Hopping/tunnelling transport

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... 

Transport Mechanism Redox exchange variable range hopping Tunneling, off resorumce injectirm, ... 

Besides its temperature dependence, hopping transport is also characterized by an electric field-dependent mobility. This dependence becomes measurable at high field (namely, for a field in excess of ca. 10d V/cm). Such a behavior was first reported in 1970 in polyvinylcarbazole (PVK) [48. The phenomenon was explained through a Poole-ITenkel mechanism [49], in which the Coulomb potential near a charged localized level is modified by the applied field in such a way that the tunnel transfer rale between sites increases. The general dependence of the mobility is then given by Eq. (14.69)... 

The mechanism of ion transport in such systems is not fully elucidated, but it is presumably dependent on the degree of crystallinity of the polymeric complex (which further depends on the temperature and the salt type). The ionic conductivity was initially attributed to cation hopping between fixed coordination sites in the depicted helical tunnel, i.e. in the crystalline part of the polymer. 

The conductance of the perylenebisimide (PBI) 15 was measured by the STM-BJ technique as 1 nS [73]. Note that the thiophenol handles are not conjugated to the central core, contributing to the small value. Electron transport was temperature-independent, indicating a tunneling mechanism. However, when a gate electrode reduced the core to its radical anion, the conductance became thermally activated, indicating that electron transport then follows a hopping mechanism into and out of the core. 

Keywords Charge transport Electrochemical molecular junctions Electron tunnelling Electron hopping Molecular electronics Photoactive molecular junctions... 

The experiments just discussed made it clear that the motion of the hole on the series of As represents a different mechanism of transport than tunneling. Giese [13] and Bixon and Jortner [18] suggested that this mechanism is incoherent hopping of the hole between neighboring bases. This means that the hole wavefunction is Hmited to one base. The wavefunctions of the remaining electrons on that base would of course be distorted by the presence of the hole. Thus in this view of the transport process the base on which the hole sits could be called a molecular polaron, or a small polaron because it is limited to one site. 

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