?-substituted ZnII-tetrarylporphyrinates: dyes with native shielding architecture for Porphyrin-Sensitized Solar Cells

Porphyrin-Sensitized Solar Cells (PSSCs) have attracted considerable interest exceeding the 12% of solar energy conversion efficiency.[1]The best-performing porphyrinic dyes appear to have push-pull architectures based on variously mesodisubstituted ZnII-diarylporphyrinates, which require multistep synthetic pathways drastically reducing overall yields. A microwaveassisted synthetic procedure was developed, by some of us, to successfully obtain ßsubstituted tetrarylporphyrinic dyes with good yields and in few reaction steps. Contrary to all expectations, these latter dyes provided similar or better conversion efficiencies in PSSCs[2]when compared to the push-pull meso disubstituted ZnII-diarylporphyrinic counterparts. The kinetic fundamentals, which play the key role on interfacial charge separation and collection processes onto the TiO2 photoanode, were investigated by time-dependant spectroscopy and EIS analysis.[3]It was found that the unexpected better efficiency of the ßsubstituted tetrarylporphyrinic dyes cannot be ascribed to a better injection process but rather to the more hindered macrocyclic core which promotes a superior passivation of the TiO2 surface against charge recombination with the oxidized specie (I3-) of the electrolyte. Moreover, the panchromatic effect on IPCE spectra of ZnII-porphyrinic dyes, when a dithienylethylene (DTE) system was introduced in ß position through different ?-delocalized linkers, was also investigated.[4]Finally we also reported the differences in conversion efficiencies obtained when ß-substituted tetrarylporphyrinic dyes were wrapped by long alcoxy chains linked in various positions on the meso aryl rings.[5] The present contribution summarizes the main results obtained by our research group in the field of PSSCs, with a particular attention on the better, and unexpected, conversion efficiencies recorded for ß-substituted ZnII-porphyrinates with respect to those of meso disubstituted homologues.