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

Nonmetal elements have remarkable potential in modulating the metal-insulator transition behavior of VO, but little is known about their impact on thermochromic performance. Here, phosphorus (P) is successfully doped into VO by the precursor decomposition diffusion. XPS spectra, together with DFT calculations, demonstrate V-substitutional doping (Pv). The onset transition temperature of P-doped VO decreases from 62.8 to 50.2 °C. Theoretical calculations reveal that P interferes with strong V-V interactions, decreasing the 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 (Δ) of 11.8%, a 36% enhancement over the undoped one (8.7%). Attractively, a P-gradient VO 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.