Unveiling and Refining an Interfacial-Resonant Exciplex for Ultrathin Emitters in High-Efficiency Organic Light-Emitting Diodes

Here, an interfacial resonant exciplex (IRE) composed of 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP) as a donor and 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) as an acceptor is proposed. The mCBP/TmPyPB IRE serves as a host in a nondoped ultrathin emissive layer structure, offering distinct advantages over conventional single-host and cohost systems in organic light-emitting diodes (OLEDs) based on 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (DMAC-TRZ). A comparative investigation with well-studied thermally activated delayed fluorescence-enabled mCBP/PO-T2T and conventional mCBP/TPBi exciplexes demonstrates that using an IRE host with a DMAC-TRZ emitter enhances efficient Förster energy transfer from the S₁ state of mCBP/TmPyPB to DMAC-TRZ, while simultaneously suppressing exciton losses from the T₁ state of the IRE host to the emitter due to the exceptionally small energy gap (0.01 eV). As a consequence, the mCBP/TmPyPB IRE-based OLED achieves a state-of-the-art maximum external quantum efficiency approaching 20%, substantially outperforming reference exciplex devices. This work reveals the critical role of modulating energy transfer dynamics in interfacial exciplexes, paving a new pathway toward highly efficient OLEDs.