Ultrafast spectroscopic characterization of dye sensitized H2 evolving photocathodes : towards optimized devices Dye sensitized photoelectrochemical cells (DSPEC) are a promising approach for the production of solar fuels such as hydrogen. However so far DSPEC performance is still far below their theoretical possibilities. Especially the photocathodes lack behind their photoanode counterparts in activity and stability. In this thesis dye sensitized NiO photocathodes were constructed with a series of newly synthesized covalent dye catalyst assemblies featuring an organic dye and a cobalt catalyst and a comprehensive characterization was carried out in order to be able to rationally improve upon the systems for the next generation of dye catalyst assemblies. To this end their activity was assessed in photoelectrochemical proton reduction and a 40 fold increase in hydrogen production was observed after 22h compared to the previous generation of dyads. Operando and post operando measurements yielded information about the degradation mechanism and kinetics during operation. The instability of the cyanoacrylate acceptor group on the dye in its reduced state was revealed which led to a loss of activity over time. Furthermore the light induced processes in the assemblies were studied in solution and on NiO using time resolved spectroscopy especially the electron transfer from the dye to the catalyst unit. A novelty of this work was to carry out these measurements while applying a potential allowing the study under quasi operando conditions. To this end a spectroelectrochemical cell was developed for transient absorption spectroelectrochemical experiments on sensitized films. Using this technique the electron transfer on the best performing system could be observed when immobilized on NiO while in solution electron transfer was absent. This could be correlated to the rather slow (ns µs scale) kinetics of the electron transfer which cannot compete with the fast deactivation of the dye excited state in solution (<2 ns).