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 its semiconductor material and configuration. In particular, the availability of high carrier mobility and high on/off ratio makes it possible to obtain a large current, thereby not only enabling to drive an organic EL device or the like but also enabling 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, 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 high fabrication cost. 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 difficulty in forming thin-film transistors on plastic substrates or plastic films.
Organic thin-film transistors, which make use of organic semiconductor thin films made of an organic semiconductor material, are fabricated by a vapor deposition process or a solution process, and have the possibility of lower cost, larger area and lighter weight. Further, organic semiconductor layers can be formed at a lower temperature compared with inorganic semiconductor layers, can realize cost reduction and can be formed on plastic substrates or plastic films, and therefore, can be applied to lightweight and flexible, electronic devices or the like.
Many organic semiconductor materials have been studied to date, and those making use of conjugated high-molecular compounds or low-molecular compounds as organic semiconductor layers are known. Semiconductor materials include n-type semiconductor materials and p-type semiconductor materials. 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 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 both n-type and p-type organic semiconductor materials require, such as complementary metal-oxide semiconductors (CMOS), are needed.
As n-type organic semiconductor materials, naphthalene imide, naphthalene diimide, and derivatives thereof are known to date. However, none of these organic semiconductor materials have been reported to have high performance as thin-film transistors. Further, potential utility of low-molecular compounds having the perylene skeleton inorganic thin-film transistors capable of exhibiting high performance is described (NPL 1: 0.6 cm2/Vs electron mobility)(NPL 2: 2.1 cm2/Vs electron mobility). However, these materials have large aromatic rings, respectively. Therefore, they have substantially no solubility in solvents, thereby making it difficult to fabricate thin-film transistors by a solution process.
Further, organic thin-film transistors making use of fullerene (C60) are known to exhibit n-type characteristics. Fabrication of a thin-film transistor making use of a vapor-deposited thin film of fullerene has been reported (NPL 3: 0.56 cm2/Vs electron mobility).
Also reported are thin-film transistors with organic semiconductor thin films, each of which make use of a fullerene derivative solubilized by introducing a substituent group into a fullerene and has been formed by a solution process. For example, a thin-film transistor making use of a fullerene with phenyl C61-butyric acid methyl ester introduced therein as an organic semiconductor layer is reported to have an electron mobility of 0.0035 cm2/Vs (NPL 4), and in the case of a C60 derivative with a long-chain alkyl group introduced therein, specifically C60-fused N-methylpyrrolidine-meta-C12 phenyl, an electron mobility of 0.067 cm2/Vs has been reported (PTL 1).
However, organic thin-film transistors fabricated by using fullerene or fullerene derivatives as organic semiconductor materials have a drawback in that fullerene is a costly material, organic semiconductor materials as its derivatives are also expensive, and therefore, economical devices can be hardly fabricated.
In addition, the possibility of formation of a film with a π-electron compound, which has a skeleton structure containing a π-electron ring and has perfluoroalkylphenyl groups at both ends of the skeleton, as an n-type organic semiconductor material by a solution process, a printing process such as inkjet printing or a vapor deposition process is described. A description is made about the fabrication of an organic thin-film transistor by a solution process, but no description is made of a fabrication example of an organic thin-film transistor by a solution process (PTL 2).
Also described are organic thin-film transistors fabricated by using, as n-type semiconductor materials, compounds having carbonyl groups at ends of oligothiophenes. In each of these compounds, however, the carbonyl groups are directly bound to the oligothiophene. To obtain stable performance, four or more thiophene rings have to be coupled together so that high solubility is hardly obtainable. Their use in a solution process, therefore, requires to employ precursors, leading to a problem that a step such as acid treatment is needed (PTL 3).
As has been described above, no n-type organic semiconductor material has been found yet to be economical, to have solubility in a solvent, and hence, to permit the formation of an organic thin film by a solution process. Further, no report has been made on an organic semiconductor thin film or organic thin-film transistor having high electron mobility and high on/off ratio.