State-of-the-art, high-performance perovskite solar cells (PSCs) contain a large amount of iodine to realize smaller bandgaps. However, the presence of numerous iodine vacancies at the surface of the film formed by their evaporation during the thermal annealing process has been broadly shown to induce deep-level defects, incur non-radiative charge recombination and induce photocurrent hysteresis, all of which limit the efficiency and stability of PSCs. In this work, modifying the defective surface of perovskite films with cadmium iodide (CdI2) effectively reduces the degree of surface iodine deficiency, and stabilizes iodine ions via the formation of strong Cd-I ionic bonds. This largely reduces the interfacial charge recombination loss, yielding a high efficiency of 21.9% for blade-coated PSCs with an open-circuit voltage of 1.20 V, corresponding to a record small voltage deficit of 0.31 V. The CdI2 surface treatment also improves the operational stability of the PSCs, retaining 92% efficiency after constant illumination at 1 sun intensity for 1000 h. This work provides a promising strategy to optimize the surface/interface optoelectronic properties of perovskites for more efficient and stable solar cells and other optoelectronic devices.