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The photovoltaic effect is one of the fundamental light-matter interactionsin light energy harvesting. In conventional photovoltaic solar cells, thephotogenerated charge carriers are extracted by the built-in electric field ofa PN junction, typically defined by ionic dopants in a semiconductor. Inatomically thin semiconductors, the doping level can be controlled by thefield-effect without need of implanting dopants in the lattice, which makes 2Dsemiconductors prospective materials to implement electrically tunable PNjunctions. However, most 2D semiconducting materials do not show ambipolarP-type and N-type field-effect transport, necessary to realize PN junctions.Few-layer black phosphorus is a recently isolated 2D semiconductor thatpresents a direct bandgap, high mobility, current on/off ratio and ambipolaroperation. Here, we fabricate few-layer black phosphorus (b-P) field-effecttransistors with split bottom gates and crystalline hexagonal boron nitride(h-BN) dielectric. We demonstrate electrostatic control of the local chargecarrier type and density above each gate in the device, tuning its electricalbehaviour from metallic to rectifying. Illuminating a gate-defined PN junction,we observe zero-bias photocurrents and significant open-circuit voltage, whichwe attribute to the separation of electron-hole pairs driven by the internalelectric field at the junction region. Due to the small bandgap of thematerial, we observe photocurrents and photovoltages for illuminationwavelengths up to 940 nm, attractive for energy harvesting applications in thenear-infrared region

Photovoltaic effect in few-layer black phosphorus PN junctions defined by local electrostatic gating