Photochromic materials, which exhibit reversible changes in transparency upon light exposure, have gained significant interest for their potential applications in smart window technologies. Among these materials, yttrium oxyhydride (YOH) stands out due to its stability and promising photochromic properties. A critical challenge in developing practical photochromic windows lies in balancing two key properties: high photochromic contrast (∆T) and short bleaching time (τB). These properties are strongly influenced by the pressure during the thin film’s deposition. This study investigates how thicker YOH films deposited at higher pressures (0.8Pa) affect photochromic performance and explores the impact of combining two layers in double-layer configurations to optimize the Photochromic properties.

YOH thin films were prepared using reactive DC magnetron sputtering at two deposition pressures: 0.5Pa and 0.8Pa. Single-layer samples were deposited at 0.8Pa with varying thicknesses (400nm, 600nm, and 800nm), while double-layer samples were created by stacking layers deposited at different pressures in two configurations: substrate/0.8Pa/0.5Pa (DL-1) and substrate/0.5Pa/0.8Pa (DL-2). The optical and structural properties of these films were characterized using a combination of transmission and reflection measurements, spectroscopic ellipsometry, X-ray diffraction (XRD), and Doppler Broadening Positron Annihilation Spectroscopy (DB-PAS). These techniques provided insights into the relationship between the material’s oxidation state, atomic structure, and photochromic response.

The results revealed that single-layer films deposited at 0.8Pa exhibited lower photochromic contrast (11.4 ± 0.4% for 400nm) but faster bleaching time constants (0.6 hours) compared to those deposited at 0.5Pa, which achieved higher contrast (53.0 ± 0.4%) but slower bleaching time constants (5.1 hours). This trade-off aligns with previous findings, confirming the inverse relationship between deposition pressure and photochromic performance. While thicker films (600nm and 800nm) at 0.8Pa showed increased photochromic contrast, the 800nm sample suffered from inhomogeneity and a lower-than expected contrast, highlighting the challenges of scaling up film thickness for practical applications.

In double layer configurations, the properties of the bottom layer significantly influenced the overall photochromic performance. The DL-1 sample, with a 0.8Pa bottom layer, exhibited increased oxidation and inhomogeneity in the 0.5Pa top layer, resulting in a reduced photochromic contrast of 29.7 ± 0.4% and significantly lower transparent transmission (31.6 ± 0.2%). In contrast, the DL-2 sample, with a 0.5Pa bottom layer, allowed the 0.8Pa top layer to retain its intrinsic photochromic properties, achieving a contrast of 55.2 ± 0.4%, closely matching theoretical predictions of ≈52%. XRD measurements further supported these findings, showing a lattice constant increase of 0.026 ± 0.005Å in the DL-1 sample compared to 0.5Pa single-layer thin films, indicative of higher oxygen incorporation in the top layer. Additionally, both double layer samples exhibited longer bleaching time constants compared to their respective bottom layers, suggesting that the capping of the bottom layer may influence hydrogen mobility within the film.

This study underscores the critical role of deposition pressure, film thickness, and layer sequencing in determining the photochromic properties of YOH thin films. Higher deposition pressures and thicker films lead to increased porosity and inhomogeneity, which can degrade photochromic performance. Additionally, careful control of layer stacking, particularly by using a less porous bottom layer (e.g., 0.5Pa), can result in a photochromic contrast similar to the stacking of layers. However, the bleaching time constant in negatively impacted.