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

Abstract

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.

Quantifying Doping Efficiency to Probe the Effects of Nanoscale Morphology and Solvent Swelling in Molecular Doping of Conjugated Polymers

“…We use KTFSI as an electrolyte for electrochemical doping as TFSI – exhibits the lowest threshold voltage among possible counterions for electrochemical doping, thus facilitating reversible oxidations. , As with molecular doping, both polymers exhibit bleaching and an increase of polaron/bipolaron bands upon electrochemical doping (Figure S2b,c). As the P3HT is doped, the π–π* transition tends to lose the vibronic shoulder and exhibits a slight blue shift, likely due to polarons preferentially residing in more ordered regions during the electrochemical doping as reported previously . We again quantify the amount of bleaching resulting from electrochemical doping as we did in molecular doping (Figure S3a).…”

“…As the P3HT is doped, the π−π* transition tends to lose the vibronic shoulder and exhibits a slight blue shift, likely due to polarons preferentially residing in more ordered regions during the electrochemical doping as reported previously. 32 We again quantify the amount of bleaching resulting from electrochemical doping as we did in molecular doping (Figure S3a). At the same time, we calculate the density of accumulated charges inside the conjugated polymers by integrating the current density with time (Figure S3b,c).…”

Using Bulky Dodecaborane-Based Dopants to Produce Mobile Charge Carriers in Amorphous Semiconducting Polymers

“…Many studies on regioregular (RR) and regiorandom (RRa) samples of the workhorse semiconducting polymer poly­(3-hexylthiophene-2,5-diyl) (P3HT) have demonstrated that RRa P3HT is largely unable to form crystalline packing regions. − Relative to RR P3HT, RRa P3HT has an increased bandgap energy due to breaks in conjugation at sites of polymer backbone rotation. This lowers the energy of the valence band, so that RRa P3HT is harder to dope .…”

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

“…1,17 The process of electrochemical doping is complex, involving couplings between ion motion, electron transport, and structural changes within the polymer, which depend on many factors, including the electrolyte identity, 9,18 the chemistry of the polymer side chains, 7,8,19 and the nanoscale morphology of the conjugated polymer OMIEC. 6,20,21 While the impact of electrochemical doping on the crystalline regions of OMIECs has been extensively characterized, 6,22,23 the behavior and evolution of the amorphous regions remain less explored.…”

“…26 Additional studies indicate that disordered regions of conjugated polymers are advantageous for ion transport. 18,20,27,28 However, the specifics of how disordered regions evolve upon electrochemical doping remains largely unexplored.…”