Protic Ionic Liquids: A General Strategy for Enhancing Electrical Conductivity and Stretchability of Conducting Polymer Thin Films

Ionic liquids (ILs) are promising materials for enhancing the electrical conductivity and stretchability of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-based stretchable transparent conductors. However, the relationship between the chemical structures of ILs and the electrical and mechanical properties of PEDOT:PSS/IL composites remains unclear. In this study, the impact of protic ILs (p-ILs) on the electrical conductivity and stretchability of PEDOT:PSS/IL thin films is investigated via a comparative analysis with aprotic ILs (ap-ILs). By synthesizing a series of p-ILs and ap-ILs based on imidazolium (IM) and bis(trifluoromethanesulfonyl)imide ions, it is demonstrated that p-ILs significantly enhance electrical conductivity and stretchability, outperforming ap-ILs. In addition, these properties further improve with decreasing alkyl chain length of IM cations, achieving maximum electrical conductivity and stretchability of ≈2200 S cm⁻¹ and 65%, respectively. Notably, the crystalline structures of PEDOT:PSS/IL thin films are elucidated, revealing that p-ILs with shorter alkyl chains facilitate the formation of PSS crystallites due to hydrogen bonding between p-ILs and PSS, which in turn enhance electrical conductivity and stretchability. Leveraging these insights, PEDOT:PSS/p-IL-based strain sensors with broad dynamic ranges and tunable gauge factors are developed. The findings of this study provide valuable design guidelines for developing high-performance ILs in stretchable and wearable electronics.