Cai-Jun Zheng

Management of Singlet and Triplet Excitons in a Single Emission Layer: A Simple Approach for a High‐Efficiency Fluorescence/Phosphorescence Hybrid White Organic Light‐Emitting Device

A high-efficiency single-emission-layer (EML) hybrid white organic light emitting device is fabricated based on an ideal sky-blue fluorophor, DADBT, using a novel doping concentration regulation strategy, which effectively separates and respectively utilizes the singlet and triplet excitons in the single-EML. The white device shows excellent electroluminescence performance with maximum total efficiencies of 26.6%, 53.5 cd A(-1) and 67.2 lm W(-1) .

Super-elastic titanium alloy with unstable plastic deformation

Here we report a non-toxic β-type titanium alloy exhibiting unstable elastic and plastic deformation behavior. Elastic instability leads to remarkable elastic softening, i.e., the decrease of incipient Young’s modulus with slight pre-straining. In spite of partial recovery during room-temperature aging, a stable modulus of 33GPa matching that of human bone can be maintained. Plastic instability causes highly-localized deformation which is very effective in grain refinement but contributes little to strength. We thus obtain soft nanostructured metallic materials (NMMs): The flow stress increases by only ∼5.5% as coarse grains are reduced to below 50nm, in contrast with several times increase for previously-reported NMMs.

Novel Efficient Blue Fluorophors with Small Singlet‐Triplet Splitting: Hosts for Highly Efficient Fluorescence and Phosphorescence Hybrid WOLEDs with Simplified Structure

The exact hosts for F-P hybrid WOLEDs have been first demonstrated following a new design strategy for blue fluorophors with small singlet-triplet splitting. Two novel compounds DPMC and DAPSF exhibit efficient blue fluorescence, high triplet energies and good conductivities. These merits allow us to use new simplified device designs to achieve high efficiency, slow efficiency roll-off and stable emission color.

Prediction and Design of Efficient Exciplex Emitters for High‐Efficiency, Thermally Activated Delayed‐Fluorescence Organic Light‐Emitting Diodes

High-efficiency, thermally activated delayed-fluorescence organic light-emitting diodes based on exciplex emitters are demonstrated. The best device, based on a TAPC:DPTPCz emitter, shows a high external quantum efficiency of 15.4%. Strategies for predicting and designing efficient exciplex emitters are also provided. This approach allow prediction and design of efficient exciplex emitters for achieving high-efficiency organic light-emitting diodes, for future use in displays and lighting applications.

Nearly 100% Triplet Harvesting in Conventional Fluorescent Dopant‐Based Organic Light‐Emitting Devices Through Energy Transfer from Exciplex

Nearly 100% triplet harvesting in conventional fluorophor-based organic light-emitting devices is realized through energy transfer from exciplex. The best C545T-doped device using the exciplex host exhibits a maximum current efficiency of 44.0 cd A(-1) , a maximum power efficiency of 46.1 lm W(-1) , and a maximum external quantum efficiency of 14.5%.

Highly efficient non-doped deep-blue organic light-emitting diodes based on anthracene derivatives

Three deep-blue-emitting anthracene derivatives, 2-tert-butyl-9,10-bis(9,9-dimethylfluorenyl) anthracene (TBMFA), 2-tert-butyl-9,10-bis[4-(2-naphthyl)phenyl] anthracene (TBDNPA), and 2-tert-butyl-9,10-bis[4-(9,9-dimethylfluorenyl)phenyl] anthracene (TBMFPA), with naphthalene or 9,9-dimethylfluorene side units, have been designed, synthesized, and characterized. The anthracene derivatives show strong deep-blue emission both in solution and in thin films. The three derivatives also have high glass-transition temperatures (Tg ≥ 133 °C) due to the presence of sterically congested terminal groups. Organic light-emitting diodes (OLEDs) prepared using these anthracene derivatives as non-doped emitters exhibit bright and saturated deep-blue emissions. OLEDs based on TBDNPA give the best performance with a low turn-on voltage (3.0 V with a brightness of 1 cd m−2), and a high efficiency (5.17% external quantum efficiency at 8.4 mA cm−2). These results are among the best ever reported for saturated deep-blue OLEDs with a CIE coordinate of y < 0.10.

Novel Carbazol-Pyridine-Carbonitrile Derivative as Excellent Blue Thermally Activated Delayed Fluorescence Emitter for Highly Efficient Organic Light-Emitting Devices

A novel blue thermally activated delayed fluorescence (TADF) emitter, CPC (2,6-di(9H-carbazol-9-yl)-4-phenylpyridine-3,5-dicarbonitrile), was designed and synthesized. By directly linking carbazole (to serve as electron-donor) and pyridine-3,5-dicarbonitrile (to serve as electron-acceptor), and distributing cyanogroups and carbazole groups at the para-position of pyridine core, CPC successfully achieves an extremely small singlet-triplet splitting and fairish photoluminescence quantum yield, thus can act as the highly efficient blue TADF emitter. The optimized organic light-emitting diode (OLED) based on 13 wt % CPC doped in mCP (1,3-bis(9H-carbazol-9-yl)benzene) host exhibits maximum current efficiency, power efficiency, and external quantum efficiency of 47.7 cd A(-1), 42.8 lm W(-1), and 21.2%, respectively, which are the best results in reported blue TADF-based devices up to date and even comparable with the best reported blue phosphorescent OLEDs.

Synthesis, Structure, and Photophysical Properties of Two Four-Coordinate CuI–NHC Complexes with Efficient Delayed Fluorescence

Two luminescent cationic heteroleptic four-coordinate Cu(I) complexes supported by N-heterocyclic carbene ligand and diphosphine ligand were successfully prepared and characterized. These complexes adopt typical distorted tetrahedral configuration and have high stability in solid state. Quantum chemical calculations show carbene units have contributions to both highest occupied molecular orbitals and lowest unoccupied molecular orbitals of these Cu(I)-NHC complexes, the lowest-lying singlet and triplet excitations (S0 → S1 and S0 → T1) of [Cu(Pyim)(POP)](PF6) are dominated by metal-to-ligand charge transfer (MLCT) transition, while the S0 → S1 and S0 → T1 excitations of [Cu(Qbim)(POP)](PF6) are mainly MLCT and ligand-centered transitions, respectively. These Cu(I)-NHC complexes show efficient long-lifetime emissions (λem = 520 nm, τ = 79.8 μs, Φ = 0.56 for [Cu(Pyim)(POP)](PF6), λem = 570 nm, τ = 31.97 μs (78.99%) and 252.2 μs (21.01%), Φ = 0.35 for [Cu(Qbim)(POP)](PF6)) in solid state at room temperature, which are confirmed as delayed fluorescence by investigating the emissions at 77 K.

Aggregation-Induced Near-Infrared Absorption of Squaraine Dye in an Albumin Nanocomplex for Photoacoustic Tomography in Vivo

Photoacoustic tomography (PAT) is a newly emerging noninvasive imaging modality that could be further enhanced using near-infrared (NIR)-absorbing materials as contrast agents. To date, the most extensively studied photoacoustic imaging agents are inorganic nanomaterials because organic materials with NIR-absorption capabilities are limited. In this study, a NIR-absorbing nanocomplex composed of a squaraine dye (SQ) and albumin was prepared based on the aggregation-induced NIR absorption of SQ. Through aggregation, the absorption spectrum of SQ was widened from the visible-light region to the NIR region, which facilitated photoacoustic signal generation in the tissue-transparent NIR optical window (700-900 nm). Blood analysis and histology measurements revealed that the nanocomplex can be used for PAT applications in vivo without obvious toxicity to living mice.

Multifunctional terpyridine/diphenylamine derivatives as highly efficient blue fluorescent emitters and red phosphorescent hosts

Three terpyridine (TPY)/diphenylamine (DPA) derivatives, with DPA functioning as the electron donor and TPY as the electron acceptor, were designed and synthesized. By switching the position of the nitrogen atom in the substituted pyridine of TPY acceptors, we can adjust the electron-drawing strength of the TPY group, and hence, further modify the fluorescence, lowest unoccupied molecular orbital energy levels, carrier transporting properties of three compounds, but barely influence triplet energy levels. Three compounds satisfy the requirements of multifunctional blue fluorophores and are successfully used as highly efficient blue fluorescent emitters and red phosphorescent hosts in organic light-emitting devices (OLEDs). Non-doped blue fluorescent OLEDs that use TPY22DPA, TPY33DPA, and TPY44DPA as emitters exhibit maximum external quantum efficiencies (EQEs) of 4.9%, 3.8%, and 2.7%, respectively. Meanwhile, red phosphorescent OLEDs that use TPY22DPA, TPY33DPA, and TPY44DPA as host materials exhibit maximum EQEs of 19.1%, 20.9%, and 17.2%, respectively. These results are among the best reported multifunctional blue fluorophore efficiencies.