Secondary Acceptor Optimization for Full-Exciton Radiation: Toward Sky-Blue Thermally Activated Delayed Fluorescence Diodes with External Quantum Efficiency of ≈30

Excited-State and Steric Hindrances Engineering Enable Fast Spin-Flip Narrowband Thermally Activated Delayed Fluorescence Emitters with Enhanced Quenching Resistance

Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials have great potential for applications in ultrahigh-definition (UHD) organic light-emitting diode (OLED) displays, that benefit from their narrowband emission characteristic. However, key challenges such as aggregation-caused quenching (ACQ) effect and slow triplet-to-singlet spin-flip process, especially for blue MR-TADF materials, continue to impede their development due to planar skeletons and relatively large ΔESTs. Here, an effective strategy that incorporates multiple carbazole donors into the parent MR moieties is proposed, synergistically engineering their excited states and steric hindrances to enhance both the spin-flip process and quenching resistance. As expected, the designed materials namely 5Cz-BNO and 5Cz-BN exhibit bright blue and green emissions with narrow full-width at half-maximums (FWHMs) around 23 nm, together with significantly improved reverse intersystem crossing (RISC) rates. The OLEDs based on 5Cz-BNO and 5Cz-BN with doping concentrations from 5 to 20 wt % achieve high maximum external quantum efficiency (EQEmax) values exceeding 30 % with suppressed efficiency roll-offs and improved operational stability. This work offers an effective approach for designing doping-insensitive blue and green MR-TADF materials with fast spin-flip processes by integrating the engineering of excited states and steric hindrances.

The Golden Age of Thermally Activated Delayed Fluorescence Materials: Design and Exploitation

Since the seminal report by Adachi and co-workers in 2012, there has been a veritable explosion of interest in the design of thermally activated delayed fluorescence (TADF) compounds, particularly as emitters for organic light-emitting diodes (OLEDs). With rapid advancements and innovation in materials design, the efficiencies of TADF OLEDs for each of the primary color points as well as for white devices now rival those of state-of-the-art phosphorescent emitters. Beyond electroluminescent devices, TADF compounds have also found increasing utility and applications in numerous related fields, from photocatalysis, to sensing, to imaging and beyond. Following from our previous review in 2017 ( Adv. Mater. 2017, 1605444), we here comprehensively document subsequent advances made in TADF materials design and their uses from 2017-2022. Correlations highlighted between structure and properties as well as detailed comparisons and analyses should assist future TADF materials development. The necessarily broadened breadth and scope of this review attests to the bustling activity in this field. We note that the rapidly expanding and accelerating research activity in TADF material development is indicative of a field that has reached adolescence, with an exciting maturity still yet to come.

Excited-State Engineering Enables Efficient Deep-Blue Light-Emitting Diodes Exhibiting BT.2020 Color Gamut

Organic luminescent materials that exhibit thermally activated delayed fluorescence (TADF) can convert non-emissive triplet excitons into emissive singlet states through a reverse intersystem crossing (RISC) process. Therefore, they have tremendous potential for applications in organic light-emitting diodes (OLEDs). However, with the development of ultra-high definition 4K/8K display technologies, designing efficient deep-blue TADF materials to achieve the Commission Internationale de l'Éclairage (CIE) coordinates fulfilling BT.2020 remains a significant challenge. Here, an effective approach is proposed to design deep-blue TADF molecules based on hybrid long- and short-range charge-transfer by incorporation of multiple donor moieties into organoboron multiple resonance acceptors. The resulting TADF molecule exhibits deep-blue emission at 414 nm with a full width at half maximum (FWHM) of 29 nm, together with a thousand-fold increase in RISC rate. OLEDs based on the champion material achieve a record maximum external quantum efficiency (EQE) of 22.8% with CIE coordinates of (0.163, 0.046), approaching the coordinates of the BT.2020 blue standard. Moreover, TADF-assisted fluorescence devices employing the designed material as a sensitizer exhibit an exceptional EQE of 33.1%. This work thus provides a blueprint for future development of efficient deep-blue TADF emitters, representing an important milestone towards meeting the blue color gamut standard of BT.2020.

Extended Conjugation Strategy Enabling Red-Shifted and Efficient Emission of Orange-Red Thermally Activated Delayed Fluorescence Materials

In account of the energy gap law, the development of efficient narrow-band gap thermally activated delayed fluorescence (TADF) materials remains a major challenge for the application of organic light-emitting diodes (OLEDs). The orange-red TADF materials are commonly designed with either large π-conjugated systems or strong intramolecular donor-acceptor (D-A) interactions for red-shift emission and small singlet-triplet energy gap (ΔEST). There are rare reports on the simultaneous incorporation of these two strategies on the same material systems. Herein, two orange-red emitters named 1P2D-BP and 2P2D-DQ have been designed by extending the conjugation degree of the center acceptor DQ and increasing the number distribution of the peripheral donor PXZ units, respectively. The emission peak of 1P2D-BP is red-shifted to 615 nm compared to 580 nm for 2P2D-DQ, revealing the pronounced effect of the conjugation extension on the emission band gap. In addition, the distorted molecular structure yields a small ΔEST of 0.02 eV, favoring the acquisition of a high exciton utilization through an efficient reverse intersystem crossing process. As a result, orange-red OLEDs with both 1P2D-BP and 2P2D-DQ have achieved an external quantum efficiency (EQE) of more than 17%. In addition, the efficient white OLED based on 1P2D-BP is realized through precise exciton assignment and energy transport modulation, showing an EQE of 23.6% and a color rendering index of 82. The present work provides an important reference for the design of high-efficiency narrow-band gap materials in the field of solid-state lighting.

Dynamic Ultra-long Room Temperature Phosphorescence Enabled by Amorphous Molecular "Triplet Exciton Pump" for Encryption with Temporospatial Resolution

Organic ultra-long room-temperature phosphorescence (RTP) materials in the amorphous state have attracted widespread attention due to their simple preparation and flexibility to adopt various forms in sensors, bioimaging, and encryption applications. However, the amorphous molecular host for the host-guest RTP systems is highly demanded but limited. Here, a universal molecular host (DPOBP-Br) has been designed by integration of an amorphous moiety of diphenylphosphine oxide (DPO) and an intersystem crossing (ISC) group of 4-bromo-benzophenone (BP-Br). Various commercial fluorescence dyes were doped into the tight and transparent DPOBP-Br film, respectively, resulting in amorphous host-guest systems with ultra-long RTP colors from green to red. It was found that DPOBP-Br acted as a universal "triplet exciton pump" for promoting the generation of triplet excitons in the guest, through energy transfer processes and external heavy-atom effect based on DPOBP-Br. Interestingly, dynamic RTP was achieved by controlling residual oxygen concentration in the amorphous matrix by UV irradiation. Therefore, multi-dimensional anti-counterfeiting coatings were realized even on curved surfaces, simultaneously exhibiting spatial and 2D-time dependence. This work provides a strategy to design new amorphous molecular hosts for RTP systems and demonstrates the advanced information encryption with tempo-spatial resolution based on the dynamic ultra-long RTP of an amorphous system.

Exploring Robust Delayed Fluorescence Materials via Structural Rigidification for Realizing Organic Light-Emitting Diodes with High Efficiencies and Small Roll-Offs

Thermally activated delayed fluorescence (TADF) materials have been widely studied for the fabrication of high-performance organic light-emitting diodes (OLEDs), but the serious efficiency roll-offs still remain unsolved in most cases. Herein, it is wish to report a series of robust green TADF compounds containing rigid xanthenone acceptor and acridine-based spiro donors. The enhancement in molecular rigidity not only endows the compounds with improved thermal stability but also results in reduced geometric vibrations and thus lowered reorganization energies. These compounds exhibit distinct merits of high thermal stabilities, excellent photoluminescence quantum efficiencies (96%-97%), large horizontal dipole orientation ratios (87.4%-92.1%) and fast TADF rates (1.4-1.5 × 106 s-1 ). The OLEDs using them as emitters furnish superb electroluminescence performances with outstanding external quantum efficiencies (ηext s) of up to 37.4% and very small efficiency roll-offs. Moreover, highly efficient hyperfluorescence OLEDs are obtained by using them as sensitizers for the green mutilresonance TADF emitter BN2, delivering excellent ηext s of up to 34.2% and improved color purity. These results disclose the high potential of these TADF compounds as emitters and sensitizers for OLEDs.

2D Covalent Organic Frameworks Based on Heteroacene Units

Covalent organic frameworks (COFs) are a unique new class of porous materials that arrange building units into periodic ordered frameworks through strong covalent bonds. Accompanied with structural rigidity and well-defined geometry, heteroacene-based COFs have natural advantages in constructing COFs with high stability and crystallinity. Heteroacene-based COFs usually have high physical and chemical properties, and their extended π-conjugation also leads to relatively low energy gap, effectively promoting π-electron delocalization between network units. Owing to excellent electron-withdrawing or -donating ability, heteroacene units have incomparable advantages in the preparation of donor-acceptor type COFs. Therefore, the physicochemical robust and fully conjugated heteroacene-based COFs solve the problem of traditional COFs lacking π-π interaction and chemical stability. In recent years, significant breakthroughs are made in this field, the choice of various linking modes and building blocks has fundamentally ensured the final applications of COFs. It is of great significance to summarize the heteroacene-based COFs for improving its complexity and controllability. This review first introduces the linkages in heteroacene-based COFs, including reversible and irreversible linkages. Subsequently, some representative building blocks are summarized, and their related applications are especially emphasized. Finally, conclusion and perspectives for future research on heteroacene-based COFs are presented.

Solution-Processed Pure Red TADF Organic Light-Emitting Diodes With High External Quantum Efficiency and Saturated Red Emission Color

In spite of recent research progress in red thermally activated delayed fluorescence (TADF) emitters, highly efficient solution-processable pure red TADF emitters are rarely reported. Most of the red TADF emitters reported to date are designed using a rigid acceptor unit which renders them insoluble and unsuitable for solution-processed organic light-emitting diodes (OLEDs). To resolve this issue, a novel TADF emitter, 6,7-bis(4-(bis(4-(tert-butyl)phenyl)amino)phenyl)-2,3-bis(4-(tert-butyl)phenyl)quinoxaline-5,8-dicarbonitrile (tBuTPA-CNQx) is designed and synthesized. The highly twisted donor-acceptor architecture and appropriate highest occupied molecular orbital/lowest unoccupied molecular orbital distribution lead to a very small singlet-triplet energy gap of 0.07 eV, high photoluminescence quantum yield of 92%, and short delayed fluorescence lifetime of 52.4 µs. The peripheral t-butyl phenyl decorated quinoxaline acceptor unit and t-butyl protected triphenylamine donor unit are proven to be useful building blocks to improve solubility and minimize the intermolecular interaction. The solution-processed OLED based on tBuTPA-CNQx achieves a high external quantum efficiency (EQE) of 16.7% with a pure red emission peak at 662 nm, which is one of the highest EQE values reported till date in the solution-processed pure red TADF OLEDs. Additionally, vacuum-processable OLED based on tBuTPA-CNQx exhibits a high EQE of 22.2% and negligible efficiency roll-off.

Theoretical perspective for substitution effect on luminescent properties of through space charge transfer-based thermally activated delayed fluorescence molecules

Recently, through space charge transfer (TSCT)-based thermally activated delayed fluorescence (TADF) molecules have shown advantages in achieving high efficiencies and tunable emissions. However, the relationships between basic molecular structures and luminescent properties are unclear. Theoretical investigations to reveal the substitution effects with different numbers and positions on excited-state properties are highly desired. Herein, by taking TSCT-based TADF molecules S-CNDF-S-tCz, S-CNDF-D-tCz and T-CNDF-T-tCz as skeletons, a series of promising TADF molecules are designed by adopting ortho, meta and para substitutions with different numbers and positions. Photophysical properties of total 16 molecules are theoretically studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods in chloroform combined with polarizable continuum model. Results indicate that molecules with ortho-substitution possess small geometric changes and short Donor-Acceptor distances which are induced by the intramolecular van der Waals interactions. Decreased non-radiative consumption and increased TSCT ratio and therefore excellent performance for them can be expected. For molecules with large substitution numbers, twist structures facilitate them to realize small adiabatic energy gaps between the lowest singlet excited state (S₁) and the lowest triplet excited state (T₁), this designing strategy is consistent with the TADF dendrimers. Thus, the relationships between molecular structures and luminescent properties are revealed and promising TSCT-based TADF molecules with high efficiencies are theoretically proposed. Our investigations provide theoretical perspectives for inner mechanisms of substitution effect, which could further afford meaningful guidance to design new efficient TSCT-based TADF molecules.

Precise Functionalization of a Multiple-Resonance Framework: Constructing Narrowband Organic Electroluminescent Materials with External Quantum Efficiency over 40

It is of important strategic significance to develop high-efficiency narrowband organic electroluminescent materials that can be employed to fabricate ultrahigh-definition displays with wide color gamut. This topic implies a great challenge to molecular design and synthesis, especially for the development of universality, diversity, scalability, and robustness of molecular architectonics. In this work, a synthetic methodology is demonstrated for functionalizing brominated BN-containing multiple-resonance (MR) frameworks with multifarious functional groups, such as donors, acceptors, and moieties without obvious push-pull electron properties. The m-DPAcP-BNCz-based organic light-emitting diode (OLED) exhibits green emission with a full-width at half-maximum (FWHM) of 28 nm and a maximum external quantum efficiency (EQE) of 40.6%. The outstanding performance of m-DPAcP-BNCz is attributed to the perfect integration of the inherent advantages of the MR framework and the donor-acceptor configuration, which can not only achieve bathochromic shift and narrowband emission, but also obtain high photoluminescence (PL) quantum yield (ΦPL ) and horizontal emitting dipole orientation ratio (Θ// ). This straightforward and efficient approach provides insightful guidance for the construction and enrichment of more high-efficiency narrowband emitters.

White Fluorescent Organic Light-Emitting Diodes with 100% Power Conversion

Energy-efficient lighting sources are desired to provide another solution of carbon emission reduction. White organic light-emitting diodes are promising, because of theoretical internal quantum efficiencies for 100% electric-to-light conversion. However, pure organic fluorescent materials still face a challenge in harvesting triplet excitons for radiation. Herein, we report a white fluorescent organic light-emitting diode having an external quantum efficiency of 30.7% and a power efficiency of 120.2 lm W-1. In the single emissive layers, we use blue thermally activated delayed fluorescent emitters to sensitize a yellow fluorescent emitter. Transient photoluminescence and electroluminescence analyses suggest that a blue thermally activated delayed fluorescent molecule with ~100% reverse intersystem crossing efficiency and negligible triplet nonradiative rate constant completely converts triplet to singlet, suppressing triplet quenching by a yellow fluorescent emitter and ensuring 100% power conversion.

Highly Effective Thermally Activated Delayed Fluorescence Emitters Based on Symmetry and Asymmetry Nicotinonitrile Derivatives

In this study, we developed two thermally activated delayed fluorescence (TADF) emitters, ICzCN and ICzCYP, to apply to organic light-emitting diodes (OLEDs). These emitters involve indolocarbazole (ICz) donor units and nicotinonitrile acceptor units with a twisted donor-acceptor-donor (D-A-D) structure for small singlet (S1) and triplet (T1) state energy gap (ΔEST) to enable efficient exciton transfer from the T1 to the S1 state. Depending on the position of the cyano-substituent, ICzCN has a symmetric structure by introducing donor units at the 3,5-position of isonicotinonitrile, and ICzCYP has an asymmetric structure by introducing donor units at the 2,6-position of nicotinonitrile. These emitters have different properties, such as the maximum luminance (Lmax) value. The Lmax of ICzCN reached over 10000 cd m-2. The external quantum efficiency (ηext) was 14.8% for ICzCN and 14.9% for ICzCYP, and both achieved a low turn-on voltage (Von) of less than 3.4 eV.

Synergetic Insulation and Induction Effects Selectively Optimize Multiresonance Thermally Activated Delayed Fluorescence

Multiresonance (MR) emitters featuring narrowband emissions and theoretically 100% exciton harvesting are great potential for organic light-emitting diode (OLED) applications. However, how to functionalize MR molecules without scarifying emission color purity is still a key challenge. Herein, we report a feasible strategy for selective optimization of MR molecules, which is demonstrated by a blue MR emitter tCBNDASPO substituted with a diphenylphosphine oxide (DPPO) group. Compared to its DPPO-free parent molecule, tCBNDASPO preserves narrowband feature with full widths at half maximum (FWHM) values of 28 nm in film and 32 nm in OLEDs and achieves 40% increased photoluminescence (92%) and electroluminescence quantum efficiencies (28%). It is showed that insulation effect of P=O effectively confines the singlet excited state on MR core to keep emission color purity, and its induction effect enhances singlet radiation and triplet-to-singlet conversion. This synergism for selective optimization is based on rational linkage between MR core and functional groups.

Ambipolar Self-Host Functionalization Accelerates Blue Multi-Resonance Thermally Activated Delayed Fluorescence with Internal Quantum Efficiency of 100

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-interaction-induced 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 × 10⁸ s^-1 and exponentially reduced nonradiative rate constants. Consequently, at the same time as preserving narrowband blue emission with an FWHM of ≈28 nm at a high doping concentration of 30%, tCBNDADPO reveals state-of-the-art photoluminescence and electroluminescence quantum efficiencies of 99% and 30%, respectively. The corresponding 100% internal quantum efficiency of tCBNDADPO supported by an ultrasimple trilayer and heavily doped device demonstrates the feasibility of the ambipolar self-host strategy for constructing practically applicable MR materials.

Resonance-Induced Stimuli-Responsive Capacity Modulation of Organic Ultralong Room Temperature Phosphorescence

Organic ultralong room temperature phosphorescence (OURTP) materials having stimuli-responsive attributes have attracted great attention due to their great potential in a wide variety of advanced applications. It is of fundamental importance but challengeable to develop stimuli-responsive OURTP materials, especially such materials with modulated optoelectronic properties in a controlled manner probably due to the lack of an authentic construction approach. Here, we propose an effective strategy for OURTP materials with controllably regulated stimuli-responsive properties by engineering the resonance linkage between flexible chain and phosphor units. A quantitative parameter to demonstrate the stimuli-responsive capacity is also established by the responsivity rate constant. The designed OURTP materials demonstrate efficient photoactivated OURTP with lifetimes up to 724 ms and tunable responsivity rate constants ranging from 0.132 to 0.308 min^-1 upon continuous UV irradiation. Moreover, the applications of stimuli-responsive resonance OURTP materials have been illustrated by the rewritable paper for snapshot and Morse code for multiple information encryption. Our works, which enable the accomplishment of OURTP materials capable of on-demand manipulated optical properties, demonstrate a viable design to explore smart OURTP materials, giving deep insights into the dynamically stimuli-responsive process.

Controllable Synthesis and Performance Modulation of 2D Covalent-Organic Frameworks

Covalent-organic frameworks (COFs) are especially interesting and unique as their highly ordered topological structures entirely built from plentiful π-conjugated units through covalent bonds. Arranging tailorable organic building blocks into periodically reticular skeleton bestows predictable lattices and various properties upon COFs in respect of topology diagrams, pore size, properties of channel wall interfaces, etc. Indeed, these peculiar features in terms of crystallinity, conjugation degree, and topology diagrams fundamentally decide the applications of COFs including heterogeneous catalysis, energy conversion, proton conduction, light emission, and optoelectronic devices. Additionally, this research field has attracted widespread attention and is of importance with a major breakthrough in recent year. However, this research field is running with the lack of summaries about tailorable construction of 2D COFs for targeted functionalities. This review first covers some crucial polymeric strategies of preparing COFs, containing boron ester condensation, amine-aldehyde condensation, Knoevenagel condensation, trimerization reaction, Suzuki CC coupling reaction, and hybrid polycondensation. Subsequently, a summary is made of some representative building blocks, and then underlines how the electronic and molecular structures of building blocks can strongly influence the functional performance of COFs. Finally, conclusion and perspectives on 2D COFs for further study are proposed.

Boosting external quantum efficiency to 38.6% of sky-blue delayed fluorescence molecules by optimizing horizontal dipole orientation

To achieve high electroluminescence efficiency, great efforts are devoted to enhancing photoluminescence quantum yield (Φ_PL) and exciton utilization of luminescent molecule, while another important factor, light out-coupling efficiency (η_out), receives less attention in molecule design. Here, we focus on horizontal dipole orientation engineering of the molecule to increase η_out and external quantum efficiency (η_ext). A series of tailor-made luminescent molecules consisting of an electron-accepting carbonyl core plus double electron-donating groups of spiro[acridine-9,9′-fluorene] and carbazole derivatives [e.g., 1,3,5-tri(carbazol-9-yl)benzene] are developed and systematically investigated. These molecules hold distinguished merits of strong sky-blue delayed fluorescence with excellent Φ_PL values, large horizontal dipole ratios, and balanced bipolar carrier transport, which furnish record-high η_ext values of up to 26.1 and 38.6% in nondoped and doped sky-blue organic light-emitting diodes (OLEDs), respectively. Moreover, the state-of-the-art nondoped hybrid white OLED and all-fluorescence single-emitting layer white OLED are also realized, demonstrating great potentials in OLED industry of these molecules.

Electroluminescent materials toward near ultraviolet region

Near ultraviolet (NUV) light-emitting materials and devices are significant due to unique applications in anti-counterfeit, manufacturing industries, and hygienic treatments. However, the development of high-efficiency NUV electroluminescent devices encounters great challenges and is far behind their RGB emitter counterparts. Besides the photoluminescence quantum yields (PLQYs) of NUV materials being higher than 40%, charge injection and lopsided carrier transport also determine the device performance, leading to great efforts in optimizing the frontier molecular orbitals to fit the adjacent function layer. In the exploration of NUV materials, organic molecules are one of the primary candidates, given their preparative facility and structural variability. Recently, all-inorganic quantum-dot light-emitting diodes (QLEDs) of Cd-based, ZnSe, graphene and inorganic perovskite emitters and organic-inorganic hybrid lead halide perovskite nanocrystals (NCs) were demonstrated for achieving NUV electroluminescence. Owing to the great efforts devoted to NUV material engineering and device configuration, NUV materials and devices have achieved great advances over the last two decades. In this review, we retrospect the development of NUV materials and devices covering all promising systems, which may inspire the enthusiasm of researchers to explore the huge potential in the NUV region.

High-Throughput Virtual Screening of Host Materials and Rational Device Engineering for Highly Efficient Solution-Processed Organic Light-Emitting Diodes

The appropriate choice of host and electron-transporting material (ETM) plays a very crucial role in the generation and collection of radiative excitons in the desired recombination zone of organic light-emitting diodes (OLEDs). Due to the sustainable development of material organic chemistry, there is a big library of functional materials that leads to uncountable combinations of device structures, which might achieve a desirable high device performance. However, there is no appropriate methodology available for the fast virtual screening of organic materials and designing a suitable device structure. Here, we have used the electrical software package SETFOS 4.5 for high-throughput virtual screening of host materials and developed a highly efficient multistack OLED device structure. To further enhance the device performance, a co-host approach has been used, and the final device structure has also been optimized with two different ETMs. The best-optimized Ir(ppy)₃-based solution-processed green OLED device exhibited a maximum power efficiency (PE) of 83.20 lm/W and brightness of 61,362 cd/m² with a driving voltage of 2.1 V without using any light extraction outcoupling techniques, which is the best among the OLEDs in its own category. The developed device structure has also been utilized to fabricate highly efficient blue hazard-free low-color temperature OLEDs for a physiologically friendly light at night. The resultant 2083 K OLED device displayed a maximum PE of 51.4 lm/W and luminance of 44,548 cd/m² with a turn-on voltage of 2.1 V that is also 42 and 104 times safer in terms of retinal protection and ∼4 and ∼11 times safer in terms of melatonin generation when compared with those of a real candle and incandescent bulb, respectively. The observed excellent device performance may be attributed to the balanced charge carrier in the recombination zone, broader emissive layer due to a mixed-host system, less accumulation of charges at the injecting surfaces, well-aligned triplet energy and molecular orbital energy level of the host and guest, and high electron mobility and enhanced hole blocking ability of the employed ETM in the designed OLED device structure.

Computational insight into newly anomalous delayed fluorescence emitters based on D-A-A structures

In variety of skeleton structures of delayed fluorescence molecular materials, the D-A-A type has been widely concerned recently for its improved double efficiency of reverse intersystem crossing process (RISC). Based on the D-A-A structure, eight new D-TRZ-nPO molecules (D = dihydrophenazine (DHPZ), phenothiazine (PTZ), phenoxazine (PXZ) and 9,9-dimethyl-9,10-dihydroacridan (DMAC), TRZ = triphenyltriazine, n = 1 or 2) with potential performance improvement have been deeply investigated by theoretical calculations. Interestingly, these molecules with the closing energy levels of high-lying excited states and charge transfer characters may perform rare high-lying excited state delayed fluorescence. Meanwhile, the changes of RISC and the corresponding effects caused by D-A-A structure from low energy level to high energy level are analyzed in detail. Furthermore, DHPZ-TRZ-2PO with blue emission (452 nm) is expected to be a potential high-lying excited state delayed fluorescence material candidate.

Triazolotriazine-based thermally activated delayed fluorescence materials for highly efficient fluorescent organic light-emitting diodes (TSF-OLEDs)

Thermally activated delayed fluorescence (TADF) sensitized fluorescent organic light-emitting diodes (TSF-OLEDs) have shown great potential for the realization of high efficiency with low efficiency roll-off and good color purity. However, the superior examples of TSF-OLEDs are still limited up to now. Herein, a trade-off strategy is presented for designing efficient TADF materials and achieving high-performance TSF-OLEDs via the construction of a new type of triazolotriazine (TAZTRZ) acceptor. The enhanced electron-withdrawing ability of TAZTRZ acceptor, fused by triazine (TRZ) and triazole (TAZ) together, enables TADF luminogens with small singlet-triplet energy gap (ΔE_ST) values. Meanwhile, the increased planarity from the TRZ-phenyl linkage (6:6 system) to the TAZ-phenyl linkage (5:6 system) can compensate the decrease of oscillator strength (f) while lowing ΔE_ST, thus achieving a trade-off between small ΔE_ST and high f. As a result, the related TSF-OLED achieved an extremely low turn-on voltage of 2.1 V, an outstanding maximum external quantum efficiency (EQE_max) of 23.7% with small efficiency roll-off (EQE₁₀₀₀ of 23.2%; EQE₅₀₀₀ of 20.6%) and an impressively high maximum power efficiency of 82.1 lm W^-1, which represents the state-of-the-art performance for yellow TSF-OLEDs.

Effects of Secondary Acceptors on Excited-State Properties of Sky-Blue Thermally Activated Delayed Fluorescence Molecules: Luminescence Mechanism and Molecular Design

The development of efficient sky-blue thermally activated delayed fluorescence (TADF) emitters is highly desired. However, the types and amounts of sky-blue TADF are far from meeting the requirements, and effective molecular design strategies are expected. Herein, the photophysical properties and excited-state dynamics of 12 molecules are theoretically studied based on the thermal vibration correlation function method. Distributions of holes and electrons are analyzed by the heat maps. The frontier molecular orbital distribution, adiabatic singlet-triplet energy gap, and reorganization energy are analyzed in detail. Furthermore, the radiative and non-radiative as well as the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes are studied, and the up-conversion process is illustrated. Our results indicate that different substitution positions and numbers play an important role in the luminescence properties of TADF molecules. The meta-position substitutions restrict the geometry variations, hinder the non-radiative energy consumption process, and promote the radiative process of TADF molecules. Meanwhile, molecules with ortho-position substitutions possess the smallest energy gaps (ΔE_st) and the largest RISC rates. Moreover, molecules with the substitutions of one tBCz group and two PO groups have the smallest ΔE_st and the largest spin orbital coupling. Thus, a wise molecular design strategy, namely, ortho-position substitutions as well as substitutions with one tBCz group and two PO groups, is proposed to facilitate the RISC process. Based on this rule, new efficient TADF molecules are theoretically designed and proposed. Our work reasonably elucidates the experimental measurements, and the effects of different substitution numbers and positions of secondary acceptors on TADF properties are highlighted, which could provide a theoretical perspective for designing efficient sky-blue TADF molecules.

Rational Design of π-Conjugated Tricoordinated Organoboron Derivatives With Thermally Activated Delayed Fluorescent Properties for Application in Organic Light-Emitting Diodes

A series of donor–acceptor (D–A) tricoordinated organoboron derivatives (1–10) have been systematically investigated for thermally activated delayed fluorescent (TADF)-based organic light-emitting diode (OLED) materials. The calculated results show that the designed molecules exhibit small singlet-triplet energy gap (ΔE_ST) values. Density functional theory (DFT) analysis indicated that the designed molecules display an efficient separation between donor and acceptor fragments because of a small overlap between donor and acceptor fragments on HOMOs and LUMOs. Furthermore, the delayed fluorescence emission color can be tuned effectively by introduction of different polycyclic aromatic fragments in parent molecule 1. The calculated results show that molecules 2, 3, and 4 possess more significant Stokes shifts and red emission with small ΔE_ST values. Nevertheless, other molecules exhibit green (1, 7, and 8), light green (6 and 10), and blue (5 and 9) emissions. Meanwhile, they are potential ambipolar charge transport materials except that 4 and 10 can serve as electron and hole transport materials only, respectively. Therefore, we proposed a rational way for the design of efficient TADF materials as well as charge transport materials for OLEDs simultaneously.

Rational Molecular Design of Highly Efficient Yellow-Red Thermally Activated Delayed Fluorescent Emitters: A Combined Effect of Auxiliary Fluorine and Rigidified Acceptor Unit

Molecular design strategies are crucial to develop highly efficient and long-wavelength thermally activated delayed fluorescent (TADF) emitters because the inherent limitation of the energy gap law degrades the efficiency of the red or orange TADF emitters. To resolve the low efficiency issue, we designed and synthesized two TADF emitters, 4,4'-(6-(9,9-dimethylacridin-10(9H)-yl)-7-fluoroquinoxaline-2,3-diyl)dibenzonitrile (FDQCNAc) and 11-(9,9-dimethylacridin-10(9H)-yl)-12-fluorodibenzo[a,c]phenazine-3,6-dicarbonitrile (FBPCNAc), by utilizing fluorine and peripheral cyano-substituted quinoxaline and phenazine acceptors of 4,4'-(6-fluoroquinoxaline-2,3-diyl)dibenzonitrile (FDQCN) and 11-fluorodibenzo[a,c]phenazine-3,6-dicarbonitrile (FBPCN), respectively. A fluorine atom at the ortho position of the acridine donor acts as an auxiliary acceptor to minimize the singlet-triplet energy gap (ΔE_ST) below 0.1 eV and promotes the reverse intersystem crossing (RISC) process. Organic light-emitting diodes (OLEDs) fabricated with FDQCNAc and FBPCNAc emitters demonstrated high external quantum efficiencies (EQEs) of 27.6 and 23.8% in the yellow-red TADF OLEDs, respectively. In particular, the combination of the F auxiliary acceptor unit and the rigidified FBPCN acceptor unit enabled red-shifted emission by about 58 nm without much sacrifice of the EQE in the red region.

Highly Efficient and Color-Stable Thermally Activated Delayed Fluorescence White Light-Emitting Diodes Featured with Single-Doped Single Emissive Layers

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 (ptBCzPO₂ TPTZ) 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.

Utilization of Multi-Heterodonors in Thermally Activated Delayed Fluorescence Molecules and Their High Performance Bluish-Green Organic Light-Emitting Diodes

We report a series of pentacarbazolyl-benzonitrile derivatives such as 2,4,6-tri(9H-carbazol-9-yl)-3,5-bis(3,6-di(pyridin-3-yl)-9H-carbazol-9-yl)benzonitrile (mPyBN), 3,5-bis(3,6-bis(4-(trifluoromethyl)phenyl)-9H-carbazol-9-yl)-2,4,6-tri(9H-carbazol-9-yl)benzonitrile (pCF3BN), 2,4,6-tri(9H-carbazol-9-yl)-3-(3,6-di(pyridin-3-yl)-9H-carbazol-9-yl)-5-(3,6-diphenyl-9H-carbazol-9-yl)benzonitrile (PyPhBN), 3-(3,6-bis(4-(trifluoromethyl)phenyl)-9H-carbazol-9-yl)-2,4,6-tri(9H-carbazol-9-yl)-5-(3,6-di(pyridin-3-yl)-9H-carbazol-9-yl)benzonitrile (PyCF3BN), and 3-(3,6-bis(4-(trifluoromethyl)phenyl)-9H-carbazol-9-yl)-2,6-di(9H-carbazol-9-yl)-5-(3,6-di(pyridin-3-yl)-9H-carbazol-9-yl)-4-(9H-pyrido[3,4-b]indol-9-yl)benzonitrile (CbPyCF3BN) in which some of the carbazoles are substituted with modified 3,5-diphenyl carbazoles, exhibiting thermally activated delayed fluorescence (TADF) properties. These emitters comprised two, three, and four different types of donors, capable of bluish-green emission of around 480 nm with relatively high photoluminescence quantum yields over 90% in solution. Emitters, namely, PyPhBN, PyCF3BN, and CbPyCF3BN, composed of three and four different types of donors endowed a rather short delayed lifetime (τ_d) of 4.25, 5.01, and 3.65 μs in their film state, respectively. Bluish-green organic light-emitting diodes based on PyPhBN, PyCF3BN, and CbPyCF3BN exhibit a high external quantum efficiency of 20.6, 19.5, and 19.6%, respectively, with unsurpassed efficiency roll-off behavior. These results indicate that the TADF properties of multidonor type molecules can be manipulated by controlling the types and number of electron donor units.

Asymmetric Thermally Activated Delayed Fluorescence Materials With Aggregation-Induced Emission for High-Efficiency Organic Light-Emitting Diodes

The exploitation of thermally activated delayed fluorescence (TADF) emitters with aggregation-induced emission is highly prerequisite for the construction of highly efficient electroluminescent devices in materials science. Herein, two asymmetric TADF emitters of SFCOCz and SFCODPAC with charming aggregation-induced emission are expediently designed and prepared based on highly twisted strong electron-withdrawing acceptor (A) of sulfurafluorene (SF)-modified ketone (CO) and arylamine donor (D) in D1-A-D2 architecture by simple synthetic procedure in high yields. High photoluminescence quantum yields up to 73% and small singlet-triplet splitting of 0.03 eV; short exciton lifetimes are obtained in the resultant molecules. Strikingly, efficient non-doped and doped TADF organic light-emitting diodes (OLEDs) facilitated by these emitters show high luminance of 5,598 and 11,595 cd m-2, current efficiencies (CEs) of 16.8 and 35.6 cd/A, power efficiencies (PEs) of 9.1 and 29.8 lm/W, and external quantum efficiencies (EQEs) of 7.5 and 15.9%, respectively. This work furnishes a concrete instance in exploring efficient TADF emitter, which is highly conducive and encouraging in stimulating the development of TADF OLEDs with high brightness and excellent efficiencies simultaneously.

Achieving Enhanced Thermally Activated Delayed Fluorescence Rates and Shortened Exciton Lifetimes by Constructing Intramolecular Hydrogen Bonding Channels

A fast radiative rate, highly suppressed nonradiation, and a short exciton lifetime are key elements for achieving efficient thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) with reduced efficiency roll-off at a high current density. Herein, four representative TADF emitters are designed and synthesized based on the combination of benzophenone (BP) or 3-benzoylpyridine (BPy3) acceptors, with dendritic 3,3″,6,6″-tetra-tert-butyl-9'H-9,3':6',9″-tercarbazole (CDTC) or 10H-spiro(acridine-9,9'-thioxanthene) (TXDMAc) donors, respectively. Density functional theory simulation and X-ray diffraction analysis validated the formation of CH···N intramolecular hydrogen bonds regarding the BPy3-CDTC and BPy3-TXDMAc compounds. Notably, the construction of intramolecular hydrogen bonding within TADF emitters significantly enhances the intramolecular charge transfer (ICT) strength while reducing the donor-acceptor (D-A) dihedral angle, resulting in accelerated radiative and suppressed nonradiative processes. With short TADF exciton lifetimes (τ_TADF) and high photoluminescence quantum yields (ϕ_PL), OLEDs employing BPy3-CDTC and BPy3-TXDMAc dopants realized maximum external quantum efficiencies (EQEs) up to 18.9 and 25.6%, respectively. Moreover, the nondoped device based on BPy3-TXDMAc exhibited a maximum EQE of 18.7%, accompanied by an extremely small efficiency loss of only 4.1% at the luminance of 1000 cd m^-2. In particular, the operational lifetime of the sky-blue BPy3-CDTC-based device was greatly extended by 10 times in contrast to the BP-CDTC-based counterpart, verifying the idea that the in-built intramolecular hydrogen bonding strategy was promising for the realization of efficient and stable TADF-OLEDs.

Achieving Efficient Blue Delayed Electrofluorescence by Shielding Acceptors with Carbazole Units

The design and synthesis of blue thermally activated delayed fluorescence (TADF) emitters that have high electroluminescence efficiency and low efficiency roll-off features remain a great challenge. Herein, we developed a facile and efficient strategy by shielding acceptors with carbazole units for constructing high-performance blue TADF emitters. Benzonitrile (BN), 9,9-diphenylacridan (DPAc), and carbazole (Cz) were adopted as the acceptor, donor, and protector, respectively, to build two TADF emitters named DPAc-DCzBN and DPAc-DtCzBN. The nondoped organic light-emitting diodes (OLEDs) of DPAc-DCzBN as the emitter exhibited a standard sky-blue emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.26), high external quantum efficiency (EQE) of 20.0%, and low efficiency roll-off (EQEs of 19.5, 16.1, and 12.6% at 100, 500, and 1000 cd m^-2, respectively), which is an outstanding nondoped blue TADF OLED. The doped device of DPAc-DtCzBN displayed a pure blue emission and the corresponding CIE coordinates are (0.16, 0.15). Meanwhile, it also demonstrated high and stabilized EQE values of 23.1, 18.3, and 11.5% at maxima, 100 and 500 cd m^-2, respectively, which is a quite high level among the pure blue TADF OLEDs. This study testifies the feasibility of our strategy in constructing high-performance TADF electroluminescent materials.

Tricomponent Exciplex Emitter Realizing over 20% External Quantum Efficiency in Organic Light-Emitting Diode with Multiple Reverse Intersystem Crossing Channels

With the naturally separated frontier molecular orbitals, exciplexes are capable of thermally activated delayed fluorescence emitters for organic light-emitting diodes (OLEDs). And, the current key issue for exciplex emitters is improving their exciton utilization. In this work, a strategy of building exciplex emitters with three components is proposed to realize multiple reverse intersystem crossing (RISC) channels, improving their exciton utilization by enhancing upconversion of nonradiative triplet excitons. Accordingly, a tricomponent exciplex DBT-SADF:PO-T2T:CDBP is constructed with three RISC channels respectively on DBT-SADF, DBT-SADF:PO-T2T, and CDBP:PO-T2T. Furthermore, its photoluminescence quantum yield and rate constant of the RISC process are successfully improved. In the OLED, DBT-SADF:PO-T2T:CDBP exhibits a remarkably high maximum external quantum efficiency (EQE) of 20.5%, which is the first report with an EQE over 20% for the OLEDs based on exciplex emitters to the best of our knowledge. This work not only demonstrates that introducing multiple RISC channels can effectively improve the exciton utilization of exciplex emitters, but also proves the superiority of the tricomponent exciplex strategy for further development of exciplex emitters.

Solution-Processed Highly Efficient Bluish-Green Thermally Activated Delayed Fluorescence Emitter Bearing an Asymmetric Oxadiazole-Difluoroboron Double Acceptor

Difluoroboron (BF₂)-containing dyes have attracted great interest owing to their exceptionally high luminescence efficiency and good electron-withdrawing properties. However, only a few reports on difluoroboron-based thermally activated delayed fluorescence (TADF) have been addressed. In this contribution, a novel BF₂-containing TADF molecule of BFOXD, which contains two acceptor fragments of oxadiazole (OXD) and BF₂ and one donor unit of 9,9-dimethylacridine, was synthesized and characterized. For comparison, the precursor of OHOXD bearing one acceptor unit was also investigated. Both molecules clearly show TADF characteristics with sky-blue emission in solution and film state. Additionally, OHOXD undergoes excited-state intramolecular proton transfer-coupled intramolecular charge transfer processes. Using 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) as the host, the organic light-emitting diodes fabricated via a solution process show maximum external quantum efficiency (EQE) of 2.98 and 13.8% for OHOXD- and BFOXD-based devices, respectively. While the bipolar TADF host of 10-(4-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (CzAcSF) is utilized instead of CzSi, the OHOXD- and BFOXD-based devices exhibit better performances with the maximum EQEs of 12.1 and 20.1%, respectively, which render the most efficient and the bluest emission ever reported for the BF₂-based TADF molecules. This research demonstrates that introduction of one more acceptor unit into the TADF molecule could have a positive effect on emission efficiency, which opens a new way to design high-efficiency TADF molecules.

High-efficiency blue thermally activated delayed fluorescence from donor-acceptor-donor systems via the through-space conjugation effect

The photophysical optimization of donor (D)-acceptor (A) molecules is a real challenge because of the intrinsic limitation of their charger transfer (CT) excited states. Herein, two D-A-D molecules featuring blue thermally activated delayed fluorescence (TADF) are developed, in which a homoconjugated acceptor 5,10-diphenyl-5,10-dihydrophosphanthrene oxide (DPDPO2A) is incorporated to bridge four carbazolyl or 3,6-di-t-butyl-carbazolyl groups for D-A interaction optimization without immoderate conjugation extension. It is shown that the through-space conjugation effect of DPDPO2A can efficiently enhance intramolecular CT (ICT) and simultaneously facilitate the uniform dispersion of the frontier molecular orbitals (FMO), which remarkably reduces the singlet-triplet splitting energy (ΔE ST) and increases FMO overlaps for radiation facilitation, resulting in the 4-6 fold increased rate constants of reverse intersystem crossing (RISC) and singlet radiation. The maximum external quantum efficiency beyond 20% and the state-of-the-art efficiency stability from sky-blue TADF OLEDs demonstrate the effectiveness of the "conjugation modulation" strategy for developing high-performance optoelectronic D-A systems.

Enhancing Spin-Orbit Coupling by Introducing a Lone Pair Electron with p Orbital Character in a Thermally Activated Delayed Fluorescence Emitter: Photophysics and Devices

Reverse intersystem crossing (RISC) is the rate-limited step for the radiative process of thermally activated delayed fluorescence (TADF) materials, which is important to achieve ideal photoluminescence and electroluminescence efficiency. Herein, we propose a new strategy of introducing a lone pair (n) electron with p orbital character to enhance spin-orbit coupling (SOC) for promoting the RISC process. A proof-of-concept TADF molecule with p orbital lone pairs, namely, MoCz-PCN, was developed, and three counterparts without any p lone pairs, namely, DMAc-PCN, DPAc-PCN, and SpiroAc-PCN, were constructed for comparison. The experimental data revealed that MoCz-PCN exhibits a ca. 1.9 times higher RISC rate than the counterparts, which can be ascribed to enhanced SOC. Moreover, a significant increase in external quantum efficiency is observed in the MoCz-PCN-based OLED device. These findings provide a feasible strategy to develop highly efficient TADF emitters by introducing a lone pair (n) electron with p orbital character.

Triggering Thermally Activated Delayed Fluorescence by Managing the Heteroatom in Donor Scaffolds: Intriguing Photophysical and Electroluminescence Properties

Establishment of the structure-property relationships of thermally activated delayed fluorescence (TADF) materials has become a significant quest for the scientific community. Herein, two new donors, 10H-benzofuro[3,2-b]indole (BFI) and 10H-benzo[4,5]thieno[3,2-b]indole (BTI), have been developed and integrated with a aryltriazine acceptor to design the green TADF emitters benzofuro[3,2-b]indol-10-yl)-5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzonitrile (BFICNTrz) and 2-(10H-benzo[4,5]thieno[3,2-b]indol-10-yl)-5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzonitrile (BTICNTrz), respectively. The physicochemical and electroluminescence properties of the compounds were tuned by exchanging the heteroatom in the donor scaffold. Intriguingly, the electronegativity of the heteroatom and the ionization potential of the donor unit played vital roles in control of the singlet-triplet energy splitting and TADF mechanism of the compounds. Both compounds showed similar singlet excited states that originated from the charge transfer (CT) states (¹ CT), whereas the triplet excited states were tuned by the heteroatom in the donor unit. The origin of phosphorescence in the BTICNTrz emitter was CT emission from the triplet state (³ CT), whereas that in the BFICNTrz emitter stemmed from the local triplet excited state (³ LE). Consequently, BTICNTrz showed a small singlet-triplet energy splitting of 0.08 eV, compared with 0.26 eV for BFICNTrz. Thus, BTICNTrz showed efficient delayed fluorescence with a high quantum yield and a short delayed exciton lifetime, whereas BFICNTrz displayed weak delayed fluorescence with a relatively long lifetime. Furthermore, a BTICNTrz-based device exhibited a maximum external quantum efficiency (EQE) of 15.2 % and reduced efficiency roll-off (12 %) compared with its BFICNTrz-based counterpart, which showed a maximum EQE of 6.4 % and severe efficiency roll-off (55 %) at a practical brightness range of 1000 cd m^-2 . These results demonstrate that the choice of subunit plays a vital role in the design of efficient TADF emitters.

Thermally Activated Delayed Fluorescence Carbonyl Derivatives for Organic Light-Emitting Diodes with Extremely Narrow Full Width at Half-Maximum

Two novel thermally activated delayed fluorescence (TADF) emitters, 3-phenylquinolino[3,2,1- de]acridine-5,9-dione (3-PhQAD) and 7-phenylquinolino[3,2,1- de]acridine-5,9-dione (7-PhQAD), were designed and synthesized based on a rigid quinolino[3,2,1- de]acridine-5,9-dione (QAD) framework. With the effective superimposed resonance effect from electron-deficient carbonyls and electron-rich nitrogen atom, both emitters realize significant TADF characteristics with small Δ E_STs of 0.18 and 0.19 eV, respectively. And, molecular relaxations were dramatically suppressed for both emitters because of their conjugated structure. In the devices, 3-PhQAD realizes superior performance with a maximum external quantum efficiency (EQE) of 19.1% and a narrow full width at half-maximum (FWHM) of 44 nm, whereas a maximum EQE of 18.7% and an extremely narrow FWHM of 34 nm are realized for 7-PhQAD. These superior results reveal that apart from nitrogen and boron-aromatic systems, QAD framework can also act as a TADF matrix with effective resonance effect, and QAD derivatives are ideal candidates to develop TADF emitters with narrow FWHMs for practical applications.

Thermally Activated Delayed Fluorescence Materials: Towards Realization of High Efficiency through Strategic Small Molecular Design

Thermally activated delayed fluorescence (TADF) is one of the most intriguing and promising discoveries towards realization of highly-efficient organic light emitting diodes (OLED) utilizing small molecules as emitters. It has the capability of manifesting all excitons generated during the electroluminescent processes, consequently achieving 100 % of internal quantum efficiency. Since the report of the first efficient OLED based on a TADF small molecule in 2012 by Adachi et al., the quest for optimal TADF materials for OLED application has never stopped. Various TADF molecules bearing different design concepts and strategies have been designed and produced, with the aim to boost the overall performances of corresponding OLEDs. In this minireview, the general principles of TADF molecular design based on three basic categories of TADF species: twisted intramolecular charge transfer (TICT), through-space charge transfer (TSCT) and multi-resonance induced TADF (MR-TADF) are discussed in detail. Several key aspects with respect to each category, as well as some effective methods to enhance the efficiency of TADF materials and corresponding OLEDs from the molecular engineering perspectives, are summarized and discussed to exhibit a general landscape of TADF molecular design to a wide variety of scientific researchers within this particular disciplinary area.

Alleviating Efficiency Roll-Off of Hybrid Single-Emitting Layer WOLED Utilizing Bipolar TADF Material as Host and Emitter

Hybrid single-emitting layer (SEML) white organic light-emitting diodes (WOLEDs) incorporating blue thermally activated delayed fluorescent (TADF) or fluorescent materials and yellow phosphors have been widely utilized for solid-state lighting. Nonetheless, developing appropriate host materials to reduce the large efficiency roll-off at high luminance is still an unsolved issue. Here, two TADF materials denoted as TRZ-CF and TRZ-CzF were synthesized, with electroluminescent emission peaking at 476 and 460 nm, respectively. In particular, TRZ-CF, using 7,7-dimethyl-5,7-dihydroindeno[2,1- b]carbazole (CF) as donor moiety, maintained both highly efficient blue emission (EQE_max = 20.0%) and excellent charge transport abilities. The WOLED utilizing TRZ-CF as host material, doped by 0.8 wt % iridium(III) bis(4-phenylthieno[3,2- c]pyridinato- N, C2') (PO-01), has EQE_max of 20.3%, realizing the lowest roll-off to date of less than 2% at a luminance of 10 000 cd/m². The efficiency roll-off is alleviated through the reduction the exciton quenching and triplet-triplet annihilation (TTA) within the light-emitting layer, benefited from the TADF effect and bipolar property. The hybrid SEML WOLED exhibits Commission Internationale de L'Eclairage (CIE) coordinates of (0.38, 0.45), providing a practical way to simplify the production complexity and to reduce efficiency roll-off for solid-state lighting.