Butterfly Wing Inspires Photovoltaics

Nanostructures of the wing of Pachliopta aristolochiae can be transferred to solar cells and enhance their absorption rates by up to 200 percent. (Source: R. H. Siddique, KIT / Caltech)

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 of Karlsruhe Insti­tute of Tech­nology KIT 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 systems was found to be much higher than expected,” says 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. “This is parti­cularly interesting under European conditions. Frequently, we have diffuse light that hardly falls on solar cells at a vertical angle,” Hölscher says. However, this does not automatically imply that effi­ciency of the complete PV system is enhanced by the same factor, says Guillaume Gomard of IMT. “Also other components play a role. Hence, the 200 percent are to be considered a theo­retical limit for effi­ciency enhance­ment.”

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

The scientists demon­strated that light yield can be enhanced consi­derably by removing material. In the project, they worked with hydro­genated amorphous silicon. According to the researchers, however, any type of thin-film PV tech­nology can be improved with such nano­structures, also on the indus­trial scale. (Source: KIT)

Reference: R. H. Siddique et al.: Bioinspired phase-separated disordered nanostructures for thin photovoltaic absorbers, Sc. Adv., online 18. Oktober 2017; DOI: 10.1126/sciadv.1700232

Link: Biomimetic Surfaces and Scanning Probe Technologies (H. Hölscher), Inst. for Microstructure Technology, Karlsruhe Inst. for Technology KIT, Karlsruhe, Germany

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