**1. Introduction**

A dye-sensitized solar cell (DSSC) relies on the interaction of the sensitizer and the flow of electrons from the mediator to the sensitizer to complete the circuit and convert photo energy to electrical energy. As a result, the sensitizer-mediator interaction in every DSSC is critical to its efficiency and stability. The reduced and oxidized forms of the sensitizer, as well as the reduced and oxidized forms of the mediator, must have adequate stability in the reaction medium to ensure a successful electrontransfer process. Meanwhile, the solubility of the oxidized and reduced sensitizers and mediators is an important metric to consider while building a DSSC. A photoanode, sensitizer, mediator, solvent, and the counter electrode are all common components of a DSSC, and the electrolyte is sometimes employed as well.

Scientists and engineers prefer DSSCs over other first and second-generation solar cells, such as thin-film and silicon-based solar cells, since they are less expensive, more stable, and environmentally friendly. Because DSSCs' maximum efficiency is lower than that of first and second-generation solar cells, which is 12–14% under ideal conditions of materials and structure using Ru(II) dyes; ruthenium in the 2+ oxidation state, compared to 20–30% for the latter two types of cells, increasing their efficiency in a cost-effective and environmentally benign manner is still a hot topic. As a result, there is still a lot to learn about DSSCs and how to increase their efficiency, stability, and longevity by employing better conditions, materials, and structures.

Researchers have examined a number of materials for the photoanode and counter electrode to improve the efficacy of a DSSC, including nanomaterials, their composites, and nanocomposites of the same materials with varied morphologies [1–5]. In this investigation, heteroatom-doped graphene catalysts were used [6]. Heterogeneous FeNi3/NiFe2O4 nanoparticles with modified graphene were also explored as electrocatalysts for dye-sensitized solar cells [7]. Studies on making ecologically acceptable and stable DSSCs using natural dyes and NHC-based iron as sensitizers [8–10], copper and other transition metal-based mediators [11, 12], and electrolytes based on gels and polymers [13–17] have been published. The steric, structural, and compositional effects of solvent on the efficiency of DSSC have also been investigated electrochemically [15, 18].

The use of potential Fe-based sensitizers and mediators has been revealed in this chapter based on their comparative kinetic study. The rate of the electron-transfer reactions is influenced by the structure of the sensitizer that has been discussed in this chapter. Iron-based sensitizers are less expensive and environmentally benign than ruthenium-based sensitizers, making them attractive to the socio-economic impact. The solubility of iron-based sensitizers in an aqueous medium, as well as the use of an aqueous medium rather than inflammable, volatile, poisonous, and expensive chemical solvents, are two further advantages. Iodate is produced by the electrolytic solution of iodide/triiodide, which is corrosive to stainless steel and a source of DSSC instability. Another approach to a stable DSSC is the use of an iron-based coordination complex such as ferrocyanide as a mediator that has one electron transfer chemistry and comparable redox potential to iodide/triiodide electrolyte. The one-electron transfer chemistry helps to reduce the recombination losses.

The effect of structure, such as extended π-conjugation, on the rate of the electrontransfer between the sensitizer-mediator in an aqueous medium, is the discussion in this chapter. Ferricyphen and ferricypyr are the names for dicyanobis(1,10 phenanthroline)iron(III) and dicyanobis(2,2<sup>0</sup> -dipyridyl)iron(III), respectively, where "ferri" stands for Fe(III) oxidation state, "cy" for cyanide, and "phen" or "pyr" for the chelate. Reduced variants are known as ferrocyphen and ferrocypyr. Both ferricyphen and ferricypyr are substitution inert outer sphere oxidants with octahedral geometry and similar Fe(III) transition metal coordination sites (**Figure 1**). Their reduction potentials are 0.80 V and 0.76 V, respectively, however, they were initially synthesized in the 1960s [19–22]. They are potential sensitizers because of their photosensitive nature and their solubility in an aqueous medium in the oxidized form and comparatively low solubility in the reduced form which may be helpful for their adsorption on the photoanode. Each of the potential sensitizers easily oxidizes the selected potential mediator such as ferrocyanide in an aqueous medium without the need for any external triggering to initiate the reaction. Each of the redox reactions starts spontaneously after mixing the aqueous solutions of both reactants such as ferricyphen-ferrocyanide or ferricypyr-ferrocyanide. The reduction potential of ferrocyanide is comparable to the iodide electrolyte, hence displays its replacement over iodide. The oxidation of iodide by ferricyphen and ferricypyr has been studied in

*Catalytic Behavior of Extended π-Conjugation in the Kinetics of Sensitizer-Mediator… DOI: http://dx.doi.org/10.5772/intechopen.106511*

**Figure 1.** *(a) Dicyanobis(1,10-phenanthroline)iron(III). (b) Dicyanobis(2,2*<sup>0</sup> *-dipyridyl)iron(III).*

acetonitrile, aqueous tertiary butyl alcohol, and aqueous 1,4-dioxane [23–26]. The comparative kinetic analysis shows the rapid kinetics of ferricyphen over ferricypyr in binary solvent media under optimized experimental conditions. The following section of the chapter will help to identify the role of pi-conjugation in reaction kinetics.
