An Insight into the Effects of SnF(2) Assisting the Performance of Lead-Free Perovskite of FASnI(3): A First-Principles Calculations

[Image: see text] It is an effective method to use SnF(2) and SnF(4) molecules to assist in enhancing the performance of FASnI(3) perovskite. However, the mechanism in this case is not clear as it lacks a certain explanation to specify the phenomenon. Through first-principles calculations, this paper constructed several modes of SnF(2) and SnF(4) adsorbed on the surfaces of FASnI(3) and explored adsorption energies, band structures, photoelectric properties, absorption spectra, and dielectric functions. The SnF(2) molecule adsorbed at the I5 position on the FAI-T surface has the lowest adsorption energy for the F atom, which is 0.5376 eV. The Sn–I bond and Sn–F bond mainly affect the photoelectric properties of FASnI(3) perovskite solar cells, and the SnF(2) adsorption on the FAI-T surface can effectively strengthen the bond energies, which shortens the bond lengths of the Sn–I and Sn–F bond, and eliminate surface unsaturated bonds to passivate the surface defects. Furthermore, the probability of energy transfer was lower between the SnF(2) molecule and the ion around it than between SnF(4) and its ion. Especially, in the aspect of optical properties, we found that the intensity of the absorption peak of SnF(2) adsorption increase was larger than that of SnF(4) adsorption. Additionally, the static dielectric constants of SnF(4) adsorption on the two surfaces, denoted SnF(4), made the perovskite respond more slowly to the external electric field. Based on this work, we found that SnF(2) had a greater positive effect on the optical property of perovskite than SnF(4). We consider that our results can help to deeply understand the essence of SnF(2) assistance in the performance of FASnI(3) and help researchers strive for lead-free perovskite solar cells.