Simple oligothiophene-based dyes for dye-sensitized solar cells (DSSCs): Anchoring group effects on molecular properties and solar cell performance

Simple oligothiophene-based dyes with a cyanoacrylic acid anchor were synthesized and evaluated as a photosensitizer for dye-sensitized solar cells (DSSCs). Although the molecular properties of the dyes at a glance are quite similar to those of related oligothiophene dyes with a carboxylic acid anchor, the DSSC performance of the former (up to η = 4.8%) is fairly improved over the latter (1.2%). Absorption spectra of the oligothiophene dyes with the cyanoacrylic acid are red-shifted compared to those with the carboxylic acid, resulting in the improved photocurrent conversion efficiency at above 600nm in the DSSCs. With the aid of theoretical calculations, electronic properties of the oligothiophene dyes were investigated, and it turned out that the excited states of the dyes are significantly affected by the anchoring groups. The cyanoacrylic acid anchor brings about a stabilized LUMO with localized electron density on the cyanoacrylic acid moiety, whereas the carboxylic acid dye has relatively a high-lying LUMO with delocalized electron density over the oligothiophene moiety. Thus, the difference in the DSSC performance is rationalized by the different electronic structure at the excited states caused by the anchoring groups on the oligothiophene cores.