Understanding the Carrier Dynamics of Complex Heterojunctions for Water Splitting through Wavelength-Dependent Intensity Modulated Photocurrent Spectroscopy
Authors: Vecchi Pierpaolo , Piccioni Alberto , Carrai Irene , Mazzaro Raffaello , Mazzanti Michele , Cristino Vito , Caramori Stefano , Pasquini Luca 
Published: November 2022

Metal oxide semiconductors are promising materials to be employed as photoanodes in photoelectrochemical (PEC) devices to drive the oxygen evolution reaction (OER) for the conversion of solar energy into chemical fuels. These materials need to efficiently absorb light, have good bulk transport properties to separate the photogenerated charges, and induce fast charge transfer from the electrode inside the solution. To this aim, the best selected materials are usually combined to form efficient heterojunction structures, often with the help of surface catalysts.
The understanding of charge carrier dynamics in complex heterojunctions is of the utmost importance for the performance optimization of photoelectrochemical cells and “
operando” techniques are most valuable, because they provide information about the charge separation, bulk transport, charge transfer at each junction, and at the semiconductor-electrolyte interface of the system upon external stimuli, such as illumination, applied bias and chemical environment.
In the presence of multiple competing processes and complex interfaces, which are often found in photoelectrodes for water splitting, the construction of an electrical model that describe the carrier dynamics under a light stimulus can be difficult, since the frequency response of the PEC system may depend on several interlaced resistive and capacitive contributions, which are sometimes difficult to separate or to interpret in an unambiguous fashion. The characterization of the relevant kinetic processes occurring at junctions and semiconductor/electrolyte interfaces can be effectively carried out by implementing wavelength-dependent Intensity Modulated Photocurrent Spectroscopy (WD-IMPS). This innovative approach allows to selectively probe different layer of the heterojunction and identify the electron transport properties in the bulk and at the interface.
We employed this straightforward technique to study the carrier dynamics of a WO3/BiVO4/CoFe-Prussian blue heterojunction in a conventional three electrode cell for water splitting, and we identified interfacial recombination processes affecting the semiconductor heterojunction, as well as the positive contribution of the inorganic catalyst on the charge separation efficiency of the BiVO4 layer.[1]

Herein, the proposed methodology is used to separate and address the role of each active component within complex interfaces coupled with different catalysts and in different electrolytic environments and represents a valuable tool for improving the understanding of dynamic processes relevant to PEC water splitting.
References
[1] Vecchi, P., Piccioni, A., Mazzaro, R., Mazzanti, M., Cristino, V., Caramori, S. and Pasquini, L. (2022), Charge Separation Efficiency in WO3/BiVO4 Photoanodes with CoFe Prussian Blue Catalyst Studied by Wavelength-Dependent Intensity Modulated Photocurrent Spectroscopy. Sol. RRL. Accepted Author Manuscript. https://doi.org/10.1002/solr.202200108