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.