Interplay of orientational order and electronic structure in methylammonium lead iodide: Implications for solar cell operation

We report on ab initio electronic structure and Car-Parrinello molecular dynamics simulations for several structural models of the prototype MAPbI perovskite for solar cells applications. We considered both configurations having a preferred orientation of the MA cations, giving rise to a net dipole alignment, and configurations with an isotropic distribution of the MA cations, respectively representative of polar (ferroelectric) and apolar (antiferroelectric) structures. Our calculations demonstrate the preferred stability of a set of polar structures over apolar ones, with an energy difference within 0.1 eV and a conversion barrier within 0.2 eV per unit cell (four MAPbI), thus possibly accessible at room temperature. Ferroelectric-like orientations lead to a quasi I4cm structure for the inorganic component, characterized by lack of inversion symmetry, while the antiferroelectric-like orientations are associated to a quasi I4/mcm structure. Ab initio molecular dynamics simulations on the polar structures show no molecular rotations in the investigated time-scale, while several MA rotations are observed in the same time scale for the considered apolar structure, which is thus characterized by a higher disorder. The I4cm and I4/mcm types of structure have markedly different band structures, despite showing a relatively small band gap variation. Simulations carried out on finite surface slabs demonstrate that a net orientation of the MA cations gives rise to a strong bending in the valence and conduction bands, which could definitely assist charge separation and reduce carrier recombination, provided one is able to effectively stabilize polar compared to apolar structures. We believe our results could contribute an important step toward an in-depth comprehension of the basic properties of organohalide perovskites, assisting a further optimization of their photovoltaic response.