High‐Performance Narrowband Pure‐Red OLEDs with External Quantum Efficiencies up to 36.1% and Ultralow Efficiency Roll‐Off

Abstract

High‐color‐purity blue and green organic light‐emitting diodes (OLEDs) have been resolved thanks to the development of B/N‐based polycyclic multiple resonance (MR) emitters. However, due to the derivatization limit of B/N polycyclic structures, the design of red MR emitters remains challenging. Herein, a series of novel red MR emitters is reported by para‐positioning N–π–N, O–π–O, B–π–B pairs onto a benzene ring to construct an MR central core. These emitters can be facilely and modularly synthesized, allowing for easy fine‐tuning of emission spectra by peripheral groups. Moreover, these red MR emitters display excellent photophysical properties such as near‐unity photoluminescence quantum yield (PLQY), fast radiative decay rate (kr) up to 7.4 × 107 s−1, and most importantly, narrowband emission with full‐width at half‐maximum (FWHM) of 32 nm. Incorporating these MR emitters, pure red OLEDs sensitized by phosphor realize state‐of‐the‐art device performances with external quantum efficiency (EQE) exceeding 36%, ultralow efficiency roll‐off (EQE remains as high as 25.1% at the brightness of 50 000 cd m−2), ultrahigh brightness over 130 000 cd m−2, together with good device lifetime.

Organic molecules with inverted singlet-triplet gaps

“…Over the past decade, TADF molecules have been widely studied in light of the merits of high efficiency as well as low cost, and a number of TADF emitters have been developed ( Jeon et al, 2019 ; Li et al, 2021c ). Up to now, there are several pathways to realize TADF, such as traditional single molecule-based TADF ( Uoyama et al, 2012 ; Zhang et al, 2012 ; Li et al, 2021d ), exciplex-based TADF ( Goushi et al, 2012 ; Li et al, 2014a ; Oh et al, 2015 ; Li et al, 2021a ; Li et al, 2021b ; Xue and Xie, 2021 ; Li et al, 2022c ; Gu et al, 2022 ), aggregation-induced emission (AIE)-based TADF ( Zhao et al, 2018 ; Liu et al, 2020 ), excited-state intramolecular proton transfer (ESIPT)-based TADF ( Mamada et al, 2017 ; Long et al, 2020 ) and multiple resonance-based TADF (MR-TADF) ( Lee et al, 2020 ; Stavrou et al, 2021 ; Wu et al, 2021 ; Yang et al, 2022 ; Zou et al, 2022 ). Particularly, MR-TADF molecules have been considered as the most promising TADF materials on account of the attainment of both high efficiencies and high color purity.…”

Advances in Blue Exciplex–Based Organic Light-Emitting Materials and Devices

“…Since the realization of the first organic light-emitting diode (OLED) possessing a high brightness of >1,000 cd m −2 and a low-driving voltage ( V on ) of <10 V ( Tang and VanSlyke, 1987 ), OLEDs based on small molecules ( Tang et al, 1989 ; Adachi et al, 1990 ), polymers ( Burroughes et al, 1990 ; Peng et al, 1998 ), and metal–organic complexes ( Baldo et al, 1998 ; Chang et al, 2013 ) have attracted tremendous attention in the fields of lighting and displays over the past few decades owing to their fascinating merits such as thinness, fast response, and flexibility ( Hong et al, 2021 ). Among these OLEDs, several different kinds of luminescence mechanisms, including traditional fluorescence ( Friend et al, 1999 ; Huang et al, 2012 ), phosphorescence ( Bernhard et al, 2002 ; Zhou et al, 2014 ), triplet–triplet annihilation (TTA) ( Fukagawa et al, 2012 ; Jankus et al, 2013 ), traditional thermally activated delayed fluorescence (TADF) ( Endo et al, 2011 ; Uoyama et al, 2012 ; Zhang et al, 2012 ; Li et al, 2013 ; Li et al, 2021d ), hyperfluorescence ( Nakanotani et al, 2014 ; Chan et al, 2021 ), singlet–triplet inversion ( Ehrmaier et al, 2019 ; Pollice et al, 2021 ; Li et al, 2022 ), exciplex-based TADF ( Goushi et al, 2012 ; Li et al, 2014 ; Oh et al, 2015 ; Li et al, 2021c ; Gu et al, 2022 ), aggregation-induced emission (AIE)–based TADF ( Peng and Shuai, 2021 ; Suman et al, 2021 ), and multiple resonance (MR)–based TADF ( Lee et al, 2020 ; Stavrou et al, 2021 ; Wu et al, 2021 ; Zou et al, 2022 ) have been reported. Thus, the exciplex used to be considered an important reason for poor OLED performance, and it thus should be avoided and eliminated ( Adachi et al, 1990 ; Jenekhe, 1995 ; Morteani et al, 2003 ).…”

Ultrapure Blue Thermally Activated Delayed Fluorescence (TADF) Emitters Based on Rigid Sulfur/Oxygen-Bridged Triarylboron Acceptor: MR TADF and D–A TADF

“…Since the epoch-making work by Adachi and co-workers in 2012, thermally activated delayed fluorescence (TADF) materials have undergone extensive development as the third-generation of organic light-emitting diodes (OLEDs) because they are capable of achieving high internal quantum efficiencies (η int ) of nearly 100% for electroluminescence (EL) through the advantageous upconversion of triplet excited state (T 1 ) to singlet excited state (S 1 ) via reverse intersystem crossing (RISC). − Over the past decade, lots of high external quantum efficiencies (EQEs) over 20% TADF materials have been reported in devices. − In general, creating a donor–acceptor-type (D–A-type) molecular skeleton with intramolecular charge-transfer (ICT) characteristics is the most commonly used approach for TADF blue emitters. This would, therefore, lead to broad emission spectra of TADF emitters with full width at half-maximum (fwhm) of over 70 nm, which eventually results in decreased color purity and high energy consumption in OLED displays with a color filter. , To satisfy the needs of high color purity in OLED displays, Hatakeyama and co-workers proposed a novel TADF emitters design strategy based on the multiresonance (MR) effect, which can simultaneously realize high EQEs and ultrapure blue emissions in a single molecule .…”

“…Meanwhile, the MR effect of the rigid polycyclic aromatic framework enhanced the oscillator strength ( f ) between the singlet ground (S 0 ) and singlet excited (S 1 ) states, which is conducive to reduce fwhm of TADF emitters. Owing to the above advantage, many nitrogen-bridged cyclized boron-based TADF emitters with good device performance have been reported in recent studies. − , However, a series of recent studies has indicated that the current approach is only appropriate for specific molecular constitutions, of which most are MR TADF emitters focused on polycyclic aromatic hydrocarbon with boron and nitrogen, as well as carbonyl- and nitrogen-containing aromatics. − …”

“…Recently, various triarylboron-based emitters consisting uncyclized and nitrogen/oxygen-bridged cyclized configurations have revealed excellent blue TADF-OLED performance. − ,− Particularly, oxygen-bridged cyclized boron-based emitters revealed groundbreaking performance, with EQEs in blue and sky-blue TADF–OLEDs as high as 38.0% . In addition to high performance, nitrogen/oxygen-bridged cyclized boron-based emitters have also achieved deep blue EL and small fwhm as a result of their good structural rigidity.…”