There has been increasing interest in organic semiconductors owing to their potential for cost-effective electronic devices for nontraditional applications. In order to enhance the performance of such devices, it is important to understand the fundamental energy and charge transport mechanisms, and in particular, to control the chemical and structural properties that affect transport processes and dopant concentration. This research is based on organic crystalline materials, where intrinsic properties can be studied in isolation from extrinsic ones. The research also focuses on trap sites, intrinsic charge mobility, and doping processes, which are poorly understood in organic thin films at present.
For the study of trapping processes, we develop a new spectroscopic method, GAte Modulated Activation Energy Spectroscopy (GAMEaS), to measure the density of localized states in various organic crystalline systems, tetracene, pentacene, and rubrene. This is motivated by the need for a simple spectroscopy that works for high resistive materials at low temperature since the conventional spectroscopic methods, commonly used in inorganic semiconductors, are incompatible with organics, which dictate a lower-temperature approach. We find there are two distinct types of band tails, deep and shallow, depending on the purification process, which implies the localized states can be reduced by material processing. The exponential band tails of the density of localized states are characterized by their slopes, with field effect transistors of the highest mobility having a value 29eV-1 and field effect transistors of the lowest mobility having a value 13eV-1 at 300K. With the aid of GAMEaS, we estimate intrinsic mobilities of polyacene crystals, up to 200cm2/Vs, which can be reached by further reduction of trapping sites.
Along with the experimental measurement of the density of localized states, we derive a new analytical model of the channel potential energy function, which gives a clear interpretation of device operation. Based on the model, we conclude that high resistivity in the depletion region does not determine the saturation current, and thus its effect can be effectively disregarded in simplified treatment of the electronic transport in crystalline field effect devices. In addition, we find that the channel potential function is affected by the presence of localized states and the dependency on the density of states is lumped into a correction parameter, to improve the accuracy of GAMEaS analysis.
We also apply photoluminescence to the study of defects in rubrene crystals. We observe the presence of oxygen dopant energy state through photoluminescence and GAMEaS analysis, and measure its energy state at about 0.25eV above the valence band edge. In addition, we demonstrate capability of controlling carrier concentration by extrinsic oxygen without changing the mobility. Thus, we have demonstrated the ability to control carrier density in organics in a manner similar to inorganics. Finally, we show the reversibility of oxygen dopants and stability of dopant state at room temperature.
The results presented in the dissertation deal with electrical and optoelectrical properties of organic crystals. The research will help answer fundamental questions regarding charge and energy transport in organic materials.
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