Electrolyte Tuning in Iron(II)-Based Dye-Sensitized Solar Cells: Different Ionic Liquids and I(2) Concentrations

The effects of different I(2) concentrations and different ionic liquids (ILs) in the electrolyte on the performances of dye-sensitized solar cells (DSCs) containing an iron(II) N-heterocyclic carbene dye and containing the I(–)/I(3)(–) redox shuttle have been investigated. Either no I(2) was added to the electrolyte, or the initial I(2) concentrations were 0.02, 0.05, 0.10, and 0.20 M. The short-circuit current density (J(SC)), open-circuit voltage (V(OC)), and the fill factor (ff) were influenced by changes in the I(2) concentration for all the ILs. For 1-hexyl-3-methylimidazole iodide (HMII), low V(OC) and low ff values led to poor DSC performances. Electrochemical impedance spectroscopy (EIS) showed the causes to be increased electrolyte diffusion resistance and charge transfer resistance at the counter electrode. DSCs containing 1,3-dimethylimidazole iodide (DMII) and 1-ethyl-3-methylimidazole iodide (EMII) showed the highest J(SC) values when 0.10 M I(2) was present initially. Short alkyl substituents (Me and Et) were more beneficial than longer chains. The lowest values of the transport resistance in the photoanode semiconductor were found for DMII, EMII, and 1-propyl-2,3-dimethylimidazole iodide (PDMII) when no I(2) was added to the initial electrolyte, or when [I(2)] was less than 0.05 M. Higher [I(2)] led to decreases in the diffusion resistance in the electrolyte and the counter electrode resistance. The electron lifetime and diffusion length depended upon the [I(2)]. Overall, DMII was the most beneficial IL. A combination of DMII and 0.1 M I(2) in the electrolyte produced the best performing DSCs with an average maximum photoconversion efficiency of 0.65% for a series of fully-masked cells.