V vs. Ag/AgCl, WE1) at a distance of approx. 50 µm above the photocathode (biased at -0.8 V vs. Ag/AgCl, WE2) using a stepper motor and the help of an inverted microscope. The photocathode samples were illuminated with monochromatic light (420 nm LED) for 10 min in a 1.5 mL phosphate buffer solution (50 mM, pH 7). For measurements in oxygen-free conditions, the solution was purged 5 min prior with argon and a constant argon stream was kept over the measurement cell during the entire experiment. An optical fiber (diam. 1000 µm, M59-L01, Thorlabs GmbH, Bergkirchen, Germany) connected to a 110 mW blue LED (420 nm, M420F3, Thorlabs GmbH) was used for the illumination of the samples.