Photochromic smart windows form a promising class of energy-saving technologies for the built environment. In recent years, sputter-deposited yttrium oxyhydride (YO_xH_y) coatings have been gaining attention due to their desirable photochromic properties such as colour-neutral darkening, high contrast, and apparent durability. For commercial application in smart windows several issues remain to be addressed, e.g their yellowish colour and bleaching time scales of the order of hours. Tailoring these photochromic properties is difficult because the mechanisms involved are largely unknown.

In this thesis, the influence of deposition pressure on photochromic contrast and film stability is linked to film microstructure to optimise the photochromic properties for smart window applications. Both contrast and stability are improved by obtaining a densest-possible microstructure. As a first step towards achieving colourless transparency, proof of concept is provided for the ability to tune the band gap by co-sputtering with dysprosium.

A metallic yttrium dihydride (YH_y) domain growth darkening model is introduced to describe the photochromic behaviour of YO_xH_y. The model appears to be consistent with observations on film porosity, contrast, kinetics, and irreversible darkening of YH_y. This might introduce possibilities to limiting bleaching times, as well as giving better insights into the photochromic mechanisms for YO_xH_y.