Colloquium

Hierarchically nanostructured thermoelectric materials: challenges and opportunities for improved power factors

¹ School of Engineering, University of Warwick, Coventry CV4 7AL, UK
² Consiglio Nazionale delle Ricerche–Istituto per lo Studio dei Materiali Nanostrutturati, CNR–ISMN, via Gobetti 101, 40129 Bologna, Italy
³ Department of Chemistry, University of Wyoming, Laramie 82071, USA
⁴ Institute for Microelectronics, Vienna University of Technology, Vienna, Austria
⁵ Centre for Advanced Manufacturing, Faculty of Engineering and Information Technology, University of Technology Sydney, Sydney NSW 2007, Australia
⁶ Department of Information Engineering, University of Pisa, Pisa 56122, Italy
⁷ University of Milano Bicocca, Dept. Materials Science, via R. Cozzi 55, 20125 Milan, Italy

^a e-mail: N.Neophytou@warwick.ac.uk

Received: 4 September 2020
Accepted: 7 October 2020
Published online: 23 November 2020

Abstract

The field of thermoelectric materials has undergone a revolutionary transformation over the last couple of decades as a result of the ability to nanostructure and synthesize myriads of materials and their alloys. The ZT figure of merit, which quantifies the performance of a thermoelectric material has more than doubled after decades of inactivity, reaching values larger than two, consistently across materials and temperatures. Central to this ZT improvement is the drastic reduction in the material thermal conductivity due to the scattering of phonons on the numerous interfaces, boundaries, dislocations, point defects, phases, etc., which are purposely included. In these new generation of nanostructured materials, phonon scattering centers of different sizes and geometrical configurations (atomic, nano- and macro-scale) are formed, which are able to scatter phonons of mean-free-paths across the spectrum. Beyond thermal conductivity reductions, ideas are beginning to emerge on how to use similar hierarchical nanostructuring to achieve power factor improvements. Ways that relax the adverse interdependence of the electrical conductivity and Seebeck coefficient are targeted, which allows power factor improvements. For this, elegant designs are required, that utilize for instance non-uniformities in the underlying nanostructured geometry, non-uniformities in the dopant distribution, or potential barriers that form at boundaries between materials. A few recent reports, both theoretical and experimental, indicate that extremely high power factor values can be achieved, even for the same geometries that also provide ultra-low thermal conductivities. Despite the experimental complications that can arise in having the required control in nanostructure realization, in this colloquium, we aim to demonstrate, mostly theoretically, that it is a very promising path worth exploring. We review the most promising recent developments for nanostructures that target power factor improvements and present a series of design ‘ingredients’ necessary to reach high power factors. Finally, we emphasize the importance of theory and transport simulations for materialoptimization, and elaborate on the insight one can obtain from computational tools routinely used in the electronic device communities.