“…Ideally, holes/electrons should be confined by the electron/hole transport layer owing to the deep HOMO (highest occupied molecular orbital) of electron transport layer/the shallow LUMO (lowest unoccupied molecular orbital) of hole transport layer [55,56,57,58,59]. For the emitting layer, the ligand density of NPLs have a vital influence on NPL-LEDs, since ligands have double-side effect: (i) vast ligands are necessary to give surface passivation to eliminate surface defects, resulting in high photoluminescence quantum efficiency (PLQY) and ink stability; (ii) excessive ligands can form insulating layers, preventing the charge injection in NPL-LEDs [60,61,62,63,64]. Hence, how to effectively control the ligand density is significant for NPL-LEDs.…”