Drastic Modification of Lattice Thermal Conductivity in Thermoelectrics Induced by Electron-Hole Pairs

Low lattice thermal conductivity (κ_L) is desirable for advancing thermoelectric (TE) materials and devices. However, κ_L depends on many factors such as phonon scattering sources, electron-phonon interactions, etc. It remains not fully understood how κ_L is influenced by excited electron-hole (e-h) pairs ubiquitously present in thermoelectrics. To address this issue, the constrained density functional theory (CDFT) simulations are performed to investigate the phonon transport properties of two typical TE materials, Mg₂Si and PbTe. Surprisingly, at high e-h concentrations of ∼10²¹ cm^-3, the κ_L of Mg₂Si is reduced significantly by ∼30% due to the softening phonon modes. In contrast, the κ_L of PbTe first decreases by ∼18% at a relatively low e-h concentration of 2.34 × 10²⁰ cm^-3, but it is enhanced by ∼3.4% as the carrier concentration increases to 7.02 × 10²⁰ cm^-3. The different behaviors of these two materials arise from the different distributions of excess electrons and holes under e-h excitation. The simulation results suggest that controlling e-h pairs may offer a promising approach to design high-performance TE devices.