Study on Developing High-performance Organic Single Crystal Devices and Alkali Metal Doped Organic Molecular Systems with Enhanced Electrical Properties

Abstract

Organic molecular material is a key electronic component in Organic light-emitting diodes (OLEDs), Organic field effect transistors (OFETs) and Organic solar cells (OSCs). These materials have several advantages such as physical flexibility, optical transparency, low cost and ease of chemical modification. Therefore, they are potentially useful as a replacement for existing silicon-based electronic materials or inorganic-based electronic materials and for application as flexible electronics in cases where existing materials are unsuitable. Despite the wide range of potential applications, there are many challenges that must be addressed before the electrical characteristics of organic molecular materials can be exploited to realize devices, except in a few cases. Before organic molecular materials can be utilized as electronic materials, it is necessary to improve not only the electrical conductivity or charge mobility, but also, to maximize their electrical characteristics. In this study, we investigated the electrical properties of organic molecules and improved their electrical characteristics by using organic molecular single crystals that can minimize structural defects and grain boundaries that interfere with charge transfer among organic molecular materials. In this dissertation, I will discuss the fabrication of organic single crystal devices that can be used to observe and understand the electrical characteristics of the constituent molecules, in addition to studies on the improvement of the electrical properties by alkali metal doping of organic single crystals. In Part І, the fundamentals of organic molecular materials and their electrical properties will be introduced. The difference between existing inorganic materials and organic molecular materials has been investigated from the perspective of the electrical characteristics and the cause of the observed difference is discussed. In addition, the electrical conductivity and charge mobility that was experimentally measured is defined and the measurement procedure is explained. The mechanisms related to the charge behavior in the organic molecular system are also described. In this regard, there is an emphasis on the importance and merits of organic molecule single crystals used as a core electronic material, the organic single crystal growth methods, and the current problems associated with the fabrication of the organic single crystal devices. The use of alkali metal doping as a way to improve the inherently poor electrical properties of organic molecular systems is described. Moreover, the key related issues are also addressed. In Part ІІ, a study on the easy and efficient growth of disk-shaped C60 single crystals at desired locations on a substrate using C60 molecules is presented. To utilize organic molecular single crystals for large-area device fabrication and device application, it is necessary to selectively grow single crystals at desired positions. Therefore, C60 seed crystals were deposited on a solid substrate in advance as a seed for crystallization via the deposition of a C60 seed thin film. Single crystals of C60 were then grown at the desired position using a drop drying method. Subsequently, field effect transistors were fabricated using these crystals to measure the charge carrier mobility and semiconductor characteristics. Potassium doping was also performed to confirm the superconducting characteristics in K3C60 single crystal at positions of interest. In Part ІІІ, the focus in on the fabrication of organic single crystal devices using exfoliated graphite as an electrode that is well suited to use with organic molecule single crystals with various structures, sizes, and shapes. In addition, exfoliated graphite has excellent physical flexibility and a work function that is well-matched with the molecular orbital energy level of most organic molecules. This facilitates good physical contact with organic single crystals and effectively lowers the charge injection barrier to realize excellent device performance. The fabricated devices using exfoliated graphite exhibit almost similar or slightly improved performance compared to devices made using conventional metal deposition. However, it is not only time and economically efficient relative to conventional methods, but also has the advantage of controlling the work function by increasing the -OH group on the surface of graphite by using oxygen plasma for exfoliation, which is a promising electrode that is suitable for use in organic single crystal devices. Part ІV describes the development of an alkali metal doping system that can efficiently maximize the electrical characteristics of organic single crystals in which the inherent electrical properties (electrical conductivity or charge mobility) are lower than those of existing inorganic or silicon-based devices. Secondary thermal activation was induced by increasing the temperature of the sample during the doping process to improve the doping of the alkali metal into densely arranged organic single crystals. This method can induce efficient alkali metal diffusion into the crystal and synthesize uniformly doped alkali metal-organic single crystal samples. As a result, K2picene and Kxcoronene single crystals could be synthesized using this method; K2picene has semiconducting properties with a small bandgap. Further analysis of the doped K2picene revealed that the crystal structure was changed without decomposition of the picene molecules. Part V introduces a study on the synthesis of materials with new electrical properties by utilizing an alkali metal doping system that is suitable for use with the organic single crystals developed in Part ІV. To compare the electrical characteristics according to the organic molecular structure, single crystals of PCBM(Phenyl-C61-butyric acid methyl ester)with a structure similar to that of the conventional C60 fullerene molecule were grown and alkali metal doping was performed. The observed changes in the electrical conductivity and optical properties associated with the PCBM doping process are very similar to those of the conventional C60 case and a phase change was observed for alkali metal doping. The electrical properties of KxPCBM doped to a specific state were measured and the electrical conductivity was observed to greatly improve with semiconducting behavior.