Conductive materials, charge carrier transport

Figure 1. Diagram of charge carrier transport in conductive materials...

Time-of-llight (TOF) electrical conductivity measurements of ion-based materials Ic CP-TATA, 2c CP-TATA, and Sc CP-TATA" indicated ambipo-lar charge-carrier transport behavior with well-balanced values at high mobilities (10 —10 cm s ) for both holes and electrons without special purification... 

The process of charge carrier transport may occur in two steps (1) the injection of charge carriers into the material and (2) the movement of charge carriers through the material via hopping, tunneling, ballistic transport, diffusion, or metallic conduction. For composites with at least two phases, several aspects have to be considered the conduction in the polymer matrix, in the filler material, between adjacent filler... 

The first comprehensive reviews on photoconductive polymers were published by Stolka [10] and in coauthorship with Pai [11]. Chemical aspects of the topic were widely discussed in these reviews. Mainly from the chemical point of view, photoconductive polymers were also reviewed by Wiedmann [12] and by Biswas and Uryu [13]. A recent review of this kind was published by Naarman and Strohriegl in which both photoconducting and conducting polymers were described [14]. In the work of Mylnikov the photoconductivity of polymers was reviewed within the framework of semiconductor physics [15,16]. The review of Haarer [16] has also appeared in which he concentrates mainly on charge carrier transport properties of photoconductive polymers. In their comprehensive book, Borsenberger and Weiss recently described all aspects of photoconductive materials. [17]. 

As for the charge carrier transport properties of the nematic phase, there are few reports available, because of the very recent discovery of electronic conduction in the nematic phase [23, 24]. Considering the molecular alignment of the nematic phase it has no positional order, like an amorphous aggregate, so it is very possible that conduction takes place in the distributed localized states, similar to that for conduction in amorphous aggregates, and that conduction can be three-dimensional. Therefore, it is very possible that the intrinsic carrier transport properties are determined by carrier-dipole interactions as in the case of amorphous materials, i.e. there is a Poole-Frenkel type of carrier transport as is reported in the chiral nematic phase of a phenylquaterthiophene derivative [45]. 

As described in the previous section, both electronic and ionic conductions take place in mesophases when the material contains trace amounts of chemical impurities. These types of conduction exhibit different charge carrier transport properties, as demonstrated by the transient photocurrents the different mesophases measured under the same conditions as shown in Fig. 2.10. In these transient photocurrents, the fast transit times are shifted to shorter times as the molecular order in the mesophases is increased from SmA to SmE and from Coin to the plastic phase, while the slow transits stay in the same time range of 1,000 (xs irrespective of the mesophase in both smectics and discotics. Figiue 2.11 shows Arrhenius plots of the mobilities for fast and slow transits of the 2-phenylnaphthalene derivative 8PNP-012. The mobility for the fast transit hardly depends on the temperature, while the mobility for the slow transit does depend on temperature, with an activation energy... 

The experimental techniques which are used for characterization of polymer conductivity are in most cases the techniques adopted from classical physics of insulators, semiconductors, and conductors however, the interpretation of the results requires an imderstanding of the different nature of organic and disordered materials. In particular one has to take into account that in most polymers the charge carrier density and mobility are very low the charge carrier transport has dispersive character and also that polymers are sensitive to enviromnen-tal conditions and can be easily degraded by light, heat, oxygen, moisture, and so Development of new techni-... 

Although different representations may facilitate interpretations of some phenomena, one should bear in mind that they contain only these information on electrical properties of the investigated material which are contained in the capacitance C(cu) and in the conductance G(cu). The measurements of the frequency dependence of conductivity performed at different temperatures provide a very important means to distinguish the charge carrier transport mechanisms. 

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