Hybrid Frenkel-Wannier excitons facilitate ultrafast energy transfer at a 2D-organic interface

Two-dimensional transition metal dichalcogenides and organic semiconductors have emerged as promising material platforms for optoelectronic devices. Combining the two is predicted to yield emergent properties while retaining the advantages of each. In organic semiconductors, the optoelectronic response is typically dominated by localized Frenkel-type excitons, whereas transition metal dichalcogenides host delocalized Wannier-type excitons. However, much less is known about the characteristics of excitons at hybrid interfaces between these materials, which determine the possible energy- and charge-transfer pathways. Here we identify a hybrid exciton at one such interface using ultrafast momentum microscopy and many-body perturbation theory. We show that this hybrid exciton, formed predominantly via resonant Förster energy transfer, has both Frenkel- and Wannier-type contributions: intralayer electron-hole transitions within the organic semiconductor layer and interlayer transitions across the interface give rise to an exciton wavefunction with mixed character. This work advances our understanding of charge and energy transfer processes across 2D-organic heterostructures.