Multiresonance thermally activated delayed fluorescence (MR-TADF) emitters manifest great potential for organic light-emitting diodes (OLEDs) due to their high exciton-utilization efficiency and narrowband emission. Nonetheless, their tendency toward self-quenching caused by strong interchromophore interactions would induce doping sensitivity and deteriorate the device performances, and effective strategy to construct quenching-resistant emitters without sacrifycing color purity is still to be developed. By segregating the planar MR-TADF skeleton using two bulky carbazolyl units, herein a highly emissive molecule with enhanced quenching resistance is reported. The steric effect largely removes the formation of detrimental excimers/aggregates, and boosts the performance of the corresponding devices with a maximum external quantum efficiency (EQEmax ) up to 40.0% and full width at half maximum (FWHM) of 25 nm, representative of the only example of single OLED that can concurrently achieve narrow bandwidth and high EL efficiency surpassing 40% to date. Even at doping ratio of 30 wt%, the EQEmax is retained to be 33.3% with nearly unchanged emission spectrum. This work provides a viable approach to realize doping-insensitive MR-TADF devices with extreme EL efficiency and color purity for high-end OLED displays.

Quenching-Resistant Multiresonance TADF Emitter Realizes 40% External Quantum Efficiency in Narrowband Electroluminescence at High Doping Level.

Organic light emitters based on multiresonance-induced thermally activated delayed fluorescent materials have great potential for realizing efficient, narrowband organic light-emitting diodes (OLEDs). However, at high brightness operation, efficiency roll-off attributed to the slow reverse intersystem crossing (RISC) process hinders the use of multiresonance-induced thermally activated delayed fluorescent materials in practical applications. Here we report a heavy-atom incorporating emitter, BNSeSe, which is based on a selenium-integrated boron–nitrogen skeleton and exhibits 100% photoluminescence quantum yield and a high RISC rate (kRISC) of 2.0 × 106 s−1. The corresponding green OLEDs exhibit excellent external quantum efficiencies of up to 36.8% and ultra-low roll-off character at high brightnesses (with very small roll-off values of 2.8% and 14.9% at 1,000 cd m−2 and 10,000 cd m−2, respectively). Furthermore, the outstanding capability to harvest triplet excitons also enables BNSeSe to be a superior sensitizer for a hyperfluorescence OLED, which shows state-of-the-art performance with a high excellent external quantum efficiency of 40.5%, power efficiency beyond 200 lm W−1, and luminance close to 20,0000 cd m−2. Green OLEDs based on BNSeSe offer high operational efficiency and reduced efficiency roll-off. 

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Efficient selenium-integrated TADF OLEDs with reduced roll-off

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.

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

Herein, a ternary boron-oxygen-nitrogen embedded polycyclic aromatic hydrocarbon with multiple resonance thermally activated delayed ﬂuorescence (MR-TADF), namely DBNO, is developed by adopting the para boron-π-boron and para oxygen-π-oxygen strategy. The designed molecule presents a vivid green emission with high photoluminescence quantum yield (96%) and extremely narrow full width at half maximum (FWHM) of 19 nm/0.09 eV, which surpasses all previously reported green TADF emitters to date. In addition, the long molecular structure along the transition dipole moment direction endows it with a high horizontal emitting dipole ratio of 96%. The organic light-emitting diode (OLED) based on DBNO reveals a narrowband green emission with a peak at 504 nm and a FWHM of 24 nm/0.12 eV. Particularly, a significantly improved device performance is achieved by the TADF-sensitization ( hyperfluorescence ) mechanism, presenting a FWHM of 27 nm and maximum external quantum efficiency of 37.1%.

Achieving 37.1% Green Electroluminescent Efficiency and 0.09 eV Full Width at Half Maximum Based on a Ternary Boron-Oxygen-Nitrogen Embedded Polycyclic Aromatic System.

Emerging multi‐resonance (MR) thermally activated delayed fluorescence (TADF) emitters can combine 100% exciton harvesting and high color purity for their organic light‐emitting diodes (OLED). However, the highly planar configurations of MR molecules lead to intermolecular‐interaction‐induced quenching. A feasible way is integrating host segments into MR molecules, namely a “self‐host” strategy, but without involving additional charge transfer and/or vibrational components to excited states. Herein, an ambipolar self‐host featured MR emitter, tCBNDADPO, is demonstrated, whose ambipolar host segment (DADPO) significantly and comprehensively improves the TADF properties, especially greatly accelerated singlet radiative rate constant of 2.11 × 108 s−1 and exponentially reduced nonradiative rate constants. Consequently, at the same time as preserving narrowband blue emission with an FWHM of ≈28 nm at a high doping concentration of 30%, tCBNDADPO reveals state‐of‐the‐art photoluminescence and electroluminescence quantum efficiencies of 99% and 30%, respectively. The corresponding 100% internal quantum efficiency of tCBNDADPO supported by an ultrasimple trilayer and heavily doped device demonstrates the feasibility of the ambipolar self‐host strategy for constructing practically applicable MR materials.

Ambipolar Self‐Host Functionalization Accelerates Blue Multi‐Resonance Thermally Activated Delayed Fluorescence with Internal Quantum Efficiency of 100%

Achieving Narrow FWHM and High EQE Over 38% in Blue OLEDs Using Rigid Heteroatom-Based Deep Blue TADF Sensitized Host

Narrow Band Red Emission Fluorophore with Reasonable Multiple Resonance Effect

Efficiency enhancement in orange red thermally activated delayed fluorescence OLEDs by using a rigid di-indolocarbazole donor moiety

Two novel, CN-substituted ortho-terphenyl (OTP) core based bipolar type host materials were designed and synthesized for application in blue thermally activated delayed fluorescence (TADF) devices. Both hosts, 3,3′′-di(9H-carbazol-9-yl)-[1,1′:2′,1′′-terphenyl]-3′-carbonitrile (oCN-OTP) and 3,3′′-di(9H-carbazol-9-yl)-[1,1′:2′,1′′-terphenyl]-4′-carbonitrile (mCN-OTP) with a CN moiety substituted on the ortho- and meta-position of the terphenyl core brought an impressive result in photophysical properties and charge carrier transporting ability. Moreover, the introduction of the CN moiety over the ortho-terphenyl core which connected with carbazole donor units not only enables achieving high triplet energy level but also enhances the bipolar characteristics. As expected, our new design strategy leads to enhancing the device efficiencies when employed with a stable blue TADF dopant of 5-(5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-10,15-diphenyl-10,15-dihydro-5H diindolo [3,2-a:3′,2′-c]carbazole (DBA-DI). The device with oCN-OTP as a host material (30% DBA-DI) exhibited excellent current and external quantum efficiencies of 58.2 cd A−1 and 29.5%, respectively. Interestingly, the oCN-OTP based blue TADF device recorded a longer device lifetime (LT90) of 28.8 hours at an initial luminance of 1000 cd m−2.

CN-substituted ortho-terphenyl core based high triplet energy bipolar host materials for stable and efficient blue TADF devices

Abstract Efficient conversion of light from short wavelengths to longer wavelengths using color conversion layers (CCLs) underpins the successful operation of numerous contemporary display and lighting technologies. Inorganic quantum dots, based on CdSe or InP, for example, have received much attention in this context, however, suffer from instability and toxic cadmium or phosphine chemistry. Organic nanoparticles (NPs), though less often studied, are capable of very competitive performance, including outstanding stability and water-processability. Surfactants, which are critical in stabilizing many types of nano-structures, have not yet been used extensively in organic NPs. Here we show the utility of surfactants in the synthesis and processing of organic NPs by thoroughly characterizing the effect of ionic and non-ionic surfactants on the properties of fluorescent organic NPs. Using this information, we identify surfactant processing conditions that result in nearly 100 % conversion of organic fluorophores into sub-micrometer particles, or nano-dots, with outstanding performance as CCLs. Such water dispersions are environmentally benign and efficiently convert light. They can be used for a range of fluorophores covering a full spectral gamut, with excellent color purity, including full-width at half-maximum (FWHM) values as low as 21 nm. Compared to inorganic (InP) reference CCLs, the organic nano-dot based CCLs show superior color conversion efficiency and substantially improved long-term stability. 

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Soon Jae Hwang

Papers

Achieving Narrow FWHM and High EQE Over 38% in Blue OLEDs Using Rigid Heteroatom-Based Deep Blue TADF Sensitized Host

Narrow Band Red Emission Fluorophore with Reasonable Multiple Resonance Effect

Efficiency enhancement in orange red thermally activated delayed fluorescence OLEDs by using a rigid di-indolocarbazole donor moiety

CN-substituted ortho-terphenyl core based high triplet energy bipolar host materials for stable and efficient blue TADF devices

Synthesis of fluorescent organic nano-dots and their application as efficient color conversion layers