High-efficiency hybrid halide perovskites: a complex material with a simple start in the crystallization process

Halide perovskites (HPs) have attracted significant attention due to their versatility and outstanding performance in optoelectronic devices, particularly in photovoltaics, where power conversion efficiencies of 27.3% have recently been reported. Despite this progress, the influence of the A-site cation on the early-stage of crystallisation of HPs from solution is still not fully understood. In this study, small-angle X-ray scattering (SAXS) is employed to investigate the atomic arrangement of iodoplumbate species in precursor solutions of NaPbI₃, KPbI₃, RbPbI₃, CsPbI₃, and the high-efficiency mixed-cation composition Cs_0.05(FA_0.98MA_0.02)_0.95Pb(I_0.98Br_0.02)₃, prepared using γ-butyrolactone (GBL), dimethylformamide (DMF) and DMF : DMSO (dimethyl sulfoxide) (80 : 20) solvents. The results show that alkali-metal A-cations significantly modify the atomic arrangement in solution, yielding shorter interparticle distances than those observed in hybrid perovskites containing organic A-cations formamidinium (FA⁺) or methylammonium (MA⁺). To explain these findings, the previously developed core-shell model, where the core is formed by either single [PbI₆]^4- octahedra or corner-sharing [Pb₂I₁₁]^7- units, surrounded by solvent molecules, is extended to HPs with inorganic A-cations. Here, in the case of fully inorganic systems, the core can be surrounded by alkali cations as well. Applying the extended core-shell model, it is shown that fully inorganic compositions favour corner-sharing octahedra within the core, whereas in precursor solutions of the mixed-cation (organic-inorganic)-mixed-halide HPs, the single-octahedron core arrangement is favoured. This indicates that the halide composition exerts a stronger influence than the A-site cation on the atomic arrangement of species in the HP precursor solution. Analysis of the SAXS form factor further reveals higher polydispersity in fully inorganic systems due to the greater variety of species in solution, while the mixed-cation composition exhibits reduced polydispersity, highlighting the impact of precursor chemistry on crystallisation pathways and, ultimately, device performance.