# On the Use of PEDOT as a Catalytic Counter Electrode Material in Dye-Sensitized Solar Cells
## 3 PEDOT in DSSCs
### PEDOT Based Alternative Counter Electrodes in DSSCs
#### 3.2.2 Alternative Redox Electrolytes
It was clear that a new generation of devices was now accessible and that to improve the overall efficiency, it is necessary to focus on all aspects of the DSSC. In addition, catalytic materials different than noble metals (Au, Pt) could be used to fabricate cost-effective high surface area CEs, which could reduce the concentration polarization losses in cobalt mediated DSSCs. [PERSON] and co-workers compared the photovoltaic performances of DSSCs assembled with either Pt or chemically oxidized PEDOT CEs and cobalt mediators. In this case, Pt outperformed PEDOT leading to an overall efficiency of about 2% for the former and 1.3-2% for the latter [62]. It should be noted that the poor activity of PEDOT:PSS when cast from a chemically prepared dispersion was already observed for iodide-based redox mediators [51]. Nevertheless, it was also reported that Pt, unlike gold, is not the material of choice to account for a fast reduction of Co(III) species, especially when bulky polypyridine ligands are used [63]. [PERSON] et al. published a detailed study that compared the electrocatalytic behavior of different electrochemically prepared PEDOT CEs toward the most relevant cobalt-based redox shuttles of that time [17]. These authors also compared the various PEDOT coated FTO electrodes they fabricated to the Au coated ones since these latter were considered the best substrates to catalyze the Co(II/III) redox chemistry. PEDOT CEs were prepared by electrochemical deposition in the presence of the anionic dopants perchlorate (CIO\({}_{4}\)-), PSS, and a mixture of perchlorate and sodium dodecyl sulfate (SDS). It was found that PEDOT/CIO\({}_{4}\) was the best electrocataly thanks to its highly porous structure, which maximized the contact area with the electrolyte, and brought the smallest diffusional and charge transfer resistances within the explored series. Importantly, PEDOT/CIO\({}_{4}\) outperformed both Pt and Au based CEs as confirmed by the analysis of thin-layer symmetric cells containing [Co(phen)\({}_{3}\)]\({}^{2+/3+}\) or [Co(bpy)\({}_{3}\)]\({}^{2+/3+}\) couples (phen = 1,10-phenanthroline; bpy = 2,2\({}^{\prime}\)-bipyridine). The comparison between the different PEDOT CEs outlined a strong dependence on the chemical structure of the cobalt complexes. In particular, while the less sterically hindered [Co(bpy)\({}_{3}\)]\({}^{2+/3+}\) showed fast kinetic response on all PEDOT based CEs, relatively less facile charge transfer kinetic was observed at the interface between PEDOT:PSS and [Co(phen)\({}_{3}\)]\({}^{2+/3+}\), or, more importantly, with the bulky [Co(DTB)\({}_{3}\)]\({}^{2+/3+}\). Similar results were also observed for PEDOT:SDS CEs, albeit less pronounced if compared to PEDOT:PSS. The authors suggested that sluggish charge transfer kinetics could be due to the hydrophobic nature of [Co(phen)\({}_{3}\)]\({}^{2+/3+}\) and [Co(DTB)\({}_{3}\)]\({}^{2+/3+}\), which resulted in unfavorable interactions with hydrophilic sulfonate or sulfate groups of PSS and SDS, causing a higher activation barrier for the electron transfer to occur, if compared, for example, to PEDOT:CIO\({}_{4}\). Other authors demonstrated that for DSSCs employing the cobalt complexes [Co(bpy)\({}_{3}\)]\({}^{2+/3+}\) and [Co(phen)\({}_{3}\)]\({}^{2+/3+}\), the use of PEDOT-coated CEs can effectively reduce the series resistance of the cell to such an extent to mitigate the mass transport limitation of Co(III) which often affects these chemical species [64]. [PERSON] et al. electrodeposited PEDOT:SDS CEs on large area conducting glass and plastic substrates (9 cm \(\times\) 9 cm) from a micellar aqueous solution of sodium dodecyl sulfate and EDOT [65]. The electrocatalytic properties of PEDOT:SDS CEs were assessed in DSSCs assembled with the organic dye LEG4 and a [Co(bpy)\({}_{3}\)]\({}^{2+/3+}\) based electrolyte, confirming that PEDOT:SDS counter electrodes exceeded the performance of thermally platinized conducting glass electrodes (Pt-FTO) (see Figure 7).