Wavelength-Dependent Photoelectrochemical Impedance Spectroscopy for Probing Charge Transport in Photoanodes

Although photoelectrochemical impedance spectroscopy (PEIS) has been widely used to analyze charge transport and transfer processes in such systems, conventional measurements under broadband illumination inherently average wavelength-dependent phenomena and obscure their individual contributions. In this study, we report the development of wavelength-dependent photoelectrochemical impedance spectroscopy, which extends the potential-dependent PEIS (pot-PEIS) by incorporating monochromatic illumination. This method enables systematic investigation of charge carrier-transport mechanisms as a function of excitation wavelength, providing insight into wavelength-dependent bulk electron transport, and interfacial hole-transfer processes. The technique was applied to hematite photoanodes fabricated using different preparation strategies, including TiO2 underlayers, alcohol-assisted deposition, and microwave-assisted calcination. The wavelength-selective pot-PEIS measurements revealed distinct impedance responses depending on the excitation wavelength and fabrication method, reflecting differences in charge carrier resistivity and defect state distributions with a two-dimensional map of the wavelength and potential. These results demonstrate that charge transport and recombination pathways in hematite are strongly influenced by the photocarrier generation region in the depth direction. The developed wavelength-dependent pot-PEIS provides a powerful and generally applicable platform for disentangling wavelength-and-potential-dependent charge transport phenomena and offers a mechanistic understanding of photoelectrochemical materials, which establishes a multidimensional analytical framework capable of generating state-sensitive electronic fingerprints of semiconductor photoelectrodes.