Stretchable conducting polymer PEDOT:PSS treated with hard-cation-soft-anion ionic liquid designed from molecular modeling

PEDOT:PSS, an ionic polymer mixture of positively-charged poly-3,4-ethylenedioxythiophene (PEDOT⁺) and negatively-charged poly-styrenesulfonate (PSS⁻), is a water-processable and environmentally-benign organic semiconductor and electrochemical transistor, which plays a key role in organic (bio)electronic devices. However, pristine PEDOT:PSS films form 10-to-30-nm granular domains, where conducting-but-hydrophobic PEDOT-rich cores are surrounded by hydrophilic-but-insulating PSS-rich shells. Such morphology makes PEDOT:PSS water-soluble and thermally stable but very poor in conductivity. A tremendous amount of effort has been made to enhance the conductivity of PEDOT:PSS by restoring the extended conduction network of PEDOT. Recently, remarkable ~5000-fold improvements of conductivity have been achieved by mixing PEDOT:PSS with proper ionic liquids (ILs). In a series of free energy estimations using density functional theory calculation and molecular dynamics simulation, we have demonstrated that the classic hard-soft acid–base (or cation-anion) principle of chemistry plays an important role in such improvements. Ion exchange between PEDOT⁺:PSS⁻ and A⁺:X⁻ ILs helps PEDOT⁺ to decouple from PSS⁻ and to grow into large-scale conducting domains of π-stacked PEDOT⁺ decorated by IL anions X⁻. Thus, the most spontaneous decoupling between soft (hydrophobic) PEDOT⁺ and hard (hydrophilic) PSS⁻ would be induced by strong interaction with soft anions X⁻ and hard cations A⁺, respectively. Such hard-cation-soft-anion principles have led us to design ILs containing extremely hydrophilic (i.e., protic) cations and hydrophobic anions. Not only they indeed improve the conductivity of PEDOT:PSS but also enhance its stretchability as well. In summary, our modeling offered molecular-level insights on the morphological, electrical, and mechanical properties of PEDOT:PSS and a molecular-interaction-based enhancement strategy for intrinsically stretchable conductive polymers.