AbstractsComputer Science

Understanding and optimization of electrical characteristics of organic devices

by Akash Nigam

Institution: Monash University
Department: Department of Electrical and Computer Systems Engineering and Indian Institute of Technology Bombay
Year: 2015
Keywords: Organic electronics; Charge transport; Strain; High temperature; Capacitance voltage; Current voltage; Mobility; Injection barrier; C60; Pentacene; Stability; Reliability
Record ID: 1062945
Full text PDF: http://arrow.monash.edu.au/hdl/1959.1/1162304


The electrical characteristics of organic semiconductors have been studied intensively ever since conductivity in organic materials has been discovered. The focus has been the understanding of factors that affect charge transport so that the performance of organic devices can be improved. This work focuses on the electrical characterization of organic semiconductors. We have investigated a series of problems related to capacitance voltage measurements and current voltage measurements of organic devices. The physics of organic electronic devices are often interpreted by invoking the concept of ― “unintentional doping”. However, the applicability and usefulness of this controversial concept is not very clear and is under much debate, recently. In this thesis, we have reevaluated the validity of this concept through careful experiments and detailed numerical simulations. Specifically, we have used Capacitance Voltage (CV) measurements of pentacene devices as a test bed to unravel the role of injecting electrodes and unintentional doping (if any). Our results have indicated that the CV of pentacene capacitors can be solely understood in terms of properties of the contact electrodes. The unintentional doping, if present, has an inconsequential role in device performance. Our conclusions have indicated that, often, an incorrect interpretation of CV results leads to unphysical values of unintentional doping. It has obvious implications in the fundamental understanding of organic semiconductor device physics, modeling, and characterization thus resolving many ambiguities in literature by providing a consistent interpretation through a coherent conceptual framework. The impact of atmospheric exposure on pentacene devices has been explained based on the contact barrier degradation at the metal-semiconductor interface. An analytical model based on the timing analysis of the capacitance frequency measurements has been proposed in order to extract the injection barrier. It was found that on atmospheric exposure, the pentacene gold injection barrier is reduced to 0.51 eV limiting the number of carriers transporting in the devices. The extracted value is close to different values reported in various photoelectron spectroscopic studies. Mechanical flexibility is one of the key advantages of organic semiconducting films in applications such as wearable-electronics or flexible displays. We have studied the electrical characteristics of C60-based top gate organic field effect transistors (OFET). The devices were characterized by curling the substrates in a concave and convex manner, to apply varying values of compressive and tensile strain, respectively. Electron mobility was found to increase with compressive strain and decrease with tensile strain. The observed strain effect was found to be strongly anisotropic with respect to the direction of the current flow. The results are quantified using the Fishchuk/Kadashchuk model for the hopping charge transport. We suggest that the observed strain dependence of the electron transport is…