Adjusting high carrier concentration and dense crystal defects to realize n-type lanthanum-doped tin-selenide-based thermoelectric materials

【introduction】

Thermoelectric materials can directly convert thermal energy and electrical energy, so they have considerable application potential in the fields of waste heat recovery, air conditioning and refrigeration. As a typical representative of a new generation of thermoelectric materials, single crystal selenium tin (SnSe) blocks have attracted much attention due to their high thermal power figure (ZT, which can reach 2.6 at 923 K). However, due to its poor mechanical properties and harsh crystal growth conditions, single crystal selenide is difficult to apply to actual thermoelectric devices.

In order to solve this problem, polycrystalline selenium tin has become a new research topic. To date, the ZT value of p-type polycrystalline tin-selenide-based thermoelectric materials has been greatly improved. However, the premise of composing thermoelectric modules is that both p-type and n-type thermoelectric materials are required. Therefore, the synthesis of n-type polycrystalline selenide tin with high thermoelectric properties has become the research focus.

Since the holes in the common tin selenide semiconductor are majority carriers, the n-type polycrystalline tin selenide block is generally difficult to realize. The preliminary work pointed out that its ZT value is difficult to exceed 1.0, which is significantly lower than the p-type polycrystalline selenium tin block. In addition, the current research on the atomic substitution law of n-type doping elements is not thorough, and the valence state of the dopants is not very clear. Therefore, there is an urgent need to study the n-type doping behavior based on polycrystalline selenium tin bulk, which is very important for exploring suitable doping elements to further improve their thermoelectric properties.

[Introduction]

Recently, Associate Professor Chen Zhigang of the University of Southern Queensland and Professor Zou Jin of the University of Queensland for the first time realized the n-type lanthanum element doped tin-selenide micron-sized lath-like crystal by solvothermal method. The ZT value of the sintered bulk material is It can reach 1.1 under 773 K. The excellent thermoelectric properties exhibited by this material are due to its high power factor (2.4 μWcm-1 K-2) and its ultra-low thermal conductivity (0.17 W m-1 K-1) ).

The high power factor of the bulk material comes from the high electron carrier concentration (3.94×1019 cm-3) achieved by effective erbium doping, and its ultra-low thermal conductivity is due to erbium doping. Dense crystal defects, including strong lattice distortion, dislocations, and macroscopic crystal curvature, which effectively scatter phonons and reduce thermal conductivity.

In addition, for the study of the doping mechanism of lanthanum elements, through XRD, XPS, SEM and TEM characterization methods, the team found that in the process of solvothermal synthesis of tin selenide crystallites, the yttrium element incorporated shows a -3 valence It can replace the position of selenium and generate additional selenium vacancies, thus allowing the material system to exhibit n-type semiconductor properties.

This work fills the blank of the n-type erbium doping mechanism in tin-selenide block thermoelectric materials, and provides a new solution for further improving the high thermoelectric performance of n-type polycrystalline selenium tin.

[Graphic introduction]

figure 1. (a): solvothermal synthesis of bismuth-doped tin-selenide crystallites, (b): Schematic diagram of the doping mechanism of lanthanum and photographs of obtained tin-selenide microcrystals (optical, scanning, and transmission electron microscopy) (c): Schematic diagram of the erbium element doped to cause dislocations and stress regions to scatter phonons, (d): Sintering process and cutting process diagram of bismuth-doped tin selenide crystallites, (e): The thermoelectric figure of merit of the bulk material.

figure 2. (a): XRD results of tin selenide crystallites at different cerium doping levels, (b): amplified 400 peaks, (c): calculated lattice of tin selenide at different cerium doping levels Parameters, (d): Calculated unit cell volume of tin selenide at different cerium doping levels.

image 3. (a): XPS full spectrum of SnSb0.03Se0.94 crystallites, and XPS peaks of (b):Sn 3d, (c):Se 3d, and (d) :Sb 3d at high resolution.

Figure 4. (a) optics of SnSb0.03Se0.94 crystallites and (b) scanning electron micrographs, (c): enlarged photo of flower-like SnSb0.03Se0.94 microcrystals, (d): on the basis of (c) Further details of the obtained SnSb0.03Se0.94 crystallites were shown to reveal 100 faces, (e): a scanning electron micrograph of a complete SnSb0.03Se0.94 crystallite to show its macroscopic crystal defects, including uneven surfaces And slight crystal bending, (f): A broken SnSb0.03Se0.94 microcrystal photo showing its potential crystal growth.

Figure 5. (a): a transmission electron micrograph of a typical SnSb0.03Se0.94 crystallite, (b): the corresponding SAED pattern indicates the crystal characteristics of tin selenide, and (c): the edge of SnSb0.03Se0.94 crystallite High-resolution transmission electron micrograph of a thin part, (d): a high-resolution photograph further enlarged on the basis of (c), (e): a photograph further enlarged in a darker area in (a) to display Dense crystal defects, (f): High resolution photographs obtained by magnifying the black strip region in (e) to show a wide range of lattice distortion due to erbium doping, (g): (f) further The high resolution photographs obtained were magnified to show lattice distortion at the nanoscale due to erbium doping, and (h): EDS spectroscopy results to confirm the homogeneous doping of yttrium elements at the micrometer scale.

Figure 6. Temperature-dependent thermoelectric properties of bulk materials sintered with different cerium doping amounts of tin selenide SnSbxSe1-2x microcrystals: (a) Seebeck coefficient, (b) conductivity, (c) power factor, and (d) )Thermal conductivity.

The experimental results show that the absolute value of Seebeck coefficient decreases and the conductivity increases with the increase of erbium doping amount. Therefore, the power factor of SnSb0.02Se0.96 is the highest. At the same time, as the amount of antimony doped increases, the thermal conductivity decreases.

Figure 7. Other important parameters of the bulk material sintered by different cerium doping amounts of tin selenide SnSbxSe1-2x microcrystals: (a) increased carrier concentration with increasing temperature (caused by thermal excitation), (b) The carrier mobility as a function of temperature, (c) the electronic thermal conductivity as a function of temperature, (d) the lattice thermal conductivity as a function of temperature, (e) the lattice thermal conductivity as a function of 1000/T, And (f) the κl /κ ratio as a function of temperature.

The experimental results show that as the amount of antimony doping increases, the carrier concentration increases, the carrier mobility decreases, the electron thermal conductivity increases, and the lattice thermal conductivity decreases.

Figure 8. (a): Comparison of XRD results of bulk materials sintered with pure SnSe and antimony doped SnSb0.03Se0.94 crystallites (compared with results in parallel and vertical sintering pressure directions), (b): amplified ( 111) and (400) peaks to show the peak shift caused by doping, (c): transmission electron micrograph of the sheet cut through the SnSb0.03Se0.94 block, (d): high resolution of the orientation Photo and its FFT pattern, (e): high resolution photo of the direction as well as its FFT pattern and a typical dislocation. (f): High-resolution transmission electron micrograph of spherical aberration obtained along the a-axis to show the positional shift of the atom caused by doping of yttrium element, (g): EDS line scan result on (f) to show 锑 atom Substituted selenium atoms.