High DimensionlessFigure of Merit and EfficiencyThermoelectrics for Operation below Sub-250 °COriginof Colossal Seebeck Coefficient with Phonon Assisting, Coexistencewith Electrical Conductivity, and Device Application for Their MultilayeredStructure

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-<i>p</i>-type GaN/TiN/GaN and <i>n</i>-type TiO₂/TiN/TiO₂ TE one-leg devices, where heterozygous of outer/inner layers demonstrates the functions of a colossal Seebeck coefficient (<i>S</i> = +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–<i>S</i> coexistence over singular curve by the asymmetric electrode configuration. <i>S</i> 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-<i>p</i>-type) or -down (<i>n</i>-type) over potential barriers at GBs are investigated. The devices showed high <i>ZT</i> (∼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%).