To gain insight into the mechanism of graphene top electrode for carrier collection, device simulation was conducted on the MoSe2/Si heterojunction by using a 2D semiconductor simulation package (ISE‐TCAD). Figure 6 a shows the simulated total current density distribution of the device with graphene top electrode at a forward bias of +1 V. The current distributes uniformly with high current density in the MoSe2 layer with the use of graphene electrode. In contrast, if only side part of the MoSe2 layer contacts with Ag electrode, corresponding to the case without graphene top electrode, the current distribution is very inhomogeneous (Figure 6b); only the region closes to the electrode shows higher current density, while the region (I section) away from the electrode has very low current density, indicating the poor carrier collection capability for the device. This result further confirms the important role of graphene transparent electrode in enhancing the device performance.