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Planar Black Silicon: disentangling optical and electrical properties of textured interfaces using transformation optics

Subject Area Experimental Condensed Matter Physics
Term from 2019 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 413644979
 
Final Report Year 2025

Final Report Abstract

This research project investigated a novel approach to enhance the overall efficiency of silicon solar cells by developing advanced dielectric nanostructures for light management that can reduce reflection from the front interface of the solar cell over a broad spectral region and, ideally, also scatter the light into a non-specular direction. While various approaches have been developed in the past years to serve the same purpose, these more conventional methods for reducing reflection from the system rely on texturing the light-absorbing layers, which has a detrimental impact on the electrical performance of solar cells. Therefore, planar interfaces are more desirable, but an additional structure should be placed on top of it that scatters the light into the active layer of the solar cell. To overcome this limitation, our project investigated dielectric nanostructures that can be integrated into existing silicon solar cell technologies. Specifically, the research focused on designing and fabricating anti-reflection coatings with a smoothly varying refractive index and a suitable lateral structure to scatter light, derived from the principles of transformation optics. This allowed us to preserve the geometrical flatness of the semiconductor material to leave it electrically intact while tailoring the optical properties through the metasurface in a manner similar to a textured absorbing medium with optimized scattering properties. The optical and electronic properties of the textured interface are, therefore, decoupled, and the resulting structure can be integrated onto a planar silicon interface. The final structures that we have been targeting consist of laterally structured materials combined with a graded-index profile. The idea behind the innovation builds on the existing notion that a smoothly varying refractive index coating is an ideal in-coupler of light for a broad spectral region, as it is well impedance-matched to its surroundings. Still, out of reciprocity, it also ends up being an ideal out-coupler of light, thus not necessarily ensuring optimal light absorption in the semiconductor. Using insights about designing inhomogeneous media from transformation optics, we design a material whose refractive index not only varies along the normal direction like a standard anti-reflection coating but also along lateral (in-plane) directions. Experimentally, we achieved large-scale fabrication of this inhomogeneous medium by atomic layer deposition of thin layers with varying refractive indices over high-index nanodisks, which were deposited through a colloidal self-assembly process. Both numerical simulations and experimental measurements confirmed that metasurfaces offer a significant enhancement in light absorption and an anti-reflection effect. This project contributes to the broader goal of making solar energy more efficient and accessible, which is crucial for combating climate change and supporting sustainable development.

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