A Black Butterfly’s Wings Point the Way Toward Better Solar Cells

By mimicking the nanostructures of black-colored butterfly wings, researchers doubled the amount of energy that solar panels convert to electricity.

A microscopic pattern on the wings of a butterfly has shown scientists how to capture more of the sun’s energy in solar cells.

The scales of the black-colored common rose butterfly are topped with an irregular lattice of chitin and melanin. Those structures drew the attention of Radwan Siddique, an engineer trying to develop a technique for building 3D nanostructures as part of his doctoral work at Germany’s Karlsruhe Institute of Technology.

Siddique told Seeker he came across a description of the butterfly’s wings in the course of his research. The lattice helps the cold-blooded insect regulate its body temperature, keeping it warm enough to fly in cool weather, he said.

“I was so intrigued that I literally went to a lot of butterfly nurseries and gathered several butterflies,” said Siddique, now a post-doctoral researcher at Caltech. “The black butterfly was one of them. I was putting them under SEM [an electron microscope] and looking at the structures.” It openings are less than a millionth of a meter wide, but they scatter light and help the butterfly absorb more of the sun’s heat.

“They use those nanostructures to improve absorption,” he said. “So I asked, ‘Can we use the same nanostructres in this type of solar cell, which is not highly used because the absorption isn’t that good?’”

By mimicking the butterfly’s structure in a sheet of hydrogenated amorphous silicon, he and his colleagues at Karlsruhe were able to capture more low-frequency light — at wavelengths near the infrared end of the spectrum — that wouldn’t have been converted to energy otherwise. A layer of polymer pockmarked with circular indentations of various sizes, transferred to a silicon base, was able to pick up about double the amount of energy that a smooth surface produced and convert it to electricity. Angling those indentations might improve those efficiencies even more, he said.

The findings were published Oct. 18 in the research journal Science Advances. They’re part of a growing body of research aimed at improving the efficiency and reducing the size of solar cells.

The ability to produce solar power with a thin film, as opposed to the larger, more typical crystal-based cells, holds the promise of making them more useful. They could be used to power personal electronics or for larger-scale applications, such as being incorporated into windows or other building materials.

And Siddique and his colleagues aren’t the first to find inspiration in the natural world: Earlier this year, researchers in Australia etched a fractal pattern inspired by the leaves of a fern onto sheets of graphene to increase the surface area available for storing and conducting energy.

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Siddique said producing his butterfly-mimicking material is quick and easy. The indentations are made using droplets of a binary polymer solution that doesn't mix with the type used to produce the sheet.

“The way we produce the structure is so simple,” he said. “We need just 5 minutes to 10 minutes to make the nanostructures on a six-inch wafer of silicon.”

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