Phase Distribution Management of Methylamine Chloride‐Assisted Formamidinium‐Cesium Perovskite Photovoltaics

Abstract Methylamine chloride (MACl)‐assisted crystallization can increase the grain size of formamidinium‐cesium (FACs) perovskite film at the buried interface, but it will aggravate the separation of FA and Cs phases. Guided by density functional theory calculations, π ‐conjugated hippuric acid (HPA) molecules are introduced into the perovskite system, anchoring the perovskite precursor and increasing the formation barrier of α ‐phase perovskite. Thus, perovskite components achieve synchronous phase transition, and the resulting film exhibits homogeneous phase distribution and constant energy level arrangement. Under the synergistic effect of MACl‐assisted crystallization and HPA‐induced homogeneous phase transition, a large‐size, low‐defect, pure α ‐phase perovskite film is obtained. As a result, the HPA‐treated perovskite solar cell achieves an improved power conversion efficiency (PCE) of 26.05%, which is one of the highest PCEs among MACl‐assisted FACs perovskite systems. The released lattice strain increases the ion migration barrier, and the resulting device enhances long‐term stability. The unencapsulated HPA‐treated device maintains more than 90% of its initial PCE after 1000 h under continuous 1‐sun illumination conditions.