Tailored Design of Mixed Ionic-Electronic Conductors for both P-Type and N-Type OECTS

Mixed ionic electronic conductors (MIECs) were designed by various strategies of polymer architecture. For all materials, Cyclic voltammetry (CV), spectroelectrochemistry (SEC), ultraviolet photoelectron spectroscopy (UPS), Ultraviolet-Visible (UV-Vis) absorption spectroscopy, and organic field effect transistor (OFET) characterization were carried out and these properties demonstrate the suitability for OECT (organic electrochemical transistor) devices.<br/>For p-type OECTs, conjugated polyelectrolytes and their copolymers based on polythiophene family was first designed and studied [1-2]. The concept was extended to hydrophilic polythiophenes carrying different oligoethylene glycol (OEG) substituents [3]. The advantage of the latter class is that no crosslinking is necessary to prevent the thin films from delamination. The concept of OEG substitution and the required degree of OEG substitution was further applied in the class of poly(diketopyrrolopyrrole) copolymers using different comonomers [4]. For this, four Polydiketopyrrolopyrroles (PDPPs) with increasing ethylene glycol (EG) content and varying nature of comonomer were synthesized. The studies of these hydrophilic PDPPs in NaCl electrolyte-gated OECTs reveal that a high amount of OEG on the DPP moiety is essential for MIEC. The PDPP containing 52 wt.% EG exhibits a high volumetric capacitance of 338 Fcm^-3 (at 0.8 V), a high hole mobility in aqueous medium (0.13 cm²V^-1s^-1) and a µC* product of 45 Fcm^-1V^-1s^-1. OECTs using this polymer retained 97 % of its initial drain-current after 1200 cycles (90 min of continuous operation). In cell-growth medium, the OECT-performance was fully maintained as in NaCl electrolyte. In vitro cytotoxicity and cell viability assays reveal the excellent cell compatibility of these novel systems, showing no toxicity after 24 h of culture. Due to the excellent OECT performance with a considerable cycling stability for 1200 cycles and an outstanding cell compatibility, these PDPPs render themselves viable for in vitro and in vivo bioelectronics.<br/>For the n-type OECTs, two different classical acceptor motifs diketopyrrolopyrrole (DPP) and thienopyrrolodione (TPD) are copolymerized to yield the acceptor-acceptor polymer, Poly(DPP-TPD) [5]. The fundamental design idea is to maximize the electron affinity, thus increasing the ambient stability of the reduced state against oxygen and water while ensuring high ion compatibility through the incorporation of hydrophilic oligoethylene glycol N-substituents. Additionally, a highly planarized polymer structure is anticipated, due to the extended non-covalent interactions (conformational locking) between the carbonyl oxygen and the thiophene protons. The acceptor-acceptor system poly(DPP-TPD) for n-type devices exhibit high electron affinity (EA), and good electron mobility (µ_e(OFET)), in addition to the electrochemical reduction of polymer films in aq. electrolyte. In n-type OECTs, poly(DPP-TPD) demonstrates a moderate threshold voltage of Vth = 0.58 V, and an outstanding µC* value of 7.62 F cm^-1 V^-1 s^-1. Cycling studies consisting of pulsed on- and off-switching of the device at gate voltages between Vg = 0.6 - 0.8 V in the saturation regime reveal high stability for more than 2700 cycles with rapid switching kinetics.<br/>[1] Brendel et al. Chem. Mater. 2014, 26, 1992–1998<br/>[2] Schmode et al. Chem. Mater. 2019, 31, 14, 5286-5295<br/>[3] Schmode et al., ACS Appl. Mater. Interfaces, 2020 12, 13209-13039<br/>[4] Krauss et al., Adv. Funct. Mater. 2021, 31, 2010048<br/>[5] M Thelakkat et al., Adv. Electron.Mater.2023, 2300026