The role of sodium in stabilizing tin-lead (Sn-Pb) alloyed perovskite quantum dots

Narrow-bandgap CsSn Pb_1- I₃ perovskite quantum dots (QDs) show great promise for optoelectronic applications owing to their reduced use of toxic Pb, improved phase stability, and tunable band gaps in the visible and near-infrared range. The use of small ions has been proven beneficial in enhancing the stability and photoluminescence quantum yield (PLQY) of perovskite QDs. The introduction of sodium (Na) has succeeded in boosting the PLQY of CsSn_0.6Pb_0.4I₃ QDs. Unfortunately, the initial PLQY of the Na-doped QDs undergoes a fast degradation after one-day storage in solution, hindering their practical applications. Using density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations, we study the effect of Na ions on the strength of surface bonds, defect formation energies, and the interactions between surface ligands and perovskite QDs. Our results suggest that Na ions enhance the covalent bonding of surface tin-iodine bonds and form strong ionic bonding with the neighboring iodine anions, thus suppressing the formation of I and Sn vacancies. Furthermore, Na ions also enhance the binding strength of the surface ligands with the perovskite QD surface. However, according to our AIMD simulations, the enhanced surface ligand binding is only effective on a selected surface configuration. While the position of Na ions remains intact on a CsI-terminated surface, they diffuse vigorously on an MI₂-terminated surface. As a result, the positive effect of Na vanishes with time, explaining the relatively short lifetime of the experimentally obtained high PLQYs. Our results indicate that engineering the surface termination of the QDs could be the next step in maintaining the favorable effect of Na doping for a high and stable PLQY of Sn-Pb QDs.