Thermochromic undoped and Mg-doped VO2 thin films and nanoparticles: Optical properties and performance limits for energy efficient windows

Undoped and Mg-doped thermochromic VO2 films with atom ratios z equivalent to Mg/(Mg + V) of 0 <= z < 0.21 were deposited by reactive DC magnetron sputtering onto heated glass and carbon substrates. Elemental compositions were found by Rutherford backscattering spectrometry. Optical constants were determined from transmittance and reflectance measurements and were used for modeling the optical properties of thin films and dilute nanoparticle composite layers below and above the critical temperature for thermochromic switching between a low-temperature infrared transparent state and a high-temperature infrared reflecting or absorbing state. Mg-doped films showed superior luminous transmittance T-lum and solar transmittance modulation Delta T-sol compared to undoped VO2 films, and both of these parameters could be further enhanced by anti-reflection. VO2-containing nanocomposites had much larger values of T-lum and Delta T-sol than VO2-based films. Mg-doping was found to erode the properties of the nanocomposites. Approximate performance limits are given on T-lum and Delta T-sol for thermochromic VO2 films, with and without Mg doping and antireflection coating, and also for VO2-containing dilute nanocomposites.