Defect Tolerance of Lead-Halide Perovskite (100) Surface Relative to Bulk: Band Bending, Surface States, and Characteristics of Vacancies

We characterized the formation of vacancies at a surface slab model and contrasted the results with the bulk of lead-halide perovskites by using cubic and tetragonal CsPbI3 as representative structures. The defect-free CsI-terminated (100) surface does not trap charge carriers. In the presence of defects (vacancies), the surface is expected to exhibit a p-type behavior. The formation energy of cesium vacancies VCs– is lower at the surface than in the bulk, while iodine vacancies VI+ have a similar energy (around 0.25–0.4 eV) within the range of chemical potentials compatible with solution processing synthesis conditions. Lead–iodine divacancies (VPbI–) are expected to dominate lead-only vacancies at the surfaces. Major surface vacancies create shallow host-like energy states with a small Franck–Condon shift, making them electronically harmless (same as in bulk). The spin–orbit coupling contributes to the defect tolerance of lead-halide perovskite surfaces by causing delocalization of electronic states associated with n-type defects and retraction of the lowest unoccupied states from the surface due to a mixing of Pb-px,y,z orbitals. These results explain a high optoelectronic performance of two-dimensional structures, nanoparticles, and polycrystalline thin films of lead-halide perovskites despite the abundance of interfaces in these materials.