ZnO is a promising thermoelectric material for high-temperature applications; however, the strong correlation between the electrical and thermal transport properties has limited their simultaneous optimization to achieve superior thermoelectric performance. In this work, defect engineering was applied to solve this problem. The results revealed that by eliminating the intrinsic acceptor defects at the grain boundaries, the Schottky barrier disappeared, which led to a huge increase in the Hall mobility. Meanwhile, an increased solid solution of the trivalent dopant Al was achieved to increase the carrier concentration. The increased Hall mobility and carrier concentration gave rise to a maximum electrical conductivity (σ310K) of 1.9 × 105 S m−1, showing a metallic-like behavior. Owing to the ultrahigh σ with moderate Seebeck coefficient, a maximum power factor of 8.2 × 10−4 W m−1 K−2 was obtained at 980 K. Moreover, by introducing large numbers of lattice defects in the grains, the lattice thermal conductivity was simultaneously decreased. Therefore, the multiple-doped ZnO ceramic with defect engineering of both grains and grain boundaries optimized the electrical and thermal transport properties in a relatively independent way which provided a new and effective route to optimize the performance of the ZnO-based thermoelectric materials.
