Ultralow Thermal Conductivity, Enhanced Mechanical Stability, and High Thermoelectric Performance in (GeTe)1-2x(SnSe)x(SnS)x.

Abstract

Thermoelectric (TE) energy conversion demands high performance crystalline inorganic solids that exhibit ultralow thermal conductivity, high mechanical stability, and good TE device properties. Pb-free germanium telluride (GeTe)-based material has recently attracted significant attention in TE power generation in mid temperatures, but pristine GeTe possesses significantly higher lattice thermal conductivity (κlatt) compared to that of its theoretical minimum (κmin) of ∼0.3 W/mK. Herein, we have demonstrated the reduction of κlatt of (GeTe)1-2x(SnSe)x(SnS)x very near to its κmin. The (GeTe)1-2x(SnSe)x(SnS)x system behaves as a coexistence of point-defect rich solid solution and phase separation. Initially, the addition of equimolar SnSe and SnS in the GeTe reduces the κlatt by effective phonon scattering because of the excess point defects and rich microstructures. In the second step, introduction of Sb-doping leads to additional phonon scattering centers and optimizes the p-type carrier concentration. Notably, 10 mol % Sb-doped (GeTe)0.95(SnSe)0.025(SnS)0.025 exhibits ultralow κlatt of ∼0.30 W/mK at 300 K. Subsequently, 10 mol % Sb-doped (GeTe)0.95(SnSe)0.025(SnS)0.025 exhibits a high TE figure of merit (zT) of ∼1.9 at 710 K. The high-performance sample exhibits a Vickers microhardness (mechanical stability) value of ∼194 HV that is significantly higher compared to the pristine GeTe and other state-of-the-art thermoelectric materials. Further, we have achieved a high output power, ∼150 mW for the temperature difference of 462 K, in single leg TE device based on 10 mol % Sb-doped (GeTe)0.95(SnSe)0.025(SnS)0.025.