Mixed formamidinium-methylammonium lead iodide perovskite from first-principles: hydrogen-bonding impact on the electronic properties.

Hybrid perovskites with mixed organic cations such as methylammonium (CHNH, MA) and formamidinium (CH(NH), FA) have attracted interest due to their improved stability and capability to tune their properties varying the composition. Theoretical investigations in the whole compositional range for these mixed perovskites are scarce in part due to the limitations of modeling cationic orientation disorder. In this work, we report on the local variation of the structural and electronic properties in mixed A-site cation MA/FA lead iodide perovskites FAMAPbI evaluated from static first-principles calculations in certain structures where the orientations of organic cations result from examining the energy landscape of some compositions. The cation replacement at the A-site to form the solid solution causes an increased tilting of the inorganic PbI octahedra: in the FA-rich compounds the replacement of FA by a smaller cation like MA is to compensate for the reduced space filling offered by the smaller cation, whereas in the MA-rich compounds it is to expand the space needed for the larger cation. In fact, the effect of octahedron tilting exceeds that of unit-cell size in determining the band gap for these organic cation mixtures. Our calculations indicate that the key role played by hydrogen bonds with iodine anions in the pure compounds, MAPbI and FAPbI, is preserved in the cation mixed perovskites. It is found that MA-I bonds remain stronger than FA-I bonds throughout the composition range regardless of the unit-cell expansion as the FA content increases. Finally, from the analysis of electronic structures we unravel how the hydrogen bonds stabilize the non-bonding I-5p orbitals, spatially perpendicular to the Pb-I-Pb bond axis, lowering their energy when the H-I interaction occurs, which would explain the well-known role of hydrogen bonding in the structural stabilization of hybrid perovskites. These results contribute to the understanding of the role played by cation mixing at A sites in the physics of lead halide perovskites.