Leveraging Deep Levels in Narrow Bandgap Bi0.5Sb1.5Te3 for Record‐High zTave Near Room Temperature

Abstract Deep levels in a narrow bandgap semiconductors are considered detrimental to their electrical performance. Here the constructive role of Indium‐induced deep levels in regulating the majority and minority carriers for state‐of‐the‐art average thermoelectric figure‐of‐merit zT ave between 300 and 500 K in narrow bandgap p‐type (Bi,Sb) 2 Te 3 is reported. Two compositional series in the pseudo‐ternary Bi 2 Te 3 ‐Sb 2 Te 3 ‐In 2 Te 3 phase diagram: Bi 0.475− x Sb 1.525 In x Te 3 (0 ≤ x ≤ 0.15) and Bi 0.475 Sb 1.525− y In y Te 3 (0 ≤ y ≤ 0.10), namely, the x ‐series and y ‐series are explored. In the x ‐series, the combined experimental and theoretical study shows that Indium doping induced donor‐like and acceptor‐like deep levels, enlarges the band gap, and flattens the conduction band edge, thereby weakening the temperature dependence of Seebeck coefficient and the bipolar heat conduction. Further doping the x ‐series with copper (aka shallow acceptors) to optimize the majority carrier concentration leads to a state‐of‐the‐art zT ≈ 1.61 at 390 K and record‐high average zT ave ≈ 1.47 between 300 and 500 K in p‐type Bi 0.396 Sb 1.525 In 0.075 Cu 0.004 Te 3 . These results attest to the efficacy of deep levels in narrow bandgap thermoelectrics for both power generation and solid‐state refrigeration near room temperature.