Interfacial electron-transfer dynamics in Ru(tcterpy)(NCS)3-sensitized TiO2 nanocrystalline solar cells

The anchoring of the ruthenium dye {(C4H9)(4)N}[Ru(Htcterpy)(NCS)(3)] (with tcterpy = 4,4',4"-tricarboxy2,2':6',2"-terpyridine), the so-called black dye, onto nanocrystalline TiO2 films has been characterized by UV-vis and FT-IR spectroscopies. FT-IR spectroscopy data suggest that dye molecules are bound to the surface by a bidentate binuclear coordination mode. The interfacial electron-transfer (ET) 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, respectively. Nanosecond kinetic data suggest the existence of two distinguishable regimes (I and II) for the rates of reactions between injected electrons and oxidized dye molecules or oxidized redox species (D+ or I-2(.-)). The frontier between these two regimes is defined by the number of injected electrons per particle (Ne), which was determined to be about 1. The present kinetic study was undertaken within regime I (N-e > 1). Under these conditions, the back-electron-transfer kinetics is comparable to that in systems with other ruthenium complexes adsorbed onto TiO2. The reduction of oxidized dye molecules by iodide results in the formation of I-2(.-) on a very fast time scale (<20 ns). Within regime 1, the decay of I-2(.-) occurs in similar to100 ns via reaction with injected electrons (I-2(.-) + e(-) --> 2I(-)). In regime II (N-e less than or equal to 1), which corresponds to the normal operating conditions of dye-sensitized solar cells, the decay of I-2(.-) is very slow and likely occurs via the dismutation reaction (2I(2)(.-) --> I- + I-3(-)). Our results predict that, under high light intensity (N-e > 1), the quantum efficiency losses in dye-sensitized solar cells will be important because of the dramatic acceleration of the reaction between I-2(.-) and injected electrons. Mechanisms for the ET reactions involving injected electrons are proposed. The relevance of the present kinetic studies for dye-sensitized nanocrystalline solar cells is discussed.