Role of hydrogen dynamics and deposition conditions in photochromic YHO/MoO3 bilayer films

Abstract

Oxygen-containing yttrium hydride (YHO) and molybdenum trioxide (MoO₃) bilayer films (YHO/MoO₃) are produced using reactive magnetron sputtering, and their photochromic properties are investigated in relation to the thickness and density of the MoO₃ layer. Compared to single YHO films, the YHO/MoO₃ films exhibit faster coloration and larger contrast, with both parameters adjustable by varying the thickness or deposition pressure of the MoO₃ layer. Transparent YHO/MoO₃ films (∼75 % at 550 nm) demonstrate a photochromic contrast of up to 60 %, significantly higher than the 25–30 % contrast observed for single YHO films after 20 h of UVA-violet light exposure. This enhancement arises from hydrogen intercalation from the (200)-textured polycrystalline YHO film into the X-ray amorphous MoO₃, leading to the formation of molybdenum bronze (H_xMoO₃), as confirmed by X-ray photoelectron and optical spectroscopies. However, the darkened YHO/MoO₃ films do not fully recover to their initial transparency after illumination due to the irreversible nature of the coloured MoO₃ layer. Most of the hydrogen intercalated into MoO₃ originates from the YHO layer during the initial darkening process. Furthermore, the bilayer films are chemically unstable, exhibiting gradual darkening over time even without intentional UV illumination, as confirmed by nuclear reaction analysis.