Charge transport concentration effects

It is obvious, and verified by experiment [73], that above a critical trap concentration the mobility increases with concentration. This is due to the onset of intertrap transfer that alleviates thermal detrapping of a carrier as a necessary step for charge transport. The simulation results presented in Figure 12-22 are in accord with this notion. The data for p(c) at ,=0.195 eV, i.e. EJa—T), pass through a minimum at a trap concentration c—10. Location of the minimum on a concentration scale depends, of course, on , since the competition between thermal detrapping and inter-trap transport scales exponentially with ,. The field dependence of the mobility in a trap containing system characterized by an effective width aeff is similar to that of a trap-free system with the same width of the DOS. 

Secondly, we describe the site-selective introduction of a functional molecule, tetrakis-5,10,15,20-(4-carboxyphenyl)porphyrin (TCPP), into the microphase separation structure of a diblock copolymer film of PS-fo-P4VP. Since porphyrin derivatives show various functionalities such as sensitization, redox activity, and nonlinear optical effect, a polymer nanodot array containing a porphyrin at a high concentration would be applicable to a light-harvesing and charge transporting nanochannel. 

The work of Mensfoort et al. is a striking test of the importance of charge carrier density effects in space-charge-limited transport studies. For a given applied voltage the space charge concentration is inversely proportional to the device thickness. This explains why in Fig. 9 the deviation from the In cx... 

The distributed resistor model neglects the effect of mobile electrolyte ions. Much of our following discussion of the electrolyte s influence neglects, for simplicity, the distributed resistance. In a real dye cell, both effects operate simultaneously. Both tend toward the same result An applied potential will be more or less confined near the substrate electrode, depending on the relative rates of charge transport and interfacial charge transfer and on the concentration of electrolyte. 

Table 5.4 Effects of redox-site loading and sulfuric acid concentration on the charge-transport properties as obtained by cyclic voltammetry of [Os(bpy)2(PVP) Cl]Cl films.

Both categories of transport discussed above involve the motion of defects relative to an otherwise ordered array of ions, so the transport is referred to as defect transport. The concentrations of such defects (as contrasted with the total atom concentrations within the oxide lattice) are the important quantities for mass transport, charge transport, and space—charge effects, so it is the species defect concentration which appears in the diffusion equation. Likewise, the diffusion coefficients... 

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