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

Abstract 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.