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 …