Electroluminescence of Perovskite Nanocrystals with Ligand Engineering

Colloidally synthesized metal halide perovskite nanocrystals (MHP NCs) have a unique defect-tolerant nature, with extremely high color purity and high photoluminescence quantum yield, affording them high potential as light sources in next-generation displays. The dynamics of organic ligands on the MHP NC surface limits efficiency and stability, and their insulating nature impedes charge transport in MHP NC light-emitting diodes. Increase in efficiency and stability of MHP NC light-emitting diodes requires tight-bound passivation with minimal content of insulating components. Surface engineering that exploits understanding of surface chemistry and binding state of ligands is critical for surface passivation and efficient charge transport to achieve efficient and stable light-emitting diodes. Light-emitting diodes (LEDs) based on metal halide perovskite nanocrystals (MHP NCs) have been rapidly developed to reach external quantum efficiencies of up to 22% with their defect-tolerant nature and extremely high color purity (full width half maxima <25 nm) that are superior to traditional inorganic colloidal quantum dots. However, highly dynamic binding of ligands impedes further increase in efficiency and induces intrinsic instability. In this review, we discuss the light emission in MHP NCs, surface chemistry regarding the surface termination of MHP crystal, and the binding of ligands to crystals. We also discuss strategies to overcome the instability of ligands to improve efficiency and stability of MHP NCs and finally achieve high efficiency of their LED devices. Light-emitting diodes (LEDs) based on metal halide perovskite nanocrystals (MHP NCs) have been rapidly developed to reach external quantum efficiencies of up to 22% with their defect-tolerant nature and extremely high color purity (full width half maxima <25 nm) that are superior to traditional inorganic colloidal quantum dots. However, highly dynamic binding of ligands impedes further increase in efficiency and induces intrinsic instability. In this review, we discuss the light emission in MHP NCs, surface chemistry regarding the surface termination of MHP crystal, and the binding of ligands to crystals. We also discuss strategies to overcome the instability of ligands to improve efficiency and stability of MHP NCs and finally achieve high efficiency of their LED devices. EB, minimum energy needed to separate an exciton into free charge carriers. two times exciton Bohr radius (RB), which is the most probable distance between the electrostatically bound electron and hole in a pair. In quantum size