Removing the Oxygen-Induced Donor-like Effect for High Thermoelectric Performance in n-Type BiTe-Based Compounds.

Bismuth telluride-based alloys are the best performing thermoelectric materials near room temperature. Grain size refinement and nanostructuring are the core stratagems for improving thermoelectric and mechanical properties. However, the donor-like effect induced by grain size refinement strongly restricts the thermoelectric properties especially in the vicinity of room temperature. In this study, the formation mechanism for the donor-like effect in BiTe-based compounds was revealed by synthesizing five batches of polycrystalline samples. We demonstrate that the donor-like effect in BiTe-based compounds is strongly related to the vacancy defects ( and ) induced by the fracturing process and oxygen in air for the first time. The oxygen-induced donor-like effect dramatically increases the carrier concentration from 2.5 × 10 cm for the zone melting ingot and bulks sintered with powders ground under an inert atmosphere to 7.5 × 10 cm, which is largely beyond the optimum carrier concentration and seriously deteriorates the thermoelectric performance. Moreover, it is found that both avoiding exposure to air and eliminating the thermal vacancy defects ( and ) via heat treatment before exposure to air can effectively remove the donor-like effect, producing almost the same carrier concentration and Seebeck coefficient as those of the zone melting ingot for these samples. Therefore, a defect equation of oxygen-induced donor-like effect was proposed and was further explicitly corroborated by positron annihilation measurement. With the removal of donor-like effect and improved texturing via multiple hot deformation (HD) processes, a maximum power factor of 3.5 mW m K and a reproducible maximum value of 1.01 near room temperature are achieved. This newly proposed defect equation of the oxygen-induced donor-like effect will provide a guideline for developing higher-performance VVI polycrystalline materials for near-room-temperature applications.