# 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
In addition, it was observed that increasing the thickness of PEDOT films is not linked to an improvement in the catalytic performance. Figure 6: IV curves in symmetric dummy cells containing cobalt-based electron-transfer mediators: (**A**,**D**) [Co(bpy)\({}_{3}\)]\({}^{3+/2+}\), (**B**,**E**) [Co(phen)\({}_{3}\)]\({}^{3+/2+}\), (**C**,**F**) [Co(dtb)\({}_{3}\)]\({}^{3+/2+}\). Adapted with permission from Ref [17]. Copyright 2013 American Chemical Society. Figure 7: JV curves of cobalt mediated DSSCs sensitized by the organic dye LEG4, assembled with different PEDOT counter electrodes (PEDOT:SDS) where the thickness of the polymer films was varied. Adapted from Ref. [65], Copyright (2013), with permission from Elsevier. The group of [PERSON] fabricated PEDOT-TSO CEs through chemical oxidation of EDOT in the presence of iron(III) tris-p-toluenesulfonate, and the resulting films were deposited by spin coating on both FTO and non-conductive glass, respectively. Interestingly, they reported a power conversion efficiency of 6.3% in the case of DSSCs assembled with cobalt-based redox electrolyte and PEDOT:TSO/glass CEs, very similar to the efficiency of 6.1% obtained with Pt/FTO CEs [18]. Despite the good results achieved by careful cell and material engineering, the main drawbacks of cobalt redox mediators remained related to their diffusional limitation, together with the high inner-sphere reorganization energy between high spin (Co\({}^{2+}\), d\({}^{7}\)) and low spin (Co\({}^{3+}\), d\({}^{6}\)) states which could slow down the process of dye regeneration [66]. More recent in their application to DSSCs, Cu(I/II) couples based on small tetragonally distorted copper complexes showed very good results as alternative redox mediators in DSSCs when used in conjunction with PEDOT DESCEs. The typical copper complexes used as an alternative redox mediator in DSSCs, shown in Figure 8, have a smaller size and thus higher diffusion coefficients than the most used Co (II)/(III) octahedral complexes, offering a better mass transport and smaller reorganization energy than the latter and resulting in an efficient regeneration of the oxidized dye [67]. As a result, remarkably high open-circuit voltages above 1 V under full sun illumination have been reached in DSSCs sensitized by organic dyes [68]. Record PCE of 28.9% under indoor conditions (1000 lux irradiance) and 11.3% in a standard AM1.5G sunlight were achieved by [PERSON] et al. using a Cu(II/I) redox mediator and electrochemically deposited PEDOT films as counter electrodes [70]. Furthermore, DSSCs fabricated through a new architecture achieved efficiencies of 13.1%. This new approach was aimed at a further reduction in the diffusional path of a Cu(II)/ Cu(I) redox mediator, and the cell was fabricated by placing in direct contact the sensitized photoanode with the PEDOT based counter electrode without any spacer [71]. The potential for scale-up of the electrochemical fabrication of the counter electrode was demonstrated by the group of [PERSON]. They prepared relatively large area (5 cm \(\times\) 5 cm) PEDOT counter electrodes, which were sealed against the counter electrode with epoxy resin and an additional thin layer of ZrO\({}_{2}\) to avoid shorting between the electrodes. This architecture, used in conjunction with the Cu\({}^{1/\text{II}}\)(dmp)\({}_{2}\) redox couple, reproducibly afforded high-performance solar cells with an efficiency of close to 8% (see Figure 9) [72]. Figure 8: Molecular structures of Cu complexes studied as redox mediators in references [67; 68; 69]. Besides metal-based redox shuttles, another promising family of alternative electron mediators for DSNCs is represented by organic thiolate/disulfide (T\({}^{-}\)/T\({}_{2}\)) redox couples (see Figure 10). In 2010, [PERSON] et al. reported the first example of a DSSC employing this new type of redox mediator in conjunction with Z907 dye and Pt CEs [73]. The authors achieved an unprecedented efficiency of 6.4% with this iodide-free redox electrolyte.