Investigation of ionic liquid induced doping in all-solid-state organic electrochemical transistors

Abstract

The momentous discovery of conducting polymers in 1977 completely shattered the outlook of the polymer science as a branch of insulators and set forth monumental developments leading to the evolution of a comprehensive library of π conjugated polymers. Notwithstanding their menial charge carrier mobility, they have unique advantages over the conventional semiconductors such as the potential for altering their properties for specific applications through molecular engineering, their compatibility with various fabrication technologies such as solution processing and vacuum processing, and their physical attributes such as softness, flexibility, and low weight. Organic electrochemical transistors (OECTs) are a new generation of transistors in the subfield of bioelectronics which are structured like field effect transistors, except the drain current is controlled by the volumetric injection of ions into the channel from an electrolyte. This distinction is one of the defining characteristics of OECTs which is responsible for their unique ion-to-electron transduction properties. However, the use of liquid electrolyte limits the application of OECTs as the doping process is complicated by the presence of water and changes in the hydration radius of the ions present during doping. Moreover, they are susceptible to electrolyte leakage, and thus not suitable for wearable sensors. Solid electrolytes, however, are not susceptible to these drawbacks, and opens other applications for OECTs beyond bioelectronics such as pressure sensors, photodetectors etc. In the present study, a solid gel electrolyte made from the polymer poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-co-HFP), and ionic liquids (ILs) 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIM BF4), and 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM OTF) are utilized to study the performance of solid state OECTs (SSOECTs). The effect of IL concentration on the static and dynamic response of devices is studied in detail. It is found that the electrochemical doping ability of the electrolyte is affected by the ionicity of the ionic liquids used in the electrolyte. Further, the performance gap between the ILs is bridged by improving the electrochemical accessibility through additive engineering the channel layer Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS). Towards that end, the effects of an ionic additive 1-Ethyl-3-methylimidazolium chloride (EMIM Cl), and a non-ionic additive Polyethylene glycol (PEO) are studied using various material characterisations such as insitu Raman, spectroelectrochemistry, X-ray photoelectron spectroscopy (XPS), GIWAX, cyclic voltammetry, electrochemical impedance spectroscopy, and AFM.