Interfacial Electron-Transfer Dynamics in Ru(tcterpy)(NCS)3-Sensitized TiO2 Nanocrystalline Solar Cells

The anchoring of the ruthenium dye {(C4H9)4N}[Ru(Htcterpy)(NCS)3] (tcterpy = 4,4',4''-tricarboxy-2,2':6',2''-terpyridine), the so-called black dye, onto nanocryst. TiO2 films has been characterized by UV-visible and FTIR spectroscopies. FTIR spectroscopy data suggest that dye mols. are bound to the surface by a bidentate binuclear coordination mode. The interfacial electron transfer dynamics has been investigated by femtosecond pump-probe transient absorption spectroscopy and nanosecond laser flash photolysis. The electron injection process from the dye excited state into the TiO2 conduction band is biexponential with a fast component (200 ± 50 fs) and a slow component (20 ps). These two components can be attributed to the electron injection from the initially formed and the relaxed dye excited states, resp. Nanosecond kinetic data suggest the existence of two distinguishable regimes (I and II) for the rates of reactions between injected electrons and oxidized dye mols. or oxidized redox species (D+ or I2•-). The frontier between these two regimes is defined by the no. of injected electrons per particle (Ne), which was detd. to be about 1. The present kinetic study was undertaken within regime I (Ne > 1). Under these conditions, the back-electron-transfer kinetics is comparable to that in systems with other ruthenium complexes adsorbed onto TiO2. The redn. of oxidized dye mols. by iodide results in the formation of I2•- on a very fast time scale ( 1), the quantum efficiency losses in dye-sensitized solar cells will be important because of the dramatic acceleration of the reaction between I2•- and injected electrons. Mechanisms for the electron transfer reactions involving injected electrons are proposed. The relevance of the present kinetic studies for dye-sensitized nanocryst. solar cells is discussed.