Operando Analysis of Semiconductor Junctions in Multi-Layered Photocathodes for Solar Water Splitting by Impedance Spectroscopy

Although electrochemical impedance spectroscopy (EIS) is a powerful technique for investigating optoelectronic devices, realistic equivalent circuit (EC) models suitable for multi-layered water splitting electrodes have rarely been reported due to their complex nature. In the present study, the utility of the EIS method for investigating multi-layered photocathodes for photoelectrochemical water splitting is demonstrated. By analyzing the EIS data of TiO₂-coated Sb₂Se₃ photocathodes, one is able to obtain information about the constituent semiconductors and interfaces such as recombination processes, carrier lifetimes, doping densities, and flat band potentials under operando conditions. The charge transfer time to the electrolyte is also extracted from the EIS data and confirmed by transient photocurrent decay measurements. In addition, the method is successfully applied to other photocathodes with different classes of light absorber, such as metal oxides (Cu₂O) and crystalline Si, to compare the device characteristics under real operational conditions. It is shown that the lifetime of photo-generated carriers in the Si photocathode is much higher than those of the Sb₂Se₃ and Cu₂O photocathodes. It is believed that the EIS analysis method presented in this study will become a powerful routine characterization technique for discovering the limiting factors in a wide range of photo-electrosynthetic as well as photovoltaic devices.