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, we demonstrate the utility of the EIS method for investigating multi-layered photocathodes for photoelectrochemical water splitting. By analyzing the EIS data of TiO₂-coated Sb₂Se₃ photocathodes, we were 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 was also extracted from the EIS data and confirmed by transient photocurrent measurements. In addition, the method was 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 under band bending conditions is much higher than those of the Sb₂Se₃ and Cu₂O photocathodes. We believe that the EIS analysis method presented in this study has made significant progress in understanding the charge carrier behavior in multi-layered photoelectrodes and will become a powerful routine characterization technique for discovering the limiting factors in a wide range of photo-electrosynthetic devices.