Energy filtering and thermoelectrics: Artifact or artifice?

The possibility of selecting carrier energies by using suitable potential barriers has played a longstanding role in the physics of thermionic devices. It entered instead the arena of thermoelectricity only in the Nineties through the pioneering work of Rowe and Min and of Nishio and Hirano. Since then, the virtuous use of energy barrier in thermoelectricity has gone through alternating fortunes, with analyses sustaining its capacity as a tool to decouple the adverse interdependency between Seebeck coefficient and electrical conductivity; and papers disproving instead such a possibility. In spite of a yet uncomplete theoretical framework, over the last years an impressive number of papers has been published reporting unusual dependencies of thermopower and conductivity upon carrier densities, mostly in nanocomposites - and attributing them to energy filtering. Aim of this paper is to discuss to which extent and under which physical constraints energy filtering may be actually invoked to explain enhanced power factors - and which alternate possibilities of explanation may be considered instead.