**2.3 Dye/electrolyte interface**

In order to elucidate DSSC working phenomenon, the physical kinetics and dynamics of charge transfer motion have been investigated in detail by many researchers, both experimentally and via computational modeling. Here, the electron transfer dynamics taking place at the oxide/dye/electrolyte interface for DSSCs are presented in **Figure 2** [7–9]. This figure also contains the information for several competing loss pathways, shown as red arrows. These loss pathways include decay of the dye excited state to ground, and charge recombination of injected electrons

*State diagram of kinetics and energetics of a traditional DSSC. The positive charge transfer processes are indicated by blue color. The competing loss pathways of recombination with TiO2 trap sites and dye surface are*

*Schematic diagram and principle of operation of proto-typed dye sensitized solar cells (inserted in the*

whith dye cations and with the redox couple.

*Solar Cells - Theory, Materials and Recent Advances*

**Figure 1.**

**Figure 2.**

**188**

*shown in red. Reprinted from [8, 10].*

*nanocrystalline TiO2 film).*

After ultrafast electron injection from the photoexcited dye into the conduction band of TiO2, the dye is in its oxidized state and must be reduced by an electron donor in the electrolyte for regeneration. The standard electron donor is iodide. For many typed dye, high value of regeneration efficiency, which give the probability that an oxidized dye is regenerated by an electron donor in the electrolyte rather than by recombination with an electron in the TiO2, have been estimated in iodide. The reaction mechanism from reduction of oxidized sensitizer (D<sup>+</sup> ) by iodide follows:

$$\mathbf{D}^+ + \mathbf{I}^- \to (\mathbf{D} \cdots \mathbf{I}) \tag{6}$$

$$(\mathbf{D}\cdots\mathbf{I}) + \mathbf{I}^- \to (\mathbf{D}\cdots\mathbf{I}\_2 - \bullet) \tag{7}$$

$$(\mathbf{D}\cdots\mathbf{I}\_2\mathbf{\!^{\star\star}}) \to \mathbf{D} + \mathbf{I}\_2 - \mathbf{\!^{\star\star}} \tag{8}$$

$$\text{I}\_2-\bullet \rightarrow \text{I}\_3^- + \text{I}^- \tag{9}$$

Eq. (6) is most likely a one-electron transfer reaction between D<sup>+</sup> and I�. (ε (I• /I�) = +1.33 V vs. NHE in aqueous solution.) The dye regeneration is firstly occured from the formation of a (D���I) complex since the redox potential (*ε*) of the iodine radical reacted to the sensitizer (ε (D���I) = +1.23 V vs. NHE in acetonitrile) shows a relatively lower potential of 0.1 V. The dye regeneration is firstly occured from the formation of a (D���I) complex since the redox potential (*ε*) of the iodine radical reacted to the sensitizer (ε (D���I) = +1.23 V vs. NHE in acetonitrile) shows a lower potential of 0.1 V. Dissociation of ground-state dye D and I2 �• is secondly occurred from the formation of a (D���I2 �• ) complex.Finally, I2 �• is by the composition of triiodide and iodide. The second-order rate constant for this reaction is about 2.3 � <sup>10</sup><sup>10</sup> <sup>M</sup>�<sup>1</sup> <sup>s</sup> �<sup>1</sup> in acetonitrile.

The oxidized sensitizer (specially, the most commonly reported Ru complex dye, named as N719) is regeneated by the iodide/iodine redox couple in the liquid electrolyte in the range of 100 ns to 10 μs. The different regenation time can be explained by iodide concentration, the presence of additivies such as identity of caion salt (lithium ions) and tertbutylpyridine (tBP). To be the effective DSSC, the recombination process of electrons with either oxidized dye molecules or acceptors in the electrolyte must be minimized compared with regeneration process, which usually happens on a time scale of about 1 μs [7, 13–15].
