A new antireflective coating inspired by the compound lenses in moth eyes could help boost the efficiency of solar cells and sharpen the view of image sensors. Using a simple method to stamp patterned lenses over large areas, researchers in Singapore have come up with a process that could make manufacturing such coatings easier (ACS Nano 2015, DOI: 10.1021/nn5051272).
Antireflective coatings help solar cells collect as much of the sun’s light as possible, boosting the power output. But typically, these thin-film coatings work best at preventing the reflection of a specific wavelength of light, hitting perpendicular to the surface. They don’t catch light of different wavelengths, coming in at other angles. Layering films of different materials of varying thicknesses yields more absorptive coatings, but that approach is expensive and difficult to do over large areas, says Peng Jiang, a chemical engineer at theUniversity of Florida, Gainesville, who was not involved with the work.
Nature provides an alternative design strategy for an affordable, broadband antireflective coating. Nocturnal moths navigate under the dim light of the moon and stars thanks to eyes made of arrays of microsized lenses called ommatidia, which are further patterned with dome-shaped nanostructures. This hierarchical design reduces reflection and also prevents water from beading up on the creatures’ eyes. But re-creating such a design in the lab, by using moth eyes as tiny stamps or by plasma etching, has proved laborious. Although this earlier research has shown that the designs are useful, the methods are not compatible with large-scale manufacturing, says Hemant Kumar Raut of Singapore University of Technology & Design.
To solve this problem, Raut and Mohammad S. M. Saifullah of the Agency for Science, Technology & Research, in Singapore, turned to nanoimprint lithography, a method for stamping high-resolution, nanoscale patterns over large areas. To create a reusable stamp, Raut and Saifullah first made two sets of nickel molds, one patterned with 200-nm-diameter domes and one with microlenses 2 to 25 μm in diameter. The researchers then used these molds to pattern films of polycarbonate. First they stamped the nanodomes and protected that pattern by spinning a thin coating of a sacrificial polymer on top. Afterwards, they stamped the larger microlenses. Finally, they washed away the sacrificial polymer, leaving a polycarbonate microlens array.
The researchers then tested the moth-inspired arrays, comparing them with microlens arrays without the nanodomes, to see how much light they reflected. From 400 to 1,000 nm in wavelength, the moth-inspired arrays reflected just 4.8% of light, compared with 8.7% reflected by the simple microlenses. When they varied the incident angle of the light, the nanodome-decorated arrays continued to perform about twice as well. The nanodomes also repelled water, which could help keep solar cells clean.
The antireflective coatings perform impressively, says Jiang, who is also making bioinspired antireflective coatings. Jiang says the Singapore group now needs to demonstrate that these methods can scale up to make coatings square meters in size. Saifullah says he’s currently adapting the imprinting technique to a roller printer that should be able to pattern large areas at high speeds.