Tin(IV) oxide (SnO₂) is a promising electron transport layer for n-i-p perovskite solar cells (PSCs) due to its high transmittance, excellent charge mobility, and strong chemical stability. However, surface defects such as oxygen vacancies and hydroxyl groups at the SnO₂/perovskite interface degrade the device performance by increasing carrier recombination and accelerating degradation. While alkali halide salts offer a simple yet effective method for passivation, their enhancement mechanisms at the atomic level remain unclear, as most studies focus on bulk or surface effects rather than the heterointerface itself. Here, we introduce a potassium halide salt (PHS: KI, KCl, and KI+KCl) post-treatment to passivate the SnO₂/MAPbI₃ interface. Our combined experimental and density functional theory (DFT) analyses demonstrate that K⁺ and halide ions facilitate the removal of oxygen vacancies and extrinsic hydroxyl groups through the formation of KOH. This process effectively reduces the bond strength of surface hydroxyls and enhances interfacial ordering. This results in a smoother interface, larger perovskite grain sizes, improved adhesion, and enhanced charge extraction. The formation of Sn-Cl-Pb and Sn-I-Pb bonds, along with electrostatic interactions among interfacial K⁺, I^- in the perovskite structure, and O^2- in SnO₂, strengthens the interface and reduces ion migration. KI-modified and mixed KI+KCl devices achieved power conversion efficiencies (PCEs) of 19.86% and 19.15%, respectively, outperforming untreated SnO₂, which had a PCE of 18.41%. More importantly, the mixed KI+KCl treatment shows superior stability improvement compared to individual PHS treatments, retaining over 96% of the initial PCE after 1000 h under 40-50% relative humidity. These findings highlight the critical role of potassium salts in improving both efficiency and stability, offering an effective strategy for advancing PSC technology.