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

TL;DR

This paper demonstrates a gate-defined PN junction in few-layer black phosphorus that exhibits photovoltaic effects, including photocurrent and open-circuit voltage, suitable for near-infrared energy harvesting.

Contribution

It introduces a method to create electrically tunable PN junctions in black phosphorus using local electrostatic gating, enabling photovoltaic effects in a 2D material without chemical doping.

Findings

Gate-controlled ambipolar conduction in black phosphorus.

Observation of photocurrent and open-circuit voltage under illumination.

Photovoltaic response extends up to 940 nm wavelength.

Abstract

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