Moth Eyes and Cicada Wings for the Solar Cells and Windows of the Future

Insects and nanotechnology

What's the link between insects and solar cells? Bugs hold the secret - improved photocells.

Peng Jiang, an assistant professor of chemical engineering at the University of Florida, gets his inspiration from the eyes of moths and the wings of cicadas in developing new anti-reflective and water-repellent coatings, with huge potential for new more effective and self-cleaning solar cells, car and home windows, computer screens and other devices. "Nature is an amazing innovator. What I'm interested in doing is mimicking the structure of some remarkable biological systems for real-world use.", said Jiang.

Moth eyes are built up of adjacent hexagonal eyelets, each one filled with thousands of neat rows of tiny bumps (nipple-like protrusions). As each bump is less than 300 nanometers (300 billionths of a meter), only an electron microscope can visualize them.

When light hits the eye, the bumps manipulate its reflection to zero, as moths, mostly nocturnal, don't reflect moon or starlight, since by doing so they could betray their position to possible predators. "Engineers have sought to replicate the eyes' microscopic structure using a printing technique called lithography, but it is expensive and ill-suited to creating the extremely tiny rows of protrusions that make the moth eyes so effective." said Jiang.

Jiang appealed to spin coating, which does not try to carve out the pattern on a surface, but constructs the pattern up from building blocks.

Jiang added a liquid suspension of nanoparticles on a circular silicon wafer, similar to that employed in photovoltaic cells. The wafer spanned the suspension, so that the centrifugal force distributed the liquid equally. By drying the liquid, an ordered particle pattern was left behind. This relatively low-tech technique delivered a moth eye-like anti-reflective coating for glass and plastic layer. Jiang even managed to use the technique to silicon wafer surfaces by applying a specific cicada wing trait. "Cicada wings are amazingly effective at rapidly shedding water and dirt, apparently because the insects often need to fly in humid environments," said Jiang.

The wings resemble the structure of the moth eyes, but it is orientated in such a way not to deflect the light, but to expel water droplets, using tiny air pockets surrounding each bump. When the researchers placed a drop of water on a stamp-sized wafer coated with the novel material, the drop moved over the surface till it got to its edge. "The anti-reflective coating may improve the performance of solar cells because it would increase the amount of light the cells receive," said Jiang.

Current solar cells reflect over 10 % of the light, while with the new coating, this could plummet to under 2 %. "The water-repelling element would be useful for keeping the cells clean - a necessity because dirt or dust easily dulls their performance. Rain or simply hosing the coated cells down would clean them adequately. The coatings could also improve the performance of ordinary windows on cars or homes, increasing visibility and reducing the need for cleaning. Numerous challenges remain, including learning how to "scale up" the spin coating process so that it could be used for industrial production", he said.

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