Ultrathin broadband solar absorbers for UV and UV-NIR based on a scalable plasmonic metamaterial

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In a joint experimental and theoretical approach, we will investigate scalable plasmonic metamaterials for applications as ultrathin broadband solar absorbers either in the UV range or in the entire range from UV to NIR. The present metamaterials consist of highly filled particulate plasmonic nanocomposites with a broad particle size distribution that are deposited on a reflective metallic film coated with a dielectric spacer layer. Our concept is based on tailored inhomogeneities in lengths scales, composition, and other parameters. In particular, it combines the huge broadening of the plasmonic absorption of the individual nanoparticles due to strong near field coupling across the nanogaps between the particles with other broadening effects. For the UV-NIR absorbers, these range from the combination of different plasmonic nanoparticles to damping in alloy nanoparticles and nanoscale roughness to quantum mechanical damping phenomena. For the UV absorbers, only Al particles of different size and shapes will be employed. We aim for broadband absorption in the UV range and from the UV to the NIR range, respectively, with simultaneous low emittance in the IR range and insensitivity to light polarization and incidence angle. The nanocomposites are fabricated by vapor phase deposition involving high-rate gas aggregation cluster sources and magnetron sputtering. The cluster sources, inter alia, allow precise tailoring of the nanostructure through independent control of the nanoparticle filing factor and size distribution. In contrast to other nanostructured plasmonic absorbers, the nanostructure forms during deposition by self-organization and does not require lithography techniques. The method thus lends itself to large-scale fabrication. We finally aim for robustness against harsh environmental conditions and thermal stability. Using cluster sources, we plan to explore the unique potential of combining nanoparticles of different materials and alloy nanoparticles. The design parameters of the absorber such as particle filing factor, size distribution, thickness of the dielectric spacer etc. will be obtained from theoretical analysis and simulations. The analysis includes a multiple scattering formalism, large scale simulations based on finite difference time domain method, and a finite element method to obtain the temperature profiles of the nanocomposites upon absorption of light. The input parameters for the simulations will be provided by various characterization techniques including UV-vis spectroscopy and spectroscopic ellipsometry. We already demonstrated the general feasibility of our concept in preliminary experimental work. While the focus of the present application is clearly on fundamental understanding of the broadband plasmonic absorbers, we expect our results to be highly relevant for future applications in solar energy harvesting.