Modulating contact resistance of organic electrochemical transistors via Cu_xO interlayer

Organic electrochemical transistors (OECTs) have garnered significant attention in biosensing and ion detection due to their high transconductance in aqueous conditions and flexible device architectures. However, the nonlinear behavior of the contact resistance under lateral ion drift remains poorly understood. Herein, we introduce a Cu_xO thin-film interlayer, prepared via sol-gel and annealed at various temperatures, to modulate the Au-semiconductor contact resistance of OECTs. Increasing the annealing temperature raises the Cu²⁺ content and continuously shifts the interlayer HOMO level from -7.11 to -5.02 eV, achieving improved energy-level alignment. With the optimized Cu_xO interlayer (annealed at 250 °C), the OECT exhibits an order-of-magnitude higher peak current, a materials-system figure of merit μC* of 21.16 F cm^-1 V^-1 s^-1, stable pulsed operation up to 3600 s, and a 5-fold reduction in contact resistance. Then, a drift-diffusion model for contact resistance was developed and reproduces the observed current-voltage response, providing a framework for mitigating these losses. Furthermore, the resulting lower contact resistance also allows reduction of the channel length, improving the ionic limit of detection (LOD) from 10^-5 to 10^-9 M. These results, consistent with our model, provide new insights into OECT operation and source-drain interface engineering, offering a design strategy for high-sensitivity and miniaturized OECT sensors.