Tungsten-Coated Silicon Nanopillars as Ultra-Broadband and Thermally Robust Solar Harvesting Materials

Solar harvesting materials or structures, with broadband optical absorption, high-temperature tolerance, and scalable fabrication processes, have attracted much attention for real-life optical applications, such as in solar energy harvesting and solar thermophotovoltaic devices. In the present work, a refractory tungsten-based absorber is proposed that exhibits a high optical absorption above 95% over the AM 1.5 G spectrum (220–2600 nm) and up to 98.5% in the visible light range. This broadband absorber is realized by combining a nanopillar structure, lossy tungsten film, and an Al2O3 encapsulation layer through simple fabrication processes. The excellent optical absorption performance is not sensitive to the thickness of the tungsten layer or the type of nanopillar substrate. An analysis of 3D full-wave simulations indicates that the physical origin of the enhanced broadband absorption is the coupling effect of the cavity mode resonance of the nanopillar structure and the intrinsic absorption of tungsten. Remarkably, a thin Al2O3 encapsulation layer deposited over the whole structure can protect tungsten from oxidation and thus extremely enhance the thermal stability of the absorber, which can survive high-temperature annealing up to 1300 K. Finally, such a hybrid tungsten nanopillar absorber can provide great potential applications ranging from high-temperature photonic devices to solar energy harvesting and conversion devices.