Investigating HOMO Energy Levels of Terminal Emitters for Realizing High-Brightness and Stable TADF-Assisted Fluorescence Organic Light-Emitting Diodes

By simple modification of the functional groups on the boron–nitrogen-containing skeleton, the energy level of the highest occupied molecular orbital (E_HOMO) of emitters can be easily adjusted. Blue-emission derivatives are developed, which are capable of showing small full width at half maximums and high photoluminescence quantum yields. Blue thermally activated delayed fluorescence (TADF)-assisted fluorescence organic light-emitting diodes (TAF-OLEDs) based on two new emitters as the terminal emitter are fabricated, resulting in high external quantum efficiency (EQE) of up to 21.9%, high color purity, and high brightness (L_max = 63 777 cd m⁻²). By analyzing the transient electroluminescence spectra of the TAF-OLEDs, it is found that a smaller E_HOMO difference between TADF-assistant dopant (TADF-AD) and terminal emitter efficiently helps to decrease hole trapping inside the emitting layer, hence resulting in a lower efficiency rolloff and a longer operational device lifetime. TAF-OLEDs based on CzBNCz as a terminal emitter having the closest E_HOMO to that of TADF-AD show a maximum EQE of 21.9% together with a reduced efficiency rolloff (EQEs of 21.2% and 19.8% at 100 and 1000 cd m⁻², respectively). This research provides a designing principle for a terminal emitter in TAF-OLEDs with well-matched energy levels towards reaching the requirements of commercial displays.