  Exciton binding energies of hundreds of meV and strong light absorption in
the optical frequency range make transition metal dichalcogenides (TMDs)
promising for novel optoelectronic nanodevices. In particular, atomically thin
TMDs can be stacked to heterostructures enabling the design of new materials
with tailored properties. The strong Coulomb interaction gives rise to
interlayer excitons, where electrons and holes are spatially separated in
different layers. In this work, we reveal the microscopic processes behind the
formation, thermalization and decay of these fundamentally interesting and
technologically relevant interlayer excitonic states. In particular, we present
for the exemplary MoSe$_2$-WSe$_2$ heterostructure the interlayer exciton
binding energies and wave functions as well as their time- and energy-resolved
dynamics. Finally, we predict the dominant contribution of interlayer excitons
to the photoluminescence of these materials.
