Modulation of Structure and Optical Property of Nitrogen-Incorporated VO₂ (M1) Thin Films by Polyvinyl Pyrrolidone

VO₂, as a promising material for smart windows, has attracted much attention, and researchers have been continuously striving to optimize the performance of VO₂-based materials. Herein, nitrogen-incorporated VO₂ (M1) thin films, using a polyvinylpyrrolidone (PVP)-assisted sol-gel method followed by heat treatment in NH₃ atmosphere, were synthesized, which exhibited a good solar modulation efficiency (ΔT) of 4.99% and modulation efficiency of 37.6% at 2000 nm (ΔT), while their visible integrated transmittance (T) ranged from 52.19% to 56.79% after the phase transition. The crystallization, microstructure, and thickness of the film could be regulated by varying PVP concentrations. XPS results showed that, in addition to the NH₃ atmosphere-N doped into VO₂ lattice, the pyrrolidone-N introduced N-containing groups with N-N, N-O, or N-H bonds into the vicinity of the surface or void of the film in the form of molecular adsorption or atom (N, O, and H) filling. According to the Tauc plot, the estimated bandgap of N-incorporated VO₂ thin films related to metal-to-insulator transition (E₁) was 0.16-0.26 eV, while that associated with the visible transparency (E₂) was 1.31-1.45 eV. The calculated E₁ and E₂ from the first-principles theory were 0.1-0.5 eV and 1.4-1.6 eV, respectively. The Tauc plot estimation and theoretical calculations suggested that the combined effect of N-doping and N-adsorption with the extra atom (H, N, and O) decreased the critical temperature (τ) due to the reduction in E₁.