2D　perovskites　have　recently　emerged　as　promising　materials　for　solar　cells　due　to　their　appealing　ambient　stability　and　structural　diversity.　The　organic　spacer　cation　plays　an　important　role　in　the　stability　and　properties　of　the　2D　perovskites.　Herein,　a　new　organic　ammonium　cation,　2-furfurylammonium　(FuMA+),　is　developed　as　the　spacer　cation　for　2D　Ruddlesden–Popper　perovskites.　Thin　films　of　(FuMA)2(MA)4Pb5I16　with　increased　crystal　size　and　enhanced　vertical　orientation　are　obtained　via　an　additive-assisted　film　forming　technique,　which　dramatically　improves　the　efficiency　of　2D　perovskite　solar　cells　from　4.90%　to　15.66%.　Moreover,　the　perovskite　films　based　on　FuMA+　can　be　prepared　in　ambient　air,　and　the　devices　based　on　the　perovskite　films　prepared　in　ambient　air　with　30%　relative　humidity　can　perform　a　power　conversion　efficiency　(PCE)　of　15.24%.　©　2022　Wiley-VCH　GmbH.