Ultra-broadband UV–VIS–NIR absorbers via nanostructured refractory nitrides

Abstract: The sun provides a widely available and clean source of energy that can support the world in addressing its energy demands with the help of powerful, ultra-broadband ideal solar absorbers. In the quest for the realization of perfect absorption in the visible range for applications in solar energy harvesting, we investigated the absorption effects of refractory nitride materials using numerical simulations. We studied three different materials (NbN, HfN, and TaN) with a circular nanoring structure for their highly efficient absorption of visible light. The meta-absorbers comprise subwavelength periodic patches whose top layer is made of refractory nitrides with a thin dielectric spacer over a highly reflecting thick layer of the same nitride material. All of them behaved as broadband absorbers in the visible region with absorptivity greater than 95% in each of the 376.65–860.09 nm, 413.63–536.00 nm, and 539.57–648.54 nm ranges. The simulated absorptance showed peak values of 99.95%, 99.75%, and 99.21% at 585 nm, 375 nm, and 417 nm for TaN, NbN, and HfN, respectively, whereas the broadband average values were 90.35%, 85.57%, and 86.14%, respectively. The devices had a subwavelength profile with thickness of 320 nm, 319 nm, and 298 nm, respectively. The absorptance characteristics were studied from both the microscopic and macroscopic electromagnetic point of view, i.e., by using excitation of localized electromagnetic resonances in the nanostructures and by applying effective-medium techniques. The presented design is found to be insensitive to variation of the incident angle as well as polarization, with the highest absorption remaining at 90.07%, 95.79%,, and 93.52% for angles as large as 70°. Thus, the proposed design has excellent potential for application in solar thermal energy harvesting, thermoelectrics, and thermal emitters owing to its strong broadband absorption. Graphical abstract: (Figure presented.) Highlights: A broadband absorber designed using refractory metal nitrides for solar thermophotovoltaics (STPV) application was studied. The choice of the refractory metal nitrides (TaN, NbN, and HfN) was based on their superior thermal stability and better lossy characteristics. The design turned out to be compact, symmetric, wide-angle, polarization insensitive, and thermally stable. The presented absorber holds promise for commercial applications in cost-effective solar thermal energy harvesting, thermoelectrics, and thermal emitters. Discussion: Energy-deficient regions around the world are constantly looking for cheaper yet sustainable alternatives to conventional power generation methods that pose serious environmental concerns. Solar thermophotovoltaics have the potential to reduce the supply–demand gap in this context through their higher cell efficiencies and feasibility for large-area applications. While the technology is welcomed with somewhat natural hesitation by the general public owing to higher capital costs, more research is warranted on how to improve the productivity of solar cells and make this alternative even more affordable and thus more acceptable.