Selective Incorporation of Organic Spacer Cations Enables Efficient and Spectrally Stable Sky‐Blue Quasi‐2D Perovskite Light‐Emitting Diodes

Quasi‐2D (Q‐2D) perovskites offer strong light emission via energy transfer from low‐ n to high‐ n perovskite domains, and superior stability due to enhanced moisture resistance and tolerance to structural stress compared with their three‐dimensional (3D) counterparts. Nevertheless, the selection of suitable organic spacer cations with appropriate chain lengths and functional groups is crucial for cascade energy transfer and defect passivation. Herein, the selective incorporation of two small‐molecule ligands, i ‐butylammonium bromide and 1‐phenylbiguanide hydrochloride, into CsPbBr 3 precursor solution is demonstrated, rendering Q‐2D perovskite films with optimized domain distribution for efficient light emission. Sky‐blue Q‐2D perovskite films with a carefully tuned ligand ratio exhibit a high photoluminescence quantum yield of 89% with significantly suppressed defect density. Transient absorption spectroscopy unveils efficient energy transfer from donor (low‐ n ) to emitting (high‐ n ) Q‐2D perovskite domains, rendering strong sky‐blue emission. Consequently, sky‐blue perovskite light‐emitting diodes based on optimized Q‐2D perovskite films yield a high external quantum efficiency of 10.6% with a low turn‐on voltage of 2.6 V. Notably, the devices exhibit spectrally stable emission at 494 nm with a narrow full‐width at half‐maximum of ≈21 nm. This work provides a facile approach to developing Q‐2D perovskites with desired domain distribution for efficient, stable optoelectronic devices.