Twist angle can expand charge carrier diffusion length in bilayer black phosphorus: ab initio quantum dynamics

Abstract

Focusing on bilayer black phosphorus (BP) and performing nonadiabatic molecular dynamic (NA-MD) simulations, we demonstrate that appropriate twist angles can significantly improve the charge carrier lifetime and diffusion length. Interlayer coupling is determined by the interlayer distance rather than wavefunction overlap between the two-layer BP. Compared to AB stacking, the weakened interlayer couplings, presented in the twisted systems, soften the interlayer breathing modes and inhibit the out-of-plane motions of P atoms by increasing the interlayer distances, which decreases NA coupling from the 38.94° to 11.54° system. Spontaneous charge separation happens in the 11.54° system due to the mismatched potentials of electrons and holes in high-symmetry stacking-induced moiré patterns, thereby generating the smallest NA coupling. Overall, non-adiabatic couplings (NACs) dominate nonradiative electron–hole recombination and extend the carrier lifetimes to 776 ps in the 38.94° and 1223 ps in the 11.54° system, which is one order of magnitude longer than that in the AB stacking. Correlating with the modulated charge carrier mobilities, the electron and hole diffusion lengths are extended by factors of 4.2 and 3.6 in the 11.54° system and 4.7 and 1.5 in the 38.94° system, compared with that in AB stacking. Our work provides atomic insights on twist angle-dependent carrier dynamics, including recombination and transport.