Photovoltaic properties of self-assembled monolayers of porphyrins and porphyrin-fullerene dyads on ITO and gold surfaces

A systematic series of ITO electrodes modified chemically with self-assembled monolayers (SAMs) of porphyrins and porphyrin-fullerene dyads have been designed to provide valuable insight into the development of artificial photosynthetic devices. First the ITO and gold electrodes modified chemically with SAMs of porphyrins with a spacer of the same number of atoms were prepared to compare the effects of energy transfer (EN) quenching of the porphyrin excited singlet states by the two electrodes. Less EN quenching was observed on the ITO electrode as compared to the EN quenching on the corresponding gold electrode, leading to remarkable enhancement of the photocurrent generation (ca. 280 times) in the porphyrin SAMs on the ITO electrode in the presence of the triethanolamine (TEA) used as a sacrificial electron donor. The porphyrin (H2P) was then linked with C₆₀ which can act as an electron acceptor to construct H₂P-C₆₀ SAMs on the ITO surface in the presence of hexyl viologen (HV²⁺) used as an electron carrier in a three electrode system, denoted as ITO/H₂P-C₆₀/HV²⁺/Pt. The quantum yield of the photocurrent generation of the ITO/H₂P-C₆₀/HV²⁺/Pt system (6.4%) is 30 times larger than that of the corresponding system without C₆₀: ITO/H₂P-ref/HV²⁺/Pt (0.21%). Such enhancement of photocurrent generation in the porphyrin-fullerene dyad system is ascribed to an efficient photoinduced ET from the porphyrin singlet excited state to the C₆₀ moiety as indicated by the fluorescence lifetime measurements and also by time-resolved transient absorption studies on the ITO systems. The surface structures of H₂P and H₂P-C₆₀ SAMs on ITO (H₂P/ITO and H₂P-C₆₀/ITO) have been observed successfully in molecular resolution with atomic force microscopy for the first time.