Breathing bands due to molecular order in CH3NH3PbI3
  CH3NH3PbI3 perovskite is nowadays amongst the most promising photovoltaic materials for energy conversion. We have studied by ab-initio calculations, using several levels of approximation - namely density functional theory including spin-orbit coupling and quasi-particle corrections by means of the GW method, as well as pseudopotential self-interaction corrections -, the role of the methylammonium orientation on the electronic structure of this perovskite. We have considered many molecular arrangements within 2x2x2 supercells, showing that the relative orientation of the organic molecules is responsible for a huge band gap variation up to 2 eV. The band gap sizes are related to distortions of the PbI3 cage, which are in turn due to electrostatic interactions between this inorganic frame and the molecules. The strong dependence of the band gap on the mutual molecular orientation is confirmed at all levels of approximations. Our results suggest then that the coupling between the molecular motion and the interactions of the molecules with the inorganic cage could help to explain the widening of the absorption spectrum of CH3NH3PbI3 perovskite, consistent with the observed white spectrum. 
Malgorzata Wierzbowska, Juan Jose Melendez, Daniele Varsano
cond-mat.mtrl-sci