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 two-dimensional 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 (κlat) 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 κlat. Our analysis indicates the activation of multi-conduction band transport driven by halogen substitution. The PbSnS1.96Br0.04 has a high power factor of five times that of intrinsic PbSnS2. Halogen-doping weaken the Pb/Sn-S bondsand enhanced the phonon scattering, leading to an ultralow κlat of 0.29 W m-1 K-1 at 873 K for PbSnS1.96Br0.04. Consequently, PbSnS1.96Br0.04 achieved a maximum ZT value of 0.82 at 873 K.