Effect of ionic conductivity of electrolyte on printed planar and vertical organic electrochemical transistors.

Conducting polymers with mixed electronic/ionic transport are attracting a great deal of interest for applications in organic electrochemical transistors (OECTs). Ions play a crucial role in OECT performance. The concentration and mobility of ions in the electrolyte influence the current flow in the OECT and its transconductance. This study examines the electrochemical properties and ionic conductivity of two semi-solid electrolytes, iongels, and organogels, with diverse ionic species and properties. Our results indicate that the organogels exhibited higher ionic conductivities than the iongels. Furthermore, the geometry of OECTs plays an important role in determining their transconductance. Thus, this study employs a novel approach for fabricating vertical-configuration OECTs with significantly shorter channel lengths planar devices. This is achieved through a printing method that offers advantages, such as design versatility, scalability, expedited production time, and reduced cost relative to traditional microfabrication methods. The transconductance values obtained for the vertical OECTs were significantly (approximately 50 times) higher than those of the planar devices because of their shorter channel lengths. Finally, the impact of different gating media on the performance of both planar and vertical OECTs was studied, and devices gated by organogels demonstrated improved transconductance and switching speed (almost two times higher) than those gated by iongels.

All-printed and stretchable organic electrochemical transistors using a hydrogel electrolyte.

Stretchable electronic devices are expected to play an important role in wearable electronics. Solution-processable conducting materials are desirable because of their versatile processing. Herein, we report the fabrication of fully stretchable organic electrochemical transistors (OECTs) by printing all components of the device. To achieve the stretchability of the whole body of the devices, a printed planar gate electrode and polyvinyl alcohol (PVA) hydrogel electrolyte were employed. Stretchable silver paste provided a soft feature to drain/source, gate and interconnect, without any additional strategies needed to improve the stretchability of the metallic components. The resulting OECTs showed a performance comparable to inkjet or screen-printed OECTs. The maximum transconductance and on/off ratio were 1.04 ± 0.13 mS and 830, respectively. The device was stable for 50 days and stretched up to 110% tensile strain, which makes it suitable for withstanding the mechanical deformation expected in wearable electronics. This work paves the way for all-printed and stretchable transistors in wearable bioelectronics.

Human sweat monitoring using polymer-based fiber.

Lightweight nano/microscale wearable devices that are directly attached to or worn on the human body require enhanced flexibility so that they can facilitate body movement and overall improved wearability. In the present study, a flexible poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) fiber-based sensor is proposed, which can accurately measure the amount of salt (i.e., sodium chloride) ions in sweat released from the human body or in specific solutions. This can be performed using one single strand of hair-like conducting polymer fiber. The fabrication process involves the introduction of an aqueous PEDOT:PSS solution into a sulfuric acid coagulation bath. This is a repeatable and inexpensive process for producing monolithic fibers, with a simple geometry and tunable electrical characteristics, easily woven into clothing fabrics or wristbands. The conductivity of the PEDOT:PSS fiber increases in pure water, whereas it decreases in sweat. In particular, the conductivity of a PEDOT:PSS fiber changes linearly according to the concentration of sodium chloride in liquid. The results of our study suggest the possibility of PEDOT:PSS fiber-based wearable sensors serving as the foundation of future research and development in skin-attachable next-generation healthcare devices, which can reproducibly determine the physiological condition of a human subject by measuring the sodium chloride concentration in sweat.