Significant dependence of the efficiency of energy-saving thermochromic VO$_2$ on slight changes of its properties in the visible due to strain and/or vacancies

TL;DR

This study investigates how minor changes in strain and vacancies in VO$_2$ influence its visible transmittance modulation, affecting the efficiency of energy-saving thermochromic coatings, with potential for enhanced energy savings.

Contribution

It demonstrates that slight lattice strain and off-stoichiometry can control visible transmittance modulation in VO$_2$, impacting energy-saving performance.

Findings

Visible transmittance modulation varies with strain and vacancies.

Lattice modifications can enhance energy-saving efficiency.

Reproducible preparation could lead to improved thermochromic coatings.

Abstract

There are worldwide efforts to maximize the energy saving achieved by VO$_2$-based thermochromic coatings. In particular, there are very different values of the modulation of integral solar energy transmittance, reported by various laboratories on various templates even for seemingly very similar coatings. A detailed analysis reveals that this is largely due to the combination of the intentional and well understood transmittance modulation in the infrared (always beneficial) with not yet understood slight transmittance modulation in the visible (sometimes beneficial, sometimes harmful, and always multiplied by strong solar irradiance). Ab-initio calculations are used to examine the hypothesis that the transmittance modulation in the visible can be controlled by lattice strain and/or slightly off-stoichiometric [O]/[V] ratio. The presented phenomenon opens a pathway which may lead, in a case of reproducible preparation of correctly altered VO$_2$, to significantly enhanced energy saving.