Solar Energy Harvesting by Perfect Absorbers Made of Natural Hyperbolic Material

Abstract

A perfect absorber, with pyramidal nanostructures made of a natural hyperbolic material, for solar energy harvesting is proposed in this chapter. A numerical investigation is first carried out for regularly arranged bismuth telluride (Bi2Te3, an anisotropic and natural hyperbolic material) pyramidal nanostructures placed on top of a Ag substrate, and the metamaterial is submerged in water. The calculated results show that the absorptance of the absorber exceeds 99.9% in the wavelength range of 300–2400 nm. The underlying mechanisms are revealed by the electric field and power dissipation density distribution in the absorber. It is found that the slow light effect in the type-II hyperbolic region (300–1000 nm) and the gradient index effect in the long wavelength range (1000–2400 nm) contribute to the perfect absorption of the solar energy for the proposed absorber. Effects of geometry parameters of nano-pyramids and the substrate on optical properties of the proposed absorber are illustrated. In addition, a rough surface with sharp nanostructures made of Bi2Te3 is also numerically studied. Based on simulation results of rough Bi2Te3 surface, samples with nanostructures made of Bi2Te3 are experimentally manufactured and optical properties of the samples are measured by using an integrating sphere with a grating monochromator. The absorptance of the samples can be as high as 97.5%, and the lowest absorptance of the sample is still higher than 94% in the wavelength range of 380–1800 nm. Moreover, other samples are also fabricated and studied to validate underlying mechanisms of the perfect absorption of solar energy. The results of the present study open a new revenue for effectively harvesting solar energy by using metamaterials with nanostructures made of natural hyperbolic materials submerged in water.