Studies on Low-Voltage Flexible Electronics and Applications

Abstract

Flexible electronics have been demonstrated the potential applications for touch-on displays, electronic skin (e-skin), healthcare monitoring devices and so on. However, the poor stability, sophisticated technology, expensive cost, and relatively high operating voltage have limited their development for practical applications. This thesis mainly focuses on the fabrication of high-performance, low-voltage flexible electronics including organic filed-effect transistors (OFETs) and pressure sensors with a cost-effective yet efficient approach. To obtain the high stable OFETs, the effect of semiconductor/dielectric interface on device performance was investigated. We have successfully obtained devices with bilayer dielectrics that show high mobility and good operational stability. The field-effect mobility (µFE) increases from 0.19 (without surface modification) to 0.35, 0.51, and 0.97 cm2V-1s-1, respectively, by employing a thin layer of hexamethyldisilazane (HMDS), polymethylmethacrylate (PMMA), and polystyrene (PS). The PS-modified device exhibits the superior mobility, operational, and ambient stability in comparison with the other devices. We ascribe these results to the reduced traps caused by the OH group or OH-induced water molecule after the modification. In our study, the morphology of the semiconductor layer shows limited influence on the mobility and operational stability.
High-k metal oxide dielectrics, especially for the solution-processed ones, have motivated the research interests by virtue of their excellent properties. We have successfully fabricated low-voltage OFETs based on the solution-processed high-k metal oxide dielectrics including aluminum oxide (Al2O3), hafnium dioxide (HfO2), and yttrium oxide (Y2O3) on commercially available aluminum foil substrate. A thin polystyrene (PS) layer was spin-coated on top of the metal oxide dielectric to reduce the leakage current (IGS) and interfacial traps. The transistors based on the metal oxide dielectrics exhibit a high µFE, a large on/off current ratio (Ion/Ioff), a low threshold voltage (VT) and a small subthreshold swing (SS) at a low operation voltage (4 V), which is comparable with the device performance on rigid Si substrate. The excellent electrical performance of the OFETs on commercially available Al foil substrate, together with the virtue of light-weight, suggested the promising potential for flexible electronics.
We have successfully fabricated large-scale printed Al2O3 dielectric on Al foil substrate for low-voltage OFETs and tactile sensors. For the low-voltage OFETs, the µFE is estimated to be 0.65 cm2V-1s-1, the Ion/Ioff is up to 105, and the SS is 90 mV/decade, which is close to the ideal value 60 mV/decade at room temperature. The proposed tactile sensor is able to detect pressure as low as 500 Pa, which fulfills the requirement of the electronic skin application in future. This technique presents a feasible approach to fabricate low-cost flexible metal oxide dielectrics in large-scale. Such printed metal oxide dielectrics on Al foil have great potential applications in future flexible electronics.
We have successfully demonstrated a cost-effective, relatively simple, and high-yield approach to fabricate the low-voltage, highly sensitive pressure sensors with the graphite and polydimethylsiloxane (PDMS) composite as the active layer. The pressure sensor exhibits fascinating performance including an ultra-high sensitivity of 64.3 kPa-1, a detection limit of 0.9 Pa, a fast response time of 8 ms, long-term durability over 100,000 cycles and fascinating ambient stability over 1 year. By virtue of its compelling performance, its applications in sensing radial artery pulses, speech recognition, and human body motion are demonstrated, exhibiting its enormous potential applications in real-time healthcare monitoring and artificial intelligence.

Date of Award	4 Sept 2017
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Arul Lenus Roy VELLAISAMY (Supervisor)