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This perspective offers insights from discussions conducted during the Telluride Science meeting on organic mixed ionic and electronic conductors, outlining the challenges associated with understanding the behavior of this intriguing materials class.

 

The gradual channel approximation forms the foundation for the analysis of field‐effect transistors. It has been used to discuss transistors that are not necessarily based on the field‐effect as well, such as the organic electrochemical transistor (OECT). Here, the applicability of the gradual channel approximation for OECTs is studied by a 2D drift‐diffusion model. It is found that OECT switching can be described by two separate effects—a doping/dedoping mechanism and the formation of an electrostatic double layer at the interface between the mixed conductor and the electrolyte. The balance between these two mechanisms is determined by the morphology of the mixed conductor, in particular the question if ions move in the same phase and electric potential as the holes, or if separate ion and hole phases are formed. It is argued that the gradual channel approximation can only be used to describe electrostatic switching at the mixed conductor/electrolyte interface (the two‐phase model), but cannot be employed to analyze devices operating on a doping/de‐doping mechanism (the one‐phase model).

 

Organic electrochemical transistors (OECTs) transduce ionic into electric signals, which makes them a promising candidate for a wide range of bio-electronic applications. However, despite their promise, the influence of their device geometry on performance is still not fully understood. Here, two different device geometries—top contact and bottom contact OECTs—are compared in terms of their contact resistance, reproducibility, and switching speed. It is shown that bottom contact devices have faster switching times, while their top-contact counterparts are superior in terms of slightly reduced contact-resistance and increased reproducibility. The origin of this trade-off between speed and reproducibility is discussed, which provides optimization guidelines for a particular application. 

Organic electrochemical transistors (OECTs) are highly versatile in terms of their form factor, fabrication approach that can be applied, and freedom in the choice of substrate material. Their ability to transduce ionic into electric signals and the use of bio-compatible organic materials makes them ideally suited for a wide range of applications, in particular in areas where electronic circuits are interfaced with biologic matter. OECT technology has attracted widespread interest in recent years, which has been accompanied by a steady increase in its performance. However, this progress was mainly driven by device optimization and less by targeting the design of new device geometries and OECT materials. To narrow this gap, this review provides an overview on the different device models that are used to explain the underlying physics governing the steady and transient behavior of OECTs. We show how the models can be used to identify synthetic targets to produce higher performing OECT materials and summarize recently reported materials classes. Overall, a road-map of future research in new device models and material design is presented summarizing the most pressing open questions in the understanding of OECTs. 

The first study of the flexo-ionic effect, i.e., mechanical deformation-induced electric signal, of the recently discovered ionic liquid crystal elastomers (iLCEs) is reported. The measured flexo-ionic coefficients were found to strongly depend on the director alignment of the iLCE films and can be over 200 µC/m. This value is orders of magnitude higher than the flexo-electric coefficient found in insulating liquid crystals and is comparable to the well-developed ionic polymers (iEAPs). The shortest response times, i.e., the largest bandwidth of the flexo-ionic responses, is achieved in planar alignment, when the director is uniformly parallel to the substrates. These results render high potential for iLCE-based devices for applications in sensors and wearable micropower generators.