Monolayer InISe: A Promising Thermoelectric Material with an Ultralow Phonon Thermal Conductivity and a High Power Factor

This work reports the novel thermoelectric (TE) material InISe, which has a FeOCl-type monolayer structure. The crystal and dynamic stabilities, electronic and phonon structures, and transport properties of InISe are systematically investigated by density functional theory and Boltzmann transport theory. The study of phonon spectra and free energy fluctuation shows that InISe has excellent mechanical and thermal stability, which are important in practical applications. The calculated electronic structure shows that near the Fermi level, the conduction band of InISe exhibits two energy valleys with very close energy values. This valley degeneracy phenomenon enhances the Seebeck coefficient without obviously reducing the electrical conductivity and is beneficial for the power factor. We found that the power factor of n-type InISe can reach 27.56 mW·m^-1·K^-2. Remarkably, the study of crystal transport properties shows that InISe has an ultralow phonon thermal conductivity, and the value is only 0.6 W/mK at 300 K, which is valuable for thermoelectric conversion. We found that the ultralow phonon thermal conductivity of InISe is attributed to the short phonon lifetime (<10² ps), which is the result of its FeOCl-type crystal structure. Our results show that n-type monolayer InISe is a promising candidate as a TE material.