The mechanism of the stability improvement of the Bγ-CsSnI3 perovskite doped with fluorine

Determining the stability mechanism of perovskite materials is one of the key factors to promote the industrialization of perovskite solar cells. In this paper, we calculated and compared the lattice and electron structure, the differential charge density of intrinsic and fluorine (F) doped B-gamma-CsSnI3 phases by Density Functional Theory (DFT) method to explain the mechanism of stability. The results showed that as the volumes of the B-gamma-CsSnI3-xFx unit cell decrease, the distortions of the three-dimensional charge densities and electron localization functions become stronger, the formation energies reduce and the band gaps enlarge with the increases of F ratios. The transition from Sn2+ to Sn4+ is delayed because the strong electronegativity of F atom attracts the 3 electrons of Sn 5p. The phase transformations from the B-gamma, Y phase to Cs2SnI6-xFx phases are delayed as the F ratio increases, which is proved by differing colour change timings of CsSnI3-xFx films with different doping levels of fluorine. The mechanism of the stability improvement of the B-gamma-CsSnI3-xFx perovskite is the strong influence of F hybridization, by partial substitution I with F, which results in a wider VBM and moving to low energy level, meanwhile the CBM moved to a higher energy level greatly, the electron densities and electron localizations strongly deform and electrons of Sn are tightly bound.