Flower-inspired texture to boost photovoltaics

November 13, 2017 // By Julien Happich
As a follow up research on bio-inspired light-harvesting textures mimicking the surface of roses' petals, researchers from the Karlsruhe Institute of Technology (KIT) have investigated how a photoresist coating imprinted with the texture of Viola petals could further improve light input into photovoltaic cells.

In a paper titled "Texture of the Viola Flower for Light Harvesting in Photovoltaics" published in the ACS Photonics Letter, lead author Raphael Schmager notes that after investigating the light harvesting of various biomimetic plant textures (featuring conical surface elements), those with the highest aspect ratios correlate with reduced reflection losses at the front side of solar cells.

The viola surface texture (red layer) applied to a
planar heterojunction silicon solar cell.
Credit KIT

Stepping up their game from the texture of Rosa “El Toro” (by R. Hünig et al.) which they reproduced with interesting results (with an aspect ratio of 0.6), the researchers turned their attention to another plant species, Viola wittrockiana, whose petal surface texture has an aspect ratio of around 1.2, boasting significantly lower reflection losses at arbitrary angle compared to any petal investigated previously.

The idea being to maximize light harvesting at the surface of solar cells, the researchers first cast a viola petal into a polydimethylsiloxane (PDMS) mould to get a template. Once cured, the silicone rubber viola stamp was used as a master to imprint the texture into a transparent ultraviolet-curable photoresist, on top of a one square centimetre c-Si solar cell.

External Quantum Efficiency plotted versus incident
wavelengths on viola-textured (red) and non-textured
(blue) silicon solar cells. Inserts, a photograph of the
original viola petal and the solar cells side by side.
Credit KIT.

This trick alone accounted for a 6% improvement in power conversion efficiency (PCE) compared to an untreated c-Si cell encapsulated with a flat transparent resist layer of same thickness. The scientists measured that the imprinted coating reduced the reflectance at the resist/air interface, from 4.5% to 1% for close to normal incidence. And at a more oblique incidence angle of 80°, the viola texture reflected 30% less light than the planar reference, exhibiting a broadband reduction of reflectance. But reducing the surface's reflectance is not all. The researchers further observed that the complex viola texture behaved as a retro-reflector for back side illumination, with transmittance for front side lighting more than 30% higher than for the illumination of the back side.

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