Removing Instability-Caused Low-Frequency Features in Small Perturbation Spectra of Perovskite Solar Cells

Small-perturbation frequency-domain techniques such as impedance spectroscopy and intensity-modulated photocurrent spectroscopy (IMPS) have become important methods for the investigation of the physical working mechanisms of the perovskite solar cell (PSC). The validity of these methods relies on assuming sample stability at the given steady state. Through a series of IMPS measurements, we identify that this assumption is invalid in certain cases for both iodide and bromide-based PSCs that show strong time drift in their IMPS response, noticeable in particular at low frequencies, which are usually connected with the kinetics of ionic motion and interaction with outer electrodes. Using time course interpolation and a corrective Z-HIT algorithm that connects the modulus of the IMPS transfer function and its phase, we identify that the low-frequency arc/tail is in certain cases an artifact generated by time drift of the sample. Since the low-frequency data in an IMPS measurement of the PSC provide important information regarding its differential external quantum efficiency, care must be taken to ascertain the origin and validity of the low-frequency phenomena. Validity test is performed by using a combination of corrective algorithms mentioned above and several measurements over time to obtain stabilized spectra virtually free of time drift.