Effect of substrate temperature on the near-infrared shielding properties of WO3-x thin films deposited by RF magnetron sputtering

Near-infrared (NIR) shielding materials emerge as potential candidates for energy conservation applications. In this work, tungsten oxide (WO3-x, 0 < x < 1) thin films were deposited on soda-lime glass substrate by RF magnetron sputtering at various substrate temperatures and NIR shielding properties of the films were studied. Structural, morphological and compositional properties of the films were also investigated. Optical constants, thickness and roughness of the films were extracted from spectroscopic ellipsometric (SE) measurements. The correlativity of SE with UV–Vis–NIR spectrophotometry, cross-sectional field emission scanning electron microscopy and atomic force microscopy was investigated. Of all the films, WO3-x thin films deposited at 500 °C showed the lowest NIR transmittance of about 23% with the highest visible transmittance of above 70% corresponding to a Tvis/Tsol ratio of 1.35 and NIR shielding value of 75%. WO3-x coated soda lime glass shows better NIR shielding performance compared to commercial soda lime glass in the thermal shielding test. The underlying reason for NIR absorption is the presence of oxygen vacancies which promote formation of W5+ ions and the resultant combined effects of polaron absorption and surface plasmon resonance. This work is significant in utilizing a facile technique for developing undoped WO3-x thin films for efficient NIR shielding applications.