# 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 Dyes  
In a dye-sensitized solar cell, the dye is absorbed in the semiconductor layer. The dye in the cell has a crucial function due to its enhancement of the ability to generate electrons and holes enabling charge transport [18]. In order to do so, the dyes used in solar cells should [19]:  
* Absorb electromagnetic radiation in the visible light range to the greatest possible extent;
* Have energy levels matching the energy levels of the redox mediator and the conduction band edge of TiO\({}_{2}\), so as to ensure efficient electron transport;
* Be chemically accessible and (photo)chemically stable. Currently, copper compounds in DSSCs are widely employed as dyes. Depending on its oxidation level from 0 to +4, a copper atom can coordinate different amounts of ligands, with the resultant coordination compounds exhibiting various geometries. Most commonly, coordination compounds of Cu\({}^{+}\) and Cu\({}^{2+}\) are encountered, exhibiting tetrahedral and linear, trigonal or tetragonal geometries, respectively. Among such coordination compounds, polymeric species formed on the framework of cuprous iodide with organic ligands, such as methyl nicotinate [20], have been reported to have potential application in solar cells as down-shifters alternative to lanthanides [21]. A recent study on copper compounds with 2,9-disubstituted-1,10-phenanthroline ligands--**3**-**7** (Figure 2) used in DSSCs indicated that high conversion efficiencies ranging from 6 to 11% with high V\({}_{\text{OC}}\) approaching above 1 V could be achieved [22]. Particularly, complexes based on Cu\({}_{2}\)I\({}_{2}\) and **39** posses double chains such as CuI-**39** and Cu\({}_{2}\)I\({}_{2}\)-**39**. Selected photovoltaic parameters of the DSSC compounds are shown in the Table (Table 1). The examples of Cu-based complexes were shown in the figure below (Figure 2):  
Figure 1: Processes occurring in DSSC: (1) electron injection, (2) dye regeneration, (3) recombination by electrolyte and (4) dye. Reprinted from [1] with permission from John Wiley and Sons. One interesting example of dye-sensitized solar cells is the use of alizarin yellow GG-CuCl\({}_{2}\) mixed with polyvinyl alcohol/ITO as a thin film. The main advantage of this type of cells is their low cost of fabrication by low-temperature processes, and they can be used in various types of sensors such as NO\({}_{2}\) sensors or temperature sensors. The use of polyvinyl alcohol with azo dyes improves the physical properties with good flexibility of the film, providing it with electrical and dielectric properties in an organic cell. The work [24] shows the basic properties of an alizarin yellow GG-CuCl\({}_{2}\) cell blended with polyvinyl alcohol that changes depending on the light color used. The reported cell shows the highest efficiency (Table 2) when exposed to green light. Furthermore, it appears that the electrical resistivity of the film is not temperature-dependent [24]. The low efficiency of this type of photocells may be explained by the formation of a limited number of holes and electrons as a result of exception splitting at the phase boundary or as a consequence of excitons encountering impurities in the sample and, as a \begin{table}
\begin{tabular}{c c c c c c} \hline
**Electrolyte** & **Dye** & **J\({}_{\text{SC}}\) (mA\(\cdot\)cm\({}^{-2}\))** & **V\({}_{\text{OC}}\) (V)** & **FF (\%)** & **PCE (\%)** \\ \hline
3 & LEG4 & 10.00 & 1.03 & 60.9 & 6.29 \\
4 & LEG4 & 10.06 & 0.84 & 66.4 & 5.90 \\
5 & LEG4 & 7.52 & 0.82 & 52.6 & 3.25 \\
6 & LEG4 & 6.77 & 0.81 & 46.4 & 2.56 \\
7 & LEG4 & 4.18 & 0.85 & 62.4 & 2.21 \\
8 & Y123 & 9.75 & 0.92 & 69 & 6.20 \\
9 & D203 & 6.12 & 0.92 & 82 & 4.60 \\
9 & D205 Si & 6.40 & 0.95 & 78 & 4.75 \\
9 & WS-72 & 13.8 & 1.07 & 79 & 11.7 \\ \hline \end{tabular}
\end{table} 
Table 1: Selected photovoltaic parameters of DSSCs of electrolyte/dye compositions. Reprinted from [23] with the permission of the Royal Society of Chemistry. Figure 2: Examples of Cu coordination compounds envisioned for application in DSSCs.