First-Principles Investigations of Few-Layer and Bulk Orthorhombic B₂N₂: Exploring Structural and Optoelectronic Properties for Photovoltaic Applications

In 2020, Demirci et al. [1] predicted a two-dimensional monolayer polymorph of boron nitride with an orthorhombic structure (o-B₂N₂) using first-principles calculations. Subsequently, Li et al. [2] showed that the band gap of monolayer o-B₂N₂, calculated at the GW level, is 2.446 eV. Recently, several groups have proposed applications for monolayer o-B₂N₂ in renewable energy. Defective and metal-decorated o-B₂N₂ have been proposed for hydrogen storage [3-5] and as hydrogen evolution reaction catalysts [6]. Additionally, the potential of o-B₂N₂ for applications in batteries has been explored [7-10]. Nevertheless, the properties of few-layer and bulk-layered o-B₂N₂ remain largely unexplored.

Here we investigate the structural and optoelectronic properties of few-layer and bulk-layered o-B₂N₂ using first-principles calculations. We use density functional theory (DFT) calculations, including van der Waals corrections, to show that the energetically favorable stacking order is AB, with B atoms sitting above N atoms and vice versa. We also studied the electronic properties of these systems using many-body GW calculations and solved the Bethe-Salpeter equations to include excitonic effects. The band gaps calculated at the GW level decrease as the number of layers increases, reaching 1.28 eV for bulk o-B2N2. Exciton binding energies also decrease as a function of the number of layers. The band gap we predict for bulk o-B2N2, along with the fact that the calculated optical absorption closely matches solar irradiance, makes this material a promising candidate for photovoltaic applications.

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