(Invited) Smart Optical Coatings for Energy Saving, Anticounterfeiting and More
Authors: Baloukas Bill , Blanchard Francis , Loquai Simon , Martinu Ludvik 
Published: February 2020

Optical coating (OC) applications represent a multibillion dollar market worldwide. Their applications range from antireflective (AR) coatings found on most optical components, to narrowband optical interference filters used in telecommunication to low emissivity (low-e) coatings for buildings and automobiles. As the range of applications of OCs continuously broadens and the need for an increased performance/versatility increases, it is becoming increasingly important to develop thin film materials with novel nanostructures or based on unconventional materials to supply them with multifunctional properties.

In this context, we have been exploring the characteristics of new smart thin film devices that integrate active materials in passive OC systems, specifically electrochromic (EC) WO3 and thermochromic (TC) VO­2. Regarding VO2, we have recently demonstrated the efficient use of new fabrication technologies such as High Power Impulse Magnetron Sputtering (HiPIMS) for low-temperature deposition of high performance coatings [1] which subsequently have been deposited onto polymeric substrates [2]. This approach could potentially lead to the possibility of retrofitting existing windows. The HiPIMS process has also been shown to result in VO2 films which are three times more durable in stringent high temperature and humidity conditions (80°C and ~100% relative humidity) [3].

Another issue with VO2, aside from its high deposition temperature and durability, is its low visible transmittance (T
lum). In this respect, we have recently explored the integration of VO2 into a passive low-e type OC, such as dielectric|VO2|Ag|dielectric, to obtain a multifunctional architecture with tunable characteristics. Specifically, the use of silver allows one to lower the VO2’s thickness while maintaining a respectable solar transmission variation (ΔT
sol) as a function of temperature; a lower thickness (and thus absorption) consequently results in a higher T
lum. Indeed, we have shown a T
lum of 58.2% with a ΔT
sol of 7.1%. The presence of a thin Ag film also procures low emissivity properties, and prototype samples show values in the 0.1 range. This unique parameter combination could bring such TC films closer to commercial implementation.

On the WO3 side, we have recently developed new single material porous/dense WO3 electrochromic interference filters [4] where both EC as well as interference effects are combined into a single structure. This offers the possibility to further control the transmittance and reflection spectra of such coatings for various applications: color shifting active security devices, variable AR coatings, advanced glazings for architectural glass and more. Controlling the refractive index is of importance, however, WO3 films are also often plagued with low durability. In this respect, we are also presently exploring different film fabrication approaches. Preliminary results indicate that the resulting films can be deposited up to five times faster and display a significantly enhanced durability.

[1] J.-P. Fortier, B. Baloukas, O. Zabeida, J.E. Klemberg-Sapieha, L. Martinu, "Thermochromic VO2 thin films deposited by HiPIMS," Sol. Energ. Mat. Sol. C. 125, 291–296 (2014).

[2] S. Loquai, B. Baloukas, O. Zabeida, J.E. Klemberg-Sapieha, L. Martinu, "HiPIMS-deposited thermochromic VO2 films on polymeric substrates," Sol. Energ. Mat. Sol. C. 155 (2016) 60-69.

[3] S. Loquai, B. Baloukas, J.E. Klemberg-Sapieha, L. Martinu, "HiPIMS-deposited thermochromic VO2 films with high environmental stability," Sol. Energ. Mat. Sol. C. 160 (2017) 217-224.

[4] B. Baloukas, J.-M. Lamarre, L. Martinu, "Electrochromic interference filters fabricated from dense and porous tungsten oxide films," Sol. Energ. Mat. Sol. C. 95 (2011) 807-815.