High Mobility Organic Field-Effect Transistor Based on Hexamethylenetetrathiafulvalene with Organic Metal Electrodes Yukihiro Takahashi, †, * Tatsuo Hasegawa, †, * Sachio Horiuchi, † Reiji Kumai, † Yoshinori Tokura, †,‡ and Gunji Saito § Correlated Electron Research Center (CERC), National AdVanced Institute of Industrial Science and Technology (AIST), Tsukuba 305-8562, Department of Applied Physics, The UniVersity of Tokyo, Tokyo 113-8656, and DiVision of Chemistry, Kyoto UniVersity, Kyoto, 606-8502, Japan ReceiVed September 19, 2007 In recent years, a large number of molecular materials with relatively high carrier mobilities have been reported for the use as channels of organic field-effect transistors (OFETs), which are envisioned as key components of low-cost, large- area, and flexible electronic devices. 1–7 Attention is being focused on modifications of polycyclic aromatic hydrocar- bons like pentacene to realize a high performance and easy processability. 2–4 Other potential candidates are tetrathiaful- valene (TTF) analogues, which are well-known as electron- donor molecules in organic charge-transfer (CT) complexes. 8 The ability of these compounds to form a wide variety of organic metals and superconductors should permit their use in high-performance, single-component semiconducting ma- terials. In fact, a mobility of about 0.1–1.4 cm 2 /Vs has been reported for some TTF analogues. 6,7 In addition, the higher solubility of these molecules, compared with polyacenes, is advantageous as it permits low-cost solution processing. In this communication, we report that the OFETs based on solution-grown as well as vapor-transport grown hexam- ethylenetetrathiafulvalene 1 (HMTTF; C 12 H 12 S 4 ) 9 single crystals exhibit a field-effect mobility exceeding 7–10 cm 2 / Vs. To achieve this high device performance we found it necessary to optimize carrier injections at the source/ drain contacts by using TTF-TCNQ thin-film electrodes (TCNQ ) tetracyanoquinodimethane). 10 Yellow-colored and elongated-block single crystals of HMTTF with a typical size of 0.8 × 0.2 × 0.2 mm 3 were obtained both by vapor transport with N 2 gas and by recrystallization from chlorobenzene solution in darkness. In the former process, care was taken to halt the crystal growth after about 2 days, because the color of the crystal surface turned into dark brown possibly as a result of thermal degradation of the molecules in longer runs of crystal growth. In the latter process, on the other hand, crystals were obtained over a longer term of several weeks at room temperature. The solution- as well as vapor-transport grown products formed the identical monoclinic crystal structure with space group P2 1 /c and unit cell parameters of a ) 6.376(6) Å, b ) 14.54(1) Å, c ) 12.92(1) Å, and  ) 94.689(16)° (Z ) 4). The packing motif is shown in Figure 1. An intermo- lecular side-by-side arrangement along the a-axis affords large π–π interactionswithcloseS–Scontactsof3.545(3)–3.647(3) Å. On the other hand, molecules are stacked along the b-axis in a brickwork arrangement in which molecules are slipped by half their length along the molecular long axes. For crystallographically independent three intermolecular contacts shown in Figure 1, the transfer integrals were estimated by extended Hückel molecular orbital calculations; 8 t 1 ) 0.1151 eV (side-by-side), t 2 ) –0.0113 eV (slipped stack), and t 3 ) 0.0176 eV (slipped stack). Single-crystal OFETs with parylene C gate dielectric layers were fabricated as reported previously. 10 The dielectric thickness we used is 1.0 µm with gate capacitance of about 1.90–2.12 nF/cm 2 . We used thermally evaporated Au, Ag, or TTF-TCNQ metallic thin films as the source/drain electrodes. The sheet resistance of the TTF-TCNQ films is 1–1.5 kΩ at the thickness of about 300 nm. Typical channel length L and width W is about 100 µm. All the properties of the transistors were measured with a source/drain current along the a-axes which correspond to the direction of the longest dimension of the crystal. Transfer characteristics of the devices are shown altogether in Figure 2 in linear scale. All the devices exhibited p-type feature, the size of which depended significantly on the kinds of electrodes. The mobility of the device was calculated by using the standard formula in the linear region; µ lin ) (dI D / dV G )[L/(WC i V D )], where I D , V D , V G , and C i are drain current, † Advanced Institute of Industrial Science and Technology (AIST). ‡ The University of Tokyo. § Kyoto University. (1) Würthner, F.; Schmidt, R. ChemPhysChem 2006, 7, 793–797. (2) Menard, E.; Podzorov, V.; Hur, S.-H.; Gaur, A.; Gershenson, M. E.; Rogers, J. A. AdV. Mater. 2004, 16, 2097–2101. (3) Payne, M. M.; Parkin, S. R.; Anthony, J. E.; Kuo, C.-C.; Jackson, T. N. J. Am. Chem. Soc. 2005, 127, 4986–4987. (4) Schmidt, R.; Gottling, S.; Leusser, D.; Stalke, D.; Krause, A.; Würthner, F. J. Mater. Chem. 2006, 16, 3708–3714. (5) Yamamoto, T.; Takimiya, K. J. Am. Chem. Soc. 2007, 129, 2224– 2225. (6) (a) Mas-Torrent, M.; Durkut, M.; Hadley, P.; Ribas, X.; Rovira, C. J. Am. Chem. 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