We demonstrate that white coatings consisting of silicone embedded with randomly distributed microbubbles provide highly efficient daytime radiative cooling with inexpensive materials and fabrication processes. In our material system, sunlight is strongly scattered with minimal absorption, and heat is effectively removed through mid-infrared (IR) radiation. In our previous study, solid microsphere-based coatings outperformed commercial solar-rejection white paint in cooling efficiency, but their mechanical robustness needed improvement for practical applications. The material system in our work substantially enhances the mechanical robustness while providing superior cooling performance to commercial solar-rejection paint. For ease of processing, we use nonoptimized structures with reduced optical scattering strength. Strong solar rejection is yet achieved by increasing coating thickness. This strategy is desirable for practical rooftop applications where coating thickness is of minor importance in comparison to cooling performance and materials cost. In addition, silicone is stable in extraterrestrial environments and efficiently radiates heat over broad mid-IR spectrum. These material properties of our silicone coatings promise great potential for radiative cooling in space applications.
We examine light-trapping in thin crystalline silicon periodic nanostructures for solar cell applications. Using group
theory, we show that light-trapping can be improved over a broad band when structural mirror symmetry is broken. This
finding allows us to obtain surface nanostructures with an absorptance exceeding the Lambertian limit over a broad band
at normal incidence. Further, we demonstrate that the absorptance of nanorod arrays with symmetry breaking not only
exceeds the Lambertian limit over a range of spectrum but also closely follows the limit over the entire spectrum of
interest for isotropic incident radiation. These effects correspond to a reduction in silicon mass by two orders of
magnitude, pointing to the promising future of thin crystalline silicon solar cells.