Triple-Conduction-Band Activation Enables High Out-of-Plane Thermoelectric Efficiency in n-type SnS Crystals

The increasing interest for high-performance and earth-abundant thermoelectrics has motivated attention toward sulfide-based compounds. Among them, tin sulfide (SnS) has emerged as a promising candidate, especially in its p-type form. However, progress in n-type SnS has been hindered by poor electrical transport. In this study, Se and Pb are dual-alloyed into n-type Pnma-phase SnS crystals to achieve a considerable boost in carrier concentration, and the resultant triple-conduction-band activation significantly decouples carrier mobility and effective mass, facilitating a substantial enhancement in the out-of-plane three-dimensional (3D) charge transport. Meanwhile, the 2D phonon transport is further strengthened by the reduced group velocities and low-lying optical phonon modes, significantly suppressing the lattice thermal conductivity. Consequently, an exceptional out-of-plane ZT exceeding 2.0 at 698 K and an average ZT higher than 1.1 at 300-723 K are achieved in n-type Sn_0.63Pb_0.37S_0.55Se_0.45 crystals within the Pnma-phase region. Furthermore, the as-fabricated single-leg thermoelectric device demonstrates a power generation efficiency of ∼8.3% under a temperature difference (ΔT) of 472 K, showing promise for n-type thermoelectric sulfides. This work marks a significant advancement in the earth-abundant n-type SnS thermoelectrics through triple-conduction-band engineering, promoting the construction and potential application of all SnS-based devices.