Successful perovskite process engineering and quality control will require the ability to monitor the contents of a perovskite precursor ink after the initial mixing. Perovskite precursor solutions are known to consist of dynamically coordinated complexes formed between lead halide species and solvent molecules. Currently, monitoring of the precursor constituents is typically realized by highly sophisticated measurement techniques like X-ray scattering, cryonic transmission electron microscopy   or nuclear magnetic resonance. The solvate complexes present in the perovskite precursor are charged species that exist in the liquid phase, which, at higher concentrations, aggregate into perovskite-like clusters with reduced charge density compared to the smaller units such as [PbI₆]⁴⁻ or MA⁺.[1] Eventually, such an aggregation mechanism is expected to affect ionic conductivity of the precursor in two ways: On the one hand a reduction of the charged species density in the ink will straightforwardly impact the ionic conductivity, while on the other hand, the clustering effects will reduce charge carrier mobility as the charge-to-weight and charge-to-volume ratio decreases. We confirmed this expected trend in electrical conductivity in our recent work.[1]

Building on our observations in electrical conductivity, in this work we introduce electrochemical impedance spectroscopy as a powerful tool to investigate subtle changes in the perovskite precursor ink by providing frequency-resolved access to ionic and dipolar relaxation processes. Impedance spectroscopy allows us to obtain deeper insights through analysis of the frequency depending imaginary and real parts of the complex impedance. Building on models developed for battery research, we perform a step-by-step approach driven by physical and chemical insight, to establish an equivalent circuit model that is able to fit the obtained spectra. Notably we were able to identify parts of the spectrum responsible for the clustering process in the ink. Intriguingly, impedance spectroscopy proved highly sensitive to degradation phenomena such as thermally induced precursor ink degradation or water contamination—influences that are known causes of perovskite device failure. Therefore, we prove electrochemical impedance spectroscopy to be a potent diagnostic tool to monitor perovskite ink properties, that will be beneficial for both perovskite research and industrialization.