Thermally activated delayed fluorescence (TADF) light-emitting electrochemical cells (TADF-LECs) are appealing due to their simple sandwich structure and potential applications in wearable displays and sensors. However, achieving high performance remains challenging. In this paper, we demonstrate that the use of TADF emitters with a low aggregated-caused quenching (ACQ) tendency is crucial to address this challenge. To verify it, two types of TADF-LECs are compared in parallel using different kinds of TADF emitters. The control device uses 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) as the dopant, which suffers from a serious ACQ issue and thus dramatically limits the doping concentrations of 4CzIPN in these TADF-LECs. At the best doping condition (0.5 wt %), insufficient host-to-dopant energy transfer (ET) does exist, thereby displaying very limited efficiency and luminance, i.e., 2.43% and 1483 cd m-2. By contrast, the TADF-LECs using 3,6-di(tert-butyl)-1,8-di(4-(bis(4-(tert-butyl)phenyl)amino)phenyl)-9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenyl) carbazole (BPAPTC) can tolerate a much higher doping concentration because BPAPTC is a satisfactory TADF emitter featuring a low ACQ tendency. At the optimized doping condition of 18 wt %, the BPAPTC-based emissive layer possesses the best TADF property, including the longest τDF (2646 ns), the largest rDF (69%), and the highest kRISC of 7.50 × 105 s-1. Moreover, the corresponding TADF-LEC simultaneously displays the most efficient host-to-dopant ET. It thus achieves unprecedented performance, e.g., the highest external quantum efficiency (EQEmax.) of 7.6%, the highest luminance (Lmax.) of 3696 cd m-2, and an EQE of 7.01% at a practical high luminance of 1000 cd m-2.
Keywords: Thermally activated delayed fluorescence; charge transporter; energy transfer; light-emitting electrochemical cells; poly(ethylene oxide).