Realizing 20% external quantum efficiency in electroluminescence with efficient thermally activated delayed fluorescence from an exciplex.

Herein we report the investigation of non-doped exciplex formed in blends of 2,4,6-Tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T), working as the one-electron acceptor molecule, with different electron donors. The emission of these exciplexes spans from blue to orange-red, and shows clear contribution from thermally activated delayed fluorescence (TADF). We focus our studies on the properties of TADF in these systems, covering the physical meaning of the different transient components observed in the luminescence decays of these blends. Our results unravel the intricate role of reverse intersystem crossing due to spin-orbit coupling and hyperfine interactions and internal conversion, which affect the efficiency of the TADF mechanism. Organic light-emitting diodes fabricated with these blends show in some cases remarkable performances. Green exciplex blends, in particular, exhibited the current efficiency of 60 cd A-1, power efficiency of 71 lm W-1 and external quantum efficiency of 20%. We believe our results will contribute significantly to highlight the potential advantages of intermolecular exciplexes in the area of organic light-emitting diodes