  Vertical heterostructures (HS) of transition metal dichalcogenides (TMDs)
host interlayer excitons (ILX), with electrons and holes residing in different
layers. With respect to their intralayer counterparts, ILX feature much longer
lifetimes and diffusion lengths, paving the way to excitonic optoelectronic
devices operating at room temperature. While the recombination dynamics of ILX
has been intensively studied, the formation process and its underlying physical
mechanisms are still largely unexplored. Here we use ultrafast transient
absorption spectroscopy with a white-light probe, spanning both intralayer and
interlayer exciton resonances, to simultaneously capture and time-resolve
interlayer charge transfer and ILX formation dynamics in a MoSe$_2$/WSe$_2$ HS.
We find that the ILX formation timescale is nearly an order of magnitude (~1
ps) longer than the interlayer charge transfer time (~100 fs). Microscopic
calculations attribute the relative delay to an interplay between a
phonon-assisted interlayer exciton cascade and subsequent cooling processes,
and excitonic wave-function overlap. Our results provide an explanation to the
efficient photocurrent generation observed in optoelectronic devices based on
TMD HS, as the ILX have an opportunity to dissociate during their
thermalization process.
