SURFACE PHOTOVOLTAGE OF PORPHYRIN LAYERS USING THE KELVIN PROBE TECHNIQUE

Citation
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
Citations number
49
Categorie Soggetti
Chemistry Physical
Journal title
JOURNAL OF PHYSICAL CHEMISTRY B
ISSN journal
15206106 → ACNP
Volume
101
Issue
42
Year of publication
1997
Pages
8492 - 8498
Database
ISI
SICI code
1089-5647(1997)101:42<8492:SPOPLU>2.0.ZU;2-Y
Abstract
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.