Common Defects Accelerate Charge Carrier Recombination in CsSnI3 without Creating Mid-Gap States

Lead-free metal halide perovskites are environmentally friendly and have favorable electro-optical properties; however, their efficiencies are significantly below the theoretical limit. Using ab initio nonadiabatic molecular dynamics, we show that common intrinsic defects accelerate nonradiative charge recombination in CsSnI₃ without creating midgap traps. This is in contrast to Pb-based perovskites, in which many defects have little influence on and even prolong carrier lifetimes. Sn-related defects, such as Sn vacancies and replacement of Sn with Cs are most detrimental, since Sn removal breaks the largest number of bonds and strongly perturbs the Sn-I lattice that supports the carriers. The defects increase the nonadiabatic electron-vibrational coupling and interact strongly with free carrier states. Point defects associated with I atoms are less detrimental, and therefore, CsSnI₃ synthesis should be performed in Sn rich conditions. The study provides an atomistic rationalization of why lead-free CsSnI₃ exhibits lower photovoltaic efficiency compared to some lead-based perovskites.