Spectroscopic Ellipsometry and Microstructure Characterization of Photochromic Oxygen‐Containing Yttrium Hydride Thin Films

Abstract

Oxygen‐containing yttrium hydride, YHx:Oy, is an inorganic phothochromic material which exhibits promising features for its application in smart windows, ophthalmic lenses, and many more. In the present work the optical properties of photochromic and non‐photochromic YHx:Oy thin films prepared by magnetron sputtering are studied by variable angle ellipsometry and spectrophotometry. These include the study of the complex refractive index, the transmittance, and luminous transmittance, as well as the reflectance of a selection of samples of diverse structural properties obtained under different deposition conditions. In the case of the photochromic YHx:Oy thin films, the refractive index in the near infrared and visible regions is found similar to the one corresponding to yttrium oxide, Y2O3. However, the YHx:Oy films exhibit a smaller band gap and hence higher absorption in the near ultraviolet than Y2O3. The photodarkening potential of the films are also quantified and characterized by the study and modeling of their optical properties before and after illumination. Our results show that photochromic YHx:Oy films, with a luminous transmittance above 80 % in the clear state, can reduce its transparency up to a 30% after 1 h illumination at 1 Sun.

Light-induced breathing in photochromic yttrium oxyhydrides

“…Since the filling factor ff of the YHO 1−x domains is predicted to be very small [13], the factor η must be η 1. Dilution of YHO 1−x domains in the dielectric YHO structure is necessary to achieve higher optical absorption rather than higher optical reflectance, which is consistent with experimental observations [10,22].…”

“…In order to achieve YHO upon air exposure, precursor YH 2 films have to be deposited when the chamber pressure is above a certain critical value, which results in films with large structural disorder [21]. Further details on the synthesis process of photochromic YHO, both intrinsic and doped with Zr, can be found elsewhere [3,21,22]. A cold white LED array from Thorlabs (color temperature 4600-9000 K) was used as an illumination source for the photodarkening experiments.…”

“…YHO thin films exhibit photochromic properties, that is, YHO films undergo a reversible decrease of their optical transmittance when illuminated with light of adequate energy and intensity [22]. Figure 1(a) shows the transmittance in the clear and photodarkened state for a 1400-nm-thick YHO film.…”

“…Very few oxygen atoms need to be released under illumination to produce a large optical contrast. In fact, ff = 0.02 causes a drop of the visible transmittance larger than 30% [22]. In addition, not all the released oxygen atoms necessarily need to leave the sample, and the material is able to host the outdiffused O atoms [56].…”

“…1(c). Therefore, although the effective medium approximation works very well for modeling the optical properties [22], it is very difficult to confirm the release of oxygen in the experiment.…”