Thermoelectric technology allows conversion between heat and electricity. Many good thermoelectric materials contain rare or toxic elements, so developing low-cost and high-performance thermoelectric materials is warranted. Here, we report the temperature-dependent interplay of three separate electronic bands in hole-doped tin sulfide (SnS) crystals. This behavior leads to synergistic optimization between effective mass (m*) and carrier mobility (μ) and can be boosted through introducing selenium (Se). This enhanced the power factor from ~30 to ~53 microwatts per centimeter per square kelvin (μW cm−1 K−2 at 300 K), while lowering the thermal conductivity after Se alloying. As a result, we obtained a maximum figure of merit ZT (ZTmax) of ~1.6 at 873 K and an average ZT (ZTave) of ~1.25 at 300 to 873 K in SnS0.91Se0.09 crystals. Our strategy for band manipulation offers a different route for optimizing thermoelectric performance. The high-performance SnS crystals represent an important step toward low-cost, Earth-abundant, and environmentally friendly thermoelectrics.

The simple binary compound SnSe has been reported as a robust thermoelectric material for energy conversion by showing strong anharmonicity and multiple electronic valence bands. Herein, we report a record-high average ZT value of ∼1.6 at 300−793 K with maximum ZT values ranging from 0.8 at 300 K to 2.1 at 793 K in p-type SnSe crystals. This remarkable thermoelectric performance arises from the enhanced power factor and lowered lattice thermal conductivity through crystal structure modification via Te alloying. Our results elucidate that Te alloying increases the carrier mobility by making the bond lengths more nearly equal and sharpening the valence bands; meanwhile, the Seebeck coefficient remains large due to multiple valence bands. As a result, a record-high power factor of ∼55 μW cm −1 K −2 at 300 K is achieved. Additionally, Te alloying promotes Sn atom displacements, thus leading to a lower lattice thermal conductivity. Our conclusions are well supported by electron localization function calculations, the Callaway model, and structural characterization via aberration-corrected scanning transmission electron microscopy. Our approach of modifying crystal structures could also be applied in other low-symmetry thermoelectric materials and represents a new strategy to enhance thermoelectric performance.

Enhanced electrical transport properties and low thermal conductivity lead to high figure of merit (ZT) over the whole temperature range in Na-doped SnS crystals.

Entropy engineering is one of the powerful approaches to suppress phase transitions.

Thermoelectric technology enables direct conversion between heat and electricity. The conversion efficiency of a thermoelectric device is determined by the average dimensionless figure of merit ZTave. Here, a record high ZTave of ≈1.34 in the range of 300–723 K in n‐type SnSe based crystals is reported. The remarkable thermoelectric performance derives from the high power factor and the reduced thermal conductivity in the whole temperature range. The high power factor is realized by promoting the continuous phase transition in SnSe crystals through alloying PbSe, which results in a higher symmetry of the crystal structure and the correspondingly modified electronic band structure. Moreover, PbSe alloying induces mass and strain fluctuations, which enables the suppression of thermal transport. These findings provide a new strategy to enhance the thermoelectric performance for the continuous phase transition materials.