VO2 nanostripe-based thin film with optimized color and solar characteristics for smart windows

This study presents a structuration approach to reach a high luminous transmittance (T-lum) and solar modulation efficiency (Delta T-sol) of thermochromic vanadium dioxide (VO2) thin films. Before starting optical simulations, we confirm that the optical properties of monoclinic VO2 continuous films deposited by reactive magnetron sputtering show a good correspondence with numerical results. Then, calculations on ordered VO2 nanostripes demonstrate an enhanced transmittance due to the presence of vertical openings, leading to a variety of photonic effects. A series of optimizations by varying the column width, period, and film thickness establishes that 20 nm wide and 10 nm separated nanostripes have a Delta T-sol of 14.2% and a T-lum of 47.6% for a film thickness of 250 nm. In comparison to a dense film without nanostructuring, the film transparency (related to T-lum) is significantly enhanced, while Delta T-sol remains unchanged. Furthermore, this also translates into a favorable, less opaque color, so the geometry could be useful for various thermochromic applications, such as smart windows.