# Transition Metal Coordination Compounds as Novel Materials for Dye-Sensitized Solar Cells
## 2 Characterization and Principle of Operation of Dye-Sensitized Solar Cells
### Alternative Electrodes  
In terms of their commercial application, one of the key objectives of DSSCs is to replace the counter electrode (CE) constituting Pt with any other electrode to achieve as high electrocatalytic activity or high conductivity [15]. Literature sources report the use of carbide materials based on WC, W\({}_{2}\)C [42] or W\({}_{\rm x}\)C [43] forms (W-based carbides); transition metal compounds (TMCs); or metal-organic frameworks (MOFs). For instance, complex com  
Figure 6: Examples of DSSCs containing Co\({}^{2+/3+}\) redox shuttles used in DSSCs. Reprinted from [41] with permission of John Wiley and Sons. pounds based on polyoxotungstate (H\({}_{3}\)PW\({}_{12}\)O\({}_{40}\)), which contained varied amounts of pyrrole (W\({}_{2}\)C@C composites), were reported to PCE values above 7% and open circuit voltages (V\({}_{\text{OC}}\)) above 0.77 V [15]. Another example to replace the Pt electrode (energy conversion efficiency of 7.67%) is the use of a Cu-MOF/PEDOT (poly(3,4-ethylenedioxythiophene)) composite electrode, which, under sunlight illumination, achieved a solar cell conversion efficiency of 9.45% [44]. The following table indicates the selected photovoltaic characteristics with the use of different electrodes (compared to the platinum electrode (Table 4):  
The paper of [44] confirms that the used Cu-MOF/PEDOT composite electrodes with appropriate redox pairs of I\({}^{-}\)/I\({}^{3-}\), Co-phen\({}^{2+}\)/Co-phen\({}^{3+}\) and Cu-dmp\({}^{+}\)/Cu-dmp\({}^{2+}\) exhibit high electrochemical capabilities, which may be applicable to electrochemical process devices. The presence of Cu-MOF (the main electrocatalyst) in the composite electrode provides a conductive character, while PEDOT functions as an auxiliary electrocatalyst [44]. Another advantage of replacing Pt as CEs by PEDOT is the achievement of stability and high conductivity at room temperature [45]. PEDOT-based material should exhibit good electrocatalytic behavior used for dye regeneration. The authors of [45] report that the use of unmodified PEDOT allowed a cell conversion efficiency of 0.1%, while platinized FTOs and a counter electrode containing PEDOT allowed a conversion efficiency of over 4%. The following table shows the basic photovoltaic performance of dye-sensitized solar cells, including CEs with PEDOT (Table 5) [45]:  
An interesting literature report in DSSCs appears to be the employment of carbon nanotubes (CNTs) as counter electrodes (CEs) in exchange for platinum. The use of graphene in the form of single and multi-walled carbon tubes (MWCNTs) allows for comparable catalytic activity or conversion efficiency and possibly better conductivity than those based on Pt/TCO [46]. In addition to carbon nanotubes or graphene, conductive polymers and also semiconductors made of metal oxides can be employed [47]. Carbon is a commonly found raw material on Earth, and its lower price compared to platinum, as well as its good chemical resistance, enables it to be classified as a good candidate for the production of DSSCs. For example, the use of Cu-based electrolytes with Pt can achieve solar-to-electric \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Counter Electrode** & **Jsc (mA\(\cdot\)cm\({}^{-2}\))** & **V\({}_{\text{OC}}\) (V)** & **FF (\%)** & **PCE (\%)** \\ \hline Pt 1  & 16.08 \(\pm\) 0.13 & 748 \(\pm\) 2 & 0.64 \(\pm\) 0.01 & 7.67 \(\pm\) 0.04 \\ Cu-MOF 1  & 0.88 \(\pm\) 0.88 & 618 \(\pm\) 166 & 0.10 \(\pm\) 0.03 & 0.04 \(\pm\) 0.04 \\ PEDOT 1  & 15.49 \(\pm\) 0.48 & 769 \(\pm\) 7 & 0.64 \(\pm\) 0.01 & 7.58 \(\pm\) 0.25 \\ Cu-MOF/PEDOT 1  & 16.36 \(\pm\) 0.27 & 777 \(\pm\) 3 & 0.65 \(\pm\) 0.02 & 8.26 \(\pm\) 0.06 \\ Cu-MOF/PEDOT 2  & 17.48 \(\pm\) 0.92 & 786 \(\pm\) 9 & 0.69 \(\pm\) 0.01 & 9.45 \(\pm\) 0.35 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Selected photovoltaic parameters using different electrodes. Reprinted with permission from [44]. Copyright 2021 American Chemical Society.