Subject classification: 72.20.Ee; 72.80.Ng; S12Percolation approach is used to study the dc hopping conductivity and thermopower in systems with a Gaussian density of localized states typical for disordered organic materials. It is shown that the theoretical methods developed earlier for the description of hopping transport in disordered inorganic solids, such as amorphous semiconductors, can also be successfully applied to description of hopping transport in organic disordered solids, such as conjugated or molecularly doped polymers. Calculations within the percolation approach give results in excellent agreement with those obtained by using a more transparent, though less rigorous approach based on the concept of the transport energy.Introduction In various disordered inorganic and organic materials, the transport of charge carriers at low temperatures is related to incoherent hopping between localized states. While the development of theory for transport in inorganic disordered solids, such as doped crystalline semiconductors, mixed crystals, semiconductor glasses, amorphous and microcrystalline semiconductors, was logically consistent, the situation is different for disordered organic solids, such as molecularly doped polymers, conjugated polymers and organic glasses. Efficient theoretical methods have been developed for inorganic systems. Among the most successful methods, one can note the percolation approach and the approach based on the concept of the transport energy. However, these methods are rarely applied to organic systems. On the contrary, description of hopping transport in organic materials is often based on the ensemble averaging of hopping rates, which is known to be quite inappropriate for the description of hopping processes. Such situation is unsatisfactory, since the physics of hopping processes in organic and inorganic solids and the basic models for their description are rather similar [1,2]. In both cases it is assumed that the hopping rate of a charged carrier G ij between two localized states i and j with energies e i and e j separated by the distance R ij is determined by the standard expression [1]

The electron hopping in presence of both an electric field and a temperature gradient is considered. Expressions for the transport coefficients are obtained, and the temperature clependence of the hopping thermopower is discussed for different temperature regions.Der Elektronentransport durch ,,Hopping" wird bei Anwesenheit eines elektrischen Feldes sowie eines Temperaturgradientes betrachtet. Ausdriicke fur die Transportkoeffizienten werden abgeleitet, und die Temperaturabhkngigkeit der Hopping-Thermospannung wird fur verschiedene Temperaturbereiche diskutiert.

Basic physical ideas and methods that are used to improve the quality of modern thermoelectric materials and to increase the thermoelectric figure-of-merit are reviewed, with special emphasis on how nanostructure affects the thermoelectric properties of materials.

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A new model for avalanche carrier multiplication in amorphous semiconductors is suggested. In contrary to previous considerations, the model does not employ the Shockley's lucky-electron ansatz according to which a free carrier gains the energy from electric field in a ballistic motion. We show that the majority of free carriers reaching the ionization threshold energy do so by drift, not ballistically.