October 19, 2017 report
Black butterfly wings offer a model for better solar cells
As scientists seek ways to improve the efficiency of solar cells, some have increasingly turned to thin film solar cells. Not only are such cells lighter and more manageable than traditional crystal-based cells, but they are expected to be more efficient if engineers can find a way to get them to work for longer periods of time. One of the roadblocks to improving the efficiency of solar cells in general is the high expense of motion hardware that tracks the sun. In this new effort, the researchers took inspiration from the rose butterfly, found commonly in India. It has soft black wings that warm the cold-blooded insect during cool periods.
To learn more about the butterfly wings, the group collected some samples and looked at them under an electron microscope. They found that the wings were covered with scales pockmarked with holes. In addition to making the wings lighter, the holes, the researchers found, scattered the light striking them, which allowed the butterfly to absorb more of the sun's heat.
Intrigued by the butterfly design, the researchers created similar structures in their lab using sheets of hydrogenated amorphous silicon sheets. A top layer with extremely tiny holes of various sizes caused light to scatter and strike the silicon base below. The design, the team found, allowed for picking up roughly twice as much light as previous designs.
The group reports that creating the cells was quick and easy. They made the holes by dropping bits of a binary polymer solution that did not mix with the type of polymer used to create the sheet. The process took just five to 10 minutes.
The wings of the black butterfly, Pachliopta aristolochiae, are covered by micro- and nanostructured scales that harvest sunlight over a wide spectral and angular range. Considering that these properties are particularly attractive for photovoltaic applications, we analyze the contribution of these micro- and nanostructures, focusing on the structural disorder observed in the wing scales. In addition to microspectroscopy experiments, we conduct three-dimensional optical simulations of the exact scale structure. On the basis of these results, we design nanostructured thin photovoltaic absorbers of disordered nanoholes, which combine efficient light in-coupling and light-trapping properties together with a high angular robustness. Finally, inspired by the phase separation mechanism of self-assembled biophotonic nanostructures, we fabricate these bioinspired absorbers using a scalable, self-assembly patterning technique based on the phase separation of binary polymer mixture. The nanopatterned absorbers achieve a relative integrated absorption increase of 90% at a normal incident angle of light to as high as 200% at large incident angles, demonstrating the potential of black butterfly structures for light-harvesting purposes in thin-film solar cells.
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