As the optimization of multi-layered photoelectrodes relies on the understanding of the underlying photophysical and electrochemical processes, researchers have sought to develop various characterization methods for investigating PEC devices. However, it is challenging to develop a diagnostic tool under real operational conditions (i.e. with full device configuration under light and applied potential) due to the high complexity of PEC devices. Electrochemical impedance spectroscopy (EIS), which is a frequency-based small perturbation and nondestructive characterization technique, is capable of extracting information about the multiple kinetic steps in the semiconductor electrodes and/or the electrolyte. Due to its versatile nature, the EIS technique has been applied to various research fields including solar cells, batteries, fuel cells, and sensors. Importantly, the interpretation of the EIS data requires a valid equivalent circuit (EC) model reflecting the underlying photophysical and electrochemical processes, which makes it challenging to apply to systems with several processes, such as a multi-layered PEC system.