Bifunctional Electron-Transporting Agent for Red Colloidal Quantum Dot Light-Emitting Diodes.

Indium phosphide (InP) quantum dots have enabled light-emitting diodes (LEDs) that are heavy-metal-free, narrow in emission linewidth, and physically flexible. However, ZnO/ZnMgO, the electron-transporting layer (ETL) in high-performance red InP/ZnSe/ZnS LEDs, suffers from high defect densities, quenches luminescence when deposited on InP, and induces performance degradation that arises due to trap migration from the ETL to the InP emitting layer. We posited that the formation of Zn2+ traps on the outer ZnS shell, combined with sulfur and oxygen vacancy migration between ZnO/ZnMgO and InP, may account for this issue. We synthesized therefore a bifunctional ETL (CNT2T, 3',3'″,3'″″-(1,3,5-triazine-2,4,6-triyl)tris(([1,1'-biphenyl]-3-carbonitrile)) designed to passivate Zn2+ traps locally and in situ and to prevent vacancy migration between layers: the backbone of the small molecule ETL contains a triazine electron-withdrawing unit to ensure sufficient electron mobility (6 × 10-4 cm2 V-1 s-1), and the star-shaped structure with multiple cyano groups provides effective passivation of the ZnS surface. We report as a result red InP LEDs having an EQE of 15% and a luminance of over 12,000 cd m-2; this represents a record among organic-ETL-based red InP LEDs.

Multidentate Molecule Anchoring Halide Perovskite Surface and Regulating Crystallization Kinetics toward Efficient Light-Emitting Diodes.

Functional passivators are conventionally utilized in modifying the crystallization properties of perovskites to minimize the non-radiative recombination losses in perovskite light-emitting diodes (PeLEDs). However, the weak anchor ability of some commonly adopted molecules has limited passivation ability to perovskites and even may desorb from the passivated defects in a short period of time, which bring about plenty of challenges for further development of high-performance PeLEDs. Here, a multidentate molecule, formamidine sulfinic acid (FSA), is introduced as a novel passivator to perovskites. FSA has multifunctional groups (S≐O, C≐N and NH2 ) where the S≐O and C≐N groups enable coordination with the lead ions and the NH2 interacts with the bromide ions, thus providing the most effective chemical passivation for defects and in turn the formation of highly stable perovskite emitters. Moreover, the interaction between the FSA and octahedral [PbBr6 ]4- can inhibit the formation of unfavorable low-n domains to further minimize the inefficient energy transfer inside the perovskite emitters. Therefore, the FSA passivated green-emitting PeLED exhibits a high external quantum efficiency (EQE) of 26.5% with fourfold enhancement in operating lifetime as compared to the control device, consolidating that the multidentate molecule is a promising strategy to effectively and sustainably passivate the perovskites.

Carbonyl-Containing Thermally Activated Delayed Fluorescence Emitters for Narrow-Band Electroluminescence.

Carbonyl-containing derivatives show enduring vitality in the field of thermally activated delayed fluorescence (TADF) materials; they can realize high device efficiency by using both singlet and triplet excitons for electroluminescence. Recently, a system based on fused ketone/amine exhibited huge potential for constructing multi-resonance TADF (MR-TADF) emitters, which exhibit higher narrow-band emission than conventional TADF emitters with twisted donor-acceptor (D-A) structure. Herein, we summarize current research progress in both traditional and MR-type ketone derivatives with TADF characteristics for introducing the molecular design strategy of maintaining high device efficiency while keeping narrow-band emission profile. We hope this review can inspire the emergence of more high-performance narrow-band materials.

Introducing Spiro-locks into the Nitrogen/Carbonyl System towards Efficient Narrowband Deep-blue Multi-resonance TADF Emitters.

The current availability of multi-resonance thermally activated delayed fluorescence (MR-TADF) materials with excellent color purity and high device efficiency in the deep-blue region is appealing. To address this issue in the emerged nitrogen/carbonyl MR-TADF system, we propose a spiro-lock strategy. By incorporating spiro functionalization into a concise molecular skeleton, a series of emitters (SFQ, SOQ, SSQ, and SSeQ) can enhance molecular rigidity, blue-shift the emission peak, narrow the emission band, increase the photoluminescence quantum yield by over 92 %, and suppress intermolecular interactions in the film state. The referent CZQ without spiro structure has a more planar skeleton, and its bluer emission in the solution state redshifts over 40 nm with serious spectrum broadening and a low PLQY in the film state. As a result, SSQ achieves an external quantum efficiency of 25.5 % with a peak at 456 nm and a small full width at half maximum of 31 nm in a simple unsensitized device, significantly outperforming CZQ. This work discloses the importance of spiro-junction in modulating deep-blue MR-TADF emitters.

Efficient Near-Infrared Electroluminescence from Lanthanide-Doped Perovskite Quantum Cutters.

Perovskite nanocrystals (PeNCs) deliver size- and composition-tunable luminescence of high efficiency and color purity in the visible range. However, attaining efficient electroluminescence (EL) in the near-infrared (NIR) region from PeNCs is challenging, limiting their potential applications. Here we demonstrate a highly efficient NIR light-emitting diode (LED) by doping ytterbium ions into a PeNCs host (Yb3+ : PeNCs), extending the EL wavelengths toward 1000 nm, which is achieved through a direct sensitization of Yb3+ ions by the PeNC host. Efficient quantum-cutting processes enable high photoluminescence quantum yields (PLQYs) of up to 126 % from the Yb3+ : PeNCs. Through halide-composition engineering and surface passivation to improve both PLQY and charge-transport balance, we demonstrate an efficient NIR LED with a peak external quantum efficiency of 7.7 % at a central wavelength of 990 nm, representing the most efficient perovskite-based LEDs with emission wavelengths beyond 850 nm.

Highly Efficient Blue Thermally Activated Delayed Fluorescence Emitters Based on Multi-Donor Modified Oxygen-Bridged Boron Acceptor.

The employment of thermally activated delayed fluorescence (TADF) emitters is one of the most promising ways to realize the external quantum efficiency (EQE) of over 25% for organic light-emitting diodes (OLEDs). In addition, the TADF emitter based on oxygen-bridged boron (BO) fragment can maintain blue emission with high color purity. Herein, we constructed two blue TADF emitters, 3TBO and 5TBO, for OLEDs application. Both emitters consist of three donors linked at the oxygen-bridged boron acceptor. OLED devices based on 3TBO and 5TBO exhibited both high excellent device efficiency and high color purity with a maximum EQE; full-width at half-maximum (FWHM); and CIE coordinates of 17.3%, 47 nm, (0.120, 0.294), and 26.2%, 57 nm, (0.125, 0.275), respectively.

Organic Branched Heterostructures with Optical Interconnects for Photonic Barcodes.

Optical interconnects exhibit superior potential in the precise regulation of photon transmission for organic photonic circuits. However, the rational design of well-defined organic heterostructures toward active optoelectronics remains challenging. Herein, we designed organic branched heterostructures (OBHs) with accurate spatial organization for optical interconnection. Notably, the precise regulation of OBHs has been controllably achieved including the trunk morphologies and the branched microwire number. Significantly, these as-prepared OBHs inherently exhibit the multichannel coupling outputs and the excitation position-dependent waveguide characteristics, leading to various outcoupling signals with tunable intensity and emission colors. The optical interconnects are realized due to the occurrence of exciton conversion and photon propagation between branch and trunk at the heterojunction, benefiting the application possibilities of two-dimensional (2D) optical barcodes.

Highly Efficient Sky-Blue π-Stacked Thermally Activated Delayed Fluorescence Emitter with Multi-Stimulus Response Properties.

Organic materials with multi-stimulus response (MSR) properties have demonstrated many potential and practical applications. Herein, a π-stacked thermally activated delayed fluorescence (TADF) material with multi-stimulus response (MSR) properties, named SDMAC, was designed and synthesized using distorted 9,9-dimethyl-10-phenyl-9,10-dihydroacridine as a donor. SDMAC possesses a rigid π-stacked configuration with intramolecular through-space interactions and exhibits aggregation-induced emission enhancement (AIEE), solvatochromic, piezochromic, and circularly polarized luminescence (CPL) under different external stimuli. The rigid molecular structure and efficient TADF properties of SDMAC can be used in displays and lighting. Using SDMAC as an emitter, the maximum external quantum efficiency (EQE) of the fabricated organic light-emitting diodes (OLEDs) is as high as 28.4 %, which make them the most efficient CP-TADF OLEDs based on the through-space charge transfer strategy. The CP organic light-emitting diodes (CP-OLEDs) exhibit circularly polarized electroluminescence (CPEL) signals.

Multi-Layer π-Stacked Molecules as Efficient Thermally Activated Delayed Fluorescence Emitters.

Multi-layer π-stacked emitters based on spatially confined donor/acceptor/donor (D/A/D) patterns have been developed to achieve high-efficiency thermally activated delayed fluorescence (TADF). In this case, dual donor moieties and a single acceptor moiety are introduced to form two three-dimensional (3D) emitters, DM-BD1 and DM-BD2, which rely on spatial charge transfer (CT). Owing to the enforced face-to-face D/A/D pattern, effective CT interactions are realized, which lead to high photoluminescence quantum yields (PLQYs) of 94.2 % and 92.8 % for the two molecules, respectively. The resulting emitters exhibit small singlet-triplet energy splitting (ΔEST ) and fast reverse intersystem crossing (RISC) processes. Maximum external quantum efficiencies (EQEs) of 28.0 % and 26.6 % were realized for devices based on DM-BD1 and DM-BD2, respectively, which are higher than those of their D/A-type analogues.

Circularly Polarized Thermally Activated Delayed Fluorescence Emitters in Through-Space Charge Transfer on Asymmetric Spiro Skeletons.

This work describes a strategy to produce circularly polarized thermally activated delayed fluorescence (CP-TADF). A set of two structurally similar organic emitters SFST and SFOT are constructed, whose spiro architectures containing asymmetric donors result in chirality. Upon grafting within the spiro frameworks, the donor and acceptor are fixed proximally in a face-to-face manner. This orientation allows intramolecular through-space charge transfer (TSCT) to occur in both emitters, leading to TADF properties. The donor units in SFST and SFOT have a sulfur and oxygen atom, respectively; such a subtle difference has great impacts on their photophysical, chiroptical, and electroluminescence (EL) properties. SFOT exhibits greatly enhanced EL performance in doped organic light-emitting diodes, with external quantum efficiency (EQE) up to 23.1%, owing to the concurrent manipulation of highly photoluminescent quantum efficiency (PLQY, ∼90%) and high exciton utilization. As a comparison, the relatively larger sulfur atom in SFST introduces heavy atom effects and leads to distortion of the molecular backbone that lengthens the donor-acceptor distance. SFST thus has lower PLQY and faster nonradiative decay rate. The collective consequence is that the EQE value of SFST, i.e., 12.5%, is much lower than that of SFOT. The chirality of these two spiro emitters results in circularly polarized luminescence. Because SFST has a more distorted molecular architecture than SFOT, the luminescence dissymmetry factor (|glum|) of circularly polarized luminescence of one enantiomer of the former, namely, either (S)-SFST or (R)-SFST, is almost twice that of (S)-SFOT/(R)-SFOT. Moreover, the CP organic light-emitting diodes (CP-OLEDs) show obvious circularly polarized electroluminescence (CPEL) signals with gEL of 1.30 × 10-3 and 1.0 × 10-3 for (S)-SFST and (S)-SFOT, respectively.

Sky-Blue Thermally Activated Delayed Fluorescence with Intramolecular Spatial Charge Transfer Based on a Dibenzothiophene Sulfone Emitter.

Intramolecular spatial charge transfer (ISCT) plays a critical role in determining the optical and charge transport properties of thermally activated delayed fluorescence (TADF) materials. Herein, a new donor/acceptor-type TADF compound based on rigid dibenzothiophene sulfone (DBTS) moiety, STF-DBTS, was designed and synthesized. Fluorene unit was used as a rigid linker to position the rigid acceptor and donor subunit in close vicinity with control over their spacing and molecular structure and to achieve high photoluminescence quantum yield (∼53%) and TADF property. For comparison purposes, we constructed the more flexible STF-DPS with a less rotationally constrained diphenylsulphone (DPS) acceptor instead of the rigid DBTS units, and STF-DPS showed no TADF properties and lower PLQY (16.0%). Organic light-emitting diodes (OLEDs) based on STF-DBTS achieve an external quantum efficiency (EQE) of 10.3% at 488 nm, which is a fivefold improvement in EQE with respect to STF-DPS.

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

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

Enhancing Light Outcoupling Efficiency via Anisotropic Low Refractive Index Electron Transporting Materials for Efficient Perovskite Light-Emitting Diodes.

Thanks to the extensive efforts toward optimizing perovskite crystallization properties, high-quality perovskite films with near-unity photoluminescence quantum yield are successfully achieved. However, the light outcoupling efficiency of perovskite light-emitting diodes (PeLEDs) is impeded by insufficient light extraction, which poses a challenge to the further advancement of PeLEDs. Here, an anisotropic multifunctional electron transporting material, 9,10-bis(4-(2-phenyl-1H-benzo[d]imidazole-1-yl)phenyl) anthracene (BPBiPA), with a low extraordinary refractive index (ne) and high electron mobility is developed for fabricating high-efficiency PeLEDs. The anisotropic molecular orientations of BPBiPA can result in a low ne of 1.59 along the z-axis direction. Optical simulations show that the low ne of BPBiPA can effectively mitigate the surface plasmon polariton loss and enhance the photon extraction efficiency in waveguide mode, thereby improving the light outcoupling efficiency of PeLEDs. In addition, the high electron mobility of BPBiPA can facilitate balanced carrier injection in PeLEDs. As a result, high-efficiency green PeLEDs with a record external quantum efficiency of 32.1% and a current efficiency of 111.7 cd A-1 are obtained, which provides new inspirations for the design of electron transporting materials for high-performance PeLEDs.

Design and Synthesis of Red Through-Space Charge Transfer Thermally Activated Delayed Fluorescence Emitters with Donor/Acceptor/Donor Stacking.

Red through-space charge transfer thermally activated delayed fluorescence (TSCT TADF) materials named SAF36DCPP and SAF27DCPP with sandwiched structures were synthesized. Single crystals indicated that the intramolecular C-H···π interactions play a vital role in rigidifying the sandwiched structure, which results in a fluorescence yield of 63% for SAF36DCPP compared to 40% for SAF27DCPP. Organic light-emitting diodes with SAF36DCPP as the emitter realized a maximum external quantum efficiency of 16.12%.

In Situ Inorganic Ligand Replenishment Enables Bandgap Stability in Mixed-Halide Perovskite Quantum Dot Solids.

Instability in mixed-halide perovskites (MHPs) is a key issue limiting perovskite solar cells and light-emitting diodes (LEDs). One form of instability arises during the processing of MHP quantum dots using an antisolvent to precipitate and purify the dots forming surface traps that lead to decreased luminescence, compromised colloidal stability, and emission broadening. Here, the introduction of inorganic ligands in the antisolvents used in dot purification is reported in order to overcome this problem. MHPs that are colloidally stable for over 1 year at 25 °C and 40% humidity are demonstrated and films that are stable under 100 W cm-2 photoirradiation, 4× longer than the best previously reported MHPs, are reported. In LEDs, the materials enable an EQE of 24.4% (average 22.5 ± 1.3%) and narrow emission (full-width at half maximum of 30 nm). Sixfold-enhanced operating stability relative to the most stable prior red perovskite LEDs having external quantum efficiency >20% is reported.

Harvesting triplet excitons for near-infrared electroluminescence via thermally activated delayed fluorescence channel.

Near-infrared (NIR) emission is useful for numerous practical applications, such as communication, biomedical sensors, night vision, etc., which encourages researchers to develop materials and devices for the realization of efficient NIR organic light-emitting devices. Recently, the emerging organic thermally activated delayed fluorescence (TADF) emitters have attracted wide attention because of the full utilization of electron-generated excitons, which is crucial for achieving high device efficiency. Up to now, the TADF emitters have shown their potential in the deep red/NIR region. Considering the color purity and efficiency, however, the development of NIR TADF emitters still lags behind RGB TADF emitters, indicating that there is still much room to improve their performance. In this regard, this perspective mainly summarizes the past progress of molecular design on constructing TADF NIR emitters. We hope this perspective could provide a new vista in developing NIR materials and enlighten breakthroughs in both fundamental research and applications.

Fully Bridged Triphenylamine Derivatives as Color-Tunable Thermally Activated Delayed Fluorescence Emitters.

Three emissive bridged-triphenylamine derivatives are designed and synthesized by incorporating carbon (DQAO), oxygen (OQAO), and sulfur (SQAO) atoms with two carbonyl groups. The fully bridged geometry and unique frontier molecular orbital distribution reveal its potential as narrowband thermally activated delayed fluorescence emitters. DQAO-, OQAO-, and SQAO-based organic light-emitting diodes exhibit the maximum external quantum efficiency (EQEmax) of 15.2%, 20.3%, and 17.8% for blue, green, and yellow, respectively.

Over 800 nm Emission via Harvesting of Triplet Excitons in Exciplex Organic Light-Emitting Diodes.

Triplet excitons can be utilized upon introduction of phosphors into exciplexes, and such a scenario has been studied in the development of high-performance near-infrared (NIR) organic light-emitting diodes (OLEDs). To generate exciplexes in an emitting layer (EML) in the device, commercially available phosphors bis(2-phenylpyridinato-N,C2')iridium(acetylacetonate) [Ir(ppy)2acac] and iridium(III) bis(4-phenylthieno[3,2-c]pyridinato-N,C2')acetylacetonate (PO-01) were selected as donor components; in addition, a new designed fluorescent molecule, 3-([1,1':3',1″-terphenyl]-5'-yl)acenaphtho[1,2-b]quinoxaline-9,10-dicarbonitrile (AQDC-tPh), and recently reported 3-([1,1':3',1″-terphenyl]-5'-yl)acenaphtho[1,2-b]pyrazine-8,9-dicarbonitrile (APDC-tPh) were selected as acceptor components. An OLED with PO-01:AQDC-tPh blends as the EML has realized NIR emission at 750 nm and a maximum external quantum efficiency (EQE) of >0.23%. Furthermore, an OLED containing a PO-01:APDC-tPh blend realizes a maximum EQE of 0.16% at 824 nm. The high performance of these devices underlying phosphor-based exciplexes proves the potential and feasibility of our strategy for the construction of efficient NIR OLEDs.

Efficient and Spectrally Stable Blue Perovskite Light-Emitting Diodes Employing a Cationic π-Conjugated Polymer.

Metal halide perovskite semiconductors have demonstrated remarkable potentials in solution-processed blue light-emitting diodes (LEDs). However, the unsatisfied efficiency and spectral stability responsible for trap-mediated non-radiative losses and halide phase segregation remain the primary unsolved challenges for blue perovskite LEDs. In this study, it is reported that a fluorene-based π-conjugated cationic polymer can be blended with the perovskite semiconductor to control film formation and optoelectronic properties. As a result, sky-blue and true-blue perovskite LEDs with Commission Internationale de l'Eclairage coordinates of (0.08, 0.22) and (0.12, 0.13) at the record external quantum efficiencies of 11.2% and 8.0% were achieved. In addition, the mixed halide perovskites with the conjugated cationic polymer exhibit excellent spectral stability under external bias. This result illustrates that π-conjugated cationic polymers have a great potential to realize efficient blue mixed-halide perovskite LEDs with stable electroluminescence.

Synergistic protective effect of picroside II and NGF on PC12 cells against oxidative stress induced by H2O2.

Epidemiological studies suggest that nerve growth factor (NGF) is associated with a reduced risk of acute or chronic neuropathies. We studied the synergistic protective effect of picroside II and NGF against the oxidative stress in PC12 cells induced by hydrogen peroxide (H2O2). The fluorescent probe CDCFH was used to assess the intracellular reactive oxygen species (ROS) level, and MTT assay, morphological observation as well as LDH leakage test were conducted to measure cellular injury. The H2O2-induced cytotoxicity was significantly attenuated in the presence of picroside II (25 microg/ml ) and NGF (2 ng/ml). Cultures with this combined treatment possessed decreased level of ROS while increased cell survival, as compared to that of picroside II or NGF alone-treated cells. Accordingly, it was concluded that their synergistic protective activities against oxidative stress in vitro were demonstrated in various aspects, including reversing morphological changes, enhancing the ability of cell proliferation and ROS scavenging. Such action supports the therapeutic potential of picroside II and NGF in treating nervous disorders based on their synergistic effect.