All-Oxide p-n Junction Thermoelectric Generator Based on SnO and ZnO Thin Films

Achieving thermoelectric devices with high performance based on low-cost and nontoxic materials is extremely challenging. Moreover, as we move toward an Internet-of-Things society, a miniaturized local power source such as a thermoelectric generator (TEG) is desired to power increasing numbers of wireless sensors. Therefore, in this work, an all-oxide p-n junction TEG composed of low-cost, abundant, and nontoxic materials, such as n-type ZnO and p-type SnO thin films, deposited on borosilicate glass substrate is proposed. A type II heterojunction between SnO and ZnO films was predicted by density functional theory (DFT) calculations and confirmed experimentally by X-ray photoelectron spectroscopy (XPS). Moreover, scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDS) show a sharp interface between the SnO and ZnO layers, confirming the high quality of the p-n junction even after annealing at 523 K. ZnO and SnO thin films exhibit Seebeck coefficients (α) of ∼121 and ∼258 μV/K, respectively, at 298 K, resulting in power factors (PF) of 180 μW/m K² (for ZnO) and 37 μW/m K² (for SnO). Moreover, the thermal conductivities of ZnO and SnO films are 8.7 and 1.24 W/m K, respectively, at 298 K, with no significant changes until 575 K. The four pairs all-oxide TEG generated a maximum power output (P_out) of 1.8 nW (≈126 μW/cm²) at a temperature difference of 160 K. The output voltage (V_out) and output current (I_out) at the maximum power output of the TEG are 124 mV and 0.0146 μA, respectively. This work paves the way for achieving a high-performance TEG device based on oxide thin films.