Defect-engineered competition between exciton annihilation and trapping in MOCVD WS₂

Exciton dynamics critically influence the optoelectronic performance of two-dimensional transition metal dichalcogenides (TMDCs). In large-scale WS₂ monolayers grown via metal-organic chemical vapor deposition (MOCVD), intrinsic sulfur vacancies introduce in-gap states that promote nonradiative recombination through defect trapping (DT). Under elevated excitation conditions, the decay behaviour changes as exciton-exciton annihilation (EEA) emerges as a competing nonradiative process. To investigate these mechanisms across excitation regimes, we combine steady-state quantum efficiency measurements with femtosecond broadband transient absorption spectroscopy on samples with varying defect concentrations. These complementary measurements provide an unprecedented quantitative disentanglement of these decay pathways, a level of analysis not previously reported for MOCVD-grown monolayer WS₂. The induced defect states are partially occupied, as first revealed by sub-bandgap excitation, and variations in defect density exert a pronounced influence on the photo-induced band renormalization. After establishing these DT-specific properties, we apply a rate-equation model including both DT and EEA to extract constants of 0.02 cm² s^-1 and 0.1 cm² s^-1, followed by an in-depth exploration of their fundamentally diffusion-limited behaviour. The competition between DT and EEA can be set by a critical defect-to-exciton density ratio (≈3.5), which serves as the threshold for EEA activation. Moreover, at high exciton densities, defect saturation suppresses DT, reshaping the decay landscape. Overall, our findings provide detailed insights into defect-modulated exciton decay mechanisms and establish a quantitative framework for tailoring the optoelectronic properties of TMDCs via controlled defect engineering.