Formation and Passivation\nof Surface Defects in Lead-Free\nTin Halide Perovskites

Lead-based hybrid perovskite materials are promising\ncandidates\nfor photovoltaic applications, but concerns about the toxicity of\nlead motivate the exploration of lead-free alternatives. In this study,\nwe examined surface defect formations in formamidinium tin iodide\n(FASnI<sub>3</sub>), methylammonium tin iodide (MASnI<sub>3</sub>),\nand mixed cation formamidinium-methylammonium tin iodide (FAMASnI<sub>3</sub>) perovskite materials as lead-free options. We systematically\ninvestigated a wide range of defect structures, such as vacancies,\ninterstitials, and antisites to identify detrimental defects that\ncan impact device performance. We found that iodine interstitial,\nFA antisite, and iodine vacancy are harmful defects with low formation\nenergy and charge traps in the bandgap. Also, we were able to eliminate\nthese charge traps using cationic passivating agent molecules. Our\nfindings provide a critical foundation for understanding the surface\nand defect interaction landscape in these materials, which could guide\nexperimental efforts in synthesizing and optimizing stable, efficient,\nand environmentally friendly perovskites for photovoltaic applications.