Janus Rashba semiconductor of RbKNaBi: High thermoelectric performance and ultralow lattice thermal conductivity

The Rashba effect, induced by the spin–orbit coupling and the broken inversion symmetry, has been regarded as an effective strategy to increase thermoelectric performance due to enhanced band degeneracy. We herein use first-principles calculations and Boltzmann transport theory to explore the thermoelectric properties of 2D Janus RbKNaBi. It is found that 2D RbKNaBi is a Rashba semiconductor with a narrow bandgap of 0.215 eV and a large Rashba constant of 0.263 eVÅ. The p-type doping exhibits a higher power factor than the n-type one due to the Rashba effect and longer relaxation time of the hole. Strong anharmonicity is observed due to the weak chemical bond and heavy atomic mass in RbKNaBi, which leads to an ultralow lattice thermal conductivity of 1.46 (0.87) Wm−1K−1 at 300 (500) K. Ultimately, a high p-type thermoelectric figure of merit of 2.50 at 500 K is achieved in 2D RbKNaBi, which is rare in narrow-gap semiconductors or at medium temperature. These results demonstrate that the 2D Janus Rashba semiconductor of RbKNaBi is a promising candidate for medium-temperature thermoelectric applications.