Ultrastable N‐Type Semiconducting Fiber Organic Electrochemical Transistors for Highly Sensitive Biosensors

Abstract Organic electrochemical transistors (OECTs) have attracted increasing attention due to their merits of high transconductance, low operating voltage, and good biocompatibility, ideal for biosensors. However, further advances in their practical applications face challenges of low n‐type performance and poor stability. Here, it is demonstrated that wet‐spinning the commercially available n‐type conjugated polymer poly(benzimidazobenzophenanthroline) (BBL) into highly aligned and crystalline fibers enhances both OECT performance and stability. Although BBL is only soluble in high‐boiling‐point strong acids, it can be wet‐spun into high‐quality fibers with adjustable diameters. The BBL fiber OECTs exhibit a record‐high area‐normalized transconductance (g_m,A) of 2.40 µS µm⁻² and over 10 times higher figure‐of‐merit (µC*) than its thin‐film counterparts. More importantly, these fiber OECTs exhibit remarkable stability with no noticeable performance attenuation after 1500 cycles over 4 h operation, outperforming all previously reported n‐type OECTs. The superior performance and stability can be attributed to shorter π–π stacking distance and ordered molecular arrangement in the fibers, endowing the BBL fiber OECT‐based biosensors with outstanding sensitivity while keeping a miniaturized form factor. This work demonstrates that, beyond new material development, developing new fabrication technology is also crucial for addressing the performance and stability issues in n‐type OECTs.