Thiocyanate-free ruthenium(II) cyclometalated complexes containing uncommon thiazole and benzothiazole chromophores for dye-sensitized solar cells

Four novel thiocyanate-free ruthenium(II) complexes based on thiazole and benzothiazole cyclometalating ligands were synthesized, characterized and applied in dye-sensitized solar cells (DSSCs). Their photophysical, electrochemical, thermal and photovoltaic properties have been investigated and the experimental results can be correlated well with the computational results obtained by density functional theory (DFT) calculations. Based on the DFT results, the HOMO of the complexes is localized on the electron-donating carbazole or triphenylamine unit whereas the LUMO mainly lies on 4,4′-dicarboxyl-2,2′-bipyridine. These dyes exhibit good thermal stability with the onset decomposition temperature at 5% weight-loss (Td) over 360 C. For the DSSC device using the dye with the benzothiazole/carbazole-based cyclometalating ligand, the power conversion efficiency (η) can reach up to 2.98%, with a short-circuit photocurrent density (Jsc) of 6.25 mA cm-2, an open-circuit photovoltage (Voc) of 0.659 V and a high fill factor (ff) of 0.724 under illumination of an AM 1.5 solar cell simulator.