The behavior of the quasi-Fermi levels of electrons and holes at various semiconductor/liquid interfaces has been probed through the use of thin, high purity, low dopant density single crystal Si photoelectrodes. Since standard Air Mass 1.5 illumination is sufficient to produce high level injection conditions in such samples, minimal electric fields can be present near the solid/liquid interface. Under these conditions, efficient charge separation relies on establishment of kinetic asymmetries at the back contacts while effectively sustaining photogenerated carrier concentration gradients in the photoelectrode. These conditions were achieved for Si/CH3OH interfaces in contact with the 1,1′-dimethylferrocene+/0, cobaltocene+/0, methyl viologen2+/+, and decamethylferrocene+/0 redox couples. For redox couples having energies near the top of the Si valence band, such as 1,1′-dimethylferrocene+/0, the sample acted like an n-type photoelectrode, yielding large photovoltages for collection of electrons at the back contact and small photovoltages for collection of holes. For redox couples having energies near the bottom of the Si conduction band, such as cobaltocene+/0, the sample acted like a p-type photoelectrode, yielding large photovoltages for collection of holes at the back contact and small photovoltages for collection of electrons. The Si sample exhibited both photoanodic and photocathodic currents in contact with redox couples having electrochemical potentials in the middle of the Si band gap. A simple explanation, based on the fundamental carrier statistics of semiconductor/liquid contacts under illumination relative to the situation at equilibrium, is advanced to describe this behavior. This explanation is also applicable to a description of the photovoltage behavior of semiconductor particles and to undoped photoconductive semiconductor electrodes that are operated under high level injection conditions. In additional experiments, measurement of the apparent electrochemical potentials of electrons and holes in contact with various redox couples has allowed quantification of the amount of recombination and experimental determination of the separation of the quasi-Fermi levels for various redox couples at the semiconductor/ liquid contact. These measurements are important to verification of key elements of the Shockley-Read-Hall and Marcus-Gerischer theories for semiconductor/liquid junctions.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry