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• Hosts with TADF, conventional fluorescence and RTP characters are developed. • Exciton utilization efficiency is improved by the host with TADF character. • An EQE of 36.1% was achieved for sky blue OLED with DspiroAc-TRZ as emitter. • An EQE of 34.0% was achieved for the sensitized device with v -DABNA dopant. Spiral donor units have attracted extensive research focuses in designing highly emissive thermally activated delayed fluorescence (TADF) emitters. Herein, to reveal the significance of spiral donor units for constructing highly efficient host materials, several spiral donor-based host materials are developed. With the incorporation of spiral donor, increased intermolecular distance is achieved to reduce the concentration quenching effect of dopant. Moreover, the high triplet energy level of spiral donor can provide an additional energy transfer channel to further improve the device performance. Noteworthy, by connecting spiral donors or conventional carbazole unit with different electron-withdrawing units phenoxathiine 10,10-dioxide (PX) or phenoxathiine (PA), these hosts are given different photophysical properties of TADF, room-temperature phosphorescence (RTP) and conventional fluorescence (CF). The strong electron-withdrawing ability of PX is beneficial to the TADF character of spiral donor-based molecules, while the weak electron-withdrawing ability of PA is beneficial to the RTP property. Consequently, the blue OLEDs with DspiroAc-TRZ as a dopant emitter based on the hosts with TADF characters exhibit higher performance than those with RTP and CF properties, and excellent external quantum efficiency (EQE) of 36.1 % and current efficiency of 84.6 cd/A are achieved in a simple device structure. Furthermore, peak EQEs of 34.0 % and 33.0 % are achieved, when DspiroS-PX and DspiroS-PA hosted DspiroAc-TRZ are used to sensitize blue multi-resonance TADF emitter v -DABNA. Our work may shed new light on designing highly efficient host materials for TADF emitters. [ABSTRACT FROM AUTHOR]

Incorporating pyridyl uniquely from its 2-position onto 10-site of acridone ring generated intramolecular hydrogen bond in a group of acridone-carbazole hybrid thermally activated delayed fluorescence emitters, which universally pulled down non-radiative transition rates by three orders and elevated reverse intersystem crossing rates by two orders, and thus boosted PLQYs to almost unity and EQEs of OLEDs to 30+%, representing the state-of-the-art efficiencies of deep-blue TICT-TADF materials and devices with CIEy≤0.10. [Display omitted] • Intramolecular-hydrogen-bond boosts performance of deep blue TADF emitters. • H-bond reduces k nr by 3 orders and increases k RISC by 2 orders. • PLQYs≈100 %, k RISC s ≥ 105 s−1 and horizontal orientation ratios ≥90 % are obtained. • Deep blue OLEDs exhibit record-high EQEs of 31–33 % with CIE y of 0.09–0.12. A group of D-A-D type thermally activated delayed fluorescence (TADF) emitters were developed with 10-(pyridin-2-yl)acridin-9(10 H)-one as acceptor and substituted carbazoles as donors. The design was focused on incorporating pyridyl to the 10-site nitrogen atom of acridin-9(10 H)-one ring, which improved the reverse intersystem crossing rate constant k RISC from 103 s−1 to 105 s−1. More importantly, the linkage of pyridyl uniquely from its 2-position to acridin-9(10 H)-one ring induced intramolecular hydrogen bond, which suppressed nonradiative transition rates by three to four orders of magnitude and universally increased the photoluminescence quantum yields from 54 % to nearly 100 %. In combination with the intrinsic high horizontal orientation ratios over 90 % of acridin-9(10 H)-one based molecules, such pyridyl decoration boosted the external quantum efficiencies of simple-structure deep blue organic light-emitting diodes to 31–33 % with CIE x of 0.15 and CIE y of 0.09 ∼ 0.12, being double of that for hydrogen-bond-free parent emitter and representing the state-of-the-art efficiencies for the deep blue twisted intramolecular charge transfer type TADF materials with CIEy ≤ 0.10 so far. These TADF emitters are comparable with the prevailing multiresonance blue emitters in terms of distinctive efficiencies, but advantageous in much facile syntheses. [ABSTRACT FROM AUTHOR]

[Display omitted] • Coexistence of intra-CT and inter-CT processes is realized for DPA-XA-TRZ. • Dual CT processes can be simultaneously enhanced by planar molecular skeleton. • Organic light-emitting diodes with external quantum efficiency of 21.9% is realized. • Efficiency of 31.7% is realized by DPA-XA-TRZ sensitized device with DMAc-BN dopant. Intramolecular charge transfer (intra-CT) and intermolecular charge transfer (inter-CT) are two general molecular design strategies of thermally activated delayed fluorescence (TADF) materials for organic light-emitting diodes (OLEDs). Herein, a diphenylamine (DPA) donor and a 2,4-diphenyl-1,3,5-triazine (TRZ) acceptor are connected by two planar units, non-conjugated 9,9-dimethyl-9 H -xanthene (XA) and conjugated dibenzo[ b,d ]furan (FR), to give D-σ-A type emitter DPA-XA-TRZ and D-π-A type emitter DPA-FR-TRZ. Coexistence of intra- and inter-CT is found for DPA-XA-TRZ and is readily tuned by various doping concentrations owing to the non-conjugated linkage. In contrast, DPA-FR-TRZ shows a dominant intra-CT characteristic due to the strong molecular conjugation. The planar molecular skeleton of DPA-XA-TRZ could not only enhance intramolecular electronic coupling, but also reduce steric hindrance and shorten intermolecular distance to enhance inter-CT process. A maximum external quantum efficiency (EQE max) of 21.9 % is achieved for the OLED device based on DPA-XA-TRZ, which is 2.5 times higher than that of DPA-FR-TRZ (8.4 %). DPA-XA-TRZ can also be utilized as an assistant host to sensitize multiple resonance TADF emitter DMAc-BN, and an EQE max of 31.7 % is realized owing to its dual CT processes and high reverse intersystem crossing rate (k RISC), which is significantly superior to the conventional mCBP host and mCBP:PO-T2T exciplex cohost. [ABSTRACT FROM AUTHOR]

A multi-sensitization strategy is proposed to develop highly efficient and low efficiency roll-off solution-processed single-emission-layer all-fluorescence white organic light-emitting diodes, resulting in significantly boosted Förster energy transfer from blue emitter and green sensitizer to the orange-red dopant and thus a recorded external quantum efficiency of 22.8%. [Display omitted] • Multi-sensitization strategy to boost FRET and block exciton loss. • Exciton utilization efficiency was improved by BDP-C-Cz with HLCT character. • An EQE of 26.8% for blue s-OLED was achieved with 70% Θ// of spin-coated EML. • An EQE of 22.8% was achieved for the multi-sensitized all-fluorescence s-WOLED. Limited by the inefficient energy transfer from blue thermally activated delayed fluorescence (TADF) material to the complementary color fluorescent dopant and unwanted charge trapping effect, it is challenging to achieve a spectrally stable, high efficiency and low efficiency roll-off solution-processed single-emission-layer (SEL) white organic light-emitting diodes (WOLEDs). Herein, a highly efficient solution-processed all-fluorescence SEL-WOLED with suppressed efficiency roll-off and stable electroluminescence spectra is achieved by introducing a hybridized local and charge-transfer (HLCT) orange-red emitter (BDP-C-Cz) and a TADF sensitizer (4CziPN) into an optimized blue TADF OLED. Specially, the adopted BDP-C-Cz emitter has a high molar absorption coefficient, whose absorption spectrum has a significant overlap with the emission spectra of 4CziPN and blue emitter. By additionally introducing 4CziPN as an assistant sensitizer of blue emitter, Förster energy transfer can be enhanced to give favorable exciton allocation and thus effectively blocks triplet exciton loss. Accordingly, a recorded external quantum efficiency of 22.8% and current efficiency of 60.0 cd/A were realized for the solution-processed SEL-WOLEDs, with color coordinates of (0.33, 0.44) and good color stability, providing a valuable guidance for the future development of simplified, high-efficiency and low-cost WOLEDs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Multiple CT channels and 3LE states contribute to the speedy k r and RISC processes. • High Θ and small FWHM are realized due to the rigid disk-like structure. • η r and η out above 30% are achieved owing to triplet exciton utilization and high Θ. • EQE above 10% was realized for the standard blue device based on BO3N. The efficiency of pure blue Organic light-emitting diodes (OLEDs) still needs to be improved, which is limited by the utilization of the forbidden transition triplet excitons and low light-out-coupling efficiency in devices. Herein, two blue emitters were developed through combining multiple diphenylamine (DPA) donors with an oxygen-bridged boron (DBA) or triphenyl triazine (TRZ) acceptor. Notably, a narrow-band deep blue emission with a peak of 423 nm and a Full-width at half-maximum (FWHM) of 34 nm is realized for the boron-based emitter in toluene solution. Thanks to their multiple Charge transfer (CT) channels and high orientated disk-like structure, high radiative transition rates (k r > 108 s−1) and high horizontal emitting dipole ratios (Θ > 80%) can be realized simultaneously. Meanwhile, the introduction of the peripheral DPA units can also provide multiple localized triplet (3LE) states for spin up-conversion of forbidden transition triplet excitons. Owing to the effective up-conversion of triplet excitons and high light-out-coupling efficiency, maximum external quantum efficiency of more than 10% is achieved for the boron emitter-based pure blue OLEDs with a Commission Internationale de l'Eclairage (CIE) coordinates of (0.14, 0.08). [ABSTRACT FROM AUTHOR]