Tuning thermochromic performance of 2D perovskites via Br/Cl doping: Enhanced optical modulation and lowered transition temperature toward building energy efficiency

Buildings account for a large portion of global energy consumption, driving the need for smart windows that can dynamically regulate solar heat. Two-dimensional (2D) thermochromic perovskites offer promising stability, but their practical application is limited by high transition temperatures and modest solar modulation ability. Here, we employ halide engineering via systematic Br/Cl doping in the 2D perovskite Tha₂MAPbI₄ (TMPI) to optimize these key properties. A systematic Br/Cl doping was conducted in TMPI and established a detailed composition-property database. It is found that optimal Cl doping (I: Cl = 4:1) enhances the solar modulation ability (Δτ_sol) to 27.7%, while Br doping effectively lowers the transition temperature to near 40 °C. Most importantly, a High-Resolution Transmission Electron Microscope (HRTEM) was employed to characterize the crystallization of 2D perovskites with different halogen dopants, providing the first atomic-scale observation of lattice fringes and SAED patterns in thermochromic metal halide perovskites and revealing the doping-dependent crystallinity evolution. Moreover, first-principles calculations reveal that enhanced water adsorption energy on doped perovskite surfaces governs the reduced transition temperature and faster response kinetics. A real-world model house demonstration confirms the superior cooling performance of the optimized smart window. This work provides not only high-performance material candidates but also a fundamental mechanistic framework for developing energy-efficient, responsive smart windows. © 2026 Elsevier B.V.