Effect of TADF Assistance on Performance Enhancement in Solution Processed Green Phosphorescent OLEDs

Abstract

Many methods have been proposed to increase the efficiency of organic electroluminescent materials applied as an emissive layer in organic light emitting diodes (OLEDs). Herein, we demonstrate enhancement of electroluminescence efficiency and operational stability solution processed OLEDs by employing thermally activated delayed fluorescence (TADF) molecules as assistant dopants in host-guest systems. The TADF assistant dopant (SpiroAC-TRZ) is used to facilitate efficient energy transfer from host material poly(N-vinylcarbazole) (PVK) to a phosphorescent Ir(III) emitter. We present the analysis of energy transfer and charge trapping-two main processes playing a crucial role in light generation in host-guest structure OLEDs. The investigation of photo-, electro- and thermoluminescence for the double-dopant layer revealed that assistant dopant does not only harvest and transfer the electrically generated excitons to phosphorescent emitter molecules but also creates exciplexes. The triplet states of formed PVK:SpiroAC-TRZ exciplexes are involved in the transport process of charge carriers and promote long-range exciton energy transfer to the emitter, improving the efficiency of electroluminescence in a single emissive layer OLED, resulting in devices with luminance exceeding 18 000 cd/m(2) with a luminous efficiency of 23 cd/A and external quantum efficiency (EQE) of 7.4%.