Photoelectrochemical properties of doubly β-functionalized porphyrin sensitizers for dye-sensitized nanocrystalline-TiO₂ solar cells

Functionalized porphyrins at meso- and β-positions with different carboxylic acid groups were prepared to investigate electronic and photovoltaic properties as dye-sensitized nanocrystalline-TiO₂ solar cells. The electronic structures of the porphyrin macrocyclic core are strongly coupled with olefinic side chains so that the absorption spectrum exhibits largely broad and red-shifted Soret and Q-bands, especially up to 475 nm at the Soret band in a porphyrin doubly functionalized with malonic diacid groups. Among porphyrin derivatives prepared in this study, 2b-bdta-Zn exhibits the maximum overall conversion efficiency of 3.03% and the maximum incident photon to current efficiency of 60.1 % in the Soret band region, superior to the others. From such photovoltaic performances, we can suggest that multiple pathways through olefinic side chains at two β-positions enhance the overall electron injection efficiency and the moderate distance between the porphyrin sensitizer and the TiO₂ semiconductor layer is important, retarding the charge recombination processes. As a consequence, these combined effects give rise to higher photovoltaic efficiency in photovoltaic regenerative solar cells.