Unraveling the Origin of Donor-Like Effect in Bismuth-Telluride-Based Thermoelectric Materials

The donor-like effect, depicting the uncontrollable increase of electron density that can significantly alter the thermoelectric performance of both p-type and n-type polycrystalline Bi2Te3-based materials, has long been an intriguing phenomenon, while its origin is still elusive. Herein, it is found that different from the common argument, the donor-like effect in Bi2Te3-based polycrystals is a result of the oxygen-adsorption-induced evolution of the point defects. The dominant point defect in stoichiometric zone-melted Bi2Te3 ingot is the acceptor-like Bi & PRIME;Te. During the fabrication of high-strength polycrystals, the exposure of the powders to the air leads to their absorption of oxygen and the formation of secondary phase Bi2TeO5 in the following sintering process. This brings about a change of local chemical equilibrium and promotes the evolution of the intrinsic point defect from acceptor-like Bi & PRIME;Te to donor-like TeBi & BULL;. Notably, if the fabrication process is strictly controlled to minimize oxygen adsorption, the evolution of the point defects will be avoided, whereby the donor-like effect disappears. Consequently, a reproducible high zT value of 1.0 at 325 K can be achieved in Bi2Te2.7Se0.3-based polycrystals. These results highlight the importance of understanding the evolution of point defects, which is crucial for developing high-performance Bi2Te3-based polycrystals and corresponding fabrication processes.