Electrochemical Doping Induced Crystallinity in Organic Mixed Ionic/Electronic Conductors

Organic mixed ionic-electronic conductors (OMIECs) are an exciting class of new materials with a range of potential applications including biosensors, electrochromics, and neuromorphic computing. These applications all require ionic transport from an electrolyte into the organic active layer during operation. Characterization of this electrolyte and ion swollen state is essential to understanding device operation on a fundamental level. However, this swollen state is particularly difficult to study because it requires development of <i>in-situ</i> techniques. Here, we utilize <i>in-situ </i>Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) to study the crystallinity of the swollen state of OMIECS as a function of applied potential. We study a variety of conjugated polymer active layers and find consistent doping induced crystallinity across a variety of different polymer backbones. Interestingly, we find a maximum in the crystallinity as a function of doping level for some polymers. At moderate doping levels the crystallinity is enhanced relative to the undoped state, but then at the highest doping levels achieved the crystallinity decreases. Finally, we use organic electrochemical transistors (OECTs) to correlate the electronic conductivity with these doping induced morphological changes. These results demonstrate the importance of <i>in-situ</i> characterization to develop a better understanding organic electronic devices.