**4. Conclusions**

In this study, we investigated the effects of linear glycols and a derivative (EG, DEG, TEG, and HEG, and EGME) as a processing additive on the electrical and optical properties of PEDOT:PSS films. We found that repeated ethylene oxides with hydroxyl groups were effective in enhancing the electrical conductivity from 3–5 S/cm in a pristine PEDOT:PSS to 270–330 S/cm with additives. We varied the concentration of the additives in the stock dispersion of PEDOT:PSS and observed sigmoidal dependence of the electrical conductivity. Through a curve fitting of the experimental conductivity–concentration data with a sigmoidal logistic function, sharp transitions were observed at the concentration of ~0.6 wt%. The PEDOT:PSS films with a thickness of 50–60 nm showed the reasonable transmittance of 94–97%. We also evaluated two different figures of merit for transparent electrodes and found that DEG and TEG among the series were effective in enhancing the performance of the PEDOT:PSS-based transparent conducting films.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/ma14081975/s1. Figure S1: SEM images of the PEDOT:PSS films. Figure S2: Relative changes in the properties after aging in air. Table S1: Comparison of the film properties with previous reports.

**Author Contributions:** Investigation, H.J.J., H.J., T.K., T.E. and F.S.K.; writing—original draft preparation, H.J.J. and H.J.; writing—review and editing, H.J.J., H.J., T.K., T.E. and F.S.K.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2014M3A7B4051749). This work was also supported by the Chung-Ang University Research Grants in 2019.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** We acknowledge supports from the National Research Foundation of Korea (NRF-2014M3A7B4051749) and the Chung-Ang University Research Grants in 2019.

**Conflicts of Interest:** The authors declare no conflict of interest.
