Cluster Light‐Emitting Diodes Containing Copper Iodine Cube with 100 % Exciton Utilization Using Host‐Cluster Synergy

Abstract Clusters combine the advantages of organic molecules and inorganic nanomaterials, which are promising alternatives for optoelectronic applications. Nonetheless, recently emerged cluster light‐emitting diodes require further excited state optimization of cluster emitters, especially to reduce population of the cluster‐centered triplet quenching state ( 3 CC). Here we report that redox‐active ligands enhance reverse intersystem crossing (RISC) of Cu 4 I 4 cluster for triplet‐to‐singlet conversion, and thermally activated delayed fluorescence (TADF) host can provide an external RISC channel. It indicates that the complementarity between TADF host and cluster in RISC transitions gives rise to 100 % triplet conversion efficiency and complete singlet exciton convergence, rendering 100‐fold increased singlet radiation rate constant and tenfold decreased triplet non‐radiation rate constant. We achieve a photoluminescence quantum yield of 99 % and a record external quantum efficiency of 29.4 %.