In the summer of 2016, Solar Novus reported on research coming out of Germany’s Karlsruhe Institute of Technology (KIT): A team had integrated transparent replicas of rose petals into an organic solar cell. In a follow-up study investigating light harvesting of various biomimetic plant textures with conical elements, KIT scientists have now discovered that those conical textures with the highest aspect ratios show significantly lower reflection losses. Thus, mimicking the texture of the viola wittrockiana flower, whose petal surface texture has an aspect ratio of around 1.2, the researchers have developed a photoresist coating imprinted on the surface of a planar heterojunction silicon solar cell.
Hierarchical surface texture boosts efficiency by 6%
“The viola flower shows a hierarchical surface texture consisting of micro- and nano-features,” says lead author Raphael Schmager, PhD student at KIT’s Institute of Microstructure Technology (IMT). “The interplay of both micro-cones and nano-wrinkles leads to excellent light-harvesting properties. By replicating and transferring the texture of viola petals on top of a silicon solar cell, we demonstrate a 6% improvement in power conversion efficiency.”
New fundamental knowledge
The KIT scientists certainly gained fundamental knowledge from experimenting with biomimetic plant textures — specifically from investigating the light-harvesting properties of the viola flower: First, we confirmed that further increase in the aspect ratio of the conical surface elements found on the petals leads to improved light incoupling,” Schmager reports. Furthermore, his team was able to describe and explain the optical working principle of the dual-scale texture. “Thereby, we identified not only the incredibly low reflection of this texture but, moreover, an additional retro-reflection effect leading to increased light-harvesting.”
Photoresist coating with the texture of viola petals
Schmager confirms that the viola texture shows an extremely low and broadband reflection with additional light recapturing abilities. And reducing frontside reflection is advantageous to increasing the amount of light reaching the solar cell. “By encapsulating the solar cell with the viola texture, light that is reflected at the encapsulation/solar cell interface is captured and redirected back to the solar cell,” Schmager illuminates. “This retro-reflection effect is beneficial for the outstanding light harvesting properties found in the texture of the viola flower.”
These latest findings reported by KIT could have a bigger-picture impact on designing future generations of photovoltaic devices. “Looking at future solar modules, textured front glass with biomimetic textures are a very promising route to achieve enhanced energy yield,” Schmager agrees, adding, however, that further research does need to address the soiling and self-cleaning aspects of these textures.
Moving along with this research endeavor at KIT, Schmager notes they first need to optimize the coating material to withstand outdoor conditions, such as extreme temperatures and UV light exposure. Besides the material aspects, he says he and his colleagues are looking into ways to upscale the texture through hot-embossing or roll-to-roll techniques. “By following the bioinspired design, artificial textures can be designed to further improve optical properties for photovoltaic applications,” he says in conclusion.
The paper "Texture of the Viola Flower for Light Harvesting in Photovoltaics” is published in the ACS Photonics Letter.
Written by Sandra Henderson, Research Editor, Solar Novus Today