Thin film transistors (TFTs) are widely used as a switching element in electronics, for example, in active-matrix liquid-crystal displays, smart cards, and a variety of other electronic devices and components thereof. The thin film transistor (TFT) is an example of a field effect transistor (FET). The best-known example of an FET is the MOSFET (Metal-Oxide-Semiconductor-FET), today's conventional switching element for high-speed applications. Presently, most thin film devices are made using amorphous silicon as the semiconductor. Amorphous silicon is a less expensive alternative to crystalline silicon. This fact is especially important for reducing the cost of transistors in large-area applications. Application of amorphous silicon is limited to low speed devices, however, since its maximum mobility (0.5-1.0 cm2/Vsec) is about a thousand times smaller than that of crystalline silicon.
Although amorphous silicon is less expensive than highly crystalline silicon for use in TFTs, amorphous silicon still has its drawbacks. The deposition of amorphous silicon, during the manufacture of transistors, requires relatively costly processes, such as plasma enhanced chemical vapor deposition and high temperatures (about 360° C.) to achieve the electrical characteristics sufficient for display applications. Such high processing temperatures disallow the use of substrates, for deposition, made of certain plastics that might otherwise be desirable for use in applications such as flexible displays.
In the past decade, organic materials have received attention as a potential alternative to inorganic materials such as amorphous silicon for use in semiconductor channels of TFTs. Organic semiconductor materials are simpler to process, especially those that are soluble in organic solvents and, therefore, capable of being applied to large areas by far less expensive processes, such as spin-coating, dip-coating and microcontact printing. Furthermore, organic materials may be deposited at lower temperatures, opening up a wider range of substrate materials, including plastics, for flexible electronic devices. Accordingly, thin film transistors made of organic materials can be viewed as a potential key technology for plastic circuitry or devices where ease of fabrication and/or moderate operating temperatures are important considerations and/or mechanical flexibility of the product is desired.
Organic semiconductor materials can be used in TFTs to provide the switching and/or logic elements in electronic components, many of which require significant mobilities, well above 0.01 cm2/Vs, and current on/off ratios (hereinafter referred to as “on/off ratios”) greater than 1000. Organic TFTs having such properties are capable of use for electronic applications such as pixel drivers for displays, identification tags, portable computers, pagers, memory elements in transaction carts, electronic signs, etc.
Organic materials for use as potential semiconductor channels in TFTs are disclosed, for example, in U.S. Pat. No. 5,347,144 to Garnier et al., entitled “Thin-Layer Field-Effect Transistors with MIS Structure Whose Insulator and Semiconductors Are Made of Organic Materials.”
A variety of materials have been considered as organic semiconductors, with the most common being fused acenes such as tetracene and pentacene, oligomeric materials containing thiophene or fluorene units, and polymeric materials like regioregular poly(3-alkylthiophene). While polymers may be coated from solution, device performance is poor when compared to well-ordered thin films prepared by high vacuum vapor deposition.
Amongst the acene class of organic semiconductors, pentacene, having five fused benzene rings, is the mainstay of this class and positive charge-carrier mobilities (p-type) have been reported for pentacene-based transistors as high as 3.3 cm2 V−1 s−1 (Kelley, T. W.; Boardman, L. D.; Dunbar, T. D.; Muyres, D. V.; Pellerite, M.; Smith, T. P., J. Phys. Chem. B 2003, 107, 5877-5881), on/off current ratios greater than 108 (Knipp, D.; Street, R. A.; Völkel, A.; Ho, J., J Appl. Phys. 2003, 93, 347-355), and sub-threshold voltages of less than 0.5 V (Klauk, H.; Halik, M.; Zschieschang, U.; Schmid, G.; Radlik, W.; Weber, W., J. Appl. Phys. 2002, 92, 5259-5263). These values are comparable or superior to those of amorphous silicon-based devices.
Pentacene has been extensively probed and modified in a search for improved performance, in particular for solubility, for organizational anchoring groups and for electronic modifications. Enhanced solubility of pentacene has been achieved by adding labile Diels-Alder adducts to the central ring (U.S. Patent Publ. No. 2003/0136964 A1) and by the addition of non-labile solubilizing groups (U.S. Pat. No. 6,690,029 to Anthony et al., issued Feb. 10, 2004). These and other strategies can enhance the solubility of pentacene. However, creating an ordered film from a disordered solution or from a vapor phase remains a challenge. C. Nuckolls et al. (J. Am. Chem. Soc. 2004, 126, 15048-15050) recognized this dilemma and sought to functionalize one end of tetracene with methoxy or hydroxyl groups. These asymmetrically placed anchoring groups were intended to organize the molecule at a dielectric surface via hydrogen-bonding attraction. Other types of surface-molecule interactions beyond hydrogen bonding could be imagined for asymmetric type molecules.
The electronic and chemical properties (band gap, HOMO-LUMO levels, oxidation potential) of the pentacene structure have been altered by, for example, replacing both of the terminal rings with thiophene rings (J. G. Laquindanum, H. E. Katz, A. J. Lovinger, J. Am. Chem. Soc. 1998, 120, 664-672). However, the placement of thiophenes at both ends of the acene inevitably leads to a cis-trans mixture. In addition, the unique directing effect of a single ended asymmetric structure is lost with a symmetrical approach. Nevertheless, there are several areas where an alternative semiconductor material could offer improvements. The device architecture, choice of materials and substrate roughness all affect device performance. In pentacene-based devices, these variations have, in part, been attributed to the existence of several polymorphs (Mattheus, C. C.; de Wijs, G. A.; de Groot, R. A.; Palstra, T. T., M. J. Am. Chem. Soc. 2003, 125, 6323-6330). The alignment or structural order of the pentacene molecules differs for each polymorph or crystallographic phase, and this structural order determines the electronic properties of the device. The crystallographic phase adopted by pentacene depends on the process and conditions under which the crystals are formed. The thin film form of pentacene can be converted to the bulk phase by exposure to solvents such as isopropanol, acetone or ethanol. (See, for example, Gundlach et al., Appl. Phys. Lett., 2000, 74(22) 3302).
Additionally, the long-term oxidative and thermal stability of pentacene is unknown, as is the lifetime of pentacene-based semiconductor devices. The ease of synthesis and purification is another factor that must be considered in regard to the utility of an organic semiconductor. In particular, soluble materials may be purified by recrystallization or chromatography, familiar techniques that are not available for fused acenes like pentacene. The ability to construct devices using solution-processing techniques is potentially key for realizing a low cost manufacturing process. And lastly, it is likely that a variety of organic semiconductor materials possessing a range of physical and chemical properties may be required for specific applications.
Pyrenes for use as potential semiconductor materials light emitting transistor devices are disclosed, in WO 2006057325 to Oyamada et al., entitled “Pyrene compound and, utilizing the same, light emitting transistor device and electroluminescence device.”
Thienyl substituted pyrene has been used as the active p-type material in thin film transistors but shows a low mobility of 0.0037 cm2/Vs, and very high threshold voltage (Zhang, et al. in Chemical Communications, 2005, 7, 755-757). Vacuum evaporated thin films of pyrene end-substituted with oligothiophene has been used as the active layer in field effect transistors but show only modest hole mobility of 10−3 cm2/Vs (in Journal of Materials Chemistry (2006), 16(24), 2380-2386).
Thin film transistors fabricated with vapor-deposited films of 1-imino nitroxide pyrene have been demonstrated to show p-type characteristics, with mobility up to 0.1 cm2/Vs but show a poor current on/off ratio of 104 (in Journal Am. Chem. Soc. (2006), 128(40), 13058-13059). Furthermore, this compound is an organic radical which makes it chemically unstable.
In view of the foregoing, we recognize there is a need for new organic semiconductors that are chemically stable and provide stable and reproducible electrical characteristics. The present invention discloses heteropyrene compounds that are useful as organic semiconductors. The compounds of the present invention are reliably prepared, advantageously purified by either gradient sublimation, and/or recrystallization, and/or chromatography, depending on the specific materials.