Printed Electronics: Oxide/Organic Composite Thin-Film Transistors, and Electrochemical Sweat Sensors

Abstract

Electronic devices and appliances have become an essential and integral part of our daily lives. These products are typically fabricated using energy intensive and environmentally harmful processes such as vacuum deposition and subtractive etching. Printing methods allow the manufacture of electronics under ambient conditions, with conservation of materials being achieved by direct additive patterning. A range of different materials can be print patterned, including conductors, insulators, and semiconductors. Therefore, printing methods are capable of producing electrode structures and active electronic devices. However, gaps in existing knowledge pose challenges for printed electronics technology. There is a lack of well performing printable materials for oxide thin-film transistor (TFT) contact electrodes. Printed In2O3 TFTs have poor optoelectronic response to frequencies lower than blue light. Scalable fabrication pathways are required for single use electrochemical sensors. This thesis work addressed these problems through two parallel research tracks. The first track investigated whether inkjet printed organic/oxide composites could improve the electrical and optoelectronic performance of inkjet printed In2O3 TFTs. The second track studied mass manufactured roll-to-roll (R2R) rotary screen-printed electrodes functionalised to form single use sensors for the analysis of biomarkers in sweat. In Publication I, a polyethyleneimine (PEI)-In2O3 composite material was inkjet printed as a charge injection layer between inkjet printed films of In2O3 and Ag contact electrodes. This interfacial engineering reduced the contact resistance by an order of magnitude, improving the saturation mobility to be comparable with devices having vacuum deposited Al contacts. It was then found in Publication II that TFT electrical performance metrics including saturation mobility and on/off ratio are improved by avoiding doping of the channel by print patterning of the PEI-In2O3 composite only in regions between the semiconductor and top contact electrodes. In Publication III, an inkjet printed rhodamine 6G-In2O3 composite material was observed to enhance the photosensitivity and responsivity of In2O3 TFTs beyond the typical limit of blue light, to green light at 565 nm. In Publication IV, a polypyrrole/Prussian blue molecularly imprinted polymer film was electrodeposited onto R2R printed electrodes to produce a sensor for the detection of cortisol stress hormone in sweat. Then in Publication V, R2R printed electrodes were functionalised with lactate oxidase enzyme and formed into a single use skin worn sensor patch that connects to a flexible wireless electronic readout tag. The sensor platform was demonstrated for monitoring of lactate in exercise induced sweat from four body locations simultaneously. Future research could explore the integration of TFT and biosensor components together as a fully printed sensor system.