Phosphorus Doping Coupled with Gradient Film Strategy to Achieve Thermochromic Performance Enhancement of VO₂

Nonmetal elements have remarkable potential in modulating the metal-insulator transition behavior of VO₂, but little is known about their impact on thermochromic performance. Here, phosphorus (P) is successfully doped into VO₂ by the precursor decomposition diffusion. XPS spectra, together with DFT calculations, demonstrate V-substitutional doping (Pv). The onset transition temperature of P-doped VO₂ decreases from 62.8 to 50.2 °C. Theoretical calculations reveal that P_V interferes with strong V-V interactions, decreasing the d orbital occupancy. However, the worse crystallinity leads to severe NIR modulation degradation even though luminance transmittance is enhanced by 6%. P-gradient films are developed to effectively address the weakening of NIR modulation caused by element doping. The film with decreased P concentration has an optimal solar-energy modulation ability (ΔT_sol) of 11.8%, a 36% enhancement over the undoped one (8.7%). Attractively, a P-gradient VO₂ smart window shows outstanding temperature response NIR modulation and thermal isolation performance; the indoor temperature of its model house decreased by 6.8 °C compared to the blank window after 25 min irradiation. This work provides a simple and effective idea to improve the element doping-induced NIR modulation degradation.