High-Transmittance Nb-Doped Vanadium Dioxide Thin Films: Crystal Structure Evolution and Thermochromic Characteristics

Utilizing VO2 thermochromic materials to construct smart energy-saving windows is an arduous point in current science research. Herein, the polymer-assisted solution method prepared ultrathin Nb-doped VO2 films with diverse doping concentrations. Subsequently, an examination was carried out to investigate the crystal structure, and first-principles further explored the mechanism underlying the reduction of the phase transition temperature (Tc). The results suggest that the electronic doping and strain effects caused by adding Nb ions in the VO2 crystal lattice cause a decline in the phase transition temperature and a growth in the optical properties. Upon the amount of Nb doping attaining 6 atom %, the Tc of the VO2 films becomes 28.5 °C, approaching room temperature. Meanwhile, the maximum transmittance in the visible region (Tvis) is 70.1%, and the direct sunlight transmittance (Tsol) is 60.6%. The morphology test showed that the prepared films are well-crystallized. XPS spectral analysis reveals the relationship between the V-ion valence transition by doping Nb and the crystal phase transition. First-principles calculations show that the addition of Nb ions changes the original bond length of the crystal and triggers lattice distortion of intrinsic VO2. The band gap of monoclinic crystalline VO2 is lessened. The formation energies of the two crystallographic systems before doping are larger. The formation energy difference is reduced upon doping, and Tc is further reduced. These results are consistent with the experimental results. The high performance of the films illustrates their great possibilities for application in the domain of optical thin films.