High Dimensionless Figure of Merit and Efficiency Thermoelectrics for Operation below Sub-250 °C─Origin of Colossal Seebeck Coefficient with Phonon Assisting, Coexistence with Electrical Conductivity, and Device Application for Their Multilayered Structure

Thermoelectric (TE) devices recycle high-temperature waste-heat efficiently, but waste-heat below sub-250 °C remains uncaptured. As promoting full autonomy for the Internet of Things (IoT), we present a TE generator using multilayered pseudo- p -type GaN/TiN/GaN and n -type TiO 2 /TiN/TiO 2 TE one-leg devices, where heterozygous of outer/inner layers demonstrates the functions of a colossal Seebeck coefficient ( S = +15,000 μV K –1 ) with phonon-assist hopping, controlling by the porosity for reducing thermal conductivity (κ), a high electric conductivity (σ) with reducing κ by outer layers, and σ– S coexistence over singular curve by the asymmetric electrode configuration. S is elucidated hopping among inner grains and the space charge (SC) grain boundary (GB) of 100 μm regions within Debye length. By assisting phonon, SC capturing, hopping-up (recoiling, pseudo- p -type) or -down ( n -type) over potential barriers at GBs are investigated. The devices showed high ZT (∼2.2), low κ (15.2 W m –1 K –1 ), and outputs at sub-250 °C, which is promising for waste heat recycling (predicted efficiency: 12.5%).