Enhanced photothermal conversion in nanometric scale MoOx multilayers with Al2O3 passivation layer

Abstract

An optically selective stack based on molybdenum oxide nanometric layers (Mo/MoOx/Mo/MoO3/Al2O3) was designed using SCOUT reflectance simulations and fabricated using balanced magnetron sputtering for applications in solar thermal systems. The material properties were experimentally optimised by varying the process parameters namely: target power, gas flow rates, and deposition time. In the stack, the bottommost Mo metal layer is used to improve adhesion, reduce diffusion while acting as an infrared reflector, the MoOx layer of the tandem stack acts as the primary light absorbing layer, and the topmost MoO3 + Al2O3 layers act as anti-reflection layers. The individual layers of the optical stack exhibited an amorphous structure as confirmed using X-ray diffraction. The existence of lower oxidation states (+4, +5) of molybdenum in the MoOx layer was revealed by X-ray photoelectron spectroscopy. The stack achieved a high absorptance in the solar spectrum region (α = 0.969) and a low thermal emissivity in the infrared region (ε = 0.15 at 82 °C) at optimal process parameters. The oxidation resistance and thermal stability were evaluated by annealing the samples in vacuum up to 500 °C.