Electron and hole transfer dynamics of a triarylamine-based dye with peripheral hole acceptors on TiO2 in the absence and presence of solvent

Abstract

We investigated photoinduced primary charge transfer processes of the sensitizer E6 on TiO₂ without solvent and in contact with the organic solvent acetonitrile and the ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate [C₂mim]⁺[B(CN)₄]⁻ using transient absorption spectroscopy, spectroelectrochemistry, and DFT/TDDFT calculations. E6, which belongs to a family of triarylamine dyes for solar cell applications, features two peripheral triarylamine units which are connected via diether spacer groups to the core chromophore and are designed to act as hole traps. This function was confirmed by spectroelectrochemistry, where the E6˙⁺ radical cation shows a considerably blue-shifted absorption compared to dyes without these two substituents. This indicates that one of the terminal triarylamine units must carry the positive charge. After photoexcitation of E6 at 520 nm (S₀ → S₁ band), electrons are injected into TiO₂ predominantly within the cross-correlation time (<80 fs), with some subsequent delayed electron injection (τ ca. 250 fs). Importantly, a transient Stark shift (electrochromism) is observed (time constants ca. 0.8 and 12 ps) which is related to a changing electric field generated by the E6˙⁺ radical cations and injected electrons. This field induces absorption shifts of the dye species on the surface. Interestingly, these dynamics are largely unaffected by solvent molecules. However, pronounced differences are observed on longer timescales. In contact with solvent, one observes an increase in the E6˙⁺ absorption band above 600 nm with a time constant of 75 ps. This is assigned to hole transfer from the core chromophore to one of the peripheral triarylamine substituents. Electron–cation recombination occurs on much longer timescales and is multiexponential, with time constants of ca. 100 μs, 1 ms and 15 ms. Because of hole trapping, it is slower than for similar dyes lacking the peripheral triarylamines. Additional experiments were performed for E6 attached to the wide band gap semiconductor ZrO₂. Here, electron injection occurs into surface trap states with subsequent recombination. Another fraction of non-injecting E6 molecules in S₁ quickly decays to S₀ (time constants 1 and 35 ps).