# 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  
#### 3.2.1. Iodide-Iodine Electrolyte  
Owing to its good conductivity, high flexibility, remarkable stability, and low cost, the use of PEDOT for different applications has significantly increased during the last twenty years. As far as counter electrodes for DSSCs are concerned, high ohmic conductivity is not necessarily a goal that should be reached since PEDOT only forms a thin layer on top of an ohmic electrode (metal or TCO charge collectors). Rather, it is very important that the PEDOT-based material exhibits a good electrocatalytic behavior toward redox mediators used to regenerate the oxidized dye. For example, bare FTO has very high conductivity, but DSSCs which employed this unmodified material as CEs exhibited PCEs of about 0.1%, whereas platinized FTO of PEDOT-based CEs easily exceeded 4% power conversion [49]. Another important key feature of PEDOT is related to its transparency over the wavelength range of 400-900 nm. The absorption spectrum of oxidized PEDOT is essentially featureless, and a progressive increase in the absorbance is observed only when approaching the NIR region. For these reasons, oxidized PEDOT exhibits a sky blue color. Of course, the transparency of PEDOT CEs decreases as the thickness of the coating increases. In general, a compromise between electrocatalytic properties and film transparency can be obtained, but it should be noted that the absorbance of iodine-based electrolytes is commonly higher than the absorbance of PEDOT CEs in the visible range. Thus, the final color of the device is minimally impacted by the color of PEDOT CE, mostly influenced by the color resulting from the dyed photoanode and the electrolyte. It was also reported that PEDOT films having a thickness on the order of 2 um provided comparable electrocatalytic properties to 50 nm thick ones, in conjunction with iodine base redox mediator [50]. Thus, a satisfactory electrochemical response can be obtained with PEDOT films having a minimal contribution to the overall light-harvesting of the device. One of the first reports of PEDOT-based CEs for DSSC was published in 2002 by the group of Prof. [PERSON] [51]. They were encouraged by preliminary results by [PERSON] et al., who outlined that PEDOT can improve the charge transfer between a metal oxide and the iodide/triiodide redox couple, which was, and still is, one of the most popular components in liquid electrolyte DSSCs [37]. About 30-50 nm of PEDOT-TsO (where TsO refers to tosylate group) and PEDOT:PSS were deposited on an ITO surface through chemical oxidation of EDOT or by spin coating a commercial solution, respec  
Figure 4: (**A**) general synthetic route to doped PEDOT:X (X = generic dopant) and (**B**) reversible switch between reduced and oxidized forms of PEDOT. tively. The cyclo-voltammetric (CV) response of iodide showed the absence of significant electrocatalytic activity when performed on bare ITO, as shown by the absence of clear oxidation/reduction diffusional waves. On the contrary, good electrocatalytic behavior was observed for PEDOT-Iso, which was comparable to Pt-based electrodes, thereby indicating PEDOT-Iso as a good candidate to substitute Pt in CEs for DSSCs. Interestingly, in the same conditions, electrodes coated with PEDOT:PSS exhibited a poorer electrochemical behavior if compared to PEDOT-TsO. Authors suggested that this could be linked to the exposure of polyanion PSS\({}^{-}\), which may sterically and electrostatically block the approach of I\({}^{-}\) and I\({}_{3}{}^{-}\) to the active sites of the PEDOT chain [51]. The results of DSSCs characterization were consistent with the preliminary indications gained by CV: comparable efficiency parameters (\(V_{\infty}\), \(J_{\infty}\) and \(FF\)) were obtained for PEDOT-TsO and Pt, but cells prepared with PEDOT:PSS based CEs provided the lowest \(FF\) values due to the larger series resistance in these cells.