Halide perovskites (HPs) have drawn the attention of the scientific community, not only for their steep increase in power conversion efficiency during the last decade, up to 26.1% in 2023, but also for using low-cost solution-based processing methods.

Despite the popularity of HPs as an absorber material, it is not fully understood the role that anions, cations and the solvent play during the early stages of the crystallisation process. For this reason, we investigated the precursor solution of different HPs (MAPbI₃, MAPbBr₃, MAPbCl₃, FAPbI₃, CsPbI₃, RbPbI₃, KPbI₃ and NaPbI₃) at room temperature using small angle X-ray scattering (SAXS) as well as the precursor solution of MAPbI₃ at increasing temperature in-situ (from room temperature up to 120°C). The binary precursors (e.g. MAI and PbI₂ to synthesise MAPbI₃) were dissolved in common solvents to synthesise HPs such as γ-butyrolactone (GBL), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP) and mixtures thereof.

SAXS is a non-destructive technique based on the scattering length difference between the scattering objects in a solution. Applying SAXS, the size and shape of nanoscale particles (scattering objects) can be investigated, as well as their adjacent distance and interactions with each other.[2] We performed SAXS experiments at BESSYII, at the PTB’s four-crystal monochromator beamline[3] using the ASAXS endstation.[4]

The SAXS patterns of all the measured samples show a peak in the scattered intensity at q-values between 2.5 and 3.3 nm^-1, except for MAPbCl₃ in DMF:DMSO 1:1, which did not show any peak. The maximum holds two essential pieces of information: it demonstrates the agglomeration of scattering objects and the peak position corresponds to the average distance between scattering objects (d_exp) in a range of 2-3 nm. In a previous study, we developed a core-shell model with [PbX₆] (X = I, Br) octahedra arranged as single or corner-sharing octahedra as the core surrounded by solvent molecules for HPs with molecular A-cation (MA⁺, FA⁺). This shows that the size of the agglomerates changes with the composition of HPs precursors and with the solvent, but not with a molecular A-cation. However, when alkali metals are used as A-cation instead (Na⁺, K⁺, Rb⁺, Cs⁺), we can demonstrate that d_exp not only depends on the solvent but also on the A-cation. This is explained by the smaller ionic radius of alkali metals compared to the molecular cations [5,6] therefore the charge density is higher. For this reason, we extended the previous model to take this phenomenon into account. Based on this information, the extended core-shell model assumes that the A-cation and the solvent molecules compete to surround the [PbI₆] octahedra. In this A-cation core-shell model, the core is composed of [PbI₆] octahedra, which can be arranged as a single octahedron or a corner-sharing octahedra. The [PbI₆] octahedra of adjacent scattering objects are surrounded by a solvent shell with molecules or by an A-cation shell. The SAXS data analysis (using SASfit[7]) shows higher polydispersity as the previous model, which indicates an increase in the heterogeneity of the solution, this is in agreement with the proposed extended model.

We will discuss the influences of the A-cation and solvent on the core as well as the solvent shell of the scattering objects since they have the potential to influence the crystallization process of the HP and therefore the performance of a device produced from solution processing.