Mitigating slow reverse ISC rates in TAPC:PBD exciplex via rapid Förster energy transfer to TTPA

Abstract

There have been many advances in the development of thermally activated delayed fluorescence (TADF) materials for organic light emitting diode (OLED) applications in recent years. In particular, intramolecular exciplex systems have been highly studied and found to produce OLED devices of high external quantum efficiency (EQE) due to triplet harvesting via TADF. The proposed next generation of OLEDs uses hyperfluorescence to overcome the problem of broad emission associated with exciplexes. This process involves Förster resonance energy transfer (FRET) from the TADF host to a fluorescent dopant. In this work we revisited the photophysics of the TAPC:PBD exciplex (formed between the electron donor di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane (TAPC) and the electron acceptor, 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD)) as a host capable of simultaneously performing triplet harvesting and work as a donor transferring energy to a bright fluorescent emitter. The aim is to investigate the interplay between energy transfer and intersystem crossing in this hyperfluorescence system. Contrarily to previous findings, films of the TAPC:PBD blend show relatively slow reverse intersystem crossing rate (RISC) and weak luminescence efficiency (PLQY). Despite this, when doped with the strong fluorescent emitter TTPA, the luminescence quantum yield is greatly improved due to the highly efficient energy transfer rate from TAPC:PBD to TTPA. The rapid FRET from the exciplex to the fluorescent emitter overcomes the non-radiative losses affecting the luminescence efficiency of the blend. This study shows that the hyperfluorescence mechanism not only allows colour purity in OLEDs to be optimised, but also facilitates suppressing major loss mechanisms affecting luminescence efficiency, thus creating conditions to maximizing EQE.