Charge transport mechanisms

Semiconducting Ceramics. Most oxide semiconductors are either doped to create extrinsic defects or annealed under conditions in which they become non stoichiometric. Although the resulting defects have been carefully studied in many oxides, the precise nature of the conduction is not well understood. Mobihty values associated with the various charge transport mechanisms are often low and difficult to measure. In consequence, reported conductivities are often at variance because the effects of variable impurities and past thermal history may overwhelm the dopant effects. 

Electron-hopping is the main charge-transport mechanism in ECHB materials. There is precedence in the photoconductivity Held for improved charge transport by incorporating a number of redox sites into the same molecule. A number of attempts to adapt this approach for ECHB materials have been documented. Many use the oxadiazole core as the electron-transport moiety and examples include radialene 40 and dendrimer 41. However, these newer systems do not offer significant improvements in electron injection over the parent PBD. 

Besides injection mechanisms, in order to describe the l/V characteristics of LEDs, the charge transport mechanisms in the bulk have to be taken into account. 

One has to consider that in Eqs. (9.15)—(9.17) the mobility /t occurs as a parameter. As it will be pointed out below, // shows a characteristic dependence on the applied electric field typical for the type of organic material and for its intrinsic charge transport mechanisms. For the hole mobility, //, Blom et al. obtained a similar log///,( ) const. [E dependency [88, 891 from their device modeling for dialkoxy PPV as it is often observed for organic semiconductors (see below). 

After a brief discussion of fundamentals of charge transport mechanisms, this chapter summarizes and discusses the most significant results obtained by using different junctions and in particular LAJs. In order to facilitate a systematic discussion, we make a functional distinction between non-active and active junctions we will refer to active junctions as those aimed at changing the electrical response by means of an external stimulus acting in situ to modify the molecular electronic structure non-active junctions are those used to measure and compare the electrical properties inherent to the different electronic structure of incorporated molecules, without any modification induced by an external signal. 

Polymerization Mechanism in Region III. In region III, all the electrons cannot be transported to the anode in a half cycle of the discharge frequency. A possible charge transportation mechanism is an ambipolar diffusion of ion and electron pairs which will cause polymerization. The diffusion of free radicals may also contribute to the polymerization. In our experiment, the contribution of these two mechanisms cannot be distinguished because the ion and electron pairs behave as neutral gases. 

The structures and charge transport mechanisms for polymer electrolytes differ greatly from those of inorganic solid electrolytes, therefore the purpose of this chapter is to describe the general nature of polymer electrolytes. We shall see that most of the research on new polymer electrolytes has been guided by the principle that ion transport is strongly dependent on local motion of the polymer (segmental motion) in the vicinity of the ion. 

The study of the dispersion of photoinjected charge-carrier packets in conventional TOP measurements can provide important information about the electronic and ionic charge transport mechanism in disordered semiconductors [5]. In several materials—among which polysilicon, a-Si H, and amorphous Se films are typical examples—it has been observed that following photoexcitation, the TOP photocurrent reaches the plateau region, within which the photocurrent is constant, and then exhibits considerable spread around the transit time. Because the photocurrent remains constant at times shorter than the transit time and, further, because the drift mobility determined from tt does not depend on the applied electric field, the sample thickness carrier thermalization effects cannot be responsible for the transit time dispersion observed in these experiments. 

The recombination current density, Jr, can be treated effectively as a Schottky barrier diode current density. Including both thermionic emission and diffusion charge transport mechanisms (13) Jr can be written as... 

Charge Transport Mechanism in Organic Polymeric and Molecular Materials... 

We have demonstrated that the donor-acceptor conjugates 18a,b exhibit efficient charge-transfer processes upon photoexcitation over distances of more than 24 A. The charge-transport mechanisms are comparable to those established for the corresponding exTTF-oPPV -C6o and exTI -oPP, systems. In view of the molecular-wire behavior of the oFLs, we have shown that in fluorene-based oligomers the ability to conduct charges lies between that of oPPVs and that of... 

The electrical conductivity of sapphire in a particular crystallographic direction was found to be 1.25mSm 1 at 1773 K. An independent experiment on the same material at the same temperature determined the oxygen tracer diffusion coefficient to be 0.4nm2s 1, the diffusion occurring by a vacancy mechanism. Do these data favour oxygen ion movement as the dominant charge transport mechanism (Relative atomic masses, A1 = 27 and 0=16 density of sapphire, 3980 kgm-3.)... 

Fig. 5.23 Speculative diagram describing the dependence of charge transport mechanisms in alumina on temperature and oxygen pressure.

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