  We investigate numerically the impact of the electron-phonon scattering on
the optical properties of a perovskite material (CH3NH3PbI3). Using
non-equilibrium Green functions formalism, we calculate the local
density-of-states for several values of the electron-phonon scattering strength
as well as in the case of ballistic transport. We report an Urbach-like
penetration of the density-of-states in the bandgap due to scattering. The
density-of-states expands deeper in the bandgap with the scattering strength.
We determine the electronic current contributions relative to photon absorption
and photon emission. This allows to estimate the Urbach energy from the
absorption coefficient below the bandgap. Values of Urbach energy up to 9.5 meV
are obtained, meaning that scattering contribution to the total experimental
Urbach energy of 15meV is quite important. The Urbach tail is generally
attributed to the disorder of the system, leaving hope for further improvement.
Yet, we show in this work that a significant contribution to the Urbach tail
comes from carriers-phonon interactions, an intrinsic properties of the
material which thus set a lower bound on the improvement margin. Finally, we
estimate the open-circuit voltage Voc for a solar cell assuming such a material
as an absorber. Voc losses increase with the scattering strength, up to 41 mV.
This study then confirms that scattering of electrons in perovskite material
modifies the optical behaviour.
