M. Zubkins¹, E. Strods¹, V. Vibornijs¹, E. Letko¹, A. Sarakovskis¹, K. Kundzins¹, I. Aulika¹, J. Gabrusenoks¹, D. Moldarev², A. Fedotovs¹, M. Salari Mehr¹, J. Purans^1
1University of Latvia, Riga, Latvia
²University of Helsinki, Helsinki, Finland
Photochromic oxygen-containing rare-earth hydrides (REHOs) have emerged as a new class of inorganic photochromic thin films. These films combine high visible transparency in the bleached state with reversible, colour-neutral photo-darkening under solar illumination, making them attractive for smart window applications.
We present photochromic YHO, LaHO, and bilayer YHO/MoO₃ films, deposited by magnetron sputtering. The films were characterised by XRD, XPS, ToF-ERDA, NRA, SEM, TEM, spectroscopic ellipsometry, and photochromic property measurements. The films were synthesised via hydride deposition followed by post-oxidation. Pulsed-DCMS and HiPIMS were compared to elucidate the influence of plasma regime, sputtering pressure, and microstructure on oxygen incorporation and photochromic performance.
Structural analysis reveals nanocrystalline cubic REHO phases whose density and texture are influenced by deposition conditions. YHO films deposited at optimal pressure and H₂/Ar gas flow ratio enable sufficient oxygen uptake, resulting in pronounced photochromic properties. Reversible photochromism in LaHO is demonstrated for the first time, extending the REHO material family. The YHO/MoO₃ films exhibit higher contrast due to the hydrogen diffusion. Building on these insights, large-area YHO coatings were deposited on flexible polymer substrates using roll-to-roll technique. These results demonstrate a viable pathway for industrial-scale production of photochromic REHO coatings.