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

Abstract Here, we investigate PbSnS 2 , 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 PbSnS 2 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 1.96 Br 0.04 has a high power factor of five times that of intrinsic PbSnS 2 . 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 1.96 Br 0.04 . Consequently, PbSnS 1.96 Br 0.04 achieved a maximum ZT value of 0.82 at 873 K.