Theoretical Study of the Molecular Passivation Effect of Lewis Base/Acid on Lead-Free Tin Perovskite Surface Defects

Extensive research has been recently conducted to improve the power conversion efficiency (PCE) of perovskite solar cells. However, the charge carriers are easily trapped by the defect sites located at the interface between the perovskite layer and the electrode, which decreases the PCE. To reduce such defect sites, the passivation technique is frequently employed to coat small molecules on the perovskite surface during the manufacturing process. To clarify the passivation mechanism from a molecular viewpoint, we performed density functional theory calculations to target Pb-free Sn perovskites (CH3NH3SnI3). We investigated the passivation effect of Lewis base/acid molecules, such as ethylene diamine (EDA) and iodopentafluorobenzene (IPFB), and discussed behaviors of the defect levels within the bandgap as they have strong negative impacts on the PCE. The adsorption of EDA/IPFB on the Sn perovskite surface can remove the defect levels from the bandgap. Furthermore, we discuss the importance of interactions with molecular orbitals.