Strongly Quantum Confined Colloidal Cesium Tin Iodide Perovskite Nanoplates: Lessons for Reducing Defect Density and Improving Stability

Within the last several years, metal halide perovskites such as methylammonium lead iodide, CH₃NH₃PbI₃, have come to the forefront of scientific investigation as defect-tolerant, solution-processable semiconductors that exhibit excellent optoelectronic properties. The vast majority of study has focused on Pb-based perovskites, which have limited applications because of their inherent toxicity. To enable the broad application of these materials, the properties of lead-free halide perovskites must be explored. Here, two-dimensional, lead-free cesium tin iodide, (CsSnI₃), perovskite nanoplates have been synthesized and characterized for the first time. These CsSnI₃ nanoplates exhibit thicknesses of less than 4 nm and exhibit significant quantum confinement with photoluminescence at 1.59 eV compared to 1.3 eV in the bulk. Ab initio calculations employing the generalized gradient approximation of Perdew-Burke-Ernzerhof elucidate that although the dominant intrinsic defects in CsSnI₃ do not introduce deep levels inside the band gap, their concentration can be quite high. These simulations also highlight that synthesizing and processing CsSnI₃ in Sn-rich conditions can reduce defect density and increase stability, which matches insights gained experimentally. This improvement in the understanding of CsSnI₃ represents a step toward the broader challenge of building a deeper understanding of Sn-based halide perovskites and developing design principles that will lead to their successful application in optoelectronic devices.