  Antiperovskites are a rich family of compounds with applications in battery
cathodes, superconductors, solid-state lighting, and catalysis. Recently, a
novel series of antimonide phosphide antiperovskites (A$_3$SbP, where A = Ca,
Sr, Ba) were proposed as candidate photovoltaic absorbers due to their ideal
band gaps, small effective masses and strong optical absorption. In this work,
we explore this series of compounds in more detail using relativistic hybrid
density functional theory. We reveal that the proposed cubic structures are
dynamically unstable and instead identify a tilted orthorhombic Pnma phase as
the ground state. Tilting is shown to induce charge localisation that widens
the band gap and increases the effective masses. Despite this, we demonstrate
that the predicted maximum photovoltaic efficiencies remain high (24-31% for
200 nm thin films) by bringing the band gaps into the ideal range for a solar
absorber. Finally, we assess the band alignment of the series and suggest hole
and electron contact materials for efficient photovoltaic devices.
