Butterfly wings help boost solar cell efficiency

Scientists have successfully reproduced nanostructures found on butterfly wings in solar cells to enhance their light absorption rate by up to 200 per cent.
Sunlight reflected by solar cells is lost as unused energy, according to the study published in the journal Science Advances.
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 by us is very dark black. This signifies that it perfectly absorbs sunlight for optimum heat management said Hendrik Holscher from Karlsruhe Institute of Technology (KIT) in Germany.

"Even more fascinating than its appearance are the mechanisms that help reaching the high absorption. The optimisation potential when transferring these structures to photovoltaics (PV) systems was found to be much higher than expected," said Holscher.
The scientists reproduced the butterfly's nanostructures in the silicon absorbing layer of a thin-film solar cell.

Compared to a smooth surface, the absorption rate of perpendicular incident light increased by 97 per cent and rose continuously until it reached 207 per cent 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," Holscher said.

However, this does not automatically imply that efficiency of the complete PV system is enhanced by the same factor, said Guillaume Gomard of KIT.
"Also other components play a role. Hence, the 200 per cent are to be considered a theoretical limit for efficiency enhancement," said Gomard.
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.
They then analysed the rates of light absorption for various hole patterns in a computer simulation.
The team 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.

The researchers introduced disorderly positioned holes in a thin-film PV absorber, with diameters varying from 133 to 343 nanometres.