Electrochemical Doping of Organic Mixed Conductors in the Anti-Ambipolar Regime— In Operando Insights

The transduction of ionic and electronic signals by organic mixed ion-electron conducting materials (OMIECs) allows interfacing soft biological tissue with conventional rigid electronics. A key feature of these materials is the control of their conductivity through electrochemical doping. Interestingly, extensive doping beyond a given threshold decreases the conductivity of these materials, leading to an anti-ambipolar response. This anti-ambipolar response has found recent application in artificial neurons, but the underlying principles are yet to be understood.

Additionally, in a broader perspective, the processes limiting (de-)doping kinetics of organic electrochemical transistors (OECTs) have been a topic of significant debate in the community recently. OMIECs operating in the anti-ambipolar regime, however, can be expected to exhibit distinct processes controlling their switching speed that differ from regular operation.

Here, we employ a combination of in-operando characterization techniques to elucidate equilibrium and kinetic processes governing the anti-ambipolar response of state-of-the-art organic mixed conducting materials. From in-operando Fourier-transformed infra-red spectroscopy, we find cation-backbone interactions govern the equilibrium state of anti-ambipolarity through inducing a multiply charged state. These same interactions are found to impose striking morphological changes, characterized by a broad selection of in-operando techniques. Additionally, employing time-dependent UV-vis spectroscopy on an OECT allowed identifying two distinct dedoping-steps upon dedoping from the anti-ambipolar charged state, where the slower second dedoping-step originates from the electrical contacts. These in-operando results are crucial in understanding device characteristics such as switching speed and bias-at-maximum-conductivity, and thereby provide directionality in further research towards optimization and/or novel concepts based on these materials.