In summary, our results show that the adsorbed ions play a more important role in the photoinduced interface charge transfer dynamics than the band bending in the dye-sensitized NiO photocathode induced by the applied potential. This can explain the different results in this work (negative potential leads to slower photoinduced hole injection in aqueous solution) and in the literature (negative potential promotes hole injection in acetonitrile). Our work highlights the key role of ions in the IHP and at the photocathode surface, controlled by the external bias potential, in the realization of efficient solar to fuel devices. reduction. We show that the main effects of the external potential likely arise from changes in surface ion adsorption in the IHP, which plays a key role as nondirectional charge transfer relay.