Achieving an Ultrahigh Power Factor in Sb₂Te₂Se Monolayers via Valence Band Convergence

An efficient approach to improve the thermoelectric performance of materials is to converge their electronic bands, which is known as band engineering. In this regard, lots of effort has been made to further improve the thermoelectric efficiency of bulk and exfoliated monolayers of Bi₂Te₃ and Sb₂Te₃. However, ultrahigh band degeneracy and thus significant improvement of the power factor have not yet been realized in these materials. Using first-principles methods, we demonstrate that the valley degeneracy of Bi₂Te₃ and Sb₂Te₃ can be largely improved upon substitution of the middle-layer Te atoms with the more electronegative S or Se atoms. Our detailed analysis reveals that in this family of materials, two out of four possible valence band valleys merely depend on the electronegativity of the middle-layer chalcogen atoms, which makes the independent modulation of the valleys' position feasible. As such, band alignment of Bi₂Te₃ and Sb₂Te₃ largely improves upon substitution of the middle-layer Te atoms with more electronegative, yet chemically similar, S and Se ones. A superior valence band alignment is attained in Sb₂Te₂Se monolayers where three out of four possible valleys are well aligned, resulting in a giant band degeneracy of 18 that holds the record among all thermoelectric materials. As a result, an outstanding power factor for the hole-doped monolayers is achieved, indicating a highly efficient p-type thermoelectric material.