Two-dimensional borocarbonitrides for photocatalysis and photovoltaics

We have designed two-dimensional borocarbonitrides (poly-butadiene-cyclooctatetraene framework BC 2 N) 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 × 10 4 cm 2 V −1 s −1 for β-BC 2 N), and strong absorption (an absorption coefficient of up to 10 5 cm −1 ). The intrinsic electric field due to the Janus geometry of α-BC 2 N reduces the recombination of photo-generated carriers. The band edge positions of α-BC 2 N and β-BC 2 N 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-BC 2 N and δ-BC 2 N 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. We have designed 2D borocarbonitrides with outstanding material properties and potential applications in a variety of important technological fields, including nano-mechanics, electronics, photocatalysis, and photovoltaics.