Rational Design of ZnO-Based Thermoelectrics through Multiscale Electron and Phonon Engineering

Abstract

ZnO is an attractive n-type oxide thermoelectric material for high-temperature energy harvesting because of its elemental abundance, environmental compatibility, chemical robustness, and thermal stability under oxidizing conditions. Its thermoelectric performance, however, remains constrained by intrinsically high lattice thermal conductivity and by the strong coupling among carrier concentration, carrier mobility, and Seebeck coefficient. Overcoming these limitations requires transport-engineering strategies that suppress phonon-mediated heat conduction, while minimizing penalties to electronic transport. This review critically examines recent advances in ZnO-based thermoelectrics through a multiscale transport-engineering framework. Rather than classifying studies solely by composition, synthesis method, or morphology, we correlate structural length scale with the dominant mechanisms governing electron and phonon transport. Compositional engineering, including cation substitution, anion modification, and defect control, is first discussed in terms of charge carrier optimization, lattice disorder, and point-defect phonon scattering. Grain-boundary and nanopore engineering are then compared as mesoscale approaches for enhancing boundary scattering, while maintaining continuous conductive pathways. Finally, multi-scale nanostructures are highlighted as an emerging design paradigm that integrates point defects, grain boundaries, pores, and heterogeneous interfaces to broaden the phonon-scattering spectrum, while enabling energy-dependent charge carrier filtering. By linking structure, characteristic length scale, transport mechanism, and thermoelectric performance, this review establishes design principles for the rational development of high-performance and thermally stable ZnO-based thermoelectric materials.