Effects of porphyrin substituents on film structure and photoelectrochemical properties of porphyrin/fullerene composite clusters electrophoretically deposited on nanostructured SnO2 electrodes

We have systematically examined the substituent effects of meso-tetraphenylporphyrins on film structures and the photoetectrochemical properties of the composite clusters of free-base porphyrin and C-60 electrophoretically deposited on nanostructured SnO2 electrodes. The photocurrent generation efficiency was found to correlate with the complexation ability of the porphyrin for C-60. Basically, the incident photon-to-current efficiency (IPCE) value was increased with increasing relative amounts of the porphyrin versus C-60 in the films. The unique molecular arrangement of the porphyrin with the simple, specific substituents (i.e., methoxy groups at the ineta-positions of the meso-phenyl rings of tetraphenylporphyrins (3,5-OMeTPP; TPP=tetraphenylporphyrin)) and C-60 on SnO2 electrodes resulted in the largest IPCE value (ca. 60%) among this type of photoelectrochemical device. The rapid formation of the composite clusters and microcrystals from the combination of 3,5-OMeTPP and C60 in a mixed solvent is unique as the association is accelerated by intermolecular interactions (i.e., hydrogenbonding and CH-pi interactions) between the methoxy groups of the porphyrins and the porphyrin/C-60, in addition to the pi-pi interactions between the porphyrins/C-60 and C-60 molecules. Both the films and single crystals composed of the porphyrin and C60 exhibited remarkably high electron mobility (7 x 10(-2) and 0.4 cm(2)V(-1) s(-1)), which is comparable to the value for highly efficient bulk heterojunction solar cells. Our experimental results have successfully demonstrated the importance of nanostructured electron- and hole-transporting pathways in bulk heterojunction solar cells. Such a finding will provide basic and valuable information on the design of molecular photovoltaics at the molecular level.