Photoelectrochemical properties of metalloporphyrins

Abstract

The photovoltaic responses of meso tetraphenyl-, meso tetra-propyl- and octaethyl-porphyrins, porphines, chlorins, cofacial diporphyrins and mesoporphyrin IX diesters were investigated using two cell configurations: Al| Porphyrin |Ag, and Al| Porphyrin | Fe(CN)6−3, Fe(CN)6−4 |Pt. We found (1) that the Al–porphyrin interface is photoactive: the action spectra closely follow the absorption spectra of the porphyrins, and this interface is best described as a semiconductor–insulator–metal diode consisting of porphyrin |A12O3 |Al; (2) that within a homologous series, in which the porphyrin skeleton is fixed but the metal is varied, the quantum yields parallel the ease of oxidation of the porphyrin in nonaqueous solvents. The more easily oxidised compounds exhibit the higher quantum yields; (3) no obvious correlations are found with the luminescent properties of the porphyrins in solution; (4) the morphology of the films influences the quantum yields: amorphous films are better than microcrystalline ones; and (5) the most efficient cells reach quantum yields of ∼0.2 and energy efficiencies of ∼1% for monochromatic light at the peak of the action spectrum in the region of 400–450 nm.