Enhanced Thermoelectric Performance in Se Alloyed SnS Through Band Engineering, Na and Ag Co-Doping, and Nanostructuring.

SnS‑based compounds are promising eco‑friendly thermoelectric materials. We investigated polycrystalline, hole‑doped Sn(1‑x‑y)NaxAgyS1‑zSez (z = 0, 0.09, and 0.5), synthesized via a solid‑state reaction, ball milling, and spark plasma sintering. The co‑doped Sn0.98Na0.01Ag0.01S0.5Se0.5 sample achieved a high figure of merit (ZT≈2.2 at 810 K), enabled by the combined effects of an enhanced power factor and ultralow lattice thermal conductivity (≈0.2 W m-1 K-1 at 810 K). This demonstrates the favorable co‑optimization of electrical and thermal transport properties in the alloyed system. HR-TEM and X-ray diffraction analyses confirm the presence of nanocrystalline domains (∼9-14 nm) that effectively scatter phonons. Electronic-structure calculations showed that Se alloying restructures the fragmented valence-band edge of SnS into a converged multivalley manifold, reducing the geometric and inertial effective masses while preserving the aggregate DOS slope. This reshaping, confirmed by DOS profiles and Pisarenko analysis, demonstrates that Se substitution simultaneously enhances carrier mobility and sustains Seebeck response, yielding a higher power factor. Experimentally, Na/Ag co‑doped Sn0.98S exhibited a 1.16 eV bandgap, narrowed to 0.99 eV in Sn0.98S0.5Se0.5, confirming electronic tuning. Specific heat and Raman spectroscopy reveal softened lattice vibrations, which, together with nanocrystalline phonon scattering, suppress lattice thermal conductivity and enable outstanding thermoelectric performance.