Defect structure regulation and thermoelectric transfer performance in n-type Bi<sub>2–<i>x</i></sub> Sb<sub><i>x</i></sub>Te<sub>3–<i>y</i></sub>Se<sub><i>y</i></sub>-based compounds

Abstract

Bi<sub>2</sub>Te<sub>3</sub>-based compounds are thermoelectric materials with the best performance near room temperature. The existence of a large number of complex defects makes defect engineering a core stratagem for adjusting and improving the thermoelectric performance. Therefore, understanding and effectively controlling the existence form and concentration of defects is crucial for achieving high-thermoelectric performance in Bi<sub>2</sub>Te<sub>3</sub>-based alloy. Herein, a series of Cl doped n-type quaternary Bi<sub>2–<i>x</i></sub> Sb<sub><i>x</i></sub>Te<sub>3–<i>y</i></sub>Se<sub><i>y</i></sub> compounds is synthesized by the zone-melting method. The correlation between defect evolution process and thermoelectric performance is systematically investigated by first-principles calculation and experiments. Alloying Sb on Bi site and Se on Te site induce charged structural defects, leading to a significant change in the carrier concentration. For Bi<sub>2–<i>x</i></sub> Sb<sub><i>x</i></sub>Te<sub>2.994</sub>Cl<sub>0.006</sub> compounds, alloying Sb on Bi site reduces the formation energy of the <inline-formula><tex-math id="M6">\begin{document}${\mathrm{S}}{{\text{b}}_{{\mathrm{Te}}}}_{_2}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M6.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M6.png"/></alternatives></inline-formula> antisite defect, which generates the antisite defect <inline-formula><tex-math id="M7">\begin{document}${\mathrm{S}}{{\text{b}}_{{\mathrm{Te}}}}_{_2}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M7.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M7.png"/></alternatives></inline-formula> and accompanied with the increase of the minority carrier concentration from 2.09×10<sup>16</sup> to 3.99×10<sup>17</sup> cm<sup>–3</sup>. The increase of the minority carrier severely deteriorates the electrical transport properties. In contrast, alloying Se in the Bi<sub>1.8</sub>Sb<sub>0.2</sub>Te<sub>2.994–<i>y</i></sub>Se<sub><i>y</i></sub>Cl<sub>0.006</sub> compound significantly lowers the formation energy of the complex defect <inline-formula><tex-math id="M8">\begin{document}${\mathrm{S}}{{\mathrm{e}}_{{\mathrm{Te}}}}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M8.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M8.png"/></alternatives></inline-formula>+<inline-formula><tex-math id="M9">\begin{document}${\mathrm{S}}{{\mathrm{b}}_{{\mathrm{Bi}}}}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M9.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M9.png"/></alternatives></inline-formula>, which becomes more energetically favorable and suppresses the formation of the antisite defect <inline-formula><tex-math id="M10">\begin{document}${\mathrm{S}}{{\text{b}}_{{\mathrm{Te}}}}_{_2}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M10.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M10.png"/></alternatives></inline-formula>. As a result, the concentration of minority carriers decreases to 1.46×10<sup>16</sup> cm<sup>–3</sup>. This eliminates the deterioration effect of the minority carrier on the electrical transport properties of the material and greatly improves the power factor. A maximum power factor of 4.49 mW/(m·K<sup>2</sup>) is achieved for Bi<sub>1.8</sub>Sb<sub>0.2</sub>Te<sub>2.944</sub>Se<sub>0.05</sub>Cl<sub>0.006</sub> compound at room temperature. By reducing thermal conductivity through intensifying the phonon scattering via alloying Sb and Se, the maximum <i>ZT</i> value of 0.98 is attained for Bi<sub>1.8</sub>Sb<sub>0.2</sub>Te<sub>2.844</sub>Se<sub>0.15</sub>Cl<sub>0.006</sub> compound at room temperature. Our finding provides an important guidance for adjusting point defects, carrier concentrations, and thermoelectric performances in Bi<sub>2</sub>Te<sub>3</sub>-based compounds with complex compositions.