## 3 Results
### Properties of PEDOT:PSS-C6 Films
We used AFM to investigate the differences in morphology of these four films for the confined area of 2 \(\upmu\)m \(\times\) 2 \(\upmu\)m, which is sufficiently large to provide simultaneously reliable analysis. As seen in Figure 7a, where AFM images of four kinds of films are compared in the height mode, the surface of PEDOT:PSS film (I) without a dopant is very smooth, while there are grains for the doped film (II). The root-mean-square roughness (RMS), the most commonly evaluated parameter representing the surface roughness, is 2.539 and 3.257 nm for (I) and (II), respectively. There are many research activities on measuring the roughness of PEDOT:PSS films and their data exhibit similar values that we have observed. For example, PEDOT:PSS films without doping and doped with 2 wt.% DMSO showed RMS values with 2.51 and 3.61 nm, respectively [36]. In contrast, both PEDOT:PSS-C6 films (III) and (IV) show high RMS values (5.141 nm and 5.757 nm, respectively) regardless of the doping process. Since [PERSON] et al. reported that incorporation of polar and high boiling-point solvents such as DMSO contributes to the improvement in conductivity [30], various attempts have been made to further increase conductivity by adding different additives or post-treatment. All of these phenomena are called secondary doping, and this second doping process is speculated to be triggered by separation of PSS chains from PEDOT [35; 37; 38]. In recent studies [36] on the relationship between the effect of various secondary dopants and the morphology of PEDOT:PSS film, the phase separation phenomenon accompanying the enhancement of conductivity was analyzed more reliably in the phase-shift mode of AFM. In this study, we also observed phase-shift images (Figure 7b) of the same four films. The PEDOT:PSS film (I) without doping shows small islands that are irregular in shape, and the doped film (II) displays globular grains with clear boundaries, which suggests the occurrence of phase separation. This result is similar to the observation reported by [PERSON] et al. [36]. On the other hand, PEDOT:PSS-C6 film (III) also shows a phase-separated image with globular grains with clear boundaries, which is similar to that of doped film (II). No significant change is observed in the doped film (IV) of PEDOT:PSS-C6, which strongly indicates that the doping of PEDOT:PSS-C6 with DMSO does not affect the morphology or the conductivity of PEDOT:PSS-C6. Figure 7: AFM image of (**a**) surface morphology and (**b**) phase shift of PEDOT:PSS films undoped (I), doped (II) with DMSO, PEDOT:PSS-C6 film undoped (III), and doped (IV) with DMSO. In summary, the surface resistance of the undoped PEDOT:PSS-C6 film (III) was significantly lower than that of pristine PEDOT:PSS (I), and showed a value similar to that of the PEDOT:PSS film (II) doped with DMSO. The study on morphology of film (III) clearly showed phase separation similar to the film (II). Accordingly, in PEDOT:PSS-C6, it is understood that the introduction of an alkyl group into the sulfuric acid of the free PSS of PEDOT:PSS causes phase separation more clearly and leads to an improvement in conductivity.