  Van-der-Waals heterostructures show many intriguing phenomena including
ultrafast charge separation following strong excitonic absorption in the
visible spectral range. However, despite the enormous potential for future
applications in the field of optoelectronics, the underlying microscopic
mechanism remains controversial. Here we use time- and angle-resolved
photoemission spectroscopy combined with microscopic many-particle theory to
reveal the relevant microscopic charge transfer channels in epitaxial
WS$_2$/graphene heterostructures. We find that the timescale for efficient
ultrafast charge separation in the material is determined by direct tunneling
at those points in the Brillouin zone where WS$_2$ and graphene bands cross,
while the lifetime of the charge separated transient state is set by
defect-assisted tunneling through localized sulphur vacanices. The subtle
interplay of intrinsic and defect-related charge transfer channels revealed in
the present work can be exploited for the design of highly efficient light
harvesting and detecting devices.
