Significant dependence of the efficiency of energy-saving thermochromic VO2 on slight changes of its properties in the visible due to strain and/or vacancies

• Efforts to maximize modulation of solar energy transmittance (Δ T sol ) of VO 2. • Well known contribution of the infrared range (desired positive sign) • Less known contribution of the visible range (both signs) • Enhanced Δ T sol from 2.7–6.7 % to 11.5–18.7 % by desired modulation in the visible. • Possible explanation, strain and/or vacancies, supported by ab-initio calculations. 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.