Strong Electron-Phonon and Phonon-Phonon Interactions Lead to High Thermoelectric Performances in Lead Phosphorene via Symmetry Breaking

Understanding the full electron-phonon and phonon-phonon interaction effects in semiconductors is crucial for tuning electronic and phonon transport properties with the purpose to the enhancement of thermoelectric performances. In this work, based on the detailed investigations on 2D lead phosphorene (PbP) by using the first-principle calculations, it is demonstrated that, in & alpha;-$\alpha -$, & beta;-$\beta -$, and & gamma;-$\gamma -$PbP, the electron-phonon interactions are significantly stronger than conventional group-IV monolayers such as silicene by five orders in magnitude, and intravalley scatterings dominate over the intervalley scattering due to less degenerate band extrema near Fermi level. Based on the selection-rule analysis, it is found that the symmetry breaking in the janus & gamma; phase compared to the higher-symmetry & alpha; phase allows fewer symmetry operations, leading to more allowed scattering channels for electrons and phonons, subsequently suppressing the electron and phonon transport in & gamma;-$\gamma -$PbP. Due to the low Debye temperatures, and strong four-phonon interactions, the lattice thermal conductivities at room temperature in these monolayers are sharply decreased. Finally, it is found that the maximum thermoelectric figure of merits for these three monolayers reach 0.90/0.24/1.25 at room temperature, respectively. This work not only paves the way for the promising applications of PbP monolayers in thermoelectric devices, but also suggests that symmetry breaking and strong high-order phonon-phonon interactions are crucial to identifying excellent thermoelectric materials. The work highlights the dominance of intravalley scattering over intervalley scattering in electron relaxation times. Group theory analysis reveals restrictions on certain phonons in assisting electron transition, resulting in improved carrier mobility in a-PbP. Ultra-low lattice thermal conductivities are observed in PbP monolayers. Promising thermoelectric figure of merit values are predicted for PbP monolayers at room temperature.image