Crystallinity Determines Ion Injection Kinetics and Local Ion Density in Organic Mixed Conductors

Conjugated polymer organic mixed ionic–electronic conductors (OMIECs) consist of a complex arrangement of crystalline and amorphous regions at the nanoscale. The arrangement of ions in this heterogeneous environment upon electrochemical doping influences the performance of devices that leverage mixed conductivity. This study investigates how varying the ratio of amorphous and crystalline content affects the distribution of ions in blends of regiorandom (RRa) and regioregular (RR) poly­(3-hexylthiophene) (P3HT). By correlating changes in the polymer lattice spacing upon ion uptake with spectroelectrochemistry measurements of polaron formation, we find that ions enter the crystalline regions of the polymer first. Using nanoscale infrared imaging with photoinduced force microscopy (PiFM) of the infrared-active PF6 – ions, we map the location of ions at the nanoscale and show that ions cluster in crystalline regions of P3HT thin films. We correlate the infrared images of ion locations with visible PiFM (785 nm) to map the presence of polarons in the film and find that the locations of ions and polarons are correlated. This study also reveals that balancing the distribution of amorphous and crystalline regions can enhance ion injection kinetics. These results underline the importance of controlling the nanoscale morphology of conjugated polymers for high-performing OMIECs.