Nb-Doped VO2-Based Coatings on Glass: Substrate Effects, Thermochromic Performance, and an Effective Transition Temperature for Smart-Glazing Applications

Abstract

Niobium-doped VO2 thin films were deposited on soda-lime and borosilicate glass substrates to investigate the coupled effects of dopant concentration, substrate chemistry, and thermal processing on microstructure, thermochromic performance, and metal–insulator transition (MIT) kinetics. Coatings with nominal Nb contents of 3, 4.5, and 6 at. % were fabricated using controlled mono- and multilayer codeposition strategies followed by rapid thermal annealing. Structural and morphological analyses confirm VO2(M) as the dominant phase on both substrates, while revealing a strong substrate-dependent stability of secondary phases driven by Na diffusion, especially in soda-lime glass. Advanced scanning-transmission electron microscopy image and spectroscopic-related techniques demonstrate that Nb exhibits a less homogeneous spatial distribution than other dopants (such as W) under comparable processing conditions, leading to compositional modulations and multistep MIT behavior, particularly on soda-lime substrates. In contrast, borosilicate glass enables enhanced grain coalescence, reduced Na-induced phase instability, and a more uniform Nb incorporation. These microstructural differences directly impact the optical response. Borosilicate-supported coatings exhibit a more favorable balance between luminous transmittance (Tlum) and solar modulation efficiency (ΔTsol), while films on soda-lime glass show a progressive degradation of ΔTsol at higher Nb contents. Despite a lower Tc reduction efficiency compared to W, Nb doping preserves or even improves Tlum at elevated concentrations. Finally, an application-oriented effective transition temperature, Teff, is proposed as a more realistic metric for assessing the suitability of VO2-based coatings for smart-glazing applications.