Impedance Spectroscopic Indication for Solid State Electrochemical Reaction in (CH₃NH₃)PbI₃ Films

Halide perovskite-based solar cells still have limited reproducibility, stability, and incomplete understanding of how they work. We track electronic processes in [CH₃NH₃]PbI₃(Cl) ("perovskite") films in vacuo, and in N₂, air, and O₂, using impedance spectroscopy (IS), contact potential difference, and surface photovoltage measurements, providing direct evidence for perovskite sensitivity to the ambient environment. Two major characteristics of the perovskite IS response change with ambient environment, viz. -1- appearance of negative capacitance in vacuo or post-vacuo N₂ exposure, indicating for the first time an electrochemical process in the perovskite, and -2- orders of magnitude decrease in the film resistance upon transferring the film from O₂-rich ambient atmosphere to vacuum. The same change in ambient conditions also results in a 0.5 V decrease in the material work function. We suggest that facile adsorption of oxygen onto the film dedopes it from n-type toward intrinsic. These effects influence any material characterization, i.e., results may be ambient-dependent due to changes in the material's electrical properties and electrochemical reactivity, which can also affect material stability.