Ion Trapping and Induced Ordering of Regiorandom Poly(3-hexylthiophene) upon Repeated Electrochemical Doping

Conjugated polymer organic mixed ionic electronic conductors (OMIECs) are promising materials for bioelectronics and other organic electronic devices. How these materials behave with repeated electrochemical doping matters for the stability and longevity of these devices. In this work, we employ a combination of grazing incidence wide-angle X-ray scattering (GIWAXS), UV-vis spectroelectrochemistry, and nanoscale imaging of the ions with photoinduced force microscopy (PiFM) to monitor film morphology changes in regiorandom poly(3-hexylthiophene) (RRa P3HT) when exposing the polymer film to various aqueous electrolytes upon repeated electrochemical doping. When using potassium bis(trifluoromethansulfonyl)imide (KTFSI) as our electrolyte, we see increased ion trapping and an increase in crystalline order in RRa P3HT, as well as the crystallization of TFSI^- ions, upon repeated electrochemical doping. Upon thermal annealing at 120 °C after continuous doping of RRa P3HT with TFSI^-, we see a significant increase in lamellar stacking intensity compared to the film without thermal annealing. When comparing less chaotropic ions to TFSI^-, we see these ions do not order RRa P3HT to the same extent as TFSI^-. Overall, this study shows that both ion trapping and structural reorganizations occur in disordered conjugated polymers upon repeated electrochemical cycling.