Non-doped Devices Research Articles

Near-Infrared Electroluminescence beyond 800 nm with High Efficiency and Radiance from Anthracene Cored Emitters.

Derivatives based on anthryleno[1,2-b]pyrazine-2,3-dicarbonitrile (DCPA) are used as luminescent materials, to realize near-infrared (NIR) electroluminescence. By functionalizing DCPA with aromatic amine donors, two emitters named DCPA-TPA and DCPA-BBPA are designed and synthesized. Both molecules have large dipole moments owing to the strong intramolecular charge transfer interactions between the amine donors and the DCPA acceptor. Thus, compared with doped films, the emission of neat films of DCPA-TPA and DCPA-BBPA can fully fall into the NIR region (>700 nm) with increasing surrounding polarity by increasing doping ratio. Moreover, the non-doped devices based on DCPA-TPA and DCPA-BBPA provide NIR emission with peaks at 838 and 916 nm, respectively. A maximum radiance of 20707 mW Sr-1 m-2 was realized for the further optimized device based on DCPA-TPA. This work provides a simple and efficient strategy of molecular design for developing NIR emitting materials.

Bipolar TADF interlayer for high performance hybrid WOLEDs with an ultrathin non-doped emissive layer architecture

Abstract White organic light-emitting diodes (WOLEDs) with non-doped construction have compelling advantages, such as simplifying manufacturing process and reducing costs, thus attracting more attention. To improve the efficiency of non-doped WOLEDs, the selection of suitable interlayer is still a big problem to balance carriers. Here, for the first time, we employed a thermally activated delayed fluorescence (TADF) material with bipolar transport characteristics as the interlayer to modulate the exciton distribution, and fabricated efficient and ultrathin non-doped devices. The optimized monochromatic and white OLED exhibited maximum external quantum efficiencies of 18.1% and 15.6%, current efficiencies of 55.7 cd/A and 46.6 cd/A, as well as power efficiencies of 43.8 lm/W and 41.8 lm/W, respectively. More importantly, the devices adopted an ultrathin non-doped emissive layer architecture, and its thickness was only 3.6 nm, which was beneficial to reduce costs and simplify preparation process. Our results illustrated a new strategy that the bipolar TADF interlayer can suppress triplet-triplet annihilation (TTA) effect and improve the exciton utilization, which presents unprecedented opportunities for fabricating highly efficient, low cost and commercial WOLEDs.

Triptycene-imbedded thermally activated delayed fluorescence emitters with excellent film morphologies for applications in efficient nondoped and doped organic light-emitting devices

Thermally activated delayed fluorescence (TADF) materials with high luminescence efficiencies, short lifetimes, excellent thermal stability and high-quality film formation are highly desired to realize efficient organic light emitting diodes (OLEDs) with low efficiency roll-off. Herein, two asymmetric TADF molecules consisting of triptycene-fused benzophenone serving as an electron acceptor, and dimethylacridin and phenoxazine as electron donors are reported. The comparative study reveals that the introduction of triptycene scaffold strengthens the molecular rigidity and the intermolecular interactions in aggregated states, which endow the titled materials with significantly improved thermal stability, film formation quality and mechanofluorochromism behaviors. The triptycene-imbedded compound TCO-DMAC emits fast efficient delay fluorescence with photoluminescence quantum yield (PLQY) of 0.92 and short lifetime of 4.94 μs in doped evaporated film, based on which the bluish-green doped OLED achieved an external quantum efficiency (EQE) of 21.2%, a current efficiency (CE) of 57.3 cd/A, a power efficiency (PE) of 62.3 lm/W, and a maximum brightness of 15780 cd/m2. It is worth mentioning that the evaporated neat film of TCO-DMAC exhibit high PLQY of 0.69 and short delay fluorescence lifetime of 4.37 μs. The non-doped OLED achieved an external quantum efficiency (EQE) of 15.6%, a turn-on voltage of 2.8 V and a maximum brightness of 13830 cd/m2 with only minor efficiency roll-off of 3.8% at a high luminance of 1000 cd/m2.

High external quantum efficiency and low efficiency roll-off achieved simultaneously in nondoped pure-blue organic light-emitting diodes based on a hot-exciton fluorescent material

“Hot-exciton” fluorescent materials can efficiently convert triplet excitons into singlet excitons through a path from high-lying triplet excited states (Tn, n > 1) to singlet excited states (Sm, m ≥ 1). The fast reverse intersystem crossing (RISC) process of the hot-exciton channel promotes a high exciton utilization efficiency (EUE) and reduces the efficiency roll-off (ηroll-off) caused by the accumulation of low-lying triplet excitons (T1). Herein, a pure-blue-emitting molecule, PICNAnCz, exhibiting hot-exciton fluorescent emission is proposed. The optimized PICNAnCz-based nondoped organic light-emitting diode (OLED) device achieves a high external quantum efficiency of 9.05% corresponding to a large EUE of 87% and a low ηroll-off of 13%, achieving both high efficiency and a small ηroll-off. The maximum current efficiency and power efficiency of the nondoped device are 9.07 cd A−1 and 5.76 lm W−1, respectively. The nondoped device shows a novel blue electroluminescence (EL) emission with a peak wavelength of 448 nm and Commission Internationale de l’Eclairage coordinates of (0.16, 0.11). These results are among the best reported for hot-exciton blue-emitting materials for nondoped blue fluorescent OLEDs. The excellent EL performance is attributed to the nanosecond-scale RISC process from the high-lying triplet excited state (T2) to the lowest singlet excited state (S1).

A purely organic D-π-A-π-D emitter with thermally activated delayed fluorescence and room temperature phosphorescence for near-white OLED

A purely organic D-π-A-π-D type emitter showing thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) was designed and synthesized by utilizing the benzophenone as an acceptor and the N-phenyl-2-napthylamine as a donor moiety. It exhibits considerable TADF character in doped PMMA film and room temperature phosphorescence with a long lifetime of 74 ms at 466 nm in solid state. The devices with the configuration of ITO/Mo2O3 (4 nm)/mCP (30 nm)/mCP: x wt% NP2BP/TmTyPB (60 nm)/LiF (1.5 nm)/Al (100 nm) were prepared by vacuum evaporation to explore their electroluminescent performance. Interestingly, the non-doped device has obtained near-white emission with a fluorescence emission peak at 475 nm and a phosphorescence emission peak at 563 nm having the CIE coordinate of (0.23, 0.32) and the maximum external quantum efficiency of 1.09%.

Non-doped long-wave red electroluminescence for fumaronitrile with fluorenyl or biphenyl group

The long-wave red emitters SFPAFN (bis(4-(N-(9,9′-spirobifluorenyl-2)-3,5- dimethylphenylamino)phenyl)fumaronitrile), BPFAFN (bis(4-(N-(9,9′-diethyl-2- fluorenyl)diphen-4,4′-ylamino)phenyl)fumaronitrile), and BBPAFN (bis(4-(N,N′-bis(diphen-4,4′-yl)amino)phenyl)fumaronitrile) were synthesized. The bulky, non-planar and large conjugated spirobifluorenyl, fluorenyl or biphenyl amino substituent formed steric torsional hindrance to restrict intermolecular aggregation. These promoted hole transport ability and luminous efficiency, and tuned the long-wave red emission successfully. The non-doped red devices for donor–π–acceptor–π–donor SFPAFN, BPFAFN and BBPAFN showed electroluminescent (EL) peaks at 680, 692, and 680 nm with the maximum luminance efficiencies of 3.33, 3.61, and 3.13 cd/A, respectively. The EL emission was redshifted and EL efficiency was improved more effectively for BPFAFN than that for SFPAFN or BBPAFN. These compounds were excellent long-wave red emitters with hole-transporting properties. Furthermore, the main EL parameters of similar non-doped red OLEDs for arylamino fumaronitrile derivatives were compared briefly.

Highly efficient red fluorescent OLEDs based on diphenylacridine-naphthothiadiazole derivatives with upper level intersystem crossing

Energy ordering of the lowest singlet and triplet excited states dominates the photophysical properties of conjugated molecules, especially emitters utilizing triplet energy through some special pathways such as triplet–triplet annihilation (TTA), thermally activated delayed fluorescence (TADF) and “hot exciton” process. Here, three novel D-A type red light-emitting compounds, DPACNZP, DPACNZPCN and DPACNZPPI, are designed and synthesized, in which the lowest triplet states are all dark states and the high-level triplet states are enabled to realize the reverse intersystem crossing (RISC) process to successfully harvest triplet excitons via “hot exciton” approach. Further, unique molecular architecture of naphthothiadiazole (NZ) group with dual-side substitutions leads to the “wrapped” state of the acceptors, which is favorable for suppressing the intermolecular interactions in solid states. As a result, DPACNZP, DPACNZPCN and DPACNZPPI achieve high PLQYs, excellent thermal stabilities and appropriate energy levels of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs). Both nondoped and doped devices using the three compounds as active layers achieve high external quantum efficiencies (EQEs) with reduced efficiency roll-offs. The doped device based on DPACNZPPI achieves the highest EQE of 8.7% with CIE coordinates of (0.604, 0.392) and EQE of 8.3% at 100 cd m−2. The exciton utilization efficiency (EUE) is as high as 74%, which is among the best results of red electroluminescence to our knowledge, and is the best performance for OLEDs based on reported NZ-derivatives.

Donor engineering for diphenylsulfone derivatives with both thermally activated delayed fluorescence and aggregation-induced emission properties

Two new aggregation-induced thermally activated delayed fluorescence (AIE-TADF) luminogens based on diphenylsulfone derivatives with asymmetrical D-A-D′ configuration have been designed and synthesized, namely 10-(4-((9-phenyl-9H-carbazol-3-yl)sulfonyl)phenyl)-10H-phenoxazine (3CP-DPS-PXZ), and 9,9-dimethyl-10-(4-((9-phenyl-9H-carbazol-3-yl)sulfonyl)phenyl)-9,10-dihydroacridine (3CP-DPS-DMAC). The TADF and AIE properties of the new luminogens can be tuned by adjusting donor moiety (D′). Since phenoxazine featured stronger electron-donating capability compared with 9,9-dimethylacridine, 3CP-DPS-PXZ manifested a bathochromic-shift emission, enhanced AIE effect and higher reverse intersystem crossing (kRISC) rate constant in comparision with 3CP-DPS-DMAC. Systematic research on the thermal, electrochemical and photophysical properties for these two new compounds have been carried out. The fabricated nondoped green organic light-emitting device (OLED) utilizing 3CP-DPS-PXZ as emitter achieved a maximum external quantum efficiency (EQE) of 17.9 ± 0.3% and the EQE remained 14.5% at the luminance of 1000 cd/m2. However, the maximum EQE of 3CP-DPS-DMAC based blue nondoped OLED was only 9.1 ± 0.2%. These manifested asymmetrical D-A-D’ structure molecular is an efficient way to optimize the performance of AIE-TADF emitter.

Highly efficient nondoped bilayer organic light-emitting diodes based on triphenyl phosphine oxide protected iridium complexes

Nondoped phosphorescent organic light-emitting diodes (OLEDs) with simplified stacks are desired for practical displaying and lighting applications. However, doping emissive layers are commonly adopted due to serious triplet quenching of phosphors. Here, we demonstrate high-efficiency nondoped bilayer OLEDs based on triphenylphosphine oxide protected iridium(III) complexes. The host feature of peripheral phosphine oxide groups effectively suppresses intermolecular interaction induced quenching. As consequence, the maximum external quantum efficiency beyond 20% and near-zero roll-offs at 1000 nits were achieved, comparable to the best nondoped phosphorescence devices.

Efficient red fluorescent OLEDs based on aggregation-induced emission combined with hybridized local and charge transfer state

Limited by the energy gap law and large conjugate structures, red organic light-emitting diodes (OLEDs) have long been criticized for their efficiency problems. The aggregation-induced emission (AIE) effect and the reverse intersystem crossing process can lead to high photoluminescent quantum yields (PLQYs) and high exciton utilization efficiency (EUE), two key objectives for obtaining efficient OLEDs. In this work, we chose benzothiadiazole as the acceptor (A) and triphenylamine and phenothiazine as the donors (Ds) to design and synthesize a D–A–D red fluorescent molecule, PBTPA. Results of systematic photophysical measurements indicated PBTPA to exhibit AIE trending, manifesting as aggregation-induced enhanced emission and the characteristics of hybridized local and charge transfer (HLCT) states. These features facilitated high solid-state luminescence efficiency and high exciton utilization to be achieved. A non-doped device fabricated using PBTPA displayed a red electroluminescence peak at 656 nm, corresponding to the Commission International de L'Eclairage coordinates of (0.65, 0.32), and an external quantum efficiency of 1.62%. By calculation, the EUE reached a value of 50%. According to the results of theoretical calculations, the effective hot exciton channel between the second triplet (T2) and the lowest singlet (S1) was responsible for the high EUE. This EUE value is relatively good for this type of red OLEDs. Evidence indicates that combining AIE and HLCT has excellent potential to aid the discovery of new-generation highly efficient red OLEDs.

Stimuli-Responsive Aggregation-Induced Delayed Fluorescence Emitters Featuring the Asymmetric D-A Structure with a Novel Diarylketone Acceptor Toward Efficient OLEDs with Negligible Efficiency Roll-Off.

Multifunctional luminescent materials with aggregation-induced delayed fluorescence (AIDF) are capable of suppressing concentration-caused emission quenching and exciton annihilation when used as organic light-emitting diode (OLED) emitters. In this contribution, three stimuli-responsive AIDF luminogens, pipd-BZ-PXZ, pipd-BZ-PTZ, and pipd-BZ-DMAC, featuring a D-A asymmetric framework based on a fused N-heterocycle diarylketone acceptor (imid-azo[1,2-a]pyridin-2-yl(phenyl)methanone pipd) are designed and synthesized. Interestingly, pipd-BZ-PTZ forms two different kinds of crystals (G-crystal and O-crystal) with distinct intermolecular interactions between pipd moieties. The G-crystal with a looser packing mode presents significant morphology-dependent stimuli-responsive behavior with a shifted emission wavelength of 56 nm. Generated by a strong intramolecular charge transfer effect, pipd-BZ-PXZ and pipd-BZ-PTZ exhibit orange to red emission in solution and neat films. Both nondoped and doped devices are fabricated for comparison. Nondoped devices present moderate performance with external quantum efficiencies and current efficiency that reach 7.04% and 19.86 cd A-1, respectively, and the corresponding efficiency roll off at 1000 cd m-2 is as small as 2.3%, which is among the best records of AIDF-OLEDs with an emission wavelength over 570 nm. Doped devices show better performance with corresponding efficiencies of up to 55.41 cd A-1 and 15.77%.

Different RISC rates in benzoylpyridine-based TADF compounds and their implications for solution-processed OLEDs

Reverse intersystem crossing (RISC) rate is among the key parameters of TADF compounds affecting triplet upconversion and annihilation processes, and thus determining efficiency roll-off in TADF-OLEDs. Herein, control of RISC rate (kRISC) in a series of benzoylpyridine-based TADF compounds via carbazole donor linking positions is demonstrated. Regardless of the similar singlet-triplet energy splitting found in the compounds with meta- or para-carbazole linking topologies, meta-Cz compounds exhibited stronger singlet-triplet state coupling resulting in 10-fold larger kRISC (up to ~2 × 106 s−1) as that in para-Cz counterparts. Different kRISC was shown to have important implications for the solution-processed TADF-OLEDs based on the benzoylpyridines. Explicitly, in the non-doped devices based on neat meta-Cz emissive layers, larger kRISC was crucial for the reduced roll-off. Conversely, in 1.2 wt% rubrene-doped hyperfluorescent devices employing benzoylpyridines as TADF hosts, insignificant roll-off was observed irrespective of kRISC, provided it is above 105 s−1. The obtained results give insight into the possibilities of controlling kRISC in benzoylpyridines and demonstrate its importance for efficiency roll-off of TADF and hyperfluorescent OLEDs.

Non-doped blue fluorescent emitting materials with donor-π-acceptor-π-donor structures based 1,2,4-triazole/carbazole derivatives

Two blue emitters based 1,2,4-triazole acceptor, carbazole and tert-butyl carbazole donor, namely 2pCzTAZ and 2tCzTAZ, were designed and synthesized. They exhibited obvious charge-transfer excited state, and blue emission with photoluminescent peaks at 425 nm in thin films. The highest occupied molecular orbital energy levels of 2pCzTAZ and 2tCzTAZ were -5.77 and -5.66 eV, respectively. The non-doped devices employing 2pCzTAZ as emitters achieved deep-blue emission with CIE coordinates (0.16, 0.08) and the maximum luminance of 2 438 cd/A, indicating two novel emitters have huge potential applications in non-dope blue fluorescent OLEDs.

High-Performance Solution-Processed Red Thermally Activated Delayed Fluorescence OLEDs Employing Aggregation-Induced Emission-Active Triazatruxene-Based Emitters

Two novel red thermally activated delayed fluorescence (TADF) emitters (TAT-DBPZ and TAT-FDBPZ) were developed by incorporating triazatruxene (TAT) as the electron donor (D) and dibenzo[a,c]phenazine (DBPZ) or fluorine-substituted dibenzo[a,c]phenazine (FDBPZ) as the electron acceptor (A). Both compounds showed aggregation-induced emission (AIE) behaviors and bright red emission in neat films. Benefited from the rigid and large planar conjugated structures of TAT and DBPZ, TAT-DBPZ and TAT-FDBPZ realized high photoluminescence quantum yields (PLQYs) in solid states. Meanwhile, the large steric hindrance between TAT and DBPZ segments produced small singlet-triplet energy splitting (EST), leading to short delayed fluorescence lifetimes and high reverse intersystem crossing (RISC) rate (106 s-1) for both compounds. The solution-processable doped organic light-emitting diodes (OLEDs) based on TAT-DBPZ achieved a high external quantum efficiency (EQE) of 15.4% with a red emission peak at 604 nm, which was one of the highly efficient solution-processable red TADF OLEDs. TAT-FDBPZ based doped devices also showed a red emission peak at 611 nm with a maximum EQE of 9.2% and low efficiency roll-off ratios of 1.0% at 100 cd m-2 and 19% at 1000 cd m-2. Furthermore, their solution-processable non-doped devices displayed EQEs of 5.6% and 2.9% with the red-shifted emission peaks at 626 nm and 641 nm, respectively. These results indicate the huge potential of utilization of triazatruxene as donor unit to achieve highly efficient and low efficiency roll-off solution-processable red TADF OLEDs.

Novel self-host heteroleptic green iridium dendrimers based on carbazole dendrons for solution-processable non-doped phosphorescent organic light-emitting diodes

Several novel green-emitting iridium dendrimers has been successfully designed and synthesized with carbazole as the dendron, which is covalently attached to the emissive bis[2-(2-dibenzothienyl)pyridine]picolate iridium(III) [(DBTPy)2Irpic] core through a nonconjugated link to form an efficient self-host system for nondoped electrophosphorescent device applications. All the iridium dendrimers display green emission, possess good thermal stability and high photoluminescence quantum yields. As well as, non-doped solution-processed OLEDs were successfully fabricated based on the resulting Ir-1, Ir-2 and Ir-3 as green-emitting phosphors. By increasing the density of carbazole dendrons at the edge of the emissive core, luminescence self-quenching caused by the intermolecular interactions in the solid state can be significantly reduced. Therefore, the OLEDs based on Ir-3 show the best performances with a maximal luminous efficiency of 0.24 cd/A and a maximum brightness of 2520 cd/m2. This work will shed light on the development of novel self-host phosphorescent iridium dendrimers for solution processable nondoped electrophosphorescent devices.

Anthracene derivatives as highly efficient deep-blue emitters with extremely low driving voltages, Von ≤2.7 V

In this work, we report a feasible and versatile method to synthesize bis(N1, N1, N3, N3-tetraphenylbenzene-1,3-diamine) (TPDA) group as a donor group for OLEDs. A series of novel tetraphenyl-diamine-phenyl end-capped anthracene derivatives, 5-(anthracene-9-yl)-N1, N1, N3, N3-tetraphenylbenzene-1,3-diamine (TPDAAn), N1, N1, N3, N3-tetraphenyl-5-(10-phenylanthracen-9-yl)benzene-1,3-diamine (TPDAPhAn), 5,5'-(anthracene-9,10-diyl)bis(N1, N1, N3, N3-tetraphenylbenzene-1,3-diamine) (dTPDAAn) were synthesized through a facile route. The doped devices based on TPDAAn, TPDAPhAn and dTPDAAn exhibit high EQE up to 4.01% and deep-blue emission with CIE coordinates of (0.15, 0.05), which fully meet the requirements of European Broadcasting Union (EBU) standard blue CIE coordinates of (0.15, 0.06). The non-doped devices based on TPDAAn and TPDAPhAn also display efficient deep-blue emission with CIEy coordinates of 0.08, matching very well with the National Television System Committee (NTSC) standard blue CIEy coordinate for display applications. More importantly, the non-doped devices show extremely low driving voltages (Von ≤ 2.7 V), making them one of the lowest driving voltage devices with CIEy ≤ 0.8.

Highly Emissive Dinuclear Platinum(III) Complexes.

Dinuclear Pt(III) complexes were commonly reported to have short-lived lowest-lying triplet states, resulting in extremely weak or no photoluminescence. To overcome this obstacle, a new series of dinuclear Pt(III) complexes, named Pt2a-Pt2c, were strategically designed and synthesized using donor (D)-acceptor (A)-type oxadiazole-thiol chelates as bridging ligands. These dinuclear Pt(III) complexes possess a d7-d7 electronic configuration and exhibit intense phosphorescence under ambient conditions. Among them, Pt2a exhibits orange phosphorescence maximized at 618 nm in degassed dichloromethane solution (Φp ≈ 8.2%, τp ≈ 0.10 μs) and near-infrared (NIR) emission at 749 nm (Φp ≈ 10.1% τp ≈ 0.66 μs) in the crystalline powder and at 704 nm (Φp ≈ 33.1%, τp ≈ 0.34 μs) in the spin-coated neat film. An emission blue-shifted by more than 3343 cm-1 is observed under mechanically ground crystalline Pt2a, affirming intermolecular interactions in the solid states. Time-dependent density functional theory (TD-DFT) discloses the lowest-lying electronic transition of Pt2a-Pt2c complexes to be a bridging ligand-metal-metal charge transfer (LMMCT) transition. The long-lived triplet states of these dinuclear platinum(III) complexes may find potential use in lighting. Employing Pt2a as an emitter, high-performance organic light-emitting diodes (OLEDs) were fabricated with NIR emission at 716 nm (η = 5.1%), red emission at 614 nm (η = 8.7%), and white-light emission (η = 11.6%) in nondoped, doped (in mCP), and hybrid (in CzACSF) devices, respectively.

Efficient deep-blue thermally activated delayed fluorescence emitters based on diphenylsulfone-derivative acceptor

Efficient deep-blue thermally activated delayed fluorescence (TADF) emitters are still very scarce. It is available to render the color bluer by reducing the intensity of intramolecular charge transfer (ICT) between acceptor and donor. Here, attaching the acceptor of six-membered heterocyclic diphenylsulfone-derivative (2PTO) with donors (DMAc and DPAc), two compounds, 2-(9,9-dimethyl-acridin-10(9H)-yl)-10-phenyl-10H-phenothiazine 5,5-dioxide (DMAC2PTO) and 2-(9,9-diphenylacridin-10(9H)-yl)-10-phenyl-10H-phenothiazine 5,5-dioxide (DPAC2PTO) were designed and synthesized. Due to the largely twisted configuration and weak ICT, DMAC2PTO with small singlet-triplet energy gap (ΔEST) displayed TADF feature and deep-blue emission. Moreover, DPAC2PTO exhibited much lower photoluminescence quantum yield (PLQY) than DMAC2PTO. The doped device of DMAC2PTO achieved a maximum external quantum efficiency (EQEmax) of 15.2% with a deep-blue color coordinate (CIEx,y) of (0.154, 0.108), and showed a narrow emission spectra with a 52 nm full width at half maximum (FWHM). In addition, the non-doped device of DMAC2PTO reached the EQEmax of 8.36% with CIEx,y of (0.158, 0.157).

Solid-State Fluorophore Based on π-Extended Heteroaromatic Acceptor: Polymorphism, Mechanochromic Luminescence, and Electroluminescence

A novel light-emitting material 4-(4-diphenylaminophenyl)-benzo[d,e]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one (4-TPA-BBI) is synthesized and characterized. 4-TPA-BBI exhibits excellent polymorphism, mechanochromic luminescence (MCL), and electroluminescence properties. It can form three different aggregate species: yellow rod-like crystal (Y-crystal), orange needle-like crystal (O-crystal), and red solid (R-solid). It is observed that the Y-crystal can be reversibly changed to R-solid upon external stimuli, while O-crystal can be irreversibly transformed to R-solid via sublimation under a vacuum. Crystallographic and photophysical studies indicate that the Y-crystal featuring loose molecular packing demonstrates a high photoluminescence quantum yield (ΦPL ≈ 1) and obvious MCL property, while the compactly packed O-crystal shows a lower ΦPL of 0.646 with negligible MCL. A nondoped light-emitting device based on 4-TPA-BBI as an emitter exhibits bright orange emission with a peak at 588 nm, a high external quantum efficiency of 4.4%, and a maximum luminance of 22470 cd m–2.

Achieving 21% External Quantum Efficiency for Nondoped Solution-Processed Sky-Blue Thermally Activated Delayed Fluorescence OLEDs by Means of Multi-(Donor/Acceptor) Emitter with Through-Space/-Bond Charge Transfer.

Although numerous thermally activated delayed fluorescence (TADF) organic light‐emitting diodes (OLEDs) have been demonstrated, efficient blue or even sky‐blue TADF‐based nondoped solution‐processed devices are still very rare. Herein, through‐space charge transfer (TSCT) and through‐bond charge transfer (TBCT) effects are skillfully incorporated, as well as the multi‐(donor/acceptor) characteristic, into one molecule. The former allows this material to show small singlet–triplet energy splitting (ΔE ST) and a high transition dipole moment. The latter, on the one hand, further lights up multichannel reverse intersystem crossing (RISC) to increase triplet exciton utilization via degenerating molecular orbitals. On the other hand, the nature of the molecular twisted structure effectively suppresses intermolecular packing to obtain high photoluminescence quantum yield (PLQY) in neat flims. Consequently, using this design strategy, T‐CNDF‐T‐tCz containing three donor and three acceptor units, successfully realizes a small ΔE ST (≈0.03 eV) and a high PLQY (≈0.76) at the same time; hence the nondoped solution‐processed sky‐blue TADF‐OLED displays record‐breaking efficiency among the solution process‐based nondoped sky‐blue OLEDs, with high brightness over 5200 cd m−2 and external quantum efficiency up to 21.0%.