A Comparison of Carboxypyridine Isomers as Sensitizers for Dye-Sensitized Solar Cells: Assessment of Device Efficiency and Stability

Three novel organic dyes (DF13A-C) carrying regioisomeric carboxypyridine anchoring groups were synthesized by means of a multistep synthetic sequence involving a Pd-catalyzed Stille coupling as the key step. The new compounds underwent full spectroscopic, electrochemical, and computational characterization, and their properties were compared with those of a reference compound endowed with a classic cyanoacrylic acid acceptor (DF15). Photovoltaic measurements showed that dye-sensitized solar cells built with dyes DF13A-C as photosensitizers yielded power conversion efficiencies corresponding to 54-63% of those obtained with the reference compound. Determination of desorption pseudo-first order rate constants indicated that isomers DF13B-C, having the nitrogen atom in neighboring position relative to the carboxylic moiety, were removed from TiO2 more slowly than isomer DF13A or cyanoacrylic derivative DF15, suggesting a possible cooperative effect of the two functional groups on semiconductor binding: such hypothesis was supported by device stability tests carried out on transparent, larger area cells.