High‐Performance Multi‐Resonance Thermally Activated Delayed Fluorescence Emitters for Narrowband Organic Light‐Emitting Diodes

Abstract

Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters have drawn considerable attention because of their remarkable optoelectronic properties of high emission efficiency and narrow emission profile, and represent an active subject of cutting‐edge research in the organic electroluminescence (EL). However, the slow reverse intersystem crossing (RISC) rate of MR‐TADF emitter caused by the large energy gap (ΔEST) and small spin‐orbit coupling (SOC) matrix elements between the singlet and triplet excited states limits their further development in organic EL devices. Currently, innovative molecular design strategies have been developed including heavy atom integration, π‐extended MR framework and metal perturbation, and so on to improve the RISC process of MR‐TADF emitters for high‐performance EL devices. Here, an overview is presented on the recent progress of MR‐TADF emitters with fast RISC rate ( > 10−5 s−1), with particular attention to the molecular design, optoelectronic properties, and device performance of organic light‐emitting diodes (OLEDs), which intends to systematize the knowledge in this subject for the thriving development of highly efficient MR‐TADF emitters. Finally, the challenges and future prospects of MR‐TADF materials are discussed comprehensively.

Boron- and Nitrogen-Embedded Polycyclic Arenes as an Emerging Class of Organic Semiconductors

“…The advantages of tunable emissions, small FWHMs, and high EQEs of MR-TADF materials make them the most promising candidates to meet the BT.2020 standard . To date, a number of MR-TADF materials with different emission colors and high color purity have been reported, which have been comprehensively reviewed recently. − In 2019, Hatakeyama and co-workers reported the ultrapure blue OLED using 55 as the emitter with CIE coordinate of (0.12, 0.11), and the device exhibited λ EL at 469 nm, FWHM of 18 nm, and EQE of 34.4%. − In 2022, Wang, Chen, Adachi, Zhang, and co-workers achieved the ultrapure green OLED by developing 56 as the emitter with CIE coordinate of (0.19, 0.74) . The OLED device exhibited a λ EL at 520 nm, FWHM of 29 nm, and EQE of 35.2%.…”

Marching Toward Long‐Wavelength Narrowband Emissive Multi‐Resonance Delayed Fluorescence Emitters for Organic Light Emitting Diodes

“…As a result, the HOMO orbital is constituted by the donor atom and the neighboring carbon (C) atoms relative to the ortho/para-positions of aromatic units, while the LUMO orbital is composed by the electron withdrawing unit and the relative ortho/para-carbon atoms in the PAH framework. [54][55][56]122,127] Thus, the separation of HOMO and LUMO orbitals into different atomic units are feasible in MR-emitters without D-A structure. By virtue of this MR-feature, the bonding/antibonding character between the adjacent atoms can be suppressed, the resulting non-bonding orbitals can suppress the structural changes and vibronic coupling of the molecule in the excited state.…”

Carbazole‐Decorated Organoboron Emitters with Low‐Lying HOMO Levels for Solution‐Processed Narrowband Blue Hyperfluorescence OLED Devices

“…Since the first report of MR-TADF emitter with B, N-doped polycyclic aromatic frameworks by Hatakeyama et al. in 2016, [3] efficient vacuum-deposited narrowband red, green and blue OLEDs covering the full color have been developed, [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] revealing extremely high external quantum efficiency (EQE) of 36.1 %, 40.1 % and 43.9 %, respectively. [20][21][22] Compared with evaporation, solution-processing methods have been intensively demonstrated as more economical approaches for the mass production of large-area OLED displays.…”