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

Abstract

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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>B</mml:mi> <mml:msub><mml:msup><mml:mi>i</mml:mi> <mml:mo>'</mml:mo></mml:msup> <mml:mrow><mml:mi>T</mml:mi> <mml:mi>e</mml:mi></mml:mrow> </mml:msub> </mml:mrow> </mml:math> . 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>B</mml:mi> <mml:msub><mml:msup><mml:mi>i</mml:mi> <mml:mo>'</mml:mo></mml:msup> <mml:mrow><mml:mi>T</mml:mi> <mml:mi>e</mml:mi></mml:mrow> </mml:msub> </mml:mrow> </mml:math> to donor-like <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>T</mml:mi> <mml:msubsup><mml:mi>e</mml:mi> <mml:mrow><mml:mi>B</mml:mi> <mml:mi>i</mml:mi></mml:mrow> <mml:mo>•</mml:mo></mml:msubsup> </mml:mrow> </mml:math> . 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 <i>zT</i> value of 1.0 at 325 K can be achieved in Bi₂Te_2.7Se_0.3-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.