Light photocells more effective by replicating leaf surface structures

Humans may be able to imitate leaves from the natural plant library of more than 350,000 species, and tailor optical properties of artificial film with careful selections of their mimicked masters.  This has potential applications in energy harvesting to alleviate fossil fuel depletion.  The foils imprinted with leaf structure can also be used in other industries such as in mosaic roof in greenhouses for agriculture.

Leaf mimicking structures have been studied in an interdisciplinary collaboration by the Nano and Molecular Systems Research Group of the University of Oulu, and Chinese institutes. The work is considered as a possible strategic route in optimization of anti-reflection covers for existing photovoltaics. The research was recently released in Scientific Reports, a journal of the Nature Publishing Group.

A close look at a green leaf's surface reveals complicated structures containing veins, blades, and coarse surface. In evolution, leaves have optimized structures on upper epidermises to adapt themselves in different environments. Solar energy is the most clean and abundant source of energy within our disposal. Beside the natural energy harvesting units of leaves, man-made photocells such as silicon photovoltaics (PV) are widely used for power generation and heating. However, bare silicon has a high surface reflection of over 30%. To decrease this rate, anti-reflection coatings need to be applied on the PV cells. Finding proper coating structures is important for increasing the efficiencies of artificial light harvesting systems.

Inspired by leaf surface structures and their functions in light harvesting, leaf morphologies were replicated on to poly-(methyl methacrylate) (PMMA) polymers with the aim of increasing light photocell efficiencies. Investigations were emphasized on optical properties of the leaf-mimicking poly-(methyl methacrylate) (LM-PMMAs) and changes of photovoltaic efficiencies after folding the biomimetic polymers onto the solar cells. In total, leaves from 32 master plants were mimicked. The LM-PMMAs have high transmission rates and haze rates, denoting a key trade-off between light transmission and transmission haze. Due to the optical properties and rich textured 3D surfaces, the LM-PMMAs substantially increased light harvesting efficiencies. It is found that a replica of the corn leaf structure can bring 17% gains to a studied photovoltaic cell power. Optical properties and efficiency can be adjusted, depending on the master species.

To understand origins of the LM-PMMAs' magic in light harvesting, sunlight propagation simulations  were carried out and discussed to tailor light guiding films in light harvesting systems. The LM-PMMA structures were inherited from their corresponding master leaves. Microscopic images showed that the morphologies have micro-patterns decorated with nano structures. These morphologies were further abstracted into regular geometric models which are applied as inputs in the Monte-Carlo simulations of light transport. The microspheres covered by nano-bars were found to have best performances as the AR shape among the mimicked structures. The results are in accordance with the observed optical and light harvesting experiments.

Replication of Leaf Surface Structures for Light Harvesting, Scientific Reports, 5, 14281, 2015 [10.1038/srep14281]

Dos Wei Cao
Prof. Marko Huttula

Nano and Molecular Systems Research Group
Faculty of Science

Last updated: 23.10.2015