VO2-based thermally active low emissivity coatings

Abstract

Vanadium dioxide is one of the likeliest candidates for future smart window applications due to its self-regulating nature and potentially simple implementation. However, the material is plagued with multiple drawbacks which need to be addressed before its commercialization. Amongst these, its low luminous transmittance has been the subject of many studies. In the present work, we propose to integrate a VO2 film into a traditional silver-containing low emissivity coating architecture. We first model and discuss the theoretical performance of such a coating in comparison with more standard configurations and demonstrate that the addition of silver offers many advantages for thin VO2 coatings; mainly an increase of luminous transmittance due to the presence of antireflective coatings while maintaining a high solar transmittance variation vs. temperature. The latter is shown to be the result of a displacement of the maximum transmission variation to lower wavelengths where the solar intensity is higher. We then fabricate prototype samples which confirm the predicted performance. Indeed, a silver-containing sample based on the following architecture Si3N4 [57 nm] | VO2 [27 nm] | Ag [11 nm] | Si3N4 [66 nm] is shown to possess the unique combination of a high luminous transmittance of 58.2% in its low temperature state, a solar transmittance variation of 7.1% with the added benefit of a low emissivity of 10%.