Mx. Tan et al., EXPERIMENTAL-MEASUREMENT OF QUASI-FERMI LEVELS AT AN ILLUMINATED SEMICONDUCTOR LIQUID CONTACT/, Journal of physical chemistry, 98(19), 1994, pp. 4959-4962
A novel electrode geometry and contacting procedure has allowed measur
ement of the quasi-Fermi levels, i.e., the apparent electrochemical po
tentials, of electrons and holes at an illuminated semiconductor/liqui
d contact. The key feature of our experiments is the use of a lithogra
phically patterned, high purity (100-400 Ohm.cm n-type float zone mate
rial), low dopant density Si sample in contact with CH3OH-dimethylferr
ocenec(+/0) solutions. The photogenerated carriers can be collected at
the back side of the Si sample through a series of diffused n(+) and
p(+) points. The lifetime of photogenerated carriers approaches 2 ms i
n this sample, indicating that electron-hole recombination is minimize
d in the bulk of the semiconductor. Furthermore, surface recombination
is minimized by use of low saturation current density, ohmic-selectiv
e contacts at the back of the sample. The solid/liquid contact also ha
s a low recombination rate. Therefore, the potentials measured at the
diffused points yield values for the quasi-Fermi levels of electrons a
nd holes under illumination of the semiconductor/liquid contact. Trans
ient photovoltage measurements have also been performed to confirm qua
ntitatively that the quasi-Fermi levels are flat across the Si samples
used in this work.