  Perovskites have attracted much attention due to their remarkable optical
properties. While it is well established that excitons dominate their optical
response, the impact of higher excitonic states and formation of phonon
sidebands in optical spectra still need to be better understood. Here, we
perform a theoretical study on excitonic properties of monolayered hybrid
organic perovskites -- supported by temperature-dependent photoluminescence
measurements. Solving the Wannier equation, we obtain microscopic access to the
Rydberg-like series of excitonic states including their wavefunctions and
binding energies. Exploiting the generalized Elliot formula, we calculate the
photoluminescence spectra demonstrating a pronounced contribution of a phonon
sideband for temperatures up to 50 K -- in agreement with experimental
measurements. Finally, we predict temperature-dependent linewidths of the three
energetically lowest excitonic transitions and identify the underlying
phonon-driven scattering processes.
