Materials selection rules for optimum power factor in two-dimensional thermoelectrics

Abstract

Two-dimensional (2D) materials have emerged as the ideal candidates for many applications, including nanoelectronics, low-power devices, and sensors. Several 2D materials have been shown to possess large Seebeck coefficients, thus making them suitable for thermoelectric (TE) energy conversion. Whether even higher TE power factors can be discovered among the ≈2000 possible 2D materials (Mounet et al 2018 Nat. Nanotechnol. 13 246–52) is an open question. This study aims at formulating selection rules to guide the search for superior 2D TE materials without the need for expensive atomistic simulations. We show that a 2D material having a combination of low effective mass, higher separation in the height of the step-like density of states, and valley splitting, which is the energy difference between the bottom of conduction band and the satellite valley, equal to 5 kBT will lead to a higher TE power factor. Further, we find that inelastic scattering with optical phonons plays a significant role: if inelastic scattering is the dominant mechanism and the energy of the optical phonon equals 5 kBT, then the TE power factor is maximized. Starting from a model for carrier transport in MoS2 and progressively introducing the aforementioned features results in a two-orders-of-magnitude improvement in the power factor. Compared to the existing selection rules or material descriptors, features identified in this study provide the ability to comprehensively evaluate TE capability of a material and helps in identifying future TE materials suitable for applications in waste-heat scavenging, thermal sensors, and nanoelectronics cooling.