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 (<i>A</i>₁ and <i>A</i>₂) were controlled by the rate coefficient of the ¹MLCT → ³MLCT intersystem crossing; the variations of <i>A</i>₁ and <i>A</i>₂ 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.