Band degeneracy and convergence in high performing thermoelectric materials

Thermoelectric technology is a promising clean energy harvesting approach that enables solid-state collection of waste heat. However, its energy conversion efficiency remains low compared with conventional heat engines. Improving thermoelectric conversion efficiency is challenging because the governing parameters are strongly interdependent. Commonly adopted strategies, including reducing lattice thermal conductivity and optimizing carrier density, are mainly optimization approaches that cannot address the fundamental question of which materials possess intrinsically high thermoelectric performance. Microscopic analysis and transport studies have revealed that band degeneracy is a critical parameter determining the intrinsic merit of thermoelectric materials, a conclusion supported by numerous experimental reports. In this review, we analyze the band degeneracy characteristics of state-of-the-art high-performance thermoelectric materials, classified according to their crystal structures, as crystal symmetry is closely related to band degeneracy. We also discuss the widely explored band convergence strategies in engineering materials’ degeneracy. By highlighting the important role of band degeneracy in thermoelectric materials, this review aims to aid the discovery of new material candidates and the design of effective strategies to enhance thermoelectric performance.