We demonstrate the possibility of engineering the optical properties of
transition metal dichalcogenide heterobilayers when one of the constitutive
layers has a Janus structure. This has important consequences for the charge
separation efficiency. We investigate different MoS2​@Janus layer
combinations using first-principles methods including electron-hole
interactions (excitons) and exciton-phonon coupling. The direction of the
intrinsic electric field from the Janus layer modifies the electronic band
alignments and, consequently, the energy separation between interlayer exciton
states -- which usually have a very low oscillator strength and hence are
almost dark in absorption -- and bright in-plane excitons. We find that
in-plane lattice vibrations strongly couple the two states, so that
exciton-phonon scattering may be a viable generation mechanism for interlayer
excitons upon light absorption. In particular, in the case of MoS2​@WSSe, the
energy separation of the low-lying interlayer exciton from the in-plane exciton
is resonant with the transverse optical phonon modes (40 meV). We thus identify
this heterobilayer as a prime candidate for efficient electron-hole pair
generation with efficient charge carrier separation