Research progress of tin telluride based thermoelectric materials

The energy problem is a significant factor constraining the development of human society. Seeking cleaner and more sustainable new energy sources has become a crucial direction in contemporary academic research and industrial development. Thermoelectric materials, which can directly convert heat energy into electric energy and vice versa, have garnered widespread attention due to their simple structure, noiseless operation, pollution-free nature, and high reliability. Unlike solar and wind energy, which require specific external conditions, heat energy is ubiquitous, enabling thermoelectric devices to be applied in a wide range of fields, including extreme environments such as deep space, oceans, deep seas, and deserts, thus supporting energy supply in these areas. Among mid-temperature thermoelectric materials, PbTe boasts the best thermoelectric performance and the most mature development. However, the toxicity of lead limits its broader application. SnTe, with its environmental friendliness as well as the same crystal structure and similar band structure with PbTe, is considered a potential alternative. Nonetheless, the thermoelectric performance of pristine SnTe is inherently inferior to that of PbTe for some reasons. Intitially, SnTe suffers from high hole carrier concentration due to its numerous Sn vacancies in the lattice, which leads to a low Seebeck coefficient and a high electronic thermal conductivity. Additionally, compared to PbTe, SnTe has a smaller band gap between the conduction and valence bands, and a larger energy offset between the light and heavy valence bands. The heavy valence band contributes minimally to electrical transport, which limits the Seebeck coefficient and negatively impacts the thermoelectric figure of merit (ZT). Many research efforts have been dedicated to addressing these issues and optimizing the thermoelectric properties of p-type SnTe, yielding a series of significant results. By regulating intrinsic defects or doping with external elements, the carrier concentration of p-type SnTe can be effectively reduced, improving various thermoelectric transport characteristics. To further enhance its electrical transport performance, band engineering methods such as band flattening, valence band convergence, and resonance levels have shown promising optimization effects, increasing the Seebeck coefficient and power factor of p-type SnTe materials. Moreover, introducing nanostructures to create barriers and achieve energy filtering of carriers has also proven to be an effective strategy for improving the Seebeck coefficient. In terms of thermal transport properties, the introduction of crystal defects such as point defects, dislocations, grain boundaries, and nanostructures can significantly enhance phonon scattering and reduce lattice thermal conductivity, thus improving the ZT value of p-type SnTe thermoelectric materials. In recent years, significant breakthroughs have also been made in the study of n-type SnTe. High-performance n-type SnTe has been successfully synthesized through the method of high-ratio PbTe alloying combined with halogen doping, and corresponding all-SnTe-based thermoelectric devices have been prepared. However, compared to p-type SnTe, the research progress of n-type SnTe still lags behind. Developing lead-free n-type SnTe thermoelectric materials and exploring more strategies to optimize the thermoelectric properties of n-type SnTe remain significant challenges in this field. This review pays attention to the crystal structure and band structure of SnTe materials, the research progress on optimization strategies for the thermoelectric performance of p-type SnTe, and recent explorations of n-type SnTe materials and all-SnTe-based thermoelectric devices. To promote the practical application of SnTe, more new technologies and strategies need to be employed, and further research is required to develop and optimize n-type SnTe materials.