  Ultrafast charge separation after photoexcitation is a common phenomenon in
various van-der-Waals (vdW) heterostructures with great relevance for future
applications in light harvesting and detection. Theoretical understanding of
this phenomenon converges towards a coherent mechanism through charge transfer
states accompanied by energy dissipation into strongly coupled phonons. The
detailed microscopic pathways are material specific as they sensitively depend
on the band structures of the individual layers, the relative band alignment in
the heterostructure, the twist angle between the layers, and interlayer
interactions resulting in hybridization. We used time- and angle-resolved
photoemission spectroscopy combined with tight binding and density functional
theory electronic structure calculations to investigate ultrafast charge
separation and recombination in WS$_2$-graphene vdW heterostructures. We
identify several avoided crossings in the band structure and discuss their
relevance for ultrafast charge transfer. We relate our own observations to
existing theoretical models and propose a unified picture for ultrafast charge
transfer in vdW heterostructures where band alignment and twist angle emerge as
the most important control parameters.
