Quasi-Fermi levels for an n-type semiconductor illuminated from the electrolyte side (cf. Figure 1.6). The potential is held at the redox potential. No current flows in the dark (equilibrium). Under illumination, photogenerated holes move to the surface, where they oxidize R to O. The position of the hole QFL (_pE_F) at the surface is determined by the illumination intensity and the kinetics of interfacial charge transfer.