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

Abstract

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.