The Intermolecular Long‐Range Coupled Charge Transfer Excited State in CsCu 2 I 3 Light‐Emitting Diodes

Low‐dimensional ternary copper iodides are highly promising for future health displays and eco‐friendly lighting due to their structural stability, excellent optoelectronic properties, high abundance, and low cost. However, the lack of a deep understanding of the complex charge transfer (CT) mechanism in their excited state dynamics hinders the efficiency progress of copper‐based optoelectronic devices. Here, we provide a comprehensive analysis of the intermolecular CT excited‐state dynamics in CsCu 2 I 3 light‐emitting diodes (LEDs) and demonstrate the long‐range coupled characteristics of CT excitons. We report a strategy to achieve long‐range coupled CT excited state by spatially separating electron‐hole pairs, thereby enhancing the electroluminescence performance of CsCu 2 I 3 LEDs. Carrier dynamics analysis reveals that the long‐range coupled CT excited state achieves the highest delayed fluorescence contribution at the optimal coupling intensity, facilitating more efficient triplet energy harvesting. Accordingly, the CsCu 2 I 3 LEDs exhibit a maximum external quantum efficiency of 4.3% and a peak luminance of 3666 cd/m 2 , representing the highest luminance reported to date. Furthermore, the universality of the long‐range coupled CT excited state has been demonstrated in other device structures. This study establishes a new method to pave the way for enhanced performance of CsCu 2 I 3 LEDs toward practical applications.