Perovskite　materials　have　demonstrated　many　excellent　properties　in　next-generation　photovoltaic　devices,　but　the　intrinsic　defects　and　the　quality　of　perovskite　film　still　limit　the　performance　and　stability　of　PSCs.　Here,　1,3-dimethylimidazolium　iodide　(DMII)　ionic　liquid　was　employed　as　an　additive　to　passivate　the　various　defects　and　produce　the　high-quality　perovskite　film　with　enlarged　grain　sizes.　DMII　could　act　as　an　＂ionic　stabilizer＂　for　passivating　the　point　defects　including　the　vacancies　defects　of　organic　cations　and　halogen　anions　of　perovskite.　At　the　same　time,　the　extra　problematic　PbI2　on　surfaces　and　at　grain　boundaries　of　the　perovskite　film　could　also　be　reacted　by　DMII,　leading　to　the　reduction　of　recombination　centers　and　trap　states.　On　the　other　hand,　the　DMII　ionic　liquid　with　a　＂Ostwald　ripening　effect＂　could　retard　the　crystallization　process　of　perovskite　crystals　and　yield　better　film　quality　with　higher　crystallinity,　smoother　morphology　and　larger　grains.　As　a　result,　the　optimal　device　achieved　a　champion　power　conversion　efficiency　(PCE)　of　20.4　%.　Particularly,　the　modified　devices　demonstrated　a　significant　elevation　in　open-circuit　voltage　from　1.03　to　1.10　V.　The　hydrophobicity　of　perovskite　films　modified　by　DMII　was　enhanced　and　the　un-encapsulated　DMII　devices　retained　91　%　of　their　initial　PCE　after　aging　60　days　under　15　+/-　5　%　relative　humidity.