Photoelectrochemical studies of thin films of several tetraphenylporphyrins deposited on tin(IV) oxide electrodes

Abstract

The photoresponses of thin films of various metallotetraphenylporphyrins, MTPP, free-base tetraphenylporphyrin, TPP, and tetra(4-carboxyphenyl)porphyrin, (o-C₂H₃O₂)TPP, have been investigated. The films were deposited on SnO₂ electrodes and placed in a wet cell. The photoresponse for the water-splitting cell was very low for all compounds, presumably because of the unfavourable relative positions of the energy levels involved. With hydroquinone in the electrolyte solution the photocurrent quantum efficiencies for films 50 Å thick were 8.6 × 10^–3, 1.1 × 10^–2, 1.0 × 10^–2 and 1.6 × 10^–2 for MgTPP, ZnTPP, TPP and (o-C₂H₃O₂)TPP, respectively. No photoresponses were observed for CoTPP, CuTPP and NiTPP. This is attributed to the occurrence of fast radiationless processes, which prevent electron injection into the semiconductor. Some data are presented which may indicate that the singlet excited state is the precursor for charge separation. The photoresponse as a function of the film thickness shows clearly that multilayer activity takes place, although it could not be described using a simple exciton-diffusion model. The photoresponse of thin films of (o-C₂H₃O₂)TPP is attributed to a strong interaction between these molecules and the semiconductor surface.