Combining high optical transmission, water-repellency and self-cleaning is of great interest for optoelectronic devices operating in outdoor conditions, such as photovoltaics where shading can significantly reduce the power output. The surface of water plant Pistia stratiotes combines these functionalities through a dense layer of transparent microhairs. It renders the surface superhydrophobic without affecting absorption of sunlight necessary for photosynthesis. Inspired by this surface, we fabricated a superhydrophobic flexible thin nanofur film made from optical grade polycarbonate using a scalable combination of hot embossing and hot pulling techniques. During fabrication, heated sandblasted steel plates locally elongate softened polymer, thus covering its surface in microcavities surrounded by high aspect ratio micro- and nanohairs. The superhydrophobic nanofur exhibits contact angles of (166±6°), low sliding angles (<6°) and is self-cleaning against various contaminants. The overall transmission of the self-standing nanofur film stands above 85% over the visible range, with 97% of the transmitted light scattered forward. Reflection drops below 4% when coated on a polymeric substrate, which can enhance light extraction in organic light emitting diodes (OLEDs). We report an increase of more than 10% in luminous efficacy for a nanofur coated OLED compared to a bare device. Finally, the nanofur film can be used for enhancing the incoupling of light to solar cells, while additionally providing self-cleaning properties. Optical coupling of the nanofur to a multi-crystalline silicon solar cell results in a 5.8% gain in photocurrent compared to a bare device under normal incidence.
Felix Vuellers, Guillaume Gomard, Jan B. Preinfalk, Efthymios Klampaftis, Matthias Worgull, Bryce S. Richards, Hendrik Hölscher, and Maryna N. Kavalenka, "Biomimetic hairy surfaces as superhydrophobic highly transmissive films for optical applications (Conference Presentation)," Proc. SPIE 10115, Advanced Fabrication Technologies for Micro/Nano Optics and Photonics X, 1011505 (Presented at SPIE OPTO: January 29, 2017; Published: 28 April 2017); https://doi.org/10.1117/12.2251461.5395719089001.
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