  Hybrid halide perovskites exhibit nearly 20% power conversion efficiency, but
the origin of their high efficiency is still unknown. Here, we compute the
shift current, a dominant mechanism of bulk photovoltaic (PV) effect for
ferroelectric photovoltaics, in CH$_3$NH$_3$PbI$_3$ and
CH$_3$NH$_3$PbI$_{3-x}$Cl$_{x}$ from first principles. We find that these
materials give approximately three times larger shift current PV response to
near-IR and visible light than the prototypical ferroelectric photovoltaic
BiFeO$_3$. The molecular orientations of CH$_3$NH$_3^{+}$ can strongly affect
the corresponding PbI$_3$ inorganic frame so as to alter the magnitude of the
shift current response. Specifically, configurations with dipole moments
aligned in parallel distort the inorganic PbI$_3$ frame more significantly than
configurations with near net zero dipole, yielding a larger shift current
response. Furthermore, we explore the effect of Cl substitution on shift
current, and find that Cl substitution at the equatorial site induces a larger
response than does substitution at the apical site.
