Influence of sulphur vacancies on ultrafast charge separation in WS$_2$-graphene heterostructures

Understanding how defects influence charge separation in WS$_2$-graphene heterostructures is crucial for future applications in light harvesting and detection. Previous studies have reported widely varying lifetimes for the charge-separated state, all supposedly linked to electron trapping at sulphur vacancies. The exact impact of these defects, however, has remained unclear. Here, we deliberately introduce sulphur vacancies by annealing the heterostructures at high temperatures in ultrahigh vacuum. Angle-resolved photoemission spectroscopy (ARPES) reveals that these vacancies modify both the band alignment and doping level of the heterostructure. Time-resolved ARPES (trARPES) further shows that increasing the sulphur vacancy concentration prolongs the lifetime of electrons in the WS$_2$ conduction band but shortens the lifetime of the charge-separated state. Guided by model calculations, we attribute this behaviour to shifts in the energy alignment between sulphur vacancy states and graphene's Dirac point, combined with a reduced excitonic absorption. The model also yields a transfer time for electrons tunneling from sulphur vacancies into graphene's Dirac cone of $\sim$4ps, consistent with our trARPES measurements. Our study clarifies the role of sulphur vacancies in WS$_2$-graphene heterostructures, further improving our microscopic understanding of charge dynamics for future optoelectronic applications.