Climate change and energy issues are prominent global challenges that affect the common interests of the international community and the future of the planet. The large population of China leads to increasing energy demand, which makes it urgent to improve energy utilization and reduce energy consumption. Building energy consumption accounts for about 40% of the world’s total energy consumption, with windows—among the least energy-efficient components—responsible for up to 60% of energy loss. Since windows are vital transparent components in buildings, it is essential to research energy-efficient window technologies. To meet the demand for energy savings in daily life, designing smart windows is particularly important. These windows need to provide adequate lighting while dynamically regulating solar irradiation entering the room and the heat transfer between indoor and outdoor environments. This helps maintain comfortable indoor temperatures, reduce the need for heating and cooling, and boost the efficiency of energy-saving buildings.

Nowadays, research on smart windows has been extensive. The main objectives are heating in winter and cooling in summer, with a core focus on regulating solar radiation band energy and blackbody radiation band emissivity. Early research concentrates on the regulation of the spectral properties of the solar radiation band, primarily centering on the refractive index of vanadium dioxide (VO₂) materials and its matching with the surrounding environment. The introduction of anti-reflection films leads to the development of multilayer film structures, which ranges from simple double-layer films to complex five-layer or even more layers structures. These have been experimentally verified (Fig. 4). The construction of a refractive index gradient has successfully increased visible light transmittance to about 50%, while also providing significant thermal regulation ability (more than 10%). With refined research, nanoscale studies are conducted. Embedding VO₂ nanoparticles into a transparent matrix for a nanocomposite film eliminates reflection caused by refractive index mismatch at low temperatures, which successfully increases visible light transmittance to about 60% (Fig. 5). At the same time, the significant thermal regulation ability is achieved due to strong scattering at high temperatures. By constructing a microstructure on the surface and reducing the content of VO₂, the transmittance increases significantly up to 95.4% (Fig. 6). Additionally, the presence of LSPR allows the thermal regulation ability to be maintained at more than 10%. In a later stage, the smart window undergoes multi-band performance modulation. By combining VO₂ with commercially available Low-E glass, photo-thermal modulation of the solar band is achieved while creating lower emissivity to reduce heat transfer between indoor and outdoor spaces (Fig. 7). However, static emissivity alone is insufficient for optimal thermal modulation performance. Dynamic modulation of solar radiation and blackbody radiation bands is achieved by introducing the Fabry-Perot (F-P) resonant cavity structure, which leads to considerable energy savings (Fig. 8).

Although the existing optical design greatly improves the performance of VO₂ thermochromic smart windows, there is still room for development in the synergy between visible transmittance, thermal regulation ability, and phase transition temperature. Since the VO₂ thermochromic smart window has weak transmittance regulation ability in the visible band, its thermal regulation ability heavily depends on changes in transmittance in the near-infrared band. Additionally, there is a limit to the thermal regulation ability of a single VO₂ film, which results in a significant difference in its energy-saving effect compared to other types of smart windows. Therefore, to strengthen the performance of VO₂ thermochromic smart windows, one approach is to combine them with other types of smart windows to achieve dual-band regulation in both the visible and near-infrared bands and to create graded energy-saving modes under different external field stimuli. Another is to leverage VO₂’s advantages in the infrared band and further enhance its energy-saving effect through optimized infrared optical design. Furthermore, from an industrialization perspective, the single color of VO₂ thermochromic smart windows does not meet modern society’s demand for rich colors. Thus, it is worth improving their color characteristics by integrating existing color modulation technology. Additionally, the aging performance and cost issues of VO₂ films limit their practical use, so it is essential to focus on continued improvements in materials, optical design, and cost control for the further development of VO₂ smart window technology.