LAUSR

Organic‐inorganic hybrid perovskites have caused great interest owing to their ever‐increasing power conversion efficiencies and extensive application prospects. However, the instability under ambient environment hinders their large‐scale commercialization. In this paper, the geometries, electronic structures, photoelectric properties, and stabilities of FAPbI3 (001) surfaces, graphene sheet, and graphene/FAPbI3 heterojunctions were explored using first‐principles calculations. Our calculated results show that a stable heterojunction can be constructed from graphene sheet and FAPbI3 (001) surface by van der Waals interaction. The construction of heterojunction has little influence on the effective masses of carriers, band structure, and optical absorbance of the perovskites. The graphene sheet of heterojunction can block the infiltration of O2 and H2O molecules and protect the perovskite from the degradation in light and humid air. Therefore, assembling a heterojunction from graphene sheet and FAPbI3 surface is an effective strategy for improving the stability and photoelectric properties of perovskite materials. Our study provides an atomic‐level insight into the fundamental properties of the graphene/FAPbI3 heterojunctions and a guideline for designing more stable perovskite solar cells.