Field-induced carriers, transport

In 1964, Moore and Pressman [16] discovered that vaiinomydn induces K" -uptake in mitochondria. By various methods it was then demonstrated that in alcoholic solution the depsipeptide forms very stable complexes with K, Rb" and Cs -ions. Since then the investigation of mechanisms by which certain substances facilitate ion transport in lipid membranes has developed into a major field in biophysics. Besides the carrier transport mentioned here, there also exists a channel mechanism. 

This proposal was in line with the improved luminance efficiency identified as the result of an enhanced balance in the charge carrier transport properties. In this case, it was proposed that the electrons would spread into the semiconductor, thereby causing the halide to be detached from the polymer backbone on contact formation between the cathode material and the PPV. The anions produced in this way would drift either in the self-induced electric field established by the cathode close to the electron reservoir, or in the external electric field. The subsequent precipitation that would be expected to occur would result in a chemical modification of the interface region of the contact materials that, ultimately, would cause aging and fatigue of the device. 

However, the values for the current that are obtained with the actual device parameters using the Fowler-Nordheim equation are several orders of magnitude higher than the values for the measured current in real devices. This is due to the fact that the I-V characteristics of PLEDs are determined not only by the injection mechanisms but also by the charge transport mechanism in the active polymer layer (see Section V). The discrepancy between the measured and calculated values for the current in the model for field-induced tunneling can be accounted for by a backflow current of the injected charge carriers into the injection contact. This effect then reduces the net device current and seems to be especially important in low mobility conjugated polymers [93]. 

The authors use optical spectroscopy of gate-induced charge carriers to show that, at low temperature and small lateral electric field, charges become localized onto individual molecules in shallow trap states, but that at moderate temperatures an electric field is able to detrap them, resulting in transport that is not temperature-activated. This work demonstrates that transport in such systems can be interpreted in terms of classical semiconductor physics and there is no need to invoke onedimensional Luttinger liquid physics [168]. 

Mathematical analysis of this problem has been performed by Belyakov et al. (1979a) on the basis of the equations of transport along the surface with due account for carrier migration in the self-induced electric field S (y). 

Fig. 26. Schematic design of field flow fractionation (FFF) analysis. A sample is transported along the flow channels by a carrier stream after injection and focusing into the injector zone. Depending on the type and strength of the perpendicular field, a separation of molecules or particles takes place the field drives the sample components towards the so-called accumulation wall. Diffusive forces counteract this field resulting in discrete layers of analyte components while the parabolic flow profile in the flow channels elutes the various analyte components according to their mean distance from the accumulation wall. This is called normal mode . Particles larger than approximately 1 pm elute in inverse order hydrodynamic lift forces induce steric effects the larger particles cannot get sufficiently close to the accumulation wall and, therefore, elute quicker than smaller ones this is called steric mode . In asymmetrical-flow FFF, the accumulation wall is a mechanically supported frit or filter which lets the solvent pass the carrier stream separates asymmetrically into the eluting flow and the permeate flow which creates the (asymmetrical) flow field...

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