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
Authors: Li Rui-Ying , Luo Ting-Ting , Li Mao , Chen Shuo , Yan Yong-Gao , Wu Jin-Song , Su Xian-Li , Zhang Qing-Jie , Tang Xin-Feng ,   Longzhong Laboratory in Hubei Province, Xiangyang Demonstration Zone of Wuhan University of Technology, Xiangyang 441000, China,   State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China,   Nanostructure Research Center, Wuhan University of Technology, Wuhan 430070, China
Published: March 2024

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.