Impact of Processing Temperature and Composition on the Formation of Methylammonium Lead Iodide Perovskites

A delicate control of the stoichiometry, crystallographic phase, and grain structure of the photoactive material is typically required to fabricate high-performance photovoltaic (PV) devices. Organo-metal halide perovskite materials, however, exhibit a large degree of tolerance in synthesis and can be fabricated into high efficiency devices by a variety of different vacuum and solution-based processes, with a wide range of precursor ratios. This suggests that the phase field for the desired material is wider than expected or that high device efficiency may be achieved with a range of phases. Here, we investigate the structural and optical properties of the materials formed when a range of compositions of methylammonium iodide (MAT) and lead iodide (PbI2) were reacted at temperatures from 40 to 190 degrees C. The reactions were performed according to a commonly employed synthetic approach for high efficiency PV devices, and the data was analyzed to construct a pseudobinary, temperature-dependent, phase-composition processing diagram. Escape of MAT vapor at the highest temperatures (150-190 degrees C) enabled a PbI2 phase to persist to very high MAI concentrations, and the processing diagram was not representative of phase equilibrium in this range. Data from reactions performed with a fixed vapor pressure of MAI allowed the high temperature portion of the diagram to be corrected and a near-equilibrium phase diagram to be proposed. The perovskite phase field is wider than previously thought under both processing conditions and extended by the existence of stacked perovskite sheet phases. Several aspects of the diagrams clarify why the organo-halide perovskite materials are compatible with solution processing.