The quality of the buried interface between the self-assembled molecules (SAM) and the perovskite layer directly governs the processes of charge carrier transport and non-radiative recombination, which ultimately dictates the efficiency and stability of the inverted perovskite solar cells. However, the simultaneous mitigation of poor SAM layer adhesion and perovskite substrate interface defects remains a significant challenge. Herein, low-cost and readily available 2‑formylbenzenesulfonic acid sodium salt (2‑FAS) is employed as a bifunctional interlayer to molecularly bridge the SAM and perovskite. The benzene ring of 2‑FAS interacts via π–π stacking with the SAM, strengthening adhesion and promoting hole transfer, while its sulfonate group (-SO3−) coordinates with Pb2+ to regulate crystallization and passivate surface defects. As a result, the 2-FAS-modified devices deliver a champion power conversion efficiency of 26.21%, with a significant fill factor of 86.15%. Furthermore, Na+ from 2‑FAS occupies A‑site vacancies in the perovskite lattice, which effectively suppresses ion migration and phase transition, thereby enhancing structural integrity. Benefiting from these combined effects, unencapsulated devices retain over 90% of their initial PCE after 4500 h of storage in a nitrogen atmosphere, demonstrating exceptional long-term stability.