Ligand-Mediated Surface Carrier Modulation in Perovskite Nanocrystals for Charge-Symmetric LEDs

Metal halide perovskite nanocrystals (MHP NCs) hold great promise for light-emitting diodes (LEDs), but face challenges due to weakly-bound, insulating long-chain ligands that impede charge transport and compromise stability. We introduce a hydrolysis-assisted ligand-exchange strategy that uses a multifunctional π-conjugated pyridine carboxamide (PCA). This approach replaces native ligands to achieve strong surface binding and to strengthen electrical coupling, while imparting n-type surface functionalization of MHP NCs. As a result, colloidal stability is improved, trap densities are reduced, and charge balance can be precisely tuned within LEDs. LEDs fabricated using ligand-exchanged MHP NCs had a current efficiency of 121.4 cd A^-1 and an external quantum efficiency of 31.7%, both of which are among the highest reported for green-emitting LEDs that use MHP NCs. This study demonstrates the ligand-exchange as a strategic approach for surface functionalization that enables control over charge balance and recombination dynamics, thereby significantly improving the efficiency of LEDs that use MHP NCs.