The　poor　performance　of　tin　(Sn)-based　perovskite　solar　cells　(PSCs)　is　mainly　due　to　the　easy　oxidation　of　Sn2+　to　Sn4+,　the　low　formation　energy　of　Sn　vacancies,　and　the　unbalanced　rates　between　nucleation　and　crystal　growth　in　Sn-based　perovskite　films.　In　this　study,　a　small　amount　of　two-dimensional　tetra-n-butylammonium　(TBA)　cation　is　doped　in　Sn-based　perovskite　for　the　first　time,　which　effectively　passivates　film　defects.　TBA　cation　doping　leads　to　improved　surface　coverage,　suppressed　Sn2+/Sn4+　oxidation,　and　reduced　background　charge　carrier　density　in　mixed　perovskite　films.　Consequently,　the　efficiency　of　TBA-doped　formamidinium　tin　triiodide　(FASnI3)　and　cesium　tin　triiodide　(CsSnI3)　cells　is　greatly　improved　from　4.0　to　7.0%　and　from　1.3　to　2.8%,　respectively.　©　2020　American　Chemical　Society.