  The correlation between the shift current mechanism for the bulk photovoltaic
effect (BPVE) and the structural and electronic properties of ferroelectric
perovskite oxides is not well understood. Here, we study and engineer the shift
current photovoltaic effect using a visible-light-absorbing ferroelectric
Pb(Ni$_{x}$Ti$_{1-x}$)O$_{3-x}$ solid solution from first principles. We show
that the covalent orbital character dicates the direction, magnitude, and onset
energy of shift current in a predictable fashion. In particular, we find that
the shift current response can be enhanced via electrostatic control in layered
ferroelectrics, as bound charges face a stronger impetus to screen the electric
field in a thicker material, delocalizing electron densities. This
heterogeneous layered structure with alternative photocurrent generating and
insulating layers is ideal for BPVE applications.
