Sustainable approaches in utilizing solar energy to produce chemical fuels would provide a way to meet the world increasing energy demand without the negative environmental effects of burning fossil fuels. One particularly interesting method is solar water splitting, where energy from sunlight is used to produce hydrogen and oxygen from water. In this process, an aqueous-stable semiconducting photoelectrode is required in order to sufficiently absorb visible light and efficiently oxidize/reduce water on its surface. Since water oxidation is typically the limiting reaction, many semiconductor photoelectrodes need to be decorated with additional water oxidation co-catalysts, such as CoP_i, FeOOH, NiFeO_x, and MnO_x, in order to improve the performance [1-5]. However, the mechanisms at the semiconductor/co-catalyst interface and how they are influenced by the interface energetics are not yet fully understood. Here, we use a well-defined MnO_x-based co-catalysts deposited by atomic layer deposition (ALD) and semiconducting Ta-O-N thin films as a model system in order to investigate the process. We successfully prepared different phases (e.g. Ta₂O₅, TaO_xN_y, Ta₃N₅) with varying valence band maximum positions by systematically controlling the partial pressure of NH₃, H₂ and H₂O during post-annealing of Ta thin films [6]. The valence band position of the MnO_x co-catalyst was also shifted by introducing Ni as dopant (Ni:MnO_x). Photoelectrochemical studies with and without hole scavenger reveal that MnO_x and Ni:MnO_x enhances the photocurrent of Ta-O-N films by a factor of ~4 without affecting the charge injection efficiency. Therefore, higher charge separation efficiency should play the crucial role in the photocurrent increase. Open circuit potential (OCP) and in-line X-ray photoelectron spectroscopy (XPS) data suggested that this is attributed to the formation of a band bending at the Ta-O-N/MnO_x and Ta-O-N/Ni:MnO_x interface. This is further supported by analyzing Ta-O-N films with varying thickness of the co-catalysts; thicker co-catalysts result in increasing band bending, which correlates with the photocurrent trend as well as the effective carrier diffusion length measured by time-resolved microwave conductivity (TRMC). Finally, the influence of the band offsets between the different photoanode in the Ta-O-N systems and the co-catalysts is discussed.