Understanding Ion Pairing in CsPbI₃ Perovskite Precursor Solutions Through X-Ray and Electrochemical Techniques

Controlling perovskite film formation requires a deep understanding of concentration-dependent chemical interactions in precursor solutions. As the precursor concentration increases, the solute-to-solvent ratio rises, leading to enhanced molecular interactions. Near saturation, ion pairing may occur, where non-covalent electrostatic forces dominate over solvation effects. In this study, we employ X-ray pair distribution function (xPDF), small-angle X-ray scattering (SAXS), X-ray absorption spectroscopy (XAS), and electrochemical impedance spectroscopy (EIS) to investigate the A⁺ to BX₃^- interactions in highly concentrated CsPbI₃ solutions. EIS data reveals a conductivity trend indicating ion pair formation above 0.5 M CsPbI₃ in DMSO. The xPDF analysis shows an average Cs-I ion pair distance of ~4.3 Å. SAXS provides insights into the cluster sizes and distributions of ion pairs. Additionally, x-ray reflectivity (XRR) explores the ion pair organization within the electrochemical double layer. These findings offer critical insights into the ordering of ions in solution and the early stages of crystallization, advancing our understanding of perovskite film formation.