Blocking the charge recombination with diiodide radicals by TiO₂ compact layer in dye-sensitized solar cells

The addition of a compact titanium dioxide (TiO2) layer between the fluorine-doped tin oxide (FTO) coated glass substrate and the mesoporous TiO₂ layer in the dye-sensitized solar cell (DSC) based on the iodide/triiodide redox couple (I⁻/I₃⁻) is known to improve its current-voltage characteristics. The compact layer decreases the recombination of electrons extracted through the FTO layer with I₃⁻ around the maximum power point. Furthermore, the short-circuit photocurrent was improved, which previously has been attributed to the improved light transmittance and/or better contact between TiO₂ and FTO. Here, we demonstrate that the compact TiO₂ layer has another beneficial effect: it blocks the reaction between charge carriers in the FTO and photogenerated diiodide radical species (I₂^−•). Using photomodulated voltammetry, it is demonstrated that the cathodic photocurrent found at bare FTO electrodes is blocked by the addition of a compact TiO₂ layer, while the anodic photocurrent due to reaction with I₂^−• is maintained.