Bulk Photovoltaic Effect in the Elemental Blue Phosphorus-Based Polar Homojunction and Heterojunction

Two-dimensional (2D) polar materials can exhibit a pronounced bulk photovoltaic effect (BPVE). Here, we report single-element polarity and BPVE in the bilayer blue phosphorus (blue-P) homojunction and blue/black phosphorus (blue-P/black-P) heterojunction, where the individual layers are nonpolar due to the centrosymmetric structures. For the homojunction, bilayer blue-P exhibits sliding ferroelectricity with switchable out-of-plane polarization and a small shift current susceptibility. For the heterojunction, we obtain an in-plane polarization and a stronger shift current response by vertical stacking blue-P and black-P with the overlapped mirror plane, where the strong BPVE is due to the enhanced charge transfer. Combining a k·p model with first-principles calculations, we further show that the photovoltaic performance of the heterojunction depends on the interlayer coupling, which has a significant effect on the shift vector. These findings indicate that 2D phosphorus sheets can be used as unique building units for designing configurable layered nonlinear optical materials.