Estimating the quasi-Fermi level of holes at the surface of semiconductor photoanodes using outer-sphere redox couples

Semiconductor electrodes can catalyze photo-induced redox reactions with light illumination. Photoexcitation produces excited carriers that subsequently transfer to the front and back contacts as determined by the bulk and surface properties of the photoelectrodes. This transfer defines the resultant quasi-Fermi levels of the photo-generated carriers at the photoelectrode surface, which, in turn, impacts the efficiency of surface photoelectrochemical reactions. However, determining such quasi-Fermi levels is not a simple task. In this study, we introduce a method for estimating the quasi-Fermi level of holes using outer-sphere electron transfer reactions. The quasi-Fermi level of holes is estimated by linking the oxidation photocurrent on photoanodes to the separately measured electrode potential on a stable metal electrode. Using this method, the quasi-Fermi level of holes at the surface is monitored in response to variations in applied potential and light intensity. This approach effectively separates the photocurrents of the CdS model electrode between surface redox reaction and photocorrosion, while concurrently quantifying the dynamic quasi-Fermi level at the surface. This work facilitates quantitative understanding of photoelectrochemical reactions on semiconductor electrodes to design green chemical transformation systems. © The Author(s) 2025.