Excitonic, Optical, and Photovoltaic Properties of the 1T-NiO<sub>2</sub> Monolayer.

Abstract

Developing new two-dimensional materials for photovoltaics is a central strategy to address the world's growing energy demands. Herein, we made a multilevel, first-principles computational investigation focused on the characterization of the 1T NiO₂ monolayer, evaluating its structural stability, vibrational modes and Raman spectrum, electronic, mechanical, and optical properties. Our investigation was done through first-principle calculations based on density functional theory for structural and ground state properties, complemented by many-body perturbation theory to accurately capture quasiparticle (<i>G</i> ₀ <i>W</i> ₀) and excitonic effects, the latter being calculated with a maximally localized Wannier function-based tight-binding framework to describe the single particle states to solve the Bethe-Salpeter equation. Our calculations confirm that the 1T-NiO₂ monolayer is energetically, dynamically, thermally (at 300 K), and mechanically stable. We found an indirect electronic band gap of 2.20 eV at the <i>G</i> ₀ <i>W</i> ₀ level. Furthermore, the optical properties are dominated by strong electron-hole interactions, resulting in a direct excitonic state at 1.34 eV and an exceptionally high exciton binding energy of 880 meV. Although this optical gap is ideally positioned for solar absorption, leading to a theoretical power conversion efficiency (PCE^SQ) limit of 32.66%, the high exciton binding energy makes exciton dissociation into free charge carriers unfavorable. Despite the strong light absorption, the highly excitonic nature of the 1T-NiO₂ monolayer makes it unsuitable for conventional photovoltaic applications but potentially promising for exciton-based optoelectronics or photocatalytic devices.