Nitroxide radicals for highly efficient redox mediation in dye-sensitized solar cells

A robust nitroxide radical, 2,2,6,6-tetramethylpiperidine-loxyl (TEMPO) and its 4-subsituted derivatives (R-TEMPO) exhibit electrochemically reversible redox properties with tunable half-wave potentials (£1/2) based on the substituent effect. Dye-sensitized solar cells using R-TEMPO as mediators with electron-withdrawing R groups showed significantly enhanced photovoltages compared with those obtained using iodides. The linearity between the open-circuit voltage (Vac) and £i/2 revealed that V00 was dominated by the energy gap between the Fermi level of TiO2 and the SOMO level of R-TEMPO.