IR-spectrophotoelectrochemical characterization of mesoporous semiconductor films.

A combined IR-spectroscopic and electrochemical approach for the study of photo- and bias-induced reactions at the semiconductor/electrolyte interface is presented. Information on the electronic structure of a mesoporous semiconductor nanoparticle network, concretely the density and distribution of band gap states, as well as the nature of solution species are analyzed in situ. It has been shown that under appropriate conditions the electrode potential determines the quasi-Fermi level throughout the mesoporous film and thus the occupation of IR-active band gap states, independently of the type of external perturbation, i.e., application of a bias voltage or electrode exposure to photons exceeding the semiconductor band gap at open circuit. Importantly, electronic properties of the semiconductor and vibrational properties of solution species can be addressed simultaneously by IR-spectroscopy. In addition, electrochemical methods provide a means for the active manipulation (in potentiostatic measurements) or the passive tracking (during open circuit potential decay) of the quasi-Fermi level in the mesoporous film together with the possibility of electron quantification (by charge extraction experiments).