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An efficient approach to improve the thermoelectric performance of materialsis to converge their electronic bands, which is known as band engineering. Inthis regard, lots of effort have been made to further improve thethermoelectric efficiency of bulk and exfoliated monolayers of Bi$_{2}$Te$_{3}$and Sb$_{2}$Te$_{3}$. However, ultra-high band degeneracy and thus significantimprovement of power factor have not been yet realized in these materials.Using first-principles methods, we demonstrate that the valley degeneracy ofBi$_{2}$Te$_{3}$ and Sb$_{2}$Te$_{3}$ can be largely improved upon substitutionof the middle layer Te atoms with the more electronegative S or Se atoms. Ourdetailed analysis reveals that in this family of materials two out of fourpossible valence band valleys merely depend on the electronegativity of themiddle layer chalcogen atoms, which makes the independent modulation of thevalleys position feasible. As such, band alignment of Bi$_{2}$Te$_{3}$ andSb$_{2}$Te$_{3}$ largely improves upon substitution of the middle layer Teatoms with more electronegative, yet chemically similar, S and Se ones. Asuperior valence band alignment is attained in Sb$_{2}$Te$_{2}$Se monolayerswhere the three out of four possible valleys are well-aligned, resulting in agiant band degeneracy of 18 that holds the record among all thermoelectricmaterials.

Achieving ultra-high power factor in Sb2Te2Se via valence band convergence