Defect Modulation via SnX₂ Additives in FASnI₃ Perovskite Solar Cells

Tin halide perovskites suffer from high defect densities compared with their lead counterparts. To decrease defect densities, SnF₂ is commonly used as an additive in tin halide perovskites. Herein, we investigate how SnF₂ compares to other SnX₂ additives (X = F, Cl, Br) in terms of electronic and ionic defect properties in FASnI₃. We find that FASnI₃ films with SnF₂ show the lowest Urbach energies (E_U) of 19 meV and a decreased p-type character, as probed with ultraviolet photoemission spectroscopy. The activation energy of ion migration, as probed with thermal admittance spectroscopy, for FASnI₃ with SnF₂ is 1.33 eV, which is higher than with SnCl₂ and SnBr₂, which are 1.22 and 0.79 eV, respectively, resulting in less ion migration. Because of improved defect passivation, the champion power conversion efficiency of FASnI₃ with SnF₂ is 7.47% and only 1.84% and 1.20% with SnCl₂ and SnBr₂, respectively.