Role of hydrogen dynamics and deposition conditions in photochromic YHO/MoO$_3$ bilayer films

TL;DR

This study investigates how hydrogen dynamics and deposition conditions influence the photochromic behavior of YHO/MoO$_3$ bilayer films, revealing enhanced contrast and faster response compared to single YHO films, with implications for smart window applications.

Contribution

It demonstrates that bilayer YHO/MoO$_3$ films exhibit superior photochromic properties due to hydrogen intercalation, with controllable effects via layer thickness and deposition parameters, and highlights their chemical instability.

Findings

YHO/MoO$_3$ films show up to 60% contrast after UVA exposure.

Hydrogen intercalation forms molybdenum bronze, enhancing photochromic response.

Films gradually darken over time even without illumination.

Abstract

Oxygen-containing yttrium hydride (YHO) and molybdenum trioxide (MoO$_3$) bilayer films (YHO/MoO$_3$) are produced using reactive magnetron sputtering, and their photochromic properties are investigated in relation to the thickness and density of the MoO$_3$ layer. Compared to single YHO films, the YHO/MoO$_3$ films exhibit faster coloration and larger contrast, with both parameters adjustable by varying the thickness or deposition pressure of the MoO$_3$ layer. Transparent YHO/MoO$_3$ 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 hours of UVA-violet light exposure. This enhancement arises from hydrogen intercalation from the (200)-textured polycrystalline YHO film into the X-ray amorphous MoO$_3$, leading to the formation of molybdenum bronze (HxMoO$_3$), as confirmed by X-ray photoelectron and optical spectroscopies. However, the darkened YHO/MoO$_3$ films do not fully recover to their initial transparency after illumination due to the irreversible nature of the coloured MoO$_3$ layer. Most of the hydrogen intercalated into MoO$_3$ 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.