The Effect of Organic Semiconductor Electron Affinity on Preventing Parasitic Oxidation Reactions Limiting Performance of n‐Type Organic Electrochemical Transistors

A key challenge in the development of organic mixed ionic‐electronic conducting materials (OMIEC) for high performance electrochemical transistors is their stable performance in ambient. When operating in aqueous electrolyte, potential reactions of the electrochemically injected electrons with air and water could hinder their persistence, leading to a reduction in charge transport. Here, the impact of deepening the LUMO energy level of a series of electron‐transporting semiconducting polymers is evaluated, and subsequently rendering the most common oxidation processes of electron polarons thermodynamically unfavorable, on organic electrochemical transistors (OECTs) performance. Employing time resolved spectroelectrochemistry with three analogous polymers having varying electron affinities (EA), it is found that an EA below the thermodynamic threshold for oxidation of its electron polarons by oxygen significantly improves electron transport and lifetime in air. A polymer with a sufficiently large EA and subsequent thermodynamically unfavorable oxidation of electron polarons is reported, which is used as the semiconducting layer in an OECT, in its neutral and N‐DMBI doped form, resulting in an excellent and air‐stable OECT performance. These results show a general design methodology to avoid detrimental parasitic reactions under ambient conditions, and the benefits that arise in electrical performance. N‐type electrochemical transistors generally exhibit lower performance than their p‐type analogs. The origins of the lower charge carrier mobility in electron‐transporting organic semiconducting polymers are explored and observed that parasitic reactions involving ambient oxygen are a contributing factor. It is shown that a molecular design strategy to increase the polymer electron affinity successfully eliminates these reactions.