Conversion of single crystalline PbI₂ to CH₃NH₃PbI₃: Structural relations and transformation dynamics

The realization of high-quality optoelectronic properties in halide perovskite semiconductors through low-temperature, low energy processing is unprecedented. Understanding the unique aspects of the formation chemistry of these semiconductors is a critical step toward understanding the genesis of high quality material via simple preparation procedures. The toolbox of preparation procedures for halide perovskites grows rapidly. The prototypical reaction is that between lead iodide (PbI₂) and methylammonium iodide (CH₃NH₃I, abbr. MAI) to form the perovskite CH₃NH₃PbI₃ (MAPbI₃), which we discuss in this work. We investigate the conversion of small, single-crystalline PbI₂ crystallites to MAPbI₃ by two commonly used synthesis processes: reaction with MAI in solution or as a vapor. The single crystal nature of the PbI₂ precursor allows definitive conclusions to be made about the relationship between the precursors and the final product, illuminating previously unobserved aspects of the reaction process. From in situ photoluminescence microscopy, we find that the reaction in solution begins via isolated nucleation events followed by growth from the nuclei. We observe via X-ray diffraction and morphological characterization that there is a strong orientational and structural relationship between the final stage of the solution-reacted MAPbI₃ product and the initial PbI₂ crystallite. In all these measurements, we find that the reaction does not proceed below a certain MAI threshold concentration, which allows the first experimental determination of a free energy of formation for a widely used synthetic procedure of ∼0.1 eV. From these conclusions, we present a more detailed hypothesis about the reaction pathway than has yet been proposed: Our results suggest that the reaction in solution begins with a topotactic nucleation event followed by grain growth by dissolution-reconstruction. By similar techniques, we find the reaction via vapor phase produces material lacking a preferred orientation, suggesting the transformation is dominated by a deconstruction-reconstruction process due to the higher thermal energy involved. We also find that the crystal lattice structure of the vapor-reacted material is clearly different from that of the solution-phase reaction due to the temperature conditions of the synthesis.