Efficient organic emitters in the deep‐red region are rare due to the “energy gap law”. Herein, multiple boron (B)‐ and nitrogen (N)‐atoms embedded polycyclic heteroaromatics featuring hybridized π‐bonding/ non‐bonding molecular orbitals are constructed, providing a way to overcome the above luminescent boundary. The introduction of B‐phenyl‐B and N‐phenyl‐N structures enhances the electronic coupling of those para‐positioned atoms, forming restricted π‐bonds on the phenyl‐core for delocalized excited states and thus a narrow energy gap. The mutually ortho‐positioned B‐ and N‐atoms also induce a multi‐resonance effect on the peripheral skeleton for the non‐bonding orbitals, creating shallow potential energy surfaces to eliminate the high‐frequency vibrational quenching. The corresponding deep‐red emitters with peaks at 662 and 692 nm exhibit narrow full‐width at half‐maximums of 38 nm, high radiative decay rates of ca. 108 s−1, ≈100 % photo‐luminescence quantum yields and record‐high maximum external quantum efficiencies of ca. 28 % in a normal planar organic light‐emitting diode structure, simultaneously.
Multiple donor–acceptor‐type carbazole–benzonitrile derivatives that exhibit thermally activated delayed fluorescence (TADF) are the state of the art in efficiency and stability in sky‐blue organic light‐emitting diodes. However, such a motif still suffers from low reverse intersystem crossing rates (kRISC) with emission peaks <470 nm. Here, a weak acceptor of cyanophenyl is adopted to replace the stronger cyano one to construct blue emitters with multiple donors and acceptors. Both linear donor–π–donor and acceptor–π–acceptor structures are observed to facilitate delocalized excited states for enhanced mixing between charge‐transfer and locally excited states. Consequently, a high kRISC of 2.36 × 106 s−1 with an emission peak of 456 nm and a maximum external quantum efficiency of 22.8% is achieved. When utilizing this material to sensitize a blue multiple‐resonance TADF emitter, the corresponding device simultaneously realizes a maximum external quantum efficiency of 32.5%, CIEy ≈ 0.12, a full width at half maximum of 29 nm, and a T80 (time to 80% of the initial luminance) of > 60 h at an initial luminance of 1000 cd m−2.
Pure green emitters are essential for realizing an ultrawide color gamut in next‐generation displays. Herein, by fusing the difficult‐to‐access aza‐aromatics onto B (boron)–N (nitrogen) skeleton, a hybridized multi‐resonance and charge transfer (HMCT) molecule AZA‐BN was successfully synthesized through an effective one‐shot multiple cyclization method. AZA‐BN shows pure green fluorescence with photoluminance quantum yield of 99.7 %. The corresponding green device exhibits a maximum external quantum efficiency and power efficiency of 28.2 % and 121.7 lm W−1, respectively, with a full width half maximum (FWHM) of merely 30 nm and Commission Internationale de l'Eclairage (CIE) coordinate y of 0.69, representing the purest green bottom‐emitting organic light‐emitting diode.
of the generated photons to the number of electrons injected into the devices. Utilizing emitters with phosphorescence or thermally activated delayed fluorescence (TADF), all excitons can be converted into photons and thus afford theoretical 100% IQE. [4][5][6] However, owing to the difference in the refractive indices of the organic layers, the electrodes, the substrate, and the surrounding air, most of the light generated cannot escape the device into air, leading to a quite limited light outcoupling efficiency (η out ) of only ~20% when using conventional glass substrates. [7,8] A suitable figure of merit to evaluate the ability of emitting photons into air is the external quantum efficiency (EQE), calculated by IQE × η out . [7] Numerous lightextraction techniques have been developed to improve η out to minimize the gap between IQE and EQE. [8,9] Recently, emitters with horizontal emitting dipole orientation (EDO) have been proved to be effective to improve η out , overcoming the limit set under the assumption of the isotropic or random orientation of the molecules. [7,9,[10][11][12] Utilizing emitters with preferred horizontal EDOs, devices with three primary colors have realized EQEs above 30% without any additional external light extraction techniques. [13] Owing to the great potential, developing strategies to control EDO has been an exigent task and hot-topic in the research field of OLEDs.Heretofore, considerable progresses have been made on phosphorescent emitters with preferentially horizontal EDO. [10] By adopting heteroleptic iridium complexes and planar platinum complexes as well as optimizing host-guest interactions, high fractions of horizontal EDO (Θ // ) of over 90% have been achieved for phosphors, realizing EQE of nearly 40% in the range of full visible colors. [12,[14][15][16] Besides phosphors, TADF emitters have also exhibited fascinating ability in controlling molecular orientation for a high Θ // , given their more flexible platforms. The most widely adopted strategy for a TADF emitter to obtain a high Θ // is elongating the molecular length. Wu, Wong, and co-workers developed a series of spiroacridinetriazine hybrids with high Θ // s in the range of 72%-83% and Thermally activated delayed fluorescence (TADF) emitters featuring preferential horizontal emitting dipole orientation (EDO) are in urgent demand for enhanced optical outcoupling efficiency in organic light-emitting diodes (OLEDs). However, simultaneously manipulating EDO and optoelectronic properties remains a formidable challenge. Here, an extended linear D-A-D structure with both enlarged donor (D) and acceptor (A) π-systems is established, not only elaborately manipulating parallel horizontal molecular orientation and EDO along its long axis by multi-driving-forces for a high horizontal dipole ratio (Θ // ), but also delocalizing distribution of frontier energy levels for optimized electronic properties. The proof-of-the-concept emitter simultaneously affords a high Θ // of 92%, a high photoluminescence quantum yield of 95%,...
Multiple resonance (MR) emitters are promising for highly efficient organic light-emitting diodes (OLEDs) with narrowband emission;h owever,t hey still face intractable challenges with concentration-caused emission quenching, exciton annihilation, and spectral broadening. In this study, sterically wrapped MR dopants with af luorescent MR core sandwiched by bulk substituents were developed to address the intractable challenges by reducing intermolecular interactions. Consequently,h igh photo-luminance quantum yields of ! 90 %a nd small full width at half maximums (FWHMs) of 25 nm over aw ide range of dopant concentrations (1-20 wt %) were recorded. In addition, we demonstrated that the sandwiched MR emitter can effectively suppress Dexter interaction when doped in at hermally activated delayed fluorescence sensitizer,eliminating exciton loss through dopant triplet. Within the above dopant concentration range,t he optimal emitter realizes remarkably high maximum external quantum efficiencies of 36.3-37.2 %, identical small FWHMs of 24 nm, and alleviated efficiency roll-offs in OLEDs.
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