Nanostructured thermoelectrics

Thermal to electrical energy conversion, through thermoelectric and thermionic materials, has been proposed to be much more efficient in lower dimensional materials at the nanoscale. In this paper, we review the underlying materials physics of nanostructured thermoelectrics which gives rise to such enhanced efficiency. We first study the basic phenomenology of the contributing terms to the power factor in the thermoelectric figure of merit, i.e., the Seebeck coefficient (S) and the electrical conductivity (σ), which are analyzed through the Boltzmann transport formalism and then thoroughly compared to recent experiments in nanostructures. Additional factors, hitherto not given much consideration, such as carrier scattering time approximations vis-à-vis dimensionality and the density of states (DOS) are also studied. Through such a study, we postulate that it is the sheer magnitude and not the specific shape of the DOS that is important in enhancing the thermoelectric power factor. We then see that most of the understood increase in the figure of merit of nanostructured thermoelectrics has been accomplished through a drastic reduction of the lattice thermal conductivity (κL)—by two orders of magnitude from the bulk values through the introduction of scattering at different length scales. Such a reduction has provided a large impetus for the use of nanostructures and will be reviewed. We next consider solid state implementations of thermionic structures, which seem to be converging towards thermoelectric devices and hence can be described by a similar figure of merit. Approaches for energy filtering and further increasing efficiencies are also described.