Optoelectronic properties of defective MoS$_2$ and WS$_2$ monolayers

TL;DR

This theoretical study investigates how metal and sulfur vacancies affect the electronic and optical properties of MoS$_2$ and WS$_2$ monolayers, revealing defect-induced states that could enhance photovoltaic efficiency.

Contribution

The paper introduces a detailed theoretical analysis of vacancy effects on MoS$_2$ and WS$_2$ monolayers using a Slater-Koster tight-binding model with spin-orbit coupling, highlighting defect-induced electronic and optical modifications.

Findings

Vacancies create flat bands and shift Fermi levels, acting as acceptors.

Defective monolayers show additional optical peaks from midgap states.

Certain vacancies enhance optical conductivity in the visible range.

Abstract

We theoretically explore the effect of metal and disulphur vacancies on electronic and optical properties of MoS$_2$ and WS$_2$ monolayers based on a Slater-Koster tight-binding model and including the spin-orbit coupling. We show that the vacancy defects create electronic flat bands by shifting the Fermi level towards the valence band, indicating that both types of vacancies may act as acceptor sites. The optical spectra of the pristine monolayers show step-like features corresponding to the transition from spin split valence band to the conduction band minimum, whereas the defective monolayers exhibit additional peaks in their spectra arising from induced midgap states in their band structures. We find that Mo and W vacancies contribute mostly in the low-energy optical spectrum, while the S$_2$ vacancies enhance the optical conductivity mainly in the visible range of the spectrum. This suggests that depending on the type of vacancy, the atomic defects in MoS$_2$ and WS$_2$ monolayers may increase the efficiency of solar cells used in photovoltaic systems.