BEHAVIOR OF SI PHOTOELECTRODES UNDER HIGH-LEVEL INJECTION CONDITIONS .1. STEADY-STATE CURRENT-VOLTAGE PROPERTIES AND QUASI-FERMI LEVEL POSITIONS UNDER ILLUMINATION

The behavior of the quasi-Fermi levels of electrons and holes at vario us semiconductor/liquid interfaces has been probed through the use of thin, high purity, low dopant density single crystal Si photoelectrode s. Since standard Air Mass 1.5 illumination is sufficient to produce h igh level injection conditions in such samples, minimal electric field s can be present near the solid/liquid interface. Under these conditio ns, efficient charge separation relies on establishment of kinetic asy mmetries at the back contacts while effectively sustaining photogenera ted carrier concentration gradients in the photoelectrode. These condi tions were achieved for Si/CH3OH interfaces in contact with the 1,1'-d imethylferrocene(+/0), cobaltocene(+/0), methyl viologen(2+/+), and de camethylferrocene(+/0) redox couples. For redox couples having energie s near the top of the Si valence band, such as 1,1'-dimethylferrocene( +/0), the sample acted like an n-type photoelectrode, yielding large p hotovoltages for collection of electrons at the back contact and small photovoltages for collection of holes. For redox couples having energ ies near the bottom of the Si conduction band, such as cobaltocene(+/0 ), the sample acted like a p-type photoelectrode, yielding large photo voltages for collection of holes at the back contact and small photovo ltages for collection of electrons. The Si sample exhibited both photo anodic 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 semicondu ctor/liquid contacts under illumination relative to the situation at e quilibrium, is advanced to describe this behavior. This explanation is also applicable to a description of the photovoltage behavior of semi conductor particles and to undoped photoconductive semiconductor elect rodes that are operated under high level injection conditions. In addi tional experiments, measurement of the apparent electrochemical potent ials 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-Ha ll and Marcus-Gerischer theories for semiconductor/liquid junctions.