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

Abstract

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 (κlat) 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 κlat. 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.