Electrochemical Impedance Spectroscopy on Perovskite Inks to Probe Ion Pairing in Solution

Hybrid perovskite solar cells (PCS) are a disruptive technology with their rapid improvement in power conversion efficiencies over the last decade. At the laboratory scale, solution processed PCSs are studied to inform the eventual commercialization which is likely to use a slot dye coating process. Many steps in the PSC device fabrication process have been optimized, however there is still a lack of understanding of the solution chemistry and chemical species prior to the crystallization and annealing to perovskite phase. During the crystallization, solvent is expelled with the addition of anti-solvent or with a purge of nitrogen. Initially, the ratio of solvent molecules to ions is dominated by the solvent, especially in low concentration solutions. As solvent is driven off, the ratio of solvent molecules to ions begins to be dominated by ions until a solubility limit is reached and precipitate is formed then an eventual crystallization. In the interim, before any precipitation or crystallization, the solution will undergo concentration dependent regions of ion pairing which directly influence the kinetics of crystallization. Here, we use electrochemical impedance spectroscopy (EIS) to probe the ion pairing in different concentrations of perovskite inks and complement our results with pair distropution function from the total scattering of x-rays.