Anti-Heavy-Atom Effect Boosts Electroluminescence in Copper Cluster-Based LEDs
Fei-Fan Wang1, Tao-Tao Xia1, Zi-Cong Dong1
1Henan Key Laboratory of Crystalline Molecular Functional Materials, Key Laboratory of Special Functional Molecular Materials (Zhengzhou University), Ministry of Education, Pingyuan Laboratory, Zhengzhou University, Zhengzhou, China.
None:
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, [Cu4S(dppm)4](PF6)2 (Cu4S) and [Cu4Se(dppm)4](PF6)2 (Cu4Se), which differ solely by a single-atom substitution of the central S2- (Z = 16) with Se2- (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 Cu4S consistently outperforms its heavier analog Cu4Se across three distinct host matrices. In particular, the Cu4S-based device employing the thermally activated delayed fluorescence (TADF) hosts achieved a maximum EQE of 20.9% at λEL = 608 nm, significantly surpassing that of devices with Cu4Se (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.
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