Transport charge

Although solitons and bipolarons are known to be the main source of charge carriers, the precise mechanism is not 

Ikeda and his coworkers studied a mechanism of acetylene polymerization in connection with olefin polymerization by various Ziegler-Natta catalysts. They found that polymerization yields not only highly polymerized polyacetylene but also benzene, which is a cyclic trimer of acetylene, and that the ratio of these two products depends 

The configuration of the double bonds strongly depends on the temperature of polymerization. The trans content of polyacetylene prepared by the Ziegler-Natta catalysts decreases with decreasing polymerization temperature, as listed in Table 29.3, determined by infrared spectroscopy. 

A thermal study by Ito et al. [39] indicated that irreversible isomerization of the cis form occurs at 

TABLE 29.3 The Trans Contents of Polyacetylene Prepared at Different Temperatures  

Three different contributions determine the carrier concentration in organic semiconductors used as active layers in an electronic device  

FIGURE 8.11. Femtosecond field-induced pump/probe spectra of ladder-type PPP at different pump/probe delays (rd). Within a few hundred picoseconds, the electric field leads to the dissociation of excitons which can be seen by (1) the decrease in stimulated emission at 2.4 and 2.55 eY and (2) the absorption of the created polarons at 1.9 and 2.1 eV. (Reproduced from Ref. 166.) 

For an intrinsic semiconductor, the effective density of states Nc,v of the valence (V) or conduction (C) band is given by the following relation 29 

Again taking the effective masses to be equal to the electron mass, using 300 K for room temperature, and using 2.7 eV for the energy gap of the ladder-type PPPs, we obtain an intrinsic carrier concentration of 10 4 cm 3. This value will rise to 41 cm-3 upon increasing the temperature to 400 K. These values have to be compared to common inorganic semiconductors at room temperature 2.7 

From picosecond transient photoconductivity measurements on PPP films,22 we know that mobile charged states decay within 110 ps. In conventional routes to PPPs, defects like branched chains and large torsion angles of neighboring rings are known to occur. These defects act as shallow or deep traps for positive and negative polarons,38,39 which limit the mobility of charge carriers.40 The synthetic route toward the PPP-type ladder-polymers prevents the described defects and leads to a trap concentration of less than 1 trap per 1000 monomer units,28 whereas substi- 

Mechanbm Thickness Voltag Temperature Notes/ References 

Coherent tunneling exp(—bii) Linear (low V), exponential (highV) Weak ) [157] 

Redox exchange (hopping) rf-i Linear (low V) Strong, activated [162] 

Nonresonant tunneling can be identified by several features, including an exponential decay of the tunneling current as the thickness of the molecular layer increases, a very weak temperature dependence (with essentially zero activation over a fairly wide temperature range), and a somewhat distinctive 

The prevalent feature of all known photoconductive polymers is that they all either 

have an extended jr-electron system in the backbone or in groups pendant to the chain or 

are a-conjugated, as in the case of silicon backbone polymers (poly-silylenes). 

The transport-active groups can be part of the polymer backbone structure, they can be covalently linked as pendant groups to a vinyl or similar chain such as carbazole groups (substituted aromatic amines) in PVK, or need not be covalently attached to the polymer backbone at all. Indeed, solid solutions of NIPC in polycarbonate display hole mobilities that are comparable to those in PVK [19, 20]. The polymer backbone in PVK does not contribute to transport, but merely ties the transport-active groups together. Similarly, both solid solutions of triphenylamine (TPA) in polycarbonate [21] and poly (methacrylate) with pendant triphenylamine groups [22] display photoconductivity and charge transport. 

The velocity v of migrating charge carriers in organic polymers depends on the electric field , the temperature T and the intersite distance g [21]  

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Niu S and Mauzerall D 1996 Fast and efficient charge transport across a lipid bilayer is electronically mediated by Cyf, fullerene aggregates J. Am. Chem. Soc. 118 5791-5... 

Henderson P T, Jones D, Hampikian G, Kan Y Z and Schuster G B 1999 Long-distance charge transport in dupiex DNA the phonon-assisted poiaron-iike hopping mechanism Proc. Natl Acad. Sc/., USA 96 8353-8... 

Nanoclusters/Polymer Composites. The principle for developing a new class of photoconductive materials, consisting of charge-transporting polymers such as PVK doped with semiconductor nanoclusters, sometimes called nanoparticles, Q-particles, or quantum dots, has been demonstrated (26,27). 

According to the Scher-MontroU model, the dispersive current transient (Fig. 5b) can be analyzed in a double-log plot of log(i) vs log(/). The slope should be —(1 — ct) for t < and —(1 + a) for t > with a sum of the two slopes equal to 2, as shown in Figure 5c. For many years the Scher-MontroU model has been the standard model to use in analyzing dispersive charge transport in polymers. 

In moleculady doped polymers, charge transport is carried out by the hole-transporting molecular dopants, usually aromatic amines. The polymer merely acts as a binder. The hole mobiUty is sensitive to the dopant concentrations. For example, the hole mobiUty of... 

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. 

This article addresses the synthesis, properties, and appHcations of redox dopable electronically conducting polymers and presents an overview of the field, drawing on specific examples to illustrate general concepts. There have been a number of excellent review articles (1—13). Metal particle-filled polymers, where electrical conductivity is the result of percolation of conducting filler particles in an insulating matrix (14) and ionically conducting polymers, where charge-transport is the result of the motion of ions and is thus a problem of mass transport (15), are not discussed. 

Charge Transport. Side reactions can occur if the current distribution (electrode potential) along an electrode is not uniform. The side reactions can take the form of unwanted by-product formation or localized corrosion of the electrode. The problem of current distribution is addressed by the analysis of charge transport ia cell design. The path of current flow ia a cell is dependent on cell geometry, activation overpotential, concentration overpotential, and conductivity of the electrolyte and electrodes. Three types of current distribution can be described (48) when these factors are analyzed, a nontrivial exercise even for simple geometries (11). 

Most of the known charge-transport layers are -type or hole transporting. Thus this type of layered photoconductor must be charged negatively. 

Fig. 8. (a) The four basic classes of charge-transporting polymers, and (b) corresponding examples. Class 4 polymers may be either CJ-bonded or... 

Transport numbers are intended to measure the fraction of the total ionic current carried by an ion in an electrolyte as it migrates under the influence of an applied electric field. In essence, transport numbers are an indication of the relative ability of an ion to carry charge. The classical way to measure transport numbers is to pass a current between two electrodes contained in separate compartments of a two-compartment cell These two compartments are separated by a barrier that only allows the passage of ions. After a known amount of charge has passed, the composition and/or mass of the electrolytes in the two compartments are analyzed. Erom these data the fraction of the charge transported by the cation and the anion can be calculated. Transport numbers obtained by this method are measured with respect to an external reference point (i.e., the separator), and, therefore, are often referred to as external transport numbers. Two variations of the above method, the Moving Boundary method [66] and the Eiittorff method [66-69], have been used to measure cation (tR+) and anion (tx ) transport numbers in ionic liquids, and these data are listed in Table 3.6-7. 

In accordance with Ohm s law, if we were to double the intensity X of the electric field, the current would be doubled that is to say, the plane CD would have to be placed at twice the distance from AB. If the number of conduction electrons per unit volume is p, and the distance between the planes CD and AB is denoted by v, we have n = pv, since we are discussing the unit area. Hence the net resultant charge transported in unit time across AB, that is, the current density, is given by... 

Miller, I. R. Structural and energetic aspects of charge transport in lipid layers and in biological membranes, in Topics in Bioelectrochemistry and Bioenergetics, Vol. 4 (ed.) Milazzo, G., New York, Wiley 1981... 

Multilayer Devices The Incorporation of Charge-Transporting Layers... 

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