Layer-Dependent Optical Properties of Orthorhombic B₂N₂: Prospects for Photovoltaics

Fundamental understanding of exciton formation is of utmost importance for a wide variety of optoelectronic applications, as this elementary quasiparticle strongly influences the absorption, charge separation, and photocurrent generation processes. While hexagonal boron nitride stands out for its high thermal stability and chemical inertness, its wide band gap hampers its use in several optoelectronic applications, including photovoltaics. Here, by employing ab initio many-body excited-state methods, we elucidate how the electronic and optical properties of orthorhombic B₂N₂ evolve with layer thickness, from the three-dimensional bulk to intermediate multilayers and down to the monolayer limit. The results indicate that the quasiparticle gap can be tuned from 2.41 eV, for the monolayer, down to 1.28 eV in the bulk limit. Interestingly, the studied excitonic response exhibits prominent peaks in the near-infrared range, going from 1.4 to 1.7 eV, which highlights their potential as an active sunlight absorber material. Finally, we model a prototypical single-junction solar cell based on bulk B₂N₂, finding that a 150-nm-thick active layer achieves power conversion efficiencies between 16.8% and 24.9% in the nonradiative and radiative limits, respectively. Our calculations suggest the potential of B₂N₂-based thin films for the design of flexible solar cells.