# 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
#### 3.2.3 PEDOT as a Solid State Hole Conductor  
PEDOT has also been successfully used in a variant of DSSCs, known as solid-state DSCs (ss-DSSCs), in which the liquid electrolyte has been replaced by a solid hole-transporting material (HTM). In this type of device, the introduction of the HTM may represent a further step towards efficient solar cell operation. Indeed, ss-DSSCs must face some possible issues, such as inadequate nanopore filling, low conductivity, and crystallization of HTM infiltrated in the mesoscopic TiO\({}_{2}\) scaffolds, leading to low performances [81]. In general, the HTM is deposited on the top of the sensitized photoanode, and then pore filling is obtained by melting the HTM film above the melting point. This protocol can be adopted especially in the case of low molecular weight HTM or, in general, when the required temperature should not be too high for the dye to decompose. Another common approach consists of casting and evaporation of a liquid solution of the HTM in a volatile solvent. However, the complete filling of the pores of the oxide is not always obtained through these methods. An interesting and more promising alternative is represented by the possibility of in situ electro-polymerization of a layer of HTM. In general, the most followed approach is represented by the in situ photoelectrochemical polymerization (PEP) of PEDOT.  
For example, the [PERSON] group investigated the photoelectrochemical polymer deposition under low light intensity in order to optimize the infiltration of the HTM in the nanoporous TiO\({}_{2}\). Indeed, the photoanode coated with the organic dye D35 was immersed into the monomer solution, which consisted of a EDOT derivative (bis-EDOT), to facilitate the introduction of the monomer into the pores of TiO\({}_{2}\). Finally, the conductive PEDOT film was obtained by photoelectrochemical oxidation by applying a constant potential on the working electrode under light bias [82]. As summarized in Figure 11, upon photoexcitation of the dye, electrons are injected into the conduction band of TiO\({}_{2}\), generating the oxidized dye, which triggers EDOT polymerization. In the working cell, the conduction mechanism occurs through hole injection in PEDOT and transport along the polymer bulk in the form of polarons and bipolarons charge carriers.  
In situ photoelectrochemical polymerization of bis-EDOT was also obtained the Z907 Ru(II) dye, obtaining a ss-DSSC conversion efficiency of 2.6% with high open-circuit voltage over 0.8 V [83]. [PERSON] et al. investigated the role of the doping anions ClO4\(-\), CF3 SO3\(-\), BF4\(-\), and TFSI- on PEDOT as a hole conductor in ss-DSSCs. They found that a careful selection of different counterions can lead to an effective way to modulate the conductivity of PEDOT when used as HTM in solid-state devices. In addition, these doping anions are directly linked to the conversion efficiency of ss-DSSCs and, in particular, it was observed that the dopant TFSI\(-\) is the most effective in reducing the series and shunt resistance of the cell, thereby representing a promising approach to the scalable preparation of solid-state transparent photoelectrochemical devices [84]. Following a slightly different approach,long-term stable and highly efficient ss-DSSC was achieved very recently through the enhanced interfacial contact properties of PEDOT as HTM in conjunction with the well-known amphiphilic ruthenium sensitizer Z907. In this study, the PEDOT-based HTM layer was prepared by thermal polymerization of a monomer derivative that was previously infiltrated through the nanoporous metal oxide. The authors compared the stability of a typical liquid DSSC with the new ss-DSSC and were able to confirm the stable response resulted from the intimate contact between the dye sensitizer and the PEDOT HTM, which ensured efficient regeneration of the former [85].