The application of self‐assembled molecules (SAM) allows inverted‐structural perovskite solar cells to accomplish high efficiencies, by virtue of their high hole conductivity and negligible parasitic absorption. However, amphiphilic SAM tend to form spherical micelles in commonly used alcoholic solvents, leading to aggregation in the resulting film. In addition, hydrophobic groups of SAM are exposed to the surroundings after being deposited on transparent substrates, responsible for the poor crystallinity of perovskite and interface quality. Therefore, it is important to study the treatment of SAM layers or SAM/perovskite interfaces for better device performance. Herein, dual regulation of SAM and perovskite is achieved via a facile posttreatment by formamidinium chloride (FACl) solution in N,N‐Dimethylformamide. First, by taking advantage of solvent rinsing, aggregations at the surface of SAM layer are alleviated. Second, FACl on the top of the SAM layer assists the crystallization of perovskite through interaction with PbI₂ and reduces the defect concentration via passivating the halide vacancies at the buried interface of the device. With a more uniform SAM layer, better‐crystallized perovskite, and suppressed nonradiative recombination between them, the power conversion efficiency of the target device is improved by 20%.