s Insights into the Photoelectric Properties of the SnF2 and SnF4-doped FASnI3 Perovskite NanoFilm

Background: In this study, experimentally, we fabricated the FASnI3 perovskite solar cells based on the SnF2 and SnF4-doped FASnI3 nano-thin film materials, and obtained the photoelectric conversion efficiency (PCE) as 6.5 % and 5.59 %, respectively. Theoretically, we wanted to know why the PCE of SnF2-doped FASnI3 is higher than the SnF4-doped FASnI3. Methods: We built three kinds of model structures by the CASTEP; they were undoped and SnF2 and SnF4 doped FASnI3 perovskite structure models, respectively. The method was ultrasoft to calculate the interaction between electron and ion, including an electron exchange correction method of generalized gradient approximation and Perdew-Burke-Emzerhof method. Results: We found the probabilities of energy transfer between SnF2 molecules and the surrounding molecules and these were found to be the lowest among the three structures. By analyzing optical properties, band structures, effective masses, and density of states (DOS), etc., we found SnF2 doping to be superior to SnF4 doping in maintaining photoelectric properties of FASnI3. In addition, SnF2- doped FASnI3 possessed smaller hole effective mass than SnF4-doped FASnI3, adding Sn4+ ion into perovskite, as a shallow acceptor energy level can effectively reduce the optical absorption properties, however, adding Sn²⁺ ion into perovskite at an appropriate proportion enhanced photoelectric performance of FASnI3. Conclusion: Sn⁴⁺ doping exhibited a negative effect, while Sn²⁺ doping showed a positive effect in promoting the photoelectric performance of FASnI3 perovskite. We found SnF2 doping to be superior to SnF4 doping in maintaining photoelectric properties of FASnI3 . Our results may help to deeply understand the role of Sn²⁺ and Sn⁴⁺ ions in promoting the stability and high efficiency of FASnI3, and help in developing lead-free perovskite solar cells.