The Big Bang of Halide Perovskites: The Starting Point of Crystallisation

Halide perovskites (HPs) are extremely popular amongst the scientific community, not only for their astonishing increase in power conversion efficiency up to 26%[1], but also for using low-cost solution-based processing methods.<br/>Some of the distinctive properties of HPs are the decrease in solubility with increasing temperature[2], and the bandgap tunability via anion and cation substitution[3]. With this study we aim to elucidate the role of the A-cation in the crystallisation process as well as how the precursors are arranged prior to crystallisation. For this reason, we investigated the precursor solution of different HPs (RbPbI₃, KPbI₃ and NaPbI₃) using small angle X-ray scattering (SAXS) as well as the precursor solution of MAPbI₃ at increasing temperature (from room temperature up to 120°C). The binary precursors (e.g. RbI and PbI₂ to synthesise RbPbI₃) were dissolved in common solvents to synthesise HPs such as γ-butyrolactone (GBL), dimethylformamide (DMF) and mixtures thereof.<br/>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.[4,5] We performed SAXS experiments at BESSYII, at the PTB’s four-crystal monochromator beamline[6] using the ASAXS endstation.[7]<br/>The SAXS pattern of all the measured samples show a peak in the scattered intensity at q-values between 2.5 and 3.3 nm^-1. The presence of such peak is a clear indication of the existence of interacting scattering objects in solution. The position of the peak correlates with the distance between adjacent scattering objects (<i>d_exp</i>). In a previous study[8], we developed a core-shell model with [Pb<i>X</i>₆] (<i>X</i> = I, Br) octahedra surrounded by randomly oriented solvent molecules to explain <i>d_exp</i> in HPs with a molecular A-cation (MA⁺, FA⁺), showing that the size of the agglomerates changes with the composition of HPs precursors and with the solvent, but not with the A-cation. However, when alkali metals are used as A-cation, we can demonstrate that <i>d_exp</i> 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 [9,10] 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 randomly oriented molecules or by an A-cation shell. The SAXS data analysis (using SASfit[11]) 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.<br/>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.<br/>[1] https://www.nrel.gov/pv/cell-efficiency.html<br/>[2] Saidaminov, M. et al, Nat. Commun., 2015, 6<br/>[3] Kulkarni et al., J. Mater. Chem. A, 2014, 2, 9221<br/>[4] Schnablegger et al., Anton Paar GmbH, 2013<br/>[5] Flatken et al., J.Mater.Chem.A, 2021, 9, 13477<br/>[6] Krumrey et al., Nucl.Instrum.Methods Phys.Res. Sect.A, 2001, 467, 1175<br/>[7] Hoell et al., DE102006029449, 2007<br/>[8] https://www.mrs.org/meetings-events/presentation/2022_mrs_fall_meeting/3784328-202211290915<br/>[9] Kieslich et al., Chem. Sci., 2014, 5, 4712<br/>[10] Shannon, R. D., Acta Cryst., 1976, A32, 751<br/>[11] Breßler et al., J. Appl. Cryst., 2015, 48, 1587