Two-terminal organic electrochemical transistors and their applications

Abstract

Flexible bioelectronics is gaining interest as a vital medium for connecting electronics and biological systems. Organic electrochemical transistors (OECTs), a part of flexible electronics, are highly effective in constructing biochemical and bioelectric sensors due to their intrinsic amplification function and ion-to-electron conversion property. However, conventional OECT sensors seldom combine with other flexible components due to the lack of integration technology. Novel fabric OECT sensor has emerged as a versatile sensing platform due to its portability, low cost, and integration capabilities. In this thesis, a literature review on the profiles and integration technologies of OECT will first be presented. The fundamental information about OECT, including typical three-terminal design, working principles, advantages, and functions, will be introduced in detail. Besides, a widely used theoretical model based on the depletion mechanism will be offered. Furthermore, the characteristics and integration technologies of flexible fiber-shaped and planar OECTs are summarized. Secondly, a novel fabric two-terminal OECT by integrating gate and drain terminals is investigated thoroughly. This unique design combines the gate and drain terminals, allowing the sensor to be fabricated on a single wire rather than the multiple wires required by a typical three-terminal structure. This character significantly reduces fabrication complexities and reliability risks. The two-terminal design has an excellent on/off ratio, low subthreshold swing, and fast response for both P- and N-type OECTs. Besides, a general de-doping/doping model is built successfully to explain the underlying mechanism. Additionally, the fabric two-terminal OECTs are applied to fabricate high-gain inverters and fast-response rectifiers, showing their high potential for wearable electronic textile systems. Lastly, a two-terminal n-type OECT sensor based on BBL organic semiconductor is developed to detect ion concentration. Different from conventional PEDOT:PSS based p-type OECT sensors, n-type sensors have an obvious advantage in low power consumption. The two-terminal configuration of the sensor avoids the decrease in conductivity at high gate voltage due to the anti-ambipolarity property of BBL material. This sensor has an impressive voltage response of nearly 100 mV/dec for NaCl and KCl electrolytes, which largely breaks the theoretical Nernst limit (59.2 mV/dec). Upon this foundation, a highly sensitive fiber-shaped K+-selective sensor has been demonstrated using PVC-based ion selective membrane and hydrogel. The design of this sensor is compatible with traditional weaving techniques and has shown potential for use in wearable electronics with multiple functions. In summary, fiber-shaped OECT sensors with two terminals have been successfully created and used in bioelectronics. Remarkably, an impressive n-type OECT sensor has been developed with low power consumption and high voltage response used for detecting K+ selectively. The fabric OECT sensors can be integrated into a multifunctional e-textile system by weaving them with other fiber-shaped components, such as energy-harvesting devices, energy storage devices, and displays. This development could potentially revolutionize current diagnostic and physiological monitoring methods.