Disorder-to-order transition of regiorandom P3HT upon electrochemical doping

Abstract

The microstructure of organic mixed ionic–electronic conductors (OMIECs) plays a crucial role in their functionality. OMIECs are generally polycrystalline conjugated polymers consisting of a complex arrangement of disordered and ordered regions. While considerable research has focused on structural transformations in the ordered regions of OMIECs upon ion insertion, the evolution of disordered regions is difficult to probe. Here, we explore changes in the disordered conjugated polymer regiorandom (RRa) poly(3-hexylthiophene) (P3HT), focusing on its response to repeated electrochemical doping/dedoping cycles and thermal annealing. By performing a series of cyclic voltammograms (CVs) followed by grazing incidence wide angle X-ray scattering (GIWAXS), we show how the molecular packing and microstructure of RRa P3HT change. Additionally, we employ nanoscale infrared imaging via photoinduced force microscopy (PiFM) to visualize the changes in polymer morphology and the distribution of ions within the polymer matrix. Our results indicate that RRa P3HT experiences significant structural reorganization after repeated doping/dedoping cycles. These large structural rearrangements lead to partially irreversible doping due to ion trapping in electronically disconnected polymer regions. Moreover, we observe that electrochemical cycling followed by thermal annealing promotes crystallinity in RRa P3HT, yielding GIWAXS diffractograms akin to those of polycrystalline regioregular (RR) P3HT. Our research conclusively shows that disordered conjugated polymers can undergo substantial structural and morphological transformations through repeated electrochemical doping, leading to enhanced ordering and ion trapping within the polymer matrix.