Advances in thermochromic smart window materials

Thermochromic smart windows can intelligently regulate indoor solar radiation in response to surrounding temperature changes. This effectively reduces energy consumption in buildings and is a promising technology for building energy saving. Thermoresponsive materials, the integral components in thermochromic smart windows, undergo reversible phase changes near the critical transition temperature. In this review, we summarize thermoresponsive materials, including VO2, hydrogels, perovskites, ionic liquids, and liquid crystals. A critical review of the thermochromic mechanisms and recent progress made in these materials is presented. Furthermore, current challenges and developing-trends in thermoresponsive materials are also discussed. (1) VO2 is the most widely studied material for thermochromic smart windows due to its unique properties, as it undergoes a metal-to-insulator transition at a critical temperature of 68 degrees C. Various methods have been proposed to enhance specific properties of VO2, such as optical properties and transition temperatures, including element doping, multilayer film structures, nanocomposite films, and nano-/microstructures. However, simultaneously enhancing tau(c), Delta T-sol and T-lum remains a challenge for VO2. (2) Hydrogels are regarded as highly promising materials for thermochromic smart windows due to their reversible phase transition between hydrophilic and hydrophobic phases. Well-known thermochromic hydrogels include poly(N-isopropylacrylamide) (PNIPAm), hydroxypropyl cellulose (HPC), and poly(N-vinylcaprolactam) (PNVCL). However, thermochromic hydrogels often exhibit weak mechanical properties and undesirable freezing at cold temperatures, which need to be addressed in future research to facilitate their applications in smart windows. (3) Thermochromic perovskites have apparent solubility variations at different temperatures, accompanied by the shift of absorbance band gaps, leading to optical change from transparency to opaqueness. However, there are some problems that hinder the development of perovskites in the field of thermochromic smart windows, including high phase transition temperature, poor stability, and limited ability to modulate solar radiation. (4) Ionic liquids have also been extensively studied in the field of thermochromic smart windows due to their unique conformational transformation in response to temperature changes. As the temperature increases, the configuration of ionic liquids changes from octahedra to tetrahedra, accompanied by transparency variations. Additionally, ionic liquids exhibit good ionic conductivity, making them suitable for constructing electrochromic smart windows. Consequently, thermochromic ionic liquids can be used to fabricate dual thermal/electrical-response smart windows for both passive and active temperature control. (5) Liquid crystals are new materials in thermochromic smart windows, which exhibit significant optical changes characterized by the molecular orientation. When the temperature exceeds the critical temperature, the aligned structure of liquid crystals transforms into focal conic orientation, offering high solar modulation capability. However, the response time of thermochromic liquid crystals in smart windows is currently long, so future research efforts should focus on enhancing the sensitivity of liquid crystals to improve their response time to temperature changes.