Designing Planar, Ultra-Thin, Broad-Band and Material-Versatile Solar Absorbers via Bound-Electron and Exciton Absorption

Abstract: Ultrathin planar absorbing layers, including semiconductor and metal films, and 2D materials, are promising building blocks for solar energy harvesting devices but poor light absorption has been a critical issue. Although interference in ultrathin absorbing layers has been studied to realize near perfect absorption at a specific wavelength, achieving high broadband absorption still remains challenging. Here, we both theoretically and experimentally demonstrated a method to tune not only reflection phase shift but also electromagnetic energy dissipation to design broadband solar absorber with simple planar structure consisting of an ultrathin absorbing layer separated from the metallic substrate by a transparent layer. We explicitly identified by deriving a new formulism that the absorbing material with refractive index proportional to the wavelength as well as extinction coefficient independent of the wavelength, is the ideal building block to create ultrathin planar broadband absorbers. To demonstrate the general strategy for naturally available absorbing materials in both high-loss (refractory metals) and weak-absorption (2D materials) regimes, we leveraged the bound-electron interband transition with a broad Lorentz oscillator peak to design a solar thermal absorber based on a ultrathin Cr layer; and leveraged the strong exciton attributed to the spin-orbit coupling for the spectrum near the band edge, and the bound-electron interband transition for shorter wavelengths, to design a solar photovoltaic absorber based on a atomically thin MoS2 layer. Furthermore, our designed ultrathin broadband solar absorbers with planar structures have comparable absorption properties compared to the absorbers with nanopatterns. Our proposed design strategies pave the way to novel nanometer thick energy harvesting and optoelectronic devices with simple planar structures.