THE ROLE OF FRÖHLICH AND INTERVALLEY SCATTERING IN THE THERMOELECTRIC TRANSPORT OF 2D PENTAGONAL BI2X (X= Ge, Sn) NETWORKS

Abstract

Two-dimensional (2D) penta-materials exhibiting inherently buckled structures and often highly degenerate multivalley electronic structures are promising thermoelectric candidates. However, most current computational investigations on these materials treat charge-carrier scattering using analytical models based on continuum approximations. Here, we elucidate the potential importance of an ab initio mode-resolved approach to carrier transport for these systems by revisiting the thermoelectric performance of recently predicted penta-Bi2X (X=Ge,Sn) monolayers. Under acoustic deformation potential theory, these materials when suitably hole- doped were previously reported to yield extraordinary peak ZT values of 7.0 and 5.8 at 700 K. However, our calculations reveal that charge transport in these monolayers is significantly restricted when long-range Fröhlich and intervalley scattering processes are incorporated. Consequently, the predicted peak ZT for p-type Bi2Ge at 700 K undergoes a substantial correction from 7.0 to 1.0. Conversely, this correction is far less pronounced for p-type Bi2Sn, which maintains a highly competitive peak ZT of 4.4. This relative robustness is attributed to the notably low lattice thermal conductivity on the order of 10-2 W m-1 K-1 reported in the prior study, which allows a concurrent drop in the electronic thermal conductivity to partially compensate for a heavily decreased electrical conductivity. These insights underscore the importance of advanced scattering models for predicting the performance of non-centrosymmetric materials featuring multiple band and valley degeneracies.