Epitaxial 2D PbS Nanosheet‐Formamidinium Lead Triiodide Heterostructure Enabling High‐Performance Perovskite Solar Cells

Abstract Nanomaterials such as quantum dots and 2D materials have been widely used to improve the performance of perovskite solar cells due to their favorable optical properties, conductivity, and stability. Nevertheless, the interfacial crystal structures between perovskites and nanomaterials have always been ignored while large mismatches can result in a significant number of defects within solar cells. In this work, cubic PbS nanosheets with (200) preferred crystal planes are synthesized through anisotropy growth. Based on the similar crystal structure between cubic PbS (200) and cubic‐phase formamidinium lead triiodide ( α ‐FAPbI 3 ) (200), a nanoepitaxial PbS nanosheets‐FAPbI 3 heterostructure with low defect density is observed. Attribute to the epitaxial growth, PbS nanosheets‐FAPbI 3 hybrid polycrystalline films show decreased defects and better crystallization. Optimized perovskite solar cells perform both improved efficiency and stability, retaining 90% of initial photovoltaic conversion efficiency after being stored at 20 °C and 20% RH for 2500 h. Notably, the significantly improved stability is ascribed to the interfacial compression strain and chemical bonding between (200) planes of PbS nanosheets and α ‐FAPbI 3 (200). This study provides insight into high‐performance perovskite solar cells achieved by manipulating nanomaterial surfaces.