Improved thermoelectric performance in n-type BiTe facilitated by defect engineering

Abstract

BiTe is a potentially attractive candidate for thermoelectric applications because it is the structural analogue of the state-of-the-art thermoelectric material, bismuth telluride (Bi2Te3). However, BiTe has attracted little attention due to its small band gap and high electron concentration. In this study, remarkable increase in thermoelectric performance in the n-type Bi1−xSbxTe compounds through tuning the carrier concentration with chemical doping is shown. The Seebeck coefficient of Bi1−xSbxTe increases remarkably while the electronic thermal conductivity decreases gradually as Sb content is increased, leading to enhanced thermoelectric figure of merit (ZT). Moreover, the simultaneous optimization of the electrical and thermal transport properties leads the peak temperature of the figure of merit to shift toward lower temperature with Sb content increasing in Bi1−xSbxTe, thus making Bi1−xSbxTe a potential alternative to bismuth telluride for near-room-temperature thermoelectric applications. In addition, the presence of multiple low-frequency optical phonons and their coupling with the long-wavelength heat carrying acoustic phonons in all the Bi1−xSbxTe investigated are revealed based on the combined Debye−Einstein model. The present results provide the underlying mechanism connecting thermoelectric performance and lattice dynamics in Bi1−xSbxTe compounds. 因其与成熟的室温热电材料Bi2Te3具有类似的结构, BiTe被认为在热电材料领域具有潜在的应用价值。但是, 受限于禁带宽度较小、载流子浓度过高等特点, BiTe的热电性能依旧处于很低的水平。本工作通过在BiTe样品中掺入Sb元素的方法来调节材料的载流子浓度, 从而显著提高了n型Bi1−xSbxTe化合物的热电性能。研究发现, 随着Sb含量的增加, Bi1−xSbxTe的塞贝克系数显著增加、电子热导率逐渐降低, 从而使得Bi1−xSbxTe材料的热电优值得以提高。进一步发现, 电、热输运行为的协同优化会导致Bi1−xSbxTe的热电优值峰值温度随着Sb含量的增加而逐渐向低温端移动, 这使得Bi1−xSbxTe成为近室温热电应用领域的明星材料Bi2Te3的潜在替代品。此外, 基于德拜—爱因斯坦模型, 本工作揭示了Bi1−xSbxTe材料中多个低频光学声子的存在以及这些光学声子与载热的声学声子之间的耦合关系, 从而将Bi1−xSbxTe化合物的热电性能与晶格动力学联系起来。