Direct transfer of thermochromic tungsten-doped vanadium dioxide thin-films onto flexible polymeric substrates

Highlights: • Flexible W-doped VO₂ films are fabricated via direct transfer technique. • VO₂ films on mica is transferred onto adhesive-coated PET to form flexible films. • Phase transition temperature is reduced to near-ambient temperature by adding W dopants. • Superior T_lum and ΔT_sol are achieved in flexible W-doped VO₂ films. In this study, we demonstrate the preparation of flexible thermochromic vanadium dioxide (VO₂) thin films via the direct film transfer technique using adhesive polymer films. Phase-change VO₂ films are prepared via solution-based deposition using sub-stoichiometric colloidal VO_x nanoparticles (NPs) on mica substrates; subsequently, a high-temperature rapid thermal annealing process is employed to induce phase transformation to monoclinic VO₂ (VO₂(M)). Thereafter, the direct film transfer of VO₂(M) on mica substrates is conducted using PET substrates coated with an acrylate adhesive to form flexible and transparent mica/VO₂(M)/PET films. The reversible phase transition behaviors of the flexible VO₂(M) films are observed, such as a high luminous transmittance (T_lum > 50%) and solar modulation ability (ΔT_sol > 15%). In addition, doping with tungsten ions systematically decreases the phase transition temperature (T_c) such that it is approximately equal to the ambient temperature, depending on the doping concentration. We observed that VO₂(M) doped with 1.3 at% of W exhibits a T_c of 29 °C with T_lum and ΔT_sol values of 53% and 10%, respectively; these are the highest T_lum and ΔT_sol values reported thus far for flexible VO₂(M) thin films at a near-ambient T_c.