In-Situ Characterization of Ordering/Disordering in Electrochemically Doped Oligo-Ether Sidechain Modified Polythiophenes

Organic mixed ionic and electronic conductors (OMIECs) are a novel material class, emerging as potential enablers for devices as diverse as actuators, biosensors and neuromorphic computing. In an OMIEC, volumetric doping (gating) of the active material is achieved through ingress of electrolyte ions, under potential control. This opens unique transduction modalities due to the mixed (ionic and electronic) modes of conduction. However, the complex changes in film structure: swelling by both electrolyte and counter ion coupled with electrochemical doping, requires in-situ and in-operando measurements to develop quantitative process-structure-function relationships. Side-chain modification, typically by the introduction of oligoethylene glycol (oEG) units, of traditional semiconducting polymers, has emerged as a powerful paradigm for OMIEC material design. We have applied in-situ grazing incidence X-ray diffraction (GIXD) to the study of 3 oEG modified polythiophenes, p3MEEMT[1], p(g3T2) and p(g1t2-g5t2).[2] All are regio-regular, but differ in terms of the length of the oEG side chain (2 units for P3MEEMT, 3 units for p(g3T2), and up to 5 units for p(g1T2-g5T2)) and in repeat unit symmetry. While the optoelectronic properties of these polymers are similar, consistent with the common backbone, the swelling behavior of the polymers were notably distinct. With moderate thermal annealing, all polymers exhibit semicrystalline behavior, typical of polythiophenes, with an edge-on habit of pi-stacked lamella. When characterized in-situ we find a remarkable disordering of the p(g3T2) in the neutral, dedoped state. Effectively no diffraction can be observed. However, a degree of order comparable to that observed ex-situ (dry) is obtained upon doping. This severe doping induced ordering is similar to that observed in doped regio-random P3HT[3] and is consistent with the enhanced pi-pi interaction (evidenced by a decrease in the interchain separation) typically observed for doped polythiophenes. This electrolyte induced “melting” is also present in the asymmetric sidechain p(g1T2-g5T2) but is absent in the shorter side chain polymer. While significant effort has been placed on characterization of swelling in OMIECs, the observation of nominal melting of semicrystalline material has not been extensively discussed and may limit the oEG content. In general, order within the polymer is considered important to electronic transport [4] thus the doping induced ordering may be essential to the final device performance.<br/>[1] Flagg, L.Q. et al. J. Am. Chem. Soc. 2019, 141, 4345, DOI:10.1021/jacs/8b12640<br/>[2] Moser, M. et al. Adv. Mater 2020, 32, 2002748, DOI: 10.1002/adma.202002748<br/>[3] Lim, E. et al. Adv. Elec. Mater 2019, 5, 1800915, DOI: 10.1002/aelm.201800915<br/>[4] Jacobs, I.E. J. AM. Chem. Soc. 2022, 144, 3005, Doi: 10.1021/jacs.1c10651