Structural design of ruthenium sensitizer compatible with cobalt electrolyte for a dye-sensitized solar cell

Abstract

Two heteroleptic ruthenium complexes, SJW-B18 and CYC-B11H, with four alkyl groups and two alkyl groups, respectively, on the terminal of their ancillary ligand were prepared. These two complexes have a similar absorption profile, energy level and orbital distribution of the frontier orbitals. Therefore, they are good model compounds to explore the effect of the molecular structure of a ruthenium sensitizer on their photovoltaic performance, especially when cobalt electrolyte was used in assembling the device. When I⁻/I₃⁻ electrolyte was employed, the dye-sensitized solar cell based on SJW-B18 and CYC-B11H sensitizers achieved similar power conversion efficiencies of 9.54% and 9.64%, respectively, because the two dyes have similar optical and electronic properties. Nevertheless, when Co^(II)/(III) electrolyte was used, the SJW-B18 sensitized device has a higher efficiency (7.30%) than the CYC-B11H-based cell (6.65%), mainly in higher open-circuit voltage (0.829 V vs. 0.784 V). Electrochemical impedance and intensity modulated photocurrent/photovoltage spectroscopic data reveal that the SJW-B18 dyed titanium dioxide (TiO₂) anode has better surface protection compared to the CYC-B11H adsorbed TiO₂ film. The physical insulation of the TiO₂ surface with the four alkyl groups on the SJW-B18 dye can reduce the charge recombination between the electron on TiO₂ and Co^(III) to increase the open circuit voltage, and the power conversion efficiency of the device. These results provide a new strategy for designing ruthenium sensitizers which are compatible with a cobalt-based electrolyte.