# 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  
The \(\text{I}_{3}-/\text{I}^{-}\) couple generally accounts for fast dye regeneration kinetics coupled to slow recombination with photoinjected electrons, resulting in a long-lived charge separation upon photoexcitation at the sensitized photoanode/electrolyte junction. While this is the main requisite for achieving high power conversion efficiency in the DSSC, \(\text{I}^{-}/\text{I}_{3}-\) also displays a list of regrettable properties: (1) the triiodide ion absorbs a significant portion of visible light, competing with the dye in light absorption in the blue region of the visible spectrum; (2) \(\text{I}^{-}/\text{I}_{3}-\) is highly corrosive towards metallic conductors typically used in large area DSSC modules; (3) it is volatile; (4) it limits the open-circuit voltage of the cell due to an unnecessarily low redox potential which brings a large overvoltage loss for dye regeneration. Initial attempts to substitute the \(\text{I}_{3}-/\text{I}^{-}\) couple with other redox shuttles were reported by the [PERSON] group at the EPFL (Ecole polytechnique federale de Lausanne). One-electron, outer-sphere redox couples with an oxidation potential more positive than \(\text{I}_{3}-/\text{I}^{-}\) couple, were first considered. In particular, the behavior of the Co(II)/(III) couple of polypyridine, phenanthroline, and imidazole complexes was evaluated [59]. It appeared that while successful, these monoelectronic redox couples suffered from faster charge recombination than the iodide/iodine couple, which required a careful synthetic design. The [PERSON]'s group screened some substituted bipyridine ligands to understand the structure/property relationship of these mediators (some of them are reported in Figure 6), finding the best PCE under one-sun illumination with the sterically hindered [Co(DTB)\({}_{3}\)]\({}^{2+/3+}\) (where DTB is 4,4'-di-tert-butyl-2,2'-dipyridyl) and gold counter electrodes [60]. This was explained by the steric hindrance of tert-butyl groups in DTB, which helped decouple Co(III) from the photoanode surface, thus reducing the rate of the recombination reaction. The need for  
Figure 5: **(A)** cyclic voltammetry of an iodide/iodine-based electrolyte at different working electrodes, including PEDOT coated FTO, Pt, and bare FTO; (**B**) Comparison between current density-voltage (JV) curves of the DSSCs solar cells fabricated with different PEDOT counter electrodes and with the reference material Pt. Readapted with permission of Royal Society of Chemistry from reference [49]. -bulky mediators to counter recombination implied the increase in the hydrodynamic radius of these new shuttles, which impacted negatively on the PCE of the DSSC due to mass transport limitations. [PERSON] et al. compared the photovoltaic performances of several Co-based DSSCs and found that the 8% efficiency obtained at 100 W/m\({}^{2}\) was almost halved under full sunlight (1000 W/m\({}^{2}\)), and this was mainly ascribed to the diffusional limitation of the photocurrent due to slow mass transport of Co(III) at the counter electrode [61]. These disadvantages related to the use of coordination compounds as fast single electron shuttles were circumvented in 2010 by the [PERSON] group, which introduced, for the first time, a combination of a sterically hindered organic triphenylamine-based sensitizer (D35) with selected cobalt polypyridine complex. The organic dye D35 was able to efficiently suppress the recombination thanks to its sterically hindered structure which, in turn, enabled the use of less bulky polypyridine ligands, thereby improving both the electron transfer rate and the mass transport from the resulting cobalt complexes [26]. With overall conversion efficiencies of 6.7% and open-circuit potentials of more than 0.9 V under 1000 W m\({}^{-2}\) AM1.5 G illumination, cobalt mediators were able to beat the iodide/iodine couple under full sun illumination.