Energy-efficient W-doped VO2-based smart glasses by glancing angle co-deposition and subsequent rapid thermal annealing

Abstract

This work reports on an original approach to the fabrication of W-doped VO2-based coatings of 25 and 50 nm nominal layer thickness which comprises the initial co-deposition, at glancing angles and in the presence of reactive oxygen, of alternating VOy/V0.85W0.15Oy multilayers with average W/V ratios between 1/99 and 5/95, and their subsequent rapid thermal annealing in air atmosphere. Exhaustive micro- and nanostructural, compositional, and optical characterizations of samples treated at reaction temperatures of 475 and 550 °C for times below 30 s confirmed the synthesis of homogeneous VxW1-xO2-based films with unique thermochromic features strongly dependent on the dopant amounts. In general, a gradual lowering of the critical temperature to 37 °C during heating was achieved for a maximum intended doping of x = 0.95, albeit at the expense of some detriments in optical performances. The best and most balanced results were accomplished in a 50 nm nominal thickness layer deposited with overall x = 0.97 (i.e. ∼1 at% W) subjected to flash annealing at 550 °C, presenting a remarkable balance of light transmittance (52.0 %), solar modulation capability (5.2 %) and critical temperature drop (44 °C), which leads to a temperature drop rate of about 20 °C per at% W attempted, taking as reference its undoped counterpart fabricated under the same conditions. In addition, the hysteresis width and smoothness ​​achieved make this coating potentially suitable for application in glazing exposed to direct solar radiation. The proposed methodologies, involving doping by purely metallic precursors and green-like reactions, postulate as an attractive, plausible and cost-effective alternative for large-scale manufacturing of VO2(M)-based smart coatings.