Study of Electron Injection Efficiency of N719/TiO2 System in Different Room Temperature Ionic Liquid (IL) Environments By Using Femtosecond Transient Absorption Spectroscopy: Effect of Varying Viscosity

Introduction:

Use of room temperature ionic liquid (RTIL) instead of conventional solvents in dye sensitized solar cells (DSSCs) has attracted many attentions among scientific as well as technical communities because of its negligible vapor pressure, high boiling points, low flammability, high ionic conductivity and electrochemical stability. Electron injection efficiency from excited state dye to the conduction band (CB) of  semiconductor nanoparticles is one of the important elementary steps to determine the overall cell efficiency.

Experimental:

In this work, a series of room temperature ionic liquids (ILs) of varying viscosity have been employed as an environment for Ru-complex dye (N719) sensitized TiO₂ system to study electron injection efficiency using femtosecond transient absorption spectroscopy. The investigated ILs are (Figure 1): i) with [EMIm]⁺ cation and [TCB]^-, [SCN]^-, [TCM]^-, [DCA]^-, [FSA]^-, [BF₃CF₃]^-, [TFSA]^- and [TfO]^- as different anionic counterparts and ii) with [DMEIm]⁺ cation and [TFSA]^- anion (DTFSA) . These ILs are categorized as –CN ([TCB]^-, [SCN]^-, [TCM]^- and [DCA]^-) and –F ([FSA]^-, [BF₃CF₃]^-, [TFSA]^- and [TfO]^-and DTFSA) containing ILs. The details of the experimental procedure are described elsewhere [1].

Results adn discussions:

Figure 2 shows the transient time profiles of adsorbed N719/TiO₂ system in -CN and -F containing IL environments using  532 nm excitation femtosecond laser pulse and probed at 800 nm, which is the absorption band of the oxidized dye molecules (N719⁺). The trend of absorbance (ΔOD) value of N719/TiO₂in –CN containing IL environments at 1ns delay time is [TCB]>[SCN]>[TCM]>[DCA] which reflects the electron injection efficiency (Figure 2a). For –F containing IL environments, the trend of absorbance value (ΔOD) at 1ns delay is [FSA]>[BF₃CF₃]>[TFSA]>[TfO]>[DTFSA] (Figure 2b) which also reflects the electron injection efficiecny trends. It intrigues us that the trend of electron injection efficiency is completely different with respect to viscosity of these –CN and –F containing IL environments. It is observed that the electron injection efficiency increases with increasing viscosity for –CN containing IL environments, however reverse trend is found for -F containing IL environments (Figure. 3). Modulation of CB edge energy is the main factor for influencing the electron injection efficiency for –CN containing ILs [1] whereas orientation of the dye moleucles along TiO₂ nanoparticles changes the electronic coupling between the excited state of the dye moleucles and CB of TiO₂for -F containing ILs of N719/TiO₂ system.

Acknowledgement:

 This work is supported by NEDO Japan

 References:

[1] Mahanta, S. Furube, A. Matsuzaki, H. Murakami, T, Matsumoto, H. J. Phys. Chem. C 2012, 116, 20213.  [2] Pastore, M. Fantacci, S. Angelis, F. D. J. Phys. Chem. C 2013, 117, 3685.