To date, energy demand in commercial and residential buildings accounts for around 40% of the total primary energy consumption, while nearly half of the energy in buildings is consumed by heating, ventilation and air-conditioning (HVAC) systems. It should be noted that the huge energy consumption by HVAC systems is mainly caused by the heat loss/gain through building envelopes, especially windows. Therefore, considerable endeavors have been devoted to develop energy-efficient glazing techniques. Among various window technologies, passive thermochromic smart windows are most attractive because of their specific capability to regulate the solar radiation between transparent and opaque states in response to the dynamic ambient temperature, a process that does not require any energy input. So far, different thermochromic materials such as vanadium dioxide (VO₂), hydrogel and perovskite have been investigated and their potential explored as window materials. However, the undesirable optical performance including low luminous transmittance (τ_lum) and solar modulation ability (Δτ_sol), along with a high transition temperature still hinders the wide application of thermochromic smart windows. The major goal of this thesis is to develop new thermochromic materials and explore their application potential to overcome the existing drawbacks of current thermochromic smart windows and fill the technology gap.

VO₂ is the most investigated thermochromic material. To improve the τ_lum and Δτ_sol of VO₂, we seek the solution from nature where the evolution of various species has enabled them to survive. Investigations into the morphology of moth eyes has shown that their unique nanostructures provide an excellent antireflection optical layer that helps the moth sharply capture the light in each wavelength from a wide angle. Inspired by this mechanism, a VO₂ thermochromic smart window coated with a TiO₂ antireflection layer with a novel nano-cone structure is, for the first time, presented to achieve high τ_lum and Δτ_sol. The nanoscale array provides a gradual refractive index change from the top to the bottom of the cone where the refractive index is higher, which enables the nano-cone coating to exhibit a relatively smooth change of refractive index, thereby suppressing the reflection of incidence in a broad range of wavelength. This study provides an alternative method to enhance the optical performance of VO₂ smart windows. However, the natural yellow appearance of the VO₂, and the reversible phase change of VO₂ that is only in the near-infrared (NIR) transmittance, results in the difficulty to further improve the optical performance of VO₂ smart windows. Thus, I explore a new thermochromic material and develop a new window, namely hydrated MAPbI_3-xCl_x thermochromic perovskite smart window (H-MAPbI_3-xCl_x TPSW). This window undergoes a reversible phase transition between a visually transparent state and a dark reddish-brown tinted state with a high solar modulation ability. By tuning the Gibbs energy with assistance of chlorine doping, the H-MAPbI_3-xCl_x TPSW achieves the tunable low transition temperature, controllable and narrow transition hysteresis width as well as a short transition time. In particular, a mathematical model based on the Clausius-Clapeyron relation was developed to realize the customized transition temperature of the H-MAPbI_3-xCl_x TPSW. Additionally, by coating with cesium-doped tungsten trioxide, I further developed a near-infrared (NIR) activated thermochromic perovskite window. The localized surface plasmon resonances and polaron absorption mechanism result in high-efficient heat generation under sunlight that enable reversible transition cycles of thermochromic perovskite at room temperature. The near-room-temperature color change, multispectral thermal management, outstanding energy-saving ability and climate adaptability, and solution-based process make thermochromic perovskite smart windows unique and promising for real applications. Finally, I investigated the potential of using optically transparent wood as thermochromic window materials, because of the renewability, high optical transmittance, strong mechanical properties and excellent thermal insulation capability of transparent wood. To achieve thermochromism and high transparency, the bleached wood is impregnated with an optically refractive index matching thermochromic polymer with solar regulation in visible light or near infrared light range. The proposed thermochromic transparent wood shows advanced optical properties, is mechanically robust, has low thermal conductivity, hydrophobic self-cleaning and anti-dust functions.

Finally, to examine the real performance of the proposed thermochromic windows in buildings, the heat transfer process was analyzed and energy consumption was numerically simulated. I also conducted several real field tests in Hong Kong to demonstrate practical applications. The results show that the proposed thermochromic smart windows can effectively regulate the indoor air temperature and significantly mitigate the energy demand of HVAC systems. This research showcases thermochromic smart windows as a technology primed to lower the energy and carbon footprint of buildings, while contributing to carbon neutrality by 2050.