Two-dimensional borocarbonitrides for photocatalysis and photovoltaics

Abstract

We have designed two-dimensional borocarbonitrides (poly-butadiene-cyclooctatetraene framework BC₂N) with hexagonal unit cells, which are stable according to the cohesive energy, phonon dispersion, ab initio molecular dynamics, and elastic modulus results. They are n-type semiconductors with strain-tunable direct band gaps (1.45–2.20 eV), an ultrahigh electron mobility (5.2 × 10⁴ cm² V⁻¹ s⁻¹ for β-BC₂N), and strong absorption (an absorption coefficient of up to 10⁵ cm⁻¹). The intrinsic electric field due to the Janus geometry of α-BC₂N reduces the recombination of photo-generated carriers. The band edge positions of α-BC₂N and β-BC₂N are suitable for photocatalytic hydrogen production, achieving high solar-to-hydrogen efficiencies of 17% and 12%, respectively, in excess of the typical target value of 10% for industrial application. Both γ-BC₂N and δ-BC₂N can be used as electron donors in type-II heterostructures with two-dimensional transition metal dichalcogenides, and the power conversion efficiency of a solar cell based on these heterostructures can be as high as 21%, approaching the performance of perovskite-based solar cells.