Efficient power generation near room temperature in earth-abundant thermoelectric tin sulfide via lattice ordering

The inefficient carrier transport of earth-abundant tin sulfide (SnS), arising from its intrinsically low crystal symmetry, limits its thermoelectric performance at low temperatures. Here, the substitutional Ag⁺ doping modifies the Na-doped SnS crystal structure by reinforcing lattice dynamical anisotropy and enhancing in-plane transport properties. Concurrently, the reconstructed valence-band structure evolves into a modulated multiband configuration, increasing the density-of-states effective mass while preserving high carrier mobility. The resulting highly metallic conductivity combined with high thermopower yields the room-temperature power factor of ~101 µW cm^-1K^-2 in Sn_0.96Na_0.01Ag_0.03S single crystals. Coupled with the intrinsically low thermal conductivity of SnS, the optimized crystal achieves the figure of merit (zT) of ~1.06 at 300 K and ~2.0 at 573 K, delivering a conversion efficiency of ~3.3% and an output power density of ~0.31 W cm^-2 in a single-leg module under a 243 K temperature gradient. These results demonstrate the effectiveness of crystal structure modification and multiband engineering in SnS and establish it as a highly promising low-cost thermoelectric material for near-room-temperature energy conversion.