E. Moons et al., SURFACE PHOTOVOLTAGE OF PORPHYRIN LAYERS USING THE KELVIN PROBE TECHNIQUE, JOURNAL OF PHYSICAL CHEMISTRY B, 101(42), 1997, pp. 8492-8498
The Fermi energy of spin-coated layers of pyridinium porphyrins on ind
ium tin oxide (ITO) was determined by the Kelvin probe technique. The
work function of zinc tetrakis(N-methyl-4-pyridinium)porphyrin (ZnTMPy
P) was found to be 500 meV larger than the one of metal-free tetrakis(
N-methyl-4-pyridinium)porphyrin (H2TMPyP), which indicates that the Fe
rmi levels ascribed to these molecular layers do not lie in the middle
of their energy gap but that the layers show a p-type and an n-type c
haracter, respectively. Furthermore, from the dependence of the work f
unction on the layer thickness, we deduce that an electric field is pr
esent near the porphyrin/ITO interface. This field extends over 10-20
nm and is oriented such that the porphyrin layers are depleted from ma
jority mobile carriers near the ITO interface. However, this field doe
s not contribute significantly to the photovoltage, which indicates a
high density of interface traps at the HTO interface. Surface photovol
tage measurements under illumination of the porphyrin molecules indica
te that an additional depletion region is present at the porphyrin/air
interface. We therefore suggest for the porphyrin/ITO system a double
junction model. This double junction was simulated numerically and va
lues for the surface charge density and the interface charge density w
ere extracted.