Youyou Bian

Luminescent materials with thermally activated delayed fluorescence (TADF) can harvest singlet and triplet excitons to afford high electroluminescence (EL) efficiencies for organic light-emitting diodes (OLEDs). However, TADF emitters generally have to be dispersed into host matrices to suppress emission quenching and/or exciton annihilation, and most doped OLEDs of TADF emitters encounter a thorny problem of swift efficiency roll-off as luminance increases. To address this issue, in this study, a new tailor-made luminogen (dibenzothiophene-benzoyl-9,9-dimethyl-9,10-dihydroacridine, DBT-BZ-DMAC) with an unsymmetrical structure is synthesized and investigated by crystallography, theoretical calculation, spectroscopies, etc. It shows aggregation-induced emission, prominent TADF, and interesting mechanoluminescence property. Doped OLEDs of DBT-BZ-DMAC show high peak current and external quantum efficiencies of up to 51.7 cd A⁻¹ and 17.9%, respectively, but the efficiency roll-off is large at high luminance. High-performance nondoped OLED is also achieved with neat film of DBT-BZ-DMAC, providing excellent maxima EL efficiencies of 43.3 cd A⁻¹ and 14.2%, negligible current efficiency roll-off of 0.46%, and external quantum efficiency roll-off approaching null from peak values to those at 1000 cd m⁻². To the best of the authors' knowledge, this is one of the most efficient nondoped TADF OLEDs with small efficiency roll-off reported so far.

A highly efficient nondoped organic light-emitting diode affording excellent maxima electroluminescence efficiencies of 43.3 cd A⁻¹ and 14.2%, negligible current efficiency roll-off of 0.46%, and external quantum efficiency roll-off approaching null from peak values to those at 1000 cd m⁻², is attained based on a robust luminogen featuring aggregation-induced emission and thermally activated delayed fluorescence.

It has been challenging to find stable blue organic light emitting diodes (OLEDs) that rely on thermally activated delayed fluorescence (TADF). Lack of stable host materials well-fitted to the TADF emitters is one of the critical reasons. The most popular host for blue TADF, bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO), leads to unrealistically high maximum external quantum efficiency. DPEPO is however an unstable material and has a poor charge transporting ability, which in turn induces an intrinsic short OLED operating lifespan. Here, an alternative host material is introduced which educes the potential efficiency and device lifespan of given TADF emitters with the appropriateness of replacing the most popular host material, DPEPO, in developing blue TADF emitters. It simultaneously provides much longer device lifespan and higher external quantum efficiency at a practical brightness due to its high material stability and electron-transport-type character well-fitted for hole-transport-type TADF emitters.

An alternative host material is introduced with the appropriateness of replacing the most popular host material, DPEPO, in developing blue thermally activated delayed fluorescent (TADF) emitters. It simultaneously provides much longer device lifespan and higher external quantum efficiency at a practical brightness due to its high material stability and electron-transport-type character well-fitted for hole-transport-type TADF emitters.

The fluorophores with long-lived fluorescent emission are highly desirable for time-resolved fluorescence imaging (TRFI) in monitoring target fluorescence. By embedding the aggregates of a thermally activated delayed fluorescence (TADF) dye, 2,3,5,6-tetracarbazole-4-cyano-pyridine (CPy), in distearoyl-sn-glycero-3-phosphoethanolamine-poly(ethylene glycol) (DSPE-PEG2000) matrix, CPy-based organic dots (CPy-Odots) with a long fluorescence lifetime of 9.3 μs (in water at ambient condition) and high brightness (with an absolute fluorescence quantum efficiency of 38.3%) are fabricated. CPy-Odots are employed in time-resolved and confocal fluorescence imaging in living Hela cells and in vivo. The green emission from the CPy-Odots is readily differentiated from the cellular autofluorescence background because of their stronger emission intensities and longer lifetimes. Unlike other widely studied DSPE-PEG2000 encapsulated Odots which are always distributed in cytoplasm, CPy-Odots are located mainly in plasma membrane. In addition, the application of CPy-Odots as a bright microangiography agent for TRFI in zebrafish is also demonstrated. Much broader application of CPy-Odots is also prospected after further surface functionalization. Given its simplicity, high fluorescence intensity, and wide availability of TADF materials, the method can be extended to develop more excellent TADF Odots for accomplishing the challenges in future bioimaging applications.

The nanotechnology toward thermally activated delayed fluorescence 2,3,5,6-tetracarbazole-4-cyano-pyridine-organic dots with excellent photostability, low cytotoxicity, and ultrabright luminescence and fluorescent lifetime of 9.3 μs was developed. Utilizing this nanoprobe, its application is successfully realized in time-resolved and confocal fluorescence imaging in vitro and in vivo.

Highly efficient bright green-emitting ZnAgInS (ZAIS)/ZnInS (ZIS)/ZnS alloy core/inner-shell/shell quantum dots (QDs) are synthesized using a multistep hot injection method with a highly concentrated zinc acetate dihydrate precursor. ZAIS/ZIS/ZnS QD growth is realized via five sequential steps: a core growth process, a two-step alloying–shelling process, and a two-step shelling process. To enhance the photoluminescence quantum yield (PLQY), a ZIS inner-shell is synthesized and added with a band gap located between the ZAIS alloy-core and ZnS shell using a strong exothermic reaction. The synthesized ZAIS/ZIS/ZnS QDs shows a high PLQY of 87% with peak wavelength of 501 nm. Tripackage white down-converted light-emitting diodes (DC-LEDs) are realized using an InGaN blue (B) LED, a green (G) ZAIS/ZIS/ZS QD-based DC-LED, and a red (R) ZnCuInS/ZnS QD-based DC-LED with correlated color temperature from 2700 to 10 000 K. The red, green, and blue tripackage white DC-LEDs exhibit high luminous efficacy of 72 lm W⁻¹ and excellent color qualities (color rendering index (CRI, R_a) = 95 and the special CRI for red (R₉) = 93) at 2700 K.

Bright green-emitting ZnAgInS (ZAIS)/ZnInS (ZIS)/ZnS alloy core/inner-shell/shell quantum dots (QDs) with high efficiency are synthesized via a multistep hot injection method. Tripackage white down-converted light-emitting diodes with a correlated color temperature of 2700-10 000 K, high luminous efficacies and excellent color qualities are realized.

Upconversion (UC) luminescence of lanthanide ions (Ln³⁺) has been extensively investigated for several decades and is a constant research hotspot owing to its fundamental significance and widespread applications. In contrast to the multiple and fixed UC emissions of Ln³⁺, transition metal (TM) ions, e.g., Mn²⁺, usually possess a single broadband emission due to its 3d⁵ electronic configuration. Wavelength-tuneable single UC emission can be achieved in some TM ion-activated systems ascribed to the susceptibility of d electrons to the chemical environment, which is appealing in molecular sensing and lighting. Moreover, the UC emissions of Ln³⁺ can be modulated by TM ions (specifically d-block element ions with unfilled d orbitals), which benefits from the specific metastable energy levels of Ln³⁺ owing to the well-shielded 4f electrons and tuneable energy levels of the TM ions. The electric versatility of d⁰ ion-containing hosts (d⁰ normally viewed as charged anion groups, such as MoO₆^6- and TiO₄^4-) may also have a strong influence on the electric dipole transition of Ln³⁺, resulting in multifunctional properties of modulated UC emission and electrical behaviour, such as ferroelectricity and oxide-ion conductivity. This review focuses on recent advances in the room temperature (RT) UC of TM ions, the UC of Ln³⁺ tuned by TM or d⁰ ions, and the UC of d⁰ ion-centred groups, as well as their potential applications in bioimaging, solar cells and multifunctional devices.

This review focuses on recent advances in the photon upconversion of transition metal ions with an emphasis on room temperature upconversion, the upconversion of Ln³⁺ tuned by TM or d⁰ ions (anion groups), and the upconversion of d⁰ ions (anion groups), as well as their potential applications in bioimaging, solar cells and multifunctional devices.

A new class of cyclometalated tetradentate alkynylgold(III) complexes has been successfully synthesized by post-synthetic modification. Through the judicious design and choice of pincer ligands, post-synthetic cyclization could be achieved to produce the robust and structurally rigid class of tetradentate gold(III) C^N^C^C complexes with high photoluminescence quantum yields of up to 0.49 in solution and 0.78 in doped thin films at room temperature, at least an order of magnitude higher than those of the structurally related uncyclized tridentate alkynylgold(III) analogues. High-performance yellow to orange-red emitting solution-processable organic light-emitting devices have also been achieved with external quantum efficiency of 11.1 %. This work describes for the first time of the use of post-synthetic ligand modification approach to overcome the synthetic challenge for tetradentate alkynylgold(III) complexes.

Rigid emitters: A new class of highly luminescent bis-cyclometalated tetradentate alkynylgold(III) complexes has been synthesized by post-synthetic intramolecular cyclization that enables the rigidification of the molecules to yield high photoluminescence quantum yields in thin films of up to 0.78. High performance solution-processable OLEDs with EQEs of 11.1 % have also been realized.

Two types of transition metal dichalcogenide (TMD) transistors are applied to demonstrate their possibility as switching/driving elements for the pixel of organic light-emitting diode (OLED) display. Such TMD materials are 6 nm thin WSe₂ and MoS₂ as a p-type and n-type channel, respectively, and the pixel is thus composed of external green OLED and nanoscale thin channel field effect transistors (FETs) for switching and driving. The maximum mobility of WSe₂-FETs either as switch or as driver is ≈30 cm² V⁻¹ s⁻¹, in linear regime of the gate voltage sweep range. Digital (ON/OFF-switching) and gray-scale analogue operations of OLED pixel are nicely demonstrated. MoS₂ nanosheet FET-based pixel is also demonstrated, although limited to alternating gray scale operation of OLED. Device stability issue is still remaining for future study but TMD channel FETs are very promising and novel for their applications to OLED pixel because of their high mobility and I_D ON/OFF ratio.

2D transition metal dichalcogenides (TMD), WSe₂, and MoS₂ are used to demonstrate their performances in pixels of organic light-emitting diode (OLED) displays. The pixel is composed of an external green OLED and TMD-channel driving/switching transistors. Digital (ON/OFF-switching) and gray-scale analogue operations of OLED pixels are nicely demonstrated.

An all-solution-processed quantum dots (QDs) light emitting diode (QLED) consists of different layers deposited from various orthogonal solvents. Here, the authors develop a general solvent selection strategy to obtain orthogonal solubility properties as well as high film quality. It is found that a “poor” QDs film morphology with striation defects often occurs when the QDs film is deposited from “bad” solvent. A physical model is presented to rationalize the observed striation defects, and then a general solvent selection strategy is proposed to prevent both surface striation defects and the dissolving of the underlying layers by carefully choosing the “good” solvent for QDs. A compact QDs film can be fabricated without altering the original morphology of underlying functional layers in a QLED device, leading to significant device performance improvement. An external quantum efficiency of 15.45% is achieved in a green QLED with uniform emitting region. This solvent selection strategy provides a general way to deposit high quality films for most of the solution-processed multilayer optoelectronic devices.

A general solvent selection strategy for all-solution-processed light-emitting quantum dots is proposed in which a “good” orthogonal solvent for quantum dots should prevent both surface striation defects and the dissolving of the underlying layers. Based on this strategy, an external quantum efficiency of 15.45% is achieved for a quantum dots light-emitting diodes device with uniform green emitting region.

Andrey L. Rogach and co-workers describe the enhanced emission of photoluminescent copper nanoclusters in solution and in powder state, in article 1600182. These blue- and orange-emitting Cu nanoclusters are combined to create white light-emitting devices.

Efficient and tunable thermally activated delayed fluorescence emitters are presented by C.-C. Wu, K.-T. Wong, and co-workers on page 7560. A toolbox composed of acridine donor units and cyano-substituted acceptor units allows for systematically probing acceptor strengths as well as tunable acceptor confirmations like twist angles. Highly efficient blue-green to yellow OLEDs with up to 31.3% external quantum efficiency are achieved.

Organic light-emitting diodes (OLEDs) able to directly emit circularly polarized (CP) electroluminescence (CP-OLEDs) are rapidly gaining much interest, due to their possible applications in displays with antiglare filters and 3D displays. Development of more efficient CP-OLEDs can open their use also in point-of-care and personalized diagnostic tools, since CP light alteration can be related to health state of irradiated tissues. In this work it is shown that the performance of chiral europium complex-based CP-OLEDs can be improved both in terms of external quantum efficiency (measured on all the Eu bands) and degree of polarization of emitted photons (as measured by the dissymmetry factor g_EL), by proper active layer formulation and through a fine tuning of the architecture of the device. Polarization performances (g_EL = −1) are obtained about three times higher than for any other CP-OLED reported so far. Moreover, for the first time, it is shown that the position of the recombination zone (RZ) plays a major role on the polarization outcomes. In order to rationalize these results the level of light polarization is related to the position of the RZ allied with the reflection on the cathode through a simple mathematical model. The values predicted by this model are in qualitative agreement with the experimental ones.

Lanthanide-based organic light-emitting diodes (OLEDs) with remarkable circularly polarized electroluminescence are designed. 75% of the emitted photons at 595 nm are circularly polarized with enhanced external quantum efficiency with respect to our earlier proof-of-concept achievement. For the first time the main factors affecting circularly polarized OLEDs performance are identified by combining experimental data and modeling to bring the technology to the next readiness level.

A new crosslinking concept based on a thermally activated one-component building block with thermally initiated crosslinkable ynol ether is introduced. For polystyrene matrices with glass transition temperatures below the reaction temperature, full conversion is reached within 30 min at 160 °C without employing any catalysts or co-reactants. The ynol ethers are chemically inert toward a variety of reaction conditions (e.g., radicals and strong bases) and consequently applicable to a wide range of materials for organic electronics. The crosslinkable solid compounds are bench-stable over more than a year. The broad applicability is demonstrated with a liquid model compound and a specifically designed crosslinking monomer introduced successfully as building block into polystyrenes with pending hole transporting groups. A detailed study of crosslinking kinetics by infrared measurements as well as an alternative method of crosslinker content determination utilizing differential scanning calorimetry is presented. The crosslinkable polymer and the corresponding noncrosslinkable molecule tris(4-(3,6-dibutoxy-9H-carbazol-9-yl)phenyl)amine (BuO₆TCTA) are synthesized and investigated as hole transport layers (HTLs) in phosphorescent organic light emitting diodes (OLEDs). OLEDs with crosslinked and noncrosslinked HTLs show efficiencies around 80 cd A⁻¹, indicating negligible influence of the crosslinking process on the device performance while yielding better HTL durability against solvent rinsing.

1-Ethynyl ethers are introduced as crosslinkers for solution processable hole transport materials in organic light emitting diode (OLED) applications. Thermally initiated crosslinking proceeds at 160 °C within 30 min quantitatively. 1-Ethynyl ethers are introduced as polymerizable styrene monomers and co-polymerized with styrene functionalized hole transport groups (m-BuO₄TCTA). The co-polymers are fully bench-stable and the corresponding OLEDs yield high efficiencies.

Cesium lead halide quantum dots (QDs) have tunable photoluminescence that is capable of covering the entire visible spectrum and have high quantum yields, which make them a new fluorescent materials for various applications. Here, the synthesis of CsPbX₃ (X = Cl, Br, I, or mixed Cl/Br and Br/I) QDs by direct ion reactions in ether solvents is reported, and for the first time the synergetic effects of solvent polarity and reaction temperature on the nucleation and growth of QDs are demonstrated. The use of solvent with a low polarity enables controlled growth of QDs, which facilitates the synthesis of high-quality CsPbX₃ QDs with broadly tunable luminescence, narrow emission width, and high quantum yield. A QD white LED (WLED) is demonstrated by coating the highly fluorescent green-emissive CsPbBr₃ QDs together with red phosphors on a blue InGaN chip, which presents excellent warm white light emission with a high rendering index of 93.2 and color temperature of 5447 K, suggesting the potential applications of highly fluorescent cesium lead halide perovskite QDs as an alternative color converter in the fabrication of WLEDs.

CsPbX₃ quantum dots (QDs) have superior photophysics properties, including high quantum yield (up to 72%), wide color gamut (>NTSC standard), and narrow emission width (18–38 nm). The WLED based on CsPbBr₃ QDs present excellent warm white light emission with a high rendering index of 93.2 and CIE coordinate of (0.3339, 0.3617).

“Chemistry-on-the-complex” synthetic methods have allowed the selective addition of 1-ethynylpyrene appendages to the 3-, 5-, 3,8- and 5,6-positions of Ir^III-coordinated 1,10-phenanthroline via Sonogashira cross-coupling. The resulting suite of complexes has given rise to the first rationalization of their absorption and emission properties as a function of the number and position of the pyrene moieties. Strong absorption in the visible region (e.g. 3,8-substituted Ir-3: λ_abs=481 nm, ϵ=52 400 m⁻¹ cm⁻¹) and long-lived triplet excited states (e.g. 5-substituted Ir-2: τ_T=367.7 μs) were observed for the complexes in deaerated CH₂Cl₂. On testing the series as triplet sensitizers for triplet–triplet annihilation upconversion, those Ir^III complexes bearing pyrenyl appendages at the 3- and 3,8-positions (Ir-1, Ir-3) were found to give optimal upconversion quantum yields (30.2 % and 31.6 % respectively).

“Chemistry-on-the-complex” synthetic methods enabled the selective addition of 1-ethynylpyrene appendages to the 3-, 5-, 3,8- and 5,6-positions of Ir^III-coordinated 1,10-phenanthroline by Sonogashira cross-coupling. The absorption and emission properties of the resulting complexes were systematically investigated as a function of the number and position of the pyrene moieties (see picture, DPA=9,10-diphenylanthracene).