Enhanced Horizontal Dipole Orientation by Novel Penta-Helicene Anthracene-Based Host for Efficient Blue Fluorescent OLEDs.

Due to the relatively low photoluminescence quantum yield (PLQY) and horizontal dipole orientation of doped films, anthracene-based fluorescent organic light-emitting diodes (F-OLEDs) have faced a great challenge to achieve high external quantum efficiency (EQE). Herein, a novel approach is introduced by incorporating penta-helicene into anthracene, presented as linear-shaped 3-(4-(10-phenylanthracen-9-yl)phenyl)dibenzo[c,g]phenanthrene (BABH) and 3-(4-(10-(naphthalen-2-yl)anthracen-9-yl)phenyl)dibenzo[c,g]phenanthrene (NABH). These blue hosts exhibit minimal intermolecular overlap of π-π stacking, effectively suppressing excimer formation, which facilitates the effective transfer of singlet energy to the fluorescent dopant for PLQY as high as 90%. Additionally, the as-obtained two hosts of BABH and NABH have effectively demonstrated major horizontal components transition dipole moments (TDM) and high thermal stability with glass transitional temperature (Tg) surpassing 188 °C, enhancing the horizontal dipole orientation of their doped films to be 89% and 93%, respectively. The OLEDs based on BABH and NABH exhibit excellent EQE of 10.5% and 12.4% at 462 nm and device lifetime up to 90% of the initial luminance over 4500 h at 100 cd m-2, which has firmly established them as among the most efficient blue F-OLEDs based on anthracene to date to the best knowledge. This work provides an instructive strategy to design an effective host for highly efficient and stable F-OLEDs.

Creating Efficient Red Thermally Activated Delayed Fluorescence Materials with Cyano-Substituted 11,12-Diphenyldipyrido[3,2-a:2',3'-c]phenazine Acceptors.

Red luminescent materials are essential components for full color display and white lightening based on organic light-emitting diode (OLED) technology, but the extension of emission color towards red or deep red region generally leads to decreased photoluminescence and electroluminescence efficiencies. Herein, we wish to report two new luminescent molecules (2CNDPBPPr-TPA and 4CNDPBPPr-TPA) consisting of cyano-substituted 11,12-diphenyldipyrido[3,2-a:2',3'-c]phenazine acceptors and triphenylamine donors. As the increase of cyano substituents, the emission wavelength is greatly red-shifted and the reverse intersystem crossing process is promoted, resulting in strong red delayed fluorescence. Meanwhile, due to the formation of intramolecular hydrogen bonds, the molecular structures become rigidified and planarized, which brings about large horizontal dipole ratios. As a result, 2CNDPBPPr-TPA and 4CNDPBPPr-TPA can perform as emitters efficiently in OLEDs, furnishing excellent external quantum efficiencies of 28.8 % at 616 nm and 20.2 % at 648 nm, which are significantly improved in comparison with that of the control molecule without cyano substituents. The findings in this work demonstrate that the introduction of cyano substituents to the acceptors of delayed fluorescence molecules could be a facile and effective approach to explore high-efficiency red or deep red delayed fluorescence materials.

Boosting External Quantum Efficiency to 12.0 % of an Ultraviolet OLED by Engineering the Horizontal Dipole Orientation of a Hot Exciton Emitter.

Currently, much research effort has been devoted to improving the exciton utilization efficiency and narrowing the emission spectra of ultraviolet (UV) fluorophores for organic light-emitting diode (OLED) applications, while almost no attention has been paid to optimizing their light out-coupling efficiency. Here, we developed a linear donor-acceptor-donor (D-A-D) triad, namely CDFDB, which possesses high-lying reverse intersystem crossing (hRISC) property. Thanks to its integrated narrowband UV photoluminescence (PL) (λPL: 397 nm; FWHM: 48 nm), moderate PL quantum yield (ϕPL: 72 %, Tol), good triplet hot exciton (HE) conversion capability, and large horizontal dipole ratio (Θ//: 92 %), the OLEDs based on CDFDB not only can emit UV electroluminescence with relatively good color purity (λEL: 398 nm; CIEx,y: 0.161, 0.040), but also show a record maximum external quantum efficiency (EQEmax) of 12.0 %. This study highlights the important role of horizontal dipole orientation engineering in the molecular design of HE UV-OLED fluorophores.

Realizing Record-High Electroluminescence Efficiency of 31.5 % for Red Thermally Activated Delayed Fluorescence Molecules.

Tailor-made red thermally activated delayed fluorescence (TADF) molecules comprised of an electron-withdrawing pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile core and various electron-donating triarylamines are developed. They can form intramolecular hydrogen-bonding, which is conducive to improving emission efficiency and promoting horizontal orientation and show near infrared (NIR) emissions (692-710 nm) in neat films and red delayed fluorescence (606-630 nm) with high photoluminescence quantum yields (73-90%) in doped films. They prefer horizontal orientation with large horizontal dipole ratios in films, rendering high optical out-coupling factors (0.39-0.41). Their non-doped OLEDs exhibit NIR lights (716-748 nm) with maximum external quantum efficiencies (ηext,max ) of 1.0-1.9%. And their doped OLEDs radiate red lights (606-648 nm) and achieve record-beating ηext,max of up to 31.5%. These new red TADF materials should have great potentials in display and lighting devices.

Improving Electroluminescence Efficiency by Linear Polar Host Capable of Promoting Horizontal Dipole Orientation for Dopant.

In doped organic light-emitting diodes (OLEDs), the host materials play an important role in emitting layers. Most studies about host materials mainly focus on their energy levels and carrier transport behaviors, while less attention is paid to their influence on the dipole orientation of dopants, which closely associate with the light out-coupling efficiency (ηout ) of the device. Herein, a linear polar host material (l-CzTRZ) consisting of carbazole donor, triazine acceptor, and the conjugated para-terphenyl skeleton is developed and its crystal and electronic structures, thermal and electrochemical stabilities, optical property, and carrier transport ability are investigated. l-CzTRZ prefers ordered horizontal orientation and favors electron transport in neat film. More importantly, it can promote horizontal dipole orientation for the dopants via dipole-dipole interaction, furnishing an excellent horizontal dipole ratio of 91.5% and thus a high ηout of 43% for the phosphorescent dopant (PO-01-TB). Consequently, the OLED with l-CzTRZ host and PO-01-TB dopant attains state-of-the-art electroluminescence efficiencies of 135.5 cd A-1 , 135.7 lm W-1 and 41.3%, with a small roll-off of 9.7% at 5000 cd m-2 luminance. The presented significant impact of the host on the dipole orientation of the dopant shall enlighten the design of host materials to improve OLED performance.

High-Performance Orange-Red Organic Light-Emitting Diodes with External Quantum Efficiencies Reaching 33.5% based on Carbonyl-Containing Delayed Fluorescence Molecules.

Developing orange to red purely organic luminescent materials having external quantum efficiencies (ηext s) exceeding 30% is challenging because it generally requires strong intramolecular charge transfer, efficient reverse intersystem crossing (RISC), high photoluminescence quantum yield (ΦPL ), and large optical outcoupling efficiency (Φout ) simultaneously. Herein, by introducing benzoyl to dibenzo[a,c]phenazine acceptor, a stronger electron acceptor, dibenzo[a,c]phenazin-11-yl(phenyl)methanone, is created and employed for constructing orange-red delayed fluorescence molecules with various acridine-based electron donors. The incorporation of benzoyl leads to red-shifted photoluminescence with accelerated RISC, reduced delayed lifetimes, and increased ΦPL s, and the adoption of spiro-structured acridine donors promotes horizontal dipole orientation and thus renders high Φout s. Consequently, the state-of-the-art orange-red organic light-emitting diodes are achieved, providing record-high electroluminescence (EL) efficiencies of 33.5%, 95.3 cd A-1 , and 93.5 lm W-1 . By referring the control molecule without benzoyl, it is demonstrated that the presence of benzoyl can exert significant positive effect over improving delayed fluorescence and enhancing EL efficiencies, which can be a feasible design for robust organic luminescent materials.