BEHAVIOR OF SI PHOTOELECTRODES UNDER HIGH-LEVEL INJECTION CONDITIONS .1. STEADY-STATE CURRENT-VOLTAGE PROPERTIES AND QUASI-FERMI LEVEL POSITIONS UNDER ILLUMINATION
Mx. Tan et al., BEHAVIOR OF SI PHOTOELECTRODES UNDER HIGH-LEVEL INJECTION CONDITIONS .1. STEADY-STATE CURRENT-VOLTAGE PROPERTIES AND QUASI-FERMI LEVEL POSITIONS UNDER ILLUMINATION, JOURNAL OF PHYSICAL CHEMISTRY B, 101(15), 1997, pp. 2830-2839
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.