The organic semiconductor material refers to an organic material exhibiting semiconductor properties which change the electrical conductivity according to the applied electric field, as compared with most of the conventionally known organic materials having the properties of electrical insulating materials which do not so. Various electric/electronic devices to which these organic semiconductor materials are applied are known, and examples thereof include an organic light emitting diode, an organic solar cell, and an organic thin film transistor.
The organic thin film transistor refers to a thin film transistor fabricated using a channel comprising an organic material having semiconductor properties. The organic thin film transistors typically consist of electrodes comprising a source, a drain, and a gate, an organic semiconductor channel, a gate insulating layer, and a substrate. These organic thin film transistors are advantageous in that they can be applied to a large area at low cost by using a solution process without a vacuum deposition process, and can also be used for flexible devices which are bendable, as compared with inorganic transistors using silicone, or the like.
The performance of the organic thin film transistors are determined, based on various factors including how effectively the potentials of the interface of the organic semiconductor are changed according to the gate electrode voltages, whether there exists any energy barrier blocking the carrier from being injected from a source electrode to an organic semiconductor, how many scattering elements exist while the carrier is transmitted along the interface of the organic semiconductor, and whether the gate insulating layer has sufficient insulating properties for preventing the carrier transmitted from the organic semiconductor from leaking to the gate electrode. Accordingly, in the development of the organic thin film transistors, the development of a gate insulating layer having excellent characteristics is also needed, as well as the development of a high-performance organic semiconductor channel material.
Conventionally known materials of a gate insulating layer for an organic thin film transistor can be divided into two types, that is, inorganic materials such as silicon oxide, silicon nitride, and aluminum oxide (Al2O3), and organic materials such as polyvinylphenol [Klauk et al., Journal of Applied Physics 92, 5259 (2002)], polyvinylalcohol [Schroeder et al., Applied Physics Letters 83, 3201 (2003)], polymethylmethacrylate [Ficker et al., Journal of Applied Physics 94, 2638 (2003)], and polyimide (Korean Patent Application Publication No. 2005-0081824).
If an inorganic insulating layer is used for the organic thin film transistor, the inorganic insulating layer should be prepared using a vacuum deposition process as in the conventional silicone semiconductors. As a result, it is disadvantageous in terms of the process and cost, and it is difficult to apply it for a large-area substrate. Further, it has low flexibility derived from its intrinsic property as the inorganic material, and relatively high process temperature. Accordingly, it is difficult to apply it for a flexible device such as a plastic substrate. Further, the inorganic insulating layer generally has a relatively low affinity with an organic semiconductor channel, which leads to undesirable performance of the organic thin film transistor device.
On the other hand, if an organic insulating layer is used in the organic thin film transistor, an insulating layer film can be prepared using a solution process. Thus, it has an advantage in that it can be applied on a large area at low cost, and it can be easily applied for a flexible device using a relevant low-temperature process and flexibility of the organic material. Further, the organic insulating layer is also superior to a predetermined kind of the inorganic insulating layer in that insulating layers having various characteristics can be prepared by introducing various designs and modifications of the chemical structures of the organic materials.
However, the materials for the organic gate insulating layer have been limited in their chemical structures and types until now. Accordingly, the development of the materials for the novel organic gate insulating layer is very important for realization of an organic thin film transistor. A number of insulating organic materials have been known, but there are still few insulating organic materials which are suitable for forming a gate insulating layer of an organic thin film transistor.