Two-step-processed perovskite solar cells show superior reproducibility in terms of stepwise crystallization management. However, the device performance is limited due to the buried-interface defects that are highly dependent on the diffusion process of organic salts into PbI2. Herein, 1,8-octanediamine dihydroiodide (ODADI) is adopted to develop an alkylammonium predeposition strategy for the high-quality perovskite film. It is found that the pre-deposited ODADI layer not only facilitates the diffusion of organic salts via interaction with PbI2, but also passivates the buried-interface defects, resulting in a perovskite film with low defect density, high crystallinity, and superior electronic properties. Consequently, the fabricated devices deliver a significant enhancement on power conversion efficiency (PCE) from 19.87 to 22.07%. In addition, a superior long-term stability in glovebox atmosphere, maintaining 96% of the initial PCE after 1000 h, is demonstrated.