Imidazolium (IM) and cesium (Cs) were treated as A-site cationic additives for a triiodide tin perovskite solar cell (FASnI 3 ; FA, formamidinium) with sulfamic acid (SA) as a bifunctional additive in varied proportions. IM has a tight aromatic structure that is feasible to passivate the crystal defects and help in perovskite crystallization. Cs can stabilize the crystal structure and decrease trap state densities. TOF-SIMS measurements show the passivation effect of both IM and SA occurring on the surface of the film. The bifunctional SA additive can occupy the iodine vacancies in tin perovskites to passivate the uncoordinated Sn atoms and to passivate the surface defects effectively. Furthermore, SA has the effect of reducing Sn 4+ back to Sn 2+ via its ammonia/ acid form converted from its zwitterionic form upon irradiation, as observed from in situ XPS measurements. The hybrid device was optimized to attain a PCE value of 12.5% for the perovskite structure Cs 0.02 IM 0.1 FA 0.88 SnI 3 +1% SA with superior, long-lasting stability.