Thermoelectric Properties of Hexagonal M₂C₃ (M = As, Sb, and Bi) Monolayers from First-Principles Calculations.

Hexagonal M₂C₃ compound is a new predicted functional material with desirable band gaps, a large optical absorption coefficient, and ultrahigh carrier mobility, implying its potential applications in photoelectricity and thermoelectric (TE) devices. Based on density-functional theory and Boltzmann transport equation, we systematically research the TE properties of M₂C₃. Results indicate that the Bi₂C₃ possesses low phonon group velocity (2.07 km/s), low optical modes (2.12 THz), large Grüneisen parameters (4.46), and short phonon relaxation time. Based on these intrinsic properties, heat transport ability will be immensely restrained and therefore lead to a low thermal conductivity (4.31 W/mK) for the Bi₂C₃ at 300 K. A twofold degeneracy is observed at conduction bands along Γ-M direction, which gives a high n-type electrical conductivity. Its low thermal conductivity and high Seebeck coefficient lead to an excellent TE response. The maximum thermoelectric figure of merit (ZT) of n-type can approach 1.41 for Bi₂C₃. This work shows a perspective for applications of TE and stimulate further experimental synthesis.