Ionic Liquid Crystal Elastomer as Solid Electrolyte for Organic Electrochemical Transistors

Abstract

The study of Organic Electrochemical Transistors (OECTs) has been growing since its discovery in the early 1980s. OECTs have garnered considerable interest for chemical and biological sensing and bioelectronics due to their low cost, the tunability of the organic molecules, low temperature processing, the ease of scaling, high sensitivity, and their stability in an aqueous environment. Due to their low gate voltage operating, they are considered as efficient switches and powerful amplifiers. Present OECTs mainly use liquid electrolytes in OECTs, although there is a growing need for solid electrolytes since they can be easily integrated into wearable devices. Recently our group developed ionic liquid crystal elastomers (iLCEs) and demonstrated that they can be used as solid electrolytes of OECTs This will broaden the range of applications toward medical health monitoring and soft robotics. In my dissertation, I further studied iLCEs as solid electrolytes for OECTs. I describe the role of various ionic liquids in the performance of the iLCE using the same LCE. It is found that the ionic liquid that phase separates from the reactive mesogenic monomers and forms micron size ionic channels performs better than the less phase separating ionic liquid. The switching time of the order of a second was found to be possible using the ionic liquid with smaller ions and larger channels. These results provide many exciting opportunities in various areas and impact future applications, although they are far from optimized. Fine tuning of the high-performance iLCEs requires an intricate compatibility between the LCE and the electrodes and appears to be laborious. Secondly, I demonstrated that iLCE based OECTs can function as highly sensitive bending sensor. The transfer curve of the iLCE-based OECT shows opposite variation with upward and downward bending representing a directional sensitive bending sensor. The change in the drain current is 4 orders of magnitude larger than the flexo-ionic current. This high sensitivity is due to the OECT that amplifies the flexo-ionic current. iLCE based OECTs therefore present a new class of curvature sensors that may find applications in soft robotics and biosensing. In addition to improving understanding, I studied the influence of device geometry on the performance of iLCEs based OECTs. The results indicate excellent electrical response with a higher switching ratio of 105. Also, the normalized maximum transconductance gm/w of the most sensitive iLCE was found to be 33 Sm-1. Overall, the dissertation provides an understanding of iLCE-based OECTs and will facilitate optimized design paving the way for the future research in this novel technology.