Surface Photovoltage of Porphyrin Layers Using the Kelvin Probe Technique

The Fermi energy of spin-coated layers of pyridinium porphyrins on indium tin oxide (ITO) was determined by the Kelvin probe technique. The work function of zinc tetrakis( N -methyl-4-pyridinium)porphyrin (ZnTMPyP) was found to be 500 meV larger than the one of metal-free tetrakis( N -methyl-4-pyridinium)porphyrin (H 2 TMPyP), which indicates that the Fermi 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 character, respectively. Furthermore, from the dependence of the work function on the layer thickness, we deduce that an electric field is present near the porphyrin/ITO interface. This field extends over 10−20 nm and is oriented such that the porphyrin layers are depleted from majority mobile carriers near the ITO interface. However, this field does not contribute significantly to the photovoltage, which indicates a high density of interface traps at the ITO interface. Surface photovoltage measurements under illumination of the porphyrin molecules indicate 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 values for the surface charge density and the interface charge density were extracted.