Anti-reflective (AR) coating on plastic has many practical applications, including reducing glare on glasses, computer monitors, and the display of a smartphone on the street. Pennsylvania researchers have now developed an AR coating that improves existing coatings to such an extent that it can make clear plastics, such as plexiglass, virtually invisible.
“This discovery happened when we tried to create more efficient solar cells,” said Chris Gibink, assistant professor of electrical engineering, Penn State. "Our approach was to concentrate light on small high-performance solar cells using plastic lenses, and we needed to minimize the loss of reflection."
They needed an anti-reflective coating that worked well across the solar spectrum and at different angles when the sun crossed the sky. They also needed coverage that could withstand the weather for long periods of time outdoors.
“We would like to find a ready-made solution, but there wasn’t one that would meet our performance requirements,” he said. "So, we started to look for our own solution."
It was a high order. Although it was relatively easy to make a coating that would eliminate the reflection at a certain wavelength or in a certain direction, one that could meet all their criteria did not exist. For example, AR coatings for glasses are focused on a narrow visible part of the spectrum. But the spectrum of the Sun is about five times wider than the visible spectrum, so this coverage will not work well for a concentrating system of solar cells.
Reflections occur when light travels from one medium, such as air, to a second medium, in this case plastic. If the difference in their refractive index, which determines the speed of light in a particular material, is large – the air has a refractive index of 1, and plastic – 1.5 – then there will be a lot of reflections. The lowest rate for a natural coating material, such as magnesium fluoride or Teflon, is about 1.3. The refractive index can be changed – slowly vary – from 1.3 to 1.5 by mixing different materials, but the gap between 1.3 and 1 remains.
A recently published article published an article in a journal. Nano LettersGibink and co-authors describe a new process in order to bridge the gap between teflon and air. They used the sacrificial molecule to create nanosized pores in evaporated Teflon, thereby creating a Teflon film with an air filter with a gradient index that deceives the light and gives a smooth transition from 1 to 1.5, eliminating almost all reflections.
“The interesting thing about Teflon, which is a polymer, is that when you heat it in a crucible, large polymer chains are broken down into smaller fragments that are small enough to evaporate and emit a stream of steam. can re-polymerize and form Teflon, "Gibink said.
When the sacrificial molecules are added to the flow, Teflon is reformed around the molecules. The dissolution of the sacrificial molecules leaves a nanoporous film that can be sorted by adding more pores.
“We are working with a number of companies that are looking for improved antireflection coatings for plastics, and some applications have been unexpected,” he said. "They range from eliminating glare from plastic domes that protect security cameras, to eliminating random reflections inside virtual / augmented reality headsets."
One of the unexpected uses are high altitude drones or unmanned aerial vehicles. This aircraft with giant wingspan, which are covered with solar panels. Used mainly for reconnaissance, these planes rely on sunlight to stay in almost endless flight, and therefore the large amount of light they receive is at an angle where the reflections are highest. One of the companies that manufactures these solar cells is researching the AR coating to see if it can improve the amount of light collected by the UAV.
Since the technology is compatible with modern production technologies, Gibink believes that the coating technology is scalable and widely applicable. At this point, his test samples were weathering in central Pennsylvania for two years, with slight changes in properties. In addition, the coating is also anti-fog.
“The coating adheres well to various types of plastics, but not to glass,” he said. “Thus, it will not be useful for your typical solar panel on the roof with a protective glass cover. But if the concentrating photovoltaic cells come back, plastic Fresnel lenses are a critical part of this, and we could make a difference. ”
Anti-reflective coatings of a new generation can increase the efficiency of solar photovoltaic cells
Baoming Wang, et al., Fluoropolymer antireflection coatings with a gradient index for invisible plastic optics, Nano Letters (2019). DOI: 10.1021 / acs.nanolett.8b03886