The same natural phenomenon that helps moths navigate in the dark could help scientists create more efficient solar panels.
Most moth species are nocturnal, and have evolved an elegant strategy that both improves their night vision and helps them stay out of sight from predators. Researchers have mimicked moths' technique of coating their eyes in nanoscale "spikes" that allow them to absorb nearly all the light that hits them, to increase the amount of light that solar cells can absorb, according to a paper published in the journal Nature Communications.
"Our solar cell collects more than 99 percent of the light across all wavelengths," said senior study author Charles Black of the Brookhaven National Laboratory to Tech Times. "So no matter what color the light is, we're going to capture it."
Black is the absence of color, because it's what we perceive when a surface absorbs all or nearly all of the light that reaches it. When researchers create these nanoscale spikes on the highly reflective surface of a silicon solar cell, it turns an extremely dark black.
This is advantageous for a solar cell, because the more light it captures, the more light that it can possibly turn into electricity. Capturing as much light as possible is useful for moths because it helps them use what little light is available at night to navigate. It also has the added advantage of making their eyes appear extremely dark in color. One of the classic ways to spot an animal in the dark is to catch their eyes as light reflects of them, and moth eyes do not afford predators this opportunity.
The trick is in extremely tiny spikes just tens of nanometers across (a nanometer is one millionth of a millimeter).
"If you look at the surface of the eye of a moth, it's covered with a kind of forest of them, and that imparts this nice property to the eye of the moth — it becomes highly antireflective, meaning that it kind of takes in all of the light that encounters it," said Black. "This kind of moth eye scheme is mathematically the best way to do antireflection. Physics tells you that you can't do better than this — so it's kind of the ultimate."
Creating precise patterns on such a fine scale is however no easy task. Machines with this ability do exist – the machines that print circuits on microchips, for example – but this method can get pricey. To avoid the need for expensive equipment, Black and his colleagues focused on using self-assembling materials called block copolymers to guide the formation of the spikes.
"The power of this approach is that you can coat a surface with this polymer and then you put it in an oven and voila: the pattern forms on its own and it forms all over the surface with a high degree of regularity and it did it all by itself and you didn't have to do it with any machine," he said.
The researchers still had to do some fine-tuning, carving the spikes into shape on the surface using a sort of stencil, but the end result is a surface covered in the nanoscale spikes shown below.
Black said "there's no question that the performance of our solar cell is better [in terms of capturing light] and there's nothing else that could be as good." But he cautions that the issue they're "keen on trying to understand is, is it worth it?"
Capturing light is an important part of a solar cell's job, but generating electricity from that light is also critical. It is possible that this moth eye technique could interfere with electricity generation, though Black said that early tests suggest this is not the case. They also must determine whether the amount of electricity gained from capturing this extra light is enough to justify using the expense of using this method over existing methods for creating antireflective surfaces.
"We've shown what's possible, but there are many more practical questions that need to be answered," Black said.
Photo: Lee Bonnifield | Flickr
