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.
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.
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.
This means that once light has been efficiently harvested by the texture (through reduced reflectance), any light that may be reflected at the resist/solar cell interface is retro-reflected back toward the solar cell again.
Scanning electron microscopy reveals micro- and nanoscale textures, with micro-cones in the order of tens of micrometres, ribbed with nano-wrinkles well under a micron wide, coinciding with visible wavelengths. The researchers performed separate ray tracing simulations of the micro- and nano-textures to get a better understanding of the optical effects at play.
First, the incident light is scattered by the micro-cones to higher propagation angles inside the resist layer, then any transmitted light reflected at the resist/solar cell interface is efficiently retro-reflected toward the solar cell (by reaching the micro-cones from below). In effect the micro-cones not only reduce the front side reflection but also limit light out-coupling.
Then the visible wavelength-scale nano-wrinkles adorning the micro-cones act as an additional antireflection coating. Asked if they expect to be able to design their own optimized antireflective and internal retro-reflective microstructures, lead author Schmager is positive.
“We are currently working on artificially designed textures for even better light harvesting in photovoltaics. The goal here is to maintain the impressive optical properties found in nature and gain an optimized texture for solar cell applications”, Schmager briefly told eeNews Europe.
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