Butterfly wing inspires photovoltaics: Light absorption can be enhanced by up to 200 percent

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.

The butterfly studied 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. The butterfly’s nanostructures in the silicon absorbing layer of a thin-film solar cell.

FEATURES:

  • 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 does not automatically imply that efficiency of the complete PV system is enhanced by the same factor. Also other components play a role.

  • Hence, the 200 percent are to be considered a theoretical limit for efficiency enhancement.

  • The 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.

  • The thin-film PV absorber can be designed with diameters varying from 133 to 343 nanometers.

Thin-film PV modules represent an economically attractive alternative to conventional crystalline silicon solar cells, as the light-absorbing layer is thinner by a factor of up to 1000 and, hence, material consumption is reduced.

Still, absorption rates of thin layers are below those of crystalline silicon cells. Hence, they are used in systems needing little power, such as pocket calculators or watches. Enhanced absorption would make thin-film cells much more attractive for larger applications, such as photovoltaics systems on roofs.

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