Insight into Carrier and Phonon Transports of PbSnS₂ Crystals

The simultaneous optimization of n-type and p-type thermoelectric materials is advantageous to the practical application of the device. As an emerging thermoelectric material, PbSnS₂ exhibits highly competitive thermoelectric properties due to its unique carrier and phonon transport characteristics. To promote the utilization of this low-cost thermoelectric material, p-type PbSnS₂ crystals are synthesized and optimized through Na doping and Se alloying. The resulting thermoelectric transport properties differ significantly from those reported for n-type crystals, prompting us to compare and analyze both n-type (Cl-doped) and p-type (Na-doped) PbSnS₂ crystals from various perspectives. Cl doping is subject to weaker "Fermi pinning" and lower impurity ionization energy compared with Na doping, leading to higher doping efficiency. The different optimal performance directions in n-type and p-type crystals can be attributed to the distinct charge density distributions near the conduction band minimum (CBM) and the valence band maximum (VBM). Additionally, both n-type and p-type crystals exhibit ultralow lattice thermal conductivity due to the low symmetry of their twisted NaCl structure combined with the strong anharmonicity. This comprehensive analysis of PbSnS₂ crystals provides a solid foundation for further performance optimization and device assembly, while also sheds light on the investigation of layered thermoelectric materials.