Interplay between Dye Coverage and Photovoltaic Performances of Dye-Sensitized Solar Cells Based on Organic Dyes

A high dye coverage on the TiO₂ surface diminishes dye-sensitized solar cell (DSSC) performances because of the significant degree of excited electron and hole recombination that results from the strong dye–dye intermolecular interactions. In this study, interplay between dye coverage and photovoltaic performances of DSSCs was systematically investigated and discussed using adsorption isotherms, photovoltaic measurements, and impedance analyses. Commercially available P25 and laboratory synthesized {010}-faceted TiO₂ nanoparticles were used in mesoporous electrodes, and MK-2 organic dye was used as a sensitizer. Estimated adsorption constant (<i>K</i>_ad) and maximum adsorption density (<i>Q</i>_m) were 1.03 × 10⁵ dm³/mol and 1.39 × 10^–6 mol/m² for P25 and 1.50 × 10⁵ dm³/mol and 8.62 × 10^–7 mol/m² for {010}-faceted TiO₂, respectively. The maximum TiO₂ surface coverage of about 60% with {010}-faceted TiO₂ and near 100% with P25 were observed in adsorption isotherms. <i>I</i>–<i>V</i> characteristics curves showed the continuous enhancement of open-circuit potential (<i>V</i>_oc) with increasing coverage by confirming its high dependency on coverage. The P25 cell exhibited the maximum short-circuit photocurrent density (<i>J</i>_sc) at 84% of coverage which corresponded to the optimum coverage of MK-2 dye. At the optimum coverage the distance between dye molecules was estimated as 1.2 nm. Compared with P25, {010}-faceted TiO₂ showed about 81% of <i>J</i>_sc and 75% of η enhancements although its maximum coverage (60%) was lower than the optimum coverage (84%). High performances of {010}-faceted TiO₂ can be explained by the effective conversion of the irradiated light to photocurrent by strongly adsorbed dye molecules on the {010}-facet