  Since the seminal work on MoS2 monolayers, photoexcitation in atomically-thin
transition metal dichalcogenides (TMDCs) has been assumed to result in excitons
with large binding energies (~ 200-600 meV). Because the exciton binding
energies are order-of-magnitude larger than thermal energy at room temperature,
it is puzzling that photocurrent and photovoltage generation have been observed
in TMDC-based devices, even in monolayers with applied electric fields far
below the threshold for exciton dissociation. Here, we show that the
photoexcitation of TMDC monolayers results in a substantial population of free
charges. Performing ultrafast terahertz (THz) spectroscopy on large-area,
single crystal WS2, WSe2, and MoSe2 monolayers, we find that ~10% of excitons
spontaneously dissociate into charge carriers with lifetimes exceeding 0.2 ns.
Scanning tunnelling microscopy reveals that photo-carrier generation is
intimately related to mid-gap defect states, likely via trap-mediated Auger
scattering. Only in state-of-the-art quality monolayers14, with mid-gap trap
densities as low as 10^9 cm^-2, does intrinsic exciton physics start to
dominate the THz response. Our findings reveal that excitons or excitonic
complexes are only the predominant quasiparticles in photo-excited TMDC
monolayers at the limit of sufficiently low defect densities.
