Long-term operational stability remains the primary concern for perovskite solar cells. Consequently, there is a quest for searching for new compositions that enable stable and efficient perovskites. We report a new molecular-level interface engineering strategy using a multifunctional ligand that augments longterm operational and thermal stability by chemically modifying the formamidinium lead iodide rich photoactive layer. The surface derivatized solar cells exhibited high operational stability (maximum powering point tracking at 1 sun) with a stabilized T80 (the time over which the device efficiency reduces to 80% of its initial value of post-burn-in) of ≈5950 h at 40 ºC and stabilized efficiency over 23%. The origin of high device stability and performance is correlated to the nano/sub-nanoscale molecular level interactions between ligand and perovskite layer, which is corroborated by comprehensive multiscale characterization. Our results provide key insights into the modulation of the grain boundaries, local density of states, surface bandgap, and interfacial recombination. Chemical analysis of the aged devices showed that interface passivation inhibited ion migration and prevented photoinduced I2 release that irreversibly degrades the perovskite. This study shows that passivating ligands have the potential to overcome stability issues associated with the high performing hybrid perovskite compositions, thus allowing a step closer to achieving long-standing stability of perovskite-based solar cells.