High-efficiency white organic light-emitting diodes based on a blue thermally activated delayed fluorescent emitter combined with green and red fluorescent emitters

Cascade Radical Cyclization of Propargylamines for Functionalized 3-Arylthioquinoline Formation

A novel and efficient strategy for the direct synthesis of 3-arylthioquinoline derivatives via radical induced tandem cyclization of propargylamines with diaryl disulfides was developed. This protocol undergoes a cascade sulfuration/ cyclization/ oxidation/ aromatization pathway to afford the desired products in a broad substrate scope using readily available starting materials under mild conditions. Based on this strategy, we further modified 3-arylquinolines to obtain two novel deep blue fluorescent molecules, QLSCz and QLSTCz, with good optical properties through two-step synthesis by oxidation and electron donor modification.

Study of the molecular design and synthesis status of metal complexes as unimolecular luminescent materials for white light emission

White organic light-emitting diodes (WOLEDs) are promising light-emitting devices. A typical method for generating white light is to superimpose the three primary colours of light - red, green, and blue - or the two colours of light - blue and yellow. These colours are generated from each emitting material doped into the emission layers of the device. To achieve high-quality white light, the emission colours and intensities should be properly adjusted in the device. Apart from the superimposition of colours of light, white light can also be generated by doping with a single molecule that emits visible light in the wavelength range of 380-780 nm. In this review, we have listed some white-light-emitting complexes that are expected to drastically simplify the device fabrication process for OLEDs. We have shed light on these metal complexes and outlined the current status of their synthesis and device applications, looking toward promising future prospects.

A perspective on next-generation hyperfluorescent organic light-emitting diodes

Hyperfluorescence, also known as thermally activated delayed fluorescence (TADF) sensitized fluorescence, is known as a next-generation efficient and innovative process for high-performance organic light-emitting diodes (OLEDs). High external quantum efficiency (EQE) and good color purity are crucial parameters for display applications. Hyperfluorescent OLEDs (HF-OLEDs) take the lead in this respect as they utilize the advantages of both TADF emitters and fluorescent dopants, realizing high EQE with color saturation and long-term stability. Hyperfluorescence is mediated through Förster resonance energy transfer (FRET) from a TADF sensitizer to the final fluorescent emitter. However, competing loss mechanisms such as Dexter energy transfer (DET) of triplet excitons and direct charge trapping on the final emitter need to be mitigated in order to achieve fluorescence emission with high efficiency. Despite tremendous progress, appropriate guidelines and fine optimization are still required to address these loss channels and to improve the device operational lifetime. This perspective aims to provide an overview of the evolution of HF-OLEDs by reviewing both molecular and device design pathways for highly efficient narrowband devices covering all colors of the visible spectrum. Existing challenges and potential solutions, such as molecules with peripheral inert substitution, multi-resonant (MR) TADF emitters as final dopants, and exciplex-sensitized HF-OLEDs, are discussed. Furthermore, the operational device lifetime is reviewed in detail before concluding with suggestions for future device development.

Promising interlayer sensitization strategy for the construction of high-performance blue hyperfluorescence OLEDs

Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials are promising candidates for organic light-emitting diodes (OLEDs) with narrow electroluminescence (EL) spectra. Current researches focus on fabricating hyperfluorescence OLEDs to improve EL efficiencies of MR-TADF emitters by co-doping them with TADF sensitizers in a single host layer. However, in many cases, the polarity of the single host could be not suitable for both blue MR-TADF emitters and blue TADF sensitizers, resulting in broadened EL spectra in high-polar hosts or decreased EL efficiencies in low-polar hosts. Herein, we wish to report an efficient sensitization strategy for blue MR-TADF emitters by constructing an interlayer-sensitizing configuration, in which the blue TADF sensitizers and blue MR-TADF emitters are separated into two closely aligned host layers with high polarity and low polarity, respectively. Based on this strategy, efficient blue hyperfluorescence OLEDs are realized and verified by employing various TADF sensitizers and different MR-TADF emitters, furnishing outstanding external quantum efficiencies of up to 38.8% and narrow EL spectra. These results validate the feasibility and universality of this interlayer sensitization strategy, which provides an effective alternative to high-performance blue hyperfluorescence OLEDs.

White Light Emission via Dual Thermally Activated Delayed Fluorescence from a Single-Component Phenothiazines-Diphenyl Quinoline Conjugate

White light emission (WLE) via dual thermally activated delayed fluorescence (TADF) from a single-component-based organic system remains challenging as a result of the difficulty in design. Here, we introduce a conformational isomerization approach to achieve WLE from a twisted donor-acceptor (PTzQP1) that comprises two phenothiazines covalently attached to the 6,8-isomeric positions of 2,4-diphenyl quinoline via two C-N single bonds. Spectroscopic studies and quantum chemistry calculations revealed that PTzQP1 shows WLE via simultaneous blue TADF and orange TADF covering the visible range (420-800 nm) with a photoluminescence quantum yield of 45 ± 2% and Commission Internationale de l'Éclairage (CIE) coordinates of 0.30, 0.33. The dual TADF features with high rates of reverse intersystem crossing (kRISC1 = 1.38 × 107 ± 0.24 s-1 and kRISC2 = 5.04 × 106 ± 0.32 s-1) are realized as a result of the low singlet-triplet gaps (S1EQ-T1EQ = 0.04 eV and S1QA-T1QA = 0.05 eV) of the quasi-axial (QA) and quasi-equatorial (QE) conformers. This finding is expected to provide a new direction for designing high-energy-efficient WLE emitters.

Reproducible and High-Performance WOLEDs Based on Independent High-Efficiency Triplet Harvesting of Yellow Hot-Exciton ESIPT and Blue TADF Emitters

Hot exciton organic light-emitting diode (OLED) emitters can balance the high performance of a device and reduce efficiency roll-off by fast reverse intersystem crossing from high-lying triplets (hRISC). In this study, an excited-state intramolecular proton transfer (ESIPT) fluorophore of 2-(benzo[d]thiazol-2-yl)-4-(pyren-1-yl)phenol (PyHBT) with the typical characteristic properties of a hot exciton is developed. With high efficiency of utilization of the exciton (91%), its yellow OLED exhibited high external quantum efficiency (EQE) of 5.6%, current efficiency (CE) of 16.8 cd A-1 , and power efficiency (PE) of 17.3 lm W-1 . The performance of the yellow emissive "hot exciton" ESIPT fluorophores is among the highest recorded. Due to the large Stokes shift of the ESIPT emitter, non-energy-transferred high-performance white OLEDs (WOLEDs) are developed, which are reproducible and highly efficient. This is possible because of the independent harvesting of most of the triplets in both complementary-color emitters without the interference of energy transfer. The PyHBT-based WOLEDs exhibit a maximum EQE of 14.3% and CE of 41.1 cd A-1 , which facilitates the high-yield mass production of inexpensive WOLEDs.

High-efficiency all-fluorescent white organic light-emitting diode based on TADF material as a sensitizer

The use of TADF materials as both sensitizers and emitters is a promising route to achieve high-efficiency all-fluorescent white organic light-emitting diodes (WOLEDs). In this study, the thermally-activated delayed-fluorescent (TADF) material DMAC-TRZ (9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine) was selected as a sensitizer for the conventional fluorescent emitter DCJTB (4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran), which was co-doped in a wide bandgap host of DPEPO (bis[2-(diphenylphosphino)phenyl]ether oxide) to fabricate WOLEDs. For the emitting layer of DPEPO:DMAC-TRZ:DCJTB, the DPEPO host can dilute the exciton concentration formed on the DMAC-TRZ sensitizer, which benefits the suppression of exciton quenching. The effect of the doping concentration of DCJTB on the carrier recombination and energy transfer process was investigated. With an optimized doping concentration of DCJTB as 0.8%, highly efficient WOLED was achieved with a maximum external quantum efficiency (EQE), power efficiency (PE), and current efficiency (CE) of 11.05%, 20.83 lm W-1, and 28.83 cd A-1, respectively, corresponding to the Commission Internationale de I' Eclairage (CIE) coordinates of (0.45, 0.46). These superior performances can be ascribed to the fact that the hole-trapping effect of the emitter and Dexter energy transfer (DET) from sensitizer to emitter can be suppressed simultaneously by the extremely low doping concentration.

Förster resonance energy transfer involving the triplet state

Triplet harvesting is important for high-efficiency optoelectronics devices, time-resolved bioimaging, sensing, and anti-counterfeiting devices. Förster resonance energy transfer (FRET) from the donor (D) to the acceptor (A) is important to efficiently harvest the triplet excitons after a variety of excitations. However, general explanations of the key factors of FRET from the singlet state (FRET_S-S) via reverse intersystem crossing and FRET from the triplet state (FRET_T-S) have not been reported beyond spectral overlap between emission of the D and absorption of the A. This feature article gives an overview of FRET involving the triplet state. After discussing the contribution of the radiation yield from the state of the D considering spin-forbidden factors of FRET, a variety of schemes involving triplet states, such as FRET_S-Svia reverse intersystem crossing from the triplet state, dual FRET_S-S and FRET_T-S, and selective FRET_T-S, are introduced. Representative examples, including the chemical structure and FRET for triplet harvesting, are highlighted using emerging applications in optoelectronics and afterglow imaging. Finally, recent developments of using FRET involving triplet states for high-efficiency optoelectronic devices and time-resolved bioimaging are discussed. This article provides crucial information for controlling state-of-the-art properties using FRET involving the triplet state.

Thermally activated delayed fluorescent small molecule sensitized fluorescent polymers with reduced concentration-quenching for efficient electroluminescence

Thermally activated delayed fluorescence (TADF) small molecule bis-[3-(9,9-dimethyl-9,10-dihydroacridine)-phenyl]-sulfone (m-ACSO2) was used as a universal host to sensitize three conventional fluorescent polymers for maximizing the electroluminescent performance. The excitons were utilized via inter-molecular energy transfer and the non-radiative decays were successfully refrained in the condensed states. Therefore, the significant enhancement of the electroluminescent efficiencies was demonstrated. For instance, after doping poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) into m-ACSO2, the external quantum efficiency (EQE) was improved by a factor of 17.0 in the solution-processed organic light-emitting device (OLED), as compared with the device with neat F8BT. In terms of the other well-known fluorescent polymers, i.e., poly (para-phenylene vinylene) copolymer (Super Yellow, SY) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), their EQEs in the devices were respectively enhanced by 70% and 270%, compared with the reference devices based on the conventional host 1,3-di(9H-carbazol-9-yl) benzene (mCP). Besides the improved charge balance in the bipolar TADF host, these were partially ascribed to reduced fluorescence quenching in the mixed films.

Harvesting Triplet Excitons in High Mobility Emissive Organic Semiconductor for Efficiency Enhancement of Light-Emitting Transistors

Organic light-emitting transistors (OLETs), a kind of highly integrated and minimized optoelectronic device, demonstrate great potential applications in various fields. The construction of high-performance OLETs requires the integration of high charge carrier mobility, strong emission, and high triplet exciton utilization efficiency in the active layer. However, it remains a significant long-term challenge, especially for single component active layer OLETs. Herein, the successful harvesting of triplet excitons in a high mobility emissive molecule, 2,6-diphenylanthracene (DPA), through the triplet-triplet annihilation process is demonstrated. By incorporating a highly emissive guest into the DPA host system, an obvious increase in photoluminescence efficiency along with exciton utilization efficiency results in an obvious enhancement of external quantum efficiency of 7.2 times for OLETs compared to the non-doped devices. Moreover, well-tunable multi-color electroluminescence, especially white emission with Commission Internationale del'Eclairage  of (0.31, 0.35), from OLETs is also achieved by modulating the doping concentration with a controlled energy transfer process. This work opens a new avenue for integrating strong emission and efficient exciton utilization in high-mobility organic semiconductors for high-performance OLETs and advancing their related functional device applications.

Toward highly efficient hyperfluorescence-based emitters through excited-states alignment using novel optimally tuned range-separated models

Hyperfluorescence has recently been introduced as a promising strategy to achieve organic light-emitting diodes (OLEDs) with high color purity and enhanced stability. In this approach, fluorescent emitters (FEs) with strong and narrow band fluorescence are integrated in thin films containing sensitizers exhibiting thermally activated delayed fluorescence (TADF). Toward highly efficient hyperfluorescence-based emitters, the excited-states ordering of the FEs should be well-aligned. Given some recent endeavors in this context, the related theoretical explorations are relatively limited and have proven to be challenging. In this work, alignments of the corresponding excited-states, crucial for both the fast Förster resonance energy transfer and suppression of the Dexter energy transfer from TADF sensitizers to FEs, have theoretically been investigated using optimally tuned range-separated hybrid functionals (OT-RSHs). We have proposed and validated several variants of the models including OT-RSHs, their coupled versions with the polarizable continuum model, OT-RSHs-PCM, as well as the screened versions accounting for the screening effects by the electron correlation through the scalar dielectric constant, OT-SRSHs, for a reliable description of the excited-states ordering in the FEs of the hyperfluorescence-based materials. Particular attention is paid to the influence of the underlying density functional approximations as well as the short- and long-range Hartree-Fock (HF) exchange contributions and the range-separation parameter. Considering a series of experimentally known hyperfluorescence-based emitters as working models, it is unveiled that any combination of the ingredients in the proposed models does not render the correct order of the excited-states of the FEs, but a particular compromise among the involved parameters is needed to more accurately account for the relevant excited-states alignment. Perusing the results of our developed methods, the best ones are found to be the generalized gradient approximation-based OT-RSHs-PCM with the correct asymptotic behavior and incorporating no (low) HF exchange contribution at the short-range regime. The proposed models show superior performances not only with respect to their standard counterparts with the default parameters but also as compared to other range-separated approximations. Accountability of the best-proposed model is also put into broader perspective, where it has been employed for the computational design of several molecules as promising FE candidates prone to be utilized in hyperfluorescence-based materials. Summing up, the proposed models in this study can be recommended for both the theoretical modeling and confirming the experimental observations in the field of hyperfluorescence-based OLEDs.

All-fluorescence white organic light-emitting diodes with record-beating power efficiencies over 130 lm W‒1 and small roll-offs

Improving power efficiency (PE) and reducing roll-off are of significant importance for the commercialization of white organic light-emitting diodes (WOLEDs) in consideration of energy conservation. Herein, record-beating PE of 130.7 lm W⁻¹ and outstanding external quantum efficiency (EQE) of 31.1% are achieved in all-fluorescence two-color WOLEDs based on a simple sandwich configuration of emitting layer consisting of sky-blue and orange delayed fluorescence materials. By introducing a red fluorescence dopant, all-fluorescence three-color WOLEDs with high color rendering index are constructed based on an interlayer sensitization configuration, furnishing ultrahigh PE of 110.7 lm W⁻¹ and EQE of 30.8%. More importantly, both two-color and three-color WOLEDs maintain excellent PEs at operating luminance with smaller roll-offs than the reported state-of-the-art WOLEDs, and further device optimization realizes outstanding comprehensive performances of low driving voltages, large luminance, high PEs and long operational lifetimes. The underlying mechanisms of the impressive device performances are elucidated by host-tuning effect and electron-trapping effect, providing useful guidance for the development of energy-conserving all-fluorescence WOLEDs.

High power efficiency and low roll-off values are essential to the commercialization of white organic light-emitting diodes. Here, the authors construct all-fluorescence devices with an orange emitting layer sandwiched between two sky-blue emitting layers, achieving figure-of-merit of 130.7 lm/W.

Color-Tunable Supramolecular Luminescent Materials

Constructing multicolor photoluminescent materials with tunable properties is an attractive research objective on account of their abundant applications in materials science and biomedical engineering. By comparison with covalent synthesis, supramolecular chemistry has provided a more competitive and promising strategy for the production of organic materials and the regulation of their photophysical properties. By taking advantage of dynamic and reversible noncovalent bonding interactions, supramolecular strategies can, not only simplify the design and fabrication of organic materials, but can also endow them with dynamic reversibility and stimuli responsiveness, making it much easier to adjust the superstructures and properties of the materials. Occasionally, it is possible to introduce emergent properties into these materials, which are absent in their precursor compounds, broadening their potential applications. In an attempt to highlight the state-of-the-art noncovalent strategies available for the construction of smart luminescent materials, an overview of color-tunable materials is presented in this Review, with the emphasis being placed on the examples drawn from host-guest complexes, supramolecular assemblies and crystalline materials. The noncovalent synthesis of room-temperature phosphorescent materials and the modulation of their luminescent properties are also described. Finally, future directions and scientific challenges in the emergent field of color-tunable supramolecular emissive materials are discussed.

Exciplex-Forming Systems of Physically Mixed and Covalently Bonded Benzoyl-1H-1,2,3-Triazole and Carbazole Moieties for Solution-Processed White OLEDs

Using the newly designed exciplex-forming 1,2,3-triazole-based acceptors with fast and efficient singlet → triplet intersystem crossing (ISC) processes, carbazole and benzoyl-1H-1,2,3-triazole derivatives were synthesized by Dimroth-type 1,2,3-triazole ring formation and Ullmann-Goldberg C-N coupling reactions. Due to the exciplex formation between covalently bonded electron-donating (carbazole) and 1,2,3-triazole-based electron-accepting moieties with small singlet-triplet splitting (0.07-0.13 eV), the compounds exhibited ISC-assisted bluish-green thermally activated delayed fluorescence. The compounds were characterized by high triplet energy levels ranging from 2.93 to 2.98 eV. The most efficient exciplex-type thermally activated delayed fluorescence was observed for ortho-substituted carbazole-benzoyl-1H-1,2,3-triazole which was selected as a host in the structure of efficient solution-processed white light-emitting diodes. The best device exhibited a maximum power efficiency of 10.7 lm/W, current efficiency of 18.4 cd/A, and quantum efficiency of 7.1%. This device also showed the highest brightness exceeding 10 thousand cd/m². Usage of the exciplex-forming host allowed us to achieve a low turn-on voltage of 3.6 V. High-quality white electroluminescence was obtained with the close to nature white color coordinates (0.31, 0.34) and a color rendering index of 92.

High-efficiency hyperfluorescent white light-emitting diodes based on high-concentration-doped TADF sensitizer matrices via spatial and energy gap effects

Despite the success of monochromatic hyperfluorescent (HF) organic light-emitting diodes (OLEDs), high-efficiency HF white OLEDs (WOLEDs) are still a big challenge. Herein, we demonstrate HF WOLEDs with state-of-the-art efficiencies, featuring a quasi-bilayer emissive layer (EML) composed of an ultrathin (0.1 nm) blue fluorescence (FL) emitter (TBPe) layer and a layer of thermally activated delayed fluorescence (TADF) sensitizer matrix heavily doped with a yellow FL emitter (TBRb, 3%). Based on an asymmetric high-energy-gap TADF sensitizer host (PhCzSPOTz), such an “ultrathin blue emitting layer (UTBL)” strategy endowed the HF WOLEDs with a record power efficiency of ∼80 lm W⁻¹, approaching the level of fluorescent tubes. Transient photoluminescence (PL) and electroluminescence (EL) kinetics demonstrate that the spatial separation of TBPe from the TADF sensitizer and TBRb, and the large energy gap between the latter two effectively suppress triplet leakage, in addition to suppressing triplet diffusion in the PhCzSPOTz matrix with anisotropic intermolecular interactions. These results provide a new insight into the exciton allocation process in HF white light-emitting systems.

A thermally activated delayed fluorescence host was developed to realize high-efficiency fluorescence white organic light-emitting diodes (WOLED) through spatial and energy gap effects.

Phototriggered Aggregation-Induced Emission and Direct Generation of 4D Soft Patterns

Microscopic control of macroscopic phenomena is one of the core subjects in materials science. Particularly, the spatio-temporal control of material behaviors through a non-contact way is of fundamental importance but is difficult to accomplish. Herein, a strategy to realize remote spatio-temporal control of luminescence behaviors is reported. A multi-arm salicylaldehyde benzoylhydrazone-based aggregation-induced emission luminogen (AIEgen)/metal-ion system, of which the fluorescence can be gated by the UV irradiation with time dependency, is developed. By changing the metal-ion species, the fluorescence emission and the intensity can also be tuned. The mechanism of the UV-mediated fluorescence change is investigated, and it is revealed that a phototriggered aggregation-induced emission (PTAIE) process contributes to the behaviors. The AIEgen is further covalently integrated into a polymeric network and the formed gel/metal-ion system can achieve laser-mediated mask-free writing enabled by the PTAIE process. Moreover, by further taking advantage of the time-dependent self-healing property of hydrazone-based dynamic covalent bond, transformable 4D soft patterns are generated. The findings and the strategy increase the ways to manipulate molecules on the supramolecule or aggregate level. They also show opportunities for the development of controllable smart materials and expand the scope of the materials in advanced optoelectronic applications.

Hyperfluorescent polymers enabled by through-space charge transfer polystyrene sensitizers for high-efficiency and full-color electroluminescence

Fluorescent polymers are suffering from low electroluminescence efficiency because triplet excitons formed by electrical excitation are wasted through nonradiative pathways. Here we demonstrate the design of hyperfluorescent polymers by employing through-space charge transfer (TSCT) polystyrenes as sensitizers for triplet exciton utilization and classic fluorescent chromophores as emitters for light emission. The TSCT polystyrene sensitizers not only have high reverse intersystem crossing rates for rapid conversion of triplet excitons into singlet ones, but also possess tunable emission bands to overlap the absorption spectra of fluorescent emitters with different bandgaps, allowing efficient energy transfer from the sensitizers to emitters. The resultant hyperfluorescent polymers exhibit full-color electroluminescence with peaks expanding from 466 to 640 nm, and maximum external quantum efficiencies of 10.3–19.2%, much higher than those of control fluorescent polymers (2.0–3.6%). These findings shed light on the potential of hyperfluorescent polymers in developing high-efficiency solution-processed organic light-emitting diodes and provide new insights to overcome the electroluminescence efficiency limitation for fluorescent polymers.

Hyperfluorescent polymers with high efficiency and full-color electroluminescence are developed by using through-space charge transfer polystyrenes as sensitizers for exciton utilization and fluorescent chromophores as emitters for light emission.

A robust stimuli responsive Eu3+ - Metalo organic hydrogel and xerogel emitting white light

Recently, there is incredible growth on optoelectronic properties of new supramolecular gels and white-light-emitting (WLE) metalo-organic gel comprised with single lanthanide metal ion having stimuli-responsive property is not yet reported. Here, we report a mandelic acid (MA)-triethylene tetraamine (TETA)-Eu-acetate conjugate (4.5:1:0.4 mol ratio), producing stimuli-sensitive WLE hydrogel exhibiting thermoreversible, thixotropic, pH-switchable, self-standing and self-healing properties. Energy minimized structure suggests complexation between MA-TETA conjugate and Eu³⁺ ion. Fluorescence intensity of MA-TETA conjugate decreases with increasing Eu³⁺ concentration indicating energy transfer from MA-TETA to Eu³⁺. Decay of donor fluorescence intensity follows Stern-Volmer equation and energy transfer efficiency is 42%. WLE gel has Quantum yield 11.4% and Förster distance 1.7 Å. Hydrogel and xerogel show WLE on excitation at 330 and 350 nm having CIE coordinates (0.34, 0.33) and (0.28, 0.32), respectively. WLE gel has Correlated colour temperature 5148 K, appropriate for cool day light emission and on coating over UV-LED bulb it emits bright white light.

A π-D and π-A Exciplex-Forming Host for High-Efficiency and Long-Lifetime Single-Emissive-Layer Fluorescent White Organic Light-Emitting Diodes

Exciplex-forming hosts with thermally activated delayed fluorescence (TADF) provide a viable opportunity to unlock the full potential of the yet-to-be improved power efficiencies (PEs) and stabilities of all-fluorescent white organic light-emitting diodes (WOLEDs), but this, however, is hindered by the lack of stable blue exciplexes. Here, an advanced exciplex system is proposed by incorporating bipolar charge-transport π-spacers into both the electron-donor (D) and the electron-accepter (A) to increase their distance for hypsochromic-shifted emission while maintaining the superior transporting ability. By using spirofluorene as the π-spacer, 3,3'-bicarbazole as the D-unit, and 2,4,6-triphenyl-1,3,5-triazine as the A-unit, a π-D and π-A exciplex with sky-blue emission and fast reverse intersystem crossing process is thereof constructed. Combining this exciplex-forming host, a blue TADF-sensitizer, and a yellow conventional fluorescent dopant in a single-emissive-layer, the fabricated warm-white-emissive device simultaneously exhibits a low driving voltage of 3.08 V, an external quantum efficiency of 21.4%, and a remarkable T80 (time to 80% of the initial luminance) of >8200 h at 1000 cd m^-2 , accompanied by a new benchmark PE of 69.6 lm W^-1 among all-fluorescent WOLEDs.

Thermally Assisted Fluorescent Polymers: Polycyclic Aromatic Materials for High Color Purity and White-Light Emission

Thermally activated delayed fluorescence (TADF) sensitization of fluorescence is a promising strategy to improve the color purity and operational lifetime of conventional TADF organic light-emitting diodes (OLEDs). Here, we propose a new design strategy for TADF-sensitized fluorescence based on acrylic polymers with a pendant energy-harvesting host, a TADF sensitizer, and fluorescent emitter monomers. Fluorescent emitters were rationally designed from a series of homologous polycyclic aromatic amines, resulting in efficient and color-pure polymeric fluorophores capable of harvesting both singlet and triplet excitons. Macromolecular analogues of blue, green, and yellow fourth-generation OLED emissive layers were prepared in a facile manner by Cu(0) reversible deactivation radical polymerization, with emission quantum yields up to 0.83 in air and narrow emission bands with full width at half-maximum as low as 57 nm. White-light emission can easily be achieved by enforcing incomplete energy transfer between a deep blue TADF sensitizer and yellow fluorophore to yield a single white-emissive polymer with CIE coordinates (0.33, 0.39) and quantum yield 0.77. Energy transfer to the fluorescent emitters occurs at rates of 1-4 × 10⁸ s^-1, significantly faster than deactivation caused by internal conversion or intersystem crossing. Rapid energy transfer facilitates high triplet exciton utilization and efficient sensitized emission, even when TADF emitters with a low quantum yield are used as photosensitizers. Our results indicate that a broad library of untapped polymers exhibiting efficient TADF-sensitized fluorescence should be readily accessible from known TADF materials, including many monomers previously thought unsuitable for use in OLEDs.

Exciplex energy transfer through spacer: White electroluminescence with enhanced stability based on cyan intermolecular and orange intramolecular thermally activated delayed fluorescence

Capability of exciplex energy transfer through a spacer was investigated using three exciplex-forming solid mixtures which contained the well-known electron accepting 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine and appropriately designed bipolar cyanocarbazolyl-based derivatives functionalized by attachment of carbazolyl, acridanyl or phenyl units. These novel cyanocarbazolyl-based derivatives were used as both the spacer and exciplex-forming donor. Efficient organic light-emitting diodes with electroluminescence in cyan-yellow region and maximum external quantum efficiency of up to 7.7% were fabricated owing to efficient thermally activated fluorescence (TADF) of the newly discovered exciplexes. An approach of exciton separation by the spacer between the studied exciplexes and selected orange TADF emitter was proposed for the fabrication of white electroluminescent devices with prolonged lifetime comparing to that of single-color exciplex-based devices. Exciplex-forming systems were tested for exciton separation between inter- and intramolecular TADF. Exciplex energy transfer through a spacer was observed on relatively long distance for one system due to the energy resonance between triplet levels of the exciplex and spacer. First time observed here exciplex energy transfer through a spacer can be useful for both improvement of device stability and obtaining of white electroluminescence.

Management of Delayed Fluorophor-Sensitized Exciton Harvesting for Stable and Efficient All-Fluorescent White Organic Light-Emitting Diodes

White organic light-emitting diodes (WOLEDs) using thermally activated delayed fluorescence (TADF)-based single emissive layer (SEL) have attracted enormous attention because of their simple device structure and full exciton utilization potential for high efficiency. However, WOLEDs made of an all-TADF SEL usually exhibit serious efficiency roll-off and poor color stability due to serious exciton-annihilation and unbalanced radiative decays of different TADF emitters. Herein, a new strategy is proposed to manipulate the TADF-sensitized fluorescence process by combining dual-host systems of high triplet energy with a conventional fluorescent emitter of complementary color. The multiple energy-funneling paths are modulated and short-range Dexter energy transfer is largely suppressed due to the steric effect of peripheral tert-butyl group in the blue TADF sensitizer. The resulting all-fluorescent WOLEDs achieve an unprecedentedly high external quantum efficiency of 21.8% with balanced white emission of Commission Internationale de l'Eclairage coordinate of (0.292, 0.343), accompanied with good color stability, reduced efficiency roll-off, and prolonged operational lifetime. These findings demonstrate the validity of this strategy for precisely allocating the exciton harvesting in SEL WOLEDs.

Multiple-State Emissions from Neat, Single-Component Molecular Solids: Suppression of Kasha's Rule

Three rigid and structurally simple heterocyclic stilbene derivatives, (E)-3H,3'H-[1,1'-biisobenzofuranylidene]-3,3'-dione, (E)-3-(3-oxobenzo[c] thiophen-1(3H)-ylidene)isobenzofuran-1(3H)-one, and (E)-3H,3'H-[1,1'-bibenzo[c] thiophenylidene]-3,3'-dione, are found to fluoresce in their neat solid phases, from upper (S₂ ) and lowest (S₁ ) singlet excited states, even at room temperature in air. Photophysical studies, single-crystal structures, and theoretical calculations indicate that large energy gaps between S₂ and S₁ states (T₂ and T₁ states) as well as an abundance of intra and intermolecular hydrogen bonds suppress internal conversions of the upper excited states in the solids and make possible the fluorescence from S₂ excited states (phosphorescence from T₂ excited states). These results, including unprecedented fluorescence quantum yields (2.3-9.6 %) from the S₂ states in the neat solids, establish a unique molecular skeleton for achieving multi-colored emissions from upper excited states by "suppressing" Kasha's rule.

Single-Molecular White-Light Emitters and Their Potential WOLED Applications

White organic light-emitting diodes (WOLEDs) are superior to traditional incandescent light bulbs and compact fluorescent lamps in terms of their merits in ensuring pure white-light emission, low-energy consumption, large-area thin-film fabrication, etc. Unfortunately, WOLEDs based on multilayered or multicomponent (red, green, and blue (RGB)) emissive layers can suffer from some remarkable disadvantages, such as intricate device fabrication and voltage-dependent emission color, etc. Single molecules, which can emit white light, can be used to replace multiple emitters, leading to a simplified fabrication process, stable and reproducible WOLEDs. Recently, the performance of WOLEDs by using single molecules is catching up with that of the state-of-the-art devices fabricated by multicomponent emitters. Therefore, an increasing attention has been paid on single white-light-emitting materials for efficient WOLEDs. In this review, different mechanisms of white-light emission from a single molecule and the performance of single-molecule-based WOLEDs are collected and expounded, hoping to light up the interesting subject on single-molecule white-light-emitting materials, which have great potential as white-light emitters for illumination and lighting applications in the world.

TADF and exciplex emission in a xanthone-carbazole derivative and tuning of its electroluminescence with applied voltage

Materials showing white light emission have found applications in a variety of solid state devices especially in display technology. For white light emission, doping of red (R), green (G) and blue (B) emitters in a host matrix is commonly practised. However, finding RGB emitters of similar stability with homogenous doping is challenging. Furthermore, such devices suffer from color purity in the long run. Small organic light emitters, capable of colour tuning and having a broad emission spectrum are in high demand as they provide colour stability, reproducibility, a simple device geometry and high efficiency. Recently, it has been shown that the efficiency of OLEDs can be enhanced by employing thermally activated delayed fluorescence (TADF) materials. Here, we designed and synthesised a xanthone-carbazole based D-A-D material (Xan-Cbz) for TADF properties. Blue TADF emission, in neat thin films, at 470 nm was observed and further investigated by studying delayed fluorescence and lifetime measurements. In addition, a blend of Xan-Cbz with NPD shows exciplex emission at 525 nm in thin film. OLEDs based on Xan-Cbz were fabricated using several device configurations. OLEDs having the device configuration ITO/PEDOT:PSS/NPD/Xan-Cbz/Bphen/LiF-Al showed a luminance of 1.96 × 104 Cd m-2 (at a current density of 50 mA cm-2) and V ON at ∼6 V. Electroluminescence showed the features of both neat emission (470 nm) of Xan-Cbz and its exciplex (525 nm) with NPD. Further, colour tuning was observed as a function of applied voltage and the ratio of light intensity (I 525/I 470) of neat and exciplex emission was found to decrease with increasing voltage. Greenish-blue emission (CIE coordinates: 0.202, 0.382) from Xan-Cbz OLEDs was obtained. Xan-Cbz showed its neat emission (at 470 nm) in ITO/PEDOT:PSS/CBP/Xan-Cbz/Bphen/LiF-Al and pure exciplex emission (at 525 nm) in ITO/PEDOT:PSS/NPD:Xan-Cbz/Bphen/LiF-Al device configurations. Thus in this article we showed blue TADF emission, exciplex emission and voltage dependent color tuning in OLEDs based on a small organic emitter.

Organic Light-Emitting Diodes Based on Conjugation-Induced Thermally Activated Delayed Fluorescence Polymers: Interplay Between Intra- and Intermolecular Charge Transfer States

In this work, interactions between different host materials and a blue TADF polymer named P1 are systematically investigated. In photoluminescence, the host can have substantial impact on the photoluminescence quantum yield (PLQY) and the intensity of delayed fluorescence (Φ_DF), where more than three orders of magnitude difference of Φ_DF in various hosts is observed, resulting from a polarity effect of the host material and energy transfer. Additionally, an intermolecular charge-transfer (CT) emission with pronounced TADF characteristics is observed between P1 and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T), with a singlet-triplet splitting of 7 meV. It is noted that the contribution of harvested triplets in monochrome organic light-emitting diodes (OLEDs) correlates with Φ_DF. For devices based on intermolecular CT-emission, the harvested triplets contribute ~90% to the internal quantum efficiency. The results demonstrate the vital importance of host materials on improving the PLQY and sensitizing Φ_DF of TADF polymers for efficient devices. Solution-processed polychrome OLEDs with a color close to a white emission are presented, with the emission of intramolecular (P1) and intermolecular TADF (PO-T2T:P1).

White light emitting diode based on purely organic fluorescent to modern thermally activated delayed fluorescence (TADF) and perovskite materials

White organic/polymer light emitting diode (WOLED/WPLED) processed from solution has attracted significant research interest in recent years due to their low device production cost, device flexibility, easy fabrication over large area including roll to roll and ability to print in various designs and shapes providing enormous design possibilities. Although WOLEDs fabricated using solution process lack their thermally evaporated counterparts in terms of device efficiency, remarkable progress has been made in this regard in recent years by utilizing new materials and device structures. In the present review, we have summarized and extrapolated an excellent association of old and modern concept of cost-effective materials and device structure for realization of white light. In particular, this article demonstrated and focused on design, and development of novel synthesis strategy, mechanistic insights and device engineering for solution process low cost WOLEDs device. Herein, an overview of the prevailing routes towards white light emitting devices (WLEDs) and corresponding materials used, including polymer based WLED, small molecules emitters based thermally activated delayed fluorescence (TADF), perovskite light-emitting diodes (PeLEDs) and hybrid materials based LEDs, color down-converting coatings with corresponding best efficiencies ever realized. We presume that this exhaustive review on WLEDs will offer a broad overview of the latest developments on white SSL and stonework the approach en route for innovations in the immediate future.

Triazine-Acceptor-Based Green Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes

High-efficiency thermally activated delayed fluorescence (TADF) is leading the third-generation technology of organic light-emitting diodes (OLEDs). TADF emitters are designed and synthesized using inexpensive organic donor and acceptor derivatives. TADF emitters are a potential candidate for next-generation display technology when compared with metal-complex-based phosphorescent dopants. Many studies are being conducted to enhance the external quantum efficiencies (EQEs) and photoluminescent quantum yield of green TADF devices. Blue TADF reached an EQE of over 35% with the support of suitable donor and acceptor moieties based on a suitable molecular design. The efficiencies of green TADF emitters can be improved when an appropriate molecular design is applied with an efficient device structure. The triazine acceptor has been identified as a worthy building block for green TADF emitters. Hence, we present here a review of triazine with various donor molecules and their device performances. This will help to design more suitable and efficient green TADF emitters for OLEDs.

Recent Progress of the Lifetime of Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescent Material

Recently, the external quantum efficiency and lifetime of organic light-emitting diodes (OLEDs) have been dramatically upgraded due to development of organic materials and device structure. In particular, an intramolecular or intermolecular complex based on thermally activated delayed fluorescent (TADF) materials has greatly contributed to improving OLED device performance. Although high external quantum efficiency has been the main objective of the development of TADF materials as hosts and emitters, recent interest has been directed towards the lifetime of TADF-material-based OLEDs. For the past several years, remarkable advances in the lifetime of phosphorescent and TADF OLEDs have been made using TADF materials as hosts or emitters in the emitting layer. Therefore, since TADF materials are useful as both hosts and emitters for a long lifetime, this work discusses the recent progress made in developing TADF materials for long-lifetime OLEDs.

Recent Progress of Singlet-Exciton-Harvesting Fluorescent Organic Light-Emitting Diodes by Energy Transfer Processes

The external quantum efficiency (EQE) of organic light-emitting diodes (OLEDs) has been dramatically improved by developing highly efficient organic emitters such as phosphorescent emitters and thermally activated delayed fluorescent (TADF) emitters. However, high-EQE OLED technologies suffer from relatively poor device lifetimes in spite of their high EQEs. In particular, the short lifetimes of blue phosphorescent and TADF OLEDs remain a big hurdle to overcome. Therefore, the high-EQE approach harvesting singlet excitons of fluorescent emitters by energy transfer processes from the host or sensitizer has been explored as an alternative for high-EQE OLED strategies. Recently, there has been a big jump in the EQE and device lifetime of singlet-exciton-harvesting fluorescent OLEDs. Recent progress on the materials and device structure is discussed herein.

Strategic-tuning of radiative excitons for efficient and stable fluorescent white organic light-emitting diodes

The emerging thermally activated delayed fluorescence materials have great potential for efficiencies in organic light-emitting diodes by optimizing molecular structures of the emitter system. However, it is still challenging in the device structural design to achieve high efficiency and stable device operation in white organic light-emitting diodes. Here we propose a universal design strategy for thermally activated delayed fluorescence emitter-based fluorescent white organic light-emitting diodes, establishing an advanced system of “orange thermally activated delayed fluorescence emitter sensitized by blue thermally activated delayed fluorescence host” combined with an effective exciton-confined emissive layer. Compared to reference single-layer and double-layer emissive devices, the external quantum efficiency improves by 31 and 45%, respectively, and device operational stability also shows nearly fivefold increase. Additionally, a detailed optical simulation for the present structure is made, indicating the validity of the design strategy in the fluorescent white organic light-emitting diodes.

Demonstrating white organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters with high performance and operational stability remains a challenge. Here, the authors show efficient and stable white OLEDs based on a double-dopant TADF system.

Recent Applications of Interfacial Exciplex as Ideal Host of Power-Efficient OLEDs

Currently, exploring the applications of intermolecular donor-acceptor exciplex couple as host of OLEDs with phosphorescence, thermally activated delayed fluorescence (TADF) or fluorescence emitter as dopant is a hot topic. Compared to other host strategies, interfacial exciplex has the advantage in various aspects, such as barrier-free charge injection, unimpeded charge transport, and the energy-saving direct exciton formation process at the “Well”-like heterojunction interface region. Most importantly, due to a very fast and efficient reverse intersystem-crossing (RISC) process, such a host is capable of regulating singlet/triplet exciton populations in itself as well as in the dopant emitters both under photoluminescent (PL) and electroluminescent (EL) driving conditions. In this mini-review, we briefly summarize and comment on recent applications of this ideal host in OLEDs (including both thermal-evaporation OLEDs and solution-processed OLEDs) with diverse emitters, e.g., fluorescence, phosphorescence, delayed fluorescence, or others. Special attention is given to illustrate the peculiar achievement of high overall EL performance with superiorities of low driving voltages, slow roll-off rate, high power efficiencies and satisfied device lifetime using this host strategy, which is then concluded by personal perspectives on the relevant next-step in this field.

Efficient Organic Light Emitting Diodes Using Solution-Processed Alkali Metal Carbonate Doped ZnO as Electron Injection Layer

In this study, we demonstrate highly efficient, inverted organic light-emitting diodes (IOLEDs) using solution-processed alkali metal carbonate doped ZnO as an electron injection layer (EIL) and tris-(8-hydroxyquinoline) aluminum (Alq₃) as an emitter layer. In order to enhance the electron injection efficiency of the IOLEDs, the ZnO EIL layers were modified by doping various alkali metal carbonate materials, including Li₂CO₃, Na₂CO₃, K₂CO₃, and Cs₂CO₃, using the low-temperature wet-chemical method. Compared to the control neat ZnO EIL-based IOLEDs, the alkali metal carbonate doped ZnO EIL-based IOLEDs possess obviously improved device performance. An optimal current efficiency of 6.04 cd A⁻¹ were realized from the K₂CO₃ doped ZnO EIL based IOLED, which is 54% improved compared to that of the neat ZnO EIL based device. The enhancement is ascribed to the increased electron mobility and reduced barrier height for more efficient electron injection. Our results indicate that alkali metal carbonate doped ZnO has promising potential for application in highly efficient solution-processed OLEDs.

Design of efficient thermally activated delayed fluorescence blue host for high performance solution-processed hybrid white organic light emitting diodes

Developing a solution-processible blue thermally activated delayed fluorescence (TADF) emitter for hybrid white organic light emitting diodes (WOLEDs) is still a challenge. In this work, two TADF blue emitters are designed and synthesized to explore a common strategy to qualify the small molecular TADF material as a solution-processible blue host. Systematic studies find that the molecular encapsulation by introducing unconjugated carbazoles as steric shields not only keeps the intrinsic TADF feature unchanged, but also effectively suppress the intermolecular interaction induced exciton quenching, which makes the material more efficient for solution-processing. The optimized solution-processed hybrid WOLEDs based on the encapsulated TADF blue host realized a highly efficient device performance with a maximum current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) of 45.6 cd A-1, 40.9 lm W-1 and 17.0%, respectively, which are three times higher in device efficiency and twenty times higher in device lifetime than the corresponding device with an unencapsulated TADF blue host. Furthermore, the obtained device exhibits a high electroluminescence (EL) above 20 000 cd m-2 and a stable EL spectrum with nearly unchanged Commission International de L'Eclairage (CIE) coordinate at a wide range of applied voltages. These results clearly demonstrate that the molecular encapsulation of the TADF blue host is a superior and promising strategy to achieve high performance and color stable solution-processed hybrid WOLEDs.

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.

White-Light-Emitting Decoding Sensing for Eight Frequently-Used Antibiotics Based on a Lanthanide Metal-Organic Framework

Developing multi-selective luminescence sensing technology to differentiate serial compounds is very important but challenging. White-light-emitting decoding sensing based on lanthanide metal-organic frameworks (Ln-MOFs) is a promising candidate for multi-selective luminescence sensing application. In this work, three isomorphic Ln-MOFs based on H₃dcpcpt (3-(3,5-dicarboxylphenyl)-5-(4-carboxylphenl)-1H-1,2,4-triazole) ligand, exhibiting red, blue, and green emission, respectively, have been synthesized by solvothermal reactions. The isostructural mixed Eu/Gd/Tb-dcpcpt is fabricated via the in-situ doping of different Ln³⁺ ions into the host framework, which can emit white light upon the excitation at 320 nm. It is noteworthy that this white-light-emitting complex could serve as a convenient luminescent platform for distinguishing eight frequently-used antibiotics: five through luminescence-color-changing processes (tetracycline hydrochloride, yellow; nitrofurazone, orange; nitrofurantoin, orange; sulfadiazine, blue; carbamazepine, blue) and three through luminescence quenching processes (metronidazole, dimetridazole, and ornidazole). Moreover, a novel method, 3D decoding map, has been proposed to realize multi-selective luminescence sensing applications. This triple-readout map features unique characteristics on luminescence color and mechanism. The mechanism has been systematically interpreted on the basis of the structural analysis, energy transfer and allocation process, and peak fitting analysis for photoluminescence spectra. This approach presents a promising strategy to explore luminescent platforms capable of effectively sensing serial compounds.

Doping-Free White Organic Light-Emitting Diodes

Doping-free white organic light-emitting diodes (WOLEDs) have great potential to the next-generation solid-state lighting and displays due to the excellent properties, such as high efficiency, bright luminance, low power consumption, simplified structure and low cost. In this account, our recent developments on doping-free WOLEDs have been summarized. Firstly, fundamental concepts of doping-free WOLEDs have been described. Then, the effective strategies to develop doping-free WOLEDs have been presented. Particularly, the manipulation of charges and excitons distribution in different kinds of doping-free WOLEDs have been highlighted, including doping-free fluorescent/phosphorescent hybrid WOLEDs, doping-free thermally activated delayed fluorescent WOLEDs and doping-free phosphorescent WOLEDs. In the end, an outlook for the future development of doping-free WOLEDs have been clarified.

High-Performance White Organic Light-Emitting Diodes with Simplified Structure Incorporating Novel Exciplex-Forming Host

It is a challenge to engineer white organic light-emitting diodes (WOLEDs) with high efficiency, low operating voltage, good color quality, and low efficiency roll-off, simultaneously. Herein, we employ a novel exciplex to solve this problem, which mixes a bipolar host material 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) with a common electron-transporting material 4,6-bis[3,5-(dipyrid-4-yl)phenyl]-2-methylpyrimidine (B4PyMPM) to form the host for a blue emitter iridium(III)bis(4,6-(difluorophenyl)-pyridinato- N,C²') picolinate (FIrpic). The blue OLED with maximum power efficiency (PE) over 48 lm W^-1 and Commission International de I'Eclairage chromaticity diagram (0.17, 0.36) was achieved. To obtain white light emission, a complementary orange emission layer is used, which consists of the bis(4-phenylth-ieno[3,2- c]pyridine)(acetylacetonate)iridium(III) (PO-01) doped into the single host of 26DCzPPy adjacent to the blue emission layer. Benefiting from the exciplex and effective utilization of the excitons by using the optimized multifunctional device structure, the WOLEDs remarkably exhibit maximum external quantum efficiency, PE, and current efficiency of 28.5%, 95.5 lm W^-1, and 82.0 cd A^-1, respectively. At the luminance of 100 cd m^-2, it maintains the values of 27.2%, 90.2 lm W^-1, and 78.4 cd A^-1, respectively. Furthermore, the WOLEDs have a low threshold voltage of about 2.6 V and remain around 4.0 V at 10 000 cd m^-2. These results indicate that the exciplex-forming co-host 26DCzPPy:B4PyMPM can provide an effective strategy to fabricate high-efficiency WOLEDs for potential applications.

Strategy for the Realization of Highly Efficient Solution-Processed All-Fluorescence White OLEDs-Encapsulated Thermally Activated Delayed Fluorescent Yellow Emitters

Fabrication of highly efficient all thermally activated delayed fluorescence (TADF) white organic light-emitting diodes (WOLEDs) through solution-process still remains a big challenge. Here, two encapsulated TADF molecules with a small singlet-triplet energy gap (Δ E_ST) and high photoluminescence quantum yield (PLQY) were designed and synthesized as yellow emitters for solution-processed WOLEDs. The high current, power, and external quantum efficiencies of 41.6 cd A^-1, 30.4 lm W^-1, and 17.3% were achieved for the solution-processed all-fluorescence WOLEDs with a single-emission layer. In contrast, even with the same Δ E_ST and PLQY, the corresponding unencapsulated parent emitters will account for nearly 50% loss of the potential device efficiency. This is for the first time that the small molecular TADF blue host and TADF yellow guest are used to construct solution-processed all-fluorescence WOLEDs, which exhibit high efficiency comparable with most of the vacuum-deposited all-fluorescence white devices. These results not only demonstrate the great potential of TADF emitters in achieving highly efficient solution-processed WOLEDs, but also testify the key role of molecular encapsulation in reducing polar-exciton quenching and enhancing electroluminescence performance.

Control of Dual Conformations: Developing Thermally Activated Delayed Fluorescence Emitters for Highly Efficient Single-Emitter White Organic Light-Emitting Diodes

In this work, we propose a novel concept to develop two fluorophores 2-(10 H-phenothiazin-10-yl)thianthrene 5,5,10,10-tetraoxide (PTZ-TTR) and 2-(4-(10 H-phenothiazin-10-yl)phenyl)thianthrene 5,5,10,10-tetraoxide (PTZ-Ph-TTR) showing dual conformations for highly efficient single-emitter white organic light-emitting diodes (WOLEDs). Both molecules exist in two stable conformations. Their nearly orthogonal forms own lower energy levels and show thermally activated delayed fluorescence (TADF) characteristics, whereas their nearly planar conformers possess higher energy levels and show only prompt fluorescence. These dual conformers were exploited for fabricating WOLEDs with complementary emission colors contributed by the two conformations. Moreover, the originally wasted triplet energy on the nearly planar conformation can be transferred to the nearly orthogonal one and then harvested via the TADF channel, realizing full exciton utilization. A PTZ-TTR-based single-emitter device exhibits standard white emission with a CIE coordinate of (0.33, 0.33) and a high color rendering index value of 92. On the other hand, the PTZ-Ph-TTR-based single-emitter device realizes an emission approaching warm white light and a high maximum external quantum efficiency of 16.34%. These results demonstrate an alternative approach for designing high-performance WOLEDs based on single TADF emitters.

Emergence of Nanoplatelet Light-Emitting Diodes

Since 2014, nanoplatelet light-emitting diodes (NPL-LEDs) have been emerged as a new kind of LEDs. At first, NPL-LEDs are mainly realized by CdSe based NPLs. Since 2016, hybrid organic-inorganic perovskite NPLs are found to be effective to develop NPL-LEDs. In 2017, all-inorganic perovskite NPLs are also demonstrated for NPL-LEDs. Therefore, the development of NPL-LEDs is flourishing. In this review, the fundamental concepts of NPL-LEDs are first introduced, then the main approaches to realize NPL-LEDs are summarized and the recent progress of representative NPL-LEDs is highlighted, finally the challenges and opportunities for NPL-LEDs are presented.

Convenient fabrication of conjugated polymer semiconductor nanotubes and their application in organic electronics

Organic heterojunction is indispensable in organic electronic devices, such as organic solar cells, organic light-emitting diodes and so on. Fabrication of core-shell nanostructure provides a feasible and novel way to prepare organic heterojunction, which is beneficial for miniaturization and integration of organic electronic devices. Fabrication of nanotubes which constitute the core-shell structure in large quantity is the key for the realization of application. In this work, a simple and convenient method to prepare nanotubes using conjugated copolymer of perylene diimide and dithienothiophene (P(PDI-DTT)) was demonstrated. The relationship between preparation conditions (solvent atmosphere, solution concentration and pore diameter of templates) and morphology of nanostructure was studied systematically. P(PDI-DTT) nanotubes could be fabricated in regular shape and large quantity by preparing the solution with appropriate concentration and placing anodic aluminium oxide template with nanopore diameter of 200 nm in the solvent atmosphere. The tubular structure was confirmed by scanning electron microscopy. P(PDI-DTT) nanotubes exhibited electron mobility of 0.02 cm2 V-1 s-1 in field-effect transistors under ambient condition. Light-emitting nanostructures were successfully fabricated by incorporating tetraphenylethylene into polymer nanotubes.