On the effectiveness of the thermoelectric energy filtering mechanism in low-dimensional superlattices and nano-composites

TL;DR

This study investigates how reduced dimensionality in superlattices and nanocomposites enhances thermoelectric energy filtering, showing 1D systems outperform 2D in power factor improvement due to quantum effects and material properties.

Contribution

It demonstrates that 1D channels more effectively utilize energy filtering than 2D, providing insights into optimizing thermoelectric materials through dimensionality control.

Findings

1D channels achieve higher power factors at smaller well sizes.

Energy filtering effectiveness is maximized when carrier energy varies significantly.

1D systems can be up to three times more effective than 2D systems.

Abstract

Electron energy filtering has been suggested as a promising way to improve the power factor and enhance the ZT figure of merit of thermoelectric materials. In this work we explore the effect that reduced dimensionality has on the success of the energy-filtering mechanism for power factor enhancement. We use the quantum mechanical non-equilibrium Green's function (NEGF) method for electron transport including electron-phonon scattering to explore 1D and 2D superlattice/nanocomposite systems. We find that, given identical material parameters, 1D channels utilize energy filtering more effectively than 2D as they: i) allow one to achieve maximal power factor for smaller well sizes / smaller grains (which is needed to maximize phonon scattering), ii) take better advantage of a lower thermal conductivity in the barrier/boundary materials compared to the well/grain materials in both: enhancing the Seebeck coefficient; and in producing a system which is robust against detrimental random deviations from optimal barrier design. In certain cases we find that the relative advantage can be as high as a factor of 3. We determine that energy-filtering is most effective when the average energy of carrier flow varies the most in the wells and the barriers along the channel, an event which appears when the energy of the carrier flow in the host material is low and when the energy relaxation mean-free-path of carriers is short. Although the ultimate reason these aspects, which cause a 1D system to see greater relative improvement than a 2D, is the 1D system's van Hove singularity in the density-of-states, the insights obtained are general and inform energy-filtering design beyond dimensional considerations.