Mn-In-Cu co-doping to optimize thermoelectric properties of SnTe-based materials

Abstract

Lead-free chalcogenide SnTe has a similar crystal structure and energy band structure to high performance thermoelectric material PbTe, which has been widely concerned in recent years. However, due to its low Seebeck coefficient, high intrinsic Sn vacancy concentration and high thermal conductivity, its intrinsic thermoelectric performance is poor. In this study, Mn-In-Cu co-doping SnTe-based thermoelectric materials are prepared by hot pressing sintering at high-temperature and high-pressure. Indium (In) doping brings the resonant level in SnTe and increases the density of states which greatly improves Seebeck coefficient at room temperature; the Seebeck coefficient of Sn<sub>1.04</sub>In<sub>0.01</sub>Te(Cu<sub>2</sub>Te)<sub>0.05</sub> reaches 70 μV·K<sup>–1</sup> at room temperature. With adding manganese (Mn), the Seebeck coefficient at room temperature is well preserved, indicating that Mn doping has little effect on the resonant level brought by In doping. In addition, due to the band convergence brought by Mn doping, the high temperature Seebeck coefficient of the material is improved, the maximum Seebeck coefficient reaches 215 μV·K<sup>–1</sup> for the sample with 17% Mn doping amount at 873 K. Owing to the combination of band convergence and resonant level, the Seebeck coefficient of the whole temperature range of the material increases, the power factor of the material is also greatly optimized, and all samples have a power factor of more than 1.0 mW·m<sup>–1</sup>·K<sup>–2</sup> at room temperature. On the other hand, the point defects brought by Mn alloying and the interstitial defects introduced by copper (Cu) enhance the phonon scattering and effectively reduce the lattice thermal conductivity of the material, the lattice thermal conductivity decreases to 0.68 W·m<sup>–1</sup>·K<sup>–1</sup> at 873 K. The electrical and thermal properties of the materials are optimized simultaneously under the combination of various strategies, the peak <i>zT ≈ </i>1.45 is obtained at 873 K in the p-type Sn<sub>0.89</sub>Mn<sub>0.15</sub>In<sub>0.01</sub>Te(Cu<sub>2</sub>Te)<sub>0.05</sub> sample and the average <i>zT</i> of 300–873 K reaches 0.76. In the process of multi-strategy coordinated regulation of SnTe-based thermoelectric materials, the excellent properties of single strategy can be well maintained, which provides a possibility for further improving the performance of SnTe-based thermoelectric materials.