The design of efficient and concentration-insensitive metal-free thermally activateddelayed fluorescence (TADF) materials is reported. Blue and green organic light-emitting diodes (OLEDs) containing a hole-transport layer, an undoped TADF emissive layer, and an electron-transport layer achieve maximum external quantum efficiencies of 19%, which is comparable to the best doped OLEDs.

A new series of luminescent 1,4-diazatriphenylene (ATP) derivatives with various peripheral donor units, including phenoxazine, 9,9-dimethylacridane and 3-(diphenylamino)carbazole, is synthesized and characterized as thermally activated delayed fluorescence (TADF) emitters. The influence of the donor substituents on the electronic and photophysical properties of the materials is investigated by theoretical calculations and experimental spectroscopic measurements. These ATP-based molecules with donor–acceptor–donor (D–A–D) structures can reduce the singlet–triplet energy gap (0.04–0.26 eV) upon chemical modification of the ATP core, and thus exhibit obvious TADF characteristics in solution and doped thin films. As a demonstration of the potential of these materials, organic light-emitting diodes containing the D–A–D-structured ATP derivatives as emitters are fabricated and tested. External electroluminescence quantum efficiencies above 12% and 8% for green- and sky-blue-emitting devices, respectively, are achieved.

Synopsis : Electrodeposition of Ni-W alloys was conducted from an unagitated sulfate solution containing citric acid at pH 5 and 60 ºC under coulostatic (3.44×10 5 -6.22×10 5 C/m 2 ) and galvanostatic (30-5000 A/m 2 ) conditions. Before annealing, the lattice constant of Ni increased linearly with an increase in the W content up to 40.7 mass% in accordance with Vegard's law, showing the W supersaturated solid solution into Ni. At W contents of < 37.1 mass%, the deposits showed a morphology of field oriented texture, which a preferred orientation of specific plane occurs toward the electric field in deposition, and the edges of platelet crystals were exposed at surface. At W contents of > 40.7 mass% of solid solubility limit, the cross section of deposits showed a layered morphology, while the surface became smooth with small granular crystals. After annealing, Ni 4 W precipitated in deposits of W contents of 32.6 and 37.1 mass%, while both Ni 4 W and NiW precipitated entire surface finely in deposits of W contents of 40.7 to 45.3 mass%. Before annealing, the hardness of deposits increased with W content, and the increase was particularly large at W content of 40.7 mass%. The hardness was almost constant regardless of current density at W contents of > 40.7 mass%. The alloy composition to change the hardness of deposits significantly corresponded with that to change the structure of deposits. The hardness of deposits increased at all W contents by annealing, and the degree of increase was particularly large at W contents of > 40.7 mass%.

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