Effect of side chain substituents on the electron injection abilities of unsymmetrical perylene diimide dyes

Three near-infrared (NIR) absorbing unsymmetrical perylene diimide D-A-D type dyes containing 6-undecanoxy as donor group were utilized in dye-sensitized nanocrystalline TiO₂ solar cells. Structure of the acceptor side of the molecules were improved by adding 4-[2-methyl-5-(cyanoacrylic acid)-3-thienyl]-phenyl (V), 3-carboxy-2-pyridil (VI) and 3-carboxy-2-pyrazyl (VII) moieties attached to one of the N-side of the dye. The relationship between the molecular structure of the acceptor sites of the dyes and the photovoltaic performances were discussed. Electrochemical measurements indicated that band gaps of the dyes were energetically favorable for electron injection from the excited state of the dyes to the conduction band of TiO₂ nanoparticles. However, three dyes gave lower conversion efficiency on DSSC applications. Strong electron-withdrawing nature of perylene core might not permit to transfer the photo-generated electrons to the carboxyl groups anchoring to TiO₂ surface, and then solar-to-electricity conversion efficiencies of the dyes were reduced.