In Situ Probing of the Effects of Doping on Mechanical Properties in oRganic Mixed Ionic-Electronic Materials via Atomic Force Microscopy

Organic mixed ionic-electronic conductors (OMIECs) transduce ionic uptake/expulsion to electronic carrier motion (“doping” and “dedoping”), and this process impacts the structure of the host polymer. Mechanical properties in these materials change significantly with doping, be it via molecular or electrochemical doping. Doping in polymers is generally associated with a stiffness change in the material, which can impact device stability and affect carrier transport. Here, we use electrical and mechanical atomic force microscopy (AFM) to probe the adhesion and modulus of OMIECs during the doping and dedoping process. We acquire force measurements in situ under bias in different electrolytes, and on ex situ doped systems using high-resolution bimodal AFM. We use these AFM methods to probe a wide range of polymers, from hydrophobic poly(3-hexylthiophene) of varying crystallinities to glycol-sidechain variants to polymers with structural phase transitions. Surprisingly, different OMIEC materials can exhibit opposite trends with doping, where adhesion can increase or decrease with doping, implying that explanations of plasticization with electrochemical oxidation are not universal to OMIECs. At the same time, the rate from time-dependent force measurements, and the magnitude of changes in adhesion, vary as a function of average crystallinity in different polymers. These results indicate that OMIECs can exhibit complex relationships between ion choice, polymer crystallinity and structure, and mechanical properties that can ultimately impact device operation.