Ultra-high thermoelectric performance achieved in only ternary lead sulfide through unconventional halogen element doping

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The abundant presence of lead sulfide on Earth, as PbTe and PbSe homologous compounds, has sparked increasing research interest in the field of thermoelectrics. However, the low carrier transport properties and high lattice thermal conductivity of PbS, contributing to its low thermoelectric properties. Conventional doping methods often fail to consider the simultaneous optimization of both aspects. Herein, an unconventional Br doping strategy is explored to solve both problems simultaneously through optimizing the carrier concentration and mobility, while simultaneously reducing the lattice thermal conductivity by forming unique and complex defect structures. Therefore, significant enhancements are achieved in the thermoelectric properties of n-type PbS0.99Br0.02. The feasibility of this method is further validated through an investigation into the doping impact of the Cl element. It is found that the excessive doping of halogen elements following the introduction of sulfur vacancies not only significantly enhances carrier concentration, but also effectively suppresses the degradation of carrier mobility with increasing temperature, thereby enhancing the power factor. Additionally, the incorporation of X-enrich precipitates (X = Br, Cl) can efficiently scatter phonons and reduce lattice thermal conductivity in the materials. Consequently, a maximum zTmax ∼1 was achieved in the PbS0.99Cl0.02 sample at 873 K, representing the highest value reported for non-chemically synthesized ternary n-type PbS. The present finding provides a novel route to optimize PbS materials, which should be applicable for other thermoelectric materials.