A Study on Charge Storage mechanism using gold nanoparticles to understand the Electrical Bistability in two-terminal Organic Memory Devices

Organic electronics has taken leaps and bounds in the past few years. From LED screens to wearable technology, it has enabled the emergence of technologies that would have been unimaginable a few decades earlier. The organic memory devices have emerged to be a perfect alternative in the non-volatile memory sector due to its compatibility in the printed technology, low power application and device performance too. Despite these advantages progress in the field has been slow due to the lack of clarity around the device operation. The field has seen several works that have focussed on the same device or even similar structures, only to arrive in vastly different conclusions. A persistent lack of agreement in this regard, despite some excellent work having been done in the field, point to a gap in knowledge in the physics of the device operation. The focus of this work has been to address this uncertainty of device operation in two terminal memory devices. This work was focussed on two terminal gold nanoparticle devices that have been fabricated by drop casting on gap cells. The problem definition was based on a literature survey of several work that had been conducted in this area for around two decades. Several mechanisms that were proposed as a result of several investigations have been identified and arranged in a manner to obtain a holistic view of the subject. Several studies have been investigated closely to recognise gaps in the experiment design and isolate factors that might have contributed to the final result. As a result, gold nanoparticle devices were carefully designed to avoid any parasitic contribution from these external factors. These devices were investigated under varying electric field to study the behavioural response. The result has shown that the device operation relies heavily on the charge storage to bring about a change in the resistance. The device properties were tested by Impedance Spectroscopy measurement, in order to verify if the change in device conduction was due to resistive switching (like filamentary conduction, valence change switching etc.) or due to charge storage. The results demonstrated that the charge storage in the nanoparticles were responsible for the resistance change of the device. This was further confirmed by conducting the impedance measurement on the device at zero-DC-bias after applying a non-zero bias to measure the remnant charge that are trapped in the devices. These tests revealed the presence of trapped charges in the device. However, a coexisting relationship of resistance and capacitance was observed too, where the ligands and the gold nanoparticle dispersion played an important part in the baseline resistance of the system. This was due to the nanoparticle dispersion and the electrode contact- varying which demonstrated a change in the baseline resistance, while the capacitance remained comparable. The retention and the ON/OFF ratio were also investigated for the devices, demonstrating the need for further isolation of the nanoparticles in the devices.