Achieving Record Efficiency and Luminance for TADF Light-Emitting Electrochemical Cells by Dopant Engineering

Abstract

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 r DF (69%), and the highest k RISC 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 (L max.) of 3696 cd m–2, and an EQE of 7.01% at a practical high luminance of 1000 cd m–2.

Studies on Annihilation and Coreactant Electrochemiluminescence of Thermally Activated Delayed Fluorescent Molecules in Organic Medium

“…Since the subsequent CV and ECL studies are performed in the DCM solvent, we measured the corresponding photophysical properties in DCM. As it is shown in Figure 1 a,c, the absorption spectra of those two emitters are basically unchanged as compared to the earlier reports measured in toluene [ 42 ]. Either of them show their intrinsic absorption and clear charge-transfer absorption features at the long wavelength range, indicating that the charge-transfer features of these two molecules are maintained in DCM.…”

White Light‐Emitting Electrochemical Cells Employing Phosphor‐Sensitized Thermally Activated Delayed Fluorescence to Approach All‐Phosphorescent Device Efficiencies

“…Recently, a novel type of fluorescent materials with thermally activated delayed fluorescence (TADF) [32] has been demonstrated to be capable of recycling the triplet excitons on them via efficient reverse intersystem crossing (RISC), leading to phosphorescent EL efficiencies. In view of such a high potential, neat-film [33][34][35][36] and host-guest [37][38][39][40][41][42][43] TADF LECs have been proposed. The host-guest TADF LECs generally showed higher device efficiencies than the neat-film TADF LECs due to reduced self-quenching and a lower possibility of triplet-triplet annihilation on the TADF guest molecules, which is a strong competing process against RISC.…”

Efficient Tandem White OLED/LEC Hybrid Devices

“…[8][9][10][11][12][13] Recently, neat-film and host-guest LECs based on small molecules with thermally activated delayed fluorescence have also been demonstrated to exhibit good device efficiencies by recycling the triplet excitons via the efficient reverse intersystem crossing process. [14][15][16][17][18][19][20] Given the high potential for white LECs in lighting and display applications, they have been intensively studied since the first publication [21] and the device performance has been improved significantly. [22] In addition to employing efficient iTMCs, sophisticated device engineering technologies for white LECs have also been utilized to take full advantage of efficient materials.…”