Asymmetric phonon-drag effect and thermoelectric performance in PbTe under strain modulation

Abstract

The intricate interplay between electrical and thermal transport in thermoelectric materials has long garnered significant theoretical interest. Despite long-standing theoretical interest in thermoelectric materials, conventional approaches neglect phonon-drag effect and explicit electron-phonon coupling. Here, using the first-principles calculation in combined with coupled electron-phonon Boltzmann transport equations, we investigated strain-dependent thermoelectric transport property in PbTe. We found a pronounced asymmetry effect of strain on Seebeck coefficient and identified a significant difference in phonon-drag contribution. Specifically, phonon-drag contribution on Seebeck coefficient is negligible at strainless and compressive condition, however, strongly activated at tensile strain. Moreover, we show that neglecting phonon-drag effect can yields an unphysical Seebeck signal for p-type PbTe, while including phonon-drag effect restores a positive Seebeck signal. This behaviour primarily results from the reduced band gap under tensile strain, which expands the scattering phase space and enhances the scattering between low-frequency phonons and valence-band electrons. These results show that phonon drag must be included in the tensile-strain regime with a markedly softened phonon mode to interpret the Seebeck response reliably. Overall, our results uncover a pronounced strain-induced asymmetry in the thermoelectric transport of PbTe, which is crucial for accurate prediction of the Seebeck coefficient.