Surface Compensation Principles of Soft‐Lattice Nanocrystals toward High‐Performance Electroluminescence

Surface reconstruction in electroluminescent nanocrystals (NCs) requires a delicate balance between defect passivation and preserving lattice integrity. The structural damage is exacerbated in soft-lattice perovskite systems by the conventional ligand exchange process. Herein, a surface compensation paradigm is presented employing quaternary phosphonium iodides that synergistically repair halogen vacancies and stabilize the surface without perturbing the ionic lattice. These molecular compensators are delicately designed to be soluble in weak-polar solvents, enabling non-destructive ionic surface compensation. Besides, the sterically optimized ionic coordination suppresses defect generation during surface reconstruction. Resulting pure-red light-emitting diodes (LEDs) achieve remarkable electroluminescence performance with 28.8% peak external quantum efficiency, 1.75 V sub-bandgap turn-on voltage, 30.1% power conversion efficiency, and 70.0 h operational half-lifetime at 100 cd m^-2, establishing new benchmarks for Rec.2020-compliant devices. The relationship between the steric factor of ligands and the compensation effect is further elucidated, providing molecular design principles for soft-lattice optoelectronic NCs.