Organic Hybrid Perovskite (MAPbI_3−xCl_x) for Thermochromic Smart Window with Strong Optical Regulation Ability, Low Transition Temperature, and Narrow Hysteresis Width

Recently, organic hybrid halide perovskites have been found to show thermochromism with good optical performance, which can be applied in smart windows to reduce building energy consumption. However, these perovskites have shortcomings regarding their thermochromic performance, namely long transition time, high transition temperature, and large transition hysteresis width. In this study, a hydrated MAPbI_3−xCl_x thermochromic perovskite smart window (H-MAPbI_3−xCl_x TPSW) is proposed, which undergoes a reversible transition between a transparent state and a dark reddish-brown tinted state with a high solar modulation ability of 23.7%. Most importantly, the H-MAPbI_3−xCl_x TPSW possesses a tunable low transition temperature of 29.4 to 51.4 °C, a controllable and narrow transition hysteresis width (7.7–13.2 °C) and a short transition time (1–4 min). Additionally, a mathematical model is developed to predict the transition temperature of the H-MAPbI_3−xCl_x TPSW. A field test is also conducted, demonstrating that the H-MAPbI_3−xCl_x TPSW fitted to a model house can reduce the indoor air temperature by 3.5 °C compared to using a quartz glass window. Overall, the H-MAPbI_3−xCl_x TPSW can yield excellent optical properties, while simultaneously providing remarkable transition properties, making it potentially useful for a wide range of applications in energy-efficient buildings.