  Strong many-body interactions in two-dimensional (2D) semiconductors give
rise to efficient exciton-exciton annihilation (EEA). This process is expected
to result in the generation of unbound high energy carriers. Here, we report an
unconventional photoresponse of van der Waals heterostructure devices resulting
from efficient EEA. Our heterostructures, which consist of monolayer transition
metal dichalcogenide (TMD), hexagonal boron nitride (hBN), and few-layer
graphene, exhibit photocurrent when photoexcited carriers possess sufficient
energy to overcome the high energy barrier of hBN. Interestingly, we find that
the device exhibits moderate photocurrent quantum efficiency even when the
semiconducting TMD layer is excited at its ground exciton resonance despite the
high exciton binding energy and large transport barrier. Using ab initio
calculations, we show that EEA yields highly energetic electrons and holes with
unevenly distributed energies depending on the scattering condition. Our
findings highlight the dominant role of EEA in determining the photoresponse of
2D semiconductor optoelectronic devices.
