Rational Device Design and Doping‐Controlled Performance in Fast‐Response π‐Ion Gel Transistors

ABSTRACT Organic electrochemical transistors (OECTs) are attracting great attention for wearable and bioelectronic devices due to their mechanical flexibility, biocompatibility, and low‐voltage operation. In our previous report, we introduced a new class of OECTs, π‐ion gel transistors (PIGTs), as fast‐response supramolecular transistors. Despite this proof‐of‐concept demonstration, the structure–property relationships governing charge transport and electrochemical performance have remained unclear. Moreover, key factors such as device geometry, material selection, and dopant effects have not been systematically investigated. Here, we comprehensively examine the structural and electronic parameters that determine PIGT performance. Volumetric capacitance spectroscopy clarifies the electrochemical doping mechanism and reveals distinct regimes of charge injection under gate bias. Using regio‐regular poly(3‐hexylthiophene) (P3HT) as a high‐mobility channel polymer, PIGTs exhibit enhanced transconductance (2.96 mS). However, a relatively low on/off ratio arises from offset current caused by intrinsic oxidation. To address this, 1,3‐dimethyl‐2‐phenyl‐2,3‐dihydro‐1H‐benzimidazole (BIH) is employed as an n‐type dopant, yielding substantial improvements in device stability and switching behavior (on/off ratio ∼3.4 × 10 4 ). Charge‐transport dynamics are further elucidated by terahertz time‐domain spectroscopy, which links electrochemical modulation to carrier conduction. Overall, these optimizations establish a robust device platform for PIGTs and highlight their potential for practical applications.