Point-like photovoltaic junction in 2D semiconductor homobilayer.

Atomically thin semiconductor junctions offer a platform for probing optoelectronic processes beyond the continuum limit, where reduced screening and strong exciton binding make local fields especially important for charge separation. Yet it remains unclear how individual dopants contribute to the photovoltaic response when the junction thickness becomes comparable with the atomic length scales and smaller than conventional depletion or diffusion lengths. Here we demonstrate this concept by probing the microscopic photoresponse of a van der Waals semiconductor homobilayer containing ionizing acceptors. Using photoconductive atomic force microscopy on vanadium-doped WSe2 (V:WSe2) bilayers, we directly visualize nanometre-scale photocurrent hotspots centred on single dopants, which have opposite current polarities for dopants in the top and bottom layers. Vertical WSe2/V:WSe2 homobilayer devices show that the macroscopic photocurrent scales linearly with dopant concentration and exhibits a compensation voltage that is independent of illumination power and dopant density, in contrast to bulk homojunction devices. Photocurrent spectroscopy and quasi-classical modelling indicate that charged dopants locally convert tightly bound intralayer excitons into charge-separated interlayer states, thereby enabling efficient exciton dissociation within a region of about 1 nm. These results establish dopant-defined point-like junctions as the elementary photovoltaic units in atomically thin homobilayers.