Dye-Sensitized Solar Cells using Fluorone-based Ionic Liquids with improved Cell Efficiency
Six trihexyltetradecylphosphonium chloride (P6,6,614Cl) 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 Erythrosine B (f). Spectroscopic characterization of these viscous salts indicated the presence of the expected 1-2 strong absorption bands. A total of 12 compounds, as sodium (from a to f) and trihexytetradecylphosphonium dianions (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/cm2 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 Eosyn B the presence of electron-donating groups and an auxiliary electron-withdrawing group, -NO2, allows reduction of the nitro group to -NO2-. This provides a new interaction between the nitro and the TiO2 surface, reflecting an improvement in the overall DSSC performance reaching its maximum of 0.85% 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 was achieved by simple cationic exchange.