Key Determining Factors for the Emitting Dipole Orientation of Exciplexes

Exciplexes have attracted interest as hosts and emitters that provide high efficiency and long lifetimes in organic light-emitting diodes (OLEDs). However, the emitting dipole orientation (EDO) of exciplexes has been overlooked despite its importance for high efficiency. In this work, the EDO of exciplexes is investigated by changing the p-type host. Various p-type hosts are mixed with 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine for exciplex formation. The EDO of the exciplexes is aligned vertically with p-type hosts having hydrophobic or electron-poor central units while dispersing the highest occupied molecular orbitals (HOMOs) at the end of the molecules, whereas it is isotropic with p-type hosts having widely distributed HOMO on the whole molecular structure without any orientation-managing central unit. The results show that EDO can be correlated with the outcoupling efficiency of exciplex OLEDs.