[Abstract]
Although polycrystalline SnSe is a promising lead-free thermoelectric material, its performance has remained constrained until recently by low electrical conductivity and unexpectedly higher lattice thermal conductivity compared with single-crystal SnSe. Here, we introduce a hierarchical compositional and structural design strategy that integrates Ge alloying, Na and Ta doping, and a combined post-processing of mechanical ball-milling and H₂-reduction purification (BM/H₂–R) to address such challenges. Based on the enhanced carrier concentration at ~1019 cm−³ by Na doping, the Ge substitution at the Sn site slightly decreases hole carrier concentration and suppresses lattice heat transport through mass-fluctuation and strain-field scattering, giving a ZT of ~1.4 at 800 K. The subsequent Ta doping induces microscale TaSe₂ precipitates within the SnSe matrix, facilitating charge carrier transport and introducing additional phonon-scattering centers. The resulting mate- rials exhibit simultaneously increased power factor and decreased thermal conductivity, giving a ZT of ~1.5 at 800 K. Further microstructural engineering through the BM/H₂–R process reduces the particle size of TaSe₂ precipitates to a nanoscale and effectively removes surface oxides, markedly suppressing the lattice thermal conductivity, especially at the high temperature regime. The optimized sample exhibits the ultralow lattice thermal conductivity of ~0.13 W m−¹ K−¹ and a very high ZT of ~2.1 at 800 K. These results highlight that simultaneous control of chemical compositions and microstructural hierarchy provides an effective approach to decouple charge and phonon transport, offering a practical pathway for developing high-performance thermo- electric materials.