If imitation is the highest form of flattery, there should be plenty of blushing moths. A group of scientists is working on a new way to create a structure similar to the moth eye, albeit with several important differences, to build a better solar cell.
Unlike human eyes, the compound moth’s eyes have a collection of miniature posts across their surface.
These posts allow the moth to absorb a wide range of light without reflecting it back. This prevents the “moth in the headlights” appearance, enabling the insect to blend in without sending a reflection predators might notice.
While Lord Rayleigh worked out the mathematics for why the moth eye geometry eliminates reflection in the 1800s, a team at Brookhaven National Laboratory has come up with a new approach to creating an anti-reflective silicon.
“Our advance is in coming up with a tricky new way” to make a silicon surface that absorbs instead of reflects light, said Chuck Black, a scientist and group leader at the Center for Functional Nanomaterials at BNL. “We think it has practical advantages in applying this” to things like solar cells or even, some day, anti-reflective windshields on cars or windows in buildings.
Companies have been using multilayer coatings to increase the ability of silicon solar cells to absorb light. By etching a nanoscale texture onto the material, researchers including Black and Atikur Rahman, a postdoctoral researcher, were able to create an anti-reflective surface that works as well as multilayer coatings, while outperforming single antireflective film by about 20 percent.
The researchers coated the top of a silicon solar cell with a substance Black has worked with for more than 15 years, called a block copolymer. The advantage to this substance is that it can self-organize into a surface pattern with dimensions of only about 10 nanometers. This pattern enabled the development of posts that are similar to those of a moth’s eyes, even though the features in their structures are much smaller than those in the insect eye.
The challenge in trying to reduce reflections is that sunlight has a wide range of colors at different wavelengths. Substances designed to absorb one color won’t be as effective at capturing a different one.
That’s where the moth enters the picture.
“Nature has learned how to create this anti-reflection,” by using spikes, Black said. “This promotes anti-reflection not just in one but in all wavelengths of light.”
The way this works is somewhat akin to the proverbial frog in a pot of water. In the frog story, a frog sitting in a pot of water that slowly heats up doesn’t jump out of the water even when it’s boiling because it’s adjusted to the changing temperature. Similarly, these spikes draw light of different wavelengths in because the distances between them are all smaller than the wavelength of light. The light effectively reacts to their average properties, Black explained.
When the light travels through these spikes, which are not cylindrical but, rather are thinner at the top and flair at the bottom, it reacts as if it hits something that is a combination of something small and insignificant and air. As the light travels towards the silicon surface, it interacts less with air and more with the spike, where it becomes absorbed by the thicker base before it can reflect back out.
“You’re softening this transition between air and whatever you’re trying to couple the light into,” Black said. Instead of a sharp boundary between the air the light is traveling through and the surface, the spikes ease that interaction, gradually capturing the light.
To demonstrate its effect, Black held a small photograph of his lab above a reflecting surface in which a small square is coated with the anti-reflective material. In the reflection, the square with the anti-reflective substance appears black.
Black and Rahman, who was the lead author on the study, published their results in Nature Communications. They don’t know whether this approach is more economical or efficient than the current multilayer coating for solar cells. They are working with external partners to understand the economic or performance advantages of this approach, he said.
Black and his wife Theresa Lu, who is a physician scientist at the Hospital for Special Surgery, live in Manhattan with their two primary school children, Marina and Charlotte.
As for his work, Black and Rahman filed a patent for this technology last year. “It’s something we’re very proud of,” he said.