Studies on flexible organic transistors, inverters and memories

Abstract

Flexible electronics is a technology for assembling electronic devices on flexible substrates. Organic flexible electronics are being developed for integrated circuits, solar cells, large area displays, radio frequency identification (RFID) tags, sensors, memories and devices that have not been dreamt of yet. This thesis is mainly concerning various organic logic electronic devices such as transistors, inverters and memories with different kinds of dielectrics on plastic substrate. High-k dielectric materials for electronic applications have stimulated important research activities. We have studied the electrical performances and mechanical properties of organic semiconductor based transistors and inverters on high-k dielectric layers including aluminum oxide (Al2O3), n-octadecylphosphonic acid (ODPA)/Al2O3 bilayer, poly (4-vinylphenol) (PVP), barium zirconate (BZ) nanocomposite and ODPA functionalized aluminum titanate (AT) nanocomposite. We have also developed organic memory devices on Al2O3 with gold (Au) nanocomposite and Au nanoparticle monolayer as the charge trapping layer and systematically studied the electrical and mechanical properties of the fabricated devices. Organic transistors with Al2O3 and ODPA/Al2O3 have been fabricated on flexible PET substrates. We have systematically compared the device performance on the two dielectrics. The transistors on ODPA/Al2O3 exhibit better electrical property and mechanical stability upon bending due to the existence of self-assembled monolayer ODPA. The saturated load inverters on ODPA/Al2O3 have also been fabricated and the device could operate at supply voltage as low as -2 V and achieve a gain up to 23 at -5V. Organic transistors and unipolar inverters with BZ nanocomposite dielectrics have been fabricated. The solution processed dielectric films possess excellent insulating quality and the dielectric constant can achieve 6.7. The effects of incorporating different concentration of BZ nanoparticles into PVP dielectrics for flexible organic transistors have been systematically studied. The morphologies of dielectric and semiconductor films have been investigated and shown to correlate closely with device performances. Two kinds of unipolar inverters based on these transistors have been fabricated and characterized. The depleted load inverter shows large signal gain and sharp switching compared with saturated load inverter. The electrical properties under various compressive and tensile strains evidence the high mechanical stability of the dielectric films hence the transistors and inverters. Organic transistors and complementary inverters with nanocomposites dielectric based on ODPA functionalized AT nanoparticles as dopants in PVP have been fabricated. The dielectric properties of the resulted composite films can be improved by the robust surface layer covering the nanoparticles due to better interfacial adhesion. The dielectric constant of the nanocomposite layer can be enhanced to about 8.2 at 4 vol% doping. Air-stable p-type and n-type transistors fabricated on the composite dielectric layer exhibited superior electrical performance than those on the pristine polymer dielectric layer. The complementary inverters made from these transistors possessed large signal gain and sharp switching. We have systematically studied the electrical performance of the capacitors, transistors and inverters under different applied strain on flexible substrates. All these devices have been found to be mechanically stable in the flexibility test. Air-stable low voltage flexible nonvolatile memory transistors by embedding Au nanoparticles in poly(methyl methacrylate) (PMMA) as charge storage element have been fabricated. The solution processability of this kind of charge trapping layer is suitable for low-cost large area processing on flexible substrates. The memory transistor can work at -5 V and exhibits a memory window of 2.1 V, long retention time (> 105 s). The memory behavior has been tuned by varying the composition of the Au nanoparticles, which offers relatively easy processability for different flexible electronics application. During the bending the electrical properties of the memory devices are found to be stable. Organic memory transistors with Au nanoparticle monolayer on plastic substrates have been investigated at ambient conditions. The devices exhibit low operation voltage, reliable memory characteristics and long data retention time. We have systematically studied the programming and erasing behavior at various bending states. Thermal-induced effects on these memory devices have also been analyzed. The mobility of the memory transistor shows ~200% rise and the memory window increases from 1.48 V to 1.8 V when the temperature rises from 20 ºC to 80 ºC. The retention capability of the devices decreases with the increased working temperature. Our findings provide a better understanding of flexible organic memory transistors under various operating temperatures and bending state.

Date of Award	15 Jul 2013
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Arul Lenus Roy VELLAISAMY (Supervisor) & Shuit Tong LEE (Co-supervisor)