Light-induced charge separation in thin tetraphenyl-porphyrin layers deposited on Au

Charge separation processes in metal-free tetraphenyl-porphyrin (H2 TPP) layers deposited on Au substrate have been studied using surface photovoltage spectroscopy. The results show that dissociation of excitons at the H2 TPP Au interface causes charge separation by holes escaping from the interface region. This interface features a potential barrier due to the ground state of the H2 TPP being 0.81 eV below the Fermi level of the Au substrate. Surface photovoltage is built up by internal photoemission at the H2 TPP Au interface, which leads to electron injection into the H2 TPP layer with a slow electron backtransfer. The latter is thermally activated with an activation energy of 0.4 eV, which can be attributed to negative polarons in H2 TPP.