In-Plane Orientation of Organic Mixed Ionic-Electronic Conductors to Enhance Charge Transport Properties

Organic mixed ionic-electronic conductors (OMIECs) are perfectly suited to interface biology and electronic devices as they enable both ionic and electronic transport; feature mechanical flexibility; can be successfully fabricated in versatile processing conditions; and are synthetically tunable.¹ While most recent progress in bioelectronics has been focused on device fabrication and comparing performance of different materials, there is still a lack of a more in-depth understanding of the fundamental processes occurring in functioning OMIECs.² The charge transport is happening mainly on the π-conjugated backbone of the organic semiconductor, while ions penetrate the bulk material. In PBTTT, a polymer with high charge-carrier mobility, ion uptake in the polymer matrix can be facilitated by incorporating alkyl side chains with an ether group, further improving the performance.^3,4Therefore, in this study we use high temperature rubbing to orient PBTTT-⁸O films helping to unravel the fundamental processes of both the charge and ion transport. For comparison the same experiments were carried out with an OMIECs model material P3HT, which was already characterized by multiple different research groups.⁵<br/>Combining electrochemical and chemical doping with spectroscopic techniques and chronoamperometry the oxidation behavior of the OMIECs was investigated. Foremost, with <i>in-situ </i>terahertz (THz) spectroscopy we can detect the scattering frequencies of charge carriers in semiconductors, this allows us to get the intrinsic nanoscale conductivity and short-range mobility of the studied OMIECs, which is not affected by any grain boundaries or electrodes. The analysis of the complex THz conductivity unveils the mobility and density of charges altogether. We were able to get conductivities of more than 1000 Scm^-1 for P3HT and PBTTT-⁸O for electrochemical doping. Furthermore, with chemical doping conductivities of more than 2000 Scm^-1 were obtained and band like transport behavior was observed for PBTTT-⁸O, showing the large effect of high in plane orientation of OMIECs.<br/><br/>[1] J. Rivnay, R. M. Owens and G. G. Malliaras, <i>Chem. </i><i>Mater.</i>, 2014,<b> 26</b>, 679-685.<br/>[2] J. Chung, A. Khot, B. M. Savoie and B. W. Boudouris, <i>ACS Macro Letters</i>, 2020, <b>9 </b>(5), 646-655.<br/>[3] J. E. Cochran, M. J. N. Junk, A. M. Glaudell, P. L. Miller, J. S. Cowart, M. F. Toney, C. J. Hawker, B. F. Chmelka and M. L Chabinyc, <i>Macromolecules</i>, 2014, <b>47</b>, 19, 6836-6846.<br/>[4] P. Durand, H. Zeng, T. Biskup, V. Vijayakumar, V. Untilova, C. Kiefer, B. Heinrich, L. Herrmann, M. Brinkmann and N. Leclerc, <i>Adv. </i><i>Energy Mater</i>, 2022, <b>12</b>, 2103049.<br/>[5] D. Khodagholy, J. Rivnay, M. Sessolo, M. Gurfinkel, P. Leleux, L. H. Jimison, E. Stavrinidou, T. Herve, S. Sanaur, R. M. Owens and G. G. Malliaras, <i>Nat. Commun.</i>, 2013, <b>4</b>, 2133.