Antireflection coatings on plastic have a myriad of practical applications such as glare reductions on the display on smartphones when outdoors, computer monitors, and eyeglasses.
Recently, a team of researchers at Penn State have created an Antireflection Coatings that advances on existing coatings to such an extent that it can make transparent plastics, like Plexiglas, virtually invisible.
The team made the new discovery when trying to build higher-efficiency solar panels. The work involved focusing light onto small, yet high-efficiency solar cells by using plastic lenses and the researchers needed to reduce their loss of reflection.
According to Chris Giebink, Penn State’s associate professor of electrical engineering, the team needed an Antireflection Coatings that works well over the entire solar spectrum as well as at several angles as the sun travels across the sky. Further, a coating was required that could withstand weather over long periods of time outdoors.
Although it was comparatively simple to make coating that will terminate reflection at a specific wavelength or at a specific direction, the one that would meet all the requirements did not exists, Giebink said.
It is known that reflection occurs when light travels from one medium like air into a second medium, in this case, plastic. The difference in their refractive index specifies the speed of light travelling in a particular medium; large difference indicates air has an R.I. of 1 and plastic 1.5, and there will be a lot of reflection. The lowest R.I. for natural coating material such as Teflon or magnesium fluoride is 1.3. By blending different materials, the R.I. can be graded between 1.3 and 1.5, however, the gap between 1.3 and 1 remains.
The research team developed a new method to bridge the gap between Teflon and air, which has been described in the journal Nano Letters.
Using a sacrificial molecule, the researchers could create nano-scale pores in evaporated Teflon, thereby, developing Teflon-air film of graded index that fools light into viewing a smooth transitions between 1 and 1.5, essentially eliminating all reflections.
When Teflon is heated up in a crucible, the large polymer chains break into smaller fragments, so small that they volatize and send up a vapor flux. When these flux lands on a substrate, they repolymerize and form Teflon.
Adding sacrificial molecules to the flux allows the reformation of Teflon around the molecules. Further, dissolving these molecules produce nanoporous film that can be graded with the addition of more pores.