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 (MAI) and lead iodide (PbI2) were reacted at temperatures from 40 to 190 °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-compositio...