A "Cocktail" Approach to Effective Surface Passivation of Multiple Surface Defects of Metal Halide Perovskites Using a Combination of Ligands.

Surface passivation of metal halide perovskites (MHPs) is essential for their stability and various properties as well as functionalities, including optical and electronic. Passivation is important for both stabilizing intrinsic defects and preventing extrinsic damaging species from reaching the perovskite (PVK), such as water and oxygen. Because of the ternary nature of their chemical composition, multiple surface defects exist for both bulk and nanostructured PVKs, with the latter particularly prominent because of their extremely large surface-to-volume ratio. To effectively passivate the different surface defects, a multitude of different ligands are necessary because each type of defect likely requires a different ligand for optimal passivation, as has been successfully demonstrated in a number of systems in essentially a "cocktail" approach. Characteristics of the ligands that affect effectiveness of passivation include size, shape, charge and charge distribution, orientation, conductivity, and interligand interaction. Examples of ligands for MHPs include both cationic and anionic or zwitterionic species with varied valences. The challenge is to identify the most effective ligand for each type of defect, and addressing this will require further experimental and theoretical study.