Avoiding oxygen induced Pb vacancies for high thermoelectric performance of n-type Bi-doped Pb1-xBixTe compounds
Graphical abstract
Introduction
In order to reduce dependence on fossil fuels and pursue green, efficient and sustainable development, it is urgent to find and develop environmentally friendly new energy sources. Thermoelectric conversion technology utilizing a direct reversible conversion between heat and electricity has attracted great attention all over the world [1,2]. The conversion efficiency of thermoelectric devices is determined by the material's dimensionless Figure of Merit, expressed as ZT = α2σT/κ, where α and σ are the Seebeck coefficient and electrical conductivity, respectively, T is the absolute temperature, and κ is the thermal conductivity, which is composed of the carrier thermal conductivity κe and the lattice thermal conductivity κL. Therefore, increasing the power factor (PF = α2σ) of the material and reducing its thermal conductivity (κ = κe+κL) are important ways to achieve high thermoelectric performance. However, the above transport parameters are not independent and are coupled with each other, which brings considerable challenges to optimizing the thermoelectric performance [[3], [4], [5], [6], [7], [8], [9]]. Through the unrelenting efforts of researchers, the following strategies were found beneficial to synergistically optimize the electronic and thermal transport performance of materials: (1) Carrier concentration optimization combined with electronic band structure engineering to improve the power factor, such as dynamic doping, electronic band convergence, electronic band flattening, forming resonance levels and deep-level impurity states [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19]]; (2) Introduction of additional scattering centers to reduce the lattice thermal conductivity, such as forming solid solutions to introduce point defects, nano-sized second-phases, and all-scale hierarchical structures [[20], [21], [22], [23], [24]].
PbTe-based compounds have demonstrated extraordinary thermoelectric performance in the intermediate temperature region (500–900 K) due to the advantages of a small band gap, complex band structure, and low lattice thermal conductivity [25]. With the application of various strategies, the peak ZT and the average ZT of p-type PbTe compounds exceeds 2.5 and 1.6 respectively [26], while the performance of n-type PbTe compounds is still unimpressive [[27], [28], [29]]. This mismatch in performance is largely determined by the band structure: the energy offset between the heavy valence band and the light valence band of PbTe compounds is 0.15 eV at room temperature, which is easy to form multi-band transport, while the energy offset between the heavy conduction band and the light conduction band is 0.55 eV, which results in a single-band transport. Therefore, n-type PbTe compounds have relatively low optimal carrier concentration nopt and effective mass m* [26,[30], [31], [32], [33], [34], [35], [36], [37], [38]]. This makes n-type PbTe compounds have theoretically high carrier mobility and are more sensitive to defects.
Doping with alien valent element is the most common method to optimize the carrier concentration [33,39,40]. Ideally, the doping element only occupies the host position to form a perfect lattice. However, practically, when the alien atoms substitute on the lattice of matrix, it is usually accompanied with the formation of some other types of structural defects i.e. vacancy defects, interstitial defects and anti-site defects etc. For the system with high concentration (above 1020 cm−3), the charge transport properties are dominated by the substitutional defects in the lattice and the impact of this accompanied structural defects with low concentration can be ignored. In contrast, for the system such as n-type PbTe based compounds, with relatively low optimal concentration (1019 cm−3), the accompanied structural defects play pivotal role on the charge transport, since the concentration of the accompanied structural defects is comparable with that of the designed substitutional defects in the structure. The existence of these defects poses a great challenge to obtain high carrier mobility and thermoelectric performance [14,41,42]. Therefore, understanding the evolution mechanism of structural defects in n-type PbTe compounds is crucial for improving the carrier mobility and thermoelectric properties of the materials.
In this study, two batches of Pb1-xBixTe samples vacuum sintered with powders ground in a glove box under the protection of Ar and, independently, ground in air were prepared. We have systematically studied the role of the atmospheres during the grinding process on the thermoelectric transport properties. For Pb1-xBixTe samples vacuum sintered with powders ground in air, the theoretical calculation and positron annihilation measurement demonstrate that Pb vacancies form in the structure. The underlying mechanism for the evolution of structural defects is revealed via experimental results combined with theoretical calculations. Such charged Pb vacancies exert a strong long-range coulombic repulsion force on electrons, resulting in a low doping efficiency, a low carrier mobility (200 cm2 V−1 s−1), and inferior electronic properties. By grinding the powders in Ar effectively avoids the generation of O–Bi related Pb vacancies, and a high carrier mobility of 1300 cm2 V−1 s−1 is achieved in the slightly doped Pb0·9995Bi0·0005Te sample at room temperature. Consequently, the Pb0·9995Bi0·0005Te sample attains the highest power factor of 40 μW cm−1 K−2 at room temperature and the Pb0·9992Bi0·0008Te sample has the highest average power factor exceeding 25 μW cm−1 K−2 in a wide temperature range of 300–723 K. As a result, a high ZT value of 1.12 at 673 K, and ZTave value of 0.84 in the temperature range of 300–873 K are achieved in the Pb0·9992Bi0·0008Te sample prepared from powders processes under the protection of Ar. Based on our research results, by grinding the powders under an inert gas, a contact with oxygen is avoided during the crushing process, which prevents the generation of cation vacancies, and effectively optimizes the performance of n-type PbTe-based compounds.
Section snippets
The dimensionless figure of merit ZT for two batches of Pb1-xBixTe compounds sintered with the powders ground in different atmospheres
In order to uncover the underlying mechanism for the inferior thermoelectric performance of Bi-doped Pb1-xBixTe compounds, two batches of Pb1-xBixTe samples were prepared by vacuum sintering with powders ground under the protection of Ar in a glove box and in the air, respectively. The temperature-dependent figure of merit ZT for two batches of Pb1-xBixTe samples are shown in Fig. 1(a) and 1(b). All samples crushed and ground in the air have lower ZT, especially in the vicinity of room
Conclusion
An effective strategy for improving the electronic performance via defects design is achieved in Bi-doped PbTe compounds. The defect evolution mechanism during grinding of Bi-doped PbTe compounds is revealed via positron annihilation measurements and calculations of the formation energy. During the fracturing process of Bi-doped PbTe in air, oxygen atoms preferentially occupy the generated Te vacancies, and doping with Bi lowers the formation energy of Pb vacancies. Bi-doped PbTe samples
Credit author statement
Cong Wang: Data curation, Formal analysis, original draft and Visualization; Keke Liu: Formation energy calculation; Qirui Tao: Supervision and Validation; Xiaodie Zhao: Data curation and Formal analysis; Suiting Ning: Formal analysis; Yingfei Tang: Investigation; Zhiquan Chen: Data curation and Formal analysis; Jinsong Wu: Funding acquisition and review & editing; Xianli Su: Conceptualization, Formal analysis, Project administration, Funding acquisition and review & editing; Ctirad Uher:
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors wish to acknowledge the support from the Natural Science Foundation of China (52122108, 51972256), the National Key Research and Development Program of China (Grant No. 2019YFA0704900), the 111 Project of China (Grant No. B07040), and the Fundamental Research Funds for the Central Universities (WUT: 2020IVB056, 20201h0028a). The SEM/EPMA work was performed at the Nanostructure Research Center (NRC), which is supported by the Fundamental Research Funds for the Central Universities (
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