Balancing the Defect Passivation and Quantum Confinement Effect toward Efficient Ligand‐Free Blue CsPbBr ₃ Perovskite Light‐Emitting Devices

ABSTRACT Organic ligand‐free Cs‐Pb‐Br perovskites exhibit promising blue emission through Cs 4 PbBr 6 ‐confined CsPbBr 3 composites, yet suffer from low photoluminescence quantum yield (PLQY) due to intrinsic defects. While additive strategies can passivate defects, they may disrupt crystal growth and quantum confinement effects (QCE). Here, we explored additive engineering to modulate the optical properties of CsPbBr 3 films for defect passivation while preserving the quantum confinement effect (QCE). We found that a strong additive‐perovskite bond hinders CsPbBr 3 transformation into Cs 4 PbBr 6 , weakening the QCE and causing a redshift in the spectrum. Conversely, a weaker additive‐perovskite interaction allows defect passivation with minimal QCE loss. The sky‐blue device (@489 nm) incorporating guanidine sulfamate achieved an external quantum efficiency (EQE) of 10.5% and operational stability (T 50 ) of ≈56 min, surpassing lifespans of many quasi‐2D and mixed halides sky‐blue PeLEDs with higher efficiency. This work offers new insights into balancing defect passivation and QCE in ligand‐free blue perovskites toward efficient and stable blue PeLEDs.