# On the Use of PEDOT as a Catalytic Counter Electrode Material in Dye-Sensitized Solar Cells
## 3 PEDOT in DSSCs  
### The Conductive Polymer PEDOT  
PEDOT was synthesized for the first time in 1988 by [PERSON] and [PERSON] by chemical oxidation of EDOT in the presence of polystyrene acid [36]. They obtained a water dispersion of PEDOT:PSS paving the way to transform this conductive polymer into one of the most used for different applications. PEDOT can also be prepared by electrochemical synthesis, but in this case, a conductive coating is deposited on the electrode surface [37; 38; 39]. The reaction pathway from EDOT to PEDOT consists of initial monoelectronic oxidation of EDOT. Subsequently, a sequence of C-C coupling of two EDOT radical cations, proton losses, and re-aromatic occur until a certain length of newly formed PEDOT oligomers is reached (see Figure 4) [40]. While the exact chain length is not known in detail, some authors reported that in the case of PEDOT:PSS, the molecular weight of the PEDOT oligomers is between 1000 and 2500 Da, which is consistent with about 6 to 18 repeating units of EDOT [41]. During the chemical or electrochemical polymerization, neutral oligomers are further oxidized to yield PEDOT in its doped and conductive state. It should be noted that the main charge carriers across PEDOT in its doped state are polarons and bipolarons [40]. In addition, the ratio between EDOT units of PEDOT chains and dopants strongly depends on the ability of the negative charges of the counterions present in the electrolyte to stabilize the positive charge on PEDOT. For monomeric dopants, this parameter is also known as the _doping ratio_, whereas for polymeric dopants, not all the negative groups are expected to counterbalance the positive charges generated on PEDOT. In the case of chemically prepared PEDOT:PSS (PSS = polystyrene sulfonate), for example, it was reported that only a small fraction of sulfonic acid groups across the PSS chains directly interact with positively charged PEDOT while the other part interacts with water [42]. In general, the real doping ratio can be established through spectroscopic techniques, such as XPS (X-ray photoelectron spectroscopy) or EDS (Energy Dispersive Spectroscopy), but some authors have used electrochemical analysis as well [42; 43; 44; 45]. The solubility of PEDOT oligomers is strongly influenced by the synthetic protocol adopted. In particular, if one is interested in obtaining a water dispersion of the conductive polymer, surfactants, such as PSS or Nafion, are preferred [46; 47]. Thus, the possibility of obtaining a dispersion/solution of PEDOT:X, where is X represents a general dopant, is limited by the solubility of the conductive polymer, which depends on the nature of the dopant as well as of the solvent. The electrochemical deposition of PEDOT is less restricted in the choice of counter anions since the goal of this process is not a stable dispersion but the precipitation of the PEDOT:X composite on the electrode surface in order to generate a solid conductive polymer thin film. Thus, one can access many options in the composition of the electrolyte, generating materials whose electrochemical response is strongly influenced by the nature of the doping counter anions, as will be shown in the next section [48].