Understanding the operational lifetime expansion methods of thermally activated delayed fluorescence sensitized OLEDs: a combined study of charge trapping and exciton dynamics

Abstract

Organic light-emitting diodes (OLEDs) still face the problem of insufficient operational stability. Devices adopting host materials with thermally activated delayed fluorescence (TADF) have outperformed their conventional phosphorescence and TADF emitter counterparts in terms of operational lifetime; however, the intrinsic reason for this still remains unclear. In this article, mechanisms of intrinsic degradation in TADF sensitized OLEDs were investigated both theoretically and experimentally. From current density–voltage characteristics and photoelectrical aging of single-carrier devices, shallow trap assisted charge transport was supposed to enhance the device operational stability. Numerical simulations on exciton dynamics were carried out to calculate the efficiency loss in various types of OLEDs. Fast reverse intersystem crossing and singlet radiation were both found to be necessary to accomplish the long device lifetime. Thus, TADF sensitizing OLEDs could on the one hand shorten the polaron residing time on the hosts by the shallow trap assisted charge transport, and on the other hand reduce the host triplet concentration by virtue of the fast reverse intersystem crossing and efficient Förster resonant energy transfer. By this means, long operational lifetime could be expected in TADF sensitized OLEDs by the reduction of host singlet–triplet splitting, the precise adjustment of the host–guest energy gap and doping concentration, and the enhancement of the host emission–guest absorption overlap.