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.