft with giant wings that are coated with solar cells. Used primarily for reconnaissance, these airplanes rely on sunlight to remain in close perpetual flight and thus most of the light they receive is at an angle of view where the reflections are higher. One of the companies that manufactures these solar cells is exploring the AR coating to see if it can improve the amount of light captured by a UAV.
As the technology is compatible with current manufacturing techniques, Giebink believes that coating technology is scalable and widely applicable. At this point, his test samples resisted the central climate of Pennsylvania for two years, with little change in properties. In addition, the coating is also antifogging.
"The coating sticks well to different types of plastics, but not glass," he said. "So it will not be useful for your typical rooftop solar panel with a protective glass cover. But if concentrated photovoltaic makes a comeback, a critical part of them is the plastic Fresnel lenses, and we could make a difference there."
Next-generation anti-glare coatings can improve the efficiency of photovoltaic solar cells
Baomin Wang et al., Anti-reflective coatings of gradual index fluoropolymer for invisible plastic optics, Nano Letters (2019) DOI: 10.1021 / acs.nanolett.8b03886