Photoelectrochemical water oxidation is attracting significant interest because it is an environmentally friendly energy conversion method that harnesses abundant solar energy for hydrogen production. An efficient photoanode capable of catalysing the intrinsically sluggish water oxidation reaction is essential to make solar-assisted water splitting a viable technology. Among candidate materials, the n-type semiconductor hematite (Fe₂O₃) stands out as a stable, earth-abundant, and low-cost option. When coupled with a conductive substrate such as fluorine-doped tin oxide (FTO), it represents a promising photoanode system.

Understanding charge transport and recombination mechanisms under realistic operating conditions is crucial for the rational design of efficient and durable photoelectrochemical systems. In this regard, operando small-perturbation techniques such as Intensity Modulated Photocurrent Spectroscopy (IMPS) and Intensity Modulated Photovoltage Spectroscopy (IMVS), together with their time-domain counterparts Transient Photocurrent (TPC) and Transient Photovoltage (TPV), provide powerful insight into transport and recombination dynamics. Despite their widespread use, the interpretation of these measurements is often based on simplified models restricted to specific frequency-domain techniques and not consistently extendable to steady-state conditions or time-domain analyses.

In this work [1], we present a novel, physically consistent model capable of predicting the steady-state current-voltage curves, IMPS/IMVS spectra, and the corresponding TPC/TPV responses of a Sn-doped hematite photoanode. The model overcomes key limitations of existing approaches by explicitly accounting for the difference between the internal voltage (quasi-Fermi level splitting) and the externally applied bias. This aspect, previously shown to be essential for correctly describing resistive losses in solar cells [2], is demonstrated here to play a central role in photoelectrochemical systems as well. A comprehensive analysis of the key parameters that govern the physics of the system reveals how recombination, charge transport, and interfacial transfer processes shape the dynamic response. Drift-diffusion simulations show strong agreement with experimental data from a Sn-doped hematite photoanode, thereby validating the analytical model. Time constants extracted under typical operating conditions characterized by slow hole charge transfer are assigned to distinct physical processes, and the electron extraction velocity within the hematite layer is quantified. The proposed model can be universally extended to explain the response of the frequency-modulated and time domain lab-scale techniques across a wide range of photoanode systems.