Nanostructured assembly of porphyrin clusters for light energy conversion

Free base porphyrin molecules form well-defined and ordered nanoclusters in mixed solvents with absorption characteristics that significantly differ from those of the monomer form. These self-assembled crystallites in acetonitrile/toluene (9:1, v/v) can be deposited as thin films on a nanostructured TiO₂ electrode using an electrophoretic technique. This porphyrin cluster assembly is highly photoactive and capable of undergoing charge separation under visible light excitation. Photoexcitation of the porphyrin film electrode assembly in a photoelectrochemical cell with visible light produces relatively high photocurrent generation. A maximum photocurrent of 0.15 mA cm^-2 and a photovoltage of 250 mV were attained using the I₃^-/I^- redox couple. The electron flow to the TiO₂ electrode can be facilitated by application of a positive potential. An incident photon-to-photocurrent generation efficiency (IPCE) of 2.0% has been achieved at an applied bias potential of 0.2 V vs. SCE. The broad photoresponse of these crystallites throughout the visible range as well as the ease of assembling them on the electrode surface opens up new avenues for harvesting a wide wavelength range of solar light.