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Halide ion diffusion in organolead halide perovskites has raised great concern in recent years and been considered as the reason for the hysteresis of current-voltage curves and degradation of perovskite solar cells. In this work, X-site halide ion diffusion in lead-free double perovskites CsAgBiX (X = Cl, Br), CsAgSbCl and CsAgInCl is investigated by first-principles calculations. The formation energies of X-site vacancies are calculated for these double perovskites, and predicted to be related to the electronic configurations of B-site cations. CsAgInCl is found to have the lowest vacancy formation energy among these double perovskites due to the unfilled s-orbitals of In. Using the climbing-image nudged elastic band method, these double perovskites are found with different barriers for halide ion migrations around Ag- and B-octahedrons due to different migration bottleneck radii and B-X bond characters. In our calculations CsAgBiBr shows the lowest energy barrier for X-site ion migration, which can explain the phenomenon of the huge hysteresis in CsAgBiBr solar cells reported. These results reveal X-site vacancy formation and diffusion properties in these lead-free halide double perovskites, which could be useful theoretical support for their optoelectronic application.

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