Butterfly Wings Provide Clues To Building Better Solar Cells

The bright colors of butterfly wings bring much beauty into the world. However, for practical lessons, ones that might even help to protect that beauty, scientists have turned to the pure black wings of Pachliopta aristolochiae, which manage to do an extraordinarily good job of absorbing light.

In theory, a black object absorbs all the light that falls on it, reflecting none back. That’s never entirely true though. Even vantablack, the blackest substance ever invented, reflects 0.035 percent of the light it meets, and normal objects we think of as black reflect much more.

However, P. aristolochiae has spent a long time evolving in a niche where the less light it reflects, the more likely it is to survive. As a result, Dr Radwanul Siddique of California Institute of Technology notes in Science Advances, its wings are covered in scales shaped at the micro and nanoscale to trap sunlight across its full spectrum.

Siddique and co-authors examined these structures in great detail and then attempted to replicate them in solar cells. Better still, they managed to make the structures self-assembling through a mixture of two polymers, making manufacturing much easier.

The solar market is dominated by silicon crystal cells. Theoretically, thin-film cells offer a lot of advantages over silicon crystals but have been held back by various obstacles, including their failure to absorb all available light. A lot of work has been done to try and resolve this, with structures with a controlled level of disorder outperforming both those with a regular pattern and random texturing. Since controlled disorder is what butterflies use to make their wings black, Siddique decided to learn from the masters.

The wings have both matt and dull black areas, featuring tiny holes with diameters of around 300 nanometers (a hundred thousandth of an inch). The density of the holes influences the different types of blackness across the wing.

Using information gained from the butterflies, Siddique made thin-film absorbers that proved very effective at capturing more light – more than tripling absorption of photons striking the material at an angle of 50 degrees. The authors added that further improvements are possible, such as using the inverted pyramids already etched into many solar cells, rather than the cylindrical shapes used by the butterflies and replicated in their study.

In the famous example given for Chaos Theory, the flapping of a butterfly’s wings in Brazil could cause a hurricane to strike North America. If P. aristolochiae’s secrets can be applied to building better solar cells, however, this could be one butterfly that stops a lot more hurricanes than it starts.

A cell inspired by the butterfly wings under optical simulation, showing the irregular distribution of hole sizes. Radwanul Hasan Siddique, KIT/Caltech


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