Ultralow Thermal Conductivity in Halogen-Doped PbSnS₂ with Optimized Thermoelectric Properties

Here, we investigate PbSnS₂, a wide band gap (1.13 eV) compound, as a promising thermoelectric material for power generation. Single crystal X-ray diffraction analysis reveals its 2D-layered structure, akin to the GeSe structure type, with Pb and Sn atoms sharing the same crystallographic site. The polycrystalline PbSnS₂ exhibits an intrinsically ultralow lattice thermal conductivity (κ_lat) of 0.37 W m^-1 K^-1 at 573 K. However, the low carrier concentration (n) leads to suboptimal electrical conductivity (σ), capping the ZT value at 0.1. Accordingly, the halogen elements (Cl, Br, and I) are employed as the n-type dopants to improve the n. The DFT results indicate a significant weakening of Pb/Sn─S bonds upon halogen-doping, contributing to the observed reduction in κ_lat. Our analysis indicates the activation of multiconduction band transport driven by halogen substitution. The PbSnS_1.96Br_0.04 has a high power factor of five times that of intrinsic PbSnS₂. Halogen-doping weakens the Pb/Sn─S bonds and enhances the phonon scattering, leading to an ultralow κ_lat of 0.29 W m^-1 K^-1 at 873 K for PbSnS_1.96Br_0.04. Consequently, PbSnS_1.96Br_0.04 achieved a maximum ZT value of 0.82 at 873 K.