Highly transparent smart thermal emitter realized by thin vanadium dioxide

Global energy shortage necessitates enhancing environmentally friendly and energy-saving technologies. This study demonstrates a smart thermal emitter (STE) with high visual transparency and temperature-dependent emissivity to realize smart thermal management in buildings or automobiles. The STE consisting of layered transparent conductive oxide (TCO), zinc sulfide (ZnS), and thin vanadium dioxide (VO2) films was fabricated using the sputtering process. The STE exhibited high transmittance in the visible region owing to the extremely thin VO2 film and the large optical bandgaps of TCO and ZnS. The average transmittance in the wavelength range of 400–800 nm was 0.57. Owing to the metal–insulator transition of VO2, a metal–insulator–metal structure resonating in the infrared region was formed above 70 °C, which increased the thermal emissivity. The thermal emissivity values at 29 and 91 °C in the infrared region within 2.5–15 μm were 0.29 and 0.66, respectively. The appearance of the STE remained almost unchanged even when the emissivity increased with temperature because the refractive index spectra of VO2 in the visible range hardly change by the metal–insulator transition. In addition, W-doped VO2 allowed the STE to operate under summer temperatures (48 °C). This scalable fabrication process and high visual transparency have significant potential in practical applications, such as windows or exterior coatings, for smart thermal management.