The Impact of the Crystallization Processes on the Structural and Optical Properties of Hybrid Perovskite Films for Photovoltaics

We investigate the relationship between structural and optical properties of organo-lead mixed halide perovskite films as a function of the crystallization mechanism. For methylammonium lead tri-iodide, the organic cations rearrange within the inorganic cage, moving from crystals grown in a mesoporous scaffold to larger, oriented crystals grown on a flat substrate. This reduces the strain felt by the bonds forming the cage and affects the motion of the organic cation in it, influencing the electronic transition at the onset of the optical absorption spectrum of the semiconductor. Moreover, we demonstrate that in mixed-halide perovskite, though Cl- ions are not present in a detectable concentration in the unit cell, they drive the crystallization dynamics. This induces a preferential order during crystallization, from a molecular, i.e., organic-inorganic moieties arrangement, to a nano-mesoscopic level, i.e., larger crystals with anisotropic shape. Finally, we show that while Cl is mainly expelled from flat films made of large crystals, in the presence of an oxide mesoporous scaffold they are partially retained in the composite.