Realizing 22.5% External Quantum Efficiency for Solution‐Processed Thermally Activated Delayed‐Fluorescence OLEDs with Red Emission at 622 nm via a Synergistic Strategy of Molecular Engineering and Host Selection

Abstract

Developing high‐efficiency solution‐processable thermally activated delayed‐fluorescence (TADF) emitters, especially in longer wavelength regions, is a formidable challenge. Three red TADF emitters, namely NAI_R1, NAI_R2, and NAI_R3, are developed by phenyl encapsulation and tert‐butyl substitution on a prototypical 1,8‐naphthalimide‐acridine hybrid. This design strategy not only grants these molecules high solubility, excellent thermal stability, and good film‐forming ability, but also pulls down their charge‐transfer (CT) energy levels excited states. Furthermore, dispersing these emitters into two different host materials of mCP and mCPCN finely tailors their CT‐state energy levels. More importantly, a synergistic combination of molecular engineering and host selection can effectively manipulate the competition between the radiative and nonradiative decay rates of the CT singlet states of these emitters and the reverse intersystem crossing from their triplet to singlet states. Consequently, the optimal combination of NAI_R3 emitter and mCP host successfully results in a state‐of‐the‐art external quantum efficiency (EQE) of 22.5% for solution‐processed red TADF organic light‐emitting diodes (OLEDs) with an emission peak above 620 nm. This finding demonstrates that a synergistic strategy of molecular engineering and host selection with TADF emitters could provide a new pathway for developing efficient solution‐processable TADF systems.

High‐Performance Quantum‐Dot Light‐Emitting Diodes Using NiOx Hole‐Injection Layers with a High and Stable Work Function

“…The current efficiency of this device reaches a maximum of 19.4 cd A −1 at a brightness of 3400 cd m −2 and maintains to be above 18.5 cd A −1 at a brightness of 10 000 cd m −2 . Remarkably, the driving voltages for the luminance of the device reaching 1000 cd m −2 and 10 000 cd m −2 are 2.1 and 3.3 V, respectively, both of which are the lowest for red solution‐processed LEDs and red vacuum‐deposited OLEDs (see Table S2, Supporting Information) . A histogram of 45 devices shows an average current efficiency of 17.4 cd A −1 (see Figure S12, Supporting Information) with a relative standard deviation of 12%.…”

Structural Design of Blue‐to‐Red Thermally‐Activated Delayed Fluorescence Molecules by Adjusting the Strength between Donor and Acceptor

“…In 2019, this group built three solution-processable red TADF materials, named as NAI R1, NAI R2, and NAI R3, by performing phenylencapsulation and tert-butyl substitution on the similar main structure (Figure 18). [83] The introduction of phenyl-encapsulated units at the 2-and 7-positions of DMAC improved the electron-donating ability and reduced the HOMO level. Further connecting the tert-butyl substituent to the peripheral phenyl can improve the solubility and fine-tuned their emission color.…”

Single White‐Emitting Polymers with High Efficiency, Low Roll‐Off, and Enhanced Device Stability by Using Through‐Space Charge Transfer Polymer with Blue Delayed Fluorescence as Host for Yellow Phosphor

“…The second kind is phosphorescent SWPs consisting of metal complex phosphors that can harness triplet excitons via spin–orbital coupling, which are able to achieve 100% IQEs by proper selection of phosphors and polymer hosts with matched energy levels. [ 9–11,18–22 ] Recently, a new kind of SWPs based on thermally activated delayed fluorescence (TADF) [ 23–36 ] emitters, which utilizes the twisted donor–acceptor structure with reduced singlet‐triplet energy splitting (∆ E ST ) to upconvert triplet excitons to singlet ones via reverse intersystem crossing (RISC), has been reported, [ 4–7 ] providing an alternative approach to realize 100% IQEs for SWPs. However, up to now, device performance of SWPs is still behind the requirement of practical application.…”