Excitonic Effects on Two-Dimensional Transition-Metal Dichalcogenide Monolayers: Impact on Solar Cell Efficiency

The search for two-dimensional (2D) systems for applications in solar cells has continuously challenged our community. Here, we report screening of 2D monolayers from group IV to XI transition-metal dichalcogenides, MQ2, searching for candidates for high-performance photovoltaic devices, where M = 3d, 4d, and 5d transition metals and Q = S, Se, and Te, i.e., it yields 72 MQ2 monolayers. Through a robust theoretical framework that combines density functional theory, tight binding based on maximum localized Wannier functions, and the Bethe–Salpeter equation, we investigate the electronic, optical, and excitonic properties of the thermodynamic stable 2H-MQ2 monolayers. Furthermore, we employ a linear regression analysis of our data to identify nontrivial correlations between different optical and excitonic properties. From the 72 2H-MQ2 monolayers, we found from phonon calculations that only 22 are dynamically stable in the 2H form, of which 14 are semiconductors with a wide range of energy band gaps. Our results show that the presence of excitons affects the band alignment and the power conversion efficiency. In these compounds, the valley degeneracy breaking driven by spin-orbit coupling and the excitonic correction in the optical properties may be explored, through control of optical helicity, to tune the efficiency of the MQ2-based photovoltaic devices. We find high-efficiency van der Waals (vdW) heterostructures for solar cells and observe a strong linear correlation between exciton energy and the energy band gap for the stable semiconductors.