To elucidate the photoinduced dynamics of NiO/P1 in PBS electrolyte under various external bias potentials, the spectrotemporal photoluminescence (PL) behavior was measured by streak camera detection (Figure ). As bare NiO does not show any PL in this wavelength range following excitation at 532 nm, the PL signal in Figure primarily originates from the excited P1 dye (P1*). On an insulating ZrO2 support, the PL lifetime of P1* equals ca. 250 ps. Lightinduced hole injection from P1* into NiO is known to occur in a few hundred fs to several ps. Although as a result the PL decays of P1 on NiO are within the instrumental response time of the streak camera, the PL intensity as function of applied potential shown in Figure be understood by the dual function of surface-adsorbed OH -in the IHP, which we recently observed to promote both hole injection and recombination, with the concentration increasing with more positive potential. Considering the dimensions of the P1 dye, we assume band bending in NiO/PBS and NiO/P1/PBS to be similar. NiO shows a very weak PL signal at 0 V to -0.8 V following excitation at 267 nm, while the PL is more intense at positive potentials (Figure ). According to the dead layer model, the thickness of the dead layer increases with more band bending (Figure ), leading to a lower PL intensity. Hence, the higher PL intensity of NiO at more positive potential indicates less band bending. Therefore, a significant effect of band bending in the NiO on the trend shown in Figure Femtosecond transient absorption (TA) studies were performed to further investigate the role of the external potential in the interfacial photodynamics of NiO/P1 in PBS electrolyte. Figures shows the TA spectra at various applied potentials. These data have been recorded on the same NiO/P1 sample to avoid any potential effect of sample to sample variations, although these are minor. The broad negative signal is due to the photoinduced ground state bleach (GSB) of the P1 dye. P1* is known to give a strong and broad positive absorbance around 550-560 nm. Due to hole injection from P1* into NiO, the P1* signal decreases and the characteristic absorbance around 610 nm of P1 •-arises, causing a red-shift in spectrum with time. The decay in signal is due to charge recombination. Hole injection from P1* into NiO is typically a biphasic process, with the fastest component within the TA instrumental response time (IRT, 100-150 fs). The early-time spectra can hence be used to assess whether the applied potential affects the extent of ultrafast hole injection within the IRT. However, the spectrum at 250 fs at +0.8 V is clearly blue-shifted and broader than the spectra at less positive potentials (Figure ), which seems in contradiction with the faster hole injection indicated by stronger PL quenching (Figure ). Ni 2+ oxidation at +0.8 V into Ni 3+ / Ni 4+ by the external potential/hole injection is the likely reason, which can lead to an increase in absorption above 560 nm. and P1 •-formed due to hole injection (positive signal), while the signal at 660 nm is primarily due to P1 •-. When NiO/P1 is exposed to an aqueous solution or to air with a high relative humidity, most of the hole injection occurs ultrafast and is finished within the IRT (100-150 fs), only a minor part of the hole injection occurs slower (1-2 ps). The strong PL quenching especially at +0.8 V (Figure ) already indicates that hole injection from P1* into NiO occurs the fastest at these conditions, and becomes gradually slower on moving towards a negative applied potential, which is supported by the TA kinetic traces at 660 nm. At positive potentials the TA signal fully develops <IRT, while at negative applied potentials a minor further ~1 ps rise is observed indicating a slower hole injection component. Comparison of the transient signals at 570 nm at different potentials (Figure ) shows that charge recombination becomes slower with more negative potential. Retardation of charge recombination at negative potential was reported earlier and explained by filling of intra-gap trap states. However, in that study performed in acetonitrile hole injection is also accelerated by a more negative potential, in contrast to the trend shown in Figures and. A likely reason for this contrast is a strong dependency of the NiO surface termination on the working environment. Considering the dual role of surface OH -we unraveled recently, accelerating both hole injection and charge recombination, the dependency of the photodynamics on the external potential observed here is likely due to a change in ions (H + or OH -) in the IHP. The quantity of surface OH -ions is the highest at positive potential, resulting in both fast photoinduced hole injection and charge recombination. This dual role of surface OH -becomes gradually less when moving to negative potentials. We cautiously assign the small difference between -0.4 V and -0.8 V to a saturation in the quantity of surface-absorbed H + in the IHP. Figure presents a model that can explain the effect of the different external potentials on the interface photodynamics of NiO/P1 in PBS observed here. At the electrode/electrolyte interface, two types of electrochemical processes can occur. One is the Faradaic process, i.e. the oxidation or reduction reaction. The second type is the non-Faradaic process, in which surface adsorption and desorption occur and the structure of the electrode/electrolyte interface changes with applied bias potential and electrolyte composition. For NiO the Faradaic process leading to Ni 2+ /Ni 3+ oxidation predominantly occurs at ~1.4 V vs. RHE, i.e. ~0.7 V vs. Ag/AgCl (see also Figure for a cyclic voltammogram of NiO in PBS), implying that at lower potential cation (H + ) and anion (OH -) adsorption onto the NiO surface and desorption into the PBS are the dominant processes. Other ions like PO4 2-or HPO4 -might also play a role, but we observed earlier that their effect on the photodynamics is insignificant compared to OH -, which we observed to promote both photoinduced hole injection and charge recombination. The present work clearly demonstrates that the interfacial photodynamics change with applied external potential before oxidation of Ni 2+ into Ni 3+ occurs. Therefore, we propose that the non-Faradaic process, i.e. ion adsorption and desorption onto the NiO surface, plays an essential role as a relay in the light-induced charge transfer and recombination processes. This effect has not gained attention yet due to the lack of ultrafast spectroscopy studies under in-situ conditions.