Electron extraction and transport represent the fundamental functions of the electron transport layer (ETL) in perovskite solar cells (PSCs). In this work, a novel ETL precursor integrates n-propylamine hydrochloride (PACl) with tin oxide (SnO₂), which significantly enhances the electron extraction efficiency and provides in-situ passivation for both SnO₂ and perovskite materials. The interaction between deprotonated PA⁰ and surface hydroxyl (-OH) groups on SnO₂ nanoparticles reduces the density of dangling bonds, thereby decreasing non-radiative recombination losses and improving the electron extraction efficiency of the ETL. Moreover, during the annealing process, the PACl additive 2 partially penetrates the perovskite layer, effectively slowing the crystallization rate, promoting grain growth, and improving the overall quality of the perovskite films. As a result, the power conversion efficiency (PCE) of the modified device increases to 24.39 %, compared to 23.55 % for the unmodified device. This improvement is primarily attributed to an enhancement in the device's fill factor, with the champion device achieving a fill factor of up to 83.44 %. In comparison with conventional interface modification techniques, the composite ETL strategy proposed in this study provides a promising pathway for defect passivation, thereby enabling further efficiency improvements in PSCs.