Dye-sensitized solar cells using fluorone-based ionic liquids with improved cell efficiency

Six trihexyltetradecylphosphonium chloride (P_6,6,6,14Cl) based ionic liquids (IL) with dianionic fluorone derivatives were synthesized with total exchange of chloride from the dianionic dye: Fluorescein (a), Rose Bengal (b), Phloxine B (c), Eosin B (d), Eosin Y (e) and Erythrosin B (f). Spectroscopic characterization of these viscous salts indicated the presence of the expected 1 or 2 strong absorption bands. A total of 12 compounds, as sodium (from a to f) or as trihexyltetradecylphosphonium dianion salts (from a′ to f′), were used for sensitization of nanocrystalline TiO₂. Here, we report the sensitization activity of these metal free dyes in terms of current-potential curve, open-circuit potential, fill factor, and overall solar energy conversion efficiency which have been evaluated under 100 mW cm^-2 light intensity. We developed a strategy to improve the light harvesting of these conventional dyes by simple cationic exchange which was accompanied by a minimum of 30% increase in the cell photovoltaic conversion efficiency. Also, for Eosin B the binding to TiO₂ apparently allows reduction of the-NO₂ electron-withdrawing group to-NO₂^2-. This provides a new interaction between the reduced nitro group and the TiO₂ surface, reflecting an improvement in the overall DSSC performance reaching its maximum of 0.65% efficiency after light DSSC soaking. Factors that improve DSSC performance like aggregate inhibition, increment of the electrode's quasi-Fermi level and slight red shift in the absorption spectra of the tested anionic dyes were achieved by simple cationic exchange.