Scientists at KIT and ZSW have replicated the epidermis of a rose petal in a transparent material that is integrated to the front of a solar cell. Image: Guillaume Gomard, KIT.
Scientists at KIT and ZSW have replicated the epidermis of a rose petal in a transparent material that is integrated to the front of a solar cell. Image: Guillaume Gomard, KIT.

Providing solar cells with a surface that resembles flower petals can enhance their light-harvesting abilities and thus allow them to generate more power, say scientists at the Karlsruhe Institute of Technology (KIT) in Germany. The scientists reproduced the epidermal cells of rose petals, which have particularly good anti-reflection properties, in a transparent material, which they then incorporated into an organic solar cell. A paper on this work appears in Advanced Optical Materials.

Like plants, solar cells absorb light energy and convert it into a different form. To do this efficiently, it is important to use as large a portion of the sun's light spectrum as possible and to trap light coming in from various incidence angles as the sun's position changes. Evolution has ensured that plants are able to do this, which is reason enough for scientists to look closely at nature when developing solar cells with a broad absorption spectrum and a high incidence angle tolerance.

So scientists at KIT and the Center for Solar Energy and Hydrogen Research (ZSW), also in Germany, decided to investigate the optical and anti-reflection properties of the epidermal cells in different plant species. These properties are particularly pronounced in rose petals, where they provide strong color contrasts and thus increase the chance of pollination. By studying the epidermis of rose petals with an electron microscope, the scientists discovered that it is made up of a disorganized arrangement of densely packed microstructures, with additional ribs formed by randomly-positioned nanostructures.

In order to produce a synthetic replica of this structure, the scientists first created a negative mold of the epidermis in a silicon-based polymer called polydimethylsiloxane, and then pressed this negative mold into transparent optical glue that was 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 Guillaume Gomard, group leader ‘Nanopothonics’ at KIT's Light Technology Institute.

The scientists then integrated the transparent replica of the rose petal epidermis into an organic solar cell, increasing the cell’s power conversion efficiency by 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 anti-reflection properties of the replicated epidermis, which is able to reduce surface reflection to below 5% even for a light incidence angle of nearly 80°.

In addition, investigations with a confocal laser microscope revealed that every single replicated epidermal cell works as a microlens. This microlens effect extends the optical path within the solar cell, enhancing the light-matter-interaction and so increasing the probability that photons will be absorbed.

"Our method is applicable to both other plant species and other photovoltaic technologies," explains 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."

This story is adapted from material from the Karlsruhe Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.