Analysis of exciton annihilation in high-efficiency sky-blue organic light-emitting diodes with thermally activated delayed fluorescence

We study external quantum efficiency (eta(EQE)) roll-off in organic light-emitting diodes (OLEDs) using thermally-activated delayed fluorescence (TADF) of 4,5-di (9H-carbazol-9-yl) phthalonitrile (2CzPN). Using 2CzPN intramolecular rate constants from optical analyses, we construct an exciton quenching model incorporating intersystem crossing and reverse intersystem crossing. The model indicates that singlet-triplet annihilation and triplet-triplet annihilation dominate eta(EQE) roll-off because of the relatively long 2CzPN triplet lifetime of 273 mu s. This work yields a method to relax the exciton quenching process in TADF based OLEDs. (C) 2013 Elsevier B. V. All rights reserved.