This lecture covers the dynamic photoelectrochemical responses used as diagnostic tools for stability, charge transfer, and recombination losses in solar absorbers and semiconductor electrolyte junctions. The speaker, Prof. David Fermin from the University of Bristol, discusses the band edge pinning in CdS, space charge capacitance, and the Gartner expression. The lecture also delves into surface recombination losses, semiconductor photocorrosion, and the intensity modulated photocurrent spectroscopy. Furthermore, it explores semiconductor electrolyte junctions, GaFeO3 thin-film photoelectrochemical responses, and the promotion of photoelectrochemical active sites by cation substitution.