  Spontaneous polarization and bulk photovoltaic effect (BPVE) are two
concomitant physical properties in ferroelectric materials. The flipping of
ferroelectric order usually accompanies with the switching of BPVE as both of
them are reversed under the inversion symmetry. In this study, we report the
distinctive BPVE characters in two-dimensional (2D) interlayer sliding
ferroelectric materials featuring unswitchable in-plane BPVE (light-induced
photocurrent in the xy plane) and switchable out-of-plane BPVE (light-induced
polarization along the z-direction). Symmetry analysis within abstract bilayer
crystal model and first-principles calculations validate these BPVE properties.
It is because the positive and negative ferroelectric states caused by
interlayer sliding are related by mirror symmetry which cannot flip all the
BPVE tensor elements. This finding extends the understanding of the
relationship between ferroelectricity and BPVE. On one hand, the switchable
out-of-plane BPVE can be used to design switchable photoelectric devices. On
the other hand, the in-plane BPVE is robust against the ferroelectric flipping,
and the unswitchable character is beneficial to construct larger-scale
photoelectric devices.
