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Laser diodes based on organic semiconductor materials have high threshold current densities that require the suppression of various inherent loss processes. One way to study such loss processes is to analyze the external quantum efficiency (EQE) roll-off in organic light-emitting diodes (OLEDs). In this work, we used electrical simulations to analyze the origin of the experimental EQE roll-off of an OLED based on 4,4′-bis[(N-carbazole)styryl]biphenyl (BSBCz) under extremely high current injection (∼1 kA/cm2). We considered various singlet exciton annihilations and quenching processes (i.e., singlet-singlet annihilation, singlet-triplet annihilation, singlet-polaron annihilation, singlet-heat quenching, and electric field quenching of singlet excitons). These results showed that the EQE roll-off can be attributed to Joule heating and/or singlet-triplet annihilation and/or the dissociation of singlet excitons under a high applied electric field. The electric field quenching of singlet excitons was confirmed by a field-induced photoluminescence (PL) quenching experiment. By applying an electric field-induced charge dissociation model to both the EQE and field-induced PL quenching, we estimated the singlet exciton binding energy of a BSBCz film to be in the range of 0.64–0.71 eV. [ABSTRACT FROM AUTHOR]

Direct numerical simulation of a temporally developing shearless turbulent mixing layer is performed. Two quasi-homogeneous isotropic turbulent (HIT) regions with different turbulent kinetic energies (TKEs) and a mixing-layer region temporally develop. The small-scale properties are analyzed with the velocity gradient tensor. The statistics on the velocity variances show that the development of the mixing layer is divided into two stages. In the first stage, grid turbulence in the large-TKE region has not fully developed, and the center of the mixing layer hardly moves. Large-scale intermittency grows in the mixing-layer region at this stage. In the second stage, grid turbulence in the large-TKE region has fully developed, and the center of the mixing layer moves toward the small-TKE region. The small-scale intermittency is most significant in the mixing-layer region in both stages. The statistics on the velocity gradient tensor show that stronger vortex compression occurs more frequently in the mixing-layer region than in the quasi-HIT regions at late times. In addition, the extensive and compressive eigenvalues of the rate-of-strain tensor exhibit the strongest intermittency in the mixing-layer region at late times. [ABSTRACT FROM AUTHOR]

We report on the cavity numerical characterization of a second order one-dimensional distributed feedback organic laser. The gain medium containing 6wt. % of 4,4'-bis[(N-carbazole) styryl]biphenyl) in a 4,4'-bis[9-dicarbazolyl]-2,20-biphenyl) host is vacuum deposited to form an organic thin film on a SiO2 grating. The influence of the organic film thickness on the properties of the resonant cavity is investigated through numerical calculations of both the confinement factor Γ and the Q-factor. The Q-factor is obtained using two methods, one by calculating the eigenmodes of the resonant cavity and the other by calculating the reflection spectrum. It was found that while the Γ increases with the organic film thickness, the Q-factor shows a non-monotonic function with a maximum value for a thickness of 200 nm. Published by AIP Publishing. [ABSTRACT FROM AUTHOR]

In recent years, the method of x-ray computer tomography imaging has been used to characterize the microstructure of asphalt concrete, cement concrete, soil, and rock. This paper focuses on developing an algorithm that links the particle identity at different load steps and thus provides sequential movements of particles. The measured data from a specimen under triaxial load is analyzed to study the nonuniform strain field and to identify shear band formation. Packing structure change and the evolution of fabric tensors due to external load are discussed. The validity of a commonly used kinematic hypothesis in granular mechanics is also evaluated. CE Database subject headings: Radiography; Granular materials; Strain; Kinematics; Bonding.