Hierarchical nanostructuring approaches for thermoelectric materials with high power factors

Abstract

The thermoelectric power factor of hierarchically nanostructured materials is investigated using the non-equilibrium Green's function method for quantum transport, including interactions of electrons with acoustic and optical phonons. We describe hierarchical nanostructuring by superlatticelike potential barriers/wells, combined with quantum dot barriers/wells nanoinclusions as well as voids in the intermediate region. We show that these structures can be designed in a way that the power factor is not only largely immune to the presence of the nanostructure features, but under certain conditions benefits can be achieved as well. Interestingly, we show that these design approaches are linked to the energy relaxation of the current flow and whether charge carrier scattering is limited by elastic or inelastic processes. In particular, when nanostructures form potential barriers, the power factor can be substantially enhanced under elastic scattering conditions, irrespective of nanostructuring density and potential barrier heights. When inelastic scattering processes dominate, however, the power factor is inevitably degraded. In the case in which nanostructures form potential wells, despite a slight decrease, the power factor is quite resilient under either elastic or inelastic scattering processes. These nanostructuring design approaches could help open the path to the optimization of new generation nanostructured thermoelectric materials by not only targeting reductions in thermal conductivity, but simultaneous improvements in the power factor as well.

Nanostructured potential well/barrier engineering for realizing unprecedentedly large thermoelectric power factors

“…Thus, an optimal compromise is achieved under semi-relaxation conditions. The advantage of energy semi-relaxation to the Seebeck coefficient is described by us and others in several works [27,26,62,63,64]. 4d shows the mobility of p-type Si (dashed line), and the mobility of the carriers that travel above VB alone, which in this case is more than double (solid line).…”

“…Interestingly, from Fig. 7b we can extract that to achieve 5×10 19 [26,27,64]. Energy relaxation in semiconductors is dominated by inelastic scattering processes, primarily electron-optical phonon scattering.…”

“…As a side note, we have shown in the past that nanoinclusions (NI) in the well regions, having barrier heights up to the VB of the superlattice, can push the energy of the current flow upwards and further increase the Seebeck coefficient, also allowing for small, but noticeable power factor improvements [26,66]. That would be something to also provide lower thermal conductivities, with additional benefits for ZT.…”

“…8a). For this, we have used our in-house 2D simulator that we have developed with details explained in Refs [26,64,66]…”

Materials selection rules for optimum power factor in two-dimensional thermoelectrics

“…Controlled engineering of materials by introducing grain boundaries [3,4], surface roughness [5][6][7], interface scattering [8][9][10], and alloy mass disorder [11][12][13] reduces κ p without significantly changing the electronic transport properties. For maximum enhancement, all of these techniques can be combined in hierarchical nanostructuring [14,15]. Many TE materials studied thus far rely on some variant of this strategy for reducing κ p to improve ZT.…”