Photoelectrochemical (PEC) devices offer a promising route for sustainable energy conversion by harnessing solar energy to drive electrochemical reactions such as water splitting. The PEC process involves light absorption, generation and transport of photoinduced charge carriers, and their participation in electrochemical reactions at semiconductor/solution interface. Understanding the charge carrier dynamics of the PEC cells is essential for guiding the design of efficient and selective photoelectrodes. In this talk, I will present charge carrier dynamics of copper oxide (CuO) films, p-type semiconductors, during photoelectrochemical hydrogen evolution reaction (HER). We employed intensity-modulated photocurrent spectroscopy (IMPS), frequency-resolved analytical technique, to measure the photocurrent response to sinusoidal light modulation. The frequency-dependent response provides insights into the distinct time scales of PEC processes. Conventionally, IMPS data are interpreted using a trap-assisted recombination model pioneered by Laurence Peter, which assumes charge recombination via trap states while neglecting their associated capacitances. However, our IMPS results reveal that the trap state capacitance, often overlooked in the standard models, plays a significant role in charge accumulation and cannot be neglected. To address this limitation, we developed an extended model that incorporates trap state capacitance and its charging/discharging dynamics. This model enables more accurate interpretations of charge carrier behavior and yields analytical expressions that capture the frequency-dependent IMPS results. Using this model, we demonstrate that trap state capacitance and corresponding charge carrier dynamics are influenced by operation conditions (e.g., light wavelength and intensity, electrode potential), semiconductor electrode properties (e.g., surface compositions and doping density), and the electrolyte solution composition. These findings underscore the crucial role of surface charge accumulation in governing charge carrier dynamics and elucidate how trap states impact the interfacial charge transfer and overall performance. This improved understanding offers a foundation for tailoring semiconductor electrodes in diverse PEC energy conversion systems.
