Red Organic Light‐Emitting Diode with External Quantum Efficiency beyond 20% Based on a Novel Thermally Activated Delayed Fluorescence Emitter

Abstract

A novel thermally activated delayed fluorescence (TADF) emitter 12,15‐di(10H‐phenoxazin‐10‐yl)dibenzo[a,c]dipyrido[3,2‐h:2′,3′‐j]phenazine (DPXZ‐BPPZ) is developed for a highly efficient red organic light‐emitting diode (OLED). With rigid and planar constituent groups and evident steric hindrance between electron‐donor (D) and electron‐acceptor (A) segments, DPXZ‐BPPZ realizes extremely high rigidity to suppress the internal conversion process. Meanwhile, the highly twisted structure between D and A segments will also lead to an extremely small singlet–triplet energy split to DPXZ‐BPPZ. Therefore, DPXZ‐BPPZ successfully realizes an efficient fluorescent radiation transition and reverse intersystem crossing process, and possesses an extremely high photoluminescence quantum efficiency of 97.1 ± 1.1% under oxygen‐free conditions. The OLED based on DPXZ‐BPPZ shows red emission with a peak at 612 nm and a Commission Internationale de L'Eclairage (CIE) coordinate of (0.60, 0.40), and it achieves high maximum forward‐viewing efficiencies of 20.1 ± 0.2% (external quantum efficiency), 30.2 ± 0.6 cd A−1 (current efficiency), and 30.9 ± 1.3 lm W−1 (power efficiency). The prepared OLED has the best performance among the reported red TADF OLEDs. These results prove that DPXZ‐BPPZ is an ideal candidate for red TADF emitters, and the designing approach is valuable for highly efficient red TADF emitters.

Recent Advancements in and the Future of Organic Emitters: TADF‐ and RTP‐Active Multifunctional Organic Materials

“…TADF emitters typically cover the green–yellow region, including 4CzIPN , DPTZ‐DBTO2 , [Cu I (POP)(tmbpy)][BF 4 ] , and TPA‐QXN(CN)2 , and there have been reports of efficient blue TADF systems, but their stability is limited . There are not many reports of orange‐to‐red TADF emitters and they are mainly based on anthraquinone spironaphthalenone ( DCPSO ), phenazine ( POZ‐DBPHZ and DPXZ‐BPPZ ), or naphthalenediimide ( TPA‐cNDI ) acceptors (Figure ). The problem with red TADF emitters is fluorescence quenching and the difficulty of distinguishing between the TTA and TADF mechanisms, owing to the small energy differences.…”

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

“…Zhang et al developed a novel red TADF material DPXZ-BPPZ with PXZ as the donor and BPPZ as acceptor (Figure 15). [75] Compared with DPPZ, BPPZ possesses a larger, more flat structure and higher conjugation degree, avoiding the detrimental affection from molecular configuration relaxation. PXZ is more rigid in structure than that of TPA, and it also has an appropriate HOMO energy level, thus the connection of PXZ and BPPZ can achieve a deep red emission.…”

Simply Structured Near‐Infrared Emitters with a Multicyano Linear Acceptor for Solution‐Processed Organic Light‐Emitting Diodes

“…In recent years, as one of typical purely organic small molecules, thermally activated delayed fluorescence (TADF) materials have shown great potential in OLED applications on the basis of their ability to harvest both singlet and triplet excited states through the reverse intersystem crossing (RISC) process for 100 % internal quantum efficiency . To date, the external quantum efficiencies (EQEs) of blue, green, and red TADF OLEDs have reached about 30 %, 30 %, and 20 %, respectively. In contrast, the development of NIR TADF materials lags behind.…”