Separating electron-hole pairs: photocurrent imaging of interlayer exciton dissociation in van der Waals heterojunctions

In van der Waals heterostructures, the separation of interlayer excitons requires that the oppositely charged electron and hole overcome an electrostatic binding energy before undergoing free charge carrier motion. We report optoelectronic transport imaging studies of WS₂-MoSe₂ heterojunctions over a range of device layer thickness. Using Multi-Parameter Dynamic Photoresponse Microscopy, we generate ~10^x photocurrent images of the heterostructure response as a function of source-drain and gate voltages. In bulk heterojunctions these images reveal rectifying behavior within the overlap region, modulated by a reduction in the interlayer photoconduction that spatially evolves with increasing source drain voltage. Monolayer junctions of the same materials reveal negative differential photoresistance that evolves with the same gate and source drain voltage dependence as the photosuppression in bulk junctions. We attribute these effects to electric field-assisted dissociation of interlayer excitons, which we analyze as a function of device thickness. Our measurements indicate that strong two-body correlations arise precisely at the onset to a reduction in interlayer conductance, which unambiguously establishes the crossover from bound interlayer exciton to free electron and hole.