Backbone Tailoring Enables High‐Performance and Stable n‐Type Organic Mixed Ionic‐Electronic Conductors for Synaptic Simulation and Biosensor

High‐performance and stable n‐type organic mixed ionic‐electronic conductors (OMIECs) are crucial for advancing organic electrochemical transistors (OECTs)‐based low‐power complementary circuits and biosensors, yet their development remains a great challenge. Herein, the study presents a series of donor‐acceptor polymers incorporating bithiophene (BTI) and fused BTI derivatives with varying conjugation backbone lengths as acceptors. The mid‐size fused BTI dimer enables polymer PBTI2g‐DTCN with simultaneously improved ion‐uptake capability, film structural order, and ion/electron transport capability. Consequently, an impressive electron mobility of 0.84 cm2 V−1 s−1 and a record figure‐of‐merit (µC*) of 287.8 F cm−1 V−1 s−1 are achieved for PBTI2g‐DTCN‐based n‐type conventional OECT in accumulation mode, while the vertical OECTs (vOECTs) attain a state‐of‐the‐art area‐normalized transconductance (gm,A) of 71.8 µS µm−2 with remarkable operational stability. Through finely manipulating the channel components, the vOECTs demonstrate dual‐mode operation, switching between non‐volatile and volatile states. In non‐volatile mode, vOECT‐based artificial synapses with excellent ambient stability enable dynamic learning and are employed in convolutional neural networks for image recognition. In volatile mode, they excel in biosensing, monitoring electrocardiography and electromyography signals. These remarkable results demonstrate that backbone tailoring is a powerful strategy for developing high‐performance n‐type OMIECs for synaptic and sensor applications.