Advanced GeSe-based thermoelectric materials: Progress and future challenge

GeSe, composed of ecofriendly and earth-abundant elements, presents a promising alternative to conventional toxic lead-chalcogenides and earth-scarce tellurides as mid-temperature thermoelectric applications. This review comprehensively examines recent advancements in GeSe-based thermoelectric materials, focusing on their crystal structure, chemical bond, phase transition, and the correlations between chemical bonding mechanism and crystal structure. Additionally, the band structure and phonon dispersion of these materials are also explored. These unique features of GeSe provide diverse avenues for tuning the transport properties of both electrons and phonons. To optimize electrical transport properties, the strategies of carrier concentration engineering, multi-valence band convergence, and band degeneracy established on the phase modulation are underscored. To reduce the lattice thermal conductivity, emphasis is placed on intrinsic weak chemical bonds and anharmonicity related to chemical bonding mechanisms. Furthermore, extra-phonon scattering mechanisms, such as the point defects, ferroelectric domains, boundaries, nano-precipitates, and the phonon mismatch originating from the composite engineering, are highlighted. Additionally, an analysis of mechanical properties is performed to assess the long-term service of thermoelectric devices based on GeSe-based compounds, and correspondingly, the theoretical energy-conversion efficiency is discussed based on the present zT values of GeSe. This review provides an in-depth insight into GeSe by retrospectively examining the development process and proposing future research directions, which could accelerate the exploitation of GeSe and elucidate the development of broader thermoelectric materials.