Ultralow Thermal Conductivity, Enhanced Mechanical Stability, and High Thermoelectric Performance in (GeTe)_{1–2<i>x</i>}(SnSe)_<i>x</i>(SnS)_<i>x</i>

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 (κ_latt) compared to that of its theoretical minimum (κ_min) of ∼0.3 W/mK. Herein, we have demonstrated the reduction of κ_latt of (GeTe)_1-2<i>x</i>(SnSe)_<i>x</i>(SnS)_<i>x</i> very near to its κ_min. The (GeTe)_1-2<i>x</i>(SnSe)_<i>x</i>(SnS)_<i>x</i> 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 κ_latt 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 <i>p</i>-type carrier concentration. Notably, 10 mol % Sb-doped (GeTe)_0.95(SnSe)_0.025(SnS)_0.025 exhibits ultralow κ_latt 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 <i>H</i>_V 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.