All Printed Resistive Switching Memory Devices Based on Solution Processed Materials for Wearable Electronics

Abstract

In recent, wearable and printed memory devices have got tremendous attention because it is highly desirable for the next-generation technologies and customized applications. Printed electronics have the potential to revolutionize the spread of electronic applications and can be easily interfaced with human machine interface devices and systems. In this thesis, we address the solution processed based flexible printed resistive memory devices through a spin coating technology. These devices can be mounted on any desire surface. To realize these devices, we utilize a spin coating technology to fabricate memory device on ITO coated flexible PET substrate. To summarize, we fabricated three printed resistive switching devices using PEDOT: PSS, ZnO, PVP, ZrO2, TPD and PVOH. We also optimized the memristor design and materials for high ROFF/RON ratio, long retention time, high stability and endurance cycles. We also investigated different memristor mechanisms to support its performance. Different structures were investigated based on heterojunction and composite of two different materials to optimize following parameters like device stability, memory device with high charge density and to control the sneak current problem in cross bar array of memory devices as given in section 2.2, 2.3 and 2.4. In section 2.2, initially we propose a stable non-volatile resistive switching based on nano-composite of inorganic and organic materials, Zirconium dioxide (ZrO2) and Poly (4-vinylphenol) (PVP), respectively. We improved the stability of the memory device by mixing different blending ratios of PVP with ZrO2 and we achieved a stable memory function. The novel heterojunction junction memory device is proposed in section 2.3, which is based on N,N′-Bis (3-methylphenyl)-N,N′- diphenylbenzidine (TPD) and Poly(3,4-ethylenedioxythiophene)- poly(styrenesulfonate)/ Poly(vinyl alcohol) (PEDOT:PSS/PVOH) composite. This memory device is highly stable and at some instant and we reduced the sneak current in negative axis. In order to completely block sneak current in memory device and to realize an asymmetric function, we propose a novel schottky diode resistive switching device based on zinc oxide (ZnO) and poly(3,4- ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) heterojunction in section 2.4. For high density integrations of nonvolatile memory devices, crossbar arrays with single bit memristor cells are demonstrated. However, those crossbar designs incur severe sneak currents problems, which are resulted in high read/write error and large power consumption during the individual cell access. In a passive crossbar resistive array, each row-column pair is connected by a resistor that can be either in the HRS or the LRS corresponding to the logic value stored in the cell. The sneak path problem occurs when a memory cell in the HRS is being read, while a series of cells exist in parallel to it, if the neighboring cells are in LRS the sneak current issue is more exacerbated thereby causing it to be erroneously read. The sneak currents not only make the data erroneous but also dissipate extra power because the currents are flown through the neighbor memristors and added to the element current at the same terminal. Especially, in large size arrays, the sneak currents problem becomes more pronounced, thus individual bits cannot be explicitly accessed. As techniques to overcome the sneak currents problem, a set of different approaches have been reported in the literature including a rectifying element connected to a memristor in series at each cross-point and anti-serial memristors complementary resistive switches. All these attempts would minimize the sneak currents, but until now the sneak current problem in a passive crossbar array has not been addressed satisfactorily. So this research opens a way to next-generation printed and flexible resistive switching devices and its customized applications. Hence, the proposed schottky diode based memory device can be applied in flexible resistive switching devices to blocking sneak current problem. The fabricated resistive switching devices were tested for their electrical, optical, chemical, structural and mechanical behavior to conform and verify the solution processed based device fabrication approach.