The influence of deposition pressure, thickness and thermal effects on the photochromic response in YHO : The effect of growth conditions and illumination wavelength

Sammanfattning

Smart windows can contribute to reducing the energy consumption in buildings. Such windows contain at least one chromic material: electrochromic, thermochromic, photochromic or gasochromic. These materials change their optical properties upon stimulation from an external source, applied voltage, heat, photons or gas. A promising photochromic material is yttrium oxyhydride (YHO) which is a part of the rare-earth oxyhydrides group. When deposited under the right conditions, YHO becomes transparent and shows a photochromic response. The exact nature of the switching mechanism remains even today unclear. In this work, YHO films have been synthesized by magnetron sputtering at different deposition pressures and deposition times. The films were deposited at deposition pressures between 0.5 ‑ 2.5 Pa and thicknesses between 278 – 964 nm. The films have been characterized by profilometry, XRD, ToF-ERDA, SEM and optical measurements. The films deposited below 1 Pa are partially or fully opaque, while films at 1 Pa or higher are transparent. Increasing the deposition makes the film increasingly porous and leads to film with higher oxygen concentration. The lattice constant expands by 2 % when the films becomes completely transparent and increasing the oxygen concentration further does not increase the lattice constant. Increasing the deposition pressure makes the switching speed between the two optical states faster but the photochromic contrast is reduced. The photochromic contrast increases when the film thickness increases and the switch speed slightly reduced. Illuminating the films with different illumination wavelengths have shown that higher energy photons produce a larger photochromic response. All films respond to light from UV to green light and only the 2 Pa films responds noticeably to red light. Annealing the film reduces the photochromic response to a barely detectable level and after the annealing the film to 190 °C the film has lost its yellow tint. A phenomenon was discovered where films did not saturate in the darkened state and instead bleached during illumination. The peaks in the transmission spectra permanently blueshift when the phenomenon appears, indicating a permanent change in the material.