Highly Horizontal Oriented Tricomponent Exciplex Host with Multiple Reverse Intersystem Crossing Channels for High-Performance Narrowband Electroluminescence and Eye-Protection White Organic Light-Emitting Diodes.

Despite multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters with small full-width at half maximum are attractive for wide color-gamut display and eye-protection lighting applications, their inefficient reverse intersystem crossing (RISC) process and long exciton lifetime induce serious efficiency roll-off, which significantly limits their development. Herein, a novel device concept of building highly efficient tricomponent exciplex with multiple RISC channels is proposed to realize reduced exciton quenching and enhanced upconversion of nonradiative triplet excitons, and subsequently used as a host for high-performance MR-TADF organic light-emitting diodes (OLEDs). Compared with traditional binary exciplex, the tricomponent exciplex exhibits obviously improved photoluminescence quantum yield, emitting dipole orientation and RISC rate constant, and a record-breaking external quantum efficiency (EQE) of 30.4% is achieved for tricomponent exciplex p-PhBCzPh: PO-T2T: DspiroAc-TRZ (50: 20: 30) based OLED. Remarkably, maximum EQEs of 36.2% and 40.3% and ultralow efficiency roll-off with EQEs of 26.1% and 30.0% at 1000 cd m-2 are respectively achieved for its sky-blue and pure-green MR-TADF doped OLEDs. Additionally, the blue emission unit hosted by tricomponent exciplex is combined with an orange-red TADF emission unit to achieve a double-emission-layer blue-hazard-free warm white OLED with an EQEmax of 30.3% and stable electroluminescence spectra over a wide brightness range.

Sulfone-Embedded Heterocyclic Narrowband Emitters with Strengthened Molecular Rigidity and Suppressed High-Frequency Vibronic Coupling.

Sulfone-embedded heterocyclics are of great interest in organic light-emitting diodes (OLEDs), however, exploring highly efficient narrowband emitters based on sulfone-embedded heterocyclics remains challenging. Herein, five emitters with different sulfur valence state and molecular rigidity, namely tP, tCPD, 2tCPD, tPD and tPT, are thoroughly analysed. With restricted twisting of flexible peripheral phenyl by strengthening molecular rigidity, molecular emission spectra can be enormously narrowed. Further, introducing the sulfone group with bending vibration in low-frequency region that suppresses high-frequency vibration, sharp narrow full-widths at half-maximum of 28 and 25 nm are achieved for 2tCPD and tPD, respectively. Maximum external quantum efficiencies of 22.0 % and 27.1 % are successfully realized for 2tCPD- and tPD-based OLED devices. These results offer a novel design strategy for constructing narrowband emitters by introducing sulfone group into a rigid molecular framework.

Novel Polycyclic Fused Amide Derivatives: Properties and Applications for Sky-blue Electroluminescent Devices.

Aromatic imide derivatives play a critical role in boosting the electroluminescent (EL) performance of organic light-emitting diodes (OLEDs). However, the majority of aromatic imide-based materials are limited to long wavelength emission OLEDs rather than blue emissions due to their strong electron-withdrawing characteristics. Herein, two novel polycyclic fused amide units were reported as electron acceptor to be combined with either a tetramethylcarbazole or acridine donor via a phenyl linker to generate four conventional fluorescence blue emitters of BBI-4MeCz, BBI-DMAC, BSQ-4MeCz and BSQ-DMAC for the first time. BSQ-4MeCz and BSQ-DMAC based on a BSQ unit exhibited higher thermal stability and photoluminescence quantum yields than BBI-4MeCz and BBI-DMAC based on a BBI unit due to their more planar acceptor structure. The intermolecular interactions that exist in the BSQ series materials effectively inhibit the molecular rotation and configuration relaxation, and thus allow for blue-shifted emissions. Blue OLED devices were constructed with the developed materials as emitters, and the effects of both the structure of the polycyclic fused amide acceptor and the electron donor on the EL performance were clarified. Consequently, a sky-blue OLED device based on BSQ-DMAC was created, with a high maximum external quantum efficiency of 4.94% and a maximum luminance of 7761 cd m-2.

Deep-Blue OLEDs with Rec.2020 Blue Gamut Compliance and EQE Over 22% Achieved by Conformation Engineering.

To achieve high-efficiency deep-blue electroluminescence satisfying Rec.2020 standard blue gamut, two thermally activated delayed fluorescent (TADF) emitters are developed: 5-(2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-10,10-diphenyl-5,10-dihydrodibenzo[b,e][1,4]azasiline (TDBA-PAS) and 10-(2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-9,9-diphenyl-9,10-dihydroacridine (TDBA-DPAC). Inheriting from their parented organoboron multi-resonance core, both emitters show very promising deep-blue emissions with relatively narrow full width at half-maximum (FWHM, ≈50 nm in solution), high photoluminescence quantum yield (up to 92.3%), and short emission lifetime (≤2.49 µs) with fast reverse intersystem crossing (>106 s-1 ) in doped films. More importantly, replacing the spiro-centered sp3 C atom (TDBA-DPAC) with the larger-radius sp3 Si atom (TDBA-PAS), enhanced conformational heterogeneities in bulky-group-shielded TADF molecules are observed in solution, doped film, and device. Consequently, OLEDs based on TDBA-PAS retain high maximum external quantum efficiencies ≈20% with suppressed efficiency roll-off and color index close to Rec.2020 blue gamut over a wide doping range of 10-50 wt%. This study highlights a new strategy to restrain spectral broadening and redshifting and efficiency roll-off in the design of deep-blue TADF emitters.

Constructing Highly Efficient Blue OLEDs with External Quantum Efficiencies up to 7.5 % Based on Anthracene Derivatives.

Acquiring desirable device performance with deep-blue color purity that fulfills practical application requirements is still a challenge. Bipolar fluorescent emitters with hybrid local and charge transfer (HLCT) state may serve to address this issue. Herein, by inserting anthracene core in the deep-blue building blocks, the authors successfully developed two highly twisted D-π-A fluorescent emitters, ICz-An-PPI and IP-An-PPI, featuring different acceptor groups. Both exhibited superb thermal stabilities, high photo luminescent quantum yields and excellent bipolar transport capabilities. The non-doped OLEDs using ICz-An-PPI and IP-An-PPI as the emitting layers showed efficient blue emission with an external quantum efficiency (EQEmax ) of 4.32 % and 5.41 %, and the CIE coordinates of (0.147, 0.180) and (0.149, 0.150), respectively. In addition, the deep blue doped device based on ICz-An-PPI was achieved with an excellent CEmax of 5.83 cd A-1 , EQEmax of 4.6 % and the CIE coordinate of (0.148, 0.078), which is extremely close to the National Television Standards Committee (NTSC) standard. Particularly, IP-An-PPI-based doped device had better performance, with an EQEmax of 7.51 % and the CIE coordinate of (0.150, 0.118), which was very impressive among the recently reported deep-blue OLEDs with the CIEy <0.12. Such high performance may be attributed to the hot exciton HLCT mechanism via T7 to S2 . Our work may provide a new approach for designing high-efficiency deep-blue materials.

Multifunctional Materials Serving as Efficient Non-Doped Violet-Blue Emitters and Host Materials for Phosphorescence.

Efficient multifunctional materials acting as violet-blue emitters, as well as host materials for phosphorescent OLEDs, are crucial but rare due to demand that they should have high first singlet state (S1 ) energy and first triplet state (T1 ) energy simultaneously. In this study, two new violet-blue bipolar fluorophores, TPA-PI-SBF and SBF-PI-SBF, were designed and synthesized by introducing the hole transporting moiety triphenylamine (TPA) and spirobifluorene (SBF) unit that has high T1 into high deep blue emission quantum yield group phenanthroimidazole (PI). As the results, the non-doped OLEDs based on TPA-PI-SBF exhibited excellent EL performance with a maximum external quantum efficiency (EQEmax ) of 6.76 % and a violet-blue emission with Commission Internationale de L'Eclairage (CIE) of (0.152, 0.059). The device based on SBF-PI-SBF displayed EQEmax of 6.19 % with CIE of (0.159, 0.049), which nearly matches the CIE coordinates of the violet-blue emitters standard of (0.131, 0.046). These EL performances are comparable to the best reported non-doped deep or violet-blue emissive OLEDs with CIEy<0.06 in recent years. Additionally, the green, yellow and red phosphorescent OLEDs with TPA-PI-SBF and SBF-PI-SBF as host materials achieved a high EQEmax of about 20 % and low efficiency roll-off at the ultra-high luminance of 10 000 cd m-2 . These results provided a new construction strategy for designing high-performance violet-blue emitters, as well as efficient host materials for phosphorescent OLEDs.

Imidazo[1,2-a]pyridine as an Electron Acceptor to Construct High-Performance Deep-Blue Organic Light-Emitting Diodes with Negligible Efficiency Roll-Off.

Two novel bipolar deep-blue fluorescent emitters, IP-PPI and IP-DPPI, featuring different lengths of the phenyl bridge, were designed and synthesized, in which imidazo[1,2-a]pyridine (IP) and phenanthroimidazole (PI) were proposed as an electron acceptor and an electron donor, respectively. Both of them exhibit outstanding thermal stability and high emission quantum yields. All the devices based on these two materials showed negligible efficiency roll-off with increasing current density. Impressively, non-doped organic light-emitting diodes (OLEDs) based on IP-PPI and IP-DPPI exhibited external quantum efficiencies (EQEs) of 4.85 % and 4.74 % with CIE coordinates of (0.153, 0.097) and (0.154, 0.114) at 10000 cd m-2 , respectively. In addition, the 40 wt % IP-PPI doped device maintained a high EQE of 5.23 % with CIE coordinates of (0.154, 0.077) at 10000 cd m-2 . The doped device based on 20 wt % IP-DPPI exhibited a higher deep-blue electroluminescence (EL) performance with a maximum EQE of up to 6.13 % at CIE of (0.153, 0.078) and maintained an EQE of 5.07 % at 10000 cd m-2 . To the best of our knowledge, these performances are among the state-of-the art devices with CIEy ≤0.08 at a high brightness of 10000 cd m-2 . Furthermore, by doping a red phosphorescent dye Ir(MDQ)2 (MDQ=2-methyldibenzo[f,h]quinoxaline) into the IP-PPI and IP-DPPI hosts, high-performance red phosphorescent OLEDs with EQEs of 20.8 % and 19.1 % were achieved, respectively. This work may provide a new approach for designing highly efficient deep-blue emitters with negligible roll-off for OLED applications.