The vast market demands for applications of organic light‐emitting diodes (OLEDs) have quickened the pace of the search for future high‐performance materials, emphasizing the importance of exploring ...blue light‐emitting materials, which determine the performance bottleneck of OLEDs. Moreover, actualizing highly efficient, pure‐blue, stable, and purely organic electroluminescence will pave the way toward the realization of cost‐effective, high‐quality, and long‐lasting commercialized OLED displays and illumination applications. Without the aid of noble heavy metal atoms, the newly emerging thermally activated delayed fluorescent (TADF) materials can effectively utilize triplet excitons owing to the small singlet–triplet splitting energy (ΔEST) for rapid reverse intersystem crossing (RISC) process, leading to the achievement of 100% internal quantum efficiency under electrical operation. Nevertheless, fundamental scientific challenges with respect to simultaneously achieving stable pure‐blue emission, large radiative recombination rates with short exciton lifetimes and small ΔEST continue to hinder the popularization of blue TADF materials. A review of the current state of blue TADF emitters is timely and underscores the key challenges that must be overcome toward the development of a stable, true‐blue TADF‐based electroluminescent application in the future.
The realization of future purely organic, highly efficient, pure‐blue, and stable thermally activated delayed fluorescent organic light‐emitting diodes is ongoing!
Purely organic emitters that can efficiently utilize triplet excitons are highly desired to cut the cost of organic light‐emitting diodes (OLEDs), but most of them require complicated doping ...techniques for their fabrication and suffer from severe efficiency roll‐off. Herein, we developed novel luminogens with weak emission and negligible delayed fluorescence in solution but strong emission with prominent delayed components upon aggregate formation, giving rise to aggregation‐induced delayed fluorescence (AIDF). The concentration‐caused emission quenching and exciton annihilation are well‐suppressed, which leads to high emission efficiencies and efficient exciton utilization in neat films. Their nondoped OLEDs provide excellent electroluminescence efficiencies of 59.1 cd A−1, 65.7 lm W−1, and 18.4 %, and a negligible current efficiency roll‐off of 1.2 % at 1000 cd m−2. Exploring AIDF luminogens for the construction of nondoped OLEDs could be a promising strategy to advance device efficiency and stability.
Neat films of luminogens with aggregation‐induced delayed fluorescence (AIDF) were employed in nondoped OLEDs. These systems afford remarkable current, power, and external quantum efficiencies as the concentration‐caused emission quenching and exciton annihilation are well suppressed, which leads to high emission efficiencies and efficient exciton utilization in the neat films.
Quasi‐2D perovskites have long been considered to have favorable “energy funnel/cascade” structures and excellent optical properties compared with their 3D counterparts. However, most quasi‐2D ...perovskite light‐emitting diodes (PeLEDs) exhibit high external quantum efficiency (EQE) but unsatisfactory operating stability due to Auger recombination induced by high current density. Herein, a synergetic dual‐additive strategy is adopted to prepare perovskite films with low defect density and high environmental stability by using 18‐crown‐6 and poly(ethylene glycol) methyl ether acrylate (MPEG‐MAA) as the additives. The dual additives containing COC bonds can not only effectively reduce the perovskite defects but also destroy the self‐aggregation of organic ligands, inducing the formation of perovskite nanocrystals with quasi‐core/shell structure. After thermal annealing, the MPEG‐MAA with its CC bond can be polymerized to obtain a comb‐like polymer, further protecting the passivated perovskite nanocrystals against water and oxygen. Finally, state‐of‐the‐art green PeLEDs with a normal EQE of 25.2% and a maximum EQE of 28.1% are achieved, and the operating lifetime (T50) of the device in air environment is over ten times increased, providing a novel and effective strategy to make high efficiency and long operating lifetime PeLEDs.
A dual‐additive strategy is developed to prepare quasi‐core/shell‐structure perovskite nanocrystals by using 18‐crown‐6 and poly(ethylene glycol) methyl ether acrylate as the additives. State‐of‐the‐art external quantum efficiency of 28.1% and increased operating lifetime are achieved for the light‐emitting diodes, owing to the synergetic effect for reduced defect density and improved environmental stability of the perovskite emissive layer.
Multiple resonance thermally activated delayed fluorescence (MR‐TADF) compounds have set off an upsurge of research because of their tremendous application prospects in the field of wide color gamut ...display. Herein, we propose a novel MR‐TADF molecular construction paradigm based on polycyclization of the multiple resonance parent core, and construct a representative multiple resonance polycyclic aromatic hydrocarbon (MR‐PAH) based on the para‐alignment of boron and nitrogen atoms into a six‐membered ring (p‐BNR). Through the retrosynthesis analysis, a concise synthesis strategy with wide applicability has been proposed, encompassing programmed sequential boron esterification, Suzuki coupling and Scholl oxidative coupling. The target model molecule BN‐TP shows green fluorescence with an emission peak at 523 nm and a narrow full‐width at half‐maximum (FWHM) of 34 nm. The organic light‐emitting diode (OLED) employing BN‐TP as an emitter exhibits ultrapure green emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.26, 0.70), and achieves a maximum external quantum efficiency (EQE) of 35.1 %.
A multiple resonance thermally activated delayed fluorescence (MR‐TADF) molecular construction paradigm based on polycyclization of the MR parent core has been proposed, and a representative MR‐PAH based on para‐alignment of boron and nitrogen atoms into a six‐membered ring (p‐BNR) has been constructed. The OLED employing the target model molecule BN‐TP as an emitter exhibits ultrapure green emission with CIE coordinates of (0.26, 0.70) and a maximum EQE of 35.1 %.
Purely organic electroluminescent materials, such as thermally activated delayed fluorescent (TADF) and triplet–triplet annihilation (TTA) materials, basically harness triplet excitons from the ...lowest triplet excited state (T1) to realize high efficiency. Here, a fluorescent material that can convert triplet excitons into singlet excitons from the high‐lying excited state (T2), referred to here as a “hot exciton” path, is reported. The energy levels of this compound are determined from the sensitization and nanosecond transient absorption spectroscopy measurements, i.e., small splitting energy between S1 and T2 and rather large T2–T1 energy gap, which are expected to impede the internal conversion (IC) from T2 to T1 and facilitate the reverse intersystem crossing from the high‐lying triplet state (hRISC). Through sensitizing the T2 state with ketones, the existence of the hRISC process with an ns‐scale delayed lifetime is confirmed. Benefiting from this fast triplet–singlet conversion, the nondoped device based on this “hot exciton” material reaches a maximum external quantum efficiency exceeding 10%, with a small efficiency roll‐off and CIE coordinates of (0.15, 0.13). These results reveal that the “hot exciton” path is a promising way to exploit high efficient, stable fluorescent emitters, especially for the pure‐blue and deep‐blue fluorescent organic light‐emitting devices.
A pure‐blue fluorescent organic light‐emitting device (OLED) based on phenanthroimidazole−anthracene derivative obtains a maximum external quantum efficiency of 10.5% with excellent stability. Experimental investigations reveal that the high efficiency is attributed to triplet exciton harvesting by reverse intersystem crossing from the high‐lying triplet state. The results demonstrates that “hot exciton” channels represent a promising way to construct high‐performance fluorescent OLEDs.
Two novel evaporation‐ and solution‐process‐feasible thermally activated delayed fluorescence emitters, green‐light‐emission ACRDSO2 and yellow‐light‐emission PXZDSO2, based on a brand‐new ...electron‐acceptor moiety thianthrene‐9,9′,10,10′‐tetraoxide, are developed for organic light‐emitting diodes. The solution‐processed devices, without any hole‐transport layer, exhibit competitive performance and reduced efficiency roll‐off compared with corresponding vacuum‐deposited devices.
In vector space model (VSM), text representation is the task of transforming the content of a textual document into a vector in the term space so that the document could be recognized and classified ...by a computer or a classifier. Different terms (i.e. words, phrases, or any other indexing units used to identify the contents of a text) have different importance in a text. The term weighting methods assign appropriate weights to the terms to improve the performance of text categorization. In this study, we investigate several widely-used unsupervised (traditional) and supervised term weighting methods on benchmark data collections in combination with SVM and kNN algorithms. In consideration of the distribution of relevant documents in the collection, we propose a new simple supervised term weighting method, i.e. tf.rf, to improve the terms' discriminating power for text categorization task. From the controlled experimental results, these supervised term weighting methods have mixed performance. Specifically, our proposed supervised term weighting method, tf.rf, has a consistently better performance than other term weighting methods while other supervised term weighting methods based on information theory or statistical metric perform the worst in all experiments. On the other hand, the popularly used tf.idf method has not shown a uniformly good performance in terms of different data sets.
Blue thermally activated delayed fluorescence (TADF) emitters that can simultaneously achieve high efficiency in doped and nondoped organic light‐emitting diodes (OLEDs) are rarely reported. Reported ...here is a strategy using a tri‐spiral donor for such versatile blue TADF emitters. Impressively, by simply extending the nonconjugated fragment and molecular length, aggregation‐caused emission quenching (ACQ) can be greatly alleviated to achieve as high as a 90 % horizontal orientation dipole ratio and external quantum efficiencies (EQEs) of up to 33.3 % in doped and 20.0 % in nondoped sky‐blue TADF‐OLEDs. More fascinatingly, a high‐efficiency purely organic white OLED with an outstanding EQE of up to 22.8 % was also achieved by employing TspiroS‐TRZ as a blue emitter and an assistant host. This compound is the first blue TADF emitter that can simultaneously achieve high electroluminescence (EL) efficiency in doped, nondoped sky‐blue, and white TADF‐OLEDs.
TADF, Tada! By using a tri‐spiral donor, the thermally activated delayed fluorescence (TADF) emitter TspiroS‐TRZ can achieve a 90 % horizontal orientation dipole ratio and greatly alleviate aggregation‐caused emission quenching (ACQ). The emitter demonstrates state‐of‐the‐art external quantum efficiencies (EQEs) of 33.3, 20.0, and 22.8 % in purely organic doped, nondoped sky‐blue, and white TADF‐OLEDs, respectively. HTAU=hole transporting adjusting unit, OLED=organic light‐emitting diode.
Ni‐promoted electrocatalytic biomass reforming has shown promising prospect in enabling high value‐added product synthesis. Here, we developed a novel hybrid catalyst with Ni nanosheet forests ...anchored on carbon paper. The hybrid catalyst exhibits high efficiency in electrooxidation of HMF to FDCA coupling with H2 production in high purity. The Ni nanosheets have small crystal grain sizes with abundant edges, which is able to deliver an efficient HMF oxidation to FDCA (selectivity >99 %) at low potential of 1.36 VRHE with high stability. The post‐reaction structure analysis reveals the Ni nanosheets would transfer electrons to carbon and readily turn into NiOx and Ni(OH)x during the reaction. DFT results suggest high valence Ni species would facilitate the chemical adsorption (activation) of HMF revealing the reaction pathway. This work emphasizes the importance of the precise control of Ni activity via atomic structure engineering.
Highly efficient and stable electro‐reforming of HMF into FDCA on a Ni nanosheet/carbon paper electrode is achieved at a potential of 1.36 VRHE with 99.7 % conversion. A precursor‐limiting electrodeposition method creates a unique Ni nanosheet forest structure with vastly different physical appearance and catalytic activity comparing with conventional Ni catalysts.
Purely organic compounds served as promising materials for organic electronics have intrigued extensive research focuses owing to their unique photophysical and electronic properties. In the field of ...organic light‐emitting diodes (OLEDs), the utilization of triplet excitons is of great significance for realizing high‐performance devices. In contrast to the traditional aromatic amine‐based counterparts, sulfur atom with a high‐level outer orbit simultaneously provides powerful electron‐donating ability and striking spin‐orbit coupling effect to facilitate the utilization of theoretically spin‐forbidden triplet excitons, demonstrating great prospect in constructing highly emissive purely organic emitters for OLED applications. Herein, we summarize the currently developed sulfur‐decorated nonaromatic amine‐based emitters exhibiting attractive photoelectronic characteristics, and gain insight into the understanding of molecular design and photophysical processes of these emitters, providing new perspectives for enriching the existing luminescent material systems and designing high‐performance emitters.
Possessed powerful electron‐donating ability and strong spin‐orbit coupling, sulfur atom is particularly attractive for boosting triplet exciton utilization in electroluminescence. Here, the sulfur‐decorated nonaromatic amine compounds served as thermally activated delayed fluorescence and room‐temperature phosphorescence emitters for highly efficient OLED applications are highlighted.
