Blue thermally activated delayed fluorescence (TADF) dyes are basically combinations of strong acceptors and weak donors. In this work, a weak acceptor PO group was employed to construct a series of weak acceptor− strong donor (WASD)-type emitters with a phenoxazine donor, namely 10-(4-(diphenylphosphoryl)phenyl)-10H-phenoxazine (SPXZPO), 10,10′-(4,4′-(phenylphosphoryl)bis(4,1-phenylene))bis(10H-phenoxazine) (DPXZPO), and 10,10′,10″-(4,4′,4″-phosphoryltris(benzene-4,1-diyl))tris(10H-phenoxazine) (TPXZPO). Owing to the insulating effect of PO on conjugation extension and intramolecular electronic communications, the photoluminescence spectra of these molecules are almost identical, manifesting the superiority of WASD structure in emission color preservation. Simultaneously, the multi-dipolar characteristics of TPXZPO enhance the intramolecular charge transfer (ICT), facilitating reverse intersystem crossing for higher TADF efficiency and shorter lifetime. As a consequence, TPXZPO realized the desired pure-blue electroluminescence peak at 464 nm, accompanied by a favorable external quantum efficiency (η ext ) up to 15.3%, 100% exciton utilization, and reduced efficiency roll-offs. Its complementary full-TADF white organic light-emitting diodes also achieved η ext as high as 16.3%, among the best results reported so far for white TADF devices. The success of TPXZPO, the first example of a PO-based WASD-type blue TADF dye, is attributed to the comprehensive and harmonized effects of the PO joint on controlling conjugation and intramolecular electronic communication and the multi-dipolar structure on enhancing ICT.

Despite their merits of environmental friendliness, low cost, and large‐scale production, thermally activated delayed fluorescence (TADF) based white organic light‐emitting diodes (WOLEDs) for daily lighting applications still face the formidable challenges of structural simplification and controllable exciton allocation. Here, the state‐of‐the‐art full‐TADF WOLEDs with features of the single‐doped single emissive layers (EMLs) and ultrasimple trilayer structure are demonstrated. The EMLs are binary systems as yellow TADF emitter (4CzTPNBu) doped blue TADF matrix (ptBCzPO2TPTZ) with the large steric hindrance and mismatched frontier molecular orbital energy levels to effectively restrain excessive blue‐to‐yellow triplet exciton transfer and host‐dopant interaction induced triplet quenching. Simultaneously, Förster resonance energy transfer is utilized to optimize exciton allocation for the balance of blue and yellow emissions, giving rise to the photoluminescence quantum yield beyond 90%. Consequently, these single‐doped EMLs endow their cool white, pure white, and warm white diodes with the high‐quality and ultrastable white light and the 100% exciton utilization efficiencies through the extremely simple structures, making them competent for the diverse daily lighting applications.

Dual emissive copper(I) halide complexes TTPPCuX (X = Cl, Br, I) with a triphosphine ligand and stable tetrahedral geometries are constructed, in which TTPPCuI successfully achieves the balanced dual thermally activated delayed fluorescence and phosphorescence (PH) emissions with PH fraction of 39% at ambient temperature, supporting the equal triplet exciton reallocation for the state-of-the-art device performance.

theoretical internal quantum efficiency (η IQE ) increasing from 25% to 100%. It is believed that the emerging TADF technology can integrate the advantages of its two predecessors in low cost and 100% exciton harvesting, respectively. [5] Nevertheless, different to FL and PH only including down transitions, as the key transition for triplet harvesting in TADF, reverse intersystem crossing (RISC) from the first triplet (T 1 ) to the first singlet (S 1 ) excited states is a unique upconversion process with the thermodynamic limitation and the competition to dominant intersystem crossing (ISC), [6] therefore requiring the rational structural design and excited-state optimization, especially for high-energy blue TADF emitters. [7] It is rational that when the energy barrier from the T 1 to the S 1 states, namely singlet-triplet energy splitting (ΔE ST ), is as low as it can be overcome by thermal fluctuation (<0.3 eV), RISC process can be remarkably facilitated. [8] As twofolds of electron exchange energy, ΔE ST is in direct proportion to the overlap integral of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). [4,9] Therefore, in most cases stronger intramolecular charge transfer (ICT) interaction can render the smaller ΔE ST , however, accompanied by the significant emission red shift. [10] In this situation, the distorted donor (D)-acceptor (A) configuration for blue TADF emitters is widely adopted to almost thoroughly separate the HOMO and the LUMO, but by the cost of near-zero singlet oscillator strength (f S ) and low photoluminescence quantum yield (PLQY, φ PL ). [11] To alleviate this contradiction, dendritic donor groups [12] and D-A-D type structures [13] were recently employed to delocalize HOMO distributions for ΔE ST reduction and concurrently preserve a small HOMO-LUMO overlap for high φ PL , which further improved the maximum η EQE of blue TADF diodes to ≈20%. [14] Nevertheless, for blue TADF dyes, not only the relationships between ΔE ST and f S and electroluminescence (EL) performance are not always congruent, but also the relatively complicated molecular structures seem indispensable for performance improvement. Therefore, it is still a big challenge to develop simple but efficient blue TADF emitters based on a clear and universal molecular design. Efficient blue emitters are indispensable for organic light-emitting diodes (OLEDs) with respect to display and lighting applications. Because of their high-energy excited states, both radiation enhancement and non-radiation suppression should be simultaneously optimized to realize 100% exciton utilization. Here, it is shown that the excited-state characteristics of blue thermally activated delayed fluorescence emitters can be precisely controlled by a secondary acceptor having moderate electronic effects on increasing the singlet charge-transfer component and preserving the triplet locally excited-state component. In addition of planar configuration between the donor and the primary acceptor, the radia...

The primary concern on high power conservation lead to the development of organic electroluminescent (EL) materials from polycyclic aromatic fluorescence (FL), [3] noble-metal-involved phosphorescence (PH) [4] to donor-acceptor (D-A)-featured thermally activated delayed fluorescence (TADF) molecules. [5] Different to 25% electrogenerated excitons, namely singlet excitons, utilized by FL emitters, both PH and TADF materials can harvest 100% excitons in virtue of mutual singlet-triplet conversion through intersystem crossing (ISC) [6] and reverse ISC (RISC). [7] However, FL molecules characteristic of locally excited (LE) states commonly reveal high chromatic purities, whose FWHM values are less than 50 nm. In contrast, charge-transfer (CT) excitedstate components of PH and TADF molecules markedly broaden their emissions by 50-100 nm.Recently, multi-resonance (MR) featured TADF emitters emerge, because of their potential to overcome the challenge in combining high efficiency and emission color purity. [8] This kind of compound has polycyclic aromatic structures with electron-donating and withdrawing atoms, for example, nitrogen and boron, at ortho-positions to form accordant and amplified resonance effects on electron-cloud distribution. [9] In this case, on the one hand, frontier molecular orbitals (FMOs) of MR molecules are separated to facilitate RISC and achieve TADF characteristics; on the other hand, their fused-ring structures limit vibrational relaxation, leading to unique narrowband emissions with FWHM < 30 nm. [10] To balance optoelectrical properties, MR cores were further substituted with functional groups, [11], for example, cyan, [12] fluorine, [13] carbazole, [14] and diphenylamine, [15] which significantly improved carrier injection and transport, resulting in increased luminance and reduced roll-offs. However, to avert spectral broadening, these functional groups were mostly simple, rigid, and finitely extended, which limit the optimal space of MR materials. [16] Nonetheless, it is noticed that most MR diodes should adopt extremely low doping concentrations (commonly <5%) [17] to avoid bimolecular quenching processes, for example, triplet-triplet annihilation (TTA) and triplet-polaron quenching 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-interactioninduced 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 × 10 8 s −1 and exponentially reduced nonradiative rate constants. C...

Thermally activated delayed fluorescence (TADF) organic lightemitting diodes arise from the development of high-performance host materials and carrier transporting materials fitting for TADF dyes with optimized respective properties and interplays, making simultaneous performance improvement and device structure simplification feasible. In this work, a highly efficient blue TADF diode with simplified four-layer structure was successfully achieved by utilizing bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS) as blue emitter, 4,6-bis(diphenylphosphoryl)dibenzothiophene (DBTDPO) as host, and 4,6-bis(diphenylphosphoryl)dibenzothiophene sulfone (46DBSODPO) as electron-transporting layer. The compatibilities between DBTDPO and DMAC-DPS and DBTDPO and 46DBSODPO were optimized with respect to configuration, polarity, energy level, and interfacial interaction, resulting in the unchanged roughness of ∼0.25 nm before and after doping, high photoluminescence quantum yield over 85%, and reduced interfacial exciplex emissions. With the similar triplet excited energy (T 1 ) of ∼3.0 eV but inferior electrical properties compared to its analogues 28DBSODPO and 37DBSODPO, besides the homogeneity with DBTDPO, 46DBSODPO suppressed the formation of interfacial exciplex and dipole for efficient exciton confinement and electron injection and transportation, in virtue of the steric effects of its orthosubstituted phosphine oxide groups. Consequently, DBTDPO and 46DBSODPO endowed their DMAC-DPS based four-layer devices with the state-of-the-art performance, for example, the maximum external quantum efficiency over 16%, which was more than two-fold of those of conventional electron-transporting material 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB). This design strategy about material compatibility could pave a way for developing high-performance blue TADF diodes with simplified configurations.

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Red TADF emitter oTPA-DPPZ employs dipyridophenazine with gradient multi-inductive effect as acceptor, which enhances intramolecular charge transfer and radiative transition, resulting photo- and electro-luminescence quantum yields of 75% and 18.5%.