Defect-Enhanced Exciton–Exciton Annihilation in Monolayer Transition Metal Dichalcogenides at High Exciton Densities

The exciton–exciton annihilation (EEA) process easily occurs in monolayer transition metal dichalcogenides (TMDs) because of the strong Coulomb interaction and quantum confinement effect, which enhance the many-body interaction of excitons. This process can affect the performance of the optoelectronic devices. It is crucial to examine the effect of defect states on the EEA process and determine whether it is comparable to that at the low excitation intensities, particularly when applied to laser devices at a high exciton density. In this study, femtosecond transient absorption spectroscopy was used to explore the EEA process of four types of CVD-grown monolayer TMDs (i.e., WS2, WSe2, MoS2, and MoSe2). We demonstrated that the defect-assisted EEA process of local excitons is enhanced and plays a key role in the exciton relaxation process at high exciton densities of approximately 1012 cm–2 below a Mott density of approximately 1013 cm–2. The measured EEA rates for WS2, WSe2, MoSe2, and MoS2 were 0.016, 0.026, 0.049, and 0.102 cm2/s, respectively, implying that EEA is enhanced as defect states increase in monolayer TMDs. Our results provide a profound insight into the effect of defect states on the EEA process in monolayer TMDs at high exciton densities.