The progress of a high-level information-oriented society in recent years is remarkable, and the development of digital technologies has led to the penetration of computers and communication technologies such as computer networks in everyday life. Keeping in step with this penetration, flat-screen TV sets and notebook-size personal computers have become increasingly popular, resulting in an increasing demand for displays such as liquid crystal displays, organic EL displays and electronic paper displays. Especially in recent years, there is an outstanding move toward larger displays of higher definition, and therefore, it is required to assemble an ever increasing large number of field-effect transistors corresponding to the number of pixels. In a liquid crystal display, the liquid crystal can be driven by providing the respective pixels with field-effect transistors as active elements and performing ON/OFF control of signals.
As field-effect transistors for use as active elements, thin-film transistors can be used. The performance of a thin-film transistor is determined by the kind and structure of its semiconductor material. In particular, the availability of high carrier mobility and high ON/OFF ratio makes it possible to obtain a large current. The availability of such a large current enables not only to drive an organic EL device or the like with a higher degree of accuracy but also to miniaturize the thin-film transistor and to provide an improved contrast.
For thin-film transistors useful as active elements, a silicon-based semiconductor material such as amorphous silicon or polysilicon can be used. A thin-film transistor is fabricated by forming such a silicon-based semiconductor material in a multilayered structure such that source, drain and gate electrodes are successively formed on a substrate.
For the fabrication of thin-film transistors making use of a silicon-based semiconductor material, however, large-scale and costly fabrication facilities are needed, and because of the use of photolithography, many process steps have to be gone through, resulting in an economical problem that the fabrication cost becomes higher. Furthermore, the fabrication requires high temperatures of from 300° C. to 500° C. or even higher, which lead not only to still higher fabrication cost but also to a technical problem that thin-film transistors can be hardly formed on plastic substrates or flexible plastic films.
On the other hand, organic thin-film transistors, which make use of organic semiconductor thin films comprised of an organic semiconductor material, are fabricated by various film-forming processes such as vapor deposition, printing and coating, and have the possibility of lower cost, larger area and lighter weight. Further, organic semiconductor thin films can be formed at a lower temperature compared with inorganic semiconductor layers, can realize cost reduction and can be formed on plastic substrates or flexible plastic films, and therefore, can be applied to lightweight and flexible, electronic devices or the like.
Many organic semiconductor materials have, therefore, been studied to date, and those making use of conjugated high-molecular compounds or low-molecular compounds as organic semiconductor thin films are known. Semiconductor materials include n-type semiconductor materials and p-type semiconductor materials, and there is a long-awaited desire for the development of n-type semiconductor materials and p-type semiconductor materials that exhibit still better transistor characteristics or the like. In an n-type semiconductor material, electrons move as main carriers to produce an electric current. In a p-type semiconductor material, on the other hand, holes move as main carriers to produce an electric current.
As organic semiconductor materials that exhibit high performance as organic thin-film transistors, pentacene materials and thiophene materials are known. These materials are semiconductor materials that exhibit p-type characteristics. However, reports on n-type organic semiconductor materials of high performance are limited. There is, accordingly, an outstanding desire for the development of n-type organic semiconductor materials of high performance. For further developments of organic electronics, lower power consumption, simpler circuits and the like are essential, and organic complementary MOS circuits which require both n-type and p-type organic semiconductor materials, such as complementary metal-oxide semiconductors (CMOS), are needed. There is, accordingly, an ever-increasing desire for the development of n-type organic semiconductor materials of high performance.
As n-type organic semiconductor materials, naphthaleneimide, naphthalenediimide, and derivatives thereof are known to date. However, none of these n-type organic semiconductor materials have been reported to have high performance as thin-film transistors. Further, Non-patent Document 1 describes the potential utility of low-molecular compounds, which have the perylene skeleton, in organic thin-film transistors capable of exhibiting high performance (Non-patent Document 1: 1.7 cm2/Vs electron mobility).
As to organic thin-film transistors making use of organic semiconductor films comprised of perylene tetracarboxylic acid derivatives and formed by vapor deposition, there are, for example, disclosures as will be described next. Patent Document 1 describes that a thin film transistor comprised of an organic semiconductor material layer, which contains a perylene tetracarboxylic diimide derivative having a carbocyclic or heterocyclic aromatic ring system substituted with fluorine-containing groups, has a mobility of from 0.05 to 0.2 cm2/Vs and an ON/OFF ratio of from 104 to 105 and exhibits stability in air and excellent reproducibility. Patent Document 2 describes that a thin film transistor comprised of an organic semiconductor material layer, which contains a perylene tetracarboxylic diimide derivative having substituted or unsubstituted phenylalkyl groups, has a mobility of from 0.04 to 0.7 cm2/Vs and an ON/OFF ratio of from 104 to 105 and exhibits stability in air and excellent reproducibility.
On the other hand, organic semiconductor thin films formed by the above-mentioned various film-forming processes generally have a polycrystalline structure formed of microcrystals aggregated together. Such organic semiconductor thin films each contain numerous grain boundaries (contacts between microcrystals), deficiencies and defects whichever material is used. These crystal grain boundaries, deficiencies and defects inhibit the transport of charges. These film-forming processes are, therefore, accompanied by a fabrication problem that they can hardly form an organic semiconductor thin film uniformly over a large area and are practically difficult to fabricate organic semiconductor devices having stable device performance. Organic semiconductor thin films formed especially by vapor deposition out of such various film-forming processes have a strong tendency to include crystal grain boundaries, deficiencies and defects.
To overcome such problems, the present inventors have already made a proposal as will be described below. Described specifically, the present inventors have solved the above-described problems by providing an organic thin-film transistor having an organic semiconductor thin film of N,N′-ditridecyl-3,4:9,10-perylene dicarboxylic acid imide, said organic semiconductor thin film having been formed by vapor deposition and having been subjected to heat treatment around a temperature at which N,N′-ditridecyl-3,4:9,10-perylene dicarboxylic acid imide presents a smectic liquid crystal phase (Non-patent Document 2:2.1 cm2/Vs electron mobility). It is, therefore, possible to decrease the above-described crystal grain boundaries, deficiencies and defects and to form a uniform film by including the heat treatment and processing through the liquid crystal state.