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.

► We synthesize NIR absorbing perylene dyes showing good light harvesting properties.
► Pyridine and pyrazine structures in PDI improve in dye absorption of TiO2 electrode.
► Polarizability of the dyes focused on perylene core reduce electron transfer to TiO2.