  The bulk photovoltaic effect (BPE) leads to the generation of a photocurrent
from an asymmetric material. Despite drawing much attention due to its ability
to generate photovoltages above the band gap ($E_g$), it is considered a weak
effect due to the low generated photocurrents. Here, we show that a remarkably
high photoresponse can be achieved by exploiting the BPE in simple planar
BaTiO$_3$ (BTO) films, solely by tuning their fundamental ferroelectric
properties via strain and growth orientation induced by epitaxial growth on
different substrates. We find a non-monotonic dependence of the responsivity
($R_{\rm SC}$) on the ferroelectric polarization ($P$) and obtain a remarkably
high BPE coefficient ($\beta$) of $\approx$10$^{-2}$ 1/V, which to the best of
our knowledge is the highest reported to date for standard planar BTO thin
films. We show that the standard first-principles-based descriptions of BPE in
bulk materials cannot account for the photocurrent trends observed for our
films and therefore propose a novel mechanism that elucidates the fundamental
relationship between $P$ and responsivity in ferroelectric thin films. Our
results suggest that practical applications of ferroelectric photovoltaics in
standard planar film geometries can be achieved through careful joint
optimization of the bulk structure, light absorption, and electrode-absorber
interface properties.
