Void Dynamics and Crystal Reconstruction in Double Perovskite Nanocrystals Revealed by In Situ TEM

Double perovskites are considered for future photovoltaic and electro-optic applications as a toxic-free alternative to lead halide perovskites. Alas, due to the lower efficiency of lead-free devices, material properties need to improve to compete. Microstructure and crystal defects are known to hinder electronic properties. Experiments are conducted on nanocrystals in-situ a TEM microscope. The setup enables creation of crystal voids, and to monitor their dynamics and crystal reconstruction in real time. Void trajectories and velocities are extracted from TEM videos using costumed computer aided image analysis. An inaccessible, protected volume for migration near the nanocrystal outer surface is discovered, confining the migration of voids to inner crystal parts. Control of the organic ligand passivation determines the change in void dynamics. It is determined that surface ligand protection against void migration is extending several atomic layers below the crystal surface. Modeling based on these results predicts equilibrium positions for the voids, which are discovered in the data. The study suggests that tuning of organic ligand density influences structural stability and crystal defect tolerance in double perovskites. Engineering surfaces with inherent crystal reconstruction tendency will increase efficiencies in future devices based on these materials.