  Two-dimensional (2D) semiconducting materials are promising building blocks
for optoelectronic applications, many of which require efficient dissociation
of excitons into free electrons and holes. However, the strongly bound excitons
arising from the enhanced Coulomb interaction in these monolayers suppresses
the creation of free carriers. Here, we probe and identify the main exciton
dissociation mechanism through time- and spectrally-resolved photocurrent
measurements in a monolayer WSe2 p-n junction. We find that under static
in-plane electric field, excitons dissociate at a rate corresponding to the one
predicted for the tunnel ionization of 2D Wannier-Mott excitons. This study is
essential for the understanding of the optoelectronic photoresponse in of 2D
semiconductors, and offers design rules for the realization of efficient
photodetectors, valley-dependent optoelectronics and novel quantum coherent
phases.
