Nanostructured Thermoelectric Thin Films for Low-Grade Waste Heat Recovery

Abstract

The increasing demand for energy and the ongoing waste-heat discharge from industrial, transportation, electronic and domestic systems have raised the urgency to develop efficient waste-heat recovery technologies. In particular, a portion of the waste heat that has been regarded as low-yield and of low value is low-grade waste heat with a small temperature difference and near ambient temperature. Thermoelectric generators provide a route for direct conversion of thermal energy to electrical power via use of the Seebeck effect. However, thermoelectric materials often show poor performance at low temperature due to the intrinsic correlation between electrical conductivity, Seebeck coefficient and thermal conductivity. Nanostructured thermoelectric thin films present an attractive opportunity to address this challenge by enabling simultaneous modulation of charge-carrier and phonon transport via grain-boundary engineering, defect engineering, interface engineering, quantum confinement, energy filtering and band-structure modification. This review describes the underlying principles of nanostructured thermoelectric thin films and investigates the relations between structure, carrier transport, phonon scattering and thermoelectric performance. The main classes of thermoelectric materials that have been explored for low-temperature applications are Bi2Te3-based compounds, Sb2Te3, Bi2Se3, SnSe, Cu2Se, oxide thermoelectrics, Mg-based compounds and half-Heusler materials. Physical and chemical fabrication approaches such as magnetron sputtering, thermal evaporation, pulsed laser deposition, chemical vapor deposition, atomic layer deposition, molecular beam epitaxy and solution processing are critically compared. Strategies for enhancing the thermoelectric figure of merit are investigated on the basis of nanostructuring, doping, defect-engineering, band-convergence, superlattice formation and interface engineering. The review highlights that future research on thermoelectric thin films should be focused on the combination of nanoscale transport engineering and scalable thin-film fabrication, while the low-grade heat sources with reasonable temperature differences should be exploited for achieving practical power generation