Cis-bis(isothiocyanato)-bis(2,2′-bipyridyl-4,4′dicarboxylato)-Ru(II) (N719) dark-reactivity when bound to fluorine-doped tin oxide (FTO) or titanium dioxide (TiO2) surfaces

The solar cell sensitizer cis-bis(isothiocyanato)-bis(2,2′-bipyridyl-4,4′dicarboxylato)-ruthenium(II) (N719) is adsorbed and investigated at two electrode surfaces: (i) at a bare fluorine-doped tin oxide (FTO) and (ii) at a nano-particulate anatase (TiO2) film in contact with FTO. N719 is adsorbed from acetonitrile onto FTO surfaces giving poor quality partial or multi-layer coverage commencing at 10−7 M concentration. In contrast, from 50% acetonitrile 50% tBuOH solution of N719 Langmuirian adsorption occurs with well-defined mono-layer coverage and a binding constant ca. 2 × 105 mol−1 dm3. The adsorbed N719 exhibits voltammetric oxidation/back-reduction responses with Emid ≈ 0.56 (weaker) and 0.68 (dominant) V vs. Ag/AgCl (3 M KCl) and with chemically reversible characteristics at sufficiently high scan rates (ca. 16 V s−1). A chemical reaction step involving oxidised N719 at the electrode surface leads to the loss of electrochemical activity at slower scan rates with a first order chemical rate constant of ca. 2.4 s−1. The electro-catalytic oxidation of iodide is demonstrated for both the intact metal complex N719 and the reaction product formed after oxidation. When adsorbed onto TiO2 (porous films made from approximately 9 nm diameter TiO2 particles, Langmuirian binding constant ca. 105 mol−1 dm3), immersed in acetonitrile (0.1 M NBu4PF6), and at sufficiently fast scan rates (ca. 16 V s−1), the N719 metal complex exhibits reversible voltammetric responses (with Emid ≈ 0.68 V vs. Ag/AgCl (3 M KCl)). At slower scan rates, the voltammetric response again appears irreversible, however, this time without significant degradation of the N719 metal complex at the TiO2 surface. It is shown that the conduction mechanism via electron hopping becomes ineffective due to degradation of FTO-adsorbed N719. In the presence of iodide, the electro-catalytic iodide oxidation process (dark electro-catalysis) is shown to occur predominantly at the N719-modified FTO electrode surface. Implications of this dark-reactivity for the solar cell performance are discussed.