# On the Use of PEDOT as a Catalytic Counter Electrode Material in Dye-Sensitized Solar Cells
## 2 Dye-Sensitized Solar Cells: An Overview  
DSSCs are photoelectrochemical solar cells based on the principle of sensitization of wide band-gap mesoporous semiconductors. In a DSSC, the photoanode is typically  
Figure 1: (**A**) Number of published articles by keyword search: dye-sensitized solar cells” and “perovskite solar cells” from 2015 to 2020. (**B**) Percentage of publications including the keyword search “counter electrode” in the topic “dye-sensitized solar cell”; inset is the share distribution of publications for counter electrodes from 2015 to 2020. Source: ISI Web of Science, Thomson Reuters. composed of nanometer-sized particles of an n-type mesoporous oxide layer, as represented in Figure 2. The most used material has been anatase TiO\({}_{2}\), although other oxides have been explored, including ZnO and Nb\({}_{2}\)O\({}_{5}\)[24]. These wide band gap metal oxides are transparent to visible light. Hence, only a modest power conversion can be achieved under solar illumination by exploiting a small portion of the UV wavelengths. Molecular sensitization was introduced in order to extend the light-harvesting efficiency of these wide gap semiconductors to low-energy photons. A molecular absorber, known as a sensitizer (S in Figure 2), is chemically linked to the mesoporous oxide through an anchoring group, typically one or more carboxylic functions. Following light absorption, the dye reaches an electronically excited state (S*), which undergoes charge injection into the conduction band (CB) of the semiconductor (Equations (1) and (2)) [25]. \[\text{Photoexcitation S}+\text{hv}\rightarrow\text{S}^{*} \tag{1}\]  
\[\text{Electron injection S}^{*}+\text{TiO}_{2}\rightarrow\text{S}+\text{(e} -,\text{TiO}_{2}) \tag{2}\]  
In Figure 3, the chemical structures of some of the most common sensitizers used in DSSCs are reported. Among these structures, we have the most successful isothiocyanate (NCS) substituted ruthenium dyes, such as N3, N719, and Z907, which represent the first generation of highly efficient molecular sensitizers, thanks to broad light absorption populating charge transfer (CT) states undergoing injection on ultrafast time scales [1]. Later entirely organic dyes based on the donor-\(\pi\)-acceptor architecture were developed, which  
Figure 2: Schematic drawing of DSSCs based on liquid redox mediators. Transparent glass-coated transparent conductive oxides (TCO), such as fluorine doped tin oxide (FTO) or indium tin oxide (ITO), are usually employed to fabricate the charge collecting terminals of the cell, on which TiO\({}_{2}\) and the catalytic counter electrode layer are deposited. In solid-state (ss-DSSCs), the liquid electrolyte is replaced by the hole transport material (HTM). are also endowed with strongly absorbing CT states. Thanks to the electron-withdrawing cyano acrylic linker, dyes, such as D35, Y123, and MK2, achieve high surface coverage and fast injection [26; 27; 28]. Following charge injection in the semiconductor, the oxidized dye S\({}^{+}\) is generated, which needs to be reduced before electron recombination occurs. The redox mediator is essential to regenerate the reduced form of the sensitizer S, finally closing the entire cycle by collecting the electron at the counter electrode (Equations (3) and (4)). \[\text{Dye regeneration: S${}^{+}$ + (redox shuttle)}_{\text{red}}\rightarrow\text{S $+ (redox shuttle)}_{\text{ox}} \tag{3}\]  
\[\text{Redox mediator regeneration: (redox shuttle)}_{\text{ox}}+(e^{-}\cdot,\text{CE}) \rightarrow(redox shuttle)_{\text{red}} \tag{4}\]  
The photocurrent produced by the DSSC can be expressed as the flux of charge carriers (electrons, having concentration \(n\)) along a given \(x\)-direction by means of the electrochemical potential gradient generated inside the semiconductor film upon illumination, according to  
\[J=eu_{n}F+eD_{n}\frac{\partial n}{\partial x} \tag{5}\]  
Figure 3: Some of the most popular inorganic and organic dyes used in DSSCs [1; 26; 27; 28].