Sunlight reflected by solar cells is lost as unused energy. The wings of the butterfly Pachliopta aristolochiae are drilled by nanostructures (nanoholes) that help absorbing light over a wide spectrum far better than smooth surfaces. Researchers have now succeeded in transferring these nanostructures to solar cells and, thus, enhancing their light absorption rate by up to 200 percent.
“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 reaching the high absorption. The optimization potential when transferring these structures to photovoltaics (PV) systems was found to be much higher than expected,” says Dr. Hendrik Hölscher of KIT’s Institute of Microstructure Technology (IMT). The scientists reproduced the butterfly’s nanostructures in the silicon absorbing layer of a thin-film solar cell. Subsequent analysis of light absorption yielded promising results: Compared to a smooth surface, the absorption rate of perpendicular incident light increases by 97% and rises continuously until it reaches 207% at an angle of incidence of 50 degrees.
Prior to transferring the nanostructures to solar cells, the researchers determined the diameter and arrangement of the nanoholes on the wing of the butterfly by means of scanning electron microscopy. Then, they analyzed the rates of light absorption for various hole patterns in a computer simulation. They found that disordered holes of varying diameters, such as those found in the black butterfly, produced most stable absorption rates over the complete spectrum at variable angles of incidence, with respect to periodically arranged monosized nanoholes. Hence, the researchers introduced disorderly positioned holes in a thin-film PV absorber, with diameters varying from 133 to 343 nanometers.