We present device model calculations for the current–voltage (I–V) characteristics of organic diodes and compare them with measurements of structures fabricated using MEH-PPV. The measured I–V characteristics have a Fowler–Nordheim (FN) functional form, but are more than three orders of magnitude smaller than the calculated FN tunneling current. We find that the low mobility of the organic materials leads to a large backflow of injected carriers into the injecting contact. These results account for the experimental observations and also demonstrate how transport layers in multilayer organic light-emitting diodes can be used to improve carrier injection.

A new class of simple, highly efficient, cylindrical acoustic concentration devices has been developed based upon cylindrical (or near cylindrical) geometries [Kaduchak et al., Rev. Sci. Instrum. 73, 1332–1336 (2002)] for aerosol concentration applications. The concentrators are constructed from single PZT tubes driven at or near the breathing mode resonance. Acoustic concentration of aerosols is performed within the tube cavity. It has been found that slight modifications to the cylindrical cavity geometry can significantly increase the collection efficiency and assist in precise particle positioning. This paper analyzes the theoretical framework for the acoustic concentration of particles in these devices for various geometrical perturbations. The cavity geometries studied are (1) hollow cylindrical piezoelectric tube, (2) hollow piezoelectric tube with an inner concentric solid cylinder insert, (3) a hollow piezoelectric tube with a concentric elliptic insert which breaks the circular-cylindrical symmetry, and (4) a hollow elliptic cylindrical piezoelectric tube. It is shown that breaking the circular symmetry within the cavity localizes the particles in small spatial regions within the cavity. This localization of particles may be very useful in applications requiring aerosol collection or particle stream positioning.

We present an analysis of the effect of surface acoustic waves ͑SAW's͒ on the optical properties of III-V semiconductor multiple quantum wells ͑MQW's͒. Modulation spectra at the fundamental and second harmonic of the SAW frequency are presented. The SAW modulates the optical properties of the MQW primarily by changing optical transition energies. The SAW generates both strains, which modulate the transition energies by deformation potential effects, and electric fields, which modulate the transition energies by the quantum confined Stark effect. We find that modulation of the transition energies by strain effects is usually more important than by electric-field effects. If large static electric fields occur in the MQW, the SAW-generated electric field can mix with the static field to give optical modulation, which is comparable in magnitude to modulation from the deformation potential effect. If there are no large static electric fields, modulation by the SAW-generated fields is negligible. A large static electric field distributes oscillator strength among the various optical transitions so that no single transition is as strong as the primary allowed transitions without a static electric field. To achieve the maximum modulation for fixed SAW parameters, it is best to modulate a strong optical transition. Thus optimum modulation occurs when there are no large static electric fields present and that modulation is primarily from deformation potential effects. We specifically consider Ga x In 1Ϫx As/Ga x Al 1Ϫx As MQW's grown on ͑100͒ and ͑111͒ oriented substrates, but our general conclusions apply to other type I MQW's fabricated from III-V semiconductors.

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