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Bi2Te3-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 Bi2Te3-based alloy. Herein, a series of Cl doped n-type quaternary Bi2−xSbxTe3–ySey 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 Bi2–xSbxTe2.994Cl0.006 compounds, alloying Sb on Bi site reduces the formation energy of the
${\mathrm{S}}{{\text{b}}_{{\mathrm{Te}}}}_{_2}$ antisite defect, which generates the antisite defect${\mathrm{S}}{{\text{b}}_{{\mathrm{Te}}}}_{_2}$ and accompanied with the increase of the minority carrier concentration from 2.09×1016 to 3.99×1017 cm–3. The increase of the minority carrier severely deteriorates the electrical transport properties. In contrast, alloying Se in the Bi1.8Sb0.2Te2.994–ySeyCl0.006 compound significantly lowers the formation energy of the complex defect${\mathrm{S}}{{\mathrm{e}}_{{\mathrm{Te}}}}$ +${\mathrm{S}}{{\mathrm{b}}_{{\mathrm{Bi}}}}$ , which becomes more energetically favorable and suppresses the formation of the antisite defect${\mathrm{S}}{{\text{b}}_{{\mathrm{Te}}}}_{_2}$ . As a result, the concentration of minority carriers decreases to 1.46×1016 cm–3. 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·K2) is achieved for Bi1.8Sb0.2Te2.944Se0.05Cl0.006 compound at room temperature. By reducing thermal conductivity through intensifying the phonon scattering via alloying Sb and Se, the maximum ZT value of 0.98 is attained for Bi1.8Sb0.2Te2.844Se0.15Cl0.006 compound at room temperature. Our finding provides an important guidance for adjusting point defects, carrier concentrations, and thermoelectric performances in Bi2Te3-based compounds with complex compositions.-
Keywords:
- Bi2Te3-based compound /
- defect engineering /
- thermoelectric properties
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图 1 (a) 区熔法制备Bi2–xSbxTe2.994Cl0.006和Bi1.8Sb0.2Te2.994–ySeyCl0.006样品粉末X射线衍射图谱; (b) Bi1.8Sb0.2Te2.694Se0.3Cl0.006样品垂直于提拉方向自由断裂截面的场发射扫描电子显微镜照片; (c) Bi1.8Sb0.2Te2.694Se0.3Cl0.006样品抛光表面背散射电子图像(单位原子百分比)以及对应区域Bi, Sb, Se和Te等元素的面分布图像
Figure 1. (a) Powder XRD patterns of Bi2–xSbxTe2.994Cl0.006 and Bi1.8Sb0.2Te2.994–ySeyCl0.006 samples prepared by zone melting method; (b) field emission scanning electron microscope images of freshly fractured surfaces of the Bi1.8Sb0.2Te2.694Se0.3Cl0.006 sample measured perpendicular to the travel direction during zone melting; (c) backscattered electron image of polished surfaces for Bi1.8Sb0.2Te2.694Se0.3Cl0.006 sample and the corresponding elemental mapping images for Bi, Sb, Se and Te, respectively.
图 2 Bi2–xSbxTe2.994Cl0.006和Bi1.8Sb0.2Te2.994–ySeyCl0.006样品的电输运性能 (a), (b) 电导率; (c), (d) Seebeck系数; (e), (f) 功率因子
Figure 2. Temperature-dependent electronic transport properties for Bi2–xSbxTe2.994Cl0.006 and Bi1.8Sb0.2Te2.994–ySeyCl0.006 samples: (a), (b) Electrical conductivity; (c), (d) Seebeck coefficient; (e), (f) power factor.
图 3 (a), (b) 室温载流子浓度及迁移率随Sb, Se含量的变化; (c) 室温下样品的塞贝克系数与载流子浓度的关系, 图中紫色实线、红色实线、黄色实线为不同态密度有效质量下基于单抛带模型计算的塞贝克系数与载流子浓度关系曲线
Figure 3. (a), (b) Hall carrier mobility and concentration change with respect to the Sb, Se content; (c) Seebeck coefficients as a function of the charge carrier concentration at 300 K, where the colored dash lines are Pisarenko plots based on the single parabolic band model with different effective mass.
图 4 Bi2Te3基化合物中不同点缺陷形成能 (a) Bi2–xSbxTe3固溶体; (b) Bi2–xSbxTe3–ySey固溶体
Figure 4. Theoretically calculated formation energies of different point defects in (a) Bi2–xSbxTe3 and (b) Bi2–xSbxTe3–ySey solid solution as a function of the Fermi level under the Te-poor condition.
图 5 在200 K下, 不同样品 (a) 两种载流子浓度及其比值随组分变化情况; (b) 两种载流子迁移率及其比值随组分变化情况; (c) Bi2–xSbxTe3–ySey固溶体缺陷演化过程示意图
Figure 5. Composition dependence of (a) ne, nh, and nh/ne, (b) μe, μh, and μe/μh of different samples at 200 K. (c) Defect evolution process for Bi2–xSbxTe3–ySey solid solution.
图 6 Bi1.8Sb0.2Te2.994–ySeyCl0.006样品的(a) 总热导率κ、(b) 热导率κ －κe和(c) 无量纲热电优值ZT随温度的变化
Figure 6. Temperature-dependent (a) Total thermal conductivity, (b) thermal conductivity κ － κe, (c) ZT for Bi1.8Sb0.2Te2.994–ySeyCl0.006 samples.
表 1 Bi2–xSbxTe2.994Cl0.006和Bi1.8Sb0.2Te2.994–ySeyCl0.006样品的室温物理性能参数
Table 1. Room-temperature physic properties of Bi2–xSbxTe2.994Cl0.006 and Bi1.8Sb0.2Te2.994–ySeyCl0.006 samples.
Samples σ/(104 S·m–1) S/(μV·K–1) n/(1019 cm–3) μ/(cm2·V–1·s–1) κ/(W·m–1·K–1) m*/m0 x = 0 11.2 –214.0 1.06 568 1.84 0.51 x = 0.05 11.0 –209.6 0.91 750 1.75 0.45 x = 0.10 9.94 –216.4 0.88 706 1.72 0.45 x = 0.15 9.16 –223.2 0.82 698 1.58 0.45 x = 0.20 5.79 –210.0 0.64 661 1.66 0.36 y = 0.05 8.97 –223.1 1.16 481 1.42 0.57 y = 0.10 8.46 –223.6 1.95 271 1.41 0.80 y = 0.15 9.94 –209.9 2.35 264 1.35 0.86 y = 0.20 8.82 –215.8 2.57 214 1.45 0.94 y = 0.25 8.68 –217.4 1.92 282 1.33 0.66 y = 0.30 9.15 –207.2 1.80 318 1.30 0.70 
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