  Hybrid organic/inorganic perovskites have emerged as efficient semiconductor
materials for applications in photovoltaic solar cells with conversion
efficiency above 20 \%. Recent experiments have synthesized ultra-thin
two-dimensional (2D) organic perovskites with optical properties similar to
those of 2D materials like monolayer MoS$_2$: large exciton binding energy and
excitonic effects at room temperature. In addition, 2D perovskites are
synthesized with a simple fabrication process with potential low-cost and
large-scale manufacture.
  Up to now, state-of-the-art simulations of the excitonic states have been
limited to the study of bulk organic perovskites. A large number of atoms in
the unit cell and the complex role of the organic molecules make inefficient
the use of \textit{ab initio} methods. In this work, we define a simplified
crystal structure to calculate the optical properties of 2D perovskites,
replacing the molecular cations with inorganic atoms. We can thus apply
state-of-the-art, parameter-free and predictive \textit{ab initio} methods like
the GW method and the Bethe-Salpeter equation to obtain the excitonic states of
a model 2D perovskite. We find that optical properties of 2D perovskites are
strongly influenced by excitonic effects, with binding energies up to 600 meV.
Moreover, the optical absorption is carried out at the bromine and lead atoms
and therefore the results are useful for a qualitatively understanding of the
optical properties of organic 2D perovskites.
