Bulk Passivation Enables Hundredfold‐Enhanced Electroluminescence of Monophosphine Cu4I4 Cubes

Electroluminescent (EL) clusters emerged rapidly, owing to their organic–inorganic hybrid character useful for comprehensive performance integration and the potential for large‐scale display and lighting applications. However, despite their good photoluminescent (PL) properties, until present, no efficient EL monodentate ligand‐based clusters were reported due to structural variation during processing and excitation and exciton confinement on cluster‐centered quenching states. Here we demonstrate an effective bulky passivation strategy for efficient cluster light‐emitting diodes with a monophosphine Cu4I4 cube named [TMeOPP]4Cu4I4. With terminal pyridine groups, an active matrix named TmPyPB supports an effective host‐cluster interplay for configuration fixation, structural stabilization, and exciton‐confinement optimization. Compared to common inactive hosts, the passivation effects of TmPyPB markedly reduce trap‐state densities by 24–40 % to suppress nonradiative decay, resulting in state‐of‐the‐art PL and EL quantum yields reaching 99 % and 15.6 %, respectively, which are significantly improved by about 7‐fold. TmPyPB simultaneously increases EL luminance to 104 nits, which is ≈100‐fold that of the non‐doped analogue. A cluster light‐emitting diode based on a monodentate‐ligand‐based Cu4I4 cube realized a record luminance and external quantum efficiency (EQE) beyond 104 nits and 15 %, respectively, owing to a bulk passivation effect of the active host matrix through non‐covalent N⋅⋅⋅Cu interactions. This markedly reduces nonradiative decay, leading to a doubled photoluminescence quantum yield of the film (99 %) and a 100‐fold and sevenfold increased luminance and EQE of the devices, respectively.