  In optically excited two-dimensional phototransistors, charge transport is
often affected by photodoping effects. Recently, it was shown that such effects
are especially strong and persistent for graphene/h-BN heterostructures, and
that they can be used to controllably tune the charge neutrality point of
graphene. In this work we investigate how this technique can be extended to h
BN encapsulated monolayer MoSe_2 phototransistors at room temperature. By
exposing the sample to 785 nm laser excitation we can controllably increase the
charge carrier density of the MoSe_2 channel by {\Delta}n {\approx} 4.45
{\times} 10^{12} cm^{-2}, equivalent to applying a back gate voltage of 60 V.
We also evaluate the efficiency of photodoping at different illumination
wavelengths, finding that it is strongly correlated with the light absorption
by the MoSe_2 layer, and maximizes for excitation on-resonance with the A
exciton absorption. This indicates that the photodoping process involves
optical absorption by the MoSe_2 channel, in contrast with the mechanism
earlier described for graphene/h-BN heterostroctures.
