# Transition Metal Coordination Compounds as Novel Materials for Dye-Sensitized Solar Cells
## 2 Characterization and Principle of Operation of Dye-Sensitized Solar Cells
### Compounds Used as Redox Mediators  
Recent literature reports indicate that electrolytes containing copper or cobalt ions together with a suitable dye, e.g., N3 or N719, can be used in addition to the previously dis  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Dye** & **J\({}_{\mathbf{SC}}\) (mA\(\cdot\)cm\({}^{-2}\))** & **V\({}_{\mathbf{OC}}\) (V)** & **FF (\%)** & **PCE (\%)** \\ \hline
**12** & 5.63 \(\pm\) 0.30 & 0.76 \(\pm\) 0.01 & 0.66 \(\pm\) 0.1 & 2.83 \(\pm\) 0.12 \\
**13** & 6.59 \(\pm\) 0.28 & 0.74 \(\pm\) 0.02 & 0.65 \(\pm\) 0.1 & 3.17 \(\pm\) 0.12 \\
**14** & 8.02 \(\pm\) 0.21 & 0.71 \(\pm\)cussed electrolytes used in DSSCs devices. It seems important to select the appropriate ligand together with the metal considering their effect on the redox potential. Some examples of electrolytes can be compounds 1-substituted with 2-(pyridin-2-yl)-1\(H\)-benzo[\(d\)]imidazole ligands, such as in 37-38 (Figure 5). By simplified alkylation of the NH\({}_{2}\)-(2-pyridyl)benzimidazole group, this group may be capable of attaching various substituents to the metal-dimine core. The paper [36] provides information about the use of copper-based compounds in liquid DSSCs with ruthenium dyes, allowing conversion efficiencies of about 5% (4.99 and 4.82%)--complexes of 4-nitro-benzyl-substituted 2-(pyridin-2-yl)-1\(H\)-benzo[\(d\)]-imidazole ligands [36]. Coordination compounds of copper can serve as redox mediators. An example of copper-containing coordination compounds are those involving bipyridine-bearing ligands, such as in 40-43 (Figure 5) [37]. By introducing alkoxy groups into them, it was possible to reach power conversion efficiency of more than 10% [30]. Currently, bidentate ligands, i.e., pyridine and phenanthroline derivatives, are a major group of investigated redox mediator [38]. These compounds serving as a redox mediator in DSSCs (in combination with Y123 dye), due to their higher solubility, lead to a rise in concentration in the solar cells up to 0.3 M, causing an increase in the photocurrent. The following table shows the basic photovoltaic parameters of the above-mentioned redox mediators (Table 3). Figure 5: Structures of ligands and coordination compounds used in DSSCs. Reprinted from [30] with permission of Elsevier. The work [38] shows further examples of copper-containing complex compounds with applications as redox mediators are compounds with diamino-tripyridine ligands **44** and **45**. The chemical structure of the ligands is presented in the following graph (Figure 5). The systems based on **46** depending on the DSSCs liquid or solid state attained maximum efficiencies of 13.5% and 11% [38]. The work of [39] reports the use of Cu compounds with the phenanthroline ligand and LEG4 dye as redox mediators in electrolyte solution, enabling them to achieve a conversion rate of 8.3% (Voc = 1020 mV, Jsc = 12.6 mA\({}^{-2}\), CE-PEDOT) [39]. Regarding the available redox pairs, transition metal coordination compounds are often used as cobalt-based redox pairs due to their several advantages. These complexes are non-corrosive, and in addition, they exhibit more positive potentials than the \(\Gamma^{-}\)/I\({}^{3-}\) redox pair, which can increase the value of the Voc potential [40]. Such compounds are used for DSSCs, and the literature reports that the efficiency of solar cells using cobalt compounds increased up to 13% (Co\({}^{2+/3+}\) redox couple). These cobalt compounds can be divided based on the type of ligand. These are [41]:  
1. Co\({}^{2+/3+}\) tris(bipyridine) complexes;
2. Co\({}^{2+/3+}\) bis(terpyridine) complexes;
3. Co\({}^{2+/3+}\) (tris)phenanthroline complexes;
4. Co\({}^{2+/3+}\) bis(benzimidazole-2-yl)pyridine complexes;
5. Co\({}^{2+/3+}\) pyrazole-based complexes;
6. Co\({}^{2+/3+}\) complexes with pentadentate ligands;
7. Co\({}^{2+/3+}\) complexes with non-pyridyl ligands. Examples of transition metal coordination compounds containing cobalt are shown in the figure below (Figure 6).