Organic bioelectronics deals with the study of organic electronic devices which are operating at the interface of biology and electronics. Its applications range from wearable to implantable devices, which e.g., can work as sensitive biomedical sensors. A perfectly suites material for these applications are organic mixed ionic-electronic conductors (OMIECs) as they enable both ionic and electronic transport. Additionally, they feature mechanical flexibility, can be successfully fabricated in versatile processing conditions, and are synthetically tunable.[1] While recent progress in bioelectronics has focused on device fabrication and comparing efficiency of different materials, there is still a lack of a deeper understanding of the fundamental processes occurring in operating OMIECs.[2] Charge transport happens mainly on the π-conjugated backbone of the organic semiconductor, while ions penetrate the bulk material. PBTTT is a polymer with high charge-carrier mobility. The polymer’s performance can be further improved by incorporating alkyl side chains with an ether group to the backbone. This change in polymer structure facilitates the ion uptake in the polymer matrix, hence enhancing the device efficiency.[3, 4] Therefore, in this study we use high temperature rubbing to orient PBTTT-⁸O films helping to unravel the fundamental functioning of both ion and charge transport. For comparison, the same experiments were carried out with an OMIECs workhorse material P3HT, which has already been characterized by multiple different research groups.[5]

Combining electrochemical and chemical doping with spectroscopic techniques and chronoamperometry, we have investigated the oxidation behavior of the OMIECs. In particular, the scattering frequencies of charge carriers in semiconductors were detected with in-situ terahertz (THz) spectroscopy. This method allows us to acquire the intrinsic nanoscale conductivity and short-range mobility of the studied OMIECs, which is not affected by any grain boundaries or electrodes. Thereby the analysis of the complex THz conductivity unveils the overall mobility and density of charges. We were able to obtain high conductivities of more than 1000 Scm^-1 for oriented P3HT and PBTTT-⁸O using different doping methods. With electrochemical doping, conductivities around 1200 Scm^-1 were obtained for both P3HT and PBTTT-⁸O and with chemical doping conductivities of more than 2000 Scm^-1 were obtained. Furthermore, temperature dependent THz measurements were carried out on the chemically doped samples. With this data, band-like transport behavior in PBTTT-⁸O was confirmed, showing the large effect of high in-plane orientation of OMIECs.