# 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
Reprinted from [17] under a Creative Commons CC-BY license and from [23] with the permission of the Royal Society of Chemistry. consequence, shortening their lifetime [24]. Another example of a copper-containing dye is C218 (including tris(1,10-phenanthroline)) with a 6.5 um TiO\({}_{2}\) layer. Its efficiency is 7.0% at V\({}_{\text{OC}}\) 0.93 V [25]. According to most recent literature reports, there are substitutes for inorganic dyes employed in cells as sensitizers based on organic dyes, such as bixin **10** (Figure 3) [26]. The use of bixin in combination with ZnSO\({}_{4}\cdot\)H\({}_{2}\)O and CuSO\({}_{4}\cdot\)H\({}_{2}\)O to obtain complexes with Zn(II)-ZnBx and Cu(II)-CuBx has also been reported [26]. The addition of **10** (MPII) (Figure 3 to the electrolyte system (containing I\({}^{-}\)/I\({}^{3-}\)) enabled increasing the performance of dye-sensitized solar cells simultaneously with increasing the conductivity and viscosity of the electrolyte [26]. However, the presence of conjugated double bonds in the structure may contribute to the decrease in bixin stability, which remains an unresolved issue [26]. Another group of compounds used in dye-sensitized solar cells are porphyrin zinc sensitizers ZnPi (i = 1-3) **11-13** (Figure 4) due to their ability to obtain high efficiency in cells. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Light** & **JSC (mA\(\cdot\)cm\({}^{-2}\))** & **VOC (V)** & **FF (\%)** & **PCE (\%)** \\ \hline White & 0.021 & 0.37 & 0.47 & 3.96 \(\times\) 10\({}^{-3}\) \\ Yellow & 0.020 & 0.39 & 0.44 & 3.44 \(\times\) 10\({}^{-3}\) \\ Green & 0.051 & 0.40 & 0.43 & 8.67 \(\times\) 10\({}^{-3}\) \\ Blue & 0.022 & 0.40 & 0.44 & 3.8 \(\times\) 10\({}^{-3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Selected photovoltaic performance of a solar cell using alizarin yellow GG-CuCl\({}_{2}\) blended with PVA)/ITO under illumination for white, blue, yellow and green. Reprinted from [24] with permission of Elsevier. Figure 3: Structure of _cis_-bixin. In contrast to ZnPA, these cells possess higher short-circuit current densities (\(\rm{J_{sc}}\)) and lower \(\rm{V_{OC}}\) due to the increase in the charge recombination rate in ZnPi cells at the electrode. As a result, ZnPi-based devices (i = 1-3) have higher photoelectric conversion efficiency (PCE) [27]. The high efficiency of porphyrin-based cells is attributed to the strong absorption of radiation in the visible and near-infrared light range [29]. The following table shows the selected photovoltaic parameters of ZnPi (i = 1-3)-ZnPA dyes (Table 3). Figure 4: Examples of porphyrin zinc sensitizers (**12–14**) and photofunctional iron coordination compounds (**15–36**). Reprinted from [27] with permission of Elsevier and from [28] under a Creative Common CC BY license. Literature reports indicate that coordination compounds containing Fe\({}^{2+/3+}\) with N-donors with a pyridine ring may also be useful. However, the performance of such solar cells was found to be rather limited, as seen by the reported power conversion efficiency (PCE) values on the order of 2% [31; 32]. On the other hand, the use of transition metal coordination compounds with special emphasis on iron (Fe) is a new development trend of DSSCs due to its widespread occurrence on Earth. The previous use of this metal in polypyridyl complexes did not work, as these compounds were difficult to maintain in the excited state. By the subsequent introduction of iron carbene, the photocatalytic and photophysical properties were improved, making this group of compounds promising in terms of achieving longer excited state lifetimes (Figure 4). These compounds utilize N-heterocyclic carbene ligands, which can be further tailored, e.g., by replacing the central pyridyl group by a diazinyl moiety surrounded by benzimidazolium and imidazolium carbenes [28]. In the following table, the basic parameters for iron-containing DSSCs are presented (Table 3).