Surface Ligand Engineering for Efficient Perovskite Nanocrystal-Based Light-Emitting Diodes

Lead halide perovskites (LHPs) are emerging as promising materials for light-emitting device applications because of the tunability of the band gap, narrow emission, solution processability, and flexibility. Typically, LHP nanocrystals (NCs) with surface ligands show high photoluminescence quantum yields because of charge-carrier confinement with higher exciton binding energy ( E_b). However, the conventionally used oleylamine (OAm) ligands result in the low electrical conductivity and stability of perovskite NCs (PNCs) because of a long carbon chain without conjugation bonds and weak interaction with the surface of NCs. Here, we report the effect of bulkiness and chain length of ligand materials on the properties and stability of CsPbBr₃ PNCs by replacing OAm with other suitable ligands. The effect of the bulkiness of quaternary ammonium bromide (QAB) ligands was systemically studied. The less bulky QAB ligands surrounded the surface of NCs effectively, and brought better surface passivation and less aggregation compared to bulky QAB ligands, and finally the optical property and stability of CsPbBr₃ PNCs were enhanced. Furthermore, the electrical property of CsPbBr₃ PNCs was optimized by tuning the long-chain length of QAB ligands for balanced charge-carrier transport. Finally, we achieved highly efficient green emissive CsPbBr₃ PNC light-emitting diodes (LEDs) by using PNCs with optimized didecyldimethyl ammonium bromide ligands with a current efficiency of 31.7 cd A^-1 and external quantum efficiency of 9.7%, which were enhanced 16-fold compared to those of CsPbBr₃ LEDs using PNCs with conventional OAm ligands.