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

## TL;DR

This study uses first-principles calculations to show that substituting Se for Te in Sb2Te2Se significantly enhances band degeneracy, leading to ultra-high power factor in thermoelectric materials.

## Contribution

It demonstrates a novel band engineering strategy to achieve record-high band degeneracy in Sb2Te2Se by substituting chalcogen atoms, improving thermoelectric performance.

## Key findings

- Achieved a giant band degeneracy of 18 in Sb2Te2Se monolayers.
- Substitution of Te with Se enhances valence band alignment.
- Record-high power factor potential in thermoelectric materials.

## Abstract

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 have been made to further improve the thermoelectric efficiency of bulk and exfoliated monolayers of Bi$_{2}$Te$_{3}$ and Sb$_{2}$Te$_{3}$. However, ultra-high band degeneracy and thus significant improvement of power factor have not been yet realized in these materials. Using first-principles methods, we demonstrate that the valley degeneracy of Bi$_{2}$Te$_{3}$ and Sb$_{2}$Te$_{3}$ 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 Bi$_{2}$Te$_{3}$ and Sb$_{2}$Te$_{3}$ 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 Sb$_{2}$Te$_{2}$Se monolayers where the 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.