The synergistic effect of Ta-doping and antireflective TaOx layer on the thermochromic VO2 thin films for smart windows

Abstract

Current efforts to promote the use of VO₂ materials as a promising candidate for thermochromic smart windows continue to be hindered by the low luminous transmittance (T_lum), the limited solar modulation ability (ΔT_sol), the high phase transition temperature (T_t, ∼68 °C) and the wide thermal hysteresis (ΔT). The present work addresses these issues by fabricating an antireflective TaO_x layer on the Ta-doped VO₂ (i.e., VO₂(Ta)) thin films. The VO₂(4.35 at% Ta) thin films with low T_t of 29.87 °C and narrowed ΔT of almost zero were grown on glass substrates by magnetron sputtering at room temperature followed by rapid thermal annealing. However, the VO₂(Ta) thin films show slightly enhanced T_lum of 38.31 % compared with undoped VO₂ films (35.46 %) and decreased the ΔT_sol by 1.83 %. The thermochromic properties are further enhanced by depositing the antireflective TaO_x layer onto the VO₂(Ta) thin film by reactive magnetron sputtering, which significantly increases the T_lum from 38.31 % to 48.58 %, marginally decreases the ΔT_sol from 4.44 % to 4.12 %, and maintain the T_t and ΔT constant. These results verify the possibility of developing VO₂ films with simultaneously increased optical transmittance, decreased T_t to suitable values, and extremely narrow ΔT by using the synergistic effect of element doping and constructing an antireflective layer.