Photovoltaic Cells Replicate Rose Petals

Biomimetics: The epidermis of a rose petal is replicated in a transparent layer which is then integrated into the front of a solar cell. (Source:  G. Gomard, KIT)

Biomimetics: The epidermis of a rose petal is replicated in a transparent layer which is then integrated into the front of a solar cell. (Source: G. Gomard, KIT)

With a surface resembling that of plants, solar cells improve light-harves­ting and thus generate more power. Scientists of Karlsruhe Institute of Technology KIT reproduced the epidermal cells of rose petals that have parti­cularly good anti­reflection properties and integrated the transparent replicas into an organic solar cell. This resulted in a relative efficiency gain of twelve percent.

Photo­voltaics works in a similar way as the photo­synthesis of plants. Light energy is absorbed and converted into a different form of energy. In this process, it is important to use a possibly large portion of the sun’s light spectrum and to trap the light from various incidence angles as the angle changes with the sun’s position. Plants have this capa­bility as a result of a long evolution process – reason enough for photo­voltaics researchers to look closely at nature when developing solar cells with a broad absorption spectrum and a high incidence angle tolerance.

Scientists at the KIT and the ZSW Center for Solar Energy and Hydrogen Research Baden-Württem­berg now suggest to replicate the outermost tissue of the petals of higher plants, the so-called epidermis, in a transparent layer and integrate that layer into the front of solar cells in order to increase their effi­ciency.

First, the researchers 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 and thus 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 Guillaume Gomard.

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 twelve percent 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,” Guillaume Gomard explains. “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.” The results of this research lead to another basic question: What is the role of disor­ganization in complex photonic structures? Further studies are now examining this issue with the perspective that the next generation of solar cells might benefit from their results. (Source: KIT)

Reference: R. Hünig et al.: Flower Power: Exploiting Plants’ Epidermal Structures for Enhanced Light Harvesting in Thin-Film Solar Cells, Adv. Opt. Mat., online 30 May 2016; DOI: 10.1002/adom.201600046

Links: Center for Solar Energy and Hydrogen Research ZSW, Stuttgart, Germany • Light Technology Institute, Karlsruhe Institute of Technology KIT, Karlsruhe, Germany

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