  Inorganic perovskite oxide ferroelectrics have recently generated substantial
interest for photovoltaic applications; however, existing materials suffer from
excessive electronic band gaps and insufficient electric polarization. The
recent resurgence of hybrid perovskite ferroelectrics addresses the
aforementioned deficiencies, but they are highly unstable against environmental
effects and an inorganic resolution may still be optimal. Here we use
first-principles calculations to design a low band gap, planar ferroelectric by
leveraging the complexity of layered double perovskite oxides AA$^\prime
$BB$^\prime$O$_6$. We exploit A$\ne$A$^\prime$ size mismatch, a nominally empty
$d$-shell on the B-site, and A cations bearing lone-pair electrons to achieve a
large ferroelectric polarization. Additionally, B$^\prime\ne$B is chosen to
achieve full charge transfer and a Mott susceptible filling on the
B$^\prime$-site, yielding a low band gap. These principles are illustrated in
BaBiCuVO$_6$, BaBiNiVO$_6$, BaLaCuVO$_6$, and PbLaCuVO$_6$, which could be
realized in layer-by-layer growth. This new class of materials could lead to
stable, high-efficiency photovoltaic.
