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.