High-Performance All-Printed Vertical Step Organic Electrochemical Transistors for Flexible Bioelectronics and Logic Circuit Integration

Printed organic electrochemical transistors (OECTs) are promising for flexible bioelectronics due to their low operating voltage, high transconductance, and mechanical flexibility, which enable seamless integration with soft biological tissues. However, printed planar-channel OECTs typically suffer from a slow transient response, mainly owing to the printing resolution, which restricts their use in high-speed logic circuits and high-throughput sensing. This work presents all screen-printed vertical step OECTs (VS-OECTs) on a flexible substrate, using poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as a channel material, poly(sodium 4-styrenesulfonate) (PSSNa)-based conductive hydrogel as an electrolyte, and Ag/AgCl paste as a top gate. In this vertical design, the source and drain electrodes are separated by an insulating layer, forming a vertical step structure. This vertical structure offers advantages over conventional planar-channel structures, where higher source-drain current Ids (∼0.45 mA), higher transconductance gm (∼1 mS), higher ON/OFF ratio (2.6 × 104), faster switching time (1.27 ms to turn on and 8.4 ms to turn off), and better pulsing stability (>96% after 1000 gate pulse) can be attained. Bending tests and various substrate printing validate the flexibility and universal printability of the vertical structures. Additionally, a unipolar inverter based on printed VS-OECTs operates at a high frequency (∼100 Hz), and effective signal amplification for electrocardiogram (ECG) and wrist artery pulse monitoring has been demonstrated, highlighting the potential of printed VS-OECTs for personal health monitoring. These findings propose a promising approach for producing large-area and high-performance printed OECTs, paving the way for the development of all-printed transistors with fast response times for various applications.