Two-dimensional borocarbonitrides for photocatalysis and photovoltaics

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 x 10(4) cm(2) V-1 s(-1) for beta-BC2N), and strong absorption (an absorption coefficient of up to 10(5) cm(-1)). The intrinsic electric field due to the Janus geometry of alpha-BC2N reduces the recombination of photo-generated carriers. The band edge positions of alpha-BC2N and beta-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 gamma-BC2N and delta-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.