Based on the results of first‐principles calculations of the electronic properties of blue light‐emitting materials, the molecular structures of oligofluorenes are optimized by incorporating electron‐withdrawing groups into the molecules to balance hole and electron injection and transport for organic light‐emitting diodes (OLEDs). The result is a remarkable improvement in the maximum external quantum efficiency (EQE) of the undoped device from 2.0% to 4.99%. Further optimization of the device configurations and processing procedures, e.g., by changing the thickness of the emitting layer and through thermal annealing treatments, leads to a very high maximum EQE of 7.40% for the undoped sky‐blue device. Finally, by doping the emitter in a suitable host material, 4,4’‐bis(carbazol‐9‐yl)biphenyl (CBP), at the optimal concentration of 6%, pure blue emission with extremely high maximum EQE of 9.40% and Commission Internationale de l’Eclairage (CIE) coordinates of (0.147, 0.139) is achieved.
Synchrotron-radiation topographic observations have shown that the Cu-doped (Ba0.25Sr0.75)0.9(K0.5Na0.5)0.2Nb2O6 crystal has a strong ferroelectric x-ray anomalous-scattering effect at the wavelength near the absorption edge of Ba atoms. The reversal of anomalous contrast of antiparallel domains in the crystal was directly revealed in the topographs of hkl and hkl̄ reflections. It was found that the domain walls are all polar and are formed to compensate the charges resulting from the inhomogeneous distributions of the metal atoms in the bulk crystal.
The Andreev-like levels and the free energy of the spin superconductor/insulator/spin superconductor junction are obtained by using the Bogoliubov-de Gennes equation. The phase dependence of the spin supercurrents exhibits a 0π transition by changing the barrier strength. The dependences of the critical current on the barrier strength and the temperature are also presented.
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