  The exciton dynamics in monolayer black phosphorus is investigated over a
very wide range of photoexcited exciton densities using time resolved
photoluminescence. At low excitation densities, the exciton dynamics is
successfully described in terms of a double exponential decay. With increasing
exciton population, a fast, non-exponential component develops as
exciton-exciton annihilation takes over as the dominant recombination mechanism
under high excitation conditions. Our results identify an upper limit for the
injection density, after which exciton-exciton annihilation reduces the quantum
yield, which will significantly impact the performance of light emitting
devices based on single layer black phosphorus.
