Effect of the solvent in the early stage of crystallisation of hybrid halide perovskites

Abstract Hybrid halide perovskites are promising absorber materials for photovoltaic applications due to their high efficiency and low-temperature solution processability. The choice of solvent is therefore critical, as it can strongly influence the crystallisation pathway and ultimately the efficiency of the solar cell. To investigate this, an in-depth study of the early stage of crystallisation of MAPbI ₃ from solution was performed by analysing precursor solutions with small-angle x-ray scattering (SAXS). These solutions were prepared by mixing the precursors PbI ₂ and MAI in different solvents as GBL, DMF, NMP, DMSO and binary mixtures. These solvents differ in Gutmann’s donor number (DN) and in molecular size, which can influence the arrangement of species in the solution. Detailed analysis of the SAXS data revealed that solvents with high DN (e.g. NMP and DMSO) favour homogeneous precursor solutions i.e. all the scattering objects have the same size, whereas solvents with low DN favour the presence of species with a variety of sizes in solution. Applying the scattering objects’ core–shell model, where the core is formed by [PbI ₆ ] ^4- octahedra surrounded by a shell of solvent molecules, the agglomerates present in the solution could be described. Solvents with high DN favour the single octahedron arrangement in the core, whereas solvents with low DN favour a mixture of objects whose core is a single or a corner-sharing octahedron. This observation agrees well with the polydispersity behaviour derived from the structural analysis of the SAXS data. These deep insights and understanding on how the nature of the solvent influences the precursor arrangement in solution lay the groundwork for designing an optimised solution-based processing of halide perovskite thin films.