Interactions at tetraphenyl-porphyrin/InP interfaces observed by surface photovoltage spectroscopy

Illumination induced charge separation processes at tetraphenyl-porphyrin (H2TPP)/InP interfaces are characterized. The results indicate that upon illumination an electronic transition takes place between the organic highest occupied molecular orbital (HOMO) and the InP conduction band. A 70 meV blue shift in the characteristic modulated surface photovoltage spectrum of a 50 nm H2TPP film was observed when n-InP substrate was used. Such blue shift was not observed when Au, SnO2:F or p-InP substrates were used. The shift may suggest a presence of an ordered interfacial sub-layer in an almost lying orientation, due to chemical or physical interactions at the interface. The results indicate that the Fermi level is unpinned at the interface. An exciton diffusion barrier in close proximity to the H2TPP/n-InP interface has been demonstrated.