Enhancing Stability: Two‐Dimensional Thermochromic Perovskite for Smart Windows in Building Applications

Abstract Recently, there is rapid development of thermochromic metal halide perovskite (MHPs) for smart window applications due to their competitive optical performance and cost‐effective synthesis. However, existing MHP smart windows predominantly feature 3D perovskite, which exhibits a deficiency in environmental stability, presenting persistent challenges for practical applications. This study introduces a novel and more durable 2D thermochromic perovskite, Tha 2 MAPbI 4 (TMPI, Tha = thiourea, MA = methylamine), wherein Tha acts as a Lewis acid‐base adduct. TMPI demonstrates a reversible transition, achieving 83.7% luminous transmittance in the cold state and 35.2% in the hot state, thereby showcasing a substantial solar modulation ability of 24.7%. Further analysis of the crystal structure reveals that the thermochromic behavior of TMPI arises from a phase transition between 0D perovskite and 2D perovskite, induced by a dehydration‐hydration process. Notably, TMPI maintains thermochromic properties even after direct exposure to 75% relative humidity and 25 °C air for up to 28 days, a stark contrast to traditional 3D perovskites that lose their thermochromic capabilities within a few days under similar conditions. This research unveils TMPI as a thermochromic 2D perovskite that marks a significant advancement in environmental stability, indicating promising prospects for thermochromic smart windows.