Highly Sensitive Two‐Dimensional Ruddlesden‐Popper Perovskites for Thermochromic Smart Windows

With the growing demand for high‐performance smart windows, the discover of a class of thermochromic materials with reversible cycling and rapid response characteristics has become urgent. In this work, we have uncovered a two‐dimensional (2D) Ruddlesden‐Popper (RP) phase halide perovskite, (PMA)2MAPb2I7−xClx, where PMA = C6H5CH2NH3 and MA = CH3NH3, exhibiting exceptional reversible thermochromic properties. The 2D RP phase perovskite thin film features a low transition temperature of 30 °C from the hydrated state to the hot state, along with a fast transition time of 40 s. Furthermore, the addition of 0.5 times excess MAI significantly enhances the visible light transmittance of the hydrated state. Characteristic hydration peaks in X‐ray diffraction patterns and O‐H bond absorption peaks in Fourier‐transform infrared spectra are observed in the thin film in its hydrated state, which disappear in the hot state, validating its reversible thermochromic properties. Additionally, a solar cell based on the thermochromic 2D RP phase thin film achieves a power conversion efficiency of 2.31%, offering a promising solution for advanced smart window technologies.