## 3 Materials and Fabrication Techniques
### Strategies for Enhancement of PEDOT:PSS Properties  
#### 3.2.1 Enhancement of Electrical Conductivity and Adhesion of PEDOT:PSS  
A good organic conducting polymer is characterized by its ability to keep a stable charge injection capacity as well as good charge mobility with time. The conductivity enhancement can be related to the presence of different counter-ions or to the intrinsic structure of the polymer. PEDOT:PSS is by far the most used organic conducting polymer involved in OECTs. Many studies were performed to increase the stability, the wettability and the electrical properties of PEDOT:PSS polymer. Few years ago, [PERSON] et al. [67] reviewed the different techniques that were reported in literature to improve the electrical conductivity of the PEDOT:PSS films. A brief summary is presented here. The easiest way to enhance the conductivity of PEDOT:PSS films is a thermal treatment performed after deposition. [PERSON] et al. [68] studied the influence of the temperature (range of temperature from 100 to 250 \({}^{\circ}\)C) as well as the time of curing in an inert atmosphere as well as in air. They concluded that the thermal treatment is an effective method to improve the conductivity of PEDOT:PSS of at least one order of magnitude (10\({}^{-3}\) to 10\({}^{-2}\) S cm\({}^{-1}\)). The influence of UV or UV-ozone post-deposition treatments has also been studied [69; 70; 71; 72]. Authors made the hypothesis that the enhancement of conductivity is a consequence of a structural change that, on the one hand, is able to prevent the loss due to charge-trapping defects and, on the other hand, favors the formation of extended-coil polymeric chains (quinoid structure instead of benzoic structure) which increases the \(\pi\)-\(\pi\) stacking (Figure 5). For the last two decades, other techniques have been employed in order to enhance the conductivity of PEDOT:PSS. [PERSON] et al. [73] were the first to report an enhancement of the PEDOT:PSS conductivity by adding a co-solvent to the aqueous PEDOT:PSS suspension. They found that the addition of dimethylsulfoxide (DMSO) allows the enhancement of conductivity by about 2 orders of magnitude compared to pristine PEDOT:PSS. Based on these observations, many other polar organic solvents have been used (ethylene glycol (EG), dimethylformamide (DMF), tetrahydrofuran (THF), glycerol, etc.) but with less remarkable effects than with DMF or THF. It must be noticed that reported conductivity values are in a broad range from 0.6 S cm\({}^{-1}\) (methoxyethanol: [PERSON] et al. [74]) to 677 S cm\({}^{-1}\) (DMSO: [PERSON] et al. [75]). Besides the addition of a co-solvent in the PEDOT:PSS aqueous suspension before processing, [PERSON] et al. observed an enhancement of conductivity by performing a post-deposition treatment of the film with EG or DMSO. Since, many examples of PEDOT:PSS post-deposition treatment were reported allowing for instance the enhancement of the conductivity by up to 3 orders of magnitude (from 0.3 S cm\({}^{-1}\) to 1164 S cm\({}^{-1}\)) by using hexafluoroacetone [76]. Some low boiling point solvents were also tested and [PERSON] et al. [77] reported an enhancement of conductivity by 3 orders of  
Figure 5: Difference between benzoid and quinoid forms (extended-coil structure) of PEDOT. magnitude (from 0.3 to 1362 S cm\({}^{-1}\)) using methanol. As for physical treatments, the supposed mechanism that is involved is an intercalation of the solvent molecules between the PEDOT and the PSS moieties leading to a stabilization of a linear form of the PEDOT chain as well as the removal of excess PSS that causes a small loss of conductivity. The same authors also reported conductivity enhancement using PEG (Figure 6). From the literature, the pre-deposition treatments and more commonly the post-deposition treatments of PEDOT:PSS films using acidic solutions allow the largest increase in conductivity (up to 4380 S cm\({}^{-1}\)). According to the authors [73], best results are obtained by immersing the PEDOT:PSS films in pure sulfuric acid.