Separating Crystal Growth from Nucleation Enables the In Situ Controllable Synthesis of Nanocrystals for Efficient Perovskite Light‐Emitting Diodes

Colloidal perovskite nanocrystals (PNCs) display bright luminescence for light‐emitting diode (LED) applications; however, they require post‐synthesis ligand exchange that may cause surface degradation and defect formation. In situ‐formed PNCs achieve improved surface passivation using a straightforward synthetic approach, but their LED performance at the green wavelength is not yet comparable with that of colloidal PNC devices. Here, it is found that the limitations of in situ‐formed PNCs stem from uncontrolled formation kinetics: conventional surface ligands confine perovskite nuclei but fail to delay crystal growth. A bifunctional carboxylic‐acid‐containing ammonium hydrobromide ligand that separates crystal growth from nucleation is introduced, leading to the formation of quantum‐confined PNC solids exhibiting a narrow size distribution. Controlled crystallization is further coupled with defect passivation using deprotonated phosphinates, enabling improvements in photoluminescence quantum yield to near unity. Green LEDs are fabricated with a maximum current efficiency of 109 cd A−1 and an average external quantum efficiency of 22.5% across 25 devices, exceeding the performance of their colloidal PNC‐based counterparts. A 45.6 h operating half‐time is further documented for an unencapsulated device in N2 with an initial brightness of 100 cd m−2. Carboxylic acid‐containing ammonium ligands separate nucleation and crystal growth in in‐situ‐formed perovskite nanocrystals, enabling quantum‐confined nanocrystals with narrow size distributions. Deprotonated phosphonates further enhance photoluminescence quantum yields to unity through defect passivation. These allow for perovskite light‐emitting diodes with a maximum current efficiency of 109 cd A−1 and a 45.6 h operating half‐time with an initial brightness of 100 cd m−2.