Triplet Exciton Confinement in Green Organic Light‐Emitting Diodes Containing Luminescent Charge‐Transfer Cu(I) Complexes

The temperature dependence of luminescence from Cu(dnbp)(DPEPhos)]BF₄ (dnbp = 2,9‐di‐n‐butylphenanthroline, DPEPhos = bis2‐(diphenylphosphino)phenyl]ether) in a poly(methyl methacrylate) (PMMA) film indicates the presence of long‐life green emission arising from two thermally equilibrated charge transfer (CT) excited states and one non‐equilibrated triplet ligand center (³LC) excited state. At room temperature, the lower triplet CT state is found to be the predominantly populated excited state, and the zero‐zero energy of this state is found to be 2.72 eV from the onset of its emission at 80 K. The tunable emission maximum of Cu(dnbp)(DPEPhos)]BF₄ in various hosts with different triplet energies is explained in terms of the multiple triplet energy levels of this complex in amorphous films. Using the high triplet energy charge transport material as a host and an exciton‐blocking layer (EBL), a Cu(dnbp)(DPEPhos)]BF₄ based organic light‐emitting diode (OLED) achieves a high external quantum efficiency (EQE) of 15.0%, which is comparable to values for similar devices based on Ir(ppy)₃ and FIrpic. The photoluminescence (PL) and electroluminescence (EL) performance of green emissive Cu(μI)dppb]₂ (dppb = 1,2‐bisdiphenylphosphino]benzene) in organic semiconductor films confirmed its ³CT state with a zero‐zero energy of 2.76 eV as the predominant population excited state.