Identification and Characterization for Spherulization in Methylammonium-Guanidinium Lead Iodide Perovskite Material—A Structural and Morphological Investigation of Microstructural Evolution

Hybrid organic-inorganic perovskite materials (HOIP) have gained tremendous popularity in the past decade for their unique optoelectronic properties for photovoltaic applications. This report presents comprehensive structural and morphological analysis which infuses new insights into the evolution of polycrystalline self-assembled spherulitic microstructures with different hierarchical architectures in HOIP material which are hitherto unattempted. X-ray diffraction (XRD) analysis confirms the formation of guanidinium (GUAI)-rich MAI-GUAI-PbI₂-DMF pure intermediate phase of the dendritic perovskite spherulites and transformation to stable α-tetragonal or quasi-cubic perovskite converted phase of Ma_0.75Gua_0.25PbI₃ material composition with calculated crystal structure parameters; a=b=6.31074 Å and c=6.33810 Å, α=β=γ=90° and a cell volume of 252.418 ųvia slow crystallization in extreme humidity levels of 92±4% and temperature 24.5±1.5°C. This work also shed light on microstructural heterogeneity of these materials in their intermediate phase. The Field Emission Scanning Electron Microscope (FESEM) analysis unravels the fundamental microstructure of embryonic spherulitic nuclei in perovskites; lamellar arrangements, categorize the mature perovskite spherulitic microislands (PSMs); transcrystalline layers (TCL), spherulite bridges, nucleation of hole, impingement behavior, coupled-PSMs, and defects such as pits and spherulitic microcracks. The inner-core and outer area of all the freshly prepared spherulitic forms are evaluated to range between 203.6-380.5 μm² and 974.4-1900.0 μm² respectively. To assess the stability of this spherulitic perovskite and understand its phase transformation, XRD and FESEM study has been done after 10 days. New zero-dimensional (0-D) orthorhombic Pmmm(47) polymorph with blood-cell like morphology have been observed in the wheat sheave branches of the aged perovskite sample. Optical microscopic images also confirms the wheat-sheaf like morphology. The UV-Visible results have shown the absorbance peak of 3-D quasi-cubic perovskite phase around 788 nm. Energy-dispersive X-ray analysis (EDX) confirms the stoichiometric homogeneity of intermediate phase with similar value of C, Pb and I atomic ratios from different regions of the thin-film. As from the viewpoint of crystallography and performance, the individual perovskite grains may display very different properties. Thus findings of this work not only enhances the understanding on organic-inorganic spherulitic microstructures but also an attempt to evident the polymeric aspect of HOIPs, intermediate phase, and phase transformation that can serve to improve the morphology and phase stability for higher device performance of HOIPs.