We report on a fabrication route based on hot embossing lithography to replicate the surface texture of plants into large-area polymeric foils, with a view to developing multifunctional photovoltaic cover layers. Our approach is demonstrated by reproducing the complex texture of rose petals with a high fidelity from the nano-to the centimeter scale into different polymers, including poly(methyl methacrylate) (PMMA), polycarbonate (PC) and fluorinated ethylene propylene (FEP). We show that the hot embossed PMMA and PC foils, applied as a cover layer onto a copper indium gallium diselenide (Cu(In,Ga)Se2) thin-film solar cell, improve its light harvesting properties compared to (unencapsulated) devices based on an optimized MgF2 antireflective coating, especially at large angles of incidence (>50°). By employing a low surface free energy polymer like FEP, we further demonstrate that the textured cover layer can achieve strong hydrophobicity (with water contact angle around 135°) without any additional surface treatment. This wetting property results in droplets hitting the foil rapidly bouncing off its surface, which can be exploited for self-cleaning purposes. The bioreplication route presented herein can be extended to other biological surfaces, polymers, photovoltaic technologies or to other optical systems to introduce a light harvesting scheme operating efficiently in outdoor conditions.
In transformation optics, coordinate transformations are usually mapped onto equivalent (meta-)material parameter distributions. In 2015, we introduced an approach mapping coordinate transformations onto dielectric free-form surfaces. We presented model experiments on cloaking of reflective contact fingers on solar cells. We now report on the fabrication of masters by 3D laser lithography used for soft imprinting. For prototype silicon heterojunction solar cells investigated under 1-sun illumination, we demonstrate the predicted 9% relative efficiency increase. We additionally show that our approach is adaptable to Lambertian sources, thereby cloaking light-emitting diode contacts to achieve spatially homogeneous emission.