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, CHNHPbI, 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, (CsSnI), perovskite nanoplates have been synthesized and characterized for the first time. These CsSnI 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 CsSnI do not introduce deep levels inside the band gap, their concentration can be quite high. These simulations also highlight that synthesizing and processing CsSnI in Sn-rich conditions can reduce defect density and increase stability, which matches insights gained experimentally. This improvement in the understanding of CsSnI 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.