Anti‐Heavy‐Atom Effect Boosts Electroluminescence in Copper Cluster‐Based LEDs

ABSTRACT The heavy‐atom effect plays a pivotal role in promoting intersystem crossing and enhancing phosphorescence. However, its impact on electroluminescence in light‐emitting diode (LED) devices remains largely unexplored, and a clear molecular‐level understanding is still lacking. Herein, we report a nearly isostructural pair of copper(I) clusters, [Cu 4 S(dppm) 4 ](PF 6 ) 2 ( Cu 4 S ) and [Cu 4 Se(dppm) 4 ](PF 6 ) 2 ( Cu 4 Se ), which differ solely by a single‐atom substitution of the central S 2− ( Z = 16) with Se 2− ( Z = 34). Despite exhibiting nearly identical photoluminescence (PL) characteristics and comparable external quantum efficiencies (EQEs) in non‐doped devices (5.8% vs. 5.5%), the lighter‐atom‐incorporated Cu 4 S consistently outperforms its heavier analog Cu 4 Se across three distinct host matrices. In particular, the Cu 4 S ‐based device employing the thermally activated delayed fluorescence (TADF) hosts achieved a maximum EQE of 20.9% at λ EL = 608 nm , significantly surpassing that of devices with Cu 4 Se (12.9%). Systematic studies reveal that the S‐centered cluster exhibits stronger resistance to concentration quenching, more enhanced charge transport, and a significantly reduced trap‐state density, thereby effectively circumventing heavy‐atom‐induced non‐radiative losses during electroluminescence. These findings demonstrate that single‐atom variations within the cluster core decisively govern EL efficiency via an anti‐heavy‐atom effect and provide a new strategy for improving LED performance by exploiting this effect.