Transport of charge carriers

The transport of charge carriers through a solid is characterized by the drift mobility which is defined as the hole or electron velocity per unit electric field strength. //, frequently given in units of cm s , can be obtained with the 

Chart 2.3 Chemical structure of tris(8-oxyquinolato)-aluminum, Alqa. 

From Table 2.3, which lists typical // values, it can be seen that the hole mobility in conjugated polymers is lower than that in organic crystals and amorphous silicon, but much larger than that in undoped poly(N-vinyl carbazole). Therefore, conjugated polymers have potential for applications in conducting opto-electronic and photonic devices. In principle, this also applies to liquid-crystal systems that can exhibit enhanced molecular order due to their self-organizing ability, as has been pointed out in a progress report [42]. 

For a lamellae orientation parallel to the substrate, hole mobility values as high as 0.1 cm T s were found. In this context, work with isolated linear polymer chains (molecular wires) is also noteworthy [44]. It revealed that the hole transport mobility along isolated polymer chains can exceed 0.1 cm s , as can be seen in 

Interestingly, electron transport has been observed with a diene compound of the structure shown in Chart 2.4. 

The filled upper and free lower orbitals in PVC belong to the carbazole groups. So one can consider PVC as a disordered organic medium. The side chromophore groups play the role of traps for hopping charge carrier transport. 

It was actually shown by the time of flight method [32-34] that coulomb type traps control the drift mobility. The concentration of such traps is 1015 m 3. The real mobility (without traps) was estimated [35] to be of the order 5x10 8 m2 V 1 s 1 with a thermal activation energy of 0.28 eV. There are no correct data as yet confirming the impurity hopping model in PVC. The drift mobility is due rather to the jumps between neighbouring molecules and not shallow traps of the semiconductor. 

Dispersive transport in PVC was investigated. The results of Pfister and Griffits obtained by the transit method are shown in Fig. 6. The hole current forms at temperatures 400 K clearly show a bend corresponding to the transit time of the holes. At lower temperature the bend is not seen and transit time definition needs special methods. The pulse form shows the broad expansion during transition to the opposite electrodes. This expansion corresponds to the dispersive transport [15]. The super-linear dependence of the transit time versus sample thickness did not hold for pure PVC. This is in disagreement with the Scher-Montroll model. There are a lot of reasons for the discrepancy. One reason may be the influence of the system dimensions. It is quite possible that polymer chains define dimension limits on charge carrier transfer. 

The influence of the light intensity on the hole transfer was marked [36]. The good agreement between experimental and theoretical data for dispersive transport was observed only for a low intensity of the exciting light. 

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Blum T, Bassler H (1988) Reinvestigation of generation and transport of charge-carriers in crystalline polydiacetylenes. Chem Phys 123 431... 

In the assumptions that were made in this chapter up to the beginning of this section, it was assumed that transport of charge carriers to and from the electrode played no part in rate control because it was always plentiful. Thus, in the evolution of hydrogen from acid solutions, the current density in most experimental situations is less than 10 times the limiting diffusion current and for this reason there is a negligible contribution to the overpotential due to an insufficiency of charge carriers. Like water from the tap in a normal city, the rate of supply of carriers is both tremendously important but seldom considered, for there is always plenty available. 

The transport of charge carriers is evidently also facilitated by the lowering of the barrier potentials between molecules due to the intermolecular bridges in n complexes of copper with acetylenic bonds. 

When an electric field is applied to an ideal dielectric material there is no long-range transport of charge but only a limited rearrangement such that the dielectric acquires a dipole moment and is said to be polarized. Atomic polarization, which occurs in all materials, is a small displacement of the electrons in an atom relative to the nucleus in ionic materials there is, in addition, ionic polarization involving the relative displacement of cation and anion sublattices. Dipolar materials, such as water, can become polarized because the applied electric field orients the molecules. Finally, space charge polarization involves a limited transport of charge carriers until they are stopped... 

Under these circumstances, then, the rate of the photoelectrochemical reaction is determined by the rate of transport of charge carriers to the interface. For p-typc photocathodes, this would refer to the photoactivated electrons that will have been produced from the valence band and are now in the conduction band. The electrons there are impelled to the surface both by diffusion (dependent on the concentration gradient of electrons) and by means of the electric field resulting from the potential gradient near the surface (Fig. 10.8), that depends on the electrode potential that shifts the Fermi level. For a given photoillumination intensity, there will be a fixed limiting current where (at best) the rate of transport to the surface becomes equal to the maximum rate of electron production due to photoactivation (Fig. 10.9). In reality, the numerical value of the limiting current will be also determined by various accidents (e.g., collisional deactivations) that destroy photoelectrons on their way to the interface. 

The generation and transport of charge carriers in these and other charge transfer complexes is determined by electron affinities and ionization potentials of the components involved and by the stabilization energies of cation radicals of the polymeric species and anion radicals of the acceptor, electron transporting species147. ... 

Much of the modeling of devices requires knowledge of the mechanism of the transport of charge carriers in the conducting organics. Because of the importance of transport properties, extensive work has been done on this topic. Transport properties are also discussed in detail in Chapter 3. 

To investigate the dependence of the polarization on molecular quantities, it is convenient to assume the polarization P to be divided into two parts the induced polarization Pa, caused by translation effects, and the dipole polarization P, caused by the orientation of the permanent dipoles. Note that in ionic polarization the transport of charge carriers and their trapping can also create induced polarization. 

Entropy flux in the absence of a net particle flow is equivalent to Jq/T where Jq is the heat flux. Thus, Eq. (6.9.4) is a formulation of Fourier s Law for heat conduction, Jq — —kVT, thereby identifying the thermal conductivity associated with the transport of charge carriers as... 

An equivalent circuit of resonant-tunneling nanostructures taking into account spin-polarized transport of charge carriers is proposed. It is based on the approximation of I-V characteristics and represents spin shifted energy levels in the quantum well. 

Within the bulk heterojimction, the donor and acceptor domains are generally disordered in volume. For exciton dissociation and charge generation a fine nanoscale intermixing is required, whereas for the efficient transport of charge carriers percolation and a certain phase separation are needed to ensure imdisturbed transport. Hence the optimization of the nanomorphology of the photoactive blend is a key issue for improving the efficiency of the photovoltaic operation [62,66,67]. 

The operation of organic light-emitting diodes (OLEDs) involves charge injection from electrodes, transport of charge carriers, recombination of holes and electrons to generate electronically excited states or excitons, followed by their deactivation by emission of either fluorescence or phosphorescence. The main factors that determine luminous and external quantum efficiencies are the following ... 

One-dimensional nanostructures such as the titania nanotubes shown in Fig. 6 and ZnO nanorods have been the focus of much recent interest [19,21,22,26]. These studies are motivated by the expectation that the transport of charge carriers along the tubes is more facile than within a random network of nanoparticles where the... 

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