From PbI2 to MAPbI(3) through Layered Intermediates

In the present work we describe the process of formation of methylammonium lead iodide perovskite (MAPbI(3)) by reaction between [001] oriented PbI2 thin films and MAI. The reaction leads to a rapid change in the optical properties with the formation of a brown phase having a large band in the visible range and a contribution in the near-UV range (375 nm). X-ray diffraction analyses indicate that the MAI + PbI2 as-obtained material mainly consists of a highly oriented matrix of MAI that intermixes with the Pb-I precursor. A chemical exfoliation of the initial PbI2 structure with the massive intercalation of the methylammonium iodide ions suitably describes the optical and structural findings. Short thermal treating (100 degrees C) of this as-obtained sample results in the formation of a second oriented intermediate phase, attributed to the presence of a low-dimensional perovskite structure. Such low-dimensional metastable phase is no longer visible by increasing the annealing time or the annealing temperature (150 degrees C), with the 3D MAPbI(3) tetragonal phase left in the XRD pattern.