Speaker
Description
Yttrium oxyhydride (YHxOy) is a promising mixed-anion semiconductor in view of its photochromic properties, offering prospects for applications in smart windows. The origin of its pronounced color-neutral photodarkening upon UV illumination and the subsequent process of bleaching back to the transparent state is topic of our investigations. We present in-situ illumination studies based on Positron Annihilation Lifetime Spectroscopy (PALS) and Doppler Broadening Positron Annihilation Spectroscopy (DB-PAS).
In-situ illumination DB-PAS shows that the Doppler S parameter increases progressively upon subsequent photodarkening-bleaching cycling, indicating the formation of open volume defects. Detailed insights are gained from PALS in combination with DFT calculations, demonstrating that yttrium vacancies are present in the as-deposited films, while yttrium-hydrogen divacancies are formed during illumination, pointing to the liberation of hydrogen from octahedral sites in the lattice. Upon cycling, a clear correlation between the evolution of the S parameter and the bleaching kinetics is observed, suggesting that the formation of hydrogen vacancies and/or hydrogen loss is key to understanding the slowing down of the bleaching process. Simultaneously, partially reversible shifts in the Doppler S-W parameters during photodarkening-bleaching are observed. These support the Anderson-Mott insulator-metal transition model for the photochromism, in which electrons localize at hydrogen vacancies and lead to a local metallic electronic structure at sufficiently high concentrations, transforming about 4-8 vol% of the films. During bleaching, in-situ illumination PALS reveals a strong correlation between the lifetime of the first positron annihilation component and the optical transmittance, providing further insights into the evolution of the metallic regions during bleaching.
