Thermochromic fenestration with VO2-based materials: Three challenges and how they can be met

Thermochromic VO2-based films have higher infrared transmittance below a “critical” temperature τc than above this temperature and can be used for regulating the solar energy transmittance Tsol in energy efficient windows. Pure VO2 is not practical, though, since (i) the modulation of Tsol is too small at τc, (ii) the luminous transmittance is too low, and (iii) τc is too high. This paper discusses how these three challenges can be met. Specifically, using VO2-based nanoparticle composites rather than films makes it possible to significantly alleviate (i) and (ii), Mg doping of VO2 can give further improvements of (ii), and W doping (and to some extent also Mg doping) is important for (iii) and can bring τc to a comfort temperature. The paper hence delineates a path towards practically useful thermochromic fenestration.