Copying roses boosts solar cell efficiency
Researchers at the Karlsruhe Institute of Technology (KIT) have replicated the epidermal cells of rose petals that have particularly good antireflection properties and integrated them into an organic solar cell to boost the efficiency of the power conversion.
Photovoltaics researchers have been looking closely at nature when developing solar cells with a broad absorption spectrum and a high incidence angle tolerance. The researchers at KIT’s Light Technology Institute (LTI), the Institute of Microstructure Technology (IMT), the Institute of Applied Physics (APH), and the Zoological Institute (ZOO) of KIT as well as the Center for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) investigated the optical properties, and above all, the antireflection effect of the epidermal cells of different plant species. These properties are particularly pronounced in rose petals where they provide stronger color contrasts to increase the chance of pollination. As the scientists found out under the electron microscope, the epidermis of rose petals consists of a disorganized arrangement of densely packed microstructures, with additional ribs formed by randomly positioned nanostructures.
In order to exactly replicate the structure of these epidermal cells over a larger area, the scientists transferred it to a mold made of polydimethylsiloxane, a silicon-based polymer, pressed the resulting negative structure into optical glue which was finally left to cure under UV light. “This easy and cost-effective method creates microstructures of a depth and density that are hardly achievable with artificial techniques,” says Dr. Guillaume Gomard, Nanopothonics Group Leader at KIT’s LTI.
The scientists then integrated the transparent replica of the rose petal epidermis into an organic solar cell. This resulted in power conversion efficiency gains of 12% for vertically incident light. At very shallow incidence angles, the efficiency gain was even higher. The scientists attribute this gain primarily to the excellent omnidirectional antireflection properties of the replicated epidermis that is able to reduce surface reflection to a value below five percent, even for a light incidence angle of nearly 80 degrees. In addition, as examinations using a confocal laser microscope showed, every single replicated epidermal cell works as a microlense. The microlense effect extends the optical path within the solar cell, enhances the light-matter-interaction, and increases the probability that the photons will be absorbed.
“Our method is applicable to both other plant species and other PV technologies,” said Gomard. “Since the surfaces of plants have multifunctional properties, it might be possible in the future to apply multiple of these properties in a single step.”
More information about the KIT Energy Center is at https://www.energy.kit.edu
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