Understanding and optimization of electrical characteristics of organic devices
thesisposted on 01.03.2017 by Nigam, Akash
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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 dominated by a change in the effective charge hopping distance over the grain boundaries in polycrystalline C60 films. Most studies on charge transport are focused around low temperature electrical measurements. We have electrically characterized pentacene based OFETs between the temperature ranges of 25 °C to 190 °C in ambient conditions. Material characterization studies such as X-ray photoelectron (XPS), X-ray diffraction (XRD) and atomic force microscopy (AFM) prove the stability of pentacene as a semiconductor in ambient conditions at elevated temperatures. The crystallinity of pentacene films is retained up to 110 °C; its phase changes around 150 °C. Charge transport studies reveal a strong dependence of mobility on the gate field and interface states. The degradation of device parameters is attributed to the deterioration of dielectric and phase transformation in pentacene at higher temperatures. At an above-room-temperature range, mobility is found to be thermally activated in the presence of traps, whereas, for a trap-free interface, it is temperature independent. These results validate the performance and stability of organic devices in practical environmental conditions. The different experimental works reported in this thesis have been wrapped under the thesis title, ― ”Understanding and Optimization of Electrical Characteristics of Organic Devices”. Thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy of the Indian Institute of Technology Bombay, India and Monash University, Australia.