The new precursors MAI_3+x and FAI_3+x exhibit exceptionally high reactivity towards lead precursors and result in high quality lead-halide perovskite films.

Solution methods remain the most popular means for the fabrication of hybrid halide perovskites. However, the solubility of hybrid perovskites has not yet been quantitively investigated. In this study, we present accurate solubility data for MAPbI₃, FAPbI₃, MAPbBr₃ and FAPbBr₃ in the two most widely used solvents, DMF and DMSO, and demonstrate huge differences in the solubility behavior depending on the solution compositions. By analyzing the donor numbers of the solvents and halide anions, we rationalize the differences in the solubility behavior of hybrid perovskites with various compositions, in order to take a step forward in the search for better processing conditions of hybrid perovskites for solar cells and optoelectronics.

At a temperature of 100 K, CH₅N₂⁺·I⁻(I), crystallizes in the monoclinic space groupP2₁/c. The formamidinium cation adopts a planar symmetrical structure [the r.m.s. deviation is 0.002 Å, and the C—N bond lengths are 1.301 (7) and 1.309 (8) Å]. The iodide anion does not lie within the cation plane, but deviates from it by 0.643 (10) Å. The cation and anion ofIform a tight ionic pair by a strong N—H...I hydrogen bond. In the crystal ofI, the tight ionic pairs form hydrogen-bonded zigzag-like chains propagating toward [20-1]viastrong N—H...I hydrogen bonds. The hydrogen-bonded chains are further packed in stacks along [100]. The thermal behaviour ofIwas studied by different physicochemical methods (thermogravimetry, differential scanning calorimetry and powder diffraction). Differential scanning calorimetry revealed three narrow endothermic peaks at 346, 387 and 525 K, and one broad endothermic peak at ∼605 K. The first and second peaks are related to solid–solid phase transitions, while the third and fourth peaks are attributed to the melting and decomposition ofI. The enthalpies of the phase transitions at 346 and 387 K are estimated as 2.60 and 2.75 kJ mol⁻¹, respectively. The X-ray powder diffraction data collected at different temperatures indicate the existence ofIas the monoclinic (100–346 K), orthorhombic (346–387 K) and cubic (387–525 K) polymorphic modifications.