Femtosecond Infrared Transient Absorption Dynamics of Benzimidazole-Based Ruthenium Complexes on TiO2 Films for Dye-Sensitized Solar Cells

By means of femtosecond infrared transient absorption spectra, we measured the interfacial electron-transfer dynamics for benzimidazole-based heteroleptic ruthenium dyes (RD5, RD12, RD15–RD18) sensitized on TiO2 thin films. For all measurements, the first singlet metal-to-ligand charge-transfer states (1MLCT) of the ruthenium complexes were excited at 519 nm and the injected electrons in the conduction band of TiO2 were probed at 4.3 μm. All transient signals featured two rising components on a femtosecond–picosecond scale due to a two-step electron injection and an offset (N719, RD16–RD18) or a slow-decay (RD5, RD12, and RD15) component on a nanosecond–microsecond scale due to a back electron transfer. A complicated two-step kinetic model was derived analytically to interpret the observed two rising components for which the rapid (τ1 < 300 fs) and slow (τ2 = 10–20 ps) electron injections arose from the singlet 1MLCT and triplet 3MLCT states, respectively. The amplitudes of the two electron-injection components (A1 and A2) were controlled by the rate coefficient of the 1MLCT → 3MLCT intersystem crossing; the variations of A1 and A2 are consistent with the trend of the corresponding Stokes shifts rationalized with a conventional energy-gap law for nonradiative transitions. Compared with the kinetics observed for the N719 dye, the involvement of a benzimidazole ligand in RD dyes had the effect of accelerating the two electron injections, thus improving the short circuit current of the device. RD dyes substituted with fluorine atoms and/or thiophene units in the benzimidazole ligands showed a retardation of 3MLCT electron injection relative to that of the nonsubstituted RD5 dye. Acceleration of the BET process was observed for the RD5 dye (9 ns), and both fluoro-substituted dyes (14 ns for RD12 and 21 ns for RD15) and thiophene-substituted dyes (nonobservable for RD16–RD18) had significantly retarded BET kinetics. The observed kinetics of the 3MLCT electron injection for all RD dyes is satisfactorily simulated with the Marcus theory.