Enhanced covalency and nanostructured-phonon scattering lead to high thermoelectric performance in n-type PbS

High thermoelectric performance can be achieved either by tuning the electronic structure or by enhancement in scattering the heat-carrying phonons, which often affect each other. Thereby, a leap in the performance can be achieved by simultaneous modulation of electronic structure and lowering of thermal conductivity. Herein, we demonstrate a high thermoelectric figure of merit (zT) of 1.45 at 900 K for Ge doped (4–10 mol%) n-type PbS, which is the one of the highest values among all n-type PbS-based thermoelectric materials. This high performance is achieved by simultaneous (a) enhancement of covalency in chemical bonding which increases the electrical conductivity, and (b) reduction of lattice thermal conductivity (κ_lat) to an ultra-low value of 0.56 W m⁻¹K⁻¹ at 900 K by the introduction of nanometer-sized (5–10 nm) precipitates of Pb₂GeS₄ in PbS matrix which strongly scatter the heat-carrying phonons. The presence of low-lying transverse acoustic (TA) and longitudinal acoustic (LA) phonon modes at 48.24 cm⁻¹ and 91.83 cm⁻¹, respectively are experimentally revealed from inelastic neutron scattering (INS) experiments. The softening of low-frequency modes at a higher temperature and ultra-short phonon lifetime (1–4.5 ps) further explain the ultra-low κ_lat. Electron localization function (ELF) analysis confirms chemical bonding hierarchy and increased covalent bonding due to the presence of Ge in PbS. The increase in bond covalency upon Ge doping in n-type PbS weakens electron–phonon coupling, thereby increasing the electrical transport.