Molecular design and synthesis of ruthenium(II) sensitizers for highly efficient dye-sensitized solar cells

We synthesized hydrophobic ruthenium(II) sensitizers (SY-04 and SY-05) with high molar extinction coefficient by extending the pi-conjugation of 3,4- or 3-alkylthiophene-substituted bipyridine ligands. Both dyes displayed a remarkably high molar extinction coefficient of 21.7 x 10(3) M-1 cm(-1) arising from red-shift of their metal-to-ligand charge transfer band when compared to a commonly used N3 sensitizer. The solar-to-electrical energy conversion efficiency (eta) of the SY-04 based dye-sensitized solar cell (DSC) was 7.70%, which is 27% higher than that (6.05%) of the N3-based DSC under the same cell fabrication conditions. The increased eta was attributed to the increase in life time and recombination half-life measured by electrochemical impedance and transition absorption spectroscopy, respectively. Density functional theory and time-dependent density functional theory calculations of two dyes in both gas phase and solution were performed. The calculated values of the highest occupied and the lowest unoccupied molecular orbitals and absorption spectra are in good agreement with the experimental results.