Valley Degeneracy-Enhanced Thermoelectric Performance in In-Based FeOCl-Type Monolayers

A strategy to enhance the energy conversion efficiency of thermoelectric (TE) materials is to ensure that their band structure has as many energy valleys with high degeneracy as possible. In this work, this strategy is tested in the systems of In-based FeOCl-type monolayers. The TE performance of the InBrSe monolayer with an FeOCl-type structure is fully studied. Detailed calculations show that a higher valley degeneracy can lead to a larger Seebeck coefficient. An ultra-high power factor (PF) of 56.22 mW m–1 K–2 along the x-axis for the n-type doped InBrSe monolayer is found, which is higher than the maximum PF value of previously reported FeOCl-type materials. Such an excellent PF originates from the high energy valley degeneracy of the conduction band and the high connectivity of electronic conduction channels along the x-axis, which simultaneously enhance the Seebeck coefficient and the electrical conductivity. Meanwhile, the InBrSe monolayer has a short phonon lifetime and strong phonon anharmonicity, resulting in low phonon thermal conductivity. Combining the ultra-high PF and low phonon thermal conductivity, the ZT values of the InBrSe monolayer are 2.34, 3.85, and 5.12 at 300, 500, and 700 K, respectively. When the temperature of the cold end is 300 K and that of the hot end is 700 K, the maximum energy conversion efficiency of the InBrSe monolayer can reach 26.1%, which is comparable to that of the traditional heat engine. This study demonstrates that the InBrSe monolayer is a promising medium-temperature TE material and confirms that increasing valley degeneracy is a valuable way to improve the TE performance of 2D materials.