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Overcoming exciton quenching and improving electroluminescence (EL) performances continue to pose persistent challenges in the development of efficient deep blue thermally activated delayed fluorescence (TADF) emitters. Herein, an asymmetric multi‐donors strategy is employed that provides the potential of establishing multiple radiative transition channels for fluorescence emission and reverse intersystem crossing (RISC), while allowing flexible modulation of packing modes in single‐crystal state and optimization of morphology in films by modifying the substitution site of the donor units. As a result, intermolecular π–π interactions are effectively alleviated and a very smooth surface with a reduced defect density is simultaneously achieved, which can significantly reduce the density of triplet excitons, accelerate the charge transfer rate, and enhance carrier injection efficiency. Consequently, a super‐high photoluminescence quantum yield (PLQY) of 99% and fast kRISC of 1.1 × 105 s−1 are concurrently achieved for the proof‐of‐concept emitter 3,4‐Cz‐SF‐SFAC. The corresponding deep‐blue organic light‐emitting diode (OLED) demonstrates an exceptional external quantum efficiency (EQE) of 31.1%, accompanied by an emission peak at 457 nm, one of the preeminent TADF materials within the deep blue region, ranking among the most efficient OLEDs with an EL spectra below 460 nm on record. [ABSTRACT FROM AUTHOR]

Multi‐resonance (MR) based organoboron emitters exhibiting high‐efficiency narrowband thermally activated delayed fluorescence (TADF) have become a critical material component for constructing high‐performance organic light‐emitting diodes (OLEDs) with high color purity and high color gamut. However, most of the MR‐TADF devices suffer from severe efficiency roll‐off at high current density due to the relatively large singlet–triplet splitting energy, long excited state lifetime, and slow reverse intersystem crossing rate (kRISC). Herein, by utilizing a sulfur atom‐fused donor unit, 7H‐benzo[4,5]thieno[2,3‐b]carbazole, an organic boron emitter (BTC‐BNCz) capable of high efficiency and low efficiency roll‐off narrowband TADF emission is developed. Benefitting from the heavy atom effect of the sulfur atom, kRISC up to 1.6 × 105 s−1 is achieved, which could greatly reduce the undesired quenching effect. Consequently, the OLED using BTC‐BNCz as dopant exhibits high external quantum efficiency (EQE) of 27.0% with full width at half maximum (FWHM) of 35 nm. Moreover, by utilizing a TADF material with a high horizontal dipole ratio as assistant host, outstanding device performance is achieved with a maximum EQE of 35.2% and FWHM of 37 nm, featuring slow efficiency roll‐off. [ABSTRACT FROM AUTHOR]

Solely through‐space charge transfer (TSCT)‐type thermally activated delayed fluorescent (TADF) emitters exhibit low radiative decay rates (krs$k_{\rm{r}}^{\rm{s}}$), while solely through‐bond charge transfer (TBCT) type TADF emitters generally suffer from low reverse intersystem crossing (RISC) rates (kRISC). Here, PhCzSpiroS‐TRZ with versatile spiro‐heterocyclic architecture and an extra donor of 9‐phenyl‐9H‐carbazole are developed as emitter and ideal host for TADF‐ and TADF‐sensitized fluorescence (TSF) organic light‐emitting diodes (OLEDs) in detail. Abandoning solely TSCT or TBCT process, PhCzSpiroS‐TRZ with multiple charge transfer characteristic exhibits photoluminescence quantum yield of 96.3% and krs$k_{\rm{r}}^{\rm{s}}$ of 2.2 × 107 s−1, which is considerably higher among all the reported TSCT‐TADF molecules. Noticeably, state‐of‐the‐art blue OLEDs using PhCzSpiroS‐TRZ as emitter and sensitizer show maximum external quantum efficiencies (EQEs) of 33.6% and 32.8%, ranking among TADF materials for achieving EQEs >30% in both TADF‐ and TSF‐OLEDs. [ABSTRACT FROM AUTHOR]

Two novel bipolar isomers, namely 3,5-bis(3-(9 H -carbazol-9-yl)phenyl)isonicotinonitrile ( m -DCzPyCN), and 3,5-bis(9-phenyl-9 H -carbazol-3-yl)isonicotinonitrile (3-DCzPyCN), are designed and synthesized by selecting cyanopyridine as n-type unit and N -phenylcarbazole as p-type unit. Two peripheral carbazoles are conjugated through their C-3 atom with the central cyanopyridine in 3-DCzPyCN but are indirectly linked through the N-9 atom via meta -phenylene bridges in m -DCzPyCN. The linking strategy had significant effect on the physical properties of the isomers. The single-carrier device study reveals that 3-DCzPyCN has much better positive and negative charge balancing ability than m -DCzPyCN. The 3-DCzPyCN hosted green phosphorescent organic light-emitting diode exhibits superior performance over m -DCzPyCN, with the maximum current efficiency of 84.1 cd A −1 and external quantum efficiencies of 24.2% and low efficiency roll-off. At an extremely high brightness of 10000 cd m −2 , the efficiency still remained at 78.4 cd A −1 , which corresponds to a roll-off of 6.7% from the maximum value. Even at an ultra-high brightness of 100000 cd m −2 , the efficiency is reduced by 47.0%. The superior performance of 3-DCzPyCN is ascribed to its remarkable charge balancing capability, which is the combined result of the cyanopyridine as n-type unit and the conjugated linking strategy between p- and n-type moieties. [ABSTRACT FROM AUTHOR]

• 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]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Mixed halide perovskites are widely investigated as emitters for efficient light‐emitting diodes owing to their excellent optoelectronic properties. Compared with red and green emissions, encouraging progress in efficiency for blue emission has only been achieved in sky‐blue region. A high Cl/Br ratio is indispensable to achieve deep‐blue emission, which often leads to pronounced halogen migration in perovskite light‐emitting diodes (PeLEDs) and thus diminished efficacy. To avoid the negative effects of excessive chloride necessary to achieve deep‐blue emission, alloyed perovskite quantum dotes (QDs) passivated by dual‐functional guanidine sulfamate are synthesized, where S═O of sulfamate coordinates with Pb, and guanidine can form N‐H‐X (Cl/Br) with halogen ions to complete passivation. As a result, deep‐blue emission is achieved for the alloyed perovskite QDs. Owing to the suppressed defects and the reduced surface electrostatic potential of the perovskite QDs film, the carrier recombination region in the PeLEDs is regulated. Finally, deep‐blue PeLEDs with a spectrally stable electroluminescence peak at 458 nm, peak external quantum efficiency of 3.65% and maximum brightness of 223 cd m−2 are realized. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbon (PAH) compounds are prospective materials for highly efficient narrowband organic light‐emitting diodes. Specifically, boron‐nitrogen doped PAHs exhibit multiple resonance (MR) effect toward thermally activated delayed fluorescence (TADF) and narrow full width at half‐maximum (FWHM). Currently, the molecular design strategy for bathochromic‐shifted narrowband MR‐TADF emitters is a hotspot and challenging puzzle. Herein, a carbonyl is fused in the boron‐nitrogen doped PAH for constructing a boron/nitrogen/carbonyl MR‐TADF emitter. Benefiting from the carbonyl, the developed emitter exhibits enhanced intramolecular charge transfer and decreased reorganization energy, which displays bathochromic‐shifted narrowband emission pertinently. As a consequence, the proof‐of‐concept compound TCZBAO achieves an emission peak at 522 nm and narrow FWHM of 39 nm, while the carbonyl‐free TCZBAC exhibits an emission peak at 502 nm and FWHM of 46 nm. Maximum external quantum efficiencies of 36.1 % and 32.4 % are realized for the devices based on TCZBAO and TCZBAC, respectively, indicating the guiding significance of the proposed design strategy for developing bathochromic‐shifted narrowband MR‐TADF emitters. [ABSTRACT FROM AUTHOR]

White organic light-emitting diodes (WOLEDs) based on all thermally activated delayed fluorescence (TADF) emitters are especially attractive due to the merits of high efficiency and low cost. Novel TADF emitters (TFM-QP and CN-QP) were developed by selecting 6-(trifluoromethyl)quinoxaline or 6-(cyano)quinoxaline as the electron acceptor and phenoxazine as the electron donor. Aggregation induced emission is detected. Small singlet and triplet energy splitting values of 0.03–0.04 eV and long fluorescence lifetimes of 1.6–5.0 μs confirm their TADF nature. The chromaticity of monochromic OLEDs with TFM-QP and CN-QP as doped emitters was adjustable from yellow to orange–red by varying the doping concentration, and the yellow OLEDs exhibit external quantum efficiencies (ηext) of 14.4% and 9.7% at a low doping level. Full-TADF WOLEDs were fabricated using TFM-QP or CN-QP in combination with the sky-blue TADF emitter DMAC-DPS. The single-emitting-layer doped warm-white OLED (CN-QP:DMAC-DPS) exhibits excellent performance with a low turn-on voltage of 2.8 V, a stable color rending index (CRI) of 72, and high efficiencies of 48.22 cd A−1, 50.47 lm W−1, and 20.16%, which should be the highest efficiency records ever reported for full-TADF warm-white OLEDs so far. The non-doped full-TADF WOLED with an emitting layer structure of DMAC-DPS/TFM-QP/DMAC-DPS achieves a high CRI of 86 and a moderate ηext of 8.7%. [ABSTRACT FROM AUTHOR]

High‐quality hosts are indispensable for simultaneously realizing stable, high efficiency, and low roll‐off blue solution‐processed organic light‐emitting diodes (OLEDs). Herein, three solution processable bipolar hosts with successively reduced triplet energies approaching the T1 state of thermally activated delayed fluorescence (TADF) emitter are developed and evaluated for high‐performance blue OLED devices. The smaller T1 energy gap between host and guest allows the quenching of long‐lived triplet excitons to reduce exciton concentration inside the device, and thus suppresses singlet‐triplet and triplet‐triplet annihilations. Triplet‐energy‐mediated hosts with high enough T1 and better charge balance in device facilitate high exciton utilization efficiency and uniform triplet exciton distribution among host and TADF guest. Benefited from these synergetic factors, a high maximum external quantum efficiency (EQEmax) of 20.8%, long operational lifetime (T50 of 398.3 h @ 500 cd m−2), and negligible efficiency roll‐off (EQE of 20.1% @ 1000 cd m−2) are achieved for bluish‐green TADF OLEDs. Additionally introducing a narrowband emission multiple‐resonance TADF material as terminal emitter to accelerate exciton dynamic and improve exciton utilization, a higher EQEmax of 23.1%, suppressed roll‐off and extended lifetime of 456.3 h are achieved for the sky‐blue sensitized OLEDs at the same brightness. [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]

Modulating the emission wavelength in the range of ultraviolet to deep‐blue with high color purity remains elusive. Herein, a versatile strategy is proposed from the perspective of rationally regulating nonbonding/π‐bonding character of frontier molecular orbitals to finely modulate emission colors while retaining superior color purity. By modifying the fused modes of indolo[3,2,1‐jk]carbazole (ICz) units, the proof‐of‐the‐concept emitters emphasize adjustable hybridization of nonbonding and π‐bonding characteristics, achieving a wide‐range color tuning range from ultraviolet (380 nm) to deep‐blue (428 nm) and small full‐width at half‐maximum (FWHM) of 8–33 nm (63–222 meV). Organic light‐emitting diodes based on the new emitter exhibit electroluminescence emission at 430 nm, a small FWHM of 44 nm (253 meV), maximum external quantum efficiency of 10.9%, and CIE color coordinates of (0.16, 0.05). The CIE coordinates are close to the National Television System Committee (NTSC) standard blue of (0.15, 0.06). [ABSTRACT FROM AUTHOR]

Homoleptic fac‐substituted Ir(III) carbene complexes exhibit higher emission energy (in purple region) in comparison to their mer‐counterparts, prohibiting them to be employed in fabrication of blue emissive organic light‐emitting diode (OLED) devices. Now, the design of two distinctive CF3‐functionalized purin‐8‐ylidene Ir(III) complexes, namely, m‐ and f‐CF3 and m‐ and f‐PhCF3, from new carbene motifs, 9‐(3‐(tert‐butyl)phenyl)‐7‐isopropyl‐2‐(trifluoromethyl)‐7,9‐dihydro‐8H‐purin‐8‐ylidene (A4) and 9‐(3‐(tert‐butyl)phenyl)‐7‐methyl‐6‐phenyl‐2‐(trifluoromethyl)‐7,9‐dihydro‐8H‐purin‐8‐ylidene (B7), having notably stabilized lowest unoccupied molecular orbital energy levels is reported. Hence, the corresponding f‐isomers f‐CF3 and f‐PhCF3 exhibit electroluminescence with peak max. at 478 and 495 nm, max. external quantum efficiencies (EQEs) of 10.4% and 12.8%, respectively. By using f‐CF3 as assistant dopant to convey its energy to terminal emitter t‐DABNA and from f‐PhCF3 donor to 2TCzBN acceptor, two hyper‐OLED devices are successfully fabricated, giving high max. EQE of 23.8%, full‐width at half‐maximum (FWHM) of 30 nm, and CIEx,y coordinate of (0.13, 0.14) for the acceptor t‐DABNA, and max. EQE of 24.0%, FWHM of 28 nm, and CIEx,y of (0.11, 0.36) for the acceptor 2TCzBN, confirming the advantages of these purin‐8‐ylidene Ir(III) complexes. [ABSTRACT FROM AUTHOR]

Four isomeric, homoleptic iridium(III) metal complexes bearing 5‐(trifluoromethyl)imidazo[4,5‐b]pyridin‐2‐ylidene and 6‐(trifluoromethyl)imidazo[4,5‐b]pyridin‐2‐ylidene‐based cyclometalating chelates are successfully synthesized. The meridional isomers can be converted to facial isomers through acid induced isomerization. The m‐isomers display a relatively broadened and red‐shifted emission, while f‐isomers exhibit narrowed blue emission band, together with higher photoluminescent quantum yields and reduced radiative lifetime relative to the mer‐counterparts. Maximum external quantum efficiencies of 13.5% and 22.8% are achieved for the electrophosphorescent devices based on f‐tpb1 and m‐tpb1 as dopant emitter together with CIE coordinates of (0.15, 0.23) and (0.22, 0.45), respectively. By using f‐tpb1 as the sensitizing phosphor and t‐DABNA as thermally activated delayed fluorescence (TADF) terminal emitter, hyperluminescent OLEDs are successfully fabricated, giving high efficiency of 29.6%, full width at half maximum (FWHM) of 30 nm, and CIE coordinates of (0.13, 0.11), confirming the efficient Förster resonance energy transfer (FRET) process. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) materials that can exhibit high efficiencies in non‐doped organic light‐emitting diodes (OLEDs) are highly desired. On the base of a reference TADF molecule (4‐(9,9‐dimethylacridin‐10(9H)‐yl)phenyl)(phenyl)methanone (BP‐DMAC) containing benzoyl/acridine as acceptor/donor, 9‐phenyl‐9H‐3,9"‐bicarbazole (PBCz) is incorporated to the benzoyl side to design and develop a novel emitter, (4‐(9,9‐dimethylacridin‐10(9H)‐yl)phenyl)(9‐phenyl‐9H‐[3,9″‐bicarbazol]‐6‐yl)methanone (PBCz‐BP‐DMAC). The introduction of PBCz unit is proved to endow PBCz‐BP‐DMAC the additional self‐host feature, in addition to its original aggregation‐induced emission and TADF characters. As a result, the non‐doped OLED of PBCz‐BP‐DMAC exhibits sky‐blue emission with remarkable external quantum efficiency (EQE) of 23.0% and rather low efficiency roll‐off (6%) at a brightness of 1000 cd m−2, while its doped device reveals a maximum EQE of 27.5%; the spectra and color coordinates of the doped and non‐doped devices are almost identical. It also acts as a host to sensitize green iridium phosphor (fac‐tris(2‐phenylpyridine) iridium(III), Ir(ppy)3) with high EQE of 23.6%. More fascinatingly, the simplified single‐emitting‐layer all‐TADF white OLED utilizing PBCz‐BP‐DMAC as host emitter in combination with an orange TADF emitter (4CzTPN‐Ph) realizes a state‐of‐the‐art EQE of 20.6%. Evidently PBCz‐BP‐DMAC is one of the best sky‐blue TADF materials and exhibits multi‐functional applications in OLEDs with excellent performance. [ABSTRACT FROM AUTHOR]

Blue iridium phosphors are always precious and desirable for both fundamental research and industrial applications in organic light-emitting diodes (OLEDs). Three heteroleptic cyclometalated iridium(III) complexes, namely, Ir1, Ir2 and Ir3, incorporating 2-(3,5-bis(trifluoromethyl)phenyl)-pyrimidine (tfmphpm) or 2-(2,4-difluorophenyl)-pyrimidine (dfppm) as the main ligand, and 2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine (fppz) or 2-(5-(4-(trifluoromethyl)phenyl)-2H-1,2,4-triazol-3-yl)pyridine (Htaz) as the ancillary ligand, were developed for application in OLEDs. Owing to the introduction of the strong-field CF3-containing ancillary ligand and the incorporation of electron-withdrawing F or CF3 groups on the HOMO-lying C-related aromatic rings of the C^N cyclometalating ligands, the phosphorescence of these Ir(III) complexes peaked at 457–459 nm. In comparison with the well-known commercial sky-blue iridium complex FIrpic (bis(4′,6′-difluorophenylpyridinato) iridium(III) picolinate, 475 nm), the phosphorescence of these complexes is hypsochromically shifted by 16–18 nm, more closely approaching standard blue light. They have high phosphorescence quantum yields in solution. The phosphorescent OLEDs employing these complexes as doped emitters display impressive performance. In particular, the PhOLED based on Ir3 achieves an outstanding maximum external quantum efficiency of 21.23% with Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.26), which is about 27% higher than that of the control device using FIrpic as a dopant (16.63%). [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]

The non-doped organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) materials are attracting increasing attentions due to the excellent performance and simple fabrication. We report an effective design strategy of TADF emitters to improve the non-doped performance by adjusting the valence state of the S atom in TADF molecules and utilizing the steric hindrance effect. Two novel TADF materials PT-BZ-DMAC and PS-BZ-DMAC were designed and synthesized with benzophenone acceptor and acridine donor. In particular, the thioether or sulfone group was attached to benzophenone. Remarkably, the bluish-green TADF-OLED with non-doped PT-BZ-DMAC turned on at 2.5 V and realized a maximum external quantum efficiency (EQE) of 17.34%, which was comparable with its doped OLED (17.71%) but showed much slower roll-off. At 100 and 1000 cd m−2, the non-doped PT-BZ-DMAC device still maintained acceptable efficiencies at 14.65% and 12.59%. In contrast, sulfone-modified PS-BZ-DMAC exhibited good efficiency of 20.55% only in doped OLED. The single crystal analysis and DFT calculation revealed that the phenyl thioether unit in PT-BZ-DMAC hardly contributes to the LUMO or the electronic transitions, instead determines the highly twisted geometry and acts as the barrier group, finally improving the performance of PT-BZ-DMAC in neat film device. Two TADF emitters are developed by attaching thioether or sulfone groups to the benzophenone acceptor. Unlike the strong electron withdrawing sulfone group that contributes to the electronic transition and TADF, the phenyl thioether blocked the conjugation of molecular backbone. The steric hindrance effect of phenyl thioether moiety along with the intermolecular hydrogen bondings can effectively prevents exciton quenching, thus further raising the EQE of non-doped TADF-OLED to 17.34% with low efficiency roll-off. [Display omitted] • Thioether and sulfone were incorporated as substituents to design TADF molecules PT-BZ-DMAC and PS-BZ-DMAC. • Thioether was found to strongly suppress intermolecular interactions and prevent exciton quenching via steric hindrance effect. • Non-doped OLED of thioether based PT-BZ-DMAC exhibited comparably high efficiency (17.34%) to its doped device (17.71%), but with much slower roll-off. • Sulfone based PS-BZ-DMAC exhibited high efficiency of 20.55% only in doped OLED, while non-doped device did not work. [ABSTRACT FROM AUTHOR]