Healing Trap States of Anomalous Antisite Defects via Surface Passivation in Lead Iodide Perovskites: A...

Undesirable loss of open-circuit voltage and current of metal halide perovskite (MHP) solar cells are closely associated with defects, so theoretical calculations have been often performed to scrutinize the nature of defects in bulk of MHPs. Yet, exploring the properties of defects at surfaces of MHPs is severely lacking given the complexity of the surface defects with high concentrations. In this study, I_Pb (Pb_I) antisite defects, namely one Pb (I) site being occupied by one I (Pb) atom at the surfaces of the FAPbI₃ (FA=CH(NH₂)₂) material, are found to create electron (hole) traps when the surfaces with I_Pb (Pb_I) antisite defects are negatively (positively) charged. These are in sharp contrast to the conventional viewpoint that electron (hole) traps are induced by positively (negatively) charged defects. The reasons are discovered through Bader charge analysis, suggesting that there is deficient (excessive) electron charge in the vicinity of the I_Pb (Pb_I) defects with respect to the case in pristine surface of FAPbI₃ material. Such understanding is then used as guidelines for effectively healing the anomalous I_Pb (Pb_I) antisite defects according to Lewis acid-base chemistry, which is exampled by passivating theophylline (C60) molecules on defective surfaces of the FAPbI₃ material.