  Chalcogenide perovskites have received considerable interest in photovoltaic
research community owing to their stability (thermal and aqueous), non-toxicity
and lead free composition. However, to date a theoretical study mainly focusing
on the excitonic and polaronic properties are not explored rigorously, due to
its huge computational demand. Herein, we capture the excitonic and polaronic
effects in a series of chalcogenide perovskites ABS$_3$ where A=Ba, Ca, Sr, and
B=Hf, Sn by employing state-of-the art hybrid density functional theory and
many body perturbative approaches viz. GW and BSE. We find that these
perovskites possess a large exciton binding energy than 3D inorganic-organic
hybrid halide perovskites. We examine the interplay of electronic and ionic
contribution to the dielectric screening, and conclude that electronic
contribution is dominant over the ionic contribution. Further using Feynman
polaron model, polaron parameters are computed, and we observe that charge
separated polaronic states are less stable than bound excitons. Finally, the
theoretically calculated spectroscopic limited maximum efficiency (SLME)
suggests that among all chalcogenide perovskites, CaSnS$_3$ could serve as a
best choice for photovoltaic applications.
