In the natural photosynthesis of all plants and bacteria, porphyrin-based chromophores play important roles not only as strongly light-harvesting pigments but also as sequential intermolecular photoinduced energy and electron transfer components. Many researchers have been attracted to the porphyrin-based chromophores in the wide field of science, such as biology, chemistry, photophysics, and so on. In this study, novel directly-linked heterometallic porphyrin dimers were synthesized and used for the artificial electrochemical solar cells based on titanium oxide nanocrystals as sensitizers adsorbed on the surface bearing high photon-to-electron conversion efficiency.
Though some porphyrin-based chromophores have been applied to the dye-sensitized solar cells (DSSCs) previously, it was reported that the efficiency of many of the DSSCs was not so high. In order to improve the efficiency of the DSSC, directly-linked heterometallic porphyrin dimers which work as intramolecular charge separators have been prepared by cross-coupling reaction. Generally, it is well known that the redox potential of metalloporphyrin shifts depending on the metal ion inserted into the porphyrin core. It is expected that photoinduced intramolecular charge separation occurs between the two metalloporphyrin cores and then the electron is injected from the negatively-charged metalloporphyrin core into the titanium oxide conduction band effectively. In the DSSC adsorbed the heterometallic porphyrin dimers, relatively high photon-to-electron conversion efficiency (2.8% under AM 1.5 solar condition) was observed. The IPCE of the DSSC in 400-600 nm visible light range was over 50%. Improvement of performance of the DSSC using the synthesized porphyrin dimer is now in progress.