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 (TiO 2) surfaces

Abstract

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 (TiO 2) 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% t BuOH solution of N719 Langmuirian adsorption occurs with well-defined mono-layer coverage and a binding constant ca. 2 × 10 5 mol −1 dm 3. The adsorbed N719 exhibits voltammetric oxidation/back-reduction responses with E mid ≈ 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 TiO 2 (porous films made from approximately 9 nm diameter TiO 2 particles, Langmuirian binding constant ca. 10 5 mol −1 dm 3), immersed in acetonitrile (0.1 M NBu 4PF 6), and at sufficiently fast scan rates (ca. 16 V s −1), the N719 metal complex exhibits reversible voltammetric responses (with E mid ≈ 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 TiO 2 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.