Red electroluminescent Cu4I4 bipyramids with external quantum efficiency beyond 40.

The development of red cluster light-emitting devices (CLED) lags far behind blue and green congeners with respect to efficiencies, since multiple excited states of red cluster molecules are involved in non-radiation during electroluminescence. Herein, we accurately optimize excited states of a red bipyramidal [PXZDPPQ]2Cu4I4, whose ligand integrates conjugation-extended 2-diphenylphosphineylquinoline (DPPQ) and strong electron-donating phenoxazine (PXZ), giving rise to desired excited state locations of the outer intraligand charge transfer (nLCT)-featured first singlet (S1) and triplet (T1) excited states and the inner high-lying metal-ligand charge transfer (nMLCT) states. Its outer and highly radiative nLCT states are spatially and energetically advantageous in carrier capture and exciton confinement; meanwhile, its inner and high-lying nMLCT states are protected from collisional quenching, and support cross transitions between nLCT states to realize exactly balanced dual emissions with a ratio of 51/49. [PXZDPPQ]2Cu4I4 achieves eightfold increased photoluminescence quantum yield of 93.6%, tenfold increased external quantum efficiency reaching 43.7% as a new record for all kinds of planar red light-emitting devices, and 20% improved exciton utilization efficiency in comparison to the congener with the reverse excited state location. These results demonstrate the unique merit of cluster materials in exciton engineering and their potential for next-generation full-color displays.