The nanostructures that optimize light absorption on a black butterfly’s wing have led researchers to a way to improve light harvesting in thin-film solar cells, potentially increasing light absorption rate in cells by as much as 200 percent. 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. Researchers at the Karlsruhe Institute of Technology (KIT) observed that these structures absorb light far better than smooth surfaces. Nanostructures of the wing of Pachliopta aristolochiae can be transferred to solar cells and enhance their absorption rates by up to 200 percent. Courtesy of Radwanul H. Siddique, KIT/Caltech. The researchers analyzed the micro- and nanostructures, focusing on the structural disorder observed in the wing scales. In addition to microspectroscopy experiments, they conducted three-dimensional optical simulations of the exact scale structure. To determine the diameter and arrangement of the nanoholes on the wing of the butterfly, the KIT team used scanning electron microscopy (SEM). Then, they analyzed the rates of light absorption for various hole patterns using computer simulation. They found that disordered holes of varying diameters, such as those found in the black butterfly, produced the most stable absorption rates over the complete spectrum at variable angles of incidence, with respect to periodically arranged monosized nanoholes. Based on these results, researchers introduced disordered nanoholes with diameters varying from 133 to 343 nm in a thin-film photovoltaic (PV) absorber, combining efficient light in-coupling and light-trapping properties with a high angular robustness. Inspired by the phase separation mechanism of the butterfly nanostructures, the researchers fabricated bioinspired absorbers using a scalable, self-assembly patterning technique based on the phase separation of a binary polymer mixture. The nanopatterned absorbers demonstrated a relative integrated absorption increase of 90 percent at a normal incident angle of light, to as high as a 200 percent increase at large incident angles, thus showing the potential of adapting black butterfly structures for light-harvesting purposes in thin-film solar cells. The researchers said that this does not automatically imply that efficiency of the complete PV system is enhanced by the same factor. “Other components also play a role,” said researcher Guillaume Gomard. “Hence, the 200 percent is to be considered a theoretical limit for efficiency enhancement.” Nevertheless, by combining bioinspired nanostructures with thin-film PV absorbers, the KIT researchers were able to improve the functionality of the absorbers by a factor of two. Their work could provide a pathway for further systematic study of nature-inspired nanostructures for optimal design and function of PV devices. “The butterfly studied by us is very dark black. This signifies that it perfectly absorbs sunlight for optimum heat management. Even more fascinating than its appearance are the mechanisms that help reach the high absorption. The optimization potential when transferring these structures to PV systems was found to be much higher than expected,” said researcher Hendrik Hölscher. In reproducing the butterfly nanostructures in the silicon absorbing layer of a thin-film solar cell, the researchers worked with hydrogenated amorphous silicon. They believe that any type of thin-film PV technology could be improved with such nanostructures, and on an industrial scale. The research was published in Science Advances (doi: 10.1126/sciadv.1700232).