  The intertwined ferroelectricity and photovoltaics in two-dimensional (2D)
materials will enable the favorable improvement and control of photovoltaic
preformances. In this paper, we take 2D 3R bilayer BX (X=P, As, Sb) as model
systems to study the photovoltaic characteristics of intrinsic 2D out-of-plane
(OOP) ferroelectric material, and try to explore a strategy to regulate the
photoelectric properties by changing the strength of ferroelectric
polarization. Due to the spatial inversion symmetry broken caused by special 3R
stacking, spontaneous OOP ferroelectric polarization will appear in the 3R
bilayer BX, which can be swichable through a specific interlayer sliding. The
OOP ferroelectricity leads to charge transfer between layers, realizes
efficient spatial separation between holes and electrons, and forms the
characteristics of type-II band alignment. Moreover, due to perfect lattice
match on account of two identical layers, the 3R bilayer BX is more stable and
easy to realize in experiments than most of traditional 2D heterostructures
made up of different materials. The 3R bilayer BX shows moderate band gap,
ultra-high carrier mobility and efficient optical absorption, and its
nano-devices present large photocurrent, high photon responsivity and excellent
external quantum efficiency. More importantly, all these photoelectric
parameters depend on the intrinsic OOP ferroelectric strength. By changing the
interlayer distance of bilayer BX, the ferroelectric polarization can be
regulated effectively to achieve the optimal photoelectric performance.
Finally, we emphasize the importance and universality of spatial inversion
symmetry broken in layered materials beyond the 3R bilayer BX to realize the
co-regulation of ferroelectric and photovoltaics.
